晚中新世植被变更与光合作用演化

光合作用酶Rubisco出现于太古代，对于以后地质时期里大气的CO2降低并不适应。晚中新世大幅度扩展的C4植物，就是光合作用演化的一种途径，适应于CO2浓度较低的大气，也适应于温暖而季节性干旱的季风气候。C4与C3植物碳同位素的重大差异，又为利用古土壤和哺乳类牙齿珐琅质的δ13C分辨C3、C4植物在植被中的比例提供了条件。自从发现巴基斯坦晚中新世古土壤层δ13C突变以来的10余年，围绕着C4植物扩展究竟反映季风气候，CO2浓度下降，还是干旱化，国内外学术界展开了热烈的讨论，至今尚属未解之谜，但从中可以吸取研究地球系统演变的经验教训。

VEGETATION CHANGE IN LATE MIOCENE AND EVOLUTION OF PHOTOSYNTHESIS

Rubisco, an ancient photosynthetic enzyme of Archean origin, became increasingly inefficient with reduction of atmospheric CO2 levels in the later geological history. The remarkable expansion of C4 biomass in late Miocene was an adaptive evolution in photosynthesis, as C4 photosynthesis was more adaptive to the lower concentration of CO2, as well as to monsoonal climate with warmer temperature and seasonal aridity. Given the significant difference in carbon isotope between C3 and C4 plants, the δ13C of paleosols and mammalian tooth enamels was widely used as an indicator of proportions of C3 to C4 plants in vegetation. Since the discovery of the dramatic shift of δ13C in the late Miocene paleosol from Pakistan over 10 years ago, the expansion of C4 biomass then has become a matter of hot debate within the scientific community: whether it was caused by monsoon climate, decreasing atmospheric CO2 level, or aridification？Though it remains an enigma up to now, there are useful lessons to be drawn from a brief review of the story as presented in this paper.