(Table 1) Age, thickness of sedimentary cover, depth into volcanic basement, calcite content, and carbon and oxygen isotope data of glassy and crystalline samples from the Atlantic Ocean and the Lau Basin

Textural and DNA studies of pillow lavas in DSDP/ODP cores from the Atlantic Ocean, the Lau Basin and the Costa Rica Rift indicate that microbes had a significant role in the alteration of basaltic glasses. Carbon isotopes (d13C) in carbonates from glassy and crystalline basalts from these locations also show differences that may relate to microbial activity during alteration. The generally low d13C values (< -7 per mil) in the basaltic glass of a high proportion of samples from the Atlantic, and most of the Costa Rica Rift material were attributed to alteration influenced by Bacteria and oxidation of organic matter. Positive d13C values of some samples from the Atlantic suggest lithotrophic utilization of CO2, in which methanogenic Archaea produced CH4 from H2 and CO2. This may result from higher abiotic production of H2 in the slow-spreading, fault-dominated Atlantic crust, due to more extensive serpentinization than at the intermediate-spreading Costa Rica Rift.