Occurrence of varved lake sediment sequences in Varmland, west central Sweden: lake characteristics, varve chronology and AMS radiocarbon dating

Varved lake sediments can be used to set multiple environmental proxies within a calendar year time scale. We undertook a systematic survey of lakes in the Province of Värmland, west central Sweden, with the aim of finding continuous varved lake sediment sequences covering the majority of the Holocene. In Fennoscandia, such sediments have previously only been recorded in northern Sweden and in southern and central Finland. By following a selective process and fieldwork we discovered three new varved sites (i.e. Furskogstjärnet, Mötterudstjärnet and Kälksjön). We found that lakes with varved sediments have several common lake morphometry properties and lake catchment characteristics such as maximum water depth, maximum water depth/lake surface area ratio, catchment soil types, altitude and number of inflows. Varve chronologies, supported by AMS-¹⁴C dating and tephrochronology were established for two of the sediment profiles. These varve chronologies are the longest geological records with an annual resolution known to exist in Sweden. In Furskogstjärnet, the AMS- ¹⁴C dates based on terrestrial plant macrofossils at several levels deviate significantly from the varve based time-depth curve. In Motterudstjarnet, a fully reasonable time-depth model based on the ¹⁴C dates gives older ages in the lower part of the sequence compared to the varve chronology. These results highlight that seemingly acceptable AMS radiocarbon dates may be erroneous. They also point to the fact that varved lake sediments are reliable geological archives with respect to chronological control and accuracy. Thus, these archives should be of prime interest for studies of climate and environmental change undertaken with the aim of providing sub-decadal resolution proxy data sets.     

Radiocarbon wiggle matching of Swedish lake varves reveals asynchronous climate changes around the 8.2 kyr cold event

Snowball, I., Muscheler, R., Zillén, L., Sandgren, P., Stanton, T. & Ljung, K. 2010: Radiocarbon wiggle matching of Swedish lake varves reveals asynchronous climate changes around the 8.2 kyr cold event. Boreas, Vol. 39, pp. 720–733. 10.1111/j.1502‐3885.2010.00167.x. ISSN 0300‐9483. A series of nine radiocarbon measurements obtained at increments of 50 years was used to wiggle match varved sediments in Lake Kälksjön (west central Sweden) to the calendar year time scale provided by the tree‐ring‐derived radiocarbon calibration curve. The anchor point for a series of 400 varves known to span the ‘8.2 kyr cold event’ was derived from a combination of correlation analysis, Monte Carlo statistics and Bayesian age modelling. The GRIP and NorthGRIP ice‐cores were matched to the same absolute time scale by comparing ¹⁰Be data and tree‐ring Δ¹⁴C. The radiocarbon‐based wiggle match, organic carbon measurements, mineral magnetic parameters and XRF data reconstruct a distinct period of enhanced erosion in Kälksjön's catchment between 8066 ± 25 and 7920 ± 25 cal. yr BP. Prior to human impact, the increased erosion in the boreal environment is assigned to increased winter precipitation in the form of snow, which caused spring meltwater discharge to intensify. Our results suggest that the abrupt onset of increased winter precipitation in west central Sweden started at least 50 years after the onset of the ‘8.2 kyr cold event’ as defined by oxygen isotope data from Greenland. The study highlights the value of synchronized time scales in the reconstruction of abrupt climate changes based on proxy data.     

象牙海岸Issia地区铌钽砂矿的研究(英文)

在西非的象牙海岸Issia地区分布着许多铌钽矿床,包括原生矿和砂矿。原生矿产在分带很好的伟晶岩中。这个地区发育着许多花岗岩和花岗伟晶岩。伟晶岩可分为:(1)分异不好的伟晶岩,无矿化;(2)含Be伟晶岩,分带较好,见绿柱石;(3)含Be-Nb-Ta的伟晶岩,分带很好,是矿体;(4)Be-Nb-Ta-Li伟晶岩,分带很好,是矿体。除了10多个原生矿外,还有许多砂矿。砂矿有两种类型:一种近原生矿的坡积砂矿,另一种是经河流冲刷搬运后的冲积砂矿。我们选择了Etienne-Meguhe砂矿作为近原生矿的坡积砂矿的代表,Bemadi矿作为河流冲积型的砂矿的代表,对这两种类型的砂矿的风化、搬运、沉积机理进行了比较研究。研究发现不同的搬运距离和搬运模式对铌钽砂矿的主要矿物——铌钽铁矿的化学性质没有什么影响,而其物理形态则发生了变化。坡积型砂矿的铌钽铁矿常与石英、长石等矿物连生,其颗粒大小与原生的Nb-Ta伟晶岩中的铌钽铁矿一致;经河流冲积而形成的砂矿中,铌钽铁矿颗粒变小、轮廓变得圆滑,几乎无连生矿物,但矿体集中,常在河流的冲积扇中分布,易开采。     

