bluuebunny/crossref_metadata_2025 huggingface dataset:
Summertime partitioning and budget of NOy compounds in the troposphere over Alaska and Canada: ABLE 3B
Authors: Sandholm S. , Olson J. , Bradshaw J. , Talbot R. , Singh H. , Gregory G. , Blake D. , Anderson B. , Sachse G. , Barrick J. , Collins J. , Klemm K. , Lefer B. , Klemm O. , Gorzelska K. , Herlth D. , O'Hara D.
Published: February 2004

As part of NASA's Arctic Boundary Layer Expedition 3A and 3B field measurement programs, measurements of NOx, HNO3, PAN, PPN, and NOy were made in the middle to lower troposphere over Alaska and Canada during the summers of 1988 and 1990. These measurements are used to assess the degree of closure within the reactive odd nitrogen (NxOy) budget through the comparison of the values of NOy measured with a catalytic convertor to the sum of individually measured NOy (i) compounds (i.e., ΣNOy (i) = NOx + HNO3 + PAN + PPN). Significant differences were observed between the various study regions. In the lower 6 km of the troposphere over Alaska and the Hudson Bay lowlands of Canada a significant fraction of the NOy budget (30 to 60%) could not be accounted for by the measured ΣNOy (i). This deficit in the NOy budget is about 100 to 200 parts per trillion by volume (pptv) in the lower troposphere (0.15 to 3 km) and about 200 to 400 pptv in the middle free troposphere (3 to 6.2 km). Conversely, the NOy budget in the northern Labrador and Quebec regions of Canada is almost totally accounted for within the combined measurement uncertainties of NOy and the various NOy (i) compounds. A substantial portion of the NOy budget's “missing compounds” appears to be coupled to the photochemical and/or dynamical parameters influencing the tropospheric oxidative potential over these regions. A combination of factors are suggested as the causes for the variability observed in the NOy budget. In addition, the apparent stability of compounds represented by the NOy budget deficit in the lower‐altitude range questions the ability of these compounds to participate as reversible reservoirs for“active”odd nitrogen and suggest that some portion of the NOy budget may consist of relatively unreactive nitrogen‐containing compounds.
Assessment of the temperature and precipitation of 1990 during the Northern Wetlands Study (NOWES)
Authors: Mortsch Linda D.
Published: February 2004

Temperature and precipitation statistics are reviewed to assess the climate during the field season of the Northern Wetlands Study (NOWES). Significant climatic events are highlighted using different temporal and spatial scales. Long‐term meteorological observing stations at Moosonee, Ontario, Schefferville, Quebec, and Churchill, Manitoba, were selected to reflect the climate of nearby NOWES research sites. Climate statistics for 1990 are compared to the 1951–1980 reference period or “normal.” The summer of 1990, on average, was warm and wet throughout most of the study area. Total summer precipitation was above the normal accumulation for Churchill and Moosonee, while precipitation was below normal in Schefferville. At all three sites the mean daily temperature for June to August was higher than the 1951–1980 normal but not significantly higher. Climatic conditions for individual months at selected stations deviated from normal. For example, at Moosonee, June 1990 was the second wettest month on record since 1932. The 163‐mm accumulation was 2 times greater than the 1951–1980 mean of 78 mm, although 63 mm fell on June 3 alone. In contrast, July precipitation, at Moosonee, was 69% of normal. In Schefferville, only 44 mm of precipitation fell in August, half the 98 mm normal. At Moosonee, the July mean daily temperature of 16.9°C was the warmest since 1981, although it was well below the record of 18.9°C set in 1959.
11‐μm emissivities and droplet radii for marine stratocumulus
Authors: Luo Gang , Lin Xijian , Coakley James A.
Published: February 2004

The results of a new multispectral infrared retrieval scheme for obtaining fractional cloud cover and 11‐μm emissivity are compared with those of the spatial coherence method which obtains fractional cloud cover assuming that the clouds are opaque at infrared wavelengths. Both methods are applied to 4‐km NOAA advanced very high resolution radiometer global area coverage data for 250‐km‐scale regions containing single‐layered marine stratocumulus off the coast of South America. The average 11‐μm emissivity for low‐level clouds is found to be between 0.70 and 0.85. The low emissivity is evidently due to the thinning of clouds at their edges. Semitransparent cloud edges evidently make up a substantial portion of the area covered by such clouds. This result indicates that cloud cover obtained using the spatial coherence method is underestimated by 0.1 to 0.2, as has been claimed in a previous study. The fractional cloud cover for the ensemble of 250‐km‐scale regions studied here increased slightly from 0.60 for daytime observations to 0.63 for nighttime observations. The 11‐μm emissivity also increased slightly, but about half of the increase was related to the increase in cloud cover and a decrease in the relative area covered by cloud edge material. Presumably, the other half was due to an increase in cloud liquid water. Cloud height showed no significant change. The average effective droplet radius increased from 9.3 μm for daytime observations to 10.2 μm at night.
Contrasting behavior of gas and aerosol scavenging in convective rain: A numerical and experimental study in the African equatorial forest
Authors: Cautenet S. , Lefeivre B.
Published: February 2004

