Roland C. Warner

A dynamic early East Antarctic Ice Sheet suggested by ice-covered fjord landscapes

The first Cenozoic ice sheets initiated in Antarctica from the Gamburtsev Subglacial Mountains and other highlands as a result of rapid global cooling ∼34 million years ago. In the subsequent 20 million years, at a time of declining atmospheric carbon dioxide concentrations and an evolving Antarctic circumpolar current, sedimentary sequence interpretation and numerical modelling suggest that cyclical periods of ice-sheet expansion to the continental margin, followed by retreat to the subglacial highlands, occurred up to thirty times. These fluctuations were paced by orbital changes and were a major influence on global sea levels. Ice-sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of Antarctic topographic lowlands. Here we show that the basal topography of the Aurora Subglacial Basin of East Antarctica, at present overlain by 2–4.5u2009km of ice, is characterized by a series of well-defined topographic channels within a mountain block landscape. The identification of this fjord landscape, based on new data from ice-penetrating radar, provides an improved understanding of the topography of the Aurora Subglacial Basin and its surroundings, and reveals a complex surface sculpted by a succession of ice-sheet configurations substantially different from today’s. At different stages during its fluctuations, the edge of the East Antarctic Ice Sheet lay pinned along the margins of the Aurora Subglacial Basin, the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1u2009km below sea level. Although the timing of the channel incision remains uncertain, our results suggest that the fjord landscape was carved by at least two iceflow regimes of different scales and directions, each of which would have over-deepened existing topographic depressions, reversing valley floor slopes.

Properties of a marine ice layer under the Amery Ice Shelf, East Antarctica

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.

Modeling the basal melting and marine ice accretion of the Amery Ice Shelf

The basal mass balance of the Amery Ice Shelf (AIS) in East Antarctica is investigated nusing a numerical ocean model. The main improvements of this model over previous nstudies are the inclusion of frazil formation and dynamics, tides and the use of the latest nestimate of the sub-ice-shelf cavity geometry. The model produces a net basal melt rate of n45.6 Gt year�1 (0.74 m ice year�1) which is in good agreement with reviewed observations. nThe melting at the base of the ice shelf is primarily due to interaction with High Salinity nShelf Water created from the surface sea-ice formation in winter. The temperature difference nbetween the coldest waters created in the open ocean and the in situ freezing point of ocean nwater in contact with the deepest part of the AIS drives a melt rate that can exceed 30 m of nice year�1. The inclusion of frazil dynamics is shown to be important for both melting and nmarine ice accretion (refreezing). Frazil initially forms in the supercooled water layer nadjacent to the base of the ice shelf. The net accretion of marine ice is 5.3 Gt year�1, ncomprised of 3.7 Gt year�1 of frazil accretion and 1.6 Gt year�1 of direct basal refreezing.

Evidence of a hydrological connection between the ice divide and ice sheet margin in the Aurora Subglacial Basin, East Antarctica

Subglacial hydrology in East Antarctica is poorly understood, yet may be critical to nthe manner in which ice flows. Data from a new regional airborne geophysical survey n(ICECAP) have transformed our understanding of the topography and glaciology nassociated with the 287,000 km2 Aurora Subglacial Basin in East Antarctica. Using these ndata, in conjunction with numerical ice sheet modeling, we present a suite of analyses nthat demonstrate the potential of the 1000 km-long basin as a route for subglacial water ndrainage from the ice sheet interior to the ice sheet margin. We present results from nour analysis of basal topography, bed roughness and radar power reflectance and from our nmodeling of ice sheet flow and basal ice temperatures. Although no clear-cut subglacial nlakes are found within the Aurora Basin itself, dozens of lake-like reflectors are observed nthat, in conjunction with other results reported here, support the hypothesis that the nbasin acts as a pathway allowing discharge from subglacial lakes near the Dome C ice ndivide to reach the coast via the Totten Glacier.

Ice sheet mass balance and sea level

Abstract 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 mm 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.

Mass balance of the Lambert Glacier-Amery Ice Shelf system, East Antarctica : a comparison of computed balance fluxes and measured fluxes

We combine European Remote-sensing Satellite (ERS-1) radar altimeter surface elevations (Fricker and others, 2000) with six different accumulation distributions to compute balance fluxes for the Lambert Glacier-Amery Ice Shelf drainage system. These interpolated balance fluxes are compared with fluxes derived from in situ measurements of ice thickness and velocity at 73 stations of the Lambert Glacier basin traverse and at 11 stations further downstream, to assess the systems state of balance. For the upstream line we obtain a range of imbalance estimates, from -23.8% to +19.9% of the observed flux, reflecting the sensitivity to the accumulation distributions. For some of the accumulation distributions the imbalance estimates vary significantly between different parts of the line. Imbalance estimates for the downstream line range from -17.7 % to +70.2%, with four of the estimates exceeding +30%, again reflecting the sensitivity of the result to input accumulation, and strongly suggesting that the mass balance of the region between the two lines is positive. Our results confirm the importance of accurate estimates of accumulation in ice-sheet mass-balance studies. Furthermore, they suggest that it is not possible to accurately determine the state of balance of large Antarctic drainage basins on the basis of currently available accumulation distributions.

