bluuebunny/crossref_metadata_2025 huggingface dataset:
Uniform accretion of oceanic crust south of the Garrett transform at 14°15′S on the East Pacific Rise
Authors: Kent Graham M. , Harding Alistair J. , Orcutt John A. , Detrick Robert S. , Mutter John C. , Buhl Peter
Published: February 2004

Using migrated common depth point reflection profiles, we find the structural differences along the ultrafast spreading (>150 mm/yr) East Pacific Rise south of the Garrett fracture zone are second‐order, suggesting a remarkably uniform process of crustal accretion. The rise axis south of the Garrett transform is underlain by a narrow (<1.0 km) melt lens which shows great along‐strike continuity. The depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading East Pacific Rise at 9°30′N. This observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent. Melt sill width, however, shows little variation along the East Pacific Rise, suggesting no dependence of magma chamber size on spreading rate. The melt reservoir decreases in width toward/across the 14°27′S ridge axis discontinuity by a modest 250–300 m and appears to be continuous across this feature. Given the small aspect ratio (∼1.0 km by ∼50 m by tens of kilometers) of the axial melt lens, the previously recorded jump in MgO content across the 14°27′S offset is likely the result of a mixing boundary which is sustained through an along‐strike impedance in convection. Wide‐angle reflections originating at the base of seismic layer 2A, assumed to coincide with the extrusive layer, reveal a twofold to threefold increase (200–250 to 500–600 m) in thickness within 1–2 km of the rise axis. The pattern of extrusive thickening imaged south of the Garrett transform is similar to that observed along the slower spreading (110–120 mm/yr) East Pacific Rise at 9°N. Outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile. This implies a degree of temporal stability of the along‐strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone. The inferred uniformity of off‐axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14°27′S discontinuity are caused by long‐term reductions in magma supply. Second‐order differences in the style of extrusive thickening may be related to structural differences within the low‐velocity zone underlying the rise axis and/or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis. Images of Mono on cross‐axis profiles may be traced to within ∼1.0 km of the melt sill edge; this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber.
Small‐scale stress heterogeneity in the Anza seismic gap, southern California
Authors: Hartse Hans E. , Fehler Michael C. , Aster Richard C. , Scott Jennifer S. , Vernon Frank L.
Published: February 2004

Focal mechanism inversions reveal significant lateral variations in stress orientations along the Anza segment of the San Jacinto fault zone. The most notable stress anomaly is within the 20‐km aseismic (seismic gap) portion of the fault zone, where , the maximum compressive stress, is nearly horizontal and is oriented at 74°±13° relative to the fault strike. This contrasts with orientations ranging from 62°±11° to 49°±7° along the more seismically active portions of the fault zone immediately to the northwest and southeast of the seismic gap. Regional stress results, found by inverting all focal mechanisms simultaneously, indicate that
is horizontal and trends north‐south, while
is horizontal and trends east‐west. Approximately, 15 km west of the seismic gap, in the off‐fault Cahuilla swarm area,
and
solutions are rotated clockwise by about 25° relative to the regional model. Roughly, 10 km southeast of the seismic gap near the Buck Ridge fault,
and
are rotated counterclockwise by about 10° relative to the regional solution. Northwest of the seismic gap along the fault zone,
plunges about 30° from the horizontal, correlating with a local increase in reverse faulting between the Hot Springs and San Jacinto faults. Southeast of the seismic gap,
plunges about 45° from the horizontal, correlating with a local increase in normal faulting in the trifurcation region of the Buck Ridge, Clark, and Coyote Creek faults. We propose a simple mechanical model in which a block rotation superimposed on the dominant right‐lateral strikeslip motion of the fault zone satisfies the first‐order observations of stress orientation, faulting, and horizontal surface strain. Under this model the Anza seismic gap is the region of zero convergence between the northeast and southwest sides of the fault, and the fault zone strength within the seismic gap is either comparable to or exceeds the fault zone strength adjacent to the gap.
North Atlantic volcanic margins: Dimensions and production rates
Authors: Eldholm Olav , Grue Kjersti
Published: February 2004

Early Tertiary lithospheric breakup between Eurasia and Greenland was accompanied by a transient (∼3 m.y.) igneous event emplacing both the onshore flood basalts of the North Atlantic Volcanic Province (NAVP) and huge extrusive complexes along the continent‐ocean transition on the rifted continental margins. Seismic data show that volcanic margins extend >2600 km along the early Eocene plate boundary, in places underlain by high‐velocity (7.2–7.7 km/s) lower crustal bodies. Quantitative calculations of NAVP dimensions, considered minimum estimates, reveal an areal extent of 1.3×106 km2 and a volume of flood basalts of 1.8×106 km3, yielding a mean eruption rate of 0.6 km3/yr or 2.4 km3/yr if two‐thirds of the basalts were emplaced within 0.5 m.y. The total crustal volume is 6.6×106 km3, resulting in a mean crustal accretion rate of 2.2 km3/yr. Thus NAVP ranks among the world's larger igneous provinces if the volcanic margins are considered. The velocity structure of the expanded crust seaward of the continent‐ocean boundary differs from standard oceanic and continental crustal models. Based on seismic velocities this “volcanic margin” crust can be divided into three units of which the upper unit corresponds to basaltic extrusives. The regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust, including the high‐velocity lower crust which extends some distance landward of the continent‐ocean boundary, was emplaced during and subsequent to breakup. The volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling, probably reflecting the interaction of a mantle plume and a lithosphere already extending.
Scavenging of 234Th and phosphorus removal from the hydrothermal effluent plume over the North Cleft segment of the Juan de Fuca Ridge
Authors: Kadko David , Feely Richard , Massoth Gary
Published: February 2004

