bluuebunny/crossref_metadata_2025 huggingface dataset:
Recent tectonic evolution and present stress in the Northern Apennines (Italy)
Authors: Mariucci Maria Teresa , Amato Alessandro , Montone Paola
Published: September 2002

The present‐day stress field and its recent tectonic evolution in the Northern Apennines are reconstructed from borehole breakout analysis and focal mechanisms of crustal earthquakes and through the comparison with paleostress data. We have considered 86 wells for breakout analysis, with depths down to 6–7 km, 125 fault plane solutions of crustal earthquakes with M<5 that occurred between 1988 and 1995 in the Northern and Central Apennines, and data of stronger earthquakes (M≤6) reported in other studies. The Tyrrhenian coastal region and the Apenninic belt are characterized by Shmin direction mainly trending NE–SW, with predominantly normal fault plane solutions. Along the outer front of the belt and the Adriatic offshore, Shmin is oriented NW–SE, and focal solutions are thrust or strike‐slip, with maximum compression around NE–SW. Conversely, south of 43°N, breakouts evidence an orthogonal direction of horizontal compression (NW–SE), following the Southern Apennine trend, where a widespread NE–SW extension was recognized by previous investigations. Comparing these results to the recent tectonic evolution inferred from structural geology, we argue that the extension‐compression pair, characteristic of the post‐Tortonian evolution of the mountain belt, has been migrating in time from late Miocene to Present only in the northern sector of the arc, whereas the southern sector underwent a generalized extension, at least since middle Pleistocene. The striking correspondence between the active compression front and the region with evidence of a remnant subducted slab suggests that the migrating extension‐compression pair has been controlled by progressive retreat of the slab.
Numerical modeling of simultaneous extension and compression: The Valencia trough (western Mediterranean)
Authors: Negredo A. , Fernàndez M. , Torné M. , Doglioni C.
Published: September 2002

A two‐dimesional kinematic model, which incorporates lateral accommodation of extension and differential stretching, is used to investigate the geodynamic evolution of the Valencia trough. This Neogene basin, located between the Iberian Peninsula and the Balearic Promontory, underwent two rifting episodes, the first one being coeval with compression. Model predictions are compared with observations of tectonic subsidence, Bouguer gravity anomaly, surface heat flow, and the present‐day crustal structure. The best fitting model assumes three stages in the evolution of the basin. During the first stage (late Oligocene‐early Miocene), extension is mainly restricted to the central and northwestern parts of the basin, whereas its southeastern margin is locked by the Alpine deformation front. The material expelled from the axial area is partly accommodated under the Balearic Promontory, producing compression and crustal thickening. During the second rifting stage (middle Miocene), extension affects a much broader area, including the Balearic Promontory and the Algero‐Provençal basin. The third stage (late Miocene to Recent) corresponds to a postrift thermal relaxation phase. Our results put further constraints on the geodynamic evolution of the area.
Tectonic evolution of the Priest River complex, northern Idaho and Washington: A reappraisal of the Newport fault with new insights on metamorphic core complex formation
Authors: Doughty P. Ted , Price Raymond A.
Published: September 2002

New geologic mapping, 40Ar/39Ar thermochronometry, and geobarometry in the Middle Eocene Priest River metamorphic core complex provide the basis for unraveling the role of en echelon fault systems in core complex formation and for determining the scale of crustal fragments that form during continental extension. Four faults occur in the Priest River complex. The east verging Purcell Trench fault zone on the eastern side consists of two distinct en echelon fault segments separated by an unfaulted homocline. The U‐shaped Newport fault system on the northwestern side is a conjugate normal fault set. The west verging eastern Newport fault terminates within the Silver Point Wrencoe pluton, which was intruded syntectonically into the fault zone. The east verging western Newport fault merges with the east verging Spokane dome mylonite zone in the underlying infrastructure. New geobarometric data show that this midcrustal shear zone, which evidently forms part of the regional basal décollement of the Cordilleran fold and thrust belt, also records significant Eocene extensional shearing. Rocks that formed beneath the mylonite zone at a depth of 30–35 km are juxtaposed against rocks that formed at a depth of 10 km above the zone. Eocene 40Ar/39Ar chrontours in the southern part of the infrastructure record progressive exhumation and quenching that becomes younger eastward. In the northern fragment of the infrastructure, alternating domains of progressive westward exhumation/quenching and progressive eastward exhumation/quenching occur beneath the eastern Newport fault and the northern Purcell Trench fault, respectively. These relationships form the basis for a new model of the evolution of the Priest River complex. The southern part of the infrastructure was exhumed by a major east verging detachment system comprising the western Newport fault and the reactivated eastern part of the Spokane dome mylonite zone, into which the western Newport fault merges. This master detachment fault, which roots in the southern Purcell Trench on the east side of the core complex, plunges beneath the northern part of the infrastructure (the Selkirk Crest block). Below the detachment, the infrastructure appears to be intact; above the detachment, the crust extended along a set of relaying conjugate detachment faults. These faults are the eastern Newport fault and the north and south Purcell Trench faults. Kinematic analysis shows that the Selkirk Crest block is a crustal‐scale extensional horse that was stranded as continued extension moved the underlying metamorphic infrastructure out from beneath it and toward the west along the master detachment. This study shows that large tracts of midcrustal rocks can be translated and stranded as allochthonous fragments during continental crustal extension.
Multiaffine random field model of rainfall
Authors: Menabde Merab , Seed Alan , Harris Daniel , Austin Geoff
Published: September 2002

A new method for the statistical description and simulation of two‐dimensional rainfall radar images is proposed. It is based on the theory of multiaffine random fields and the bounded lognormal cascade model. The model has three free parameters, which are shown to provide a parsimonious and robust statistical description of rainfall images. The parameters retrieved from the analysis of the real rainfall data by fitting the one‐ and two‐point statistics are used for simulation. The simulated rainfall fields are in good statistical and visual agreement with their real counterparts.
Three‐dimensional analysis of the capture of contaminated leachate by fully penetrating, partially penetrating, and horizontal wells
Authors: Steward David R.
Published: September 2002

The capture of a contaminated leachate by wells is examined for a fully penetrating well, a partially penetrating well, series of partially penetrating wells, and a horizontal well. The capture zones for these wells are illustrated in figures. Charts are used to analyze the minimum pumping rates required to capture a contaminated leachate over a range of aquifer parameters. It is shown that fully penetrating and partially penetrating wells have the same minimum pumping rates when the ratio of the aquifer thickness to the size of the contaminated region is <0.2; when this ratio is >0.5, a partially penetrating well has a much lower pumping rate. A horizontal well that is oriented perpendicularly to flow has the lowest pumping rate; this well can capture a contaminant with a pumping rate of ∼1.5–2 times the contaminated discharge.
Long‐term trends in stream water and precipitation chemistry at five headwater basins in the northeastern United States
Authors: Clow David W. , Mast M. Alisa
Published: September 2002

Stream water data from five headwater basins in the northeastern United States covering water years 1968–1996 and precipitation data from eight nearby precipitation monitoring sites covering water years 1984‐1996 were analyzed for temporal trends in chemistry using the nonparametric seasonal Kendall test. Concentrations of SO4declined at three of five streams during 1968–1996 (p < 0.1), and all of the streams exhibited downward trends in SO4 over the second half of the period (1984–1996). Concentrations of SO4 in precipitation declined at seven of eight sites from 1984 to 1996, and the magnitudes of the declines (−0.7 to −2.0 µeq L−1 yr−1) generally were similar to those of stream water SO4. These results indicate that changes in precipitation SO4 were of sufficient magnitude to account for changes in stream water SO4. Concentrations of Ca + Mg declined at three of five streams and five of eight precipitation sites from 1984 to 1996. Precipitation acidity decreased at five of eight sites during the same period, but alkalinity increased in only one stream. In most cases the decreases in stream water SO4 were similar in magnitude to declines in stream water Ca + Mg, which is consistent with the theory of leaching by mobile acid anions in soils. In precipitation the magnitudes of SO4 declines were similar to those of hydrogen, and declines in Ca +Mg were much smaller. This indicates that recent decreases in SO4 deposition are now being reflected in reduced precipitation acidity. The lack of widespread increases in stream water alkalinity, despite the prevalence of downward trends in stream water SO4, suggests that at most sites, increases in stream water pH and acid‐neutralizing capacity may be delayed until higher soil base‐saturation levels are achieved.
Improved method for the determination of nonpurgeable suspended organic carbon in natural water by silver filter filtration, wet chemical oxidation, and infrared spectrometry
Authors: Burkhardt Mark R. , Brenton Ronald W. , Kammer James A. , Jha Virenda K. , O'Mara‐Lopez Peggy G. , Woodworth Mark T.
Published: September 2002

Precision and accuracy are reported for the first time for the analysis of nonpurgeable suspended organic carbon by silver membrane filtration followed by wet chemical oxidation. A water sample is pressure filtered through a 0.45‐μm‐pore‐size, 47‐mm‐diameter silver membrane filter. The silver membrane filter then is cut into ribbons and placed in a flame‐sealable glass ampule. The organic material trapped on the membrane filter strips is acidified, purged with oxygen to remove inorganic carbonates and volatile organic compounds, and oxidized to carbon dioxide (CO2) using phosphoric acid and potassium persulfate in the sealed glass ampule. The resulting CO2 is measured by a nondispersive infrared CO2 detector. The amount of CO2 is proportional to the concentration of chemically oxidizable nonpurgeable organic carbon in the environmental water sample. The quantitation and method detection limit for routine analysis is 0.2 mg/L. The average percent recovery in five representative matrices was 97 ± 11%. The errors associated with sampling and sample preparation of nonpurgeable suspended organic carbon are also described.
Upscaling transmissivity under radially convergent flow in heterogeneous media
Authors: Sánchez‐Vila Xavier , Axness Carl L. , Carrera Jesús
Published: September 2002

Most field methods used to estimate transmissivity values rely on the analysis of drawdown under convergent flow conditions. For a single well in a homogeneous and isotropic aquifer and under steady state flow conditions, drawdown s is directly related to the pumping rate Q through transmissivity T. In real, nonhomogeneous aquifers, s and Q are still directly related, now through a value called equivalent transmissivity Teq. In this context, Teq is defined as the value that best fits Thiem's equation and would, for example, be the transmissivity assigned to the well location in the classical interpretation of a steady state pumping test. This equivalent or upscaled transmissivity is clearly not a local value but is some representative value of a certain area surrounding the well. In this paper we present an analytical solution for upscaling transmissivities under radially convergent steady state flow conditions produced by constant pumping from a well of radius rw in a heterogeneous aquifer based upon an extension of Thiem's equation. Using a perturbation expansion, we derive a second‐order expression for Teq given as a weighted average of the fluctuations in log T throughout the domain. This expression is compared to other averaging formulae from the literature, and differences are pointed out. Teq depends upon an infinite series which may be expressed in terms of coefficients of the finite Fourier transform of the log transmissivity function. Sufficient conditions for convergence of this series are examined. Finally, we show that our solution agrees with existing analytical ones to second order and test the solution with a numerical example.
Lateral expansion of preferential flow paths in sands
Authors: DiCarlo David A. , Bauters Tim W. J. , Darnault Christophe J. G. , Steenhuis Tammo S. , Parlange J.‐Yves
Published: September 2002

The stability and persistence of preferential flow paths in sands can determine the flow paths of subsequent infiltration events. We have measured the evolution of preferential flow paths in a slab of sandy soil using an array of tensiometers and light transmission. The pressure and water content measurements show that the nonuniform moisture content exists even when the potentials are equalized horizontally and that then are the result of hysteresis in the soil's pressure‐saturation relationship. The equalization of potential takes place over several days, if at all, and is consistent, initially, with estimates of vapor transport out of the finger cores. Once the soil is wet enough, the remainder of water movement takes place in liquid films. Hysteresis produces another interesting situation when the pack is drained. We find that the wetter portions of the soil can be at a lower potential than the drier portions, resulting in a horizontal driving force for a flow of water from the drier to the wetter soil.
On the asymmetric response of aquifer water level to floods and droughts in Illinois
Authors: Eltahir Elfatih A. B. , Yeh Pat J.‐F.
Published: September 2002

Here we analyze observed characteristics of the natural variability in the regional‐scale hydrological cycle of Illinois, including the soil and atmospheric branches. This analysis is based on a consistent data set that describes several hydrological variables: the flux of atmospheric water vapor, incoming solar radiation, precipitation, soil moisture content, aquifer water level, and river flow. The climatology of the average regional hydrological cycle has been estimated. Variability in incoming solar radiation, not precipitation, is the main forcing of the seasonal variability in evaporation, soil moisture content, aquifer water level, and river flow. While precipitation plays a minor role in shaping the natural variability in the regional hydrological cycle at the seasonal timescale, variability in precipitation is the major factor in shaping the natural variability in the regional hydrological cycle at the interannual timescale. The anomalies in the different variables of the regional hydrological cycle have been computed and the persistence patterns of extreme floods and droughts have been compared. The 1988 drought left a signature in the aquifer water level that is significantly more persistent than the corresponding signature for the 1993 summer flood. The discharge from unconfined groundwater aquifers to streams (base flow) provides an efficient dissipation mechanism for the wet anomalies in aquifer water level. However, the nonlinear dependence of the groundwater discharge on aquifer water level (groundwater rating curve) may explain why droughts leave a significantly more persistent signature on groundwater hydrology, in comparison to the signature of floods. This nonlinearity has been attributed to the increasing degree by which the unconfined aquifers get connected to the channels network, as the aquifer water level rises leading to higher drainage density. The potential implications of these results regarding the impact on regional water resources due to any future climate change are discussed.
An analytical solution of groundwater response to tidal fluctuation in a leaky confined aquifer
Authors: Jiao Jiu Jimmy , Tang Zhonghua
Published: September 2002

An analytical solution is derived to investigate the influence of leakage on tidal response in a coastal leaky confined aquifer system. The analytical solution developed here is more general than the traditional solution obtained by Ferris [1951], which can be regarded as a special case of the solution presented in this paper. This solution is based on a conceptual model under the assumption that the groundwater level in the confined aquifer fluctuates in response to sea tide while that of the overlying unconfined aquifer remains constant. This conceptual model is supported by numerous field studies by previous researchers which have demonstrated that the tidal response in an unconfined aquifer may be negligible compared to that in a confined aquifer. The leakage has a significant impact on the tidal behavior of the confined aquifer. Hypothetical studies indicate that both tidal amplitude of groundwater head in the aquifer and the distance over which the aquifer can be disturbed by the sea tide will be considerably reduced because of the existence of leakage. This analytical solution is used to investigate the tidal and piezometer data at the Chek Lap Kok airport, Hong Kong Special Administrative Region, People's Republic of China.
Combined visual and quantitative techniques in the study of natural turbulent flows
Authors: Roy André G. , Biron Pascale M. , Buffin‐Bélanger Thomas , Levasseur Myléne
Published: September 2002

An important difficulty in understanding turbulence in natural flows is that one does not see what is being measured by devices recording velocity fluctuations. This study shows how visualization of turbulent structures can be effectively implemented in natural river flows. In order to match turbulent flow structures with their velocity signatures, flow visualization is combined with high‐frequency (20 Hz) velocity measurements from electromagnetic current meters. Two fluvial environments are examined. In a river confluence where the difference in turbidity provides a natural tracer, very strong negative velocity fluctuations for the streamwise component are concomitant with an upwelling motion. A video recording made from a bridge of the passage of turbulent structures delineated by turbid and clear water allowed us to relate the marked fluctuations in velocity to eddies of a duration of 1–2 s in the mixing layer. In a gravel‐bed river we have examined patterns of eddy shedding in the lee of a pebble cluster. These patterns were highlighted by dye (milk) injected in the separation zone downstream. Black and white images were obtained from an underwater lens linked to a video camera recorder. We observed a complex dynamic characterized by two dominant patterns of eddy shedding: one with a clear motion of eddies toward the water surface and another with a downward expansion toward the bed of the structures. This pattern is related to the passage of intermittent wedges of low‐speed fluid separated by wedges of fast moving fluid.
Boundary effects on solute transport in finite soil columns
Authors: Schwartz R. C. , McInnes K. J. , Juo A. S. R. , Wilding L. P. , Reddell D. L.
Published: September 2002

This study investigates the influence of inlet and outlet disturbances and formulated boundary conditions on the estimation of the dispersion coefficient and retardation factor for short soil columns. Unsaturated miscible displacement experiments utilizing a Br− tracer were carried out on undisturbed columns of a fine‐textured Ultisol. Solutions were applied using either a fritted plate or an array of dispensing tips that produced droplets at a prescribed flow rate. One‐ and two‐layer analytical solutions of the advective‐dispersive equation were fitted to effluent concentrations using nonlinear least squares parameter optimization. Comparison of two‐layer simulations with experimental data indicated that the analytical solution with a semi‐infinite interface boundary best approximated effluent concentrations under the conditions of this study. This solution corresponds to a continuous flux concentration and a macroscopically discontinuous resident concentration at the interface between the soil and porous plates. Parameter estimates were not significantly different with respect to the application method used at the inlet. This may be attributed to a less uniform distribution of solution onto the soil surface by the drip apparatus and/or by the presence of stagnant regions within the inlet reservoir and hence increased dispersion within the inlet platen apparatus. Two‐layer simulations indicated that the dispersion coefficient was underestimated by 14–27% when the influence of the inlet and outlet apparatus were not included in the fitted solution of the advective‐dispersive equation. In addition, use of one‐layer analytical solutions caused the retardation factor to be overestimated by no more than the fractional increase in pore volume imparted by the platen apparatus.
Revisiting steady state water flow in a saturated, inclined soil slab
Authors: Prunty Lyle , Padmanabhan G.
Published: September 2002

Mathematical models of flow in a saturated, inclined soil slab with an impermeable lower boundary have contributed substantially to understanding of important processes in the environment. One such process is movement of chemicals induced by interflow in sloping soil. Certain aspects of previous mathematical treatment regarding the inclined soil slab problem have, however, escaped scrutiny. The original 1965 treatment includes a peculiar quadratic boundary condition term that applies over a vanishingly small fraction of the soil surface. The quadratic term is shown to be unjustified and unnecessary in obtaining practical solutions. Mathematical solutions of two more general inclined slab geometries, three‐dimensional and parallelogram slabs, are presented and solved. The solutions are illustrated in terms of figures showing selected flow paths and equipotentials.
Isotopic variability of N2O emissions from tropical forest soils
Authors: Pérez T. , Trumbore S. E. , Tyler S. C. , Davidson E. A. , Keller M. , de Camargo P. B.
Published: September 2002

We report measurements of the 15N and 18O signature of N2O emitted from tropical rain forest soils at the La Selva Biological station in Costa Rica and in the Fazenda Vitoria in the State of Pará, Brazil. The δ15N values ranged from −34 to 2‰ with respect to atmospheric N2, while δ18O values had a smaller range, from −4 to 18‰ with respect to atmospheric O2. We attribute these large variations to differences in microbial production, consumption, and transport of N2O. In general the δ15N of N2O emissions from an Oxisol soil in Brazil were consistently enriched by ∼20‰ in 15N compared to those from Ultisol and Inceptisol soils in Costa Rica. Denitrification is the most likely source of N2O in both locations during the rainy season, and the 15N of nitrate was similar in both locations. We attribute the overall variability in emitted 15N to differences in the ratio of N2O:N2 escaping from the soil to the atmosphere, with a larger fraction of the N2O reduced to N2 at the Brazilian sites. We found light δ15N‐N2O values associated with high N2O emissions in a fertilized agricultural site in Costa Rica and in a “hot spot” of high emissions in the forest site in Brazil. This result suggests that the increase of substrate availability might increase the fractionation associated with N2O production. Overall, the Brazilian Oxisol soils had the most enriched δ15N‐N2O emissions yet measured from soils. If these are more representative of tropical soil emissions than the Costa Rica emissions, then the globally averaged δ15N‐N2O tropical rain forest soil source is more enriched than previously estimated. The large variations in isotopic signature for N2O emissions demonstrate the potential utility of stable isotopes as tools for understanding the processes of N2O production and consumption in soils.
On the use of dissolved aluminum in surface waters to estimate dust deposition to the ocean
Authors: Measures C. I. , Vink S.
Published: September 2002

The concentration of dissolved Al in surface waters from various oceanic regimes is used in a simple model to calculate the annual amount of dust deposited to the surface ocean. Calculated values range from 0.015 to 9.9 g dust m−2 yr−1. Comparison of these calculated dust depositions with independent dust deposition estimates obtained from direct measurements, suspended atmospheric dust loads, or sediment traps show remarkably good agreement over approximately 3 orders of magnitude. In regions where the agreement between the model and other estimates is weakest, it is anticipated that local scaling of model parameters such as mixed layer depth and surface water residence time, will lead to improved agreement. Since surface water Al concentrations appear to be driven primarily by dust deposition, the distribution of dissolved Al in surface waters can be used to investigate the systematics of the delivery of other biologically important trace elements, for example, Fe, to the surface of the remote ocean by this route. In addition, temporal variations in surface water Al concentrations can be used to investigate the biogeochemical consequences to the surface ocean of large‐scale changes in atmospheric dust loads driven by decadal‐scale climatic variations.
Relationships between methane production and emission to lacunal methane concentrations in rice
Authors: Byrd G. T. , Fisher F. M. , Sass R. L.
Published: September 2002

We measured lacunal methane concentrations in field‐grown rice plants as a correlative to both methane production and emissions. Using a gas‐tight syringe, 100‐μL samples were withdrawn from plant lacunar spaces below the water level and diluted to provide enough volume for analysis by gas chromatography. Lacunal methane concentrations increased throughout the season and, for each sampling date, were usually significantly higher in the cultivars Mars and Cypress (high emitters) when compared with Lemont and Della (low emitters). The field site influenced lacunal methane concentrations, wherein greater lacunal methane concentrations corresponded with greater methane. Methane emission rates were positively correlated with plant lacunal methane concentrations for each cultivar, with an improvement in the relationship during the preheading season. With increases in methane production determined by emissions following field‐induced anoxia, lacunal methane concentrations increased accordingly. Lacunal methane concentrations also clearly increased as plant biomass increased, but the relationship depended on field location, which also influenced emissions. Sampling lacunal methane concentrations of rice plants, although labor intensive, is quite flexible, using little field equipment, and may provide an effective alternative to large‐scale flux measurements in areas not easily accessible.
Changes in CH4 emission from rice fields From 1960 to 1990s: 2. The declining use of organic inputs in rice farming
Authors: van der Gon Hugo Denier
Published: September 2002

Previous studies have shown that organic amendments used in paddy rice cultivation strongly enhance methane emission from rice fields. Because of the increased availability of mineral fertilizer and the labor‐intensive nature of many organic amendments, it is likely that the use of organic amendments in rice farming has decreased in recent years. If so, a significant decrease in CH4 emissions from rice fields could be the result, but quantification is difficult because data on organic amendments are scarce. In this paper data on green manure application in China are used to quantify the impact of changes in an organic amendment on CH4 emission. The trend in CH4 emissions over time, taking green manure application explicitly into account, suggests that CH4 emissions from Chinese rice fields from the 1970s to the early 1980s may have been considerably higher than at present. The results presented here and in a previous study [Denier van der Gon, 1999] indicate that changes in agricultural management and technology need to be considered in assessments of the CH4 source strength of wetland rice fields and their role in the global CH4 budget of the past, present, and future.
Iron supply and demand in the upper ocean
Authors: Fung Inez Y. , Meyn Stephanie K. , Tegen Ina , Doney Scott C. , John Jasmin G. , Bishop James K. B.
Published: September 2002

Iron is hypothesized to be a limiting micronutrient for ocean primary production. This paper presents an analysis of the iron budget in the upper ocean. The global distribution of annual iron assimilation by phytoplankton was estimated from distributions of satellite‐derived oceanic primary production and measured (Fe:C)cellular ratios. The distributions of iron supply by upwelling/mixing and aeolian deposition were obtained by applying (Fe:NO3)dissolved ratios to the nitrate supply and by assuming the soluble fraction of mineral aerosols. A lower bound on the rate of iron recycling in the photic zone was estimated as the difference between iron assimilation and supply. Global iron assimilation by phytoplankton for the open ocean was estimated to be 12 × 109 mol Fe yr−1. Atmospheric deposition of total Fe is estimated to be 96×109 mol Fe yr−1 in the open ocean, with the soluble Fe fraction ranging between 1 and 10% (or 1‐10 ×109 mol Fe yr−1). By comparison, the upwelling/entrainment supply of dissolved Fe to the upper ocean is small, ∼0.7×109 mol Fe yr−1. Uncertainties in the aeolian flux and assimilation may be as large as a factor of 5‐10 but remain difficult to quantify, as information is limited about the form and transformation of iron from the soil to phytoplankton incorporation. An iron stress index, relating the (Fe:N) demand to the (Fe :N) supply, confirms the production in the high‐nitrate low‐chlorophyll regions is indeed limited by iron availability.
Methane sources and sinks in a periodically flooded South African savanna
Authors: Otter Luanne B. , Scholes Mary C.
Published: September 2002

