Mid-Holocene relative sea-level reconstruction from digital surface models of coral microatolls at Pulau Semakau, southwestern Singapore

Coral microatolls are coral colonies that grow with distinct morphologies consisting of living polyps on their outer perimeters and dead upper surfaces with concentric rings in planform. Their upward growth is limited by the lowest tides, allowing them to be used as precise indicators of relative sea-level (RSL) change. Therefore, detailed morphological investigation of fossil microatolls provides an important proxy for the reconstruction of past RSL. We present a preliminary RSL reconstruction from Pulau Semakau (Semakau Island), southwestern Singapore, based on digital surface models (DSMs) of fossil corals captured by portable iPhone LiDAR integrated with field survey data and radiocarbon analysis. Pulau Semakau is the largest field site in Singapore, with an intertidal flat extending more than 2 km long by 0.4 km wide, on which we observed 79 living and 65 fossil microatolls containing well preserved, concentric rings. In this study, we reconstruct mid-Holocene RSL using seven fossil, Diploastrea heliopora microatolls, relative to living counterparts on the island. DSMs indicate that three of these fossil corals are lower in elevation at the center with higher outer rings, indicating gradual RSL rise between ~7700 and 7500 cal yr BP. Conversely, three fossil corals are observed to decrease in elevation from the innermost to outermost rings, indicative of RSL fall between ~7350 and 7200 cal yr BP. These observations are consistent with but more well constrained than the existing sea-level curve of Singapore based on sea-level index points (SLIPs) and limiting dates from intertidal mangrove and shallow marine sediments. RSL records between ~7500 and 7350 cal yr BP are largely uncertain due to erosion of a fossil coral, and this remains as future work. The initial results reflect mid-Holocene RSL fluctuations at Pulau Semakau, from ~7700 to 7200 cal yr BP.