ALBERT

Notizen: Abstract This survey of the current status of research into Earth's orbitally forced paleoclimatic record summarizes recent developments in the theory of Earth's orbital parameters, and reviews how various techniques of data collection and analysis have fared in the search and recovery of orbital signals in ancient stratigraphy. The emerging significance of the quasi-periodicity of Earth's orbital variations as a principal tool in the analysis of orbitally forced stratigraphy is discussed in detail. Five case studies are presented that illustrate new directions in research: (a) time series analysis of discontinuous strata; (b) measurement of ultra-high resolution stratigraphic signals; (c) new perspectives on the 100 kyr Pleistocene glaciation problem; (d) stratigraphic evidence for solar system resonance modes; and (e) evaluating Phanerozoic length of day from orbitally forced stratigraphy.

Notizen: Abstract Theories of scaling apply wherever similarity exists across many scales. This similarity may be found in geometry and in dynamical processes. Universality arises when the qualitative character of a system is sufficient to quantitatively predict its essential features, such as the exponents that characterize scaling laws. Within geomorphology, two areas where the concepts of scaling and universality have found application are the geometry of river networks and the statistical structure of topography. We begin this review with a pedagogical presentation of scaling and universality. We then describe recent progress made in applying these ideas to networks and topography. This overview leads to a synthesis that attempts a classification of surface and network properties based on generic mechanisms and geometric constraints. We also briefly review how scaling and universality have been applied to related problems in sedimentology-specifically, the origin of stromatolites and the relation of the statistical properties of submarine-canyon topography to the size distribution of turbidite deposits. Throughout the review, our intention is to elucidate not only the problems that can be solved using these concepts, but also those that cannot.

Notizen: Abstract This paper reviews recent research focused on the Earth's inner core. Large inner-core traveltime anomalies and the anomalous splitting of core-sensitive free oscillations strongly suggest that the inner core is anisotropic. Initial models involved a simple, constant or depth-dependent cylindrical anisotropy at a level less than a few percent. Recent observations suggest that its eastern hemisphere is largely isotropic, whereas its western hemisphere is highly anisotropic, and there are indications that its top 100 km may be isotropic. The coda of inner-core reflected phases has been used to infer strong heterogeneities with a length scale of just a few kilometers. Thus, a complicated three-dimensional picture of the inner core is beginning to emerge, although it has been suggested that much of this complexity may be the misinterpretation of signals that have their origin in the lowermost mantle. Numerical models of the geodynamo suggest that the inner core may rotate at a slightly different rate than the mantle. Recent seismological estimates based upon traveltime and normal-mode data limit inner-core differential rotation to less than +0.2 degrees per year.

Notizen: Abstract The recycling of elements by plants and plant-induced biological activity cause the rates and products of weathering to be markedly different from what would result in abiotic processes. Plants directly control water dynamics, weathering, and the chemistry of weathering solutions, which is clearly exhibited in equatorial areas where old weathering mantles are greatly influenced by biological activity. Depending on the dynamics of plant-induced organometallic compounds, this weathering results in either clayey soils, which are in a dynamic equilibrium sustained by the forest's cycling of elements, or sandy soils. In most places (tropical as well temperate areas), the weathering mantle can be regarded as being in a dynamic equilibrium sustained by plants.

Notizen: Abstract Molecular and paleontological data provide independent means of estimating when groups of organisms evolved in the geological past, but neither approach can be considered straightforward. The single most fundamental obstacle to developing an accurate estimate of times of origination from gene sequence data is variation in rates of molecular evolution, both through time and among lineages. Although various techniques have been proposed to circumvent this problem, none unambiguously allow the components of time and rate to be separated. Furthermore, problems of establishing accurate calibration points, correctly rooted phylogenies, and accurate estimates of branch length remain formidable. Conversely, paleontological dates fix only the latest possible time of divergence, and so probabilistic methods are required to set a lower boundary on origination dates. Realistic confidence intervals that take preservational biases into account are only just becoming available. Although molecular and paleontological approaches to dating often agree reasonably well, there are two notable areas of disagreement; when mammal and bird orders originated and when the major phyla originated. The discrepancy in dating bird/mammal ordinal origins probably reflects a global rock-record bias. Paleontological sampling in the Late Cretaceous is still too restricted geographically to draw any firm conclusions about the existence of a pre-Tertiary record for modern orders of bird or mammal from anywhere other than North America. Dating the time of origin of phyla is more complicated, and is confounded by both preservational biases and problems of molecular clock estimation.

Notizen: Abstract The subducted lithosphere is composed of a complex pattern of chemical systems that undergo continuous and discontinuous phase transformation, through pressure and temperature variations. Volatile recycling plays a major geodynamic role in triggering mass transfer, melting, and volcanism. Although buoyancy forces are controlled by modal amounts of the most abundant phases, usually volatile-free, petrogenesis and chemical differentiation are controlled by the occurrence of minor phases, most of them volatile-bearing. Devolatilization of the subducted lithosphere is a continuous process distributed over more than 300 km of the slab-mantle interface. Melting of the subducted crust, if any, along sufficiently hot P-T paths, is governed by fluid-absent reactions, even though the difference between fluid and melt vanishes at pressures above the second critical end point. The density distribution at a depth of 660 km suggests episodic penetration in space and time of subducted slabs into the lower mantle and sinking down to the D" region at the core-mantle boundary.

Notizen: Abstract Pluto and Charon, once thought to be a singular system in an odd orbit at the edge of the solar system, are now known as members of a vast population of icy bodies beyond Neptune. Models for the occurrence of the odd orbit and formation of these bodies in the context of the total population are reviewed. Pluto's orbital characteristics, coupled with the existence of volatiles on the surface, suggest that large-scale seasonal change should occur on the surface. Models of seasonal variability are discussed, past and current observations are examined for evidence of variability, and a straw-man model of seasonal changes is proposed. Finally, recent observations of the surface composition of Charon are discussed and compared with observations of other similarly sized icy bodies in the outer Solar System.

Notizen: Abstract We present preliminary evidence for a ~10,000-year earthquake record from two major fault systems based on sediment cores collected along the continental margins of western North America. New stratigraphic evidence from Cascadia demonstrates that 13 earthquakes ruptured the entire margin from Vancouver Island to at least the California border since the eruption of the Mazama ash 7700 years ago. The 13 events above this prominent stratigraphic marker have an average repeat time of 600 years, and the youngest event ~300 years ago coincides with the coastal record. We also extend the record of past earthquakes to the base of the Holocene (at least 9800 years ago), during which 18 events correlate along the same region. The sequence of Holocene events in Cascadia appears to contain a repeating pattern of events, a tantalizing first look at what may be the long-term behavior of a major fault system. The northern California margin cores show a cyclic record of turbidite beds that may represent Holocene earthquakes on the northern segment of the San Andreas Fault. Preliminary results are in reasonably good agreement with onshore paleoseismic data that indicate an age for the penultimate event in the mid-1600s at several sites and the most likely age for the third event of ~AD 1300.

Notizen: The nation has over 40,000 metric tonnes (MT) of nuclear waste destined for disposal in a geologic repository at Yucca Mountain. In this review, we highlight some of the important geoscience issues associated with the project and place them in the context of the process by which a final decision on Yucca Mountain will be made. The issues include understanding how water could infiltrate the repository, corrode the canisters, dissolve the waste, and transport it to the biosphere during a 10,000-year compliance period in a region, the Basin and Range province, that is known for seismic and volcanic activity. Although the site is considered to be "dry," a considerable amount of water is present as pore waters and as structural water in zeolites. The geochemical environment is oxidizing, and the present repository design will maintain temperatures at greater than 100oC for thousands of years. Geoscientists in this project are challenged to make unprecedented predictions about coupled thermal, hydrologic, mechanical, and geochemical processes governing the future behavior of the repository and to conduct research in a regulatory and legal environment that requires a quantitative analysis of repository performance.

Notizen: The migration of the Mendocino triple junction through central and northern California over the past 25-30 million years has led to a profound change in plate interactions along coastal California. The tectonic consequences of the abrupt change from subduction plate interactions north of the triple junction to the development of the San Andreas transform system south of the triple junction can be seen in the geologic record and geophysical observations. The primary driver of this tectonism is a coupling among the subducting Juan de Fuca (Gorda), North American, and Pacific plates that migrates with the triple junction. This coupling leads to ephemeral thickening of the overlying North American crust, associated uplift and subsequent subsidence, and a distinctive sequence of fault development and volcanism.

Notizen: Bedrock rivers set much of the relief structure of active orogens and dictate rates and patterns of denudation. Quantitative understanding of the role of climate-driven denudation in the evolution of unglaciated orogens depends first and foremost on knowledge of fluvial erosion processes and the factors that control incision rate. The results of intense research in the past decade are reviewed here, with the aim of highlighting remaining unknowns and suggesting fruitful avenues for further research. This review considers in turn (a) the occurrence and morphology of bedrock channels and their relation to tectonic setting; (b) the physical processes of fluvial incision into rock; and (c) models of river incision, their implications, and the field and laboratory data needed to test, refine, and extend them.

Notizen: Abstract Considerable progress has been made over the past decade in understanding the static rheological properties of granitic magmas in the continental crust. Changes in H2O content, CO2 content, and oxidation state of the interstitial melt phase have been identified as important compositional factors governing the rheodynamic behavior of the solid/fluid mixture. Although the strengths of granitic magmas over the crystallization interval are still poorly constrained, theoretical investigations suggest that during magma ascent, yield strengths of the order of 9 kPa are required to completely retard the upward flow in meter-wide conduits. In low Bagnold number magma suspensions with moderate crystal contents (solidosities 0.1 〈=phi〈= 0.3), viscous fluctuations may lead to flow differentiation by shear-enhanced diffusion. AMS and microstructural studies support the idea that granite plutons are intruded as crystal-poor liquids (phi〈= 50%), with fabric and foliation development restricted to the final stages of emplacement. If so, then these fabrics contain no information on the ascent (vertical transport) history of the magma. Deformation of a magmatic mush during pluton emplacement can enhance significantly the pressure gradient in the melt, resulting in a range of local macroscopic flow structures, including layering, crystal alignment, and other mechanical instabilities such as shear zones. As the suspension viscosity varies with stress rate, it is not clear how the timing of proposed rheological transitions formulated from simple equations for static magma suspensions applies to mixtures undergoing shear. New theories of magmas as multiphase flows are required if the full complexity of granitic magma rheology is to be resolved.

