Redox element record shows that environmental perturbations associated with the T-OAE were of longer duration than the carbon isotope record suggests – the Aubach section, SW Germany

Abstract

The Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE) with its associated carbon-isotope excursion (CIE) was possibly one of the most pronounced periods of widespread oxygen deficiency in the Mesozoic ocean. The event has been extensively studied in order to understand the processes triggering the environmental perturbations and the extreme oxygen depletion in many marine basins. However, comparatively little focus has been placed on the end of the positive CIE and the stratigraphic coherent end of anoxiceuxinic conditions. In the present study, we constrain the stratigraphic extent of anoxic-euxinic conditions and define the termination of the positive CIE in the Swabo-Franconian Basin covering the Lower Toarcian strata using carbon-isotope ratios, organic matter pyrolysis and redox-sensitive element concentrations of outcrop samples from the Aubach section. Bulk organic carbon-isotope values, corrected for changes in type of organic matter using the Hydrogen Index (HI), suggest that the amplitude of the negative CIE in organic matter is as little as 3.3–3.5 ‰, in contrast to 4.5 ‰ change in δ13 Ccarb in the same section. Enrichment in redox-sensitive proxies (V/Al and DOP-T) and %TOC suggest that environmental perturbations associated with the T-OAE continued until the upper falciferum Zone in the Aubach section. This indicates that anoxic–euxinic conditions terminated in the same stratigraphic interval in which δ13 C values return to steady, light values at ~–28 ‰ (termination of positive CIE). This synchronism in the return to normal marine conditions is also observed in the southern Paris Basin, but not in the Cleveland Basin.