bluuebunny/crossref_metadata_2025 huggingface dataset:
Laboratory measurement of the temperature dependence of gaseous sulfur dioxide (SO2) microwave absorption with application to the Venus atmosphere
Authors: Suleiman Shady H. , Kolodner Marc A. , Steffes Paul G.
Published: February 2004

High‐accuracy laboratory measurements of the temperature dependence of the opacity from gaseous sulfur dioxide (SO2) in a carbon dioxide (CO2) atmosphere at temperatures from 290 to 505 K and at pressures from 1 to 4 atm have been conducted at frequencies of 2.25 GHz (13.3 cm), 8.5 GHz (3.5 cm), and 21.7 GHz (1.4 cm). Based on these absorptivity measurements, a Ben‐Reuven (BR) line shape model has been developed that provides a more accurate characterization of the microwave absorption of gaseous SO2 in the Venus atmosphere as compared with other formalisms. The developed BR formalism is incorporated into a radiative transfer model. The resulting microwave emission spectrum of Venus is then used to set an upper limit on the disk‐averaged abundance of gaseous SO2 below the main cloud layer. It is found that gaseous SO2 has an upper limit of 150 ppm, which compares well with previous spacecraft in situ measurements and Earth‐based radio astronomical observations.
The stability of climate on Venus
Authors: Bullock Mark A. , Grinspoon David H.
Published: February 2004

The climate of Venus is to a large degree controlled by the radiative properties of its massive atmosphere. However, volcanic outgassing and surface‐atmosphere interactions may moderate the atmospheric abundances of CO2 and other radiatively important volatiles. Recently, work on the interpretation of the impact cratering record has placed limits on the magnitude of the volcanic flux. Additionally, recent laboratory data on the equilibria and kinetics of possible surface‐atmosphere reactions under Venus‐like conditions provide important insights into the role that they may play in moderating Venus' climate. The surface temperature and pressure on Venus coincide approximately with the P‐T equilibrium of the calcite‐wollastonite mineral reaction, and atmospheric sulfur species are probably involved in rapid heterogeneous reactions with the surface. Perturbations to the atmospheric inventory of radiatively active species may have a significant impact on the climate of Venus and upon the stability of the greenhouse effect. Through the use of a Venus climate model that couples atmospheric radiative‐convective equilibrium with surface processes, we show that it is likely that Venus' climate is at or near a state of unstable equilibrium. Furthermore, we show that only moderate perturbations in the abundances of radiatively active volatiles may be sufficient to precipitate changes to new climate regimes.
A model for variation in the chemistry of planktonic foraminifera due to secondary calcification and selective dissolution
Authors: Lohmann G. P.
Published: February 2004

A quantitative model for describing the within‐population variation in planktonic foraminifer shell chemistry that results from secondary calcification and selective dissolution is presented. The objective is to construct a basis for inferring the chemistries of different shell components and estimating the extent of shell dissolution. Variation is modeled as mixtures of two kinds of shell calcite, a primary calcite that forms the chambers and a secondary calcite that forms crust. Bulk shell chemistries are intermediate between the chemistries of these calcites and lie on mixing lines between them. For two component systems, mass balance relationships can be reformulated as a linear regression and solved for the chemistries of the primary and secondary calcites and for the uncertainties associated with these estimates. To apply this model, one needs measurements of bulk shell chemistries and estimates of the relative proportions of secondary and primary calcites. For most planktonic foraminifer species the proportion of secondary calcite can be estimated from differences in the relationship between shell size and mass before and after crusting. Preliminary results are consistent with previous work showing that secondary calcites are added at depth. However, even deep‐dwelling species appear to grow most of their primary shell in surface waters and some surface‐dwelling species add secondary calcite in the deep ocean. In contrast to the model's simple description of secondary calcification, the variation in chemistry from selective dissolution is more complicated because undissolved shells are themselves mixtures of primary and secondary calcites and therefore present a wide range of initial shell compositions. Nevertheless, the model allows both the compositions of different components to be inferred and the amount of dissolution to be estimated. Preliminary results indicate that dissolution of planktonic foraminifera is apparent nearly 2 km above the foraminifer lysocline and even apparently well‐preserved shells may be over 50% dissolved.
A trace metal perspective on the evolution of Antarctic Circumpolar Deep Water chemistry
Authors: Lea David W.
Published: February 2004

