bluuebunny/crossref_metadata_2025 huggingface dataset:
Three‐dimensional groundwater flow near narrow surface water bodies
Authors: Meigs Lucy C. , Bahr Jean M.
Published: February 2004

A series of natural gradient tracer tests was conducted to delineate groundwater flow patterns near a drainage ditch in glacial outwash of central Wisconsin. Water level and precipitation data were collected to document factors that could contribute to temporal variations in the flow field. Year‐to‐year variations in recharge generated shifts in flow paths between replicate tracer experiments. The field data provided the basis for numerical modeling designed to elucidate the sensitivity of flow paths to variations in recharge. A three‐dimensional aspect of flow paths identified in this study is a “wrap‐around” pattern, in which groundwater passes under a surface water body prior to turning in the downstream direction and continuing to migrate as subsurface flow. This feature was suggested by the path of a tracer injected deep in the aquifer and was simulated at all recharge rates employed in the modeling. The three‐dimensional nature of the flow field and temporal variability of flow paths can have important implications for design of monitoring networks near narrow surface water bodies.
Chlorine 36 in Great Basin Waters: Revisited
Authors: Phillips Fred M. , Rogers David B. , Dreiss Shirley J. , Jannik Nancy O. , Elmore David
Published: February 2004

Inspired by recently published chloride balance chronologies of closed‐basin lakes in the Great Basin, researchers of the early 1960s attempted to estimate residence times of chloride in these lakes using 36Cl. Unfortunately, the analytical methods of the period were not capable of measuring the 36Cl levels found in these waters. About 20 years after the early research, advances in accelerator mass spectrometry permitted 36Cl measurement at the required sensitivity. In this study we follow up on those pioneering efforts by remeasuring and reevaluating the 36Cl content at several of the previously studied sites, focusing on Mono Lake in eastern California. Our data show that in general the streams in the region have 36Cl/Cl ratios similar to those expected in present‐day atmospheric fallout, but that the terminal lakes into which the streams flow have much lower ratios. These lower ratios could result from either a very long (>1 million years) residence time of the chloride in the basin sinks or from subsurface influx of low‐36Cl chloride. In the case of Mono Lake, a mass balance model based on the 36Cl data and on independent estimates of chloride fluxes and reservoirs indicates major subsurface chloride input, presumably from volcanic sources, and an accumulation time in the range of 100–450 kyr. The upper bound of this range is similar to the timing of a shift from long‐term humid to arid climate in the region and may indicate that hydrological closure of the basin was triggered by this event.
Pebble Counts and The Role of User‐Dependent Bias in Documenting Sediment Size Distributions
Authors: Marcus W. Andrew , Ladd Scott C. , Stoughton Julie A. , Stock Joseph W.
Published: February 2004

Replicate subaqueous pebble counts show that measurement and selection bias can significantly affect sediment size distribution data. The standard deviation about replicate means increases linearly with sediment size and is independent of the sediment percentile (e.g., d50, d84, etc.). For samples collected by one person, sampling error varies from 2 ± 2.6 mm to 300 ± 35.4 mm within one standard deviation. For samples collected by different individuals, sampling bias increases the error from 2 ± 4.2 mm to 300 ± 63.8 mm. Whenever possible, only one individual should select and measure pebbles, which improves the ability of the pebble count to delineate trends between sites or over time (although the absolute size values may be in error due to bias). Bias can be reduced by rigorous training of individuals, by using templates to measure sediment size, and by using grids on the stream bottom to select particles. If two or more individuals collect samples, they should conduct replicate samples to provide a basis for later calibration of results.
Stochastic‐Convective Transport with Nonlinear Reaction: Mathematical Framework
Authors: Simmons C. S. , Ginn T. R. , Wood B. D.
Published: February 2004

A stochastic‐convective reactive (SCR) transport method is developed for one‐dimensional steady transport in physically heterogeneous media with nonlinear degradation. The method is free of perturbation amplitude limitations and circumvents the difficulty of scale dependence of phenomenological parameters by avoiding volume‐averaged specifications of diffusive/dispersive fluxes. The transport system is conceptualized as an ensemble of independent convective‐reactive streamlines, each characterized by a randomized convective velocity (or travel time). Dispersive effects are treated as a component of the randomness in the streamline velocity ensemble, so no explicit expression for hydrodynamic dispersive flux is written in the streamline transport equation. The expected value of the transport over the stream tube ensemble is obtained as an average of solutions to the reactive convection equation according to the stream tube (travel time) probability distribution function. In this way, transport with reaction can be expressed in terms of global‐scale random variables, such as solute travel time and travel distance, which are integrals of the stochastic variables such as velocity. Derivations support the hypothesis that via the SCR the decay process can be factored out of the mechanical transport behavior (as reflected by movement of a passive tracer) and scaled independently. Solution strategies are presented for general linear and nonlinear kinetic reactions. Demonstration simulations show that for Fickian transport with nonlinear reactions the SCR and convection dispersion equation can give different results. Ginn et al. (this issue) extend the SCR solution to coupled nonlinear equations, to accommodate Michaelis‐Menten biodegradation of solute with an accounting of microbial growth.
The SAWAH Riceland Hydrology Model
Authors: ten Berge H. F. M. , Metselaar K. , Jansen M. J. W. , de San Agustin E. M. , Woodhead T.
Published: February 2004

