bluuebunny/crossref_metadata_2025 huggingface dataset:
Isotopic evolution of snowmelt 1. A physically based one‐dimensional model
Authors: Feng Xiahong , Taylor Susan , Renshaw Carl E. , Kirchner James W.
Published: November 2002

The 18O/16O ratio of snowmelt from a seasonal snowpack typically increases with time as the melting process progresses. This temporal evolution is caused by isotopic exchange between liquid and ice as meltwater percolates down the snow column. Consequently, hydrograph separations of spring runoff using the bulk snow composition as the new water end‐member will be erroneous. Accurate determinations of the new water input should take into account the temporal variation of the snowmelt. Here we present a one‐dimensional (1‐D) physically based model for the isotopic evolution of snowmelt. Two parameters, the effective rate of isotopic exchange between water and ice and the ice to liquid ratio of the exchange system, are important for controlling the range and temporal pattern of the isotopic variation in snowmelt. For all plausible values of these parameters the modeled isotopic signature of snowmelt changes by 1–4‰ as snowmelt progresses. These isotopic shifts will affect the results of hydrograph separations.
Isotopic evolution of snowmelt 2. Verification and parameterization of a one‐dimensional model using laboratory experiments
Authors: Taylor Susan , Feng Xiahong , Renshaw Carl E. , Kirchner James W.
Published: November 2002

Three controlled cold room experiments were conducted to verify and parameterize a one‐dimensional (1‐D) model that simulates the isotopic composition of meltwater exiting the base of a snowpack. In the model, snow melts at the surface at a constant rate, and water percolates down the column while exchanging isotopically with ice. The effective rate of isotopic exchange and hence the isotopic composition of the melt at a given time is determined by the exchange rate constant kr, the height of the original snowpack, the percolation velocity u*, and the liquid to ice ratio in the exchange system. The experiments were designed to have different effective rates of exchange by varying the height of the snow column and the melt rate. Fitting the model to each of the experiments yielded kr values that fall in a narrow range, 0.14 to 0.17 hr−1, confirming that kr is an intrinsic rate constant for isotopic exchange. Knowing this value is important for developing future models, in which more complicated hydrological conditions are considered.
A similarity approach to predict landscape saturation in catchments
Authors: Aryal Santosh K. , O'Loughlin Emmett M. , Mein Russell G.
Published: November 2002

An investigation into landscape saturation has been carried out analytically using similarity parameters derived from topographic, soil, and climatic attributes. Three parameters (an input index (q/Z)(L/KS), convergence ratio (CR), and profile factor (B)) define the extent of hillslope saturation. From these, a single dimensionless parameter has been derived to describe the occurrence of saturation in planar, concave, and convex profile hillslope shapes, and with parallel, convergent, and divergent planforms. The relationship can be represented by a single dimensionless curve. The theory derived in this paper provides a basis for combining the individual parameters into a similarity criterion for saturation in landscapes. The theory and ensuing similarity hypothesis are applied to three natural catchments near Canberra, Australia, and are tested against observed rainfall‐runoff data. The results support the validity of the similarity approach. There is a clear link between the distribution of hillslope similarity parameters and catchment saturation behavior.
Downstream changes in the channel geometry of a large gravel bed river
Authors: Pitlick John , Cress Robert
Published: November 2002

Field data obtained on a nearly contiguous segment of the Colorado River in western Colorado and eastern Utah are used to examine the mechanisms driving downstream changes in channel geometry. Measurements characterizing the bank‐full hydraulic geometry, bed material grain size, and average channel gradient were made at closely spaced intervals in 10 alluvial and quasi‐alluvial reaches covering 260 km of the river. These data indicate that changes in surface and subsurface grain sizes are small in relation to the change in channel slope: over the full length of the study area, the median grain size of the surface sediment decreases by a factor of a little more than 2, whereas the average channel slope decreases by a factor of about 5. The decreases in slope and median grain size are offset by a large increase in bank‐full depth relative to width, such that the bank‐full Shields stress, τ*b, is constant downstream. For the reach as a whole, τ*b averages 0.049, which is roughly 50% higher than the threshold for bed load transport.
Analysis of rainfall variability using generalized linear models: A case study from the west of Ireland
Authors: Chandler Richard E. , Wheater Howard S.
Published: November 2002

In the early 1990s a cluster of extreme flood events occurred in the south Galway region of western Ireland, and this led to speculation of changing rainfall patterns in the area. In this paper we illustrate the use of generalized linear models (GLMs) to test for such changes and quantify their structure. GLMs, long established in the statistical literature, provide a flexible and rigorous formal framework within which to distinguish between possible climate change scenarios and are able to deal with high levels of variability, such as those typically associated with daily rainfall sequences. The study indicates that the GLM approach provides a powerful tool for interpreting historical rainfall records.
Dispersion of groundwater age in an alluvial aquifer system
Authors: Weissmann Gary S. , Zhang Y. , LaBolle Eric M. , Fogg Graham E.
Published: November 2002

