2200万年来风尘沉积的粒度变化及古气候意义

Three eolian deposit formations, including Quaternary loess (QL, Liu et al.3 1985), Hipparion red earth (HRE, also called red clay, Liu et al., 1985) and Miocene loess (ML, Guo et al., 2002) constitute a set of unique paleoclimatic archives in northern China dated back to at least 22Ma ago. The Miocene loess is a recently discovered loess-soil sequence. Detailed investigation has been made on its origin, chronology and paleoclimatic significance (Guo et al., 2002), but further work is still needed to obtain detailed paleoclimate information, and mechanical links behind paleoclimatic changes. In this study, grain size analysis of QL, HRE and ML has been conducted on two sections: Qinan and Xifeng. The objective is focused on comparison of the grain size distribution characteristics (GSDC) among different eolian deposit formations, and reconstruction of the Asian monsoon circulation in the past 22 Ma. Results show that GDSC of ML resembles that of QL and HRE, and GDSC of ML is especially similar to HRE. Both ML and HRE contain a significant proportion of fine fraction, however, QL has a large amount of coarse sediments. This is mainly due to that the wind system transported aeolian dust was weaker in the late Tertiary than that in the Quaternary. Grain size difference between loess and paleosol in ML is much smaller than that in QL, indicating that the climatic fluctuations during the late Tertiary were much smaller than that happened in the Quaternary The grain size records of the past 22 Ma reveal several evolution phases of the Asia winter monsoon. -2.7 Ma BP is the most important boundary in the process of the winter monsoon evolution: the wind strength have significantly enhanced since 2.7 Ma ago. During a period between -22.0 and -3.6 Ma, three periods with relatively stronger winter monsoon are recorded in the QA-I section, between 21.2 and 19.9, and 16.0 and 13.3, and 8.7 to 6.9 Ma, respectively. From 3.6 to 2.7, the winter monsoon was enhanced gradually. In the Miocene time, the intensified winter monsoon phases (between 21.2 and 19.9, and 16.0 and 13.3, and 8.7 and 6.9 Ma) seemed to have a close relationship with the uplift of the Tibetan Plateau and/or the ongoing global cooling, but the forcing mechanism behind the Asia winter monsoon evolution need to be further investigated. During the Pliocene-Pleistocene time, the Asia winter monsoon strengthened at 3.6 and 2.7Ma ago are in good agreement with the ongoing global cooling and the Arctic ice sheet development. In the mean time, much evidence suggests that an intense uplift of the Tibetan Plateau occurred at ~3.6 Ma, which is synchronous with a major increase in Asia winter monsoon. Therefore, two major factors may be invoked to explain the winter monsoon enhancement: Arctic ice sheet development and Tibetan uplift. We propose that changes in location and intensity of the Siberian-Mongolian high that were caused by the Tibetan uplift and Arctic ice sheet development might be an important factor for Asian winter monsoon evolution in the Pliocene-Pleistocene.

浑善达克沙地与河北沽源地区土地沙漠化研究

The Otindag sandy land and the Guyuan region of Hebei Province lie in the agro-pastoral zone, where sandy desertification is serious. So they are typical for us to study on. In this paper, detail investigation were made on the Remote Sensing, Hydrochemistry, Chronology, grain size analyzing of research region to monitor sandy desertification and environmental background. The main conclusions are presented as following: 1. According to the diverse natural condition, the research area is divided into three types as sandy land desertification, cultivated land desertification and desertification reflected by lake change. The monitoring result of the first type shows that the main performance way of the sandy desertification in Otindag sandy land is that (1) the expansion of both the shifting dune and the half fixed sandy dune, (2) the reduce of the fixed sandy dune. While the result of the second type shows (1) the desertification land in the Guyuan region has first increasing then reducing change for about 30 years. (2) The sand mainly concentrates west of the research area and small part of wind-drift sand distributes northeast the research area with the spot shape. (3) The meadow area increases obviously. As far as the third type, the Dalai Nur lake area occurs first expanding then reducing change and the wind-drift sand around the lake first reduces then increases. 2. The land cover of the different types change with the same law. It is worth notice that the lake area changes oppositely with that of the wind-drift sand. 3. For about 5,000 a B.P. -2800 a B.P., the well developed palaeosols emerged. After that, three layer palaeosols were founded in the profile of Otindag sandy land. The analyses of grain size show that the sand grains of the south were coarser than that of the north. The sand in the north and middle were well sorted, while the south poor sorted. 4. Both the natural and human impact on the process of sandy desertification. On this research result, different regions have different influences. So the measures to improve sandy desertification should be choosed respectively.

