The most commonly used reference material for I/Ca analyses is coral JCp-1 with reported values ranging between $3.70 \pm 0.27 \mu mol mol^{-1}$ but with values as high as $4.33 \pm 0.36 \mu mol mol^{-1}$ (compiled in Lu et al., 2020), suggesting the potential for some heterogeneity and/or the need for a multi-lab intercalibration. A limestone standard (ECRM752-1) that is commonly used for Mg/Ca for inter-laboratory comparison may have a similar potential for Mn/Ca, although extensive datasets are still missing (Greaves et al., 2008).

Techniques with higher spatial resolution, such as nanoSIMS and SIMS, have also been used to understand I/Ca distribution in foraminiferal tests (Glock et al., 2016; Glock et al., 2019a), but there has been limited subsequent application. For Mn, in situ measurements include laser ablation (LA)-ICP-MS and SIMS for quantitative measurements, while µXRF and conventional EPMA are usually used as semi-quantitative or relative measurements. For such high-spatial-resolution measurements, cleaning processes are minimal. Ca⁴³ is used as the internal standard and the NIST SRM 610 glass standard as the external calibration material (using established values from Jochum et al., 2011) for LA-ICP-MS. NIST SRM 612 glass (Jochum et al., 2011) and calcium carbonate pellets of MACS-3 (Jochum et al., 2012), JCp1, JCt1, and now NFHS (Boer et al., 2022) can be used as quality control material. For U, Skinner et al. (2019) showed that core-top and downcore measurements of U/Ca using LA-ICP-MS are also possible. We note that high-resolution ICP-MS is not required, and a more accessible quadrupole ICP-MS may also be used. However, due to the low concentration of U in foraminiferal coatings (compared to, for example, Mg or Sr), ICP-OES is not sensitive enough to carry out U/Ca measurements.

## 8.4 Future directions

There are important knowledge gaps for each of the proxies discussed above. Addressing these gaps requires an overview of water column and pore water geochemistry obtained through oceanographic and sediment and pore water sampling. Though multi-element applications show potential (Hu et al., 2023), cross-proxy calibration comparisons of I, Mn, and U in sediments or water column transects are currently lacking. We also note that there are gaps in the calibration of the various element/Ca ratios relative to oxygen. For planktic and benthic foraminifera, I/Ca ratio gaps are found around oxygen levels between 80 and $140 \mu mol kg^{-1}$, respectively (Fig. 9). Mn/Ca measurements from oceanic settings are concentrated at oxygen levels below $70 \mu mol kg^{-1}$ (Fig. 9c, d). The U/Ca proxy is specific to coatings and is thus bottom-water-specific, but there are only a few calibrations in low-oxygen environments (Fig. 9e). Finally, the application of Mn/Ca to planktic foraminifera has hardly been explored, but some recent studies show promise that

Mn/Ca could record redox conditions in OMZs (Vedamati et al., 2015; Davis et al., 2023a).

It is important to understand the impacts that vital effects may exert on element/Ca ratios. Controlled culture experiments allow for the assessment of direct relationships between foraminiferal element/Ca (e.g. I/Ca and Mn/Ca) and ambient seawater and pore water elemental concentrations. Planktic foraminifera migrate through the water column during their life cycle (Schiebel and Hemleben, 2017). As such, the element/Ca signal represents an integrated signal from environments with potentially variable elemental compositions. In order to constrain pathways of element incorporation into planktic foraminiferal calcite, we need to study the life cycle of living planktic foraminifera (i.e. from plankton tows), settling dead planktic foraminifera (i.e. from sediment traps), and planktic foraminifera in sediments (e.g. from core-top samples), all across the variable ambient element concentrations. This integrated framework allows us to track the proxy from seawater elemental values to the fossil record, thus integrating vital effects and diagenesis. Benthic foraminifera can actively migrate between the top of sediments and different redox zones within the sediments. To monitor the effect of their migratory behaviour on element incorporation, controlled culture experiments are needed, tracking the benthic foraminifera depth habitat, calcification, and bottom and pore water chemistry.

On a final note, analysis of successive chambers from single foraminiferal shells has shown promise for Mn/Ca when using high-resolution techniques such as LA-ICP-MS (Guo et al., 2019; Petersen et al., 2018; Brinkmann et al., 2023). This approach may be further developed for I/Ca, U/Ca, and other redox proxies to reconstruct short-term (seasonal to annual, since foraminifera can live up to 1–2 years) variations in paleo-records.

# 9 Foraminiferal assemblages

## 9.1 Introduction

Like all organisms, foraminifera thrive when environmental conditions match their requirements. Among the environmental parameters that drive the presence and abundance of benthic foraminifera species, the most important are export productivity (supply of organic matter and nutrients to the seafloor) and bottom water oxygenation (Jorissen et al., 1995). The tolerance of specific species to different oxygen levels has made benthic foraminifera assemblages an especially useful tool for qualitative and quantitative reconstructions of past bottom water oxygenation variability, with planktic foraminifera also emerging as a tool for understanding water column oxygenation (Sen Gupta and Machain-Castillo, 1993; Kaiho, 1994; Alve and Bernhard, 1995; Bernhard et al., 1997; Baas et al., 1998; Nordberg et al., 2000; Jannink, 2001; Schmiedl et al., 2003; Leiter and Altenbach,
<table><caption>**Table 3.** Summary of cleaning methods and analytical techniques for foraminifera I / Ca, Mn / C, and U / Ca analyses.</caption><thead><tr><th>Trace element</th><th>Cleaning method</th><th>Instrument for measurement</th><th>Lattice bound/coating</th><th>References</th></tr></thead><tbody><tr><td>I / Ca</td><td>Clay and organic matter</td><td>Solution ICP-MS</td><td>Lattice bound</td><td>Lu et al. (2010), Glock et al. (2014), Winkelbauer et al. (2021), Cook et al. (2022), Zhou et al. (2022)</td></tr><tr><td>Mn / Ca</td><td>Clay and organic matter</td><td>Solution ICP-MS/ICP-OES, LA-ICP-MS</td><td>Lattice bound</td><td>Boyle and Keigwin (1985), Groeneveld and Filipsson (2013), Ni et al. (2020), Brinkmann et al. (2023)</td></tr><tr><td>U / Ca (Mn / Ca) (U / Mn)</td><td>Clay only</td><td>Solution ICP-MS/ LA-ICP-MS (coatings)</td><td>Coating</td><td>Boiteau et al. (2012), Gottschalk et al. (2016a, 2020a), Skinner et al. (2019), Chen et al. (2017), Umling and Thunell (2018)</td></tr></tbody></table>

2010; Ohkushi et al., 2013; Tetard et al., 2017; Sharon et al., 2021; Erdem et al., 2020; Tetard et al., 2021a, 2024).

## 9.2 Historical perspective on foraminifera assemblages

The use of benthic foraminiferal assemblages as indicators of environmental conditions began with their application as proxies for paleobathymetry (Bandy, 1953). Since then, numerous studies have established connections between the distribution of benthic foraminifera and various environmental parameters, with particular emphasis on oxygenation in open ocean settings (e.g. Phleger and Soutar, 1973; Lutze and Coulbourn, 1984; Mackensen and Douglas, 1989; Sen Gupta and Machain-Castillo, 1993; Bernhard et al., 1997; Den Dulk et al., 1998; Jannink et al., 1998; Levin et al., 2002; Schumacher et al., 2007; Cardich et al., 2012, 2015; Mallon et al., 2012; Caulle et al., 2014). Certain species inhabit regions characterized by sustained low-oxygen concentrations, such as the OMZs in the Pacific Ocean (Smith, 1964; Bernhard et al., 1997; Bernhard and Sen Gupta, 1999; Cardich et al., 2012; 2015; Mallon et al., 2012; Erdem et al., 2020; Castillo et al., 2021; Tavera Martínez et al., 2022) and Arabian Sea (Hermelin and Shimmield, 1990; Caulle et al., 2015; Gooday et al., 2000; Jannink et al., 1998), as well as in restricted basins and fjords (Bernhard and Alve, 1996; Leiter and Altenbach, 2010; Nordberg et al., 2000; Bouchet et al., 2012; Fontanier et al., 2014). Culture studies have confirmed the ability of certain species of both benthic and planktic foraminifera to survive (Alve and Bernhard, 1995; Moodley and Hess, 1992; Moodley et al., 1997; Geslin et al., 2004, 2014; Bernhard and Alve, 1996), calcify (Kuroyanagi et al., 2013; Nardelli et al., 2014), and thrive (Bernhard, 1993; Enge et al., 2016; Orsi et al., 2020) under very low oxygen and even euxinic conditions (Fig. 10).

Oxygen-depleted marine environments are often characterized by a high organic carbon rain rate, and these two factors together influence the habitat depth, abundance, and assemblage composition of benthic foraminifera (e.g. Lutze and Coulbourn, 1984; Corliss and Emerson, 1990; Loubere,

1994; Altenbach et al., 1999; Gooday and Rathburn, 1999; Geslin et al., 2004). The Trophic-Oxygen (or TROX) model was the first to consider food and oxygen availability to be key factors in determining benthic foraminiferal assemblages and microhabitats (Barmawidjaja et al., 1995; Jorissen et al., 1995), though other considerations, such as food quality and nitrate concentrations, are also predictive (Jorissen et al., 2022). The oxygen concentration in pore water is influenced not only by bottom water oxygen concentration but also by respiration in pore waters, which relates to the organic matter content in the sediments. Thus, bottom water oxygen concentration is a key driver of infaunal foraminifera living deeper in the sediments (Jorissen et al., 1995), particularly because several infaunal species have been shown to denitrify (a form of respiration without oxygen; Risgaard-Petersen et al., 2006). A more detailed discussion of this phenomenon follows.

Ecological studies have identified benthic foraminiferal species that can serve as indicator taxa of low-oxygen conditions and the association of specific species with quantitative ranges of bottom water oxygen. Several studies have developed indices to reconstruct paleo-oxygen levels based on benthic foraminiferal assemblages (e.g. Kaiho, 1994, 1999; Jannink, 2001; Schmiedl et al., 2003; Jorissen et al., 2007; Ohkushi et al., 2013; Tetard et al., 2017; Sharon et al., 2021; Erdem et al., 2020; Tetard et al., 2021a, Kranner et al., 2022, Tetard et al., 2024), which are summarized here (Fig. 10). The benthic foraminiferal oxygen index (BFOI) developed by Kaiho (1994, 1999) considers the relative proportion of taxa indicative of low-oxygen conditions for bottom or pore water compared to the total fauna. Another index developed by Jannink (2001) focuses on the presence of oxyphilic species, which consistently inhabit the top centimetre of sediment. Schmiedl et al. (2003) proposed a method based on a combination of the relative proportion of low-oxygen-tolerant species and a diversity index. Ohkushi et al. (2013) developed a new index using thresholds of oxygen tolerance for different species and paleoenvironment assessments through principal component analysis (PCA) of assemblages.
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Timeline of key discoveries and advances regarding physiology, ecology, and proxy development based on benthic and planktic foraminifera since the 1960s, including field and laboratory studies, oxygen indices, and review papers. (1) Smith (1964), (2) Phleger and Soutar (1973), (3) Corliss (1985), (4) Bernhard (1992), (5) Moodley and Hess (1992), (6) Sen Gupta and Machain-Castillo (1993), (7) Bernhard (1993), (8) Kaiho (1994), (9) Jorissen et al. (1995), (10) Bernhard and Alve (1996), (11) Bernhard et al. (1997), (12) Kaiho (1999), (13) Bernhard et al. (2000), (14) Alve and Murray (2001), (15) Schmiedl et al. (2003), (16) Fontanier et al. (2005), (17) Risgaard-Petersen et al. (2006), (18) Schumacher et al. (2007), (19) Jorissen et al. (2007), (20) Piña-Ochoa et al. (2010a), (21) Koho and Piña-Ochoa (2012), (22) Ohkushi et al. (2013), (23) Nardelli et al. (2014), (24) Caulle et al. (2015), (25) Orsi et al. (2020), (26) Erdem et al. (2020), (27) Sharon et al. (2021), (28) Tetard et al. (2021a,b), (29) Davis et al. (2021), (30) Kranner et al. (2022), and (31) this review.</figcaption></figure>

Tetard et al. (2017, 2021a) also used PCA and calibrated transfer functions to semi-quantitatively reconstruct benthic oxygen offshore California during the last 80 000 years. Sharon et al. (2021) presented downcore applications using a modification of the Ohkushi et al. (2013) index, updated through comparison with modern assemblages from core-top studies and cross-checked with redox-sensitive trace metals. Erdem et al. (2020) employed a transfer function approach, using living rose-bengal-stained benthic assemblages and prevailing oxygen concentrations as a regional analogue for downcore environments. More recently, Tetard et al. (2021b) proposed the use of the relative abundance of *Eubuliminella exilis* (*Buliminella tenuata*) as a proxy, calibrated with average bottom water oxygenation and core-top samples predominantly from the Pacific Ocean. A recent attempt to quantify paleo-oxygen updates the original BFOI from Kaiho (Kranner et al., 2022). The enhanced BFOI (EBFOI) considers multiple indicator species and transfer functions for

both bottom water and pore water oxygen concentrations. Finally, Tetard et al. (2024) published an update of the benthic foraminiferal assemblage (BFA) index, the BFAex, which extends its range of applicability, based on the compilation of census data of over 1500 benthic foraminifera species from ~1700 samples recovered from the BENFEP database (Diz et al., 2023). Continued improvement and implementation of these approaches are active areas of research.

All assemblage-based proxies are empirical, with weaknesses identified by several authors (e.g. Buzas et al., 1993; Gooday, 2003; Jorissen et al., 2007). Buzas et al. (1993) identified a lack of replicate observations prior to assignment of species to a microhabitat preference, while Gooday (2003) and Jorissen et al. (2007) noted that the availability of food might be as important for the distribution of foraminifera as oxygen. Furthermore, when applied to extinct foraminifera species, these approaches will lack any observation of microhabitat. In such cases, microhabitat assignment is based
on the morphology (i.e. size and shape) (e.g. Kaiho, 1994), though violations of morphologically based assumptions are known even in modern foraminiferal species. For example, while large trochospiral forms are used as indicators of oxic conditions (Kaiho, 1994), several *Ammonia* species match this morphology but thrive in anoxic shallow-water environments (e.g. *Ammonia confertitesta*; see Richirt et al., 2022). Further refinement of indices and calibrations will increase the reliability of these proxies, while careful consideration of limitations, depositional environment, and specific questions can improve applications.

Identifying where whole assemblages or indicator taxa are most useful will also improve reconstructions. There is active debate whether it is advantageous to use full assemblages (e.g. Sharon et al., 2021), only the dominant species (e.g. Tetard et al., 2017), or only indicator species (see also Fig. 11). For example, a comparison of indices applied to a sediment core from the California margin demonstrates that while different indices follow the same trends, large offsets exist in quantitative oxygen reconstructions (Fig. 11). It is likely that different approaches are appropriate for different environments and questions. For example, in environments with a high degree of temporal or spatial averaging, use of indicator species may be more successful in positively identifying low-oxygen events. However, use of indicator species alone may limit the range of oxygen conditions reconstructed. These hypotheses deserve more rigorous testing.

Planktic foraminifera inhabit the water column above the seafloor. As most species occupy a near-surface habitat, planktic foraminiferal assemblages have primarily been used to reconstruct sea surface or mixed-layer temperature and, to a lesser extent, conditions such as productivity, salinity, and stratification (e.g. Imbrie and Kipp, 1971; Kipp, 1976; Cayre et al., 1999; Kucera et al., 2005; Morey et al., 2005; Kucera, 2007; Siccha et al., 2009). However, certain extant species of planktic foraminifera, including *Hastigerinella digitata* and *Globorotaloides hexagonus*, have been observed in low-oxygen pelagic environments (Hull et al., 2011; Davis et al., 2021), with the latter taxon subsequently being used to infer past distribution of low-oxygen water in the open ocean (Davis et al., 2023b). Some extinct species of planktic foraminifera, particularly those with digitate morphologies such as the clavigerinellids, have also been associated with low-oxygen environments (reviewed in Coxall et al., 2007). As planktic foraminiferal assemblages integrate across multiple distinct depth habitats, the use of indicator species rather than relative abundance is generally more appropriate for interpreting paleo-oxygenation. A notable example is the use of biserial chiloguembelinids, interpreted as OMZ dwellers based on isotope records (Boersma et al., 1987; Boersma and Silva, 1989; Luciani et al., 2020). The presence of their shells has been used to infer the presence of a pelagic OMZ in Paleocene to Oligocene sediments (e.g. Corfield and Shackleton, 1988; Pardo et al., 1997; Luciani et al., 2020).

## 9.3 Analyses and required resources

The majority of foraminiferal assemblages relevant to Cenozoic oxygen reconstructions is obtained from sedimentary records, with a smaller portion coming from lithified outcrops. For the purposes of this review, we focus on the sedimentary records. Collecting samples from subtidal stations, ranging from coastal to abyssal environments, often requires the use of seagoing vessels. However, the study of foraminiferal assemblages is relatively inexpensive once sediment samples have been collected. Prior to analysing assemblages, sediment samples typically undergo various laboratory treatments. These may include disaggregation using hexametaphosphate, wet or dry sieving to separate different size fractions, and sample splitting. Standard size fractions include 63, 125, or 150 µm and sometimes 32 or 250 µm, with the choice of size influencing the composition of the represented assemblage such that careful attention needs to be paid to ensure that these match size fractions used by a transfer function, if applicable. The subsequent steps involve “picking”, isolating, and sorting foraminiferal tests using a fine brush and black tray and gathering the specimens in mi- cropaleontological slides. Standard collection and counting procedures have been reviewed by Schönfeld (2012).

It is important to highlight that conducting robust counts requires an even split of the sample. Typically, a split containing 200–300 specimens is considered sufficient for evaluating species assemblages with a reasonable degree of accuracy (Patterson and Fishbein, 1989). This number could be lower if only the most abundant species are being considered but may be insufficient if rare species are important to the analyses (Fatela and Taborda, 2002). This work is carried out using an optical stereomicroscope equipped with a light source. Best practices also require documenting taxonomic attribution using either a high-resolution camera or a scanning electron microscope (SEM). Ultimately, the most crucial resource needed for these analyses is a trained researcher or technician, skilled in taxonomic identification of foraminiferal species. It is worth noting that acquiring this ability is relatively accessible compared to the taxonomy of macrofauna.

## 9.4 Recent advances

Several recent advances in analytical methods have shown promise in automating labour-intensive aspects of foraminiferal assemblage analysis. This is exemplified by the utilization of high-throughput imaging and artificial intelligence (AI) neural networks to generate species-level identifications (Mitra et al., 2019; Hsiang et al., 2019; Marchant et al., 2020). These methods have been combined with mechanical shell sorting to further enhance efficiency (Mitra et al., 2019; Richmond et al., 2022). While the current applications of automatization techniques have primarily been focused on planktic foraminifera, Marchant et al. (2020) have
<figure><img src="image_13.png" /></figure>

**Figure 11.** Comparison of past oxygen reconstructions for core site MD02-2503 (Ohkushi et al., 2013) comparing main species (determined by a PCA on the census data, usually corresponding to species with an averaged relative abundance > 5%) with the complete assemblage. This comparison was performed using four dissolved oxygen estimation methods: the BFOI (Kaiho, 1994), the BDO (Ohkushi et al., 2013), the BFA (Tetard et al., 2021a), and the eBFOI (Kranner et al., 2022). The black star shows the modern oxygen value at the core site. Note that there is significant overlap between the “main species only” and “full assemblage” approaches in all four methods, which makes the curves hard to distinguish.

also included a few benthic taxa. Although these approaches are still in their early stages, the use of AI to generate assemblages holds tremendous potential, especially as models can be trained on an increasing number of species.

Another important advancement is the increased availability of imaging technology. This includes the use of high-resolution digital cameras, enabling high-quality, true-colour imaging (e.g. Erdem and Schönfeld, 2017; Wilfert et al., 2015), as well as the growing popularity of desktop or environmental SEMs. Both approaches enhance the speed and cost-effectiveness of generating and sharing assemblage data. Alongside improved data storage capabilities, both internally and in online repositories, the sharing of images can and should continue to increase the transparency and reproducibility of assemblage work. The best practice for assemblage work is that all images, or at least a representative subset of images, should be published alongside papers. Furthermore, efforts should be made to ensure that images are uploaded to publicly available archives or repositories and stored as supplementary media whenever permitted by journals. The inclusion of images in publications and their archival in digital repositories offer several benefits to the scientific community. Firstly, it increases reproducibility, as other researchers can refer to the images for verification, validation, and comparison. Secondly, it facilitates resolution of taxonomic and nomenclature issues related to key indicator species. This is particularly important considering that taxonomic revision is ongoing.

## 9.5 Environmental influences

Most benthic foraminifera have the ability to survive at relatively low oxygen concentrations of around 45–89 µmol kg⁻¹ (Jorissen et al., 1995; Bernhard et al., 1997; Van der Zwaan et al., 1999; Levin et al., 2001; Murray, 2001; Geslin et al., 2011). Some species are even able to survive, calcify, and reproduce under anoxia conditions (Langlet et al., 2013; Nardelli et al., 2014), possessing diverse adaptations to and survival strategies for oxygen-depleted conditions (reviewed in Glock, 2013). Thus, it is only below this threshold that oxygen is expected to be a key driver of assemblage composition and robustly quantifiable. While oxygen concentrations and the tolerance of individual species are important factors, designating species as low-oxygen indicators is an ecological oversimplification but a useful one. Other factors, such as food supply, biogeography, differing metabolic strategies, habitat depth, and other environmental gradients including temperature and salinity, all contribute to defining ecological ranges. Some species may be opportunistic rather than specifically adapted to low-oxygen conditions. Other species may possess a unique tolerance to very low oxygen or have metabolic adaptations that allow them to utilize alternate electron receptors. Although low-oxygen-adapted species may survive in oxic environments, their relative abundance increases when oxygen becomes a limiting factor and competition with other species decreases. Tetard et al. (2024) detail dissolved oxygen affinities and ranges of 200 common occurring species from the Pacific.
There is a particularly complex interplay between organic matter flux and oxygenation of bottom and pore waters. In general, higher organic matter content leads to higher mineralization rates, i.e. through decomposition of organic material and subsequent oxygen consumption, resulting in lower dissolved oxygen concentrations in pore waters. This covariation can make it challenging or impossible to differentiate between the two drivers. Indeed, many indicator species can be viewed as specialists adapted to high productivity that also happen to tolerate low-oxygen conditions. This is likely the case for taxa such as *Bulimina*, *Bolivina*, and *Nonionella* (Margreth et al., 2009), which are rare in oligotrophic low-oxygen conditions but found in highly productive yet oxic regions.

Discrepancies can also be observed between geographical areas with similar bottom water oxygenation, particularly in defining specific indicator taxa. For example, *Uvigerina peregrina* is typical of OMZ environments in the Pacific and Arabian Sea, where it can be used in oxygen reconstructions (e.g. Moffitt et al., 2014; Schumacher et al., 2007). However, the same species is considered a high-productivity indicator in the relatively well oxygenated waters of the northeastern Atlantic (e.g. Lutze and Coulbourn, 1984; Fontanier et al., 2005; Mojtahid et al., 2017). Similar observations apply to *Nonionella stella* (Moffitt et al., 2014; Diz and Francés, 2008). In some cases, these geographic differences may highlight the complexity of food and oxygen as co-drivers, or they may suggest the presence of unknown environmental drivers, cryptic species, or adaptive differences between populations.

Benthic foraminifera are distributed throughout the upper centimetres of the sediment column, and their depth habitat is influenced by the availability of food and oxygen, ecosystem stability, bioturbation, and competition for resources (Jorissen et al., 2007, and references therein). In oligotrophic conditions, particularly open ocean settings, most species tend to occupy the uppermost sediment levels to maximize access to food exported from the photic zone (Linke and Lutze, 1993). However, other factors such as the quality of food, with fresh phytodetritus sometimes preferred, can also have an impact (Smart et al., 1994; Gooday and Hughes, 2002). In eutrophic conditions, where oxygen levels are low, the depth habitat of benthic foraminifera is determined by the capability of the species or assemblage to tolerate low oxygen (Jorissen et al., 1995). A key consequence of variability in habitat depth is that not all benthic foraminifera at a particular site will experience the same environmental conditions (Tetard et al., 2021a). This variability in sediment depth habitat not only impacts assemblage composition, but also plays a fundamental role in other oxygen proxies such as $\Delta\delta^{13}C$ (Sect. 11) and determines the chemical microenvironment that is recorded by trace metal proxies (Sect. 8).

While oxygen concentration may be limiting for many species, certain adaptations, including anaerobic metabolisms like denitrification, enable some foraminifera to persist even in euxinic conditions (Orsi et al., 2020).

