ANOX-SEA project

**Role of continental configuration in the development of worldwide ANOXia in SEAwater.

Recent increase of atmospheric CO2 levels is driving the Earth climate and ocean chemistry toward conditions that have not existed for million years. The impact on ocean of recent global warming (ocean acidification, expansion of the oxygen minimum zone) recalls the model initially invoked for the onset of Oceanic Anoxic Events (OAEs). These brief extreme and global events seem to occur during periods of warm, greenhouse climate and are associated to major crises of calcifying organisms, possible ocean acidification, and increase of organic carbon burial in sediments, inducing major perturbations of the carbon cycle, central in climate evolution. These events are usually associated to an abrupt warming through (1) an acceleration of hydrological cycle and continental alteration enhancing nutrient supply and hence primary productivity, and (2) a slowdown of the oceanic circulation, favoring the development of anoxia in deep waters and preservation of organic matter.
However abrupt warming events have occurred after the middle of the Cretaceous, at the end of the Maastrichtian ( 65 Ma) or at the Paleocene-Eocene transition ( 55 Ma), that have not resulted in the occurrence of OAEs. In this project we propose to study the role of continental configuration, through its impact on oceanic circulation and continental weathering, on the thresholds for ocean-climate system to shift toward a state of worldwide anoxia in deep waters. We focus on a key period, the Late Cretaceous, that covers the last occurrence of worldwide anoxia in deep waters and the transition toward a continental configuration more alike the modern one.

The structure of oceanic circulation and its changes during the Cretaceous remain debated, mainly because of the insufficient spatial and temporal coverage of neodymium isotopic data (Nd), a proxy of oceanic circulation. We plan to reconstruct the Nd evolution during the Late Cretaceous of both deep water masses in the Atlantic and Indian part of the Austral Ocean, providing a link between published data, and of the surface waters at potential sites of deep water sinking. The inferred oceanographic changes will then be confronted to the evolution of proxies of marine redox conditions gathered through compilations in different oceanic basins and newly acquired : iron speciation (tracking the extend of anoxic conditions in the water column) and cross-plots of Mo and U enrichment factors (identifying the restriction in water mass exchanges). Simulations using the general circulation model FOAM coupled to the geochemical model GEOCLIM-reloaded will be performed using different widths and depth of marine passages and different CO2 levels to discuss the origin of the changes in oceanic circulation and oxygenation state of the different basins.

In a second time, we will establish the latitudinal distribution of humid/arid conditions using clay mineralogical assemblages (both through published and new data) for the mid- and latest Cretaceous. Changes in weathering rates will also be explored using a new proxy, the combined Hf, Pb and Nd isotope systems, that allows to track both the evolution of continental weathering and the sources provenance and has never been applied so far to the Cretaceous period.

Both slight changes in paleogeography and CO2 levels will be used to obtain simulations with FOAM coupled to GEOCLIM-reloaded in agreement with the data (of oceanic circulation, continental weathering, and oxygenation) for the mid- (apart from OAE2) and latest Cretaceous. Once the situation is correctly reproduced for both periods, a warming event of different amplitude (using different CO2 levels in the models) will be simulated. The response of the ocean in terms of turnover rates and development of anoxia will then be determined in order to explore the sensitivity of the ocean-climate system to develop anoxia in different paleogeographic and paleoclimatic contexts, therefore fulfilling the main aim of the project.

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