3.  The geochemistry of sulfur during serpentinization of oceanic ultramafic rocks  Slow-spreading mid-ocean ridges undergo tectonic extension, resulting in large exposures of ultramafic rocks.  My past work has examined hydrothermal exchange of sulfur between ocean crust and seawater, and this project fills a missing gap. Results on three sample suites (Hess Deep, Iberian Margin, and MARK) reveal serpentinization is a function of temperatures and fluid fluxes (Alt and Shanks, 1998a,b). Where temperatures and fluid fluxes are high, inorganic sulfate reduction leads to sulfide and 34S enrichment (Alt and Shanks, 1998b).  At low temperatures microbial sulfate reduction can be significant, utilizing hydrogen and methane produced during serpentinization, and resulting in addition of 34S-depleted sulfide to the rocks (Alt and Shanks, 1998a).  Serpentinization results in an estimated sink for seawater sulfur of 0.2-6.0 x 1012 g S y-1 (Alt and Shanks, 1998b).  This is comparable to sulfur exchange in hydrothermal systems in mafic ocean crust at mid-ocean ridges and on ridge flanks (2.1-2.5 x 1012 g S y-1; Alt, 1995), making serpentinization a significant part of the sulfur budget for ocean crust.