Climatic influence on peatland formation and lateral expansion in sub‐arctic Fennoscandia

Weckström, J., Seppä, H. & Korhola, A. 2010: Climatic influence on peatland formation and lateral expansion in sub‐arctic Fennoscandia. Boreas, Vol. 39, pp. 761–769. 10.1111/j.1502‐3885.2010.00168.x. ISSN 0300‐9843. The initiation and lateral expansion patterns of five small sub‐arctic peatlands in the Fennoscandian tree‐line region were studied by 21 accelerator mass spectrometry (AMS) ¹⁴C‐dated basal‐peat samples representing three to six dates per site. The radiocarbon dates were converted to calendar years and are based on the median probability. When combined with earlier basal‐peat dates from the region, four distinctive periods can be observed in the cumulative record of the dates. The early Holocene, from c. 10 000 to 8000 cal. yr BP, was characterized by the fast initiation and rapid expansion of peatlands, whereas at 8000–4000 cal. yr BP lateral expansion was modest. The most intensive period of peatland expansion occurred at the beginning of the late Holocene at c. 4000 to 3000 cal. yr BP, after which it slowed down towards the present. All these periods are in rough agreement with the main Holocene climatic periods in the area, namely the relatively warm and moist early Holocene, the warm and dry Holocene thermal maximum (HTM) at 8000–4000 cal. yr BP, and the start of the cooler and moister trend (neoglacial cooling) from c. 4000 cal. yr BP to the present, indicating a broad‐scale climatic control on the lateral growth of sub‐arctic peatlands in Fennoscandia. In order to study the lateral expansion of peatlands and to evaluate their Holocene succession patterns, more studies based on multiple dates from the same peatland are needed.     

Holocene development and permafrost history in sub‐arctic peatlands in Tavvavuoma,northern Sweden

Under changing climatic conditions permafrost peatlands can play an important role in the global carbon budget through permafrost carbon feedbacks and shifts in carbon assimilation. To better predict future dynamics in these ecosystems an increased understanding of their Holocene carbon and permafrost history is needed. In Tavvavuoma, northern Sweden, we have performed detailed analyses of vegetation succession and geochemical properties at six permafrost peatland sites. Peatland initiation took place around 10 000 to 9600 cal. a BP, soon after retreat of the Fennoscandian Ice Sheet, and the peatlands have remained permafrost‐free fens throughout most of the Holocene. At the four sites that showed a continuous accumulation record during the late Holocene radiocarbon dating of the shift from wet fen to dry bog vegetation, characteristic of the present permafrost peatland surface, suggests that permafrost developed at around 600–100 cal. a BP. At the other two sites peat accumulation was halted during the late Holocene, possibly due to abrasion, making it more difficult to imply the timing of permafrost aggradation. However also at these sites there are no indications of permafrost inception prior to the Little Ice Age. The mean long‐term Holocene carbon accumulation rate at all six sites was 12.3±2.4 gC m⁻² a⁻¹ (±SD), and the mean soil organic carbon storage was 114±27 kg m⁻².     

Evidence of ameliorated Middle Weichselian climate and sub‐arctic environment in the western Baltic region: coring lake sediments at Klintholm,Møn,Denmark

Coring through glaciotectonically stacked Quaternary sediments situated below sea level on the island of Møn, Denmark, recovered a succession of interstadial sediments of Middle Weichselian age. Plant and animal remains including insects found in laminated sand and mud indicate deposition in a lake surrounded by dwarf shrubs, herbs, mosses and rare trees. The insect fauna indicates a mean July temperature of 8–12 °C, suggesting an arctic to sub‐arctic environment, while winter temperatures around ?8 to ?22 °C suggest periglacial conditions with permafrost. Luminescence dating of sediment samples gave ages from 48–29 ka, and radiocarbon dating indicates deposition of plant fragments between 45 and 36 ka BP. The fossil assemblage from Møn shows close resemblance to those from other sites with similar ages found in the vicinity of the western Baltic Basin.     

