陕西关中盆地大气降水同位素研究

大气降水作为地下水的主要补给源之一,它是自然界水循环过程中的一个重要环节.查明大气降水中同位素分布特征及其影响因素,不仅有助于定性或定量地研究地下水的起源和形成等问题,更有助于揭示"三水"转化关系.根据全球大气降水同位素观测网 1985～1993 年大气降水氢氧同位素资料并结合所采集样品的分析结果对陕西关中盆地大气降水环境同位素特征、变化规律及其影响因素进行了探讨,建立了西安地区雨水线,为地下水的起源及运移研究奠定了基础.     

干酪根化学降解产物的碳同位素组成研究

采用一系列温和的化学降解法对松辽盆地南部嫩江组烃源岩的干酪根进行连续的选择性化学降解，并对不同降解产物进行色谱—同位素比值质谱分析。研究表明碱性水解和脱硫产物以一元脂肪酸和正构烷烃为主，其中正构烷烃碳同位素分布曲线呈现一定的“负倾”（即随碳数增加呈现逐渐贫13C）趋势；一元脂肪酸以C16和C18为主，具有明显的偶碳优势，与同碳数的正构烷烃具有相似的碳同位素组成。氧化产物则以一元脂肪酸和α,ω 二元脂肪酸为主，一元脂肪酸低碳数部分呈现“负倾”趋势，高碳数部分则呈现“正倾”的趋势；α,ω 二元脂肪酸与低碳数的一元脂肪酸具有相似的分布，表明它们可能具有相同的母源。对比研究表明JL 30抽提物中正构烷烃可能存在混源的影响，其碳同位素组成是多源混合的结果。     

Kirchhoff积分法叠前时间偏移技术在三江盆地XDLZ地区的应用

三江盆地前进坳陷XDLZ地区构造较复杂，以往开展的二维叠后时间偏移成像精度低和空间位置不够准确，为此进行了二维叠前时间偏移处理研究，分析了Kirchhoff积分法叠前时间偏移处理中关键环节技术参数（叠前去噪、振幅补偿、反褶积、静校正、均方根速度建模和偏移孔径选取）对研究区复杂构造成像的影响及处理技巧。通过叠前时间偏移和叠后时间偏移在该区的应用效果对比分析，Kirchhoff积分法叠前时间偏移处理结果包含地震信息更加丰富，深层复杂构造成像在同相轴的连续性和断层及断点空间位置更趋合理，成像相位和振幅误差较小，构造成图精度提高了约4％。     

塔里木盆地于奇地区三叠纪孢粉组合

对新疆塔里木盆地于奇地区钻孔中三叠纪泥岩孢粉的研究,以优势属建立了3个孢粉组合。下部Limatulasporites-Lundbladispora-Taeniaesporites组合产于柯吐尔组,蕨类植物孢子占优势成分,以具纹饰的三缝孢子及腔状三缝孢子为主,裸子植物花粉以具肋双气囊花粉为主,地质时代为早三叠世;中部Punctatisporites-Aratrisporites组合产于阿克库勒组,蕨类植物孢子以光面圆形三缝孢子和单缝孢类为优势成分,裸子植物花粉以无肋双气囊花粉为主,地质时代为中三叠世;上部Dictyophyllidites-Cyclogranisporites-Alisporites组合产于哈拉哈塘组,蕨类植物孢子常见光面三角形三缝孢子和具纹饰的三缝孢子,裸子植物花粉以无肋双气囊花粉为主,地质时代为晚三叠世。     

Inverse groundwater modelling in the Willunga Basin,South Australia

A new inverse technique for modelling groundwater flow, based on a functional minimization technique, has been used to calibrate a groundwater flow model of a subregion of the Port Willunga aquifer within the Willunga Basin in South Australia. The Willunga Basin is the location of extensive viticulture, irrigated primarily by groundwater, the levels and quality of which have declined significantly over the last 40 years. The new method is able to generate estimates of transmissivity, storativity and groundwater recharge over the whole subregion as a time-varying continuous surface; previous methods estimate local discrete parameter values at specific times. The new method has also been shown to produce accurate head values for the subregion and very good estimates of groundwater recharge. Its ultimate goal will be to provide a new and invaluable tool for significantly improved groundwater resource management. Supported in part by US National Science Foundation grants, DMS-0107492 and DMS-0079478.     

The geochemical study of fluvio-lacustrine aquifers in the Kathmandu Basin (Nepal) and the implications for the mobilization of arsenic

The geochemical analyses of fluvial-lacustrine aquifer sediments of the Kathmandu Valley have been made as a step in assessing the environment for the mobilization of arsenic in groundwater. Elements measured by X-ray fluorescence (XRF) include 4 major oxides (Fe₂O₃, TiO₂, CaO, P₂O₅) and 14 trace elements (As, Pb, Zn, Cu, Ni, Cr, V, Sr, Y, Nb, Zr, Th, Sc and TS). Elution tests of 15 selected core samples were also carried out to determine the potential leaching of arsenic from the sediments. The XRF results show that average bulk concentrations of the major oxides and trace elements are similar to modern unconsolidated sediments and average upper continental crust. However, the abundance of elements varies with grain size, with higher concentrations in finer-size grades. Variations in elemental abundances within the basin are strongly controlled by sediment facies. The elution tests show that greater amounts of arsenic are generally eluted from the fine sediments, although the rates are variable. The results overall suggest that As concentrations in the bulk sediments are not a controlling factor for elevated As in the Kathmandu Basin groundwater, and the roles of other factors such as redox conditions and organic matter contents are likely to be more significant.     

