# Benthic Foraminiferal Mn/Ca as Low-Oxygen Proxy in Fjord Sediments

Inda Brinkmann, Christine Barras, Tom Jilbert, K. Mareike Paul, Andrea

Somogyi, Sha Ni, Magali Schweizer, Joan Bernhard, Helena Filipsson

► To cite this version:

Inda Brinkmann, Christine Barras, Tom Jilbert, K. Mareike Paul, Andrea Somogyi, et al. Benthic Foraminiferal Mn/Ca as Low-Oxygen Proxy in Fjord Sediments. Global Biogeochemical Cycles, 2023, 37 (5), 10.1029/2023GB007690. http://dx.doi.org/10.1029/2023GB007690

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Distributed under a Creative Commons Attribution 4.0 International License
# Global Biogeochemical Cycles®

## RESEARCH ARTICLE

10.1029/2023GB007690

### Special Section:

Fjords: Estuaries on the Front-line of Climate Change

### Key Points:

* The Mn/Ca oxygen proxy is explored in the context of pore-water redox- and variable bottom-water oxygen conditions in a fjord

* Foraminiferal Mn/Ca is promising as low-oxygen proxy at up to 130 µmol/L [O₂], but insensitive at well-oxygenated conditions

* Foraminiferal in-sediment migration and biological processes can significantly alter Mn/Ca, causing species-specific proxy limitations

### Supporting Information:

Supporting Information may be found in the online version of this article.

### Correspondence to:

I. Brinkmann,
inda.brinkmann@geol.lu.se

### Citation:

Brinkmann, I., Barras, C., Jilbert, T., Paul, K. M., Somogyi, A., Ni, S., et al. (2023). Benthic foraminiferal Mn/Ca as low-oxygen proxy in fjord sediments. *Global Biogeochemical Cycles*, *37*, e2023GB007690. https://doi.org/10.1029/2023GB007690

Received 5 JAN 2023

Accepted 22 APR 2023

© 2023. The Authors.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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## Benthic Foraminiferal Mn/Ca as Low-Oxygen Proxy in Fjord Sediments

Inda Brinkmann¹,² ID, Christine Barras³, Tom Jilbert⁴ ID, K. Mareike Paul⁴ ID, Andrea Somogyi⁵ ID, Sha Ni¹,⁶ ID, Magali Schweizer³ ID, Joan M. Bernhard⁷ ID, and Helena L. Filipsson¹ ID

¹Department of Geology, Lund University, Lund, Sweden, ²Now at Department of Glaciology and Climate, Geological Survey of Denmark and Greenland, Copenhagen, Denmark, ³Laboratoire de Planétologie et Géosciences, LPG UMR 6112, CNRS, University of Angers, Nantes University, Le Mans University, Angers, France, ⁴Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland, ⁵Nanoscopium Synchrotron SOLEIL Saint-Aubin, Gif-sur-Yvette Cedex, France, ⁶Now at Institute for Geology, University of Hamburg, Hamburg, Germany, ⁷Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA

### Abstract
Fjord systems are typically affected by low-oxygen conditions, which are increasing in extent and severity, forced by ongoing global changes. Fjord sedimentary records can provide high temporal resolution archives to aid our understanding of the underlying mechanisms and impacts of current deoxygenation. However, such archives can only be interpreted with well-calibrated proxies. Bottom-water oxygen conditions determine redox regime and availability of redox-sensitive trace elements such as manganese, which in turn may be recorded by manganese-to-calcium ratios (Mn/Ca) in biogenic calcium carbonates (e.g., benthic foraminifera tests). However, biological influences on Mn incorporation (e.g., species-specific Mn fractionation, ontogeny, living and calcification depths) are still poorly constrained. We analyzed Mn/Ca of living benthic foraminifera (*Bulimina marginata*, *Nonionellina labradorica*), sampled at low- to well-oxygenated conditions over a seasonal gradient in Gullmar Fjord, Swedish West coast (71–217 µmol/L oxygen (O₂)), by laser-ablation ICP-MS. High pore-water Mn availability in the fjord supported Mn incorporation by foraminifera. *B. marginata* recorded contrasting Mn redox regimes sensitively and demonstrated potential as proxy for low-oxygen conditions. Synchrotron-based scanning X-ray fluorescence nanoimaging of Mn distributions across *B. marginata* tests displayed Mn/Ca shifts by chambers, reflecting bottom-water oxygenation history and/or ontogeny-driven life strategy preferences. In contrast, Mn/Ca signals of *N. labradorica* were extremely high and insensitive to environmental variability. We explore potential biologically controlled mechanisms that could potentially explain this species-specific response. Our data suggest that with the selection of sensitive candidate species, the Mn/Ca proxy has potential to be further developed for quantitative oxygen reconstructions in the low-oxygen range.

### Plain Language Summary
One of the impacts of global change on fjords and coastal zones is the decline of oxygen levels in seawater (“deoxygenation”), with serious environmental consequences. Important insights into the processes and consequences of deoxygenation can be gained through indicator records of past oxygen variability in seawater. An emerging indicator is the trace element manganese (Mn) due to its reduction and oxidation chemistry. We investigated the effects of low- to well-oxygenated seawater on Mn concentrations, measured as manganese-to-calcium ratio (Mn/Ca) in shells of marine micro-organisms (i.e., foraminifera), in a seasonal field-study of two sites in GF, Swedish west coast. We found that foraminiferal Mn/Ca varied with oxygen and Mn levels in the seawater. However, also dependent on biological factors. Of the two investigated candidate species (*Bulimina marginata* and *Nonionellina labradorica*), one showed promise as indicator of low-oxygen conditions. Our research shows that the foraminiferal Mn/Ca indicator can be used to better understand changes in oxygen levels in coastal environments.

## 1. Introduction

Fjord systems are highly vulnerable to environmental and climate changes (e.g., Bianchi et al., 2020), representing a system at the interface of terrestrial, marine, and atmospheric processes. One of the pressures aggravated by anthropogenic influences over the last century is the increase in periods of oxygen depletion observed in fjords globally (e.g., Aksnes et al., 2019; Boone et al., 2018; Jackson et al., 2021). At the same time, fjords host continuous sedimentary records, which archive Holocene environmental variability in high temporal resolution (e.g.,
Howe et al., 2010; Polovodova Asteman et al., 2018). Such records may reveal natural fluctuations in dissolved oxygen variability and resolve past periods of decreased oxygen availability (e.g., Filipsson & Nordberg, 2004; Nordberg et al., 2000, 2017), paramount to the evaluation of ongoing oxygenation trends. Interpretation of these archives relies on well-calibrated, quantitative proxies.

The contrasting influence of riverine input and seawater typically causes a strong salinity stratification of fjord water masses, broadly categorized in a thin, fresh surface layer, deeper intermediate water to sill depth, and fully marine waters in the deep basin below the sill (e.g., Arneborg, 2004). The stratification is disrupted on fjord-specific timescales by water mass exchanges with the open ocean, in response to wind directions, upwelling conditions and/or, in high-latitude fjords, thermal convection. The introduction of well-oxygenated seawater, and subsequent gradual oxygen consumption through organic matter respiration results in widely ranging oxygenation cycles in deep waters (e.g., Gullmar Fjord (GF) monitoring data by the Swedish Meteorological and Hydrological Institute, SMHI).

Due to its redox-sensitivity, manganese (Mn) has received attention as a potential geochemical proxy for bottom-water (BW) oxygen concentrations (e.g., Groeneveld & Filipsson, 2013; Limburg & Casini, 2018; Schöne et al., 2021). Seasonal low-oxygen conditions, inherent in many fjords, facilitate steep redox gradients in surficial sediments. Sedimentary manganese oxides (MnOx) are a thermodynamically favorable electron acceptor in microbially mediated organic matter degradation after depletion of oxygen and nitrate (e.g., Froelich et al., 1979; Luther et al., 1997). Hence, during remineralization in the upper sediments, solid Mn(III/IV) oxides are reduced to dissolved Mn(II,III). Conversely, the dissolved species are mobile and can diffuse upwards until reaching oxic conditions, where they re-precipitate as MnOx (e.g., Thamdrup, Fossing, & Jørgensen, 1994). The depth of Mn reduction and oxidation processes in the sediment column is sensitive to BW oxygen concentrations (e.g., Froelich et al., 1979; Shaw et al., 1990), hence fluctuating concentrations of dissolved Mn in upper sediment pore-waters (PW) inform indirectly about BW oxygenation (e.g., Koho et al., 2015).

