An optimized estimate of glacial melt from the Ross Ice Shelf using noble gases, stable isotopes, and CFC transient tracers

摘要

Isotopes of helium and neon and the H_2~(18)O/H_2~(16)O ratio of water areproven proxies for melt from glacial ice beneath floating ice shelves and at ice shelf fronts. Their high concentrations in glacial meltwater, compared to other environmental sources, make them ideal tracers for studies of the pathways of glacial meltwater from its origins into the ocean interior. We combine noble gas and stable isotopes with temperature, salinity, and dissolved oxygen measurements from three cruises (along the Ross Ice Shelf during the austral summers of 1993-1994 and 1999-2000 and to the Ross Sea in 2000- 2001) and use optimal multiparameter analysis to compute the water mass concentration, including glacial meltwater. The distribution of meltwater at the front of the Ross Ice Shelf extended east from 180°W, with the highest concentrations found near 165°W in both 1994 and 2000. The mean meltwater concentration at the ice shelf front was 2.0 ± 0.33% in 1994, 2.2 ± 0.36% in 2000, and 0.25 ± 0.1% in the western Ross Sea in 2001. Water mass concentrations are used to correct for bias in the CFC age, introduced by mixing with CFC-free waters, an effect revealed by comparing CFC age with transit time distribution curves. The water residence time within the ice shelf cavity, using CFCs and the mean meltwater concentration, implies a basal melt rate of 33-50 km~3 a~(-1).