Antarctica Ice Sheet Research Articles

Tracking Earth's attraction

Very small changes in the Earth's gravity field—which might result from shifts in deep ocean currents, melting of ice sheets in Antarctica, or accumulation of seasonal snow pack in the mountains—could be detected and mapped using newly developed instruments on future satellite missions. A new report from the National Research Council recommends the satellite missions, which would detect gravity changes that are 100 to 100,000 times smaller than those measured previously, to map the Earth's gravity field and provide new data for a variety of scientific fields.

Here today, gone tomorrow

Shorelines are ephemeral boundaries that might serve as a forest floor or a seafloor thousands of years from now. What will happen to present shorelines is determined thousands of miles away, by the behavior of continental ice sheets in Antarctica and Greenland. To investigate the timing and magnitude of past sealevel changes and their relationship to the waxing and waning of ice sheets over the past 25 million years, scientists working for the Ocean Drilling Program are beginning to collect sediment and rock samples below the ocean floor off the coast of New Jersey, near the edge of the continental shelf. Scientists from seven countries will extract more than two miles of cores from an immense pile of sediments that accumulated on the ocean floor over millions of years. By analyzing the sedimentary layers, researchers will document the precise timing and scale of sea level changes, which are hypothesized to result from the advance and retreat of continentsized ice sheets that stored and released water during climate cycles.

Antarctica Ice Sheet Curvature and its relation with ice flow and boundary conditions

The map of the Antarctica ice sheet surface curvature in the across‐slope direction shows a very coherent pattern, related to ice flow anomalies of each flowline with respect to adjacent ones. Near the coast, these anomalies are correlated to bedrock features which suggests that they result from outlet flow conditions that are transmitted from coast to domes. As a consequence, they should represent the drainage pattern, glacier positions and 3‐dimensional flowline directions. Thus a large part of the ice sheet flow pattern seems controlled by outflow boundary conditions which can be empirically estimated.

Rapid Sea-Level Rise Soon from West Antarctic Ice-Sheet Collapse?

There has been recent concern over whether shrinkage of ice sheets in Antarctica caused by climate change could lead to rapid global sea-level rise. In his Perspective, Bentley discusses the processes at work in ice-sheet behavior and concludes that, in view of the evidence for ice-sheet stability, catastrophic changes are unlikely in the next century or two.

Dynamics of the Katabatic Wind Confluence Zone near Siple Coast, West Antarctica

Abstract The surface wind pattern over the ice sheets of Antarctica is irregular with marked areas of airflow confluence near the coastal margins. Where cold air from a large interior area of the ice sheet converges (a confluence zone), an anomalously large supply of air is available to feed the coastal katabatic winds, which, as a result, are intensified and more persistent. The confluence zone inland of Siple Coast, West Antarctica, differs from its East Antarctic counterparts in that the terrain slopes become gentler rather than steeper as the coast is approached. In addition, synoptic processes exert substantially more impact on the behavior of the surface winds. A month-long field program to study the dynamics of the springtime katabatic wind confluence zone has been carried out near Siple Coast. Two sites, Upstream B (83.5°S, 136.1°W) and South Camp (84.5°S, 134.3°W), were established roughly perpendicular to the downslope direction. The field program involved the use of the ground-based remote sens...

The high sensitivity of the palynological record in the Vico maar lacustrine sequence (Latium, Italy) highlights the climatic gradient through Europe for the last 90 ka

