bluuebunny/crossref_metadata_2025 huggingface dataset:
Stresses and plate boundary forces associated with subduction plate margins
Authors: Whittaker A. , Bott M. H. P. , Waghorn G. D.
Published: February 2008

Primary tectonic stress in the lithosphere is predominantly caused by lateral density variations within the Earth and associated topographical loading. When such a stress system caused by major sublithospheric density anomalies is intersected by a weak zone which cuts across the lithosphere, plate boundary forces develop and modify the plate interior stresses. In this paper, unite element analysis is used to model the stresses and plate boundary forces associated with subduction plate boundaries, with dipping and vertical slabs extending to about 270 and 400 km depths having the subduction fault both locked and unlocked. In such regions, several types of horizontal deviatoric stress may occur, including (1) local compression in the trench‐arc region caused by the dense sinking slab and the associated surface downflexure; (2) plate interior tension which occurs when this compression is intersected by a weak subduction fault; (3) local tension associated with thickening of the crust at the arc and elsewhere; (4) local tension in the back arc region produced by the underlying low density upper mantle; and (5) downbending stresses in the subducting slab, thermal stresses, and transmitted ridge push, which are not included in the modelling here. A gradient from compression in the forearc to tension in the back arc can be modelled in terms of these stresses when the fault is partially locked. It is, however, the intersection of the local compression (1) by an unlocked or partially locked subduction fault that modifies the plate interior stresses and gives rise to the slab pull and trench suction plate boundary forces. The state of stress in the interior of the overriding plate is also crucially influenced by back arc spreading where this occurs. Plate boundary forces have been evaluated for each of the models. It is shown that slab pull and trench suction may be of comparable magnitude.
Mid‐Cenozoic stress evolution and magmatism in the Southern Cordillera, Texas and Mexico: Transition from continental arc to intraplate extension
Authors: Henry Christopher D. , Price Jonathan G. , James Eric W.
Published: February 2008

Orientations of dikes, veins, faults, and slickenlines reveal the evolution of stress during Eocene to Miocene magmatism in the southern Cordillera. Where most thoroughly studied by us in Trans‐Pecos Texas, magmatism began at about 48 Ma shortly after the cessation of Laramide folding. Dikes and veins that formed from then until about 32 Ma strike dominantly east‐northeast. This indicates that the least principal stress (σ3) was north‐northwest; additional data suggest that the maximum principal stress (σ1) was east‐northeast. The stress field changed to σ1 vertical and σ3 east‐northeast (i.e., east‐northeast extension) at least by 28 Ma and probably by 31 Ma. Dikes and veins that formed between 31 and 17 Ma, when all magmatism ceased in Texas, strike north‐northwest. This change marks the beginning of regional, Basin and Range extension; however, major normal faulting, exclusively of high‐angle type, did not begin until about 24 Ma. A similar stress change, marked by a similar change in dike and vein orientations, occurred throughout the southwestern United States and northern Mexico. The time of change is not well constrained in Texas, but available information allows it to have occurred at the same time thoughout the southern Cordillera. We suggest the earlier stress field is related to east‐northeast convergence between the Farallon and North American plates. The change in stress is approximately coincident with collision of the East Pacific Rise and paleotrench. Extension may be related to the change from a convergent to a transform margin along the western edge of North America. The changes in the stress field are accompanied by changes in the sources and compositions of magmas erupted in Texas. Contemporaneity of the changes in stress and magmatism indicates that they are related. Combined with regional age patterns, paleostress and geochemical data indicate that pre‐31 Ma magmatism in the southern Cordillera occurred in a subduction‐related, continental volcanic arc. Subsequent magmatism occurred in an environment of intraplate extension of the Basin and Range province.
Domain states in fine particle magnetite and titanomagnetite
Authors: Moon T. S.
Published: February 2008

The domain state (single domain versus multidomain) of fine magnetic particles has long been associated with their remanent and hysteresis properties. Both domain observations and micromagnetic theory indicate that a large number of local energy minimum (LEM) domain states are available to single domain and multidomain particles. Theoretical calculations based on a simple model of domain structure are made to determine the particle size, mineralogical, stress and temperature dependence of the LEM domain states available to fine particles of magnetite and titanomagnetite (TM56). The global (or absolute) energy minimum (AEM) domain state depends primarily on the balance between magnetostatic and exchange energies and shows only a weak temperature dependence. The range of local energy minima (LEM) domain states depends primarily on the anisotropy energies (magnetocrystalline and magnetoelastic) and is strongly temperature dependent. From the evolution of domain structure as a function of temperature and grain size implied from the domain state calculations, the relative stabilities of Thermoremanent Magnetization (TRM), grain growth Chemical Remanent Magnetization (CRM) and trans‐domain Viscous Remanent Magnetization (VRM) are discussed. The magnetic behavior of an ensemble of fine particles will depend on the LEM states initially occupied by the particles.
Parameter estimation in a one‐dimensional anelastic medium
Authors: Dahl Terje , Ursin Bjørn
Published: February 2008

