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.
