Oceanic deoxygenation linked to enhanced water column stratification in the eastern Tethys during Cretaceous Oceanic Anoxic Event 2

Intensified water column hypoxia drives disproportionate benthic P release and N:P imbalance via enhanced sedimentary sulfate reduction.

Abstract Oxygen in the global oceans has declined since the 1960s, including in the Eastern Equatorial Pacific (EEP). Reconstructions of EEP glacial oxygenation help advance understanding of current and projected ocean deoxygenation. Previous estimates of the glacial oxygen deficient zones (ODZs) in the upper EEP are poorly constrained. Here we include sediment cores that extend shallower into the ODZ and quantitatively reconstruct the glacial oxygen depth profile. We find glacial oxygen levels were similar or slightly lower than modern in the upper‐intermediate ocean and much lower in the deep EEP. We estimate ∼95% of the lower glacial oxygen occurred in the deep EEP. This contrasts with modern, where only 67% of oxygen loss is in the deep ocean, and could be due to a smaller role of temperature‐dependent oxygen solubility in modulating oxygen on longer timescales and/or a longer ocean response time to record oxygen changes at depths during glacial times.

ABSTRACTClimate change alters the oceans' temperature, pH, and oxygen concentration. These changes are expected to increase globally over the coming decades, affecting a wide range of marine organisms. Coastal upwelling zones, characterized by their high environmental variability, serve as ideal natural laboratories to study the potential impacts on marine organisms and ecosystems of temperature change, acidification, and ocean deoxygenation. The estimation of survival using capture‐mark‐recapture (CMR) data has been commonly applied to vertebrates, and to date, very few studies have been done on marine invertebrate organisms. In this study, we combined field CMR data and laboratory measurements to assess the physiological responses (metabolic rate and heart rate) and survival probability of individuals in two populations of intertidal mollusks, Chiton granosus and Scurria zebrina, in contrasting upwelling environments (i.e., semi‐permanent vs. seasonal). We found that (1) there are no differences between the two studied populations for heart rate in both species, (2) the S. zebrina population subjected to seasonal upwelling has a higher metabolism, (3) there are no differences in the calcification rate between the two studied populations of both species, and (4) survival is significantly higher in the semi‐permanent upwelling location for both species. Our findings highlight species‐specific responses to contrasting upwelling regimes, suggesting that phenotypic plasticity and survival differences may influence resilience under ongoing climate change.

Introduction Coastal ecosystems are vital blue carbon sinks that are increasingly threatened by climate change. Their vulnerability and sensitivity are strongly influenced by ecological structures and local conditions. Methods Using the Ecopath model, we evaluated the responses of three bays in Fujian, China (Sansha Bay, Fuqing Bay, and Xinghua Bay) under scenarios of increased precipitation, ocean deoxygenation, and warming. The analysis focused on how differences in food web structures shape ecosystem responses to climate pressures and determine their sensitivity and vulnerability. Results The results revealed differentiated response patterns dictated by baseline food web characteristics, including constant, linear, and non-linear threshold collapse, in which the baseline ecological structure of a bay dictates its degree of vulnerability. Fuqing Bay, despite having the lowest total system throughput (2405 t/km²/year), showed the highest resilience and bivalve ecological carrying capacity (22.80 t/km²). In contrast, Xinghua Bay, a high biomass system (Total Biomass: 39.93 t/km²), exhibited the highest sensitivity, with its food web structure collapsing even under low deoxygenation stress (shrimp EE > 1). Under severe warming, bivalve ecological carrying capacity declined linearly by up to 50% across all the bays, with absolute losses being the greatest in the most productive systems. Discussion Our findings underscore the critical role of baseline ecosystem structure in shaping divergent climate responses and provide a scientific basis for site-specific adaptive management and blue carbon conservation strategies.

It is currently debated whether Earth system models (ESMs) can reproduce observation-based long-term changes in global and regional deoxygenation rates. Both models and observations include uncertainties, which must be considered when evaluating their consistency. Based on 14 ESMs and 6 observational datasets, the models’ climatological annual mean oxygen matches observations well near the surface. However, significant biases remain in the tropics and in the thermocline. Based on the same set of models and three time-varying observation-based datasets, the models tend to underestimate deoxygenation trends from 1965 to 2014, except for the North Atlantic basin. However, the small number of observational datasets limits this conclusion. One dataset appears to significantly underestimate deoxygenation due to sparse data coverage. This review highlights the need for improvements in model process representations and the development of more observation-based, quality-controlled datasets to better constrain and interpret oxygen changes in the ocean. ▪ Uncertainties of dissolved oxygen field in CMIP6 ESMs and observational reconstructions are quantified. ▪ The ESMs can skillfully reproduce long-term average oxygen near the surface, but challenges remain in the thermocline and tropics. ▪ The ESMs underestimates the deoxygenation trends except for the North Atlantic basin.