Deep infaunal *Globobulimina* spp. (i.e. *G. turgida*, *G. pacifica*, *G. affinis*) and *Chilostomella* spp. (i.e. *C. ovoidea*, *C. oolina*) inhabit anoxic sediments below the oxygen and nitrate penetration depth (e.g. Jorissen et al., 1998; Schönfeld, 2001; Fontanier et al., 2002, 2003, 2005; Koho et al., 2007, 2008; Glud et al., 2009). These species are capable of storing and respiring nitrate in the absence of oxygen (Risgaard-Petersen et al., 2006; Piña-Ochoa et al., 2010a, b; Koho and Piña-Ochoa, 2012). They can be considered facultative (an)aerobes. The foraminiferal denitrification pathway is a rare example of eukaryotic denitrification (Woehle et al., 2018; Gomaa et al., 2021), and while foraminifera appear to perform only incomplete denitrification, the missing steps are likely completed by bacterial symbionts (Bernhard et al., 2010; Woehle et al., 2022). Intracellular nitrate accumulation has been found in species including *Nonionella* cf. *stella*, *Uvigerina akitaensis*, and *Bolivina spissa* (Høgslund et al., 2008; Glud et al., 2009). Some species even exhibit a metabolic preference for nitrate over oxygen, with nitrate concentrations rather than oxygen concentrations being limiting (Glock et al., 2019b; Suokhrie et al., 2020). Dormancy has also been identified as an adaptive response to short-term oxygen depletion (LeKieffre et al., 2017; Ross and Hallock, 2016). A comprehensive review of the survival strategies of foraminifera under low-oxygen conditions is given in Glock (2023). However, due to the limited numbers of species that have evolved these adaptations, oxygen-depleted conditions are characterized by lower species diversity and higher dominance compared to oxygenated settings (Phleger and Soutar, 1973; Sen Gupta and Machain-Castillo, 1993; Gooday et al., 2000; Levin, 2003; Koho and Piña-Ochoa, 2012).

The role of oxygen as a driver of planktic foraminiferal abundances is not as well established as in benthic foraminifera; however it is likely that the key environmental drivers in low-oxygen environments are similar. A combination of drivers, including accessibility of food and either tolerance of or adaptations to low oxygen, is probable. Some planktic low-oxygen specialists, such as *H. digitata* and *G. hexagonus*, do not seem to occur in oxic environments and may possess special adaptations to low oxygen. It remains unknown whether more widely distributed deep-dwelling species, such as *Globorotalia scitula* or *Globorotalia truncatulinoides*, whose habitat intersects low-oxygen waters, can opportunistically persist across both oxic and low-oxygen environments.

## 9.6 Marine archives and limitations

Reconstructions based on foraminiferal assemblages can be applied in any marine environment where foraminiferal shells are deposited and preserved in relatively undisturbed sediments. This includes fjord, estuarine, supratidal shelf, and deep-sea (including continental shelf) environments above the lysocline. While foraminiferal archives are widely available, they are most useful as oxygen indicators in
low-oxygen environments, where oxygen limitation is a
meaningful driver of assemblage composition. The use of
foraminiferal assemblages may be more limited in settings
where it is more difficult to deconvolve oxygen and produc-
tivity as drivers.

Low-carbonate-saturation-state environments also pose a limitation, where living calcareous species are rare or absent due to unfavourable conditions (Bernhard et al., 2009; Dias et al., 2010; Pettit et al., 2013; Martinez et al., 2018). Taphonomic processes in such environments can further complicate interpretations. Corrosive bottom or pore waters can lead to preferential dissolution of small, thin-walled species or species with a spiral arrangement of chambers (Berger, 1973; Hecht et al., 1975; Nguyen et al., 2009, 2011; Nguyen and Speijer, 2014) or can even lead to the complete loss of calcareous foraminifera (Gutiérrez et al., 2009), potentially altering the preserved assemblage. Thus, while dissolution could impact different methods differently (i.e. the use of only indicator species vs. diversity metrics), it should always be considered. Dissolution can pose limitations, particularly in very low oxygen or high productivity sites where high respiration rates can result in both low-carbonate-saturation states and oxygen-depleted conditions.

Fossil assemblages contain individuals that lived over a
period of time, depending on factors such as sedimentation
rate and degree of sediment mixing. Within a single centime-
tre of sediment, it is possible to find foraminiferal species that
lived contemporaneously or thousands of years apart (e.g.
Hupp et al., 2019, 2022, 2023; Hupp and Kelly, 2020). This
complication is common to all marine sedimentary records,
but it poses specific concerns for foraminifera. One impor-
tant factor to consider is the vertical migration of infaunal
species within the sediment column, which can occur over
several centimetres (Alve and Murray, 2001; Jorissen et al.,
1995; Duijnstee et al., 2003). As a result, older fossil epiben-
thic foraminifera can be recovered alongside younger infau-
nal specimens. This vertical movement of species contributes
to the phenomenon of time averaging in fossil assemblages.
In environments with seasonal or short-term variation in oxy-
gen levels, the presence of both oxic and low-oxygen taxa as
part of the same death assemblage can limit high-temporal-
resolution reconstructions using foraminiferal assemblages
rather than indicator taxa. However, this time-averaging ef-
fect can also offer opportunities to investigate seasonal con-
trasts. For instance, when species from contrasting environ-
mental settings occur together in the same samples, it could
indicate large seasonal (e.g. Stassen et al., 2015; Wagner et
al., 2022) or other short-term differences in oxygenation at
the site. Foraminifera populations can fluctuate over interan-
nual and seasonal timescales, even in the deep sea (Heinz
and Hemleben, 2003; Goineau and Gooday, 2019). Due to
these temporal variabilities and the spatial variations (patch-
iness) within the foraminiferal assemblages, replicate sam-
ples might be required to provide a realistic assessment of
the species-level composition of modern assemblages. Nev-

ertheless, in intertidal to nearshore environments, log-normal
distributions of foraminiferal standing stocks appear to be a
pervasive feature (Schönfeld et al., 2023), and internal repro-
ducibility of the analysis of foraminiferal assemblage com-
positions has been shown to be within ~2% (Schönfeld et
al., 2013).

Another challenge in interpreting foraminiferal assem-
blages is the occurrence of “no-analogue” fauna, where taxa
that are not observed together in modern environments co-
exist in the sediment record. This poses a potential limita-
tion as most identification and quantification of relationships
between assemblages or indicator species and oxygenation
are based on observations in the modern ocean. The pres-
ence of no-analogue fauna can have different explanations.
Time averaging can create assemblages that do not reflect
the present-day co-occurrence patterns. In other cases, the
limited range of modern sites used for calibration may not
capture the full spectrum of environmental conditions that
have existed throughout space and time. Consequently, there
may be gaps in our understanding of the relationships be-
tween foraminiferal assemblages and oxygenation in certain
environments. This highlights the need for further research
and a broader spatial and temporal sampling of modern envi-
ronments to improve our understanding of foraminiferal as-
semblage dynamics and their relationship with oxygenation.

9.7 Future directions

Open questions

To improve reconstructions of past oxygen levels based on foraminiferal assemblages, it is crucial to enhance our understanding of foraminiferal ecology. By doing so, we can distinguish between opportunistic species driven by food availability and true low-oxygen specialists. Exploring ecological questions related to seasonality and physiological oxygen tolerances in key species is also important. Understanding the constraints and variations in seasonality can provide insights into the temporal dynamics of foraminiferal assemblages and their response to changing oxygen conditions. Investigating the physiological oxygen tolerances of different species while considering interactions between oxygen, food availability, and carbonate chemistry stresses will contribute to a more comprehensive understanding of their ecological responses.

The exploration of metabolic adaptations and the role of
oxygen and nitrogen respiration in different species and en-
vironments is another avenue of research. New technolo-
gies, such as the increasing availability of genetic and ge-
nomic analyses, hold promise for addressing these metabolic
questions (Woehle et al., 2018; Orsi et al., 2020; Gomaa
et al., 2021) and explaining the variations in optimal oxy-
gen ranges observed among geographically disparate popu-
lations. These advanced techniques can provide valuable in-
sights into the underlying mechanisms and adaptations that
allow foraminifera to thrive in low-oxygen environments. It is important to note that while this discussion has focused primarily on benthic foraminifera, addressing the same suite of questions for planktic foraminifera is equally important and will improve the use of planktic assemblages for reconstructing pelagic oxygenation. Environmental DNA (eDNA) metabarcoding of foraminifera is another evolving method that could even be applied to paleo-reconstructions of foraminiferal assemblages in environments where ancient DNA can be preserved in sediments (Pawlowska et al., 2022; Barrenechea Angeles et al., 2023). Continued focus on foraminiferal ecology can facilitate the integration of traditional assemblage-based proxies with emergent proxies, leading to more comprehensive reconstructions. Understanding the ecological and physiologic drivers of foraminiferal oxygen tolerance can help researchers to determine where regionally specific or global calibrations are appropriate and which species should be considered. This will be crucial when applying calibrations developed in specific times and regions, such as the modern Pacific, to other times and places. It is especially relevant when comparing restricted basins and fjord environments to open ocean OMZs. Understanding the species-specific responses to varying oxygen content and co-stressors can be achieved through advances in culturing techniques, enabling analyses of assemblage-level changes (Bernhard et al., 2021). Furthermore, denitrifying species capable of living across large oxygen gradients can serve as target species for complementary proxy approaches, such as foraminiferal morphometrics and geochemistry. These approaches can provide additional insights into environmental conditions and further enhance our ability to reconstruct past oxygen levels.

While foraminiferal assemblages form the basis of some of the more established approaches for constraining marine oxygenation, a lack of sensitivity to oxygen higher than 45–89 µmol kg⁻¹ remains a key limitation. The inclusion of other fossilizing organisms in addition to foraminifera is one solution for extending oxygen reconstructions into more oxic conditions. This potential is reviewed briefly in Gooday et al. (2009) and discussed in Myhre et al. (2017). By incorporating groups such as brachiopods, molluscs, ostracods, brittle stars, sponges, and other organisms with hard parts recoverable in marine sediments, a composite index could be developed that is sensitive to intermediate oxygen concentrations. This expanded approach would provide a more comprehensive understanding of past oxygen levels.

The use of foraminiferal assemblages for reconstructing oxygenation has a robust, decades-long history. Due to the time-consuming nature of this approach and recent advances in genetic methods, traditional morphology-based taxonomists are becoming rarer, although the distinction between morphologically similar species is often crucial for geochemical approaches as well. However, studying foraminiferal assemblages remains an area of active research. Advancements in both knowledge and technological appli-

cations continue to expand the utility of foraminiferal assemblages as an oxygen proxy. Lessons from this approach have already informed and will continue to inform other foraminifera-based proxies, and it is likely that it will remain in the toolkit of paleoceanographers for years to come.

# 10 Foraminiferal morphometrics

## 10.1 Introduction

The principles behind the use of foraminiferal assemblages have sometimes been distilled into a few morphological traits, primarily size and shape. Most extant low-oxygen-tolerant species share several features. Calcareous hyaline foraminifera often have thin, porous test walls such as those seen in *Bolivina* and *Globobulimina* species (e.g. Kaiho, 1994; Sen Gupta and Machain-Castillo, 1993; Bernhard and Sen Gupta, 1999; Gooday, 2003; Boltovskoy et al., 1991; Caulle et al., 2014). Conversely, larger Nodosariida species like *Dentalina*, *Lagena*, and *Nodosaria* are associated with well-oxygenated conditions in modern settings. In terms of shape, more circular epifaunal species like *Cibicides* and *Planulina* dominate during oxygenated conditions, while elongated infaunal species like *Eubuliminella*, *Bolivina*, and *Brizalina* migrate to the water-sediment interface and dominate the benthic foraminiferal record during oxygen-impoverished periods (Jorissen et al., 1995, 2007; Palmer et al., 2020). These observations of size and shape have been crucial to the development of indices, based on test morphology and individual size. Today, the morphology of foraminiferal shells, both within and across species, is widely understood to reflect environmental conditions, including oxygenation (Fig. 12 and Table 4). Specific features include those influenced by the in situ calcification environment, such as shell porosity, size, ornamentation features (e.g. spines, costae, and keels), shape, and coiling direction. It also includes features influenced by the carbonate saturation state of the depositional environment where the shells are deposited. These can manifest as shell density changes, pitting, or other dissolution features. Some features like shell thickness or size-normalized weight can be affected by both growth and depositional environments. Out of these metrics, porosity (percentage of pore area of total shell surface) and pore density (number of pores per surface area) have received particular attention as proxies for redox conditions, and they are the primary focus of this section (Petersen et al., 2016; Rathburn et al., 2018; Glock et al., 2011, 2018, 2022; Tetard et al., 2017; Richirt et al., 2019; Lu et al., 2022). Shell size and circularity, especially within the context of the Major Axis and Roundness INdex (MARIN; Tetard et al., 2021a), are discussed as well.
<figure><img src="image_14.png" /><figcaption>**Figure 12.** Illustration showing the response of different foraminiferal morphometrics to changes in oxygen concentrations. Typical conditions at a continental margin OMZ are used as an example. For an overview of methods, see Tables 4 and 5.</figcaption></figure>

## 10.2 Historical perspective

### 10.2.1 Porosity and pore density

Porosity is defined as the percentage of pore area relative to the surface area of the foraminiferal shell. Pore density is defined as the number of pores per surface area. The two measures are distinct but closely linked: larger pores at a constant pore density result in a higher porosity. A detailed review of pores as an oxygen proxy in foraminifera can be found in Glock et al. (2012b) and is summarized here. The first studies of the morphology and fine structure of pores in foraminiferal tests go back to the middle of the 20th century. Advances in electron microscopy during this period allowed scientists to describe “pore plates” or “sieve plates” that cover the pores of many benthic foraminiferal species (Le Calvez, 1947; Jahn, 1953; Arnold, 1954; Angell, 1967; Sliter, 1974; Berthold, 1976; Leutenegger, 1977). There was also early evidence of strong environmental influences on pore density and other morphological features in *Bolivina spissa* and other closely related bolivinids (Lutze, 1962; Hartmann, 1964). In the 1980s and 1990s more studies described the correlation between pore size and pore density in benthic foraminifera and ambient oxygen concentrations, with species with higher porosity having been suggested as indicators of oxygen-depleted conditions (Bernhard, 1986; Perez-Cruz and Machain-Castillo, 1990; Moodley and Hess, 1992; Sen Gupta and Machain-Castillo, 1993; Kaiho, 1994). Since

then, several studies have shown that in some species of benthic foraminifera, individuals living in oxygen-depleted environments have higher pore density and porosity than specifics in well-oxygenated conditions (Glock et al., 2011; Kuhnt et al., 2013; Petersen et al., 2016; Rathburn et al., 2018; Richirt et al., 2019; Lu et al., 2022). Over the past decade, the porosity and pore density of foraminifera have evolved from a qualitative indicator of redox conditions towards a quantitative proxy.

Shell porosity as an oxygen proxy has received less attention in planktic foraminifera. Bé (1968) initially interpreted the porosity of planktic foraminifera as a potential temperature proxy due to a significant correlation between the porosity and latitudinal temperature gradients. A correlation between temperature and porosity was more recently validated in culture, with higher porosity interpreted as the result of the increase in metabolic rates with increasing temperature (Burke et al., 2018). Differences in porosity have also been observed with oxygen (Kuroyanagi et al., 2013; Davis et al., 2021). Notably, Kuroyanagi et al. (2013) report smaller pores in the final spherical chamber of the shallow, symbiont-bearing foraminifer *Orbulina universa* cultured under low-oxygen conditions. Davis et al. (2021) report higher porosities in the youngest chambers of the deep-dwelling, low-oxygen-affiliated species *Globorotaloides hexagonus*. The differences in these observations could reflect inter-species variability, ecological specificity, or a difference in the metric (pore size vs. porosity) used.
<table><caption>**Table 4.** Foraminiferal morphometrics that can be assessed to estimate past oxygen concentrations. Morphometrics are divided into benthic and planktic foraminifera. The column "Low-oxygen conditions" refers to the response of the correspondent morphometric under low-oxygen concentrations. "Other controlling factors" list other environmental parameters that might influence the morphometric.</caption><thead><tr><th>Morphometric</th><th>Low-oxygen conditions</th><th>Other controlling factors</th><th>Example species</th></tr></thead><tbody><tr><td>Benthic</td><td></td><td></td><td></td></tr><tr><td>Test pore density/Porosity</td><td>High</td><td>Temperature (affecting metabolic rates), nitrate availability, age (decreases with older chambers)</td><td>Infaunal – <i>Bolivina spissa</i>, <i>Bolivina seminuda</i> (Glock et al., 2011); epifaunal – <i>Cibicidoides</i> and <i>Planulina</i> spp. (Rathburn et al., 2018; Lu et al., 2022)</td></tr><tr><td>Test size</td><td>Small (between different species), small or large (within the same species) – respondent but variable direction of effect</td><td>Nitrate and food availability, depth in sediment (increases with depth)</td><td>The small <i>Nonionella stella</i> dominate low-oxygen sediments of the Santa Barbara Basin (Bernhard et al., 1997); <i>Bolivina spissa</i> adults (larger size) tolerate lower oxygen concentrations deeper in Peruvian margin sediment than juveniles (smaller size) (Glock et al., 2011). <i>Bolivina (seminuda, spissa, argentea, subadvena)</i>, <i>Takayanagia delicata</i>, offshore California (Tetard et al., 2017; Moffitt et al., 2015; Ohkushi et al., 2013)</td></tr><tr><td>Test circularity</td><td>Size and circularity used for MARIN to reconstruct oxygen: low oxygen, small size, and elongated specimen (low MARIN); high oxygen, big size, and round specimen (high MARIN)</td><td></td><td>Low oxygen: elongated specimen (bolivinids, buliminids) High oxygen: cibicidids, planulinids (Tetard et al., 2021a)</td></tr><tr><td>Test surface area/volume ratio</td><td>High or low – respondent but variable direction of effect</td><td>Age (increases with older chambers), pollution</td><td><i>U. peregrina</i>, <i>B. tunata</i>, and <i>L. psuedobeyrichii</i> have lower SA : V during the low-oxygen period in the Gulf of Alaska (Belanger, 2022)</td></tr><tr><td>Test size-normalized weight</td><td>Uncertain</td><td></td><td></td></tr><tr><td>Test thickness</td><td>Unknown</td><td>Acidification</td><td><i>Elphidium clavatum</i> (Choquel et al., 2023)</td></tr><tr><td>Planktic</td><td></td><td></td><td></td></tr><tr><td>Test pore density/porosity</td><td>High or low – respondent but variable direction of effect</td><td>Temperature (affecting metabolic rates)</td><td>Low in <i>Orbulina universa</i> (Kuroyanagi et al., 2013); high in <i>Globorotaloides hexagonus</i> (Davis et al., 2021)</td></tr><tr><td>Test size</td><td>Small or large (within the same species) – respondent but variable direction of effect</td><td></td><td></td></tr><tr><td>Test circularity</td><td>Unknown</td><td></td><td></td></tr><tr><td>Test surface area : volume ratio</td><td>High or low – respondent but variable direction of effect</td><td></td><td>Lower for <i>G. hexagonus</i> in the lowest-oxygen conditions of the eastern tropical North Pacific (Davis et al., 2021)</td></tr><tr><td>Test size-normalized weight</td><td>Low (high dissolution)</td><td>Mainly controlled by carbonate chemistry</td><td><i>G. sacculifer</i>, <i>P. obliquiloculata</i>, and <i>N. dutertrei</i> (Broecker and Clark, 2001)</td></tr><tr><td>Test thickness</td><td>Unknown</td><td></td><td></td></tr></tbody></table>
Several equations are available to calculate environmental oxygen and [NO₃⁻], using the porosity or pore density of benthic foraminifera. Rathburn et al. (2018) found that the porosity of Cibicides spp. and Planulina spp. within a 5000 µm² window at the centre of the ultimate or penultimate chamber can be used to calculate bottom water oxygen concentrations ([O₂]BW) according to Eq. (3):

$$[O_2]_{BW} = e^{((\text{Pore \%} - 47.237)/( -8.426))} \quad (3)$$

Similarly, the pore density (PD; in pores per µm²) of Cibicides spp. and Planulina spp. within a 5000 µm² window at the centre of the ultimate or penultimate chamber on the spiral side can be used with slightly lower accuracy (see Eq. 4; Glock et al., 2022):

$$[O_2]_{BW} = e^{((\text{PD} - 0.008[+/-0.0002]) / (-0.00142[+/-0.00006]))} \quad (4)$$

Within a limited [O₂]BW range of 2–14 µmol kg⁻¹, the PD of Planulina limbata on a size-normalized area of the older part on the spiral side can be used to calculate [O₂]BW with higher accuracy according to Eq. (5) (Glock et al. 2022):

$$[O_2]_{BW} = -6027[+/-652] \cdot \text{PD} + 22.0[+/-1.7] \quad (5)$$

Glock et al. (2011, 2018) found that the pore density of some denitrifying benthic foraminifera can be used to calculate bottom water NO₃⁻ concentrations ([NO₃⁻]BW). The most recent equations used to reconstruct [NO₃⁻]BW have been found for Bolivina spissa and Bolivina subadvena (Govindankutty-Menon et al., 2023). [NO₃⁻]BW can be calculated using the pore density from a size-normalized area that includes the ~10 oldest chambers according to Eq. (6):

$$[NO_3^-]_{BW} = -3896[+/-350] \cdot \text{PD} + 61[+/-1] \quad (6)$$

### 10.2.2 Size and morphotype

A predominance of smaller benthic foraminifera species in low-oxygen environments has frequently been observed (Bernhard, 1986; Sen Gupta and Machain-Castillo, 1993; Bernhard and Sen Gupta, 1999). From the perspective of gas exchange, a decrease in size is an efficient way to increase the shell surface area/volume ratio and thus maximize the relative surface available for gas diffusion. However, this trend is not universally observed within species. Some studies report no consistent relationships between specimen size and oxygen levels (Keating-Bitonti and Payne, 2017). Some low-oxygen-adapted species, such as *Uvigerina peregrina* and *Buliminella tenuata*, even seem to increase in size and decrease in surface area/volume ratios in lower-oxygen settings (Keating-Bitonti and Payne, 2017; Davis et al., 2021; Belanger, 2022). This counter-intuitive observation may be explained by reliance on nitrogen rather than oxygen for respiration (Glock et al., 2019b) or the influence of high food availability (Belanger, 2022). Starting in the 1990s, multiple authors have mentioned the relationship between the test

morphotype or shape of benthic foraminiferal tests and environmental parameters such as bottom water oxygenation (e.g. Corliss, 1991; Kaiho, 1994; Kaiho et al., 2006). These observations were combined with size metrics to develop the Major Axis and Roundness INdex (MARIN; Eq. 7; Tetard et al., 2021a).

$$\text{MARIN} = \text{Major axis} \times \text{roundness} \quad (7)$$

The major axis corresponds to the primary axis of the best-fitting ellipse. The roundness can be calculated according to Eq. (8):

$$\text{Roundness} = 4 \times \text{area} \times \pi^{-1} \times \text{major axis}^{-2} \quad (8)$$

Tetard et al. (2021a) calibrated MARIN as an oxygen proxy for the eastern North Pacific according to Eq. (9):

$$[O_2]_{BW} = 0.0000266 \times \exp^{0.0354 \times \text{MARIN}} \quad (9)$$

Thus, by measuring size and shape using image analysis software, such as ImageJ (Petersen et al., 2016; Tetard et al., 2021), past oxygen values for OMZ conditions can be estimated without the need for species-level identification. An assemblage characterized by small, elongated tests (Fig. 12) would indicate low-oxygen bottom water conditions, while an assemblage dominated by large, spirally arranged tests would indicate well-oxygenated conditions. The impact of changing oxygen conditions on foraminiferal morphology is discussed in more detail in the next section.

### 10.2.3 Size-normalized weight and dissolution

Size-normalized weight (SNW) is a metric derived from normalizing the weight of individual or pooled shells by their length, area, or volume. It serves as a measure of how heavily calcified a foraminifera is and has been frequently linked to carbonate ion concentration in both pelagic and benthic environments. High input of organic matter results in higher oxygen consumption and release of dissolved inorganic carbon (DIC) by remineralization. The resulting coupling between oxygen and the carbonate system in many marine environments makes SNW worth mentioning here. While the direct driver of SNW (and shell dissolution) is likely carbonate chemistry, it could act as a viable supporting proxy. Lohmann (1995), followed by Broecker and Clark (2001), proposed the use of planktic foraminiferal shell weights to assess dissolution and therefore bottom water carbonate chemistry. However, the use of shell weights as a carbonate chemistry (and, indirectly, oxygen) proxy was rapidly complicated by evidence which shows that carbonate ion concentration influences the SNW of planktic foraminifera during growth as well (e.g. Bijma et al., 1999; Barker and Elderfield, 2002; de Moel et al., 2009; Moy et al., 2009; Manno et al., 2012; Marshall et al., 2013). Results from size-normalized weight studies in foraminifera from low-oxygen, high-carbon environments have been equivocal in benthic foraminifera (Davis
et al., 2016) but show some promise when applied to planktic foraminifera (Davis et al., 2021). Use of micro-CT to differentiate dissolution from calcification (Iwasaki et al., 2019a) may make the application of SNW as an indirect proxy for oxygen more feasible in the future.

## 10.3 Analyses and required resources

Sample preparation is normally as described for foraminiferal assemblages (see Sect. 9). Morphometric analyses then proceed using one of several microscope and imaging approaches. In most cases, analyses of basic morphology (size, circularity, ornamentation, and sometimes porosity, pore size, and pore density) can be carried out using a stereo microscope, equipped with a camera and/or micrometer. This approach can also be automated using a motorized microscope stage and image acquisition and processing software (e.g. NI Vision software, ImageJ, the R package forImage; Freitas et al., 2021) for automatically reconstructing and measuring pore density, pore surface area, volume, and various test measurements (e.g. ImageJ, the MorFo_ijm plugin available at https://doi.org/10.5281/zenodo.4740079; Tetard et al., 2021a; Freitas et al., 2021). Higher-resolution analyses or precise measurements of small features such as pores sometimes require SEM imaging. The SEM images usually include the entire specimen and in some processing methods also need to include higher-magnification images of a specific chamber. Processing the SEM images can be performed using the open-source software ImageJ (https://imagej.nih.gov/ij/download.html) (Petersen et al., 2016) or Adobe Photoshop and ArcGIS software (Rathburn et al., 2018). A detailed manual for semi-automated pore measurements of benthic foraminifera can be found in Petersen et al. (2016).

