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Glaciology
 



Ice is the defining characteristic of Antarctica, indisputably. The entire continent (with a few exceptional areas such as the McMurdo Dry Valleys and some lakes and mountains) is covered by a "sheet" of ice that has been laid down over eons, if the term sheet can be used to describe a dynamic mass several thousand meters (m) thick, larger than most countries, rising over 2,000 m above sea level (peaking in an ice dome in the east nearly twice that high), and heavy enough to depress the bedrock beneath it some 600 m. Actually there are two sheets: The East Antarctic Ice Sheet is much the larger, covering the bedrock core of the continent. The smaller West Antarctic Ice Sheet overlays a group of islands and waters.

The Glaciology Program is concerned with the history and dynamics of the antarctic ice sheet; this includes research on near-surface snow and firn, floating glacier ice (ice shelves), glaciers, ice streams and continental and marine ice sheets. These species of ice facilitate studies on ice dynamics, paleoenvironments (deduced from ice cores), numerical modeling, glacial geology, and remote sensing. Some current program objectives include:

correlating antarctic climatic fluctuations (from ice core analysis) with data from arctic and lower-latitude ice cores;

integrating the ice record with the terrestrial and marine records;

investigating the physics of fast glacier flow with emphasis on processes at glacier beds;

investigating ice-shelf stability; and

identifying and quantifying the relationship between ice dynamics and climate change.

(II-163-O)
History and evolution of the West Antarctic Ice Sheet, Marie Byrd Land.
Charles F. Raymond, University of Washington.

The West Antarctic Ice Sheet (WAIS) has been an object of intense study for years, yet much remains to be specified about its evolution and dynamics - and therefore its possible futures. And, almost certainly, those potential futures are vital to the Earth's global climate and its ocean systems. Because its base consists of a series of archipelagos, the WAIS is a marine ice sheet. The Siple Coast Ice Stream system is a principal dynamic process by which the ice sheet drains ultimately into the Ross Sea. This seaward movement runs primarily through the Byrd subglacial trough, its flanks defined by the Ellsworth Mountains; such movement will usually leave behind tell-tale scars in the ice.

This project focuses on scar-like features in this region; some are well known, other margin scars are poorly constrained and need better dating, and still other as-yet unvisited scars require primary identification and exploration. To locate and map these features, we will use Advanced Very High Resolution Radiometer (AVHRR) and Radarsat image data, which will enable us to place them more exactly within the region's known topography.

Our goal for these initial data is a better description of the recent history of the Siple Coast glaciers and a more coherent account of the history of their configuration. For this, we will use low-frequency RES and high-frequency ground-penetrating radar (GPR) profiles to image internal layers and measure the depths of buried crevasses or disrupted layering. These depths, seen in the context of accumulation rates determined from shallow ice cores, will provide "shutdown" ages for when the margin features ceased actively flowing; that is, times after which they could not have formed. The field data should allow us to develop simple ice-flow models - for the margins and inter-ice stream ridges - during active shearing and after shutdown. One primary output of such models would be closer estimates than we have at present of the initial elevation of a given scar, and the corresponding ice-stream elevation, at the time of shut down. (II-163-O)

(II-168-O)
West Antarctic Ice Sheet surface melting: Recognition, controls, and significance.
Richard Alley, Pennsylvania State University, State College, Pennsylvania.

Glaciologists work to discover the history of dynamic processes, such as ice melting and climate. For example, surface melting on polar ice sheets can be said to occur when the temperature increases above some threshold. With data on these parameters, scientists try to link observed patterns to detectable changes in the macro glacial terrain, in hopes of developing models that may predict the future of antarctic ice.

This project focuses on the critical Ross Ice Shelf and Siple Dome regions of West Antarctica. There are currently in use several different procedures to measure melting:

space-based microwave sensors record the occurrence of liquid water or refrozen ice layers in the near surface;

Automatic Weather Stations (AWS) record the high temperatures that are linked to development of liquid water; and

snow-pit and ice-core studies show layers where re-freezing of sufficient liquid water has caused a visibly distinct layer to form.

