, 黄春长Age and provenance of Younger Dryas paleo-aeolian sandlayers in the Jin-Shaan Gorges of the Yellow River
ZHANGYuzhu, HUANGChunchang收稿日期:2016-12-15
修回日期:2017-03-20
网络出版日期:2017-07-12
版权声明:2017《地理学报》编辑部本文是开放获取期刊文献，在以下情况下可以自由使用：学术研究、学术交流、科研教学等，但不允许用于商业目的.
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1 引言
第四纪古风成沙是指现代风成沙出现以前,在第四纪期间干旱多风环境下形成的沙质沉积物,其存在表明流沙出现或沙地扩张。因此,它既是干旱多风气候的标志,又是沙地发展正过程的标志[1-2]。第四纪古风成沙的沉积学、地球化学、微形态学和年代学等多学科交叉研究,具有极为重要的科学价值[3-12]。目前,国内外****通过对河流阶地上沉积的古风成沙的上述研究,揭示出其作为沙漠环境下的标型沉积物,沉积过程往往表明大江大河在第四纪时期某些时间段,流域处于干旱多风的环境之中,记录了流域发生的极端干旱事件[13-17]。黄河晋陕峡谷段处于干旱半干旱季风气候区,素有“十年九旱”之称。干旱历来都是本河段最严重的自然灾害,也是本河段最显著的气候特点。这里干旱灾害不仅发生频率高,而且历时长、范围广、危害大[18]。诸如“焦地流金,大地生烟,野绝青草,寸粒不收,骨肉相食,十室九空”等对黄河中游旱灾灾情的记述不胜枚举。在全球暖干化影响的新形势下,为更加合理的开发利用黄河中游水资源,更加有效的防旱减灾,都需要准确掌握黄河中游长尺度极端干旱事件发生的时间规律及其与季风气候变化的关系[19-23]。
然而,从黄河晋陕峡谷河谷两侧的沉积地层中准确鉴别出古风成沙层及其沉积物源,是深入理解黄河中游流域极端干旱事件发生的时间性规律和气候背景的前提和关键。本文选择黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）为重点研究对象,通过细致的野外宏观特征和室内沉积学和地球化学元素等指标分析,以准确鉴别出剖面中所夹古风成沙层的沉积学和地球化学特征及其物源。根据光释光（OSL）测年数据,并结合典型剖面地层对比,建立剖面的地层年代框架,确定古风成沙层的沉积年代。这对于进一步揭示流域极端干旱事件发生的时间规律及其与季风气候变化的关系,具有重要的科学意义。
2 研究区概况
黄河中游河口镇至龙门区间（简称晋陕峡谷）干流长723 km,谷坡陡峻,谷道狭窄,除河曲、府谷河谷段较宽外,其余河段宽度多在400~600 m 之间,河槽深切入黄土高原的基岩之中,大部分河段由二叠纪、三叠纪沙页岩组成。晋陕峡谷河段黄河干流落差为607 m,河道平均比降0.84‰,水力资源较丰富。晋陕峡谷段黄河支流众多,干沟密布,水系发达,以皇甫川、窟野河、无定河、延水、红河、三川河、昕水河等河流为骨干,加上纵横交错的大小支流及毛支沟,形成了树枝状的地表水系网。集水面积111586 km2,占黄河流域面积的14.8%,大部分属于黄土高原地区。该区域地处温带干旱半干旱季风气候区,总的气候特点是：春季短促,多风沙、常发生干旱;夏季南长北短,湿度大、高温多雨;秋季较短,天气温和;冬季漫长,寒冷、干燥、少雨雪。其年平均降水量在300~550 mm之间,从东南向西北递减。全年的降水量高度集中在7月、8月、9月3个月,占年降水量的61.6%。降水的年际变化大,多雨年份比少雨年份的降水量大2~3倍。区间年平均气温在3.6~11.8 ℃之间,大体以低纬度向高纬度递减。
3 研究剖面与研究方法
3.1 研究剖面
通过对黄河晋陕峡谷壶口至龙门段进行广泛细致的野外考察,发现该河段地貌以中山和低山为主,地质构造简单,整个岩层倾角平缓,岩性以砂页岩为主,黄河深切于二叠纪、三叠纪砂页岩中,河槽形态规整。在该河段乡韩大桥东端偏上游200 m的缓坡台地上,发现并选择了夹有古风成沙层的晚更新世以来黄土—土壤剖面,进行细致的野外宏观特征和室内磁化率、粒度、石英颗粒形态和地球化学元素指标分析,并采集了研究河段黄河现代河床相沙层和毛乌素沙地东南缘现代沙丘沙层（BTW）进行对比分析,以准确鉴别剖面所夹古风成沙层的沉积学和地球化学特征,并准确揭示出古风成沙的物质来源（图1）。
显示原图|下载原图ZIP|生成PPT图1黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面位置
-->Fig. 1Map showing the location of the Late Pleistocene and Holoceneloess-soil profile at the Beisangyu site (BSY) in the Hukou-Longmenreach in the Jin-Shaan Gorges of the Yellow River
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由于该研究剖面位于北桑峪村附近,故命名为北桑峪剖面（BSY）。该剖面顶部海拔405~408 m,高出黄河平水位15~18 m,剖面层次清楚,未经人为扰动,其土壤学和沉积学特征明显（图2）。基于野外沉积剖面的宏观特征进行细致观察分析的基础上,确定了其基本的地层划分框架（表1）。然后,在古风成沙层的上部和下部、马兰黄土层,共采集沉积学样品3个,并且利用直径为5 cm的不锈钢管,在剖面的上述关键层位采集了4个光释光样品。对比沉积学样品主要分别采自研究河段黄河现代河床相沙层和毛乌素沙地东南缘现代沙丘沙层,共2个。
显示原图|下载原图ZIP|生成PPT图2黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面及其所夹古风成沙层
-->Fig. 2The Late Pleistocene and Holocene loess-soil profile containing paleo-aeolian sand layers at theBeisangyu site (BSY) in the Hukou-Longmen reach in the Jin-Shaan Gorges of the Yellow River
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Tab. 1
表1
表1黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面地层划分及描述
Tab. 1Pedo-stratigraphic subdivision and description of the Late Pleistocene and Holocene loess-soil profile at the Beisangyu site (BSY) in the Hukou-Longmen reach in the Jin-Shaan Gorges of the Yellow River
| 沉积地层 | 地层符号 | 颜色 | 地层描述 |
|---|---|---|---|
| 坡积土层 (20~0 cm) | Slope deposit (SD) | 灰黄色 | 坡积石渣土,分选极差,含有中间夹有坡积物角砾石块,厚度在10~20 cm之间。 |
| 古风成沙层 (500~20 cm) | Aeolian sand | 灰黄色 | 中沙质细沙,分选极好,很疏松,马兰黄土地形面延展,厚度在50~400 cm之间,水平延伸距离达10 m,高度由顶部至两坡侧逐渐降低,直至尖灭,古风成沙层中可见到明显的风成层理,包括了厚度极薄(小于或等于1~2 mm)的加积纹层构成的水平层理,厚度在几毫米至数厘米前积纹层构成的倾向偏南的平板状或楔形交错层理,以及由加积纹层与前积纹层组成的水平—交错层理。 |
| 马兰黄土层 (> 500 cm) | Malan loess (L1) | 浊黄橙色 | 极细沙质粉沙,分选好,块状构造,疏松多孔,厚度大于500 cm,未见底。 |
| 现代河床相沙层 | Riverbed sand | 灰黄色 | 粉沙质中沙,分选较好,很疏松。 |
| 现代沙丘沙层 | Modern dune sand | 棕黄色 | 中沙质细沙,分选极好,很疏松。 |
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3.2 研究方法
为了确保实验结果能够真实反映古风成沙的沉积学和地球化学特征,对于采集的古风成沙样品,在实验室内对所采样品进行充分混合,并且均匀取样分析测试。分别进行了磁化率、粒度、地球化学元素和石英表面形态指标的测定。磁化率采用英国Bartington公司生产的MS-2B型磁化率仪测定,取研磨后的风干样品10 g,将样品放入小塑料盒中,分别对每个样品进行高频和低频测量,将3次测量结果进行平均后取值。粒度分析是取约1 g（精度为0.001 g）的风干样品,先用10%的H2O2和10%HCl去除有机质和钙质胶结物,经过72 h后加入适量六偏磷酸钠使颗粒充分分散,用英国Malvern公司生产的MS-2000型激光粒度仪测量,测量精度为1%。石英表面形态特征是将去除碳酸盐矿物、氧化铁和有机质的石英颗粒干燥后,放入日立台式扫描电子显微镜TM3000下观察分析。样品化学元素分析采用荷兰Panalytical公司生产的PW2403型X-Ray荧光光谱仪。在测量过程中,为了保证数据质量,加入国家标准土壤参比物质样品,保证测定误差小于5%。3.3 年代确定
黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）年代框架的建立,主要通过典型剖面地层对比和对剖面沉积物的OSL测年来解决。黄土高原晚更新世以来黄土—土壤剖面地层以及环境变迁年代框架已相当成熟[24]。黄河晋陕峡谷壶口至龙门段BSY剖面古风成沙沉积层直接覆盖于晚更新世马兰黄土层（L1）之上,这说明其沉积于晚更新世以来。在陕西师范大学环境变迁实验室用Ris? TL/OSL DA-20型释光断代仪,直接对剖面关键沉积层位的光释光样品进行了OSL年龄测定（表2）。可知黄河晋陕峡谷壶口至龙门段BSY剖面古风成沙沉积层中上部22.5 cm和52.5 cm处,OSL年龄分别为11.6±0.7 ka和11.9±0.5 ka,古风成沙层下部497.5 cm处,OSL年龄为12.5±0.5 ka。这样就比较准确的确定了古风成沙层的沉积年代为12.5-11.6 ka。即剖面中覆盖于马兰黄土层（L1）之上的古风成沙层沉积于末次冰消期,即末次冰期向冰后期（全新世）过渡的一个关键时段[25]。Tab. 2
表2
表2黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面样品的OSL测年结果
Tab. 2OSL dating results of the samples of the Late Pleistocene and Holocene loess-soil profile at the Beisangyu site (BSY) in the Hukou-Longmen reach in the Jin-Shaan Gorges of the Yellow River
| 样品 编号 | 地层 层位 | 深度 (cm) | U (ppm) | Th (ppm) | K (%) | 含水量 (%) | 等效剂量 De (Gy) | 环境剂量 Dy (Gy ka-1) | 年龄 (ka) |
|---|---|---|---|---|---|---|---|---|---|
| BSY-1 | 古风成沙层 | 22.5 | 1.02±0.05 | 4.97±0.17 | 2.16±0.06 | 14.0 | 30.39±1.64 | 2.64±0.06 | 11.6±0.7 |
| BSY-2 | 古风成沙层 | 52.5 | 0.81±0.05 | 3.89±0.14 | 2.21±0.06 | 14.0 | 27.98±1.02 | 2.34±0.06 | 11.9±0.5 |
| BSY-3 | 古风成沙层 | 497.5 | 1.10±0.06 | 4.72±0.17 | 2.18±0.06 | 14.0 | 32.02±1.02 | 2.55±0.06 | 12.5±0.5 |
| BSY-4 | 马兰黄土层 | 502.5 | 2.46±0.10 | 11.30±0.32 | 1.76±0.06 | 15.8 | 36.73±1.20 | 2.92±0.07 | 12.6±0.5 |
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4 结果分析
4.1 沉积学特征
