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Holocene reef growth in the Maldives: Evidence of a mid-Holocene sea-level highstand in the central Indian Ocean
P.S. Kench1, S.G. Smithers2, R.F. McLean3, and S.L. Nichol1
1School of Geography, Geology and Environmental Science, University of Auckland, Private Bag 92019, Auckland, New Zealand 2School of Earth and Environmental Sciences, James Cook University, Townsville, QLD 4811, Australia
3School of Physical, Environmental and Mathematical Sciences, University of New South Wales, ACT 2600, Australia
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 sea-
level history. First, the ambiguity of a late Holocene highstand has been resolved with clear evi- dence of its existence reported here. Second, the uncertainty of the regional pattern of sea-level B 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 C
Chagos INDIAN OCEAN
central Indian Ocean, though the observed level (+0.50 ± 0.1 m) is lower than that projected.
The Holocene sea-level history of the Mal- dives has been a source of speculation and sci- entific debate for more than a century. There have been two major issues: (1) was there a sea- level highstand in the middle to late Holocene in the central Indian Ocean, and (2) is the pattern of relative sea level similar to, or different from, the contrasting sea-level histories of the eastern and western Indian Ocean?
This study presents an 8 k.y. history of reef growth in the central Maldives, resolving aspects of sea-level history in the archipelago and the central Indian Ocean. The Maldives provides an ideal location to examine sea-level change in the Indian Ocean for three reasons. First, the 850-km-long archipelago is located in a tectoni- cally stable mid-ocean setting, midway along the Laccadive-Chagos aseismic submarine ridge (Fig. 1). The atolls are founded on as much as 3 km of limestone overlying Eocene volcanic basement and have been subject to high-ampli- tude sea-level fluctuations during the Pliocene– Pleistocene with alternate periods of exposure and vertical reef growth (Aubert and Droxler, 1992; Purdy and Bertram, 1993). The Holo- cene is the latest of these reef growth periods. Second, the Maldives is located in the farfield, where detailed studies of sea-level change are needed to delimit the ice melt history since the Last Glacial Maximum, the equatorial siphon- ing mechanism (Camoin et al., 2004; Mitrovica and Milne, 2002), and geophysical models that
retrodict local sea-level histories (Peltier, 1999;
Lambeck et al., 2000). Finally, understanding
the pattern of sea-level change and how Maldiv-
ian reefs and islands have responded to past sea- 0 level rise can provide important analogues in Addu assessing the future stability of low-lying atoll
Male Hulhudhoo Funadhoo
islands (Kench et al., 2005a).
Previously published field evidence for a
higher Holocene sea level in the Maldives is ambiguous (Woodroffe, 2005). Early interpre- tations of a highstand were based on the pres- ence of conglomerate rocks (Gardiner, 1903) and subsequently discounted because of their storm-deposited origin (Sewell, 1936; Stod- dart et al., 1966). Later, outcrops of lagoonal Heliopora reef in Addu atoll were used to sug- gest that sea level was at or slightly above its present level in the late Holocene (Stoddart et al., 1966; Woodroffe, 1993), although possible moating and uncertain elevation with respect to living coral prevented definitive interpreta- tion (Woodroffe, 2005). Mörner et al. (2004) presented a sea-level curve for the Maldives that included sea level as much as 1.0–1.2 m above present at 3900 yr B.P. However, con- cerns regarding the nature of the material dated (reworked coral clasts) and poor elevation control on samples have cast serious doubt on the veracity of this curve (Kench et al., 2005b; Woodroffe, 2005). Gischler et al. (2008) found no indication of a higher than present Holo- cene sea level in their study of Rasdhoo atoll, central Maldives.
Figure 1. Location of study islands in South Maalhosmadulu Atoll, Maldives, central In- dian Ocean.
The second issue relates to the different Holo- cene sea-level histories established between the western and eastern Indian Ocean. In this con- text, because of its central position, the Maldiv- ian archipelago is seen as a key site “at which to clarify the regional pattern of Holocene sea- level variation in the central Indian Ocean” (Woodroffe, 2005, p. 134). There is evidence for a higher Holocene sea level to the east. Woodroffe et al. (1990) showed that sea level reached 0.6 ± 0.2 m above present 3300–2500 yr B.P. on the Cocos (Keeling) Islands. In the western Indian Ocean, sea-level history is more equivocal. Camoin et al. (2004) established that sea level rose from its glacial lowstand to stabilize around present level 3 k.y. ago. Their sea-level reconstructions from the granitic Sey- chelles and the volcanic islands of Reunion, Mauritius, and Comores do not reflect higher stands during the late Holocene. Montaggioni (2005, p. 52) also noted that “emergent in situ reef buildups are missing” on these and other
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GEeoOloLgOy,GMY,aMy 2a0y0290;0v9. 37; no. 5; p. 455–458; doi: 10.1130/G25590A.1; 3 figures; 1 table. 455
Cocos (Keeling) Island
60 E 90
C Sth Maalhosmadulu atoll
1000 m contour
Image | Holocene reef growth in the Maldives
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