Dissolved and total 234Th (t1/2 = 24.1 days) profiles were measured over the hydrothermal field located within the North Cleft segment of the Juan de Fuca Ridge. Above the hydrothermal effluent plume, which rose approximately 200 m from the seafloor, ≥90% of the 234Th was dissolved, and the total 234Th activity was in equilibrium with its parent uranium. This is typical of deep water for which the residence time of dissolved 234Th with respect to particle uptake is greater than 200 days. Within the effluent plume, however, as little as 30% of the 234Th was in the dissolved state and the residence time of dissolved 234Th with respect to particle uptake was then of the order of only a few weeks. The flux (JTh) of dissolved 234Th to the particle phase was about 300 disintegrations per minute (dpm) cm−2 yr−1. The total 234Th was still approximately in equilibrium with U, indicating that the residence time of the 234Th‐bearing particles with respect to sinking was greater than 100 days. Particulate P, V, and As, while insignificant above the plume, attained concentrations as high as 60, 0.65, and 0.45 nmol/L within the plume. The particulate [P]/[234Th] ratio and the 234Th flux (JTh) are used to calculate a removal flux of P resulting from hydrothermal scavenging of ∼50 μg cm−2 yr−1 or approximately 5.0× 106 g P yr−1 per kilometer of active ridge crest. Similarly, 1.0×105 g km−1 of V and As are removed per year from the water column by hydrothermal scavenging over this region. Extrapolation to a global scale suggests that ridge crest scavenging removes ≤10% of these elements from the ocean.
A thermodynamic assessment of silica phase diagram
Authors: Swamy V. , Saxena Surendra K. , Sundman Bo , Zhang J.
Published: February 2004

An internally consistent data set on the thermodynamic properties of the silica polymorphs stable up to 15 GPa (α‐quartz, β‐quartz, tridymite, cristobalite, coesite, and stishovite) and the liquid phase is presented. The data set was produced through a computer‐based assessment of the properties in which the available thermochemical (calorimetric), physical (bulk modulus and thermal expansion), and solid‐state and melting transition data (including some newly determined data on the high‐pressure polymorphs cosite and stishovite) were considered. The data set can be used to calculate phase relations at pressures of 0.1 MPa to 15 GPa and temperatures of 300 to 3200 K. The calculated phase diagram using these data agrees quite well with the phase equilibrium determinations except for the high‐temperature part of the coesitestishovite boundary. The properties of the liquid phase obtained are also in good agreement with the available data.
Seismicity at Mount St. Helens, 1987–1992: Evidence for repressurization of an active magmatic system
Authors: Moran Seth C.
Published: February 2004

Several changes have occurred in seismicity associated with Mount St. Helens since it last erupted in October 1986. Perhaps the most significant is the recurrence of earthquakes deeper than 3 km, previously observed only in the months following the cataclysmic eruption of May 18, 1980, and briefly in March 1982. Events located below 6.5 km define a circular aseismic zone, similar in location to one observed after May 18, 1980. Focal mechanisms are computed for two depth ranges, 4–6.5 km (Intermediate) and 6.5–10 km (Deep), using both polarity and amplitude ratio data. Mechanisms for Intermediate events are mostly strike‐slip, and many P and T axes point in directions ∼80° offset from the regional stress field, indicating that earthquakes in this zone are caused by magmatic as well as tectonic forces. Mechanisms for Deep events are also mostly strike‐slip, and P and T axes form a pattern suggestive of a pressure increase within the aseismic zone. The observed Deep patterns are compared with theoretical stress fields generated by a numerical model of a pressurized hole in an elastic plate. Results from modeling support the pressure increase hypothesis for 1987–1992 seismicity, in contrast to a pressure drop for post‐May 18, 1980, earthquakes. This apparent repressurization is proposed to be a result of the sealing of the shallow conduit system.
Regional variation of attenuation and travel time in China from analysis of multple‐ScS phases
Authors: Sipkin Stuart A. , Revenaugh Justin
Published: February 2004

