Abstract:

The Cretaceous period was characterized by numerous globally significant tectonic and climatic events that are well preserved in the Liupanshan Basin,where the exceptionally continuous and widely distributed Cretaceous strata,predominantly composed of fluvial-lacustrine deposits,host exquisitely preserved Lycoptera fossils,making this basin an ideal site for studying paleoclimatic and paleoenvironmental changes during the Cretaceous. Through an integrated approach combining sedimentology,stratigraphy,paleontology,and geochemistry,this study investigates the Liwaxia Formation in the Liupanshan Basin,employing organic carbon isotope(${\delta }^{13}{\mathrm{C}}_{\mathrm{o}\mathrm{r}\mathrm{g}}$)analysis,major and trace element geochemistry,and X-ray fluorescence(XRF)to examine the paleoclimate and paleoenvironment of Lycoptera-bearing intervals and to assess their relationship to global events. Geochemical results reveal three distinct ${\delta }^{13}{\mathrm{C}}_{\mathrm{o}\mathrm{r}\mathrm{g}}$ evolutionary stages: a significant negative excursion(from-30.21‰ to-27.19‰,amplitude 3.02‰)in intervals B3-B7,a sharp positive shift(from-28.05‰ to-24.13‰,amplitude 3.92‰)in intervals B11-B15,followed by an oscillatory positive trend in intervals B17-B25. The fossil-bearing horizons exhibit notable geochemical anomalies,including marked fluctuations in major elements(Ca,Mg)and trace elements(Sr,Th)relative to adjacent strata,while micro-XRF analysis of fish fossil bones reveals marked enrichments of heavy metals and toxic elements(Mn,Cr,As,S). These Lycoptera-bearing strata record lacustrine environmental conditions during the C3 stage of Oceanic Anoxic Event 1a(OAE1a),characterized by the diagnostic ${\delta }^{13}{\mathrm{C}}_{\mathrm{o}\mathrm{r}\mathrm{g}}$ negative excursion marking OAE1a onset and reflecting hot,arid,hypersaline,and anoxic conditions in the Liupanshan Basin. The mass mortality of Lycoptera likely resulted from the combined effects of persistent bottom-water anoxia caused by Early Cretaceous volcanic-induced greenhouse conditions and the toxicological impact of heavy metals and toxic elements enriched in volcanic ash. This research not only sheds light on the response mechanisms of the paleoclimate and paleoenvironment within the Liupanshan Basin to global events,but also provides crucial biogeochemical evidence and paleontological records essential for understanding the climatic and environmental evolution of Cretaceous terrestrial ecosystems and associated biological extinction events.

Key words: Oceanic Anoxic Events, Lycoptera, Liwaxia Formation, Cretaceous, Liupanshan Basin

摘要：

白垩纪发生了许多具有全球性影响的构造和气候事件,这些事件在六盘山盆地也有很好的响应。六盘山盆地白垩系为河流—湖泊相沉积,地层连续性好,分布范围广,尤其重要的是,地层中还保存了完美的狼鳍鱼化石,是研究白垩纪古气候、古环境变化的理想场所。依据沉积学、地层学、古生物学、地球化学等理论,以六盘山盆地白垩系李洼峡组为研究对象,通过有机碳同位素(${\mathrm{\delta }}^{13}{\mathrm{C}}_{\mathrm{o}\mathrm{r}\mathrm{g}}$)、主量元素、微量元素、XRF(X射线荧光光谱分析)等分析测试,对狼鳍鱼化石赋存层位的古气候、古环境进行研究,以揭示其与全球性事件的响应关系。地球化学分析结果显示,李洼峡组岩石样品的${\mathrm{\delta }}^{13}{\mathrm{C}}_{\mathrm{o}\mathrm{r}\mathrm{g}}$结果呈现出3个明显的演化阶段: B3-B7层位急剧负偏(-30.21‰~-27.19‰,幅度3.02‰),B11-B15层位显著正偏(-28.05‰~-24.13‰,幅度3.92‰),B17-B25层位转为振荡式正偏移。化石赋存层位中主量元素Ca、Mg和微量元素Sr、Th相较于其上、下层位呈现显著差异。鱼化石骨骼微区XRF分析显示,Mn、Cr、As、S等重金属和有毒元素显著异常。李洼峡组狼鳍鱼化石层位记录了OAE1a(Oceanic Anoxic Event 1a,大洋缺氧事件)的C3阶段(OAE1a的起始阶段,${\mathrm{\delta }}^{13}{\mathrm{C}}_{\mathrm{o}\mathrm{r}\mathrm{g}}$呈显著负偏),该阶段六盘山盆地表现为炎热干旱、高盐缺氧的沉积环境。狼鳍鱼大规模死亡主要是由于早白垩世火山活动诱发的温室效应导致底层水体形成持续性缺氧环境,同时叠加火山灰中富集的重金属和有毒元素的毒害作用。该研究不仅揭示了六盘山盆地古气候、古环境对全球事件的响应机制,也为理解白垩纪陆相生态系统的气候环境演变和生物灭绝事件提供了关键的地球化学证据和古生物记录。

关键词: 大洋缺氧事件, 狼鳍鱼, 李洼峡组, 白垩纪, 六盘山盆地

CLC Number: 

• P591