Evolution of supraglacial lakes on the Larsen B ice shelf in the decades before it collapsed

The Larsen B ice shelf collapsed in 2002 losing an area twice the size of Greater London to the sea (3,000 km 2), in an event associated with widespread supraglacial lake drainage. Here we use optical and radar satellite imagery to investigate the evolution of the ice shelf's lakes in the decades preceding collapse. We find (1) that lakes spread southward in the preceding decades at a rate commensurate with meltwater saturation of the shelf surface; (2) no trend in lake size, suggesting an active supraglacial drainage network which evacuated excess water off the shelf; and (3) lakes mostly refreeze in winter but the few lakes that do drain are associated with ice breakup 2–4 years later. Given the relative scale of lake drainage and shelf breakup, however, it is not clear from our data whether lake drainage is more likely a cause, or an effect, of ice shelf collapse

A spatial framework for assessing current conditions and monitoring future change in the chemistry of the Antarctic atmosphere

This is the first study to measure more than 25 chemical constituents in the surface snow and firn across extensive regions of Antarctica. It is also the first to report total- Cs concentrations. We present major ion, trace element, heavy metal, rare earth element 5 and oxygen isotope data from a series of surface snow samples and shallow firn sections collected along four US ITASE traverses across East and West Antarctica. In each sample we measure dissolved concentrations of Na+, K+, Mg2+, Ca2+, Cl−, NO− 3 , SO2− 4 , and MS− using ion chromatography and total concentrations of Sr, Cd, Cs, Ba, La, Ce, Pr, Pb, Bi, U, As, Al, S, Ca, Ti, V, Cr, Mn, Fe, Co, Na, Mg, Li, and K using 10 inductively coupled plasma sector field mass spectrometry. We also measure 18O by isotope ratio mass spectrometry. The 2002/2003 traverse began at Byrd Surface Camp, West Antarctica, and ended close to South Pole, East Antarctica. The 2003/2004 traverse began at South Pole, passed through AGO4 in central East Antarctica before turning north and finishing at 15 Taylor Dome. The combined 2006/2007 and 2007/2008 traverses started out at Taylor Dome and headed south, passing through the Byrd Glacier drainage basin and ending at South Pole. In this study, we utilize satellite remote sensing measurements of microwave backscatter and grain size to assist in the identification of glaze/dune areas across 20 Antarctica and show how chemical concentrations are higher in these areas, precluding them from containing useful high-resolution chemical climate records. The majority of the non-glaze/dune samples in this study exhibit similar, or lower, concentrations to those from previous studies. Consequently, the results presented here comprise a conservative baseline for Antarctic surface snow chemical concentra25 tions. The elements Cd, Pb, Bi, As, and Li are enriched across Antarctica relative to both ocean and upper crust elemental ratios. Global volcanic outgassing accounts for the majority of the Bi measured in East and West Antarctica and for a significant fraction of the Cd in East Antarctica. Nonetheless, global volcanic outgassing cannot account for the enriched values of Pb or As. Local volcanic outgassing from Mount Erebus may account for a significant fraction of the As and Cd in West Antarctica and for a significant fraction in East Antarctic glaze/dune areas. However, despite potential 5 contributions from local and global volcanic sources, significant concentrations of Pb, Cd, and As remain across much of Antarctica. Most importantly, this study provides a baseline from which changes in the chemistry of the atmosphere over Antarctica can be monitored under expected warming scenarios and continued intensification of industrial activities in the Southern Hemisphere

Surface structure and stability of the Larsen C ice shelf, Antarctic Peninsula

A structural glaciological description and analysis of surface morphological features of the Larsen C ice shelf, Antarctic Peninsula, is derived from satellite images spanning the period 1963-2007. The data are evaluated in two time ranges: a comparison of a 1963 satellite image photomosaic with a modern digital mosaic compiled using 2003/04 austral summer data; and an image series since 2003 showing recent evolution of the shelf. We map the ice-shelf edge, rift swarms, crevasses and crevasse traces, and linear longitudinal structures (called 'flow stripes' or 'streak lines'). The latter are observed to be continuous over distances of up to 200km from the grounding line to the ice-shelf edge, with little evidence of changes in pattern over that distance. Integrated velocity measurements along a flowline indicate that the shelf has been stable for similar to 560years in the mid-shelf area. Linear longitudinal features may be grouped into 12 units, each related to one or a small group of outlet feeder glaciers to the shelf. We observe that the boundaries between these flow units often mark rift terminations. The boundary zones originate upstream at capes, islands or other suture areas between outlet glaciers. In agreement with previous work, our findings imply that rift terminations within such suture zones indicate that they contain anomalously soft ice. We thus suggest that suture zones within the Larsen C ice shelf, and perhaps within ice shelves more generally, may act to stabilize them by reducing regional stress intensities and thus rates of rift lengthening