Long‐period multiple‐ScS waveforms from intermediate‐ and deep‐focus earthquakes surrounding China, recorded by the Chinese Digital Seismograph Network, are used to estimate the multiple‐ScS attenuation operator and differential travel times using both frequency (phase equalization and stacking) and time domain (waveform inversion) techniques. Theoretical considerations, augmented by synthetic testing, indicate that for continental regions the frequency domain approach yields a more stable estimate of whole mantle attenuation (QScS) than the time domain approach, which is better for whole mantle travel time (τScS). When teamed together, the two afford unprecedented success with continental paths. The results, 16 path‐averaged estimates of QScS and τScS constitute a significant addition to the catalog of multiple‐ScS studies of subcontinental mantle and provide sufficiently dense coverage to invert “path” estimates of attenuation and travel time for “regional” quantities of direct tectonic relevance. Across the frequency band of 10 to 50 mHz we obtain high (>215) values of regionalized QScS τScS the Tarim platform and the fold systems of northeastern China and low (<170) values elsewhere on the continent, defining a pattern that is well correlated with present‐day levels of tectonic activity. The corresponding two‐way travel time residuals with respect to the Jeffreys‐Bullen (J‐B) tables, ΔτScS (= τScS ‐ 935.7 s), vary from −1.3±1.4 to +0.8±4.3 s and are typical of other “fast” continental regions. Unlike QScS, ΔτScS varies little from region to region and is correlated poorly with tectonic activity, indicating that QScS is much more sensitive to changes in the current tectonic environment. Offshore, ΔτScS for the Sea of Japan is +2.7±3.6 s and for the combined East China and Philippine seas region is +9.6±2.7 s. The latter, very large residual likely reflects the low velocities associated with back arc spreading. Throughout the study region, ΔτScS correlates well with the residuals predicted by recent upper mantle tomographic models, although the observed variance exceeds the model predictions by roughly a factor of 3.
The size‐isotopic evolution connection among layered mafic intrusions: Clues from a Sr‐Nd isotopic study of a small complex
Authors: Poitrasson Franck , Pin Christian , Duthou Jean‐Louis , Platevoet Bernard
Published: February 2004

Several theoretical and experimental works have focused on the processes occurring in continental mafic magma chambers. In contrast, systematic isotopic studies of natural remnants of these latter remain scarce, although they can give fundamental constraints for theoretical studies. This is especially true if different layered complex with contrasting characteristics (e.g., different size) are compared. For this reason, we present the results of a Sr‐Nd isotopic profile across a small layered mafic intrusion of Permian age exposed near Fozzano (SW Corsica). In the main zone of the layered section, decreasing 87Sr/86Sri and increasing 143Nd/144Ndi are observed from less evolved (bottom) to more evolved (top) rocks. This peculiar pattern precludes assimilation and fractional crystallization (AFC) as a dominant mechanism in the petrogenesis of this body. Instead, we interpret this trend as reflecting the dilution of an early stage contaminated magma by several reinjections of fresh basalt in the chamber. In agreement with mineralogical and structural data, every cyclic unit is interpreted as a new magmatic input. On the basis of rough refill and fractional crystallization (RFC) calculations, the average volume for each reinjection is estimated to have been about 0.04 km3. The cumulative volume of these injections would amount to about 75% of the total volume of the layered complex. This implies that reinjections were accompanied by an important increase of the volume of the chamber or by magma withdrawal by surface eruptions. The RFC mechanism documented within this small layered body contrasts with the isotopic pattern observed between several intrusions at the regional scale in SW Corsica, and within large continental mafic magma chambers elsewhere. In these cases the isotopic evolution is dominated by AFC processes, and there is no clear isotopic evidence for reinjections, unless major influx of fresh magma occurred. It is suggested that there is a close relationship between the assimilation rate and the magma chamber volume. Small magma chambers are quickly isolated from their country rocks and better preserve the subtle isotopic signature of reinjection processes.
Geometrical effects of curvature in axisymmetric spherical models of mantle convection
Authors: Vangelov Valentin I. , Jarvis Gary T.
Published: February 2004

A two‐dimensional axisymmetric model of convection in a spherical shell has been developed in order to investigate the effects of curvature on model predictions of heat flow and temperature in the Earth's mantle. We restrict the solution domain to a belt centered midway between the poles of the axisymmetric coordinate system. This facilitates direct comparisons with models in cylindrical shells and plane layers, and avoids the physically unrealistic near‐pole effects associated with the axial symmetry at the poles. A boundary layer argument is suggested which accounts for the observed variations in heat flow and mean temperature from one level of curvature to another. In particular, plane layer results may be scaled to any desired degree of curvature. Similarities between previous model results in cylindrical shells and our results in spherical shells are noted, and it is shown that results in one geometry may be mapped into the other provided the ratio of the surface areas of the upper and lower boundaries is the same.
Toward the discrimination of fine‐grained ferrimagnets by magnetic measurements in lake and near‐shore marine sediments
Authors: Oldfield F.
Published: February 2004

The magnetic properties of low field susceptibility (χ), frequency dependent susceptibility (χfd), and susceptibility of anhysteretic remanent magnetisation (χARM) are used to define sets of natural soil and sediment samples within which, by the criteria of King et al. (1982) and Maher (1988), the modal ferrimagnetic grain diameter is less than ∼0.07 ‐ 0.1 μm. Within this sample set, two distinct groups with low and high values, respectively, for the quotients χARM/χ and χarm/χfd are defined. The first group includes sediment samples from sites where published studies propose a detrital origin for the fine‐grained ferrimagnetic content. Where catchment samples are available for comparison, they fall within the same envelope, as do clays from palaeosol samples within Chinese loess. This envelope of low quotient values also overlaps with the values for the fine grained synthetic magnetite samples within Maher's (1988) New MT series. The high quotient envelope of values includes sediments from the Adriatic Sea and clays from saltmarsh and shallow water marine sediments in NW Britain. In these cases, no catchment source is postulated for the fine‐grained magnetite. The size range of magnetite in this set appears to be almost exclusively stable single domain (SSD), and a bacterial origin seems likely. Sample sets from estuarine environments between river inflow sites and the open sea show intermediate values which, in the case of the Potomac, are ordered by distance down river. Although at this stage, magnetic measurements alone cannot discriminate between bacterial and fine‐grained detrital ferrimagnets in sediments, they hold out some promise of doing so provided the distinctions proposed here can be substantiated by transmission electron microscopy.
Initial reference models in local earthquake tomography
Authors: Kissling E. , Ellsworth W. L. , Eberhart‐Phillips D. , Kradolfer U.
Published: February 2004

