IMPLICATIONS OF MASS-INDEPENDENT FRACTIONATIONS FOR THE EARTH'S EARLY SULFUR CYCLE

Sulfur isotope measurements for a variety of different types of Archean samples reveal mass-independent compositions that are absent in the recent rock record. These anomalous compositions are interpreted to reflect the influence of atmospheric photochemistry on the Archean sulfur cycle. The implications for the Archean Earth are three-fold. Archean atmospheric conditions (UV transmission, oxidation state, composition) enabled the source photochemical reactions to occur; The Archean atmospheric environment permitted transfer this mass-independent signature to Earth's surface; and Oxidative weathering and biological activity did not homogenize the signature once it was imparted to the surface sulfur cycle. Our analyses to date indicate a range of D³³S values (+3.5 to -1.3 ‰). These analyses show that the anomalous signature is widespread and of global significance. We have identified a possible source reaction (deep-ultraviolet photolysis of sulfur dioxide). The occurrence of this reaction, the transfer of its signature to the surface, and the preservation of this signature in the Archean rock record imply much lower oxygen contents - an upper limit of at most, a few percent of that in the present atmosphere. All barite analyzed to date has negative D³³S. Much of this barite has been suggested to be of hydrothermal origin, but the negative D³³S precludes a magmatic source of sulfate, leading to the suggestion that sulfate derives from surficial sources (possibly oceanic). (Negative D³³S sulfate is also generated by deep UV photolysis of SO₂.) The alternative to an oceanic source is the possibility that the sulfur in barite reflects incorporation of locally-derived atmospheric sulfate rather than the global oceanic signature. Resolution of this question may come from analysis of trace sulfate in carbonates.