The palynological study of the Vico maar lacustrine sequence (Italy) is realized from the correlation of three drillholes. The time span covers a period from the end of Oxygen Isotope Substage 5b till the beginning of Stage 1 (from ca. 90 to ca. 10 ka BP). At the base of the pollen diagram (pollen zone 0), a Younger Dryas-like fluctuation is observed. After pollen zone 1, which is a well developed forest period (Substage 5a), the forests of pollen zones 3 and 5 correspond to extensive climate improvements. These last two zones are correlated to the Ognon complex of the Grande Pile (France) and to interstadial periods 20 and 19 defined in the GRIP ice core (Greenland) (72.6−66.2 ka) before the Stage 5/4 transition. These two interstadial periods are global events found in the Greenland and Antarctica ice sheets, as well as in North Atlantic marine cores. In other pollen records from north of the Alps, changes have been recorded but they are less pronounced than in Vico. Pollen zone 7 (Stage 3) shows ca. 7 warm/humid fluctuations, that might have a link to the Dansgaard-Oeschger events (Greenland). During zone 9, the Lateglacial interstadial is recorded as well as the Younger Dryas event. The localization of Vico near plant refugia and far from the ice cap enables its pollen diagram to record abrupt and intense response to climate changes, possibly through a link to the Atlantic Ocean via westerly winds.

Arctic and Antarctic precipitation simulations produced by the NCAR community climate models

Precipitation predictions from globai-climate models (GCMs) for the ice-covered Arctic Ocean and the ice sheets of Antarctica are among the most important aspects of the inferred response of the polar areas to climate change. It is generally recognized that the atmospheric hydrologic cycle, which includes precipitation as a key part, is one of the components of the climate system that GCMs do not handle particularly well. The present-day atmospheric-moisture budget poleward of 70° latitude in both hemispheres, as represented by two versions of the NCAR (U.S. National Center for Atmospheric Research) community climate model (CCM1 and CCM2), is compared with observational analyses. The quantities examined on the seasonal and annual timescales are precipitation, evaporation/sublimation and atmospheric poleward moisture transport. The results are discussed in terms of the physiographic and climatic characteristics of both polar regions and how the particular models handle moisture transport: CCM1 uses the positive-moisture fixer and CCM2 the semi- Lagrangian transport. A particularly important test both for models and for observations is the degree to which the independently determined moisture-budget quantities actually balance. Deficiencies of both observations and models are discussed.

Arctic and Antarctic precipitation simulations produced by the NCAR community climate models

Precipitation predictions from globai-climate models (GCMs) for the ice-covered Arctic Ocean and the ice sheets of Antarctica are among the most important aspects of the inferred response of the polar areas to climate change. It is generally recognized that the atmospheric hydrologic cycle, which includes precipitation as a key part, is one of the components of the climate system that GCMs do not handle particularly well.The present-day atmospheric-moisture budget poleward of 70° latitude in both hemispheres, as represented by two versions of the NCAR (U.S. National Center for Atmospheric Research) community climate model (CCM1 and CCM2), is compared with observational analyses. The quantities examined on the seasonal and annual timescales are precipitation, evaporation/sublimation and atmospheric poleward moisture transport. The results are discussed in terms of the physiographic and climatic characteristics of both polar regions and how the particular models handle moisture transport: CCM1 uses the positive-moisture fixer and CCM2 the semi- Lagrangian transport. A particularly important test both for models and for observations is the degree to which the independently determined moisture-budget quantities actually balance. Deficiencies of both observations and models are discussed.

Global‐mean temperature and sea level consequences of greenhouse gas concentration stabilization

The Intergovernmental Panel on Climate Change (IPCC) has defined a set of scenarios for future CO2 concentrations stabilizing at levels of 350 to 750 ppmv. Using models previously employed by IPCC, the implied global‐mean temperature and sea level changes are calculated out to 2500. While uncertainties are large, the results show that even with concerted efforts to stabilize concentrations of greenhouse gases, substantial temperature and sea level increases can be expected to occur over the next century. Increases in sea level are likely to continue for many centuries after concentration stabilization because of the extremely long time scales associated with the deep ocean (which influences thermal expansion) and with the large ice sheets of Greenland and Antarctica.

