Identifying impact events within the lunar cataclysm from Ar–Ar ages and compositions of Apollo 16 impact melt rocks

Lunar impact melt breccias provide a unique record of the timing and frequency of collisional events during the early history of the inner Solar System prior to the development of a significant rock record on Earth. The predominance of ages clustering between 3.8 and 4.0 Ga was a major, unexpected discovery obtained from geochronological studies of lunar impact melts, and is the basis of the concept that a cataclysmic bombardment of large planetesimals struck the Earth and Moon, and possibly the entire inner Solar System, about 3.85 ± 0.10 billion years ago. As a test of the cataclysm hypothesis, we measured high-resolution (20–50 steps) ⁴⁰Ar–³⁹Ar age spectra on 25 samples of Apollo 16 impact melt breccias using a continuous laser heating system on sub-milligram fragments. Twenty-one of these 25 breccias produced multi-step plateaus that we interpret as crystallization ages, with 20 of these ages falling in the range 3.75–3.96 Ga. We propose that at least four different melt-producing impact events can be distinguished based on the ages, bulk compositions, and petrographic characteristics of Apollo 16 melt breccias. The recognition of multiple impact events within the Apollo 16 melt breccia suite shows that numerous impact events occurred on the lunar surface within a relatively narrow time interval, providing additional evidence of a heavy bombardment of the Moon during the early Archean.     

Water and astrobiology

Water is formed from two of the three most abundant elements in the universe and so is abundant in interstellar space, in our Solar System, and on Earth, where it is an essential compound for the existence of life as we know it. Water ice acts as a substrate and reactant in interstellar clouds of gas and dust, enabling the formation of organic compounds that are important precursors to life and that eventually became incorporated into comets and asteroids in the early Solar System. Laboratory experiments have allowed us to infer the reaction pathways and mechanisms by which some of these compounds are formed. In these reactions, water can act as an energy transfer medium, increasing product yields, or it can lower yields by diluting reaction centers. Water can also destroy organic compounds when water ice decomposes under ionizing radiation and the decomposition products attack the compounds; whether this happens depends critically on temperature and structure of the ice, whether crystalline or amorphous. Ice structure and temperature also largely determine its gas content. As the solar nebula collapsed, icy mantles on interstellar grains probably sublimated and then recondensed onto other grains, thus influencing the transport of energy, mass, and angular momentum in the disk. Icy grains also influenced the temperature structure of the disk because they influence mean disk opacity. Outside the “snow line” at 3–5 AU icy grains accreted to become part of comets and planetesimals that occupy the region of the outer planets, the Kuiper belt, and the Oort cloud. Water was acquired by the growing Earth by several mechanisms. Evidence from noble gas isotopes indicates that Earth achieved sufficient mass fast enough to capture an early H-rich atmosphere from the Solar nebula itself. Although the remnant of this primary atmosphere is now found only in the mantle, it may also reside in the core, which could contain most of the H on Earth (or none at all). The bulk silicate Earth contains only 500–1100 ppm H₂O, an amount small enough to explain by “wet” accretion, although most of it probably accumulated with the latter half of Earth's mass from wetter planetary embryos originating beyond 1.5 AU. Degassing on impact delivered water to Earth's surface, where it dissolved into a magma ocean, a process that likely saved it from loss during subsequent catastrophic impacts such as the Moon-forming giant impact, which resulted in >99% loss of the noble gas inventory. Although most of Earth's water probably came from meteoritic material, the depletion on Earth of Xe relative to Kr strongly suggests a role for comets. The role of water in supporting life is an essential one on Earth and probably elsewhere, given the unusual properties of water compared with other potentially abundant compounds. Its dipolarity, high boiling point and heat of vaporization and, for ice, melting temperature; its expansion on freezing; and its solvent properties make it an ideal medium for life. Life originated early on Earth, indicating an abundance of water, nutrients, precursor molecules, substrates, and appropriate physical and chemical conditions. Life adapted quickly to (and may have originated in) extreme environments, of heat, cold, dryness, saltiness, and acidity. This adaptation to extreme conditions bodes well for the prospect of finding life elsewhere in our Solar System and in planetary systems around other stars.     

