Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans

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 oxygenation 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 [...]