粤北石灰岩化学风化过程中的Sr-Nd同位素
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摘要
石灰岩化学风化显著控制着其所在流域盆地的河水Sr同位素组成,进而制约全球Sr循环。然而,石灰岩化学风化过程中Sr同位素组成的变化特征及其控制机制的研究报道依旧很少。本文通过报道粤北地区一个典型石灰岩风化剖面的Sr同位素组成变化特征,试图探讨石灰岩化学风化过程中Sr同位素变化的控制机制。结果显示:该剖面上,~(87)Sr/~(86)Sr比值介于0.70979~0.72216之间;自剖面底部到顶部,~(87)Sr/~(86)Sr比值呈现出逐渐增大的趋势;Nd同位素组成比较均一(εNd(t)=–15.0±0.5)。研究区的潜在源区(如大气降水、地表水),其~(87)Sr/~(86)Sr比值明显低于该剖面上部的最大值,加之,该剖面的风化产物与典型的中国黄土剖面具有截然不同的Sr-Nd同位素组成,暗示了这些潜在源区的输入不大可能造成该剖面上Sr同位素组成发生显著的变化。该剖面上~(87)Sr/~(86)Sr比值与Sr含量之间呈现显著的负相关性(r=–0.95),暗示粤北石灰岩化学风化过程中Sr同位素的演化受控于碳酸盐矿物与硅酸盐矿物在抗风化强度以及Sr同位素组成方面的显著差异:相对硅酸盐矿物,碳酸盐矿物(如方解石)抗风化强度较弱,于是随着风化作用的进行,原岩中具有较低~(87)Sr/~(86)Sr比值的方解石中的Sr大量迁出剖面,而具有较高~(87)Sr/~(86)Sr比值的硅酸盐矿物中的Sr残留在风化产物中,因此,随着风化强度的增加,风化产物中的~(87)Sr/~(86)Sr比值呈现出逐渐增大的趋势。
Limestones are broadly outcropped on Earth's surface. They are very easily weathered and contribute most of the dissolved alkaline earth elements such as Ca and Sr to rivers, and hence to the oceans. The budget of Sr isotopes, an important isotope system in tracing variable surficial processes, is largely controlled by weathering of limestones. In addition to the amount of weathered limestones, variation of Sr isotopes during limestone weathering likely influences the Sr isotopic composition of river water and regulates the global Sr isotope budget. However, consecutive variations of the ~(87) Sr/~(86) Sr ratio in weathering profiles have not been investigated, making it difficult to understand the mechanism for the variation of Sr isotopic composition during limestone weathering. We herein report the Sr and Nd isotopic compositions of a weathering profile developed on limestone in northern Guangdong province, South China. The results show that the Sr isotopes exhibited a large variation range, with the ~(87) Sr/~(86) Sr ratio varying from 0.70979 at the bottom of the profile to 0.72216 at the top, whereas the Nd isotopes are relatively homogeneous, with εNd(t) values ranging from –15.53 to –14.46, with an average of –15.0 ± 0.5. Sr from river water and meteoric water are two potential extraneous inputs, and their ~(87) Sr/~(86) Sr ratios vary from 0.70741 to 0.71583, which are obviously lower than the maximum in the upper weathering profile(~0.72216). This implies that river water and rainwater do not contribute much extraneous Sr to the saprolites in the profile. In addition, the studied profile has quite a different distribution from that of China Loess in the ~(87) Sr/~(86) Sr–εNd(t) diagram, suggesting that dust deposits from China Loess also contribute very little to the large variation of Sr isotopes in this profile. It is worth noting that a significant negative correlation between the ~(87) Sr/~(86) Sr ratios and Sr concentrations appears in the profile, implying a potential scenario that the variation of Sr isotope composition can be largely attributed to incongruent weathering of minerals during limestone weathering, as there are dramatic differences in the resistance to weathering and ~(87) Sr/~(86) Sr ratios of silicate and carbonate minerals. As a result, dissolved Sr sourced from the weathering of calcite was obviously removed from the weathering profile before Sr occurred in silicate minerals. Finally, the ~(87) Sr/~(86) Sr ratios of the saprolites move toward values that are similar to those of silicate minerals as the chemical weathering becomes stronger.
