sea-level changes and palaeoclimate

sea-level changes and palaeoclimate Sea level is intimately linked to climate. At the maximum of the last ice age, some 20 000 years ago, large ice caps were present on the great northern hemisphere continents. At this time the Laurentide and Cordilleran ice sheets merged, covering over half the land area of northern North America with over 30 million km³ (cubic kilometres) of ice. The Antarctic ice sheet, which today has a volume of about 24 million km³, expanded to 38 million km³, and other continental ice sheets on Scandinavia, Greenland, and Iceland also grew. This massive build-up of ice resulted in an estimated eustatic sea-level fall of about 120 m below present levels. The immense Laurentide ice sheet contributed about half to this total, and Antarctica about 20 m. The British ice sheet, which reached as far south as north Norfolk, the Midlands, and South Wales, was small by comparison. It had a volume of only 0.8 million km³ and contributed about 1.0 m to global lowering of sea level.

Ice and sea level are also of utmost concern today. If all the remaining continental ice were to melt, the resulting rise in sea level would be about 90 m, with the Antarctic ice sheet contributing about 65 m. It is clear, therefore, that the major determinant of global (eustatic) sea-level change over the past 2 million years is the amount of ice present on the continents. This suggests a direct link with climatic change. There are, however, many other factors, operating on a variety of scales in time and space, that influence the exact level of the sea, and not all of them are linked to climatic change. These other factors can conveniently be divided into those that affect ocean basin volume (such as sea-floor spreading and sediment infill), those that affect ocean water volume (such as the amount of water stored on land), those that affect the geoid (which is the surface of the undisturbed ocean), and finally dynamic sea-level changes which can have a meteorological, hydrological, or oceanographic origin.

The direct link between climate and sea level during the Quaternary can be illustrated by comparing sea-level records with other information on climatic change. It is generally accepted that the oxygen isotope record, which is recovered by analysing benthonic or planktonic foraminifera from deep-sea cores, chronicles the history of worldwide continental ice volume. It should also therefore act as a first approximation to glacio-eustatic sea-level change. In 1987, N. J. Shackleton of Cambridge attempted to compare the oxygen isotope and sea-level record. Sea level was estimated using data from the remarkable ‘staircase’ of raised coral reefs on the Huon Peninsula of New Guinea. In this area more than 20 coral reef complexes have been uplifted during the past quarter of a million years. The ‘eustatic’ sea-level curve from New Guinea is derived from uranium-series dating of coral and has been corrected for the effects of land uplift. The oxygen isotope record used for comparison was combined from two cores, V19–30 from the eastern Pacific, where benthonic foraminifera were analysed, and RC17–177 from the western Pacific, which was based on planktonic foraminifera.

Both sets of data show good agreement. In particular the high sea-level stands at 125 000, 105 000, and 80 000 years BP can be equated with marine isotope stages 5e, 5c, and 5a. In both records, stage 5e shows eustatic sea level approximately 20 m above that attained in stages 5c and 5a. Both curves show a gradual decline to the last glacial maximum at 10 000–20 000 bp and a large rise thereafter. The agreement between 80 000 and 30 000 bp is not so good, which is interesting in the light of the rapid alternations of climate from glacial to interstadial recorded in the Greenland ice cores from this period.

Using the extremes of the oxygen isotope record for the past 750 000 years, Shackleton suggested that marine isotope stages 12 and 16 record significantly more global ice volume and hence sea-level lowering than the last glacial maximum (stage 2). In addition, interglacial stages 7, 13, 15, 17, and 19 did not attain Holocene (stage 1) oxygen isotope values, suggesting that during these periods sea level did not reach its present level. The extreme values of stages 1, 5e, 9, and 11 were considered so similar that it was not possible to suggest that any one was significantly higher than any of the others.

A more recent study from 1992 by Peter Smart and David Richards (Bristol University) used a compilation of the frequency distribution of 320 published uranium-series dates on corals from Quaternary marine terraces relating to high sea levels in more than 30 areas worldwide. They found that since the peak of marine isotope stage 5e at 123 000 bp there were six other maxima recorded, at 102 500, 81 500, 61 500, 50 000, 40 500, and 33 000 bp. These dates should not be seen as exact—there is a degree of uncertainty about each one—but when they are compared with independent records of climate change the degree of similarity is evident.

There is therefore convincing evidence that, over larger time and space scales of the Quaternary, the main determinant of sea level has been continental ice volume, which is influenced by climatic change. When smaller time and space scales are investigated, however, the link is obscured by the many other factors that can influence the position of sea level.

Some of the methodology and operational definitions of Holocene sea-level research were improved by Ian Shennan of Durham University. Defining dated points in a consistent manner, he looked at the chronology of sea-level movements between the ‘uplifted’, ‘stable’, and ‘subsiding’ regions of the Tay estuary in Scotland, north-west Lancashire, and the Fenland. He demonstrated, using simple sequence matching that regionally significant events could be detected and matched in these areas.

With improvements in the climatic proxy record and with the use of comparable methodologies, it may soon be possible on smaller time and space scales to separate the climate signal from the noise caused by palaeotidal changes, compaction of sediments, and variations in coastal processes, sediment supply, and sedimentation rates.

B. A. Haggart

Bibliography

Devoy, R. J. N. (ed.) (1987) Sea surface studies: a global view. Croom Helm, London.
Smith, D. E. and Dawson, A. G. (eds) (1983) Shorelines and isostasy. Academic Press, London.