Paleoenvironment in the circum-arctic region from the Middle Jurassic to Early Cretaceous: Trace element and stable isotope geochemistry of the Agardhfjellet Formation, Svalbard

Abstract

From the Middle Jurassic to Early Cretaceous, the mudstone-dominated Agardhfjellet Formation was deposited during the prolonged shelf dysoxic-anoxic event (SDAE) recorded predominantly in the circum-arctic polar regions. Commonly, oceanic anoxic events (OAEs) throughout Earth's history are linked to the emplacement of large igneous provinces (LIPs). OAEs and LIPs, alone or jointly, are inferred to be triggers for severe biotic crises in Earth's history. However, the causes for SDAEs remain uncertain. To elucidate these causes, we document the geochemical features of the Agardhfjellet Formation using trace element concentrations and stable isotope ratios. The Agardhfjellet Formation, which consists of the Oppdalen, Lardyfjellet, Oppdalssåta, and Slottsmøya members in ascending order, formed during ∼13 Myr of prolonged dysoxic, anoxic, and euxinic conditions. The onset of shelf anoxia took place between deposition of the Oppdalen and Lardyfjellet Members. The extent of sea floor anoxia in the basin varied within the Lardyfjellet Member but likely persisted throughout most of the Slottsmøya Member. Mo/TOC and MoEF/UEF ratios in the sedimentary rocks are consistent with the anoxic events. Enhanced continental runoff marked the Oppdalen, Oppdalssåta, and Slottsmøya members, contrasting with most of the Lardyfjellet Member, in which authigenic phases were widespread under anoxic conditions. Our results show that, compared to other OAEs, such as the Toarcian OAE (∼183 Ma) or the OAE2 (∼94 Ma), the Late Jurassic – Early Cretaceous SDAE recorded in the Agardhfjellet Formation is characterized by prolonged suboxic–anoxic condition, substantial water-mass stratification, and increased continental runoff.