Showing papers in "Annual Review of Earth and Planetary Sciences in 2008"
TL;DR: In this article, the authors show that deformation mechanisms and rheology strongly vary as a function of stress, grain size, and fluids in the Earth's crust and upper mantle.
Abstract: Rock-mechanics experiments, geodetic observations of postloading strain transients, and micro- and macrostructural studies of exhumed ductile shear zones provide complementary views of the style and rheology of deformation deep in Earth's crust and upper mantle. Overall, results obtained in small-scale laboratory experiments provide robust constraints on deformation mechanisms and viscosities at the natural laboratory conditions. Geodetic inferences of the viscous strength of the upper mantle are consistent with flow of mantle rocks at temperatures and water contents determined from surface heat-flow, seismic, and mantle xenolith studies. Laboratory results show that deformation mechanisms and rheology strongly vary as a function of stress, grain size, and fluids. Field studies reveal a strong tendency for deformation in the lower crust and uppermost mantle in and adjacent to fault zones to localize into systems of discrete shear zones with strongly reduced grain size and strength. Deformation mechanisms ...
899 citations
TL;DR: In this article, the upper mantle oxygen fugacity at the top of the spinel peridotite rocks is shown to fall within 2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer.
Abstract: Oxygen thermobarometry measurements on spinel peridotite rocks indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. Measurements on garnet peridotites from cratonic lithosphere reveal a general decrease in fo2 with depth, which appears to result principally from the effect of pressure on the controlling Fe3+/Fe2+ equilibria. Modeling of experimental data indicates that at approximately 8 GPa, mantle fo2 will be 5 log units below FMQ and at a level where Ni-Fe metal becomes stable. Fe-Ni alloy and an Fe2O3-garnet component will be formed as a result of the disproportionation of FeO, which is experimentally demonstrated through observations of high Fe3+/ΣFe ratios in minerals in equilibrium with metallic Fe. In the lower mantle, the favorable coupled substitution of Al and Fe3+ into (Fe,Mg)SiO3 perovskite results in very high perovskite Fe3+/ΣFe ratios in equilibrium with metallic Fe. As a result, the lower mantle sh...
887 citations
TL;DR: In this article, the author's version of the work is posted here by permission of Annual Reviews for personal use, not for redistribution, and the definitive version was published in Annual Review of Earth and Planetary Sciences 36 (2008): 601-647, doi:10.1146/annurev.35.031306.140139.
Abstract: Author Posting. © Annual Reviews, 2007. This is the author's version of the work. It is posted here by permission of Annual Reviews for personal use, not for redistribution. The definitive version was published in Annual Review of Earth and Planetary Sciences 36 (2008): 601-647, doi:10.1146/annurev.earth.35.031306.140139.
729 citations
TL;DR: In this paper, the role of water as well as stress, temperature, pressure, and partial melting has been addressed, and new results require major modifications to the geodynamic interpretation of seismic anisotropy in tectonically active regions such as subduction zones, asthenosphere and plumes.
Abstract: Seismic anisotropy is caused mainly by the lattice-preferred orientation of anisotropic minerals. Major breakthroughs have occurred in the study of lattice-preferred orientation in olivine during the past ∼10 years through large-strain, shear deformation experiments at high pressures. The role of water as well as stress, temperature, pressure, and partial melting has been addressed. The influence of water is large, and new results require major modifications to the geodynamic interpretation of seismic anisotropy in tectonically active regions such as subduction zones, asthenosphere, and plumes. The main effect of partial melting on deformation fabrics is through the redistribution of water, not through a change in deformation geometry. A combination of new experimental results with seismological observations provides new insights into the distribution of water associated with plume-asthenosphere interactions, formation of the oceanic lithosphere, and subduction. However, large uncertainties remain regarding the role of pressure and the deformation fabrics at low stress conditions.
644 citations
TL;DR: The largest Fe isotope fractionations occur during redox changes, as well as differences in bonding, but these are expressed only in natural environments in which significant quantities of Fe may be mobilized and separated as discussed by the authors.
