Early diagenesis of coral rubble on the high-energy platform of Ximen Reef in the southern South China Sea

Abstract

Coral rubble is extensively distributed across high-energy coral reef platforms in the southern South China Sea (SCS), with certain areas forming unvegetated shingle cays. The rubble has undergone significant early diagenesis during its formation and accumulation; however, the specific early diagenetic processes affecting the rubble remain underexplored. Ximen Reef, a representative high-energy reef platform in the southern SCS, is characterized by a broad reef flat and a fully exposed unvegetated shingle cay predominantly composed of coral rubble. Coral rubble samples collected from both the reef flat and the cay of Ximen Reef were analyzed for chronology, mineralogy, petrography, and geochemistry to elucidate their early diagenetic characteristics. The rubble consists mainly of unconsolidated contemporary coral skeletal fragments and has undergone carbonate cementation, a key process in early diagenesis. The cementation is primarily characterized by the precipitation of secondary acicular aragonite and micritic high-Mg calcite. Additionally, other diagenetic processes, including bioerosion, encrustation, and internal deposition, have also occurred in the rubble. Early diagenesis has induced notable geochemical alterations in the rubble, with high δ¹³C, δ¹⁸O and Mg/Ca ratios, as well as low Sr/Ca ratios, compared to living corals. Both mineralogically and geochemically, the rubble on the reef flat and cay reflects a typical marine-meteoric mixing diagenetic environment. The early diagenesis of the rubble at Ximen Reef is relatively weak and remains in its initial stages, primarily due to the inherent instability of the rubble, which is influenced by high-energy hydrodynamic conditions. This may adversely impact the consolidation and development of coral reefs and islands in the southern SCS.