Paleosalinity determination in ancient epicontinental seas: A case study of the T-OAE in the Cleveland Basin (UK)

Abstract

The Early Jurassic Toarcian oceanic anoxic event (T-OAE) was marked by deposition of organic-rich sediments and a pronounced negative carbon-isotope excursion across the Northwest European Sea (NWES), representing a signal that was stronger regionally than globally. Various models for the T-OAE have been proposed, some focusing on general atmospheric-oceanic controls to reflect its global character and others emphasizing local or regional factors to account for its strong expression in the NWES. In this study, we evaluate watermass dynamics of the Early Jurassic Cleveland Basin (UK) during the T-OAE using a combination of paleosalinity proxies (B/Ga, Sr/Ba, and S/TOC). These proxies suggest that brackish conditions existed prior to onset of the T-OAE, with a shift to nearly freshwater conditions during the T-OAE, reflecting a large increase in terrestrial runoff due to a stronger hydrological cycle, possibly combined with shallowing of the marginal sill of the Cleveland Basin through sea-level fall. Following the T-OAE, basinal salinity conditions first returned to brackish and then gradually shifted to fully marine. This hydrographic history accounts for low Mo concentrations in T-OAE sediments of the Cleveland Basin, demonstrating control by watermass restriction and local (rather than global) aqueous Mo drawdown. These findings demonstrate the importance of local hydrographic controls on the sedimentary expression of the T-OAE. The more general significance of this study is that it highlights the need for application of paired salinity and redox proxies in order to correctly interpret paleoenvironmental variations in marginal-marine basins.