Interpretation of groundwater ages typically rests on assumptions of minimal mixing of different water ages in the water samples. The effects of three‐dimensional, geologic heterogeneity on groundwater mixing and tracer concentrations, however, have not been evaluated. In this study, we use a series of 10 detailed geostatistical realizations along with high‐resolution numerical groundwater flow and contaminant transport simulation to model distributions of groundwater ages and chlorofluorocarbon (CFC) ages at wells within a heterogeneous stream‐dominated alluvial fan aquifer system. Results show that groundwater reaching a well in the heterogeneous aquifer system typically consists of a wide distribution of groundwater ages (often spanning >50 years), even over short (<1.5 m) screened intervals. Additionally, simulated arithmetic mean groundwater ages do not correspond to mean ages estimated from simulated CFC concentrations. Results emphasize the potential ambiguity of “mean” groundwater ages estimated from environmental tracer concentrations in typically heterogeneous geologic systems. The significant dispersion of groundwater ages also implies that ultimate, maximum effects of nonpoint source, anthropogenic contamination of groundwater may not be reached until after many decades or centuries of gradual decline in groundwater quality.
Physical pictures of transport in heterogeneous media: Advection‐dispersion, random‐walk, and fractional derivative formulations
Authors: Berkowitz Brian , Klafter Joseph , Metzler Ralf , Scher Harvey
Published: November 2002

The basic conceptual picture and theoretical basis for development of transport equations in porous media are examined. The general form of the governing equations is derived for conservative chemical transport in heterogeneous geological formations for single realizations and for ensemble averages of the domain. The application of these transport equations is focused on accounting for the appearance of non‐Fickian (anomalous) transport behavior. The general ensemble‐averaged transport equation is shown to be equivalent to a continuous time random walk (CTRW) and reduces to the conventional forms of the advection‐dispersion equation (ADE) under highly restrictive conditions. Fractional derivative formulations of the transport equations, both temporal and spatial, emerge as special cases of the CTRW. In particular, the use in this context of Lévy flights is critically examined. In order to determine chemical transport in field‐scale situations, the CTRW approach is generalized to nonstationary systems. We outline a practical numerical scheme, similar to those used with extended geological models, to account for the often important effects of unresolved heterogeneities.
Understanding the anomalous temperature data from the Large Block Test at Yucca Mountain, Nevada
Authors: Mukhopadhyay S. , Tsang Y. W.
Published: November 2002

The Large Block Test (LBT) at Yucca Mountain, Nevada, is unique because of its size, which is large enough to include a number of large fractures, while still small enough so that boundary conditions and heterogeneities can be adequately controlled or characterized. Extensive mapping of the test block has established the presence of many small and large fractures. Preheat air injection testing has also revealed that the block is highly heterogeneous, with fracture permeability varying more than four orders of magnitude. We hypothesize that these large fractures and the resulting heterogeneity play a significant role in the development of coupled thermal‐hydrological (TH) processes in the LBT. A large volume of TH data, including temperature and saturation measurements, has been collected from the LBT. Some of these temperature data from the LBT, particularly those recorded by sensors TT1‐14 and TT2‐14, appear anomalous. We show that these anomalous temperature data can be understood only if heterogeneity is invoked. As an example, we show that the heat pipe signature, an indication of the extent of TH coupling, in TT2‐14 is significantly longer in duration than that in TT1‐14. We show that such a difference can be explained only if one includes a high‐permeability layer, the existence of which has been indicated by air injection testing, through the location of sensor TT2‐14. Similarly, the oscillating temperature pattern in all sensors of borehole TT2 during 4470–4500 hours of heating is shown to be the result of rainwater flowing down a high‐permeability fracture. The irregular temperature pattern in borehole TT1 happens because of rainwater reaching the source of heat quickly through a highly permeable, inclined fracture and vapor rising upwards through a less permeable vertical fracture. We also demonstrate that the anomalous temperature pattern recorded by TT1‐14 during 2500–3500 hours of heating can be explained in terms of continuous upward movement of vapor and the downward flow of condensate through the fractures. We conclude that the temperature data from the LBT are the results of coupled TH processes occurring in a heterogeneous environment.
Analytical expressions for transient specific yield and shallow water table drainage
Authors: Nachabe Mahmood H.
Published: November 2002

New closed‐form expressions are introduced to capture the dependence of specific yield on time and depth to water table. The expressions allow the user to convert observations of water table fluctuations to volumes of water released from storage in a shallow water table aquifer. Whereas a linear relationship between water table fluctuations and released volumes holds for a deep water table aquifer, this relationship is nonlinear for shallow water table aquifers. The dependence of specific yield on time stems from the slow drainage of soil water from pores above the water table. The new expressions allow determination of transient specific yield and time to drain the soil water profile for a given water table fluctuation. If the time step in a numerical groundwater model is longer than the time for limiting specific yield, then a constant (time independent) specific yield can be justifiably adopted. The new expressions are easy to use and require knowledge of soil hydraulic properties which are readily available from soil water retention data and surveys.
Electrical‐hydraulic relationships observed for unconsolidated sediments
Authors: Slater Lee , Lesmes David P.
Published: November 2002

