bluuebunny/crossref_metadata_2025 huggingface dataset:
Reconciling down core anticorrelation of diatom carbon and nitrogen isotopic ratios from the Southern Ocean
Authors: Crosta X. , Shemesh A.
Published: October 2002

Oceanic nutrient cycling in the Southern Ocean is supposed to have an important impact on glacial‐interglacial atmospheric CO2 changes and global climate. In order to characterize such nutrient cycling over the last two climatic cycles we investigated carbon and nitrogen isotopic ratios of diatom‐bound organic matter (δ13Cdiat and δ15Ndiat, respectively) in two cores retrieved form the Atlantic and Indian sectors of the Antarctic Ocean. The two cores show the same isotopic patterns. The δ13Cdiat values are depleted during glacial periods and enriched during interglacial periods, indicating lower productivity during cold times. The δ15Ndiat values are enriched during glacial periods and depleted during interglacial periods, arguing for greater nitrate utilization during cold times. Taken at face value, this apparent contradiction leads to opposite conclusions on the role of the Southern Ocean biological pump on the atmospheric CO2 changes. However, the two sets of data can be reconciled by a “sea ice plus mixing rate scenario” that calls upon a balance between the effect of cutting off gas transfer at the ocean‐atmosphere boundary and the effect of reducing vertical transport of nutrients through the pycnocline.
Sensitivity of the thermohaline circulation to tropical and high latitude freshwater forcing during the last glacial‐interglacial cycle
Authors: Schmittner Andreas , Clement Amy C.
Published: October 2002

Recent studies indicate that the tropical freshwater budget of the Atlantic is modulated by changes in El Niño‐Southern Oscillation (ENSO). If anomalies of the Atlantic freshwater balance persist on the order of decades, these might have a large influence on the North Atlantic thermohaline circulation (THC). Here we present a sensitivity study in which we use a model scenario for ENSO behavior during the last 120 kyr to force a simplified model of the THC with corresponding freshwater exchange between the tropical Atlantic and Pacific. If the steady state strength of the North Atlantic THC is similar to the present‐day, its response to the forcing is of the order of 1–3 Sv (1 Sv = 106 m3 s−1). No mode changes of the THC are simulated for reasonable values of the coupling constant between freshwater exchange and ENSO. If, on the other hand, the steady state overturning is significantly weaker, a collapse of the THC occurs due to forcing from the tropics. The modeled THC variation due to tropical freshwater perturbations are compared to those resulting from middle‐ and high‐latitude freshwater forcing due to long‐term growth and decay of Northern Hemisphere ice sheets. It is found that both forcings are of similar amplitude except during the deglaciation when high latitude forcing dominates. A possible out‐of‐phase relationship between deep water formation in the North Pacific and North Atlantic is also explored.
Antarctic Holocene climate change: A benthic foraminiferal stable isotope record from Palmer Deep
Authors: Shevenell Amelia E. , Kennett James P.
Published: October 2002

The first moderate‐ to high‐resolution Holocene marine stable isotope record from the nearshore Antarctic continental shelf (Ocean Drilling Program (ODP) Hole 1098B) suggests sensitivity of the western Antarctic Peninsula hydrography to westerly wind strength and El Niño‐Southern Oscillation (ENSO)‐like climate variability. Despite proximity to corrosive Antarctic water masses, sufficient CaCO3 in Palmer Deep sediments exists to provide a high‐quality stable isotopic record (especially in the late Holocene). Coherence of benthic foraminifer δ18O, δ13C, sedimentologic, and CaCO3 fluctuations suggests that rapid (<20 years) Palmer Deep bottom water temperature fluctuations of 1°–1.5°C are associated with competitive interactions between two dominant oceanographic/climatic states. An abrupt shift from a warmer, stable Upper Circumpolar Deep Water (UCDW) state to a cooler, variable shelf water state occurred at ∼3.6 ka. Palmer Deep bottom waters oscillated between UCDW and shelf water‐dominated states between ∼3.6 and 0.05 ka. Cool shelf water intervals correlate with Neoglacial events, the most recent and largest being the Little Ice Age (LIA; ∼0.7–0.2 ka). Similarities between Palmer Deep and global Holocene records and the rapidity of inferred bottom water fluctuations suggest that western Antarctic Peninsula shelf hydrography has not been controlled by thermohaline reorganizations but by variable strength and/or position of the Southern Hemisphere westerly wind field. We suggest that these atmospheric perturbations may have originated in the low‐latitude tropical Pacific.
Precessionally forced productivity variations across the equatorial Pacific
Authors: Perks Helen M. , Charles Christopher D. , Keeling Ralph F.
Published: October 2002

