土壤颗粒组成影响137Cs含量的初步实验结果

放射性核素¹³⁷Cs作为人类核试验的产物,以其独特的理化性质而成为研究土壤侵蚀和泥沙沉积一种良好的示踪源。在¹³⁷Cs的再分配过程中,土壤颗粒组成是影响土壤¹³⁷Cs的重要因子。一般认为,土壤中的¹³⁷Cs活度主要与粘粘含量有关,但这要取决于原生沉积物的颗粒组成。通过选取共和盆地主要的三种土壤类型,采集0~30cm的混合样品,进行入工筛分,测定不同粒径成分的¹³⁷Cs含量。初步实验表明,砂质土的¹³⁷Cs活度与平均粒径之间呈负指数关系,0.01~0.1mm组分对¹³⁷Cs含量的影响最为显著;壤质土的¹³⁷Cs活度主要与粗粉砂(0.01~0.05mm)含量有关,与137Cs活度主要与原生沉积物的颗粒特性和粒度有关.¹³⁷Cs一旦被原生沉积物颗粒吸附,在以后的土壤颗粒再分配过程中,是很难被置换掉的。     

Three new distal tephras in sediments spanning the Last Glacial–Interglacial Transition in Scotland

Three new microtephras are reported from a number of lake sites from the Inner Hebrides and Scottish mainland. One occurs stratigrapically in the middle of Greenland Interstadial 1 (GI‐1) and has been named the Penifiler Tephra. It is rhyolitic and possesses a geochemical signature that is very similar to that of the Borrobol Tephra, which also occurs in three of the sequences reported here, but which lies close to the lower boundary of GI‐1. The second occurs stratigraphically in the early Holocene below the Saksunarvatn Ash and is named the Ashik Tephra. This tephra is geochemically bimodal, with a rhyolitic component comparable to the An Druim Tephra that occurs later in the Holocene, and a basaltic component which is similar to the Saksunarvatn Ash. A third tephra occurs stratigraphically above the Saksunarvatn Ash and is provisionally named the Breakish Tephra. The consistent inter‐site correlation demonstrated for these new tephras at several sites enhances the regional tephrostratigraphic framework, and increases the potential for correlating palaeoenvironmental events during GI‐1 and the early Holocene. However, the occurrence of multiple tephras with similar geochemistry in close stratigraphic and temporal proximity has implications for the rigour with which tephrostratigraphic investigations must be performed. Copyright © 2006 John Wiley & Sons, Ltd.     

Late Quaternary cryptotephra detection and correlation in loess in northeastern Japan using cummingtonite geochemistry

We detected late Pleistocene cummingtonite-bearing cryptotephras in loess deposits in NE Japan and correlated them with known tephras elsewhere by using major-element compositions of the cummingtonite. This is the first time cryptotephras have been identified by analysis of a crystal phase rather than glass shards. In central NE Japan, four cummingtonite-bearing tephras, the Ichihasama pumice, the Dokusawa tephra, the Naruko–Nisaka tephra, and the Adachi–Medeshima tephra, are present in late Pleistocene loess deposits. Because the cummingtonite chemistry of each tephra is different and characteristic, it is potentially a powerful tool for detecting and identifying cryptotephras. An unidentified cummingtonite-bearing cryptotephra previously reported to be present in the late Pleistocene loess deposits at Kesennuma (Pacific coast) did not correlate with any of the known cummingtonite-bearing tephras in central NE Japan, but instead with the Numazawa–Kanayama tephra (erupted from the Numazawa caldera, southern NE Japan), although Kesennuma is well beyond the previously reported area of the distribution of the Numazawa–Kanayama tephra. Three new cummingtonite-bearing cryptotephras in the mid and late Pleistocene loess deposits (estimated to be less than 82 ka, 100–200 ka, and ca. 250 ka) on the Isawa upland were also detected.     

锡林浩特－阿巴嘎火山群内的玛珥式火山

Tephra fallout is an important type of hazard caused by explosive volcanic eruption,and numerical simulation has become a fast and effective approach to assess the dispersion and deposition of tephra fallout.According to the improved 2D diffusion model of Suzuki( 1983),we edited a tephra diffusion program that can run in the Windows system.Based on previous data,we simulated the diffusion scope of the Jinlongdingzi volcanic eruption,which is the latest eruption in the Longgang volcanic cluster.The simulated results are in good agreement with the results from measurement in situ,indicating that the model is reliable and the parameters used in the model are suitable.By using wind profiles of ten years,7,021 simulations under different wind profiles were carried out,and then probabilistic hazard maps of tephra fallout were constructed for tephra thickness thresholds,1cm and 0.5cm.This study can provide an important scientific basis for volcanic hazard analysis,risk mitigation plans and countermeasures in the Longgang volcanic area.     

Multiple approaches to date Japanese marker tephras using optical and ESR methods

The present study aimed to test reliability of luminescence and electron spin resonance (ESR) methods to date tephra. We investigated on three Japanese marker tephras, Ikeda-ko (6.4 ka), Aira-Tn (30 ka) and Aira-Iwato (45–50 ka). A systematic studies were performed using different minerals (quartz and feldspar), different grain fractions (75–250 and 250–500 μm), different luminescence and ESR signals, like optically stimulated luminescence (OSL) of quartz, infrared stimulated luminescence (IRSL) of feldspar, including recently developed least faded post infrared IRSL (pIR-IRSL), and ESR signals from paramagnetic centers Al and Ti–Li of quartz. Ages obtained using pIR-IRSL signal of plagioclase with preheat of 320 °C, 60 s and stimulation at 300 °C are consistent with the reference ages. High dose detection range (up to ∼600 Gy) and accurate age estimation enable pIR-IRSL of feldspar a promising methodology to date quaternary tephra. ESR ages from quartz are grossly correlated with the reference ages but large deviation and large associated errors are observed, possibly due to either low signal to noise ratio or heterogenous dose response of different aliquot in multiple aliquot additive dose (MAAD) approach.     

