RESPONSE OF GLOBAL OCEAN OXYGENATION TO EARLY CENOZOIC CLIMATE EXTREMES (RESPIRE)

Lead Research Organisation: Open University
Department Name: Faculty of Sci, Tech, Eng & Maths (STEM)

Abstract

The evolution of life on Earth has been tightly linked to the development of the planet's oceans and to its climate system. Scientists have built up a picture of Earth history, and of the animals and plants of past times, through detailed examination of ancient rock strata that have accumulated on the continents and in the oceans over the ages. Long periods of relative quiescence and of gradual change were punctuated by shorter intervals when Earth's environment changed abruptly. Intervals of rapid environmental change were often accompanied by unusually high levels of species extinctions and of changes in diversity, and were often followed by new patterns of species evolution.

One well established aspect of Earth history is that past climates were often much warmer than at present. Furthermore, it is almost universally accepted that climate and mean global temperature are intimately related to the level of atmospheric CO2, albeit in a complex way. But no matter what the precise nature of the climate-CO2 relationship, one consequence of global warmth is that seawater oxygen levels are expected to be relatively low, for two reasons. The first is that all gases - including oxygen - are less soluble in warmer liquids than in cooler ones; the second is that the primary productivity of the oceans affects oxygen levels directly, as higher productivity leads to greater levels of oxygen consumption. Thus there is the reasonable expectation that seawater oxygenation will decline in the future, as the oceans warm and as rivers supply more nutrients. This expectation is backed up by the direct observation of substantially decreasing oxygen levels in many parts of the oceans over the last 50 or 60 years. Although a global phenomenon, oxygen levels are most sensitive in continental shelf waters. This is a concern, because most marine species live on the continental shelf, and they are highly susceptible to changes in seawater oxygenation. Humankind is acutely at risk from the consequences of shelf deoxygenation: more than one billion people depend on marine food as their primary protein source.

However, it is notoriously difficult to predict accurately the speed, severity and trajectory of future deoxygenation. One very powerful way of improving the reliability of forecasts is to refine predictive models by 'tuning' them using observations of past seawater oxygenation. This project (RESPIRE) will define the oxygenation history of seawater covering a period of just over 30 million years, from around 56 million years ago to 25 million years ago. The Earth's surface environment cooled substantially during this period, both gradually and also in a few discrete jumps. Because there are no direct records of past seawater oxygenation, we will use geochemical proxies whose values reflect oxygenation levels. Although these geochemical measurements are very difficult and time consuming, we have many years' experience in their development and application and we have shown that the proxies can act as robust archives of past oxygenation for short time intervals. The challenge now is to generate longer-term records that will help us to better understand the controls on past - and future - seawater oxygenation.

An additional and highly important aspect of low-oxygen marine environments is that they are a pre-requisite for the formation of hydrocarbon source rocks, which supply most of the world's current energy demand. Because RESPIRE will involve close co-operation between field geologists, geochemists, climate modellers and industry geologists, the project will provide a forum to better define the relationship between past seawater deoxygenation and the accumulation of organic matter from which hydrocarbons are derived. RESPIRE will be the first study to establish the longer-term oxygenation history of seawater by providing an integrated, interdisciplinary assessment of how seawater oxygenation is linked to global Earth System processes.

Planned Impact

Our proposed research will impact four main groups:
Academics: The proposal fits into three of NERC's current science themes ('Earth System Science', 'Biodiversity' and 'Climate System') and will complement NERC research that has been funded under the 'Ocean Acidification' and 'The Long-term Co-Evolution of Life and the Planet' thematic programmes. Results from this proposal will also complement a current Integrated Ocean Drilling Program (IODP) expedition that aims to explore changes in ocean chemistry over the same time interval that is the focus of this study. Oxygen concentrations in the oceans have a direct effect on marine life and biodiversity, biogeochemical cycling and organic carbon burial. Our work will have an impact on the work of an international, multi-disciplinary group of academics (including palaeontologists, stratigraphers, organic and inorganic geochemists, marine biologists and ecologist, biogeochemical modellers and atmospheric chemists) researching the mechanics and impacts of these aspects of the marine environment. Lastly, the development of isotope-based proxies within this proposal will add to the portfolio of isotopic techniques available to UK and international academics, particularly those working to determine past global marine oxygenation levels, and for those investigating the controls on seawater oxygenation.

