bluuebunny/crossref_metadata_2025 huggingface dataset:
Methane stable isotopic ratios and concentrations as indicators of methane dynamics in estuaries
Authors: Sansone Francis J. , Holmes M. Elizabeth , Popp Brian N.
Published: December 2003

Mixing diagrams of methane (CH4) concentration and stable isotopic ratio (δ13C‐CH4) were used to examine the fate of river‐borne CH4 as it crosses a variety of estuaries: Columbia River (Oregon/Washington), Parker River (Massachusetts), Great Bay (New Hampshire), Kaneohe Bay (Hawaii), and Elkhorn Slough (California). Unlike the surface of the open ocean, these systems are not in near atmospheric equilibrium with respect to concentration or δ13C‐CH4 value. The range of observed CH4 concentrations and δ13C‐CH4 values were 33–440 nM and −36 to −58 per mil, respectively, for the freshwater end‐members for these systems, 12–330 nM and −48 to −60 per mil for water at the mouths of the estuaries, and 1.6–6 nM and −45 to −60 per mil for the seawater end‐members. In the Kaneohe Bay estuary, CH4 concentration and δ13C‐CH4 displayed near‐conservative behavior. In the Columbia River estuary, there was loss of riverine CH4 coupled with shifts to heavier isotopic values, apparently the result of in situ CH4 oxidation; this oxidation exhibited an apparent kinetic isotopic fractionation factor of 1.0042−1.012. In contrast, the other estuaries showed elevated concentrations and more negative δ13C‐CH4 values apparently resulting from inputs of biogenic CH4 from midestuary marshes and sediments. The upper reaches of all these systems were well out of equilibrium with the atmosphere on a concentration basis, indicating that they are atmospheric CH4 sources. However, these first δ13C‐CH4 measurements show that there is a wide range of isotopic variation in these waters, which indicates that it will be difficult to estimate the collective isotopic contribution of estuaries to the global methane budget.
A one‐dimensional model of dissolved organic carbon cycling in the water column incorporating combined biological‐photochemical decomposition
Authors: Anderson Thomas R. , Williams Peter J. le B.
Published: December 2003

A one‐dimensional model incorporating separate labile, semilabile, and refractory fractions of dissolved organic carbon (DOC) is used to study the vertical distribution of dissolved organics in the ocean and the downward flux of organic carbon into the water column. The modeled vertical gradient of DOC is generated almost entirely by the semilabile fraction, which has predicted lifetimes of 0.4 years at the ocean surface increasing to 6.3 years at 1000 m, owing to diminishing bacterial numbers. Although predicted fluxes of DOC and sinking particles exiting the upper mixed layer are similar, labile and semilabile DOC are mostly degraded by bacteria before being mixed below 250 m. The simple one‐dimensional scheme employed by the modeling may, however, underestimate downward transport of DOC by physical mechanisms because it does not capture three‐dimensional processes such as subduction. The inclusion of a biologically inert refractory pool is a first step to try and incorporate the growing awareness that photochemical processes play an important role in the dynamics of organic carbon in the ocean. However, the predicted rate of change of refractory material in response to altered climatic forcing (e.g., ultraviolet radiation at the ocean surface) is so slow that it may not be necessary to include it dynamically in ocean models used to examine climatic change within the next 200 years.
Global patterns of terrestrial biological nitrogen (N2) fixation in natural ecosystems
Authors: Cleveland Cory C. , Townsend Alan R. , Schimel David S. , Fisher Hank , Howarth Robert W. , Hedin Lars O. , Perakis Steven S. , Latty Erika F. , Von Fischer Joseph C. , Elseroad Adrien , Wasson Matt F.
Published: December 2003

Human activities have clearly caused dramatic alterations of the terrestrial nitrogen cycle, and analyses of the extent and effects of such changes are now common in the scientific literature. However, any attempt to evaluate N cycling processes within ecosystems, as well as anthropogenic influences on the N cycle, requires an understanding of the magnitude of inputs via biological nitrogen fixation (BNF). Although there have been many studies addressing the microbiology, physiology, and magnitude of N fixation at local scales, there are very few estimates of BNF over large scales. We utilized >100 preexisting published estimates of BNF to generate biome‐ and global‐level estimates of biological N fixation. We also used net primary productivity (NPP) and evapotranspiration (ET) estimates from the Century terrestrial ecosystem model to examine global relationships between these variables and BNF as well as to compare observed and Century‐modeled BNF. Our data‐based estimates showed a strong positive relationship between ecosystem ET and BNF, and our analyses suggest that while the model's simple relationships for BNF predict broad scale patterns, they do not capture much of the variability or magnitude of published rates. Patterns of BNF were also similar to patterns of ecosystem NPP. Our “best estimate” of potential nitrogen fixation by natural ecosystems is ∼195 Tg N yr−1, with a range of 100–290 Tg N yr−1. Although these estimates do not account for the decrease in natural N fixation due to cultivation, this would not dramatically alter our estimate, as the greatest reductions in area have occurred in systems characterized by relatively low rates of N fixation (e.g., grasslands). Although our estimate of BNF in natural ecosystems is similar to previously published estimates of terrestrial BNF, we believe that this study provides a more documented, constrained estimate of this important flux.
Nitrogen in crop production: An account of global flows
Authors: Smil Vaclav
Published: December 2003

