Osmium-isotope evidence for the interplay between submarine volcanism and continental weathering during the early Aptian Oceanic Anoxic Event (OAE 1a)

In the early Aptian, the ocean experienced a global phenomenon of widespread deposition of organic carbon-rich sediments under oxygen-poor conditions: the Oceanic Anoxic Event 1a (OAE1a or Selli Event: ~120 Ma). Triggering mechanisms for OAE1a are thought to be an exceptional episode of submarine volcanism, namely the emplacement of the Ontong Java Plateau (OJP), associated with global warming and enhanced primary productivity. Osmium- isotope ratios are used as a high-resolution geochemical proxy because they potentially offer information on the timing, duration and proportion of globally significant volcanic/hydrothermal activity versus continental weathering. Here, we consider new and published geochemical, isotopic and biotic data collected for the OAE1a interval. New Os-isotope analyses have been produced from two sections: DSDP Site 463 (Mid-Pacific Mountains, situated relatively close to the Ontong-Java province at the time of its emplacement; and the Cismon core (Southern Alps, N. Italy) that accumulated in the margins of the Tethyan Ocean. Both sections are well dated through bio-, magneto-, chemo-, and cyclostratigraphy, providing high-resolution time control and correlation. Variations in the 187Os/188Os ratios are suggestive of both massive magmatic pulses and accelerated continental weathering. The Ontong Java emplacement started before and then persisted very intensely for most of OAE1a (as conventionally defined). The dominant feature is a ~880 kyr-long unradiogenic Os-isotope interval, requiring a large contribution from a mantle source, and implying a homogenous Os-isotope composition of seawater. A short-lived (~34 kyrs) radiogenic Os-isotope spike has been detected in the early phase of OAE1a. These high-resolution records also show how volcanism and the associated marine anoxia were related to changes in marine biota that took place before and during the interval of global anoxia.