Contents volume 131

Abstract

Microbes have been identified within altered parts of the glass rims of pillow lavas in the upper oceanic crust, 237 m below the top of the volcanic basement of ODP Hole 896A at the Costa Rica Rift. Their presence is verified by spherical and vermicular microbodies containing DNA. The elemental composition of the microbially processed areas differ from the parent glass. Further, the microbially processed parts, showing different morphological forms also show different elemental composition. Extreme K-enrichment indicate that microbes are presently active in the alteration process.The carbon isotopic composition of disseminated carbonates within the basaltic section of the Hole 504B (1 km distance from Hole 896A) also give evidence for microbial activity during rock alteration. The σ13C values of most of these trace carbonates are very low, reflecting metabolic control of the carbon cycle in these rocks.Microbial alteration of basaltic glass, comprising a substantial volume and surface area of the upper oceanic crust, may thus play an important role in the element exchange between oceanic crust and seawater.