bluuebunny/crossref_metadata_2025 huggingface dataset:
The formation of Pliocene sapropels and carbonate cycles in the Mediterranean: Diagenesis, dilution, and productivity
Authors: Van Os B. J. H. , Lourens L. J. , Hilgen F. J. , De Lange G. J. , Beaufort L.
Published: February 2004

High‐resolution micropaleontological (planktonic foraminifera and calcareous nannofossils) and geochemical (stable isotopes, organic carbon, Fe, P, S, Ca, Ba, Mn, and Al) records are presented for the first sapropel‐containing carbonate cycle in the Pliocene of Sicily. The carbonate cycle is characterized by a gray to white to beige to white color layering typical of the marls of the Trubi formation. A faintly laminated sapropel is intercalated in the gray‐colored bed of the carbonate cycle. CaCO3 content varies from 40% in the beige to 45‐50% in the white layers. Lowest CaCO3 content of 25–30% is found in the gray layer and sapropel. Variations in carbonate and organic matter percentages can best be explained by changes in paleoproductivity rather than by variations in dilution and dissolution. Total productivity was highest during deposition of the gray layer and sapropel, as indicated by high organic carbon and Ba contents and high abundance of Globorotalia puncticulata. Carbonate production reached its highest values, however, during deposition of the white layers, as evidenced by enhanced abundances of planktonic foraminifera and nannofossils. The low carbonate content in the gray layer and sapropel is explained in terms of a collapse in carbonate production caused by extreme changes in the physical and biochemical properties of the water column, which in turn resulted in siliceous plankton and opportunistic foraminifers such as Globorotalia puncticulata outcompeting most calcareous organisms. The beige layer represents a low‐productivity environment similar to the present‐day eastern Mediterranean basin. Several mechanisms have previously been proposed to explain variations in productivity in the eastern Mediterranean. Both sapropels and gray layers were deposited at times when perihelion occurred in northern hemisphere summer. We envisage that the increase in seasonal contrast resulting from this orbital configuration enhanced winter mixing and stabilization of the water column during summer, both leading to favorable conditions for intensification of the spring bloom. In addition, a decrease in excess evaporation, as can be deduced from the δ18O record, led to shoaling of the pycnocline and reduced circulation, thus enhancing the availability of nutrients in the photic zone. Finally, enhanced precipitation and associated runoff should have caused an increase in river‐borne nutrients.
Orbital forcing of bottom‐current enhanced sedimentation on Feni Drift, NE Atlantic, during the mid‐Pleistocene
Authors: Robinson Simon G. , McCave I. Nicholas
Published: February 2004

Feni Ridge is a plastered sediment drift, deposited along the northwestern flank of Rockall Trough, NE Atlantic, under the influence of deep, geostrophic currents formed by intermittent overflows of Arctic Intermediate Water from the Norwegian Sea across the Iceland‐Scotland Ridge (NSOW). Lower Deep Water (LDW), a derivative of Antarctic Bottom Water from the South Atlantic mixed with old North Atlantic Deep Water (NADW), also bathes the lower flanks of the drift, below 2500 m. Deep Sea Drilling Project site 610 was drilled in a sediment wave field on the surface of Feni Drift (2417 m), though the sediments recovered proved to be typically pelagic, preserving a high‐resolution (>5 cm kyr−1) Quaternary record of orbitally modulated ice‐rafting and biogenic productivity cycles but showing little evidence of current affected deposition. From particle size and compositional analyses of the biogenic and lithogenic < 63 µm fractions of samples taken at ∼2.5 kyr intervals between 0.5 and 1.2 Ma, we derived indices of bottom current enrichment and flux of noncohesive (> 10 µm) silt, which we then compared with records of the percentage and flux of ice‐rafted detrital (IRD) sand, foraminifera, and coccoliths for the same interval. Cross‐spectral comparisons were then made between these records and variations in a model of northern hemisphere ice volume (ice sheet growth and decay in response to orbitally forced insolation changes at 65°N). Variations in the relative proportions and fluxes of IRD sand, foraminifera, and coccoliths all covary closely with orbitally forced ice volume fluctuations, but with differing amplitude, coherency and phase relationships at each Milankovitch frequency. However, there is a significant component of variance in pelagic and ice‐rafted sedimentation which is controlled by bottom currents, and this shows excellent phase and coherency agreement with the ice volume model at all orbital frequencies. The relative enrichment, coarseness, and flux of the contourite silt fraction of Feni Drift sediments all increase during glacial or cold stages of the mid‐Pleistocene, relative to warm stages, suggesting that currents were stronger during these periods. Possible explanations for such changes in current activity include: (1) an increased flux of NSOW via the Wyville‐Thompson Ridge into the Rockall Trough during glacials, but not due to increased production of NSOW; (2) LDW, presently lying below 2500 m, rises during glacials by at least 50–60 m, thereby flooding over the surface of Feni Ridge at site 610; (3) the path of a major recirculation gyre in the lower‐NADW/LDW system, presently situated marginally south of site 610, encroaches upon the southern tip of Feni Ridge during glacials; and (4) the Rockall Trough itself is a site of deep water formation during glacials (analogous to Labrador Sea Deep Water formation today).
Sedimentation processes within channel‐ridge systems, southeastern Weddell Sea, Antarctica
Authors: Weber M. E. , Bonani G. , Fütterer K. D.
Published: February 2004