We use complex conductivity measurements to predict the hydraulic conductivity (K) of unconsolidated materials. The samples include natural sediments and artificial sand/clay mixtures. We apply the Börner et al. [1996] model, which is based on the Kozeny‐Carman equation and incorporates electrical estimates of formation factor (F) and specific surface area per unit volume‐to‐porosity ratio (Spor), from the real (σ′) and imaginary (σ″) conductivity components respectively. We find that K correlates with σ″ but shows no correlation with F, which we attribute to the wide range in grain size for these materials. The Börner model appears primarily dependent on the K ‐ σ″ relation. The relationship between σ″ and Spor is nonlinear and appears to depend upon material type. Further examination shows that σ″ is well correlated with effective grain size (d10) and is relatively independent of the material type. We propose a simple Hazen‐type equation in which the effective grain size is estimated from σ″. This simple model provides order of magnitude estimates of K for a range of unconsolidated sediments.
Variational inverse parameter estimation in a long‐term tidal transport model
Authors: Yang Zhaoqing , Hamrick John M.
Published: November 2002

A variational data assimilation scheme for parameter estimation is developed and tested with a long‐term tidal transport model. Long‐term advective transport in a tidal environment is represented by the Lagrangian mean transport velocity that can be decomposed into two parts: the Eulerian transport velocity and the curl of a three‐dimensional vector potential A. In the present study, the vector potential A is treated as a poorly known parameter field, and the optimal long‐term advection transport field is obtained through adjusting the vector potential using a variational inverse data assimilation method to obtain the best fit between the model output and the data. Experiments were performed in an idealized estuary. Results showed that variational inverse data assimilation could successfully retrieve poorly known parameters in a long‐term tidal transport model. The results also showed that the smooth best fit model state could be retrieved using a penalty method even when observed data are too sparse or contain noisy signals.
On the spatial variation of resistance to flow in upland channel networks
Authors: Orlandini Stefano
Published: November 2002

Detailed field measurements of channel properties and flow characteristics collected in the Ashley and Cropp catchments (New Zealand) are used to investigate the spatial variation of resistance to flow across upland channel networks. The application of the Darcy‐Weisbach equation and semilogarithmic flow resistance relationships reveals that mean flow velocities calculated from local measurements of bed material particle size, hydraulic depth, and channel bed slope may be inaccurate. The Manning‐Gauckler‐Strickler equation with resistance coefficient independent of bed material particle size is found to be relatively more reliable but not sufficiently general to reproduce the spatial variation of resistance to flow across a complex channel network. A new methodology is developed by combining a hydraulic equation of the Manning‐Gauckler‐Strickler type, a flow discharge‐upstream drainage area relationship, and geomorphological fluvial relationships for mean flow velocity, Gauckler‐Strickler resistance coefficient, hydraulic depth, and friction slope. This methodology is found to improve the reproduction of the spatial variation of mean flow velocity across the Ashley catchment and appears of general applicability for the parameterization of resistance to flow in distributed catchment models.
Theoretical prediction of poroelastic properties of argillaceous rocks from in situ specific storage coefficient
Authors: Cosenza P. , Ghoreychi M. , de Marsily G. , Vasseur G. , Violette S.
Published: November 2002

An integrated and theoretical approach is proposed to determine poroelastic properties of isotropic or transversely isotropic argillaceous rocks from conventional mechanical and hydraulic field tests. It uses values of specific storage coefficient, “undrained” elastic parameters, and porosity. No special hydromechanical test is required. In the isotropic case, a simple binomial equation is derived, whereby the poroelastic properties can be rapidly obtained. An application to deep argillaceous formations has shown that calculated poroelastic property values are in satisfactory agreement with direct experimental measurements. In the transversely isotropic case, which is more representative of sedimentary rocks, the approach is generalized from a theoretical relationship between the specific storage coefficient and the anisotropic poroelastic properties. Nevertheless, in both cases, sensitivity analyses have shown that the method is very sensitive to input data, especially the specific storage coefficient and the “undrained” mechanical parameters, which have to be measured with accuracy.
Saturation‐matric potential relations in gravel
Authors: Tokunaga Tetsu K. , Wan Jiamin , Olson Keith R.
Published: November 2002

Some environmentally sensitive unsaturated zone sediments, such as those underlying radioactive waste tanks in Hanford (Washington State), contain large fractions of gravels and coarse sands. Coarse, granular media are also included in designs of engineered capillary barriers for subsurface waste isolation. Thus knowledge of the unsaturated hydraulic properties of gravels is needed to understand flow and transport in these critical settings. When standard methods for measuring moisture characteristics or water retention relations are used for gravels, corrections are needed in the near‐zero region of matric (pressure) potentials. The need for correction results from gravity stratification of saturation profiles within even short sample columns. Such a correction method was developed and used to determine drainage curves for Hanford gravels having characteristic grain sizes of 8.0–9.5, 4.8–5.3, and 2.0–2.4 mm. In 30 mm tall sample columns, gravity corrections were essential for the 9 and 5 mm gravels and less significant for the 2 mm gravel. Validity of the correction method was demonstrated through accurately reconstructing average column saturation‐potential relations from their predicted local saturation‐potential relations. The method and results presented here are part of an ongoing study on Hanford gravels and on limits to classical unsaturated hydraulic scaling encountered at large grain sizes.
Inorganic carbon in the Indian Ocean: Distribution and dissolution processes
Authors: Sabine Christopher L. , Key Robert M. , Feely Richard A. , Greeley Dana
Published: November 2002

