Late Quaternary Reef Growth and Sea-Level Rise in the Maldives (Indian Ocean)

Eberhard Gischler¹ and Harold Hudson^2
1Geosciences, J.W. Goethe-University, Frankfurt am Main, Germany
²Florida Keys National Marine Sanctuary, N.O.A.A., Key Largo, FL

The Maldives is among the largest tropical shallow-water carbonate areas of the world. Reef growth and sea-level rise are fundamental topics for the inhabitants of the archipelago, as it is for people living on similar low-lying reef island nations in the Indo-Pacific. Even so, Holocene sea-level data from the Indian Ocean is controversial, and data on Holocene reef-growth and sea-level in the Maldives is fragmentary at best. Based on drilling and radiometric and U-series dating, we present the first data on Holocene reef anatomy and sea-level rise as well as nature and age of underlying Pleistocene limestone in the Maldives. Holocene reefs in Rasdhoo Atoll are composed of four facies including robust branching coral, coralline algal, domal coral, and detrital sand facies. Branching coral and coralline algal facies predominate the marginal reefs and domal corals and detrital facies preferentially occur in a lagoon reef. In addition, microbialite crusts are found in reef cavities in lower core sections of marginal reefs, where they formed during the early Holocene. Submarine cementation in Holocene reefs studied is weak, presumably as a consequence of high accretion-rates, i.e., short time available for consolidation. Holocene reef thickness ranges from 14.5 m to >22 m. Pleistocene coral grainstone was encountered in one core at 14.5 m below present level and U-series dates indicate deposition during isotope stage 5e ca. 135 kyrs BP. Holocene reef growth started as early as 8.5 kyrs BP. Age-depth plots show that marginal Holocene reefs accreted in the keep-up mode with rates of >15 m/kyr from 9-7 kyrs BP. Rate of sea-level rise significantly slowed down from 7-6 kyrs BP and subsequently gradually rose to present level with rates <1 m/kyr. No indications of a higher-than-present or recently falling Holocene sea-level were found during this study.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas