bluuebunny/crossref_metadata_2025 huggingface dataset:
A synthetic seismicity model for the Middle America Trench
Authors: Ward Steven N.
Published: February 2008

This paper develops a new iterative technique, based on the concept of fault segmentation and computed using two‐dimensional static dislocation theory, to build models of seismicity and fault interaction which are physically acceptable and geometrically and kinematically correct. The technique is applied in two steps to seismicity observed at the Middle America Trench. The first step constructs generic models which randomly draw segment strengths and lengths from a two‐dimensional probability distribution. The second step constructs predictive models in which segment lengths and strengths are adjusted to mimic the actual geography and timing of large historical earthquakes. Both types of models not only reproduce the statistics of seismicity over five units of magnitude but also duplicate other aspects including foreshock and aftershock sequences, migration of foci, and the capacity to produce both characteristic (when an isolated segment fails) and noncharacteristic earthquakes (when several adjacent or nearby segments fail in concert). Over a period of about 150 years the complex interaction of fault segments and the nonlinear failure conditions conspire to transform an apparently deterministic model into a chaotic one in that small changes in initial conditions ultimately lead to very different patterns of events. At least for the Middle America Trench, it seems doubtful that successful predictive models of earthquakes could span a much longer duration.
Ablative subduction: A two‐sided alternative to the conventional subduction model
Authors: Tao Winston C. , O'connell Richard J.
Published: February 2008

Subduction is conventionally modelled as a convergence and overlapping of two semirigid plates; such a convergence is one‐sided, in that one lithospheric plate descends while the other remains on the surface. We suggest an alternative model, one based on the dynamics of fluids. In this model, the viscous lower lithosphere flows downward, and the brittle upper lithosphere deforms in passive response. This process is potentially double‐sided, for we find that even a buoyant plate can be dragged downward by a dense, descending neighbor. Thus an apparent overriding plate may be worn away by a process of viscous ablation, with the rate of ablation a function of plate buoyancy. We call this process “ablative subduction”. Ablative subduction allows us to simply interpret observations concerning slab profiles, interplate seismicity, back arc tectonics, and complex processes such as double subduction and subduction polarity reversal. In performing experiments modelling the evolution of simple, fluid “slabs”, we find that slab profile is strongly influenced by ablation in the overriding plate. When ablation is weak, as when a buoyant continent borders the trench, deformable slabs adopt shallow, Andean‐style profiles. These profiles develop over time from an initially steep shape. More vigorous ablation rates, as might occur in an ocean‐ocean convergence, yield steeper, Marianas‐style profiles. Thus differences between Marianas‐style and Andean‐style slabs may result from differences in ablation rate and subduction duration. The occurrence of ablation might not be easily detected at depth, because material from subducting and ablating plates adhere closely as a single slab. These slabs show downdip deformations that are consistent with seismic focal mechanisms. Plate deformations associated with ablative subduction are also consistent with observed patterns of seismicity. In particular, the relative aseismicity of Marianas‐style plate boundaries is consistent with the ablative model; we may thus explain how plates can converge aseismically, without requiring that these plates be decoupled. Ablation should be weakest in Andean‐style subduction, although pulses of rapid ablation might lead to episodes of crustal shortening; in oceanic regions, the manifestation of ablation may depend upon tectonic regime. In an extensional area like the Marianas, ablation may be temporally self‐limiting, and ablative cycles may explain conflicting observations of tectonic erosion and accretion. In a compressional regime, vigorous ablation might lead to observations of double subduction and subduction reversal. In each case, the behavior of the lower lithosphere is relatively simple; what varies is the rate of ablation and the response of the upper lithosphere.
The electrical conductivity of an isotropic olivine mantle
Authors: Constable Steven , Shankland Thomas J. , Duba Al
Published: February 2008

In order to extend the useful temperature range of interpretation of olivine electrical conductivity σ we have used the nonlinear iterative Marquardt technique to fit experimental data over the range 720°–1500°C to the parametric form σ = σ1e−A1/kT + σ2e−A2/kT, where k is Boltzmann's constant and T is absolute temperature. The model describes conduction by migration of two different thermally activated defect populations with activation energies A1 and A2, and preexponential terms σ1 and σ2 that depend on number of charge carriers and their mobility and that may be different for each crystallographic direction. A combined interpretation of recent high (San Carlos olivine) and low (Jackson County dunite) temperature measurements has been made that demonstrates that a single activation energy A1 for all three crystallographic directions adequately fits the data. The parametric fits show that the high‐temperature conduction mechanism has far greater anisotropy than the low‐temperature mechanism, consistent with previous assignments to ionic and electronic conduction, respectively. The geometric mean of the conductivity in the three directions is approximately
S/m and is presented as a model for isotropic olivine, SO2, appropriate from 720°C to above 1500°C, at oxygen fugacities near the center of the olivine stability field. It is observed that the magnitudes of σ1 for the three crystal directions are similar to the ratios of the inter‐ionic distances between the M1 magnesium sites in olivine, to within 5%, consistent with Fe3+ preferring the M1 site below 1200°C.
A method for automatically determining normal fault geometry at depth
Authors: White Nicky
Published: February 2008

