Ancient Meteorite Finds and the Earth's Surface Environment

2000 ◽  
Vol 53 (2) ◽  
pp. 131-142 ◽  
Author(s):  
Philip A. Bland ◽  
Alex W. R. Bevan ◽  
A. J. Tim Jull
Keyword(s):  
Age Distribution ◽  
Cosmogenic Radionuclides ◽  
Cold Desert ◽  
Frequency Distributions ◽  
Carbon Isotopic ◽  
The Earth ◽  
History Of ◽  
Tropical Areas ◽  
Surface Environment ◽  
Terrestrial Age

AbstractThe flux of meteorites to the Earth over the last 50,000 yr has remained approximately constant. Most meteorites that fall in temperate or tropical areas are destroyed on a time scale which is short compared to the rate of infall; however, in arid regions (both “hot” deserts and the “cold” desert of Antarctica) weathering is slower and accumulations of meteorites may occur. The initial composition for many meteorite groups is well known from modern falls, and terrestrial ages may be established from analyses of the abundance of cosmogenic radionuclides, providing an absolute chronology for recording terrestrial processes. As samples are falling constantly, and are distributed approximately evenly over the Earth, meteorites may thus be thought of as an appropriate “standard sample” for studying aspects of the terrestrial surface environment. Studies involving 14C and 36Cl terrestrial ages of meteorites, 57Fe Mössbauer spectroscopy (to quantify the degree of oxidation in samples), stable isotopes, and determination of halogen abundances are yielding information on the terrestrial history of meteorites: (i) terrestrial age and oxidation-frequency distributions for populations of samples allow the ages of surfaces to be estimated; (ii) differences in the weathering rate of samples between sites allows constraints to be imposed on the effect of climate on rock weathering rates; (iii) carbon isotopic compositions of generations of carbonate growth within meteorites allows, in some cases, temperatures of formation of carbonates to be estimated; (iv) structure in the oxidation–terrestrial age distribution for meteorites from some arid accumulation sites (specifically, the Nullarbor of Australia) appears to be linked to previous humid/arid cycles; (v) meteorite accumulations in Antarctica have been used to constrain aspects of the Quaternary evolution of the ice sheet, and terrestrial age and oxidation data have been used to constrain ice flow.

Analytical thermochronometric models of Earth’s crust during the late accretionary bombardment epoch (4.5-3.5 Ga)

2020 ◽  
Author(s):  
Stephen J. Mojzsis ◽  
Oleg Abramov
Keyword(s):  
Age Distribution ◽  
Three Dimensional ◽  
Plate Boundary ◽  
Impact Angle ◽  
Asteroid Belt ◽  
Size Frequency Distribution ◽  
Frequency Distributions ◽  
The Earth ◽  
Size Frequency ◽  
The Impact

<p>Late accretionary bombardments in the first billion years of solar system history strongly affected the initial physical and chemical states of the Earth. Evidence of ancient impacts can be preserved in the oldest known terrestrial zircons with ages up to ca. 4.4 Ga. Here, we use the Hadean zircon record to directly assess the thermal effects of impact bombardment on the early Earth’s crust, couple the results to models of closure temperature-dependent diffusive loss and U-Pb age-resetting in zircon, derive zircon ages, and compare them to published ages.</p><p>The impact bombardment model consists of (i) a stochastic cratering model which populates the surface with craters within constraints derived from the lunar cratering record, the size/frequency distribution of the asteroid belt, and dynamical models; (ii) analytical expressions that calculate a temperature field for each crater; and (iii) a three-dimensional thermal model of the terrestrial lithosphere, where craters are allowed to cool by conduction and radiation. Equations for diffusion in zircon are coupled to these thermal models to estimate the amount of age-resetting.</p><p>We present modeling results for the Earth between 4.5 Ga and 3.5 Ga based new mass-production functions. Mean surface temperatures and geothermal gradients were assumed as 20 °C and 70 °C/km. Total delivered mass was estimated at 0.0013(M<sub>planet</sub>), or 7.8 × 10<sup>21</sup> kg. The size-frequency distributions of the impacts were derived from dynamical modeling. We begin model runs with a global magma ocean, which would have been formed by the Moon-forming impact. Mean impactor density of 3000 kg/m<sup>3</sup> and impactor velocity distribution from [1,2] was used, and impact angle of each impactor was stochastically generated from a gaussian centered at 45 degrees. The typical impact velocity of the Earth is ~21 km s<sup>-1</sup>.</p><p>It is important to note that the model age outputs we report omit normal processes of generation of zircon-saturated magmas that were operative in the Hadean. We find that as the impact flux decreases with time and becomes negligible for the purposes of thermal modeling by ca. 3.5 Ga. We find that the probability of randomly selecting a zircon of a given age increases with increasing age, predicting a large number of very old zircons. This contrasts with the actual age distribution of Hadean zircons, which, for >4 Ga, indicates the opposite case: the probability of selecting a zircon of a given age decreases with increasing age. We interpret this discrepancy to mean that impacts were not the dominant process in determining the ages of Hadean zircons. This is consistent with observations that the majority of Hadean zircons had formation temperature significantly lower than those expected for melt sheets and thermobarometry measurements suggesting formation of some Hadean zircons in a plate boundary environment.</p><p>[1] Mojzsis, S.J. et al. (2019). Astrophys. J., 881, 44. [2] Brasser, R. et al. (2020) Icarus 338, 113514. </p>

George Hoggart Toulmin: politics and geology

1978 ◽  
Vol 8 (4) ◽  
pp. 481-481
Author(s):  
ROY PORTER
Keyword(s):  
French Revolution ◽  
Social Order ◽  
Lower Class ◽  
Great Distance ◽  
The Earth ◽  
Christian View ◽  
The Social ◽  
French Enlightenment ◽  
Political Conservatism ◽  
History Of

