bluuebunny/crossref_metadata_2025 huggingface dataset:
CO2‐rich impact craters on Callisto
Authors: Hibbitts C. A. , Klemaszewski J. E. , McCord T. B. , Hansen G. B. , Greeley R.
Published: November 2002

Carbon dioxide is present over most of Callisto's surface and is often concentrated in and around morphologically fresh and bright impact craters. The depths of the 4.26‐μm absorption band in reflectance spectra returned by the Near‐Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft imply a CO2 abundance in the top ∼1 mm of the surface that is ≤0.3 wt %. This absorption band is due to CO2 that is contained in the nonice material. Ice on Callisto may also contain CO2, but the ice appears to be large‐grained and therefore is not sufficiently reflective for an absorption band at 4.26‐μm to be visible. The depth of the CO2 absorption band has at most only a very slight photometric dependence. The distribution of CO2 around morphologically fresher impact craters suggests its origin there is related to the impact process, but there is no indication that impactors are the source of the impact‐related CO2, (e.g., known comet chains are not CO2‐rich). These distributions of CO2 also suggest that the enrichment of CO2 in and around impact craters is a transient phenomenon. The increased CO2 abundance around impact craters on Callisto is associated with (1) the dark, less icy material within and nearby the craters, (2) occasionally the icy material inside the craters and their icy rims, (3) their continuous ejecta blankets, and, if existing, (4) their icy ejecta.
Survival of impact‐induced thermal anomalies in the Martian mantle
Authors: Reese C. C. , Solomatov V. S. , Baumgardner J. R.
Published: November 2002

A number of geophysical arguments point out the possibility that global mantle convection could have been extremely sluggish or even absent during a large part of Martian history. This implies that early mantle thermal anomalies produced during planetary formation might not have been quickly erased by vigorous convection. One likely mechanism of early thermal inhomogeneity is large impacts at the end of heavy bombardment. We suggest that Tharsis province might be related to such an impact‐induced thermal anomaly rather than a convective plume. The shape of the present‐day geoid for our preferred model is in good agreement with the geoid measured by Mars Global Surveyor (MGS).
Calculating iron contents of lunar highland materials surrounding Tycho crater from integrated Clementine UV‐visible and near‐infrared data
Authors: Le Mouélic Stéphane , Lucey P. G. , Langevin Yves , Hawke B. Ray
Published: November 2002

Optical properties of lunar surface materials are modified by exposure to the space environment, including bombardment by solar wind particles and micrometeorites. One of the main problems in the interpretation of lunar spectral observations is to separate the effects of exposure (“space weathering”) and those due to the composition of the soils (mainly iron and titanium content). Here we use Clementine near‐infrared (NIR) data to investigate this problem for highland‐type soils in the Tycho crater area. Our approach is based on the methods developed for an analysis of a mare region near Aristarchus Plateau [Le Mouélic et al., 2000]. We show that the systematic relationships between spectral parameters (1‐μm band depth and continuum slope), which were previously observed in a mare area with varying maturity degree, are also valid for highland‐type soils. This technique aimed at evaluating the iron content of the surface materials from Clementine UV‐visible (UVVIS) and near‐infrared data can therefore be applied globally. This approach complements the widely used method of Lucey and coworkers, which relies on the UV‐visible bands only. The proposed method relies mostly on band ratios. The iron map produced from this method is therefore less dependent on topography than the iron estimates obtained with UV‐visible data alone, where brightness is one of the controlling parameters. Therefore the proposed approach should be particularly useful at middle to high latitudes, where local topography generates large variations in brightness and therefore hampers the interpretation of iron maps produced from UV‐visible bands only.
A multiproxy approach to reconstructing sea surface conditions using coral skeleton geochemistry
Authors: Quinn Terrence M. , Sampson Daniel E.
Published: November 2002