Notizen: Abstract Is El Nino one phase of a continual, self-sustaining natural mode of the coupled ocean-atmosphere that has La Nina as the complementary phase? Or is El Nino a temporary departure from "normal" conditions "triggered" by a random disturbance such as a burst of westerly winds? A growing body of evidence-stability analyses, studies of the energetics, simulations that reproduce the statistics of sea surface temperature variations in the eastern equatorial Pacific-indicates that reality corresponds to a compromise between these two possibilities: The observed Southern Oscillation between El Nino and La Nina corresponds to a weakly damped mode that is sustained by random disturbances. This means that the predictability of El Nino is limited by the continual presence of "noise" so that forecasts should be probabilistic. The Southern Oscillation is also subject to decadal modulations. How it will be influenced by global warming is a matter of considerable uncertainty.

Notizen: Computer models are used to mimic the early evolution of ancient vascular plants (tracheophytes). These models have three components: (a) an N-dimensional domain of all mathematically conceivable ancient morphologies (a morphospace); (b) a numerical assessment of the ability (fitness) of each morphology to intercept light, maintain mechanical stability, conserve water, and produce and disperse spores; and (c) an algorithm that searches the morphospace for successively more fit variants (an adaptive walk). Beginning with the most ancient known plant form, evolution is simulated by locating neighboring morphologies that progressively perform one or more tasks more efficiently. The resulting simulated adaptive walks indicate that early tracheophyte evolution involved optimizing the performance of many tasks simultaneously rather than maximizing the performance of one or only a few tasks individually, and that the requirement for optimization accelerated the tempo of morphological evolution in the Silurian and Devonian.

Notizen: Models of processes in the alpine snow cover fundamentally depend on the spatial distribution of the surface energy balance over areas where topographic variability causes huge differences in the incoming solar radiation and in snow depth because of redistribution by wind. At a spatial scale commensurate with that of the terrain, we want to know which areas are covered by snow, and we want to estimate the snow's spectral albedo, along with other properties such as grain size, contaminants, temperature, liquid water content, and depth or water equivalent. From multispectral and hyperspectral remote sensing at wavelengths from 0.4-15 mum, the retrievable properties include snow-covered area, albedo, grain size, liquid water very near the surface, and temperature. Spectral mixture analysis allows the retrieval of the subpixel variability of snow-covered area, along with the snow's albedo. Remaining research challenges include the remote sensing of absorbing impurities; accounting for variability in the bidirectional-reflectance distribution function and the variability of grain size with depth; retrieving snow cover in forested regions; reconciling field measurements of emissivity with snow properties; and adapting the algorithms to frequent, large-scale processing.

Notizen: We examine the genetics of marine diversification along the West Coast of North America in relation to the Late Neogene geology and climate of the region. Trophically important components of the diverse West Coast fauna, including kelp, alcid birds (e.g., auks, puffins), salmon, rockfish, abalone, and Cancer crabs, appear to have radiated during peaks of upwelling primarily in the Late Miocene and in some cases secondarily in the Pleistocene. Phylogeographic barriers associated with Mio-Pliocene estuaries of the mid-California coast, the Pliocene opening of the Gulf of California, tectonic and eustatic evolution of the California Bight, as well as the influence of Pleistocene and Holocene climate change on genetic structure are assessed in a geologic context. Comparisons to East Coast and western freshwater systems, as well as upwelling systems around the globe, provide perspective for the survey.

Notizen: Measurements of cosmogenic nuclides, predominately 10Be, allow new insights into the ways in which and the rates at which sediment is generated, transported, and deposited over timescales ranging from 103 to 106 years. Samples from rock exposures are used to estimate erosion rates at points on the landscape, whereas samples of fluvial sediment provide estimates of basin-scale rates of denudation integrated over 〈1 to 〉104 km2. Nuclide data show that hilltop, bare rock outcrops erode more slowly than basins as a whole, suggesting the potential for relief to increase over time as well-drained outcrops grow higher. More elaborate experiments and interpretive models provide insight into the distribution of hillslope processes, including the bedrock-to-soil conversion rate, which appears to increase under shallow soil cover and then decrease under deeper soils. Changes in average nuclide activity down slopes can be used to estimate grain speed over millennia, suggesting, for example, that sediment on desert piedmonts moves, on average, decimeters to meters per year. In other cases, changes in nuclide activity down river networks or along shorelines can be interpreted with mixing models to indicate sediment sources. Sediment deposition rates in otherwise undateable deposits can now be estimated by analyzing samples collected from depth profiles. Over the past decade, the analysis and interpretation of cosmogenic nuclides has given geomorphologists an unprecedented opportunity to measure rates and infer the distribution of geomorphic processes across Earth's varied landscapes. Long-standing models of landscape change can now be tested quantitatively.

Notizen: Avulsion is the natural process by which flow diverts out of an established river channel into a new permanent course on the adjacent floodplain. Avulsions are primarily features of aggrading floodplains. Their recurrence interval varies widely among the few modern rivers for which such data exist, ranging from as low as 28 years for the Kosi River (India) to up to 1400 years for the Mississippi. Avulsions cause loss of life, property damage, destabilization of shipping and irrigation channels, and even coastal erosion as sediment is temporarily sequestered on the floodplain. They are also the main process that builds alluvial stratigraphy. Their causes remain relatively unknown, but stability analyses of bifurcating channels suggest that thresholds in the relative energy slope and Shields parameter of the bifurcating channel system are key factors.

Notizen: Abstract Non-avian dinosaur reproductive and parenting behaviors were mostly similar to those of extant archosaurs. Non-avian dinosaurs were probably sexually dimorphic and some may have engaged in hierarchical rituals. Non-avian coelurosaurs (e.g. Troodontidae, Oviraptorosauria) had two active oviducts, each of which produced single eggs on a daily or greater time scale. The eggs of non-coelurosaurian dinosaurs (e.g. Ornithischia, Sauropoda) were incubated in soils, whereas the eggs of non-avian coelurosaurs (e.g. Troodon, Oviraptor) were incubated with a combination of soil and direct parental contact. Parental attention to the young was variable, ranging from protection from predators to possible parental feeding of nest-bound hatchlings. Semi-altricial hadrosaur hatchlings exited their respective nests near the time of their first linear doubling. Some reproductive behaviors, once thought exclusive to Aves, arose first in non-avian dinosaurs. The success of the Dinosauria may be related to reproductive strategies.

Notizen: Abstract Synthetic aperture radar interferometry (InSAR) from Earth-orbiting spacecraft provides a new tool to map global topography and deformation of the Earth's surface. Radar images taken from slightly different viewing directions allow the construction of digital elevation models of meter-scale accuracy. These data sets aid in the analysis and interpretation of tectonic and volcanic landscapes. If the Earth's surface deformed between two radar image acquisitions, a map of the surface displacement with tens-of-meters resolution and subcentimeter accuracy can be constructed. This review gives a basic overview of InSAR for Earth scientists and presents a selection of geologic applications that demonstrate the unique capabilities of InSAR for mapping the topography and deformation of the Earth.

Notizen: Abstract Vredefort, Sudbury, and Chicxulub are the largest known terrestrial impact structures. All have been cited as multi-ring basins. The available data indicate that all have some form of multiple-ring attributes, most commonly structural features. Chicxulub, however, is the only example with morphological ring features. There are also commonalities in the structural and lithological features of Vredefort and Sudbury, and it is possible to construct a generalized compilation of the character of 200-300 km diameter impact basins on Earth. It is not clear, however, that any of these structures had the original morphological characteristics of large lunar multi-ring basins. Additional data and synthesis are required to fully characterize these structures in order to realize their potential to constrain large-scale cratering processes. If this is not sufficient incentive for further studies (the environmental effects of Chicxulub aside), the Vredefort, Sudbury, and Chicxulub impact events are also the reason for the existence of world-class mineral and hydrocarbon deposits.

Notizen: Abstract The mantle plume hypothesis was proposed thirty years ago by Jason Morgan to explain hotspot volcanoes such as Hawaii. A thermal diapir (or plume) rises from the thermal boundary layer at the base of the mantle and produces a chain of volcanoes as a plate moves on top of it. The idea is very attractive, but direct evidence for actual plumes is weak, and many questions remain unanswered. With the great improvement of seismic imagery in the past ten years, new prospects have arisen. Mantle plumes are expected to be rather narrow, and their detection by seismic techniques requires specific developments as well as dedicated field experiments. Regional travel-time tomography has provided good evidence for plumes in the upper mantle beneath a few hotspots (Yellowstone, Massif Central, Iceland). Beneath Hawaii and Iceland, the plume can be detected in the transition zone because it deflects the seismic discontinuities at 410 and 660 km depths. In the lower mantle, plumes are very difficult to detect, so specific methods have been worked out for this purpose. There are hints of a plume beneath the weak Bowie hotspot, as well as intriguing observations for Hawaii. Beneath Iceland, high-resolution tomography has just revealed a wide and meandering plume-like structure extending from the core-mantle boundary up to the surface. Among the many phenomena that seem to take place in the lowermost mantle (or D"), there are also signs there of the presence of plumes. In this article I review the main results obtained so far from these studies and discuss their implications for plume dynamics. Seismic imaging of mantle plumes is still in its infancy but should soon become a turbulent teenager.

Notizen: Abstract There has been considerable controversy concerning the role of chemical weathering in the regulation of the atmospheric partial pressure of carbon dioxide, and thus the strength of the greenhouse effect and global climate. Arguments center on the sensitivity of chemical weathering to climatic factors, especially temperature. Laboratory studies reveal a strong dependence of mineral dissolution on temperature, but the expression of this dependence in the field is often obscured by other environmental factors that co-vary with temperature. In the field, the clearest correlation is between chemical erosion rates and runoff, indicating an important dependence on the intensity of the hydrological cycle. Numerical models and interpretation of the geologic record reveal that chemical weathering has played a substantial role in both maintaining climatic stability over the eons as well as driving climatic swings in response to tectonic and paleogeographic factors.