Characterisation of Apatites as Potential Uranium Reference Materials for Fission‐track Dating by LA‐ICP‐MS

We report homogeneity tests on large natural apatite crystals to evaluate their potential as U reference materials for apatite fission‐track (AFT) thermochronology by laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS). The homogeneity tests include the measurements of major element concentrations by electron probe microanalysis (EPMA), whereas for U concentration, isotope dilution (ID) ICP‐MS and laser ablation (LA) ICP‐MS were employed. Two apatite crystals are potential reference materials for LA‐ICP‐MS analysis: a 1 cm³ fraction of a Durango crystal (7.5 μg g^?1 U) and a 1 cm³ Mud Tank crystal (6.9 μg g^?1 U). The relative standard deviation (1 RSD) of the U concentration determined by ID‐ICP‐MS of both apatite crystals was ≤ 1.5%, whereas 1 RSD for the LA‐ICP‐MS results was better than 4%, providing sufficient homogeneity for fission‐track dating. The results on the U homogeneity for two different apatite samples are an important step towards establishing in situ dating routines for AFT analysis by LA‐ICP‐MS.     

The First Halite Age Revealed by MC‐ICP‐MS U‐series Dating in Lop Nur,Northwestern China

正Objective Quaternary saline lakes are significant for paleoclimatic reconstruction due to the high sensitivity of salt minerals to environmental changes.However,salts in these areas with great thickness such as halite or potash representing extreme environments have severely blocked the application of traditional chronological methods.This is typified by Lop Nur,since accurate age to constrain salt     

Holocene colluvial chronology in a sub‐arctic esker landscape at Kuttanen,Finnish Lapland: kettleholes as geo‐ecological archives of interactions amongst fire,vegetation, soil,climate and geomorphological instability

Excavations were made in the colluvial deposits of seven kettleholes in a sandy esker at Kuttanen, northwestern Finnish Lapland. The Holocene history of forest fires and the associated colluvial activity initiated on the slopes of the kettleholes were reconstructed based on 131 radiocarbon dates from charcoal layers. These dates were supplemented by luminescence dating of colluvial sand layers. The first forest fires occurred ～9000 years ago following the immigration of Pinus sylvestris about 1000 years after deglaciation. Evidence of forest fires and colluvial activity increased with the density of the pine forest, reaching a maximum during the Holocene Thermal Maximum between ～8000 and 4000 cal. a BP, declining thereafter to a minimum in the last ～500 years. This multimillennial‐scale pattern corresponds with forest fires being triggered by lightning strikes during the warmest summer weather of the Holocene, which also produced heavy rain and slope wash from convective storms. The 50 forest fires identified over the Holocene indicate a long cycle in fire frequency of 1 in ～200 years, which appears to reflect the average successional recovery time of the forest. Complex interactions amongst vegetation, fire and climate may account for little or no association between 23 centennial‐ to millennial‐scale clusters of forest fires/colluvial events and Holocene temperature or precipitation anomalies. Buried podzols indicate five phases of soil formation and hence low levels of landscape disturbance. The kettleholes and their colluvial deposits therefore provide a unique geo‐ecological archive that has led to new insights into the geo‐ecological interactions that affect environmental change in the sub‐arctic landscape.     

Advance in Matrix Effect Study of LA‐MC‐ICPMS U‐Pb Dating on U‐Bearing Oxide Minerals

正Objective With the development of analytic technologies,in-situ dating on U-bearing oxide minerals(e.g.,cassiterite,rutile and baddeleyite)has been widely used in geological chronological researches and has attracted remarkable attention to explore evolution of the earth and obtain age information of various geological processes.Matrix effect     

In Situ U‐Th‐Pb Dating and Sr‐Nd Isotope Analysis of Bastnäsite by LA‐(MC)‐ICP‐MS

Bastnäsite is the end member of a large group of carbonate–fluoride minerals with the common formula (REE) CO₃F·CaCO₃. This group is generally widespread and, despite never occurring in large quantities, represents the major economic light rare earth element (LREE) mineral in deposits related to carbonatite and alkaline intrusions. Since bastnäsite is easily altered and commonly contains inclusions of earlier‐crystallised minerals, in situ analysis is considered the most suitable method to measure its U‐Th‐Pb and Sr‐Nd isotopic compositions. Electron probe microanalysis and laser ablation (multi‐collector) inductively coupled plasma‐mass spectrometry of forty‐six bastnäsite samples from LREE deposits in China, Pakistan, Sweden, Mongolia, USA, Malawi and Madagascar indicate that this mineral typically has high Th and LREE and moderate U and Sr contents. Analysis of an in‐house bastnäsite reference material (K‐9) demonstrated that precise and accurate U‐Th‐Pb ages could be obtained after common Pb correction. Moreover, the Th‐Pb age with its high precision is preferable to the U‐Pb age because most bastnäsites have relatively high Th rather than U contents. These results will have significant implications for understanding the genesis of endogenous ore deposits and formation processes related to metallogenic geochronology research.     