A mathematical model of melt lake development on an ice shelf

The accumulation of surface meltwater on ice shelves can lead to the formation of melt lakes. Melt lakes have been implicated in ice shelf collapse; Antarctica's Larsen B Ice Shelf was observed to have a large amount of surface melt lakes present preceding its collapse in 2002. Such collapse can affect ocean circulation and temperature, cause habitat loss and contribute to sea level rise through the acceleration of tributary glaciers. We present a mathematical model of a surface melt lake on an idealised ice shelf. The model incorporates a calculation of the ice shelf surface energy balance, heat transfer through the firn, the production and percolation of meltwater into the firn, the formation of ice lenses and the development and refreezing of surface melt lakes. The model is applied to the Larsen C Ice Shelf, where melt lakes have been observed. This region has warmed several times the global average over the last century and the Larsen C firn layer could become saturated with meltwater by the end of the century. When forced with weather station data, our model produces surface melting, meltwater accumulation, and melt lake development consistent with observations. We examine the sensitivity of lake formation to uncertain parameters, and provide evidence of the importance of processes such as lateral meltwater transport. We conclude that melt lakes impact surface melt and firn density and warrant inclusion in dynamic-thermodynamic models of ice shelf evolution within climate models, of which our model could form the basis for the thermodynamic component

Influence of Persistent Wind Scour on the Surface Mass Balance of Antarctica

Accurate quantification of surface snow accumulation over Antarctica is a key constraint for estimates of the Antarctic mass balance, as well as climatic interpretations of ice-core records. Over Antarctica, near-surface winds accelerate down relatively steep surface slopes, eroding and sublimating the snow. This wind scour results in numerous localized regions (< or = 200 sq km) with reduced surface accumulation. Estimates of Antarctic surface mass balance rely on sparse point measurements or coarse atmospheric models that do not capture these local processes, and overestimate the net mass input in wind-scour zones. Here we combine airborne radar observations of unconformable stratigraphic layers with lidar-derived surface roughness measurements to identify extensive wind-scour zones over Dome A, in the interior of East Antarctica. The scour zones are persistent because they are controlled by bedrock topography. On the basis of our Dome A observations, we develop an empirical model to predict wind-scour zones across the Antarctic continent and find that these zones are predominantly located in East Antarctica. We estimate that approx. 2.7-6.6% of the surface area of Antarctica has persistent negative net accumulation due to wind scour, which suggests that, across the continent, the snow mass input is overestimated by 11-36.5 Gt /yr in present surface-mass-balance calculations

The spatial distribution and temporal variability of föhn winds over the Larsen C Ice Shelf, Antarctica

The eastern side of the Antarctic Peninsula (AP) mountain range and the adjacent ice shelves are frequently affected by föhn winds originating from upwind of the mountains. Six automatic weather stations (AWS) and archived model output from 5km resolution Antarctic Mesoscale Prediction System (AMPS) forecasts have been combined to identify the occurrence of föhn conditions, and their spatial distribution over the Larsen C Ice Shelf (LCIS) from 2009 to 2012. Algorithms for semi‐automatic detection of föhn conditions have been developed for both AWS and AMPS data. The frequency of föhn varies by location, being most frequent at the foot of the AP and in the north of the ice shelf. They are most common in spring, when they can prevail for 50% of the time. The results of this study have important implications for further research, investigating the impact of föhn on surface melting, and the surface energy budget of the ice shelf. This is of particular interest due to the collapse of Larsen A and B ice shelves in 1995 and 2002 respectively, and the potential instability issues following a large calving event on Larsen C in 2017

Extent of Low-accumulation 'Wind Glaze' Areas on the East Antarctic Plateau: Implications for Continental Ice Mass Balance

Persistent katabatic winds form widely distributed localized areas of near-zero net surface accumulation on the East Antarctic ice sheet (EAIS) plateau. These areas have been called 'glaze' surfaces due to their polished appearance. They are typically 2-200 square kilometers in area and are found on leeward slopes of ice-sheet undulations and megadunes. Adjacent, leeward high-accumulation regions (isolated dunes) are generally smaller and do not compensate for the local low in surface mass balance (SMB). We use a combination of satellite remote sensing and field-gathered datasets to map the extent of wind glaze in the EAIS above 1500m elevation. Mapping criteria are derived from distinctive surface and subsurface characteristics of glaze areas resulting from many years of intense annual temperature cycling without significant burial. Our results show that 11.2 plus or minus 1.7%, or 950 plus or minus 143 x 10(exp 3) square kilometers, of the EAIS above 1500m is wind glaze. Studies of SMB interpolate values across glaze regions, leading to overestimates of net mass input. Using our derived wind-glaze extent, we estimate this excess in three recent models of Antarctic SMB at 46-82 Gt. The lowest-input model appears to best match the mean in regions of extensive wind glaze

Oceanic Controls on the Mass Balance of Wilkins Ice Shelf, Antarctica

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 \u3c w(b)\u3e = 1.3 +/- 0.4 m a(-1). Interannual variability was large, associated with changes in both surface mass accumulation and \u3c w(b)\u3e. 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 \u3c w(b)\u3e 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