研究论文 正式出版 版本 2 Vol 10 (5) : 453-464 2019
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西宁盆地总有机碳同位素记录的~39 Ma亚洲内陆急剧干旱事件
An Asian inland aridification enhancement event at ~39 Ma recorded by total organic carbon isotopes from Xining Basin
: 2018 - 11 - 26
: 2019 - 03 - 27
: 2019 - 04 - 11
296 10 0
摘要&关键词
摘要:始新世亚洲内陆干旱环境演化(简称干旱化)的研究比较匮乏,驱动机制存在争议。位于青藏高原东北缘的西宁盆地含有连续的始新世地层沉积序列,是研究上述科学问题的理想材料。通过对西宁盆地中央西宁东和水湾平行剖面(古地磁年代均为~43 Ma—~35 Ma)详细的总有机碳同位素记录,研究始新世的干旱生态环境变化进行了研究,结果显示:上述两个平行剖面总有机碳同位素平均值均在~39 Ma发生了急剧的~2.5‰变正(分别从-27.5‰和-28.4‰变正为-25.0‰和-26.0‰),结合盆地内耐旱植物麻黄属和白刺属孢粉的百分含量急剧升高,共同指示该时期的西宁盆地发生了急剧干旱化。副特提斯海退却导致的水汽输送的减少可能是驱动西宁盆地始新世干旱生态环境变化的主要原因,全球长期变冷可能起到了背景叠加作用。
关键词:西宁盆地;~39 Ma;总有机碳同位素;亚洲内陆干旱化;副特提斯海退却
Abstract & Keywords
Abstract: Background, aim, and scope The Cenozoic evolution of the aridification of the Asian interior has attracted major research interest in recent years. However, the history of aridification during the Eocene has been neglected and the forcing mechanism remains controversial. Xining Basin, located on the northeastern margin of the Tibetan Plateau, contains a thick sequence of Cenozoic fine grained sedimentary sequences which have been dated using high-resolution paleomagnetic studies. Thus, Xining Basin is well suited for studying the history of regional aridification and its forcing mechanisms. In this study, we selected two parallel early Cenozoic sections in the Xining Basin spanning the interval from ~43 Ma to ~35 Ma, for analysis of the stable isotope composition of total organic carbon (δ13 CTOC). Our aim was to reconstruct the history of aridification during the Eocene. Materials and methods The mudstone was sampled at intervals of 1—2 m with 96 samples being obtained from the East Xining Section, and 94 samples from the Shuiwan Section. The samples were pretreated with 2 mol/L HCl to remove carbonate and the stable carbon isotope composition f the resulting CO2 was measured on a Thermo Delta V mass spectrometer interfaced with a Flash EA 1112 elemental analyzer. Results The δ13 CTOC records of the two parallel sections in Xining Basin both exhibit an abrupt ~2.5‰ increase at ~39 Ma. Discussion The distribution of n-alkanes and the δ13 CTOC values indicate the organic matter is mainly derived from terrestrial C3 plants; provenance of the sediments remained constant. Thus, the increase of δ13 CTOC at ~39 Ma in Xining Basin can be mainly attributed to environmental factors that have influenced the carbon isotopic composition of terrestrial C3 plants. The concentration and carbon isotopic values of atmospheric CO2 can be excluded from the list of potential causal factors. Precipitation and temperature both have a negative relationship with the carbon isotopic composition of terrestrial C3 plants, the effect of the former is stronger. Notably, there was no abrupt decrease in global temperature at ~39 Ma and therefore, a decrease in precipitation was the principal cause of the carbon isotopic shift at ~39 Ma in the Xining Basin. An aridification enhancement event at ~39 Ma is also recorded by other proxies, such as pollen and spore assemblages, from other sections in Xining Basin as well as Qaidam Basin. It is likely that the retreat of Paratethys Sea was the main factor responsible for the reduction in precipitation in Xining Basin, against the background of global cooling. Conclusions The stable carbon isotope records of the total organic fraction of the sediments of Xining Basin reveals an abrupt enhancement of aridification happened at ~39 Ma, which is confirmed by other proxy records from the region.. The retreat of Paratethys Sea was likely the principle cause, while global cooling played a less important role. Recommendations and perspectives The explanation for Asian inland aridification at ~39 Ma still remains controversial and further research is needed to determine the cause.
