Annual Review of Earth and Planetary Sciences - Volume 37, 2009
Volume 37, 2009
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Where Are You From? Why Are You Here? An African Perspective on Global Warming
Vol. 37 (2009), pp. 1–18More LessGlobal warming, although usually associated with imminent environmental disasters, also presents splendid opportunities for research and education and for collaboration between the rich and the poor that will benefit both groups. Unfortunately, efforts to take advantage of these opportunities are handicapped by the misperception that scientific disputes concerning imminent global climate changes have been settled—that the science “is over”—in addition to a failure to appreciate that some people face problems more urgent than adaptation to and mitigation of the climate changes scientists predict. Changes over the past few decades in the way we conduct our affairs in the Earth sciences, specifically the atmospheric and oceanic sciences, contribute to these misunderstandings. This is a subjective, personal account of those changes.
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Stagnant Slab: A Review
Vol. 37 (2009), pp. 19–46More LessA stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above, across, or below the 660 km discontinuity. This phenomenon has now been widely recognized beneath subduction zones around the circum-Pacific and in the Mediterranean. Collaboration of seismic and electromagnetic observations, mineral physics measurements, and geodynamic modeling has begun to provide a consistent picture of stagnant slab.
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Radiocarbon and Soil Carbon Dynamics
Vol. 37 (2009), pp. 47–66More LessResearch over the past several decades has clarified the mechanisms and timescales involved in stabilizing organic matter in soils, but we still lack process-based understanding sufficient for predicting how vulnerable soil carbon (C) is, given climatic or environmental change across a range of soil types and landscapes. Part of the problem is the emphasis on short-term studies and processes that dominate C balance at the point or soil profile scale, whereas other processes that dominate over longer timescales and larger spatial scales may actually be more important for determining the carbon balance of soils in a region. Radiocarbon is one of the only tools to study the dynamics of C in soils on decadal to millennial timescales. It provides a means for directly testing models of organic matter dynamics in ecosystems and, when measured in respired CO2 or dissolved organic carbon (DOC), provides evidence of shifts in microbial metabolism. This review explores the application of this underutilized tool, with an emphasis on conceptual advances made using the state-factor approach and on detecting processes causing abrupt change in soil C stores.
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Evolution of the Genus Homo
Vol. 37 (2009), pp. 67–92More LessDefinition of the genus Homo is almost as fraught as the definition of Homo sapiens. We look at the evidence for “early Homo,” finding little morphological basis for extending our genus to any of the ∼2.5–1.6-myr-old fossil forms assigned to “early Homo” or Homo habilis/rudolfensis. We also point to heterogeneity among “early African Homo erectus,” and the lack of apomorphies linking these fossils to the Asian Homo erectus group, a cohesive regional clade that shows some internal variation, including brain size increase over time. The first truly cosmopolitan Homo species is Homo heidelbergensis, known from Africa, Europe, and China following 600 kyr ago. One species sympatric with it included the >500-kyr-old Sima de los Huesos fossils from Spain, clearly distinct from Homo heidelbergensis and the oldest hominids assignable to the clade additionally containing Homo neanderthalensis. This clade also shows evidence of brain size expansion with time; but although Homo neanderthalensis had a large brain, it left no unequivocal evidence of the symbolic consciousness that makes our species unique. Homo sapiens clearly originated in Africa, where it existed as a physical entity before it began (also in that continent) to show the first stirrings of symbolism. Most likely, the biological underpinnings of symbolic consciousness were exaptively acquired in the radical developmental reorganization that gave rise to the highly characteristic osteological structure of Homo sapiens, but lay fallow for tens of thousands of years before being “discovered” by a cultural stimulus, plausibly the invention of language.
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Feedbacks, Timescales, and Seeing Red
Vol. 37 (2009), pp. 93–115More LessFeedback analysis is a powerful tool for studying the Earth system. It provides a formal framework for evaluating the relative importance of different interactions in a dynamical system. As such, its application is essential for a predictive or even a mechanistic understanding of the complex interplay of processes on the Earth. This paper reviews the basic principles of feedback analysis and tries to highlight the importance of the technique for the interpretation of physical systems. The need for clear and consistent definitions when comparing different interactions is emphasized. It is also demonstrated that feedback analyses can shed light on how uncertainty in physical processes translates into uncertainty in system response, and that the strength of the feedbacks has a very tight connection to the dynamical response time of the system.