The inverse problem of three‐dimensional (3‐D) local earthquake tomography is formulated as a linear approximation to a nonlinear function. Thus the solutions obtained and the reliability estimates depend on the initial reference model. Inappropriate models may result in artifacts of significant amplitude. Here, we advocate the application of the same inversion formalism to determine hypocenters and one‐dimensional (1‐D) velocity model parameters, including station corrections, as the first step in the 3‐D modeling process. We call the resulting velocity model the minimum 1‐D model. For test purposes, a synthetic data set based on the velocity structure of the San Andreas fault zone in central California was constructed. Two sets of 3‐D tomographic P velocity results were calculated with identical travel time data and identical inversion parameters. One used an initial 1‐D model selected from a priori knowledge of average crustal velocities, and the other used the minimum 1‐D model. Where the data well resolve the structure, the 3‐D image obtained with the minimum 1‐D model is much closer to the true model than the one obtained with the a priori reference model. In zones of poor resolution, there are fewer artifacts in the 3‐D image based on the minimum 1‐D model. Although major characteristics of the 3‐D velocity structure are present in both images, proper interpretation of the results obtained with the a priori 1‐D model is seriously compromised by artifacts that distort the image and that go undetected by either resolution or covariance diagnostics.
Confidence regions for mantle heterogeneity
Authors: Pulliam R. Jay , Stark Philip B.
Published: February 2004

Least squares tomographic models of mantle P and S velocity structure from travel times have shown large‐scale variations correlated with surface tectonic features, as well as coherent structures in the lowermost mantle. The reliability of these global features of velocity models depends on whether velocity throughout the features can be estimated well simultaneously: one needs to be able to say with confidence that a feature involving many voxels is likely to be real. We find a lower bound on how wide a 95% simultaneous confidence region for least squares estimates of mantle P or S velocity perturbations must be, as a function of position in the mantle. Suppose (perhaps optimistically) that summary ray travel time errors are independent with mean zero and standard deviation 0.25 s for direct P phases, and standard deviation 0.5 s for all other phases, regardless of distance. If summary travel time residuals are fitted by least squares to a mantle model parametrized by 4872 10° by 10° voxels, the half width of a 95% confidence region for P velocity (using 889,909 summary P rays, 84,046 summary PP rays, 67,228 summary pP rays, 23,024 summary PcP rays, 11,239 summary PKPab rays, 47,227 summary PKPbc rays, and 141,843 summary PKPdf rays) ranges from ±0.025 to ±99.8 km s−1. It is wider than 0.193 km s−1 in half of the mantle and wider than 0.593 km s−1 in a quarter of the mantle, by volume. Under the same assumptions and using the same parametrization, a 95% confidence region for S velocity based on 163,354 summary S rays, 13,781 summary SS rays, 7441 summary ScS rays, and 12,494 summary sS rays ranges from ±0.031 to ±∞ km s−1, is wider than 0.254 km s−1 in half of the mantle and is wider than 0.554 km s−1 in a quarter of the mantle. With this ray set, parametrization, and error model, the 95% confidence region around the least squares estimate of P velocity includes the radially symmetric iasp91 model for 86.6% of the mantle's volume. A 95% confidence region around the least squares S estimate includes iasp91 for 88.9% of the mantle's volume. On a global scale, the mantle's velocity structure is nearly consistent with the iasp91 radially symmetric model. Smaller voxels, more realistic assumptions about the errors, more accurate modeling of the physics, and three‐dimensional structure outside the mantle would make the confidence regions still wider.
Combined paleointensity and40Ar/39Ar age spectrum data from volcanic rocks of the West Eifel field (Germany): Evidence for an early Brunhes geomagnetic excursion
Authors: Schnepp Elisabeth , Hradetzky Helmuth
Published: February 2004

