| Abstract: |
The Malay-Thai Peninsula is part of Sundaland, a geological region of relative crustal stability. Therefore, the positions of old shorelines (of the past 20,000 years or so) can be considered to indicate positions of actual past sea levels. Over 200 shoreline indicators at positions below as well as above present sea level have been radiocarbon dated. These dated positions indicate that prior to 6 ka the sea rose from at least 90 m below present sea level at rates that initially amounted to 15 mm/year and later became 6 mm/year. At ca. 6 ka, the rising sea surpassed its current position and at ca. 5 ka, it reached 5 m. During this maximum marine inundation, the peninsula was narrower, since the low coastal plains did not yet exist. In the period following this maximum transgression, the available shoreline data indicate that regional sea level subsided through a series of fluctuations that were characterized by amplitudes of 2 m and periods of about 2000 years. The peaks and troughs of the fluctuations became progressively lower. The latest peak was at about +1.7 m (or 0.7 m above present high tide) a few hundred years ago. The cause of these fluctuations is not yet known, but the trend suggests that, in the near future, regional sea level may subside at annual rates between 1.5 mm and 2 mm. This rate of descent is 25 to 30% of the estimated rate of sea-level rise that may result from global warming. In other words, for the Malay-Thai Peninsula, the effect of sea-level rise on account of manmade greenhouse gases will be partly compensated by actual subsidence of regional sea level.
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