In fjords, Mn is naturally enriched relative to typical coastal sedimentary environments (e.g., for Loch Goil, Fyne and Etive, Scotland see Overnell et al., 1996; for Kongsfjorden, Van Keulenfjorden and Van Mijenfjorden, Western Svalbard, see Ardini et al., 2016; Herbert et al., 2020; Wehrmann et al., 2014; for GF, Sweden, see Engström et al., 2005; Goldberg et al., 2012). This is due to riverine input and salinity-driven flocculation of MnOx during estuarine mixing processes at the freshwater-marine interface (Sholkovitz, 1978), followed by focusing of Mn by redox shuttling into deeper areas (e.g., Lenstra et al., 2020, 2021; Lenz et al., 2015). In the deep basins of fjords, Mn may become trapped in a “reflux” cycle whereby low-oxygen conditions support effluxes of dissolved Mn into BWs, while sediments re-capture Mn oxides during intermittent periods of oxygenation (e.g., Sulu-Gambari et al., 2017). The inherent oxygen variability and abundance of Mn present in the system, in addition to sensitivity to environmental and climatic changes, make fjords an ideal environment for the calibration and application of Mn-based redox proxies (e.g., Groeneveld & Filipsson, 2013; Wehrmann et al., 2014).

Manganese is incorporated into biogenic calcium-carbonates (CaCO₃) in relation to its ambient availability at the time of precipitation, such as into tests of foraminifera (e.g., Barras et al., 2018; Munsel et al., 2010), mollusc shells (e.g., Zhao et al., 2017) or fish otoliths (e.g., Limburg et al., 2015). As opposed to sediment-based proxies, the manganese-to-calcium (Mn/Ca) signal of the primary calcite material remains unchanged after burial, independent of subsequent geochemical conditions (e.g., Koho et al., 2015; McKay et al., 2015). Hence, biogenic Mn/Ca can be used as a potentially reliable archive of past redox conditions and thus indirectly BW oxygen availability, if well calibrated.

Here we studied Mn/Ca in extant (living) benthic foraminifera from GF, Swedish west coast, on three occasions, capturing different periods of the fjord's seasonal oxygenation cycle. A previous study attempted the calibration of Mn/Ca as low-oxygen indicator based on living foraminifera sampled in GF, suggesting proxy potential (Groeneveld & Filipsson, 2013). However, the lack of PW Mn data prevented the quantitative assessment and validation of the proxy relationship at that time. In this study, sedimentary and PW Mn concentrations for the studied sites and dates are well constrained (Brinkmann et al., 2022), allowing interpretations of the biogenic Mn/Ca signals in the context of ambient environmental and geochemical conditions. Further, we address if species-specific biological effects facilitate or compromise the proxy potential, comparing two benthic foraminiferal species—*Bulimina marginata* D'Orbigny, 1826 and *Nonionellina labradorica* (Dawson, 1869).
<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) Study area on the west coast of Sweden (upper left inset) with locations of study sites (Gullmar Fjord (GF) 71 and GF 117) and monitoring sites (Släggö, Alsbäck, and Björkholmen; SMHI) in GF indicated (b) Average BW oxygenation cycle with standard deviation at Alsbäck over reference period 1980–2014 (data compiled from SMHI) with indication of BW [O₂] at GF 117 during sampling dates. (c) Transect of GF with water masses (S = salinity, t = typical residence time in days (d) and years (y); modified from Arneborg, 2004).</figcaption></figure>

## 2. Material and Methods

### 2.1. Site Descriptions and Sampling

This study is based on sediment samples from two sites in GF, collected during three campaigns (September 2018, February, and June 2019; Brinkmann et al., 2022): the outer fjord site “shallow fjord” GF 71 (69–71 m water depth, approx. 58°17.116′N, 11°30.546′E) and the inner fjord site located close to the Alsbäck Deep “deep fjord” GF 117 (115–117 m water depth, approx. 58°19.695′N, 11°33.147′E; Figure 1).

Gullmar Fjord is a sill fjord located on the Swedish west coast (Figure 1a) and connected with the Skagerrak (North Sea) to the west via a sill of 43 m depth (fjord max. depth 118 m). The Skagerrak and Baltic waters are of major influence on the fjord's hydrography, resulting in strong vertical stratification due to the contrasting contribution of high- and low-salinity waters, respectively (Figure 1c; e.g., Arneborg, 2004). While surface and mid-level waters (i.e., Baltic and Skagerrak water masses, respectively) exchange more readily with the open ocean, the sill prevents deep-water circulation of the basin. Water exchange events are tightly linked to the wind conditions and winter North Atlantic Oscillation (e.g., Filipsson & Nordberg, 2004; Nordberg et al., 2000; Polovodova Asteman & Nordberg, 2013), ventilating the fjord's basin approximately annually, typically in later winter/early spring. This results in strong temporal variability in BW dissolved oxygen concentrations, ranging from hypoxic (<63 µmol/L O₂) to fully oxygenated conditions after BW renewal (Figure 1b). The hydrographic and nutrient conditions are monitored at several sites within the fjord, including Släggö (65-m water depth), Björkholmen (70-m water depth) and Alsbäck (117-m water
depth) (Figure 1; SMHI). Släggö is located close to the sill on the sea-ward end of the fjord. The Alsbäck site is in the stagnant deep basin of the fjord, close to GF 117. The three sampling campaigns during 2018–2019 covered different time points in the water oxygenation cycle (Figure 1b; Table S1 in Supporting Information S1).

The surface sediments of the fjord's deep basin, including site GF 117, are characterized by high solid and dissolved Mn concentrations with >300 µmol/g and >200 µmol/L, respectively (Brinkmann et al., 2022; Engström et al., 2005; Goldberg et al., 2012), suggesting active Mn cycling. Here, a strong gradient in dissolved Mn concentrations across the sediment-water interface indicates upward vertical diffusion of Mn (Table S1 in Supporting Information S1). The sediment in the deep basin is foremost clay and silt with organic carbon content of ~3% (e.g., Filipsson & Nordberg, 2004).

The sediment sampling procedure is detailed in Brinkmann et al. (2022). In short, duplicate sediment cores were collected with a GEMAX™ twin barrel corer (modified Gemini corer, 9-cm inner diameter) in two casts. In the first core, dissolved oxygen concentrations of the water-sediment interface and PW were measured using micro-sensors (Unisense O₂ Microsensor OX-500) in a temperature-regulated room (6°C) at the Kristineberg Center for Marine Research and Innovation, Sweden. The motor-driven profiler allowed measurements at 500-µm vertical resolution. Two to three profiles were taken per core in multiple positions, and the profiling depth was mandated by the oxygen-penetration depth in the sediment (typically <1 cm).

The top 2 cm of each micro-profiled core was sliced in four 0.5 cm sections for foraminiferal sampling (ca. 40 ml sediment each) in the same temperature-regulated room (6°C). The first core from the second cast was sampled to supplement sediment for foraminiferal sampling if specimen abundance was low. All sediment samples were treated with 36 µl of CellTracker™ Green (CTG) CMFDA dye (5-chloromethylfluorescein diacetate; with dimethyl sulfoxide (DMSO); both ThermoFisher Scientific), which allows the identification of foraminifera (and other organisms) living at the time of sampling by epifluorescence microscopy during later observations (e.g., Bernhard et al., 2003, 2006).

The second core from the primary cast was sampled for BW and PW trace-elemental (trace element (TE)) analyses. Data for GF 117 are published in Brinkmann et al. (2022), and the TE sampling procedure—used for both sites—outlined therein. In short, Rhizons™ were used to collect water samples into 10-ml polyethylene syringes (Jokinen et al., 2020) at 2-cm resolution, which were subsequently transferred into centrifuge tubes and acidified (1 M HNO₃). The acidified BW and PW samples were analyzed for elemental concentrations of Mn, iron (Fe), sulfur (S), and Ca by inductively coupled plasma-optical emission spectrometry (ICP-OES; Thermo Scientific iCAP 6000; Helsinki University, Ecosystems and Environment Research Program) (Brinkmann et al., 2022). The GF 71 cores were profiled to a depth of 5 cm in September 2018, and 3 cm in February and June 2019.

## 2.2. Hydrographic Data

Bottom-water temperature, dissolved oxygen concentration and salinity data measured with a Seabird CTDO₂ at the times of sampling are published in Brinkmann et al. (2022). Further hydrographic data of GF are publicly available from SMHI (SMHI, 2022) and its “SHARK” hydrographic database (https://sharkweb.smhi.se/hamta-data/; last access 01 February 2022). We used [O₂] across 2018 and 2019 from three fjord stations, Släggö (58°25.980 N, 11°43.570 E, 70 m water depth), Alsbäck (58°19.400 N, 11°32.800 E, 120 m water depth), and Björkholmen (58°23.260 N, 11°37.600 E, 50 m water depth) (Figure 1a).

## 2.3. Foraminiferal Trace-Element Geochemistry

Living (CTG-labeled) specimens of *B. marginata* and *N. labradorica*, each of approximately consistent size, from the sediment intervals 0–0.5 cm, 0.5–1.0 cm, 1.0–1.5 cm, 1.5–2.0 cm were selected from one or two cores for Laser-Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) TE analysis. The specimens were wet-picked from the >100 µm sediment fraction under a stereomicroscope with epifluorescence filters (Nikon SMZ 1500 with Nikon Intensilight C-HGFI) to identify CTG labeling. The dried tests were treated with NaOCl (5%) for 2–5 hr to clean the tests and remove the cytoplasm (e.g., Lea et al., 1999; Mashiotta et al., 1999) and subsequently rinsed with MilliQ water prior to LA-ICP-MS analyses.