Methane (CH4) fluxes were measured over a 2‐year period at subtropical savanna and floodplain sites located in South Africa. No significant differences were detected in fluxes from the nutrient‐rich and nutrient‐poor savannas. Savannas were not always sinks and showed some CH4 emissions during the wet, summer season with fluxes in the range of −1.6 to 1.68 mg m−2 d−1. Soil temperature did not have a significant effect on the flux, while CH4 consumption decreased and emissions increased with an increase in soil moisture between 5% and 50% water‐filled‐pore‐space (WFPS). Understanding the factors which control the production and emission of CH4 in aerobic environments is extremely important for predicting net emissions from a region. Dry floodplain soil fluxes were near zero and did not differ significantly from savanna fluxes. During a flood, saturated sites emitted CH4 at an average rate of 69.4 mg m−2 d−1. Flooded areas with a water depth of 0.3 ‐ 0.4 m emitted CH4 from the water surface at rates between 0.48 and 466.3 mg m−2 d−1 with the highest emission occurring during summer. Emission rates were exponentially related to sediment temperature, which had a greater influence on the emission rate than the flood regime. The length of the dry period preceding the flood and the extent of a flood did not have a significant effect on CH4 fluxes from saturated and flooded sites. Emission rates were highest when the water level was between 0.1 m below the soil surface and 0.4 m above the soil surface, with emission rates declining to near zero as the water became deeper than 0.4 m. Savannas were estimated to consume an average of 0.04 g CH4 m−2 d−1, with southern African savannas consuming 0.23 Tg CH4 yr−1. Saturated and flooded sites were estimated to produce 25.3 and 57.2 g CH4 m−2 d−1, respectively. Southern African floodplains are estimated to produce between 0.2 and 10 Tg CH4 yr−1(excluding the effects of vegetation‐mediated emissions) and therefore produce more CH4 than the savannas consume.
Carbon inputs of the Rhône River to the Mediterranean Sea: Biogeochemical implications
Authors: Sempéré Richard , Charrière Bruno , Van Wambeke France , Cauwet Gustave
Published: September 2002

Since the damming of the Nile, the Rhône River is the main freshwater and sediment supplier to the Mediterranean Sea. We estimated for the period 1987–1996, the dissolved and particulate organic carbon (DOC and POC), dissolved inorganic carbon (DIC), and total suspended matter (TSM) fluxes of the Rhône River to the Mediterranean Sea to be 1.1 ± 0.2, 1.6 ± 0.5, 16.2 ± 0.3 × 1010 moles C yr−1, and 9.9 ± 6.4 × 106 t yr−1, respectively. PIC flux was estimated to be 8.2 ± 5.4 × 109 moles C yr−1. On the basis of literature data, we estimated that nearshore bacterial respiration of Rhône derived labile‐POC and ‐DOC (LPOC and LDOC) might produce in a few days ∼0.21 and 0.12 × 1010 moles CO2 yr−1, respectively. Extended to the whole Mediterranean, this study suggests that bacterial respiration of labile organic carbon derived from Mediterranean rivers might rapidly (days) produce 2.6‐11 × 1010 moles CO2 yr−1. On the continental shelf, up to 4.7 × 1010 moles of organic carbon introduced by primary production and Rhône export would escape each year to sedimentation and bacterial mineralization and would be exported off the shelf. Moreover, as total carbon fixed by phytoplankton exceeds (+ 5.2 × 1010 moles C yr−1) the CO2 produced by bacterial respiration (on average), the biological system on the shelf, could be considered as an autotrophic system and then a sink for atmospheric CO2. However, these numbers need further examination because of the large uncertainties associated currently to the bacterial growth efficiency values (±100%).
Intensive field measurements of nitrous oxide emissions from a tropical agricultural soil
Authors: Crill P. M. , Keller M. , Weitz A. , Grauel B. , Veldkamp E.
Published: September 2002

The amount of nitrous oxide (N2O) continues to increase in the atmosphere. Agricultural use of nitrogen fertilizers in the tropics is thought to be an important source of atmospheric N2O. High frequency, highly precise measurements of the N2O flux were made with an automated system deployed in N fertilized and unfertilized agricultural plots of papaya and corn in Costa Rica for an entire corn crop growth to harvest cycle. N2O fluxes were as high as 64 ng N‐N2O cm−2 h−1 from fertilized versus 12 ng N‐N2O cm−2 h−1 from unfertilized corn and 28 ng N‐N2O cm−2 h−1 from fertilized versus 4.6 ng N‐N2O cm−2 h−1 from unfertilized papaya. Fertilized corn released more N2O than fertilized papaya over the 125 days of the crop cycle, 1.83 kg N ha−1 versus 1.37 kg N ha−1. This represents a loss as N2O of 1.1 and 0.9% of the total N applied as ammonium nitrate to the corn and papaya, respectively. As has often been observed, N2O fluxes were highly variable. The fastest rates of emission were associated with fertilization and high soil moisture. A diurnal cycle in the fluxes was not evident probably due to the minimal day/night temperature fluctuations. Each chamber was measured between 509 and 523 times over the course of the experiment. This allows us to evaluate the effect on constructed mean fluxes of lowered sampling frequencies. Sampling each collar about once a day throughout the crop cycle (25% of the data set) could result in a calculated mean flux from any individual chamber that can vary by as much as 20% even though the calculated mean would probably be within 10% of the mean of the complete data set. The uncertainty increases very rapidly at lower sampling frequencies. For example, if only 10% of the data set were used which would be the equivalent of sampling every other day, a very high sampling frequency in terms of manual measurements, the calculated mean flux could vary by as much as 40% or more at any given site.
Variations in hydrothermal methane and hydrogen concentrations following the 1998 eruption at Axial Volcano
Authors: McLaughlin‐West E. A. , Olson E. J. , Lilley M. D. , Resing J. A. , Lupton J. E. , Baker E. T. , Cowen J. P.
Published: September 2002

