第四纪研究  2016, Vol.36 Issue (5): 1103-1113   PDF    
阿尔金新近纪红粘土的古地磁年代及古气候记录
李建星 , 王建强 , 岳乐平 , 潘峰 , 郭琳 , 奚仁刚 , 过磊     
(① 中国地质调查局西安地质调查中心, 西安 710054;
② 浙江省水文地质工程地质大队, 宁波 315012;
③ 西北大学地学系, 大陆动力学国家重点实验室, 西安 710069)
摘要: 黄土-古土壤序列、红粘土等风尘堆积由于其本身蕴含着丰富的气候信息而成为研究新生代古气候(东亚季风)演化的理想载体。已有的研究表明大面积连续分布的风尘堆积主要出露在黄土高原及邻区。新近,在青藏高原北缘阿尔金山地区发现了一套厚约88.4m的新近纪红粘土堆积(彩虹沟组),红粘土本身沉积特征及其与黄土高原典型风成沉积物的诸多相似性表明其为风成产物。生物化石约束下的磁性地层学表明该风尘堆积形成于13.0~2.6Ma。根据沉积物的载磁矿物、磁化率系统差异可将该序列分为上下两段:下段(13.0~11.5Ma)载磁矿物以磁铁矿为主,磁化率值总体较高(平均值为15×10-8m3/kg)且变化频繁,指示了总体湿润但干湿交替明显的古气候;上段(10.3~2.6Ma)稳定的磁化率低值(平均值为9.5×10-8m3/kg,主要载磁矿物为赤铁矿)为干旱少雨背景下的产物。11.5~10.3Ma(过渡段)的磁化率降低及载磁矿物的转变为区域干旱化增强的反映。该次干旱化事件是在全球变冷与副特提斯洋退出的总体背景下,高原北部逐渐向北扩展引起的。
主题词阿尔金     红粘土     磁性地层学     磁化率     古气候    
中图分类号     P534.62;P318.4;P539.3                    文献标识码    A

1 引言

自Heller和Liu[1]发现黄土磁化率与深海氧同位素变化的耦合关系以来,中国东部黄土高原黄土-古土壤序列等风尘堆积物一直作为古气候重建的重要载体而备受关注[24];随后,古气候学的研究对象扩大到了同为风成成因的红粘土[5, 6]。经数十年研究,基于磁性地层年代学框架控制的风成堆积磁化率[713]研究不但重建了晚中新世以来东亚季风区的气候演化过程,而且识别出了16.5~13.8 Ma[14]、 8 Ma[15]、 3.6 Ma[15, 16]及2.6 Ma[15, 16]等若干重要气候事件。

与东部黄土高原相比,中国西部红粘土出露较少,这在一定程度上影响了高分辨率古气候研究。近年来,先后在准噶尔盆地[17]、阿尔金[18, 19]发现了风尘堆积——红粘土。但准噶尔红粘土垂向上不连续[17],不利于基于气候指标的古气候恢复;而阿尔金红粘土地层连续,是西部地区区域古气候重建的重要载体[19]。本文在野外调查基础上,结合古生物化石,标定了阿尔金红粘土的古地磁年代并初步探讨了晚中新世以来的区域气候演化特征及其可能的驱动因素。

2 区域地质

阿尔金山位于青藏高原东北缘,南接高原内陆盆地柴达木盆地,北邻中国二级地势区之塔里木盆地。坐落于阿尔金山之上的索尔库里盆地将阿尔金山分为南、北两部分;盆地中东部的索尔库里北山将盆地分割为北部盆地和南部走廊两部分(图 1)。

图 1 研究区及邻区地貌图 紫色部分为红粘土分布区,五角星为彩红沟剖面位置 Fig. 1 Schematic tectonic map of the studied area and its adjacent regions. Purple areas show distribution of red clay, star shows location of the section

