第四纪研究  2018, Vol.38 Issue (4): 874-886   PDF    
沙漠/黄土过渡带6 ka B.P.以来气候环境变化及其对人类活动的影响
张贵林1,2,3, 周新郢1, 赵克良1, 杨庆江1,2,3, 李小强1,2,3     
(1 中国科学院古脊椎动物与古人类研究所, 中国科学院脊椎动物演化与人类起源重点实验室, 北京 100044;
2 中国科学院生物演化与环境卓越创新中心, 北京 100044;
3 中国科学院大学, 北京 100049)
摘要:文章选择沙漠/黄土过渡带的榆林市白河庙村钻孔(38°35'9.89″N,109°24'0.71″E)泥炭沉积,通过建立良好的年代序列和高分辨率花粉记录分析,重建研究区内植被演替和气候环境变化过程。在此基础上,结合考古学资料,探讨适宜的气候环境阶段和极端气候事件对人类活动的影响。结果表明,6 ka B.P.以来研究区植被类型经历典型草原、荒漠草原、干草原、荒漠草原4个阶段,指示6.0~4.2 ka B.P.气候较为暖湿,为全新世适宜期;4.2 ka B.P.时发生极端干旱事件,之后(4.2~1.1 ka B.P.)气候向干旱化发展;1.1~0.5 ka B.P.,气候经历短暂的湿润期;0.5 ka B.P.之后,气候再次转干。另外,通过统计沙漠/黄土过渡带6~3 ka B.P.考古遗址数量及其空间分布变化信息,发现史前人类活动与气候、生态系统变化紧密相关。东亚夏季风带来的降水量控制干旱/半干旱地区水资源变化,从而影响依赖水资源的旱作农业发展、人类活动强弱和考古学文化兴衰。6.0~4.2 ka B.P.降水较多,植被盖度高,毛乌素沙地固定,沙漠/黄土过渡带发育古土壤,并向西北移动。适宜的气候环境和良好的生态条件有利于旱作农业生产,导致人口增加,遗址点数量增多,人类活动范围扩大,仰韶时期和龙山时期的考古学文化繁荣发展。而4.2~3.0 ka B.P.遗址点大幅减少,人类活动规模减小,考古学文化衰退。4.2 ka B.P.极端干旱事件及之后持续的干旱,导致植被退化,毛乌素沙地沙丘活化,沙漠/黄土过渡带向东南方向摆动。水资源短缺和生态环境破坏,对以粟为主的旱作农业生产和人类生存造成严重影响,可能是导致该地区史前考古学文化大范围衰退的重要原因。
主题词沙漠/黄土过渡带     泥炭花粉记录     4.2 ka B.P.     极端干旱事件     人类活动    
中图分类号     Q913.84;P532;P941.73                     文献标识码    A

0 引言

我国沙漠/黄土过渡带地处东亚夏季风尾闾区,水资源匮乏,生态系统脆弱,对气候变化响应敏感[1]。毛乌素沙地东南缘与黄土高原北部丘陵沟壑区接壤地带[2],是我国沙漠/黄土过渡带的典型区域[3](图 1a1b)。独特的自然地理环境,使其成为全球变化研究的理想地区,受到广泛关注。科学家通过对沙漠/黄土过渡带内广泛分布的风成砂/黄土/古土壤沉积进行沉积相分析和地层空间对比,同时结合年代学、气候环境代用指标等研究方法,科学论证了冰期/间冰期长时间尺度上毛乌素沙地形成与演化、沙地时空分布、环境演变、季风环流变迁及与全球气候变化的联系等重要科学问题[1, 4~8]。近年来基于大范围沉积相调查工作和大量研究点位的批量测年结果,已基本确立毛乌素沙地在末次冰盛期(Last Glacial Maximum,简称LGM)和全新世大暖期(Holocene Megathermal)时的空间分布范围,这对特殊地质历史时期的古气候模拟、粉尘排放强度估算等有重要科学意义[9~11]

