Toarcian climate and carbon cycle perturbations – its impact on sea-level changes, enhanced mobilization and oxidation of fossil organic matter

Abstract

Organic geochemistry and palynology reveal major organofacies changes in the marginal marine Polish Basin during the early Toarcian linked to changes in environmental conditions. Increasing water salinity and enhanced biomass contributions from phytoplankton, adopted to brackish conditions, attest to periodic marine ingressions into the paralic, hyposaline Polish Basin. Marine ingressions occurred upon reoccurring sea level rises concomitant with i) the Pliensbachian-Toarcian boundary carbon isotope excursion (Pl-Toa-CIE) and ii) the stepped Toarcian carbon isotope excursion (Toa-CIE), the latter marking the Toarcian Oceanic Anoxic Event (Toa-OAE). Strata with marine influences indicate an intermittent connection of the Polish Basin with the Tethys and attest to a close correlation between regional shoreline progradation and evolution of global climate and sea-level. Organic geochemistry indicated the link between sea level fluctuations and the Toa-CIEs, although in some areas of the Polish Basin the sea level rise was balanced by massively increased sediment delivery, caused by enhanced hydrological cycling that lead to intensified weathering and erosion in the hinterland. Enhanced hydrological cycling further led to the remobilization of organic matter of pre-Toarcian age. This points to erosion of older organic-rich rocks containing significant abundances of marine and terrestrial organic matter that was partly oxidized and re-deposited along with autochthonous Toarcian organic matter. Provenance areas identified point to two main sources, most likely Carboniferous terrestrial/paralic and Ordovician/Silurian marine sediments. Fossil organic matter should be considered as additional source in Toarcian carbon cycling, particularly in times of enhanced hydrological cycling, erosion and weathering. Remobilization of sedimentary fossil carbon has not been hitherto considered as a carbon source during the Toa-OAE.