The paper presents a robust Darcy‐based one‐dimensional soil water balance model (SAWAH) for simulating riceland hydrology. Experimental data from an upland rice toposequence showing highly variable groundwater and volumetric soil water contentsθare used to calibrate the model and to validate it by a modified cross‐validation method. Analysis of variance is applied to estimate the mean squared prediction error and to split it into errors in predicting horizontal, vertical, and temporal gradients ofθ. Root mean squared error of prediction (RMSEP) of soil water content at strip‐depth‐time points was about equal to the standard deviation of unreplicated measurements. Measured strip and field means were more accurate than predicted means, but RMSEP remained below 0.03 m3m−3. Ranking of crop and soil parameters for their effects on root zone water content and seasonal flux totals depended strongly on the imposed hydrology regime. Capillary rise contributed 5–55% of seasonal evapotranspiration, depending on season and topographic position.
Characterization of leaky faults: Study of water flow in aquifer‐fault‐aquifer systems
Authors: Shan Chao , Javandel Iraj , Witherspoon Paul A.
Published: February 2004

Leaky faults provide important flow paths for fluids to move underground. It is often necessary to characterize such faults in engineering projects such as deep well injection of waste liquids, underground natural gas storage, and radioactive waste isolation. To provide this characterization, analytical solutions are presented for groundwater flow through saturated aquifer‐fault‐aquifer systems assuming that both the aquifers and the fault are homogeneous and that the fault has an insignificant effect on aquifer hydraulic properties. Three different conditions are considered: (1) drawdown in the unpumped aquifer is negligibly small; (2) drawdown in the unpumped aquifer is significant, and the two aquifers have the same diffusivity; and (3) drawdown in the unpumped aquifer is significant, and the two aquifers have different diffusivities. Methods are presented to determine the fault transmissivity from pumping test data.
Chemical fingerprints of hydrological compartments and flow paths at La Cuenca, Western Amazonia
Authors: Elsenbeer Helmut , Lack Andreas , Cassel Keith
Published: February 2004

A forested first‐order catchment in western Amazonia was monitored for 2 years to determine the chemical fingerprints of precipitation, throughfall, overland flow, pipe flow, soil water, groundwater, and streamflow. We used five tracers (hydrogen, calcium, magnesium, potassium, and silica) to distinguish “fast” flow paths mainly influenced by the biological subsystem from “slow” flow paths in the geochemical subsystem. The former comprise throughfall, overland flow, and pipe flow and are characterized by a high potassium/silica ratio; the latter are represented by soil water and groundwater, which have a low potassium/silica ratio. Soil water and groundwater differ with respect to calcium and magnesium. The groundwater‐controlled streamflow chemistry is strongly modified by contributions from fast flow paths during precipitation events. The high potassium/silica ratio of these flow paths suggests that the storm flow response at La Cuenca is dominated by event water.
Episodic and constant flow models for the origin of low‐chloride waters in a modern accretionary complex
Authors: Bekins Barbara A. , McCaffrey Anne M. , Dreiss Shirley J.
Published: February 2004

Some low‐chloride pore waters observed in accretionary complexes are thought to result from clay dehydration and subsequent migration of the released water along faults or sand layers. We test this hypothesis with a two‐dimensional flow and transport model for a cross section of the northern Barbados accretionary complex. The model flow system is driven by consolidation of the accreted sediments and by fluids from smectite clay dehydration. Steady state simulations result in concentrations that are too high along the décollement fault and too low near the seafloor. In a transient model we simulate buildup and release of fluids by assuming that strain or hydrofracture along the fault causes an instantaneous increase in décollement permeability of 2–3 orders of magnitude. With such an increase, the observed concentrations can be achieved in 100–1000 years. Also pressures along the fault rise to near lithostatic values in 10–100 years and remain high for 1000–10,000 years. This pressure rise may represent a mechanism for sustaining high fault permeabilities long after the initial increase.
Design and Test of a Sonic Permeameter for Dry Unconsolidated Porous Materials
Authors: Dai Renshou , Young Charles T.
Published: February 2004

We developed a sonic permeameter that can quickly measure the intrinsic permeability of dry unconsolidated porous materials in the laboratory. It consists of a control box, a loudspeaker, and an external waveguide filled with the sample. Sound wave attenuation and dispersion are measured by two microphones recessed in the inner wall of the waveguide. They receive a sound pulse that is sent down to the waveguide from the loudspeaker. The permeability of the sample is then calculated from the attenuation and dispersion spectra using our mathematical model. Test results obtained over several dry Ottawa sand and glass bead samples agree very well with the results obtained using a conventional constant‐head permeameter. The advantage of this apparatus is its rapid and automated measurement, easy operation, and potential workability in the field. Theoretical analysis is also extended to a hypothetical case where the porous material is totally saturated with water.
Nonlinear Dynamics of the Great Salt Lake: Dimension Estimation
Authors: Sangoyomi Taiye B. , Lall Upmanu , Abarbanel Henry D. I.
Published: February 2004

