bluuebunny/crossref_metadata_2025 huggingface dataset:
Dissolution and mass transfer of multiple organics under field conditions: The Borden emplaced source
Authors: Frind E. O. , Molson J. W. , Schirmer M. , Guiguer N.
Published: September 2002

The process that transfers mass from a subsurface source zone of residual dense nonaqueous phase liquid (DNAPL) to the flowing groundwater is a controlling factor in determining the time required to dissolve the source by noninvasive means. While mass transfer can be kinetic or equilibrium under laboratory conditions, aqueous concentrations in the field are generally found to be below equilibrium levels. To gain insight into the mass transfer process under field conditions, we simulated the dissolution of the emplaced DNAPL source at the Canadian Forces Base Borden, Ontario, which contains a mixture of three DNAPLs. The simulations clearly show that mass transfer at this site is equilibrium‐controlled during the 1000‐day observation period and that apparent tailing of one of the organic components is due to its declining solubility, rather than mass transfer kinetics. Flow lines passing through the source are focused in a narrow streamtube downstream of the source, and equilibrium concentrations are therefore observed only at the center of the effluent plume. Since the concentration peaks can be easily missed in the sampling, streamline focusing can explain the low concentrations observed in the field.
Five‐minute, 1/2°, and 1° data sets of continental watersheds and river networks for use in regional and global hydrologic and climate system modeling studies
Authors: Graham S. T. , Famiglietti J. S. , Maidment D. R.
Published: September 2002

A major shortcoming of the land surface component in climate models is the absence of a river transport algorithm. This issue becomes particularly important in fully coupled climate system models (CSMs), where river transport is required to close and realistically represent the global water cycle. The development of a river transport algorithm requires knowledge of watersheds and river networks at a scale that is appropriate for use in CSMs. These data must be derived largely from global digital topographic information. The purpose of this paper is to describe a new data set of watersheds and river networks, which is derived primarily from the TerrainBase 5′ Global DTM (digital terrain model) and the CIA World Data Bank II. These data serve as a base map for routing continental runoff to the appropriate coast and therefore into the appropriate ocean or inland sea. Using this data set, the runoff produced in any grid cell, when coupled with a routing algorithm, can easily be transported to the appropriate water body and distributed across that water body as desired. The data set includes watershed and flow direction information, as well as supporting hydrologic data at 5′, 1/2°, and 1° resolutions globally. It will be useful in fully coupled land‐ocean‐atmosphere models, in terrestrial ecosystem models, or in stand‐alone macroscale hydrologic‐modeling studies.
Microgravity effects on water flow and distribution in unsaturated porous media: Analyses of flight experiments
Authors: Jones Scott B. , Or Dani
Published: September 2002

Plants grown in porous media are part of a bioregenerative life support system designed for long‐duration space missions. Reduced gravity conditions of orbiting spacecraft (microgravity) alter several aspects of liquid flow and distribution within partially saturated porous media. The objectives of this study were to evaluate the suitability of conventional capillary flow theory in simulating water distribution in porous media measured in a microgravity environment. Data from experiments aboard the Russian space station Mir and a U.S. space shuttle were simulated by elimination of the gravitational term from the Richards equation. Qualitative comparisons with media hydraulic parameters measured on Earth suggest narrower pore size distributions and inactive or nonparticipating large pores in microgravity. Evidence of accentuated hysteresis, altered soil‐water characteristic, and reduced unsaturated hydraulic conductivity from microgravity simulations may be attributable to a number of proposed secondary mechanisms. These are likely spawned by enhanced and modified paths of interfacial flows and an altered force ratio of capillary to body forces in microgravity.
Simulations of snow distribution and hydrology in a mountain basin
Authors: Hartman Melannie D. , Baron Jill S. , Lammers Richard B. , Cline Donald W. , Band Larry E. , Liston Glen E. , Tague Christina
Published: September 2002

We applied a version of the Regional Hydro‐Ecologic Simulation System (RHESSys) that implements snow redistribution, elevation partitioning, and wind‐driven sublimation to Loch Vale Watershed (LVWS), an alpine‐subalpine Rocky Mountain catchment where snow accumulation and ablation dominate the hydrologic cycle. We compared simulated discharge to measured discharge and the simulated snow distribution to photogrammetrically rectified aerial (remotely sensed) images. Snow redistribution was governed by a topographic similarity index. We subdivided each hillslope into elevation bands that had homogeneous climate extrapolated from observed climate. We created a distributed wind speed field that was used in conjunction with daily measured wind speeds to estimate sublimation. Modeling snow redistribution was critical to estimating the timing and magnitude of discharge. Incorporating elevation partitioning improved estimated timing of discharge but did not improve patterns of snow cover since wind was the dominant controller of areal snow patterns. Simulating wind‐driven sublimation was necessary to predict moisture losses.
A dual‐porosity model for water level response to atmospheric loading in wells tapping fractured rock aquifers
Authors: Desbarats A. J. , Boyle D. R. , Stapinsky M. , Robin M. J. L.
Published: September 2002

