研究论文 正式出版 版本 1 Vol 11 (1) : 45-65 2020
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中中新世气候适宜期西北内陆干旱区气候演化特征
Climate evolution of the Northwestern China during Middle Miocene Climatic Optimum
: 2019 - 02 - 19
: 2019 - 06 - 26
: 2019 - 06 - 30
45 1 0
摘要&关键词
摘要:中中新世适宜期是距离现今最近的一个气候适宜阶段,现有记录大多显示全球在中中新世适宜期期间气候以暖湿为特征。我国西北内陆干旱区作为北半球中纬度面积最大的干旱带的重要组成部分,其气候如何响应全球中中新世适宜期的气候变化对预测全球变暖背景下西北内陆干旱区气候变化趋势及实现生态建设具有重要的指导意义。本文总结了西北内陆干旱区具有年代控制的15个剖面的孢粉、同位素、磁化率等19项记录,并选取各记录中指示意义明确的气候代用指标进行集成、分析,发现多数剖面在中中新世适宜期的不同时段出现了湿润化特征,但湿润化的出现、结束时间不同,自西向东,湿润事件结束(或适宜期后干旱化增强事件出现)的时间变晚。推测西北内陆干旱区中中新世适宜期的气候演化主要受全球气候变化影响,但湿润事件结束或适宜期后干旱化增强事件出现及其呈现出的时空差异则受区域构造活动及气候系统等因素控制。
关键词:中中新世气候适宜期;西北内陆干旱区;气候代用指标;全球变化;构造活动
Abstract & Keywords
Abstract: Background, aim, and scope The Middle Miocene Climatic Optimum is the most recent climatic optimum, when the climate was characterized as warm and humid. Northwestern China is an important part of the largest arid region in the mid-latitude of Northern Hemisphere, and how did its climate respond to the global climatic change during Middle Miocene Climatic Optimum is of great guiding significance, based on which we can predict the climatic change in Northwestern China on the background of the global warming and build ecological construction. Materials and methods We summarize 19 sedimentary records including sporopollen, isotope and susceptibility from 15 sections with age control in Northwestern China, from which we select the climatic proxies with clearly indicative meanings for integration and analysis. Results The majority of the selected sections presented humid climatic characteristic at different stages of Middle Miocene Climatic Optimum, and the ending time of wet episode generally becomes later from west to east, in other word, the occurrence time of the aridification enhancement that after Middle Miocene Climatic Optimum becomes later from west to east. Discussion Humid climatic stage can be identified in almost all of these Middle Miocene sections in the study area, which shows great similarity with global climate and implying its evolution was influenced by global climate change during Middle Miocene. However, the environmental gradient from west to east during Middle Miocene Climatic Optimum in the study area is possibly caused by climatic system differences and the gradually weakening of the East Asian summer monsoon. In addition, the temporal and spatial differences of the aridification occurrence or enhancement that after Middle Miocene Climatic Optimum may be result from regional tectonic activities, varied intensity of monsoon induced by global cooling and Paratethys sea retreat. Conclusions The majority of these sections presented humid climatic feature in Middle Miocene Climatic Optimum with different occurrence and ending time. The ending time of wet episode or the occurrence time of aridification enhancement that after Middle Miocene Climatic Optimum becomes later from west to east. The climatic change of Northwestern China during Middle Miocene Climatic Optimum is mainly influenced by global climate change, while the ending of the wet episode or enhancement of aridification that after Middle Miocene Climatic Optimum and its temporal and spatial differences are controlled by the regional tectonic activities and climate systems.Recommendations and perspectives Although this paper provides a review of the climatic evolution during the Middle Miocene Climatic Optimum in Northwestern China, it is still having some uncertainties that is urgent to continue to carry out relevant research with precise age control and indicative climatic proxies.
Keywords: Middle Miocene Climatic Optimum; Northwestern China; climatic proxies; global climate change; tectonic activities
中中新世气候适宜期(Middle Miocene Climatic Optimum,MMCO)是距离现今最近的一个气候适宜期,也是新生代自晚始新世以来最温暖的时期,期间的深海氧同位素值较低(δ18O值范围为1.4‰—2‰),据推测当时全球地表年均温(18.4℃)较现今(15.5℃)高约3℃,大气CO2含量(480—560 ppmv)高于现今水平,适宜期后即快速降温(Flower and Kennett,1994;Zachos et al,2001;Zachos et al,2008;You et al,2009)。了解MMCO期间的气候变化过程,对预测全球变暖背景下未来气候变化具有重要意义。
西北内陆干旱区是北半球中纬度面积最大的干旱带——亚洲内陆干旱区的重要组成部分。亚洲内陆干旱区延伸范围极广,影响着众多国家的生活与发展,因而成为学术界研究的重点区域,其形成时间争议较大:最初通过黄土高原洛川剖面的磁学研究,认为始于约2.4 Ma(Heller and Liu,1982);随后通过黄土高原多个剖面的磁学、元素组成等研究,认为在6 Ma左右(孙东怀等,1997;Guo et al,2001)、7 Ma左右(Ding et al,1999;Liu et al,2009)或中新世中期(刘东生等,1998);近些年来,随着研究范围和研究方法的扩展,认为干旱化在中新世早期(Guo et al,2002;Guo et al,2008;Lu et al,2010;柯学等,2013)或渐新世晚期就已形成(Garzione et al,2005;洪汉烈等,2007;Sun et al,2010;Qiang et al,2011),甚至是始新世(Licht et al,2014;Caves et al,2015)。其形成原因现今认为主要有青藏高原隆升、副特提斯海退及全球变冷等(Kutzbach et al,1993;Ramstein et al,1997;An et al,2001;Zhang et al,2007;Miao et al,2012)。
我国西北内陆干旱区,位于亚洲内陆干旱区腹地,具有更独特的水汽和构造条件:随着中纬度西风的逐渐前进,携带的水汽逐渐消耗殆尽,导致西北内陆干旱区较亚洲内陆其他地区更干;同时,西北内陆干旱区毗邻青藏高原,且边界被高山限定,受构造隆升的影响更显著。因此,西北内陆干旱区成为近些年来研究干旱区气候演化如何响应全球变化和区域构造隆升的重点地区(郭正堂等,1999;刘晓东等,2001;Graham et al,2005;Miao et al,2012;Wang et al,2013)。
现有同位素(Wright et al,1992;Flower and Kennett,1994;Zachos et al,2001;Feakins et al,2012)、植物学(Sun et al,2002;Mosbrugger et al,2005;Utescher et al,2007;Yang et al,2007;Ivanov et al,2011;Liu et al,2011)、动物化石(Böhme,2003)、矿物学(Hauptvogel and Passchier,2012;Song et al,2018a)及岩石磁学(Zan et al,2015)等记录显示全球大部分地区MMCO期间温度和降水量都较现今高得多。西北内陆干旱区作为北半球中纬度面积最大干旱带的重要组成部分,研究其气候如何响应全球中中新世适宜期的气候变化不仅有助于认识干旱区气候对全球变化和区域构造活动的响应情况,更对预测全球变暖背景下干旱区气候演化趋势及进一步部署干旱区的生态建设具有重要的指导意义。本文拟选择西北内陆干旱区中新世具有年代控制的沉积记录,对比、分析各项记录中意义明确的代用指标并进行整合,以期得到西北内陆干旱区MMCO期间的气候演化特征,研究其与全球变化及区域构造活动间的关系,为全面认识干旱区气候演化过程奠定基础。
1   研究区概况
西北内陆干旱区,位于欧亚大陆的腹地(31°—50°N,73°—111°E)(Shi et al,2007)(图1),是亚洲内陆干旱区的重要组成部分。现今主要受西风控制(Caves et al,2015),仅东南部可能受到夏季风的影响(Sun and Wang,2005;Chen et al,2008)。年均温变化较大,因距大洋较远且受到周围高山的阻挡作用(Guo et al,2002),年平均降水量通常低于400 mm,降雨量自东向西呈降低趋势,但山脉区较盆地高。研究区的植被自东向西划分为:温带森林草原植被带(主要为桦、栎、樟子松等乔木及羊草、贝加尔针茅、大针茅、长茅草等灌木)、温带草原植被带(主要为大针茅、克氏针茅、羊草及长茅草等)、温带草原荒漠植被带(包含荒漠草原及各类草原化荒漠,分别以短花针茅、戈壁针茅、沙生针茅并混生红沙、盐生草、木霸王等和红沙荒漠、盐爪爪、猪毛菜、合头草等为主)和温带荒漠带(主要为藜科、蒺藜科、紫菀属、禾本科、蒿属及麻黄等旱生的植物)(宋永昌,2001;Sun and Wang,2005)。研究区毗邻青藏高原地区,区内构造活动强烈,是研究干旱区气候如何响应全球变化和区域高原隆升的理想场所。


