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# Ocean Oxygenation Changes in the Arabian Sea Oxygen Minimum Zone During the Penultimate Glacial Cycle

Fang Qian, Yi Wang, Kassandra Costa, Sune Nielsen

► To cite this version:

Fang Qian, Yi Wang, Kassandra Costa, Sune Nielsen. Ocean Oxygenation Changes in the Arabian Sea Oxygen Minimum Zone During the Penultimate Glacial Cycle. Paleooceanography and Paleoclimatology, 2025, 40 (6), pp.e2024PA005059. 10.1029/2024PA005059. hal-05344383

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# Paleoceanography and Paleoclimatology®

## RESEARCH ARTICLE

10.1029/2024PA005059

**Key Points:**

* The Arabian Sea OMZ was slightly weaker during MIS 6 compared to MIS 5e, paralleling patterns observed during the LGM and the Holocene

* The weaker OMZ during MIS 6 was likely driven by productivity, oxygen solubility, and Southern-sourced intermediate waters delivery

* BWO levels from porosity are similar during MIS 6 and LGM, but higher oxygen levels are observed during MIS 5e compared to the Holocene

**Supporting Information:**

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

**Correspondence to:**

F. Qian,
fqian@tulane.edu;
fang.qian@whoi.edu

**Citation:**

Qian, F., Wang, Y., Costa, K. M., & Nielsen, S. G. (2025). Ocean oxygenation changes in the Arabian Sea oxygen minimum zone during the penultimate glacial cycle. *Paleoceanography and Paleoclimatology, 40*, e2024PA005059.
https://doi.org/10.1029/2024PA005059

Received 4 NOV 2024

Accepted 6 JUN 2025

© 2025. The Author(s).
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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## Ocean Oxygenation Changes in the Arabian Sea Oxygen Minimum Zone During the Penultimate Glacial Cycle

Fang Qian¹,²,³ ID, Yi Wang¹ ID, Kassandra M. Costa² ID, and Sune G. Nielsen²,³,⁴

¹Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA, USA, ²Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA, ³NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, MA, USA, ⁴Centre de Recherches Pétrographiques et Géochimiques, CNRS, Université de Lorraine, Vandoeuvre lès Nancy, France

**Abstract** Reconstructing past oxygen fluctuations in oxygen minimum zones (OMZs) is crucial for understanding their response to climate change. Numerous studies suggest better oxygenation in the Arabian Sea OMZ during the Last Glacial Maximum (LGM) compared to the Holocene. However, bottom water oxygen (BWO) variability during the Penultimate Glacial Cycle (Marine Isotope Stage [MIS] 6 to MIS 5e, ~140–115 ka B.P.) remains poorly constrained. This study reconstructs BWO variations during this period from sediment core TN041-8JPC in the western Arabian Sea OMZ, utilizing proxies including benthic foraminiferal surface porosity, redox-sensitive trace metal enrichment factors (e.g., U~EF~), and U/Ba ratios. Bottom water oxygen concentrations were 24.4 ± 5.9 µmol/kg during MIS 6 and 16.8 ± 6.5 µmol/kg during MIS 5e, with all proxies indicating higher BWO in MIS 6 than in MIS 5e. However, these proxies show different patterns within MIS 5e, indicating that U~EF~ and U/Ba ratios may be limited to recording average BWO in glacial and interglacial (quasi)steady states. We propose that the intensified OMZ during MIS 5e, relative to MIS 6, was driven by higher productivity, temperature-induced reductions in oxygen solubility, and reduced delivery of Southern-sourced intermediate waters. In contrast, the intensified OMZ during the Holocene, compared to the LGM, was likely influenced by lower oxygen solubility, reduced Southern water delivery, and winter convective mixing rather than productivity. This study highlights a general trend of weaker OMZs in glacial than interglacial periods, though the mechanisms may not be identical, offering insights into OMZ dynamics under climate change in the past.

## 1. Introduction

Oceanic dissolved oxygen variability significantly influences marine productivity, biodiversity, and biogeochemical cycles (Breitburg et al., 2018). It supports the respiration of heterotrophic organisms (e.g., bacteria, zooplankton, and fish) and regulates the cycling of essential nutrients such as nitrogen, carbon, and trace elements (Breitburg et al., 2018; Diaz & Rosenberg, 2008). Recent instrumental records suggest decadal oceanic deoxygenation, particularly marked by the intensification and expansion of Oxygen Minimum Zones (OMZs) (Keeling et al., 2010). This decline in dissolved oxygen may be driven by reduced oxygen solubility due to ocean warming and/or increased oxygen consumption from higher nutrient inputs (Breitburg et al., 2018; Schmidtko et al., 2017). However, the duration of dissolved oxygen observations (since ~1950s) limits our understanding of longer-term ocean oxygenation variability and its controlling mechanisms. Additionally, climate model simulations inconsistently predict future oceanic oxygenation responses to long-term climate change (e.g., Frölicher et al., 2020; Fu et al., 2018). Therefore, constraining the historical changes and underlying mechanisms of OMZs can help guide future projections.

The Arabian Sea contains one of the world's major OMZs, with midwater oxygen minima <20 µmol/kg dissolved O₂, and so it presents an ideal region for exploring oceanic oxygen responses to climate change in the past. Numerous studies have suggested that the OMZ in the Arabian Sea was weaker during the Last Glacial Maximum (LGM) compared to the Holocene (Altabet et al., 2002; Gaye et al., 2018; Jaccard & Galbraith, 2012; Lu et al., 2023; Pichevin et al., 2007; Reichart et al., 1998; Schmiedl & Leuschner, 2005). Notably, Lu et al. (2023) provided a quantitative reconstruction of bottom water oxygen (BWO) that demonstrated that oxygen concentrations were 10–15 µmol/kg higher in the LGM compared to the modern. However, high-resolution reconstructions of BWO for Marine Isotope Stages (MIS) 6 to 5e (140–115 thousand years before the present [ka B. P.]) remain limited. Reichart et al. (1998) suggested that periods of a weakened or even absent OMZ in the
northern Arabian Sea correspond to climatic cooling in the North Atlantic over the past 225 ka, based on proxies for surface water productivity, water column denitrification, and winter mixing. MIS 5e, one of the warmest interglacial periods in the last 800 ka, saw sea surface temperatures ~2°C warmer than pre-industrial levels in the Pacific (Lea, 2004) and ~1.5°C higher in the Arabian Sea (Saher et al., 2009), with global sea level 6–9 m higher than present (Dutton & Lambeck, 2012; Kopp et al., 2009). Therefore, constructing high-resolution quantitative BWO records in the Arabian Sea from MIS 6 across the termination into MIS 5e period will provide a benchmark for the mechanisms driving BWO variability during glacial/interglacial cycles as well as projections of future ocean oxygenation under anthropogenic forcing.

