# Controls on barium incorporation into tests of benthic foraminifera from the Aegean Sea (Eastern Mediterranean Sea) – Towards a species-specific Ba/Ca calibration

Jassin Petersen, Gerhard Schmiedl, Jacek Raddatz, André Bahr, Jörg Pross, Meryem Mojtahid

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## To cite this version:

Jassin Petersen, Gerhard Schmiedl, Jacek Raddatz, André Bahr, Jörg Pross, et al. Controls on barium incorporation into tests of benthic foraminifera from the Aegean Sea (Eastern Mediterranean Sea) – Towards a species-specific Ba/Ca calibration. Marine Micropaleontology, 2024, 194, pp.102431. 10.1016/j.marmicro.2024.102431. hal-05168720

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Submitted on 18 Jul 2025

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# Controls on barium incorporation into tests of benthic foraminifera from the Aegean Sea (Eastern Mediterranean Sea) – Towards a species-specific Ba/Ca calibration

Jassin Petersen<sup>a,∗</sup> <sup>ID</sup>, Gerhard Schmiedl<sup>b</sup>, Jacek Raddatz<sup>c,d,1</sup>, André Bahr<sup>e</sup>, Jörg Pross<sup>e</sup>, Meryem Mojtahid<sup>f</sup>

## ARTICLE INFO

**Keywords:**

Proxy development

Past primary productivity

Benthic foraminiferal barium incorporation

Biomineralisation

Barium cycle Mediterranean Sea

## ABSTRACT

We have analysed Ba/Ca ratios of live and dead benthic foraminifera (Uvigerina mediterranea and Melonis affinis) from core tops of seven sites located in the Aegean Sea by laser ablation ICP-MS. There are no significant Ba/Ca differences in live and dead specimens of U. mediterranea. For M. affinis we apply a threshold criterion for extremely high Ba/Ca in live specimens in order to highlight the overall uniform Ba/Ca signal. The Ba/Ca intra-test variability varies between 16 and 24 % relative standard deviation per specimen. It is attributed to biomineralisation processes i.e., vital effects. The Ba/Ca ratios of M. affinis are significantly higher than those of U. mediterranea throughout most of the sites and their respective samples from different sediment depths. The influence on Ba/Ca of both species by biomineralisation processes and/or microhabitat effects remains open. Linking the Ba/Ca ratios to measured Ba concentration of bottom waters from one of the studied sites, shows partition coefficients of DBa = 0.34, and DBa = 0.49 for U. mediterranea and M. affinis, respectively. We could not identify trends of Ba/Ca ratios to observed/ modelled gradients of relevant environmental factors between the seven analysed sites, such as primary productivity and associated Corg fluxes or TOC concentrations. Despite the demonstrated limited proxy potential in these parts of the Aegean Sea, we suggest that it is possible to employ Ba/Ca of infaunal benthic species in order to reconstruct export productivity in deep-sea areas with a less complex linkage between primary productivity and nutrient distribution in bottom waters.

## 1. Introduction

The processes controlling the marine carbon and barium cycles are closely coupled, therefore allowing the reconstruction of export productivity through the assessment of the Ba content in the geological record (Bishop, 1988; Paytan and Kastner, 1996; Carter et al., 2020). Models and global ocean data show that an export of organic matter (OM) from the upper ocean into the deep ocean is correlated to the flux of dissolved Ba from upper to deep oceans, and also to barite formation in the sediment (Dymond and Collier, 1996; Paytan and Kastner, 1996; Dickens et al., 2003). Some of these models show that to maintain such a

scenario, an additional source of dissolved Ba to the upper water column of oceans is needed, i.e., riverine input (Paytan and Kastner, 1996; Carter et al., 2020). Historically, biogenic Ba fluxes normalized to $^{230}$Th, as well as Ba/Th and Ba/Al ratios of sediment records have been used to link the Ba and carbon cycles and reconstruct export productivity (e.g., Dehairs et al., 2000; Moller et al., 2012; Jaccard et al., 2013). Generally, sedimentary signatures are subject to primary and secondary diagenetic alteration, and in the case of Ba, barite dissolution occurs when $SO_4^{2-}$ concentrations decrease below a threshold in surface sediments, and barite can (re)precipitate diagenetically (Church and Wolgemuth, 1972; McManus et al., 1998; Schenau et al., 2001). For organic-rich sediments

<footnote><sup>a</sup> Institute of Geology and Mineralogy, University of Cologne, Otto-Fischer-Straße 14, 50674 Cologne, Germany</footnote>

<footnote><sup>b</sup> Center for Earth System Sciences and Sustainability, Institute for Geology, University of Hamburg, Bundesstraße 55, 20146 Hamburg, Germany</footnote>

<footnote><sup>c</sup> Institute of Geosciences, Goethe University Frankfurt, Altenhöferallee 1, 60438 Frankfurt am Main, Germany</footnote>

<footnote><sup>d</sup> Frankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, 60438 Frankfurt, Germany</footnote>

<footnote><sup>e</sup> Institute of Earth Sciences, Heidelberg University, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany</footnote>

<footnote><sup>f</sup> Université d'Angers, Université de Nantes, Université Le Mans, CNRS, LPG, UMR 6112, 2 Bd Lavoisier, 49045 Angers Cedex, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: jassin.petersen@uni-koeln.de (J. Petersen).</footnote>

<footnote n="1">¹ present address: GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr.1-3, 24,148 Kiel, Germany</footnote>
such as sapropel layers from the Mediterranean Sea (Section 1.1), there is the phenomenon of a burndown effect, altering the geochemical composition of most recently deposited layers (Higgs et al., 1994; Thomson et al., 1995; Möbius et al., 2010). Regarding Ba, it was shown that post-depositional processes such as oxidation do not decrease its content in sapropelic sediments, underlining its potential as proxy for palaeo-export productivity (Thomson et al., 1995, 2006; Moller et al., 2012).

Benthic foraminifera colonize the surface sediment and precipitate their carbonate tests with an elemental and isotopic composition reflecting the surrounding bottom water and pore water composition, allowing for the reconstruction of palaeoenvironmental parameters (e. g., McCorkle et al., 1997; Schmiedl et al., 2004; Koho et al., 2015). The Ba/Ca ratios of fossil benthic foraminiferal tests may conserve the signal of Ba²⁺ of the bottom and pore waters, from which the foraminifera have calcified their test, and which is thought to reflect the input of Ba and thus OM fluxes from the water column (e.g., Ní Fhlaithearta et al., 2010; Mojtahid et al., 2019). The further development of this proxy requires a profound understanding of the underlying processes for the incorporation of Ba in foraminiferal carbonate, partly stemming from calibration studies on core tops or undisturbed surface sediment samples, as well as culturing experiments in the laboratory. In fact, such calibration studies have been performed for Ba/Ca of benthic foraminifera from culturing experiments (Havach et al., 2001; de Nooijer et al., 2017) and core tops (Lea and Boyle, 1989; Groeneveld et al., 2018; Brinkmann et al., 2022; Guo et al., 2023). These studies often focus on species of benthic foraminifera that are 1) abundant in the studied samples, and 2) valuable as

proxy carriers since they have been used in numerous palaeoceanographic reconstructions.

Here, we analyse *in situ* Ba/Ca ratios from two species of rotaliid benthic foraminifera abundant in modern sediments of the Mediterranean Sea (including the Aegean Sea), i.e., *Uvigerina mediterranea* and *Melonis affinis*. The Aegean Sea is suitable for this study since primary productivity shows gradients with increasing values along S–N transects (Section 1.1, Fig. 1). Seven stations from several basins (water depths of 600–1500 m) within the Northern and Central Aegean seas have been selected, and core tops from surface sediments have been sampled in 0.5 to 1.0 cm vertical resolution. We investigate biologic and ecologic influences on the Ba/Ca signal from live (Rose Bengal) and dead specimens by quantifying variability of Ba/Ca across these two different species using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The primary research questions related to the main hypotheses tested are: 1. Do biomineralisation processes lead to species-specific Ba/Ca?, and 2. Is Ba/Ca of infaunal benthic foraminifera species-specific due to contrasting microhabitats? We further assess the differences and similarities in signals from live and dead specimens and quantify intra-test variability of Ba/Ca, hypothesizing an influence of biomineralisation processes. Then, we compare the species-specific Ba/Ca<sub>foram</sub> to measured bottom water Ba/Ca from the same locality in the Northern Aegean Sea, and place these new partition coefficients in a context of existing core top and laboratory calibrations. In summary, we address biomineralisation processes and ecological aspects as underlying processes of Ba incorporation into benthic foraminiferal carbonate.

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Map of the Northern and Central Aegean seas (NAS and CAS, respectively) within the Mediterranean Sea. Sampling sites for this study are displayed by circles. M592, M594, M595, and M596 are located in the CAS whereas M599, M601, and M602 are located in the NAS. Locations of gravity cores from the Aegean Sea are displayed by triangles. Created with GeoMapApp (version 3.6.15) using topography and bathymetry of SRTM30 (Becker et al., 2009). Primary productivity data is described in Table 1, and was taken from Theodor et al. (2016a). Vertical hydrographic T and S profiles (CTD) in the Athos basin, station M599, from cruise M144 (Pross et al., 2021).</figcaption></figure>
## 1.1. The Northern and Central Aegean seas as natural laboratory

The Northern and Central Aegean seas (NAS and CAS, respectively, Fig. 1) are located in the Eastern Mediterranean Sea (EMS), in connection to the Levantine Sea, and represent continental margin environments with several deep basins (Poulos, 2009). The marine environment receives freshwater inflow through several major rivers such as the Evros in the North (Poulos, 2009). Moreover, low-saline surface water enters through the Strait of the Dardanelles and the Marmara Sea from the Black Sea (Androulidakis et al., 2012). These inputs of fresh and brackish waters influence the surface water composition of the NAS (and CAS) in terms of salinity and temperature (Pazi, 2008). Productivity and terrigenous influx form a gradient from the South Aegean Sea to the CAS and NAS, where primary production seems to be mainly influenced by Black Sea Water inflow and riverine inputs (Lykousis et al., 2002; Poulos, 2009; Skliris et al., 2010). Also within the study area, the primary production is increasing from the CAS to the NAS, i.e., from S to N (Table 1; Theodor et al., 2016a). In general, bottom waters of the study area are well oxygenated (> 4.1 mL/L or 5.9 mg/L or 183.1 µmol/kg) for the time period prior to sampling of the surface sediment (MedAtlas, 1997; Theodor et al., 2016a). Local upwelling events seem to concern only shallow waters from above the thermocline (Androulidakis et al., 2017). The intermediate water layer is characterized by the inflow of Levantine Intermediate Water from the south, which is warmer and more saline than the surface water (Androulidakis et al., 2012; Fig. 1). The relatively cold deep water in the NAS is formed locally and with seasonal and interannual variability leading to ventilation events in the deep basins interrupting episodes of stagnation (Zervakis et al., 2003; Pazi, 2008; Androulidakis et al., 2012). Formation of deep water in the NAS has wider implications for the Mediterranean Sea, and for a defined time period its subsequent export to the EMS is described as Eastern Mediterranean Transient (e.g., Velaoras and Lascaratos, 2005; Roether et al., 2007; Tsiaras et al., 2012).

