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To advance the understanding of the spatial variability of sediment composition and its controlling factors in a large hydrologically closed lake, the spatial distributions in the mineralogy and geochemistry of 18 surface sediment samples along three transects across the deepest part of Lake Qinghai on the northeastern Tibetan Plateau were investigated. The data show (1) that the surface lake sediments are almost the same in the XRD patterns, (2) that the sediments do not have considerable changes in geochemical compositions along three transects and (3) that the standard deviations of the lake sediments are low in grain size, elemental abundances, δC and δO values of carbonates, and total organic carbon and nitrogen, with the exception of one sample (QH-11) from the westernmost part of the northern sub-basin. These characteristics indicate that the spatial variability of the surface sediment across Lake Qinghai is very low in geochemical and silicate mineralogical compositions, with limited differences in the chemical composition of carbonates associated with river water dilution. Sample QH-11 has anomalously high levels of MgO, a high carbonate content, and positive δC and δO values in bulk and fine-grained carbonates, probably because of the effect of calcareous tufa. Spatial uniformity in the mineralogical and geochemical compositions of surface sediments in Lake Qinghai can be attributed to the flat topography of the lakebed and stable sediment provenances, the latter dominated by dust deposition and authigenic carbonates. The spatial uniformity and dominance of dust and authigenic aragonite of surface sediments across Lake Qinghai provide a potential record of the hydroclimate in the past, which is associated with changes in the Westerlies and the summer monsoons. [ABSTRACT FROM AUTHOR]

Otoliths are biogenic carbonate minerals in the inner ear of teleost fish, whose compositions can record the physical and chemical conditions of the ambient water environment inhabited by individual fish. In this research, the fishbones and otoliths of naked carp sampled near the Bird Island, offshore Lake Qinghai, were dated and analyzed for mineralogy and microchemical compositions. Comparing the microchemical compositions of ancient otoliths with those of modern otoliths, we conclude that the ancient naked carps inhabited a relict lake formed when the lake shrank from a high lake level, by combining with the AMS-C ages of fishbones and otoliths, the stratigraphy and surrounding topography of the sample site. AMS-C dating results of ancient fishbones and otoliths show that these naked carps lived from 680 to 300 years ago, i.e. during the Ming Dynasty of China. The X-ray diffraction (XRD) patterns demonstrate that the ancient lapillus is composed of pure aragonite, identical to modern one, indicating that the mineral of lapillus didn't change after a long time burial and that the ancient lapillus is suitable for comparative analysis thereafter. Microchemical results show that both ratios of Mg/Ca ((70.12±18.50)×10) and δO ((1.76±1.03)‰) of ancient lapilli are significantly higher than those of modern lapilli (average Mg/Ca=(3.11±0.41)× 10 and δO=(−4.82±0.96)‰). This reflects that the relict water body in which the ancient naked carp lived during the Ming Dynasty was characterized by higher Mg/Ca and δO ratios than modern Lake Qinghai, resulting from strong evaporation after being isolated from the main lake, similar to today's Lake Gahai. Based upon the stratigraphy and altitude of naked carp remains, it can be inferred that the altitude of lake level of Lake Qinghai reached at least 3202 m with a lake area of 4480 km during the Ming Dynasty, approximately ∼5% larger than it is today. [ABSTRACT FROM AUTHOR]

Conventional hydrochemical techniques and statistical analyses were applied to better understand the solute geochemistry and the hydrochemical process of shallow groundwater in the Qinghai Lake catchment. Shallow groundwater in the Qinghai Lake catchment is slightly alkaline, and is characterized by a high ion concentrations and low water temperature. The total dissolved solids (TDS) in most of the samples are <1,000 mg/L, i.e. fresh water and depend mainly on the concentration of SO, Cl and Na. Groundwater table is influenced directly by the residents' groundwater consumption. Most of the groundwaters in the Qinghai Lake catchment belong to the Ca(Na) -HCO type, while the Qinghai Lake, part of the Buha (BHR) and the Lake Side (LS) samples belong to the Na-Cl type. The groundwater is oversaturated with respect to aragonite, calcite and dolomite, but not to magnesite and gypsum. Solutes are mainly derived from strong evaporite dissolution in Daotang, BHR and LS samples and from strong carbonate weathering in Hargai and Shaliu samples. Carbonate weathering is stronger than evaporite dissolution with weak silicate weathering in the Qinghai Lake catchment. Carbonate weathering, ion exchange reaction and precipitation are the major hydrogeochemical processes responsible for the solutes in the groundwater in the Qinghai Lake catchment. Most of the shallow groundwaters are suitable for drinking. More attention should be paid to the potential pollution of nitrate, chloride and sulfide in shallow groundwater in the future. [ABSTRACT FROM AUTHOR]