放射性钚和铯在湖泊中的时空分布特性及近期沉积年代学应用

短半衰期的137Cs具有高放射毒性，而半衰期较长的Pu的化学毒性极强，它们在环境中富集，进而在食物链中迁移，将直接威胁人类健康，国内在Pu和137Cs的分析方法、环境剂量以及湖泊地质等方面已展开了一些研究，但仍处于起步阶段，内容不够系统全面。湖泊和水库是生活饮用水或农用灌溉水源地，与人类生活息息相关，因此研究湖泊中人为放射性核素的污染状况、来源及归宿显得尤为重要；而且湖泊沉积物作为湖泊中大多污染物的最终归宿地，能够有效地保存湖泊区域的污染历史，研究湖泊沉积物有助于认清湖泊污染历史、探明污染机理，为污染现状评价、水质调查及治理提供可靠的基础依据，钚和铯的特殊性质可为环境科学等方面的研究提供关键的沉积年代学基础数据。 本研究在全国范围内选取具有代表性的不同区域分布、湖泊类型、人为扰动状况的湖泊为研究对象，应用基于γ谱的137Cs分析方法和本文建立的针对淡水湖泊沉积物的Pu同位素分析方法，研究了人为放射性核素Pu和137Cs在全国范围内的分布特征、近源传输及在湖泊中的沉积与迁移机理，并探讨了239+240Pu计年方法的有效性。通过上述研究，得到了如下主要结论： 1、建立了基于阴离子交换树脂分离的Pu同位素ICP-SF-MS分析方法，采用AG1-X8树脂分离Pu与杂质元素以及AG MP-1M纯化Pu同位素。高的仪器灵敏度、良好的U去除效率、足够的Pu同位素化学回收率表明该方法能够有效地检测湖泊沉积物样品中的痕量239Pu和240Pu。此外，纯化Pu同位素时，采用HBr洗脱Pu同位素以排除PbCl+对Pu同位素的干扰，因此该方法除了适应239Pu、240Pu分离分析之外，还可为后期分析浓度更低的241Pu提供技术保证。 2、通过分析沉积物岩芯中的239+240Pu沉积通量和137Cs沉积通量、240Pu/239Pu同位素平均比、239+240Pu/137Cs活度平均比及Pu、137Cs在沉积物岩芯中的剖面分布特征发现博斯腾湖有中国核试验基地的人为放射性核素污染，其表现形式为核爆试验产生的可溶性气溶胶颗粒和热核颗粒沉降后再悬浮造成的偶然性污染和源自罗布泊的地表径流引起的普遍性污染。博斯腾湖的Pu同位素结果表明平流层沉降的中国核试验的核碎片的240Pu/239Pu同位素原始比值可能为0.18左右，非常接近全球大气沉降值。苏干湖和双塔水库有源自中国核试验的可溶性气溶胶颗粒经大气沉降后再悬浮造成的人为放射性核素污染，苏干湖中大约有40%左右的137Cs以及27%左右的239+240Pu源自中国核试验，双塔水库中源自中国核试验的239+240Pu和137Cs所占的比例分别为27.1%和30.6%。 3、单一的239+240Pu和137Cs蓄积峰不能完全解析出沉积速率较慢的青海湖是否有源自在罗布泊进行的中国核武器爆炸试验的污染，但1963年全球大气沉降高峰下端的低240Pu/239Pu同位素比值表明青海湖可能有源自始建于1957年的中国原子城的地表径流泄漏或地下渗透泄漏的人为放射性核素污染。 4、西南地区的湖泊中，云南程海部分137Cs可能源于长距离迁移的中国核试验，但明确中国核试验对西南地区的影响还需要中国核试验源Pu和137Cs长距离迁移的信息，如青藏高原湖泊和西川盆地湖泊沉积物中人为放射性核素来源。其它湖泊中人为放射性核素的污染均源于全球大气沉降。 5、火山成因封闭式玛珥湖，在没有人为扰动的条件下，137Cs和239+240Pu主要以微弱的分子扩散迁移，迁移速率不明显，能较好地保持沉积历史；对人为活动比较频繁的浅水湖泊而言，人为活动、强风等恶劣气候以及上层水体和底层水体的交互对流等因素引起的沉积物再悬浮作用加剧了239+240Pu和137Cs主要以分子扩散迁移作用，造成水体寄宿时间短的浅水湖泊中239+240Pu和137Cs的不同步损失，导致239+240Pu和137Cs峰型的无序化。 6、受人为扰动较弱的湖泊中137Cs比活度和239+240Pu比活度成良好的线性相关，表明239+240Pu蓄积峰的信息能够为此类型湖泊沉积物提供有效的近期沉积年代学数据；对受人为扰动比较大的湖泊来说，137Cs和239+240Pu蓄积峰在近期沉积物计年的方面各有优劣，计年数据的可靠性不强；青海湖各沉积物岩芯的沉积物计年结果表明湖泊的沉积物累积率或沉积速度存在区域性差异。

