Understanding the origin of sediments and the direction of sediment transport is a prerequisite for accurate reconstruction of reef island evolution. In tropical settings, island development depends on the accumulation of sediments on the reef flat, which are produced by calcifying organisms such as corals, foraminifera, and green macro-algae of the genus Halimeda. In certain areas of tropical regions, calcareous segments from these algae might account for more than 50 wt% of the coarser sediment fraction. Halimeda-rich sediments typically contain complete segments that become widely distributed to a range of sedimentary settings. However, the exact sedimentary pathway is either mostly unknown or was neglected. Here we show that the alteration of Halimeda segments from the Spermonde Archipelago (SW Sulawesi, Indonesia) is related to processes during transport and deposition. Using field observations and scanning electron microscopy we observed that Halimeda segments, sampled from the reef flat to the shore on four uninhabited islands, without exception belong to the grain-size class of pebble gravel (1.6 > 0.4 cm; medium-to-fine pebbles) and reveal characteristic patterns of external and internal microstructural alteration with respect to sampling area. Furthermore, Halimeda species, from which most of these segments originate, preferably inhabit hard substrates in the reef, on the reef crest, and the outer reef flat. Thus, the observed distinct microstructural alteration of the segments allows drawing conclusions on sediment transport and deposition. Particularly, rapid secondary cementation may preserve the segments as a sedimentary component for the development and maintenance of reef islands and coastlines in tropical shallow seas.