4.1.1 磁化率 磁化率能够指示沉积物中铁磁性矿物的多少,反映沉积物风化成壤作用的强弱[24]。在黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）中（表3）,马兰黄土层（L1）的磁化率呈现低值,结合野外宏观结构构造特征,可知其堆积于气候相对冷干的时期,风尘堆积作用旺盛,几乎未受到风化成壤作用的影响。古风成沙层上部和下部磁化率相较于马兰黄土层,则呈现极低值,可知其作为沙漠环境下的标型沉积物,堆积于气候更加寒冷干旱多风的时期。晋陕峡谷壶口至龙门段黄河现代河床相沙层磁化率值呈现低值,这主要是受到其物源磁学特征的影响,同样说明其作为新鲜的河流沉积物,沉积之后尚未受到风化成壤作用的影响。毛乌素沙地现代沙丘沙层磁化率呈现最低值,这说明其作为现代沙漠环境下的标型沉积物,气候干旱多风,也未受到风化成壤作用的影响。Tab. 3
表3
表3黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面样品与其他沉积物磁化率对比
Tab. 3A comparison of magnetic susceptibility between the samples of the Late Pleistocene and Holoceneloess-soil profile at the Beisangyu site (BSY) in the Hukou-Longmen reach in theJin-Shaan Gorges of the Yellow River and other sediments
| 沉积地层 | 低频磁化率(×10-8 m3/kg) | 高频磁化率(×10-8 m3/kg) | 频率磁化率(%) |
|---|---|---|---|
| 古风成沙层上部 | 27.12 | 26.46 | 2.43 |
| 古风成沙层下部 | 28.54 | 26.38 | 7.57 |
| 马兰黄土层 | 34.65 | 33.24 | 4.07 |
| 现代河床相沙层 | 25.80 | 24.43 | 5.31 |
| 现代沙丘沙层 | 14.52 | 14.33 | 1.31 |
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4.1.2 粒度 粒度能够反映沉积物的性质、成因和沉积环境。结合粒度和磁化率指标,可以揭示沉积物的物质来源、沉积动力和次生条件变化的影响[26]。按照沉积学分级标准,在黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）中（表4）,马兰黄土层粒度成分以粉沙（2~63 μm）为主,其次是极细沙（63~125 μm）和粘土（< 2μm）,细沙（125~250 μm）、中沙（250~500 μm）和粗沙（>500 μm）极少。中值粒径（Md）和平均粒径（Mz）分别为38.02 μm和31.34 μm,质地为极细沙质粉沙。但是,黄河晋陕峡谷壶口至龙门段BSY剖面古风成沙层上部和下部粒度成分以细沙为主,其次是中沙和极细沙,粉沙和粗沙含量较少,不含粘土。中值粒径和平均粒径分别介于216.21~219.05 μm和209.30~213.17 μm之间,质地为中沙质细沙。综合磁化率和粒度指标分析表明,这表明古风成沙层与马兰黄土层的形成,应该具有不同的风动力条件和物质来源。晋陕峡谷壶口至龙门段黄河现代河床相沙层粒度成分以中沙为主,其次是粉沙和细沙,极细沙、粗沙和粘土含量也较高。中值粒径和平均粒径分别为203.49 μm和140.98 μm,质地为粉沙质中沙。这是由于研究地点处于黄河峡谷河段,河流比降较大,水流湍急,河流的搬运动力较强,河床相沉积物中粗颗粒含量就会较高。毛乌素沙地现代沙丘沙层粒度成分以细沙为主,其次是中沙和极细沙,粉沙、粗沙和粘土含量极少。中值粒径和平均粒径分别为196.50 μm和178.70 μm,质地为中沙质细沙。该研究结果与前人对毛乌素沙地流动沙丘沙的粒度分析结果相一致,即其主要由细沙和中沙组成,几乎不含极粗和极细组分[27-29]。
Tab. 4
表4
表4黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面样品与其他沉积物粒度成分对比
Tab. 4A comparison of grain size distribution between the samples of the Late Pleistocene andHolocene loess-soil profile at the Beisangyu site (BSY) in the Hukou-Longmen reach in theJin-Shaan Gorges of the Yellow River and other sediments
| 沉积地层 | 粘土 (<2 μm, %) | 粉沙 (2~63 μm, %) | 极细沙 (63~125 μm, %) | 细沙 (125~250 μm, %) | 中沙 (250~500 μm, %) | 粗沙 (>500 μm, %) |
|---|---|---|---|---|---|---|
| 古风成沙层上部 | 0.00 | 2.87 | 12.20 | 45.86 | 36.82 | 2.25 |
| 古风成沙层下部 | 0.00 | 2.79 | 13.80 | 45.59 | 35.19 | 2.63 |
| 马兰黄土层 | 5.92 | 71.07 | 20.77 | 1.71 | 0.41 | 0.12 |
| 现代河床相沙层 | 2.97 | 24.94 | 12.03 | 17.39 | 31.58 | 11.09 |
| 现代沙丘沙层 | 0.83 | 2.26 | 11.70 | 58.80 | 25.60 | 0.83 |
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在研究沉积环境时,标准偏差（σ）或者分选系数（S）可用于分析沉积环境的动力条件和沉积物的物质来源。这常用于分析沉积环境的动力条件和沉积物的物质来源。偏度（SK）表示沉积物粗细分布的对称程度,是偏态的定量描述。峰态（Kg）反映粒度分布曲线的尖锐或钝圆的程度,是衡量频率曲线尖峰凸起程度的参数。在黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）中（表5）,马兰黄土层标准偏差和分选系数呈现较低值,分选程度好,偏态为极正偏,峰态很尖窄。黄河晋陕峡谷壶口至龙门段BSY剖面全新世古风成沙层上部和下部标准偏差和分选系数呈现极低值,分选程度极好,偏态为近对称,峰态中等。黄河晋陕峡谷壶口至龙门段黄河现代河床相沙层标准偏差和分选系数呈现较高值,分选程度较好,偏态为极正偏,峰态中等。毛乌素沙地现代沙丘沙层标准偏差和分选系数呈现最低值,分选程度极好,偏态为负偏,峰态中等。
Tab. 5
表5
表5黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面样品与其他沉积物粒度参数值对比
Tab. 5A comparison of grain size parameters between the samples of the Late Pleistocene andHolocene loess-soil profile at the Beisangyu site (BSY) in the Hukou-Longmen reach in theJin-Shaan Gorges of the Yellow River and other sediments
| 沉积地层 | 中值粒径(Md, μm) | 平均粒径(Mz, μm) | 标准偏差(σ) | 分选系数(S) | 偏态(SK) | 峰态(Kg) |
|---|---|---|---|---|---|---|
| 古风成沙层上部 | 219.05 | 213.17 | 0.73 | 0.49 | 0.09 | 0.99 |
| 古风成沙层下部 | 213.17 | 228.12 | 0.75 | 0.51 | 0.09 | 0.99 |
| 马兰黄土层 | 38.02 | 31.34 | 1.60 | 0.79 | 0.41 | 1.56 |
| 现代河床相沙层 | 203.49 | 140.98 | 2.04 | 1.33 | 0.49 | 1.11 |
| 现代沙丘沙层 | 196.50 | 178.70 | 0.60 | 0.39 | 0.10 | 1.09 |
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粒度自然分布频率曲线更好地呈现了这些不同类型沉积物样品在粒度特征上的区别 （图3）。在黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）中（图2）,古风成沙层上部和下部粒度自然分布频率曲线较马兰黄土层,明显偏向右侧粗颗粒级。其与晋陕峡谷壶口至龙门段黄河现代河床相沙层和毛乌素沙地现代沙丘沙层粒度自然分布频率曲线也有着很大差异。
显示原图|下载原图ZIP|生成PPT图3黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面样品与其他沉积物粒度自然分布频率曲线对比
-->Fig. 3A comparison of grain size distribution frequency curves between the samples of Late Pleistocene andHolocene loess-soil profile at the Beisangyu site (BSY) in the Hukou-Longmen reach in theJin-Shaan Gorges of the Yellow River and other sediments
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4.2 石英颗粒形态特征
坚硬的石英表面能够很好的记录作用过它的动力过程,其具有高处侵蚀、低处沉淀的基本特征。同时,不同的动力过程所形成的石英表面特殊形态组合特征与相互关系,能够清晰地反映出颗粒所经历的不同沉积环境[30-31]。扫描电镜观测结果显示（图4）,在黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）中,马兰黄土层的石英颗粒磨圆度较好,多为流线型、水滴形或者次棱角状的长形颗粒,颗粒表面具有一些撞击坑、撞击点以及次生硅质沉淀,其具有典型风成黄土的特征形态（图4d、4e和4f）。尤其是流线型的颗粒,表明了沙粒悬浮于气流之中,经过长时间（长距离）的搬运,颗粒本身亦沿长轴螺旋形转动,经风力摩擦而渐渐形成这种旋转形态特征[20-21]。黄河晋陕峡谷壶口至龙门段BSY剖面古风成沙层的石英颗粒磨圆度很好,多为圆形、次圆形和具有光滑的次圆脊的长条形颗粒,颗粒表面具有风力搬运作用过程中形成的碟形坑等撞击痕迹。同时颗粒表面保存了河流沙才具有的一些浅洼坑和贝壳状断口（图4a、4b和4c）,这意味着沙地物源曾经流水搬运作用到达该区域[32]。晋陕峡谷壶口至龙门段黄河现代河床相沙层石英颗粒磨圆度很好,多为光滑的次圆至圆形颗粒,颗粒表面具有一些浅碟形的凹面、V形坑和三角痕等（图4g、4h和4i）,这表明其经过了流水长距离搬运磨圆,且由于水流的冲击与沙粒间的碰撞产生了一些宽约数十微米的三角形撞击痕[31]。毛乌素沙地现代沙丘沙层颗粒磨圆度较好,多为光滑的长形（边角呈浑圆状）、次棱角至次圆形颗粒,颗粒表面具有明显的碟形坑、新月形坑、麻坑以及次生硅质沉淀（图4j、4k和4l）,这都很好地反映出当时风沙流的沉积环境特征,在干旱且昼夜温差较大的环境下,石英砂颗粒曾经历过较长时间（长距离）的风力搬运分选,搬运过程中颗粒间相互磨蚀撞击频繁[33]。
显示原图|下载原图ZIP|生成PPT图4黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面样品与其他沉积物石英颗粒表面形态特征对比（a, b, c-古风成沙层;d, e, f-马兰黄土层;g, h, i-现代河床相沙层;j, k, l-现代沙丘沙层）
-->Fig. 4A comparison of surface textures of quartz sand between the samples of the Late Pleistocene and Holoceneloess-soil profile at the Beisangyu site (BSY) in the Hukou-Longmen reach in the Jin-ShaanGorges of the Yellow River and other sediments
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4.3 地球化学特征
沉积物的元素地球化学特征在指示古环境、古气候变化和物源示踪研究中,具有十分重要的意义[35-36]。在黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）中（表6）,马兰黄土层的常量元素Fe2O3、SiO2、Al2O3、MgO、CaO、Na2O和微量元素Ba、Rb、Sr、Cu、Cr、Zr的含量与黄土高原黄土的平均化学成分含量非常接近[37]。这说明它们作为典型的风成沉积物,其物质皆来源于西北内陆荒漠区[24]。黄河晋陕峡谷壶口至龙门段BSY剖面古风成沙层上部和下部的常量元素Fe2O3、SiO2、Al2O3、MgO、CaO、Na2O和微量元素Ba、Rb、Sr、Cu、Cr、Zr的含量显著区别于马兰黄土层和毛乌素沙地现代沙丘层,却与晋陕峡谷壶口至龙门段黄河现代河床相沙层非常接近。沉积物地球化学元素与UCC比率变化曲线也很好的指示了黄河晋陕峡谷壶口至龙门段BSY剖面古风成沙层上部和下部与晋陕峡谷壶口至龙门段黄河现代河床相沙层的曲线非常相似,而显著区别于剖面中马兰黄土层和毛乌素沙地现代沙丘沙层（图5）。Tab. 6
