Sources and flux of trace elements in river water collected from the Lake Qinghai catchment,NE Tibetan Plateau

River waters play a significant role in supplying naturally- and anthropogenically-derived materials to Lake Qinghai, northeastern Tibetan Plateau. To define the sources and controlling processes for river water chemistry within the Lake Qinghai catchment, high precision ICP-MS trace element concentrations were measured in water samples collected from the Buha River weekly in 2007, and from other major rivers in the post-monsoon (late October 2006) and monsoon (late July 2007) seasons. The distributions of trace elements vary in time and space with distinct seasonal patterns. The primary flux in the Buha River is higher TDS and dissolved Al, B, Cr, Li, Mo, Rb, Sr and U during springtime than those during other seasons and is attributed to the inputs derived from both rock weathering and atmospheric processes. Among these elements, the fluxes of dissolved Cr, B and Rb are strongly influenced by eolian dust input. The fluxes of dissolved Li, Mo, Sr and U are also influenced by weathering processes, reflecting the sensitivity of chemical weathering to monsoon conditions. The anthropogenic sources appear to be the dominant contribution to potentially harmful metals (Ni, Cu, Co, Zn and Pb), with high fluxes at onset of the main discharge pulses due, at least partially, to a runoff washout effect. For other major rivers, except for Ba, concentrations of trace elements are higher in the monsoon than in the post-monsoon season. A total of 38.5 ± 3.1 tons of potentially harmful elements are transported into the lake annually, despite human activities within the catchment being limited. Nearly all river water samples contain dissolved trace elements below the World Health Organization guidelines for drinking water, with the exception of As and B in the Daotang River water samples collected in late July probably mobilized from underlying lacustrine sediments.     

Major ion geochemistry of shallow groundwater in the Qinghai Lake catchment, NE Qinghai-Tibet Plateau

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 SO₄ ^2?, 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 Ca²⁺(Na⁺) ?CHCO₃ ^? type, while the Qinghai Lake, part of the Buha (BHR) and the Lake Side (LS) samples belong to the Na⁺?CCl^? 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.     

Seasonally chemical weathering and CO<Subscript>2</Subscript> consumption flux of Lake Qinghai river system in the northeastern Tibetan Plateau

The major cation and anion compositions of waters from the Lake Qinghai river system (LQRS) in the northeastern Tibetan Plateau were measured. The waters were collected seasonally from five main rivers during pre-monsoon (late May), monsoon (late July), and post-monsoon (middle October). The LQRS waters are all very alkaline and have high concentrations of TDS (total dissolved solids) compared to rivers draining the Himalayas and the southeastern Tibetan Plateau. Seasonal variations in the water chemistry show that, except the Daotang River, the TDS concentration is high in October and low in July in the LQRS waters. The forward models were used to quantify the input of three main rivers (Buha River, Shaliu River, and Hargai River) from rain, halite, carbonates, and silicates. The results suggest that (1) atmospheric input is the first important source for the waters of the Buha River and the Shaliu River, contributing 36–57% of the total dissolved cations, (2) carbonate weathering input and atmospheric input have equal contribution to the Hargai River water, (3) carbonate weathering has higher contribution to these rivers than silicate weathering, and (4) halite is also important source for the Buha River. The Daotang River water is dominated by halite input owing to its underlying old lacustrine sediments. The water compositions of the Heima River are controlled by carbonate weathering and rainfall input in monsoon season, and groundwater input may be important in pre-monsoon and post-monsoon seasons. After being corrected the atmospheric input, average CO₂ drawdown via silicate weathering in the LQRS is 35 × 10³ mol/km² per year, with highest in monsoon season, lower than Himalayas and periphery of Tibetan Plateau rivers but higher than some rivers draining shields.     

