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Shepherd, A, Fricker HA, Farrell SL.  2018.  Trends and connections across the Antarctic cryosphere. Nature. 558:223-232.   10.1038/s41586-018-0171-6   AbstractWebsite

Satellite observations have transformed our understanding of the Antarctic cryosphere. The continent holds the vast majority of Earth's fresh water, and blankets swathes of the Southern Hemisphere in ice. Reductions in the thickness and extent of floating ice shelves have disturbed inland ice, triggering retreat, acceleration and draw-down of marine-terminating glaciers. The waxing and waning of Antarctic sea ice is one of Earth's greatest seasonal habitat changes, and although the maximum extent of the sea ice has increased modestly since the 1970s, inter-annual variability is high, and there is evidence of longer-term decline in its extent.

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.

Moholdt, G, Padman L, Fricker HA.  2014.  Basal mass budget of Ross and Filchner-Ronne ice shelves, Antarctica, derived from Lagrangian analysis of ICESat altimetry. Journal of Geophysical Research: Earth Surface.   10.1002/2014JF003171   Abstract

Traditional methods of deriving temporal variability of Antarctic ice-shelf elevation from satellite altimetry use a fixed (“Eulerian”) reference frame, where the measured changes include advection of ice thickness gradients between measurement epochs. We present a new method which removes advection effects by using an independent velocity field to compare elevations in a moving (“Lagrangian”) reference frame. Applying the technique to ICESat laser altimetry for the period 2003-2009 over the two largest Antarctic ice shelves, Ross and Filchner-Ronne, we show that the Lagrangian approach reduces the variability of derived elevation changes by about 50% compared to the Eulerian approach, and reveals clearer spatial patterns of elevation change. The method simplifies the process of estimating basal mass budget from the residual of all other processes that contribute to ice-shelf elevation changes. We use field data and ICESat measurements over ice rises and the grounded ice sheet to account for surface accumulation and changes in firn air content, and remove the effect of ice-flow divergence using surface velocity and ice thickness data. The results show highest basal melt rates (>5 m a-1) near the deep grounding lines of major ice streams, but smaller melt rates (<5 m a-1) near the ice-shelf fronts are equally important to total meltwater production since they occur over larger areas. Integrating over the ice-shelf areas, we obtain basal mass budgets of -50 ± 64 Gt a-1 for Ross and -124 ± 66 Gt a-1 for Filchner-Ronne, with changes in firn air content as the largest error source.

Walker, CC, Bassis JN, Fricker HA, Czerwinski RJ.  2013.  Structural and environmental controls on Antarctic ice shelf rift propagation inferred from satellite monitoring. Journal of Geophysical Research-Earth Surface. 118:2354-2364.   10.1002/2013jf002742   AbstractWebsite

Iceberg calving from ice shelves accounts for nearly half of the mass loss from the Antarctic Ice Sheet, yet our understanding of this process is limited. The precursor to iceberg calving is large through-cutting fractures, called rifts, that can propagate for decades after they have initiated until they become iceberg detachment boundaries. To improve our knowledge of rift propagation, we monitored the lengths of 78 rifts in 13 Antarctic ice shelves using satellite imagery from the Moderate Resolution Imaging Spectroradiometer and Multiangle Imaging Spectroradiometer between 2002 and 2012. This data set allowed us to monitor trends in rift propagation over the past decade and test if variation in trends is controlled by variable environmental forcings. We found that 43 of the 78 rifts were dormant, i.e., propagated less than 500 m over the observational interval. We found only seven rifts propagated continuously throughout the decade. An additional eight rifts propagated for at least 2 years prior to arresting and remaining dormant for the rest of the decade, and 13 rifts exhibited isolated sudden bursts of propagation after 2 or more years of dormancy. Twelve of the fifteen active rifts were initiated at the ice shelf fronts, suggesting that front-initiated rifts are more active than across-flow rifts. Although we did not find a link between the observed variability in rift propagation rate and changes in atmospheric temperature or sea ice concentration correlated with, we did find a statistically significant correlation between the arrival of tsunamis and propagation of front-initiated rifts in eight ice shelves. This suggests a connection between ice shelf rift propagation and mechanical ocean interaction that needs to be better understood.

