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June 2017 Archives - Elements
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Posts Tagged ‘June 2017’

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Rock and mineral coatings, the thin and fascinating varnishes portrayed in this issue of Elements, are found in deserts, soils, sediment, oceans, Mars and on buildings or other manmade structures. They record the legacy of millions of years of ocean circulation, thousands of years of climate change, and tens of years of anthropogenic contaminant dispersal. Rock coatings teach us how life can both dissolve and form minerals and how it can survive some of the most extreme environmental conditions. Our early ancestors used petroglyphs carved into these coatings to depict hunting scenes, religious and cultural context, and to communicate geographical, and even astronomical, information. On every continent (except Antarctica), we can view some of our cultural heritage, preserved over thousands of years. Rock coatings have even been spotted on Mars by the NASA rovers Spirit and Opportunity.

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Minerals: A Very Short Introduction

Minerals: A Very Short Introduction by David J. Vaughan is one of a series of over 400 volumes of “very short introductions” on a large variety of topics published by Oxford University Press. Begun in 1995, the volumes are meant to have an expert author present an introduction to a topic “for anyone wanting a stimulating and accessible way into a new subject”. This book certainly hits that mark and is a very effective introduction to mineralogy; indeed, the book should be required reading for the lay public and, particularly, for scientists outside of the broad discipline of mineralogy.

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RIMG 82: Non-Traditional Stable Isotopes

Thirteen years ago, in 2004, the book Geochemistry of Non-Traditional Isotopes [Volume 55 of the Reviews in Mineralogy and Geochemistry (RiMG) series] was published. Since then, tremendous advances in multi-collector inductively coupled plasma (ICP) mass spectrometry has made the precise measurement of additional isotope systems possible. This, together with advances in the calculation of equilibrium isotope fractionation using ab initio methods, has led to an unbelievable increase of publications, making it hard for the interested reader to keep up. Therefore, the publication of Non-Traditional Stable Isotopes (RiMG 82) is highly welcome.

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OSIRIS-REX: The Journey to Asteroid Bennu and Back

In May 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third of its New Frontiers program missions. The previous, yet ongoing, two New Frontiers missions are New Horizons—which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on 1 January 2019—and Juno—an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The OSIRIS-REx spacecraft departed for near-Earth asteroid (101955) Bennu aboard a United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. eastern daylight time (EDT) on 8 September 2016 for a seven-year journey to return samples from Bennu. Bennu is an Earth-crossing asteroid that has an orbital semi-major axis of 1.1264 AU, which is greater than that of the Earth, but a perihelion distance of 0.89689 AU, less than the Earth’s aphelion distance.

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Rock Coatings and the Potential for Life on Mars

Rock coatings observed on the surface of Mars since the Viking landers of the mid-1970s continue to generate considerable interest in the field of astrobiology. Terrestrial rock coatings are associated with diverse microbial life, raising the question as to whether martian rock coatings might also be partly of biogenic origin. Rock coating formation can be mediated by microbes, which in turn are shielded from harmful radiation and can obtain nutrients from the coating. In addition, coatings may be indicative of life and can preserve its signatures. Pragmatically, rock coatings are surface environments accessible by Mars rovers. This article focuses on the habitability, preservation capability, and practicality of rock coatings as astrobiological targets on Mars.

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Climate Change, Rock Coatings, and the Archaeological Record

Rock varnish commonly occurs in arid environments. It can coat not only rock surfaces but also any exposed prehistoric stone artifacts and rock art. Thin sections of varnish coatings reveal microlaminations that reflect major changes between wet versus dry paleoclimates. These microlaminations can be used to assign minimum ages to the underlying rock surface, providing dates for otherwise undateable stone artifacts and rock art. This dating approach has provided important information about the peopling of the Americas (North and South America), including how and when the native American populations adapted to changing climatic conditions.

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Marine Ferromanganese Encrustations: Archives of Changing Oceans

Fungi are ubiquitous inhabitants of rock and mineral surfaces and are significant geoactive agents. Capable of numerous transformations of metals and minerals, fungi can prosper in the most adverse of environments, their activities underpinned by growth form and metabolism. Free-living filamentous species, microcolonial fungi and lichens can significantly change a rock’s surficial structure and appearance, ranging from discolouration and staining to biodeterioration and the formation of new biogenic minerals and rock coatings. The presence and activity of fungi should be considered in any study of rock and mineral transformations that seeks to understand the biotic and abiotic processes that underpin geochemical change in the biosphere.

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Fungi, Rocks, and Minerals

Fungi are ubiquitous inhabitants of rock and mineral surfaces and are significant geoactive agents. Capable of numerous transformations of metals and minerals, fungi can prosper in the most adverse of environments, their activities underpinned by growth form and metabolism. Free-living filamentous species, microcolonial fungi and lichens can significantly change a rock’s surficial structure and appearance, ranging from discolouration and staining to biodeterioration and the formation of new biogenic minerals and rock coatings. The presence and activity of fungi should be considered in any study of rock and mineral transformations that seeks to understand the biotic and abiotic processes that underpin geochemical change in the biosphere.

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Case Hardening: Turning Weathering Rinds into Protective Shells

Case hardening is the process by which the outer shell of an exposed rock surface hardens due to near-surface diagenesis. Rock coatings and weathering rinds are distinct phenomena: rock coatings accrete on surfaces; weathering rinds derive from mineral dissolution and mechanical fracturing of the outer millimeters of a rock to create porosity. Ongoing reaction with rain, dew, or melted snow results in the downward migration of rock-coating components into weathering-rind pores. Initially, pore infilling protects the outer surface of the rock from flaking. As case hardening progresses, however, ongoing mineral dissolution underneath the case-hardened zone eventually leads to detachment. This sudden loss can destroy rock art, the surfaces of stone monuments, and facing stones of buildings.

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Mineral Surface Coatings: Environmental Records at the Nanoscale

Past and present (a)biotic soil processes can be preserved by mineral surface coatings, which can sequester contaminants in soils and sediments. The coatings can contain complex assemblages of nanometer-size minerals and organic components. The formation, composition, and morphology of these complex mineral assemblages depend on, and hence reflect, the mineralogical and chemical composition of the substrate they develop on and the environmental factors in the surrounding soils and sediments. Mineral surface coatings typically contain complex and variable porosities, many with regions of limited fluid flow. Low-flow conditions, combined with different nanometer-size phases in the interior of mineral surface coatings, allow coatings to sequester contaminant-bearing solutes, complexes, and nanoparticles.

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