Eustasy is defined as the rise and fall of sea level. From the standpoint of glaciology, the implications of the changing of relative sea level are global in scope. This page examines some of the causes of change in terms of glaciers.

Eustasy could be broken down into transgression and regression. Transgression is the increase in sea level due to more precipitation returning to the ocean (which indicates glacial ablation or interglaciation). Regression is the decrease in sea level due to less precipitation returning to the ocean (indicates glacial advance).

Sea level represents ultimate base level, so the implications for eustatic adjustments in terms of rivers and coastal processes are global in scope. Unfortunately, much of the evidence of past sea levels is not preserved; it either has been eroded away or is currently submerged.

It is important to recognize two points in terms of glacial eustasy. First, global sea level is a response to isostatic loads placed on the lithosphere by the overlying ice sheets, which can depress the surface enough to induce localized flooding in coastal areas. Second, glaciers are interruptions in the hydrologic cycle, and water that is locked up in glacial ice cannot return to the oceans. Thus, when water is removed from the ocean basin, the oceanic crust rebounds isostatically, thereby reducing the amount of local change.

Although there are many ways to change sea level, both locally and globally, there are four primary methods associated with glaciation and deglaciation. These include the following:

     1) Glacioeustasy: During the time of glacier build-up, moisture is removed from the atmosphere in the form of snow and locked up in glacial ice. The moisture in the atmosphere is replaced by evaporation from the oceans, thereby reducing global sea level. Water is removed by glacial expansion, and is returned by glacial melt.

     2) Glacioeustasy: The load placed on the crust by the ice causes it to sink into the underlying mantle, depressing the land surface relative to sea level. The addition of more mass causes the crust to sink deeper into the mantle, thus depressing the surface more and causing a peripheral depression around the ice sheet, resulting in the flooding of coastal sites. Glacioeustasy is a function of ice sheet thickness; the amount of depression is approximately 0.3 times the ice thickness (this is variable of course with lithology, structure, pre-existing topography, etc.). Glacioeustatic change can also be estimated using the simple equation E=I-R, where (E) is equal to the eustatic curve, (I) equals the amount of isostatic rebound, and (R) is equal to the relative emergence or submergence.

     3) Hydroisostasy: Changes in sea level cause isostatic adjustments on the crust. In other words, a drop in sea level will cause the oceanic crust to rebound as water is stored in the ice sheets, thereby reducing the amount of localized change.

     4) Geoidal Eustasy: A build-up of glacial ice caused variations in the earth's gravitational field, which in turn affects the shape of the geoid. There is a measurable gravitational attraction of sea water to a large ice mass, causing a localized rise in sea level exponentially closer to the ice.

     Other factors eustatic adjustments include steric sea level change. When ocean water warms, sea level rises. This could happen during times of glacial ablation, changes in oceanic or atmospheric circulation patterns, and other ways.

• E = I - R where (E) = eustatic curve, (I) = Isostatic Rebound, and (R) = Relative emergence or submergence

Quaternary sea level oscillations are best preserved along continents and coastlines which are rising tectonically or isostatically. Relatively stable coastlines are not conducive to the preservation and subsequent recognition of geomorphic landforms. The landforms that are most helpful in the estimation of former sea levels are typically deltas, beaches, and erosional marine-cut terrace platforms. Also, since entrenchment of rivers should occur during times of glaciation (when sea level is lower), terrace formation should propagate upstream, and should be recognizable for great distances upstream from the mouth of the river. This could also have implications of estuary formation and subsequent drowning of estuaries during interglacial times.

The process of Sequence Stratigraphy is an approach used with the Deep-Sea Drilling Program. When this technology first became available, it was the first direct evidence in support of eustatic changes in sea level linked to glacial and interglacial cycles. Essentially, sequence stratigraphic techniques analyze the alternating cold and warm cycles, which are preserved in the rock record. From those, correlations can be drawn with Milankovich cycles.

Much of the evidence of eustatic change is found in radiocarbon dating of sediment, coastal and oceanic foraminifera, and in the Oxygen-18 isotope. The O-18 technique is most successful in sites of the North Atlantic, because there were glaciers on both sides (the Greenland Ice Sheet to the east, and the Laurentide Ice Sheet to the west). Andrews (Quaternary Research, 1998) suggested that the O-18 isotope studies may not yield accurate results, however, because the radiocarbon techniques used in
the dating process cannot detect if the change in the record was due to ocean temperature or salinity of the seawater.

Blanchon (Geology, 1995) did a study of reef drowning during the last deglaciation as evidence of ice sheet collapse. His study correlated elevations and ages of Acropora palmata from reefs in the Caribbean/Atlantic region, and he was able to deduce three catastrophic events of sea level rise in the last deglaciation:

    1) Collapse of the Laurentide and part of the Antarctic Ice Sheets;

    2) Reorganization of oceanic and atmospheric circulation patterns following the collapse;

    3) Releases of subglacial and proglacial meltwater from the decaying ice sheets.

This study demonstrates a positive feedback process in the deglaciation stages in terms of eustatic change. The destabilization of an ice sheet causes destabilization in other ice sheets. In terms of climatic effects, the reorientation of oceanic circulation cells causes warmer water to be forced further northward than previously possible, thereby destabilizing other ice sheets. The same holds true for atmospheric circulation cells on a hemispheric level. Another process that could aid in the decay of other ice sheets would be the eustatic water rise itself, with the associated latent heat in the water.

In his study of late Quaternary meltwater in the northwest Labrador Sea, Andrews (Quaternary Research, 1994) found that this region was most affected by glacierization and associated isostatic and eustatic effects, due to the region being surrounded by the Greenland and the Laurentide Ice Sheets. The amount of isostatic depression in the area was sufficient to cause flooding of ocean water some 30 degrees of longitude inward from the coast. It is also noteworthy that the Hudson Bay in this area drained as much as 4*10^6 square kilometers of the Laurentide Ice Sheet.

The Tertiary saw the glacierization on Antarctica and Greenland. There was an estimated 80 meters of sea level drop associated with the water locked up in the ice sheets, although isostatic adjustment of the oceanic crust reduced that amount to approximately 70 meters of total relief.

Pleistocene Eustasy:

The maximum amount of eustatic sea-level change in the Pleistocene Epoch is unknown, but the best estimates range from 100 to 159 meters below the present-day. These estimates were calculated based on the estimated ice volume locked up in the Illinoian glacial episode. The numbers given above represent the high estimate; most range from about 100 meters to about 135 meters below the present day volume.

An important aspect during the Pleistocene is that the exposed continental shelves added about 8% to the total subaerial land mass, thus allowing the first indigenous peoples of North and South America to cross the Bering Land Bridge through Siberia and Alaska.

At this timescale, radiocarbon dating techniques (Carbon-14) can be accurately used to date former shorelines, using driftwood, peat, and other foraminifera from both marine and continental origins.

Holocene Eustasy:

Current sea level was reached about 5000 years ago, although fluctuations of about 30 centimeters are relatively common. Sea level is continuing to rise at the present time, and has risen 12 to 30 centimeters since the turn of the century due to a recession of glaciers worldwide.

Andrews, John T. Late Quaternary Meltwater and Heinrich Events, Northwest Labrador Sea. Quaternary Research, Vol. 41, No. 1, January 1994, p. 26-34.

Blanchon, Paul. Reef Drowning During the Last Deglaciation: Evidence for Catastrophic Sea-Level Rise and Ice Sheet Collapse. Geology, Vol. 23, No. 1, January, 1995, p. 4-8.