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Atmospheric Lifetime of Fossil Fuel Carbon Dioxide
Vol. 37 (2009), pp. 117–134More LessCO2 released from combustion of fossil fuels equilibrates among the various carbon reservoirs of the atmosphere, the ocean, and the terrestrial biosphere on timescales of a few centuries. However, a sizeable fraction of the CO2 remains in the atmosphere, awaiting a return to the solid earth by much slower weathering processes and deposition of CaCO3. Common measures of the atmospheric lifetime of CO2, including the e-folding time scale, disregard the long tail. Its neglect in the calculation of global warming potentials leads many to underestimate the longevity of anthropogenic global warming. Here, we review the past literature on the atmospheric lifetime of fossil fuel CO2 and its impact on climate, and we present initial results from a model intercomparison project on this topic. The models agree that 20–35% of the CO2 remains in the atmosphere after equilibration with the ocean (2–20 centuries). Neutralization by CaCO3 draws the airborne fraction down further on timescales of 3 to 7 kyr.
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Evolution of Life Cycles in Early Amphibians
Vol. 37 (2009), pp. 135–162More LessMany modern amphibians have biphasic life cycles with aquatic larvae and terrestrial adults. The central questions are how and when this complicated ontogeny was established, and what is known about the lives of amphibians in the Paleozoic. Fossil evidence has accumulated that sheds light on the life histories of early amphibians, the origin of metamorphosis, and the transition to a fully terrestrial existence. The majority of early amphibians were aquatic or amphibious and underwent only gradual ontogenetic changes. Developmental plasticity played a major role in some taxa but was restricted to minor modification of ontogeny. In the Permo-Carboniferous dissorophoids, a condensation of crucial ontogenetic steps into a short phase (metamorphosis) is observed. It is likely that the origin of both metamorphosis and neoteny falls within these taxa. Fossil evidence also reveals the sequence of evolutionary changes: apparently, the ontogenetic change in feeding, not the transition to a terrestrial existence per se, made a drastic metamorphosis necessary.
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The Fin to Limb Transition: New Data, Interpretations, and Hypotheses from Paleontology and Developmental Biology
Vol. 37 (2009), pp. 163–179More LessAfter a brief historical review of the fin to limb transition and consideration of a theoretical “prototetrapod,” this article considers new ideas generated from recent fossil finds and from developmental biology that bear on the question of how limbs, digits, limb joints, and pentadactyly evolved. Among the first changes to take place were those to the humerus, in concert with those to the breathing apparatus, and these adaptations were acquired while the animals were still basically aquatic with the evolution of digits occurring during this phase. Studies from developmental biology of modern taxa can be integrated with information from fossils to produce a fuller picture. The acquisition of pentadactyly was among the last changes to occur in the modification of a fin into a limb. This vision differs radically from older theoretical ideas which perceived land locomotion as the prime evolutionary force driving the transition.
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Mammalian Response to Cenozoic Climatic Change
Vol. 37 (2009), pp. 181–208More LessMultiple episodes of rapid and gradual climatic changes influenced the evolution and ecology of mammalian species and communities throughout the Cenozoic. Climatic change influenced the abundance, genetic diversity, morphology, and geographic ranges of individual species. Within communities these responses interacted to catalyze immigration, speciation, and extinction. Combined they affected long-term patterns of community stability, functional turnover, biotic turnover, and diversity. Although the relative influence of climate on particular evolutionary processes is oft debated, an understanding of processes at the root of biotic change yields important insights into the complexity of mammalian response. Ultimately, all responses trace to events experienced by populations. However, many such processes emerge as patterns above the species level, where shared life history traits and evolutionary history allow us to generalize about mammalian response to climatic change. These generalizations provide the greatest power to understand and predict mammalian responses to current and future global change.
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Forensic Seismology and the Comprehensive Nuclear-Test-Ban Treaty
Vol. 37 (2009), pp. 209–236More LessOne application of forensic seismology is to help verify compliance with the Comprehensive Nuclear-Test-Ban Treaty. One of the challenges facing the forensic seismologist is to discriminate between the many thousands of earthquakes of potential interest each year and potential Treaty violations (underground explosions). There are four main methods: (a) ratio of body- to surface-wave magnitudes, (b) ratio of high-frequency P to S energy, (c) model-based methods, and (d) source depth. Methods (a) and (b) have an empirical basis. The weakness of methods (a)–(c) is the lack of an equivalent elastic source for an underground explosion fired in the range of geological media found around the world. Reliable routine source-depth determination has proved difficult. However, experience gained in the past decade at identifying suspicious seismic sources suggests that although no single method works all of the time, intelligent and original application of complementary methods is usually sufficient to satisfactorily identify the source in question.
