西藏达则错盐湖沉积背景与有机沉积结构

Abstract

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 estuarine 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.Phytoplankton debris 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 for20. 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.