Pb-Pb age of sedimentary phosphorite reworking in Lower Riphean carbonate sediments,the Satka Formation of Southern Urals     

G. V. Ovchinnikova  A. B. Kuznetsov  I. M. Vasil’eva  I. M. Gorokhov  M. T. Krupenin  A. V. Maslov  T. L. Turchenko 《Stratigraphy and Geological Correlation》2008,16(2):138-142

The mineral composition and U-Pb and Rb-Sr systematics of phosphorites from the Satka Formation of Lower Riphean carbonates, the Burzyan Group of Southern Urals, are studied. Phosphorites occurring as small lenses between stromatolite layers are composed largely of fluorapatite with admixture of detrital quartz, feldspars, illite, and chlorite. Phosphorite samples have been subjected to stepwise dissolution in 1 N (fraction L-1) and 2 N (fraction L-2) HCl. As is established, the maximum apatite content is characteristic of fraction L-1, while fraction L-2 is enriched in products of dolomite and sulfide dissolution and in elements leached from siliciclastic components. The Sr content in the Satka apatites (280–560 ppm) is substantially lower as compared with that in unaltered marine apatite. The ⁸⁷Sr/⁸⁶Sr “initial ratio in the phosphorites studied (0.71705–0.72484) and host dolomites from the lower part of the Satka Formation is significantly higher than in the Early Riphean seawater that indicates a reset of the Rb-Sr original systems in sediments. The Pb-Pb age of 1340 ± 30 Ma (MSWD = 6.4) estimated based on 7 data points characterizing fractions L-1 and L-2 is younger than the formation time of overlying Burzyan sediments, being consistent, within the error range, with date of the Mashak rifting event recorded at the Early-Middle Riphean boundary. The comparative U-Pb characteristics of two soluble fractions (L-1 and L-2) and silicate residue of phosphorites show that epigenetic redistribution of Pb and U was characteristic of the phosphorite horizon only. The initial Pb isotope composition and μ (²³⁸U/²⁰⁴Pb) estimated according to model by Stacey and Kramers for the early diagenetic fluids in carbonate and phosphate sediments of the Satka Formation suggest that they were in isotopic equilibrium with erosion products of the Taratash crystalline complex.  相似文献   

Drought events in Southern Portugal from the 12th to the 19th centuries: integrated research from descriptive sources     

Afonso Do Ó  Maria J. Roxo 《Natural Hazards》2008,47(1):55-63

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Soil erosion susceptibility assessment and validation using a geostatistical multivariate approach: a test in Southern Sicily   总被引：2，自引：0，他引：2  

Christian Conoscenti  Cipriano Di Maggio  Edoardo Rotigliano 《Natural Hazards》2008,46(3):287-305

A certain number of studies have been carried out in recent years that aim at developing and applying a model capable of assessing water erosion of soil. Some of these have tried to quantitatively evaluate the volumes of soil loss, while others have focused their efforts on the recognition of the areas most prone to water erosion processes. This article presents the results of a research whose objective was that of evaluating water erosion susceptibility in a Sicilian watershed: the Naro river basin. A geomorphological study was carried out to recognize the water erosion landforms and define a set of parameters expressing both the intensity of hydraulic forces and the resistance of rocks/soils. The landforms were mapped and classified according to the dominant process in landsurfaces affected by diffuse or linear water erosion. A GIS layer was obtained by combining six determining factors (bedrock lithology, land use, soil texture, plan curvature, stream power index and slope-length factor) in unique conditions units. A geostatistical multivariate approach was applied by analysing the relationships between the spatial distributions of the erosion landforms and the unique condition units. Particularly, the density of eroded area for each combination of determining factors has been calculated: such function corresponds, in fact, to the conditional probability of erosion landforms to develop, under the same geoenvironmental conditions. In light of the obtained results, a general geomorphologic model for water erosion in the Naro river basin can be depicted: cultivated areas in clayey slopes, having fine-medium soil texture, are the most prone to be eroded; linear or diffuse water erosion processes dominate where the topography is favourable to a convergent or divergent runoff, respectively. For each of the two erosion process types, a susceptibility map was produced and submitted to a validation procedure based on a spatial random partition strategy. Both the success of the validation procedure of the susceptibility models and the geomorphological coherence of the relationships between factors and process that such models suggest, confirm the reliability of the method and the goodness of the predictions.  相似文献   

藏南地区中生代硅质岩的地球化学特征及其成因意义   总被引：6，自引：1，他引：5  

周永章  付伟  杨志军  何俊国  聂凤军  李文  赵文霞 《岩石学报》2008,24(3)