图 1 (a) 现代亚洲季风边界线;(b)沙漠/黄土过渡带白河庙钻孔地理位置示意图 Fig. 1 (a)Modern Asian summer monsoon limit; (b)The geographical location of Baihemiao core in desert/loess transition zone

植被是自然生态系统中最活跃的因子,花粉指标重建的区域性植被类型演替和空间格局变迁,是反映气候变化、沙地收缩与扩张、生态环境演变的直接证据。但是因为沙漠/黄土过渡带内缺乏可提供高分辨率花粉记录分析的地质档案,使得已有研究对全新世以来百年时间尺度上的植被演变历史关注较少[12~13]

史前人类活动和自然环境变化之间的关系,一直是备受关注的科学问题[14~15]。环境考古学家通过对新石器时代考古遗址的数量、空间分布范围和气候变化进行关联性分析,认为气候变化对史前考古学文化的兴衰有重要影响[16~31]。例如,在北方甘青地区,降水变化而非温度,是影响马家窑文化演替的主要因素[19];全新世适宜期夏季风增强,降水增多,湿润的气候有利于考古聚落数量增加和人类活动空间范围扩大[17, 20, 28~29],反之亦然。但在中国南方福建地区,人类活动加强,农业文明发展是在夏季风衰弱,降水减少的气候背景下完成的,与全新世适宜期暖湿的气候形成明显滞后性[32];另外,极端气候突变事件引发的干旱、洪水等自然灾害是导致史前考古学文化衰退的重要自然因素[15, 33~35]。总体来看,目前气候变化影响人类活动的机制和过程还不清晰[16],需要结合自然科学和考古学对具体区域进行分析查验,才能为解释这一问题提供更多视角和科学依据。石峁、寨峁、新华等重要遗址的考古发掘及研究[36~38],充分证明沙漠/黄土过渡区是中华文明形成时重要的组成部分。较为清晰的考古学文化发展脉络和丰富的考古遗址,为开展典型生态脆弱区学科交叉研究,探索自然环境变化与人类活动之间的联系提供了丰富的素材。

泥炭沉积敏感地记录了区域气候环境信息,为建立高分辨率气候变化序列提供了良好的地质载体[39~40]。本文选择的白河庙钻孔位于沙漠/黄土过渡带中心地区,通过对泥炭沉积进行可靠的测年和高分辨率花粉指标分析,重建区域内中晚全新世古植被演替、生态系统演变及气候环境变化;同时结合考古资料,统计与气候变化序列处于同一时间框架内的考古遗址点分布信息,以此来尝试探索适宜气候环境阶段内和极端气候事件发生时考古遗址的时空分布变化规律,揭示人类活动变化的气候环境背景及其影响机制。

1 研究区自然环境概况

2015年9月,使用冲击钻在榆林市白河庙村榆溪河滩地钻取岩芯。白河庙钻孔(38°35′9.89″N,109°24′0.71″E)位于黄土高原北部丘陵地貌向毛乌素沙地过渡地带,榆溪河下游,东距榆林市36 km(图 1b)。研究区属半干旱中温带大陆性季风气候,冬季受蒙古-西伯利亚高压影响,气候寒冷干燥,风沙活动频繁,夏季接受少量夏季风携带的水汽,气候暖湿。距离研究点最近的乌审旗气象站1981~2010年气象统计资料显示,该区域年均降水量约337 mm,降水集中在5~9月份,占全年降水总量的84 %,年均温约7.1 ℃(图 2)。研究点附近现代植被以禾本科、蒿类草原群落为主,低洼滩地为草甸、盐生草甸,柽柳(Tamarix)、沙柳(Salix psammophila)、柠条(Caragana korshinskii)等灌木被广泛栽培,形成大范围的人工灌木林。

图 2 乌审旗气象站1981~2010年降水和温度月值数据集 Fig. 2 Monthly average data set of precipitation and temperature in Wushenqi Weather Station in 1981~2010
2 白河庙钻孔岩性描述、年代序列及实验室方法 2.1 岩性描述