We study the possibility that variations in the volume of the Great Salt Lake (GSL), a large, closed basin lake, may be described as a low‐dimensional nonlinear dynamical system. There is growing evidence for structure in the recurrence patterns of climatic fluctuations that drive western United States hydrology. Moreover, the time behavior of such lakes is generally more regular than that of the climatic forcing. This suggests the possibility that an analysis of the 144‐year, biweekly time series of the GSL volume may shed some light on the underlying dynamics of lake variations. Three methods (correlation dimension, nearest neighbor dimension, and false neighbor dimension) of estimating attractor dimension are applied and compared. The analysis suggests that the GSL dynamics may be described by a dimension of about four. Implications of such analyses relative to low‐frequency variations and colored noise and limitations of such analyses are discussed.
Fate and Transport Modeling using a Numerical Tracer
Authors: Lung Wu‐Seng
Published: February 2004

One of the most recurrent questions in eutrophication control is the extent that nutrients from wastewater discharges are transported and transformed in a receiving water system. For example, phosphorus from point and nonpoint sources could be incorporated into the biomass of phytoplankton in the water column, deposited into the sediments, or transported downstream. Perhaps a more meaningful question is, How much phosphorus in the algal biomass at a certain location in the receiving water is from a given source? A modeling technique has been developed to track the fate and transport of phosphorus from various sources using a numerical tracer. Model results from an application to the upper Mississippi River provide insight into the impact on the receiving water of phosphorus loads from various sources.
Prediction of Wetting Front Movement During One‐Dimensional Infiltration into Soils
Authors: Kao Cindy S. , Hunt James R.
Published: February 2004

The Green‐Ampt and Philip infiltration models are simplified with a capillary tube representation of porous media that can predict wetting front movement during horizontal and vertical infiltration into dry or initially moist soil. The simple model captures the fundamental physics of unsaturated fluid flow and reduces the number of parameters that must be measured. The relevant parameters are the air‐liquid interfacial tension, the density and viscosity of the liquid, the initial saturation, and the permeability. Comparisons with published data show good predictions for infiltration under nonnegative source pressures but are less successful under negative source pressures. The model applies to imbibition of both water and nonaqueous phase liquids.
The modeling of reactive solute transport with sorption to mobile and immobile sorbents: 2. Model discussion and numerical simulation
Authors: Totsche K. U. , Knabner P. , Kögel‐Knabner L.
Published: February 2004

We analyze mathematically and numerically a model derived in part 1 of this paper [Knabner et al., this issue]. The model deals with carrier‐influenced transport and besides advective and dispersive transport and equilibrium and nonequilibrium sorption takes into account both carrier facilitation and cosorption. Guidelines are derived, whether the overall mobility is enhanced or reduced and various other properties of the model elucidated, in particular for varying carrier concentrations. We indicate how to modify existing numerical codes for the usual adsorption model and discuss simulations for experimental data sets.
Effects of Spatial Variability of Saturated Hydraulic Conductivity on Hortonian Overland Flow
Authors: Woolhiser D. A. , Smith R. E. , Giraldez J.‐V.
Published: February 2004

Many field investigations have documented the existence of significant spatial variability of the saturated hydraulic conductivity Ks in typical catchments, and several modeling studies have examined the effects of this variability on Hortonian surface runoff. However, these modeling studies had limitations, and some could not deal adequately with possible runoff‐run‐on phenomena where areas of low saturated hydraulic conductivities Ks are upstream of areas of higher Ks. The kinematic wave equations with interactive Smith‐Parlange infiltration are solved by a finite‐difference scheme on a characteristic computational net. It is shown that runoff hydrographs are strongly affected by trends in hydraulic conductivity, particularly for small runoff events. The results also show that Smith and Hebbert [1979] mislabeled outflow hydrographs, resulting in incorrect conclusions about the effects of linear trends in Ks. A simple technique is introduced to account for flow concentration in rills and the resulting reduced effective wetted areas subject to infiltration after rainfall ceases. It is shown that this mechanism has a major impact on runoff peaks, volumes, and time to peak for cases where Ks increases downslope.
Application of Similar Media Scaling and Conditional Simulation for Modeling Water Flow and Tritium Transport at the Las Cruces Trench Site
Authors: Rockhold Mark L. , Rossi Richard E. , Hills Richard G.
Published: February 2004

Similar media scaling and geostatistical analyses are used to characterize the spatial variability of soil hydraulic properties at the Las Cruces Trench Site in New Mexico. A simple method is described for conditioning the hydraulic properties used for unsaturated water flow and solute transport modeling, based on the spatial distributions of initial field‐measured water contents and a set of scale‐mean hydraulic parameters determined from the scaling analysis. This method is used to estimate hydraulic properties for numerical simulations of the latest field‐scale flow and transport experiment conducted at the Las Cruces Trench Site. Relatively good matches between the observed and simulated flow and transport behavior are obtained without model calibration. The results of this study suggest that using similar media scaling in conjunction with the described conditioning procedure can significantly reduce the uncertainty in predictions of water flow and solute transport in spatially variable soils.
An Improved Approximation for Spherical Diffusion of Tritium in Relatively Dry Soil
Authors: Knight J. H.
Published: February 2004

