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 H 2 S-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.