甘肃合水地区全新世以来土壤剖面黑碳记录及其气候变化
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摘要
针对甘肃合水马家村(MJC)全新世黄土剖面进行野外采样,分析磁化率和黑碳(焦炭和烟炱)等古气候指标,试图探讨黄土高原北部全新世以来生物质燃烧特征以及自然生态环境演变过程,结果表明:末次冰期和全新世早期(11500—8500 a BP)气候寒冷而干旱,区域野火事件时而发生,植被生物量是此阶段野火发生与蔓延的限制因素;距今8000年以来,黑碳和炭屑浓度整体呈现下降趋势。由于季节性降水增多从而抑制大范围火灾发生几率。然而,8000—7000 a BP和3500—2500 a BP野火频率明显上升,这可能与仰韶时期老官台文化和寺洼文化时期的古人类放火烧荒和开垦农田等活动有关;全新世晚期(3100 a BP至今)气候进一步干旱,生物质燃烧增强与人口增加和人类土地利用水平密切相关,其中1500—1000 a BP出现一次较为明显的峰值,这可能与隋唐以来该区农耕活动加强有关。可见,近2000年以来黄土高原北部地区生物质燃烧特征受到气候和人类活动复合驱动所控制。
Background, aim, and scope The study site is located in the climate-sensitive regions of the semi-arid and subhumid Asian monsoon belt, which is in the southwestern part of the ecotone between Chinese traditional dry farming and nomadic pastoral practice. The region has also experienced a long and complex regional landuse history, in which slash-and-burn cultivation by means of ignited fire and deforestation for dry farming has been traced back to at least 8000 year. Meanwhile, many of the Neolithic cultures that practiced dry farming by means of ignited fire arose on the loess plateau since the Holocene, such as the Laoguantai Culture(8000 —7000 a BP) and the Siwa Culture(~(14)C cal 3350 —2650 a BP). In this research, the Holocene loess-paleosol profiles were selected for its proximity to archaeological sites in the region. An interdisciplinary approach has been adopted to study wildfire history and its interactions with changes in monsoonal climate and human activities on the loess tableland during the Holocene. Materials and methods A high-resolution sedimentary charcoal and black carbon(char and soot) records from the loesssoil profile in the middle of loess plateau, combined with Magnetic susceptibility, Total organic carbon(TOC) analyses, pollen counts and other paleoclimaticproxies reveal past wildfire history and the evolution of natural ecological landscape. Results Black carbon(BC) preserved in accretionary loessesoil profiles recorded changes in fire, which may be due to climatic variations, changes in vegetation, and human activities. EC-soot contents showed a stable rising trend since 12000 years, although a marked decline occurred in the past 100 years. EC-char contents ranges from 0.031 mg ? g~(-1)to 0.179 mg ? g~(-1); EC-soot contents ranges from 0.072 mg ? g~(-1) to 0.149 mg ? g~(-1) since 12000 years BP. More specifically, EC-char values increased gradually from the early Holocene to relatively high during 8000 —7000 a BP and 3500 —2500 a BP and then gradually decrease. In the MJC profile, higher values of the micro-charcoal content occur at 8000 —7000 a BP and 3500 — 2500 a BP, and lower values occur during at 7000 — 4000 a BP,indicating that regional fire activity was high in the early and late Holocene, whereas fire was less frequent and pervasive in the middle Holocene. Peaks in the macro-charcoal content occur during 8000 — 7000 a BP and 3500 —2500 a BP, suggesting that local fires were frequent during those periods. Discussion The results suggest that charcoal and BC influxes with peak fluctuations consistently attest to important changes in seasonal precipitation variability and mirror with the gradual climate aridity trend of the entire region during the Holocene which are be strongly controlled by global and regional climate dynamics. Meanwhile, the detail analysis of the BC and charcoal signals inconsistently were attributed to the differences in transportation mechanisms and biomass burning process at regional and local scale. Drier and colder than present the late glacial and the early Holocene climates, local and regional wildfire infrequently occurred with Artemisia and Gramineae-dominated desert steppe landscape at the north of loess plateau registered by lower value of EC and micro-charcoal content. Wetter and warmer than present during the mid-Holocene(8500 —3100 a BP), Fire episodes largely reduced with the mixed forest and foreststeppe growth and woodland cover, However, localized wildfires in connection with human activities frequently occurred during 8000 —7000 a BP. Fire seemed to be applied to vegetation clearance for land reclamation for millet cultivation in the early Neolithic Age. and the subsequent increase in fire-episode frequency