第四纪研究  2018, Vol.38 Issue (1): 54-66   PDF    
柴达木盆地早始新世大暖期古土壤中赤铁矿和针铁矿的定量分析及其气候意义
赵晨蕾, 王朝文, 姬凯鹏, 赵璐璐, 宋博文, 殷科, 洪汉烈     
( 中国地质大学(武汉)地球科学学院, 湖北 武汉 430074;中国地质大学(武汉)珠宝学院, 湖北 武汉 430074;中国地质大学(武汉)地质调查研究院, 湖北 武汉 430074)
摘要:赤铁矿和针铁矿是古土壤中常见的两种风化产物,定量研究赤铁矿和针铁矿有利于了解成壤时期古气候特征。文章利用可见光波段的漫反射光谱法对早始新世大暖期期间柴达木盆地大红沟剖面路乐河组古土壤中的铁氧化物矿物进行了定量分析。测试结果清晰显示了赤铁矿和针铁矿的存在,其中赤铁矿含量约在0.09~17.16 g/kg之间,针铁矿的含量大致为0.13~24.6 g/kg;针铁矿(Gt)相对赤铁矿(Hm)平均含量较高(Gt/Hm变化范围0.04~34.34,平均值约为2.18),且数据存在较大波动。古土壤在早始新世大暖期时经历了3个阶段的古气候变化,早期古降雨量偏高,针铁矿/赤铁矿平均值为3.06,数据波动明显,指示相对温暖湿润且干湿波动大的气候条件;中期时古降雨量稍低,针铁矿/赤铁矿平均值降低至1.75,指示相对干旱的气候条件;晚期时古降雨量增加,针铁矿/赤铁矿平均值升高至1.80,指示相对温暖湿润的气候条件。其中早期的路乐河组古土壤对应了全球早始新世气候大暖期的峰值,路乐河组古土壤整体的古气候特征是对全球早始新世气候大暖期的区域响应。
主题词漫反射光谱     赤铁矿     针铁矿     古土壤    
中图分类号     P534.61+3;P575.9;P591;P532                     文献标识码    A

0 引言

早始新世(55~50 Ma)是新生代以来地球最温暖的时期,大陆和海洋表面年平均温度显著高于当今水平[1~2];极地常年无冰,冬季温度比现今高约10 ℃[2~3]。尽管对大气中CO2的来源存在争议[4~5],但大气中CO2浓度升高造成的温室效应被认为是导致全球温度升高的主要因素[6~7]。这与人类正面临的由于CO2及其他温室气体(CH4、N2O等)浓度的增加而导致的全球环境问题在形成机制上具有相似性[8~9]。这一全球性的气候变化在海洋记录中有很好的记载和研究[10~11],由于对古气候的重建可以为研究现代的全球变暖趋势提供很好的研究模型,近年来对陆地系统内气候变化的研究也渐趋热烈。

稳定沉积的古土壤可以对气候变化而导致的生态环境的变化提供更为直观地记录[12~14]。而古土壤所呈现的颜色,有助于对古气候特征进行定性研究[15~16],尤其是在土壤中广泛存在的针铁矿(α-FeOOH)和赤铁矿(α-Fe2O3)这两种有色的铁氧化物矿物[17],它们的分布情况和含量受气候条件变化的影响[17~22],长期以来被用做衡量古气候演化的指标[23~25]。可见光波段的漫反射光谱法(DRS)相对于较常规的检测手段,如X射线衍射(XRD)、化学方法(如选择性化学交换)具有较快速而准确的优势[26~28],可以便捷有效地对土壤中含量较低的赤铁矿和针铁矿进行定量分析,误差可以低至0.1%以下[17, 27, 29]

柴达木盆地作为青藏高原东北部最大的山间盆地,沉积了从53.5 Ma以来跨越整个新生代巨厚的河湖相沉积物[30],为研究青藏高原北部长期的古气候变化提供很好的依据[14, 31~32]。关于花粉序列[33~34]、构造[35~36]、碳、氧同位素[37]、粘土矿物[38~39]以及磁性地层学、沉积速率、磁化率[40~42]已经有了研究,但是利用古土壤进行高分辨率的古气候的重建却鲜有研究。而柴达木盆地的古气候变化对于了解青藏高原北部早始新世大暖期间的气候变化特征至关重要[39]。本文主要研究路乐河组的古土壤样品,希望能通过精细的地层对比、土壤采集工作,对柴达木盆地早始新世沉积物中赤铁矿和针铁矿含量的分析,而对柴达木盆地北缘早始新世的气候变化有较为精确和深入的了解。

