bluuebunny/crossref_metadata_2025 huggingface dataset:
Soil moisture redistribution and infiltration in frozen sandy soils
Authors: Stähli Manfred , Jansson Per‐Erik , Lundin Lars‐Christer
Published: September 2002

Infiltration into frozen soil is a result of the whole climate dynamics of the preceding winter with all its importance for the freezing of the soil. Therefore a predictive infiltration model needs to include a proper description of the main processes of soil water and heat transfer during season‐long periods. Such a model may assume two water‐conducting flow domains. A lysimeter experiment was set up with the aim of studying these processes in two different sandy soils. Frequent measurements of total and liquid soil water content, soil temperature, and groundwater level were made during two winters with contrasting meteorological conditions. The main problems in the simulation of the two winters were (1) frost‐induced upward water redistribution, (2) rate of infiltration in the initially air‐filled pores, and (3) heat transfer caused by snowmelt refreezing in the frozen soil. An extensive calibration of the model suggested that some key empirical parameters were not constant for the two soils and the two seasons. Complementary methods for determining the hydraulic conductivity of frozen unsaturated field soils are necessary to further improve the model.
Probabilistic optimization for conceptual rainfall‐runoff models: A comparison of the shuffled complex evolution and simulated annealing algorithms
Authors: Thyer Mark , Kuczera George , Bates Bryson C.
Published: September 2002

Automatic optimization algorithms are used routinely to calibrate conceptual rainfall‐runoff (CRR) models. The goal of calibration is to estimate a feasible and unique (global) set of parameter estimates that best fit the observed runoff data. Most if not all optimization algorithms have difficulty in locating the global optimum because of response surfaces that contain multiple local optima with regions of attraction of differing size, discontinuities, and long ridges and valleys. Extensive research has been undertaken to develop efficient and robust global optimization algorithms over the last 10 years. This study compares the performance of two probabilistic global optimization methods: the shuffled complex evolution algorithm SCE‐UA, and the three‐phase simulated annealing algorithm SA‐SX. Both algorithms are used to calibrate two parameter sets of a modified version ofBoughton's [1984] SFB model using data from two Australian catchments that have low and high runoff yields. For the reduced, well‐identified parameter set the algorithms have a similar efficiency for the low‐yielding catchment, but SCE‐UA is almost twice as robust. Although the robustness of the algorithms is similar for the high‐yielding catchment, SCE‐UA is six times more efficient than SA‐SX. When fitting the full parameter set the performance of SA‐SX deteriorated markedly for both catchments. These results indicated that SCE‐UA's use of multiple complexes and shuffling provided a more effective search of the parameter space than SA‐SX's single simplex with stochastic step acceptance criterion, especially when the level of parameterization is increased. Examination of the response surface for the low‐yielding catchment revealed some reasons why SCE‐UA outperformed SA‐SX and why probabilistic optimization algorithms can experience difficulty in locating the global optimum.
Large‐time behavior of concentration variance and dilution in heterogeneous formations
Authors: Pannone Marilena , Kitanidis Peter K.
Published: September 2002

Consider the advection and dispersion of a conservative nonsorbing solute in a spatially variable but statistically homogeneous velocity field. A Lagrangian approach leads to expressions for the large‐time mean and variance of concentration. The expressions require only the macroscopic velocity vector Um, the macrodispersion tensor Dm, and an additional tensor Θ. The macroscopic velocities and macrodispersivities are well known from numerous previous studies, but Θ is introduced here for the first time. The tensor Θ is needed to describe the kinetics of dilution of a plume: Two cases with the same Um andDm have different dilution characteristics depending on Θ. The characteristic times of dilution are given by tensor ΘD−1m. It is demonstrated, for the first time through a Lagrangian approach, that the coefficient of variation of concentration at the center of the plume becomes proportionate to 1/t, as was previously shown in the Eulerian theory ofKapoor and Gelhar [1994a, b]. Expressions for the geometric mean of the dilution index and the reactor ratio are derived. Numerical simulations support the validity of the approach.
Observed spatial organization of soil moisture and its relation to terrain indices
Authors: Western Andrew W. , Grayson Rodger B. , Blöschl Günter , Willgoose Garry R. , McMahon Thomas A.
Published: September 2002

We analyze the degree of spatial organization of soil moisture and the ability of terrain attributes to predict that organization. By organization we mean systematic spatial variation or consistent spatial patterns. We use 13 observed spatial patterns of soil moisture, each based on over 500 point measurements, from the 10.5 ha Tarrawarra experimental catchment in Australia. The measured soil moisture patterns exhibit a high degree of organization during wet periods owing to surface and subsurface lateral redistribution of water. During dry periods there is little spatial organization. The shape of the distribution function of soil moisture changes seasonally and is influenced by the presence of spatial organization. Generally, it is quite different from the shape of the distribution functions of various topographic indices. A correlation analysis found that ln(a), where a is the specific upslope area, was the best univariate spatial predictor of soil moisture for wet conditions and that the potential radiation index was best during dry periods. Combinations of ln(a) or ln(a/tan(β)), where β is the surface slope, and the potential solar radiation index explain up to 61% of the spatial variation of soil moisture during wet periods and up to 22% during dry periods. These combinations explained the majority of the topographically organized component of the spatial variability of soil moisture a posteriori. A scale analysis indicated that indices that represent terrain convergence (such as ln(a) or ln(a/tan(β))) explain variability at all scales from 10 m up to the catchment scale and indices that represent the aspect of different hillslopes (such as the potential solar radiation index) explain variability at scales from 80 m to the catchment scale. The implications of these results are discussed in terms of the organizing processes and in terms of the use of terrain attributes in hydrologic modeling and scale studies. A major limitation on the predictive power of terrain indices is the degree of spatial organization present in the soil moisture pattern at the time for which the prediction is made.
Drag, turbulence, and diffusion in flow through emergent vegetation
Authors: Nepf H. M.
Published: September 2002