In general, the oceanography and palaeoceanography of the Aegean Sea is well constrained (Aksu et al., 1995; Triantaphyllou et al., 2016; Bozyiğit et al., 2022). In the recent past, records of Holocene sapropel formation were found to contain evidence for oxygen-depleted bottom waters (however less severely than in other Eastern Mediterranean waters), and high export productivity (Sperling et al., 2003; Abu-Zied et al., 2008; Schmiedl et al., 2010). The palaeoenvironmental changes of the Aegean Sea, including changes in atmospheric circulation, precipitation and related riverine input, and transient development of dysoxic conditions during sapropel deposition are well understood (Casford et al., 2003; Rohling et al., 2015; Tachikawa et al., 2015). In this context, the sediment record in the NAS and CAS constitutes a robust framework to test and validate the relatively unexplored palaeo-export productivity proxy Ba/Ca of benthic foraminifera. In fact, an earlier study on benthic foraminiferal test geochemistry has investigated samples from the same surface sediments in order to calibrate the $\Delta\delta^{13}C$ signal of epifaunal (Planulina ariminensis/ Cibicides pachyderma/ Cibicides lobatulus) and

infaunal (Uvigerina mediterranea) benthic foraminifera for reconstructing changes in palaeo-export productivity (Theodor et al., 2016a). We are using the same set of environmental parameters (e.g., $C_{org}$ for export flux of organic carbon to bottom waters) for evaluating the variability in surface and export productivity in the study area (Table 1).

## 2. Material & methods

### 2.1. Sampling of benthic foraminifera

During R/V METEOR cruise M51/3 in November/December 2001 (Hemleben et al., 2001), a multicorer was deployed to retrieve cores from sediment surfaces at multiple locations in the different basins of the Aegean Sea. For this study we selected cores from seven stations in the Northern and Central Aegean seas (Fig. 1). Onboard the ship, the multicores were sampled for benthic foraminifera using slices of sediment. The upper 3 cm of sediment were sliced into the following intervals: 0–0.5, 0.5–1, 1–2, 2–3 cm. To each sample a solution of Rose Bengal and ethanol (1.5 g of Rose Bengal per 1 L of ethanol, 96%) was added in order to stain specimens of benthic foraminifera alive at the time of sampling (Walton, 1952; Lutze and Altenbach, 1991; Bernhard, 2000). For our study, we chose samples which have been treated by Theodor et al. (2016a) in the following way: Samples were washed and wet sieved over a sieve with a mesh size of 63 µm before being left to dry (40 °C). The residues were sieved with a mesh size of 150 µm in order to separate coarse and fine fractions. For our study we use benthic foraminifera from the coarse fraction (> 150 µm). Specimens of *Uvigerina mediterranea* and *Melonis affinis* (supp. mat.; Table S1) were picked under a stereo microscope. In order to identify living specimens of both species, the following criterion was applied: only specimens with cytoplasm stained brightly by Rose Bengal in at least three subsequent chambers were considered alive (supp. mat.; Fig. S16). Consequently, the remaining specimens not fulfilling this criterion were considered dead. In total, 136 specimens were selected for subsequent analyses of elemental composition. Of those 136 specimens, there are 30 live, and 38 dead specimens of *U. mediterranea*, as well as 29 live, and 39 dead specimens of *M. affinis* (see Table 2 for the number of specimens per site in the final data set).

### 2.2. Cleaning of benthic foraminiferal tests

The protocol for the cleaning procedure of individual benthic foraminiferal tests is based on published protocols for preparation of elemental analyses by Barker et al. (2003), Glock et al. (2012, 2016), Mojtahid et al. (2019), van Dijk et al. (2019), Petersen (2017), and Petersen et al. (2018, 2019). Furthermore, this protocol agrees with cleaning procedures employed for LA-ICP-MS analyses or other micro-analytical techniques employing precise targeting of parts of foraminiferal shells (with the exception of the first named study which used a wet chemical measurement method) of planktic foraminifera (from

<caption>**Table 1**</caption>

Name and location (decimal degrees, WGS84) as well as water depth of sampling stations. Environmental parameters from Theodor et al. (2016a): $C_{org}$ flux and primary production (PP), Total organic carbon (TOC); $CaCO_3$ was removed from samples, analytical precision of 0.02%, data from Theodor et al. (2016a) and Möbius et al. (2010), and redox boundary depth. $C_{org}$ export fluxes after Betzer et al. (1984) and Antoine and Morel (1996). PP values (annual averages for the year prior to sampling); data from GlobColour project. The mean living depth (MLD) was calculated by Theodor et al. (2016a) for living specimens of *Uvigerina mediterranea* following the procedure presented by Theodor et al. (2016b).

<table><thead><tr><th>Station</th><th>Lat.<br>(° N)</th><th>Long.<br>(° E)</th><th>Depth<br>(m)</th><th>C<sub>org</sub><br>(g C m<sup>-2</sup>a<sup>-1</sup>)</th><th>PP<br>(g C m<sup>-2</sup>a<sup>-1</sup>)</th><th>TOC<br>(%)</th><th>Redox depth<br>(cm)</th><th>MLD<sub>U. med.</sub><br>(cm)</th></tr></thead><tbody><tr><td>M(MUC)592</td><td>37.794</td><td>26.262</td><td>1148</td><td>5.81</td><td>151.46</td><td>0.63</td><td>16</td><td>0.38</td></tr><tr><td>M(MUC)594</td><td>37.902</td><td>26.218</td><td>991</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>M(MUC)595</td><td>38.261</td><td>25.103</td><td>662</td><td>8.84</td><td>159.63</td><td></td><td>19</td><td>0.56</td></tr><tr><td>M(MUC)596</td><td>38.955</td><td>24.753</td><td>884</td><td>7.43</td><td>160.5</td><td>0.73</td><td>30</td><td>0.41</td></tr><tr><td>M(MUC)599</td><td>39.756</td><td>24.094</td><td>1084</td><td>8.66</td><td>195.88</td><td>0.58</td><td>16.5</td><td>0.47</td></tr><tr><td>M(MUC)601</td><td>40.087</td><td>24.610</td><td>977</td><td>9.97</td><td>206.68</td><td>0.75</td><td>6</td><td>0.27</td></tr><tr><td>M(MUC)602</td><td>40.217</td><td>25.240</td><td>1466</td><td>9.36</td><td>236.78</td><td>0.82</td><td>4</td><td>0.78</td></tr></tbody></table>
<caption>**Table 2**</caption>

Ba/Ca ratios in µmol/mol for all analysed chambers and specimens after Laser Ablation-ICP-MS data processing (section 2.3.1). Separated for species, station, and between live and dead specimens. Reported are median and MAD (see section 2.5) with the respective number of data points, i.e., single chamber measurements (n) for all analysed specimens per station. Also reported are the number of analysed specimens (N) for each station (after LA-ICP-MS data processing). The total number of analyses represented in this table is 396.

<table><thead><tr><th rowspan="2"></th><th colspan="2">Ba/Ca U. med. live</th><th rowspan="2">n</th><th rowspan="2">N</th><th colspan="2">Ba/Ca U. med. dead</th><th rowspan="2">n</th><th rowspan="2">N</th><th colspan="2">Ba/Ca M. aff. live</th><th rowspan="2">n</th><th rowspan="2">N</th><th colspan="2">Ba/Ca M. aff. dead</th><th rowspan="2">n</th><th rowspan="2">N</th></tr><tr><th>median</th><th>MAD</th><th>median</th><th>MAD</th><th>median</th><th>MAD</th><th>median</th><th>MAD</th></tr></thead><tbody><tr><td>MUC592</td><td>1.57</td><td>0.25</td><td>13</td><td>3</td><td>1.82</td><td>2.42</td><td>63</td><td>13</td><td>2.44</td><td>0.36</td><td>6</td><td>2</td><td>2.76</td><td>0.39</td><td>19</td><td>8</td></tr><tr><td>MUC594</td><td>2.03</td><td>0.41</td><td>9</td><td>3</td><td>1.56</td><td>0.14</td><td>12</td><td>4</td><td>3.68</td><td>4.84</td><td>5</td><td>2</td><td>5.41</td><td>2.17</td><td>9</td><td>5</td></tr><tr><td>MUC595</td><td>1.98</td><td>1.06</td><td>20</td><td>4</td><td></td><td></td><td></td><td></td><td>2.88</td><td>0.16</td><td>4</td><td>3</td><td></td><td></td><td></td><td></td></tr><tr><td>MUC596</td><td>1.93</td><td>0.98</td><td>21</td><td>7</td><td>1.94</td><td>0.46</td><td>18</td><td>6</td><td>16.90</td><td>9.54</td><td>6</td><td>2</td><td>4.65</td><td>1.12</td><td>11</td><td>6</td></tr><tr><td>MUC599</td><td>1.70</td><td>0.22</td><td>30</td><td>6</td><td>1.87</td><td>0.44</td><td>25</td><td>5</td><td>2.58</td><td>0.58</td><td>5</td><td>2</td><td>2.60</td><td>0.71</td><td>17</td><td>6</td></tr><tr><td>MUC601</td><td>2.57</td><td>0.12</td><td>9</td><td>3</td><td>2.33</td><td>0.45</td><td>21</td><td>7</td><td>8.62</td><td>2.02</td><td>8</td><td>4</td><td>3.31</td><td>1.38</td><td>12</td><td>7</td></tr><tr><td>MUC602</td><td>2.11</td><td>0.25</td><td>20</td><td>4</td><td>1.87</td><td>0.15</td><td>15</td><td>3</td><td>4.05</td><td>1.13</td><td>12</td><td>4</td><td>3.36</td><td>0.25</td><td>6</td><td>3</td></tr><tr><td>all stations</td><td>1.93</td><td>0.52</td><td>122</td><td>30</td><td>1.88</td><td>0.50</td><td>154</td><td>38</td><td>3.90</td><td>2.60</td><td>46</td><td>19</td><td>3.18</td><td>0.96</td><td>74</td><td>35</td></tr></tbody></table>