Surface seismic data with a known source pulse are used to estimate parameters in a stack of homogeneous layers. The unknown parameters in each layer are the P wave velocity, the S wave velocity, the density, the layer thickness, and the quality factor. The forward problem is solved by dynamic ray tracing. Primary and multiple P wave reflections are considered. The S wave velocities are estimated from the PP transmission and reflection coefficients. The model is fitted to the data using a stabilized least squares procedure. The Jacobian is computed analytically in each step. By introducing the layer thicknesses as unknown parameters, it is possible to keep the number of unknowns at a tractable level, and the Gauss‐Newton approximation to the Hessian matrix can be inverted exactly at each iteration. When only one event is considered, the scheme provides a nonlinear solution to the amplitude versus offset problem, where offset dependent corrections for absorption, geometrical spreading, and transmission losses are automatically included. All parameters are successfully estimated from synthetic data. We also perform a sensitivity analysis for the different parameters and discuss the reliability for the estimates in each parameter group.
Analysis of local seismic waveforms from underground nuclear explosions: Forward modeling for effective source functions at Pahute Mesa
Authors: Barker Jeffery S. , Hartzell Stephen H. , Burdick L. J. , Helmburger D. V.
Published: February 2008

In this study we use forward modeling of surface particle velocity data recorded within 15 km of underground nuclear explosions at Pahute Mesa in the Nevada Test Site. A plane‐layered velocity structure model is developed, and effective source functions are obtained for six events with yields of 155–1300 kt (kilotons), recorded at ranges presumed to be well outside the zone of inelastic deformation. We then use the effective source functions determined from well outside the elastic radius to compute synthetic seismograms for comparison with recordings within the spall zone. Prior to the onset of spall the comparison is good, enabling us to estimate effective source functions for two additional events for which only recordings from the spall zone are available. As surface zero is approached, however, the first half cycle of the observed velocity records includes an inflection, which appears as a shoulder in acceleration records. This suggests that, in addition to the elastic arrival, we are seeing secondary arrivals due to spall opening or inelastic shock wave propagation. Elastic wave propagation predicts the observed peak velocities well beyond a scaled slant range of about 175 m/(kt)⅓ but overpredicts the first phase at closer ranges. This limit of applicability of elastic wave propagation provides a useful estimate of the elastic radius for Pahute Mesa explosions. Yield scaling relations are determined using three events of known yield, and these are then used to estimate yields for the eight Pahute Mesa explosions modeled in this study. Finally, implications for the attenuation of teleseismic body waves are discussed.
Thermal properties of β‐Mg2SiO4 at mantle pressures derived from vibrational spectroscopy: Implications for the mantle at 400 km depth
Authors: Chopelas A.
Published: February 2008

The pressure dependences of 25 Raman modes of β‐Mg2SiO4 were measured to 235 kbar. The thermodynamic parameters, heat capacity, GV, and entropy, S, of β‐Mg2SiO4 were calculated using a statistical approach from information derived from the spectra. CV and S calculated from the 1‐atm data agree well with the available previous calorimetric data and previous lattice dynamical calculations. The high pressure data were used to calculate the pressure dependence of CV and S. From these data and those from previous spectroscopic measurements on forsterite at high pressures, previous enthalpy data, and the equations of state of both phases, the equilibrium phase boundary was calculated to be P(kbar) = 101 + 0.027 kbar‐K−1×T(K), agreeing exactly with new phase equilibrium data. The Clapeyron slope, ΔS/ΔV, for the forsterite to β‐Mg2SiO4 transition was found to be nearly constant for a large P‐T range, indicating that the pressure and temperature induced volume changes compensate for the pressure and temperature induced entropy changes for the transition. Using the equation of state of both minerals at equilibrium conditions and the thermal expansion systematics of δT = 5.5 ± 0.5, the sound velocity increases across the forsterite to β‐phase transition by approximately 7.5%. Thus, a mantle of ∼ 60% olivine, i.e., a pyrolitic mantle, could account for the average seismic sound velocity change of +4.5% at 400 km depth. In addition, at 92 kbar, strong decreases in the pressure derivative of 7 Raman modes indicate changes in the compression mechanism of the β‐Mg2SiO4 lattice. Similar observations for forsterite at the same pressure imply that similar elastic limits were reached within the respective lattices. This phenomenon is interpreted as a second‐order phase transition.
Estimating aquifer parameters from analysis of forced fluctuations in well level: An example from the Nubian Formation near Aswan, Egypt: 2. Poroelastic properties
Authors: Beavan John , Evans Keith , Mousa Sameh , Simpson David
Published: February 2008