The LA-ICP-MS analyses were carried out at the Department of Geology, Lund University, using a Bruker Aurora Elite (quadrupole) ICP-MS and a 193-nm Cetac Analyte G2 excimer laser installed with a two-volume HelEx2 sample cell. Helium was used as carrier gas (approximately 0.8 L/min) in combination with a down-stream addition of argon (approximately 0.95 L/min) after the sample chamber (for operating conditions, see Table S2 in Supporting Information S1; see also Brinkmann et al., 2022). The analyzed specimens of the 0–1 cm interval are
<table><caption>**Table 1**

Laser-Ablation Analyzed Specimens per Species and Sediment Interval for Each Site and Sampling Occasion, Before and After Quality Control of Data Points</caption><thead><tr><th rowspan="2">Site</th><th rowspan="2">Species</th><th rowspan="2">Sampling date</th><th colspan="4">Number of analyzed n-chambers per depth interval (number of n-chamber spots after quality control)</th><th colspan="2">Median Mn/Ca<sub>foram</sub> of n-chambers with MAD (mmol/mol)</th></tr><tr><th>0–0.5</th><th>0.5–1.0</th><th>1.0–1.5</th><th>1.5–2.0</th><th>0–1 cm</th><th>1–2 cm</th></tr></thead><tbody><tr><td rowspan="6">GF 117</td><td rowspan="3">N. labradorica</td><td>17 September 2018</td><td>8 (7)*</td><td>13 (11)</td><td>n.a.</td><td>n.a.</td><td>6.67 ± 2.48</td><td>n.a.</td></tr><tr><td>24 February 2019</td><td>10 (7)</td><td>8 (8)</td><td>n.a.</td><td>n.a.</td><td>2.86 ± 2.18</td><td>n.a.</td></tr><tr><td>11 June 2019</td><td>7 (7)</td><td>8 (6)</td><td>n.a.</td><td>n.a.</td><td>8.03 ± 2.36</td><td>n.a.</td></tr><tr><td rowspan="3">B. marginata</td><td>17 September 2018</td><td>16 (14)*</td><td>10 (9)</td><td>8 (6)</td><td>7 (6)</td><td>0.071 ± 0.108</td><td>0.352 ± 0.218</td></tr><tr><td>24 February 2019</td><td>6 (4)</td><td>6 (6)</td><td>8 (6)</td><td>7 (4)</td><td>0.142 ± 0.091</td><td>0.100 ± 0.061</td></tr><tr><td>11 June 2019</td><td>11 (4)</td><td>10 (6)</td><td>9 (5)</td><td>9 (6)</td><td>0.074 ± 0.094</td><td>0.032 ± 0.022</td></tr><tr><td rowspan="6">GF 71</td><td rowspan="3">N. labradorica</td><td>17 September 2018</td><td>10 (9)</td><td>10 (10)</td><td>9 (9)</td><td>5 (5)</td><td>5.98 ± 1.42</td><td>4.49 ± 1.98</td></tr><tr><td>24 February 2019</td><td>9 (8)</td><td>10 (10)</td><td>13 (11)</td><td>12 (10)</td><td>3.85 ± 1.58</td><td>5.01 ± 1.08</td></tr><tr><td>11 June 2019</td><td>12 (11)</td><td>11 (11)</td><td>12 (12)</td><td>10 (10)</td><td>5.48 ± 2.20</td><td>4.78 ± 2.61</td></tr><tr><td rowspan="3">B. marginata</td><td>17 September 2018</td><td>13 (7)</td><td>9 (8)</td><td>6 (5)</td><td>8 (7)</td><td>0.020 ± 0.011</td><td>0.016 ± 0.007</td></tr><tr><td>24 February 2019</td><td>8 (3)</td><td>8 (8)</td><td>n.a.</td><td>n.a.</td><td>0.014 ± 0.005</td><td>n.a.</td></tr><tr><td>11 June 2019</td><td>9 (9)</td><td>10 (8)</td><td>n.a.</td><td>n.a.</td><td>0.013 ± 0.009</td><td>n.a.</td></tr></tbody></table>

*Note.* Median Mn/Ca with median average deviation (MAD) of n-chambers in mmol/mol per 0–1 and 1–2 cm interval. *indicates samples compiled from two duplicate cores; n.a. = not available.

those measured for barium-to-calcium ratios (Ba/Ca) in Brinkmann et al. (2022). Here we present the specimens' Mn/Ca, magnesium-, and strontium-to-Ca (Mg/Ca, Sr/Ca) data for the first time, as well as TE data for specimens from 1 to 2 cm sediment depth. Spots on individual foraminiferal chambers were ablated for TE profiles (i.e., from the outside to the inside of the chamber wall) on the final (n), penultimate (n − 1), antepenultimate chambers (n − 2), as well as the area covering the proloculus (i.e., first chamber; P) of *B. marginata* (749 ablation profiles), and n, n − 1, and n − 2 of *N. labradorica* specimens (531 ablation profiles) (Table 1). The P signal is considered to represent the juvenile chambers as well as subsequently added layers of calcite with each new chamber addition (e.g., Erez, 2003), thus a bulk signal of primary and secondary calcite layers over an extended growth period.

Raw counts were converted to elemental concentrations using the software package Igor Pro 6.37 with Iolite v. 3.5 (Paton et al., 2011). The element concentrations were calculated as averages over selections of the ablation profiles (>15 laser ablation data points per selection). These selections were made by manually screening the profiles for ablation intervals of constant ⁴³Ca counts, corresponding to the primary CaCO₃ material of the foraminifera test walls (e.g., Brinkmann et al., 2022; Guo et al., 2019; Petersen et al., 2018). Ablation intervals enriched with elements such as aluminum (Al) or zinc (Zn) at the beginning and/or end of the profiles were deselected, as these represent contaminations. Trace elemental ratios (TE/Ca) were calculated with the assumption that the TEs were bound as carbonate (assuming 40% wt Ca in the CaCO₃ matrix) and normalized to Ca.

The raw data were processed according to the following quality control criteria: (a) exclusion of data with [Mn] lower than the limit of detection (LOD = baseline signal +3.3 × σ; σ = standard deviation of background signal) (63 selections removed), (b) exclusion of ablation selections with potential clay contamination (i.e., Al/ Ca >0.4 mmol/mol; Wit et al., 2010) (76 selections removed), (c) application of a 95% confidence limit on Mn/ Ca, for each species, site and season, to remove outliers (i.e., >2 × standard deviation), for (3a) all chambers (33 selections removed, *B. marginata* and *N. labradorica*) and (3b) n, n − 1, and n − 2 chambers for *B. marginata* data points specifically (20 selections removed, *B. marginata*). In total, 1088 of 1280 (85%) ablation profile selections were retained for further analyses. For each species, median Mn/Ca values (with median average deviation) and averages were calculated per chamber, sediment interval and site.

## 2.4. Synchrotron Scanning X-Ray Fluorescence Imaging

For synchrotron scanning X-ray fluorescence (XRF) imaging, four specimens (I-IV) of *B. marginata* from the 0–0.5 cm interval of the September 2018 sampling data in GF 117 were selected. The specimens were embedded in epoxy (EPO-TEK® 301) in a mold of 1 cm diameter and dried at room temperature for 48 hr. The mount
surface was ground down (Struers LaboSystem with a LaboPol-60 and LaboForce-Mi specimen mover head, DiaDuo diamond emulsion 9-µm grain size) at the Department of Geology to expose central test cross-sections. The backside of the mount was abraded until an overall thickness of less than 1 mm was reached. Finally, the specimen-side surface was polished using a diamond emulsion of decreasing grain size (6-, 3-, and 1-µm grain size; DiaDuo). A mount thickness of <1 mm is preferable to decrease the epoxy's scattering-effects on the XRF spectra. SEM images were taken in a low vacuum without coating (Tescan Mira3 High Resolution Schottky Field Emission SEM) at the Department of Geology in Lund to identify areas of interest.

To investigate the spatial distribution of Ca and Mn across tests of *B. marginata*, XRF imaging was performed at the NANOSCOPIUM hard X-ray nanoprobe beamline (150-nm beam size; Somogyi et al., 2015) hosted at the 2.75-GeV high-brilliance third-generation Synchrotron SOLEIL facility (Saint-Aubin, France). The samples were mounted vertically with an angle of 90° to the incident beam (11.2-keV X-ray energy) and the XRF spectra were collected by two SDD XRF detectors positioned at 20° to the reflecting sample surface. Elemental concentrations and distributions were mapped using the Flyscan continuous sample scanning technique (Medjoubi et al., 2013). Fast maps of entire specimens were acquired with 1-µm spatial resolution and 20–100 ms dwell time per pixel to guide the selection of regions of interest for higher resolution and longer dwell time zoom-in mapping (300–500 nm, 100 ms/pixel). The detection limit of [Mn] per pixel was ~5–10 ppm for these high-resolution maps. The information depth was several tens of µm.

PyMca software was used to fit the XRF sum-spectra to identify the elements present in the sample and possible spectral overlapping (Solé et al., 2007). The elemental distribution maps were reconstructed by a custom Matlab code including dead-time correction, showing intensities above the detection limit (95% confidence limit).

