ISOTOPE GEOCHEMISTRY OF OXYGEN IN THE SEDIMENTARY SULFATE CYCLE

A re-evaluation of the isotopic geochemistry of oxygen involved in the exogenic sulfur cycle leads to a model in which δ 18OSO of seawater is determined by a dynamic balance of four fluxes: inputs by erosion of evaporite sulfate and by oxidative erosion of sulfide, and outputs of sulfate into evaporites and of carbon dioxide into seawater by bacterial reduction of sulfate to sulfide. The sulfate deposition-erosion loop is closed, but the sulfide loop is open, connected through the fixed reservoir of 18OH of seawater. The level of δ 18OSO is apparently not appreciably affected by either equilibration with 18OH, or by Lloyd's proposed fast reduction-oxidation cycle on the sea floor. The possible effect of equilibration during a subsea hydrothermal circulation is unclear from available data. Calculations of hypothetical equilibration of 18O 16OSO with 18O 16OH, either from the dehydration of gypsum or from formation waters, give δ 18OSO values much higher than any observed in old evaporites. Consequently, these processes were probably not significant in altering the δ 18OSO of seawater that is recorded in evaporites. © 1979.