Padman, L, Costa DP, Dinniman MS, Fricker HA, Goebel ME, Huckstadt LA, Humbert A, Joughin I, Lenaerts JTM, Ligtenberg SRM, Scambos T, van den Broeke MR.  2012.  Oceanic controls on the mass balance of Wilkins Ice Shelf, Antarctica. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007301   AbstractWebsite

Several Antarctic Peninsula (AP) ice shelves have lost significant fractions of their volume over the past decades, coincident with rapid regional climate change. Wilkins Ice Shelf (WIS), on the western side of the AP, is the most recent, experiencing a sequence of large calving events in 2008 and 2009. We analyze the mass balance for WIS for the period 1992-2008 and find that the averaged rate of ice-shelf thinning was similar to 0.8 m a(-1), driven by a mean basal melt rate of < w(b)> = 1.3 +/- 0.4 m a(-1). Interannual variability was large, associated with changes in both surface mass accumulation and < w(b)>. Basal melt rate declined significantly around 2000 from 1.8 +/- 0.4 m a(-1) for 1992-2000 to similar to 0.75 +/- 0.55 m a(-1) for 2001-2008; the latter value corresponding to approximately steady-state ice-shelf mass. Observations of ocean temperature T obtained during 2007-2009 by instrumented seals reveal a cold, deep halo of Winter Water (WW; T approximate to - 1.6 degrees C) surrounding WIS. The base of the WW in the halo is similar to 170 m, approximately the mean ice draft for WIS. We hypothesize that the transition in < w(b)> in 2000 was caused by a small perturbation (similar to 10-20 m) in the relative depths of the ice base and the bottom of the WW layer in the halo. We conclude that basal melting of thin ice shelves like WIS is very sensitive to upper-ocean and coastal processes that act on shorter time and space scales than those affecting basal melting of thicker West Antarctic ice shelves such as George VI and Pine Island Glacier.

Chen, X, Shearer PM, Walter F, Fricker HA.  2011.  Seventeen Antarctic seismic events detected by global surface waves and a possible link to calving events from satellite images. Journal of Geophysical Research-Solid Earth. 116   10.1029/2011jb008262   AbstractWebsite

We detect 17 seismic events in Antarctica from 1997 to 2009 by applying a surface wave detector to global seismic data. We locate these events using a waveform cross-correlation method and find that most occurred near the coast of Antarctica and are clustered in three regions: four events are on the Ronne Ice Shelf, close to the location of a 1998 calving event; five events are near the Vanderford Glacier; and eight events are near the Ninnis Glacier. The observed Rayleigh and Love waves for these events have similar amplitudes and a two-lobed radiation pattern, matching the expected amplitude behavior of a single-force source model. Using such a model, we obtain best fitting horizontal force directions for the 14 events that have relatively better signal-to-noise ratios. Analysis of coastline changes from MODIS images before and after the detected events show that two events on Vanderford Glacier and one event near Ninnis Glacier are likely associated with calving events. Moreover, the inferred force directions for the seismic events appear consistent with local ice flow directions. Both satellite observations and modeling results strongly suggest a link between seismic events and calving processes in the two regions. However, the force directions on the Ronne Ice Shelf are aligned with observed rift propagation directions, suggesting that these events may arise from rifting processes.

Brunt, KM, Fricker HA, Padman L.  2011.  Analysis of ice plains of the Filchner-Ronne Ice Shelf, Antarctica, using ICESat laser altimetry. Journal of Glaciology. 57:965-975. AbstractWebsite

We use repeat-track laser altimeter data from the Ice, Cloud and land Elevation Satellite (ICESat) to map the grounding zone (GZ) of the Filchner-Ronne Ice Shelf, Antarctica. Ice flexure in the GZ occurs as the ice shelf responds to ocean-height changes due primarily to tides. We have identified three 'ice plains', regions of low surface slope near the GZ where the ice is close to hydrostatic equilibrium: one on Institute Ice Stream; another to its east; and another west of Foundation Ice Stream. The vertical information from repeated ICESat tracks enables us to study the topography, state of flotation and flexure characteristics across these features. In regions of ephemeral grounding, tidal migration of the grounding line allows us to estimate bed slope (similar to 1-2 x 10(-3)). From these studies we develop a classification scheme for ice plains, expressed in terms of the evolution, or 'life cycle', of these features. A lightly grounded ice plain progresses to a state of ephemeral grounding as the ice sheet thins near the GZ. Once sufficient thinning has occurred, the ice plain becomes a fully floating, relict ice plain with an undulated surface topography similar to that of lightly grounded ice; we expect viscous relaxation to a smooth ice-shelf surface to occur over a timescale of decades. Our improved insight into ice-plain evolution suggests added complexity in modeling ice in the vicinity of the GZ, and a role for ice-plain observations as a guide to relatively rapid changes in ice-sheet mass balance.

