bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Earth albedo on the LAGEOS I satellite
Authors: Martin Chreston F. , Rubincam David P.
Published: February 2004

For several years, Earth albedo has been one of the leading candidates for explaining the anomalous along‐track acceleration experienced by the LAGEOS satellites, with reasonable reflection models having been shown to give acceleration peaks comparable to those observed. The effects of Earth albedo on LAGEOS I have been studied for the period March 1985 to June 1989, during which time measurements of reflected radiation from the Earth were made by one to three Earth Radiation Budget Experiment /ERBE) satellites. Data taken during this experiment were processed into hourly radiation exitances for the entire Earth divided into 2.5° × 2.5° blocks. These ERBE data were used, along with detailed models of reflectance characteristics of the Earth, to calculate the accelerations which reflected radiation would induce on LAGEOS I. The along‐track component of the acceleration was averaged over 10 revolutions in order to average out the short‐period effects. The results showed that the albedo effect on along‐track acceleration did not exceed 0.5 pm/s2, or about 20% of the anomalous acceleration. Even this acceleration showed no correlation in phase with the observed acceleration. The albedo accelerations were also used to calculate the effects on the LAGEOS Keplerian orbital elements. The effects on the LAGEOS node and inclination excitations were significant, having amplitudes of several milliarc seconds /mas) per year. The largest effects were found to be on the eccentricity excitation function, having amplitudes at the 50–100 mas/yr level. The patterns also correlate well with the observed eccentricity vector excitations, but with a difference in sign, suggesting that some parameters estimated with LAGEOS data, such as ocean tide parameters, may have been corrupted by albedo effects.
Transition zone velocity gradients and the 520‐km discontinuity
Authors: Shearer Peter M.
Published: February 2004

Stacks of long‐period Global Digital Seismograph Network (GDSN) seismograms at 110° to 180° epicentral distance reveal precursors to SS that result from underside reflections off upper mantle seismic discontinuities. The 410‐ and 660‐km discontinuities are obvious in these stacks, but identification and modeling of other transition zone discontinuities are complicated by sidelobes from the 410‐ and 660‐km reflections. These sidelobes result from the limited bandwidth of the GDSN instrument responses and the effect of crustal reverberations on the SS reference phase. The crustal effects can be minimized by restricting the records to oceanic bounce points where the ∼6‐km‐thick crust has little effect on the long‐period waveforms. Over 2000 long‐period, transverse‐component seismograms with oceanic SS bounce points recorded by the GDSN from 1976 to 1991 are manually edited, aligned on SS, and then stacked using a new procedure that weights the records by data quality. The resulting image shows a clear reflection from a 520‐km discontinuity that cannot be explained as a sidelobe artifact, confirming earlier results of Shearer [1990, 1991] and Revenaugh and Jordan [1991]. By stacking along the expected travel time curves for discontinuity phases, the time versus range image of the precursor wave field is reduced to a single trace that measures upper mantle reflectivity versus time. The features in this reflectivity profile are sensitive to the brightness and depth of the transition zone discontinuities and to the steepness of the velocity gradients between the interfaces. Using geometrical ray theory and assuming a constant velocity versus density scaling relationship, I fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion. The inverse problem is addressed by performing a deconvolution of the profile with the SS reference phase (after a correction for attenuation), followed by a direct mapping of reflectivity versus time into velocity versus depth. Velocity‐depth profiles resulting from these procedures are roughly in agreement with standard upper mantle velocity models, except that the SS precursor data require a minor discontinuity near 520 km and a steeper gradient just below the 660‐km discontinuity. Estimated discontinuity shear impedance changes are 6.7 ± 1.1% at 420 km, 2.9 ± 0.7% at 519 km, and 9.9 ± 1.5% at 663 km. The impedance change near 520 km is consistent with current mineral physics results for the olivine β to γ phase change and places constraints on the fraction of olivine in the transition zone.
Elastic wave velocities in partially saturated rocks: Saturation hysteresis
Authors: Le Ravalec Mickaële , Guéguen Yves , Chelidze Tamaz
Published: February 2004

An effective medium approach (differential self‐consistent scheme) has been used to calculate elastic wave velocities in rocks containing spherical pores and ellipsoidal cracks with two fluid phases. The results are applied to the case of imbibition/drainage processes performed in the laboratory (at high frequencies). For that purpose, two scales of heterogeneities are accounted for. The first one corresponds to the size of pores or cracks. It is the scale of a dispersion mechanism termed “local flow” because it depends on local variations in pore/crack compressibility. The second scale, much larger, is controlled by the size of the heterogeneities of the fluid phase distribution. These heterogeneities are assimilated to pockets whose saturation is different from that of the surrounding medium. The pocket scale involves a second dispersion mechanism, often called “global flow” or “pocket flow,” which depends on fluid pressure equilibration among the regions which have distinct saturation levels. Our calculations include both mechanisms, each having its own characteristic scale. Imbibition is described through simple relations involving two parameters. The same is true for drainage. The predicted behavior for P wave velocities against saturation appears to be in good agreement with experimental data reported in the literature, for both drainage and imbibition.
Medial axis analysis of void structure in three‐dimensional tomographic images of porous media
Authors: Lindquist W. Brent , Lee Sang‐Moon , Coker David A. , Jones Keith W. , Spanne Per
Published: February 2004

