Postglacial changes in level of land and sea

Abstract

Four postglacial high sea levels, separated by intervals of low water, are recognized at 7.5 ft, 4,5 ft, 3.0 ft and 1 ft above present mean sea level in East Gippsland, Victoria, Australia. Direct estimation of shoreline age is made impracticable by an absence of datable material in the stranded beach sediments, but a few dates are available for lagoonal beds. These dates, and correlations with New Zealand made possible by the common occurrence of similar sequences of coastal dunes resulting from changes in postglacial climate, suggest that the four stages of high sea level occurred about 4,700, 3,000, 1,500 and 750 calendar years ago. In particular the East Gippsland shoreline sequence is similar to that observed in the Firth of Thames, New Zealand. The stranded shorelines are believed to result from combination of long-term land uplift with real changes in sea level. This conclusion follows comparison of the Firth of Thames data with observations reported for 15 other localities in New Zealand, western Europe, North America, Africa and Oceania. Contradictory theses concerning postglacial sea-level changes in these localities are reconciled if it is assumed that each locality has been affected by earth movements. Such movements are already recognized in some areas affected by isostatic rebound or by mountain-building, and are presumed in other where shorelines appear excessively high or excessively low. To facilitate comparison, two simple models that relate shoreline elevation to implied rate of uplift are used. These models are based on the supposition that the earth movements actually experienced were continuous, or can be reasonably represented as small fluctuations about a regular long-term trend. It is not yet possible to decide whether any of the localities studied has been stable in postglacial time. Earth movements may not have affected the Cook Islands, South Pacific, however. If this is so, it would seem that the sea g:ained its present level several thousand years ago. Subsequent fluctuations have been in response to global climatic changes. The highest level actually attained by the sea (5.5 ft) was reached briefly in Sub-boreal time.