Results of virtual geomagnetic pole position, paleointensity and isotopic age determinations have been derived from lavas of the Quaternary West Eifel volcanic field (Germany). The characteristic remanent magnetization (CARM) directions and corresponding positions of virtual geomagnetic poles (VGP) have mainly been determined by Böhnel et al. They are exclusively of normal polarity. The VGPs form a strongly non‐Fisherian distribution characterized by about 30% pole positions with latitudes below 60°N, which are mainly confined to the longitude sector from 20°W to 120°E. Eight independent poles are clustered at 40°N and 45°E. Geomagnetic paleointensity values from 37 volcanic units of the West Eifel have been determined. Paleointensities range from 7 to 62 μT with an unexpectedly large number of paleointensity values below 30 μT. About half of the low paleointensity values are associated with intermediate VGP latitudes. Isotopic age determinations from West Eifel volcanic rocks have been published (Fuhrmann and Lippolt). Additional isotopic ages are presented here. The West Eifel volcanic field is usually devided into an older part in the NW and a younger one in the SE, based on petrological models and geological evidence. All available isotopic ages of the West Eifel volcanic rocks which are exclusively from the northwestern part of the field lie in the interval between 460 and 660 kyr. For seven of the clustered low‐latitude VGPs it was possible to determine paleointensities and40Ar/39Ar ages. Within their error limits six poles have identical virtual dipole moments (VDM) at 3.5×1022A m2. Furthermore, the individual isotopic age intervals of the40Ar/39Ar data overlap. The assigned age of 510±30 kyr is considered to be the age of a geomagnetic excursion. For higher‐latitude VGP positions (>45°N), VDM values range from 2 to 10×1022A m2. The mean VDM value (about 7.5×1022A m2) of the younger volcanoes (ages estimated to be below 100 kyr) is not remarkably different from the present dipole moment of the Earth. In contrast, the older volcanoes with higher‐latitude VGP positions yielded a mean value of only 5.3×1022A m2and a bimodal frequency distribution of the VDM values. Isotopic ages determined for two of these volcanic units are close to the inferred excursion age suggesting a decrease of the geomagnetic dipole moment during a time interval which surpassed the duration of the excursion.
The estimation of diffuse strains in the toe of the western Nankai accretionary prism: A kinematic solution
Authors: Morgan Julia K. , Karig Daniel E. , Maniatty Antoinette
Published: February 2004

Arcward changes in the vertical spacing of seismically defined stratigraphic reflectors and sediment porosities determined from drill cores and seismic interval velocities suggest a compact kinematic solution for estimating diffuse deformation within accretionary prism toes. The method involves the solution of the Lagrangian form of the conservation of mass equation for the two‐dimensional finite deformation gradient field within discrete deforming domains. We apply this technique to the unfaulted protothrust zone (PTZ) along a western transect across the Nankai accretionary prism, in the vicinity of Deep Sea Drilling Project sites 582 and 583. The results yield horizontal shortening estimates of about 1.7 km, or about 20%. This shortening appears to be accommodated through different strain responses depending on the consolidation state of the sediments at the time of accretion. Shallow sediments respond to horizontal shortening dominantly by expelling pore fluids, producing volume strains up to 30%, while deeper sediments accommodate horizontal shortening dominantly through vertical extension reaching 70% in the footwall of the frontal thrust. Horizontal shear strains within the PTZ are not well resolved by this solution due to uncertainties in the porosity distribution, but they generally appear to be quite small.
The continent‐ocean crustal transition across the southwest Greenland margin
Authors: Chian Deping , Louden Keith E.
Published: February 2004

This paper examines the complete crustal transition across the nonvolcanic, southwest Greenland continental margin of the Labrador Sea using wide‐angle and coincident vertical‐incidence seismic profiles. Six ocean bottom seismometers and a sonobuoy record P and S wave first and multiple arrivals from the crust and upper mantle, which are analyzed by two‐dimensional dynamic ray tracing and one‐dimensional reflectivity modeling. The resulting seismic velocity model requires that the preexisting 30‐km thick continental crust is thinned abruptly to ∼3 km across the continental slope, primarily by removal of the lower crust. Farther seaward, the crust thickens to ∼6 km primarily through the addition of a high‐velocity (7.0–7.6 km/s) layer in the lower crust. This lower crustal layer is 4–5 km thick, extends for a horizontal distance of ∼80 km, and is interpreted as partially serpentinized upper mantle. It is overlain by a low‐velocity (4.0–5.0 km/s), upper layer which is interpreted as highly fractured upper continental crust. Our model suggests that seafloor spreading did not start until chrons 27–28, 13 Ma younger than previously suggested. This interpretation is supported by two‐dimensional modeling of gravity and magnetic data along the refraction line. Our results are consistent with a simple shear mechanism for the initial rifting, with the SW Greenland margin as the upper plate. However, a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin, east of Labrador.
Shear wave anisotropy beneath the Tibetan Plateau
Authors: McNamara Daniel E. , Owens Thomas J. , Silver Paul G. , Wu Frances T.
Published: February 2004

Eleven broadband digital seismic stations were deployed across the central Tibetan Plateau in the first extensive passive‐source experiment attempted within the Tibetan Plateau. One year of recording resulted in 186 event‐station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array. Measurements of the fast polarization direction (ϕ) and delay time (δt) for SKS and direct S arrivals reveal systematic variations along the north‐south oriented array. In the north central region of the plateau, very large delay times are observed at three stations, the largest of which is BUDO with of δt=2.4 s. However, at TUNL, which is off the northern edge of the plateau and 110 km from BUDO, and at sites in the south central plateau, δt decreases by nearly a factor of 3. We also observe systematic rotation of ϕ from about 45° (NE) to 90° (E‐W) from south to north along the array. A previously identified zone of inefficient Sn propagation correlates well with our region of large δt observations. The large delay times suggest that a relateively high number of anisotropic crystals are preferentially alligned within the mantle‐lid, beneath the north central portion of the Tibetan Plateau. In most cases, fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the Tibetan Plateau.
Isotopic evidence for closed‐system anatexis at midcrustal levels: An example from the Acadian Appalachians of New England
Authors: Lathrop Alison S. , Blum Joel D. , Chamberlain C. Page
Published: February 2004