表6
表6黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面样品与其他沉积物地球化学元素含量对比
Tab. 6A comparison of geochemical elements between the samples of Late Pleistocene andHolocene loess-soil profile at the Beisangyu site (BSY) in the Hukou-Longmen reach in theJin-Shaan Gorges of the Yellow River and other sediments
| 沉积地层 | Fe2O3 (%) | SiO2 (%) | Al2O3 (%) | MgO (%) | CaO (%) | Na2O (%) | Ba (ppm) | Rb (ppm) | Sr (ppm) | Cu (ppm) | Cr (ppm) | Zr (ppm) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 古风成沙层上部 | 1.53 | 61.42 | 8.76 | 0.64 | 2.58 | 3.37 | 631.60 | 66.00 | 269.60 | 2.80 | 22.80 | 183.40 |
| 古风成沙层下部 | 1.58 | 59.08 | 8.47 | 0.68 | 2.72 | 3.15 | 603.90 | 65.90 | 272.60 | 2.30 | 25.90 | 166.90 |
| 马兰黄土层 | 4.12 | 55.51 | 11.31 | 1.92 | 5.96 | 1.46 | 498.80 | 84.60 | 180.30 | 17.60 | 59.20 | 256.60 |
| 现代河床相沙层 | 1.38 | 66.84 | 6.43 | 0.57 | 2.71 | 2.17 | 530.70 | 62.10 | 168.40 | 4.00 | 21.50 | 158.50 |
| 现代沙丘沙层 | 1.49 | 81.29 | 7.13 | 0.62 | 1.06 | 1.48 | 497.10 | 55.80 | 113.30 | 6.40 | 119.30 | 119.80 |
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显示原图|下载原图ZIP|生成PPT图5黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面样品与其他沉积物样品/UCC[
-->Fig. 5A comparison of sample/UCC[
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5 讨论
5.1 古风成沙层的鉴别
第四纪古风成沙是在第四纪期间干旱多风环境下形成的沙质沉积物。结合黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）中古风成沙层的保存状态,可将其划分为埋藏古风成沙[1-2]。河谷地区埋藏古风成沙层的准确鉴别,是深入理解和探究大江大河流域极端干旱事件发生的时间规律和气候背景的前提和关键。根据本文对黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面 （BSY）中古风成沙层野外实际调查和室内分析结果,将其沉积地点所处的地貌条件、分布条件和沉积物特征,总结如下：① 地貌条件：古风成沙位于黄河晋陕峡谷左侧的缓坡台地之上（图2a）;② 分布条件：古风成沙层沿马兰黄土地形面延展,厚50~400 cm之间,水平延伸距离达10 m,高度由顶部至两坡则逐渐降低,直至尖灭（图2a）;③ 沉积物特征：古风成沙层呈灰黄色,颗粒分选极好,很疏松,不含有砾石和钙结核（图2a）;可见到明显的风成层理,包括了厚度极薄（小于或等于1~2 mm）的加积纹层构成的水平层理,厚度在几毫米至数厘米前积纹层构成的倾向偏南的平板状或楔形交错层理,以及由加积纹层与前积纹层组成的水平—交错层理（图2a、2b）;粒度组成以细沙为主,其次是中沙和极细沙,粉沙和粗沙含量较少,不含粘土（图3）;石英颗粒磨圆度很好,多为圆形、次圆形和具有光滑的次圆脊的长条形颗粒,颗粒表面具有风力搬运作用过程中形成的碟形坑等撞击痕迹。同时颗粒表面保存了河流沙才具有的一些浅洼坑和贝壳状断口（图4d、4e和4f）。
黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）中古风成沙层的上述的某些沉积地点的地貌条件、分布条件和沉积物特征,与前人总结的第四纪古风成沙的特征十分相似[1-2],这表明其为典型的古风成沙沉积物,是在干旱多风环境下形成的沙质沉积物。
基于OSL测年数据,可知黄河晋陕峡谷壶口至龙门段BSY剖面古风成沙层的沉积年代为12.5-11.6 ka,记录了黄河中游发生在末次冰消期的极端干旱事件 （表2）。这次极端干旱事件在时间坐标上与其与相邻区域沙漠/黄土过渡带中湖沼沉积和风成黄土—古土壤剖面记录的新仙女木事件（Younger Dryas, YD）很好对应。即在相邻区域沙漠/黄土过渡带的上述沉积剖面中,共同记录了YD时期东亚季风气候存在着干冷特征,沙地具有总体扩张的趋势[38-40]。
5.2 古风成沙层的物源
通过广泛细致的野外考察,可知黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）中古风成沙层为中沙质细沙,呈现出灰黄色,其与剖面中风成黄土土壤层构成的景观色调差异明显。在实验室内,通过磁化率、粒度、石英颗粒形态特征和地球化学元素等指标分析,表明黄河晋陕峡谷壶口至龙门段BSY剖面中所夹古风成沙层呈现极低值,这说明它作为沙漠环境下的标型沉积物,堆积于气候更加干旱寒冷多风的时期,沉积之后未受到风化成壤作用的影响。该古风成沙层粒度组成以细沙为主,其次是中沙和极细沙,粉沙和粗沙含量较少,不含粘土,粒度自然分布频率曲线偏向右侧粗颗粒级,分选极好,这与剖面中的马兰黄土层和毛乌素沙地现代沙丘沙层相比较,粒度自然分布频率曲线明显偏向右侧粗颗粒级。但是,该古风成沙层与晋陕峡谷壶口至龙门段现代河床相沙层相比较,后者粒度自然分布频率曲线更偏向右侧粗颗粒级。古风成沙层的常量元素Fe2O3、SiO2、Al2O3、MgO、CaO、Na2O和微量元素Ba、Rb、Sr、Cu、Cr、Zr的含量显著区别于马兰黄土层和毛乌素沙地现代沙丘层,却与晋陕峡谷壶口至龙门段黄河现代河床相沙层非常接近,尤其是通过对石英颗粒磨圆度很好,多为圆形、次圆形和具有光滑的次圆脊的长条形颗粒,颗粒表面具有风力搬运作用过程中形成的碟形坑等撞击痕迹,同时颗粒表面保存了河流沙才具有的一些浅洼坑和贝壳状断口。
据此,可以推断黄河晋陕峡谷壶口至龙门段BSY剖面所夹古风成沙沉积物的物质来源,主要来自黄河晋陕峡谷的古河床相沙层物质。这是因为该河段处于干旱半干旱季风气候区,对于全球变化响应敏感。在新仙女木时期,黄河晋陕峡谷段处于在极端干冷的环境之中,冬季风强盛、夏季风衰弱,黄河水位下降明显,河漫滩和江心洲大面积出露,大量的沙物质在风力作用下向岸边输移,成为河谷两侧缓坡台阶上古风成沙沉积层出现的主要沙源地[41]。而且,通过对黄河晋陕峡谷壶口至龙门段BSY剖面所夹古风成沙层与晋陕峡谷壶口至龙门段现代河床相沙层的粒度分析结果对比,表明其也存在风成沙沉积物的沉积分选规律,即古风成沙的源区物质（古黄河河床相沙层）经风力吹蚀搬运发生沉积分异,在风力这种有效分选介质的作用下,古黄河河床相沙层的尘埃状物质被搬运到远处,细颗粒粉沙含量减小,中等颗粒百分比相对增加,粗颗粒物质留在原地或搬运不远,古风成沙层的粒度特征在空间上表现为沿主导风向由物源区向沉积区沙粒变细,分选性变好[28, 42]。
6 结论
结合不同类型沉积物的野外宏观特征、磁化率、粒度成分、石英颗粒表面特征和地球化学元素的对比分析,对黄河晋陕峡谷壶口至龙门段北桑峪晚更新世以来黄土—土壤剖面（BSY）中古风成沙层的沉积学和地球化学特征及其沉积物源获得了明确的认识。剖面中古风成沙层沉积地点的地貌条件、分布条件和沉积物特征,符合典型第四纪古风成沙的特征,这表明其为典型的古风成沙沉积物,是在干旱多风环境下形成的沙质沉积物。基于OSL测年数据,确定了剖面中古风成沙的沉积年代为12.5-11.6 ka,处于末次冰消期,即末次冰期向冰后期（全新世）过渡一个关键时段。进而揭示出该古风成沙层记录了黄河中游发生在末次冰消期的极端干旱事件,其与相邻区域沙漠/黄土过渡带中湖沼沉积和风成黄土—古土壤剖面记录的新仙女木事件记录的YD事件相对应,即在相邻区域沙漠/黄土过渡带的上述沉积剖面中,共同记录了YD时期东亚季风气候存在着干冷特征,沙地具有总体扩张的趋势。黄河晋陕峡谷壶口至龙门段BSY剖面所夹古风成沙沉积物的物质来源,主要来自黄河晋陕峡谷的古河床相沙层物质。这是因为该河段处于干旱半干旱季风气候区,在YD时段,黄河中游晋陕峡谷段处在极端干冷的环境之中,冬季风强盛、夏季风衰弱,黄河水位下降明显,河漫滩和江心洲大面积出露,大量的沙物质在风力作用下向岸边输移,成为河谷两侧缓坡台阶上古风成沙沉积层出现的主要沙源地。
本文研究成果对于揭示黄河晋陕峡谷古风成沙的沉积学和地球化学特征,鉴别其沉积物源,对于进一步揭示流域极端干旱事件发生的时间规律及其与东亚季风气候变化的关系,具有重要的科学意义。
The authors have declared that no competing interests exist.
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| [3] | The paleoclimatic evolution of the Maghreb since 40,000 yr B.P. is not yet known in great detail. For that purpose, data from more than 35 sites have been collected, although only about 110 14C dates and a few prehistoric remains can be used to correlate these data. However, the sites are well distributed over the Maghreb, and the convergent results from paleosoils, paleolakes, alluvian or eolian deposits, pollen analysis, etc., induce one to draw up a certain pattern in climatic evolution. For the period between 40,000 and 20,000 yr B.P., the data are still fragmentary, but are concordant in indicating wetter environments, generalized as far as the northern Sahara. The effect of dropping temperatures on the increasing water budget and soil moisture is not yet clear, but rainfall could have been more regular and more abundant as a consequence of the shifting of middle-latitude cyclone paths towards the South in both summer and winter. The environmental indicators show that between about 20 and 18,000 yr B.P. rainfall progressively became more irregular. However, proofs of aridity in the South Maghreb and the northern Sahara is evident only for the period between 12-10,000 yr B.P. At that time, the Saharan high-pressure belt was reinforced and well established at higher latitudes than at present. This dry climate lasted more or less until the Thermal Optimum. After 6000 yr B.P., there is some evidence that the climate became wetter again, but the environmental conditions were rather different from those of before 20,000 yr B.P. For instance, the Middle Holocene dark humic sois are the result of a pedogenesis different from that of the Late Pleistocene reddish paleosoils. For the last two millennia, it is not easy to distinguish between a natural trend towards aridification and desertification through man's intervention. |
