Atoll-scale comparisons of the sedimentary structure of coral reef rim islands, Huvadhu Atoll, Maldives

Abstract

Coral reef islands are low-lying (typically <5 m above mean sea level, MSL) accumulations of wave deposited bioclastic sediments. These sediments are produced within the surrounding coral reef habitats and reef islands are therefore intrinsically linked to reef ecology (Perry et al. 2011). As a result of their dependence upon locally generated sediment, low elevations and largely unconsolidated structure, reef islands are regarded as extremely vulnerable to environmental change, particularly to sea-level rise. This is of concern given their high ecological and socioeconomic value, not least because they offer the only habitable land in regions including the Maldives, Kiribati and the Marshall Islands. However, assertions of vulnerability are largely made without a full understanding of how and when islands formed, the processes controlling island formation and inter- and intra-regional variations in island-building processes. Understanding reef island accretionary histories and the controls on island development is thus crucial for assessing their morphological stability and future resilience. To date, research has focused largely upon a few discrete localities in the Pacific and within the Great Barrier Reef Shelf/Torres Strait region (e.g. Kench et al., 2014; Woodroffe et al., 2007; Yamano et al., 2014). In other major reef island regions such as the Maldives (a nation comprised of >1,200 reef islands inhabited by a population of ~345,000), our knowledge of island building processes is far more limited. Maldivian reef islands may be divided into two key types: (i) rim islands, which form around the atoll perimeters; and (ii) interior islands, which are located on the reef platforms within atoll lagoons. However, research of reef island sedimentology and the modes of island-building in the Maldives is restricted to two main datasets developed for interior islands within just one atoll (South Maalhosmadulu Atoll in the northern-central part of the archipelago – Kench et al., 2005; Perry et al., 2013). Knowledge of rim island stratigraphy is even more limited and based on qualitative descriptions of one pit in the centre of Feydhoo island, Addu Atoll (Woodroffe, 1992). However, it is the rim islands that dominate spatially (82.4% of land area), host the majority of the population (88.93%), and therefore support the nation’s key infrastructure (all regional administrative capitals, hospitals, and designated ‘safe islands’). Here, we present the first detailed sedimentary study of Maldivian rim islands. Textural, compositional and topographical datasets are used to infer the major sources of reef island sediment, the key controls upon island building, and the degree of intra-regional (at the atollscale) variability in island building. Study Site Two sites were selected on the rim of Huvadhu atoll – a leeward site, with respect to wave energy, in the north-east (Galamadhoo and Baavanadhoo islands), and a windward site in the south-west (Mainadhoo, Boduhini and Kudahini islands; Figure 1).