Monitoring continental ice sheets by satellite altimetry

Altimeter data are helpful in monitoring the evolution of polar caps as well as constraining their dynamics. In the recent years, our group has quantified the impact of the so-called volume echo on the determination of the surface height, analyzed the effect of surface roughness on the intensity of the radar signal, adapted an inverse technique for mapping the topography of the ice sheets and its error, including correction of the surface slope effect and finally used the Antarctica ice sheet topography deduced from Seasat data to estimate the rheological parameters of the ice flow.

Interpretation of short-term ice-sheet elevation changes inferred from satellite altimetry

Satellite altimetry offers means of directly measuring changes in surface elevation over the polar ice sheets of Greenland and Antarctica. By relating these changes to variations in ice mass, it becomes possible to detect short-term changes in the Earth's ice sheets. However, it is not immediately obvious that short-term changes in surface elevation are indicative of any (long-term) trend in ice mass. An increase in ice thickness may very well reflect the response of the glacier to random fluctuations in precipitation. The spectrum of this response is dominated by low frequencies, with the majority of the variance contained in the longer time scales. As a result, the ice-thickness record may exhibit trends that have no climatic significance, but are due to a low-frequency response to random forcing. A simple model for the interpretation of observed elevation changes is developed and applied to measurements made over the Greenland Ice Sheet. It appears to be unlikely that the difference between the rate of thickening derived by Zwally and others (1989) using repeat satellite altimetry, and significantly smaller previous estimates, can be explained as being the response of the ice sheet to random climatic forcing or that this difference can be attributed to a recent increase in accumulation rate.

Changes in the longitudinal profiles of glaciers during advance and retreat

AbstractThe pattern of elevation change along the length of glaciers caused by retreat from Neoglacial maximum to present is investigated and described in terms of a profile-shape factorfdefined as the ratio of the average thickness change to the local thickness change at the present terminus. The factorfis relevant to estimation of the change in volume of a glacier in relation to its change in length and the response time to a change in climate. Thickness-change profiles on a selection of 15 mountain valley glaciers givefin the range 0.1–0.4 with an average of 0.28. The elevation changes are in all cases largest near the terminus and decrease headward more rapidly than a linear variation with distance that would correspond tofof 0.50, a value assumed to be characteristic in the past. The smaller values offindicate reduced estimates of changes in volume versus change in lengths and smaller response times for glaciers. Total volume change for glaciers and ice caps (excluding ice sheets in Greenland and Antarctica) as estimated roughly by these means, is consistent with previous estimates from mass-balance measurements and supports the view that melting of these ice masses has made a significant contribution to sea-level change over the last century. The observed values offdo not explain an apparent discrepancy between alternative theoretical methods for estimating the response time of glaciers based on length and terminus velocity or based on the thickness and terminus-ablation rate. The discrepancy may be caused in part by the relationship between terminus velocity and ablation rate associated with strongly non-steady-state conditions during advance or retreat. The patterns of elevation change measured for short time intervals of a few decades or less correspond to a wide spread of profile-shape factorffrom less than 0.1 to more than 1.2 indicative of transients in the short time-scale response.

Changes in the longitudinal profiles of glaciers during advance and retreat

AbstractThe pattern of elevation change along the length of glaciers caused by retreat from Neoglacial maximum to present is investigated and described in terms of a profile-shape factor f defined as the ratio of the average thickness change to the local thickness change at the present terminus. The factor f is relevant to estimation of the change in volume of a glacier in relation to its change in length and the response time to a change in climate. Thickness-change profiles on a selection of 15 mountain valley glaciers give f in the range 0.1–0.4 with an average of 0.28. The elevation changes are in all cases largest near the terminus and decrease headward more rapidly than a linear variation with distance that would correspond to f of 0.50, a value assumed to be characteristic in the past. The smaller values of f indicate reduced estimates of changes in volume versus change in lengths and smaller response times for glaciers. Total volume change for glaciers and ice caps (excluding ice sheets in Greenland and Antarctica) as estimated roughly by these means, is consistent with previous estimates from mass-balance measurements and supports the view that melting of these ice masses has made a significant contribution to sea-level change over the last century. The observed values of f do not explain an apparent discrepancy between alternative theoretical methods for estimating the response time of glaciers based on length and terminus velocity or based on the thickness and terminus-ablation rate. The discrepancy may be caused in part by the relationship between terminus velocity and ablation rate associated with strongly non-steady-state conditions during advance or retreat. The patterns of elevation change measured for short time intervals of a few decades or less correspond to a wide spread of profile-shape factor f from less than 0.1 to more than 1.2 indicative of transients in the short time-scale response.

