4000aB.P.前后降温事件与中华文明的诞生

4000aB.P.前后降温事件是世界上许多地区全新世人类演化史上的重要转折点之一,也是导致古埃及、两河流域和印度河流域等地区世界古文明衰落的主要原因.在中国,该降温事件导致了中原周围地区五大新石器文化的衰落和终结,但却加速和促进了中原地区以夏朝建立为标志的中华文明的诞生.

4 000aB. P. EVENT AND ITS IMPLICATIONS FOR THE ORIGIN OF ANCIENT CHINESE CIVILIZATION

Carbon isotopic composition of pedogenic carbonate can be used as an indicator of paleoenvironmental change. Carbon isotopes yield paleoenvironmental information because carbon in soil carbonate forms in isotopic equilibrium with local soil CO 2, whose δ 13 C is determined by local plant cover. Plants can be grouped into two major groups which have different isotop ic values. The C 3 and C 4 plants use different photosynthetic pathways and have very different δ 13 C values averaging about -27‰ and -13‰, respectively. C 3 plants include nearly all trees, most shrubs, and cool season grasses. C 4 plants include some shrubs and warm season grasses. In this paper, carbon isotopic composition of soil carbonate in Weinan, Jixian, Changwu and Huining during last interglacial (S 1) and last glacial (L 1) was analyzed to examine the main factor for the variations of δ 13 C values of soil carbonate formed in different climatic conditions and its implications on paleoenvironmental reconstruction in the red earth sequence at Xifeng.Results from four different sites have shown that S 1 is characterized by more negative δ 13 C values of carbonate while L 1 by more positive δ 13 C values, implying that C 3 plants dominated on the Loess Plateau during last interglacial (S 1), and C 4 plants favored the climatic condition during last glacial (L 1). The humidity is most likely the main factor influencing δ 13 C values of soil carbonate in the Loess Plateau.Carbon isotopic composition of pedogenic carbonate was analyzed on paleosols of a red earth sequence at Xifeng. An increase in δ 13 C values occurs around 4.0 Ma B.P., implying increase of C 4 plants biomass. The fact that this C 4 vegetation event occurred simultaneously in the Loess Plateau, but different from the C 4 expansion in Pakistan and other localities in terms of timing and nature, may suggest that local climatic changes were main factors for driving the expansion of C 4 vegetation. We argued that the Pliocene expansion of C 4 plants in northern China might have been caused by increased aridity, which in turn might be related to the uplift of Tibetan Plateau.