Late Miocene-Pliocene Asian summer monsoon variability linked to both tropical Pacific temperature and Walker Circulation

Abstract

The Pliocene climate, characterized by sustained global warmth and weakened Walker Circulation, might be a potential analogue of our future climate. How Asian Summer Monsoon (ASM) evolved over this period and its controlling mechanisms remain controversial, which limits our understanding of future monsoon behavior. Here, we present alkenone- and GDGT-based records reconstructed from the northern coast of the South China Sea, interpreted to document the strength of ASM-induced upwelling/mixing effect, to infer Late Miocene-Pliocene ASM variability. Cooler local sea surface temperatures occurred between ∼5.6 million years ago (Ma) and 4.3 Ma, corresponding to Pliocene warmer conditions, effectively capturing the cooling signal induced by summer upwelling. Our multiple biomarker records consistently indicate three phases of Late Miocene-Pliocene ASM evolution: weak ASM strength at ∼6.5–5.6 Ma, substantially enhanced strength at 5.6–4.3 Ma, and reduced strength but with a slightly strengthening trend after ∼4.3 Ma. The three features have been reported collectively from previous terrestrial and marine records. The close correspondence of the ASM strength to tropical temperature before ∼4.3 Ma suggests a dominant tropical temperature control, whereas the increasing ASM strength after that, opposite to temperature changes, implies that the developed Walker Circulation started to influence ASM strength over the Late Pliocene. Hence, tropical temperature and Walker Circulation appear to have exerted competing roles in the Late Miocene-Pliocene ASM evolution.