Limitations of access have long restricted exploration and investigation of the cavities beneath ice shelves to a small number of drillholes. Studies of sea-ice underwater morphology are limited largely to scientific utilization of submarines. Remotely operated vehicles, tethered to a mother ship by umbilical cable, have been deployed to investigate tidewater-glacier and ice-shelf margins, but their range is often restricted. The development of free-flying autonomous underwater vehicles (AUVs) with ranges of tens to hundreds of kilometres enables extensive missions to take place beneath sea ice and floating ice shelves. Autosub2 is a 3600 kg, 6.7 m long AUV, with a 1600 m operating depth and range of 400 km, based on the earlier Autosub1 which had a 500 m depth limit. A single direct-drive d.c. motor and five-bladed propeller produce speeds of 1–2 m s−1. Rear-mounted rudder and stern-plane control yaw, pitch and depth. The vehicle has three sections. The front and rear sections are free-flooding, built around aluminium extrusion space-frames covered with glass-fibre reinforced plastic panels. The central section has a set of carbon-fibre reinforced plastic pressure vessels. Four tubes contain batteries powering the vehicle. The other three house vehicle-control systems and sensors. The rear section houses subsystems for navigation, control actuation and propulsion and scientific sensors (e.g. digital camera, upward-looking 300 kHz acoustic Doppler current profiler, 200 kHz multibeam receiver). The front section contains forward-looking collision sensor, emergency abort, the homing systems, Argos satellite data and location transmitters and flashing lights for relocation as well as science sensors (e.g. twin conductivity–temperature–depth instruments, multibeam transmitter, sub-bottom profiler, AquaLab water sampler). Payload restrictions mean that a subset of scientific instruments is actually in place on any given dive. The scientific instruments carried on Autosub are described and examples of observational data collected from each sensor in Arctic or Antarctic waters are given (e.g. of roughness at the underside of floating ice shelves and sea ice).

ABSTRACT. This paper compares estimates of susp ended-sediment yield and discha rge from two glacier basins in Svalbard ex hibiting contrasting glacial thermal regimes: Austre Br0ggerbreen (",12 km 2), which is almost entirely cold-based, and Finsterwalderbreen (~4 km2), dominated by warm basal ice. There are marked differences in th e magnitude a nd temporal pattern o[ mean d ail y discharge a nd mean daily suspended-sediment concentration from the two glacier basin s. Specific suspended-sediment yields from Finsterwalderbreen (710-2900 t km 2 a I) were more than one order ofmagnitude greater than at Austre Br0ggerbreen (81 -110 t km 2 a-I). Th ese differences are ascribed to the influence o[ thermal regime upon the meltwater drainage system and the predominant sources of suspended sediment. The potential significance of glacier thermal regime is furth er explored using studies from other glacier basins in Svalbard. Variations in thermal regime resulting from mass-balance adjustments since the termination o[ the Little Ice Age are also examined.

ABSTRACT. Antarctic radio-echo so unding (RES) data at 60 MHz have b een used to determine a n independent stratigraphy for the ice core at Vostok station, based on internal radio-echo layering. A-scope RES data a llow the amplitude of reflected electromagnetic (elm ) waves to be meas ured and, by accounting for geometric spreading a nd absorption losses of the elm wave, power refl ection coefficients (PRCs) to be calculated. This information is compared with time-co ntinuous Z-scope RES data in order to trace continuous elm reflectors across the ice sheet. Internal ice-sheet hori zons deeper than 800 m are caused by layers of ice that possess distinctly different dielect ric properties (i.e. acidic layers) compared with ice above and/or below. Comparison offour PRC samples, located", 5 km from Vostok station, revealed five distinct internal reflections between 1000 a nd 2200 m. Z-scope data from directly over the Vostok station site show the same five prominent internal radio-echo layers. The depth-related radio-echo signals were then compared with chemical records from the Vostok ice core, including the H 2 S0 4 signal, a major component of which is derived from volcanic events. From this procedure, internal radio-echo reflectors a nd Vostok ice-core acid measurements were correlated. A very good match was made between Z-scope and ice-core data. However, vertical offse ts observed between A-scope-derived RES layers a nd peaks in the chemical signal of up to 100 m are probably due to the general folding of the ice-sheet laye ring between the core site a nd the RES flight-lin e. We conclude that 60 MHz RES layering may be regarded as a stratig raphy indep endent of palaeoclimate, a nd may be used to correlate other deep Southern H emisphere ice cores.

