Cascadia Basin Sediments as a Source for the North Pacific Silica Plume

The North Pacific silica plume extends from the North American margin almost to Siberia at an approximate mid-water depth of 2300 meters, and may be the largest single chemical anomaly in the global ocean. The plume contains 164 Teramols of silica and is sustained by a flux equivalent to one third of the total riverine silica input (1.5 Tmol/year) into the oceans. Cascadia Basin source estimates are 15-35 percent of the total flux that sustains the plume. A cruise in August 2006 examined potential source regions for dissolved silica within the small Cascadia/Gorda Basins adjacent to the Oregon/Washington/BC margins. During this cruise, 43 CTD and Niskin bottle casts, 11 sediment multi-core stations and 17 heat flow probe measurements were made. Water column samples for Rn-222 and Ra-228 were also taken to estimate vertical mixing rates and residence times. Sampling strategy focused on profiles within topographic 'gateways' that control entrance/egress of near-bottom water to/from the topographically restricted Cascadia/Gorda Basins. Sediment multi-core and heat flow stations were located to provide broad geographic coverage of the two Basins. Benthic fluxes of Si were estimated from core incubations and pore water profiles. Bottom water from the deep North Pacific enters into the Gorda/Cascadia Basins via the southern Mendocino/Gorda gateway with silica concentrations near 185 micromol/L, enriched by 20 micromol/L from the passage over the 4000 m deep western flank of the Juan de Fuca Ridge. Spatial distribution of dissolved silica within Cascadia Basin is non-uniform, with two large areas showing concentrations in excess of 200 micromol/L. These two areas include the western mid-Basin region and a N-S elongated area near, but not directly adjacent to, the OR/WA/BC continental margin. The two high concentration regions are divided by a tongue of cold and relatively low silica water that enters the Basin from the south through Cascadia Channel. The new data suggests that the primary contribution from the Basin may be derived from sediment remineralization of biogenic opal. The final exit of silica-enriched bottom water from Cascadia Basin into the North Pacific plume is through the topographic gap formed by the propagating ridge offset lying just south of the Endeavour Segment at 47° 30' N. While analysis of this new data set is not yet complete, it appears that sediments from Cascadia Basin are a significant contributor to the North Pacific silica budget.