The Toarcian Oceanic Anoxic Event in southwestern Gondwana: an example from the Andean Basin, northern Chile

Abstract : Palaeoenvironmental and palaeoclimatic changes during the Toarcian Oceanic Anoxic Event (T-OAE) are reported from two successions deposited in the Andean Basin (Chile), based on a high-resolution biostratigraphic (calcareous nannofossils), sedimentological, mineralogical and geochemical analysis. A large negative carbon-isotope excursion was identified in a lower Toarcian storm-dominated marl-limestone alternation, underlining the global extent of the carbon-cycle perturbation and the occurrence of episodic high-energy conditions during the T-OAE. Low total organic carbon content and absence of redox-sensitive trace-element enrichment indicate that the dynamic depositional conditions did not favour the development of oxygen-depleted conditions and the preservation of organic matter. The decrease in kaolinite reflects a shift to more arid conditions, whereas the increase in the chemical index of alteration at the onset of the T-OAE is probably related to a change in the sediment source. Less hydrolytic activity probably promoted a decrease in nutrient input in the Andean Basin. Mercury anomalies support the link between the onset of the Karoo-Ferrar large igneous province activity, the carbon cycle perturbation and the generated environmental change. This paper presents one of the most complete T-OAE studies from the southern hemisphere, and reveals that regional conditions strongly modulated the expression of this global event in the Andean Basin.