Lake level changes of central-northern Eurasia and their indicative significance for paleoclimate since Last Glacial Maximum
ZHANG Fengju,1, XUE Bin,2, YU Ge21. School of Geography, Geomatics and Planning, Jiangsu Normal University, Xuzhou 221116, Jiangsu, China 2. State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, CAS, Nanjing 210008, China
National Key Research and Development Program of China(2019YFA0607100) National Natural Science Foundation of China(41807281)
作者简介 About authors 张风菊(1988-), 女, 博士, 讲师, 主要从事湖泊沉积与环境演化研究。E-mail: fjzhang8899@163.com
摘要 湖泊水位高低通常能有效地指示湖盆内湿润条件的变化,进而反映区域有效降水(降水—蒸发)变化,成为重建第四纪古气候演变的重要指标之一。通过对苏联和蒙古国古湖泊数据库以及中国晚第四纪古湖泊数据库中149个湖泊水位变化资料的梳理总结,探讨了末次盛冰期(18 cal. ka BP)以来该地区干湿变化规律及区域分异。根据研究区气候特征、地理位置及已有研究成果将其分为东欧湖泊区、中东亚干旱区和中国北方季风区三大湖区。根据不同水位记录在整个湖泊历史中出现的频率,采用3级重新分类区分出高、中、低3级水量,并把每个湖泊数字化的3级古水量表示成与现代的差值,得到每个湖泊样点每千年时间间隔内相对现代的5级水量变化(很湿润、湿润、无变化、干旱和很干旱)。结果表明,三大湖区末次盛冰期以来可能经历了不同的干湿变化过程:东欧地区湖泊水量记录在晚冰期之前较少,至全新世逐渐增多,且基本表现为早全新世干旱、中晚全新世相对湿润的状况。中东亚干旱区整体呈现出末次盛冰期至中全新世均较湿润而晚全新世干旱的气候状况,但区域内部不同湖泊在起讫时间和强度上存在显著差异。中国北方季风区的湿润期主要发生在早中全新世,但是不同湖泊有所不同。对比分析显示,早全新世时东欧地区东部气候随着斯堪的那维亚冰流的逐渐消退而逐渐变湿润,中全新世由于夏季北欧反气旋东翼的气旋气流增强而达到最湿润状态,西部地区早全新世由于强劲的西伯利亚热高压存在而整体偏干旱,中全新世由于夏季亚洲季风的渗透而转为湿润。中东亚干旱区冰期内的湿润条件可能主要与西风带降水及低温低蒸发有关,而全新世则可能主要与夏季风深入内陆导致降水增加有关。中国北方季风区全新世湿度变化可能主要受东亚季风控制。 关键词:末次盛冰期;湖泊水位变化;古气候变化;欧亚大陆中北部
Abstract Fluctuations of lake level are good indicators of broad-scale wetness and thus can reflect the changes in the moisture balance (precipitation minus evaporation), which has been an important index in reconstructing the paleoclimate of late Quaternary. Spatial and temporal patterns of humidity changes since Last Glacial Maximum (LGM, 18 cal. ka BP) have been reconstructed for central and northern Eurasia based on the compilation of 149 lake records from the database documentation of the Soviet Union and Mongolia as well as China. The study areas were divided into three regions according to the climate differences, geographic locations and existing research results, that are, the Eastern Europe lake region, the arid Central Asian lake region and the East Asian monsoon lake region of northern China. A three-category scheme (collapsed coding) was used here to reclassify different lake statuses in order to standardize the lake