Cryosphere General Contributions II
Presiding: C Shuman, NASA Goddard Space Flight Center; J Minster, Scripps Institution of Oceanography
C43B-01 13:30h
Is Lake Vostok Really Sealed?
The waters of Lake Vostok are both replenished and drained by the ice sheet that is dragged across its surface. We address the question of the larger-scale cycling of water through this system, i.e., how long does it take new snow to enter the lake and how long does it take water removed from the lake to enter the ocean? Employing a numerical ice sheet model that includes anisotropic flow, we find that it takes approximately one million years both to transport surface snow into the lake and an additional one million years to transport accreted lake water to the ocean. We examine a number of flow trajectories. Water frozen from the south side of the lake is routed to the Ross Ice Shelf where it eventually melts off the bottom of the ice shelf. Ice transiting the north side of the lake is routed towards Wilkes Land where calving returns it to the southern Pacific Ocean. Thus most of the accreted ice is delivered to the Ross Ice Shelf and some of it may remain as ice until calved off the front of the ice shelf. Assuming that the lake has been covered by a thick ice sheet for 14 million years and that accretion rates withdraw a volume equal to the lake's volume every 40,000 years (correcting an earlier published estimate with a revised lake volume), even modest circulation leads to the conclusion that most of the original water of the lake has been replaced. There remains the possibility that a connected network of subglacial water channels hinted at by satellite altimetry analysis could provide a much quicker connection from the lake to the ocean.
C43B-02 13:45h
Observational Constraints on Ice Shelf Mechanics from GLAS
The Geoscience Laser Altimeter System (GLAS) on the ICESat satellite is the first spaceborne laser altimeter to acquire data over Antarctica's ice shelves. The small (65m) footprint of GLAS enables it to sample the ice shelves with horizontal resolution heretofore unattainable with radar altimeters. This capability, combined with its high vertical accuracy, makes it a powerful tool for studying ice shelf features. We use GLAS surface elevation profiles of ice shelf rifts to constrain mechanical models of the rifting process. The GLAS profiles reveal vertical structure on either side of the rift, as well as within the rift-filling "melange." Repeat GLAS profiles provide us with information on the time dependence of these structures. We find evidence of a deformation zone that extends for several ice thicknesses on either side of the rift, and interpret this in terms of a plastic necking instability that amplifies small initial perturbations in the structure of the ice shelf. The GLAS data also reveal topographic profiles across the ice shelf fronts that are consistence with hydrostatic forces acting at the ice front.
C43B-03 14:00h
Mass budgets of the Lambert, Mellor and Fisher glaciers and basal fluxes beneath their flowbands on Amery Ice Shelf
We use RADARSAT InSAR velocity data, the RAMP image mosaic, OSU digital elevation model (DEM), GLAS laser altimeter data, AIS-DEM (Fricker and others, 2000), ANTARCTIC ATLAS-DEM (Herzfeld, 2004), BEDMAP ice thickness, a new surface accumulation data set (Giovinetto and Zwally (2000), modified by Giovinetto), and in situ measurements collected by ANARE and CHINARE to: 1) assess the mass budgets of the Lambert, Mellor and Fisher glaciers, upstream and downstream of the ANARE LGB traverse and 2) investigate melting and freezing beneath the Amery Ice Shelf (AIS) in the flowbands originating from those glaciers. The total accumulation over the grounded part of the Lambert, Mellor and Fisher glaciers is 55.0±1.6 Gt ice a-1. The total ice flux into the AIS across the southern grounding line from these glaciers is 58.8±4.7 Gt ice a-1. Thus the grounded system as a whole could be in a slight negative budget. However, there are distinct mass budget differences for the higher and lower elevation parts of the grounded ice, and the three glaciers. The whole upstream region has a positive imbalance of 6.4±3.1 Gt ice a-1, or 15.2±7.2% compared to its total accumulation, and the whole downstream region has a significant negative imbalance of -10.3±7.1 Gt ice a-1, or -21.2±14.6%. The mass budgets of the grounded Fisher, Mellor and Lambert glaciers are -1.0±0.9, 1.7±2.6, -4.5±2.9 Gt ice a-1, or -14.7±12.2 %, 7.0±10.4%, -19.4±12.6%, thus Fisher and Lambert glaciers are possibly in negative mass balance, whereas the Mellor is in slight positive mass balance. Beneath the AIS of the three flowbands, the mean melting rate is -23.2±4.6 m ice a-1 near the southern grounding line (~530 km from the ice front), which decreases rapidly downstream, and transitions to refreezing at around 250 km from the ice front. Mean freezing rates of the flowbands are around 0.5±0.2 to 1.5±0.4 m ice a-1. The total basal melting is -50.4±5.0 Gt ice a-1 (which includes part of the refrozen marine ice). The total marine ice accreted is 7.2±1.0 Gt ice a-1. The total loss of the ice from the interior of the three glaciers by basal melting under the ice shelf is 80±5%.