We introduce the medial axis as a tool in the analysis of geometric structure of void space in porous media. The medial axis traces the fundamental geometry of the void pathways. We describe an algorithm for generating the medial axis of the void structure from digitized three dimensional images of porous media obtained from X ray CAT scans. The medial axis is constructed during an iterative erosion procedure which, at each step, replaces the image of the void structure with a smaller version obtained by eroding its surface layer of voxels. The algorithm is applied to high (5 μm) resolution microtomographic images of two rock chips (Berea sandstone and Danish chalk) and a sample of uniform (100 μm) diameter, packed glass beads. We statistically investigate several geometrical properties of the structure of the medial axes obtained. The first is the distribution of relative volumes in each erosion layer of the void space. We find the distributions to be exponential for the two real rock samples and normal for the packed glass beads. The second property investigated is the distribution of volumes of disconnected segments of the medial axis which are in one‐to‐one correspondence with disconnected void segments of the sample. We find indications for a universal power law behavior governing the distribution of volumes of the smallest disconnected pieces. The final behavior studied is a geometric tortuosity as measured by shortest paths through the medial axis. This tortuosity distribution appears well described by a gamma distribution.
Effects of temperature on fracture in dry slab avalanche release
Authors: McClung D. M.
Published: February 2004

Field experience shows that snow temperatures have important influences on snow stability and the potential for propagating shear fractures. Part of the influence is with respect to metamorphism and bond formation. However, in this paper I consider temperature effects on fracture and the relation to snow slab stability at two scales: (1) microscale fractures of individual bonds or chains of bonded grains which lead to strain‐softening behavior prior to organization of failure into large‐scale failure planes or zones and (2) large‐scale imperfections already undergoing strain‐softening in failure zones in weak layers underneath snow slabs. When fracture at both scales is considered, two important results emerge. First, increasing slab temperatures may decrease slab stability by decreasing the effective slab stiffness (modulus). Second, increasing temperature in the weak layer (strain‐softening) failure zone decreases failure toughness (but not strength) also decreasing stability. The first result shows how slab avalanches can release by temperature increases without the weak layer being directly affected by warming. The second result gives a partial explanation of decreasing frequency of slab avalanche failures as weak (failure) layer temperatures decrease. Both results are supported by field measurements.
Haskell [1935] revisited
Authors: Mitrovica Jerry X.
Published: February 2004

The Haskell [1935] value of 1021 Pa s for mantle viscosity is a classic and enduring constraint on the rheology of the Earth's interior. We revisit this inference using spherically symmetric, self‐gravitating, Maxwell viscoelastic Earth models. Our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by Haskell, Angerman River, Sweden, and Oslo, Norway, rather than the raw relative sea level (RSL) data at these sites. We demonstrate that predictions of the decay time associated with the Angerman River data are insensitive to variations in both the late Pleistocene ice load history and the lithospheric thickness of the Earth model (the predictions at Oslo are sensitive to both these inputs), and hence decay times at this site provide a remarkably robust constraint on mantle viscosity. We derive a constraint on the “average” viscosity of the mantle of 0.65 − 1.10 × 1021 Pa s, where the “average” resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km. This indicates that many previous analyses which have invoked the Haskell value of 1021 Pa s as a constraint on the average upper mantle (i.e., above 670 km depth) viscosity alone have misinterpreted the resolving power of the inference. Furthermore, our analysis indicates that a number of apparently contradictory inferences of viscosity based on Fennoscandian data satisfy the new, rigorous, interpretation of the Haskell constraint. Finally, we demonstrate how the ambiguity in the upper mantle/lower mantle viscosity contrast associated with the Haskell “average” may be reduced by invoking decay time constraints estimated from RSL curves in Hudson Bay. A preliminary inversion of decay times at Angerman River and Richmond Gulf (in Hudson Bay) suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle; however, a conclusive analysis in this regard must await the determination of consistent decay time estimates for the Hudson Bay region.
Paleoseismicity of the Ovindoli‐Pezza fault, central Apennines, Italy: A history including a large, previously unrecorded earthquake in the Middle Ages (860–1300 A.D.)
Authors: Pantosti Daniela , D'Addezio Giuliana , Cinti Francesca R.
Published: February 2004