| [4] | There is generally a dearth of evidence of the nature of Quaternary climate change within desert systems, which has limited previous interpretations of past environmental change at low latitudes. The Last Glacial Maximum has previously been identified as the peak of Late Quaternary aridity, when desert systems expanded to five times their present extent, and low-latitude aridity has been described for previous glaciations. But little evidence has been derived directly for large desert basins, particularly southern Africa. Here we report new chronological (optical dating) evidence of arid episodes recorded in aeolian sediments from the Mega Kalahari sand sea. Episodic aeolian activity is recorded at the northeastern desert margin, whereas more sustained activity is evident from the southwestern desert core. Several significant arid events are apparent since the last interglacial period, with dune-building (arid) phases at ~95-115, 41-46, 20-26 and 9-16kyr before present. Existing atmospheric general circulation model simulations and independent palaeoclimate data indicate that the changes in aridity are related to changes in the northeast-southwest summer rainfall gradient, which are in turn related to sea surface temperatures in the southeastern Atlantic Ocean. |
| [5] | A 76.7m-thick section of aeolian sediment is exposed in the transition zone between the Mu Us Desert and the Loess Plateau of northern China. The aeolian sediments contain sand beds, loess layers, and palaeosols. The particle-size distribution data and the SEM examination of quartz grains suggest the sand beds are wind-blown origin. Palaeomagnetic measuring combined with thermoluminescence (TL) dating and pedostratigraphic correlation suggests the studied aeolian sequences were deposited within about 580kyr. The sand layers appear to reflect the episodic extensions of Mu Us Desert during phases of active dune formation and intensified winter monsoon. Both the stratigraphy and the proxy climatic curves such as bulk magnetic susceptibility and grain size indicate that large amplitude second-order climatic oscillations occur during both glacial and interglacial periods. It is worth noting that nearly all the interglacial soils can be sub-divided by the intercalated sand or loess beds, this second-order climatic oscillations are on the order of 10,000 of years which may reflect the precession-scale variations of southeast summer monsoon driven by the orbital pacing of insolation in low latitudes. |
| [6] | Sediment records from the Tarim Basin of western China are of great importance for understanding Late Quaternary climatic variability in Central Asia. A chronology of aeolian and lacustrine deposits from the centre and southern margin of the Taklamakan Desert, central Tarim Basin, has been established using optical dating methods. Distinct variations in humidity during the last 40,000 a in this extremely arid inland basin have been identified. Lacustrine sediments were deposited in the centre of the Taklamakan during two periods of wetter than present day conditions at around 2000 and 30,000 a ago. Another humid period is recorded between 40,000 and 30,000 a ago. Aeolian processes, the development of large migrating dune fields dominated during periods of more arid conditions. Sand wedges at the southern margin of the Taklamakan are dated at ca 40,000 a and ca 18,000 a, and imply a significant temperature decrease in that area. Sedimentological evidence for a late Holocene humid period are consistent with records in ancient Chinese literature. Wetter environmental conditions in the past within the Taklamakan, as indicated by the presence of lacustrine deposits, are also supported by data from adjacent regions. It is assumed that changes of global westerlies and of the mobile polar high triggered the fluctuations of precipitation in the study area. However, variations in temperature in the Taklamakan Desert are presumed to be mainly controlled by the intensity of the winter monsoon. |
| [7] | The sand-loess transition zone in north China is sensitive to climate change, and is an ideal place to investigate past environmental changes. However, past climate change at millennial-centennial timescales in this region has not been well reconstructed because of limited numerical dating. Alternations of sandy loam soils with aeolian sand layers in the Mu Us and Otindag sand fields, which lie along the sand-loess transition zone, indicate multiple intervals of dune activity and stability. This change is probably a response to variations of the East Asian monsoon climate during the late Quaternary. The single aliquot regeneration (SAR) optically stimulated luminescence (OSL) dating protocol, which has been successfully applied to aeolian deposits worldwide, is applied to these two sand fields in this study. The OSL ages provide reliable constraints for reconstruction of past climate changes at suborbital timescale. Sections in both sand fields contain aeolian sand beds recording millennial-scale episodes of dry climate and widespread dune activation, including episodes at about the same time as Heinrich Event 5 and the Younger Dryas in the North Atlantic region. These results demonstrate the potential of aeolian sediments in semi-arid north China to record millennial-scale climatic events, and also suggest that dry-wet climate variation at the desert margin in China may be linked to climatic change elsewhere in the Northern Hemisphere, through atmospheric circulation. This article was published online on 27 November 2008. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected (16 December 2008). Copyright 2008 John Wiley & Sons, Ltd. |
