Sea-Level Change and Shore-Line Evolution in Aegean Greece since Upper Paleolithic Time

By Lambeck, Kurt | Antiquity, September 1996 | Go to article overview

Sea-Level Change and Shore-Line Evolution in Aegean Greece since Upper Paleolithic Time


Lambeck, Kurt, Antiquity


'As the glaciation ended, the ice melted and the sea-level rose.' Yes - but it has not been as simple as that, as the Earth has adjusted in several ways to the changing surface-loads it suffers under ice and under weight of water. The important issues are set out in a simple mathematical treatment, and their varied consequences are shown for Greece and especially for the Greek coastal plains and the Greek islands, where the impact on human settlement has been large.

Nature and consequences of Postglacial sea-level change

Sea-levels have changed significantly since Late Palaeolithic time, primarily in response to the melting of the great ice-sheets that covered northern Europe and North America. These ice-sheets were of a sufficiently large volume that, upon melting, sea-level rose globally by about 120-130 m. Release of this ice into the oceans was initiated at about 18,000 years before present (b.p.), although the majority of the melting occurred between about 16,000 and 8000 b.p.(1) Rates of global sea-level rise reached 15-20 mm per year during this interval. As the sea-levels rose, so did the shorelines migrate with time, at rates that for some low-lying regions reached about a kilometre per year. Examples of where such rapid encroachment of the sea occurred include the Persian Gulf and the Gulf of Carpentaria of northern Australia, between about 12,000 and 10,000 years b.p. In some areas of the world the sea-levels peaked at about 6000 years b.p., inundating now low-lying areas before falling slowly to their present position. The consequences of these changes on human settlement and movement have been recognized in the archaeological and pre-historic records. Thus it is widely accepted that levels during the Last Glacial Maximum, about 20,000 to 18,000 years ago, were sufficiently lower than today to leave exposed coastal plains that have since flooded. But less attention appears to have been focused on the timing and rates of change after the onset of melting of the great ice-sheets. What discussion there is - with the exception of the important paper by van Andel & Shackleton (1982) (see also van Andel 1989) - often leaves the distinct impression that this change in level occurred early and quickly with rather minimal human impact.

This paper sets out to describe, using the Aegean Sea region as an example, a realistic model of sea-level change and shoreline migration for the past 20,000 years, one that can provide a frame-work for discussing impacts of such change on human movements and settlement.

If, during the decay of the ice sheets, the meltwater volume is distributed uniformly over the oceans, then the sea-level change at time t would be

[Delta][[Zeta].sub.e](t) = change in ocean volume/ocean surface area (1)

This 'eustatic sea-level change' is a function of time. It represents only a zero-order approximation because sea-level does not respond uniformly to the melting of the ice caps: the rates of rise are spatially variable, and in some localities sea-level may actually be falling relative to the land. This is a consequence of the adjustment of the Earth to the changing surface-loads of ice and meltwater. The Earth's response can be described as that of an elastic layer (the lithosphere which includes the crust) overlying a viscoelastic mantle. When changes in the mass distribution occur on the Earth's surface, the lithosphere and mantle respond to the new stress state in different ways: elastic deformation primarily occurs in the lithosphere and viscous flow occurs primarily in the underlying mantle. The characteristic time-scale of this flow is of the order of a few thousand years. The deformation of the Earth's surface under a changing lead therefore exhibits both an instantaneous elastic response and a viscous response. Such behaviour is well documented by other geophysical observations: gravitational attraction of the Sun and Moon raises tides in the solid Earth; ocean tides lead the sea floor and contribute to the deformation of the Earth's surface; atmospheric pressure fluctuations over the continents induce deformations in the solid Earth.

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