Human activities have roughly doubled the amount of reactive N that enters the element's biospheric cycle. Crop production is by far the single largest cause of this anthropogenic alteration. Inorganic fertilizers now provide 80 Tg N yr−1(Tg = 1012g), managed (symbiotic) biofixation adds about 20 Tg N yr−1, and between 28 and 36 Tg N yr−1are recycled in organic wastes. Anthropogenic inputs (including N in seeds and irrigation water) now supply about 85% of 170 (151–186) Tg N reaching the world's cropland every year. About half of this input, 85 Tg N yr−1, is taken up by harvested crops and their residues. Quantification of N losses from crop fields is beset by major uncertainties. Losses to the atmosphere (denitrification and volatilization) amount to 26–60 Tg N yr−1, while waters receive (from leaching and erosion) 32–45 Tg N yr−1. These N losses are the major reason behind the growing concerns about the enrichment of the biosphere with reactive N. The best evidence suggests that in spite of some significant local and regional losses, the world's agricultural land accumulates N. The addition of 3–4 billion people before the year 2050 will require further substantial increases of N input in cropping, but a large share of this demand can come from improved efficiency of N fertilizer use.
Net CO2 and H2O fluxes of terrestrial ecosystems
Authors: Buchmann Nina , Schulze Ernst‐Detlef
Published: December 2003

Using 139 flux studies, we addressed the variability of net ecosystem surface assimilation (Asmax), net ecosystem surface respiration (Rsmax), as well as net surface evapotranspiration (Esmax) among and within vegetation types. While forests and C3 crops, particularly in the northern hemisphere, have been preferentially investigated, information on tropical forests, C4 grasslands or wetlands is rather limited. Almost no data are available for disturbed sites. Despite large variations within a vegetation type, enclosure studies tended to give highest Asmax rates compared to micrometeorological techniques. Excluding enclosure studies, we tested the effect of stand age and leaf area index (LAI) on net ecosystem gas exchange. For grasslands, Asmax increased by 7 μmol m−2 s−1 per unit LAI, for C4 crops by 11 μmol m−2 s−1, and for coniferous forests by 0.9 μmol m−2 s−1 per unit LAI. In contrast, Asmax of broad‐leaved forests and C3 crops as well as Rsmax stayed constant over a wide range of LAI. Asmax and Rsmax of forests were lowest in young stands (< 20 years old) and highest in stands of age 30–80 years. Asmax of old forests (> 160 years) was within the same range as those of 30‐ to 80‐year‐old forests, and always higher than those of regenerating stands. Rsmax seemed to decrease with age. Asmax increased linearly with ecosystem surface conductance for all vegetation types (r2 = 0.65). Asmax of forests and grasslands was closely related to Esmax (r2 = 0.87), with a slope of 0.082 μmol CO2 m−2 s−1/mmol H2O m−2 s−1. The results clearly illustrated where gaps in our knowledge exist and how ecosystem properties affect the capacity of net ecosystem gas exchange.
Lateral mixing and advection of reactive isotopetracers in ocean basins: Numerical modeling
Authors: Igel Heiner , von Blanckenburg Friedhelm
Published: December 2003

Ocean isotope tracers of intermediate residence time are becoming of increasing importance in studies of paleo‐ocean circulation, global weathering fluxes, and trace pollutant dispersion in ocean basins. While analytical models exist that predict the way in which conservative tracers are dispersed in ocean gyres, the lateral movement of reactive tracers has not yet received much attention. Here we present time‐dependent numerical simulations of the lateral mixing process (advection‐diffusion‐reaction) of reactive tracers with different residence times in gyres with varying Péclet number, diffusivity, western boundary intensification, and source geometry. Our main conclusions are that in our model gyre (1) homogenization of tracer concentrations and also of isotopic ratios in those cases where the isotopes have different pathways (e.g., atmospherically deposited cosmogenic 10Be, continent‐derived9Be) depends largely on the residence time and eddy diffusivity and isonly a weak function of Péclet number; (2) western intensification influences homogenization only in the case of short residence times and large Péclet numbers; (3) homogenization of isotopic ratios of continent‐sourced tracers (e.g., 206Pb/204Pb, 143Nd/144Nd) is favored by high water velocities and hence shallow ocean levels; (4) boundary scavenging can result in significant lateral redistribution of output fluxes for even very short residence time tracers (e.g., 230Th) if Péclet numbers are high; (5) the width of source and scavenging regions exerts a strong control over the actual tracer concentrations, but the spatial distribution and degree of tracer homogenization is not affected. The space dependence of input and scavenging is applicable to several reactive isotope tracers such as Be, Pb, Nd, Hf, Th, or Pa. Relative uniformity in their isotope ratios has been observed by mapping of these tracers in individual ocean basins (von Blanckenburg and Igel, 1999). Such uniformity would not be necessarily expected since the isotopes concerned enter through entirely different pathways. The simulations presented here demonstrate that homogenization of such short residence time tracers in ocean gyres is a feasible mechanism that can account for these observations.
Variation of foraminiferal Sr/Ca over Quaternary glacial‐interglacial cycles: Evidence for changes in mean ocean Sr/Ca?
Authors: Martin P. A. , Lea D. W. , Mashiotta T. A. , Papenfuss T. , Sarnthein M.
Published: December 2003

