The case for significant numbers of extraterrestrial impacts through the late Holocene

When astronomers tell us that there should have been numerous impacts from space during the last five millennia, when impact craters exist on land and more impacts can be assumed over the oceans, why are historians, archaeologists and palaeoecologists not diligently seeking evidence for these impacts, and their effects? This article reviews just some of the relevant evidence for impacts. In turn this suggests that ablation material, background material from space, and micro‐tektites, should all be present in ocean cores, ice cores, peat, and lake sediments. It seems that almost no efforts have been made to find evidence that might link to the known crater fields, or to identify and date periods of enhanced cosmic activity. The question must be, why? Copyright © 2007 John Wiley & Sons, Ltd.     

Styles of mantle convection and their influence on planetary evolution

Mantle convection is the method of heat elimination for silicate mantles in terrestrial bodies, provided they are not too small or too hot. Bodies that are small (～Moon or smaller, possibly even Mercury) may rely largely on conduction or melt migration, and bodies that are very hot (Io, very early Earth) may use massive melt migration (magma oceans) and heat pipes. In the standard, simple picture, we can use scaling laws to determine the secular cooling of a planet, likelihood and form of volcanism, and the possibility of a core dynamo. Contrary to popular belief, small planets do not cool faster than larger planets (provided they convect) but they do tend to have a slightly lower internal temperature at all times and thus may cease to be volcanically active at an earlier epoch. On the other hand, a larger volume fraction of a small planet may be involved in melt generation. However, our understanding of heat transfer by mantle convection is limited by three very important, largely unsolved problems: The complexities of rheology, the effects of compositional gradients, and the effects of phase transitions, especially melting. The most striking manifestation of the role of rheology lies in the difference between a mobile lid mode (plate tectonics for Earth) and a stagnant lid mode (other large terrestrial bodies). This difference may arise because of the role of water, but perhaps also because of melting, or size (gravity), or the vagaries of history. It has profound effects for the differences in history of Earth, Venus and Mars, including their surface geology, volatile reservoirs and magnetic fields. Since thermal convection is driven by small density differences, it can also be greatly altered or limited by compositional or phase effects. Melt migration introduces additional complications to the heat transport as well as being a source for the irreversible differentiation that might promote layering. Our limited understanding and ability to model these processes continues to limit the development of a predictive framework for the differences among the terrestrial planets.     

Study of melt and a clast of 546 Ma magmatic arc rocks in the 65 Ma Chicxulub bolide breccia,northern Maya block,Mexico: western limit of Ediacaran arc peripheral to northern Gondwana

The basement of the Maya block of eastern Mexico is generally covered by Mesozoic and Cenozoic platformal carbonate rocks. However, the 65.5 Ma Chicxulub meteorite impact in the northern Yucatan Peninsula excavated deep into the crust and brought crystalline basement fragments into the impact breccias. Common Pb isotopic data from impact melt and a granitic clast from drill core (Y6) are highly radiogenic, consistent with the Archaean derivation. A granodiorite clast in this breccia from drill core (Yaxcopoil-1) yielded a continuous range of concordant ²⁰⁶Pb/²³⁸U laser ablation inductively coupled plasma mass spectrometry zircon ages between 546 ± 5 Ma and 465 Ma, with three discordant zircons having ²⁰⁶Pb/²³⁸U ages between 130 Ma and 345 Ma. The ca. 546 Ma age is interpreted as the age of granodiorite intrusion, with younger ages representing variable Pb loss during melting associated with the meteorite impact. This is consistent with previous U–Pb zircon data that gave an upper intercept age of 550 ± 15 Ma at Chicxulub, which becomes 545 ± 5 Ma when combined with the zircon data from distal ejecta. Such arc rocks are absent in the southern Maya block, and in the neighbouring Oaxaquia terrane (s.s.) they are replaced by a 546 ± 5 Ma plume-related dike swarm. On the other hand, Ediacaran arc rocks continue through the peri-Gondwanan terranes of the Appalachians and Europe (Florida, Carolinia, Avalonia, Iberia, Armorica, Massif Central, Bohemia, and NW Africa). Arc magmatism in these areas ended between 570 Ma (Newfoundland) and 540 Ma (Carolinia/UK) as the subduction zone was replaced by a transform fault along the northern Gondwanan margin. This age range is synchronous with the two-stage birth of Iapetus, suggesting that both are related to major plate reorganization. The source of plume-related dikes may have been located at the rift–rift–transform triple junction between Laurentia, Baltica, and Gondwana.     