A large number of methods have been described which relate the shape of a normal fault at depth to the shape of a sedimentary horizon in its hanging wall. Unfortunately, the calculated fault geometry is strongly dependent upon the method used. Here, arguments are put forward in favor of a general two‐dimensional kinematic model which assumes that the hanging wall deforms by a combination of arbitrarily inclined simple shear and differential compaction. The method should be applicable to areas where it can be assumed that the fault plane does not change shape as deformation proceeds. This assumption may not always be valid, especially in the case of large‐scale, basement‐extending normal faults. Fault geometry, the inclination of simple shear, and compaction parameters may all be determined from N beds using a simple inversion scheme based on the general method. Throughout it is assumed that hanging wall and footwall stratigraphies are accurately known, though uncertainty in either could be included as a bounded variable within a formal inversion scheme. The most important feature of the model is that it is testable. It has first been tested on synthetic data using a range of fault geometries. The results of such modeling are encouraging and imply that all unknown parameters including fault geometry can, in general, be uniquely determined provided that the geometries of two or more beds within the hanging wall are known. The greater the number of beds used, the greater the degree of confidence in the solution. The method has also been applied to published sand‐box models with surprisingly good results. Ultimately, it is hoped that the method can be applied routinely to depth‐converted seismic reflection data. The preliminary applications presented here suggest that bulk, antithetically inclined, simple shear combined with differential compaction provide a good approximation for hanging wall deformation. In the future, the scheme will be generalized to allow for deformation in three dimensions. Other complexities such as horizontal and vertical variation in the degree of compaction could also be included, although at this stage the data do not appear to justify them.
Variations in the reflectivity of the moho transition zone beneath the Midcontinent Rift System of North America: Results from true amplitude analysis of GLIMPCE data
Authors: Hutchinson D. R. , Lee M. W. , Behrendt John , Cannon W. F. , Green A. G.
Published: February 2008

True amplitude processing of The Great Lakes International Multidisciplinary Program on Crustal Evolution seismic reflection data from the Midcontinent Rift System of North America shows large differences in the reflectivity of the Moho transition zone beneath the axial rift, beneath the rift flanks, and outside of the rift. The Moho reflection from the axial rift has a discontinuous, diffractive character marginally stronger (several decibels) than an otherwise transparent lower crust and upper mantle. Beneath the axial rift, Moho is interpreted to be a synrift igneous feature. Beneath the rift flanks, the reflectivity of the Moho transition is generally well developed with two identifiable boundaries, although in places it is weakly reflective to nonreflective, similar to Moho outside the rift. The two boundaries are interpreted as the base of essentially intact, although stretched, prerift Archean crust (upper boundary) and new synrift Moho 1–2 s (6–7 km) deeper (lower boundary). Beneath the rift flanks, the layered reflection Moho transition results from the preexisting crustal composition and fabric modified by synrift igneous processes and extensional tectonic/metamorphic processes. The geologic evidence for extensive basaltic magmatism in the rift is the basis for interpreting the Moho signature as a Keweenawan structure that has been preserved for 1.1 b.y. Extension and magmatism appear to enhance reflectivity in the lower crust and Moho transition zone only where stretching factors are moderate (rift flanks) and not where they are extreme (axial rift). This leads to the prediction that the reflectivity across analogous volcanic passive continental margins should be greatest beneath the moderately stretched continental shelves and should decrease towards the ocean‐continent boundary.
High‐temperature elasticity of iron‐bearing olivines
Authors: Isaak Donald G.
Published: February 2008

The first high‐temperature data on the nine adiabatic elastic moduli for iron‐bearing olivine are reported. These measurements are on two single‐crystal specimens of natural olivine at ambient pressure and from room temperature to a maximum of 1500 K. The two specimens contain 8 and 9 modal percent fayalite, which required the oxygen fugacity be controlled at high temperature to preserve their chemical stability. The rectangular parallelepiped resonance apparatus was adapted to buffer the specimens from the atmosphere with a mixture of CO and CO2 gas. A small increase (∼1–2 GPa) in the adiabatic bulk modulus of each specimen, over that of end‐member forsterite, was found. The data are high quality to extreme temperatures, with good agreement found when comparing the temperature derivatives of the elastic moduli of the two specimens. Neither specimen exhibits measurable nonlinear temperature dependence in the computed isotropic bulk and shear moduli, which is in contrast to published forsterite data. The temperature derivatives of the isotropic bulk modulus KS are (−1.69, −1.80) × 10−2 GPa K−1 for the two olivine specimens, and the shear modulus G derivatives are (−1.38, −1.36) × 10−2 GPa K−1. These derivatives are only slightly larger in magnitude than |(∂KS/∂T)P| = 1.56 × 10−2 and |(∂G/∂T)P| = 1.30 × 10−2 GPa K−1 found previously for iron‐bearing olivine over a very small temperature range. There are also no significant differences between the temperature derivatives found here and the average derivatives of end‐member forsterite from data retrieved over a slightly larger temperature range. Several dimensionless parameters have been calculated from these results and are discussed in view of systematics which bear on high‐pressure phases in Earth's transition zone. One result from these systematics related to the seismic velocities in the Earth, and especially the shear wave velocities, is that an olivine content of less than 50% is implied at the 400‐km discontinuity if Earth's upper mantle is isochemical. Furthermore, the substitution of almost 10% iron for magnesium at the forsterite end of the olivine solid solution series has little effect on the dimensionless parameters or on the temperature derivatives of the elastic moduli at high temperature.
Further studies of carbonate remagnetization in the northern Appalachian Basin
Authors: Saffer Barbara , McCabe Chad
Published: February 2008