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How the Continents Deform: The Evidence From Tectonic Geodesy*
Vol. 37 (2009), pp. 237–262More LessSpace geodesy now provides quantitative maps of the surface velocity field within tectonically active regions, supplying constraints on the spatial distribution of deformation, the forces that drive it, and the brittle and ductile properties of continental lithosphere. Deformation is usefully described as relative motions among elastic blocks and is block-like because major faults are weaker than adjacent intact crust. Despite similarities, continental block kinematics differs from global plate tectonics: blocks are much smaller, typically ∼100–1000 km in size; departures from block rigidity are sometimes measurable; and blocks evolve over ∼1–10 Ma timescales, particularly near their often geometrically irregular boundaries. Quantitatively relating deformation to the forces that drive it requires simplifying assumptions about the strength distribution in the lithosphere. If brittle/elastic crust is strongest, interactions among blocks control the deformation. If ductile lithosphere is the stronger, its flow properties determine the surface deformation, and a continuum approach is preferable.
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The Tropics in Paleoclimate
Vol. 37 (2009), pp. 263–297More LessThe past decade saw a surge in interest in the role of the Tropics in paleoclimate changes. This was motivated by the emergence of outstanding questions in paleoclimate that pointed to a role for the Tropics in addition to advances in tropical climate dynamics. This article reviews these developments, starting from a historical perspective. Three properties of tropical dynamics are prominent in paleoclimate: the sensitivity of the tropical climate to change; the ability of the tropical climate to reorganize; and the ability of the tropical climate to project its influence globally. The El Niño-Southern Oscillation (ENSO) system exemplifies these properties, making ENSO particularly prominent in paleoclimate. Summaries of three paleoclimate cases in which the science developed over the past decade—mid-Holocene ENSO, abrupt climate change during the most recent glacial period, and the mid-Pliocene permanent El Niño scenario—illustrate how the tropical hypothesis worked its way into paleoclimate research. This review closes with a discussion of prevailing views of the Tropics in the paleoclimate changes.
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Rivers, Lakes, Dunes, and Rain: Crustal Processes in Titan's Methane Cycle
Vol. 37 (2009), pp. 299–320More LessTitan exhibits ample surface and crustal processes including lakes and seas, fluvial erosive features, possibly subsurface reservoirs of liquid, and rainfall. Together these constitute strong evidence for a multicomposition hydrological system, composed mostly of methane and ethane as well as trace amounts of other alkanes. Estimates of the volume of liquid methane required in streams and rainfall to produce erosional features suggest that these could be relatively recent phenomena, perhaps periodically renewed as the overall climate cycles between dry and wet periods. The end state of the longer-term chemical processing of methane in the upper atmosphere is expressed on the surface in the form of deposits of solid organics organized into dunes, and lighter hydrocarbons such as ethane (in the lakes), acetylene, and other hydrocarbons and nitriles. The long-term evolution of the methane cycle may have involved episodic resupply of methane to the surface or gradual depletion of a larger surface reservoir of methane, but in either case, removal of large amounts of ethane from the surface remains an unresolved problem.
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Planetary Migration: What Does It Mean for Planet Formation?
Vol. 37 (2009), pp. 321–344More LessGravitational interactions between a planet and its protoplanetary disk change the planet's orbit, causing the planet to migrate toward or away from its star. Migration rates are poorly constrained for low-mass bodies but reasonably well understood for giant planets. In both cases, significant migration will affect the details and efficiency of planet formation. If the disk is turbulent, density fluctuations will excite orbital eccentricities and cause orbits to undergo a random walk. Both processes are probably detrimental to planet formation. Planets that form early in the lifetime of a disk are likely to be lost, whereas late-forming planets will survive and may undergo little migration. Migration can explain the observed orbits and masses of extrasolar planets if giants form at different times and over a range of distances. Migration can also explain the existence of planets orbiting close to their star and of resonant pairs of planets.
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The Tectonic Framework of the Sumatran Subduction Zone
Vol. 37 (2009), pp. 345–366More LessThe great Aceh-Andaman earthquake of December 26, 2004 and its tragic consequences brought the Sumatran region and its active tectonics into the world's focus. The plate tectonic setting of Sumatra has been as it is today for tens of millions of years, and catastrophic geologic events have likely been plentiful. The immaturity of our understanding of great earthquakes and other types of geologic hazards contributed to the surprise regarding the location of the 2004 earthquake. The timing, however, is probably best understood simply in terms of the inevitability of the infrequent events that shape the course of geologic progress. Our best hope is to improve understanding of the processes involved and decrease our vulnerability to them.