Notizen: Abstract Epizonal base and precious metal deposits makeup a range of familiar deposit styles including porphyry copper-gold, epithermal veins and stockworks, carbonate-replacement deposits, and polymetallic volcanic rock-hosted (VHMS) deposits. They occur along convergent plate margins and are invariably associated directly with active faults and volcanism. They are complex in form, variable in their characteristics at all scales, and highly localized in the earth's crust. More than a century of detailed research has provided an extensive base of observational data characterizing these deposits, from their regional setting to the fluid and isotope chemistry of mineral deposition. This has led to a broad understanding of the large-scale hydrothermal systems within which they form. Low salinity vapor, released by magma crystallization and dispersed into vigorously convecting groundwater systems, is recognized as a principal source of metals and the gases that control redox conditions within systems. The temperature and pressure of the ambient fluid anywhere within these systems is close to its vapor-liquid phase boundary, and mineral deposition is a consequence of short timescale perturbations generated by localized release of crustal stress. However, a review of occurrence data raises questions about ore formation that are not addressed by traditional genetic models. For example, what are the origins of banding in epithermal veins, and what controls the frequency of oscillatory lamination? What controls where the phenomenon of mineralization occurs, and why are some porphyry deposits, for example, so much larger than others? The distinctive, self-organized characteristics of epizonal deposits are shown to be the result of repetitive coupling of fracture dilation consequent on brittle failure, phase separation ("boiling"), and heat transfer between fluid and host rock. Process coupling substantially increases solute concentrations and triggers fast, far-from-equilibrium depositional processes. Since these coupled processes lead to localized transient changes in fluid characteristics, paragenetic, isotope, and fluid inclusion data relate to conditions at the site of deposition and only indirectly to the characteristics of the larger-scale hydrothermal system and its longer-term behavior. The metal concentrations (i.e. grade) of deposits and their internal variation is directly related to the geometry of the fracture array at the deposit scale, whereas finer-scale oscillatory fabrics in ores may be a result of molecular scale processes. Giant deposits are relatively rare and develop where efficient metal deposition is spatially focused by repetitive brittle failure in active fault arrays. Some brief case histories are provided for epithermal, replacement, and porphyry mineralization. These highlight how rock competency contrasts and feedback between processes, rather than any single component of a hydrothermal system, govern the size of individual deposits. In turn, the recognition of the probabilistic nature of mineralization provides a firmer foundation through which exploration investment and risk management decisions can be made.

Notizen: Abstract Growth of the Japanese arc system, which has mainly taken place along the continental margin of Asia since the Permian, is the result of subduction of the ancient Pacific ocean floor. Backarc basin formation in the Tertiary shaped the present-day arc configuration. The neotectonic regime, which is characterized by strong east-west compression, has been triggered by the eastward motion of the Amur plate in the Quaternary. The tectonic evolution of the Japanese arc system includes formation of rock assemblages common in most orogenic belts. Because the origin and present-day tectonics of these assemblages are better defined in the case of the Japanese arc system, study of the system provides useful insight into orogenesis and continental crust evolution.

Notizen: Abstract Radiocarbon dating is the method most frequently used to date Holocene deltaic sequences, but less than one quarter of 14C dates are within +-500 years of predicted age. Such dates tend to be unreliable, in other words, often too old and commonly inverted upsection, and core sample dates obtained near deltaic plain surfaces may be as old as mid- to late Holocene. Stratigraphic irregularities result primarily from downslope reworking of upland alluvial sediment, with displacement of "old carbon" in the sediment that accumulates in lower valleys and deltaic plains. Use of dates that are too old results in inaccurately calculated rates (most often too low) of relative sea-level rise and/or land subsidence. More reliable timing of deltaic sediment requires a multiple-method dating approach, including, where possible, identification of associated archaeological material. Developing an accurate dating strategy is a critical step for implementing reliable coastal protection measures needed for the rapidly increasing human populations in these low-lying, vulnerable nearshore settings.

Notizen: Abstract In this paper, we present a review of seismic-wave propagation in fluid-saturated and partially saturated porous media. Seismic-wave velocity and attenuation are affected by the degree of saturation and the spatial distribution of fluids within the medium. Attenuation mechanisms include local and global flow as well as energy loss caused by scattering. We also present results from acoustic tomography of unconsolidated porous media with residual paraffin saturation. The acoustic attenuation was found to be sensitive to the grain- and subgrain-scale (microscale) distribution of residual saturation; in other words, the residual saturation behaves like soft cement that locally stiffens grain contacts and creates heterogeneity that results in scattering. The effect of microscale phenomena on multigrain scale (macroscale) measurements of seismic-wave attenuation and velocity cannot be ignored.

Notizen: Abstract Fossil deposits that preserve soft-bodied organisms provide critical evidence of the history of life. Usually, only more decay resistant materials, e.g., cuticles, survive as organic remains as a result of selective preservation and subsequent diagenesis to more resistant biopolymers. Permineralization, the permeation of tissues by mineralizing fluids, may preserve remarkable detail, particularly of plants. However, evidence of more labile tissues, e.g., muscle, normally requires the replication of their morphology by rapid in situ growth of minerals, i.e., authigenic mineralization. This process relies on the steep geochemical gradients generated by decay microbes. The minerals involved, and the level of detail preserved (which may be subcellular), depend on a number of factors, including the nature of microbial activity and amount of decay, availability of ions, and the type of organism that is fossilized. Understanding these controls is essential to determining the conditions that favor exceptional preservation.

Notizen: Abstract The anthropogenic production of greenhouse gases and their consequent effects on global climate have garnered international attention for years. A remaining challenge facing scientists is to unambiguously quantify both sources and sinks of targeted gases. Microbiological metabolism accounts for the largest source of nitrous oxide (N2O), mostly due to global conversion of land for agriculture and massive usage of nitrogen-based fertilizers. A most powerful method for characterizing the sources of N2O lies in its multi-isotope signature. This review summarizes mechanisms that lead to biological N2O production and how discriminate placement of 15N into molecules of N2O occurs. Through direct measurements and atmospheric modeling, we can now place a constraint on the isotopic composition of biological sources of N2O and trace its fate in the atmosphere. This powerful interdisciplinary combination of biology and atmospheric chemistry is rapidly advancing the closure of the global N2O budget.

Notizen: Abstract Decades of seabed mapping, reflection profiling, and seabed sampling reveal that throughout the past two million years the Black Sea was predominantly a freshwater lake interrupted only briefly by saltwater invasions coincident with global sea level highstand. When the exterior ocean lay below the relatively shallow sill of the Bosporus outlet, the Black Sea operated in two modes. As in the neighboring Caspian Sea, a cold climate mode corresponded with an expanded lake and a warm climate mode with a shrunken lake. Thus, during much of the cold glacial Quaternary, the expanded Black Sea's lake spilled into to the Marmara Sea and from there to the Mediterranean. However, in the warm climate mode, after receiving a vast volume of ice sheet meltwater, the shoreline of the shrinking lake contracted to the outer shelf and on a few occasions even beyond the shelf edge. If the confluence of a falling interior lake and a rising global ocean persisted to the moment when the rising ocean penetrated across the dividing sill, it would set the stage for catastrophic flooding. Although recently challenged, the flood hypothesis for the connecting event best fits the full set of observations.

Notizen: The Cordilleran orogen of western North America is a segment of the Circum-Pacific orogenic belt where subduction of oceanic lithosphere has been underway along a great circle of the globe since breakup of the supercontinent Pangea began in Triassic time. Early stages of Cordilleran evolution involved Neoproterozoic rifting of the supercontinent Rodinia to trigger miogeoclinal sedimentation along a passive continental margin until Late Devonian time, and overthrusting of oceanic allochthons across the miogeoclinal belt from Late Devonian to Early Triassic time. Subsequent evolution of the Cordilleran arc-trench system was punctuated by tectonic accretion of intraoceanic island arcs that further expanded the Cordilleran continental margin during mid-Mesozoic time, and later produced a Cretaceous batholith belt along the Cordilleran trend. Cenozoic interaction with intra-Pacific seafloor spreading systems fostered transform faulting along the Cordilleran continental margin and promoted incipient rupture of continental crust within the adjacent continental block.

Notizen: Wrinkle ridges accommodate very low amounts of shortening strain around immense volcanic constructs and within impact basins on Mars. They originate from stresses distributed uniformly throughout the brittle lithosphere and are consistently located in stratified deposits, including lava flows and sediment. Most recent models interpret wrinkle ridges as the surface manifestation of folding above underlying blind thrusts that accommodate similarly low strain and likely penetrate tens of kilometers into the brittle crust. Alternative models suggest shortening accommodated by some wrinkle ridges is confined to only the upper few kilometers of the crust. The interpretation of the geometry of blind thrusts on Mars appears to be quite varied and remains controversial, although some models suggest they may ultimately flatten into the brittle-ductile transition in the middle to lower crust. Small-scale crenulations superposed on ridges are interpreted as produced by high-level back thrusts nucleating at a weak layer in the upper crust, or by flexural slip faults that facilitate bending of layered materials. Wrinkle ridges are related to their structural cousins, lobate scarps that accommodate shortening in older Noachian cratered highlands as surface fault ruptures. Wrinkle ridges thus form when displacement across upwardly propagating blind thrusts is consumed by folding of layered material near the surface. Conversely, lobate scarps are formed by blind thrusts that are not impeded by folding of overlying layered deposits. Broad, low-amplitude arches associated with ridges on the Tharsis rise also accommodate shortening, but their relationship to adjacent or superposed ridges remains enigmatic. The evenly spaced nature of wrinkle ridges that appears to vary systematically between the ridged plains and northern lowlands may be related to the depth of the brittle-ductile transition, which is respectively located in the middle crust and upper mantle in these regions.