Measurements of combustion oxygen demand (COD) in two sediment cores provide a record of paleoproductivity driven by surface‐ocean dynamics in the equatorial eastern and western Pacific for the past 400,000 years. The COD time series are well correlated with each other over this time span and show pronounced precessionally forced peaks of higher productivity during globally colder periods. The phase of this signal in the two cores is identical, to within chronological uncertainties, suggesting a common insolation forcing mechanism for the upper ocean across the equatorial Pacific. COD is also in phase with the precessionally forced component of global ice volume, as indicated by oxygen isotopes, and with atmospheric methane in the Vostok ice core. These relationships imply that the COD relative paleoproductivity index provides an important diagnostic measure of the mechanisms of tropical ocean dynamics and climate change.
North Atlantic Deep Water export to the Southern Ocean over the past 14 Myr: Evidence from Nd and Pb isotopes in ferromanganese crusts
Authors: Frank Martin , Whiteley Nicholas , Kasten Sabine , Hein James R. , O'Nions Keith
Published: October 2002

The intensity of North Atlantic Deep Water (NADW) production has been one of the most important parameters controlling the global thermohaline ocean circulation system and climate. Here we present a new approach to reconstruct the overall strength of NADW export from the North Atlantic to the Southern Ocean over the past 14 Myr applying the deep water Nd and Pb isotope composition as recorded by ferromanganese crusts and nodules. We present the first long‐term Nd and Pb isotope time series for deep Southern Ocean water masses, which are compared with previously published time series for NADW from the NW Atlantic Ocean. These data suggest a continuous and strong export of NADW, or a precursor of it, into the Southern Ocean between 14 and 3 Ma. An increasing difference in Nd and Pb isotope compositions between the NW Atlantic and the Southern Ocean over the past 3 Myr gives evidence for a progressive overall reduction of NADW export since the onset of Northern Hemisphere glaciation (NHG). The Nd isotope data allow us to assess at least semiquantitatively that the amount of this reduction has been in the range between 14 and 37% depending on location.
RADIOGENIC ISOTOPES: TRACERS OF PAST OCEAN CIRCULATION AND EROSIONAL INPUT
Authors: Frank Martin
Published: October 2002

The radiogenic isotope composition of dissolved trace metals in the ocean represents a set of relatively new and not yet fully exploited tracers with a large potential for oceanographic and paleoceanographic research on timescales from the present back to at least 60 Ma. The main topic of this review are those trace metals with oceanic residence times on the order of or shorter than the global mixing time of the ocean (Nd, Pb, Hf, and, in addition, Be). Their isotopic composition in the ocean has varied as a function of changes in paleocirculation, source provenances, style and intensity of weathering on the continents, as well as orogenic processes. The relative importance of these processes for each trace metal is evaluated, which is a prerequisite for reliable interpretation of their time series in terms of changes in paleocirculation or weathering inputs. This analysis of processes includes a discussion of the long‐term isotopic evolution of Sr and Os, which are well mixed in the ocean and have thus not been influenced by circulation changes. The radiogenic isotope evolution of those trace metals with intermediate oceanic residence times can be used as paleoceanographic proxies to reconstruct paleocirculation and weathering inputs into the ocean. This is demonstrated by studies from different ocean basins, mainly carried out on ferromanganese crusts, which show that radiogenic trace metal isotopes provide important new insights and can complement results obtained by other well‐established paleoceanographic tracers such as carbon isotopes.
TIDAL‐TECTONIC PROCESSES AND THEIR IMPLICATIONS FOR THE CHARACTER OF EUROPA'S ICY CRUST
Authors: Greenberg Richard , Geissler Paul , Hoppa Gregory , Tufts B. R.
Published: October 2002

Jupiter's satellite Europa is covered by an icy crust, likely overlying a deep, global liquid water ocean. The mutually dependent relationship between the orbit and tidal processes controls Europa's rotation, heating, and stress. Observations of the surface and investigation of tidal‐tectonic processes indicate that the dominant types of surface terrain have been created by frequent repeated exposure of an underlying ocean to the surface. Surface lineaments, including ubiquitous cycloidal (chains of arcs) features, are correlated with tidal stress patterns, demonstrating that they form by crustal cracking but only if a substantial ocean is present to give adequate tidal amplitude. Stratigraphy of tectonic features, combined with models of how and where they formed, demonstrates a moderate rate of nonsynchronous rotation. Tidal driving of strike‐slip displacement (in which surface areas shear passed one another) by daily (85 hours on Europa) tidal stresses suggests that cracks penetrate to a liquid layer. The characteristic ridge sets that cover tectonic terrain may be built by tidal pumping of fluid and slush to the surface on a daily basis. Widespread tectonic dilation creates new surface as material rises from below. In addition to those tectonic terrains, nearly half of the surface is chaotic terrain, with morphology and other characteristics indicative of melt‐through from below, consistent with the tectonic indications of thin ice and plausible tidal heating rates. Formation of both chaotic and tectonic terrains has continually resurfaced the satellite, while connecting the ocean to the surface. Surface colorants correlate with locations where ocean water reached the surface, such as along large‐scale ridge systems and around chaotic terrain. As a result of tides, liquid water may have bathed crustal cracks and surfaces with heat, transporting and mixing substances vertically. Such daily transported materials would include substances from the oceanic reservoir, oxidants, and fuels created at the surface, as well as any organisms and their chemical products. Thus a variety of habitable environments likely exist in the crust. Deactivation of individual cracks after thousands of years (due to the tidally driven nonsynchronous rotation) constrains processes like ridge building and strike‐slip displacement and would require any living organisms to adapt to such change.
Global Ionospheric Propagation Model (GIM): A propagation model for scintillations of transmitted signals
Authors: Béniguel Y.
Published: October 2002

