Holocene variability of the East Asian summer monsoon from Chinese cave records: a re-assessment

Abstract

Oxygen isotope records from stalagmites in caves in southern China, interpreted as proxy rainfall records reflecting the intensity of the East Asian summer monsoon, indicate gradual monsoon weakening for the last ~9000 years, as also documented for the Indian monsoon. Coupled with high-precision dating, the speleothem proxy records have been used to test monsoon links with orbital forcing, solar changes, iceberg discharges in the North Atlantic, ocean currents and atmospheric methane. However, these `benchmark' cave records do not match other published, dated E Asian proxy rainfall records (specifically here, independently calibrated rainfall records from loess/palaeosol magnetic properties, and cave oxygen isotope intercomparisons), which show variable E Asian monsoon intensity through the entire Holocene. The strong correlation of the cave records with the extraregional Indian monsoon record yet their mismatch with these other dated Chinese rainfall records might be reconciled if the speleothem isotope variations reflect not changes in Holocene rainfall amount but in rainfall source. Declining Holocene influence of isotopically lighter, Indian monsoon-sourced moisture over China would have resulted in increasing proportions of isotopically heavier rainfall, sourced from the more oceanic E Asian monsoon. Individual speleothems may thus regionally record Holocene changes in Indian monsoon intensity and isotopic influence. Conversely, the other Chinese proxy records described here reflect changes in rainfall amount, and thus in E Asian summer monsoon intensity. For the Holocene, the E Asian and the Indian monsoon responses to orbital forcing are likely to have differed, specifically due to E Asian internal feedbacks and the seasonal contrasts between the two monsoon systems.