白河庙岩芯总厚6.19 m,根据不同的岩性,将岩芯从上至下划分如下(图 3a):

图 3 (a) 白河庙岩芯岩性及测年材料选择点;(b)Bacon年龄-深度模式建立的白河庙岩芯年代序列 Fig. 3 (a)Lithological characters and layers of dating materials of Baihemiao core; (b)Age sequence of Baihemiao core reconstructed by Bacon age-depth model

0~ 88 cm,岩芯上部为黄褐色粗砂,含现代植物根系;

88~ 150 cm,灰褐色粗砂,偶见植物残体;

150~ 155 cm,黑色粉砂质泥炭,含丰富的植物花序;

155~ 235 cm,灰褐色粗砂;

235~ 366 cm,黑色粉砂质泥炭,分解度较差,保留大量植物茎秆、叶片、种子;

366~ 472 cm,灰黑色细砂,其中390~420 cm含较多植物茎秆;

472~ 615 cm,灰褐色粗砂;

615~ 619 cm,黑色粉砂质泥炭,含大量炭屑。

2.2 年代序列

为建立白河庙泥炭沉积良好的年代框架,共选择植物种子、花序、炭屑、陆生植物残体等4个可靠的测年材料,送交Beta实验室进行AMS 14C定年(图 3a)。在获得14C年代后,使用OxCal v4.3.2和IntCal13软件将其校正为日历年龄[41~42]。以已测定的4个可靠年龄点为年代标尺,使用贝叶斯年龄模型软件Bacon 2.0年龄-深度模式[43],将泥炭沉积深度转换为年龄,建立岩芯的年代序列(图 3b)。

2.3 花粉分析

以间隔10 cm或20 cm厚度为间距,共采集42块孢粉样品。泥炭样品用量为2 g,其余样品为50~100 g。综合使用筛选、酸碱处理、重液浮选等方法提取花粉[44~45],外加石松孢子以计算花粉浓度。在Zeiss AxioImager. A2生物显微镜400X下对样品中提取出的花粉进行鉴定和统计,花粉形态鉴定参照已发表的花粉图谱[46~48]。其中37个样品统计花粉至300粒,5个样品统计花粉至100~150粒。根据陆生植物花粉的统计数量,使用Tilia软件计算不同类型花粉百分比,同时通过CONISS地层约束聚类分析对花粉组合进行分带,绘制花粉图谱。因花粉浓度在中国北方草原区与植被盖度并不一定呈线性关系[49~50],因此本文计算的花粉浓度只做参考,并未在图谱中列出。选择干旱-半干旱地区具有可靠生态指示意义的蒿属/藜科(A/C)花粉含量比值,来反映气候的干湿变化[49, 51~53]

3 史前考古遗址点的统计

考古遗址点的时空分布模式,可证明史前人类活动与自然环境的联系。因此,为查验适宜气候环境阶段和极端气候事件对人类活动的影响,本文参照《中国文物地图集——陕西分册》[54]以及相关考古学研究成果[55],首先使用ArcGIS软件对相关考古遗址点进行数字化处理,之后围绕107°59′36.04″E以东和37°26′33.77″N以北沙漠/黄土过渡带核心区域,统计仰韶时期庙底沟文化到夏商时期(6~3 ka B.P.)不同考古学时代考古遗址点的数量、空间分布范围、海拔高度等地理信息。3 ka B.P.之后考古遗址点受周边地区不同经济基础国家的政治、军事、交通、屯垦政策等社会因素影响大,因此未做统计。毛乌素沙地内部的考古遗址受风沙活动、沙漠扩张等埋藏条件限制,保存状况差,目前发现的数量极少且考古学文化背景不清晰,因此本文未统计沙地内部的考古遗址。