Geomorphic and trenching investigations along the Ovindoli‐Pezza fault show that this normal fault is one of the major seismogenic faults in the central Apennines. We found clear geological evidence for three Holocene surface‐faulting earthquakes on this fault: the most recent earthquake occurred in the Middle Ages between 860 and 1300 A.D., the penultimate occurred about 1900 B.C. or shortly after, and the oldest probably occurred between 3300 and 5000 B.C. Although the most recent surface faulting earthquake occurred during historical time, no evidence for it has been found in the historical record. Slip per event ranges between 2 and 3 m, and the length of the rupture is at least 12–20 km, suggestingM6.5–7.0 for the paleoearthquakes. The dip‐slip rate determined from trenching is 0.7–1.2 mm/yr and is consistent with the long‐term slip rate of 0.9–2.5 mm/yr (lower values preferred) obtained from displaced geomorphic features. Trench data combined with long‐term slip rate estimates suggest the recurrence interval is longer than a millennium and possibly as long as 3300 years. The time elapsed since the most recent earthquake is 700–1130 years. The seismic behavior of the Ovindoli‐Pezza fault is consistent with other well‐known seismogenic faults of the central and southern Apennines. The lack of mention or mislocation of the most recent event on the Ovindoli‐Pezza fault in the historical record of the past two millennia should be attributed mainly to the unsettled cultural and social conditions and poor economic state that characterized the Middle Ages, especially in thinly populated regions such as the central Apennines. This example highlights an intrinsic limit of the historical data and raises the possibility that other regions considered “seismically quiet” on the basis of a long historical record may in reality have had large earthquakes that were not recorded.
Stress/strain changes and triggered seismicity at The Geysers, California
Authors: Gomberg Joan , Davis Scott
Published: February 2004

The principal results of this study of remotely triggered seismicity in The Geysers geothermal field are the demonstration that triggering (initiation of earthquake failure) depends on a critical strain threshold and that the threshold level increases with decreasing frequency, or, equivalently, depends on strain rate. This threshold function derives from (1) analyses of dynamic strains associated with surface waves of the triggering earthquakes, (2) statistically measured aftershock zone dimensions, and (3) analytic functional representations of strains associated with power production and tides. The threshold is also consistent with triggering by static strain changes and implies that both static and dynamic strains may cause aftershocks. The observation that triggered seismicity probably occurs in addition to background activity also provides an important constraint on the triggering process. Assuming the physical processes underlying earthquake nucleation to be the same, Gomberg [this issue] discusses seismicity triggered by the MW 7.3 Landers earthquake, its constraints on the variability of triggering thresholds with site, and the implications of time delays between triggering and triggered earthquakes. Our results enable us to reject the hypothesis that dynamic strains simply nudge prestressed faults over a Coulomb failure threshold sooner than they would have otherwise. We interpret the rate‐dependent triggering threshold as evidence of several competing processes with different time constants, the faster one(s) facilitating failure and the other(s) inhibiting it. Such competition is a common feature of theories of slip instability. All these results, not surprisingly, imply that to understand earthquake triggering one must consider not only simple failure criteria requiring exceedence of some constant threshold but also the requirements for generating instabilities.
Thermal equation of state of CaSiO3 perovskite
Authors: Wang Yanbin , Weidner Donald J. , Guyot François
Published: February 2004

A comprehensive pressure‐volume‐temperature data set has been obtained for CaSiO3 perovskite up to 13 GPa and 1600 K, using synchrotron X ray diffraction with a cubic‐anvil, DIA‐6 type apparatus (SAM‐85). For each volume measurement, nonhydrostatic stress is determined from the relative shift in the diffraction lines of NaCl, within which the sample was embedded. Heating to above 973 K greatly reduced the strength of NaCl (to below 0.05 GPa), making the measurements hydrostatic. At room temperature the cubic perovskite structure remains metastable at pressures as low as 1 GPa, below which the sample transforms into an amorphous phase as indicated by a large background, a marked decrease in diffraction signals, and an anomalous volume decrease of the remaining crystalline phase. Because our experimental uncertainties are significantly smaller than those in previous measurements, the new data provide a tighter constraint on the zero pressure bulk modulus for CaSiO3 perovskite. A new set of room temperature equation of state parameters are identified so that both our data and the diamond cell data of Mao et al. [1989] are compatible [KT0 = 232(8) GPa, K′T0 = 4.8(3), and V0 = 45.58(4) Å3]. Volume measurements along several isotherms under both stable and metastable pressure conditions allow isochoric and isobaric interpolations within the range of experimental pressure and temperature conditions. Analyses using various approaches yielded consistent results for (∂KT/∂T)P of −0.033(8) GPa K−1, and (∂α/∂P)T of −6.3 × 10−7 GPa−1 K−1, with a zero‐pressure thermal expansion α0 of 3.0 × 10−5 K−1. The thermal pressure is found to be virtually independent of volume, and thus the Anderson‐Grüneisen parameter δT = K′T0 = 4.8. These results are used to predict the bulk modulus and density of CaSiO3 perovskite under lower mantle conditions. Along an adiabat with the foot temperature of 2000 K, the density of the perovskite agrees with that of the preliminary reference Earth model (PREM) within 1% throughout the lower mantle. The bulk modulus shows a smaller pressure dependence along the adiabat; it matches that of PREM at the top of the lower mantle but is about 10% too low near the core‐mantle boundary.
Helium isotopic evolution of Mauna Kea Volcano: First results from the 1‐km drill core
Authors: Kurz Mark D. , Kenna Timothy C. , Lassiter J. C. , DePaolo D. J.
Published: February 2004