| [8] | The Mu Us Desert, located in the northwestern fringe of the East Asian monsoon region, is sensitive to climate variability. The desert is characterized by mobile, semi-fixed and fixed sand dunes. Alternating units of dune sands and sandy palaeosols in the Mu Us Desert imply multiple episodes of dune building and stabilization, in response to the ebb and flow of the East Asian monsoon. Desert evolution and climatic change of high-resolution in the Mu Us Desert are still poorly understood due to limited numerical dating results. In the present study, 19 samples collected from five sand dune sections along a northwest–southeast transect in the Mu Us Desert were dated using quartz optically stimulated luminescence (OSL) and single aliquot regenerative-dose (SAR) protocol. Internal checks of the OSL dating indicate that the SAR protocol is appropriate for equivalent dose ( D e ) determination for the samples under study. Combined with the lithologic stratigraphy and the luminescence chronology, the sand dune development in the Mu Us Desert during the Late Pleistocene is discussed. Our results indicate that the sand was mobilized approximately at 9102ka, 7102ka, 48–2202ka, 502ka, 102ka, and 0.4402ka; the sand was fixed approximately at 6502ka and Holocene Optimum period in the interior Mu Us Desert. The Mu Us Desert formed at least before 6514402ka, and has shown increasing aridity in the Late Pleistocene. |
| [9] | Aeolian deposits from the Hulun Buir Desert of northeastern China are studied with optically stimulated luminescence dating methods to establish the chronology of dune building phases and climatic changes since the last deglaciation. Our results indicate that wet climate, marked by dune stabilization and soil development in the Hulun Buir Desert, commenced at 11 ka ago, and this early episode of dune stabilization lasted until 4.4 ka ago. This optimum climate between 11 and 4.4 ka ago is mostly the response to the strengthened monsoon circulation and increased precipitation in the Northern Hemisphere. The environment generally became arid after 4.4 ka ago, but the dry climate was interrupted by three phases of weak soil development occurring at 1.8-1.4, 1.2-1.0 and 0.84-0.5 ka ago, respectively. Such short events of dune stabilization were associated with the warm and humid climate in historical time. However, the present dune mobilization in the Hulun Buir Desert is mainly the result of poor land-use practices (land cultivation and overgrazing) since about 300 years ago. 2006 Edward Arnold (Publishers) Ltd. |
| [10] | Dune systems represent important palaeoenvironmental archives and therefore provide valuable information on past climate conditions of dryland regions. This study reports on several palaeodunes (thickness ca. 10–2002m) intercalated in thick loess accumulations, located in the Lanzhou area of the western Chinese Loess Plateau. The intercalated dune sands are well-sorted and have a typical aeolian dune structure, with distinct cross-bedded sand layers. The chronology for these palaeodunes and the related loess sediments was established by quartz optically stimulated luminescence (OSL) dating of 19 samples. The dating results indicate that sand dune accumulation occurred at ca. 35–2502ka, which implies a period of increased aridity during dune formation. This interpretation is in contrast to previously retrieved data from lake archives of the nearby deserts (e.g. Tengger and Badain Jaran desert), indicating a humid episode during marine isotope stage 3 (MIS 3). The previously proposed humid MIS 3a may not be universal over all of northwest China. |
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| [12] | The Ulan Buh Desert (UBD), in southwestern Inner Mongolia, is one of the main dune fields and dust source areas in northern China. The formation of the desert and associated environmental changes since the middle Pleistocene are still unclear due to a lack of depositional records and environmental proxy index analyses. In this study, quartz and K-feldspar optical dating, environmental proxy indexes of grain size, loss on ignition, pollen, and ostracod analysis were employed to supplement the sediment record of a 120.5m drill core, WL12ZK-1, from the southern UBD. In combination with previous stratigraphic records obtained from drill cores WL10ZK-1 (35m deep) and WL10ZK-2 (32m deep) from the northern UBD, and drill core WL12ZK-2 (80m deep) from the northeastern UBD, these proxies indicate there has been essentially an arid environment in the UBD, with desert or steppe vegetation, since the middle Pleistocene, and that sand dunes were widely distributed in the UBD beginning at least 65230ka ago. The Yellow River filled a freshwater paleolake beginning 6515ka ago that covered both the UBD and the adjacent Hetao Plain. The paleolake lasted until 6587ka, and was associated with wetlands along its margins. Steppe vegetation was present in the surrounding region. An arid environment appeared again after 6587ka, and there is no evidence of a large stable lake in the UBD at any time thereafter. Sand dune deposition and a very arid desert environment were present throughout the last glacial period and lasted into the early Holocene. During the Holocene these arid conditions were interrupted by minor wetland intervals. Deserts in southern Inner Mongolia formed at least since the middle Pleistocene, expanded during the last glaciation and into the early Holocene and again after 652ka. We suggest that a combination of tectonic activity and climate change may be responsible for desert formation and environmental changes in southern Inner Mongolia since the middle Pleistocene, with additional human influence exacerbating these conditions in the late Holocene. |