Aquatic plants convert mean kinetic energy into turbulent kinetic energy at the scale of the plant stems and branches. This energy transfer, linked to wake generation, affects vegetative drag and turbulence intensity. Drawing on this physical link, a model is developed to describe the drag, turbulence and diffusion for flow through emergent vegetation which for the first time captures the relevant underlying physics, and covers the natural range of vegetation density and stem Reynolds' numbers. The model is supported by laboratory and field observations. In addition, this work extends the cylinder‐based model for vegetative resistance by including the dependence of the drag coefficient, CD, on the stem population density, and introduces the importance of mechanical diffusion in vegetated flows.
A new method of analysis of water‐level response to a moving boundary of a longwall mine
Authors: Karaman Abdullah , Akhiev Seyidali S. , Carpenter Philip J.
Published: September 2002

Longwall coal mining is an economical method for coal extraction that allows most of the coal to be extracted from a wide rectangular panel. The roof of the working face area is temporarily held up by supports which advance as the mine face advances. A basin‐like subsidence trough develops at the ground surface over the panel as the panel roof behind the supports collapses. A dynamic subsidence front causes a water‐level drop at wells ahead of the panel. We examine the effects of subsidence on water‐level by introducing a sink that moves with the mining face, using the one‐dimensional flow equation. To test the validity of this method, we estimated aquifer parameters of Trivoli Sandstone aquifer over a longwall coal mine in the Illinois Basin by analyzing water‐level versus time data measured from three observation wells. The presented method predicts a value of transmissivity and storage coefficient that is reasonably close to the average of pumping test results. With this method we provide solutions to two significant problems: (1) Presubs(2) water‐level drops can be predicted for a planned longwall mine if the aquifer parameters are known.
Experimental and theoretical investigation of vaporization of liquid hydrocarbon mixtures in water‐wetted porous media
Authors: Liang Han‐Chih , Udell Kent S.
Published: September 2002

The evaporation of a separate‐phase, multicomponent liquid hydrocarbon mixture in water‐wetted porous media was studied to determine the role of water in the vaporization of contaminants during soil venting. Experiments were conducted in one‐dimensional (1‐D) water‐wetted sand columns containing liquid hydrocarbons under both through‐flow and bypass‐flow configurations. One‐dimensional through‐flow and bypass‐flow multicomponent drying models were applied to predict the removal rates of the hydrocarbons, as well as the effluent gas concentrations, liquid component concentration distribution in the sand, and the position of the evaporation front. Theoretical predictions agreed well with experimental data when water was present in residual amounts. Experimental findings show that the presence of water does not influence the hydrocarbon vaporization rates in both through‐flow and bypass‐flow drying cases, although the bypass‐flow theory predicts lower effluence in a system with high water content.
Chemical weathering and lithologic controls of water chemistry in a high‐elevation river system: Clark's Fork of the Yellowstone River, Wyoming and Montana
Authors: Horton Travis W. , Chamberlain C. Page , Fantle Matt , Blum Joel D.
Published: September 2002

Seasonal analyses of surface water geochemistry were conducted in the Clark's Fork of the Yellowstone watershed to determine whole‐rock weathering rates. The Clark's Fork of the Yellowstone is a high‐elevation catchment with distinct bedrock lithologies. Using dissolved solute concentrations and stream flow data, we calculated cation denudation rates of 119 g m−2 yr−1 (65,900 eq ha−1 yr−1) for carbonate‐rich sedimentary rocks, 16.6 g m−2 yr−1 (8200 eq ha−1 yr−1) for andesitic volcanics, and 9.8 g m−2 yr−1(5300 eq ha−1 yr−1) for granitic gneisses. Ca/Na ratios indicate that chemical weathering of disseminated calcite in granitic rocks contributes to the total solute load in these subcatchments. Removal of this c(2100 eq ha−1 yr−1).
Geographic Information Systems (GIS)‐based spatially distributed model for runoff routing
Authors: Olivera Francisco , Maidment David
Published: September 2002

A method is proposed for routing spatially distributed excess precipitation over a watershed to produce runoff at its outlet. The land surface is represented by a (raster) digital elevation model from which the stream network is derived. A routing response function is defined for each digital elevation model cell so that water movement from cell to cell can be convolved to give a response function along a flow path and responses from all cells can be summed to give the outlet hydrograph. An example application of analysis of runoff on Waller Creek in Austin, Texas, is presented.
Transport of sequentially decaying reaction products influenced by linear nonequilibrium sorption
Authors: Khandelwal Ashutosh , Rabideau Alan J.
Published: September 2002

