Annual Review of Earth and Planetary Sciences - Volume 42, 2014
Volume 42, 2014
- Preface
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Falling in Love with Waves
Vol. 42 (2014), pp. 1–6More LessDesire to understand the physics of natural grandeur and the beauty of wave theories brought me to geophysics and seismology. Advancements in theory, practice, and instrumentation have allowed seismologists to unravel the mysteries carried by seismic waves and to use the acquired knowledge for the benefit of society.
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The Diversity of Large Earthquakes and Its Implications for Hazard Mitigation
Vol. 42 (2014), pp. 7–26More LessWith the advent of broadband seismology and GPS, significant diversity in the source radiation spectra of large earthquakes has been clearly demonstrated. This diversity requires different approaches to mitigate hazards. In certain tectonic environments, seismologists can forecast the future occurrence of large earthquakes within a solid scientific framework using the results from seismology and GPS. Such forecasts are critically important for long-term hazard mitigation practices, but because stochastic fracture processes are complex, the forecasts are inevitably subject to large uncertainty, and unexpected events will continue to surprise seismologists. Recent developments in real-time seismology will help seismologists to cope with and prepare for tsunamis and earthquakes. Combining a better understanding of earthquake diversity with modern technology is the key to effective and comprehensive hazard mitigation practices.
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Broadband Ocean-Bottom Seismology
Vol. 42 (2014), pp. 27–43More LessBroadband ocean-bottom seismographs (BBOBSs) were first developed in the 1980s and have since been used to explore the structure of Earth's interior beneath oceanic regions—for example, mid-oceanic ridges, subduction zones, hot spots, and the oceanic lithosphere-asthenosphere boundary. The best approach for broadband seismic observations in oceanic regions is that of a borehole seismic observatory attached to the ocean-floor cable, which is realized in several near-coast networks. Considering the high cost of such networks, there is still a need to develop autonomous BBOBSs with a better signal-to-noise ratio for temporary observations of focused scientific targets far from the coast.
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Extrasolar Cosmochemistry
M. Jura, and E.D. YoungVol. 42 (2014), pp. 45–67More LessEvidence is now compelling that elements heavier than helium in many white dwarf atmospheres have accumulated by accretion from orbiting rocky bodies, often larger than 100 km in diameter, such as asteroids. Consequently, we now possess a powerful tool to measure the elemental constituents of extrasolar minor planets. To zeroth order, the accreted extrasolar parent bodies resemble bulk Earth: They are at least 85% by mass composed of oxygen, magnesium, silicon, and iron; carbon and ice are only trace constituents. Assembled data for white dwarf pollutions suggest that differentiation of extrasolar planetesimals, leading to iron-rich cores and aluminum-rich crusts, is common. Except for instances of unexpectedly high calcium abundances, the compositions of extrasolar planetesimals can be understood as resulting from processes similar to those controlling the formation and evolution of objects in the inner Solar System.
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Orbital Climate Cycles in the Fossil Record: From Semidiurnal to Million-Year Biotic Responses
Vol. 42 (2014), pp. 69–102More LessUnderstanding climate change, its effect on terrestrial and marine ecosystems, and possible ways to prevent future climate disasters is a major challenge for society, involving specialists in climate science, terrestrial and marine ecology, paleontology, and sedimentary geology. One approach is to study the deep-time record, especially when the time involved in a particular climatic change can be calibrated. Cyclostratigraphy is a useful tool for this. Throughout Earth's history, different scales of orbital cycles have had significant impacts on atmosphere-ocean dynamics; these impacts are preserved in the ecological and sedimentary record. Most characterizations of these cycles are based on the sedimentary record. But fossil records of past biota, corresponding to individual organisms and communities, have proven very useful in cyclostratigraphic research: From semidiurnal cycles mainly recorded in fossil skeletons to million-year-scale cycles involving mass extinctions, various cases illustrate their worth. This article reviews the use of the fossil record to recognize several cycles, from ecological timescales (≤1.0 yr to 10 kyr cycles; calendar and solar bands) to geological timescales (>10 kyr cycles; Milankovitch and galactic bands).