| [13] | This investigation uses a multidisciplinary approach including geomorphic surface mapping, soil stratigraphic analysis, radiocarbon dating and optically stimulated luminescence (OSL) dating to investigate the chronology of soil formation and aeolian activation periods recorded in ridge dune deposits as well as the spatial variability of aeolian processes during active aeolian episodes. This study shows that aeolian landforms superimposed on fluvial terraces adjacent to the Cimarron River in southeastern Major County and northwestern Kingfisher County, Oklahoma are the product of distinct late Holocene climatic conditions, in which periods of dune activity are episodic and accumulation of sediments is relatively rapid. Soil-forming processes operated on the stabilized dunes during intervening periods. We also demonstrate that the synthesis of geomorphic techniques, soil stratigraphy, 14C dating and OSL dating can provide complimentary and high-resolution data about aeolian activity in the Southern Plains. |
| [14] | The Kimmeridgian Quebrada del Sapo Formation in the southernmost Neuqu茅n Basin in Argentina represents a succession up to 40m thick of coarse- to fine-grained fluvial deposits overlain by aeolian deposits. These fluvial eolian deposits reflect a significant palaeogeographic change in the basin and are related to a major, tectonically enhanced, relative sea-level fall. The fluvial section is dominated by braided-channel, fine-grained ephemeral, and sheetflood deposits. Aeolian facies are dominated by dune deposits, with minor sandsheet and interdune units. Changes in the nature of both fluvial and aeolian sedimentation within the studied area suggest a regional variability of accommodation/sediment supply conditions. The regional changes of the aeolian succession likely reflect different relative positions within a major erg. In the upwind margin of the erg, a shallow water table promoted water-lain sedimentation in interdune areas, whereas in the central parts of the erg, dry sediment accumulation took place above the regional water-table level. The vertical transition observed in the Quebrada del Sapo Formation, from fluvial to aeolian deposits, may be the result of a local climatic change to drier conditions due to the development of a climatic barrier imposed by growth of a magmatic arc to the west. Alternatively, the vertical transition could be related to a lowering of the water table associated with the compartmentalization of the basin during a period of low sea level. |
| [15] | Late Pleistocene to Holocene Morava River valley-fill of the eastern Czech Republic reflects the geomorphic evolution of the valley as forced by climate change. Valley-fill stratigraphy was studied through measured sections, optically stimulated luminescence (OSL) and radiocarbon dating, ground-penetrating radar surveys of relict sand dunes, archived drill-hole data, and a comparison of elevations and ages of stratigraphic units. Fluvial systems evolved from meandering with floodplains to braided during MIS 3. Braided fluvio-aeolian systems dominated through MIS 2 and the Last Glacial Maximum (LGM). Valley aggradation occurred during arid glacial times of a low water-to-sediment discharge ratio. Most valley-fill was removed at 13 ka with incision by a large-bend meandering river with an estimated bankful paleodischarge 3 larger than the modern Morava River. The Holocene Morava River has varied from meandering to anabranching with low rates of floodplain aggradation. The Bzenec sand body, up to 36 m thick, represents an erosional remnant bypassed during late Pleistocene incision and consists of interpreted lacustrine turbidites overlain by braided stream and aeolian dune strata. The turbidites consist of laterally continuous, thin, normally graded beds of rounded and frosted sand grains of aeolian origin. Dates and elevation data argue that the valley lake formed during the LGM through downstream damming by a braided terminal fan and sand dune complex. The turbidites are interpreted to have formed through fluvial undercutting and slumping of dune accumulations as lake level rose. This process forced an erosional unroofing of aeolian accumulations, reflected in inverted OSL dates for the turbidites. |
| [16] | 根据地表沉积相特征及其对气候变化的反映,着重讨论了西藏雅鲁藏布江中游沙地形成时代、演化过程和气候变化。沙地早在中更新世中期的0.8MaB.P.就已出现,经历了强烈发展、缓慢发展或相对稳定和缩小与固定的多次迭覆更替,并呈不断扩大趋势。与此同时,气候也经历了冷干和暖湿的多次变化,并表现出不断向干冷化方向发展的趋势。 . 根据地表沉积相特征及其对气候变化的反映,着重讨论了西藏雅鲁藏布江中游沙地形成时代、演化过程和气候变化。沙地早在中更新世中期的0.8MaB.P.就已出现,经历了强烈发展、缓慢发展或相对稳定和缩小与固定的多次迭覆更替,并呈不断扩大趋势。与此同时,气候也经历了冷干和暖湿的多次变化,并表现出不断向干冷化方向发展的趋势。 |
| [17] | 对山陕峡谷张家湾村蔚汾河阶地上沉积的黄土地层粒度一磁化率进行 分析发现：沉积物的粒度一磁化率变化曲线能够很好地反映黄土一古土壤地层序列的变化．粒径小于5μm和大于40μm组份的体积分数曲线分别是夏季风和冬季 风的良好替代性指标，通过粒度分析所绘制的各粒级组份体积分数曲线显示，古土壤S2、S1和Sm发育时期，东亚夏季风加强，粒径小于5μm组份的体积分数 增大；黄土L2和L1堆积时期，冬季风有所加强，粒径大于40μm组份的体积分数增大．张家湾剖面的黄土记录只能在万年尺度上与深海氧同位素记录进行对 比，整体上反映了最近230ka以来山陕峡谷东亚季风的演化过程． . 对山陕峡谷张家湾村蔚汾河阶地上沉积的黄土地层粒度一磁化率进行 分析发现：沉积物的粒度一磁化率变化曲线能够很好地反映黄土一古土壤地层序列的变化．粒径小于5μm和大于40μm组份的体积分数曲线分别是夏季风和冬季 风的良好替代性指标，通过粒度分析所绘制的各粒级组份体积分数曲线显示，古土壤S2、S1和Sm发育时期，东亚夏季风加强，粒径小于5μm组份的体积分数 增大；黄土L2和L1堆积时期，冬季风有所加强，粒径大于40μm组份的体积分数增大．张家湾剖面的黄土记录只能在万年尺度上与深海氧同位素记录进行对 比，整体上反映了最近230ka以来山陕峡谷东亚季风的演化过程． |