Limestones are broadly outcropped on Earth's surface. They are very easily weathered and contribute most of the dissolved alkaline earth elements such as Ca and Sr to rivers, and hence to the oceans. The budget of Sr isotopes, an important isotope system in tracing variable surficial processes, is largely controlled by weathering of limestones. In addition to the amount of weathered limestones, variation of Sr isotopes during limestone weathering likely influences the Sr isotopic composition of river water and regulates the global Sr isotope budget. However, consecutive variations of the ~(87) Sr/~(86) Sr ratio in weathering profiles have not been investigated, making it difficult to understand the mechanism for the variation of Sr isotopic composition during limestone weathering. We herein report the Sr and Nd isotopic compositions of a weathering profile developed on limestone in northern Guangdong province, South China. The results show that the Sr isotopes exhibited a large variation range, with the ~(87) Sr/~(86) Sr ratio varying from 0.70979 at the bottom of the profile to 0.72216 at the top, whereas the Nd isotopes are relatively homogeneous, with εNd(t) values ranging from –15.53 to –14.46, with an average of –15.0 ± 0.5. Sr from river water and meteoric water are two potential extraneous inputs, and their ~(87) Sr/~(86) Sr ratios vary from 0.70741 to 0.71583, which are obviously lower than the maximum in the upper weathering profile(~0.72216). This implies that river water and rainwater do not contribute much extraneous Sr to the saprolites in the profile. In addition, the studied profile has quite a different distribution from that of China Loess in the ~(87) Sr/~(86) Sr–εNd(t) diagram, suggesting that dust deposits from China Loess also contribute very little to the large variation of Sr isotopes in this profile. It is worth noting that a significant negative correlation between the ~(87) Sr/~(86) Sr ratios and Sr concentrations appears in the profile, implying a potential scenario that the variation of Sr isotope composition can be largely attributed to incongruent weathering of minerals during limestone weathering, as there are dramatic differences in the resistance to weathering and ~(87) Sr/~(86) Sr ratios of silicate and carbonate minerals. As a result, dissolved Sr sourced from the weathering of calcite was obviously removed from the weathering profile before Sr occurred in silicate minerals. Finally, the ~(87) Sr/~(86) Sr ratios of the saprolites move toward values that are similar to those of silicate minerals as the chemical weathering becomes stronger.
引文
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[2]Pett-Ridge J C,Derry L A,Kurtz A C.Sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed,Luquillo Mountains,Puerto Rico[J].Geochim Cosmochim Acta,2009,73(1):25–43.
[3]Erel Y,Torrent J.Contribution of Saharan dust to Mediterranean soils assessed by sequential extraction and Pb and Sr isotopes[J].Chem Geol,2010,275(1/2):19–25.
[4]Wei G J,Liu Y,Ma J L,Xie L H,Chen J F,Deng W F,Tang S.Nd,Sr isotopes and elemental geochemistry of surface sediments from the South China Sea:Implications for Provenance Tracing[J].Mar Geol,2012,319:21–34.
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[6]Chadwick O A,Derry L A,Bern C R,Vitousek P M.Changing sources of strontium to soils and ecosystems across the Hawaiian Islands[J].Chem Geol,2009,267(1/2):64–76.
[7]Hodell D A,Mead G A,Mueller P A.Variation in the strontium isotopic composition of seawater(8 Ma to present):Implications for chemical weathering rates and dissolved fluxes to the oceans[J].Chem Geol Isotop Geosci,1990,80(4):291–307.
[8]Burke W H,Denison R E,Hetherington E A,Koepnick R B,Nelson H F,Otto J B.Variation of seawater 87Sr/86Sr throughout Phanerozoic time[J].Geology,1982,10:516–519.
[9]Krishnaswami S,Trivedi J R,Sarin M M,Ramesh R,Sharma K K.Strontium isotopes and rubidium in the Ganga-Brahmaputra river system:Weathering in the Himalaya,fluxes to the Bay of Bengal and contributions to the evolution of oceanic87Sr/86Sr[J].Earth Planet Sci Lett,1992,109(1/2):243–253.
[10]Palmer M R,Edmond J M.The strontium isotope budget of the modern ocean[J].Earth Planet Sci Lett,1989,92(1):11–26.
