Showing papers in "Journal of Geophysical Research in 1992"

Relationship between wind speed and gas exchange over the ocean

Abstract: Relationships between wind speed and gas transfer, combined with knowledge of the partial pressure difference of CO2 across the air-sea interface are frequently used to determine the CO2 flux between the ocean and the atmosphere. Little attention has been paid to the influence of variability in wind speed on the calculated gas transfer velocities and the possibility of chemical enhancement of CO2 exchange at low wind speeds over the ocean. The effect of these parameters is illustrated using a quadratic dependence of gas exchange on wind speed which is fit through gas transfer velocities over the ocean determined by the natural-14C disequilibrium and the bomb-14C inventory methods. Some of the variability between different data sets can be accounted for by the suggested mechanisms, but much of the variation appears due to other causes. Possible causes for the large difference between two frequently used relationships between gas transfer and wind speed are discussed. To determine fluxes of gases other than CO2 across the air-water interface, the relevant expressions for gas transfer, and the temperature and salinity dependence of the Schmidt number and solubility of several gases of environmental interest are included in an appendix.

GPS Meteorology: Remote Sensing of Atmospheric Water Vapor Using the Global Positioning System

Abstract: We present a new approach to remote sensing of water vapor based on the global positioning system (GPS). Geodesists and geophysicists have devised methods for estimating the extent to which signals propagating from GPS satellites to ground-based GPS receivers are delayed by atmospheric water vapor. This delay is parameterized in terms of a time-varying zenith wet delay (ZWD) which is retrieved by stochastic filtering of the GPS data. Given surface temperature and pressure readings at the GPS receiver, the retrieved ZWD can be transformed with very little additional uncertainty into an estimate of the integrated water vapor (IWV) overlying that receiver. Networks of continuously operating GPS receivers are being constructed by geodesists, geophysicists, government and military agencies, and others in order to implement a wide range of positioning capabilities. These emerging GPS networks offer the possibility of observing the horizontal distribution of IWV or, equivalently, precipitable water with unprecedented coverage and a temporal resolution of the order of 10 min. These measurements could be utilized in operational weather forecasting and in fundamental research into atmospheric storm systems, the hydrologic cycle, atmospheric chemistry, and global climate change. Specially designed, dense GPS networks could be used to sense the vertical distribution of water vapor in their immediate vicinity. Data from ground-based GPS networks could be analyzed in concert with observations of GPS satellite occultations by GPS receivers in low Earth orbit to characterize the atmosphere at planetary scale.

First‐ and second‐order patterns of stress in the lithosphere: The World Stress Map Project

Abstract: To date, more than 7300 in situ stress orientations have been compiled as part of the World Stress Map project. Of these, over 4400 are considered reliable tectonic stress indicators, recording horizontal stress orientations to within <±25°. Remarkably good correlation is observed between stress orientations deduced from in situ stress measurements and geologic observations made in the upper 1–2 km, well bore breakouts extending to 4–5 km depth and earthquake focal mechanisms to depths of ∼20 km. Regionally uniform stress orientations and relative magnitudes permit definition of broad-scale regional stress patterns often extending 20–200 times the approximately 20–25 km thickness of the upper brittle lithosphere. The “first-order” midplate stress fields are believed to be largely the result of compressional forces applied at plate boundaries, primarily ridge push and continental collision. The orientation of the intraplate stress field is thus largely controlled by the geometry of the plate boundaries. There is no evidence of large lateral stress gradients (as evidenced by lateral variations in stress regime) which would be expected across large plates if simple resistive or driving basal drag tractions (parallel or antiparallel to absolute motion) controlled the intraplate stress field. Intraplate areas of active extension are generally associated with regions of high topography: western U.S. Cordillera, high Andes, Tibetan plateau, western Indian Ocean plateau. Buoyancy stresses related to crustal thickening and/or lithospheric thinning in these regions dominate the intraplate compressional stress field due to plate-driving forces. These buoyancy forces are just one of several categories of “second-order” stresses, or local perturbations, that can be identified once the first-order stress patterns are recognized. These second-order stress fields can often be associated with specific geologic or tectonic features, for example, lithospheric flexure, lateral strength contrasts, as well as the lateral density contrasts which give rise to buoyancy forces. These second-order stress patterns typically have wavelengths ranging from 5 to 10+ times the thickness of the brittle upper lithosphere. A two-dimensional analysis of the amount of rotation of regional horizontal stress orientations due to a superimposed local stress constrains the ratio of the magnitude of the horizontal regional stress differences to the local uniaxial stress. For a detectable rotation of 15°, the local horizontal uniaxial stress must be at least twice the magnitude of the regional horizontal stress differences. Examples of local rotations of SHmax orientations include a 75°–85° rotation on the northeastern Canadian continental shelf possibly related to margin-normal extension derived from sediment-loading flexural stresses, a 50°–60° rotation within the East African rift relative to western Africa due to extensional buoyancy forces caused by lithospheric thinning, and an approximately 90° rotation along the northern margin of the Paleozoic Amazonas rift in central Brazil. In this final example, this rotation is hypothesized as being due to deviatoric compression oriented normal to the rift axis resulting from local lithospheric support of a dense mass in the lower crust beneath the rift (“rift pillow”). Estimates of the magnitudes of first-order (plate boundary force-derived) regional stress differences computed from modeling the source of observed local stress rotations magnitudes can be compared with regional stress differences based on the frictional strength of the crust (i.e., “Byerlee's law”) assuming hydrostatic pore pressure. The examples given here are too few to provide a definitive evaluation of the direct applicability of Byerlee's law to the upper brittle part of the lithosphere, particularly in view of uncertainties such as pore pressure and relative magnitude of the intermediate principal stresses. Nonetheless, the observed rotations all indicate that the magnitude of the local horizontal uniaxial stresses must be 1–2.5+ times the magnitude of the regional first-order horizontal stress differences and suggest that careful evaluation of such local rotations may be a powerful technique for constraining the in situ magnitude stress differences in the upper, brittle part of the lithosphere.

