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Since Singer (1984) reviewed clay minerals as they relate to paleoclimates, clay minerals have been used to reconstruct paleoclimates and environments. However, the paleoclimate information recorded by the paleo-clays in sediments will be overprinted or changed if the clay minerals undergo diagenetic transformations or post-depositional diagenetic changes, which are quite common. However, numerous studies still use clay minerals to reconstruct paleoclimates and paleo-environments. Uplift of the Tibetan Plateau since the Late Miocene has significantly impacted the sediments of the Qaidam Basin. However, due to the scarcity of fully dated sediment records, we know little about whether or not and/or how these clay minerals responded to this uplift. In order to understand their response to the uplift, in this study, we present the results of high-resolution profiling of the clay minerals from a 938.5-m borehole core (SG-1) from the Qaidam Basin (NE Tibetan Plateau). These clay minerals are mainly composed of illite, mixed layers of illite/smectite (I/Sm) and chlorite, with minor kaolinite and smectite. The clay minerals may have undergone early diagenesis. The δ 18 O-δD values of the interlayer water and the clay minerals imply the following. (a) The primary amounts of illite and chlorite were lower than those observed, while the primary amounts of smectite and kaolinite were higher than those observed; the amounts of clay minerals should be careful to be used to reconstruct paleo-environment singly; the changes in the illite “crystallinity” (Kübler Index) and chlorite “crystallinity” (Árkai Index) were related to changes in salinity and temperature. (b) Chemical weathering from 2.77 to 0.1 Ma was stronger than that at present. (c) The δ 18 O and δD of the interlayer water ranged from −11.8 to 82.95‰ and from −114.3 to 165.5‰, respectively, which was related to pore water and/or lake water undergoing strong evaporation. The isotopic composition of the interlayer water reflects variations in the pore water/lake water or of the reacting solutions, and records changes in environment. (d) The clay minerals recorded climate changes and uplift in the core, with a dramatic shift at 1.2 Ma and two different environment stages at 2.77–1.2 Ma and 1.2–0.1 Ma. [ABSTRACT FROM AUTHOR]

The oxygen and hydrogen isotopic compositions of gypsum hydration water can be useful for determining the isotopic composition of the original brine from which gypsum precipitated. However, relatively few long-term and continuous records of the stable isotope geochemistry of gypsum hydration water in arid regions have been reported. We measured the δ 18 O and δD of primary gypsum hydration water from a 938.5 m-long deep core (SG-1) in the western Qaidam Basin to study the mechanisms that contributed to gypsum formation and to reconstruct potential paleoclimatic change. The measured δ 18 O and δD ranged from − 4.21‰ to 8.69‰ and from − 72.77‰ to 49.73‰, respectively. The linear relationship between δ 18 O and δD indicates that meteoric water was the original source of the gypsum hydration water. The gradient of 5.39 for the δ 18 O and δD plots is lower than that of global meteoric water, suggesting that paleo-lakewater evaporated and became a CaSO 4 -rich brine leading to gypsum deposition. The evaporation/precipitation (E/P) ratio played an important role in determining δ 18 O and δD. The oscillations noted in the δ 18 O and δD of the gypsum hydration water imply that: (a) there was a long-term and stepwise aridification after ~ 2.2 Ma in the western Qaidam Basin; and (b) there were three increasingly dry phases at 2.2–1.2 Ma, 1.2–0.6 Ma, and 0.6–0.1 Ma, with two cold and dry events at ~ 1 Ma and ~ 0.6 Ma. Global cooling, especially during the Mid Pleistocene Climate Transition event (MPT), may have been the primary cause of the aridification recorded in core SG-1 in the Asian inland. [ABSTRACT FROM AUTHOR]

The Qaidam Basin is an arid, closed, intermontane basin located on the northern margins of the Tibetan Plateau in China. A 938.5 m-long core (SG-1), dated between ∼2.8 Ma–0.1 Ma, was obtained from the western Qaidam Basin. Gypsum is one of major evaporative minerals in the core. A total of 201 gypsum samples were selected and their crystal morphologies were carefully investigated. The crystal habits are lenticular, prismatic, tabular, twinned, stubby, and aggregate. Prismatic crystals occur in the upper 723 m, while lenticular crystals exist almost entirely in the upper 523 m. Tabular crystals are the most common morphology. The total abundance of gypsum crystal (including tabular, lenticular, and prismatic forms) tends to increase with decreasing depth. The relationship between crystal habits and depositional environments is complex, because same morphology can appear in different environments, and different morphologies can also occur in the same depositional environment. However, the occurrence of different morphologies can be attributed to the variable growth rates caused by the inhibition effect of additional cations (e.g. Mg 2+ , Na + , K + and Sr 2+ ) on particular lattice parameters and the variation in precipitation experiments. The lattice parameters of the unit cell of gypsum, measured by X-ray diffraction method, are: a = 5.854–12.57 Å; b = 9.654–16.231 Å; and c = 5.163–14.024 Å; β = 113.54–119.88°; Volume = 601.55–712.65 Å 3 . The substitution of K + , Na + and Mg 2+ for Ca 2+ can reduce c -axis, while Sr 2+ can reduce a and b axes. a / c , b / c , a / b tend to increase with K + , Na + and Mg 2+ and decrease with Sr 2+ . The Mg 2+ , Na + , K + and Sr 2+ contents in gypsum show significant variability, with K/Ca molar ratios varying from 1.03 × 10 −4 to 32.08 × 10 −4 , Mg/Ca molar ratios from 0.0023 to 0.1629, Na/Ca molar ratios from 0.031 to 0.575, and Sr/Ca molar ratios from 5.59 × 10 −4 to 19.67 × 10 −4 . Sr 2+ , Mg 2+ , Na + , and K + concentrations in gypsum mainly depend on growth rate and brine concentration. [ABSTRACT FROM AUTHOR]

