Trace gas disequilibria during deep-water formation

We present high-precision measurements by a new isotope dilution technique of a suite of inert gases in the North Pacific. Remarkably smooth gradients in Ar, Kr and Xe from near equilibrium in intermediate waters to several percent undersaturated in deep waters were observed. The general pattern in the deepest waters was that Ar, Kr and Xe were undersaturated (Ar least and Xe most), while N2 was close to equilibrium, and Ne was supersaturated. We propose that this pattern was produced by the interaction between the different physical properties of the gases (solubility and the temperature dependence of solubility) with the rapid cooling and high wind speeds that characterize deep-water formation regions. In a simple model of deep-water formation by convection, the saturations of the more temperature-sensitive gases were quickly driven down by rapid cooling and could not reequilibrate with the atmosphere before the end of the winter. In contrast, the gas exchange rate of the more bubble-sensitive gases (Ne and N2) was able to meet or exceed the drawdown by cooling. Our simple convection model demonstrates that the heavier noble gases (Ar, Kr and Xe) are sensitive on seasonal timescales to the competing effects of cooling and air–sea gas exchange that are also important to setting the concentration of CO2 in newly formed waters.