during 3500 —2500 a BP is consistent with cooler and drier conditions in the late Holocene and the Neolithic anthropogenic burning practices such as deforestation, land reclamation, and crop cultivation during Siwa Cuture(cal 3350 —2650 a BP). Therefore, land-use was shifted from arable farming to nomadic in the upper and middle reaches of the Jinghe River drainage. After 1500 a BP the amount of charcoal and black carbon sharply decreased and fire occurrence disappeared gradually because of the development to mixed agriculture of cultivation and semi animal husbandry. Conclusions Combining results from macro-charcoal, micro-charcoal and BC records can provide information about fire history at local, regional and broader scales. In eastern Asia, where there is a long history of relatively intensive human land-use, all types of paleofire data may be expected to reflect the importance not only of fire, but of changes in the seasonal monsoon and associated aridification during the Holocene, as well as shifts in cultivation and other human activities. Recommendations and perspectives We also must further explore paleofire records to disentangle the respective roles of climate, vegetation and humans in shaping fire activity the north of loess plateau, especially, the role of anthropogenic activities in present and past fire histories by charcoal and black carbon records. A broad and rigorous use of palaeoenvironmental mult-proxies will allow a better understanding of the mechanic link between fires activity, vegetation dynamics and monsoon climatic variability at hundred-millennial scales.
Background, aim, and scope The study site is located in the climate-sensitive regions of the semi-arid and subhumid Asian monsoon belt, which is in the southwestern part of the ecotone between Chinese traditional dry farming and nomadic pastoral practice. The region has also experienced a long and complex regional landuse history, in which slash-and-burn cultivation by means of ignited fire and deforestation for dry farming has been traced back to at least 8000 year. Meanwhile, many of the Neolithic cultures that practiced dry farming by means of ignited fire arose on the loess plateau since the Holocene, such as the Laoguantai Culture(8000 —7000 a BP) and the Siwa Culture(~(14)C cal 3350 —2650 a BP). In this research, the Holocene loess-paleosol profiles were selected for its proximity to archaeological sites in the region. An interdisciplinary approach has been adopted to study wildfire history and its interactions with changes in monsoonal climate and human activities on the loess tableland during the Holocene. Materials and methods A high-resolution sedimentary charcoal and black carbon(char and soot) records from the loesssoil profile in the middle of loess plateau, combined with Magnetic susceptibility, Total organic carbon(TOC) analyses, pollen counts and other paleoclimaticproxies reveal past wildfire history and the evolution of natural ecological landscape. Results Black carbon(BC) preserved in accretionary loessesoil profiles recorded changes in fire, which may be due to climatic variations, changes in vegetation, and human activities. EC-soot contents showed a stable rising trend since 12000 years, although a marked decline occurred in the past 100 years. EC-char contents ranges from 0.031 mg ? g~(-1)to 0.179 mg ? g~(-1); EC-soot contents ranges from 0.072 mg ? g~(-1) to 0.149 mg ? g~(-1) since 12000 years BP. More specifically, EC-char values increased gradually from the early Holocene to relatively high during 8000 —7000 a BP and 3500 —2500 a BP and then gradually decrease. In the MJC profile, higher values of the micro-charcoal content occur at 8000 —7000 a BP and 3500 — 2500 a BP, and lower values occur during at 7000 — 4000 a BP,indicating that regional fire activity was high in the early and late Holocene, whereas fire was less frequent and pervasive in the middle Holocene. Peaks in the macro-charcoal content occur during 8000 — 7000 a BP and 3500 —2500 a BP, suggesting that local fires were frequent during those periods. Discussion The results suggest that charcoal and BC influxes with peak fluctuations consistently attest to important changes in seasonal precipitation variability