A two‐dimensional convective cloud model has been coupled with a chemical model consisting of the explicit prediction of five chemical species: SO2, SO42−, NH4+, O3, and H2O2. The model takes the scavenging processes into account. We examine the relationship between the liquid water content (LWC) and the chemical concentrations of atmospheric trace elements in convective precipitation. The model results compare favorably with observations (ABLE 2B and DECAFE experiments). The modeled dilution curves were found to be nonlinear, in agreement with the DECAFE data. The model also accounts for the large differences in dilution effects that exist between gases and aerosols. More generally, this study shows that within the African equatorial forest there are (1) a reduction of aerosol scavenging efficiency with increasing rain intensity (or LWCg); (2) a strong impact of vertical profiles of atmospheric trace elements on ground rain concentrations; (3) a difference in scavenging efficiencies according to the origin of the elements (gas or aerosol) and (4) a depletion of atmospheric concentrations during rainfall.
Radon 222 tracing of soil and forest canopy trace gas exchange in an open canopy boreal forest
Authors: Ussler William , Chanton Jeffrey P. , Kelley Cheryl A. , Martens Christopher S.
Published: February 2004

A set of continuous, high‐resolution atmospheric radon (222Rn) concentration time series and radon soil flux measurements were acquired during the summer of 1990 at a micrometeorological tower site 13 km northwest of Schefferville, Quebec, Canada. The tower was located in a dry upland, open‐canopy lichen‐spruce woodland. For the period July 23 to August 1, 1990, the mean radon soil flux was 41.1±4.8 Bq m−2 h−1. Radon surface flux from the two end‐member forest floor cover types (lichen mat and bare soil) were 38.8±5.1 and 61.8±15.6 Bq m−2 h−1, respectively. Average total forest canopy resistances computed using a simple “flux box” model for radon exchange between the forest canopy and the overlying atmosphere range from 0.47±0.24 s cm−1 to 2.65±1.61 s cm−1 for daytime hours (0900–1700 LT) and from 3.44±0.91 s cm−1 to 10.55±7.16 s cm−1 for nighttime hours (2000–0600) for the period July 23 to August 6, 1990. Continuous radon profiling of canopy atmospheres is a suitable approach for determining rates of biosphere/atmosphere trace gas exchange for remote field sites where daily equipment maintenance is not possible.
Five years of continuous observations of PAN and ozone at a rural location in eastern Canada
Authors: Bottenheim Jan W. , Sirois Alain , Brice Kenneth A. , Gallant Alan J.
Published: February 2004

Five years of measurements of PAN and O3 at a rural site in eastern Canada are reported. The measurements were made year‐round and only interrupted due to instrumental failures. The data are analyzed by advanced statistical methods to determine temporal trends, both for PAN and O3 individually and with respect to their covariance. Since the data are clearly lognormally distributed, this analysis is performed on the logarithmic transform of the data. PAN shows a statistically significant long‐term trend but O3 does not. It is argued that this trend may be the result of a change in the overall hydrocarbon composition of the air, since no trend in NOx emissions appears to have taken place over the same time period in eastern North America. Strong seasonal trends in PAN and O3 are found that are similar in overall pattern but differ in detail. The similarity in shape leads to a significant covariance between PAN and O3. The differences in detail can be interpreted as indicative of well‐known atmospheric processes, such as the impact of stratospheric folding occurrences leading to additional O3 with respect to PAN in the spring and reduced photochemical activity and lower temperatures in the winter months, which results in a decoupling of the covariance between O3 and PAN. On a day‐to‐day basis the covariance between PAN and O3 is strong due to the fact that it is essentially determined by meteorological variation. Episodes of alternatively clean and polluted air are observed, each lasting circa 3 days. Seasonal trends in the diurnal patterns are also evident for both compounds: diurnal variation in the summer is considerable but becomes negligible in the winter. PAN displays a larger diurnal variation than O3 in the summer, which can be explained by their relative dry deposition velocities.
Aqueous reaction kinetics of ozone and dimethylsulfide and its atmospheric implications
Authors: Lee Yin‐Nan , Zhou Xianliang
Published: February 2004

In order to understand and predict the budgets and distributions of ozone (O3) and dimethylsulfide (DMS) in the marine atmosphere, it is necessary to determine the rates of aqueous reactions in clouds and surface water that may influence the lifetimes of these species. We studied the kinetics of the aqueous phase reaction of O3 and DMS in the laboratory using a bubbler‐type gas‐liquid reactor at sub‐parts‐per‐billion (ppb) gas phase concentrations. The steady state reaction rate was determined from the decrease of gas phase DMS concentration in the effluent gas as ozone was simultaneously added. The reaction stoichiometry was found to be 1:1 with respect to the two reagents. Fitting the data to a kinetics scheme appropriate for a gas‐liquid reaction in the slow reaction regime where the reaction rate is governed by both chemical reaction and convective mass transfer, we determined an intrinsic second‐order aqueous rate coefficient of O3‐DMS reaction of 4×108(±40%) M−1 s−1 at 15.0°C. The implications of this reaction include (1) the atmospheric lifetime of DMS against in‐cloud O3‐DMS reaction at 30 ppb of O3 and 1 g m −3 liquid water concentration is calculated to be ∼3 days and is comparable to that against the gas phase reaction with OH radicals at 1×106 molecules cm−3; (2) the deposition of atmospheric O3 to the ocean surface is only slightly enhanced by the O3‐DMS reaction at the highest reported surface DMS concentrations, (∼40 nM); and (3) the sea to air flux of DMS is not affected by this reaction at O3 concentrations typical of the marine atmosphere.
Influence of the heterogeneous reaction HCl + HOCl on an ozone hole model with hydrocarbon additions
Authors: Elliott Scott , Cicerone Ralph J. , Turco Richard P. , Drdla Katja , Tabazadeh Azadeh
Published: February 2004

