The activities of comets related to their aging and origin     

Fred L. Whipple 《Celestial Mechanics and Dynamical Astronomy》1992,54(1-3):1-11

Two indices have been developed for the purpose of comparing the natures of various classes of comets. The first is the Activity Index (AI), measuring the inherent magnitude increase in brightness from great solar distances to maximum near perihelion. The second, or Volatility Index (VI), measures the variation in magnitude near perihelion. Tentative determinations of these two indices are derived from observations by Max Beyer over more than 30 years for long-period (L-P) and short-period (S-P) comets near perihelion and from other homogeneous sources. AI determinations are made for 32 long-period (L-P) comets and for 14 short-period (S-P). The range of values of AI is of the order of 3 to 10 magnitudes with a median about 6. An expected strong correlation with perihelion distance q, is found to vary as q ^–2.3. Residuals from a least-square solution (AI) are used for comparing comets of different orbital classes, the standard deviation of a single value of AI is only ±1^m.1 for L-P comets and ±1^m.2 for S-P comets.Among the L-P comets, 19 of period P larger than 10⁴ years yield AI = 0^m.27 ± 0^m.25 compared to 0^m.39 ± 0^m.26 for 13 of period between 10² years and 10⁴ years. This denies any fading with aging among the L-P comets. Also no systematic change with period occurs for the VI index, leading to the same conclusions. Weak correlations are found with the Gas/Dust ratio of comets. No correlations are found between the two indices, nor of either index with near-perihelion magnitudes or orbital inclination.The various data are consistent with a uniform origin for all types of comets, the nuclei being homogeneous on the large scale but quite diverse on a small scale (the order of a fraction of kilometer in extent). Small comets thus may sublimate away entirely, leaving no solid core, while huge comets may develop a less volatile core by radioactive heating and possibly become inactive like asteroids after many S-P revolutions about the Sun. When relatively new, huge comets may be quite active at great solar distances because of volatiles from the core that have refrozen in the outer layers.  相似文献   

黑河流域水环境演变及其驱动机制研究进展   总被引：10，自引：0，他引：10  

肖生春  肖洪浪 《地球科学进展》2008,23(7):748-755

基于近几十年来诸多学者对西北干旱区典型内陆河流域——黑河流域水环境演变诸方面研究成果,综述了上游山区成水环境、中游绿洲用水环境和下游荒漠天然绿洲水成环境在不同时间尺度上的演变过程及其驱动机制。通过对各种古环境代用指标的对比研究表明：在大尺度上,流域上下游表现出阶段性的一致性水环境演变趋势;在小尺度上,则表现水环境演变的时空异质性。驱动机制方面,在空间上,流域上、中、下游水环境演变的气候变化和人类活动影响驱动作用各有侧重;在时间上,表现为不同尺度上气候变化与流域人类活动耦合作用的驱动机制。  相似文献   

湖北辽瓦店遗址地层记录的环境变迁与人类活动的关系研究   总被引：6，自引：4，他引：2       下载免费PDF全文

李中轩  朱诚  张广胜  欧阳杰  王然 《第四纪研究》2008,28(6):1145-1159

用XRF和ICP-AES方法分别测试了湖北辽瓦店遗址地层样品的氧化物含量和地球化学元素含量,恢复了遗址地层中夏代文化层、东周文化层和明清文化层记录的环境变化和人类活动特征。结果表明:1)夏代文化层时期,气候温暖湿润,生态环境良好,人类活动与自然环境和谐度高,有原始的手工业活动记录。2)东周时期气候开始进入干旱期,干旱程度在上东周文化层中部逐渐加深(LOI=6.98%),到了上东周文化层顶部气候进入暖湿期。该层Cu含量的异常高值(0.31mg/g)和出土的青铜器表明本文化层反映了青铜社会的繁荣期,而P含量的高值(22.73mg/g)和动物骨屑则反映了该期人们驯养、捕猎动物的普遍性。3)明清时期的气候趋于干凉,明代文化层中部有短暂暖湿期,后又进入冷干期。清代文化层中下部有暖湿特征,之后重新进入干冷期。该层Ca(均值101.5mg/g),Mg(均值60.27mg/g),Zn和Pb的高含量表明遗址曾有过制陶(瓷)业的历史。另外,Mg和Ca含量的高值与耕作区、墙壁灰浆、生活垃圾堆积等人类活动相关。Ca和Mg含量在近现代层逐步升高揭示了人类活动逐渐增强的事实。地层的平均粒径自上东周文化层(<142cm)开始出现振荡,暗示汉江洪水和季节性山洪发生的频率不断增加,表明恶劣环境条件虽然促进了人类生产力水平的提高,但原始生态系统却遭到破坏,人类活动对自然环境的正反馈效应日益凸显。  相似文献   