Constraints on paleowater dissolved loads and on catchment weathering over the past 16 ka from Sr/Sr ratios and Ca/Mg/Sr chemistry of freshwater ostracode tests in sediments of Lake Constance, Central Europe

Sr isotope and Ca/Mg/Sr chemical compositions of freshwater ostracode tests separated from a sediment core represent the last 16 ka of sedimentation in Lake Constance, Central Europe. The chemical evolution of the paleowater's dissolved load of Lake Constance was estimated by correcting the ostracode data for Ca/Mg/Sr fractionation due to biogenic calcification. Since the Late Pleistocene deglaciation, the Ca/Sr molar ratios of paleowaters increased systematically from about 100 (a near marine signature) to about 200. Ca/Mg molar ratios varied in the range of 1–25. The ⁸⁷Sr/⁸⁶Sr ratios indicate Late Pleistocene paleowater compositions of 0.7086–0.7091, significantly more radiogenic than present day waters (0.7085). Sr isotopes and Ca/Mg/Sr chemical data together show that weathering of Mesozoic evaporites consistently dominated the dissolved Sr load (80–90%). Carbonate and silicate weathering were less important (1–10%). Trends of Sr dissolved loads were therefore not related to Mg which was mainly mobilized by carbonate weathering. Biotite weathering was an important source of radiogenic Sr in the paleowaters. The short-term release (duration about 600–800 years) of radiogenic Sr during glacier retreat started 15.2 ka ago and was due to enhanced biotite weathering at the glacier base. Long-term release of radiogenic Sr was due to biotite weathering in glacial soils and silicate rocks, and has gradually declined since the Late Pleistocene/Holocene transition.     

Ostracod Mg/Sr/Ca and 87Sr/86Sr geochemistry from Tibetan lake sediments: Implications for early to mid‐Pleistocene Indian monsoon and catchment weathering

Jin, Z. D., Bickle, M. J., Chapman, H. J., Yu, J., An, Z., Wang, S. & Greaves, M. J. 2010: Ostracod Mg/Sr/Ca and ⁸⁷Sr/⁸⁶Sr geochemistry from Tibetan lake sediments: Implications for early to mid‐Pleistocene Indian monsoon and catchment weathering. Boreas, 10.1111/j.1502‐3885.2010.00184.x. ISSN 0300‐9483 Lacustrine sediment serves as a valuable archive for tracing catchment weathering processes associated with past climatic and/or tectonic changes. High‐resolution records of fossil ostracod Mg/Ca, Sr/Ca and ⁸⁷Sr/⁸⁶Sr ratios from a lake sediment core from the central Tibetan Plateau reveal a temporal link between lake‐water chemistry and catchment weathering and distinct monsoonal oscillations over the early to mid‐Pleistocene. Between 2.01 and 0.95 Ma, lake‐water chemistry was dominated by a high proportion of carbonate weathering related to variations in the Indian monsoon, resulting in relatively low and constant ostracod ⁸⁷Sr/⁸⁶Sr but obvious fluctuations in Mg/Ca, Sr/Ca and δ¹⁸O. Across the mid‐Pleistocene transition (MPT), a significant increase in ⁸⁷Sr/⁸⁶Sr and frequently fluctuating ratios of ostracod Mg/Ca, Sr/Ca and δ¹⁸O are coincident with increases in both Chinese loess grain size and Arabian Sea lithogenic flux. This correlation indicates an increased glaciation and a strong monsoon seasonal contrast over the plateau. The increase in lake‐water ⁸⁷Sr/⁸⁶Sr across the MPT highlights a change in catchment weathering patterns, rather than one in climate‐enhanced weathering intensity, with an increased weathering of ⁸⁷Sr‐rich minerals potentially induced by marked extensive glaciation and strong seasonality in the central plateau.     

Age constraints on the late Quaternary evolution of Qinghai Lake, Tibetan Plateau

Dating and geomorphology of shoreline features in the Qinghai Lake basin of northwestern China suggest that, contrary to previous interpretations, the lake likely did not reach levels 66-140 m above modern within the past ∼ 90,000 yr. Maximum highstands of ∼ 20-66 m above modern probably date to Marine Isotope Stage (MIS) 5. MIS 3 highstands are undated and uncertain but may have been at or below post-glacial highs. The lake probably reached ∼ 3202-3206 m (+ 8-12 m) during the early Holocene but stayed below ∼ 3202 m after ∼ 8.4 ka. This shoreline history implies significantly different hydrologic balances in the Qinghai Lake basin before ∼ 90 ka and after ∼ 45 ka, possibly the result of a more expansive Asian monsoon in MIS 5.     