The characteristics of random ionospheric inhomogeneities are described, including their dependency on the geophysical parameters. They are used in a propagation model to estimate the propagation errors and the fades of the transmitted signals. Margins to be included in a budget link are subsequently determined. Features of our propagation model, Global Ionospheric Propagation Model (GIM), are presented with typical results for VHF and L band links.
Comparison of wind estimation errors for the spaced antenna technique: A case study for the MU radar
Authors: Kawano Noriyuki , Tahara Yukihiro , Fukao Shoichiro
Published: October 2002

In this paper we estimate the random errors in the spaced antenna (SA) winds by two techniques and compare them with measurements made by the Doppler technique. With a one spatial dimension computer simulation, we apply typical parameters for the middle and upper atmosphere (MU) radar without noise. Briggs [1984] full correlation analysis (FCA) is commonly applied to the SA technique, while Doviak et al. [1996] and Holloway et al. [1997] used another simple analytic form based on scattering theory to derive generalized expressions. This simple analytic form should have equivalent results to the Briggs FCA for the horizontal wind estimation. With our simulation results, this form generally shows similar results to the FCA, and sometimes shows better performance than the FCA although it needs simpler calculation than the FCA. The estimation error of this simple analytic form is better than that of the Doppler for large wind, and better than that of the FCA for small antenna spacing. MU radar observations are compared with these two analyses for the SA technique, and the simple analytic form also shows similar performance to the FCA.
First spaceborne observation of an Earth‐reflected GPS signal
Authors: Lowe Stephen T. , LaBrecque John L. , Zuffada Cinzia , Romans Larry J. , Young Larry E. , Hajj George A.
Published: October 2002

We present the first spaceborne observation of a Global Positioning System (GPS) signal reflected from the Earth's surface, specifically from the Pacific Ocean. This result is scaled to obtain the expected voltage signal‐to‐noise ratio (SNR) and altimetric accuracy for a generic GPS reflections altimetry mission and the current SAC‐C and CHAMP missions. Cross‐correlating a three‐parameter phase model with both a 1‐s and 4‐s segment of spaceborne imaging radar‐C (SIR‐C) calibration data, recorded before and after a Galapagos Islands imaging pass, results in beam‐limited signals having voltage SNRs of 10 and 334, respectively. Evidence for these results being reflected GPS signals includes: (1) The signals' temporal shapes agree closely with that predicted using a detailed scattering model, at two different observation geometries, and differ significantly from the expected direct signal shapes. (2) The signal in the 4‐s data has a measured coherence time of 1.0 ms, which agrees closely with that expected for a reflected signal and is completely inconsistent with the direct signal's coherence properties. (3) The 1‐ and 4‐s signals' voltage SNR is maximized by shifting the model frequency −2740 Hz and 497 Hz, respectively from that expected from their respective specular reflection points, or −2875 Hz and 690 Hz from the expected direct signal frequencies. These values agree with the −2900 Hz and 510 Hz Doppler frequency shifts expected from those points on the surface corresponding to the antenna's pointing direction, thus illustrating beam‐steering effects on the surface. (4) Plausible hypotheses for the detected waveform being a corrupted direct signal, including second‐order mismodeling effects, shuttle multipath effects, or a band‐pass cutoff of the GPS spread spectrum, are shown to be inconsistent with the data. Space‐based observations of reflected GPS signals, like the ones presented here, may enable a new class of ocean topography measurements unavailable from traditional altimeters, such as TOPEX/Poseidon, and perhaps surface wind vector measurements. Making such observations with sufficient SNR will require unusually large, high‐gain antennas. The measurement presented here is scaled to assess the expected SNR for those applications. Because this result lies in a nonlinear scaling regime, the correct scaling equations are presented, and the expected signal strength from a generic GPS reflections altimetry mission is derived to illustrate the most important contributions to the signal SNR. An SNR estimate is also derived for the SAC‐C and CHAMP missions, which are expected to make GPS reflection measurements in the near future. Finally, a qualitative wind speed determination is extracted from the observed signal.
Measurement and modeling of auroral absorption of HF radio waves using a single receiver
Authors: Greenberg E. M. , LaBelle J.
Published: October 2002

We develop a new method of measuring auroral absorption using only a single passive HF receiver detecting a large number of existing HF transmissions. Although the locations, strengths, and timetables of these sources are not known, their large number allows them to be treated statistically. Modeling the distribution of the sources and using the Foppiano and Bradley [1983] model for the propagation of the waves leads to a simulated pattern of received signals at the receiver as a function of time of day which agrees well with the observed pattern.
Evaluation of mesospheric VHF echoes observed with the middle and upper atmosphere radar
Authors: Kubo Koji , Fukao Shoichiro , Sugiyama Takuya
Published: October 2002

