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

[摘要] During the oceanic anoxic event 2 (OAE 2) in the Mid-Cretaceous Period, widespreadblack shale (BS) formation occurred, reflecting perturbations in majorbiogeochemical cycles. Here we present geochemical and biomarker data of theOAE 2 from a shelf setting situated at about 100–150 m water depth(Wunstorf, Germany). Our data support that processes inducing BS depositionwere related to orbital cyclicity in Wunstorf and that they were notrestricted to the time of the OAE 2 carbon isotope excursion. Correlationsof total organic carbon (TOC) and δ¹⁵N and high relativeabundances of functionalized hopanoids (including 2-methylated structures)suggest that BS were formed during times of imbalanced nutrients with highphosphorus inputs and increased cyanobacterial nitrogen fixation. Periodsof BS formation were also characterized by enhanced growth ofdinoflagellates and bacteriovorous ciliates, the latter supporting thepresence of a stratified water body. The lack of biomarkers specific forgreen sulfur bacteria excludes photic zone euxinia during OAE 2 in Wunstorf.Conflicting maturities and biomarker distributions in kerogen andextractable organic matter and, interestingly, a negative correlation of thediagenetically resistant 2-methyl hopane hydrocarbons with TOC indicate acomplex depositional setting at Wunstorf. This might have beeninduced by high continental runoff during BS formation and the accompanyingmobilisation of refractory OM from the shelves and near shore areas.

[学科分类] 地球化学与岩石