Annual Review of Earth and Planetary Sciences - Volume 45, 2017
Volume 45, 2017
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Researching the Earth—and a Few of Its Neighbors
Vol. 45 (2017), pp. 1–29More LessDuring my career, our knowledge of erupting geysers and volcanoes in the Solar System has exploded. In this prefatory, I tell how I became fascinated with high-speed processes through studying meteorite impact dynamics, and then how my initial idea of studying Old Faithful geyser as a volcanic analog led me to work not only on the dynamics of eruption of Mount St. Helens in 1980 but also on geysers erupting on Io (a fiery satellite of Jupiter), Triton (a frigid satellite of Neptune), and Enceladus (an active satellite of Saturn). Unforeseeably, the study of these events also led to work on mineral thermodynamics and the hydraulics and geomorphic evolution of rapids in the Grand Canyon. This is a narrative, not a formal review article, but the reader can find references in the Related Resources section to explore topics in more detail.
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The Fascinating and Complex Dynamics of Geyser Eruptions
Vol. 45 (2017), pp. 31–59More LessGeysers episodically erupt liquid and vapor. Despite two centuries of scientific study, basic questions persist—why do geysers exist? What determines eruption intervals, durations, and heights? What initiates eruptions? Through monitoring eruption intervals, analyzing geophysical data, taking measurements within geyser conduits, performing numerical simulations, and constructing laboratory models, some of these questions have been addressed. Geysers are uncommon because they require a combination of abundant water recharge, magmatism, and rhyolite flows to supply heat and silica, and large fractures and cavities overlain by low-permeability materials to trap rising multiphase and multicomponent fluids. Eruptions are driven by the conversion of thermal to kinetic energy during decompression. Larger and deeper cavities permit larger eruptions and promote regularity by isolating water from weather variations. The ejection velocity may be limited by the speed of sound of the liquid + vapor mixture.
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Plant Evolution and Climate Over Geological Timescales
Vol. 45 (2017), pp. 61–87More LessThe terrestrial vegetation is unambiguously an important factor in the climate system, modulating the exchange of energy, momentum, water vapor, and other trace gases between land and atmosphere. Here, we review the evolution of the terrestrial flora from the Proterozoic through to the Neogene at three distinct scales—the overall evolution of floral composition, the evolution of plant physiology, and the evolution of landscape occupation both spatially and seasonally—all in the context of how the vegetation may have influenced climate through time and which deep-time evolutionary transitions may have had the greatest effect. Our focus is upon the direct impacts of the vegetation on temperature and precipitation, but we also consider the indirect impacts of plants on climate via atmospheric composition. We argue that the times of greatest change in plant climate feedbacks are likely to have been the Carboniferous and the early Paleogene.
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Origin and Evolution of Water in the Moon's Interior
Vol. 45 (2017), pp. 89–111More LessNearly forty years after the return of the first lunar samples to Earth, improvements in laboratory detection limits made possible the first definitive discovery of magmatic water in lunar volcanic samples. The intervening decade has seen an exponential increase in the amount of data on the abundance of magmatic water, and its hydrogen isotope composition, in various rock types recovered from the Moon. Here we review these data and describe how the abundance of water in the lunar interior places important constraints on models for the high-temperature origin and evolution of the Moon.
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Major Questions in the Study of Primate Origins
Vol. 45 (2017), pp. 113–137More LessNumerous factors have stimulated new enthusiasm for understanding the process of primate origins, including new fossil discoveries, improvements to methods for analyzing molecular data, and technological advances. These novel approaches have led to a better appreciation of the complexities of early primate evolution. Eight fundamental questions provide a framework for thinking about these issues. Among these topics are the phylogenetic position of Primates in Mammalia and the membership of particular fossil groups in the order. Also of central interest are questions about early primate ecology and anatomy such as the ancestral body mass, diet, locomotor mode, interactions with predators, and brain size and form. And finally, considerations of the paleontological record need to be informed by the most relevant living models, which help flesh out the story that is being told by fossils. Although much is known about all of these areas, fundamental questions still remain.
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Seismic and Electrical Signatures of the Lithosphere–Asthenosphere System of the Normal Oceanic Mantle
Vol. 45 (2017), pp. 139–167More LessAlthough plate tectonics started as a theory of the ocean basins nearly 50 years ago, the mechanical details of how it works are still poorly known. Our understanding of these details has been hampered partly by our inability to characterize the physical nature of the lithosphere–asthenosphere system (LAS) beneath the ocean. We review the existing observational constraints on the seismic and electrical properties of the LAS, particularly for normal oceanic regions away from mid-oceanic ridges, hot spots, and subduction zones, where plate tectonics is expected to present its simplest form. Whereas a growing volume of seismic data on land has provided remarkable advances in large-scale pictures, seafloor observations have been shedding new light on essential details. By combing through these observational constraints, researchers are unveiling the nature of the enigmatic LAS. Future directions for large-scale seafloor observations are also discussed.
