GEOTRACES Pacific Section: Resolving Silicon Isotope Anomalies in the Eastern Pacific

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A scientist from the University of California, Santa Barbara will obtain water column samples collected during the 2013 GEOTRACES cruise from Peru to Tahiti. The samples will be analyzed for silicon (Si) isotopes to obtain the first major section of Si isotope distributions in the Pacific Ocean basin. Because the transect covers a large gradient in primary productivity from the upwelling zone off Peru to the oligotrophic subtropical gyre near Tahiti the researcher will determine how Si behaves within high productivity waters relative to oligotrophic waters. Specifically, the hypothesis to be tested is that Si isotope distributions are controlled by fractionation of Si isotopes during silica production and silica dissolution coupled to movement of Si by the biological pump and the meridional overturning circulation. In addition, the scientist plans to collaborate with researchers from Stanford University, the University of Massachusetts, Dartmouth, and Brown University who will measure the stable isotopic composition of nitrate on the same section, to ascertain whether Si and nitrogen isotopes can be used to predict nutrient depletion patterns within the gradient in productivity observed between Peru and Tahiti. The role of Fe in Si dynamics will be evaluated in collaboration with a University of California, Santa Cruz researcher who will measure total [Fe] in surface waters collected during the cruise. Lastly, with the data, the scientist plans to test the prediction capabilities of the PANDORA model, as well as improve the model by incorporating a benthic source of Si in the North Pacific to simulate the inputs of the Northeast Pacific Silicic Acid Plume. As regards broader impacts, one undergraduate student would be supported and trained as part of this project.