华北地台中元古代碳酸盐岩中的微生物成因构造及其生烃潜力   总被引：6，自引：3，他引：3  

史晓颖  张传恒  蒋干清  刘娟  王议  刘典波 《现代地质》2008,22(5)

华北地台中元古界主要由环潮坪石英砂岩、浅海碳酸盐岩和浅海—泻湖相暗色页岩3种沉积相组合构成,以陆表海浅水碳酸盐岩占主导。碳酸盐岩中除含有丰富的微古植物、宏观藻类和微生物建隆外,还发育大量的微生物成因构造(MISS)和微生物诱发的碳酸盐沉淀(MMCP)。微生物席和MISS构造在高于庄组上部(约1.6 Ga)和雾迷山组下部(约1.45 Ga)碳酸盐岩中尤为发育,表明活跃的微生物活动和高有机质产量。在石化微生物席中,发现有丝状、球状细菌化石和草莓状黄铁矿;围岩中发现有针状文石、花瓣状重晶石、放射状菱铁矿、铁白云石和葡萄状碳酸盐胶结物等多种自生碳酸盐矿物,指示甲烷厌氧氧化(AOM)导致的自生碳酸盐沉淀。中元古代的温暖气候和海洋分层、缺氧、硫化条件有利于微生物的高生产量和高有机质埋藏率。气隆构造和核形石状碳酸盐结核反映浅埋藏条件下活跃的成烷作用和甲烷排放,围岩和MMCP中富沥青质。华北地台中元古界富微生物席碳酸盐岩有良好的生烃潜力,有可能形成重要的烃源岩。据微生物席、MISS构造及MMCP的研究,初步估算华北地台中元古代碳酸盐岩的概略生烃潜力约为10×108t石油当量。  相似文献   

胶东地区晚中生代岩浆活动及对大地构造的制约   总被引：16，自引：1，他引：15  

邱连贵  任凤楼  曹忠祥  张岳桥 《大地构造与成矿学》2008,32(1):117-123

应用近十几年来胶东地区及其周边岩浆岩体的同位素测年资料,结合几个岩体的新的SHRIMP测年数据(牙山岩体117.7Ma,院格庄岩体113.4Ma,正长斑岩脉115.7Ma),系统研究了该区晚中生代以来的岩浆活动规律,结果表明,该区可分为4个岩浆活动期;(1)晚侏罗世构造-岩浆热事件(150~160Ma),以玲珑-滦家河型岩体为代表,是在区域挤压构造应力作用下侵入的,属同构造或同造山时期的岩浆作用。(2)早白垩世早期构造-岩浆热事件(135~125Ma),以郭家岭岩体为典型,该时期的岩浆活动均具有双峰式岩浆作用特征,反映了伸展动力学背景。(3)早白垩世中晚期构造-岩浆热事件(125~105Ma),以崂山花岗岩为特征,该阶段对应于中国东部岩石圈大规模的减薄时期,是大陆裂谷作用的高峰期。(4)晚白垩世-古新世基性-超基性火山作用。本文还探讨了岩浆活动的地球动力学因素及其对大地构造的制约。  相似文献   

中国西部现今地壳形变与地震活动特征     

李金平 《测绘工程》2008,17(1):6-9

以1999-2004年间中国西部地区近650个GPS站点的观测资料为基础,采用二维“高张力样条”函数内插算法获得了连续地壳形变场。结果表明,相对于稳定欧亚参考框架,中国西部现今地壳运动西强东弱,北向运动从西向东逐渐减弱,东向运动逐渐增强,青藏高原东缘及附近地区是东向运动的消减区带;主应变从西向东由压缩应变转变为拉伸应变;最大剪应变主要位于喜马拉雅地块及其东部地区;中国西部地区地震活动在空间分布上似乎集中发生在印度板块北东向挤压欧亚板块的两条共扼带内,与地壳形变场有一定对应关系,地壳形变的高应变率区为中强地震多发地带。  相似文献   

贝加尔裂谷系及邻区断裂现今活动变化及成因(英文)     