Further, the images were analyzed using the Fiji distribution of ImageJ (Rueden et al., 2017; Schindelin et al., 2012). The Mn and Ca distribution maps were merged into color-coordinated composite images illustrating the co-localization of Mn and Ca. A sample mask was defined for each scan based on the Ca intensity maps of the entire test cross-section and, subsequently, semi-quantitative Mn to Ca ratio maps calculated within those masks based on the corrected images. The average TE signal with standard deviation was calculated for the entire test wall area as well as parts thereof. The semi-quantitative concentration ratios (percentage mass and molar mass; reported as Mn/Ca<sub>XRF</sub>) were calculated from the relative intensity ratios by taking measurement geometry, XRF yields and detection efficiencies of XRF spectral lines into account and assuming a calcite sample matrix (Jenkins, 1995).

## 2.5. Statistical Analyses

All statistical analyses were performed using the software PAST 4.05 (Hammer et al., 2001), considering a *p* value of <0.05 as significant. Correlations between TE/Ca ratios were calculated using the Pearson correlation coefficient. Medians, median average deviations (MAD) and relative standard deviation (RSD, in %) were calculated for TE data of foraminiferal n-chambers (i.e., final chambers) for the sediment intervals 0–1 and 1–2 cm. The non-normally distributed data sets (Shapiro-Wilk's test), and those comprising variable specimen or sample numbers (heteroscedastic data) were tested using the non-parametric Kruskal-Wallis and associated Mann-Whitney post-hoc tests or the Wilcoxon signed-rank test, for pairwise comparisons, to test for differences between different subsets of our data (e.g., TE/Ca, Mn/Ca by sediment intervals). For correlation tests of Mn/Ca<sub>foram</sub> to geochemical variables, only ratios of each n-chamber were considered, assuming close temporal proximity of the precipitation of the most recent chamber (n) and measurements of environmental parameters at the time of sampling. To test relationships between two variables, Spearman's rank correlation coefficient was used (e.g., TE data and [O<sub>2</sub>]<sub>BW</sub>).

In addition to measured values at the time of sampling, theoretical values of ambient PW [Mn] during calcification were estimated from foraminiferal Mn/Ca values using previously established partition coefficients (also referred to as distribution coefficients) for *N. labradorica* (Koho et al., 2017) and *B. marginata* (Barras et al., 2018), using the following equation (e.g., Morse & Bender, 1990):

$$D_{Mn} = (Mn/Ca)_{foram} / (Mn/Ca)_{pore-water} \left[ \frac{\text{mmol/mol}}{\text{mmol/mol}} \right]$$

# 3. Results

## 3.1. Pore-Water Mn and O₂ in Gullmar Fjord

Pore-water [Mn] of surface sediments were distinctly lower in the shallow-fjord than deep-fjord site during all sampling dates (Figure 2a; Tables S1 and S3 in Supporting Information S1; for GF 117 see also Brinkmann et al., 2022). The
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Dissolved Mn profiles of deep fjord site Gullmar Fjord (GF) 117 (data from Brinkmann et al., 2022) and shallow fjord site GF 71 for each sampling date. (a) Average BW and pore-water dissolved oxygen profiles of sites GF 117 and GF 71 at each sampling date. Note the difference in sediment depth scale between (a, b).</figcaption></figure>

clear surface-sediment redox zonation evident in the deep-fjord site (Brinkmann et al., 2022) is lacking in the shallow fjord, indicating a deeper Mn chemocline—beyond the interval sampled for foraminifera—and/or generally low Mn availability. Although [Mn] is consistently high (>200 µM) in the deeper part of the sediment column at GF 117, the concentration gradient close to the sediment-water interface differs between seasons. In September 2018, the 1 cm depth interval showed very low Mn concentrations (<10 µM), while the corresponding value in February 2019 was >100 µM. The value in June 2019 was intermediate between these endpoints (approx. 65 µM).

Concentrations of PW Fe, Ca and S are reported in Table S3 in Supporting Information S1.

Bottom-water oxygen measured by micro-profiling within a few mm above the sediment-water interface corresponded well to [O₂]~BW~ measured c. 1 m above the fjord bottom by CTDO₂ (Figure 2b). Oxygen became undetectable within the first 1 cm of the sediment at each site and sampling date (Figure 2b). At the site GF 117, the sampling date with the lowest [O₂]~BW~ and shallowest O₂ penetration into sediments (February 2019) was also the date with the highest PW [Mn] in the uppermost 1 cm.

## 3.2. Foraminiferal Mn/Ca in Gullmar Fjord

In our data set, Mn/Ca in *B. marginata* ranged from 0.004 to 2.883 mmol/mol and in *N. labradorica* from 0.016 to 27.521 mmol/mol (Figure 3; Brinkmann (2023)). Compiled medians for the intervals 0–1 cm and 1–2 cm are given in Table 1. The two species differed in their Mn/Ca in all samples (Kruskal-Wallis test of n-chambers, 0–1 cm interval: *p* < 0.001; medians of n-chambers per sediment-interval: *p* < 0.001), with median *N. labradorica* Mn/Ca signals being on average 100-times higher (median of 4.906 mmol/mol) than those of *B. marginata* (median of 0.053 mmol/mol; Figure 3a).

Also, Mg/Ca and Sr/Ca differed (Wilcoxon test of n-chamber averages per sediment interval: *p* = 0.007 and *p* = 0.001, respectively), albeit to a much lesser degree than Mn/Ca. Overall, both species' values were in comparable ranges, with Mg/Ca being on average slightly higher (2.20 vs. 1.90 mmol/mol) and Sr/Ca slightly lower (1.16 vs. 1.22 mmol/mol) in *N. labradorica* than *B. marginata*, respectively (Figure S1 in Supporting Information S1).

Mn/Ca values of neither species showed a significant difference between sediment intervals of respective sampling dates and sites (Kruskal-Wallis and Mann-Whitney test; Table S4 in Supporting Information S1). Therefore, we combined measurements and focused on the sediment interval 0–1 cm going forward. Further, no chamber-specific trends were evident for either species comparing the most recently formed chambers *n*, *n* − 1, and *n* − 2 (testing 0–1 cm interval; Table S5, Figures S2 and S3 in Supporting Information S1). Patterns of Mn/Ca
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Mn/Ca of *Nonionellina labradorica* and *Bulimina marginata* plotted along BW oxygen concentration representing different sampling dates and sites (data from surface sediments; 0–1 cm). (a) Mn/Ca of the n-chambers. Note the logarithmic scale on y-axis. (b, c) Individual and median Mn/Ca of n-chambers by species and sites. Note the difference in scale for Mn/Ca of different species.</figcaption></figure>

across the BW oxygen gradient appear similar regardless of whether only n-chambers are considered (Figure 3) or all measured adult chambers combined (Figure S4 in Supporting Information S1).

Interestingly, the initial chamber area (i.e., *P*) of *B. marginata* had consistently different Mn/Ca signals than more recently formed chambers, especially n-chambers (Table S4 and Figure S2 in Supporting Information S1). A trend of distinctly higher Mn/Ca in the proloculus area is particularly evident in samples of the deep-fjord site from September 2018 (on average 0.3 ± 0.1 mmol/mol vs. 0.06 ± 0.08 mmol/mol in n-chambers), but to a lesser degree seen for other sampling dates as well.

Differences by sampling dates between foraminiferal Mn/Ca persisted only for the deep fjord site GF 117 (Kruskal-Wallis test *p* = 0.03 and *p* < 0.001 for *B. marginata* and *N. labradorica*, respectively). There, for both species, samples from February 2019 ([O₂]~BW~ < 100 µmol/L) differed significantly from samples from September 2018 and June 2019 ([O₂]~BW~ > 100 µmol/L; *B. marginata*: *p* = 0.01 and 0.04, respectively; *N. labradorica*: both *p* < 0.001). Mn/Ca signals of *B. marginata* (RSD = 102 ± 30%) were overall more variable than those of *N. labradorica* (RSD = 53 ± 10%), but no clear trends by sampling site or date were evident.

Using established species-specific partition coefficients for *N. labradorica* (*D*<sub>Mn</sub> = 1.24; Koho et al., 2017) and *B. marginata* (*D*<sub>Mn</sub> = 0.621; Barras et al., 2018), we could calculate the theoretical PW Mn/Ca at which the analyzed specimens calcified and compare it to the in situ PW Mn/Ca values actually measured at the time of sampling. Mn/Ca values of n-chambers of *N. labradorica* suggested that specimens calcified at ambient PW Mn/Ca of 3.2 ± 1.7 to 4.4 ± 1.8 mmol/mol in the shallow fjord and 2.5 ± 1.8 to 8.1 ± 4.4 mmol/mol in the deep-fjord site. Such PW values were reached at sediment depths >3 cm in GF 71 in September 2018 (no data available beyond surface 3 cm for other dates) and within the immediate surface sediment (c. 1 cm) of GF 117 (Figure 2 and Table S3 in Supporting Information S1). For *B. marginata* n-chambers, the calculated theoretical PW Mn/Ca was 0.04 ± 0.03 to 0.05 ± 0.05 mmol/mol in GF 71 and 0.11 ± 0.17 to 0.23 ± 0.15 mmol/mol in GF 117, corresponding to BW and immediate surface [Mn]~PW~ and [Ca]~PW~ (Figure 2 and Table S3 in Supporting Information S1).