图1   西北地区自然地理图(据安芷生等(2006)、郭正堂(2017)改绘)
Fig.1 The physical geography map of Northwestern China (modified from An et al (2006), Guo (2017))
2   西北内陆干旱区MMCO气候记录
近年来部分学者已对西北内陆干旱区MMCO期间的沉积记录开展了系列研究(表1),下文将自西向东对各剖面的指标变化情况进行描述。
表1   西北地区MMCO沉积记录
序号剖面年代/Ma位置指标参考文献
1库车塔吾
Kuchetawu
13.3—741°55.097′N,83°03.280′E孢粉
Pollen
Zhang and Sun,2011
2金沟河
Jingouhe
28—4.244°10.4′N,85°27.4′E孢粉
Pollen
Tang et al,2011
3塔西河
Taxihe
26.5—2.644°′N,86°20′E孢粉
Pollen
Sun and Zhang,2008
4花土沟
Huatugou
~27.8—1138°25.6′N,90°53.8′E-38°22.0′N,90°52.9′E碳酸盐氧同位素
Carbonate δ18O
Chang et al,2015;Li et al,2016
5-1红沟子
Honggouzi
17—538°34′ N ,91°3′ E地化(TOC、SO42-、Cl-及碳酸盐含量)
Geochemical proxy records (TOC,SO42-,Cl-,carbonate)
Song et al,2014
5-2红沟子
Honggouzi
17—538°34′ N ,91°3′ E叶蜡烷烃δ2H、δ13C
δ2H,δ13C of leaf wax n-alkane
Wu et al,2019
6柴西ChaixiS23新生代Cenozoic38.5°N,91°E孢粉
Pollen
Wang et al,1999;沈振区等,1990;王建等,1996
ZK40239°N,91.5°E
F238.25°N,92°E
7KC-118—538° 03′ N,91° 45′ E孢粉
Pollen
Miao et al,2011
8西水沟、铁匠沟
Xishuigou,Tiejianggou
渐新世—上新世
Oligocene—Pliocene
40°N,94°43′E孢粉
Pollen
Sun and Wang,2005;马玉贞,1991
9-1大红沟
Dahonggou
52—6.537°29′11″N ,95°12′42″E磁化率
Susceptibility
柯学,2014;柯学等,2013
9-2大红沟
Dahonggou
15—12.137°29′11″N ,95°12′42″E介形虫、稳定同位素、风化指数
Ostracod fauna,stable isotope records and chemical index of weathering
Song et al,2017
10S26
S27
~53.5—3.237.7°N,95.8E
37.8°N,95.2E
矿物学
Mineralogy
Wang et al,2013
11-1怀头他拉
Huaitoutala
15.7—1.837.2°N,96.8°E碳酸盐氧同位素
δ18O of carbonate
Zhuang et al,2011
11-2怀头他拉
Huaitoutala
15—1.837.2°N,96.8°E叶蜡烷烃氢同位素
δ2H of leaf wax n-alkane
Zhuang et al,2014
11-3怀头他拉
Huaitoutala
15.3—1.837.2°N,96.8°E黏土组分的地球化学组分
Geochemical compositions of the clay components
Bao et al,2019
12瑙格等65个东亚剖面
Naoge et al
23—0瑙格为36.9°N,97.2°E伏平粉属
Fupingopollenites
Miao et al,2016b
13老君庙
Laojunmiao
13—2.2139°47′N ,97°32′E孢粉
Pollen
Ma et al,2005
14塔山钻孔
Tashan Borehole
18.5—13.536°32′N,101°50′E岩石磁学
Rock magnetism
Zan et al,2015
15毛沟
Maogou
30.6—535°N, 104°E孢粉
Pollen
马玉贞等,1998
2.1   库车塔吾
Zhang and Sun(2011)对南天山地区年代为13.3—2.6 Ma的库车塔吾剖面(图1中1)的孢粉记录进行了分析(图2),整个剖面的孢粉以乔木类(以松属、桦属为主,含冷杉属、栎属、云杉属、榆属等)为主,草本类则以蒿属和藜科为主,自下而上划分为三带:13.3—7 Ma,喜温属种含量较高,气候相对暖湿,受全球气候变化影响;7—5.23 Ma,冷杉含量增加,气候较之前变冷,受区域隆升影响较大;5—2.6 Ma,耐旱属种含量增加,干旱化增强,受全球气候和构造影响。