地质体方法圈定化探异常——以张家口某地区为例

如何确定元素异常下限是化探工作的关键问题。由于元素在不同的地质体内具有不同的分布形式和背景含量,因此在地质情况较复杂地区采取统一值作为元素异常下限值存在一定缺陷。文章以张家口某地区1？20万水系沉积物数据为例,在分析不同地质体内元素分布规律的基础上,确定了不同地质体内元素异常区域,研究表明该方法可有效弥补全区采用统一值圈定异常所造成的不足。     

Tracing growing degree‐day changes in the cuticle morphology of Betula nana leaves: a new micro‐phenological palaeo‐proxy

Changing growing‐season properties in the northern latitudes are among the most obvious consequences of ongoing global change. Available techniques including satellite monitoring and phenological observations enable the detection of changes over the last few decades to centuries, but the full range of natural variability is still difficult to capture. Here we introduce a new approach to reconstruct growing season properties, by studying imprints of prolonged growing season on epidermal cell growth in Betula nana. A high correlation between cell expansion determined in annually collected B. nana leaves and subfossil leaf fragments collected from recent peat sections in northern Scandinavia, and climatic indices such as budburst date, growing‐season degree‐days and May–September mean temperatures, enable the establishment of a new micro‐phenological proxy for growing season characteristics. The applicability of the epidermal cell undulation index (UI) is tested by comparison with historical instrumental records of growing‐season degree‐days for the last 200 a. The results demonstrate the potential of the new leaf‐morphology‐based technique to reconstruct and quantify past changes in growing degree‐days beyond instrumental data series. Applied to abundant B. nana leaf remains from peat and lake sediments, the UI may enable a reconstruction of growing degree‐days throughout the Holocene and other parts of the late Quaternary. Copyright © 2010 John Wiley & Sons, Ltd.     

Zircon U‐Pb Dating of Leucogranite in Lhozag and Its Geological Significance

正Objective The Himalayan leucograite,which is typical production of continent-continent collision orogenic belt,has become a research hotspot of the Tibetan Plateau.The research on the leucogranite would help to verify and improve the continent-continent collision orogenic theory.(Huang et al.,2017;Fig.1a).Previous studies show the Himalayan leucogranite was mainly melted from crust materials(Guo     

Compositional tomography of a gold‐bearing sample by Laser‐induced breakdown spectroscopy

This contribution presents the first results of compositional tomography of a geological sample. The volume render of 6 × 8 × 1 mm³ was constructed by assembling 63 compositional maps acquired in 21 min (19.5 s/layer) by laser‐induced breakdown spectroscopy (LIBS), which determines the chemical composition of the analysed spot from the light emitted by a plasma produced by the laser. This technique is, therefore, able to directly reveal the 3D distribution of chemical elements in a sample. As an example, the spatial distribution and 3D geometry of visible gold in an ultramafic schist are presented. Inasmuch as this newly developed portable LIBS instrument is able to the rapidly characterize the 3D geometry of any geological materials, it has a high potential to be useful for the mining industry and for a wide range of geosciences, such as structural geology, petrology, sedimentology and economic geology.     

Re‐Os Isotopic Dating of a W‐Be Polymetallic Deposit in the Southern Qinling Region,China

正Objective In recent years,a series of tungsten prospecting breakthroughs have been made in the southern Qinling Mountains.Especially,a new deposit type with a scheelite-beryl-molybdenite assemblage in the Zhen’an area of Shaanxi Province was firstly discovered.This deposit is currently in a detailed investigation stage,and no detailed     

The distribution and utility of sea‐level indicators in Eurasian sub‐Arctic salt marshes (White Sea,Russia)

