Using Seafloor Heat Flow as a Tracer to Map Subseafloor Fluid Flow in the Ocean Crust

Abstract

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 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.

Surface heat flow measurements from the East Siberian continental slope and southern Lomonosov Ridge, Arctic Ocean

“…In addition to downhole changes in thermal conductivity, other causes of nonlinear gradients include subsurface fluid flow [Fisher and Harris, 2010;Vanneste et al, 2003] and changing bottom water temperatures (BWT) on seasonal and/or multiyear timescales [Davis et al, 2003]. These BWT variations can distort the thermal gradient in the upper few meters of sediments, as in situ temperatures reequilibrate to the new boundary conditions at the seafloor.…”

“…Geochemistry, Geophysics, Geosystems 10.1002/2016GC006284 broader Chukchi Sea region, and penetrate into Tertiary sediments, sometimes extending to a few tens of meters below the seabed [Grantz and Eittreim, 1981;Thurston and Lothamer, 1991;Khain et al, 2009]. An alternate possibility is that the in-situ temperature profile does not reflect a steady state conductive system disrupted by transient changes in BWT, but is influenced by advective fluid flow [Fisher and Harris, 2010]. The anomalously high heat flow at this site warrants a more thorough investigation combining other geophysical and pore water geochemical data.…”