北京西山下苇甸第三统/芙蓉统界线附近碳酸盐岩地球化学特征及古环境意义

Geochemistry of Carbonates Near the Cambrian Series 3-Furongian Boundary and Its Paleoenvironmental Constraints

芙蓉世早期全球发生的生物绝灭事件与该时期古环境的变化可能存在着密切联系,但未有定论;相对于深水相区,浅水台地沉积物地球化学特征对古环境的变化有着更敏感的响应,有利于深化这一问题的理解。选取北京西山下苇甸剖面第三统崮山组和芙蓉统长山组碳酸盐岩沉积为研究对象,通过元素地球化学和同位素的分析,重建这一时期古环境的变迁过程,并探讨其与生命演化的关系。结果表明:崮山组和长山组沉积更替时期,古气候和古海洋环境有大的波动。海水由海平面低点快速海侵,水体还原性急剧增强,海水温度降至最低点,盐度升高至最高点,可能这是三叶虫生态崩溃及其灭绝的重要原因。长山组早期干热气候和温湿气候交替频繁,大规模海侵促使陆源碎屑物质输入的增加,海水中营养元素的富集促进了生物勃发;同时深水的还原性条件有利于有机质的保存,最终导致碳酸盐岩碳同位素的正漂。研究还表明,这次碳同位素正漂的发生与长山中晚期海平面的下降没有明显的成因联系。

The early Furongian mass extinction was likely related to paleoenvironmental changes, yet their relations remain unconstrained. Geochemical signals of shallow-water platform sediments are more sensitive to paleoenvironmental changes than deep-water sediments, which is conducive to understanding this issue. Carbonates in the Cambrian Series 3 Gushan Formation and the Furongian Changshan Formation in the Xiaweidian section of the Xishan(Beijing) were selected for element geochemical and isotopic analyses. The aim is to trace the paleoenvironmental evolution during the Kushanian-Changshanian transition, and to assess its effects on life evolution. The results reveal a major fluctuation in the paleoclimate and paleo-marine environment during the transition period. The sea-level lowstand at the end of Kushanian Stage was followed by rapid transgression in the early Changshanian Stage, during which seawater may have become significantly reducing. These changes, combining with the seawater temperature drop(to minimum) and salinity rise(to maximum), could explain the collapse of trilobite ecology and its subsequent extinction. During this period, the frequent alternation of dry-hot and warm-humid climate and the large-scale transgression may have facilitated the terrigenous detrital(and nutrients) input into the seawater, which promoted biological growth. The presence of deep reducing water is favorable for organic matter preservation, which eventually led to the Positive Carbon Isotope Excursion in the carbonates. We propose that strong correlation is unlikely between the Positive Carbon Isotope Excursion and the Middle to Late Changshanian sea-level drop.