Annual Review of Earth and Planetary Sciences - Volume 32, 2004

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 δ-MnO2 or acid birnessite. Metal sequestration by the Mn biooxides occurs predominantly at vacant layer octahedral sites.

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.

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 100°C 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.

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.

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.

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 μm, 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.

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.

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.

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.