A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic

Abstract: We have constructed a magnetic polarity time scale for the Late Cretaceous and Cenozoic based on an analysis of marine magnetic profiles from the world's ocean basins. This is the first time, since Heirtzler et al. (1968) published their time scale, that the relative widths of the magnetic polarity intervals for the entire Late Cretaceous and Cenozoic have been systematically determined from magnetic profiles. A composite geomagnetic polarity sequence was derived based primarily on data from the South Atlantic. Anomaly spacings in the South Atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles. Fine-scale information was derived from magnetic profiles on faster spreading ridges in the Pacific and Indian Oceans and inserted into the South Ariantic sequence. Based on the assumption that spreading rates in the South Atlantic were smoothly varying but not necessarily constant, a time scale was generated by using a spline function to fit a set of nine age calibration points

Oceanic crustal thickness from seismic measurements and rare earth element inversions

Abstract: Seismic refraction results show that the igneous section of oceanic crust averages 7.1±0.8 km thick away from anomalous regions such as fracture zones and hot-spots, with extremal bounds of 5.0–8.5 km. Rare earth element inversions of the melt distribution in the mantle source region suggest that sufficient melt is generated under normal oceanic spreading centers to produce an 8.3±1.5 km thick igneous crust. The difference between the thickness estimates from seismics and from rare earth element inversions is not significant given the uncertainties in the mantle source composition, though it is of the magnitude that would be expected if partial melt fractions of about 1% remain in the mantle and are not extracted to the overlying crust. The inferred igneous thickness increases to 10.3±1.7 km (seismic measurements) and 10.7±1.6 km (rare earth element inversions) where spreading centers intersect the regions of hotter than normal mantle surrounding mantle plumes. This is consistent with melt generation by decompression of the hotter mantle as it rises beneath spreading centers. Maximum inferred melt volumes are found on aseismic ridges directly above the central rising cores of mantle plumes, and average 20±1 and 18±1 km for seismic profiles and rare earth element inversions respectively. Both seismic measurements and rare earth element inversions show evidence for variable local crustal thinning beneath fracture zones, though some basalts recovered from fracture zones are indistinguishable geochemically from those generated on normal ridge segments away from fracture zones. This is consistent with a model where the melt generated beneath spreading ridges is redistributed to intrusive centers along the ridge axis, from where it may flow laterally along the axis at crustal or surface levels. The melt may sometimes flow into the bathymetric lows associated with fracture zones. Oceanic crust created at very slow-spreading ridges, and in regions adjacent to some continental margins where rifting was initially very slow, exhibits anomalously thin crust from seismic measurements and unusually small amounts of melt generation from rare earth element inversions. We attribute the decreased mantle melting on very slow-spreading ridges to the conductive heat loss that enables the mantle to cool as it rises beneath the rift.

A model of nitrous oxide evolution from soil driven by rainfall events: 1. Model structure and sensitivity

Abstract: Simulations of N2O and CO2 emissions from soils were conducted with a rain-event driven, process-oriented model (DNDC) of nitrogen and carbon cycling processes in soils. The magnitude and trends of simulated N2O (or N2O + N2) and CO2 emissions were consistent with the results obtained in field experiments. The successful simulation of these emissions from the range of soil types examined demonstrates that the DNDC will be a useful tool for the study of linkages among climate, soil-atmosphere interactions, land use, and trace gas fluxes.

A bidirectional reflectance model of the Earth's surface for the correction of remote sensing data

Abstract: A surface bidirectional reflectance model has been developed for the correction of surface bidirectional effects in time series of satellite observations, where both sun and viewing angles are varying. The model follows a semiempirical approach and is designed to be applicable to heterogeneous surfaces. It contains only three adjustable parameters describing the surface and can potentially be included in an algorithm of processing and correction of a time series of remote sensing data. The model considers that the observed surface bidirectional reflectance is the sum of two main processes operating at a local scale: (1) a diffuse reflection component taking into account the geometrical structure of opaque reflectors on the surface, and shadowing effects, and (2) a volume scattering contribution by a collection of dispersed facets which simulates the volume scattering properties of canopies and bare soils. Detailed comparisons between the model and in situ observations show satisfactory agreement for most investigated surface types in the visible and near-infrared spectral bands. The model appears therefore as a good candidate to reduce substantially the undesirable fluctuations related to surface bidirectional effects in remotely sensed multitemporal data sets.

Tomographic imaging of P and S wave velocity structure beneath northeastern Japan

Abstract: The seismic body wave tomography method has been improved and extended to adapt to a general velocity structure with a number of complexly shaped seismic velocity discontinuities (SVDs) and with three-dimensional variations in the velocities in the modeling space. An efficient three dimensional ray tracing algorithm which iteratively uses the pseudobending technique and Snell's law is developed. The large and sparse system of observation equations is solved by using the LSQR algorithm. This method is applied to 18,679 arrival times from 470 shallow and intermediate-depth earthquakes in order to study P and S wave tomographic images beneath northeastern Japan. In addition to first P and S wave arrivals, clear later arrivals of SP waves converted at the Moho and PS and SP waves converted at the upper boundary of the subducted Pacific plate (UBPP) are also used in the inversion. The UBPP, Conrad and Moho are taken as three SVDs, and their depth distributions obtained by previous studies are used. High-resolution P and S wave tomographic images down to a depth of 200 km have been determined. Large velocity variations amounting to 6% for P wave and 10% for S wave are revealed in the crust and upper mantle. In the crust low-velocity (low-V) zones exist beneath active volcanoes. In the upper mantle the low-V zones dip toward the west from the volcanic front. A high-velocity (high- V) zone corresponding to the subducted Pacific plate is clearly delineated. Most earthquakes in the lower plane of the double-planed deep seismic zone are found to occur in relatively high-V areas. The obtained tomographic images are also found to explain other seismological observations well.

