Project: GEOTRACES Arctic Section: Diagnosing the unique silicon isotope composition of the Arctic Ocean

NSF Award Abstract:
An investigator will participate in the 2015 U.S. GEOTRACES Arctic Ocean expedition and measure silicon isotopes composition of silicic acid in seawater samples, as well as diatoms and sea ice. In common with other multinational initiatives in the International GEOTRACES program, the goals of the U.S. Arctic expedition are to identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions. Some trace elements are essential to life, others are known biological toxins, and still others are important because they can be used as tracers of a variety of physical, chemical, and biological processes in the sea. As silicon is a key component of diatom shells, the measurement of their concentrations in this study can provide an important indicator of primary productivity, both in the present day, as well as past oceans. The project will provide training for undergraduate students and a post-doctoral researcher in silicon chemistry, as well as include outreach to K-12 students.

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 role in Earth's climate. Application of the proxy requires knowledge of the silicon 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 silicon isotopes in marine waters suggest that silicon isotopes in subsurface waters are not uniform, but vary systematically driven by interactions between silicon isotope fractionation, the biological pump and thermohaline circulation. In addition, significant anomalies exist between model predictions and observations. In this study, researchers will test hypotheses regarding the origin of this signal and the mechanisms controlling silicon isotope distributions within the Arctic Ocean by greatly expanding the silicon isotope data set for this region. The overarching hypothesis to be tested is that silicon isotope distributions are controlled by the coupling of silicon fractionation during silica production and during silica dissolution to the biological pump and to the meridional overturning circulation.