Abstract: Major ions, isotopic ratios of strontium (87Sr/86Sr), hydrogen (δD) and oxygen (δ18O) of groundwater samples were analyzed to decipher spatial variation, controlling factors, solute sources, and the recharge source of shallow groundwaters within the Lake Qinghai catchment. Shallow groundwaters in this area are slightly alkaline, with 97% being fresh water of good quality, though there are high concentrations of nitrate and sulfide in Buha and lakeside groundwaters. Most of the shallow groundwaters are of the Ca2+–HCO3 − type, whereas part of groundwaters surrounding the lake (LS) belongs to the Na+–Cl− type as lake water (QHL). Groundwater geochemistry is controlled by regional lithological association, ion exchange, and mineral precipitation. The dissolved Sr concentrations and 87Sr/86Sr ratios vary from 1.0 to 15.6 μmol/L, and from 0.709859 to 0.715779, respectively. The first quantitative calculation in groundwater using a forward model shows that 40% of dissolved Sr is from carbonate weathering, 33% from evaporite dissolution, 17% from silicate weathering, and the remainder from atmospheric input for the whole catchment. Carbonate weathering dominates groundwater geochemistry in Shaliu (SL), Hargai (HG) and Buha (BH) samples, while evaporite dissolution dominates LS and Daotang (DT) samples. δD and δ18O data show that rain water is the major recharge source of both river water and shallow groundwater within the Lake Qinghai catchment. Qinghai Lake water is characterized by Buha-type water, but its Sr geochemistry is different from the shallow groundwater due to carbonate precipitation. Although shallow groundwater contributes ∼5% of the dissolved Sr to the QHL, groundwater must be taken into account when the chemistry and budget of lake water are characterized. [Copyright &y& Elsevier]

Abstract: Major ion compositions were analyzed in 65 groundwater samples to decipher water quality, solute geochemistry and sources of groundwater within the Qinghai Lake catchment, NE Tibetan Plateau. Groundwaters were slightly alkaline with pH varying from 7.2 to 8.7. The total dissolved solids (TDS) varied over two orders of magnitude from fresh (88%) to brackish (12%) with a mean value of 672mg/L, higher than river waters within the Qinghai Lake catchment and river waters draining the Himalayas and the southeastern Tibetan Plateau. Most of the samples, approximately 80%, were the Ca2+–Mg2+– type and suitable for drinking and irrigation. Some of the Lakeside, the Buha and the Qinghai Lake water were the Na+–Cl− type and not suitable for drinking and irrigation. Water quality of Hargai and Shaliu samples was better than the others. Rock weathering, ion exchange and precipitation are the major geochemical processes responsible for the solutes in the groundwater within the Qinghai Lake catchment. Anthropogenic input to the groundwater is minor. The forward models on the input of groundwaters from various sources showed that the contributions were 40.1% for carbonate weathering, 29.0% for evaporite dissolution, 16.8% for atmospheric input and 14.1% for silicate weathering of the total dissolved cations for the whole catchment. Evaporite dissolution was dominating in the Lakeside, the Buha and the Daotang samples, contributing 15.3–45.9%, 20.4–61.1% and 21.3–63.9% of the total dissolved cations, respectively. Carbonate weathering was dominated in the Shaliu and the Hargai samples, contributing 23.9–71.7% and 31.8–95.4% of the total dissolved cations, respectively. The result also demonstrated that carbonate weathering had higher contribution to the groundwaters than silicate weathering. [Copyright &y& Elsevier]

An understanding of topsoil geochemistry within a watershed helps to identify the influence of natural processes and anthropogenic activities in the environment. Fifteen elements were measured in 73 topsoil samples (top 30 cm) from the Lake Qinghai catchment, NE Qinghai–Tibetan Plateau (QTP) in 2009, to investigate geochemical patterns, identify possible sources and explore the implications of chemical weathering. Compared with their content in the upper continental crust (UCC), a deficit of Cu and Zn and an enrichment of Pb were identified in the study area. Enriched Ca levels were observed in alpine soil and pedocal, solonchak and aeolian sand soils, whereas there was a loss in hydromorphic and semi-hydromorphic soils. High Na in marsh solonchak indicated the possible presence of evaporites. Cluster analysis (CA) and principal component analysis (PCA) indicated the dominance of carbonate and the accumulation of P, possibly from grassland litterfall in topsoil. Some human activities have exerted an influence on virgin topsoil, as revealed by GIS mapping. The implications for chemical weathering, based on a chemical index of alteration ( CIA ) and an Al 2 O 3 –(CaO* + Na 2 O)–K 2 O (A–CN–K) diagram were characterised by a major carbonate control of the primary weathering stage under cold-dry climatic conditions with Ca and/or Na removal. [ABSTRACT FROM AUTHOR]