The Extent of the Oceanic Biosphere Throughout Geologic Time

The extent of microbial habitability in the ocean crust is a subject of considerable interest. Current data suggests that microbial activity ceases above approximately 120° C, and we use this isotherm as a proxy for determining the depth of the biosphere in the ocean crust. We first estimate the depth of this isotherm as a function of lithospheric age between 1 to 180 Ma by using theoretical conductive heat loss estimates. This model predicts that the maximum depth of the biosphere ranges from approximately 0.5 km at 1 Ma to nearly 5 km at 180 Ma. We then take into account the effect of hydrothermal activity in lithosphere less than 65 Ma. We first approximate this effect by using a close fit to observed conductive heat flow versus age to construct a geotherm. This approach suggests that the 120° C isotherm may be more than 1 km deeper in younger lithosphere than estimated from the theoretical conductive cooling curve. We then attempt to account for hydrothermal cooling more realistically by constructing temperature profiles that would result from either fluid upflow or downflow at constant velocity and that fit observed conductive heat flow data versus age between 1 and 65 Ma. These profiles suggest that estimates based on conductive geotherms in young crust provide over estimates of the depth of the biosphere as a function of lithospheric age. Consequently, the models provide good end-member constraints on the depth of the biosphere as a function of lithospheric age even in the presence of hydrothermal heat loss. By multiplying the estimate for the depth of the biosphere by the rate of plate production and integrating over time, we obtain estimates of the volume of the biosphere in the present oceanic crust. Finally, we extend our results backward through geologic time by considering the effects of greater rates of plate production, younger lithospheric subduction ages, and models of continental growth.