A study by Smiles et al. [1995] investigated the spread of tritium in relatively dry soil, using a spherically symmetrical diffusion equation with radioactive decay to model the evolution of tritium profiles from the initial release of tritium in a uniform concentration over a small spherical volume of soil. They found that numerical cancellation errors affected the accurate evaluation of the analytical solution of Carslaw and Jaeger [1959], and they used a point source solution as an approximation to the solution for the correct initial conditions. We derive a new and more accurate approximation, based on the principle that the concentration distribution “forgets” the precise initial conditions and is determined mainly by the total quantity of tritium and the spatial variance of the distribution. The actual distribution is approximated by that from a point source which started spreading some time before the actual release time, and the value of the time shift which gives the correct variance is found.
Random‐Walk Simulation of Transport in Heterogeneous Porous Media: Local Mass‐Conservation Problem and Implementation Methods
Authors: LaBolle Eric M. , Fogg Graham E. , Tompson Andrew F. B.
Published: February 2004

The random‐walk method for simulating solute transport in porous media is typically based on the assumption that the velocity and velocity‐dependent dispersion tensor vary smoothly in space. However, in cases where sharp interfaces separate materials with contrasting hydraulic properties, these quantities may be discontinuous. Normally, velocities are interpolated to arbitrary particle locations when finite difference or finite element methods are used to solve the flow equation. The use of interpolation schemes that preserve discontinuities in velocity at material contacts can result in a random‐walk model that does not locally conserve mass unless a correction is applied at these contacts. Test simulations of random‐walk particle tracking with and without special treatment of material contacts demonstrate the problem. Techniques for resolving the problem, including interpolation schemes and a reflection principle, are reviewed and tested. Results from simulations of transport in porous media with discontinuities in the dispersion tensor show which methods satisfy continuity. Simulations of transport in two‐dimensional heterogeneous porous media demonstrate the potentially significant effect of using a nonconservative model to compute spatial moments and breakthrough of a solute plume.
Application of a Generalized TOPMODEL to the Small Ringelbach Catchment, Vosges, France
Authors: Ambroise Bruno , Freer Jim , Beven Keith
Published: February 2004

This paper describes the application of the revised parabolic transmissivity profile form of TOPMODEL, leading to a second‐order hyperbolic base flow recession curve, to four years of 18 minute data from the 36 ha Ringelbach research catchment in the Vosges, France. The model is completed by evapotranspiration, snowmelt and runoff routing components. As far as possible the parameters of this form of the model, or their ranges, are estimated a priori. Results are given that compare the original and revised forms of TOPMODEL, with and without optimization. One particular feature of this catchment is the availability of measurements of saturated areas. These are taken into account by using a modified distribution of the soil‐topographic index, in a first attempt to relax the assumption of uniform transmissivity in the catchment. It is shown that the reasonably good results obtained with the original model are improved with the revised model, particularly in the simulation of dry weather flows, and that further improvements are obtained with the modified soil‐topographic index distribution.
Causes of Concavity in Longitudinal Profiles of Rivers
Authors: Sinha Sanjiv K. , Parker Gary
Published: February 2004

A smooth concave configuration represents the most common quasi‐equilibrium profile of alluvial rivers. In the present paper a generalized, unified set of models for the longitudinal profile of rivers is presented. Various independent effects that drive concavity in longitudinal rivers are considered and classified. The driving mechanisms considered here are (1) horizontal wavelike progradation of the river profile, (2) abrasion of bed particles, (3) aggradation of the river profile balancing subsidence at a constant speed, and (4) the effect of tributaries adding sediment and water to the main stem of the river. For simplicity it is assumed that the long‐term supply of water and sediment is constant; hence a single discharge, that is, bank‐full discharge, is used as an approximate characterization of the hydrologic regime for each point along the river. Sediment sorting has been studied elsewhere and is excluded from this analysis. The river is assumed to have uniform bed material, although the mean size may vary downstream. It is concluded that the quasi‐equilibrium longitudinal profile created by each of the first three driving mechanisms mentioned above is concave. The degree of concavity varies, however, depending on a set of physical conditions that can be identified by a set of dimensionless numbers. A comparison of these numbers reveals the relative importance of the driving mechanisms for concavity. A condition for concavity driven by the fourth mechanism, that is, tributary input, is delineated.
Oil Mound Spreading and Migration with Ambient Groundwater Flow in Coarse Porous Media
Authors: Corapcioglu M. Yavuz , Tuncay Kagan , Ceylan B. Kagan
Published: February 2004