We use elemental ratio (Mg/Ca, Sr/Ca, U/Ca and Ba/Ca) and oxygen isotope data from a Porites lutea coral head collected from offshore Amédée Island, New Caledonia (22°29′S, 166°28′E) to assess the degree to which changes in these geochemical variables reflect variations in sea surface conditions. We have assessed the robustness of each geochemical proxy by comparing 25 years (1992–1968) of monthly geochemical variations with a local record of sea surface temperature (SST) and with the appropriate 1° by 1° grid box from a global gridded SST data set. We conclude from our comparison that the most consistent proxies of monthly SST variations at this site are δ18O (r2 = −0.84; p = <.0001) and Sr/Ca (r2 = −0.84; p = <.0001). The fidelity of the coral‐based Sr/Ca‐SST proxy was assessed via calibration‐verification exercises at New Caledonia and Rarotonga. The “paleoclimate” accuracy of the coral Sr/Ca‐SST technique is judged to be robust, and the precision of the technique is estimated to be ∼0.3° ± 0.5 (2σ) based on the calibration‐verification exercise. In contrast, monthly variations in Mg/Ca and U/Ca ratios are only moderately correlated with monthly SST variations (r2 = 0.55; p = <.0001 for Mg/Ca and r2 = 0.47; p = <.0001). The strength of the correlation between SST and Mg/Ca and U/Ca varies as a function of time, suggesting that variations in these ratios are not a simple function of SST variations. This variability in goodness of fit between U/Ca‐SST and Mg/Ca‐SST may ultimately limit the use of these ratios as coral paleothermometers. Averaging or stacking individual proxy SST records (Sr/Ca, Mg/Ca and U/Ca) to generate a composite proxy SST record does not necessarily improve the proxy record if the signal‐to‐noise ratio in each proxy is highly variable. For example, stacking of Sr/Ca‐Mg/Ca‐U/Ca records at New Caledonia produces a proxy SST time series that has greater standard error than the individual Sr/Ca‐SST time series (cf. ordinary least squares standard error of 1.16°C versus 0.86°C). Ba/Ca variations have little correlation with SST, Sr/Ca, Mg/Ca, or U/Ca variations at New Caledonia. In conclusion, variations in oxygen isotopic composition of seawater, calculated by removing the Sr/Ca‐based temperature component of the δ18O signal, agree reasonably well with observed variations in sea surface salinity, especially at the interannual timescale.
Sequence of events during the last deglaciation in Southern Ocean sediments and Antarctic ice cores
Authors: Shemesh A. , Hodell D. , Crosta X. , Kanfoush S. , Charles C. , Guilderson T.
Published: November 2002

The last glacial to interglacial transition was studied using down core records of stable isotopes in diatoms and foraminifera as well as surface water temperature, sea ice extent, and ice‐rafted debris (IRD) concentrations from a piston core retrieved from the Atlantic sector of the Southern Ocean. Sea ice is the first variable to change during the last deglaciation, followed by nutrient proxies and sea surface temperature. This sequence of events is independent of the age model adopted for the core. The comparison of the marine records to Antarctic ice CO2 variation depends on the age model as 14C determinations cannot be obtained for the time interval of 29.5–14.5 ka. Assuming a constant sedimentation rate for this interval, our data suggest that sea ice and nutrient changes at about 19 ka B.P. lead the increase in atmospheric pCO2 by approximately 2000 years. Our diatom‐based sea ice record is in phase with the sodium record of the Vostok ice core, which is related to sea ice cover and similarly leads the increase in atmospheric CO2. If gas exchange played a major role in determining glacial to interglacial CO2 variations, then a delay mechanism of a few thousand years is needed to explain the observed sequence of events. Otherwise, the main cause of atmospheric pCO2 change must be sought elsewhere, rather than in the Southern Ocean.
South Asian monsoon climate change and radiocarbon in the Arabian Sea during early and middle Holocene
Authors: Staubwasser Michael , Sirocko Frank , Grootes Pieter M. , Erlenkeuser Helmut
Published: November 2002

The 14C ages of planktonic foraminifers Globigerinoides sacculifer bracketing the Younger Dryas in a δ18O record of Globigerinoides ruber from a laminated sediment core on the Pakistani continental margin suggest that surface reservoir ages in the Arabian Sea were in excess of 1000 years during the deglaciation. A least squares error fit of a detailed 14C chronology to the (atmospheric) tree ring record gave variable early Holocene reservoir ages between 780 and 1120 years, well above the prebomb value of 640 years. Mid‐Holocene reservoir ages are less well constrained but were probably closer to the prebomb value. The method used to fit individual core sections to the tree ring record was designed to require only a rough a priori estimate of the time spans, which in the core presented here were taken from each section's range of 14C ages. A significant 220‐year quasi‐oscillation was present in the δ18O record during the early Holocene but not thereafter. This frequency and amplitude pattern resembles an early Holocene 207‐ to 227‐year oscillation previously observed in the atmospheric 14C record, which is generally interpreted as reflecting solar irradiance variability. An early Holocene climate event at 8150–8400 calendar years B.P. observed elsewhere within the Asian‐East African monsoon system was again found in our record, suggesting a reduction in precipitation over NW India and Pakistan.
Oxygen minimum zone controlled Mn redistribution in Arabian Sea sediments during the late Quaternary
Authors: Schenau S. J. , Reichart G. J. , De Lange G. J.
Published: November 2002

