We present Bedmap2, a new suite of gridded products describing surface elevation, ice-thickness and the seafloor and subglacial bed elevation of the Antarctic south of 60° S. We derived these products using data from a variety of sources, including many substantial surveys completed since the original Bedmap compilation (Bedmap1) in 2001. In particular, the Bedmap2 ice thickness grid is made from 25 million measurements, over two orders of magnitude more than were used in Bedmap1. In most parts of Antarctica the subglacial landscape is visible in much greater detail than was previously available and the improved data-coverage has in many areas revealed the full scale of mountain ranges, valleys, basins and troughs, only fragments of which were previously indicated in local surveys. The derived statistics for Bedmap2 show that the volume of ice contained in the Antarctic ice sheet (27 million km³) and its potential contribution to sea-level rise (58 m) are similar to those of Bedmap1, but the mean thickness of the ice sheet is 4.6% greater, the mean depth of the bed beneath the grounded ice sheet is 72 m lower and the area of ice sheet grounded on bed below sea level is increased by 10%. The Bedmap2 compilation highlights several areas beneath the ice sheet where the bed elevation is substantially lower than the deepest bed indicated by Bedmap1. These products, along with grids of data coverage and uncertainty, provide new opportunities for detailed modelling of the past and future evolution of the Antarctic ice sheets

We present Bedmap2, a new suite of gridded products describing surface elevation, ice-thickness and the seafloor and subglacial bed elevation of the Antarctic south of 60° S. We derived these products using data from a variety of sources, including many substantial surveys completed since the original Bedmap compilation (Bedmap1) in 2001. In particular, the Bedmap2 ice thickness grid is made from 25 million measurements, over two orders of magnitude more than were used in Bedmap1. In most parts of Antarctica the subglacial landscape is visible in much greater detail than was previously available and the improved coverage of data has in many areas revealed the full scale of mountain ranges, valleys, basins and troughs, only fragments of which were previously indicated in local surveys. The derived statistics for Bedmap2 show that the volume of ice contained in the Antarctic ice sheet (27 million km³) and its potential contribution to sea-level rise (58 m) are similar to those of Bedmap1, but the mean thickness of the ice sheet is 4.6 % greater, the mean depth of the bed beneath the grounded ice sheet is 72 m lower and the area of ice sheet grounded on bed below sea level is increased by 10 %. The Bedmap2 compilation highlights several areas beneath the ice sheet where the bed elevation is substantially lower than the deepest bed indicated by Bedmap1. These products, along with grids of data coverage and uncertainty, provide new opportunities for detailed modelling of the past and future evolution of the Antarctic ice sheets

Understanding the flow of water through the body of a glacier is important, because the spatial distribution of water and the rate of infiltration to the glacier bottom is one control on water storage and pressure, glacier sliding and surging, and the release of glacial outburst floods. According to the prevailing hypothesis, this water flow takes place in a network of tubular conduits. Here we analyse video images from 48 boreholes drilled into the small Swedish glacier Storglaciären, showing that the glacier's hydrological system is instead dominated by fractures that convey water at slow speeds. We detected hydraulically connected fractures at all depths, including near the glacier bottom. Our observations indicate that fractures provide the main pathways for surface water to reach deep within the glacier, whereas tubular conduits probably form only in special circumstances. A network of hydraulically linked fractures offers a simple explanation for the origin and evolution of the englacial water flow system and its seasonal regeneration. Such a fracture network also explains radar observations that reveal a complex pattern of echoes rather than a system of conduits. Our findings may be important in understanding the catastrophic collapse of ice shelves and rapid hydraulic connection between the surface and bed of an ice sheet.

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Radar profiles of bed echo intensity can survey conditions at the ice-bed interface and test for the presence or absence of water. However, extracting information about basal conditions from bed echo intensities requires an estimate of the attenuation loss through the ice. We used the relationship between bed echo intensities from constant-offset radar data and ice thickness to estimate depthaveraged attenuation rates at several locations on and near Kamb Ice Stream (KIS), West Antarctica. We found values varying from 29 dB km -1 at Siple Dome to 15 dB km -1 in the main trunk region of KIS, in agreement with a previous measurement and models. Using these attenuation-rate values, we calculated the relative bed reflectivity throughout our KIS surveys and found that most of the bed in the trunk has high basal reflectivities, similar to those obtained in the location of boreholes that found water at the bed. Areas of lower bed reflectivity are limited to the sticky spot, where a borehole found a dry bed, and along the margins of KIS. These results support previous hypotheses that the basal conditions at locations like the sticky spot on KIS control its stagnation and possible reactivation.

ABSTRACT. The non-linea rit y of the ice-flow la w o r a loca l acc umula tion low over an ice di vide can cause isochrones (inte rnal layers) to b e shallower under the divide relative La the fl a nks, forming a "di vide bump" in the intern a l laye r pat tern . Thi s di vide sign ature is ana lyzed using ice-flow models a nd inverse tech niques to detec t a nd q u a ntif y motion o f th e Siple Dome ice divide, Wes t Antarc tica. The principal feature indicating that migrati o n h as occ urred is a distinc t tilt of th e axi s of the peaks of th e wa rped inte rna l laye rs beneath th e di v ide. The calcula ted migration ra te is 0.05-0.50 J11 a I towa rd Ice Stream D a nd dep end s slightl y on wh eth er the di vide bump is caused by the non-linearit y o f ice now or by a loca l acc umul ati on low. Our calcul a tion s also suggest a stro ng so uth-north accumul ati o n g r adient of 5-10 x 10-6 a I in a na rro w zon e north of th e di vide. A co n equ ence of di v ide mig ration is th a t pre-Holocen e ice is thickest abo ut 0.5 km south of the present di vide p osition. Divide m o ti on indicates that n o n-steady processes, poss ibl y assoc ia ted with ac tivit y o f the bounding ice streams, a re a flecting th e geom e tr y o f Siplc D om e. The migrati on r a te is suffi cientl y slow that the di vide bump is maintain e d in th e intern a l laye r pattern a t a ll obse rvable d epth s. This suggests tha t m aj or asy nchrono us cha nges in th e eleva ti on or p os ition of the bounding ice stream s a r e unlikely ove r a t least the pas t 10 3 -10 I yea rs. ( r(z ) ::::(x),lieal1il1gAcc umul ati on rat(' Length sca les for pa rtiti oning fun cti on

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