藏南地区中生代硅质岩包括蛇绿岩套硅质岩(与蛇绿岩共生)和非蛇绿岩套硅质岩两大类.本文重点分析日喀则地区彭错林、夏鲁以及泽当地区的罗布莎、江孜盆地宗卓组及四个剖面的硅质岩.其中,彭错林、夏鲁和罗布莎硅质岩与蛇绿岩共生,江孜盆地宗卓组为非蛇绿岩套硅质岩.分析表明:(1)藏南地区硅质岩剖面地球化学特征鲜明,具有一致性和多样性特点;(2)与蛇绿岩共生的彭错林、夏鲁、罗布莎硅质岩普遍具有高Si、高Fe、低Al特征,大部分微量元素相对于克拉克值亏损,稀土元素总量低,经北美页岩标准化后,Ce异常明显或不明显,重稀土相对轻稀土富集.硅质泥岩的∑REE要明显高于硅质岩;(3)非蛇绿岩套宗卓组硅质岩SiO2含量稍低,Al2O3、TiO2则相反.V、Th、Hf、Ta等不相容元素上亏损程度较小,部分样品含量可接近克拉克值.稀土总量相对较高,页岩标准化配分模式上体现为弱Ce正异常,负Eu异常,轻重稀土分异不明显的平坦型曲线图;(4)地球化学特征指示了,藏南地区硅质岩多数具有明显的热水沉积成因属性,同时有正常陆源组分的加入.其中,夏鲁硅质岩的热水沉积地球化学特征较为典型,而宗卓组硅质岩则表现出受陆源物质加入的影响显著的地球化学特点.  相似文献   

新疆塔里木北缘色日牙克依拉克一带泥盆纪花岗质侵入体的确定及其地质意义   总被引：15，自引：7，他引：8  

朱志新  李锦轶  董连慧  王克卓  刘国忠  李亚萍  刘振涛 《岩石学报》2008,24(5)

位于新疆南天山色日牙克依拉克一带发育较多的中酸性侵入岩,前人均划为石炭纪侵入岩,作者对其中花岗闪长岩锆石进行SHRIMP定年,获得侵入岩形成年龄387±8Ma,另有一组捕获锆石的年龄为418.4±6.5Ma,通过对这些侵入岩的岩石学研究,结果显示该区泥盆纪侵入岩均具较典型活动陆缘侵入岩的特征.结合区域岩浆活动资料及前人成果,初步确定在塔里木北缘木札尔特、色日牙克依拉克、虎拉山南缘野云沟至库尔勒一带在古生代为一活动陆缘.  相似文献   

拉萨地体东南缘的多期深熔作用及动力学   总被引：13，自引：7，他引：6  

王金丽  张泽明  石超 《岩石学报》2008,24(7)

位于青藏高原班公湖-怒江蛇绿岩带与印度-雅鲁藏布江蛇绿岩带之间拉萨地体东南缘的前寒武纪结晶基底经历了角闪岩相-麻粒岩相区域变质作用和强烈的混合岩化.研究区可识别出两期深熔作用,第一期为斜长角闪岩部分熔融形成的花岗闪长岩脉体,其与围岩一起发生了变质与变形再造,转变成了黑云斜长片麻岩.第二期为含夕线石石榴石片麻岩部分熔融形成的含石榴石斜长石花岗岩脉体.岩石化学成分显示,第一期深熔脉体具有埃达克岩的典型地球化学特征,其SiO2=63.81%,Al2O3=16,92%,MgO=1.86%,Na2O=4.22%,K2O=1.81%,K2O/Na2O=0.43,并显示出LREE富集、无Eu异常的BEE配分模型,同时明显富Sr(366×10-6),贫Y(12×10-6)和Yb(0.4×10-6),Sr/Y=30.舍石榴石斜长石花岗岩主要由斜长石、石英和石榴石组成,含少量白云母和黑云母,其全岩SiO2=71.14%,Al2O3=14.26%,K2O=0.26%,Na2O=2.79%.CaO=2.94%.过铝指数A/CNK=1.40,为典型的过铝花岗岩.黑云斜长片麻岩脉体中的大多数锆石具有同心韵律状结晶环带和较高的Th//U比值(0.433～1.167),为典型的岩浆结晶锆石.锆石岩浆结晶域U-Pb原位定年给出了64.0±1.0Ma(MSWD=8.7)加权平均年龄;个别锆石变质交生边给出了27.9 Ma的谐和年龄,同时具有较低的Th/U比值(0.019),应代表后期叠加的变质热事件年龄.含石榴石斜长石花岗岩中的锆石均发育同心韵律状环带,而且具有较高的Th/U比值(0.196～1.212).所获得的谐和年龄在27.0～34.1Ma之间(加权平均年龄为29.3±0.9 Ma),应代表过铝花岗岩的结晶年龄.因此,我们认为拉萨地体东南缘变质基底在古近纪经历了两期深熔作用,第一期发生在约65Ma,在特提斯洋壳俯冲和印度板块与拉萨地体碰撞的动力学体制下,拉萨地体下地壳加厚和升温,发生了麻粒岩相变质和部分熔融,形成了埃达克岩质的花岗闪长岩浆;第二期混合岩化作用发生在约30Ma,在印度板块与拉萨地体碰撞后伸展的动力学体制下,高角闪岩相泥质变质岩中的含水矿物脱水熔融形成了过铝质花岗闪长岩浆.  相似文献   