Injection of ethane or propane has been suggested as a means for reducing ozone loss within the Antarctic vortex because alkanes can convert active chlorine radicals into hydrochloric acid. In kinetic models of vortex chemistry including as heterogeneous processes only the hydrolysis and HCl reactions of ClONO2 and N2O5, parts per billion by volume levels of the light alkanes counteract ozone depletion by sequestering chlorine atoms. Introduction of the surface reaction of HCl with HOCl causes ethane to deepen baseline ozone holes and generally works to impede any mitigation by hydrocarbons. The increased depletion occurs because HCl + HOCl can be driven by HOx radicals released during organic oxidation. Following initial hydrogen abstraction by chlorine, alkane breakdown leads to a net hydrochloric acid activation as the remaining hydrogen atoms enter the photochemical system. Lowering the rate constant for reactions of organic peroxy radicals with ClO to 10−13 cm3 molecule−1 s−1 does not alter results, and the major conclusions are insensitive to the timing of the ethane additions. Ignoring the organic peroxy radical plus ClO reactions entirely restores remediation capabilities by allowing HOx removal independent of HCl. Remediation also returns if early evaporation of polar stratospheric clouds leaves hydrogen atoms trapped in aldehyde intermediates, but real ozone losses are small in such cases.
Total reactive nitrogen (NOy) as an indicator of the sensitivity of ozone to reductions in hydrocarbon and NOx emissions
Authors: Milford Jana B. , Gao Dongfen , Sillman Sanford , Blossey Peter , Russell Armistead G.
Published: February 2004

For areas in the United States not meeting the federal air quality standard for ozone, an issue of continuing controversy is the emphasis to be placed on controlling nitrogen oxides (NOx) in addition to emissions of reactive organic gases (ROG). To assess conditions under which ROG or NOx controls would be most effective, we have analyzed predictions from four studies that represent different locations and meteorological conditions, distinct chemical inputs, e.g., with or without significant biogenic emissions, and different air quality models. A consistent association is found between the sensitivity of ozone to reductions in ROG versus NOx emissions and the simulated total reactive nitrogen (NOy) at the time and place of peak ozone. In the studies examined, ozone was predicted to be reduced most effectively by ROG controls at locations where NOy concentrations exceeded a threshhold value falling in the range of 10 to 25 ppb, whereas NOx controls were predicted to be more effective where NOy concentrations were below that threshhold. The NOy level explains much of the difference in ozone sensitivity at different locations and provides a basis for comparison of predicted sensitivity from different models. In contrast, the morning concentration ratio of ROG to NOx that has been used in the past is a less reliable indicator of O3 sensitivity. Measurement of NOy concentrations along with ozone would assist in empirical testing of model predictions of responses to emission reductions.
Approaches of comparison for clear‐sky radiative fluxes from general circulation models with Earth Radiation Budget Experiment data
Authors: Zhang M. H. , Cess R. D. , Kwon T. Y. , Chen M. H.
Published: February 2004

In order to compare the clear‐sky greenhouse effect and cloud‐radiative forcing from general circulation models with Earth Radiation Budget Experiments (ERBE) data, it is necessary to calculate the general circulation model (GCM) clear‐sky radiative fluxes in a way consistent with ERBE. This study discusses problems associated with the available methods for clear‐sky radiative flux computations in GCMs and proposes a new approach, which uses a statistical relationship between the grid cloud cover and the availability of ERBE clear‐sky measurement, established from ERBE pixel data, to sample the model radiative fluxes. Calculations with version 2 of the NCAR Community Climate Model using observed SST show good agreement of clear‐sky sampling from the proposed method with ERBE sampling. It is also shown that large improvements are achieved in the spatial variability of the model clear‐sky radiative fluxes over ocean, with reference to ERBE, by using the new clear‐sky sampling method.
Parameterization of moist convection in the National Center for Atmospheric Research community climate model (CCM2)
Authors: Hack James J.
Published: February 2004

The National Center for Atmospheric Research (NCAR) community climate model (CCM) has historically made use of a moist adiabatic adjustment procedure for parameterizing the effects of moist convection. The most recent version of the NCAR CCM, CCM2, has abandoned this approach in favor of a stability‐dependent mass‐flux representation of moist convective processes. This scheme physically constrains the process of moist convection with the use of a simple bulk cloud model, which provides a basis for estimating convective‐scale transports of heat, moisture, and other atmospheric constituents as well as the diabatic heating associated with condensation and the fallout of precipitation. This paper presents the formalism associated with this simple mass‐flux approach and contrasts its behavior with the moist adiabatic adjustment procedure used in earlier models. The inclusion of this scheme significantly moistens and warms the model troposphere at all latitudes but particularly in the tropics. Additionally, the simulated magnitude, structure, and location of the large‐scale mean circulations are generally improved. The sensitivity of the simulated climate to the formulation of the cloud model is also presented.
Diagnostic study of climate feedback processes in atmospheric general circulation models
Authors: Zhang M. H. , Hack J. J. , Kiehl J. T. , Cess R. D.
Published: February 2004