A fractured rock aquifer may be viewed as a permeable fracture network embedded within a less permeable porous matrix. This permeability contrast gives rise to distinct pressure fields in the pore space and fractures, and therefore to a response to atmospheric loading different from that of nonfractured porous media. Assuming that fracture sizes and orientations are uncorrelated and sufficiently random to ensure homogeneous and isotropic elastic behavior of the rock mass at the continuum scale, an effective stress model proposed by Tuncay and Corapcioglu [1995] is used to develop a linear elastic stress‐strain relationship for dual‐porosity media. Elastic constants of the fractured rock mass, required by the model, are estimated using theoretical relations derived by Budiansky and O'Connell [1976]. The stress‐strain relationship and the coupled continuity equations for flow in deformable fractured media are combined to derive expressions for the undrained, static‐confined barometric efficiency of a dual‐porosity medium. Two limiting cases are considered: the initial, instantaneous pressure response in the pore space and fractures due to a change of confining stress, and the later stage of pressure equilibrium between the two phases. The dual‐porosity response model developed here may be used to characterize hydraulic properties in fractured aquifers, including low‐porosity and low‐permeability media. The model is evaluated on water level hydrograph data from a borehole tapping fractured volcanic rocks. Water level and barometric data are supplemented by independent measurements of matrix porosities, fracture spacing, and rock elastic properties, allowing an assessment of total porosity and specific storage at the site. For the observed static‐confined barometric efficiency of 0.78 the instantaneous pressure response model yields plausible values for both parameters.
Retrieving near‐surface soil moisture from Radarsat SAR data
Authors: Biftu Getu Fana , Gan Thian Yew
Published: September 2002

On the basis of six Radarsat synthetic aperature radar (SAR) images acquired over the Paddle River Basin, Alberta, and 1350 field soil moisture data collected from nine selected sites of the basin in the same days, this study demonstrated the feasibility of retrieving near‐surface soil moisture (5–10 cm depth) from Radarsat SAR data. Between two retrieval algorithms, a linear regression and the theoretical integral equation model (IEM) of Fung et al. [1992], the results indicate that IEM performed consistently better than the regression model. However, the performance of both models generally deteriorates when the basin is nearly saturated, which is the case for September 10 and 27, 1996, with a basin saturation level of 82 and 75%, respectively. With respect to Radarsat's configuration and IEM's surface parameters, it is found that radar backscattering σ0 is highly sensitive to the surface rms height, especially for a relatively smooth surface and a Gaussian correlation function. From the semivariogram of measured soil moisture and σ0 it is found that the homogeneity assumption of spatial interpolation as a function of distance lag breaks down only at a distance lag of about 200–275 m. Apparently, spatial interpolation methods based on this assumption can only interpolate soil moisture meaningfully for single land use plots of several hundred meters in size.
Nonstationary stochastic analysis of transient unsaturated flow in randomly heterogeneous media
Authors: Zhang Dongxiao
Published: September 2002

In this study, a first‐order, nonstationary stochastic model of transient flow is developed which is applicable to the entire domain of a bounded vadose zone in the presence of sink/source. We derive general equations governing the statistical moments of the flow quantities by perturbation expansions. Owing to the mathematical complexity of the equations, in general we need to solve them numerically. The numerical moment equation approach, however, has the flexibility in handling different boundary conditions, flow configurations, input covariance structures, and soil constitutive relationships. The moment equations presented in this study are simpler and easier to solve than those in the literature for transient unsaturated flow. We solve these moment equations by the method of finite differences and demonstrate the developed model through some one‐ and two‐ dimensional examples under various transient conditions.
Marsh nutrient export supplied by groundwater discharge: Evidence from radium measurements
Authors: Krest J. M. , Moore W. S. , Gardner L. R. , Morris J. T.
Published: September 2002

We use 228Ra and 226Ra to determine the mass balance of dissolved inorganic nitrogen (DIN) and dissolved reactive phosphorus (DRP) in the North Inlet salt marsh‐estuarine system. While this system has only minor freshwater inputs of nutrients or radium, it is an extremely productive ecosystem. In addition, there are significant exports of these dissolved species to the coastal ocean. Saline groundwater in this estuarine system contains nutrient and radium concentrations more than an order of magnitude greater than surface waters. Using a radium mass balance, we estimate the groundwater discharge necessary to support the export of radium to the coastal ocean and the corresponding flux of nutrients from the groundwater. From these calculations, we show that the underlying aquifer supplies nutrients sufficient to support the net primary productivity of the salt marsh ecosystem and to account for the known export of nutrients from the marsh. We conclude that the major nutrient source to the North Inlet, South Carolina, salt marsh is the saline aquifer lying just beneath the surface of the marsh. Furthermore, extrapolation of the nutrient export to include other South Carolina marshes suggests that nutrient fluxes from salt marshes to the coastal ocean rival riverine nutrient fluxes for the region.
Equatorial nutrient trapping in biogeochemical ocean models: The role of advection numerics
Authors: Oschlies Andreas
Published: September 2002