We discuss the in situ estimation of aquifer poroelastic properties from observed water level fluctuations in five wells sampling the 0.30 porosity Nubian sandstone near Aswan Reservoir, Egypt. The study area is close to ideal for the application of poroelastic theory, because of the relatively simple stratigraphy and the absence of cultural and climatological noise. In this paper we use approximately 2 years data sampled at 0.2‐hour intervals to calculate frequency‐dependent admittances between the well levels and the recognized forcing functions of the earth tides and barometric fluctuations; the effect of variations in lake level is treated in a companion paper. The time series analysis techniques that we employ are described in detail. Three of the wells show equilibrium‐confined response at tidal frequencies, which allows various in situ poroelastic parameters of the aquifer to be derived from the tidal and barometric admittance values. We make extensions to earlier treatments of poroelastic theory in order to include the effect of horizontal strain resulting from barometric loading; the modifications proved of minor importance for our data set because of the high compliance of the aquifer material and the shallow depth to basement. In situ shear modulus μ is determined as 5.4 ‐ 6.4 × 109 Pa, and uniaxial strain loading efficiency as 0.33. Each of these parameters depends on the field measurements of well level and air pressure, and μ also depends strongly on a theoretical estimate of the solid earth areal strain tides; the principal source of error in μ lies in these areal strain estimates. We measured Poisson's ratio in the laboratory and found that it lies in the range 0.13–0.18. This, together with the admittance values, allows estimates of uniaxial strain modulus (13 ‐ 19 × 109 Pa, with the smaller values at shallower depth), specific storage (1.5–1.9×10−6 m−1, with the larger values at shallower depth), porosity (0.2–0.3), Skempton's constant (0.54–0.58), and Biot's constant (0.78–0.85). Laboratory measurements of uniaxial strain modulus are about 50% higher than the in situ estimates, probably reflecting scale effects. Despite the high porosity, use of a theory of aquifer response in which the solid constituent is considered incompressible produces significant errors in these estimates.
The high‐pressure electronic structure of magnesiowustite (Mg, Fe)O: Applications to the physics and chemistry of the lower mantle
Authors: Sherman David M.
Published: February 2008

The electronic structure of magnesiowustite is investigated using self‐consistent field Xα scattered wave (SCF‐Xα‐SW) molecular orbital calculations on (FeO6)10− and (FeMg12O14)2− clusters. Calculated one‐electron transition energies are used to interpret the optical spectrum of (Mg, Fe)O. The results are applied to the electrical and thermal conductivity of the lower mantle. The spin pairing of Fe2+ and the effect of pressure on bonding in magnesiowustite, with some inferences regarding the incorporation of oxygen in the outer core, is also addressed. The approach used here appears to give a reliable description of the energy and pressure dependence of the spin‐allowed 5T2g → 5Eg ligand field transition and the spin‐pairing transition of Fe2+ in (Mg, Fe)O. However, the oxygen to metal charge transfer transitions in (Mg, Fe)O are not as reliably determined insofar as the p‐d band gap varies with cluster size and the energies of the charge transfer states cannot be found without including configurational interaction. Nevertheless, it is argued that the charge transfer transitions that are intrinsic to (Fe, Mg)O are of a sufficiently high energy to be irrelevant to the electrical and thermal conductivity of the lower mantle. This is especially true if Fe2+ adopts the low‐spin configuration. The geophysically significant properties of (Fe, Mg)O probably result from defect Fe3+.
A theory of ideal bodies for seamount magnetism
Authors: Parker Robert L.
Published: February 2008

Recent studies of samples from seamounts indicate that the distribution of magnetic intensity is approximately lognormal, which implies that the commonly adopted models of interior magnetization based upon a constant vector with an isotropic perturbation are inappropriate. We develop a unidirectional model in which the direction of magnetization is fixed and the intensity is of one sign, with no upper limit on magnitude, which, if the seamount is built during a period of single magnetic polarity, is likely to be a better approximation. We show that models of this class fitting the data best in the two‐norm sense conform to the ideal‐body pattern comprising unidirectional, point dipoles in the surface of the seamount. Practical methods are developed for discovering the best data misfit associated with paleopole position. The methods are first tested on simple artificial magnetic anomalies and are found to be capable of recovering the true pole position with high accuracy when such a solution is possible; also when a mixed polarity artificial model is analyzed, it is found that there are no unidirectional solutions, just as would be hoped. The method is next applied to three seamount surveys. In the first it is found that every direction of magnetization is in accord with the data, so that apparently nothing useful can be learned from the survey without a stronger assumption; this result is in contrast with the results of an earlier solution based upon a statistical model, which yielded a high accuracy in the position of the paleopole. The second investigation provides a reasonably compact location of the paleopole of the seamount. The third magnetic anomaly is complex and earlier studies concluded this was necessarily the product of mixed polarity magnetization. We find that in fact unidirectional magnetizations can satisfy observation.
Multifractal analysis of the microdistribution of elements in sedimentary structures using images from scanning electron microscopy and energy dispersive X ray spectrometry
Authors: Block A. , von Bloh W. , Klenke T. , Schellnhuber H. J.
Published: February 2008