Northern latitude chemical weathering rates: clues from the Mackenzie River Basin, Canada

The main scope of this study is to investigate parameters controlling chemical weathering rates for a large river system submitted to subarctic climate. More than 110 river water samples from the Mackenzie River system (northern Canada) have been sampled and analyzed for major and trace elements and Sr isotopic ratios in the dissolved phase. The three main morphological units are reflected in water chemistry. Rivers from the Canadian Shield are very dilute, dominated by silicate weathering (Millot et al., 2002), whereas the rivers of the Rocky and Mackenzie Mountains as well as the rivers of the sedimentary Interior Platform are dominated by carbonate weathering and are SO₄ rich. Compared to the rivers of the Mackenzie and Rocky Mountains, the rivers of the interior plains are organic, silica, and Na rich and constitute the dominant input term to the Mackenzie River mainstream. Rivers of the Canadian Shield area do not significantly contribute to the Mackenzie River system. Using elemental ratios and Sr isotopic ratios, a mathematical inversion procedure is presented that distinguishes between solutes derived from silicate weathering and solutes derived from carbonate weathering. Carbonate weathering rates are mostly controlled by runoff, which is higher in the mountainous part of the Mackenzie basin. These rates are comparable to the carbonate weathering rates of warmer areas of the world. It is possible that part of the carbonate weathering is controlled by sulfide oxidative weathering, but its extent remains difficult to assess. Conversely to what was stated by Edmond and Huh (1997), overall silicate weathering rates in the Mackenzie basin are low, ranging from 0.13 to 4.3 tons/km²/yr (Na + K + Ca + Mg), and confirm the negative action of temperature on silicate weathering rates for river basins in cold climates. In contrast to what has been observed in other large river systems such as the Amazon and Ganges Rivers, silicate weathering rates appear 3 to 4 times more elevated in the plains than in the mountainous headwaters. This contradicts the “Raymo hypothesis” (Raymo and Ruddiman, 1992). Isotopic characterization of suspended material clearly shows that the higher weathering rates reported for the plains are not due to the weathering of fine sediments produced in the mountains (e.g., by glaciers) and deposited in the plains. Rather, the relatively high chemical denudation rates in the plains are attributed to lithology (uncompacted shales), high mechanical denudation, and the abundance of soil organic matter derived from incomplete degradation and promoting crystal lattice degradation by element complexation. The three- to fourfold factor of chemical weathering enhancement between the plains and mountains is similar to the fourfold factor of enhancement found by Moulton et al. (2000) between unvegetated and vegetated watershed. This study confirms the negative action of temperature on silicate weathering for cold climate but shows that additional factors, such as organic matter, associated with northern watersheds are able to counteract the effect of temperature. This acceleration by a factor of 4 in the plains is equivalent to a 6°C increase in temperature.     

Optical dating of sand wedges and ice‐wedge casts from Qinghai Lake area on the northeastern Qinghai‐Tibetan Plateau and its palaeoenvironmental implications

Sand wedges and ice‐wedge casts in the Qinghai Lake area on the northeastern Qinghai‐Tibetan Plateau (QTP) occur within alluvial gravel or river terrace deposits. In this study, we report the results of quartz Optically Stimulated Luminescence (OSL) dating of the infill of five relict sand wedges and one ice‐wedge cast. Combining our dating results with previously published luminescence ages of permafrost wedges in the Qinghai Lake area, we show that sand/ice wedges formed at c. 62 ka, c. 45 ka and between 30 and 15 ka, and that the mean annual air temperature (MAAT) was depressed by at least ～3°C relative to present during the sand/ice‐wedge formation periods. This new work is partially corroborated by post‐LGM proxy records from lakes and aeolian deposits reported from the northeastern QTP. It also significantly extends the palaeoenvironmental record in the region in the period before the LGM, when other proxy records are rare, allowing a better understanding of the palaeoenvironmental conditions on the northeastern QTP.     