The unrealistic accumulation of nutrients below the equatorial upwelling regions, termed “nutrient trapping” by Najjar et al. [1992], has been a major problem of first‐generation oceanic carbon cycle models. Suggestions to eliminate this modeling problem include the consideration of dissolved organic nutrients and improvements to the underlying circulation field. This study shows that neither of these suggestions can be complete without a careful investigation of advection numerics. Using a high‐resolution coupled biological‐physical model of the North and equatorial Atlantic, it is demonstrated for standard central‐difference advection (if a too coarse vertical resolution is used) as well as for upstream advection that numerical artifacts alone can lead to a systematic overestimation (by as much as 50%) of simulated subsurface nutrient concentrations in the equatorial upwelling region. A corollary of these results is that without a critical examination of the underlying advection numerics, nutrient trapping alone can no longer present an unchallenged justification for including dissolved organic matter in biogeochemical ocean models.
Reconstructing the oceanic 13C Suess Effect
Authors: Sonnerup Rolf E. , Quay Paul D. , McNichol Ann P. , Bullister John L. , Westby Tania A. , Anderson Heather L.
Published: September 2002

The anthropogenic δ13C change for the time period 1968 to 1991 was determined based on calculations of the preformed 13C/12C of dissolved inorganic carbon (DIC) distributions on isopycnal surfaces in the main thermocline of the Pacific, North Atlantic and South Indian Oceans. The time rate of change of preformed δ13C (the 13C Suess effect) along isopycnals was calculated using CFC‐derived water ages and yields a time history of the surface water δ13C change at the isopycnal outcrop location. The surface ocean Suess effect recorded on isopycnals decreased with increasing outcrop latitude from approximately −0.2‰ decade−1 within the subtropics to around −0.1‰ decade−1 in the subpolar oceans. In the Pacific Ocean these surface δ13C change rate reconstructions agree, both in magnitude and meridional trend, with direct observations of surface ocean δ13C changes reported from time series measurements and from comparisons of surface water δ13C of DIC measurements in 1970 and 1993. A global ocean average surface δ13C rate of change of −0.15 ± 0.04 ‰ decade−1 is determined, which is slightly smaller than a previous time series data and model‐based estimate (−0.171‰ decade−1 , [Bacastow etal., 1996]). Depth integrations of the 13C reconstructions in the Pacific, Indian, and Atlantic Oceans, when combined with these previous individual depth profile comparisons and Geochemical Ocean Sections Study bomb 14C inventories [Quay et al, 1992], imply a global depth‐integrated δ13C change rate of −9.7 ± 2.4‰ m yr−1 over the time period 1970–1990. These results imply a net oceanic CO2 uptake rate of 1.9 ± 0.9 Gt C yr−1 over the time period 1970–1990 when applied to an atmospheric 13CO2 and 12CO2 budget.
Factors governing the oceanic nitrous oxide distribution: Simulations with an ocean general circulation model
Authors: Suntharalingam P. , Sarmiento J. L.
Published: September 2002

A global model of the oceanic nitrous oxide distribution is developed to evaluate current understanding of the processes governing nitrous oxide formation and distribution in the open ocean. N2O is treated as a nonconserved tracer in a global ocean general circulation model subject to biological sources in the oceanic interior and gas exchange at the ocean surface. A simple scalar parameterization linking N2O production to oxygen consumption (and based on observed correlations between excess N2O and apparent oxygen utilization) is successful in reproducing the large‐scale features of the observed distribution, namely, high surface super‐saturations in regions of upwelling and biological productivity, and values close to equilibrium in the oligotrophic subtropical gyres. The majority of the oceanic N2O source is produced in the upper water column (over 75% above 600 m) and effluxes directly to the atmosphere in the latitude band of formation. The observed structure at depth is not as well reproduced by this model, which displays excessive N2O production in the deep ocean. An alternative source parameterization, which accounts for processes which result in a depth variation in the relationship between N2O production and oxygen consumption, yields an improved representation of the deep distribution. The surface distribution and sea‐air flux are, however, determined primarily by the upper ocean source and, therefore, are relatively insensitive to changes in the nature of deep oceanic N2O production.
Modeling the geochemical cycle of iron in the oceans and its impact on atmospheric CO2 concentrations
Authors: Lefèvre Nathalie , Watson Andrew J.
Published: September 2002

Iron occurs at very low concentrations in seawater and seems to be a limiting factor for primary production in the equatorial Pacific and the Southern Ocean. The global distribution of iron is still not well understood because of a lack of data and the complex chemistry of iron. We develop a 10‐box model to study the oceanic distribution of iron and its effect on atmospheric CO2 concentration. Subject to our assumptions, we find that a lack of interocean fractionation of deep sea iron concentrations, as suggested by Johnson et al. [1997a], is not readily explained by a balance of eolian deposition, scavenging, and regeneration. Incorporation of organic complexation in the model, as suggested by Johnson et al., to reduce the scavenging rate of iron when concentrations fall below some ligand‐stabilized concentration, is one solution to this difficulty. Alternatively, the deep‐sea concentration may be more variable than the current, rather sparse data coverage suggests. In the model, deep‐sea iron concentrations are responsive to the atmospheric source, even if we adopt stabilization of concentrations by a ligand as modeled by Johnson et al. [l997a]. In the Southern Ocean, where the model suggests iron supply has an important limiting effect on the biota, more than 99% of the iron supply to the surface in the present day comes from upwelling and not from the local atmospheric flux. In the context of glacial‐interglacial changes to atmospheric CO2 the model suggests that increasing atmospheric iron to the entire global ocean by a factor of 2, leads to decreases in atmospheric CO2 of 10–30 ppm, depending on assumptions. However, in our model, CO2 concentrations are almost unaffected by changes in Southern Ocean atmospheric fluxes alone, unless these are unrealistically large (> 100 times present day). The effect on atmospheric carbon dioxide is slightly stronger if accompanied by increased stratification of the Southern Ocean. The model suggests that eolian “iron fertilization” of the ocean could have importantly influenced glacial atmospheric CO2 concentrations but that other processes must also be at work to account for the full magnitude of the glacial‐interglacial change.
Differences in CH4 oxidation and pathways of production between rice cultivars deduced from measurements of CH4 flux and δ13C of CH4 and CO2
Authors: Bilek Rebecca S. , Tyler Stanley C. , Sass Ronald L. , Fisher Frank M.
Published: September 2002