A novel method for the quantitative characterization of density distributions of elements in sedimentary geosystems is presented. This general technique is based on the multifractal analysis of image‐processed elemental maps obtained by scanning electron microscopy combined with energy dispersive X ray spectrometry. Applications to microdistributions of Si, Fe, and Al in recent bioactive siliciclastic marine sediments are reported. Inhomogeneous scaling behavior of these elemental distributions is observed in all cases. Two main conclusions can be drawn: (1) The sedimentary matrix exhibits true fractal geometry and (2) the processes allocating and rearranging the elements are not of pure stochastic type. Therefore the identification of genetic processes may be possible on the basis of their “multifractal fingerprints”.
Constraints on upper mantle discontinuities from observations of long‐period reflected and converted phases
Authors: Shearer Peter M.
Published: February 2008

Stacked images combining over 5 years of long‐period Global Digital Seismograph Network data reveal many phases associated with reflections and conversions from upper mantle discontinuities. These images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking. Results obtained for P, S, SS, and PP reference arrivals resolve numerous phases from discontinuities near 410 and 660 km, while some of the stacks also show evidence for a weaker discontinuity near 520 km. Phases of particular interest include P and SH multiples resulting from topside reflections, precursors to SS from underside reflections, and P‐to‐SV converted phases. These phases can be clearly seen both in the waveform stacks and in cross‐correlation analysis of individual seismograms. Travel times for these arrivals are converted to discontinuity depths relative to velocities in the Preliminary Reference Earth Model, and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle. Average apparent depths to the discontinuities for the different phases agree to within ±3 km for the 410‐km discontinuity, to within ±4 km for the 520‐km discontinuity, and to within ±8 km for the 660‐km discontinuity. The best global averages are obtained from the SS precursor data which indicate discontinuities at 415, 519, and 659 km. Discontinuity depths obtained from the P‐to‐SV converted phases at over 35 individual seismic stations exhibit variations of less than ±20 km. Apparent depths to the 660‐km discontinuity consistently show greater variability than depths to the 410‐km discontinuity, supporting recent laboratory results which indicate that the Clapeyron slope for the 660‐km discontinuity is significantly larger in magnitude than the slope for the 410‐km discontinuity. Precursors to SS (seen between 110° and 180°) are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point Apparent discontinuity depths computed from SS precursors for different tectonic regions agree to within about ±5 km. The SS precursors have especially good coverage near the subducting slabs in the northwest Pacific. Analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000‐ to 1500‐km‐wide region near the slab in which the 660‐km discontinuity is depressed by about 20 km. Measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking. Relative amplitude analysis suggests that the P and S wave impedance changes at 410 km are about 0.8–1.1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0.3 and 0.6 of the changes at 410 km.
How does an asperity break? New elements from the waveform inversion of accelerograms for the 2319 UT, October 15, 1979, Imperial Valley Aftershock
Authors: Zollo Aldo , Bernard Pascal
Published: February 2008

The 16 accelerograms obtained at less than 30 km during the 2319 UT, October 15, 1979, Imperial Valley aftershock were analyzed, after having been corrected for the crack‐induced anisotropy in the shallow sediments. Both P and S waveforms are used to study the earthquake source process and to infer an estimate of attenuation and site amplification effects on the waveforms at frequencies ≃ 0.6–1 Hz and ≃ 2–4 Hz. The comparison between the 0.6–1 Hz P and S wave seismic moment measures and their dependence on distance has provided an estimate for this parameter (1.9(±40%) 1023 dyn cm) and the bound limits for attenuation (QS(ƒ = 0.6 ‐ 1Hz) ≃20–25) and near‐site amplification of shear waves at low frequency. The inversions of the 2–4 Hz S velocity waveforms for different values of rupture velocities and attenuation parameters have been preliminarily performed to investigate the variability of static slip and static stress drop estimates. A circular growth of the rupture at a constant velocity and a final elliptical shape are assumed. The bipolar character of the velocity waveform excludes the asperity models consisting of a small, highly stressed area inbedded in a large stress‐free crack. Therefore, only the slip distribution of a quasidynamic crack with healing phases has been considered. The dynamic stress release has been independently measured from the amplitude level of the first acceleration pulses for various rupture velocities and attenuation. The constraint that the static stress drop does not significantly exceed the dynamic stress drop suggests relatively high Qs values (≥200) at ƒ=2–4 Hz and small rupture velocities uR ≃0.5β‐0.7β), β is the shear wave velocity). The elliptical source is about 1.2 km long and 0.7 km wide (±0.2 km), elongated in the NW‐upward direction. The nucleation point location indicates a dominant propagation toward the NW top fault direction. For the inferred values of attenuation parameter and rupture velocity the minimum values of the dynamic and static stress drop are ≈300–700 bars and are associated with about l m of average slip on the fault. The nucleation of the rupture is explained as an effect of the high loading stress on a larger size barrier (left unbroken by the mainshock) in which the aftershock fault area is embedded. The hetereogeneous stress and rock strength distribution on the barrier region would therefore have controlled the rupture development and arrest. An estimate of the S amplification in the considered frequency bands has been obtained through the seismic moment computed from the P wave records, which are less sensitive to site amplification and attenuation effects for the sites under study at frequencies below 5 Hz. The results show a differential amplification of S waves (≃3 at 1 Hz and ≃2 at 2–4 Hz), which is attributed to the propagation in the very shallow, low‐velocity sediments of the Imperial Valley.
Anticyclonic current rings in the eastern tropical Pacific Ocean
Authors: Hansen Donald V. , Maul George A.
Published: February 2008