我们新近完成的“1:25万巴什库尔干幅区域地质调查”项目依据地层接触关系、岩石组成及古生物证据将索尔库里北盆地的新生代地层分别划分为始新世溪水沟组、新近纪彩虹沟组、早-中更新世七个泉组、上更新统及全新统。新近纪彩虹沟组为本文研究重点。彩虹沟组平行不整合于始新世溪水沟组之上,其间缺失渐新世及中新世早期沉积物。受晚新生代断裂逆冲影响,溪水沟组及彩虹沟组被整体掀斜形成以北缓南陡的不对称向斜;早-中更新世七个泉组砂砾石层角度不整合于已经掀斜溪水沟与彩虹沟组之上,其本身也被小角度掀斜;上更新统及全新统是现今地表出露地层的主体。受第四纪地层覆盖影响,彩虹沟组及溪水沟组地表出露较少,仅在深切割的沟道中可见其天然露头。彩虹沟剖面就是其中的典型,本文研究剖面位于新疆维吾尔自治区若羌县若羌镇索尔库里北盆地,剖面起点坐标为38°57′09″N,91°30′41″E,高程为3652m;终点坐标为38°55′30.65″N,91°30′57.21″E,高程为3635 m。

彩虹沟组厚度达94.2 m,可分为上、下两部分:上部为5.8 m薄层状泥岩,下部为88.4 m厚的红粘土夹钙质结核,为本文研究对象。前期研究显示[18, 19]下部红粘土为典型风尘堆积。彩虹沟组下部物质组成单一,主要由红粘土及钙质结核层交互组成,自上而下可见40个厚度不一的旋回。红粘土成壤作用较弱,铁锰胶膜、钙质结核的发育程度较低:铁锰胶膜稀疏分布于各红粘土层中;钙质结核呈不规则状,但直径(最大轴)一般小于3 cm;密集分布或呈钙板状者较为少见。局部可见水成砂砾石层夹层,产中中新世哺乳动物化石Stephanocemas?sp.[20](中国科学院古脊椎动物与古人类研究所王伴月研究员鉴定)。根据颜色及红粘土与钙质结核的比例,彩虹沟组下部88.4 m厚的红粘土又可以将红粘土进一步划分成两个部分:上段主要以红褐色粘土夹棕黄色钙质结核为主(二者比例约为8:1~10:1),部分结核层在走向上尖灭;下段主要由黄棕色粘土和灰色钙质结核层组成(4:1~5:1)。

此外,新近纪彩虹沟组红粘土还在贝壳滩盆地、巴什考供盆地零星出露(图 1)。索尔库里盆地公路建设中新挖掘出来的人工剖面中也可见典型红粘土露头,这表明新近纪彩虹沟组红粘土在阿尔金地区是广泛分布的,仅因为第四系覆盖而地表露头较少。

3 采样及实验测试

本论文研究时所选剖面的沉积物大部分是半固结的掀斜地层。野外采样时先将剖面表层的坡积物和风化层清理干净直至露出原始地层的新鲜面后,以20 cm为间距采集样品,共采集剩磁分析样品820个(多数层位采集两个平行样品,但部分层位较为破碎不适于采样)。本次工作分别采用古地磁采样钻机及任意面采样两种方法。钻机采样时钻孔沿地层走向方向尽可能水平方向取样,同时记录地层产状、钻孔倾向和入孔方向。钻出样品在实验室按照截取1英寸作为测试样品。任意面采样法采样时需记录任意面产状及地层产状,采集样品在实验室整理成2 cm×2 cm×2 cm样品供测试。需要特别强调的是,在整个采样和加工过程中都要避免外界强磁场对磁罗盘和样品的干扰。

为了判定剩磁携带的矿物特征,选取了部分典型样品(10个样品)进行了岩石磁学测试,岩石磁学实验在中国科学院地质与地球物理研究所岩石磁学与古地磁实验室完成的。磁化率-温度(χ-T)曲线在卡帕桥KLY-3及其附带的CS-3型高温装置上完成。为了降低样品由于加热引起氧化作用,整个加热过程均在氩气环境中进行。热磁分析和磁滞回线使用居里秤(VFTB)完成。样品用量大约为0.2 g。将粉末样品置于石英管中,用耐火棉塞紧、压实,从35℃缓慢升温,并测试每一个温度点样品的剩余磁化强度,升温至700℃后缓慢降低到室温。

