Manping Xie

Comparisons between the chemical compositions of lake water, inflowing river water, and lake sediment in Nam Co, central Tibetan Plateau, China and their controlling mechanisms

ABSTRACT The chemical composition of lake water and inflowing river water was investigated in Nam Co (lake), Tibetan Plateau, in September 2005. Lake water samples (n = 76) were collected at different depths along a south-north transect of the eastern part of the lake while water samples were collected from 69 rivers flowing into the lake; a sediment core was collected at 64 m midway along the water survey transect to investigate salt precipitation. Na+ and Ca2+ were the dominant cations, accounting for 76.2% and 60.6% of the lake and river water cations, respectively while was the dominant anion accounting for 70.8% and 93.4% of lake and river anions, respectively. CaCO3 precipitation from the water column decreased Ca2+ in the lake water, with the relative proportion of other ions increasing significantly. Evaporation-crystallization processes largely control Nam Co lake-water chemistry, while rock weathering is the dominant processes influencing the chemistry of river water; carbonate and silicate weathering are the major sources of ions in these rivers.

Further discussion about the features of Lake Puma Yum Co, South Tibet, China

Further discussion about the limnological features of Lake Puma Yum Co, South Tibet, China, is provided based on the results of several investigations. By using depth data from all over the lake, the whole submarine topography has been compiled. Horizontal analysis of the waters physicochemical features indicates that compared with the relatively uniform water features at other lake areas, apparent spatial heterogeneity exists in the water of the subaquatic alluvial fan induced by the Jiaqu River, the biggest inflow. Vertical analysis of water characteristics using two-factor analysis of variance with no re-experiment indicates that temperature, dissolved oxygen, and pH of the water vary with water depth rhythmically, whereas other parameters demonstrate no evident vertical variation, which shows that chemical stratification is not obvious. But this does not exclude slightly higher concentrations of Ca2+ induced by lower pH at the bottom of deep lake water. The hydrochemistry difference between inflow water and lake water reveals the loss of Ca2+ in lake water, which indicates calcite deposition may be an important characteristic of lake sediment.

Ostracoda Assemblages in Core Sediments and Their Environmental Significance in a Small Lake in Northwest Tibet, China

ABSTRACT A 107-cm-long lake core was drilled in South Hongshan Lake (35°10′N, 80°04′E, 5060 m a.s.l.) shows 150 yr of environmental change as inferred from ostracoda assemblages. Four species of ostracoda, which belong to four genera, have been identified. The ostracoda distribution in the core can be divided into six ostracoda assemblages as follows: OA1—Ilyocypris biplicata–Limnocythere inopinata assemblage; OA2—Limnocythere inopinata–Leucocythere mirabilis assemblage; OA3—Leucocythere mirabilis–Limnocythere inopinata assemblage, in which dominant species is Limnocythere inopinata; OA4—Nearly ostracoda free; OA5—Eucypris inflata–Limnocythere inopinata assemblage; and OA6—Limnocythere inopinata changes from dominant to sole species. These ostracoda assemblages, together with other proxies, imply the following environmental sequence for the past 150 yr in the study area. Shallow moving water appears in A.D. 1850–1884, followed by a warm-humid climate in 1884–1922. Lake level decreased and water temperature supported maximum ostracoda growth in 1922–1946, followed by cooling conditions in 1946–1960. From 1960 to 1980, declining lake level is closely linked with local precipitation. The period since 1982 is characterized by less precipitation and continuous aridity under warming climatic conditions.