On the continental margin of the southeastern Weddell Sea, Antarctica, several channel‐ridge systems can be traced on the eastern side of the Crary Fan. Swath mapping of the bathymetry reveals three southwest‐northeast trending ridges up to 300 m high with channels on their southeastern side. The structures occur on a terrace of the continental slope in water depths of 2000 – 3300 m. We carried out sedimentological studies on cores from three sites. Two of the studied cores are from ridges, one is from the northwestern part of the terrace. The stratigraphy of the recovered sediments is based on accelerator mass spectrometer 14C determinations, stable oxygen and carbon isotopes analyses and paleomagnetic measurements. The sediments represent a period from the last glacial maximum (LGM) to recent time. They are composed predominantly of terrigenous components. We distinguish four different sedimentary facies and assign them to processes controlling sedimentation. Microlaminated muds and cross‐stratified coarse‐silty sediments originated from contour currents. Bioturbated sediments reflect the increasing influence of hemipelagic sedimentation. Structureless sediments with high contents of ice‐rafted debris characterize slumps. The inferred contour currents shaping the continental slope during the LGM were canalized within the channels and supplied microlaminated mud to the western sedimentary ridges due to deflection to the left induced by the Coriolis force. The lamination of the sediments is attributed to seasonal variations of current velocities. The thermohaline bottom currents were directed to the northeast and hence opposite to the Weddell Gyre. Cross‐stratified coarse‐silty contourites on the ridges are intercalated with the muds and indicate spillover of faster thermohaline flows. Average sedimentation rates on the terrace of the continental slope were unusually high (250 cm/ka) during the LGM, indicating active growth phases of the Crary Fan during glacial intervals. A substantial environmental change at 19.5 – 20 ka is documented in the sediments by a gradual change from lamination to bioturbation. During the recent interglacial, bioturbated sediments were deposited in all parts of the terrace. Because of a reduction of the contour current velocities (4–7 cm/s), the water masses of the Weddell Gyre, supplying fine‐grained sediments from northeast, gain a greater influence on sedimentation on the continental slope. Higher percentages of microfossils indicate enhanced biogenic productivity. Increased iceberg activity is documented by greater amounts of ice‐rafted debris. The interglacial sedimentation rates decrease to a few cm/ka and indicate that the Crary Fan became relatively sediment‐starved during interglacial intervals.
Development of sediment drifts approaching an active plate margin under the SW Pacific Deep Western Boundary Current
Authors: Carter L. , McCave I. N.
Published: February 2004

The abyssal Pacific Ocean is fed by a 1000 km wide, deep western boundary current (DWBC) that flows northward along the continental margin, east of New Zealand. Between the passive margin of Chatham Rise and the subduction zone of Kermadec Trench, a distance of 1200 km, the DWBC has formed a suite of sediment drifts over a depth range of 2200–5700 m. Airgun and 3.5‐kHz profiles record a variety of drift types that reflect regional variations in bathymetry, sediment supply, and the tectonic/volcanic framework. On Chatham Rise the DWBC has deposited a sinuous, linear body along the south flank (3000 m), an extensive apronlike drift on the north flank (2200–4500 m), and a ridgelike drift about the rise base (4500–5200 m). The flow has also deposited a body of sediment over 400 km long within a moat at the base of the nearby Louisville Seamount Chain. Further downcurrent, the 250 km long Rekohu Drift (3600–4190 m) has developed northward to 39°S. South of this latitude, drifts comprise mainly reworked pelagic/hemipelagic material and sediment transported from distant southerly sources. In contrast, drifts north of 39°S have received a major injection of terrigenous sediment from Hikurangi Channel which runs 1400 km from New Zealand, eastward across the Hikurangi Plateau to disgorge on to the abyssal floor at the plateau edge. En route, turbidity current overspill from the channel has moved north under the influence of the shallow DWBC to contribute to a series of small ridge and patch drifts among the numerous seamounts on the plateau at 3500–4200 m. Off Hikurangi Channel mouth, a large fan has accumulated. The DWBC has extended the fan into a drift running over 250 km along the base of Hikurangi Plateau (5150–5770 m) toward Kermadec Trench. Here drift sediment becomes increasingly disrupted by mass wasting associated with the active subduction in this area. The seismic stratigraphy reveals the drifts to rest mainly on a widespread erosional surface that is interpreted to mark the inception of the DWBC in the region with the late Oligocene opening of the Australian‐Antarctic seaway. Drift construction commenced during the Miocene but was punctuated in the late Miocene by another period of erosion that coincided with increased bottom water production in Antarctica. Deposition resumed in Plio‐Pleistocene times when large quantities of sediment from the rapidly rising landmass of New Zealand were injected into the boundary current. The modern flow continues to affect drift deposition as manifest by an active boundary channel along the foot of Hikurangi Plateau and widespread scour zones and sediment wave fields.
The oceanic phosphorus cycle and continental weathering during the Neogene
Authors: Filippelli Gabriel M. , Delaney Margaret Lois
Published: February 2004