These analyses are relatively cheap and highly accessible. In addition, because most SEM analyses are non-destructive, specimens can be reused in geochemical analyses, which provides potential for a multi-proxy approach to paleo-oxygen reconstruction.

## 10.4 Recent advances

While interest in studying foraminiferal morphology in 3D emerged in the mid-20th century (Bé et al., 1969; Schmidt, 1952), quantifications and paleoclimatic reconstructions using such methods have become possible only recently. Since the pioneering micro-CT work of Speijer et al. (2008), the number of studies dealing with 3D reconstructions of foraminiferal tests has been increasing in a variety of areas, such as taxonomy and ontogeny (e.g. Briguglio, 2010; Görög et al., 2012; Schmidt et al., 2013; Caromel et al., 2016, 2017; Burke et al., 2020), ocean acidification and test dissolution processes (e.g. Johnstone et al., 2010, 2011; Iwasaki et al., 2015, 2019b; Prazeres et al., 2015; Ofstad et al., 2021; Kuroyanagi et al., 2021; Charrieau et al., 2022; Choquel et

al., 2023), effects of temperature (e.g. Kinoshita et al., 2021; Titelboim et al., 2021), and paleoclimate reconstruction (e.g. Fox et al., 2020; Zarkogiannis et al., 2022; Todd et al., 2020; Schmidt et al., 2018).

Foraminiferal 3D reconstruction is a promising, non-destructive approach for accessing the morphology of the entire shell (inner and outer walls). However, it remains technically and methodologically challenging as well as costly to scan a large number of shells with high resolution (< 1µm). To date, conventional µCT scanners have primarily been used, supplemented with particle accelerator facilities and atomic force microscopy (AFM). Studies using 3D foraminifera constructions with scanner-based or synchrotron light-based µCT were reviewed in Choquel et al. (2023). Most studies reconstructing foraminifera in 3D are performed with costly software such as Avizo (e.g. Fox et al., 2020), Amira (e.g. Schmidt et al., 2013), Molcer Plus, and ConeCT express (e.g. Iwasaki et al., 2015). These software packages are adapted to the problems of test reconstruction but can also induce limitations on access to data processing and lack guidelines on how to analyse test morphometrics. Some authors are developing 3D post-data analyses with free software such as ImageJ (Fiji) (Belanger, 2022), MeshLab (Choquel et al., 2023), and Gwyddion (Giordano et al., 2019).

Reconstruction of pore patterns in 3D is challenging. Few studies have addressed porosity from µCT images, mostly focusing on porosity from planktic foraminifera only (Burke et al., 2018; Davis et al., 2021). Davis et al. (2021) demonstrated that in *Globorotaloides hexagonus*, the porosity of the most recent chamber, measured by µCT scans and light microscope images, captures the same trend; however the porosity from µCT images is higher. This difference could be due to the accuracy of the pore segmentation (the delineation of the automatic pore contour) and the lower resolution of the µCT images compared to conventional microscopy. Many efforts have been made in the last decade to automate or semi-automate the acquisition of pore measurements (number of pores, pore density, and porosity). Automated pore measurements have the advantage of acquiring data rapidly, facilitating the production of a large amount of representative data (Kuhnt et al., 2014). These automatic methods are often made from SEM images of a small part or fragments of the test to limit the pore deformation linked to the test curvature. The difficulty lies in standardizing pore measurements between individuals of the same species, e.g. *Ammonia tepida* (Petersen et al., 2016; Giordano et al., 2019), *Orbulina universa* (Morard et al., 2009), or *Bolivina seminuda* (Tetard et al., 2017). The pores are mainly studied with a 2D view, but 3D analyses allow access to more parameters such as pore depth and roughness (Giordano et al., 2019). Interest in studying 3D pore patterns from µCT scans is increasing (Burke et al., 2018; Davis et al., 2021). Indeed, the 3D reconstruction of the test is a promising tool that could give us
access to the pore patterns chamber by chamber and could especially be adaptable to different shapes of the tests.

## 10.5 Proxy drivers

While porosity and/or pore density are empirically useful proxies in some species of foraminifera, understanding the parameters that directly drive this correlation is a work in progress. Several authors have hypothesized that larger pores act to facilitate increased gas exchange across the shell in low-oxygen environments (Leutenegger and Hansen, 1979; Corliss, 1985). A clustering of mitochondria behind the pores and the exchange of labelled CO₂ through pores suggest that pores are involved in respiratory processes in several species (Leutenegger and Hansen, 1979; Bernhard et al., 2010). Leutenegger and Hansen (1979) argue that the clustering of mitochondria below the pores will create a deficiency of oxygen and thereby a diffusion gradient across the pores. *Patellina corrugata* has been shown to actively pump dissolved organic dyes through its pores into the cytoplasm (Berthold, 1976). Moreover, pores have been found to be adequate for the exchange of gases in both low- and high-oxygen conditions (Moodley and Hess, 1992). A gas exchange function would lead to a prediction of increased porosity and/or pore density under conditions of increased demand for oxygen diffusion due to either increased metabolic demand or decreased oxygen.

The studies reviewed above (Sect. 9.5) show that some foraminifera are able to denitrify and complicate this interpretation. Some denitrifying foraminifera such as *Bolivina spissa* most likely take up nitrate through pores. The pore density of denitrifying species is more strongly related to nitrate than oxygen, suggesting it can be used as a paleoproxy for nitrate (Glock et al., 2011, 2018, 2019b). Other potential functions of surface pores include taking up dissolved organic matter as food resources and releasing metabolic CO₂ (Glock et al., 2012b).

Direct drivers of porosity may therefore be species- and environment-specific. For example, epifaunal or very shallow infaunal species such as *Cibicidoides* and *Planulina* spp. do not live or migrate to anoxic pore waters, and no studies have suggested their use of nitrate for respiration. Thus, it can be inferred that bottom water nitrate does not influence the porosity of these species (Rathburn et al., 2018). However, there is a possibility that some *Cibicides* spp. might be able to denitrify under severe oxygen depletion, since they cluster next to the known denitrifiers within their phylogenetic tree (Woehle et al., 2022). Another recent study from the Arabian Sea found no significant correlation between the surface porosity of *Cibicidoides* spp., dissolved organic carbon, and CO₂ concentrations in seawater, suggesting that bottom water oxygen is likely the major control on surface porosity for *Cibicidoides* spp. (Lu et al., 2022).

## 10.6 Influence of temperature, ontogeny, and dimorphism on morphological characteristics

There are several other parameters, in addition to oxygen and nitrate, which can influence the porosity of foraminifera. This includes temperature (Bé, 1968; Glock et al., 2011; Kuhnt et al., 2013; Burke et al., 2018), which might influence porosity via changes to metabolic rates and oxygen solubility in the environment. Since these parameters often covary, it can be difficult to unravel the main factors that control porosity. Ontogeny can also influence foraminiferal morphology in various ways. Usually, surface to volume ratios decrease as foraminifera grow. This results in an increase in pore density and porosity in the youngest chambers, as the organism compensates for a decreased surface to volume ratio (Glock et al., 2011). During ontogeny, benthic foraminifera can migrate in the sediment (Glock et al., 2011) and planktic foraminifera in the water column (Hemleben et al., 1989; Schiebel and Hemleben, 2017; Meilland et al., 2021), though this appears to be species-specific. For example, the size (and likely age) of living *Bolivina spissa* from the Peruvian OMZ increases with sediment depth (Glock et al., 2011). By contrast, there is no significant correlation between pore density in congener *Bolivina pacifica* and its depth in sediments from the Arabian Sea, indicating minimal ontogenetic effects on the living depth of this species (Kuhnt et al., 2013). The latter study also focused on analysing porosity in small areas of the shell, which may reduce ontogenetic effects if parts of the test from a similar ontogenetic state are analysed (Kuhnt et al., 2013). Generational dimorphism may also impact porosity. At least in some species there are systematic differences in the pore patterns between megalospheric and microspheric specimens (Fig. 13), and some studies have exclusively focused on megalospheric specimens (Glock et al., 2011, 2018). One final consideration when analysing foraminiferal porosity is shell stability (Richirt et al., 2019). As porosity increases ontogenetically (Glock et al., 2011), the last chamber usually shows the highest porosity. The last chamber is also usually the thinnest due to the laminar calcification mechanism in Rotaliid species (Erez, 2003) and may be broken in many fossil specimens. There might be different strategies to preserve stability while at the same time increasing porosity. One strategy is to build larger but fewer pores (Richirt et al., 2019). Another strategy might be to increase wall thickness.

## 10.7 Marine archives and limitations

Similar to oxygen assessments based on benthic foraminiferal assemblages, morphometrics can potentially be used wherever marine carbonates are preserved. Features such as size and test morphology should be robust to minor dissolution, and image analyses typically should reveal if extensive dissolution has occurred. Corrosive environments could lead to preferential dissolution of smaller tests and/or enlargement of pores, but this has yet to be
<figure><img src="image_15.png" /><figcaption>**Figure 13.** Comparison of the pore patterns of a megalospheric (left) and microspheric (right) specimen of *Bolivina spissa*. Note the larger but fewer pores in the old parts of the test of the microspheric specimen (bottom part of the image, close to the proloculus). Location: Mexican margin. Image taken on a Hitachi Tabletop SEM TM4000 series at the University of Hamburg.</figcaption></figure>

directly tested in the context of low-oxygen proxies, such as porosity, pore density, and MARIN.

The correlation between pore density/porosity and oxygen concentrations or other environmental parameters is species-specific (Kuhnt et al., 2013, 2014; Glock et al., 2011, 2022). The distribution of foraminiferal species is often restricted to a certain oxygen range, and the species-specific calibrations thus often cover only a limited oxygen range (Kuhnt et al., 2013, 2014; Tetard et al., 2017; Glock et al., 2011, 2022; Sect. 9.2). One potential solution to this limitation is a multi-species pore density/porosity calibration. For example, the global multi-species calibration of *Cibicidoides* and *Planulina* spp. suggests a strong negative logarithmic relationship between porosity and bottom water oxygen from oxygen concentrations as low as 2 µmol kg⁻¹ to completely oxygenated environments, but the correlation is very flat at oxygen concentrations > 100 µmol kg⁻¹ (Rathburn et al., 2018; Lu et al., 2022). The porosity of these species is usually > ~10% when bottom water oxygen is < 100 µmol kg⁻¹. In oxic environments, few or no pores are found on the surface (Rathburn et al., 2018). The porosity proxy is thus most useful for indicating low bottom water oxygen conditions (Fig. 1).

The porosity of *Bolivina seminuda* has been suggested as an oxygen proxy for lower-oxygen environments (~1 to 45 µmol kg⁻¹; Tetard et al., 2017, 2021a), although *B. seminuda* is a denitrifying species, and it is possible that differences in oxygen concentrations play only a minor role regarding its porosity (Piña-Ochoa et al., 2010a; Glock et al., 2019b). Multiple environmental parameters have been shown to correlate with the porosity or pore density of foraminifera, such as oxygen, nitrate, and temperature, and these often covary. This can result in significant correlation of the pore characteristics with multiple parameters (Glock et al., 2011; Kuhnt et al., 2013). Oxygen and nitrate are coupled to both denitrification and remineralization. Nitrate loss through denitrification is increased when oxygen is depleted, while remineralization consumes oxygen and increases nitrate concentrations (Anderson and Sarmiento, 1994; Johnson et al., 2019). Similar opposing trends are found for the correlation between oxygen and temperature. Oxygen solubility decreases with increasing temperatures, and both parameters often covary with water depth (Keeling and García, 2002; Schmidtko et al., 2017). In addition, temperature has an influence on metabolic rates, which might be a factor influencing foraminiferal porosity (Burke et al., 2018). For now, the covariation in all these parameters limits interpretation of porosity proxies, but future studies using controlled laboratory cultures might unravel their specific influences.

## 10.8 Future directions

### 10.8.1 Emerging developments

High-throughput imaging and other forms of automation are rapidly increasing the scope of what can be done with morphological proxies but bring new challenges as well. Automation reduces historically intensive work. This means that larger sample sizes, higher-resolution records, and attention to an increasing number of species and environments are becoming more feasible. However, key questions remain, such as how automated analyses compare to manual analyses and what trade-offs are associated with high throughput versus accuracy. One concern is that the accuracy of high-throughput automated image analyses may suffer due to a decrease in human oversight, though this might improve with further development of algorithms and AI. High-throughput methods and use of traditional equipment, such as optical microscopy, may also provide different benefits, with the former saving labour and the latter saving instrumental cost.

Other directions include wider application of existing methods and ground truthing through laboratory culture. Application of morphological methods developed in benthic foraminifera to other organisms would also increase the number of environments from which oxygen can be constrained. This most readily applies to planktic foraminifera, which would expand proxies into the pelagic realm, but may also apply to other hard-bodied organisms such as ostracods, in
which morphology has been linked to environmental param-
eters including oxygen (e.g. McKenzie et al., 1989).

As mentioned above, the stability of tests is an important
factor when studying foraminiferal porosity and pore den-
sity (Richirt et al., 2019). One factor related to test stability
that has not been analysed yet is the thickness of the walls.
The effects of mechanical stress on morphological character-
istics such as porosity and wall thickness are currently un-
known. Other factors to be considered in future studies in-
clude variations in mechanical stresses in the environment
induced by varying sediment grain size, bioturbation, and the
intensity of bottom water currents. Future directions could
include culturing experiments to isolate the influence of dif-
ferent environmental parameters on the pore density/porosity
of both benthic and planktic species, including both denitri-
fying and oxygen-respiring species. Previous culture studies
have demonstrated pore plasticity responding to environmen-
tal conditions within the lifespan of a single individual in
multiple benthic (Sliter, 1974; Moodley and Hess, 1992) and
planktic species (Bijma et al., 1990; Kuroyanagi et al., 2013;
Burke et al., 2018). Porosity changes during a foraminifera
lifetime are a potential metabolic response to environmental
drivers (Kuroyanagi et al., 2013; Burke et al., 2018).

The evolution of 3D methods such as micro-computed to-
mography (µCT) provides access to morphological features
of the full test. However, the resulting datasets are very large.
This can be problematic, since full datasets cannot be easily
shared or published, and storage space can be a limited re-
source. Further work is needed to automate and streamline
some data handling and processing.

Finally, little attention has been given to studying ecophe-
notypic variability, or the potential for adaptive responses
to environmental forcing, in the frequency of ornamenta-
tions and test deformations in response to oxygen depletion.
Lutze (1964) and Harmann (1964) showed that bolivinids
from lower-oxygen sites in the Santa Barbara Basin typically
have less ornamentations such as spines, costae, and keels. In
addition, growth disruption and test deformation can appear
under unfavourable environmental conditions (Lutze, 1964).
Future research might address this issue and include system-
atic studies on the frequency and size of ornamentations and
test deformation under variable oxygen concentrations.

10.8.2 Open questions: resolving methodological
differences

The literature includes many methods for determining
foraminiferal pore characteristics, such as porosity, pore size,
and pore density. A relatively widespread method is focusing
on the centre of the ultimate or penultimate chambers and
using a small window to approximate a flat surface (Kuhnt
et al., 2013, 2014; Petersen et al., 2016; Rathburn et al.,
2018; Richirt et al., 2019; Glock et al., 2022). An alternative
method suggests using a larger size-normalized part of the
older chambers (Glock et al., 2011, 2018, 2022). Another ap-

proach is automated image acquisition and analysis of shards
after cracking open the shell in order to investigate penulti-
mate chamber vs. whole test porosity (Tetard et al., 2017).
Finally, one study used atomic force microscopy to automat-
ically analyse foraminiferal morphometrics (Giordano et al.,
2019). Each method has advantages and disadvantages (see
Sect. 10.2). A recent study on the pore density of epifaunal
*Planulina limbata* found the best correlation between oxygen
and pore density by using a size-normalized area on the older
parts of the spiral side (Glock et al., 2022). In other epifau-
nal foraminifera, Rathburn et al. (2018) found that there is a
better correlation between porosity and oxygen than between
the pore density and oxygen. Finally, to minimize problems
through dissolution effects or overgrown pores of planktic
foraminifera, Constandache et al. (2013) suggested breaking
the shells and determining the pore characteristics on the in-
ner surface. The wide variability in methods that have been
used in existing studies shows a need for the development of
a common approach. The ongoing automation of data acqui-
sition may provide a suitable way to achieve this in future
work, although the approaches may vary slightly, depending
on the shape of the shells (i.e. spatulate, plano-convex, ellip-
tic, etc.).

11 Benthic foraminiferal carbon isotope offsets

11.1 Theory and proxy driver(s)

The carbon isotopic offset between specific benthic foraminifera species (Δδ¹³C) can be used to reconstruct bottom water oxygen concentrations. Application of the proxy relies on the principle that oxic remineralization of organic matter releases isotopically light DIC into the pore waters. Within the top few centimetres of the sediment, aerobic respiration of organic matter dominates. This generates a δ¹³C gradient between bottom and pore waters from the sediment-water interface to the anoxic boundary (McCorkle et al., 1985). It is thought that greater aerobic remineralization in pore waters, associated with higher bottom water oxygen concentrations, enhances this δ¹³C gradient, and the availability of oxygen in marine pore waters is set by downward diffusion of bottom water oxygen across the sediment-water interface (McCorkle et al., 1985).

The basis of the Δδ¹³C proxy is the difference in δ¹³C between epifaunal and deep infaunal benthic foraminifera species, which theoretically reflects the carbon isotopic composition in their habitats (Eq. 10). The carbon isotopic composition of epifaunal foraminifera such as *C. wuellerstorfii* reflects that of DIC in bottom water, and the carbon isotopic composition of deep infaunal foraminifera of the genus *Globobulimina* reflects that of DIC in pore waters near the anoxic boundary (Duplessy et al., 1984; Zahn et al., 1986; Fontanier et al., 2002; Geslin et al., 2004; Schmittner et
<table><caption>**Table 5.** Different methods used to determine pore characteristics (e.g. shell porosity and pore density) of foraminifera with a list of different advantages and disadvantages of those methods.</caption><thead><tr><td>Description of method</td><td>Advantages</td><td>Disadvantages</td><td>References</td></tr></thead><tbody><tr><td>Focusing on a small window with a smooth surface in the centre of the ultimate/penultimate chamber</td><td>Relatively fast, minimizes artefacts due to the curvature of the specimens, normalizes regarding the ontogenetic stage, negates problems with overgrown pores</td><td>Dataset is limited due to the small window size; ultimate and penultimate chambers usually have the highest porosity, which can reduce test stability</td><td>Kuhnt et al. (2013, 2014), Petersen et al. (2016), Rathburn et al. (2018), Richirt et al. (2019), Glock et al. (2022)</td></tr><tr><td>Using a larger size-normalized area on the older parts of the test</td><td>Normalization for ontogenetic effects, larger datasets per specimen; lower porosity in these parts causes fewer stability restrictions in the test, and porosity might thus be better adapted to environmental changes</td><td>Possible artefacts by overgrown pores and curvature of the test More effort to acquire the data</td><td>Glock et al. (2011, 2018, 2022)</td></tr><tr><td>Automated image acquisition and analysis of shards from crushed foraminifera</td><td>Large datasets with relatively low effort</td><td>Method is destructive</td><td>Tetard et al. (2017)</td></tr><tr><td>Automated morphometric analyses using atomic force microscopy</td><td>Most metadata of all methods, including depth and 3D shape of the pores</td><td>Accessibility of atomic force microscopy</td><td>Giordano et al. (2019)</td></tr><tr><td>Analysis of porosity from the inside after breaking the test</td><td>Avoids problems with pores that are overgrown or shows evidence of dissolution from the outside</td><td>Method is destructive</td><td>Constandache et al. (2013)</td></tr><tr><td>3D image of the whole test using X-ray</td><td>All pores of the test can be counted, and the surface area and pore sizes can be calculated in 3D</td><td>Beam time is constrained, and mCT can be costly</td><td>Burke et al. (2018), Davis et al. (2021), Choquel et al. (2023)</td></tr></tbody></table>

al.,2017).

$$ \Delta\delta^{13}\text{C} = \delta^{13}\text{C}_C \cdot \text{wuellerstorfi} - \delta^{13}\text{C}_{\text{Globobulimina spp.}} \quad (10) $$

Robust quantification of bottom water oxygen concentrations based on $\Delta\delta^{13}\text{C}$ rests on three fundamental assumptions. First, foraminifera species record the $\delta^{13}\text{C}$ of the DIC of bottom waters and pore waters near the anoxic boundary within subsurface marine sediments. Second, degradation of organic matter above the sedimentary anoxic boundary and associated deviation of pore water $\delta^{13}\text{C}$ of DIC from bottom waters are predominantly driven by aerobic respiration. Lastly, pore water $\delta^{13}\text{C}$ gradients in DIC are largely determined by aerobic respiration of organic matter, which is mediated by the diffusion of dissolved oxygen from bottom waters and thus directly scales with the availability of dissolved oxygen in bottom waters. If overlying bottom waters are characterized by low oxygen concentrations, a balance between consumption of oxygen through mainly aerobic respiration and diffusion of oxygen from the overlying bottom waters is reached at shallow sediment depths, with small deviations of

pore water $\delta^{13}\text{C}$ of DIC at the anoxic boundary from the $\delta^{13}\text{C}$ of DIC in bottom waters. Low $\delta^{13}\text{C}$ gradients are thus associated with low bottom water oxygen levels (Fig. 14). The greater the oxygen concentration in overlying bottom waters, the greater the release of low $\delta^{13}\text{C}$ DIC into pore waters during respiration of organic matter, leading to a larger $\delta^{13}\text{C}$ difference between the DIC at the sediment-water interface and the DIC at the anoxic boundary and a larger $\delta^{13}\text{C}$ gradient.

There is a strong relationship between $\delta^{13}\text{C}$ of seawater DIC and oxygen in the water column today (Hoogakker et al., 2016). However, epifaunal benthic foraminiferal $\delta^{13}\text{C}$ alone cannot be used to reconstruct past bottom water oxygen due to large uncertainties relating to preformed $^{13}\text{C}$, air-sea fractionation, mixing with other water masses, and terrestrial biomass changes (Lynch-Stieglitz et al., 1995; Schmitner et al., 2013; Gruber et al., 1999; Curry and Oppo, 2005; Oliver et al., 2010). Instead, the carbon isotope gradient pairs epifaunal $\delta^{13}\text{C}$ measurements with those of a deep infaunal species.
<figure><img src="image_16.png" /><figcaption>**Figure 14.** Relationship between $\Delta\delta^{13}\text{C}$ of *C. wuellerstorfi* versus *Globobulimina* (spp.) and bottom water oxygen. The white circles represent outliers that were excluded from the regression. Sample details can be found in Supplement Table S2.</figcaption></figure>

## 11.2 History of development and use

McCorkle and Emerson (1988) and McCorkle et al. (1990) first proposed that bottom water oxygen concentrations could be quantified using $\delta^{13}\text{C}$ in benthic foraminifera. They recognized that as remineralization of organic matter in pore waters consumes oxygen and releases isotopically light DIC in relatively fixed ("Redfield ratio") proportions, $\delta^{13}\text{C}$ in pore waters relative to overlying bottom waters will reflect the amount of oxygen consumed in pore waters as a function of bottom water oxygen concentrations. They further suggested that paired specimens of benthic foraminifera that record bottom water and deep pore water carbon isotopes could be used to record this gradient and thus bottom water oxygen concentrations in the past.

McCorkle et al. (1990) verified that the $\Delta\delta^{13}\text{C}$ of deep infaunal living benthic foraminifera *Globobulimina affinis* and the overlying bottom water are similar to the $\Delta\delta^{13}\text{C}$ between the pore water around the anoxic boundary and overlying bottom waters. Schmiedl and Mackensen (2006) showed that the $\Delta\delta^{13}\text{C}$ between *Globobulimina* species and bottom water DIC correlates well with bottom water oxygen concentrations, and they applied the derived relationship to reconstruct past bottom water oxygen in the Arabian Sea. Hoogakker et al. (2015) used existing (Mackensen and Licari, 2004; Schmiedl and Mackensen, 2006; Fontanier et al., 2008) and new data to show that the $\Delta\delta^{13}\text{C}$ of epifaunal *C. wuellerstorfi* and deep infaunal *Globobulimina* species from six locations of the Atlantic, Indian, and Pacific oceans show a similar relationship to $\Delta\delta^{13}\text{C}$ estimates between bottom water and pore water around the anoxic boundary from these basins (but at different locations) (Fig. 14). They showed that

$\Delta\delta^{13}\text{C}$ between *C. wuellerstorfi* and *Globobulimina* spp. is linearly correlated with bottom water oxygen levels in the range of 50–235 µmol kg$^{-1}$ and used this new calibration to reconstruct bottom water oxygen variations in the North Atlantic. The proxy has been applied in several studies to quantitatively reconstruct past bottom water oxygen concentrations in various ocean regions, with an estimated total error of 17 µmol kg$^{-1}$, including the North Atlantic (Hoogakker et al., 2016; Thomas et al., 2022), South Atlantic (Gottschalk et al., 2016b), Indian Ocean (Schmiedl and Mackensen, 2006; Gottschalk et al., 2020b), and Pacific Ocean (Hoogakker et al., 2018; Umling and Thunell, 2018; Jacobel et al., 2020). Figure 14 shows the most up to date core-top calibration between $\Delta\delta^{13}\text{C}$ obtained from *C. wuellerstorfi* and *Globobulimina* spp. and bottom water oxygen, with bottom water oxygen and new data points from Umling and Thunnell (2018), Jacobel et al. (2020), and Thomas et al. (2022), as detailed in the Supplement.

