Title: Tracer Studies of Air/Sea Gas Exchange, Mean Residence Times, and Stable Isotope Fractionation in the Arctic Ocean
Description: In this dissertation, I explore elements of the changing Arctic Ocean through the application of Stable Isotope, Noble Gas Isotopes, and sulfur hexafluoride (SF6) to better understand ice dynamics for freshwater balance, air/sea gas exchange and ocean circulation. For the tracer studies of stable isotope fractionation, our approach is to use sea ice core data to determine the stable oxygen isotope effective fractionation coefficient. The result is an average value close to 2.2 ‰, which is compared to literature values. For the tracer studies of air/sea gas exchange, we use Neon (Ne) and Helium (He) isotope data sets collected in the ‘Switchyard’ region of the Arctic Ocean between 2005 and 2013 and in the Greenland and Norwegian seas between 1994 and 1999. The Switchyard data show a distinct excess in Ne concentrations in the upper waters. We hypothesize that rejection of Ne during sea ice formation accounts for the Ne excess in the Switchyard area of the Arctic Ocean. Based on this hypothesis we estimate sea-ice formation rates by integrating the Ne excess from the surface to the Atlantic Water layer.

The resulting amount of excess Ne corresponds to formation of a nearly 4 m thick sea ice layer. We compare the sea ice formation obtained from the Ne excess method with an independent estimate based on oxygen isotope ratio anomalies ?18O, which is nearly 6.07 m. The difference in the sea ice formation estimated by these two methods indicates loss of Ne through leads. We estimate that the gas exchange rate through the sea-ice cover is ca. 11.3 percent per year. The gas exchange rate through sea-ice covered water would be 0.015 meters per day. For the tracer studies of mean residence times, we analyzed tritium (3H), helium isotope (3He and 4He) and sulfur hexafluoride (SF6) samples collected in the ‘Switchyard’ region of the Arctic Ocean between 2008 and 2013. We calculated apparent tracer ages using the 3H/3He ratios and the partial pressure of SF6 and compare their values for the depth interval between the surface and the core of the Atlantic Water layer. The apparent tracer ages range from zero to about 30 years.

Generally, the linear correlation between the 3H/3He and SF6 apparent ages was strong, with the coefficient of determination R2 of 0.94. We explore deviations from this linear trend and discuss them in the context of mixing, air-sea gas exchange, and the impact of sea ice formation on the helium and SF6 gas balances in the surface mixed layer.