青海共和盆地中三叠世花岗岩组合岩石成因:地球化学、锆石U-Pb年代学及Hf同位素约束

青海共和盆地位于青藏高原东北缘,以往有关共和盆地及邻区早-中三叠世岩浆岩成因机制的认识分歧较大,且研究主要集中在露头岩石方面。本文以共和盆地干热岩GR1井深部花岗岩岩芯样品作为研究对象,对其进行岩石学、长石电子探针、主微量元素地球化学、锆石U-Pb年代学及Lu-Hf同位素研究。矿物组成及长石电子探针测试结果显示花岗岩主要为奥长花岗岩、英云闪长岩及花岗闪长岩。锆石U-Pb测年结果表明,奥长花岗岩的结晶年龄为236.5±1.7Ma,英云闪长岩的岩浆结晶年龄为241.6±3.0Ma。主、微量元素地球化学显示这些花岗岩主要为准铝质,属于高钾钙碱性系列。全岩Ta-Nb-Hf等不相容元素图解及锆石Hf同位素数据表明共和盆地的236.5~241.6Ma花岗岩显示火山弧及同碰撞花岗岩成分特征,说明中三叠世共和地区发生俯冲-碰撞转换。结合本文数据与区域背景资料,作者认为：共和盆地早-中三叠世花岗岩组合的形成与印支期宗务隆洋的南向俯冲作用密切相关,此时,在宗务隆-青海南山-西秦岭北缘存在统一的大陆边缘弧环境;在236~241Ma时发生俯冲-碰撞转换;晚三叠世时宗务隆-青海南山-西秦岭北缘已处于碰撞期和后碰撞期。     

Marine Sr (Ca) input in Quaternary volcanic rock weathering profiles from the Mediterranean coast of Morocco: Sr isotopic approach

Carbonate commonly accumulates in weathering profiles developed on noncalcareous parent material under arid and semiarid climatic conditions. Such weathering profile types are widespread in Morocco, making it critical to know the sources of the calcium in order to understand the genesis of carbonate-rich soils. In this study, we use ⁸⁷Sr/⁸⁶Sr as a geochemical tracer of the Ca sources in weathering profiles developed on volcanic rocks (basalt, tuff, andesite and latite) in the Mediterranean coast of Morocco. The results show that the most of the Sr, and hence Ca, is of an external origin, derived from (i) sea-water for the profiles located near to the coast or (ii) sea-water plus Liassic carbonate outcropping in the catchment for the profiles more distant from the coastline. The contribution of the host rock is small, being less than 25%. The ratio of the Sr (Ca) from the parent material to that from the external sources is controlled by the degree of porosity developed in the profile.     

Li/Ca ratios of ostracod shells at Lake Qinghai, NE Tibetan Plateau, China: a potential temperature indicator

Acquiring continuous and high-resolution natural records in recent 2,000?years is the hot issue in the palaeoclimate research. Recent studies revealed that Li/Ca and Mg/Li ratio of carbonate is a potential tracer of past environmental changes in the oceans, and is seldom applied for palaeoclimate reconstruction in lakes. To make full understanding of the potential of Li/Ca ratios of carbonates in lakes, Li/Ca ratios of monospecific ostracod shell Eucypris inflate with similar size in the lacustrine sediment core from Lake Qinghai, NE Tibetan Plateau have been analysed for the first time. Single species can effectively avoid the interspecies effects. In combination with the sedimentation rate that existed, data derived from ²¹⁰Pb and ¹³⁷Cs, downcore variations of Li/Ca ratios of ostracod shells during the past 800?years in Lake Qinghai have been reconstructed successfully. By comparing Li/Ca ratios and temperature inferred from meteorological records and tree ring widths in Dulan and Qilianshan in adjacent regions, Li/Ca ratios of ostracod shells negatively correlate with temperature. Higher temperature corresponds with lower Li/Ca ratios, and vice versa, indicating that Li/Ca ratio of ostracod shells is an effective indicator for temperature variations. Therefore, ongoing in-depth investigation using Li/Ca ratios of carbonates in more lakes to further reveal its palaeotemperature implications would be deserved.     