Abstract. Ocean deoxygenation, driven by climate change, poses significant challenges to marine ecosystems and can profoundly alter nutrient and carbon cycling. Quantifying the rate and regional patterns of deoxygenation relies on spatio-temporal interpolation tools to fill gaps in observational coverage of dissolved oxygen. However, this task is challenging due to the sparsity of observations, and classical interpolation methods often lead to high uncertainty and biases, typically underestimating long-term deoxygenation trends. In this work, we develop a novel gridded dissolved oxygen product by integrating direct oxygen observations with machine-learning-based emulated oxygen estimates derived from temperature and salinity profiles. The gridded product is then generated through optimal interpolation of both the observed and emulated data. The resulting product (Ouala et al., 2025; https://doi.org/10.5281/zenodo.15478088) shows strong agreement with baseline climatology and captures well-known patterns of seasonal variability and long-term deoxygenation trends. It also outperforms current state-of-the-art products by more accurately capturing dissolved oxygen variability at synoptic and decadal scales, and by reducing uncertainty around long-term changes. This study highlights the potential of combining machine learning with classical interpolation methods to generate improved gridded biogeochemical products, enhancing our ability to study and understand ocean biogeochemical processes and their variability under a changing climate.

Due to global warming and the excessive input of nutrients resulting from human activities, ocean deoxygenation is gradually intensifying. However, the sparse spatiotemporal distribution of in situ global dissolved oxygen (DO) observations poses a significant challenge to understanding the spatiotemporal variations of DO in the world’s oceans. In this study, we employed a SOM-FFNN approach to reconstruct a global monthly dissolved oxygen dataset from 1960 to 2021, with high spatial (1° × 1°) and vertical (to 2000 m) resolution. Compared to existing products, our model exhibits a lower RMSE (15.36 μmol/kg), a higher R² (0.96), and strong agreement with long-term observations, outperforming other available datasets. Analysis of this dataset shows that the global ocean has been experiencing a steady loss of total oxygen content since the late 1980s, with the rate of depletion over the past decade nearly twice as high as that during 1980–2010. Further investigation reveals that low-oxygen zones have expanded both horizontally and vertically, with this expansion also intensifying in the past decade. These findings highlight the accelerating nature of ocean deoxygenation and the growing extent of low-oxygen habitats.

Ocean deoxygenation, driven by warming and eutrophication, poses an escalating threat to marine biodiversity. In tropical coastal ecosystems, diel cycles naturally produce substantial oxygen variability, with nighttime deoxygenation driven by community respiration and daytime oxygenation from intense photosynthesis, regularly reaching levels of supersaturation (>100% air saturation). Critically, these supersaturation peaks coincide with peak daily temperatures, raising the question of whether elevated oxygen levels can buffer organisms against acute thermal stress. To test this, we exposed newly hatched Amphiprion bicinctus larvae to three oxygen regimes-deoxygenation (approx. 4 mg l⁻¹), air saturation (approx. 6 mg l⁻¹) and supersaturation (approx. 12 mg l⁻¹)-under ramping temperatures (+0.5°C h⁻¹, 30-34°C). Larvae under deoxygenation exhibited the lowest lethal temperature for 50% mortality (LT₅₀= 30.2°C), while both air-saturated (31.3°C) and supersaturated (31.54°C) treatments improved thermal resistance. Mortality remained lower under supersaturation up to 31.5°C, with slower declines than in other treatments. Additionally, parental care increased daytime oxygen in egg masses by approximately 51%, enhancing oxygenation. These results show that oxygen supersaturation-naturally occurring during thermal peaks-can mitigate physiological stress during development. This highlights an underrecognized role of diel oxygen dynamics in climate resilience, with implications for the management of marine nurseries, fisheries and aquaculture under warming conditions.