4 研究结果 4.1 年代序列

白河庙岩芯的测年结果见表 1,日历年龄显示白河庙岩芯底部619 cm深处的年代为6 cal.ka B.P. (表 1),基本涵盖了中晚全新世新石器时代考古学文化发展的关键时间段。另外,从Bacon模型[43]建立的年代序列和花粉样品量可计算出本文每个样品平均时间分辨率约为143年,基本符合高分辨率花粉分析的要求(图 3b)。

表 1 白河庙岩芯AMS 14C测年结果 Table 1 AMS 14C dating results of botanical materials of Baihemiao core
4.2 花粉分析结果

42个样品共鉴定花粉12921粒,分属30个科属(图 4),包括松属(Pinus)、云杉属(Picea)、冷杉属(Abies)、桦属(Betula)、栎属(Quercus)、柽柳属(Tamarix)、蒿属(Artemisia)、藜科(Chenopodiaceae)、禾本科(Poaceae)、豆科(Leguminosae)、唐松草属(Thalictrum)、莎草科(Cyperaceae)、麻黄属(Ephedra)、葎草属(Humulus)、狐尾藻属(Myriophyllum)、卷柏科(Selaginellaceae)等。其中,以蒿属、藜科为主的草本植物花粉百分含量(75 % ~99 %)占绝对优势,而以松属植物花粉为主的乔木花粉所占百分含量少(1 % ~24 %)(图 4)。

图 4 白河庙泥炭沉积中主要陆生植物花粉百分比图谱 Fig. 4 Percentage diagram of main terrestrial pollen types from Baihemiao peat

以岩芯年代序列为基础,依据花粉百分比含量聚类分析,从下至上可将花粉谱划分为4个不同的花粉组合带(图 4)。

带Ⅰ(619~350 cm,6.0~4.2 ka B.P.):草本植物花粉含量高(75 % ~99 %),其中以蒿属(21 % ~87 %)花粉含量占优势,藜科(3 % ~44 %)、禾本科(1 % ~23 %)以及喜湿的唐松草属(0~15 %)花粉含量也较高。乔木花粉含量以松属(1 % ~17 %)为主,桦属、栎属、胡桃科(Juglandaceae)、木犀科(Oleaceae)、榛属(Corylus)花粉等在短时期内有零星出现。A/C比值由6.0~5.8 ka B.P.的0.6~1.6,持续升高到2.3~25.0。

带Ⅱ(350~121 cm,4.2~1.1 ka B.P.):草本植物花粉含量含量(83 % ~98 %)仍然很高,以藜科花粉为主(17 % ~76 %),蒿属(21 % ~52 %)、禾本科(0~4 %)、唐松草属(0~2 %)花粉含量低,较带Ⅰ明显下降。灌木麻黄属(0~3 %)花粉含量达到全序列最高值。莎草科花粉含量上升(0~9 %)。乔木植物花粉含量(0~17 %)小,以松属为主(0~14 %)。桦属(0~2 %)、栎属(0~4 %)花粉有一定含量,其余乔木花粉含量极少,均小于1 %。A/C比值为0.6~2.6,较带Ⅰ显著降低。

带Ⅲ(121~50 cm,1.1~0.5 ka B.P.):草本植物花粉含量高达99 %,蒿属花粉(58 % ~78 %)含量较带Ⅱ明显上升,藜科花粉(15 % ~20 %)含量下降。菊科紫菀属(Aster)(0~10 %)和蒲公英属(Taraxacum)花粉(1 % ~3 %)含量增加。乔木花粉极少出现或消失。A/C比值升高,在2.9~5.6之间。

带Ⅳ(50~0 cm,0.5 ka B.P.以来):草本植物花粉含量(97 % ~99 %)占绝对优势,以藜科花粉为主(58 % ~79 %),蒿属花粉(13 % ~55 %)较少。乔木花粉只零星出现在接近地表的沉积中,含量极低(0~3 %)。A/C比值下降,在0.1~1.9之间。