Helium concentrations and isotopic ratios have been measured in a suite of basaltic olivines from the Hawaii Scientific Drilling Project (HSDP) core at Hilo, Hawaii, which allows a characterization of the temporal helium isotopic evolution of Mauna Kea volcano. Typically more than 85% of the helium within the olivines is released by crushing, which demonstrates that helium is dominantly contained within the melt inclusions, and strongly suggests that helium behaves as an incompatible element during silicate melting. Lavas from the base of the Mauna Loa section of the core (240–270 m depth) have 3He/4He ratios between 13.9 and 15.8 times atmospheric (Ra). These values, when combined with Sr and Nd isotopic data, correspond to those previously found in subaerial Mauna Loa lavas that are greater than 30 ka in age, place a minimum age of 30 ka for the top of the Mauna Kea section, and are consistent with radiocarbon chronology. The upper part of the Mauna Kea lava section (290–620 m) has 3He/4He ratios of 7.0 to 6.8 Ra, similar to most mid‐ocean ridge basalts and is thought to represent the normal asthenospheric mantle signature. Between 620 and 670 m depth there is a transition to higher values, with 3He/4He ratios between 10.2 and 12.5 Ra at depths greater than 670 m, suggesting a greater contribution of undegassed plume material to the older Mauna Kea tholeiites. The correlations between He, Nd, and Pb isotope ratios demonstrate that the helium isotopic variations are related to mixing of source materials rather than shallow contamination effects (i.e., via addition of radiogenic 4He). The transition from high to low 3He/4He ratios occurs within the tholeiitic shield of Mauna Kea. The highest 3He/4He ratios in the Mauna Kea lavas (up to 12.5 Ra) are significantly lower than the values found in the shield tholeiites of the adjacent volcanoes (Loihi Seamount, Mauna Loa, and Kilauea); this relates to a smaller hot spot contribution to the Mauna Kea lavas. The temporal helium isotopic evolution of Hawaiian shield volcanos (with highest 3He/4He ratios found in the oldest lavas) and the inter‐volcano variations (with the Mauna Kea shield having the lowest 3He/4He ratios on the island) are attributed to movement of the Pacific plate over a radially zoned hot spot.
Complexities in high‐frequency seismic waveforms due to three‐dimensional structure in the New Madrid Seismic Zone
Authors: Xie Jiakang , Cong Lianli , Mitchell B. J. , Chiu J. M.
Published: February 2004

Effects of three‐dimensional structure on 2 to 10 Hz seismic signals in the New Madrid Seismic Zone (NMSZ) are investigated using waveforms from similar microearthquakes that occurred near Ridgely, Tennessee. Slowness power spectral (SPS) analysis is used to study the plane wave composition of P, S, and coda waves. The observed SPS at four stations suggest that the early S coda is composed mainly of wavelets leaving the source with slownesses of the direct S wave. At greater lapse times, coda is increasingly affected by random scattering. SPS observed on the vertical components and/or near the S wave nodes are more comlplex due to practical limitations of the algorithm. P coda and early S coda at a fifth station, located southwest of the junction, show the most complex SPS. Analysis of that complexity, together with waveform synthetics and travel time analysis, strongly suggests that scattering/multipathing due to source zone velocity heterogeneities occurs near the path, probably due to dilatancy occurring southwest of the Ridgely junction, or due to internal fault zone complexities. Coda Q, when averaged over three components, show no station variations. We infer that (1) coda at 1 Hz is mainly caused by scattering in the sedimentary layer where Q is frequency independent, (2) at higher frequencies the coda contains an increasing amount of scattered body waves, and (3) the source radiation pattern and path variable scattering/multipathing effects in early coda do not affect coda Q.
Heat flow in the western abyssal plain of the Gulf of Mexico: Implications for thermal evolution of the old oceanic lithosphere
Authors: Nagihara S. , Sclater J. G. , Phillips J. D. , Behrens E. W. , Lewis T. , Lawver L. A. , Nakamura Y. , Garcia‐Abdeslem J. , Maxwell A. E.
Published: February 2004