S.I. Sherman　E.A.Gorbunova 《地学前缘》2008,15(3):337-347

已有的地质和地球物理方法无法对年、月、旬等现今时间尺度上的断层活动变化进行有效预测。运用基于地震活动定量指数运算程序的地理信息技术可以解决这个问题。这种方法被应用于贝加尔裂谷系(BRS)及邻区的研究当中。研究发现,断层活动变化发生在以几年为周期的时间尺度上,这无法用区域应力场的变化进行合理解释。沿着穿越贝加尔裂谷系的剖面编制了活动断层图以及地震活动定量指数曲线。提出的这种方法可以根据地震活动定量指数对活动断层进行仔细的分类,从而为解决地震中期预报相关问题开辟了重要途径。这种方法还被用来研究断层现今活动的时空变化和形成机制。研究发现,在实时尺度上断层活动具有高频发生的特征,这种现象可能是由于脆性岩石圈板内和块体内部运动产生的慢变形波的干扰导致的。利用变形波的通过速度可以对活动断层进行分组,各组具有不同的地质和地球物理特征参数,同时还可以对变形波波前的方向以及某一区域内主要断层在实时间隔内(地质上的瞬间)的活动情况进行预测。  相似文献   

西藏冈底斯矿带成矿作用及远景分析   总被引：5，自引：1，他引：4  

梁华英  喻亨祥  莫济海  曾提  胡光黔 《矿物岩石地球化学通报》2008,27(3)

冈底斯带矿床众多,类型复杂,主要有斑岩型铜（金钼）矿床、矽卡岩型铁铜-铅-锌（银）矿床、层控铅-锌-银矿床、火山岩型金-银矿床及雄村式铜-金矿床。矿床地质特征和同位素年龄表明,冈底斯带南部的矿床与新特提斯洋壳向北俯冲-陆陆碰撞及碰撞期后的构造岩浆事件有关;冈底斯带北部的矿床与班公湖-怒江洋壳向南俯冲-陆陆碰撞及碰撞期后构造岩浆事件有关。冈底斯带与洋壳的俯冲-碰撞有关的岩浆活动强烈,成矿条件优越。西藏高原在碰撞后发生了快速抬升剥蚀,部分矿床顶部出现低温组合矿化,多数矿床保存良好。  相似文献   

Fluid–Sediment Interaction in a Marine Shallow-Water Hydrothermal System in the Wakamiko Submarine Crater, South Kyushu, Japan     

Miwako Nakaseama    Jun-Ichiro Ishibashi    Keita Ogawa    Hiroshi Hamasaki    Keiko Fujino  Toshiro Yamanaka 《Resource Geology》2008,58(3):289-300

The Wakamiko submarine crater is a small depression located in Kagoshima Bay, southwest Japan. Marine shallow‐water hydrothermal activity associated with fumarolic gas emissions at the crater sea floor (water depth 200 m) is considered to be related with magmatic activity of the Aira Caldera. During the NT05‐13 dive expedition conducted in August 2005 using remotely operated vehicle Hyper‐Dolphine (Japan Agency for Marine‐Earth Science and Technology), an active shimmering site was discovered (tentatively named the North site) at approximately 1 km from the previously known site (tentatively named the South site). Surface sediment (up to 30 cm) was cored from six localities including these active sites, and the alteration minerals and pore fluid chemistry were studied. The pore fluids of these sites showed a drastic change in chemical profile from that of seawater, even at 30 cm below the surface, which is attributed to mixing of the ascending hydrothermal component and seawater. The hydrothermal component of the North site is estimated to be derived from a hydrothermal aquifer at 230°C based on the hydrothermal end‐member composition. Occurrence of illite/smectite interstratified minerals in the North site sediment is attributed to in situ fluid–sediment interaction at a temperature around 150°C, which is in accordance with the pore fluid chemistry. In contrast, montmorillonite was identified as the dominant alteration mineral in the South site sediment. Together with the significant low potassium concentration of the hydrothermal end‐member, the abundant occurrence of low‐temperature alteration mineral suggests that the hydrothermal aquifer in the South site is not as high as 200°C. Moreover, the montmorillonite is likely to be unstable with the present pore fluid chemistry at the measured temperature (117°C). This disagreement implies unstable hydrothermal activity at the South site, in contrast to the equilibrium between the pore fluid and alteration minerals in the North site sediment. This difference may reflect the thermal and/or hydrological structure of the Wakamiko Crater hydrothermal system.  相似文献   

应用探地雷达探测活动断层   总被引：1，自引：0，他引：1  

薛建  贾建秀  黄航  易兵  张良怀  张羽 《吉林大学学报(地球科学版)》2008,38(2):347-350

在浅覆盖区采用探地雷达探测地震活动断层。通过实验确定出最佳的采集参数和数据处理流程,在雷达剖面上,能够清晰地显示出断层上部的形态特征、上断点埋深和岩土分层。结合钻孔资料,利用雷达剖面上对第四系覆盖的分层结果和上断点所在的层位可以分析和评价断层的活动性。  相似文献