Observations from satellite‐tracked drifting buoys, expendable bathythermograph and conductivity‐temperature‐depth data, and Geosat altimeter data are used to describe anticyclonic eddies that occur in small numbers off the Pacific coast of Central America. These eddies are similar in many respects to the well‐known warm‐core rings that are observed north of the Gulf Stream off the Atlantic coast of North America, except that they occur in an environment that also is warm, and they contain considerably greater kinetic energy. It is hypothesized that they are formed as a result of conservation of potential vorticity when the North Equatorial Countercurrent (NECC) turns northward upon approaching the eastern boundary during its autumnal maximum. The rings so formed have a strongly nonlinear character which causes them to propagate westward between 9°N and 14°N with a speed in excess of that of long Rossby waves. Due to a relatively small available potential energy content, these rings have a dissipation time scale of about 6 months and perhaps end by collision with and reabsorbtion into the NECC. The rings account for the observed enhancement of surface kinetic energy, and probably for the seaward transport of waters enriched in copper.
Impact of the Geosat altimeter wet tropospheric range correction in the Greenland‐Iceland‐Norwegian Sea
Authors: May Douglas A. , Hawkins Jeffrey D.
Published: February 2008

Atmospheric water vapor data derived from the special sensor microwave imager (SSM/I) are used to make time‐coincident wet tropospheric range corrections to Geosat altimeter data in the Greenland‐Iceland‐Norwegian (GIN) Sea. The SSM/I sensor is onboard a polar orbiting Defense Meteorological Satellite Program platform and provides water vapor data at 25‐km spatial resolution and an accuracy of 0.24 g cm−2. Water vapor height corrections were calculated and analyzed along all Geosat tracks crossing the area of interest in July and September 1987 and January and March 1988. Results indicate that the horizontal spatial variations in the altimeter height corrections can sometimes equal or exceed the magnitude (10–20 cm/100 km) of true GIN Sea sea surface height gradients, especially in the summer and autumn seasons. SSM/I and Geosat data must be closely matched in time, due to the potential rapid movement of atmospheric systems through this region (35–40 km h−1). The water vapor data must be timely enough to adequately describe atmospheric conditions at the time of the altimeter pass, or significantly false signals may be introduced into the sea surface height data. Substantial differences in water vapor gradients were observed in SSM/I data sets 12 hours apart. Because of this, it is suggested that future satellite altimeter platforms include a nadir‐viewing radiometer to provide time‐coincident atmospheric water vapor data.
Modeled and observed mesoscale circulation and wave‐current refraction during the 1988 Norwegian Continental Shelf Experiment
Authors: Haugan Peter M. , Evensen Geir , Johannessen Johnny A. , Johannessen Ola M. , Pettersson Lasse H.
Published: February 2008

A simple two‐layer, quasi‐geostrophic model with bottom topography is shown to reproduce several features of the mesoscale circulation on the Norwegian continental shelf between 62°N and 66°N, as observed with in situ and remote sensing instruments during the Norwegian Continental Shelf Experiment (NORCSEX '88). The widening of the shelf, its steplike deepening northeastward along the coast, and the presence of the Haltenbank generate intense eddy activity. Further northeast, Atlantic Water is steered anticyclonically around the Haltenbank, and the Norwegian Coastal Current is squeezed toward the coast. The variability of the modeled and observed near‐surface current is shown by ray‐tracing calculations to be able to generate significant wave‐current refraction. In addition to its importance for wave modeling and prediction, this fact opens possibilities for indirect surface current measurements from directional wave observations, particularly from synthetic aperture radar.
Measurements of diffusive sublayer thicknesses in the ocean by alabaster dissolution, and their implications for the measurements of benthic fluxes
Authors: Santschi Peter H. , Anderson Robert F. , Fleisher Martin Q. , Bowles Walter
Published: February 2008