In this study, we employ three BWO proxies—benthic foraminiferal surface porosity, bulk elemental enrichment factors (e.g., U<sub>EF</sub>), and U/Ba ratio in bulk sediments—to reconstruct BWO changes from sediment core TN041-8JPC in the western Arabian Sea OMZ during the Penultimate Glacial Cycle (140–115 ka B.P.). Each of these proxies (benthic foraminiferal surface porosity, U enrichment factors, and U/Ba ratio) offer unique insights, with varying sensitivity and limitations, and combining multiple proxy types is recommended to achieve relatively robust qualitative or semi-quantitative BWO assessments (Jaccard et al., 2014). By assessing the applicability and limitations of these proxies and comparing OMZ variability in the western Arabian Sea across different glacial and interglacial periods, we aim to deepen our understanding of the driving mechanisms behind oxygen changes during the Last and Penultimate Glacial Cycles.

## 2. Proxy Descriptions and Applications

This study employs three proxies for BWO reconstruction: Benthic foraminiferal surface porosity, bulk elemental enrichment factors (e.g., U<sub>EF</sub>), and U/Ba ratio in bulk sediments.

First, the surface porosity of *Cibicidoides* spp. (Lu et al., 2021, 2022; Rathburn et al., 2018) has been mecha-
nistically linked directly to BWO changes. Benthic foraminiferal tests exhibit high surface porosity in low-
oxygen environments, likely as a biotic response to facilitate gas exchange, while under oxic conditions, respi-
ration primarily occurs through the aperture, the main opening on the test (Kaiho, 1994). Recent studies have
revealed a strong negative logarithmic correlation between BWO and surface porosity of *Cibicidoides* spp. when
BWO concentrations fall below 100 µmol/kg, with this pattern observed both globally and regionally in the
Arabian Sea (Lu et al., 2022; Rathburn et al., 2018). Additionally, benthic foraminiferal surface porosity appears
to be independent of other pore functions, such as organic carbon uptake and respiratory CO₂ release, indicating
that BWO is likely the primary factor controlling surface porosity for *Cibicidoides* spp. (Lu et al., 2022).

Second, enrichments of sedimentary trace metals (such as U, Mo, V, Re, Cd, and Mn) serve as qualitative in-
dicators of sedimentary oxygen conditions and may occur by precipitation and/or adsorption of metals in reducing
environments (Calvert & Pedersen, 2007; Froelich et al., 1979; Jaccard et al., 2009; Tribovillard et al., 2006). For
example, in oxygenated seawater, uranium (U) exists as soluble U(VI); as oxygen concentrations decrease in the
ambient bottom and/or pore waters, U(VI) is reduced to insoluble U(IV) and precipitates as solid UO₂, leading to
U enrichment in sediments (Anderson, 1982; Anderson et al., 1989; Morford & Emerson, 1999). However, trace
metal enrichments can be influenced by additional factors, such as organic carbon fluxes and post-depositional
diagenesis. High organic carbon fluxes, associated with high sedimentation rates, promote reducing conditions
that enhance the preservation of authigenic trace metal signals (e.g., Bradtmiller et al., 2010; Costa et al., 2018).
Conversely, when sedimentation rates are low and BWO concentrations increase, authigenic trace metals are
remobilized, leading to poor preservation (e.g., Bradtmiller et al., 2010; Costa et al., 2018). Post-depositional
diagenetic processes can also alter sedimentary trace metal concentrations (e.g., Anschutz et al., 2002; Deflan-
dre et al., 2002; Mangini et al., 2001). For instance, increased porewater oxygen can lead to U remobilization,
facilitating its diffusion into overlying bottom or pore waters and/or re-precipitation at deeper depths (e.g.,
Morford et al., 2009). These processes may decouple sedimentary trace metal signals from contemporaneous
water column oxygen variability. Consequently, trace metal data provide a qualitative, rather than quantitative,
indication of past BWO changes.

Finally, U/Ba ratios have recently been proposed as a quantitative oxygen proxy on local to regional scales,
including in the Arabian Sea (Costa et al., 2023). This proxy is best applied to the uppermost sedimentary layers
with porewater sulfate to avoid barite diagenesis interference (Costa et al., 2023). The regional calibration for the
Arabian Sea covers an oxygen range of 0~150 µmol/kg, and preliminary results show U/Ba-based oxygen
reconstructions align with those based on *Cibicidoides* sp. Surface porosity in the Arabian Sea (Costa et al., 2023). This suggests that U/Ba is a promising redox proxy for use in the Arabian Sea.

# 3. Materials and Methods

## 3.1. Materials and Study Area

The sediment core TN041-8JPC (17°48.76′N, 57°30.34′E, 761 m water depth) was retrieved from the center of the modern Arabian Sea OMZ (Figure 1) through the US Joint Global Ocean Flux Study (JGOFS) Arabian Sea Process Study. The core was split and subsampled at 4 cm intervals, covering a depth range of 166–900 cm. It is carbonate-rich and exhibits a high organic carbon content (>1 wt.%; Lu et al., 2022), with most foraminifera being well-preserved (this study). However, the uppermost section of this core may have experienced some material loss during coring, and previous studies from the last deglaciation also utilized sediment from the corresponding trigger core (TN041-8PG; Lu et al., 2023). A total of 91 samples of *G. ruber* were analyzed for oxygen isotopes to establish the age model. Additionally, 28 samples were processed for bulk sediment trace elements, and 27 samples (comprising 171 individuals) for benthic foraminifer porosity analysis, covering the period from 142 to 60 ka. Detailed sampling intervals are provided in the Table S1 in Supporting Information S1.

