四川盆地宣汉渡口和重庆北碚下三叠统海相碳酸盐碳同位素组成与演化

doi:10.7605/gdlxb.2016.01.008

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早三叠世海水的碳同位素组成与演化是二叠纪—三叠纪生物群体灭绝事件之后全球生态萧条与重建时期地球碳循环的重要记录,为地学界长期高度关注。文中测试了四川盆地宣汉渡口和重庆北碚下三叠统及相邻地层258个海相碳酸盐样品的碳、氧同位素组成,以便探讨早三叠世海水的碳同位素组成与演化。这些样品对海水碳同位素组成具有不同的保存性,飞仙关组及嘉陵江组第一、三段保存较好,而嘉陵江组第二、四段保存较差。基于氧同位素可以更为敏感地反映海相碳酸盐成岩蚀变性的原理,在碳、氧同位素相关性分析的基础上,以δ¹⁸O<-7.5‰为剔除标准,筛选出近200个有效数据建立了碳同位素演化曲线,该曲线与前人在贵州建立的同期曲线具有很好的可对比性。曲线显示出海水碳同位素组成的巨大波动,变化幅度达8.55‰,包括2个完整的上升—下降旋回,第1个旋回由飞仙关组和嘉陵江组第一段组成,第2个旋回由嘉陵江组第二段、第三段和部分第四段组成,同时还包括若干短周期的次级旋回。利用曲线形态与前人建立的同期曲线进行了岩石地层与年代地层单位的对比,飞仙关组可大致与Induan阶对应,嘉一段可大致与Smithian亚阶对应,嘉二段、嘉三段和嘉四段下部可大致Spathian亚阶对应;绿豆岩作为下—中三叠统界线其位置可能偏高,下—中三叠统界线应该位于绿豆岩之下的嘉四段内部。早三叠世海水碳同位素组成高频率大幅度的波动指示了生物大灭绝后环境的不稳定性,其中菌藻类等微生物灾后过度繁盛,可能导致了有机碳在短时间内的快速埋藏和海水碳同位素的正漂移;而碳同位素大规模的负漂移可能与甲烷水合物释放带来的轻碳有关。研究认为,白云岩所在的地层段往往具有更高的δ¹³C值,这可能说明微生物活动、白云岩形成和海水δ¹³C值升高之间的有机联系。下三叠统的白云岩、尤其是嘉二段和嘉四段的白云岩可能是微生物成因的,嘉陵江组有机碳的生产和埋藏可能主要是与白云岩形成有关的微生物的贡献,具最高δ¹³C值的嘉二段的烃类在一定程度上可能具有自生自储的特点。

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关键词 ：下三叠统, 生态重建, 海相碳酸盐, 碳同位素, 四川盆地

Abstract：

The Early Triassic,as an interval following the mass extinction at the Permo-Triassic boundary,representing the ecosystem reconstruction and recovery after the end-Permian biotic crisis,has been garnered much attention in the geological fields. In order to approach carbon isotopic composition and evolution of Early Triassic seawater,we examined the δ¹³C value of 258 carbonate samples from the Early Triassic(including adjacent strata)from Dukou section in Xuanhan County and Beibei section in Chongqing City,Sichuan Basin. Most of the samples preserve the initial carbon isotopic composition of seawater,but the original signal in the second and fourth Members of Jialingjiang Formation is much more poorly preserved than that of Feixianguan Formation and first and third Members of Jialingjiang Formation. Based on the principle that the oxygen isotope is more sensitive to alternation during the diagenetic process,the relationship between δ¹³C and δ¹⁸O was analyzed,and 200 samples with the δ¹⁸O <-7.5‰ were selected to construct the carbon isotopic evolution curve of the Early Triassic. This isotopic evolution curve shows finely comparable patterns with the coeval curve originating from Guizhou Province. The curve presents larger fluctuations in δ¹³C value of Early Triassic seawater. There are two complete ascending and descending cycles. The cycle 1 consists of Feixianguan Formation,and the first Member of Jialingjiang Formation;while the cycle 2 consists of the second and third Members and part of the fourth Member of Jialingjiang Formation,as well as several sub-cycles. According to the pattern of the curve,the lithostratigraphic unit is correlated with chronostratigraphic unit from the coeval curve of Guizhou Province originating from literatures,and further shows that the Feixianguan Formation is in accordance with the Induan Stage,the first Member of Jialingjiang Formation is corresponding to the Smithian substage,the second and third Members and part of the fourth Member of Jialingjiang Formation would be correlated to Spathian substage. Thus,the green-bean rock,as the boundary of Middle and Lower Triassic,is probably distributed in the Middle Triassic. The fluctuations of Early Triassic carbon isotopic curve,both in frequency and amplitude,would result from the perturbations of ecologic environments. The thriving of bacteria and algae in ocean ecosystems results in high burial efficiency of organic carbon and positive excursion of δ¹³C value. The negative excursion of δ¹³C might be the consequences of the hydrates methane release. The carbon isotopic evolution shows the tendency of commonly higher δ¹³C value in dolomite beds than in limestone beds,suggesting the interrelation among activity of microorganism,formation of dolomite and elevation of δ¹³C of seawater. The formation of dolomites in the Early Triassic,especially in the second Member of Jialingjiang Formation,might be originated from activity of microorganism,also as the main source for organic carbon burial. The hydrocarbon in dolomites of the second Member of Jialingjiang Formation is characterized by “in situ reservoir” to some degrees.

Key words： State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation Institute of Sedimentary Geology Chengdu University of Technology Chengdu 610059 Sichuan

收稿日期: 2015-04-23 出版日期: 2016-02-01

ZTFLH:

P588.2

基金资助:

国家自然科学基金项目(41272130)资助

作者简介: 黄可可,女,1983年生,博士,成都理工大学沉积地质研究院讲师,从事沉积地质学的教学与科研工作。E-mail: h-k-k@yeah.net。通讯作者简介:黄思静,男,1949年生,硕士,成都理工大学沉积地质研究院教授,博士生导师,从事沉积地质学的教学与科研工作。E-mail: hsj@cdut.edu.cn。

引用本文:

黄可可,黄思静,胡作维等. 四川盆地宣汉渡口和重庆北碚下三叠统海相碳酸盐碳同位素组成与演化[J]. 古地理学报, 2016, 18(1): 101-114.

Huang Keke,Huang Sijing,Hu Zuowei et al. Carbon isotopic composition and evolution of the Lower Triassic marine carbonates from Dukou of Xuanhan and Beibei of Chongqing[J]. JOPC, 2016, 18(1): 101-114.

链接本文:

http://journal09.magtechjournal.com/gdlxb/CN/10.7605/gdlxb.2016.01.008 或 http://journal09.magtechjournal.com/gdlxb/CN/Y2016/V18/I1/101

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