Records of Sr/Ca changes in planktonic and benthic foraminifera from diverse hydrographic settings reveal coherent variability (5 ± 1%) between ocean basins and between surface and deep waters over the past 300 kyr. There is a general increase in foraminiferal Sr/Ca over the penultimate glaciation declining to minimum values during stage 5 and an increase in Sr/Ca from stage 5 through stage 2. Coincident changes in benthic foraminifera records from the Atlantic and Pacific basins imply that Sr/Ca variations are not dominated by dissolution. Planktonic culturing data provide evidence that the downcore Sr/Ca variations are not controlled by temperature changes and suggest only a small influence of salinity and pH. Variation common to the records is most readily explained by changes in mean ocean Sr/Ca. If fossil foraminifera reliably record higher glacial seawater Sr/Ca, coral Sr paleothermometry would underestimate sea surface temperature during glacialepisodes.
Crustal motion north and south of the Arica deflection: Comparing recent geodetic results from the central Andes
Authors: Bevis Michael , Kendrick Eric C. , Smalley Robert , Herring Thomas , Godoy Jorge , Galban Fernando
Published: December 2003

An intercomparison of recent velocity solutions for the Global Positioning System (GPS) networks constructed by the South American‐Nazca Plate Project (SNAPP), based in Bolivia and Peru, and our Central Andes GPS Project (CAP), based in Chile and Argentina, indicates a velocity discontinuity of order 10 mm/yr near the boundary between these networks. We suggest that this velocity jump manifests measurement bias in the SNAPP velocity field. Our results indicate that no major slip partioning occurs within the forearc of northern Chile in response to the obliquity of subduction but that Nazca‐South America plate convergence is partitioned between the forearc and the backarc regions. The present rate of shortening across the southern part of the sub‐Andean belt in Argentina is 8.9 ± 1.6 mm/yr.
Assessing the presence of garnet‐pyroxenite in the mantle sources of basalts through combined hafnium‐neodymium‐thorium isotope systematics
Authors: Stracke Andreas , Salters Vincent J. M. , Sims Ken W. W.
Published: December 2003

The existence of an enriched component in the mantle with a pyroxenitic or eclogitic composition and its importance for basalt genesis has been discussed for over two decades. Inferences about the depth of melting as well as the dynamics of melting based on the presence of garnet and the location of the spinel‐garnet transition are different if garnet‐pyroxenite is present in the peridotitic mantle. Trace element partition coefficients are dependent on composition, and the differences between garnet‐pyroxenite and peridotite are large enough to produce significant differences in trace element fractionation between melts derived from these different lithologies. Melts derived from garnet‐pyroxenite or eclogite‐bearing sources will have small or no 230Th excesses, which are largely independent of melting and upwelling rate. Melts derived from garnet‐peridotite will have significant 230Th excesses, which are dependent on melting and upwelling rate. We show that the combined hafnium‐neodymium‐thorium (Hf‐Nd‐Th) isotope and trace element data can distinguish between melts derived from peridotitic and pyroxenitic or eclogitic sources. We also present new Hf isotope data for Hawaiian basalts and use the combined Hf‐Nd‐Th isotope and trace element systematics to argue against the existence of garnet‐pyroxenite or eclogite in the source of Hawaiian basalts. It is especially the large variation in degree of melting for relatively constant isotopic composition that allows us to rule out garnet‐pyroxenite in the source of the Hawaiian basalts.
Δ14C, ΣCO2 and salinity of the Western Indian Ocean Deep Waters: Spatial and temporal variations
Authors: Somayajulu B. L. K. , Bhushan R. , Narvekar P. V.
Published: December 2003

Indian Ocean Deep waters (IDW) show an increase in salinity and ΣCO2 between 64°S and 16±4°N by 0.106–0.110‰ and 82–104 µM kg−1 respectively with a corresponding decrease in Δ14C by 42‰ as revealed by the GEOSECS data (1977–78). The Δ14C‐ΣCO2‐Salinity relationships show better correlation in the western sector. High biological productivity induced changes and corrosive deepwaters could account for ΣCO2 increase in the northern regions. Reoccupation of GEOSECS stations during 1985–95 do not show distinctly discernible short‐term temporal changes in Δ14C, ΣCO2 and salinity.
A possible link between off‐equatorial warm anomalies propagating along the NECC path and the onset of the 1997–98 El Niño
Authors: Zhang Rong‐Hua , Busalacchi Antonio J.
Published: December 2003

By examining sea surface height (SSH) from the TOPEX/Poseidon (T/P) altimeter and upper‐ocean temperature/current data from the NCEP reanalysis product, a well‐defined, shallow pathway has been identified of subsurface temperature anomalies along the Pacific North Equatorial Countercurrent (NECC). A physical picture is developed to demonstrate how oceanic‐atmospheric anomalies leading to the 1997–98 El Niño may be generated in the tropical Pacific climate system. Prior to its onset, a preconditioned oceanic state is set in the west during 1995–96, characterized by elevated sea level, depressed thermocline and warm temperature anomalies in the western boundary along 6–10°N. With the seasonal intensification of the NECC in the fall, the warm anomalies propagated coherently eastward along the relatively shallow NECC subsurface pathways (centered about 100 m), from the western boundary in mid‐1996 to near the date line in late 1996 and early 1997. As the thermocline shoals eastward and upward along the NECC, the subsurface anomalies were exposed to the surface near the date line, likely initiating warm sea surface temperature (SST) anomalies north of the equator in late 1996. Due to the convergence of mean subsurface circulation onto the equator, the equatorward flows helped move these initially subsurface‐produced SST anomalies southward onto the equator, which would induce westerly wind anomalies.
On the sensitivity of projected oceanic thermal expansion to the parameterisation of sub‐grid scale ocean mixing
Authors: Weaver Andrew J. , Wiebe Edward C.
Published: December 2003