We have investigated the Sr and O isotope systematics of granitoid and metasedimentary samples from the Central Maine Terrane (CMT) of New England. Granitoid samples were taken from interior and contact zones within the Acadian‐aged (∼410 m.y.), synmetamorphic and syntectonic Kinsman Quartz Monzonite (KQM), which is a member of the New Hampshire Plutonic Series. Metasedimentary samples were taken from Silurian and Devonian formations hosting the KQM. Initial Sr isotope ratios (Sri) and δ18O values for the KQM range from 0.70799 to 0.71246 and 1.6‰ to 12.9‰, respectively, and Sriand δ18O values of the metasedimentary rocks range from 0.70770 to 0.75008 and 6.2‰ to 14.1‰, respectively. We observe a linear and slightly positive correlation between Sriand δ18O for interior KQM samples that can be duplicated by a mixing curve calculated for metasedimentary endmembers, whereas the Sriand δ18O values of contact KQM samples cluster near the Sriand δ18O values of the metasedimentary rocks with which they are in contact. Mixing calculations provide no evidence for a measurable primitive mantle component in either interior or contact KQM samples, and we conclude that the Sr‐O isotopic composition of the KQM is most likely a reflection of isotopic heterogeneities inherited from a complex package of midcrustal metasedimentary source rocks. We propose that the KQM is the product of midcrustal partial melting that was initiated due to excess thermal energy from the decay of anomalously high concentrations of heat‐producing elements in Silurian source rocks within the CMT. Because we see no isotopic evidence for a lower‐crustal or mantle component in the KQM, we suggest that midcrustal anatexis may have occurred as a closed‐system process, requiring no accompanying mantle‐derived magma or above normal mantle heat flow.
Thermal‐infrared remote sensing and Kirchhoff's law: 1. Laboratory measurements
Authors: Salisbury John W. , Wald Andrew , D'Aria Dana M.
Published: February 2004

By the end of this century the Earth Observing System will provide worldwide, thermal infrared, multispectral images of the Earth, presenting geologists with a new kind of remote sensing data for interpretation. Thus it has become essential to understand the spectral emittance behavior of terrestrial surface materials. Perhaps the most fundamental question to be answered is the extent to which such materials follow Kirchhoff's law (ε = 1 −R) under laboratory and field conditions, especially when a sample displays a thermal gradient. We present the first rigorous quantitative comparison of directional hemispherical reflectance and directional emittance measurements of rock and soil samples in the laboratory, with thermal gradients induced by heating them from below and allowing them to radiate to a colder background. The results show that only an extremely low density sample composed of fine particles sifted into a “fairy castle” structure displays a thermal gradient steep enough within the infrared skin depth to cause significant (6%) departure from Kirchhoff's law. There is no detectable effect on the more normal terrestrial samples, such as soils and rocks measured in the laboratory, even when semitransparent coatings are involved. Thus both emittance and reflectance measurements can be used to calculate sample emissivity for most terrestrial surface materials. However, the effect on Kirchhoffian behavior of different field environments, which may induce a steeper thermal gradient in particulate samples, has yet to be determined, and some low‐density surface materials like newly fallen snow, frost, and efflorescent salts on playas have yet to be measured in emittance.
Magnetic susceptibility of bottom sediments and suspended particulates from Mulki‐Pavanje River, estuary, and adjoining shelf, west coast of India
Authors: Karbassi A. R. , Shankar R.
Published: February 2004

Magnetic susceptibility has been determined for sediments from the riverine, estuarine, and marine environments near Mulki, west coast of India, to understand the behaviour of magnetic minerals as they enter the oceanic realm through the estuary. In the riverine environment, magnetic minerals are bimodally distributed. Magnetic susceptibility is positively correlated with the magnetite weight percentage. The magnetic mineral content decreases from the river head to the river mouth. Well‐sorted sediments are rich in magnetic minerals. It is seen that the susceptibility of samples is governed both by the type of minerals present and by their relative abundances. Several instances of the environmental regime/process influencing the magnetic susceptibility and hence the magnetic mineral content are discussed: deposition of magnetic minerals in meander loops, anoxic conditions in the inner shelf, etc.
Crystal structure, stable isotopes (δ18O), and development of sea ice in the Ross, Amundsen, and Bellingshausen seas, Antarctica
Authors: Jeffries Martin O. , Shaw Raymond A. , Morris Kim , Veazey Alice L. , Krouse H. Roy
Published: February 2004

