Using Digital Surface Models to Reconstruct Sea Level from Coral Microatolls

Coral microatolls are precise indicators of relative sea-level (RSL) change as their upward growth is controlled by lowest tides, hence their morphologies reflect RSL trends as the corals grew. Established field methods to study microatolls involve surveying and slabbing. Slabbing, however, involves the extraction of a vertical slab of the coral, is not permitted at many sites. Additionally, due to the short periods during which corals are exposed at low tide, coral microatoll field work must be completed within tight time constraints. This can pose a challenge for slabbing, which is time-consuming and logistically difficult.

We are developing and comparing techniques to acquire high-resolution digital surface models (DSMs) as a novel tool to study and make quantitative observations of coral microatolls. These techniques include structure-from-motion photogrammetry, terrestrial LiDAR scans, and the iPhone's recent built-in LiDAR scanner to create DSMs.

With established methods, microatoll ring elevations are typically surveyed in the field with a limited number of points on each ring, but if the microatoll morphology is misinterpreted in the field or if subtle features are missed, it may be impossible to rectify the issues later. In contrast, the DSMs objectively capture a million data points or more per coral, allowing for thorough quantitative analysis and reanalysis of the microatoll morphology. This DSM approach is inexpensive, quick, and non-invasive. However, due to its non-invasive nature, some details about the coral's growth history and internal structure must be inferred from its surface morphology, and age control on fossil corals still requires small sample cores to be drilled for radiometric dating. Nonetheless, we show that DSM-derived metrics are accurate and replicable when compared to measurements made with previously established field methods.