A new benthic foraminiferal Ba/Ca and Cd/Ca data set from core RC13‐229 in the deep Cape Basin indicates only small variations in bottom water nutrient concentrations in Circumpolar Deep Water (CPDW) over the last 450 kyr. Variability in the Ba record is characterized by somewhat higher values during glacial periods, consistent with a reduction in the flux of Ba‐depleted North Atlantic Deep Water to the Southern Ocean during glacial periods. The small changes in the Ba and Cd records contrast with the large and systematic increase in CPDW nutrients during glacial periods implied by the benthic δ13C record. This discrepancy, essentially an extension of the well‐known Southern Ocean Cd‐δ13C conflict, is evaluated by transforming RC13‐229 paleochemical data into carbonate parameters using the modern oceanic relationships between δ13C, Cd, and ΣCO2 and between Ba and alkalinity. Calculations using Cd/Ca to estimate past variations in CPDW ΣCO2 and Ba/Ca to estimate past variations in CPDW alkalinity yield carbonate ion concentrations that exceed calcite saturation throughout the record length, with generally higher carbonate ion values associated with glacial intervals (opposite in sense to the RC13‐229 %CaCO3 record). Substituting δ13C to estimate ΣCO2 leads to extreme calcite undersaturation at this site during glacial periods, clearly inconsistent with the preservation of calcite throughout the length of RC13‐229. Accepting the carbon isotope record as a direct measure of past variations in CPDW ΣCO2 concentrations requires that both the Cd and Ba evidence for limited nutrient and alkalinity changes be disregarded.
Calcitic foraminiferal data confirmed by cadmium in aragonitic Hoeglundina: Application to the Last Glacial Maximum in the northern Indian Ocean
Authors: Boyle Edward A. , Labeyrie Laurent , Duplessly Jean‐Claude
Published: February 2004

A core‐top study of cadmium uptake into the shells of the aragonitic benthic foraminifera Hoeglundina elegans demonstrates that the Hoeglundina Cd/Ca partition coefficient is close to 1.0 throughout the ocean. Cd uptake by Hoeglundina is far less depth dependent than that of calcitic benthic foraminifera. Furthermore, manganese carbonate does not precipitate on these aragonitic shells, allowing for the recovery of Cd estimates in some samples where calcitic species are spoiled by contaminating overgrowths. Because Cd incorporation into Hoeglundina shows little depth dependence, a comparison of calcitic and Hoeglundina Cd data can be used to verify the assumption that the depth dependence observed for calcitic benthic foraminifera is time invariant. This comparison has been undertaken in downcore and last glacial maximum (LGM) samples from the northern Indian Ocean. Aragonitic and calcitic foraminiferal estimates for Cd in the LGM ocean are in excellent agreement. This result indicates upper ocean estimates of LGM Cd are reliable despite a factor‐of‐two variation in Cd uptake by calcitic species over this depth range. Cd and δ13C data indicate that the uppermost waters of the LGM Arabian Sea were strongly nutrient depleted relative to today but that the deepest waters of the northern Bay of Bengal had nutrient concentrations at least as high as or higher than modern levels. A strong vertical gradient of increasing nutrients with increasing depth existed in both basins during the LGM. A west to east gradient of increasing nutrient concentrations also is evident at all depths. In the Bay of Bengal, there is evidence for an enhanced gradient of nutrients increasing from south to north. The LGM deep Arabian Sea and intermediate‐depth Bay of Bengal were nutrient depleted compared to the modern ocean; however, Cd and δ13C data indicate that nutrient depletion in the northern Indian Ocean did not exceed that of the deep eastern tropical Pacific except during the latest part of oxygen isotope stage 2 and (according to Cd data only) the early stages of deglaciation. During oxygen isotope stage 3 and early stage 2, Cd in the northern Indian Ocean and Cd in the eastern tropical Pacific were indistinguishable, although both regions have lower Cd at these times compared to the Holocene.
Sedimentary record of rapid climatic variability in the North Atlantic Ocean during the Last Glacial Cycle
Authors: Cortijo Elsa , Yiou Pascal , Labeyrie Laurent , Cremer Michael
Published: February 2004

Comparisons between a Greenland ice core isotopic record and marine sediment grey level records of the North Atlantic Ocean show that rapid temperature variations witnessed by the ice core (Dansgaard‐Oeschger events) during the last glacial period can also be detected in marine sediments. This shows that the resolution obtained in marine cores can be sufficiently high to record rapid climatic fluctuations. Several spectral analyses of those grey level marine records consistently indicate that significative fluctuations of the climatic response exist with periodicities of 5 to 1 kyr in addition to orbital forcing. These high frequencies are close to those predicted by various dynamic ocean models.
Middle Miocene deepwater paleoceanography in the southwest Pacific: Relations with East Antarctic Ice Sheet development
Authors: Flower Benjamin P. , Kennett James P.
Published: February 2004