Origin of Triassic magmatism of the Southern Alps (Italy): Constraints from geochemistry and Sr-Nd-Pb isotopic ratios

During Middle-early Late Triassic (∼243–235?Ma) a diffuse igneous activity developed in the Southern Alps (Italy). Sparse lava flow and pyroclastic succession remnants of such Southern Alps Triassic Igneous Rocks (SATIR) crop out in the Brescian Prealps, the Vicentinian Alps (Recoaro-Schio-Posina), Non Valley, Dolomites and Julian Alps. Plutonic rocks are found in two main plutonic complexes (Monzoni and Predazzo) and a small stock (Cima di Pape). Coeval igneous products can be traced eastward to Austria and Dinarides, for a total length of ∼450?km.The coeval formation of major late Anisian-late Ladinian carbonate platforms in the subsiding eastern Dolomites, and significant uplift and subaerial erosion in the western Dolomites suggest, for these areas, the occurrence of large-scale strike-slip and extensional tectonics with the development of horst-and-graben structures.This study reports the first complete review of the SATIR activity, including new mineral chemical data on 14 samples, 61 major and trace element whole rock analyses and 7 Sr-Nd-Pb isotopic ratios for volcanic and plutonic samples from Dolomites (lavas plus Monzoni and Predazzo plutonic rocks) and Vicentinian Alps lavas. Despite the variable post-magmatic modifications, the large areal distribution of the products and their wide spectrum of chemical compositions, these samples show rather common geochemical and mineralogical characteristics and define major and trace element trends that can be associated with nearly close-system magmatic evolution. Minor upper crustal contaminations can be observed in specific cases, mostly in the most differentiated products (SiO₂ >70?wt%).A specific characteristic of SATIR is their calcalkaline to shoshonitic affinity, resembling the derivation from subduction-modified mantle sources, a feature at odds with the coeval strike-slip and extensional tectonics. Geochemical modelling, petrological considerations and geological constraints allow us to propose a model in which the SATIR mantle sources reflect previous subduction metasomatism (likely occurred during Variscan times). These metasomatised mantle sources were reactivated ∼90–100 Myr after the end of the subduction, when continental rifting caused a raise of the geotherms and passive upwelling of asthenospheric mantle.     

藏东义敦地体早古生代构造格局：来自碎屑锆石U-Pb-Hf同位素的约束

位于藏东的义敦地体是研究青藏高原和古特提斯构造演化的关键区域,其在早古生代时期的古地理位置及构造演化过程尚不明确。沉积岩中碎屑锆石记录了物源区丰富的地质信息,被广泛应用于示踪沉积物源和古地理重建。本文对义敦地体三件下古生界浅变质沉积岩样品开展了碎屑锆石LA-ICP-MS U-Pb年代学和Hf同位素研究,结果显示：三件样品均具有“多峰”的碎屑锆石年龄分布特征,其U-Pb年龄主要集中在约2535～2350 Ma、约1000～900 Ma、约890～750 Ma和约590～520 Ma四个区间,对应的ε_Hf(t)值分别为-8.8～13.1、-11.8～10.0、-20.1～12.6和-27.6～6.1。综合本次研究结果和前人数据,提出义敦地体下古生界变沉积岩中约2535～2350 Ma和约890～750Ma年龄段的锆石主要来自邻区松潘-甘孜地体和华南地块,而约1000～900 Ma和约590～520 Ma年龄段的碎屑锆石主要源自东冈瓦纳大陆Rayner-Eastern Ghats、Prydz-Darling和Kuunga造山带的岩浆岩。对比该地层与区内新元古界碎屑岩及相...     

Origin of Halite in the Lanping–Simao Basin, Southeastern Tibetan Plateau, China: Evidence from Sr and Cl Isotopes

The Lanping?Simao Basin is located on the southeastern Tibetan Plateau, China, and contains massive evaporites. The origin of evaporites in the basin has been hotly debated because of the strong transformation by tectonic movement. Forty halite samples from borehole MK-3 in the Mengyejing area of the basin were collected and analyzed using XRD, Cl-Sr isotopes and chemical compositions to trace the origin of the evaporites in the basin. The Br × 103/Cl ratios of the halite samples are between 0 and 0.55, most of which are synchronized with the law of seawater evaporation and at the stage of halite precipitation from seawater, indicating that the evaporites are mainly of marine origin. The 87Sr/86Sr ratios range from 0.707489 to 0.711279; after correction, the 87Sr/86Sr 145 Ma ratios range from 0.704721 to 0.707611, equivalent with the 87Sr/86Sr ratios of seawater at 145 Ma, indicating a marine origin. The decay of 87Rb in the evaporite during deposition, change of the depositional environment and the unsealed environment at a later period resulted in the present 87Sr/86Sr ratios of some samples being high. The δ37Cl value compositions range from ?0.38‰ to 0.83‰, which is consistent with the δ37Cl value composition of the world marine halite (?0.6‰ to 0.4‰), further confirming that seawater is the main origin. In addition, the high δ37Cl value of some samples at the boundary of the upper and lower evaporite layers might be related to the influence of δ37Cl-rich brine and the incomplete dissolution of the halite.     

