EVIDENCE OF MEGAHUMID CLIMATE OF THE MID-HOLOCENE IN THE WESTERN PART OF CHINESE LOESS PLATEAU

Mid-Holocene climate in the arid-semiarid China was interpreted to be megathermal (Shi et al., 1993). But, an increased wetness was speculated to be responsible for producing the "megathermal" signals (Wu et al., 1994). Here, we are presenting the stratigraphic evidence of a ubiquitously distributed sandwich layer of swamp/wetland deposits exposed in the first terrace cliffs in the western part of the Chinese Loess Plateau. The evidence shows that the mid-Holocene climate was megahumid rather than megathermal at least in the western part of the Chinese Loess Plateau. Fourteen radiocarbon dates (including two earlier dates) from different sites show that the swamp/wetland layer was formed between ~10,000 and 4,000 cal. yr. B.P. The field-observed stratigraphic characteristics and laboratory-obtained proxy data show that an unequivocal wet period occurred between ~8,000 and 6,000 cal. yr B.P. Four possible mechanisms are proposed here to account for the mid-Holocene Megahumid climate. Firstly, the insolation peak at 65 oN occurred around ~9,000-8,000 cal. yr B.P. might have warmed up the oceans to set up the stage for the mid-Holocene Megahumid climate. Secondly, the increased late summer insolation about 6,000 cal. yr B.P. in the Northern Hemisphere might have enhanced the summer precipitation. Thirdly, the shift of the long-term El Nino-like system towards the Asian side of the Pacific might have further enhanced the precipitation. Finally, the warm and wet mid-Holocene climate must have resulted in better vegetation coverage, which might have in turn induced more precipitation.