Radar studies at the mouths of ice streams D and E, Antarctica

Ice thickness measurements have been carried out at the mouths of ice streams D and E, West Antarctica using a surface-based impulse radar. These studies have been undertaken as a part of the continuing effort to understand the state of the West Antarctica ice sheet and its response to climate change. Thickness measurements will be used in the mass balance calculation currently in progress and to better understand features in the surface topography seen at low angle sun illumination in the satellite imagery. Results show that the discharge areas of ice streams D and E are thickening by approximately 1 m per year, and thus that these ice streams are probably loosing mass. Aperiodic wavelike features in the surface topography are described which pose interesting questions about migration of the grounding line and ice-stream dynamics.

Radar studies at the mouths of ice streams D and E, Antarctica

Ice thickness measurements have been carried out at the mouths of ice streams D and E, West Antarctica using a surface-based impulse radar. These studies have been undertaken as a part of the continuing effort to understand the state of the West Antarctica ice sheet and its response to climate change. Thickness measurements will be used in the mass balance calculation currently in progress and to better understand features in the surface topography seen at low angle sun illumination in the satellite imagery. Results show that the discharge areas of ice streams D and E are thickening by approximately 1 m per year, and thus that these ice streams are probably loosing mass. Aperiodic wavelike features in the surface topography are described which pose interesting questions about migration of the grounding line and ice-stream dynamics.

Quaternary and Tertiary microfossils from beneath Ice Stream B: Evidence for a dynamic West Antarctic Ice Sheet history

Some glaciologists have suggested that the West Antarctic Ice Sheet, which is grounded well below sea level, may be susceptible to rapid grounding-line retreat and disintegration. However, until now, geologic evidence of previous ice sheet “collapses” has been lacking. Sediments that have recently been collected from beneath the West Antarctic Ice Sheet at Ice Stream B contain direct evidence of ice-free conditions in the West Antarctic interior during certain Cenozoic intervals, both prior to and subsequent to the development of grounded ice sheets in West Antarctica. The sediments contain rare but diverse microfossils that represent a wide variety of biostratigraphic ages and depositional environments. Microfossils present include relatively common marine and non-marine diatoms and sponge spicules, plus rare foraminifera, nannofossils, radiolarians, silicoflagellates, chrysophyte cysts and palynomorphs. Clasts of Neogene freshwater diatomite demonstrate the former presence of large lake systems in West Antarctica, possibly as part the Cenozoic West Antarctic rift system. Age-diagnostic marine fossils in the sediment include Late Paleogene calcareous nannofossils and planktonic foraminifera, Miocene marine planktonic diatoms and, significantly, late Pleistocene marine diatoms. Relatively common late Miocene diatoms probably reflect marine deposition prior to initiation of a dominantly glacial phase in West Antarctica. It is likely that Pliocene and early Pleistocene diatoms were deposited in the West Antarctic interior during certain warm interglacials, but these have been eroded and transported toward the continental shelf edge during repeated ice sheet expansions. The late Pleistocene diatoms from Upstream B were deposited in the West Antarctic interior basins during a marine phase, subsequent to an ice sheet collapse, during at least one late Pleistocene interglacial. This discovery provides an indication of the complex history of the West Antarctic Ice Sheet, and calls into question some previous interpretations of ice sheet stability based on paleoceanographic proxy data. The results of this study may lend credence to the concern that global warming and rising sea levels in a “greenhouse” earth could lead to collapse of this marine ice sheet during the current interglacial period.

