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How much dissolved oxygen was present in the mid-Proterozoic oceans between 1.8 and 1.0 billion years ago is debated vigorously. One model argues for oxygénation of the oceans soon after the initial rise of atmospheric oxygen ~2.3 billion years ago. Recent evidence for H^sub 2^S in some mid-Proterozoic marine basins suggests, however, that the deep ocean remained anoxic until much later. New molybdenum isotope data from modern and ancient sediments indicate expanded anoxia during the mid-Proterozoic compared to the present-day ocean. Consequently, oxygenation of the deep oceans may have lagged that of the atmosphere by over a billion years.
Understanding how and when the Earth's early atmosphere and oceans became rich in oxygen is essential to our understanding of early life on Earth and to the search for life beyond the solar system (1-3). A large body of geochemical and sedimentological evidence points to a substantial rise in atmospheric oxygen ~2.3 billion years ago (Ga) (4-6). The history of ocean oxygenation is less clear. The occurrence of banded iron formations (BIFs) until ~1.8 Ga suggests deep oceans with high concentrations of ferrous iron until this time and therefore, because of the low solubilities of ferric oxides, little dissolved O^sub 2^ (4). The subsequent disappearance of BIFs may indicate the beginning of persistent deep-sea oxygenation, which continued with only brief interruptions to the present day (4). This model of ocean redox evolution is challenged by the recent proposal that euxinic (anoxie and sulfidic) conditions were common in the deep oceans from ~1.8 Ga to at least 1.0 Ga, after which atmospheric and deep-sea oxygen levels approached modern values (7-9). Euxinic conditions could also account for the disappearance of BIFs because of the low solubilities of ferrous sulfides (8). This debate has important implications for evolution linked to the availability of redox-sensitive, bioessential trace metals (2) and for climate, because the flux of biogenie methane from oceans to atmosphere is sensitive to ocean oxygenation (10, 11).
Sulfur isotopes and sedimentary iron chemistry indicate that some mid-Proterozoic sedimentary basins were oxic at the surface and euxinic at depth (9, 12, 13). Sulfur isotope data further suggest that sulfate concentrations in mid-Proterozoic oceans were < 1/10th the modern value (9, 12-14). However, low sulfate does not uniquely indicate...