Characteristics of stable carbon isotope of the Early Devonian plants from Yunnan Province and its palaeoclimatic significance

Because of the availability of plant fossil materials in the deep past,palaeoenvironment inversion using δ13C value of Paleozoic plant fossils is relatively poorly studied. In this paper,the stable carbon isotopes of some fossil plants are analyzed from the Lower Devonian Xujiachong Formation in Qujing area and from the Posongchong Formation in Wenshan area,Yunnan Province,China. The δ13C values of Drepanophycus from the Xujiachong Formation range from-29.0‰ to -22.8‰ with an average value of-24.5‰. The δ13C values of Zosterophyllum and Eophyllophyton from the Posongchong Formation range from -24.8‰ to -22.1‰ and -23.0‰ to -22.6‰,and their average values are -23.2‰ and -22.7‰,respectively. All values fall within the distribution range of the δ13C values of modern C3 plants. The δ13C values of different organs of Drepanophycus are dissimilar,and the δ13C value of the axis is 0.1‰ to 0.3‰ higher than that of the leaf. From the 57th to the 79th sedimentary cycles of the Xujiachong Formation,the δ13C value of Drepanophycus increases by about 2‰,which is interpreted as relating to a decrease in the palaeoatmospheric CO2 concentration and to the climate cooling. The calculated Ci/Ca values of Drepanophycus and Zosterophyllum are relatively stable across the studied interval. Thus,it is inferred that these two plants are sensitive to the change of palaeoatmospheric CO2 concentration and the palaeoclimate in Qujing area may be the present tropical savanna climate or tropical monsoon climate.