北京石花洞疏松-致密型石笋微层形成机理  

The fabric and its formation mechanism of porous-compact laminae in the stalagmite from Shihua Cave, Beijing

作　　者：班凤梅 谭明[3] 韩晋仙[1] 翟艳峰 赵旭红 孙会国[3,4] Fengmei Ban;Ming Tan;Jinxian Han;Yanfeng Zhai;Xuhong Zhao;Huiguo Sun(School of Resources and Environment,Shanxi University of Finance & Economics,Taiyuan 030006,China;Institute of Global Environmental Change,Xi'an Jiaotong University,Xi'an 710049,China;Key Laboratory of Cenozoic Geology Environment,Institute of Geology and Geophysics,Chinese Academy of Science,Beijing 100029,China;Institutions of Earth Science,Chinese Academy of Sciences,Beijing 100029,China)

机构地区：[1]山西财经大学资源环境学院,太原030006 [2]西安交通大学全球环境变化研究所,西安710049 [3]中国科学院地质与地球物理研究所新生代地质与环境重点实验室,北京100029 [4]中国科学院地球科学研究院,北京100029

出　　处：《科学通报》2019年第20期2134-2140,共7页Chinese Science Bulletin

基　　金：国家自然科学基金(41402161,41877402);山西省自然科学基金(2015011085);国家重点研发计划(2017YFA0603400)资助

摘　　要：北京石花洞石笋物质旋回微层及其气候重建研究在古气候领域已有广泛报道.而本文所研究的石笋(XMG),自20世纪90年代初后突变发育了疏松-致密型微层,这种微层类型以前在石花洞从未发现.扫描电子显微镜(scanning electron microscope, SEM)观测显示,这种疏松-致密层偶是由方解石组构(calcite fabric)差异引起,不透光疏松层是粒状镶嵌结构微晶方解石,而透光致密层是晶簇状纤维晶方解石.在对该石笋观测站点的现生碳酸盐SEM分析后,发现其可分为3种类型, I型是旱季1~3月沉积的晶粒较大且晶面较为平整的菱面体及其组合体;II型是旱-雨季过渡时期(4~5和10~12月)生长的大、小晶体混生型;III型是雨季生成的晶粒细小(粒径<4μm)的微晶.菱面晶体所含杂质少,形成条件为滴水慢且稳定、方解石饱和指数偏高、洞穴CO2浓度低以及洞穴扰动最小,这类晶体构成了透光纤维晶方解石层;而微晶伴有较多杂质,当季新水补给、洞穴CO2浓度升高以及旅游活动增加等因素均利于其生成.由于疏松的微晶方解石亚层形成于洞穴开放之后,故推测洞穴开放后人为干扰所引起的洞穴环境改变,很可能是其突变发育的关键因素.The stalagmites in Shihua Cave, Beijing have been intensely studied as archives of regional palaeoclimate. They yield high resolution time series of multi-proxy records which have greatly improved the understanding of the relationship between speleothem formation and climate changes. The stalagmites studied in this cave were composed of fibrous calcite with clear visible and fluorescent laminae. Recently, porous-compact layers at the top 4 mm were found on an actively growing stalagmite(XMG). It consists of visible transparent-opaque layers. A noteworthy observation is that the laminae of calcite fabrics occurred on the top of XMG which formed after the cave open for tourism. The results from scanning electron microscopy analysis on this stalagmite indicates that the opaque microcrystalline calcite(MCC) porous sub-layers are composed of granular mosaic structures, comparing to the compact transparent fibrous columnar calcite(FCC) sub-layers with structures almost oriented perpendicular to the growth surface. Further analysis on the modern carbonate of this site was carried to investigate its formation mechanisms. Based on the size and habit of crystal, these modern carbonate crystallite formations can be categorized into three types. Type I contains larger rhombohedral grains, most of which are formed in the dry season of January–March. Most crystals with c-axis(along the growth direction) represent scales larger than 10 μm and structures of FCC. Type II includes the mixture of large rhombohedrons and small microcrystalline with a scale of less than 4 μm during the transition period of dry to rainy season, with deposition in April-May and October-December. During this period, the scale of large crystals with a diameter of more than 10 μm gradually decreases and the microcrystalline crystals begin growing. Type III consists of calcite precipitates with a spherical habit in the rainy season of June–September. Crystallites with diameters of less than 1 μm can be observed in this period, which is highly relat

关 键 词：石笋 疏松-致密微层 微晶方解石 人类活动 石花洞 

分 类 号：P532[天文地球—古生物学与地层学] P642.25[天文地球—地质学]