THE EVOLUTION OF THE EARTH SURFACE SULFUR RESERVOIR

The Earth-surface sulfur reservoir is connected to the Earth’s interior through various channels including volcanic outgassing, mid ocean ridge volcanism, basalt-seawater exchange, and subduction. The absolute and relative magnitudes of these sources and sinks have likely changed through history in response to decreasing heat flow and variations in ocean chemistry. A strong sulfur sink by subduction, which would have been maintained through periods of iron-rich and sulfide-rich ocean bottom water, likely limited the build-up of the surface sulfur reservoir until the oceans became persistently oxic. With this scenario, most of the present-day surface sulfur reservoir accumulated during the Phanerozoic. This model is consistent with the sulfur isotope record, which does not support separation of a substantial oxidized sulfate reservoir until the late Precambrain or early Phanerozoic. Peculiarities of late Neoproterozoic ocean chemistry, and traffic between the mantle and the surface sulfur reservoir, may also help to explain a large sulfur isotope mass imbalance during the Neoproterozoic.