culturing experiments and sediment trap samples; Lea and Spero, 1992; Vetter et al., 2013; Spero et al., 2015; Branson et al., 2016; Richey et al., 2022). Regarding Na/Ca, similar sample preparation and cleaning methods have been used by Mezger et al. (2018) when analysing planktic foraminifera from core tops. First, specimens were put in vials and rinsed three times with reverse osmosis water (ROW). During each rinsing step, the vials were put into an ultrasonic bath for approx. 20 s. Afterwards, the vials were rinsed three times with methanol and put into the ultrasonic bath for 1 min during each rinsing step. The vials were rinsed again two times with ROW to remove residual methanol (in an ultrasonic bath for approx. 2\*20 s). In order to remove organic matter adhering to the tests, an oxidative reagent was freshly mixed by adding 100 µL 30 % H₂O₂ to 10 mL of a 0.1 M NaOH solution. Subsequently, 350 µL of this reagent were added to each vial. The vials were transferred into an approx. 75 °C warm water-bath (with ultra-sonication) for 15 min to remove contamination by organic matter. During this oxidative cleaning step, samples were removed from the water-bath in 5 min intervals to remove gas bubbles by tapping the bottom of the vials. After 15 min the foraminiferal specimens were rinsed with ROW. The oxidative cleaning was repeated once. Finally, the vials were rinsed twice with ROW to remove residues of the reagent. All specimens were picked from vials and left to dry on slide.

## 2.3. Analyses by Laser Ablation ICP-MS

We measured the elemental composition of single chambers of the calcite tests using LA-ICP-MS at the Institute of Geosciences, at the University of Mainz, Germany. The analytical setup involves an ArF excimer laser (193 nm, ESI New wave) which is coupled to a quadrupole ICP-MS (Agilent 7500ce). Ablations were performed in a TwoVol² 2-Volume Cell with He as carrier gas, a laser energy density of 2–3 J/cm² and a 7 Hz repetition rate. Foraminiferal samples were ablated with an energy density of 2 J/cm² and, reference materials at a slightly higher energy density of ~3 J/cm². Spot sizes of spot ablations, sampling vertically through the sample material, were 50 µm in diameter for foraminifera and reference materials. The ICP-MS settings included an RF Power of 1200 W, a sampling depth of the torch of 6 mm, and a gas-flow rate (Ar) of 0.86 (L/min). Before the start of each measurement session (n = 2 different sessions), the ICP-MS was tuned to monitor elemental fraction effects by ensuring a U/Th of ~1. The oxide formation was monitored as well by ensuring a low ThO⁺/Th⁺ ratio (< 0.5–0.8 %). A background signal of 15 s was recorded before the measurement of each laser ablation profile which lasted for 30 s. The analysed isotopes were: ²³Na, ²⁵Mg, ²⁶Mg, ²⁷Al, ⁴³Ca, ⁵⁵Mn, ⁵⁶Fe, ⁶⁶Zn, ⁸⁸Sr, ¹³⁷Ba, ¹³⁸Ba, ¹⁴⁰Ce, and ¹⁰⁸Pb. The dwell time for each analyte (isotope) was set to 10 ms.

The data were calibrated against NIST SRM 610. The calibration using NIST SRM 610, or NIST glasses in general, was shown to yield accurate results for carbonates (e.g., Jochum et al., 2012, 2019).

Fractionation effects were monitored using different reference materials (USGS MACS-3, NIST SRM 612, and BCR-2G), which were analysed after each block of 30 ablations of sample material. For the calculation of element concentrations, the GLITTER software was used with ⁴³Ca as internal standard, assuming samples contained 40 wt% Ca of CaCO₃ (e. g., Petersen et al., 2018). External reproducibility for Ba/Ca was determined using carbonate reference materials USGS MACS-3 as 1.1 % (calculated as 2xRSE, relative standard error, based on the average value of 59.4 ppm and a standard deviation of 2.4 ppm for n = 51 measurements of all analytical sessions). A table for external reproducibility of Ba, Mg, Sr, and Na is given in the supplementary material (supp. mat.; Table S2).

Accuracy of Ba/Ca was also ensured via comparison to the reference value of LA-ICP-MS measurements of USGS MACS-3 (mean ± SD: 58.9 ± 1.9 ppm) established by Jochum et al. (2012). Analyses were performed in the penultimate (n-1), antepenultimate (n-2), and n-3 chambers of individuals of *U. mediterranea* and *M. affinis*, as well as up to n-7 chambers in a subset of individuals of *U. mediterranea*.

### 2.3.1. Laser Ablation ICP-MS data processing

Laser ablation data processing includes several steps of data reduction. Since the data were acquired by ablating vertically through the shell in a time-resolved way (Figs. S10 and S11), a stable count of Ca implies that these parts of the ablation profiles included the analysis of the entire test wall, and such zones were selected for further interpretation (e.g., Petersen et al., 2018; Brinkmann et al., 2022). Having performed these steps for all analysed profiles, we have excluded parts of profiles with elevated raw data values for elements that can or could be associated with coatings on the outside and inside of shell walls (Al, Fe, Zn, and to a lesser extent Mn). This step was performed within the software Glitter based on visual inspection of the profiles. Implicitly, this data reduction proofs that final data sets show Ba/Ca ratios of the foraminiferal carbonate which are not influenced by other phases of Ba, also validating our cleaning method described in section 2.2 (e.g., Spero et al., 2015; Mojtahid et al., 2019; Ni et al., 2020; Brinkmann et al., 2022; Guo et al., 2023). Moreover, in the resulting data set of calibrated concentration values (ppm) we have removed all profiles with 1) remaining (after data reduction and selection of integration interval within the Glitter software) exceptionally high concentrations of Al (> 200 ppm, or ~ 700 µmol/mol), Fe (> 300 ppm, or ~ 500 µmol/mol), and Mn (> 200 ppm, or ~ 350 µmol/mol), and 2) profiles for which the integration interval within the ablation profile included not sufficient data points for reliable calibration/quantification (< 10 sweeps, i.e., data points, of the ICP-MS). These steps of scrutinizing laser ablation profiles were adapted on the basis of previous protocols of LA-ICP-MS data of benthic and planktic foraminifera from culturing experiments, core tops and down core records published in Petersen (2017), Petersen et al. (2018), Petersen et al. (2019), Barras et al. (2018), Mojtahid et al. (2019), Nairn et al. (2021), Guo et al. (2019), Brinkmann et al. (2022),
de Nooijer et al. (2017), Ní Fhlaithearta et al. (2010), Wit et al. (2010), Marr et al. (2011), Leduc et al. (2014), and Reichart et al. (2003). Moreover, for setting the thresholds in Al, Fe, and Mn, we employed histograms and boxplots shown in Figs. S12 to S15 of the supplementary material. In this way, the data set was reduced from originally 456 to 396 data points. We ensured that all data were above limits of quantification for Na, Mg, Ba, and Sr.

## 2.4. Sea water analyses

Sea water samples from R/V METEOR cruise M144 (December 2017) sampled at station M599 of R/V METEOR cruise M51 (December 2001) were analysed for concentrations of Ca, Mg, Na, and Sr by a Thermoscientific iCap ICP-OES Duo 6300 at the Institute of Geosciences, University of Frankfurt. Samples were diluted 40:1 with HNO₃. Yttrium was used as an internal standard element with a concentration of 1 mg/L. Intensity data was background corrected and standardized to Y. IAPSO and NASS-7 seawater reference materials were measured to allow for drift correction and monitor measurement quality. Accuracy is reported as the deviation of the measured reference materials from reported values and amounts to 97 %, 101 %, 99 % and 108 % for Ca, Mg, Na and Sr. Precision is reported as the RSD of the measured reference materials (n = 3) and amounts to 1.5 %, 1.1 %, 2.4 % and 1 % for Ca, Mg, Na and Sr.

Seawater samples were analysed for Ba by a Thermoscientific Sector Field Element ICP-MS. Samples were diluted 1:1 with HNO₃. Yttrium was used as an internal standard element with a concentration of 0.025 mg/L. Intensity data was background corrected and standardized to Y. IAPSO, NASS-7 and Multi-Element Standard Solution were measured after every 8th sample to allow for drift correction and monitor measurement quality. Accuracy is reported as the deviation of the measured reference materials from reported values and amounts to 93 % for Ba. Precision is reported as the RSD of the measured reference materials (n = 4) and amounts to 2.8 % for Ba.

## 2.5. Statistical analyses

Statistical analyses were performed in R (R Core Team, 2016) version 3.5.2 with RStudio (version 1.1.463) and graphical representations were

generated using ggplot2 (Wickham, 2009). Statistical tests included testing against normal distribution using Kolmogorov-Smirnov tests. For non-normally distributed data, subsets were tested for significant differences using Kruskal-Wallis tests (including post-hoc tests). Non normally distributed data is reported with median values and median average distances (MAD) in order to compare subsets. When comparing Ba/Ca ratios to the measured environmental parameters, in order to check for potential correlations of both parameters, we used Spearman rank correlation. In all cases, a *p* value below 0.05 was considered as significant.

