Holocene reef growth in the Maldives: evidence of a mid-Holocene sea level highstand in the central Indian Ocean

Abstract

Radiometrically calibrated ages from three reef cores are used to develop a Holocene reef growth chronostratigraphy and sea-level history in the Maldives, central Indian Ocean. Last interglacial reef (U-series age 122 ± 7 ka) was encountered at 14.1 m below mean sea level. An age of ca. 8100 calibrated (cal) yr B.P. immediately overlying this Pleistocene surface records the initiation of Holocene reef growth. Massive in situ corals occur throughout the cores and the consistency of the three age-depth plots indicate that the reef grew steadily between 8100 and 6500 cal yr B.P., and at a decreasing rate for the next 2 k.y. The position of modern sea level was first achieved ca. 4500 cal yr B.P. and sea level reached at least 0.50 ± 1 m higher from 4000 to 2100 cal yr B.P. before falling to present level. Emergent fossil microatolls provide evidence of this higher sea level. Results are significant to two long-standing issues relating to Maldivian sealevel history. First, the ambiguity of a late Holocene highstand has been resolved with clear evidence of its existence reported here. Second, the uncertainty of the regional pattern of sea-level change in the central Indian Ocean has been clarified, the Maldivian results broadly agreeing with island records in the eastern, rather than western Indian Ocean. Our results provide the first field evidence confirming geophysical model projections of a highstand 4–2 k.y. ago in the central Indian Ocean, though the observed level (+0.50 ± 0.1 m) is lower than that projected.