Much of the variability in methane seep macrofaunal communities has been attributed to seepage activity (i.e., fluid flux regime); however, more attention is needed to other environmental factors that might be playing a role in structuring methane seep communities. A primary goal was to understand how depth and bottom-water dissolved oxygen concentration affect the influence of seep activity on the diversity and trophic structure of carbonate macrofauna and their recovery and resilience. We conducted mensurative and manipulative experiments on the Costa Rican Pacific margin at three seep locations with varied hydrographic conditions: (1) Quepos Landslide, at 400 m deep, within the oxygen minimum zone (OMZ); (2) Mound 12, at 1,000 m deep, just below the OMZ; and (3) Jaco Scar, at 1,850 m deep, well below the OMZ. Within locations, experiments were conducted at active, transition, and background seep habitats. Habitat was the main factor influencing macrofauna at the deeper seeps, where chemosynthetic production supplies the primary food source. Seep-specialist species found in active habitats exhibited faster responses to colonization and transplant experiments mimicking seep activation than species found in transition habitats. Species in transition habitats, relying on both photo- and chemosynthetic production, appeared to have higher recovery and resilience rates in experiments mimicking seep cessation than seep specialists. Within the OMZ, low oxygen conditions overrode the effects of habitat, yielding low densities and low diversity to the point of limiting colonization and community retention, as observed through manipulated changes in seep habitat. Our study highlights how environmental factors (i.e., seep habitat, depth, and oxygen concentration) promote macrofaunal heterogeneity on carbonates at methane seeps and might control resilience. With the expansion of OMZs and seeps due to ocean deoxygenation and warming, respectively, an understanding of how environmental factors affect the resilience and recovery of these communities is important.

Ocean deoxygenation, increasingly driven by human activities and climate change, severely threatens ocean health. Although manganese (Mn) plays a pivotal role in sediment biogeochemistry, the dynamics of Mn in sediments associated with water column hypoxia (WCH) are understudied. To elucidate the impacts of seasonally recurring WCH on the Mn distributions and benthic flux and precipitation of Mn minerals in coastal sediments enriched with Mn, we combined sediment incubation experiments with X-ray absorption near-edge structure (XANES) analysis. Under severe WCH conditions, enhanced sulfate reduction (SR) stimulated abiotic Mn reduction (MnR) coupled with H2S oxidation, which promoted Mn2+ release into the overlying water column. An inverse relationship between benthic Mn flux and bottom water dissolved oxygen concentrations further suggested that severe WCH induces high dissolved Mn persistence in the water column for at least a quarter of the year, ultimately affecting the health of coastal ecosystems. The XANES analysis revealed simultaneous Mn(IV)-oxides depletion and MnCO3 formation under severe WCH conditions. In particular, the increase in MnCO3 precipitation is likely a result of enhanced SR and MnR generating bicarbonate and Mn2+, highlighting the potential enhancement of microbially induced carbonate precipitation and hence carbon sequestration in Mn-rich coastal sediments underlying hypoxic water columns.

The nitrogen isotopic composition of shell-bound organic matter in planktonic foraminifera (FB-δ15N) is widely used as a proxy for past ocean deoxygenation because water-column denitrification in oxygen-deficient zones (ODZs; [O2] < 5 µmol/kg) preferentially removes 14N, enriching the remaining nitrate in 15N. Typically, increases in FB-δ15N records from ODZ-influenced regions are interpreted as evidence of ODZ expansion or intensification. However, planktonic foraminifera predominantly feed on organic nitrogen derived from the subsurface nitrate immediately below the euphotic zone, often above ODZ core depths. It remains unclear if the δ15N maxima observed within ODZ cores, reflecting denitrification intensity at a given location, directly correlates with the FB-δ15N values recorded above. Here, we combine new and published data from the eastern tropical Pacific ODZs to examine relationships among subsurface nitrate δ15N, ODZ δ15N maxima, and ODZ upper-boundary depths. Our analysis reveals a strong correlation between subsurface nitrate δ15N and ODZ δ15N maxima (R2 = 0.56-0.79), supporting the use of FB-δ15N as an indicator of denitrification intensity within ODZ regions. However, subsurface nitrate δ15N also correlates strongly with the ODZ upper-boundary depth (R2 = 0.57-0.59), with lower δ15N values observed where ODZs are deeper. For example, at our new study sites in the Eastern Tropical North Pacific (5 – 8°N), where the ODZ upper-boundary depth is ~300 m, the δ15N maxima (>10‰) at the ODZ core decrease upward to subsurface nitrate δ15N values of ~6.5‰ — only slightly higher than the global pycnocline nitrate δ15N. These results suggest that variations in ODZ depth should be accounted for when interpreting FB-δ15N records (and other δ15N archives) from ODZ regions. Under warmer conditions, organic matter remineralization may become shallower due to the temperature dependence of respiration, shifting ODZs upward and elevating FB-δ15N even without changes in denitrification rates. To more robustly reconstruct ODZ history using FB-δ15N, we recommend using multiple sites from the ODZ interior to regions beyond their modern boundaries. Cores situated outside modern ODZs, where thermocline nitrate δ¹5N still carries the ODZ signature, are ideal for tracing ODZ expansions and contractions, while cores from within the modern ODZs provide complementary constraints on ODZ intensity and vertical structure.

