ES 331/767 Lecture 2 MODERN GLACIERS AND ICE SHEETS

James S. Aber

Table of Contents

Glacier ice	Glacier budget
Glacier movement	Columbia Ice Field
Greenland Ice Sheet	Antarctic Ice Sheets
Ice-sheet meteorites	Polar science online

World Data Center for Glaciology.

Satellite Image Atlas of Glaciers.

Landsat 7 glacier inventory.

	Ice on Lake Kahola, east-central Kansas. The swirling, frothy appearance was created by freezing of foam (bubbles) into the ice. Photo date 1/96; © by J.S. Aber.
	Broken shards of ice piled on shore of Melvern Reservoir, east-central Kansas. Lake ice froze during an extended cold period--temperature <10°F, and then was broken up by wind and wave action. Photo date 1/97; © by J.S. Aber.
	Closeup view of ice shards piled on shore of Melvern Reservoir. Fragments are 3-4 inches (10 cm) thick. Individual ice crystals are oriented in a vertical position within shards, which indicates the ice formed by freezing from the lake surface downward. Photo date 1/97; © by J.S. Aber.

First, ice behaves as a plastic material under relatively low pressure. This accounts for internal deformation and flowage that takes place in all glaciers. The hardness of ice varies with temperature. At 0°C, ice has a hardness of 1.5 on the Mohs scale; at -70°C the hardness is 6 (Nesje and Dahl 2000). However, temperature below -30°C is rarely achieved in glaciers. Next ice is subject to pressure melting In other words, ice can be melted at temperatures below 0°C by application of pressure--see Fig. 2-1. Because of this and because water has a higher density than ice, water is able to exist under high pressure beneath many glaciers. Finally ice and water have relatively high heat capacities, and phase changes between ice and water involve large heat transfers. Thus the nature of a glacier depends in large part on its thermal regime.

Glacier ice is formed by a process of snow metamorphism, which may be accompanied by seasonal melting and refreezing in some glaciers. The transition from new-fallen snow to glacier ice takes place over a period of years as the snow becomes buried and gradually recrystallized. The process is evident in three main stages:

1. New snow: Density varies from 0.05 to 0.4 g/cm³, and porosity is generally high. Distinctly layered for individual storms. Highly variable in texture and crystal form--see Fig. 2-3.

2. Firn: "Old snow" that has survived at least one summer melt season. Density between 0.4 and 0.8 g/cm³ with porosity around 50%. Granular texture with annual layering clearly visible.

   View of firn exposed on the margin of accumulation zone in Vatnajökull, Iceland. Notice banding and bright white color. This firn section represents several years of snow accumumlation. Photo date 8/94; © by J.S. Aber.

3. Glacier ice: Solid, impermeable ice with density >0.8 g/cm³ and little porosity. Isolated air bubbles under high pressure; large crystals up to 30 cm long aligned in preferred fabric parallel to flow. Glacier ice often displays a distinctive aquamarine (cyan) color--see ice spectrum and Fig. 2-2.

   View inside glacier cave on Mt. Rainier, Washington. Typical blue-green (cyan) color of dense glacier ice is visible. Photo © PANA-VUE (S1979).

Glacier budget

Glacier movement

Columbia Ice Field

Greenland Ice Sheet

	SeaWiFS satellite image over the whole of Greenland. Date 07/15/2000; obtained from NASA's Visible Earth.
	Space-shuttle photograph of southern Greenland. Low-oblique view toward north, color-visible, 70-mm format, 12/88. All land areas are snow covered in this mid-winter scene. The interior ice sheet is smooth, and coastal mountains appear rugged. Deep glacier-carved fjord valleys cut through the coastal mountain ranges. NASA Johnson Space Center, Imagery Services, STS027-32-17.
	Southwestern margin of the Greenland Ice Sheet. View toward the northeast. Ice sheet and outlet glaciers visible in upper right portion of scene; coastal mountains and fjords toward lower left. Photo taken from a commercial airplane at ~11,000 m altitude. Image © J.S. Aber, Sept. 2005.

The Greenland Ice Sheet is scientifically important because it is the only remaining ice sheet in the northern hemisphere, and it is located at the approximate center of ice-sheet development during the Pleistocene. Greenland is relatively accessible, because of the permanent human population located mainly in the southwestern coastal region. The Greenland Ice Sheet represents a bridge to understanding other northern hemisphere ice sheets of the Pleistocene.

