bluuebunny/crossref_metadata_2025 huggingface dataset:
Crustal structure beneath Orphan Basin and implications for nonvolcanic continental rifting
Authors: Chian D. , Reid I. D. , Jackson H. R.
Published: February 2004

A wide‐angle seismic profile was acquired across the northeast Newfoundland margin, seaward to Orphan Knoll. The profile is complemented by existing multichannel reflection data, two online deep wells, and other deep seismic reflection and refraction lines. Modeling of data from 15 ocean bottom seismometers provides the seismic velocity structure for a significant portion of the ∼400‐km‐wide stretched continental crust on this margin. Prerift metasedimentary rocks have a topographic relief of up to 5 km, with a velocity of ∼5.2 km/s increasing to 5.9 km/s. Unaltered crystalline crust below the metasedimentary section has a velocity range of 6.1–6.5 km/s. Most of the lower crust landward of Orphan Knoll has a typical continental lower crustal velocity (6.8–7.0 km/s), and the lower crust shallows significantly beneath Orphan Knoll. Our model shows no evidence for the 5‐km‐thick, 7.35 km/s layer modeled from data of an earlier experiment and interpreted as magmatic underplating but instead indicates that thinned continental crust extends seaward for 360 km without underplating, implying nonvolcanic rifting for the NE Newfoundland margin. We suggest that continental stretching persisted from ∼180 Ma to Chron 34N time (∼101 Ma) when the final breakup between Canada and western Europe took place, leaving a 400‐km‐wide zone of thinned continental crust underneath the shelf and deep water area of the Orphan Basin. On the landward side of the basin, gravity modeling indicates a zone of very thin (6–8 km) crust, possibly a failed rift center formed as a result of the northward progression of nonvolcanic rifting between Canada and Europe. A possible sub‐Moho reflector suggests generally uniform stretching of the crust and upper mantle, and the absence of volcanics and underplating is in contrast to the observations on related rift basins of the Grand Banks.
Temperature gradients in the laser‐heated diamond anvil cell
Authors: Panero W. R. , Jeanloz R.
Published: February 2004

A semianalytic heat transfer model reproduces the temperature distributions measured inside the laser‐heated diamond cell and constrains the temperature dependence of the thermal conductivity of dielectric mineral samples at high pressures and temperatures. Both axial and radial heat conduction determine the observed temperature distributions obtained from the TEM00 (Gaussian) and TEM01* laser modes. Measurements on the high‐pressure perovskite‐dominated assemblage of peridotite are consistent with the expected T−1 dependence of thermal conductivity at 45 GPa and temperatures between 1500 and 3800 K.
Reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits: 1. Methods
Authors: Koyaguchi Takehiro , Ohno Marekazu
Published: February 2004

Two inversion methods are developed to reconstruct eruption column dynamics from granulometric data of Plinian deposits. They are based on the steady tephra dispersal model from umbrella clouds which spread as gravity currents. The methods provide (1) expansion rate of an umbrella cloud, (2) grain‐size distribution at the top of the eruption column, and (3) total amount of ejecta including very fine particles which are extensively dispersed in the atmosphere. The first quantity is correlated to the magma discharge rate. The second quantity provides information about sorting processes during column rise, and hence, about deceleration in the “gas thrust region” of the eruption column and the timing of column collapse. The third quantity, together with the estimated magma discharge rate, provides the effective duration of the eruption. The two methods are based on the same dispersal model but use different averaging. Combining the two methods can provide additional information about eruption dynamics, such as minor flow structures of the umbrella cloud and fluctuations of eruption rate. The methods are applicable to large‐scale eruptions where the radial expansion velocity of the umbrella cloud exceeds wind velocity. The applicability of the methods is tested from the viewpoint of comparison with real‐time observations in the companion paper (paper 2).
Atmospheric excitation of the Chandler wobble, 1983–1998
Authors: Aoyama Yuichi , Naito Isao
Published: February 2004

The excitation of the Chandler wobble (CW) by atmospheric wind and pressure variations is studied for the period 1983–1998. The data used here are the atmospheric angular momentum (AAM) functions computed from the operational objective analysis data of the Japan Meteorological Agency, compared with observed Earth wobble data. In the vicinity of the Chandler frequency (0.847 cycle per year), the AAM function has a broad spectral peak with somewhat excessive power and about 30° phase lead to that inferred from the observed wobble, with a coherence exceeding 95% confidence threshold. In addition, the temporal variation in the CW excitation calculated from the shifted 8‐year data of the AAM function is found to have nearly the same pattern as that inferred from observation, in which the wind and inverted barometer (IB) pressure contributions complement each other. They reveal that the atmospheric wind and IB pressure variations by themselves maintain a major part of the observed CW during the analysis period.
Effects of volcano loading on dike propagation in an elastic half‐space
Authors: Muller Jordan R. , Ito Garrett , Martel Stephen J.
Published: February 2004