A method is proposed to diagnose climate feedbacks of water vapor, temperature lapse‐rate, and cloud variations in atmospheric general circulation models. It is then applied to study differences in sensitivity of the National Center for Atmospheric Research community climate model (CCM2) and two hybrid versions of CCM2 with different cumulus‐convection schemes. Water vapor feedback and temperature lapse‐rate feedback differ among the models due to different efficiencies of heat and moisture transport by cumulus convections. A large compensation occurs between water vapor feedback and temperature lapse‐rate feedback. This leads to similar clear‐sky sensitivities in the models. Cloud‐radiative feedback is negative in CCM2 with a ΔSST climate change due to the vigorous cumulus‐convective scheme. Stronger convection warms the upper troposphere and reduces its cloudiness more, resulting in negative longwave cloud‐radiative feedback. In models where a moist‐adiabatic‐adjustment scheme and then a decoupling of the atmospheric boundary layer are subsequently used, intensity of cumulus convection is successively reduced and cloud‐radiative feedback changes to either neutral or positive.
Coupling of volatile transport and internal heat flow on Triton
Authors: Brown Robert H. , Kirk Randolph L.
Published: February 2004

Recently Brown et al. (1991) showed that Triton's internal heat source could amount to 5–20% of the absorbed insolation on Triton, thus significantly affecting volatile transport and atmospheric pressure. Subsequently, Kirk and Brown (1991a) used simple analytical models of the effect of internal heat on the distribution of volatiles on Triton's surface, confirming the speculation of Brown et al. that Triton's internal heat flow could strongly couple to the surface volatile distribution. To further explore this idea, we present numerical models of the permanent distribution of nitrogen ice on Triton that include the effects of sunlight, the two‐dimensional distribution of internal heat flow, the coupling of internal heat flow to the surface distribution of nitrogen ice, and the finite viscosity of nitrogen ice. From these models we conclude that: (1) The strong vertical thermal gradient induced in Triton's polar caps by internal heat‐flow facilitates viscous spreading to lower latitudes, thus opposing the poleward transport of volatiles by sunlight, and, for plausible viscosities and nitrogen inventories, producing permanent caps of considerable latitudinal extent; (2) It is probable that there is a strong coupling between the surface distribution of nitrogen ice on Triton and internal heat flow; (3) Asymmetries in the spatial distribution of Triton's heat flow, possibly driven by large‐scale, volcanic activity or convection in Triton's interior, can result in permanent polar caps of unequal latitudinal extent, including the case of only one permanent polar cap; (4) Melting at the base of a permanent polar cap on Triton caused by internal heat flow can significantly enhance viscous spreading, and, as an alternative to the solid‐state greenhouse mechanism proposed by Brown et al. (1990), could provide the necessary energy, fluids, and/or gases to drive Triton's geyser‐like plumes; (5) The atmospheric collapse predicted to occur on Triton in the next 20 years (Spencer, 1990) may be plausibly avoided because of the large latitudinal extent expected for permanent polar caps on Triton.
Mars aeolian sand: Regional variations among dark‐hued crater floor features
Authors: Edgett Kenneth S. , Christensen Philip R.
Published: February 2004

Different regions on Mars appear to have low‐albedo intracrater deposits that have distinct regional thermophysical and/or aeolian dune characteristics. Thermal inertia derived from a carefully selected set of Viking infrared thermal mapper observations of the dark features obtained in 1977–1978 supports this conclusion. The observed similarities and differences among dark intracrater features on Mars is probably a function of the combined influences of sand availability and regional wind conditions.
Topography of Valles Marineris: Implications for erosional and structural history
Authors: Lucchitta B. K. , Isbell N. K. , Howington‐Kraus A.
Published: February 2004

