The influence of Ontong Java Plateau volcanism on oceanic deoxygenation during Oceanic Anoxic Event 1a: evidence from Mo isotope

Oceanic Anoxic Event 1a (OAE 1a, ~120Ma) during the early Aptian represents a dramatic perturbation of the global carbon cycle and is associated to widely deposition of black shale [1]. The emplacement of the Ontong Java Plateau is considered to be the trigger of OAE 1a, which led to a series of environmental perturbations including marine anoxia [2]. However, whether Ontong Java Plateau volcanism is the direct driving factor of oceanic deoxygenation is still under debate. Most research suggested that the eruption of Ontong Java Plateau injected enormous CO2 into the atmosphere-ocean system and accelerated continental weathering, which eventually resulted in ocean anoxic/euxinic conditions [1]. Few argued that oceanic deoxygenation was an immediate response to the Ontong Java Plateau volcanism [3]. Here, we select DSDP Site 463 and ODP Site 866A in the Pacific Ocean near Ontong Java Plateau as study sections. We present high-resolution δ98Mo records to reflect changes in ocean redox conditions and use Mn and Fe abundances and Eu/Eu* ratios to resolve volcanic phases. Combined with other redox, biological productivity, and weathering proxies, we reconstruct the history of Ontong Java Plateau eruption and ocean redox environment across OAE 1a. Our data reveal that oceanic deoxygenation started before OAE 1a and was driven by Ontong Java Plateau volcanism instead of continental weathering. Volcanically sourced nutrients fluxed into the ocean and stimulated local organic productivity, resulting in ocean deoxygenation. During OAE 1a, ocean maintained anoxic/euxinic conditions. The coeval global seawater δ98Mo was probably around or greater than 2.1‰.   [1] Jenkyns, 2010, Geochemistry Geophysics Geosystems 11. [2] Percival et al., 2021, Global and Planetary Change 200. [3] Bauer et al., 2021, Geology 49, 1452-1456.