Investigation of an Ongoing Sequence of Coral Microatoll Diedowns in Singapore (20202024?)

Coral microatolls record low water levels through diedowns, which can be used to reconstruct relative sea level (RSL). Diedowns are generally understood to be caused by instances of extreme low water. However, the causes of diedowns have not yet been fully explored. How uniform is each diedown on a single coral, and how replicable are coeval diedowns on different corals at a single site or across nearby sites? What might lead to (centimeter-scale) variability in the elevations to which different corals die down? An existing limitation on the use of coral microatolls as RSL proxies is that the main driving factors of diedowns are not fully understood. Our study aims to overcome these limitations by documenting coral diedowns in progress. Beginning in early 2020, a sequence of coral diedowns has been observed on living Porites coral microatolls in Singapore. These diedowns are expected from simple modeling of tides over the 18.61-year lunar nodal cycle, with successively lower tides each year until 2024. We are capitalizing on this window of opportunity to document coral diedowns in unprecedented detail, and to investigate factors that potentially influence these diedowns, including tide levels, insolation angle, water and air temperatures, as well as exposure time out of the water. Work is ongoing to explain variability in diedowns within and between corals. Additionally, we are testing the hypothesis that insolation angle exerts a considerable control on diedown magnitudes. When the lowest tide occurs at sunrise (April to September in Singapore), we might expect larger diedowns on the east face of the coral; when the lowest tide occurs at sunset (October to March), we might expect larger diedowns on the west face of the coral. This poster will focus on the diedown process and its driving factors in Singapore. Related contributions to this session will focus on RSL histories reconstructed from corals at several sites in Singapore as well as methodological advances to interpret RSL changes from surface morphologies of corals.