A hydroclimate regime shift around 270Â ka in the western tropical Pacific inferred from a late Quaternary n-alkane chain-length record

The inert leaf wax deposited in deep sea sediment preserves clues of the past climate change. In particular, the n-alkane distribution patterns characterized by the n-alkane average chain length index (ACL) provide useful information on the hydrological changes, as longer chain alkanes are more frequently biosynthesized under relatively arid condition to prevent water loss. Our data from the western South China Sea reveal the superimposition of a systematic regime shift in the regional mean precipitation at ca. 420-270 ka on the glacial-interglacial changes in the Quaternary. The dominance of shorter n-alkane chain length between 420 ka and 270 ka is interpreted as overall wetter conditions during this period compared to the older or younger intervals. A similar timing of hydrological regime shifts has also been observed in other paleoclimate records, including major changes in productivity in the eastern Pacific and pollen records from the western Pacific. We suggest that the changes in the regional mean hydroclimate states near 420 ka and 270 ka are most likely related to a regime change in the frequency and/or intensity of the El Niño-Southern Oscillation, with the overall wetter hydroclimate prior to 270 ka resulting in a more prevalent La Niña-like condition in the Pacific.