1 区域地质背景和剖面地质情况 1.1 区域地质背景

柴达木盆地位于青藏高原北缘,平均海拔约3000 m,总面积约12×104 km2,总体地势呈现西北高东南低的特点[43]。地形上,柴达木盆地被北部的祁连山、西部的阿尔金山和南部的昆仑-祁漫塔格山环绕[44]。构造上,柴达木盆地周围有三大断裂体系:南部昆仑山逆冲带、西北部阿尔金左旋走滑断裂带、东北部南祁连-南山逆冲带[36](图 1a),形成了菱形的山间盆地[45~46]。受山体的阻挡作用,携带湿润水汽的东南季风和印度洋季风仅可到达盆地东部,加之地形、海拔、地理位置及大气环流等因素,导致目前盆地内气候干旱,降水稀少,年蒸发量高,盆地气候具有典型的大陆性荒漠气候特征[45, 47]

图 1 柴达木盆地构造地质简图(修改自文献[39]) (a)柴达木盆地区域构造简图;(b)研究区地层分布简图 Fig. 1 A generalized structural and geological map of the Qaidam Basin, modified after reference [39]. (a) Generalized structure and the location of the Qaidam Basin; (b) Generalized structural and geology map showing the locations of the study area

研究区位于柴达木盆地的北部(图 1b)。始新世至更新世以来,柴达木盆地为印度板块和欧亚板块碰撞产生长期的、自西南-东北方向的推挤作用而形成的断拗盆地[48]。复杂的构造作用和长期的沉积作用使得柴达木盆地的沉积物对地质事件有很好的记载[39, 49]。研究表明,柴达木盆地新生代地层主要被划分为7个组,从老到新依次为路乐河组(E1-2)、干柴沟组(E3N1)、油砂山组(N1)、狮子沟组(N1-N2)和七个泉组(图 1b),岩性以河湖相的砾岩、砂岩和泥岩为主[33~34]。所以,柴达木盆地的连续的沉积序列是研究新生代以来的全球气候演化的重要载体[35, 50~53]

1.2 研究剖面地质概况

大红沟剖面位于柴达木盆地的北缘(起点坐标:37.29°N,95.12°E)[51],隶属青海省大柴旦行政区,地层呈背斜产出。新生代以来,大红沟剖面总厚度约6172 m,研究剖面位于背斜的南翼。其中路乐河组分布于大红沟剖面的底部,全长约460 m[41],岩性以紫红色调的砾岩、含砾砂岩为主。岩层厚度通常在1~10 m左右,从底部到顶部具有沉积粒度变细的特征。路乐河组的底部和下伏的白垩系地层呈不整合接触,由于受褶皱影响,二者的地层界线不清晰[53~55]

本次研究对象主要为发育在路乐河组中底部的古土壤(图 2),地层产状倾向约200°~220°,倾角约在25°~40°。由野外实测地层产状并结合全部取样剖面的总长度,以及路乐河组底部地层的矿物学和岩相学特征,仔细厘定了本研究的古土壤层在50~176 m深度处,总厚度约126 m(图 2),岩性以砾岩、含砾泥岩、含砾粉砂岩、粉砂岩和泥岩为主,夹少量薄层状膏泥岩和膏岩层,为冲积扇相沉积[37~38]

图 2 路乐河组古土壤地层及岩性图(磁性地层年龄引自文献[41]) (a)7号探槽古土壤A层和B层界线图;(b)古土壤B层中碳酸盐瘤;(c)古土壤B层中潜穴遗迹;(d)古土壤B层中滑移;(e)古土壤B层中点状和网脉状的杂斑;(f)13号探槽野外图 Fig. 2 Stratigraphy and lithology of paleosols of the Lulehe Formation(magnetostratigraphic data from reference [41]). (a) The boundary line between A and B horizons of No.7 trench; (b) Carbonate nodules in B horizon; (c) Vertical burrows in B horizon; (d) Slickensides in B horizon; (e) Mottlings in the shape of spots or veins in B horizon; (f) The generalized picture of No.13 trench