### 3.3. Correlation of Foraminiferal Mn/Ca to Ambient [Mn] and Bottom-Water [O₂]

For correlative comparisons of Mn/Ca<sub>foram</sub> to geochemical variables only ratios of n-chambers were considered, with the assumption of chamber precipitation and measurement of environmental parameters being approximately contemporaneous.

Mn/Ca<sub>foram</sub> of neither species was negatively correlated to BW oxygen in a simple manner. At the deep-fjord site GF 117, *B. marginata* showed noticeably higher Mn/Ca signals after the low-oxygen period prior to sampling in Feb. 2019 (i.e., <100 µmol/L O₂; c. 60% of Mn/Ca >0.1 mmol/mol), and lower Mn/Ca signals after well-oxygenated conditions (i.e., >100 µmol/L O₂; c. 80% of Mn/Ca <0.1 mmol/mol) in September 2018 and June 2019 (Figure 4). The same trend is true considering all analyzed chambers—n, *n* − 1, *n* − 2 and *P* (see
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Boxplots of foraminiferal Mn/Ca (n-chamber only) versus BW oxygen concentrations for (a) *Bulimina marginata* and (b) *Nonionellina labradorica* in the deep fjord site Gullmar Fjord 117. Inset of positive correlation between foraminiferal Mn/Ca (y-axis) and BW [O₂] (x-axis; data points at 71, 104 and 217 µmol/L O₂) (r = 0.47, *p* = 0.002). Whiskers show extreme values; the white line denotes medians. Note the difference in scale for Mn/Ca of different species.</figcaption></figure>

Figure S5 in Supporting Information S1). In the more oxic shallow-fjord site GF 71, Mn/C<sub>foram</sub> was consistently <0.1 mmol/mol (97% of individual signals across all dates; Figure 4).

In contrast, Mn/Ca of *N. labradorica* correlated positively with [O₂]ᵣₘ₉₀ in the deep-fjord site GF 117 (*r* = 0.47, *p* = 0.002, Figure 4), showing the highest Mn/Ca signals under well-oxygenated conditions.

Across all measurements, neither species' Mn/C<sub>foram</sub> showed a significant correlation to surface-sediment [Mn]<sub>PW</sub> at the time of sampling. However, the highest median Mn/C<sub>foram</sub> values of *B. marginata* were observed during the sampling in February 2019, when surface-sediment [Mn]<sub>PW</sub> was at its highest measured level, and [O₂]<sub>BW</sub> was at its lowest measured level.

### 3.4. Trace-Element Distribution Within *B. marginata* Tests by Scanning XRF Nano-Imaging

Four specimens of *B. marginata* were investigated for Ca and Mn distributions by XRF imaging (Figure 5). The Ca-fluorescence intensity maps showed high intensity Ca areas of 10–15 µm horizontal thickness, corresponding to the CaCO₃ chamber walls exposed in cross-section on the sample surface. The XRF signals penetrating the sample by several tens of micrometers created a projection image of parts of the test area within the epoxy on the sample surface, represented by weaker Ca signals. Manganese intensities were distinctly lower than Ca intensities. Manganese was distributed in two distinct patterns: (a) homogeneously distributed in areas coinciding with high Ca intensities, that is, Mn incorporated within the test wall; (b) heterogeneously distributed in a blotchy pattern, only partially overlaying high-Ca areas, that is, discrete Mn-phases not associated primarily with the CaCO₃ walls. Creating masks based on the Ca high-intensity areas largely removed regions of Mn in distribution pattern (b).

The semi-quantitative Mn/C<sub>XRF</sub> molar ratios averaged over the test wall area of all specimens ranged from 0.065 to 0.310 mmol/mol, thus corresponding well to the range of Mn/Ca measured in *B. marginata* by LA-ICP-MS. In some areas of the chamber walls, Mn was below the detection limit (i.e., represented by a signal of 0), so the average Mn/C<sub>XRF</sub> might slightly underestimate the true ratio. When averaging Mn/C<sub>XRF</sub> only over test areas with Mn signals above detection limit, the ratios range higher (0.106–0.311 mmol/mol), overestimating the true signal. Maximum Mn/Ca values were up to 87 mmol/mol, likely representing contamination signals of non-CaCO₃-bound Mn (e.g., in the cytoplasm). The higher-resolution zoom images of selected chamber parts had comparable average Mn/C<sub>XRF</sub> with 0.015–0.078 mmol/mol (Figures 5c1 and 5c3).

Comparing the initial chambers (approximately corresponding to P signal of LA-ICP-MS measurements) to the signal of the following adult chambers, a trend of higher and lower Mn/C<sub>XRF</sub>, respectively, was observable. This
<figure><img src="image_7.png" /><figcaption>**Figure 5.** X-ray fluorescence (XRF) trace elemental maps of a cross-section of *B. marginata* specimens I, II, III and IV. (a) XRF intensity maps of Ca. Brightness of signal indicates the intensity of TE signal. (b, c) Semi-quantitative Mn/Ca<sub>XRF</sub> maps (mmol/mol) within sample mask based on Ca intensity (corresponding to test wall): (b) entire specimen, (c1) wall area of second, fourth and sixth chambers showing a high- and low-Mn/Ca layer on the inside and outside of the chamber wall (location of zoom indicated in a–b1), (c3) proloculus area showing elevated Mn/Ca<sub>XRF</sub> in some chambers (location of zoom indicated in a–b3; colors as in b3).</figcaption></figure>

was particularly evident in specimens I (average of 0.576 vs. 0.0913 mmol/mol) and II (0.263 vs. 0.029 mmol/mol) (Figures 5b1–5c1 and 5b2). In specimen III, the initial chambers had an average Mn/Ca<sub>XRF</sub> of 0.087 mmol/mol, whereas Mn/Ca<sub>XRF</sub> averaged over the entire test was 0.065 mmol/mol. The transition from “high” to “low-Mn” chambers was in all cases abrupt, with the fourth or fifth chamber being the last one with elevated Mn concentrations (i.e., P + 3, P + 4). In specimen III, only the second and third chambers had elevated Mn concentrations in comparison to the remaining chambers (Figures 5b3 and 5c3). In specimen IV, the proloculus area was not exposed on the sample surface. The depth integration through overlying epoxy affects the measured TE intensity negatively, resulting in a reduced signal (Figures 5a4–5b4). Still, the fourth chamber, which was exposed, showed elevated Mn/Ca signals compared to the subsequent chambers. The maps further showed a layering of CaCO<sub>3</sub> within the chamber walls, which was particularly evident in the walls of the proloculus area where different layers showed distinct Mn/Ca<sub>XRF</sub> signals (Figure 5c1).

# 4. Discussion

## 4.1. Proxy Potential of Biogenic Mn/Ca in Fjords

### 4.1.1. Biogenic Mn/Ca in the Context of Manganese Availability in Fjords

Manganese enrichment is a typical feature of restricted coastal basins, such as fjords. The level of Mn accumulation is a function of both the contrasting influence of land runoff versus open-ocean-derived water masses
(i.e., inner vs. outer fjord) and water depth, where deep basins act as sinks for Mn oxides (see, e.g., Thamdrup, Fossing, & Jørgensen, 1994; Thamdrup, Glud, & Hansen, 1994). Due to these dynamics, Mn concentrations in the water column and in sediments are typically the highest in inner fjord environments and/or the deep basin. In GF, specifically, the inner and deep fjord zones coincide, and PW [Mn] is particularly high there compared to the shallow outer fjord (Brinkmann et al., 2022).

In the inner Van Keulenfjorden, Western Svalbard, concentrations of up to 210 µmol/L Mn were measured in PWs of surface sediments (150–210 µmol/L in top 10 cm), whereas the mid- and outer fjord sites had distinctly lower values around 50 µmol/L (top 10 cm) (Wehrmann et al., 2014), and in the neighboring Van Mijenfjorden even higher [Mn]~PW~ of up to 780 µmol/L were measured in the inner zone (vs. <100 µmol/L [Mn] in outer fjord; Herbert et al., 2020). This pattern of elevated signals in the inner fjord relative to the entrance region is also recorded by biogenic Mn/Ca, such as *N. labradorica* in Kongsfjorden (Skirbekk et al., 2016) and bivalves in the Doubtful Sound, southwest New Zealand (Beer et al., 2011).

