A marine Mo-isotope record across OAE1a

Abstract

Although the oceans have remained well oxygenated for most of the Phanerozoic, the stratigraphic record is punctuated by transient events reflecting an apparent worldwide increase in marine anoxia. The Cretaceous oceans appear to have been particularly prone to the development of marine anoxia, and feature a number of discrete oceanic anoxic events (OAEs; [1]). Research has shown that OAEs were broadly contemporaneous with the emplacement of large igneous provinces (LIPs), abrupt global warming and significant marine biotic changes. OAEs were also associated with substantial changes in the global carbon cycle, intensification of the hydrological cycle and ocean acidification. Whilst it is clear from direct observation that unusually high levels of marine organic carbon accumulated globally during OAEs, it is much harder to quantify the lateral increase in marine anoxia. Most redox proxies reflect local conditions either within the water column or beneath the sediment water interface. In contrast, the Mo-isotope system has, under certain circumstances, the unique potential to reflect the areal extent of marine anoxia globally [2, 3]. Here we present new Mo-isotope results and a comprehensive suite of other geochemical data derived from a section across Cretaceous OAE 1a (the Selli event) at Gorgo a Cerbera, in the Umbria-Marche region of Italy. Integration of our new data with an existing Os-isotope stratigraphy [4] enables us to track the course and development of marine redox fluctuations during the OAE in relation to the emplacement of the coeval Ontong-Java LIP. We observe significant shifts in the Mo-isotopic composition of seawater to light isotopic ratios. Taken together, the isotopic and trace element data provide unique insights into how the chemical composition and redox state of the Cretaceous oceans, and the contemporaneous marine biota, were affected by the emplacement of the Ontong-Java volcanics