Notizen: Abstract The synoptic coverage offered by satellites provides unparalleled opportunities for monitoring active volcanoes, and opens new avenues of scientific inquiry. Thermal infrared radiation can be used to monitor levels of activity, which is useful for automated eruption detection and for studying the emplacement of lava flows. Satellite radars can observe volcanoes through clouds or at night, and provide high-resolution topographic data. In favorable conditions, radar inteferometery can be used to measure ground deformation associated with eruptive activity on a centimetric scale. Clouds from explosive eruptions present a pressing hazard to aviation; therefore, techniques are being developed to assess eruption cloud height and to discriminate between ash and meterological clouds. The multitude of sensors to be launched on future generations of space platforms promises to greatly enhance volcanological studies, but a satellite dedicated to volcanology is needed to meet requirements of aviation safety and volcano monitoring.

Notizen: Abstract A review of the geologic history of the Himalayan-Tibetan orogen suggests that at least 1400 km of north-south shortening has been absorbed by the orogen since the onset of the Indo-Asian collision at about 70 Ma. Significant crustal shortening, which leads to eventual construction of the Cenozoic Tibetan plateau, began more or less synchronously in the Eocene (50-40 Ma) in the Tethyan Himalaya in the south, and in the Kunlun Shan and the Qilian Shan some 1000-1400 km in the north. The Paleozoic and Mesozoic tectonic histories in the Himalayan-Tibetan orogen exerted a strong control over the Cenozoic strain history and strain distribution. The presence of widespread Triassic flysch complex in the Songpan-Ganzi-Hoh Xil and the Qiangtang terranes can be spatially correlated with Cenozoic volcanism and thrusting in central Tibet. The marked difference in seismic properties of the crust and the upper mantle between southern and central Tibet is a manifestation of both Mesozoic and Cenozoic tectonics. The former, however, has played a decisive role in localizing Tertiary contractional deformation, which in turn leads to the release of free water into the upper mantle and the lower crust of central Tibet, causing partial melting in the mantle lithosphere and the crust.

Notizen: Abstract Twenty years after the Viking Mission, Mars is again being scrutinized in the light of a flood of information from spacecraft missions to Mars, the Hubble Space Telescope, and the SNC meteorites. This review provides an overview of the current understanding of Mars, especially in light of the data being returned from the Mars Global Surveyor Mission. Mars does not now have a global magnetic field, but the presence of crustal anomalies indicates that a global field existed early in Martian history. The topography, geodetic figure, and gravitational field are known to high precision. The northern hemisphere is lower and has a thinner and stronger crust than the southern hemisphere. The global weather and the thermal structure of the atmosphere have been monitored for more than a year. Surface-atmosphere interaction has been investigated by observations of surface features, polar caps, atmospheric dust, and condensate clouds. The surface has been imaged at very high resolution and spectral measures have been obtained to quantify surface characteristics and geologic processes. Many questions remain unanswered, especially about the earliest period of Mars' history.

Notizen: Abstract Substantial volumes of methane gas are trapped below the seafloor and in permafrost by an ice-like solid called clathrate hydrate. Global estimates of the methane in clathrate hydrate may exceed 1016 kg, which represents one of the largest sources of hydrocarbon on Earth. Speculations about large releases of methane from clathrate hydrate have raised serious but unresolved questions about its possible role in climate change. Progress in our understanding of clathrate hydrate has been made through integrated geophysical and geochemical surveys of known clathrate occurrences. Details from these surveys have motivated new investigations of the physical, chemical, and biological processes that contribute to growth and breakdown of clathrate hydrate in natural settings. In this article, I give an overview of recent advances and future challenges.

Notizen: Abstract As volcanoes grow, they become ever heavier. Unlike mountains exhumed by erosion of rocks that generally were lithified at depth, volcanoes typically are built of poorly consolidated rocks that may be further weakened by hydrothermal alteration. The substrates upon which volcanoes rest, moreover, are often sediments lithified by no more than the weight of the volcanic overburden. It is not surprising, therefore, that volcanic deformation includes-and in the long term is often dominated by-spreading motions that translate subsidence near volcanic summits to outward horizontal displacements around the flanks and peripheries. We review examples of volcanic spreading and go on to derive approximate expressions for the time volcanoes require to deform by spreading on weak substrates. We also demonstrate that shear stresses that drive low-angle thrust faulting from beneath volcanic constructs have maxima at volcanic peripheries, just where such faults are seen to emerge. Finally, we establish a theoretical basis for experimentally derived scalings that delineate volcanoes that spread from those that do not.

Notizen: Abstract An overview is given of the main anthropogenic influences on the chemistry of the atmosphere. Industrial and agricultural activities have altered the chemical composition of the atmosphere in many important ways, which is reflected especially in the distribution and concentrations of ozone in the troposphere and stratosphere. On one hand, as a result of industrial chlorofluorocarbon emissions, ozone has been depleted in unexpected major ways in the polar stratosphere. On the other hand, especially as a result of NO emissions, tropospheric ozone has increased both in the industrial mid-latitude regions and at low latitudes, in the latter mostly because of tropical biomass burning. In the future, growing anthropogenic emissions by developing nations will have an additional effect on the climate and the self-cleaning (oxidation) power of the atmosphere.

Notizen: Abstract Spring water provides a unique opportunity to study a range of subsurface processes in regions with few boreholes or wells. However, because springs integrate the signal of geological and hydrological processes over large spatial areas and long periods of time, they are an indirect source of information. This review illustrates a variety of techniques and approaches that are used to interpret measurements of isotopic tracers, water chemistry, discharge, and temperature. As an example, a set of springs in the Oregon Cascades is considered. By using tracers, temperature, and discharge measurements, it is possible to determine the mean-residence time of water, infer the spatial pattern and extent of groundwater flow, estimate basin-scale hydraulic properties, calculate the regional heat flow, and quantify the rate of magmatic intrusion beneath the volcanic arc.

Notizen: Abstract The icy moons of the outer solar system have not been quiescent bodies, in part because many have a substantial water component and have experienced significant internal heating. We can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice (in several of its polymorphic forms) flows under deviatoric stress at planetary conditions. Details of laboratory-derived flow laws for pure, polycrystalline ice are reviewed in detail. One of the more important questions at hand is the role of ice grain size. Grain size may be a dynamic quantity within the icy moons, and it may (or may not) significantly affect rheology. One recent beneficiary of revelations about grain-size-sensitive flow is the calculation of the rheological structure of Europa's outer ice shell, which may be no thicker than 20 km.

Notizen: Abstract The global soil C reservoir, ~1500 Gt of C (1 Gt = 1012 kg of C), is dynamic on decadal time scales and is sensitive to climate and human disturbance. At present, as a result of land use, soil C is a source of atmospheric CO2 in the tropics and possibly part of a sink in northern latitudes. Here I review the processes responsible for maintaining the global soil C reservoir and what is known about how it responds to direct and indirect human perturbations. "I am fire and air; my other elements I give to baser life" -William Shakespeare, Antony and Cleopatra

Notizen: Abstract In 1997, after almost forty years since the initial attempt by Benioff et al (1959), continuous free oscillations of the Earth were discovered. Spheroidal fundamental modes between 2 and 7 millihertz are excited continuously with acceleration amplitudes of about 0.3-0.5 nanogals. The signal is now commonly found in virtually all data recorded by STS-1 type broadband seismometers at quiet sites. Seasonal variation in amplitude and the existence of two coupled modes between the atmosphere and the solid Earth support that these oscillations are excited by the atmosphere. Stochastic excitation due to atmospheric turbulence is a favored mechanism, providing a good match between theory and data. The atmosphere has ample energy to support this theory because excitation of these modes require only 500-10000 W whereas the atmosphere contains about 1017 W of kinetic energy. An application of this phenomenon includes planetary seismology, because other planets may be oscillating owing to atmospheric excitation. The interior structure of planets could be learned by determining the eigenfrequencies in the continuous free oscillations. It is especially attractive to pursue this idea for tectonically quiet planets, since quakes may be too infrequent to be recorded by seismic instruments.

Notizen: Abstract The observed properties of extrasolar planets and planetary systems are reviewed, including discussion of the mass, period, and eccentricity distributions; the presence of multiple systems; and the properties of the host stars. In all cases, the data refer to systems with ages in the Ga range. Some of the properties primarily reflect the formation mechanism, while others are determined by postformation dynamical evolutionary processes. The problem addressed here is the extraction of information relevant to the identification of the formation mechanism. The presumed formation sites, namely disks around young stars, therefore, must provide clues at times much closer to the actual formation time. The properties of such disks are briefly reviewed. The amount of material and its distribution in the disks provide a framework for the development of a model for planet formation. The strengths of, as well as the problems with, the two major planet formation mechanisms-gravitational instability and core accretion-gas capture-are then described. It is concluded that most of the known planetary systems are best explained by the accretion process. The timescales for the persistence of disks and for the formation time by this process are similar, and the mass range of the observed planets, up to approximately 10 Jupiter masses, is naturally explained. The mass range of 5-15 Jupiter masses probably represents an overlapping transition region, with planetary formation processes dominating below that range and star formation processes dominating above it.

Notizen: Abstract The past seven years have seen significant advances in computational simulations of convection and magnetic field generation in the Earth's core. Although dynamically self-consistent models of the geodynamo have simulated magnetic fields that appear in some ways quite similar to the geomagnetic field, none are able to run in an Earth-like parameter regime because of the considerable spatial resolution that is required. Here we discuss some of the subtle compromises that have been made in current models and propose a grand challenge for the future, requiring significant improvements in numerical methods and spatial resolution.

Notizen: Abstract Geological investigations of major fault scarps ("tectonic windows") and DSDP/ODP Drill Holes provide direct views of the uppermost oceanic crust generated at fast- to intermediate-rate spreading centers. These areas reveal a consistent upper crustal structural geometry with basaltic lava flows defining a pattern of downward increasing ("inward") dip toward the spreading center at which they formed and dikes in the lavas and underlying sheeted dike complex showing a similar degree of "outward" dip. Widespread fracturing, faulting, and hydrothermal metamorphism accompanied magmatic construction. These geological relationships can be interpreted in terms of dramatic, asymmetrical, subaxial subsidence of upper crustal rock units that diminishes across the very narrow (few kilometers wide) zone of lava accumulation and dike intrusion at the ridge axis. This type of crustal structure is in accord with some existing models of spreading but augments these idealized views with more realistic geological complexity.