Correspondingly, in GF, Mn/Ca of both investigated species—*B. marginata* and *N. labradorica* (Table 1; Figure 3, Figures S2 and S3 in Supporting Information S1)—showed distinctly higher values in specimens from the inner versus outer fjord site (i.e., deeper vs. shallower site). This is congruent with the expected PW Mn availability, with up to 30 times higher [Mn] in the inner deep than outer shallow site (Figure 2; Brinkmann et al., 2022). Further, compared to the species' respective Mn/Ca of nearby coastal regions (in Kattegat: 0.03 and 1.9 mmol/mol Mn/Ca at 104 µmol/L [O₂]~BW~, respectively; Groeneveld et al., 2018), foraminiferal Mn/Ca was elevated in the GF. Due to the apparent Mn accumulation in the inner/deep fjord, specifically, these settings present suitable conditions for the possible calibration and application of an Mn-based proxy for BW oxygenation.

### 4.1.2. Biogenic Mn/Ca in the Context of Bottom-Water Oxygenation Conditions

The vertical zonation of Mn cycling in the sediment column is sensitive to oxygen conditions in the BWs. Oxygen concentrations are generally the lowest in the deep basin of GF, compared to shallower levels. There, water masses are only renewed annually during late winter to early spring (see Figure 1b), after which [O₂] gradually decreases until the next water exchange event. While [O₂]~BW~ in the deep fjord spanned from 71 to 217 µmol/L (Figure 1b, Table S1 in Supporting Information S1), the shallow-fjord site GF 71 was relatively well-oxygenated during all sampling dates (125–211 µmol/L [O₂]). Also, monitoring data of [O₂]~BW~ documented well-oxygenated conditions in the shallow fjord during our sampling period, with the latest low-oxygen period being in the end of 2017 (based on the sites Släggö and Björkholmen, see Figure 1a for location; SMHI). Thus, higher temporal variability of dissolved Mn in surface sediments is expected for the deep basin in the inner fjord in response to changes in oxygen availability. Indeed, only at the deep fjord site did our foraminiferal Mn/Ca differ by sampling dates (i.e., contrasting BW oxygen conditions; Figure 3) (Kruskal-Wallis test of n-chambers, 0–1 cm interval: *B. marginata* *p* = 0.03, *N. labradorica* = *p* < 0.05 in GF 117; both n.s. in GF 71).

Similarly, in the Yangtze River Estuary, China, Mn/Ca measured in benthic foraminifera at dissolved [O₂] between c. 156 and 344 µmol/L was approximately stable but increased distinctly when [O₂]~BW~ dropped below 130 µmol/L (measurements available from 67 to 130 µmol/L; Guo et al., 2019). Assuming diagenetic in-sediment conditions (i.e., [Mn²⁺] and carbon fluxes) are comparable in GF and the Yangtze River Estuary, [O₂]~BW~ of around 130 µmol/L or 2.9 ml/L may generally present the upper limit of utility of the Mn/Ca proxy.

In well-oxygenated BW conditions, O₂ presumably penetrates deeper into the sediment and accordingly Mn oxide reduction will occur at greater depths—potentially beyond the living and/or calcification depth of foraminifera. Hence, foraminiferal Mn/Ca may not reflect quantitatively on [O₂]~BW~ but remain stable over a wide range of [O₂]. Also, in the shallower, outer part of GF, where BW conditions were consistently well-oxygenated (>125 µmol/L), median foraminiferal Mn/Ca was generally stable (Figure 3).

Hence, quantitative reconstructions of BW oxygenation using the Mn/Ca proxy might not be possible beyond [O₂]~BW~ of around 130 µmol/L for estuarine or even marine settings. While presenting an upper limit of utility, it does not restrict the potential of the Mn/Ca proxy to inform about the early onset of declining oxygen availability and hypoxia in marine settings (dissolved [O₂] < 62.5 µmol/L or 1.4 ml/L; e.g., Levin et al., 2009).

### 4.1.3. Proxy Potential of *Bulimina marginata* and *Nonionellina labradorica*

In GF, the Mn/Ca record of *B. marginata* supports the hypothesis of enhanced Mn accumulation in foraminiferal calcite due to elevated PW concentrations under low-oxygen conditions (i.e., February 2019 samples; Figure 4a).
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Box and whisker plots of benthic foraminiferal Mn/Ca from surface sediments of coastal areas: deep and shallow Gullmar Fjord (this study; Groeneveld & Filipsson, 2013), the Skagerrak (Groeneveld & Filipsson, 2013), the Kattegat (Groeneveld et al., 2018), the Öresund, Baltic Sea (Gabrielsson, 2014) and Kongsfjorden, Svalbard (Skirbekk et al., 2016). Gray boxes represent “low-Mn” species (i.e., *B. marginata*, and spp.**, which comprise *Islandiella helenae* Feyling-Hanssen & Buzas, 1976*I. norcrossi* (Cushman, 1933), *Buccella frigida* (Cushman, 1922), *Elphidium clavatum* Cushman, 1930 and *Cassidulina reniforme* Nørvang, 1945); white boxes “high-Mn” species (i.e., *Nonionellina labradorica* and *Globobulimina turgida*). Number of analyzed specimens (for LA-ICP-MS analyses) or bulk samples (for ICP-OES analyses, marked by *) given in brackets. b.d.l. = below detection limit.</figcaption></figure>

After periods of high [O₂]ᵣ*BW*, the majority of specimens had Mn/Ca of <0.1 mmol/mol (Brinkmann, 2023). The trend between Mn/Ca and BW oxygenation persisted even when combining measurements of all chambers measured for *B. marginata* (n, *n*−1, *n*−2, *P*), although the variability per sampling date increased (Figure S5 in Supporting Information S1). This is expected considering the duration of the calcification period spanning proloculus to *n*-chambers throughout, presumably, variable Mn availability.

In contrast, *N. labradorica* had the opposite directionality in its Mn/Ca to BW oxygen relationship in the deep fjord site (Figure 4b). The same trend of comparatively higher Mn/Ca in *N. labradorica* under well-oxygenated BW conditions has previously been observed in samples from the Kattegat (1.3–1.9 vs. 2.9 mmol/mol Mn/Ca at 104 vs. 193 µmol/L [O₂]; Groeneveld et al., 2018), indicating that other environmental and/or biological parameters control Mn/Ca in this species (further discussed in Section 4.2). As the positive trend cannot be explained by the geochemical relationship of [O₂] and [Mn] (e.g., Froelich et al., 1979), the use of this species' Mn/Ca as an oxygenation indicator is not recommended.

## 4.2. Biological Influences on the Potential of Mn/Ca as a Proxy for Low-Oxygen Conditions

### 4.2.1. Species-Specific Differences in Foraminiferal Mn/Ca and Comparison Across Environments

In addition to environmental Mn availability, so called “vital effects” play a role in determining foraminiferal Mn incorporation. Vital effects encompass all influences of biological nature that affect the incorporation of TEs during test precipitation, such as biomineralization processes or fractionation, and that alter the TE availability in the site of calcification controlled by foraminifera (e.g., Bentov & Erez, 2005; de Nooijer et al., 2009). If unconstrained, such effects can compromise the potential of the Mn/Ca proxy by adding variability stemming directly or indirectly from biological processes that cannot be separated easily from the primary environmental signal.

*N. labradorica* has particularly high Mn/Ca (Figure 6, and references therein). Similarly high Mn/Ca has also been measured in *Globobulimina turgida* (Bailey, 1851) (Figure 6, and references therein), a deep-infaunal species (Murray, 2006) tolerant to oxygen depletion (Koho et al., 2011; Piña-Ochoa, Koho, et al., 2010; Risgaard-Petersen et al., 2006; Woehle et al., 2022). The difference in Mn/Ca between these “high-Mn” species and other shallow-dwelling benthic foraminifera, incorporating less Mn, is consistently resolved over a variety of coastal environments. In GF (this study; Groeneveld & Filipsson, 2013), Mn/Ca of *N. labradorica* is, on average, 100-times higher than that of *B. marginata*.

Also, in samples from the continental slope along the NE Japanese margin, western Pacific, the Mn/Ca signal of a species morphologically identified as *N. labradorica* exceeded that of all other investigated species (*Elphidium*
*batialis*, *Uvigerina* spp., *Bolivina* spissa, *Chilostomellina fimbriata*; Koho et al., 2017). Still, *N. labradorica* had Mn/Ca distinctly below the coastal signals (Mn/Ca = 0.023–0.277 mmol/mol at [Mn]~PW~ < 2 µmol/L; Koho et al., 2017), in accordance with the opposing geochemical regime of deep waters and sediments of open ocean regions versus fjords and other coastal settings. Similarly, the closely related species *Nonionella stella* displayed very low Mn/Ca (average of c. 0.102–0.159 mmol/mol) under suboxic oxygen conditions (<10 µmol/L O~2~) in the Santa Barbara Basin, a silled feature off southern California (Brinkmann et al., 2021). Open-ocean settings that derive water masses from oxygen minimum zones frequently have low ambient Mn availability. This is, as persistent low-oxygen conditions in the anoxic to hypoxic range in BWs (i.e., <63 µmol/L) may lead to the Mn redoxcline moving above the sediment-water interface, facilitating dissolved Mn to leave the system by “escap- ing” into the water column (e.g., Law et al., 2009; van der Weijden et al., 1999). Hence, Mn bioavailability in those types of surface sediments is low.