We use laboratory experiments and numerical models to examine the effects of volcano loading on the propagation of buoyant dikes in a two‐dimensional elastic half‐space. In laboratory experiments we simulate the propagation of buoyant dikes in an isotropic regional stress field by injecting air into tanks of solidified gelatin. A weight resting on the surface of the gelatin represents a volcanic load. A numerical model is used to simulate these experiments. Both experiments and numerical simulations show that as a dike ascends, it begins to curve toward the load in response to the local stress field imposed by the load. The lateral distance over which dikes curve to the load increases with the ratio of average pressure at the base of the load to the dike driving pressure. For realistic volcano and dike dimensions this pressure ratio is going to be large, suggesting that dikes can converge to a volcano over lateral distances several times the load width. Numerical calculations involving an anisotropic regional stress field, however, predict that the lateral extent of dike attraction shrinks as the regional horizontal compressive stress decreases relative to the vertical compressive stress. Dike focusing will be substantial if the regional differential stresses are less than the average pressure at the base of the load. If this is the case, then our models predict a positive feedback between the size of volcanoes and the area of dike attraction. This feedback may promote the development of large discrete volcanoes and also predicts a positive correlation between the spacing and sizes of adjacent volcanoes. To test this prediction, we examine nearest‐neighbor pairs of the 21 largest volcanoes in the Cascade Range. The 14 pairs examined show a large range in volcano spacing (6–115 km) and a statistically significant correlation between spacing and average volcano height. This result is consistent with our model results and suggests that the local compressive stress induced by these volcanoes may be an important factor in controlling magma transport in the lithosphere.
Source parameters and three‐dimensional attenuation structure from the inversion of microearthquake pulse width data: Method and synthetic tests
Authors: Zollo Aldo , de Lorenzo Salvatore
Published: February 2004

We propose a new method to determine source parameters and attenuation structure of a three‐dimensional medium based on first P rise time and total pulse width measurements from microearthquake data. The effects of fault finiteness on seismic radiation are taken into account by assuming the rupture model for a circular crack of Sato and Hirasawa (1973). Ray theory synthetic seismograms in a constant Q anelastic medium are computed to derive a set of nonlinear equations which relate the source and attenuation parameters (fault radius, orientation of the fault plane, and quality factor) to the pulse width data (half and total duration of the P waveforms).The numerically built relationships are used to compute the direct problem in the framework of a nonlinear inversion scheme, based on the modified downhill Simplex method. The validity and robustness of the inversion method are tested by synthetic simulations by assuming the sources and receivers configuration of the seismic passive experiment conducted in the Campi Flegrei caldera (southern Italy) during the last microearthquake crisis (1982–1984). Different heterogeneous Q models have been considered in order to assess the uncertainty and resolution of source and attenuation parameters for the given acquisition layout. The results of this simulation study indicate that first pulse width data from a local network permit retrieval with sufficient accuracy of the heterogeneous Q structure and fault radii. A rather dense azimuthal coverage of the sources is instead needed to recover the angles (in particular, the fault strike) which define the fault orientation.
On the effective friction law of a heterogeneous fault
Authors: Campillo Michel , Favreau Pascal , Ionescu Ioan R. , Voisin Christophe
Published: February 2004

Numerical simulation of the rupture process is usually performed under an assumption of scale invariance of the friction process although heterogeneous fault properties are shown by both direct observations of surface crack geometry and slip inversion results. We investigate if it is possible to define an effective friction law for a finite fault with a small‐scale heterogeneity, that is, with a distribution of narrow segments with a resistance to rupture higher than the rest of the fault. We consider a model where the local boundary condition corresponds to a linear slip‐dependent friction law. We define the effective slip‐dependent friction law by analogy with the theoretical spectral solution for the initiation phase in the case of a homogeneous infinite fault. We use finite difference simulations to test the validity of this approach. The results show a surprisingly good agreement between the calculations for the complete heterogeneous fault model and for a homogeneous fault with an effective friction law. The time of initiation and the average of the slip velocity on the fault are well predicted by the effective model. The effective friction law exhibits a nonlinear slip dependence with an initial weakening rate different from the one of the local laws. This initial weakening rate is related to the geometry of the heterogeneity and can be obtained by an eigenvalue analysis. The effective law shows a kink at a slip that corresponds to the average slip on the fault for which the stress concentration of the strong segments is sufficient to trigger their rupture. While based on a rather simple model of a fault, these results indicate that an effective friction can be defined and used for practical purposes. The heterogeneity of a fault tends to decrease the initial weakening rate of the local weak patches. Since the initial weakening rate controls the initiation duration, this last point indicates that the duration of initiation expected from actual heterogeneous faults is much larger than the one deduced from small‐scale laboratory measurements. The actual fracture energy is not conservative in the rescaling of the friction law.
Laboratory studies of high Rayleigh number circulation in an open‐top Hele‐Shaw cell: An analog to mid‐ocean ridge hydrothermal systems
Authors: Cherkaoui Abdellah S. M. , Wilcock William S. D.
Published: February 2004

