The Drowning of the Urgonian Platform as a Response to Early Aptian Greenhouse Conditions




Byerly, Jacob

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The Early Cretaceous Epoch (145-110 Ma) is characterized by the widespread development of shallow marine carbonate platforms in epicontinental seas thanks to configuration of continental landmasses, and an overall warm climate. The health of these carbonate platforms requires specific environmental conditions; for instance, coral and rudist bivalve-dominated ecosystems thrived in warm, nutrient-poor, well-lit water. There is a lack of understanding on why the north and central Tethys carbonate platforms reacted differently to climate change during Aptian time (125-113 Ma): carbonate platforms at higher latitudes (ca. 30° N Latitude) died off while lower latitude platforms (ca. <10° N/S Latitude) transitioned to include more competitive ecosystems and were able to adapt (Huck et al., 2010). Older, Early Cretaceous deposits adapted to sea level changes until the Urgonian Formation group was drowned in the middle Aptian and never recovered. This disappearance is coeval with the occurrence of the Oceanic Anoxic Event 1a (OAE1a), which corresponds to the first worldwide development of oceanic anoxia during the Cretaceous (Schlanger and Jenkyns, 1976). The drowning event and the OAE1a can be identified using microfacies, petrography, and Carbon and Oxygen stable isotope data from rock samples. The microfacies analysis based on the type of fauna and flora observed in thin section shows that ecosystems adapted to nutrient-rich conditions during a period of lowering sea level. Carbon isotope stratigraphy indicates that the OAE1a unfolded synchronously with this adaptation phase. Together, these data show that the OAE1a and associated paleoenvironmental changes impacted the Urgonian platform and led to its demise. Understanding the platform's interaction with the environment could give insight to modern carbonate platforms and their interaction with future climate change.


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OAE1a, Urgonian, Aptian greenhouse conditions