Keywords: Xining Basin; ~39 Ma; total organic carbon isotope; Asian inland aridification; retreat of Paratethys Sea
亚洲内陆干旱区横跨我国西北至中亚、蒙古和俄罗斯北纬约55°以南的广大中纬度地区(图1a),是地球上独一无二的温带干旱区。亚洲内陆干旱化是新生代北半球环境演化的重大事件,它的起始时间、演化历史和驱动机制是古环境研究的热点之一。新生代亚洲内陆干旱化重大事件起源时间最早可追溯至始新世(Caves et al,2015;Bougeois et al,2018)。目前干旱化的研究主要集中在晚中新世(Rea et al,1998;Sun et al,2015;Yang et al,2016)、早中新世(Guo et al,2002;Zhang et al,2015)、晚渐新世(Sun et al,2010;Zheng et al,2015)和始新世—渐新世转变时期(Dupont-Nivet et al,2007;Miao et al,2013;Zhang and Guo,2014;Sun and Windley,2015)。由于早新生代的地层保存比较少,始新世亚洲内陆干旱化的研究比较匮乏,驱动机制仍不明确。可能的驱动机制主要包括全球变冷(Dupont-Niviet et al,2007;Bosboom et al,2014;Fang et al,2015)、副特提斯海的退却(Caves et al,2015;Bougeois et al,2018;Meijer et al,2019)和青藏高原的隆起(Song et al,2013;Li et al,2018)。
西宁盆地位于西北内陆干旱区、东亚湿润季风区和青藏高原高寒区交汇带,对气候变化十分敏感,盆地保存有完好的新生代连续沉积序列和丰富的哺乳动物化石(李传夔和邱铸鼎,1980;李传夔等,1981),且沉积地层有高精度的古地磁年龄制约(Dai et al,2006;Yang et al,2017;Fang et al,2019),是研究亚洲内陆干旱环境演化以及季风演化历史和驱动机制的理想区域(Dupont-Nivet et al,2007;Hoorn et al,2012;Xiao et al,2012;Chi et al,2013;Licht et al,2014;Fang et al,2015,2019;Zan et al,2015;Zhang et al,2015;Meijer et al,2019)。然而,目前对于该区域始新世亚洲内陆干旱化重大事件记录却相当匮乏,制约了对于早新生代青藏高原东北缘古气候演化历史、季风影响和驱动机制的认识。
植被对周围环境的变化非常敏感,是研究古环境的理想材料。植物进行光合作用合成有机物,该过程碳同位素的分馏受周围环境的影响,从而使得植物在不同环境下自身的碳同位素组成产生差异,因此植被的碳同位素组成可以反映生态环境信息(Farquhar et al,1982,1989;Diefendorf and Freimuth,2017)。尽管植物凋谢物进入地层之后发生降解导致碳同位素的分馏,但总有机碳同位素(δ13 CTOC)仍能保存原始植被信息(Melillo et al,1989;Meyer et al,1993;Rao et al,2017),可以被用来重建古生态和古环境。此外,总有机碳同位素指标在地层相对其他生态指标如孢粉和正构烷烃,存在如下优势:一是测试方法样品消耗量少,实验方法简单高效,数值结果可靠;二是它的有机质来源于区域内所有的植被,记录的环境信息更加全面综合,可被用来综合评价流域内植被的变迁历史(Diefendorf and Freimuth,2017)。因此该指标被广泛应用于在第四纪黄土高原风成沉积和早新生代河湖相地层的古生态和古气候重建工作中(Schouten et al,2007;Bechtel et al,2008;Holdgate et al,2009;Chi et al,2013;吴福莉等,2015;Yang et al,2015)。本研究拟通过对西宁盆地两个有精确年代控制的早新生代平行剖面总有机碳同位素的研究,重建青藏高原东北缘始新世时期的古环境演化历史,并尝试对其驱动机制进行探讨。