When a light, immiscible oil leaks above an unconfined aquifer, it spreads and forms a floating mound on the water table. The oil mound migrates in the direction of ambient groundwater flow. In this study we present a governing equation for the migrating mound thickness by averaging the oil phase mass balance equation. Analytical and numerical solutions to an advective‐dispersive type equation are presented to estimate the temporal and spatial distribution of the migrating oil mound thickness for two problems of practical importance: formation, spreading, and migration of an oil mound on the water table and spreading and migration of an established layer of oil with ambient groundwater flow. The model results compare favorably with test data obtained by laboratory flume experiments. Although the model has some simplifying assumptions such as the absence of capillary pressure gradients, sharp saturation changes across the phase interfaces, and single mobile phase (i.e., oil flow only), it can be useful as a screening or site assessment tool because of its relative simplicity.
Controls on CH4 flux from an Alaskan boreal wetland
Authors: Moosavi Sadredin C. , Crill Patrick M. , Pullman Erik R. , Funk Dale W. , Peterson Kim M.
Published: February 2004

Factors controlling the flux of the radiatively important trace gas methane (CH4) from boreal wetlands were examined at three sites along a moisture gradient from a treed low‐shrub bog to an open floating graminoid bog in Fairbanks, Alaska. In the summer of 1992 average static chamber flux measurements were −0.02, 71.5, and 289 mg CH4/m2/d in dry, wet, and floating mat communities, respectively. In contrast, the warmer, drier 1993 field season flux measurements were −0.02, 42.9 and 407 mg CH4/m2/d. The data indicate that despite net oxidation of CH4 in the dry regions of the bog, the wetland is a net source of CH4, with fluxes ranging across three orders of magnitude between different plant communities. Comparison with water levels suggests that CH4 flux is turned on and off by changes in site hydrology. In sites where sufficient moisture is present for methanogenesis to occur, CH4 flux appears to be temperature limited, responding exponentially to soil temperature changes. The combined effects of hydrology and temperature create hot spots of CH4 flux within boreal wetlands. The plant communities within Lemeta Bog respond differently to changes in temperature and moisture availability, creating both positive and negative feedbacks to potential global climate change.
CH4 and N2O fluxes in the Colorado shortgrass steppe: 1. Impact of landscape and nitrogen addition
Authors: Mosier A. R. , Parton W. J. , Valentine D. W. , Ojima D. S. , Schimel D. S. , Delgado J. A.
Published: February 2004

A weekly, year‐round nitrous oxide (N2O) and methane (CH4) flux measurement program was initiated in nine sites within the Central Plains Experimental Range in the Colorado shortgrass steppe in 1990 and continued through 1994. This paper reports the observed intersite, interannual, and seasonal variation of these fluxes along with the measured variation in soil and air temperature and soil water and mineral nitrogen content. We found that wintertime fluxes contribute 20–40% of the annual N2O emissions and 15–30% of CH4 consumption at all of the measurement sites. Nitrous oxide emission maxima were frequently observed during the winter and appeared to result from denitrification when surface soils thawed. Interannual variation of N2O maximum annual mean fluxes was 2.5 times the minimum during the 4‐year measurement period, while maximum annual mean CH4 uptake rates were 2.1 times the minimum annual mean uptake rates observed within sites. Generally, site mean annual flux maxima for CH4 uptake corresponded to minimum N2O fluxes and vice versa, which supports the general concept of water control of diffusion of gases in the soil and limitations of soil water content on microbial activity. We also observed that pastures that have similar use history and soil texture show similar N2O and CH4 fluxes, as well as similar seasonal and annual variations. Sandy loam soils fertilized with nitrogen 5–13 years earlier consumed 30–40% less CH4 and produced more N2O than unfertilized soils. In contrast, the N addition 13 years ago does not affect CH4 uptake but continues to increase N2O emissions in a finer‐textured soil. Our long‐term data also show that soil mineral N concentration is not a reliable predictor of observed changes, or lack of changes, in either N2O efflux or CH4 uptake. Finally, from our data we estimate that annual global N2O emission rates for native, temperate grasslands are about 0.16 Tg N2O‐N yr−1, while CH4 consumption totals about 3.2 Tg CH4‐C yr−1.
A data‐driven model of the global calcite lysocline
Authors: Archer David
Published: February 2004

Gridded maps of sediment calcium carbonate (calcite) concentration and overlying water saturation state [Archer, 1996] are combined with maps of benthic oxygen fluxes and sediment accumulation rates from Jahnke [1996] and Cweink [1986] to drive a diagenetic model of calcium carbonate preservation in deep‐sea sediments. The only model input for which we cannot draw a detailed map is the rain rate of calcite to the seafloor, so I use the model to calculate the calcite rain rate required to simulate the observed distribution of calcite concentration on the seafloor. The predictive power of the model can be checked by searching for input parameters to which the model is sensitive and comparing the model requirements with independent data. The model is sensitive to the ratio of organic carbon to calcite rain rates (the rain ratio) and, within the constraint of pelagic sediment trap rain ratio data, is unable to reproduce the calcite field without respiratory dissolution, the promotion of calcium carbonate dissolution by the oxic degradation of organic carbon. However, relative to variability in sediment trap data, the required model rain ratio is insensitive to the extent of anoxic respiration, the stoichiometric ratio of O:C during respiration, the bioturbation rate, the dissolution rate constant, the effect of borate chemistry, and small offsets in the saturation state for CaCO3. The model is sensitive to the accumulation rate of non‐CaCO3 material and predicts an Atlantic/Pacific difference in non‐CaCO3 rain rate which is consistent with observations. The model predicts that the dissolution flux of CaCO3 from sediments is 25–40 × 1012 mol yr−1, roughly half of the total deep sea CaCO3 dissolution rate estimated from the water column alkalinity distribution [Broecker and Peng, 1987; Archer and Maier‐Reimer, 1994; Wollast, 1994]. The model also predicts that only 20–30% of the flux of CaCO3 to the seafloor globally escapes dissolution.
Change in net primary production and heterotrophic respiration: How much is necessary to sustain the terrestrial carbon sink?
Authors: Thompson Matthew V. , Randerson James T. , Malmström Carolyn M. , Field Christopher B.
Published: February 2004

