扬子地区中—晚奥陶世转折期的碳同位素漂移事件及其成因探讨

Carbon Isotope Excursions Near the Middle-Late Ordovician Transition in the Yangtze Area and Their Possible Genesis

中—晚奥陶世转折期的两次全球性碳同位素正漂——Middle Darriwilian Inorganic Carbon Excursion(MDICE)和Guttenberg Inorganic Carbon Excursion(GICE)——在发生时间上与生物辐射、全球大规模海侵和地球磁极的倒转等重大地质事件,以及海水锶、氧、硫同位素的显著变化相吻合,说明它们可能受相同形成机制的控制。在报道了扬子地区陈家河剖面中—晚奥陶世界线附近的这两次碳同位素正漂事件,并与塔里木盆地进行对比之后发现,扬子地区和塔里木盆地的MDICE和GICE与海水锶同位素比值的快速剧烈下降出现在同一层位,指示这两次碳同位素的正漂的形成可能和当时洋中脊热液活动的加剧有关。海底热液系统能够通过向海水中注入铁等生命必需元素,刺激海洋生物的繁盛,提高海洋的生产力和有机碳的埋藏,进一步引起了MDICE和GICE这两次碳同位素正漂。

Two global positive carbon isotope excursions near the Middle to Late Ordovician transition—the Middle Darriwilian Inorganic Carbon Excursion（ MDICE） and the Guttenberg Inorganic Carbon Excursion（ GICE） ——have been documented approximately in parallel with the major pulses of Great Ordovician Biodiversification Events（ GOBE）,great global sea level rises,geomagnetic reversals and significant variations of strontium,oxygen,sulfur isotopes in the ocean,indicating a possible causal link. Here,we reported the MDICE and GICE at Chenjiahe section in Yangtze area and compared them with their equivalents in Tarim Basin. Both the MDICE and GICE are found at the same stratigraphic level of rapid decrease of oceanic strontium isotope ratios in Yangtze Block and Tarim Basin,suggesting that the two carbon isotope excursions may be related to the enhanced hydrothermal activities at the middle ocean ridges,which could have increased releasing of essential nutrient elements such as iron into the seawater; this may have stimulated the biological blooms and increased oceanic primary productivity and organic burial,further inducing the positive carbon isotope excursions,i.e.,the MDICE and GICE.