and mirror with the gradual climate aridity trend of the entire region during the Holocene which are be strongly controlled by global and regional climate dynamics. Meanwhile, the detail analysis of the BC and charcoal signals inconsistently were attributed to the differences in transportation mechanisms and biomass burning process at regional and local scale. Drier and colder than present the late glacial and the early Holocene climates, local and regional wildfire infrequently occurred with Artemisia and Gramineae-dominated desert steppe landscape at the north of loess plateau registered by lower value of EC and micro-charcoal content. Wetter and warmer than present during the mid-Holocene(8500 —3100 a BP), Fire episodes largely reduced with the mixed forest and foreststeppe growth and woodland cover, However, localized wildfires in connection with human activities frequently occurred during 8000 —7000 a BP. Fire seemed to be applied to vegetation clearance for land reclamation for millet cultivation in the early Neolithic Age. and the subsequent increase in fire-episode frequency during 3500 —2500 a BP is consistent with cooler and drier conditions in the late Holocene and the Neolithic anthropogenic burning practices such as deforestation, land reclamation, and crop cultivation during Siwa Cuture(cal 3350 —2650 a BP). Therefore, land-use was shifted from arable farming to nomadic in the upper and middle reaches of the Jinghe River drainage. After 1500 a BP the amount of charcoal and black carbon sharply decreased and fire occurrence disappeared gradually because of the development to mixed agriculture of cultivation and semi animal husbandry. Conclusions Combining results from macro-charcoal, micro-charcoal and BC records can provide information about fire history at local, regional and broader scales. In eastern Asia, where there is a long history of relatively intensive human land-use, all types of paleofire data may be expected to reflect the importance not only of fire, but of changes in the seasonal monsoon and associated aridification during the Holocene, as well as shifts in cultivation and other human activities. Recommendations and perspectives We also must further explore paleofire records to disentangle the respective roles of climate, vegetation and humans in shaping fire activity the north of loess plateau, especially, the role of anthropogenic activities in present and past fire histories by charcoal and black carbon records. A broad and rigorous use of palaeoenvironmental mult-proxies will allow a better understanding of the mechanic link between fires activity, vegetation dynamics and monsoon climatic variability at hundred-millennial scales.
引文
安芷生,吴国雄,李建平,等.2015.全球季风动力学与气候变化[J].地球环境学报,6(6):341-381.[An Z S,Wu G X,Li J P,et al.2015.Global monsoon dynamics and climate change[J].Journal of Earth Environment,6(6):341-381.]
甘肃省博物馆.1960.甘肃古文化遗存[J].考古学报,2:11-51.[Gansu Museum.1960.The ancient culture remains of Gansu[J].Journal of Archaeology,2:11-51.]
何凡能,李科,刘浩龙.2010.历史时期气候变化对中国古代农业影响的研究若干进展[J].地理研究,29(12):2189-2197.[He F N,Li K,Liu H L.2010.The influence of historical climate change on agriculture in ancience China[J].Geographical Research,29(12):2189-2197.]
庆阳县志编纂委员会.1993.庆阳县志[M].兰州:甘肃人民出版社.[Qingyang County Compilation Committee.1993.Qingyang County[M].Lanzhou:Gansu People’s Publishing House.]
谭志海,黄春长,庞奖励,等.2008.陇东黄土高原北部全新世野火历史的木炭屑记录[J].第四纪研究,33(4):733-739.[Tan Z H,Huang C C,Pang J L,et al.2008.Charcoal recorded Holocene fire history in the northern part of the Longdong Loess Plateau[J].Quaternary Sciences,33(4):733-739.]
谭志海,黄春长,庞奖励,等.2010.渭河流域全新世土壤剖面木炭屑记录及其古环境意义[J].中国生态农业学报,18(1):25-30.[Tan Z H,Huang C C,Pang JL,et al.2010.Charcoal records of Holocene loess-soil sequences and its palaeoenvironmental significance in Weihe River Drainage[J].Chinese Journal of EcoAgriculture,18(1):25-30.]
王守春.1992.河西走廊及其邻近地区历史时期人文变化和环境变化序列[M]//尹泽生,杨逸畴,王守春.西北干旱地区全新世环境变迁与人类文明兴衰.北京:地质出版社:64-82.[Wang S C.1992.Humanities sequence changes and environmental changes in the Hexi Corridor and adjacent areas[M]//Yin Z S,Yang Y C,Wang S C.Holocene environmental change and the rise and fall of human civilization in arid area of Northwest China.Beijing:Geological Publishing House:64-82.]