A suite of middle Miocene Deep Sea Drilling Project sites in the southwest Pacific reveals large‐scale changes in deepwater circulation associated with East Antarctic Ice Sheet (EAIS) variations from ∼16.5 to 13.8 Ma. Oxygen and carbon isotopic records based on Cibicidoides benthic foraminifera from a depth transect (sites 590B, 588A, 591B, and 206 from 1200‐ to 3150‐m paleodepth at ∼35°S paleolatitude) and from a meridional transect (sites 588A, 590B, 593, and 594 from 30° to 48°S paleolatitude at intermediate water depth) allow detailed examination of southwest Pacific deepwater circulation from ∼17.5 to 12 Ma. Significantly, intervals of low δ18O from 16.5 to 16.3 Ma and perhaps at 15.7 Ma were marked by similar δ18O values at upper bathyal (∼1200–1500 m; sites 588A and 590B) and midbathyal (∼2100 m; site 591B) water depths. Small vertical δ18O gradients during δ18O minima may indicate warm saline deep water (WSDW) at midbathyal depths in the southwest Pacific during intervals of inferred global warmth and low global ice volume. Increased vertical δ18O gradients after ∼15.6 Ma and especially after 13.8 Ma indicate increased production of Southern Component Water (SCW) in association with EAIS growth. These data are consistent with the hypothesis (Woodruff and Savin, 1989) that major EAIS growth was fostered by diminished meridional heat transport to the high southern latitudes related to the termination of Tethyan Indian Saline Water (TISW) and an increase in SCW production during the early middle Miocene after ∼15.6 Ma. Further, a maximum vertical carbon isotopic gradient of ∼0.8 ‰ at 13.6 Ma suggests that Southern Component Intermediate Water (SCIW) production and Pacific Deep Water (PDW) strength were each at a maximum at this time and were critical to major EAIS growth. The establishment of near‐modern δ13C and δ18O gradients following major EAIS growth from ∼14.0 to 13.8 Ma marks a major step in the development of the Neogene ocean/cryosphere system.
Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene
Authors: Dickens Gerald R. , O'Neil James R. , Rea David K. , Owen Robert M.
Published: February 2004

Isotopic records across the “Latest Paleocene Thermal Maximum“ (LPTM) indicate that bottom water temperature increased by more than 4°C during a brief time interval (<104 years) of the latest Paleocene (∼55.6 Ma). There also was a coeval −2 to −3‰ excursion in the δ13C of the ocean/atmosphere inorganic carbon reservoir. Given the large mass of this reservoir, a rapid δ13C shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere. However, a direct consequence of warming bottom water temperature from 11 to 15°C over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic CH4 hydrate at locations with intermediate water depths. In terms of the present‐day oceanic CH4 hydrate reservoir, thermal dissociation of oceanic CH4 hydrate during the LPTM could have released greater than 1.1 to 2.1 × 1018 g of carbon with a δ13C of approximately −60‰. The release and subsequent oxidation of this amount of carbon is sufficient to explain a −2 to −3‰ excursion in δ13C across the LPTM. Fate of CH4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the LPTM.
Sr and Nd isotopes as tracers of North Atlantic lithic particles: Paleoclimatic implications
Authors: Revel Marie , Sinko Jean A. , Grousset Francis E. , Biscaye Pierre E.
Published: February 2004

Sr and Nd isotopic compositions have been measured on the lithic fraction of last climatic cycle sediments from the North Atlantic (≈40°N/≈60°N), in order to identify the origins of the particles. From the reconstruction of their transport pathways, we deduce the mechanisms that explain their distributions. The main source regions are the Canadian shield (mostly the area of Baffin Bay and western Greenland), the Scandinavian shield, the European region (British Isles and Bay of Biscay), and Iceland. We observe a significant glacial/interglacial contrast, characterized by a dominant Icelandic input via near‐bottom transport by North Atlantic Deep Water (NADW) during the interglacials and a largely continent‐derived contribution of surface‐transported, ice‐rafted detritus (IRD) during the glacial period. During the last glacial period, the Heinrich events (abrupt, massive discharges of IRD) originated not only from the Laurentide ice sheet as heretofore envisioned but also from other sources. Three other major North Atlantic ice sheets (Fennoscandian, British Isles, and Icelandic) probably surged simultaneously, discharging ice and IRD into the North Atlantic. As opposed to theories implying a unique, Laurentide origin [Gwiazda et al., 1995] driven by an internal mechanism [McAyeal, 1993], we confirm that the Icelandic and the Fennoscandian ice sheets also surged as recently proposed by other authors, and we here also distinguish a possible detrital contribution from the British Isles ice sheet. This pan‐North Atlantic phenomenon thus requires a common regional, external forcing.
Variability in the earth climate system: Decadal and longer timescales
Authors: Schimel David , Sulzman Elizabeth
Published: February 2004

