| Abstract: | The oceanic upper crustal reservoir is a 600‐m thick layer of porous and permeable basaltic rock that forms the uppermost igneous basement underlying the global ocean. Pore spaces within this fluid aquifer contain a significant fraction of the global seawater, and active circulation through this reservoir has profound influence on the chemical composition of the ocean, strongly impacting the biological environment near the sea surface. Because of the relative inaccessibility of the deep seafloor, where hydrothermal fluid discharges and seawater re‐charges the oceanic crustal aquifer, our understanding of the dynamic physical, chemical and biological processes is strongly dependent on our ability to obtain uncontaminated samples from this challenging environment. Recent technological advances have addressed some, but certainly not all of these sampling problems, providing new data and samples that test our current hypotheses about the crustal fluid reservoir. Current scientific interest in the sub‐seafloor biosphere has focused on the uppermost igneous oceanic crust as likely to be one of the most habitable environments, because of its porosity and locus of hydrothermal circulation of chemical nutrients. Recent observations indicate that sub‐seafloor crustal environments harbor novel CO2‐utilizing bacteria (primary producers) that could be a significant source of carbon‐fixation in the ocean, thus broadening possible habitable zones both on Earth and elsewhere where microbial life could exist independent of nutrient input from photosynthesis. |