图2   库车塔吾剖面13.3—2.6 Ma孢粉图谱(据Zhang and Sun(2011)修改)
Fig.2 Pollen diagram from 13.3 Ma to 2.6 Ma within the Kuchetawu section, modified from Zhang and Sun (2011)
2.2   金沟河
Tang et al(2011)对北天山年代为28—4.2 Ma的金沟河剖面(图1中2)的孢粉记录进行了描述(图3),发现自23.8 Ma起,孢粉含量和乔木种类减少;17.3—16.2 Ma,孢粉含量、乔木类和水生类含量较高,蒿属和藜科含量保持较低水平;16.2—13.5 Ma,孢粉含量降低,主要以藜科、蒿属、菊科和禾本科为主,水生类含量较低。进一步对孢粉结果进行主成分分析,认为23.8—23.3 Ma时,中亚地区气候特征转为干旱,并持续至17.3 Ma;17.3—16.2 Ma,变为湿润;16.2 Ma—晚中新世-早上新世气候持续变干,在13.5 Ma达到峰值。其研究认为长期的气候变化受控于全球变冷,而短期的干旱事件受控于区域活动。


图3   金沟河剖面28—4.2 Ma孢粉图谱(据Tang et al(2011)修改)
Fig.3 Pollen diagram from 28 Ma to 4.2 Ma within the Jingouhe section, modified from Tang et al (2011)
2.3   塔西河
Sun and Zhang(2008)对北天山年代为晚渐新世—上新世(26.5—2.6 Ma)的塔西河剖面(图1中3)进行了孢粉分析(图4),整个剖面以乔木类为主。其中:18—15 Ma,温带桦属、暖温带栎属和胡桃属含量较高,对应中中新世适宜期;6—2.6 Ma,蒿属、藜科等干旱草原属种增加,气候变更干冷,其研究认为期间的气候变化主要受控于全球气候变冷。


图4   塔西河剖面26.5—2.6 Ma孢粉图谱(据Sun and Zhang(2008)修改)
Fig.4 Pollen diagram from 26.5 Ma to 2.6 Ma within the Taxihe section, modified from Sun and Zhang (2008)
2.4   花土沟
Li et al(2016)对柴达木盆地年代为晚渐新世—中中新世(Chang et al,2015)、厚4435 m的花土沟剖面(图1中4)泥灰岩/灰岩、成壤碳酸盐和碳酸盐胶结物的δ18O、δ13C值进行了分析(图5),发现在~20 Ma至15—14 Ma,湖相碳酸盐的δ18O值变化范围较小;15—14 Ma至13—12Ma,碳酸盐胶结物δ18O值整体负偏;13—12 Ma至~11 Ma,δ18O值整体为负值,但发生正偏。进而认为:在~20 Ma时,古水文条件发生变化,盆地由局限变为开放;15—14 Ma时,构造活动和周围山脉隆升;13—12 Ma时,干旱化增强。