引用前人研究中得出的较为精确年龄控制,如Ji等[41]对大红沟剖面高分辨率的磁性地层学研究、柯学等[51]确定位于大红沟剖面西北约40 km锡铁山路乐河组在约53.5 Ma开始沉积,并将地层年龄与Gradstein等[56]的古地磁极性年代表(GPTS)中C23n~C22r对比,最终确定路乐河组古土壤在52~50 Ma(图 2),在时间上与全球的早始新世大暖期(Early Eocene Climatic Optimum,简称EECO)对应[10]

含砾泥岩和含砾粉砂岩是路乐河组的主要岩性,二者颜色相近,多为深红色调(Hue10R 6/4、Hue10R 6/6),含砾粉砂岩较含砾泥岩砂质感略强,砾石含量多在5%~30%之间变化;泥岩常呈深红色调(Hue10R 5/4、Hue10R 6/4)。古土壤中砾岩主要分布在各层古土壤之间,颗粒支撑,粒径大小不一,颜色多为砖红色(Hue10R 5/6)。泥岩中偶见有河流或湖泊蒸发或地下水或地表径流蒸发而沉淀的盐类矿物[38, 54]

路乐河组古土壤均遭受了一定程度的风化,古土壤发育程度较好时可见淋溶层(A层,以遭受较强烈的风化、富含有机质、颜色较为深暗为特征,见图 2a),多数出露淀积层(B层,以遭受中等程度风化、土壤颜色较浅为特征,见图 2a)和母质层(C层,以遭受较弱的风化为主要特征)。此处对古土壤的分类标准依据Mack等[57]修改的古土壤分类方案。淋溶层、淀积层和母质层古土壤野外特征见表 1

表 1 路乐河组古土壤A、B、C层野外特征描述 Table 1 Paleosol descriptions of A、B and C horizons of the Lulehe Formation

古土壤中B层分布最为广泛,且层内普遍发育有次生碳酸盐瘤(图 2b),碳酸盐瘤大小不一,粒径多数在1~5 cm。另外土壤中偶尔见有潜穴(图 2c)和较弱的滑移(图 2d)。此外,灰白色调(Hue10R 7/1)的网纹在古土壤中普遍存在,多呈圆点状和脉状(图 2e)。以上现象可以证明古土壤经历了成土作用[12]

1.3 样品采集

古土壤样品主要采于路乐河组底部约50~176 m以内。为了采集新鲜且未受表面风化的样品,人工挖了19个取样探槽,探槽长度从3~15 m不等,宽度约70 cm,深度约为30~50 cm(图 2f)。根据露头和岩性特征,以40 cm为间距采集样品,在岩性变化频繁的地方适当加密采样。取样时,用毛刷清除表面浮土后,采集岩壁上新鲜的古土壤样品,对砾岩层未进行系统采样。每处样品重约300~500 g,样品总数共计228个。采样的同时,对于古土壤样品岩性特征进行总体描述,其中颜色特征严格按照蒙赛尔颜色体系(Munsell Color System)的标准界定[58]

2 样品处理和实验方法 2.1 样品处理和测定

将样品置于70 ℃的烘箱中烘干后,用玛瑙研钵将样品研磨至200目以下并混合均匀,样品制备和分析方法主要依据Deaton和Balsam[59]描述的方法。将样品以 > 500 kPa的压力压入黑色的塑料器皿中,以减少粒度变化的影响。

样品在Persee TU-109紫外分光光度计仪器上进行测试,将光谱带宽设置为2 nm,光谱扫描范围设置在380~750 nm(包括近紫外区380~400 nm、可见光区400~700 nm和近红外区700~750 nm)。机器自检正常进入后,用随机配置的黑挡板对全波段(850~ 230 nm)进行暗电流校正,校正完后,将黑挡板更换为BaSO4标准白板在工作波段范围内(380~ 750 nm)做基线校正。基线校正结束后,将标准白板更换为待测样品,即可进行光谱扫描。最终选取可见光范围内的数据进行处理,是因为可见光范围内的光谱对于沉积物中导致颜色差异的铁氧化物最为敏感[41, 60~61]

2.2 数据处理方法

DRS曲线的一阶导数,即曲线的斜率,已被证实代表了不同的矿物组成,可对土壤或沉积物中的铁氧化物进行检测[58, 61]。一阶导数的数值可由UVWin 5.0软件自动算出。

而Scheinost等[17]的研究成果表明,根据DRS曲线的二阶导数,可以对铁氧化物矿物的含量进行厘定,从而可以定量研究土壤或沉积物中的针铁矿和赤铁矿含量,计算方法如下:

(1)
(2)

