云南早泥盆世植物稳定碳同位素特征及其古气候意义*

受到植物化石材料局限性与时代久远性的限制,目前用古生代植物化石δ¹³C值来反演古环境的研究程度较低。文中通过对云南省曲靖市下泥盆统徐家冲组和文山州下泥盆统坡松冲组的部分植物化石进行稳定碳同位素测试,分析了早泥盆世植物的光合作用途径以及古环境。结果表明: 徐家冲组镰蕨属(Drepanophycus)δ¹³C值为-29.0‰~-22.8‰,平均值为-24.5‰,其中镰蕨属茎轴的δ¹³C值高于叶片0.1‰~0.3‰;坡松冲组工蕨属(Zosterophyllum)和始叶蕨属(Eophyllophyton)δ¹³C值分别为-24.8‰~-22.1‰和-23.0‰~-22.6‰,平均值分别为-23.2‰和-22.7‰;三者的δ¹³C值分布范围都与现代C3植物一致。自徐家冲组第57至第79沉积旋回,δ¹³C值总体呈上升趋势,达到2‰左右,推测可能与早泥盆世大气CO₂浓度下降和气候变冷相关。镰蕨属和工蕨属的C_i/C_a值较为稳定,说明这2种植物可能对大气CO₂浓度变化较为敏感,曲靖市古气候可能为当今的热带疏林草原气候或热带季风气候。

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 C_i/C_a 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.