Compilation of a simplified geologic/geomorphic map onto digital terrain models of the Valles Marineris permitted an evaluation of elevations in the vicinity of the troughs and the calculation of depth of troughs below surrounding plateaus, thickness of deposits inside the troughs, volumes of void spaces above geologic/geomorphic units, and volumes of deposits. The central troughs north Ophir, north and central Candor, and north Melas Chasmata lie as much as 11 km below the adjacent plateaus. In Ophir and Candor Chasmata, interior layered deposits reach 8 km in elevation. If the deposits are lacustrine and if all troughs were interconnected, lake waters standing 8 km high would have spilled out of Coprates Chasma onto the surrounding plateaus having surface elevations of only 4–5 km. In this case, interior deposits above about 4 km in the central troughs would not be lacustrine. They could be volcanic. On the other hand, the troughs may not have been interconnected at the time of interior‐deposit emplacement; they may have formed isolated ancestral basins. The existence of such basins is supported by independent structural and stratigraphic evidence. The ancestral basins may have eventually merged, perhaps through renewed faulting, to form northern subsidiary troughs in Ophir and Candor Chasmata and the Coprates/north Melas/Ius graben system. The peripheral troughs are only 2–5 km deep, shallower than the central troughs. They may have formed from a combination of erosional collapse and structural activity. Chaotic terrain is seen in the peripheral troughs near a common contour level of about 4 km on the adjacent plateaus, which supports the idea of release of water under artesian pressure from confined aquifers. The layered deposits in the peripheral troughs may have formed in isolated depressions that harbored lakes and predated the formation of the deep outflow channels. If these layered deposits are of volcanic origin, they may have been emplaced beneath ice in the manner of table mountains. Areal and volumetric computations show that erosion widened the troughs by about one‐third and that deposits occupy one‐sixth of the interior space. Even though the volume eroded is larger than the volume deposited, topographic and geologic considerations imply that material eroded from trough walls was probably part of the interior layered deposits but not their sole source. Additional material may have come from subterranean piping, from reworking of local disintegration products on the floors, such as chaotic materials, or from eolian influx. But overall it is likely that the additional material is volcanic and that it forms mostly the upper, more diversely bedded layers of the interior deposits.
New evidence for graben origin of Oceanus Procellarum from lunar sounder optical imagery
Authors: Cooper Bonnie L. , Carter James L. , Sapp Clyde A.
Published: February 2004

The Procellarum Basin is thought by some to be the result of a gargantuan impact that occurred very early in lunar history. It is suggested that later impacts, such as Imbrium, would have partially obliterated the outline of that ancient “Gargantuan” basin in most areas, leaving only its western margin with a circular shape. An alternative idea is that the Procellarum Basin is the result of faulting associated with the Imbrium event. We have detected a subsurface horizon in Oceanus Procellarum using the Apollo lunar sounder experiment optical‐format data. The horizon is deepest at the western edge of the basin, and shallows eastward. We did not observe peak ring structure in the areas where peak rings were predicted to exist according to the “Gargantuan impact” hypothesis. This observation implies that the alternative (graben) formation hypothesis for Oceanus Procellarum may be more likely. In this paper, we also review other geophysical and geologic evidence that supports the graben hypothesis. Finally, we report on some new radar evidence of a 500‐km‐diameter pre‐Imbrian impact basin beneath Mare Insularum, in the Copernicus region.
Thermal studies of Martian channels and valleys using Termoskan data
Authors: Betts Bruce H. , Murray Bruce C.
Published: February 2004

The Termoskan instrument on board the Phobos '88 spacecraft acquired the highest spatial resolution thermal infrared emission data ever obtained for Mars. Included in the thermal images are 2 km/pixel, midday observations of several major channel and valley systems including significant portions of Shalbatana, Ravi, Al‐Qahira, and Ma'adim Valles, the channel connecting Valles Marineris with Hydraotes Chaos, and channel material in Eos Chasma. Termoskan also observed small portions of the southern beginnings of Simud, Tiu, and Ares Valles and some channel material in Gangis Chasma. Simultaneous broadband visible reflectance data were obtained for all but Ma'adim Vallis. We find that most of the channels and valleys have higher thermal inertias than their surroundings, consistent with previous thermal studies. We show for the first time that the thermal inertia boundaries closely match flat channel floor boundaries. Also, buttes within channels have inertias similar to the plains surrounding the channels, suggesting the buttes are remnants of a contiguous plains surface. Lower bounds on typical channel thermal inertias range from 8.4 to 12.5 (10−3 cal cm−2 s−1/2 K−1) (352 to 523 in SI units of J m−2 s−1/2 K−1). Lower bounds on inertia differences with the surrounding heavily cratered plains range from 1.1 to 3.5 (46 to 147 SI). Atmospheric and geometric effects are not sufficient to cause the observed channel inertia enhancements. We favor nonaeolian explanations of the overall channel inertia enhancements based primarily upon the channel floors' thermal homogeneity and the strong correlation of thermal boundaries with floor boundaries. However, localized, dark regions within some channels are likely aeolian in nature as reported previously. Most channels with increased inertias have fretted morphologies such as flat floors with steep walls. Eastern Ravi and southern Ares Valles, the only major channel sections observed that have obvious catastrophic flood bedforms, do not have enhanced inertias. Therefore, we favor fretting processes over catastrophic flooding for explaining the inertia enhancements. We postulate that the inertia enhancements were caused either by the original fretting process or by a process involving the bonding of fines due to an increased availability of water, either initially or secondarily.
Cd/Ca changes in a Deep Cape Basin Core over the past 730,000 years: Response of circumpolar deepwater variability to northern hemisphere ice sheet melting?
Authors: Oppo Delia W. , Rosenthal Yair
Published: February 2004