Bursty Bulk Flows in the Inner Central Plasma Sheet

Abstract: High-speed flows in the inner central plasma sheet (first reported by Baumjohann et al. (1990) are studied, together with the concurrent behavior of the plasma and magnetic field, by using AMPTE/IRM data from about 9 to 19 R(E) in the earth magnetotail. The conclusions drawn from the detailed analysis of a representative event are reinforced by a superposed epoch analysis applied on two years of data. The high-speed flows organize themselves in 10-min time scale flow enhancements called here bursty-bulk flow (BBF) events. Both temporal and spatial effects are responsible for their bursty nature. The flow velocity exhibits peaks of very large amplitude with a characteristic time scale of the order of a minute, which are usually associated with magnetic field dipolarizations and ion temeperature increases. The BBFs represent intervals of enhanced earthward convection and energy transport per unit area in the y-z GSM direction of the order of 5 x 10 exp 19 ergs/R(E-squared).

An ISEE/Whistler model of equatorial electron density in the magnetosphere

Abstract: Attention is given to an empirical model of equatorial electron density in the magnetosphere covering the L range 2.25-8. Although the model is primarily intended for application to the local time interval 00-15 MLT, a way to extend the model to the 15-24-MLT period is presented. The model describes, in piecewise fashion, the 'saturated' plasmasphere, the region of steep plasmapause gradients, and the plasma trough. Within the plasmasphere the model profile can be expressed as logne - Sigma-xi, where x1 = -0.3145L + 3.9043 is the principal or 'reference' term, and additional terms account for: a solar cycle variation with a peak at solar maximum; an annual variation with a December maximum; and a semiannual variation with equinoctial maxima.

Seismicity and shape of the subducted Nazca Plate

Abstract: An updated compilation of earthquake locations and focal mechanism solutions from the International Seismological Centre and Preliminary Determination of Earthquakes is the basis of a comprehensive study of the geometry of the Wadati-Benioff zone beneath western South America. The new data support previous mapping of a sharp flexure rather than a tear in the subducted Nazca plate beneath southern Peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with ∼30° dip at latitude 33°S. In contrast, the transition from 30° slab dip beneath Bolivia to a nearly horizontal dip in the region between 28°S–32°S is more gradual, occurring over several hundred kilometers of along-strike distance between 20°S and 32°S. This southward flattening corresponds to a broadening of a horizontal, benchlike part of the subducted plate formed between 100 and 125 km depth. The transition in continental tectonic style near 27°S–28°S, from a wide, volcanically active plateau to a narrow, nonvolcanic cordillera, appears not to be associated with the main slab flattening, which begins to the north of these latitudes, but with a more abrupt change in curvature of the subducted slab, from convex upward to concave upward, immediately below the plate boundary interface. The concept of Gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the South American plate. We hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin. Focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses. Results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths.

Mid-ocean ridge magma chambers

Abstract: Geophysical evidence precludes the existence of a large, mainly molten magma chamber beneath portions of the East Pacific Rise (EPR). A reasonable model, consistent with these data, involves a thin (tens to hundreds of meters high), narrow (<1–2 km wide) melt lens overlying a zone of crystal mush that is in turn surrounded by a transition zone of mostly solidified crust with isolated pockets of magma. Evidence from the superfast spreading portion of the EPR suggests that the composition of the melt lens is mainly moderately fractionated ferrobasalt. These results have important implications for magmatic processes occurring beneath mid-ocean ridges and are consistent with a model that effectively separates the processes of magma mixing and fractionation into different parts of a composite magma chamber. Magma mixing, as evidenced by disequilibrium relations between host liquids and included phenocrysts, is especially apparent in samples from low magma supply ridges and probably mainly arises from interactions between crystals of the mush zone and new injections of primitive magma rising out of the mantle. Magmatic differentiation beneath mid-ocean ridges occurs in two parts. Migration of melts through the transition and mush zones can produce chemical trends consistent with in situ fractionation processes. Segregation of melt into molten horizons near the top of a composite magma chamber promotes the more extensive differentiation characteristic of fast spreading ridges. The optimum conditions for the formation of highly differentiated abyssal lavas is where small, discontinuous melt lenses occur, such as at intermediate spreading rates, in the vicinity of propagating rifts, and near ridge offsets at fast spreading ridges. Along-axis homogenization of subaxial magma is inhibited by the thin, high aspect ratio of the melt lens and by the high viscosities expected in the mush and transition zones. Low magma supply ridges are unlikely to be underlain by eruptable magma in a steady state sense, and eruptions at slow spreading ridges are likely to be closely coupled in time to injection events of new magma from the mantle. Extensional events at high magma supply ridges, which are more likely to be underlain by significant volumes of low-viscosity melt, can produce eruptions without requiring associated injection events. The critical magma supply necessary for the development of a melt lens near the top of a composite magma chamber is similar to that of normal ridges spreading at rates of about 50–70 mm/yr, a rate approximately corresponding to that marking an abrupt change in the morphology and gravity signal at the ridge axis. A composite magma chamber model can explain several previous enigmas concerning mid-ocean ridge basalts, including why slow spreading ridges dominantly erupt a narrow range of relatively undifferentiated lavas, why magma mixing is most evident in lavas erupted from slow spreading ridges, why fast spreading ridges erupt a wide range of generally more differentiated compositions, why bimodal lava populations occur in the vicinity of some propagating rifts, and how along-axis geochemical segmentation can occur at a scale shorter than the major tectonic segmentation of ridge axes.