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Heterogeneity and Anisotropy of Earth's Inner Core
Vol. 42 (2014), pp. 103–126More LessSeismic observations provide strong evidence that Earth's inner core is anisotropic, with larger velocity in the polar than in the equatorial direction. The top 60–80 km of the inner core is isotropic; evidence for an innermost inner core is less compelling. The anisotropy is most likely due to alignment of hcp (hexagonal close-packed) iron crystals, aligned either during solidification or by deformation afterward. The existence of hemispherical variations used to be controversial, but there is now strong evidence from both seismic body wave and normal mode observations, showing stronger anisotropy, less attenuation, and a lower isotropic velocity in the western hemisphere. Two mechanisms have been proposed to explain the hemispherical pattern: either (a) inner core translation, wherein one hemisphere is melting and the other is solidifying, or (b) thermochemical convection in the outer core, leading to different solidification conditions at the inner core boundary. Neither is (yet) able to explain all seismically observed features, and a combination of different mechanisms is probably required.
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Detrital Zircon U-Pb Geochronology Applied to Tectonics
Vol. 42 (2014), pp. 127–149More LessDetrital zircon geochronology is rapidly developing into an essential tool in Earth science research because of the widespread occurrence of zircon in sedimentary systems; the wide range of information that can be extracted from zircon crystals; the ability to determine ages with reasonable precision, accuracy, and efficiency; and the wide range of new ideas about how to use detrital zircon geochronologic information. The U-Pb system is particularly powerful because three chronometers are available (238U→206Pb, 235U→207Pb, and 232Th→208Pb), but challenges arise because of complexities from inheritance and Pb loss. Ages can be used to constrain the age of deposition of the host sediment, reconstruct provenance, characterize a sedimentary unit, and characterize many different aspects of source regions. Detrital zircon geochronology has an exciting future given the growth history recorded in individual crystals; the variety of detrital minerals that can provide complementary information; and the large number of geochemical, isotopic, and chronologic systems that can be applied to these minerals.
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How Did Early Earth Become Our Modern World?
Vol. 42 (2014), pp. 151–178More LessSeveral features of Earth owe their origin to processes occurring during and shortly following Earth formation. Collisions with planetary embryos caused substantial melting of the growing Earth, leading to prolonged core formation, atmosphere outgassing, and deepening of the magma ocean as Earth grew. Mantle noble gas isotopic compositions and the mantle abundance of elements that partition into the core record this very early Earth differentiation. In contrast, the elements that are not involved in either core or atmosphere formation show surprisingly muted evidence of the fractionation expected during magma ocean crystallization, and even this minimal evidence for early intramantle differentiation appears to have been erased by mantle convection within ∼1.5 billion years of Earth formation. By 4.36 Ga, Earth's surface and shallow interior had reached temperatures similar to those of the present Earth, and mantle melting, and perhaps plate subduction, was producing crustal rock types similar to those seen today. Remnants of early Earth differentiation may still exist in the deep mantle and continue to influence patterns of large-scale mantle convection, sequestration of some trace elements, geomagnetic reversals, vertical motions of continents, and hot-spot volcanism.
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The Stardust Mission: Analyzing Samples from the Edge of the Solar System
Vol. 42 (2014), pp. 179–205More LessComet samples returned to Earth by the NASA Stardust mission have provided a surprising glimpse into the nature of early Solar System materials and an epiphany on the origin of the initial rocky materials that once filled the cold regions of the solar nebula. The findings show that the cold regions of the early Solar System were not isolated and were not a refuge where interstellar materials could commonly survive. Wild 2, the sampled comet, appears to be a typical active Jupiter family comet, and yet most of its sampled micron and larger grains are familiar high-temperature meteoritic materials, such as chondrule fragments, that were transported to cold nebular regions. The rocky components in primitive asteroids and comets may differ because asteroid formation was dominated by local materials, whereas comets formed from mixed materials, many of which were transported from very distant locations.