Global change—the cumulative impact of anthropogenic changes to water, energy, and biogeochemical cycles—is one of the defining issues for the geophysical and life sciences in the 1990s and beyond. The increasing emphasis on understanding the potential future climatic consequences of anthropogenic interference in the global carbon and trace gas cycles has driven an increased effort to understand the behavior of the climate system on decadal‐to‐centennial and longer timescales. The issues related to the long‐term behavior of the climate system are both of fundamental scientific interest and of importance to society, as a reading of the Climate Convention [United Nations, 1992] will immediately reveal. Variability on long timescales is challenging and exciting because much of the climate system's low frequency behavior and many long‐term changes arise from the coupling of the atmosphere to the ocean, biogeochemical cycles, and the cryosphere, as well as from internal dynamics [Pielke and Zeng, 1994]. Variability on decadal and longer timescales must be studied in an interdisciplinary fashion and challenges existing models and data sets, which have typically been developed or collected to address more limited problems.
Ocean biogeochemical fluxes: New production and export of organic matter from the upper ocean
Authors: Ducklow Hugh W.
Published: February 2004

Studies of ocean biogeochemical fluxes have been energized in this decade, by the urgency of our need to understand and predict the effects of continued CO2 accumulation in the atmosphere, by the global perspectives offered by satellite views of ocean color and related physical fields (McClain et al. 1991; Yoder et al. 1992; Mitchell 1994), and by the successful implementation of the Joint Global Ocean Flux Study (JGOFS; Bowles and Livingston, 1993). In this review, I focus on oceanic new production, originally defined as the fraction of primary production supported by inputs of ‘new’ nitrogen from outside the euphotic zone. With a growing appreciation of the role of this fundamental biogeochemical flux in the global carbon cycle, it has become more common to refer interchangeably to new production so defined, and to the export of organic matter from the upper ocean (e.g.. Sarmiento and Siegenthaler 1992). New production, the driving process of the ocean carbon cycle, is responsible for maintaining over half the vertical gradient in total inorganic carbon. In this review I refer to nitrate‐based new production in the open sea, and not to new production supported by other N compounds as observed in the coastal zone. Eppley (1992) gives a personal view of the modern formulation of the concept of equivalence between new production and upper ocean export. This review is dedicated to the memory of John Martin, a friend, colleague, leader and teacher who contributed mightily to our field.
Rock deformation: Ductile and brittle
Authors: Karato Shun‐ichiro , Wong Teng‐fong
Published: February 2004

Rock deformation studies are conducted to constrain the non‐elastic behavior of rocks subjected to non‐hydrostatic stresses in various geological and geophysical situations including long term creep responsible for large scale tectonics such as mantle convection and brittle failure related to earthquakes. Significant advancements made in these areas during the last quadra‐annual period are reviewed. In ductile deformation, important progress has been made toward understanding rheological properties under deep mantle conditions. These studies provide important insights into the dynamics of the Earth's deep interior. In addition, significant progress has been made toward understanding the role of chemical environment on rheology and the influence of microstructures such as grain‐size. In the brittle field, important advances were made in the observation of the spatio‐temporal evolution of faulting. The complex interplay of microcracking, crystal plasticity, diffusive mass transfer and pore collapse in controlling the brittle‐plastic transition under crustal conditions have been elucidated. Recent investigations demonstrate the importance of fluid flow on frictional strength. New measurements have clarified the influence of normal stress, temperature and mineralogy on friction constitutive behavior. Some of these experimental observations are being incorporated into crustal faulting models that provide physical insights into the dynamics of the earthquake cycle.
Recent advances in pore scale models for multiphase flow in porous media
Authors: Celia Michael A. , Reeves Paul C. , Ferrand Lin A.
Published: February 2004

In the last decade, multi‐phase flow in porous media has become a prominent topic in hydrologic research. This has been motivated by the widespread occurrence of subsurface contamination problems involving sparingly soluble liquids, often referred to as non‐aqueous phase liquids (NAPLs). NAPL contamination problems require analysis of the simultaneous movement of at least two fluid phases, NAPL and water. This is somewhat different than the traditional multiphase flow problem in hydrology, namely water movement in unsaturated soils. In the traditional treatment of unsaturated systems, the movement of air, the non‐aqueous phase, is not of interest, and its movement is ignored. Both NAPL‐water and air‐water systems are similar in that capillary forces, acting at the pore scale, usually dictate the pore‐scale distribution of each fluid phase. Pore‐scale fluid distributions then dictate the continuum‐scale properties such as relative saturation and relative permeability.
The auroral acceleration region: The world of beams, conies, cavitons, and other plasma exotica
Authors: Shelley E. G.
Published: February 2004