We report measurements of CH4 flux and δ13C and δD values of emitted CH4 and sediment CH4 and CO2 during the 1995 rice growing season in Beaumont, Texas. Four rice plant cultivars, Lemont, Mars, Cypress, and Della, and an unplanted plot were studied to provide possible explanations for the differences in CH4 emissions between cultivars. Using the measured isotope values, along with data of CH4 and CO2 concentrations and other ecosystem data, we determined differences between cultivars in the processes of oxidation and production throughout the growing season. For instance, rhizospheric CH4 oxidation increased as the season progressed in both Mars and Lemont cultivars. Late in the season, however, 71±10% of CH4 produced in the Mars plot was oxidized compared to only 39±10% in the Lemont plot. The contribution of acetate fermentation to methanogenesis at specific times during the season was calculated using measured isotopic values and assuming identical isotopic fractionation factors in methanogenic pathways for the cultivars. In these calculations a range of values for the contribution to CH4 production from acetate fermentation and CO2 reduction with H2 was estimated by considering different fractionation factors for the methanogenic CO2 reduction pathway and the possibility of a 10% contribution to CH4 production from acetate produced by homoacetogenesis. In general, a steady increase in the CH4 portion produced by acetate fermentation was noted in the Lemont cultivar, while an increase followed by a decrease near the end of the season was observed for the Mars cultivar.
Changes in freshwater carbon exports from Canadian terrestrial basins to lakes and estuaries under a 2×CO2 atmospheric scenario
Authors: Clair T. A. , Ehrman J. M. , Higuchi K.
Published: September 2002

Rain water running through soils and wetlands will leach decomposing plant organic matter into streams and lakes in the form of dissolved organic carbon (DOC). In streams, lakes and eventually estuaries, DOC can be mineralized to CO2, precipitated to sediments or taken up in biological matter, and is thus an important part of many aquatic ecosystems. Using hydrological, climatological and geographical data from 32 sites located in Canada, we developed a neural network model which allowed us to estimate DOC export from the Canadian land mass. We reapplied the model to the 32 sites plus a further 43 basins to estimate area normalized exports for various regions of the country. We estimated that 14.3×106 t of DOC are currently exported from Canadian terrestrial ecosystems. We then modified climatological inputs to the model to reflect the predicted temperature and precipitation conditions under a doubled atmospheric CO2 regime. Our model suggests that DOC exports will increase by approximately 14% under a doubled CO2 atmosphere, mostly owing to increases in runoff. Our analysis also shows that DOC export is greatest in the spring in southern Canada and summer in the north.
Dynamic replacement and loss of soil carbon on eroding cropland
Authors: Harden J. W. , Sharpe J. M. , Parton W. J. , Ojima D. S. , Fries T. L. , Huntington T. G. , Dabney S. M.
Published: September 2002

Links between erosion/sedimentation history and soil carbon cycling were examined in a highly erosive setting in Mississippi loess soils. We sampled soils on (relatively) undisturbed and cropped hillslopes and measured C, N, 14C, and CO2 flux to characterize carbon storage and dynamics and to parameterize Century and spreadsheet 14C models for different erosion and tillage histories. For this site, where 100 years of intensive cotton cropping were followed by fertilization and contour plowing, there was an initial and dramatic decline in soil carbon content from 1870 to 1950, followed by a dramatic increase in soil carbon. Soil erosion amplifies C loss and recovery: About 100% of the original, prehistoric soil carbon was likely lost over 127 years of intensive land use, but about 30% of that carbon was replaced after 1950. The eroded cropland was therefore a local sink for CO2 since the 1950s. However, a net CO2 sink requires a full accounting of eroded carbon, which in turn requires that decomposition rates in lower slopes or wetlands be reduced to about 20% of the upland value. As a result, erosion may induce unaccounted sinks or sources of CO2, depending on the fate of eroded carbon and its protection from decomposition. For erosion rates typical of the United States, the sink terms may be large enough (1 Gt yr−1, back‐of‐the‐envelope) to warrant a careful accounting of site management, cropping, and fertilization histories, as well as burial rates, for a more meaningful global assessment.
Influence of a [CO2(aq)] dependent biological C‐isotope fractionation on glacial 13C/12C ratios in the ocean
Authors: Hofmann Matthias , Broecker Wallace S. , Lynch‐Stieglitz Jean
Published: September 2002