Currently, consensus on the specific foraminiferal Mn uptake pathways during calcification has not been reached (e.g., Barras et al., 2018; van Dijk et al., 2019, 2020), which could explain the contrasting incorporation of Mn between “high-Mn” species such as *N. labradorica* and *G. turgida*, and “low-Mn” species such as *B. marginata*. In high-Mg species, high Mg incorporation is typically simultaneously associated with higher incorpora- tion of other TEs, such as Sr, Na, Ba, and Zn (tropical large benthic foraminifera; Evans et al., 2015; van Dijk et al., 2017). Therefore, if species-specific TE fractionation determined the generally high Mn/Ca signals of *N. labradorica*, a similar trend of distinctly higher other TE/Ca would be expected. This is not evident in our data: *N. labradorica* and *B. marginata* differ in Mn/Ca on average 144-times, whereas in Mg/Ca only by 1.2-times higher, and Sr/Ca 0.95-times. We hypothesize that the high Mn/Ca measured in *N. labradorica* is not a consequence of species-specific Mn biomineralization mechanisms but does reflect ambient Mn availability (further discussion in Sections 4.2.2 and 4.2.3).

The contrasting incorporation of Mn by the investigated species narrows the application potential of established calibrations to the species-level, and limits quantitative comparisons between Mn/Ca records of different species or genera to those with similar Mn/Ca ranges. We highlight the need for considerations of species-specific effects—or other biological effects (discussed below)—if attempting interpretations of Mn/Ca records with regards to environmental conditions.

### 4.2.2. Effects of Micro-Habitat Differences on Mn Incorporation

Previously, the offset in Mn/Ca between *B. marginata* and *N. labradorica* has been attributed to their micro-habitat separation (Koho et al., 2017; Skirbekk et al., 2016), as Mn/Ca~foram~ of different calcification depths should reflect the ambient in-sediment Mn redox zonation (compare also e.g., Koho et al., 2015). Several studies have discussed the influence of vertical migration on a foraminiferal species' proxy potential, suggesting different micro-habitat distributions to be ideal for respective marine settings according to the Mn chemocline (e.g., Guo et al., 2021; Koho et al., 2017; Ní Fhlaithearta et al., 2018; Petersen et al., 2018). In estuarine and coastal settings, epi- or shallow infaunal foraminifera are expected to show generally high and variable Mn/Ca—opposed to high and constant Mn/Ca expected for deep infaunal taxa—, due to active Mn “refluxing” across the sediment-water inter- face (Lenstra et al., 2020; Sulu-Gambari et al., 2017), aiding the proxy potential (e.g., Guo et al., 2021).

In GF, *B. marginata* was comparably more variable in its Mn/Ca signal than *N. labradorica* and followed the expected negative trend with increasing BW oxygenation (see previous section). *B. marginata* is often considered shallow-infaunal (e.g., Alve & Bernhard, 1995), and indeed showed its highest densities in the deep GF in the surface, at c. 5 mm depth (comparing densities in 0–5 cm depth at 2–5 mm resolution; Choquel et al., 2021). However, in some settings, it can have a bimodal in-sediment distribution, with abundance maxima both in the immediate surface and deeper sediment (e.g., Jorissen, 1999), potentially as a response to changing geochemistry as is known for foraminifera in general (e.g., Alve & Bernhard, 1995; Geslin et al., 2004). Such may hamper the proxy potential if Mn/Ca signals represent the PW Mn availability of two distinct geochemical environments.

On the other hand, *N. labradorica*, considered deep-infaunal (e.g., Alve & Bernhard, 1995), did not correlate inversely with [O~2~]~BW~, supporting the hypothesis of deep-infaunal species showing reduced proxy potential in coastal settings. The average living depth of *N. labradorica* was estimated at 4 cm (in Northeast Japan margin sediments; Koho et al., 2017), although in GF its maximum abundance is around 1–2 cm depth (Choquel et al., 2021). This comparatively shallow depth distribution is supported by the calculated calcification depth based on [Mn]~PW~ values inferred from the Mn partitioning coefficient and lies within the Mn chemocline of GF
sediments. Hence, *N. labradorica*'s micro-habitat cannot fully explain the lack of proxy response to BW oxygenation, nor contrasting Mn/Ca signals in comparison to *B. marginata*.

### 4.2.3. Influence of Other Biological Processes on Foraminiferal Mn/Ca

In addition to the environment-controlled geochemical in-sediment zonation, biological processes of foraminifera themselves may influence Mn concentrations and/or speciation in their micro-environment. Such are generally not regarded in geochemical proxy calibration studies. Pore-water TE concentrations are typically measured at resolutions too low to resolve potential alterations of [Mn] in the foraminifers' micro-environment during biomineralization (e.g., 2 cm intervals in this study; 0.5 cm intervals in field-study of Koho et al., 2017; aquarium scale in laboratory-based study of Barras et al., 2018), and speciation of dissolved Mn is not investigated. Thus, while a higher Mn partitioning coefficient is assumed for *N. labradorica* compared to *B. marginata* (*D*<sub>Mn</sub> of 1.24 vs. 0.621; Barras et al., 2018; Koho et al., 2017), this relies on the assumption that ambient [Mn] inferred from (pore)-water TE measurements are identical to the microenvironment where the foraminifera were actually living, whereas several physiological mechanisms could alter the chemical composition of the latter.

Here we discuss potential influences of (a) the presence of diatom chloroplasts in *N. labradorica* (i.e., kleptoplasty; e.g., Cedhagen, 1991; Jauffrais et al., 2019), (b) cysts enveloping foraminiferal tests (e.g., Cedhagen, 1996; Heinz et al., 2005), and (c) the denitrification process used by some foraminifera for anaerobic respiration (e.g., Piña-Ochoa, Høgslund, et al., 2010; Risgaard-Petersen et al., 2006).

(a) The sequestration of functional chloroplasts of diatom origin, a process generally referred to as kleptoplasty, has been documented for *N. labradorica* (Cedhagen, 1991; Jauffrais et al., 2019), and other foraminiferal species (e.g., Bernhard & Bowser, 1999; Grzymski et al., 2002; Jauffrais et al., 2018, 2019). The exact function of kleptoplasts in *N. labradorica* is not fully understood. While Cedhagen (1991) reported a detectable photosynthetic activity, a more recent study excluded a photosynthetic pathway in this species and suggested possible functioning in ammonium or sulfate assimilation pathways (Jauffrais et al., 2019). For the closely related *N. stella*, an active use of kleptoplasts in trophic pathways has been proposed (Grzymski et al., 2002) and demonstrated via transcriptomics (Gomaa et al., 2021), facilitating their survival under euxinic (anoxic, sulfidic) conditions.

The possible role of kleptoplasts on foraminiferal uptake or Mn availability in the foraminifers calcification environment during chamber addition has, to our knowledge, not been investigated to date. However, it has been shown that Mn plays a key role in diatom photosynthetic activity (i.e., manganese superoxide dismutase, essential to suppress oxidative stress associated with photosynthesis), the pathway of which is localized in the chloroplasts and can account for 10%–20% of the total cellular Mn in diatoms (Wolfe-Simon et al., 2006). Hence, the potential involvement of kleptoplasts in foraminiferal Mn availability is conceivable if foraminifera use the kleptoplastic photosynthetic pathway. Given the infaunal lifestyle of *N. labradorica*, however, photosynthetic activity is unlikely.

(b) The formation of cysts, typically composed of detritus or other foreign particles and temporally enveloping foraminiferal tests (e.g., Cedhagen, 1996; Heinz et al., 2005), has been documented for some foraminifera. The function of these cysts is not fully understood, although a connection to either feeding, growth, protection and/or reproduction has been proposed (e.g., Groß, 2002; Heinz et al., 2005 and references therein). Cysts create a micro-environment closely surrounding the foraminiferal tests, and processes occurring within this cyst environment may potentially alter TE concentrations. Similarly, markedly high and, on inter-specimen-level, variable Ba/Ca have been observed for the planktonic foraminifer *Neogloboquadrina dutertrei* and associated with its calcification in organic aggregate micro-environments of elevated [Ba] (i.e., marine snow; Fehrenbacher et al., 2018). In the fjord, cysts have been observed around tests of living (authors' observation) and rose bengal stained *N. labradorica* specimens (C. Choquel, personal communication), but, to our knowledge, not *B. marginata*—although it has been observed to form cysts in laboratory setups (authors' observations).

