Drowning of a carbonate platform as a precursor stage of the Early Toarcian global anoxic event (Southern Provence sub-Basin, South-east France).

Carbonate platforms: archives of past global change Special Issue; 29 pagesInternational audienceThe Toarcian Oceanic Anoxic Event is well-known as coinciding with a carbonate crisis, coupled with organic matter accumulation and perturbation of the carbon cycle expressed by carbon-isotope excursions. In this palaeoenvironmental setting, the present research attempts to better constrain the palaeoenvironmental conditions leading to the drowning of a carbonate platform during Late Pliensbachian to Early Toarcian times. This study is based on the integrated sedimentological, diagenetic and geochemical (stable isotopes and Rock-Eval pyrolysis) analysis of several stratigraphic successions located in the Southern Provence sub-Basin (South-east France). Eodiagenetic ferroan calcite cements below two hardground surfaces at the Pliensbachian-Toarcian transition record episodic eutrophication events. Such events were precursor stages of a global palaeoenvironmental perturbation (i.e. a global carbon cycle perturbation) that led to the demise of the carbonate platform and occurred prior to the Toarcian Oceanic Anoxic Event. Subsequently, the acme of the palaeoenvironmental deterioration that coincided with the Toarcian Oceanic Anoxic Event is recorded by a pronounced negative carbon-isotope excursion -3‰) followed by a positive one (2.87‰), organic matter preservation and inferred high temperature of surface sea water (ca 25°C) as given by oxygen-isotope values from chondrichthyan tooth enamel. Sedimentological comparisons, as well as chemostratigraphic and biostratigraphical correlations between the Early Toarcian sedimentary series of the Southern Provence and Eastern Dauphinois sub-basins, indicate that tectonic tilting occurred earlier in the Southern Provence sub-Basin; this resulted in the preservation of organic-rich deposits in the first sub-basin and depositional hiatus in the second sub-basin. Thus, local conditions, such as contrasting structural setting, could markedly modify the sedimentary signature of a global climatic event