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Earth's Continental Lithosphere Through Time
Vol. 45 (2017), pp. 169–198More LessThe record of the continental lithosphere is patchy and incomplete; no known rock is older than 4.02 Ga, and less than 5% of the rocks preserved are older than 3 Ga. In addition, there is no recognizable mantle lithosphere from before 3 Ga. We infer that there was lithosphere before 3 Ga and that ∼3 Ga marks the stabilization of blocks of continental lithosphere that have since survived. This was linked to plate tectonics emerging as the dominant tectonic regime in response to thermal cooling, the development of a more rigid lithosphere, and the recycling of water, which may in turn have facilitated plate tectonics. A number of models, using different approaches, suggest that at 3 Ga the volume of continental crust was ∼70% of its present-day volume and that this may be a minimum value. The continental crust before 3 Ga was on average more mafic than that generated subsequently, and this pre-3 Ga mafic new crust had fractionated Lu/Hf and Sm/Nd ratios as inferred for the sources of tonalite-trondhjemite-granodiorite and later granites. The more intermediate composition of new crust generated since 3 Ga is indicated by its higher Rb/Sr ratios. This change in composition was associated with an increase in crustal thickness, which resulted in more emergent crust available for weathering and erosion. This in turn led to an increase in the Sr isotope ratios of seawater and in the drawdown of CO2. Since 3 Ga, the preserved record of the continental crust is marked by global cycles of peaks and troughs of U-Pb crystallization ages, with the peaks of ages appearing to match periods of supercontinent assembly. There is increasing evidence that the peaks of ages represent enhanced preservation of magmatic rocks in periods leading up to and including continental collision in the assembly of supercontinents. These are times of increased crustal growth because more of the crust that is generated is retained within the crust. The rates of generation of continental crust and mantle lithosphere may have remained relatively constant at least since 3 Ga, yet the rates of destruction of continental crust have changed with time. Only relatively small volumes of rock are preserved from before 3 Ga, and so it remains difficult to establish which of these are representative of global processes and the extent to which the rock record before 3 Ga is distorted by particular biases.
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Aerosol Effects on Climate via Mixed-Phase and Ice Clouds
Vol. 45 (2017), pp. 199–222More LessClouds in Earth's atmosphere can be composed of liquid droplets, ice crystals, or a combination of the two. Clouds’ thermodynamic phase is largely controlled by temperature, but other factors can also have a significant effect. Aerosols—i.e., particles suspended in Earth's atmosphere—affect cloud properties differently depending on cloud phase and can potentially have a strong influence on climate via any cloud type. Aerosol-cloud-climate interactions have been a topic of active research for more than two decades, but these interactions nevertheless currently represent one of the most uncertain forcings of climate change over the past century. Most research to date has focused on how aerosols can impact climate via liquid clouds, which are better understood and observed than their ice-containing counterparts. Thus, the problem of how liquid clouds mediate aerosols’ effects on climate is a more tractable one. However, there is no a priori reason to think that mixed-phase and ice clouds are any less affected by changes in atmospheric aerosol composition than liquid clouds, and estimates of how aerosols can influence these ice-containing clouds have started to emerge. Laboratory and field work, as well as satellite observations, is now shifting attention to this new frontier in the field of aerosol-cloud-climate interactions, allowing for improved representation of ice processes in numerical models. Here, we review this recent progress in our understanding of aerosol effects on mixed-phase and ice clouds, focusing on the four underpinning research pillars of laboratory experiments, field observations, satellite retrievals, and numerical modeling of global climate. Evident from this review is the possibility of a powerful yet poorly constrained climate forcing, which is uncertain in terms of both its magnitude and its sign.
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Hydrogeomorphic Ecosystem Responses to Natural and Anthropogenic Changes in the Loess Plateau of China
Bojie Fu, Shuai Wang, Yu Liu, Jianbo Liu, Wei Liang, and Chiyuan MiaoVol. 45 (2017), pp. 223–243More LessChina's Loess Plateau is both the largest and deepest loess deposit in the world, and it has long been one of the most severely eroded areas on Earth. Since the 1970s, numerous soil- and water-conservation practices have been implemented: terracing, planting of vegetation, natural vegetation rehabilitation, and check-dam construction. With the implementation of the Grain-for-Green Project in 1999, the Loess Plateau has become the most successful ecological restoration zone in China. However, these large-scale restoration measures and drought have significantly reduced both runoff and sediment from the Loess Plateau. This situation has both advantages and disadvantages for the lower Yellow River. Some local soil erosion has been successfully controlled, but the whole regional ecosystem remains very fragile. Therefore, it is necessary to balance each ecosystem service, for example, by determining the region's vegetation capacity and its spatial distribution for the sustainable development of the socioecological system of the Loess Plateau.
