ABSTRACT                        

The middle Miocene climatic optimum (MMCO) to climate transition (MMCT) is a critical Cenozoic turning point, yet its imprint on shallow-water carbonate platforms remains poorly understood. We conducted systematic geochemical analyses (major/trace elements, δ13C, 87Sr/86Sr) on 13.5–18 Ma carbonates from Well CK2 in the Xisha Islands, South China Sea, to reconstruct the paleoenvironmental evolution of an isolated carbonate platform. Using Al, Ti, Zr to trace terrigenous input, Ni, Cu, δ13C for marine productivity, and Mn for redox conditions, we identify three evolutionary stages. During 18–17.42 Ma, a weak East Asian summer monsoon (EASM) limited source-area weathering and erosion, maintaining a stable, oligotrophic, and oxic environment with minimal terrigenous input. From 17.42 to 14.87 Ma, intensified EASM precipitation under warm MMCO conditions enhanced chemical weathering and physical erosion, increasing terrigenous detrital flux, as corroborated by elevated 87Sr/86Sr. The synergy of high sea levels and monsoon-driven upwelling significantly boosted primary productivity. Subsequent organic matter oxygen consumption triggered localized suboxic conditions. Between 14.87 and 13.5 Ma, EASM weakening reduced weathering and erosion, limiting terrigenous material supply. Aeolian sorting during long-distance transport kept Al high while Ti and Zr declined. Concurrently, sealevel fall and weakened upwelling caused a sharp productivity drop. Notably, Mn enrichment persisted, indicating the dominant driver of suboxia shifted from productivity-induced oxygen consumption to restricted water circulation caused by sea-level fall. This study demonstrates that high-resolution geochemical records from isolated platforms effectively capture the coupled responses of terrestrial input, marine productivity, and redox states to middle Miocene monsoon and climate forcing.