Ridge crest hydrothermal systems are generally characterized by high heat fluxes and a permeable top which allows fluids to flow freely across the seafloor. The basic patterns of flow and heat transport in such systems are poorly known especially at high Rayleigh numbers. We present the results of a laboratory study using a wide‐aspect ratio, open‐top Hele‐Shaw cell heated from below. The model is an analog for two‐dimensional hydrothermal convection. The onset of convection occurs at a Rayleigh number Ra = 20.4 in good agreement with numerical predictions. Above onset, the spacing of upwelling plumes decreases quickly with the Rayleigh number before reaching an asymptotic limit in the range 0.2–0.3 above Ra ≈ 125. Periodic unsteady flow is first observed at Ra ≈ 47 and is a result of small thermals that are advected horizontally in the bottom boundary layer. The amplitude, frequency, and irregularity of oscillations increase with the Rayleigh number. Unsteadiness can also result from the repeated formation of new plumes accompanied by the merging of plumes elsewhere in the model. Above Ra ≈ 590, the flow patterns show no dominant frequency and are chaotic. In the unsteady regime the frequency of oscillations scales as Ra1.92 and the Nusselt number scales as Ra0.91 above Ra ≈ 200. These relationships are in close agreement with theoretical scalings for high Rayleigh number porous convection, Nu ∝ Ra and ƒ ∝ Ra2. Assuming vigorous mid‐ocean ridge systems can be approximated by flow through a Darcy porous media, our results predict that upwelling sites will be spaced at about half the depth of circulation. This is compatible with observations on the Endeavour segment of the Juan de Fuca Ridge. Our results also predict that ridge crest hydrothermal systems with Nusselt numbers of 10–500, will exhibit unsteady flow at periods of ∼500–0.2 years. Unsteady convection has not yet been observed, perhaps because highly permeable flow channels stabilize the flow in real systems.
Attenuation tomography in the western central Andes: A detailed insight into the structure of a magmatic arc
Authors: Haberland Christian , Rietbrock Andreas
Published: February 2004

High‐quality data from 1498 local earthquakes recorded by the PISCO ’94 (Proyecto de Investigatión Sismológica de la Cordillera Occidental, 1994) and ANCORP ’96 (Andean Continental Research Project, 1996) temporary seismological networks allowed the detailed determination of the three‐dimensional (3‐D) attenuation structure (Qp−1) beneath the recent magmatic arc in the western central Andes (20° to 24°S). Assuming a frequency‐independent Qp−1 in a frequency band between 1 and 30 Hz, whole path attenuation (t*) was estimated from the amplitude spectra of the P waves using spectral ratios and a spectral inversion technique. The damped least squares inversion (tomography) of the data reveals a complex attenuation structure. Crust and mantle of the forearc and subducting slab are generally characterized by low attenuation (Qp > 1000). Crust and mantle beneath the magmatic arc show elevated attenuation. The strongest anomaly of extremely low Qp is found in the crust between 22° and 23°S beneath the recent volcanic arc (Qp < 100). N‐S variations can be observed: The western flank of the crustal attenuation anomaly follows the curved course of the volcanic front. North of 21°S the attenuation is less developed. In the northern part of the study area the low‐Qp zone penetrates in the forearc mantle down to the subducting slab. In the south a deeper zone of high attenuation is resolved between 23° and 24°S directly above the subducting slab. Low Qp in the mantle correlates with earthquake clusters. The strong crustal attenuation is confined to the distribution of young ignimbrites and silicic volcanism and is interpreted as a thermally weakened zone with partial melts. The attenuation pattern in the upper mantle might reflect the variable extent of the asthenosphere and maps variations of subduction‐related hydration processes in the mantle wedge from slab‐derived fluids.
Influence of volatile loss on thickness and density profiles of active basaltic flow lobes
Authors: Baloga Stephen M. , Glaze Lori S. , Peitersen Matthew N. , Crisp Joy A.
Published: February 2004

A bulk density increase due to degassing during emplacement may have a significant influence on the thickness of a lava flow and the rate at which it advances. We present a theoretical model of a lava flow that loses enough volatiles to cause density changes along the path of the flow. We assume that the flow is emplaced as a single, isolated unit and the bulk rheology (e.g., viscosity) is a function of distance from the vent. This type of model is applicable to solitary lobes of basaltic aa and isolated sheets of pahoehoe that advance as a fluid continuum with bulk lava density changes as a function of distance along the flow path. Equations for the flow thickness and the bulk density profiles are derived from mass and volume conservation. Formulas are tabulated for thickness and density profiles for various combinations of flow rates, rheologic changes, and degassing rate functions. We also tabulate formulas for estimating parameters associated with the form and rate of degassing from field data. The Mauna Loa 1984 “1 flow” is a typical example of a flow showing evidence of a bulk density increase and is used to estimate the model parameters. Thickness and density profiles are then computed for a range of plausible lava densities, two different rate functions for the loss of volatiles, and two different models of viscosity change. Results indicate that the thickness profile of a lava flow can be significantly affected when there is a large difference between the density at the vent and at the flow front. For relatively high rates of degassing, the flow profile has a maximum thickness located progressively closer to the vent as the rate of degassing increases. For depth‐dependent degassing, an increase in viscosity acts to thicken the flow, which increases the rate of degassing, thus mitigating the thickening influence of the increasing viscosity. Degassing while a flow is active can increase the duration of emplacement by as much as 60%. We find that the flow thickness profiles are sensitive to the choice of flow rate and the initial density, regardless of the form of the degassing function. The nature of the flow rate can significantly affect the shape of the profile as well as the flow front thickness. When the rate of volatile loss depends on the flow thickness, the density profile depends explicitly on the way the rheology changes along the flow path. In all cases, density increases during emplacement counter the tendency of a flow to thicken due to increases in viscosity or resistance to flow with distance. Thus the parameters that define the rate of degassing, and the consequent density change along the path of a flow, emerge as important variables for a quantitative understanding of flow emplacement.
A two‐phase approach to the simulation of the combined isotope/salinity signal of marine ice
Authors: Tison J.‐L , Khazendar A. , Roulin E.
Published: February 2004