Gravito-electrodynamics and the structure of planetary ring systems

Recent spacecraft observations of the Saturnian and Jovian ring systems have highlighted a plethora of interesting new phenomena associated with those regions containing fine (micron and sub-micron sized) dust. Recognizing that these dust grains, by virtue of being immersed within the planetary magnetospheres, are electrostatically charged to the point that they experience comparable gravitational and electric forces, a new “gravito-electrodynamic” theory has been developed to describe their dynamics. This theory has been successful in explaining all these phenomena in a systematic way. In this review, the basic model and its range of validity are outlined, and its application to the Saturnian and Jovian ring systems are discussed.     

Buried impact basins,the evolution of planetary surfaces and the Chicxulub multi‐ring crater

Impact craters are distinctive landforms on Moon, Mars, Venus and other bodies of the Solar System. In contrast, the Earth has few craters, due to the dynamic nature of the planet, where craters and other geological structures are destroyed, modified or covered. Planetary missions have also shown that in other worlds where craters are numerous and well preserved, the crater record has been modified, through the identification of buried structures. Studies of the concealed crater record have major implications for the crater‐size frequency distribution and crater‐counting chronologies. On Earth, Chicxulub is an example of a large multi‐ring buried basin. Its study provides clues for the investigation other planetary surfaces. In addition, geophysical surveys have unravelled its deep 3‐D structure, providing data and constraints for new planetary missions.     

Geological evolution of offshore pollution and its long-term potential impacts on marine ecosystems

Populations and metropolitan centers are accumulated in coastal areas around the world. In view of the fact that they are geographically adjacent to coasts and intense anthropogenic activities, increasing global offshore pollution has been an important worldwide concern over the past several decades and has become a very serious problem that needs to be addressed urgently. Due to offshore pollution, various geological disasters occur in high frequency, including intensified erosion and salinization of coastal soils, frequent geological collapses and landslides and increasing seismic activities. Moreover, offshore pollution shows increasingly serious impacts on the topography and geomorphology of offshore and coastal areas, including coastal degradation, retreating coastlines and estuary delta erosion. Offshore sedimentation processes are strongly influenced by the pH changes of terrestrial discharges, and sedimentary dynamics have become extremely acute and complex due to offshore pollution. The seabed topography and hydrodynamic environment determine the fate and transport of pollutants entering offshore regions. Coastal estuaries, port basins and lagoons that have relatively moderate ocean currents and winds are more likely to accumulate pollutants. Offshore regions and undersea canyons can be used as conduits for transporting pollutants from the continent to the seabed. It is particularly noteworthy that the spatial/temporal distribution of species, community structures, and ecological functions in offshore areas have undergone unprecedented changes in recent decades. Due to increasing offshore pollution, the stable succession and development trend of marine ecosystems has been broken. It is thus important to identify and regulate the quantity, composition and transportation of pollutants in offshore regions and their behavior in marine ecosystems. In particular, crucial actions for stabilizing marine ecosystems, including increasing species and biodiversity, should be implemented to enhance their anti-interference capabilities. This review provides an overview of the current situation of offshore pollution, as well as major trends of pollutant fate and transportation from continent to marine ecosystems, transformation of pollutants in sediments, and their bioaccumulation and diffusion. This study retrospectively reviews the long-term geological evolution of offshore pollution from the perspective of marine geology, and analyses their long-term potential impacts on marine ecosystems. Due to ecological risks associated with pollutants released from offshore sediments, more research on the influence of global offshore pollution based on marine geology is undoubtedly needed.     