Notizen: Abstract Observations of the geochemical diversity of mid-oceanic ridge and ocean-island basalts have traditionally been attributed to the existence of large-scale mantle heterogeneity. In particular, the layered convection model has provided an important conceptual basis for discussing the chemical evolution of the Earth. In this model, a long-term boundary is assumed between a well-mixed and depleted upper mantle and a heterogeneous and more primitive lower mantle. The existence of high 3He/4He in ocean-island sources has been used to argue for the preservation of a primitive component in the deep mantle. Nevertheless, a primitive deep layer is difficult to reconcile with the abundant lithophile isotopic evidence for recycling of oceanic crust and the lack of preservation of primitive mantle. In addition, the widespread acceptance of geophysical evidence for whole mantle flow has made straightforward application of the layered convection model problematic. Model calculations show that whole mantle convection with present day heat flow and surface velocities is sufficiently vigorous to mix large-scale heterogeneity to an extent that is incompatible with the geochemical observations. Several concepts have been proposed in recent years to resolve the apparent conflicts between the various observational constraints and theoretical interpretations. The suggestions include the presence of deeper layering, preservation of highly viscous blobs, core mantle interaction, and strong temporal variations in mantle dynamics. Although these models generally appear to solve parts of the puzzle, at present no single model is able to account for all of the major observations. The reconciliation of conflicting evidence awaits improvements in observational and experimental techniques integrated with better model testing of hypotheses for the generation and destruction of mantle heterogeneity.

Notizen: Abstract Accretion models for the Earth and terrestrial planets are based on the distribution of siderophile (iron-loving) elements between metal and silicate. Extensive experimental studies of the partitioning of these elements between metallic liquid and silicate melt have led to a better understanding and a more sophisticated application to planetary problems. Siderophile element metal/silicate partition coefficients are a function of temperature, pressure, oxygen fugacity, and metal and silicate composition. Quantification of these effects for a limited subset of siderophile elements has led to the idea that early Earth had a 700-km or deeper magma ocean. This new understanding of siderophile element partitioning has also led to applications to the kinetics of metal-silicate equilibrium, links to the timing of core formation, and a better understanding of core formation and metal-silicate equilibrium in the Moon and Mars. Key issues for future consideration include the role of water in early Earth, consideration of the core as a reservoir for noble gases and/or traditionally lithophile elements, siderophile element concentrations in the deep mantle, oxygen fugacity at high pressures, and further evaluation of the need for a late accretional veneer. The strongest approach to improving accretion models for the terrestrial planets is one that combines geochemistry, geophysics, and planetary dynamics.

Notizen: Abstract We review the present status of global mantle tomography and discuss two main classes of models that have been developed in the past 10 years: P velocity models based on large datasets of travel times from the International Seismological Centre bulletins, often referred to as "high resolution" models, and S velocity models based on a combination of surface wave and hand picked body wave travel times, or waveforms, referred to as "long wavelength" models. We discuss their respective strengths and weaknesses, as well as progress in the resolution of other physical parameters, such as anisotropy, anelasticity, density, and bulk sound velocity using tomographic approaches. We present the view that future improvements in global seismic tomography require the utilization of the rich information contained in complete broadband seismic waveforms. This is presently within our reach owing to theoretical progress as well as the increase in computational power in recent years.

Notizen: Abstract For over 300 years, the monsoon has been viewed as a gigantic land-sea breeze. It is shown in this paper that satellite and conventional observations support an alternative hypothesis, which considers the monsoon as a manifestation of seasonal migration of the intertropical convergence zone (ITCZ). With the focus on the Indian monsoon, the mean seasonal pattern is described, and why it is difficult to simulate it is discussed. Some facets of the intraseasonal variation, such as active-weak cycles; break monsoon; and a special feature of intraseasonal variation over the region, namely, poleward propagations of the ITCZ at intervals of 2-6 weeks, are considered. Vertical moist stability is shown to be a key parameter in the variation of monthly convection over ocean and land as well as poleward propagations. Special features of the Bay of Bengal and the monsoon brought out by observations during a national observational experiment in 1999 are briefly described.

Notizen: The history of the theory and experimental evidence that the natural catalytic and reactive qualities of transition metal sulfides are linked to primitive metabolism is reviewed. In the late 1980s, a hypothesis arose that proposed that transition metal sulfides (in particular pyrrhotite and pyrite) might play a significant role promoting abiotic organic chemistry. As an outgrowth of this hypothesis, elaborate theories were presented, including proposals for earliest life being structurally distinct from extant prokaryotic life. During the 1990s and into the twenty-first century, experimental evidence has emerged that supports certain aspects of these theories; in other cases, the experiments reveal chemistry that diverges significantly from that which was proposed theoretically. In either case, however, there is clear evidence that transition metal sulfide minerals exhibit catalytic qualities for the promotion of reactions that have obvious metabolic utility and, therefore, could have provided the primitive Earth with valuable biochemical intermediates.

Notizen: Accumulation rates of terrestrial sediment have increased in the past few million years both on and adjacent to continents, although not everywhere. Apparently, erosion has increased in elevated terrain regardless of when last tectonically active or what the present-day climate. In many regions, sediment coarsened abruptly in late Pliocene time. Sparser data suggest increased sedimentation rates at ~15 Ma, approximately when oxygen isotopes in benthic foraminifera imply high-latitude cooling. If climate change effected accelerated erosion, understanding how it did so remains the challenge. Some obvious candidates, such as lowered sea level leading to erosion of continental shelves or increased glaciation, account for increased sedimentation in some, but not all, areas. Perhaps stable climates that varied slowly allowed geomorphic processes to maintain a state of equilibrium with little erosion until ~3-4 Ma, when large oscillations in climate with periods of 20,000-40,000 years developed and denied the landscape the chance to reach equilibrium.

Notizen: Visible and near-infrared spectra of reflected sunlight from asteroid surfaces exhibit features that hold the promise for identifying surface mineralogy. However, the very surfaces that are observed by remote-sensing are also subject to impingement by micrometeoroids and solar wind particles, which are believed to play the dominant role in space weathering, which is the time-dependent modification of an asteroid's reflectance spectrum. Such space weathering has confused the interpretations of telescopic spectra of asteroids, especially concerning the possible association of common ordinary chondritic meteorites with so-called S-type asteroids. Recent spacecraft studies of asteroids (especially of Eros by NEAR-Shoemaker) have documented aspects of space weathering processes, but we still do not understand the physics of space weathering well enough to confidently assay mineralogy of diverse asteroids by remote-sensing. A review of the intellectual history of this topic reveals the complexity of interdisciplinary research on far-away astronomical bodies.

Notizen: Manganese(IV) oxides produced through microbial activity, i.e., biogenic Mn oxides or Mn biooxides, are believed to be the most abundant and highly reactive Mn oxide phases in the environment. They mediate redox reactions with organic and inorganic compounds and sequester a variety of metals. The major pathway for bacterial Mn(II) oxidation is enzymatic, and although bacteria that oxidize Mn(II) are phylogenetically diverse, they require a multicopper oxidase-like enzyme to oxidize Mn(II). The oxidation of Mn(II) to Mn(IV) occurs via a soluble or enzyme-complexed Mn(III) intermediate. The primary Mn(IV) biooxide formed is a phyllomanganate most similar to delta-MnO2 or acid birnessite. Metal sequestration by the Mn biooxides occurs predominantly at vacant layer octahedral sites.

Notizen: Abstract To understand the complex collisional history of asteroids and explain their observed characteristics, it is necessary to be able to describe the fundamental physics of large-scale impact events. Because data collected on collisional outcomes is derived from small-scale laboratory experiments, success at describing asteroid evolution hinges sensitively on methods devised to "scale" this data to the appropriate size ranges. The recent use of numerical modeling to study collisional fragmentation is a valuable asset for calculating important impact parameters: energies required for catastrophic fragmentation, and resulting fragment size and velocity distributions. The current size distribution of main belt asteroids provides an important constraint of model predictions, as do comparisons of numerical impact calculations to asteroid families. Agreement between theory and observations is reasonable, but improvements in collision simulations are required for the modeling of more realistic asteroid shapes and structures.

Notizen: Abstract Strong heterogeneity at a variety of scale lengths has been imaged in the lowermost mantle using different forward and inverse methods. Coherent patterns in differential travel times of waves that sample the base of the mantle-such as diffracted shear waves (Sdiff) and compressional waves (Pdiff)-are readily apparent, and are compared with results from tomographic studies. Travel time and waveform modeling studies have demonstrated the presence of intense lateral variations in a variety of mapped features, such as a regionally detected high velocity D" layer, ultra-low velocity zones, D" anisotropy, strong scattering and heterogeneity. Such short-wavelength variations currently preclude confident mapping of D" structure at smaller scales. Issues of seismic resolution and uncertainties are emphasized here, as well as the limitations of one-dimensional modeling/averaging in highly heterogeneous environments.

Notizen: During the last 35 years the planets and moons of our solar system have been visited for the first time, and the plate tectonics paradigm has revolutionized earth science and led to the acceptance of mantle convection as the cause of plate tectonics. The author has been a fortunate participant in these extraordinary events and he offers some reminiscences and recollections of his involvement. He also recalls his former colleague William M Kaula and dedicates this prefatory chapter to him.

Notizen: Abstract Recent partial melting experiments on peridotite indicate that mantle peridotites with Mg# 〈 88 can produce primitive mid-ocean ridge basalt by 15-25% partial melting at 1.0-1.5 GPa followed by small amounts (a few to 10%) of olivine fractionation, whereas those with Mg# 〉 89 can produce primitive mid-ocean ridge basalt by partial melting at pressures greater than 1.5 GPa followed by extensive olivine fractionation. Polybaric incremental batch melting or stepwise fractional melting experiments indicate that the compositions of accumulated incremental melts formed along a mantle adiabat in the pressure range 2.0-1.0 and 2.0-0.5 GPa are close to, but slightly more olivine-rich than, those of primitive mid-ocean ridge basalt, and that the amount of accumulated incremental melts is significantly smaller than that of batch partial melts formed for the same potential temperature. The melting in adiabatically ascending mantle beneath mid-ocean ridges would cease at depths 〉15 km because of the inflection of the solidus of mantle peridotite.