其中Y1代表针铁矿(Gt)处于415~445 nm之间的漫反射曲线的二阶导数差值,Y2代表赤铁矿(Hm)处于535~580 nm之间的漫反射曲线的二阶导数差值。

Hyland等[62]总结了来自全球不同纬度和地区的13个不同研究项目中的70个现代土壤样品,建立了针铁矿和赤铁矿的比值Gt/Hm和年平均降水量之间的线性关系,可以用来估算古降水量(MAP)和土壤类型,公式如下:

(3)

此处Y代表B层土壤中针铁矿/赤铁矿的比值(Gt/Hm)位于0.05~2.75之间的值。由于针铁矿可以氧化成赤铁矿,对于明显泛红色的古土壤,此处由针铁矿含量和赤铁矿含量比值估算的古降水量被认为是最低值[62]。需要强调的是,这一指数不适用于以下情况:1)有明显的土壤扰动痕迹的土壤;2)发育十分弱的土壤或邻近河道的土壤;3)经历了特殊成岩条件的土壤,比如地底深处( > 10 km)厚实的埋藏作用或埋藏之前经历了潜育环境。由于本文对赤铁矿和针铁矿采用的测试方法与Hyland等[62]的测试方法不同,所以为了避免极端情况,尽管这一公式可以对古降雨量进行定量,本文仅对路乐河组的古降雨量数据的变化趋势进行定性分析。

3 结果 3.1 赤铁矿和针铁矿的识别

由于针铁矿(α-FeOOH)显黄色调,赤铁矿(α-Fe2O3)显紫红色调,它们色调的不同可由光谱分析技术精准地检测出来,可以对复杂样品中的针铁矿、赤铁矿进行鉴别[24, 27, 63~65]。以往研究也表明,土壤的红色调主要由三价的铁氧化物,尤其是赤铁矿所控制[29]

赤铁矿的一阶导数通常在565~575 nm之间有特征峰,而针铁矿的一阶导数通常在435 nm和535 nm处有两个特征峰[19, 24, 61, 66]。但是由于针铁矿漫反射曲线的一阶导数中,535 nm处的吸收峰常受基体的影响或和赤铁矿的峰重叠而变得不明显,所以常用435 nm处的特征峰来鉴别针铁矿。本文对路乐河组古土壤不同层位漫反射光谱的一阶导数在435 nm和575 nm处可以看出明显的反射峰,分别为典型针铁矿和赤铁矿的峰,535 nm处针铁矿峰不明显(图 3)。实验结果清晰的显示了赤铁矿和针铁矿的存在,和野外观察的以红黄色调为主的颜色特征一致。Ji等[41]对路乐河组样品热退磁的研究结果也表明这一阶段沉积物中含有大量赤铁矿,几乎不含磁铁矿等其他磁性矿物。

图 3 路乐河组古土壤可见光漫反射曲线一阶导数图 (a)淋溶层(A层)样品的一阶导数,n=4;(b)淀积层(B层)样品的一阶导数,n=135;(c)母质层(C层)样品的一阶导数,n=57
灰色矩形分别标注了对应赤铁矿、针铁矿的峰
Fig. 3 First-derivative (FD) DRS curves of samples from the A, B and C horizon of paleosols of the Lulehe Formation. (a) First-derivative DRS curves of samples from A horizon (n=4);(b)First-derivative DRS curves of samples from B horizon (n=135);(c) First-derivative DRS curves of samples from C horizon (n=57). Gray bars denote notable changes of the first derivative values, corresponding to the presence of goethite and hematite

从整体看,淋溶层(图 3a)、淀积层(图 3b)和母质层(图 3c)反射光谱的一阶导数曲线中在575 nm处的峰值处样品平均值显示了逐步降低的趋势,在一定程度上表明赤铁矿含量在这3个土壤发生层中有逐步降低的趋势[58],和淋溶层风化最强、淀积层次之和母质层风化最弱的野外特征相符。

3.2 早始新世赤铁矿和针铁矿的含量变化

利用公式(1)和(2)计算可得,赤铁矿的含量大致为0.09~17.16 g/kg,平均含量约为4.61 g/kg;针铁矿的含量变化范围为0.13~24.6 g/kg,平均含量约为5.85 g/kg。从二者的含量变化情况来看,针铁矿的含量变化的标准差为5.48 g/kg,而赤铁矿含量的标准差为3.14 g/kg,赤铁矿的含量变化相对平稳,这一点二者的数据变化曲线上也可体现(图 4b4c)。