Little is known about the history of dissolved phosphorus (P) input to the ocean during the Cenozoic, an important factor in reconstructing global change because of the role of P in controlling net oceanic productivity and organic carbon burial. We present P accumulation rates from the eastern (Neogene) and western (Cenozoic) equatorial Pacific, a region chosen to reflect oceanic P input trends because of its importance in global models of new production and biogenic sedimentation. P accumulation rates range from 5 to 60 µmol P cm−2kyr−1and exhibit a positive correlation with mass sediment accumulation rates, calcium carbonate being the major sediment component. The influences of surface productivity patterns, site migration through time relative to equatorial upwelling, and water depth are observed in the P accumulation rate records. These site‐specific effects are relatively minor, however, compared to synchronous, significant trends in P accumulation rates in these equatorial Pacific sites. The most notable event occurred in the late Miocene, when mean P accumulation rates exhibited a strong peak at 5–6 Ma, to the highest mean value of the Neogene (37 µmol P cm−2kyr−1), followed by a sharp decrease to 1–2 Ma (14 µmol P cm−2kyr−1), after which values increased slightly to the present. These changes in P accumulation rates in the equatorial Pacific translate into substantial changes in P burial; the decrease from 5–6 to 1–2 Ma signified a 62% decrease in P burial in the equatorial Pacific, and the difference is equivalent to 14% of the estimated modern P input rate to the oceans. Some of the late Neogene change in the equatorial Pacific P accumulation rate record may have been due to redistribution of P burial to high‐latitude regions. However, on the basis of P mass balance considerations, redistribution alone cannot account for the bulk of the change in P accumulation and burial in the equatorial Pacific. The P accumulation rate record is markedly different from the unidirectional increases in continental weathering rates over the last 40 m.y. interpreted from the Sr isotope record, suggesting a decoupling of nutrient input fluxes from input fluxes of other dissolved constituents.
Oceanic front in the Greenhorn Sea (Late Middle through Late Cenomanian)
Authors: Fisher C. G. , Hay W. W. , Eicher D. L.
Published: February 2004

An abrupt lithofacies change between calcareous shale and noncalcareous shale occurs in strata deposited in the mid‐Cretaceous Greenhorn Seaway in the southeastern corner of Montana. The facies were correlated lithostratigraphically using bentonites and calcarenites. The lithocorrelations were then refined using ammonites, foraminifera, and calcareous nannofossils. Twenty‐five time slices were defined within the upper middle and lower upper Cenomanian strata. Biofacies analysis indicate that lithofacies changes record the boundary or oceanic front between two water masses with distinctly different paleoceanographic conditions. One water mass entered the seaway from the Arctic and the other from the Gulf of Mexico/Tethys. The microfauna and microflora permit interpretation of the environmental conditions in each water mass. At times when the front was near vertical, the two water masses were of the same density but of different temperatures and salinities.
Eastern Mediterranean Quaternary paleoclimates from pollen and isotope records of marine cores in the Nile Cone Area
Authors: Cheddadi Rachid , Rossignol‐Strick M.
Published: February 2004

Pollen spectra from three eastern Mediterranean cores have been used to document the paleoclimates of the Levantine Basin borderlands over the last 250 kyr to establish the relationship between this regional climate data set and the global climate as recorded by foraminiferal δ18O and to compare it with proximal land pollen records. Core MD 84 642 with eight sapropels covers the last two climatic cycles up to the early Holocene, MD 84 627 with four sapropels goes back to 125 kyr, and MD 84 629 with one sapropel covers the last 70 kyr. The sedimentation rate decreases from core 629, located at the shallowest depth beneath the Nile River plume, to cores 627 and 642. During the interglacials defined by a low 18O/16O ratio, the abundance of tree pollen is maximum and points to an optimum Mediterranean climate with greatest humidity, including some summer rainfall. During glacial maxima, with highest 18O/16O ratio, the pollen abundance is high for steppe and semidesert plants and low for trees, indicating a definitely more arid, more continental, and probably colder climate. The variations of pollen abundance occur in phase with those of the foraminifer δ18O record. This signifies that the regional climate of the Levantine Basin borderlands had the same temporal pattern as the global ice volume documented by the ice volume curve.
Sortable silt and fine sediment size/composition slicing: Parameters for palaeocurrent speed and palaeoceanography
Authors: McCave I. N. , Manighetti B. , Robinson S. G.
Published: February 2004

