Holocene East Asian summer monsoon records in northern China and their inconsistency with Chinese stalagmite delta O-18 records

摘要

Monsoon precipitation over China exhibits large spatial differences. It has been found that a significantly enhanced East Asian summer monsoon (EASM) is characterized by increased rainfall in northern China and by reduced rainfall in southern China, and this relationship occurs on different time scales during the Holocene. This study presents results from a diverse range of proxy paleoclimatic records from northern China where precipitation variability is traditionally considered as an EASM proxy. Our aim is to evaluate the evolution of the EASM during the Holocene and to compare it with all of the published stalagmite delta O-18 records from the Asian monsoon region in order to explore the potential mechanism(s) controlling the Chinese stalagmite delta O-18. We found that the intensity of the EASM during the Holocene recorded by the traditional EASM proxy of moisture (or precipitation) records from northern China is significantly different from the Chinese stalagmite delta O-18 records. The EASM maximum occurred during the mid-Holocene, challenging the prevailing view of an early Holocene EASM maximum mainly inferred from stalagmite delta O-18 records in eastern China. In addition, all of the well-dated Holocene stalagmite delta O-18 records, covering a broad geographical region, exhibit a remarkably similar trend of variation and are statistically well-correlated on different time scales, thus indicating a common signal. However, in contrast with the clear consistency in the delta O-18 values in all of the cave records, both instrumental and paleoclimatic records exhibit significant spatial variations in rainfall on decadal-to-centennial time scales over eastern China. In addition, both paleoclimatic records and modeling results suggest that Holocene East Asian summer monsoon precipitation reached a maximum at different periods in different regions of China. Thus the stalagmite delta O-18 records from the EASM region should not be regarded as a reliable indicator of the strength of the East Asian summer monsoon. Furthermore, modern observations indicate that the moisture for precipitation in the East Asian monsoon region is mainly derived from the Indian Ocean. The moisture transport route from the Indian Ocean to the East Asian monsoon region during the Holocene is almost identical to that of modern precipitation. Therefore the strong correlation of delta O-18 records in the EASM and Indian summer monsoon (ISM) regions, and the similarity of the pattern of latitudinal changes in delta O-18 values in stalagmites and in modern meteoric precipitation along the water vapor transport route, further suggests that the stalagmite delta O-18 records from the EASM region are essentially a signal of the isotopic composition of precipitation, which is determined mainly by changes in the delta O-18 of atmospheric vapor in the upstream source region over the Indian Ocean and Indian Monsoon region via the upstream depletion mechanism. Finally, the main trends of the stalagmite delta O-18 records are strongly correlated with the known evolution of the ISM, and therefore these records reflect the history of the ISM rather than that of the EASM.