bluuebunny/crossref_metadata_2025 huggingface dataset:
A compositional study of asteroid 243 Ida and Dactyl from Galileo NIMS and SSI observations
Authors: Granahan James
Published: November 2002

On 28 August 1993 the Galileo spacecraft encountered the asteroid 243 Ida and its moon Dactyl. A variety of observations of this asteroid system were collected, including visible wavelength (0.4–1.0 μm) imagery with the Solid State Imager (SSI) instrument and infrared wavelength (0.7–5.2 μm) with the Near Infrared Mapping Spectrometer (NIMS) for both objects. These data indicate that 243 Ida has an orthopyroxene/(orthopyroxene + olivine) ratio of 0.35, a value consistent with that of LL chondrites. The ratio does not vary significantly for the portions of 243 Ida observed by the Galileo NIMS instrument. Asteroid 243 Ida is a SIV subtype of the S type asteroid population. Two very similar spectral units were identified in a combined SSI and NIMS spectral data set. The primary discriminator between the spectral units is a small difference in the amount of “red” slope. A larger “red” slope value corresponds to regions of 243 Ida where ejecta from the crater Azurra are present. Dactyl has a relatively deep absorption centered around 0.97 μm. This suggests that Dactyl has larger mineral grains on its surface in comparison to 243 Ida and thus was formed by a younger collision than that which formed 243 Ida.
Martian clouds observed by Mars Global Surveyor Mars Orbiter Camera
Authors: Wang Huiqun , Ingersoll Andrew P.
Published: November 2002

We have made daily global maps that cover both polar and equatorial regions of Mars for Ls 135°–360° and 0°–111° using the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) red and blue wide‐angle swaths taken from May 1999 to January 2001. We study the seasonal distribution of condensate clouds and dust clouds during roughly 1 Martian year using these daily global maps. We present the development and decay of the tropical cloud belt and the polar hoods, the spatial and temporal distributions of lee waves and spiral clouds, and an unusual “aster” cloud above the volcanoes, consisting of rays around a central disk, like the flower. The tropical cloud belt contains mostly fibrous clouds during northern spring/early summer and convective clouds during middle/late northern summer. The detailed development and decay of the tropical cloud belt is nonuniform in longitude. Two distinct stormy periods in late summer precede the formation of the north and south circumpolar hoods. The north polar storms in late summer resemble baroclinic frontal systems on Earth but contain both dust and condensate clouds. Spiral clouds occur only in the northern high latitudes and only during northern spring and summer. The north polar hood displays a stationary wave number two structure during the fall and winter. The south polar hood has fewer streak clouds and lee wave clouds than the north polar hood. During this particular year the lee wave cloud abundance in the south had two peaks (in early fall and late winter), and the lee wave cloud abundance in the north had one peak (in early fall).
Polarization properties of ULF waves in the Jovian middle magnetosphere
Authors: Petkaki Panagiota
Published: November 2002

Polarization analysis of ultralow frequency waves in the ion cyclotron wave range is presented. The waves were observed with the Ulysses magnetometer during the flyby of the Jovian magnetosphere in 1992. During the inbound pass of Ulysses in the dawnside of Jupiter and in low to intermediate latitudes, the wave signatures show either elliptically left‐ or right‐handed polarization and in one occasion linear polarization. During the outbound pass when the spacecraft was south of the magnetic equator in the duskside and in intermediate latitudes, the polarization of the events is found to be mostly right handed in the frequency range of ion cyclotron waves. Even though the statistical sample of events is limited, the relevance of crossover frequencies in the Jovian multicomponent plasma is strongly indicated by the observations. Further investigation into a more complete set of data covering both day and midnight sectors of the Jovian magnetosphere along with meridian variation is considered to be necessary in order to resolve the propagation properties of ion cyclotron waves in the cold multi‐ion plasma of Jupiter's magnetosphere.
Morphology, stratigraphy, and surface roughness properties of Venusian lava flow fields
Authors: Byrnes Jeffrey M. , Crown David A.
Published: November 2002

Morphologic characteristics, flow stratigraphy, and radar backscatter properties of five lava flow fields on Venus (Turgmam Fluctus, Zipaltonal Fluctus, Tuli Mons/Uilata Fluctus, Var Mons, and Mylitta Fluctus) were examined to understand flow field emplacement mechanisms and relationships to other surface processes. These analyses indicate that the flow fields studied developed through emplacement of numerous, thin flow units, presumably over extended periods of time. Although the Venusian fields display flow morphologies similar to those observed within terrestrial flow fields, the Venusian flow units are significantly larger and have a larger range of radar backscatter coefficients. Both simple and compound flow emplacement appear to have occurred within the flow fields. A potential correlation between flow rheology and radar brightness is suggested by differences in planform morphology, apparent flow thickness, and apparent sensitivity to topography between bright and dark flows. Distributary flow morphologies may result from tube‐fed flows, and postemplacement modification by processes such as flow inflation and crustal foundering is consistent with discrete zones of increased radar brightness within individual flow lobes. Mapping of these flow fields does not indicate any simple evolutionary trend in eruptive/resurfacing style within the flow fields, or any consistent temporal sequence relative to other tectonic and volcanic features.
Volcanic plume heights on Mars: Limits of validity for convective models
Authors: Glaze Lori S. , Baloga Stephen M.
Published: November 2002