Fricker, HA, Powell R, Priscu J, Tulaczyk S, Anandakrishnan S, Christner B, Fisher AT, Holland D, Horgan H, Jacobel R, Mikucki J, Mitchell A, Scherer R, Severinghaus J.  2011.  Siple Coast subglacial aquatic environments; the Whillans ice stream subglacial access research drilling project. Geophysical Monograph. 192:199-219.   10.1029/2010gm000932   AbstractWebsite

The Whillians Ice Stream Subglacial Access Research Drilling (WISSARD) project is a 6-year (2009-2015) integrative study of ice sheet stability and subglacial geobiology in West Antarctica, funded by the Antarctic Integrated System Science Program of National Science Foundation's Office of Polar Programs, Antarctic Division. The overarching scientific objective of WISSARD is to assess the role of water beneath a West Antarctic Ice Stream in interlinked glaciological, geological, microbiological, geochemical, hydrological, and oceanographic systems. The WISSARD's important science questions relate to (1) the role that subglacial and ice shelf cavity waters and wet sediments play in ice stream dynamics and mass balance, with an eye on the possible future of the West Antarctic Ice Sheet and (2) the microbial metabolic and phylogenetic diversity in these subglacial environments. The study area is the downstream part of the Whillans Ice Stream on the Siple Coast, specifically Subglacial Lake Whillans and the part of the grounding zone across which it drains. In this chapter, we provide background on the motivation for the WISSARD project, detail the key scientific goals, and describe the new measurement tools and strategies under development that will provide the framework for conducting an unprecedented range of scientific observations.

Allison, I, Alley RB, Fricker HA, Thomas RH, Warner RC.  2009.  Ice sheet mass balance and sea level. Antarctic Science. 21:413-426.   10.1017/s0954102009990137   AbstractWebsite

Determining the mass balance of the Greenland and Antarctic ice sheets (GIS and AIS) has long been a major challenge for polar science. But until recent advances in measurement technology, the uncertainty in ice sheet mass balance estimates was greater than any net contribution to sea level change. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (AR4) was able, for the first time, to conclude that, taken together, the GIS and AIS have probably been contributing to sea level rise over the period 1993-2003 at an average rate estimated at 0.4 nine yr(-1). Since the cut-off date for work included in AR4, a number of further studies of the mass balance of GIS and AIS have been made using satellite altimetry, satellite gravity measurements and estimates of mass influx and discharge using a variety of techniques. Overall, these studies reinforce the conclusion that the ice sheets are contributing to present sea level rise, and suggest that the rate of loss from GIS has recently increased. The largest unknown in the projections of sea level rise over the next century is the potential for rapid dynamic collapse of ice sheets.

Scambos, T, Fricker HA, Liu CC, Bohlander J, Fastook J, Sargent A, Massom R, Wu AM.  2009.  Ice shelf disintegration by plate bending and hydro-fracture: Satellite observations and model results of the 2008 Wilkins ice shelf break-ups. Earth and Planetary Science Letters. 280:51-60.   10.1016/j.epsl.2008.12.027   AbstractWebsite

Satellite remote sensing observations of three break-up events in 2008 for the Wilkins Ice Shelf (28 February to 6 March, 27 May to 31 May, and 28 June to mid-July) provide unprecedented detail of ice shelf calving during rapid break-up. The observations reveal that the Wilkins break-ups occur through a distinctive type of shelf calving, which we term 'disintegration', as well as more typical rifting and calving. Here we focus on the disintegration process, which is characterized by repeated rapid fracturing that creates narrow ice-edge-parallel blocks, with subsequent block toppling and fragmentation forming an expanding iceberg and ice rubble mass. We use these data to develop and test a model of floating ice plate disintegration in which ice plate bending stresses at the ice front arising from buoyancy forces can lead to runaway calving when free (mobile) water is available. High-resolution satellite images and laser altimetry of the first break-up event provide details of fracture spacings, ice thicknesses, and plate bending profiles that agree well with our model predictions. We suggest that surface or near-surface meltwater is the main pre-condition for disintegration, and that hydro-fracture is the main mechanism. Brine layers from near-waterline brine infiltration can support a similar process, but this is less effective unless regional ice stress patterns contribute to the net stress available at the crack tip for fracturing. A combination of brine-enhanced fracturing and changing internal net extensional stresses was the likely mechanism behind the latter two Wilkins events. (C) 2008 Elsevier B.V. All rights reserved.

Fricker, HA, Padman L.  2006.  Ice shelf grounding zone structure from ICESat laser altimetry. Geophysical Research Letters. 33   10.1029/2006gl026907   AbstractWebsite

We present a technique for investigating the grounding zone (GZ) of Antarctic ice shelves using laser altimetry from the Ice, Cloud and land Elevation Satellite (ICESat). Most surface height variability in the GZ is easily resolved by the ICESat laser's similar to 65 m footprint and similar to 172 m along-track spacing. Comparisons of repeated tracks sampled at different phases of the ocean tide identify the landward and seaward limits of tide-forced ice flexure, providing GZ location and width information for each track. Using ICESat data in the Institute Ice Stream region of southern Ronne Ice Shelf, we demonstrate that the location of the GZ based on feature identification in satellite imagery or digital elevation models may be in error by several km. Our results show that ICESat will contribute significantly to improving knowledge of GZ structure and to studies requiring accurate GZ locations, e. g., ice mass balance calculations and ice-sheet/ocean modeling.