Paleomagnetic and rock magnetic studies of sedimentary carbonates in the northern Appalachian Basin demonstrate that the late Paleozoic remagnetization in this area is probably related to authigenesis of single domain magnetite. Fifteen sites within Middle Devonian carbonates were sampled along a transect extending from Albany, New York, to the Algonquin Arch in southeastern Ontario. Paleomagnetic studies show that all sites except the two westernmost sites on the Algonquin Arch were remagnetized during Alleghenian time. The very weakly magnetic unremagnetized sites retain a dual‐polarity magnetization of probable Devonian age. In the remagnetized sites, both the size of the Kiaman components and magnetite concentration, estimated from low‐field susceptibility (X), are highest in the central part of the transect and lower on either side. X ray powder diffraction analyses demonstrate that K‐feldspar is also most abundant in the middle of the transect, with lower concentrations to the west and east. We suggest that formation of authigenic magnetite and K‐feldspar may be related in this setting. Sites with the largest Kiaman components have the highest unblocking temperatures and the largest ratios of anhysteretic remanent magnetization (ARM) to bulk susceptibility (ARM/X), indicating that these sites have the greatest amounts of single‐domain magnetite. Several remagnetized sites exhibit northerly, shallow magnetic components which may be Kiaman “normal” directions. We conclude that remagnetization along the New York‐Ontario transect is a chemical remanent magnetization resulting primarily from authigenesis of single‐domain magnetite. We present a model in which pulses of heated fluids triggered magnetite formation. Scanning electron microscope observation of magnetic extracts from 14 sites show that most of the resolvable magnetite grains are multidomain grains of irregular shape; many sites also contain a small percentage of magnetic spheres and/or euhedral magnetite particles. A majority of the magnetite grains contain Fe as the only cation, but many also contain minor amounts of other cations including various combinations of Cr, Ni, Ti, Zn, Mn, Si, Al, Ca, and Mg. Single domain magnetite is not resolvable with our observational techniques, but its presence is inferred from rock magnetic tests. Much of this magnetite may be of authigenic origin.
A Late Quaternary geomagnetic secular variation record from Lake Waiau, Hawaii, and the question of the Pacific nondipole low
Authors: Peng Lei , King John W.
Published: February 2008

A continuous paleomagnetic record for the last 13,000 years was obtained from the sediments of Lake Waiau, located near the summit of Mauna Kea Volcano on the island of Hawaii. The secular variation (SV) of the geomagnetic field was studied using spectral analysis and precession analysis of the magnetic vector. The hypothesis that there has been anomalously low geomagnetic SV in the central Pacific was tested using angular dispersion analysis. The inclination record has a mean value of 27° that is much lower than the 35° predicted by the geocentric axial dipole hypothesis. The spectrum obtained from spectral analysis shows major inclination peaks at periodicities of approximately 3600 and 7900 years and major declination peaks at periodicities of approximately 1100, 2000, 3300, and 7800 years. The approximately 8000‐ and 3500‐year periodicities shown in both inclination and declination spectrums may represent the dipole and nondipole variation, respectively. The approximately 1100‐ and 2000‐year periodicities are probably derived from either the sources that affect declination only or the offsets between the core sections. In addition, Bauer plots of the magnetic vector are dominated by clockwise motion. The angular dispersion calculated from lake sediments is 5.9° with respect to the mean virtual geomagnetic pole (VGP) and 9.1° with respect to the geographic pole. Both values are much lower than the 13.5° expected from paleosecular variation model G which separates the angular dispersion into components from the dipole and the quadruple families. Our results support the hypothesis that an anomalously low secular variation is a characteristic feature of the central Pacific. Furthermore, this geomagnetic behavior persists on time scales of 104 years. Regional anomalies in temperature, topography or electrical conductivity at the core‐mantle boundary are inferred to cause the low SV.
Numerical simulation of fluid flow and silica transport and deposition in boiling hydrothermal solutions: Application to epithermal gold deposits
Authors: Cline Jean S. , Bodnar Robert J. , Rimstidt J. Donald
Published: February 2008