In recent years, the chief approaches used to describe the terrestrial carbon sink have been either (1) inferential, based on changes in the carbon content of the atmosphere and other elements of the global carbon cycle, or (2) mechanistic, applying our knowledge of terrestrial ecology to ecosystem scale processes. In this study, the two approaches are integrated by determining the change in terrestrial properties necessary to match inferred change in terrestrial carbon storage. In addition, a useful mathematical framework is developed for understanding the important features of the terrestrial carbon sink. The Carnegie‐Ames‐Stanford Approach (CASA) biosphere model, a terrestrial carbon cycle model that uses a calibrated, semimechanistic net primary production model and a mechanistic plant and soil carbon turnover model, is employed to explore carbon turnover dynamics in terms of the specific features of terrestrial ecosystems that are most important for the potential development of a carbon sink and to determine the variation in net primary production (NPP) necessary to satisfy various carbon sink estimates. Given the existence of a stimulatory mechanism acting on terrestrial NPP, net ecosystem uptake is expected to be largest where NPP is high and the turnover of carbon through plants and the soil is slow. In addition, it was found that (1) long‐term, climate‐induced change in heterotrophic respiration is not as important in determining long‐term carbon exchange as is change in NPP and (2) the terrestrial carbon sink rate is determined not by the cumulative increase in production over some pre‐industrial baseline, but rather by the rate of increase in production over the industrial period.
A gridded global data set of daily temperature and precipitation for terrestrial biospheric modeling
Authors: Piper Stephen C. , Stewart Elisabeth F.
Published: February 2004

The first global terrestrial gridded data set of the daily average and range of temperature and daily precipitation has been developed, intended for use in terrestrial biospheric modeling. Data for the year 1987 are shown to illustrate our methodology. Daily station data, primarily from the World Meteorological Organization global synoptic surface network of stations, have been extensively quality checked and interpolated to a 1×1 degree grid by using a nearest neighbors interpolation scheme. Annual averages of the daily average temperatures have been compared with 1987 temperatures constructed from data supplied by P.D. Jones (personal communication, 1996). Agreement between these two data sets is good, except in some areas of the southern hemisphere where station coverage is poor. Monthly and annual totals of the daily precipitation data have been compared with the monthly 1987 data set produced by the Global Precipitation Climatology Centre. Agreement between the two data sets is good over much of the northern hemisphere and South America; however, large discrepancies are seen in east‐central and south‐central Africa and in most of Australia, primarily due to the poor station coverage there. Comparison of the time series from individual stations with those from the gridded data set indicate that the day‐to‐day variation of temperature and the fraction of wet days are preserved, except in the tropics where wet days are overestimated. Station densities have been tabulated in terms of total annual net primary productivity to identify countries where increases in station data will be most effective for terrestrial biospheric modeling.
Climatically induced lake level changes at Lake Van, Turkey, during the Pleistocene/Holocene Transition
Authors: Landmann Günter , Reimer Andreas , Kempe Stephan
Published: February 2004

Sediment core K10 from Lake Van (eastern Turkey) provides a continuous varve record back to 14,570 calendar years B.P. (before present, 1950), the longest unbroken and nonfloating lake varve sequence yet described. The underlying sediment is unvarved and hard. Changes in the aragonite/calcite ratio, the presence of protodolomite and magnesite in certain profile sections, the annual record of the sedimentation rate, the water content of the sediment, the concentrations of organic carbon and opal, and the texture of the sediments from this core provide a record of the lake level history. The new chronology enabled us to redate the old pollen profile [van Zeist and Woldring, 1978a, b] and to establish an accurate timescale for the reconstructed lake level change. Carbon 14 dates show that the highest lake terrace corresponds to high lake level at around 19,000 years B.P. during the Last Glacial, >70 m above its present level. Before 15,000 years B.P. the lake must have been completely dry, marking a reduction of lake level by 500 m in maximum 4000 years. Beginning at 14,600 years B.P. and ending at 12,040 years B.P., the lake level recovered by 250 m to fall again during the next 1400 years. By 10,600 years B.P. the lake began to rise and reached, following another regression between 9000 and 8100 years B.P., the Holocene highstand by about 7500 years B.P., dropping to today's level at about 3000 years B.P.
Remote determination of SiF4 in volcanic plumes: A new tool for volcano monitoring
Authors: Francis Peter , Chaffin Charles , Maciejewski Adam , Oppenheimer Clive
Published: February 2004

