Effects of Depositional Environment on Pollen Assemblages—A case study in the Shiyang River Basin
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摘要: 西部干旱区小型内陆流域 (石羊河流域 )内三个地点孢粉分析显示不同地点全新世孢粉组合差异极大。三角域剖面孢粉组合完全以针叶树为主;洪水河剖面孢粉组合下部以旱生草本为主,上部以针叶树为主;终闾区表土孢粉组合中 90 %为旱生草本、灌木。其差异已远远超出气候环境变化所导致的差异。结合沉积相、沉积环境、孢粉在不同沉积环境中的传播、扩散特性的综合分析,笔者认为这种差异是沉积环境、搬运介质差异造成的。三角城剖面处于半封闭环境,接纳储存来自上游的所有沉积物的信息,孢粉组合反映的是流域上游祁连山上的植被状况。洪水河剖面下部冲积地层发育时,剖面处于开放环境,孢粉组合反映当地植被;上部湖相层发育时,剖面处于半封闭环境,孢粉组合的环境指示意义与三角城剖面一致。终闾区表土孢粉组合反映现代风媒环境终闾区植被状况。古环境重建不能仅从孢粉组合出发,必须结合沉积环境综合分析,否则,古环境重建的结果是不可靠的.Abstract: The differences of the pollen assemblages of three sites in Shivang River basin, an internal river system in the arid area of Northwest China are so sharp that they can' t he explained by climatic change.The early to mid-Holocene pollen assemhlage of Sanjiaocheng section located at the end of Shivang River basin is dominated by the conifer. The surface samples of a site closed by the Sanjiaocheng Section has over 90% of xeromophte in the pollen assemblages.The pollen in the lower part of Hongshui River Secton is mainly xeric herb.In the upper part of the same section however conifer is dominant. Based on the analyses of sedimentary facies, depositional environments and the transportion characteristics of modern pollen we proposed that the obvious difference of pollen assemblages in the catchment are caused by the differences of depositional environments.As the Sanjiaocheng Section is near to the inlet of Shivang River to the paleo-lake with a semi-enclosed environmenu the sediments including pollen transported by water from the upper reaches would settle down there.The pollen assemhlage, therefore represents the vegetation of the upper reaches of the drainage.the Hongshui River Section is located near the end of the Hongshui River a tributary of Shivang River. The lower part of the section is alluvial sediments indicating an open flowing environment where only a small portion of the pollen carried by water can he kept in the sediments.There is no environmental significance in this pollen assemblage and it can' t he used to explain the vegetation.The upper part of the section is lacustrine sediments suggesting an enclosed environment similar to that in Sanjiaocheng Section.The pollen assemblage also reflects the vegetation in the uplands.The prevailing xeromophte in the surface sample near the Sanjiaocheng section implies the local desert vegetation because there is no water to the lowest reaches of the Shivang River as several reservoirs were built for irrigation in the upper and middle reaches.We have to he very careful when reconstructing or explaining palaeoenvironment with pollen assemblages only.Sedimentary fades and environment should also he given enough consideration as it is useful in judging the origin of pollen in the sediments.
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Key words:
- depositional environment /
- pollen assemblage /
- Shiyang River basin /
- Holocene
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