Notizen: Abstract Ground penetrating radar (GPR) is a near-surface geophysical technique that can provide high resolution images of the dielectric properties of the top few tens of meters of the earth. In applications in contaminant hydrology, radar data can be used to detect the presence of liquid organic contaminants, many of which have dielectric properties distinctly different from those of the other solid and fluid components in the subsurface. The resolution (approximately meter-scale) of the radar imaging method is such that it can also be used in the development of hydrogeologic models of the subsurface, required to predict the fate and transport of contaminants. GPR images are interpreted to obtain models of the large-scale architecture of the subsurface and to assist in estimating hydrogeologic properties such as water content, porosity, and permeability. Its noninvasive capabilities make GPR an attractive alternative to the traditional methods used for subsurface characterization.

Notizen: Abstract Near the end of the Late Ordovician, in the first of five mass extinctions in the Phanerozoic, about 85% of marine species died. The cause was a brief glacial interval that produced two pulses of extinction. The first pulse was at the beginning of the glaciation, when sea-level decline drained epicontinental seaways, produced a harsh climate in low and mid-latitudes, and initiated active, deep-oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths. Following that initial pulse of extinction, surviving faunas adapted to the new ecologic setting. The glaciation ended suddenly, and as sea level rose, the climate moderated, and oceanic circulation stagnated, another pulse of extinction occurred. The second extinction marked the end of a long interval of ecologic stasis (an Ecologic-Evolutionary Unit). Recovery from the event took several million years, but the resulting fauna had ecologic patterns similar to the fauna that had become extinct. Other extinction events that eliminated similar or even smaller percentages of species had greater long-term ecologic effects.

Notizen: Abstract Late Carboniferous and Early Permian strata record the transition from a cold interval in Earth history, characterized by the repeated periods of glaciation and deglaciation of the southern pole, to a warm-climate interval. Consequently, this time period is the best available analogue to the Recent in which to study patterns of vegetational response, both to glacial-interglacial oscillation and to the appearance of warm climate. Carboniferous wetland ecosystems were dominated by spore-producing plants and early gymnospermous seed plants. Global climate changes, largely drying, forced vegetational changes, resulting in a change to a seed plant-dominated world, beginning first at high latitudes during the Carboniferous, reaching the tropics near the Permo-Carboniferous boundary. For most of this time plant assemblages were very conservative in their composition. Change in the dominant vegetation was generally a rapid process, which suggests that environmental thresholds were crossed, and involved little mixing of elements from the wet and dry floras.

Notizen: Abstract Until the early 1990s, numerical simulations of Solar System dynamics were done using accurate but slow integrators. The typical timescales were on the order of a million years, apart from exceptions achieved by considering averaged equations or using specifically designed supercomputers. In the last decade, new numerical integration methods for Solar System dynamics have been introduced. The mixed variable symplectic method (Wisdom & Holman 1991) has permitted the study, in the absence of close encounters, of the evolution of planets and small bodies on timescales comparable to the age of the Solar System. The regularized mixed variable scheme (Levison & Duncan 1994) has allowed the compilation of statistics on the evolution of thousands of near-Earth asteroids and comets, from their source regions to their dynamical elimination. The Symba and the Mercury codes (Duncan et al. 1998, Chambers 1999), which treat close encounters between massive bodies in a symplectic way, have permitted the simulations of planetary accretion and of the early phase of the highly chaotic evolution of the Solar System. This paper reviews the most exciting results obtained with these new integrators. Emphasis is given to the conceptual steps that these works represent in our understanding of Solar System dynamics.

Notizen: Abstract In the last decades, insights from the fields of ecology, geomorphology, and hydrology have been applied to the question of the streamflows necessary for environmental maintenance. For instance, determining the streamflow needed for spawning by salmon or trout requires ascertaining how much water, for how long, and at what time it will be needed? And what flows are necessary for the sustenance of streamside vegetation? Answers to these and similar questions have been sought to minimize environmental degradation in the development or relicensing of water projects, in restoring riverine ecosystems, and in balancing multiple uses for limited water resources. In this contribution, the varieties of environmental maintenance flows applied to rivers are described, as are their fundamental principles. These environmental maintenance flows include flows to maintain aesthetics and recreation, streambed sediment size and its mobility, the channel, its features and continuity, and the floodplain, its wetness regime, and riparian vegetation.

Notizen: Abstract The Prelude began with the origin of Metazoa, perhaps between 720 and 660 million years ago (mya), and ended with the geologically abrupt appearance of crown bilaterian phyla that began between 530 and 520 mya. The origin and early evolution of phyla cannot be tracked by fossils during this interval, but molecular phylogenetics permits reconstruction of their branching topology, whereas molecular developmental evidence supports hypotheses for the evolution of the metzoan genome during the rise of complex bodyplans. A flexible architecture of genetic regulation was in place even before the appearance of crown sponges, permitting increases in gene expression events as bodyplan complexity rose. Neoproterozoic bilaterians were chiefly small-bodied but likely diverse, whereas in the earliest Cambrian, between 543 and approximately 530-520 mya, bodies that were complex by marine invertebrate standards evolved in association with body-size increases.

Notizen: Abstract The recent recognition of a potentially vast, unexplored hot microbial biosphere associated with active volcanism along the global mid-ocean ridge network has fundamentally shifted concepts of how planets and life coevolve. Many processes intrinsic to the dynamics of the spreading center volcanic system provide partial or complete nutritional fluxes that support diverse microbial communities that thrive under extreme conditions on and beneath the seafloor. Mantle melting, volcanism, and fluid-rock reactions transport volatiles from the asthenosphere to the hydrosphere. Volcanic heat and exothermic reactions drive circulation of nutrient-rich fluids from which chemosynthetic organisms gain metabolic energy. In turn, many of these organisms symbiotically support macrofaunal communities that populate the vents. Long-term seafloor observatories will allow exploration of linkages between volcanism and this newly discovered biosphere. Such approaches may provide essential new information about our own planet while providing critically needed insights into how we can explore other planets for life.

Notizen: Abstract Theoretical calculations, based on both the chemical and isotopic composition of sedimentary rocks, indicate that atmospheric O2 has varied appreciably over Phanerozoic time, with a notable excursion during the Permo-Carboniferous reaching levels as high as 35% O2. This agrees with measurements of the carbon isotopic composition of fossil plants together with experiments and calculations on the effect of O2 on photosynthetic carbon isotope fractionation. The principal cause of the excursion was the rise of large vascular land plants and the consequent increased global burial of organic matter. Higher levels of O2 are consistent with the presence of Permo-Carboniferous giant insects, and preliminary experiments indicate that insect body size can increase with elevated O2. Higher O2 also may have caused more extensive, possibly catastrophic, wildfires. To check this, realistic burning experiments are needed to examine the effects of elevated O2 on fire behavior.

Notizen: Abstract Plants and animals exploit the soil for food and shelter and, in the process, affect it in many different ways. For example, uprooted trees may break up bedrock, transport soil downslope, increase the heterogeneity of soil respiration rates, and inhibit soil horizonation. In this contribution, we review previously published papers that provide insights into the process of bioturbation. We focus particularly on studies that allow us to place bioturbation within a quantitative framework that links the form of hillslopes with the processes of sediment transport and soil production. Using geometrical relationships and data from others' work, we derive simple sediment flux equations for tree throw and root growth and decay.

Notizen: The 100 kyr quasiperiodic variation of continental ice cover, which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of Earth history, has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n-body effects in the Solar System on the geometry of Earth's orbit around the Sun. The impacts of the changing surface ice load upon both Earth's shape and gravitational field, as well as upon sea-level history, have come to be measurable using a variety of geological and geophysical techniques. These observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid Earth and on the detailed spatiotemporal characteristics of glaciation history. This review focuses upon the most recent advances that have been achieved in each of these areas, advances that have proven to be central to the construction of the refined model of the global process of glacial isostatic adjustment, denoted ICE-5G (VM2). A significant test of this new global model will be provided by the global measurement of the time dependence of the gravity field of the planet that will be delivered by the GRACE satellite system that is now in space.

Notizen: This review describes the changes in the understanding of turbulence and mixing in the upper layers of the ocean, the abyssal ocean, and in continental shelf seas that have come in the past 25 years, largely through advances in instruments and methodology.

Notizen: Abstract Volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of plate-pull (as the plates are pulled apart) associated with the mid-ocean ridges and the magma dynamics of the Iceland Mantle Plume. Most systems are 40-150 km long, 5-20 km wide, and develop a central volcano. They supply magma to all eruptions in Iceland. Data obtained in the last few years have greatly improved our knowledge of their volcanotectonic environment; as a result, the geometry of the plume is better constrained, and the crust, previously considered thin (~10 km), is now modeled as thick (~20-40 km). Depending on the location of the volcanic systems, their activity either decreases or increases faulting in the two main seismic zones. From this, we can infer that emplacement of the feeder-dike to the largest historical eruption in Iceland (that of Laki in 1783) increased shear stress in the South Iceland Seismic Zone and almost certainly triggered the largest (M~7.1 in 1784) historical earthquake in Iceland.

Notizen: Abstract Pollen grains preserved in lake and bog sediment provide a record of past vegetation that has been an important source of information about climate and land cover during the Quaternary Period. Yet from the beginning, questions have been raised about the source area of pollen in sediment. Interpretation has been hampered by the lack of well-developed theory treating the relationship between the spatial distribution of trees on the landscape and the percentages of pollen in sediment. Within the past decade, however, new theory, models, and empirical data show how heterogeneous vegetation is represented by pollen. The distinction between "local" and "regional" pollen is explained by the Prentice-Sugita dispersal/deposition models, which predict how the ratio of regional to local pollen changes with lake size. Sugita's model simulating a landscape with heterogeneous vegetation predicts the size of the relevant source area-the area of vegetation reflected in between-lake variations in pollen loading-while demonstrating that regional pollen from beyond this distance is homogeneous at all lakes of similar size. By predicting the way landscape patterns will be reflected in pollen records, simulation models can improve research design and lead to more detailed and spatially precise records of past vegetation, enhancing continental-scale climate reconstructions.