图5   花土沟剖面晚渐新世-早中新世氧、碳同位素变化图(据Li et al(2016)修改)
Fig.5 Diagram of stable isotopic variations (δ18O and δ13C) of carbonates from Oligocene to Pliocene within the Huatugou section, modified from Li et al (2016)
2.5   红沟子
Song et al(2014)对柴达木盆地西部年代为17—5 Ma的红沟子剖面(图1中5)进行了地球化学指标(TOC、碳酸盐含量及可溶性盐含量(SO42-,Cl-))分析(图6),发现在17—11.1 Ma,TOC和碳酸盐含量均逐渐降低、可溶性离子SO42-,Cl-含量持续增加,盆地呈长期干旱特征;11.1 Ma起,碳酸盐含量明显降低,可溶性离子SO42-,Cl-含量较高(降低),表明盆地干旱化增强,气候变更干。推测中新世时全球变冷影响着盆地的干旱化进程;中新世—上新世,青藏高原周期性、持续隆升对盆地古气候变化影响较大。
Wu et al(2019)则分析了红沟子剖面(Song et al,2014)和七个泉剖面(Zhang et al,2013)叶蜡烷烃的δ13C和δ2H变化情况,得到了长约14 Ma的沉积序列(图7)。其研究表明:14.6—13 Ma,δ13C值减少3‰,δ2H值低,气候湿润;13 Ma开始,δ2H值增加,干旱化增强;8 Ma时,δ2H值达到峰值,为极度干旱事件。其研究认为自13 Ma开始的干旱化增强是阿尔金山及祁连山等周围山脉隆升导致的。其研究指出了盆地东-西气候演化在15—10 Ma时不同,并认为其是因东、西部气候条件具有差异(东部受到东亚季风影响)而导致的。


图6   红沟子剖面17—5 Ma地球化学指标记录图(据Song et al(2014)修改)
图6 红沟子剖面17—5 Ma地球化学指标记录图(据Song et al(2014)修改) Fig.6 Diagram of geochemical records from 17 Ma to 5 Ma within the Honggouzi section, modified from Song et al (2014)


图7   红沟子剖面叶蜡烷烃的δ13C和δ2H记录图(据Wu et al(2019)修改)
Fig.7 Diagram of leaf wax n-alkane δ2H and δ13C records within the Honggouzi section, modified from Wu et al (2019)
2.6   柴西
王建等(1996,1999)和沈振区等(1990)根据狮23孔、风2孔和ZK402的孢粉记录(图1中6),建立了柴达木盆地西部的新生代孢粉序列(图8):主要包含亚热带及热带阔叶树(漆树属、冬青属、栎属、栗属、楝科)、亚热带及热带山地针叶树(油杉属、雪松属、铁杉属和罗汉松科)、温带阔叶植物(桦科、榆科、胡桃科、木樨科、槭属和忍冬等)和旱生植物(凤尾蕨科、麻黄科、蒺藜科、藜科、菊科等)。从孢粉序列中看,新生代干旱属种孢粉含量较高,推测盆地新生代整体较干。晚古新世和早始新世,亚热带阔叶树花粉含量为新生代整个序列中的最高值,随后减小,表明新生代早期至晚期,气候逐渐变冷。


图8   柴达木盆地西部中新世以来的孢粉图谱(据Wang et al(1999)修改)
Fig.8 Pollen diagram of western Qaidam since Miocene , modified from Wang et al (1999)
2.7   KC-1
Miao et al(2011,2016a)对柴达木盆地西部年代为18—5 Ma的KC-1钻孔(图1中7)的孢粉记录进行了描述(图9):主要为云杉属、松属、罗汉松属、铁杉属和雪松属等针叶类及藜科、麻黄属、菊科、蒿属、白刺属及禾本科等灌木及草本,栎属、胡桃科、榆科、桦木科等阔叶树类较少,藻类和孢子较少。记录显示18—14 Ma,喜温属种含量高,对应中中新世气候适宜期(MMCO),14 Ma后喜温属种含量降低,耐旱属种含量增加,表明14 Ma后变冷、变干。进一步与全球气候变化对比,认为盆地中新世气候变化主要受全球变冷驱动,青藏高原隆升也具一定作用。


