Oceanic crust is a plausible but unproven habitat for a microbial biosphere of planetary proportions. The upper 500 meters of igneous basement is warm, porous and permeable, forming a global-scale aquifer hosting the circulation of nutrient-rich hydrothermal fluids. Heat from the lower intrusive crustal rocks drives fluid convection cells within this aquifer, even in crust many tens of millions of years old. This thermal convection is strongly modulated by tidal forces, increasing fluid velocities and stirring the sub-surface incubator more vigorously than previously believed. Two unique methods will be used for sampling uncompromised crustal fluids from two contrasting hydrothermal areas on the Juan de Fuca Ridge. Axial Seamount is currently forming directly on the ridge axis, with recent eruptions on the summit and flanks, and has considerable geological and surface biological variability with periods of less than a year. At Axial caldera a sampling facility with a re-entry cone was cemented directly to the seafloor in 1998, providing uncontaminated access to a 50 degrees C diffuse vent. In contrast, Baby Bare (BB) Seamount sits on 3.5 My crust east of the ridge axis, with a small exposed summit of altered rock completely surrounded by low permeability sediments. This project will sample Baby Bare fluids (40 degrees C) below the water/rock interface, using a newly developed break-away coring instrument to insert a 3-meter long titanium tube into the sub-surface, below the biologically active surface zone. The samples obtained from both Axial and Baby Bare will be studied for the entire spectrum of bacteria and archea populations, the fluids will be analyzed for their inorganic and organic chemistry, and the data placed in their full environmental context using temperature and fluid flow measurements. This project attempts to integrate for the first time inorganic and organic chemistry with geophysical parameters including porosity and fluid flow rates, so as to better understand the microbial ecology of these sub-surface biotopes. By examining the chemical and microbial ecology and energetics of the sub-surface, and particularly the sub-surface associated with hydrothermal systems, a framework for studying the prospects of extraterrestrial life may be developed.