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Paleobiology of Herbivorous Dinosaurs
Vol. 42 (2014), pp. 207–230More LessHerbivorous dinosaurs were abundant, species-rich components of Late Triassic–Cretaceous terrestrial ecosystems. Obligate high-fiber herbivory evolved independently on several occasions within Dinosauria, through the intermediary step of omnivory. Anatomical character complexes associated with this diet exhibit high levels of convergence and morphological disparity, and may have evolved by correlated progression. Dinosaur faunas changed markedly during the Mesozoic, from early faunas dominated by taxa with simple, uniform feeding mechanics to Cretaceous biomes including diverse sophisticated sympatric herbivores; the environmental and biological drivers causing these changes remain unclear. Isotopic, taphonomic, and anatomical evidence implies that niche partitioning reduced competition between sympatric herbivores, via morphological differentiation, dietary preferences, and habitat selection. Large body size in dinosaur herbivores is associated with low plant productivity, and gave these animals prominent roles as ecosystem engineers. Although dinosaur herbivores lived through several major events in floral evolution, there is currently no evidence for plant-dinosaur coevolutionary interactions.
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Spin Transitions in Mantle Minerals
Vol. 42 (2014), pp. 231–248More LessMantle minerals at shallow depths contain iron in the high-spin electronic state. The crystal-field splitting energy increases with increasing pressure, which can favor the low-spin state. Hence, pressure-driven transitions from the high-spin to the low-spin state were proposed as early as the 1960s, and minerals in the lower mantle were suggested to contain iron in the low-spin state. Only in the past 10 years did experiments and calculations prove that iron in mantle minerals transforms from high-spin to low-spin at lower-mantle pressures. This transition has important consequences for volume, thermodynamics, and bonding. In a geophysical framework, the transition would affect the dynamics and thermochemical state of the lower mantle, through combined effects on density, elasticity, element partitioning, and transport properties. These observations provide the basis for a new paradigm of the physics and chemistry in Earth's lower(most) mantle.
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Mercury Isotopes in Earth and Environmental Sciences
Vol. 42 (2014), pp. 249–269More LessVirtually all biotic, dark abiotic, and photochemical transformations of mercury (Hg) produce Hg isotope fractionation, which can be either mass dependent (MDF) or mass independent (MIF). The largest range in MDF is observed among geological materials and rainfall impacted by anthropogenic sources. The largest positive MIF of Hg isotopes (odd-mass excess) is caused by photochemical degradation of methylmercury in water. This signature is retained through the food web and measured in all freshwater and marine fish. The largest negative MIF of Hg isotopes (odd-mass deficit) is caused by photochemical reduction of inorganic Hg and has been observed in Arctic snow and plant foliage. Ratios of MDF to MIF and ratios of 199Hg MIF to 201Hg MIF are often diagnostic of biogeochemical reaction pathways. More than a decade of research demonstrates that Hg isotopes can be used to trace sources, biogeochemical cycling, and reactions involving Hg in the environment.
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Investigating Microbe-Mineral Interactions: Recent Advances in X-Ray and Electron Microscopy and Redox-Sensitive Methods
Vol. 42 (2014), pp. 271–289More LessMicrobe-mineral interactions occur in diverse modern environments, from the deep sea and subsurface rocks to soils and surface aquatic environments. They may have played a central role in the geochemical cycling of major (e.g., C, Fe, Ca, Mn, S, P) and trace (e.g., Ni, Mo, As, Cr) elements over Earth's history. Such interactions include electron transfer at the microbe-mineral interface that left traces in the rock record. Geomicrobiology consists in studying interactions at these organic-mineral interfaces in modern samples and looking for traces of past microbe-mineral interactions recorded in ancient rocks. Specific tools are required to probe these interfaces and to understand the mechanisms of interaction between microbes and minerals from the scale of the biofilm to the nanometer scale. In this review, we focus on recent advances in electron microscopy, in particular in cryoelectron microscopy, and on a panel of electrochemical and synchrotron-based methods that have recently provided new understanding and imaging of the microbe-mineral interface, ultimately opening new fields to be explored.