An experimental set-up for carbon isotopic analysis of atmospheric CO 2 and an example of ecosystem response during solar eclipse 2010

We present here, an experimental set-up developed for the first time in India for the determination of mixing ratio and carbon isotopic ratio of air-CO₂. The set-up includes traps for collection and extraction of CO₂ from air samples using cryogenic procedures, followed by the measurement of CO₂ mixing ratio using an MKS Baratron gauge and analysis of isotopic ratios using the dual inlet peripheral of a high sensitivity isotope ratio mass spectrometer (IRMS) MAT 253. The internal reproducibility (precision) for the δ ¹³C measurement is established based on repeat analyses of CO₂±0.03‰. The set-up is calibrated with international carbonate and air-CO₂ standards. An in-house air-CO₂ mixture, ‘OASIS AIRMIX’ is prepared mixing CO₂ from a high purity cylinder with O₂ and N₂ and an aliquot of this mixture is routinely analyzed together with the air samples. The external reproducibility for the measurement of the CO₂ mixing ratio and carbon isotopic ratios are ±7 (n?=?169) $\upmu $ mol·mol^???1 and ±0.05 (n?=?169) ‰ based on the mean of the difference between two aliquots of reference air mixture analyzed during daily operation carried out during November 2009–December 2011. The correction due to the isobaric interference of N₂O on air-CO₂ samples is determined separately by analyzing mixture of CO₂ (of known isotopic composition) and N₂O in varying proportions. A +0.2‰ correction in the δ ¹³C value for a N₂O concentration of 329 ppb is determined. As an application, we present results from an experiment conducted during solar eclipse of 2010. The isotopic ratio in CO₂ and the carbon dioxide mixing ratio in the air samples collected during the event are different from neighbouring samples, suggesting the role of atmospheric inversion in trapping the emitted CO₂ from the urban atmosphere during the eclipse.     

Organic Geochemistry of Sedimentary Rock-hosted Disseminated Gold Deposits in Southwestern Guizhou Province, China

Sedimentary rock-hosted disseminated gold (SRHDG) deposits in the Youjiang-Nanpanjiang Basin,southwestem Guizhou Province are commonly hosted by the same fold crests that commonly contain a remarkable amount of organic material. The total organic carbon (TOC) contents of the ores and host rocks are usually less than 1%. The reflectance of vitrinite and pyrobitumen in the ores and the host rocks ranges from 1.5% to 4.5%, often in the range of 2% to 3%. In the Lannigou deposit, the reflectance of vitrinite and pyrobitumen in the ores is usually somewhat higher than those within the host rocks, indicating a hydrothermal impact on the organic matter in the altered host rocks. On the contrary, the estimated maximum paleotemperatures of the Getang and Zimudang deposits are higher than the homogenization temperatures of the fluid inclusions in the ores, signifying that the organic matter maturation predated Au mineralization. No correlation between the organic matter contents and Au concentrations were recognized in the ores.However, the most striking observation is that there is a positive correlation between the $2 (a parameter of Rock-Eval analysis), Au and As contents of the ores in the Lannigou deposit. Organic matter maturation and migration is apparent from the TOC vs. HCI diagram. Furthermore, group analysis of the dichloromethane extractable organic component of the ores and host rocks shows that the maturation degree of the organic matter in the ores is slightly higher than that of the host rocks in the Lannigou gold deposit. However, the compositions of their alkanes, steranes and terpenes, which serve as biomarkers, are quite similar; this suggests that the organic matter found in the ores and host rocks has a common marine source. Organic matter probably contributed to the preconcentration of Au in the host rocks. Hydrocarbons in the system,on the other hand, clearly contributed to the emplacement of the gold mineralization through thermal sulfate reduction.Organic matter in the solution might have increased the potential of the hydrothermal solution to transport Au.     

