UC Santa Cruz

Abstract Subseafloor hydrothermal circulation drives biogeochemical exchange between the solid Earth and its ocean, and could play a similar role in other ocean worlds. We assess the scale of subseafloor temperature ( T ) variations during hydrothermal circulation in ∼3.5 Ma seafloor on the eastern flank of Juan de Fuca Ridge. This well‐studied, moderate T system (∼65°C–70°C in the upper ocean crust) represents a warm end‐member on Earth, with slow flow across 10s of kilometers. Modest lateral flow between recharge and discharge sites is accompanied by small‐scale convection, resulting in variable local T and flow conditions, even for simple, layered models. The flow and T fields are time‐varying, with the magnitudes of subseafloor variation oscillating around quasi‐steady conditions. The fractions of the crustal aquifer that are in T ranges relevant to different microbial habitats were calculated. In simulations with discharging fluids at ∼70°C, temperatures of 40°C–80°C were found within 65%–90% of the aquifer. The same simulation, run with low gravity appropriate for Europa, has 50%–70% of the aquifer at temperatures of 40°C–80°C, and 30%–50% at T  > 80°C. Tracking of fluid parcels suggests that fluids can heat and cool at rates of 0.2°C∼10°C/1,000 yr due to secondary convection. Geochemical modeling based on these simulations indicates that some nutrients and organic molecules in the fluids for this end‐member case could vary in concentration up to an order of magnitude as a consequence of such T variability. These findings highlight implications for variations in water–rock reactions and, hence, a potentially transient nature of habitable subseafloor niches. Plain Language Summary Subseafloor circulation below Earth's ocean, which heats seawater as it flows through the porous oceanic crust, drives chemical and biological exchange between the rocky interior and the ocean. Low‐moderate temperature fluids transport the products of chemical reactions between the rock and fluid, and can deliver nutrients that support microbial life. These fluids can travel long distances in the crust before eventually discharging back into the ocean, supporting microbial life at depth and near the seafloor. This study investigates the magnitude of subseafloor temperature variations that characterize a slow‐flow hydrothermal system, based on novel analyses of results from prior computer simulations. Calculations of variations in geochemical and nutrient concentrations, both consequential for microbial growth and abundance, show significant differences in habitability for different hydrothermal species. This particular type of hydrothermal system, which can explain the warm end‐member case observed on the young, eastern flank of the Juan de Fuca ridge on Earth, could also occur on small ocean moons such as Europa and Enceladus. Key Points Indirect flow paths during hydrothermal circulation between inflow and outflow sites can result in spatial and temporal variability of subseafloor temperature Simulations of a ridge‐flank hydrothermal system predict areas of distinct biogeochemical temperature ranges in a shallow crustal aquifer This variability can influence organic chemical and nutrient concentrations, and thus the nature of microbial habitats