Collaborative Research: Examination of the Potential Sources for the Mid-Water Silica Anomaly in the NE Pacific

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ABSTRACTOCE-0452565 / OCE-0452088Previous studies have shown that the North Pacific contains the highest deep-water concentrations of dissolved silica in the world ocean, and Cascadia Basin appears to be the source region for this global-scale anomaly. Data compilations indicate that a .plume. of dissolved silica originates from within this small marginal Basin, which can be traced for several thousand miles, from an origin in the NE Pacific to the Hawaiian Ridge in the south and almost to Siberia in the west. Box model calculations based on compiled data in the area indicate that the North Pacific deep-water silica anomaly requires a flux from Cascadia Basin of nearly 2 Tmols/year to sustain the observed concentration levels. If correct, this dissolved silica flux is an astonishing 40% of the total riverine silica input to the oceans, but this interpretation requires substantial verification. Although there are several identified sources for dissolved silica within the Basin, including ridge-axis vent fields, ridge-flank hydrothermal circulation, compression of sediments at the accretionary wedge and the upward diffusive flux from sediment pore waters, none of these processes, either individually or in aggregate, appear to be adequate to produce the observed deep-water silica anomaly. In this study, researchers at the University of Washington and the University of Southern California will conduct a field program to provide the required spatial distribution of CTD and Niskin/nutrient measurements needed to construct a reasonable silica inventory and geostrophic bottom water flux. They plan to (a) compile all existing CTD/nutrient data from Cascadia Basin, which is substantial in number, but limited in location, (b) add new data from cruises-of-opportunity in 2004, (c) map the geographic distribution and inventory of deep-water silica with 40 new hydrocast profiles from areas not sampled by existing data, (d) use the new data to determine the basin-scale pattern of bottom water circulation and (e) determine the Ge/Si ratios of the bottom water, to distinguish biological from geological source regions. The primary goals of the program are to quantify the flux of dissolved silica into the Basin and identify the source regions that are responsible.Broader Impacts: Large ocean-going field programs at UW support graduate and undergraduate students in their academic research, allow participation of additional non- Oceanography students from UW and from other institutions on the cruises, acquire data for student theses, and provide substantial out-reach in public awareness about scientific research. Further, although Cascadia Basin was poorly sampled by WOCE, the few data collected suggest a disproportionate role for this small basin in determining properties of the entire Pacific Deep Water.