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Mineralogy of the Martian Surface
Vol. 42 (2014), pp. 291–315More LessThe past fifteen years of orbital infrared spectroscopy and in situ exploration have led to a new understanding of the composition and history of Mars. Globally, Mars has a basaltic upper crust with regionally variable quantities of plagioclase, pyroxene, and olivine associated with distinctive terrains. Enrichments in olivine (>20%) are found around the largest basins and within late Noachian–early Hesperian lavas. Alkali volcanics are also locally present, pointing to regional differences in igneous processes. Many materials from ancient Mars bear the mineralogic fingerprints of interaction with water. Clay minerals, found in exposures of Noachian crust across the globe, preserve widespread evidence for early weathering, hydrothermal, and diagenetic aqueous environments. Noachian and Hesperian sediments include paleolake deposits with clays, carbonates, sulfates, and chlorides that are more localized in extent. The late Hesperian to Amazonian mineralogic record of water is sparser, though sulfates and silica in some locations indicate local availability of ground and surface waters even in the most recent geologic epoch.
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The Uses of Dynamic Earthquake Triggering
Vol. 42 (2014), pp. 317–339More LessDynamic triggering of earthquakes by seismic waves is a robustly observed phenomenon with well-documented examples from over 30 major earthquakes. We are now in a position to use dynamic triggering as a natural experiment to probe the reaction of faults to the known stresses from seismic waves. We show here that dynamic triggering can be used to investigate the distribution of stresses required for failure on faults. In some regions, faults appear to be uniformly distributed over their loading cycles with equal numbers at all possible stresses from failure. Regions under tectonic extension, at the interface between locked and creeping faults, or subject to anthropogenic forcing are most prone to triggered failure. Predictions of future seismicity rates based on seismic wave amplitudes are theoretically possible and may provide similar results to purely stochastic prediction schemes. The underlying mechanisms of dynamic triggering are still unknown. The prolonged triggered sequences require a multistage process such as shear failure from rate-state friction coupled to aseismic creep or continued triggering through a secondary cascade. Permeability enhancement leading to drainage or pore pressure redistribution on faults is an alternative possibility.
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Short-Lived Climate Pollution
Vol. 42 (2014), pp. 341–379More LessAlthough carbon dioxide emissions are by far the most important mediator of anthropogenic climate disruption, a number of shorter-lived substances with atmospheric lifetimes of under a few decades also contribute significantly to the radiative forcing that drives climate change. In recent years, the argument that early and aggressive mitigation of the emission of these substances or their precursors forms an essential part of any climate protection strategy has gained a considerable following. There is often an implication that such control can in some way make up for the current inaction on carbon dioxide emissions. The prime targets for mitigation, known collectively as short-lived climate pollution (SLCP), are methane, hydrofluo-rocarbons, black carbon, and ozone. A re-examination of the issues shows that the benefits of early SLCP mitigation have been greatly exaggerated, largely because of inadequacies in the methodologies used to compare the climate effects of short-lived substances with those of CO2, which causes nearly irreversible climate change persisting millennia after emissions cease. Eventual mitigation of SLCP can make a useful contribution to climate protection, but there is little to be gained by implementing SLCP mitigation before stringent carbon dioxide controls are in place and have caused annual emissions to approach zero. Any earlier implementation of SLCP mitigation that substitutes to any significant extent for carbon dioxide mitigation will lead to a climate irreversibly warmer than will a strategy with delayed SLCP mitigation. SLCP mitigation does not buy time for implementation of stringent controls on CO2 emissions.