Previous studies have overestimated volcanic plume heights on Mars. In this work, we demonstrate that volcanic plume rise models, as currently formulated, have only limited validity in any environment. These limits are easily violated in the current Mars environment and may also be violated for terrestrial and early Mars conditions. We indicate some of the shortcomings of the model with emphasis on the limited applicability to current Mars conditions. Specifically, basic model assumptions are violated when (1) vertical velocities exceed the speed of sound, (2) radial expansion rates exceed the speed of sound, (3) radial expansion rates approach or exceed the vertical velocity, or (4) plume radius grossly exceeds plume height. All of these criteria are violated for the typical Mars example given here. Solutions imply that the convective rise model is only valid to a height of ∼10 km. The reason for the model breakdown is that the current Mars atmosphere is not of sufficient density to satisfy the conservation equations. It is likely that diffusion and other effects governed by higher‐order differential equations are important within the first few kilometers of rise. When the same criteria are applied to eruptions into a higher‐density early Mars atmosphere, we find that eruption rates higher than 1.4 × 109 kg/s also violate model assumptions. This implies a maximum extent of ∼65 km for convective plumes on early Mars. The estimated plume heights for both current and early Mars are significantly lower than those previously predicted in the literature. Therefore, global‐scale distribution of ash seems implausible.
High‐resolution X‐ray computed tomography of impactites
Authors: Koeberl Christian , Denison Cambria , Ketcham Richard A. , Reimold Wolf Uwe
Published: November 2002

High‐resolution X‐ray computed tomography (HRXCT) is a nondestructive method used to study the interiors of opaque solid objects. Here we present the results of a first application of the HRXCT method to imaging the interior of impactites, in particular, suevites (glass‐bearing impact breccias) from the Bosumtwi (Ghana) and Ries (Germany) craters and a Muong Nong–type tektite from Thailand. The aim of these studies was to determine the three‐dimensional (3‐D) distribution of clasts within the matrix of the suevites, to test this technique with respect to its suitability for the recognition of different clast types of different densities, and to determine textural characteristics of the tektite. The main part of the study concentrated on a large suevite sample (about 10 × 7 × 5 cm) from the Bosumtwi impact structure. Target rock fragments in the Bosumtwi sample consist of greywacke and sandstone/quartzitic rocks, shale and phyllite, and granites. Another large clast component is composed of impact melt and glass fragments. Macroscopic petrography and thin section petrography were used to identify the clast types in the specimen for correlation with its HRXCT signatures. The results show that HRXCT allows the easy discrimination of the relatively frothy inclusions of glassy melt in the suevites, as they are darker than the matrix in the raw X‐ray scans and can be traced through the whole sample. Color or gray scale applications allow the distinction of at least four different clast types based on density differences. The size of the smallest discernable clasts (about 0.5 mm) is determined by the resolution of the measurements, which, in turn, is a function of scan slice thickness and field of view. In the case of the tektite sample, we were also able to image the 3‐D distribution of vesicles, possibly indicating glass flow. The HRXCT method allows us to determine the three‐dimensional distribution of clast populations in impact breccias by image processing techniques and to quantify their abundances in volume percent.
An improved 50‐degree spherical harmonic model of the magnetic field of Mars derived from both high‐altitude and low‐altitude data
Authors: Arkani‐Hamed Jafar
Published: November 2002

This paper presents a 50‐degree spherical harmonic model of the magnetic field of Mars derived from both low‐ and high‐altitude magnetic data through covariance analysis. Three sets of models are first calculated by using the high‐altitude data alone, which include harmonics of degree 55, 60, and 65. Each set consists of four models, depending on whether the north‐south and west‐east components data are used in a model in addition to the radial component. The models correlate strongly over harmonics of degree up to about 50, except the model that is calculated by using the radial component data alone, which shows appreciable differences from the others. The high‐altitude models are then correlated with the corresponding models that were previously derived from the low‐altitude data. The correlation between the corresponding models is again very high over harmonics of degree 4–50, confirming the reliability of the spherical harmonic coefficients over these harmonic degrees. The slightly lower correlation, ∼0.7, over harmonics of degree lower than 4 implies some contribution from quasi‐steady external field. A final 50‐degree model is determined from the covarying harmonics of the low‐ and high‐altitude models that use all three components of the magnetic data. The corresponding radial and tangential components of the magnetic anomalies are presented at the surface of Mars, along with their error limits.
A model for large‐scale convective storms in Jupiter
Authors: Hueso Ricardo , Sánchez‐Lavega A. , Guillot T.
Published: November 2002