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Interface Kinetics, Grain-Scale Deformation, and Polymorphism
Vol. 45 (2017), pp. 245–269More LessDeviatoric stress generated within subducting slabs by the olivine–spinel transformation has been modeled by assuming the phases to be simply connected rather than comprising a mixture in which one phase is embedded within another. Here, we use a simplified model to explain how transformation strain is incorporated into a continuum model, and we then use the simplified model to explain quantitatively the origin of the unreasonably large deviatoric stresses predicted by existing slab models. We review experiments on the transformation of single-crystal samples and argue that they are consistent with the occurrence, at the grain scale, of deviatoric stresses comparable with those predicted (erroneously) to exist at the slab scale by those slab models. Using a simple example, we show that although large deviatoric stresses can exist at the grain scale, their average over a sample containing many grains can be hydrostatic. This leads us to the problem of modeling the microscale structure. We outline the thermodynamics needed for such nonhydrostatic systems, and we illustrate their use and implications with examples.
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Back-Projection Imaging of Earthquakes
Eric Kiser, and Miaki IshiiVol. 45 (2017), pp. 271–299More LessBack-projection analysis of earthquakes is a type of array processing that images the source of seismic waves coherently recorded at stations throughout the seismic network. The method was developed following the magnitude 9.2 Sumatra-Andaman earthquake in 2004. Although properties of earthquakes have been investigated using array data prior to the introduction of the back-projection method, this technique differs from other approaches because it makes limited assumptions and allows detailed and complex rupture propagation to be examined. These advantages have led several researchers to apply the method to many of the largest earthquakes to occur this century. The method has also been effective for the detection of smaller events. A critical component of the success of back-projection has been the development of large-scale, dense seismic arrays. Further improvements and future applications of the method will depend greatly on the continued maintenance and development of these networks.
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Photochemistry of Sulfur Dioxide and the Origin of Mass-Independent Isotope Fractionation in Earth's Atmosphere
Vol. 45 (2017), pp. 301–329More LessArchean sulfide and sulfate minerals commonly exhibit anomalous ratios among four stable sulfur isotopes, 32S, 33S, 34S, and 36S. These anomalous relationships, referred to as sulfur mass-independent fractionation (S-MIF), provide strong evidence for an early anoxic atmosphere. Correlated variations among three isotope ratios (δ33S, δ34S, and δ36S) can be observed in rocks throughout the Archean and are a key clue toward identifying the source reaction of S-MIF. Studies to investigate the origin of Archean S-MIF so far have primarily focused on the photochemistry of sulfur dioxide (SO2). Photolysis of SO2 at wavelengths <220 nm and photoexcitation at 240–340 nm both yield large-magnitude S-MIF. Proposed mechanisms of S-MIF include isotopologue-dependent self-shielding, cross-sectional amplitudes, and vibronic coupling during intersystem crossing. This review discusses the emerging picture of the physical origins of S-MIF and their implications for the chemistry of the early Earth's atmosphere.
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Southeast Asia: New Views of the Geology of the Malay Archipelago
Vol. 45 (2017), pp. 331–358More LessSoutheast (SE) Asia is surrounded by subduction zones causing intense seismicity and volcanic activity. Subduction has been the principal tectonic driver of collisions that caused the growth of continental SE Asia, and most recently the collision of Australia with SE Asia. The western part of SE Asia, Sundaland, is a heterogeneous and weak region, reflecting processes that can be observed today in the east, where there are subduction zones in different stages of development. A close relationship between subduction rollback and extension has caused dramatic elevation of land, exhumation of deep crust, and spectacular subsidence of basins, observable with remotely acquired images and seismic and multibeam data obtained from oil exploration. New dating indicates that subsidence and uplift occurred at high rates during short time intervals. Laboratory studies, modeling, and reconstructions provide valuable insights, but field-based studies continue to present surprises and new discoveries essential for interpretations of the geological history of the region.
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Forming Planets via Pebble Accretion
Vol. 45 (2017), pp. 359–387More LessThe detection and characterization of large populations of pebbles in protoplanetary disks have motivated the study of pebble accretion as a driver of planetary growth. This review covers all aspects of planet formation by pebble accretion, from dust growth over planetesimal formation to the accretion of protoplanets and fully grown planets with gaseous envelopes. Pebbles are accreted at a very high rate—orders of magnitude higher than planetesimal accretion—and the rate decreases only slowly with distance from the central star. This allows planetary cores to start their growth in much more distant positions than their final orbits. The giant planets orbiting our Sun and other stars, including systems of wide-orbit exoplanets, can therefore be formed in complete consistency with planetary migration. We demonstrate how growth tracks of planetary mass versus semimajor axis can be obtained for all the major classes of planets by integrating a relatively simple set of governing equations.