图 4 路乐河组古土壤赤铁矿和针铁矿含量图 (a)路乐河组古土壤地层图;(b)针铁矿含量图(Gt);(c)赤铁矿含量图(Hm);(d)针铁矿相对赤铁矿含量(Gt/Hm);
灰色矩形标记了无数据的砾岩层
Graphs showing the concentrations of hematite and goethite of paleosols of Lulehe Formation. (a) Stratigraphy of paleosols of Lulehe Formation; (b) Line graph of Gt concentration; (c) Line graph of Hm concentration; (d) Line graph of Gt/Hm. Gray bars marked the conglomerate layers without data

路乐河组古土壤的Gt/Hm数值变化范围约为0.04~34.34,平均值2.18。从曲线的整体波动来看(图 4d图 5b),下半部分(约50.5 Ma前,标准差5.51)数据波动范围相对中上部(标准差1.83)的数值波动较大,且数值整体偏高(约50.5 Ma之前平均值为2.94,约50.5 Ma之后平均值为1.60)。约83%的数据落在1~4的区间内,其中也含Gt/Hm比值数低于1的情况。

图 5 路乐河组古土壤年平均降水量图 (a)路乐河组古土壤地层图;(b)针铁矿/赤铁矿数据;(c)古降雨量数据
(b)和(c)中细线代表原始数据,粗线为取5点平均后的数据;图中虚线矩形框标注了数据变化的3个阶段;条形阴影标记了无数据的砾岩层
Fig. 5 Mean annual precipitation of paleosols in Lulehe Formation. (a) Stratigraphy of paleosols of Lulehe Formation; (b) Line graph of Gt/Hm; (c) Line graph of MAP. The fine lines in graph (b) and (c) represent the original values, and the thick lines are mean values after smoothing by five points. The dashed rectangle marked three stages of the data. The shadow bars marked the conglomerate layers without data
3.3 早始新世年平均降水量的估算

采集于路乐河组古土壤B层的Gt/Hm值位于0.05~2.75之间的样品总计113个,利用公式(3)计算得出的实验结果显示大红沟剖面在EECO期间年平均降水量(MAP)显示了清晰的波动性(图 5c)。

从古降雨量的整体变化特征来看,曲线具有中部低(约为EECO中期)、两端高(约为EECO早期和晚期)的趋势。为了更清晰的看出这一变化趋势,笔者对3个阶段的数据进行了详细对比,发现与早始新世早期和晚期的古降雨量相比,中期的古降雨量整体数值明显较低(图 6)。为了进一步验证这一变化趋势,笔者对Gt/Hm数值取5点做平均值,并将曲线在3个阶段的变化进行了对比,发现在早、中、晚3个时间段内,Gt/Hm的平均值分别为3.06、1.75和1.80(图 5b),同样具有两端高中部低的特征。且两组数据同样都具有在早期波动振幅最大,中期的数据变化次之,晚期数据波动最为稳定的特征。

图 6 路乐河组古降雨量含量变化图 Fig. 6 Profiles of mean annual precipitation of Lulehe Formation
4 讨论 4.1 赤铁矿和针铁矿含量变化对气候的指示意义

土壤中的赤铁矿和针铁矿因为其对不同温度变化和降水量的季节性变化敏感,常被用作成土的风化指标[24]。季峻峰等[25]利用DRS方法分析了多个黄土剖面的赤铁矿和针铁矿含量,发现黄土-古土壤剖面的赤铁矿和针铁矿的相对含量可作为东亚季风干/湿变化的敏感指标。其中赤铁矿相对含量较高时反映了干燥土壤环境,而较低时指示了潮湿土壤环境[20, 25, 29]。Cornell和Schwertmann[64]的研究结果也表明,针铁矿和赤铁矿的相对含量和原始矿物的水合状态有关,针铁矿/赤铁矿越高(Gt/Hm值越大)表明平均水合速率越高(越潮湿)。它们的相对含量被认为和土壤形成过程中湿度的变化有着直接关系[64~66]

