植硅体14C测年研究:过去、现在与未来

文章回顾了植硅体14C测年发展的历史,对当前其研究的现状进行了全面的总结和评述,对近年来其研究的争议进行了梳理。植硅体测年研究中存在的问题(如污染和老碳效应等)在其他材料测年过程同样存在,不能基于少量现代植物植硅体测年的异常数据而完全否定其意义。大量的测年数据显示,并不是所有的植硅体年代都偏老,相反地层中的植硅体年代与同层位其他材料的年代重合或接近。因此,在一些缺少其他有机质测年材料的地层,植硅体14C测年仍然是一种极具潜力的年代学方法,在构建一些重要环境和考古文化时间标尺上将会发挥更重要的作用。     

北京房山岩体的磁组构特征及其 对岩体侵位的约束

岩浆岩的磁组构与岩浆流动、侵位时应力和冷凝后遭受后期构造改造作用有关.本文分析了北京房山岩体东山口-凤凰亭剖面岩石磁化率各向异性(AMS)变化.热磁曲线和磁滞回线分析显示,岩石的主要载磁矿物为多畴磁铁矿颗粒.AMS磁化率椭球体呈压扁状,磁面理发育且产状陡倾,磁化率各向异性度P值平均值高达1.189.自边缘相(东山口)到中心相(凤凰亭),岩石磁化率椭球的扁率E值和形状因子T值逐渐减小,说明磁组构主要反映岩浆流动和岩体侵位时近NNE-SSW方向的挤压作用,在侵位冷凝后并没有遭受到明显的构造改造.     

Test of AMS 14C dating of pollen concentrates using tephrochronology

Previous attempts to radiocarbon date sediments >10 kyr from the high rainfall region of Westland, New Zealand, a critical location for investigation of interhemispheric patterns of climate change, have been problematic. This study, building on recent work by Vandergoes and Prior ( 2003 ), shows that AMS ¹⁴C dating of pollen concentrates has potential to provide more reliable ages than other sediment constituents, including plant macrofossils. The method was applied to sediments from three sites containing the 22.6k ¹⁴C yr Kawakawa Tephra, which provided an independent test of the ¹⁴C ages. Although some minor laboratory contamination was detected in tests on background standards, the modelled relationship between sample mass and measured ¹⁴C content permitted an appropriate correction to be determined. Improved pollen concentrations derived by density separation between 1.4 and 1.2 specific gravity and sieving in the range 10–50 μm provided either older ages than other fractions of the same sample or, where in situ contamination was not evident, equivalent ages. Differences in degree of in situ contamination between depositional environments indicated that, in Westland, lake sites may be less susceptible to contamination by younger carbon than peat sites, where this process may be facilitated by root penetration into underlying sediments. Copyright © 2006 John Wiley & Sons, Ltd.     

Magnetic fabric and modeled strain distribution in the head of a nested granite diapir,the Melechov pluton,Bohemian Massif

The Melechov pluton, Bohemian Massif, is interpreted as a mid-crustal nested granitic diapir with an apical part exposed at the present-day erosion level. The diapir head exhibits a concentric structure defined by lithologic zoning and by the anisotropy of magnetic susceptibility (AMS). In concert with theoretical models, outward-dipping margin-parallel magnetic foliations are associated with oblate shapes of the susceptibility ellipsoids and higher degree of anisotropy, passing inward into weaker triaxial to prolate fabric. By contrast, magnetic fabric in an inner granite unit is in places oriented at a high angle to internal contacts and is interpreted as recording an internal diapir circulation. We use inverse modeling to calculate strain variations across the diapir from the AMS data. The magnetic fabric parameters and calculated strains are in agreement with strain distribution in heads of model Newtonian diapirs traveling a distance of two body radii and suggest granitic magma ascent as a crystal-poor suspension followed by crystallization of fabric markers and their response to strain near the final emplacement level. The intrusive fabric thus formed late but, though generally weak, was still capable of recording incremental strain gradient in the granite diapir.     

AMS ~(14)C测年揭示的渤海湾沿海低地第Ⅱ海相层年龄

在渤海湾沿海低地地区获得4个机械钻孔,结合AMS14C测年方法,对Ⅱ海的年龄进行了研究。结果显示,在Ⅱ海所在层位获得的14C样品年龄均大于43.5ka BP,说明渤海湾沿海低地地区Ⅱ海形成于MIS 3早期或MIS 5,而不是前人认为的23~39ka。建议在前人地层结构研究成果的基础上,系统开展Ⅱ海年龄研究。     