[11]Bickle M J,Chapman H J,Bunbury J,Harris N B W,Fairchild I J,Ahmad T,Pomies C.Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges[J].Geochim Cosmochim Acta,2005,69(9):2221–2240.
[12]Donnini M,Frondini F,Probst J L,Probst A,Cardellini C,Marchesini I,Guzzetti F.Chemical weathering and consumption of atmospheric carbon dioxide in the Alpine region[J].Global Planet Change,2016,136:65–81.
[13]Gaillardet J,Dupre B,Louvat P,Allegre C J.Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers[J].Chem Geol,1999,159(1/4):3–30.
[14]Kump L R,Brantley S L,Arthur M A.Chemical weathering,atmospheric CO2,and climate[J].Ann Rev Earth Planet Sci,2000,28(1):611–667.
[15]Li G J,Elderfield H.Evolution of carbon cycle over the past100 million years[J].Geochim Cosmochim Acta,2013,103:11–25.
[16]Martin J B.Carbonate minerals in the global carbon cycle[J].Chem Geol,2017,449:58–72.
[17]Kirstein J,Hellevang H,Haile B G,Gleixner G,Gaupp R.Experimental determination of natural carbonate rock dissolution rates with a focus on temperature dependency[J].Geomorphology,2016,261:30–40.
[18]Wei G J,Ma J L,Liu Y,Xie L H,Lu W J,Deng W F,Ren Z Y,Zeng T,Yang Y H.Seasonal changes in the radiogenic and stable strontium isotopic composition of Xijiang River water:Implications for chemical weathering[J].Chem Geol,2013,343(3):67–75.
[19]Jacobson A D,Grace Andrews M,Lehn G O,Holmden C.Silicate versus carbonate weathering in Iceland:New insights from Ca isotopes[J].Earth Planet Sci Lett,2015,416:132–142.
[20]Peucker-Ehrenbrink B,Miller M W,Arsouze T,Jeandel C.Continental bedrock and riverine fluxes of strontium and neodymium isotopes to the oceans[J].Geochem Geophy Geosy,2010,11(3):153–164.
[21]虎贵朋,韦刚健,马金龙,曾提,刘志锋.粤北碳酸盐岩化学风化过程中的元素地球化学行为[J].地球化学,2017,46(1):33–45.Mao Gui-peng,Wei Gang-jian,Ma Jin-long,Zeng Ti,Liu Zhi-feng.Mobilization and re-distribution of major and trace elements during the process of moderate weathering of carbonates in northern Guangdong,South China[J].Geochimca,2017,46(1):33–45(in Chinese with English abstract).
[22]韦刚健,刘颖,涂湘林,梁细荣,李献华.利用选择性特效树脂富集分离岩石样品中的锶、钐和钕[J].岩矿测试,2004,23(1):11–14.Wei Gang-jian,Liu Ying,Tu Xiang-lin,Liang Xi-rong,Li Xian-hua.Separation of Sr,Sm and Nd in mineral and rock samples using selective specific resins[J].Rock Mineral Anal,2004,23(1):11–14(in Chinese with English abstract).
[23]Ma J L,Wei G J,Liu Y,Ren Z Y,Xu Y G,Yang Y H.Precise measurement of stable neodymium isotopes of geological materials by using MC-ICP-MS[J].J Anal Atom Spectr,2013,28(12):1926–1931.
[24]韦刚健,梁细荣,李献华,刘颖.(LP)MC-ICPMS方法精确测定液体和固体样品的Sr同位素组成[J].地球化学,2002,31(3):295–299.Wei Gang-jian,Liang Xi-rong,Li Xian-hua,Liu Ying.Precise measurement of Sr isotopic composition of liquid and solid base using(LP)MC-ICPMS[J].Geochimica,2002,31(3):295–299(in Chinese with English abstract).
[25]Mcarthur J M.Recent trends in strontium isotope stratigraphy[J].Terra Nova,1994,6(4):331–358.