level data over the past 18000 years. Then we used five-scheme status anomaly within each 1000-yr interval and the present to map and analyze the spatial patterns of five levels, namely, much drier, drier, no change, wetter and much wetter than today. The results demonstrated that moisture conditions have experienced diverse changes since LGM in different lake regions. There were relatively few lake records in Eastern Europe before late glacial period and the records increased substantially since the Holocene. Arid climate prevailed during the early Holocene in Eastern Europe, and relatively wet conditions dominated the middle and late Holocene. The arid Central Asian lake region as a whole experienced a wet climate from the LGM to the middle Holocene and turned dry during the late Holocene. But the patterns of climate change differed from lake to lake. In the East Asian monsoon region of northern China, the wet period was mainly observed in the early and middle Holocene and then the climate became drier in the late Holocene. The comparative analysis revealed that the eastern part of Eastern Europe lake region presented a wet trend with the retreat of the Scandinavian ice sheet during the early Holocene and the humidity reached a maximum in the mid-Holocene associated with cyclone activity on the downstream limb of the summer anticyclone. However, the western part of Eastern Europe experienced drier conditions in the early Holocene due to the development of a strong thermal high-pressure cell over central and western Siberia, and then returned to wetter conditions in the mid-Holocene as the result of the penetration of the Asian monsoon. The relatively wet condition in the arid Central Asian lake region during the glacial period was mainly produced by the precipitation brought by the westerlies and the significant decrease in evaporation, while the wet mid-Holocene was probably related to the strengthening of summer monsoon precipitation. The regional climate may have been mainly controlled by East Asian monsoon in the monsoon region of northern China. Keywords:LGM;lake level changes;paleoclimate changes;the central-northern Eurasia
