浙江煤山二叠-三叠系界线剖面有机和无机碳同位素变化与古环境

Organic and inorganic carbon-isotope shift and paleoenvironment at the P-T boundary section in Meishan,Zhejiang Province

浙江长兴煤山D剖面196个碳酸盐岩样品无机和有机碳同位素δ13Ccarb和δ13Corg变化趋势表明,煤山地区晚二叠世末为比较封闭的局限海沉积环境,早三叠世初期经局限和开阔海环境交替出现渐变为海水循环良好的广海沉积环境。长兴期的海进作用使海洋初始生产率升高,可能是引起长兴组葆青段碳同位素变重的主要原因;海洋CO2浓度增高可能是长兴组煤山段δ13Corg出现负漂移的原因。二叠纪末期全球性海平面下降以及火山作用可能是使生态系统崩溃、引起二叠-三叠系(P-T)界线附近生物绝灭的外部原因,也是使海洋中12C输入量增加导致过渡层碳同位素变轻的主要原因。二叠纪晚期生物绝灭经历了由渐变到突变的过程。δ13Corg在生物绝灭后才出现大幅度负漂移,可能是由于营光合作用的浮游植物不一定因海洋缺氧以及其他生物的绝灭而停止生长,当海水酸度超过浮游生物忍耐限度时,才导致浮游生物大量死亡。殷坑组δ13Corg和δ13Ccarb呈上升趋势,说明经历过P-T生物大绝灭后,早三叠世海洋生产率逐渐升高,生物开始复苏。

The variations of ?13Ccarb and ?13Corg of 196 carbonate samples fr om Meishan section D, Zhejiang Province, show that the depositional environments might change from restricted sea during Late Permian and P T transitional peri od, through an alternate restricted and open sea, to the open sea in Early Trias sic. The carbon isotopic fractionation in Baoqing Member of Changxing Formation is probably caused by the increase of primary productivity owing to the transgre ssion. However, the higher concentration of dissolved CO2 in surface water migh t cause in the negative excursion of ?13Corg in Meishan Member of Changxing For mation. The mass extinctions at the P T boundary were resulted from the global sea level drop and volcanic activity in Late Permian, of which the changes might be from gradual to sudden. The maximum negative excursion of ?13Corg occurred after the mass extinction indicates that the photosynthesis planktons continued to grow up not due to the anoxia of ocean and/or the death of other organisms. T he planktons would die only at that time when they were driven beyond the forbea rance to the acidity of seawater. The rising of ?13Corg and ?13Ccarb in Yinken g Formation implies that the primary productivity in surface water increased and the organisms started to be recovery after the P T mass extinction.