洞庭湖流域全新世孢粉记录的植被、气候变化及人类活动

HOLOCENE PALYNOLOGICAL RECORDS OF VEGETATION,CLIMATE CHANGE AND HUMAN ACTIVITIES IN THE DONGTING LAKE WATERSHED

本研究选择洞庭湖流域内湖南十万古田泥炭(SWGT)、贵州梵净山九龙池湿地(JLC)和湖北江陵剖面(JL)三处研究点。SWGT钻孔深度为250 cm,通过AMS 14C测年使用Bacon软件进行线性插值建立年龄-深度模型,最底部年代达到约11.6 cal.ka B.P.;JLC钻孔深度为400 cm,通过AMS 14C测年使用Clam软件进行线性插值建立年龄-深度模型,最底部年代达到约11.9 cal.ka B.P.;JL剖面深度为594 cm,常规14C测年后经过树轮校正,用线性插值法内插和外推获得各对应深度的年代,最底部年代外推达到约9.4 cal.ka B.P.。通过对SWGT进行孢粉分析,集合已发表的JLC和JL的孢粉分析结果,获得了早中全新世洞庭湖流域的植被和气候变化特征,并利用现代类比法(MAT)重建了三处研究点的人类活动指数(HII),探讨了全新世人类活动的影响。研究结果显示:1)全新世早期,花粉类型以Evergreen Quercus、Cyclobalanopsis、Castanopsis、Fagus、Carpinus等亚热带植物花粉类型为主,草本植物的花粉含量整体较低,此时植被主要是亚热带常绿阔叶林,气候温暖湿润,东亚夏季风(EASM)强度较强。2)全新世中期,约9.0~4.0 cal.ka B.P.,乔木花粉含量最高,草本植物花粉含量最低,森林覆盖度最高。该阶段最主要特征是以Evergreen Quercus、Cyclobalanopsis为主的亚热带常绿阔叶树含量升高,气候更加暖湿,揭示了EASM强度进一步增强;尤其是6.0~4.0 cal.ka B.P.之间,Evergreen Quercus、Cyclobalanopsis含量达到最高,气候最为暖湿,标志着全新世适宜期的出现。3)晚全新世,4.0 cal.ka B.P.以来,HII出现最高值,反映了人类对环境影响更加剧烈;此时花粉数据显示较低的栎属、栲属、青冈栎属等植被含量,结合研究区出现较高的松属花粉百分比含量,表明人类大量砍伐阔叶森林,导致次生松树林扩张,与此同时,禾本科植物含量升高,揭示了人类活动开始在低海拔地区进行农业活动。4)太阳辐射、北半球高纬冰量和冰盖融化导致的融水通量等因素可能对早中全新世气候变化产生较大影响,而该区晚全新世记录更多受人类活动影响。本研究认为,在该区域利用孢粉进行全新世古气候定量重建时,很有必要对晚全新世人类活动信号进行识别。

The Dongting Lake watershed(24°38′29″~30°24′25″N,107°16′56″~114°14′50″E)is situated in the subtropical region of China,significantly influenced by monsoons,rendering it highly responsive to climate and vegetation changes.With a history of rice cultivation dating back to the Early Holocene,this watershed has become an ideal region for studying Holocene paleoclimatic changes,vegetation evolution,and ancient human activities.This study focuses on three research sites within the Dongting Lake watershed,namely the Shiwangutian peat(SWGT:26°05′N,110°21′E;1700 m a.s.l.)in southern Hunan,Jiulongchi wetland(JLC:27°55′N,108°41′E;2213 m a.s.l.)in Fanjing Mountain,Guizhou,and the Jiangling profile(JL:30°08′N,112°20′E;30 m a.s.l.)in Hubei.The SWGT core has a depth of 250 cm,and AMS 14C dating,coupled with Bacon model for linear interpolation,establishes an age-depth model,reaching an approximate age of 11.6 cal.ka B.P.at the bottom.The JLC core,with a depth of 400 cm,undergoes AMS 14C dating,utilizing Clam model for age-depth model interpolation,reaching an approximate age of 11.9 cal.ka B.P.at the bottom.The JL profile,with a depth of 594 cm,undergoes conventional 14C dating,followed by tree-ring calibration.Linear interpolation is used for age-depth model interpolation and extrapolation,reaching an approximate age of 9.4 cal.ka B.P.at the bottom.Palynological analysis of the SWGT,combined with published results from JLC and JL,provides insights into the vegetation and climate changes in the Dongting Lake watershed during the Early to mid-Holocene.Additionally,a Human Influence Index(HII)is reconstructed using the Modern Analog Technique(MAT)to explore the impact of human activities during the Holocene.Results indicate the following:(1)Early Holocene:Dominance of subtropical plant pollen types such as Evergreen Quercus,Cyclobalanopsis,Castanopsis,Fagus,and Carpinus,suggesting a warm and humid climate with a strong East Asian summer monsoon(EASM)intensity.(2)Mid-Holocene(approx.9.0~4.0 cal.ka B.P.):Highest tree pollen content,lowest herbaceous plant pollen content,and maximum forest coverage.Increased content of subtropical evergreen broadleaf trees,primarily Evergreen Quercus and Cyclobalanopsis,indicating a warmer and wetter climate with further strengthened EASM.Notably,between 6.0~4.0 cal.ka B.P.,Evergreen Quercus and Cyclobalanopsis reach their highest content,signifying the onset of an optimum period during the Holocene.(3)Late Holocene(4.0 cal.ka B.P.onwards):Highest HII values,reflecting intensified human impact on the environment.Decreased pollen content of Evergreen Quercus,Castanopsis,and Cyclobalanopsis,coupled with a higher percentage of pine pollen,indicates extensive deforestation by human activities,leading to the expansion of secondary pine forests.Simultaneously,an increase in Poaceae pollen suggests the initiation of agricultural activities in low-altitude areas.(4)Factors such as solar radiation,Northern Hemisphere high-latitude ice volume,and meltwater flux from ice sheet melting may have exerted significant influences on Early to mid-Holocene climate changes.However,the Late Holocene record in this region appears more strongly influenced by human activities.It is emphasized that identification of signals related to human activities is crucial when utilizing pollen for quantitative reconstruction of Holocene paleoclimate in this region.