Planktonic foraminiferal shells buried in deep ocean sediments record lower δ13C values of surface water dissolved inorganic carbon during glacial times than during Holocene. In the Southern Ocean and at high northern latitudes, a drop of between 0.3‰ and 0.9‰ is observed, whereas the mean change in low and middle latitudes is only ±0.15‰. However, a stronger biological carbon pump sufficient to explain the 80 ppmv lower atmospheric pCO2 values during glacial times would raise the surface ocean δ13C values of dissolved inorganic carbon by about 1.0‰. Here the results of a three‐dimensional ocean circulation model study are presented which demonstrate that the increase of δ13C values in the sea surface due to a strengthening of the biological carbon pump is counteracted by processes which drive the δ13C values in the opposite direction. This was found by performing simulations employing the three‐dimensional Hamburg Model of the Oceanic Carbon Cycle (HAMOCC) combined with a [CO2(aq)] dependent parameterization of the biological carbon isotope fractionation. The difference in the biological carbon isotope fractionation between Glacial and Holocene is responsible for a lowering of δ13C values in surface water dissolved inorganic carbon by about 0.3‰. The additional effects of the glacially elevated CO32− concentration (0.25–0.50‰) combined with the 0.35‰ lowering of δ13C values for the whole ocean due to a transfer of terrestrial organic carbon from the biosphere to the ocean‐atmosphere reservoir also contribute to a further δ13C drop of 0.6–0.85‰. Hence a small glacial decrease of the planktonic foraminifera δ13C of the order of 0.25‰ instead of an increase is predicted.
A reassessment of the global isotopic budget of atmospheric nitrous oxide
Authors: Rahn Thom , Wahlen Martin
Published: September 2002

A simple box model is developed which accounts for the natural and anthropogenic sources and sinks of tropospheric nitrous oxide. Stable isotopic terms are included as well to investigate what additional insight they might provide concerning the overall picture of the global nitrous oxide budget. It is seen that fractionation associated with ultraviolet photolysis in the stratosphere plays a key role in balancing the isotopic budget. It is also noted that increased production due to human influence should have a unique isotopic signature which should provide observable differences between preindustrial air trapped in polar firn or ice and modern air. This build‐up of isotopically light, anthropogenic nitrous oxide can be thought of as an N2O “Suess effect” and should be observable in time series measurements of clean baseline atmospheric samples. High‐precision records of such changes could lead to a better estimate of the preindustrial N2O isotopic signature as well as provide insight into the pathways of the anthropogenic sources.
Atmospheric depositional fluxes of trace elements, 210Pb, and 7Be to the Sargasso Sea
Authors: Kim Guebuem , Alleman Laurent Y. , Church Thomas M.
Published: September 2002

Atmospheric wet and bulk depositional fluxes of trace elements (Fe, Al, Mn, Zn, Pb, Cu, Cd, Ni, and Cr), 210Pb, and 7Be (bulk only) to the Sargasso Sea were measured at Bermuda from September 1996 to September 1997. Wet deposition was the dominant process of trace element deposition over most of the sampling seasons, based on the measured bulk versus wet depositional fluxes. Although about half of the Mn in Bermuda precipitation was calculated to originate from noncrustal sources, its seasonal trend is similar to the Fe and Al, suggesting primary sources from continental crustal materials. On the other hand, Cd, Zn, Pb, Cu, Ni, and Cr in Bermuda precipitation originate mainly from noncrustal sources (presumably anthropogenic) based on enrichment factor analyses. The noncrustal sources were highest during the fall‐winter due to more frequent intrusion of U.S. continental air as the Bermuda High weakens. Atmospheric depositional fluxes of 210Pb showed a good correlation with the 7Be fluxes (r2=0.84) at Bermuda, indicating major transport of 210Pb via the upper troposphere from continents, along with other trace elements. Annual depositional fluxes of noncrustal elements (Cd, Zn, Pb, Cu, Ni, and Cr) at Bermuda were a factor of 2 to 3 lower than those at the mid‐Atlantic Bight and were about 2–20 fold lower during the sampling year than those during the early 1980s. This confirms the rapid evolution of trace element inputs to the upper Sargasso Sea during the last few decades resulting from industrial emission controls in the surrounding continents.
Dynamics of isotopic exchange of carbon dioxide in a Tennessee deciduous forest
Authors: Bowling David R. , Baldocchi Dennis D. , Monson Russell K.
Published: September 2002

The combination of isotopic measurements and micrometeorological flux measurements is a powerful new approach that will likely lead to new insight into the dynamics of CO2 exchange between terrestrial ecosystems and the atmosphere. Since the biological processes of photosynthesis and respiration modify the stable isotopic signature of atmospheric CO2 in different ways, measurements of the net fluxes of CO2, 13CO2, and C18OO can be used to investigate the relative contribution of each process to net ecosystem CO2 exchange. We used two independent approaches to measure isotopic fluxes of CO2 over a Tennessee oak‐maple‐hickory forest in summer 1998. These approaches involved (1) a combination of standard eddy covariance with intensive flask sampling, and (2) a modification to the relaxed eddy accumulation technique. Strong isotopic signals associated with photosynthesis and respiration were observed and persisted in forest air despite the potential for mixing due to atmospheric turbulence. Calm nights allowed a buildup of respiratory CO2 below the canopy and were associated with isotopically depleted forest air in the morning. Windy nights were followed by a relatively more enriched early‐morning isotopic signal. Entrainment of air from above the decaying nocturnal boundary layer during daytime mixed layer growth exerted strong control on isotopic composition of forest air, resulting in similar isotope ratios in the late afternoon despite different isotopic starting points following calm or windy nights. The influences of the convective boundary layer and turbulent mixing within the forest cannot be ignored when using isotopes of CO2 investigate biological processes.
Redfield stoichiometry in Arabian Sea subsurface waters
Authors: Hupe Axel , Karstensen Johannes
Published: September 2002