445个样品的剩磁分析(部分层位进行了2次分析)在西北大学大陆动力学国家重点实验室完成。剩磁测量选用捷克Agico公司产的JR6-A旋转磁力仪。退磁选用英国Magnetic Measurement公司产的TTMD-80型热退磁仪,样品的测试和退磁均在磁屏蔽空间中进行。样品在测完天然剩磁(NRM)后,按50℃或25℃分档退磁,退到600℃或675℃。对出现异常或者磁性不稳定样品,对其平行样品进行了补测,以检验原来测试样品值是否可靠。

4 测试结果

整体上彩虹沟剖面新近纪红粘土磁化率值偏低,平均值仅为11.61×10-8 m3/kg,其最小值仅为5.69×10-8 m3/kg,最大值为23.92×10-8 m3/kg。根据磁化率绝对值变化,可将新近纪红粘土分为:1)下段94.2~78.0 m(13.0~11.5 Ma),绝对值较高且振荡幅度较大,变化在8.77×10-8~23.92×10-8 m3/kg,平均值为15×10-8 m3/kg;2)上段64.0~5.8 m(10.3~2.6 Ma),绝对值较低且变化幅度较小,5.69×10-8 m3/kg到14.76×10-8m3/kg(平均值为9.5×10-8 m3/kg);3)磁化率急剧减小的中间过渡段78~64 m(11.5~10.3 Ma)[19]

彩虹组红粘土样品的χ-T曲线(图 2)差异明显:剖面上段样品的加热曲线在500℃时开始发生明显的下降趋势,逐渐到700℃时到达零值,并且在居里点(580℃)处没有明显的下降,这种现象表明样品中的主要载磁矿物为赤铁矿;而下段样品的加热曲线在580℃时陡然下降,说明该段主要的载磁矿物为磁铁矿。

图 2 阿尔金新近纪红粘土典型样品χ-T曲线 (e)和(f)来自文献[19] Fig. 2 Typical χ-T curves of Neogene red clay in Altun, and (e) and (f) are from reference[19]

彩虹沟组磁滞回线测试样品均具有“蜂腰”形态特征,并在900~1000 mT以上才闭合(图 3),指示样品含有高、低矫顽力两种磁性矿物。上段样品具有高矫顽力(Bc、 Bcr)及中部较宽磁滞回线形态,表明赤铁矿是其主要载磁矿物[21];而下段样品则表现为低矫顽力及窄磁滞回线特征,说明其中主要载磁矿物为磁铁矿。

图 3 阿尔金新近纪红粘土典型样品磁滞回线 (e)和(f)来自文献[19],磁滞回线已经过顺磁校正 Fig. 3 Typical hysteresis loops corrected for paramagnetic contributions of Neogene red clay in Altun. (e)and (f) are from reference[19], hysteresis loops have been adjusted

图 4可以看出,彩虹沟剖面新近纪阿尔金红粘土的IRM曲线可以明显的分为两组,下段样品多在300~400 mT即已达到饱和剩磁的80 %,但是上段红粘土则在近500 mT才开始达到饱和剩磁的80 %,暗示了红粘土上段比下段的硬磁性矿物含量更高。

图 4 阿尔金新近纪红粘土典型样品IRM曲线 红色曲线来自剖面上段,蓝色曲线来自剖面下段 Fig. 4 Typical IRM of Neogene red clay in Altun. Red/blue curves are from samples of upper/lower part of section respectively

在典型样品退磁曲线中,彩虹沟组下段样品当热退磁温度达到580℃时,就能分离出稳定的特征剩磁,表明磁铁矿是特征剩磁的主要载体;而上段样品需加热到675℃时才能分离出稳定的特征剩磁(图 5),表明具有高矫顽力的赤铁矿的存在。