Qaidam Basin is a tectonically controlled Mesozoic–Cenozoic depression on the northern margin of the Tibetan Plateau. A 938.5 m-long core was drilled in the Qahansilatu sub-basin in the western Qaidam Basin, with an average core recovery of 95%. It revealed alternating salt layers and carbonate clay layers. Samples were collected at 10–40 cm intervals for mineralogical analysis by XRD and chemical analysis by ICP-OES. The lower 538.5 m sediments are composed of halite, gypsum, anhydrite, gaylussite, calcite, aragonite, ankerite, dolomite, and an unnamed mineral (Mg0.92Ca0.08CO3·3H2O), with trace eugsterite. The mother brines could be Na-type, Na–Ca-type, Na–Ca–(Mg)-type, Ca–(Na)-type, Ca–(Mg)–(Na)-type, and trace Ca–Mg–(Na). Reflux and bacterial activity could be suitable for the formation of dolomite. Deep burial diagenesis could have played an active role in the formation of ankerite and anhydrate. The continuous presence of halite suggested the paleo-lake water was highly brackish or saline, with high evaporation since 2.77 Ma. Salt layers in the lower 538.5 m-long sediments were present from 2221 ka to 2052 ka, corresponding to Pleistocene salt formation in the Qaidam Basin. As hydrated carbonate minerals, both gaylussite and the unnamed mineral are deposited under high precipitation rates with microbial activity. Gaylussite was deposited from Na–Ca-enriched solutions with molar ratios of Na/Ca more than 2 from 691 m (2226 ka) to 413.6 m (1222 ka). The un-named mineral (Mg0.92Ca0.08CO3·3H2O) was found from 523.4 m (1728 ka) to 724.2 m (2308 ka). Anhydrite could be transformed from gypsum under deep burial from 657.42 m (2052 ka) to 867 m (2556 ka). Alternating salt and clay layers in the lower part of the core recorded arid and relatively wet climatic oscillations and the evolution of brine. As a tectonic sub-basin, tectonic activities could change the local climate during episodes of uplift and subsidence. The dominant minerals in the Chahansilatu sub-basin are similar to those of the other sub-basins in the western Qaidam Basin, but have asynchronous evolutionary stages of brine. [ABSTRACT FROM AUTHOR]

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]

Uranium (U) provides especially useful information for reconstructing paleoredox conditions due to its unique geochemical properties and behavior in supergene environments. Its proxy potential under suboxic –oxic conditions associated with salinization of surface water in closed hydrologic systems, however, has not been adequately explored. This study examined the abundance and distribution of U in a sedimentary sequence of calcareous lacustrine clastics and salt layers from a high quality (938.5 m-long) core drilled in the western Qaidam Basin, NE Tibetan Plateau. This study examined U concentrations in calcareous clastic sediments (including carbonates, acid insoluble fractions, as well as the amounts of iron and manganese oxides/oxyhydroxides) and salt layers. In the calcareous clastic sediments, variations in U concentrations within the acid insoluble fraction follow the fluctuations of reconstructed lake levels based on lithologic investigations, providing a tool for tracing redox conditions in paleolake bottom water. Correlations between the variations in carbonate content with U and Sr concentrations in the carbonate fractions demonstrate that the types and abundance of carbonate minerals control the U distribution in carbonates. In the salt layers, U and Al concentrations correlate closely, implying that both derived mainly from the detrital fraction in the salt layers. However, enrichments of U and Mg are systematically higher in salt minerals formed in more concentrated brine and lower in those formed in less concentrated brine. These results indicate that during the process of carbonate precipitation, carbonate might effectively remove U from lake water. During the subsequent stage of salt mineral precipitation, U behavior is conservative, and U resides for a long period in dissolved forms within the saline water, giving rise to enhanced U enrichment in the more concentrated brines and the associated precipitated salt minerals. Our study suggests, however, that U-related redox proxies should be interpreted with caution in suboxic–oxic environments during the long-term paleolake evolution. [ABSTRACT FROM AUTHOR]

Abstract: Phosphorus (P) provides an important biologically limiting nutrient in lake ecosystems, and its concentrations furnish significant information on lake nutrient status, climatic change, and landscape development within a drainage area. This study explores the potential of calcium-bound P and iron-bound P in calcareous lacustrine sediments to reveal variations of the paleolake nutrient status and its related climatic changes. Data come from the nearly 1000-m long core (SG-1) from the western Qaidam Basin (NE Tibetan Plateau), which has been well-dated by paleomagnetism to between 2.77 and 0.1 Ma. Calcium-bound P and iron-bound P make up the two most frequent phases of inorganic phosphorus pools in the calcareous sediment. This study compares the variations of calcium-bound P and iron-bound P with the independent proxy for biologic production proxy Total Organic Carbon (TOC) as well as a climatic proxy for lake level fluctuation from the borehole. Those results indicate that calcium-bound P responds sensitively to paleolake nutrients and climatic changes, while the iron-bound P responds to more complex factors, more specific, to the content of iron oxides. Variations of the calcium-bound P in the core SG-1 show a long-term, stepwise decrease, with shifts at ∼2.5 Ma, 1.8 Ma, 1.2 Ma, and 0.6 Ma, indicating a long-term stepwise decrease of the paleolake nutrient supply in the drainage. This suggests that the ecologic systems experienced a long-term deterioration, and the climate in central Asia became increasingly arid since after the late Pliocene. [Copyright &y& Elsevier]