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Siegfried, MR, Fricker HA, Carter SP, Tulaczyk S.  2016.  Episodic ice velocity fluctuations triggered by a subglacial flood in West Antarctica. Geophysical Research Letters. 43:2640-2648.   10.1002/2016gl067758   AbstractWebsite

Height change anomalies in satellite altimeter data have been interpreted as the surface expressions of basal water moving into and out of subglacial lakes. These signals have been mapped throughout Antarctica on timescales of months to years, but only broad connections have been made between active lakes and ice dynamics. We present the first high-frequency observations of ice velocity evolution due to a cascading subglacial lake drainage event, collected over 5years (2010-2015) using Global Positioning System data on Whillans and Mercer ice streams, West Antarctica. We observed three episodic ice velocity changes over 2years, where flow speed increased by up to 4%, as well as an 11month disruption of the tidally modulated stick-slip cycle that dominates regional ice motion. Our observations reveal that basal conditions of an Antarctic ice stream can rapidly evolve and drive a dynamic ice response on subannual timescales, which can bias observations used to infer long-term ice sheet changes.

Massom, RA, Giles AB, Warner RC, Fricker HA, Legresy B, Hyland G, Lescarmontier L, Young N.  2015.  External influences on the Mertz Glacier Tongue (East Antarctica) in the decade leading up to its calving in 2010. Journal of Geophysical Research-Earth Surface. 120:490-506.   10.1002/2014jf003223   AbstractWebsite

The Mertz Glacier Tongue (MGT) in East Antarctica lost similar to 55% of its floating length in February 2010, when it calved large tabular iceberg C28 (78x35km). We analyze the behavior of the MGT over the preceding 12years using a variety of satellite data (synthetic aperture radar and Landsat imagery and Ice, Cloud, and land Elevation Satellite laser altimetry). Contact of its northwestern tip with the eastern flank of shoals from 2002/2003 caused eastward deflection of the ice flow by up to similar to 47 degrees. This change contributed to opening of a major rift system similar to 80km to the south, along which iceberg C28 eventually calved. Paradoxically, the seabed contact may have also held the glacier tongue in place to delay calving by similar to 8years. Our study also reveals the effects of other, more localized external influences on the MGT prior to calving. These include an abrupt sideways displacement of the glacier tongue front by at least similar to 145m following an apparent collision with iceberg C08 in early 2002 and calving of numerous small icebergs from the advancing northwestern front due to the chiseling action of small grounded icebergs and seabed contact, resulting in the loss of similar to 36km(2) of ice from 2001 to 2006. The example of the MGT confirms the need for accurate bathymetry in the vicinity of ice shelves and glacier tongues and suggests that the cumulative effect of external factors might be critical to understanding and modeling calving events and ice shelf stability, necessarily on a case-specific basis.

Carter, SP, Fricker HA, Siegfried MR.  2013.  Evidence of rapid subglacial water piracy under Whillans Ice Stream, West Antarctica. Journal of Glaciology. 59:1147-1162.   10.3189/2013JoG13J085   AbstractWebsite

The subglacial water system of lower Whillans Ice Stream on the Sip le Coast, West Antarctica, contains numerous connected subglacial lakes in three hydrological basins (northern, central and southern). We use Ice, Cloud and land Elevation Satellite (ICESat) data to derive estimates of lake volume change and regional thickness changes. By combining these results with a water budget model, we show that a uniform, localized thickness increase perturbed the hydropotential, resulting in a change in course of a major flow path within the system in 2005. Water originating from upper Whillans and Kamb Ice Streams that previously supplied the southern basin became diverted toward Subglacial Lake Whillans (SLW). This diversion led to a tenfold filling rate increase of SLW. Our observation suggests that water piracy may be common in the Sip le Coast region, where the gentle basal relief makes the basal hydropotential particularly sensitive to small changes in ice thickness. Given the previously inferred connections between water piracy and ice-stream slowdown elsewhere in the region, the subtle and complex nature of this system presents new challenges for numerical models.

Massom, RA, Giles AB, Fricker HA, Warner RC, Legresy B, Hyland G, Young N, Fraser AD.  2010.  Examining the interaction between multi-year landfast sea ice and the Mertz Glacier Tongue, East Antarctica: Another factor in ice sheet stability? Journal of Geophysical Research-Oceans. 115   10.1029/2009jc006083   AbstractWebsite

The Mertz Glacier tongue (MGT), East Antarctica, has a large area of multi-year fast sea ice (MYFI) attached to its eastern edge. We use various satellite data sets to study the extent, age, and thickness of the MYFI and how it interacts with the MGT. We estimate its age to be at least 25 years and its thickness to be 10-55 m; this is an order of magnitude thicker than the average regional sea-ice thickness and too thick to be formed through sea-ice growth alone. We speculate that the most plausible process for its growth after initial formation is marine (frazil) ice accretion. The satellite data provide two types of evidence for strong mechanical coupling between the two types of ice: The MYFI moves with the MGT, and persistent rifts that originate in the MGT continue to propagate for large distances into the MYFI. The area of MYFI decreased by 50% following the departure of two large tabular icebergs that acted as pinning points and protective barriers. Future MYFI extent will be affected by subsequent icebergs from the Ninnis Glacier and the imminent calving of the MGT. Fast ice is vulnerable to changing atmospheric and oceanic conditions, and its disappearance may have an influence on ice tongue/ice shelf stability. Understanding the influence of thick MYFI on floating ice tongues/ice shelves may be significant to understanding the processes that control their evolution and how these respond to climate change, and thus to predicting the future of the Antarctic Ice Sheet.

Bassis, JN, Coleman R, Fricker HA, Minster JB.  2005.  Episodic propagation of a rift on the Amery Ice Shelf, East Antarctica. Geophysical Research Letters. 32   10.1029/2004gl022048   AbstractWebsite

We investigate ice shelf rift propagation using a combination of GPS and seismic measurements near the tip of an active rift in the Amery Ice Shelf. These measurements reveal that propagation occurs in episodic bursts, which were identified based on swarms of seismicity accompanied by rapid rift widening. The bursts last approximately 4 hours and are separated by 10-24 days. In between bursts, the rift widens at a rate comparable to that of ice shelf spreading. Comparison of automatic weather station data and tidal amplitudes show that the propagation bursts are not directly triggered by winds or tides, suggesting that rift propagation is driven by the background glaciological stress in the ice shelf. We show that the ice debris that partly fills the rift may play a role in controlling the rate of propagation.