A coupled model of intermediate complexity is used to examine the importance of the parameterisation of sub‐grid scale ocean mixing on the global mean steric sea level rise in global warming simulations. It is shown that when mixing associated with mesoscale eddies is treated in a more physically realistic way than the commonly used horizontal/vertical scheme, quasi‐equilibrium projected steric sea level rise is more than two times lower in both 2 × CO2 and 4 × CO2 climates. This occurs despite the invariance of the coupled model climate sensitivity to the particular sub‐grid scale mixing scheme employed. During the early phase of the transient integrations thermal expansion differences are smaller, although experiments using the Gent and McWilliams parameterisation for mixing associated with mesoscale eddies approach equilibrium more rapidly once the radiative forcing is held fixed. This reduced expansion commitment reflects a greater decoupling of the surface ocean from the deep ocean, due to a reduction in spurious high latitude convection that occurs when a horizontal/vertical mixing scheme is used.
Dual spacecraft observations of lobe magnetic field perturbations before, during and after plasmoid release
Authors: Slavin J. A. , Hesse M. , Owen C. J. , Taguchi S. , Fairfield D. H. , Lepping R. P. , Kokubun S. , Mukai T. , Lui A. T. Y. , Anderson R. R. , Matsumoto H. , Sutcliffe P. R.
Published: December 2003

This study examines a data set returned by IMP 8 and Geotail on January 29, 1995 during a substorm which resulted in the ejection of a plasmoid. The two spacecraft (s/c) were situated in the north lobe of the tail and both observed a traveling compression region (TCR). We show that in this instance dual s/c measurements can be used to model all three dimensions of the underlying plasmoid and to estimate its rate of expansion. For this event plasmoid dimensions of ΔX ∼18, ΔY ∼30, and ΔZ ∼10 Re are determined from the IMP 8 and Geotail observations. Furthermore, a factor of ∼2 increase in the amplitude of the TCR occurred in the 1.5 min it took to move from IMP 8 to Geotail. Modeled using conservation of magnetic flux, this increase in lobe compression implies that the underlying plasmoid was expanding at a rate of ∼140 km/s. Finally, a reconfiguration of the lobe magnetic field followed plasmoid ejection which moved magnetic flux tubes into the wake behind the plasmoid where they would become available to feed the reconnection region.
Gravity evidence for Mid Crustal Domal Structure below Delhi Fold Belt and Bhilwara Super Group of western India
Authors: Bansal Abhey Ram , Dimri Vijay P.
Published: December 2003

A scheme to interpret nonstationary gravity profile is proposed in two steps (1) divide the entire nonstationary gravity profile into piecewise stationary sub‐profiles, and (2) apply the scaling power spectral analysis to each sub‐profile. The scheme is applied to gravity data along a well studied Nagaur‐ Jhalawar transect in western India. The profile has been subdivided into three sub‐profiles; each sub‐profile corresponds to a geological unit. Gravity data delineated a 2‐D mid crustal domal structure below the Delhi fold belt and Bhilwara super group. Further, forward gravity modelling is carried out to model the subsurface along the entire profile.
Fractal analysis of ULF geomagnetic data associated with the Guam Earthquake on August 8, 1993
Authors: Hayakawa Masashi , Ito Tetsuya , Smirnova Natalia
Published: December 2003

This paper presents a fractal analysis of the ULF electromagnetic data obtained at Guam observatory during a strong (Ms=8.0) earthquake occurred on August 8, 1993 near the Guam island. We have already reported the presence of seismogenic ULF emissions for this quake (Hayakawa et al., GRL, 1996). The chosen period covered four months before and two months after the earthquake. It was found that the spectrum of emissions at noon exhibited a power law behavior f−β which is typical for self‐organized critical dynamics. The slope of the spectrum in the noon sector fluctuates in a range from 2.5 to 0.7 with a tendency to gradually decrease during the process of the earthquake preparation. We consider this to be a candidate for a precursory signature. The similar effect was found for the night sector. We suggest that the focal zone was at the critical stage of self‐organized criticality process before the quake, related to active microfracturing processes followed by the generation of ULF emissions as we detected.
One slope or two? Detecting statistically significant breaks of slope in geophysical data, with application to fracture scaling relationships
Authors: Main I. G. , Leonard T. , Papasouliotis O. , Hatton C. G. , Meredith P. G.
Published: December 2003

The scaling of displacement as a function of length is important for a variety of applications which depend on the mechanical and hydraulic properties of faults and fractures. Recently it has been suggested that the power‐law exponent ν which has been found to characterise this relationship may change significantly at a characteristic length for a variety of reasons, for example when cracks begin to interact, or when faults grow to a length comparable to a characteristic size in the brittle layer. Such a break of slope requires a second straight line, requiring two extra model parameters. Here we present a new method for analysing such data, which penalises the extra parameters using a modified form of Schwarz's Information Criterion, and a Bayesian approach which represents uncertainty in the unknown parameters. We apply the method to data from the Krafla fissure zone in the north of Iceland, and find a significant break of slope, from ν ≈ 3/2 to ν ≈ 2/3, at a characteristic length of 12m.
Imaging the lithospheric mantle in northwestern Canada with seismic wide‐angle reflections
Authors: Viejo Gabriela Fernandez , Clowes Ron M. , Amor John R.
Published: December 2003

