Late Holocene Relative Sea-Level Changes from Coral Microatolls in Sentosa, Singapore

Holocene relative sea-levels in the far-field location of Singapore are sensitive to ice-equivalent sea-level changes and provide an independent dataset against which glacial isostatic adjustment models can be validated. However, the existing Singapore sea-level record is temporally incomplete, with a lack of data points in the mid to late Holocene. We present new constraints for the late Holocene from in-situ fossil corals (Diploastrea heliopora, Porites sp.) at Sentosa, Singapore.

Coral microatolls track sea level as their upward growth is restricted by extreme low water. Changes in the elevations of successive concentric rings within a microatoll provide morphological expressions of sea-level tendencies, indicating if relative sea levels were stable, rising or falling throughout the coral's lifetime.

We determined relative sea level by surveying the elevations of fossil and living corals. To quantify the vertical uncertainty in our sea-level reconstructions, we surveyed the indicative range of living coral microatolls in relation to tidal datums, validated against nearly two years of continuous tidal measurements at the site (from mid-2020 to mid-2022). Modern Porites sp. at the site were found between the lowest observed tide and mean low water spring tide level. As living Diploastrea microatolls were absent at Sentosa, we relied on the difference in elevations between living Diploastrea and Porites sp. microatolls that we surveyed at the nearby Kusu and Semakau Islands. Elevation profiles from 3D models of the best-preserved microatolls were used to complement our inferences of relative sea-level change. We show that relative sea level in Singapore has been close to present-day levels for the past ~3.2 ka. The calibrated radiocarbon dates suggest a possible relative sea-level fluctuation at ~2 ka that is independently supported by the coral microatoll morphologies.