204 / 2022-02-28 22:34:41

晚三叠世卡尼期湿润暮：来自黔西南地区沉积学和地球化学证据

黔西南 CPE 竹杆坡段 瓦窑段 陆源碎屑

卡尼期湿润幕（Carnian Pluvial Episode, CPE）作为近年来三叠纪古气候研究前沿与热点，是继二叠纪-三叠纪之交全球气候显著变化后又一次重要的气候变化转折期。此次事件最显著的标志为短时内降雨量增加；陆源碎屑大量输入引发环特提斯域碳酸盐岩工厂危机。同时，CPE还伴随着全球气候变暖，碳同位素幕次负偏，水体缺氧及生物的灭绝与演替等诸多地质事件。

位于特提斯东缘的黔西南地区，卡尼阶海相地层出露较完全，且沉积类型多样，分异明显，保存有丰富的海洋生物化石，是开展卡尼期气候变化与生态响应协同研究的理想场所。本文对区域内关岭永宁剖面三叠系法郎组进行了系统沉积学研究；并对CPE界线处（瓦窑段-竹杆坡段边界）附近16m进行高精度地球化学测试。结果显示：研究区竹竿坡段顶部为中层含生屑泥晶灰岩，瓦窑段底部为薄层灰黑色泥灰岩与钙质页岩互层，至瓦窑段下部15m处变为黑色钙质页岩夹泥灰岩，至赖石科组灰岩沉积消失，变为砂岩沉积。通过对比贞丰龙场剖面（Sun et al.，2016），永宁剖面CPE起始时期为竹杆坡段与瓦窑段界线处，岩性上的逐渐变化指示碳酸盐台地沉积逐渐消亡被陆源碎屑沉积取代；竹杆坡段正常海相沉积泥晶灰岩中碳同位素值为2.03‰-3.54‰，平均2.58‰，而覆盖之上的瓦窑段泥灰岩、钙质页岩中碳同位素值为0.86‰-0.92‰，平均0.89‰，两者界线处碳同位素发生了较为明显的负偏移，其变化与西特提斯（奥地利）Lunz-Polzberg剖面、（意大利）Dolomites剖面、（匈牙利）Transdanubian Range剖面有一定相似性，即从碳酸盐岩到黑色页岩，碳同位素值有变小、更趋于负值的趋势。表明大量亏损¹³C的碳元素被注入到表层地球系统中，使得大气中二氧化碳浓度增加，导致地表温度升高，海洋表层水体蒸发量也随之增加。在季风气候影响的地区，由于陆地海洋之间比热容的巨大差异，水文活动的增强引发大规模的降雨。相对应地，竹杆坡段与瓦窑段界线处，多种氧化还原替代指标发生明显波动，瓦窑段底部8m灰岩中U/Th均值1.42，V/Cr均值2.46，V/(V+Ni)均值0.624，与下伏竹杆坡段相比较，水体环境由富氧向贫氧-缺氧环境转变；此外，研究区竹杆坡段上部至瓦窑段下部，受到水体环境贫氧的影响，遗迹化石丰度和分异度快速降低，底栖生物明显减少。

全球性的卡尼期湿润幕是三叠纪最为显著的气候变化事件（Preto et al.，2010），也是中生代最大的灭绝事件之一（Dal Corso et al., 2020），然而其研究主要集中在西特提斯地区和美国西南部等区域，国内有关CPE研究还十分有限。国内CPE事件的发生次序和发展过程并不清晰，卡尼期地层年代框架还需要进一步厘清，岩相古地理、生物地层学和沉积学的研究等仍有待进一步开展。特提斯东缘地区CPE研究，有助于深刻认识全球古气候变化，为气候变化与生态响应协同研究提供更多中国的对比方案。



As the frontier and hotspot of Triassic paleoclimate research in recent years, Carnian pluvial episode (CPE) is another important turning period of climate change after the significant global climate change at the turn of the Permian-Triassic transition. The most significant sign of this event was an increase of rainfall in a short time. The massive input of terrigenous debris caused the carbonate plant crisis in the Circumtethys domain. CPE is also accompanied by many geological events, such as global warming, carbon isotope episodic negative deviation, water hypoxia and biological extinction and succession.

Located in the eastern margin of The Tethys, southwest Guizhou, the Carnian Marine strata are completely exposed, with diversified sedimentary types and obvious differentiation, and abundant Marine fossils are preserved. It is an ideal place to carry out collaborative research on the climate change and ecological response of the Carnian. The Carnian Marine strata are completely exposed, sedimentary types are diverse and distinct, and abundant biological fossils are preserved. It is an ideal place for the study of Carnian extreme climate change and ecological response process. The systematic sedimentology of the Falang Formation in the Guanling Yongning section was studied in this paper. A high-precision geochemical test was carried out at 16m near the boundary of CPE (Wayao member-Zhuganpo member boundary). The results showed that the top of Zhuganpo member in the study area was the middle beded bioclastic wackestones, and the bottom of Wayao member was the interbedding of thinly beded gray black packstones and calcareous shales. It became black calcareous shales with interbeds of packstones at 15m below Wayao member, and turned into sandstone of Laishike Formation after the limestone deposition disappeared. By comparing the Longchang section of Zhenfeng (Sun et al., 2016), the CPE of Yongning section started at the boundary between Zhuganpo member and Wayao member, and the gradual change of lithology indicated that the carbonate platform deposition gradually disappeared and was replaced by terrigenous detrital deposition; The carbon isotope values of the normal Marine sedimentary micritic limestones in Zhuganpo member were 2.03‰-3.54‰, with an average of 2.58‰, while the carbon isotope values of the packstones and calcareous mudstones in Wayao member were 0.86‰-0.92‰, with an average of 0.89‰. The carbon isotope values at the boundary between the two were obviously negative migration. It was similar to the Lunz-Polzberg profile (Austria), the Dolomites profile (Italy) and the Transdanubian Range profile (Hungary), that was, from carbonate rock to black shale, the carbon isotope values tended to be smaller and more negative. It indicated that massive ¹³C-depleted carbon was injected into the earth surface system, the concentration of carbon dioxide in the atmosphere increased, resulting in higher surface temperature and the evaporation of marine surface water. In areas affected by monsoon climate, due to the great difference of specific heat capacities between land and ocean, the enhancement of hydrological activities led to massive rainfall. Correspondingly, a variety of redox substitution indexes fluctuated obviously at the boundary between Zhuganpo member and Wayao member. The average value of U/Th, V/Cr and V/(V+Ni) in the 8m limestone at the bottom of Wayao member were 1.42, 2.46 and 0.624 respectively. Compared with the underlying Zhuganpo member, the water environment changed from oxygen-rich to oxygen-poor and anoxic. In addition, from the upper part of Zhuganpo member to the lower part of Wayao member, the abundance and diversity of trace fossils decreased rapidly, and benthos decreased significantly due to the influence of oxygen deficiency in water environment.

CPE was the most significant climate change event in the Triassic (Preto et al., 2010) and one of the largest Mesozoic extinction events (Dal Corso et al., 2020). However, its research mainly focused on the west Tethys region and the Southwestern United States. Domestic research on CPE was very limited. The occurrence sequence and development process of CPE events in China was not clear, the stratigraphic chronological framework of the Carnian needs to be further clarified, and the study of lithofacies palaeogeography, biostratigraphy and sedimentology needs to be further carried out. The study of CPE in the eastern Margin of Tethys contributed to a deep understanding of global paleoclimate change and provided more comparative schemes in China for the collaborative study of environment biological co-evolution relationship.