Subseafloor Temperature Variability During Ridge‐Flank Hydrothermal Circulation: Implications for a Moderate‐Temperature Earth System and Other Ocean Worlds

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.