In support of efforts to reconstruct relative sea level (RSL), we investigated the utility of foraminifera, diatoms and bulk‐sediment geochemistry (δ¹³C, C:N and parameters measured by Rock‐Eval pyrolysis) as sea‐level indicators in Eurasian sub‐Arctic salt marshes. At three salt marshes (<15 km apart) in Dvina Bay (White Sea, Russia), we collected surface sediment samples along transects from subtidal to Taiga forest environments. Foraminifera at all sites formed bipartite assemblages, where elevations below mean high higher water (MHHW) were dominated by Miliammina spp. and elevations between MHHW and the highest occurrence of foraminifera were dominated by Jadammina macrescens and Balticammina pseudomacrescens. Five high‐diversity groups of diatoms were identified and they displayed pronounced variability amongst the study sites. Bulk‐sediment geochemistry recognized two groups (clastic‐dominated environments below MHHW and organic‐rich environments above MHHW). As one group included subtidal elevations and the other included supratidal elevations, we conclude that the measured geochemical parameters are not stand‐alone sea‐level indicators. Core JT2012 captured a regressive sediment succession of clastic, tidal‐flat sediment overlain by salt‐marsh organic silt and freshwater peat. The salt‐marsh sediment accumulated at 2804±52 years before present and preserved foraminifera (Jadammina macrescens and Balticammina pseudomacrescens) with good analogy to modern assemblages indicating that RSL was +2.60±0.47 m at this time. Diatoms confirm that marine influence decreased through time, but the lack of analogy between modern and core assemblages limited their utility as sea‐level indicators. Geochemical parameters also indicate a reduction in marine influence through time. We conclude that RSL reconstructions derived from salt‐marsh sediment preserved beneath Eurasian sub‐Arctic peatlands can provide valuable insight into the spatio‐temporal evolution of the Fennoscandian and Eurasian ice sheets.     

Indicator value of oribatid mites in determining past permafrost dynamics in northern European sub‐Arctic peatlands

Permafrost dynamics play an important role in the surface hydrology and carbon balance of northern peatlands. Plant macrofossil analysis with radiocarbon dating has been widely used in detecting past permafrost dynamics in peatlands; however, there is a lack of permafrost‐specific plant indicator species, which makes it challenging to determine the exact timing of historical permafrost aggradation. We investigated the indicator value of oribatid mites in determining past permafrost dynamics in sub‐Arctic peatlands. Analyses of subfossil oribatid mite assemblages of Holocene peat profiles from two mires, one in northern Finland and one in northeastern European Russia, were carried out and interpreted using modern calibration data from the same study areas. The results were compared with previously published reconstructions of permafrost history based on plant macrofossil analyses from the same locations. The results suggest that the oribatid mites Carabodes labyrinthicus, Chamobates borealis and Neoribates aurantiacus are promising indicator species to detect past permafrost occurrence in peatlands. In addition, N. aurantiacus is clearly associated with the presence of lichens, which is particularly useful because lichen remains are rarely preserved in peat deposits. Results are in accordance with earlier studies showing that oribatid mites are useful indicators of past environmental change.     

Root‐related rhodochrosite and concretionary siderite formation in oxygen‐deficient conditions induced by a ground‐water table rise

Sedimentological, mineralogical, stable carbon and oxygen isotope determinations and biomarker analyses were performed on siderite concretions occurring in terrestrial silts to understand their formation and to characterize the sedimentary and diagenetic conditions favouring their growth. High δ¹³C values (6·4‰ on average) indicate that siderite precipitated in an anoxic environment where bacterial methanogenesis operated. The development of anoxic conditions during shallow burial was induced by a change in sedimentary environment from flood plain to swamp, related to a rise of the ground‐water table. Large amounts of decaying plant debris led to efficient oxygen consumption within the pore‐water in the peat. Oxygen depletion, in combination with a decrease in sedimentation rate, promoted anoxic diagenetic conditions under the swamp and favoured abundant siderite precipitation. This shows how a change in sedimentary conditions can have a profound impact on the early‐diagenetic environment and carbonate authigenesis. The concretions contain numerous rhizoliths; they are cemented with calcium‐rhodochrosite, a feature which has not been reported before. The rhodochrosite cement has negative δ¹³C values (?16·5‰ on average) and precipitated in suboxic conditions due to microbial degradation of roots coupled to manganese reduction. The exceptional preservation of the epidermis/exodermis and xylem vessels of former root tissues indicates that the rhodochrosite formed shortly after the death of a root in water‐logged sediments. Rhodochrosite precipitated during the initial stages of concretionary growth in suboxic microenvironments within roots, while siderite cementation occurred simultaneously around them in anoxic conditions. These suboxic microenvironments developed because oxygen was transported from the overlying oxygenated soil into sediments saturated with anoxic water via roots acting as permeable conduits. This model explains how separate generations of carbonate cements having different mineralogy and isotopic compositions, which would conventionally be regarded as cements precipitated sequentially in different diagenetic zones during gradual burial, can form simultaneously in shallow burial settings where strong redox gradients exist around vertically oriented permeable root structures.     

