Annual Review of Earth and Planetary Sciences - Volume 33, 2005

This paper reviews the Precambrian history of atmospheric oxygen, beginning with a brief discussion of the possible nature and magnitude of life before the evolution of oxygenic photosynthesis. This is followed by a summary of the various lines of evidence constraining oxygen levels through time, resulting in a suggested history of atmospheric oxygen concentrations. Also reviewed are the various processes regulating oxygen concentrations, and several models of Precambrian oxygen evolution are presented. A sparse geologic record, combined with uncertainties as to its interpretation, yields only a fragmentary and imprecise reading of atmospheric oxygen evolution. Nevertheless, oxygen levels have increased through time, but not monotonically, with major and fascinating swings to both lower and higher levels.

Dedicated to the memory of three pioneers, İhsan Ketin, Sırrı Erinç and Melih Tokay, and a recent student, Aykut Barka, who burnt himself out in pursuit of the mysteries of the North Anatolian Fault.

The North Anatolian Fault (NAF) is a 1200-km-long dextral strike-slip fault zone that formed by progressive strain localization in a generally westerly widening right-lateral keirogen in northern Turkey mostly along an interface juxtaposing subduction-accretion material to its south and older and stiffer continental basements to its north. The NAF formed approximately 13 to 11 Ma ago in the east and propagated westward. It reached the Sea of Marmara no earlier than 200 ka ago, although shear-related deformation in a broad zone there had already commenced in the late Miocene. The fault zone has a very distinct morphological expression and is seismically active. Since the seventeenth century, it has shown cyclical seismic behavior, with century-long cycles beginning in the east and progressing westward. For earlier times, the record is less clear but does indicate a lively seismicity. The twentieth century record has been successfully interpreted in terms of a Coulomb failure model, whereby every earthquake concentrates the shear stress at the western tips of the broken segments leading to westward migration of large earthquakes. The August 17 and November 12, 1999, events have loaded the Marmara segment of the fault, mapped since the 1999 earthquakes, and a major, M ≤ 7.6 event is expected in the next half century with an approximately 50% probability on this segment. Currently, the strain in the Sea of Marmara region is highly asymmetric, with greater strain to the south of the Northern Strand. This is conditioned by the geology, and it is believed that this is generally the case for the entire North Anatolian Fault Zone. What is now needed is a more detailed geological mapping base with detailed paleontology and magnetic stratigraphy in the shear-related basins and more paleomagnetic observations to establish shear-related rotations.

Orogenic collapse is a process that transfers gravitational potential energy from regions of high potential energy to regions of lower potential energy. This transfer is classically considered to be accomplished by extension in the orogenic core and by synchronous shortening in foreland regions of the orogen. Not all extensional features in collisional mountain belts need, however, reflect orogenic collapse. Normal faulting, thrust faulting, and strike-slip faulting are all active in different parts of the Alps today and reflect complex local responses to ongoing Europe-Adria convergence. The Western Alps is the only area today where extension and shortening radial to orogen trend occur synchronously and where orogenic collapse may be an important process. Elsewhere in the Alps, normal faults are oriented at a high angle to orogen trend and were primarily active in Oligocene and Miocene time. Most present-day activity in the Central and Eastern Alps is on strike-slip faults that are accommodating lateral extrusion of material rather than orogenic collapse.

The bulk of the ∼50-km-thick Martian crust formed at ∼4.5 Gyr B.P., perhaps from a magma ocean. This crust is probably a basaltic andesite or andesite and is enriched in incompatible and heat-producing elements. Later additions of denser basalt to the crust were volumetrically minor, but resurfaced significant portions of the Northern hemisphere. A significant fraction of the total thickness of the crust was magnetized prior to 4 Gyr B.P., with the magnetization later selectively removed by large impacts. Early large impacts also modified the hemispheric contrast in crustal thickness (the dichotomy), which was possibly caused by long-wavelength mantle convection. Subsequent Noachian modification of the crust included further impacts, significant fluvial erosion, and volcanism associated with the formation of the Tharsis rise. Remaining outstanding questions include the origin of the dichotomy and the nature of the magnetic anomalies.

