150ka BP以来萨拉乌苏河流域粒度分布与气候变化剖析

【作者】 张宇红；

【导师】 李保生； 丘世钧；

【作者基本信息】 华南师范大学 ， 自然地理学， 2002， 硕士

【摘要】 位于鄂尔多斯高原毛乌素沙漠东南边缘的萨拉乌苏河流域的米浪沟湾剖面，记录了150 ka BP以来的38个风成的砂丘砂与河湖相和古土壤交替演化的沉积旋回。本文对该剖面这一时期143个样品的粒度结果进行了计算、分析与研究，提出：1．米浪沟湾剖面150 ka BP以来风成砂的粒度累积百分含量以古流动砂丘砂物质为主，其中以细砂含量最高，极细砂为次，几乎不含粉砂与粘土；河流相、特别是湖沼相与古土壤以极细砂含量占优势，且具一定含量的粉砂和粘土。2．该剖面不同沉积相粒度的一般分布特征尤其是粒度参数—Mz、σ的变化明显表现出，河湖相或古土壤通常要较风成砂颗粒细化，分选相对较差。就Sk和Kg变化而言，河湖相或古土壤值较之风成砂明显增高，前者呈正偏，后者常常表现近对称分布，仅少数呈负偏。3．在剖面的垂直方向，粒度的粗细韵律分布随沉积相的变化非常明显。这不仅在剖面上粒度的百分含量可以清楚地反映出来，且各个粒度参数也同样具有显著响应。河湖相和古土壤与风成砂的Mz、σ、Sk、Kg以及SC／D（粉砂粘土之和与砂的比值）构成一峰谷交替、犬牙交错的锯齿状多波动过程线：常常是由风成砂至上覆河湖相或和古土壤，Mz的φ值、σ、Sk值相应增大，Kg相应增高，SC／D值亦与之变化步骤基本一致。4．粒度参数的大小取决于粒度特征值，粒度特征值随不同沉积相相互交替而“跳动”，其规律与粒度参数的变化一致。但以φ50为界，其上的“活性特征值”φ75、φ84、φ95跳动幅度大，对于沉积相响应的敏感度较高，而其下的“惰性特征值”φ5、φ16、φ25则相反。 文中根据粒度分析及古今砂丘的Mz—σ散点图比较结果，认为古代风成砂丘特别是古流动砂丘是砂丘活化的体现，视为“沙漠期”，而砂丘上覆的河湖相沉积与古土壤发育则可视为“间沙漠期”。从这一认识出发，将米浪沟湾剖面150 ka以来的砂丘堆积划分为38个旋回的沙漠期与间沙漠期交替演化过程，并认为，该剖面自那时以来发生的38个砂丘沉积与河湖相或古土壤的演替过程，实际上是这一长期地质时代毛乌素沙漠受东亚冬夏古代季风影响交替演化所致。而粒度特征值的跳动及其导致的粒度参数的波动图象，则是这种环境响应的最为直接的体现。 为进一步探讨砂丘沉积及其演变的原因，本文还将米浪沟湾地层剖面150 ka以来不同时段样品的Mz和SC／D的平均值与相同时期氧同位素诸阶段进行了对比。结果发现：1.δ¹⁸O1、δ¹⁸O3和δ¹⁸O5暖阶段的Mz（φ值）和 SC／D值分别比其前冷阶段 6‘’OZ、6‘’O4和 6‘8O6的明显增高；2．6’8O5诸亚段的 MZ（。值）和 SC／D值呈现出三峰夹两谷的特点，也即暖阶段 6“O5a、5“O5c及 6’‘O5e的 Mz（O值）和 SC／D值表现为高值，而与其交错相邻的冷阶段 6”O5b、6‘’O5d表现为低值。由此可见，米浪沟湾剖面 150 ha来各主要时段地层的粒度同与之相对应的氧同位素各阶段的气候性质具有良好的耦合关系。这说明，剖面所在的萨拉乌苏河流域 150 ha以来多旋回冷暖干湿的气候变化，与过去全球冷暖气候波动存在着成因上的联系，是全球气候变化在沙漠地区的反映。更多还原

【Abstract】 Situated at the southeast of the Mu Us Desert, Ordos Plateau, the Milanggouwan stratigraphical section in the Salawusu River Valley keeps a record of 38 sedimentary cycles of alternate evolution of the aeolian dune layers with the fluvio-lacustrine facies and palaeosols since 150 ka BP. This paper, based on the analysis of 142 samples of grain-size and on the calculation and research results of the stratigraphical sequences in the section, has put forward the following views: 1. Grain-size accumulation percentage of the aeolian sands in the past 150 ka in Milanggouwan section is mainly characterized by the aeolian palaeo-mobile dune sands, which are composed of majority fine sands and minority very fine sands. There isn’t almost any silt and clay. Those of the fluvial facies, especially the lacustrine facies and palaeosols, are mainly composed of very fine sands and also contain some quantities of silt and clay. 2. The general characteristics of the grain-size distribution of different sedimentary facies, especially the variations of the parameters Mz (mean particle diameter) and o (standard deviation) clearly show that the fluvio-lacustrine facies and palaeosols usually become finer in particles and worse in sorting than the aeolian sands. As regards Sk (bias angle) and Kg (kurtosis) values, the fluvio-lacustrine facies or palaeosols increases considerably compared with the aeolian sands. The former displays positive bias and the latter often approximately symmetrical distribution with only a minority negative bias. 3.At the vertical direction of the section, the grain-size distributional coarse and fine rhythms vary very pronouncedly with the alternation of sedimentary facies, which is clearly shown not only in the grain-size percentage in this section but also in the response from different grain-size parameters. Mz, a , Sk, Kg and SC/D (ratio of the sum of silt and clay to sand) of fluvio-lacustrine, palaeosols and aeolian sands constitute an interlocking, multi-fluctuation process curve with peak and valley values: the <& value of Mz, o and Sk values often increase accordingly from aeolian sands to its overlying fluvio-lacustrine facies or/and palaesols, the Kg value heightens, too, and the SC/D value also basically consists with its change steps. 4.The values of grain-size parameters lie on their eigenvalues. Therefore, the grain-size eigenvalues "bounce" with alternation of the different sedimentary facies in the section, whose changeregularity is coincident with that of grain-size parameters. Relatively speaking, however, taking the eigenvalue 50 as a boundary, "active eigenvalues" 75, 84 and 95 above the eigenvalue have wide bounce range and high sensitivity responding to the sedimentary facies ; in contrast with them, those "inert eigenvalues" 5, 16 and O25 below the above-cited eigenvalue, have relatively narrow bounce range and low sensitivity.This paper, based on the grain-size analysis and the comparative result of the Mz - o scatter gram of ancient aeolian sands in Milanggouwan section with the modern dunes in Mu Us Desert, holds that fossil eolian dunes are a proxy of the dune activation, and be regarded as "desert process" , while the overlying fluvio-lacustrine and palaeosols on the dunes are regarded as inter-desert process. Accordingly, the dune deposits of the section since 150 ka BP can be divided into 38 alternate evolvement processes of desert and inter-desert periods, which accord with the sedimentary cycles of the section. It is further suggested from now climate in the area that the 38 inter-succession processes of dune deposits and fluvio-lacustrine or palaeosols in past 150 ka actually result from the alternate evolvement of the ancient East Asia winter and summer monsoons in the Mu Us Desert during the long geologic times. The most direct embodiment is the fluctuant images of grain-size parameters brought about by the bounce of grain-size eigenvalues.In order to discuss dune sedimentary and the cause of its evolvement, in this paper we also contrast every oxygen isoto更多还原