Quaternary and Tertiary microfossils from beneath Ice Stream B: Evidence for a dynamic West Antarctic Ice Sheet history

Some glaciologists have suggested that the West Antarctic Ice Sheet, which is grounded well below sea level, may be susceptible to rapid grounding-line retreat and disintegration. However, until now, geologic evidence of previous ice sheet “collapses” has been lacking. Sediments that have recently been collected from beneath the West Antarctic Ice Sheet at Ice Stream B contain direct evidence of ice-free conditions in the West Antarctic interior during certain Cenozoic intervals, both prior to and subsequent to the development of grounded ice sheets in West Antarctica. The sediments contain rare but diverse microfossils that represent a wide variety of biostratigraphic ages and depositional environments. Microfossils present include relatively common marine and non-marine diatoms and sponge spicules, plus rare foraminifera, nannofossils, radiolarians, silicoflagellates, chrysophyte cysts and palynomorphs. Clasts of Neogene freshwater diatomite demonstrate the former presence of large lake systems in West Antarctica, possibly as part the Cenozoic West Antarctica rift system. Age-diagnostic marine fossils in the sediment include Late Paleogene calcareous nannofossils and planktonic foraminifera, Miocene marine planktonic diatoms and, significantly, late Pleistocene marine diatoms. Relatively common late Miocene diatoms probably reflect marine deposition prior to initiation of a dominantly glacial phase in West Antarctica. It is likely that Pliocene and early Pleistocene diatoms were deposited in the West Antarctic interior during certain warm interglacials, but these have been eroded and transported toward the continental shelf edge during repeated ice sheet expansions. The late Pleistocene diatoms from Upstream B were deposited in the West Antarctic interior basins during a marine phase, subsequent to an ice sheet collapse, during at least one late Pleistocene interglacial. This discovery provides an indication of the complex history of the West Antarctic Ice Sheet, and calls into question some previous interpretations of ice sheet stability based on paleoceanographic proxy data. The results of this study may lend credence to the concern that global warming and rising sea levels in a “greenhouse” earth could lead to collapse of this marine ice sheet during the current interglacial period.

Geological evidence for changes in the East Antarctica ice sheet (60°–120°E) during the last glaciation

AbstractThe East Antarctica ice sheet advanced onto the continental shelf during the last glaciation but appears to have been thinner (<300 m) than previously hypothesised and probably did not everywhere extend to the edge of the continental shelf. Where the shelf is wide, the ice probably terminated against shallow banks on its outer edge. There may not have been time for the sheet to develop the maximum profile form and thickness predicted by Hollin (1962) and Hughes and others (1981) of about 1000–500 moverthe shelf. Large outlet glaciers occupied deep troughs that conveyed most of the ice towards the edge of the shelf: intervening areas were less intensely glaciated. Much of Prince Charles Mountains and Amery Oasis were not ice-covered: Vestfold, Bunger and Casey oases were glaciated. Vestfold and Bunger oases became ice-free after 10 ka BP under the influence of the Holocene marine transgression, which was complete by about 6 ka BP. During at least the last 5–6 ka these oases have been approximately their present size. Since then the margins of the Antarctic continental ice sheet have maintained almost steady state conditions against the landward edges of the hill masses.

