The surface hydrographic conditions of the South China Sea (SCS) are influenced strongly by fluctuations of the East Asian Monsoon (EAM). Long-term fluctuations of the EAM have been previously investigated through analysis of several proxy records obtained from SCS sediment cores. However, in this marginal sea, interpretation of these records is complicated by ice volume/sea level effects. Here, we have extracted east-west hydrographic gradients from two IMAGES core records in the SCS, which are proposed to indicate monsoon variability. This strategy minimizes the effect of ice volume/sea level on the monsoon signal. High-resolution records of planktonic foraminifer faunal assemblages and sea surface temperature (SST) of the last ∼ 135 kyr have been generated for two cores, taken off the Vietnam margin (MD012394) in the western SCS and near Palawan Island (MD972142) in the southeastern SCS. These core sites represent contrasting hydrographic conditions in modern monsoon climates, where the winter EAM drives cold and strong mixing conditions in the western SCS, but maintains warm and deep-thermocline conditions in the southeastern SCS. A Planktonic Foraminifera Hydrographic Index (PFHI) is established based on the abundance ratios of warm/thermocline and cold/mixing species assemblages and SST estimates from a Modern Analogue Technique (MAT). The records of the east-west gradients for PFHI (ΔPFHI) and for SST (ΔSST) between the two cores (MD012394 and MD972142) show a dominance of 23-kyr precession cycles. In the precession cycles, the ΔPFHI and ΔSST increased at the December 21 perihelion, which is associated with seasons of minimum Northern Hemisphere winter and maximum Southern Hemisphere summer insolation. Cross-spectral analysis between ΔPFHI/ΔSST and the precession cycles indicates coherent and in-phase relationships. Considering that the December 21 perihelion of the cycle of precession accorded with the time of maximum summer insolation in the Southern Hemisphere, we suggest that the SCS variability of hydrographic gradients for the last 135 kyr has been dominated by winter monsoon dynamics in response to geographic forcing controlled by the strength of the Australian summer monsoon.
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