# 3. Results

## 3.1. Ba/Ca of live and dead specimens

For *Uvigerina mediterranea*, there are no significant differences between live and dead specimens (median ± MAD: 1.93 ± 0.52 for *n* = 122, and 1.88 ± 0.50 µmol/mol for *n* = 154, respectively; Table 2, and Figs. 2, 3). For *Melonis affinis*, there are significant differences between live and dead specimens (median ± MAD: 3.90 ± 2.6 for *n* = 46, and 3.18 ± 0.96 µmol/mol, for *n* = 74, respectively; Table 2, and Figs. 2, 4). Altogether, the Ba/Ca of live *M. affinis* is elevated compared to dead specimens (significant difference, Kruskal Wallis rank sum test *p* value = 0.0081). This effect is due to results from two stations (M596 and M601) of which one (M596) shows values >9.1 µmol/mol (median + 2*MAD* for all data of live specimens of *M. affinis*, i.e., threshold for identifying high values) for live specimens. When these high values are omitted (*n* = 10), there is no significant difference between live and dead specimens of *M. affinis* (*p* value = 0.1371).

## 3.2. Foraminiferal Ba/Ca between sampling sites

For live specimens of *U. mediterranea*, average (median of all single chamber Ba/Ca) values per station range from 1.57 ± 0.25 for *n* = 13 (M592) to 2.57 ± 0.12 for *n* = 9 (M601) µmol/mol (Table 2, and Figs. 3 and 5). For dead specimens of *U. mediterranea*, average values per station show a variability from 1.56 ± 0.14 for *n* = 12 (M594) to 2.33 ± 0.45 for *n* = 21 (M601) µmol/mol (Table 2, and Figs. 3 and 5). For the live specimens of *M. affinis*, the range of average values is from 2.44 ± 0.36

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Ba/Ca ratios of all analysed chambers and specimens (see Table 2). Results are reported as boxplots per species, and separated between live and dead specimens. Number of analyses per boxplot are given, i.e., single chamber values.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Ba/Ca ratios of *U. mediterranea*. Results are reported as boxplots per station, and separated between live and dead specimens (see Table 2). For panels on the right half the data was cropped to values ≤5 µmol/mol. Number of analyses per boxplot are given, i.e., single chamber values.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Ba/Ca ratios of *M. affinis*. Results are reported as boxplots per station, and separated between live and dead specimens (see Table 2). For panels on the right half the data was cropped to values ≤10 µmol/mol. Number of analyses per boxplot are given, i.e., single chamber values.</figcaption></figure>

for *n* = 6 (M592) to 16.90 ± 9.54 for *n* = 6 (M596) µmol/mol (Table 2, and Figs. 4 and 5). For the dead specimens of *M. affinis*, the average values range from 2.60 ± 0.71 for *n* = 17 (M599) to 5.41 ± 2.17 for *n* = 9 (M594) µmol/mol (Table 2, and Figs. 4 and 5). The range of Ba/Ca within single stations is relatively high, and the inter-station comparison is not resulting in a marked south to north gradient of Ba/Ca for *U. mediterranea* (Fig. 5). Thus, there are no meaningful correlations to the environmental parameters shown in Table 1 for which a S to N increasing gradient may be apparent (e.g., Ba/Ca vs. Corg flux; Spearman rank correlation; rho = 0.27 with *p* value <0.001, for *n* = 276 data points of combined live and dead Ba/Ca; note that individual inter-station comparisons of Ba/Ca ratios yield significant differences, without a repeated pattern between live and dead specimens). Similarly, the dead specimens of *M. affinis* show no particular trend with latitude in inter-station comparison of Ba/Ca and thus no significant correlation to any observed environmental variable (e.g., Corg flux; rho = 0.19 with *p* value <0.04, for *n* = 120 data points of combined live and dead Ba/Ca). However, the median Ba/Ca of live specimens of *U. mediterranea* is highest (and Ba/Ca of live specimens of *M. affinis* is affected by high values at station M596 and) at station M601 which shows a relatively low value in redox boundary depth (6 cm) and a high value in Corg flux (9.97 g C m⁻²a⁻¹; Table 1).

### 3.3. Species-specific Ba/Ca

*U. mediterranea* has a lower range of Ba/Ca than *M. affinis*. For *U. mediterranea*, the majority of values is in the range ≤ 5 µmol/mol, while for *M. affinis* values stretch further up with numerous counts in the range ≤ 10 µmol/mol, as shown in the histograms for live and dead specimens separately (Fig. 6). When using non-parametric tests (Kruskal Wallis rank sum test), the difference between both species in Ba/Ca for live specimens is significant (p value <6 × 10⁻¹⁶, for *n* = 168 data points of live specimens from both species) as well as for dead specimens (p value <2.2 × 10⁻¹⁶, for *n* = 228 data points of dead specimens from both species). Even if high Ba/Ca values are excluded from the live data (as the data for *M. affinis* suggests that outliers could influence the result, we removed all values exceeding the median + 2*MAD* for this subset: all data <9.1 µmol/mol remained in the new dataset, *n* = 10 data points are removed), the difference between both species remains significant (p value 8 × 10⁻¹³).

For each station, separated between live and dead specimens, the Ba/Ca of *M. affinis* displays elevated average values compared to *U. mediterranea* (Fig. 5). At individual stations, the Ba/Ca has been plotted per sampling depth (Figs. 7, 8, for stations M599 and M602, respectively; for all other stations plots are found in the supplementary
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Ba/Ca ratios of live specimens (above) and dead specimens (below) reported as boxplots per station, and per species (n for number of analyses per boxplot, i.e., single chamber values, cf., Table 2). Indicated above is the gradient of relatively increasing productivity (PP and $C_{org}$ flux, cf., Table 1).</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Histogram for all single chamber Ba/Ca ratios of live specimens (left) and dead specimens (right).</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** All single chamber Ba/Ca ratios of specimens from station M599 (this study). Different colours and symbols apply to different species. The δ¹³C ratios of *U. mediterranea*, and δ¹³C ratios of epifaunal species are shown as well (data from Theodor et al., 2016a).</figcaption></figure>

material; Figs. S1–S5), also per sampling depth the values for Ba/Ca of
*M. affinis* have a range of elevated values compared to those of
*U. mediterranea* (both for live and dead specimens) at most stations.

50.3 ± 1.4 nmol/L for a depth of 10 m, 53.2 ± 1.5 nmol/L at 110 m depth, and 69.9 ± 2.0 nmol/L in the bottom water at 1064 m depth (corresponding to a bottom water Ba/Ca of 5.3 µmol/mol with measured [Ca], Table 4).

**3.4. Ba/Ca intra-test variability**

The Ba/Ca intra-test variability expressed as relative standard devi-
ation in % (RSD=SD\*100/average) for each specimen is shown in
Table 3. For *U. mediterranea*, 3 to 5 chambers per individual have been
analysed. For *M. affinis*, 2 to 3 chambers per specimen have been ana-
lysed (specimens with only 1 chamber passing criteria of data processing
have been omitted for this comparison). For live and dead specimens of
*U. mediterranea*, intra-test variability is 16 ± 12 and 19 ± 19 % on
average, respectively (Table 3). For live and dead specimens of
*M. affinis*, intra-test variability is 24 ± 17 and 16 ± 17 % on average,
respectively. In order to compare the Ba/Ca intra-test variability to
intra-test variability of other elements, we show in Table 3 the results of
Mg/Ca and Sr/Ca for our data. Ba/Ca and Sr/Ca chamber-to-chamber
variability for live and dead specimens of *U. mediterranea* from station
M599 are shown in Fig. 9 (similar plots for Ba/Ca of *U. mediterranea*
from all stations in the supplementary material; Fig. S6), and show no
consistent trend for Ba/Ca with ontogeny.

**3.5. Sea water Ba concentration at sampling site M599**

Samples of sea water at station M599 exhibit Ba concentrations of

**4. Discussion**

4.1. Ba/Ca intra-test variability

In order to quantify Ba/Ca intra-test variability and provide new
insights on the influence of biomineralisation processes on this factor,
we have compiled the LA-ICP-MS data of individual chambers of
*U. mediterranea* and *M. affinis* per specimen. We showed in Section 3.4
and Table 3 that the Ba/Ca intra-test variability is similar between live
and dead specimens of *U. mediterranea* as well as for live and dead
specimens of *M. affinis*. Therefore, it seems that the Ba/Ca signal from
dead specimens is reflecting the same variability as live specimens, and
is representative for the total fauna. While Mg/Ca intra-test variability is
similarly high and variable when compared to Ba/Ca intra-test vari-
ability in both species, the Sr/Ca intra-test variability is lower by an
order of magnitude (Table 3). For cultured and core top specimens of
different species of benthic foraminifera, intra-test variability of Mg/Ca
of 20–50 % RSD has been reported (Curry and Marchitto, 2008; de
Nooijer et al., 2014a; Petersen et al., 2018). Petersen et al. (2018)
concluded that 20 % Mg/Ca intra-test variability could be attributed to
intrinsic factors (i.e., the combined variability resulting from vital
<figure><img src="image_12.png" /><figcaption>**Fig. 8.** All single chamber Ba/Ca ratios of specimens from station M602 (this study). Different colours and symbols apply to different species. The δ¹³C ratios of *U. mediterranea*, and δ¹³C ratios of epifaunal species are shown as well (data from Theodor et al., 2016a).</figcaption></figure>

<caption>**Table 3**</caption>

Ba/Ca, Sr/Ca, and Mg/Ca intra-test variability expressed as RSD %, with average for all analysed specimens and respective SD. Number of specimens (N), as well as number of chambers per specimen (n) applies to Ba/Ca, Sr/Ca, and Mg/Ca.

