Role of continental configuration in the development of worldwide ANOXia in bottom ocean SEAwater. – Anox-Sea
Role of continental configuration in the development of worldwide ANOXia in SEAwater
Understanding the mechanisms and the thresholds that trigger global anoxia in the oceans during abrupt warming events represents a major issue in the current context of atmospheric CO2 increase. These events disappear during the major paleogeographic changes that characterize the Late Cretaceous, highlighting a possible role of paleogeography through changes in oceanic circulation and continental weathering.
Understanding the role of paleogeography on the sensitivity of the ocean-climate system to abrupt warming events and on the CO2 levels required to trigger an oceanic anoxic event.
This project relies on three main aims : 1) to identify the changes in oceanic circulation during the Late Cretaceous and their links to the oxygenation state of the oceans and to the paleogeography; 2) to constrain the evolution of continental weathering during the Late Cretaceous; 3) to identify the sensitivity of the ocean-climate system to develop anoxia in different continental configurations using numerical models.
The oceanic circulation during the Late Cretaceous is tracked using the Nd isotope signature (eNd) of fossil fish remains or of authigenic ferromanganese oxides, that both record the eNd of seawater, a tracer of water mass exchanges. To explore the oxygenation state of the water column, two approaches involving trace elements are adopted : concentrations in total organic carbon (TOC) and Mo (TOC vs. Mo/Al crossplots) that allows to derive information on the intensity of the development of anoxia into water columns and the size of the dissolved-Mo marine reservoirs, and an approach based on the contrasting modalities of authigenic enrichment observed for U and Mo. Redox sensitive iron species ratio such as highly reactive iron/total iron, pyrite iron/total iron ratios and the degree of pyritisation used as cross plots is compared to modern examples and used with sulfides ?34S variations to explore the extent of anoxia throughout the entire water column and type of anoxia. Chemical weathering intensity is accessed using the combined Lu-Hf and Sm-Nd isotope systems, as while Nd isotopes track the source of eroded sediments, the departure of bulk sediment eHf from the “Mantle array” for a given eNd depends on the relative proportion of zircon-rich phase (sands) and clays, related to chemical weathering intensity. Clay mineral assemblages are also used to track local changes in hydrolyzing conditions and therefore chemical erosion. The general circulation ocean atmosphere model FOAM is used to investigate the ocean dynamics during the Late Cretaceous. To explore the conditions for the onset of anoxic conditions in the ocean, the GEOCLIM reloaded tool is used, that is a fully coupled model simulating the climate (temperature, runoff and oceanic overturning simulated by the FOAM GCM), the weathering rates of the continental surfaces, the atmospheric level of CO2 and O2, the chemistry of the water column and the burial of organic and inorganic carbon in the sediments.
The project begun with a kick off meeting in november 2012, that allowed (1) to discuss the uncertainties on Late Cretaceous paleogeography and to define several configurations to test with numerical simulations using the ocean-atmosphere model FOAM that will be used solely at first, and (2) to define the sites to be treated in priority for the acquisition of the different geochemical and mineralogical data along with additional sites. In the following 6 months, samples from all the selected sites have been requested to the ODP/DSDP curator department and have been received. Samples already available for the neritic sites of Japan along with those from the Western Interior Seaway have been treated to extract fish remains and most of them have been analyzed for their neodymium isotopic composition. Whole rock samples have also been analyzed for the site in Japan. The results, very radiogenic for the Japan, are still being interpreted. A high resolution multiproxies geochemical study is currently in process on the requested samples of the selected DSDP site. The samples are currently being analyzed for their organic carbon and nitrogen isotopic composition and their concentration in trace elements and iron speciation.
To be continued.
To be continued.
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 (eNd), a proxy of oceanic circulation. We plan to reconstruct the eNd 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.
Project coordination
Emmanuelle Puceat (Biogéosciences UMR 5561 CNRS) – emmanuelle.puceat@u-bourgogne.fr
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partner
UMR5275 Institut des Sciences de la Terre
UMR 8212 Laboratoire des Sciences du Climat et de l'Environnement
UMR 5563 Géosciences Environnement Toulouse
Biogéosciences Biogéosciences UMR 5561 CNRS
Help of the ANR 378,000 euros
Beginning and duration of the scientific project:
October 2012
- 48 Months