The physician George Hoggart Toulmin (1754–1817) propounded his theory of the Earth in a number of works beginning with The antiquity and duration of the world (1780) and ending with his The eternity of the universe (1789). It bore many resemblances to James Hutton's "Theory of the Earth" (1788) in stressing the uniformity of Nature, the gradual destruction and recreation of the continents and the unfathomable age of the Earth. In Toulmin's view, the progress of the proper theory of the Earth and of political advancement were inseparable from each other. For he analysed the commonly accepted geological ideas of his day (which postulated that the Earth had been created at no great distance of time by God; that God had intervened in Earth history on occasions like the Deluge to punish man; and that all Nature had been fabricated by God to serve man) and argued they were symptomatic of a society trapped in ignorance and superstition, and held down by priestcraft and political tyranny. In this respect he shared the outlook of the more radical figures of the French Enlightenment such as Helvétius and the Baron d'Holbach. He believed that the advance of freedom and knowledge would bring about improved understanding of the history and nature of the Earth, as a consequence of which Man would better understand the terms of his own existence, and learn to live in peace, harmony and civilization. Yet Toulmin's hopes were tempered by his naturalistic view of the history of the Earth and of Man. For Time destroyed everything — continents and civilizations. The fundamental law of things was cyclicality not progress. This latent political conservatism and pessimism became explicit in Toulmin's volume of verse, Illustration of affection, published posthumously in 1819. In those poems he signalled his disapproval of the French Revolution and of Napoleonic imperialism. He now argued that all was for the best in the social order, and he abandoned his own earlier atheistic religious radicalism, now subscribing to a more Christian view of God. Toulmin's earlier geological views had run into considerable opposition from orthodox religious elements. They were largely ignored by the geological community in late eighteenth and early nineteenth century Britain, but were revived and reprinted by lower class radicals such as Richard Carlile. This paper is to be published in the American journal, The Journal for the History of Ideas in 1978 (in press).

Computers in Geology - 25 Years of Progress

1994 ◽  
Keyword(s):  
Computer Modeling ◽  
Quantitative Methods ◽  
State Of The Art ◽  
International Association ◽  
Mathematical Geology ◽  
25Th Anniversary ◽  
The Earth ◽  
Wide Range ◽  
History Of ◽  
Mapping Techniques

This volume vividly demonstrates the importance and increasing breadth of quantitative methods in the earth sciences. With contributions from an international cast of leading practitioners, chapters cover a wide range of state-of-the-art methods and applications, including computer modeling and mapping techniques. Many chapters also contain reviews and extensive bibliographies which serve to make this an invaluable introduction to the entire field. In addition to its detailed presentations, the book includes chapters on the history of geomathematics and on R.G.V. Eigen, the "father" of mathematical geology. Written to commemorate the 25th anniversary of the International Association for Mathematical Geology, the book will be sought after by both practitioners and researchers in all branches of geology.

The Thermal History of the Earth

Nature ◽  
1956 ◽  
Vol 177 (4500) ◽  
pp. 155-157 ◽  
Author(s):  
J. A. JACOBS ◽  
D. W. ALLAN
Keyword(s):  
Thermal History ◽  
The Earth ◽  
History Of

Early accretional history of the Earth and the Moon-forming event

Lithos ◽  
1993 ◽  
Vol 30 (3-4) ◽  
pp. 207-221 ◽  
Author(s):  
Stuart Ross Taylor
Keyword(s):  
The Moon ◽  
The Earth ◽  
History Of

scholarly journals In-Situ Cosmogenic 14C: Production and Examples of its Unique Applications in Studies of Terrestrial and Extraterrestrial Processes

Radiocarbon ◽  
2001 ◽  
Vol 43 (2B) ◽  
pp. 731-742 ◽  
Author(s):  
D Lal ◽  
A J T Jull
Keyword(s):  
Half Life ◽  
Earth Science ◽  
Chemical Properties ◽  
Radioactive Isotopes ◽  
Earth’S Atmosphere ◽  
The Earth ◽  
Wide Range ◽  
History Of ◽  
Earth's Atmosphere

Nuclear interactions of cosmic rays produce a number of stable and radioactive isotopes on the earth (Lai and Peters 1967). Two of these, 14C and 10Be, find applications as tracers in a wide variety of earth science problems by virtue of their special combination of attributes: 1) their source functions, 2) their half-lives, and 3) their chemical properties. The radioisotope, 14C (half-life = 5730 yr) produced in the earth's atmosphere was the first to be discovered (Anderson et al. 1947; Libby 1952). The next longer-lived isotope, also produced in the earth's atmosphere, 10Be (half-life = 1.5 myr) was discovered independently by two groups within a decade (Arnold 1956; Goel et al. 1957; Lal 1991a). Both the isotopes are produced efficiently in the earth's atmosphere, and also in solids on the earth's surface. Independently and jointly they serve as useful tracers for characterizing the evolutionary history of a wide range of materials and artifacts. Here, we specifically focus on the production of 14C in terrestrial solids, designated as in-situ-produced 14C (to differentiate it from atmospheric 14C, initially produced in the atmosphere). We also illustrate the application to several earth science problems. This is a relatively new area of investigations, using 14C as a tracer, which was made possible by the development of accelerator mass spectrometry (AMS). The availability of the in-situ 14C variety has enormously enhanced the overall scope of 14C as a tracer (singly or together with in-situ-produced 10Be), which eminently qualifies it as a unique tracer for studying earth sciences.

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