The primary goal for our LEXEN program is to test the hypothesis that there is a substantial microbial biosphere within oceanic crust. A critical component of this study is to learn how this biosphere varies under different crustal-geothermal regimes, and how the organic carbon evidence of the sub-surface microbial activity is manifested at sites of widely different crustal age and sediment cover. We will be determining the processes that control the distribution of life within the crust, specifically within (a) young crust with high temperature fluids vs old crust with warm fluids, (b) sedimented vs un-sedimented crustal sections, oxygenated crustal fluids vs anoxic fluids, and (d) long fluid residence times vs short residence times. A plausible answer to the question regarding the existence of a sub-surface biosphere requires that we characterize the source fluids in terms of their organic composition and microbial taxonomy (lipids and molecular genetic probes), identify the fluid transit paths, the depth of fluid circulation and residence times in order to determine the area/volume of the crustal rocks that we are sampling. Finally, it is important that we make a first-order estimate of how globally we can extrapolate our results, by comparison of data from both old and young sites at the same spreading center.