Accumulation of labile Mn is related to bottom water oxygen concentrations. In this study we investigate the relation between oxygen minimum zone (OMZ) intensity and Mn burial in Arabian Sea during the late Quaternary. Mn carbonates in sediments of the deep Arabian Basin indicate that this part of the basin remained oxygenated for at least the last 185 kyr. Sequential leaching showed manganese carbonate enrichments with concentrations up to 3750 ppm in deep basin sediments. These Mn spikes must have formed diagenetically during periods of increased Mn oxide fluxes to the deep basin, associated with vertical extension of the OMZ. This extension caused remobilization of Mn oxides from a previously oxic slope, resulting in increased Mn accumulation in the deep basin. Mn relocation is evidenced by synchronous Mn depletions in organic‐rich intervals in slope sediments. Mn records from the deep basin can thus be used as a qualitative proxy to reconstruct intermediate water oxygenation.
Ionospheric correction and statistical optimization of radio occultation data
Authors: Gorbunov Mikhail E.
Published: November 2002

One of the most significant error sources in radio occultation soundings are residual errors of the ionospheric correction originating from small scale ionospheric turbulence and receiver noise. These errors degrade the quality of retrievals above 35–40 km. We describe a combined algorithm of the ionospheric correction and noise reduction including the following important details: (1) fitting of climatological model, (2) dynamical estimation of signal and noise covariances using radio occultation signals above 50 km, and (3) statistically optimized retrieval of neutral atmospheric and ionospheric refraction angles from L1 and L2 measurements using background refraction angles and signal and noise covariances (optimal linear combination). This method allows for the estimation of the error covariances of the retrieved neutral atmospheric and ionospheric refraction angles. We show some examples of GPS/MET data processing and perform a statistical comparison with the retrievals obtained by UCAR and DMI/IGAM algorithms. Our algorithm was previously used in the statistical validation of GPS/MET data on the basis of ECMWF analyses.
Prediction of the degradation of the carrier‐to‐noise plus interference ratio concerning a site diversity system suffering from differential rain attenuation
Authors: Livieratos S. N. , Kanellopoulos J. D.
Published: November 2002

Rain‐induced attenuation is a factor of utmost importance in designing a satellite link since it can strongly deteriorate the availability and performance of an Earth‐space path. Availability and performance are key criteria for the reliable design process. In the present paper the statistics of carrier‐to‐noise plus interference ratio under rain fade conditions are examined, given that the interference is caused by an adjacent satellite operating at the same frequency, and taking also under consideration double site diversity protection as a countermeasure technique. The method presented herein is based on a model of convective rain cells and the lognormal assumption for the point rainfall statistics. Numerical results have been obtained for heavy rain climatic regions and very large availability times, to examine how effective site diversity can be, not only for lowering fade margins but for compensating severe interference effects. Moreover, the elaboration of the numerical results has revealed how the optimum design of the satellite link can be achieved by selecting proper values for some critical parameters of the system.
Liquid phase structure within an unsaturated fracture network beneath a surface infiltration event: Field experiment
Authors: Glass Robert J. , Nicholl Michael J. , Ramirez Abelardo L. , Daily William D.
Published: November 2002

We conducted a simple field experiment to elucidate structure (i.e., geometry) of the liquid phase (water) resulting from ponded infiltration into a pervasive fracture network that dissected a nearly impermeable rock matrix. Over a 46 min period, dyed water was infiltrated from a surface pond while electrical resistance tomography (ERT) was employed to monitor the rapid invasion of the initially dry fracture network and subsequent drainage. We then excavated the rock mass to a depth of ∼5 m, mapping the fracture network and extent of dye staining over a series of horizontal pavements located directly beneath the pond. Near the infiltration surface, flow was dominated by viscous forces, and the fracture network was fully stained. With increasing depth, flow transitioned to unsaturated conditions, and the phase structure became complicated, exhibiting evidence of fragmentation, preferential flow, fingers, irregular wetting patterns, and varied behavior at fracture intersections. ERT images demonstrate that water spanned the instrumented network rapidly on ponding and also rapidly drained after ponding was terminated. Estimates suggest that our excavation captured from ∼15 to 1% or less of the rock volume interrogated by our infiltration slug, and thus the penetration depth from our short ponding event could have been quite large.
Oscillatory dynamics of the biologically active zone in in situ bioremediation
Authors: Murray Regan E. , Luce Benjamin P.
Published: November 2002