Notizen: Abstract The Lachlan Fold Belt (Lachlan Orogen) of eastern Australia was part of a Paleozoic convergent plate margin that stretched around the supercontinent of Gondwana from South America to Australia. Lower Paleozoic (545-365 Ma) deep-water, quartz-rich turbidites, calcalkaline volcanic rocks, and voluminous granitic plutons dominate the Lachlan Orogen. These rocks overlie a mafic lower crust of oceanic affinity. Shortening and accretion of the Lachlan occurred through stepwise deformation and metamorphism from Late Ordovician (~450 Ma) through early Carboniferous times, with dominant events at about 440-430 Ma and 400-380 Ma. The development and accretion of the Lachlan Orogen and other related belts within the Tasmanides added about 2.5 Mkm2 to the surface area of Gondwana. The sedimentary, magmatic, and deformational processes converted an oceanic turbidite fan system into continental crust of normal thickness. The addition of this recycled continental detritus and juvenile material to Australia represents an under-recognized continental crustal growth mechanism that has been important thoughout earth history.

Notizen: Abstract Natural impacts in which the projectile strikes the target vertically are virtually nonexistent. Nevertheless, our inherent drive to simplify nature often causes us to suppose most impacts are nearly vertical. Recent theoretical, observational, and experimental work is improving this situation, but even with the current wealth of studies on impact cratering, the effect of impact angle on the final crater is not well understood. Although craters' rims may appear circular down to low impact angles, the distribution of ejecta around the crater is more sensitive to the angle of impact and currently serves as the best guide to obliquity of impacts. Experimental studies established that crater dimensions depend only on the vertical component of the impact velocity. The shock wave generated by the impact weakens with decreasing impact angle. As a result, melting and vaporization depend on impact angle; however, these processes do not seem to depend on the vertical component of the velocity alone. Finally, obliquity influences the fate of the projectile: in particular, the amount and velocity of ricochet are a strong function of impact angle.

Notizen: Abstract Temperature changes at the Earth's surface propagate downward into the subsurface and impart a thermal signature to the rocks. This signature can be measured in boreholes and then analyzed to reconstruct the surface temperature history over the past several centuries. The ability to resolve surface temperature history from subsurface temperatures diminishes with time. Microclimatic effects associated with the topography and vegetation patterns at the site of a borehole, along with local anthropogenic perturbations associated with land use change, can obscure the regional climate change signal. Regional and global ensembles of boreholes reveal the broader patterns of temperature changes at the Earth's surface. The average surface temperature of the continents has increased by about 1.0 K over the past 5 centuries; half of this increase has occurred in the twentieth century alone.

Notizen: Abstract The two most common low-temperature iron(III) oxides on Earth are goethite (alpha-FeOOH) and hematite (alpha-Fe2O3). The delta18O values of natural goethites range from -15.5% to +3.3%, whereas delta18O values of low-temperature hematites range from -16.7% to +4.7%. Plots of deltaD against delta18O for continental goethites are approximately parallel to the meteoric water line of Craig (H Craig. 1961. Science 133:1702-3). This suggests that goethite-water fractionation factors are systematic over a wide range of surficial environments and may indicate that isotopic equilibrium is commonly attained or closely approached. Several experimental or calculated mineral-water, oxygen isotope fractionation curves have been determined for both goethite and hematite. Although there is not yet a consensus on which of these curves best approximates isotopic fractionation in natural samples, oxygen isotope measurements of both goethite and hematite have provided evidence of significant continental climate change on time scales that range from thousands to millions of years. The concentration and delta13C values of an Fe(CO3)OH component in apparent solid solution in goethite are proxies for the partial pressure and delta13C values, respectively, of CO2 in the environment at the time of goethite crystallization. Biological productivity, CO2 pressures in soil or groundwater, and partial pressures of atmospheric CO2 in ancient environments have been estimated from measurements of the mole fractions and delta13C values of Fe(CO3)OH in goethite.

Notizen: Abstract The mechanisms of exchange of hydrogen between the deep interior and surface of Earth, as well as the means of retention and possible abundance of hydrogen deep within the Earth, are examined. The uppermost several hundred kilometers of Earth's suboceanic upper mantle appear to be largely degassed, but significant primordial hydrogen could be retained within the transition zone, lower mantle, or core. Regassing of the planet occurs via subduction: Cold slabs are likely particularly efficient at transporting hydrogen to depth within the planet. Marked changes in hydrogen cycling have taken place throughout Earth's history: Evidence of hydrated ultramafic melts in the Archean and probable hydrogen retention within a Hadean magma ocean indicate that early in its history, the deep Earth was substantially wetter. The largest enigma associated with hydrogen in the deep Earth lies in the core: This region could represent the dominant reservoir of hydrogen on the planet, with up to ~100 hydrospheres of hydrogen present as a high-pressure iron-alloy.

Notizen: Abstract Earth, Venus, and Mars all exhibit populations of giant (radiating, linear, and arcuate) mafic dike swarms hundreds to 〉2000 km in length. On Earth the dikes are exposed by erosion, while on Venus and Mars their presence is mainly inferred from associated volcanic morphology and surface deformation. The apparent absence of plate tectonics in the geologic record of Venus and Mars means that the observed population of swarms remains geometrically intact, while on Earth plate tectonics has fragmented swarms. About 30 giant radiating swarms have so far been identified on Earth, but with further study the number is expected to rise and may eventually coincide with the hundreds of mantle plume head events now being proposed. On Venus, at least 118 radiating swarms are distributed across the planet, and new high resolution mapping is revealing additional swarms. On Mars, up to 16 giant dike swarms are observed, most associated with the Tharsis region.

Notizen: Abstract We review selected geological processes to which numerical modeling has been applied, with the aim of describing some of the general approaches and applications of the modeling. All of these examples involve multiphase fluid flow, in some cases coupled with heat transport and phase changes. First, we describe modeling approaches to a human-made geological system-a potential underground radioactive waste repository. Next, we describe recent advances in modeling two-phase flow through random heterogeneous porous media. We review recent modeling studies of fluid processes in magmatic systems, especially focusing on melting and crystallization induced by magma chambers. Finally, several research directions are suggested, including improving our understanding of the linkage between small-scale and field-scale processes, coupling across regimes (e.g., surface water and ground water), and further developments in the modeling of stochastic geological processes.

Notizen: Abstract Soil moisture is spatially and temporally highly variable, and it influences a range of environmental processes in a nonlinear manner. This leads to scale effects that need to be understood for improved prediction of moisture dependent processes. We provide some introductory material on soil moisture, and then review results from the literature relevant to a variety of scaling techniques applicable to soil moisture. This review concentrates on spatial scaling with brief reference to results on temporal scaling. Scaling techniques are divided into behavioral techniques and process-based techniques. We discuss the statistical distribution of soil moisture, spatial correlation of soil moisture at scales from tens of meters to thousands of kilometers and related interpolation and regularization techniques, and the use of auxiliary variables such as terrain indices. Issues related to spatially distributed deterministic modeling of soil moisture are also briefly reviewed.

Notizen: Abstract Imaging using seismic reflection data has undergone tremendous advances over the past few years. The advances have been fostered in part by the availability of faster computers that have made more reliable algorithms for migration imaging feasible. The conventional approach to migration imaging, ray-based Kirchhoff migration, has been improved by the use of multiple-valued traveltime tables, ray amplitudes, and ray phases that can be calculated from various ray-tracing implementations. Wave-equation imaging, based on implementations of solutions of the wave equation, one-way wave equation, and approximations to the Lippmann-Schwinger equation, has become tractable. Wave-equation methods take account of wave phenomena such as focusing, defocusing, and diffraction that are important in many geological environments where imaging is used for petroleum exploration. There have also been applications of various types of migration imaging in basic studies of Earth structure. Such studies have been made to investigate deep Earth structure and large-scale lithospheric structure using waveforms from teleseisms as sources.

Notizen: Abstract Tropical cyclones encompass virtually every subdiscipline of geophysical fluid dynamics, including cumulus convection, boundary layers, thermodynamic cycles, surface wave dynamics, upper ocean wind-driven circulations, barotropic instability, Rossby waves, and air-sea interaction. After briefly reviewing what is known about the structure, behavior, and climatology of these fascinating storms, the author provides an overview of their physics, focusing on the unique and poorly understood nature of the air-sea interface, and discusses several of the most interesting avenues of ongoing research.

Notizen: Abstract Neither geologists nor biologists have a definition that is capable of classifying Madagascar unambiguously as an island or a continent; nor can they incorporate Malagasy natural history into a single model rooted in Africa or Asia. Madagascar is a microcosm of the larger continents, with a rock record that spans more than 3000 million years (Ma), during which it has been united episodically with, and divorced from, Asian and African connections. This is reflected in its Precambrian history of deep crustal tectonics and a Phanerozoic history of biodiversity that fluctuated between cosmopolitanism and parochialism. Both vicariance and dispersal events over the past 90 Ma have blended a unique endemism on Madagascar, now in decline following rapid extinctions that started about 2000 years ago.

Notizen: Abstract Mantle plumes are recognized by domal uplift, triple junction rifting, and especially the presence of a large igneous province (LIP), dominated in the Phanerozoic by flood basalts, and in the Proterozoic by the exposed plumbing system of dykes, sills, and layered intrusions. In the Archean, greenstone belts that contain komatiites have been linked to plumes. In addition, some carbonatites and kimberlites may originate from plumes that have stalled beneath thick lithosphere. Geochemistry and isotopes can be used to test and characterize the plume origin of LIPs. Seismic tomography and geochemistry of crustal and subcrustal xenoliths in kimberlites can identify fossil plumes. More speculatively, plumes (or clusters of plumes) have been linked with variation in the isotopic composition of marine carbonates, sea-level rise, iron formations, anoxia events, extinctions, continental breakup, juvenile crust production, magnetic superchrons, and meteorite impacts. The central region of a plume is located using the focus of a radiating dyke swarm, the distribution of komatiites and picrites, etc. The outer boundary of a plume head circumscribes the main flood basalt distribution and approximately coincides with the edge of domal uplift that causes shoaling and offlap in regional sedimentation.