PDF (3432KB)元数据多维度评价相关文章导出EndNote|Ris|Bibtex收藏本文 本文引用格式 张风菊, 薛滨, 于革. 欧亚大陆中北部末次盛冰期以来的湖泊水位变化及其古气候指示意义. 地理学报, 2021, 76(11): 2673-2684 doi:10.11821/dlxb202111006 ZHANG Fengju, XUE Bin, YU Ge. Lake level changes of central-northern Eurasia and their indicative significance for paleoclimate since Last Glacial Maximum. Acta Geographica Sinice, 2021, 76(11): 2673-2684 doi:10.11821/dlxb202111006
本文基于苏联和蒙古国古湖泊数据库[6]以及中国晚第四纪古湖泊数据库(第二版)[7]中已有的古湖泊水位记录资料,对末次盛冰期(18 cal. ka BP)以来欧亚大陆中北部地区湖泊水位变化进行分析,揭示不同区域湖泊水位变化历史及其空间分布特征,并探讨气候驱动机制下湖泊水位变化的原因,以期为客观认识和理解欧亚地区各气候系统之间的内在联系及深入解析全球变化机制提供一定参考。
CLSDB.2中的所有湖泊点年代均已校正为日历年龄(cal. a BP)。考虑到FSUDB.2中的湖泊点年代均为原始14C年龄,本文针对该数据库中的每一个湖泊点年代数据,采用OxCal 4.30程序校正为日历年龄,因此本文中所有时间体系均采用日历年。值得一提的是,为便于湖泊间的对比研究,本文中所采用的年代均暂未考虑其复杂的碳库效应。
FSUDB.2和CLSDB.2中湖泊不同时期水位数字化等级划分主要根据沉积岩芯代用指标、湖区地貌调查以及考古信息综合汇总等。例如,如果湖泊沉积物为粉泥沉积,纹理发育,且硅藻组合中以浮游种占优势,则这一时段湖泊水位就相对较高。如果某一时期湖泊沉积物为有大量附生硅藻的沼泽沉积,则该阶段湖水位就相对较低。以内蒙古白素海为例,钻取于湖岸的岩芯资料表明[7],自960 cal. a BP以来,钻孔处缺失沉积,且顶部有现代植物根系,表明湖泊在该时期并未到达钻孔位置处,因此这一时段是白素海历史记录以来水位最低时期(即水位级别划为1,极低)。9840—5710 cal. a BP时期为泥炭层沉积,说明水位仍较低(即水位级别划为2,低)。16100—8710 cal. a BP及6730—3230 cal. a BP时期为灰绿色黏土或粉砂质黏土沉积,粒度相对较粗,且出现软体动物壳及大量的植物大化石,说明水位相对较高(此时水位级别划为3,中等)。3230—960 cal. a BP期间为灰绿色黏土沉积,粒度较细,软体动物壳数量较少且无植物大化石出现,表明该阶段为白素海水位最高时期(即水位级别划为4,高)。具体到每个湖泊的水位划分级别及依据,可参考FSUDB.2和CLSDB.2。
Fig. 7Lake status, expressed as differences from present during the late Holocene
3 末次盛冰期以来不同湖区湖泊水位变化特征
18—16 cal. ka BP的末次盛冰期保留下来的地质证据表明,东欧地区有水位记录的湖泊点相对较少,且零星记录显示湖泊大多表现为中湖面状况,与现今水位类似。中东亚干旱区湖泊记录点主要集中在中国西北地区,且主要以高水位为主,偶见个别低水位湖泊,表明当时气候相对湿润,这和现今该地区气候格局形成较大反差。中国西部地区古降水量重建结果显示该时期年降水量总体比现代高100 mm以上[12],也表明该时期有效湿度较大,对应着湖泊高水位时期。中国北方季风区此时湖泊记录点相对较少,已有资料显示此时湖面呈现高低并存的状况,如华北地区的宁晋泊为高湖面,而东北地区的二龙湾则为低湖面状态,反映同一区域内不同湖泊水位波动仍存有较大差异。
晚冰期(15—12 cal. ka BP),东欧地区湖泊水位记录在前期(13 cal. ka BP之前)仍较少,后期有所增加。尽管该阶段东欧湖泊以中高湖面为主,但低湖面所占比例也相对较高,表明此时不同湖泊地面有效湿度差异较大。中东亚干旱区此时湖泊记录有所增加,特别是蒙古境内开始出现湖泊记录。湖泊水量距平显示以高水位为主,反映该时期有效湿度状况仍较好,只是在晚冰期的后期低水位湖泊记录有所增加。中国北方季风区湖泊记录此时以中低湖面为主,但后期(约13 cal. ka BP)高湖面比例有所增加,预示着有效湿度状况呈逐渐增加趋势。
全新世以来不同地区的湖泊水量记录更加丰富,不仅在东欧及中东亚干旱区,中国北方季风区也有较多的记录。早全新世时(11—9 cal. ka BP),东欧地区湖泊水位发生了显著变化,总体以中低湖面为主,表明湿润状况较前期有一定程度的减弱。中东亚干旱区尽管总体反映了比现代水量大的特征,但不同湖泊所反映的干、湿状况差异显著。比如蒙古境内的Gun-Nuur湖及中亚的Borovoe湖均显示为低湖面,中国的呼伦湖、乌伦古湖等也表现为低水量特征,但中国北方非季风区大多湖泊仍以中高水位为主。这种不同地点气候所表现出的不一致性,一方面可能与不同湖泊地理位置的变化及其相应的水热组合差异有关[13,14],另一方面也可能与不同区域对气候变化的响应存在差异有关[10]。早全新世中东亚干旱区湖泊记录仍以中国北方非季风区点位为主,导致该时期整体仍表现为相对湿润的气候状况。早全新世后期(9 cal. ka BP),中东亚干旱区高水量湖泊比例显著增加。中国北部季风区在该阶段尽管夹杂零星的“无变化”和“干旱”的记录,但湖泊水量距平总体反映的是较现在湿润的状况,说明全新世早期降水总体比现在偏高。