A numerical fluid flow model quantifying silica precipitation in hydrothermal systems as a function of temperature, permeability, and mass flux has been employed to evaluate the role of boiling in precipitating quartz in the epithermal environment. Relative permeability relationships of liquid and vapor phases incorporated into the physical model have been modified to be temperature dependent and are consistent with current understanding of fluid flow in one‐component, two‐phase systems. The modifications extend the height of the boiling column and eliminate discontinuities in steam content on initiation of boiling. Results indicate that the degree to which boiling contributes to quartz precipitation is dependent on three related factors: the temperature of the fluid entering the base of the boiling system, the rate of fluid temperature decrease with respect to the distance of fluid travel, and the extent of fluid vaporization, particularly in regions of gradual temperature decline. Boiling contributes significantly to quartz precipitation in systems with high‐temperature fluids, and in deeper portions of systems in which extensive vaporization occurs. Temperature reduction is the dominant quartz precipitation mechanism in regions where temperature reduction is rapid, and in lower temperature systems. Owing to the smaller absolute difference in quartz solubility between the liquid and vapor phases at low temperatures as compared to higher temperatures, boiling is a less important precipitation mechanism in low temperature, nearsurface regions. Quartz precipitation is most intense in systems with short column heights, i.e., systems with high mass flux/permeability ratios, and low initial fluid temperatures. Vertical permeability variations within the flow channel produce steam jumps, or discontinuities, in mass fraction steam profiles, resulting in local zones of enhanced quartz precipitation or dissolution. Ore grade calculations indicate that high mass flux rates, low permeabilities, and low initial fluid temperatures promote precipitation of high gold grades in response to boiling. Under these conditions gold precipitates over a smaller interval of temperature decline and over a shorter ore horizon height.
“Turbulent mixing” induced by upgoing near‐inertial waves in the seasonal thermocline of the Norwegian Sea
Authors: Marmorino G. O. , Trump C. L.
Published: February 2008

Acoustic Doppler current profiler and towed thermistor chain measurements made in the seasonal thermocline in the Norwegian Sea are examined for evidence that near‐inertial shear leads to small‐scale “turbulent mixing” (as inferred from meter‐scale temperature fluctuations). For the ≈1 day of data examined here, spatial variability in shear is dominated by near‐inertial waves having upward energy propagation, ≈30‐m vertical wavelength, and 5‐ to 10‐km horizontal extent ( ≈20‐km wavelength). Repeated samples of one of these waves had high Froude number and enhanced small‐scale variability dominated by ≈20‐m‐long waveforms which we conjecture are Kelvin‐Helmholtz instabilities arising from the near‐inertial shear.
A Gulf Stream model and an altimetry assimilation scheme
Authors: Mellor George L. , Ezer Tal
Published: February 2008

A continuous data assimilation scheme and a multilayer, primitive equation, numerical model are described. The model is an eddy‐resolving, coastal ocean model that has been extended to include the Gulf Stream region. It has complete thermohaline dynamics, a bottom‐following, sigma, vertical coordinate system, and a coastal‐following, curvilinear orthogonal, horizontal coordinate system. Calculated model fields are used to provide a model climatology and correlations between subsurface temperature and salinity anomalies and surface elevation anomalies. An optimal interpolation method, the surface to subsurface correlations, and estimated model and data errors are the basis of the assimilation technique. Altimetry anomaly data extracted from the model calculations according to the Geosat orbital schedule are used to test the assimilation scheme and to provide nowcasts and forecasts. Sensitivity studies are performed to test the effects of various parameters of the scheme. It is found that the scheme is less efficient in the shallow continental shelf area than in the deeper regions of the model. The results show significant nowcast skill, with area‐averaged rms error for surface elevation and subsurface properties of about 40–50% of the corresponding error of the unassimilated case. Good forecast skill, better than persistence, is demonstrated for 10–20 days; there is little skill after 30–40 days. Increasing the density of the satellite altimetry data (especially by decreasing the separation distance between tracks) should decrease the nowcast rms error to about 15% and improve the forecast.
A Geosat altimeter wind speed algorithm and a method for altimeter wind speed algorithm development
Authors: Witter Donna L. , Chelton Dudley B.
Published: February 2008