The seafloor depth of an oceanic basin reflects the average temperature of the lithosphere. Thus the western abyssal plain of the Gulf of Mexico, which has tectonically subsided much (>1 km) deeper than other basins of comparable ages (late Jurassic), should be underlain by an anomalously cold lithosphere. In order to examine this hypothesis, we made suites of high‐accuracy heat flow measurements at 10 sites along a line connecting Deep Sea Drilling Project (DSDP) sites 90 and 91 in the Sigsbee abyssal plain. The new heat flow sites were initially surveyed by 3.5‐kHz echo sounding, 4‐channel seismic reflection, seismic refraction with eight ocean bottom seismometers, and nine piston cores. We occupied a total of 48 heat flow stations along the seismic survey line (3 to 6 at each site), including 28 where we measured in situ thermal conductivities over the practical depth interval (4 m) of the new multioutrigger bow heat flow probe. We determined the heat flow associated with the lithosphere by correcting the values measured at the seafloor (41 to 45 mW/m2) for (1) the thermal effect of the sedimentation and (2) the additional heat from the radioactive elements within the sediments. The sedimentation history, required for the first, was reconstructed at each heat flow site based on ages and thicknesses of the major seismic stratigraphical sequences, age data from the DSDP cores, 3.5‐kHz subbottom reflectors, and correlation of turbidite units found in the piston cores. Radiogenic heat production was measured for 55 sediment samples from four DSDP holes in the gulf, whose age ranged from present to Early Cretaceous (0.83 μW/m3on the average). This provided the correction for the second. The effects of these two secondary factors approximately cancel one another. The lithospheric heat flow under the abyssal plain thus estimated ranges from 40 to 47 mW/m2. These heat flow values are among the lowest in the Mesozoic ocean basins where highly reliable data (45 to 55 mW/m2) have been reported. Therefore the lithosphere under the gulf seems indeed colder than that under other old ocean basins. However, it is not as cold as expected from the large tectonic subsidence. The inconsistency between the depth and heat flow may imply an anomaly in the regional thermal isostasy.
High‐frequency isotopic variations in the Mauna Kea tholeiitic basalt sequence: Melt zone dispersivity and chromatography
Authors: DePaolo Donald J.
Published: February 2004

A stratigraphically controlled sequence of Sr, Nd, Pb, and He isotope ratio measurements on tholeiitic basalt cored by the Hawaii Scientific Drilling Project is analyzed to constrain the dispersivity and chromatographic efficiency of the melt‐producing zone under Hawaii. The data are interpreted using a simplified model for the transport of isotopic signals through the melt zone as magma separates from the upwelling solid by buoyancy‐driven porous flow. A constant lava accumulation rate is assumed for the 620‐m‐thick section of analyzed basalt; a total duration of lava accumulation of 120 kyr is based on Ar‐Ar dating. The amplitude‐“period” spectrum of the isotopic variations in the lava sequence is determined by Fourier analysis and compared with a predicted amplitude‐period spectrum for the mantle magma sources, based on an assumed amplitude‐wavelength spectrum for the mantle and a plume upwelling velocity of 20 cm/yr. The isotopic variations in the mantle appear to be substantially attenuated in the lava record. If the attenuation is due to hydrodynamic dispersion in the melting zone, then the melt zone dispersivity is estimated to be in the range 100–1000 m, depending mainly on the value assumed for the melt‐matrix velocity contrast. Dispersivity is a crude measure of effective grain size in the melt zone; the large values, although strictly only upper limits, suggest that permeability in the melt zone is inhomogeneous and some amount of channel development is present. The ratios 3He/4He and 206Pb/204Pb in the Mauna Kea section describe convoluted loops when plotted against one another, consistent with significant chromatographic separation of He and Pb in the melt zone. The evidence for chromatography of isotopic signals is consistent with the porous flow model predictions and indicates that the melt is maintained moderately close to chemical equilibrium with the solid as it separates. The modeling suggests that generally, chromatography in the melting zone causes isotope ratio‐ratio relationships observed in the lava record to be severely out‐of‐phase with those present in the mantle source if the period of the variability is of order 105 years; for periods greater than 106 years the ratios are close to being “in phase,” whereas for periods less than 104 years the lava record is hopelessly scrambled.
On using ambient internal waves to monitor Brunt‐Väisälä frequency
Authors: Jones R. Michael
Published: February 2004

The practicality of obtaining significant information about profiles of Brunt‐Väisälä frequency from surface observations of internal waves is investigated. The inversion method investigated uses a three‐dimensional Fourier transform (two spatial and one temporal) to détermine the dispersion relations of the internal‐wave modes and Abel transforms to estimate those aspects of the Brunt‐Väisälä frequency profile to which internal‐wave dispersion relations are sensitive. If classical Fourier transforms are used to estimate the modal dispersion relations, observations over a 200‐km square region for 17 hours would be necessary to obtain about a 15% error for one profile. However, if nonlinear methods, such as maximum entropy, could be extended to three dimensions, it might be practical to obtain significant information about profiles of Brunt‐Väisälä frequency by observing a 40‐km square region for only 3 hours. This technique cannot be used when there are no significant surface manifestations of internal waves, such as during rough seas or when a significant mixed layer is present. The ambiguity of the inversion is increased when the Brunt‐Väisälä frequency profile has multiple maxima.
The pH of the North Atlantic Ocean: Improvements to the global model for sound absorption in seawater
Authors: Brewer Peter G. , Glover David M. , Goyet Catherine , Shafer Deborah K.
Published: February 2004

