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Overview– This is a proposal to examine the mechanisms controlling silicon isotope dynamics in the Arctic Ocean as part of the US GEOTRACES Arctic section scheduled for 2015. Full-ocean depth profiles of the silicon isotopic composition of silicic acid in seawater are proposed along with measures of the Si isotopic composition of diatoms from the water column and from sea ice. These data will be used to test hypotheses regarding the biogeochemical controls on Si isotope distributions in the Arctic Ocean as well as the role of sea ice diatoms in regional Si isotope dynamics. Among the water masses sampled will be surface melt water and Pacific halocline waters that are influenced by the Bering Sea and Chukchi Sea, Atlantic waters which dominate intermediate depths throughout the Arctic, and bottom waters of the Canada and Makarov Basins. Two full-depth profiles of the isotopic composition of diatoms will test for the effects of fractionation of Si isotopes during silica dissolution.
Intellectual Merit -The silicon isotope proxy is increasingly being used to assess the role of diatoms and silicic acid supply to 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 these values translates directly into uncertainty in the level of productivity implied by the proxy. The growing global data set of Si isotopes in marine waters suggest that Si isotopes in subsurface waters are not uniform, but vary systematically driven by interactions between Si isotope fractionation, the biological pump and thermohaline circulation. In addition, significant anomalies exist between model predictions and observations. Of particular relevance is the recent discovery from the Canadian GEOTRACES cruise to the Arctic that the deep Canada Basin contains the heaviest Si ever observed in the deep ocean. This feature was not predicted by published models. Here we propose to test hypotheses regarding the origin of this signal and the mechanisms controlling Si isotope distributions within the Arctic by greatly expanding the Si isotope data set for this ocean.
Broader Impacts - While we can test hypotheses regarding the controls on Si isotope distributions in the Arctic Ocean using the proposed ocean section, it is clear that a comprehensive understanding of the factors controlling global marine Si isotope distributions will require spatially resolved δ30Si data from multiple oceans basins. International GEOTRACES sections completed or planned by the United States, Japan, Canada, Great Britain, the Netherlands, France, Germany, Sweden and India include Si isotope measurements that complement those proposed here. All data will be submitted to the US GEOTRACES and to the International GEOTRACES databases to aid the larger global synthesis. The PI supplies secondary Si isotope standards to all International GEOTRACES PI’s measuring Si isotopes and has worked with the International GEOTRACES Standards and Intercalibration Committee to establish a set of seawater reference waters for δ30Si(OH)4 analysis.
The research will also involve undergraduate students at UCSB who will be trained in dissolved Si concentration analysis, data entry and data management. A post-doctoral scholar will be trained in Si isotope methods to increase the intellectual capacity in this field of study. The PI will also participate in the PolarTREC teacher-at-sea activates funded through the management proposal by linking the teacher’s blogs and broadcasts of ship activities to local K-12 classrooms through UCSB’s Oceans-to-Classrooms program.