试论月球科学与类地行星研究的学科属性

我国月球探测一期工程获国家批准立项和“嫦娥一号”卫星成功发射及运行这一历史性事件标志着我国已继人造地球卫星、载人航天阶段之后,开始进入人类航天活动的第三领域——深空探测。无疑此时探讨人类这一伟大社会实践活动的自然科学意义,特别是深空探测对象的科学内涵具有重要的现实意义。随着相关技术的开发、创新和成熟,工作重点将逐渐转移到深空探测对象本身的科学研究上来,即认识地球之外的行星体以及更深刻地重新审视地球。本文讨论了现阶段深空探测对象——月球及类地行星的基本物质属性,回顾美国和前苏联两国探月计划实施历史的启示,Apollo计划的演变．科学内容的纳入及宇航员地学素养的要求,Apollo登月计划的实施对月球科学本身的促进,以及对20世纪地球科学发展、人才队伍建设和分析技术进步等的重大影响,总结了前苏联探月计划的成败及其教训,并对我国月球探测工程立项历史等方面进行了分析。在此基础上,论述了月球科学及类地行星研究的地球科学属性,并强调指出地球科学家应将月球及整个类地行星研究纳入到地球系统科学的框架之中。     

Potential impacts of global warming on the frequency and magnitude of heavy precipitation

It is now widely recognised that the most significant impacts of global warming are likely to be experienced through changes in the frequency of extreme events, including flooding. This paper reviews physical and empirical arguments which suggest that global warming may result in a more intense hydrological cycle, with an associated increase in the frequency and/or magnitude of heavy precipitation. Results derived from enhanced-greenhouse experiments using global climate models (GCMs) are shown to be consistent with these physical and empirical arguments. Detailed analysis of output from three GCMs indicates the possibility of substantial increases in the frequency and magnitude of extreme daily precipitation, with amplification of the effect as the return period increases. Moreover, return period analyses for locations in Australia, Europe, India, China and the USA indicate that the results are global in scope. Subsequent discussion of the limitations of GCMs for this sort of analysis highlights the need for caution when interpreting the precipitation results presented here. However, the consistency between physically-based expectations, empirical observations, and GCM results is considered sufficient for the GCM results to be taken seriously, at least in a qualitative sense, especially considering that the alternative seems to be reliance by planners on the fundamentally flawed concept of a stationary climate.     

On the implications of the coupled evolution of the deep planetary interior and the presence of surface ocean water in hydrous mantle convection

We investigate the influence of the deep mantle water cycle incorporating dehydration reactions with subduction fluxes and degassing events on the thermal evolution of the Earth as a consequence of core–mantle thermal coupling. Since, in our numerical modeling, the mantle can have ocean masses ∼12 times larger than the present-day surface ocean, it seems that more than 13 ocean masses of water are at the maximum required within the planetary system overall to partition one ocean mass at the surface of the present-day Earth. This is caused by effects of water-dependent viscosity, which works at cooling down the mantle temperature significantly so that the water can be absorbed into the mantle transition zone and the uppermost lower mantle. This is a result similar to that without the effects of the thermal evolution of the Earth's core (Nakagawa et al., 2018). For the core's evolution, it seems to be expected for a partially molten state in the deep mantle over 2 billion years. Hence, the metal–silicate partitioning of hydrogen might have occurred at least 2 billion years ago. This suggests that the hydrogen generated from the phase transformation of hydrous-silicate-hosted water may have contributed to the partitioning of hydrogen into the metallic core, but it is still quite uncertain because the partitioning mechanism of hydrogen in metal–silicate partitioning is still controversial. In spite of many uncertainties for water circulation in the deep mantle, through this modeling investigation, it is possible to integrate the co-evolution of the deep planetary interior within that of the surface environment.     

基于最优分割分级法的月球撞击坑分级及其演化分析

撞击坑是月球表面广泛分布的重要构造形态,占据了月球表面的大部分面积。撞击坑的直径差别很大,从几微米到数百千米,其退化程度与形成年代具有密切关系。为了研究不同地质年代形成的撞击坑直径大小及其演化规律,需采用量化分级方法对大小不同的撞击坑进行定量分级和统计分析。本文在月表撞击坑数据库LU60645GT和Lunar_Impact_Crater_Database(2011)的基础上,结合数据库中撞击坑的直径、深度和年代信息,利用最优分割分级法对撞击坑直径进行定量化分级,并根据分级结果,综合分析撞击坑几何形态特征及其演化规律。研究结果表明,撞击坑形态特征的演化与年代有密切的关系。在相同级别、相同地体下,撞击坑形成的年代越早,其形态特征的精细结构退化程度越明显,只保留了大体的几何形状;而在不同级别、相同地体、相同年代下的撞击坑形态特征则由简单逐渐变为复杂,坑物质也逐渐变得复杂。     