The aurora is the most visible manifestation of the energetic plasma in the Earth's magnetosphere and its interaction with the atmosphere. For a brief history of aurora and excellent references to early theories one is referred to Chapman [1967]. It has long been recognized that the direct cause of the aurora is the precipitation of energetic (keV) electrons and ions into the atmosphere leading to excitation of the ambient atmospheric gases which then emit light characteristic of their chemical composition. However, early in the history of space physics it was recognized that the auroral process was far more complex than the simple guiding of energetic charged particles from the sun to the atmosphere by the Earth's magnetic field lines. Balloon measurements of x‐rays with energies up to 70 keV in association with active aurorae [Winckler et al., 1958] implied that electrons were accelerated to energies large compared to those of solar wind electrons. Figure 1 shows schematically many of the features and processes that we now associate with active aurorae.
Recent advances in the representation of land‐atmosphere interactions in general circulation models
Authors: Avissar R.
Published: February 2004

The turbulent heat fluxes near the ground surface are strongly affected by the ability of the surface to redistribute the radiative energy absorbed from the sun and the atmosphere into sensible and latent heat. On a bare dry land, the absorption of this energy results in a relatively strong heating of the surface, which usually generates a strong turbulent sensible heat flux in the atmospheric surface layer and a large soil heat flux. In that case, there is no evaporation (i.e., no latent heat flux) and the Bowen ratio (i.e., the ratio of sensible to latent heat flux) is infinite. By contrast, in wet land, as is common in irrigated agricultural areas and/or after rain events, the incoming radiation is mostly used for evaporation. In that case, the turbulent sensible heat flux and the soil heat flux are usually much smaller than the latent heat flux. As a result, the Bowen ratio is close to zero. When the ground is covered by a dense vegetation, water is extracted mostly from the plant root zone by transpiration. Thus, latent heat flux is dominant even if the soil surface is dry, but as long as there is enough water available in the root zone and plants are not under stress conditions.
The geochemical evolution of the continental crust
Authors: Taylor Stuart Ross , McLennan Scott M.
Published: February 2004

A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post‐Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present‐day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond‐hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction‐related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post‐Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.
Recent advances associated with soil water in the unsaturated zone
Authors: Sposito Garrison
Published: February 2004

Perhaps the most compelling theme to emerge in the discipline of vadose‐zone hydrology during the past decade is the scale invariance of soil water behavior. The numerous practical hydrologic consequences of this type of symmetry were summarized recently in a volume edited by Hillel and Elrick [1990]. The vadose zone, i.e., the porous earth material below the land surface whose water saturation fluctuates, is spatially heterogeneous to a degree difficult to capture in routine measurements, and there is a need to reduce this complexity for purposes of prediction and management. Scale invariance offers a way to do that. One imagines a heterogeneous field soil to be the union of approximately homogeneous spatial domains, each of which can be associated with a small number of characteristic length scales that are related to the equilibrium properties and movement of water. Heterogeneity then simplifies into the spatial variability of these local length scales, while the generic functional relationships that describe soil water properties remain uniform. These generic functional relationships include not only the dependence of water content and hydraulic conductivity on matric potential (the analog of pressure head for water in the unsaturated zone), but also the partial differential equations of transport and the empirical flux laws they contain. Russo [1991] has summarized and illustrated this important simplifying approach in the context of stochastic models of nonsteady infiltration. Roth et al. [1990] have compiled both research practice and the practitioners' consensus on how scale‐invariance assists in the quantification and modeling of vadose‐zone hydrologic processes. Some of the same scaling issues, of course, arise in groundwater hydrology; their conceptual underpinnings were discussed in detail in a compilation of research position papers edited by Cushman [1990].
Dynamics of SLR tracked satellites
Authors: Marshall J. A. , Klosko S. M. , Ries J. C.
Published: February 2004

This report summarizes recent progress in orbit determination and the role of Satellite Laser Ranging (SLR) in advancing the state of its art. In the past four years, major improvements in our ability to position a near‐Earth SLR satellite within a well understood and stable geocentric frame have occurred. Key enabling factors are upgraded gravitational and non‐conservative force models and extensive advancement in tracking technologies. In combination, these capabilities allowed orbit analysts to reach their most ambitious goal, that of providing better than 4 cm RMS radial orbit knowledge in support of the joint USA/French TOPEX/POSEIDON Mission (T/P) (Fu et al., [1994]).
Theoretical and experimental studies of magnetic domains
Authors: Merrill Ronald T. , Halgedahl Susan L.
Published: February 2004

The primary purpose of this paper is to review recent contributions from North American scientists to theory and observations of magnetic domain structures in magnetic minerals common to rocks.

