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

Abstract

[1] Isotopes of helium and neon and the H 2 18 O/H 2 16 O ratio of water are proven 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 À1 .

The Transient Response of Ice Shelf Melting to Ocean Change

“…Therefore, real cold-water cavities may have very long residence times, both because their slow melting induces a weak cavity flushing and because the cavity to be flushed is larger. The residence times of the real coldwater ice shelves are estimated at 4-8 years (Loose et al 2009;Nicholls and Østerhus 2004), an order of magnitude longer than the residence times examined here. There is no reason to believe that the conclusions of this study are inapplicable to larger cavities.…”

“…However, this information can only be used to derive ice front flux residence times, which are shown here to be significantly shorter than the mean cavity measure. In principle, mean cavity residence times can be derived from observations of transient tracers at ice fronts (Loose et al 2009;Smethie and Jacobs 2005), though detailed knowledge of the ice front flow field is also needed. Residence times have been inferred from measurements within subice cavities (Michel et al 1979;Nicholls and Østerhus 2004), but this requires both a transient external forcing and knowledge of the cavity flow field.…”

Environmental controls of marine productivity hot spots around Antarctica

“…Binding by increased ligand concentrations keeps this Fe in bioavailable form so that the bloom can persist long distances from the Fe source [ Thuróczy et al ., ]. By contrast, in the Ross Sea, basal meltwater from the Ross Ice Shelf deepens as it advects away from the shelf face [ Loose et al ., ], so it contributes little or nothing to either surface stratification or surface dFe concentrations. Thus, the data from these two contrasting polynyas indicate that phytoplankton biomass and NPP in coastal polynyas is controlled largely by Fe availability, both released from melting ice shelves and resuspended from continental shelf sediments, rather than by surface stratification by ice sheet meltwater.…”

Ice Sheet Model Intercomparison Project (ISMIP6) contribution to CMIP6

“…These datasets include Argo, expendable bathythermograph (XBT) and conductivity/temperature/depth (CTD) vertical temperature and salinity profiles (e.g., Dong et al, 2008), sea ice extent products sourced from passive microwave instruments (e.g., Bjørgo et al, 1997;Cavalieri and Parkinson, 2012;Parkinson and Cavalieri, 2012), sea surface temperature (SST) from WindSat and AMSR-E over the open ocean, satellite altimetry (Jason-1 and Jason-2) over the open ocean, and World Ocean Atlas 2009 climatological temperatures. For ocean models that include ice-shelf cavities and ice-ocean interactions, sub-iceshelf basal melting can be compared with glaciological estimates of ice-shelf melting around Antarctica Depoorter et al, 2013) derived from remote-sensing observations, as well as independent tracer-oceanographic estimates (Loose et al, 2009;Rodehacke et al, 2006). Just as regional atmospheric models will be key for evaluating the atmospheric component of climate models, regionally focused ocean models (e.g., Timmermann et al, 2012) and ocean reanalysis products (e.g., Menemenlis et al, 2008) are likely to provide valuable insight for evaluating CMIP ocean models.…”