Abyssal Circulation in the Panama Basin Driven by Geothermal Fluxes?

Theoretical investigations suggest that geothermal (hydrothermal and diffusive) fluxes are important in sustaining the deep ocean stratification and driving the abyssal circulation. Observational evidence in support of this thesis has been obtained during fieldwork in the Panama Basin as part of the UK funded project OSCAR (Oceanographic and Seismic Characterisation of heat dissipation and alteration by hydrothermal fluids at an Axial Ridge). The Panama Basin is a miniature basin (~1000 x 1000 km) in the eastern Pacific which, below approximately 2300 m, communicates with the open ocean only through a 20 km wide passage: the Ecuador Trench. The basin contains three fast spreading ridges, the Costa Rica, Ecuador and Galapagos rifts, where heat flow through the crust is ~1 W m^-2 (10 times larger than the global ocean average). Full water column hydrography (temperature, salinity and currents) and vertical microstructure profiling at 130 stations in the basin, combined with ADCP measurements at the Ecuador Trench, all gathered during OSCAR, have allowed us to create a first order, reduced gravity model of the basin’s circulation and of the role of geothermal forcing in driving it. Inflow of water from the Pacific proper into the basin occurs as a northbound bottom boundary layer flow through the Ecuador Trench. At a rate of between 0.35 and 0.7 Sv, this inflow would fill the basin in less than 50 years. To leave the basin, the inflowing water must either be lifted above the 2300 m depth horizon or return into the Pacific as a southbound flow through the Ecuador Trench. The buoyancy input necessary to drive the outflow is contributed by geothermal fluxes from below and by downward heat diffusion from above. Our calculations indicate that about 55% of the buoyancy input is provided geothermally and the remaining 45% is due to downward diffusion with a diffusivity of around 1 x 10^-4 m² s^-1, a value which 10 times larger than background ocean mixing rates.