ES 331/767 Exercise I
GLACIER ICE VOLUME AND
SEA-LEVEL EQUIVALENT

James S. Aber

Table of Contents

Last great ice sheets Arctic glaciation
Timing of glaciation Ice volume & sea level
Exercise References

Note: see ice_labs.pdf for figures.

Last great ice sheets

The last major glaciation took place during Late Wisconsin/Vistulian time, roughly 25,000 to 10,000 years ago. Deposits and landforms related to this glaciation are widely preserved on land and on the sea floor. Based on more than a century of field observations, it is now possible to reconstruct the northern hemisphere ice sheets. Reconstructions generally fall in two categories--minimum and maximum--according to overall ice coverage and thickness--see Figs. I-1 and I-2.

The minimum reconstruction includes large ice sheets in North America, Greenland, and Europe that were confined mainly to existing land areas and marginal continental shelves. Smaller, thinner ice sheets/caps were located in several other areas: Iceland, Alps, Svalbard, Rockies, Andes, etc. The Laurentide Ice Sheet in North America consisted of two domes, one over southeastern Hudson Bay, the other over Keewatin. Ice thickness in the dome areas exceeded 4 km.

The minimum Scandinavian Ice Sheet had a single dome located over the northern Baltic with a thickness exceeding 3 km. It may have joined with a smaller ice sheet over the British Isles. Crustal depression beneath the Scandinavian Ice Sheet reached 800 m, whereas depression under the Laurentide Ice Sheet exceeded 1 km. The total (global) volume of glacier ice in the minimum reconstruction is estimated to be equivalent to about 130 m of sea-level decline (Hughes et al. 1981).

Ice sheets in the maximum reconstruction were thicker and extended far out onto the continental shelves. This is most noticeable in Arctic Canada and on the Barents Sea and Kara Sea shelves of northern Europe. The Laurentide Ice Sheet consisted of a single dome over Hudson Bay that was nearly 5 km thick, and crustal depression exceeded 1.2 km. The Scandinavian Ice Sheet was only slightly thicker compared with the minimum reconstruction, however, with as much as 1 km of crustal depression. The total volume of glacier ice in the maximum reconstruction is equivalent to around 160 m of sea-level decline.

Glaciation of the Arctic region

The minimum and maximum reconstructions differ most in the circum-Arctic continental shelf areas. As with many scientific controversies, the true nature of the last great ice sheets is probably somewhere between these two extremes. The minimum reconstruction is supported mainly by geomorphic evidence for multiple-dome, ice-sheet glaciation on land. The maximum model is, conversely, favored by the record of postglacial crustal rebound around individual centers of glaciation. Better reconstruction of the Laurentide and Scandinavian Ice Sheets will depend to a large extent on increased knowledge of glaciation from the continental shelf areas.

In Scandinavia, it now appears that an extensive, marine-based ice sheet was present in the Barents Sea region. Widespread glacial deposits, ice-push deformation, and submarine ridges (moraines) demonstrate continuous ice-sheet coverage from Norway to Svalbard during the last glaciation--see Fig. I-3. The distribution of Holocene crustal rebound indicates that the ice sheet was probably thickest on the shelf between Svalbard and Franz Josef Land--see Fig. I-4.

On the other hand, both maximum and minimum reconstructions have been proposed for the North Sea region. Based on latest evidence from sea-floor sediments, it now appears that an ice-free region existed on the North Sea shelf between Norway and Scotland after 22,000 years BP--see Fig. I-5. During the previous interval of 29,000 to 22,000 years BP, the North Sea experienced its maximum Vistulian glaciation, as the British and Scandinavian ice sheets coalesced (Sejrup et al. 1994). In North America, a minimum reconstruction for the Laurentide Ice Sheet seems to be favored at present, although debate continues. The two-dome model for structure of the ice sheet is certainly accepted by many, but not all, on the basis of geomorphic zonation of glacial landforms and deposits (see Fig. 8-7 from lectures).

