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Abstract: Rare Earth elements (REEs) from a 332 cm-long core, surface sediments and soils of the lake shores were examined with the aim of evaluating their suitability as a paleo-environmental proxy. REEs were transported into the Nam Co Lake not only by runoff but also by wind. Concentrations of REE in the core range from 97.57 to 162.98 ppm with an average of 126.5 ppm, from 171.3 to 475.7 ppm in the soils of shores, and from 65.03 to 354.03 ppm in the surface sediments with an average of 130.64 ppm. The REE average of soils is 158.9 ppm for the northern shore, 214.72 ppm for the southern shore, 244 ppm for the eastern shore, and 289.69 ppm for the western shore. The REE average of surface sediments is 114.45 ppm for the northern lake, 115.73 ppm for the eastern lake, 129.69 ppm for the western lake, 144.38 ppm for the southern lake, and 150.95 ppm in the center of the lake, suggesting an accumulation of REEs in the center of the lake. The ratios of LREE/HREE range from 8 to 11.85 for the core, from 7.8 to 11.8 for the surface sediments and from 6.5 to 14.5 for the soils, which suggesting that LREE are richer than HREE. The REE distribution patterns of all samples are characterized by “V-shape” curves with slightly negative Ce anomalies (from 0.83 to 0.99) and obviously negative Eu anomalies (from 0.28 to 0.72). Mn played an active role on the Ce anomaly. The values of δCe showed almost no variance, suggesting a stable post depositional environment in the core. The REEs peaks of >140 ppm occurred at 6.9 ka (307 cm), 5.9–5.5 ka (292–287 cm), 2.2–2.0 ka (262–252 cm) and 1.78 ka (212 cm) corresponding to the beginning of environmental shifts and/or events, while the REEs peak at 0.91–0.87 ka (107–102 cm) and the REEs valley of <100 ppm at 7.8–7.5 ka (317–322 cm) corresponded to the end of Zepu Glacier advance and a cold event, respectively. Lagging behind climate changes, this suggests a careful use of REEs can be used as a proxy of paleo-environment in the Nam Co area. [Copyright &y& Elsevier]

Abstract: Qaidam Basin is a tectonically controlled depression on the northern margin of the Tibetan Plateau. In 2008, a long core was drilled in the Qahansilatu sub-basin in the western Qaidam basin. The sediment layers in the upper 400m alternate between evaporite mineral layers and carbonaceous clay layers. The detailed mineralogical investigation focused on evaporite minerals including halite, gypsum, mirabilite, thenardite, glauberite, eugsterite, and bloedite. Gypsum and halite make up the majority of the evaporate minerals. Environmentally induced variations in the mineralogy and crystal habit of the sulfates have been extensively investigated. Gypsum has prismatic and pyramid habits, such as disc pyramid, stubby prismatic, slender prismatic. Visible isolated gypsum and aggregates (rosette/radial and twins) are mostly scattered in carbonaceous clay layers, suggesting that secondary gypsum is well developed. Gypsum may be a precursor mineral of glauberite, and thenardite is the precursor of bloedite. As a metastable and rare mineral, eugsterite does not appear in other Tibetan areas. It forms at the expense of pre-existing gypsum or thenardite in the core at an experimental temperature of higher than room temperature. The presence of eugsterite indicates a warm and/or hot climate at its deposition time. Mineralogical variations have been explained by the brine evolution of Na–Cl, Na–Ca–SO4, Na–SO4, Na–SO4–Cl, Na–Ca–SO4–Cl, Ca–SO4, and Na–Mg–SO4. For instance, Na–Mg–SO4 corresponds to bloedite, while Na–Ca–SO4–Cl to the assemblage of halite, gypsum and glauberite. The evaporite minerals and carbonaceous clay layers'' alternation indicates the shift between dry and wet climate. According to the thickness, 18 evaporite stages and/or dry climate stages were identified from 0.97Ma to about 0.03Ma. The two early dry climate stages are identified at 0.96–0.97Ma and 0.87Ma. The other 16 evaporite stages occurred from 0.78Ma to about 0.03Ma. The evaporate-rich stages suggested that evaporation was high and groundwater inflow was sufficient at the sub-basin. [Copyright &y& Elsevier]