## 11.3 Description of analyses and resources required

Stable carbon isotope ratios from epifaunal (*Cibicidoides* spp.) and deep infaunal benthic foraminifera (*Globobulimina* spp.) can be measured using a mass spectrometer, commonly a Thermo Kiel V automated preparation device coupled with a Thermo Delta V Plus or Thermo MAT 253 mass spectrometer. Measurements are calibrated to the international Vienna Pee Dee Belemnite (VPDB) standard. The sample mass required for analysis varies and depends on the mass spectrometer setup. Modern mass spectrometers typically require 40–80 µg CaCO$_3$ but can provide measurements for samples as small as 5 µg CaCO$_3$. Specimen size does not seem to matter for the $\delta^{13}\text{C}$ of *C. wuellerstorfi* (Franco-Fraguas et al., 2011), whereas size effects on *Globobulimina* spp. have not yet been explored. *Cibicidoides* spp. specimens tend to weigh more compared to *Globobulimina* spp. specimens. Selecting foraminifera from the fraction larger than 250 µm, about 1 to 4 specimens of *Cibicidoides* and 2 to 20 specimens of *Globobulimina* spp. are needed for stable isotope analysis. Dirty specimens may need some treatment prior to analyses, including crushing, ultrasonication, and/or methanol rinses, to remove clay particles.

The most commonly used deep infaunal *Globobulimina* species for bottom water oxygen reconstructions via the $\Delta\delta^{13}\text{C}$ proxy is *Globobulimina affinis*. In the absence of *G. affinis*, other *Globobulimina* spp. have been used as mono- or multispecific samples (e.g. Hoogakker et al., 2018), assuming all share a similar deep infaunal depth habitat: *G. pacifica*, *G. turgida*, and *G. auriculata* (Supplement Fig. S1). A deep infaunal species of the genus *Chilostomella*, a potential candidate for monitoring conditions at the anoxic boundary, does not form its test in equilibrium with pore water DIC (McConnaughey et al., 1997; Nomaki et al., 2021). When *C. wuellerstorfi* is absent, other foraminiferal species thought to live epifaunally, such as *Cibicides kullenbergi* (synony-
mously used with *Cibicides mundulus*), have also been used to reconstruct bottom water oxygen levels using the Δδ¹³C proxy (e.g. Gottschalk et al., 2016a, 2020a; Bunzel et al., 2017; Lu et al., 2022) with some notable caveats for their use (Supplement Fig. S2).

## 11.4 Proxy limitations

The infaunal habitat of *Globobulimina* spp. may extend below the depth of sedimentary anoxia in some locations (and/or time periods) (Geslin et al., 2004). This may potentially lead to the incorporation of additional isotopically light carbon generated at depth via anoxic processes, including denitrification by *Globobulimina* spp. and/or sulfate reduction (McCorkle and Emerson, 1988). Variations in depth habitat are of concern because denitrification and sulfate reduction are known to play a significant role in the mineralization of organic matter, releasing isotopically light carbon to pore waters after oxygen has been consumed. Sulfate reduction and other early diagenetic reactions are of particular concern in margin settings that are shallower than 1500 m (Sarmiento and Gruber, 2006) as more organic matter is generally deposited in these settings. Sulfate reduction can lead to the shallowing of the early diagenetic zones (Egger et al., 2018) and an increased diffusive flux of DIC into the zone of aerobic respiration. Thus, there may be a variable contribution of anaerobic processes to the pore water DIC from which *Globobulimina* spp. calcify. If one or more of these influences are at play, Δδ¹³C is expected to be elevated, and the calibration would overestimate bottom water oxygen concentrations.

Recent work has shown that at least four species of *Globobulimina* spp. (including *affinis*, *pacifica*, *turgida*, and *pseudospinescens*) concentrate nitrate for use as an electron acceptor in the absence of oxygen (Risgaard-Petersen et al., 2006; Nomaki et al., 2015; Piña-Ochoa et al., 2010a, b). Metabolic and genetic data corroborate the capability of *Globobulimina* species to denitrify, one of the main reasons they are successful in anoxic settings (Piña-Ochoa et al., 2010b; Woehle et al., 2018, 2022). These results may imply that *Globobulimina* spp. may thrive and calcify successfully well below the anoxia boundary, meaning they could be influenced by the addition of low-δ¹³C DIC during sulfate and nitrate reduction, although to date there is no direct evidence of this. Furthermore, if *Globobulimina* spp. contribute isotopically light carbon to the sedimentary pore waters, this could potentially decouple the relationship between bottom water oxygen and Δδ¹³C. The influence of shallow denitrification has been invoked to explain observations of inconsistent Δδ¹³C between five contemporaneous records of Δδ¹³C from the EEP ocean (Jacobel et al., 2020) and between co-located records of U/Ba and Δδ¹³C, where U/Ba yields consistently lower estimates of bottom water oxygen concentrations (Costa et al., 2023). Significant inconsistencies between core-top bottom water oxygen

concentrations, determined using Δδ¹³C, and measured bottom water oxygen have also been identified at some equatorial Pacific sites (Jacobel et al., 2020). Separately, higher Δδ¹³C values found in the OMZ of the Arabian Sea (Lu et al., 2022) have been attributed to sulfate reduction in sediments. Specifically, Δδ¹³C-based bottom water oxygen estimates at these sites are more than 60 µmol kg⁻¹ higher than those reconstructed using other proxies (i.e. benthic surface porosity, benthic I / Ca, and aU) (Lu et al., 2022). Indeed, the three outliers in the updated Δδ¹³C–bottom water oxygen relationship in Fig. 14 are from areas where sulfate reduction is known to play an important role (Bradbury et al., 2021, 2024; Thomas et al., 2022).

Another factor that may drive variations in Δδ¹³C independent of bottom water oxygen is changes in the carbon isotopic composition of organic material (δ¹³Corg) that is mineralized in the pore space of marine sediments (Hoogakker et al., 2015). A decrease in δ¹³Corg would enhance Δδ¹³C and cause an apparent increase in reconstructed bottom water oxygen. It is therefore important to assess δ¹³Corg alongside Δδ¹³C-based bottom water oxygen quantifications.

Finally, although previous work has shown a generally strong correspondence between the δ¹³C of DIC in bottom waters and the δ¹³C of *C. wuellerstorfi* (e.g. Schmittner et al., 2017), there is some evidence that seasonal pulses of organic material (the phytodetritus effect) may decrease the δ¹³C of epifaunal species by as much as 0.4‰ (Zarriess and Mackensen, 2011), perhaps due to the development of benthic “fluff” layers in which *C. wuellerstorfi* may calcify (Mackensen et al., 1993). This effect has not been found in all locations experiencing seasonally variable production (Corliss et al., 2006), emphasizing the need for further work to develop the regional and/or time-variant conditions under which these effects develop. Insights into this open question could be derived from core-top samples where organic carbon flux, δ¹³CC.org, δ¹³CC.wuellerstorfi, and bottom water oxygen are directly measured to evaluate and quantify relationships.

## 11.5 Species relevant for calibration

Application of the Δδ¹³C proxy is limited by the availability of epifaunal (*C. wuellerstorfi* or *C. mundulus*) and deep infaunal (*Globobulimina* spp.) species in the sediment samples. However, some epifaunal species can adopt a shallow infaunal habitat and can therefore be influenced by the pore water environment (Gottschalk et al., 2016b; Wollenburg et al., 2021). Figure S2 shows an extended version of Fig. 14 including other *Cibicidoides* species. Temporal variations in the δ¹³C offsets of *Cibicides* species and *C. wuellerstorfi* could be an indication of a change in habitat (e.g. Gottschalk et al., 2016b). If there is an indication of temporal variations in this offset or information about offsets is unavailable, the application of the Δδ¹³C proxy based on presumably shallow infaunal species such as *C. mundulus* may lead to a bias
in bottom water oxygen concentrations towards lower values (e.g. Gottschalk et al., 2016a).

Because *Globobulimina* spp. has a deeper habitat compared with *C. wuellerstorfi*, there is the possibility that the measurements are age-offset. Upon death, sediment stirring through benthic organisms will mix the sediments with a bioturbation depth unique to the sedimentary environment at the time of deposition. It is unlikely that the two species in the fossil record maintain the depth offsets observed in living specimens. Comparison of the stable oxygen isotope records of both species is thus critical for ruling out or determining the appropriate, possibly time-variant depth offset between species (Hoogakker et al., 2015).

## 11.6 Future directions and open questions

Direct comparison and correlation between living and dead *C. wuellerstorfi* and *Globobulimina* spp. $\delta^{13}$C, measurements of bottom and pore water oxygen, and the $\delta^{13}$C of bottom and pore water DIC would be ideal for rigorously quantifying calibration uncertainties. The calibration could also be strengthened by quantitative assessment of covarying environmental parameters, such as the flux of organic carbon, the $\delta^{13}$C of $C_{org}$, and the influence of sulfate reduction and denitrification at calibration sites. This could be achieved through coring campaigns and culturing studies following the methods of Wollenburg et al. (2015).

Several possibilities exist for expanding the use of the $\Delta\delta^{13}$C proxy. For example, the improvement of analytical techniques now allows for the analysis of single specimens with respect to oxygen and carbon isotopes (e.g. Ishimura et al., 2004). Analysis of single-specimen $\delta^{13}$C may provide insights into the natural variability of $\delta^{13}$C of communities and improve our interpretation of $\Delta\delta^{13}$C. Additionally, application of the $\Delta\delta^{13}$C proxy could be expanded if other deep infaunal species are found to record pore water $\delta^{13}$C of DIC at the anoxic boundary.

## 12 Concluding summary statement and future directions

In this review, we summarize the current state of knowledge about proxies for reconstructing Cenozoic marine oxygen levels. Sediments are the carriers for all proxies associated with seawater oxygen reconstructions. Sedimentological and other non-destructive methods, as well as the presence, relative abundance, and potential trace element compositions of pyrite, provide important information about depositional redox conditions.

Bulk geochemical methods that can be used to reconstruct bottom water redox/oxygen conditions, as well as methods that involve fossil foraminiferal abundance, appearance, and geochemistry, are described:

1. Redox-sensitive elements that are preserved under various redox potentials have provided key insights into deep-ocean oxygenation on a variety of timescales. However, challenges remain, and redox element research continues to refine the interpretations of these proxies by constraining variations in other environmental variables (notably the rain of organic carbon) that affect the redox state of sediments. Recent technical advances have allowed for the development of novel non-traditional stable metal isotope systems, which open new possibilities for more quantitative redox reconstructions and globally integrated estimates of ocean oxygenation through time.

2. Lipid biomarkers provide a wealth of paleoceanographic information. Their source specificity and excellent preservation potential allow for detailed and comprehensive reconstruction of water column (and sediment) redox conditions. Taxonomically specific biomarkers are available for a range of microorganisms thriving in different ecological redox niches, providing insights into past changes in the ocean's carbon, nitrogen, and sulfur cycles. Instrumental advancements and increased resolution continue to widen the analytical window, reveal novel biomarkers, and – in combination with (meta)genomics – aid identification of source organisms. Moreover, biomarker proxies are becoming more and more quantitative, and the community strives to develop tools that allow for inferring absolute oxygen concentrations.

3. Bulk nitrogen isotopes offer insights into bacterial denitrification processes that are closely linked to water column oxygen concentrations below $< 5 \mu mol kg^{-1}$. The strong isotopic discrimination by denitrifying bacteria can be measured in bulk sediments. Isotopic discrimination by denitrifying bacteria can also be measured in foraminifera-bound $\delta^{15}$N, and this method shows great promise for understanding dynamics of OMZs. We highlight the need for integrated biogeochemical models in order to refine interpretations of nitrogen isotope records.

4. Foraminiferal trace elements, especially I/Ca, Mn/Ca, and U/Ca, show promise as proxies for reconstructing past oxygen conditions within the constraints of the complexities arising from various environmental factors and potential interferences. I/Ca values are linked to the presence of $IO_3^-$ and its reduction to $I^-$ in low-oxygen settings. U/Ca utilizes the formation of authigenic U coatings on foraminiferal tests buried in marine sediments. Higher U/Ca concentrations are indicative of reducing oceanic bottom water conditions. Higher Mn/Ca in foraminiferal calcite indicates increased free $Mn^{2+}$ incorporation under low-oxygen bottom/pore water
conditions. Foraminiferal trace element proxies require careful consideration of carbonate chemistry, variable oxygen thresholds, vital effects, ontogenetic effects, and potential diagenetic effects that can distort the signals in the geologic record. Further work is needed to establish robust calibrations for the relationships between proxies and oxygen conditions in different environmental settings and for different foraminiferal species.

5. Foraminiferal assemblages have a long tradition as paloproxies. Benthic foraminiferal assemblages are particularly sensitive to changes in bottom water oxygenation due to specific adaptations of some benthic species, including use of anaerobic metabolisms, to survive in O₂-depleted environments. In some cases, it is difficult to decouple changes in bottom water oxygenation from changes in organic matter input. One bottleneck in using foraminiferal assemblages is the high workload involved in sample processing, including picking foraminifera and taxonomic classification of each sample. The main advantages of this method are the low instrumental and resource requirements for this approach. Future directions include AI-based automation of species recognition, which will greatly reduce the amount of time involved, and the more routine use of planktic foraminiferal assemblages to reconstruct O₂ concentrations in the water column.

6. Various morphological features of foraminiferal shells reflect the environmental conditions in which they grew, including oxygen concentrations. Shell porosity specifically reflects (a) the availability of oxygen for oxygen-respiring species and (b) nitrate availability for species specialized in denitrification. Although a lot of focus has been placed on morphological features of benthic foraminifera, advances in understanding planktic foraminiferal morphology open new windows for oxygen reconstruction within different layers of the water column. The recent improvement in automated image analysis facilitates the quick generation of large datasets, while non-destructive methods for image acquisition preserve the analysed specimens for further analyses. In addition, the development of image acquisition methods and broader availability of µCT and nanoCT techniques allow for 3D analyses of specimens to further provide access to morphological details that were hidden before.

7. There has been significant progress in employing the carbon isotope gradient between epifaunal and infaunal benthic foraminifera as a proxy to reconstruct bottom water oxygen concentrations. Multi-proxy work has been key in identifying sources of proxy uncertainty that are currently not well quantified and in highlighting depositional environments where Δδ¹³C may not work

well, specifically areas where sedimentary denitrification and sulfate reduction are prevalent. Our review emphasizes that Δδ¹³C-based reconstructions are likely to provide estimates that represent an upper bound of past bottom water oxygen concentrations. Further research into uncertainties has the potential to improve the quantitative nature of the proxy. Specifically, we recommend focusing on the fidelity of different species in recording the δ¹³C values of bottom and pore waters, the role of carbon isotope composition of organic carbon, the significance of biases arising from contributions from anaerobic metabolic processes, and how changes in the rain rate of organic carbon may influence the proxy.

Proxies are by definition indirect measurements, each with their own sources of uncertainty, biases, limitations, and drivers, as detailed in the sections above. For this reason, we recommend applying a multi-proxy approach wherever possible, in which two or more proxy records are generated in tandem from the sample set. Ideally, the design of a multi-proxy study should incorporate multiple proxies for the same or related parameters with different sources of uncertainty.

Multi-proxy approaches are particularly appropriate in the field of paleo-oxygenation, where most available proxies are semi-quantitative and cover different ranges of redox chemistries (Fig. 1) and, in some cases, may have only been recently developed. They may also differ in their drivers, with some proxies having multiple drivers, which may be independent of oxygen. The layering of multiple, semi-quantitative proxies can allow researchers to assign more quantitative paleo-oxygen estimates (e.g. FB-δ¹⁵N and planktic foraminiferal I/Ca in Hess et al., 2023), an exercise that may be pivotal in generating paleo-oxygen reconstructions that can inform models. The rapid development of novel paleo-oxygen proxies, such as the use of biomagnetic particles (Chang et al., 2023), not discussed here, will continue to benefit the field. However, the limitations of and uncertainties in more recently developed proxies need to be further explored. Multi-proxy approaches can serve to validate these proxies (e.g. comparing benthic I/Ca, U, and foraminiferal porosity as bottom water oxygen proxies in Lu et al., 2022) and increase our understanding of their application in the sedimentary record (e.g. δ¹³C and U in Jacobel et al., 2020). Finally, inclusion of multiple proxies may allow researchers to disentangle multiple drivers in the paleo-record, constraining not only oxygen but also related environmental factors, such as export productivity or carbon fluxes, redox structure of the water column and sediment, or depositional settings.

## 13 Recommendations for data management and transparency

The interdisciplinarity of the communities generating oxygen proxy data and model outputs and the presently increasing number of applications make the implementation
of FAIR data practices critical. Data availability from publications and data repositories (i.e. PANGAEA, NOAA) for proxy reconstructions (long term) and validation of benthic and pelagic proxies are increasing rapidly. Moving forward, it is important that data are easily accessible to the wider scientific community using the guiding principles of findability, accessibility, interoperability, and reusability (FAIR; Wilkinson et al., 2016).

Proxy data produced for paleo-oxygen reconstructions are heterogeneous in terms of material (e.g. sediment, calcite, organic matter), methodology, chronology, and data formats. It is important that we standardize oxygen proxy datasets, including qualitatively, semi-quantitatively, and quantitatively, with assessed and reported error margins. As part of the FAIR principles, it is important that metadata (sample identifier, core name and sections, location, depth, etc.) and raw data (original analyses counts, etc.) are reported. Through networking activities, scientists can promote FAIR principles, and several institutes and journals are already doing so. It is important that this effort is also reflected in the paleo- and oceanographic communities. Following FAIR principles in oxygen proxy data management will improve data accessibility for scientists from the same discipline, those in other disciplines, and policymakers.

Below we provide recommendations for data management that follow the guidelines for modern proxy validation data and reconstructions as proposed by Khider et al. (2019), Morrill et al. (2021), Jonkers et al. (2021), Mulitza et al. (2022), Muglia et al. (2023), and Paradis et al. (2023).

Organize files following the *findability (F)* principle and provide a *unique identifier* for the files (see example in the Supplement).

Deposit data files in a public database/repository in order to follow the *accessibility (A)* principle. The most commonly used data repositories for paleoceanographic data are PANGAEA and NCEI, and GitHub is most commonly used for code. Recently, Zenodo has also emerged as an alternative repository. To make data widely accessible, add the unique repository link. Data can be submitted to repositories at any time prior to paper submission, and authors can determine when data become available to the public. Data should be made available upon publication of papers.

Organize data and save the file in a format accessible with different operating systems (e.g. Linux, Windows, iOS). The file needs to be in a format that is accessible and can be edited without altering the order of the data to adopt the *interoperability (I)* principle. We recommend files in comma-separated values (CSVs). Files with a CSV format can be easily opened/read by data visualization and statistical software such as e.g. Excel, RStudio, Python, PaleoDataView (PDW), and Ocean Data View (ODV). Provide auxiliary information (metadata, depth model, age, proxy data) for each site.

To optimize data use, follow the principle of *reusability (R)*. There needs to be a dataset descriptor containing all the

necessary details to ensure reusability and/or replication. The original references should be cited when reusing data.

**Data availability.** All data referenced in the text is openly available and can be found in the cited sources.

**Supplement.** The supplement related to this article is available online at https://doi.org/10.5194/bg-22-863-2025-supplement.

**Author contributions.** JC, CD, BH, KNK, and DRM organized the workshop in September 2022 that initiated this review, supported by PAGES. BH, CD, YW, SK, DSH, AJ, DRM, NG, SN, JS, AR, AA, AH, KM, and JC took the lead in writing various sections of the paper, with BH and CD moderating the final text. All further authors (RA, ChrB, ChaB, HB, IB, AC, MC, KC, CC, PD, JD, FE, ZE, HF, SG, JG, AGM, JG, CH, IH, RH, WL, JLS, LM, AMG, GM, PM, SM, JM, SN, SR, MR, CS, AS, KT, RT, MT, TV, SW, YaZ, XYZ, and YuZ) contributed to the discussions, writing of the text, and creation of the figures.

**Competing interests.** The contact author has declared that none of the authors has any competing interests.

**Disclaimer.** Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors.

**Special issue statement.** This article is part of the special issue "Low-oxygen environments and deoxygenation in open and coastal marine waters". It is not associated with a conference.

**Acknowledgements.** We thank ICP14 and PAGES (https://pastglobalchanges.org/, last access: 11 February 2025) for providing logistic and travel support that facilitated the writing of this paper. The paper benefitted from discussions with Trinity Ford, Philip Froelich, Zunli Lu, Tim Sweere, and Qingchen Wang and proofreading of an earlier version of the text by Alex Poulton. The paper benefitted from reviews from Thomas J. Algeo, Andy Gooday, and one anonymous referee. Furthermore, the paper benefitted from comments by Ellen Thomas.

**Financial support.** Babette Hoogakker was supported by a UK Research and Innovation (UKRI) FLF (grant no. MR/S034293/1). Nicolaas Glock was supported by the Deutsche Forschungsgemeinschaft (DFG) via the grants GL 999/3-1 and GL 999/4-1. Julia Gottschalk was supported by the Deutsche Forschungsmeinschaft (DFG) through grant GO 2294/3-1. Felix Elling was
supported by the Deutsche Forschungsgemeinschaft through grant 441217575. Alexis Castillo was supported by the FONDECYT project 11230555. Dalton Hardisty was supported by an Alfred Sloan Fellowship. Morgan Raven was supported by NSF grant no. OCE-2143817. Christopher Somes was supported by the Deutsche Forschungsgemeinschaft (project no. 445549720). Raul Tapia was supported by the National Science and Technology Council of Taiwan (112-2116-M-002-003, 113-2611-M-002-008, 113-2811-M-002-039). Lelia Matos was supported by FCT through project nos. UIDB/04326/2020, UIDP/04326/2020, LA/P/0101/2020, and 2022.05765.PTDC. Simone Moretti was supported by a DFG grant (MO 4402/2-1). Helena L. Filipsson, Inda Brinkmann, Constance Choquel, and Sha Ni were supported by the Swedish Research Council VR (grant no. 2017-04190).

*Review statement.* This paper was edited by Lisa Levin and reviewed by Thomas J. Algeo, Andy Gooday, and one anonymous referee.

## References

Abken, H. J., Tietze, M., Brodersen, J., Bäumer, S., Beifuss, U., and Deppenmeier, U.: Isolation and characterization of methanophenazine and function of phenazines in membrane-bound electron transport of *Methanosarcina mazei* Göl, J. Bacteriol., 180, 2027–2032, https://doi.org/10.1128/jb.180.8.2027-2032, 1998.

Adam, P., Schneckenburger, P., Schaeffer, P., and Albrecht, P.: Clues to early diagenetic sulfurization processes from mild chemical cleavage of labile sulfur-rich geomacromolecules, Geochim. Cosmochim. Ac., 64, 3485–3503, https://doi.org/10.1016/S0016-7037(00)00443-9, 2000.

Ahrens, J., Beck, M., Böning, P., Degenhardt, J., Schnetger, B., and Brumsack, H.: Thallium cycling in pore waters of intertidal beach sediments, Geochim. Cosmochim. Ac., 306, 321–339, https://doi.org/10.1016/j.gca.2021.04.009, 2021.

Akintomide, O. A., Adebayo, S., Horn, J. D., Kelly, R. P., and Johannesson, K. H.: Geochemistry of the redox-sensitive trace elements molybdenum, tungsten, and rhenium in the eu-xinic porewaters and bottom sediments of the Pettaquams-cutt River estuary, Rhode Island, Chem. Geol., 584, 120499, https://doi.org/10.1016/j.chemgeo.2021.120499, 2021.

Al-Farawati, R. and van den Berg, C. M. G.: Metal-sulfide complexation in seawater, Mar. Chem., 63, 331–352, https://doi.org/10.1016/S0304-4203(98)00056-5, 1999.

Alfken, S., Wörmer, L., Lipp, J. S., Napier, T., Elvert, M., Wendt, J., Schimmelmann, A., and Hinrichs, K. -U.: Disrupted coherence between upwelling strength and redox conditions reflects source water change in Santa Barbara Basin during the 20th century, Paleocean. Paleoclim., 36, e2021PA004354, https://doi.org/10.1029/2021PA004354, 2021.

Algeo, T. J.: Can marine anoxic events draw down the trace element inventory of seawater?, Geology, 32, 1057–1060, https://doi.org/10.1130/G20896.1, 2004.

Algeo, T. J. and Maynard, J. B.: Trace-element behaviour and redox facies in core shales of Upper Pennsylvanian Kansas-type cyclothems, Chem. Geol., 206, 289–318, 2004.