1   研究区域概况
西宁盆地位于青藏高原东北缘,地貌上处于青藏高原和黄土高原的衔接地带,盆地北侧是大坂山,南以拉脊山为界,西临日月山,东延入甘肃境内陇中盆地(图1)(Dai et al,2006;Long et al,2011)。盆地内大部分地区海拔在1750—2600 m,周围一些高山的海拔在3000 m以上,现代年平均温4.8—7.1℃,年均降水量196.2—541.2 mm,降水主要集中在夏季(曹生奎等,2011)。
西宁地区现代植被以温带植物为主,主要植被类型有:(1)森林:海拔2300—2600 m的山地阴坡主要是以青海云杉为建群种的寒温性针叶林,海拔2100—2900 m的山地阴坡、半阴坡和沟谷地带则分布着温带落叶阔叶林;(2)灌丛:高寒常绿灌丛主要分布在海拔2800—3400 m的山地阴坡,高寒落叶灌丛分布在2600—3500 m的山地阴坡和沟谷地带,温性灌丛分布在海拔2100—2800 m的山地阳坡、半阴坡和林缘;(3)温性草原:主要在西宁市海拔2170—2900 m的山前干旱阴坡;(4)草甸:高寒草甸分布在海拔3100—4100 m的山地、滩地和宽谷,沼泽草甸仅分布在水滩地、河流两岸洼地(孙海群和李长慧,1995)。
西宁盆地新生代地层与下伏太古宙、中生代基岩呈不整合接触,与上覆第四系为不整合接触(Dai et al,2006),沉积环境从盐湖相沉积逐渐转变为河流相沉积再转变为洪积扇沉积(Dai et al,2006;Zhang et al,2017)。新生代地层被划分为古近纪的西宁群和新近纪的贵德群:西宁群下部发育非常显著的厚层石膏组合,呈厚度(~1.5 m—~2.5 m)近似的韵律交替产出的白色(绿色)石膏层和红色石膏质泥岩/粉砂质泥岩。上部是紫红色—红色石膏质红层组合,呈现为厚层的块状紫红色—红色石膏质砂质泥岩,偶夹薄层灰绿色石膏/泥质石膏;贵德群几乎不含石膏,岩性特征是褐色到褐黄色—褐红色的块状泥岩和粉砂岩,偶夹薄层淡绿色泥质石膏/石膏质泥岩—粉砂岩和中等—薄层的透镜状浅灰色砂岩,砂岩中时有交错层理发育或者块状含砾(Fang et al,2019;方小敏等,2007)。西宁群进一步被分为古新世的祁家川组,始新世的洪沟组和始新世—渐新世的马哈拉沟组(Fang et al,2019;方小敏等,2007)。


图1   亚洲近50年来的年降水量分布图及研究区在亚洲自然地理格局中的位置(红色方框指示研究区域西宁盆地)(a,改自张鹏等(2016)),西宁盆地新生代地层分布和剖面位置图(b,改自Fang et al(2019)和Zhang et al(2017))
Fig.1 The map showing the distribution of annual precipitation in Asia in recent 50 years and the location of the Xining Basin (red box) in the configuration of the Center Asian arid inland and the Monsoon humid region (a, modified from Zhang Peng et al (2016)), geological map of the Xining Basin showing the distribution of Cenozoic stratigraphy and locations of the studied and related sections (b, modified from Fang et al (2019) and Zhang et al (2017))
EXN:西宁东剖面;SW:水湾剖面;XJ:谢家剖面;TF:铁佛剖面。EXN: East Xining section; SW: Shuiwan section; XJ: Xiejia section; TF: Tiefo secetion.