广西钦防海槽迁移与沉积-构造转换面

广西的钦州－防城－带，素以钦防海槽称之，系指加里东期构造运动后，扬子与华夏陆块间的“残留海”。其两侧为古隆所夹持，西为大明山古隆起，东为云开大山古隆起，其间划分为四个构造单元，由东向西依次为：博白坳陷，六万大山隆起，钦州坳陷和十万大山坳陷。现构造形迹的排列，反馈防海槽在早古生代至中生代间深海盆或浅海深水盆地在构造和沉积上有自东向西迁移的特点。晚古生代盆地迁移过程至少有八个沉积－构造转换面可记录盆地的构造演化：第1转换面为早奥陶世与晚寒武世间的沉积界面；第2转换面为早志留世与晚奥陶世间的沉积界面；第3转换面为早泥盆世早期与晚志留世间的海侵上超面；第4转换面为中泥盆世的海侵上超面；第5转换面为中二叠世与晚二叠世间的沉积界面；第6转换面为早三叠世的海侵面；第7转换面为中晚三叠世与早三叠世间的沉积界面；第8转换面为早侏罗世与晚三叠世间的沉积界面。前两个界面为盆山转换面，与华南加里东构造运动过程相耦合，为挤压的构造背景；第3界面为水下间断面，下泥盆统与上志留统为不连续沉积，在构造上应是挤压机制下的破裂不整合，也是加里东期构造运动的响应；第4界为海西期的海侵上超面，与盆地走滑拉张同步；第5界面则反馈于印支期造山的初始阶段，第6界面为中生代盆地迁移转换面；第7界面为印支期造山过程的盆地转换面；第8界面为燕山期造山造盆转换面。其转换面性质的转化，代表钦防海槽可能是个复杂大陆边缘前陆盆地演化史。     

冰冻圈变化对经济社会系统的综合影响及其适应性管理策略

冰冻圈是气候系统影响最直接和最敏感的圈层。在全球变化背景下,冰冻圈变化与生物圈、岩石圈、水圈、大气圈、人类圈之间的相互作用日趋加剧。特别地,冰冻圈快速变化对其影响区经济社会系统产生了广泛而深刻的负面影响,甚至危及人类福祉、经济社会系统的健康、和谐、持续发展。鉴于冰冻圈区位劣势及其较为落后的经济水平,其应对冰冻圈变化的能力极为有限。目前,适应仍然是冰冻圈应对气候变化的主要途径。冰冻圈变化对经济社会系统的综合影响分析是适应冰冻圈快速变化的基础,其适应性管理战略则是减轻冰冻圈快速变化不利影响、降低自然和经济社会系统防御其不利影响脆弱性的最终目标。     

北极快速增暖背景下冰冻圈变化及其影响研究综述

北极具有独特的地理位置和战略地位,是当前全球变化研究的热点区域之一.北极增暖是全球平均值的两倍以上,被称为"北极放大"现象.在北极快速增暖背景下,冰冻圈尤其是海冰显著萎缩,对北极乃至中纬度天气气候产生深远影响.对北极快速增暖背景下冰冻圈主要要素(包括海冰、冰盖、冰川、积雪和冻土)时空变化特征及未来预估进行了综述,同时总...     

山西地堑系甸子梁期低起伏高海拔地貌面成因及新构造演化

在山西地堑系的山地顶部广泛发育着一级低起伏高海拔地貌面,即甸子梁面。认识甸子梁面的形成模式,是能否以其作为地貌标志进行区域地貌演化和构造变形研究的关键。文章在阐述低起伏高海拔地貌面形成模式的基础上,以蔚县-广灵盆地南缘山地上发育的甸子梁面为例,探索该级低起伏高海拔地貌面的成因机制。结果表明,甸子梁面是残留的准平原,其被准平原化后分别经历了新近纪早期构造抬升、新近纪晚期断裂形成、唐县期山麓剥蚀以及第四纪断陷等4个时期。蔚县-广灵盆地南缘断裂自开始正断运动以来,该断裂总的水平断错量约902±71 m,总的垂直断错量约1567±5 m,其中在唐县期之前（>3 Ma）的垂直断错量约276±16 m,在唐县期之后（< 3 Ma）的垂直断错量约1291±15 m。蔚县-广灵盆地南缘断裂在距今3 Ma以来的活动性稳定,不同时间尺度的垂直断错速率均约0.4~0.5 mm/a。

