Microatolls and the record of Holocene sea level that can be derived from them

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Australian Geosciences Council
The Great Barrier Reef expedition in 1973 described living and fossil microatolls on the Great Barrier Reef and recognised their significance as sea-level indicators. John Chappell subsequently surveyed and dated Porites microatolls along the mainland and interpreted them in the context of hydro-isostatic adjustment, inferring gradual falling sea level during the past 6000 years. We re-examine the significance of microatolls, providing a detailed account of more than 100 fossil microatolls from Christmas Island in the central Pacific. We relate their elevations to living modern equivalents and compare the sea-level record derived from them with the modelled Holocene isostatically-corrected sea level for this equatorial location. These long-lived massive corals, up to 9 metres in diameter, preserve a biologically-mediated multi-decadal record of sea-level over the past 5000 years. The upper surface of fossil microatolls on the reef flat lie in a narrow elevation range similar to that observed for their modern, living counterparts, implying negligible change of sea level. Further evidence for the lack of major fluctuations in sea level over the period 5000–1000 years BP is provided by corals from the interior of the island. This second population grew prolifically in a large lagoonal setting and is offset from those in open water by about 50 cm. The continuity of microatoll growth precludes significant oscillations of sea level during this time, and accords with geophysical modelling for this site, implying that the ‘eustatic’ contribution from post-6ka ice melt and the isostatic adjustment of the ocean floor to loading cancel each other at this site.
Data, Sea level, Quaternary period, Coral reefs, Queensland, Australia, Fossils, Seas
Woodroffe, C. D., Smithers, S. G., McGregor, H. V., Fink, D., & Lambeck, K. (2012). Microatolls and the record of Holocene sea level that can be derived from them. Paper presented to the 34th International Geological Congress 2012, "Unearthing our Past and Future - Resourcing Tomorrow", Brisbane, Australia, 5-10 August 2012, (pp. 1688).