Response of the Corbieres Carbonate Platform to Early Cretaceous Super Greenhouse Conditions




Sharpe, Justin

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The world's oceans and shallow marine reef systems host some of the most diversified ecosystems of our planet. However, these ecosystems are vulnerable to climate change: when stressed by thermal anomaly coral bleaches, as seen recently and repeatedly in the Great Barrier Reef of Australia where 91% of the ecosystem have been affected, causing high mortality rates. During the Earth's history, paleoclimatic and paleoenvironmental conditions deteriorated and stressed shallow-marine carbonate ecosystems; for instance, Mesozoic Oceanic Anoxic Events (OAEs) correspond to perturbations of the global carbon cycle triggered by climate instability and enhanced volcanism on the Ontong Java plateau and can serve as models for carbonate platform demise. During OAEs, atmospheric greenhouse gases increased dramatically, causing carbonate-producing ecosystems to survive, adapt, or disappear. However, it is unclear how, why, and when these ecosystems adjust to environmental stresses, and how carbonate production can continue at a lower rate with less diversity. This research links environmental deteriorations associated with the early Aptian OAE1a with the adaptation of benthic carbonate-producing ecosystems by investigating a carbonate platform that developed at a high latitude along the northern Tethyan margin (25°N), and that is preserved in the present day Corbieres region of southern France. The OAE1a has not been identified in this region, as it has so far been underexplored. In the rock record the OAE1a is identified by well-defined, peculiar segments in the evolution of carbon isotope composition (δ13C) values and is often associated with enhanced preservation of organic matter in deep marine settings. By identifying perturbations in the carbon cycle and in fossil assemblages associated with the OAE1a, parameters that favor carbonate-producing ecosystems can be identified. It is hypothesized that in latitudes with an arid, warm climate, continental weathering and the transfer of siliciclastic particles and dissolved nutrients are deterred, and the resilience of carbonate platforms are enhanced. Results show that in the Corbières region, seven of the eight segments of an OAE1a defined by Menegatti et al. (1998) are present, and microfacies FT to F8 described by (Godet et al., 2010) are also present. Prior to the OAE1a, during deposition of the Urgonian 1, carbonate production was not impacted and there was no change in fauna and flora assemblages. A microfacies change characterized by a change in fauna and flora occurs at the contact between the Urgonian 1 and the Urgonian Marls. This corresponds to an interpreted drowning of the platform with a change from photozoan to heterozoan carbonate production and a drastic increase in detrital input. However, in the aftermath of the OAE1a, carbonate production surprisingly not only recovered, but flourished in the upper Urgonian Marls and Urgonian 2.



Organic matter, Carbonate production, Environmental stresses, Ecosystems, Marine reef systems