图 5 阿尔金新近纪红粘土典型样品退磁曲线 空心/实心符号代表垂直/水平投影;(a)、 (d)、 (e)和(f)来自文献[19] Fig. 5 Typical demagnetization curves of Neogene red clay in Altun. Circle/dot denote vertical/horizontal project, and (a), (d), (e) and (f) are from reference[19]

综合χ-T曲线、磁滞回线、 IRM及热退磁曲线可知,阿尔金新近纪红粘土剖面上、下段载磁矿物上下略有不同,下段以磁铁矿为主体,而上段以赤铁矿为主体。

5 讨论 5.1 阿尔金新近纪红粘土的古地磁年代

新近纪彩虹沟组红粘土平行不整合于中始新世溪水沟组之上,角度不整合于早更新世七个泉组之下。剖面下部70m附近处发现的哺乳动物Stephanocemas?sp.为中中新世重要生物分子[20],限定了化石所在层位之上的地层沉积于中中新世-上新世。

通过系统退磁获得了新近纪红粘土记录的磁极性序列,共26个正极性带和27个负极性带。在地层接触关系及古生物化石约束下,将所获得的磁极性序列与地磁极性年表[22]进行对比(图 6),结果表明,彩虹沟剖面记录了从C5An.4r到C2r的沉积,时间跨度为13.0~2.6 Ma,时代为新近纪中中新世至上新世。

图 6 阿尔金新近纪红粘土磁性地层结果(来自文献[19]) Fig. 6 Magnetostratigraphy of the Neogene red clay sequence in Altun from reference[19]
5.2 磁化率记录的古气候演化

Heller和Liu[1]开创了风尘堆积磁化率用于古气候研究的先河。对黄土高原而言,夏季风携带的水汽对冬季风搬运而来的黄土进行了不同程度的成壤作用改造,该过程中新生的纳米级磁赤铁矿颗粒大大提高提高风成沉积物的磁化率[23, 24]。因此,增强的磁化率已经被用于重建黄土高原夏季风的演变[2, 15, 17]。这一原理也适用于同属风成成因的新近纪红粘土[25]。与第四纪相比,中新世及上新世气候相对比较温暖及湿润,部分成壤作用形成的中间磁赤铁矿会转化为赤铁矿,从而导致更红的颜色,但比上覆古土壤磁化率值低[25, 26]

古气候包括古温度、古湿度等多方面信息,磁化率主要记录了与古降雨量相关的湿度信息[27, 28]。黄土高原的湿润水汽主要由夏季风携带而来,因此,磁化率主要记录了夏季风携带水汽的相对强度[25]。氧同位素研究显示,自始新世以来,中国西部乃至亚洲大陆大部分地区的湿润水汽主要由西部西风携带而来[29]。因此,阿尔金红粘土磁化率记录了西风携带水汽能力的大小,这与黄土高原红粘土存在明显差异。

氧同位素高度计的研究显示青藏高原中部及南部地区在早始新世[3032],至少中中新世之前[3335]已经隆起并达到相当高度,该高度足以阻挡来自于印度洋的湿润水汽[29]。但是此时高原向外扩展尚未波及到阿尔金[33, 36],阿尔金山地区仍处于中南部高原的雨影区(rain shadow),其地势可能与今日之塔里木盆地相似[19]。在全球变冷的总体背景下,副特提斯洋退出及中南部高原的阻挡导致了包括阿尔金、柴达木在内的亚洲内陆广大地区非常干旱[29, 3739],这是新近纪阿尔金红粘土磁化率较低的主要原因。新近纪彩虹沟组红粘土剖面上部物质的磁化率值远低于同时期黄土高原红粘土磁化率值[4042]也证实了这一点。