The ecology of the oxyscape in coastal ecosystems.

Abstract Mesopelagic fish are integral to ocean food webs and play an important role in carbon transport through their vertical migration behavior. Ocean deoxygenation caused by anthropogenic warming is expected to pose severe threats to mesopelagic fauna by enhancing physical stress and changing predator-prey relationships. In agreement with this expectation, our fish otolith record in a Mediterranean sediment core shows near absence of mesopelagic species during Sapropel deposition between ~7 and ~10 thousand years ago, concurrent with high surface productivity and low oxygenation of mid-depth waters. Instead, the otolith record is dominated by fish species adapted to epipelagic habitats, including European anchovy (Engraulis encrasicolus) and silvery lightfish (Maurolicus muelleri). Subsequent reoxygenation starting ~7 thousand years ago is accompanied by a three-fold increase in total otolith abundance. The large majority of these are mesopelagic lanternfish (Myctophidae) that dominate the otolith assemblage from the middle-Holocene to the present. Our findings corroborate expectations that future expansion of midwater deoxygenation could severely deplete mesopelagic fish communities over the coming centuries, with major impacts on marine fisheries, marine conservation, ocean food web structure, carbon storage and other marine ecosystem services.

Abstract Bioavailable nitrogen governs ocean productivity and carbon fixation by regulating phytoplankton growth and community composition. Nitrogen input primarily results from fixation, while denitrification and anammox remove bioavailable nitrogen in oxygen‐depleted conditions. Traditionally considered limited to highly suboxic (i.e., <5 μM) waters, recent studies suggest that fixed‐nitrogen removal processes may extend beyond, elevating global nitrogen loss estimates. This study directly quantifies fixed‐nitrogen loss across oxygen gradients (from 140 to 32 μM) along the Estuary and Gulf of St. Lawrence using N cycle tracers (, , and ). Notably, we observe significant production when ambient concentrations fall below a threshold value of 58.9 ± 1.1 μM, including potential water column fixed‐nitrogen removal processes above suboxia. We hypothesis that ambient deoxygenation eases the formation of suboxic microareas in suspended organic matter. Benthic production remains unaffected under intensifying water column deoxygenation from 50 down to 32 μM, but the contribution of produced through nitrification in the sediment to denitrification diminishes as deoxygenation intensifies. Combined, water column and benthic fixed‐nitrogen removal processes drive anomalies and strong deficiency in bottom waters. Additionally, the observed threshold also triggers production. Overall, our study highlights the profound impact of coastal ocean deoxygenation on nitrogen cycling, suggesting unexpected shifts even at ambient oxygen concentrations traditionally considered well above suboxic conditions.

Abstract Continued human activity is expected to accelerate ocean deoxygenation, leading to the expansion and shallowing of oxygen‐deficient zones (ODZs). This decline in oxygen may impact both phytoplankton growth and trace metal uptake. We conducted culture experiments with Prochlorococcus MIT9313 and Synechococcus XM‐24, two numerically dominant genera of marine picocyanobacteria, under varying O2 levels (36–242 μM). These experiments provide some of the first data on trace metal quotas in phytoplankton under varying O2 conditions in culture. Moderate deoxygenation, above hypoxic levels, did not negatively impact the growth and trace metal quotas of either Prochlorococcus MIT9313 or Synechococcus XM‐24 (80 and 85 μM O2, respectively). Indeed, some parameters (e.g., growth rate, carbon fixation and chlorophyll fluorescence) suggest slight growth enhancement at these moderate levels of deoxygenation. However, hypoxic conditions (36 μM O2) significantly inhibited the growth of Prochlorococcus MIT9313, suggesting a minimum O2 threshold needed to prevent disruptions in O2‐dependent processes. Synechococcus XM‐24 exhibited no significant response to elevated pCO₂ (1054 μatm), indicating possible adaptation to variable coastal CO₂ environments. Changes in trace‐metal quotas (metal‐to‐phosphorous ratios; metal/P) were also observed when Prochlorococcus MIT9313 was cultured under hypoxia, including up to 48% increases in Fe/P, 48% decreases in Ni/P, and 27% decreases in Co/P, compared to oxic conditions. These results contrast with elevated metal/P ratios in autotrophic‐associated particles from ODZs, suggesting either distinct metal uptake strategies by phytoplankton in low‐O2 regions or that elevated particulate metals are influenced by other plankton types such as heterotrophs.