Fluxes of reactive chemical species across the sediment‐water interface can profoundly influence the dominant biogeochemical cycles in the worlds ocean. However, reliable in‐situ measurements of benthic fluxes of many reactive species cannot be carried out without adjustment of stirring rates inside benthic flux chambers to match boundary layer conditions prevailing outside. A simple method to compare flow levels consists of measurements of gypsum dissolution rates inside benthic chambers and on the seafloor. The measurement of the diffusion‐controlled dissolution rate of gypsum allows the estimation of the diffusive sublayer thickness and the time‐averaged bottom stress on the seafloor. This method had previously been intercalibrated with the stress sensor method in flumes and inside benthic chambers. We describe here free‐vehicle deployments of alabaster plates on the bottom of the ocean which gave results consistent with hydrodynamic theory. Errors in the calculated diffusive sublayer thicknesses were estimated to be about 10–15% for typical deployment conditions in the ocean. Current velocities 5 m off the bottom, which were measured concurrently during two deployments, allowed for comparisons with hydrodynamic predictions of diffusive sublayer thicknesses. The values obtained this way agreed within 15%. The measured mass transfer velocity was found to correlate with the plate dimension L, to the power of ⅓. This confirms the theoretical procedure for extrapolating to infinite plate size when calculating the sublayer impedance of solute fluxes from sediments (where L is large). Typical values of diffusive sublayer thicknesses, corrected to infinite plate size, were 1200 μm for current velocities, U100, of 2 cm s−1, and 500 μm at 8 cm s−1. Furthermore, values of friction velocities calculated from alabaster dissolution were compared with those using stress sensors. Gypsum plate values of u* were 0 and 30% lower than skin friction values of u*, at u* values of 0.6 and 0.2 cm s−1, respectively, possibly as a result of bottom roughness effects. A case is made for the importance of considering boundary layer dynamical conditions when benthic fluxes are to be measured correctly. When rates of oxygen uptake, carbonate dissolution, and nutrient species uptake or release in regions near the sediment‐water interface are fast compared with diffusion rates across the diffusive sublayer, overall rates can become mass transport controlled.
Net diffusivity in ocean general circulation models with nonuniform grids
Authors: Yin F. L. , Fung I. Y.
Published: February 2008

The numerical vertical diffusivity Knum, embedded in a numerical ocean general circulation model with nonuniform vertical grid, is estimated. It is shown that in a downwelling region, Knum is negative for a grid with grid size increasing with depth. When the grid size increment, or the downward vertical velocity, is large, Knum may exceed the vertical diffusivity specified and may result in a negative effective vertical diffusivity. Therefore care needs to be taken to specify the vertical diffusivity in a numerical model with nonuniform grid, and a lower bound is generally imposed in order to avoid an unphysical negative value. Some possible effects of the negative effective diffusivity are discussed.
Simple climatic indices for the tropical Atlantic Ocean and some applications
Authors: Servain Jacques
Published: February 2008

Two indices related to the sea surface temperature (SST) variability in the tropical Atlantic are proposed. One index describes the SST averaged over the whole basin (30°N to 20°S, 60°W to 15°E), and the other illustrates a meridional dipole between the northern and southern hemispheres. The computational method for obtaining these indices is intentionally kept simple, the objective being to reproduce the signature of the main results previously provided from more complicated statistical analyses. Monthly time series for both indices are produced from 1964 up to the present time. The whole basin index exhibits principally a sustained warming which has intensified since about 1975, and it has a significant periodicity close to that of the quasi‐biennial oscillation. The dipole index exhibits a decadal‐scale variation, and its building up seems to be related to other worldwide climatic changes, as for instance El Niño / Southern Oscillation extreme episodes, rainfall variabilities over the Brazilian Nordeste and African Sahel.
Shear and strain in Santa Monica Basin
Authors: Gregg M. C. , Kunze E.
Published: February 2008

By injecting a cloud of SF6 into Santa Monica Basin at a depth of 790 m and observing its vertical spread, Ledwell and Watson (1991) estimate the average diapycnal diffusivity between 750 and 850 m as Kp = 2.5 × 10−5 m2 m−1 This diffusivity is higher than inferred from microstructure measurements in the open‐ocean thermocline for typical internal wave levels (Gregg and Sanford, 1988). To examine shear and strain in the basin, we dropped expendable current profilers (XCPs) and calculated strain from conductivity, temperature, and depth (CTD) casts. Shear and strain are above the GM76 model spectrum (Garrett and Munk, 1975; Cairns and Williams, 1976), and comparisons with published parameterizations indicate more than enough shear variance to account for the high diffusivity reported by Ledwell and Watson. Thus, the eddy diffusivity in Santa Monica Basin is not at odds with lower values estimated from microstructure measurements in the open ocean, but is due to an elevated internal wave field. The spectral shapes, however, differ from typical open‐ocean spectra.
Lower continental slope cyclonic eddies in the central Gulf of Mexico
Authors: Hamilton Peter
Published: February 2008

Current meters, an inverted echo sounder, hydrography, drifters, and satellite imagery are used to characterize relatively small (100‐ to 150‐km diameter) cold cyclonic eddies in the central basin and on the lower Louisiana slope of the Gulf of Mexico. These cyclones are shown to be long‐lived (6 months or more), have limited movements when compared with Loop Current anticyclones, and be fairly vigorous, with upper layer currents of 30–50 cm s−1. They usually have only small temperature differences with surrounding water masses in the upper 50 to 100 m of the water column and are therefore not readily apparent in satellite thermal imagery. The largest isotherm displacements occur over depths of 200–800 m. In one case, a cyclone was traced from the deep basin near 92°W to the northern slope as a major anticyclone propagated southwestward through the gulf. The presence of cyclones and anticyclones on the Louisiana slope is consistent with observed current meter measurements in the upper half of the water column that have long (several months) periods and approximately equal variances in the cross‐slope and along‐slope directions.
A multitracer study of radionuclides in Lake Zurich, Switzerland: 2. Residence times, removal processes, and sediment focusing
Authors: Wieland E. , Santschi P. H. , Beer J.
Published: February 2008