Large‐scale convective storms are a common phenomenon in Jupiter's atmosphere. They are apparent in ground‐based and spacecraft images and may strongly affect the dynamics of the global atmosphere as well as the energy transport in the meteorological layer. In this paper we analyze the outburst of a large convective storm system (core ∼5000 km) in the South Equatorial Belt (SEB) that was observed at high spatial and temporal resolution by the Voyager 1 spacecraft in 1979. We use a two‐dimensional (2‐D) model to study the interaction between cloud material brought up by moist convection and the environmental wind. Aided by previous 3‐D models of Jovian storms, we can draw several qualitative and quantitative conclusions. The evolution of this storm can be characterized by three phases: (1) onset of the perturbation, well reproduced by the growth of a single‐cell storm; (2) an expanding phase in which the number of convective cells increases to ∼200, with updraft velocities limited to 50 m/s; and (3) a relatively sudden suppression of the convective activity leading to the disruption of large structures by the environmental wind. Furthermore, we interpret the observations of the inner bright core as a well‐defined anticyclonic vortex, with darker cloud material preferentially left southwest of the system. Finally, we show that ∼1016 W are released by such a storm over its life cycle of 12 days and that the direct formation of a large‐scale anticyclonic vortex after the moist convective source has been removed is prevented by the environmental wind shear.
Variation in surface and deep water circulation in the Denmark Strait, North Atlantic, during marine isotope stages 3 and 2
Authors: Hagen Sveinung , Hald Morten
Published: November 2002

The paleoceanography in the Denmark Strait, North Atlantic, is reconstructed based on down core analysis of planktonic foraminifera, carbon and oxygen isotopes, carbonate, and ice rafted debris (IRD). The stratigraphy spans 57 to 6 calibrated (cal) ka years and reveals sea surface temperatures that covary with Greenland ice core temperatures throughout marine isotope stages (MIS) 3 and 2. Planktonic δ18O minima, interpreted as meltwater events, follow sea surface warming. IRD from both the Greenland and Iceland Ice Sheets was delayed by ∼2 ka relative to the North Atlantic Heinrich events. Ventilation of bottom water was mostly sustained through open ocean convection associated with North Atlantic Deep Water production or by convection of glacial North Atlantic Intermediate Water, except for two episodes at ∼54 cal ka (MIS 3.31) and 18–15 cal ka (termination I). During these two periods we suggest that deep water ventilation was accomplished mainly by brine formation.
Simulated warm polar currents during the middle Permian
Authors: Winguth A. M. E. , Heinze C. , Kutzbach J. E. , Maier‐Reimer E. , Mikolajewicz U. , Rowley D. , Rees A. , Ziegler A. M.
Published: November 2002

During Permian stage 6 (Wordian, Kazanian) the Pangaean supercontinent was surrounded by a superocean: Panthalassa. An ocean general circulation model has been coupled to an atmospheric energy balance model to simulate the sensitivity of the Wordian climate (∼265 million years ago) to changes in greenhouse gas concentrations, high‐latitude geography, and Earth orbital configurations. The model simulates significantly different circulation features with different levels of greenhouse gas forcing, ranging from a strong meridional overturning circulation in the Southern Hemisphere at low CO2concentration (present level) to more symmetric overturning circulation cells with deep water formation in polar latitudes of both hemispheres at high CO2concentration (8 times present level). The simulated climate with 4 times present level CO2concentration agrees generally well with climate‐sensitive sediments and phytogeographic patterns. The model simulates strong subtropical gyres with similarities to the modern South Pacific circulation and moderate surface temperatures on the southern continent Gondwana, resulting from a strong poleward heat transport in the ocean. An even more moderate climate is generated if high‐latitude land is removed from the southern continent so that ocean currents can penetrate into the polar regions of Gondwana.
Nitrogen isotope and productivity variations along the northeast Pacific margin over the last 120 kyr: Surface and subsurface paleoceanography
Authors: Kienast Stephanie S. , Calvert Stephen E. , Pedersen Thomas F.
Published: November 2002

Glacial‐interglacial changes in sedimentary δ15N over the last 120 kyr display a remarkably similar pattern in timing and amplitude in core records extending from the denitrification zone in the eastern tropical North Pacific (ETNP), where subsurface denitrification is active, to the Oregon margin, where no denitrification occurs today. Low δ15N values (4–6‰) generally characterize glacial stages 2 and 4, and higher δ15N values (7–10‰) are representative of the Holocene, millennial‐scale periods within stage 3, and stage 5. The inferred synchroneity of δ15N variations along the entire margin implies that the nitrate isotopic signal produced in the oxygen‐poor subsurface waters in the ETNP is rapidly advected northward and recorded at sites far beyond the boundaries of the modern denitrification zone. Similar to δ15N, primary production indicators (percent Corg, Ba/Al, and percent opal) show glacial‐interglacial as well as millennial‐scale variations along the NE Pacific margin, with higher primary production during warm periods. However, the relative phasing between δ15N and paleoproduction tracers within individual records changes latitudinally. Whereas δ15N and primary production vary approximately synchronously in the midlatitudes, production lags δ15N in the ETNP by several kiloyears. This lag calls for a new understanding of the processes driving denitrification in the ETNP. We suggest that oxygen input by the Equatorial Undercurrent as well as local organic matter flux controls denitrification rates in the ETNP. Moreover, the differences in relative timing point to a time‐transgressive development of upwelling‐favorable winds along the NE Pacific margin after the last glaciation, with those in the north developing several kiloyears earlier.
Feedback between aeolian dust, climate, and atmospheric CO2 in glacial time
Authors: Ridgwell A. J. , Watson A. J.
Published: November 2002

