Variations in thermocline depth and their response to the East Asian Monsoon in the Northern South China Sea during the Early-Middle Miocene: Evidence from planktonic foraminifera

This study reconstructs thermocline depth variations and their response to the East Asian Monsoon (EAM) in the northern South China Sea (SCS) during the early-middle Miocene (18–12 Ma) based on planktonic foraminifera analyses from IODP Hole U1505C. Stable oxygen isotope analyses were conducted on all planktonic foraminiferal species from four key time slices (11.99 Ma, 13.01 Ma, 14.87 Ma, and 17.35 Ma). By integrating cluster analysis results, the vertical distribution of planktonic foraminifera in the water column was reconstructed. Variations in thermocline depth were then inferred from the relative abundance ratio of shallow-dwelling to deep-dwelling species. When combined with other marine and terrestrial proxies, the upper water column evolution in the northern SCS primarily reflects EAM dynamics, showing consistency with multi-proxy records: (1) 18.0–17.2 Ma: weakening of the East Asian summer monsoon (EASM) with slight East Asian winter monsoon (EAWM) intensification; (2) 17.2–14.7 Ma: EASM peak intensity with stable EAWM conditions; (3) 14.7–13.8 Ma: EASM decline accompanied by EAWM intensification; and (4) 13.8–12.0 Ma: concurrent weakening of both EAWM and EASM. Furthermore, our synthesis suggests that the dominant forcing mechanism of the monsoon system shifted from regional tectonic control during the warm MCO to global climatic forcing during the MMCT, underscoring the differential sensitivity of the EAM to varying drivers across major climate thresholds. • Isotopic analysis revealed planktonic foraminiferal habitat depths in the northern South China Sea upper water column during the early-middle Miocene (18–12 Ma). • The thermocline depth changes record the evolution history of the East Asian monsoon during this interval. • The dominant forcing of monsoon intensity shifted from regional tectonic to global climatic control across the Middle Miocene Climatic Transition.