东秦岭北部富碱侵入岩钕、锶、铅同位素特征及构造意义

东秦岭北部富碱侵入岩主要分布在陕西省洛南与河南省卢氏－确山一带,岩石有碱性岩、碱性花岗岩和石英正长岩三大类。岩石Ｎｄ、Ｓｒ和Ｐｂ同位素地球化学的研究表明,该区富碱侵入体的源区应是以下地壳为主,并通过碰撞造山作用带入少量地幔和上地壳物质。源区同位素组成横向与垂向的不均一性说明华北块体存在成熟的下地壳层,而北秦岭块体是造山带环境。古大陆边缘经历了多次裂解作用和多期碱性岩浆活动;碱性岩浆是在减压扩容环境     

皖南旌德花岗闪长岩与暗色包体的成因: 地球化学、锆石U-Pb年代学与Hf同位素制约

旌德复式岩体位于安徽南部,主体相花岗闪长岩中发育暗色包体。本文对花岗闪长岩与暗色包体进行了岩相学观察、全岩主微量元素分析、锆石U-Pb定年与Hf同位素测试。岩相学观察发现暗色包体为典型岩浆岩结构,且发育针状磷灰石。主量元素分析数据表明花岗闪长岩的SiO₂含量为66.04%~67.80%;暗色包体的SiO₂含量为54.63%~54.77%,为二长闪长岩。花岗闪长岩的Mg^#=38~40;暗色包体的Mg^#=44~45。微量元素分析数据表明花岗闪长岩与暗色包体的REE球粒陨石标准化图呈右倾型,Eu负异常;大离子亲石元素富集,高场强元素亏损。锆石U-Pb年代学与Hf同位素研究表明,花岗闪长岩与暗色包体的年龄分别为139.7±1.3Ma和142.3±1.7Ma,在误差范围内一致。花岗闪长岩锆石的ε_Hf(t)为-2.5~0.4,地壳模式年龄(t_DM^C)为1170~1350Ma;暗色包体锆石的ε_Hf(t)为-5.2~1.8,地壳模式年龄(t_DM^C)为1090~1530Ma。两者的t_DM^C峰值都在1.2~1.3Ga。这些数据表明花岗闪长岩中的暗色包体为同源岩浆混合成因,源区为年轻地壳,有可能为中新元古代古华南洋壳俯冲扬子板块形成的火山岛弧。旌德花岗闪长岩在Pearce et al.(1984)的构造判别图上落在岛弧花岗岩区。在Sr/Y-Y图解上落在经典岛弧岩浆岩区。花岗闪长岩的岩浆Zr饱和温度低(630~680℃),与锆石钛温度计(630~720℃)给出的结果基本一致。锆石的Ce(Ⅳ)/Ce(Ⅲ)高(240~530),指示岩浆具有高的氧逸度。旌德岩体的低温与高氧逸度特征说明岩体的源区物质受到过洋壳俯冲的影响。旌德岩体的成因可能与太平洋板块后撤诱发的地壳部分熔融有关。     

Quantitative climate and vegetation trends since the late glacial on the northeastern Tibetan Plateau deduced from Koucha Lake pollen spectra

Quantitative information on vegetation and climate history from the late glacial-Holocene on the Tibetan Plateau is extremely rare. Here, we present palynological results of a 4.30-m-long sediment record collected from Koucha Lake in the Bayan Har Mountains, northeastern Tibetan Plateau. Vegetation change has been traced by biomisation, ordination of pollen data, and calculation of pollen ratios. The application of a pollen-climate calibration set from the eastern Tibetan Plateau to Koucha Lake pollen spectra yielded quantitative climate information. The area was covered by alpine desert/steppe, characteristic of a cold and dry climate (with 50% less precipitation than today) between 16,700 and 14,600 cal yr BP. Steppe vegetation, warm (∼ 1°C higher than today) and wet conditions prevailed between 14,600 and 6600 cal yr BP. These findings contradict evidence from other monsoon-influenced areas of Asia, where the early Holocene is thought to have been moist. Low effective moisture on the northeastern Tibetan Plateau was likely due to high temperature and evaporation, even though precipitation levels may have been similar to present-day values. The vegetation changed to tundra around 6600 cal yr BP, indicating that wet and cool climate conditions occurred on the northeastern Tibetan Plateau during the second half of the Holocene.     

