NEW EVIDENCE FOR EPISODIC POSTGLACIAL SEA-LEVEL RISE, CENTRAL GREAT-BARRIER-REEF, AUSTRALIA

We present an extensive database of 364 radiocarbon dates from coastal and marine sediments of the central Great Barrier Reef (GBR) shelf, of which 110 are previously unpublished. The elevation data have been reduced to a common datum (Australian Height Datum, AHD) and the various sources of error have been assessed. Using modern lithological and biological relationships with sea level, the elements of the radiocarbon database have been converted into sea-level indicators. The upper bound of the assembled dataset corresponds to a best-estimate sea-level curve, and the full dataset provides a narrow envelope for sea-level rise on the GBR shelf for the last 11-12 kyr (not including hydro-isostatic crustal flexing). The envelope is consistent with episodic rise in post-glacial sea levels. The rising post-glacial sea level probably included stillstands (or minor falls), at ca. -45 m AHD (at ca. 10.5 kyr B.P.), -5 m (7.8 kyr B.P.), -2 m (ca. 6 kyr B.P.) and +1.7 m (5.5 kyr B.P.). There is evidence for a significant fall in sea level between stillstands at -11 m (8.5 kyr B.P.) and -17 m (8.2 kyr B.P.). Stillstand durations apparently ranged between <200 years and ca. 1800 years. Rates of sea-level change varied between a fall of 20 mm/yr (20 m/kyr, pre-8.2 kyr B.P.) and a rise of 30 mm/yr (30 m/kyr, post-8.2 kyr B.P.). The vertical spread in the derived sea-level data is very wide. The use of shell material for dating seems unreliable and prone to large and unpredictable errors. Data from bulk mangrove muds appear reliable for determination of ancient sea level, but may at times result in sea level being placed up to 4 m below the true level. In-situ biogenic carbonates such as preserved oyster beds and coral micro-atolls are the most reliable indicators of sea-level position, while deposits of mangrove mud give a useful first-order approximation of ancient sea levels. Caution should be used in drawing 'sea-level curves' from few data points. We conclude that the post-11-12 kyr B.P. relative rise in sea level was episodic on the central GBR continental margin. More data are required to define clearly sea-level change up to ca. -20 m at 9 kyr B.P.