Enhanced aeolian supply of iron to the biota of the Southern Ocean during glacial periods is suspected to be an important contributory mechanism to the concurrently low observed mixing ratios of atmospheric CO2. Declining rates of dust deposition prior to the glacial terminations may be critical in driving the initial deglacial rise in CO2, but the reasons behind the dust decline itself are as yet unknown. Here we show that the dust record from the Vostok ice core can be qualitatively derived from a few general assumptions regarding the formation and aging of Patagonian sources of aeolian material and the efficiency with which it is transported through the atmosphere. We suggest that during glacial periods this dust supply becomes particularly sensitive to changes in global climate and that, in turn, climate is responsive to the dust due to iron fertilization. This positive feedback may mean that during glacial periods the carbon cycle exhibits two quasi steady states, characterized by distinct CO2 concentrations. Recognition of this “glacial subcycle” may help to account for the timing and sequence of events at the terminations.
HF radio signal fading and atmospheric radio noise measurements at low latitudes
Authors: Rao S. Vijaya Bhaskara , Rao T. Rama , Reddy V. Gunasekhara , Lakshmi D. R. , Veenadhari B. , Dabas R. S. , Ahmed Iqbal , Gupta M. M. , Reddy B. M.
Published: November 2002

Two HF broadcast transmissions, one originating from Sri Lanka (7.1°N, 79.9°E) at 11.800 MHz around the geomagnetic equator and the other from BBC, Masirah (20.6°N, 58.9°E), at 15.310 MHz several degrees north of the geomagnetic equator were monitored at Tirupati (13.5°N, 79.5°E) in southern India to compare the fading characteristics of the links. It is found that the link over the geomagnetic equator exhibits much higher fade rates as compared to the other link. The fade ranges also are, in general, higher for Sri Lanka. Fade allowances derived from these data do indicate that margins higher than those recommended by the International Telecommunication Union Radiocommunication Sector (ITU‐R) need to be provided at these latitudes. The diurnal variation in the atmospheric radio noise (ARN), which sets a lower limit to the satisfactory reception of signal, over the Indian subcontinent is in total disagreement with the variations obtained at higher latitudes. This rather puzzling phenomenon has been interpreted as being due to path attenuation differentials between the LOS mode and the ionospheric mode from the lightning source because of significantly greater cloud height in the Indian region.
First in situ observations of equatorial ionospheric bubbles by Indian satellite SROSS‐C2 and simultaneous multisatellite scintillations
Authors: Paul A. , Ray S. , DasGupta A. , Garg S. C.
Published: November 2002

The first observation of equatorial ionospheric irregularities by RPA probe of the Indian low Earth orbiting satellite SROSS‐C2 is presented in this paper. Amplitude scintillations of medium Earth orbiting Global Positioning System (GPS) satellites and geostationary FLEETSATCOM (244 MHz, 73°E) and INMARSAT (1.5 GHz, 65°E) signals recorded simultaneously at Calcutta (lat: 22.97° N, long: 88.50°E geographic; dip: 32°N) are used for a coordinated study of equatorial F region irregularities in the Indian zone. Cases of ionospheric irregularities identified from the SROSS‐C2 records obtained during the initial one‐and‐a‐half years since its launch in May 1994 have been analyzed. Some events of in situ ion density irregularities are compared with scintillations simultaneously observed on the transionospheric satellite links. Intense bite‐outs of ion density (maximum relative irregularity amplitude ΔN/N ∼ 65%) were detected on one occasion (October 29, 1994) coupled with deep fadings (S4 ∼ 1 at VHF, ∼0.52 at L‐band, and ∼0.69 at GPS L1 frequency) on ground‐based satellite links. An estimate of scintillation indices from the observed in situ density deviations compares well with the ground‐based measurements. The development of intense equatorial bubbles even on a day like October 29, 1994, under low solar activity conditions, may be attributed to a prompt penetration of magnetospheric electric field equatorwards during the main phase of a magnetic storm in progress [maximum negative excursion of Dst ∼ −127 nT at 1600UT (2100MLT) with a dDst/dt rate −37 nT/hr at 1300–1400UT (1800–1900MLT)]. The drift velocity and spatial extent of these irregularities have been estimated from ground‐based observations.
Spatial‐temporal statistics of rainrate random fields
Authors: Paulson Kevin S.
Published: November 2002