Predicting the migration of sequentially degrading reaction products is important for the accurate analysis of subsurface treatment walls and other environmental systems. Recently, it has also been realized that consideration of rate‐limited sorption may be necessary to quantify the transport of contaminants in reactive treatment walls. In this paper, analytical and semianalytical solutions are developed for the one‐dimensional transport of a parent compound and three sequentially decaying reaction products influenced by nonequilibrium sorption. The solutions are based on the Laplace transformation technique and incorporate a different sorption distribution coefficient and mass transfer rate for each reaction product. Two types of exit boundary conditions are considered: semi‐infinite and finite zero‐gradient. The developed solutions are used to simulate the migration of trichloroethylene and its reaction products in a zero‐valent iron treatment wall. The effect of nonequilibrium sorption on the time to reach steady state is investigated, and it is shown that a zero‐valent iron treatment wall may exhibit a significant transient period. However, concentrations reach 80% of the steady state values within the first 100 pore volumes. It is also shown that neglecting dispersion may lead to nonconservative predictions of the effluent contaminant concentrations.
Measurement of 18O/16O in the soil‐atmosphere CO2 flux
Authors: Miller John B. , Yakir Dan , White James W. C. , Tans Pieter P.
Published: September 2002

Measurements of 18O in atmospheric CO2 can be used to trace gross photosynthetic and respiratory CO2 fluxes between the atmosphere and the terrestrial biosphere. However, this requires knowledge of the 18O signatures attributable to the fluxes from soil and leaves. Newly developed methods were employed to measure the 18O of soil‐respired CO2 and depth profiles of near‐surface soil CO2, in order to evaluate the factors influencing isotopic soil‐atmosphere CO2 exchange. The 18O of soil‐respired CO2 varied predominantly as a function of the 18O of soil water which, in turn, changed with soil drying and with seasonal variations in source water. The 18O of soil‐respired CO2 corresponds to full isotopic equilibrium with soil water at a depth ranging between 5 and 15 cm. The 18O of respired CO2, in reality, results from a weighted average of partial equilibria over a range of depths. Soil water isotopic enrichment of up to 10‰ in the top 5 cm did not appear to strongly influence the isotopic composition of the respired CO2. We demonstrate that during measurements “invasion” of atmospheric CO2 (the diffusion of ambient CO2 into the soil, followed by partial equilibration and retrodiffusion) must be considered to accurately calculate the 18O of the soil‐respired CO2. The impact of invasion in natural settings is also considered. We also have determined the effective kinetic fractionation of CO2 diffusion out of the soil to be 7.2 ± 0.3‰. High‐resolution (1 cm) depth profiles of 18O of near‐surface (top 10 cm) soil CO2 were carried out by gas chromatography—isotope ratio mass spectrometry (GC‐IRMS). This novel technique allowed us to observe the competitive diffusion‐equilibration process near the soil surface and to test simulations by a diffusion and equilibration model of the soil CO2 18O content.
An assessment of vegetation fire in Africa (1981–1991): Burned areas, burned biomass, and atmospheric emissions
Authors: Barbosa Paulo Marinho , Stroppiana Daniela , Grégoire Jean‐Marie , Cardoso Pereira José Miguel
Published: September 2002

This paper presents the first published time series of burned area maps of Africa, covering an 8 year period, 1981–1983 and 1985–1991. These maps were derived from the analysis of the advanced very high resolution radiometer (AVHRR) global area coverage (GAC) images at 5 km resolution. The burned area maps for the period 1985–1991 were used with biomass density and burning efficiency figures, to estimate the quantity of burned biomass during this 6 year period. Emission factors were further used to estimate the trace gas and aerosol emissions produced by vegetation fires. Biomass density was estimated based on values found in the literature and on the accumulated normalized difference vegetation index (NDVI) as derived from the remote sensing images. Burning efficiency was assessed with a dryness index that was based on the relative greenness index (RGI), also derived from the NDVI. Average emission factors were retrieved from the literature. The uncertainties in the burned area, biomass density, combustion efficiency, and emission factors were considered, with a total error of 51% for the burned biomass and 58% for the emission estimates. The results obtained for the burned biomass in Africa were compared with other values found in the literature and showed values lower by a factor of 1.1–3.3. The annual burned biomass from vegetation fires in Africa on average was estimated between 704 and 2168 Tg . In the same way, the atmospheric emissions on average ranges are as follows: CO2(990–3726 Tg), CO (40–151 Tg), CH4(1.2–4.4 Tg), NOx (2.8–10.6 Tg), and PM (< 2.5 μm) (3.3–12.4Tg).
Human impacts on the large world rivers: Would the Changjiang (Yangtze River) be an illustration?
Authors: Zhang J. , Zhang Z. F. , Liu S. M. , Wu Y. , Xiong H. , Chen H. T.
Published: September 2002

The “Three Gorges Project” (TGP) in the upstream of Changjiang (Yangtze River) has resulted in great concern of scientific society and public conversations on the economic, environmental, and human health issues. Data of nutrients from main stream and 15 large tributaries indicate that Changjiang receives a large part of its nutrient burden from the drainage area upstream the “Three Gorges Dam” (TGD). A model calculation shows that the construction of TGD may cause further eutrophication in the upstream region with phosphate as a limiting factor relative to nitrogen for photosynthesis. The estimated carbon fixation within the reservoir may equal to 10–20% of the actual particulate organic carbon (POC) budget for the Changjiang. Taking into account the fact that dissolved inorganic nitrogen (DlN) in the Changjiang has increased by a factor of 2 in the last 10–20 years, the expected N:P ratio of the river would reach 300–400 after the year 2010. Such a change in nutrient and organic carbon budgets of the Changjiang will significantly influence the environment and health of ecosystems of the adjacent shelf region.
The southern ocean at the Last Glacial Maximum: A strong sink for atmospheric carbon dioxide
Authors: Moore J. Keith , Abbott Mark R. , Richman James G. , Nelson David M.
Published: September 2002

Analysis of satellite ocean color, sea surface temperature, and sea ice cover data reveals consistent patterns between biological production, iron availability, and physical forcings in the Southern Ocean. The consistency of these patterns, in conjunction with information on physical conditions during the last glacial maximum (LGM), enables estimates of export production at the LGM. The LGM Southern Ocean experienced increased wind speeds, colder sea surface and atmospheric temperatures, increased deposition of atmospheric dust, and a greatly expanded winter sea ice cover. These variations had strong effects on Southern Ocean ecology and on air‐sea fluxes of CO2. The seasonal ice zone (SIZ) was much larger at the LGM (30 million km2) than at present (19 million km2). The Antarctic Polar Front (PF) likely marked the northern boundary of this expanded SIZ throughout the Southern Ocean, as it does today in the Drake Passage region. A large northward shift in the position of the PF during glacial times is unlikely due to topographic constraints. North of the PF, the increased flux of aeolian dust during glacial times altered phytoplankton species composition and increased export production, and as a result this region was a stronger sink for atmospheric CO2 than in the modern ocean. South of the PF, interactions between the biota and sea ice strongly influence air‐sea gas exchange over seasonal timescales. The combined influence of melting sea ice and increased aeolian dust flux (with its associated iron) increased both primary and export production by phytoplankton over daily‐monthly timescales during austral spring/summer, resulting in a strong flux of CO2 into the ocean. Heavy ice cover would have minimized air‐sea gas exchange over much of the rest of the year. Thus, an increased net flux of CO2 into the ocean is likely during glacial times, even in areas where annual primary production declined. We estimate that export production in the Southern Ocean as a whole was increased by 2.9‐3.6 Gt C yr−1 at the LGM, relative to the modern era. Altered seasonal sea ice dynamics would further increase the net flux of CO2 into the ocean. Thus the Southern Ocean was a strong sink for atmospheric CO2 and contributed substantially to the lowering of atmospheric CO2 levels during the last ice age.
Precise atmospheric oxygen measurements with a paramagnetic oxygen analyzer
Authors: Manning Andrew C. , Keeling Ralph F. , Severinghaus Jeffrey P.
Published: September 2002

A methodology has been developed for making continuous, high‐precision measurements of atmospheric oxygen concentrations by modifying a commercially available paramagnetic oxygen analyzer. Incorporating several design improvements, an effective precision of 0.2 ppm O2 from repeated measurements over a 1‐hour interval was achieved. This is sufficient to detect background changes in atmospheric O2 to a level that constrains various aspects of the global carbon cycle. The analyzer was used to measure atmospheric O2 in a semicontinuous fashion from air sampled from the end of Scripps Pier, La Jolla, California, and data from a 1‐week period in August 1996 are shown. The data exhibit strongly anticorrelated changes in O2 and CO2 caused by local or regional combustion of fossil fuels. During periods of steady background CO2 concentrations, however, we see additional variability in O2 concentrations, clearly not due to local combustion and presumably due to oceanic sources or sinks of O2. This variability suggests that in contrast to CO2, higher O2 sampling rates, such as those provided by continuous measurement programs, may be necessary to define an atmospheric O2 background and thus aid in validating and interpreting other O2 data from flask sampling programs. Our results have also demonstrated that this paramagnetic analyzer and gas handling design is well suited for making continuous measurements of atmospheric O2 and is suitable for placement at remote background air monitoring sites.
Stability of elemental carbon in a savanna soil
Authors: Bird M. I. , Moyo C. , Veenendaal E. M. , Lloyd J. , Frost P.
Published: September 2002

