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Climate and environmental instability during the early Aptian culminated with the unfolding of the Oceanic Anoxic Event (OAE) 1a, which resulted in the deposition of black shales in deep marine settings and a typical negative spike followed by a positive excursion in δ13C values. In Vercors (southern France) the Urgonian platform developed prior to and coeval to the OAE1a, but the impact of this paleoenvironmental crisis on the ecology of benthic ecosystems is yet to be quantified. We gathered field and petrographic data to identify sequence boundaries and maximum flooding surfaces that are biostratigraphically dated and correlated between four localities within the study area. A composite δ13C curve is built where the C3 to C7 isotope segments from the literature are identified, pinpointing the onset of the OAE1a above the last episode of deposition of Urgonian facies rich in rudist bivalves. Furthermore, thin section point counting data are used to quantify the proportion of allochems in samples and to trace changes in the ecology of ecosystems. The principal component analysis of point counting data helps define ecological tiers: a diversified, photozoan association with rudists, green algae, and benthic foraminifera dominated ecosystems before the OAE1a and up to the C7 segment, while a less diversified heterozoan association with bryozoans and crinoids developed after the OAE1a. To explore the triggers for this change, the principal component analysis of elemental geochemical data highlights an increased nutrient and detrital input as major triggering mechanisms for ecological adjustments and changes in the biodiversity of ecosystems. In particular after the OAE1a, an increase in detrital and nutrient input leads to the replacement of photozoan by heterozoan assemblages more adapted to these stressful conditions. This research directly links paleoenvironmental deterioration to paleoecological changes and quantifies the amount of adaptation of ecosystems. 

Shallow marine reef systems are the most diversified ecosystems of modern oceans but face a severe threat from climate change: 91% of ecosystems in the Great Barrier Reef suffer from coral bleaching. To better understand how such ecosystems cope with environmental stress, a carbonate platform from the Corbières region of southern France serves as ancient analog as it developed during the Early Aptian OAE1a, a period marked by significant climate and volcanic activity. The study sought to uncover how benthic carbonate-producing ecosystems adapted during this challenging period. The OAE1a is typically identified by distinctive shifts in carbon isotope composition (δ13C) values and increased organic matter preservation in deep marine settings. Identifying these shifts can shed light on factors favoring carbonate production. The research proposes that warm, arid climates promoted reduced continental weathering and limited transfer of siliciclastic particles and dissolved nutrients that might enhance carbonate platform resilience. We identified seven out of eight segments of the OAE1a and specific microfacies in the Corbières region. Prior to the OAE1a, carbonate production was sustained by a photozoan assemblage with rudists and [insert main biota], with no changes in fauna and flora. A significant shift occurred at the interface between the Urgonian Marl that consists of siliciclastic-rich deposits with bryozoan and crinoid, indicating platform drowning and altered carbonate production. In the aftermaths of the OAE1a, carbonate production not only rebounded but thrived in the upper Urgonian Marl and Urgonian 2 with the return of a photozoan assemblage. This research provides an understanding into the adaptability of carbonate ecosystems to environmental stress, potentially offering lessons for mitigating similar crisis in the future. 

Climate and environmental instability during the Early Aptian culminate with the unfolding of the Oceanic Anoxic Event (OAE) 1a, which consists of the deposition of black shales in deep marine settings and a typical negative spike in δ13C values followed by a positive excursion. In the Vercors, southern France, the Urgonian platform developed coeval to the OAE1a, but the impact of this paleoenvironmental crisis on the ecology of benthic ecosystem is yet to be quantified. First, field and petrographic data allow to identify sequence boundaries and maximum flooding surfaces; these are biostratigraphically dated and correlated within the study area. Second, a composite δ13C curve permits to identify the C3 to C7 isotope segments from the literature, thus pinpointing the onset of the OAE1a above the Urgonian Limestone, in the Upper Orbitolina Beds. Third, thin section point counting data permit to quantify the proportion of allochems, thus illuminating the ecology of ecosystems. Principal component analysis helps define three ecological tiers: diversified, photozoan associations with rudists, green algae, and benthic foraminifera dominate ecosystems prior to the OAE1a and up to the C7 segment, while a less diversified heterozoan association with bryozoan and crinoid developed in the aftermaths of the OAE1a. Fourth, elemental geochemical data identify an increased nutrient and detrital input (C7 segment) as the major triggering mechanisms for ecological adjustments and changes in the biodiversity of ecosystems. Our research indicate that these changes are initiated in the aftermaths of the OAE1a but culminate after it. 

The Kootenai Formation of Western Montana records the Aptian- Albian (121.4Ma-100.5Ma), a significant interval in Earth’s history. The Early Cretaceous is notable for a multitude of changes in both the geologic and biotic realm. Significant events that occurred during this time include the tectonic evolution of the Western Interior Basin (WIB) and the displacement of gymnosperms by angiosperms. Given the significance of this time, previous and ongoing research seek to better understand the timing and interactions between these changes. The focus of this study is to refine stratigraphic constraint of the Kootenai Formation using carbon isotope chemostratigraphy. The depositional age of the lower clastic unit of the Kootenai formation has been debated over the past decade. Detrital zircon U-Pb analyses by Laskowski et al. (2013) indicated an Albian age with a U-Pb detrital zircon maximum depositional age (MDA) of 109Ma. However, more recent studies (Finezl and Rosenblume, 2020 and Rosenblume et al. 2021) using LA-ICP-MS-generated detrital zircon U-Pb analyses indicate MDAs of the lower clastic unit as old as Valanginian to Aptian (MDAs ~135-115Ma) with the upper units of the Kootenai having MDAs from Albian (~105 Ma). Detrital zircon U-Pb analyses have generally been limited in the lower units of the Kootenai particularly because syndepositionally formed zircon grains are not common in the lower units (Quin et al. 2018, Finzel and Rosenblume 2020).Additionally, previous flora in the Kootenai suggests predominately Aptian and older ages(Brown 1946). Given the limited geochronologic constraint of the lower clastic unit of the Kootenai formation, addition data is needed. For this study, approximately 60 samples from just above the basal conglomerate to the top of the lower clastic unit were collected and processed to determine bulk organic carbon isotope values. The prior MDAs suggest C isotope excursions such as those associated with OAE1a and even as old as the Valanginian Weissert event could be preserved in the strata of the lower clastic unit. The new stable isotope data will provide an opportunity to refine the age of these Cretaceous units leveraging the existing U-Pb data.