A modified boundary layer box diffusion model is used to simulate the production of the combined oxygen isotope/salinity signal of a body of marine ice. The final signal is considered to be the result of fractionation and segregation processes occurring first upon the formation of frazil crystals in suspension and second during their accumulation and consolidation due to the advancement of a freezing front. The model results are compared to oxygen isotope and salinity values in a 45‐m marine ice core from the Nansen Ice Shelf, Antarctica. Parameters more suitable to the modeling of marine ice (as opposed to sea ice) are introduced and conclusions are drawn on the origin of the impurity characteristics of marine ice, including the still unresolved question of its anomalously low salinity.
Particle transport through a narrow tidal inlet due to tidal forcing and implications for larval transport
Authors: Brown Cheryl A. , Jackson George A. , Brooks David A.
Published: February 2004

For estuarine‐dependent species, especially those that spawn offshore and whose larvae must reach estuarine nursery areas, advective transport through tidal inlets may be a major factor influencing recruitment variability. We examined the role of tidal forcing on particle transport through a narrow, microtidal inlet along the Texas coast by using a three‐dimensional hydrodynamic and particle transport model. Although tidal forcing is relatively small in the study area, tidal currents through the inlet effectively transport passive particles a distance of about 15 km landward of the inlet. The majority of the particles that enter the inlet are transported to regions that are not suitable for larval settlement. There is limited tidal dispersion of the particles into the bays due to shoreline geometry and bathymetry. Most of the particles that enter the inlet are expelled offshore in the ebb tidal jet resulting in estuarine‐shelf exchange of particles. When acting alone, tidal forcing is not effective at retaining particles in a suitable estuarine habitat, suggesting that other physical or biological mechanisms are required to maintain larvae in an estuarine habitat or that there is substantial along‐shelf transport of larvae.
An Arctic Ocean cold core eddy
Authors: Muench Robin D. , Gunn John T. , Whitledge Terry E. , Schlosser Peter , Smethie William
Published: February 2004

Detailed physical and chemical observations were obtained during September 1997 of a cold core eddy situated in the southern central Canada Basin of the Arctic Ocean. The eddy was about 20 km in diameter, had maximum current speeds exceeding 20 cm s−1, and extended from the base of the upper mixed layer, near 40 m, down to ∼400 m. Excess salt in the eddy core was consistent with addition of brine from ∼1 m of ice formation. Core tracer distributions indicated a lifetime exceeding 1 year and were consistent with an origin as near‐surface Pacific water with some admixed terrestrial runoff. The eddy was probably formed in association with a polynya along the Alaskan Chukchi Sea coast through local water densification from surface ice formation followed by development of frontal instabilities. Formation and subsequent migration of such eddies, which may have lifetimes of several years, provide a mechanism for transporting water from the Beaufort and Chukchi Sea coastal and shelf regions into the interior Canada Basin and for contributing to maintenance of the permanent halocline. Present light ice conditions throughout the Arctic Ocean favor the formation of such eddies and may contribute to enhanced ventilation of the halocline waters.
Evolution of freely propagating, two‐dimensional surface gravity current fronts
Authors: Cooper Arnold L. , Mied Richard P. , Lindemann Gloria J.
Published: February 2004

This work addresses the two‐dimensional propagation and shape evolution of surface gravity current fronts with a surface density outcrop frontal line. The problem is formulated using the reduced gravity shallow water equations, and the gravity currents are assumed to advance into a fluid at rest. We formulate a nonlinear analytical model for the gravity current plume front morphology by applying the shock tube theory of compressible fluids, which casts the problem in the form of an initial value calculation to be solved numerically. The simulations are initiated by assuming three different plan forms for the initial plume front and their subsequent evolutions followed in time. The paper is concerned exclusively with gravity current fronts having initially a uniform frontal propagation speed locally normal to the plume front, and a number of interesting results emerge. We find that an initially concave region of the front can lead to a nonlinear focusing that results in an energetic bulge in the frontal plan view. These bulges form sharp angles, or kinks, where they are joined to the front at their edges on either side. As they evolve, these angles increase toward 180° (a straight line), and the front becomes smoother in time. The orientation of the bulge and kink features predicted by the model is in agreement with visual and radar imagery observations. The kinks are always oriented toward the lighter plume material. When a plume has two or more such concave regions, the resulting energetic bulges can interact at a later time. The issue of determining plume speeds by tracking these features on sequential images of gravity currents is also dealt with.
Propagation of Big Island eddies
Authors: Holland Christina L. , Mitchum Gary T.
Published: February 2004