We have investigated the stability of oxidation‐resistant elemental carbon (OREC) in a sandy savanna soil at the Matopos fire trial site, Zimbabwe. The protection of some soil plots from fire for the last 50 years at this site has enabled a comparison of OREC abundances between those plots which have been protected from fire and plots which have continued to be burnt. The total 0–5 cm OREC inventory of the soil protected from fire is estimated to be 2.0±0.5 mg cm−2; approximately half the “natural” OREC inventory at the study site of 3.8±0.5 mg cm−2 (the mean for plots burnt every 1–5 years). The associated half‐life for natural OREC loss from the 0–5 cm interval of the protected plots is calculated to be <100 years, with the half‐life for large carbonized particles (>2000 μm) in the soil being considerably <50 years. These results suggest that at least in well‐aerated tropical soil environments, charcoal and OREC can be can be significantly degraded on decadal to centennial timescales. OREC abundance and carbon‐isotope data suggest that OREC in coarse particles is progressively degraded into finer particle sizes, with a concomitant increase in resistance to oxidative degradation of OREC in the finer particle sizes due to the progressive loss of more readily degraded OREC. It remains unclear whether the OREC that is degraded is oxidized completely to CO2 and subsequently emitted from the soil, reduced to a sufficiently small particle size to be illuviated to deeper parts of the soil profile, solubilized and lost from the profile as dissolved organic carbon or transmuted into a chemical form which is susceptible to attack by the acid‐dichromate reagent. The conclusion that a significant proportion of OREC can undergo natural degradation in well‐aerated environments on decadal/centennial timescales suggests that only a fraction of the total production of OREC from biomass burning and fossil fuel combustion is likely to be sequestered in the slow‐cycling “geological” carbon reservoir.
An eddy‐permitting coupled physical‐biological model of the North Atlantic: 2. Ecosystem dynamics and comparison with satellite and JGOFS local studies data
Authors: Oschlies Andreas , Koeve Wolfgang , Garçon Veronique
Published: September 2002

A model of biological production in the euphotic zone of the North Atlantic has been developed by coupling a Nitrate, Phytoplankton, Zooplankton, Detritus (NPZD) nitrogen‐based ecosystem model with an eddy‐permitting circulation model. The upper ocean physical and biological results are presented for an experiment with monthly climatological forcing. A comparison with satellite ocean color data shows that the model is capable of a realistic description of the main seasonal and regional patterns of surface chlorophyll. Agreement is also good for primary production except in the subtropical gyre where the model produces values more than an order of magnitude smaller than derived from satellite observations. In situ data available at Joint Global Ocean Flux Study (JGOFS) time series and local study sites (Bermuda Atlantic Time‐series Study (BATS), 32°N, 65°W; North Atlantic Bloom Experiment (NABE), 47°N, 2O°W; EUMELI oligotrophic, 21°N, 31°W) are used for a more detailed analysis of the model's capability to simultaneously reproduce seasonal ecosystem dynamics in different biological provinces of the North Atlantic Ocean. The seasonal cycle of phytoplankton biomass and nitrate is simulated quite realistically at all sites. Main discrepancies between model and observations are a large zooplankton peak, required by the model to end the phytoplankton spring bloom at the 47°N, 20°W site, and the underestimation of primary production at EUMELI and under oligotrophic summer conditions at BATS. The former model deficiency can be related to the neglect of phytoplankton aggregation; the latter is caused by too inefficient recycling of nutrients within the euphotic zone. Model improvements are suggested for further steps toward a realistic basin‐wide multiprovinces simulation with a single ecosystem model.
Soil C:N ratio as a predictor of annual riverine DOC flux at local and global scales
Authors: Aitkenhead J. A. , McDowell W. H.
Published: September 2002

Dissolved organic carbon (DOC) is important in a wide variety of chemical, physical, and biological processes in surface waters. We examined the relationship between DOC flux and soil C:N ratio on a biome basis. DOC fluxes for 164 rivers were subdivided into 15 biome types including tropical rain forest, coniferous forests, peatland, deciduous forests, mixed forests, and grasslands. A database of soil C:N ratios was constructed and subdivided into biome types. At a global scale, mean soil C:N ratio of a biome accounts for 99.2% of the variance in annual riverine DOC flux among biomes. The relationship between soil C:N ratio and DOC flux at the biome scale was used to predict annual riverine DOC flux at the watershed scale for three test watersheds not included in the original model. Predicted flux of each watershed was within 4.5% of the actual DOC flux. Using the C:N model, we estimated the total export of carbon from land to the oceans to be 3.6×1014 g yr−1. This empirical model should be useful in predicting changes in DOC flux under changing climatic conditions.
Mean annual cycle of the air‐sea oxygen flux: A global view
Authors: Najjar Raymond G. , Keeling Ralph F.
Published: September 2002

A global monthly‐mean climatology of the air‐sea oxygen flux is presented and discussed. The climatology is based on the ocean oxygen climatology of Najjar and Keeling [1997] and wind speeds derived from a meteorological analysis center. Seasonal variations are characterized by outgassing of oxygen during spring and summer and ingassing of oxygen during fall and winter, a pattern consistent with thermal and biological forcing of the air‐sea oxygen flux. The annual mean flux pattern is characterized by ingassing at high latitudes and the tropics and outgassing in middle latitudes. The air‐sea oxygen flux is shown to exhibit patterns that agree well with patterns seen in a marine primary productivity climatology, in model generated air‐sea O2 fluxes, in estimates of remineralization in the shallow aphotic zone based on seasonal oxygen variations, in observed seasonal nutrient‐temperature relationships, and in independent estimates of meridional oxygen transport in the Atlantic ocean. We also find that extratropical mixed layer new production during the spring‐summer period, computed from biological seasonal net outgassing of oxygen, is equivalent to the production of 4.5–5.6 Gt C, much lower than previous estimates based on atmospheric O2/N2 measurements.
Biological pump and vertical mixing in the southern ocean: Their impact on atmospheric CO2
Authors: Popova Ekaterina E. , Ryabchenko Vladimir A. , Fasham Mike J. R.
Published: September 2002