C43B-04 14:15h
Space-based Swath Imaging Laser Altimeter for Cryospheric Topographic and Surface Property Mapping
Uncertainties in the response of the Greenland and Antarctic polar ice sheets to global climatic change inspired the development of ICESat/GLAS as part of NASA's Earth Observing System. ICESat's primary purpose is the measurement of ice sheet surface elevation profiles with sufficient accuracy, spatial density, and temporal coverage so that elevation changes can be derived with an accuracy of <1.5 cm/year for averages of measurements over the ice sheets with areas of 100 x 100 km. The primary means to achieve this elevation change detection is spatial averaging of elevation differences at cross-overs between ascending and descending profiles in areas of low ice surface slope. Insights gained during the development of GLAS, its orbital operations, and the calibration and scientific utilization of ICESat data have contributed to an approach for a next-generation laser altimeter for global measurements. Cryospheric scientific drivers for the approach include a greater recognition of (1) the importance of documenting processes leading to ice mass change that vary on short spatial scales (e.g., snowfall, melt events and runoff, rainfall, iceberg discharge, snow drift removal), (2) elevation changes in the higher-relief margins of ice sheets near major outlet glaciers, (3) the importance of ice stream dynamics within ice sheets, (4) the role of accelerated ice cap and mountain glacier mass loss to sea level rise, and (5) energy-balance feedbacks of sea ice loss and its potential disruptive impact on climate in the high northern latitudes. We are developing a new swath imaging laser altimeter mission concept for topographic mapping from space, with a mission lifetime goal of 7-10 years, to provide precise elevation and surface character image data for global measurements of glaciers, ice sheets, and sea ice. The measurement technique produces measurements in ~10 m spots, which are contiguous along and cross track, in a swath approximately 1 km wide. The approach measures an echo pulse waveform for each spot, via modulated fiber laser transmitters and photon-counting detectors, in a highly redundant push-broom configuration operating near 1060 nm. The pulse code modulated approach is well suited for the output characteristics of fiber lasers. Calculations show the vertical elevations will be measured to < 10 cm for each spot. The swath width, in combination with precision spacecraft pointing, allows well-overlapped repeat coverage of temporally variable cryospheric features, a key to determining trends in mass balance. Measuring the range-resolved intensity and depolarization of the backscatter signal at 1060 nm, and potentially also at a frequency-doubled wavelength of 530 nm, enables additional estimates to be made about the physical state of snow, ice, water and land surfaces.
C43B-05 14:30h
Antarctic Peninsula Ice Sheet Grounding Events on the Pacific-Margin Continental Shelf During the Late Miocene/Early Pliocene Compared to two Drift-Derived Proxies on the Adjacent Continental Rise
We compared the record of Antarctic Peninsula Ice Sheet (APIS) grounding events derived from the outer-continental-shelf stratigraphy with the glacial history deduced from two drift-sedimentology proxies within coeval section on the adjacent continental rise. On the outer continental shelf, detailed seismic-stratigraphic mapping and correlations (Bart et al., in review) to glaciogenic sediments at ODP Leg 178 Site 1097 (Eyles et al., 2001) shows that there were at least 13 major cycles of APIS advance and retreat on the Pacific-margin during the late Miocene/early Pliocene. The age constraints are based on diatom biozonations from Winter and Iwai (2001). At ODP Leg 178 Site 1095 on the adjacent continental rise, the coeval section sampled from a large drift exhibits distinct meter-scale bioturbation events capping thick (up to 20 m) unbioturbated terrigenous sections that were interpreted by Pudsey (2001) to result from 36 glacial/interglacial cycles. Within the same section, Hillenbrand and Ehrmann (2001) recognize two distinct clay-mineralogy assemblages (based on ratios of smectite and chlorite) from which they deduce that there were 14 glacial/interglacial cycles. Our preliminary comparison (on a diatom biozone-to-biozone basis) of continental-shelf and continental-rise data suggests that there is no clear relationship between the number of APIS grounding events and these two sedimentologic-based proxies (bioturbation and clay mineralogy) on the adjacent continental rise.
C43B-06 14:45h
Tracing Pollutants in Northwest Alaskan Snow
Northwest Alaska has two primary storm tracks: one coming from the South, bringing the Pacific air mass, and the other from the North bringing the Arctic air mass to the area. In general, the precipitation is cleaner in the southern source than the northern one because the latter involves the Arctic air mass contaminated by industrial pollutants from Northern Russia (AMAP Report, 2002; Pollisar et al. 1998). As a result, the storm tracks may significantly affect the contaminant deposition in Northwest Alaska. In spring of 2003, a sampling team traveled inland from Kotzebue towards Kobuk, completing a 400-mile loop, collecting snow samples, and interacting with the local residents about related science and cultural experiences. Along the route, nine snow pits were dug and a series of physical observations and measurements was made including temperature, snow depth, snow density, presence of ice layers, and crystal grain classes. For each identified snow layer, samples were taken for quantitative microcopy and chemical and isotopic analyses. We examined hydrogen isotopic compositions, trace metal concentrations (including Cr, Co, As, Pb, V and Cd), and the tracks of storms from surface weather maps provided by the US Navy. The winter of 2002-2003 marked an unusually mild year for Alaska. A detailed examination of 12-hour weather maps for the snow accumulation period shows that most storms were derived from the Pacific Ocean, and these storms have variable ΔD values, ranging from -123 to -207. Only one storm, which may be carrying pollutants from Russia was from the Bering Sea. This storm occurred during the sampling trip and only four out of nine sites were sampled. Isotopically, this storm is significantly more enriched in deuterium (-100 permil) than the other storms. We conducted a discriminant analysis separating the samples into three groups -- the Bering Sea storm, the Pacific storms with relatively low ΔD values, and the Pacific storms with relative high ΔD values. These groups have different patterns in metal variations and thus significant separations are obtained along the first canonical axis. This result suggests that the contaminant loading in snow is not only affected by the moisture source of the storm, but also by the pathway of the storm tracks.