Microbial fractionation of carbon isotopes in altered basaltic glass from the Atlantic Ocean, Lau Basin and Costa Rica Rift

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 (δ13C) in carbonates from glassy and crystalline basalts from these locations also show differences that may relate to microbial activity during alteration. The generally low δ13C values (<−7‰) 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 δ13C 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.