Mesospheric radar echoes in midlatitudes, which are generated by refractive index irregularities at half the radar wavelength, are considered to be caused by electron density irregularities induced by neutral turbulence. In this paper we construct a model to evaluate intensities of VHF mesospheric echoes due to electron irregularities with the help of an empirical model profile of mean electron densities and temperatures. We simulate the electron density deviations in a parcel displaced by neutral turbulence, the intensity of which is evaluated from the Doppler spectral width measured with the mesosphere‐stratosphere‐troposphere radar. Our results show that the estimated reflectivity increases up to the height range of 70–80 km mainly because of the increase of the electron density gradients and decreases above 80 km because of the suppression of turbulence at half the radar wavelength by increasing molecular viscosity. Here 70–80 km is the critical height for the mesospheric echoes where the inner scale of inertial turbulence is equal to half the radar wavelength. Our model reproduces well the characteristics of observed echo intensities. Particularly mesospheric echoes are most frequently found around and just below the critical height, which is much higher than those estimated in the earlier studies.
Seasonal variation of solar tides in the troposphere and lower stratosphere over Gadanki: Comparisons with the global scale wave model
Authors: Dutta Gopa , Bapiraju B. , Balasubrahmanyam P. , Aleem Siddiqui M. A. , Aleem Basha H.
Published: October 2002

Diurnal cycle hourly horizontal wind measurements by VHF radar in the altitude range of 3–20 km made at a low‐latitude station, Gadanki (13.5°N, 79.2°E), are used to investigate seasonal variation of tidal parameters. The observations are compared with the recent updated version of the global scale wave model (GSWM‐2000). Measured amplitudes are found to be much larger than model values, the discrepancy being larger in convective seasons, particularly for zonal winds. Observations suggest superposition of nonmigrating modes on normal migrating tides. The comparison between the observed and model is good in phase behavior, especially for meridional winds. The terdiurnal component is found to have significant amplitude. Energy densities of tidal components become maximum in the upper troposphere and get reduced in the lower stratosphere. However, the seasonal variations deserve further investigation with longer data sets and with a stress on interannual variability.
Radioholographic analysis of radio occultation data in multipath zones
Authors: Gorbunov Mikhail E.
Published: October 2002

Numerical simulations of radio occultation experiments indicate that the effect of multipath propagation and diffraction must play a very significant role in the lower troposphere. Two methods of processing radio occultation data in the multipath zone were previously discussed: the radioholographic (radio‐optics) method and the back propagation method. Both methods, however, have their restriction. The radio‐optics method cannot accurately take into account the effects of diffraction in subcaustic zones. The back propagation method does not work for complicated caustic structures. In this paper we describe a combined method. The radio‐optics analysis is applied both for the radio occultation data measured along the low Earth orbiter orbit and for the back propagated data. The computed spatial spectra are then averaged. This allows one to get rid of diffraction effects in subcaustic zones to a significant extent. The method is validated in numerical simulations on the material of global fields of atmospheric variables from analyses of the European Centre for Medium‐Range Weather Forecasts. An example of processing a real GPS/MET occultation is given.
Ray tracing in curved confined spaces: A performance evaluation based on measurements
Authors: Didascalou Dirk , Schäfer Thomas M. , Wiesbeck Werner
Published: October 2002

This work is concerned with the comparison of measurements and predictions of high‐frequency electromagnetic wave propagation in curved confined spaces. For this purpose, measurements have been performed at 120 GHz (D‐band) in curved prefabricated stoneware tubes. A ray‐tracing tool based on geometrical optics and ray density normalization is used for the simulations. The comparisons reveal that an appropriate ray‐tracing approach is able to precisely predict wave propagation in curved geometry. It is shown that the actual shape of the boundaries has a major impact on the propagation characteristics compared to, for example, the materials parameters. Furthermore, the effect of the focusing of energy was observed in both measurements and simulations. In this case, the ray density normalization allows us to overcome one of the major disadvantages of conventional geometrical optics: its failure at caustics.
Performance of the generalized minimum residual (GMRES) iterative solution for the magnetic field integral equation
Authors: Makarov Sergey , Vedantham Ramanuja
Published: October 2002

The paper discusses a generalized minimum residual (GMRES) iterative solution of the magnetic field integral equation (MFIE) applied to frequency domain scattering problems at medium and high frequencies. First, the performance of the original MFIE is studied, for the perfectly electrically conducting (PEC) sphere. It is shown that the residual error and the solution error do not correlate with each other. Whereas the solution error has already reached a limiting value or even increases, the residual error continues to decrease very fast, typically exponentially. Second, the MFIE is combined with the normal projection of the primary integral equation for the surface magnetic field. Such a technique does not increase the computational complexity of the MFIE. At the same time, it gives a termination criterion for GMRES iterations since the residual error of the combined equation has a typical saturation behavior. In the saturation zone, the residual error and the solution error have approximately the same small value (a typical relative RMS error for the sphere is 1%). A very similar saturation behavior of the residual error has been observed for other tested PEC scatterers including a cube, a cylinder, and a sphere with one segment cut off (the so‐called cat eye) at different frequencies.
On the use of an effective ionospheric height in electron content measurement by GPS reception
Authors: Birch M. J. , Hargreaves J. K. , Bailey G. J.
Published: October 2002