Ozone precursor relationships in the ambient atmosphere

Abstract: The concentrations of ozone, nitrogen oxides, and nonmethane hydrocarbons measured near the surface in a variety of urban, suburban, rural, and remote locations are analyzed and compared in order to elucidate the relationships between ozone, its photochemical precursors, and the sources of these precursors. While a large gradient is found among remote, rural, and urban/suburban nitrogen oxide concentrations, the total hydrocarbon reactivity in all continental locations is found to be comparable. Apportionment of the observed hydrocarbon species to mobile and stationary anthropogenic sources and biogenic sources suggests that present-day emission inventories for the United States underestimate the size of mobile emissions. The analysis also suggests a significant role for biogenic hydrocarbon emissions in many urban/suburban locations and a dominant role for these sources in rural areas of the eastern United States. As one moves from remote locations to rural locations and then from rural to urban/suburban locations, ozone and nitrogen oxide concentrations tend to increase in a consistent manner while total hydrocarbon reactivity does not.

Line-by-Line Calculations of Atmospheric Fluxes and Cooling Rates: Application to Water Vapor

Abstract: A model for the accelerated calculation of clear sky fluxes based on the line-by-line radiance model FASCODE has been developed and applied to the calculation of cooling rates for atmospheric water vapor. The model achieves computational accuracies for the longwave upwelling and downwelling fluxes of the order of 0.2%, an accuracy well within current limitations imposed by uncertainties in the spectral parameters, the line shape, and the associated continua. For the same treatment of line shape, the Voigt profile with a 10 cm−1 cutoff and no continuum, the results from the present model are in acceptable agreement with those from two other line-by-line models reported as part of the intercomparison of radiation codes used in climate models (ICRCCM). For this line profile and the mid-latitude summer atmosphere, the largest difference between the results from our model and the Goddard Laboratory for Atmospheres (GLA) model occurs for the downwelling flux at the surface, with the present model providing a value greater than that from GLA. The differences are generally consistent with greater atmospheric opacity from the present model, attributable to the inclusion of a self-broadening component for the half width for water and to finer spectral sampling in the lower-pressure regime. Utilization of the line shape and associated continuum model included in FASCODE gives results that are significantly different from those provided by the other two models. This radiance model, including contributions from the foreign continuum as well as from a modified self-continuum, has received extensive validation against measured radiance spectra, an example of which is provided. For the mid-latitude summer atmosphere the principal contribution from the foreign continuum occurs in the upper troposphere in the 250–350 cm−1 spectral region, whereas the contribution from the self-continuum, dependent on the square of the water vapor density, is greatest in the lower troposphere. For the mid-latitude summer atmosphere the foreign continuum contributes 0.4 Kd−1 or 20% to the cooling in the upper troposphere and the self-continuum contributes 1.9 K d−1 to the cooling rate at the surface due to water vapor. The latter is 0.17 K d−1 less than the cooling rate from the GLA model which is principally due to a modification of the self-continuum. A significant result that has developed from the present work is the insight into atmospheric radiative processes provided by spectral profiles of the cooling rate. In the spectral domain there exists a mapping between the altitude and the molecular absorption strength as weighted by the Planck function. The extremely high correlation between the outgoing spectral radiance at the top of the atmosphere and the spectral cooling rate profile suggests that measurement of the outgoing spectral radiance can provide important information about atmospheric state that is not available from spectrally integrated quantities. Our results also indicate the critical importance of the spectral region from 100 to 600 cm−1 for the radiative transfer associated with atmospheric water vapor.

A simple lightning parameterization for calculating global lightning distributions

Abstract: A simple parameterization has been developed to simulate global lightning distributions. Convective cloud top height is used as the variable in the parameterization, with different formulations for continental and marine thunderstorms. The parameterization has been validated using two lightning data sets: one global and one regional. In both cases the simulated lightning distributions and frequencies are in very good agreement with the observed lightning data. This parameterization could be used for global studies of lightning climatology; the earth's electric circuit; in general circulation models for modeling global lightning activity, atmospheric NO(x) concentrations, and perhaps forest fire distributions for both the present and future climate; and, possibly, even as a short-term forecasting aid.

Subducting slabs stagnant in the mantle transition zone

Abstract: The P wave velocity structure beneath the Western Pacific is found from the International Seismological Center first arrival data. Special attention was paid to the deep structure beneath the Wadati-Benioff zone. We discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks. This way of discretization minimizes a problem with tomographic studies of regional scale: difficulty in making corrections for the effects outside the region of interest. Our solution is iterative with the alternate step of the relocation of earthquakes, using the whole mantle model of Inoue et al. (1990) as a starting model. A first-order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model. The essential result depends little on whether the reference spherical model is smooth or discontinuous near 400- and 670-km depths. We examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere. The resolution is depth dependent but in general good enough to see the slab configuration beneath the Southern Kurile-Japan-Izu-Bonin arcs and the Java arc. It is relatively poor beneath the Northern Kurile and Mariana arcs. The seismic image of subducting slab beneath the Southern Kurile to Bonin arcs bends to subhorizontal near the leading edge of the Wadati-Benioff zone and extends continentward over a distance of more than 1000 km. The subhorizontal portion of the slab connects a high-velocity blob to the bottom that reaches a depth of at least 800 km across the 670-km discontinuity under the Japan arc. Although the image of the Java slab directly penetrates the 670-km discontinuity, it then bends to a shallow dip with a considerable spread, reaching a depth of about 1200 km. These results suggest that descending slabs of lithosphere in the Western Pacific tend to be stagnant in the transition zone under a subtle control of the 670-km discontinuity. Although stagnant slab materials eventually descend into the lower mantle, they no longer maintain their original configuration below the 670-km discontinuity.