Fine sediment size (<63 µm) is best measured by a sedimentation technique which records the whole size distribution. Repeated size measurement with intermediate steps of removal of components by dissolution, allows inference of the size distribution of the removed component as well as the residue. In this way, the size of the biogenic and lithogenic (noncarbonate) fractions can be determined. Observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 µm. The dynamics of sediment erosion, deposition, and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10‐µm grain size, and noncohesive above that size. Thus silt coarser than 10 µm displays size sorting in response to hydrodynamic processes and its properties may be used to infer current speed. Silt that is finer than 10 µm behaves in the same way as clay (<2 µm). Useful parameters of the distribution are the 10–63 µm mean size and the percentage 10–63 µm in the fine fraction. We cannot use size distributions to distinguish the nature of the currents. Therefore, to infer water mass advection speeds (i.e., the mean kinetic energy of the flow, KM), regions of high eddy kinetic energy (KE) must be avoided. At the present, such abyssal regions lie under the high surface KE of major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic Circumpolar Current, and Brazil/Falkland currents in the Argentine Basin. This is probably a satisfactory guide for the Pleistocene. With regard to the carbonate subfraction of the size spectrum, size modes due to both coccoliths and foraminiferal fragments can be recognized and analyzed, with the boundary between them again at about 10 µm. The flux of less than 10 µm carbonate, at pelagic sites above the lysocline, is another candidate for a productivity indicator.
Late Pleistocene oxygen isotope records of biogenic silica from the Atlantic sector of the Southern Ocean
Authors: Shemesh A. , Burckle L. H. , Hays J. D.
Published: February 2004

We determined the isotopic composition of oxygen in marine diatoms in eight deep‐sea cores recovered from the Atlantic sector of the Southern Ocean. The analytical reproducibility and core‐to‐core consistency of the isotopic signal suggests that diatom δ18O can be used as a new paleocenographic tool to reconstruct past variations in surface water characteristics and to generate 18O ‐isotope‐based stratigraphy for the Southern Ocean. The data indicate that diatom δ18O reflects sea surface temperature and seawater isotopic composition and that diatoms retain their isotopic signal on timescales of a least 430 ka. The δ18O analyses of different diatom assemblages reveal that the isotopic signal is free of species effects and that the common Antarctic species have the same water‐opal fractionation. The transition from the last glacial maximum (LGM) to the Holocene is fully recorded in high sedimentation rate cores. An 18O enrichment during the LGM, a post‐LGM meltwater spike and an input of meltwater during the late Holocene are the main isotopic features observed in down core records. The origin of this meltwater was very likely melting icebergs and/or continental ice or by melting sea ice that had accumulated snow. The most pronounced meltwater effects are recorded in cores that are associated with the Weddel gyre. Our results provide the basis for extending isotope studies to oceanic regions devoid of carbonate; further, isotopic stratigraphies may be constructed for records and regions where they were previously not possible.
Ertel's potential vorticity theorem in physical oceanography
Authors: Müller Peter
Published: February 2004

Under certain circumstances the equations of motion of a fluid yield theorems that provide powerful aids for comprehension of the character and physics of a wide variety of motions. Perhaps the most powerful of such theorems, especially in geophysical fluid dynamics, are the vorticity theorems that specify how the angular velocity of fluid particles changes with time and position. For large‐scale water motions the relevant form of vorticity is what is called potential vorticity, which incorporates inhomogeneities of the constituent elements of seawater. Ertel's theorem specifies the dynamical evolution of potential vorticity. Most other vorticity theorems can be derived from it. For ideal one‐component fluids, potential vorticity is materially conserved, that is, it is conserved when a particle of fluid is followed as its location changes, which is a powerful constraint for analyzing fluid motion. Seawater is a two‐component fluid with its components being water and salt. Potential vorticity becomes materially conserved for ideal oceanic motions when the thermobaric coefficient (dependent upon the variations with pressure of the thermal expansion and haline contraction coefficients) is assumed to be zero as, for example, for incompressible seawater (incompressibility being a quite valid assumption for many oceanic motions). Approximate forms of potential vorticity are illustrated for standard oceanographic approximations, including a flat Earth with zero, constant, or varying rotation; a spherical Earth; shallow water; stratification; quasi‐geostrophy; and others. The concept of potential vorticity naturally defines forms of motion (“vortical” modes) which exhibit nonzero amounts of potential vorticity. The best known forms of “vortical” modes are planetary geostrophic motion (where Coriolis and pressure forces are in approximate balance over the globe), quasi‐geostrophic motion (where the forces deviate somewhat from the geostrophic balance), and two‐dimensional stratified turbulence. The absence of potential vorticity defines the inertia‐gravity mode of motion, that is, gravity waves dependent upon the ocean's stratification. An arbitrary flow can be separated into a part that carries linear motionally induced potential vorticity and a part that does not.
Comparison between calculated and observed F region electron density profiles at Jicamarca, Peru
Authors: Preble Amanda J. , Anderson David N. , Fejer Bela G. , Doherty Patricia H.
Published: February 2004

Electron density profiles and isodensity contours derived from Jicamarca incoherent scatter radar observations in Peru for October 1–2, 1970, are compared in detail with results from the Phillips Laboratory global theoretical ionospheric model. This model solves the ion continuity equation for O+ concentration through production, loss, and transport of ionization. The primary factor controlling the peak plasma density at Jicamarca is the vertical E×B drift, which drives the ionization upward during the day and downward at night. When we use the measured drift in the model, we achieve excellent results with the measured electron density profiles. We illustrate the sensitivity of the low‐latitude plasma density calculations to changes in the vertical E×B drift and changes in the neutral winds. We also compare the calculated profiles and peak parameters with an empirical model, the International Reference Ionosphere (IRI). We illustrate several limitations associated with the IRI that contribute to its limited capability at the magnetic equator.
New insights into Hertz's theory of electromagnetism
Authors: Tai Chen‐To , Bryant John H.
Published: February 2004