4.3 研究区6~3 ka B.P.考古遗址点时空分布格局

图 5表 2的统计信息表明,仰韶中晚期(6~5 ka B.P.)考古遗址点有65处,集中分布于河流谷地或阶地(图 5a)。龙山时期(5~4 ka B.P.)遗址点显著增多,共有509处,数量约为仰韶时期的8倍,遗址密度明显增大。遗址点密集分布于黄河、窟野河、秃尾河等主要河流的谷地、阶地。另外,遗址点向毛乌素沙地东南缘、海拔较高的丘陵地区大规模扩展(图 5b)。这表明龙山时期人口数量大幅增加,人类活动空前加强,人类活动范围达到最大。夏商时期(4~3 ka B.P.),遗址点数量急剧下降,仅有29处,遗址点密度大幅降低。遗址较为分散,多分布于海拔较低、水资源条件较好的地区(图 5c)。表明夏商时期人口急剧减少,人类活动空间范围大幅缩小。

图 5 6~3 ka B.P.考古遗址点空间分布信息 (a)仰韶中晚期6~5 ka B.P.;(b)龙山时期5~4 ka B.P.;(c)夏商时期4~3 ka B.P. Fig. 5 Spatial and temporal distribution information of archaeological sites in 6~3 ka B.P. (a)Middle and late Yangshao Period 6~5 ka B.P.; (b)Longshan Period 5~4 ka B.P.; (c)Xia and Shang Period 4~3 ka B.P.

表 2 沙漠/黄土过渡区6~3 ka B.P.考古遗址点数量、空间分布范围、海拔高度统计信息 Table 2 Counting numbers of archaeological sites and their geographical information during 6~3 ka B.P.
5 讨论 5.1 区域植被和气候环境记录

根据白河庙钻孔沉积的花粉组合以及重要花粉属种的生态特征(图 4),可将该地区中晚全新世气候变化划分为4个阶段:

阶段Ⅰ(6.0~4.2 ka B.P.),蒿属花粉含量上升,并保持高值,A/C比值随着蒿属花粉的增加而持续上升。藜科花粉含量下降,5.8 ka B.P.后百分比含量小于25 %。喜湿的唐松草属花粉含量达到整个年代序列的峰值。乔木植物花粉含量虽少,但种类多样,以松属(最高含量17 %)为主。松属花粉产量大,传播能力强,具有超代表性。当孢粉组合带中松属花粉含量小于30 %时,指示研究区内可能无松林发育[56]。因此,白河庙研究点周围可能并无松属植物生长,松属花粉可能通过风力或水流作用,从黄土高原丘陵或河谷地区搬运而来。花粉组合反映研究区植被为典型草原,可能伴生有少量乔木,指示夏季风较为强盛,降水较多,温暖湿润的气候条件。

阶段Ⅱ(4.2~1.1 ka B.P.),孢粉组合在4.2 ka B.P.后发生明显变化,蒿属花粉含量较阶段Ⅰ迅速下降,藜科花粉含量相应快速上升,之后保持高值,A/C比值小于2。指示干旱气候的麻黄属花粉含量明显增加。花粉组合显示区域范围内植被为荒漠草原,而快速扩张的耐干旱植被,揭示4.2 ka B.P.突变的气候干旱事件,以及之后持续的干旱化。但花粉组合中喜湿的莎草科花粉含量增加,榆属、栎属等乔木有零星出现,结合该阶段发育以莎草科植物为主的泥炭等证据,证明研究区附近湖泊和河漫滩等局地环境较为湿润。

阶段Ⅲ(1.1~0.5 ka B.P.),蒿属花粉含量及A/C比值较阶段Ⅱ快速上升,藜科花粉含量的变化趋势与蒿属花粉相反。花粉组合表明,1.1~0.5 ka B.P.期间,研究区植被由阶段Ⅱ的荒漠草原短暂恢复为干草原,指示气候条件改善,降水增多,湿度增大。