图9   KC-1钻孔18—5 Ma孢粉图谱(据Miao et al(2011)修改)
Fig.9 Pollen diagram from 18 Ma to 5 Ma within KC-1 borehole, modified from Miao et al (2011)
2.8   西水沟、铁匠沟
马玉贞(1991)对敦煌盆地南部西水沟和铁匠沟附近的第三纪红层剖面(图1中8)的孢粉记录进行了研究,自老至新划分为:晚渐新世晚期-早中新世,麻黄粉属-白刺粉属-藜粉属和栎粉属;中中新世,藜粉属-蒿粉属-榆粉属-杉粉属;晚中新世,藜粉属-蒿粉属-藜粉属-麻黄粉属;上新世,藜粉属-蒿粉属-麻黄粉属。该记录显示盆地第三纪整体为温带干旱性气候,仅中中新世时出现短时暖湿气候。
2.9   大红沟
柯学等(2013,2014)对柴达木盆地北部年代为52—6.5 Ma的大红沟剖面(图1中9)进行了磁化率研究(图10),认为早—中中新世(22—14 Ma),磁化率增加,表明气候呈冷干特征,期间,19—15 Ma时磁化率略有降低,对应MMCO,15 Ma后变冷;中—晚中新世(14—6.5 Ma),磁化率再次增加且波动剧烈,表明气候进一步冷干,同时西北干旱进一步扩大。其研究认为盆地气候演化受全球变冷和构造隆升共同影响。
Song et al(2017)则对剖面中年代为15—12.1 Ma的沉积序列进行了介形虫种类及同位素分析(图11),并结合已有的蒸发盐类矿物含量(Wang et al,2013),将期间的环境演化划分为:15—13.3 Ma,δ18O和δ13C值较低且相关性较低、化学风化指数(CIW)及K/Na较高、介形虫主要为Ilyocypris,表明湖泊开放且发生扩张,气候较为湿润;13.3—12.1 Ma,出现砾石、δ18O和δ13C值较高且变化范围窄、相关性高、介形虫变为喜盐型-Cyprideis;高石膏+岩盐含量、CIW即K/Na降低,表明湖泊封闭,自13.3 Ma开始,柴达木盆地干旱化增强。


图10   大红沟剖面中新世磁化率记录图(据柯学(2014)修改)
Fig.10 Diagram of Miocene susceptibility within the Dahonggou section, modified from Ke (2014)


图11   大红沟剖面15—12.1 Ma生物、化学记录图(据Song et al(2017)修改)
Fig.11 Diagram of biological and geochemical records from 15 Ma to 12.1 Ma within the Dahonggou section, modified from Song et al (2017)
2.10   S26、S27
Wang et al(2013)对柴达木盆地北部年代为~53.5—2.5 Ma的S26、S27剖面(图1中10)沉积物中的石膏和石盐含量、石英与长石含量的比值(Q/F)及矿物形态学进行了分析(图12),认为~26—13.5 Ma,石盐和石膏突然消失、Q/F增加、伊利石结晶度18—15 Ma较~26—18 Ma高,气候暖湿,其中18—15 Ma对应MMCO;~13—2.5 Ma,石盐和石膏含量持续增加、伊利石结晶程度低、坡缕石出现,气候更冷、干。


图12   S26、S27剖面~26—2.5 Ma矿物学记录图(据Wang et al(2013)修改)
Fig.12 Diagram of mineralogic records from ~26 Ma to 2.5 Ma within the S26 and S27, modified from Wang et al (2013)
2.11   怀头他拉
Zhuang et al(2011)对柴达木盆地东北缘年代为15.7—1.8 Ma的怀头他拉剖面(图1中11)土壤和湖相沉积物中的碳酸盐的氧、碳同位素进行了测定(图13),发现在12—10.7 Ma时,δ18O值正偏约2.5‰,推测是亚洲内陆干旱事件的反应,认为此次干旱事件与副特提斯海退、晚中新世存在的高海拔喜马拉雅山和青藏中南部及青藏高原的北向生长等有关。
Zhuang et al(2014)分析了剖面中15—1.8 Ma叶蜡烷烃的δD特征,将15—1.8 Ma划分为四个阶段:15—10.4 Ma,构造活跃;10.4—6.9 Ma,极度干旱;6.9—4.1 Ma,气候较湿期;4.1—1.8 Ma,气候变干(图14)。
Bao et al(2019)则分析了剖面中15.3—1.8 Ma沉积物中黏土组分的地球化学组成,并对期间的硅酸盐风化史进行了探讨(图15),认为15.3—12.6 Ma,化学蚀变指数(CIA)相对高、活动与稳定性氧化物比值较低,指示化学风化强烈;12.6—6 Ma,CIA值降低,活动与稳定性氧化物比值增加,指示源区硅酸盐风化强度降低。推测盆地化学风化强度持续降低并在12.6 Ma时经历持续性干旱,主要受中中新世全球变冷影响,而晚新生代高原快速隆升也具一定作用。


图13   怀头他拉剖面15.7—1.8 Ma碳酸盐δ18O、δ13C记录图(据Zhuang et al(2011)修改)
Fig.13 Diagram of stable isotopic variations (δ18O and δ13C) of carbonates from 15.7 Ma to 1.8 Ma within the Huaitoutala section, modified from Zhuang et al (2011)


图14   怀头他拉剖面15—1.8 Ma δD记录图(由a至d依次为nC27、nC29、nC31、nC33)(据Zhuang et al(2014)修改)
Fig.14 Diagram of δ2D from 15 Ma to 1.8 Ma within the Huaitoutala section, from a to d is nC27, nC29, nC31 and nC33 respectively, modified from Zhuang et al (2014)