(c) Lastly, several foraminiferal species have been recognized for their ability to denitrify—reduce nitrate to nitrous oxide and/or dinitrogen—as a strategy of anaerobic respiration (e.g., Risgaard-Petersen et al., 2006). Transcriptomics of known denitrifying foraminifera recently showed the involvement of resident bacterial communities, specifically Desulfobacteraceae, in the reduction of nitrate (Woehle et al., 2022). Intracellular nitrate storage capacity or denitrification activity itself is known for representatives of rotaliid clades 1 and 3 (Piña-Ochoa, Høgslund, et al., 2010), to which also *Nonionellina* and *Bulimina*, respectively, belong (Schweizer et al., 2008). Nevertheless, the denitrification process and/or pathway have not been shown yet for either of the here studied
genera specifically, but for close relatives of *Nonionellina* (i.e., *Nonionella auris*, *N. stella* and *Nonionella* sp. T1) (Choquel et al., 2021; Glock et al., 2019; Gomaa et al., 2021; Woehle et al., 2022). Additionally, *G. turgida*, which exhibits similarly high Mn/Ca as *N. labradorica* (see earlier), has been shown to denitrify (Koho et al., 2011; Risgaard-Petersen et al., 2006), with a denitrification pathway phylogenetically related to that in nonionel-lids (Gomaa et al., 2021). Nitrate reduction and Mn oxidation—as electron donors—have been observed to be coupled in marine sediments (Luther et al., 1997), and Mn, as well as other redox trace metals, can have a positive effect on bacterial denitrification activity (e.g., Labbé et al., 2003). Thus, it is conceivable that the denitrifica-tion pathway in foraminifera does affect Mn incorporation, for example, by increasing [Mn] in the specimen's micro-environment and/or site of calcification.

Dedicated studies using high spatial resolution techniques to explore Mn concentrations and speciation in the micro-environment of foraminifera during calcification have the potential to elucidate biological effects on TE/Ca signals. Before such uncertainties are resolved, Mn/Ca in *N. labradorica* should not be used for direct inferences on [Mn]~PW~ and oxygenation histories. Species-specific geochemical applications may be preferred for quantita-tive reconstructions of past changes in Mn availability (i.e., oxygenation extent) to avoid biases introduced by contrasting biological effects on the proxy species' Mn/Ca.

### 4.2.4. Ontogenetic Trends in *Bulimina marginata*

Usually, ontogenetic (i.e., age and size-related effects), which have been shown to affect several foraminiferal TE/Ca (such as B/Ca, Ba/Ca, Mg/Ca; e.g., de Nooijer et al., 2017; Raitzsch et al., 2011), are interpreted to be a consequence of higher precipitation rates in initial chambers (e.g., Diz et al., 2012; Elderfield et al., 2002; Hintz et al., 2006). This is because faster precipitation compromises the control of the incorporation of specific elements, often resulting in comparatively higher TE/Ca. Barras et al. (2018) observed increasing Mn/Ca from older to younger chambers (i.e., from n-2 to n) in *B. marginata* under controlled laboratory conditions. There, ambient [Mn] were 10 and 100 µmol/L, comparable to our study. To explain this trend (and the inverted trend in another morphospecies, *Ammonia tepida* (Cushman, 1926)), a model was proposed describing the contribution of abiotic phases (i.e., adsorption of Mn²⁺ ions) between biotic calcite layers. In both scenarios, the observed ontogenetic trend was gradual. This is not the case in our specimens, nor did we observe statistically different Mn/Ca ratios in the last three chambers (i.e., *n* to n-2). The only significant difference was observed between the proloculus area (higher Mn/Ca) and the last cham-bers (lower Mn/Ca), with changes being clearly abrupt, as highlighted by the XRF mapping (Figure 5). Accord-ingly, these drastic changes—elevated Mn/Ca in initial chambers by a factor of up to 5–10—cannot be explained by known biomineralization mechanisms but indicate a predominantly environmental control.

Ontogenetic migration across redox gradients could potentially affect Mn/Ca suddenly if the foraminifer is exposed to changing ambient [Mn] in PWs throughout the precipitation of successive chambers. Similarly, Koho et al. (2017) suggested that Mn/Ca is not influenced by ontogenetic trends in benthic foraminifera but solely by spatial or temporal variability in ambient [Mn] based on observations of *n* − 2 to *n* − 12 chambers of *Elphidium batialis* Saidova, 1961, *Uvigerina akitaensis* Asano, 1950 and *Bolivina spissa* Cushman, 1926.

We hypothesize that the partially very high Mn signals in the proloculus reflect precipitation under high ambient Mn concentrations. Such may have been induced by highly reductive conditions in surface sediments, as facili-tated by extended periods of low-oxygen conditions in BWs of the fjord. While *B. marginata* has been described as tolerant to oxygen depletion, abundance data suggest that it responds positively to well-oxygenated condi-tions (e.g., Alve, 2003; Alve & Bernhard, 1995; Bouchet et al., 2018; Ernst et al., 2005; Filipsson et al., 2010). However, these observations are based on adult size-fractions, thus not necessarily reflecting on the ambient conditions during the growth of initial chambers. Indeed, Alve and Bernhard (1995) recorded a reproduction event of *B. marginata* under low-oxygen conditions. Accordingly, the last recorded period of extended low-oxygen conditions in the fjord at the end of 2017 (SMHI) may correspond to the time of precipitation of the juvenile chambers, particularly of those specimens sampled in September 2018. Specimens sampled at later dates during 2019, presumably calcified later and, indeed, show comparatively less elevated P chamber signals (Figure S2 in Supporting Information S1). We do not have data on *B. marginata* reproduction events during 2017–2019 to confirm this hypothesis. Alternatively, *B. marginata* may exhibit ontogeny-specific in-sediment migration patterns and calcify young life stages deeper in the sediment with high ambient Mn availability due to perma-nently anoxic conditions. This is conceivable, considering the bi-modal in-sediment distribution of the species (see earlier), and generally rapid migratory activity being known for *Bulimina* species (e.g., Ernst et al., 2005; Mackensen et al., 2000). For either scenario, a calcification of juvenile chambers under anoxic or low-oxygen
conditions is assumed—either by timing or active migration—, which is surprising. Less competition than during oxic conditions could be a factor.

Life-stage (and thus specimen-size) dependent effects are important to consider, particularly if TE/Ca is measured by bulk geochemical analyses, as signals combine potentially different calcification periods. However, the influence of the initial high-Mn chambers in *B. marginata* is negligible compared to the overall Mn/Ca signal (Figure S5 in Supporting Information S1). Potential life-stage-dependent trends should therefore not compromise the species' proxy potential with regards to inferring low-oxygen conditions (see earlier in Section 4.1.3), even based on TE data derived from several pooled specimens.

## 5. Conclusions

Fjords are highly sensitive settings with regard to environmental and geochemical variabilities. Our study demonstrates the potential of fjords for investigating the potential of geochemistry-based proxies for BW oxygenation conditions using biogenic Mn/Ca signals. In particular, the high Mn availability and BW oxygen variability of the land-ward fjord zone with its deep basin in GF provided conditions ideal for the calibration and application of the Mn/Ca proxy.

We highlight that benthic foraminiferal Mn/Ca has large potential as an indicator of BW oxygenation under low-oxygen conditions, but propose that it may only be effective in distinguishing changes in $[O_2]_{BW}$ up to c. 130 µmol/L $[O_2]$. This adds an upper oxygen limit for the robust application of the proxy, which utility is already suggested to be limited on the lower end of low-oxygen conditions (i.e., sub-, or anoxic) (e.g., Brinkmann et al., 2021; Koho et al., 2015). As the value of the proxy depends on both Mn availability and redox conditions, interpretations of oxygenation histories may vary regionally, and quantitative inferences may only be feasible for settings where specific redox geochemistry and Mn availability can be constrained.

The response of Mn/Ca to ambient [Mn] and $[O_2]$ is specific to the two investigated species, *B. marginata* and *N. labradorica*, with only the former showing the expected negative trend between Mn/Ca and BW oxygenation. It is imperative to consider biological effects on the incorporation of Mn in foraminifera; in-sediment migration, the presence of kleptoplasts, denitrification ability and/or cyst formation may be of particular significance in determining foraminiferal Mn/Ca. Further studies exploring potential biologically mediated alterations of Mn availability in the foraminiferal micro-environment are needed to further constrain and to better understand species-specific Mn/Ca variability.

## Data Availability Statement

The data generated during this study is available on the Swedish National Data Service (SND) under Brinkmann (2023) via https://doi.org/10.5878/50fg-7x12.

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## Acknowledgments

The authors thank the captain and crew of the R/V *Oscar von Sydow* and R/V *Skagerak* for technical assistance; Sami Jokinen and Hanna Nilsson, as well as the staff of the *Kristineberg* Center for Marine Research and Innovation for their support during the field campaigns; Tomas Næraa (Department for Geology, Lund University, Sweden) for technical guidance during LA-ICP-MS analyses and data processing. We thank the Editor and Reviewers for their thoughtful and valuable comments on a previous version of this paper. We acknowledge SOLEIL for the provision of synchrotron radiation facilities and the beamline NANO-SCOPIUM (proposal number 20181115). The synchrotron-based experiments were supported by CALIPSOPlus under the EU Framework Programme for Research and Innovation HORIZON 2020 (grant agreement 730872). This research has been supported by the Swedish Research Council VR (Grant 2017-04190), the Royal Physiographic Society in Lund, and the Crafoord Foundation. TJ acknowledges funding from the Academy of Finland. JMB acknowledges funding from The Investment in Science Fund at WHOL. The hydrographic data used in this project are from SMHI's database SHARK, funded by the Swedish Environmental Protection Agency.
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