Fricker, HA, Bassis JN, Minster B, MacAyeal DR.  2005.  ICESat's new perspective on ice shelf rifts: The vertical dimension. Geophysical Research Letters. 32   10.1029/2005gl025070   AbstractWebsite

The small footprint (similar to 70 m) and similar to 172 m along-track spacing of the Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud and land Elevation Satellite (ICESat) provides unprecedented horizontal resolution for a satellite altimeter. This enables ICESat to map many previously unresolved features on ice shelves, such as crevasses, rifts, grounding zones and ice fronts. We present examples of ICESat-derived elevation data showing topography over rifts on the Amery and Ross ice shelves, widths of rifts and as estimates of the thickness of melange (a collection of ice and snow trapped inside the rifts). We show that melange thickness remains constant over the ICESat data period and tends to be thicker in older rifts. We validate the ICESat-derived melange depth estimate with an in situ measurement on the Ross Ice Shelf.

Jacka, TH, Abdalati W, Allison I, Carsey F, Casassa G, Fily M, Frezzotti M, Fricker HA, Genthon C, Goodwin I, Guo Z, Hamilton GS, Hindmarsh RCA, Hulbe CL, Jacka TH, Jezek KC, Scambos TA, Shuman C, Skvarca P, Takahashi S, van de Wal RSW, Vaughan DG, Wang WL, Warner RC, Wingham DJ, Young NW, Zwally HJ, Comm I.  2004.  Recommendations for the collection and synthesis of Antarctic Ice Sheet mass balance data. Global and Planetary Change. 42:1-15.   10.1016/j.gloplacha.2003.11.008   AbstractWebsite

Recent unexpected changes in the Antarctic Ice Sheet, including ice sheet thinning, ice shelf collapse and changes in ice velocities, along with the recent realization that as much as one third of ice shelf mass loss is due to bottom melt, place a new urgency on understanding the processes involved in these changes. Technological advances, including very new or forthcoming satellite-based (e.g. ICESat, CryoSat) remote sensing missions, will improve our ability to make meaningful determinations of changes in Antarctic Ice Sheet mass balance. This paper is the result of a workshop held to develop a strategy for international collaboration aimed at the collection and synthesis of Antarctic Ice Sheet mass balance data, and at understanding the processes involved so that we might predict future change. Nine sets of recommendations are made, concerning the most important and sensitive measurements, temporal ranges and study areas. A final tenth recommendation calls for increased synthesis of ice sheet data and communication between the field measurement, satellite observation and modelling communities. (C) 2004 Published by Elsevier B.V.

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, Padman L.  2002.  Tides on Filchner-Ronne Ice Shelf from ERS radar altimetry. Geophysical Research Letters. 29   10.1029/2001gl014175   AbstractWebsite

[1] We use harmonic analysis of 8 years of ERS satellite radar altimeter (RA) data at orbital crossovers to retrieve complex amplitude (amplitude and phase) coefficients for several major tidal harmonics over the Filchner-Ronne Ice Shelf (FRIS), Antarctica. We describe a method for estimating the accuracy of this method, which ranges from similar to2 to 8 cm per harmonic. A comparison between M-2 complex amplitude from a recent ocean model and from our ERS RA analyses identifies two regions of the FRIS where the RA data are inconsistent with the model. In both regions the differences can be attributed to incorrect specification of the grounding line location in the model. Our study demonstrates the value of ERS RA data in Antarctic ice shelf tide modeling, and the potential for future altimeter satellites with high polar orbits to contribute to the definition of global tide height variations.

Fricker, HA, Popov S, Allison I, Young N.  2001.  Distribution of marine ice beneath the Amery Ice Shelf. Geophysical Research Letters. 28:2241-2244.   10.1029/2000gl012461   AbstractWebsite

We present a map of the marine ice accreted to the base of the Amery Ice Shelf (AIS), East Antarctica. This map is obtained by converting a Digital Elevation Model (DEM) of the AIS generated from satellite radar altimeter data to an ice thickness map, assuming hydrostatic equilibrium, and subtracting from that a second ice thickness map, derived from airborne radio-echo sounding (RES) measurements. The RES signal does not penetrate the marine ice, so the measurement is only to the meteoric-marine ice boundary, and therefore the difference between the two maps is the marine ice thickness. The marine ice is up to 190 m thick and accounts for about 9% of the shelf volume. It is concentrated in the northwest of the shelf, a result of the clockwise ocean circulation in the cavity below.