[26]梁细荣,韦刚健,李献华,刘颖.利用MC-ICPMS精确测定143Nd/144Nd和Sm/Nd比值[J].地球化学,2003,32(1):91–96.Liang Xi-rong,Wei Gang-jian,Li Xian-hua,Liu Ying.Precise measurement of 143Nd/144Nd and Sm/Nd ratios using multiple-collectors inductively coupled plasma-mass spectrometry(MC-ICPMS)[J].Geochimica,2003,32(1):91–96(in Chinese with English abstract).
[27]Tanaka T,Togashi S,Kamioka H,Amakawa H,Kagami H,Hamamoto T,Yuhara M,Orihashi Y,Yoneda S,Shimizu H.JNdi-1:A neodymium isotopic reference in consistency with La Jolla neodymium[J].Chem Geol,2000,168(3/4):279–281.
[28]Jahn B M,Gallet S,Han J M.Geochemistry of the Xining,Xifeng and Jixian sections,Loess Plateau of China:Eolian dust provenance and paleosol evolution during the last 140 ka[J].Chem Geol,2001,178(1/4):71–94.
[29]Ma J L,Wei G J,Xu Y G,Long W G.Variations of Sr-Nd-Hf isotopic systematics in basalt during intensive weathering[J].Chem Geol,2010,269(3/4):376–385.
[30]Banner J L.Radiogenic isotopes:Systematics and applications to earth surface processes and chemical stratigraphy[J].Earth-Sci Rev,2004,65(3/4):141–194.
[31]Li J W,Zhang G L,Ruan L,Yang J L,Wang H L.Sr-Nd elements and isotopes as tracers of dust input in a tropical soil chronosequence[J].Geoderma,2016,262:227–234.
[32]Cheng M C,You C F,Lin F J,Chung C H,Huang K F.Seasonal variation in long-range transported dust to a subtropical islet offshore northern Taiwan:Chemical composition and Sr isotopic evidence in rainwater[J].Atmos Environ,2010,44(28):3386–3393.
[33]Han G L,Liu C Q.Strontium isotope and major ion chemistry of the rainwaters from Guiyang,Guizhou Province,China[J].Sci Total Environ,2006,364(1/3):165–174.
[34]Xu Z F,Li Y S,Tang Y,Han G L.Chemical and strontium isotope characterization of rainwater at an urban site in Loess Plateau,Northwest China[J].Atmos Res,2009,94(3):481–490.
[35]Rao W B,Han G L,Tan H B,Jiang S.Chemical and Sr isotopic compositions of rainwater on the Ordos Desert Plateau,Northwest China[J].Environ Earth Sci,2015,74(7):5759–5771.
[36]Xu Z F,Han G L.Chemical and strontium isotope characterization of rainwater in Beijing,China[J].Atmos Environ,2009,43(12):1954–1961.
[37]覃小群,蒋忠诚,张连凯,黄奇波,刘朋雨.珠江流域碳酸盐岩与硅酸盐岩风化对大气CO2汇的效应[J].地质通报,2015,34(9):1749–1757.Qin Xiao-qun,Jiang Zhong-cheng,Zhang Lian-kai,Huang Qi-bo,Liu Peng-yu.The difference of the weathering rate between carbonate rocks and silicate rocks and its effects on the atmospheric CO2 consumption in the Pearl River Basin[J].Geol Bull China,2015,34(9):1749–1757(in Chinese with English abstract).
[2]Pett-Ridge J C,Derry L A,Kurtz A C.Sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed,Luquillo Mountains,Puerto Rico[J].Geochim Cosmochim Acta,2009,73(1):25–43.
[3]Erel Y,Torrent J.Contribution of Saharan dust to Mediterranean soils assessed by sequential extraction and Pb and Sr isotopes[J].Chem Geol,2010,275(1/2):19–25.
[4]Wei G J,Liu Y,Ma J L,Xie L H,Chen J F,Deng W F,Tang S.Nd,Sr isotopes and elemental geochemistry of surface sediments from the South China Sea:Implications for Provenance Tracing[J].Mar Geol,2012,319:21–34.
[5]Capo R C,Stewart B W,Chadwick O A.Strontium isotopes as tracers of ecosystem processes:Theory and methods[J].Geoderma,1998,82(1/3):197–225.
[6]Chadwick O A,Derry L A,Bern C R,Vitousek P M.Changing sources of strontium to soils and ecosystems across the Hawaiian Islands[J].Chem Geol,2009,267(1/2):64–76.