Normal‐moveout‐corrected and stacked sections of wide‐angle reflection data collected in the Archean Slave province and Paleoproterozoic Wopmay orogen in northwestern Canada image the crust‐mantle boundary as a subhorizontal discontinuity at about 32–36 km depth along a 450 km lateral extent, and, for the first time from such data, layering within the lithospheric mantle at depths between 75 and 90 km over a lateral distance of 750 km. These images correspond to Moho and mantle reflections observed on a colocated multichannel seismic (MCS) reflection profile. Structural features and/or the basalt‐eclogite phase change, associated with subduction of oceanic and/or delaminated lower crust, are possible sources of the deep reflectivity. One mantle reflection group lies west of the MCS data and east of the Cordilleran deformation front. It could have similar sources or result from the Mesoproterozoic Racklan orogen or Meso‐Neoproterozoic rifting of the ancient margin.
Identifying an asperity through 3‐D Mapping of the frequency‐magnitude distribution
Authors: Sylvander Matthieu
Published: December 2003

Following the idea introduced by Wiemer and Wyss [1997], here is an attempt to map the seismic hazard, in terms of the b value in the frequency‐magnitude distribution, in a region with moderate seismic activity (Béarn region, French Pyrenees). In this area, the complexity of the geology makes it necessary to perform a 3‐D mapping, b values are computed within 5 km radius spherical volumes, that contain an average of 140 events. Strong lateral variations, between 0.45 and 1.8, are observed at short scales in the whole investigated volume. The minimum of the b value is clearly correlated with a historically active zone, which last ruptured in 1980 (Mb=5.1). Interpretating low b values in terms of high stresses may lead us to consider this minimum‐b patch as a 5 km radius asperity. This study allows to define its location and dimensions.
Three‐dimensional velocity structure of the Kilauea Caldera, Hawaii
Authors: Dawson P. B. , Chouet B. A. , Okubo P. G. , Villaseñor A. , Benz H. M.
Published: December 2003

High‐resolution velocity models (0.5 km resolution) of the Kilauea caldera region are obtained by the tomographic inversion of both P‐ and S‐wave arrival times. Data are from the permanent Hawaiian Volcano Observatory (HVO) seismic network, a broadband seismic network, and a temporary array of stations centered on the southern boundary of the caldera. A low‐velocity P‐wave anomaly is imaged centered on the southeastern edge of the caldera, with a velocity contrast of about 10% and a volume of 27 km³. The VP/VS model mimics the spatial extent of the P‐wave anomaly, but is partitioned into two discrete anomalous volumes centered on the southern boundary of the caldera and on the upper east rift of the volcano. The corresponding Poisson's ratio in these zones is high (ν=0.25–0.32) which is consistent with a densely‐cracked, hot volume which may contain partial melt. The large‐scale features of the models are consistent with results obtained from an earlier, larger‐scale (2 km resolution) tomographic image of Kilauea Volcano based on HVO network data.
A self‐organized model of earthquakes with constant stress drops and the b‐value of 1
Authors: Kumagai Hiroyuki , Fukao Yoshio , Watanabe Sei‐ichiro , Baba Yuito
Published: December 2003

The magnitude‐frequency relation and the constant stress drop are fundamental features of earth‐quakes, to which a full physical explanation has yet to be given. We present a model that can reproduce the above two fundamental features simultaneously and spontaneously. The model is two‐dimensionally configured spring‐loaded blocks with a velocity‐weakening friction law. We change widely the dynamic friction parameter, which results in the frequency distributions showing the critical, subcritical and supercritical behaviors. Seismicity near the critical state is characterized by almost constant stress drops and the b‐value of 1, in which a self‐healing pulse maintains its frontal dynamic stress at a level near the static friction in an environmental stress heterogeneity that has evolved through the healing process itself.
Synergistic feedbacks from ocean and vegetation on the African Monsoon response to Mid‐Holocene insolation
Authors: Braconnot P. , Joussaume S. , Marti O. , de Noblet N.
Published: December 2003

We used a coupled ocean‐atmosphere General Circulation Model without flux corrections, iteratively coupled to a biome model, to explore the impact of vegetation and ocean feedbacks on mid‐Holocene monsoon change in northern Africa. We show that both feedbacks significantly increase the northward penetration and the intensity of the monsoon rainfall. The major contribution of the ocean is to enhance the inland moisture advection, whereas that of vegetation is to increase local recycling. Synergistic interactions between ocean and vegetation feedbacks play a critical role in the African monsoon changes, yielding to model results in much better agreement with observations.
A methodology for estimation of surface evapotranspiration over large areas using remote sensing observations
Authors: Jiang Le , Islam Shafiqul
Published: December 2003

We propose a simple scheme to estimate surface evaporation over large heterogeneous areas using remote sensing observations. Our approach is based on a relationship between easily measured surface parameters (e.g. radiometric surface temperature) and a surrogate for effective surface resistance. Preliminary results, using remotely sensed data sets from AVHRR NOAA‐14 over the Southern Great Plains, show good agreement. The proposed approach appears to be more reliable and easily applicable for operational estimation of evaporation over large areas.
Impact craters on geologic units of northern Venus: Implications for the duration of the transition from Tessera to regional plains
Authors: Basilevsky A. T. , Head J. W. , Ivanov M. A. , Kryuchkov V. P.
Published: December 2003

Using Magellan SAR images and the Schaber et al. [1998] crater data base we examined impact craters in the area north of 35°N and determined the geologic units on which they are superposed. The crater density of the regional plains with wrinkle ridges (Pwr) was found to be very close to the global average and thus the mean surface age of the plains is close to the mean surface age of the planet (T). About 80 to 97% of the craters superposed on a composite unit that includes materials of Tessera terrain (Tt), Densely fractured plains (Pdf), Fractured and ridged plains (Pfr), and Fracture Belts (FB), also postdate the regional plains. Thus, the time interval between the formation of these older units and emplacement of the regional plains (ΔT) should be geologically short, from a few percent to about 20% of T, or approximately 40 to 150 m.y. This means that in the area under study, volcanic and tectonic activity in the beginning of the morphologically recognizable part of the geologic history of Venus (about the last 750 m.y.) was much more active than in the subsequent time.
Dust devil vortices seen by the Mars Pathfinder Camera
Authors: Metzger Stephen M. , Carr James R. , Johnson Jeffrey R. , Parker Timothy J. , Lemmon Mark T.
Published: December 2003

