Periodic Reporting for period 1 - OxyQuant (Quantitative reconstruction of past seawater oxygen concentrations)
Reporting period: 2023-03-01 to 2025-09-30
The world’s oceans act as Earth’s primary climate regulator, storing over 90% of the planet’s excess heat and climatically active carbon while sustaining half of its biological productivity. Yet, one of the most critical and least understood aspects of ocean-climate interactions is the long-term variability of deep ocean oxygenation. Bottom water oxygen (BWO) concentrations are intrinsically linked to physical circulation, biological productivity, and carbon sequestration, making them a vital indicator of past ocean ventilation and climate feedbacks. Despite their importance, quantitative reconstructions of past BWO remain scarce, limiting our ability to validate climate models, constrain historical carbon cycling, and predict future responses to anthropogenic warming and deoxygenation.
The OxyQuant project directly addresses this gap by developing a geochemical toolkit capable of quantitatively reconstructing past BWO. The project focuses on three complementary proxies preserved in marine sediments: redox-sensitive metals and rare earth elements (e.g. manganese, uranium, cerium) in authigenic phases, organic-bound iodine (I/TOC ratios), and stable cerium isotopes (δ¹⁴²Ce) in marine phosphates like fish debris. While some of these proxies have been explored individually in prior studies, a systematic, multi-proxy calibration across diverse environmental conditions has been missing. Such calibration is essential because the redox signals preserved in sediments are influenced not only by BWO but also by organic carbon flux and respiration, which vary with surface productivity and sediment dynamics. To disentangle these competing effects, OxyQuant employs a comprehensive approach, analysing sediments from a wide range of oceanographic settings with varying combinations of oxygenation levels and export productivity.
During the first phase of the project, these proxies were investigated in surface and shallow subsurface sediments from 57 globally distributed sites. By correlating geochemical signals with modern bottom water oxygen data, OxyQuant aims to empirically calibrate each proxy and define their realms of applicability. This calibration phase is foundational to achieving the project’s overarching goal: determining the most robust combination of proxies for reconstructing past ocean oxygenation with unprecedented accuracy and spatial resolution.
The results of OxyQuant are expected to make significant contributions to both scientific understanding and societal challenges. By providing quantitative records of past BWO, the project will improve our knowledge of ocean ventilation, carbon storage, and climate feedbacks, particularly during critical intervals such as the Last Glacial Maximum, when ocean circulation and biological pumps operated differently than today. These reconstructions will offer ground-truth data to validate and refine Earth system models, reducing uncertainties in projections of future ocean deoxygenation, a pressing concern given the expansion of oxygen-minimum zones due to warming and pollution.
Beyond its scientific impact, OxyQuant aligns with global initiatives such as the UN Sustainable Development Goals (SDGs) 13 (Climate Action) and 14 (Life Below Water), as well as the UNESCO Ocean Decade and the Global Ocean Oxygen Decade (GO2NE). By advancing our understanding of ocean-climate interactions, the project supports evidence-based policies for marine conservation and fisheries management in a changing world. Ultimately, OxyQuant’s findings will help to fill a fundamental gap in paleoceanography and provide critical insights for mitigating the impacts of modern ocean deoxygenation on marine ecosystems and coastal communities.
The OxyQuant project directly addresses this gap by developing a geochemical toolkit capable of quantitatively reconstructing past BWO. The project focuses on three complementary proxies preserved in marine sediments: redox-sensitive metals and rare earth elements (e.g. manganese, uranium, cerium) in authigenic phases, organic-bound iodine (I/TOC ratios), and stable cerium isotopes (δ¹⁴²Ce) in marine phosphates like fish debris. While some of these proxies have been explored individually in prior studies, a systematic, multi-proxy calibration across diverse environmental conditions has been missing. Such calibration is essential because the redox signals preserved in sediments are influenced not only by BWO but also by organic carbon flux and respiration, which vary with surface productivity and sediment dynamics. To disentangle these competing effects, OxyQuant employs a comprehensive approach, analysing sediments from a wide range of oceanographic settings with varying combinations of oxygenation levels and export productivity.