     

人类活动对全球淡水循环影响与水行星边界评估研究进展

近几十年来,人类活动对淡水循环的影响已经超出了流域尺度。解决目前所面临的水资源问题,不应只着眼于流域过程,而应从更大的空间尺度——全球尺度对地球系统过程加以研究。文章总结了人类活动对全球淡水循环的影响途径,论述了水行星边界方法框架与研究进展,分析了淡水利用管理从流域尺度到全球尺度的实现方法。水行星边界方法以地球系统科学理论为指导,以保持地球系统稳定运行为目标,将淡水循环研究视野从流域尺度扩展至全球尺度,定量描述了全球淡水利用安全运行空间,成为评价全球水资源可持续性的重要方法。水资源管理既要考虑流域尺度的水资源、生态与经济社会发展关系,也要考虑全球尺度的水循环、地球系统稳定与全球可持续发展关系。水行星边界是对现有区域水资源承载能力评价方法的重要补充,为全球与区域淡水资源治理提供了重要的方法框架。根据我国自然资源管理新制度下水资源管理需要,针对水行星边界评估,建议加强全球淡水循环过程研究,完善全球水文模型与模拟预测平台;加强水行星边界与土地、生物、大气等其他行星边界的相互影响与作用研究;探索应用水行星边界框架完善资源环境承载能力评价和国土空间开发适宜性评价方法。     

Age and impacts of the caldera-forming Aniakchak II eruption in western Alaska

The mid-Holocene eruption of Aniakchak volcano (Aniakchak II) in southwest Alaska was among the largest eruptions globally in the last 10,000 years (VEI-6). Despite evidence for possible impacts on global climate, the precise age of the eruption is not well-constrained and little is known about regional environmental impacts. A closely spaced sequence of radiocarbon dates at a peatland site over 1000 km from the volcano show that peat accumulation was greatly reduced with a hiatus of approximately 90–120 yr following tephra deposition. During this inferred hiatus no paleoenvironmental data are available but once vegetation returned the flora changed from a Cyperaceae-dominated assemblage to a Poaceae-dominated vegetation cover, suggesting a drier and/or more nutrient-rich ecosystem. Oribatid mites are extremely abundant in the peat at the depth of the ash, and show a longer-term, increasingly wet peat surface across the tephra layer. The radiocarbon sample immediately below the tephra gave a date of 1636–1446 cal yr BC suggesting that the eruption might be younger than previously thought. Our findings suggest that the eruption may have led to a widespread reduction in peatland carbon sequestration and that the impacts on ecosystem functioning were profound and long-lasting.     

基于滑带强度参数动态演化的滑坡稳定性研究

基于滑带应力状态分析和室内试验,提出了基于滑带强度参数动态演化的滑坡稳定性评价方法,并将应力状态作为评价滑面力学参数的主要指标,以合理地反映滑面强度状态对滑坡稳定性的影响。通过室内试验,获得了滑面强度参数与工程性状、力学参数之间的量化关系,在该基础上,通过数值计算,对滑面的力学参数和滑坡的稳定性进行三维计算。计算结果表明,利用提出的方法能较好地反映出滑带强度参数的空间分区特征。同时,对工况变动情况下滑带的强度参数进行量化取值,并对滑坡的稳定性进行动态计算,提出的计算方法更能体现出滑坡力学参数与稳定性的动态变化过程。     

华北盆地地震剖面地质解释及其构造演化

从地震剖面显示华北盆地发育为手风琴式演化史,大体上形成有三层“断-坳”结构：（l）中新元古界的“断”古生界—三叠系的“坳”;（2）侏罗系的“断”上白垩统的“坳”;（3）古近系的“断”新近系的“坳”。断陷与坳陷分别由拉张与挤压应力场所致,这种应力场的变化是由相邻洋壳板块俯冲倾角由小到大的变化所引起的。由于地壳的多旋回运动,多次发生构造沉积演变,构成了多套生储盖组合,多领域、多种圈闭类型的油气藏,从中新元古界—古生界—中生界—新生界,各断陷-坳陷结构的盆地都有可能形成油气藏的地质条件。     