西宁东剖面(EXN)和水湾剖面(SW)均含有较好的古近纪地层沉积(图1b)。西宁东剖面(36°34.77′N,101°53.81′E至36°35.05′N,101°53.56′E)位于西宁城东边互助河的入口处,厚350 m;水湾剖面(36°39.48′N,101°52.31′E至36°40.55′N,101°50.24′E)位于水湾村的西北边,总厚720 m。这两个平行剖面均位于西宁盆地中部(图1b),有约150 m的马哈拉沟组平行地层重叠。西宁东剖面和水湾剖面马哈拉沟组岩性均是石膏层与红色含膏泥岩层的几乎等厚的旋回沉积(图2b、2f),石膏为浅盐湖沉积,红色含膏泥岩为盐湖边的泥滩-洪泛平原远端沉积物(Dupont-Nivet et al,2007;Abels et al,2011)。本文研究样品分别采自西宁东剖面200—350 m,水湾剖面-12—140 m,古地磁年龄均为43.4—35.6 Ma(图2)(Fang et al,2019)。
2   实验材料和方法
本文主要选取石膏—泥岩沉积旋回中的细粒泥岩沉积进行研究(图2)。野外采样去除表面风化层,尽量选择膏质泥岩/粉砂质泥岩层新鲜面上取样,采样间距约为1—2 m。西宁东剖面采集样品96个,水湾剖面样品94个。样品在烘箱45℃环境下烘干,磨成粉末,称取1 g粉末状样品装在15 mL PET离心管中,加入约2 mol/L的过量盐酸,反应48 h以保证碳酸盐被完全去除,期间每隔12 h震荡一次。离心加超纯水洗至中性,在45℃环境下烘干磨成粉末(Schubert and Nielsen,2000)。实验采用EA-IRMS在线分析技术,氧化温度设定为1020℃。根据样品有机质含量高低适量称取10—40 mg样品装入锡杯,包裹严实放入自动载样盘中开始样品有机质总有机碳同位素值的测定,每隔十个样品放置一个标样。测试仪器是Thermo公司生产的Delta V型气体稳定同位素比质谱仪,通过CONFLO Ⅲ连接附件Flash EA 1112元素分析仪。标样和样品总有机碳同位素值计算均采用VPDB标准,标样和样品重复分析误差范围是±0.2‰。样品前处理在中国科学院青藏高原研究所大陆碰撞与高原隆升重点实验室完成,测试工作在中国科学院青藏高原研究所拉萨部环境变化与地表过程重点实验室完成。


图2   研究剖面岩性柱、磁性地层(Fang et al,2019)和总有机碳碳同位素记录(a—c:EXN;e—g: SW)
Fig.2 Lithology (b, f), magnetostratigraphy (c—e) (Fang et al, 2019) and total organic carbon isotopic records (a, g) of the studied sections (a—c: EXN; e—g: SW)
3   实验结果
研究剖面δ13 CTOC变化的最大特征,是两个剖面均在~39 Ma同时开始出现~2.5‰的明显偏正,其中西宁东剖面的δ13 CTOC值从-27.5‰变化到-25.0‰,水湾剖面的δ13 CTOC值从-28.4‰变化到-26.0‰。可以将记录明显分为上下两段(图2):
阶段Ⅰ:43.4—~39.0 Ma,西宁东剖面在剖面200—266 m的δ13 CTOC值为-28.22‰—-25.50‰,平均值约为-26.97‰;水湾剖面在剖面-12—52 m的δ13 CTOC值为-20.02‰—-23.26‰,平均值约为-27.37‰。δ13 CTOC整体呈现变负的趋势。
阶段Ⅱ:~39.0—35.6 Ma,西宁东剖面在厚度266—350 m的δ13 CTOC值为-25.79—-24.25‰,平均值约为-24.96‰;水湾剖面在厚度52—140 m的δ13 CTOC值为-27.65—-21.87‰,平均值约为-25.82‰。δ13 CTOC值在~39 Ma开始急剧变正~2.5‰,在~38.6 Ma之后无明显变化趋势。
4   结果讨论
4.1   地层有机质来源