Abstract: The largest Fe isotope fractionations occur during redox changes, as well as differences in bonding, but these are expressed only in natural environments in which significant quantities of Fe may be mobilized and separated. At the circumneutral pH of most low-temperature
398 citations
TL;DR: The authors examined modern and ancient euxinic environments and the range of hypotheses for these sulfidic episodes, concluding that periods of global warmth promoted anoxia because of reduced solubility of oxygen, not because of ocean stagnation.
Abstract: Euxinic ocean conditions accompanied significant events in Earth history, including several Phanerozoic biotic crises. By critically examining modern and ancient euxinic environments and the range of hypotheses for these sulfidic episodes, we elucidate the primary factors that influenced the generation of euxinia. We conclude that periods of global warmth promoted anoxia because of reduced solubility of oxygen, not because of ocean stagnation. Anoxia led to phosphate release from sediments, and continental configurations with expansive nutrient-trapping regions focused nutrient recycling and increased regional nutrient buildup. This great nutrient supply would have fueled high biological productivity and oxygen demand, enhancing oxygen depletion and sulfide buildup via sulfate reduction. As long as warm conditions prevailed, these positive feedbacks sustained euxinic conditions. In rare, extreme cases, euxinia led to biotic crises, a hypothesis best supported by evidence from the endPermian mass extinction.
381 citations
TL;DR: The basement of the Central Andes provides insights for the dispersal of Rodinia, the reconstruction of Gondwana, and the dynamics of terrane accretion along the Pacific as discussed by the authors.
Abstract: The basement of the Central Andes provides insights for the dispersal of Rodinia, the reconstruction of Gondwana, and the dynamics of terrane accretion along the Pacific. The Paleoproterozoic Arequipa terrane was trapped during collision between Laurentia and Amazonia in the Mesoproterozoic. Ultrahigh-temperature metamorphism correlates with the collapse of the Sunsas-Grenville orogen after ∼1000 Ma and is related to slab break-off and dispersal of Rodinia. The Antofalla terrane separated in the Neoproterozoic, forming the Puncoviscana basin. Its closure was coeval with the collision of the eastern Sierras Pampeanas. The rift-drift transitions of the early Paleozoic clastic platform showed a gradual younging to the north, in agreement with counterclockwise rotation based on paleomagnetic data of Antofalla. North of Arequipa arc magmatism and high-grade metamorphism are linked to collision of the Paracas terrane in the Ordovician, during the Famatinian orogeny in the Sierras Pampeanas. The early Paleozoic ...
265 citations
TL;DR: A new way of thinking about groundwater age is proposed in this paper, where a groundwater sample is seen not as water that recharged the flow regime at a point in the past, but as a mixture of waters that have resided in the subsurface for varying lengths of time.
Abstract: A new way of thinking about groundwater age is changing the field of groundwater age dating. Following a rigorous definition of age, a groundwater sample is seen not as water that recharged the flow regime at a point in the past, but as a mixture of waters that have resided in the subsurface for varying lengths of time. This recognition resolves longstanding inconsistencies encountered in age dating and suggests new ways to carry out age dating studies. Tomorrow’s studies will likely employ sets of marker isotopes and molecules spanning a broad spectrum of age and incorporate a wide range of chemical and physical data collected from differing stratigraphic levels. The observations will be inverted using reactive transport modeling, allowing flow to be characterized not in one direction along a single aquifer, but in two or three dimensions over an entire flow regime.
262 citations
TL;DR: In this article, the authors have focused on understanding how rheologic variations, geometry (2D versus 3D), and plate motions (trench roll-back or advance) influence the evolution of slabs in the upper mantle and how they sink into the lower mantle.
Abstract: Cold, dense subducting lithosphere provides the primary force driving tectonic plates at Earth’s surface. The force available to drive the plates depends on a balance between the buoyancy forces driving subduction and the mechanical and buoyancy forces resisting subduction. Because both the buoyancy and rheology of the slab and mantle depend on temperature, composition, grain size, water content, and melt fraction, unraveling which of these processes exert a first-order control on slab dynamics and under what circumstances other processes become first-order effects can be challenging. Laboratory and numerical models of slab dynamics provide a powerful method for testing the combined effects of buoyancy and strength changes that accompany the slab evolution in the upper mantle, transition zone, and lower mantle. Recent studies have focused on understanding how rheologic variations (Newtonian versus non-Newtonian viscosity or water content), geometry (2D versus 3D), and plate motions (trench roll-back or advance) influence the evolution of slabs in the upper mantle and how they sink into the lower mantle. These models suggest that spatial and temporal variations in slab strength and the history of subduction determine whether slabs sink directly into the lower mantle or are trapped in the transition zone.
