ISSN: 1525‐2027 Extent of the microbial biosphere in the oceanic crust

[1] We estimate the depth of the 120°C isotherm by constructing crustal thermal gradients based on theoretical and observed conductive heat flux as a function of lithospheric age.We chose the 120°C isotherm because it is close to the upper limit for prokaryotic life, and therefore, the isotherm approximates the maximum depth at which life can persist in the ocean crust. The depth of the potential microbial biosphere increases with lithospheric age from approximately 0.5 to 1 km for 1 Ma lithosphere to as much as 5 km at a subduction age of 180 Ma. We use global models of oceanic plate creation to estimate the volume of crust occupied by the biosphere today and throughout geologic time. Presently, the volume of the ocean crust that is capable of containing life is similar to the volume of the oceans (∼1018 m3). Depending on the model used for the growth of continental crust, the volume of rock available to house the subsurface biosphere may have remained constant or doubled since the Archean. Although the thermal models presented here provide estimates for the potential depth and volume of rock in which microbes may live, the biomass in this volume is not well constrained. Using a previously published model, the prokaryotic biomass in the igneous ocean crust is estimated to exceed that in all aquatic and soil environments and is similar to that in the continental subsurface and in marine sediment. Most of the crustal biomass beneath the sediments is likely contained within the extrusive layer, and this has probably been the case since microbes first colonized the oceanic crus