从赤铁矿和针铁矿的成因来看,温度较高且较干旱的环境会使得土壤温度升高,从而促使水铁矿脱水而形成赤铁矿[67~68],所以蒸发量高于降雨量的干旱环境有利于赤铁矿的形成;而针铁矿是由硅酸盐中的铁氧化物或者其他母质直接溶解沉淀而成[67~68],所以持续潮湿的环境有利于针铁矿的形成[24, 67~69]。中国南方红土经历了整体温暖伴随季节性干旱的环境,所以铁氧化物矿物中以赤铁矿为主;而北方的黄土古土壤中则由于寒冷湿润的气候条件而富含针铁矿[29, 60]。所以,针铁矿/赤铁矿高表明了一种较为湿润的气候状态,低时则表明气候相对干旱。

实验结果清晰显示了路乐河组古土壤样品中赤铁矿和针铁矿的存在(图 3),与样品的野外红黄色调的特征相一致,且绝大多数样品中针铁矿含量较高(Gt/Hm平均值为2.18)。从针铁矿/赤铁矿曲线(图 4d)中可以看出有明显的波动,从古土壤的顶部至底部数值波动变得剧烈,但从整体来看,Gt/Hm数值高,但期间也存在数值多次小于1的情况,这在一定程度表明该地气候有波动。

对大红沟剖面的地球化学和粘土矿物学的分析对比也表明,路乐河组底部FeO/Fe2O3的比值和绿泥石的含量相对其他地层较低[14],由于铁氧化物矿物对于沉积环境的敏感性以及绿泥石倾向于保存于干旱和偏碱性的环境的特性,它们的含量低时都可以在一定程度上反映相对湿润的环境条件[70~71],且该段的化学指数CIW′(即[Al2O3/(Al2O3+Na2O)]×100)较其他层位也明显偏高[14],反映了原岩经历了较为强烈的化学风化,即经历了温暖而湿润的气候条件[72]。对路乐河组的孢粉化石研究表明,路乐河组所在的花粉组合带中含较多的阔叶植物(包括楝Melia、栎Quercus、栗Castanea、桦Betula、榆Ulmus等)、针叶植物(包括松Pinus、罗汉松Podocarpus、雪松Cedrus、破隙杉Taxodia hiatus等)和干旱植物(主要为麻黄Ephedra),代表了整体较为温暖湿润,但是存在干湿波动的气候环境[73~74]。前人的数据均表明路乐河组底部早始新世的地层整体以炎热湿润为特征,中间存在干湿波动,但是未对数据波动的细节做详细的研究[14, 40, 73~74]

4.2 古降雨量对柴达木盆地早始新世气候特征的指示

关于全球变化时降水量的模拟和水循环的变化研究对于了解陆地气候系统,预测未来气候变化至关重要[62]。笔者根据Hyland等[62]总结的公式对路乐河组古土壤成岩过程中的古降水量进行了重建后发现,古降雨量曲线变化显著的低谷和峰值处分别和针铁矿/赤铁矿曲线(图 5b5c)的峰值和低谷处相对应,这也从侧面验证了年平均降水量数据的适用性。笔者将前人研究中路乐河组的孢粉特征和粘土矿物学特征[39, 73~74]与本文中路乐河组底部古降雨量进行综合对比(图 7),确认了3个明显不同的阶段,即早始新世大暖期早期和晚期的相对湿润期,以及中期的相对干旱期。

图 7 路乐河组与气候相关的指数图 (a)针铁矿/赤铁矿变化曲线;(b)古降雨量变化曲线;(c)路乐河组孢粉组合图(引自路晶芳等[73~74]),其中虚线矩形框标记了与研究区域同期的年龄段;(d)路乐河组古土壤粘土矿物含量变化图(引自Wang等[39]),其中Ch——绿泥石(Chlorite),I——伊利石(Iilite),S——蒙脱石(Smectite),I-S mixed layers——伊蒙混层(Iilite and smectite mixed layers),Kao——高岭石(Kaolinite) Fig. 7 The indexes graph relating to paleoclimate of Lulehe Formation. (a) Line graph of Gt/Hm; (b) Line graph of MAP; (c)The pollen grapy of Lulehe Formation (according to Lu et al.[73~74]); (d) The change of concentration of clay minerals of paleosols in Lulehe Formation (according to Wang et al.[39]). The rectangle in dashed line in graph (c) marked the same stage with the study area

(1) 早始新世大暖期早期相对湿润期,此时古降雨量较高,针铁矿/赤铁矿高且变化较剧烈,此期间孢粉组合中耐旱的植物种类总体较少(图 7c);粘土矿物含量波动较大,在前期时指示干旱环境的伊利石和绿泥石占多数,后半段指示湿润环境的高岭石、蒙脱石有所增加(图 7d);反映了此期间气候比较湿润,但干湿波动大的特征。