239 citations
Journal Article•
TL;DR: In this paper, the authors present a new orbital calibration of the Early Toarcian carbon isotope excursion, based on spectral analyses of two independent datasets generated from the sedimentary record of two hemipelagic sections from Portugal (Peniche) and SW Germany (Dotternhausen), to better constrain the timescale and hence the origin of this excursion.
Abstract: The marked 3–8‰ negative carbon isotope excursion associated with the Early Toarcian oceanic anoxic event (OAE; ~ 183 myr ago) in the Early Jurassic period is thought to represent one of the most important perturbations of the C-cycle in the last 200 myr. However, the origin of this excursion remains strongly debated, primarily due to uncertainties in the estimation of its duration, which ranges from ~ 200 kyr to 1 myr. Here we present a new orbital calibration of the Early Toarcian carbon isotope excursion, based on spectral analyses of two independent datasets generated from the sedimentary record of two hemipelagic sections from Portugal (Peniche) and SW Germany (Dotternhausen), in order to better constrain the timescale and hence the origin of this excursion. These analyses reveal that orbital cycles exert a strong influence on both the calcium carbonate content in Portugal and on the greyscale of black shales in Germany, which allow us to propose a duration of ≥ 1.9 myr for the Early Toarcian and of ~ 900 kyr for the entire carbon isotope excursion. The shift towards lower carbon isotope values lasted ~ 150 kyr, and carbon isotope values remained low for ~ 450 kyr; the subsequent increase of carbon isotope values lasted ~ 300 kyr. This calibration suggests that the sustained input of isotopically light carbon at the origin of the excursion occurred over ~ 600 kyr and thus dismisses causal mechanisms implying relatively small source reservoirs such as the massive dissociation of methane hydrates. In the light of our new cyclostratigraphic timescale, the massive input of isotopically light carbon associated with the emplacement of the Karoo–Ferrar basaltic province appears as the most likely cause of the Toarcian global carbon isotope excursion. We also show that the C-isotope perturbation coincided with a transition from precession–eccentricity-dominated cycles to obliquity–eccentricity-dominated cycles, suggesting that the OAE was marked by a fundamental change in the response of the climate system, which allowed the obliquity signal, normally better recorded at high latitudes, to be a dominant forcing factor of short-term sedimentary cycles at tropical latitudes.
207 citations
TL;DR: In this paper, the authors show that the current climate has a distinct and generally repeatable pattern of seasonal and spatial variations, and that the greatest year-to-year variations are caused by intermittent large dust storms, which also affect atmospheric temperature and circulation patterns on a global scale.
Abstract: Recent spacecraft observations of the Martian atmosphere show that the current climate has a distinct and generally repeatable pattern of seasonal and spatial variations. The half of the year that Mars is closest to the Sun is characterized by warm, dusty conditions, whereas the opposite season is relatively cool, free of dust, and dominated by a low-latitude band of water ice clouds. Water vapor is most abundant at high latitudes in the summertime of both hemispheres, whereas a polar hood of clouds forms over the winter polar region. The greatest year-to-year variations are caused by intermittent large dust storms, which also affect atmospheric temperature and circulation patterns on a global scale.
TL;DR: The North American Cordilleran Orogen is the result of a two-stage process: (a) Triassic-Jurassic accretion within Panthalassa forming SAYBIA, a composite ribbon continent, and (b) Late Cretaceous collision of SayBIA with North America as mentioned in this paper.