A linear inverse mixing model is applied to hydrographic, nutrient, and carbon data collected during Joint Global Ocean Flux Study and World Ocean Circulation Experiment cruises in 1995 to estimate the ΔCorg/ΔN/ΔP/ΔSi/‐ΔO2 remineralization ratios within the Arabian Sea between 550 and 4500 m. The observed concentrations are separated into mixing fractions of source water masses and changes caused by remineralization processes, while the effect of denitrification is considered. In contrast to earlier investigations, diapycnal mixing, which plays an important role in dissolved matter fluxes in the Arabian Sea, is accounted for. The ratios are found to be variable with depth, especially in the upper 2000 m of the water column. We suppose that in general nutrients are released faster than carbon dioxide during remineralization. The ΔCorg/ΔCinorg decrease from ∼4 ± 1 at 550 m to 2 ± 0.2 at 2000 m and 1.2 ± 0.3 at 4000 m, suggesting that the dissolution of calcium carbonate above the calcite lysocline is a potentially important process within the Arabian Sea.
Isotopic constraints on the transpiration, evaporation, energy, and gross primary production Budgets of a large boreal watershed: Ottawa River Basin, Canada
Authors: Telmer Kevin , Veizer Jan
Published: September 2002

The residual signals of the physical and chemical processes undergone in a river basin are stored in the isotopic composition of river water and are used here to isolate and quantify fluxes of water, energy and carbon for a large boreal river basin. The integrated nature of the river signal is exploited to provide meaningful basin‐wide annual averages for fluxes difficult to quantify and extrapolate by studying highly variable interface exchanges at discrete locations. The slope of the linear regression of deuterium (δD) and oxygen (δ18O) isotopes for the Ottawa River is ∼6.0, considerably less than the slope of the local meteoric water line (7.7). This discrepancy is a consequence of evaporative loss from open water bodies and soils and, through a new method, is calculated to be 8.1% of annual precipitation. As well, on the basis of thirty years of daily meteorological and discharge data, annual evapotranspiration for the Ottawa River basin is calculated to be 53.1%. Combining the evaporation and evapotranspiration calculations apportions 45% of the water losses to transpiration. The energy required to drive these cycles is calculated to be 8% of annual solar radiation for total evapotranspiration and 13% of growing season solar radiation for transpiration. These energies are transformed into latent heat. The water use efficiency ratio is used to estimate total fixation of carbon (gross primary production (GPP)) for the basin at 15.6 mol C m−2 yr−1. This rate is substantially greater than the export of carbon via rivers plus rates estimated for carbon respiration in the literature, indicating that the boreal forest is a plausible component of the postulated “missing” carbon sink. Comparison of accumulation rates of C in peatlands and the rates required to account for the missing sink suggest that peat accumulation rates are ∼20 times too slow to account for the missing sink flux. Speculatively, the living biomass of the boreal forest is the dominant sink. Accepting this, the respiration rate needed for a steady state balance between the calculated boreal forest GPP and the missing global carbon sink is found to be around 5.6 mol C m−2 yr−1.
Seasonal variation in methane oxidation in a landfill cover soil as determined by an in situ stable isotope technique
Authors: Chanton Jeffrey , Liptay Karen
Published: September 2002

Seasonal variations in the oxidation of methane during its transport across the soil cap of a landfill in Leon County, Florida, were determined in situ with a stable isotopic technique. The approach contrasted the δ13C values of emitted and anoxic zone CH4 and utilized measurements of the isotopic fractionation factor α, which varied inversely with temperature from 1.025 to 1.049. Anoxic zone CH4 did not vary seasonally and had a δ13C average value of −55.18 ± 0.15‰. Methane emitted from the landfill soil surface and captured in chambers ranged in δ13C from −54‰ in winter, when emission rates were high, to −40‰ in summer, when emission rates were lower. The antipathetic variation between the δ13C of emitted CH4 and the rate of CH4 emission is consistent with control of the emission rate by bacterial oxidation. Our interpretation of the isotope data indicates that methane oxidation consumed from 3 to 5% of the total flux in winter to a maximum of 43 ± 10% in summer. There was variation in the extent of methane oxidation in soil types, with mulch/topsoil averaging 55 ± 14% and clay averaging 33 ± 13% in summer. The seasonally integrated value for methane oxidation for areas of the landfill covered with mulch/topsoil was 26 ± 4% of the flux toward the soil surface, while for clay soil it was only 14 ± 2%. The overall annual average, which includes both types of soil, was 20 ± 3%. Covering landfills with additional mulch, which can be generated from yard waste, may attenuate methane emission by providing a loose noncompact substrate for bacterial attachment and an environment with moisture, methane, and oxygen. At specific sites within the landfill we studied, temperature was the main factor controlling methane oxidation.
Sea level impact on nutrient cycling in coastal upwelling areas during deglaciation: Evidence from nitrogen isotopes
Authors: Bertrand Philippe , Pedersen Thomas F. , Martinez Philippe , Calvert Stephen , Shimmield Graham
Published: September 2002

