Abstract
The bottom water of Cascadia Basin, in the Northeast Pacific, has long been associated with a geothermal temperature anomaly signal and has also been thought to be a significant source of silica to the Pacific-wide mid-depth plume. A geostrophic inverse model, using data from a field program in 2006 and a compilation of available historical data, provides a clearer picture of the basins deep circulation and fluxes from the seafloor into the bottom water. Below 2000 m, two water masses (CBBW: Cascadia Basin Bottom Water and CBDW: Cascadia Basin Deep Water) that have distinct temperature-salinity characteristics are separated by a transition zone at about 2400 m depth. Below the depth where it freely communicates with the broader North Pacific, Cascadia Basin is renewed by cold bottom water inflow from the south and diapycnal water mass conversion that produces an overturning cell within the CBBW. Geothermal heating plays an important role in the conversion of lower CBBW to lighter density within the basin. And, although covering only about 0.05% of the global seafloor, the combined effects of bottom heat flux and diapycnal mixing within Cascadia Basin provide about 2-3% of the total required global input to the upward branch of the global thermohaline circulation. Bottom silica fluxes were determined in three ways: direct measurement at selected locations, inference from basin-wide budgets, and estimation using the inverse model advection/diffusion fields. Cascadia Basin supplies only a minor percentage of the silica required by the larger Pacific plume, suggesting that the ultimate source may lie just upstream of the basin, on the western flank of the Juan de Fuca Ridge. Additional work is in progress to test the ability of the inverse model to constrain budgets for oxygen and 228Ra.