<table><thead><tr><th rowspan="2"></th><th colspan="4">live</th><th colspan="4">dead</th></tr><tr><th>RSD % average</th><th>RSD % SD</th><th>n = number of chambers per specimen</th><th>N = number of specimens</th><th>RSD % average</th><th>RSD % SD</th><th>n = number of chambers per specimen</th><th>N = number of specimens</th></tr></thead><tbody><tr><td rowspan="2">U. med.</td><td>16</td><td>12</td><td>3 to 5</td><td>30</td><td>19</td><td>19</td><td>3 to 5</td><td>38</td></tr><tr><td>Ba/Ca</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">M. aff.</td><td>24</td><td>17</td><td>2 to 3</td><td>16</td><td>16</td><td>17</td><td>2 to 3</td><td>28</td></tr><tr><td>Sr/Ca</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">U. med.</td><td>5</td><td>2</td><td></td><td></td><td>5</td><td>3</td><td></td><td></td></tr><tr><td>2</td><td>1</td><td></td><td></td><td>3</td><td>2</td><td></td><td></td></tr><tr><td rowspan="2">M. aff.</td><td>Mg/Ca</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td></td><td>21</td><td>13</td><td></td><td></td></tr><tr><td rowspan="2">U. med.</td><td>22</td><td>11</td><td></td><td></td><td>16</td><td>17</td><td></td><td></td></tr><tr><td>16</td><td>19</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

effects related to biomineralisation processes during chamber growth and from ontogenetic trends, which are size-related vital effects, meaning chamber-to-chamber variability) when referring exclusively to the results of culturing experiments of de Nooijer et al. (2014a). Furthermore, Petersen et al. (2018) resumed, based on data of previous studies by Bentov and Erez (2005, 2006) and Nehrke et al. (2013) as well as following the suggestions made by de Nooijer et al. (2014b) that the Sr/Ca intra-test variability was reported with lower values (< 10 % RSD) than those for Mg/Ca in all of the aforementioned studies. This

observation allows postulating that biomineralisation processes may lead to higher intra-test variability for elements that are strongly discriminated against during the chamber forming process (e.g., Mg) than for rather passively transported ions from seawater to the site of calcification (e.g., Sr). Since we can show that the intra-test variability of Ba/Ca is of the same order of magnitude as for Mg/Ca, and higher than for Sr/Ca, we speculate that most of the intra-test variability of Ba/Ca (and Mg/Ca) of the studied specimens is related to intrinsic factors of the biomineralisation process. Support for this hypothesis comes from the
<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Ba/Ca and Sr/Ca intra-test variability expressed as single chamber values per specimen for all specimens of *U. mediterranea* sampled at station M599 (live and dead specimens shown separately). Different symbols and colours apply to different specimens. The external reproducibility is applied as error bar for single chamber el/Ca.</figcaption></figure>

results of Ba/Ca (and Mg/Ca, as well as Sr/Ca) ratios from a culturing experiment with large hyaline benthic foraminifera by de Nooijer et al. (2017), for which these authors associate similar biomineralisation processes as for small miliolid benthic foraminifera. A Ba/Ca intrinsic intra-test variability of 9 ± 4 %, and 13 ± 4 % can be associated with *Heterostegina depressa* and *Amphistegina lessonii*, respectively, based on Ba/Ca single chamber analyses of multiple specimens within single aquaria with constant [Ba] (de Nooijer et al., 2017). For A. lessonii, the Mg/Ca and Sr/Ca intra-test variability fit the pattern described above:

Mg/Ca intra-test variability is 20 ± 9 % whereas Sr/Ca intra-test vari-
ability is 6 ± 3 % (compared to a Ba/Ca intra-test variability of 13 ± 4 %).
However, for *H. depressa*, Mg/Ca and Sr/Ca intra-test variability are
on the same order of magnitude (6 ± 3 % and 7 ± 3 % for Mg/Ca and Sr/
Ca, respectively, compared to a Ba/Ca intra-test variability of 9 ± 4 %).
De Nooijer et al. (2017) argue that the Ba/Ca intra-test variability in
their cultured specimens could be explained by vital effects and/or
ontogenetic trends. In fact, they describe a significant decrease of Ba/Ca
with chamber size (i.e., latest built chambers), and thus ontogeny, for
<caption>**Table 4**</caption>

Seawater analyses for three different water depths at station M599 sampled during cruise M144 in December 2017. Reported values are the mean of two analyses.

<table><thead><tr><th>Depth (m)</th><th>Ca (mmol/L)</th><th>Mg (mmol/L)</th><th>Na (mmol/L)</th><th>Sr (µmol/L)</th><th>Ba (nmol/L)</th><th>Ba/Ca (µmol/mol)</th></tr></thead><tbody><tr><td>10</td><td>13.2</td><td>61.5</td><td>518.6</td><td>99.6</td><td>50.3</td><td>3.8</td></tr><tr><td>110</td><td>13.3</td><td>62.9</td><td>522.6</td><td>101.9</td><td>53.2</td><td>4.0</td></tr><tr><td>1064</td><td>13.2</td><td>62.2</td><td>514.2</td><td>101.1</td><td>69.9</td><td>5.3</td></tr></tbody></table>

their cultured specimens of *A. lessonii* and *H. depressa*. For the data presented here, we argue that it is rather vital effects than ontogenetic trends leading to the amount of Ba/Ca intra-test variability, since for our specimens, there are no consistent ontogenetic Ba/Ca trends within single specimens (Fig. 9 for specimens from station M599, and supp. mat. Fig. S6 for specimens from all stations).

Lea and Spero (1992) analysed single shell Ba/Ca from cultured planktonic foraminifera (*Orbulina universa*) by solution ICP-MS and found that intra-test variability was within the analytical error of the method (4.4 % standard deviation) for three of five different culturing conditions. Richey et al. (2022) analysed planktic foraminifera (e.g., *Globorotalia truncatulinoides*) by LA-ICP-MS and reported Ba/Ca intra-test variability from ablation profiles of different chambers of the same specimen. The data suggests that Ba/Ca intra-test variability may range over one order of magnitude (<50–>200 µmol/mol). Therefore, it seems likely that also within planktic foraminifera, intrinsic variability assessed by LA-ICP-MS is at least as high as for benthic hyaline foraminifera. Recently, Ba/Ca measured with LA-ICP-MS was evaluated in terms of intra-test variability of live specimens from near-coastal environments, i.e. *Bulimina marginata* and *Nonionellina labradorica* (Brinkmann et al., 2022), as well as *Cribrononion subincertum* and *Florilus decorus* (Guo et al., 2024), respectively. In both studies, the authors have employed different approaches to assess Ba/Ca intra-test variability compared to the approach discussed here using a RSD per specimen. The results of Brinkmann et al. (2022) and Guo et al. (2024) show the presence of ontogenetic trends that can be overprinted by, or can be related to, temporal variability of the driving environmental factor responsible for Ba distribution in the bottom waters of the respective coastal areas.

Inorganic carbonate growth experiments (Kitano et al., 1971; Angus et al., 1979; Pingitore, 1986; Tesoriero and Pankow, 1996) indicate that the calcite lattice is less compatible with ions larger than Ca (such as Ba and Sr). These larger ions may be incorporated in an uncoordinated way in the calcite lattice (Lorens, 1981), between the actual lattice positions, leading to deformation of the calcite lattice (pointed out by Pingitore and Eastman, 1984), or be adsorbed to surfaces (Pingitore, 1986). From this point of view, we could expect to a certain extent similar behaviour and potential variability of Ba/Ca and Sr/Ca in inorganically grown calcite at similar concentrations. Since we observe a higher intra-test variability for Ba/Ca than for Sr/Ca (16–24 % RSD for Ba/Ca vs. 2–5 % RSD for Sr/Ca, on average), we may speculate that this higher intra-test variability for Ba/Ca is related to factors deriving from biomineralisation processes rather than from factors that are universal to inorganic and biogenic carbonates, and/or that the different concentrations of Ba and Sr in seawater lead to different variability in the calcite (cf., Table 4).

In conclusion, it seems that most of the Ba/Ca intra-test variability observed in our deep-sea core top specimens of benthic foraminifera is explained by intrinsic factors since it is similarly high as Mg/Ca intra-test variability, and since results from culturing experiments show similarly high values of intra-test variability. Intrinsic factors are related to biomineralisation processes (vital effects), and short-term variability (i.e., during the lifespan of a foraminifer) of environmental factors such

as export productivity seems to be of minor importance for deep-sea Ba/Ca intra-test variability.

## 4.2. Species-specific Ba/Ca

There are several possible hypotheses to explain the species-specific differences in Ba/Ca, and here we want to specifically address three of them: 1. methodological aspects, 2. biomineralisation processes, and 3. microhabitat effects. The data from this study shows significantly higher Ba/Ca for *M. affinis* than for *U. mediterranea* when all single-chamber measurements are grouped per species, and are separated between live and dead specimens (Section 3.3).

### 4.2.1. Methodological aspects

The test walls of the two species show slight differences in ablation behaviour. In general, the ablation until perforation of the test wall lasted longer for *U. mediterranea* than for *M. affinis*. It could be argued that in the case of *M. affinis*, parts of coatings on the outside and inside of the test wall (Section 2.3.1) were included in the integration interval, leading to a bias of higher Ba/Ca for *M. affinis* compared to *U. mediterranea*. However, since laser ablation necessitates stepwise data reduction and screening for indicators of such coatings, we want to point out that these steps ensure the comparability between results of foraminiferal species that show different ablation behaviour. For our initial dataset of 456 ablation profiles of both species, the stepwise detection of anomalous data for 1) profiles with not sufficient data points (i.e., sweeps of the ICP-MS) in the integration profile and 2) extraordinary high values in the elements indicating surface contamination/ coatings, resulted in the exclusion of 60 profiles, so that 396 profiles remained. The excluded 60 profiles exclusively concern data of *M. affinis*. By removing these profiles, comparability between the remaining Ba/Ca data of both species is ensured.

Examples for such ablation profiles are given in the supplementary material (Figs. S10 and S11). For these examples, the raw data of Al (indicator of coatings) shows higher values by one order of magnitude for *M. affinis* than for *U. mediterranea*. However, the raw Ba signal for these specimens is higher in *U. mediterranea* compared to *M. affinis*. After the selection of the integration interval and calculation of elemental composition with the GLITTER software, the selected example profile for *U. mediterranea* shows a Ba/Ca of 15.2 µmol/mol compared to 0.8 µmol/mol for *M. affinis*. It should be noted that these selected examples originate from different sampling stations (M592 for *U. mediterranea*, and M601 for *M. affinis*), and in both cases, examples from the subset of dead specimens were selected.