A common feature in δ15N profiles downcore in continental margin sediments is that the heaviest δ15N values are frequently observed during the deglaciation (i.e., between 12,000 calendar years BP and the climatic optimum at 6000 yrs BP), not at the warmest stage. Using a conceptual model across the northwestern Africa margin, a region of pronounced modern upwelling, as well as data from a core in the area, we show that this feature can be explained as a consequence of postglacial sea level rise. The model is based on a simplified twodimensional physical circulation scheme orthogonal to the margin and uses the topographic profile at the latitude of the core as well as a simplified biological model for nitrate utilization and nitrogen isotope fractionation. Shore‐parallel influences are ignored. The most recently published age model of sea level rise for the last deglaciation is used [Bard et al., 1996]. The trangression causes a progressive increase in the area of shallow regions where large amounts of nutrients are recycled relative to deep regions, to which a significant portion of the nutrients is exported. This causes first an increase and then a decrease in the δ15N of the organic matter accumulating at a fixed point on the upper slope. Although the deglacial δ15N maximum is more pronounced in areas where there is not a marked oxygen minimum layer [Holmes et al., 1997; this paper], it does exist in areas where an oxygen minimum layer is present in the water column [Altabet et al., 1995; Ganeshram et al., 1995]. In such areas, the major δ15N contrast between glacial and interglacial episodes is explained by higher denitrification during interglacial stages, but it is probable that transgressing sea level contributes to this effect. The model has implications for the changes of vertical oceanic nutrient fractionation [Boyle, 1988] and the respiratory dissolution of deep carbonates [Archer and Maier‐Reimer, 1994] and hence could have important potential implications for the timing of global CO2 exchanges between ocean and atmosphere and their feedback to climate.
Dynamical and observational constraints on tropical Pacific sea surface temperatures at the Last Glacial Maximum
Authors: Liu Z. , Shin S. , Behling P. , Prell W. , Trend‐Staid M. , Harrison S. P. , Kutzbach J. E.
Published: September 2002

Asynchronously coupled atmosphere and ocean general circulation model simulations are used to examine the consequences of changes in the west/east sea‐surface temperature (SST) gradient across the equatorial Pacific at the last glacial maximum (LGM). Simulations forced by the CLIMAP SST for the LGM, where the west/east SST gradient across the Pacific is reduced compared to present, produce a reduction in the strength of the trade winds and a decrease in the west/east slope of the equatorial thermocline that is incompatible with thermocline depths newly inferred from foraminiferal assemblages. Stronger‐than‐present trade winds, and a more realistic simulation of the thermocline slope, are produced when eastern Pacific SSTs are 2°C cooler than western Pacific SSTs. Our study highlights the importance of spatial heterogeneity in tropical SSTs in determining key features of the glacial climate.
1998 eruption of axial volcano: Multibeam anomalies and sea‐floor observations
Authors: Embley R. W. , Chadwick W. W. , Clague D. , Stakes D.
Published: September 2002

Lava flows erupted during the January/February 1998 seismic swarm at Axial Volcano on the Juan de Fuca Ridge have been identified by differencing of pre‐ and post‐event multibeam bathymetric surveys and by seafloor observations. A sheet flow more than 3 km in length and 500–800 m wide erupted from the uppermost south rift is the site of a robust hydrothermal system. 1998 lavas occur over about 9 km of the upper south rift zone, or about 20% of the along‐axis length of the seismicity event (∼50 km). The estimated volume of lava erupted is 18‐76 ×106 m³ for the extrusion and 100‐×106 m³ for the intrusion. The total volume is consistent with the volume from modeling of seafloor strain measurements recorded during the event.
Plume separation by transient thermohaline convection in porous media
Authors: Oldenburg Curtis M. , Pruess Karsten
Published: September 2002

Research on thermohaline convection in porous media has borrowed heavily from the field of viscous liquid thermohaline convection. Transient thermohaline convection in porous media differs dramatically from that in viscous liquids because of thermal retardation. The effects of thermal retardation are well known in geothermal reservoir engineering for forced convection processes such as liquid reinjection. Thermal retardation causes heat to move at approximately the Darcy velocity, while solute moves at the pore velocity. The difference in transport of heat and solute makes transient thermohaline convection in porous media double‐advective rather than double‐diffusive. For upward moving plumes, dense solute is advected in front of the thermal plume and creates a density lid. For downward moving plumes, the dense solute plume separates from the thermal plume.
The satellite altimeter data derived mean sea surface GSFC98
Authors: Wang Yan M.
Published: September 2002

