Dissolved argon, krypton and xenon demonstrate the importance of rapid cooling in causing gas undersaturations in the deep ocean

The deviation of dissolved inert gases from solubility in the ocean reflects the relative importance of temperature change, diffusive and bubble-mediated gas exchange, atmospheric pressure variation, and ice processes occurring at the ocean surface. In particular, previous results showing that Ar is undersaturated in the deep ocean indicate that deep-water formation creates an imbalance between the rate at which gas saturations are driven downward by rapid cooling and the rate at which gas exchange can bring saturations back toward equilibrium. Here we present depth profiles of argon, krypton and xenon, which were collected at the Hawaii Ocean Time-series station ALOHA in August 2004 and analyzed by a new technique. Gas was extracted from the water by equilibration with a headspace and then gettered to remove interfering gases. Argon was measured by isotope dilution using a ³⁸Ar spike on a stable isotope ratio mass spectrometer, while Kr and Xe were measured relative to Ar on the same instrument by peak jumping. Precisions of 0.1% were achieved for all three gases based on duplicate samples, while repeated processing and analysis of an air standard demonstrated a laboratory precision of 0.002%. These new measurements show that Kr is even more undersaturated than Ar in the deep ocean. Previous solubility data for Xe appears to be about three percent high, complicating the interpretation of absolute Xe measurements. All three gases reveal small gradients, becoming more undersaturated from 1000 to 4800m, that were undetectable by previous methods. These gradients likely indicate that a difference in the balance between cooling and gas exchange exists between the formation regions of Pacific intermediate waters and Circumpolar Deep Water (CDW). Moreover, the slope of the Ar vs. Kr saturation relationship in this dataset implies that temperature change is the overriding process controlling the Ar/Kr ratio in the ocean.