US GEOTRACES Pacific Meridional Transect (GP-15): Resolving Silicon Isotope Anomalies in the Northest Pacific

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The silicon isotope proxy is increasingly being used to assess the role of diatoms and of silicic acid supply in driving modern day and past shifts in ocean productivity and their implications for climate.  Application of the proxy requires knowledge of the Si isotopic composition of ventilating water masses. Uncertainty in those values translates directly into uncertainty in the level of productivity implied by the proxy.  The growing δ30Si(OH)4 database implies that a first principle understanding of the controls on Si isotope distributions is attainable which would improve inferences in both modern- and paleo- reconstructions.  Current data show systematic variations in the isotopic signature of major water masses that appear to be driven by interactions between biological Si isotope fractionation in surface waters, the biological pump and the MOC.  The deep waters of the northeast Pacific are of interest for testing this concept as they contain the highest [Si(OH)4] and oldest waters at the ‘end’ of the MOC.  Moreover, the northeast Pacific may be unique due to the presence of the North Pacific Silicic Acid Plume that originates in the Cascadia Basin.  Current data imply that the plume constitutes the largest silicon isotope anomaly in the global ocean as the relationship between Si isotopes and Si concentration across the Plume are the opposite of model predictions challenging our current understanding of the mechanisms controlling Si isotope distributions.  Outside the plume the section will sample several major water masses (North Pacific Intermediate Water, North Pacific Deep Water, and Antarctic Intermediate Water and Circumpolar Deep Water) allowing further tests of predictions of how Si isotopes vary along the MOC.

We will also work with GEOTRACES P.I.s measuring N isotopes in nitrate to investigate whether Si and N isotopes can predict the high silicic acid:nitrate drawdown ratios in the HNLC waters of the equatorial Pacific and the Subarctic Pacific that the section will sample. In addition, prior US GEOTRACES sections suggest that a comparison of Si and Zn isotopes may provide a new novel constraint on the mechanisms leading to the correlation between Si and Zn concentrations in the sea.