UK and European Industry partners: This proposal will involve geochemical analyses of organic-carbon and hydrocarbon-rich source rocks which have hitherto not been characterised fully in this manner. Our results will help to characterise the depositional palaeoenvironments of these economically important hydrocarbon source-rocks, which will impact on the knowledge base of industry partners (BP, Statoil, RAG (Rohöl-Aufsuchungs Aktiengesellschaft) Vienna. These industry partners have been involved in the formulation of the project, and will be fully involved as science results are produced.

Policy makers: This proposal will improve our understanding of the rate, magnitude, and direction of seawater oxygenation in response to global environmental changes, both past and present. Our findings will thus indirectly affect the ability of policy makers to make the best decisions about how to manage marine resources and natural environments as these evolve under future scenarios of global environmental change. The communication of the impacts of this project on policy makers will be facilitated by involvement of co-I Edwards on a European FP7 consortium project aimed at improving integration of climate, environmental and economic modelling tools for more robust environmental policy assessments; by the contribution of Edwards and researcher Holden to the IPCC AR5 assessment process as contributors to the long-term C-cycle and climate change projections; and by the long-term involvement of PI Cohen with the Centre for Science and Policy at Cambridge University.

General public/students: The broad science base of this project, together with its widespread relevance to past and present-day environmental change is such that it will impact on public perceptions of the sensitivity of marine environments to anthropogenic environmental change. In this regard, our proposal has the potential to be highly influential due to a general public interest in climate-change related issues and because results from this study will be the first of their kind for a period of Earth history frequently mooted as an analogue for a future warmer Earth. The impact of this proposal on students and the general public will be facilitated through a wide range of activities, including summer schools, outreach seminars at local schools (through the STEM Network), science fairs and geological societies; and through a range of media communications such as podcasts, online forums, and project website.

These themes are expanded upon in the 'Academic beneficiaries' section and 'Pathways to Impact' document.

Publications

10 25 50
 
Description We used an Earth system model to study the high-CO2 'greenhouse' climate of the early Eocene, from about 56 million years ago, a possible analogue for Earth's future climate. We found that although CO2 exerts a dominant control on most climatic features, the Earth's orbit around the sun also strongly influences important components of the ocean-atmosphere system in a greenhouse Earth. In our experiment, atmospheric CO2 accounts for over 90 % of the effects on mean air temperature, southern winter high-latitude ocean-land temperature contrast and northern winter tropical-polar temperature difference. However, the variation of orbital precession accounts for over 80 % of the influence of the forcing parameters on the Asian and African monsoon rainfall, and orbital obliquity variation accounts for over 65 % of the effects on winter ocean-land temperature contrast in high northern latitudes and northern summer tropical-polar temperature difference. Our results indicate climate sensitivity may have been higher than today if the atmospheric CO2 concentration was above 1000 ppm. These results were published in Climate of the Past in 2018.

The Paleocene Eocene Thermal Maximum (PETM) around 55 million years ago represents a major carbon cycle and climate perturbation that was associated with ocean de-oxygenation, in a qualitatively similar manner to the more extensive Mesozoic Oceanic Anoxic Events earlier in Earth history. Although indicators of ocean de-oxygenation are common for the PETM, and linked to biotic turnover, the global extent and temporal progression of de-oxygenation is poorly constrained. We analysed uranium isotope data associated with carbonate sediment records for the PETM. A lack of resolvable perturbation to the U-cycle during the event suggests a limited expansion of seafloor anoxia on a global scale. We use this result, in conjunction with a biogeochemical model, to set an upper limit on the extent of global seafloor de-oxygenation. The model suggests that the new U isotope data, whilst also being consistent with plausible carbon emission scenarios and observations of carbon cycle recovery, permit a maximum ~10-fold expansion of anoxia, covering <2% of seafloor area. A paper on these results was published in Nature Communications in 2021.

