浑善达克沙地碱湖表层沉积物的粒度、沉积有机质变化特征与指示意义

本文选择浑善达克沙地南缘濒临干涸的碱湖为研究对象,综合分析湖泊及流域表层沉积物的粒度、总有机碳(TOC)及其同位素(δ¹³C_TOC)指标的空间分布特征及环境指示意义。结果表明,湖泊水域区及滩地区的黏土与粉砂组分含量高,TOC含量多低于1%,而东缘的盐生草甸砂组增加,同时TOC含量升高至10.75%;δ¹³C_TOC值表现为从西向东,即水域区(-24.88‰)、滩地区(-25.17‰)、盐生草甸(-27.93‰)呈逐渐偏负的趋势。水域区表层沉积物粒度端员组分分析表明,粗粒端员(近源风成组分及洪积组分)含量的增加指示湖泊退缩及流域植被退化;水域区低TOC指示水生植物基本消失,陆源C₃植物成为湖泊沉积物有机质的主要来源,控制了δ¹³C_TOC的波动。当湖泊彻底干涸,裸露滩地的粒度初始以细粒组分(黏土、粉砂)为主,而后强烈的风蚀作用将富盐细粒带走,沉积物的粒度变粗,盐度降低,盐生植被入侵。此后沉积物的TOC含量显著增加,而δ¹³C_TOC值逐渐偏负可能与上覆盐生植被演替过程中C₃植物生物量增加有关。因此,乌日图音淖尔现代沉积过程及机理的研究,加深了对浑善达克沙地碱湖干涸过程中沉积及生态演化过程的认识,也为古环境的重建提供了重要依据。

Surface sediments of an alkaline lake in the Otindag sandy land:Grain size and sedimentary organic matter variations and their environmental significance

The spatial characteristics of grain size distribution, total organic matter (TOC) and stable carbon isotopic composition (δ¹³C_TOC) in surface sediments of a desiccating alkaline lake in the southern Otindag sandy land were studied to explore their environmental implications. We found that clay and silt fractions were dominated in water surface and desiccated lakebed while TOC concentrations were generally less than 1% in most surficial samples. In the halophytic meadow to the east, the sand fraction moderately increased while TOC increased up 10.75% on average. From west to east, δ¹³C_TOC became more negative from the lake (-24.88‰) to desicacated lakebed (-25.17‰) and halophytic meadow (-27.93‰) areas. End-member mixing analysis (EMMA) showed that the increases in the saltation and creeping end members were correlated to lake regression and vegetation degradation in the catchment. For Qrdin Nuur, the relatively low TOC and depleted δ¹³C_TOC might indicate the absence of hydrophytes and major contributions of terrestrial C₃ plants to sedimentary organic matter. Furthermore, when the seasonal lake dries up, the bared lakebed, composed mainly of clay and silt fractions, would experience strong wind erosion, leading to increased sand fraction and reduced salinity therefore the invasion of halophytes. Then TOC increased significantly following the appearance of halophytes, and the subsequent succession of vegetation and the increasing C₃ biomass would likely lead to a decline in δ¹³C_TOC. Thus, by studying the modern sedimentary processes and mechanism of Qrdin Nuur, we have deepened our understanding of the sedimentation and ecological evolution in desiccating alkaline lakes in the Otindag sandy land, thereby providing an important basis for paleoenvironmental reconstruction.