图15   怀头他拉剖面15.3—1.8 Ma地球化学指标记录图(据Bao et al(2019)修改)
Fig.15 Diagram of geochemical records from 15.3 Ma to 1.8 Ma within the Huaitoutala section, modified from Bao et al (2019)
2.12   瑙格
Miao et al(2016)根据伏平粉属含量在柴达木盆地东缘瑙格剖面(图1中12)及东亚等地区的65个剖面的变化情况,归纳了新生代以来亚洲夏季风的演化过程:早中新世(23—17 Ma),伏平粉属的北东边界为42°N、135°E、北西边界为36°N、103°E;中中新世(17—11 Ma),伏平粉属向西扩展达到柴达木盆地东部,亚洲夏季风最强,随后季风减弱;晚中新世(11—5.3 Ma),伏平粉属的分布界线明显撤退,北西边界为33°N、105°E;上新世进一步收缩;更新世期间(2.6—0 Ma)几乎消亡。
2.13   老君庙
Ma et al(2005)对青藏高原北缘河西走廊酒西盆地年代为13—2.21 Ma老君庙剖面(图1中13)的孢粉记录进行了分析(图16):13—8.6 Ma,植被为草原、柏林,气候较湿润;8.6—8.4 Ma,干旱化增强;8.4—6.93 Ma,植被为森林草原,较之前暖湿;6.93—6.64 Ma,干旱化增强;6.64—5.67 Ma,植被为开放的森林-草原,气候温暖、半湿润;5.67—3.3 Ma,整体较为干旱,存在短期相对湿润阶段;3.3—2.2 Ma,气候较干。




图16   老君庙剖面13—2.21 Ma孢粉图谱(据Ma et al(2005)修改)
Fig.16 Pollen diagram from 13 Ma to 2.21 Ma within the Laojunmiao section, modified from Ma et al (2005)
图a、b分别代表6.93—2.2 Ma和13.8—6.93 Ma的孢粉序列。 Fig. a and b represent the pollen sequence from 6.93 Ma to 2.2 Ma and from 13.8 Ma to 6.93 Ma respectively.
 

图a、b分别代表6.93—2.2 Ma和13.8—6.93 Ma的孢粉序列。
Fig. a and b represent the pollen sequence from 6.93 Ma to 2.2 Ma and from 13.8 Ma to 6.93 Ma respectively.
图16 老君庙剖面13—2.21 Ma孢粉图谱(据Ma et al(2005)修改)
Fig.16 Pollen diagram from 13 Ma to 2.21 Ma within the Laojunmiao section, modified from Ma et al (2005)
2.14   塔山钻孔
Zan et al(2015)对西宁盆地年代为18.5—13.5 Ma的塔山钻孔(图1中14)的岩性和岩石磁学参数进行了研究(图17):17 Ma之前及14 Ma之后,岩性以红色黏土岩和粉砂质黏土岩为主,岩石磁学指标的值较高且变化较小;~17—14 Ma,岩性以灰白色或蓝灰色黏土岩、泥质粉砂岩及细粒砂岩为主,岩石磁学指标的值变化较大,其中薄层红色黏土岩的值大,灰白色或蓝灰色黏土岩层的值较低。其研究表明~17—14 Ma西宁盆地为暖湿气候,对应MMCO。


图17   塔山钻孔18.5—13.5 Ma磁学参数记录图(据Zan et al(2015)修改)
Fig.17 Magnetic parameter records from 18.5 Ma to 13.5 Ma within the Tashan Borehole, modified from Zan et al (2015)
2.15   毛沟
马玉贞等(1998)对临夏盆地年代为30.6—5 Ma的毛沟剖面(图1中15)的孢粉记录进行了描述(图18),并划分为疏林-草原(30.6—21.8 Ma)、森林(21.8—8.5 Ma)、草原(8.5—6 Ma)和森林(6—5 Ma)四个阶段。根据孢粉组合,认为19.5—18.6 Ma、13—8.5 Ma和6—5 Ma为湿润期,19.5—18.6 Ma为最暖湿阶段;18.6—13 Ma为降温期;8.5—6 Ma发生旱化。