At frequencies below 1 kHz, sound absorption coefficients in the ocean are a function of pH, and at higher frequencies they are dependent upon MgSO4. The pH dependent terms are attributable to relaxation of B(OH)3 and MgCO3 species, and the ensemble effect has been approximated (Mellen et al., 1987a) as α = α1(MgSO4) + α2(B(OH)3) + α3(MgCO3), where α is the absorption coefficient in decibels per kilometer, and αn = (S/35)An[ƒ2ƒn/(ƒ2 + ƒn2)], where S is the salinity, An is the amplitude of each component and depends on pH, ƒ is the frequency, and ƒn is the relaxation frequency. Overall accuracy of ±15% in a requires that pH be known to 0.05 pH units. The presently used oceanic pH field for sound absorption models is derived from a combination of Geochemical Ocean Sections Study (GEOSECS) data and Soviet data from the 1978 Gorshkov atlas for the North Atlantic where GEOSECS data are absent. We compare the North Atlantic fields with the well‐constrained Transient Tracers in the Ocean North Atlantic CO2 data set and find large differences. We further show that sufficiently strong correlations exist between CO2 system variables and other more commonly available hydrographic properties and that improved renditions of the pH field in other regions of the ocean are possible once equivalent local correlations are established, thus leading to greatly improved estimates of the sound absorption field.
The underside topography of sea ice over the continental shelf of the Beaufort Sea in the winter of 1990
Authors: Melling Humfrey , Riedel David A.
Published: February 2004

A spatial section of draft across 348 km of sea ice was obtained by moored subsea sonars in the Beaufort Sea in the winter of 1990. The ice field was a mixture of first‐year and multiyear types with mean draft of the order of 3 m. Level ice of 0‐ to 4‐m draft occupied approximately 50% of the section; within this fraction, draft values of 0 and 1.5 m were most common, while at 2‐ to 3‐m draft, a noticeable contribution was made by rafted first‐year ice. Deformed ice was observed to a maximum draft of nearly 27 m. Keels within the section were identified using both the Rayleigh criterion and a new criterion based on level ice. Both criteria defined exponentially distributed populations but with different e‐folding scales. The observations revealed an ice cover whose statistical properties are consistent with other data from the periphery of the polar pack in the Beaufort Sea, but which are much more severe than those of ice found in the seasonal ice zone only a few tens of kilometres to the south. The cross‐sectional shape of individual keels was highly variable, but on average the distribution of ice within a keel was an exponential function of draft. The topography of both first‐year and old ice subsections of the data was fractal in character, with a dimension of about 1.5 for both ice types. The average mass of sea ice per unit area was estimated as 2540–2970 kg m−2, with 68–73% of this mass contributed by deformed ice. The large uncertainties in these estimates arise because the porosity of keels and the burden of snow or melt water upon the ice are not known.
Interocean exchange of water masses
Authors: Döös Kristofer
Published: February 2004

A new method for calculating water mass transport between different ocean basins from the velocity fields obtained by numerical models is presented. The method is applied to the velocity field of the Southern Ocean simulated by a primitive equation model (fine resolution Antarctic model). With this method it is possible to judge whether a water mass has been ventilated or not, to estimate how many times it has circled around Antarctica, and to calculate the time it has spent in the Southern Ocean. Calculations have also been undertaken revealing to what extent the changes of temperature, salinity, and density have been caused by mixing and by ventilation. Two major ways to redistribute the water through the Southern Ocean are identified. The first one redistributes 53% of the water and involves an un ventilated direct exchange between the oceans, the second one redistributes 33% by going around Antarctica. It is found that, on average, the water mass makes six circuits before the water is ventilated and subsequently driven to the north by the Ekman transport. A heat transport study is carried out for the Atlantic, showing that the northward heat transport into the Atlantic comes 85% from the Indian Ocean and the rest from the Drake Passage.
Currents and mixing in the Irminger Sea and in the Iceland Basin
Authors: Krauss W.
Published: February 2004

On the basis of a hydrographic section from the Faeroe Islands toward Greenland along 62°N and the deployment of about 20 satellite‐tracked buoys, it is shown that the Irminger Current branches at the Reykjanes Ridge. Its total transport amounts to 17–18 Sv. The eastern branch of the Irminger Current (8 Sv) turns east and enters the Norwegian Sea both through the Faeroe‐Shetland Channel (3 Sv) and by passing northeast of the Faeroe Islands. Maximum speeds of 70 cm s−1 are reached at the outer edge of the East and West Greenland Current. Very intensive mixing is observed between the four water masses that flow into the Irminger Basin. Stirring by eddies plays a dominant role for strong interleaving, subsequent salt fingering, and double‐diffusive layering. Conditions for these processes are optimal nearly everywhere in the Irminger Sea.
Simultaneous measurements of the ocean wave—Radar modulation transfer function at L, C, and X bands from the research platform Nordsee
Authors: Schmidt Anke , Wismann Volkmar , Romeiser Roland , Alpers Werner
Published: February 2004