(2) 早始新世大暖期中期相对干旱期,此时古降雨量相对减少,针铁矿/赤铁矿也偏低,含量变化较为平稳;虽然气候环境的变化反映在生物环境中会有一定的延迟[62],不过仍可以看出这一阶段喜湿的植物总量明显减少,耐旱植物如麻黄粉(Ephedripites)的数量也显示了一定的增加(图 7c);粘土矿物组合也显示在中期的后半段,指示干旱环境的绿泥石、伊利石含量增幅显著(图 7d);综上,笔者判断这一期间该地区经历了相对干旱的气候条件;

(3) 早始新世大暖期晚期相对湿润期,此时古降雨量相对较多,针铁矿/赤铁矿也较中期稍高,二者波动都比较平稳;植物类型以热带、亚热带植物为主,如喜湿的植物罗汉松粉属(Podocarpidites)等(图 7c),这些均可以指示这一期间温暖湿润的气候特征。Kent-Corson等[37]对距离路乐河组东部约20 km的小柴旦湖同期的泥岩/粉砂岩样品进行碳氧同位素研究,结果表明氧同位素值偏正(δ18O≈19‰),碳同位素值为负(δ13C≈-6‰),也证明该段时间内温暖湿润的气候条件。

对大红沟剖面沉积速率的重建也显示路乐河组底部在早始新世期间压实作用下的沉积速率存在差异,其中早期沉积速率约为85 m/Ma,中期沉积速率约为80 m/Ma,晚期沉积速率约为45 m/Ma。Ji等[41]将约49.6~52.0 Ma沉积速率整体较高的现象归因于早始新世柴达木盆地复杂的构造环境,笔者认为,古降雨量的变化可能也是使得沉积速率呈现差异的原因之一。

4.3 柴达木盆地对早始新世大暖期的气候响应

路乐河组高分辨率的古土壤记录在时间上与早始新世大暖期(EECO)一致[10]。研究较为成熟的大洋序列显示这一全球性的早始新世大暖期持续时间约在52~50 Ma之间[10],以海洋和大气平均温度的升高和碳同位素的负漂为特征[10~11]。然而,这一事件的峰值在陆地系统的发生时间略有不同。如Hyland等[12, 75]对美国Wind River盆地的研究显示EECO事件峰值发生于约51 Ma,整体持续时间少于1 Ma;Wang等[39]对柴达木盆地锡铁山路乐河组样品的研究显示EECO事件主要体现在约52.9~51.0 Ma。

大红沟剖面路乐河组古土壤记录在底部早期时,针铁矿/赤铁矿含量较高(Gt/Hm平均值为3.06),古降雨量也较之后显著偏高,这一结果也得到了地球化学数据、粘土矿物学研究和孢粉记录的支持[14, 73, 76]。这些和湿度有关的记录所指示的气候的变化特征与其他研究较为成熟的陆地系统的EECO峰值的相关记录表现了相似性,所以路乐河组底部的古土壤序列位于早始新世大暖期的峰值附近。虽然研究区域中未能体现出EECO事件的起始端,但由路乐河组整段的土壤记录表现出的湿热温暖的古气候特征是对全球气候变化的区域响应。

从地理背景来看,早始新世时,印度板块开始与欧亚大陆碰撞,青藏高原中部及南部地区隆起的高度有限,阿尔金山尚未隆起[77],塔里木盆地和柴达木盆地可能还是联通的[78];而此时特提斯洋尚未从塔里木盆地退出[78~80]。所以,此时柴达木盆地的气候变化未受青藏高原隆起的影响,也可以说明路乐河组古土壤所记录的气候变化是受全球气候变化控制的。

尽管气候条件变化的背景和速率不同,但是柴达木盆地沉积物所记载的早始新世大暖期仍然可以为研究全球变暖机制提供很好的范例,这对于理解和预测全球气候变化和全球变暖趋势下生态环境的变化至关重要[11, 75]。后期工作中利用多指标进行协同分析,可以为我们提供更为客观可靠的依据。

5 结论

本文利用可见光分析技术对大红沟剖面路乐河组古土壤样品中赤铁矿和针铁矿的含量进行了定量分析,并对比前人所作的工作成果,得出如下结论:

(1) 柴达木盆地大红沟剖面路乐河组古土壤中清晰地显示了赤铁矿和针铁矿的存在,其中赤铁矿含量约在0.09~17.16 g/kg之间,平均含量约为4.61 g/kg;针铁矿的含量大致为0.13~24.6 g/kg,平均含量约为5.85 g/kg。针铁矿/赤铁矿整体较高,数据存在明显波动(数据变化范围为0.04~34.34,平均值约为2.18),表明了路乐河组古土壤经历了整体较为湿润,伴随季节性干旱的气候条件;

(2) 结合前人相关气候指数和本文的古降雨量信息,判断路乐河组古土壤大致经历了3个气候阶段:1)早始新世大暖期早期相对湿润期,此时古降雨量相对增高,针铁矿/赤铁矿高且数值变化剧烈,平均为3.06,指示了湿润温暖、干湿波动大的气候状况;2)早始新世大暖期中期相对干旱期,此时古降雨量相对偏低,针铁矿/赤铁矿稍低,平均为1.75,指示了相对干旱的气候状况;3)早始新世大暖期晚期相对湿润期,古降雨量相对之前略有增加,针铁矿/赤铁矿较之前平均值增加至为1.80,指示了相对湿润的气候状况;

(3) 通过古降雨量的重建以及和针铁矿/赤铁矿变化曲线与其他陆地系统的对比,认为路乐河组古土壤的底部经历了早始新世气候大暖期(EECO)的峰值时期,整段古土壤是全球气候变暖趋势的区域响应。

致谢 感谢艾承志同学在野外样品采集工作中的帮助,感谢课题组方谦、程柳菱和杨鹏涛在论文撰写期间的帮助和支持。衷心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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The quantification of the hematite and goethite and its climate significance in the paleosols during Early Eocene Climatic Optimum, Qaidam Basin
Zhao Chenlei, Wang Chaowen, Ji Kaipeng, Zhao Lulu, Song Bowen, Yin Ke, Hong Hanlie     
( School of Earth Sciences, China University of Geosciences(Wuhan), Wuhan 430074, Hubei; Gemmological Institute, China University of Geosciences(Wuhan), Wuhan 430074, Hubei; Geological Survey, China University of Geosciences(Wuhan), Wuhan 430074, Hubei)

Abstract

Qaidam Basin is the largest intermont basin in the northeast of Qinghai-Tibet Plateau. It's continuous sedimentary sequence provides a good example for studying the global climate evolution since the Cenozoic. Our research mainly focus on the paleosols developed at the bottom of Luluhe Formation, Dahonggou section, Qaidam Basin. The stratigraphic tendencies are about 200°~220°, and dip angles are about 25°~40°. The total thickness of the paleosol layers were calculated to be about 126 m (the length of Lulehe Formation is about 460 m, and the paleosols were sampled from ca. 50 m to 176 m). The lithology is dominated by conglomerate, gravel, sandstone, siltstone and mudstone, with some thin laminated mudstone and paste. And the deposits are alluvial fan facies. According to the previous studying, we speculate the time span of paleosols of Luluhe Formation is during 52~50 Ma. It is during the period of Early Eocene Climatic Opitmum (EECO). Hematite (Hm)and goethite (Gt)are two common weathering products in paleosols. Their sensitivity to the environment can help us to understand the humidity of soil and pedogenesis as the paleo-climate changing. We use the diffuse reflectance spectroscopy (DRS)to quantify the iron-oxide in paleosols of Lulehe Formation. To collecte fresh samples, 19 channels were digged manually. Each sample weighs about 300~500 g, and the total number of samples is 228. The results showed the existence of hematite and goethite clearly in the paleosols. The content of hematite was about 0.09~17.16 g/kg, and the content of the goethite was approximately 0.13~24.6 g/kg. The goethite is higher than hematite in many samples (Gt/Hm fluctuated between 0.04 to 34.34, with an average of 2.18), and the data fluctuated sometimes. During the early stage of EECO, the mean annual precipitation (MAP)was high with the average of Gt/Hm is 3.06. The data fluctuated sometimes, which indicates a warmer and wetter, with seasonally humid and dry climate. The MAP goes down between middle stage of EECO, meanwhile the average of Gt/Hm come down to 1.75, indicating a dryer climate. And during the late stage of EECO, the MAP is higher than before and the average of Gt/Hm is 1.80, indicating a warmer and wetter climate again. We also correlated the early stage of the paleosols to the peak stage of EECO. The climatic features of paleosols is the response to the global climatic warming events.
Key words: DRS     hematite     goethite     paleosol