The Aptian (Early Cretaceous) oceanic anoxic event (OAE1a) in Svalbard, Barents Sea, and the absolute age of the Barremian-Aptian boundary

Abstract

The age of the Cretaceous Barremian-Aptian boundary has been a long-standing problem in the geological timescale. Age suggestions range between 126Ma and 117Ma, making causal links between environmental crises and large-scale processes such as large igneous provinces (LIPs) problematic. New stratigraphic and radiometric data from three drillcores in Svalbard show that Barremian shelf siltstones and sandstones are overlain by Aptian shallow-marine mudstones. The organic carbon content in the mudstones is higher than 1.0wt%, with values ranging up to 70wt% in coal-dominated intervals. Carbon isotope measurements of the TOC show a negative δ13C excursion of 4.5‰ followed by a positive excursion of 6.7‰ within a 20-m interval, identified as the early Aptian Oceanic Anoxic Event 1a (OAE1a). This isotopic pattern of the excursion is in perfect agreement with excursions from lower latitude sections of the same age. A ~10-cm-thick zircon-bearing bentonite was discovered in the lower part of the Barremian, about 40m below the OAE1a interval. U-Pb TIMS zircon analyses of two equivalent bentonites in adjacent boreholes gave a robust age of 123.1±0.3Ma. Based on the Barremian depositional systems, we estimate that a time interval of 1–2Myr separates the bentonite and the OAE1a, providing an age of about 121–122Ma for the Barremian-Aptian boundary. Our results stress that the geological timescale needs revision, and provide a benchmark test for the correlation between the OAE1a and LIPs in general.