The Arabian Sea is one of the largest OMZs in the modern ocean, characterized by midwater oxygen minima (~200–1,200 m water depth, $O_2$ < 20 µmol/kg) that develop due to high surface productivity and a poorly ventilated thermocline (Banse et al., 2014; Olson et al., 1993; Resplandy et al., 2012). Several studies have demonstrated the dominant role of monsoon circulation in influencing the size and intensity of Arabian Sea OMZs (e.g., Gaye et al., 2018; Pichevin et al., 2007). Strong southwesterly summer monsoon winds (June–September) drive upwelling of cold, nutrient-rich water masses along the Somali and Omani coasts, enhancing primary productivity and particle flux in the Arabian Sea. In contrast, during the boreal winter (December–March), the dominant wind direction shifts to northeasterly, leading to lower sea surface temperatures and deeper convective mixing, promoting the delivery of dissolved oxygen to subsurface waters (Lachkar et al., 2018; Reichart et al., 1998; Rixen et al., 1996; Schmiedl & Leuschner, 2005). The intensified winter monsoon may also enhance productivity by promoting stronger surface cooling-driven mixing and aeolian input (e.g., Zhou et al., 2022).

Intermediate waters affecting Arabian Sea OMZs include high-density Red Sea Water, Persian Gulf Water, and the southern-sourced Indian Ocean Central Water, which is a mixture of aged Sub-Antarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and Indonesian Intermediate Water (Figures 1a; Olson et al., 1993; You & Tomczak, 1993; You, 1998). Persian Gulf Water and Red Sea Water are relatively oxygenated due to their recent contact with the atmosphere before being exported into the Arabian Sea (Sarma, 2002), with source oxygen concentrations of approximately 110 and 150 µmol/kg, respectively (Olson et al., 1993). In contrast, Indian Ocean Central Water is nutrient-rich and oxygen-poor due to oxygen consumption by organic carbon respiration in Southern-sourced intermediate waters (e.g., AAIW, SAMW) on their path to the Arabian Sea, with source oxygen concentrations (AAIW/SAMW) of approximately 250 µmol/kg (You, 1998).

## 3.2. Age Model and Stable Isotopes

All samples were freeze-dried, wet washed through a 63-µm sieve, and then oven-dried at 45°C. Approximately 5–6 specimens of the planktonic foraminifera *G. ruber* were picked from the 250–355 µm size fraction for stable isotopes. Carbon and oxygen isotope measurements were conducted using Finnigan MAT253 mass spectrometers at WHOI and Lamont-Doherty Earth Observatory, with long-term laboratory precision of ±0.05‰ for $\delta^{18}$O and ±0.02‰ for $\delta^{13}$C, based on the NBS-19 carbonate standard.

*G. ruber* $\delta^{18}$O has been widely used in paleoceanographic studies of the Arabian Sea to establish robust age models (e.g., Khan et al., 2022; Kim et al., 2018; Lathika et al., 2021; Lu et al., 2022; Saher et al., 2009). In this study, we also employed *G. ruber* $\delta^{18}$O to construct the chronology of core TN041-8JPC by correlating it with global benthic foraminiferal $\delta^{18}$O stack (Lisiecki & Raymo, 2005). To improve the accuracy of the age model, we applied a new technique using the software package BIGMACS (Bayesian Inference Gaussian Process regression and Multiproxy Alignment of Continuous Signals) (https://zenodo.org/records/8327654). This approach reduces uncertainties in both age and $\delta^{18}$O through Bayesian interference (Lee et al., 2023). The resulting age model spans the Penultimate Glacial Cycle, with an average deviation of 0.9 kyr within the 95% confidence interval, except for
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**Figure 1.** The Arabian Sea site and oceanographic setting. (a) Circulation and water masses in the Arabian Sea. The yellow star indicates the location of the studied core TN041-8JPC, while the white circles represent previously published cores discussed in this study. The red and blue dotted arrows denote summer and winter monsoon, respectively. Schematic pathways of the intermediate water masses in the Indian Ocean are shown by black arrows, adapted from You (1998). PGW, Persian Gulf Water; RSW, Red Sea Water; AAIW, Antarctic Intermediate Water; SAMW, Sub-Antarctic Mode Water; IIW, Indonesian Intermediate Water. (b) Dissolved oxygen profile of the water column for the western Arabian Sea. The yellow star indicates the modern bottom water oxygen concentrations at the core TN041-8JPC site. Map source: Ocean Data View (ODV; Schlitzer, 2015). Oxygen data sourced from the World Ocean Atlas 2018 (Garcia et al., 2019).
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Age model of core TN041-8JPC aligned to the global benthic foraminiferal $\delta^{18}$O stack LR04 (Lisiecki & Raymo, 2005) using *G. ruber* $\delta^{18}$O. (a) Age versus depth plot. The 95% confidence interval is indicated with dotted black lines and the median age model is displayed as a solid red line. (b) The 95% confidence interval width versus depth with $\delta^{18}$O ages. (c) Shifted and scaled *G. ruber* $\delta^{18}$O (red asterisk) with 95% error bars (red lines). The shaded region displays the upper and lower 2 sigma of the alignment target, with the solid black line (global $\delta^{18}$O stack) indicating the median.</figcaption></figure>

the last few data points (Figure 2). Although the deviations in the last few data points are somewhat high (~2–5 kyr), these discrepancies do not impact the chronology of MIS 6.

### 3.3. Foraminiferal Surface Porosity

Between 2 and 11 well-preserved specimens of the benthic foraminifera *Cibicidoides* spp. were selected in each sample (>250 µm size fraction). Following the methods outlined by Petersen et al. (2016) and Lu et al. (2022, 2023), we measured and calculated the surface porosity of the *Cibicidoides* spp. The preservation of *Cibicidoides* individuals was generally good, as evidenced by Scanning Electron Microscope (SEM) images (Figure 3). Additionally, *Cibicidoides* spp. have previously been used to reconstruct past BWO with no systematic porosity offsets (Lu et al., 2022; Rathburn et al., 2018). Accordingly, we calculated the average porosity values for each sample based on multiple *Cibicidoides* individuals and reported the standard deviations (1SD) for each sample to account for variability among the specimens.