The Plio-Pleistocene evaporitic lacustrine succession in the Qaidam Basin, which was recovered by drill core SG-1, provides the material for understanding the maturation of dolomite since dolomite is present in the core from 7.6 to 870 m depth, spanning a time interval of 0.1–2.8 Ma. Twenty-two samples were collected at different burial depths. The samples were examined by thin-section petrography and SEM for textures, and analyzed for mineralogy (XRD), trace elements and carbon and oxygen isotopes. The main mineral compositions of each sample are dolomite, quartz, halite and gypsum. Dolomite is mostly scattered with spheroidal to ellipsoidal shapes, several microns in diameter, partially encapsulated in gypsum. Microbial micropores and calcified microorganisms are observed on and within the dolomite crystals and in the thin sections. The degree of ordering of the dolomite gradually increases with burial depth, which basically conforms to the first-order reaction function. The crystal constants of dolomite decrease with burial depth, indicating recrystallisation with increasing overburden. The trace elements of the dolomites are significantly different from those of hydrothermal dolomites, but close to lacustrine microbial carbonate. It is concluded that the dolomites in core SG-1 are mainly authigenic, with microbial processes a likely facilitator of dolomite precipitation. The dolomite formed and then evolved in crystal structure during burial, gradually approaching ordered stochiometric dolomite. This study gives clues to link microbial dolomite to the massive ideal dolomite rock encountered in the geological record. • The Plio-Pleistocene dolomite in the core SG-1 is authigenic in the Qaidam paleo lake. • The formation of dolomite crystals in the core SG-1 was associated with microbially mediation process. • The microbial dolomite crystals continued to mature toward ideal state during burial. [ABSTRACT FROM AUTHOR]

Different from the ITCZ-driven global tropical monsoon, the development of a monsoon-dominated climate within previously subtropical arid Asia has conventionally been regarded as marking the onset of a modern-like monsoon over East Asia. This monsoon has increasingly been reported as having appeared in the Eocene or older and linking with different forcings. Here we present robust climatic records from well-dated fluviolacustrine sediments from the eastern central Tibetan Plateau which demonstrate that such a monsoon was not established in the northern subtropical area (∼32-33oN) until at least ∼35 Ma, rather a Mediterranean-type-like climate toward drier prevailed. Numerical modelling supports that neither the existence of an Andes-like Gangdese Mts. along Tibet southern margin, nor an elevated south-central Tibet, nor an increased atmospheric CO 2 concentration, were sufficient to drive the expansion of the Asian tropical monsoon northward to ∼32oN in Asia in the Eocene. • The Paleogene succession on the eastern central Tibetan Plateau was determined at ∼53–35 Ma. • Asian monsoon did not expand to northern subtropical East Asia before 35 Ma. • Forcing factor for late Eocene monsoon could not enhance monsoon to expand to northern subtropics. [ABSTRACT FROM AUTHOR]

Abstract: Manganese (Mn) in lake sediments reacts strongly to changes of redox conditions. This study analyzed Mn concentrations in oxides, carbonates, and bulk phases of the calcareous lacustrine sediments of a 938.5-m-long core (SG-1) taken from the western Qaidam Basin, well dated from 2.77Ma to 0.1Ma. Comparisons of extractions from diluted hydrochloric acid, acetic acid and citrate–bicarbonate–dithionite demonstrate that variations of Mn concentrations from acetic acid leaching (MnHOAc) are mostly responsible for Mn (II) fluctuations in the carbonate phase. Taking into account the relevant processes during weathering, transportation, deposition and post-deposition of Mn-bearing rocks, we conclude that Mn input from catchment weathering and paleolake redox condition provide the primary controls on variations in the Mn records of carbonate and oxide phases. We propose MnHOAc as a new sensitive indicator of paleolake redox evolution and catchment-scale climate change. The MnHOAc variations show a long-term upward decreasing trend, indicating a long-term decrease of Mn input from catchment weathering associated with increasing oxygen content in the paleolake bottom water. The great similarities of the MnHOAc record with other regional and global records suggest that paleolake redox changes and climatic drying in the Qaidam Basin may be largely related to global cooling. [Copyright &y& Elsevier]