Discovery of the Callovian Oceanic Anoxic Event in the Qiangtang Basin, eastern Tethys: Insights from in situ calcite UPb dating

Abstract

Oceanic anoxic events (OAE) are global carbon-cycle perturbations and major paleoenvironmental changes documented by deposition of black shales rich in organic matter. Jurassic black shales exposed in the Biluo Co Section of the Qiangtang Basin (central Tibet) display a ∼ 3 % negative carbon isotope excursion (CIE) previously regarded as documenting the Toarcian T-OAE based on poorly preserved ammonites and one detrital zircon age. New biostratigraphic data on diagnostic calcareous nannofossils and ammonite assemblages have suggested a younger, Middle Jurassic age, thus raising doubts about the presence of the T-OAE in the Qiangtang Basin. We here present the first accurate in situ LA-ICPMS calcite UPb ages for carbonate layers intercalated in the Biluo Co section. Ages of 165.6 ± 3.7 Ma, 166 ± 16 Ma, and 164.0 ± 7.2 Ma obtained from two samples analyzed in two different laboratories indicate the Late Bathonian to mid-Callovian age, which is consistent with recent age assignments based on calcareous nannofossils and ammonites. Petrographic, geochemical, and C–O isotope analyses testify to only minor diagenetic effects, indicating that geochronological data do reflect the original timing of carbonate deposition. Therefore, the new age assignment does not document the T-OAE in the Qiangtang Basin, but possibly a younger, Callovian global-warming event.