Radar backscatter measurements were performed from the German Forschungsplattform Nordsee (FPN) in the North Sea in order to determine the ocean wave‐radar modulation transfer function (MTF), which relates the backseat t ered radar power to the long surface waves. The radar operated quasi‐simultaneously at 1.0 GHz (L band), 5.3 GHz (C band), and 10.0 GHz (X band) at HH and VV polarization by using a single antenna. MTFs obtained at these radar frequencies and polarizations are compared. Our measurements of the dependence of the MTF on wind speed and long wave frequency are in agreement with earlier measurements. It is shown that the dependence of the coherence between the backscattered radar power and the long ocean wave height is a strongly decreasing function of radar frequency. This implies that a real aperture radar operating at a low radar frequency, e.g., at L band, is best suited for imaging ocean waves. Residual MTFs, Mres, are calculated by subtracting the theoretical tilt and range MTFs from the measured total MTFs. According to conventional ocean wave radar modulation theory, Mres should be identical to the hydrodynamic MTF and therefore be independent of polarization. However, the experimental data show a strong dependence of the modulus and phase of Mres on polarization. We find larger values of |Mres| for HH than for VV polarization at C and X bands. In principle, a difference between Mres for HH and VV polarization can be explained by a three‐scale composite surface model which takes into account also the modulation of the Bragg waves by intermediate‐scale waves (i.e., waves with wavelengths between the long waves and the Bragg waves). However, the differences observed in this experiment are found to be much larger than expected from this theory.
The variation in the inherent optical properties of phytoplankton near an absorption peak as determined by various models of cell structure
Authors: Zaneveld J. Ronald V. , Kitchen James C.
Published: February 2004

Optical oceanography models of attenuation and scattering properties often contain simple spectral relationships. Electromagnetic theory, however, predicts fluctuations in the spectra of the attenuation coefficients and scattering properties of substances at wavelengths near an absorption peak. We have modeled these effects for phytoplankton using homogeneous, two‐layered, and three‐layered sphere models of cell structure and using a wide range of plausible particle size distributions. The magnitude of the scattering in backward directions is affected the most. The effect on the beam attenuation spectra is relatively small compared with the effect on the absorption and scattering coefficients. The backscattering coefficient shows large variability, varying by almost a factor of 3 for some models. The results suggest that beam attenuation at any wavelength in the red shorter than the wavelength of the chlorophyll absorption peak will be insensitive to the chlorophyll content of the particles. Increases in the pigment content per unit volume of phytoplankton will increase the index of refraction in the infrared and therefore increase the attenuation and scattering coefficients there.
Optical characterization of the oceanic unicellular cyanobacterium Synechococcus grown under a day‐night cycle in natural irradiance
Authors: Stramski Dariusz , Shalapyonok Alexi , Reynolds Rick A.
Published: February 2004

The optical properties of the oceanic cyanobacterium Synechococcus (clone WH8103) were examined in a nutrient‐replete laboratory culture grown under a day‐night cycle in natural irradiance. Measurements of the spectral absorption and beam attenuation coefficients, the size distribution of cells in suspension, and microscopic analysis of samples were made at intervals of 2–4 hours for 2 days. These measurements were used to calculate the optical properties at the level of a single “mean” cell representative of the actual population, specifically, the optical cross sections for spectral absorption
, scattering
, and attenuation
. In addition, concurrent determinations of chlorophyll a and particulate organic carbon allowed calculation of the Chl a‐ and C‐specific optical coefficients. The refractive index of cells was derived from the observed data using a theory of light absorption and scattering by homogeneous spheres. Low irradiance because of cloudy skies resulted in slow division rates of cells in the culture. The percentage of dividing cells was unusually high (>30%) throughout the experiment. The optical cross sections varied greatly over a day‐night cycle, with a minimum near dawn or midmorning and maximum near dusk. During daylight hours,
and
can increase more than twofold and
by as much as 45%. The real part of the refractive index n increased during the day; changes in n had equal or greater effect than the varying size distribution on changes in
and
. The contribution of changes in n to the increase of
during daylight hours was 65.7% and 45.1% on day 1 and 2, respectively. During the dark period, when
decreased by a factor of 2.9, the effect of decreasing n was dominant (86.3%). With the exception of a few hours during the second light period, the imaginary part of the refractive index n′ showed little variation over a day‐night cycle, and
was largely controlled by variations in cell size. The real part of the refractive index at λ = 660 nm was correlated with the intracellular C concentration and the imaginary part at λ = 678 nm with the intracellular Chl a concentration. The C‐specific attenuation coefficient showed significant diel variability, which has implications for the estimation of oceanic primary production from measurements of diel variability in beam attenuation. This study provides strong evidence that diel variability is an important component of the optical characterization of marine phytoplankton.
A comparison of methods for the measurement of the absorption coefficient in natural waters
Authors: Pegau W. Scott , Cleveland Joan S. , Doss W. , Kennedy C. Dan , Maffione Robert A. , Mueller James L. , Stone R. , Trees Charles C. , Weidemann Alan D. , Wells Willard H. , Zaneveld J. Ronald V.
Published: February 2004

