安徽巢湖凤凰山晚古生代大冰期沉积特征与碳同位素变化

SEDIMENTARY FEATURES AND CARBON ISOTOPE CHANGES OF THE LATE PALEOZOIC ICE AGE RECORDED IN FENGHUANGSHAN, CHAOHU, ANHUI

地史中海相碳酸盐岩记录的无机碳同位素的波动经常与全球气候事件相互关联,但碳酸盐岩的碳同位素不仅与全球碳循环相关,也受控于区域水循环和成岩作用。晚古生代大冰期在石炭纪末—早二叠世达到最高峰,南方冈瓦纳大陆高纬度地区冰川的进退引起低纬度地区旋回性的海平面变化,并发育显著的沉积间断,在华南表现为喀斯特岩溶角砾岩等。本文选取安徽巢湖凤凰山剖面,对宾夕法尼亚亚系—乌拉尔统18m厚的碳酸盐岩地层(黄龙组和船山组)进行了详细的沉积学与碳同位素对比研究,以探索碳同位素变化的控制因素。本文共识别出厚层灰泥灰岩、厚层粒泥灰岩、中至厚层泥粒灰岩、厚层泥球颗粒灰岩、薄层鲕粒颗粒灰岩、厚至巨厚层生物碎屑颗粒灰岩、厚层核形石颗粒灰岩、薄至中层灰砾岩和炉渣状灰岩等9种沉积岩相,指示了多次暴露的碳酸盐台地环境。这些暴露特征与同时期贵州罗甸斜坡相碳酸盐岩序列(约194 m)对比,表明凤凰山剖面存在严重的地层缺失。揭示了研究层段11 m和14 m附近的碳同位素两次显著负漂(幅度约为6‰和7‰)与古喀斯特层对应,是由碳酸盐台地暴露引起的区域成岩改造导致的,而非源于引起气候变化的全球碳循环波动。因此,地层学、沉积学和地球化学等多学科综合交叉研究是深入揭示古气候和古海洋变化规律的重要前提。

Fluctuations of inorganic carbon isotopes recorded in marine carbonates in geological history are often correlated with global climate changes. However, carbonate carbon isotopes are not only related to global carbon cycle, but also influenced by regional cycling of water mass and diagenesis. Advance and retreat of continental glaciers in the southern Gondwana during the late Carboniferous-early Permian glacial apex resulted in cyclic sea-level changes and significant sedimentary discontinuities in low-latitude regions, represented by paleokarsts in the south China. In order to evaluate the carbonate carbon isotopes, we carried out detailed comparison between sedimentary features and carbonate carbon isotopes on the Pennsylvanian-Cisuralian carbonate strata(Huanglong and Chuanshan formations) of the Fenghuangshan section in Chaohu, Anhui Province. In this paper, nine sedimentary facies are identified, including thick-bedded lime mudstone, thick-bedded wackestone,medium-to thick-bedded packstone, thick-bedded peloidal grainstone, thin-bedded ooidal grainstone, thick-to very thick-bedded bioclastic grainstone, thick-bedded oncoidal grainstone, thin-to medium-bedded rudstone and paleokarst breccia, indicating a frequently exposed carbonate platform environment. Correlation with the coeval, relatively thick carbonate slope succession in Luodian, Guizhou Province and the presence of frequent subaerial exposure features in the Fenghuangshan section suggest substantial missing of strata in the Fenghuangshan section. Detailed comparison studies on sedimentary features and carbon isotopes reveal that the two significant negative excursions in carbon isotopes are correlated to two paleokarsts in the Fenghuanshan section, which most likely resulted from regional diagenetic overprint rather than global carbon cycle perturbations. The study implies that deep understanding of paleoclimate changes and paleoceanographic conditions needs integrated studies on stratigraphy, sedimentology, and geochemistry.