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Author(s): Joerg M. Schaefer (corresponding author) [1, 2]; Robert C. Finkel [1, 3]; Greg Balco [4]; Richard B. Alley [5]; Marc W. Caffee [6]; Jason P. Briner [7]; Nicolas E. [...]

A major meltwater body exists beneath a tributary of Bindschadler Ice Stream, West Antarctica, in a region where subglacial lakes have not been mapped but near where rapid vertical motion of the ice sheet surface has suggested shifting of a subglacial water body. The water is trapped by a local reversal in ice-air surface slope arising from ice flow over variable basal topography and from the positive feedback of basal lubrication from the trapped water. Strong variations in the water content of the sediments upglacier of the water body arise from a similar process. These results are revealed by a novel application of the amplitude variation with offset (AVO) seismic technique. The existence of such water bodies and of the strong spatial variation in subglacial sediment properties is not captured in current models of subglacial hydrology, lubrication of ice stream motion, and sediment transport. Keywords: subglacial meltwater, West Antarctica, ice streams, seismic amplitude.

Future sea-level rise is an important issue related to the continuing buildup of atmospheric greenhouse gas concentrations. The Greenland and Antarctic ice sheets, with the potential to raise sea level ~70 meters if completely melted, dominate uncertainties in projected sea-level change. Freshwater fluxes from these ice sheets also may affect oceanic circulation, contributing to climate change. Observational and modeling advances have reduced many uncertainties related to ice-sheet behavior, but recently detected, rapid ice-marginal changes contributing to sea-level rise may indicate greater ice-sheet sensitivity to warming than previously considered., Because a heavy concentration of the population lives along coastlines, even small amounts of sea-level rise would have substantial societal and economic impacts through coastal erosion, increased susceptibility to storm [...]

During the last glacial-interglacial transition, atmospheric [N.sub.2]O values increased front ~190 to 265 parts per billion (ppb) (3-5). Model studies suggest that the lifetime of [N.sub.2]O in the atmosphere has [...], Paleoatmospheric records of trace-gas concentrations recovered from ice cores provide important sources of information on many biogeochemical cycles involving carbon, nitrogen, and oxygen. Here, we present a 106,000-year record of atmospheric nitrous oxide ([N.sub.2])O along with corresponding isotopic records spanning the last 30,000 years, which together suggest minimal changes in the ratio of marine to terrestrial [N.sub.2.]O production. During the last glacial termination, both marine and oceanic [N.sub.2.]O emissions increased by 40 ± 8%. We speculate that our records do not support those hypotheses that invoke enhanced export production to explain low carbon dioxide values during glacial periods.

Carbon dioxide (C[O.sub.2]), nitrous oxide ([N.sub.2]O), and methane (C[H.sub.4]) concentrations of ice samples from a South American mountain glacier are larger than those in coeval polar ice cores. Of the six possible mechanisms that may cause the composition of air measured in ice cores to depart from that of the overlying troposphere (gravitational fractionation, thermal fractionation, gases trapped in refrozen meltwater, atmospheric heterogeneity, abiotic chemical reactions, and biologically mediated chemical reactions), only metabolism of a consortium of microorganisms within the ice can explain our data. Keywords: geomicrobiology, evolution, ice cores, ecology.

The flux of glacially derived bedload and the proportions of the suspended and bedload components carried by proglacial streams are highly debated. Published data indicate a large range--from 75%--in the bedload percentage of the total load. Two 'vents,' where supercooled subglacial meltwater and sediment are discharged, were sampled over the course of an entire melt season in order to quantify the flux of glacially delivered bedload at the Matanuska Glacier, Alaska. The bedload component contributed by these vents, for the one melt season monitored, is negligible. Furthermore, the bedload fluxes appear to be strongly supply limited, as shown by the poorly correlated discharge, bedload-flux magnitude, and grain-size caliber. Thus, in this case, any attempt to employ a predictive quantitative expression for coarse-sediment production based on discharge alone would be inaccurate. A nonglaciated basin proximal to the Matanuska Glacier terminus yielded higher bedload sediment fluxes and larger clast sizes than delivered by the two monitored vents. Such nonglaciated basins should not be overlooked as potentially major sources of coarse bedload that is reworked and incorporated into valley outwash. Keywords: bedload, sediment yield, sediment transport, glacial erosion, Matanuska Glacier, Alaska.