A comparison of cadmium/calcium (Cd/Ca) records of benthic foraminifera from a deep Cape Basin and a deep eastern equatorial Pacific core suggests that over the past 400,000 years, the nutrient concentration of Circumpolar Deep Water (CPDW) has always been lower than that of the deep Pacific. The data further suggest that at the 100,000‐ and 23,000‐year orbital periods, the contribution of North Atlantic Deep Water to CPDW is at a maximum during periods of ice growth and at a minimum during periods of ice decay. These results are not in agreement with results based on carbon isotope records of benthic foraminifera, which suggest intervals of CPDW nutrient enrichment relative to the deep Pacific and an approximately in‐phase relationship between CPDW nutrient concentration and ice volume. Resolution of the apparent conflict between δ13C and Cd/Ca data may provide important constraints on past deep‐ocean circulation and nutrient variability.
The ecology of living (stained) deep‐sea benthic foraminifera from the Sulu Sea
Authors: Rathburn A. E. , Corliss B. H.
Published: February 2004

The distribution of living (rose bengal stained) deep‐sea benthic foraminifera was determined in the upper 20 cm of sediments of eight Soutar box cores taken from two depth transects (510‐4515 m) in the thermospheric (> 10°C) Sulu Sea. Despite the uniformity of bottom water temperatures, salinities, and dissolved oxygen levels below 1000 m, significant faunal differences exist at different depths in the low‐oxygen (∼1.25 mL/L below 1000 m) basin. The shallowest site (510 m) is dominated (> 10% of the calcareous fauna) by Cibicidoides, Uvigerina, (> 150 µm) and Bolivina (> 63 µm), while Siphonina is codominant with Cibicidoides and Uvigerina in the 1005‐m core. The 2000‐m cores are dominated by Cibicidoides, Gyroidinoides, and Oridorsalis, while Cibicidoides bradyi and Oridorsalis umbonatus dominate the 3000‐ and 4000‐m cores. Infaunal assemblages of Valvulineria mexicana are found in the sediments of the 4515‐m core. Relatively low bottom water oxygen values do not necessarily yield "typical low‐oxygen taxa" such as Bolivina, Uvigerina, Chilostomella, Bulimina, and Globobulimina. Changes in the abundances of these taxa in fossil assemblages have been used as indicators of changes in ancient bottom water oxygen levels but may instead reflect organic carbon contents of the sediments. An examination of the vertical distributions of foraminiferal assemblages from the > 63‐µm and > 150‐µm fractions reveals that taxa have microhabitat preferences similar to those observed in other regions. Taxa found in the upper 0‐ to 1‐cm interval (epifaunal) include Cibicidoides wuellerstorfi and Hoeglundina elegans, while taxa such as Chilostomella and Globobulimina reach maximum abundances in subsurface sediments and have infaunal microhabitat preferences. Cibicidoides bradyi and O. umbonatus live in sediment depths from 0‐ to 4‐cm and have transitional preferences with both epifaunal and infaunal occurrences. Intrageneric differences in test morphologies, including pore distribution, rounded peripheries, and variable spire height, are observed in Cibicidoides and Gyroidinoides and are suggested to be related to microhabitat preferences. Vertical distributions of a number of taxa found in both the 63‐ to 150‐µm and > 150‐µm fractions are similar, suggesting that juveniles and adults live under similar microhabitat conditions. Ontogenetic changes in microhabitat preferences of most species are not observed in this study and therefore would not be expected to account for isotopic vital effects reported for some taxa in previous studies.
Late Miocene‐Early Pliocene manganese redirection in the central Indian Ocean: Expansion of the Intermediate Water oxygen minimum zone
Authors: Dickens Gerald R. , Owen Robert M.
Published: February 2004

Decomposition of organic matter combined with density stratification generate a pronounced intermediate water oxygen minimum zone (OMZ) in the northwest Indian Ocean. This zone currently lies between water depths of 200 and 2000 m and extends approximately 5000 km southeast from the Arabian coast. Based upon benthic foraminiferal assemblage changes, it has been suggested that this OMZ was even more extensive during the late Miocene‐early Pliocene (6.5–3.0 Ma), with a maximum volume and/or intensity at approximately 5.0 Ma. While this inference may contribute to an understanding of the history of northwest Indian Ocean upwelling, corroborating geochemical evidence for this interpretation has heretofore been lacking. Ocean Drilling Program (ODP) sites 752, 754, and 757 on Broken and Ninetyeast ridges are located within central Indian Ocean intermediate water depths (1086–1650 m) but outside the present lateral dimensions of the Indian Ocean OMZ. High‐resolution chemical analyses of sediment from these sites indicate significant reductions in the flux of Mn and normalized Mn concentrations between 6.5 and 3.0 Ma that are most pronounced at approximately 5.0 Ma. Because late Miocene‐Pliocene paleodepths for these sites were essentially the same as at present and because extremely low sedimentation rates (0.3–1.3 cm/ky) most likely precluded sedimentary metal oxide diagenesis, we suggest that the observed Mn depletions reflect diminished deposition of reducible Mn oxyhydroxide phases within O2 deficient intermediate waters and that this effect was most intense at approximately 5.0 Ma. This interpretation implies that waters with less than 2.0 mL/L O2 extended at least 1500 km beyond their present limits and is consistent with changes in benthic foraminifera assemblages. We further suggest this expanded Indian Ocean OMZ is related to regionally and/or globally increased biological productivity.
Production and destruction: Control of biogenous sedimentation in the tropical Atlantic 0–300,000 years B.P.
Authors: Verardo David J. , McIntyre Andrew
Published: February 2004