An attempt is made to recast a masterpiece in the theory of electromagnetism by the great scientist H. Hertz into modern notations and to supply the derivations of several important equations in the work. By invoking the radiation condition in his formulation, an amendment has been made to clarify a criticism addressed to that theory. Also, upon removing the constraint placed on the current distribution in the original theory we have demonstrated a complete union of Hertz's theory of electromagnetism with Maxwell's theory. The distinct and independent features of Hertz's theory are emphasized. The importance of this theoretical work by Hertz and its significance appear not to have been fully recognized.
The influence of random variations in the tropospheric refractive index on the focusing of an intense microwave beam from a phased array
Authors: Pan Wei‐Yan , Shen Hao‐Ming
Published: February 2004

A high‐power microwave phased array has to occupy a large area to avoid atmospheric breakdown at the height of 50 km. In traveling from each antenna element to the target, the phase delay of each ray will experience random variations. These are caused by the random variations in the refractive index of the troposphere. As a consequence, the focusing on the target will be degraded. This kind of influence is discussed.
ELF radio wave propagation in a locally nonuniform Earth‐ionosphere cavity
Authors: Nickolaenko Alexander P.
Published: February 2004

The vertical electric field component of the ELF radio wave propagating in the spherical Earth‐ionosphere cavity with a localized nonuniformity is studied. The analysis is performed in the framework of Born's approximation of the perturbation theory applied to the Stratton‐Chu integral equation. Both the vertical electric dipole and the horizontal grounded wire antenna are considered as the ELF field sources. It is shown that different effects can arise within the nonuniform waveguide depending on the disturbance position between or beyond the endpoint of the propagation path that are conditioned by the vectorial nature of the radio wave. Backscatter is the prevailing effect in the case of impedance or height disturbances. A localized nonuniformity can bring forth pattern rotation of the horizontal wire antenna and at the same time an amplitude modification of its lobes.
Fresnel filtering in HF ionospheric reflection channel
Authors: Gherm Vadim E. , Zernov Nikolay N.
Published: February 2004

Phase and amplitude fluctuations spectra of the HF point source field in the fluctuating reflection channel are investigated on the basis of Rytov's method generalized to the case of inhomogeneous ionosphere. Integral representations expressing the fluctuation spectra are transformed to a form that is more convenient for numerical estimations owing to the reduced number of integrals. The resulting expressions are evaluated numerically with controlled accuracy for the three‐dimensional power type model of dielectric permittivity fluctuations. The results of calculations show the oscillatory behavior of frequency spectra apparently having Fresnel diffraction origin. The phenomenon is investigated in detail, and results are presented.
Polarized microwave emission from water waves
Authors: Gasiewski A. J. , Kunkee D. B.
Published: February 2004

Partially polarimetric measurements of thermal emission from a striated water surface at 91.65 GHz illustrate the potential for remote sensing of water wave direction by passive microwave radiometry. The three Stokes parameter measurements were made using a precision polarimetric radiometer trained on a rotatable water wave tank at several elevation angles from near nadir to near grazing. The polarimetric measurements are well corroborated by calculations using a tilted‐facet geometrical optics model for the water surface emission and scattering. Multiple scattering of the incident background radiation is incorporated for observation angles approaching grazing. The downwelling background brightness is computed by using an atmospheric radiative transfer model. We show that azimuthal brightness variations in the third Stokes parameter are in phase quadrature with the first and second modified Stokes parameters. For observation angles near ∼60°–70° from nadir the first three parameters have particularly large azimuthal brightness variations and thus have significant potential for measuring ocean wave direction. Moreover, the azimuthal brightness variations caused by water waves are not negligible for many passive microwave atmospheric sounding and surface remote sensing purposes, even at nadir. A range of elevation angles resulting in minimal azimuthal variations is identified.
Polarimetric scattering and emission properties of targets with reflection symmetry
Authors: Yueh Simon H. , Kwok Ronald , Nghiem Son V.
Published: February 2004

This paper investigates the symmetry of polarimetric scattering and emission coefficients of media with reflection symmetry. A reflection operator is defined and is used to create the images of electromagnetic fields and sources. The image fields satisfy Maxwell's equations, meaning that Maxwell's equations are invariant under the described reflection operations. By applying the reflection operations to media with reflection symmetry, the symmetry properties of the Stokes parameters, characterizing the polarization state of thermal emissions, are shown to agree with existing experimental data. The first two Stokes parameters are symmetric with respect to the reflection plane, while the third and fourth Stokes parameters have odd symmetry. In active remote sensing, the symmetry properties of the polarimetric scattering matrix elements of deterministic targets and the polarimetric covariance matrix elements of random media or distributed targets are examined. For deterministic targets, the cross‐polarized responses are odd functions with respect to the symmetry direction, whereas the copolarized responses are even functions. For distributed targets or random media, it is found that the correlations of copolarized and cross‐polarized responses are antisymmetric with respect to the reflection plane, while the other covariance matrix elements are symmetric. Consequently, in the cases of backscatter, the copolarized and cross‐polarized components are completely uncorrelated when the incidence direction is on the symmetry plane. The derived symmetry properties of polarimetric backscattering coefficients agree with the predictions of a two‐scale surface scattering model and existing sea surface HH and VV backscatter data. Finally, the conditions for a general type of media, i.e., bianisotropic media, to be reflection symmetric are examined.
Sheath waves observed on OEDIPUS A
Authors: James H. G. , Balmain K. G. , Bantin C. C. , Hulbert G. W.
Published: February 2004

