A Multi-proxy Examination of the Toarcian Oceanic Anoxic Event in Argentina

The Toarcian, Early Jurassic, Oceanic Anoxic Event (T-OAE: ~183 Ma) was characterized by relatively high mid-latitude sea-surface temperatures (~6°C warmer than present), mass extinction, and the deposition of sediments rich in organic carbon (black shales). The T-OAE correlates with emplacement of a large igneous province (Karroo-Ferrar) and is associated with a positive carbon-isotope excursion punctuated by a negative carbon-isotope excursion, in bulk sedimentary organic matter and fossil wood, the later recording atmospheric isotopic composition. The T-OAE has been extensively studied in northern and southern Europe and new studies from the Southern Hemisphere have provided chemostratigraphic evidence for the global expression of the event. Here we present new geochemical data from the Neuquén Basin, Argentina: bulk-sediment δ13C values fall to -31.3 ‰ and δ13C values from fossil wood fall to -30.7 ‰, isotopic ratios that are comparable to those identified in Europe for the T-OAE. Hydrogen Index (HI) data for the T-OAE in Argentina give values ranging from 12 to 425 mg HC/ g TOC, indicating a mixture of terrestrial and marine organic components. Biomarkers indicative of photic-zone euxinia (diagenetic products of the green sulphur bacterial pigment isorenieratene) occur at some levels, and new δ98Mo data, with values ranging from ~ -0.2 to 0.78 ‰, contrast with previously published molybdenum-isotope values from T-OAE black shales of northern Europe. The new molybdenum-isotope data call into question the inference that the northern European seaways record global molybdenum-isotope fluctuations.