| [18] | |
| [19] | |
| [20] | Seven different tree-ring parameters were obtained from Picea schrenkianas using X-ray densitometric techniques at two chronology sites in Yili.Climate correlation analysis indicates that there is significantly positive correlation between PDSI(January~August) and latewood width(r=0.66,1970-2005).The latewood-width chronology was used to reconstruct January~August PDSI for the period of 1652-2005,and it explained the 48.6% of the total PDSI variance during their common period(1940-2005).Six wet periods and five dry periods were identified using the reconstructed PDSI series.Spatial analysis shows that the PDSI in Yili has strong common signals for central Asia.The results reveal that the six dry and five wet period changes of Yili was affected by the westerly circulation.Using the multi-taper method(MTM) spectrum analysis,it is found that the reconstructed PDSI is of 64 a(99%),10 a(95%),8.3 a(99%),6.2 a(95%),4.3 a(95%),3.3 a(95%) and 2.1 a(95%)cycles,which may potentially be the fingerprints of some proposed climate change forcings. |
| [21] | Droughts events have long-lasting effects on economic and social activities, particularly in the semi-arid region of northern China. In this study, four tree ring-width chronologies were developed based on lived Qilian Juniper ( Sabina przewalskii Kom.) trees in the southeast Qilian Mountains, China. The four chronologies were developed from 164 cores (82 trees) for the northern marginal of the Asian summer monsoon in China. Tree growth of this area is sensitive to moisture variability of the growth season. During the last 260 years, there had been extreme growth depressions in the years 1877–1878 and 1928, indicating large-scale droughts had occurred in the study area. These extreme drought events had a devastating effect not only on hydrology and agriculture, but also on society and economics. Extreme droughts in 1877–1878 and in 1928 occurring in the study region and most of China were triggered by different climate regimes. The former drought distribution pattern was probably attributable to El Ni09o events, while the latter was associated with warm and cold air masses. |
| [22] | 从干旱半干旱区气候的时空变化特征、陆气相互作用的观测试验以及气候变化的动力学机制等几个方面系统总结了近年来国内外干旱半干旱区气候变化的最新研究进展，指出目前干旱半干旱区气候变化研究以特定区域研究为主，缺乏对全球不同区域干旱半干旱区气候变化时空关联的系统性归纳研究，且野外观测试验持续时间较短，这在很大程度上限制了对干旱半干旱区气候变化机理的认识和陆面过程模式的发展。针对这些问题，从资料获取、资料分析及数值模拟3个方面提出未来干旱半干旱区气候变化研究的主要方向。 . 从干旱半干旱区气候的时空变化特征、陆气相互作用的观测试验以及气候变化的动力学机制等几个方面系统总结了近年来国内外干旱半干旱区气候变化的最新研究进展，指出目前干旱半干旱区气候变化研究以特定区域研究为主，缺乏对全球不同区域干旱半干旱区气候变化时空关联的系统性归纳研究，且野外观测试验持续时间较短，这在很大程度上限制了对干旱半干旱区气候变化机理的认识和陆面过程模式的发展。针对这些问题，从资料获取、资料分析及数值模拟3个方面提出未来干旱半干旱区气候变化研究的主要方向。 |
| [23] | 利用青海省阿尼玛卿山地区祁连圆柏树轮宽度年表资料,分析树木生长与黄河上游唐乃亥水文站平均流量的关系,结果表明前一年8月到当年7月的平均流量与当年轮宽密切相关,相关系数为0.656,在此相关关系的基础上,重建了黄河上游过去1234年以来的流量变化.经11年滑动平均后曲线,定义流量低于均值1个标准差的时段为枯水期,流量高于均值1个标准差的时段为丰水期.在重建的黄河上游过去1234年流量变化中存在18个丰水期和12个枯水期,丰水期主要分布时段为:846～873年、1375～1400年,枯水期主要分布时段为:1140～1156年、1295～1309年、1473～1500年、1820～1847年.其中15世纪末是过去1234年以来黄河上游流量最低的时段,该时期青藏高原东北部发生了大范围的干旱现象.重建的流量变化包含较多的低频信息,枯水期与都兰、乌兰、德令哈等地树轮资料和其他代用指标记录的干旱期一致.多窗谱分析表明黄河流量变化存在2～5年,22年,35～38年,55～62年和114～227年左右的周期,其中最显著的是159和36年的周期. . 利用青海省阿尼玛卿山地区祁连圆柏树轮宽度年表资料,分析树木生长与黄河上游唐乃亥水文站平均流量的关系,结果表明前一年8月到当年7月的平均流量与当年轮宽密切相关,相关系数为0.656,在此相关关系的基础上,重建了黄河上游过去1234年以来的流量变化.经11年滑动平均后曲线,定义流量低于均值1个标准差的时段为枯水期,流量高于均值1个标准差的时段为丰水期.在重建的黄河上游过去1234年流量变化中存在18个丰水期和12个枯水期,丰水期主要分布时段为:846～873年、1375～1400年,枯水期主要分布时段为:1140～1156年、1295～1309年、1473～1500年、1820～1847年.其中15世纪末是过去1234年以来黄河上游流量最低的时段,该时期青藏高原东北部发生了大范围的干旱现象.重建的流量变化包含较多的低频信息,枯水期与都兰、乌兰、德令哈等地树轮资料和其他代用指标记录的干旱期一致.多窗谱分析表明黄河流量变化存在2～5年,22年,35～38年,55～62年和114～227年左右的周期,其中最显著的是159和36年的周期. |
| [24] | |
| [25] | |
| [26] | . |
| [27] | 对毛乌素沙地不同类型41个现代沙丘的沙粒度分析结果表明,流动沙丘沙主要由细沙和中沙组成,平均粒径为2.17 Φ;固定-半固定沙丘沙的主要粒级是细沙和极细沙,平均粒径为3.09 Φ。流动沙丘沙平均粒径整体自西北到东南减小,而分选性总体表现为中等-较好,无明显空间变化规律。概率累计曲线和粒度特征参数散点图表明,毛乌素沙地沙丘沙以跃移搬运方式为主,存在着就地起沙和就地堆积的可能。平均粒径和分选性空间变化规律的差异,是在频繁强烈的风沙活动环境下物源和大气环流共同作用的结果。 . 对毛乌素沙地不同类型41个现代沙丘的沙粒度分析结果表明,流动沙丘沙主要由细沙和中沙组成,平均粒径为2.17 Φ;固定-半固定沙丘沙的主要粒级是细沙和极细沙,平均粒径为3.09 Φ。流动沙丘沙平均粒径整体自西北到东南减小,而分选性总体表现为中等-较好,无明显空间变化规律。概率累计曲线和粒度特征参数散点图表明,毛乌素沙地沙丘沙以跃移搬运方式为主,存在着就地起沙和就地堆积的可能。平均粒径和分选性空间变化规律的差异,是在频繁强烈的风沙活动环境下物源和大气环流共同作用的结果。 |
| [28] | <p>在风季发生转换期间， 根据不同方向风的持续时间， 对毛乌素沙地南缘长48m， 高8m 的 横向沙丘进行了7 次断面形态测量和表面沉积物采样分析。结果表明， 横向沙丘断面形态及表面 沉积物粒度分布随区域气流方向和强度而发生变化。在沙丘各部位， 丘顶及附近两侧的沙粒活动 性强， 表面沉积物粒径及分选参数变化显著。在粒径及分选参数中， 粒径频率曲线、平均粒径和偏 度对风的作用比较敏感， 随时间的变化也明显， 利用它们的变化可以解释区域气流变化对沙丘的 影响。风向发生转换时期的短期观测和采样分析结果进一步说明采样时间是影响沙丘表面沉积 物粒度特征的重要因素。</p> . <p>在风季发生转换期间， 根据不同方向风的持续时间， 对毛乌素沙地南缘长48m， 高8m 的 横向沙丘进行了7 次断面形态测量和表面沉积物采样分析。结果表明， 横向沙丘断面形态及表面 沉积物粒度分布随区域气流方向和强度而发生变化。在沙丘各部位， 丘顶及附近两侧的沙粒活动 性强， 表面沉积物粒径及分选参数变化显著。在粒径及分选参数中， 粒径频率曲线、平均粒径和偏 度对风的作用比较敏感， 随时间的变化也明显， 利用它们的变化可以解释区域气流变化对沙丘的 影响。风向发生转换时期的短期观测和采样分析结果进一步说明采样时间是影响沙丘表面沉积 物粒度特征的重要因素。</p> |
| [29] | |
| [30] | |
| [31] | |
| [32] | 运用扫描电镜与能谱分析,研究呼伦贝尔沙地末次冰盛期晚期以来全样风成沙的微区形态和粘土矿物成分。呼伦贝尔沙地的沙粒磨圆度较好,在扫描电镜观察下保存了水成沙粒的特征,意味着沙地物源曾经流水作用到达该区域。颗粒表面同时留有风力搬运的碟型坑等撞击痕迹,为干枯河床沙丘活化、风力搬运流动扩张的过程提供证据。末次冰盛期晚期风成沙颗粒大且表面光滑洁净,未见次生粘土矿物,反映出冰期地表无植被发育,区域沙漠活化、流动性加强。而全新世适宜期的褐色沙质土壤砂粒表面同时发育鳞片状自生粘土矿物和同沉积粘土颗粒及团块。能谱分析表明:全新世适宜期,风化自生粘土矿物有蒙脱石和伊利石,在相对温湿气候条件下,发育干旱碱性土壤,反映出冰后期植被发育、土壤化作用增强。 . 运用扫描电镜与能谱分析,研究呼伦贝尔沙地末次冰盛期晚期以来全样风成沙的微区形态和粘土矿物成分。呼伦贝尔沙地的沙粒磨圆度较好,在扫描电镜观察下保存了水成沙粒的特征,意味着沙地物源曾经流水作用到达该区域。颗粒表面同时留有风力搬运的碟型坑等撞击痕迹,为干枯河床沙丘活化、风力搬运流动扩张的过程提供证据。末次冰盛期晚期风成沙颗粒大且表面光滑洁净,未见次生粘土矿物,反映出冰期地表无植被发育,区域沙漠活化、流动性加强。而全新世适宜期的褐色沙质土壤砂粒表面同时发育鳞片状自生粘土矿物和同沉积粘土颗粒及团块。能谱分析表明:全新世适宜期,风化自生粘土矿物有蒙脱石和伊利石,在相对温湿气候条件下,发育干旱碱性土壤,反映出冰后期植被发育、土壤化作用增强。 |
