北京石花洞疏松-致密型石笋微层形成机理
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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 related to the new water recharge during this season. To investigate their elemental compositions, both the rhombohedral crystallites and microcrystalline were analyzed through an elemental spectrum analyzer. The results show that the elemental consistency is highly associated with the scales of crystallites. Trace elements, including silicon, aluminum, magnesium, etc. could be obtained on the microcrystalline specimen. On the contrary, only the basic formation elements of calcium carbonate as carbon, oxygen and calcium are detected on rhombohedral structures. Based on these observations, there are two hypotheses on this formation mechanism. Hypothesis I is that, FCC corresponded to the relatively decreased drip rates.During the dry season, the recharge contains high calcite saturation from a stable soil-water-rock interaction over the cave.On the other hand, new water inputs during the rain seasons of increased drip rates result in increased variables including lower index of calcite saturation, complex soil-water-rock interaction and variable amounts of prior calcite precipitation.Hereby, it presents a result of increased MCC with more impurities. Hypothesis II is that, the changed structure might be related to the cave tourism conditions. As the new structure is observed after the cave open to the public, it is expected to suggest a correlation to the tourist patterns. Moreover, both the increased CO2 level and aerosol levels inside cave systems are highly associated with increased visitors, which result in more venting as well as external air inputs. It is likely that the impurities from the external aerosol are the key factors of MCC Formation during the transition period. These anthropogenic factors might play a vital role in the thicker microcrystalline calcite layers formation.
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 related to the new water recharge during this season. To investigate their elemental compositions, both the rhombohedral crystallites and microcrystalline were analyzed through an elemental spectrum analyzer. The results show that the elemental consistency is highly associated with the scales of crystallites. Trace elements, including silicon, aluminum, magnesium, etc. could be obtained on the microcrystalline specimen. On the contrary, only the basic formation elements of calcium carbonate as carbon, oxygen and calcium are detected on rhombohedral structures. Based on these observations, there are two hypotheses on this formation mechanism. Hypothesis I is that, FCC corresponded to the relatively decreased drip rates.During the dry season, the recharge contains high calcite saturation from a stable soil-water-rock interaction over the cave.On the other hand, new water inputs during the rain seasons of increased drip rates result in increased variables including lower index of calcite saturation, complex soil-water-rock interaction and variable amounts of prior calcite precipitation.Hereby, it presents a result of increased MCC with more impurities. Hypothesis II is that, the changed structure might be related to the cave tourism conditions. As the new structure is observed after the cave open to the public, it is expected to suggest a correlation to the tourist patterns. Moreover, both the increased CO2 level and aerosol levels inside cave systems are highly associated with increased visitors, which result in more venting as well as external air inputs. It is likely that the impurities from the external aerosol are the key factors of MCC Formation during the transition period. These anthropogenic factors might play a vital role in the thicker microcrystalline calcite layers formation.
引文
1 Tan M,Liu T,Han J,et al.Cyclic rapid warming on centennial-scale revealed by a 2650-year stalagmite record of warm season temperature.Geophys Res Lett,2003,30:1617-1620