An important novel feature of the tethered sounding rocket experiment OEDIPUS A (Observations of Electric‐field Distributions in the Ionospheric Plasma—A Unique Strategy) was its direct excitation and detection of electromagnetic waves on conductors in space plasmas. We present quantitative evidence about sheath waves excited in the ionosphere by a high‐frequency transmitter on one end of the 1‐km tether and detected by a synchronized receiver on the other end. An important characteristic of sheath waves is their sequence of sharply defined passbands and stop bands in the frequency range 0.1–5 MHz. The lowest passband is between 0.1 MHz and the plasma frequency near 2 MHz, the bandwidth where existing theory predicts sheath waves. Resonance fringes in this band have been scaled to determine the phase and group refractive indices of sheath waves. These agree reasonably well with the theory, considering the approximations therein. Passbands and stop bands observed in the range between 2 and 5 MHz are not expected on the basis of the current theory. In this range, band limits have clear signatures of the interaction of the tether fields with electrostatic cyclotron waves. Finite wire moment method modeling of the payload shows that in the low‐frequency passband, RF coupling along the tether is increased by 20 dB over vacuum conditions. Similarly, isolation is greater than vacuum isolation in the stop bands. Because sheath waves at frequencies up to 2 MHz are guided efficiently along conductors in plasma, they are a significant design issue in the electromagnetic compatibility of avionics at frequencies up to HF on large metal space structures.
Characteristics of fading of HF signal and noise intensities on three paths between the United Kingdom and Turkey
Authors: Gibson A. J. , Yilmaz U. M. , Tulunay Y. , Erol C. B. , Özgüç A. , Ataç T.
Published: February 2004

Results from a recent measurement campaign in Turkey, pertinent to reliability and compatibility determination in HF service planning, are presented. Within‐an‐hour signal fading was found to be slightly more severe than, but day‐to‐day variability only half as large as the values (in decibels) adopted by the 1984 World Administrative Radio Conference on HF Broadcasting, or WARC‐HFBC(84). Within‐an‐hour variability of noise was negligible, and day‐to‐day variations of noise were only about one quarter of the amplitude (in decibels) given by CCIR report 322‐3 [International Radio Consultative Committee (CCIR), 1988a]. The day‐to‐day spatial correlation coefficients of both signal and noise at sites separated in range by about 400 km were found to be about 0.4. Correlation of signal with noise at each site was negligible.
Mode angles of arrival in the 55‐ to 3500‐km range
Authors: Black Q. R. , Wood J. F. , Sherrill W. M.
Published: February 2004

A direction‐finding ionosonde consisting of an interferometer using seven‐element, 50‐ and 150‐meter crossed baselines, has been developed to provide group path delay, azimuth, and elevation of each resolved mode over the sounding frequency range. Direction‐finding (DF) ionograms have been acquired automatically, using various noncooperative, compatible sounder transmissions. Diurnal sequences of DF ionograms were obtained for mid‐latitude paths at nominal ranges of 55‐, 1000‐, 2300‐ and 3500‐km. Data measured to characterize the DF properties of each path include group delay, azimuth, and elevation versus frequency, as well as an azimuth/elevation scatter plot showing the distribution of angular occupancy for the path. The results show modes arriving closest to the great circle bearing (i.e., with lowest azimuth error) arrive via the lowest elevation angles and with least group delay. Fractional degree bearing standard deviations were commonly observed for one‐hop E and one‐hop F low‐ray modes, decreasing with increasing range. Variable bearing standard deviations between 1° and 6° were observed for one‐hop F high‐ray and two‐hop F modes. Bearing standard deviation of 50° were seen at near vertical incidence (55‐km range).
Probing the ionosphere with an extremely narrow frequency separation
Authors: Lin K. H. , Yeh K. C. , Wong A. , Yang K. S.
Published: February 2004

An HF propagation link via ionospheric reflections has been established for several years between Platteville, Colorado and Urbana, Illinois at a separation distance of 1,401 km. Although the probe is wideband in nature, covering the frequency range from 5 to 15 MHz, the transmitter is capable of transmitting two sets of CW waves at a frequency separation as narrow as 2 Hz. The signals are coherently received. Since the receiver bandwidth is 50 Hz, a beat is observed if the frequency separation is less than 50 Hz. In this case the spectra so computed and plotted would show two clear and separated peaks. The phase information can be extracted which is used to investigate the propagation delay and Doppler shift. The variation of propagation delay and Doppler shift with time gives delay spread of about 17 μs and Doppler spread of about 0.1 Hz for this midlatitude path. These spreads are important characteristics for a fading channel.
Spectral analysis of gravity anomalies and the architecture of tectonic wedging, NE Venezuela and Trinidad
Authors: Russo R. M. , Speed R. C.
Published: February 2004