A global box model simulating nitrogen and carbon cycling in the ocean has been developed. The distinctive feature of the model is the detailed description of the seasonal cycles of the oceanic upper mixed layer (UML) ecosystem. Unlike other ocean regions, phytoplankton productivity in the Southern Ocean is assumed to be limited by low iron availability, leading to twofold decrease in the phytoplankton growth rate. Calculated ecosystem and carbon cycle characteristics are in a good agreement with available observational data and conceptual models of generalized phytoplankton seasonal cycles in the world ocean. The model estimates of the global ocean new production outside of shelf regions and the preindustrial atmospheric pCO2 are 9.9 Gt C/yr and 282 ppm, respectively. Results of numerical experiments with the model showed that the potential new production which might be reached by allowing phytoplankton maximum growth rate to increase is 29 Gt C/yr (76% of this increase is contributed by the Southern Ocean) and corresponds to an atmospheric pCO2 of 205 ppm; however, this would require an unrealistic tenfold increase in growth rate. The large contribution of the Southern Ocean is accounted for by the high‐nutrient, low‐chlorophyll (HNLC) conditions existing in this region caused by the high dissolved inorganic nitrogen concentrations below the UML, deep mixing during the austral summer, and iron limitation of phytoplankton productivity. A realistic (twofold) increase in the phytoplankton growth rate in the Southern Ocean which can be considered as a maximal effect of iron fertilization results in the lowering of atmospheric pCO2 by only 10 ppm. Changes in the UML depth in the Southern Ocean (a wintertime shallowing and summertime deepening of the UML in comparison with preindustrial conditions) could lead to a decrease of atmospheric pCO2 by 15 ppm at the most. The combined effect of iron fertilization and these changes in vertical mixing might constitute about 30–35 ppm, that is, less than one half of the lowering of 80 ppm during the last glaciation.
Global system of rivers: Its role in organizing continental land mass and defining land‐to‐ocean linkages
Authors: Vörösmarty C. J. , Fekete B. M. , Meybeck M. , Lammers R. B.
Published: September 2002

The spatial organization of the Earth's land mass is analyzed using a simulated topological network (STN‐30p) representing potential flow pathways across the entire nonglacierized surface of the globe at 30‐min (longitude × latitude) spatial resolution. We discuss a semiautomated procedure to develop this topology combining digital elevation models and manual network editing. STN‐30p was verified against several independent sources including map products and drainage basin statistics, although we found substantial inconsistency within the extant literature itself. A broad suite of diagnostics is offered that quantitatively describes individual grid cells, river segments, and complete drainage systems spanning orders 1 through 6 based on the Strahler classification scheme. Continental and global‐scale summaries of key STN‐30p attributes are given. Summaries are also presented which distinguish basins that potentially deliver discharge to an ocean (exorheic) from those that potentially empty into an internal receiving body (endorheic). A total of 59,122 individual grid cells constitutes the global nonglacierized land mass. At 30‐min spatial resolution, the cells are organized into 33,251 distinct river segments which define 6152 drainage basins. A global total of 133.1 × 106 km2 bear STN‐SOp flow paths with a total length of 3.24 × 106 km. The organization of river networks has an important role in linking land mass to ocean. From a continental perspective, low‐order river segments (orders 1‐3) drain the largest fraction of land (90%) and thus constitute a primary source area for runoff and constituents. From an oceanic perspective, however, the small number (n=101) of large drainage systems (orders 4‐6) predominates; draining 65% of global land area and subsuming a large fraction of the otherwise spatially remote low‐order rivers. Along river corridors, only 10% of land mass is within 100 km of a coastline, 25% is within 250 km, and 50% is within 750 km. The global mean distance to river mouth is 1050 km with individual continental values from 460 to 1340 km. The Mediterranean/Black Sea and Arctic Ocean are the most land‐dominated of all oceans with land:ocean area ratios of 4.4 and 1.2, respectively; remaining oceans show ratios from 0.55 to 0.13. We discuss limitations of the STN‐30p together with its potential role in future global change studies. STN‐30p is geographically linked to several hundred river discharge and chemistry monitoring stations to provide a framework for calibrating and validating macroscale hydrology and biogeochemical flux models.
Nitrous oxide emission from an agricultural field fertilized with liquid lagoonal swine effluent
Authors: Whalen S. C. , Phillips R. L. , Fischer E. N.
Published: September 2002