An experimental method, using simultaneous vertical and slant observations, has been derived for estimating the effective shell height for electron content measurement by GPS reception. At a latitude of approximately 53° north, Lancashire is well placed as an observing site since GPS satellite orbits are inclined at 55° to the Earth's equatorial plane and, as a result, many GPS tracks pass almost directly overhead, giving true zenithal measurements. This paper focuses on the question of oblique‐to‐zenithal correction and related matters. In particular, plasmaspheric effective height and satellite bias corrections are determined by measuring the total electron content (TEC) from pairs of satellites to a single ground station, each pair giving simultaneous observations of oblique and zenithal TEC. Additional bias corrections are determined by extracting the TEC using pairs of satellites with the same elevation at the same time. The Chapman Production Function Model and the Sheffield University Plasmasphere and Ionosphere Model are both used to determine a theoretical value for the plasmaspheric effective height. The results indicate that the plasmaspheric effective height used in the oblique‐to‐zenithal thin shell conversion is considerably greater than the commonly adopted value of 350 km. It is suggested, on the basis of all the available evidence, that a value between 600 and 1200 km is preferred. Assuming a lower value could produce an error of 15 to 30% or more in the electron content.
Characterization of Leonid meteor head echo data collected using the VHF‐UHF Advanced Research Projects Agency Long‐Range Tracking and Instrumentation Radar (ALTAIR)
Authors: Close S. , Hunt S. M. , McKeen F. M. , Minardi M. J.
Published: October 2002

The Leonid meteor shower, which was predicted to hit storm‐like activity on 17 November 1998, was observed using radar and optical sensors at the Kwajalein Missile Range in order to study potential threats to orbiting spacecraft. Meteor head echo data were collected during the predicted peak of the “storm” primarily using the Advanced Research Projects Agency Long‐Range Tracking and Instrumentation Radar (ALTAIR). ALTAIR is a dual‐frequency radar at VHF (160 MHz) and UHF (422 MHz) that is uniquely suited for detecting meteor head echoes due to high sensitivity, precise calibration, and the ability to record radar data at a high rate (Gb/min). ALTAIR transmits right‐circular (RC) polarized energy and records left‐circular (LC) sum, RC sum, LC azimuth angle difference, and LC elevation angle difference channels; these four measurements facilitate the determination of three‐dimensional target position and velocity as a function of radar cross section and time. During the predicted peak of the storm, ALTAIR detected 734 VHF head echoes in 29 min of data and 472 UHF head echoes in 17 min of data, as well as numerous specular and nonspecular ionization trails. This paper contains analysis on the head echo data, including dual‐frequency statistics and the variability of head echo decelerations. We also include results from the analysis of the radius‐density parameter, which shows a strong correlation with deceleration.
Simultaneous ambient air motion and raindrop size distributions retrieved from UHF vertical incident profiler observations
Authors: Williams Christopher R.
Published: October 2002

The raindrop size distribution is a fundamental quantity used to describe the characteristics of rain. Vertically pointing Doppler radar profilers are well suited to retrieve the raindrop size distributions because of their operating frequency and data collection methodology. Doppler radar profilers operating at UHF are sensitive to both Bragg scattering from the radio refractive index of turbulence and Rayleigh scattering from distributed targets. During light precipitation, both scattering processes are resolved in the Doppler velocity spectra. During moderate to heavy precipitation the ambient air motion is not resolved in the Doppler velocity spectra. The sans air motion (SAM) model is introduced in this study and uses only the Rayleigh scattering portion of the Doppler velocity spectrum to estimate the ambient vertical air motion, the spectral broadening, and the raindrop size distribution. The SAM model was applied to 915 MHz profiler observations in central Florida. There was good agreement between the SAM‐model‐retrieved rain rate and mass‐weighted mean diameter at an altitude of 300 m with simultaneous surface disdrometer observations. The SAM model was applied to the profile of Doppler velocity spectra to yield estimates of rain rate, mass weighted mean diameter, and ambient vertical air motion from 300 m to just under the melting level at 4 km.
Two‐phase westward encroachment of Basin and Range extension into the northern Sierra Nevada
Authors: Surpless Benjamin E. , Stockli Daniel F. , Dumitru Trevor A. , Miller Elizabeth L.
Published: October 2002