Residence and settling times of particles and particle‐reactive nuclides evaluated from in situ tracer studies can be used as diagnostic indicators of trace element pathways in lakes. Natural (e.g., 210Pb, 7Be, and 10Be ) and artificial (i.e., Chernobyl 137Cs) radionuclide fluxes through Lake Zurich (at 50 and 130 m depth) from 1983 to 1987, atmospheric fluxes of the same nuclides and fluxes of 10Be during 1987 allowed the calculation of nuclide residence times as well as particle settling and transit velocities in Lake Zurich. The residence time of 210Pb in the lake is approximately 1 month and, hence, of the same order as the residence times of particles and stable Pb. Steady state residence times for 7Be were calculated as 150–170 days, while non steady state, instantaneous removal residence times ranged from 50 to 800 days. Polonium 210 was removed from Lake Zurich with removal times of 10–26, months indicating slow removal processes or efficient recycling in the lake water. Transit velocities of particles, calculated from the attenuation of Chernobyl 137Cs fluxes at 50 m and 130 m depth during early May 1986, were 17 m d−1, while 2–4.5 m d−1 was calculated as an effective settling velocity from the attenuation of 7Be fluxes at 130 m depth as compared to 50 m depth. Model calculations reveal that the overall removal process of atmospherically deposited 7Be from the water column to the sediments is mainly controlled by the adsorption/coagulation step and not by the rate of particle settling. Extra inputs of 10Be, 210Pb, or 137Cs into the lower trap (i.e., “rebound flux”), originating from episodic lateral inputs of fine particles with a high content of 210Pb, 10Be, and 137s, were observed during the lake stratification period (i.e., summer). This novel observation of summertime radionuclide and sediment focusing is different from previously described wind‐generated resuspension and focusing effects caused by river plumes and lake sediments during the stagnation period. Boundary and focusing effects for radionuclide‐bearing particles are proposed to explain observed summertime enhancements of longer‐lived nuclide fluxes at 130 m depth as well as the decrease in calculated effective settling velocities.
Breaking waves affecting microwave backscatter: 1. Detection and verification
Authors: Jessup A. T. , Melville W. K. , Keller W. C.
Published: February 2008

This paper is the first of a two‐part series concerning detection and characterization of wave breaking when using microwave techniques. The importance of wave breaking in both microwave remote sensing and air‐sea interaction has led to this investigation utilizing a Ku band continuous wave Doppler scatterometer. Simultaneous microwave, video, and environmental measurements were made during the SAXON‐CLT experiment off Chesapeake Bay in the fall of 1988. The scatterometer was pointed upwind at an incidence angle of 45° and had an illuminated area that was small compared with the wavelength of the dominant surface waves. This first paper presents the schemes developed to detect individual breaking waves and verification of the method using video recordings. The most successful scheme is based on thresholds in both the radar cross section and the Doppler bandwidth. Microwave events consisting of a sea spike in the radar cross section accompanied by a large bandwidth were found to be associated with the steep forward face of waves in the process of breaking. The location of the illuminated area with respect to the phase of the breaking wave and the stage of breaking were found to influence the detectability of individual breaking waves. Approximately 70% of the sea spikes associated with waves that produced whitecaps were identified by the most successful detection scheme. The second paper examines how the degree of wave breaking, as measured by the microwave technique developed in this paper, depends on wind and wave conditions.
Quantitative spatial models of Atlantic primary productivity: An application of geomathematics
Authors: Herzfeld Ute Christina
Published: February 2008

The role of physical oceanographical, geochemical, and sedimentological data in the problem of estimating ocean primary productivity is analyzed using geostatistical and algebraic multivariate spatial methods. The available maps mirror difficulties in measuring productivity directly and quantifying biological observations, which result in a very spotty survey coverage of the world's oceans. This paper takes an approach of estimating the target productivity from related sedimentological, physical oceanographical, and marine geochemical variables. The variables are considered as a multivariate spatial system. Geostatistical estimation is applied to fill in the survey gaps, and the individual data sets are then integrated into a multivariate spatial model using algebraic map comparison. Results are used in the quality assessment and calibration of transform models between the proxy variables and primary productivity. For the Atlantic Ocean, case studies are carried out for phosphate distribution at the 100 m level, foraminifera abundance in sediments, and sea surface temperature. The utility of each proxy variable in the prediction of productivity is discussed. A new map of phosphate concentration at the 100 m level with full coverage of the Atlantic Ocean is compiled by application of geostatistics to an enlarged data base containing new observations. Primary productivity can be predicted from this phosphate map, using transforms that involve also distance from the coastline, and latitude (photosynthesis restriction). Foraminifera abundance is in principle closely related to productivity and is of importance in view of a paleoceanographic reconstruction, but difficulties in measuring and quantifying reflected in the quality of the data set prohibit a direct estimation. The relationship between higher fertility and lower temperature known from upwelling areas does not provide a simple global predictor; instead, sea surface temperature seasonality is an indicator of primary productivity under the constraint of regional availability of nutrients.
Isolation of marine microlayer film surfactants for ex situ study of their surface physical and chemical properties
Authors: Frew Nelson M. , Nelson Robert K.
Published: February 2008

