Pliensbachian environmental perturbations and their potential link with volcanic activity: Swiss and British geochemical records

Abstract

The mechanisms leading to environmental and climate change prior to and anticipating the well-studied Toarcian oceanic anoxic event are not completely understood. Specifically, the role of the continuing break-up of Pangea, associated palaeogeographic rearrangements, and the influence of volcanic activity are less well known. Therefore, we studied the Pliensbachian interval in the open marine, hemipelagic section of the Breggia Gorge in southern Switzerland, to elucidate the potential impact of these major changes on the environment and climate. We identified carbon isotope excursions towards lower values in whole-rock carbonates dated from the Sinemurian/Pliensbachian boundary, davoei, margaritatus, and spinatum Zones, and possibly from the Pliensbachian-Toarcian boundary. Using the whole-rock oxygen isotope record and phosphorus content, we associated these events with phases of increased runoff possibly related to climate warming. We also established the first mercury (Hg) record for the entire Pliensbachian, and in order to verify its validity, we analysed an additional section located along the Dorset coast in southern England. The presence of Hg spikes common in both sections may indicate the influence of episodes of increased volcanic activity. The Hg spike during the earliest Pliensbachian may coincide with a late phase of the central Atlantic magmatic province, and/or increased rifting linked with the opening of the Hispanic corridor. Further significant maxima in Hg contents are observed within the davoei and margaritatus Zones, and both phases may correspond to early volcanic activity of the Karoo-Ferrar large igneous province. Further analyses are, however, needed to corroborate this interpretation and evaluate the importance of organic-matter scavenging and detrital input in comparison to volcanic activity.