Evidence of the barrier layer in the surface layer of the tropics

Abstract: Comparisons between isothermal depth to the top of the thermocline, and the mixed layer depth based on a ot criterion were undertaken for the tropical world oceans. In three equatorial regions, a shallower mixed layer than isothermal layer occurs, implying the presence of a strong halocline above the thermocline. This distance separating the top of the thermocline and the bottom of the mixed layer is referred to as the "barrier layer", in relation to its impediment to vertical heat flux out of the base of the mixed layer. Different mechanisms are responsible for maintaining the barrier layer in each of the three regions. In the western equatorial Pacific Ocean a salinity budget confirmed that heavy local precipitation most likely results in the isothermal but salt-stratified layer. In the northwest equatorial Atlantic, it is hypothesized that high salinity waters are subducted at the subtropics during winter and advected westward as a salinity maximum in the upper layers of the tropics, resulting in the barrier layer. In the eastern equatorial Indian Ocean, monsoonal related rainfall and river runoff contribute significantly to the freshwater flux, producing salt stratification in the surface. These results suggest the need to include the effects of salinity stratification when determining mixed layer depth.

Physical model of source region of subduction zone volcanics

Abstract: The thermal structure of a generic subduction zone is investigated to elucidate the source region of subduction zone volcanics. The steady state thermal field is evaluated for a model subduction zone where the plates are prescribed by kinematic boundary conditions, such that the subducting slab induces a flow in the mantle wedge. The resulting model suggests that the oceanic crust of the downgoing slab is not melted extensively, if at all, and hence is not the source of subduction zone magmatism (with the possible exception of the special case of very young oceanic crust). The temperature in the mantle wedge is high enough to produce melting at the amphibole-buffered peridotite solidus. It is proposed that the combination of vertical motion of water as a free phase and the transport of hydrous phases (e.g., amphiboles) by the slab-induced mantle wedge flow lead to the net transport of water being horizontal, across the mantle wedge from the slab. Provided the subducting oceanic crust enters the asthenosphere at a velocity > 6(±2) cm/yr, the mantle wedge will be hot enough at the limit of the lateral water transport mechanism to generate melting at the amphibole-buffered solidus. The model was then extended to include the effect of localized sources of buoyancy (melt, residue, etc.) as a stationary body force, to investigate the possibility of reversing the slab-induced flow. Best estimates of the buoyancy sources and appropriate viscosity in the wedge suggest that there is likely to be only a weak modulation of the slab-induced flow unless the slab and wedge are locally decoupled, for instance by shear heating, the presence of water, or dehydration/hydration reactions. If there is decoupling, then it is possible for there to be an appreciable reversal of the slab-induced flow. Such an appreciable reversal of flow, if it persists, leads to cooling of the mantle wedge. Hence flow reversal cannot be a steady state mechanism. Instead it would lead to a cycle in the melting with a period of O(1 m.y.). The time dependence of a model with appreciable flow reversal would be reinforced by the need to clear the wedge of infertile material.

Oblique plate convergence, slip vectors, and forearc deformation

Abstract: Slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis, suggesting that oblique convergence is taken up by partial decoupling. Decoupling means that a component of arc-parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench. Partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike-slip faults that are parallel to the trench and to each other and, starting with a force equilibrium condition, a relationship between the obliquity and the earthquake slip vector orientation is derived. Assuming that either fault slips when shear stress on it reaches a yield stress, oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle. For obliquity at or greater than this angle the stress on the strike-slip fault is large enough to start it slipping, and when both faults are active, the arc-parallel motion of the forearc deflects the slip vector back toward the trench-normal. If we assume that continued slip on either fault occurs at constant stress (but the two faults can be at different stresses), the slip vector will maintain a constant angle relative to the trench-normal even when obliquity is larger than the critical angle. This limiting angle of the slip vector, called ψmax (measured relative to the trench-normal), is simply the arcsine of the ratio of the shear forces resisting slip on the strike-slip and thrust faults. A consequence is that when the obliquity exceeds ψmax the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates. Slip vectors at the Java trench southwest of Sumatra show the relationship clearly with ψmax =20°±5°, while slip vectors at the Aleutian trench show the relationship less clearly with ψmax=25° to 45°. The greater angle at the Aleutian trench suggests that the upper plate is stronger in the Aleutian arc (relative to the thrust fault) than in the Sumatran arc, consistent with the Sumatran arc being continental and having a well-developed strike-slip fault while the Aleutian arc is oceanic and without a clear transcurrent fault. Slip vectors at the Philippine trench which, like Sumatra, has a large strike-slip fault inboard of it, tend to stay within 25° of the trench-normal when obliquity is as large as 50°. If obliquity exceeds ψmax and continues to increase along a subduction zone, the rate of motion of the forearc relative to the upper plate will vary with obliquity, in which case the forearc sliver should extend or contract parallel to the arc. From the geometry of modern island arcs, arc-parallel extension should be the more common and has been hypothesized for both Sumatra and the Aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations. From estimates of ψmax and the arc-parallel gradients in obliquity, arc-parallel strain rates are estimated to be 1 to 3×10−8/yr for the Sumatran forearc, 2 to 6×10−8/yr for the Aleutian forearc, and 0.3 to 3×10−8/yr for the Philippine forearc. Oblique convergence and subsequent arc-parallel extension, if accompanied by crustal thinning, may provide an important yet little appreciated mechanism for bringing high-grade metamorphic rocks to the surface of subduction complexes.