The wavelet-smoothed distribution of young stellar objects in the Galactic plane

We have analyzed the distribution of young objects (open clusters, classical Cepheids, and HII regions) projected onto the plane of the Galaxy using wavelet smoothing. This smoothing technique enables investigation of the large-scale structure of the distribution of young objects. All the studied objects display a similar spiral structure, whose appearance essentially corresponds to a regular alternation of young and older stellar complexes along sections of the spiral arms. A four-arm spiral structure with its arms originating at the Galactic center is obtained if the major arms are taken to be the Carina-Sagittarius arm and an outer arm behind the Perseus arm. If all the observed arms are taken to be major arms, we obtain a 12-arm structure, in contradiction with the size of the region in which the structure is observed in the Galaxy. This discrepancy can be removed if the arms originate from a ring formed by a sufficiently long bar-like structure, rather then from the Galactic center.     

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.     

Volcanism in the rift‐valley system that evolved into the western margin of Australia

The Mesozoic forerunner of the western margin of Australia has been regarded tectonically as an ancient analogue of the multiple rift‐valley system of East Africa, which comprises two arms: volcanic on the E, and virtually non‐volcanic on the W. The abundance of widespread volcanics recently dredged and cored along the outermost margin, which corresponds with the volcanic arm of the East African system, contrasts with the apparent scarcity of volcanics inshore, in the inner arm of the rift system. We tested the possibility that volcanogenic material has been overlooked inshore by a petrographic study of the Late Jurassic to earliest Cretaceous Yarragadee Formation of the Perth Basin; only rare possible pyroclasts of quartz and glass (probably emplaced by air‐fall from the volcanic outer arm) were found, confirming the contrast in volcanism between the arms. This petrological evidence, together with the appropriate range of composition of the volcanism, from silicic to mafic, including alkaline and peralkaline members, reinforces the analogy with East Africa.     

On the role of Geography in Earth System Science

Geography is fundamentally a non-reductionist and holistic discipline. While we tend to focus on particular areas (Physical, Human, etc.), or we focus on specific successes (Quaternary studies for example) this paper argues that selling Geography though emphasizing these specific areas or strengths misses a major potential contribution our discipline can make. While most sciences have become reductionist over the last two centuries, they have recently discovered that the Earth is a “complex system” with “emergent” properties that cannot be explained through understanding the components parts individually. Many of these sciences are now contributing to a major effort called Earth System Science, an integrative super-discipline that accepts that biophysical sciences and social sciences are equally important in any attempts to understand the state, and future of the Earth System. This paper argues that the development of Earth System Sciences is a risk for Geography since it is, in effect, Geography with few Geographers. While representing a threat, the development of Earth System Science is also an opportunity. I argue that Geography could be a lead discipline among the other biophysical and social sciences that are now building Earth System Science to address key problems within the Earth System. While I am optimistic about the potential of Geography to take this leadership role, I am pessimistic about the likelihood that we will. I provide suggestions on how we might take on the leadership of Earth System Science including individual engagement and a refinement of tertiary training of some Geography students.     

The origin of organic matter in the solar system: evidence from the interplanetary dust particles

The detailed examination of meteorites and interplanetary dust particles provides an opportunity to infer the origin of the organic matter found in primitive Solar System materials. If this organic matter were produced by aqueous alteration of elemental (graphitic or amorphous) carbon on an asteroid, then we would expect to see the organic matter occurring preferentially in interplanetary materials that exhibit evidence of aqueous activity, such as the presence of hydrated silicates. On the other hand, if the organic matter were produced either during the nebula phase of Solar System evolution or in the interstellar medium, we might expect this organic matter to be incorporated into the dust as it formed. In that case pre-biotic organic matter would be present in both the anhydrous and the hydrated interplanetary materials. We have performed carbon X-ray absorption near-edge structure spectroscopy and infrared spectroscopy on primitive anhydrous and hydrated interplanetary dust particles (IDPs) collected by NASA from the Earth's stratosphere. We find that organic matter is present in similar types and abundances in both the anhydrous and the hydrated IDPs, and, in the anhydrous IDPs some of this organic matter is the “glue” that holds grains together. These measurements provide the first direct, experimental evidence from the comparison of extraterrestrial samples that the bulk of the pre-biotic organic matter occurs in similar types and abundances in both hydrated and anhydrous samples. This indicates that the bulk of the pre-biotic organic matter in the Solar System did not form by aqueous processing, but, instead, had already formed at the time that primitive, anhydrous dust was being assembled. Thus, the bulk of the pre-biotic organic matter in the Solar System was formed by non-aqueous processing, occurring in either the Solar nebula or in an interstellar environment. Aqueous processing on asteroids may have altered this pre-existing organic matter, but such processing did not affect in any substantial way the C=O content of the organic matter, the aliphatic C-H abundance, or the mean aliphatic chain length.     