Late Pleistocene signals of calcium carbonate, organic carbon, and opaline silica concentration and accumulation are documented in a series of cores from a zonal/meridional/depth transect in the equatorial Atlantic Ocean to reconstruct the regional sedimentary history. Spectral analysis reveals that maxima and minima in biogenous sedimentation occur with glacial‐interglacial cyclicity as a function of both (1) primary production at the sea surface modulated by orbitally forced variation in trade wind zonality and (2) destruction at the seafloor by variation in the chemical character of advected intermediate and deep water from high latitudes modulated by high‐latitude ice volume. From these results a pattern emerges in which the relative proportion of signal variance from the productivity signal centered on the precessional (23 kyr) band decreases while that of the destruction signal centered on the obliquity (41 kyr) and eccentricity (100 kyr) periods increases below ∼3600‐m ocean depth.
Late Quaternary CaCO3 stratigraphy of the eastern equatorial Pacific
Authors: Snoeckx Hilde , Rea David K.
Published: February 2004

Four patterns of CaCO3 abundance occur in cores retrieved from the eastern equatorial Pacific during the Venture 01 expedition. Core locations lie along a north‐south transect at 110°W underneath the different currents of the equatorial current system and along an east‐west transect from 110° to 90°W, at approximately 3°S, underneath the South Equatorial Current. The “classic” or central equatorial Pacific pattern of CaCO3 abundance maxima associated with glacial stages as defined by the δ18O record characterizes only a small portion of the area studied. In the area where the Peru current turns west to join the South Equatorial Current, the carbonate record is characterized by a broad low, centered on interglacial carbonate stage B‐3 (approximately stage 5 in δ18O stratigraphy) overlying a broad mid‐Brunhes maximum. Low‐amplitude variations in CaCO3 percentages are superimposed on this pattern. This pattern extends westward in a long‐wavelength pattern in which the B‐3 carbonate low becomes decreasingly pronounced away from the equator. The fourth pattern, observed north of 10°N, is erratic and may be only local in extent.
Depth profiles of δ13C in bottom water and core top C. wuellerstorfi on the Ontong Java Plateau and Emperor Seamounts
Authors: McCorkle Daniel C. , Keigwin Lloyd D.
Published: February 2004

We have measured the carbon isotopic composition of dissolved inorganic carbon in bottom waters of the Ontong Java Plateau (western equatorial Pacific) and on the northern Emperor Seamounts (northwest Pacific). Each of these locations is several hundred miles from the nearest Geochemical Ocean Sections Study (GEOSECS) stations, and the observed δ13C values at each site differ substantially from regionally averaged GEOSECS δ13C profiles. We discuss the possible causes of these differences, including horizontal variability, near‐bottom effects, and problems with the Pacific GEOSECS δ13C data. We also measured the isotopic composition (C and O) of core top C. wuellerstorfi from a depth transect of cores at each location. The δ18O data are used to verify that our samples are Holocene. Comparison of foraminiferal and bottom water δ13C values shows that this species faithfully records bottom water δ13C at both sites and demonstrates that there is no depth‐related artifact in the dissolved inorganic carbon‐C. wuellerstorfi δ13C relationship at these sites.
Secondary ion mass spectrometry (SIMS) and its application to chemical weathering
Authors: Shotyk William , Metson Jim B.
Published: February 2004

Secondary ion mass spectrometry (SIMS) is the mass spectrometry of atomic species which are emitted when a solid surface is bombarded by an energetic primary ion beam. By continually bombarding the surface of the sample with the ion beam, the atoms making up the material being studied are sputtered away. The secondary ions emitted from the surface are continually analyzed and their intensities recorded over time. The secondary ion intensities are proportional to the concentration of elements in the sample, thereby producing a semiquantitative concentration depth profile. The depth profile provides an illustration of the chemical composition of a sample as a function of depth through the surface. The SIMS technique has been applied to a wide variety of surface analytical problems and can easily be used to analyze reacted glass and mineral surfaces which have been exposed to weathering solutions. Traditional experimental studies of chemical weathering were based primarily on the analyses of aqueous solutions generated during leaching experiments. Such studies have provided valuable information concerning rates and stoichiometry of mineral dissolution reactions but have led to some confusion and much speculation regarding the mechanisms of surface processes. SIMS analyses of the surfaces of dissolving glasses and plagioclase feldspars have recently been used to help resolve a number of unanswered questions. For example, SIMS analyses of dissolving feldspars have shown how the chemical composition of reacted surfaces and depth of attack vary, depending on the composition of the mineral, the pH of the leaching solution, and the presence of dissolved salts and complex‐forming organic ligands.
Basic properties and gravity wave initiation of the midlatitude F region instability
Authors: Huang Chao‐Song , Miller C. A. , Kelley M. C.
Published: February 2004

