Unique islets in SE Asia: How Holocene climate and sea-level controlled growth of a coral reef island

Coral reef islands are unconsolidated landforms, composed of skeletal carbonate from surrounding reefs and related ecosystems. Facing the predicted rise of sea-level and a changing environment in the next decades, response of low-lying coral reef islands is of great scientific and public interest. To better estimate future behavior, it is necessary to know and classify the effects of past environmental changes on the sedimentary system. In SE Asia, sedimentological understanding of the development of these landforms is still in infancy state, despite regional dependency on coral reef islands as habitable space. Here we provide a first comprehensive sedimentological study from the Spermonde Archipelago, Sulawesi, and simultaneously a unique data set in global coral reef island research. By analyzing sediment cores down to 10 m below island surface, we can reconstruct the sedimentary processes that ultimately formed the foundation of the recent landform. The main components of the sediment are coral fragments, constituting around 60% of the spectra, which underlines their importance for carbonate budgets. Gastropods also frequently contribute with about 20% to the sedimentary material, while foraminifera, bivalves and the green alga Halimeda are present only to a minor extent. Deposits from the early Holocene appear to be connected to hydrodynamic high-magnitude events. A significant depositional hiatus coincides with the onset of a mid-Holocene sea-level highstand in the study area. Rapid sedimentation occurs in late stages of subsequent sea-level fall, demonstrating how sensitive the dynamics of the sedimentary system are to changing environmental conditions. We can conclude that a small-scale sea-level fall of around 0.5 m enabled the formation of the coral reef island. Our final evolutionary model provides new and valuable insights for a better understanding of these landforms.