A multi-million-year decrease in global temperatures during the Eocene was accompanied by large reorganisations to ocean circulation, ocean chemistry and biological productivity. These changes culminated in the rapid growth of ice on Antarctica during the Eocene-Oligocene climate transition (EOT), ~34 million years ago. However, while this probably altered the oceanic oxygen inventory, the sign and magnitude of the response is poorly constrained. We found that euxinic (highly anoxic) conditions developed in the Austrian Molasse Basin during the EOT. The isotopic compositions of molybdenum and uranium captured by sediments accumulating in the Molasse Basin at this time reveal that the global extent of sulfidic conditions (indicating poorly oxygenated seawater) during the EOT was not appreciably different to that of the Early Eocene greenhouse world. Our results suggest that the early Cenozoic oceans were buffered against extreme long-term changes in oxygenation. A paper on these results was accepted for publication in Earth and Planetary Science Letters in early 2021.
Exploitation Route The new analytical methods and modelling can be expected to be adopted by other researchers once the results are published and accepted by the community.
Sectors Environment

URL https://www.clim-past.net/14/215/2018/
 
Title U-series analysis methodology 
Description A new, high-precision d238/235U measurement protocol has been developed for analysis of black shale deposits from low-oxygen environments, and also adapted for the analysis of carbonate sample isotopic compositions. 
Type Of Material Improvements to research infrastructure 
Provided To Others? No  
Impact The U-series analysis methodology has provided new insights into the redox history of the global ocean in the Eocene 
 
Title Eocene GENIE ensemble 
Description Early Eocene palaeogeography configuration for the GENIE Earth System Model has been created, including continental and ocean configurations, increased resolution in the Arctic, main ocean gateways, and wind-fields. This has been used to create a further database comprising output from an ensemble of 50 Genie model runs investigating the early Eocene, focused on ocean biogeochemical variations including oxygenation state in response to broad variation in forcing factors, including orbital parameters. 
Type Of Material Database/Collection of data 
Provided To Others? No  
Impact The results are under investigation 
 
Title Eocene PLASIM-GENIE ensemble 
Description We have applied the Eocene palaeogeography developed for the GENIE model (see previous entry) in a 50-member ensemble of PLASIM-GENIE simulations. PLASIM-GENIE is an intermediate complexity climate model (AOGCM), with 3D dynamics in both the atmosphere and ocean, that has been developed at the Open University. The ensemble of PLASIM-GENIE simulations has been performed to investigate the role of orbital forcing on Eocene climate and to provide boundary conditions (such as wind-fields) for the ensemble of GENIE-1 biogeochemistry simulations. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
Impact This is under study 
URL https://doi.org/10.5194/cp-14-215-2018
 
Title Geochemical data 
Description We measured trace element concentrations, Mo and U stable isotope compositions, recorded in marine anoxic and euxinic sediments from the Arctic Ocean (IODP Expedition 302). Additionally, we measured Re and Os isotope compositions in the same samples. This has allowed us to construct a new age model for these sediments, and hence to get better constraints on the timing of redox variations. 
Type Of Material Database/Collection of data 
Provided To Others? No  
Impact These new d98/95Mo and d238/235U records for the Early to Middle Eocene (~56-46 Ma), combined to trace element data, can be interpreted in terms of local and global seawater redox variations. Our results suggest that anoxic and euxinic areas of marine sedimentation were more widespread at the onset of the Eocene than in the modern ocean, and that they further expanded during the long-term increase in global temperatures that culminated with the Early Eocene Climatic Optimum. Moreover, our analyses of the Arctic Ocean mudrocks highlight the importance of first understanding local sedimentary processes and their impact on isotope fractionation before any redox information can be inferred from Mo and U isotopic variations. 
 
Description RAG Austria 
Organisation RAG Rohöl-Aufsuchungs Aktiengesellschaft
Country Austria 
Sector Private 
PI Contribution Geochemical analyses of core samples.
Collaborator Contribution Provision of core samples and facilities.
Impact Work in progress.
Start Year 2013
 
Description Slovak collaboration 
Organisation Slovak Academy of Sciences
Country Slovakia 
Sector Public 
PI Contribution Geochemical analysis of core samples.
Collaborator Contribution Provision of core samples for geochemical analysis.
Impact Work is in an early stage of progress - none yet.
Start Year 2013