图18   毛沟剖面30.6—5 Ma孢粉图谱(据马玉贞等(1998)修改)
Fig.18 Pollen diagram from 30.6 Ma to 5 Ma within the Maogou section, modified from Ma et al (1998)
除上述独立剖面外,部分学者也对研究区的一些盆地的沉积指标进行了集成性的对比研究。Li et al(2017)对柴达木盆地西南部中始新世以来的多个剖面的碳酸盐碳、氧同位素特征进行了测定,发现始新世—渐新世时,δ18O和δ13C正偏;中新世时,δ18O负偏、δ13C值相对稳定,据此认为始新世—渐新世盆地气候由暖湿变干冷;中新世盆地发生隆升。
王树基和高存海(1990)对塔里木盆地中新世的孢粉和植被记录进行了概述:喀什地区中新世的孢粉以膜蕨、红豆杉、铁杉、柏科、银杏、罗汉松、栎、水青冈等常绿及落叶植物为主,含有麻黄及藜科等;库车地区,中新世则以落叶阔叶植物为主,含常绿植物,可见草原,属亚热带-暖温带针阔叶林;罗布泊地区中新世早期植物类似于同期库车地区;昆仑山山口,以斛蕨和瓦伟(蕨类、水龙骨类)为主,其次为铁杉、冬青、栎、楝和山核桃等,属亚热带常绿阔叶及落叶阔叶林,据此认为中新世时塔里木盆地为亚热带常绿阔叶及落叶阔叶林植被,气候炎热湿润。
3. 结果
本文综述的金沟河(Tang et al,2011)、塔西河(Sun and Zhang,2008)、花土沟(Li et al,2016)、红沟子(Wu et al,2019)、柴西(Wang et al,1999)、KC-1(Miao et al,2011)、西水沟和铁匠沟(马玉贞,1991)、大红沟(Song et al,2017;柯学等,2013,2014)、S26和S27(Wang et al,2013)、怀头他拉(Bao et al,2019)、瑙格(Miao et al,2016b)、塔山钻孔(Zan et al,2015)及毛沟(马玉贞等,1998)等剖面的中中新世时期的沉积记录均显示研究区在MMCO的不同时段存在一段气候湿润期,其中塔西河、KC-1、大红沟(柯学等,2013,2014)、S26和S27剖面及塔山钻孔记录了显著的MMCO事件。
选取各项记录中古气候意义明确的气候代用指标进行集成分析并整合成图(图19),发现多数剖面MMCO后干旱化增强或出现的时间不尽相同,总体来看,自西向东,干旱化增强或出现的时间变晚(即与之相应的湿润事件结束时间变晚);位置临近的剖面适宜期后干旱化增强或出现的时间相近。在塔里木盆地和柴达木盆地的最西部,中新世大部分时间呈现炎热干旱特征(王树基和高存海,1990;Song et al,2014);在柴达木盆地东部,12 Ma甚至10 Ma,出现干旱化增强事件(Zhuang et al,2011,2014);在临夏盆地,则在8.5 Ma时出现干旱化。相近的剖面,如花土沟、红沟子及KC-1干旱化增强或出现的时间为14—13 Ma;大红沟、S26和27及怀头他拉为13—12 Ma。虽然干旱化增强的时间整体自西向东变晚,但呈现出了显著的时空差异性。
进一步将研究区西部的库车塔吾、金沟河、塔西河、柴西及KC-1剖面及东部的老君庙及毛沟剖面的孢粉记录进行综合、对比,发现中新世期间西北内陆干旱区内植物种类相似,整体以针叶林为主,含一定的温带阔叶树和草本植物,但西部多云杉、冷杉等高山类型,相较而言,东部植物种类则丰富,热带、亚热带属种较多。