Photographs were taken using a SEM (Hitachi model TM3000) at WHOI. A total of 171 individuals from core TN041-8JPC were scanned, and representative SEM images of *Cibicidoides* spp. are presented in Figure 3. The dorsal side, which is exposed to bottom water, of each specimen was imaged at 500 × magnification using a consistent scale for comparison.

Surface porosity in the penultimate chamber of all specimens was calculated using a semi-automatic software (ImageJ, available at https://imagej.nih.gov/ij/), which applies a grayscale threshold to a specific frame. We used the penultimate chamber rather than the last chamber because the last chamber is not consistently suitable for
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Representative Scanning Electron Microscope photos of *Cibicidoides* spp. In core TN041-8JPC, presented as in Lu et al. (2023).</figcaption></figure>

analysis and the penultimate chamber represents the same ontogenetic stage (Petersen et al., 2016; Rathburn et al., 2018). To apply a uniform-size frame to all SEM images, we utilized a macro developed by Petersen et al. (2016), which requires manual positioning of the frame. We selected a flat, interior section of the penultimate chamber, as it more accurately represents the surface area percentage compared to the curved portions of the shell.

### 3.4. Bulk Sediment Trace Elements and Enrichment Factors

All samples were freeze-dried, and a fraction of them (~10 mg) was ground into powder for bulk elemental analyses, following the methods described by Wang et al. (2022). The samples were digested with a mixture of 1 mL concentrated nitric acid (HNO₃), 1 mL hydrofluoric acid (HF), and 0.1 mL hydrochloric acid (HCl) at 135°C. Aqua regia (3:1 mixture of HCl and HNO₃) was subsequently used to dissolve the fluorides formed during the HF digestion. The dried samples were redissolved in 2% HNO₃ and diluted prior to analysis using a Thermo iCAP Q inductively coupled plasma mass spectrometer (ICP-MS) at WHOI. Two standard calibration curves were made: one including BHVO-2 [basalt] and AGV-2 [andesite] from the United States Geological Survey and the other based on high-purity ICP-MS elemental standards. Indium served as an internal standard to monitor instrument drift, and MESS-3 (continental margin sediments) from the National Research Council of Canada was used as the standard reference material, achieving an external recovery rate of 81%–108% for all certified trace
elements. The long-term reproducibility of the elemental concentrations was monitored by the relative standard deviation of repeated measurements of MESS-3, which remained below 10%.

The elemental enrichment factors of core TN041-8JPC were calculated using the following equation, with respect to the upper continental crust (UCC, Rudnick & Gao, 2003).

$$Element_{EF} = \frac{(Element/Al)_{sample}}{(Element/Al)_{UCC}}$$

UCC is often used as a global lithogenic end member, but the composition of lithogenic material likely varies on the regional scale as a function of the source composition (e.g., mafic, felsic, or carbonate rich substrates) and the transport mechanisms. The use of UCC is a (sometimes necessary) simplification that imparts a degree of uncertainty to the interpretation of the enrichment factors. It is possible that some variability in the enrichment factors may reflect variability in lithogenic composition rather than in BWO concentrations (e.g., Calvert & Pedersen, 2007). For example, mafic sources tend to have lower U content than felsic sources, while Al content shows less systematic variation (Rudnick & Gao, 2003). Consequently, variability in ratios such as U/Al may be induced from variable mixing proportions of mafic and felsic sediment sources, rather than enrichment of U (e.g., Calvert & Pedersen, 2007).

# 4. Results

## 4.1. BWO Reconstruction Based on Foraminiferal Surface Porosity

The benthic surface porosity values in core TN041-8JPC range from 14.6% to 30.1% and remained high during 140–60 ka BP (Figure 4c). In core TN041-8JPC, surface porosity is lower during MIS 6 (140–130 ka BP, average 18.1 ± 3.3%, n = 41) compared to MIS 5e (130–115 ka BP, average 23.3 ± 4.3%, n = 73) (Figure 4c).

Lu et al. (2022) found a strong negative logarithmic correlation between BWO and benthic surface porosity when oxygen levels are below 100 µmol/kg. Given the modern BWO in core TN041-8JPC is approximately 7 µmol/kg (Garcia et al., 2019), we applied the core-top porosity calibration equation from Lu et al. (2022) for the Arabian Sea, as follows:

$$\ln(\text{Porosity}) = 3.62 - 0.03 * \text{BWO}$$

Using this equation, the estimated BWO concentrations range from 7.2 to 31.2 µmol/kg, which is higher than modern BWO concentrations that are ~7 µmol/kg. The average BWO estimates based on surface porosity are 24.4 ± 5.9 µmol/kg during MIS 6, gradually decreasing to 16.8 ± 6.5 µmol/kg during MIS 5e (Figure 4d).

## 4.2. BWO Reconstruction Based on U/Ba Ratios and Bulk Elemental Enrichment Factors

The U enrichment factor (U<sub>EF</sub>) in core TN041-8JPC was relatively low during 140–130 ka, increased at the onset of MIS 5e, peaked at approximately 121 ka, and then exhibited a decreasing trend from ~115 to 60 ka (Figure 4a). The overall trend of the U<sub>EF</sub> parallels that of surface porosity, indicating relatively lower BWO during MIS 5e compared to MIS 6 (Figures 4a and 4c).