Rainrate near the ground can be interpreted as a collection of random variables forming a random field parameterized by three coordinates: two spatial coordinates and one temporal. Interest among the radio propagation community has been focused on the first order statistics of these random variables, principally on the average annual probability distribution of rainrate as a function of location. However, increasingly interest is turning to second order statistics describing high‐resolution rainrate variation in both time and space. This information is important for the design of fade countermeasures and for efficient spectrum management. In this paper radar data from a widespread, slow moving and intense, stratiform rain event experienced by the southeastern UK on May 1, 2001, are used to calculate the second order, spectral, statistics of spatial‐temporal log rainrate variation. It is demonstrated that log rainrate is self‐similar in all three coordinates and that symmetry exists between the spatial and temporal variation. These data are used to develop an isotropic, spatial‐temporal log rainrate model assuming that log rainrate is a homogeneous, Gaussian, random field. This model is developed further to yield closed form expressions for the mean and covariance of the random fields associated with rainrate, with specific attenuation and the logarithm of specific attenuation. Furthermore, expressions for the temporal covariance of the rain attenuation experienced by pairs of radio links are developed, and these expressions are tested against measured rain attenuation data from an experimental link.
GPS‐derived ionospheric total electron content response to a solar flare that occurred on 14 July 2000
Authors: Zhang D. H. , Xiao Z. , Igarashi K. , Ma G. Y.
Published: November 2002

This paper studies the ionospheric response to an X5.7/3B solar flare that occurred at 10:03 UT on 14 July 2000. With Global Positioning System (GPS) observations, temporal evolution of the ionospheric total electron content (TEC) values was obtained within the latitude range of 30°N ∼ 45°N and the longitude range of 15°E ∼ 45°E. It was found that dayside TEC values were enhanced during the flare event, which could be as large as 5 TECU (1 TECU = 1016/m2) in regions with small solar zenith angles. The enhancement tended to depend on latitude, longitude and the solar zenith angle of the subionospheric point. However, the TEC enhancement derived from the latitude belt between 30°N and 45°N was not symmetrical about either the longitude or the local hour; it was smaller in the local morning than in the afternoon. The TEC enhancement in the Southern Hemisphere seems to be larger than that in the Northern Hemisphere for the same solar zenith angle. This implies that the background levels of the ionosphere and thermosphere had some influence on the TEC enhancement. The temporal variation of TEC shows minor correlative disturbances from 10:15 UT to 10:27 UT when the solar flare was in the maximum phase. It is likely that the minor disturbances resulted from the evolution of flare emission in the EUV domain.
Discordant paleomagnetic data for middle‐Cretaceous intrusive rocks from northern Baja California: Latitude displacement, tilt, or vertical axis rotation?
Authors: Böhnel Harald , Delgado‐Argote Luis A. , Kimbrough David L.
Published: November 2002

Paleomagnetic results and U/Pb zircon dating from the San Marcos dike swarm and the El Testerazo pluton in the Cretaceous Peninsular Ranges batholith of northern Baja California are used to evaluate alternative pre‐Neogene paleogeographic reconstructions of the Baja California peninsula. The San Marcos dike swarm is a dense, northwest striking, regional dike swarm that is exposed over an ∼100 km long segment of the batholith and has yielded a U/Pb zircon crystallization age of 120 ± 1 Ma. Dike attitudes from the swarm suggest a regionally consistent average ∼320°E strike and ∼79°NE dip. The El Testerazo pluton is a younger tonalite intrusion that truncates the northern end of the dike swarm. All but one of 36 sites sampled in this study show remanence of normal polarity. Paleopoles for the San Marcos dike swarm and El Testerazo pluton are indistinguishable and were combined into a paleopole at 248.1°E, 86.6°N, A95 = 4.8°, which is displaced with respect to the 122 Ma reference pole for stable North America at 198.2°E, 72.3°N, A95 = 3.3°. The displacement may be described by an apparent clockwise rotation of 18° ± 6° and an apparent northward shift of 8° ± 5°. Restoring a northward shift of about 3°, related to the separation of Baja California from North America since 10 Ma, only a marginal northward displacement of 5° ± 5° is left. The clockwise rotation may be the result of crustal block rotations within the right‐lateral shear systems in northern Baja California, although there is no geological evidence that supports this possibility. Alternatively, the difference between paleopole and reference pole may be due to tilting of the study area. Restoring a northeastward tilt of 11°, based on the mean dip measured for the San Marcos dike swarm in the study area, yields a paleopole at 187.6°E, 70.8°N, A95 = 5.6°, which is indistinguishable from the 122 Ma North American reference pole. The tilting hypothesis suggested previously as a potential explanation for low inclination of paleomagnetic data from Baja California therefore seems to be viable for the discordant paleopoles from this segment of the batholith. Data obtained previously about 150 km further south and along the western edge of the batholith defined a concordant paleopole without any tilt correction, indicating that regional tilting may not be valid for all of Baja California.
Multistage extensional evolution of the central East Greenland Caledonides
Authors: White Arthur P. , Hodges Kip V.
Published: November 2002