Using satellite altimetry data, we have observed a series of anticyclonic eddies as they form at the Big Island of Hawaii and have tracked them as they move away from the island. While similar eddies have been observed near the Hawaiian Islands in previous studies, the fate of the anticyclonic eddies has previously been unclear. The eddies that we observed initially propagated to the southwest but consistently changed propagation direction to the northwest later in their lifetimes. This was intriguing to us, as theoretically, the decay of isolated anticyclonic eddies on a β plane should cause them to continually move toward the southwest. Such isolated eddy dynamics are unable to account for the observed change to northwestward eddy propagation, and the presence of the westward flowing North Equatorial Current turns out to be important to the Big Island eddy dynamics. The eddies are not passively advected by the North Equatorial Current; rather, the mean flow changes the propagation characteristics of the eddies. An existing theory that includes meridionally varying, purely zonal mean flow is shown to account for the observed propagation of the Big Island eddies if the zonal variation of the mean flow is considered.
Comparison of statistical and model‐based hindcasts of subtidal water levels in Chesapeake Bay
Authors: Bosley Kathryn Thompson , Hess Kurt W.
Published: February 2004

Subtidal water levels in Chesapeake Bay, which can have amplitudes as large as 1 m at Baltimore, are an important component of total water levels. The most important forcing mechanisms for these variations are surface winds over the Bay and coastal subtidal water levels. Two methods for hindcasting subtidal water levels in the Bay were developed: statistical prediction (based on multiple linear regression) and a barotropic numerical circulation model‐based prediction. The hindcast water levels were compared with the observed values at three key locations (Chesapeake Bay Bridge Tunnel (CBBT) in the lower bay near the mouth, Solomons Island at midbay, and Baltimore in the upper bay) by a variety of statistical measures. The hindcast results show that in both annually averaged differences and in the incidence of outliers the numerical model‐based hindcasts are slightly more accurate than the statistical hindcasts, although on a monthly basis the statistical hindcast was often equal to or better than the model hindcast. Errors in both methods follow a seasonal pattern, being smallest in the summer months and largest in winter. Both hindcast methods explain significant portions of the observed variance. In addition, several techniques were used to estimate the relative importance of coastal water level forcing and wind forcing in the subtidal variability. In general, the two forcings were about of equal importance at Baltimore, while coastal forcing was dominant at Solomons Island and CBBT.
Bergy bit and growler melt deterioration
Authors: Savage S. B. , Crocker G. B. , Sayed M. , Carrieres T.
Published: February 2004

The Canadian Ice Service, Environment Canada, is currently developing an operational iceberg forecasting model; the present work forms part of that effort. While existing models predict iceberg drift and deterioration, the new model will account for calving that produces smaller ice pieces and subsequently track the drift and melt of the calved pieces. Bergy bits and growlers, which we consider here to be ice pieces in the size range from 3 to 20 m, can cause large forces upon impact with offshore structures. The probability of encountering these bergy bits and growlers is of significant interest to marine transportation and offshore resource development. Calving due to wave‐induced erosion at the waterline of a floating iceberg can produce many thousands of small ice pieces having a wide distribution of sizes. These small ice pieces then melt as individual entities and eventually disappear. Since the calving events occur periodically, there is a continual supply of small ice pieces in the neighborhood of the parent iceberg. The focus of the present paper is on the evolution of the size‐frequency distribution function for the calved ice pieces. It makes use of the initial distribution function following the calving event discussed by Savage et al. (2000). Dimensional analysis, laboratory tests, and field observations are applied to obtain simple correlations and devise a melt law for the smaller ice pieces. This melt law is then used to determine the temporal evolution of the small ice piece size‐frequency distribution function.
Phytoplankton and microzooplankton variability between the Subtropical and Polar Fronts south of Australia: Thriving under regenerative and new production in late summer
Authors: Kopczynska Elzbieta E. , Dehairs Frank , Elskens Marc , Wright Simon
Published: February 2004

Phytoplankton, microzooplankton were studied along a 42°–55°S, 141°–143°E transect in March 1998 and compared with production‐related parameters (carbon biomass, chlorophyll a, nitrogen and carbon uptake, and ƒ ratios). The transect crossed the Subtropical Front (STF), the Subantarctic Front (SAF), and the Polar Front (PF). Phytoplankton assemblages were dominated by nano‐ and pico‐sized flagellates; their peak numbers (nanoflagellates: 8.2×105 cells L−1) occurred in the areas of STF and within the Subantarctic Zone (SAZ). North of the SAF, dinoflagellates were next in abundance. Diatoms exceeded dinoflagellates in the PF area (maximum 1.26×105 cells L−1). Dinoflagellates were dominated by nano‐sized gymnodinioid forms with microplanktonic species increasing in numbers in SAZ and STF. Diatoms contained mainly Fragilariopsis pseudonana and Pseudonitzschia lineola; several abundant species exibited a latitudinally restricted distribution. Phytoplankton carbon biomass was dominated by dinoflagellates (including >20 μm heterotrophs) representing 48 to 84% of total cell carbon. Maxima of 18–26 μg C L−1 occured both at STF and PF. Heterotrophic dinoflagellates and ciliates showed similar distributions. Their peaks of cell densities and carbon in STF and SAZ were associated with phytoplankton maxima. Microzooplankton cell distribution and biomass suggest they are major grazers and contributors to carbon flow. Phytoplankton assemblages represented at least three stages with different relative contributions of regenerative and new production. Production related parameters (e.g., low ƒ ratio and high NH4+ uptake) point to the presence of regenerative community at the STF. It attracted the highest concentration of microzooplankton. The phytoplankton community associated with a frontal feature (46°–47°S) within the SAZ, thrived under increased new production (e.g., relatively higher ƒ ratio and NO3− uptake). The community along 47°–55°S was characterized by intermediate ƒ ratios, with slight predominance of regenerated production. Southward of 47°S, the relative contribution of new production increased.
Distribution of solar radiation in the Barents Sea marginal ice zone during summer
Authors: Makshtas Alexander P. , Korsnes Reinert
Published: February 2004

