Imbalanced nutrients as triggers for black shale formation in a shallow shelf setting during the OAE 2 (Wunstorf, Germany)

Abstract

Abstract. During the oceanic anoxic event 2 (OAE 2) in the Mid-Cretaceous Period, widespread black shale (BS) formation occurred, reflecting perturbations in major biogeochemical cycles. Here we present geochemical and biomarker data of the OAE 2 from a shelf setting situated at about 100–150 m water depth (Wunstorf, Germany). Our data support that processes inducing BS deposition were related to orbital cyclicity in Wunstorf and that they were not restricted to the time of the OAE 2 carbon isotope excursion. Correlations of total organic carbon (TOC) and δ15N and high relative abundances of functionalized hopanoids (including 2-methylated structures) suggest that BS were formed during times of imbalanced nutrients with high phosphorus inputs and increased cyanobacterial nitrogen fixation. Periods of BS formation were also characterized by enhanced growth of dinoflagellates and bacteriovorous ciliates, the latter supporting the presence of a stratified water body. The lack of biomarkers specific for green sulfur bacteria excludes photic zone euxinia during OAE 2 in Wunstorf. Conflicting maturities and biomarker distributions in kerogen and extractable organic matter and, interestingly, a negative correlation of the diagenetically resistant 2-methyl hopane hydrocarbons with TOC indicate a complex depositional setting at Wunstorf. This might have been induced by high continental runoff during BS formation and the accompanying mobilisation of refractory OM from the shelves and near shore areas.