Recent field investigations in the central East Greenland Caledonides (72°–74°N) resulted in the identification of an orogen‐scale extensional fault system called the Fjord Region Detachment (FRD). Previous geochronologic constraints on this deformation indicated that the FRD was active circa 430–425 Ma, a time when the Baltica‐Laurentia collision was thought to be occurring, and continued to be active for up to 80 million years. We present new40Ar/39Ar thermochronologic data from an E‐W transect that cuts across two splays of the FRD. Our data demonstrate that at least two distinct episodes of faulting were responsible for extension in the East Greenland Caledonides: an earlier phase (circa 425–423 Ma) that was synorogenic and penetrated to middle‐crustal levels, followed by a post‐Caledonian phase of reactivation (∼414 to 380 Ma) that affected even deeper structural levels. Furthermore, we present in situ UV laser40Ar/39Ar data for pseudotachylite collected along the deepest splay of the FRD that indicate this fault was active again as recently as ∼357 Ma (coeval with Devonian basin formation). Altogether, our data suggest that rather than being active continuously for 80 million years, the FRD consisted of multiple splays that were active for shorter intervals over discrete time periods separated by as much as 60 million years. Finally, our data provide evidence that young extensional deformation associated with postorogenic collapse in East Greenland was not restricted to the formation of sedimentary basins in the far eastern part of the orogen, but also resulted in deformation of the Archean‐Paleozoic crystalline basement.
A doubly vergent fan structure in the Peninsular Ranges batholith: Transpression or local complex flow around a continental margin buttress?
Authors: Schmidt Keegan L. , Paterson Scott R.
Published: November 2002

The Jura‐Cretaceous Peninsular Ranges batholith (PRB) of southern and Baja California contains a remarkable example of a crustal transition between juxtaposed oceanic and continental floored arcs. Jura‐Cretaceous deformation in the PRB is focused along this lithospheric discontinuity and defines a zone of contractional deformation that stretches at least 800 km along the batholith. The western edge of this zone is associated with a dramatic eastward increase in maximum recorded metamorphic pressures (from 2 to 6 kbar) and temperatures (from <300 to >650°C). Elsewhere in the North American Cordillera, similar lithospheric boundaries are thought to accommodate various amounts of orogen‐parallel displacements of outboard tectonic fragments. To test this proposal in the PRB, we examined an unusually well‐preserved part of the transition zone in the southern Sierra San Pedro Martir (SSPM). Here a spectacular ∼20 km wide, doubly vergent fan structure, which developed over a period >40 m.y. during both Jurassic and Cretaceous magmatism, occurs along the transition zone. This structure consists of moderately inward dipping, outward vergent, fold and thrust belts with widespread mylonitic fabrics that steepen toward the center where both mylonitic and magmatic fabrics occur in syntectonic, vertically sheeted plutons. Mineral lineations change from steeply pitching on the sides of the fan to moderately pitching in the center. A number of constraints on possible displacements, rotation of units, and duration of fabric formation limit the amount of orogen‐parallel displacement to a few kilometers across the fan structure in the SSPM. Instead, we propose that the fan structure is a local feature that results from a combination of tectonic wedging caused by a cusp of rigid, pre‐Mesozoic continental margin crust at this location and vise tectonics that arose from strong lateral rheological gradients resulting from extensive heating by the sheeted tonalite complex, focused deformation along the lithospheric discontinuity, and subsequent strain softening. We suggest that these mechanical controls on deformation during orthogonal convergence may be common in other orogens where similar fan structures have been attributed to transpression, emphasizing the need for caution in interpreting such features as zones of significant, orogen‐parallel displacement.
Effect of bed form geometry on the penetration of nonreactive solutes into a streambed
Authors: Marion Andrea , Bellinello Matteo , Guymer Ian , Packman Aaron
Published: November 2002

The contamination of riverbeds by solutes derived from the surface flow has recently received increasing attention. Channel morphological features such as bed forms are important characteristics of the stream‐subsurface interface and represent one control on the rate of solute delivery from the stream to the bed. Generally, larger bed forms are expected to produce greater rates of stream‐subsurface exchange. However, the longitudinal dimension (wavelength) of the bed form is also important, and this effect can produce penetration patterns that may be unexpected from a visual observation of the bed surface. Experimental tests in a recirculating flume demonstrate these effects. Commonly used mathematical models do not consider the bed form geometry explicitly and depend on the availability of calibration data to derive exchange parameters for each stream reach. More detailed models that consider the effect of bed form shape are capable of simulating some of the observed experimental results. However, existing physically based models are shown to be insufficient for some bed form geometries that may occur in real streams.
Modeling a two‐layer flow system at the subarctic, subalpine tree line during snowmelt
Authors: Leenders Erica E. , Woo Ming‐ko
Published: November 2002

In the subarctic it is common to encounter a two‐layer flow system consisting of a porous organic cover overlying frozen or unfrozen mineral soils with much lower hydraulic conductivities. The “simple lumped reservoir parametric,” or “semidistributed land‐use‐based runoff processes” (SLURP), model was adapted to simulate runoff generated by such a flow system from an upland shrub land to an open woodland downslope. A subalpine site in Wolf Creek, Yukon, Canada, was subdivided into two aggregated simulation areas (ASA), each being a unit characterized by a set of parameters. The model computes the vertical water balance and flow generation from several storages, and then routes the water out of the ASA. When applied to the 1999 snowmelt season, the model simulated the very low lateral flow and a large increase in storage in the mineral soil, as was observed in the field. The model was used to assess the sensitivity of the two‐layer flow system under a range of temperature, snow cover, and frost conditions. Results show that within the range of possible climatic conditions, the hydrologic system is unlikely to yield significant runoff across the subalpine tree line, but if ground ice is abundant in the soil pores, percolation will be limited and fast flow from the surface layer is enhanced.
A two‐fraction model for the transport of sand/gravel mixtures
Authors: Wilcock Peter R. , Kenworthy Stephen T.
Published: November 2002

