Tracking Euxinia in the Ancient Ocean: A Multiproxy Perspective and Proterozoic Case Study

The evolution and extinction of life are tied intimately to the oxygen state of the ocean, and particularly to the presence of anoxic and H2S-containing (euxinic) water on a global scale. Anoxia and euxinia were more common in the past, relative to today's <0.5% euxinic seafloor. We are able to constrain the distributions of these conditions through a combination of indirect numerical modeling methods and more direct geochemical proxies, with particular emphasis on Fe-S-Mo analysis of fine-grained siliciclastic rocks for the latter. Establishing the spatiotemporal pattern of oceanic redox is more difficult with very old rocks because of the limited availability of well-dated, well-preserved materials that span shallow and deep environments across time lines. Despite these difficulties, the multiple approaches synthesized in our case study point to global oxygen-deficiency in the deep ocean and perhaps euxinia during most, if not all, of the Proterozoic and likely extending into the early Paleozoic.