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Microbial Transformations of Minerals and Metals: Recent Advances in Geomicrobiology Derived from Synchrotron-Based X-Ray Spectroscopy and X-Ray Microscopy
Vol. 37 (2009), pp. 367–391More LessThe field of geomicrobiology has embraced efforts to define and quantify the role of microbial organisms in low-temperature geochemical processes. However, any mechanistic understanding of the interactions between microbial organisms and minerals or metals requires analytical tools with the appropriate chemical sensitivity and spatial resolution. In the past several years, increasing application of nanometer-, micron-, and bulk-scale synchrotron-based X-ray techniques has provided new insights regarding the feedbacks among microbial growth, mineral dissolution, redox transformations, and biomineralization processes. In this review, recent findings derived from X-ray absorption spectroscopy, X-ray microprobe mapping, and X-ray microscopy studies are integrated to provide a new view of the dynamic biogeochemistry occurring at the microbe-mineral interface.
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The Channeled Scabland: A Retrospective
Vol. 37 (2009), pp. 393–411More LessThe Channeled Scabland of east-central Washington in the United States is a complex of anastomosing rock-cut fluvial channels, cataracts, loess islands, rock basins, broad gravel deposits, and immense gravel bars. In the 1920s, J Harlen Bretz demonstrated that the Channeled Scabland formed by cataclysmic erosion and deposition from Pleistocene megaflooding derived from the margins of the Cordilleran Ice Sheet, particularly glacial Lake Missoula in western Montana and northern Idaho. Studies of this region and the high-energy flood processes that generated it are stimulating (a) discoveries of similar megaflood-related landscapes around the world and on Mars, (b) enhanced understanding of the processes involved in the fluvial erosion of bedrock, and (c) the use of paleoflood indicators for understanding the magnitudes and frequency of flooding.
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Growth and Evolution of Asteroids
Vol. 37 (2009), pp. 413–448More LessAsteroids are what is left of the precursors to the terrestrial planets. They are stunning in their diversity, ranging from charcoal-black worlds the size of a hilltop, spinning like a carnival ride, to dog-bone-shaped metallic remnants of some cataclysmically disrupted planetary core, to worlds as stately as Ceres and Vesta (and fragments thereof), to garden-variety fractured and blocky nuggets that dominate near-Earth space. Asteroid belts are common around Sun-like stars. When properly seen as unaccreted residues, as scraps on the floor of the planetary bakery, the diversity of asteroids can be fully appreciated, for to paraphrase Tolstoy, accreted planets are all alike; every unaccreted planet is unaccreted in its own way.
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Thermodynamics and Mass Transport in Multicomponent, Multiphase H2O Systems of Planetary Interest
Xinli Lu, and Susan W. KiefferVol. 37 (2009), pp. 449–477More LessHeat and mass transport in low-temperature, low-pressure H2O systems are important processes on Earth, and on a number of planets and moons in the Solar System. In most occurrences, these systems will contain other components, the so-called noncondensible gases, such as CO2, CO, SO2, CH4, and N2. The presence of the noncondensible components can greatly alter the thermodynamic properties of the phases and their flow properties as they move in and on the planets. We review various forms of phase diagrams that give information about pressure-temperature-volume-entropy-enthalpy-composition conditions in these complex systems. Fluid dynamic models must be coupled to the thermodynamics to accurately describe flow in gas-driven liquid and solid systems. The concepts are illustrated in detail by considering flow and flow instabilities such as geysering in modern geothermal systems on Earth, paleofluid systems on Mars, and cryogenic ice-gas systems on Mars and Enceladus. We emphasize that consideration of single-component end-member systems often leads to conclusions that exclude many qualitatively and quantitatively important phenomena.
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The Hadean Crust: Evidence from >4 Ga Zircons
Vol. 37 (2009), pp. 479–505More LessA review of continental growth models leaves open the possibilities that Earth during the Hadean Eon (∼4.5–4.0 Ga) was characterized by massive early crust or essentially none at all. Without support from the rock record, our understanding of pre-Archean continental crust must largely come from investigating Hadean detrital zircons. We know that these ancient zircons yield relatively low crystallization temperatures and some are enriched in heavy oxygen, contain inclusions similar to modern crustal processes, and show evidence of silicate differentiation at ∼4.5 Ga. These observations are interpreted to reflect an early terrestrial hydrosphere, early felsic crust in which granitoids were produced and later weathered under high water activity conditions, and even the possible existence of plate boundary interactions—in strong contrast to the traditional view of an uninhabitable, hellish world. Possible scenarios are explored with a view to reconciling this growing but fragmentary record with our knowledge of conditions then extant in the inner solar system.
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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)