It seems, therefore, that the slight differences in shell thickness between the two hyaline species do not necessarily lead to a bias in terms of elevated Ba/Ca ratios for *M. affinis* compared to *U. mediterranea*. However, care has to be taken when analysing and scrutinizing ablation profiles from different species in order to ensure comparability. Finally, we point out that we validated our LA-ICP-MS method against an existing Ba/Ca dataset obtained by (solution) ICP-MS (Section 4.3).

### 4.2.2. Biomineralisation processes in benthic foraminifera

For the evaluation of our data, we consider a specific biomineralisation mixing model for benthic foraminifera that is developed by Nehrke et al. (2013), and subsequent publications (Langer et al., 2016; Barras et al., 2018; Nagai et al., 2018), and which is reassessed and put into context with other biomineralisation models (using, e.g., Rayleigh fractionation as a basis) by Nehrke and Langer (2023). This biomineralisation model mixes both processes of trans-membrane transport (TMT, selective ion channels, pumps), and passive transport (incorporation of elements from surrounding seawater at the site of calcification which was not manipulated by the foraminifer, and vacuolization of seawater; Nehrke et al., 2013). The mixing model can explain species-specific differences in el/Ca because a certain benthic foraminiferal species has a characteristic amount of passively
transported seawater (PT) that distinctively alters the el/Ca (e.g., Mg/Ca as used by Nehrkę et al., 2013, and 0.04–4.4 % of PT for species used in the inter-species comparison) in comparison to a uniform el/Ca amongst different benthic foraminiferal species (and other groups of microfossils) if only TMT was driving the element incorporation.

Based on the described mixing model, we may expect for our data
significant differences in the measured el/Ca between *U. mediterranea*
and *M. affinis* if both species had each characteristic proportions of
passively transported seawater as part of biomineralisation processes.
We would expect this difference to be observable for both dead and live
specimens and to act similar for all elements in the seawater (i.e., be
relatively comparable in order of magnitude for all measured el/Ca). In
fact, our observations show that all el/Ca (Fig. 2 for Ba/Ca; for Mg/Ca,
Sr/Ca, and Na/Ca, supp. mat. Figs. S7, S8, and S9, with indicated *p*
values, and the number of data points considered) are significantly
different between both species for the dead specimens (i.e., in all cases
the values for *M. affinis* are significantly higher than those for
*U. mediterranea*). For the live specimens, there are significant differences
between *U. mediterranea* and *M. affinis* for Ba/Ca and Sr/Ca, with higher
values for the latter species (Fig. 2 and Fig. S8). The observed differences
in Ba/Ca (and Sr/Ca) between both species for our data of live and dead
specimens suggest differences in passively transported seawater to the
site of calcification according to the mixing model of Nehrkę et al.
(2013).

However, for Mg/Ca and Na/Ca of live specimens, no significant difference between both species can be observed (Figs. S7 and S9 with indicated *p* values, and the number of data points considered). Thus, the data for Mg/Ca and Na/Ca of live specimens do not confirm the here proposed hypothesis, and speak against a systematic difference in el/Ca between both species due to biomineralisation processes. We cannot rule out that other biomineralisation processes that discriminate for certain elements may be responsible for the observed Ba/Ca (and Sr/Ca) offset between the two species. Such systematic differences may exist for the comparison of other benthic foraminiferal species and genera, and biomineralisation mixing models such as proposed by Nehrkę et al. (2013) can explain many species-specific el/Ca ratios.

4.2.3. Microhabitat effects

This section considers species-specific microhabitat effects on Ba/Ca
in the sense that pore water Ba may vary over mm to cm scales within
the surface sediment. For foraminiferal species, each adapted to a
preferred microhabitat (depending on organic matter fluxes and thus
depth of the oxygenated zone in surface sediments and its redox
boundaries; e.g., Jorissen et al., 1995; Van der Zwaan et al., 1999; Koho
et al., 2008), a vertical redox zonation (and thus a gradient of pore water
Ba, e.g., McManus et al., 1998; Schenau et al., 2001) would imply
contrasting amounts of Ba²⁺ in each microhabitat. In fact, some of the
aforementioned studies have pointed out that the zone of calcification
can be much narrower than the species-specific microhabitat (McCorkle
et al., 1997), while others concluded that calcification could take place
over longer time periods in different depths of the surface sediment
(Jorissen, 2003). However, most studies imply vertical migration of
benthic foraminifera in surface sediments, and calcification in (deep)
macrofaunal burrows of (otherwise shallow) infaunal taxa is suggested
(for occasional deep infaunal occurrences of *U. mediterranea*, Schmiedl
et al., 2004). For *U. mediterranea* in comparison to *M. affinis* it has been
suggested from mesocosm experiments and live census data that shallow
and intermediate infaunal microhabitats can be associated, respectively
(Fontanier et al., 2002; Geslin et al., 2004; Mojtahid et al., 2010).
*M. affinis* is shown to be adapted to oxygen-deficient environments such
as found deep in surface sediments (e.g., Licari et al., 2003; Caulle et al.,
2014; Geslin et al., 2014).

From a palaeoceanographic point of view, it would be preferable to
be able to relate measured el/Ca from epi- and infaunal species directly
to elemental concentrations of bottom waters allowing the reconstruc-
tion of bottom water chemistry. In reality, for some elements and their

isotopic composition it has been shown that steep pore water gradients
are present, and that benthic foraminiferal proxy signals may be a
function of such gradients: trends in species-specific live benthic fora-
miniferal δ¹³C with depth in surface sediments have been related to the
preferred microhabitat depths (Fontanier et al., 2002; Tachikawa and
Elderfield, 2002; Schmiedl et al., 2004), and offsets between epi- and
inaunal species have been used to reconstruct export productivity
(Schmiedl and Mackensen, 2006; Hoogakker et al., 2015; Lynch-Stieglitz
et al., 2024). Earliest studies of fossil foraminifera have found little to no
differences in foraminiferal Ba/Ca from epi- or shallow infaunal habitats
(different species of cibicids compared to *Uvigerina* spp., e.g., Lea and
Boyle, 1989; Lea, 1993), and it has been concluded that Ba/Ca of the
epifaunal *Cibicidoides wuellerstorfi* is not influenced by pore water Ba in
deep-sea surface sediments (McCorkle et al., 1995). However, a
culturing study found differences in Ba incorporation between species;
epifaunal *Cibicides pachyderma* incorporated more Ba for the same Ba of
seawater than (shallow) infaunal *Uvigerina peregrina* as well as *Bulimina
marginata* (Section 4.3; Havach et al., 2001). Mojtahid et al. (2019) have
observed significant differences in Ba/Ca for different species of benthic
foraminifera from samples of a sediment core covering the sapropel
layer S1 (at 550 m water depth in the Eastern Mediterranean Sea). The
sampled species of *Hanzawaia boueana*, *Gyroidina altiformis*, *Gyroidina
orbicularis*, *U. peregrina*, and *Globobulimina affinis* can be associated to
very contrasting microhabitats. Yet, a systematic trend of Ba/Ca vs.
microhabitat depth was not observed, which led Mojtahid et al. (2019)
to postulate that only a combination of several factors can be responsible
for the observed species-specific Ba/Ca. To our knowledge, no study has
so far investigated the Ba/Ca of live specimens of different species from
deep-sea core tops (except for unpublished results of Martin et al. cited
in McCorkle et al., 1995).

Regarding pore water profiles of Ba, published data suggests that higher Ba can be observed at greater depth and oxygen deficiency due to sulphate reduction and associated barite dissolution, as well as dissolution of Mn/Fe oxyhydroxides (e.g., McManus et al., 1998; Schenau et al., 2001). In view of these trends in pore water Ba, we would expect deeper infaunal species (e.g., *M. affinis*) to display higher Ba/Ca than shallow infaunal species (e.g., *U. mediterranea*) if the pore water Ba was substantially influencing the Ba/Ca of infaunal benthic foraminifera. This described trend is observed for Ba/Ca of our live (and dead) specimens of *U. mediterranea* compared to *M. affinis*; at stations M599 and M602 as well as at other sampled stations there is a clear tendency for Ba/Ca ratios of the latter having higher values than for the former (Table 2, Figs. 7 and 8, and supp. mat. Figs. S1–S5). Moreover, we would expect this potential microhabitat effect to exert a larger control on Ba/Ca differences between shallow and intermediate to deep infaunal species in surface sediments with narrow redox boundary depth (i.e., were oxygen is more deficient), since such environments can show a stronger pore water gradient (McManus et al., 1998), and the microhabitat of *U. mediterranea* should be restricted to a narrower zone compared to sediments with larger depths of the redox boundary (Theodor et al., 2016a). For our stations with the shallowest redox boundary depths (6 and 4 cm for M601 and M602, respectively, Table 1), we observe the most pronounced differences (for individual sampling depths, which include low numbers of data points in some cases) in Ba/Ca ratios between live specimens of *U. mediterranea* and *M. affinis* (Fig. S5 and Fig. 8, respectively).

If our here discussed hypothesis of infaunal Ba/Ca being influenced
by the microhabitat depth holds true, then we would also expect a
relationship/gradient between the epifaunal and infaunal Ba/Ca, similar
to what has been observed for Cd/Ca of benthic foraminiferal tests
(Tachikawa and Elderfield, 2002). However, as pointed out before in
this section, there is evidence from the Eastern Mediterranean Sea that
epifaunal Ba/Ca may not always be lower than infaunal Ba/Ca, since
Mojtahid et al. (2019) observe significantly higher Ba/Ca for two species
of *Gyroidina* compared to *U. peregrina* (similar as what has been observed
for epifaunal and infaunal species in culturing experiments of Havach
et al., 2001). It can be argued that *Gyroidina* is not always occupying a truly epifaunal habitat and may also occupy a shallow infaunal habitat, being found in low oxygen environments of the NAS (Ní Fhlaithearta et al., 2010). In addition, for the geochemical composition of *U. peregrina* strong vital effects (or microhabitat effects) have to be considered (e.g., Schmiedl et al., 2004; Theodor et al., 2016b; Le Houedec et al., 2020).

In view of these (contrasting) findings, we think there is evidence for and against interpreting our observed differences in Ba/Ca between *U. mediterranea* and *M. affinis* as a microhabitat effect in the sense that the deeper living species reflects higher pore water Ba. Future research could address this question by investigating Ba/Ca of live specimens from core tops for a range of species known to inhabit contrasting microhabitats alongside *in situ* measurements of pore waters (Section 4.4.).

