苏皖地区石炭纪海相碳酸盐岩碳和氧同位素演化规律

Evolution regularities of carbon and oxygen isotopes in Carboniferous marine carbonate rocks from Jiangsu and Anhui provinces

对江苏南京孔山和安徽巢湖凤凰山石炭系碳酸盐岩C、O稳定同位素进行了测定。结果表明,苏皖地区石炭纪δ13C、δ18O记录是以4个同位素演化阶段为特征,前3个同位素演化阶段与同时代的北美石炭纪相似,仅仅是内部演化特征略有不同,这反映了区域性的沉积环境的不同变化。在金陵组下部、和州组下部、黄龙组中下部和船山组层段,δ13C表现出明显的正漂移,这可能是受植物和生物量增长、有机碳储藏量增加所引起的。δ18O记录总体上与δ13C值呈相似的变化趋势。δ18O增加记录了气候变冷和冰川作用的结果,δ18O负漂移与气候变暖和冰川消融是一致的。在浅水潮坪环境,大气淡水的淋滤作用使δ13C和δ18O值明显降低。

The carbon and oxygen isotope compositions of Carboniferous marine carbonate rocks from Jiangsu and Anhui provinces are analyzed. The results show that the evolutions of Carboniferous δ13C records in these areas can be divided into four isotopic stages, C1, C2, C3 and C4. Positive shifts of δ13C valueshappened during early Jinling, early Hezhou, early and middle Huanglong, and Chuanshan times. This shift may be caused by the increase of the terrestrial plants and organism, and the storage of organic carbon during Carboniferous. The earlier three isotopic evolution stages are similar to the coeval Carboniferous in North America, but a little difference of isotopes evolution in every stage reflects the difference of territorial sedimentary environment. The earliest C1 stagehas high positive δ13C values in Jinling time; the values of δ13C fluctuatebetween +1.76‰and +4.46‰, and decrease evidently in Gaolishan time. The high positive values of δ13C in Jinling time can be comparable with the positive δ13C peaks of North America and Western Europe in the Tournaisian. They are one of the largest known Phanerozoic δ13C values, and may have global significance. Global high δ13C values are interpreted as evidence of enhanced storage of organic carbon and lower atmospheric CO2 levels. Stage C1 ends with a 3‰increase between late Gaolishan and early Hezhou times (C1 C2 transition). Then, δ13C values decrease into negative values field (almost between -1.40‰and -2.81‰) during late Hezhou and Laohudong times, and are the minimum values during Middle Carboniferous. At the end of Stage C2, there is a positive δ13C values shift (4.27‰) between late Laohudong and early Huanglong times (C2 C3 transition). Then, δ13C values sharply decrease into the negative valuesfield in late Huanglong time. The δ13C values obviously increase and become positive values during C3 C4 transition, then keep relative stable during stageC4, δ13C values range from +1.09‰to +3.33‰mostly. The δ18O recordcan also be separated into the four stages which is general consistent with δ13C record. This resulted in cooling and glaciation recorded as increased carbonate δ18O, and warming and deglaciation recorded as δ18O value shifts towardsrelatively higher negative values. Meteoric fresh waters eluviations resulted in the decrease of δ13C and δ18O values in shallow tide environment, and the salinities, Z values, which is calculated using δ13C and δ18O, are all less than 120.