Hydrochemistry of the Lhasa River, Tibetan Plateau: Spatiotemporal Variations of Major Ions Compositions and Controlling Factors Using Multivariate Statistical Approaches

Spatiotemporal variations of the hydrochemical major ions compositions and their controlling factors are essential features of a river basin. However, similar studies in the southern Tibetan Plateau are relatively limited. This study focuses on the chemical compositions of the dissolved loads in the Lhasa River (LR) in the southern Tibetan Plateau. Two sampling campaigns were conducted during the rainy and dry seasons across the LR basin to systematically investigate the spatiotemporal variations of water chemistry and sources of the dissolved loads. The results show that the river water possesses slight alkalinity with an average pH of 8.05 ± 0.04. Total dissolved solids (TDS) and oxidation-reduction potential (ORP) range widely from 39.8 mg/L to 582.6 mg/L with an average value of 165.6 ± 7.7 mg/L and from −9.4 mV to 295 mV with a mean value of 153.7 ± 6.9 mV, respectively. The major cations follow the decreasing order of Ca2+, Mg2+, Na+, and K+ while HCO3−, SO42−, Cl−, and NO3− for anions. Ca2+ and Mg2+ account for 87.8% of the total cations, while HCO3− and SO42− accounts for 93.9% of the total anions. All the major ions show higher concentrations in the dry season. NO3−, HCO3−, and Mg2+ show significant spatial variations due to the influence of basin lithology and anthropogenic activity. Multi-variables statistical analysis reveals that the mechanisms controlling the LR hydrochemistry are mainly carbonate weathering followed by silicate weathering. Geothermal springs and anthropogenic activities also play crucial roles in altering river water ions composition in the middle stream and downstream. The relatively high NO3− value (3 ± 0.2 mg/L) suggests water quality will be under the threat of pollution with the increase of anthropogenic activities.

A high-accuracy rainfall dataset by merging multiple satellites and dense gauges over the southern Tibetan Plateau for 2014–2019 warm seasons

Tibetan Plateau (TP) is well known as Asia's water tower from where many large rivers originate. However, due to complex spatial variability in climate and topography, there is still a lack of a high-quality rainfall dataset for hydrological modeling and flood prediction. This study therefore aims to establish a high-accuracy daily rainfall product through merging rainfall estimates from three satellites, i.e., GPM-IMERG, GSMaP and CMORPH, based on a high-density rainfall gauge network. The new merged daily rainfall dataset with a spatial resolution of 0.1∘ focuses on warm seasons (10 June–31 October) from 2014 to 2019. Statistical evaluation indicated that the new dataset outperforms the raw satellite estimates, especially in terms of rainfall accumulation and the detection of ground-based rainfall events. Hydrological evaluation in the Yarlung Zangbo River basin demonstrated high performance of the merged rainfall dataset in providing accurate and robust forcings for streamflow simulations. The new rainfall dataset additionally shows superiority to several other products of similar types, including MSWEP and CHIRPS. This new rainfall dataset is publicly accessible at https://doi.org/10.11888/Hydro.tpdc.271303 (Li and Tian, 2021).

Variation of the winter mid-latitude Westerlies in the Northern Hemisphere during the Holocene revealed by aeolian deposits in the southern Tibetan Plateau

The mid-latitude Westerlies (MLW) are one of the most important atmospheric circulation systems in the Northern Hemisphere, exerting a huge influence on the climate of the region downwind, and thus on vegetation, water resources, and human wellbeing. However, the seasonal variation of the MLW during the Holocene is not yet been fully understood, especially when its contribution is the most important. Here, we used end-member (EM) modeling analysis of the grain-size distributions of a high-altitude aeolian sedimentary sequence (4452 m a.s.l.) from the Yarlung Zangbo River valley in the southern Tibetan Plateau to reveal variations in the winter MLW during the Holocene. Analysis of seasonal differences in modern atmospheric circulation suggests that the southern Tibetan Plateau was heavily influenced by the mid-latitude Westerlies at the 400, 500, and 600 hPa levels in winter, while it was seldom influenced at these levels in summer. Four grain-size end-members are identified, representing distinct aerodynamic environments, of which EM1 (modal grain size 8.1 μm) can be used as a proxy of the winter MLW. A reconstruction of the variation of the winter MLW during the Holocene based on EM1 revealed that a weaker winter MLW occurred during the Early to Middle Holocene, and a stronger winter MLW during the Middle to Late Holocene. Overall, we suggest that this change in the winter MLW was closely related to the insolation/temperature/pressure gradient between low and high latitudes in the Northern Hemisphere.

A model involving amphibolite lower crust melting and subsequent melt extraction for leucogranite generation

In the southern Tibetan Plateau, leucogranites are dominantly distributed in the Himalayan orogenic belt with minor occurrences in the southern Lhasa subterrane. In this paper, we report the first Miocene Anglonggangri leucogranites in the northern Lhasa subterrane. This finding provides important constraints on both leucogranite petrogenesis and the tectono-magmatic evolution of the Lhasa terrane. The Anglonggangri leucogranites include biotite-muscovite granite and slightly younger garnet-muscovite granite and pegmatite. Zircon U-Pb and muscovite 40Ar-39Ar dating of these leucogranites yields Miocene ages of 11.1−10.2 Ma. The biotite-muscovite and garnet-muscovite granites are characterized by high SiO2 (72.3−74.4 wt%) and Al2O3 contents (14.4−15.4 wt%) and are peraluminous. The biotite-muscovite granite displays geochemical signatures with high Sr/Y (29.2−81.0) and (La/Yb)N (37.5−98.9) ratios, low Y (4.30−7.22 ppm) and Yb contents (0.26−0.47 ppm), low to moderate initial (87Sr/86Sr)i ratios (0.7085−0.7192), and moderate εNd(t) values (−10.17 to −6.94). Furthermore, they also exhibit radiogenic Pb isotope and variable zircon εHf(t) values (−9.6 to +4.4) with Proterozoic Nd (1.1−1.4 Ga) and Hf model ages (0.8−1.7 Ga). By comparison, the garnet-muscovite granite has lower CaO, MgO, TiO2, and total FeO contents and is enriched in Rb (380−466 ppm) and depleted in Sr (24.1−38.5 ppm) and Ba (30.7−58.6 ppm) and further characterized by a significant rare earth element (REE) tetrad effect and non-charge and radius-controlled (CHARAC) trace element behaviors. The garnet-muscovite granite shows a negative Eu anomaly and positive correlations among Sr and Eu, Sr and Ba, and Th and light rare earth elements (LREEs). Pegmatite comprising Nb-Ta oxides and cassiterite occurs in the garnet-muscovite granite. Geochronological and geochemical characteristics of the Anglonggangri leucogranites indicate that the magma of the biotite-muscovite granite was derived from partial melting of amphibolite lower crust contaminated with Proterozoic-Archean upper crustal materials. The garnet-muscovite granite was generated through melt extraction from the biotite-muscovite granite crystal mush. These results confirm that partial melting of the amphibolite lower crust not only occurred in the southern and central Lhasa subterranes but also in the northern Lhasa subterrane.