Quaternary evolution, carbonate export and controlling factors of a drowning carbonate platform in the South China Sea

Carbonate platforms represent important end-member source-to-sink systems, yet their sediment production, export pathways and sinks under glacial–interglacial sea-level fluctuations remain insufficiently constrained. The Zhongsha Atoll, the largest ring-shaped atoll in the South China Sea, preserves relatively intact Quaternary reef–bank systems and provides an important archive for investigating carbonate platform evolution. This study examines the Quaternary stratigraphic evolution and carbonate export of a drowning carbonate platform based on a lagoon drilling core from Zhongsha Atoll. A sequence stratigraphic framework is established, and the responses of major geochemical proxies across sequence boundaries are evaluated. Limited U–Th and Sr isotope ages are used as absolute chronological constraints.The lagoon core can be subdivided into 11 depositional sequences corresponding to interglacial stages from MIS 1 to MIS 23. Elemental geochemical results show that mean concentrations of Sr, U and Na decrease stepwise with depth between successive reef–shoal sequences. Abrupt downward shifts in these elemental contents coincide with exposure surfaces, suggesting that such geochemical features may serve as indicators of sequence boundaries in Quaternary carbonate platform systems. Facies analysis indicates that lagoonal deposits dominate lowstand systems tracts, whereas progradational reef deposits prevail during highstand systems tracts.From a glacial–interglacial perspective, the results highlight the role of sea-level amplitude, rise rate and highstand duration in controlling reef accretion and carbonate export from the platform. Integration with regional exploration seismic data provides preliminary constraints on carbonate delivery to adjacent deep-sea areas. These findings improve the understanding of Quaternary carbonate platform evolution and offer insights into the future response of reef systems in the South China Sea to sea-level change.