Global versus local processes during the Pliensbachian–Toarcian transition at the Peniche GSSP, Portugal: A multi-proxy record

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE, ~ 183 Ma) was an episode of extreme warmth, environmental changes and carbon cycle perturbation. We present a high-resolution study of the Peniche section (Lusitanian Basin, Portugal), recently defined as the Toarcian Global Stratotype Section and Point, to provide a review of the Pliensbachian–Toarcian (Pl–To) environmental and climatic changes. A large ~7‰-negative carbon-isotope excursion (CIE) is recorded within the T-OAE interval in bulk organic matter. Distinct small-scale δ13C negative shifts, modulating the T-OAE carbon isotope signal, are correlated across different European basins, which have implications on the estimation of the T-OAE duration. The increase in kaolinite content and chemical index of alteration from the Pl–To boundary up to the T-OAE interval is interpreted as reflecting a shift towards warmer and wetter climate conditions. The common occurrence of coarse-grained turbidites within the T-OAE interval is possibly linked with changes of the hydrological cycle and storm intensity. Low total organic carbon contents together with the absence of redox-sensitive trace element enrichments indicate oxygen-deficient conditions unfavourable for organic matter preservation. Mercury data show a clear increase at the base of the T-OAE CIE followed by a return to background values, providing further evidence that Karoo-Ferrar volcanism might have triggered the series of environmental disturbances during the early Toarcian. However, the highest mercury values are recorded in samples containing abundant pyritized wood fragments, highlighting the influence of local chemical processes on mercury enrichments, which should be considered before any interpretation of the Hg records.