During the first phase of the project, these proxies were investigated in surface and shallow subsurface sediments from 57 globally distributed sites. By correlating geochemical signals with modern bottom water oxygen data, OxyQuant aims to empirically calibrate each proxy and define their realms of applicability. This calibration phase is foundational to achieving the project’s overarching goal: determining the most robust combination of proxies for reconstructing past ocean oxygenation with unprecedented accuracy and spatial resolution.
The results of OxyQuant are expected to make significant contributions to both scientific understanding and societal challenges. By providing quantitative records of past BWO, the project will improve our knowledge of ocean ventilation, carbon storage, and climate feedbacks, particularly during critical intervals such as the Last Glacial Maximum, when ocean circulation and biological pumps operated differently than today. These reconstructions will offer ground-truth data to validate and refine Earth system models, reducing uncertainties in projections of future ocean deoxygenation, a pressing concern given the expansion of oxygen-minimum zones due to warming and pollution.
Beyond its scientific impact, OxyQuant aligns with global initiatives such as the UN Sustainable Development Goals (SDGs) 13 (Climate Action) and 14 (Life Below Water), as well as the UNESCO Ocean Decade and the Global Ocean Oxygen Decade (GO2NE). By advancing our understanding of ocean-climate interactions, the project supports evidence-based policies for marine conservation and fisheries management in a changing world. Ultimately, OxyQuant’s findings will help to fill a fundamental gap in paleoceanography and provide critical insights for mitigating the impacts of modern ocean deoxygenation on marine ecosystems and coastal communities.
The scientific foundation of this project phase was established through the careful selection and analysis of a globally representative collection of deep ocean sediment cores from diverse marine environments. This required engaging with an extensive network of international collaborators across marine biogeochemistry and paleoceanography - two disciplines that OxyQuant uniquely connects. Through these scientific partnerships, we successfully assembled sediment samples from more than fifty sites worldwide, creating a comprehensive dataset that captures a wide spectrum of oceanographic conditions including varying bottom water oxygen levels and organic carbon fluxes.
The analytical work focused on high-resolution investigations of redox-sensitive metals and rare earth elements in surface and young subsurface sediments, which allowed us to examine in detail how early diagenetic processes affect the distribution of these geochemical tracers within sediment columns. Organic-bound iodine and cerium isotopes were analysed with lower resolution but sufficient coverage to represent the full environmental gradient. All geochemical parameters were extracted from small sediment aliquots using optimized chemical leaching procedures that were specifically refined to improve efficiency, safety, and reproducibility while handling the large sample volume required for this study.
Initial data analyses confirm that we have successfully met the primary objectives of this project phase. The collected sediments display a remarkably wide range of chemical compositions, ensuring our proxies cover a large range of environmental conditions. Clear evidence of early diagenetic processes, such as manganese and uranium migration within sediment profiles, can be observed in most samples. Importantly, certain element ratios appear remarkably stable despite diagenetic gradients, suggesting they may serve as particularly robust indicators for reconstructing past oxygen levels from older sediments.
Recognizing the complex interplay of factors influencing these geochemical systems, we determined that additional validation was necessary before applying these proxies to ancient sediments. To this end, we are conducting targeted analyses on sediments from three carefully selected sites that present unique opportunities for validation. A Mediterranean core containing an organic-rich sapropel layer allows us to test proxy robustness under extreme diagenetic conditions. A Southern Ocean core where past changes in oxygenation and productivity were large but uncorrelated provides an ideal natural laboratory for disentangling these competing influences. Finally, a Pacific core with independent oxygenation records will serve as the ultimate test of our proxies' ability to accurately reconstruct past ocean conditions.
Due to the complex nature of the behaviour of all these geochemical parameters, it was decided that additional tests of their robustness are necessary before they can be reliably applied to reconstruct past deep ocean oxygenation. Therefore, sediments from three sites with different properties have been or are being analysed:
Firstly, one sediment core from the Mediterranean Sea, where an organic-rich layer (“sapropel”) was deposited a few thousand years ago which led to marked changes in sedimentary diagenesis dynamics. These analyses will help to determine the robustness of the geochemical parameters to diagenetic changes.
Secondly, one sediment core from the Southern Ocean for which it has been shown that past changes in deep water oxygenation and overlying biological productivity were large and were not correlated. These analyses will help to disentangle the two competing processes affecting sedimentary redox conditions in a true paleo-context.