Abstract: The North American Cordilleran Orogen is the result of a two-stage process: (a) Triassic-Jurassic accretion within Panthalassa forming SAYBIA, a composite ribbon continent, and (b) Late Cretaceous collision of SAYBIA with North America. This model requires that a large portion of the continental foreland of the orogen is exotic. The exotic continental component of SAYBIA, Cassiar Platform, is distinguished from the autochthon on the basis of its (a) Triassic Eurasian fauna; (b) involvement in a major Late Triassic-Early Jurassic orogenic event; and (c) young, in part Grenvillian basement and mantle. A mid-Cretaceous magmatic arc records west-dipping subduction beneath the east-margin of SAYBIA. The related accretionary prism consists of imbricated shale, chert, and deep-water limestones (the Medial Basin) and overlies an isotopically juvenile mantle domain. Carbonatite complexes delineate the cryptic suture separating SAYBIA and the autochthon. Paleomagnetic and paleobotanical data place SAYBIA 2000 km to the south relative to the autochthon at 80 Ma. Late Cretaceous thrust belt development records transpression between the north-moving ribbon continent and the autochthon. Pinning against the Okhotsk-Chukotka arc in Siberia buckled SAYBIA, giving rise to the Alaskan promontory.
TL;DR: In this paper, the authors compare geodynamic models with seismic tomographic results using experimentally derived relations between velocity, attenuation, and temperature, and conclude that small melt fractions in the mantle flow can be modeled with along-strike flow in a low viscosity channel.
Abstract: Seismic imaging provides an opportunity to constrain mantle wedge processes associated with subduction, slab dehydration, arc volcanism, and backarc spreading. The mantle wedge is characterized by a low attenuation forearc, an inclined zone of low velocity and high attenuation underlying the volcanic front, and a broad region of low velocity and high attenuation beneath the backarc spreading center when present. Seismic velocities, bathymetry, and basalt chemistry suggest mantle temperature variations of ∼100°C between different backarc regions. Rock physics experiments and geodynamic modeling are essential for interpreting seismic observations. Seismic anisotropy indicates a complex pattern of mantle flow that can be modeled with along-strike flow in a low viscosity channel beneath the arc and backarc. Comparison of geodynamic models with seismic tomographic results using experimentally derived relations between velocity, attenuation, and temperature suggests the existence of small melt fractions in the ...
TL;DR: In this article, the authors discuss the causes for the limited use of this tool until recently and how these hindrances are being overcome, and illustrate that diffusion modeling can clock processes that last from only a few days to those that last over tens of millions of years, recorded in rocks that range in age from current volcanic eruptions to condensates from the early solar nebula.
Abstract: Understanding how processes occurring on a wide range of temporal and spatial scales combine to produce a stable dynamic Earth is a major goal of the Earth scientist. Determining durations of processes is a key step toward attaining that goal. Records of incomplete diffusive equilibration preserved in minerals are uniquely suited for the purpose of unraveling timescales of a variety of processes. Compositional zoning in minerals is like the tracks of a CD that can be decoded with suitable technology. This review discusses the causes for the limited use of this tool until recently and how these hindrances are being overcome. Examples are presented to illustrate that diffusion modeling can clock processes that last from only a few days to those that last over tens of millions of years, recorded in rocks that range in age from current volcanic eruptions to condensates from the early solar nebula.
TL;DR: The Dead Sea Fault is an active transform fault linking opening in the Red Sea with collision in the Taurus/Zagros Mountains as mentioned in this paper, which can be divided into two distinct structural segments.
Abstract: The Dead Sea Fault is an active transform fault linking opening in the Red Sea with collision in the Taurus/Zagros Mountains. Motion is left-lateral and estimated at approximately 5–7 mm year −1 . The fault is seismically active, and can be divided into two distinct structural segments. This study focuses on the southern segment based mainly on the wealth of geophysical data. Owing to transtention caused by oblique-slip and the overlapping of en-echelon fault strands, a series of pull-apart basins were formed along the fault’s length. These basins are long and deep-reaching in places more than 10 km deep. They are characterized by extensional, compressional, and asymmetrical structures varying in size from large-scale (defining the general structure of the Dead Sea fault valley) to small-scale (defining the internal structure). This study examines the internal structure of these basins from south to north and summarizes the state of knowledge to date.
TL;DR: Recently, the discovery of the postperovskite (PPv) transition has profound impact on our understanding of the core-mantle boundary (CMB) region.