This study uses nearly 25,000 carbon measurements from the WOCE/JGOFS global CO2 survey to examine the distribution of dissolved inorganic carbon (DIC) and total alkalinity (TA) in the Indian Ocean. Shallow and intermediate distributions of inorganic carbon do not strictly follow temperature and salinity because of differing surface gradients and vertical biological processes that work to modify the circulation derived features. Anthropogenic CO2 has increased the shallow DIC by as much as 3%, decreasing the vertical DIC gradient. Deep ocean DIC and TA increase toward the north because of the decomposition and dissolution of organic and inorganic particles. Calcite saturation depths range from 2900–3900 m with the deepest saturation depth in the central Indian Ocean. Variations of aragonite saturation depth (200–1400 m) are similar to calcite, but the deepest saturations are in the southwestern Indian Ocean. The shallowest aragonite saturation depths are found in the Bay of Bengal. In the northern Arabian Sea and Bay of Bengal, the current aragonite saturations are 100 and 200 m shallower, respectively, than in preindustrial times. Estimates of carbonate dissolution rates on isopycnal surfaces range from 0.017 to 0.083 μmol kg−1 yr−1 in deep waters. Upper water column dissolution rates range from 0 to 0.73 μmol kg−1 yr−1, with a local maximum occurring in intermediate waters just below the aragonite saturation horizon. Dissolution is also generally higher north of the Chemical Front at 10–20°S. There is some evidence for significant sedimentary sources in the northern Indian Ocean.
Hafnium isotopes in basalts from the southern Mid‐Atlantic Ridge from 40°S to 55°S: Discovery and Shona plume–ridge interactions and the role of recycled sediments
Authors: Andres Magdalena , Blichert‐Toft Janne , Schilling Jean‐Guy
Published: November 2002

Using high‐precision multiple collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS), 176Hf/177Hf was measured in 64 mid‐ocean ridge basalts (MORB) from the Mid‐Atlantic Ridge (MAR) between 40°S and 55°S and in one basalt from the Discovery Tablemount. εHf of these basalts range from −2.8 to +16.6. The low end of this range exceeds that previously reported for MORB by 3 epsilon units and is only 5 epsilon units higher than the lowest reported ocean island basalt (OIB) value. Besides showing evidence for large‐scale DUPAL‐like pollution, the South Atlantic depleted upper mantle is also a region of localized plume–ridge interactions. 176Hf/177Hf from this study confirms the interaction of the Discovery and Shona plumes with the MAR as has been proposed previously on the basis of Pb, Sr, and Nd isotope ratios in these basalts. ΔεHf deviations from the global εHf versus εNd mantle array show that Shona anomaly basalts tend to fall below the mantle array while most Discovery and LOMU anomaly basalts fall above the array. 176Hf/177Hf versus 206Pb/204Pb for basalts from the Shona anomaly indicate that the ridge‐centered heterogeneous Shona plume is mixing with the ambient upper mantle, which is locally fairly homogeneous. The Shona anomaly basalts can be modeled as three‐component mixtures of ambient depleted upper mantle (ADM) with a recycled plume component consisting of old oceanic crust (ca. 1.5 Ga) and varying proportions of ancient sediment. The proportion of sediment present in the recycled Shona plume component increases stepwise from north to south along the MAR, but calculated sediment fractions are model dependent. In contrast, according to this three‐component model, ancient sediment is probably not a significant component in the intraplate Discovery plume. Delaminated continental material, either from the subcontinental lithosphere or from the lower crust, may be present as the EM1 or LOMU‐type component in the Discovery plume. In addition, both Shona and Discovery plumes likely contain a primitive rare gas component entrained from the lower mantle in order to account for the high 3He/4He and the solar neon isotope compositions of these MORB.
Decadal‐centennial scale monsoon variations in the Arabian Sea during the Early Holocene
Authors: Jung S. J. A. , Davies G. R. , Ganssen G. , Kroon D.
Published: November 2002

An essential prerequisite for the prediction of future climate change due to anthropogenic input is an understanding of the natural processes that control Earth's climate on timescales comparable to human‐lifespan. The Early Holocene period was chosen to study the natural climate variability in a warm interval when solar insolation was at its maximum. The monsoonal system of the Tropics is highly sensitive to seasonal variations in solar insolation, and consequently marine sediments from the region are a potential monitor of past climate change. Here we show that during the Early Holocene period rapid δ18O‐changes of the summer (upwelling) dwelling foraminifer G. bulloides of up to ∼0.6% in Core 905 from the Arabian Sea occurred on decadal‐centennial timescales. This δ18O‐change predominantly translates into summer temperature variations of roughly 2–3°C. Within the resolution of the AMS14C‐dating, the isotope changes occur in phase with precipitation‐induced δ18O‐variations recorded in a stalagmite in the Hoti Cave in Oman [Neff et al., 2001]. From this relationship we conclude that solar insolation affecting the monsoonal system not only induced the precipitation changes as recorded in Oman (in the sense of Neff et al. [2001]) but also controlled temperature variations in the upwelled waters found off Somalia. The present study demonstrates that decadal‐scale climate records can be obtained from nonlaminated ocean sediment records and that coherent rapid high‐amplitude climate change is recorded in the Arabian Sea on a regional scale of >2000 km.
Volcanic evolution in the Galápagos: The dissected shield of Volcan Ecuador
Authors: Geist Dennis , White William M. , Albarede Francis , Harpp Karen , Reynolds Robert , Blichert‐Toft Janne , Kurz Mark D.
Published: November 2002

