Rapid oscillations of Chinese monsoon climate since the last deglaciation and its comparison with GISP2 record

Abstract

The geographic extent of the climate oscillations during the last deglaciation is an open ques- tion in the world. Here a high resolution climate record from a 16 000 C-14 a loess sequence in the n~rthwestern margin of the Chinese Loess Plateau is reported. Comparison with GISP2 record shows that all major climate oscillation (e. g. Oldest Dryas/ Boiling/ Older Dryas/ Allerod/ Younger Dryas) in the North Atlantic region also registered in the record. In addition, the sequence also contains some other distinctive strengthened winter monsoon periods. It is suggested that a coupling mechanism oper- ate between the two areas, and climate oscillations in the North Atlantic region may not be a local phe- nomenon. THE Greenland GISP2 ice core record revealed that the climate in the North Atlantic region experienced a series of short-lived (century to millennium scale), abrupt oscillations during the last deglaciation(ll. It is shown that Chinese monsoon also underwent abrupt changes during the same period(21, but their details and relationship with the high latitude North Atlantic region were still unclear. Here we present a high resolution record of the Chinese monsoon variations since the last deglaciation from a loess section in Caox- ian Village (36"33'N, 104'38'E), Gansu Province, Northern China. The 5.4 m long Caoxian (CX) section, with the top 3.2 m deposited since the last deglaciation, is located at the top of a loess platform. Samples were taken at 2-cm interval (corresponding to 70 a in average). Grain size and magnetic suscep- tibility are used as proxies of winter and summer monsoon variation, with a coarser grain size indicating enhanced winter monsoon and high magnetic susceptibility implying enhanced summer monsoon. Seven radiocarbon-14 dates on organic matter were obtained and calibrated, which enable us to establish a tem- poral series (figure 1 (a) ) . The climate proxies in CX section indicate a series of abrupt winter/summer monsoon oscillations since the last glacial termination. Comparison of grain size record of winter monsoon changes with GISP2 oxygen isotope record of climate oscillations shows that these two records bear great similarity during the last deglaciation. For example, the four enhanced winter monsoon events (W1 to W4 in fig. 1) docu- mented at CX section correlate well with the well-known four cold climate regimes during the last glacial termination, i. e. the Oldest Dryas, the older Dryas, IACP (the Inter-Allerod Cold Period)(31 and the Younger Dryas. Moreover, the four climate events in both records also show similarities in the amplitude, duration and abruptness of change. Several periods of stronger summer monsoon were also evident in the magnetic susceptibility curve during the last deglaciation, but their patterns and amplitudes are different from winter monsoon changes. Their correlation with GISP2 record are somewhat complicated (fig. 1). During the early Holocene, shortly after its abrupt reduction at the beginning of the Holocene, the winter monsoon intensity greatly increased ( W5 in fig. 1 ), this may document the cold period following the Younger Dryas in a number of the Greenland ice cores and in the North America lake records(41. After this, the winter monsoon intensity gradually decreased and the summer monsoon gradually increased. During the middle Holocene, especially at ca. 6.0 to 2.7 ka (14c), the intensity of the Chinese summer monsoon reached climax, while the winter monsoon strength was the weakest since the last termination, the so-called Holocene climate optimum period in China. This pattern of climate variations differs greatly from that in the Greenland. At ca. 2.7 ka (14c) both the winter and summer monsoon showed rapid changes-the winter monsoon strength increased and the summer monsoon strength declined. Howev- er, we do not know how long this change lasted because the upper 20 cm of the CX section was strongly influenced by human cultivation. Besides, our record indicates that the winter monsoon changed earlier than summer monsoon at least since the beginning of the Holocene. This may imply that winter monsoon