Researchers have developed a new technique for identifying rapid past temperature changes based on the principle of thermal diffusion. In thermal diffusion, gas mixtures are fractionated in a temperature gradient according to their mass. This approach has been used to investigate the timing of sudden climate change at the end of the Younger Dryas interval. It was established that the rise in atmospheric methane concentrations at the end of the Younger Dryas interval started between 0 and 30 years after a sudden temperature rise.

Analysis of borehole temperature and Greenland Ice Sheet Project II ice-core isotopic composition reveals that the warming from average glacial conditions to the Holocene in central Greenland was large, approximately 15°C. This is at least three times the coincident temperature change in the tropics and mid-latitudes. The coldest periods of the last glacial were probably 21°C colder than at present over the Greenland ice sheet., The Greenland Ice Sheet Project II (GISP2) deep ice core has yielded a remarkable history of the oxygen isotopic composition of central Greenland snowfall [delta.sup.18]O of ice[1]] that extends through [...]

Author(s): Gavin A. Schmidt (corresponding author) [1]; Jeff Severinghaus [2]; Ayako Abe-Ouchi [3, 4]; Richard B. Alley [5]; Wallace Broecker [6]; Ed Brook [7]; David Etheridge [8]; Kenji Kawamura [9, [...]

A major West Antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter. Acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less. Slip propagates at approximately 88 meters per second, suggestive of a shear wave traveling within the subglacial till. A model of an episodically slipping friction-locked fault reproduces the observed quasi-periodic event timing, demonstrating an ice stream's ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level., Concern that the West Antarctic Ice Sheet did, does, and will contribute to increasing sea level has driven two decades of extensive research on ice-flow dynamics (1, 2). Evidence for [...]

Ice-core records show that climate changes in the past have been large, rapid, and synchronous over broad areas extending into low latitudes, with less variability over historical times. These ice-core records come from high mountain glaciers and the polar regions, including small ice caps and the large ice sheets of Greenland and Antarctica.

Large ice sheets actively interact with the rest of the climate system by amplifying, pacing, and potentially driving global climate change over several time scales. Direct and indirect influences of ice sheets on climate cause changes in ocean surface temperatures, ocean circulation, continental water balance, vegetation, and land-surface albedo, which in turn cause additional feedbacks in the climate system and help to synchronize global climate change. The effect of the underlying geological substrate on ice-sheet dynamics may be the missing link in understanding the ice sheet--climate interactions that are integral to the middle Pleistocene transition; the 100,000-year climate cycle; high-amplitude, millennial-scale climate variability; and low--aspect ratio ice sheets of the Last Glacial Maximum., Following a long-term global cooling trend through much of the Cenozoic, large ice sheets first developed in the Northern Hemisphere ~2.54 million years ago (Ma) (Fig. 1) (1, 2). Statistical [...]

Humans are changing the amount of carbon dioxide in the atmosphere by burning coal, oil, and gas. The current atmospheric C[O.sub.2] concentration is higher than it has been for at [...]

The big ice sheets in Greenland and Antarctica are key elements of the global climate system. By storing large volumes of water as ice or ice-contact lakes and sometimes releasing [...]

Most of the last 100,000 years or longer has been characterized by large, abrupt, regional-to-global climate changes. Agriculture and industry have developed during anomalously stable climatic conditions. New, high-resolution analyses of sediment cores using multiproxy and physically based transfer functions allow increasingly confident interpretation of these past changes as having been caused by 'band jumps' between modes of operation of the climate system. Recurrence of such band jumps is possible and might be affected by human activities.

Some of the large climate changes in the North Atlantic could have been caused by the advance and retreat of the Laurentide, one of the great ice sheets feeding the region. New research indicates that the ice-rafted-debris signal of the youngest Heinrich event, which took place around 17,000 years ago, is more consistent with a complex and varied reaction to a surge in this ice sheet than with a common reaction of ice masses to an external forcing. This research reopens the debate about the meaning of ice-rafted-debris deposits and of the north-south phasing of deglaciation.

Author(s): Richard B. Alley [1] [illus. 1] Lacking an actual time machine for studying volcanoes, drifting continents, dinosaurs and other denizens of the past, Earth scientists have to read the [...]

Fabrics in Polar Ice Sheets: Development and Prediction THE VISCOSITY OF AN ICE CRYSTAL IS more than an order of magnitude less parallel to its basal plane than perpendicular to [...]

Glaciers flow under gravitationally induced stresses. The weight of the ice causes the glacier to spread and thin in a manner dictated by surface conditions, basal conditions, and the ice [...]