Volcan Ecuador, a young, active shield volcano at the northwest tip of Isabela Island, Galápagos, experienced at least one sector collapse event that removed its western half. The compositions of lavas exposed in the old caldera wall and fault scarps dissecting the outer shield together with young post‐collapse lavas provide insights into the history of this volcano. 40Ar/39Ar and cosmogenic 3He dating reveal that sector collapse occurred at <100 Ka. Prior to sector collapse most magma erupted from circumferential dikes that fed the summit carapace and other dikes that fed vents on the caldera floor. Almost all of the erupted magmas cooled and underwent fractional crystallization in the lower crust or upper mantle, where clinopyroxene crystallization dominated. Magmas then rose to a small (∼0.1 km3), steady state, subcaldera magma chamber where plagioclase, along with lesser amounts of olivine and clinopyroxene, crystallized. Sector collapse perturbed the plumbing system in a way that now allows some magmas, particularly those erupted on the East Rift, to bypass the magma chamber. Sector collapse also opened new pathways to the surface that enabled magma to erupt at low elevation, effectively cutting the magma supply to the summit. The paucity of summit eruptions over the past 100,000 years accounts for the old, eroded appearance of the shield. Subtle variations in isotopic and trace element composition occurred over the past 100,000 years, with the net change being toward slightly more depleted compositions. These variations most likely reflect small‐scale (<5 km) heterogeneity superimposed on more regular regional patterns of mantle composition beneath the Galápagos. Overall, however, the compositional variation at Volcan Ecuador has been minimal, suggesting that, unlike Hawaii, the compositions of magmas do not substantially change while the volcano is in its shield‐building phase.
V‐shaped ridges around Iceland: Implications for spatial and temporal patterns of mantle convection
Authors: Jones Stephen M. , White Nicky , Maclennan John
Published: November 2002

V‐shaped lineations in the bathymetry and in the free‐air gravity field surrounding Iceland result from crustal thickness variations caused by temporal variations in melt production rate at the Mid‐Atlantic Ridge. We have studied the record of V‐shaped ridges in the basins surrounding Iceland by plotting the short‐wavelength component of the gravity field in terms of age versus distance from Iceland. The V‐shaped ridge gravity signal is obscured by crustal segmentation and by sediment more than 1–2 km thick. The best V‐shaped ridge record is found in the unsegmented part of the Irminger Basin, where Oligocene‐Recent V‐shaped ridges occur with a primary periodicity of 5–6 Myr and a secondary periodicity of 2–3 Myr. V‐shaped ridge records from the Iceland Basin and from east of the Kolbeinsey Ridge to the north of Iceland correlate with the record from the Irminger Basin but are less complete. A record of uplift of the Greenland‐Iceland‐Faroes Ridge based on paleoceanographic data is correlated with the gravity record of V‐shaped ridges. There is less decisive evidence for V‐shaped ridges in crust of Eocene age. The observation that V‐shaped ridges propagate up to 1000 km from Iceland is compatible with a model in which the Iceland Plume head spreads out from the plume stalk below a depth of ∼100 km, as suggested by geochemical arguments and studies of mantle rheology. Time‐dependent flow in the plume head probably results from time‐dependent flow up the plume stalk from deep below Iceland. These pulses may have triggered jumps in location of the spreading axis observed in the Icelandic geological record.
Coherence of tropospheric 14CO2 with El Niño/Southern Oscillation
Authors: Dutta Koushik
Published: November 2002

The El Niño/Southern Oscillation (ENSO) events cause significant perturbation in the global carbon cycle, due to reduced upwelling in the equatorial Pacific and enhanced release of biospheric CO2 from tropical forests. Both these effects tend to enrich the atmospheric CO2 in radiocarbon (14C). To examine the relationship of interannual variations of 14C in the atmospheric CO2 with the ENSO events, statistical coherence between tropospheric 14CO2 with various ENSO indices are analyzed here. The warm ENSO phases found to correlate with positive tropospheric 14CO2 anomalies.
A 250‐year cycle in naked‐eye observations of sunspots
Authors: Vaquero J. M. , Gallego M. C. , García J. A.
Published: November 2002

The state of the sun strongly affects terrestrial phenomena. While the eleven‐year solar cycle was recognized in the XIX century, the lack of direct data concerning our star in historical times has made it difficult to determine the characteristics of solar variability on a secular scale. We here construct a series of yearly sunspot numbers as observed by eye in remote times and recorded in historical documents covering the period 165 BC–1918 AD. Using different spectral analysis methods, we find a 250‐year cycle which appears to have a solar origin.
Comment on “A new estimate for present‐day Cocos‐Caribbean plate motion: Implications for slip along the Central American volcanic arc” by Charles DeMets
Authors: Guzmán‐Speziale Marco , Gómez Juan Martín
Published: November 2002