影响地层中的δ13 CTOC的因子主要包括有机质的来源和气候环境因素(Diefendorf and Freimuth,2017)。有机质的来源对地层中δ13 CTOC有明显影响。湖相地层有机质可分为内源有机质和外源有机质,其中内源有机质主要来自水生植物(沉水植物、漂浮植物或各种藻类),外源有机质主要来自湖泊流域内周围侵蚀带来的陆源植物碎屑(Diefendorf and Freimuth,2017)。现代研究表明微生物产生的正构烷烃集中在短链部分(C14—C20),主碳峰处于C17或C18、C19,没有明显的奇偶优势(Han and Calvin,1969;Nishimura and Baker,1986);藻类、沉水/漂浮水生大型植物和泥炭藓产生的正构烷烃以中链(C21—C25)相对含量最高,多以C21,C23和C25为主碳峰,具有明显的奇偶优势(Baas et al,2000;Ficken et al,2000;Aichner et al,2010);陆生高等植物的正构烷烃集中在长链部分(C26—C35),主要以C27、C29和C31为主碳峰,具有明显的奇偶优势(Ficken et al,2000)。本文研究剖面阶段Ⅰ和Ⅱ的正构烷烃以C27、C29和C31占主导,而且高碳数部分(C26—C35)具有明显的奇偶优势(图3),表明研究地层的有机质主要来自流域周围的陆生植物,沉水/漂浮水生植物和微生物来源的有机质含量大致可以忽略(Cranwell et al,1987;Ficken et al,2000)。陆生植物根据光合作用途径的差异可分为C3植物、C4植物和CAM植物(Farquhar et al,1989;Tipple and Pagani,2007)。它们的碳同位素组成存在明显差异(Farquhar et al,1982,1989)。现代C3植物的碳同位素组成在-20‰—-34‰,C4植物的碳同位素组成在-9‰—-19‰(Rao et al,2017)。CAM植物的碳同位素组成介于C3和C4植物两者之间,主要分布在荒漠地带,种类稀少,含量相对C3和C4植物可以忽略不计。一般认为C3和C4陆生植物是地层有机质的主要来源(Diefendorf and Freimuth,2017)。C4植物被认为起源于渐新世,8 Ma之前它在植被中所占的比例几乎可以忽略不计,8 Ma之后才有显著扩展(Tipple and Pagani,2007;Edwards et al,2010),因此,有机质应该主要来自流域周围的陆生C3植被。


图3   研究剖面代表性样品正构烷烃气相色谱图
Fig.3 The n-alkanes GC spectrums of representative samples from studied sections
Fig.3 The n-alkanes GC spectrums of representative samples from studied sections
陆生植被来源的有机质被河流搬运到盆地沉积下来。当盆地的物源发生变化时,所携带的陆生植被来源也会发生变化,进而影响地层δ13 CTOC的变化。构造活动可能对物源有影响。西宁盆地始新世地层沉积速率总体上很低且保持相对稳定(图4d)(Dai et al,2006;Fang et al,2019),岩性均以泥岩和石膏沉积为主,未见有粗碎屑沉积,说明当时西宁盆地没有显著的构造活动,沉积相和沉积环境相对稳定。Zhang et al(2016)发现西宁盆地早新生代沉积物主要来自祁连山、西秦岭和三叠纪复理石沉积,沉积物物源在该时段没有发生明显变化。因此物源变化导致西宁东和水湾剖面~39 Ma碳同位素于~39 Ma发生显著变化的可能性较小。需要注意的是,植物有机质进入地层之后,可能会遭受成岩作用。Melillo et al(1989)和Meyers and Lallier-Vergès(1999)研究发现有机质进入土壤之后碳同位素没有被显著改变。高温环境的模拟实验也发现植物的碳同位素组成没有显著变化(Schleser et al,1999)。本文研究剖面正构烷烃的长链碳数分布具有明显的奇偶优势,表明沉积物中的有机质几乎没有遭受成岩作用。因此δ13 CTOC值能够记录当时的生态和环境信息。