Field measurements during the Barex‐95 expedition in the marginal ice zone of the Barents Sea provided new data of solar radiation under different conditions of cloudiness and sea ice cover during the thawing. We found a strong dependence of angular and spectral redistribution of radiation on cloud optical thickness and surface albedo. We developed a Monte Carlo model for radiative transfer through a homogeneous cloud layer above sea ice with given concentration and homogeneous snow on top. This simple Monte Carlo model reproduced the observed angular distribution of solar intensity and cloud albedo as a function of solar zenith angle, optical thickness of the clouds, and surface albedo. The model allowed us to estimate the influence of surface tilt on irradiance and to give a qualitative explanation of how the sea ice relief transforms during summer. Data of short wave radiation below clouds enabled the estimation of the effect of clouds on the distribution of radiative heating of the upper ocean for spectral ranges 350–1000 and 400–700 nm (photosynthetically active radiation range). Spectral sensitivity of absorption of solar radiation in clouds relatively decreases radiative heating in the upper centimeters of the water up to 25 and 5% for 350–1000 and 400–700 nm, respectively.
Euphotic zone variations in bulk and compound‐specific δ13C of suspended organic matter in the Subantarctic Ocean, south of Australia
Authors: O'Leary T. , Trull T. W. , Griffiths F. B. , Tilbrook B. , Revill A. T.
Published: February 2004

The carbon isotopic compositions of suspended organic matter δ13CPOC collected from Subantarctic Zone surface waters south of Australia in November 1995 decrease southward from −20 to −26‰ and display strong correlations with aqueous carbon dioxide concentration ([CO2]aq), consistent with previous studies. In contrast, vertical profiles through the euphotic zone (top ∼100 m) of δ13CPOC at six stations display decreases with depth of up to 2.4‰. These decreases in δ13CPOC cannot be fully explained by the small vertical variations in [CO2]aq or its 13C content. Carbon 13 analyses of several individual sterols revealed similar isotopic changes with depth, suggesting that they derive from a fundamental depth control on primary production, rather than from algal community variations or remineralization processes. Growth rate μ appears to be the most likely source of the depth variations. The relationship between μ/[CO2]aq and δ13CPOC derived from surface water samples can explain the vertical variations of δ13CPOC within the mixed layer, provided integrated mixed layer growth rates are used. Below the mixed layer, differences between the observed δ13CPOC and the growth rate model can be explained by recent shallowing in mixed layer depth and the subsequent effect on growth rates. These results suggest that δ13CPOC determinations can be used to provide some information on the recent history of mixed layer processes and that interpretation of sedimentary δ13CPOC records should include consideration of possible growth rate and mixed layer depth variations.
Bubbles: An estimate of their role in the global oceanic flux of carbon
Authors: Monahan Edward C. , Dam Hans G.
Published: February 2004

The amount of particulate organic carbon (POC) formed annually via the scavenging of dissolved organic carbon (DOC) by bubbles is calculated to be 1.9 Gt C yr−1. In arriving at this estimate use was made of (1) published descriptions of the bubble spectrum associated with the alpha plumes beneath spilling wave crests, (2) the published relationship of stage A whitecap coverage to wind speed, (3) data on the scale depth of alpha plumes and on the rise velocity of bubbles coated with organic film, (4) published bubble scavenging efficiencies, and (5) recorded concentrations of DOC in oceanic surface waters. This rate of DOC scavenging by bubbles represents a 1.7% yr−1 turnover of the global oceanic DOC pool, and each year produces an amount of POC equal to 17% of the POC pool in the uppermost 300 m of the ocean. Bubble scavenging turns over the DOC present in the shallow layer of bubble influence once every 67 days. Moreover, within the depth of influence of large bubbles (uppermost 1.4 m of the water column), bubble scavenging represents an annual threefold turnover of the DOC pool and a POC production equivalent to thirtyfold of the POC pool. It is noted that the amount of DOC transformed into POC each year via bubble scavenging is equal to the amount of carbon present in the carbon dioxide estimated to be absorbed by the ocean each year from the atmosphere.
Eddy development and motion in the Caribbean Sea
Authors: Andrade Carlos A. , Barton Eric D.
Published: February 2004