We comment on “A new estimate for present‐day Cocos‐Caribbean plate motion: Implications for slip along the Central American volcanic arc” by Charles DeMets. We find the following inconsistencies in his model: Components of relative motion along the arc are small and variable, not uniform. There is no single surface faulting and earthquakes occur on faults along and perpendicular to the arc. Earthquakes also stop in the middle of the arc. Geometrically, the model calls for buttressing, but there is no evidence for this.
Remnant of the ancient Farallon Plate breakup: A low‐velocity body in the lower oceanic crust off Nicoya Peninsula, Costa Rica ‐ evidence from wide‐angle seismics
Authors: Walther Christian , Flueh Ernst
Published: November 2002

A seismic wide‐angle section offshore Costa Rica is presented across the boundary between oceanic crust generated at the East Pacific rise (EPR) and at the Galápagos spreading center (GSC) as indicated by magnetic anomalies. This suture, where the Farallon plate broke up ∼23 Ma ago, is marked by pronounced velocity variations throughout the crust including a low‐velocity body in the lower crust. This body is well constrained by refracted waves above the inversion zone and by strong PmP reflections from its lower boundary. The distinctness of this body and the local gravity field point to an igneous intrusion rather than serpentinized rock. Typical oceanic crust is found adjacent to the suture zone.
ENA observations of a global substorm growthphase dropout in the nightside magnetosphere
Authors: C:son Brandt P. , Ohtani S. , Mitchell D. G. , Demajistre R. , Roelof E. C.
Published: November 2002

We report a global decrease of the energetic neutral atom (ENA) flux coming from the entire nightside magnetosphere detectable out to dipole L = 14 during the growth phase of a substorm at 08:00–09:09 UT on 4 October 2000. The ENA images were obtained in the 27–60 keV range by the high energy neutral atom (HENA) imager onboard the IMAGE spacecraft. The underlying proton distribution is derived by inverting the ENA images using a constrained linear inversion technique. The magnetosphere experienced enhanced convection and the geomagnetic field at geosynchronous was stretched. The global ENA flux decrease is found to be related to the rapid stretching of the tail inducing a dusk to dawn electric field. The phenomenon can be explained either as a resulting “choking” of the E × B drift from the tail, or, as an adiabatic de‐energization of the azimuthally drifting ring current protons.
Paleoclimate studies of minerogenic sediments using annually resolved textural parameters
Authors: Francus Pierre , Bradley Raymond S. , Abbott Mark B. , Patridge Whit , Keimig Frank
Published: November 2002

We obtained quantitative multivariate data from each varve of a minerogenic lacustrine sequence from the Canadian High Arctic, using an image analysis technique applied to thin‐sections. The information on each varve from the uppermost core section was compared with a 35 yr meteorological dataset. Snowmelt intensity, which is an index reflecting the energy available for sediment transport, correlates well with the median grain‐size measured for each varve, as well as with the weight of the 10–20 and 20–60 μm fractions. The proportion of fine silt also correlates with low intensity summer precipitation. This methodology allows us to decipher the climatic control on sedimentary processes and yield a new perspective for constructing models which link climate to sediments that contain few biological remains. This model can then be used to infer paleoclimate with annual resolution from the downcore grain‐size analysis.
Density modulated whistler mode emissions observed near the plasmapause
Authors: Moullard O. , Masson A. , Laakso H. , Parrot M. , Décréau P. , Santolik O. , Andre M.
Published: November 2002

Electron density fluctuations are regularly observed near the plasmapause together with electromagnetic waves below the electron cyclotron frequency (usually called hiss or chorus). Instruments on board CLUSTER spacecraft often observe two such emission bands with fluctuating wave intensities that suggest wave ducting in density enhancements as well as troughs. Near perigee the CLUSTER density measurements are usually limited to the electron density from 0.2 to 80 cm−3. To establish a correlation between density and wave intensity deeper inside the outer plasmasphere, we extrapolate the electron density from the spacecraft potential after fitting a relationship between observed plasma frequency and spacecraft potential. During a plasmapause crossing on June 5, 2001 (near the geomagnetic equator, L = 4 − 6, afternoon sector), density fluctuations up to hundreds cm−3 are found while whistler mode waves are observed in two separate frequency bands, at 100–500 Hz (correlated to the density fluctuations) and 3–6 kHz (anti‐correlated).
Holographic 3‐D laser imaging into sandstone
Authors: Yu P. , Mustata M. , Chen D. , Pyrak‐Nolte L. J. , Nolte D. D.
Published: November 2002

With a novel imaging technique that uses holographic recording and reconstruction to perform laser‐ranging into translucent media, we have imaged several grain layers into a sandstone sample. Three‐dimensional microscale (10 micron lateral and 30 micron depth resolution) information on grain size and geometry was obtained as deep as 400 microns into the sample without thin‐sectioning. Surficial grain features in the holographic reconstruction were associated with features observed using conventional optical and scanning electron microscopy. This imaging technique represents a new form of nondestructive evaluation of grain and pore structure in reservoir rock by acquiring full‐field images at variable depths, unlike sequential scanning microscopy that requires tomographic reconstruction.
Crustal Structure and Moho Geometry beneath the Northern Apennines (Italy)
Authors: Piana Agostinetti N. , Lucente F. P. , Selvaggi G. , Di Bona M.
Published: November 2002

