A Determination of Krypton and Xenon Solubilities in Seawater for Oceanic Air-Sea Exchange and Ventilation Studies
Recent work has revealed that there are significant errors in our knowledge of the solubilities of a number of the noble gases, with offsets in different determinations approaching 1? 2%. Ne and Ar have recently been re-determined, and appear ?well in hand?. However, there are questions about the solubilities of Kr and Xe, which is problematic as these gases comprise an important end-member in the noble gas group. In fact, the only documented determinations of seawater xenon solubility claim an uncertainty of order 2%, and appear to be systematically off according to a recent study. Such errors are large and comparable in magnitude to observed mixed layer anomalies, and hence significantly limit the value of noble gas measurements for diagnosing oceanic processes. In this study, researchers at the Woods Hole Oceanographic Institution will determine the solubility of Kr and Xe to an accuracy of 0.1% or better, using a purpose-designed equilibration apparatus and dual isotope dilution mass spectrometry combined with high-quality cryogenic separation and purification technology. They will determine the solubilities of these gases in fresh water as well as seawater salinities over a range of temperatures spanning from near-freezing to 30ºC. In all, 40 points in temperature and salinity space will be measured in replicate samples. They plan to analyze both the dissolved gases and the head-space gases so that some systematic uncertainties will cancel out in the calculation of Bunsen solubilities. In the same experiment, they will also measure the solubilities of Ne and Ar by single isotope dilution as a comparison of previous measurements. This research is expected to have a number of important broader impacts. Ocean ventilation and air-sea gas exchange processes are key components in a number of major biogeochemical cycles and play an important role in global change. For example, a significant portion of the anthropogenic fossil-fuel CO2 inventory has been transferred to the ocean by these processes. Another critical application requiring knowledge of air-sea gas exchange rates is the use of seasonally occurring shallow oxygen maxima to estimate biological production, which relies on estimating the loss of photosynthetic oxygen through the air-sea interface and accounting for the contribution of bubble trapping to oxygen supersaturation. Thus Identifying, characterizing, and quantifying the physical processes associated with air-sea exchange of gases is a vital step in building credible models of climate and carbon cycles, particularly for diagnosing biogeochemical processes and for assessing and predicting anthropogenic impacts. Despite their importance, these processes are not adequately characterized and quantified for scientific and societal/policy needs. Noble gases are potentially powerful tools for diagnosing air-sea exchange and water-mass formation processes, since their distributions are controlled by purely physical mechanisms. Moreover, as a group they span a wide range of molecular diffusivities and an approximately order of magnitude in solubilities. Molecular diffusivity plays an important role both in diffusive air-sea gas exchange, and in bubble injection processes. Solubility, and in particular its dependence on temperature, is an important driver for gas exchange when significant heat transfer occurs, either during radiative warming in the summer months, or in water mass formation processes during the winter. Noble gas distributions have recently been used to quantify air-sea gas exchange rates, constrain biological (net community) production, diagnose diapycnal mixing in the thermocline, and characterize air-sea disequilibrium associated with water mass formation. All of these are important biogeochemical and physical processes.