根据磁化率绝对值及其变化特征,可将阿尔金地区新近纪中晚期(13.0~2.6 Ma)的古气候分为两个差异明显的阶段(图 6):13.0~11.5 Ma磁化率高值且频繁变化显示该阶段气候总体相对湿润但干湿变化明显;10.3~2.6 Ma绝对值较小且稳定的磁化率值显示该时期气候总体干旱且无明显变化。值得一提的是柴达木盆地中氧同位素在10.7~1.8 Ma也无明显变化[39],记录了与阿尔金相似的古气候信息。阿尔金及柴达木盆地不同的古气候指标记录了相似的古气候信息,不但彼此印证了各气候指标的可靠性,也证实该气候特征并非局地特征,而具有一定区域性。

5.3 干旱加剧及可能的机制

11.5~10.3 Ma,阿尔金红粘土磁化率平均值从15×10-8 m3/kg降到9.5×10-8 m3/kg,系区域性干旱化增强事件所致。前已述及,剖面上、下段的主要载磁矿物及磁化率特征存在明显差异:下段高磁化率值可能与强磁性的磁铁矿为载磁矿物相关;而上段的载磁矿物为弱磁性的赤铁矿,从而导致剖面上段磁化率较低。已有的研究表明,磁铁矿易在氧化(干旱)、还原(湿润)交替环境下形成,而长期干旱(氧化)环境则有利于赤铁矿的形成[4345]。剖面上、下段红粘土的主要载磁矿物从磁铁矿向赤铁矿的转变本身也暗示了区域气候逐渐转干。综合以上可知,11.5 Ma前,在氧化、还原环境快速更替的背景下,阿尔金地区以明显干湿变化为特征,这与野外可见的棕黄色粘土与钙质结核交互的特征相吻合;10.3 Ma之后,假定物源没有发生明显变化,棕红色粘土与灰白色钙质结核交互出现可能与更强氧化环境下赤铁矿(纳米级)形成有关。

阿尔金干旱化事件得到了周缘地区氧同位素研究的证实。例如,柴达木盆地西北缘红沟子地区沉积物地球化学指标在11.1 Ma前后发生了突变被解释为一次区域性干旱化事件[46];柴达木盆地东南缘12.0~10.7 Ma明显的δ18O和δ13C正偏移也为区域性干旱化事件所致[39]。远在青藏高原东北缘的临夏盆地也由于高原的快速隆升且达到一定高度后,阻挡了来自南部的湿润水汽,进而使临夏盆地的δ18O从-10.5 ‰增加到-9 ‰ [47];相似的气候特征在塔里木盆地也有记录[48, 49]。以上东西近3000 km不同地区、不同的古气候指标都记录了同时期相似的古气候事件,暗示了该干旱化为区域性事件,这也得到了青藏高原东北缘古气候研究的证实[50]

从全球尺度看,亚洲内陆干旱化受控于青藏高原隆升、全球变冷及副特提斯洋退出[15, 41, 5169]。阿尔金地区风尘堆积记录的干旱化过程也不例外。首先,在全球变冷背景下,高纬度地区冰量增加直接到了空气中液态水汽的降低,促动了内陆干旱化[41, 51, 52, 5660];其次,在特提斯洋从塔里木盆地退出之前,一直为内陆提供着湿润水汽[51, 53]。最新的研究表明至少在37 Ma,副特提斯洋已经从塔里木盆地彻底退出而不再为内陆提供充足的湿润水汽[61];第三,诸多地质事实证明到上新世中期青藏高原已经隆起到相当的高原[34, 6265],阻挡了来自南部的湿润水汽进而推动干旱化发展[15, 41, 58, 59, 66]。此后,高原北部边缘进一步隆升(垂向上及横向上)的事实已经为岩浆侵位、低温热年代学[7074]、沉积盆地演化[75, 76]等证实;约12 Ma高原北部的地表快速隆升阻挡了南部的湿润水汽和将来自于西部的有限湿润水汽分为南北两支,进而促进了干旱化[39, 47]

6 结论

阿尔金新近纪红粘土为黄土高原之外新发现的第一个连续风尘堆积剖面。磁性地层学研究表明该序列共记录了26个正极性带和27个负极性带,结合地层接触关系及古生物化石约束,将其对应于C5An.4r到C2r,时间跨度为13.0~2.6 Ma,时代为中中新世至上新世。

