Sealevel fluctuations

As noted earlier, fluctuations in world-wide sea levels during the course of the later Pleistocene period can now be reconstructed with a remarkable degree of accuracy. The evidence comes from two major sources (Fig. 2.17): from the oxygen-isotope records in deep-sea cores, which can be used to provide an estimate of the total volume of sea water in the world's oceans (Shackleton 1977, 1987); and from the more direct evidence of varying sea-level positions preserved in the form of coral terraces and other forms of raised beach deposits in various areas of the world (Aharon & Chappell 1986; Chappell & Shackleton 1986). Estimates of sea levels from oxygen-isotope records need to be handled with caution, since some specific allowance must be made as to the particular component in

Red Sea Isotope Ratio
Figure 2.17 Estimates of world-wide sea level fluctuations over the past 140,000 years derived from studies of oxygen-isotope ratios in surface and deep-water ocean sediments (continuous line) and dated coral terraces in New Guinea (dashed line). After Shackleton 1987.

these changing isotope ratios which is attributable directly to temperature changes, rather than to the removal of isotopically light sea water from the oceans to form the continental ice sheets (Shackleton 1987). In principle the direct records of coral reefs and terraces are simpler to interpret, although in these cases some allowance must be made for the effects of local tectonic displacements in the recorded altitudes of the individual terraces. The most detailed and extensively studied traces of coral terraces are those preserved in the Huon Peninsula of New Guinea, from which large numbers of precise and internally consistent radiocarbon and uranmm-series dates have been obtained (Aharon 1983; Aharon & Chappell 1986; Chappell & Shackleton 1986). Similar terrace sequences are documented from Barbados, Timor and the New Hebrides, all of which have yielded patterns comparable with those documented from the New Guinea terraces (Aharon & Chappell 1986).

The general picture of these 'eustatic' sea-level fluctuations which emerges for the later stages of the Pleistocene period is summarized in Fig. 2.17. The period of maximum sea level can now be shown to have coincided closely with the period of oxygen-isotope stage 5e (i.e. the peak of the last interglacial), and to have reached a level around 6-10 metres higher than that of the present day. There is clear evidence for this in many parts of the world not only in the coral terraces preserved in New Guinea and elsewhere but also in the form of beach deposits located at altitudes of ca 6-10 metres in many parts of the world (for example in northern France, southern Britain and the Mediterranean) (Bowen 1978, 1990). Following this period of interglacial high sea level, there was a series of complex fluctuations during the earlier stages of the last glacial period. During the initial cold stages of isotope stages 5d and 5b it seems that world-wide sea level fell to around 50-60 metres below present levels (Shackleton 1987). During the much warmer intersta-dial episodes of stages 5c and 5a, by contrast, sea levels rose sharply to levels only perhaps 10-20 metres below those of the present day.

Clearly defined episodes of relatively high sea-level stands during these major intersta-dial episodes are well documented in several parts of the world (especially New Guinea and Barbados) and have been dated by direct uranium-series measurements to around 105-110,000 and 80-85,000 BP (Aharon & Chappell 1986). Sea levels fell to their early glacial minimum during the period of isotope stage 4, to around 60-80 metres below present levels. A succession of rapid fluctuations then occurred during the period of isotope stage 3 (between ca 60,000 and 30,000 BP) before the massive reduction in sea levels, down to around —130 metres below modern levels, occurred during the overall maximum of the glaciation at around 18,000 BP (Shackleton 1987).

The effects of these sea-level fluctuations on the coastal geography of Europe are reasonably easy to reconstruct from the patterns of submarine contours around various parts of the coastline. The major reduction of sea level during isotope stage 4 had the effect of exposing large areas of the current North Sea basin and English Channel as dry land, effectively integrating southern Britain, northern France, the Low Countries and southern Scandinavia as part of a greatly extended North European plain. At the same time the coastal plain along the Atlantic coast of France was extended by at least 30-40 km towards the west. The much higher sea levels which occurred during the warmer inter-stadial episodes brought the coastlines much closer to modern patterns. Even so, the highest sea levels recorded during the major interstadials of the last glaciation would have had a major impact on the geography of the areas bordering the North Sea basin and would have left Britain an integral part of the main European land mass throughout the whole of the last glacial period. It was only during the period of the last interglacial that Britain was fully isolated from the continent, with coastlines broadly similar to those of the present day.

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