Determining magma flow in sills,dykes and laccoliths and their implications for sill emplacement mechanisms

Three-dimensional seismic data from the Faeroe-Shetland Basin provides detailed information on the relationships between sills, dykes, laccoliths and contemporaneous volcanic activity. The data shows that sills are predominantly concave upwards, being complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat outer rim by a steeply inclined sheet. Such morphologies are only partially modified by pre-existing faults. Sills can be sourced from dykes or the steep climbing portions of deeper sills. Both sills and dykes can provide magma to overlying volcanic fissures and sills can be shown to feed shallow laccoliths. Magma flow patterns, as revealed by opacity rendering, suggest that sills propagate upwards and outwards away from the magma feeder. As an individual sill can consist of several leaves emplaced at different stratigraphic levels, and as a sill or dyke can provide magma to volcanic fissures, other sills and laccoliths, the data suggests that neutral buoyancy concepts may not provide a complete explanation for the mechanism and level of sill emplacement. Instead, the data suggests that the presence of lithological contrasts, particularly ductile horizons such as overpressured shales may permit sill formation at any level below the neutrally buoyant level. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Ken Thomson–deceased, April 2007     

The impact of hurricanes on sedimenting particulate matter in the semi-arid Bahía de La Paz,Gulf of California

From 2002 through 2004, time-series sediment trap samples were collected from a depth of 410 m in Cuenca Alfonso, Bahía de La Paz, on the SW coast of the Gulf of California. The instrument recorded the impact of the local passage of hurricanes “Ignacio” (24–26 August) and “Marty” (21–23 September) in 2003. These two events accounted for 82% of the total rainfall measured in 2003, equivalent to the annual average precipitation in years without hurricanes. Mean total mass fluxes (TMFs) of 2.88 and 3.58 g m⁻² d⁻¹ were measured during the week of each hurricane as well as the following week. This may have been enough to produce a lamina in the underlying sediment with characteristics peculiar to such events. The terrigenous component was particularly abundant, with notably higher concentrations of Fe, Sc, Co and Cs and REEs. In contrast, TMFs throughout 2002–2004 (excluding the hurricane periods) averaged only 0.73 g m⁻² d⁻¹ and had a larger marine biogenic component. The extraordinary elemental fluxes during the 29 days of hurricane-influenced sedimentation represented a great proportion of the totals over an entire “normal” year: Co (67.8%) >Sc (62.6) >Fe (59.6) >Cs (53.4)>Lu (51.5)>La (51.3)>Yb (51.0)>Ce (49.5) >Tb (48.4) >Sm (44.7)>Cr (36.5) >Ca (31.0)>Eu (25.4%). The terrigenous fraction was calculated using (a) TMF minus the sum of CaCO₃, biogenic silica and organic matter and (b) the ratio of Sc in the trap samples to the average in the Earth's crust. The latter was consistently smaller, but the two methods offered similar results following hurricanes (78% vs. 63%, respectively). For normal sedimentation, however, the difference method yielded values twice as large as the Sc method (58% vs. 30%) This suggests that the mineralogy of the terrigenous fraction may also vary, with unsorted dessert soil being carried to sea by the powerful flash floods associated with hurricanes. Eolian supply of particles, particularly Sc-free quartz grains, possibly from beyond the limited fluvial drainage basin, apparently dominates normal sedimentation.     

Contemporary versus long‐term denudation along a passive plate margin: the role of extreme events

Short‐term (contemporary) and long‐term denudation rates were determined for the Blue Mountains Plateau in the western Sydney Basin, Australia, to explore the role of extreme events (wildfires and catastrophic floods) in landscape denudation along a passive plate margin. Contemporary denudation rates were reconstructed using 40 years of river sediment load data from the Nattai catchment in the south‐west of the basin, combined with an analysis of hillslope erosion following recent wildfires. Long‐term denudation rates (10 kyr–10 Myr) were determined from terrestrial cosmogenic nuclides, apatite fission track thermochronology and post‐basalt flow valley incision. Contemporary denudation rates average several times lower than the long‐term average (5·5 ± 4 mm kyr^?1 versus 21·5 ± 7 mm kyr^?1). Erosion of sediment following wildfires accounts for only a small proportion (5%) of the contemporary rate. Most post‐fire sediment is stored on the lower slopes and valley floor, with the amount transported to the river network dependent on rainfall–run‐off conditions within the first few years following the fire. Historical catastrophic floods account for a much larger proportion (35%) of the contemporary erosion rate, and highlight the importance of these events in reworking stored material. Evidence for palaeofloods much larger than those experienced over the past 200 years suggests even greater sediment export potential. Mass movement on hillslopes along valleys incised into softer lithology appears to be a dominant erosion process that supplies substantial volumes of material to the valley floor. It is possible that a combination of infrequent mass movement events and high fluvial discharge could account for a significant proportion of the discrepancy between the contemporary and long‐term denudation rates. Copyright © 2006 John Wiley & Sons, Ltd.     

Origin of gray-green sandstone in ore bed of sandstone type uranium deposit in north Ordos Basin

Dongsheng sandstone-type uranium deposit is located in the northern part of Ordos Basin, occurring in the transitional zones between gray-green and gray sandstones of Jurassic Zhiluo Formation. Sandstones in oxidized zone of the ore bed look gray-green, being of unique signature and different from one of ordinary inter-layered oxidation zone of sandstone-type uranium deposits. The character and origin of gray-green sandstones are systematically studied through their petrology, mineralogy and geochemistry. It is pointed out that this color of sandstones is originated from secondary oil-gas reduction processes after paleo-oxidation, being due to acicular-leaf chlorite covering surfaces of the sandstone grains. To find out the origin of gray-green sandstone and recognize paleo-oxidation zones in the ore bed are of not only theoretical significance for understanding metallogenesis of this kind of sandstone-type uranium deposit, but also very importantly practical significance for prospecting for similar kind of sandstone-type uranium deposit.