Miocene isotope reference section, Deep Sea Drilling Project Site 608: An evaluation of isotope and biostratigraphic resolution

We developed an isotope (87Sr/86Sr, δ18O) reference section for the uppermost Oligocene to lower upper Miocene (ca. 25–8 Ma) at Site 608 in the northeastern North Atlantic. This site contains the least ambiguous magnetostratigraphic record of Miocene polarity changes available, providing direct correlations to the Geomagnetic Polarity Time Scale (GPTS). We integrate biostratigraphic, magnetostratigraphic, Sr isotope, and stable isotope data to provide a reference section for Miocene isotope fluctuations. The direct correlation of isotopes and biostratigraphy to the Geomagnetic Polarity Time Scale (GPTS) provides relatively precise age estimates. We use these age estimates to evaluate the timing of first and last occurrences of planktonic foraminifera, and conclude that many of these are synchronous within a 0.5 m.y. resolution between subtropical Site 563 (33°N) and high‐latitude Site 608 (43°N). In addition, we use this chronology to estimate the ages of previously established Miocene oxygen isotope Zones Mi 1 through Mi 7 and to compare the Sr isotope record at Site 608 with previously published87Sr/86Sr records. We approximate latest Oligocene to early late Miocene (25–8 Ma) Sr isotope changes with two linear regressions. The rate of increase of87Sr/86Sr was high from the latest Oligocene (∼25 Ma) to earliest middle Miocene (∼15 Ma), with an estimated rate of 0.000059/m.y. Our ability to reproduce Sr isotope measurements is ±0.000030 or better, yielding a stratigraphic resolution of as good as ±0.5 m.y. for this interval. The rate of change was much lower from about 15 to 8 Ma (on average, 0.000013/m.y.), yielding Sr isotope stratigraphic resolution of worse than ±2.3 m.y. The causes of the late Eocene to Miocene87Sr/86Sr increases are not known. We speculate that a moderate87Sr/86Sr increase (0.000030/m.y) which occurred during the late Eocene‐latest Oligocene can be explained by intermittent glaciations and deglaciations of the Antarctic continent. These pulse‐like changes in the input of glacial weathering products yield what appears to be a monotonic, linear increase. The increase in the frequency of glaciations during the latest Oligocene‐early Miocene can explain the higher rate of change of87Sr/86Sr at this time. We speculate that by the middle Miocene, the development of a permanent east Antarctica ice sheet resulted in decreased input of glacial weathering products and a lower rate of87Sr/86Sr change.Appendix 1 is available with entire article on microfiche.Order from American Geophysical Union, 2000 FloridaAvenue N.W., Washington, D.C. 20009. Document P90‐001;$2.50. Payment must accompany order.

Design, Development, Field Observations, and Preliminary Results of the Coherent Antarctic Radar Depth Sounder (CARDS) of the University of Kansas, U.S.A.

AbstractA modern coherent Antarctic radar depth sounder for probing the ice sheets of Antarctica and Greenland has been designed and developed by the University of Kansas. It was successfully tested during the austral summers of 1987 and 1988 at Downstream Β and Upstream B, Antarctica. Ground-based measurements were made with the radar in a mobile hut hauled by a Sno-cat in 1987 and in a Spryte vehicle in 1988.The coherent Antarctic radar depth sounder (CARDS) is an unfocussed synthetic-aperture chirp radar where the along-track resolution is improved by extensive coherent integration. Surface acoustic wave (SAW) devices are used to implement pulse expansion and compression. A common stable oscillator for the transmitter and the receiver establishes coherency. The system signal-to-noise ratio is enhanced by pulse compression and coherent integration. Antennas for the ground-based measurements are configured with an array of eight dipole elements, four active and four passive, the latter acting as reflectors. The aircraft antennas also consist of four active elements hung underneath the two wings. The wings serve as reflectors. A PC facilitates system control and data recording on a high-density recorder. Α-scope plots of selected records allow frequent field checks on system performance. More descriptive display facilities have been incorporated in the latest version of the system.The radar transmits 60 ns, 20 W peak power at 150 MHz. The number of coherent integrations is selectable up to 64 k. The system is capable of 5 m range resolution and 5 km range in ice. A programmable sensitivity time control (STC) increases the receiver dynamic range. System parameters such as pulse-repetition frequency, number of integrations, and display modes can be chosen during field operations by user-friendly software.This paper describes the design and field operations of the system. Some results of the 1987 operations at Downstream Β are presented.