[7]Hodell D A,Mead G A,Mueller P A.Variation in the strontium isotopic composition of seawater(8 Ma to present):Implications for chemical weathering rates and dissolved fluxes to the oceans[J].Chem Geol Isotop Geosci,1990,80(4):291–307.
[8]Burke W H,Denison R E,Hetherington E A,Koepnick R B,Nelson H F,Otto J B.Variation of seawater 87Sr/86Sr throughout Phanerozoic time[J].Geology,1982,10:516–519.
[9]Krishnaswami S,Trivedi J R,Sarin M M,Ramesh R,Sharma K K.Strontium isotopes and rubidium in the Ganga-Brahmaputra river system:Weathering in the Himalaya,fluxes to the Bay of Bengal and contributions to the evolution of oceanic87Sr/86Sr[J].Earth Planet Sci Lett,1992,109(1/2):243–253.
[10]Palmer M R,Edmond J M.The strontium isotope budget of the modern ocean[J].Earth Planet Sci Lett,1989,92(1):11–26.
[11]Bickle M J,Chapman H J,Bunbury J,Harris N B W,Fairchild I J,Ahmad T,Pomies C.Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges[J].Geochim Cosmochim Acta,2005,69(9):2221–2240.
[12]Donnini M,Frondini F,Probst J L,Probst A,Cardellini C,Marchesini I,Guzzetti F.Chemical weathering and consumption of atmospheric carbon dioxide in the Alpine region[J].Global Planet Change,2016,136:65–81.
[13]Gaillardet J,Dupre B,Louvat P,Allegre C J.Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers[J].Chem Geol,1999,159(1/4):3–30.
[14]Kump L R,Brantley S L,Arthur M A.Chemical weathering,atmospheric CO2,and climate[J].Ann Rev Earth Planet Sci,2000,28(1):611–667.
[15]Li G J,Elderfield H.Evolution of carbon cycle over the past100 million years[J].Geochim Cosmochim Acta,2013,103:11–25.
[16]Martin J B.Carbonate minerals in the global carbon cycle[J].Chem Geol,2017,449:58–72.
[17]Kirstein J,Hellevang H,Haile B G,Gleixner G,Gaupp R.Experimental determination of natural carbonate rock dissolution rates with a focus on temperature dependency[J].Geomorphology,2016,261:30–40.
[18]Wei G J,Ma J L,Liu Y,Xie L H,Lu W J,Deng W F,Ren Z Y,Zeng T,Yang Y H.Seasonal changes in the radiogenic and stable strontium isotopic composition of Xijiang River water:Implications for chemical weathering[J].Chem Geol,2013,343(3):67–75.
[19]Jacobson A D,Grace Andrews M,Lehn G O,Holmden C.Silicate versus carbonate weathering in Iceland:New insights from Ca isotopes[J].Earth Planet Sci Lett,2015,416:132–142.
[20]Peucker-Ehrenbrink B,Miller M W,Arsouze T,Jeandel C.Continental bedrock and riverine fluxes of strontium and neodymium isotopes to the oceans[J].Geochem Geophy Geosy,2010,11(3):153–164.
[21]虎贵朋,韦刚健,马金龙,曾提,刘志锋.粤北碳酸盐岩化学风化过程中的元素地球化学行为[J].地球化学,2017,46(1):33–45.Mao Gui-peng,Wei Gang-jian,Ma Jin-long,Zeng Ti,Liu Zhi-feng.Mobilization and re-distribution of major and trace elements during the process of moderate weathering of carbonates in northern Guangdong,South China[J].Geochimca,2017,46(1):33–45(in Chinese with English abstract).
[22]韦刚健,刘颖,涂湘林,梁细荣,李献华.利用选择性特效树脂富集分离岩石样品中的锶、钐和钕[J].岩矿测试,2004,23(1):11–14.Wei Gang-jian,Liu Ying,Tu Xiang-lin,Liang Xi-rong,Li Xian-hua.Separation of Sr,Sm and Nd in mineral and rock samples using selective specific resins[J].Rock Mineral Anal,2004,23(1):11–14(in Chinese with English abstract).