Structural, geophysical, and thermochronological data from the transition zone between the Sierra Nevada and the Basin and Range province at latitude ∼39°N suggest ∼100 km westward encroachment of Basin and Range extensional deformation since the middle Miocene. Extension, accommodated primarily by east dipping normal faults that bound west tilted, range‐forming fault blocks, varies in magnitude from <2% in the interior of the Sierra Nevada crustal block to >150% in the Wassuk and Singatse Ranges to the east. Geological and apatite fission track data from exhumed upper crustal sections in the Wassuk and Singatse Ranges point to rapid footwall cooling related to large magnitude extension starting at ∼14–15 Ma. Farther to the west, geological and thermochronological data indicate a younger period of extension in the previously unextended Pine Nut Mountains, the Carson Range, and the Tahoe‐Truckee depression initiated between 10 Ma and 3 Ma, and incipient post‐0.5 Ma faulting to the west of the Tahoe‐Truckee area. These data imply the presence of an extensional breakaway zone between the Singatse Range and the Pine Nut Mountains at ∼14–15 Ma, forming the boundary between the Sierra Nevada and Basin and Range at that time. In addition, fission track data imply a Miocene preextensional geothermal gradient of 27 ± 5°C km−1in the central Wassuk Range and 20 ± 5°C km−1in the Singatse Range, much higher than the estimated early Tertiary gradient of 10 ± 5°C km−1for the Sierra Nevada batholith. This might point to a significant increase in geothermal gradients coupled with a likely decrease in crustal strength enabling the initiation of extensional faulting. Apatite fission track, geophysical, and geological constraints across the Sierra Nevada‐Basin and Range transition zone indicate a two‐stage, coupled structural and thermal westward encroachment of the Basin and Range province into the Sierra Nevada since the middle Miocene.
Arc dynamics and tectonic history of Fiji based on stress and kinematic analysis of dikes and faults of the Tavua Volcano, Viti Levu Island, Fiji
Authors: Begg Graham , Gray David R.
Published: October 2002

Stress analysis of dikes and kinematic analysis of faults within the Tavua Volcano of Viti Levu, Fiji, indicate an apparent migration of the causative maximum principal stress axis (σ1) from NW toward north between ∼5.0 and <3.9 Ma. The dikes and faults are part of successive structural and magmatic events at the arc scale. We present data on the chronological succession of stress indicators in the Tavua Volcano, based on overprinting relationships and published geochronology, which also equate with a counterclockwise rotation of Fiji, assuming an approximately constant N‐S regional compressive stress orientation during this time. The latter interpretation is consistent with paleomagnetism studies that require substantial counterclockwise rotation of the Fiji Platform, since fragmentation of the formerly continuous Outer Melanesian Arc during the latest Miocene to earliest Pliocene. The stress data suggest that the Fiji platform has undergone an ∼50° rotation since ∼5 Ma. Given that rotation apparently stopped at 3 Ma, then particularly fast rotation of ∼25° Myr−1 occurred from ∼5 Ma to 3 Ma. Recent seismicity data indicate the platform is currently subject to N‐S compression. The history of late Miocene and Pliocene events in the region can be related to the consequences arising from collision of an island arc with an oceanic plateau.
Nature of extensional accretionary orogens
Authors: Collins William J.
Published: October 2002

Extensional accretionary orogens form by creation and destruction of large arc/back arc basin systems, generated by extension and sediment infilling during prolonged slab retreat, but episodically thickened by basin inversion during short‐lived (∼10 Ma), orogenic contraction events. They are characterised by widespread, syntectonic, silicic, and minor basaltic magmatism, regional low‐P, variable‐T metamorphism, and by the enigmatic development of rift basins throughout the peak orogenic history. These orogens have features associated with retreating subduction boundaries and contrast markedly with those formed by terrane accretion, such as the Canadian Cordillera. The Paleozoic Lachlan orogen example from eastern Australia shows that Silurian‐Devonian synorogenic basalts and gabbros were intimately associated with rifting and granite emplacement, but they formed during a period of repeated orogenic contraction. Moreover, primitive basaltic compositions have oceanic affinities, indicating generation under lithosphere that was <30 km thick. Only in the final stages of orogeny (Middle Devonian), after at least three major crustal contraction events, did the lithosphere thicken to ∼80 km or more, leading to stabilization of the orogen. Extensional accretionary orogens grow by magmatic and sedimentary additions during extension, caused mainly by asthenospheric melting and rift basin formation/sedimentation, augmented by localized and repeated crustal thickening events. Orogenic contraction leaves an indelible structural imprint which may obliterate the prior‐formed extensional structures. The orogen remains hot, despite repeated thickening events, because of ongoing extension, which promotes advective heat transfer into the crust by basalt injection and crustal melting. Rapid switching to contraction, possibly during intermittent arrival of buoyant oceanic plateaus, inverts the thermally softened basins and forms localized fold‐thrust belts in which the penetrative foliations record the peak metamorphism. Their most diagnostic features are the presence of basaltic rocks and rift basins throughout the orogenic contraction history.
Thermochronology of high heat‐producing crust at Mount Painter, South Australia: Implications for tectonic reactivation of continental interiors
Authors: McLaren Sandra , Dunlap W. James , Sandiford Mike , McDougall Ian
Published: October 2002