湘东北金井地区花岗岩成因及地球动力学暗示：岩石学、地球化学和Sr-Nd同位素制约

出露于湘东北地区的燕山期花岗质岩岩体,呈岩基或岩株状广泛侵位于中元古界冷家溪群浅变质碎屑沉积岩中, 部分岩体为白垩纪红层覆盖。文章详细研究了金井岩体的岩石学、地球化学及Sr-Nd同位素特征,并与其围岩冷家溪群进行了对比。该岩体具有较高的SiO2(71．44-74．31wt．％)、K2O(3．86-4．98wt．％)和Na2O(3．24-3．84wt．％),以及较低的 FeO Fe2O3 MgO TiO2(均小于3％),结合高含量的Al2O3(13．25-14．89wt．％)及高ASI值(普遍大于1．1),为典型的强过铝质(SP)高钾钙碱性花岗岩。金井花岗岩富硅、碱和高Sr、Ba丰度、高LILE／HSFE和LREE／HREE比值,低镁铁质及REE 含量,表明金井岩体为高度分异的花岗岩,其源区的残留物中黑云母含量高。REE配分图和原始地幔多元素标准化图谱上 Rb、Th、U、K和LREE相对富集,而Ba、Sr和高强场元素(Nb、Ta、P和Ti)相对亏损,Eu可变的负异常(δEu:0．53-0．78),表明存在角闪石、黑云母、堇青石、磷灰石和钛铁矿等分异演化的影响,类似于同碰撞型花岗岩地球化学特征。高Rb／Sr(3-6)、低 Sr／Ba(0．2-0．7)、高P2O5含量(0．09-0．15wt．％)、低Al2O3／TiO2比值(40-700．4和<0．2两组)以及Nd同位素的不同(高Nd和低Nd两组),类似于围岩冷家溪群砂质和泥质沉积岩特征,以及高Sr[(87Sr／86Sr)i=0．72204-0．72540]、低Nd[εNd(145 Ma)=-10--12]等均表明花岗质岩浆主要起源于中元古代冷家溪群变质砂岩(高Ca)和变质泥岩(低Ca)的部分熔融或中下地壳物质的熔融,但有少量地幔物质的加入。结合印支期以来华南及湘东北地区地球动力学演化史,作者认为金井燕山期花岗岩形成于陆内碰撞造山晚期,是160-140Ma 陆内俯冲(A型俯冲)碰撞造山作用增温减压体制达到鼎盛阶段的产物,标志湘东北地区由此全面转入陆内伸展阶段。     

腾冲新生代火山作用流体组成及其来源——火山岩流体化学组成和碳同位素制约

腾冲新生代火山区是青藏高原唯一幔源挥发份大量排放的火山-地热区,大规模火山作用的流体组成的系统研究具有多方面的科学意义。对腾冲马鞍山、老龟坡和打鹰山等地新生代火山岩进行流体化学组成和碳同位素分析,结果表明腾冲新生代火山岩的流体组成中H2O占有极高的比例,CO2、N2和O2的含量较高,而且不同火山区的流体组成有所差异。CO2的δ13C值为-27.1‰~-7.5‰,位于地壳和地幔范围之间;CH4、C2H6、C3H8和C4H10等甲烷同系物的碳同位素组成随碳数增高具有整体正序、C2H6与C3H8局部反序的分布特征,显示海洋环境I型有机质热裂解成因烃类气体的特征。腾冲火山作用中存在地幔来源的CO2,岩浆存在轻微的CO2去气作用。含碳流体挥发份主要表现为俯冲大洋板片脱出流体挥发份的加入,特别是俯冲洋壳沉积有机质热裂解产物,大量的H2O可能来源于岩浆上升过程中围岩流体或再循环流体的加入,不同火山区岩浆上升演化的差异造成了流体组成的不同。