王有清,姚檀栋.2002.冰芯记录中末次间冰期冰期旋回气候突变事件的研究进展[J].冰川冻土,24(5):550-559.[Wang Y Q,Yao T D.2002.Progresses in the studies of abrupt climatic change events recorded in ice cores during the Last Glacial-Interglacial Cycle[J].Journal of Glaciology and Geocryology,24(5):550-559.]
谢瑞琚.2002.青海省和甘肃省的原始社会考古[M].北京:文化遗物出版社:11-258.[Xie R J.2002.Pre-historical archaeology in Gansu and Qinghai provinces[M].Beijing:Cultural Relics Publishing House:11-258.]
许清海,孔昭宸,陈旭东,等.2002.鄂尔多斯东部4000余年来的环境与人地关系的初步探讨[J].第四纪研究,22(2):105-112.[Xu Q H,Kong Z C,Chen X D,et al.2002.Discussion on the environment changes and the effects of human impacts in the east Ordos Plateau since4000 a BP[J].Quaternary Sciences,22(2):105-112.]
杨英,沈承德,易惟熙,等.2001.21 ka以来渭南黄土剖面的元素碳的记录[J].科学通报,46(8):688-698.[Yang Y,Shen C D,Yi W X,et al.2001.Elemental carbon record of loess plateau in Weinan since 21 ka[J].Chinese Science Bulletin,46(8):688-698.]
张健平,吕厚远.2006.现代植物炭屑形态的初步分析及其古环境意义[J].第四纪研究,26(5):857-863.[Zhang J PLüH Y.2006.Preliminary study of Charcoal morphology and its Environmental significance[J].Quaternary Science,26(5):857-863.]
Chen J H,Chen F H,Feng S,et al.2015.Hydroclimatic changes in China and surroundings during the Medieval Climate Anomaly and Little Ice Age:spatial patterns and possible mechanisms[J].Quaternary Science Reviews107:98-111.
Dykoski C A,Edwards R L,Cheng H,et al.2005.A highresolution,absolute-dated Holocene and deglacial Asian monsoon record from Dongge Cave,China[J].Earth&Planetary Science Letters,233:71-86.
Goldberg E D.1985.Black carbon in the environment[M].New York:John Wiley and Sons.
Haberle S G,Hope G S,Kaars S V.2001.Biomass burning in Indonesia and Papua New Guinea:Natural and human induced fire events in the fossil record[J].Palaeogeography,Palaeoclimatology,Palaeoecology,171(3):259-268.
Han Y M,Cao J J,Chow J C,et al.2007.Evaluation of the thermal/optical reflectance method for discrimination between EC-char and EC-soot[J].Chemosphere,69:569-574.
Han Y M,Manrlon J R,Cao J J,et al.2012.Holocene linkages between char,soot,biomass burning and climate from Lake Daihai,China[J].Global Biogeochemical Cycles,26,GB4017.DOI:10.1029/2011GB004197.
Huang C C,Pang J L,Su H X,et al.2009.The Ustic Isohumisol(Chernozem)distributed over the Chinese Loess Plateau:Modern soil or palaeosol[J].Geoderma,150(3/4):344-358.
Huang C C,Pang J L,Zhou Q Y,et al.2004.Holocene pedogenic change and the emergence and decline of rainfed cereal agriculture on the Chinese loess plateau[J]Quaternary Science Reviews,23(23/24):2525-2535.
Jiang H C,Ding Z L.2005.Temporal and spatial changes of vegetation cover on the Chinese Loess Plateau through the last glacial cycle:Evidence from spore-pollen records[J].Review of Palaeobotany and Palynology,133(1/2):23-37.
Johnson E A.1992.Fire and vegetation dynamics:Studies from the North American boreal forest[M].Cambridge:Cambridge University Press:5-41.
Kawamura K,Parrenin F,Lisiecki L,et al.2007.Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years[J].Nature,448:912-916.
Masiello C A.2004.New directions in black carbon organic geochemistry[J].Marine Chemistry,92(1/2/3/4):201-213.
Mayewski P A,Rohling E E,Stager J C,et al.2004.Holocene climate variability[J].Quaternary Research,62(3):243-255.
Patterson W A,Edwards K J,Maguire D J.1987.Microscopic charcoal as a fossil indicator of fire[J].Quaternary Science Reviews,6:3-23.
Roberts N.1992.The Holocene:An environmental history(Paperback)[M].Oxford:Black-well:1-315.