中国冰冻圈水文未来变化及其对干旱区水安全的影响

受气候变暖持续影响, 中国冰冻圈水文过程正在发生着显著变化。在梳理已有过去冰冻圈融水变化基础上, 重点分析了冰冻圈融水径流未来变化特点, 尤其是对冰川融水拐点及温升2 ℃阈值情况下, 到2050年和21世纪末, 冰川融水的可能变化进行了辨析, 进而研判了冰冻圈水文变化对流域、 重点是干旱区内陆河流域水安全的影响。研究表明, 中国未来大部分流域冰川融水径流总体呈现减少趋势, 并呈现冰川径流持续减少、 在不久的将来出现峰值和持续增加三种情况; 单条冰川融水可能会出现拐点, 而且拐点是否出现和出现的时间与升温速率和冰川面积大小有关; 在流域尺度上, 冰川覆盖率较大、 大型冰川面积占比较高的流域, 到2050年融水径流持续稳定增加; 冰川规模较小的流域, 冰川径流峰值早已出现; 介于上述两种情况之间的大多数流域, 冰川融水峰值在2020 - 2030年相继出现或变化呈现不显著增加或减少, 相对稳定; RCP情景温升2 ℃阈值下, 到21世纪末, 西北干旱区冰川融水量减少34% ~ 74%。冰冻圈水文变化导致水源涵养能力下降、 径流补给量减少、 对水资源的调节作用减弱、 流域径流变化幅度增加、 发生旱涝的风险增加、 春汛提前进而影响用水制度。     

The melt rocks of the Vredefort impact structure - Vredefort Granophyre and pseudotachylitic breccias: Implications for impact cratering and the evolution of the Witwatersrand Basin

The unique combination of its large size (250-300 km diameter), deep levels of erosion (>7 km), and widespread regional mining activity make the Vredefort impact structure in South Africa an exceptional laboratory for the study of impact-related deformation phenomena in the rocks beneath giant, complex impact craters. Two types of impact-generated melt rock occur in the Vredefort Structure: the Vredefort Granophyre - impact melt rock - and pseudotachylitic breccias. Along the margins of the structure, mining and exploration drilling in the Witwatersrand goldfields has revealed widespread fault-related pseudotachylitic breccias linked to the impact event. There, volumetrically limited melt breccia occurs in close association with cataclasite or mylonitic zones associated with bedding-parallel normal dip-slip faults that formed during inward slumping of the crater walls, and in rare subvertical faults oriented radially to the center of the structure. This association is consistent with formation of pseudotachylites by frictional melting. On the other hand, rocks in the Vredefort Dome - the central uplift of the impact structure - contain ubiquitous melt breccias that range in size from sub-millimeter pods and veinlets to dikes up to tens of meters wide and hundreds of meters long. Like fault-related pseudotachylites in the goldfields and elsewhere in the world, they display a close geochemical relationship to their wallrocks, indicating local derivation. However, although mm/cm- to, rarely, dm-scale offsets are commonly found along their margins, they do not appear to be associated with broader fault zones, are commonly considerably more voluminous than most known fault-related pseudotachylites, and show no consistent relationship between melt volumes and slip magnitude. Recent petrographic observations indicate that at least some of these melt breccias formed by shock melting, with or without frictional melting. Consequently, the non-genetic term “pseudotachylitic breccia” has been adopted for these Vredefort occurrences. These breccias formed during the impact in rocks at temperatures ranging from greenschist to granulite facies, and were subsequently annealed to varying degrees during cooling of the central uplift.In addition to the pseudotachylitic breccias, nine clast-laden impact melt dikes (Vredefort Granophyre), each up to several kilometers long, occur in vertical radial and tangential fractures in the Vredefort Dome. Unlike the pseudotachylitic breccias, they display a remarkably uniform bulk composition and clast populations that are largerly independent of their wallrocks, and they contain geochemical traces of the impactor. They represent intrusive offshoots of the homogenized impact melt body that originally lay within the crater. U-Pb single zircon and Ar-Ar dating indicates that the Vredefort Granophyre and pseudotachylitic breccias, and the Witwatersrand pseudotachylites all formed at 2020±5 Ma - the age of the impact event, making the breccias a convenient time marker in the evolution of the structurally complex Witwatersrand basin with its unique gold deposits.