A mixed‐size sediment transport model using only two fractions, sand and gravel, offers practical advantages in terms of the required input and conceptual advantages by readily incorporating the nonlinear effects of sand content on transport rate. A two‐fraction model is developed from flume and field transport observations. Sand and gravel transport rates are collapsed to a single transport function using a scaling parameter representing incipient motion for each fraction. The incipient motion function is consistent with established values in the limit of pure sand and gravel beds and shows a sharp change with increasing sand content over the transition from a clast‐ to matrix‐supported bed. A two‐fraction model captures an important and previously unmodeled nonlinear effect of sand content on gravel transport rate and suggests important implications for a number of fluvial processes, including the response of channels to sediment supply and the mechanism for abrupt sorting in gravel/sand transitions.
Noniterative time stepping schemes with adaptive truncation error control for the solution of Richards equation
Authors: Kavetski Dmitri , Binning Philip , Sloan Scott W.
Published: November 2002

Noniterative implicit time stepping schemes with adaptive temporal truncation error control are developed for the solution of the pressure form of Richards equation. First‐ and second‐order linearizations of an adaptive backward Euler/Thomas‐Gladwell formulation are introduced and are shown to constrain the temporal truncation errors near a user‐prescribed tolerance and maintain adequate mass balance. Numerical experiments demonstrate that accurate noniterative linearizations achieve cost‐effective solutions of problems where soils are described by highly nonlinear and nonsmooth constitutive functions. For these problems many conventional iterative solvers fail to converge. The noniterative formulations are considerably more efficient than analogous time stepping schemes with iterative solvers. The second‐order noniterative scheme is found to be more efficient than the first‐order noniterative scheme. The proposed adaptive noniterative algorithms can be easily incorporated into existing backward Euler software, which is widely used for Richards equation and other nonlinear PDEs.
Evaluation of unsaturated zone water fluxes in heterogeneous alluvium at a Mojave Basin Site
Authors: Nimmo John R. , Deason Jeffrey A. , Izbicki John A. , Martin Peter
Published: November 2002

Vertical and horizontal water fluxes in the unsaturated zone near intermittent streams critically affect ecosystems, water supply, and contaminant transport in arid and semiarid regions. The subsurface near the Oro Grande Wash is typical in having great textural diversity, pronounced layer contrasts, and extremely low hydraulic conductivities associated with nearly dry media. These features prevent a straightforward application of the Darcian method for recharge estimation, which has provided high‐quality flux estimates at simpler, wetter sites. We have augmented the basic Darcian method with theoretical developments such that a small number of core sample unsaturated hydraulic property measurements, combined with additional, easily obtained data (e.g., drillers' logs) can provide useful flux estimates and knowledge of two‐dimensional water behavior beneath the wash.
Air‐water interfacial areas in unsaturated soils: Evaluation of interfacial domains
Authors: Costanza‐Robinson Molly S. , Brusseau Mark L.
Published: November 2002

A gas‐phase miscible‐displacement method, using decane as an interfacial tracer, was used to measure air‐water interfacial areas for a sand with water contents ranging from ∼2% to 20%. The expected trend of decreasing interfacial areas with increasing water contents was observed. The maximum estimated interfacial area of 19,500 cm−1 appears reasonable given it is smaller than the measured surface area of the porous medium (60,888 cm−1). Comparison of the experimental data presented herein with literature data provided further insight into the characterization of the air‐water interface in unsaturated porous media. Specifically, comparison of interfacial areas measured using gas‐phase versus aqueous‐phase methods indicates that the gas‐phase method generally yields larger interfacial areas than the aqueous‐phase methods, even when accounting for differences in water content and physical properties of the porous media. The observations are consistent with proposed differences in interfacial accessibility of the aqueous‐ and gas‐phase tracers. Evaluation of the data in light of functional interfacial domains, described herein, yields the hypothesis that aqueous interfacial tracers measure primarily air‐water interfaces formed by “capillary water,” while gas‐phase tracers measure air‐water interfaces formed by both capillary and surface‐adsorbed (film) water. The gas‐ and aqueous‐phase methods may each provide interfacial area information that is more relevant to specific problems of interest. For example, gas‐phase interfacial area measurements may be most relevant to contaminant transport in unsaturated systems, where retention at the air‐water interface may be significant. Conversely, the aqueous‐phase methods may yield information with direct bearing on multiphase flow processes that are dominated by capillary‐phase behavior.
Drag force approach to the transport of colloids in unsaturated soils
Authors: Germann Peter F. , Alaoui Abdallah , Riesen Dagmar
Published: November 2002