A Geosat altimeter wind speed algorithm is derived by cross‐calibrating Geosat and Seasat altimeter estimates of the normalized radar cross section σ0 and modifying an existing Seasat altimeter wind speed model function to obtain a model function appropriate for Geosat observations. It is argued that the σ0 distribution measured by an altimeter is relatively stable over a sufficiently large geographical region and a long enough time period. Systematic differences between σ0 estimates from two altimeters can therefore be identified based on comparisons of their σ0 histograms. Any such systematic differences can then be corrected using independent σ0 estimates. When this method is applied to the Geosat and Seasat altimeters, a systematic difference between the two σ0 histograms is shown to be consistent with differences between Seasat altimeter and nadir Seasat scatterometer estimates of σ0 deduced independently by a previous study. This supports the conclusions that (1) the σ0 distribution is stable, and (2) the Seasat altimeter estimates of σ0 were miscalibrated. After modifying the existing Seasat altimeter wind speed algorithm to account for this apparent σ0 error, the resulting Geosat estimates of wind speed agree with high‐quality buoy observations to within an rms difference of less than 2 m/s.
Pigment distribution and primary production in the western Mediterranean as derived and modeled from coastal zone color scanner observations
Authors: Morel André , André Jean‐Michel
Published: February 2008

A set of 114 coastal zone color scanner (CZCS) images of the western Mediterranean (mainly in the year 1981) have been processed and analyzed to describe the algal biomass evolution and estimate its potential carbon fixation. For that, the pigment concentration in the top layer, Csat, is used through empirical relationships to infer the depth‐integrated pigment content of the productive column, 〈C〉tot. A spectral light‐photosynthesis model driven by 〈C〉tot is operated with additional information, namely, about sea temperature and photosynthetically available radiation (computed from astronomical and atmospherical parameters then combined with a cloud climatology). This model also includes a standard set of physiological parameters which account for the light capture by algae and for the use of this radiant energy in photosynthesis. This model allows a climatology of ψ* the cross section for photosynthesis per unit of areal chlorophyll, to be produced and then convoluted with the biomass maps after they have been averaged and composited. On average and for the whole western Mediterranean, the pigment concentration in the upper layer is about 0.25 mg Chl m−3, leading to an areal mean concentration of 21 mg Chl m−2. The maximum (bloom) occurs in early May in all zones. Seasonal variations in algal biomass are much more marked in the northern part than in the southern part (apart from Alboran Sea); the south Tyrrhenian basin and the central part of the Algerian basin are steadily oligotrophic. The mean annual carbon fixation rate for the whole basin is about 94 g C m−2 yr−1, or 106 and 87, for the northern and southern basins when separately considered. The seasonality is expressed by a six‐fold change in the production rate (between February and May) within the northern zone, whereas only a two‐fold change occurs in the southern zone between the same months. These estimates, which compare well with previous episodic field data, considerably extend our knowledge of the temporal progression of productivity within the entire western Mediterranean and its various provinces.
Dynamical simulations of filament formation and evolution in the Coastal Transition Zone
Authors: Haidvogel Dale B. , Beckmann Aike , Hedström Katherine S.
Published: February 2008

Using the semispectral primitive equation model of Haidvogel et al. (1991), the evolution of a forced, surface‐intensified, eastern boundary current is studied in the presence of both finite‐amplitude topography and irregular coastline geometry. The model domain is 1000 km in alongshore length, and extends on average 700 km in the cross‐shelf direction. A representative cape, as well as smoothed continental shelf‐slope topography, are included. The model is forced by inclusion of nudging terms in the equations of motion which relax the fluid system back to a prescribed reference state on a time scale of 45 days. The reference state chosen is a broad, geostrophically balanced, equatorward flow having a maximum current at the surface of 0.45 m s−1 and a transport of approximately 10 Sv. No explicit wind forcing is included. Initialized with the smooth surface current, the model quickly approaches a turbulent, time‐dependent equilibrium featuring an intense, meandering alongshore jet with local velocities of 0.8–1.0 m s−1. A deep, poleward undercurrent also forms. Subsequent interaction with the protruding cape geometry causes an offshore deflection in the steepening frontal meanders, some of which produce elongated filaments which penetrate significant distances (400–500 km) offshore. The emerging filaments are characterized by a strong downwelling signal (maximum vertical velocities of 30–40 m d−1). The simulated filaments ultimately pinch off, typically within 40–50 days, to form a corotating pair of detached eddies. The existence of the cape geometry, as well as the southward surface flow, appears to be necessary in this model to produce filament generation; removal of the irregular coastal geometry or reversal of the sense of the surface circulation is shown to inhibit filament formation. Detailed analysis of both instantaneous and time‐mean momentum balances show dynamical similarity to observations taken during the Coastal Transition Zone experiment and elucidate the eddy transport mechanisms responsible for the formation of the poleward undercurrent in these experiments. Offshore transport of heat by the filaments, found to be O(0.06) PW, is substantial.
Standing stocks of particulate carbon and nitrogen in the equatorial Pacific at 150°W
Authors: Eppley Richard W. , Chavez Francisco P. , Barber Richard T.
Published: February 2008