Algeo, T. J., and Lyons, T. W. Mo-total organic carbon covariation in modern anoxic marine environments: Implications for analysis of paleoredox and paleohydrographic conditions, Paleocean. Paleoclim., 21, PA1016, https://doi.org/10.1029/2004PA001112, 2006.

Algeo, T. J. and Tribovillard, N.: Environmental analysis of paleoceanographic systems based on molybdenum-uranium covariation, Chem. Geol., 268, 211–225, https://doi.org/10.1016/j.chemgeo.2009.09.001, 2009.

Algeo, T. J. and Rowe, H.: Paleoceanographic applications of trace-metal concentration data, Chem. Geol., 324, 6–18, https://doi.org/10.1016/j.chemgeo.2011.09.002, 2012.

Algeo, T. J., Meyers, P. A., Robinson, R. S., Rowe, H., and Jiang, G. Q.: Icehouse-greenhouse variations in marine denitrification, Biogeosciences, 11, 1273–1295, https://doi.org/10.5194/bg-11-1273-2014, 2014.

Algeo, T. J. and Li, C.: Redox classifications and calibration of redox thresholds in sedimentary systems, Geochim. Cosmochim. Ac., 287, 8–26, https://doi.org/10.1016/j.gca.2020.01.055, 2020.

Alibo, D. S. and Nozaki, Y.: Rare earth elements in seawater: particle association, shale-normalization, and Ce oxidation, Geochim. Cosmochim. Ac., 63, 363–372, https://doi.org/10.1016/S0016-7037(98)00279-8, 1999.

Altabet, M. A., Deuser, W. G., and Honjo, S.: Seasonal and depth-related changes in the source of sinking particles in the North Atlantic, Nature, 354, 136–139, https://doi.org/10.1038/354136a0, 1991.

Altabet, M. A. and Francois, R.: Sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization, Glob. Biogeochem. Cy., 8, 103–116, https://doi.org/10.1029/93GB03396, 1994.

Altabet, M. A., Francois, R., Murray, D. W., and Prell, W. L.: Climate-related variations in denitrification in the Arabian Sea from sediment $^{15}$N/$^{14}$N ratios, Nature, 373, 506–509, https://doi.org/10.1038/373506a0, 1995.

Altenbach, A. V., Pflaumann, U., Schiebel, R., Thies, A., Timm, S., and Trauth, M.: Scaling percentages and distributional patterns of benthic foraminifera with flux rates of organic carbon, J. Foramin. Res., 29, 173–185, 1999.

Alve, E. and Bernhard, J. M.: Vertical migratory response of benthic foraminifera to controlled oxygen concentrations in an experimental mesocosm, Mar. Ecol. Prog. Ser., 116, 137–151, https://doi.org/10.3354/meps116137, 1995.

Alve, E. and Murray, J. W.: Temporal variability in vertical distributions of live (stained) intertidal foraminifera, Southern England, J. Foramin. Res., 31, 12–24, https://doi.org/10.2113/0310012, 2001.

Amos, C. L., Sutherland, T. F., Radzijewski, B., and Doucette, M.: A rapid technique to determine bulk density of fine-grained sediments by X-ray computed tomography, J. Sediment. Res., 66, 1023–1039, https://doi.org/10.1306/D4268144-2B26-11D7-8648000102C1865D, 1996.

Amrani, A. and Aizenshtat, Z.: Reaction of polysulfide anions with α,β unsaturated isoprenoid aldehydes in aquatic media: simulation of oceanic conditions, Org. Geochem., 35, 909–21, https://doi.org/10.1016/j.orggeochem.2004.04.002, 2004.

Amrani, A., Said-Ahamed, W., and Aizenshtat, Z.: The $\delta^{34}$S values of the early-cleaved sulfur upon low temperature pyrolysis of a synthetic polysulfide cross-linked polymer, Org. Geochem.,
36, 971–74, https://doi.org/10.1016/j.orggeochem.2005.02.002, 2005.

Andersen, M., Romaniello, S., Vance, D., Little, S., Herdman, R., and Lyons, T.: A modern framework for the interpretation of $ ^{238}U / ^{235}U $ in studies of ancient ocean redox, Earth Planet. Sc. Lett., 400, 184–194, https://doi.org/10.1016/j.epsl.2014.05.051, 2014.

Andersen, M. B., Stirling, C. H., and Weyer, S.: Uranium isotope fractionation, Rev. Mineral. Geochem., 82, 799–850, https://doi.org/10.2138/rmg.2017.82.19, 2017.

Andersen, M. B., Matthews, A., Vance, D., Bar-Matthews, M., Archer, C., and de Souza, G.: A 10-fold decline in the deep Eastern Mediterranean thermohaline overturning circulation during the last interglacial period, Earth Planet. Sc. Lett., 503, 58–67, https://doi.org/10.1016/j.epsl.2018.09.013, 2018.

Andersen, M., Matthews, A., Bar-Matthews, M., and Vance, D.: Rapid onset of ocean anoxia shown by high U and low Mo isotope compositions of sapropel S1, Geochem. Perspect. Lett., 15, 10–14, https://doi.org/10.7185/geochemlet.2027, 2020.

Anderson, L. and Sarmiento, J.: Redfield ratios of remineralization determined by nutrient data analysis, Global Biogeochem. Cy., 8, 65–80, https://doi.org/10.1029/93GB03318, 1994.

Anderson, R. F., LeHuray, A. P., Fleisher, M. Q., and Murray, J. W.: Uranium deposition in saanich inlet sediments, vancouver island, Geochim. Cosmochi. Ac., 53, 2205–2213, https://doi.org/10.1016/0016-7037(89)90344-X, 1989.

Anderson T. F. and Pratt L. M.: Isotopic evidence for the origin of organic sulfur and elemental sulfur in marine sediments, Acs. Sym. Ser., 612, 378–396, https://doi.org/10.1021/bk-1995-0612.ch021, 1995.

Anderson, R. F., Sachs, J. P., Fleisher, M. Q., Allen, K. A., Yu, J., Koutavas, A., and Jaccard, S. L.: Deep-Sea Oxygen Depletion and Ocean Carbon Sequestration During the Last Ice Age, Global Biogeochem. Cy., 33, 301–317, https://doi.org/10.1029/2018GB006049, 2019.

Angell, R. W.: The Test Structure of the Foraminifer Rosalina floridana, J. Protozool., 14, 299–307, https://doi.org/10.1111/j.1550-7408.1967.tb02001.x, 1967.

Anschutz, P., Jorissen, F. J., Chaillou, G., Abu-Zied, R., and Fontanier, C.: Recent turbidite deposition in the eastern Atlantic: early diagenesis and biotic recovery, J. Mar. Res., 60, 835–854, https://doi.org/10.1357/002224002321505156, 2002.

Arndt, S., Jørgensen, B. B., LaRowe, D. E., Middelburg, J. J., Pancost, R. D., and Regnier, P.: Quantifying the degradation of organic matter in marine sediments: A review and synthesis, Earth-Sci. Rev., 123, 53–86, https://doi.org/10.1016/j.earscirev.2013.02.008, 2013.

Arnold, Z. M.: A Note on Foraminiferan Sieve Plates, Contrib. Cushman Found. Foramin. Res., 5, 77–67, 1954.

Auderset, A., Moretti, S., Taphorn, B., Ebner, P.-R., Kast, E., Wang, X. T., Schiebel, R., Sigman, D. M., Haug, G. H., and Martínez-García, A.: Enhanced ocean oxygenation during Cenozoic warm periods, Nature, 609, 77–82, https://doi.org/10.1038/s41586-022-05017-0, 2022.

Azrieli-Tal, I., Matthews, A., Bar-Matthews, M., Almogi-Labin, A., Vance, D., Archer, C., and Teutsch, N.: Evidence from molybdenum and iron isotopes and molybdenum–uranium covariation for sulphidic bottom waters during Eastern Mediterranean

sapropel S1 formation, Earth Planet. Sc. Lett., 393, 231–242, https://doi.org/10.1016/j.epsl.2014.02.054, 2014.

Baas, J. H., Schönfeld, J., and Zahn, R.: Mid-depth oxygen drawdown during Heinrich events: evidence from benthic foraminiferal community structure, trace-fossil tiering, and benthic $ \delta^{13}C $ at the Portuguese Margin, Mar. Geol., 152, 25–55, https://doi.org/10.1016/S0025-3227(98)00063-2, 1998.

Bahl, A., Gnanadesikan, A., and Pradal, M. A.: Variations in ocean deoxygenation across earth system models: isolating the role of parameterized lateral mixing, Global Biogeochem. Cy., 33, 703–724, https://doi.org/10.1029/2018GB006121, 2019.

Bale, N. J., Hopmans, E. C., Zell, C., Sobrinho, R. L., Kim, J.-H., Sinninghe Damsté, J. S., Villareal, T. A., and Schouten, S.: Long chain glycolipids with pentose head groups as biomarkers for marine endosymbiotic heterocystous cyanobacteria, Org. Geochem., 81, 1–7, https://doi.org/10.1016/j.orggeochem.2015.01.004, 2015.

Bale, N. J., Hennekam, R., Hopmans, E. C., Dorhout, D., Reichart, G.-J., Meer, M. van der, Villareal, T. A., Damsté, J. S. S., and Schouten, S.: Biomarker evidence for nitrogen-fixing cyanobacterial blooms in a brackish surface layer in the Nile River plume during sapropel deposition, Geology, 47, 1088–1092, https://doi.org/10.1130/G46682.1, 2019.

Bandy, O. L.: Ecology and paleoecology of some California foraminifera, Part I. The frequency distribution of recent foraminifera off California, J. Paleontol., 27, 161–182, 1953.

Barker, S. and Elderfield, H.: Foraminifera calcification response to glacial-interglacial changes in atmospheric $CO_2$, Science, 297, 833–836, https://doi.org/10.1126/science.1072815, 2002.

Barker, S., Greaves, M., and Elderfield, H.: A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry, Geochem. Geophy. Geosy., 4, 1525–2027, https://doi.org/10.1029/2003GC000559, 2003.

Barmawidjaja, D. M., Van der Zwaan, G. J., Jorissen, F. J., and Puskaric, S.: 150 years of eutrophication in the northern Adriatic Sea: evidence from a benthic foraminiferal record, Mar. Geol., 122, 367–384, https://doi.org/10.1016/0025-3227(94)00121-Z, 1995.

Barnes, C. E. and Cochran, J. K.: Uranium removal in oceanic sediments and the oceanic U balance, Earth Planet. Sc. Lett., 97, 94–101, https://doi.org/10.1016/0012-821X(90)90101-3, 1990.

Barras, C., Mouret, A., Nardelli, M. P., Metzger, E., Petersen, J., La, C., Filipsson, H. L., and Jorissen, F.: Experimental calibration of manganese incorporation in foraminiferal calcite, Geochim. Cosmochim. Ac., 237, 49–64, https://doi.org/10.1016/j.gca.2018.06.009, 2018.

Barrenechea Angeles, I., Romero-Martinez, M.L., Cavaliere, M., Varella, S., Francescangeli, F., Schirone, A., Delbono, I., Margiotta, F., Corinaldesi, C., Chiavarini, S., Montereali, M.R., Rimauro, J., Parrella, L., Musco, L., Dell'Anno, A., Tangherlini, M., Pawlowski, J., and Frontalini, F.: Encapsulated in sediments: eDNA deciphers the ecosystem history of one of the most polluted European marine sites, Enviro. Int., 172, 107738, https://doi.org/10.1016/j.envint.2023.107738, 2023.

Batista, F. C., Ravelo, A. C., Crusius, J., Casso, M. A., and McCarthy, M. D.: Compound specific amino acid $ \delta^{15}N $ in marine sediments: A new approach for studies of the marine nitrogen cycle, Geochim. Cosmochim. Ac., 142, 553–569, https://doi.org/10.1016/j.gca.2014.08.002, 2014.
Bauersachs, T., Compaoré, J., Hopmans, E. C., Stal, L. J., Schouten, S., and Sinninghe Damsté, J. S.: Distribution of heterocyst glycolipids in cyanobacteria, Phytochemistry, 70, 2034–2039, https://doi.org/10.1016/j.phytochem.2009.08.014, 2009a.

Bauersachs, T., Hopmans, E. C., Compaoré, J., Stal, L. J., Schouten, S., and Sinninghe Damsté, J. S.: Rapid analysis of long-chain glycolipids in heterocystous cyanobacteria using high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry, Rapid Commun. Mass Sp., 23, 1387–1394, https://doi.org/10.1002/rcm.4009, 2009b.

Bauersachs, T., Speelman, E. N., Hopmans, E. C., Reichart, G.-J., Schouten, S., and Sinninghe Damsté, J. S.: Fossilized glycolipids reveal past oceanic $N_2$ fixation by heterocystous cyanobacteria, P. Nat. Acad. Sci. USA, 107, 19190–19194, https://doi.org/10.1073/pnas.1007526107, 2010.

Bé, A. W. H.: Shell Porosity of Recent Planktonic Foraminifera as a Climatic Index, Science, 161, 881–884, https://doi.org/10.1126/science.161.3844.881, 1968.

Bé, A. W. H., Jongebloed, W. L., and McIntyre, A.: X-ray microscopy of recent planktonic Foraminifera, J. Paleontol., 43, 1384–1396, 1969.

Becker, K. W., Elling, F. J., Schröder, J. M., Lipp, J. S., Goldhammer, T., Zabel, M., Elvert, M., Overmann, J., and Hinrichs, K.-U.: Isoprenoid Quinones Resolve the Stratification of Redox Processes in a Biogeochemical Continuum from the Photic Zone to Deep Anoxic Sediments of the Black Sea, Appl. Environ. Microb., 84, e02736-17, https://doi.org/10.1128/AEM.02736-17, 2018.

Behl, R. J. and Kennett, J. P.: Brief interstadial events in the Santa Barbara basin, NE Pacific, during the past 60 kyr, Nature, 379, 243–246, https://doi.org/10.1038/379243a0, 1996.

Belanger, C. L.: Volumetric analysis of benthic foraminifera: Intraspecific test size and growth patterns related to embryonic size and food resources, Mar. Micropaleontol., 176, 102170, https://doi.org/10.1016/j.marmicro.2022.102170, 2022.

Bender, M. L., Klinkhammer, G. P., and Spencer, D. W.: Manganese in seawater and the marine manganese balance, Deep-Sea Res., 24, 799–812, https://doi.org/10.1016/0146-6291(77)90473-8, 1977.

Bennett, W. W. and Canfield, D. E.: Redox-sensitive trace metals as paleoredox proxies: A review and analysis of data from modern sediments, Earth-Sci. Rev., 204, 103175, https://doi.org/10.1016/j.earscirev.2020.103175, 2020.

Berger, W. H.: Deep-sea carbonates: Pleistocene dissolution cycles, J. Foramin. Res., 3, 187–195, https://doi.org/10.2113/gsjfr.3.4.187, 1973.

Berndmeyer, C., Birgel, D., Brunner, B., Wehrmann, L. M., Jöns, N., Bach, W., Arning, E. T., Föllmi, K. B., and Peckmann, J.: The influence of bacterial activity on phosphorite formation in the Miocene Monterey Formation, California, Palaeogeogr. Palaeocl., 317, 171–181, https://doi.org/10.1016/j.palaeo.2012.01.004, 2012.

Bernhard, J. M.: Characteristic assemblages and morphologies of benthic foraminifera from anoxic, organic-rich deposits; Jurassic through Holocene, J. Foramin. Res., 16, 207–215, https://doi.org/10.2113/gsjfr.16.3.207, 1986.

Bernhard, J. M.: Benthic foraminiferal distribution and biomass related to pore-water oxygen content central California con-

tinental slope and rise, Deep-Sea Res. Pt A, 39, 585–605, https://doi.org/10.1016/0198-0149(92)90090-G, 1992.

Bernhard, J. M.: Experimental and field evidence of Antarctic foraminiferal tolerance to anoxia and hydrogen sulfide, Mar. Micropaleontol., 20, 203-213, https://doi.org/10.1016/0377-8398(93)90033-T, 1993.

Bernhard, J. M. and Alve, E.: Survival, ATP pool, and ultrastructural characterization of benthic foraminifera from Drammensfjord (Norway): response to anoxia, Mar. Micropaleontol., 28, 5–17, https://doi.org/10.1016/0377-8398(95)00036-4, 1996.

Bernhard, J. M., Sen Gupta, B. K., and Borne, P. F.: Benthic foraminiferal proxy to estimate dysoxic bottom-water oxygen concentrations; Santa Barbara Basin, US Pacific continental margin, J. Foramin. Res., 27, 301-310, https://doi.org/10.2113/gsjfr.27.4.301, 1997.

Bernhard, J. M. and Sen Gupta, B. K.: Foraminifera of oxygen-depleted environments, in: Modern Foraminifera, edited by: Sen Gupta, B. K., Springer, Dordrecht, Germany, 201–216, https://doi.org/10.1007/0-306-48104-9_12, 1999.

Bernhard, J. M., Buck, K. R., Farmer, M. A., and Bowser, S.: The Santa Barbara Basin is a symbiosis oasis, Nature, 403, 77–80, https://doi.org/10.1038/47476, 2000.

Bernhard, J. M., Barry, J. P., Buck, K. R., and Starczak, V. R.: Impact of intentionally injected carbon dioxide hydrate on deep-sea benthic foraminiferal survival, Glob. Change Biol., 15, 2078–2088, https://doi.org/10.1111/j.1365-2486.2008.01822.x, 2009.

Bernhard, J. M., Goldstein, S. T., and Bowser, S. S.: An ectobiont-bearing foraminiferan, *Bolivina pacifica*, that inhabits microxic pore waters: cell-biological and paleo-ceanographic insights, Environ. Microbiol., 12, 2107–2119, https://doi.org/10.1111/j.1462-2920.2009.02073.x, 2010.

Bernhard, J. M., Wit, J. C., Starczak, V. R., Beaudoin, D. J., Phalen, W. G., and McCorkle, D. C.: Impacts of Multiple Stressors on a Benthic Foraminiferal Community: A Long-Term Experiment Assessing Response to Ocean Acidification, Hypoxia and Warming, Front. Mar. Sci., 8, 643339, https://doi.org/10.3389/fmars.2021.643339, 2021.

Berner, R. A.: A new geochemical classification of sedimentary environments, J. Sediment. Petrol., 51, 359–365, https://doi.org/10.1306/212F7C7F-2B24-11D7-8648000102C1865D, 1981.

Berthold, W.-U.: Ultrastructure and function of wall perforations in *Patellina corrugata* Williamson, Foraminiferaida, J. Foramin. Res., 6, 22–29, https://doi.org/10.2113/gsjfr.6.1.22, 1976.

Bianchi, T. S., Cui, X., Blair, N. E., Burdige, D. J., Eglinton, T. I., and Galy, V.: Centers of organic carbon burial and oxidation at the land-ocean interface, Org. Geochem., 115, 138–155, https://doi.org/10.1016/j.orggeochem.2017.09.008, 2018.

Bijma, J., Faber, W. W., and Hemleben, C.: Temperature and salinity limits for growth and survival of some planktonic foraminifera in laboratory cultures, J. Foramin. Res., 20, 95–116, https://doi.org/10.2113/gsjfr.20.2.95, 1990.

Bijma, J., Spero, H. J., and Lea, D. W.: Reassessing foraminiferal stable isotope geochemistry: impact of the oceanic carbonate system (experimental results), in: Use of Proxies in Palaeoceanography: Examples from the South Atlantic, edited by: Fischer, G. and Wefer, G., Springer, Berlin, 489–512, https://doi.org/10.1007/978-3-642-58646-0_20, 1999.
Bindoff, N. L., Cheung, W. W. L., Kairo, J. G., Ariëstegui, J., Guinder, V. A., Hallberg, R., Hilmi, N., Jiao, N., Karim, M. S., Levin, L., O'Donoghue, S., Purca Cuicapusa, S. R., Rinkevich, B., Suga, T., Tagliabue, A., and Williamson, P.: Changing Ocean, Marine Ecosystems, and Dependent Communities, in: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate, edited by: Pörtner, H.-O., Roberts, D. C., Masson-Delmotte, V., Zhai, P., Tignor, M., Poloczanska, E., Mintenbeck, K., Alegriia, A., Nicolai, M., Okem, A., Petzold, J., Rama, B., and Weyer, N. M., Cambridge University Press, Cambridge, UK and New York, NY, USA, 447–587, https://doi.org/10.1017/9781009157964.007, 2019.

Bižić, M., Klintzsch, T., Ionescu, D., Hindiyeh, M. Y., Günthel, M., Muro-Pastor, A. M., Eckert, W., Urich, T., Kepler, F., and Grossart, H.-P.: Aquatic and terrestrial cyanobacteria produce methane, Science Advances, 6, eaax5343, https://doi.org/10.1126/sciadv.aax5343, 2020.

Blaga, C. I., Reichart, G.-J., Heiri, O., and Sinninghe Damsté, J. S.: Tetraether membrane lipid distributions in water-column particulate matter and sediments: a study of 47 European lakes along a north-south transect, J. Paleolimnol., 41, 523–540, https://doi.org/10.1007/s10933-008-9242-2, 2009.

Blair, N. E., and Aller, R. C.: The Fate of Terrestrial Organic Carbon in the Marine Environment, Annu. Rev. Mar. Sci., 4, 401–423, https://doi.org/10.1146/annurev-marine-120709-142717, 2012.

Blumenberg, M., Seifert, R., Reitner, J., Pape, T., and Michaelis, W.: Membrane lipid patterns typify distinct anaerobic methanotrophic consortia, P. Nat. Acad. Sci. USA, 101, 11111–11116, https://doi.org/10.1073/pnas.0401188101, 2004.

Blumenberg, M., Krüger, M., Nauhaus, K., Talbot, H. M., Oppermann, B. I., Seifert, R., Pape, T., and Michaelis, W.: Biosynthesis of hopanoids by sulfate-reducing bacteria (genus Desulfovibrio), Environ. Microbiol., 8, 1220–1227, https://doi.org/10.1111/j.1462-2920.2006.01014.x, 2006.

Blumenberg, M., Hoppert, M., Krüger, M., Dreier, A., and Thiel, V.: Novel findings on hopanoid occurrences among sulfate reducing bacteria: Is there a direct link to nitrogen fixation?, Org. Geochem., 49, 1–5, https://doi.org/10.1016/j.orggeochem.2012.05.003, 2012.

Bodelier, P. L. E., Gillisen, M. -J. B., Hordijk, K., Sinninghe Damsté, J. S., Rijpstra, W. I. C., Geenevasen, J. A. J., and Dunfield, P. F.: A reanalysis of phospholipid fatty acids as ecological biomarkers for methanotrophic bacteria, ISME J., 3, 606–617, https://doi.org/10.1038/ismej.2009.6, 2009.

Boer, W., Nordstad, S., Weber, M., Mertz-Kraus, R., Hönisch, B., Bijma, J., Raitzsch, M., Wilhelms-Dick, D., Foster, G. L., Goring-Harford, H., and Nürnberg, D.: New calcium carbonate nano-particulate pressed powder pellet (NFHS-2-NP) for LA-ICP-OES, LA-(MC)-ICP-MS and µXRF, Geostand. Geoanal. Res., 46, 411–432, https://doi.org/10.1111/ggr.12425, 2022.

Boersma, A., Silva, I. P., and Shackleton, N. J.: Atlantic Eocene planktonic foraminiferal paleohydrographic indicators and stable isotope paleoceanography, Paleocean. Paleoclim., 2, 287–331, https://doi.org/10.1029/PA002i003p00287, 1987.

Boersma, A. and Silva, I. P.: Atlantic Paleogene biserial heteroheericid foraminifera and oxygen minima, Paleocean. Paleoclim., 4, 271–286, https://doi.org/10.1029/PA004i003p00271, 1989.

Boetius, A., Ravenschlag, K., Schubert, C. J., Rickert, D., Widdel, F., Gieseke, A., Amann, R., Jorgensen, B. B., Witte, U., and

Pfannkuche, O.: A marine microbial consortium apparently mediating anaerobic oxidation of methane, Nature, 407, 623–626, https://doi.org/10.1038/35036572, 2000.

Boiteau, R., Greaves, M., and Elderfield, H.: Authigenic uranium in foraminiferal coatings: A proxy for ocean redox chemistry, Paleocean. Paleoclim., 27, PA3227, https://doi.org/10.1029/2012PA002335, 2012.

Bograd, S. J., Castro, C. G., Di Lorenzo, E., Palacios, D. M., Bailey, H., Gilly, W., and Chavez, F. P.: Oxygen declines and the shoaling of the hypoxic boundary in the California Current, Geophys. Res. Lett., 35, L12607, https://doi.org/10.1029/2008GL034185, 2008.

Boltovskoy, E., Scott, D. B., and Medioli, F. S.: Morphological variations of benthic foraminiferal tests in response to changes in ecological parameters: a review, J. Paleontol., 65, 175–185, https://doi.org/10.1017/S0022336000020394, 1991.

Böning, P., Brumsack, H.-J., Böttcher, M. E., Schnetger, B., Kriete, C., Kallmeyer, J., and Borchers, S. L.: Geochemistry of Peruvian near-surface sediments, Geochim. Cosmochim. Ac., 68, 4429–4451, https://doi.org/10.1016/j.gca.2004.04.027, 2004.

Böning, P., Bard, E., and Rose, J.: Toward direct, micron-scale XRF elemental maps and quantitative profiles of wet marine sediments, Geochem. Geophy. Geosy., 8, Q05004, https://doi.org/10.1029/2006GC001480, 2007.

