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Totten Ice Shelf – Rapid Basal Melt Driven by Ocean Heat

In the West Antarctic ice shelves and glaciers mass loss has been linked to ocean heat flux causing basal melt. In East Antarctica the Totten Ice Shelf, which buttresses a marine-based ice sheet that has a volume which is equivalent to a sea level rise of 3.5 m, has been found to also experience rapid basal melt, though due to a lack of observations near the ice shelf the role of ocean forcing was not known. It was confirmed by observations from the Totten calving front that (0.22 x 0.07) x 10⁶ m³ per second of warm water enters the cavity through a newly discovered deep channel. The transport of ocean heat into the cavity is sufficient to support the large rates of basal melt that is inferred by glaciological observations. Ocean heat flux change is suggested by Rintoul et al. to be a plausible mechanism to explain the changes in the past and that are projected for the future in this sector of the East Antarctic Ice Sheet and its contribution to sea level.

At the point where the Antarctic Ice Sheet reaches the ocean and begins to float, ice shelves form. The grounded ice sheet is buttressed by the rock stress that is produced by the interaction of the floating ice shelf with side walls and topographic rises, which inhibits flow of the ice into the ocean (DuPont & Alley, 2005). This back stress is reduced by thinning or weakening of the ice shelves, and this reduction increases the discharge of the grounded ice into the ocean which contributes to the rise of sea levels. The thinning that has been occurring in the Antarctic ice shelves has been attributed to basal melt by ocean heat flux (Pritchard et al., 2012; Paolo, Fricker & Padman, 2015), and the most rapid thinning, retreat of the grounding line, and accelerated flow of glaciers has been observed in the Bellingshausen Sea and the Amundsen Sea (Paolo, Fricker & Padman, 2015; Rignot et al., 2014). In this sector of Antarctica much of the ice sheet rests on bedrock that is below sea level, deepening upstream, and this is a potentially unstable configuration that may result in rapid retreat of glaciers and mass loss to the ocean (Weertman, 1974; Schoof, 2007). It is suggested by models and observations that the unstable retreat of some glaciers in West Antarctica may have already been initiated (Rignot et al., 2014; Favier, 2014; Joughin, Smith & Medley, 2014). The future evolution of the Antarctic Ice Sheet is therefore linked to change in the ocean surrounding Antarctica.

In the Bellingshausen Sea/Amundsen Sea sector warm ocean waters approach the closest to the Antarctic continent (Pritchard et al., 2012; Orsi, Whitworth III & Nowlin, 1995), and it in this area that the most rapid warming of bottom waters has occurred (Schmidtko, Heywood, Thompson & Aoki, 2014), which helps to explain the rapid mass loss from the West Antarctic Ice Sheet (WAIS). The WAIS has been marine-based for a long time and susceptible to unstable retreat, whereas the East Antarctic Ice Sheet (EAIS) was assumed to be more stable because of its bedrock configuration and isolation from warm ocean currents. Global rise of sea level in excess of 10 m during past epochs of warm climate requires a substantial contribution from East Antarctica (Nash et al., 2009; Miller et al., 2912). It has been shown by new observations that large regions of the East Antarctica Ice Sheet, which includes the Aurora Basin that is drained primarily by the Totten Glacier, are marine based, with repeated retreat and advances of the ice sheet on large scales being indicated by basal morphology (Young et al., 2011) and sediment erosion records (Aitken et al., 2016). More ice is drained from East Antarctica by the Totten Glacier than by any other glacier in the EAIS, which contains a volume of marine-based ice above flotation equivalent to that of at least 3.5 m of rise of the global sea level (Greenbaum et al., 2015), which is comparable to that of the WAIS. The Totten Glacier occupies a deep fjord connecting to inland regions of retrograde bed slope, which is conducive to rapid retreat, though the bed is flat or rises upstream immediately inland from the grounding line (Rignot et al., 2015). It is shown by satellite altimetry and gravity measurements that parts of the grounded portion of the East Antarctic Ice Sheet have thinned in recent decades, with the most rapid changes occurring in the Totten Glacier (Pritchard et al., 2009; Harig & Simons, 2015). There is mixed evidence of recent change in the Totten Ice Shelf (TIS), with thinning being indicated between 2003-2008 (Pritchard et al., 2012) by laser altimetry, while large variability over time as shown by radar altimetry, with no net loss between 1994 and 2012 (Paolo, Fricker & Padman, 2015), and it was found by a recent study that the mean basal melt rate for the period 2005-2011 was about ⅓ larger than the steady state melt rate required to balance mass (Liu et al., 2015). A substantial contribution to sea level rise in the future by both the Wilkes Subglacial Basin and the Aurora Subglacial Basin  in East Antarctica is suggested by models if emissions of greenhouse gases remain high (Golledge et al;., 2015; DeConto & Pollard, 2016). The retreat of the Totten Glacier that has been modelled is initiated by simulated or assumed ocean temperature increase, though the processes that transport ocean heart to the cavities in the ice shelf are not well represented in climate models that are of coarse resolution. No oceanographic measurements from the Totten ice front have to date been available to test the hypothesis that warm ocean waters can reach the cavity in the ice shelf and drive basal melting.

Conclusion

Rintoul et al. suggest that rapid basal melt of the Totten Ice Sheet is supported by several lines of evidence the this melting is being driven by the flux of warm Circumpolar Deep Water (wCDW) into the cavity: Warm water being present at the ice front, access being provided by a deep trough of this warm water to the cavity, the signature in the outflow of glacial meltwater, and exchange rates that are inferred from the heat budget and basal melt rates that are satellite derived. The hypothesis of a dynamic East Antarctica Ice Sheet is supported by observations of recent change in some glaciers and ice shelves in East Antarctica (Rignot et al., 2015; Pritchard, Arthem, Vaughan & Edwards, 2009; Harig & Simons, 2105; Liu et al., 2015) and studies of past (Miller et al., 2012; Young et al., 2011; Aitken et al., 2016; Cook et al., 2013; Dutton et al, 2015) and future (Golledge et al;., 2015; DeConto & Pollard, 2016) sea levels. According to Rintoul et al. their observations confirm the presence of a pathway that allows the communication of ocean anomalies to the Totten Ice Shelf cavity, thereby highlighting variability in the basal melt that is ocean-driven as a plausible mechanism to explain changes in the past and that are projected in the Totten Ice Shelf, as well as the ice sheet it buttresses.

Sources & Further reading

1. Rintoul, S. R., A. Silvano, B. Pena-Molino, E. van Wijk, M. Rosenberg, J. S. Greenbaum and D. D. Blankenship (2016). "Ocean heat drives rapid basal melt of the Totten Ice Shelf." Science Advances 2(12).