A negative carbon isotopic excursion (CIE), typically overlaid atop a long term positive pattern, and significant environmental and climatic shifts are two characteristics that identify the Coniacian–Santonian, according to a variety of sedimentary records. But there is still no evidence to support the theory that variations in oceanic deoxygenation and continental weathering input in shallow seas could contribute to carbonate-platform crisis at low latitude. Here, carbonate content and carbonate-hosted elements from the Tanuma carbonate platform in Central Iraq (East Baghdad Oilfield; EB10 well) are analyzed for the Coniacian–Early Santonian transitional phase. The OAE3 boundary is marked by a clear increase in the elements that are most water insoluble (such as Al, Sc, Th, Ti, and all of the rare earth elements), which is followed by a modest increase or comparatively high-level values throughout the OAE CIE's negative phase. This implies that the improved terrigenous input may be connected to the rapid global warming that occurred throughout this time period. The increase in the abundance of these water-insoluble elements is immediately followed by an increase in the Mn, Ce, and Ce anomaly, which are then better values throughout the negative CIE interval. These data suggest that throughout this time period, shallow water experienced the process of deoxygenation and the growth in Mn (suboxic) condition. These events were probably related to increased nutrient input and continental weathering, which favored oxygen consumption as well as primary productivity. In CIE's recovery phase, the stratigraphically elevated insoluble in water elements exhibit a gradually declining trend in parallel to heightened redox proxy values, indicating a drop in the intensity of continental weathering and associated second the deoxygenation at shallow seas. In this case, increased recycling in bioessential nutrients or a slowing of the ocean's circulation could have contributed to deoxygenation. The interdependent connection among carbonate content, geochemical data, and biotic changes indicates that:1- the Tanuma carbonate platform probably noticed a minor degradation around the OAE3 boundary period due to the beginning of increasing terrigenous input and the deoxygenation at shallow seas.2- during the CIE's negative phase, the heightened terrigenous input and deoxygenation probably contributed significantly to the more serious situation facing benthic carbonate producers.

Abstract Over the last century, increasing atmospheric carbon dioxide (CO2) concentrations, among other greenhouse gases, and resulting climate change have greatly impacted the ocean. Observed impacts include lower oxygen solubility and changes in ocean stratification, circulation and biological activity. To reduce the carbon burden in the atmosphere in the future and thereby mitigate anthropogenic climate change, carbon dioxide removal (CDR) techniques have been increasingly studied and tested. However, information on the impact of CDR on oceanic oxygen is still scarce. In the current study we explore dissolved oxygen responses from an idealized CDR implementation, with atmospheric CO2 ramp-up and ramp-down simulations following the CDR model intercomparison project protocol. We find that over the timescale of a few centuries, the degree of recovery of marine oxygen, after atmospheric CO2 has returned to pre-industrial levels, differs for different water depths. Oxygen concentrations strongly recover in the upper ocean, achieving a near reversibility within 97%–99% across models, and even overshoot pre-industrial levels at depths of 100–600 m. Conversely, oxygen responses show a long-lasting deoxygenation signal in the deep ocean, with a much smaller initial recovery signal by the end of the experiment. The main factor driving oxygen changes in the deep ocean is indicated by the apparent oxygen utilization, related to changes in circulation and ventilation, as inferred by the simulated age of deep water masses. According to our models and despite the effective recovery of oxygen in the upper ocean, the effects of time lags and hysteresis on deep ocean responses could lead to longstanding and deleterious impacts on redox-sensitive biogeochemical processes and on marine biota throughout the ocean.

The abrupt ending of the Sturtian ‘Snowball’ glaciation was characterised by enhanced chemical weathering and carbon cycle perturbations, but there is less certainty over how oxygen levels responded to those changes. Here we reconcile conflicting views using a carbonate-based multiproxy dataset from the Taishir Formation in Mongolia. The geochemical data reveal an episode of ocean deoxygenation, followed by a shift toward less reducing, but still largely anoxic conditions in a post-glacial ocean characterised by nutrient and sulfate limitation. Ocean redox dynamics and biogeochemical cycling following the Sturtian deglaciation were likely dictated by unique tectonic and climatic regimes that facilitated the buildup of a recalcitrant dissolved organic carbon pool in the deep ocean. Post-glacial eutrophication may help to explain the delayed diversification of algal clades, but the persistence of ocean anoxia, excepting transient oxidation pulses, likely hindered the emergence of obligate aerobes, such as animals, until the Ediacaran Period.

Arctic amplification drives Arctic Ocean deoxygenation