Discovery of dust devil vortices in Mars Pathfinder (MPF) images reveals a dust entrainment mechanism at work on Mars. Scattering of visible light by dust in the Martian atmosphere creates a pronounced haze, preventing conventional image processing from displaying dust plumes. Spectral differencing techniques have enhanced five localized dust plumes from the general haze in images acquired near midday, which we determine to be dust devils. Processing of 440 nm images highlights dust devils as distinct occultation features against the horizon. The dust devils are interpreted to be 14–79 m wide, 46–350 m tall, travel at 0.5–4.6 m/s, with dust loading of 7E‐5 kg m‐3, relative to the general haze of 9E‐8 kg m‐3, and total particulate transport of 2.2–700 kg. The vortices match predictions from terrestrial analog studies.
Rapid rotations about a vertical axis in a collisional setting revealed by the Palu Fault, Sulawesi, Indonesia
Authors: Stevens C. , McCaffrey R. , Bock Y. , Genrich J. , Endang  , Subarya C. , Puntodewo S. S. O. , Fauzi  , Vigny C.
Published: December 2003

Global Positioning System (GPS) measurements from 1992 to 1995 indicate that the left‐lateral Palu fault in central Sulawesi slips at a rate of 38±8 mm/a with a locking depth between 2 and 8 km. From the measured slip rate and the historic seismicity of the fault, we estimate that the Palu fault currently has stored enough strain to produce a Mw>7 earthquake. The Palu and other nearby faults accommodate rapid clockwise rotation of nearly 4°/Ma of E Sulawesi relative to eastern Sunda. The rotation of east Sulawesi transfers E‐W shortening between the Pacific and Eurasian plates to N‐S subduction of the Celebes Basin beneath Sulawesi.
Time‐lapse speckle interferometry
Authors: Schmitt Douglas R. , Hunt Roger W.
Published: December 2003

An experimental technique for recording the time dependent evolution of micron‐scale three dimensional displacement fields is developed. In the method, each frame of a motion‐picture consists of a map of the relative wrapped phase, displayed as a fringe pattern, between two images of the appropriately illuminated object acquired at different times. In an illustrative demonstration, stress‐relief displacements are obtained by drilling an acrylic block subject to uniaxial compression. The resulting fringe motion‐picture displays three time dependent regimes to the resulting displacement field: 1) a transient axi‐symmetric thermal expansion centered on the stress relief hole due to drilling, 2) elastic stress‐relief fringes at short times after drilling, and 3) continued plastic deformation over long time periods after drilling. The method will have many applications at the laboratory scale and is particularly well suited to physical modeling of geodynamic processes.
Time‐dependent IRM: A new technique for magnetic granulometry
Authors: Worm Horst‐Ulrich
Published: December 2003

The intensities of isothermal remanent magnetizations (IRM) acquired in 0.01 s and 10 s can differ by more than an order of magnitude depending on field strength and sample composition. For 15 samples containing magnetite, titanomagnetite, maghemite, hematite or pyrrhotite of distinct grain sizes, ranging from superparamagnetic (SP) over stable single domain (SSD) to multidomain (MD) behavior, the IRM acquisition has been meas‐ured for the two time constants. In addition, the viscous decay following IRM acquisition within 2000 s has been determined. In low fields (≤10 mT) the IRM(10s)/IRM(0.01s) ratios range from 1.03 for MD grains up to >20 for SP/SSD grains. For fields larger than the coercivity of remanence the IRM ratio approaches unity for all samples. Viscous decay after IRM acquisition is for samples containing SP grains much larger than for SSD and MD grains as well. Hence, a viscous decay parameter Sd = (IRMto−IRMt)/log(t/to) normalized by the saturation remanence can serve as an alternative to the frequency dependence of susceptibility, the commonly used proxy for SP grains. In feet, it may be more sensitive because remanences cannot be masked by para‐ or diamagnetic contributions and IRMs can well be measured on weakly magnetic sediments.
High resolution measurements of absolute thorium abundances on the lunar surface
Authors: Lawrence D. J. , Feldman W. C. , Barraclough B. L. , Binder A. B. , Elphic R. C. , Maurice S. , Miller M. C. , Prettyman T. H.
Published: December 2003

The Lunar Prospector (LP) Gamma‐Ray Spectrometer (GRS) has been used to map the global abundance of thorium on the lunar surface. A global map of absolute thorium abundances on a 2° by 2° equal‐area pixel scale is presented using new detector modeling results and low altitude LP data. Because thorium is relatively abundant in KREEP‐rich material, these data provide fundamental information regarding the locations and importance of terranes that are rich in KREEP bearing materials.
Pentacoordinate silicon in high‐pressure crystalline and glassy phases of calcium disilicate (CaSi2O5)
Authors: Stebbins J. F. , Poe B. T.
Published: December 2003