We used receiver functions (RFs) from broad‐band seismic stations to investigate the crustal structure of the Northern Apennines, Italy. Additionally, we use data obtained in this study to provide initial constraints for a map of the Moho depth of Italy. Ten stations were deployed along a transect [N75°E] during the 1994 GeoModAp project. RF analysis shows the presence of lateral variations in the crust. We observed patterns of symmetric and anti‐symmetric converted phases from radial and tangential RFs vs. back‐azimuth. These patterns can be explained by the presence of dipping interfaces and/or anisotropy within the crust. We then inverted RFs following the inversion scheme proposed by Sambridge [1999]. The results show the presence of S‐velocity inversions in the lower crust beneath the Apennines, the upwelling of Moho in the Tyrrhenian area and its progressive deepening from the Tyrrhenian Sea toward the Adriatic coast.
Solar‐reflectance‐based calibration of spectral radiometers
Authors: Cattrall Christopher , Carder Kendall L. , Thome Kurtis J. , Gordon Howard R.
Published: November 2002

A method by which to calibrate a spectral radiometer using the sun as the illumination source is discussed. The procedure is simple, requiring only a calibrated reflectance panel, relatively low aerosol optical depth, and accurate measurements of atmospheric transmittance. Solar‐based calibrations eliminate several uncertainties associated with applying a lamp‐based calibration to field measurements and may be performed with a comparable uncertainty. A solar‐reflectance‐based calibration (SRBC), by removing the need for extraterrestrial irradiance spectra, further reduces calibration uncertainty to approximately 2.2%. This calibration may be applied to radiometric measurements of any system illuminated by the sun, such as the atmosphere or ocean, with associated reduction of uncertainty in properties derived from such measurements (e.g., aerosol single_scattering albedo). The procedure is ideal for on_site calibration of long_term field instruments, mitigating the logistics and costs associated with transport to a calibration facility.
Increase of total alkalinity due to shoaling of aragonite saturation horizon in the Pacific and Indian Oceans: Influence of anthropogenic carbon inputs
Authors: Sarma V. V. S. S. , Ono Tsuneo , Saino Toshiro
Published: November 2002

Aragonite saturation horizon (ASH) shallowed significantly by 25 to 155 m and 16 to 124 m in the Pacific and Indian Ocean respectively in two decades. Apparent oxygen utilization (AOU) increased by 3 to 34 and 0.5 to 31.5 μmol kg−1 in the Pacific and Indian Ocean respectively at the depth of ASH during this period. DIC increased by 12.5 to 36.8 and 5.5 to 32 μmol kg−1 in the vicinity of ASH in the Pacific and Indian Ocean respectively due to combined effect of increased anthropogenic CO2 and change in AOU. TA increased significantly by 5 to 10 and 4 to 9.2 μmol kg−1 in the Pacific and Indian Oceans respectively at the ASH most likely as a result of aragonite dissolution. The upward migration of ASH solely due to anthropogenic CO2 amounted to 6 to 58 m in the Pacific and 4 to 44 m in the Indian Ocean.
Global ENA observations of the storm mainphase ring current: Implications for skewed electric fields in the inner magnetosphere
Authors: C:son Brandt P. , Ohtani S. , Mitchell D. G. , Fok M.‐C. , Roelof E. C. , Demajistre R.
Published: November 2002

The local time distribution of the ring current in the 27–119 keV range during several geomagnetic storm main phases have been investigated. The data was obtained by the high energy neutral atom (HENA) imager onboard IMAGE. Global proton distributions are derived from the observed energetic neutral atom (ENA) images using a linear inversion technique. During storms with low IMF Bythe peak of the proton distribution is around 01 MLT. For storms with high IMF Bythe peak can rotate to dawn. The rotation angle depends on solar wind velocity and interplanetary magnetic field (IMF) By, but less on IMF Bz. We discuss how this morphology implies the existence of strong and skewed equatorial electric fields in the inner magnetosphere. Our results are consistent with in‐situ ring current measurements, radar observations and with kinetic models that self‐consistently calculate the electric field produced by the closure of the partial ring current.
Modeling the 1100 bp paleotsunami in Puget Sound, Washington
Authors: Koshimura Shunichi , Mofjeld Harold O. , González Frank I. , Moore Andrew L.
Published: November 2002