Tan Z H,Han Y M,Cao J J,et al.2015.Holocene wildfire history and human activity from high-resolution charcoal and elemental black carbon records in the Guanzhong Basin of the Loess Plateau,China[J].Quaternary Science Reviews,109:76-87.
Wang X,Peng P A,Ding Z L.2005.Black carbon records in Chinese Loess Plateau over the last two glacial cycles and implications for paleofires[J].Palaeogeography,Palaeoclimatology,Palaeoecology,223(1/2):9-19.
Zhou B,Shen C D,Sun W D,et al.2007.Elemental carbon record of paleofire history on the Chinese Loess Plateau during the last 420 ka and its response to environmental and climate changes[J].Palaeogeography,Palaeoclimatology,Palaeoecology,252:617-625.
Zhou X J,Zhao P,Liu G,et al.2011.Characteristics of decadalcentennial-scale changes in East Asian summer monsoon circulation and precipitation during the Medieval Warm Period and Little Ice Age and in the present day[J].Chinese Science Bulletin,56:4651-4653.
甘肃省博物馆.1960.甘肃古文化遗存[J].考古学报,2:11-51.[Gansu Museum.1960.The ancient culture remains of Gansu[J].Journal of Archaeology,2:11-51.]
何凡能,李科,刘浩龙.2010.历史时期气候变化对中国古代农业影响的研究若干进展[J].地理研究,29(12):2189-2197.[He F N,Li K,Liu H L.2010.The influence of historical climate change on agriculture in ancience China[J].Geographical Research,29(12):2189-2197.]
庆阳县志编纂委员会.1993.庆阳县志[M].兰州:甘肃人民出版社.[Qingyang County Compilation Committee.1993.Qingyang County[M].Lanzhou:Gansu People’s Publishing House.]
谭志海,黄春长,庞奖励,等.2008.陇东黄土高原北部全新世野火历史的木炭屑记录[J].第四纪研究,33(4):733-739.[Tan Z H,Huang C C,Pang J L,et al.2008.Charcoal recorded Holocene fire history in the northern part of the Longdong Loess Plateau[J].Quaternary Sciences,33(4):733-739.]
谭志海,黄春长,庞奖励,等.2010.渭河流域全新世土壤剖面木炭屑记录及其古环境意义[J].中国生态农业学报,18(1):25-30.[Tan Z H,Huang C C,Pang JL,et al.2010.Charcoal records of Holocene loess-soil sequences and its palaeoenvironmental significance in Weihe River Drainage[J].Chinese Journal of EcoAgriculture,18(1):25-30.]
王守春.1992.河西走廊及其邻近地区历史时期人文变化和环境变化序列[M]//尹泽生,杨逸畴,王守春.西北干旱地区全新世环境变迁与人类文明兴衰.北京:地质出版社:64-82.[Wang S C.1992.Humanities sequence changes and environmental changes in the Hexi Corridor and adjacent areas[M]//Yin Z S,Yang Y C,Wang S C.Holocene environmental change and the rise and fall of human civilization in arid area of Northwest China.Beijing:Geological Publishing House:64-82.]
王有清,姚檀栋.2002.冰芯记录中末次间冰期冰期旋回气候突变事件的研究进展[J].冰川冻土,24(5):550-559.[Wang Y Q,Yao T D.2002.Progresses in the studies of abrupt climatic change events recorded in ice cores during the Last Glacial-Interglacial Cycle[J].Journal of Glaciology and Geocryology,24(5):550-559.]
谢瑞琚.2002.青海省和甘肃省的原始社会考古[M].北京:文化遗物出版社:11-258.[Xie R J.2002.Pre-historical archaeology in Gansu and Qinghai provinces[M].Beijing:Cultural Relics Publishing House:11-258.]
许清海,孔昭宸,陈旭东,等.2002.鄂尔多斯东部4000余年来的环境与人地关系的初步探讨[J].第四纪研究,22(2):105-112.[Xu Q H,Kong Z C,Chen X D,et al.2002.Discussion on the environment changes and the effects of human impacts in the east Ordos Plateau since4000 a BP[J].Quaternary Sciences,22(2):105-112.]
