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

Abstract

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 (δ 13 C org). The nitrogen isotopic composition of sedimentary organic matter (δ 15 N org) 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 δ 15 N org record across the TJ transition at the Doniford Bay section (Bristol Channel Basin, UK), combined with δ 13 C org, kerogen typology and carbon (δ 13 C min) and oxygen (δ 18 O min) isotopic composition of bulk carbonates. The end Triassic is characterized by a major negative excursion both in δ 13 C org and δ 13 C min, very low TOC (Total Organic Carbon, wt%) and high δ 15 N org values, associated with a sea level lowstand. A second δ 13 C org 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 δ 15 N org and δ 13 C org. 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. Copyright 2010 by the American Geophysical Union.