Variation of stable isotopic composition and relationship to climatic changes around 2.5 Ma B.P. on Fengyi Section, Lishi County of Shanxi Province

摘要

The 2.5 Ma B.P. was important in the history of the earth, around which global environment changed greatly. It is discovered recently that isotopic composition of carbonate in the loess and the red clay at the Fengyi Section changes obviously and canbe used as an indicator of climatic changes. The section we studied is located in the Fengyi Town, Lishi County of North China's Shanxi Province. It is 123 m thick, with Quaternary loess in the upper part, the Pliocene red clay in the lower and a lithologic transitional zone between them, The transitional zone can be divided into 3 layers without distinct boundary. The upper layer, 20 cm thick, is loess containing red clay glaebules; the middle layer, 60 cm in thickness, is red clay containing loess glaebules; and the lower layer is red clay containing fewer loess glaebules with a thickness of 120 cm. The boundary between the middle and the lower layers coincides with the M/Ga boundary of the palaeomagnetic polarity, We have obtained 49 samples with intervals of 10--50cm in the loess, 10--40 cm in the transitional zone, and 40--100 cm in the red clay in the part of the section 16m above and below the M/Ga boundary. Analyses of the composition of stable oxygen and carbon isotopes of CaCO_3 in the samples show that δ~(18)O and δ~(13)C values change greatly around the M/Ga boundary.