A seawater-sulfate origin for early Earth's volcanic sulfur

Mass-independent fractionation of sulfur isotopes (MIF-S)-as recorded primarily in pre-2.5 billion years ago (Ga) sedimentary rocks-has been interpreted as evidence of photolysis of volcanic SO(2)in an anoxic troposphere. Here, I present thermodynamic and kinetic calculations, combined with data on the geology, mineralogy and chemical and isotopic compositions of modern and Archaean (3.8-2.5 Ga) aged volcanic samples from different tectonic settings, to examine early Earth's sulfur cycle. Based partly on the similarities between submarine hydrothermal deposits and arc volcanic rocks in pyrite (FeS2) abundances and sulfur isotopic compositions (for example, the presence of both positive and negative delta S-34 values), I conclude that degassing of sulfur (mostly as SO2) into the atmosphere has been carried out primarily by subaerial eruptions of oxidized, arc-like magmas since at least 3.5 Ga. The generation of volcanic SO(2)requires plate tectonics and the involvement of sulfate-rich seawater, which requires large exposed lands and an oxygenated atmosphere. I propose that the MIF-S signatures in sedimentary rocks were created by ultraviolet photochemical reactions between SO(2)from explosive volcanic eruptions and O(2)in the stratosphere, above an oxygen-rich troposphere, or by high-temperature reactions between organic compounds and sulfate in the oceans. Formation of mass-independent isotope fractionation of sulfur signatures recorded in Archaean sedimentary rocks could have occurred in an oxygen-rich atmosphere, according to thermodynamic and kinetic calculations and analysis of Earth's early sulfur cycle.