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

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 likely related to a change in the sediment source. Less hydrolytic activity likely 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.