阶段Ⅳ(0.5 ka B.P.以来),草本植物花粉以藜科为主,植被群落单一。A/C比值低,可能与过渡放牧和人类活动有关系[49]。花粉组合表明,0.5 ka B.P.后植被退化为荒漠草原,指示降水减少,湿度减小,气候再次向干旱化转变。

5.2 沙漠/黄土过渡带气候环境变化对人类活动的影响 5.2.1 中晚全新世气候变化的区域环境效应

白河庙泥炭沉积花粉记录显示,6.0~4.2 ka B.P.东亚夏季风带来的降水较多,气候湿润,为全新世适宜期(Holocene Optimum)。适宜的气候有利于良好生态环境的建立和持续稳定发展,使得区域内发育典型草原。这种环境效应在沙漠/黄土过渡带内多个黄土/古土壤剖面中都有记录。例如,榆林剖面磁化率、总有机碳含量在6.0~4.2 ka B.P.处于高值,区域植被生产力较高[57];三道沟剖面7.5~4.0 ka B.P.气候湿润,古土壤中粉砂和粘粒成分所占比例显著增加[58]。多个风成砂/古土壤剖面沉积相变化显示,全新世适宜期毛乌素沙地水分条件较好,广泛发育较厚砂质古土壤层,沙地收缩,沙丘基本被固定,沙漠/黄土过渡带向西北中纬度干旱区移动[9~11, 59~60]。和林格尔、姬塬、韩家梁、蔡家沟等黄土/古土壤剖面孢粉记录表明沙漠/黄土过渡带在10~4 ka B.P.发育草甸草原,并伴生有乔木,植被群落发育好[61~62]。另外,季风边缘区其他重要湖泊、泥炭沉积载体中均有类似的环境效应记录,例如,岱海、察素齐地区分别在7.90~4.45 ka B.P.和5.0~4.1 ka B.P.气候温暖湿润,发育针阔叶混交林[63~64];调角海子7.1~4.4 ka B.P.孢粉通量最高,盘星藻含量高,在山地发育以松属、桦属和栎属为主的针阔叶混交林[65]。黄土高原西部在8~4 ka B.P.气候暖湿,河流谷地森林植被繁茂,植被盖度高,形成广阔的湿地/沼泽相地层[66~67]。C4植物分布与东亚夏季风强度有关,全新世适宜期黄土高原C4植物增加,并向西北扩张[68]

4.2 ka B.P.白河庙地区气候突然变干,季风降水快速减少。4.2 ka B.P.事件可能是全球性的气候突变事件,在我国季风区及全球范围内多种地质载体中均有广泛记录[69~72]。黄土高原东部莲花洞石笋δ18 O表明,东亚夏季风在全新世适宜期后快速衰退,导致中国北方全新世在4 ka B.P.出现干旱[73];多个研究点的植被综合形成的湿度曲线表明,全新世季风边缘区最大湿度出现在中全新世8~4 ka B.P.,最干旱的环境在晚全新世4 ka B.P.后[74];4.2 ka B.P.极端气候事件,导致全新世气候适宜期结束,气候由湿润向干旱化转型[75]。季风边缘区生态系统对这一重要气候转型期响应明显。由于降水减少,植被盖度降低,4.4 ka B.P.后呼伦贝尔沙地内部沉积风成砂,沙丘活化,沙地扩张加剧[76];呼伦湖在4.40~3.35 ka B.P.发育以藜科为主的荒漠草原,气候极其干旱[77];达里湖湖泊水位在4.45~3.75 ka B.P.大幅降低[78];4.5 ka B.P.后青海湖区气候变冷干,特别是在3.9 ka B.P.后孢粉浓度、碳酸盐、总有机碳、总氮含量迅速减少,蒿属草原扩张,湖泊生产力下降[79]