Weather and climate predictions are uncertain, because both forecast initial conditions and the computational representation of the known equations of motion are uncertain. Ensemble prediction systems provide the means to estimate the flow-dependent growth of uncertainty during a forecast. Sources of uncertainty must therefore be represented in such systems. In this paper, methods used to represent model uncertainty are discussed. It is argued that multimodel and related ensembles are vastly superior to corresponding single-model ensembles, but do not provide a comprehensive representation of model uncertainty. A relatively new paradigm is discussed, whereby unresolved processes are represented by computationally efficient stochastic-dynamic schemes.

Real-time seismology refers to a practice in which seismic data are collected and analyzed quickly after a significant seismic event, so that the results can be effectively used for postearthquake emergency response and early warning. As the technology of seismic instrumentation, telemetry, computers, and data storage facility advances, the real-time seismology for rapid postearthquake notification is essentially established. Research for early warning is still underway. Two approaches are possible: (a) regional warning and (b) on-site (or site-specific) warning. In (a), the traditional seismological method is used to locate an earthquake, determine the magnitude, and estimate the ground motion at other sites. In (b), the beginning of the ground motion (mainly P wave) observed at a site is used to predict the ensuing ground motion at the same site. An effective approach to on-site warning is discussed in light of earthquake rupture physics.

Airborne geophysics has been used to identify more than 100 lakes beneath the ice sheets of Antarctica. The largest, Lake Vostok, is more than 250 km in length and 1 km deep. Subglacial lakes occur because the ice base is kept warm by geothermal heating, and generated meltwater collects in topographic hollows. For lake water to be in equilibrium with the ice sheet, its roof must slope ten times more than the ice sheet surface. This slope causes differential temperatures and melting/freezing rates across the lake ceiling, which excites water circulation. The exploration of subglacial lakes has two goals: to find and understand the life that may inhabit these unique environments and to measure the climate records that occur in sediments on lake floors. The technological developments required for in situ measurements mean, however, that direct studies of subglacial lakes may take several years to happen.

Processes operating beneath glaciers can have a greater influence on flow dynamics than those operating within them. The variety and complexity of these processes, which involve interactions among ice, water, and geological solids, resist efforts to establish simple truths and can lead to surprising outcomes. Thermal conditions at the ice-bed interface (melting or nonmelting) and the mechanical properties of the glacier substrate (soft or hard) determine which processes can be activated. The warm-soft case supports the greatest variety of processes and is the most important for fast-flow dynamics and for the mobilization of subglacial sediment. Process interactions can lead to oscillations and spatio-temporal switching behavior in glaciers and ice sheets as well as to the generation of subglacial landforms.

Recent fossil discoveries from Early Cretaceous rocks of Liaoning Province, China, have provided a wealth of spectacular specimens. Included in these are the remains of several different kinds of small theropod dinosaurs, many of which are extremely closely related to modern birds. Unique preservation conditions allowed soft tissues of some of these specimens to be preserved. Many dinosaur specimens that preserve feathers and other types of integumentary coverings have been recovered. These fossils show a progression of integumentary types from simple fibers to feathers of modern aspect. The distribution of these features on the bodies of these animals is surprising in that some show large tail plumes, whereas others show the presence of wing-like structures on both fore and hind limbs. The phylogenetic distribution of feather types is highly congruent with models of feather evolution developed from developmental biology.

Microbes are recognized as important components of the Earth system, playing key roles in controlling the composition of the atmosphere and surface waters, forming the basis of the marine food web, and the cycling of chemicals in the ocean. A revolution in microbial ecology has occurred in the past 15–20 years with the advent of rapid methods for discovering and sequencing the genes of uncultivated microbes from natural environments. Initially based on sequences from the 16S rRNA gene, this revolution made it possible to identify microorganisms without first cultivating them, to discover and characterize the immense previously unsuspected diversity of the microbial world, and to reconstruct the evolutionary relationships among microbes. Subsequent focus on functional genes, those that encode enzymes that catalyze biogeochemical transformations, and current work on larger DNA fragments and entire genomes make it possible to link microbial diversity to ecosystem function. These approaches have yielded insights into the regulation of microbial activity and proof of the microbial role in biogeochemical processes previously unknown. Questions raised by the molecular revolution, which are now the focus of microbial ecology research, include the significance of microbial diversity and redundancy to biogeochemical processes and ecosystem function.