We applied the regional calibration functions for the Arabian Sea as published in Costa et al. (2023), which are applicable for ln (U/Ba) values between -4.87 and -1.31. The U/Ba values for core TN041-8JPC were low during MIS 6 (average ln(U/Ba) = -3.23) and began to increase from ~133 to 111 ka, with a mean ln(U/Ba) of -2.18 (Figure 4b). The highest U/Ba value (ln(U/Ba) = -1.53) occurred at the end of MIS 5e (~115 ka) (Figure 4b). For core TN041-8JPC during 140–60 ka, ln(U/Ba) values ranged from -3.42 to -1.53 (mean -2.54), corresponding to BWO concentrations ranging from 3.7 to 19.8 µmol/kg, with an average of 12.3 µmol/kg (Figure 4b). The BWO values inferred from ln(U/Ba) decreased during Termination II and MIS 5e (~133–115 ka) compared to MIS 6 (mean 18.2 µmol/kg). Although BWO concentrations exhibit a decreasing trend during Termination II, the specific timing of this change may not be reliable, as U can be susceptible to diagenetic remobilization during climatic transitions (Costa et al., 2023).
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Multi-proxy records of bottom water oxygen (BWO) changes from sediment core TN041-8JPC. (a) Enrichment factors of U relative to the upper continental crust. (b) ln(U/Ba) data (Purple circles) and BWO concentrations (lilac lines) based on U/Ba. Shaded regions indicate 1σ error. (c) *Cibicidoides* spp. surface porosity (red) and (d) its corresponding BWO records (blue). Error bars represent 1 standard deviation (SD). (e) Oxygen isotope data of *G. ruber* (black line) compared with the global benthic foraminiferal δ¹⁸O stack LR04 (blue line; Lisiecki & Raymo, 2005). Dash lines represent modern BWO concentration of core TN041-8JPC, extracted from WOA2018 (Garcia et al., 2019). Colored vertical shadings denote Marine Isotope Stages (MIS) 5e (interglacial period), Termination II (deglacial period) and MIS 6 (glacial period).</figcaption></figure>
# 5. Discussion

## 5.1. BWO Proxy Intercomparison During the Penultimate Glacial Cycle

We compare BWO reconstructions derived from benthic surface porosity, bulk sedimentary enrichment factors (e.g., U<sub>EF</sub>), and U/Ba in core TN041-8JPC, located in the modern Arabian Sea OMZ, to assess proxy consistency and differences during the Penultimate Glacial cycle (Figure 4). As discussed in Section 2.5, benthic foraminiferal surface porosity is mainly controlled by BWO levels, independent of other pore functions such as organic carbon uptake and respiratory CO<sub>2</sub> release (Lu et al., 2022). Therefore, we consider benthic foraminiferal surface porosity to be the most reliable indicator for reconstructing local BWO concentrations in this region. In core TN041-8JPC, BWO concentrations estimated from surface porosity are higher during MIS 6 compared to MIS 5e, indicating that the OMZ during the glacial period was better oxygenated by about 8 ± 6 µmol/kg (Figure 4d). U<sub>EF</sub> values were also lower during MIS 6 than MIS 5e (Figure 4a), suggesting higher local oxygenation during MIS 6, consistent with the results from surface porosity (Figures 4c and 4d). The BWO trend based on U/Ba in core TN041-8JPC is generally consistent with those inferred from surface porosity and sedimentary enrichment factors (Figure 4). These independent BWO proxy reconstructions (benthic surface porosity, bulk sedimentary enrichment factors, and U/Ba) collectively suggest that the western Arabian Sea OMZ was better oxygenated during the Penultimate Glacial period (MIS 6) compared to the Last Interglacial period (MIS 5e). This finding aligns with qualitative paleo-redox records from the northern Arabian Sea OMZ (Reichart et al., 1997, 1998), which also indicate a weaker OMZ during glacial periods like MIS 6.

Although the general BWO trends across these proxies are similar in core TN041-8JPC during glacial/interglacial periods, some discrepancies arise during MIS 5e. The lowest BWO based on benthic surface porosity occurs at ~127 ka, near the onset of MIS 5e, while the highest U<sub>EF</sub> values are recorded in the middle of MIS 5e (~121 ka), and the lowest BWO concentrations based on U/Ba is at the end of MIS 5e (~115 ka). Since all records are derived from the same core, these temporal offsets cannot be attributed to age model mismatches. Instead, they likely reflect the varying sensitivity and limitations of each proxy. In addition to seawater oxygen changes, sedimentary trace metal enrichments can be influenced by organic carbon flux to the sediments (e.g., Boudreau, 1994; Bradtmiller et al., 2010) and post-depositional diagenetic effects (e.g., Anschutz et al., 2002; Deflandre et al., 2002; Mangini et al., 2001). Thus, the preservation of U peaks may be influenced by subsequent variability in the oxygen penetration depth, potentially explaining the differing timing of peaks in U<sub>EF</sub> and porosity records during MIS 5e. In addition, this discrepancy might also be attributed to different dominant factors influencing U<sub>EF</sub> and porosity proxies (e.g., productivity and Southern-sourced intermediate waters delivery). For instance, given the close alignment between the general trends of U<sub>EF</sub> (Figure 4a) and the productivity record (Figure 5i; Reichart et al., 1998), it is likely that organic carbon fluxes play an important role in driving productivity changes in U<sub>EF</sub>.

For the U/Ba proxy, Costa et al. (2023) suggested that it is most applicable to BWO concentrations in regions with >50 µmol/kg oxygen and high oxygen variability (several tens of µmol/kg). Consequently, this proxy may be less effective at recording the small BWO changes in core TN041-8JPC within MIS 5e but may be more suited for recording the amplitude of mean states (over ~10s kyrs), such as during MIS 5 and MIS 6.

In summary, benthic surface porosity, bulk sedimentary enrichment factors (e.g., U<sub>EF</sub>), and U/Ba reconstructions from core TN041-8JPC all suggest higher BWO during MIS 6 and lower BWO during MIS 5e, with no significant changes during Termination II. However, these proxies exhibit different patterns during MIS 5e, indicating that bulk sedimentary enrichment factors and U/Ba proxies may be more reliable for recording average glacial and interglacial oxygen concentrations than for capturing the nuances of climate transitions.

