A southern hemisphere record of the Toarcian Oceanic Anoxic Event from the Neuquén Basin, Argentina

Abstract

The Toarcian, Early Jurassic, Oceanic Anoxic Event (T-OAE: ~183 Ma) was characterized by globally synchronous deposition of sediments rich in organic carbon (black shales), and is associated with an abrupt negative carbon isotope excursion, disrupting a positive carbon-isotope excursion during the tenuicostatum–serpentinum ammonite Zone boundary. The T-OAE has been extensively studied in northern and southern Europe, but evidence from the southern hemisphere is limited. New geochemical data from the tenuicostatum–hoelderi (~serpentinum) Andean ammonite Zone, from the Neuquén Basin, Argentina provides evidence for the global imprint of the event. The event is recorded in bulk-sediment from Arroyo Lapa (North and South) and Arroyo Serrucho with δ13Corg values of -30‰ – -32.5‰ 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. Pristane/Phytane ratios and pyrite framboid distribution data indicative of anoxic conditions occur at some levels, and new δ98/95Mo data, with values ranging from ~ -0.2 to 0.78 ‰, which contrast with previously published molybdenum-isotope values from T-OAE black shales of northern Europe. Sedimentological data also reveals a marine transgression during the tenuicostatum–hoelderi Andean ammonite zone.This thesis is not currently available via ORA