A drag force approach to the transport of colloids in unsaturated soils is introduced. It is based on film flow which is due to momentum dissipation during porous media flow. Supporting results are from three types of colloid transport experiments: (1) sprinkling of a latex bead and methylene blue suspension in situ on the surface of a forest soil with subsequent bead collection underneath, (2) application of bead suspension to a column of undisturbed soil and purging of retained beads, and (3) application of a suspension with 5 bacteriophages to a different soil column. Relative colloid breakthrough concentrations in relation to the drag forces, P/P0, versus Fd(t)/A, were well pronounced during bead breakthrough, and bead diameters had no significant impact. In contrast, phage breakthrough showed a wide range of behavior. Previous bacteria breakthrough experiments support the approach; however, site‐specific soil properties should also be considered.
Macroscale water fluxes 1. Quantifying errors in the estimation of basin mean precipitation
Authors: Milly P. C. D. , Dunne K. A.
Published: November 2002

Developments in analysis and modeling of continental water and energy balances are hindered by the limited availability and quality of observational data. The lack of information on error characteristics of basin water supply is an especially serious limitation. Here we describe the development and testing of methods for quantifying several errors in basin mean precipitation, both in the long‐term mean and in the monthly and annual anomalies. To quantify errors in the long‐term mean, two error indices are developed and tested with positive results. The first provides an estimate of the variance of the spatial sampling error of long‐term basin mean precipitation obtained from a gauge network, in the absence of orographic effects; this estimate is obtained by use only of the gauge records. The second gives a simple estimate of the basin mean orographic bias as a function of the topographic structure of the basin and the locations of gauges therein. Neither index requires restrictive statistical assumptions (such as spatial homogeneity) about the precipitation process. Adjustments of precipitation for gauge bias and estimates of the adjustment errors are made by applying results of a previous study. Additionally, standard correlation‐based methods are applied for the quantification of spatial sampling errors in the estimation of monthly and annual values of basin mean precipitation. These methods also perform well, as indicated by network subsampling tests in densely gauged basins. The methods are developed and applied with data for 175 large (median area of 51,000 km2) river basins of the world for which contemporaneous, continuous (missing fewer than 2% of data values), long‐term (median record length of 54 years) river discharge records are also available. Spatial coverage of the resulting river basin data set is greatest in the middle latitudes, though many basins are located in the tropics and the high latitudes, and the data set spans the major climatic and vegetation zones of the world. This new data set can be applied in diagnostic and theoretical studies of water balance of large basins and in the evaluation of performance of global models of land water balance.
Macroscale water fluxes 2. Water and energy supply control of their interannual variability
Authors: Milly P. C. D. , Dunne K. A.
Published: November 2002

Controls on interannual variations in water and energy balances of large river basins (10,000 km2 and greater) are evaluated in the framework of the semiempirical relation / = [1 + (/)−ν]−1/ν in which and E, P, and R are basin mean values of annual evaporation, precipitation, and surface net radiation, respectively, expressed as equivalent evaporative water flux, overbars denote long‐term means, and ν is a parameter. Precipitation is interpolated from gauges; evaporation is taken as the difference between precipitation and runoff, with the latter determined from basin discharge measurements and a simple storage‐delay model; and radiation is based on a recent analysis in which 8 years of satellite observations were assimilated into radiative transfer models. Objective estimates of precipitation errors are considered; results suggest that past estimates of ν may have been biased by systematic errors in estimates of precipitation. Under the assumption that the semiempirical relation applies also to annual values, long‐term mean observations are sufficient to predict the sensitivity of annual runoff to fluctuations in precipitation or net radiation. Additionally, an apparent sensitivity of runoff to precipitation can be inferred from the observations by linear regression. This apparent sensitivity is generally in good agreement with the predicted sensitivity. In particular, the apparent sensitivity increases with decreasing basin /; however, slightly excessive apparent sensitivity (relative to the prediction) is found in humid basins of the middle latitudes. This finding suggests a negative correlation between precipitation and net radiation: the increase in runoff caused by a positive precipitation anomaly is amplified by an accompanying decrease in surface net radiation, possibly induced by increased cloud cover. The inferred sensitivity of radiation (water flux equivalent) to precipitation is on the order of −0.1. Such a value is supported by independent direct analysis of annual precipitation and radiation data. The fraction of interannual variance in runoff explained by the annual precipitation anomaly (including any correlative influence of net radiation) varies systematically with climatic aridity, approaching unity in humid basins and falling to 40–80% in very arid basins. We conclude that the influence of seasonality of the precipitation anomaly on annual runoff is negligible under humid conditions, though it may be significant under arid conditions.
Objective, observations‐based, automatic estimation of the catchment response timescale
Authors: Morin Efrat , Georgakakos Konstantine P. , Shamir Uri , Garti Rami , Enzel Yehouda
Published: November 2002

A new characteristic timescale of a catchment is presented, the response timescale (RTS). It is a range of averaging time intervals which, when applied to catchment rainfall, yield smoothed time series that best approximate that of the resultant runoff. In determining the RTS, nothing is assumed about the nature of the rainfall‐runoff transformation. In addition, this new measure is shown to be robust against measurement errors. An objective, automatic, observations‐based algorithm is described that introduces the concept of peaks density for the estimation of RTS. Estimation is exemplified for single and multiple rainfall‐runoff events through application to small catchments in Panama and Israel. In all cases, relatively stable values of response timescale are obtained. It is concluded that at least for the case studies, the response timescale is an intrinsic characteristic of the catchment and it is generally expected to be different from the catchment lag time and time of concentration.