Ocean circulation and ice‐ocean interaction beneath the Amery Ice Shelf, Antarctica

Simulations of the ocean dynamics in the cavity under the Amery Ice Shelf, Antarctica, were carried out using a three-dimensional numerical ocean model. Two different boundary conditions were used to describe the open ocean barotropic exchange at the ice front. The simulations show that the circulation in the ocean cavity is predominantly barotropic and is generally steered by the cavity topography. The circulation is driven by the density gradient in the cavity, which is strongly influenced by the heat and salt fluxes from melting and freezing processes at the ice-ocean interface, and by the horizontal exchange of heat and salt across the open ocean boundary at the ice front. The interaction at the ice-ocean interface allows the basal component of the mass loss of the Amery Ice Shelf to be estimated. In the two simulations the computed losses were 5.8 Gt yr−1 and 18.0 Gt yr−1, values consistent with observations. The bulk of the melting occurred near the southern grounding line of the ice shelf, although substantial melting also occurred in areas where heat transport by horizontal circulation was large. Accretion was restricted to areas where water, from upstream melting, became supercooled as it ascended the ice shelf base.

Examining the interaction between multi‐year landfast sea ice and the Mertz Glacier Tongue, East Antarctica: Another factor in ice sheet stability?

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.

Analysis of velocity field, mass balance, and basal melt of the Lambert Glacier–Amery Ice Shelf system by incorporating Radarsat SAR interferometry and ICESat laser altimetry measurements

[1]xa0By incorporating recently available remote sensing data, we investigated the mass balance for all individual tributary glacial basins of the Lambert Glacier–Amery Ice Shelf system, East Antarctica. On the basis of the ice flow information derived from SAR interferometry and ICESat laser altimetry, we have determined the spatial configuration of eight tributary drainage basins of the Lambert-Amery glacial system. By combining the coherence information from SAR interferometry and the texture information from SAR and MODIS images, we have interpreted and refined the grounding line position. We calculated ice volume flux of each tributary glacial basin based on the ice velocity field derived from Radarsat three-pass interferometry together with ice thickness data interpolated from Australian and Russian airborne radio echo sounding (RES) surveys and inferred from ICESat laser altimetry data. Our analysis reveals that three tributary basins have a significant net positive imbalance, while five other subbasins are slightly positive or close to zero balance. Overall, in contrast to previous studies, we find that the grounded ice in Lambert Glacier–Amery Ice Shelf system has a positive mass imbalance of 22.9 ± 4.4 Gt a−1. The net basal melting for the entire Amery Ice Shelf is estimated to be 27.0 ± 7.0 Gt a−1. The melting rate decreases rapidly from the grounding zone to the ice shelf front. Significant basal refreezing is detected in the downstream section of the ice shelf. The mass balance estimates for both the grounded ice sheet and the ice shelf mass differ substantially from other recent estimates.

A 4‐decade record of elevation change of the Amery Ice Shelf, East Antarctica

[1]xa0We report on long-term surface elevation changes of the central Amery Ice Shelf (AIS) by comparing elevation records spanning 4 decades (1968–2007). We use elevation records acquired with the following methods: optical leveling (1968–1969); ERS radar altimetry (1992–2003); GPS (1995–2006); and Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry (2003–2007). We compute multidecadal elevation trend (dh/dt) values at crossovers between the leveling route and each of the GPS and ICESat tracks as well as shorter-period dh/dt at ERS-ERS, GPS-GPS, and ICESat-ICESat crossovers. At GPS-leveling crossovers the mean long-term dh/dt is −0.003 m a−1, and at ICESat-leveling crossovers the mean dh/dt is +0.013 m a−1; neither trend is significantly different from zero. The data do, however, exhibit variable trends: near-zero change between 1991 and mid-1996, then thickening to ∼2003, followed by thinning ∼2003–2007, with 5 year dh/dt averages exceeding ∼±0.1 m a−1. The changes in dh/dt pattern in mid-1996 and again in 2003 occur with unexpected speed. The ice shelf exhibits different dh/dt patterns than does the surrounding grounded ice, suggesting that surface mass balance variations or longer-term variations in firn densification processes are unlikely to be major causes. We conclude that these observed multiyear elevation changes must be due to currently unexplained or presently poorly quantified phenomena involving surface or basal processes and/or ice dynamics. With the multidecadal stability of the AIS established, the short-term fluctuations that we observe suggests that for other ice shelves, observed strong dh/dt signals over short time periods do not necessarily indicate ice shelf instability.