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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.

Mikucki, JA, Lee PA, Ghosh D, Purcell AM, Mitchell AC, Mankoff KD, Fisher AT, Tulaczyk S, Carter S, Siegfried MR, Fricker HA, Hodson T, Coenen J, Powell R, Scherer R, Vick-Majors T, Achberger AA, Christner BC, Tranter M, Team WS.  2016.  Subglacial Lake Whillans microbial biogeochemistry: a synthesis of current knowledge. Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences. 374   10.1098/rsta.2014.0290   AbstractWebsite

Liquid water occurs below glaciers and ice sheets globally, enabling the existence of an array of aquatic microbial ecosystems. In Antarctica, large subglacial lakes are present beneath hundreds to thousands of metres of ice, and scientific interest in exploring these environments has escalated over the past decade. After years of planning, the first team of scientists and engineers cleanly accessed and retrieved pristine samples from a West Antarctic subglacial lake ecosystem in January 2013. This paper reviews the findings to date on Subglacial Lake Whillans and presents new supporting data on the carbon and energy metabolism of resident microbes. The analysis of water and sediments from the lake revealed a diverse microbial community composed of bacteria and archaea that are close relatives of species known to use reduced N, S or Fe and CH4 as energy sources. The water chemistry of Subglacial Lake Whillans was dominated by weathering products from silicate minerals with a minor influence from seawater. Contributions to water chemistry from microbial sulfide oxidation and carbonation reactions were supported by genomic data. Collectively, these results provide unequivocal evidence that subglacial environments in this region of West Antarctica host active microbial ecosystems that participate in subglacial biogeochemical cycling.

Tulaczyk, S, Mikucki JA, Siegfried MR, Priscu JC, Barcheck CG, Beem LH, Behar A, Burnett J, Christner BC, Fisher AT, Fricker HA, Mankoff KD, Powell RD, Rack F, Sampson D, Scherer RP, Schwartz SY, Wissard Sci T.  2014.  WISSARD at Subglacial Lake Whillans, West Antarctica: scientific operations and initial observations. Annals of Glaciology. 55:51-58.   10.3189/2014AoG65A009   AbstractWebsite

A clean hot-water drill was used to gain access to Subglacial Lake Whillans (SLW) in late January 2013 as part of the Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) project. Over 3 days, we deployed an array of scientific tools through the SLW borehole: a downhole camera, a conductivity-temperature-depth (CTD) probe, a Niskin water sampler, an in situ filtration unit, three different sediment corers, a geothermal probe and a geophysical sensor string. Our observations confirm the existence of a subglacial water reservoir whose presence was previously inferred from satellite altimetry and surface geophysics. Subglacial water is about two orders of magnitude less saline than sea water (0.37-0.41 psu vs 35 psu) and two orders of magnitude more saline than pure drill meltwater (<0.002 psu). It reaches a minimum temperature of -0.55 degrees C, consistent with depression of the freezing point by 7.019 MPa of water pressure. Subglacial water was turbid and remained turbid following filtration through 0.45 mu m filters. The recovered sediment cores, which sampled down to 0.8 m below the lake bottom, contained a macroscopically structureless diamicton with shear strength between 2 and 6 kPa. Our main operational recommendation for future subglacial access through water-filled boreholes is to supply enough heat to the top of the borehole to keep it from freezing.

Carter, SP, Fricker HA, Blankenship DD, Johnson JV, Lipscomb WH, Price SF, Young DA.  2011.  Modeling 5 years of subglacial lake activity in the MacAyeal Ice Stream (Antarctica) catchment through assimilation of ICESat laser altimetry. Journal of Glaciology. 57:1098-1112. AbstractWebsite

Subglacial lakes beneath Antarctica's fast-moving ice streams are known to undergo similar to 1 km(3) volume changes on annual timescales. Focusing on the MacAyeal Ice Stream (MacIS) lake system, we create a simple model for the response of subglacial water distribution to lake discharge events through assimilation of lake volume changes estimated from Ice, Cloud and land Elevation Satellite (ICESat) laser altimetry. We construct a steady-state water transport model in which known subglacial lakes are treated as either sinks or sources depending on the ICESat-derived filling or draining rates. The modeled volume change rates of five large subglacial lakes in the downstream portion of MacIS are shown to be consistent with observed filling rates if the dynamics of all upstream lakes are considered. However, the variable filling rate of the northernmost lake suggests the presence of an undetected lake of similar size upstream. Overall, we show that, for this fast-flowing ice stream, most subglacial lakes receive >90% of their water from distant distributed sources throughout the catchment, and we confirm that water is transported from regions of net basal melt to regions of net basal freezing. Our study provides a geophysically based means of validating subglacial water models in Antarctica and is a potential way to parameterize subglacial lake discharge events in large-scale ice-sheet models where adequate data are available.