During the February response cruise to Axial Volcano, methane and hydrogen concentrations within the plume were significantly elevated above background. CH4/³He ratios were found to be 5–10 times greater than previously reported values for unsedimented ridges. An average δ13C value of −32.7±0.8‰ was determined for CH4 concentrations >10 nM and CH4/Mn ratios were determined to be 2.2. Hydrogen concentrations as high as 183 nM were measured; however most were <10 nM. Microbial oxidation of methane was negligible (<0.001 day−1) during this sampling period. However, microbial hydrogen oxidation was sufficient to explain decreased H2 concentrations measured at ‘older’ downstream stations.
El Niño 1997–98 monitoring in mixed layer at the Pacific Ocean near mexico's West Coast
Authors: Filonov A. , Tereshchenko I.
Published: September 2002

The field data collected on monthly polygonal hydrographic surveys off the West Coast of Mexico in 1996–98 are analyzed. Monthly measured temperature and salinity profiles have been computed from the experimental data. The profiles for the 1997 summer‐fall and 1998 winter periods differ essentially from profiles computed for the same months in the year 1996. The inferred cause of the difference is the ingress of large volumes of relatively warm and fresh water whose T,S‐characteristics identify these water masses as the Pacific Tropical Surface Water. By January 1998, those water masses had filled the entire upper 90‐m layer. The heat storage of a 150‐m column increased to 15.3 GJ/m², as compared with its January 1996 value of 10.1 GJ/m². The process which caused sea surface temperature anomalies in region B of the Pacific, occurred one month before the rise in temperature and drop down in salinity were observed on the site.
Multi‐year meanders and eddies in the Alaskan Stream as observed by TOPEX/Poseidon altimeter
Authors: Crawford William R. , Cherniawsky Josef Y. , Foreman Michael G. G.
Published: September 2002

The birth, life and decay of long‐lived anticyclonic eddies, or meanders, in the Alaskan Stream are observed from TOPEX/Poseidon (T/P) satellite. Their surface height anomalies approach 72 cm, average diameter is about 160km and they live for 1 to 3 years, propagating along the Stream with mean speed of 2.5 km d−1. Of the six observed, the last three were formed after the first T/P observations in September 1992. Of these, two evolved from eddies near Alaskan Panhandle in winter, drifting across the northern Gulf of Alaska before entering the Alaskan Stream, while another formed in the Stream west of Shelikof Strait.
Multipoint measurements of large DC electric fields and shears in the auroral zone
Authors: Pietrowski D. , Lynch K. A. , Torbert R. B. , Marklund G. , Ivchenko N. , Ranta A. , Danielides M. , Kelley M. C.
Published: September 2002

We present electric field and electron data from the Auroral Turbulence II (AT2) sounding rocket mission. This three pay load rocket flew through several discrete night side auroral arc structures. The pay loads traversed a large, stable arc structure containing, near its poleward edge, at least 400 mV/m of DC electric field, together with broadband waves in the electric field at frequencies from DC up to 100 Hz. Multipoint measurements allow us to determine the size of this region and to observe that it is characterized by two distinct variations: a region of spatial shear approximately 1 km in extent across the arc, immediately followed by a region of temporal change in electric field, nearly 6 km in extent. The observed broadband waves do not show a preference for the spatial shear region over the time varying region.
Sudden disruption of a thin current sheet in collisionless Hall magnetohydrodynamics due to secondary tearing and coalescence instabilities
Authors: Ma Z. W. , Bhattacharjee A.
Published: September 2002

A numerical simulation of impulsive growth and sudden disruption of a thin current sheet due to inward boundary flows in a collisionless plasma, governed by the equations of Hall magnetohydrodynamics, is presented. After a period of slow growth followed by a much shorter period of impulsive enhancement, the thin current sheet becomes unstable to secondary tearing instabilities which produce multiple islands. These secondary islands coalesce to form a large plasmoid. Much of the thin current sheet disrupts rapidly to produce magnetic turbulence in less than a characteristic Alfvén time. Possible implications for magnetotail substorms are discussed.
Equinoctial and solstitial averages of magnetospheric relativistic electrons: A strong semiannual modulation
Authors: Baker D. N. , Kanekal S. G. , Pulkkinen T. I. , Blake J. B.
Published: September 2002