## 5.2. Comparison of BWO Changes Between the Last and Penultimate Glacial Cycles

We compare published BWO reconstructions from core TN041-8PG/JPC (Lu et al., 2022) during the Last Glacial Cycle (30–0 ka B.P.) with our records from the Penultimate Glacial Cycle (140–115 ka B.P.) in the western Arabian Sea OMZ to investigate oxygenation responses across different glacial and interglacial periods. Following the methods outlined in Section 2, we calculated the U enrichment factor (U<sub>EF</sub>) and ln(U/Ba) for core TN041-8PG/JPC based on bulk element data from Lu et al. (2022). Both the U<sub>EF</sub> and ln(U/Ba) show higher values during the Holocene compared to the LGM, indicating a weaker OMZ during the LGM, and no significant millennial-scale variations (e.g., Younger Dryas) were observed during Termination I. Similarly, benthic foraminiferal surface porosity exhibits an increasing trend from the LGM to the early Holocene, remaining high
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Comparison of Bottom Water Oxygen (BWO) variations in the western Arabian Sea OMZ between the Last and Penultimate Glacial Cycles for core TN041-8PG/JPC. (a, f) Global benthic foraminiferal $\delta^{18}$O stack (LR04; Lisiecki & Raymo, 2005). (b, g) Benthic surface porosity and corresponding BWO reconstructions, with error bars representing 1 standard deviation (SD). (c, h) Sea surface temperature (SST) derived from alkenone records of core RC-27-42 and from foraminifera Mg/Ca ratios of core NIOP929 (Pourmand et al., 2007; Saher et al., 2009). (d, i) Organic carbon content ($C_{org}$) from core NIOP 464 (Reichart et al., 1998). (e, j) Neodymium isotopic composition ($\delta$Nd) data from cores SK-17 (Shukla et al., 2025), MD77-176 (Yu et al., 2018) and ODP 1087 (Hu et al., 2016). Gray shading represents glacial periods (i.e., Last Glacial Maximum, MIS 6). Blue shading represents glacial to interglacial transitions (deglacial periods), specifically Termination I (i.e., last deglaciation) and Termination II. Pink shading represents interglacial periods, including the Holocene and MIS 5e.</figcaption></figure>

throughout the Holocene (Figure 5b; Lu et al., 2022). Correspondingly, BWO concentrations declined from ~20 µmol/kg during the LGM to ~10 µmol/kg during deglaciation, stabilizing at ~6 µmol/kg during the Holocene (Lu et al., 2022). This pattern supports the interpretation of a better-oxygenated OMZ during glacial periods (Figure 5b), and it aligns with other studies that used different paleo-redox indicators (Altabet et al., 2002; Gaye et al., 2018; Pichevin et al., 2007; Reichart et al., 1998). For example, lower bulk sedimentary $\delta^{15}$N values during the LGM indicate less denitrification and a weaker OMZ in the glacial Arabian Sea (Altabet et al., 2002; Gaye et al., 2018).
Our results for the Penultimate Glacial Cycle similarly show higher BWO during MIS 6 compared to MIS 5e, using benthic surface porosity, $U_{EF}$, and U/Ba (Figures 4a–4c). Thus, BWO records from both the Last and Penultimate Glacial Cycles suggest that BWO concentrations in the western Arabian Sea were higher during glacial periods (MIS 6 and LGM) and lower during interglacial periods (MIS 5e and Holocene). Notably, the absence of significant millennial-scale BWO variations during Terminations I and II (Figure 5) suggests that these proxies may reflect stable controlling factors (e.g., delivery of Southern-sourced intermediate waters, productivity) that did not respond strongly to changing climate across the deglaciations (e.g., Hu et al., 2016; Reichart et al., 1998).

BWO concentrations derived from surface porosity show similar values during glacial periods (LGM and MIS 6), but they disagree during interglacial periods (MIS 5e and the Holocene) (Figures 5b and 5g). BWO concentrations derived from surface porosity suggest higher BWO during MIS 5e (~16.8 ± 6.5 µmol/kg) than during the Holocene (~6 ± 7 µmol/kg). As discussed in Section 4.1, we consider BWO estimates based on benthic foraminiferal surface porosity to be the most reliable, and thus we interpret this signal to reflect a real difference in mechanisms driving BWO variability in these time periods. This would imply that the Arabian Sea OMZ was more pronounced during the Holocene compared to MIS 5e. The discrepancies between proxies during the Last and Penultimate Glacial Cycle may reflect differing influences on OMZ dynamics, which we explore further below.

### 5.3. Potential Driving Mechanisms of BWO Changes in the Arabian Sea

As outlined in Section 2.1, the size and intensity of the modern Arabian Sea OMZ are influenced by both limited oxygen supply from ocean circulation and high productivity driven by the Indian monsoons (Keeling et al., 2010; Olson et al., 1993; Pichevin et al., 2007). The summer monsoon enhance productivity by upwelling nutrient-rich waters along the Somali and Omani coasts (Keeling et al., 2010; Olson et al., 1993; Pichevin et al., 2007), leading to higher oxygen consumption. The winter monsoon may promote productivity through stronger surface cooling-driven mixing and increasing aeolian input during glacial periods (e.g., Zhou et al., 2022). To investigate the potential drivers of BWO variations during the Last and Penultimate Glacial Cycles, we compare our BWO results from core TN041-8PG/JPC, located in the western Arabian Sea OMZ, with existing proxy data on productivity, sea-surface water temperature (SST), and Southern-sourced intermediate waters (e.g., AAIW).

#### 5.3.1. Influence of Productivity on BWO Changes

In the northern Arabian Sea OMZ (NIOP 464; 1,470 m water depth), multiple proxies for surface productivity (e.g., organic carbon content, abundance of the upwelling-favorable planktonic foraminifera *G. bulloides*, bulk sedimentary $\delta^{15}$N, bulk sedimentary Ba/Al) found consistent evidence that OMZ intensity is positively correlated with summer surface productivity (Reichart et al., 1998). When summer surface productivity is high, respiration of the corresponding organic-rich material at depth leads to oxygen drawdown and contributes to a stronger OMZ (Reichart et al., 1998). During MIS 5e, organic carbon content peaked, suggesting higher surface productivity compared to MIS 6 (Figure 5i; Reichart et al., 1998). Therefore, one interpretation may be that the weaker OMZ during MIS 6 could be attributed to reduced productivity relative to MIS 5e (Figures 6c and 6d).