2 Tan M,Pan G X,Wang X F,et al.Stalagmites and environment-Preliminary study on the formation of laminated stalagmites(in Chinese).Carsol Sin,1999,18:197-205[谭明,潘根兴,王先锋,等.石笋与环境--石笋纹层形成的环境机理初探.中国岩溶,1999,18:197-205]
3 Gilson J R,Macarthney E.Luminescence of speleothems from Devon,UK:The presence of organic activators.Ashford Speleol Soc J,1954,6:8-11
4 Ban F,Pan G,Zhu J,et al.Temporal and spatial variations in the discharge and dissolved organic carbon of drip waters in Beijing Shihua Cave,China.Hydrol Process,2008,22:3749-3758
5 Johnson K R,Hu C,Belshaw N S,et al.Seasonal trace-element and stable-isotope variations in a Chinese speleothem:The potential for highresolution paleomonsoon reconstruction.Earth Planet Sci Lett,2006,244:394-407
6 Hu C Y,Xie S C.Paleo-enviroment reconstruction based on speleothems from Heshang Cave,Qingjiang Valley,Hubei Province:An overview(in Chinese).Geol Sci Technol Inform,2012,31:57-64[胡超涌,谢树成.湖北清江和尚洞石笋古环境研究的回顾和展望.地质科技情报,2012,31:57-64]
7 Ban F M,Baker A,Marjo C E,et al.An optimized chronology for a stalagmite using seasonal trace element cycles from Shihua Cave,Beijing,North China.Sci Rep,2018,8:4551
8 Mattey D,Lowry D,Duffet J,et al.A 53 year seasonally resolved oxygen and carbon isotope record from a modern Gibraltar speleothem:Reconstructed drip water and relationship to local precipitation.Earth Planet Sci Lett,2008,269:80-95
9 Genty D,Quinif Y.Annually laminated sequences in the internal structure of some Belgian stalagmites-importance for paleoclimatology.JSediment Res,1996,66:275-288
10 Boch R,Sp?tl C,Frisia S.Origin and palaeoenvironmental significance of lamination in stalagmites from Katerloch Cave,Austria.Sedimentology,2011,58:508-531
11 Duan W,Cai B,Tan M,et al.The growth mechanism of the aragonitic stalagmite laminae from Yunnan Xianren Cave,SW China revealed by cave monitoring.Boreas,2012,41:113-123
12 Duan W,Kotlia B S,Tan M.Mineral composition and structure of the stalagmite laminae from Chulerasim cave,Indian Himalaya,and the significance for palaeoclimatic reconstruction.Quat Int,2013,298:93-97
13 LüJ B,Li T Y,Sun Y H,et al.Karst geology of the Shihua Cave,Beijing(in Chinese).Regional Geol China,1999,18:373-378[吕金波,李铁英,孙永华,等.北京石花洞的岩溶地质特征.中国区域地质,1999,18:373-378]
14 Liu H,Zheng M C,Duan H W,et al.Structure and evolution of the Shihua Cave in Beijing(in Chinese).Carsol Sin,2015,34:27-34[刘宏,郑明存,段洪伍,等.北京石花洞结构特征及发育演化过程研究.中国岩溶,2015,34:27-34]
15 Li X,Cheng H,Tan L,et al.The East Asian summer monsoon variability over the last 145 years inferred from the Shihua Cave record,North China.Sci Rep,2017,7:7078
16 Riechelmann S,Schr?der-Ritzrau A,Wassenburg J A,et al.Physicochemical characteristics of drip waters:Influence on mineralogy and crystal morphology of recent cave carbonate precipitates.Geochim Cosmochim Acta,2014,145:13-29
17 Gratz A J,Hillner P E,Hansma P K.Step dynamics and spiral growth on calcite.Geochim Cosmochim Acta,1993,57:491-495
18 Frisia S,Borsato A,Fairchild I J,et al.Calcite fabrics,growth mechanisms,and environments of formation in speleothems from the Italian Alps and Southwestern Ireland.J Sediment Res,2000,70:1183-1196
19 Dredge J,Fairchild I J,Harrison R M,et al.Cave aerosols:Distribution and contribution to speleothem geochemistry.Quat Sci Rev,2013,63:23-41
20 Pan Y X,Zhou Z F,Zhang J,et al.Temporal and spatial distribution characteristics of microclimate environmental elements in Karst tourism caves-A case study of Zhijin cave in Guizhou Province(in Chinese).Sci Technol Eng,2018,18:20-30[潘艳喜,周忠发,张结,等.喀斯特旅游洞穴微气候环境要素时空分布特征及影响因素--以贵州织金洞为例.科学技术与工程,2018,18:20-30]
21 Ban F M,Cai B G.Research on seasonal variations of the air’s main environmental factors in the Shihua Cave,Beijing(in Chinese).Carsol Sin,2011,30:132-137[班凤梅,蔡炳贵.北京石花洞空气环境主要因子季节性变化特征研究.中国岩溶,2011,30:132-137]
22 Zhang M L,Zhu X Y,Wu X,et al.Impact of tourism activities on the cave environment and landscape of calcium carbonate(CaCO3)deposits at Shuijinggong Cave,Bama count(in Chinese).Carsol Sin,2017,36:119-130[张美良,朱晓燕,吴夏,等.旅游活动对巴马水晶宫洞穴环境及碳酸钙沉积物景观的影响.中国岩溶,2017,36:119-130]