In March 1988 a north‐south transect at longitude 150°W was carried out in the equatorial Pacific between latitude 15°N and 15°S. Observations of suspended particulate matter in the euphotic zone included particulate carbon, nitrogen, and chlorophyll, with a few samples taken for counting picoplankton and nanoplankton. Nearly all the particulate nitrogen is accounted for by bacteria and phytoplankton. The standing stock of bacterial C and N exceeded that of phytoplankton. The residence time of particulate organic carbon in the surface water was only 4–5 days in the equatorial upwelling, but 10–15 days to the north and south, suggesting relatively rapid removal by recycling and lateral advection.
Seasonal cycling of trace elements 137Cs, 7Be, and 239+240Pu in Lake Michigan
Authors: Robbins John A. , Eadie Brian J.
Published: February 2008

Trace elements, fallout 137Cs (t½=30.2 years) and cosmogenic 7Be (t½=53.4 days) were measured in trap samples collected from two sites in southern Lake Michigan at selected times during the period from 1982 through 1987. Concentrations of 137Cs in trap material were virtually constant from top to bottom during the period when the lake was vertically well‐mixed. Fluxes of 137Cs were more than two orders of magnitude higher than atmospheric loading. With the development of the thermocline, concentrations of the isotope diminished to undetectable levels at a rate indicating a 1‐month residence time of resuspended components in the epilimnion. Declines in mass flux (10 to 0.2 g m−2 d−1) and trace element concentrations (some by 30 x) also occurred at this time. Epilimnetic concentration of 7Be increased from a prestratification value of about 30 dpm/g to a maximum of about 120 dpm/g in July. During calcite formation in August, the concentration rapidly declined mainly as a result of enhanced particle settling rates. Throughout the period of stratification, a maximum in concentration and flux of 7Be persisted in trap material between 40 and 60 m depth but only about 15% of the atmospheric flux was transferred to the benthic boundary region below 60 m. In near‐bottom trap materials, the 137Cs concentration was reduced at the onset of stratification by addition of radiocesium‐deficient authigenic materials. By November, concentrations of 137Cs had returned to prestratification levels through remineralization of these materials. A decrease of 22% in the seasonally invariant ratio of 137Cs/ 40K between 1982 and 1986 indicated a decay‐corrected removal time of about 20 years, which is consistent with the rate of decrease of Pu concentrations in the well‐mixed water column measured over more than a decade. In benthic trap samples, 7Be concentration decreased exponentially throughout the stratified period, with a time constant comparable to the half‐life of the isotope. The resuspendable sediment pool, evidently isolated from atmospheric loading during stratification, was “recharged” with 7Be following late fall overturn. Box and advection‐diffusion models describing the storage and vertical distribution of 7Be during the winter‐mixing period successfully account for observations, implying an inverse relation between water depth and winter recharging of the resuspendable pool. A plutonium‐calibrated box model for the seasonal cycle of tracers in surface waters describes the main features of the variation of Pu isotopes, 137Cs and 7Be.
Ventilation of North Pacific intermediate waters: The role of the Alaskan Gyre
Authors: Van Scoy Kim A. , Olson Donald B. , Fine Rana A.
Published: February 2008

Hydrographic data, tritium data, and potential vorticity calculations suggest that although North Pacific Intermediate Water is formed in the northwest, the Alaskan Gyre might be an additional ventilation site. The proposed ventilation is quantified by a vertical column tritium inventory, which indicates an excess of 0.08 kg of tritium in the Alaskan Gyre. An evaluation of the energy stored in the water column and of wind and buoyancy forcing shows that during winter conditions enough energy can be pumped into the system to force 26.80 σθ to outcrop in the Alaskan Gyre. Model results suggest that relatively limited outcrops in time and space (tens of days and several hundred kilometers in diameter) can account for the excess tritium.
On the formulation of open boundary conditions at the mouth of a bay
Authors: Greatbatch Richard J. , Otterson Timm
Published: February 2008

We describe our experience in formulating open boundary conditions to apply at the mouth of a reduced‐gravity model of a bay. Our objective is to find a way to calculate the response of the bay to wind forcing over the bay itself, without being concerned about the influence of regions beyond. We show that open boundaries from which Kelvin waves can propagate along the coast into the model domain (“upstream” boundaries) must be treated with care. We begin by considering an “upstream” boundary which runs perpendicular to the coast. We find that if a radiation condition is applied on such a boundary, then spurious Kelvin waves of near‐inertial period can propagate in from the boundary and contaminate the solution in the interior of the model domain. Also, if there is Ekman transport at the “upstream” boundary away from (toward) the coast, then upwelling (downwelling) will occur indefinitely and completely swamp the model solution in the bay. This is similar to the solution we expect when the coastline is straight and extends to infinity in the “upstream” direction. However, it is not the same, since the rate of upwelling (downwelling) is roughly half the theoretical value for that case. For the problem of a bay we suggest that the way to deal with this is to extend the coastline out to sea on the “upstream” side of the mouth and apply a condition on the artificial stretch of the boundary which suppresses Kelvin wave propagation but is also not prohibitively reflective to outgoing Poincaré waves. For our problem a condition of zero normal gradient in interface displacement seems to be sufficient. This condition also captures reasonably well the near‐inertial Kelvin waves that are generated by the northwest corner of the bay (which are a genuine part of the solution) as long as the other boundaries are sufficiently far from the bay. We have also experimented with using sponge layers rather than radiation conditions on the other boundaries. We find that sponging only the interface displacement and leaving the velocities undamped gives results that compare well with those obtained using radiation conditions. On the other hand, sponging the velocities as well as the interface displacement leads to significantly different results. This is the case even if only the “pressure‐driven” part of the velocities is sponged and the Ekman part is left undamped. This suggests that applying highly viscous layers around open ocean parts of model boundaries can be counter‐productive. It is sometimes better to damp only the density field and leave the velocities undamped. Finally, we discuss how our results apply to three‐dimensional general circulation type models.
Currents forced by stochastic winds with meridionally varying amplitude
Authors: Samelson R. M. , Shrayer B.
Published: February 2008

