Depth of active bio‐alteration in the ocean crust: Costa Rica Rift (Hole 504B)

Abstract

Bio‐alteration of basaltic glass in the oceanic crust has lately attained much attention. One of the many questions related to this topic is the depth at which bio‐alteration presently takes place in the oceanic crust. For this purpose we have investigated samples from the deepest drill hole, i.e. Hole 540B at the Costa Rica Rift in the eastern equatorial Pacific. The glassy rim of pillow lava samples show alteration textures and δ13C values compatible with microbial activity throughout the upper 500 m part of the volcanic succession. The concentration and distribution patterns of carbon and potassium within the microbially altered parts, however, indicate that microbes presently interact with the fresh glass to depths of about 380 m into the volcanic basement, at temperatures up to ≈100 °C.