杨英,沈承德,易惟熙,等.2001.21 ka以来渭南黄土剖面的元素碳的记录[J].科学通报,46(8):688-698.[Yang Y,Shen C D,Yi W X,et al.2001.Elemental carbon record of loess plateau in Weinan since 21 ka[J].Chinese Science Bulletin,46(8):688-698.]
张健平,吕厚远.2006.现代植物炭屑形态的初步分析及其古环境意义[J].第四纪研究,26(5):857-863.[Zhang J PLüH Y.2006.Preliminary study of Charcoal morphology and its Environmental significance[J].Quaternary Science,26(5):857-863.]
Chen J H,Chen F H,Feng S,et al.2015.Hydroclimatic changes in China and surroundings during the Medieval Climate Anomaly and Little Ice Age:spatial patterns and possible mechanisms[J].Quaternary Science Reviews107:98-111.
Dykoski C A,Edwards R L,Cheng H,et al.2005.A highresolution,absolute-dated Holocene and deglacial Asian monsoon record from Dongge Cave,China[J].Earth&Planetary Science Letters,233:71-86.
Goldberg E D.1985.Black carbon in the environment[M].New York:John Wiley and Sons.
Haberle S G,Hope G S,Kaars S V.2001.Biomass burning in Indonesia and Papua New Guinea:Natural and human induced fire events in the fossil record[J].Palaeogeography,Palaeoclimatology,Palaeoecology,171(3):259-268.
Han Y M,Cao J J,Chow J C,et al.2007.Evaluation of the thermal/optical reflectance method for discrimination between EC-char and EC-soot[J].Chemosphere,69:569-574.
Han Y M,Manrlon J R,Cao J J,et al.2012.Holocene linkages between char,soot,biomass burning and climate from Lake Daihai,China[J].Global Biogeochemical Cycles,26,GB4017.DOI:10.1029/2011GB004197.
Huang C C,Pang J L,Su H X,et al.2009.The Ustic Isohumisol(Chernozem)distributed over the Chinese Loess Plateau:Modern soil or palaeosol[J].Geoderma,150(3/4):344-358.
Huang C C,Pang J L,Zhou Q Y,et al.2004.Holocene pedogenic change and the emergence and decline of rainfed cereal agriculture on the Chinese loess plateau[J]Quaternary Science Reviews,23(23/24):2525-2535.
Jiang H C,Ding Z L.2005.Temporal and spatial changes of vegetation cover on the Chinese Loess Plateau through the last glacial cycle:Evidence from spore-pollen records[J].Review of Palaeobotany and Palynology,133(1/2):23-37.
Johnson E A.1992.Fire and vegetation dynamics:Studies from the North American boreal forest[M].Cambridge:Cambridge University Press:5-41.
Kawamura K,Parrenin F,Lisiecki L,et al.2007.Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years[J].Nature,448:912-916.
Masiello C A.2004.New directions in black carbon organic geochemistry[J].Marine Chemistry,92(1/2/3/4):201-213.
Mayewski P A,Rohling E E,Stager J C,et al.2004.Holocene climate variability[J].Quaternary Research,62(3):243-255.
Patterson W A,Edwards K J,Maguire D J.1987.Microscopic charcoal as a fossil indicator of fire[J].Quaternary Science Reviews,6:3-23.
Roberts N.1992.The Holocene:An environmental history(Paperback)[M].Oxford:Black-well:1-315.
Tan Z H,Han Y M,Cao J J,et al.2015.Holocene wildfire history and human activity from high-resolution charcoal and elemental black carbon records in the Guanzhong Basin of the Loess Plateau,China[J].Quaternary Science Reviews,109:76-87.
Wang X,Peng P A,Ding Z L.2005.Black carbon records in Chinese Loess Plateau over the last two glacial cycles and implications for paleofires[J].Palaeogeography,Palaeoclimatology,Palaeoecology,223(1/2):9-19.
Zhou B,Shen C D,Sun W D,et al.2007.Elemental carbon record of paleofire history on the Chinese Loess Plateau during the last 420 ka and its response to environmental and climate changes[J].Palaeogeography,Palaeoclimatology,Palaeoecology,252:617-625.
Zhou X J,Zhao P,Liu G,et al.2011.Characteristics of decadalcentennial-scale changes in East Asian summer monsoon circulation and precipitation during the Medieval Warm Period and Little Ice Age and in the present day[J].Chinese Science Bulletin,56:4651-4653.