We have analyzed the spectral content of free air gravity anomalies in the Caribbean‐South American plate boundary zone in order to determine better the near‐surface (0–120 km) distribution of crustal and upper mantle elements which give rise to the unusual gravity field of this region. The plate boundary zone in northeastern Venezuela and Trinidad is the site of the world's sea level continental minimum of Bouguer gravity anomalies, yet the region is also one of mild topography (mean value 43 m, maximum 1200 m). We find the mean depths to interfaces of significant density contrast at a variety of depths for portions of the plate boundary zone. We interpret interfaces at 30–35 km and 32 km beneath the Guyana Shield and the Aves Ridge, respectively, to be the Moho. Other shallow interfaces (5–14 km) are most likely sediment cover‐basement contacts in the Maturin foreland basin and southern Grenada Basin. Deeper interfaces (54–63 km) we associate with loaded and downwarped continental and oceanic South American lithosphere. The deepest boundaries, at depths of 89–120 km, may be related to detached or detaching oceanic lithosphere overridden by continental South America. We use our results to test the tectonic wedging model of the plate boundary zone recently published by Russo and Speed (1992). We find that the tectonic wedging model adequately describes many of the structural boundaries inferable from our analysis of gravity anomalies but that the model must be modified to include a thinner Guyana Shield crust.
Structural styles, chronology rates of deformation, and time‐space relationships in the Umbria‐Marche thrust system (central Apennines, Italy)
Authors: Calamita Fernando , Cello Giuseppe , Deiana Giovanni , Paltrinieri Werter
Published: February 2004

Structural interpretation of geological and geophysical data available for the central Apennines (Italy) allowed us to draw, balance, and restore three geological profiles across the external zones of the Umbria‐Marche thrust system and to evaluate the timing and rates of deformation in a roughly 4 Ma time window (from late Messinian to late Pleistocene time). From this data set we established a general correlation between spacing L and thickness D of the thrust sheets (which was used as a depth correction factor for deriving local versus regional depths to the sole thrust) and a time‐space relationship between the closing time of activity of each thrust and the distance X from a reference point within the system.
Tectonic history of the Barberton greenstone belt, South Africa: 490 million years of Archean crustal evolution
Authors: de Ronde Cornel E. J. , de Wit Maarten J.
Published: February 2004

Rocks of the 3570–3080 Ma Barberton greenstone belt record a geological history of at least 490 million years (m.y.), and were affected by regional thermal perturbations for over 1500 m.y. Small tectonic wedges of 3570–3511 Ma schistose tonalite are the oldest rocks yet recognized, but little is known about their tectonothermal history. The earliest, well‐recognized tectonothermal events (3490–3450 Ma) represent mid‐ocean ridge‐like processes. These were followed by two periods of arc‐related and trench‐related processes, separated by ∼160 m.y.; the first (3445–3416 Ma) recording an intraoceanic suprasubduction‐like environment and the second, (3260–3225 Ma) intra‐arc and interarc‐like processes, culminating in arc amalgamation. Accretion‐like convergent processes dominated between ∼3230 Ma and 3080 Ma, overlapping with the assembly of the Kaapvaal craton. At 3080 Ma the Barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system. Soon afterward, transtension became distributed across the entire Kaapvaal craton, heralding the onset of formation of the Witwatersrand basin by ∼3070 Ma.
Evolution of the eastern Waddington thrust belt and its relation to the mid‐Cretaceous Coast Mountains arc, western British Columbia
Authors: Rusmore Margaret E. , Woodsworth G. J.
Published: February 2004

The eastern Waddington thrust belt cuts the eastern margin of the Coast Mountains mid‐Cretaceous magmatic arc. Thrust faults carried Triassic rocks of the Intermontane superterrane, Early Cretaceous volcanic and clastic rocks, and volcanic and plutonic rocks of the active arc to the northeast, outward from the core of the arc. Minimum shortening across the thrust belt is estimated as 50% or 40 km. Synkinematic through postkinematic metamorphism produced an inverted metamorphic gradient with the structurally higher magmatic arc as the likely heat source. Radiometric, thermochronologic, and structural data indicate that the thrust belt was active at 84 Ma and probably at 87 Ma and suggest that the peak of postkinematic metamorphism occurred about 82–84 Ma. The thrust belt was intruded by postkinematic plutons in latest Cretaceous and early Tertiary time (68 Ma and 58 Ma). The eastern Waddington thrust belt is coeval with or slightly younger than a system of west directed thrusts in the western and southern parts of the Coast Mountains arc. The prominence of these structures suggests that synmagmatic contraction played a major role in development of the arc. The distribution and character of syn‐and postthrusting metamorphism indicate that this contraction, rather than localized loading by magma, produced the metamorphism.
Mechanics of back arc deformation in Costa Rica: Evidence from an aftershock study of the April 22, 1991, Valle de la Estrella, Costa Rica, earthquake (Mw=7.7)
Authors: Protti Marino , Schwartz Susan Y.
Published: February 2004

