Prolonged reducing and ferruginous oceanic conditions and abrupt global seawater oxidation after oceanic anoxic event 1a

Abstract During the Cretaceous, several massive volcanic episodes led to significant environmental perturbations, including oceanic anoxic events (OAEs). The volcanic eruption forming Ontong Java Nui (ca. 120 Ma) in the western Pacific Ocean is recognized as the largest volcanic event on Earth and is thought to have caused oceanic anoxic event 1a (OAE1a). Although metal emissions from this volcanism are believed to have played a crucial role in triggering the Aptian oceanic anoxia, no direct evidence has been reported so far. Here, we showed that the iron-bearing minerals in the sedimentary sequence of the Mid-Pacific Mountains, deposited near Ontong Java Nui, gradually changed from pyrite and siderite to hematite after OAE1a. Notably, the presence of enigmatic large siderite (FeCO3) grains during the early to mid-Aptian suggests that strongly ferruginous condition prevailed in the Mid-Pacific region, potentially reflecting the substantial iron input from Ontong Java Nui volcanism through hydrothermal activity and volcanic ash emissions. This iron influx likely stimulated ocean primary production by cyanobacteria and potentially contributed to the oceanic anoxia and associated biotic crises of marine plankton. The stratigraphic distribution of siderite indicates that oxygen-depleted and strongly ferruginous conditions triggered by this volcanic event persisted for ~3 m.y. in the Pacific region, even after OAE1a. The disappearance of siderite grains during the mid-Aptian corresponds to the worldwide appearance of hematite-bearing brownish layers, suggesting that global oceanic conditions abruptly became fully oxic due to the weakening of Ontong Java Nui volcanism.