Sulfur geochemistry and microbial sulfate reduction during low-temperature alteration of uplifted lower oceanic crust: Insights from ODP Hole 735B

► Sulfur geochemistry documents effects of the subsurface biosphere in lower ocean crustal gabbros altered at low temperatures. ► The gabbros gain sulfur and have elevated sulfur isotope ratios. ► Sulfur is added to the rocks via microbial reduction of seawater sulfate. ► Under appropriate conditions microbial activity in the subsurface biosphere can affect the chemistry of the lower oceanic crust.