A second tephra isochron for the Younger Dryas period in northern Europe: The Abernethy Tephra

Visible and non-visible (cryptotephra) volcanic ash layers are increasingly being used to underpin the chronology and high-precision correlation of sequences dating to the last glacial–interglacial transition (LGIT). As the number of sediment records analysed for tephra content rises, and methodological developments permit the detection, extraction and chemical analysis of increasingly scantily represented glass shard concentrations, greater complexity in shard count profiles is revealed. Here we present new evidence from sites in Scotland, and review published evidence from sites elsewhere in NW Europe, that indicate complexity in the eruptive history of Katla volcano during the mid-Younger Dryas and Early Holocene. We propose evidence for a previously-overlooked tephra isochron, here named the Abernethy Tephra, which is consistently found to lie close to the Younger Dryas/Holocene transition. It has a major-element chemical composition indistinguishable from that of the Vedde Ash, which was erupted from the Katla volcano at 12,121 ± 114 cal a BP. The new data suggest that Katla may have erupted again between 11,720–11,230 cal a BP and the subsequent ash fall increases the potential to assess environmental response to Holocene warming across north and west Europe.     

Geochronological reconstruction of Cs transport from the Coruh river to the SE Black Sea: comparative assessment of radionuclide retention in the mountainous catchment area

The deposition record of ¹³⁷Cs was traced in the SE Black Sea sediments adjacent to the Coruh river mouth in comparison with the earlier studied chronology of ¹³⁷Cs deposition in front of the Danube delta (NW Black Sea). In both cases, the ¹³⁷Cs profiles showed two subsurface peaks attributable to maximum fallout of ‘bomb’ and Chernobyl radionuclides. The Coruh profile revealed a larger contribution of ‘bomb’ ¹³⁷Cs in comparison with the Chernobyl input, suggesting different coverage of NW and SE Black Sea regions with the Chernobyl fallout. The ¹³⁷Cs-derived dating showed that maximum deposition of particulate bound ¹³⁷Cs in sediments adjacent to the Coruh river mouth was delayed for 14 yr relative to date of Chernobyl accident, reflecting a buffer effect of the watershed soils. This transit time is 3 times longer than in the Danube catchment area, indicating a difference in retention processes in these mountainous (Coruh) and lowland (Danube) river basins. The ¹³⁷Cs profile in Coruh sediments showed penetration of ¹³⁷Cs to much greater depth than would be expected from ¹³⁷Cs fallout chronology, suggesting the sediment mixing rate of 1.3 cm² yr⁻¹. This value was used to evaluate deposition chronology of ¹³⁷Cs, applying the model developed for pulse fallout case. Comparing the measured and modelled data has allowed differentiation of the flood-induced discharge of the ¹³⁷Cs-containing suspended matter and the slower transit of eroded soil particles from the contaminated catchment areas. The obtained results may be used for the prediction of period when the pollutants, deposited over the river basins, can reach the Black Sea.     

The volcanic history of Ruapehu during the past 2 millennia based on the record of Tufa Trig tephras

 Tufa Trig Formation comprises a sequence of at least 19 andesitic tephras erupted from Mt. Ruapehu (Tongariro Volcanic Centre, New Zealand). Tephras of Tufa Trig Formation are the most recent eruptives from Ruapehu, dated between ca. 1850 years B.P. and the present. Members of the Formation show restricted dispersals, principally to the east of Mt. Ruapehu. Volumes calculated for the most widespread members are all less than 0.1 km³. Compared with other Mt. Ruapehu eruptives, Tufa Trig Formation tephras represent small eruptions that have contributed little tephra to the ring plain. They do, however, show a greater frequency of eruption with one event occurring on average every 100 years. Tufa Trig Formation members Tf3–Tf18 are black to dark grey, vitric, coarse-ash and lapilli-grade tephras which mantle the relief. They contain juvenile vitric particles which exhibit varying degrees of vesicularity, together with free crystals of pyroxene and feldspar, and few lithic fragments. Several morphological types of vitric pyroclasts are recognised in these tephras, the dominant type being of equant blocky morphology with fracture-bound surfaces (type-1 morphology). Field characteristics, tephra distributions, and the morphologies and textures of constituent pyroclasts suggest that these members (Tf3–Tf18) are the products of small-volume hydrovolcanic eruptions resulting from the interaction of fresh magma and meteoric water. We propose that a source of this water was an ancestral crater lake which formed within the late Holocene ca. 3000 years B.P. The morphological, compositional, and chemical (major-element) characteristics of three Tufa Trig Formation Tephras are compared with those of two new tephras erupted from Ruapehu Volcano during the October 1995 eruptions which comprise part of a newly defined member (Tf19) of Tufa Trig Formation. The comparisons support our interpretation that the majority of the Tufa Trig Formation tephras are primarily the products of hydrovolcanic eruptions. Other members of the Formation (Tf1 and Tf2) are coarse-grained scoriaceous tephras and are interpreted to be the products of strombolian events. Received: 14 September 1996 / Accepted: 6 June 1997     

龙岗火山群四海火山渣层—来自金龙顶子火山亚普林尼式火山爆发

龙岗火山群四海玄武质火山渣层为金龙顶子火山爆发形成，而非多个火山共同喷发的结果。火灾山渣组成金龙顶子火山渣锥及以东相伴随的火山碎屑席，反映火山爆发时强劲单向西风的吹指。火山碎屑席中火山渣层厚度，火山渣粒度参数远离火山渣锥呈规律性变化。