SiF4 is probably a common trace component of fumarole gases on volcanoes, arising from interactions between magmatic HF and wall rocks. It is not distinguished from HF in conventional analyses. A strong absorption band at 1032 cm−1 makes SiF4 well suited to remote open‐path FTIR spectroscopic techniques. We report the first measurements of SiF4 in volcanic gases at Etna and Vulcano (Sicily). SiF4 concentrations were low at Etna, but higher at Vulcano, where the SO2: SiF4 ratio was consistently around 1.7×10−2: 1. Thermodynamic considerations show that the HF : SiF4 ratio increases sharply with increasing temperature. Thus, remote measurements of SiF4 may offer a qualitative way of constraining fumarole temperatures. We use our FTIR measurements of HCl: SiF4 in combination with conventional measurements of HCl: HF to infer an HF :SiF4 ratio for Vulcano of ∼10:1, consistent with gas temperatures in the range 200–450°C.
Interannual variability in the stratospheric‐tropospheric circulation in a Coupled Ocean‐Atmosphere GCM
Authors: Kitoh Akio , Koide Hiroshi , Kodera Kunihiko , Yukimoto Seiji , Noda Akira
Published: February 2004

Relationships between the northern winter stratospheric and tropospheric circulations and the sea surface temperature (SST) simulated by a global coupled ocean‐atmosphere general circulation model are investigated. Two modes are extracted for interannual variability of the zonal‐mean zonal wind by an empirical orthogonal function analysis. The first mode is related with interannual variations of the stratospheric polar vortex. This mode has a significant correlation with the North Pacific SST through tropospheric circulation changes. The second mode is the variation of the tropospheric subtropical jet and is related with the simulated El Niño. Model results imply no strong relationship between El Niño and the polar vortex.
Implications of a 400 year tree ring based 13C/12C chronology
Authors: Krishnamurthy R. V.
Published: February 2004

The significance of a 400 year (1550–1950 AD) 13C /12C chronology from a juniper from the Sinai Peninsula has been reassessed. A calibration of the isotope data covering the period 1740–1950 was carried out using the 13C/12C of CO2 trapped in ice cores and a carbon isotope fractionation model. The calibration indicated a surprising constancy of Ci/Ca (concentration of CO2 in the leaf intercellular space to that in the atmosphere) during this time period. Using this procedure of calibration, when the pre‐1740 tree ring record was examined in terms of the 13C/12C of the atmospheric CO2, significant variations were indicated. These variations, comparable to that imposed by anthropogenic causes occurred between 1700 and 1850 AD. Obviously, such variations in the 13C/12C of atmospheric carbondioxide is unlikely, but rather these should reflect variations in the Ci/Ca ratio. Interestingly, the atmospheric 13C/12C variations deduced from this tree ring record parallels the 14C variations in the atmosphere during the same period. Since the 14C variations (at least prior to 1850 AD) have been linked to solar activities and by implication to climate, covariation between 13C/12C and 14C supports the hypothesis that the marked shifts in 13C/12C prior to 1850 resulted from a climate‐induced perturbation in the Ci/Ca ratio. The combined calibration procedure and comparison with 14C provides a more useful approach to interpret 13C/12C time series in trees.
Organic carbon paleo‐pCO2 and marine‐ice core correlations and chronology
Authors: Raymo M. E. , Horowitz M.
Published: February 2004

Here we present evidence that marine organic matter δ13C measurements can closely reproduce the Vostok ice core CO2 record and, in addition, can be used to evaluate the absolute Vostok chronology and the transfer of that chronology between the marine and terrestrial realm. Our data supports recent speculations that the base of the Vostok ice core (2755 m) is still within oxygen isotope stage 7, in agreement with recent orbitally‐tuned ice core chronologies.
Heterogeneity in a self‐organized critical earthquake model
Authors: Steacy Sandra J. , McCloskey John , Bean Christopher J. , Ren Jinwei
Published: February 2004

Earthquake dynamics are believed to exhibit self‐organized criticality. This belief results from the power‐law magnitude frequency distributions of earthquake catalogues, distributions which are accurately reproduced by cellular automata, and from the occurrence of triggered earthquakes. This paper examines the effects of heterogeneity on self‐organized criticality in a two‐dimensional cellular automaton. The strength heterogeneity is distributed fractally; stress is incremented uniformly. The model produces power‐law magnitude frequency distributions. For fractal dimensions above 1.9, the slope of the power‐law decreases with increasing fractal dimension. The slope increases weakly with the range of heterogeneity.
7Be and 210Pb Indicate an upper troposphere source for elevated ozone in the summertime subtropical free troposphere of the eastern North Atlantic
Authors: Graustein William C. , Turekian Karl K.
Published: February 2004

The relative abundance of 7Be and 210Pb in aerosols, represented as the “normalized fraction”, 7Be/(7Be + n210Pb), is unaffected by aerosol scavenging. This normalized fraction is an index of the relative influence on an air parcel of stratospheric and upper tropospheric processes versus those in the continental boundary layer (CBL). The high correlation of ozone concentration with the normalized fraction observed during the summer at Izaña, Canary Islands, indicates that 1) low altitude transport of air from the CBL is associated with low ozone concentrations; 2) high altitude transport through the troposphere from the CBL is associated with net ozone production; and 3) the stratosphere is not a direct source of a significant fraction of the ozone observed at Izaña during the summer.
Lateral variation in slab orientation beneath Toba Caldera, northern Sumatra
Authors: Fauzi  , McCaffrey R. , Wark D. , Sunaryo  , Prih Haryadi P. Y.
Published: February 2004