图19   西北内陆干旱区各剖面适宜期后干旱化增强/出现时间演化图
Fig.19 Evolution map of occurrence or enhancement time of aridfication that after Middle Miocene climatic Optimum in Northwestern China
图中序号意义见表1,指标演化图中红色部分代表MMCO阶段,绿色矩形代表干旱化增强/出现的时间;箭头指示自西向东干旱化增强/出现的时间变晚。Ⅰ、Ⅱ及Ⅲ代表研究区内自西向东分布的三组剖面集合。Ⅰ包含1、2及3;Ⅱ包含4、5、6、7;Ⅲ包含9、10、11。 The meaning of the number is shown in Tab. 1. The red part of the climatic proxies evolutionary lines represents MMCO. Green rectangle means the time of aridification occurrence or enhancement. The arrow indicates that the time of aridification occurrence or enhancement becomes later from west to east. Ⅰ, Ⅱ and Ⅲ represent three section assemblages from west to east in the study area. Ⅰ including 1, 2 and 3; Ⅱ including 4, 5, 6 and 7; Ⅲ including 9, 10 and 11.
4   讨论
4.1   研究区中新世气候演化的驱动因素
本文综述的沉积记录多显示研究区在MMCO的不同时段存在一段气候湿润期,全球深海氧同位显示,在MMCO期间,δ18O值(范围为1.4‰—2‰)较早中中新世和晚中新世均低,全球大部分地区的气候较现今更较暖湿,其中中纬度海表温度较现今高6℃(Savin et al,1975;Flower and Kennett,1994)。因西北内陆干旱区MMCO期间的气候特征与全球MMCO期间的气候特征具有显著相似性,故推测西北内陆干旱区MMCO期间的气候主要受全球气候变化影响。
研究区上述剖面MMCO后干旱化增强或出现(即湿润事件结束)的时间整体自西向东变晚,但具显著的时空差异性;地理位置相近的剖面适宜期后干旱化增强或出现的时间相近。现有的孢粉、同位素及磁化率等记录显示,西北地区的干旱化的确在~14—13 Ma(Miao et al,2011;柯学等,2013,2014;Song et al,2017)、13—12 Ma(Li et al,2016)、12 Ma(Zhuang et al,2011)、~11 Ma(Song et al,2014)、10—8 Ma(Jian et al,2014)、9—8 Ma(马玉贞等,1998;An et al,2001;Ma et al,2005)、6—5 Ma(Sun and Zhang,2008;Zhang and Sun,2011)等多个时段发生了强化。青藏高原北缘13个新生代沉积剖面的碳酸盐样品的δ13C和δ18O特征也表明整个新近纪氧、碳同位素值均变重,表明新近纪时期干旱化一直增强(Kent-Corson et al,2009)。此外,干旱化的增强在空间上也呈现出了差异性,土壤碳酸盐δ13C特征显示,中新世时中亚北部更湿,而塔里木、柴达木盆地的干旱化则在新近纪期间一直增强(Caves et al,2016);柴达木盆地西部湖相沉积物与东部的土壤和湖相碳酸盐δ18O特征表明西部的湖泊长期封闭偶尔开放,而东部则为开放湖泊,表明西部较东部更干(Zhuang et al,2011;Jian et al,2014)。
造成研究区适宜期后干旱化演化存在时空演化差异性的原因可能为区域构造隆升活动具有时空差异性(Guo et al,2018)。现有研究表明研究区内主要山脉及青藏高原隆升的发生存在时空差异性:早—中中新世,碳酸盐氧同位素显示高原东北缘发生了构造隆升(Kent-Corson et al,2009);中中新世(18/17—15 Ma),热年代、碎屑锆石、植物化石、等厚度图和磁性地层学及磁化率各向异性等记录显示,高原周边地区(包含周围的祁连山、阿尔金山、昆仑山等)发生了构造活动(Ritts et al,2008;Lease et al,2011;Duvall et al,2013;Lu et al,2014;Mao et al,2014;Lu et al,2015;Wang et al,2016;Zhuang et al,2018);15—13 Ma,同位素及磁性地层学等研究表明研究区内发生了构造活动(Sun et al,2005;Zhuang et al,2014;Li et al,2016;Song et al,2017);12—-11 Ma(Dettman et al,2003;Zhuang et al,2011;Duvall et al,2013;Song et al,2014)及晚中新世时(An et al,2001;Zheng et al,2010),高原周边发生了构造活动。
造成研究区适宜期后干旱化演化存在时空差异性的原因也可能为全球变冷导致季风强度变化具有时空差异性。全球变冷一方面导致蒙古-西伯利亚高压增强,进而引起冬季风增强,最终导致干旱化增强(Guo et al,2004;Song et al,2018b);另一方面,全球变冷导致海洋变冷,进而引起到达陆地的水汽减少,最终导致干旱化增强(Guo et al,2004)。南海北部的1146钻孔的黏土矿物学分析显示在15 Ma、8 Ma及3 Ma时冬季风发生增强(Wan et al,2008);孢粉记录也显示14—5 Ma,夏季风减弱、冬季风较强(Miao et al,2011)。
此外,副特提斯海退也可能是时空差异性形成的原因,海退引起海陆分布变化,导致欧亚大陆的大陆性变强,进而影响气候的演化进程(Ramstein et al,1997;Zhang et al,2007)。一些学者根据环地中海腹足动物化石的特征认为西特提斯洋在早中新世时消亡(Harzhauser et al,2002),也有学者认为在中新世时开始逐步退却(Ramstein et al,1997)。但对海退的时间争议较大,部分学者认为在始新世时即从塔里木盆地西南部退出(Sun and Jiang,2013),因此其对研究区MMCO气候演化是否有影响尚不清晰。
4.2   研究区自西向东环境梯度的形成原因
研究区的多气候代用指标集成分析结果表明,自西向东,适宜期后干旱化增强或出现(即湿润事件结束)的时间趋于变晚(图19),推测应是由气候系统差异引起的。研究区西部主要受西风环流控制,而东部可能受到东亚夏季风的影响(Sun and Wang,2005;Chen et al,2008;Caves et al,2015),相较而言,西部较东部接受的降雨量更少,但蒸发量更大,因而干旱化增强或出现相对早,伏平粉属的分布已证明中中新世时东亚夏季风可达到柴达木盆地东南缘(Miao et al,2016b)。另一方面,夏季风边界的后退可能也是造成西—东梯度形成的原因,伏平粉属等孢粉记录及碳酸盐氧、碳同位素记录,也表明中中新世时夏季风强盛(Miao et al,2016b;Dong et al,2018),随后强度减弱且界线后退(Miao et al,2011,2016b)。
5   结论
本文总结了西北内陆干旱区具有年代控制的15个剖面的孢粉、同位素、磁化率等19项记录,发现:
(1)多数记录在中中新世适宜期的不同时段呈现出了湿润化特征,但湿润化出现、结束的时间不同,自西向东,湿润事件结束(或适宜期后干旱化增强事件出现)时间变晚。研究区自西向东环境梯度的形成是由于西部、东部气候系统差异和夏季风减弱(即夏季风界线逐渐东退)导致的。
(2)西北内陆干旱区中中新世适宜期的气候演化主要受全球气候变化影响,但湿润事件结束或适宜期后干旱化增强及其呈现出的时空差异则主要受区域构造活动及气候系统等控制,可能受副特提斯海退影响。
致谢:
感谢审稿人对稿件修改提出的意见。此外,课题组的谢宇龙、唐分俊及高守杰等在成文过程中的资料查找、图件绘制及文章修改等方面提供了宝贵建议,在此表示由衷的感谢。
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稿件与作者信息
王海涛
WANG Haitao
王海涛,E-mail: wanghaitao@itpcas.ac.cn
吴福莉
WU Fuli
方小敏
FANG Xiaomin
杨立业
YANG Liye
中国科学院战略性先导科技专项(A类)(XDA20070201);国家自然科学基金重点国际合作研究项目(41620104002);第二次青藏高原科学考察(2019QZKK0707);青藏高原地球系统基础科学中心(41988101-01)
Strategic Priority Research Program of Chinese Academy of Sciences (XDA20070201); Key International Cooperation Research Project of National Natural Science Foundation of China (41620104002); The Second Tibetan Plateau Scientific Expedition and Research (2019QZKK0707); Center for Basic Science in Tibetan Plateau Earth System (41988101-01)
出版历史
出版时间: 2019年6月30日 (版本1
参考文献列表中查看
地球环境学报
Journal of Earth Environment