干旱区内陆河流域中游地区全新世沉积相变与环境变化——以石羊河流域为例

Changes of sedimentary facies and Holocene environments in the middle reaches of inland rivers, arid China:A case study of the Shiyang River

中国干旱区内陆河流域中游地区分布了大量晚第四纪河、湖相交替的沉积地层,这些沉积物被广泛用于古气候和古环境重建。然而,以往的研究较多关注沉积物的理、化、生指标,而忽略了沉积相变的古环境意义,这可能使古环境重建结果产生偏差。针对这一问题,选取干旱区典型内陆河——石羊河中游地区的JDT全新世沉积剖面,进行年代学、沉积相、岩性和粒度的研究,并与该区域已有的HSH全新世剖面的地球化学和古生物指标进行对比,在分析沉积相的基础上讨论二者的环境代用指标,重建石羊河中游地区全新世的古环境。两个剖面在年代和岩性上具有很好的一致性,指示了石羊河中游地区约8500 cal yr BP-3000 cal yr BP以来的沉积过程。两剖面从底部到上部可分为六层:河流相沉积(F层)——河湖相沉积(E层)——湖相沉积(D层)——浅湖—风成沉积(C层)——风成沉积(B层)——风成沉积(A层),代表了不同沉积相及其转换过程。综合讨论沉积物岩性、沉积相和各种古环境代用指标认为:石羊河中游地区的沉积相体现了从河流相到湖相再到风成沉积物的变化过程,指示了全新世的干旱化过程。这与以往研究中认为石羊河流域全新世气候变化具有波动性的观点存在一定差异。因此,在研究沉积相变化的基础之上讨论沉积物的物理、化学和生物指标的含义,而非简单地研究这些指标的数值,是干旱区流域性气候变化对比研究的新视角。

There are lots of late Quaternary fluvial and lacustrine sedimentary strata in the middle reaches of inland rivers in arid China, and these sediments have already been widely used to reconstruct paleo-climate and paleo-environment. So far, most of the studies have only attached importance to the physical, chemical and biological proxies of sediments, but they are lack of the meaning of sedimentary facies, which may lead to some misunderstandings of paleo-environment reconstruction. Concerning this issue, we analyzed chronology, sedimentary facies, lithology and grain size parameters of a Holocene section named JDT in the Shiyang River's middle reaches in the arid areas of China, and compared them with the data of geochemical and pollen proxies from a section named HSH in surrounding areas. The results show that there is a coherence of these two sections, and we can reconstruct the deposition process during ~8500-~3000 cal yr BP. We can divide the section into six phases from the bottom to the top: fluvial face(part F)—fluvial and lacustrine face(part E)—lacustrine face(part D)—shallow lake-eolian deposit(part C)—eolian deposit(part B)—eolian deposit(part A). We comprehensively analyzed lithological, sedimentary facies and paleo-environment proxies, and found that the sedimentary facies change from fluvial facies to lacustrine facies and eolian deposit, and from that we can know the Holocene environment in this area was becoming drier and drier. The reason why there are some differences between this study and the previous studies is that the analyses of this study are based on the changes of sedimentary facies.