Research camp of the Free University of Amsterdam (Netherlands) on the Greenland ice sheet. The surface is hard frozen snow and ice, but percolation melt water collects about ½ m below the surface. Foot pressure is sometimes enough to break through the hard surface, and people suddenly find themselves knee-deep in slush. Photo © by J.J. Zeeberg; used here by permission.

A good way to visualize the present state of the ice sheet is to examine the glaciation limit. This represents the lowest elevation at which firn can exist on the ice sheet. This limit is generally lowest in the northwest and northeast (200-400 m) and highest at the southern tip (1600-1800 m). All parts of the ice sheet above the glaciation limit are in the accumulation area (névé). Within the névé, snow exists in various facies depending on its water/ice content--see Fig. 2-7.

Dry-snow facies covers the high, central and northern portions of the ice sheet. Percolation facies covers most of the southern portion and lower northern parts; wet-snow facies is found only in a narrow marginal zone. A narrow marginal zone of the ice sheet is subject to loss, mainly by melting on land and by calving of icebergs where ice reaches the sea in large fjords.

	Overview of margin of Greenland ice sheet. Note braided streams of melt water flowing over the broad valley. Thick outwash sediment of sand and gravel accumulates on the valley plain or sandur. Photo © by J.J. Zeeberg; used here by permission.
	Outwash valley plain almost completely covered by sediment-laden melt water. Photo © by J.J. Zeeberg; used here by permission.
	Outlet glacier on Greenland's west coast. This glacier and many others break off as icebergs, a process called glacier calving. Photograph by Preben Jensen, Denmark; reproduced here by permission.
	View of massive iceberg floating in coastal waters of western Greenland. Only about 10% of the iceberg's volume is visible, the rest is below sea level. Photograph by Preben Jensen, Denmark; reproduced here by permission.

The ice sheet occupies a shallow basin that is depressed below sea level and surrounded by mountains--see Fig. 2-8. In effect the ice sheet creates its own topography; it is the highest large plateau in the northern hemisphere. The coldest portion of the ice sheet is the high central zone (below -30°C), with warmer temperatures deeper and toward the margins, where pressure-melting conditions exist at the base of the ice sheet.

Several attempts have been made to estimate the ice-sheet budget. These attempts were difficult in the past because of the great size of the ice sheet and limited information concerning accumulation, melting, and calving of icebergs. Remote sensing observations have improved our ability to gauge changes in the ice sheet. Altimeter measurements from the ERS-1 and ERS-2 satellites documented an overall increase in elevation of the ice surface during the period 1992-2003 (Johannessen et al. 2005). Throughout the vast interior of the ice sheet, surface elevations increased at an average rate of ~6½ cm/year. The narrow marginal zone suffered a decrease at ~2 cm/year. Over the whole of the ice sheet this represents an average increase of ~5½ cm/year, or 60 cm total for the 11-year period of observation. When corrected for crustal depression, the total increase is estimated to be 54 cm. In other words, the Greenland Ice Sheet has gained more than half a meter of surface accumulation overall since the early 1990s, which suggests a strongly positive budget.

Ice movement is generally downward and outward from two major domes in the east-central and southern portions. Movement within most of the accumulation zone is slow, only a few m/year, with movement increasing to about 80 m/year near the firn line. The two domes are separated by a lower saddle from which several fleuves de glâce flow westward. These concentrated, high-velocity ice streams follow subglacial valleys leading westward to Disko and Umanak Bays, where a large volume of icebergs is discharged. Near the calving margin, ice-flow velocities are 2-10 km/year, and much melt water emerges from beneath the ice.

	Aerial view of a modern nunatak surrounded by the Greenland ice sheet. Note crevasses and pressure ridges in ice around the nunatak. Ice flow from upper right toward lower left of view is indicated by the medial moraine extending away from the nunatak. Photo © by J.J. Zeeberg; used here by permission.
	Overview of margin of Greenland ice sheet; outlet glaciers extend into valleys toward the left. The longest of these is the Russell Glacier. Photo © by J.J. Zeeberg; used here by permission.
	Russell Glacier. Note the lateral moraine (far side of valley) and push moraine (glacier snout). The push moraine is composed of sand and gravel shoved up from the sandur plain. Research camp of the University of Amsterdam and Utrecht University (Netherlands) is visible in the lower right part of view. Photo © by J.J. Zeeberg; used here by permission.

The Greenland Ice Sheet was considerably larger during times of maximum Pleistocene glaciation. It averaged about 500 m thicker and extended to near the edge of the continental shelf. In spite of its greater thickness, the ice sheet did not cover all of the marginal mountains; many nunataks remained in which some plant species may have locally survived glaciation.