Paleomagnetic constraints on the geodynamic history of the major blocks of China from the Permian to the present

Abstract: Paleomagnetic study of China and its environs has been the center of a major international effort for the last 10 years. In this paper, we critically review all available paleomagnetic poles of Upper Permian to Tertiary age from the main blocks of China with the goal of placing constraints on models of the formation and the subsequent deformation of the region. After selecting “reliable” poles by applying objective criteria, we divide our analysis into first-order (motions of blocks) and second order (deformation within blocks). For first order analysis, apparent polar wander paths are constructed for the major blocks. We discuss the compatibilities and contradictions between the geological and paleomagnetic records. A sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented. In general, the major blocks were probably in contact throughout the Permian and Triassic, but the Jurassic was the key age during which most of the movement toward China's present configuration took place. Our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events. During the Cretaceous, Chinese poles agree with poles from other continents transferred onto Eurasia. At the second order, we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region, indicating that much of China has been affected by small (< 20°) differential rotations. This we interpret as deformation caused in part by the extrusion of the Chinese blocks away from the Indian collision. The complete annotated list of poles is given as an appendix.

A land-surface hydrology parameterization with subgrid variability for general circulation models

Abstract: Most of the existing generation of general circulation models (GCMs) use so-called bucket algorithms to represent land-surface hydrology. Biosphere-atmosphere models that include the transfer of energy, mass, and momentum between the atmosphere and the land surface are a recent alternative to this highly simplified representation of the land surface in GCMs. These models require estimation of a large number of parameters for which parameter estimation methods and supporting data remain to be developed. We describe a more incremental approach to generalizing the bucket representation of land-surface hydrology based on a model that represents the variation in infiltration capacity within a GCM grid cell. The variable infiltration capacity (VIC) model requires estimation of three parameters: an infiltration parameter, an evaporation parameter, and a base flow recession coefficient. The VIC model was explored through direct comparisons with the Geophysical Fluid Dynamics Laboratory (GFDL) bucket model for the French Broad River, North Carolina, and via sensitivity analysis for the GFDL R30 grid cell which contains the French Broad River. Generally, the bucket model runoff had much greater variability than the historic streamflows for short time scales (e.g., 1 day); the VIC model was much more similar to the observed flows in this respect. The results also showed that the bucket model tended to have unrealistically high short-term variability. The sensitivity analysis showed that the base flow parameter exerted the greatest influence on both the mean and variability of most of the hydrologic variables, especially winter runoff, summer evaporation, and summer and winter soil moisture.

Delta-Eddington approximation for solar radiation in the NCAR community climate model

Abstract: Motivated by the desire for a more flexible and general solar radiation calculation in the NCAR community climate model (CCM), the δ-Eddington approximation has been employed in the CCM version 2 (CCM2). Eighteen spectral intervals span the solar spectrum from 0.2 to 5.0 μm. Absorption parameterizations for H2O, O3, CO2, and O2 were developed by making use of the latest theoretical calculations. Water droplet scattering and absorption are parameterized as shown by Slingo (1989). An accurate and efficient convolution of the H2O vapor spectrum with water droplet clouds is presented that yields good agreement with available line-by-line (LBL) calculations for single-layer clouds. A simple and efficient method to simulate partial cloud cover and cloud overlap is included. The simulated albedo-solar zenith angle dependence agrees very well with adding/doubling scattering calculations. The CCM2 δ-Eddington method will make possible many interesting applications of CCM2 in the years to come.

Rock magnetism of Late Neogene and Pleistocene deep-sea sediments: Relationship to sediment source, diagenetic processes, and sediment lithology

Abstract: We use bivariate scatter plots to illustrate variations in selected rock magnetic properties (low-field susceptibility, anhysteretic and isothermal remanence) of late Neogene and Pleistocene deep-sea sediments from 16 sites in the Arctic Ocean, North Atlantic, equatorial Atlantic and North Pacific Ocean, and the Arabian Sea. Our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology, geographical area, and the dominant mode of terrigenous sedimentation, which at these sites is via ice-rafting, via bottom currents, or via eolian processes. Overall, correlations between sediment magnetic properties and gross lithology is poor, and factors such as the source and transport path of terrigenous sediment (and detrital magnetic minerals), together with the action of reductive diagenetic processes, are the major controls on the magnetic properties. On the bivariate scatter plots, sites with major ice-rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations, relatively coarse ferrimagnetic grain-sizes, and scattered sample point distributions; in contrast, sites where we infer significant bottom-current supply of terrigenous material have tightly grouped sample point distributions. Carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration, caused by glacial-interglacial fluctuations in carbonate accumulation and eolian activity. Sediments containing significant volcanogenic material have high concentrations of relatively coarse-grained ferrimagnetic material. Reductive diagenesis is a significant determinant of sediment magnetic properties in high-productivity areas and has the effect of preferentially removing the fine-grained ferrimagnetic fraction, causing a coarsening of the ferrimagnetic grain-size distribution and a rise in the antiferromagnetic:ferrimagnetic ratio.