Notably, the productivity changes from glacial to interglacial during the Penultimate Glacial Cycle contrast with the trends observed during the LGM and Holocene (Figures 5d and 5i; Reichart et al., 1998), despite similar changes in BWO (Figures 5b and 5g). Paleo-productivity reconstructions for the Last Glacial cycle suggest generally higher productivity in the Arabian Sea during the LGM compared to the Holocene (e.g., Hernández-Almeida et al., 2019; Reichart et al., 1997; Rogalla & Andruleit, 2005; Zhou et al., 2022). For instance, organic carbon content from sediment core NIOP 464 exhibit a significant decline from the LGM to the early Holocene (Figure 5d; Reichart et al., 1998). This higher productivity during the LGM likely increased oxygen consumption in the water column, potentially intensifying the OMZ. Yet, despite this increase in productivity during the LGM, existing records show higher BWO concentrations during this period (Figure 5b), indicating that productivity may not have been the primary driver of bottom water oxygenation (Lu et al., 2023). The higher productivity during the LGM might be attributed to the combined effects of stronger surface cooling-driven mixing and stronger Aeolian input driven by the winter monsoon (Zhou et al., 2022). The stronger winter monsoon during the LGM could have promoted deeper convective mixing and transported oxygenated waters into the OMZ (Figures 6a and 6b;
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Schematic diagrams showing the potential driving mechanisms of the Arabian Sea oxygen minimum zone (OMZ) during different glacial/interglacial periods: (a) Holocene, (b) Last Glacial Maximum, (c) Marine Isotope Stage (MIS) 5e, and (d) MIS 6. Solid arrows represent intensified summer monsoon (orange), winter monsoon (blue), upwelling (green), and increased delivery of Southern-sourced intermediate waters (mauve), while dashed arrows denote their weakening. More green circles reflect higher productivity associated with the monsoon, and more yellow circles indicate increased oxygen solubility.</figcaption></figure>

Reichart et al., 1998; Lu et al., 2023). Overall, changes in productivity are unlikely to be the primary driver of BWO changes between glacial periods and interglacial periods in the Arabian Sea.

### 5.3.2. Impact of Intermediate Water Masses Delivery on BWO Changes

As noted in Section 2.1, variable inputs of Red Sea Water, Persian Gulf Water, and aged SAMW/AAIW can theoretically affect the supply of oxygen to the Arabian Sea OMZ (Figures 1a; Olson et al., 1993; You & Tomczak, 1993; You, 1998). However, during glacial periods, lower sea levels caused the Persian Gulf to dry up and reduced Red Sea outflow (e.g., Rohling & Zachariasse, 1996), limiting the contribution of Red Sea Water and Persian Gulf Water to the Arabian Sea OMZ. Several studies suggest that during glacial periods, relatively oxygen-rich AAIW may have exerted a greater influence on Arabian Sea intermediate waters, delivering more dissolved oxygen (e.g., Jung et al., 2009; Kim et al., 2018).

The neodymium isotope record (εNd) of planktonic foraminifera from core ODP 1087 (1,372 m) in the southeast Atlantic, influenced by Antarctic Intermediate Water/Upper Circumpolar Deep Water, suggests increased delivery of Southern-sourced intermediate waters (e.g., AAIW) toward the core site during MIS 6 (Figure 5j; Hu et al., 2016). Similarly, εNd record from core Y9 (1,267 m) in southwest Pacific also show increased AAIW influence during MIS 6 (Hu et al., 2016). These εNd records from different basins suggest a generally enhanced delivery of Southern-sourced intermediate waters (e.g., AAIW) further north during MIS 6 (Hu et al., 2016). Based on these findings, it appears possible that more oxygenated Southern-sourced intermediate waters could have reached the Arabian Sea OMZ during MIS 6.

Comparative evidence can be obtained from εNd records of cores MD77-176 (1,325 m water depth) in the northern Indian Ocean and SK-17 (840 m water depth) in the eastern Arabian Sea during the LGM-Holocene transition (Figure 5e; Yu et al., 2018; Shukla et al., 2025). The εNd record from MD77-176 has a clearly defined increase in the delivery of Southern-sourced intermediate waters (e.g., AAIW) during the LGM (Figure 5e; Yu et al., 2018). However, the distinct relevance of this εNd record to OMZ dynamics remains uncertain, because the additional influence of freshwater delivery from the Ganges-Brahmaputra River system may take the oxygen dynamics of the Bay of Bengal unique. Given that AAIW may follow a similar pathway into both
the northern Indian Ocean and the Arabian Sea (Figure 1), its influence on the Arabian Sea OMZ during the LGM cannot be ruled out.

The εNd record from core SK-17 also indicates that AAIW could substantially influence the Arabian Sea, particularly during the Younger Dryas (Figure 5e; Shukla et al., 2025). However, its temporal pattern is not entirely consistent with the observed BWO variations at our site (Figures 5b and 5e). Specifically, the εNd data suggest only minor differences in AAIW influence between the LGM and Holocene, with a ~0.8 εNd-unit difference between these periods. Much of the variability appears masked by the pronounced shifts during YD (Figure 5e; Shukla et al., 2025), raising questions about whether AAIW was the primary driver of oxygenation changes in the Arabian Sea OMZ. Nevertheless, all available evidence supports the possibility of AAIW advection into the Arabian Sea during LGM. Therefore, while the delivery of Southern-sourced intermediate waters (e.g., AAIW), or other large-scale circulation changes during the glacial periods (e.g., LGM and MIS 6), may have contributed to slightly higher BWO levels in the Arabian Sea OMZ, additional factors likely played a role in shaping these variations.