Each approach is different, and they are presently not well calibrated to one another. We hope to determine how the different measures of melting may be correlated, using a combination of techniques - snow-pit, ice-core, AWS, remotely sensed data, and experiments on melt generation. By looking at a variety of records of past surface melting events in Antarctica, we hope to develop a context that will pinpoint especially high temperatures. With all of this data, we hope to develop a model for a seasonally resolved paleothermometry, based on a joint approach to measuring ice melt, as well as complementary paleothermometers such as borehole temperature, and isotopes. (II-168-O)

(II-171-O)
High precision borehole temperature measurements at Siple Dome, Antarctica, for paleoclimate reconstruction and ice dynamics studies.
Edwin D. Waddington and Gary D. Clow, University of Washington.

One of the procedures involved in ice coring is high-precision borehole temperature profiling. By constructing continuous temperature logs, scientists can develop data vital to paleoclimate reconstruction and ice dynamics studies. This project will work in the 1 kilometer (km) deep fluid-filled Siple Dome borehole and in several 160 meter-deep holes along a 20 km north-south transect across Siple Dome. The borehole temperature data will be used to:

establish the conductive heat flux across the basal interface of the ice sheet;

reconstruct the surface temperature history at Siple Dome, using geophysical inverse methods, known as borehole paleothermometry;

constrain how thick the ice sheet was during the late Wisconsin, the magnitude of the Wisconsin/Holocene deglacial warming, and the background geothermal heat flux;

determine calibration constants for the oxygen-isotope paleothermometer at Siple Dome in the past; and

establish the spatial variability of surface temperature over the last century on the 20-km scale near the main drill site.

We expect the results to provide information needed to assess the short-term stability of the West Antarctic Ice Sheet; also to improve estimates of the pore close-off ages in the past, which should in turn provide a more accurate age-scale for the Siple Dome ice core. Ultimately, this work should enhance our understanding of the magnitude of past temperature changes at this significant southern hemisphere site. (II-171-O)

(IO-157-O)
Basal conditions of Ice Stream C and related borehole studies of antarctic ice stream mechanics.
Barclay Kamb and Hermann Engelhardt, California Institute of Technology

To obtain observational evidence for the cause of rapid flow of the great ice streams in the West Antarctic Ice Sheet, we have drilled a number of boreholes through ice streams B, C, and D. With them, we measure physical conditions and can sample materials at the base of the ice, where lubrication of the rapid ice-stream motion (approximately 1 meter per day) is thought to take place.

Ice Stream C poses a special problem; though it has nearly stopped streaming, the basal materials and physical conditions are scarcely if at all different from those in the other ice streams, yet those streams continue to move rapidly. This season we will return to Ice Stream C for more intensive study, trying to find what physical conditions there might differ enough from those in Ice Streams B and D to explain why C has virutally stopped, while B and D continue to stream. In particular, we need more accurate measurements of the basal water pressure and ice overburden pressure, as well as more accurate measurements of the strength of the basal till.

Using a new borehole video camera developed at JPL, we'll be able to observe the basal zone; including the ice structure, rock debris in basal ice, the basal water-conduit system in the gap between ice and till, and, finally, the basal till itself. These observations will be interpreted in terms of basal sliding, basal melting or freeze-on, and deformation of basal ice. We will study the variation in basal conditions in the transition from unfrozen to frozen bed along a traverse from Ice Stream C across the shear margin to Ridge BC, and along a traverse from UpC to a major "sticky area" about 10 kilometers north of UpC, where the flow velocity drops from about 20 meters per year to 3 meters per year. (IO-157-O)

(IO-162-O)
Climate studies using antarctic deep ice cores and firn air sampling.
Michael Bender, Princeton University.

Scientists probe many of the atmospheric processes in the high latitudes by exhuming evidence trapped in the ice. But before snow or surface condensation can become ice, it must make the transition through firn (that is, be subjected to its first summer of relative warmth and consequent melting) and then harden into glacier ice, impermeable to liquid water. In extreme cases, the conversion can take thousands of years, and the firn can extend to 100 meters. As it continues to be overlaid with new material, its depth and contents - revealed when scientists remove sediment and ice cores - will often tell a fairly detailed story about the chemistry in the atmosphere, and in some cases even the biology, that was current when the material first hit the surface. Here, for example, the variations evident in the total gas content in the Siple Dome ice core provides a framework to reconstruct aspects of the glacial history of West Antarctica during the last glacial maximum (less than 20,000 years ago).