The Mount Painter Region in the Northern Flinders Ranges, South Australia, contains a Mesoproterozoic gneissic complex characterized by extraordinary heat production (∼16 μW m−3), resulting in the development of elevated middle‐upper crustal thermal gradients through much of the Paleozoic. Early Paleozoic deformation and metamorphism attained amphibolite facies (>500°C) in the deepest parts of the metamorphic pile (∼10–12 km) during the ∼500 Ma Cambro‐Ordovician Delamerian orogeny. The subsequent thermal history of these rocks is assessed through new K/Ar and40Ar/39Ar age measurements on amphiboles and micas and multiple‐diffusion‐domain thermal modeling of K‐feldspar40Ar/39Ar data. The preferred interpretation of these data is that the deepest rocks were at ∼500°C until around 430 Ma, requiring average upper crustal thermal regimes of the order of 40°C km−1for at least 70 million years. At around 430 Ma, and again at 400 Ma, the terrane underwent periods of moderately fast cooling, possibly separated by a period of isothermal residence. Following cooling at 400 Ma, the terrane entered a second period of relative tectonic quiescence remaining essentially isothermal until ∼330 Ma. This long residence near the closure temperature of biotite resulted in variable argon loss from biotites in the 400 Ma to 330 Ma interval. Tectonic and thermal stability was terminated by a further period of moderately fast cooling (∼4°–8°C Ma−1) in the interval between 330 and 320 Ma. The three cooling episodes at around 430 Ma, 400 Ma, and 330 Ma are interpreted to be the result of exhumation resulting in a combined minimum of 6–7 km of denudation. We attribute this exhumation to the Alice Springs orogeny, a major intraplate tectonothermal event known throughout central Australia but not previously recognized as a significant tectonic event in the Adelaide Fold Belt. These new data provide compelling evidence that thermally modulated variations in lithospheric strength control the distribution of intraplate deformation at the continental scale.
Sediment trapping characteristics of a pit trap and the Helley‐Smith sampler in a cobble gravel bed river
Authors: Sterling Shannon M. , Church Michael
Published: October 2002

Although designed for granule gravels, the Helley‐Smith (HS) bed load sampler is often used in streams with much coarser beds. To compare magnitudes and grain size distributions of sediment samples collected by a pit trap and a standard HS sampler, we conducted 22 “sampling events” over a wide range of flows during a snowmelt freshet in a stream with a coarse gravel bed. Each sampling event consisted of three simultaneous measurements: one made with a pit trap installed in the bed, one with a HS sampler placed on the bed directly beside the pit trap (HS‐I), and one with a HS sampler placed on the downstream rim of the pit trap (HS‐P). We summed the catches of the pit trap and HS‐P to estimate “actual” bed load transport. The pit trap and HS‐I catches were then compared to the summed sample. The pit trap yielded a remarkably consistent, positively skewed, sigmoidal distribution of catch efficiency for all 22 measurements, with near 100% efficiency for material larger than 2.8 mm. The HS‐I sampler was more variable in its catch and trapping efficiency, exhibiting low trapping efficiency for midrange material (0.71 to 16 mm) but high efficiency for finer material. The results cast doubt on the accuracy of bed load data sets collected by Helley‐Smith samplers in coarse gravel channels.
Soil property database: Southern Great Plains 1997 Hydrology Experiment
Authors: Mohanty B. P. , Shouse P. J. , Miller D. A. , van Genuchten M. T.
Published: October 2002

Many models used in land surface hydrology, vadose zone hydrology, and hydroclimatology require an accurate representation of soil properties. Unfortunately, existing soil property databases are limited in terms of reliability, precision, and their usefulness in evolving soil‐vegetation‐atmosphere‐transfer (SVAT) schemes of general circulation models (GCMs) or regional‐scale hydrologic models. Furthermore, not many site‐specific, comprehensive soil property measurement campaigns have been carried out concurrently with large‐scale remote sensing hydrologic campaigns. To better understand the complex and interdependent geophysical processes in the near surface, we conducted an extensive soil property measurement campaign during the Southern Great Plains 1997 (SGP97) Hydrology Experiment. We measured soil physical, hydraulic, and thermal properties across the SGP97 study region. The resulting soil property database not only is useful for evaluating the SVAT schemes in GCMs and other hydrologic models but also can be used as a basis for transfer function modeling, extrapolating point estimates of soil properties to larger spatial scales, testing point and nonpoint source pollution modeling, and evaluating evolving hypotheses in water and energy transfer across the land‐atmosphere boundary. The complete data report [Shouse et al., 2002] and raw data are available upon request from the George E. Brown Salinity Laboratory. Summarized data are given by Mohanty et al. [1999].
A fuzzy risk approach for seasonal water quality management of a river system
Authors: Mujumdar P. P. , Sasikumar K.
Published: October 2002

A fuzzy optimization model is developed for the seasonal water quality management of river systems. The model addresses the uncertainty in a water quality system in a fuzzy probability framework. The occurrence of low water quality is treated as a fuzzy event. Randomness associated with the water quality indicator is linked to this fuzzy event using the concept of probability of a fuzzy event. In most water quality management models the risk level for violation of a water quality standard is constrained by a preassigned value through a chance constraint. In the fuzzy risk approach a range of risk levels is specified by considering a fuzzy set of low risk, instead of using a chance constraint. Thus the two levels of uncertainty, one associated with low water quality and the other with low risk, are quantified and incorporated in the management model. The model takes into account the seasonal variations of river flow to specify seasonal fraction removal levels for the pollutants. Fuzzy sets of low water quality are considered in each season. The membership functions of these fuzzy sets represent the degree of low water quality associated with the discrete states of water quality in a season. The fuzzy risk of low water quality in a season at a checkpoint in the river system is expressed in terms of the degree of low water quality and the steady state probabilities associated with discrete river flows. Considering the fuzzy goals of the pollution control agency and dischargers, and the crisp constraints, the water quality management problem is formulated as a fuzzy optimization model. The model solution gives seasonal fraction removal levels for the pollutants. Application of the fuzzy optimization model is illustrated with a hypothetical river system.
Quantifying recreation use values from removing dams and restoring free‐flowing rivers: A contingent behavior travel cost demand model for the Lower Snake River
Authors: Loomis John
Published: October 2002