In the spring of 1992 an optical closure experiment was conducted at Lake Pend Oreille, Idaho. A primary objective of the experiment was to compare techniques for the measurement of the spectral absorption coefficient and other inherent optical properties of natural waters. Daily averages of absorption coefficients measured using six methods are compared at wavelengths of 456, 488, and 532 nm. Overall agreement was within 40% at 456 nm and improved with increasing wavelength to 25% at 532 nm. These absorption measurements were distributed over the final 9 days of the experiment, when bio‐optical conditions in Lake Pend Oreille (as indexed by the beam attenuation coefficient cp(660) and chlorophyll a fluorescence profiles) were representative of those observed throughout the experiment. However, profiles of stimulated chlorophyll a fluorescence and beam transmission showed that bio‐optical properties in the lake varied strongly on all time and space scales. Therefore environmental variability contributed significantly to deviations between daily mean absorption coefficients measured using the different techniques.
The average cosine due to an isotropic light source in the ocean
Authors: Maffione R. A. , Jaffe J. S.
Published: February 2004

The average cosine
of the light field created by an isotropic point source (IPS) embedded in a homogeneous ocean is investigated with a Monte Carlo model. Two volume scattering functions (VSFs) are used in the model, taken from Petzold (1972), to compute the radiance distributions at various distances from the source. The simulated radiance distributions are compared with measurements of the point spread function made at Lake Pend Oreille, Idaho, during the 1992 optical closure experiment. An analytic model is presented for
which is valid to at least 15 optical lengths from the source. The model shows that the mean light path, derived from
, is a strong function of the single scattering albedo and the VSF. We found that errors in estimating the absorption coefficient by neglecting the increase in the mean light path, which is due to scattering, vary between 5% and 12% for nearly all natural waters. A mathematical proof is given that
as the distance to the IPS goes to zero. An analytic expression is derived for
close to a finite diffuse‐isotropic source which shows that
approaches one as the distance decreases, but at extremely close distances,
as the distance to the surface of the source goes to zero. At distances beyond one attenuation length, for finite sources small compared to an attenuation length,
behaves essentially as it would for a point source. An asymptotic model for
as a function of the single scattering albedo is given with coefficients that depend on the VSF. Model results and comparisons with measured PSFs reveal the surprising result that the light field from an embedded isotropic point source in the ocean does not exhibit asymptotic behavior as far as 15 attenuation lengths from the source.
Regional models for phytoplankton absorption as a function of chlorophyll a concentration
Authors: Cleveland Joan S.
Published: February 2004

Empirical relationships for predicting phytoplanktonic absorption at 676 and 436 nm from water column chlorophyll a concentration are presented for distinct geographic regions defined by latitude. The forms of the predictive equations are controlled by underlying biological mechanisms and lend insight into these mechanisms. These region‐specific models allow prediction of phytoplanktonic absorption from more easily measured parameters such as chlorophyll a concentration or in situ fluorescence and increase accuracy in modeling optical properties or primary production rates from specific absorption coefficients. Phytoplanktonic absorption can be predicted from chlorophyll a concentrations estimated from satellite‐based ocean color measurements. Temperate and tropical regions exhibited statistically indistinguishable relationships at low chlorophyll so these regions were combined and treated as one. Nonlinear relationships between phytoplanktonic absorption at both 436 and 676 nm and chlorophyll a concentration for the combined temperate/tropical region suggested that pigment packaging effects were important and variable. Higher slopes between absorption and chlorophyll a at low chlorophyll supported the concept of low pigment packaging effects (thus higher specific absorption) in oligotrophic, low chlorophyll a waters. Subpolar waters displayed a distinct pattern and were defined as a separate region. Near‐linear and linear relationships between phytoplanktonic absorption at 436 and 676 nm and chlorophyll a concentration indicated that influences of pigment packaging on phytoplankton specific absorption coefficients were relatively constant and uncoupled from water column chlorophyll a concentration in the subpolar region. Optical depth correlated inversely with specific absorption at 436 nm in the subpolar region, illustrating the role of photoadaptation in determining specific absorption and predictive relationships. Differences between predictive quadratic equations for particulate and phytoplanktonic absorption indicated that detritus made only a small contribution to the nonlinear nature of the relationships. Absorption by detritus at 436 nm ranged from 25 to 90% of the total particulate absorption. The proportion of absorption by detritus did not exhibit any strong patterns as a function of chlorophyll a concentration, arguing against the previously invoked explanation that proportional increases in absorption by detritus as environments become more oligotrophic cause the observed nonlinearity between absorption and chlorophyll a concentration.
Irreversible thermodynamics and air‐sea exchange
Authors: Doney Scott C.
Published: February 2004

Recent theoretical work has suggested that the air‐sea exchange of trace gases such as CO2 can be modified by irreversible thermodynamic coupling phenomena and that the commonly used gas exchange relationships can incorrectly predict the magnitude and, under certain circumstances, the sign of air‐sea trace gas fluxes. The physical basis for irreversible thermodynamic effects during air‐sea exchange is developed for both the liquid‐gas interface and the molecular boundary layers or sublayers adjacent to the interface. The impact of irreversible thermodynamics is shown to differ qualitatively from that predicted by an earlier analysis due to the explicit inclusion in this work of the molecular kinetic layer at the air‐water interface. For insoluble trace gases such as CO2 the temperature contrast across the limiting region for air‐sea exchange, the aqueous mass sublayer, is not great enough to have a measurable impact on the trace gas distributions, and irreversible effects do not need to be included in empirical air‐sea gas exchange relationships.