Surface‐active organic matter from natural marine microlayers has been isolated by solid phase adsorption using reversed phase (C18) cartridges. Surfactants collected from a suite of microlayer films in the California Bight and the Gulf of Maine and isolated by this technique were respread on clean seawater to reconstitute surface films which exhibited surface pressure‐area (π‐A) isotherms and static (Gibbs) surface elasticities closely approximating those measured for films formed by diffusion in the original fresh microlayer samples. Microlayer‐subsurface (10 cm depth) carbon enrichment factors were estimated to range from 3.9 to 16. The estimated mean recovery of microlayer dissolved organic carbon (DOC) was 20 ± 5%. The select hydrophobic DOC fraction isolated was the dominant surfactant class responsible for microlayer film characteristics as measured quasi‐statically, even though other relatively hydrophilic DOC fractions which may have been enriched in the microlayer were not absorbed and passed through the isolation procedure. This technique allows collection of marine microlayer films for systematic ex situ study and correlation of their surface physical and chemical properties. For the first time, the π‐A isotherms for microlayer films can be expressed on a specific area basis and, in cases where molecular weights can be estimated, according to mean molecular area.
Increased ocean heat transports and warmer climate
Authors: Rind D. , Chandler M.
Published: February 2008

We investigated the effect of increased ocean heat transports on climate in the Goddard Institute for Space Studies (GISS) general circulation model (GCM). The increases used were sufficient to melt all sea ice at high latitudes, and amounted to 15% on the global average. The resulting global climate is 2°C warmer, with temperature increases of some 20°C at high latitudes, and 1°C near the equator. The warming is driven by the decreased sea ice/planetary albedo, a feedback which would appear to be instrumental for producing extreme high‐latitude amplification of temperature changes. Resulting hydrologic and wind stress changes suggest that qualitatively, for both the wind‐driven and thermohaline circulation, the increased transports might be self‐sustaining. As such, they would represent a possible mechanism to help account for the high‐latitude warmth of climates in the Mesozoic and Tertiary, and decadal‐scale climate fluctuations during the Holocene, as well as a powerful feedback to amplify other climate forcings. It is estimated that ocean transport increases of 50–70% would have been necessary to reproduce the warmth of various Mesozoic (65–230 m.y. ago) climates without changes in atmospheric composition, while the 15% increase used in these experiments would have been sufficient to reproduce the general climatic conditions of the Eocene (40–55 Ma). A companion experiment indicates that increased topography during the Cenozoic (0–65 Ma) might have altered the surface wind stress in a manner that led to reduced heat transports; this effect would then need to be considered in understanding the beginning of ice ages. Colder climates, or rapid climate perturbations, might have been generated with the aid of such altered ocean transports. The large high‐latitude amplification associated with ocean heat transport and sea ice changes differs significantly from that forecast for increased trace gases, for which water vapor increase is the primary feedback mechanism. The different signatures might allow for discrimination of these different forcings; e.g., the warming of the 1930s looks more like the altered ocean heat transport signal, while the warming of the 1980s is more like the trace gas effect. The actual change of ocean heat transport and deep water circulation both in the past and in the future represents a great uncertainty.
Numerical simulation of the seasonal variation of mesospheric water vapor
Authors: Smith Anne K. , Brasseur Guy P.
Published: February 2008

A two‐dimensional radiative/dynamical/chemical model is used to investigate the role of varying vertical diffusivity on the distribution and temporal variability of water vapor in the mesosphere. Model runs in which the effective turbulent Prandtl number varies over 2 orders of magnitude (values of 1, 10, and 100) are compared. The results indicate that a Prandtl number of 10 or more gives a simulation of mesospheric water vapor characterized by a strong decrease with height and a seasonal cycle with maximum mixing ratios during late summer. The test with a small Prandtl number gives, in contrast, very weak vertical gradients of concentration and a semiannual cycle with maxima in late winter and late summer. The very high Prandtl number case was included to test the role of vertical diffusion in the model when the effective diffusivity coefficient is very small. Comparison of these three cases indicates that the vertical structure of water is sensitive to the effective diffusivity over the range tested. The results also indicate that meridional advection by the mean circulation has a significant influence on the vertical distribution of water in the mesosphere.
Time‐dependent transport of dust
Authors: Hassan M. H. A. , El‐Tayeb I. A.
Published: February 2008

A diffusion model for the time‐dependent transport of dust undergoing dispersion and gravitational settlement is studied analytically. A general analytic expression for the dust concentration above the roughness height is derived. The steady state results obtained by previous authors are easily deduced from this expression. The profiles of the concentration are evaluated numerically and are found to be strongly dependent on time, height above the roughness surface, and falling speed.