Timing of Magma Mixing in the Gangdisě Magmatic Belt during the India-Asia Collision： Zircon SHRIMP U-Pb Dating

Abstract  Abundant mafic microgranular enclaves (MMEs) extensively distribute in granitoids in the Gangdisê giant magmatic belt, within which the Qüxü batholith is the most typical MME‐bearing pluton. Systematic sampling for granodioritic host rock, mafic microgranular enclaves and gabbro nearby at two locations in the Qüxü batholith, and subsequent zircon SHRIMP II U‐Pb dating have been conducted. Two sets of isotopic ages for granodioritic host rock, mafic microgranular enclaves and gabbro are 50.4±1.3 Ma, 51.2±1.1 Ma, 47.0±1 Ma and 49.3±1.7 Ma, 48.9±1.1 Ma, 49.9±1.7 Ma, respectively. It thus rules out the possibilities of mafic microgranular enclaves being refractory residues after partial melting of magma source region, or being xenoliths of country rocks or later intrusions. Therefore, it is believed that the three types of rocks mentioned above likely formed in the same magmatic event, i.e., they formed by magma mixing in the Eocene (c. 50 Ma). Compositionally, granitoid host rocks incline towards acidic end member involved in magma mixing, gabbros are akin to basic end member and mafic microgranular enclaves are the incompletely mixed basic magma clots trapped in acidic magma. The isotopic dating also suggested that huge‐scale magma mixing in the Gangdisê belt took place 15–20 million years after the initiation of the India‐Asia continental collision, genetically related to the underplating of subduction‐collision‐induced basic magma at the base of the continental crust. Underplating and magma mixing were likely the main process of mass‐energy exchange between the mantle and the crust during the continental collision, and greatly contributed to the accretion of the continental crust, the evolution of the lithosphere and related mineralization beneath the portion of the Tibetan Plateau to the north of the collision zone.     

东北冻土区积雪深度时空变化遥感分析

积雪作为冰冻圈的重要组成部分,对地面有保温作用,在消融时又吸收热量降低地面温度,影响冻土发育,对气候的变化十分敏感。利用微波遥感数据1979-2014年逐日中国雪深长时间序列数据集,采用GIS空间分析和地学统计方法,分析了东北冻土区积雪深度的时空变化规律及其异常变化。结果表明,东北冻土区多年平均雪深为2.92 cm,年平均雪深最高值出现在岛状多年冻土区,最低值出现在季节冻土区。东北冻土区年平均积雪深度变化以减少为主,占区域面积的39.77%,减少速率为0.07 cm·（10a）^-1。东北冻土区年平均积雪深度在1986年发生突变,开始出现减少的趋势,这与气温突变年份较为吻合。受地形和气温变化影响,年平均积雪深度减少的敏感区域主要发生在岛状多年冻土区。气温是影响东北冻土区年平均积雪深度变化最主要的因素,降水量、风速、湿度、日照时数对积雪深度均有影响。季节冻土区积雪深度对气候的敏感性要大于多年冻土区。     

LA-ICP-MS Zircon U-Pb Geochronology of the Fine-grained Granite and Molybdenite?Re-Os?Dating in the Wurinitu Molybdenum Deposit, Inner Mongolia, China

The Wurinitu molybdenum deposit, located in Honggor, Sonid Left Banner of Inner Mongolia, China, is recently discovered and is considered to be associated with a concealed fine-grained granite impregnated with molybdenite.?The wall rocks are composed of Variscan porphyritic-like biotite granite and the Lower Ordovician Wubin’aobao Formation.?LA-ICP-MS zircon U-Pb dating of the fine-grained granite reveals two stages of zircons,?one were formed at 181.7±7.?4 Ma and?the other at 133.6±3.3 Ma. The latter age is believed to be the formation age of the fine-grained granite, while the former may reflect the age of inherited zircons, based on the morphological study of the zircon and regional geological setting. The Re-Os model age of molybdenite is 142.2±2.5?Ma, which is older than the diagenetic age of the fine-grained granite.?Therefore the authors believe that the metallogenic age of the Wurinitu molybdenum deposit should be?nearly 133.6±3.3 Ma or slightly later, i.e., Early Cretaceous.?Combined with regional geological background research, it is speculated that the molybdenum deposits were formed at the late Yanshanian orogenic cycle in the Hingganling-Mongolian orogenic belt, belonging to the relaxation epoch posterior to the compression and was associated with the closure of the Mongolia-Okhotsk?Sea.