Quaternary sea-level changes

Quaternary sea-level changes The Quaternary period (the past 1.8 million years) has been characterized by rapid and large-scale changes in sea level. The extent and nature of these changes has varied around the globe and they have resulted from an interaction between changes in the volume of the world's oceans (eustatic changes) and movements of the crust relative to the sea (tectonic movements). Since the interaction of these two factors produces a complex response, there is no agreement over the exact nature of sea-level changes during this period.

Causes of sea-level change

Quaternary eustatic changes in sea level result from a number of processes, the most important of which have been changes produced by the growth and decay of Quaternary ice sheets (glacio-eustatic changes). As ice sheets grow, water is transferred from the world's oceans on to the land; world sea levels thus fall, perhaps by as much as 150 m. When ice sheets decay, the water is returned to the oceans and the sea level rises. The period has thus been characterized by major fluctuations in sea level.

Nils-Axel Mörner of the University of Stockholm has suggested that major sea-level changes may also have been produced as the ocean's geoid (the surface of the sea) responded to environmental changes. Measurements taken from satellites indicate that the surface of the sea is not uniform, since it shows a variation in altitude from one point to the next. For example, measured from satellites, sea level near Indonesia is some 180 m higher than in the Indian Ocean. These variations in the geoid are the product of variations in the gravitational field and in the shape of the ocean basins. What Mörner has suggested is that as sea level rose and fell in response to the growth and decay of ice sheets, so the ocean basins would have changed shape and gravitational forces would have altered. He concluded that such changes could affect the geoid and thus alter the altitude of the sea surface. He was, however, unable to quantify the magnitude and form such changes may have taken. Other scientists have remained unconvinced about the importance of such processes.

Another eustatic process is the change in the level of the sea induced by changes in the volume of the ocean basins. Increased activity along constructive plate boundaries can increase the size of the mid-oceanic ridges, which in turn displace water and thus raise global sea levels. By contrast, reduced activity along constructive plate margins or mountain-building at destructive plate margins results in reduced global sea levels. Such changes are not as rapid as those associated with glacio- and geoidal eustasy but over a prolonged time period (perhaps longer than the Quaternary) they could have had a major impact on the level of the world's oceans.

More minor eustatic changes are produced by volcanic activity adding new water to the ocean basins or by temperature changes altering the volume of the world's oceans. It has been estimated that if the temperature of the world's oceans were raised by 1 °C, the thermal expansion of the water body would raise sea levels by 20 cm—a relatively minor fluctuation when compared with those produced by glacio-eustatic changes.

Sea-level changes along coastlines are also produced by movements of the land in response to tectonic processes. Some areas of the world (e.g. New Guinea) have experienced extensive uplift during the Quaternary, usually along destructive plate margins. At other sites (e.g. the Mississippi delta) subsidence has dominated, for example, as a consequence of sediment loading or sediment compaction.

More complex tectonic movements have characterized many other coastal areas. In regions that were covered in the past by ice sheets, the crust was depressed during ice-sheet growth and then recovered (rebounded) as the ice sheets decayed. Areas such as Scotland, Scandinavia, and Canada were significantly depressed, and thus the postglacial period has been characterized by rising land and generally falling sea levels. In contrast, many areas at the margins of the ice sheets underwent uplift as material was displaced from beneath the ice. Similarly, many oceanic areas experienced complex movements, since the growth of the ice sheets resulted in reduced crustal loading from ocean water, and thus crustal uplift (hydro-isostasy) occurred. As a result, the initial fall in sea level produced by the growth of ice was accentuated by crustal uplift. As the ice sheets melted and world sea levels rose, so the mass of water returned and the crust was lowered, promoting further rises in local water levels.

Nature of Quaternary sea-level changes

A detailed record of Quaternary eustatic sea-level changes has been derived from studying oceanic cores taken at selected sites where there is a continuous record of sedimentation. The tests of foraminifera are extracted from the sediments and oxygen-isotope analysis undertaken. The results (Fig. 1a) are believed to illustrate variations in ice volume through time, and thus they also represent Quaternary variations in the volume of the oceans. As a result such curves have often been used to determine eustatic sea-level changes. This analysis suggests that rapid fluctuations in sea level have occurred during the Quaternary, with 22 glaciations in the past 1.6 million years. It is noteworthy that the curve indicates that prior to 900 000 years ago sea level fluctuated by about 100 m, while since that time the fluctuations have been in the region of 180 m. The current view also suggests that sea level stood near present-day levels during each interglacial.

The oxygen isotope curve has been used to estimate the magnitude of sea-level changes for the past 140 000 years (Fig. 1b). This curve suggests that some 120 000 years ago, representing the last interglacial period, global sea levels were about 5 m higher than at present. Sea level subsequently fell to reach its lowest point at –130 m some 18 000 years ago. This fall in sea level was certainly not a gradual decline; on the contrary, it was marked by major fluctuations. The end of the last glaciation was marked by a very rapid rise in sea level (6000–18 000 years ago), and similar conditions appear to have occurred before the last interglacial (140 000–120 000 years ago).

Some scientists have suggested that Quaternary eustatic sea-level changes were characterized by a series of glacio-eustatic oscillations superimposed on a longer-term fall (Fig. 1c). It has been argued that the oxygen isotope curve reflects only the volume of water in the oceans and it does not illustrate changes in the volume of the ocean basins, which could account for the supposed fall. The proposed curve is noteworthy because it suggests that in the past eustatic sea levels associated with low stands stood at the same level as present-day sea level. Once again, this observation suggests that features of the present coastline may have developed in the past, perhaps under contrasting environmental conditions. This model is, however, based upon raised marine features from the Mediterranean region, and it is considered highly likely that many of these features have been uplifted by tectonic processes.

Sea-level changes in many areas have been dominated by tectonic processes which have been superimposed upon the eustatic fluctuations. One the best examples is provided from the Huon Peninsula of New Guinea, which is subject to uplift because it lies near a destructive plate boundary. A sequence of coral reefs and deltas rising to altitudes of 400 m illustrate sea-level changes spanning the past 400 000 years. In most instances the coastal features developed when sea levels were high, and once the tectonic component has been extracted from the data, it is possible to estimate local eustatic changes in sea level (Fig. 1b). The resulting eustatic curve is similar to that derived from the oxygen isotope data, although for the past 120 000 years the terraces generally indicate higher sea levels. The data do, however, support the view that eustatic sea level has fluctuated around a constant level and has not been associated with a gradual decline.

Many other areas of the world have been characterized by tectonic uplift. These include Barbados, the east and west coasts of the United States, many sections of the Mediterranean coastline, New Zealand, and the New Hebrides. Other areas have experienced glacio-isostatic movements (Scotland, North America, Scandinavia) and thus have seen rises and falls in the crust, while many oceanic islands have been characterized by subsidence. At one time large sections of the world's coastline were believed to have been stable (e.g. the eastern seaboard of South America, western Australia, the Great Barrier Reef), and although little tectonic movement has occurred such areas are likely to have been influenced by hydro-isostatic movements.

It is now generally accepted that at present it is impossible to construct a truly eustatic curve for the Quaternary period. The oxygen isotope record provides the best approximation for the sea-level changes, but these variations are not entirely controlled by eustatic fluctuations. As a result, one oxygen isotope curve differs from another. Some scientists have suggested that geoidal variations may have been important, but at present the significance of such processes is disputed.

Callum R. Firth

Bibliography

Bowen, D. Q. (1978) Quaternary geology. Pergamon Press, Oxford.
Dawson, A. G. (1992) Ice Age Earth: Late Quaternary geology and climate. Routledge, London.