ES 331/767 Lecture 9
GLACIAL ISOSTASY AND EUSTASY

James S. Aber

Table of Contents
Introduction Depression and rebound
Glacioeustasy Complicated responses
Related sites References

Introduction

Glacial isostasy is the process of lithospheric depression beneath the weight of an ice sheet and subsequent rebound when the ice mass is reduced or removed. Glacial eustasy, on the other hand, refers to worldwide changes in sea level as a consequence of changing volume of glacier ice on land. Both glacial isostasy and eustasy, thus, are related to the volume—thickness and areal coverage—of ice sheets, but the relationship is neither simple nor fully understood at present.

Lithospheric depression and rebound

Lithospheric depression of 100s of m takes place beneath large ice sheets due to the static weight of the ice mass. This excess loading causes elastic and plastic deformation in the lithosphere and underlying asthenosphere. Crustal rock is displaced as the mantle sinks. Given ice density of 0.9 g/cm³ and mantle rock density of about 3.3 g/cm³, the potential depression beneath an ice sheet 1000 m thick could be as much as 275 m. This maximum depression rarely occurs, however, because of the time lag between glacier loading and crustal response. Several 1000 years are required for complete isostatic adjustment to take place, by which time ice thickness has often changed.

In order to compensate for lithospheric depression beneath a crustal load, the surrounding area may rise creating a forebulge. These principles are demonstrated by the Amazon delta, where huge sediment loading has created a central depression that is surrounded by peripheral uplifts—see Fig. 9-1.

In the cases of ice sheets, the amount of forebulge uplift varies from negligible to a few 100 m at most—see Fig. 9-2. The presence of a forebulge has important implications for melt-water drainage. A drainage moat may be created between the ice margin and forebulge, in which proglacial lakes, seas, or ice-marginal streams are confined.

As an ice sheet begins to shrink, rebound of the depressed region then takes place during deglaciation and for several 1000 years thereafter—see Fig. 9-3. The rate of rebound is initially quite rapid and slows progressively. The magnitude of postglacial rebound is nicely documented in Sweden and eastern Canada—see Figs. 9-4 to 9-6. Five lines of evidence support the idea that recent uplift of formerly glaciated regions is the result of isostatic rebound following deglaciation (based on Flint 1971).

The geological evidence for postglacial rebound is mainly based on tilted or uplifted shoreline features. Shorelines of ice-marginal lakes or seas are assumed to have formed horizontally. Where such features are now tilted, this reveals crustal warping associated with rebound—see Fig. 9-7. Uplifted marine deposits are particularly important for establishing the amount of postglacial rebound of coastal sites since deglaciation—see Figs. 9-8 and 9-9. In like manner, the glacial forebulge subsides when the ice mass is reduced or removed. For example, the Atlantic Ocean is encroaching on the Chesapeake Bay region at a rate of about 30 cm per century (Colin 1996). Similar subsidence is taking place in southern portions of the Baltic and North Seas in Europe.

Beach ridges on coast of Novaya Zemlya, arctic Russia. Such ridges are formed by pushing of sea ice; older ridges have been uplifted inland (to left) as a result of Holocene glacio-isostatic rebound. Photo © by J.J. Zeeberg; used here by permission.
Skansen bog, island of Askøy, western Norway. Inge Aarseth (left) and assistant display coring tool used to recover samples of sediment within the bog. During late glacial time, this site was depressed below sea level and the basin accumulated marine sediment. With postglacial rebound, the site was uplifted, and lake and bog sediments have filled in. The age of lowest non-marine sediment in the core dates the time the basin rose above sea level.

Glacioeustasy

The ice volume of current glaciers is equivalent on melting to an increase in sea level of about 65 m (Flint 1971). During full glacial conditions of the Pleistocene, sea level was conversely about 120 m lower than at present. Thus the total range of eustatic sea level between glacial and nonglacial conditions is on the order of 180-200 m. This figure does not take into account any adjustments for crustal depression/rebound or long-term tectonic movements of continents and ocean basins.

The margins of many continents slope very gently into the sea forming wide, shallow continental shelves. The Atlantic and Gulf coasts of the United States are good examples—see Fig. 9-10. In such situations, shoreline positions migrated over 10s to 100s of km as sea level rose and fell during glacial cycles. These sea-level cycles could be preserved as intervals of marine deposits (high sea level) separated by surfaces of erosion (low sea level). A complete sedimentary record of past sea-level changes would ideally be developed in stable coastal areas far removed from glaciation.

Much of the sedimentary record for Quaternary sea level is, unfortunately, not directly accessible; it is underwater on the continental shelves—see Fig. 9-11. In some places older deposits have been uplifted tectonically, but the magnitude of uplift cannot be determined from independent evidence. As a result of these problems, a global long-term record of Quaternary sea level is not yet firmly established.

Massive coral (Pavona clavus) exposed in 1954 by tectonic uplift in the Galapagos Islands. Coral reefs of this kind form in shallow tropical seas just beneath low-tide level. They are a record of past sea level. Image obtained from the World Data Center-A for Paleoclimatology, © Educational Slide Project.

An excellent Late Quaternary record is preserved in South Florida (Enos and Perkins 1977). Six episodes of high sea level (interglaciation) are marked by coral reefs and other marine deposits.