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Himalayan Metamorphism and Its Tectonic Implications
Vol. 42 (2014), pp. 381–419More LessThe Himalayan range exposes a spectacular assemblage of metamorphic rocks from the mid- and deep crust that have fostered numerous models of how the crust responds to continental collisions. Recent petrogenetically based petrologic and geochronologic studies elucidate processes with unprecedented resolution and critically test models that range from continuum processes to one-time events. The pronounced metamorphic inversion across the Main Central Thrust reflects continuum thrusting between ca. 15 and 20 Ma, whereas exposure of ultrahigh-pressure rocks in northwestern massifs and syntaxis granulites reflects singular early (≥45 Ma) and late (≤10 Ma) exhumation events. Multiple mechanisms including wedge collapse and flow of melt-weakened midcrust are debated to explain pressure-temperature trajectories, patterns of thinning, and thermal overprinting. A geochronologic revolution is under way in which spatially resolved compositions and ages of accessory minerals are combined in a petrogenetically valid context to recover specific temperature-time points and paths. Combined chemical and chronologic analysis of monazite is now well established and titanite is particularly promising, but recent zircon data raise questions about anatectic rocks and their use for investigating tectonism.
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Phenotypic Evolution in Fossil Species: Pattern and Process
Vol. 42 (2014), pp. 421–441More LessSince Darwin, scientists have looked to the fossil record with the hope of using it to document how the phenotypes of species change over substantial periods of time. How best to interpret this record has been controversial, but empirical and methodological advances have resolved at least two issues about pattern: (a) directional transformations are seldom sustained over geological timescales, and (b) net rates of morphological change in fossil species are usually quite slow. Considerable uncertainty remains, however, about the processes responsible for these patterns, but most fruitful explanations use the framework of adaptive landscapes to consider the role of natural selection and other processes. An additional, unresolved issue is the claim that most phenotypic change is associated with speciation. A variety of methods, using data from both fossil and extant species, have supported such a link, at least in some clades and traits, but its prevalence and underlying mechanism remain unresolved.
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Earth Abides Arsenic Biotransformations
Vol. 42 (2014), pp. 443–467More LessArsenic is the most prevalent environmental toxic element and causes health problems throughout the world. The toxicity, mobility, and fate of arsenic in the environment are largely determined by its speciation, and arsenic speciation changes are driven, at least to some extent, by biological processes. In this article, biotransformation of arsenic is reviewed from the perspective of the formation of Earth and the evolution of life, and the connection between arsenic geochemistry and biology is described. The article provides a comprehensive overview of molecular mechanisms of arsenic redox and methylation cycles as well as other arsenic biotransformations. It also discusses the implications of arsenic biotransformation in environmental remediation and food safety, with particular emphasis on groundwater arsenic contamination and arsenic accumulation in rice.
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Previous Volumes
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Volume 52 (2024)
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Volume 51 (2023)
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Volume 50 (2022)
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Volume 49 (2021)
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Volume 48 (2020)
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Volume 47 (2019)
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Volume 46 (2018)
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Volume 45 (2017)
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Volume 44 (2016)
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Volume 43 (2015)
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Volume 42 (2014)
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Volume 41 (2013)
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Volume 40 (2012)
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Volume 39 (2011)
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Volume 38 (2010)
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Volume 37 (2009)
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Volume 36 (2008)
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Volume 35 (2007)
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Volume 34 (2006)
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Volume 33 (2005)
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Volume 32 (2004)
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Volume 31 (2003)
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Volume 30 (2002)
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Volume 29 (2001)
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Volume 28 (2000)
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Volume 27 (1999)
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Volume 26 (1998)
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Volume 25 (1997)
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Volume 24 (1996)
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Volume 23 (1995)
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Volume 22 (1994)
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Volume 21 (1993)
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Volume 20 (1992)
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Volume 19 (1991)
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Volume 18 (1990)
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Volume 17 (1989)
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Volume 16 (1988)
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Volume 15 (1987)
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Volume 14 (1986)
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Volume 13 (1985)
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Volume 12 (1984)
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Volume 11 (1983)
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Volume 10 (1982)
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Volume 9 (1981)
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Volume 8 (1980)
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Volume 7 (1979)
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Volume 6 (1978)
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Volume 5 (1977)
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Volume 4 (1976)
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Volume 3 (1975)
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Volume 2 (1974)
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Volume 1 (1973)
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Volume 0 (1932)