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Tungsten Isotopes in Planets
Vol. 45 (2017), pp. 389–417More LessThe short-lived Hf-W isotope system has a wide range of important applications in cosmochemistry and geochemistry. The siderophile behavior of W, combined with the lithophile nature of Hf, makes the system uniquely useful as a chronometer of planetary accretion and differentiation. Tungsten isotopic data for meteorites show that the parent bodies of some differentiated meteorites accreted within 1 million years after Solar System formation. Melting and differentiation on these bodies took ∼1–3 million years and was fueled by decay of 26Al. The timescale for accretion and core formation increases with planetary mass and is ∼10 million years for Mars and >34 million years for Earth. The nearly identical 182W compositions for the mantles of the Moon and Earth are difficult to explain in current models for the formation of the Moon. Terrestrial samples with ages spanning ∼4 billion years reveal small 182W variations within the silicate Earth, demonstrating that traces of Earth's earliest formative period have been preserved throughout Earth's history.
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Shape, Internal Structure, Zonal Winds, and Gravitational Field of Rapidly Rotating Jupiter-Like Planets
Vol. 45 (2017), pp. 419–446More LessHigh-precision gravitational measurements by orbiting spacecraft provide a means of probing the structures, fluid motions, and convective dynamos in the interiors of the rapidly rotating outer planets. Here, the classical theory of rotating homogeneous planets is briefly reviewed. Emphasis is placed on recent developments in theories and methods that relate internal structure and processes to their gravitational signatures. Whereas early theories usually treated the effects of interior density stratification and rotational distortion as perturbations to a spherical state, recent research is marked by a self-consistent perturbation approach in which the leading-order problem accounts exactly for rotational distortion, thereby determining the basic shape, internal structure, and gravitational field of the planet. The next-order problem, which is mathematically and physically coupled with the leading-order problem, describes the modifications caused by internal fluid motions. Although the theories and methods have general applicability, advances have been spurred by the need to have a basis for interpretation of the gravitational data for Jupiter and Saturn expected from the Juno and Cassini missions.
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Effects of Partial Melting on Seismic Velocity and Attenuation: A New Insight from Experiments
Vol. 45 (2017), pp. 447–470More LessThe effects of partial melting on seismic velocity and attenuation have long been studied by focusing on the direct effects of melt, such as the poroelastic effect. The direct effects are generally very small for a very small melt fraction. Because geochemical studies have shown that the melt fraction during partial melting is very small (∼0.1%), it is difficult to explain upper-mantle low-velocity regions by the direct effects of melt. Recent experimental studies, by using a rock analog, have captured a significant enhancement of polycrystal anelasticity just before partial melting in the absence of melt. This newly recognized effect enables us to interpret seismological and geochemical observations consistently. The new anelasticity model significantly changes the interpretation of upper-mantle seismic structures. This review summarizes the recent progress in the understanding of polycrystal anelasticity, starting from a basic knowledge of linear anelasticity.
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Origin and Evolution of Regional Biotas: A Deep-Time Perspective
Vol. 45 (2017), pp. 471–495More LessHistorical processes tens to hundreds of millions of years in the past have shaped not only the trajectory of life through time but also the distribution and composition of life today. Studies aimed at the origin and evolution of regional biotas promise to forge a stronger link among paleobiology, ecology, and evolutionary biology. Improvements in high-resolution stratigraphic interpretation, numerical modeling of the fossil record, and the application of phylogenetic methods to extinct groups will lead to advances in understanding of (a) assembly of regional biotas, (b) the ecology of extinct taxa, (c) the diversification and environmental expansion of major groups, (d) the processes underlying regional ecosystem persistence and pulsed change, and (e) whether or not diversity has limits over geologic time.
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Statistics of Earthquake Activity: Models and Methods for Earthquake Predictability Studies
Vol. 45 (2017), pp. 497–527More LessStatistical methods and various models in time-space-magnitude parameter space of earthquakes are being developed to analyze seismic activity based on earthquake hypocenter catalogs that are routinely accumulated. Considering complex geophysical environments and uncertainties, we seek proper stochastic modeling that depends on the history of earthquake occurrences and relevant geophysical information for describing and forecasting earthquake activity. Also, we need empirical Bayesian models with many parameters in order to describe nonstationary or nonhomogeneous seismic activity. This review is concerned with earthquake predictability research aimed at realizing practical operational forecasting. In particular, uncertainty lies in identifying whether abnormal phenomena are precursors to large earthquakes. The predictability of such models can be examined by certain statistical criteria.
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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)