A model of nitrous oxide evolution from soil driven by rainfall events: 2. Model applications

Abstract: Simulations of nitrous oxide (N2O) and carbon dioxide (CO2) emissions from soils were carried out with a rain-event model of nitrogen and carbon cycling processes in soils (Li et al., this issue). Model simulations were compared with five field studies: a 1-month denitrification study of a fertilized grassland in England; a 2-month study of N2O emissions from a native and fertilized grassland in Colorado; a 1-year study of N2O emissions from agricultural fields on drained, organic soils in Florida; a 1-year study of CO2 emissions from a grassland in Germany; and a 1-year study of CO2 emissions from a cultivated agricultural site in Missouri. The trends and magnitude of simulated N2O (or N2O + N2) and CO2 emissions were consistent with the results obtained in field experiments. The successful simulation of nitrous oxide and carbon dioxide emissions from the wide range of soil types studied indicates that the model, DNDC, will be a useful tool for studying linkages among climate, land use, soil-atmosphere interactions, and trace gas fluxes.

Cycling of dissolved and particulate organic matter in the open ocean

Abstract: Author(s): Druffel, Ellen R. M; Williams, Peter M; Bauer, James E; Ertel, John R | Abstract: Radiocarbon (Δ14C), δ13C, bulk carbon and organic constituent concentration measurements are presented for dissolved and particulate carbon pools from the North Central Pacific Ocean (NCP) and the Sargasso Sea (SS). We operationally define three overlapping pools of dissolved organic carbon (DOC): (1) DOC that is oxidizable by UV radiation (DOCuv); (2) “extra” DOC measured by Co/CoO flow-through high-temperature catalytic oxidation (DOCFt-htc), which also has low Δ14C values like DOCuv (Bauer et al., 1992a); and (3) a potential residual DOC fraction that is the difference between DOC measured by discrete-injection high-temperature catalytic oxidation (DOChtc) and DOCFt-htc, and which has unknown Δ14C signature. The distribution of a large fraction of DOC appears to be controlled by circulation of deep ocean waters between major oceans. The DOC in the SS is slightly younger than would be expected if circulation was the sole process controlling DOC cycling. We propose that there is more bomb 14C in the deep SS DOC to account for this difference. The Δ14C values of suspended, and to a lesser extent sinking particulate organic carbon (POC), decrease with depth, with the suspended POC displaying a much steeper gradient in the SS than in the NCP. These data reflect the incorporation of low-activity organic matter into the POC pool, possibly through incorporation of DOC by physical adsorption and/or biological heterotrophy.

Modeling the land surface boundary in climate models as a composite of independent vegetation stands

Abstract: An efficient strategy for modeling the land surface boundary in general circulation models (GCMs) is presented which accounts for the effects of vegetation on surface energy fluxes and allows for an arbitrary number of vegetation types to coexist in a grid square. The GCM grid square is depicted as a 'mosaic' of vegetation 'tiles', with each tile consisting of a single vegetation type. The energy balance equation for each tile follows closely that of a single vegetation version of the simple biosphere (SiB) model of Sellers et al. (1986) but is simplified enough to be written in Penman-Monteith form. Each tile in the square is coupled independently to the GCM atmosphere, and tiles affect each other only through the atmosphere. This coupling strategy differs conceptually from that of models such as SiB that assume a homogeneous mixture of vegetation types within a GCM grid square. A quantitative comparison of the two strategies is presented.

The Mars Observer laser altimeter investigation

Abstract: The primary objective of the Mars Observer laser altimeter (MOLA) investigation is to determine globally the topography of Mars at a level suitable for addressing problems in geology and geophysics. Secondary objectives are to characterize the 1064-nm wavelength surface reflectivity of Mars to contribute to analyses of global surface mineralogy and seasonal albedo changes, to assist in addressing problems in atmospheric circulation, and to provide geodetic control and topographic context for the assessment of possible future Mars landing sites. The principal components of MOLA are a diode-pumped, neodymium-doped yttrium aluminum garnet laser transmitter that emits 1064-nm wavelength laser pulses, a 0.5-m-diameter telescope, a silicon avalanche photodiode detector, and a time interval unit with 10-ns resolution. MOLA will provide measurements of the topography of Mars within approximately 160-m footprints and a center-to-center along-track foot print spacing of 300 m along the Mars Observer subspacecraft ground track. The elevation measurements will be quantized with 1.5 m vertical resolution before correction for orbit- and pointing induced errors. MOLA profiles will be assembled into a global 0.2 deg x 0.2 deg grid that will be referenced to Mars' center of mass with an absolute accuracy of approximately 30 m. Other data products will include a global grid of topographic gradients, corrected individual profiles, and a global 0.2 deg x 0.2 deg grid of 1064-nm surface reflectivity.