Although rock magnetism deals with all the magnetic properties of rocks, a major goal of rock magnetists has been to understand the origins and properties of remanent magnetization (RM). A remanent magnetization is defined as the magnetization that exists in the absence of an external magnetic field. The analyses of RMs (of which there are more than 20 kinds) have led to the numerous, and well‐known, successes of paleomagnetism.
Nearshore processes
Authors: Holman Rob
Published: February 2004

As wind‐generated waves propagate from the deep ocean toward the shore, the decreasing depths begin to influence near‐bottom orbital motions, modifying the wave kinematics. Through the resulting nonlinearities, energy is transferred from typical ocean periods of order 10 seconds to both lower and higher frequencies, until local spectra bear little resemblance to the offshore energy input. In turn, orbital velocities can cause transport of unconsolidated bottom sediments that may eventually result in changes in the beach topography. Thus, the nearshore system is usually considered composed of two components; the fluid forcing and the sediment response. At short time scales, the mobility of the sediment bed is considered to have negligible effect on the fluid dynamics and each component of the problem can be considered separately (i.e. fluid dynamics over a measured topography, and sediment transport under a measured local wave field). At longer scales, changes in bottom bathymetry can be substantial; the feedback between the two component can have a fundamental influence on system behavior.
Magnetic fabrics
Authors: Kodama Kenneth P.
Published: February 2004

Since Graham [1954; 1967] demonstrated in the 1950s and 1960s that magnetic anisotropy was a quick and non‐destructive way of measuring rock fabric, magnetic fabric measurements have become increasingly prevalent in the study of earth and environmental processes (see Table 1 for definitions of magnetic terms). The accomplishments of U.S. workers over the past 4 years show that the U.S. community continues to contribute to the growth and development of the field. In addition to the more traditional anisotropy of magnetic susceptibility (AMS) measurements work is branching out to include anisotropy of remanence studies, particularly anisotropy of anhysteretic remanence (AAR) [McCabe et al, 1985] and anisotropy of isothermal remanence (AIR), since these parameters can be tied directly to the remanence‐carrying grains in a rock and hence have relevance to the effects of magnetic fabric on natural remanent magnetization (NRM). Along with this shift is the realization, as a result of combining nonmagnetic observations (e.g.. scanning electron microscopy (SEM), finite strain measurements) with rock magnetic measurements (e.g.. hysteresis parameters), that magnetic mineral composition (paramagnetic and ferromagnetic) has an important control on magnetic anisotropy.
Melts and fluids: An overview of recent advances
Authors: Brenan James M.
Published: February 2004

Owing to their capacity to transport mass and heat melts and low viscosity fluids profoundly influence such global processes as planetary heat loss, large and small‐scale planetary differentiation as well as affecting the evolution of oceans and atmospheres. As such, these materials play a key role in the physical and chemical volution of Earth, the terrestrial planets and the meteorite parent bodies. In this context, a review chapter that deals exclusively with recent advances in our understanding of the composition, properties, origin and volution of melts and fluids is clearly relevant.1 Since 1991, a host of research advances has provided earth and planetary scientists with new and unique perspectives for understanding natural melts and low viscosity fluids. New instrumentation has provided the basis for several advances, and perhaps most notable is the development and application of reaction cells that allow measurement of melt or fluid properties in situ. Such devices have allowed workers to monitor properties at high pressure and temperature that are typically not preserved when samples are quenched to ambient conditions. In addition to the development of new machines, tried and true experimental and analytical technologies have also yielded significant new results on melts and fluids, largely as a result of their clever application to the solution of what had been longstanding problems in geochemistry and petrology. Although laboratory‐based measurements have provided the basis for many recent advances, it is also clear that the Earth still yields provocative samples for our study, and new insights have also been gained concerning the behavior of melts and fluids in natural processes based on recent documentation of previously unobserved melt and fluid compositions. Along with new technologies, clever experiments and unique samples, it is also notable that strides have been made in certain research areas enjoying a resurgence of activity following new and provocative developments. Provoked by controversial theoretical results, perhaps the most notable example of this has been an increased experimental effort aimed at evaluating whether equilibrium core segregation can account for the siderophile element abundances in Earth's upper mantle.
Substorm theories: United they stand, divided they fall
Authors: Erickson Gary M.
Published: February 2004

The magnetospheric substorm expansion involves the intermittent release of energy stored in the geomagnetic tail as a result of the interaction of Earth's magnetic field with the solar wind. Consensus on the timing and mapping of substorm features has permitted a synthesis of substorm models. Within the synthesis model the mechanism for onset of substorm expansion is still unknown. Possible mechanisms are: growth of an ion tearing mode, current disruption by a cross‐field current instability, and magnetosphere‐ionosphere coupling. While the synthesis model is consistent with overall substorm morphology, including near‐Earth onset, none of the onset theories, taken individually, appear to account for substorm expansion onset. A grand synthesis with unification of the underlying onset theories appears necessary.
Recalibration of the geomagnetic polarity timescale
Authors: Channell J.E.T.
Published: February 2004