5.2.2 适宜的气候环境阶段和极端气候事件对人类活动的影响

关中盆地鱼化寨遗址植物种子浮选结果表明,仰韶晚期(5.0~5.5 ka B.P.)甚至庙底沟文化时,以粟黍为主的北方旱作农业已取代狩猎采集,成为最主要的经济主体[80]。史前旱作农业体系的确立,农业生产增加,使先民有较为充足的食物来源,人口规模不断扩大[17, 81~82]。关中盆地的古代先民在仰韶中晚期,开始向周边地区扩散[83],而该时间段内沙漠/黄土过渡带受东亚夏季风影响,气候适宜,降水较多,毛乌素沙地向西北方向收缩。同时水资源较为丰富,植被群落发育良好,植被盖度高,成壤作用加强,发育古土壤层。适宜的自然环境和气候条件,有利于旱作农业生产和古代先民定居。考古遗址点统计信息表明(图 5表 2),6~5 ka B.P.庙底沟文化因素已经扩展到陕北沙漠/黄土过渡带。但该时间段内遗址点数量较少;到5~4 ka B.P.时,沙漠/黄土过渡带遗址点数量呈现爆发式增长,遗址密度大幅增加,这与适宜而且稳定的气候、农业经济发展、人口大量增加有密切联系。水资源是干旱/半干旱地区影响农业活动和人类生存的重要自然因子[14]。龙山时期古代先民多选择沙漠/黄土过渡带内主要河流的谷地和阶地定居[84],说明水资源的获取与利用是先民定居和开展农业活动优先考虑的因素。另外,龙山晚期古代先民开始修筑多个规模较大的石城。其中石峁城址为超大型中心聚落,面积约400万平方米,是目前发现的中国史前最大的城址[36]

东亚夏季风降水变化是影响我国北方干旱/半干旱地区生态系统最重要的自然因子[85],而大范围生态系统的突变则对人类生存策略和人类活动产生深远影响。在季风边缘区的浑善达克沙地,4.2 ka B.P.气候突变事件导致沙地内部水文系统快速转变,河流水位迅速降低,湖泊消失,地下水位大幅下降。由此造成生态系统破坏,引发不可逆的沙漠化,导致人类活动明显衰退[75]。4.2 ka B.P.后气候条件不稳定,气象灾害频发,如黄河中游泾河、七水河、北洛河等多地爆发大洪水,破坏了原有的人类生存条件,导致人类居址废弃[34, 87~88]。另外,甘青地区的齐家文化[21]、山东龙山文化[15]、长江中下游地区的石家河文化、良渚文化等在4.2 ka B.P.后大范围衰退可能均与气候突变事件及频繁的气象灾害有关[33, 35]。而沙漠/黄土过渡区4.2 ka B.P.发生的极端干旱事件,以及夏季风衰退引发的长期干旱,导致水资源匮乏,植被退化,风沙活动加强,沙漠化发展,毛乌素沙地向东南方向的沙漠/黄土过渡带扩张;同时大范围内降水持续减少,导致河流水量减小,湖泊干涸[89]。水分条件的大规模重组造成生态系统破坏,环境承载力降低,土地利用条件退化,对龙山时期水分变化响应敏感的以粟为主的旱作农业造成严重影响[90]。由于农业经济衰退,人口数量锐减,考古遗址点随之大幅减少,空间分布范围缩小,考古学文化衰退。

6 结论

白河庙泥炭沉积高分辨率孢粉记录表明,研究区6.0~4.2 ka B.P.发育典型草原,同时伴生少量乔木,植物种类多样,群落发育良好,指示夏季风强盛,降水较多,气候温暖湿润。4.2 ka B.P.时,以藜科为主的荒漠草原快速扩张,表明极端干旱事件发生。4.2 ka B.P.后,虽然在1.1~0.5 ka B.P.期间植被短暂恢复为草原,但总体上植被以荒漠草原为主,植物种类少,群落结构简单。植被类型演替过程表明4.2 ka B.P.后夏季风明显衰退,降水减少,气候逐渐干旱化。