Abstract: The discovery of the postperovskite (PPv) transition has profound impact on our understanding of the core-mantle boundary (CMB) region. Unlike perovskite (Pv), the PPv phase has a layered structure of the SiO6 octahedra, which may lead to a large contrast in some properties with Pv. Recent studies have proposed unusual properties of PPv, such as a large positive Clapeyron slope, a large sensitivity of the transition depth to iron, a decrease in bulk sound speed at the transition, and a development of significant lattice preferred orientation. Many of the proposed properties can provide explanations for the intriguing seismic observations at the CMB region. Yet significant discrepancies still exist. However, rapid developments in mineral physics will continue to improve our knowledge on the changes across the PPv transition.
TL;DR: In this article, the importance of wind erosion has been reevaluated and it has been recognized that other horizontal sorting processes have played a role, especially movement of fines by overland flow.
Abstract: Recently, the importance of wind erosion has been reevaluated. Many low-angle surfaces, developed on susceptible materials, possess closed depressions (pans), and these often have a distinctive morphology and lunette dunes on their lee sides. It has also become apparent that in drylands there are extensive areas of stone pavements, some of which have been molded by deflation; however, it is now recognized that other horizontal sorting processes have played a role, especially movement of fines by overland flow. Vertical sorting associated with wetting and drying, salt heave, bioturbation, frost action, and dust accretion have also played a role. In hyperarid areas, especially with unidirectional winds, bedrock outcrops and old lake beds have been molded to give aerodynamic forms, called yardangs. The study of dust storms by analysis of climatological data and remote sensing has revealed the importance of deflation, especially in hyperarid areas with centripetal drainage. Deflation hot spots, such as the Bo...
TL;DR: Motility, a trait critical for handling predation pressure by stalkless crinoids, is shown to play an analogous role in stalked crinoid lives, and a review of predation on crinoidal supports the view that it is ecologically significant.
Abstract: Recent studies of crinoids reveal that their connective tissue, known to be mutable, is also contractile, and that certain stalked taxa are capable of crawling and subject to predation by cidaroid sea urchin Aspects of crinoid functional morphology, ecology, and paleobiology are reviewed in the context of these findings Mutability and contractility of ligament are important to autotomy, posture maintenance, and motility, and those, in turn, are shown to play a role in how crinoids acquire nutrients, select habitats and respond to predation pressure A review of predation on crinoids supports the view that it is ecologically significant Motility, a trait critical for handling predation pressure by stalkless crinoids, is shown to play an analogous role in stalked crinoids The distribution of traits required for motility among extant and fossil crinoids reveals that their frequency increased dramatically following the Permo-Triassic extinction, with low values characterizing the Paleozoic and high values
TL;DR: For example, the degree to which such a freshening affects the deep, slow thermohaline, rather than the shallow, swift, wind-driven circulations of the ocean, differ from model to model, depending on factors such as the treatment of diffusive processes in the oceans.
Abstract: Leading hypotheses for abrupt climate changes are focused on the ocean response to a freshening of surface waters in the north Atlantic. The degree to which such a freshening affects the deep, slow thermohaline, rather than the shallow, swift, wind-driven circulations of the ocean, and hence the degree to which that freshening affects climate in high rather than low latitudes, differ from model to model, depending on factors such as the treatment of diffusive processes in the oceans. Many comprehensive climate models are biased and confine the influence mainly to the thermohaline circulation and northern climates. Simulations of paleoclimates can provide valuable tests for the models, but only some of those climates provide sufficiently stringent tests to determine which models are realistic.
TL;DR: In early years, she repeatedly was told that she did not look like a physicist, but it was physics that provided her a rewarding opportunity to elucidate the characteristics of the space plasmas of terrestrial and planetary magnetospheres and to discover unexpected properties of the Galilean moons of Jupiter.
Abstract: Should one call it serendipity to have stumbled into a career in space plasma physics within the first decade of the era of satellite exploration? The author had the good fortune to have done so. In early years, she repeatedly was told that she did not look like a physicist, but it was physics that provided her a rewarding opportunity to elucidate the characteristics of the space plasmas of terrestrial and planetary magnetospheres and to discover unexpected properties of the Galilean moons of Jupiter. Here, she describes some of her scientific contributions and introduces family members, colleagues, students, and friends who helped her along her trajectory and participated in her scientific investigations.