Böning, P., Brumsack, H.-J., Schnetger, B., Grunwald, M.: Trace element signatures of Chilean upwelling sediments at ~36S, Mar. Geol., 259, 112–121, https://doi.org/10.1016/j.margeo.2009.01.004, 2009.

Böning, P., Shaw, T., Pahnke, K., and Brumsack, H. J.: Nickel as indicator of fresh organic matter in upwelling sediments, Geochim. Cosmochim. Ac., 162, 99–108, https://doi.org/10.1016/j.gca.2015.04.027, 2015.

Bonatti, E., Fisher, D. E., Joensuu, O., and Rydell, H. S.: Post-depositional mobility of some transition elements, phosphorus, uranium and thorium in deep sea sediments, Geochim. Cosmochim. Acta, 35, 189–201, https://doi.org/10.1016/0016-7037(71)90057-3, 1971.

Bouchet, V. M., Alve, E., Rygg, B., and Telford, R. J.: Benthic foraminifera provide a promising tool for ecological quality assessment of marine waters, Ecol. Indic., 23, 66–75, https://doi.org/10.1016/j.ecolind.2012.03.011, 2012.

BouDagher-Fadel, M. K.: Biostratigraphic and Geological Significance of Planktonic Foraminifera, UCL Press, London, https://doi.org/10.14324/111.9781910634257, 2015.

Boudreau, B. P.: Is burial velocity a master parameter for bioturbation?, Geochim. Cosmochim. Ac., 58, 1243–1249, https://doi.org/10.1016/0016-7037(94)90378-6, 1994.

Boussafir, M., Gelin, F., Lallier-Vergès, E., Derenne, S., Bertrand, P., and Largeau, C.: Electron Microscopy and pyrolysis of kerogens from the Kimmeridge Clay Formation, UK: Source organisms, preservation processes, and origin of microcycles, Geochim. Cosmochim. Ac., 59, 3731–47, https://doi.org/10.1016/0016-7037(95)00273-3, 1995.

Boussafir, M. and Lallier-Vergès, E.: Accumulation of organic matter in the Kimmeridge Clay Formation (KCF): An update fossilisation model for marine petroleum source-rocks, Mar. Petrol. Geol., 14, 75–83, https://doi.org/10.1016/S0264-8172(96)00050-5, 1997.

Boyle, E. A. and Keigwin, L. D.: Comparison of Atlantic and Pacific paleochemical records for the last 215,000 years: changes
in deep ocean circulation and chemical inventories, Earth Planet. Sc. Lett., 76, 135-150, https://doi.org/10.1016/0012-821X(85)90154-2, 1985.

Boyle, R. A., Dahl, T. W., Dale, A. W., Shields-Zhou, G. A., Zhu, M. Y., Brasier, M. D., Canfield, D. E., and Lenton, T. M.: Stabilization of the coupled oxygen and phosphorus cycles by the evolution of bioturbation, Nat. Geosci., 7, 671–676, https://doi.org/10.1038/ngeo2213, 2014.

Bradbury, H. J., Turchyn, A. V., Bateson, A., Antler, G., Fotherby, A., Druhan, J. L., Greaves, M., Sevilgen, D. S., and Hodell, D. A.: The Carbon-Sulfur Link in the Remineralization of Organic Carbon in Surface Sediments, Front. Earth Sci., 9, 652960, https://doi.org/10.3389/feart.2021.652960, 2021.

Bradbury, H. J., Thomas, N. C., Mleneck-Vautravers, M., and Hodell, D. A.: Revisiting the relationship between the pore water carbon isotope gradient and bottom water oxygen concentrations, Geochim. Cosmochim. Ac., 366, 84–94, https://doi.org/10.1016/j.gca.2023.12.011, 2024.

Bradley, A. S., Fredricks, H., Hinrichs, K. -U., and Summons, R. E.: Structural diversity of diether lipids in carbonate chimneys at the Lost City Hydrothermal Field, Org. Geochem., 40, 1169–1178, https://doi.org/10.1016/j.orggeochem.2009.09.004, 2009.

Bradtmiller, L. I., Anderson, R. F., Sachs, J. P., and Fleisher, M. Q.: A deeper respired carbon pool in the glacial equatorial Pacific Ocean, Earth Planet. Sc. Lett., 299, 417–425, https://doi.org/10.1016/j.epsl.2010.09.022, 2010.

Branson, O., Bonnin, E. A., Perea, D. E., Spero, H. J., Zhu, Z., Winters, M., Honisch, B., Russell, A. D., Fehrenbacher, J. S., and Gagnon, A. C.: Nanometer-scale chemistry of a calcite biomineralization template: Implications for skeletal composition and nucleation, P. Nat. Acad. Sci. USA, 113, 12934–12939, https://doi.org/10.1073/pnas.1522864113, 2016.

Brassell, S. C., Wardroper, A. M. K., Thomson, I. D., Maxwell, J. R., and Eglinton, G.: Specific acyclic isoprenoids as biological markers of methanogenic bacteria in marine sediments, Nature, 290, 693–696, https://doi.org/10.1038/290693a0, 1981.

Brassell, S. C.: Molecular changes in sediment lipids as indicators of systematic early diagenesis, Philos. T. R. Soc. S-A, 315, 57–75, https://doi.org/10.1098/rsta.1985.0029, 1985.

Bratsch, S. G.: Standard electrode potentials and temperature coefficients in water at 298.15 K, J. Phys. Chem. Ref. Data, 18, 1–21, https://doi.org/10.1063/1.555839, 1989.

Breitburg, D., Levin, L. A., Oschlies, A., Grégoire, M., Chavez, F. P., Conley, D. J., Garçon, V., Gilbert, D., Gutiérrez, D., Isensee, K., Jacinto, G. S., Limburg, K. E., Montes, I., Naqvi, S. W. A., Pitcher, G. C., Rabalais, N. N., Roman, M. R., Rose, K. A., Seibel, B. A., Telszewski, M., Yasuhara, M., and Zhang, J.: Declining oxygen in the global ocean and coastal waters, Science, 359, eaam7240, https://doi.org/10.1126/science.aam7240, 2018.

Briggs, D. E. G. and Summons, R. E.: Ancient biomolecules: Their origins, fossilization, and role in revealing the history of life, BioEssays, 36, 482–490, https://doi.org/10.1002/bies.201400010, 2014.

Briguglio, A.: Hydrodynamic behaviour of nummulitids, Ph.D. thesis, Universität Wien, Austria, 185 pp., https://doi.org/10.25365/thesis.12131, 2010.

Brinkmann, I., Ni, S., Schweizer, M., Oldham, V. E., Quintana Krupinski, N. B., Medjoubi, K., Somogyi, A., Whitehouse, M. J., Hansel, C. M., Barras, C., Bernhard, J. M., and Filip-

son, H. L.: Foraminiferal Mn/Ca as Bottom-Water Hypoxia Proxy: An Assessment of *Nonionella stella* in the Santa Barbara Basin, USA, Paleocean. Paleoclim., 36, e2020PA004167, https://doi.org/10.1029/2020PA004167, 2021.

Brinkmann, I., Barras, C., Jilbert, T., Paul, K. M., Somogyi, A., Ni, S., Schweizer, M., Bernhard, J. M., and Filipsson, H. L.: Benthic foraminiferal Mn/Ca as low-oxygen proxy in fjord sediments, Global Biogeochem. Cy., 37, e2023GB007690, https://doi.org/10.1029/2023GB007690, 2023.

Brocks, J. J. and Schaeffer, P.: Okenane, a biomarker for purple sulfur bacteria (Chromatiaceae), and other new carotenoid derivatives from the 1640 Ma Barney Creek Formation, Geochim. Cosmochim. Ac., 72, 1396–1414, https://doi.org/10.1016/j.gca.2007.12.006, 2008.

Broecker, W. and Clark, E.: An evaluation of Lohmann's foraminifera weight dissolution index, Paleocean. Paleoclim., 16, 531–534, https://doi.org/10.1029/2000PA000600, 2001

Bruland, K. W.: Trace elements in seawater, in: Chemical Oceanography 8, edited by: Riley, J. P. and Chester, R., Academic Press, London, 1983, 158–220, ISBN: 9781483206400, 1983.

Bruland, K. and Lohan, M. C.: Controls of trace metals in seawater, in: Treatise on geochemistry, Vol. 6, edited by: Elderfield, H., Holland, H. D., and Turekian, K. K., Elsevier, 23–47, https://doi.org/10.1016/B0-08-043751-6/06105-3, 2006.

Brumsack, H. J.: The trace metal content of recent organic carbon-rich sediments: Implications for cretaceous black shale formation, Palaeogeogr. Palaeoclim., 232, 344–361, https://doi.org/10.1016/j.palaeo.2005.05.011, 2006.

Brunner, B., Contreras, S., Lehmann, M. F., Matantseva, O., Rollog, M., Kalvelage, T., Klockgether, G., Lavik, G., Jetten, M. S. M., Kartal, B., and Kuypers, M. M. M.: Nitrogen isotope effects induced by anammox bacteria, P. Nat. Acad. Sci. USA, 110, 18994–18999, https://doi.org/10.1073/pnas.1310488110, 2013.

Brüske, A., Weyer, S., Zhao, M.-Y., Planavsky, N., Wegwerth, A., Neubert, N., Dellwig, O., Lau, K., and Lyons, T.: Correlated molybdenum and uranium isotope signatures in modern anoxic sediments: Implications for their use as paleo-redox proxy, Geochim. Cosmochim. Ac., 270, 449–474, https://doi.org/10.1016/j.gca.2019.11.031, 2020.

Bunzel, D., Schmiedl, G., Lindhorst, S., Mackensen, A., Reolid, J., Romahn, S., and Betzler, C.: A multi-proxy analysis of Late Quaternary ocean and climate variability for the Maldives, Inner Sea, Clim. Past 13, 1791–1813, https://doi.org/10.5194/cp-13-1791-2017, 2017.

Burdige, D. J. and Gieskes, J. M.: A pore water/solid phase diagenetic model for manganese in marine sediments, Am. J. Sci., 283, 29–47, https://doi.org/10.2475/ajs.283.1.29, 1983.

Burdige, D. J.: The biogeochemistry of manganese and iron reduction in marine sediments, Earth-Sci. Rev., 35, 249–284, https://doi.org/10.1016/0012-8252(93)90040-E, 1993.

Burdige, D. J.: Preservation of organic matter in marine sediments: Controls, mechanisms, and an imbalance in sediment organic carbon budgets?, Chem. Rev., 107, 467–485, https://doi.org/10.1021/cr050347q, 2007.

Burke, J., Renema, W., Henehan, M. J., Elder, L. E., Davis, C. V., Maas, A. E., Foster, G. L., Schiebel, R., and Hull, P. M.: Factors influencing test porosity in planktonic foraminifera, Biogeosciences, 15, 6607–6619, https://doi.org/10.5194/bg-15-6607-2018, 2018.
Burke, J. E., Renema, W., Schiebel, R., and Hull, P. M.: Three-dimensional analysis of inter- and intraspecific variation in ontogenetic growth trajectories of planktonic foraminifera, Mar. Micropaleontol., 155, 1–12, https://doi.org/10.1016/j.marmicro.2019.101794, 2020.

Buzas, M. A., Culver, S. J., and Jorissen, F. J.: A statistical evaluation of the microhabitats of living (stained) infaunal benthic foraminifera, Mar. Micropaleontol., 20, 311–320, https://doi.org/10.1016/0377-8398(93)90040-5, 1993.

Callbeck, C. M., Canfield, D. E., Kuypers, M. M. M., Yilmaz, P., Lavik, G., Thamdrup, B., Schubert, C. J., and Bristow, L. A.: Sulfur cycling in oceanic oxygen minimum zones, Limnol. Oceanogr., 66, 2360–2392, https://doi.org/10.1002/lno.11759, 2021.

Calvert, S. E. and Pedersen, T. F.: Sedimentary geochemistry of manganese; implications for the environment of formation of manganiferous black shales, Econ. Geol., 91, 36–47, https://doi.org/10.2113/gsecongo.91.1.36, 1996.

Calvert, S. E. and Pedersen, T. F.: Elemental Proxies for Palaeo-climatic and Palaeoceanographic Variability in Marine Sediments: Interpretation and Application, in: Palaeoceanography of the Late Cenozoic, Part 1, Methods, edited by: Hillaire-Marcel, C. and de Vernal, A., Elsevier, New York, 567–644, https://doi.org/10.1016/S1572-5480(07)01019-6, 2007.

Campos, M. L. A. M., Farrenkopf, A. M., Jickells, T. D., and Luther III, G. W.: A comparison of dissolved iodine cycling at the Bermuda Atlantic Time-series Station and Hawaii Ocean Time-series Station, Deep-Sea Res. Pt. II, 43, 455–466, https://doi.org/10.1016/0967-0645(95)00100-X, 1996.

Canfield, D. E.: Biogeochemistry of sulfur isotopes, Rev. Mineral. Geochem., 43, 607–636, https://doi.org/10.2138/gsrmg.43.1.607, 2001.

Canfield, D. E. and Farquhar, J.: Animal evolution, bioturbation, and the sulfate concentration of the oceans, P. Nat. Acad. Sci. USA, 106, 8123–8127, https://doi.org/10.1073/pnas.0902037106, 2009.

Canfield, D. E. and Thamdrup, B.: Towards a consistent classification scheme for geochemical environments, or, why we wish the term “suboxic” would go away, Geobiology, 7, 385–392, https://doi.org/10.1111/j.1472-4669.2009.00214.x, 2009.

Canfield, D. E., Stewart, F.J., Thamdrup, B., Brabandere, L.D., Dalsgaard, T., DeLong, E. F., Revsbech, N. P., and Ulloa, O.: A Cryptic Sulfur Cycle in Oxygen-Minimum-Zone Waters off the Chilean Coast, Science, 330, 1375–1378, https://doi.org/10.1126/science.1196889, 2010.

Cardich, J., Morales, M., Quipúzcoa, L., Sifeddine, A., and Gutiérrez, D.: Benthic foraminiferal communities and microhabitat selection on the continental shelf off central Peru, in: Anoxia, edited by: Altenbach, A., Bernhard, J., and Seckbach, J., Springer, Dordrecht, Germany, 323–340, https://doi.org/10.1007/978-94-007-1896-8_17, 2012.

Cardich, J., Gutiérrez, D., Romero, D., Pérez, A., Quipúzcoa, L., Marquina, R., Yupanqui, W., Solís, J., Carhuapoma, W., Sifeddine, A., and Rathburn, A.: Calcareous benthic foraminifera from the upper central Peruvian margin: control of the assemblage by pore water redox and sedimentary organic matter, Mar. Ecol. Prog. Series, 535, 63–87, https://doi.org/10.3354/meps11409, 2015.

Cardich, J., Sifeddine, A., Salvatteci, R., Romero, D., Briceño-Zuluaga, F., Graco, M., Anculle, T., Almeida, C., and Gutiérrez, D.: Multidecadal Changes in Marine Subsurface Oxygenation Off Central Peru During the Last ca. 170 Years, Front. Mar. Sci., 6, 270, https://doi.org/10.3389/fmars.2019.00270, 2019.

Caromel, A. G. M., Schmidt, D. N., Fletcher, I., and Rayfield, E. J.: Morphological Change During The Ontogeny Of The Planktic Foraminifera, J. Micropalaeontol., 35, 2–19, https://doi.org/10.1144/jmpaleo2014-017, 2016.

Caromel, A. G. M., Schmidt, D. N., and Rayfield, E. J.: Ontogenetic constraints on foraminiferal test construction, Evol. Dev., 19, 157–168, https://doi.org/10.1111/ede.12224, 2017.

Carpenter, S. R., Elser, M. M., and Elser, J. J.: Chlorophyll production, degradation, and sedimentation: Implications for paleolimnology, Limnol. Oceanogr., 31, 112–124, https://doi.org/10.4319/lo.1986.31.1.0112, 1986.

Casanova-Arenillas, S., Rodríguez-Tovar, F. J. and Martínez-Ruiz, F.: Ichnological evidence for bottom water oxygenation during organic rich layer deposition in the westernmost Mediterranean over the Last Glacial Cycle, Mar. Geol., 443, 106673, https://doi.org/10.1016/j.margeo.2021.106673, 2022.

Casciotti, K. L.: Nitrogen and Oxygen Isotopic Studies of the Marine Nitrogen Cycle, Annu. Rev. Mar. Sci., 8, 379–407, https://doi.org/10.1146/annurev-marine-010213-135052, 2016.

Castillo, A., Hromic, T., Uribe, R. A., Valdés, J., Sifeddine, A., Quezada, L., Vega, S. E., Arenciba, A., Días-Ochoa, J., and Guiñez, M.: Living (stained) calcareous benthic foraminiferal assemblages (> 125µm) in a coastal upwelling zone of the Humboldt Current System, Northern Chile (~27° S), Reg. Stud. Mar. Sci., 44, 101725, https://doi.org/10.1016/j.rsma.2021.101725, 2021.

Castillo, A., Valdés, J., Sifeddine, A., Reyss, J., Bouloubassi, I., and Ortlieb, L.: Changes in biological productivity and ocean-climatic fluctuations during the last ~1.5 kyr in the Humboldt ecosystem off northern Chile (27° S): a multiproxy approach, Palaeogeogr. Palaeocl., 485, 798–815, https://doi.org/10.1016/j.palaeo.2017.07.038, 2017.

Castillo, A., Valdés, J., Sifeddine, A., Vega, S. E., Díaz-Ochoa, J., and Marambio, Y.: Evaluation of redox-sensitive metals in marine surface sediments influenced by the oxygen minimum zone of the Humboldt Current System, Northern Chile, Int. J. Sediment Res., 34, 178–190, https://doi.org/10.1016/j.ijsrc.2018.08.005, 2019.

Caulle, C., Koho, K. A., Mojtahid, M., Reichart, G. J., and Jorissen, F. J.: Live (Rose Bengal stained) foraminiferal faunas from the northern Arabian Sea: faunal succession within and below the OMZ, Biogeosciences, 11, 1155–1175, https://doi.org/10.5194/bg-11-1155-2014, 2014.

Caulle, C., Mojtahid, M., Gooday, A. J., Jorissen, F. J., and Kitzato, H.: Living (Rose-Bengal-stained) benthic foraminiferal faunas along a strong bottom-water oxygen gradient on the Indian margin (Arabian Sea), Biogeosciences, 12, 5005–5019, https://doi.org/10.5194/bg-12-5005-2015, 2015.

Cayre, O., Beaufort, L., and Vincent, E.: Paleoproductivity in the Equatorial Indian Ocean for the last 260,000 yr: a transfer function based on planktonic foraminifera, Quaternary Sci. Rev., 18, 839–857, https://doi.org/10.1016/S0277-3791(98)00036-5, 1999.
Chan, F., Barth, J. A., Lubchenco, J., Kirincich, A., Weeks, H., Pe-
terson, W. T., and Menge, B. A.: Emergence of anoxia in the Cal-
ifornia current large marine ecosystem, Science, 319, 920–920,
https://doi.org/10.1126/science.1149016, 2008.

Chance, R., Baker, A. R., Carpenter, L., and Jickells, T. D.: The distribution of iodide at the sea surface, Environ. Sci-Proc. Imp., 16, 1841–1859, https://doi.org/10.1039/C4EM00139G, 2014.

Chang, L., Hoogakker, B. A. A., Heslop, D., Zhao, X., Roberts, A., De Deckker, O., Xue, P., Zeng, F., Wang, S., Berndt, T. A., Stuut, J.-B. W., and Harrison, R. J.: Recurrence of glacial ocean deoxygenation and accumulated respired carbon storage in the Indian Ocean of the last 850,000 years, Nat. Commun., 14, 4841, https://doi.org/10.1038/s41467-023-40452-1, 2023.

Chase, Z., Anderson, R. F., and Fleisher, M. Q.: Evidence from authigenic uranium for increased productivity of the glacial subantarctic ocean, Paleocean. Paleoclim., 16, 468–478, https://doi.org/10.1029/2000PA000542, 2001.

Charrieau, L. M., Nagai, Y., Kimoto, K., Dissard, D., Below, B., Fujita, K., and Toyofuku, T.: The coral reef-dwelling *Peneroplis* spp. shows calcification recovery to ocean acidification conditions, Sci. Rep., 12, 6373, https://doi.org/10.1038/s41598-022-10375-w, 2022.

Chen, J. H., Edwards, R. L., and Wasserburg, G. J.: ²³⁸U, ²³⁴U and ²³²Th in seawater, Earth Planet. Sc. Lett., 80, 241–251, https://doi.org/10.1016/0012-821X(86)90108-1, 1986.

Chen, X., Ling, J.-F., Vance, D., Shields-Zhou, G. A., Zhu, M., Poulton, S. W., Och, L. M., Jiang, S.-Y., Li, D., Cremonese, L., and Archer, C.: Rise to modern levels of ocean oxygenation coincided with the Cambrian radiation of animals, Nat. Commun., 6, 7142, https://doi.org/10.1038/ncomms8142, 2015.

Chen, P., Yu, J., and Jin, Z.: An evaluation of benthic foraminiferal U/Ca and U/Mn proxies for deep ocean carbonate chemistry and redox conditions, Geochem. Geophy. Geosy., 18, 617–630, https://doi.org/10.1002/2016GC006730, 2017.

Chen, X., Robinson, S. A., Romaniello, S. J., and Anbar, A. D.: ²³⁸U/²³⁵U in calcite is more susceptible to carbonate diagenesis, Geochim. Cosmochim. Ac., 326, 273–287, https://doi.org/10.1016/j.gca.2022.03.027, 2022.

Chiu, C. F., Sweere, T. C., Clarkson, M. O., de Souza, G. F., Hennekam, R., and Vance, D.: Co-variation systematics of uranium and molybdenum isotopes reveal pathways for descent into euxinia in Mediterranean sapropels, Earth Planet. Sc. Lett., 585, 117527, https://doi.org/10.1016/j.epsl.2022.117527, 2022.

Choquel, C., Müter, D., Ni, S., Pirzamanbein, B., Charrieau,
L. M., Hirose, K., Seto, Y., Schmiedl, G., and Filips-
son, H. L.: 3D morphological variability in foraminifera
unravel environmental changes in the Baltic Sea entrance
over the last 200 years, Front. Earth Sci., 11, 1120170,
https://doi.org/10.3389/feart.2023.1120170, 2023.

Clarkson, M. O., Hennekam, R., Sweere, T. C., Andersen, M. B., Reichart, G.-J., and Vance, D.: Carbonate associated uranium isotopes as a novel local redox indicator in oxidatively disturbed reducing sediments, Geochim. Cosmochim. Ac., 311, 12–28, https://doi.org/10.1016/j.gca.2021.07.025, 2021a.

Clarkson, M. O., Lenton, T. M., Andersen, M. B., Bagard, M.-L., Dickson, A. J., and Vance, D.: Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes, Nat. Commun., 12, 1–9, https://doi.org/10.1038/s41467-020-20486-5, 2021b.

Clement, B. G., Luther III, G. W., and Tebo, B. M.: Rapid, oxygen-dependent microbial Mn (II) oxidation kinetics at sub-micromolar oxygen concentrations in the Black Sea suboxic zone, Geochim. Cosmochim. Ac., 73, 1878–1889, https://doi.org/10.1016/j.gca.2008.12.023, 2009.

Cline, J. D. and Kaplan, I. R.: Isotopic fractionation of dissolved nitrate during denitrification in the eastern tropical north Pacific Ocean, Mar. Chem., 3, 271–299, https://doi.org/10.1016/0304-4203(75)90009-2, 1975.

Cole, D., Zhang, S., and PLanavsky, N.: A new estimate of detrital redox-sensitive metal concentrations and variability in fluxes to marine sediments, Geochim. Cosmochim. Ac., 215, 337–353, https://doi.org/10.1016/j.gca.2017.08.004, 2017.

Colley, S. and Thomson, J.: Recurrent uranium relocations in distal turbidites emplaced in pelagic conditions, Geochim. Cosmochim. Ac., 49, 2339–2348, https://doi.org/10.1016/0016-7037(85)90234-0, 1985.

Colley, S., Thomson, J., and Toole, J.: Uranium relocations and derivation of quasi-isochrons for a turbidite/pelagic sequence in the Northeast Atlantic, Geochim. Cosmochim. Ac., 53, 1223–1234, https://doi.org/10.1016/0016-7037(89)90058-6, 1989.

Colley, S., Thomson, J., Wilson, T. R. S., and Higgs, N. C.: Post-
depositional migration of elements during diagenesis in brown
clay and turbidite sequences in the North East Atlantic, Geochim.
Cosmochim. Ac., 48, 1223–1235, https://doi.org/10.1016/0016-
7037(84)90057-7, 1984.

Colodner, D., Edmond, J., and Boyle, E.: Rhenium in the Black Sea: comparison with molybdenum and uranium, Earth Planet. Sc. Lett., 131, 1–15, https://doi.org/10.1016/0012-821X(95)00010-A, 1995.

Conley, D. J., Carstensen, J., Aigars, J., Axe, P., Bonsdorff, E., Eremina, T., Haahti, B.-M., Humborg, C., Jonsson, P., Kotta, J., Lännegren, C., Larsson, U., Maximov, A., Medina, M. R., Lysiak-Pastuszak, E., Remeikaitė-Nikienė, N., Walve, J., Wilhelms, S., and Zillén, L.: Hypoxia Is Increasing in the Coastal Zone of the Baltic Sea, Environ. Sci. Technol., 45, 6777–6783, https://doi.org/10.1021/es201212r, 2011.

