Nitrogen isotope record of a perturbed paleoecosystem in the aftermath of the end-Triassic crisis, Doniford section, SW England

International audience[1] The Triassic-Jurassic transition (TJ) is characterized by successive perturbations of the carbon cycle during a time of biotic disruption as recorded by the carbon isotopic composition of organic matter (δ13Corg). The nitrogen isotopic composition of sedimentary organic matter (δ15Norg) constitutes a key parameter to explore the functioning of the ecosystem during carbon cycle perturbations and biological crises, because it provide information on seawater redox conditions and/or nutrient cycling. Here we report the first continuous δ15Norg record across the TJ transition at the Doniford Bay section (Bristol Channel Basin, UK), combined with δ13Corg, kerogen typology and carbon (δ13Cmin) and oxygen (δ18Omin) isotopic composition of bulk carbonates. The end Triassic is characterized by a major negative excursion both in δ13Corg and δ13Cmin, very low TOC (Total Organic Carbon, wt%) and high δ15Norg values, associated with a sea level lowstand. A second δ13Corg negative excursion occurs during the lower Hettangian. This interval is characterized by phases of carbonate production increase alternated with phases of exceptional accumulations of type I organic matter (up to 12%) associated with lower δ15Norg and δ13Corg. This alternation likely reflects a succession of nutrient input increase to the basin leading to enhanced productivity and eutrophication, which promoted a primary production driven by organic-walled prokaryotic organisms. The following OM export increase generates anaerobic conditions within the basin. These events occur between periods of relatively good seawater column ventilation and nutrient recycling boosting the carbonate producer recovery. Ecosystems remain perturbed in the Bristol Channel Basin during the aftermath of the end-Triassic crisis

The Impact of Global Warming and Anoxia on Marine Benthic Community Dynamics: an Example from the Toarcian (Early Jurassic)

The Pliensbachian-Toarcian (Early Jurassic) fossil record is an archive of natural data of benthic community response to global warming and marine long-term hypoxia and anoxia. In the early Toarcian mean temperatures increased by the same order of magnitude as that predicted for the near future; laminated, organic-rich, black shales were deposited in many shallow water epicontinental basins; and a biotic crisis occurred in the marine realm, with the extinction of approximately 5% of families and 26% of genera. High-resolution quantitative abundance data of benthic invertebrates were collected from the Cleveland Basin (North Yorkshire, UK), and analysed with multivariate statistical methods to detect how the fauna responded to environmental changes during the early Toarcian. Twelve biofacies were identified. Their changes through time closely resemble the pattern of faunal degradation and recovery observed in modern habitats affected by anoxia. All four successional stages of community structure recorded in modern studies are recognised in the fossil data (i.e. Stage III: climax; II: transitional; I: pioneer; 0: highly disturbed). Two main faunal turnover events occurred: (i) at the onset of anoxia, with the extinction of most benthic species and the survival of a few adapted to thrive in low-oxygen conditions (Stages I to 0) and (ii) in the recovery, when newly evolved species colonized the re-oxygenated soft sediments and the path of recovery did not retrace of pattern of ecological degradation (Stages I to II). The ordination of samples coupled with sedimentological and palaeotemperature proxy data indicate that the onset of anoxia and the extinction horizon coincide with both a rise in temperature and sea level. Our study of how faunal associations co-vary with long and short term sea level and temperature changes has implications for predicting the long-term effects of “dead zones” in modern oceans

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