彩虹沟剖面下部88.4 m厚的风成堆积(红粘土)下段(13.0~11.5 Ma)、上段(10.3~2.6 Ma)典型样品岩石磁学参数(χ-T曲线、磁滞回线、 IRM、热退磁曲线)存在系统差异,系载磁矿物不同所致。结合磁化率随时间变化曲线可知:以磁铁矿为主要载磁矿物的剖面下段记录了总体湿润但干湿频繁变化的古气候过程;而剖面上段的主要载磁矿物为赤铁矿,其记录的古气候以干旱少雨为特征。

彩虹沟剖面红粘土上、下段之间的过渡段(11.5~10.3 Ma)磁化率(平均值)从15×10-8 m3/kg降低到9.5×10-8 m3/kg,结合红粘土上、下段的载磁矿物及其记录不同气候过程,可以推断阿尔金新近纪红粘土也记录中国西部曾普遍发生的区域性干旱事件。该次干旱化是在全球变冷与副特提斯洋退出的总体背景下,高原北部逐渐向北扩展引起的。

致谢: 感谢审稿专家和编辑部老师对文章提出的建设性修改意见。

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Magnetostratigraphic chronology and paleoenvironmental records of Neogene red clay in Altun, Western China
Li Jianxing, Wang Jianqiang, Yue Leping, Pan Feng, Guo Lin, Xi Rengang, Guo Lei     
(① Xi'an Center of Geological Survey, China Geological Survey, Xi'an 710054;
Zhejiang Institute of Hydrogeology and Engineering Geology, Ningbo 315012;
State Key Laboratory of Continental Dynamics, Department of Geological Sciences, Northwestern University, Xi'an 710069)

Abstract

Aeolian deposits, such as loess/paleosol and red clay, have been widely used for the reconstruction of paleoclimate(such as the evolution of East Asian Monsoon).Previous studies have more focused on the aeolian deposit on Chinese Loess Plateau(CLP), the largest and most extensive area of such deposits in China.Here, we describe and interpret an 88.4-m-thick Neogene aeolian sequence in the lower part of Caihonggou Formation containing 40 visually-defined, brownish-red clay and gray caliche nodule layers in the eastern Xorhol Basin, northeast of the Tibetan Plateau.The study section is located in the north Xorkol Basin of Ruoqiang Town, Ruoqiang County, Xinjiang, with starting coordinates and elevation of 38°57'09"N, 91°30'41"E and 3652m a.s.l., as well as the ending ones of 38°55'30.65"N, 91°30'57.21"E and 3635m a.s.l.Both sedimentary features and similarities to the typical aeolian deposit on the CLP point to its eaolian origin.Typical samples from different parts of the sequence have been used for rock magnetism analysis and distinguished characters have been explored along the sequence.Based on the constraints of contact relation and mammalian fossils (Stephanocemas? sp.), magnetostratigraphic results show that the sequence spans from chron C5An.4r to chron C2r, indicating that the sequence was deposited between ca.13.0Ma and 2.6Ma.The sequence can be divided into two parts with different climate conditions according to magnetic susceptibility and carriers of magnetic remanence:the lower part(13.0~11.5Ma)indicates a more changeable climate with higher magnetic susceptibility values(average 15×10-8m3/kg) dominated by magnetite; the upper part(10.3~2.6Ma) reflects a drier climate with lower, stable magnetic susceptibility values (average 9.5×10-8 m3/kg) mainly arising from hematite.Transition stage(11.5~10.3Ma)in the middle is characterized for sharp decrease of magnetic susceptibility.Our results indicate that an episode of intensified aridity in the Asian interior occurred between 11.5Ma and 10.3Ma, which have been widely reported in Western China.The intensified aridity event may resulted from the outgrowth of Tibetan Plateau, with the background of global cooling and retreat of Neotethys Ocean during the Cenozoic.
Key words: Altun     red clay     magnetostratigraphy     magnetic susceptibility     paleoclimate