Organic Matter in the Early Toarcian Shales, Paris Basin, France: a Response to Environmental Changes

A freshly drilled core from the Paris Basin contains a 16 m section of Early Toarcian Shale deposition. The organic matter and carbonates in these black-shale sediments were investigated using isotopic, petrologic and molecular organic geochemical methods. Throughout the studied section, molecular organic geochemical and organic petrographic characteristics remain remarkably uniform and suggest that the organic matter is overwhelmingly of marine origin (bacterial and algal) with only minor contributions of terrigenous organic matter. In contrast, TOC content of the sediments, hydrogen indices of the kerogens, isotopic composition of co-existing kerogens and carbonates, and isotopic fractionation between the organic and inorganic carbon vary greatly and follow systematic trends. In general, higher values of hydrogen indices and TOC content are associated with the significantly depleted 13C values in both kerogens and carbonates and greater values of carbon isotopic fractionation. The integration of carbon isotopic fractionation between carbonates and kerogens (a proxy for surface water [CO2(aq)]) and hydrogen indices (a reflection of organic matter preservation) suggests that the greatest accumulation of hydrocarbon-rich organic matter occurred at a time when surface water [CO2(aq)] was extremely high. This implies that intermediate waters enriched in 12C-CO2 and [CO2(aq)] were recycled into the photic zone of the surface water masses and provided the waters necessary to produce the observed negative isotopic excursions and maximum carbon isotopic fractionations. The settling of organic matter through a primarily anoxic water column with deposition in a highly reducing and quiescent bottom water mass was ideal for the extraordinary preservation of the organic matter. In turn, these results suggest that the depositional environment prevailing during the early Toarcian in the Paris Basin was a stratified-silled basin type environment