Fricker, HA, Scambos T.  2009.  Connected subglacial lake activity on lower Mercer and Whillans Ice Streams, West Antarctica, 2003-2008. Journal of Glaciology. 55:303-315. AbstractWebsite

We examine patterns of localized surface elevation change in lower Mercer and Whillans Ice Streams, West Antarctica, which we interpret as subglacial water movement through a system of lakes and channels. We detect and measure the lake activity using repeat-track laser altimetry from ICESat and image differencing from MODIS image pairs. A hydrostatic-potential map for the region shows that the lakes are distributed across three distinct hydrologic regimes. Our analysis shows that, within these regimes, some of the subglacial lakes appear to be linked, with drainage events in one reservoir causing filling and follow-on drainage in adjacent lakes. We also observe changes near ice raft 'a' in lower Whillans Ice Stream, and interpret them as evidence of subglacial water and other changes at the bed. The study provides quantitative information about the properties of this complex subglacial hydrologic system, and a relatively unstudied component of ice-sheet mass balance: subglacial drainage across the grounding line.

Craven, M, Allison I, Fricker HA, Warner R.  2009.  Properties of a marine ice layer under the Amery Ice Shelf, East Antarctica. Journal of Glaciology. 55:717-728. AbstractWebsite

The Amery Ice Shelf, East Antarctica, undergoes high basal melt rates near the southern limit of its grounding line where 80% of the ice melts within 240 km of becoming afloat. A considerable portion of this later refreezes downstream as marine ice. This produces a marine ice layer up to 200 m thick in the northwest sector of the ice shelf concentrated in a pair of longitudinal bands that extend some 200 km all the way to the calving front. We drilled through the eastern marine ice band at two locations 70 km apart on the same flowline. We determine an average accretion rate of marine ice of 1.1 +/- 0.2 m a(-1), at a reference density of 920 kg m(-3) between borehole sites, and infer a similar average rate of 1.3 +/- 0.2 m a(-1) upstream. The deeper marine ice was permeable enough that a hydraulic connection was made whilst the drill was still 70-100 m above the ice-shelf base. Below this marine close-off depth, borehole video imagery showed permeable ice with water-filled cavities and individual ice platelets fused together, while the upper marine ice was impermeable with small brine-cell inclusions. We infer that the uppermost portion of the permeable ice becomes impermeable with the passage of time and as more marine ice is accreted on the base of the shelf. We estimate an average closure rate of 0.3 m a(-1) between the borehole sites; upstream the average closure rate is faster at 0.9 m a(-1). We estimate an average porosity of the total marine ice layer of 14-20%, such that the deeper ice must have even higher values. High permeability implies that sea water can move relatively freely through the material, and we propose that where such marine ice exists this renders deep parts of the ice shelf particularly vulnerable to changes in ocean properties.

Fricker, HA, Allison I, Craven M, Hyland G, Ruddell A, Young N, Coleman R, King M, Krebs K, Popov S.  2002.  Redefinition of the Amery Ice Shelf, East Antarctica, grounding zone. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000383   AbstractWebsite

[1] New evidence is presented which shows that the Amery Ice Shelf, East Antarctica, extends similar to240 km upstream of the previously reported position. We combine a digital elevation model of the Amery Ice Shelf created from ERS-1 satellite radar altimetry with measured ice thicknesses and a simple density model in a hydrostatic (buoyancy) calculation to map the extent of the floating ice. This reveals that the ice is floating as far south as 73.2degreesS. The result is confirmed by static GPS measurements collected during three consecutive field campaigns on the Amery Ice Shelf where the vertical component of the GPS shows a clear tidal signal at 72.98degreesS. Other evidence for the grounding zone position comes from an analysis of satellite imagery, mass flux calculations, and ice radar data. The southward extension of the grounding line substantially alters the shape and dimensions of the ocean cavity beneath the ice shelf, which has implications for modeling studies of sub-ice shelf processes, such as basal melting and freezing, ocean circulation, and tides. The new grounding line position will also improve geophysical studies, where the computation of ocean tidal loading corrections is important for postglacial rebound estimates and correction of satellite altimetry measurements within the region.

Fricker, HA, Young NW, Allison I, Coleman R.  2002.  Iceberg calving from the Amery Ice Shelf, East Antarctica. Annals of Glaciology, Vol 34, 2002. 34( Winther JG, Solberg R, Eds.).:241-246., Cambridge: Int Glaciological Soc   10.3189/172756402781817581   Abstract

We investigate the iceberg-calving cycle of the Amery Ice Shelf (AIS), East Antarctica, using evidence acquired between 1936 and 2000. The most recent major iceberg-calving event occurred between late 1963 and early 1964, when a large berg totalling about 10 000 km(2) in area broke From the ice front. The rate of forward advance of the ice front is presently 1300-1400 m a(-1). At this rate of advance, based on the present icefront position front recent RADARSAT imagery, it would take 20-25 years to attain the 1963 (pre-calve) position, suggesting that the AIS calving cycle has a period of approximately 60-70 years. Two longitudinal (parallel-to-flow) rifts, approximately 25 km apart at the AIS front, are observed in satellite imagery acquired over the last 14+ years. These rifts have formed at suture zones the shelf where neighbouring now-bands have separated in association with transverse spreading. The rifts were 15 km (rift A) and 26 km (rift B) in length in September 2000, and will probably become the sides of a large tabular iceberg (23 km x 25 km). A transverse (perpendicular-to-flow) fracture, visible at the upstream end of rift A in 1996, had propagated 6 km towards rift B by September 2000; when it meets rift B the iceberg will calve. A satellite image acquired in 1962 shows an embayment of this size in the AIS front, hence we deduce that this calving pattern also occurred during the last calving cycle, and therefore that the calving behaviour of the AIS apparently follows a regular pattern.