Recent paleoseismic and geologic studies indicate that a tsunami occurred 1100 years ago in Puget Sound. This study aims to reproduce and validate the model tsunami, using a finite difference model based on nonlinear shallow water theory and abathymetry/topography computational grid that takes into account the 1‐meter rise in sea level that has occurred in the region during the past 1100 years. Estimates of tsunami height, the extent of inundation, and the current velocity pattern are provided at the northern head of Cultus Bay, Whidbey Island, where paleotsunami sand deposits have been found. The model demonstrates that a tsunami generated when the background water level was at mean high water or above could have surged across the then‐existing coastal marsh, penetrated the full length of the shallow bay, and deposited the observed sand layers.
Dust and sea salt variability in central East Antarctica (Dome C) over the last 45 kyrs and its implications for southern high‐latitude climate
Authors: Röthlisberger Regine , Mulvaney Robert , Wolff Eric W. , Hutterli Manuel A. , Bigler Matthias , Sommer Stefan , Jouzel Jean
Published: November 2002

A detailed record of non‐sea‐salt calcium, a proxy for dust, and sea‐salt sodium, a proxy for sea salt, covering the last 45 kyr is presented. It shows that in the first part of the transition from the last glacial period to the Holocene (18–15 kyr BP), the changes in dust flux mainly reflect changes at the dust source, namely vegetation cover and local climate. The changes in the later part of the transition (12–11 kyr BP) are similar in extent to the changes seen in sea salt and most likely reflect a reorganization of the atmospheric circulation. During the last glacial period, considerable variation of dust but not of sea salt is observed, pointing to climatic changes in Patagonia, the main dust source for Dome C. A comparison of the glacial records from Dome C and Taylor Dome suggests that similar influences controlled aerosol input at both sites during this period.
Overview of mirror mode fluctuations in the jovian dusk magnetosheath: Cassini magnetometer observations
Authors: André N. , Erdös G. , Dougherty M.
Published: November 2002

The recent Cassini flyby of Jupiter provides a new unique data set in order to allow tracking of the instability along flow lines down to the distant magnetosheath, as yet unexplored by previous missions. Signatures consistent with mirror mode activity were frequently observed during the flyby in the magnetometer data, appearing at many different locations in the jovian pre‐ and post‐dusk magnetosheath. Also reported and analysed were very distant mirror modes waves appearing as magnetic depressions observed at hundreds of Rj from Jupiter and much lower in amplitude than those observed closer to the planet. If these structures were convected from the upstream subsolar regions, such distant observations could allow an assessment of how highly stable they are. However, since some of the structures appear in sequence, they could also have been produced locally by a phenomenon yet to be identified.
Seismic evidence of longitudinal extension in the Southern Apennines chain (Italy): The 1997–1998 Sannio‐Matese seismic sequence
Authors: Milano G. , Ventura G. , Di Giovambattista R.
Published: November 2002

We present evidence of longitudinal extension in the Apennines based on data from the 1997–1998 Sannio‐Matese seismic sequence. The earthquakes occurred between the NW‐SE faults responsible for larger historical earthquakes. Inversion of focal mechanisms indicates a stress field with a prevailing normal stress characterized by a NNW–SSE sub‐horizontal σ3. This stress regime is consistent with normal movements along NE‐SW faults. These features suggest that the NE‐SW faults located in between the NW‐SE faults cannot be interpreted as transfer faults or old faults reactivated during the eastward migration of the chain. We propose that the deformation due to the eastward chain migration concentrates along NW‐SE normal faults, which are responsible for the largest earthquakes, whereas the deformation related to the chain curvature concentrates along NE‐SW normal faults, which are responsible for the lower energy seismic sequences.
Nitric acid concentrations near the tropical tropopause: Implications for the properties of tropical nitric acid trihydrate clouds
Authors: Jensen Eric , Drdla Katja
Published: November 2002

In situ measurements of NOy, NOx, and temperature confirm that nitric acid trihydrate (NAT) particles could form at the tropical tropopause. The HNO3 mixing ratio near the tropical tropopause is typically no larger than about 0.2–0.3 ppbv, and the corresponding equilibrium mass of nitric acid trihydrate (NAT) is no larger than 0.3 μg m−3. Considerably larger NAT condensed masses are required to explain the HALOE extinctions; however, localized regions of enhanced HNO3 produced by oxidation of lightning‐generated NO might exist. NAT layers would only be identified as clouds by SAGE II if the particle diameters are in the optimum range of about 0.6 to 2 μm and the condensed NAT mass is larger than about 0.2 μg m−3. The SAGE II extinction ratio measurements (0.5 μm/1.0 μm) cannot distinguish NAT clouds from mixtures of optically thin ice clouds and background aerosols.
A new SuperDARN meteor wind measurement: Raw time series analysis method and its application to mesopause region dynamics
Authors: Yukimatu Akira Sessai , Tsutsumi Masaki
Published: November 2002

SuperDARN meteor wind measurements in the mesopause region using the standard analysis of ACFs constructed from the multi‐pulse sequence have several shortcomings. To overcome them, we have developed a new method to enable raw time series analysis without degrading normal SuperDARN ACF observations. The unevenly sampled raw time series is processed to detect only underdense meteor echoes to deduce line‐of‐sight neutral wind velocities and decay time constants. We installed this new code at the SENSU Syowa radar in Antarctica. Distributions of underdense meteor echoes show clear and expected dependency on distance from the radar, local time, and diffusion coefficient. By using diffusion coefficients as height information, we successfully obtained the structure of a downward phase propagating semi‐diurnal tide, which has not previously been achieved by analysing normal SuperDARN ACF data.