| [33] | 萨拉乌苏河流域米浪沟湾剖面萨拉乌苏组含7层古流动砂丘砂、4层固定一半固定砂丘砂。 对这些砂丘砂的沉积构造观察以及粒度，扫摇电镜的石英砂表面结构特征和常量化学元素分析结果表明:1,这些砂丘砂具有与现代沙丘沙相似的风成沉积构造特 征;2、粒度及其参数---Mz, δ, Sk和Kg,以及主要化学组分SiO2,Al203和TQEE也与现代沙丘沙相近;3,Mz-δ、SiO2-Al203+TOFE散点图和概率累积曲线显 示这些砂丘砂与萨拉乌苏组中的河湖相、古土壤差别明显，而与现代沙丘沙一致;4、石英砂颗粒具有良好的磨圆以及碟形坑、新月形坑、麻坑、上翻解理薄片、硅 质沉淀和硅质裂纹等表面结... 展开 萨拉乌苏河流域米浪沟湾剖面萨拉 乌苏组含7层古流动砂丘砂、4层固定一半固定砂丘砂。对这些砂丘砂的沉积构造观察以及粒度，扫摇电镜的石英砂表面结构特征和常量化学元素分析结果表 明:1,这些砂丘砂具有与现代沙丘沙相似的风成沉积构造特征;2、粒度及其参数---Mz, δ, Sk和Kg,以及主要化学组分SiO2,Al203和TQEE也与现代沙丘沙相近;3,Mz-δ、SiO2-Al203+TOFE散点图和概率累积曲线显 示这些砂丘砂与萨拉乌苏组中的河湖相、古土壤差别明显，而与现代沙丘沙一致;4、石英砂颗粒具有良好的磨圆以及碟形坑、新月形坑、麻坑、上翻解理薄片、硅 质沉淀和硅质裂纹等表面结构特征，反映其曾受持久的风力搬运作用。沉积构造、粒度、石英砂颗粒表面结构和化学元素等多个方面具备了与现代沙丘沙类似的风成 特征，证明这些砂丘砂的成因是风成的。 收起 . 萨拉乌苏河流域米浪沟湾剖面萨拉乌苏组含7层古流动砂丘砂、4层固定一半固定砂丘砂。 对这些砂丘砂的沉积构造观察以及粒度，扫摇电镜的石英砂表面结构特征和常量化学元素分析结果表明:1,这些砂丘砂具有与现代沙丘沙相似的风成沉积构造特 征;2、粒度及其参数---Mz, δ, Sk和Kg,以及主要化学组分SiO2,Al203和TQEE也与现代沙丘沙相近;3,Mz-δ、SiO2-Al203+TOFE散点图和概率累积曲线显 示这些砂丘砂与萨拉乌苏组中的河湖相、古土壤差别明显，而与现代沙丘沙一致;4、石英砂颗粒具有良好的磨圆以及碟形坑、新月形坑、麻坑、上翻解理薄片、硅 质沉淀和硅质裂纹等表面结... 展开 萨拉乌苏河流域米浪沟湾剖面萨拉 乌苏组含7层古流动砂丘砂、4层固定一半固定砂丘砂。对这些砂丘砂的沉积构造观察以及粒度，扫摇电镜的石英砂表面结构特征和常量化学元素分析结果表 明:1,这些砂丘砂具有与现代沙丘沙相似的风成沉积构造特征;2、粒度及其参数---Mz, δ, Sk和Kg,以及主要化学组分SiO2,Al203和TQEE也与现代沙丘沙相近;3,Mz-δ、SiO2-Al203+TOFE散点图和概率累积曲线显 示这些砂丘砂与萨拉乌苏组中的河湖相、古土壤差别明显，而与现代沙丘沙一致;4、石英砂颗粒具有良好的磨圆以及碟形坑、新月形坑、麻坑、上翻解理薄片、硅 质沉淀和硅质裂纹等表面结构特征，反映其曾受持久的风力搬运作用。沉积构造、粒度、石英砂颗粒表面结构和化学元素等多个方面具备了与现代沙丘沙类似的风成 特征，证明这些砂丘砂的成因是风成的。 收起 |
| [34] | We report new estimates of abundances of rarely analyzed elements (As, B, Be, Bi, Cd, Ge, In, Mo, Sb, Sn, Te, Tl, W) in the upper continental crust based on precise ICP-MS analyses of well-characterized upper crustal samples (shales, pelites, loess, graywackes, granitoids and their composites) from Australia, China, Europe, New Zealand and North American. Obtaining a better understanding of the upper crustal abundance and associated uncertainties of these elements is important in placing better constraints on bulk crust composition and, from that, whole Earth models of element cycling and crust generation. We also present revised abundance estimates of some more commonly analyzed trace elements (Li, Cr, Ni, and Tm) that vary by > 20% compared to previous estimates. The new estimates are mainly based on significant ( r 2 > 0.6) inter-element correlations observed in clastic sediments and sedimentary rocks, which yield upper continental crust elemental ratios that are used in conjunction with well-determined abundances for certain key elements to place constraints on the concentrations of the rarely analyzed elements. Using the well-established upper crustal abundances of La (3102ppm), Th (10.502ppm), Al 2O 3 (15.40%), K 2O (2.80%) and Fe 2O 3 (5.92%), these ratios lead to revised upper crustal abundances of B = 4702ppm, Bi = 0.2302ppm, Cr = 7302ppm, Li = 4102ppm, Ni = 3402ppm, Sb = 0.075, Te = 0.02702ppm, Tl = 0.5302ppm and W = 1.402ppm. No significant correlations exist between Mo and Cd and other elements in the clastic sediments and sedimentary rocks, probably due to their enrichment in organic carbon. We thus calculate abundances of these elements by assuming the upper continental crust consists of 65% granitoid rocks plus 35% clastic sedimentary rocks. The validity of this approach is supported by the similarity of SiO 2, Al 2O 3, La and Th abundances calculated in this way with their upper crustal abundances given in Rudnick and Gao [Rudnick, R., Gao, S., 2003. Composition of the continental crust. In: Rudnick, R.L. (Ed.), The Crust. In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry, vol. 3. Elsevier–Pergamon, Oxford, pp. 1–64.]. The upper crustal abundances thus obtained are Mo = 0.602ppm and Cd = 0.0602ppm. Our data also suggest a 65 20% increase of the Tm, Yb and Lu abundances reported in Rudnick and Gao [Rudnick, R., Gao, S., 2003. Composition of the continental crust. In: Rudnick, R.L. (Ed.), The Crust. In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry, vol. 3. Elsevier–Pergamon, Oxford, pp. 1–64.]. |
| [35] | |
| [36] | 物源分析是盆地分析和古地理分析不可或缺的内容和方法,地球化学在物源及沉积背景分析中起着非常重要的作用。运用文献研究法对"地球化学在物源及沉积背景分析中的应用"有关文献进行了研究,分析阐述了常量元素分析、稀土-微量元素分析以及裂变径迹、K-Ar、Ar-Ar、Rb-Sr、Sm-Nd、U-Pb、Re-Os、S、O、Si同位素分析等方法及其优势与不足。结果表明:常量元素、稀土-微量元素分析在源区物质组成、构造背景、源区风化强度、成分成熟度及氧化-还原条件判别方面有很好的应用效果;各种同位素分析体系在不同目的的研究中具有自身优势,均得到了很好的应用。但影响岩石化学成分的因素较多,特别是对于沉积岩,很容易受外生营力的影响;物源及沉积背景分析中的地球化学方法运用单一;对研究区区域地质情况的研究不足。为此,建议在利用地球化学方法进行物源及沉积背景分析时,需要充分认识影响碎屑沉积物化学组成的因素,综合运用多种方法进行物源区分析,扬长补短,同时要特别注意对区域地质情况的研究。 . 物源分析是盆地分析和古地理分析不可或缺的内容和方法,地球化学在物源及沉积背景分析中起着非常重要的作用。运用文献研究法对"地球化学在物源及沉积背景分析中的应用"有关文献进行了研究,分析阐述了常量元素分析、稀土-微量元素分析以及裂变径迹、K-Ar、Ar-Ar、Rb-Sr、Sm-Nd、U-Pb、Re-Os、S、O、Si同位素分析等方法及其优势与不足。结果表明:常量元素、稀土-微量元素分析在源区物质组成、构造背景、源区风化强度、成分成熟度及氧化-还原条件判别方面有很好的应用效果;各种同位素分析体系在不同目的的研究中具有自身优势,均得到了很好的应用。但影响岩石化学成分的因素较多,特别是对于沉积岩,很容易受外生营力的影响;物源及沉积背景分析中的地球化学方法运用单一;对研究区区域地质情况的研究不足。为此,建议在利用地球化学方法进行物源及沉积背景分析时,需要充分认识影响碎屑沉积物化学组成的因素,综合运用多种方法进行物源区分析,扬长补短,同时要特别注意对区域地质情况的研究。 |
| [37] | . |
| [38] | . |
| [39] | . |
| [40] | 位于沙漠／黄土过渡带的杨桃峁剖面，高分辨率地启示记录了ＹｏｕｇｅｒＤｒａｙｓ时期东亚季风气候和沙漠变化历史，在ＡＭＳ＾１４Ｃ测年基础上，磁化率，粒度和有机质含量等替代性气候指标的测试结果分析表明，ＹｏｕｎｇｅｒＤｒｙａｓ时期东亚季风气候在全球气候变化的影响下，总的表现为与北半球高纬度地区相同的干寒特征，即沙漠总的具有扩大的趋势，但又与南北半球大气相互作用，特别是赤道太平洋地区气压异常引起亚洲季风气 . 位于沙漠／黄土过渡带的杨桃峁剖面，高分辨率地启示记录了ＹｏｕｇｅｒＤｒａｙｓ时期东亚季风气候和沙漠变化历史，在ＡＭＳ＾１４Ｃ测年基础上，磁化率，粒度和有机质含量等替代性气候指标的测试结果分析表明，ＹｏｕｎｇｅｒＤｒｙａｓ时期东亚季风气候在全球气候变化的影响下，总的表现为与北半球高纬度地区相同的干寒特征，即沙漠总的具有扩大的趋势，但又与南北半球大气相互作用，特别是赤道太平洋地区气压异常引起亚洲季风气 |
| [41] | 黄河北长滩-河曲段,长1200公里,沿风沙区绕行,经腾格里、河东、乌兰布和、库布齐等四大沙漠.虽毛乌素沙地距黄河有一定距离,但无定河、秃尾河、窟野河等支流又流经毛乌素沙地、汇入黄河。显而易见,黄河两岸的风成沙,源源不断地流入黄河。严重进沙段151.4公里,次严重段112公里,轻微进沙段208.6公里。黄河沿岸沙地的形成是在干旱、多风、地面富含沙物质这一特定自然条件下的产物。沙地的演变过程,拟分为两个阶段,即地质时期和历史时期。地质时期阶段,晚更新世晚期为一风沙活动强盛期。人类历史时期,至少有三个期风沙活动加强期和三个期风沙衰弱期。风沙对黄河的危害内涵危害特点及形式,前者体现在时间性、方向性和区域性等方面,后者指风沙流、沙丘移动及河岸坍塌而进入黄河,淤积河道的方式。 . 黄河北长滩-河曲段,长1200公里,沿风沙区绕行,经腾格里、河东、乌兰布和、库布齐等四大沙漠.虽毛乌素沙地距黄河有一定距离,但无定河、秃尾河、窟野河等支流又流经毛乌素沙地、汇入黄河。显而易见,黄河两岸的风成沙,源源不断地流入黄河。严重进沙段151.4公里,次严重段112公里,轻微进沙段208.6公里。黄河沿岸沙地的形成是在干旱、多风、地面富含沙物质这一特定自然条件下的产物。沙地的演变过程,拟分为两个阶段,即地质时期和历史时期。地质时期阶段,晚更新世晚期为一风沙活动强盛期。人类历史时期,至少有三个期风沙活动加强期和三个期风沙衰弱期。风沙对黄河的危害内涵危害特点及形式,前者体现在时间性、方向性和区域性等方面,后者指风沙流、沙丘移动及河岸坍塌而进入黄河,淤积河道的方式。 |
| [42] |