Most controversy concerns the extent of the last glaciation in the Arctic region--see Fig. I-6, where geomorphic evidence for glaciation is scarce, but crustal rebound is substantial. The lack of glacial geomorphic features could be a result of cold-based ice sheets, which had minimal geomorphic influence. Other regions of uncertain ice-sheet extents during the last glaciation include the High Plains of Alberta and Montana, the offshore southern New England continental shelf, and the possible connection between Greenland and Laurentide ice sheets. The existence of floating ice shelves in the Labrador Sea and Norwegian Sea is also a strongly debated possibility.

Timing of glaciation

Another problem in reconstructing the volume of ice sheets is the question of timing. It is generally thought that glaciation reached its greatest global extent during the late Wisconsin, about 18,000 to 22,000 years ago. However, it now appears that individual ice masses had quite different histories of expansion and contraction. The timing of North Sea glaciation was noted above. Another case is glaciation of the Kara Sea and Pechora Sea shelves as well as northern Russia--see Fig. I-7. Maximum expansion of the ice sheet in this region is recognized during the middle and early Vistulian, prior to 35,000 years ago (Mangerud et al. 1999; Svendsen et al. 1999; Polyak et al. 2000). The Kara Sea ice sheet experienced major advances 90,000 and 60,000 years ago according to latest information from northern Russia (Mangerud, pers. comm. 2001).

By late Vistulian time, the Kara Sea ice sheet was much reduced, whereas the Barents and Fennoscandian ice sheets reached their maximum sizes. Why the Kara Sea ice sheet should be out of phase with the Barents and Fennoscandian ice sheets is a mystery. Thus the maximum geographic expansion of ice sheets did not take place simultaneously in all parts of the world. Temporal uncertainty about ice sheet expansions makes estimates of global ice volumes and sea-level effects difficult.

Surprising [scientific] conclusions are the most interesting.
Jan Mangerud (2001).

Ice volume and sea-level equivalent

During the Quaternary, global sea level rose and fell primarily in response to the volume of water stored on land in ice sheets and glaciers. Many attempts have been made to measure the size of past ice sheets and corresponding changes in eustatic sea level. The numerical results depend on assumptions about the areas and thicknesses (volumes) of ice sheets. These estimates are complicated by isostatic depression and rebound both beneath the ice sheets and in the ocean basins. Further uncertainties are caused by asynchronous growth and decay of ice sheets in different parts of the world and by a lack of good dating control for different phases of glaciation.

In light of these uncertainties, maximum and minimum models have been developed for ice-sheet volume and sea-level changes--see Table I-1. The surface area of the modern ocean is approximately 361 million km². In order to convert glacier ice volume into sea-level equivalent, two steps are necessary.

  1. Convert ice volume into water-equivalent volume: multiply ice volume (km³) times its density, 0.9.

  2. Water equivalent (km³) of step 1 is next divided by area of ocean (km²) to yield equivalent sea-level height (km).

Note: this method does not take into account any possible crustal depression or rebound either beneath ice sheets or on the sea floor. Furthermore, it does not include correction for any change in the area of the ocean due to higher or lower sea level. Nonetheless, reasonable approximations can be determined for sea-level changes resulting from glaciation.

Table I-1. Glacier ice volumes for modern and late Wisconsin glaciations (values in million km³).
Glacier/Ice Sheet Modern Min. Wiscon. Max. Wiscon.
Laurentide 0.4 30.9 34.8
Greenland 2.6 2.9 5.6
Scandinavian 0.2 7.3 7.5
Barents-Kara 0.1 1.0 6.8
East Antarctic 20.0 24.2 24.2
West Antarctic 7.0 13.5 13.5
All others 2.6 4.4 5.4
Totals 32.9 84.2 97.8

Based on Hughes et al. (1981).

Glacial sea-level exercise

  1. Calculate the rise (rounded to nearest m) of sea level that would occur if all modern glaciers and ice sheets melted.

  2. Calculate the rise (in m) of sea level for melting of each of the following modern ice sheets: Greenland, West Antarctic, and East Antarctic. Melting of which ice sheet would best explain the Sangamon (Eemian) high sea level?

  3. Determine the fall (in m) of sea level for both minimum and maximum reconstructions of the Late Wisconsin glaciation. What is the difference in sea-level equivalent for the minimum and maximum Late Wisconsin reconstructions?

  4. What is the total possible range in sea-level change between maximum glaciation and non-glacial conditions?

References

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ES 331/767 © by J.S. Aber (2006).