Metallogeny of the Lannigou Sedimentary Rock-hosted Disseminated Gold Deposit in Southwestern Guizhou Province, China

The Lannigou deposit is a large-sized sedimentary rock-hosted disseminated gold （SRHDG） deposit located in the Youjiang Basin. It is hosted by the Middle Triassic turbidite. Wall rock alterations, including silicification, pyritization, arsenopyritization, carbonatization and argillization, commonly occur along fractures. PGE study demonstrates that either Permian basalts or Triassic ultrabasic intrnsives are unlikely to be the main source of gold mineralization. Coupled with the lack of other nmgmatic activity in the vicinity of the mining area, an amagmatic origin is proposed. Organic matter compositions and GC-MS analysis of the ores and host rocks show that the organics in the ores and the host rocks have a common source; the organic matter in the ores was mainly indigenous. The positive correlation between S2 and Au contents, along with the common occurrence of organic inclusions, suggest involvement of organic matter in the ore-forming process in terms of promoting Au leaching from the source rocks, making colloidal Au migration possible, as well as hydrocarbon reduction of sulphate. Geological and geochemical characteristics of the Lannigou deposit suggest that it was formed through circulation of meteoric water and probably less importantly organic bearing formation water driven by high geothermal gradient caused by late Yanshanian extension, which leached Au from the source bed, and then migrated as Au-bisnlfides and colloidal Au, culminating in deposition by reduction-adsorption and surface complexation of gold onto the growth surface of arsenlan pyrite.     

粤西古水剖面震旦系顶部层状硅岩的热水成因属性:岩石学和地球化学证据

古水剖面震旦系顶部硅岩主要发育层状构造、纹理状构造、块状构造和假角砾状构造,化学成份以ＴｉＯ_２、Ａｌ_２Ｏ_３和大部分微量元素亏损（相对地壳克拉克值）为特征。但Ｂａ、Ａｓ和Ｓｂ等少数已知的热水沉积特征元素显示富集。在Ａｌ－Ｆｅ－Ｍｎ三元图上,古水硅岩投映在“热水沉积”域内。因子分析和对应分析显示了大部分金属元素在代表热水淋滤作用的因子上均有反映。稀土元素以低含量为特征,在对北美页岩组合样（ＮＡＳＣ）正规化后的稀土配分模式落在典型热水沉积的上、下限之间,而在正常沉积的之外。各种岩性和地球化学证据表明,古水硅岩具有热水成因。     

Hydrothermal origin of Late Proterozoic bedded chert at Gusui, Guangdong, China: petrological and geochemical evidence

The Late Proterozoic bedded chert from Gusui in Guangdong Province, southern China, is characterized by bedded, laminated, massive and pseudobrecciated structures. The chert is depleted in TiO₂, Al₂O₃ and most trace elements. However, it is enriched in Ba, As, Sb, Hg, and Se. In Al-Fe-Mn ternary diagrams, it falls into the ‘hydrothermal field’. Factor analysis shows that many elements show up in the principal trace element factor suggesting their enrichment results from leaching of the country rock by hydrothermal solutions. The chert has low REE concentrations and displays an REE pattern consistent with a hydrothermal origin. It may have formed in a Late Proterozoic rift or extension zone developed within the Yunkai continental margin back-arc basin, with a fault system linking it to an unknown heat source at depth.     

黔西南微细粒浸染型金矿床有机地球化学研究

黔西南微细粒浸染型金矿床赋矿地层中常含有一定量的有机质。矿石和赋矿岩石有机碳含量可达 1%左右。其中镜质组和热变沥青的反射率的变化范围为 1.5 %～ 4 .5 % ,但多在 2 %～ 3%之间。烂泥沟金矿矿石中的镜质体和热变沥青的反射率一般略大于赋矿围岩 ,表明成矿热液对赋矿地层