### 5.3.3. Role of Temperature-Driven Oxygen Solubility in BWO Changes

Changes in ocean temperature and salinity can also affect oxygen solubility (e.g., Garcia & Gordon, 1992). The SST based on the *G. ruber* Mg/Ca ratio from core NIOP 929 (2,490 m water depth) in the western Arabian Sea was significantly lower during MIS 6 compared to MIS 5e (Figure 5h; Saher et al., 2009), which could have contributed to higher oxygen solubility during MIS 6, and thus potentially explain the higher BWO. The SST record derived from alkenone analysis of core RC-27-42 in the western Arabian Sea indicates cooler conditions during the LGM (~23°C) compared to the Holocene (~26°C) (Figure 5c; Pourmand et al., 2007). No bottom water temperature records exist at our site, but the ocean, on average, was ~2.5°C colder (Bereiter et al., 2018) and 3% saltier during the LGM relative to the Holocene (Adkins et al., 2002). Starting from the modern temperature (10.8°C) and salinity (35.6) at our core depth and using the solubility equation of Garcia and Gordon (1992), the mean LGM conditions would result in 13.5 µmol/kg higher oxygen, which is close to the magnitude of the glacial-interglacial oxygen variations we observe (e.g., a ~14 µmol/kg difference between LGM and Holocene). However, the mean ocean temperature reconstruction is volumetrically skewed toward the temperature change of deep waters, and it is likely that thermocline waters, such as at our site (761 m), did not cool by so much. The amplitude of subsurface temperatures changes in (sub)tropical regions during the LGM and Holocene is significantly lower than that of mean ocean temperature, with an estimated average subsurface temperature variation of only ~1°C (e.g., Qian et al., 2023). If this more muted cooling also occurred in the Arabian Sea, then the oxygen content would only have been higher by 3.4 µmol/kg, insufficient to explain the full glacial-interglacial amplitude observed (Figure S1 in Supporting Information S1).

Based on the above calculations (Figure S1 in Supporting Information S1), it is likely that there is some contribution of temperature on glacial-interglacial oxygen variability, but maximal glacial cooling would be required for oxygen solubility to be the only explanation. We propose that the higher oxygenation levels observed in the Arabian Sea OMZ during the LGM relative to the Holocene, were likely also influenced by deeper convective mixing and delivery of Southern-sourced intermediate waters (Figures 6a and 6b).

### 5.3.4. Drivers of BWO Changes During the Last Versus Penultimate Glacial Cycles

Based on the preceding discussions, we propose that the more intense Arabian Sea OMZ during MIS 5e compared to MIS 6 was likely driven by a combination of increased productivity from stronger summer monsoon activity, lower oxygen solubility due to higher ambient water temperatures, and reduced influx of Southern-sourced intermediate waters (Figures 6c and 6d). For the LGM and Holocene, changes in temperature-driven oxygen solubility, the delivery of Southern-sourced intermediate waters, and winter convective mixing all likely influenced the Arabian Sea OMZ (Figures 6a and 6b).

Notably, BWO concentrations derived from surface porosity show similar values during glacial periods (MIS 6 and LGM) but diverge during interglacial periods (MIS 5e and Holocene), with higher BWO during MIS 5e than during the Holocene (Figures 5b and 5g). These similarities/discrepancies between the Last and Penultimate Glacial Cycles likely reflect differing influences on OMZ dynamics. Considering the most probable factors affecting BWO during the Last Glacial Cycle (Southern-sourced intermediate waters, oxygen solubility, and winter convective mixing) and Penultimate Glacial Cycle (productivity, oxygen solubility, and Southern-sourced
intermediate waters), we propose that the higher BWO during the MIS 5e compared to the Holocene may be attributed to relatively increased delivery of Southern-sourced intermediate waters (e.g., AAIW) during MIS 5e (Figures 6a and 6c), which could offset the higher oxygen consumption caused by elevated productivity (Figures 5d and 5i). Similarly, despite higher productivity (and thus more oxygen consumption) during the LGM compared to MIS 6 (Figures 5d and 5i), the comparable BWO values observed between these periods suggest that either more Southern-sourced intermediate waters (e.g., AAIW) delivery during LGM, relative to MIS 6, or winter mixing, played a more significant role in controlling BWO concentrations during the LGM (Figures 6b and 6d). However, due to the lack of direct evidence for AAIW delivery (e.g., εNd record) during MIS 5e and MIS 6 in the Arabian Sea, the strength of AAIW delivery between the last and penultimate glacial cycles remains uncertain. Therefore, the above interpretation is speculative, suggesting that the delivery of Southern-sourced intermediate waters (e.g., AAIW) may play a significant role in influencing BWO variability, albeit with varying magnitudes across different glacial or interglacial periods. However, the potential contributions of other factors (e.g., oxygen solubility) cannot be ruled out.

## 6. Conclusion

Our study presents a multi-proxy reconstruction of BWO changes in the western Arabian Sea OMZ during the Penultimate Glacial Cycle. BWO concentrations, as inferred from benthic foraminiferal surface porosity, were approximately 8 ± 6 µmol/kg higher during MIS 6 compared to MIS 5e. Additionally, bulk sedimentary enrichment factors (e.g., U~EF~) and U/Ba ratios also peaked during MIS 5e, suggesting a more intense OMZ at that time. These independent proxies collectively suggest that the western Arabian Sea OMZ was better oxygenated during MIS 6 compared to MIS 5e. Discrepancies in BWO concentrations across proxies for MIS 5e may reflect the sensitivity and limitations of different BWO proxies. Our reconstructions in the Penultimate Glacial Cycle also align with patterns observed during the Last Glacial Cycle, where the OMZ was weaker during the LGM compared to the Holocene. We suggest that the weaker OMZ during MIS 6, compared to MIS 5e, was likely driven by a combination of reduced productivity, higher oxygen solubility due to lower ambient water temperatures, and enhanced oxygen supply via delivery of Southern-sourced intermediate waters (e.g., AAIW). In contrast, productivity may not have been the primary driver of enhanced oxygenation in the western Arabian Sea OMZ during the LGM; instead, it was more significantly affected by temperature-driven increases in oxygen solubility, delivery of Southern-sourced intermediate waters, and deeper winter convective mixing. Notably, BWO concentrations derived from surface porosity suggest higher values during MIS 5e (16.8 ± 6.5 µmol/kg) than during the Holocene (6 ± 7 µmol/kg), while similar values are observed during MIS 6 and LGM. This suggests that the delivery of Southern-sourced intermediate waters might have had a significant yet variable role in influencing BWO levels during different glacial or interglacial periods, though other factors, such as oxygen solubility, cannot be excluded.

## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

## Data Availability Statement

Data generated in this study are archived at Qian (2025).

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