  1. Recent (Holocene) sea level, last few 1000 years.
  2. Sangamon sea level, peak about 130,000 years ago.
  3. High sea level with peak about 180,000 years ago.
  4. High sea level with peak about 236,000 years ago.
  5. High sea level with peak about 324,000 years ago.
  6. Earliest preserved high sea level, age uncertain.

Section in Key Largo Limestone exposed in canal across Key Largo, Florida. A large branching coral is preserved in growth postion. Key Largo is a fossil reef that grew when sea level was 5-8 m higher than today, about 130,000 years ago. Section is about 5 m high.
Miami Oolite, the limestone formation that underlies much of Miami, Florida. The limestone was deposited on a shallow marine bank, much like the modern Bahama Bank. Conspicuous cross-bedding is visible at base of exposure. The Miami Oolite is a northern continuation of the Key Largo Limestone trend; they were deposited simultaneouly during high sea level about 130,000 years BP.

The highest sea level achieved during any of these cycles was by the Sangamon (Eemian) sea. Sea level stood 5-8 m higher than at present; all of southern Florida was submerged, and the shoreline was located approximately 250 km (150 miles) farther north across the Florida peninsula. In many other parts of the world, the Sangamon Sea is marked by marine deposits or erosional beach terraces a few m above present sea level. In the subarctic Pacific, sea-surface temperature reached ~2 °C higher than modern 128 to 126 thousand years ago (Max et al. 2014), which confirms the warm climate and high sea level noted around the world during the Eemian interglaciation.

Wembury Bay, near Plymouth southwestern England. Bedrock platform in foreground is a strandflat a few m above present sea level. It was eroded by wave action during the Eemian high stand of sea level about 130,000 years ago.

In the Bahamas, sea level stood about 2 m above present during the time interval 132-118 thousand years ago during oxygen isotope stage 5—more in
lecture 11. Near the end of this period, sea level rose rapidly to +6 m; this highest stand of sea level was very brief, lasting perhaps only a few centuries before declining rapidly—see Fig. 9-12. The mechanism for such a large and rapid change in sea level is thought to be related to possible glacial surging into the ocean.

Another excellent record of Late Quaternary sea-level fluctuations has been documented from uplifted reefs and deltaic sediments in New Guinea—see Fig. 9-13. Again a high sea level, about 4 m above present, took place around 120,000 years ago. Globally high Sangamon sea level can be explained only by melting of a major ice sheet, either in West Antarctica or Greenland. Minimum sea level, nearly 130 m below present, occurred 18,000 years ago, during the maximum late Wisconsin glaciation.

Until recently, the Sangamon high stand of sea level was considered to be the highest of the Pleistocene. However, new evidence suggests that sea level was even higher during oxygen-isotope stage 11, about 420-400 thousand years ago (Hearty et al. 1999; Poore and Dowsett 2001). Sea level may have exceeded 20 m above present in the Caribbean Sea, Bahamas and Bermuda, which implies that both the Greenland and West Antarctic ice sheets were absent or greatly reduced.

Complicated responses

Changes in relative sea level can become quite complicated in coastal regions adjacent to ice sheets. The effects of both crustal depression/rebound and eustatic sea-level changes are involved. The coastal area of western British Columbia shows the possibilities. Most of the shelf of Queen Charlotte Sound was ice covered and depressed during maximum glaciation, >15,000 years BP. When northern Vancouver Island was deglaciated by 13,000 years BP, the shelf and coastal area was submerged under the Pacific—see Fig. 9-14. Marine shorelines were locally as much as 200 m higher than today.

Crustal rebound then took place; the coastal region was uplifted, and parts of the present shelf were eventually exposed as dry land—see Fig. 9-15. Soil development and forest growth took place about 10,500 years BP. Meanwhile fjords to the east were still depressed well below sea level. Between 10,500 and 9000 years ago, most remaining glaciers on the mainland melted and eustatic sea level rose. The results were rebound of the fjord-head region (emergence) and drowning of the shelf area (submergence).

This region of northwestern Denmark was initially emergent land during late Pleistocene deglaciation, then was flooded by shallow sea in the Holocene, and since has emerged as a low-lying coastal environment. Hurup coast along Aalborg Bay, Kattegat Sea. Kite aerial photograph © S.W. and J.S. Aber (lower left corner of scene).

The Missisippi Delta exhibits another complicated situation for isostasy and eustasy. During times of major glaciation, when sea level was low, the Mississippi River incised a deep valley across the delta and transported sediment into the sea beyond the delta. During interglacial times, in contrast, high sea level allowed sediment accumulation and infilling of the valley. These substantial changes in sediment mass induced crustal subsidence and rebound exceeding 9 m (30 feet) along the Mississippi valley (Blum et al. 2008). Lesser depression and rebound extended >100 km to the east and west along the coast.

Mississippi River and Delta region, southern Louisiana and Mississippi, March 2001, prior to major hurricanes of 2005 and 2008. Note distinctive patterns of suspended sediment in coastal waters. MODIS image from the Visible Earth.

The timing of rebound coincided with release of melt water from continental ice sheets to the north. At present, high sea level combined with sediment accumulation and compaction lead to crustal depression and delta sinking. This example demonstrates the importance of glacially related processes for understanding delta behavior worldwide.

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ES 331/767 © J.S. Aber (2014).