This project demonstrates the wide range of such techniques, extracting information from antarctic ice cores, as well as from the overlying firn layer. How gas is trapped in ice in the first place is the subject of several ongoing studies. We will collaborate with other projects at Vostok, Siple Dome, and South Pole in probing the process of firn-air chemistry, and examining the concentration history and isotopic composition of greenhouse gases, oxygen, trace biogenic gases and trace anthropogenic gases during the last 100 years.

Another important result of such work is to better establish stratigraphic time-series records - essentially a map in time. The concentration of methane and certain oxygen-isotopes permit us to fairly precisely correlate these cores to others extracted in Greenland, as well as to other climatic records previously established. From these data, variations in the concentration and inter-hemispheric gradient of methane permit us to deduce changes in both continental climates and other biogeochemical processes dependant on atmospheric methane. (IO-162-O)

(IO-164-O)
Ice dynamics, the flow law, and vertical strain at Siple Dome.
William Harrison, University of Alaska, Fairbanks.

Ice flow near a divide such as Siple Dome is unique because it is predominantly vertical. As ice is deformed vertically, the vertical strain rate component is the dominant one and must be known to calibrate dynamic models of ice flow. This 5-year project - a collaboration among the Universities of Alaska, Washington, and UC-San Diego - is measuring the vertical strain rate (as a function of depth) at two sites on Siple Dome, Antarctica. We hope the results will help us to:

develop a better analysis of the Siple Dome ice core than was possible from recent coring sites in central Greenland,

interpret the shapes of radar-revealed internal layering as indicators of the accumulation patterns and dynamic history of Siple Dome over the past 10,000 years,

interpret deep temperatures, and

address a more fundamental problem, the appropriate form of the flow law of ice at low effective stress.

During the 1997-98 field season we installed two relatively new, high-resolution systems for measuring strain rate, using holes drilled with the CalTech hot water rig. The data are being collected in subsequent seasons. One system measures strain over a gauge length of 1 meter (m) by electrical methods, and the other over a length of 200 m by optical methods. The electrical system has the advantage of high spatial resolution but remains more subject to the effects of installation transients and therefor requires several years of data. (IO-164-O)

(IO-173-O)
West Antarctic Glaciology - V.
Robert Bindschadler, National Aeronautic and Space Administration.

The West Antarctic Ice Sheet (WAIS) shows patterns of ice flow that are not fully understood. One so-called surge hypothesis has been put forth to explain the basis of these patterns; to test it, two critical questions must be answered: "Are ice streams B, D, and E currently surging? And, "What has been the buttressing effect of an enlarging Crary Ice Rise on the flow of ice stream B?"

This 3-year project addresses these questions by collecting data from the air, from space and from the surface. Many of the studies of change in West Antarctica have been based on interpolations and the use of calculations with large uncertainties. We hope to take advantage of global positioning system (GPS) data to minimize field logistic requirements and collect more accurate data. Specifically, we plan to obtain direct measures of (what we expect to be) thinning in the upper portion of ice stream D; as well as repeated satellite image measurements at the heads of ice streams B, D, and E. Should these indicators demonstrate inland migration of the onset area, we may be able to verify sustained surging and strengthen the hypothesis.

We will also take new measurements of the thickness, surface elevation, and velocity of the ice, in order to compare the current buttressing impact (of Crary Ice Rise) on ice stream B's flow with data collected during the 1950s, 1970s, and 1980s. This part of the study should yield a time series of change in the WAIS over the last half century. (IO-173-O)

(IO-175-O)
Retreat history of the West Antarctic Ice Sheet, Marie Byrd Land.
John O. Stone, University of Washington

The Earth undergoes periodic glaciation (though the exact causes and period cycle times remain scientifically debatable) - where ice advances and appears in previously non-ice environments. The full cycle brings deglaciation, when the ice retreats. This phenomena is of vital interest for the West Antarctic Ice Sheet (WAIS), since a complete meltdown would add sufficient water to the world's oceans to raise global sea levels by tens of meters; further, the effect on the atmosphere would be dramatic, with a consequent impact on the weather. This is no theoretical scenario, because it appears that the WAIS (as opposed to its neighbor the East Antarctic Ice Sheet) disappeared completely during the last deglaciation.

