Annual Review of Earth and Planetary Sciences - Volume 29, 2001

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.

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.

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.

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.

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.

The two most common low-temperature iron(III) oxides on Earth are goethite (α-FeOOH) and hematite (α-Fe2O3). The δ¹⁸O values of natural goethites range from −15.5‰ to +3.3‰, whereas δ¹⁸O values of low-temperature hematites range from −16.7‰ to +4.7‰. Plots of δD against δ¹⁸O 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 δ¹³C values of an Fe(CO3)OH component in apparent solid solution in goethite are proxies for the partial pressure and δ¹³C 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 δ¹³C values of Fe(CO3)OH in goethite.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.