Notizen: Abstract Developments in plant physiology since the 1980s have led to the realization that fossil plants archive both the isotopic composition of atmospheric CO2 and its concentration, both critical integrators of carbon cycle processes through geologic time. These two carbon cycle signals can be read by analyzing the stable carbon isotope composition (delta13C) of fossilized terrestrial organic matter and by determining the stomatal characters of well-preserved fossil leaves, respectively. We critically evaluate the use of fossil plants in this way at abrupt climatic boundaries associated with mass extinctions and during times of extreme global warmth. Particular emphasis is placed on evaluating the potential to extract a quantitative estimate of the delta13C of atmospheric CO2 because of the key role it plays in understanding the carbon cycle. We critically discuss the use of stomatal index and stomatal ratios for reconstructing atmospheric CO2 levels, especially the need for adequate replication, and present a newly derived CO2 record for the Mesozoic that supports levels calculated from geochemical modeling of the long-term carbon cycle. Several suggestions for future research using stable carbon isotope analyses of fossil terrestrial organic matter and stomatal measurements are highlighted.

Notizen: Abstract Galileo's explorations have revealed a remarkable variety of eruptive styles among Io's diverse volcanoes. Activity at hundreds of volcanic centers ranges from dormant through sporadic to continuous over the 20-year period of spacecraft observation. High temperature volcanism is common on Io, suggesting that the lavas are made up of mafic to ultramafic silicates rather than sulfur compounds. Io's largest plumes are driven by SO2 and sulfur-rich gasses vented from the silicate interior that produce prominent red pyroclastic deposits. Red deposits flag the source regions of many other ongoing or very recent eruptions. Smaller plumes are produced near the margins of active lava flows by explosive volatilization of the underlying or surrounding SO2. These plumes produce SO2 snowflakes that mantle existing topography. Io's volcanism drives significant variations in the atmosphere and plasma torus, yet most of the heat loss occurs through lava flows and by the quiet overturning of lava lakes without large-scale explosive activity. Although only a handful of Io's volcanoes have been directly observed to produce explosive eruptions, volcanic resurfacing is efficient enough to erase even small craters from Io's youthful surface.

Notizen: Abstract A review of crocodylian phylogeny reveals a more complex history than might have been anticipated from a direct reading of the fossil record without consideration of phylogenetic relationships. The three main extant crocodylian lineages-Gavialoidea, Alligatoroidea, Crocodyloidea-are known from fossils in the Late Cretaceous, and the group is found nearly worldwide during the Cenozoic. Some groups have distributions that are best explained by the crossing of marine barriers during the Tertiary. Early Tertiary crocodylian faunas are phylogenetically composite, and clades tend to be morphologically uniform and geographically widespread. Later in the Tertiary, Old World crocodylian faunas are more endemic. Crocodylian phylogeneticists face numerous challenges, the most important being the phylogenetic relationships and time of divergence of the two living gharials (Gavialis gangeticus and Tomistoma schlegelii), the relationships among living true crocodiles (Crocodylus), and the relationships among caimans.

Notizen: Modern analog analysis, the comparison of Quaternary fossil pollen assemblages with modern assemblages, has long been a mainstay of paleoecological and paleoclimatic inference. The logic of analogical inference involves a comparative element (comparison of modern and fossil assemblages to select matches and assess goodness of fit) and a causal element (assumption that the relationships between modern vegetation and derivative pollen assemblages are matched by those between ancient vegetation and fossil pollen assemblages). An array of numerical and statistical tools have been developed to ensure objective, consistent, and quantitative assessments of similarity between pollen assemblages. Divergent or convergent relationships between vegetation and pollen assemblages can arise from a variety of sources, composing a potential source of error in analog analysis, but such errors can be anticipated and minimized. Pollen assemblages lacking modern analogs are well documented for the late-glacial period (17,000-10,000 years BP) in eastern North America and other regions. Simulated climates for this period also lacked modern analogs owing to increased seasonality of insolation, lowered CO2, and persistent ice sheets. Most pollen assemblages from the last glacial maximum (23,000-20,000 years BP) in eastern North America have modern analogs, but macrofossil and other evidence suggest that the vegetation may have lacked modern analogs, owing to unique climate realizations and perhaps direct effects of lowered CO2. Better understanding of the nature of past no-analog vegetation, and the underlying causes, will address important issues in ecology and evolutionary biology and help anticipate biotic responses to the no-analog greenhouse world of the near-future.

Notizen: A large extraterrestrial object striking Earth at cosmic velocity melts and vaporizes silicate materials, which can condense into highly spheroidal, sand-size particles that get deposited hundreds to thousands of kilometers from the point of impact. These particles, known as impact spherules, have been detected in great abundance in a relatively small number of thin, discrete layers ranging in age from less than a million years to 3.47 billion years. Unaltered impact spherules consist entirely of glass (microtektites) or a combination of glass and crystals grown in flight (microkrystites). Impact spherule layers form very rapidly and can be very extensive, even global in extent [e.g., the Cretaceous-Tertiary (K/T) boundary layer], so they form excellent time-stratigraphic markers. Because they are always found in a stratigraphic context, spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts. A record of impacts whose craters have since been obliterated, most notably those in pre-Mesozoic oceanic crust, could survive in the form of spherule layers. Secular changes in surface environments and/or the nature of the impactors striking Earth through its history could also be reflected in differences in spherules and spherule layers as a function of geologic age. In this paper, we briefly review what spherules and spherule layers are and the processes that create them, then speculate about what might be learned through wider identification of and more extensive study of impact spherule layers.

Notizen: The fossil record preserves a wide range of events that might be used to build timescales and correlate strata from place to place. The events include the originations and extinctions of species, the occurrence of distinctive faunal assemblages, magnetic field reversals, changes in ocean chemistry, and volcanic ash falls. The fundamental task is to determine the global sequence of all these events. Modern computer algorithms can build high-resolution timescales by sequencing and calibrating thousands of such events from hundreds of localities. Each successful sequencing algorithm can be understood in terms of a simple two-dimensional visual device. Graphic correlation, permutable sequences, permutable matrices, and slotting devices are each suited to a different data problem, such as contradictory evidence of sequence, a lack of information about sequence, or uncertain correspondence of events. Algorithms based upon permutable sequences and evolutionary programming heuristics have the flexibility to optimize sequences with a wide variety of event types and data problems; they are slower than the more narrowly tailored methods. All methods will be challenged to keep pace with the amount of biostratigraphic information that is accumulating in the latest generation of shared databases. Currently, it is possible to sequence sufficiently large numbers of events to imagine a potential resolution of 10,000 to 50,000 years over time spans on the order of 50 to 100 million years. The actual resolving power is less because the solutions to these sequencing problems are not unique. Multiple equally good solutions typically emerge and, to extract a consensus sequence with which all solutions agree, some runs of events must be collapsed into unresolvable clusters. Nevertheless, quantitative methods have been shown to improve resolution up to tenfold over traditional methods that discard many potentially useful events.

Notizen: The global positioning system (GPS) is used increasingly to control horizontal and vertical displacement of large civil constructions. The displacements can occur over varius time scales. For example, in case of the loading test of a bridge, the time span between measurements is on the order of 30 min. The achievable precision in the vertical component and its relation to residual multiplath effects become critically important in such aaplications. A multipath-to-noise ratio (MNR) on individual satellites is introduced to quantify the multipath effects. © 2000 John Wiley & Sons, Inc.

Notizen: In this article, initial results are presented of a method to improve fast carrier phase ambiguity resolution over longer baselines (with lengths up to about 200 km). The ionospheric delays in the global positioning system (GPS) data of these long baselines mainly hamper successful integer ambiguity resolution, a prerequisite to obtain precise positions within very short observation time spans. A way to correct the data for significant ionospheric effects is to have a GPS user operate within an active or permanently operating network use ionospheric estimates from this network. A simple way to do so is to interpolate these ionospheric estimates based on the expected spatial behaviour of the ionospheric delays. In this article such a technique is demonstrated for the Dutch Active Control Network (AGRS.NL). One hour of data is used from 4 of the 5 reference stations to obtain very precise ionospheric corrections after fixing of the integer ambiguities within this network. This is no problem because of the relatively long observation time span and known positions of the stations of the AGRS.NL. Next these interpolated corrections are used to correct the GPS data from the fifth station for its ionospheric effects. Initial conclusions about the performance of this technique are drawn in terms of improvement of integer ambiguity resolution for this baseline. © 1999 John Wiley & Sons, Inc.

Notizen: Since global positioning system (GPS) measurements are ranges (code) and biased ranges (carrier), it seems natural to model them as ranges and determine the biases. This is particularly compelling since the double-difference range biases turn out to be integers. At some level there is also an elegance, perhaps therefore a naturalness, to modeling the carrier measurements as time differences of double differences. While something is lost something else is gained. Here we apply the proven delayed-state Kalman filter to processing carrier phase measurements as triple differences. In practice we process these triple differences along with double-difference code measurements. We also treat the measurement error as, mostly, Gauss-Markov states to be determined. Many of the details are discussed and experimental results are included. These demonstrate that excellent performance can be obtained if the Kalman filter modeling is done carefully. © 2000 John Wiley & Sons, Inc.

Notizen: A third civil frequency at 1176.45 MHz will be added to the Global Positioning System (GPS). This new frequency will bring a number of benefits. The aviation user will be one of the prime beneficiaries because the new frequency is in a protected aviation band. Thus, the system will be more robust against interference and jamming. The carrier-phase differential user will also be a prime beneficiary as long as his application has a reasonably short baseline. It is this high accuracy use that is explored in some depth. The process of forming linear combinations of both the code and carrier-phase measurements is studied, and the benefits and problems are explained. © 2000 John Wiley & Sons, Inc.