ABSTRACT. Glacierized basins in the high Arctic are beli eved to be regions of low chemical weathering rates, despite the lack of pertinent data, because it is beli eved that water does not flow in sig nificant quantiti es through subgl acial drainage systems. "Ve have calc ulated chemica l weathering rates at Finsterwalderbreen, a pol y th ermal , surge-type g lacier in Svalbard. Rates of 320 and 150 meqI:+ m -2 year I were m easured in 1994 a nd 1995, resp ectively. The corres ponding water fluxes were 4.1 x 10 7 and 1.7 x 10 7 m 3 . We estimate that we have measured ",72% of th e total annual discharpe, h ence the true a nnual chemical weathering rates are ",440 a nd 210 meq I:+ m -2 yea r-, res pectively. Thi s g ives a mean annual chemical weathering rate of 330 meq L:+ m 2 year-I, which a pprox im ates the continental average of 390 meqL:+ m -2 year 1 a nd is intermedi ate betwee n chemical weathering rates m easured on cold-based:placiers (",110-160 m eqI:+ m 2 yea r I) and temperate glaciers (450-1000 meqI: + m-year I). This suggests that there may b e a direct link between chemical weathering rates and th ermal regime, a nd that glacierized basins in the high Arctic cannot necessaril y be considered as reg ion s of low chemi cal weathering and CO 2 drawdown .

A finite element model is used to calculate engineering studies, have been increasingly applied to ice temperature and velocity along a two-dimensional glacier masses [e.g., Hooke et al., 1979; Iken 1981' Sikonia, flow line. The model incorporates automatic element mesh 1982; MacAyeal and Thomas, 1982]. generation, interpolation fi'om point input data to all In this paper we examine some of the strengths and boundary nodes, comprehensive output graphics, linkage weaknesses of the finite element technique when adapted between velocity and temperature under one control program for a typical glacier flow study. Some of the problems to allow iteration between the two, and time stepping in highlighted relate to fundamental limitations of the discrete units. The model results are tested against method (e.g., grid size, discretizing of a continuous analytical solutions (for ice temperatures and velocities body) and an inability to handle complex ice conditions in simple situations) and field data (ice thickness, (e.g., basal sliding and the sliding/non sliding velocity, temperature, and mass balance). Sensitivity transition, cavitation, presence of significant bodies of analyses were conducted with respect to model resolution, basal water). Other problems arise from the inadequacy of changed time step interval, geothermal temperature glaciological data sets to provide sufficiently accurate gradient, and bedrock topography. Limitations and and numerous boundary conditions. strengths of applying finite element techniques to the A result of this study has been to indicate the modeling of glaciers are discussed. Some problems are considerable merit of finite element modeling for studies encountered with the techni•ciue and methods of computation of ice masses where flow is creep-dominated and (e.g., limitation of mesh size, discretizing of a characterized by marked gradients in temperature and continuous body, equotion approximation). Others arise velocity. Such specifications are least well handled by from an inability to handle complex ice conditions (e.g., conventional numerical techniques. basal sliding and non sliding/sliding transition, cavitation, basal water). The inadequacy of glaciological Data Sources data sets to provide sufficiently accurate and numerous boundary conditions and input values is a further major Finsterwalderbreen, the glacier selected as a case problem. Nevertheless, finite element methods hold study, is located in Svalbard (Figure I). It has been considerable promise for modeling studies of cold, creep-monitored closely by Norwegian glaciologists [Liestol, dominated ice masses where there are marked gradients in 1969, 1976] and has a useful, although asynchronous, temperature and velocity. These conditions are least well observational data set comprising velocity, temperature, handled by conventional numerical techniques. and mass balance measurements. In addition, ice thickness data from radio echo sounding are available from a 1980'

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