中朝板块旋回层序、事件和形成演化的探索

通过 10余年来对中朝板块旋回层序、事件及构造演化的调查研究和综合分析 ,笔者编制了中朝板块旋回层序、事件和形成演化综合图 ,深刻地认识到地质科学是复杂性的科学体系。复杂系统研究要求重组现有的科学划分 ,实现跨越岩石矿物学、古生物学、层序地层学、地质地球化学、磁性地层学、古地理学、古构造学、天文学、海洋学及矿床学等不同学科的协同 ,从而促进学科的交叉和融合 ,以求学科体系理论和方法论的深刻变革 ,以推动地质科学的发展。这就是笔者提出以宇地系统观来综合研究地球、层序、事件和演化的基本思路。依据宇地系统作用原理 ,研究了中朝板块从元古宙至中新生代的旋回层序、事件和演化历史。从 180 0Ma至第四纪共划分出 2 0个超旋回或巨旋回周期和百余个奥尔特周期 ;还研究了海泛事件、隆升间断事件、跷跷板运动事件、臼齿碳酸盐事件、热事件、反极性事件、岩溶事件、火山喷发事件、风暴事件、物源区类型及古构造演化等一系列重大事件。应用宇地系统周期等时原理对中朝板块拗拉谷、鲕滩台地建造、前陆盆地演化和陆相裂谷盆地 ,按长偏心率周期进行了古地理演化的精细研究 ,再现了中朝板块构造、古地理和地层格架的演变 ,并对具全球性意义的大气圈、水圈、生物圈、沉积圈及成矿作用等重大地球     

Native iron: Greenland's natural blast furnaces

We live in an oxidized world: oxygen makes up 22 percent of the atmosphere and by reacting with organic matter produces most of our energy, including the energy our bodies use to function: breathe, think, move, etc. It has not always been thus. Originally the Earth, in common with most of the Solar System, was reduced. The oxidized outer layers of the Earth have formed by two processes. Firstly, water is decomposed to oxygen and hydrogen by solar radiation in the upper parts of the atmosphere, the light hydrogen diffusing to space, leaving oxygen behind. Secondly, plants, over the course of geological time have utilized solar energy in the process of photosynthesis to produce carbon‐rich materials and release oxygen to the atmosphere. Of these, the second is by far the most important. It is a consequence of life and since about 2.4 billion years ago we have had an oxidizing atmosphere, a situation unique in the Solar System. In such a world, iron metal is unstable and, as we all know, oxidizes to the ferric iron compounds we call ‘rust’. If we require iron metal it must be produced at high temperatures by reacting iron ore, usually a mixture of ferrous (Fe²⁺) and ferric (Fe³⁺) oxides (Fe₂O₃, hematite, or FeO.Fe₂O₃, magnetite), with carbon in the form of coke. This is carried out in a blast furnace. Although the Earth's core consists of metallic iron, which may also be present in parts of the mantle, this is inaccessible to us, so we must make our own. In West Greenland, however, some almost unique examples of iron metal, otherwise called ‘native iron’ or ‘telluric iron’, occur naturally.     

On the spiral structure of the Milky Way Galaxy

We consider a possible scheme of the overall spiral structure of the Galaxy using data on the distribution of neutral (atomic), molecular, and ionized hydrogen. Our analysis assumes that the spiral structure is symmetric, i.e., that the spiral arms are translated into each other via a rotation about the Galactic center by 180° (a two-arm pattern) or 90° (a four-arm pattern). In the inner region, the observations are best represented with a four-arm spiral pattern, associated with all-Galaxy spiral density waves. The initial position is that of the Carina arm, reliably determined from distances to HII regions and from HI and H₂ radial velocities. This pattern continues in quadrants III and IV with weak outer HI arms; from their morphology, the Galaxy should be considered an asymmetric multi-arm spiral. The knee-like shape of the outer arms, which consist of straight segments, may indicate that these arms are transient formations that appeared due to gravitational instability in the gaseous disk. The distances between HI superclouds in the two arms that are brightest in neutral hydrogen, the Carina and Cygnus (Outer) arms, are concentrated near two values, suggesting the presence of a regular magnetic field in these arms.     