Eddy motion in the Caribbean Sea is described on the basis of sea level anomalies deduced from ERS‐1 altimetry data corrected with TOPEX/Poseidon data during the 15 months of the Exact Repeat Mission (October 1992 to December 1993). Both cyclones and anticyclones were observed in the satellite data as anomalies originating in the Venezuelan Basin or entering the Caribbean through the Antillean passages, mainly the St. Lucia Channel, Anegada Passage, and north of Trinidad. The diameter of the eddies ranged from a few tens of kilometers to 700 km. Advection speeds were typically 20–30 cm s−1 and the eddies were energetic (kinetic energy > 0.6 m2 s−2). Their lifetime of 3–4 months was determined, in general, by their interaction with topography. Most eddy activity was eroded and disappeared at the Central American Rise area, although a few eddies crossed into the Cayman Sea through the Chibcha Channel. Some eddies also entered the Cayman Sea from outside the Caribbean through the Windward Passage. The Panama‐Colombia Gyre was evident only during the tropical rainy season. A large cyclonic eddy was formed there during the period of maximum precipitation, when strong meridional salinity and wind speed gradients occurred. Eddy production in the central Caribbean appears to be associated with the interaction of the meandering Caribbean Current and the strong wind curl.
An s coordinate density evolving model of the northwest European continental shelf: 1. Model description and density structure
Authors: Holt Jason T. , James Ian D.
Published: February 2004

The Proudman Oceanographic Laboratory Three‐Dimensional Baroclinic B grid model (POL3DB) has been developed to incorporate features suitable for the modeling of baroclinic processes on the shelf, at the shelf slope, and in ocean regions to allow long‐term coupled ocean‐shelf simulations. We test the model on the northwest European continental shelf in the period November 1988 to October 1989 against satellite sea surface temperature measurements, against CTD sections both on and off the shelf, and (in summary) against the whole of the North Sea Project CTD data set. The model accurately reproduces the seasonal cycle in the shelf‐wide spatial temperature structure seen in the observations. Spatial correlations range from r = 0.92 in December to 0.79 in July, and the overall rms errors range from 0.8°C in April to 1.6°C in July. We demonstrate that the increase in the errors during the summer is due to uncertainties in modeling the vertical temperature structure. Compared with a climatology, the large‐scale sea surface salinity structure is also well modeled (r = 0.80), but there is a tendency for the model to overestimate the salinity in the Norwegian Trench and underestimate it in the Kattegat. The s coordinates (a modified sigma‐coordinate system) allow the formation of a seasonal mixed layer across the shelf break and into the northeast Atlantic with a modest vertical resolution. The accuracy to which this deep water region is modeled is limited by the initial and boundary conditions and by the extent of the model domain.
Thin ice impacts on surface salt flux and ice strength: Inferences from advanced very high resolution radiometer
Authors: Yu Y. , Rothrock D. A. , Zhang J.
Published: February 2004

Temperatures and albedos derived from satellite imagery are combined with a thermodynamic ice model to estimate thin ice thickness distributions over the Beaufort and the northern Greenland seas. The study shows that thin ice (thinner than 1 m) occupied over half the area in the seasonal ice zones in November and December of 1990 but dropped significantly in April 1991; the Beaufort Shelf showed the largest seasonal change. The aggregate properties of surface salt flux and compressive ice strength, both strongly dependent on the thin end of the ice thickness distribution, were estimated with data from advanced very high resolution radiometer to reveal the spatial and temporal variations over a large scale. The salt flux from growing thin ice was 1–2 orders of magnitude larger on the Beaufort and Greenland Shelves than in the deep basins. On the shelves, flux from thin ice accounted for over 90% of the total surface salt budget. These satellite‐derived estimates provided detailed spatial information on salt flux, which can be of great use in studies of surface patterns of salinity forcing and shelf‐basin interaction. Also revealed by the satellite data was the wide range in values of compressive strength, which affects how freely the ice cover can deform. Strengths were low in early winter and in seasonal ice zones and nearly doubled in spring. Both thin ice fraction and compressive ice strength estimated from satellite imagery were in good agreement with those simulated by a coupled ice‐ocean model.
Spring evolution of Arabian Sea High in the Indian Ocean
Authors: Prasad T. G. , Ikeda M.
Published: February 2004