2 Tan M,Pan G X,Wang X F,et al.Stalagmites and environment-Preliminary study on the formation of laminated stalagmites(in Chinese).Carsol Sin,1999,18:197-205[谭明,潘根兴,王先锋,等.石笋与环境--石笋纹层形成的环境机理初探.中国岩溶,1999,18:197-205]
3 Gilson J R,Macarthney E.Luminescence of speleothems from Devon,UK:The presence of organic activators.Ashford Speleol Soc J,1954,6:8-11
4 Ban F,Pan G,Zhu J,et al.Temporal and spatial variations in the discharge and dissolved organic carbon of drip waters in Beijing Shihua Cave,China.Hydrol Process,2008,22:3749-3758
5 Johnson K R,Hu C,Belshaw N S,et al.Seasonal trace-element and stable-isotope variations in a Chinese speleothem:The potential for highresolution paleomonsoon reconstruction.Earth Planet Sci Lett,2006,244:394-407
6 Hu C Y,Xie S C.Paleo-enviroment reconstruction based on speleothems from Heshang Cave,Qingjiang Valley,Hubei Province:An overview(in Chinese).Geol Sci Technol Inform,2012,31:57-64[胡超涌,谢树成.湖北清江和尚洞石笋古环境研究的回顾和展望.地质科技情报,2012,31:57-64]
7 Ban F M,Baker A,Marjo C E,et al.An optimized chronology for a stalagmite using seasonal trace element cycles from Shihua Cave,Beijing,North China.Sci Rep,2018,8:4551
8 Mattey D,Lowry D,Duffet J,et al.A 53 year seasonally resolved oxygen and carbon isotope record from a modern Gibraltar speleothem:Reconstructed drip water and relationship to local precipitation.Earth Planet Sci Lett,2008,269:80-95
9 Genty D,Quinif Y.Annually laminated sequences in the internal structure of some Belgian stalagmites-importance for paleoclimatology.JSediment Res,1996,66:275-288
10 Boch R,Sp?tl C,Frisia S.Origin and palaeoenvironmental significance of lamination in stalagmites from Katerloch Cave,Austria.Sedimentology,2011,58:508-531
11 Duan W,Cai B,Tan M,et al.The growth mechanism of the aragonitic stalagmite laminae from Yunnan Xianren Cave,SW China revealed by cave monitoring.Boreas,2012,41:113-123
12 Duan W,Kotlia B S,Tan M.Mineral composition and structure of the stalagmite laminae from Chulerasim cave,Indian Himalaya,and the significance for palaeoclimatic reconstruction.Quat Int,2013,298:93-97
13 LüJ B,Li T Y,Sun Y H,et al.Karst geology of the Shihua Cave,Beijing(in Chinese).Regional Geol China,1999,18:373-378[吕金波,李铁英,孙永华,等.北京石花洞的岩溶地质特征.中国区域地质,1999,18:373-378]
14 Liu H,Zheng M C,Duan H W,et al.Structure and evolution of the Shihua Cave in Beijing(in Chinese).Carsol Sin,2015,34:27-34[刘宏,郑明存,段洪伍,等.北京石花洞结构特征及发育演化过程研究.中国岩溶,2015,34:27-34]
15 Li X,Cheng H,Tan L,et al.The East Asian summer monsoon variability over the last 145 years inferred from the Shihua Cave record,North China.Sci Rep,2017,7:7078
16 Riechelmann S,Schr?der-Ritzrau A,Wassenburg J A,et al.Physicochemical characteristics of drip waters:Influence on mineralogy and crystal morphology of recent cave carbonate precipitates.Geochim Cosmochim Acta,2014,145:13-29
17 Gratz A J,Hillner P E,Hansma P K.Step dynamics and spiral growth on calcite.Geochim Cosmochim Acta,1993,57:491-495
18 Frisia S,Borsato A,Fairchild I J,et al.Calcite fabrics,growth mechanisms,and environments of formation in speleothems from the Italian Alps and Southwestern Ireland.J Sediment Res,2000,70:1183-1196
19 Dredge J,Fairchild I J,Harrison R M,et al.Cave aerosols:Distribution and contribution to speleothem geochemistry.Quat Sci Rev,2013,63:23-41
20 Pan Y X,Zhou Z F,Zhang J,et al.Temporal and spatial distribution characteristics of microclimate environmental elements in Karst tourism caves-A case study of Zhijin cave in Guizhou Province(in Chinese).Sci Technol Eng,2018,18:20-30[潘艳喜,周忠发,张结,等.喀斯特旅游洞穴微气候环境要素时空分布特征及影响因素--以贵州织金洞为例.科学技术与工程,2018,18:20-30]
21 Ban F M,Cai B G.Research on seasonal variations of the air’s main environmental factors in the Shihua Cave,Beijing(in Chinese).Carsol Sin,2011,30:132-137[班凤梅,蔡炳贵.北京石花洞空气环境主要因子季节性变化特征研究.中国岩溶,2011,30:132-137]
22 Zhang M L,Zhu X Y,Wu X,et al.Impact of tourism activities on the cave environment and landscape of calcium carbonate(CaCO3)deposits at Shuijinggong Cave,Bama count(in Chinese).Carsol Sin,2017,36:119-130[张美良,朱晓燕,吴夏,等.旅游活动对巴马水晶宫洞穴环境及碳酸钙沉积物景观的影响.中国岩溶,2017,36:119-130]