Observations of midlatitude spread F irregularities have shown that the most violent eruptions are often periodic and may be related to modulations of the F region bottomside. Similar aspects of equatorial spread F have been conclusively shown to be the result of seeding by atmospheric gravity waves. In this paper, we pursue a nonlinear description of the midlatitude F region instability first proposed by Perkins (1973) and show that, by itself, this instability saturates at approximately 1%. With gravity wave seeding, however, the growth of the instability is significantly enhanced and progresses until it is limited by third‐ or fourth‐order nonlinearities. In the course of the analysis we expand the Perkins linear instability study, particularly with regard to the wave propagation angles suitable for stimulating linearly unstable, and eventually, nonlinear turbulent upwellings. Although gravity waves themselves have limited spatial extent, we also show that gravity waves which propagate perpendicular to the Earth's magnetic field create electric fields which map along the field lines to considerable distances. This transmission of electric fields is dependent on both the particular characteristics of the ionosphere along the field lines and the wavelength of the perturbation, but under certain circumstances, gravity wave‐induced large‐scale electric fields can map to the F region from either the E or conjugate F region and thus influence F region electrodynamics. Finally, we note that our results are consistent with observations of midlatitude spread F irregularities by the MU radar in Japan and large‐scale perturbations of the F region over Arecibo.
Full wave calculation of ELF/VLF propagation from a dipole source located in the lower ionosphere
Authors: Yagitani Satoshi , Nagano Isamu , Miyamura Kazutoshi , Kimura Iwane
Published: February 2004

A full wave technique has been developed to calculate the field intensities both in the upper ionosphere and on the ground when a dipole source immersed in the lower ionosphere radiates ELF/VLF waves. The radiated wave is divided into a large number of elementary plane waves, for each of which the propagation in the horizontally stratified model consisting of the ionosphere, the free space and the ground is calculated by the full wave technique. Then the plane waves are summed up to give a horizontal distribution of the radiated wave intensities at any altitudes. This technique is applied to investigate the propagation characteristics of radiation from a polar electrojet (PEJ) antenna modulated at an ELF/VLF frequency, which was created by the Tromsø heating facility. Comparison between the results of experiments and the calculation is given in the companion paper [Kimura et al., this issue]. We discuss several propagation characteristics, such as the frequency dependence of the radiated wave propagation up to the upper ionosphere and down to the ground. Under the conditions that the frequency is 2.5 kHz and the ionospheric dc electric field is 5 mV/m, the calculated results are roughly consistent with the experimental results and the radiation efficiency of the PEJ antenna is found to be extremely small, 2.5×10−6 upward and 3×10−8 downward.
Efficient numerical and closed‐form asymptotic representations of the dyadic aperture Green's function for material‐coated ground planes
Authors: Somers Gary A. , Pathak Prabhakar H.
Published: February 2004

This paper provides both numerical and closed‐form uniform asymptotic representations of the aperture Green's function which determines the magnetic field due to a point magnetic current source where both the source and field points lie on the ground plane which is covered by a planar material slab. The closed‐form asymptotic representation which explicitly contains effects of space, surface, and leaky waves is shown to agree with the numerical evaluation of the exact integral representation for source and field point separations which are only a few tenths of a free space wavelength. An extended envelope extraction technique allows an efficient evaluation of the exact integral representation for source and observation separations too small for the asymptotic solution to be valid. The asymptotic solution is uniform in that it remains valid across all of the relevant surface and leaky wave transition regions. The computation time for this asymptotic result is 2–3 orders of magnitude faster than the numerical evaluation of the exact integral form, and additionally itscomputation time is independent of the source and field point separations. This aperture Green's function is very useful when computing the mutual coupling elements of the method of moments admittance matrix for an array of slots in a material covered ground plane.
Double‐passage radar cross section enhancements
Authors: Rino Charles L.
Published: February 2004

Transionospheric radar measurements reported by Knepp and Houpis (1991) verify the backscatter enhancement known to be associated with the double‐passage problem. Knepp and Reinking (1989) showed, moreover, that the enhancement is proportional to the intensity scintillation index on the one‐way path. While this result is easily derived, it is more difficult to characterize the complete angular distribution of backscattered waves of which the radar measurement represents one component. In this paper we develop a simple product expression for the backscattered spectrum from an incident plane wave. The spectrum of backscattered waves from a point source is then derived by transformation of the plane‐wave result. Motivated by a discussion in a recent paper by Fremouw and Ishimaru (1992), we show that both reciprocity and energy conservation are preserved in our wave‐space formulation, which is essentially exact. We also discuss the method of computing the scattering functions and the fourth‐order moments that appear in our results; however, the general expressions are the principal results of this paper.
Efficient computation of Fresnel zone fields associated with circular apertures
Authors: Evans G. E. , Dvorak S. L. , Fast S. A.
Published: February 2004

Efficient computation of the Fresnel‐zone fields for circular apertures plays an important role in applications where the frequency, aperture size, and observation location are such that the Fraunhofer approximation is not justified. If an adequate approximation for the aperture field distribution is known, then the standard procedure for computing the Fresnel‐zone field distribution involves a time‐consuming numerical integration over the aperture field distribution. In this paper, we will represent the Fresnel‐zone field for some commonly used aperture field distributions in terms of incomplete Weber integrals, thereby avoiding the need for numerical integration. Since the incomplete Weber integrals are related to the incomplete Lipschitz‐Hankel integral of the first kind, four Bessel series expansions are used to dramatically reduce the computation time required to compute the Fresnel‐zone electric fields.