The distribution of glacial meltwater in the Amundsen Sea, Antarctica, revealed by excess helium and neon

Abstract:

Noble gases in seawater are useful tracers of glacial melting around Antarctica because the dissolution of the air bubbles trapped in glacial ice produces significant saturation anomalies of noble gases. To evaluate the significance of glacial meltwater (GMW) fluxes, we measured the two noble gases, helium (He) and neon (Ne), in the water column of the Amundsen Sea, Antarctica in 2011 and 2012. The measured saturation anomalies of He and Ne (ΔHe = (He/He_eq – 1) × 100% and ΔNe = (Ne/Ne_eq – 1) × 100%, where He_eq and Ne_eq are at equilibrium with the atmosphere) were in the range of 4 – 25% and 2 – 15% (n = 85), respectively, near the Getz and Dotson Ice Shelves (GIS and DIS). The dissolved He and Ne in the upper 500 m of this region were largely supersaturated up to 16% and 13%, respectively, with respect to the background seawater (open ocean water). The maximum values of ΔHe and ΔNe were observed in 400 – 500 m depth where the warm Circumpolar Deep Water (CDW) melts the base of the ice shelves. These large excess He and Ne were even appeared nearly 200 km away the ice shelves, suggesting that GMW can be transported up to several hundred kilometers offshore. The calculated meltwater fraction in GIS, DIS, and continental shelf regions, based on the excess He, were 0.4 – 0.8% and 0.5 – 1.2%, 0.2 – 1.0%, respectively. The largest GMW fraction (almost 2%) was observed in the western side of the DIS due to an intensified outflow from the western side of the DIS. In 2012, the GMW fraction decreased by approximately 30 – 40% compared to 2011, demonstrating significant inter-annual variability in glacial melting. For an estimated He residence time of 0.4 – 0.9 yr on the shelf region, the GMW flux from the GIS and DIS were estimated to be about 45 – 110 Gt yr^-1, corresponding basal melting rate of 3 – 8 m yr^-1. Our regional melting rate is comparable to recent satellite- or heat flux based estimates.