The GSFC98 mean sea surface (MSS) was computed on a 2' oceanwide grid for latitudes below 80°. The data used included 3‐years of TOPEX data (Cycles 9 to 119), 1.5‐years of ERS‐1 35‐day repeat cycle (Cycles 1 to 18), 2‐years of Geosat ERM data (Cycles 1 to 42), 2 ERS‐1 168‐day repeat cycles, and 18 months of the Geosat Geodetic Mission data. All non‐TOPEX satellite altimeter data were adjusted to the mean of TOPEX data in 2° × 30° blocks. After the adjustment, the mean sea surface height was gridded into 2' nodes using least squares collocation and an iteration procedure to reduce the ocean variability. To validate the mean sea surfaces, three comparisons were made. The GSFC98 MSS, along with OSU95 [Yi, 1995] and CSR95 [Kim et al., 1995], were compared with 6‐years of TOPEX and 3‐years of ERS‐2 mean tracks which were not used in the MSS computations. Finally, the marine gravity anomalies were computed from the three MSS implied geoid undulations using the inverse Stokes integral. The marine gravity anomalies were compared with ship gravity data in selected areas. The ship gravity comparison is an independent assessment of the quality of the MSSs, especially at intermediate and short wavelengths. Finally, the inter‐comparisons between the mean sea surfaces were made. The root mean square values of the differences were 6.8, 6.8, and 7.2 cm between GSFC98/OSU95, GSFC98/CSR95, and OSU95/CSR95. The differences agree well with the error estimation of GSFC98 MSS.
Atmospheric Intertropical Convergence impacts surface ocean carbon and nitrogen biogeochemistry in the western tropical Pacific
Authors: Hansell Dennis A. , Feely Richard A.
Published: September 2002

Concentrations of inorganic and organic carbon and organic nitrogen, as well as other hydrographic and biogeochemical variables, were measured in the oligotrophic waters of the western South Pacific Ocean (5‐35°S; 170°W). With those data, we assess the impact of the intertropical convergences of the western tropical Pacific Ocean on surface ocean biogeochemistry. Low salinity, oligotrophic waters of the western tropical Pacific underlying regions of high net precipitation are characterized by concentrations of organic carbon and nitrogen that are elevated relative to higher salinity (lower net precipitation) zones. We hypothesize that water column stratification, forced by high net precipitation, favors enhanced rates of N2 fixation, with resultant elevation of organic carbon and nitrogen concentrations in the region. An effect of anthropogenic ocean warming and the associated increased hydrologic cycle in the tropical Pacific may be to alter the biogeochemical cycling of carbon and nitrogen through enhancement of N2 fixation.
Effect of flux adjustments on temperature variability in climate models
Authors: Duffy P. B. , Bell J. , Covey C. , Sloan L.
Published: September 2002

It has been suggested that “flux adjustments” in climate models suppress simulated temperature variability. If true, this might invalidate the conclusion that at least some of observed temperature increases since 1860 are anthropogenic, since this conclusion is based in part on estimates of natural temperature variability derived from flux‐adjusted models. We assess variability of surface air temperatures in 17 simulations of internal temperature variability submitted to the Coupled Model Intercomparison Project. By comparing variability in flux‐adjusted vs. non‐flux adjusted simulations, we find no evidence that flux adjustments suppress temperature variability in climate models; other, largely unknown, factors are much more important in determining simulated temperature variability. Therefore the conclusion that at least some of observed temperature increases are anthropogenic cannot be questioned on the grounds that it is based in part on results of flux‐adjusted models. Also, reducing or eliminating flux adjustments would probably do little to improve simulations of temperature variability.
Near‐Earth dipolarization: Evidence for a non‐MHD process
Authors: Lui A. T. Y. , Liou K. , Nosé M. , Ohtani S. , Williams D. J. , Mukai T. , Tsuruda K. , Kokubun S.
Published: September 2002

We have investigated a near‐Earth dipolarization event in the midnight sector using simultaneous observations of Polar and Geotail. We have found evidence for near‐Earth dipolarization to be a non‐MHD process: dipolarization occurring without significant plasma flow or with tailward flow and during dawnward electric field different from that inferred based on the frozen‐in condition. These observations are inconsistent with the idea that dipolarization is an MHD process of magnetic flux pileup from braking of sunward plasma flow. Possible variances of the flow braking scenario are considered but none is satisfactory in accounting for the observed features. On the other hand, these findings are quite consistent with the expectations from the current disruption scenario.
Extremely cold electrons in the January 1997 magnetic cloud
Authors: Larson D. E. , Lin R. P. , Steinberg J.
Published: September 2002

The 3D Plasma and Energetic Particle (3DP) instrument on the WIND spacecraft detected extremely cold solar wind electrons in the January 10–11, 1997 magnetic cloud, ∼4 times lower temperature than any previously reported measurement. Detailed fits to the electron distributions show that the core electron temperature, Tec, generally ranged from ∼1 to 4.5 eV through the cloud, dropping to a low of ∼0.7 eV in an unusually dense (n>∼150 cm−3) region in the trailing portion of the cloud. Due to the extremely low average electron temperature Te and high density, the ions and electrons are collisionally coupled as confirmed by Te=Tp, the first such observation in the solar wind. For most of the cloud the halo density is very low (neh<∼0.1 cm−3), implying magnetic disconnection from the Sun. Correlations in Te and Tp are observed throughout the cloud and are particularly evident when neh is low. We suggest the diminished halo density has reduced a significant heat source to the core population, thus allowing the electrons to cool more thoroughly. Furthermore, we show that Coulomb collisions are a significant mechanism of energy transfer between the halo and core distributions under normal solar wind conditions.