Abstract: Carbonate-associated sulfate (CAS) is a key proxy for reconstructing the isotopic evolution of seawater sulfate and tracing global sulfur cycling through geologic time. However, the fidelity of primary CAS isotopic signals in reef limestones subjected to complex diagenetic alteration remains uncertain. We present a systematic analysis of CAS abundance, sulfur isotopes (δ³⁴S_CAS), and oxygen isotopes (δ¹⁸O_CAS) in a continuous reef limestone core (NK-1) from Meiji Atoll in the southern South China Sea to evaluate the impact of diagenesis on CAS records. Our results show that early diagenesis strongly controls CAS concentrations: primary aragonite-high-Mg calcite phases contain up to 5413 ppm CAS, whereas freshwater and marine diagenetic alteration reduces CAS by 60-80%, with values dropping below 200 ppm at subaerial exposure surfaces due to meteoric leaching. Despite this substantial loss of sulfate, δ ³⁴S_CAS remains remarkably stable throughout the core (20.1‰-24.7‰; mean = 22.9‰) and aligns closely with coeval marine barite δ³⁴S, demonstrating that the original seawater sulfur isotope signature is robustly preserved even through intense recrystallization and dolomitization. In contrast, δ ¹⁸O_CAS exhibits large variability (7.1‰-15.0‰), indicating high susceptibility to diagenetic overprint. Only samples younger than ~5 Ma retain primary seawater-like δ¹⁸O_CAS values; older samples have been significantly modified by oxygen isotope exchange with diagenetic fluids. This study confirms that CAS in reef limestones provides a reliable archive of past seawater δ ³⁴S, but cautions against the uncritical use of δ ¹⁸O_CAS for paleoenvironmental reconstruction without rigorous diagenetic screening. These findings offer critical constraints for high-resolution reconstructions of tropical marine sulfur cycling using reefal carbonate successions.

Key words: Carbonate-associated sulfate, Sulfur and oxygen isotopes, Diagenesis, Reef limestone, South China Sea