Orbital-scale hydroclimate variations in the southern Tibetan Plateau over the past 414,000 years

Abstract

Speleothem oxygen isotope (δ18O) constitutes one of key proxies to reconstruct the Indian and East Asian summer monsoon (ISM and EASM) hydroclimate history on a wide range of timescales. Here, we use three new well-dated speleothem δ18O records from Tianmen cave, in the south-central Tibetan Plateau (TP), together with previous records from the same cave, to reconstruct a composite Tianmen record spanning over the past 414,000 years. The Tianmen δ18O record follows Northern Hemisphere summer insolation (NHSI), broadly coherent with speleothem δ18O records from both ISM and EASM regimes. In comparison with previous model simulations and modern hydroclimate analyses, the Tianmen δ18O was interpreted primarily as a proxy indicating the ISM intensity. The Tianmen δ18O record comprises both high and low NHSI time periods, which reveals an exceptional large amplitude (∼14‰) of δ18O variations at the precession band, possibly resulted from a combination effect of precipitation amount, moisture source, transport pathway, high elevation and rainout. Besides, a broad consistency on orbital-scale variability among Tianmen δ18O, δ13C and trace element records is found, suggesting that the ISM intensity regulates the effective rainfall at the cave site as well. Notably, Tianmen speleothem growth occurred merely during warm-wet interglacial periods, even during low NHSI times near the precession maximum, and in contrast, hiatuses persistently occurred throughout cold-dry glacial periods. This pattern is similar to that found in travertine formations in the region, both suggesting that the interglacial temperature might be a prerequisite for speleothem/travertine formations in the region, while the strong ISM precipitation in turn provides a sufficient condition. Our new Tianmen record, combined with a large set of speleothem records from both Asian Westerlies and Asian monsoon domains, gives further evidence to reinforce a previous hypothesis: a consistent temporal pattern in the δ18O of precipitation on orbital and millennial scales over the vast Asian monsoon and Westerlies regimes. As such, in order to reconcile the precipitation δ18O variations between speleothem and TP ice cores, we argue that the Guliya ice-core chronology may have a basal age of ∼70 ka BP, which is supported by recent dating results from the Guliya ice cap.