Using the Mellor–Yamada mixing scheme in seasonally ice-covered seas—Corrigendum to: Parameterization of vertical mixing in the Weddell Sea [Ocean Modelling 6 (2004) 83–100]     

Ralph Timmermann  Martin Losch 《Ocean Modelling》2005,10(3-4):369-372

A coding error in the s-Coordinate Primitive Equation Model (SPEM) has led to misleading statements about the behaviour of the Mellor–Yamada level 2 parameterization of vertical mixing. It has been claimed that the scheme removes static instability only very slowly and preserves statically unstable stratifications for an unrealistic long time. This note corrects this statement by demonstrating that the Mellor–Yamada mixing scheme, if implemented correctly, tends to overestimate rather than underestimate vertical mixing in seasonally ice-covered seas. Similar to other mixing schemes with the same behaviour, this leads to spurious open ocean deep convection, an unrealistic homogenization of the water column, and a significant reduction of sea ice volume.  相似文献   

Mechanisms controlling the temporal variation of the sea ice edge in the Southern Ocean     

Noriaki Kimura 《Journal of Oceanography》2007,63(4):685-694

This paper examines the mechanism controlling the short time-scale variation of sea ice cover over the Southern Ocean. Sea ice concentration and ice velocity datasets derived from images of the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) are employed to reveal this mechanism. The contribution of both dynamic and thermodynamic processes to the change in ice edge location is examined by comparing the meridional velocity of ice edge displacement and sea ice drift. In the winter expansion phase, the thermodynamic process of new ice production off the ice edge plays an important role in daily advances of ice cover, whereas daily retreats are mostly due to southward ice drift. On the other hand, both advance and retreat of ice edges in the spring contraction phase are mostly caused by the dynamic process of the ice drift. Based on the above mechanism and the linear relation between the degree of ice production at the ice edge and northward wind speed, the seasonal advance of ice cover can be roughly reproduced using the meridional velocity of ice drift at the ice edge.  相似文献   

Nitrogen forms and decomposition of organic carbon in the southern Bohai Sea core sediments   总被引：5，自引：0，他引：5  

Song Jinming  Ma Hongbo  L&#; Xiaoxia 《海洋学报(英文版)》2002,21(1):125-133

Study on form characteristics of nitrogen in marine sediments is the primary method to research its biogeochemical cycling and nitrogen form characteristics in core sediments can reflect the process and results of early diagenesis in a certain degree. In this paper, Sequential extraction process in natural grain size was used for studying the existent forms of nitrogen in five core sediments of the southern Bohai Sea for the first time. Nitrogen was divided into two parts -transferable and fixed based on whether it could be extracted by the reagent. Distributions and early diagenesis of transferable nitrogen forms in the southern Bohai Sea were researched integratedly. Results indicate that IEF - N and OSF-N are predominant forms in transferable part in the studied core sediments. Contents of different nitrogen forms vary differently with depth, and have different diagenesis process. Decomposition constant of organic nitrogen (ON) and OC are about 15.51 × 10~(-3)a~(-1)and 4.79× 10~(-3)a~(-1) respective  相似文献   

南大洋普里兹湾基于234Th/238U不平衡法的POC输出通量研究   总被引：2，自引：0，他引：2       下载免费PDF全文

何建华  马豪  陈立奇  项宝强  曾宪章  尹明端  曾文义 《海洋学报》2007,29(4):69-76

中国第22次南极科学考察(2005年11月至2006年3月)期间,测定了南极普里兹湾海域5个站位的从表层至150 m水深的不同层位水样中溶解态和颗粒态234Th,238U的放射性比活度以及颗粒有机碳.利用234Th/238U在上层水体中的不平衡,计算了南极普里兹湾上层水体中234Th的平均停留时间和输出通量.结果显示,随着纬度的增加,上层水体中颗粒态和溶解态234Th的平均停留时间总体趋向减小,并在中纬度站位出现了最低值,分别为1~8和29~48 d,而颗粒态和溶解态234Th的输出通量则在中纬度站位出现了最大值,分别为21~38和26~39 dpm/(m3·d).运用箱型清除模式,利用两种不同的方法估算了各水柱中从真光层底部输出的POC通量,平均值分别达到104.7 mmol/(m2·d)(E法)和120.6 mmol/(m2·d)(B法),表明南极普里兹湾夏季存在很高的新生产力,它将会对该海域碳的生物泵过程产生重要作用.  相似文献