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Eustasy

Hydrologic Cycle

Hydrologic Cycle

The hydrologic cycle is the constant circulation of water from the sea through the atmosphere, to the land, and its eventual return to the sea.
The major elements of storage in the hydrologic cycle are the oceans and glacial ice (groundwater can also be considered as storage or slow transport).  Across geologic time, groundwater volume is nearly constant, but ocean volume (thus sea level) fluctuates as more or less water is stored on the continents as ice.

Eustatic Change

Because of glacial fluctuations true, or eustatic, sea level (which those of us who live along oceanic coasts may think of as a constant, ignoring waves, tides, and storms) varies across spans of observation ranging from decades to hundreds of thousands of years.  This variation, in turn, is superimposed on sea level fluctuations across millions of years caused by changes in the volume of the ocean basins.

Effects of Eustasy

The diagram at left compares the extent of the North American continent at highest Pleistocene sea levels (dark green), modern sea levels (light green), and lowest Pleistocene sea levels (blue).  Because these fluctuations take place over a geologically short time (ten to one hundred thousand years), they are important to understanding glaciers, animals, and even people.  At the glacial maximum ice could extend well onto the present ocean floor off Alaska and Nova Scotia without floating or calving.  At those times, the Bering Strait between Alaska and Siberia was open to animals and humans for migration (it was too dry to support ice at sea level).  If the  Antarctic and Greenland ice sheets were to disappear, cities as far inland as St. Louis (80 m above present sea level) would be inundated, and even decay of the Greenland Ice Sheet alone would drown most coastal cities up to 7 m above modern sea level.

Postglacial Sea Levels

As the last great ice sheets thinned, retreated, and disappeared, their water was returned to the oceans.  Radiocarbon dating of drowned reefs, swamps, coastal features, and river channels allows the reconstruction of both the rapid decay of ice sheets and the gradual climb to present sea level.  Rapid sea level rise about 12,000 years ago may represent the major decay of the Scandinavian Ice Sheet, and about 9000 years ago, the Laurentide.

The Laurentide Ice Sheet had nearly disappeared by about 5000 years ago, thus the last 10 m of sea level rise cannot be explained by ice sheet decay.  Thermal expansion of ocean waters and isostatic uplift of the shallow North Sea and Hudson Bay (spilling their water into the larger oceans), are the most likely explanations.

Terraces

Because postglacial sea level rise has drowned most evidence of past sea levels, such evidence can only be seen clearly where tectonic uplift has raised former coasts above present sea level.  The best evidence of former coasts is erosional marine terraces, which are formed when rising water levels erode bedrock.  The cartoon at left shows how old shorelines, cut in some cases at sea levels below present, can now be found above sea level.  The ages of the terraces, determined through radiometric dating, define times of sea level rise and high water in the past.

Pleistocene Sea Levels 

Terraces on rapidly uplifting New Guinea, dated by uranium-series on coral, allow the interpretation of sea level changes across the entire span of the last glaciation.   Only the high stands are well-dated - the intervening falls are estimated.