摘要
早寒武世中―晚期是生命演化史上的重要节点。渝东地区寒武系龙王庙组海相碳酸盐岩地层记录了当时海水的碳同位素变化。在确保碳同位素组成有效保留海水原始信息基础上,分析发现龙王庙组61个碳酸盐岩样品的δ^(13)CV-PDB值分布在+2.7‰^-4.3‰之间;碳同位素变化曲线大体上呈先降低、后升高的变化趋势;地层中―下部δ^(13)C值波动幅度大,正、负漂移事件频发,出现可全球对比的幅度约为4‰的正漂移与幅度约为5‰的负漂移事件;地层上部δ^(13)C值具小幅波动,总体正偏。龙王庙组碳酸盐岩的碳同位素特征,受控于海平面变化、风暴沉积、生物演化引起的海洋生产力变化。龙王庙组沉积的早―中期,海平面变化频繁,陆源碎屑注入量波动性增大,风暴沉积发育,海洋环境的稳定性较差,生物种属数量逐渐降低,导致海洋原始生产力下降及碳同位素组成的频繁波动与多次负漂移事件。龙王庙组沉积的中―晚期,海平面长期缓慢上升,风暴沉积发育程度低,海洋环境趋于稳定,生物多样性增强,海洋原始生产力增大,使得碳同位素组成分布较为稳定,多表现为正值。渝东地区龙王庙组碳酸盐岩中δ^(13)C的急速负漂移可能归因于较浅水体、较充足陆源碎屑注入背景下的风暴沉积。
The middle-late Early Cambrian represents a key point in the history of life. Marine facies carbonatic rocks of the Lower Cambrian Longwangmiao Formation in east Chongqing preserve the record of coeval seawater carbon isotopic change. Based on carbon isotopic data which are preserved effectively in seawater, analysis about 61 samples of carbonatic rocks shows valid δ13 CVpDB values of ＋ 2. 7‰ to --4. 3‰, presenting a trend from decreasing to increasing. δ13C values in the middle to lower part of the formation shows big shifting, with frequent occurrences of positive and negative carbon isotope excursions, which correspond to positive （-4‰） and negative excursion （-5‰） globally. δ13C values at the upper part fluctuated slightly, but generally in a positive way. The carbon isotopic compositions of the Longwangmiao Formation varied with sea level change, tempestites and （or） biological evolutions. At the early-middle stage of the Longwangmiao Formation, frequent fluctuations of sea level and voluminous involvement of terrestrial elasts resulted in extensive development of tempestites. Worse stability of sea environment and progressive decrease of biodiversity led to decreasing of sea productivity and frequent fluctuation of carbon isotopic compositions and multiple δ13C negative excursions. During the middle-late stage of the Longwangmiao Formation, long-term slow rising of sea level and low development of storm deposition formed a stable oceanic environment and resulted in increasing biodiversity, which improved primitive productivity and caused even distribution of carbon isotope with most δ13C positive. The rapid negative excursion of δ13 C values in carbonatic rocks in the eastern Chongqing area might attribute to deposition of tempesite in a shallow water setting with relatively abundant terriestial clasts.
出处
《地质学报》
EI
CAS
CSCD
北大核心
2018年第2期359-377,共19页
Acta Geologica Sinica
基金
国家自然科学基金项目"白音查干凹陷下白垩统腾格尔组白云岩成因机理研究"(编号41072104)资助的成果
关键词
渝东地区
寒武系
龙王庙组
碳酸盐岩
碳同位素
海洋环境
East Chongqing
Cambrian
Longwangmiao Formation
Carbonates
Carbon isotope
Ocean environment