The crystal structure and oxygen isotopic composition of ice cores obtained from floes at the end of summer in the eastern Ross Sea, the Amundsen Sea, and the western Bellingshausen Sea were investigated to determine the ice growth processes and conditions that contribute to sea ice development in the eastern Pacific sector of the southern ocean. The isotope data indicate that a moderate amount of snow contributes to the development of the sea ice. However, even the combined use of isotopes and crystal structure analysis does not unambiguously explain the means by which all of the snow is entrained in the ice. Nevertheless, it seems clear that much of the snow is contained in granular snow‐ice that results from seawater flooding of floes and the base of the snow cover. The snow cover in the Ross‐Amundsen region was as much as 2 m deep and supported by 7‐ to 8‐m‐thick floes primarily composed of frazil ice. In the Bellingshausen region the snow cover and the floes were thinner than in the Ross‐Amundsen region. The Bellingshausen cores were composed primarily of multiple layers of frazil and congelation ice. In addition, in both regions there were numerous tipped or inclined blocks of congelation ice and layers of rafted nilas in the cores. The data indicate that the sea ice develops by multiple mechanisms in a turbulent environment.
Elemental and isotopic fluxes in the Southern California Bight: A time‐series sediment trap study in the San Pedro Basin
Authors: Thunell Robert C. , Moore Willard S. , Dymond Jack , Pilskaln Cynthia H.
Published: February 2004

Temporal variability in the fluxes of various elements and radionuclides in San Pedro Basin have been estimated using week‐long sediment trap samples collected continuously from January to July 1988. Deposition in this marginal basin is controlled by seasonal changes in both upper ocean hydrography (i.e., upwelling) and local climate (i.e., precipitation, runoff). The fluxes of elements derived primarily from continental sources (Al, Ti, Na, and V) were highest during the winter (late January to early February) in association with the highest rainfall recorded during the study period. The flux patterns for the biologically active elements (Br, Ca, and I) are highest during the spring upwelling period (April–May) and correlate well with organic carbon fluxes. Due to significant input of continentally derived Ba, the flux of this element in San Pedro Basin is dominated by terrestrial input and not biological processes. The ultimate removal of 234Th and 210Pb from the water column appears to be controlled primarily by organic carbon flux. However, the sources of organic carbon vary seasonally, with significant terrestrial organic carbon input during the winter and predominantly marine organic carbon during the spring and summer. This suggests that the removal of these two radionuclides is independent of the source of the organic carbon. In contrast, the flux of 228Th is controlled by seasonal changes in terrigenous input.
Estimates of particle‐ and thorium‐cycling rates in the northwest Atlantic Ocean
Authors: Murnane R. J. , Cochran J. K. , Sarmiento J. L.
Published: February 2004

We provide least squares estimates of particle‐cycling rate constants and their errors at 13 depths in the Northwest Atlantic Ocean using a compilation of published results and conservation equations for thorium and particle cycling. The predicted rates of particle aggregation and disaggregation vary through the water column. The means and standard deviations, based on lognormal probability distributions, for the lowest and highest rates of aggregation (β2) and disaggregation (β−2) in the water column are 8±27 y−1 < β2 < 18±90 y−1, and 580±2000 y−1 < β−2 < 3×103±104y−1. Median values for these rates are 2.1 y−1 < β2 < 3.2y−1, and 149 y−1 < β−2 < 156 y−1. Predicted rate constants for thorium adsorption (k1 = 5.0 ± 1.0 × 104 m3 kg−1 y−1) and desorption (k−1 = 3.1±1.5 y−1) are consistent with previous estimates. Least squares estimates of the sum of the time dependence and transport terms from the particle and thorium conservation equations are on the same order as other terms in the conservation equations. Forcing this sum to equal zero would change the predicted rates. Better estimates of the time dependence of thorium activities and particle concentrations and of the concentration and flux of paniculate organic matter would help to constrain estimates of β2 and β−2.
Numerical experiments of internal wave generation by strong tidal flow across a finite amplitude bank edge
Authors: Lamb Kevin G.
Published: February 2004

Results of some idealized numerical experiments of strong tidal flow of a stratified fluid across a finite amplitude bank edge are presented. These experiments were motivated by a need to develop an understanding of some of the complex internal wave phenomena observed on Georges Bank (Loder et al., 1992) and at other locations where tidal forcing is strong. The numerical model solves the fully nonlinear, nonhydrostatic Boussinesq equations on an ƒ plane. The model is two‐dimensional, with spatial variation in the vertical and cross‐bank directions only. Model forcings are based on the Georges Bank observations. A horizontally uniform stratification is used. The model successfully reproduces some observed features including the formation of a large depression and a hydraulic jump over the bank edge during off‐bank flow and two on‐bank propagating depressions every tidal period. An undular bore propagating away from the bank is in agreement with other observations (La Violette et al., 1990). Rotational effects are shown to be responsible for the formation of the second of the on‐bank propagating depressions. Sensitivity of the results to the topographic slope, tidal current strength, stratification, and model initialization is explored.
Evaluation of bio‐optical algorithms to remotely sense marine primary production from space
Authors: Berthelot Béatrice , Deschamps Pierre‐Yves
Published: February 2004