On the continental shelf, sea level was some 100-150 m lower, so some banks were exposed as dry land at the edge of the ice sheet. The ice sheet may have been confluent with the Ellesmere Ice Cap to the northwest, although this is a debatable interpretation. During maximum glaciation, the coastal areas were depressed as a result of ice loading on the crust; these areas have now rebounded by 50-200 m.

It is generally thought that the Greenland Ice Sheet formed in the late Pliocene or early Pleistocene by coalescence of ice caps and glaciers. New evidence from offshore marine deposits suggests that Greenland had a partial ice cover as far back as seven million years ago (late Miocene). The marine deposits contain large stones transported by drifting icebergs derived from Greenland glaciers.

Antarctic Ice Sheets

Landsat image mosaic of Antarctica--LIMA.

Marine-based ice sheets are inherently unstable. Such an ice sheet is held in check by back stress from the adjacent ice shelf by shear stress at basal pinning points and along lateral margins--see Fig. 2-10. A slight rise in relative sea level (crustal subsidence), decrease in ice flow, or thinning of the ice shelf would reduce the back stress and could lead to rapid collapse of the ice sheet (Alley 1989). Antarctic ice shelves are sources of enormous tabular icebergs.

The West Antarctic Ice Sheet is drained by a series of ice streams that feed into the Ross Ice Shelf--see Fig. 2-11. These ice streams flow through a region of relatively thin ice (about 1000 m), but they maintain flow rates of around 500 m per year, and they accelerate downstream. They are characterized by highly crevassed surfaces in contrast to the adjacent smooth surface of slow-moving ice. The individual ice streams appear to have complex and different histories of movement. For example, ice stream C seems to have stopped moving around 100 years ago. Most of the rapid motion of ice streams is accomplished by basal sliding over water-lubricated beds or by deformation within a thin, water-saturated sediment layer below the ice (Alley 1989).

The Ross Ice Shelf has fluctuated considerably during the past 45,000 years--see Fig. 2-12. Between 45,000 and 27,000 years ago, the ice edge was quite similar to now, then expanded reaching a maximum position ~13,000 years ago. During the Holocene, the ice margin retreated considerably reaching a minimum position in the middle Holocene between 4000 and 2000 years ago.

Antarctica was glaciated much earlier than was Greenland. A semipermanent ice sheet was established by the end of the Eocene or earliest Oligocene, about 34 million years ago (Ivany et al. 2006). Glaciation in other high-latitude southern lands also started early. Tasmania supported local glaciers in the early Oligocene, around 36 million years ago, when it was a mountainous peninsula of Australia at about 55-56°S latitude (Macphail et al. 1993). The Antarctic ice sheet experienced a major expansion during the early Pliocene, 5-4 million years ago (Bart 2001). This expansion took place in spite of relatively high sea level and warm temperature globally, and the ice volume may have been significantly larger than present ice volume.

Ice-sheet meteorites and cosmic dust

Many meteorites have been found in Greenland, especially in the Thule area. The same is true for Antarctica, where literally thousands of meteorites have been collected from the surface in "blue ice" areas. In fact, the number of meteorites found during the past few years in Antarctica is about 25% of all meteorites ever found worldwide. It is estimated that more than 750,000 meteorites lie buried within the ice sheet (Lucchitta et al. 1987). Ice flow locally concentrates meteorites at the surface in areas of strong wind ablation, where ice is locally trapped behind mountains--see Fig. 2-13.

Large nickel-iron meteorite, named Agpalilik, recovered from the edge of the Greenland ice sheet, near Thule. The meteorite is about 4 feet (1.2 m) long, and was one of several larger and smaller fragments found in the vicinity. This specimen on display at the Geological Museum, Copenhagen, Denmark. Photo date 6/79; © J.S. Aber.

The Greenland Ice Sheet has yielded a rich collection of cosmic dust (particles <1 mm in size). About 85% of the cosmic dust particles are carbonaceous chondrites, thought to be samples of the primitive solar system. Cosmic dust falls constantly on the ice sheet and is carried by ice flow to the margin. In some places, ice melts in local basins and the particles settle to the bottom of "blue ice lakes," where they accumulate year after year (Episodes 9, 1986).

• Scott Polar Research Institute, University of Cambridge, United Kingdom.
  
• Byrd Polar Research Center, Ohio State University, United States.
  
• United States in Antarctica--a thorough review of Antarctic environment, history of exploration, Antarctic Treaty; current U.S. activities, significance and recommendations.
  
• Coastal-change and glaciological maps of Antarctica. U.S. Geological Survey, Fact Sheet 050-98.

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Glossary or references.

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