西藏达则错盐湖沉积背景与有机沉积结构

The depositional environment and organic sediment component of Dagze Co,a saline lake in Tibet,China

以西藏拟溞(Daphniopsis tibetana Sars)为优势浮游动物物种的低盐度盐湖是西藏湖泊的一个重要类型,以达则错为代表,分析了其沉积背景及沉积物组成。结果如下:(1)湖泊敞水区无机沉积以内生化学沉积为主,可代表深水盐湖无机沉积物的自然沉积过程。(2)达则错盐湖浮游植物以蓝藻、硅藻、裸藻、绿藻为主,总生物量11.35 mg/L;浮游动物生物量为4.92 mg/L,其中西藏拟溞占82.30%;浮游植物残体受盐梯度影响在盐梯度层之上聚集,而浮游动物残体及粪粒(Fecal pellets)因外表有碳酸盐附着可穿过盐梯度层沉积湖底,生物残体与浮游动物代谢产物构成了沉积有机物的物质基础。(3)表层沉积物平均含水量为66.70%,粒径0.004—0.02 mm范围内的颗粒物含量最大,占20.42%,其次为<0.004 mm的粘土,占4.53%。(4)表层沉积物总有机碳(TOC)平均含量为27.99 mg/g(干重),其中颗粒有机碳(POC)约为18.11 mg/g,占TOC的64.70%;在POC中,西藏拟溞粪粒贡献最大,约占POC的60.48%,占TOC的39.06%,占沉积物总量的1.12%,其次为西藏拟溞残体,占POC的38.85%。分析结果表明盐湖因其独特的水化学和生物学特征具有较强的沉积能力,以化学沉积为主的无机沉积及以西藏拟溞粪粒和残肢碎屑为主的有机沉积构成了该类型盐湖颗粒物沉降及沉积的主要过程。

The saline lakes of the Qinghai-Tibet Plateau play an important role in the study of lakes in our country.The lakes, which are characterized by low salinity and are dominated by the zooplankton species Daphniopsis tibetana Sars 1903, are an common type in Tibet.The depositional environment and sediment composition in a representative lake, Dagze Co, were analyzed from water and surface sediment samples in sediment traps method.The majority of sediments in shallow lakes are transported by streams and rivers, or are derived from endogenous carbonate sources.Inorganic deposition in more open areas is dominated by endogenous chemical precipitation, concentrated by intense evaporation; similar processes are common in deep saline lakes. In addition to the sedimentation caused by the Bocangzangbu stream, deposition of a large number of carbonates was found in the estuafine region of Dagze Co.This was because of strong mixing between two water bodies of different properties （the fresh water from the stream and the saline water from the lake ）. Sixty-four species of phytoplankton were recorded in the lake, with a total biomass of 11.35 mg/L.The dominant phytoplankton groups observed included blue-green algae, diatoms, euglenophytes and green algae.Forty-one species of zooplankton were also observed.The lake＇s zooplankton community had a total biomass of 4.92 mg/L and was dominated （82.30%） by D.tibetana.Phytoplanktondebris aggregated above a salt gradient layer.Zooplankton residues and fecal pellets were not impeded from sinking through the salt gradient layer to the bottom of the lake. It is likely that the adhesion of carbonates to these particles aided their descent.As such, biological debris and zooplankton metabolites comprised the material basis of the lake＇s organic sediments. The average moisture content of the surface sediments was reported to be 66.70%.Size analysis of sediment particles revealed that particles within the 0.004--0.02 mm size range were the most abundant, accounting for 20. 42% （ wet weight） of the total.The next largest group, accounting for 4.53% （wet weight） of sediment particles, was characterized as clay （0.004 mm） .The total organic carbon （TOC） content in the surface sediments was 27.99 mg/g （mean dry weight）.Particulate organic carbon （POC） accounted for 64.70% of the TOC.Fecal pellets of D.tibetana were the greatest contributors to POC （60.48%, comprising 39.06% of TOC and 1.12% of sediments）.Residues of D. tibetana accounted for 38.85% of POC. D. tibetana was at the highest level of. a very simple trophic web in Dagze Co. As a result, the decomposition of benthic organisms in the lake was minimal. In addition, the reducing environment in the deeper waters of the lake promoted the preservation of organic matter.This caused the TOC content to be significantly higher in the surface sediments.These results show that the distinct hydrochemical and biological properties of saline lakes have a strong influence on their sedimentary depositional environments.In Dagze Co, the main processes affecting the precipitation and deposition of particulate matter were influenced by both inorganic （chemical precipitation ） and organic （fecal pellets and residues of D. tibetana ） mechanisms.This study provides a basis for future analysis of carbon deposition and assessments of carbon source/sink potential in lakes of this type.