行星构造:寻求地球演化的踪迹

地质构造是记录地球内、外动力地质作用过程的标志.和地球相似,太阳系其他天体上也发育丰富的地质构造.以研究天体表面的地质构造及其动力学机制为目的的"行星构造学"是建立在构造地质学、遥感地质学和地球物理学等学科基础上的一门新兴前沿学科.由于天体的大小、组分和轨道位置不同,表面构造特征及其形成机制各异.对比研究地球和其他天体...     

黄土与地球系统--李希霍芬对黄土研究的贡献及对地球系统科学研究的现实意义

李希霍芬是19世纪德国著名的地质学家,对欧洲和中国黄土的研究工作,以及后来的黄土研究有着深刻影响,其学术思想在地球系统科学研究的时代依然有重要的现实意义。文章简要回顾了李希霍芬对黄土研究的贡献,并探讨这些贡献与黄土研究在地球系统科学研究中的现实意义。从李希霍芬时代到今天,黄土作为全球宝贵的地球系统演化的历史记录,在揭示地球岩石圈、水圈、大气圈、冰冻圈、生物圈及宇宙事件对地球系统的影响发挥了重要作用,并在未来地球系统科学研究时代有旺盛的生命力。     

A pulse of the Earth:A 27.5-Myr underlying cycle in coordinated geological events over the last 260 Myr

We performed spectral analyses on the ages of 89 well-dated major geological events of the last 260 Myr from the recent geologic literature.These events include times of marine and non-marine extinctions,major ocean-anoxic events,continental flood-basalt eruptions,sea-level fluctuations,global pulses of intraplate magmatism,and times of changes in seafloor-spreading rates and plate reorganizations.The aggregate of all 89 events shows ten clusters in the last 260 Myr,spaced at an average interval of～26.9 Myr,and Fourier analysis of the data yields a spectral peak at 27.5 Myr at the≥96％confidence level.A shorter period of～8.9 Myr may also be significant in modulating the timing of geologic events.Our results suggest that global geologic events are generally correlated,and seem to come in pulses with an underlying～27.5-Myr cycle.These cyclic pulses of tectonics and climate change may be the result of geophysical processes related to the dynamics of plate tectonics and mantle plumes,or might alterna-tively be paced by astronomical cycles associated with the Earth's motions in the Solar System and the Galaxy.     

磁性示踪在土壤侵蚀研究中的应用进展

示踪技术在土壤侵蚀研究中已得到了较广泛的应用，尤其是利用核素示踪（如137Cs、7Be、210Pb和REE示踪）定量研究土壤侵蚀规律和侵蚀机理方面取得了一系列成果。近几年利用磁性示踪研究土壤侵蚀成为示踪技术在土壤侵蚀研究中应用的一种新思路。目前，在利用环境磁性的时空差异性来研究区域内侵蚀泥沙来源，以沉积物磁性作为流域环境变迁指示剂以及利用磁性参数的变化研究土壤侵蚀等方面已取得一些成果，同时也有人提出利用人工磁性示踪剂来研究土壤侵蚀强度的空间分异规律，定量描述侵蚀方式的演变及侵蚀过程的发生、发展规律，并在野外进行了小区试验。但总体上磁性示踪在土壤侵蚀研究中的应用还相对较少。就磁性示踪在土壤侵蚀研究中应用的理论基础以及目前的研究现状作了简要论述，并分析提出了其研究方向和应用前景。     

Chelyabinsk event as an astronomical phenomenon

The fall of a meteorite near the town of Chelyabinsk is considered from the viewpoint of astronomy, and the major witness facts and entry characteristics (including the measured entry velocity and the height of the explosion) are analyzed. The aerodynamic phenomena that accompanied the entry of the meteorite in the atmosphere at an ultrasonic velocity and the origin of a shock wave that induced damage on the Earth’s surface are analyzed. The paper also reports the estimated frequency of the falls of celestial bodies depending on their size, and consequences of collisions of these bodies with the Earth. It is emphasized that studies of small bodies in the Solar System can provide insight into the origin of the protoplanetary disk and the processes that produced the planets. The studies of small bodies, such as the Chelyabinsk meteorite, are directly related to the problem of asteroid and comet impact hazard (ACIH). The paper reports the sizes of potentially hazardous celestial bodies whose monitoring requires the deployment of a network of specialized telescopes on the Earth to mitigate ACH and a system of space-based systems for the identification and monitoring of such bodies in near space.     

