US GEOTRACES OCE: Measuring the distribution of stable carbon isotopes and estimating organic matter export rates

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The goal of this project is to measure the concentration of the rare isotope of carbon (13C) present in the carbon dioxide (CO2) molecules dissolved in seawater of the South Pacific Ocean. 13C atoms make up only 1% of the carbon atoms on earth whereas atoms with the common carbon isotope (12C) make up 99% of the carbon atoms. Variations in the concentration of 13C atoms are represented relative to the concentration of 12C atoms and expressed as the 13C/12C ratio. The utility of measuring spatial variations in the 13C/12C of CO2 in the ocean results from two observations. First, during photosynthesis the 13C/12C of the plant (phytoplankton in the ocean) is distinctly different from the 13C/12C of CO2 consumed during photosynthesis. Second, the 13C/12C of CO2 molecules produced during the combustion of fossil fuels (coal, oil, natural gas) is measurably different from the 13C/12C of CO2 in the atmosphere and ocean. As a result, measuring the spatial variations of 13C/12C of CO2 in the ocean can be used to estimate variations in the rate of photosynthesis and the rate at which CO2 produced by fossil fuel combustion is being adsorbed by the ocean. Estimating variations in these two rates are goals of this project. The 13C/12C of CO2 in the ocean depends in large part on the rates of photosynthesis and respiration, which results in spatial covariations in the 13C/12C of CO2, concentrations of primary nutrients (nitrate and phosphate) and trace elements that are used by phytoplankton (e.g., Cadmium). In this project, the planned measurements of the 13C/12C of CO2, nutrients and trace elements during the GEOTRACES GP17 cruise will be used to determine how the north-south variations in biological productivity (photosynthesis and respiration), elemental composition of sinking particles and water mass mixing control the regional variations in the 13C/12C of CO2, nutrients and bioactive trace element distributions in the S. Pacific Ocean. An improved understanding of the biological, chemical and physical processes that control spatial variations of 13C/12C of CO2, bioactive trace elements and nutrient distributions in the modern ocean will improve our ability to use 13C/12C and trace elements measurements on CaCO3 preserved in the sedimentary record to reconstruct past changes in the ocean circulation and CO2 cycling in the paleo ocean. The project will involve undergraduate students in the research activities and carry out public outreach through University of Washington, Program on Climate Change (PCC).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.