Ge/Si Ratio of River Water in the Yarlung Tsangpo: Implications for Hydrothermal Input and Chemical Weathering

Germanium/Silicon (Ge/Si) ratio is a common proxy for primary mineral dissolution and secondary clay formation yet could be affected by hydrothermal and anthropogenic activities. To decipher the main controls of riverine Ge/Si ratios and evaluate the validity of the Ge/Si ratio as a weathering proxy in the Tibetan Plateau, a detailed study was presented on Ge/Si ratios in the Yarlung Tsangpo River, southern Tibetan Plateau. River water and hydrothermal water were collected across different climatic and tectonic zones, with altitudes ranging from 800 m to 5000 m. The correlations between TDS (total dissolved solids) and the Ge/Si ratio and Si and Ge concentrations of river water, combined with the spatial and temporal variations of the Ge/Si ratio, indicate that the contribution of hydrothermal water significantly affects the Ge/Si ratio of the Yarlung Tsangpo River water, especially in the upper and middle reaches. Based on the mass balance calculation, a significant amount of Ge (11–88%) has been lost during its transportation from hydrothermal water to the river system; these could result from the incorporation of Ge on/into clays, iron hydroxide, and sulfate mineral. In comparison, due to the hydrothermal input, the average Ge/Si ratio in the Yarlung Tsangpo River is a magnitude order higher than the majority of rivers over the world. Therefore, evaluation of the contribution of hydrothermal sources should be considered when using the Ge/Si ratio to trace silicate weathering in rivers around the Tibetan Plateau.

Spatiotemporal Variation of Hydrogen and Oxygen Stable Isotopes in the Yarlung Tsangpo River Basin, Southern Tibetan Plateau

Characterization of spatiotemporal variation of the stable isotopes δ18O and δD in surface water is essential to trace the water cycle, indicate moisture sources, and reconstruct paleoaltimetry. In this study, river water, rainwater, and groundwater samples were collected in the Yarlung Tsangpo River (YTR) Basin before (BM) and after the monsoon precipitation (AM) to investigate the δ18O and δD spatiotemporal variation of natural water. Most of the river waters are distributed along GMWL and the line of d-excess = 10‰, indicating that they are mainly originated from precipitation. Temporally, the δ18O and δD of river water are higher in BM series (SWL: δD = 10.26δ18O+43.01, R2 = 0.98) than AM series (SWL: δD = 9.10δ18O + 26.73, R2 = 0.82). Spatially, the isotopic compositions of tributaries increase gradually from west to east (BM: δ18O = 0.65Lon (°)-73.89, R2 = 0.79; AM: δ18O = 0.45Lon (°)-57.81, R2 = 0.70) and from high altitude to low (BM: δ18O = −0.0025Alt(m)-73.89, R2 = 0.66; AM: δ18O = −0.0018Alt(m)-10.57, R2 = 0.58), which conforms to the “continent effect” and “altitude effect” of precipitation. In the lower reaches of the mainstream, rainwater is the main source, so the variations of δ18O and δD are normally elevated with the flow direction. Anomalously, in the middle reaches, the δ18Omainstream and δDmainstream values firstly increase and then decrease. From the Saga to Lhaze section, the higher positive values of δ18Omainstream are mainly caused by groundwater afflux, which has high δ18O and low d-excess values. The δ18Omainstream decrease from the Lhaze to Qushui section is attributed to the combined action of the import of depleted 18O and D groundwater and tributaries. Therefore, because of the recharge of groundwater with markedly different δ18O and δD values, the mainstream no longer simply inherits the isotopic composition from precipitation. These results suggest that in the YTR Basin, if the δ18O value of surface water is used to trace moisture sources or reconstruct the paleoaltimetry, it is necessary to rule out the influence from groundwater.

Brief communication: A ~50 Mm³ ice-rock avalanche on 22 March 2021 in the Sedongpu valley, southeastern Tibetan Plateau

On 22 March 2021, a ~50 M m3 ice-rock avalanche occurred from 6500 m asl in the Sedongpu basin, southeastern Tibet. The avalanche transformed into a highly mobile flow which temporarily blocked the Yarlung Tsangpo river. The avalanche flow lasted ~5 minutes and produced substantial geomorphological reworking. This event, and previous ones from the basin, occurred concurrently with, or shortly after recorded positive air temperature anomalies. The occurrence of future large mass flows from the basin cannot be ruled out, and their impacts must be carefully considered given implications for sustainable hydropower and associated socioeconomic development in the region.

Evaluating the Influence of Climate Change on Sophora moorcroftiana (Benth.) Baker Habitat Distribution on the Tibetan Plateau Using Maximum Entropy Model

The ecosystems across the Tibetan Plateau are changing rapidly in response to climate change, which poses unprecedented challenges for the control and mitigation of desertification on the Tibetan Plateau. Sophora moorcroftiana (Benth.) Baker is a drought-resistant plant species that has great potential to be used for desertification and soil degradation control on the Tibetan Plateau. In this study, using a maximum entropy (MaxEnt) niche model, we characterized the habitat distribution of S. moorcroftiana on the Tibetan Plateau under both current and future climate scenarios. To construct a robust model, 242 population occurrence records, gathered from our field surveys, historical data records, and a literature review, were used to calibrate the MaxEnt model. Our results showed that, under current environmental conditions, the habitat of S. moorcroftiana was concentrated in regions along the Yarlung Tsangpo, Lancang, and Jinsha rivers on the Tibetan Plateau. Elevation, isothermality, and minimal air temperature of the coldest month played a dominant role in determining the habitat distribution of S. moorcroftiana. Under future climate scenarios, the increased air temperature was likely to benefit the expansion of S. moorcroftiana over the short term, but, in the long run, continued warming may restrict the growth of S. moorcroftiana and lead to a contraction in its habitat. Importantly, the Yarlung Tsangpo River valley was found to be the core habitat of S. moorcroftiana, and this habitat moved westwards along the Yarlung Tsangpo River under future climate scenarios, but did not detach from it. This finding suggests that, with the current pace of climate change, an increase in efforts to protect and cultivate S. moorcroftiana is necessary and critical to control desertification on the Tibetan Plateau.