季风变化控制的水资源分布以及与水资源密切相关的旱作农业发展,是影响人类活动的重要机制。6.0~4.2 ka B.P.湿润的气候推动沙漠/黄土过渡带向西北移动,较为充足的水资源和良好的生态环境促进了农业发展,使得仰韶时期和龙山时期的人口数量不断增加,人类活动逐渐加强,人类活动范围明显扩大。4.2 ka B.P.极端干旱事件及之后持续的干旱化,导致生态系统退化、沙漠扩张,沙漠/黄土过渡带向东南摆动。水资源短缺和旱作农业经济体系破坏,引发夏商时期人类活动大范围衰退。

致谢: 感谢审稿专家和杨美芳老师提出的重要修改意见;感谢地理空间数据云提供榆林地区DEM高程数据,国家气象科学数据共享服务平台提供乌审旗站点气象资料。

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Climate and environment change of desert/loess transition zone and its impact on prehistoric human activities since 6ka B.P.
Zhang Guilin1,2,3, Zhou Xinying1, Zhao Keliang1, Yang Qingjiang1,2,3, Li Xiaoqiang1,2,3     
(1 Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044;
2 CAS Center for Excellence in Life and Paleoenvironment, Beijing 100044;
3 University of Chinese Academy of Sciences, Beijing 100049)

Abstract

Baihemiao core(38°35'9.89″N, 109°24'0.71″E) was drilled in desert/loess transition zone. The core is 619 cm in length, and from top to bottom, mainly consists of yellowish brown eolian sand(0~88 cm), grey brown sand(88~150 cm, 155~235 cm, and 472~615 cm), black silty peat(150~155 cm, 235~366 cm, and 615~619 cm) and sandy silt(366~472 cm). The bottom of the core(619 cm) was dated to 6 cal.ka B.P. by precise radiocarbon date. Then high resolution fossil pollen data was used for reconstructing vegetation succession and climate change. In addition, ancient archaeological sites in 6~3 ka B.P. were counted, and their spatial and temporal distribution information were analysed in order to explore how optimum climate and extreme climate event affected human activities. The result shows that 12921 pollen grains in total were counted and identified in 42 samples, mainly including Pinus, Picea, Abies, Betula, Quercus, Tamarix, Artemisia, Chenopodiaceae, Poaceae, Leguminosae, Thalictrum, Cyperaceae, Ephedra, Humulus, Myriophyllum and Selaginellaceae. Herbaceous pollen dominated by Artemisia and Chenopodiaceae account for 75%to 99%of total pollen percentage content, and tree pollen content was 1%to 24%. Vegetation types experienced four phases since 6 ka B.P., including typical steppe, desert grassland, dry steppe and desert grassland. Vegetation succession presents four phases of climate change. 6.0~4.2 ka B.P., climate was warm and moist. 4.2~1.1 ka B.P., extreme arid climate event occurred and climate distinctly became dry. After 1.1 ka B.P., the moist climate resumed and then turned to dry condition. Climate change and archaeological data indicate ancient human activities were closely related to climate and ecosystem change. East Asian summer monsoon brought rainfalls and controlled water sources change in semi-arid and arid region, and then affected rain-fed agriculture, human activities and archaeological culture succession. 6.0~4.2 ka B.P., relatively sufficient rainfall and high vegetation cover fixed Mu Us Desert. Desert/loess transition zone expanded northwest. Moist climate condition and good ecological environment contributed to agriculture development and population growth. Therefore archaeological sites increased rapidly and spatial space of human activities expanded. However, 4.2~3.0 ka B.P., archaeological sites decreased and human activities fade. The development of archaeological culture was interrupted because extreme dry climate event of 4.2 ka B.P. and continuous dry climate conditions. Water sources were lack and made a severe impact on rain-fed agricultural production and human subsistence. Therefore, climate change was testified as a significant influencing factor of archaeological culture succession.
Key words: desert/loess transition     pollen record of peat     4.2 ka B.P.     extreme dry climate event     human activities