Noble Gases in Seawater as Tracers for Physical and Biogeochemical Ocean Processes

Noble gases are biologically and chemically inert, making them excellent tracers for physical processes. There are 5 stable noble gases: He, Ne, Ar, Kr, and Xe, with a range of physicochemical properties; the diffusivities of the noble gases in seawater differ by approximately a factor of 5 and the solubilities of the noble gases in seawater differ by approximately a factor of 10. This broad range in physicochemical characteristics leads to differing response to physical forcing. Thus, measurements of multiple noble gases made concurrently allow quantification of many physical processes. In seawater studies, noble gas measurements have been used to investigate air-sea gas exchange, allowing explicit separation of the bubble component from the diffusive gas exchange component, and to study equilibration during deep water formation. Argon has been used to quantify diapycnal mixing and the heavier noble gases could be useful in such studies as well. Helium, Ne, and Ar have yielded insights on ocean-cryospheric processes such as sea ice formation and basal melting of glaciers. The isotope³ He has been used extensively in studies of ocean circulation, and also for quantifying ocean-lithospheric interactions. Additionally, noble gases can be combined with biologically active gases, such as O₂ or N₂, in order to quantify rates of biological production and denitrification.