We report 29Si MAS NMR spectra for the monoclinic and triclinic crystalline phases of CaSi2O5. The latter, which is now known from single crystal x‐ray diffraction to contain pentacoordinate silicon (SiO5 or [5]Si) units, has an NMR peak for this group near to −150 ppm, for the first time definitively confirming previous peak assignments in the spectra of ambient and high‐pressure glasses. This result, and new data on a CaSi2O5 glass quenched from a liquid at 2300°C, 10 GPa, demonstrate that [5]Si groups, along with SiO6 ([6]Si) units, form at high pressures in silicate liquids with abundant non‐bridging oxygens, and are thus likely to be important for the properties such as viscosity and density of mafic to ultramafic magmas in nature.
Imaging the moho and Vp/Vs ratio in the Western Superior Archean Craton with wide angle reflections
Authors: Kay I. , Musacchio G. , White D. , Asudeh I. , Roberts B. , Forsyth D. , Hajnal Z. , Koperwhats B. , Farrell D.
Published: December 2003

A refraction/wide‐angle reflection survey in northwestern Ontario investigated the Archean Superior Province of the Canadian Shield. Images of apparent Moho topography were produced from both the PmP and SmS Moho wide angle reflections. The average Moho depth is 41 km, and average P‐ and S‐wave velocities 6.6 and 3.8 (±0.05) km/s respectively. The Moho dips southward from 39.5 to 42 km depth over roughly 400 km. From ratios of S and P travel times to the Moho, profiles of Vp/Vs were constructed. The average Vp/Vs ratios are 1.74 and 1.76 for on the E‐W and N‐S lines (Poisson's ratio σ = 0.253 and 0.262 respectively). Vp/Vs profiles show a sharp increase from west to east and a gradual increase from north to south, correlating with the distribution of more felsic continental crust and composite crust in the Western Superior.
Wavefield separation using a volume distribution of three component recordings
Authors: Robertsson Johan O. A. , Muyzert Everhard
Published: December 2003

By making multiple three component (3C) measurements around a volume so that the rate of change of the wavefield in all directions can be estimated, it is possible to calculate the divergence and the curl of the wavefield which in turns corresponds to P‐ and S‐waves. The volume may be of arbitrary shape, but the equations for wavefield separation are simplified if the volume has a known shape such as that of a tetrahedron. The dimensions of the volume should be smaller than approximately one fifth of the minimum wavelength of the recorded wavefield.

A field test was carried out in Oman in April 1998. With a lack of distinct P‐ and S‐waves in the data, the technique was instead evaluated on the airwave‐induced ground motion. The airwave arrival was found to consist mainly of vertical motion that decays with depth. The wavefield separation technique was successfully tested on this arrival.
Modeling laser altimeter return waveforms over complex vegetation using high‐resolution elevation data
Authors: Blair J. Bryan , Hofton Michelle A.
Published: December 2003

The upcoming generation of laser altimeters record the interaction of emitted laser radiation with terrestrial surfaces in the form of a digitized waveform. We model these laser altimeter return waveforms as the sum of the reflections from individual surfaces within laser footprints, accounting for instrument‐specific properties. We compare over 1000 modeled and recorded waveform pairs using the Pearson correlation. We show that we reliably synthesize the vertical structure information for vegetation canopies contained in a medium‐large diameter laser footprint from a high‐resolution elevation data set.
Shear wave splitting observations in the Archean Craton of western Superior
Authors: Kay I. , Sol S. , Kendall J.‐M. , Thomson C. , White D. , Asudeh I. , Roberts B. , Francis D.
Published: December 2003

Shear wave splitting observations in the Archean Superior Province of the Canadian Shield show moderate to large delay times (1.1–2.1 s) with azimuths suggesting upper‐mantle anisotropy is subparallel to the structural grain of the craton. Two regions with uniform anisotropy azimuth (62°±7° and 87°±3°) are separated by a transitional zone showing strong dependence of observed azimuth on source direction, suggesting that lateral structure is being observed. No splitting was observed at stations near the coast of Hudson Bay, continuing a pattern of weak splitting in surrounding Proterozoic orogenic belts. Current absolute plate motion is consistent with the direction of anisotropy, but would not explain the regional contrasts. Instead, the anisotropy appears to be related to Archean structure and tectonic history.
On the solar wind composition during the November 1997 solar particle events: WIND/MASS observations
Authors: Wimmer‐Schweingruber Robert F. , Kern Olivier , Hamilton Douglas C.
Published: December 2003

November 1997 saw a series of very intense and unusual solar particle events that have been well studied [Cohen et al., 1999; Lario et al., 1998; Mason et al., 1999; Mazur et al., 1999; Möbius et al., 1999]. However, the composition of the solar wind has not previously been analyzed. We report the elemental and charge‐state composition of the solar wind during these events. We find unusually broad and high charge‐state distributions for O, Si, and Fe. Fe/O was enhanced by a factor >3–5 relative to the ambient solar wind in the magnetic cloud and in the following material. We speculate whether the observed compositional similarities in the solar wind and energetic particles might, in part, be explained by a common reservoir out of which both kinds of particles were accelerated.
Aircraft measurements of trace gases between Japan and Singapore in October of 1993, 1996, and 1997
Authors: Matsueda Hidekazu , Inoue Hisayuki Y.
Published: December 2003

Carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) mixing ratios were measured in discrete air samples from aircraft between Japan and Singapore in October. The mixing ratios of all trace gases at 9–12 km were enhanced over the South China Sea in 1997 compared with those in 1993 and 1996. Vertical distributions of all trace gases over Singapore in 1997 also showed largely elevated mixing ratios at all altitudes. These distributions indicate a wide outflow of trace gases from intense biomass burning in the southeast Asia regions in the very strong El Niño year. The enhanced trace gases showed a strong linear correlation between CH4and CO, and between CO and CO2, with the regression slopes of 0.051 (ΔCH4ppb/ΔCOppb) and 0.089 (ΔCOppb/ΔCO2ppb). The emission ratios are characteristic of fires with relatively lower combustion efficiency from the tropical rain forest and peat lands in Kalimantan and Sumatra of Indonesia.
Tropospheric ozone enhancements during the Indonesian Forest Fire Events in 1994 and in 1997 as revealed by ground‐based observations
Authors: Fujiwara M. , Kita K. , Kawakami S. , Ogawa T. , Komala N. , Saraspriya S. , Suripto A.
Published: December 2003

Pronounced enhancements of total and tropospheric ozone were observed with the Brewer spectrophotometer and ozonesondes at Watukosek (7.5°S, 112.6°E), Indonesia in 1994 and in 1997 when extensive forest fires were reported in Indonesia. The integrated tropospheric ozone increased from 20 DU to 40 DU in October 1994 and to 55 DU in October 1997. On October 13, 1994, most ozone mixing ratios were more than 50 ppbv throughout the troposphere and exceeded 80 ppbv at some altitudes. On October 22, 1997, the concentrations were more than 50 ppbv throughout the troposphere and exceeded 100 ppbv at several altitudes. The coincidences of the ozone enhancements with the forest fires suggest the photochemical production of tropospheric ozone due to its precursors emitted from the fires for both cases. The years of 1994 and 1997 correspond to El Niño events when convective activity becomes low in Indonesia. Thus, in this region, it is likely that pronounced enhancements of tropospheric ozone associated with extensive forest fires due to sparse precipitation may take place with a period of a few years coinciding with El Niño events. This is in a marked contrast to the situation in South America and Africa where large‐scale biomass burnings occur every year.
Delineating land/forest fire burnt scars with ERS Interferometric Synthetic Aperture Radar
Authors: Liew S. C. , Kwoh L. K. , Padmanabhan K. , Lim O. K. , Lim H.
Published: December 2003

Cloud‐penetrating C‐band synthetic aperture radar imagery acquired during two ERS‐1/2 tandem missions (April 1996 and October 1997) was used in mapping burnt areas in South Kalimantan, Indonesia, during the 1997 Southeast Asia forest fire episode. Vegetated areas were classified by their low interferometric coherence in both the 1996 and 1997 imagery while the burnt areas were delineated by their increased coherence in 1997. A total of 552 kha of land was found to be burnt scars out of the 3.6 Mha area surveyed. The results were validated using a multispectral SPOT image of the area acquired in September 1997.
Satellite and ground‐based study of optical properties of 1997 Indonesian Forest Fire aerosols
Authors: Nakajima Teruyuki , Higurashi Akiko , Takeuchi Nobuo , Herman Jay R.
Published: December 2003

Optical properties of biomass burning aerosols in the event of Indonesian forest fires in 1997 were studied by groundbased sky radiometry and satellite remote sensing with AVHRR and TOMS radiometers. The AVHRR‐derived optical thickness distribution agreed with the distribution of TOMS‐derived UV‐absorbing aerosol index and with the optical thickness measured by sky radiometry and sunphotometry. The single scattering albedo of aerosols was fairly constant as 0.9 in the September–October period. Relationship between Ångström turbidity factor and exponent supported the polydispersion consisted of aged small particles. This observation was consistent with the fact that the retrieved volume size distribution by sky radiometry has a distinct accumulation mode with a peak radius of 0.25 µm. Those optical properties of smoke aerosols seem to reflect the specific chemical structure of Indonesian forest fire aerosols, i.e., a mixture of carbonaceous and sulfate particles.
High humidities and subvisible cirrus near the tropical tropopause
Authors: Jensen Eric J. , Read William G. , Mergenthaler John , Sandor Brad J. , Pfister Leonhard , Tabazadeh Azadeh
Published: December 2003

Measurements of water vapor from the Microwave Limb Sounder on the Upper Atmospheric Research Satellite show that humidities in the extreme tropical upper troposphere are often near saturation with respect to ice. These high humidity values are supported by the available accurate in situ water vapor measurements near the tropical tropopause. A nearly saturated upper troposphere reduces the outgoing longwave radiative flux at the top of the atmosphere by about 1–2 W‐m−2 compared to the radiative flux calculated using a standard atmospheric humidity profile. Subvisible cirrus near the tropopause reduce the outgoing longwave radiation an additional few W‐m−2 as well as increasing the radiative heating near the tropopause. Changes in the humidity frequency distribution in this region of the atmosphere and the associated changes in subvisible cirrus frequency would substantially impact the radiative budget of the tropics.
Horizontal radiative fluxes in clouds and accuracy of the independent pixel approximation at absorbing wavelengths
Authors: Marshak A. , Oreopoulos L. , Davis A. B. , Wiscombe W. J. , Cahalan R. F.
Published: December 2003

For absorbing wavelengths, we discuss the effect of horizontal solar radiative fluxes in clouds on the accuracy of a conventional plane‐parallel radiative transfer calculation for a single pixel, known as the Independent Pixel Approximation (IPA). Vertically integrated horizontal fluxes can be represented as a sum of three components: the IPA accuracies for reflectance, transmittance and absorptance. We show that IPA accuracy for reflectance always improves with more absorption, while the IPA accuracy for transmittance is less sensitive to the changes in absorption: with respect to the non‐absorbing case, it may first deteriorate for weak absorption and then improve again for strongly absorbing wavelengths. IPA accuracy for absorptance always deteriorates with more absorption.