Low latitude hydro-climatic changes during the Plio-Pleistocene: evidence from high resolution alkane records in the southern South China Sea

High resolution records of long chain n-alkane biomarkers from the southern South China Sea reveal tightly response of n-alkane distribution to hydro-climate changes over the past 5 Ma, with increasing longer chain n-C31 alkanes indicating a correlation with drier conditions and increasing shorter chain n-C27 alkanes with wetter conditions. The variations of the C31/C27 ratio, or the alkane chain length index, imply humid conditions before 2.9 Ma, progressively reduced moisture since then and to bigger fluctuations between wet and dry conditions since 1.2 Ma. This long term hydro-climate trend is superimposed by glacial dry and interglacial wet patterns over the Plio-Pleistocene glacial cycles. Combined with other proxy records, our results indicate that precipitation over the tropical Asia-Pacific strengthened before the onset of the northern hemisphere glaciation and the mid-Pleistocene climate transition at about 1.2 Ma. These dramatic humidity changes over major climate transitions imply a crucial role of tropical hydrology dynamics on global climate change in the late Cenozoic.