NEW VIEW OF THE OLD OCEAN: A PREVALENCE OF DEEP IRON AND MARGINALIZED SULFIDE FROM THE LATE ARCHEAN THROUGH THE PROTEROZOIC

Recent studies are challenging the simple notion of unfailingly iron-rich (ferruginous) conditions in the anoxic Archean ocean that gave way to either an oxic or euxinic (anoxic and sulfidic) deep ocean throughout the Proterozoic. There is now convincing evidence for euxinic conditions in the late Archean, prior to the Great Oxidation Event—with and without the driving force of incipient atmospheric oxygenation and associated weathering reactions that supplied sulfate to the ocean. Emerging from this work is also a clear view of ocean stratification in the latest Archean, with iron domination in the deeper ocean and indications of at least episodic sulfide at mid depths and oxic surface waters. We assert that such conditions also prevailed throughout the Proterozoic beneath an atmosphere with oxygen at concentrations still well below the present level and support this assertion with a growing data set spanning the Proterozoic. As a corollary, accumulations of dissolved sulfide in the waters were controlled by locally high fluxes of organic matter and/or sulfate delivery, both favored along the ocean margin. The possibility of a persistently iron-rich deep ocean, but a far patchier record of banded iron formations, supports the view that almost all BIFs are anomalous sedimentary deposits linked to increased or local hydrothermal activity. Low sulfate conditions, as fingerprinted by the abundance of isotopically heavy pyrite, were maintained throughout the Proterozoic by still widespread euxinia and associated copious pyrite burial. Nevertheless, a model for a pervasively ferruginous deep ocean over much if not all of the Proterozoic, coeval with frequent euxinia concentrated along the ocean margin including restricted basins, demands that we reevaluate our estimates for past availability of bioessential trace metals. This stratified ocean model, for example, does a better job of explaining the Mo properties of Proterozoic seawater compared to previous models that assumed global euxinia.