稳定同位素记录与环境、生命演化中的重大事件

自从地球诞生以来经历了许多重大事件：早期生命的出现、大气氧化事件（Atmospheric oxygenation）、雪球地球及多次生物绝灭与复苏事件。稳定同位素记录在古环境和生命演化研究中具有重要意义,在记录环境变化和生命演化的重大事件中发挥着重大作用：碳同位素的分馏记录了最早生命的开始,硫同位素的非质量相关分馏（Independent mass fractionation of sulphur isotopes）记录了大气中氧含量的重大变化,而在显生宙的几次重大生物灭绝事件中,均有碳同位素的负向漂移。     

地球化学若干领域的回顾与展望

对地球化学的某些领域，如元素的丰度和起源；地球与太阳系的化学演化；资源的勘察、利用与成因研究；元素的循环与示踪；环境地球化学；天体化学；分析与实验地球化学等在20世纪所取得的进展作了简短回顾。并对地球化学的某些领域，如全球变化研究与生态环境保护、新的洁净能源与矿产资源、防灾减灾和保障人群健康、征服太阳系并使之为人类社会的可持续发展服务等方面的发展前景作了简要论述。     

月球晚期重轰击和Nice模式

晚期重轰击(一般又称为月球灾难,简称LHB)指的是距今约3.8~4.1 Ga时段月球受到大量陨石的轰击,于月面上形成的大量撞击坑,并推论地球、水星、金星和火星也经历了这样一次重轰击。Nice模式是关于太阳系动力学演化的一种设想:在初始原行星气体星盘消散之后很久,大行星从最初紧凑的组构迁移到目前的位置。这个行星迁移理论用来解释包括内太阳系的晚期重轰击,以及Oort云、Kuiper带、海王星和木星Trojans行星等形成的历史事件。     

白垩纪地球表层系统重大地质事件与温室气候变化研究

国家重点基础研究发展计划（973）项目“白垩纪地球表层系统重大地质事件与温室气候变化”，将以白垩纪与碳循环相关的重大地质事件和温室气候变化的关系为主线，以大洋缺氧事件—富氧作用转变过程和机制研究为突破口，进行海陆相整合研究，重点追溯东特提斯洋和我国大陆地球表层系统重大地质事件的记录，探讨这些事件与碳循环、快速气候变化的正/负反馈机制。项目将充分利用中国大陆发育完好的白垩纪海相、陆相地层及古生物记录，通过松辽盆地白垩系科学钻探全岩芯取样和多学科综合研究的途径，着眼于厘定反映地质事件和气候变化的层位及标志和时间格架，解决高分辨率海、陆相沉积事件的精确对比，分析地层记录中气候标志和古生物类群的地理分布，集中研究陆地和海洋环境对同一事件的响应机制，重溯白垩纪地球表层系统重大地质事件过程及成因，探究陆相烃源岩大规模形成、陆地生物群更替与温室气候变化和碳循环之间的正/负反馈关系和机制等科学问题，为预测全球长时间尺度上的气候变化趋势提供科学依据。     

Chronology of meteorites and the early solar system

Understanding the chronology of the chondritic and differentiated meteorites can potentially important constraints on the accretion and origin of the solar system planets, life-time of our protoplanetary disk and circumstellar disks around solar mass stars, and astrophysical setting of the solar system formation. The special issue of Geochimica et Cosmochimica Acta consists of invited and contributed papers presented at the Workshop on The Chronology of Meteorites and the Early Solar System, Kauai, 2007 and is honoring the outstanding contributions of C.J. Allégre, G.W. Lugmair, L.E. Nyquist, D.A. Papanastassiou, and G.J. Wasserburg to our understanding of the chronology of the early Solar System.