Holocene sea level variations drive formation of a coral atoll in southern South China Sea

Coral reef islands are highly vulnerable habitats to global warming. The projection of future coral reef evolution is challenging due to insufficient sedimentary records. In the southern South China Sea (SCS), numerous coral islands require a thorough understanding of their Holocene depositional history to inform predictive models. This study addresses this gap by determining the depositional history of Meiji Atoll through a detailed analysis of grain size and biological components from a new borehole, Well NK-1. Our results reveal that Unit 1 and Unit 4 with abundant corals and coralline algae, larger mean grain sizes and poor sorting, were classified as sand-apron facies. Unit 2, dominated by Halimeda, exhibits finer grains and the poorest sorting, suggesting deeper, more stagnant hydrodynamic conditions and lagoon expansion. Unit 3 and Unit 5 also contain abundant corals and coralline algae as well as more broken coral branches, showing reef flat facies with water depth under 1 m. Additionally, the coral reef's vertical growth accelerated when the Pleistocene surface was submerged by rising sea level approximately 8200 years before present (yr BP). This increase in vertical accretion was primarily driven by 8.2 ka and 7.5 ka meltwater pulses. However, a sea-level decline at 4800 yr BP triggered a shift from vertical accretion to lateral expansion, significantly impacting the island's geomorphology. These findings enhance our understanding of how reef deposition responded to sea-level changes during the Holocene, improving predictive models for the future of low-lying coral islands in the South China Sea and other tropical regions.