A travel cost demand model that uses intended trips if dams are removed and the river restored is presented as a tool for evaluating the potential recreation benefits in this counterfactual but increasingly policy relevant analysis of dam removal. The model is applied to the Lower Snake River in Washington using data from mail surveys of households in the Pacific Northwest region. Five years after dam removal, about 1.5 million visitor days are estimated, with this number growing to 2.5 million annually during years 20–100. Using the travel cost method model estimate of the value of river recreation, if the four dams are removed and the 225 km river is restored, the annualized benefits at a 6.875% discount rate would be $310 million. This gain in river recreation exceeds the loss of reservoir recreation but is about $60 million less than the total costs of the dam removal alternative. The analysis suggests this extension of the standard travel cost method may be suitable for evaluating the gain in river recreation associated with restoration of river systems from dam removal or associated with dam relicensing conditions.
Hydraulic conditions in experimental rill confluences and scour in erodible soils
Authors: Bryan Rorke B. , Kuhn Nikolaus J.
Published: October 2002

Few studies have examined processes active within rill networks or the effect of confluence geometry on rill sediment flux, and it is not clear if observations from river confluences also apply at rill scale. Laboratory experiments were carried out in artificial rectangular rill channels with fixed and erodible beds to identify the effects of confluence geometry on hydraulic conditions and scour patterns. Symmetrical and asymmetrical confluences with angles from 19° to 90° were used. Hydraulic changes in the confluence zone in fixed bed experiments included decreased flow velocity, increased shear velocity, transition from supercritical to subcritical, and from transitional to turbulent flow. Bed scour in the confluence zone resulted, generally increasing with confluence angle, but a more important influence was junction symmetry or asymmetry. Symmetrical confluences produced a symmetrical scour pattern mirroring the original channels, but in asymmetrical confluences scour was more complex, evolving to a symmetrical pattern with confluence angles higher than the original system. Results are consistent with channel evolution to an optimally branched system controlled by minimum power criteria, but validation requires testing with more precise velocity measurement.
Effects of digital elevation model resolution on derived stream network positions
Authors: McMaster Kevin J.
Published: October 2002

This study examines the effect of digital elevation model (DEM) resolution on the positional accuracy of derived hydrologic networks and quantitatively confirms that DEM resolution should be greater than the average hillslope length when used for hydrologic modeling. Seven hundred kilometers of mapped stream networks are compared to stream networks derived from 17 DEMs with resolutions ranging from 30 m to 3 km. Comparison between predicted and mapped streams reveals that accuracy of predicted stream locations decays quickly beyond a DEM resolution of 180 m. A new application of Gyasi‐Agyei et al.'s [1995] DEM resolution suitability test based on average slope and vertical resolution indicates an average hillslope length of between 150 and 180 m. Tarboton et al.'s [1991] method of determining average hillslope length based on slope and accumulation areas reconfirms the length to be 150 m and verifies the link between network accuracy and hillslope scale. Two algorithms are used to derive stream networks: the D∞ algorithm [Tarboton, 1997], which allows for flow dispersion, and the D8 algorithm [O'Callaghan and Mark, 1984], which does not. A comparison between the D8 and D∞ algorithms shows that modeling flow dispersion is not necessary in steep terrain, as both algorithms performed equally well. However, the D∞ algorithm is found to be less susceptible to modeling erroneous hillside flow convergence, making it preferable to the D8 algorithm when used by Tarboton et al.'s [1991] method of determining the average hillslope lengths. Finally, field observation indicates that predicted first‐ and second‐order streams tend to exist when predicted; however, mapped streams have a higher positional accuracy.
Geologic and hydrologic controls on the movement of water through a thick, heterogeneous unsaturated zone underlying an intermittent stream in the western Mojave Desert, southern California
Authors: Izbicki John A.
Published: October 2002

A two‐dimensional, axially symmetric, unsaturated flow model was developed to test hypotheses about geologic and hydrologic controls on the movement of water through the thick, heterogeneous, unsaturated zone underlying Oro Grande Wash in the Mojave Desert, California. Heterogeneity within the unsaturated zone was simulated with multiple realizations of subsurface geology estimated on the basis of transition probability/Markov chain statistics. Model results show lateral spreading of water away from the wash was best approximated by realizations that include thin, horizontally extensive clay layers that impede the downward movement of water. There was a wide range in model responses for these realizations, and the movement of water through unsaturated zones containing thin, horizontally extensive clay layers may be more difficult to predict than water movement through unsaturated zones where clay layers are less extensive. For realizations having less extensive clay layers, the range of model responses decreased with time, and model results became increasingly similar as water encountered larger volumes of material.