The Tibetan Plateau (TP) and surrounding areas contain the largest number of glaciers outside the polar regions. The region affects downstream water supply and food security, thereby directly influencing one-third of the world’s population. The lakes in the central TP expanded rapidly in recent decades, which has attracted growing attention. Glacier meltwater was considered as a major component in the water balance of TP lakes, although few studies quantified its contribution. Stable isotope analysis is a powerful tool to trace hydrologic circulation, while its interpretation in paleoclimate records has been controversial. To bridge the gap between hydrologic and paleoclimatic studies, we performed a hydrologic and isotopic mass balance model to simulate the lake level change of Linggo Co in the central TP. The model was forced by the meteorological data, calibrated through observed lake level changes, and validated by oxygen isotope compositions (δ18O) of lake water. Our results indicated that glacier meltwater contributed 73.94% of the inflow water to Linggo Co before 1993 but decreased thereafter. Increasing glacier meltwater together with positive water balance (precipitation/evaporation) in the catchment contributed to the rapid expansion of Linggo Co after the mid-1990s. Lake water δ18O in Linggo Co was more sensitive to changes in the precipitation amount and precipitation δ18O than temperature. Our findings could shed light on the usage of δ18O proxy in future paleoclimate research on the TP.
Hg pollution in soils surrounding the Wanshan mercury mine (WMM), once the largest Hg-producing center in China, has been confirmed, neglecting other potential toxic elements (PTEs). Better understanding of the sources and transport pathways of soil PTEs remains insufficient. To response these limitations, eight soil PTEs (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn) at two typical agricultural sites, namely AZ and WW that are located near and far from the WMM, respectively, were systemically investigated. The results showed that AZ exhibited significantly higher concentrations of all the PTEs in the surface soil than WW (p < 0.01). Hg and Cd were recognized as the priority control PTEs, with their average concentrations of 21.54 and 1.21 mg kg–1 at AZ, and 15.79 and 0.48 mg kg–1 at WW. Those affected PTEs tended to enrich in near-river areas. Atmospheric deposition contributed more to soil Hg than did regular irrigation, but these two sources could not explain the considerable soil Hg accumulation. Three sources, including natural sources, hydraulic transport (torrential floods and regular irrigation) and atmospheric deposition, were identified and quantified based on the positive matrix factorization model, statistical methods and various auxiliary information. Hydraulic transport (mainly torrential floods) dominated the soil Hg input, which could explain 83.8% and 69.8% of the soil Hg input at AZ and WW, respectively. Atmospheric deposition dominated the soil Cd input, explaining 44.3% and 59.9% of the soil Cd input at AZ and WW, respectively. More attention should be given to the safe utilization of agricultural land and long-term monitoring of atmospheric deposition of Hg and Cd. This study could provide insights to prevent PTE diffusion along the above dominant transportation pathways while developing similar mine regions.
The spatial and temporal variability, effects, and mechanisms of the Indian Summer Monsoon (ISM) have been investigated intensively during the past few decades. The pattern of a relatively strong ISM during the early to middle Holocene, and a relatively weak ISM in the late Holocene, has been widely demonstrated in both marine and continental records. However, the timing of the ISM onset during the early Holocene remains controversial. Here, we present oxygen isotope record from ostracods and hydrogen isotope record from sedimentary leaf waxes from a sediment core at Linggo Co, a glacier-fed lake on the central Tibetan Plateau, in order to investigate the onset of the ISM. The ostracod δ 18 O record indicates an early ISM onset at ~11.7 ka, whereas the leaf wax δD record indicates a later ISM onset at ~10 ka. This apparent two-step development of the ISM revealed by aquatic and terrestrial records is confirmed by principal component analysis of nine marine records from the ISM domain. The comparison between isotope records from Linggo Co and the marine records implies that the early ISM onset was likely linked to elevated temperatures in the Northern Hemisphere, while the later ISM onset may be related to intensified precipitation.
Although predators in microalgal culture can often be protozoa reducing biomass productivity and culture stability, there are few effective approaches to control them. This study investigated the effect of culture pH (i.e., 6.0, 6.5, 7.0 and 7.5) maintained by supply of compressed air bubbles containing various concentrations of CO2 on death of the flagellate Poterioochromonas malhamensis and several other protozoa in the culture of the green microalgae Chlorella sorokiniana GT-1. C. sorokiniana GT-1 grew well at pH 6.0 and 6.5 and a sustainable biomass concentration of 1.61 g L− 1 was obtained from the cultures maintained at pH 6.5. The cultures maintained at pH 7.0 and 7.5 collapsed on days 7 and 4 of culture, respectively, as a result of contamination by P. malhamensis and to less extent by other protozoa (e.g., ciliates and amoebae). Further experiments revealed that it was the actual dissolved CO2, not the low pH itself, or reduced dissolved oxygen in the culture medium that prevented the occurrence of P. malhamensis. It is speculated that increased CO2 partial pressure in the culture media may enhance diffusion of CO2 into the cytoplasm of P. malhamensis that lowers the intercellular pH, and thus results in cell death. The method developed in this study can be effective in protozoan control in pilot-scale Chlorella culture in an open raceway pond. It is suggested that a low pH maintained temporarily or constantly by supply of CO2 may be a promising approach to control P. malhamensis and alike in microalgal culture.
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