The Investigator Fracture Zone (IFZ) subducts beneath Toba Caldera, the Earth's largest Quaternary caldera, in northern Sumatra, suggesting a possible relationship between them. Locations of sub‐crustal earthquakes based on arrival times of P and S waves at a seismograph network surrounding Toba reveal the geometry of the subducted slab and the IFZ beneath Toba. A vertical tear of less than 20 km in the slab across the IFZ, as previously suggested, cannot be ruled out but the large‐scale geometry of the slab is dominated by a broad bend of slab contours parallel to the concaveseaward indentation of the trench. The slab shape is probably a response to the trench curvature, can explain the change in trend of the volcanic arc near Toba, and may cause shallowing of the forearc basin near Nias Island. The decrease in radius of curvature of the slab contours is not accompanied by an observable decrease in dip angle, possibly resulting in lateral compressive stress in the slab. The high rate of seismicity along the subducted Investigator Fracture Zone, that intersects the slab obliquely to its plunge direction, is uncommon at subducted fracture zones and is likely caused by such lateral stress in the slab.
Conjugate gradient analysis: A new tool for studying satellite magnetic data sets
Authors: Purucker Michael E. , Sabaka Terence J. , Langel Robert A.
Published: February 2004

Conjugate gradient and sparse matrix techniques are utilized in the solution of a geomagnetic inverse problem. Global crustal data sets collected from low‐earth orbit are quickly inverted (using a design matrix approach) or continued to a common altitude (using a normal matrix approach) even when using parameterizations of 10,000 or more dipoles. The sparsity results from the rapid decay of the magnetic field with distance from the dipole. Iterative techniques such as the conjugate gradient save computer time and space when compared to more direct approaches using the Householder transformation, thus allowing problems that were intractable to all but the largest supercomputers to be performed on workstations of only moderate power.
Whistler waves in 3D hybrid simulations of quasiperpendicular shocks
Authors: Hellinger P. , Mangeney A. , Matthews A.
Published: February 2004

We investigate a low Mach number supercritical quasiperpendicular shock using a three‐dimensional hybrid code. We find an out‐of‐coplanarity whistler wave mode propagating upstream, with properties similar to those of the “upstream whistlers” observed in the vicinity of planetary shocks. We discuss the generation mechanism, which turns out to be driven by the free energy provided by reflected protons.
Relative geomagnetic paleointensity across the Jaramillo Subchron and the Matuyama/Brunhes Boundary
Authors: Verosub Kenneth L. , Herrero‐Bervera Emilio , Roberts Andrew P
Published: February 2004

Analysis of old paleomagnetic data and new mineral magnetic data from a sediment core from the central equatorial Pacific Ocean indicates that this core can be used for determination of relative geomagnetic paleointensity. These data indicate that during the Jaramillo subchron, there was a gradual decrease in paleointensity from the onset of the subchron to its termination. The data from the subchron also display high‐amplitude, fine‐scale features that may be coherent over the scale of an ocean basin. Furthermore, there was a rapid rise in paleointensity immediately after the termination of the Matuyama/Brunhes polarity transition. These observations support the hypothesis that an asymmetrical saw‐tooth pattern in paleointensity may be a common characteristic of polarity transitions; however, a better understanding of the high‐amplitude features is needed before definitive conclusions can be drawn about the overall behavior of the field. The coherence of fine‐scale features across such large distances does not support the suggestion that the asymmetrical sawtooth pattern is an artifact of viscous remanent remagnetization.
Influence of variations in extratropical wintertime teleconnections on northern hemisphere temperature
Authors: Hurrell James W.
Published: February 2004

Pronounced changes in the wintertime atmospheric circulation have occurred since the mid‐1970s over the ocean basins of the Northern Hemisphere, and these changes have had a profound effect on surface temperatures. The variations over the North Atlantic are related to changes in the North Atlantic Oscillation (NAO), while the changes over the North Pacific are linked to the tropics and involve variations in the Aleutian low with teleconnections downstream over North America. Multivariate linear regression is used to show that nearly all of the cooling in the northwest Atlantic and the warming across Europe and downstream over Eurasia since the mid‐1970s results from the changes in the NAO, and the NAO accounts for 31% of the hemispheric interannual variance over the past 60 winters. Over the Pacific basin and North America, the temperature anomalies result in part from tropical forcing associated with the El Niño‐Southern Oscillation phenomenon but with important feedbacks in the extratropics. The changes in circulation over the past two decades have resulted in a surface temperature anomaly pattern of warmth over the continents and coolness over the oceans. This pattern of temperature change has amplified the observed hemispheric‐averaged warming because of its interaction with land and ocean; temperature changes are larger over land compared to the oceans because of the small heat capacity of the former.