The quest for improved time control, to estimate rates of geologic and paleoenvironmental processes, is central to the Earth sciences. Until the middle of this century, geochronology was based solely on macrofossil evolution, providing the largely unmodified definition of geologic stages. In the last few decades, with impetus from the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP), microfossil biostratigraphy has become increasingly important. Although new radiometric techniques have resulted in improved absolute age precision, the direct correlation of absolute ages to biozonations (and hence to geologic stage boundaries) remains poor. This is due to the difficulty of finding materials suitable for radiometric dating which can be directly correlated to biozonations. The bridge between biozonations and absolute ages and the interpolation between absolute ages are best accomplished, particularly for Cenozoic and late Mesozoic time, through the geomagnetic polarity timescale (GPTS). The template for Late Jurassic to Quaternary geomagnetic polarity is derived from oceanic magnetic anomalies. As demonstrated by Heirtzler et al. [1968], the GPTS can be constructed using oceanic magnetic anomaly record(s), assuming constant seafloor spreading rate(s), and interpolating between available radiometric ages. In the last quadrennium, there have been outstanding developments in radiometric dating and astrochronology which have greatly improved the absolute age control of the GPTS.
Upper atmospheric waves, turbulence, and winds: Importance for mesospheric and thermospheric studies
Authors: Killeen T. L. , Johnson R. M.
Published: February 2004

U.S. scientists interested in the physics and chemistry of the upper atmosphere (i.e., that region above the Earth's stratosphere, encompassing the mesosphere [∼50–100km], the mesopause region [∼85–100 km], the lower thermosphere [∼100–180km], and the upper thermosphere [∼180–1000km]) have increasingly come to recognize the importance of atmospheric waves of all types. Gravity/buoyancy waves, planetary waves, and tides pervade the upper atmosphere and are, in fact, ubiquitous features of all planetary atmospheres. Upper atmospheric waves generated by dynamical processes propagate vertically and horizontally, dissipate, interact nonlinearly, and profoundly influence the flows of momentum, energy, and constituents on a global basis. These waves also pass vertically through the various regions or “spheres” (troposphere, stratosphere, etc.) of the atmosphere and, in so doing, impel scientists in previously compartmentalized subdisciplines to assess the effects of the processes that couple these regions. For example, tropospheric gravity waves generated by air flow over topographic features can propagate vertically through the stratosphere and dissipate within the mesosphere [e.g., Fritts, 1994]. These “breaking” waves alter the global thermal and wind structures, producing a large mesopause temperature anomaly (cold summer pole, warm winter pole). The mesopause turbulence generated by breaking gravity waves on Earth is substantial and has been noted by astronauts during the gentle buffeting of the reentering Space Shuttle [A. England, private communication, 1994].
Marine geodynamics: Depth‐age revisited
Authors: McNutt Marcia K.
Published: February 2004

More than 20 years have passed since the subsidence of the seafloor away from midocean ridges was explained by the cooling of the lithosphere (Figure 1), a thermal boundary layer at the Earth's surface [Turcotte and Oxburgh, 1967], Although the general concept has proven quite sound and found much practical application, the details of the model have never been completely agreed upon. For example, if no process limits the growth of the thermal boundary layer, the seafloor should continue to deepen as the square root of lithospheric age. In contrast, the observations indicate that little deepening occurs for lithosphere aged greater than about 100 Ma (Ma = million years before present). Early on this fact was explained by supposing that there was a limit to the depth of cooling of the boundary layer, such that the lithosphere cools as a plate of finite thickness rather than as a semi‐infinite half space [McKenzie, 1967]. According to the plate model, the depth of old lithosphere approaches a maximum as the geothermal gradient in the plate reaches thermal equilibrium. However, this theoretical model merely provided a better fit to the observations without addressing the fundamental question of what process might limit the growth of the Earth's uppermost thermal boundary layer.
Atmospheric science in the laboratory: Progress 1991–1994
Authors: Hallett John
Published: February 2004

Laboratory study of atmospheric phenomena provides a controlled, steady environment which is usually difficult to obtain in the turbulent atmosphere. Boundary conditions for cloud formation such as homogeneous ice nucleation of water droplets near −40°C and cloud droplet formation on hygroscopic particles at less than a few percent above water saturation were first appreciated by careful laboratory experiments many years ago. These ideas are foundation‐stones of our understanding of atmospheric clouds. Such ideas came from careful laboratory studies of the behavior of particles under well controlled conditions.
Earth‐based remote sensing of solar system objects
Authors: Muhleman Duane O.
Published: February 2004