Data from SAMPEX, POLAR, and other spacecraft shows that high energy electrons (E ≳ 1 MeV) vary in a highly coherent way throughout the entire outer radiation zone of the Earth (2.5 ≲ L ≲ 6.5). These data are used to perform long‐term analysis of the flux variations of E>2 MeV electrons throughout the outer trapping zone. Averages are performed over 3‐month intervals centered on the spring and fall equinoxes and on the winter and summer solstices. Modulation is found such that equinoctial fluxes of electrons are nearly a factor of three larger than the solstitial fluxes based upon a superposed epoch analysis for 1992–1999. These semiannual modulations of relativistic electron fluxes are compared with concurrent solar wind data and are examined in terms of prior models of geomagnetic activity modulation. It is concluded that both southward IMF component merging (Russell‐McPherron) and Kelvin‐Helmholtz instability considerations (Boller‐Stolov) may play important roles.
SOHO and radio observations of a CME shock wave
Authors: Raymond John C. , Thompson Barbara J. , St. Cyr O. C. , Gopalswamy Nat , Kahler S. , Kaiser M. , Lara A. , Ciaravella A. , Romoli M. , O'Neal R.
Published: September 2002

A 1200 km s−1 Coronal Mass Ejection was observed with the SOHO instruments EIT, LASCO and UVCS on June 11, 1998. Simultaneously, Type II radio bursts were observed with the WAVES experiment aboard the Wind spacecraft at 4 MHz and by ground‐based instruments at metric wavelengths. The density in the shock wave implied by the higher frequency is close to that inferred from the SOHO/UVCS experiment. The drift rates of the Type II radio bursts suggest shock speeds lower than the speed derived from SOHO observations. The SOHO/UVCS spectrum shows enhanced emission in lines of O5+ and Si11+, consistent with modest compression in an MHD shock.
The contribution of unknown weak water vapor lines to the absorption of solar radiation
Authors: Learner R. C. M. , Zhong W. , Haigh J. D. , Belmiloud D. , Clarke J.
Published: September 2002

Thousands of unknown water vapour weak lines in the spectral region between 13200 and 22700 cm−1 are deduced from extrapolations of experimental results. These lines are then included in the HITRAN database and used in line‐by‐line calculations of atmospheric opacity with standard atmospheric profiles. The weak lines predicted by a theoretical model are also used in the line‐by‐line model to estimate their contribution to the absorption of solar radiation. The additional absorption of solar radiation is of order 1.5 to 2.5 W/m² at 45 mm precipitable water, about 8.5% to 14% of the absorption due to HITRAN lines in the spectral region. The effect is also compared with that of a continuum model.
On the basic processes forming the long‐term variations of the exospheric temperature
Authors: Vlasov Michael N.
Published: September 2002

The analytical expression for the long‐term variations of the exospheric temperature, T∞, has been derived by solving the simplified equation of the thermal balance and the hydrostatics equation. This expression describes the T∞ variations with the solar activity and season in a very good agreement with empirical and physical models. Excellent agreement has also been found between the exospheric temperatures calculated by the analytical expression and the physical model for the long‐term cooling of the thermosphere due to the CO2 infrared radiation increase. Further analysis indicates that the T∞ long‐term changes depend on the input energy, molecular thermal conductivity, and the temperature at the lower boundary of the thermosphere. The analytical expression makes it possible to compare the experimental data on the long‐term cooling of the lower thermosphere and the exosphere. A strong discrepancy between these data has been revealed.
The first daytime ground‐based optical image of the aurora
Authors: Rees David , Conde Mark , Steen Åke , Brändström Urban
Published: September 2002

Aurorae, spectacular phenomena in the polar night sky, also provide a convenient projection of effects of complex and energetic plasma processes of the outer magnetosphere. Much has been learned about the ionosphere and magnetosphere from night‐time auroral images. However, similar imaging is extraordinarily difficult by day, due to the overwhelming background of atmospherically‐scattered sunlight. This is unfortunate, since many auroral plasma processes may be unique to the sunlit ionosphere. A visible‐light image of the aurora at λ630‐nm wavelength was obtained from Kiruna, Sweden, at sunset on May 2, 1999, by an imaging spectrometer featuring excellent spectral resolution and out‐of‐band rejection. We believe this to be the first such image obtained from the ground under near‐daytime conditions. These observations were obtained as a test of principle during the development of a prototype instrument. We believe this technique holds great promise for future ground‐based studies of the daylit ionosphere and magnetosphere.
Long‐term behavior of Jovian bKOM and nKOM radio emissions observed during the Ulysses‐Jupiter encounter
Authors: Reiner M. J. , Kaiser M. L. , Desch M. D.
Published: September 2002

We present observational evidence that the long‐term behavior of Jovian bKOM and nKOM radio emissions observed during the Ulysses‐Jupiter encounter was controlled by the sector structure of the solar wind. Specifically, we found brightenings in the Jovian bKOM emission, followed by a sudden cessation of the bKOM emission and an onset of an nKOM “event” that lasted for some 120 hours. This sequence of events was observed to recur every ∼12 or 25 days from October, 1991 to mid‐January, 1992 when Ulysses was inbound toward Jupiter and at Jovian latitudes <2.3°. These observations indicate that the solar wind structures, which influence the dynamics of Jupiter's magnetosphere, in turn control the radio emissions generated inside the magnetosphere.