[23]Ma J L,Wei G J,Liu Y,Ren Z Y,Xu Y G,Yang Y H.Precise measurement of stable neodymium isotopes of geological materials by using MC-ICP-MS[J].J Anal Atom Spectr,2013,28(12):1926–1931.
[24]韦刚健,梁细荣,李献华,刘颖.(LP)MC-ICPMS方法精确测定液体和固体样品的Sr同位素组成[J].地球化学,2002,31(3):295–299.Wei Gang-jian,Liang Xi-rong,Li Xian-hua,Liu Ying.Precise measurement of Sr isotopic composition of liquid and solid base using(LP)MC-ICPMS[J].Geochimica,2002,31(3):295–299(in Chinese with English abstract).
[25]Mcarthur J M.Recent trends in strontium isotope stratigraphy[J].Terra Nova,1994,6(4):331–358.
[26]梁细荣,韦刚健,李献华,刘颖.利用MC-ICPMS精确测定143Nd/144Nd和Sm/Nd比值[J].地球化学,2003,32(1):91–96.Liang Xi-rong,Wei Gang-jian,Li Xian-hua,Liu Ying.Precise measurement of 143Nd/144Nd and Sm/Nd ratios using multiple-collectors inductively coupled plasma-mass spectrometry(MC-ICPMS)[J].Geochimica,2003,32(1):91–96(in Chinese with English abstract).
[27]Tanaka T,Togashi S,Kamioka H,Amakawa H,Kagami H,Hamamoto T,Yuhara M,Orihashi Y,Yoneda S,Shimizu H.JNdi-1:A neodymium isotopic reference in consistency with La Jolla neodymium[J].Chem Geol,2000,168(3/4):279–281.
[28]Jahn B M,Gallet S,Han J M.Geochemistry of the Xining,Xifeng and Jixian sections,Loess Plateau of China:Eolian dust provenance and paleosol evolution during the last 140 ka[J].Chem Geol,2001,178(1/4):71–94.
[29]Ma J L,Wei G J,Xu Y G,Long W G.Variations of Sr-Nd-Hf isotopic systematics in basalt during intensive weathering[J].Chem Geol,2010,269(3/4):376–385.
[30]Banner J L.Radiogenic isotopes:Systematics and applications to earth surface processes and chemical stratigraphy[J].Earth-Sci Rev,2004,65(3/4):141–194.
[31]Li J W,Zhang G L,Ruan L,Yang J L,Wang H L.Sr-Nd elements and isotopes as tracers of dust input in a tropical soil chronosequence[J].Geoderma,2016,262:227–234.
[32]Cheng M C,You C F,Lin F J,Chung C H,Huang K F.Seasonal variation in long-range transported dust to a subtropical islet offshore northern Taiwan:Chemical composition and Sr isotopic evidence in rainwater[J].Atmos Environ,2010,44(28):3386–3393.
[33]Han G L,Liu C Q.Strontium isotope and major ion chemistry of the rainwaters from Guiyang,Guizhou Province,China[J].Sci Total Environ,2006,364(1/3):165–174.
[34]Xu Z F,Li Y S,Tang Y,Han G L.Chemical and strontium isotope characterization of rainwater at an urban site in Loess Plateau,Northwest China[J].Atmos Res,2009,94(3):481–490.
[35]Rao W B,Han G L,Tan H B,Jiang S.Chemical and Sr isotopic compositions of rainwater on the Ordos Desert Plateau,Northwest China[J].Environ Earth Sci,2015,74(7):5759–5771.
[36]Xu Z F,Han G L.Chemical and strontium isotope characterization of rainwater in Beijing,China[J].Atmos Environ,2009,43(12):1954–1961.
[37]覃小群,蒋忠诚,张连凯,黄奇波,刘朋雨.珠江流域碳酸盐岩与硅酸盐岩风化对大气CO2汇的效应[J].地质通报,2015,34(9):1749–1757.Qin Xiao-qun,Jiang Zhong-cheng,Zhang Lian-kai,Huang Qi-bo,Liu Peng-yu.The difference of the weathering rate between carbonate rocks and silicate rocks and its effects on the atmospheric CO2 consumption in the Pearl River Basin[J].Geol Bull China,2015,34(9):1749–1757(in Chinese with English abstract).