In this project, we will focus on reconstructing the retreat history of the West Antarctic Ice Sheet (from the last glacial maximum to now) along a flowline through the Ford Ranges in northwest Marie Byrd Land. We plan to reconstruct the ice surface- and elevation-histories of the region, using cosmogenic-isotope exposure dating of moraine boulders and ice-abraded bedrock surfaces in the Clark [present ice surface: 1,200 meters (m)], Allegheny (800 m) and Sarnoff (200-400 m) Mountains.

By taking altitude transects at each of these three sites, we will date the thinning of the WAIS. This deglaciation chronology for Marie Byrd Land should help to resolve competing models of ice-sheet retreat (among which the more prominent are the surge theory and the disintegration theory); the basic data set will also become available for testing numerical models of the WAIS throughout the glacial cycle. Further, such indicators of how much ice was where and when will help constrain scientists' models that require the past ice load in West Antarctica; these computations can help to predict the effect of glacio-isostatic motion on geodetic surveys being undertaken in the region. (IO-175-O)

(IO-190-O)
Iceberg drift in the near-shelf environment, Ross Ice Shelf, Antarctica.
Douglas MacAyeal, University of Chicago.

Icebergs command a lot of attention. The Titanic disaster at sea illustrates only one important reason. Such a massive piece of glaciology on the move is a process that scientists would like to have better models for. One theoretical benefit entails harnessing the extraordinary freshwater volume of large tabular icebergs - possibly even harvesting it - as a natural resource of potential economic value, especially for water-poor regions of the earth. And though feasibility studies of towing icebergs to such areas in need have largely been dismissed as science fiction, it is tantalizing to realize that tabular icebergs commonly travel thousands of miles as a result of natural processes. Might a better understanding of the behavior and dynamics of icebergs one day lead to such a boon of human economic and social value?

The recent calving of an extraordinarily large iceberg (dubbed B-15) from the Ross Ice Shelf presents a unique opportunity to measure the processes - such as wind-driven and thermohaline currents, tides, sea ice, and winds - that control the drift of large tabular icebergs. Such an event rarely occurs within the logistical reach of the U.S. Antarctic Program, and provides us with the opportunity to study iceberg drift, as well as other aspects of iceberg behavior that are associated with the long-term stability of the antarctic environment.

In this project, we plan to make direct measurements of the drift of icebergs B-15a, B-15b and a smaller iceberg (either B-16, B-17 or B-18, depending on circumstances) to o constrain parameters that will improve the models of iceberg drift, by determining drag coefficients appropriate to atmospheric and oceanic interactions, including drag induced by sea ice;

improve our ability to predict calving events and the subsequent iceberg drift trajectory;

complement ongoing remote sensing studies of the iceberg and its behavior; and

measure the progress of the iceberg and its progeny toward logistically sensitive areas.

It is now in two pieces and has caused smaller bergs to calve from the Ross Ice Shelf, a situation that could complicate normal shipping to and from McMurdo Station on Ross Island, the main U.S. research and logistics station in Antarctica. (IO-190-O)

(IO-192-O)
Collecting micriometeorites form the South Pole water well.
Susan Taylor, U.S. Army Cold Regions Research and Engineering Laboratory.

Ever since Mawson discovered the first meteorite in Antarctica in 1912, scientists have harvested a rich vein of information about life in space from what is probably the best area on Earth to collect specimens of extra-terrestrial origin. This project follows up on the discovery (in 1995) of thousands of micrometeorites that were collected from the bottom of the South Pole water well. Using these samples, we were able to determine a precise flux and mass distribution for four centuries (1100-1500 A.D.) of cosmic spherules (melted micrometeorites) in the 50-700 millimeter (mm) range.

This austral summer we will collect new samples to follow up on a number of implications from that previous cache. First, we hope to verify that the polar plateau itself preserves the original surface flux of micrometeorites, and to quantify both melted and unmelted micrometeorites for the same period. Next, we will try to determine the flux for smaller pieces in the 1-50 mm range, commonly classified as interplanetary dust particles (IDP's). These data may also permit us to assess whether IDPs derive from different cosmic sources, or rather comprise a subclass of micrometeorites. We will revisit the well to recover as many micrometeorites from the well as possible. With the cumulated data, we will look for quantifiable variations (if any are to be found) in the flux or compositional distribution of micrometeorites on 10- to 100-year scales for that Middle Ages time frame.