Ocean general circulation from a global eddy‐resolving model

Abstract: A concerted effort has been made to simulate the global ocean circulation with resolved eddies, using a highly optimized model on the best available supercomputer. An earlier 20-year spin-up has been extended for 12.5 additional years: the first 2.5 with continued annual mean forcing and the final 10.0 with climatological monthly forcing. Model output archived at 3-day intervals has been analyzed into mean fields, standard deviations, products, and covariances on monthly, annual, and multiyear time scales. The multiyear results are examined here in order to give insight into the general circulation of the world ocean. The three-dimensional flow fields of the model are quite realistic, even though resolution of eddies in high latitudes is marginal with a 0.5°, 20-level grid. The use of seasonal forcing improves the simulation, especially in the tropics and high northern latitudes. Mid-latitude gyre circulations, western boundary currents, zonal equatorial flows, and the Antarctic Circumpolar Current (ACC) all show mean and eddy characteristics similar to those observed. There is also some indication of eddy intensification of the mean flow of the ACC and of separated boundary jets. A global thermohaline circulation of North Atlantic Deep Water is identified in deep western boundary currents connected by the ACC. This deep circulation rises mainly in the equatorial Pacific. Several zonal jets are an integral part of this circulation near the equator. The deep flow rises toward the surface in a series of switchbacks. Much of the thermohaline return flow then follows an eddy-rich warm-water route through the Indonesian archipelago and around the southern tip of Africa. However, some intermediate level portions of the thermohaline circulation return south into the ACC and follow a cold water route through the Drake Passage. The representation of a global “conveyor belt” circulation with narrow and relatively high-speed currents along most of its path may be the most important result of this modeling study. Statistics of scalar fields such as transport stream function and surface height are exhibited, as are time series and frequency spectra of certain variables at selected points. These provide a baseline for comparison both with observations and with other model studies at higher resolution. Mean and eddy characteristics of the near-surface temperature and salinity fields are discussed, and surface forcing fields are examined. In particular, combined thermal and hydrological forcing effects are found to drive a conveyor belt circulation between the tropical Pacific and the high-latitude North Atlantic. The effect of weak restoring terms to observed temperature and salinity at great depth and in polar latitudes is found mainly to augment the model's convective processes, which are poorly resolved with 0.5° grid spacing. However, the deep restoring terms are insignificant in both the tropics and the mid-latitudes. The geographical distributions of eddy heat and salt transport are discussed. The eddies transport heat and salt down the gradients and along the mean flow in many regions of strong currents. Net meridional transports of heat and salt by both the total currents and the eddies are computed for the Atlantic, the Indo-Pacific, and the global ocean. The total currents provide for poleward heat transport (except near 40°S, where the contribution from ACC instabilities is rather weak) and, in particular, for that needed to sustain the conveyor belt transport. Meridional eddy transports are especially important for warming the Pacific upwelling branch of the thermohaline circulation and for transporting salt across the equator into the North Pacific. Planned improvements to the model include a free-surface treatment of the barotropic mode and additions of the Arctic Basin and sea ice. A fully prognostic extension of the existing integration is intended, with subsequent transitioning of the model onto a 0.25° grid having very realistic geometry. The 0.25° version of the model will run effectively on newly available supercomputers.

Primary magmas of mid-ocean ridge basalts 2. Applications

Abstract: Variable initial mantle composition and extent of depletion during dynamic melting processes strongly influence compositions of primary basaltic magmas. The descriptions of the equilibria that pertain to melting in the upper oceanic mantle presented in the companion paper (Kinzler and Grove, this issue) are used to estimate the major element compositions and temperatures of aggregate primary magmas of mid-ocean ridge basalt (MORB) generated in the adiabatically upwelling mantle beneath oceanic spreading centers. Primary MORB magmas with high Na2O abundances that are produced from more fertile mantle compositions or represent initial melts of a depleted spinel-lherzolite have higher SiO2 and Al2O3 and lower MgO, FeO, and CaO abundances, relative to low-Na2O primary magmas. Na2O abundance variation in the mantle source during polybaric, near-fractional melting processes causes melt compositions to vary significantly. The total extents of depletion achieved by the decompression melting process to yield the observed variation in major elements of MORB range from ∼ 6 to 18%; the range of mean pressures of melting is relatively narrow, 8–15 kbar; the total range modeled for the adiabatic, near-fractional melting process is 4 to 25 kbar. Aggregate primary magmas of MORB are not picritic, nor do they resemble sampled primitive MORB (MORB with MgO > 9.0 wt%). Much of the variation in major element composition observed in sampled MORB can be explained by melting a similar depleted MORB-mantle source. The ambient temperature range of the upper mantle beneath the global ridge system required to explain the observed chemical variations is 1475°–1315°C.

Primary magmas of mid-ocean ridge basalts 1. Experiments and methods

Abstract: This paper reports experiments carried out between 9 and 16 kbar (0.9–1.6 GPa) using natural, primitive mid-ocean ridge basalt compositions and synthetic analogs of mid-ocean ridge basalts to investigate the effects of pressure, temperature, and variable bulk composition on the composition of melts multiply saturated with the minerals present in the upper oceanic mantle: olivine, orthopyroxene, augite, and plagioclase or spinel. For this low-variance, five-phase assemblage, equations involving pressure, melt NaK # ([Na2O+K2O]/[Na2O+K2O+CaO]; weight ratio), melt Mg # (Mg/[Mg+Fe2+]; total iron as Fe2+), and weight percent TiO2 in the melt predict temperature and major element compositions of magmas produced by melting spinel and plagioclase lherzolites at upper mantle pressures. The equations are estimated using a selected set of data from this experimental study and published experimental studies that report compositions of glasses coexisting with olivine, low-Ca pyroxene, augite, and plagioclase and/or spinel. An experimental test of a liquid compositionally similar to melts produced in a subset of peridotite-basalt sandwich experiments is presented. The composition tested was reported as multiply saturated (with orthopyroxene + augite + spinel + olivine) in the sandwich experiment, but it does not crystallize these phases at the conditions of the experiment. We exclude this liquid and the subset it represents (data from Fujii and Scarfe [1985] and Falloon and Green [1987]) from the data set used to constrain the melting equilibria. With estimates of the melt NaK #, melt Mg #, and weight percent TiO2 of the melt the quantitative descriptions of the melting equilibria can be used to predict the temperatures and major element compositions of melts from lherzolite. Methods are described for estimating these compositional parameters with the nonmodal batch melting equation (for Na2O, CaO, K2O, and TiO2) and a mass balance calculation (for MgO and FeO) from the initial composition and phase proportions of the mantle source, the amount of melt produced and the nature of the melting process, and the stoichiometric coefficients of the mantle melting reaction.