The Early Toarcian Oceanic Anoxic Event: A Southern Hemisphere record from Chile

The Early Toarcian was marked by important environmental changes, marine oxygen deficiency and extensive organic-rich sediment deposition (T-OAE; ∼182 Ma, Early Jurassic). The T-OAE coincides with a marked negative carbon... [ view full abstract ]

The Early Toarcian was marked by important environmental changes, marine oxygen deficiency and extensive organic-rich sediment deposition (T-OAE; ∼182 Ma, Early Jurassic). The T-OAE coincides with a marked negative carbon isotope excursion recorded in marine carbonate, and marine and terrestrial organic carbon. This is commonly attributed to the massive release of isotopically light carbon to the atmospheric and oceanic reservoirs derived from the destabilization of methane hydrates from marine sediments and/or the emissions of thermogenic methane from the eruption of the Karoo-Ferrar LIP. Moreover, in most documented marine sections, this episode is marked by a generalized crisis in carbonate production and marine invertebrate extinctions.

Only few data are available from the Southern Hemisphere, leading to large uncertainty concerning the exact expression of this event in this region. The aims of this study are to characterize the sediments deposited during the tenuicostatum and hoelderi Andean ammonite Zones, equivalent of the tenuicostatum and falciferum European Zones and establish in which way the T-OAE affected this region. In the Early Jurassic, the Andean basin was in a back-arc setting with marine corridors connected to Panthalassa. We have generated high-resolution sedimentological, mineralogical and geochemical data from the sections of El Peñon and Asiento, located in Chile, Atacama region. The biostratigraphy of these sections has been studied by von Hillebrandt and Schmidt-Effing (1981) and complemented here by a biostratigraphy based on calcareous nannofossils. The sections consist of a succession of marl, siltstone and limestone of Pliensbachian and Toarcian age. The carbon isotope composition of the sedimentary organic matter reveals a negative excursion (~5‰) in the hoelderi ammonite Zone, which is characteristic of the T-OAE interval of coeval sites. Total organic carbon (TOC) content is relatively low (< 1wt.%) during the T-OAE but secondary organic-matter degradation cannot be excluded (high geothermal gradient/burial). A stronger diagenetic overprint is also indicated by the poor preservation of smectites. Interestingly, total phosphorus (Ptot) content is remarkably high (> 10000 ppm) during the upper Pliensbachian and mainly linked to the presence of phosphatic pellets, whereas a sharp decrease is observed towards the Pliensbachian-Toarcian boundary. The Hg/TOC ratio shows an increase during the T-OAE interval, but these results should be considered with caution due to possible poor organic-matter preservation. Additional analyses of clay and bulk mineralogy are underway and will assist in our study of the impact of the T-OAE and improve our understanding of the general paleoceanographic conditions during this time.