Contemporary agriculture is characterized by the intensive production of livestock in confined facilities and land application of stored waste as an organic fertilizer. Emission of nitrous oxide (N2O) from receiving soils is an important but poorly constrained term in the atmospheric N2O budget. In particular, there are few data for N2O emissions from spray fields associated with industrial scale swine production facilities that have rapidly expanded in the southeastern United States. In an intensive, 24‐day investigation over three spray cycles, we followed the time course for changes in N2O emission and soil physicochemical variables in an agricultural field irrigated with liquid lagoonal swine effluent. The total N (535 mg L−1) of the liquid waste was almost entirely NH4+−N (>90%) and thus had a low mineralization potential. Soil profiles for nitrification and denitrification indicated that >90% of potential activity was localized in the surface 20 cm. Application of this liquid fertilizer to warm (19° to 28°C) soils in a form that is both readily volatilized and immediately utilizable by the endogenous N‐cycling microbial community resulted in a sharp decline in soil NH4+−N and supported a rapid but short‐lived (i.e., days) burst of nitrification, denitrification, and N2O emission. Nitrous oxide fluxes as high as 9200 μg N2O−N m−2h−1were observed shortly after fertilization, but emissions decreased to prefertilization levels within a few days. Poor correlations between N2O efflux and soil physicochemical variables (temperature, moisture, NO3−−N, NH4+−N) and fertilizer loading rate point to the complexity of interacting factors affecting N2O production and emission. Total fertilizer N applied and N2O−N emitted were 29.7 g m−2(297 kg N ha−1) and 395 mg m−2, respectively. The fractional loss of applied N to N2O (corrected for background emission) was 1.4%, in agreement with the mean of 1.25% reported for mineral fertilizers. The direct effects of fertilizer application appear to be more immediate and short‐lived for liquid swine waste than for manures and slurries, which have a slower release of nitrogenous nutrients.
A stable isotopic study to determine carbon and nitrogen cycling in a disturbed Southern Californian forest ecosystem
Authors: Korontzi Stefania , Macko Stephen A. , Anderson Iris C. , Poth Mark A.
Published: September 2002

This study utilized isotope analyses to contrast nitrogen and carbon dynamics at four sites located along an air pollution gradient in the San Bernardino National Forest in southern California. Natural 15N and 13C abundances along with nutritional and edaphic properties were determined in soil, litter, and vegetation samples. Mean bulk nitrogen δ15N values of soil and vegetation at Camp Paivika (CP), the most polluted site, were at least 1.7‰ more enriched than the other, less polluted sites. Mean soil δ15NH4+ was also significantly enriched in 15N at CP compared to Barton Flats (BF), the least polluted site, by 3.8‰. Soil δ15NO3− signatures were not statistically different among sites. The litter δ15NH4+ values followed a trend similar to that of the soil. Furthermore, the litter δ15NO3− at CP was significantly depleted in 15N compared to the other sites. The isotopic discrimination for the eventual production of nitrate from organic nitrogen in soil and litter was maximized at CP and minimized at BF. A stable carbon isotopic gradient of decreasing soil, litter, and foliar δ13C was also observed with increasing site pollution level. These results support the hypothesis that chronic atmospheric deposition has enhanced nitrogen cycling processes and has affected carbon metabolism at CP.
Variations in global mean sea level associated with the 1997–1998 ENSO event: Implications for measuring long term sea level change
Authors: Nerem R. S. , Chambers D. P. , Leuliette E. W. , Mitchum G. T. , Giese B. S.
Published: September 2002

The TOPEX/POSEIDON satellite has observed variations in global mean sea level with a precision of 4 mm at 10‐day intervals since late 1992. During the 1997–1998 ENSO event, a 20 mm rise, and subsequent fall, of mean sea level was observed. These changes are well correlated with global mean sea surface temperature anomalies, which exhibit a similar response for every major ENSO event since 1981, suggesting the observed mean sea level change is mostly caused by thermal expansion. An Empirical Orthogonal Function analysis of the altimeter‐derived sea level maps also suggests a connection with ENSO. We observed the same signal in global mean dynamic heights of the MOM2 ocean model and in anomalies of global mean precipitable water vapor. The presence of ENSO‐variability in global mean sea level suggests that detecting the much smaller sea level variations associated with climate change will require at least a decade of precise altimeter measurements.
An investigation of the link between lead‐induced thermohaline convection and Arctic eddies
Authors: Bush John W. M. , Woods Andrew W.
Published: September 2002

A recent laboratory study indicates that a turbulent buoyant line plume discharging into a rotating stratified fluid provides a natural mechanism for generating a series of anticyclonic geostrophic vortices [Bush and Woods 1999]. Here we examine the implications of these experimental results in the context of the thermohaline convection which develops when saline water is released as leads freeze over in the polar oceans. Using the experimental results in conjunction with a simple numerical model of plume dynamics in a non‐uniformly stratified environment, we develop a model which characterizes the geometry of the eddies which would develop owing to lead‐induced convection. The model predicts that, in the absence of strong currents, lead‐induced thermohaline convection may generate anticyclonic geostrophic vortices of characteristic radius 2–10 km at the base of the mixed layer; however, this mechanism cannot account for the arctic eddies observed at substantial depths beneath the mixed layer, cyclonic, or strongly ageostrophic eddies.
Lagrangian measurement of mid‐depth currents in the eastern tropical Pacific
Authors: Thomson Richard E. , Freeland Howard J.
Published: September 2002

Helium tracer data and basin‐scale steric height maps imply that there is an eastward‐flowing, intermediate‐depth (≈ 1000 m) countercurrent near 20°N in the eastern tropical Pacific. To measure this current directly, we deployed Profiling Autonomous Lagrangian Current Explorer (P‐ALACE) drifters at depths of 1000 and 1300 m near Hawaii (≈ 19°N, 156°W) in June 1997. The deployment site marks the summit of Loihi Seamount, an active hydrothermal venting region thought to be the source for the helium anomaly. Contrary to expectation, the drifters first moved to the southwest, then turned abruptly westward along ≈ 18°N. The drifters maintained their westward trajectories for one year before stalling in an eddying region near 165° to 168°W. The westward intermediate flow and location of the current reversal region observed by the drifters appear to be at variance with the average flow fields derived from the helium and steric height data.
metastability of gas hydrate
Authors: Buffett Bruce A. , Zatsepina Olga Y.
Published: September 2002