福建省基性岩的年代学和地球化学：晚中生代以来中国东南部地幔演化

中国东南部地处太平洋板块与欧亚板块的接合部位，其独特的地理位置和构造运动长期以来引起了地质学家的广泛关注。自印支运动以来，板内构造演化复杂而多样。最引人注目的无疑是燕山期广泛而强烈的构造一岩浆热事件。近年来随着研究的深入，逐渐凸显出两个重要的地质问题需要解决：（1）中国东南部晚中生代构造属性的转变时间及地球动力学演化过程；（2）多金属成矿作用与岩石圈伸展减薄及地壳拉张的关系。基于此，本文以中国东南部福建省的基性脉岩、岩体为研究对象，运用系统的矿物学、岩石学、岩石地球化学、同位素地球化学及同位素年代学证据，详细论述了晚中生代中国东南部板块俯冲、地慢演化、壳幔相互作用及岩石圈伸展减薄的地球动力学过程，探讨了地壳拉张期次在福建省区域构造上的响应。本研究主要获得以下几点认识：1、岱前山辉长岩体沿福建长乐一南澳大断裂带分布，侵位于沿海的绿片岩相和角闪岩相的变质岩中及内陆的中生代火山岩中。岩体的微量元素特征表现为与俯冲作用有关的岛弧特点。岱前山岩体由低程度部分熔融形成（约7％），伴有角闪石、单斜辉石的分离结晶作用和斜长石的堆晶作用。时间上与古太平洋板块低角度一歪斜俯冲、晚中生代的变质事件、平潭一东山变质带抬升和长乐一南澳剪切带运动在时间上基本一致。岩体是受俯冲流体交代的上地慢，经部分熔融沿长乐一南澳断裂一应力转换带侵入。它与中国东南部出露的其它基性岩同源，但未受到地壳的混染作用。2、闽南茅坪一晒鞍角基性侵入体具独特的地球化学属性，表现为高Al2O3、CoO、MnO，低FeOT、MgO、TiO2含量，富集轻稀土元素（LREE）和大离子亲石元素（LILE），并具正的Pb异常和负的Ti异常；Sr-Nd-Pb同位素结果显示，该基性岩有EM2组分的参与。模拟计算表明，该基性岩墙群是尖晶石二辉橄榄岩地慢5-15％部分熔融的产物；微量元素配分模式及理论模拟表明茅坪一晒鞍角基性岩体的地慢源区在熔融前曾受到1％俯冲沉积物熔体的源区混染和5％流体交代作用。基性岩浆在上升过程中还受到10％左右的地壳物质的混染作用，导致该区基性岩富放射成因Sr、Pb同位素。3、福建省晚中生代基性脉岩富Al2O3（14.0-20.4 wt%）、CaO（4.09-12.7wt％）。按地球化学特征可分为两组：第一组脉岩具较低稀土总量（53.8-145.5μg／g）和平缓的稀土配分模式L（La/Yb）n=1.68-4.651，而第二组脉岩的稀土总量较高（63-247μg／g），且轻稀土富集［（La/Yb）n＝4.63-19］。在原始地慢标准化图解上，第一组脉岩显示Pb的正异常和Ti负异常，无Nb、Ta异常，而第二组脉岩显示明显的Nb一Ta一Ti负异常和Pb正异常。两组脉岩经历了不同的源区混合和陆壳混染过程，第一组脉岩显示了以陆壳混染为主的地球化学过程，不具有岛弧特点的微量元素配分模式表明该类基性岩的地慢源区可能未受俯冲作用过程影响，第二组脉岩的岛弧地球化学特点暗示该类基性岩地慢源区是经俯冲作用改造过的富集岩石圈地慢。第一组基性脉岩来源于石榴石一尖晶石二辉橄榄岩地慢，而第二组脉岩来源于尖晶石或尖晶石一斜长石二辉橄榄岩地慢。地球化学数据显示，不同类型的地慢存在不均一性，反映了不同程度的交代作用或不同量俯冲沉积物的加入。福建省晚中生代基性脉岩产生于拉张构造背景，与岩石圈的伸展减薄及软流圈的上涌紧密联系；4、福建省基性脉岩的同位素特征显示，第一组脉岩具较高的143Na／144Nd，第二组脉岩的143Nd／144Nd较低；Th/Nd、Ba/La比值及理论模拟结果表明，第二组脉岩可能受到流体交代和俯冲沉积物的源区混合作用，而第一组脉岩基本未受俯冲组分（沉积物＋流体）的改造。地壳混染对两组脉岩可能起着重要的作用。EC-AFC理论模拟表明，福建省基性脉岩在上升过程中受到前寒武纪变质岩的影响，这种混染作用在一定程度上改变了同位素组成。因此，福建省晚中生代基性脉岩的地球化学特点是太平洋板块俯冲与壳慢相互作用共同作用的结果；5、K-Ar年龄结果表明，出露于福建省的基性脉岩年龄具周期性分布，表现为五个周期：70-75Ma士，85 Ma士，105-110Ma，125Ma士和135-140Ma。这与前人的研究结果基本一致，表明福建省与中国东南部区域上有相似的地壳拉张期次。K-Ar年龄主要集中分布在14OMa→65Ma之间，这表明：①140M。限定了中国东南部岩石圈伸展作用开始的下限，也就是说中国东南部晚中生代的岩石圈伸展至少开始于140Ma左 右，而并非90Ma；②标志着中国东南部大规模拉张作用的开始，表明构造属性由令挤压为主转变为以拉张作用为主。