Motivated by discrepancies between observations and previous theory, we present a stochastically forced model of wind‐driven current fluctuations that includes the observed northward intensification of wind stress curl, a feature absent from a previous model. The model is linear, barotropic, and quasi‐geostrophic. It is bounded by absorbing walls along lines of constant latitude in the north and south, and is unbounded east and west. The northward wind intensification causes a northward bias in the coherence of wind stress curl and currents that is similar to that observed. Art unanticipated secondary maximum occurs northeast of the mooring in the low‐frequency (37‐day period) meridional velocity coherence predictions.
Sea ice ridging in the eastern Weddell Sea
Authors: Lytle V. I. , Ackley S. F.
Published: February 2008

In August 1986, sea ice ridge heights and spatial frequency in the eastern Weddell Sea were measured using a ship‐based acoustical sounder. Using a minimum ridge sail height of 0.75 m, a total of 933 ridges were measured along a track length of 415 km. The ridge frequency varied from 0.4 to 10.5 ridges km−1. The mean height of the ridges was found to be about 1.1 m regardless of the ridge frequency. These results are compared to other ridging statistics from the Ross Sea and found to be similar. Comparison with Arctic data, however, indicates that the height and frequency of the ridges are considerably less in the Weddell Sea than in the Arctic. Whereas in the Arctic the mean ridge height tends to increase with the ridge frequency, we found that this was not the case in the Weddell Sea, where the mean ridge height remained constant irrespective of the ridge frequency. Estimates of the contribution of deformed ice to the total ice thickness are generally low except for a single 53‐km section where the ridge frequency increased by an order of magnitude. This resulted in an increase in the equivalent mean ice thickness due to ridging from 0.04 m in the less deformed areas to 0.45 m in the highly deformed section. These values were found to be consistent with values obtained from drilled profile lines during the same cruise.
Use of satellite scatterometer winds to force an ocean general circulation model
Authors: Barnier Bernard , Boukthir Moncef , Verron Jacques
Published: February 2008

The present study is an attempt to evaluate the capacity of scatterometer wind data to be provided by the forthcoming European Remote Sensing 1 (ERS 1) and NASA Scatterometer (NSCAT) satellite missions, to successfully force basin scale ocean circulations. The mid‐latitude ocean is simulated with a three‐layer, quasi‐geostrophic, eddy‐resolving model in a square box geometry. The variable wind stress, which is used to simulate scatterometer winds, is derived from European Centre for Medium‐Range Weather Forecast (ECMWF) daily analyses of the wind stress over the North Atlantic. Its curl is applied as a forcing at every grid point of the model to drive a circulation of reference. The model is forced in real time under the swath with the curl derived from the raw scatterometer winds, with a condition of persistence that keeps the forcing constant between two consecutive passages of the satellite. The winds of ERS 1 (at 3‐ and 35‐day periods) produce circulations which globally resemble that of the reference experiment. A 3‐day repeat orbit appears to be the most suitable for producing raw winds capable of realistically driving a numerical ocean model. NSCAT (3‐day period) reproduces the main features of the reference large scale circulation with more success than ERS 1, and it is clear that NSCAT raw winds are far more suitable for driving an ocean Eddy‐Resolving General Circulation Model (EGCM) than ERS 1 winds because of their denser spatial coverage. ERS 1 and NSCAT raw wind stress curl fields are then objectively analyzed to produce smoothed “interpolated” wind stress curl fields having continuous values at every model grid point. Simulations with ERS 1 and NSCAT interpolated forcings show an improvement in the structure of the mean flow, but discrepancies in the energetics lead us to conclude that our interpolated winds are not significantly better than the raw winds. A «combined» forcing is constructed consisting of daily maps containing NSCAT raw data under the swaths and the corresponding objectively analyzed field outside the swaths. The circulation driven by the combined forcing reproduces remarkably well the main features and the energetics of the reference circulation. The patchy resolution of the combined forcing appeared to have little effect on the results of the simulations. Our conclusion is that, within the framework of the present study, scatterometer winds (and particularly those of NSCAT) seem appropriate for providing a forcing function to mid‐latitude ocean models, and for that purpose, these satellite data may not require prior assimilation in meteorological models, despite their patchiness and irregular resolution.
Seasonal and interannual variations of sea surface salinity in the tropical Pacific Ocean
Authors: Delcroix Thierry , Hénin Christian
Published: February 2008