In situ bio‐optical measurements from several oceanographic campaigns have been analyzed to derive a direct relationship between water column primary production Pt ocean color as expressed by the ratio of reflectances R1 at 440 nm and R3 at 550 nm and photosynthetically available radiation (PAR). The study is restricted to the Morel case I waters for which the following algorithm is proposed: log (Pt) = −4.286 ‐ 1.390 log (R1/R3) + 0.621 log (PAR), with Pt in g C m−2 d−1 and PAR in J m−2 d−1. Using this algorithm the rms accuracy of primary production estimate is 0.17 on a logarithmic scale, i.e., a factor of 1.5. Using spectral reflectance measurements in the entire visible spectral range, the central wavelength, spectral bandwidth, and radiometric noise level requirements are investigated for the channels to be used by an ocean color space mission dedicated to estimating global marine primary production and the associated carbon fluxes. Nearly all the useful information is provided by two channels centered at 440 nm and 550 nm, but the accuracy of primary production estimate appears weakly sensitive to spectral bandwidth, which, consequently, may be enlarged by several tens of nanometers. The sensitivity to radiometric noise, on the contrary, is strong, and a noise equivalent reflectance of 0.005 degraded the accuracy on the primary production estimate by a factor 2 (0.14–0.25 on a logarithmic scale). The results should be applicable to evaluating the primary production of oligotrophic and mesotrophic waters, which constitute most of the open ocean.
Structure and dynamics of fluid muds on the Amazon Continental Shelf
Authors: Trowbridge J. H. , Kineke G. C.
Published: February 2004

Thick layers of fluid mud occur in strong tidal flows over the inner portion of the Amazon continental shelf in regions of strong salinity fronts associated with the plume discharged from the Amazon River. Detailed shipboard profile measurements obtained in this region during A Multidisciplinary Amazon Shelf Sediment Study (AMASSEDS) provide an unprecedented opportunity to examine the structure and dynamics of fluid muds under natural conditions. The analysis focuses on flows in which the motion is fully turbulent and suspended sediment dominates the stratification. Under these conditions a comparison of measurements and one‐dimensional model calculations indicates that vertical transport is controlled by suppression of turbulent mixing at gradient Richardson numbers near 1/4. This constraint produces a distinctive vertical structure and leads to an upper bound on the total amount of suspended sediment that may be carried in a turbulent suspension by a tidal flow.
Seasonal heat balance in the Gulf of California
Authors: Castro Rubén , Lavín M. F. , Ripa P.
Published: February 2004

Most of the available hydrographic data (1939–1988) from the Gulf of California are used to describe the seasonal cycle of heat storage in the upper 400 m of water. The seasonal cycle of surface heat flux is obtained by using the sea surface temperature from the hydrographic data bank and meteorological data from coastal stations along the gulf. Monthly values of surface heat flux and heat storage are fitted to a mean value plus an annual harmonic. The longitudinal (i.e., along‐gulf) heat flux is then obtained by integrating the difference between the rate of change of heat storage and the surface heat flux. It is found that the surface heat flux has a positive (into the sea) annual mean all along the gulf; the average annual net surface heat flux for the whole gulf being ∼118 W m−2. The southern part of the gulf gains more heat (∼130 W m−2) than the northern part (∼100 W m−2), owing to the larger loss by evaporation in the latter (∼0.6 m/yr versus ∼1 m/yr) caused by lower humidity and stronger winds. These values are in agreement with previous estimates for the northern gulf. The amplitude of the seasonal signal shows a maximum of ∼230 W m−2 in the central part. Winter heat losses occur only from the middle of the gulf, the Guaymas Basin, to its head. There is a strong annual signal superimposed on the average longitudinal heat flux, whose amplitude is larger than the annual harmonic of the surface heat flux. Input of heat from the Pacific Ocean occurs from mid‐March to mid‐July, with a maximum at the mouth in May (21×1012 W). Attempts to explain this flux by diffusive processes were unsuccessful. The proposition that advection is the main longitudinal heat carrier is congruent with the generally accepted circulation patterns.
Determining the mean Gulf Stream and its recirculations through combining hydrographic and altimetric data
Authors: Qiu Bo
Published: February 2004

The altimetric data from the first 2.5‐year Geosat Exact Repeat Mission were used to estimate the mean sea surface height (SSH) field in the region of the Gulf Stream and its recirculation gyres. Assuming the instantaneous surface velocity field is composed of an eastward flowing jet and two westward recirculating flows, we used the time‐varying surface data from the altimeter to determine the shape of the along‐track mean SSH profiles and the historical hydrographic data to constrain the net SSH difference across the Gulf Stream system. The two‐dimensional mean SSH field was determined by objectively mapping the mean height profiles along the ascending and descending tracks. The SSH jump across the mean Gulf Stream has a maximum of 1.15m around 65°W and drops to an almost constant 0.9m downstream of the New England Seamount Chain (NESC). While the SSH jump associated with the mean northern recirculating flow is mostly uniform, we found that the Gulf Stream's southern recirculation has two local gyres that are separated by the NESC. An attempt was then made to estimate the mean deep circulation in this region by comparing the mean SSH field derived from the altimetry data and the surface dynamic height field based on the historical hydrographic data. Despite the large uncertainties, the mean deep flow pattern thus estimated agrees favorably with the overall circulation pattern from the long‐term current meter observations. Like the well‐defined northern recirculation gyre, we found that a continuous southward flow exists along 57.5°W, which follows closely along the deep layer potential vorticity contours. To the south of the Gulf Stream, the deep circulation consists of two separated recirculation gyres; the recirculation gyre to the east of the NESC appears to be trapped around the Corner Rise.