Jianting Ju

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.

Holocene lake level fluctuations and environmental changes at Taro Co, southwestern Tibet, based on ostracod-inferred water depth reconstruction

Lake level change is an important indicator of changes in the hydrological status of lakes triggered by the influence of atmospheric circulation systems. Previous studies show that the interaction of the Indian monsoon and the Westerlies, and their influence on environmental changes in the Tibetan Plateau region, remain unclear. Here, we reconstruct the Holocene lake level history of a lake, Taro Co, in southwestern Tibet, based on the relationship between ostracod assemblages and water depth from 34 surface sediment samples spanning the depth range 0 to 125 m. Measurement and analysis of multi-proxy data from a 310 cm sediment core from Taro Co lead to the following conclusions: (1) From 10,200 to 9400 cal. a BP, the lake received large inflows of freshwater (mainly glacial melt water) resulting in a rapid rise in lake level. Following a cold and dry interval and low lake level from 9400 to 8800 cal. a BP, the lake maintained a high level between 8800 and 7500 cal. a BP. This was followed by a less warm and wet period until 6400 cal. a BP. From 6400 to 1200 cal. a BP, the lake probably overflowed after reaching a high level, and after 1200 cal. a BP, the lake level decreased and fluctuated around the modern depth of 60 m at the coring site. (2) The region experienced warm and wet conditions from 10,200 to 6400 cal. a BP, with a cold event at around 9000 cal. a BP, which probably resulted from the influence of the Indian monsoon. However, the inferred wet conditions between 6400 and 1200 cal. a BP are not observed in other records from the monsoon region; they are consistent with numerous records from the regions under the influence of the Westerlies, which probably indicate an interval of Westerlies-domination in the Taro Co region.

The Warming of Large Lakes on the Tibetan Plateau: Evidence From a Lake Model Simulation of Nam Co, China, During 1979–2012

Lakes are considered as indicators of climate change on the Tibetan Plateau (TP). In the present study, we use the General Lake Model to simulate water temperature changes in Nam Co, the second largest lake on the central TP, for the period 1979-2012. The calibration and validation results demonstrate that this model is well suited for thermal simulation of Nam Co. The simulation results indicate that Nam Co has responded to the recent warming climate. The average summer surface water temperature fluctuated yearly, but its trend is positive at a rate of 0.52 +/- 0.25 degrees C per decade. At the same time, the onset of summer stratification advanced by 4.20 +/- 2.02d per decade, and the duration increased at a rate of 6.00 +/- 3.54d per decade. To explore the roles of air temperature and longwave radiation in lake warming, three sensitivity experiments are conducted by removing long-term trends from time series of air temperature and longwave radiation in the forcing data. These experiments prove that both increased air temperature and downward longwave radiation are two driving factors responsible for the warming of Nam Co.

Estimating the contribution of glacial meltwater to Ranwu Lake, a proglacial lake in SE Tibet, using observation data and stable isotopic analyses

It is important for both current monitoring and paleoenvironmental research conducted on proglacial lakes and their adjacent glaciers to clarify the hydrological processes operating on these lakes. However, in remote regions with limited accessibility it may be difficult to study hydrological processes by direct monitoring. In this study, we use measurements of stable isotopic compositions to trace the multiple water sources contributing to Ranwu Lake, a proglacial lake in south-eastern Tibet. Using stable isotopic data from precipitation, inflowing rivers and the lake water, a water and isotope mass balance modelling method was used to calculate the ratio of evaporation to input. Subsequently, using hydrological and climatic data for the outflow, the largest inflow and precipitation, other hydrological elements of the lake water balance were also calculated. The results demonstrate that the ratio of evaporation to inflow is as low as 0.009, the lowest value observed for the Tibetan Plateau, indicating that Ranwu Lake is a through-flow lake with a very short retention time. Glacial meltwater accounts for at least 55% of total runoff, the highest value observed for the Tibetan Plateau, indicating that the sediments of Ranwu Lake may have considerable potential for reconstructing variations in the activity of the local glaciers. Finally, we note that it may be inappropriate in this glacier-fed lake to use the intersection of the local meteoric water line with the lake water line for determining the isotopic composition of the input water, and this possibility must be carefully considered when stable isotope mass modelling is used in proglacial lakes.

Mid- to late-Holocene paleoenvironmental changes inferred from organic geochemical proxies in Lake Tangra Yumco, Central Tibetan Plateau:

Investigations of the paleoclimate and paleoenvironmental changes in the Tibetan Plateau in recent years have led to many significant achievements. However, high-resolution lake records from the mid- to late-Holocene have not been determined for the Central Tibetan Plateau, which is in the transition zone between the Indian summer monsoon (ISM) system and the westerlies. In this study, we analyzed the distribution and compound-specific carbon isotope compositions (δ13C) of sedimentary n-alkanes and the bulk sediment total organic matter (TOC), total nitrogen (TN), and C/N ratios to reconstruct the paleoenvironmental changes over the past 6 cal. ka BP using a 376-cm sediment core from the southern basin of Tangra Yumco, Central Tibetan Plateau. Our results indicated that clear stepwise decreases in lake productivity and lake level occurred during the mid- to late-Holocene based on decreases in δ13C values of mid-chain n-alkanes and Paq (an n-alkane-based proxy). Additionally, the variations in the TOC, TN, and C/N ratios revealed generally increased inputs of allochthonous organic matter over the past 6 cal. ka BP. When our findings at Tangra Yumco were compared with records from the Central Tibetan Plateau and adjacent regions, we found generally consistent patterns of lake environmental changes over the past 6 cal. ka BP, suggesting that the climate of the Central Tibetan Plateau was primarily influenced by the ISM system at the millennial time scale. Therefore, our results depict the evolution of the ISM over the past 6 cal. ka BP in the Central Tibetan Plateau, which corresponds to summer solar insolation and the migrations of the Intertropical Convergence Zone (ITCZ). We also propose that the climate of the North Atlantics influenced the climate of the Central Tibetan Plateau through some indirect climatic mechanisms.