中全新世(8—5 cal. ka BP)为距今最近的一个温暖时期,也是历来古气候模型模拟的重要时段之一[15,16]。东欧地区中高水量湖泊比例较之前明显增加,对应着相当湿润的气候条件。中东亚干旱区及中国北方季风区高水量湖泊的比例也较之前有所增加,且达到末次盛冰期以来的最高值,反映这两个地区均表现为研究时段以来最为湿润的状态。蒙古湖泊孢粉记录显示中全新世气候较为暖湿[17,18],中国区域内的其他研究也记录了相近的变化过程。如北方沙漠地区从末次盛冰期到中全新世分别向北向西撤退了大约300 km和1000 km[19,20],黄土记录也表明中全新世为全新世最适宜期[21,22],此外,中国古降雨量定量重建结果也表明,中全新世黄土高原地区降雨量增加了约100~150 mm,东北平原年降雨量高于现代约100 mm,华北地区的宁晋泊、白洋淀区域降雨量增加值约为200~400 mm[23,24],均对应于此时较湿润的气候状况。但值得注意的是,该时期中东亚干旱区及中国北方季风区湖泊水位存在一定的波动,特别是中全新世后期,高水位湖泊的范围与幅度所减少,出现一些和现代湖泊环境接近甚至比现代干旱的气候条件。
晚全新世以来(4—0 cal. ka BP),三大湖区湖泊水量资料均反映出气候类型逐渐向现代过渡的状况。图7中“无变化”的记录逐渐增加,但该时期不同地区湖泊水位变化仍存在一定差异。东欧地区湖泊记录继续增加,尽管湖泊水量距平状况较前期有一定程度的减弱,但变化幅度并不十分显著,只是到近千年才出现大量“无变化”的记录。中东亚干旱区湖泊晚全新世以来整体以低水位为主,但也存在一定波动。如约在2 cal. ka BP时期该地区湖水位呈现略微上涨趋势,表明此时有一个短时间尺度的变湿波动。其后干旱化趋势明显,气候逐渐过渡为与现代相似的情况。中国北方季风区整体表现为研究时段内湖泊水位最低时期。特别是自3 cal. ka BP开始,湖泊水量出现大幅下降,整体表现为低水量和干旱的特征,只有极少数湖泊仍然维持高水量或中等水量的特点。
末次盛冰期和晚冰期东欧湖区的湖泊记录点相对较少,很难据此给出湖泊水量变化的确切解释。然而已有孢粉及植物大化石资料表明,由于斯堪的那维亚冰盖的扩张导致欧洲反气旋强盛以及劳伦冰盖的存在导致西风带南移,盛冰期时苏联大部为干冷气候[25]。全新世初期,东欧湖区总体表现为较干旱的气候,但西伯利亚东西两侧气候状况有所不同。东部显示了逐渐向湿润过渡的过程,而西部广大地区则一直比现代干旱。前者的湿润可能与斯堪的那维亚冰流的逐渐消退有关,而后者的干旱则可能主要由全新世初期夏季辐射的增加导致西伯利亚中部和西部的热高压比现在强势导致[26,27]。自8 cal. ka BP以后,东欧湖区东部湖泊水量出现显著增加,反映此时来源于夏季北欧反气旋东翼的气旋气流增强[26]。同时,西伯利亚地区的湖泊也出现比现代潮湿或与现今类似的水位状况(图6),可能为夏季亚洲季风渗透的结果[26]。晚全新世,湖泊资料显示了一个从先前相对湿润逐渐过渡到现代的气候类型。
来自中东亚干旱区的湖泊资料清楚地显示了高湖面发生在末次盛冰期,晚冰期时湖泊水位有所下降,全新世早期又逐渐上升,并于中全新世达到最高值,现代的湖面是18 cal. ka BP以来的最低阶段(图2)。冰期时,由于北极冰盖扩张以及东亚夏季风萎缩,使得北半球的西风气流全年强盛且西风带位置较现在南移,气候表现为冷湿,因而该地区此时湖面较高[5, 28]。同时,西风带的南移还可能造成地中海地区气候表现为冷湿而非现在的冬湿夏干,于是导致在空间上中东亚干旱区可能与地中海地区高湖面连成一带[29]。再者,由于冰期时温度较低,使得蒸发受到限制,从而有利于湖区有效降水增加,水位上升。有研究表明冰期时北半球中纬度地区气温平均下降约6~7 ℃[30],据此估算的水面蒸发量下降约30%[31]。晚冰期时,该地区仍保持高湖面特征,只是在晚冰期末期出现较多和现代水量类似的湖泊点,表明前期西风带位置的北移幅度和强度变化并不十分显著,冬季风仍相当强盛,但后期冬季风范围及强度可能呈现减弱趋势。进入全新世,中东亚干旱区大多数湖泊在早全新世表现为较为湿润的状态,在中全新世表现出最为湿润的状态。轨道参数变化引起的北半球太阳辐射变化可能是此时东亚夏季风增强并向西北内陆深入,进而导致大部分湖泊水位较高的重要原因[32,33]。从其他资料来看,此时孢粉资料显示中国东部地区的植被带曾整体向北迁移[31],腾格里沙漠南部边界曾向北退缩20 km[33],现在为荒漠草原的蒙古西北地区当时也被森林覆盖[17]。与此同时,东亚区域中全新世GCM的模拟结果也表明,该时段中国西北内陆、蒙古高原湖区降水均有显著增加[15]。全新世后期,随着季风区萎缩接近于现代环流形势,该地区湿润程度逐渐降低,但在2 cal. ka BP前后湿度略有增加,这可能与新冰期的降水增加以及温度下降有关。2 cal. ka BP 以后逐渐转向现代气候。
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