### 4.2.4. Species-specific Ba/Ca; evaluation of discussed aspects

Based on our data, hypothesis 1 (Section 4.2.1) regarding analytical aspects can be mostly rejected, whereas hypothesis 2 (Section 4.2.2) considering biomineralisation processes, and hypothesis 3 (Section 4.2.3) addressing species-specific microhabitat effects seem more promising in order to explain the species-specific effects on Ba/Ca. However, incorporation of other elements (Mg/Ca, and Na/Ca) may contradict hypothesis 2, and published data from multi-species benthic foraminiferal Ba/Ca does not confirm the here proposed microhabitat effect in relation to pore water Ba trends within the sediment surface (hypothesis 3). It remains difficult to disentangle the different possible factors, also due to the fact that there could be linkages between them.

## 4.3. Species-specific comparison of Ba/Ca<sub>foram</sub> and bottom water Ba from sampling station M599

Here, we present our Ba/Ca<sub>foram</sub> data in relation to the measured bottom water Ba/Ca from one of the investigated stations in the Northern Aegean Sea, in order to place our partition coefficients for *U. mediterranea* and *M. affinis* in the context of existing core top and laboratory calibrations for a range of (other) species. Station M599 is located in the Sporades Basin, which is connected and/or combined with

the Athos Basin to the Northeast (Fig. 1, Poulos, 2009). Selecting this station for the definition of a species-specific partition coefficient (representative at minimum for the Northern and Central Aegean seas) yields a first estimate about its magnitude. Since it has been shown that subsurface waters in the Aegean Sea are subject to mixing and low residence times with fast replenishment of intermediate and deep waters, the applicability of this local comparison of Ba/Ca and bottom water Ba to the entire region and beyond is certainly limited (Section 1.1, e.g., Androulidakis et al., 2012). It has to be pointed out that there are no regional gradients in Ba/Ca although productivity is subject to a S to N gradient (Section 3.2, Fig. 5, and Table 1).

The measured bottom water Ba/Ca of 5.3 µmol/mol (Table 4; based on Ba conc. of ~70 nmol/L) at station M599 agrees well with other measured bottom water Ba from the Eastern Mediterranean Sea GEO-TRACES cruise (Roy-Barman et al., 2019). Therefore, we are confident that this value adequately represents the bottom water conditions at the time of sampling (the samples for foraminifera were taken in December 2001 at station M599 whereas seawater samples were taken at exactly the same location in December 2017).

The Ba/Ca of *M. affinis* is significantly higher in live and dead specimens compared to Ba/Ca of *U. mediterranea* for the data set as a whole (Fig. 2). Also, when subsampling for Ba/Ca from station M599 it became apparent that Ba/Ca of *M. affinis* is slightly elevated compared to *U. mediterranea* for all sampling depths within the sediment surface (Table 2, Fig. 7). In consequence, the difference in Ba/Ca between the two species leads to species-specific partition coefficients when calibrated against the same bottom water value of Ba/Ca (Fig. 10). For station M599 we calculate an average D<sub>Ba</sub> for *U. mediterranea* of 0.34 (n = 55 single spot measurements, see Table 2; associated Ba/Ca<sub>foram</sub> RSD of 27 %) whereas D<sub>Ba</sub> of *M. affinis* is 0.49 (n = 22 single spot measurements, see Table 2; associated Ba/Ca<sub>foram</sub> RSD of 34 %). In fact, D<sub>Bas</sub> are based on median values for Ba/Ca data from live and dead specimens combined, estimated live and dead differences in D<sub>Ba</sub> are shown in Fig. 10. We argue that it is valid for our data to combine live and dead Ba/Ca for the calculation of D<sub>Ba</sub> because we have shown that for both species there are no significant differences for the live vs. dead

<figure><img src="image_14.png" /><figcaption>Fig. 10. Left: Ba/Ca<sub>foram</sub> of all analysed specimens from station M599 in comparison to bottom water Ba/Ca measured at the same station (from samples of cruise M144). Also shown is the core top calibration of Ba/Ca<sub>foram</sub> of Lea and Boyle (1989) for specimens of *Uvigerina* spp. and *C. wuellerstorfii*, as well as *C. kullenbergi* (orig. name; accepted name is *C. mundulus*; Brady, Parker & Jones, 1888; Hayward et al., 2023). Right: magnification of Ba/Ca range measured in this study with species-specific partition coefficients shown in legend.</figcaption></figure>

<figure><img src="image_15.png" /></figure>
comparison (if certain high values are excluded for *M. affinis*, Section 3.1). This similarity in dead and live Ba/Ca indicates that the dead fauna (in general, but specifically at station M599) has not undergone substantial transport or redeposition. For assessing the age of our dead specimens, we use sedimentation rates calculated from upper sections of cores from the Athos and Sporades basins, which show a range of 15–17 cm/ka (Kuhnt et al., 2007; Giamali et al., 2019; note that the latter authors also estimated sedimentation rates of 43 cm/ka for a core from the Athos Basin). It seems reasonable to estimate the age of our dead specimens at this station to be several hundred years at maximum, following similar age assessments for specimens from the same sites and sampling campaign as presented by Theodor et al. (2016a). Moreover, Theodor et al. (2016a) suggest that in selecting live and dead specimens from the same sediment depth, age differences should be minor, and that also effects of redeposition and bioturbation can be excluded for station M599. Therefore, it is valid to combine Ba/Ca of our live and dead specimens in order to calculate D<sub>Ba</sub> for the total fauna.

In comparison to D<sub>Ba</sub> found by other core top studies, we find similar values in the same order of magnitude (Table 5). It is striking that our D<sub>Ba</sub> for *U. mediterranea* is matching exactly the D<sub>Ba</sub> of *Uvigerina* spp. from surface sediments in deep-sea settings (water depths 830–5000 m; Lea and Boyle, 1989). This observation also suggests that to a certain extent laser ablation ICP-MS yields comparable data to solution ICP-MS (used by Lea and Boyle, 1989). The D<sub>Ba</sub> of *M. affinis* (0.49) is at the upper end of reported values, and is remarkably closely comparable to the value of coastal benthic species (D<sub>Ba</sub> of 0.57 ± 0.02) for a measured bottom water Ba of 19.49 µmol/mol (Groeneveld et al., 2018), which is substantially higher than our bottom water Ba of 5.3 µmol/mol. This similarity in D<sub>Ba</sub> might support our hypothesis 3 from Section 4.2.3 regarding the higher pore water Ba in the microhabitat of *M. affinis* compared to *U. mediterranea* leading to higher Ba/Ca for *M. affinis*. On the other hand, the relatively high D<sub>Ba</sub> of 0.5 ± 0.1 for epifaunal *C. pachyderma* from culturing experiments compared to lower D<sub>Ba</sub> for infaunal species from the same culturing experiments of Havach et al. (2001) does not support our hypothesis of higher D<sub>Ba</sub> for species with a

deeper microhabitat. It should be noted that the authors of the study suggest that pore water effects are an unlikely explanation for the observed differences in D<sub>Ba</sub> between species (Havach et al., 2001).

In comparison to D<sub>Ba</sub> from inorganic calcite (D<sub>Ba-inorganic</sub> = 0.01–0.06; as determined by growth experiments, e.g., Pingitore and Eastman, 1984; Tesoriero and Pankow, 1996) which is one order of magnitude lower than the D<sub>Ba-foram</sub>, we can show that the Ba/Ca of our live and dead specimens seems to reflect a rather pristine signal of the primary foraminiferal calcite.

With our method, we can add interesting details regarding the variability across species and specimens from the same site (and thus the same bottom water Ba/Ca) as well as intra-test variability (discussed in Section 4.1). The range of Ba/Ca of *U. mediterranea* from station M599 is almost the same as the range of Ba/Ca of *Uvigerina* spp. from multiple core tops with Ba/Ca<sub>seawater</sub> reaching up to 14 µmol/mol (compared to the 5.3 µmol/mol at station M599, Table 5, Fig. 10). This high Ba/Ca<sub>foram</sub> range for single specimens of two different benthic species and a specific value of Ba/Ca<sub>seawater</sub> highlights once more that variability in Ba/Ca (and also other elements) may hold in itself important information about vital effects, ontogenetic trends, and species-specific incorporation of elements.

### 4.4. Global implications and directions of future research

For paleoceanographic applications, such as reconstructions of bottom water conditions and Ba cycling, with the ultimate goal of estimating export productivity, it is important to point out the limitations, presented here for the Northern and Central Aegean seas. The net export of Ba to bottom waters from surface waters may be related to export productivity. However, local differences and regional gradients in surface productivity, and or riverine input, reflected possibly by differences in Ba content of surface waters, may have been masked through formation and distribution of subsurface waters with low residence times (Zervakis et al., 2003; Pazi, 2008; Androulidakis et al., 2012).

Nevertheless, we have shown that it is possible to estimate bottom

<caption>**Table 5**</caption>

Summary of partition coefficients from this study and published data, with respective methodology and associated bottom water Ba/Ca (as well as environmental parameters, if available for deep-sea and shelf core tops).