磁化率各向异性的原理及应用实例

磁化率各向异性(AMS)在地质领域中的应用极为广泛,可以用来研究古流向造成的磁性矿物的定向排列,以及构造应力作用引起的岩石内磁性矿物的定向重结晶、定向排列及韧性变形。本文介绍了AMS的基本原理和参数,并并介绍了前人及作者应用AMS详细分析研究了二个实例:(1)以假多畴(MD)高钛磁铁矿为主要载磁矿物的玄武岩样品的AMS变化及其对构造运动的响应;(2)以MD磁铁矿为主要载磁矿物的湖泊沉积物样品在沉积过程中AMS变化。AMS可以灵敏地检测样品中磁性矿物的定向排列,因此在在地质领域中具有很好的应用前景。     

现代珠江三角洲地区QZK4孔第四纪沉积年代

在地质环境变迁迅速的现代珠江三角洲地区,对于第四纪沉积的年代存在多种认识。结合南中国海的海平面变化记录,通过AMS14C和光释光测年发现,珠江三角洲腹地QZK4孔第四纪岩心底部最老年龄约为43.71ka BP。岩心记录的第四纪环境对南中国海海平面变化有着良好的响应:岩心下部陆相河流沉积和暴露风化层发育于末次冰期至早全新世的低海平面时期,上部滨海—河口湾沉积发育于早中全新世以来的高海平面时期。根据钻孔岩心的环境分析,推测现代珠江三角洲地区第四纪的底界可能较本钻孔记录的更老。     

青藏高原东缘鲜水河断裂带磁组构特征及构造意义

为了探究鲜水河断裂带的几何学、运动学特征,在野外构造、显微构造分析基础上,研究了鲜水河断裂带296块构造岩定向样品的磁组构特征和热磁特征,结果显示样品的平均磁化率km值总体较小,属微弱磁性到弱磁性;热磁实验及其显微构造表明顺磁性的页硅酸盐(如黑云母)等矿物对糜棱岩类样品磁化率贡献较大;磁化率各向异性度PJ总体较大,表明鲜水河断裂带构造变形强烈;磁化率椭球体形状参数T总体大于0,扁率E总体在1附近分布,说明鲜水河断裂带磁化率椭球体以扁圆形为主,整体上磁面理较磁线理发育,进一步显示出鲜水河断裂带构造变形样式以剪切、压扁为主,伴有拉伸的构造变形,同时也反映出鲜水河断裂带多次变形的综合特征;最小磁化率主轴Kmin方位表明鲜水河断裂带北段和南段分别受近EW向和NE-SW向主压应力控制;同时Kmin方位及其倾伏角特征显示鲜水河断裂带总体以左行走滑剪切为主,北段两侧块体在鲜水河断裂带两次不同的构造活动时期,各自有一定的相对抬升,但抬升幅度均不大;南段则是SW块体相对NE块体抬升,抬升幅度较大;整个断裂带特征显示出鲜水河断裂带在向南发展逐渐转化为挤压构造,这可能与青藏高原物质的向东逃逸受阻以及鲜水河断裂带与龙门山断裂带在此交接的地质背景不无关系.     

南京市区埋藏古河道沉积物的年代

埋藏古河道沉积物年代学框架的建立对于探讨当地水系演变过程、重建气候演化历史具有重要意义。本文用光释光(OSL)测年中的简单多片再生法(SMAR)和单片再生法(SAR)对南京市区汉府街埋藏古河道堆积物钻孔样品进行了光释光测年,对岩芯中含有的植物碎片进行了AMS 14C测年。光释光等效剂量(De)的预热坪实验表明,在 200～260℃的预热温度范围内均能获得基本一致的De值,并得到了剂量恢复实验支持。样品的光释光年龄和树轮校正的AMS 14C年龄吻合,结果显示N06S6孔秦淮河古河道沉积是不连续的,主要堆积于6.6ka至7.9ka期间和14ka至15ka期间。末次冰盛期期间,秦淮河下切形成深达至少42m的古河谷。     

蒙山峨峪口砾石堆积堤的年代学研究

准确界定蒙山峨峪口砾石堆积堤的形成时代，对于探明其成因、澄清山东中低山丘陵第四纪冰川有无之争，是一个需要解决的科学问题。峨峪口堆积垄岗砾石组构、沉积构造、地貌组合等标志，均指向其为山洪泥石流堆积物，且为暴发频率极低、发展周期较长的水石流或稀性泥石流堆积。其下伏第四纪沉积物OSL埋藏年龄和AMS14C年龄可作为砾石堆积堤形成时代的最老约束参考年龄，当地村民迁居此地的历史可作为最小约束参考年代。OSL测年结果为2. 1~2. 3 ka BP，AMS14C测年结果为951~1522 cal AD，证明砾石堆积堤为数百年前形成的历史泥石流痕迹。