The Resilience of a Northern Tethyan Carbonate Platform During the OAE1a: Geochemical Insights from Corbières, France

Sansing, Alexis
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Increased volcanism during the Cretaceous caused a rise in pCO2 which led to higher global temperatures. Warmer temperatures led to slower ocean circulation and expansion of the oxygen minimum zone, leading to ocean anoxia known as Oceanic Anoxic Events (OAEs). OAEs consist of the development of low oxygen areas that are toxic for marine ecosystems. The OAE1a is dated to the Early Aptian; the Corbières region of Southern France was located on the north margin of the Tethys Ocean at that time. There, a shallow-marine carbonate-producing ecosystem was dominated by Urgonian 1 limestones containing rudists, followed by marls containing orbitolinids, and topped by Urgonian 2 limestones containing rudists and coral. The objectives for this research include defining best practice for use of geochemical proxies in shallow-marine carbonate series that are prone to diagenesis and reconstructing Aptian paleoenvironmental conditions. This research informs on the resilience of carbonate production during periods of super greenhouse conditions by linking enhanced nutrient and detrital supply to the demise of rudist-bearing facies and their replacement by a more adapted carbonate-producing ecosystem. We use mineralogical and geochemical data along with petrographic analysis of samples from France to assess impacts of detrital input, redox conditions, paleoproductivity and nutrient supply on the efficiency of the carbonate factory of this platform in the Corbières. Geochemical data provide proxies for paleoproductivity (Sixs, Sr/Al, P/Al, P2O5xs), detrital input (%terrigenous, Ti/Al, Zr/Al), and redox conditions (Mn*, Mn/Al, Mnxs). Results show P2O5xs and %terrigenous curves vary with lithology through the section, and within the limestone sections both proxies vary little and remain low. But, in the marls both P2O5xs and %terrigenous increase, specifically %terrigenous values range from 10 - 85%. Mineralogical results from X-ray diffraction diffractometry analysis show a detrital index that is on average 97 times higher in the marls than in the Urgonian 1 limestones and 76 times higher than the Urgonian 2 limestones. Increased nutrient and terrigenous inputs were detrimental to rudist-dominated ecosystems that were replaced by biotic association with suspension feeders. Our results provide information about the resilience of the carbonate platforms to changes during the Early Aptian OAE1a and increase knowledge about how chemical elements act during diagenesis, thus enhancing our ability to identify the most reliable geochemical proxies.

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Oceanic Anoxic Events, Carbon platforms, Paleoproductivity