Carbonate-platform response to the Toarcian Oceanic Anoxic Event in the Tethys Himalaya

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) was a profound short-term environmental perturbation associated with the large-scale release of 13C-depleted carbon into the global ocean-atmosphere system, which resulted in a significant negative carbon-isotope excursion (CIE). The general lack of characteristic T-OAE records outside of the northern hemisphere means that the precise environmental effects and significance of this event are uncertain. Many biotic carbonate platforms of northern hemisphere from the western Tethys drowned or shifted to comparatively unfossiliferous oolitic platforms during the early Toarcian. However, southern hemisphere records of Toarcian carbonate platforms are rare, and thus the extent and significance of biotic platform demise during the T-OAE is unclear. Here we present high-resolution biostratigraphical, sedimentological, and geochemical data across two Pliensbachian–Toarcian shallow-water carbonate-platform sections exposed in the Tethys Himalaya. These sections were located paleogeographically on the open southeastern tropical Tethyan margin in the southern hemisphere. The T-OAE in the Tethys Himalaya is marked by a negative CIE in organic matter. Our sedimentological analysis of the two sections reveals an abundance of storm deposits within the T-OAE interval, which emphasizes a close link between warming and tropical storms during the T-OAE, in line with evidence recently provided from western Tethyan sections of the northern hemisphere. In addition, our analysis also reveals extensive biotic carbonate-platform crisis by drowning or changing to unfossiliferous carbonates coincident with the onset of the CIE where the proxies of continental weathering (e.g. Ti, Sc, Th) and redox (e.g. Mn, Ce and Ce) show obviously increase. Taken together, the drastically enhanced terrigenous flux and deoxygenation likely played a pivotal role in the more severe crisis for benthic carbonate producers during the negative phase of the CIE.