图4   研究剖面进行校正之后的总有机碳同位素记录(a:SW;b:SW)与盆地内铁佛剖面耐旱植物(麻黄属和白刺属)孢粉记录(c)(Bosboom et al,2014)、和沉积速率记录(d)(Fang et al,2019)以及柴达木盆地化学风化指数记录(e)(Song et al,2013)、塔里木海平面变化记录(f)(Bosboom et al,2011)和全球深海氧同位素记录(g)(Zachos et al,2008)的对比
Fig.4 Comparison of the revised total organic carbon isotopic records of the studied sections (a: SW; b: EXN) with records of the steppe-desert vegetation (Ephedripites and Nitrariadities) pollens from the nearby Tiefo section (Bosboom et al, 2014) (See Fig. 1 for location) (c), the sedimentation rate of the Xining Basin (d) (Fang et al, 2019), the chemical index of weathering (CIW’) from the Dahonggou section in the Qaidam Basin (Song et al, 2013) (e), the Tarim sea-level record (Bosboom et al, 2011) (f), and global deep-sea oxygen isotope record (Zachos et al, 2008) (g)
4.2   环境因子
现代研究表明影响C3植物碳同位素组成的环境因子主要包括温度、降水和大气CO2浓度及其碳同位素组成(Farquhar et al,1982,1989)。新生代以来大气碳同位素组成δ13CCO2值存在明显的波动(Tipple et al,2010),为了消除δ13 CCO2值波动的影响,根据下面公式(1)对研究剖面总有机碳同位素数据进行校正(Farquhar et al,1989):
\(∆{}^{13}\mathrm{C}\left(\mathrm{T}\mathrm{O}\mathrm{C}-{\mathrm{C}\mathrm{O}}_{2}\right)=\frac{\mathrm{\delta }{}^{13}{\mathrm{C}}_{\mathrm{T}\mathrm{O}\mathrm{C}}-\mathrm{\delta }{}^{13}{\mathrm{C}}_{\mathrm{C}\mathrm{O}2}}{1+\mathrm{\delta }{}^{13}{\mathrm{C}}_{\mathrm{T}\mathrm{O}\mathrm{C}}/1000}\)
式中:δ13 CTOC为地层总有机碳同位素组成,δ13 CCO2为地层对应时期大气CO2的稳定碳同位素组成。
校正前后数据趋势无明显改变(图2a和图4b,图2g和图4a),说明δ13 CCO2不是研究剖面~39 Ma碳同位素显著变化的主控因子。始新世大气CO2浓度远比现代大气高(Bijl et al,2010;Beerling and Royer,2011;Wolfe et al,2017)。现代研究表明CO2浓度可以在短时间尺度影响植物有机碳同位素的组成(Lammertsma et al,2011;Schubert and Jahren,2012),但是在地质历史时期,植物有足够长的时间去适应,使得植物对大气CO2浓度变化不敏感,因此大气CO2浓度不是西宁盆地早新生代地层有机碳同位素变化的主要控制因子(Franks et al,2014;Diefendorf et al,2015;Kohn,2016)。现代C3植物碳同位素与温度以及降水关系的研究均表明植物碳同位素与降水呈显著的负相关关系(Diefendorf et al,2010;Kohn,2010),温度与植物碳同位素值的关系虽然也存在负相关关系,但是相关性不显著(Kohn,2010;Rao et al,2017)。因此,δ13 CTOC在~39 Ma的变正可能与温度和降水的减少都有关,但降水的减少造成的影响可能更为显著。