Earthquake triggering is the process by which stress changes associated with an earthquake can induce or retard seismic activity in the surrounding region or trigger other earthquakes at great distances. Calculations of static Coulomb stress changes associated with earthquake slip have proven to be a powerful tool in explaining many seismic observations, including aftershock distributions, earthquake sequences, and the quiescence of broad, normally active regions following large earthquakes. Delayed earthquake triggering, which can range from seconds to decades, can be explained by a variety of time-dependent stress transfer mechanisms, such as viscous relaxation, poroelastic rebound, or afterslip, or by reductions in fault friction, such as predicted by rate and state constitutive relations. Rapid remote triggering of earthquakes at great distances (from several fault lengths to 1000s of km) is best explained by the passage of transient (dynamic) seismic waves, which either immediately induce Coulomb-type failure or initiate a secondary mechanism that induces delayed triggering. The passage of seismic waves may also play a significant role in the triggering of near-field earthquakes.

Stable cratons and stable continental platforms are salient features of the Earth. Mantle xenoliths provide detailed data on deep structure. Cratonal lithosphere is about 200 km thick. It formed in the Archean by processes analogous to modern tectonics and has been stable beneath the larger cratons since that time. Its high viscosity, high yield strength, and chemical buoyancy protected it from being entrained by underlying stagnant lid convection and by subduction. Chemically buoyant mantle does not underlie platforms. Platform lithosphere has gradually thickened with time as convection waned as the Earth's interior cooled. The thermal contraction associated with this thickening causes platforms to subside relative to cratons. At present, the thickness of platform lithosphere is comparable to that of cratonal lithosphere.

Ichthyosaurs were a group of Mesozoic marine reptiles that evolved fish-shaped body outlines. They are unique in several anatomical characters, including the possession of enormous eyeballs sometimes exceeding 25 cm and an enlarged manus with sometimes up to 20 bones in a single digit, or 10 digits per manus. They are also unique in that their biology has been studied from the perspective of physical constraints, which allowed estimation of such characteristics as optimal cruising speed, visual sensitivity, and even possible basal metabolic rate ranges. These functional inferences, although based on physical principles, obviously contain errors arising from the limitations of fossilized data, but are necessarily stronger than the commonly made inferences based on superficial correlations among quantities without mechanical or optical explanations for why such correlations exist.

The Ediacara biota (575–542 Ma) marks the first appearance of large, architecturally complex organisms in Earth history. Present evidence suggests that the Ediacara biota included a mixture of stem- and crown-group radial animals, stem-group bilaterian animals, “failed experiments” in animal evolution, and perhaps representatives of other eukaryotic kingdoms. These soft-bodied organisms were preserved under (or rarely within) event beds of sand or volcanic ash, and four distinct preservational styles (Flinders-, Fermeuse-, Conception-, and Nama-style) profoundly affected the types of organisms and features that could be preserved. Even the earliest Ediacaran communities (575–565 Ma) show vertical and lateral niche subdivision of the sessile, benthic, filter-feeding organisms, which is strikingly like that of Phanerozoic and modern communities. Later biological and ecological innovations include mobility (>555 Ma), calcification (550 Ma), and predation (<549 Ma). The Ediacara biota abruptly disappeared 542 million years ago, probably as a consequence of mass extinction andor biological interactions with the rapidly evolving animals of the Cambrian explosion.

