肇庆高要泥炭沉积揭示的全新世植被演变及人类活动历史

HOLOCENE VEGETATION CHANGES AND HUMAN ACTIVITIES REVEALED BY A PEAT SEDIMENT CORE IN GAOYAO, ZHAOQING

位于珠江三角洲西北边缘地区的肇庆高要钻孔岩芯长610cm,其泥炭沉积物记录了全新世的沉积过程、植被演变和人类活动历史.根据木屑及泥炭沉积物的AMS-^14C测年,钻孔底部年龄按沉积速率外推大约为6100cal.a B.P..在孔深536～ 175cm段发现以水松（Glyptostrobus pensilis）为主的原位腐木根茎和泥炭沉积.孢粉研究表明近6000年来钻孔所在区域植被类型发生了明显变化,经历了常绿阔叶林→水松森林沼泽→常绿阔叶林→草地沼泽→次生林及草地农田5个阶段.当地的水松沼泽群落在4900～ 2900cal.aB.P.期间十分繁盛,该阶段后期随着沼泽湿地面积的缩小呈现缓慢衰退的趋势;在2900cal.aB.P.左右水松林面积大幅缩小,水松森林沼泽快速转变成浅水及早地莎草科、禾本科为主的草地沼泽;约1700cal.aB.P.后研究区域残余的天然水松基本消失.炭屑分析表明,火灾事件始于距今3500cal.aB.P.,与该地区新石器时代末期相对应,揭示了该区域早期农业的初始时代.乔木植物的比例在2660cal.a B.P.骤减,禾本科、里白/芒箕、松属等先锋属种大量生长,这个过程伴随着炭屑浓度的大幅增加,指示人类刀耕火种活动逐渐增强.研究表明,晚全新世三角洲进积、海岸带推移、潜水位下降和人类活动等综合因素导致了研究区域水松林的消亡,近2600年三角洲早期农业的发展大大加剧了当地水松群落的衰退速度.

The study site is located in Baizhu Town of Gaoyao County, Zhaoqing, about 800m away from modern Xinxing Stream that belongs to the river basin area of Xijiang.A great number of lowland depressions occur in this region of northwest uplands（Xijiang is a main branch of upstream rivers of the Pearl River Delta）.A 610cm-length core （22°54.072＇N, 112°20.427＇E） has been drilled on a low-mountainous depression of 29m altitude.The age-model is built based on a series of calibrated AMS ^14C ages（the bottom of the core is older than 5057cal.B.P.）.By linear projection of age-model, this core reveals a history of ca.6100a local sedimentary evolution, vegetation succession and human impact.An in-situ thick wood layer with peat and abundant Glyptostrobus pensilis occurs between the depth of 536cm and 175cm. A total of 98 pollen and micro-charcoal samples have been analyzed, and more than 82092 pollen grains were counted and identified.The average of the counted pollen numbers is 838 particles per sample.Pollen and spore are abundant with the average concentration as high as 179.916×10^3grains/g.There are 90 identified pollen and spore taxa, including 70 arboreal pollen taxa, 11 non-arboreal taxa, and 9 Pteridophytes.The most popular pollen of this core types are evergreen broadleaved trees, such as Castanopsis/Lithocarpus, Quercus, Moraceae, Rutaceae, Elaeocarpaceae, Symplocaceae etc. The pollen of wetland tree Glyptostrobus is extremely abundant in the wood layer.Pollen data reveals that there was a series of obvious changes in local vegetation, successively evergreen broadleaved forest, Glyptostrobus wetland forest, again evergreen broadleaved forest, herbaceous wetland（mainly Cyperaceae）, secondary forest and grassland/cultivated land（Poaceae）.The evolution pattern of local wetland has been clarified that forest wetland highly developed especially between 4900cal.B.P.and 2900cal.B.P., and a tendency shows that the taxon of wetland Glyptostrobus gradually declined and spatially reduced at around 2900cal.B.P.The herbaceous wetland and grass replace gradually the forest wetland marked by the increase of Cyperaceae and Poaceae.By ca.1700cal.B.P., pollen of Glyptostrobus tends to disappear, indicating local vanishment of the forest wetland.Charcoal analysis demonstrates that fire events initiate at about 3500cal.B.P., which is coinstantaneous with late Neolithic human influence of early agriculture occurred in many regions of the Pearl River Delta during that time.The proportion of arboreal pollen rapidly decreases at around 2660cal.B.P., in association with an increase of Poaceae, Dicranopteris and the secondary conifer plant of pine.The upper succession of local vegetation is also marked by significant increase in micro-charcoal.The lower section of the profile beneath 360cm has few charcoal particles with only 1707 grains/g in average, whereas in the upper part above 360cm, this charcoal type rise to 22236 grains/g.The coarse charcoal particles（〉100μm） reach the highest level between 290cm and 175cm depth with 13511 grains/g, inferring the development of ＂slash-and-burn＂ farming for rice. On the basis of pollen, charcoal results and regional correlation, we conclude that Holocene shift of the coastline, regional water table change and human activity are the multiple causes resulting in local forest wetland migration.The pre-history development of agriculture aggravates since ca.2600cal.B.P., which leads to the complete vanishment of local forest of Glyptostrobus.