In addition, we will also collect diatoms, opal phytoliths, and terrestrial mineral grains from the well. Such collections will provide additional indicators for researchers who are trying to determine the sources of terrestrial particles landing on the antarctic plateau. Johnson Space Center/NASA will curate the samples and make them readily available to researchers in earth and planetary sciences. (IO-192-O)

(IO-196-L)
AMS radiocarbon chronology of glacier fluctuations in the South Shetland Islands during the last glacial/interglacial hemicycle: Implications for the role of Antarctica in global climate change.
Brenda Hall, University of Maine.

The Antarctic Ice Sheet is an integral component of the world's atmospheric and hydrologic systems. Paleoclimatologists have found a roughly predictable pattern to Earth's glacial cycles and see that over the next several millenia we are heading for another cooling. These two scientific ideas converge on the question, What would significant changes in antarctic ice auger for the rest of the planet, and how might such future changes be modeled? The search for the answer begins in the paleoclimatic record in the high latitudes.

This project joins the effort by gathering data to produce a new reconstruction of ice extent, elevation and thickness at the Last Glacial Maximum (LGM) for the South Shetland Islands in the Antarctic Peninsula. (This general area anchors an ice sheet, has active geothermal springs, and is the site of an extinct volcanic arc; clearly plate motion is occurring here.) The Drake Passage is an area of significant maritime and general scientific interest; our data should contribute to studies of ocean circulation and ice dynamics in this area. It will also contribute to the production of a deglacial chronology that will afford important clues about the mechanisms controlling ice retreat in this region of the Southern Hemisphere.

We plan to map the areal extent and geomorphology of glacial drift and to determine the elevation and distribution of trimlines on Livingston Island; also, by mapping the elevation of erosional features and the position of erratic boulders, we expect to determine the direction of the ice flow. One of our main goals is to discover if sufficient organic material exists in the South Shetland Islands to radiocarbon dating and, if so, to determine the age of the deposits by measuring the carbon-14 age of plant, algal and fungal remains that we find preserved at the base of the deposits, as well as those that have been incorporated into marine shells, seal skin, and other organic material we may find in raised beach deposits. Sea-level histories are also valuable to the overall paleoclimate studies, and we will search two or three key areas to show whether construction of such curves for the South Shetland Islands is possible. (IO-196-L)

(IO-196-M)
Deglacial chronology of the northern Scott Coast from relative sea-level curves.
Brenda Hall, University of Maine.

This project dovetails with the previous one (IO-196-L). A key unresolved question in antarctic glaciology concerns the stability of the marine-based West Antarctic Ice Sheet (WAIS). Marine-based means that (unlike the base of the East Antarctic Ice Sheet sitting on a lithospheric plate) the substratum for the WAIS is a series of archipelagoes, such that the sheet at its relatively fixed position is grounded on the continental shelf - in the northwestern Ross Sea Embayment off the northern Scott Coast - with plate boundaries nearby. As deglaciation began after the Last Glacial maximum (LGM), the WAIS eventually became unmoored. Scientists believe this was likely the first area of the shelf to become free of grounded ice. Learning how and when (and in what sequence) this has occurred in the past is a critical step for isolating the mechanisms (sea level, climate, ocean temperature, and internal dynamics) that control WAIS dynamics.

Thus the northern Scott Coast is of particular interest to researchers looking for mechanisms that may have triggered the key stages of deglaciation. But an important first step is to better constrain the age where the inquiry is focused. The Barbados coral record suggests the initial retreat from the Ross Sea Embayment may have begun as early as 17,000 years ago. In contrast, recent glacial geologic mapping and relative sea-level work from the southern Scott Coast suggests that deglaciation here is more recent, during the Holocene (between the present and 11,000 years ago), with southward grounding-line migration past Ross Island shortly before 6,500 carbon-14 years ago. This chronology suggests that rising sea level could not have driven grounding-line retreat to the Siple Coast, because deglacial sea-level rise essentially would already have occurred by mid-Holocene.

To begin to resolve this conflict, one deficiency in the southern Scott Coast work might be corrected. Those data cannot differentiate among the possible triggering mechanisms because they come from 450 kilometers south of the LGM grounding-line position. We will try to overcome this by constructing relative sea-level curves on a transect along the northern Scott Coast. We hope to get the ages for this work from accelerator mass spectrometer carbon-14 dates of seal skins and shells within raised beaches. These curves should tell us when the grounded ice from the northwestern Ross Sea Embayment cut loose. (IO-196-M)

 
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