The mathematical modeling of landform evolution consists of two components: the processes represented (i.e., considered dominant) in the model and the (typically computer) model representation of these processes. This review discusses the current debates surrounding processes represented in landform evolution. The potential impact on both evolving landforms and computer model structure is discussed. Issues specifically discussed include (a) the fundamental nature of mass conservation and the role of detachment- and transport-limited processes in mass conservation equations, (b) the interaction between detachment- and transport-limitation in channels, (c) the role of hillslope erosion and soil properties and their interaction with channel processes, (d) the interactions with tectonics when applying these models at large scale, (e) depositional structures and implications for paleo-climatic interpretation, (f) engineering applications of these models, and (g) numerical issues in the computer implementations. This review is not a model comparison. However, many applications are at the boundaries of computer capabilities so a comparison of existing models is provided.

Recent developments in volcanic seismology include new techniques to improve earthquake locations that have changed clouds of earthquakes to lines (faults) for high-frequency events and small volumes for low-frequency (LF) events. Spatial mapping of the b-value shows regions of normal b and high b anomalies at depths of 3–4 and 7–10 km. Increases in b precede some eruptions. LF events and very-long-period (VLP) events have been recorded at many volcanoes, and models are becoming increasingly sophisticated. Deep long-period (LP) events are fairly common, but may represent several processes. Acoustic sensors have greatly improved the study of volcanic explosions. Volcanic tremor is stronger for fissure eruptions, phreatic eruptions, and higher gas contents. Path and site effects can be extreme at volcanoes. Seismicity at volcanoes is triggered by large earthquakes, although mechanisms are still uncertain. A number of volcanoes have significant deformation with very little seismicity. Tomography has benefited from improved techniques and better instrumental arrays.

We know that giant planets played a crucial role in the making of our Solar System. The discovery of giant planets orbiting other stars is a formidable opportunity to learn more about these objects, what their composition is, how various processes influence their structure and evolution, and most importantly how they form. Jupiter, Saturn, Uranus, and Neptune can be studied in detail, mostly from close spacecraft flybys. We can infer that they are all enriched in heavy elements compared to the Sun, with the relative global enrichments increasing with distance to the Sun. We can also infer that they possess dense cores of varied masses. The intercomparison of presently characterized extrasolar giant planets shows that they are also mainly made of hydrogen and helium, but that they either have significantly different amounts of heavy elements, have had different orbital evolutions, or both. Hence, many questions remain and need to be answered to make significant progress on the origins of planets.

Pourquoi l'azur muet et l'espace insondable?

Pourquoi les astres d'or fourmillant comme un sable?

Arthur Rimbaud—Soleil et chair

The Earth has a radiogenic W-isotopic composition compared to chondrites, demonstrating that it formed while ¹⁸²Hf (half-life 9 Myr) was extant in Earth and decaying to ¹⁸²W. This implies that Earth underwent early and rapid accretion and core formation, with most of the accumulation occurring in ∼10 Myr, and concluding approximately 30 Myr after the origin of the Solar System. The Hf-W data for lunar samples can be reconciled with a major Moon-forming impact that terminated the terrestrial accretion process ∼30 Myr after the origin of the Solar System. The suggestion that the proto-Earth to impactor mass ratio was 7:3 and occurred during accretion is inconsistent with the W isotope data. The W isotope data is satisfactorily modeled with a Mars-sized impactor on proto-Earth (proto-Earth to impactor ratio of 9:1) to form the Moon at ∼30 Myr.

Contrary to Earth, the interior of terrestrial planets is poorly known. This is mainly related to the lack of seismic data and of planetary seismic networks on these planets. So far, despite several attempts, only the Apollo Seismic Network has returned seismic information from the Moon. But even in this case, very few seismic signals were recorded after a propagation path through the deep interior and core owing to a hemispheric distribution of the stations on the near side and to a probably strongly attenuating lower mantle. This review presents the main results achieved by the analysis of the Apollo seismic data and the associated constraints on the internal structure of the Moon. It then presents the current knowledge on the Martian interior, the seismic activity of the planet, and possible source of seismic noise. This information can be used for preparing future Martian seismic network missions. A short review on existing space-qualified instruments and on possible seismic missions toward other telluric bodies, such as Venus, the giant planets' satellites, or small bodies, is then given.