Variability of Coral Microatoll Growth Rates in Singapore and its Implications on Sea-level Reconstructions

Coral microatolls are precise proxies for tracking relative sea-level changes in the Holocene. Infrequent extreme low tides limit the height to which microatolls grow, but in between those lowest tides, a coral's growth rate limits the extent to which the coral can grow up from below its highest level of survival and influences the overall morphology of the microatoll. Coral growth rates must therefore be determined in order to use the microatoll morphologies to reconstruct sea-level histories.

In this study we use novel three-dimensional (3D) photogrammetric techniques to non-invasively capture the morphology of coral microatolls. We introduce a workflow using CloudCompare (open-source software) to process these 3D models and determine microatoll growth rates. This methodology is then applied on several living Porites microatolls, all found within ~70 m of one another on St. John's Island, Singapore, and therefore likely to have similar environmental conditions and experienced identical sea-level histories. We are generating multiple 3D models of the microatolls documented during site visits between December 2020 and August 2022. We are quantifying growth rates for each period between survey days. We will analyze the extent of inter-coral growth variability, as well as growth variability across different periods of the year. Based on preliminary results, the high variability in both parameters highlights a need for a deeper understanding of microatoll growth, if they are to be accurately used for sea-level reconstructions.