Connock, G. T., Owens, J. D., and Liu, X.-L.: Biotic induction and microbial ecological dynamics of Oceanic Anoxic Event 2, Commun. Earth Environ., 3, 136, https://doi.org/10.1038/s43247-022-00466-x, 2022.

Constandache, M., Yerly, F., and Spezzaferri, S.: Internal pore mea-
surements on macroperforate planktonic Foraminifera as an al-
ternative morphometric approach, Swiss J. Geosci., 106, 179–
186, https://doi.org/10.1007/s00015-013-0134-8, 2013

Cook, M. K., Dial, A. R., and Hendy, I. L.: Iodine stability as a function of pH and its implications for simultaneous multi-element ICP-MS analysis of marine carbonates for palaeoenvironmental reconstructions, Mar. Chem., 245, 104148, https://doi.org/10.1016/j.marchem.2022.104148, 2022.

Corfield, R. M. and Shackleton, N. J.: Productivity change as a control on planktonic foraminiferal evolution after the Cretaceous/Tertiary boundary, Hist. Biol., 1, 323–343, https://doi.org/10.1080/08912968809386482, 1988.

Corliss, B. H.: Microhabitats of benthic foraminifera within deep-sea sediments, Nature, 314, 435–438, https://doi.org/10.1038/314435a0, 1985.

Corliss, B. H.: Morphology and microhabitat preferences of ben-
thic foraminifera from the northwest Atlantic Ocean, Mar.
Micropaleontol., 17, 195-236, https://doi.org/10.1016/0377-8398(91)90014-W, 1991.

Corliss, B. H. and Emerson, S.: Distribution of Rose Bengal stained deep-sea benthic foraminifera from the Nova Scotian continental margin and Gulf of Maine, Deep-Sea Res., 37, 381-400, https://doi.org/10.1016/0198-0149(90)90015-N, 1990.

Corliss, B. H., Sun, X., Brown, C. W., and Showers, W. J.: Influence of seasonal primary productivity on δ¹³C of North Atlantic deep-sea benthic foraminifera, Deep-Sea Res. Pt. I, 53, 740-746, https://doi.org/10.1016/j.dsr.2006.01.006, 2006.

Costa, K. M., Anderson, R. F., McManus, J. F., Winckler, G., Middleton, J. L., Langmuir, C. H.: Trace element (Mn, Zn, Ni, V) and authigenic uranium (aU) geochemistry reveal sedimentary redox history on the Juan de Fuca Ridge, North Pacific Ocean, Geochim. Cosmochim. Ac., 236, 79-98, https://doi.org/10.1016/j.gca.2018.02.016, 2018.

Costa, K. M., Nielsen, S. G., Wang, Y., Lu, W., Hines, S. K., Jacobel, A. W., and Oppo, D. W.: Marine sedimentary uranium to barium ratios as a potential quantitative proxy for Pleistocene bottom water oxygen concentrations, Geochim. Cosmochim. Ac., 343, 1-16, https://doi.org/10.1016/j.gca.2022.12.022, 2023.

Cowie, G., Mowbray, S., Kurian, S., Sarkar, A., White, C., Anderson, A., Vergnaud, B., Johnstone, G., Brear, S., Woulds, C., Naqvi, S. W. A., and Kitazato, H.: Comparative organic geochemistry of Indian margin (Arabian Sea) sediments: estuary to continental slope, Biogeosciences, 11, 6683-6696, https://doi.org/10.5194/bg-11-6683-2014, 2014.

Coxall, H. K., Pearson, P. N., Wilson, P. A., and Sexton, P. F.: Iterative evolution of digitate planktonic foraminifera, Paleobiology, 33, 495-516, https://doi.org/10.1666/06034.1, 2007.

Crusius, J., Calvert, S., Pedersen, T., and Sage, D.: Rhenium and molybdenum enrichments in sediments as indicators of oxic, suboxic and sulfidic conditions of deposition, Earth Planet. Sc. Lett., 145, 65-78, https://doi.org/10.1016/S0012-821X(96)00204-X, 1996.

Crusius, J. and Thomson, J.: Comparative behaviour of authigenic Re, U and Mo during reoxidation and subsequent long-term burial in marine sediments, Geochim. Cosmochim. Ac., 64, 2233-2242, https://doi.org/10.1016/S0016-7037(99)00433-0, 2000.

Croudace, I. W., Rindby, A., and Rothwell, R. G.: ITRAX: description and evaluation of a new multi-function X-ray core scanner, Geol. Soc. Lond. Spec. Publ., 267, 51-63, https://doi.org/10.1144/GSL.SP.2006.267.01.04, 2006.

Croudace, I. W. and Rothwell, R. G. (Eds.). Micro-XRF Studies of Sediment Cores: Applications of a non-destructive tool for the environmental sciences, Vol. 17, p. 668, Dordrecht, Springer, https://doi.org/10.1007/978-94-017-9849-5, 2015.

Croudace, I. W., Löwemark, L., Tjallingii, R., and Zolitschka, B.: Current perspectives on the capabilities of high resolution XRF core scanners, Quaternary Int., 514, 5-15, https://doi.org/10.1016/j.quaint.2019.04.002, 2019.

Curry, W. B. and Oppo, D. W.: Glacial water mass geometry and the distribution of δ¹³C of ΣCO₂ in the western Atlantic Ocean, Paleoocean. Paleoclim., 20, PA1017, https://doi.org/10.1029/2004PA001021, 2005.

Cutter, G. A., Moffett, J. W., Nielsdóttir, M. C., and Sannial, V.: Multiple oxidation state trace elements in suboxic waters off Peru: In situ redox processes and advective/diffusive horizontal transport, Mar. Chem., 201, 77-89, https://doi.org/10.1016/j.marchem.2018.01.003, 2018.

Dahl, T. W., Anbar, A. D., Gordon, G. W., Rosing, M. T., Frei, R., and Canfield, D. E.: The behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic Lake Cadagno, Switzerland, Geochim. Cosmochim. Ac., 74, 144-163, https://doi.org/10.1016/j.gca.2009.09.018, 2010a.

Dahl, T. W., Chappaz, A., Hoek, J., McCenzie, C. J., Svane, S., and Canfield, D. E.: Evidence of molybdenum association with particulate organic matter under sulfidic conditions, Geobiology, 15, 311-323, https://doi.org/10.1111/gbi.12220, 2017.

Dahl, T. W., Hammarlund, E. U., Rasmussen, C. M. Ø., Bond, D. P., and Canfield, D. E.: Sulfidic anoxia in the oceans during the Late Ordovician mass extinctions—insights from molybdenum and uranium isotopic global redox proxies, Earth-Sci. Rev., 220, 103748, https://doi.org/10.1016/j.earscirev.2021.103748, 2021.

Davis, C. V., Myhre, S. E., and Hill, T. M.: Benthic foraminiferal shell weight: Deglacial species-specific responses from the Santa Barbara Basin, Mar. Micropaleontol., 124, 45-53, https://doi.org/10.1016/j.marmicro.2016.02.002, 2016.

Davis, C. V., Wishner, K., Renema, W., and Hull, P. M.: Vertical distribution of planktic foraminifera through an oxygen minimum zone: how assemblages and test morphology reflect oxygen concentrations, Biogeosciences, 18, 977-992, https://doi.org/10.5194/bg-18-977-2021, 2021.

Davis, C. V., Doherty, S., Fehrenbacker, J., and Wishner, K.: Trace element composition of modern planktic foraminifera from an oxygen minimum zone: potential proxies for an enigmatic environment, Front. Mar. Sci., 10, 1145756, https://doi.org/10.3389/fmars.2023.1145756, 2023a.

Davis, C. V., Sibert, E. C., Jacobs, P. H., Burls, N., and Hull, P. M.: Intermediate water circulation drives distribution of Pliocene Oxygen Minimum Zones, Nat. Commun., 14, 40, https://doi.org/10.1038/s41467-022-35083-x, 2023b.

Debenay, J.-P. (Ed.): A Guide to 1,000 Foraminifera from Southwestern Pacific New Caledonia, IRD Éditions, Publications scientifiques du Muséum, Muséum national d'Histoire naturelle, Paris, France, 384 pp., ISBN 978-2-7099-1729-2, 2012.

Deflandre, B., Mucci, A., Gagné, J.-P., Guignard, C., and Sundby, B.: Early diagenetic processes in coastal marine sediments disturbed by a catastrophic sedimentation event, Geochim. Cosmochim. Ac., 66, 2547-2558, https://doi.org/10.1016/S0016-7037(02)00861-X, 2002

De Lange, G. J.: Early Diagenetic Reactions in Interbedded Pelagic and Turbiditic Sediments in the Nares-Abissal-Plain (Western North-Atlantic) Consequences for the Composition of Sediment and Interstitial Water, Geochim. Cosmochim. Ac., 50, 2543-2561, https://doi.org/10.1016/0016-7037(86)90209-7, 1986.

De Lange, G. J., Thomson, J., Reitz, A., Slomp, C. P., Principato, M. S., Erba, E., and Corselli, C.: Synchronous basin-wide formation and redox-controlled preservation of a Mediterranean sapropel, Nat. Geosci., 1, 606-610, https://doi.org/10.1038/ngeo283, 2008.

Dellinger, M., Hilton, R. G., Geoff M. and Nowell, G. M.: Fractionation of rhenium isotopes in the Mackenzie River basin during oxidative weathering, Earth Planet. Sc. Lett., 573, 117131, https://doi.org/10.1016/j.epsl.2021.117131, 2021.

de Moel, H., Ganssen, G. M., Peeters, F. J. C., Jung, S. J. A., Kroon, D., Brummer, G. J. A., and Zeebe, R. E.: Planktic
foraminiferal shell thinning in the Arabian Sea due to anthro-
pogenic ocean acidification?, Biogeosciences, 6, 1917–1925,
https://doi.org/10.5194/bg-6-1917-2009, 2009.

Den Dulk, M., Reichart, G. J., Memon, G. M., Roelofs, E. M. P., Zachariasse, W. J., and Van der Zwaan, G. J.: Benthic foraminiferal response to variations in surface water productivity and oxygenation in the northern Arabian Sea, Mar. Micropaleontol., 35, 43–66, https://doi.org/10.1016/S0377-8398(98)00015-2, 1998.

De Nooijer, L. J., Sampedro, L. P., Jorissen, F. J., Pawlowski, J., Rosenthal, Y., Dissard, D., and Reichart, G. J.: 500 million years of foraminiferal calcification, Earth-Science Rev., 243, 104484, https://doi.org/10.1016/j.earscirev.2023.104484, 2023.

Detlef, H., Sosdian, S. M., Kender, S., Lear, C. H., and Hall, I. R.: Multi-elemental composition of authigenic carbonates in benthic foraminifera from the eastern Bering Sea continental margin (International Ocean Discovery Program Site U1343), Geochim. Cosmochim. Ac., 268, 1–21, https://doi.org/10.1016/j.gca.2019.09.025, 2020.

Deutsch, C., Brix, H., Ito, T., Frenzel, H., and Thompson, L.: Climate-forced variability of ocean hypoxia, Science, 333, 336–339, https://doi.org/10.1126/science.1202422, 2011.

Deutsch, C., Berelson, W., Thunell, R., Weber, T., Tems, C., McManus, J., Crusius, J., Ito, T., Baumgartner, T., Ferreira, V., Mey, J., and van Geen, A.: Centennial changes in North Pacific anoxia linked to tropical trade winds, Science, 345, 665–668, https://doi.org/10.1126/science.1252332, 2014.

Dias, B. B., Hart, M. B., Smart, C. W., and Hall-Spencer, J. M.: Modern seawater acidification: the response of foraminifera to high-CO₂ conditions in the Mediterranean Sea, J. Geol. Soc. Lond., 167, 843–846, https://doi.org/10.1144/0016-76492010-050, 2010.

Diaz, R. J. and Rosenberg, R.: Spreading Dead Zones and Consequences for Marine Ecosystems, Science, 321, 926–929, https://doi.org/10.1126/science.1156401, 2008.

Dickson, A. J. and Cohen, A. S.: A molybdenum isotope record of Eocene Thermal Maximum 2: Implications for global ocean redox during the early Eocene, Paleocean. Paleoclim., 27, PA3230, https://doi.org/10.1029/2012PA002346, 2012.

Dickson, A. J., Jenkyns, H. C., Porcelli, D., van den Boorn, S., and Idiz, E.: Basin-scale controls on the molybdenum-isotope composition of seawater during Oceanic Anoxic Event 2 (Late Cretaceous), Geochim. Cosmochim. Ac., 178, 291–306, https://doi.org/10.1016/j.gca.2015.12.036, 2016.

Dickson, A. J.: A molybdenum-isotope perspective on Phanerozoic deoxygenation events, Nat. Geosci., 10, 721–726, https://doi.org/10.1038/ngeo3028, 2017.

Dickson, A. J., Hsieh, Y.-T., and Bryan, A.: The rhe-
nium isotope composition of Atlantic Ocean sea-
water, Geochim. Cosmochim. Acta 287, 221–228,
https://doi.org/10.1016/j.gca.2020.02.020, 2020.

Diz, P. and Francés, G.: Distribution of live benthic foraminifera in the Ría de Vigo (NW Spain), Mar. Micropaleontol., 66, 165–191, https://doi.org/10.1016/j.marmicro.2007.09.001, 2008.

Diz, P., Gonzalez-Guitian, V., Gonzalez-Villanueva, R., Ove-
jero, A., Hernandez-Almeida, I.: BENFEP, a quantitative
database of BENthic Foraminifera from surface sediments
of the Eastern Pacific, Earth Syst. Sci. Data, 15, 697–722,
https://doi.org/10.5194/essd-15-697-2023, 2023.

Dorador, J., Rodríguez-Tovar, F. J., and Titschack, J.: Ex-
ploring computed tomography in ichnological analysis of
cores from modern marine sediments, Sci. reports, 10, 201,
https://doi.org/10.1038/s41598-019-57028-z, 2020.

Dubois, N., Kienast, M., Kienast, S., Normandeau, C., Calvert, S. E., Herbert, T. D., and Mix, A.: Millennial-scale variations in hydrography and biogeochemistry in the Eastern Equatorial Pacific over the last 100 kyr, Quaternary Sci. Rev., 30, 210–223, https://doi.org/10.1016/j.quascirev.2010.10.012, 2011.

Duijnstee, I. A. P., Ernst, S. R., and Van der Zwaan, G. J.: Effect of anoxia on the vertical migration of benthic foraminifera, Mar. Ecol. Progr. Ser., 246, 85–94, https://doi.org/10.3354/meps246085, 2003.

Dummann, W., Steinig, S., Hofmann, P., Lenz, M., Kusch, S., Flögel, S., Herrle, J. O., Hallmann, C., Rethemeyer, J., Kasper, H. U., and Wagner, T.: Driving mechanisms of organic carbon burial in the Early Cretaceous South Atlantic Cape Basin (DSDP Site 361), Clim. Past, 17, 469–490, https://doi.org/10.5194/cp-17-469-2021, 2021.

Dunk, R. M., Mills, R. A., and Jenkins, W. J.: A reevaluation of the oceanic uranium budget for the Holocene, Chem. Geol., 190, 45–67, https://doi.org/10.1016/S0009-2541(02)00110-9, 2002.

Duplessy, J. C., Shackleton, N. J., Matthews, R. K., Prell, W., Ruddiman, W. F., Caralp, M., and Hendy, C. H.: ¹³C record of benthic foraminifera in the last interglacial ocean: implications for the carbon cycle and the global deep water circulation, Quaternary Res., 21, 225–243, https://doi.org/10.1016/0033-5894(84)90099-1, 1984.

Egger, M., Riedinger, N., Mogollon, J. M., and Jørgensen, B. B.: Global diffusive fluxes of methane in marine sediments, Nat. Geosc., 11, 421–425, https://doi.org/10.1038/s41561-018-0122-8, 2018.

Eglinton, T. I. and Eglinton, G.: Molecular proxies for paleoclimatology, Earth Planet. Sc. Lett., 275, 1–16, https://doi.org/10.1016/j.epsl.2008.07.012, 2008.

Eglinton, T. I. and Repeta, D. J.: Organic matter in the con-
temporary ocean, in: Treatise on Geochemistry: The Oceans
and Marine Geochemistry, edited by: Mottl, M. J. and El-
derfield, H., Elsevier, Oxford, United Kingdom, 151–189,
https://doi.org/10.1016/B978-0-08-095975-7.00606-9, 2014.

Eiler, J., Cesar, J., Chimiak, L., Dallas, B., Grice, K., Griep-
Raming, J., Juchelka, D., Kitchen, N., Lloyd, M., Makarov,
A., Robins, R., and Schwieters, J.: Analysis of molecular
isotopic structures at high precision and accuracy by Orbi-
trap mass spectrometry, Int. J. Mass Spectrom., 422, 126–142,
https://doi.org/10.1016/j.ijms.2017.10.002, 2017.

Elling, F. J., Könneke, M., Lipp, J. S., Becker, K. W., Gagen, E. J., and Hinrichs, K.-U.: Effects of growth phase on the membrane lipid composition of the thaumarchaeon *Nitrosopumilus mar- itimus* and their implications for archaeal lipid distributions in the marine environment, Geochim. Cosmochim. Ac., 141, 579–597, https://doi.org/10.1016/j.gca.2014.07.005, 2014.

Elling, F. J., Becker, K. W., Könneke, M., Schröder, J. M., Keller-
mann, M. Y., Thomm, M., and Hinrichs, K.-U.: Respiratory
quinones in archaea: phylogenetic distribution and application
as biomarkers in the marine environment, Envir. Microbiol., 18,
692–707, https://doi.org/10.1111/1462-2920.13086, 2016.

Elling, F. J., Könneke, M., Nicol, G. W., Stieglmeier, M., Bayer, B.,
Spieck, E., Torre, J. R. de la, Becker, K. W., Thomm, M., Prosser,
J. I., Herndl, G. J., Schleper, C., and Hinrichs, K.-U.: Chemo-
taxonomic characterisation of the thaumarchaeal lipidome, En-
viron. Microbiol., 19, 2681–2700, https://doi.org/10.1111/1462-
2920.13759, 2017.

Elling, F. J., Hemingway, J. D., Evans, T. W., Kharbush, J. J., Spieck, E., Summons, R. E., and Pearson, A.: Vitamin B₁₂-dependent biosynthesis ties amplified 2-methylhopanoid production during oceanic anoxic events to nitrification, P. Natl. Acad. Sci. USA, 117, 32996–33004, https://doi.org/10.1073/pnas.2012357117, 2020.

Elling, F. J., Hemingway, J. D., Kharbush, J. J., Becker, K. W., Polik, C. A., and Pearson, A.: Linking diatom-diazotroph sym-bioses to nitrogen cycle perturbations and deep-water anoxia: Insights from Mediterranean sapropel events, Earth Planet. Sc. Lett., 571, 117110, https://doi.org/10.1016/j.epsl.2021.117110, 2021.

Elling, F. J., Evans, T. W., Nathan, V., Hemingway, J. D., Kharbush, J. J., Bayer, B., Spieck, E., Husain, F., Summons, R. E., and Pearson, A.: Marine and terrestrial nitrifying bacteria are sources of diverse bacteriohopanepolyols, Geobiology, 20, 399–420, https://doi.org/10.1111/gbi.12484, 2022.

Elvert, M., Suess, E., and Whiticar, M. J.: Anaerobic methane oxidation associated with marine gas hydrates: superlight C-isotopes from saturated and unsaturated C₂₀ and C₂₅ irregular isoprenoids, Naturwissenschaften, 86, 295–300, https://doi.org/10.1007/s001140050619, 1999.

Emerson, S. R. and Huested, S. S.: Ocean anoxia and the concentra-
tions of molybdenum and vanadium in seawater, Mar. Chem., 34,
177-196, https://doi.org/10.1016/0304-4203(91)90002-E, 1991.

Endrizzi, F. and Rao, L.: Chemical speciation of uranium (VI) in marine environments: complexation of calcium and magnesium ions with [(UO₂)(CO₃)₃]⁴⁻ and the effect on the extraction of uranium from seawater, Chem-Eur. J., 20, 14499-14506, https://doi.org/10.1002/chem.201403262, 2014.

Enge, A. J., Wukovits, J., Wanek, W., Watzka, M., Witte, U. F., Hunter, W. R., and Heinz, P.: Carbon and nitrogen uptake of calcareous benthic foraminifera along a depth-related oxygen gradient in the OMZ of the Arabian Sea, Front. Microbiol., 7, 71, https://doi.org/10.3389/fmicb.2016.00071, 2016.

Erdem, Z., Schönfeld, J. Glock, N., Dengler, M., Mosch, T., Sommer, S., Elger, J., and Eisenhauer, A.: Peruvian sediments as recorders of an evolving hiatus for the last 22 thousand years, Quaternary Sci. Rev., 137, 1-14, https://doi.org/10.1016/j.quascirev.2016.01.029, 2016.

Erdem, Z. and Schönfeld, J.: Pleistocene to Holocene benthic foraminiferal assemblages from the Peruvian continental margin, Palaeontol. Electron., 20, 1–32, https://doi.org/10.26879/764, 2017.

Erdem, Z., Schönfeld, J., Rathburn, A. E., Pérez, M. E., Cardich, J., and Glock, N.: Bottom-water deoxygenation at the Peruvian margin during the last deglaciation recorded by benthic foraminifera, Biogeosciences, 17, 3165–3182, https://doi.org/10.5194/bg-17-3165-2020, 2020.

Erez, J.: The source of ions for biomineralization in foraminifera and their implications for paleoceanographic proxies, Revi. Mineral. Geochem., 54, 115–149, https://doi.org/10.2113/0540115, 2003.

Erickson, B. E. and Helz, G. R.: Molybdenum (VI) specia-
tion in sulfidic waters: stability and lability of thiomolyb-

dates, Geochim. Cosmochim. Ac., 64, 1149–1158,
https://doi.org/10.1016/S0016-7037(99)00423-8, 2000.

Ettwig, K. F., Butler, M. K., Paslier, D. L., Pelletier, E., Mangenot, S., Kuypers, M. M. M., Schreiber, F., Dutilh, B. E., Zedelius, J., Beer, D. de, Gloerich, J., Wessels, H. J. C. T., Alen, T. van, Luesken, F., Wu, M. L., Pas-Schoonen, K. T. van de, Camp, H. J. M. O. den, Janssen-Megens, E. M., Francoijs, K.-J., Stunnenberg, H., Weissenbach, J., Jetten, M. S. M., and Strous, M.: Nitrite-driven anaerobic methane oxidation by oxygenic bacteria, Nature, 464, 543–548, https://doi.org/10.1038/nature08883, 2010.

Farrell, J. W., Pedersen, T. F., Calvert, S. E., and Nielsen, B.: Glacial-interglacial changes in nutrient utilization in the equatorial Pacific Ocean, Nature, 377, 514–517, https://doi.org/10.1038/377514a0, 1995.

Fatela, F. and Taborda, R.: Confidence limits of species proportions in microfossil assemblages, Mar. Micropaleontol., 45, 169–174, https://doi.org/10.1016/S0377-8398(02)00021-X, 2002.

Ferry, J. G. and Lessner, D. J.: Methanogenesis in marine sediments, Ann. NY Acad. Sci., 1125, 147–157, https://doi.org/10.1196/annals.1419.007, 2008.

Finney, B. P., Lyle, M. W., and Heath, G. R.: Sedimentation at MANOP Site H (eastern equatorial Pacific) over the past 400,000 years: climatically induced redox variations and their effects on transition metal cycling, Paleocean. Paleoclim., 3, 169–189, https://doi.org/10.1029/PA003i002p00169, 1988.

Flegal, A. R., Sañudo-Wilhelmy, S. A., and Scelfo, G. M.: Silver in the Eastern Atlantic Ocean, Mar. Chem., 49, 315–320, https://doi.org/10.1016/0304-4203(95)00021-I, 1995.

Fontanier, C., Jorissen, F.J., Licari, L., Alexandre, A., Anschutz, P., and Carbonel, P.: Live benthic foraminiferal faunas from the Bay of Biscay: faunal density, composition, and microhabitats, Deep-Sea Res. Pt. I, 49, 751–785, https://doi.org/10.1016/S0967-0637(01)00078-4, 2002.

Fontanier, C., Jorissen, F. J., Chaillou, G., David, C., Anschutz, P., and Lafon, V.: Seasonal and interannual variability of benthic foraminiferal faunas at 550 m depth in the Bay of Biscay, Deep-Sea Res. Pt. I, 50, 457–494, https://doi.org/10.1016/S0967-0637(02)00167-X, 2003.

Fontanier, C., Jorissen, F. J., Chaillou, G., Anschutz, P., Grémare, A., and Griveaud, C.: Live foraminiferal faunas from a 2800 m deep lower canyon station from the Bay of Biscay: faunal response to focusing of refractory organic matter, Deep-Sea Res. Pt. I, 52, 1189-1227, https://doi.org/10.1016/j.dsr.2005.01.006, 2005.