The April 22, 1991, Mw=7.7 Valle de la Estrella, Costa Rica, earthquake represents back arc thrusting of the Caribbean plate beneath the Panama block along the North Panama Thrust Belt. Large back arc thrusting events are quite rare, occurring in only two other locations along the Sunda Arc and Japan Sea. To better understand the mechanics of back arc thrusting, we constrain the faulting geometry associated with the 1991 Costa Rica earthquake using aftershock locations and focal mechanisms obtained from a three‐component portable digital network deployed in and around the aftershock area following the mainshock. The spatial distribution of aftershocks reveals a complicated faulting geometry in the rupture area. Focal mechanisms determined from inversion of P wave and tangentially and radially polarized S wave (SH and SV, respectively) amplitudes recorded by this temporary network confirm fault complexity and indicate active thrust, normal and strike‐slip faults in the back arc of Costa Rica. Most of the thrust events are confined to the southern portion of the aftershock zone in the vicinity of the mainshock. Their distribution suggests the existence of a near‐horizontal basal fault plane at a depth of about 15 km, with many imbricate faults having steeper dips extending from the basal plane toward the surface. Events with strike‐slip mechanisms locate northwest of the thrust events and define a SW–NE trending, left‐lateral strike‐slip fault zone that represents the NW termination of the mainshock rupture and possibly the maximum NW extension of the Panama block. The superposition of the aftershock locations on a geologic map of the region shows that aftershocks are restricted to occur in the older, more competent rock units (volcanic and volcaniclastic rocks interbedded with carbonates) of the back arc sedimentary basin. Shallow events (depth<5 km) occur only where these oldest units are exposed at the surface. This suggests that (1) exposure of the lower units results from repeated earthquake slip on the shallow crustal faults imaged by the aftershocks and (2) as much as 7 km of basin fill material, overlying the lower units, northeast of the mainshock, does not deform seismically but, instead, folds and possibly faults aseismically.
The Puglia uplift (SE Italy): An anomaly in the foreland of the Apenninic subduction due to buckling of a thick continental lithosphere
Authors: Doglioni Carlo , Mongelli Francesco , Pieri Piero
Published: February 2004

The Apenninic foreland shows two distinct structural signatures comparing the central Adriatic Sea and the Puglia region. During the Pliocene‐Pleistocene the central Adriatic underwent high subsidence rates due to the eastward rollback of the hinge of the west dipping Apenninic subduction. The Puglia region and the Bradanic foredeep are located southward along strike in the same foreland, but, in contrast with the central Adriatic, after Pliocene‐early Pleistocene subsidence they underwent uplift since the middle Pleistocene. The geometry and the kinematics of the frontal accretionary wedge and related foreland changed from that moment on between the two areas. At the front of the central northern Apennines, off scraping and subsidence continued, whereas the foredeep and foreland of the southern Apennines were buckled. Those differences are interpreted as being due to the larger subduction hinge rollback rate since middle Pleistocene of the central Adriatic lithosphere (70 km thick) with respect to the thicker Puglia (110 km). The different thicknesses of the continental crust and lithosphere were inherited from the Mesozoic rifting that disrupted the Adriatic plate. The different thicknesses appear to have controlled the variable degree of flexure of the lithosphere and its asthenospheric penetration rate. The Tremiti E–W alignment is the right‐lateral lithospheric transfer zone of those different tectonic regimes. The consequent different dip of the subduction in the two sections (steeper west of Puglia) could also explain the lower elevation of the southern Apennines, compared to their central‐northern sector.
Kinematic analysis of the Taiwan Slate Belt
Authors: Tillman Kennen S. , Byrne Timothy B.
Published: February 2004

The Taiwan Slate Belt can be subdivided into two structural domains based on fold and cleavage geometries and finite and incremental strain histories. The two belts correspond generally to two topographic ranges in Taiwan: the Hsüehshan Range and the Backbone Range. Rocks in the Hsuehshan Range are characterized by upright, symmetric folds and a steeply SE dipping, axial planar pressure solution cleavage. Synkinematic fibers in pressure shadows are typically straight, parallel to cleavage, and plunge downdip, indicating that the rocks experienced a coaxial strain history. In contrast, rocks in the Backbone Range are characterized by inclined, asymmetric folds that verge toward the foreland and display a moderately SE dipping, axial planar pressure solution cleavage. Synkinematic fibers are straight and parallel to the downdip stretching lineation in XY sections. In XZ sections, however, the fibers typically display a clockwise curvature (viewed to the NE), indicating a noncoaxial strain history. We propose that the different strain histories are due, at least in part, to differences in the original tectonostratigraphic settings of the strata in the two ranges. Strata in the Hsüehshan Range are interpreted to have been deposited in a half‐graben associated with an east dipping normal fault. The strata of the Backbone Range, in contrast, are considered to have been deposited on the unfaulted Chinese continental margin. We suggest that the basement horst on the east side of the half‐graben functioned as a steep‐faced, rigid backstop that concentrated pure shear deformation throughout most of the Hsüehshan Range; shear strains were apparently limited to zones near the base of the sedimentary sequence and adjacent to the backstop. The noncoaxial strains characteristic of the Backbone Range are interpreted to be the result of a regional‐scale shear zone that formed as Chinese basement and its sedimentary cover were underthrust southeastward beneath the Luzon Arc.