Marine occurrences of gas hydrate are normally confined to the top few hundred meters of sediments along deep continental margins. The zone of stability for gas hydrate is limited in depth by increases in temperature below the seafloor. We use thermodynamic calculations to show that gas hydrate can exist in a metastable state below the usual base of the stability zone. We estimate that gas hydrate can be overheated by several degrees and that it may persist in this metastable state in the seafloor for as long as 106 years. Sudden decomposition of metastable hydrate should produce substantial pore pressure in the sediments, contributing to slope failure in locations where gas hydrate is found. Such a mechanism might help to explain why slumping appears to be more frequent than average during the interval around the last glacial maximum.
Slow variations in mean path of the Gulf Stream east of Cape Hatteras
Authors: Rossby T. , Benway R. L.
Published: September 2002

The Gulf Stream between Cape Hatteras and the New England Seamounts varies in position over a wide range of time scales. We present evidence that interannual path changes may be governed by a time‐varying outflow of waters from the Labrador Shelf region. When the shelf and Slope Waters south of New England turn cold and fresh, the stream frequently assumes a more southerly track and conversely when the waters are warm and saline. These changes in path and surface water properties most likely result from large variations in shelf transport and thus greater or less dilution of the warm saline Gulf Stream waters that continually leak from the current into the Slope Sea. These observations point to a thermohaline rather than winddriven mechanism for governing the path of the Gulf Stream after it leaves the coast.
Accelerating carbonate dissolution to sequester carbon dioxide in the ocean: Geochemical implications
Authors: Caldeira Ken , Rau Greg H.
Published: September 2002

Various methods have been proposed for mitigating release of anthropogenic CO2 to the atmosphere, including deep‐sea injection of CO2 captured from fossil‐fuel fired power plants. Here, we use a schematic model of ocean chemistry and transport to analyze the geochemical consequences of a new method for separating carbon dioxide from a waste gas stream and sequestering it in the ocean. This method involves reacting CO2‐rich power‐plant gases with seawater to produce a carbonic acid solution which in turn is reacted on site with carbonate mineral (e.g., limestone) to form Ca2+ and bicarbonate in solution, which can then be released and diluted in the ocean. Such a process is similar to carbonate weathering and dissolution which would have otherwise occurred naturally, but over many millennia. Relative to atmospheric release or direct ocean CO2 injection, this method would greatly expand the capacity of the ocean to store anthropogenic carbon while minimizing environmental impacts of this carbon on ocean biota. This carbonate‐dissolution technique may be more cost‐effective and less environmentally harmful, and than previously proposed CO2 capture and sequestration techniques.
Arctic oscillation and Arctic sea‐ice oscillation
Authors: Wang Jia , Ikeda Moto
Published: September 2002

The variability of the sea‐ice cover in the Arctic and subpolar regions associated with the Arctic Oscillation (AO) was investigated using historical data from 1901 to 1997. Unrotating principal component analyses (or empirical orthogonal functions, EOFs) were applied to demeaned, normalized sea‐level pressure (SLP), surface air temperature (SAT), and sea‐ice area (SIA) for the periods 1901–97 and 1953–97. The leading SLP EOF mode is the AO. The leading SIA EOF mode is named the Arctic Sea‐Ice Oscillation (ASIO), which accounts for 41% of the total variance for the period of 1901‐1995. This dominant ASIO is AO‐related; its spatial and temporal patterns are consistent with the leading modes of SLP and SAT, and with the total arctic sea‐ice anomalies. The second SIA mode is North Atlantic Oscillation (NAO)‐related because sea‐ice anomalies in the Labrador Sea region and the Greenland Sea region are out of phase. During the last three decades, the arctic sea ice has significantly decreased, which may be the decreasing phase of long term variations.
Oceanographic evidence for a transient geothermal event affecting the Mid‐Atlantic Ridge
Authors: Murton Bramley J. , Redbourn Lisa J.
Published: September 2002

Long‐term monitoring has revealed variations in the temperature and salinity of bottom water overlying the axis of the Mid‐Atlantic Ridge. These are attributed to three primary mechanisms: externally introduced changes in the water mass; topography‐driven vertical mixing; and geothermal heating. Here, we focus on anomalous temperature changes that were induced within the ridge axis. Over a period of 35 days the bottom water warmed by up to 0.05°C but without changing its salinity. During the following 60 days, anomalously warm water was advected out of the southwestern end of the ridge axis at a depth of 2850 m. We attribute the temperature anomaly to a geothermal event, triggered by either magmatic or tectonic activity, during which ∼6.9 × 1015 J of heat were dissipated. This is equivalent to the emplacement and cooling of 1.5 ± 0.15 × 106 m³ of basaltic material on, or into, the shallow oceanic crust. Alternatively, it could have been caused by the release of an upper crustal reservoir of warm, 50°C above ambient, with a volume of 3.4 × 107 m³.