Toarcian Carbon Isotope Chemostratigraphy (Fossil Wood and Bulk Organic Matter), Neuquén Basin Argentina

The Early Toarcian Oceanic Anoxic Event or T-OAE (Early Jurassic, tenuicostatum-falciferum ammonite Zones: ~183 Ma) is characterized by globally near-synchronous deposition of black shales relatively enriched in organic carbon. A broad positive and intervening stepped negative excursion in δ¹³C (-2 to -7 ‰) is also recorded in marine and terrestrial organic matter and in pelagic and shallow-water carbonates deposited during the event indicating the introduction of isotopically light carbon dioxide in to the atmosphere during the T-OAE. Few records of the event exist outside of the Northern Hemisphere and confirmation of the global extent of the T-OAE and its associated Carbon Isotope Excursion (CIE) is essential to further our understanding of this event. This study expands on a pilot study from Arroyo Lapa, Argentina (Al-Suwaidi et al., 2011), and utilizes ammonite biostratigraphy, total organic carbon, and δ¹³C_org values in wood and bulk sediment from three locations in the Neuquén Basin, Argentina (Arroyo Lapa, Arroyo Serrucho and Chacay Melehue), providing a complete record of the T-OAE and the associated CIE in the Southern Hemisphere. The δ¹³C values from bulk organic matter are comparable with carbon-isotope datasets from coeval sections of black shale in England, Wales, Canada, Italy and Greece showing a stepped negative excursion of -6‰ leading up to the Andean tenuicostatum-hoelderi or its European equivalent the tenuicostatum-serpentinum (= ~falciferum) ammonite Zonal boundary. The data presented in this study contrasts with the work of Mazzini et al. (2011) from Arroyo Serrucho for the early Toarcian. Fossil wood records δ¹³C_org values as low as -32.5‰, indicating the introduction of isotopically light carbon dioxide in to the atmosphere during the excursion, and providing a record independent of any local oceanographic restriction. Combined marine and fossil wood chemostratigraphic data indicate that the T-OAE was recorded in the Southern Hemisphere and influenced the total ocean-atmosphere system.