深海氧同位素记录表明中始新世是高温时期,虽然温度呈逐渐下降的趋势(图4f)(Zachos et al,2008),但在~39 Ma并无急剧的降低趋势,这与碳同位素在~39 Ma突然变正的趋势不一致,因此全球变冷可能不是主控因子,降水的减少才是主控因子,即西宁盆地水汽来源的急剧减少可能是造成总有机碳同位素在~39 Ma急剧变正的关键原因。盆地内铁佛剖面稀树草原(麻黄属和白刺属)的孢粉含量在~40 Ma急剧升高(图4c)(Bosboom et al,2014),也表明该时期盆地降水明显减少。青藏高原东北缘其他盆地该时期干旱化的记录也证实该干旱增强事件在亚洲内陆广泛存在,如柴达木盆地基于粘土矿物硼含量重建的古盐度指标在~40 Ma增大(Ye et al,2016),化学风化指数在~40.5 Ma开始显著下降(图4e)(Song et al,2013),都表明在~40 Ma青藏高原东北缘气候干旱增强。
4.3   39 Ma亚洲内陆干旱化加剧的驱动机制
中始新世西宁盆地的水汽来源主要来自西风环流携带的水汽(Caves et al,2015;Bougeois et al,2018)。始新世时期副特提斯海占据欧洲和西亚的大部分地区以及中亚部分地区,可能是重要的西风水汽来源(Zhang et al,2007;Roe et al,2016;Bougeois et al,2018)。全球变冷和副特提斯海的退却都会影响进入西宁盆地的水汽:全球温度下降,水汽蒸发作用减弱,进入西宁盆地的水汽也会减少。44—35 Ma全球温度一直持续下降(Zachos et al,2008)(图4g),但是在~39 Ma并没有显著的降低,与西宁盆地在~39 Ma干旱化突然增强的记录不符,因此全球变冷不是主导因素;副特提斯海的退却会降低海平面,使得进入亚洲内陆的水汽传输路径变长,进入西宁盆地的水汽减少(Ramsstein et al,1997)。研究表明帕米尔高原在~40 Ma向北突刺导致副特提斯海从塔里木盆地退出,导致进入亚洲内陆的水汽减少(图4f)(Bosboom et al,2011;Carrapa et al,2015;Sun et al,2016),这与西宁盆地干旱化增强记录的时间相近,说明副特提斯海的退却可能才是西宁盆地~39 Ma干旱化增强的主导因子,全球变冷是次要因子。
5   结论
西宁盆地具有精细古地磁年龄控制的新生代地层沉积序列,是研究亚洲内陆干旱化及其驱动机制的绝佳材料。对西宁盆地西宁东和水湾两个平行剖面的马哈拉沟组进行的总有机碳同位素研究,发现~39 Ma总有机碳同位素突然变正。总有机碳同位素变正的时间与盆地内旱生植物(麻黄属和白刺属)孢粉含量急剧升高的时间基本一致同步,且与柴达木盆地该时期的急剧的干旱化也较为吻合,表明中始新世亚洲内陆发生了一次急剧的干旱化,~39 Ma降水的急剧减少可能是造成西宁盆地总有机碳同位素突然变正的主要原因。该时期副特提斯海的快速退却导致的水汽向大陆内部输送的减少可能是造成西宁盆地此时明显干旱化的主要驱动力,全球长期变冷导致的水汽蒸发减弱可能起到了背景叠加作用。
致谢:野外样品采集得到了张伟林、方小辉、卢银、栗兵帅、鲍晶、马丽芳、何鹏举、胡春华、钟思睿、杨戎生和陈炽皓等人的鼎力帮助,实验过程中得到了朱志勇老师的大力支持,在此致以诚挚的感谢。
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稿件与作者信息
方亚会
FANG Yahui
方小敏
FANG Xiaomin
方小敏,E-mail: fangxm@itpcas.ac.cn
昝金波
ZAN Jinbo
张涛
ZHANG Tao
杨一博
YANG Yibo
叶程程
YE Chengcheng
白艳
BAI Yan
中国科学院战略性先导科技专项(A类)(XDA20070201)
Strategic Priority Research Program of Chinese Academy of Sciences (XDA20070201)
出版历史
出版时间: 2019年4月11日 (版本2
参考文献列表中查看
地球环境学报
Journal of Earth Environment