The seasonal formation of a large anticyclonic circulation pattern, Southern Arabian Sea High (SAH), appears in the southern Arabian Sea during the northeast monsoon. SAH is studied on the basis of the recent observations that include the World Ocean Circulation Experiment (WOCE)/Tropical Ocean‐Global Atmosphere (TOGA) experiment drifting buoys along with TOPEX/ERS‐2 satellite altimeter data in conjunction with the climatological hydrography observations. The SAH forms west of the Laccadive High (LH), first appears in January, and grows rapidly in situ to form a well‐developed anticyclonic eddy, extending across the southern Arabian Sea by March. The center of the eddy is ∼65°E, 5°N. The SAH is bounded by a strong eastward current to the north and a weak southward flow between 65° and 70°E, which separates SAH from the LH. The westward flowing North Equatorial Current (NEC) is on the southern flank. A strong northward flow along 55°E during January and February marks the western boundary. SAH propagates westward as a Rossby wave between March and April. The arrival of SAH in the western boundary in April is accompanied by intensification of current along the coast of Somalia. In May, part of the SAH has subsequently moved northward to form an anticyclonic eddy with an apparent center 13°N, 53°E in the vicinity of the Gulf of Aden (Gulf of Aden Eddy (GAE)). The GAE extends horizontally nearly 600 km (along 13°) and is vertically extended down through the upper main thermocline to ∼250 m. Comparisons of hydrography, buoy, and altimetry reveal a good agreement in eddy size, location, and time of appearance of the GAE.
Seasonal evolution of the mixed layer in the Subantarctic zone south of Australia
Authors: Rintoul Stephen R. , Trull Thomas W.
Published: February 2004

The circumpolar Subantarctic Zone (SAZ) is a globally significant region of water mass formation and carbon dioxide uptake from the atmosphere. Here we synthesize the results of nine voyages over 8 years to describe the seasonal variation in mixed layer properties in the SAZ south of Australia for comparison with biogeochemical process studies carried out in late summer (March 1998) as part of the SAZ Project. Winter mixing extends to depths >400 m, resulting in the formation of Subantarctic Mode Water. In summer the mixed layer shoals to 75–100 m, depths which are still sufficiently deep that phytoplankton growth may be light limited. Nitrate and phosphate concentrations are reduced in summer (e.g., nitrate decreases from >15 to <5 μmol kg−1) but remain well above limiting levels. Silicate, in contrast, is low throughout the year (4 μmol kg−1 in winter and <2 μmol kg−1 in summer). Water mass properties along a north‐south hydrographic section in March 1998 suggest that near‐surface waters spread from south to north across the Subantarctic Front (SAF), supplying cool, fresh, nutrient‐rich water to the SAZ. As a consequence, the properties of the southern SAZ differ from those farther north: the mixed layer in the south is cooler, fresher, deeper, higher in nutrients, and bounded below by a halocline (rather than by a seasonal thermocline, as in the northern SAZ). The contrast between the northern and southern SAZ persists throughout the year, suggesting the cross‐front exchange occurs year‐round and likely contributes to the differences in seasonal thermal amplitude and algal biomass accumulation seen in satellite images. Density‐compensated horizontal gradients of temperature and salinity are common in the mixed layer of the SAZ and the northern SAF, consistent with the hypothesis that the vigor of lateral mixing in the mixed layer is a strong function of the magnitude of the lateral density gradient.
Seasonal and interannual variability of phytoplankton, nutrients, TCO2, pCO2, and O2 in the eastern subarctic Pacific (ocean weather station Papa)
Authors: Signorini Sergio R. , McClain Charles R. , Christian James R. , Wong C. S.
Published: February 2004

A coupled, one‐dimensional ecosystem/carbon flux model is used to simulate the seasonal and interannual variability of phytoplankton, nutrients, TCO2, O2, and pCO2 at ocean weather station Papa (OWS P at 50°N, 145°W). The 23‐year interannual simulation (1958–1980) is validated with available data and analyzed to extend seasonal and interannual variations beyond the limited observational records. The seasonal cycles of pCO2 and sea‐air CO2 flux are controlled by a combination of thermodynamics, winds, and biological uptake. There is ingassing of CO2 during the fall‐winter months when SSTs are colder and wind forcing is vigorous, while there is a much smaller ingassing of CO2 during the summer when sea surface temperatures are warmer and wind speeds are reduced. Biological production plays a major role in maintaining the air‐sea equilibrium. An abiotic simulation showed that OWS P would be a source of atmospheric CO2 (1.41 mol C m−2 yr−1) if the biological sink of CO2 were removed. The peak net community production in summer compensates for the increased temperature effect on pCO2, which prevents large outgassing in summer. Oxygen anomalies relative to the temperature‐determined saturation value show that there is a seasonal cycle of air‐sea flux, with ingassing in winter and outgassing in summer. The net surface oxygen flux is positive (0.8 mol m−2 yr−1), indicating that OWS P is a source of oxygen to the atmosphere. The average primary production is 167 g C m−2 yr−1. The 1960–1980 (1958 and 1959 spin‐up years removed) mean carbon flux is −1.8 mol C m−2 yr−1, indicating that the ocean at OWS P is a sink of atmospheric carbon. The sea‐air CO2 flux ranges from −1.2 to −2.3 mol C m−2 yr−1 during the 21‐year simulation period. This finding emphasizes the need for long‐term observations to accurately determine carbon flux budgets. A series of sensitivity experiments indicate that the seasonal variability and overall (21 years) mean of TCO2, pCO2, ΔpCO2, and air‐sea CO2 flux are strongly dependent on the gas transfer formulation adopted, the total alkalinity near the surface, and the bottom (350 m) value adopted for TCO2. The secular atmospheric pCO2 upward trend is manifested in the TCO2 concentration within the upper 100 m by an increase of 15 mmol m−3 in 20 years, consistent with observations at other locations [Winn et al., 1998; Bates, 2001].