And thirdly, one Pacific sediment core for which past bottom water oxygenation through the last glacial cycle has been determined with independent paleoceanographic methods. This will be the ultimate validation of the suggested geochemical parameters and their ability to reconstruct past ocean oxygenation. Due to the high efficiency of the sediment leaching methods, these analyses are not a major additional work burden.
The analytical work focused on high-resolution investigations of redox-sensitive metals and rare earth elements in surface and young subsurface sediments, which allowed us to examine in detail how early diagenetic processes affect the distribution of these geochemical tracers within sediment columns. Organic-bound iodine and cerium isotopes were analysed with lower resolution but sufficient coverage to represent the full environmental gradient. All geochemical parameters were extracted from small sediment aliquots using optimized chemical leaching procedures that were specifically refined to improve efficiency, safety, and reproducibility while handling the large sample volume required for this study.
Initial data analyses confirm that we have successfully met the primary objectives of this project phase. The collected sediments display a remarkably wide range of chemical compositions, ensuring our proxies cover a large range of environmental conditions. Clear evidence of early diagenetic processes, such as manganese and uranium migration within sediment profiles, can be observed in most samples. Importantly, certain element ratios appear remarkably stable despite diagenetic gradients, suggesting they may serve as particularly robust indicators for reconstructing past oxygen levels from older sediments.
Recognizing the complex interplay of factors influencing these geochemical systems, we determined that additional validation was necessary before applying these proxies to ancient sediments. To this end, we are conducting targeted analyses on sediments from three carefully selected sites that present unique opportunities for validation. A Mediterranean core containing an organic-rich sapropel layer allows us to test proxy robustness under extreme diagenetic conditions. A Southern Ocean core where past changes in oxygenation and productivity were large but uncorrelated provides an ideal natural laboratory for disentangling these competing influences. Finally, a Pacific core with independent oxygenation records will serve as the ultimate test of our proxies' ability to accurately reconstruct past ocean conditions.
Due to the complex nature of the behaviour of all these geochemical parameters, it was decided that additional tests of their robustness are necessary before they can be reliably applied to reconstruct past deep ocean oxygenation. Therefore, sediments from three sites with different properties have been or are being analysed:
Firstly, one sediment core from the Mediterranean Sea, where an organic-rich layer (“sapropel”) was deposited a few thousand years ago which led to marked changes in sedimentary diagenesis dynamics. These analyses will help to determine the robustness of the geochemical parameters to diagenetic changes.
Secondly, one sediment core from the Southern Ocean for which it has been shown that past changes in deep water oxygenation and overlying biological productivity were large and were not correlated. These analyses will help to disentangle the two competing processes affecting sedimentary redox conditions in a true paleo-context.
And thirdly, one Pacific sediment core for which past bottom water oxygenation through the last glacial cycle has been determined with independent paleoceanographic methods. This will be the ultimate validation of the suggested geochemical parameters and their ability to reconstruct past ocean oxygenation. Due to the high efficiency of the sediment leaching methods, these analyses are not a major additional work burden.
The ongoing analysis of OxyQuant's comprehensive geochemical dataset is already revealing new insights that extend significantly beyond the current state of the art in marine geochemistry. This unprecedented collection of trace element data from over fifty globally distributed sediment sites establishes a new benchmark for understanding elemental behaviour in the uppermost marine sediment layer, the sediment-water interface. To fully realize this potential, additional detailed statistical analyses and modelling will be required to elucidate the precise mechanisms governing these elements' geochemical cycling in both the water column and shallow sediments.
The meticulous chemical characterization of this diverse and globally representative sediment collection creates exceptional opportunities for future research. This well-documented sample set provides an ideal platform for investigating additional geochemical systems, enabling comparative studies across different oceanographic regimes. The comprehensive nature of this dataset ensures its value will extend far beyond the current project, serving as a reference for future paleoceanographic and biogeochemical research initiatives.
The meticulous chemical characterization of this diverse and globally representative sediment collection creates exceptional opportunities for future research. This well-documented sample set provides an ideal platform for investigating additional geochemical systems, enabling comparative studies across different oceanographic regimes. The comprehensive nature of this dataset ensures its value will extend far beyond the current project, serving as a reference for future paleoceanographic and biogeochemical research initiatives.