Using seafloor heat flow as a tracer to map subseafloor fluid flow in the ocean crust
We describe how seafloor heat flow is determined, review current understanding of advective heat loss from oceanic lithosphere, and present results from three field areas to illustrate how heat flow measurements are used (along with complementary data) to resolve fluid flow rates and patterns. Conductive heat flow through much of the seafloor is lower than predicted by lithospheric cooling models as a result of hydrothermal circulation; this discrepancy is the basis for global estimates of the
... l estimates of the magnitude of advective cooling of oceanic lithosphere. Hydrothermal circulation also redistributes heat within the ocean crustal aquifer, leading to local variability. Heat flow studies in Middle Valley, a sedimented spreading center in the northeastern Pacific Ocean, indicate multiple scales of fluid circulation, delineate conditions at the top of a hydrothermal reservoir, and show the influence of primary and secondary convection. Heat flow studies on the eastern flank of the Juan de Fuca Ridge document the thermal influence of isolated basement outcrops surrounded by thick, low-permeability sediments. Warm hydrothermal fluids seep from the crust through a small volcanic edifice, having flowed into the crust through a larger outcrop $50 km to the south. These fluids generate a local geothermal anomaly, but have little influence on regional heat loss from the plate. In contrast, heat flow surveys on part of the Cocos Plate, eastern Equatorial Pacific Ocean, indicate that regional conductive heat loss is just 10-40% of predictions from lithospheric cooling models. Basement outcrops in this area focus massive discharge of cool, hydrothermal fluid and associated heat (4-80 · 10 3 l s -1 of fluid, 0.8-1.4 GW of heat). Seafloor heat flow studies will be increasingly important in coming years for understanding marine hydrogeologic regimes and the role of fluids in a variety of Earth processes and settings.