Fontanier, C., Jorissen, F. J., Michel, E., Cortijo, E., Vidal, L., and Anschutz, P., Stable oxygen and carbon isotopes of live (stained) benthic foraminifera from Cap-Ferret Canyon (Bay of Biscay), J. Foram. Res., 38, 39–51, https://doi.org/10.2113/gsjfr.38.1.39, 2008.

Fontanier, C., Koho, K. A., Goñi-Urriza, M. S., Deflandre, B., Galaup, S., Ivanovsky, A., Gayer, N., Dennielou, B., Gremare, A., Bichon, S., Gassie, C., Anschutz, P., Duran, and Reichart, G. J.: Benthic foraminifera from the deep-water Niger delta (Gulf of Guinea): Assessing present-day and past activity of hydrate pockmarks, Deep-Sea Res. Pt. I, 94, 87–106, https://doi.org/10.1016/j.dsr.2014.08.011, 2014.

Fortin, D., Francus, P., Gebhardy, A.C., Hahn, A., Kliem, P., Lisé-
Pronovost, A., Roychowdhury, R., Labrie, J., St-Onge, J., and
The PASADO Team: Destructive and non-destructive density determination: method comparison and evaluation from the Laguna Potrok Aike sedimentary record, Quaternary Sci. Rev. 71, 147–153, https://doi.org/10.1016/j.quascirev.2012.08.024, 2013

Fox, L., Stukins, S., Hill, T., and Miller, C.G.: Quantifying the Effect of Anthropogenic Climate Change on Calcifying Plankton, Sci. Rep., 10, 1620, https://doi.org/10.1038/s41598-020-58501-w, 2020.

Franco-Fruagas, P., Badaraco Costa, K., and de Lima Toledo, F.: Stable isotope/test size relationship in *Cibicidoides wuellerstorfi*, Braz. J. Oceanogr., 59, 2870291, https://doi.org/10.1590/S1679-87592011000300010, 2011.

Francois, R.: A Study of Sulphur Enrichment in the Humic Fraction of Marine Sediments during Early Diagenesis, Geochim. Cosmochim. Ac., 51, 17–27, https://doi.org/10.1016/0016-7037(87)90003-2, 1987.

Frei, R., Gaucher, C., Døssing, L., and Sial, A.: Chromium isotopes in carbonates – a tracer for climate change and for reconstructing the redox state of ancient seawater, Earth Planet. Sc. Lett., 312, 114–125, https://doi.org/10.1016/j.epsl.2011.10.009, 2011.

Freitas, T. R., Bacalhau, E., and Bonetti, C.: ForImage: Foraminiferal Image Analysis and Test Measurement, R Package Version 0.1.0., https://doi.org/10.32614/CRAN/package.forImage, 2021.

French, K. L., Rocher, D., Zumberge, J. E., and Summons, R. E.: Assessing the distribution of sedimentary $C_{40}$ carotenoids through time, Geobiology, 13, 139–151, https://doi.org/10.1111/gbi.12126, 2015.

Fritz-Endres, T. and Fehrenbacher, J.: Preferential Loss of High Trace Element Bearing Inner Calcite in Foraminifera During Physical and Chemical Cleaning, Geochem. Geophy. Geosy., 22, e2020GC009419, https://doi.org/10.1029/2020GC009419, 2021.

Froelich P. N., Klinkhammer G. P., Bender M. L., Luedtke N. A., Heath G. R., Cullen D., Dauphin P., Hammond D., Hartman B., and Maynard V.: Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis, Geochim. Cosmochim. Ac., 43, 1075–1090, https://doi.org/10.1016/0016-7037(79)90095-4, 1979.

Freund, C., Wishard, A., Brenner, R., Sobel, M., Mizelle, J., Kim, A., Meyer, D. A., and Morford, J. L.: The effect of a thiol-containing organic molecule on molybdenum adsorption onto pyrite, Geochim. Cosmochim. Ac., 174, 222–235, https://doi.org/10.1016/j.gca.2015.11.015, 2016.

Fyke, J. G., D'Orgeville, M., and Weaver, A. J.: Drake Passage and Central American Seaway controls on the distribution of the oceanic carbon reservoir, Glob. Planet. Change 128, 72–83, https://doi.org/10.1016/j.gloplacha.2015.02.011, 2015

Galbraith, E. D., Kienast, M., Pedersen, T. F., and Calvert, S. E.: Glacial-interglacial modulation of the marine nitrogen cycle by high-latitude $O_2$ supply to the global thermocline, Paleocean. Paleoclim., 19, PA4007, https://doi.org/10.1029/2003PA001000, 2004.

Galbraith, E. D., Kienast, M., Jaccard, S. L., Pedersen, T. F., Brunelle, B. G., Sigman, D. M., and Kiefer, T.: Consistent relationship between global climate and surface nitrate utilization in the western subarctic Pacific throughout the last 500 ka, Paleocean. Paleoclim., 23, PA2212, https://doi.org/10.1029/2007PA001518, 2008.

Galbraith, E. G., Kienast, M., and The NICOPP working group members: The acceleration of oceanic denitrification during deglacial warming, Nat. Geosc., 76, 579–584, https://doi.org/10.1038/ngeo1832, 2013.

Gambacorta, A., Pagnotta, E., Romano, I., Sodano, G., and Trinccone, A.: Heterocyst glycolipids from nitrogen-fixing cyanobacteria other than *Nostocaceae*, Phytochemistry, 48, 801–805, https://doi.org/10.1016/S0031-9422(97)00954-0, 1998.

Ganeshram, R. S., Pedersen, T. F., Calvert, S. E., and Murray, J. W.: Large changes in oceanic nutrient inventories from glacial to interglacial periods, Nature, 376, 755–758, https://doi.org/10.1038/376755a0, 1995.

Geslin, E., Heinz, P., Jorissen, F., and Hemleben, C.: Migratory responses of deep-sea benthic foraminifera to variable oxygen conditions: laboratory investigations, Mar. Micropaleontol., 53, 227–243, https://doi.org/10.1016/j.marmicro.2004.05.010, 2004.

Geslin, E., Risgaard-Petersen, N., Lombard, F., Metzger, E., Langlet, D., and Jorissen, F.: Oxygen respiration rates of benthic foraminifera as measured with oxygen microsensors, J. Exp. Mar. Biol. Ecol., 396, 108–114, https://doi.org/10.1016/j.jembe.2010.10.011, 2011.

Geslin, E., Barras, C., Langlet, D., Nardelli, M. P., Kim, J. H., Bonnin, J., Metzger, E., and Jorissen, F. J.: Survival, reproduction and calcification of three benthic foraminiferal species in response to experimentally induced hypoxia, in: Approaches to Study Living Foraminifera, edited by: Kitazato, H. M., and Bernhard, J., Springer, Tokyo, 163–193, https://doi.org/10.1007/978-4-431-54388-6\_10, 2014.

Giordano, L., Ferraro, L., Salvatore, M., Oscurato, S. L., and Maddalena, P.: Morphometric analysis on benthic foraminifera through atomic force microscopy, Mar. Micropaleontol, 153, 101775, https://doi.org/10.1016/j.marmicro.2019.101775, 2019.

Glock, N., Eisenhauer, A., Milker, Y., Liebetrau, V., Schönfeld, J., Mallon, J., Sommer, S., and Hensen, C.: Environmental influences on the pore density of *Bolivina* spissa (Cushman), J. Foraminifer. Res., 41, 22–32, https://doi.org/10.2113/gsjfr.41.1.22, 2011.

Glock, N., Eisenhauer, A., Liebetrau, V., Wiedenbeck, M., Hensen, C., and Nehrke, G.: EMP and SIMS studies on Mn/Ca and Fe/Ca systematics in benthic foraminifera from the Peruvian OMZ: a contribution to the identification of potential redox proxies and the impact of cleaning protocols, Biogeosciences, 9, 341–359, https://doi.org/10.5194/bg-9-341-2012, 2012a.

Glock, N., Schönfeld, J., and Mallon, J.: The Functionality of Pores in Benthic Foraminifera in View of Bottom Water Oxygenation: A Review, in: ANOXIA: Evidence for Eukaryote Survival and Paleontological Strategies, Cellular Origin, Life in Extreme Habitats and Astrobiology, Vol. 21, edited by: Altenbach, A. V., Bernhard, J. M., and Seckbach, J., Springer, Dordrecht, Germany, 540–556, https://doi.org/10.1007/978-94-007-1896-8\_28, 2012b.

Glock, N., Liebetrau, V., and Eisenhauer, A.: I/Ca ratios in benthic foraminifera from the Peruvian oxygen minimum zone: analytical methodology and evaluation as a proxy for redox conditions, Biogeosciences, 11, 7077–7095, https://doi.org/10.5194/bg-11-7077-2014, 2014.

Glock, N., Liebetrau, V., Eisenhauer, A., and Rocholl, A.: High resolution I/Ca ratios of benthic foraminifera from the Peruvian oxygen-minimum-zone: A SIMS derived assess-
ment of a potential redox proxy, Chem. Geol., 447, 40–53,
https://doi.org/10.1016/j.chemgeo.2016.10.025, 2016.

Glock, N., Erdem, Z., Wallmann, K., Somes, C. J., Liebetrau, V., Schönfeld, J., Gorb, S., and Eisenhauer, A.: Coupling of oceanic carbon and nitrogen facilitates spatially resolved quantitative reconstruction of nitrate inventories, Nat. Commun., 9, 1217, https://doi.org/10.1038/s41467-018-03647-5, 2018.

Glock, N., Liebetrau, V., Vogts, A., and Eisenhauer, A.: Organic heterogeneities in foraminiferal calcite traced through the distribution of N, S, and I measured with nanoSIMS: a new challenge for element-ratio-based paleoproteins?, Front. Earth Sci., 7, 175, https://doi.org/10.3389/feart.2019.00175, 2019a.

Glock, N., Roy, A. S., Romero, D., Wein, T., Weissenbach, J., Revsbech, N. P., Høgslund, S., Clemens, D., Sommer, S., and Dagan, T.: Metabolic preference of nitrate over oxygen as an electron acceptor in foraminifera from the Peruvian oxygen minimum zone, P. Natl. Acad. Sci. USA, 116, 2860–2865, https://doi.org/10.1073/pnas.1813887116, 2019b.

Glock, N., Erdem, Z., and Schönfeld, J.: The Peruvian oxygen minimum zone was similar in extent but weaker during the Last Glacial Maximum than Late Holocene, Commun. Earth Environment, 3, 307, https://doi.org/10.1038/s43247-022-00635-y, 2022.

Glock, N.: Benthic foraminifera and gromiids from oxygen-depleted environments – survival strategies, biogeochemistry and trophic interactions, Biogeosciences, 20, 3423–3447, https://doi.org/10.5194/bg-20-3423-2023, 2023.

Glud, R. N., Thamdrup, B., Stahl, H., Wenzhoefer, F., Glud, A., Nomaki, H., Oguri, K., Revsbech, N. P., and Kitzazato, H.: Nitrogen cycling in a deep ocean margin sediment (Sagami Bay, Japan), Limnol. Oceanogr., 54, 723–734, https://doi.org/10.4319/lo.2009.54.3.0723, 2009.

Goineau, A. and Gooday, A. J.: Diversity and spatial patterns of foraminiferal assemblages in the eastern Clarion-Clipperton zone (abyssal eastern equatorial Pacific), Deep-Sea Res. Pt. I, 149, 103036, https://doi.org/10.1016/j.dsr.2019.04.014, 2019.

Goldschmidt, V. M. (Ed.): Geochemistry, The Clarendon Press, Oxford, 730 pp., ISBN: 0198512104, 1954.

Gomaa, F., Utter, D. R., Powers, C., Beaudoin, D. J., Edgcomb, V. P., Filipsson, H. L., Hansel, C. M., Wankel, S., Zhang, Y., and Bernhard, J. M.: Multiple integrated metabolic strategies allow foraminiferan protists to thrive in anoxic marine sediments, Sci. Adv., 7, eabf1586, https://doi.org/10.1126/sciadv.abf1586, 2021.

Gooday, A. J.: Benthic foraminifera (protista) as tools in deep-water palaeoceanography: environmental influences on faunal characteristics, Adv. Mar. Biol., 46, 1–90, https://doi.org/10.1016/S0065-2881(03)46002-1, 2003.

Gooday, A. J. and Rathburn, A. E.: Temporal variability in liv-
ing deep-sea benthic foraminifera: a review, Earth-Sci. Rev.,
46, 187–212, https://doi.org/10.1016/S0012-8252(99)00010-0,
1999.

Gooday, A. J., Bernhard, J. M., Levin, L. A., and Suhr, S. B.: Foraminifera in the Arabian Sea oxygen minimum zone and other oxygen-deficient settings taxonomic composition, diversity, and relation to metazoan faunas, Deep-Sea Res. Pt. II, 47, 25–54, https://doi.org/10.1016/S0967-0645(99)00099-5, 2000.

Gooday, A. J. and Hughes, J. A.: Foraminifera associated with phytodetritus deposits at a bathyal site in the northern Rockall Trough (NE Atlantic): seasonal contrasts and a comparison

of stained and dead assemblages, Mar. Micropaleontol., 46, 83–
110, https://doi.org/10.1016/S0377-8398(02)00050-6, 2002.

Gooday, A. J., Jorissen, F., Levin, L. A., Middelburg, J. J., Naqvi, S. W. A., Rabalais, N. N., Scranton, M., and Zhang, J.: Historical records of coastal eutrophication-induced hypoxia, Biogeosciences, 6, 1707–1745, https://doi.org/10.5194/bg-6-1707-2009, 2009.

Goossens, H., De Leeuw, J. W., Schenck, P. A., and Bras-
sell, S. C.: Tocopherols as likely precursors of pristane in
ancient sediments and crude oils, Nature, 312, 440–442,
https://doi.org/10.1038/312440a0, 1984.

Gordon, G., Lyons, T., Arnold, G., Roe, J., Sageman, B., and Anbar, A.: When do black shales tell molybdenum isotope tales?, Geology, 37, 535–538, https://doi.org/10.1130/G25186A.1, 2009.

Görög, A., Szinger, B., Tóth, E., and Viszkok, J.: Methodology of the micro-computer tomography on foraminifera, Palaeontol. Electron., 15, 1–15, https://doi.org/10.26879/261, 2012.

Gottschalk, J., Skinner, L. C., Lippold, J., Vogel, H., Frank, N., Jaccard, S. L. and Waelbroeck, C.: Biological and physical controls in the Southern Ocean on past millennial-scale atmospheric CO₂ changes, Nat. Commun., 7, 11539, https://doi.org/10.1038/ncomms11539, 2016a.

Gottschalk, J., Riveiros, N. V., Waelbroeck, C., Skinner, L. C., Michel, E., Duplessy, J.-C., Hodell, D. A., and Mackensen, A.: Carbon isotope offsets between species of the genus *Cibicides* (*Cibicidoides*) in the glacial sub-Antarctic Atlantic Ocean, Paleocean. Paleoclim., 31, 1583–1602, https://doi.org/10.1002/2016PA003029, 2016b.

Gottschalk, J., Skinner, L. C., Jaccard, S. L., Menviel, L., Nehrbass-Ahles, C., and Waelbroeck, C.: Southern Ocean link between changes in atmospheric CO₂ levels and northern-hemisphere climate anomalies during the last two glacial periods, Quaternary Sci. Rev., 230, 106067, https://doi.org/10.1016/j.quascirev.2019.106067, 2020a.

Gottschalk, J., Michel, E., Thöle, L. M., Studer, A. S., Hasenfratz, A. P., Schmid, N., Butzin, M., Mazaud, A., Martínez-García, A., Szidat, S., and Jaccard, S. L.: Glacial heterogeneity in Southern Ocean carbon storage abated by fast South Indian deglacial carbon release, Nat. Commun., 11, 6192, https://doi.org/10.1038/s41467-020-20034-1, 2020b.

Govindankutty-Menon, A., Davis, C. V., Nürnberg, D., Nomaki, H., Salonen, I., Schmiedl, G., and Glock, N.: A deep-learning automated image recognition method for measuring pore patterns in closely related bolivinids and calibration for quantitative nitrate paleo-reconstructions, Sci. Rep., 13, 19628, https://doi.org/10.1038/s41598-023-46605-y, 2023.

Granger, J., Sigman, D. M., Needoba, J. A., and Harrison, P. J.: Coupled nitrogen and oxygen isotope fractionation of nitrate during assimilation by cultures of marine phytoplankton, Limnol. Oceanogr., 49, 1763–1773, https://doi.org/10.4319/lo.2004.49.5.1763, 2004.

Greaves, M., Caillon, N., Rebaubier, H., Bartoli, G., Bohaty, S., Cacho, I., Clarke, L., Cooper, M., Daunt, C., Delaney, M., DeMenocal, P., Dutton, A., Eggins, S., Elderfield, H., Garbe-Schoenberg, D., Goddard, E., Green, D., Groeneveld, J., Hastings, D., Hathorne, E., Kimoto, K., Klinkhammer, G., Labeyrie, L., Lea, D. W., Marchitto, T., Martínez-Botí, M. A., Mortyn, P. G., Ni, Y., Nuernberg, D., Paradis, G., Pena, L., Quinn, T., Rosenthal, Y., Russell, A., Sagawa, T., Sosdian, S., Stott,
L., Tachikawa, K., Tappa, E., Thunell, R., and Wilson P. A.: Interlaboratory comparison study of calibration standards for foraminiferal Mg / Ca thermometry, Geochem. Geophy. Geosy., 9, Q08010, https://doi.org/10.1029/2008GC001974, 2008.

Grice, K., Gibbison, R., Atkinson, J. E., Schwark, L., Eckardt, C. B., and Maxwell, J. R.: Maleimides (1H-pyrrole-2,5-diones) as molecular indicators of anoxygenic photosynthesis in ancient water columns, Geochim. Cosmochim. Ac., 60, 3913–3924, https://doi.org/10.1016/0016-7037(96)00199-8, 1996.

Grigorov, I., Pearce, R. B., and Kemp, A. E. S.: Southern Ocean laminated diatom ooze: Mat deposits and potential for palaeo-flux studies, ODP Leg 177, Site 1093, Deep-Sea Res. Pt. II, 49, 3391–3407, https://doi.org/10.1016/S0967-0645(02)00089-9, 2002.

Groeneveld, J. and Filipsson, H. L.: Mg / Ca and Mn / Ca ratios in benthic foraminifera: The potential to reconstruct past variations in temperature and hypoxia in shelf regions, Biogeosciences, 10, 5125–5138, https://doi.org/10.5194/bg-10-5125-2013, 2013.

Groeneveld, J., Filipsson, H. L., Austin, W. E., Darling, K., McCarthy, D., Quintana Krupinski, N. B., Bird, C., and Schweizer, M.: Assessing proxy signatures of temperature, salinity, and hypoxia in the Baltic Sea through foraminifera-based geochemistry and faunal assemblages, J. Micropalaeontol., 37, 403–429, https://doi.org/10.5194/jm-37-403-2018, 2018.

Gruber, N., Keeling, C. D., Bacastow, R. B., Guenther, P. R., Lueker, T. J., Wahlen, M., Meijer, H. A., Mook, W. G., and Stocker, T. F.: Spatiotemporal patterns of carbon-13 in the global surface oceans and the oceanic Suess effect, Global Biogeochem. Cy., 13, 307–335, https://doi.org/10.1029/1999GB900019, 1999.

Gueguen, B., Reinhard, C. T., Algeo, T. J., Peterson, L. C., Nielsen, S. G., Wang, X., Rowe, H., and Planavsky, N. J.: The chromium isotope composition of reducing and oxic marine sediments, Geochim. Cosmochim. Ac., 184, 1–19, https://doi.org/10.1016/j.gca.2016.04.004, 2016.

Gueneli, N., McKenna, A. M., Ohkouchi, N., Boreham, C. J., Beghin, J., Javaux, E. J., and Brocks, J. J.: 1.1-billion-year-old porphyrins establish a marine ecosystem dominated by bacterial primary producers, P. Natl. Acad. Sci. USA, 115, E6978–E6986, https://doi.org/10.1073/pnas.1803866115, 2018.

Guerrero-Cruz, S., Vaksmaa, A., Horn, M. A., Niemann, H., Pijuan, M., and Ho, A.: Methanotrophs: Discoveries, environmental relevance, and a perspective on current and future applications, Front. Microbiol., 12, 678057, https://doi.org/10.3389/fmicb.2021.678057, 2021.

Gulev, S. K., Thorne, P. W., Ahn, J., Dentener, F. J., Domingues, C. M., Gerland, S., Gong, D., Kaufman, D. S., Nnamchi, H. C., Quaas, J., Rivera, J. A., Sathyendranath, S., Smith, S. L., Trewin, B., von Shuckmann, K., and Vose, R. S.: Changing State of the Climate System, in: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, edited by: Masson-Delmotte, V., Zhai, P., Pirani, A., Connors, S. L., Péan, C., Berger, S., Caud, N., Chen, Y., Goldfarb, L., Gomis, M. I., Huang, M., Leitzell, K., Lonnoy, E., Matthews, J. B. R., Maycock, T. K., Waterfield, T., Yelekçi, O., Yu, R., and Zhou, B., Cambridge University Press, https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_02.pdf (last access: 21 March 2022), 2021.

Guo, X., Xu, B., Burnett, W. C., Yu, Z., Yang, S., Huang, X., Wang, F., Nan, H., Yao, P., and Sun, F.: A potential proxy for seasonal hypoxia: LA-ICP-MS Mn / Ca ratios in benthic foraminifera from the Yangtze River Estuary, Geochim. Cosmochim. Ac., 245, 290–303, https://doi.org/10.1016/j.gca.2018.11.007, 2019.

Gupta, B. K. S. and Machain-Castillo, M. L.: Benthic foraminifera in oxygen-poor habitats, Mar. Micropaleontol., 20, 183–201, https://doi.org/10.1016/0377-8398(93)90032-S, 1993.

Gutiérrez, D., Sifeddine, A., Field, D. B., Ortlieb, L., Vargas, G., Chávez, F. P., Velazco, F., Ferreira, V., Tapia, P., Salvatteci, R., Boucher, H., Morales, M. C., Valdés, J., Reyss, J.-L., Campusano, A., Boussafir, M., Mandeng-Yogo, M., García, M., and Baumgartner, T.: Rapid reorganization in ocean biogeochemistry off Peru towards the end of the Little Ice Age, Biogeosciences, 6, 835–848, https://doi.org/10.5194/bg-6-835-2009, 2009.

Hain, M. P., Sigman, D. M., and Haug, G. H.: Carbon dioxide effects of Antarctic stratification, North Atlantic Intermediate Water formation, and subantarctic nutrient drawdown during the last ice age: Diagnosis and synthesis in a geochemical box model, Glob. Biogeochem. Cy., 24, GB4023, https://doi.org/10.1029/2010GB003790, 2010.

Hansen, J., Sato, M., Russell, G., and Kharecha, P. Climate sensitivity, sea level and atmospheric carbon dioxide, Philos. T. R. Soc. A, 371, 20120294, https://doi.org/10.1098/rsta.2012.0294, 2013.

Harayama, T. and Riezman, H.: Understanding the diversity of membrane lipid composition, Nat. Rev. Mol. Cell. Biol., 19, 281–296, https://doi.org/10.1038/nrm.2017.138, 2018.

Hardisty, D. S., Lu, Z., Planavsky, N. J., Bekker, A., Philippot, P., Zhou, X., and Lyons, T. W.: An iodine record of Proterozoic surface ocean oxygenation, Geology, 42, 619–622, https://doi.org/10.1130/G35439.1, 2014.

Hardisty, D. S., Lu, Z., Bekker, A., Diamond, C. W., Gill, B. C., Jiang, G., Kah, L. C., Knoll, A. H., Loyd, S. J., Osburn, M. R., and Planavsky, N. J.: Perspectives on Proterozoic surface ocean redox from iodine contents in ancient and recent carbonate, Earth Planet. Sc. Lett., 463, 159–170, https://doi.org/10.1016/j.epsl.2017.01.032, 2017.

Hardisty, D. S., Lyons, T. W., Riedinger, N., Isson, T. T., Owens, J. D., Aller, R. C., Rye, D. M., Planavsky, N. J., Reinhard, C. T., Gill, B. C., Masterson, A. L., Asael, D., and Johnston, D. T.: An evaluation of sedimentary molybdenum and iron as proxies for pore fluid paleoredox conditions, Am. J. Sci., 318, 527–556, https://doi.org/10.2475/05.2018.04, 2018.

Hardisty, D. S., Horner, T. J., Evans, N., Moriyasu, R., Babbin, A. R., Wankel, S. D., Moffett, J. W., and Nielsen, S. G.: Limited iodate reduction in shipboard seawater incubations from the Eastern Tropical North Pacific oxygen deficient zone, Earth Planet. Sc. Lett., 554, 116676, https://doi.org/10.1016/j.epsl.2020.116676, 2021a.

Hardisty, D. S., Riedinger, N., Planavsky, N. J., Asael, D., Bates, S. M., and Lyons, T. W.: Holocene spatiotemporal redox variations in the southern Baltic Sea, Front. Earth Sci., 9, 344, https://doi.org/10.3389/feart.2021.671401, 2021b.

Harmann, R. A.: Distribution of foraminifera in the Santa Barbara Basin, California, Micropaleontology, 10, 81–96, https://doi.org/10.2307/1484628, 1964.

Harris, P. G., Zhao, M., Rosell-Melé, A., Tiedemann, R., Sarnthein, M., and Maxwell, J. R.: Chlorin accumulation rate as a proxy