It can be said that remote sensing began with Galileo's invention of the telescope and covers the entire history of observational astronomy to this day. Recently, the subject has rapidly advanced with the development of observing techniques made possible by the invention of new detectors and telescopes over the entire electromagnetic spectrum from gamma rays to meter‐wavelength radio emission. Superbly quantitative measurements of distant planets, satellites, comets and asteroids are now routinely made using the techniques of visible, IR and microwave spectrometry in addition to radar mapping and broadband photometry in all bands. Beautiful results, often reaching the sublime, have been obtained with ingenious instruments aboard robotic spacecraft, in particular the Voyager and Magellan vehicles. Equally ingenious are observations and experiments made from Earthbound astronomical facilities operated by distant relatives of the great master and innovator. Here, we limit this 5‐year review to these latter efforts, partly because there have been few spacecraft successes in this period (except for Magellan), but also because our understanding of the solar system has continued to grow in this period with the use of Earthbased techniques. The Earth orbiting Hubble Telescope is now a magnificent success in the visible and near IR bands and is proving to be a powerful tool for solar system imaging, easily exceeding that of the Earthbound telescopes. This subject is too vast to review here. There is as yet no real progress to report on the search for other solar systems. These investigations await the development of arrayed telescopes in all wavelength bands, (such as the two Keck telescopes on Mauna Kea), a program that will probably remain “Earthbound” for the next decade or so. Preliminary work has been carried out with arrayed telescopes in the millimeter wavelength band. Many ideas and very preliminary results can be found in Beckwith and Sargent [1992], and Sargent and Beckwith [1993].
Remote sensing of comets and asteroids
Authors: McFadden L. A.
Published: February 2004

In the past Quadrennium, we have observed a fragmented comet collide with Jupiter, and discovered a population of objects of the order of hundreds of kilometers in diameter beyond the orbit of Neptune, a probable source region of short period comets. We are still in the age of discovery as studies of the solar system continue. The discovery of many small asteroids with the same surface composition as the second largest main belt asteroid, 4 Vesta, has opened our eyes to a pathway by which certain meteorites reach earth and by which asteroids collide and their orbits evolve. This evidence for delivery of meteorites was found by pushing existing observing techniques to their limits. An expanded capability for detecting returned radar echoes from asteroids permits the extraction of basic information about their position, shape, spin state, size and surface texture which is equivalent to that acquired from a flyby of a robotic spacecraft. We will never send spacecraft to as many asteroids as can be studied from the ground, so the contributions of ground‐based astronomy at all wavelengths provides insights into the chemical and physical diversity of the asteroids and comets. Remote sensing is carried out by ground‐based and Earth‐orbiting telescopic studies, and robotic spacecraft traveling through the solar system. In the past Quadrennium, we have viewed data returned from two main belt asteroid flybys missions which represent the transition of asteroids as targets of astronomical study t o subjects of geological study. Scientists using the Galileo spacecraft additionally made the serendipitous discovery of Dactyl, a satellite of 243 Ida, the first direct proof that satellites orbit asteroids. The significant advances discussed below have occurred because of 1) chance discovery (the discovery of Comet Shoemaker‐Levy 9 and Dactyl), 2) the interaction between theoretical predictions and observational programs (discovery of trans‐ Neptunian objects, 3) the use of available technology pushed to its limits of sensitivity (discovery of small asteroids with the same surface composition as 4 Vesta,) and 4) the consequence of technical upgrading of facilities. The latter provides fundamental facts about the diverse physical characteristics of small solar system bodies. A prime example is advancing knowledge derived from radar astronomy. Our new knowledge has generated a modified view of the number and nature of the small bodies in the solar system and how they formed and evolved through time.
Mineral physics of the mantle
Authors: Stixrude Lars
Published: February 2004

The last few years have seen intense interest in the global environment and climate change, and with it an increasing appreciation for the interactions between the atmosphere, biosphere, oceans and the solid earth. In solid earth geophysics, the traditional boundaries between the earth's fluid and solid spheres have been breached by the growing body of evidence that they may physically communicate on a massive scale, that atmospheric constituents, under certain conditions, may be transported to and stored within the deepest parts of the earth. Of course there has for some time been an appreciation for influence of mantle dynamics, the driving force of plate tectonics, volcanism, and seismicity, on surface processes. However, perhaps nothing illustrates the essential connections better than visualizing, now with some experimental and observational support, a tropospheric molecule, transported through sedimentary and tectonic agents 3000 km to the core mantle boundary, only to rise again, perhaps many millions of years later in a volcanic eruption.