Sea surface bucket measurements, obtained through a ship‐of‐opportunity program, are used to describe the sea surface salinity (SSS) field for the tropical Pacific during the period 1969–1988. Emphasis is placed upon the mean SSS distribution and the seasonal and interannual SSS variability occurring along four well‐sampled shipping tracks. These tracks extend from New Zealand to Japan, from New Zealand to Hawaii, from Tahiti to California, and from Tahiti to Panama. They cross the equator at 155°E, 160°W, 140°W, and 100°W, respectively. Along each track, the mean SSS distribution is characterized by SSS minima which are 4°–6° further poleward than the axes of maximum precipitation associated with the Intertropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ). It is suggested that these SSS minima owe their existence mainly to heavy rainfall and poleward Ekman salt transport associated with the trade winds. The role of zonal salt advection was found negligible for these SSS minima. Except along the eastern track, maximum seasonal SSS variations are located in the ITCZ and SPCZ regions, with minimum SSS in September‐October and March‐April, respectively. On the basis of precipitation island stations, it is demonstrated that the maximum seasonal SSS variations are closely related to the rainfall regimes of the ITCZ and SPCZ (rainfall maximum 3 months before SSS minimum; rainfall amount sufficient to account for SSS changes). Along the eastern track, a strong annual SSS cycle is found from about 4°S (110°W) to the Panama coast (minimum SSS in February–March), reflecting the combined effects of rainfall, salt advection, and vertical mixing. Notable interannual SSS variability concerns only the western half of the tropical Pacific Ocean where El Niño‐Southern Oscillation (ENSO) related SSS changes are strongly related to ENSO‐related precipitation changes. During ENSO periods, the SSS field west of about 150°W is characterized by fresher‐than‐average SSS within about 8°N to 8°S, and conversely saltier‐than‐average SSS poleward of 8° latitudes. These modifications in the SSS field are thought to result mainly from an eastward displacement in the ascending branch of the Walker and Hadley cells which induces unusually high rainfall over the western and central equatorial Pacific region bordered on all sides by rainfall deficits. Reproducing the actual SSS changes at seasonal and interannual time scales would be a very stringent test for model capability.
Study of ocean slicks by nonlinear laser processes: 1. Second‐harmonic generation
Authors: Frysinger Glenn S. , Asher William E. , Korenowski Gerald M. , Barger William R. , Klusty Mark A. , Frew Nelson M. , Nelson Robert K.
Published: February 2008

Reflected optical second‐harmonic generation (SHG) was applied for the first time to in situ study of the air‐sea interface. SHG signals were detected from the first several molecular layers of the ocean surface during the SAR X Band Ocean Nonlinearities (SAXON) Chesapeake Light Tower experiment in 1988, and during experiments on Nantucket Sound at the Woods Hole Oceanographic Institution in 1989. The SHG response of the ocean surface was observed to correlate with increased slick activity and surface tension measurements of surface water organic content. The SHG response was similar for naturally occurring slicks and for artificially created slicks of several known materials. The SHG signal intensity was also used to estimate the second‐order nonlinear optical susceptibility of the ocean surface. It was determined that the SHG nonlinear laser technique is a useful noninvasive probe for in situ studies of ocean surface chemistry.
Barotropic and baroclinic tidal currents in the eastern basins of the North Atlantic
Authors: Siedler Gerold , Paul Uwe
Published: February 2008

Data from a large‐scale moored array in the Iberian and Canary basins are used to determine the energies of barotropic and baroclinic M2 and S2 tides. An analysis of time‐varying dynamical modes is performed. The results for barotropic modes confirm the global surface tide model results of Schwiderski (1980) for this region. The barotropic modes dominate in the deep basins, but increased baroclinic contributions are usually found over rough topography. At three locations near the continental slope in the southern Canary Basin the baroclinic modes dominate the barotropic mode. Results from an array of three moorings at the northern part of the Cape Verde Rise show an inverse behavior of barotropic and baroclinic energies, such that the baroclinic energy is steadily enhanced while the barotropic energy is reduced towards the continental margin. The increase in baroclinic energy is consistent with a generation of internal tides close to the shelf by surface tidal forcing over topography. Further evidence for this process is provided by the 2‐week periodicity of the first‐order baroclinic mode at the slope, corresponding to the spring‐neap cycle of the barotropic tide.