<table><thead><tr><th></th><th>D<sub>Ba</sub></th><th>Species</th><th>live/dead</th><th>Ba/Ca method</th><th>Ba/Ca<sub>bw</sub> (µmol/mol)</th><th>PP (g C/m<sup>-2</sup>a<sup>-1</sup>)</th><th>C<sub>org</sub> (g C/m<sup>-2</sup>a<sup>-1</sup>)</th><th>Study area</th><th>Water depth (m)</th><th>Authors</th></tr></thead><tbody><tr><td rowspan="4">Deep-sea core top</td><td>0.34</td><td><i>U. mediterranea</i></td><td>live and dead</td><td>LA-ICP-MS</td><td>5.3</td><td>195.88</td><td>8.66</td><td>NAS</td><td>1084</td><td>this study</td></tr><tr><td>0.49</td><td><i>M. affinis</i></td><td>live and dead</td><td>LA-ICP-MS</td><td>5.3</td><td>195.88</td><td>8.66</td><td>NAS</td><td>1084</td><td>this study</td></tr><tr><td>0.34</td><td><i>Uvigerina</i> spp.</td><td>dead</td><td>ID flow injection ICP-MS + flame atomic absorption (for Ca)</td><td>~4.7–14.2</td><td></td><td></td><td>Atlantic, Pacific, Indian oceans</td><td>830–5000</td><td>Lea and Boyle (1989)</td></tr><tr><td>0.37–0.41</td><td><i>Cibicoides</i> spp.</td><td>dead</td><td>ID flow injection ICP-MS + flame atomic absorption (for Ca)</td><td>~4.7–14.2</td><td></td><td></td><td>Atlantic, Pacific, Indian oceans</td><td>830–5000</td><td>Lea and Boyle (1989)</td></tr><tr><td>Shelf core top</td><td>0.57 ± 0.02</td><td><i>Elphidium</i> spp. and <i>Ammonia</i> spp.</td><td>dead</td><td>ICP-OES</td><td>19.49</td><td>~57.7<sup>a</sup></td><td></td><td>Kattegat and Baltic Proper (Hanö Bay)</td><td>10–70</td><td>Groeneveld et al. (2018)</td></tr><tr><td rowspan="5">Cult. exper.</td><td>0.33</td><td><i>A. lessonii</i></td><td>live</td><td>LA-ICP-MS</td><td>50–90</td><td></td><td></td><td></td><td></td><td>de Nooijer et al. (2017)</td></tr><tr><td>0.78</td><td><i>H. depressa</i></td><td>live</td><td>LA-ICP-MS</td><td>50–90</td><td></td><td></td><td></td><td></td><td>de Nooijer et al. (2017)</td></tr><tr><td>0.24 ± 0.06</td><td><i>U. peregrina</i></td><td>live</td><td>ID ICP-MS</td><td>55 ± 2</td><td></td><td></td><td></td><td></td><td>Havach et al. (2001)</td></tr><tr><td>0.5 ± 0.1</td><td><i>C. pachyderma</i></td><td>live</td><td>ID ICP-MS</td><td>55 ± 2</td><td></td><td></td><td></td><td></td><td>Havach et al. (2001)</td></tr><tr><td>0.24 ± 0.07</td><td><i>B. marginata</i></td><td>live</td><td>ID ICP-MS</td><td>55 ± 2</td><td></td><td></td><td></td><td></td><td>Havach et al. (2001)</td></tr><tr <td></td><td>0.20 ± 0.04</td><td><i>A. beccarii</i></td><td>live</td><td>ID ICP-MS</td><td>62 ± 2</td><td></td><td></td><td></td><td></td><td>Havach et al. (2001)</td></tr></tbody></table>

<footnote><sup>a</sup> For Bornholm Basin, Baltic Proper (Filipsson et al., 2017).</footnote>
water Ba variability based on infaunal benthic foraminiferal Ba/Ca. Such applications may serve in deep-sea areas with less complex cycling of nutrients in bottom waters, and linkage to primary productivity, to assess variability in palaeo-export productivity. In order to place the results in a global context, it is important to characterise the Northern and Central Aegean seas in terms of productivity, and compare it to, e.g., open-ocean conditions. The presented primary productivity data for the study area were derived from satellite data prior to the year of sampling of the surface sediments. Theodor et al. (2016a) show that both PP and Corg fluxes can be underestimated in these marginal basins (e.g., up to an order of magnitude when only considering the spring season) based on measurements from the water column. Therefore, we point out that, although being comparably lower than other ocean areas, Corg fluxes in the Aegean Sea could in theory serve well to study the impact on Ba/Ca ratios of benthic foraminifera with potentially wider implications.

In Section 4.3 we have shown that our local partition coefficient for Ba/Ca of *U. mediterranea* is matching those values found by a global deep-sea calibration for *Uvigerina* spp. (Lea and Boyle, 1989). Therefore, our results strengthen the application of this DBa for palaeoceanographic reconstructions using the genus *Uvigerina*, since also in marginal basins with sporadic ventilation events the calibration can confidently reconstruct bottom water Ba. We could speculate that the application of our DBa to reconstruct past primary productivity would strengthen results of benthic foraminiferal faunal proxies. The latter are indicating higher seasonal marine organic matter fluxes (in the Northern Aegean Sea, for which we do not recommend the Ba/Ca<sub>foram</sub> proxy application, and) at coring sites in the Eastern Mediterranean Sea during deposition of sapropel S1 (~10.2–6.4 ka BP) compared to reconstructed most recent conditions (Schmiedl et al., 2010).

In order to suggest future directions of research on Ba/Ca calibration, we highlight once more one of the hypotheses addressed in this study. For further evaluation of the supposed microhabitat effects of the studied shallow and intermediate infaunal species (Section 4.2.3), additional species from other microhabitats should be included. Especially, Ba/Ca data from strictly epifaunal species would be useful to confirm microhabitat effects in infaunal species. An important aspect to strengthen such study designs is the availability of both *in situ* bottom and pore water Ba concentrations. Finally, in order to link the bottom water conditions to primary productivity, it is desirable to use samples originating from a study area in deep-sea settings with precise information on primary and export productivity.

## 5. Conclusions

Ba/Ca ratios of live (Rose Bengal stained) and dead specimens of *Uvigerina mediterranea* and *Melonis affinis* from surface sediments of seven stations within the Northern and Central Aegean seas have been analysed by LA-ICP-MS in order to better understand underlying processes of Ba incorporation into benthic foraminiferal tests. We observe a strong variability without specific trends related to measured environmental variables (e.g., primary productivity, and Corg fluxes) of foraminiferal Ba/Ca for both analysed species between the different stations and basins in the study area. This observation likely reflects small-scale differences in Ba cycling between the stations. The distribution of nutrients and organic matter, and in turn Ba, in the deep basins of the Northern and Central Aegean seas may be difficult to characterise on short timescales. We point out the limitation of using Ba/Ca<sub>foram</sub> to reconstruct, and estimate palaeo-export productivity in these parts of the Aegean Sea, where on millennial time scales changes in riverine input/surface productivity, and stagnating bottom waters have led to the formation of sapropelic sediments.

Nevertheless, if we can transfer the fundamental insights on Ba incorporation to deep-sea areas with less complex and dynamic distribution of nutrients in bottom waters, the proxy application is strengthened by our findings that the Ba/Ca of dead foraminiferal specimens seems to be representative for the total fauna of both analysed species

which we assessed through comparison of data from live and dead specimens and their respective intra-test variability. From the comparison of Ba/Ca intra-test variability to other el/Ca from our specimens and to published values, it seems that most of the Ba/Ca intra-test variability of 16–24 % can be attributed to intrinsic factors, and more specifically vital effects during chamber growth, since consistent onto-genetic trends could not be observed. We discussed analytical aspects, biomineralisation processes, and ecological factors (species-specific microhabitat within the sediment surface of infaunal benthic foraminiferal species) as possible reasons to explain the observed significantly higher Ba/Ca of *M. affinis* compared to *U. mediterranea*. We can rule out biases due to the used analytical method of LA-ICP-MS, since scrutinizing and data reduction lead to the comparability of data from both species. However, it remains open if a single factor such as biomineralisation processes or the specific microhabitat related to early diagenetic processes within the surface sediment is responsible for the observed consistently increased Ba/Ca of *M. affinis* compared to *U. mediterranea* throughout most stations and sediment depths.

By comparing the Ba/Ca ratios of one particular station in the Northern Aegean Sea (M599) to seawater Ba/Ca measured from bottom water samples at the same site, we can suggest that species-specific partition coefficients are comparable to published DBa from core tops and culturing experiments. We add valuable insights into possible reasons for the need of species-specific calibrations (see above), and show that live and dead specimens yield similar DBa, meaning that DBa-dead is representative for the total fauna. The Ba/Ca intra-test variability, attributed to biomineralisation processes, can lead to a large scatter of DBa from single chamber measurements (measured by LA-ICP-MS). We average the data of numerous analysed specimens of the same species for the comparison to Ba/Ca<sub>seawater</sub> to lay the foundation for calibrations in order to reconstruct bottom water conditions in deep-sea areas.

## CRediT authorship contribution statement

**Jassin Petersen:** Writing – original draft, Conceptualization, Methodology, Investigation, Formal analysis, Visualization. **Gerhard Schmiedl:** Writing – review & editing, Conceptualization, Resources. **Jacek Raddatz:** Writing – review & editing, Conceptualization, Methodology, Investigation, Resources. **André Bahr:** Writing – review & editing, Conceptualization, Visualization, Resources. **Jörg Pross:** Writing – review & editing, Resources, Funding acquisition. **Meryem Mojtahid:** Writing – review & editing, Conceptualization, Methodology, Resources.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Ba/Ca, as well as Mg/Ca, Sr/Ca, and Na/Ca single chamber data are available through Mendeley Data at doi: 10.17632/cfbmyg67fv.2.

## Acknowledgements

We thank Xavier Crosta and two anonymous reviewers for their constructive comments and suggestions. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. We are grateful to Patrick Grunert for financing the LA-ICP-MS measurements through funding to his research group at the University of Cologne, discussions on the data set, and commenting on an earlier version of the manuscript. We thank Frans Jorissen for providing some of the foraminiferal sample material. We acknowledge Regina Mertz-Kraus and the LA-ICP-MS laboratory of the Institute of
Geosciences at the Johannes von Gutenberg University Mainz for providing facility and assistance when measuring the elemental ratios of benthic foraminifera and performing part of the data treatment. The participants and crews of R/V METOE cruise nos. 51 and 144 are thanked. We are thankful for the laboratory support for seawater analyses by Alexander Schmidt and Nicolai Schleinkofer (Goethe University Frankfurt and FIERCE). FIERCE is financially supported by the Deutsche Forschungsgemeinschaft (DFG: INST 161/921-1 FUGG, INST 161/923-1 FUGG and INST 161/1073-1 FUGG), and received financial support from the Wilhelm and Else Heraeus Foundation, which is gratefully acknowledged. This is FIERCE contribution No. 180. This study is a contribution to the Center for Earth System Research and Sustainability (CEN) of the University of Hamburg.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.marmicro.2024.102431.

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