The development of late Holocene coastal cooling in the northern South China Sea

Paired high-resolution sea surface temperature (SST) records, based on the long-chain alkenone unsaturation index (U37K′), were reconstructed to decipher the SST evolution over the past 8000 years and its possible driving mechanisms at the Hong Kong coastal region. The two SST records, derived from a sediment core in the Pearl River Estuary and an offshore core in the northern South China Sea (SCS), show different patterns of SST changes. The coastal record shows relatively stable, warm temperatures during the mid-Holocene (∼7.4–2.6 ka), and substantial cooling (>2 °C) thereafter. The cooling was most pronounced in the past ∼0.6 ka, likely associated with the Little Ice Age (LIA). The offshore record generally reveals a warming trend from 8 ka to ∼3.5 ka and relatively warm temperatures since then. As a result, the SST difference (ΔSST) between the two sites increased over time, suggesting that the SST gradient gradually developed in the northern SCS. The declining coastal SST with the increasing ΔSST likely indicates and results from the intensified East Asian winter monsoon since ∼7 ka, especially during the late Holocene. Consistent with previous terrestrial records revealing winter monsoon variability and modern observations, ΔSST between coastal and offshore sites could thus be used as an indicator of winter monsoon variability.