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 (delta C-13) in carbonates from glassy and crystalline basalts from these locations also show differences that may relate to microbial activity during alteration. The generally low delta C-13 values (< - 7<parts per thousand>) 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 delta C-13 values of some samples from the Atlantic suggest lithotrophic utilization of CO2, in which methanogenic Archaea produced CH4 from H-2 and CO2. This may result from higher abiotic production of H-2 in the slow-spreading, fault-dominated Atlantic crust, due to more extensive serpentinization than at the intermediate-spreading Costa Rica Rift. (C) 2001 Elsevier Science B.V. All rights reserved.