US GEOTRACES: Inorganic Carbon Cycling in the South Pacific and Southern Oceans by Direct Measurement

The Pacific and Southern Oceans regulate the global carbon (C) cycle, predominantly by modulating atmospheric CO2 levels through the uptake and storage of carbon. The Southern Ocean accounts for ~30- 40% of the global oceanic uptake of anthropogenic C, but only about 9% of the storage. The disparity between uptake and storage results from the complex circulation surrounding Antarctica, which acts to transport carbon from the Southern Ocean into the surrounding ocean basins. Recent work has identified significant decadal and interannual variability in the uptake of CO2 from the atmosphere. The 1990s had low uptake, which rebounded in the 2000s. Resolving the mechanisms and magnitude of such variability, and implications on the global C cycle, requires regularly repeated high-quality measurements. International programs (e.g. GO-SHIP) have provided such measurements on decadal time scales, which can then be supplemented by programs such as GEOTRACES to understand sub-decadal scale variability and mechanisms. Additionally, the synergy of co-measuring C and trace metals (TEIs) provides further insights into the natural and anthropogenic cycling of C. The perennially ice-covered regions of the Southern Ocean are especially under sampled. The Amundsen Sea, not visited by programs such as GO-SHIP, is characterized by complex circulation thought to be important to C cycling in the Southern Ocean. Measurements in the Amundsen Sea are rare, and generally limited to surface fugacity of CO2 (fCO2), making it impossible to determine the influence of glacial ice melt, polynyas, and circulation on the C content and chemistry of the region and the Southern Ocean overall. The proposed research will measure full water column profiles of pH, dissolved inorganic carbon, and total alkalinity in the south Pacific, Southern Ocean, and Amundsen Sea on the already funded GEOTRACES GP17 OCE and ANT cruises. The GP17-OCE cruise will leverage prior data from GO-SHIP to determine sub-decadal scale variability of CO2 uptake and storage in the south Pacific using repeat cruise tracks. The cruise track crosses the Antarctic Circumpolar Current in several key locations for CO2 uptake. The GP17-ANT cruise will provide valuable CO2 measurements in the Amundsen Sea where very few whole water column, fully constrained CO2 system profiles exist. The measurements will significantly improve our understanding of the Amundsen Sea on the C cycle, including poorly understood regimes like glacial/sea-ice melt and highly productive polynyas. An additional advantage of the proposed research is the synergy created with the GEOTRACES program. A plethora of TEIs will be analyzed on these cruises which will provide additional context and understanding of the C measurements. For example, iron and zinc are thought to limit primary productivity in the region and therefore have a controlling factor on the C cycling in the region, including anthropogenic C uptake. The pH measurements in particular aid in understanding the TEIs as pH influences speciation, kinetics, and enzymatic activity, all important in TEI cycling.