Numerical Simulation of Cool-Hydrothermal Processes Beneath a Marine Sediment Pond: North Pond, North Atlantic Ocean

Low temperature hydrothermal systems hosted in the volcanic oceanic crust are responsible for ~20% of Earth's global heat loss. Marine sediment ponds comprise an important type-setting on young ridge flanks where hydrothermal circulation can occur, but the nature of coupled fluid-heat transport in the volcanic crust under these features remain poorly understood. North Pond is a sediment pond located on ~8 Ma oceanic crust, 140 km west of the Mid-Atlantic Ridge, with a sedimented seafloor depth of ~4400 m. This area has been extensively surveyed and contains several ODP and DSDP boreholes as well as CORK borehole observatories, used to study the thermal, chemical, hydrogeologic, and microbiological conditions of young, slow-spreading crust. Heat flux measurements in North Pond sediments are generally ~10-50% of lithospheric input values, suggesting that the majority of heat loss from the crust in this setting is advective. It has been thought that cool hydrothermal fluids may circulate under North Pond as part of a single-pass flow system, from one side to the other. We tested this hypothesis with two- and three--dimensional numerical simulations using a transient, coupled model. Simulation results show that a unidirectional, single-pass flow system is neither favored nor needed to match the spatial distribution of seafloor heat flux through North Pond sediments. Instead, simulated advective transport beneath North Pond is transient and convective with multiple spatial and temporal scales. These simulations are also consistent with very high basement permeability values in the shallow crust (10-10 to 10-9 m2), modest differential pressures in the basement aquifer, and conductive conditions in North Pond sediments. The rapid, complex, and transient nature of hydrothermal circulation in this setting may help to explain heterogeneity and dynamics in microbial ecosystems sampled using North Pond observatories.