In situ bioremediation is a promising biotechnology for removing aqueous phase contaminants from groundwater. The system of three partial differential equations used to model bioremediation has a traveling wave solution which loses stability in a Hopf bifurcation, giving rise to oscillating fronts. To understand the origin of these oscillations, we construct a simplified model of the biologically active zone, a time delay differential equation with state‐dependent delay. Despite its simplicity the new model mimics the dynamical characteristics of the bioremediation equations remarkably well and yields an approximate parametric expression for the oscillation onset point.
Anaerobic microbial biogeochemistry in a northern bog: Acetate as a dominant metabolic end product
Authors: Duddleston Khrystyne N. , Kinney Monica A. , Kiene Ronald P. , Hines Mark E.
Published: November 2002

Field measurements and incubation techniques were used to determine the dynamics of acetate formation, iron reduction, and methanogenesis in surficial peat of an Alaskan bog. Acetate concentrations were ∼100 μM early in the season and decreased to ∼20 μM in July when the water table decreased. Acetate levels increased rapidly to ∼1000 μM when the water table rose to the surface in August. Acetate production in anaerobic slurries occurred at rates of 2.8–420 nmol carbon mL−1 day−1, which was 7–120 times more rapid than CH4 production. Experiments utilizing 14C‐acetate confirmed that methanogenesis was not acetoclastic although acetate was converted very slowly to CO2. Peat incubated anaerobically for 4.5 months at 24°C never produced methane from acetate, suggesting that anaerobic acetate accumulation would have occurred all season if the water table had remained high. CO2 production was the most rapid process measured in laboratory incubations (up to 750 nmol mL−1 day−1) and appeared to be due primarily to fermentation. Acetate was the primary organic terminal product of anaerobic decomposition in the bog, and acetate was ultimately oxidized to CO2 via aerobic respiration and to a much lesser extent anaerobically by Fe reduction.
The stable carbon isotope composition of the terrestrial biosphere: Modeling at scales from the leaf to the globe
Authors: Kaplan Jed O. , Prentice I. Colin , Buchmann Nina
Published: November 2002

Global data sets of the stable carbon isotope composition of plant leaves, of CO2 in canopy air, and of CO2 in the background atmosphere were compiled and compared to results of a global vegetation model (BIOME4) that simulated, at these three scales, the magnitude, direction, and timing of fluxes of CO2 and 13C between the biosphere and the atmosphere. Carbon isotope data on leaves were classified into 12 Plant Functional Types (PFTs), and measurements from canopy flasks were assigned to 16 biomes, for direct comparison to model results. BIOME4 simulated the observed leaf δ13C values to within 1 standard deviation of the measured mean for most PFTs. Modeled δ13C for C3 grasses, tundra shrubs, and herbaceous plants of cold climates deviated only slightly more from measurements, perhaps as a result of the wide geographic range and a limited set of measurements of these PFTs. Modeled ecosystem isotopic discrimination against 13C (Δe) averaged 18.6 globally when simulating potential natural vegetation and 18.1 when an agricultural crop mask was superimposed. The difference was mainly due to the influence of C4 agriculture in areas that are naturally dominated by C3 vegetation. Model results show a gradient in Δe among C3‐dominated biomes as a result of stomatal responses to aridity; this model result is supported by canopy air measurements. At the troposphere scale, BIOME4 was coupled to a matrix representation of an atmospheric tracer transport model to simulate seasonally varying concentrations of CO2 and 13C at remote Northern Hemisphere measuring stations. Ocean CO2 and 13C flux fields were included, using the HAMOCC3 ocean biogeochemistry model [Six and Maier‐Reimer, 1996]. Model results and observations show similar seasonal cycles, and the model reproduces the inferred latitudinal trend toward smaller isotopic discrimination by the biosphere at lower latitudes. These results indicate that biologically mediated variations in 13C discrimination by terrestrial ecosystems may be significant for atmospheric inverse modeling of carbon sources and sinks, and that such variations can be simulated using a process‐based model.
Historical and future land use effects on N2O and NO emissions using an ensemble modeling approach: Costa Rica's Caribbean lowlands as an example
Authors: Reiners W. A. , Liu S. , Gerow K. G. , Keller M. , Schimel D. S.
Published: November 2002

The humid tropical zone is a major source area for N2O and NO emissions to the atmosphere. Local emission rates vary widely with local conditions, particularly land use practices which swiftly change with expanding settlement and changing market conditions. The combination of wide variation in emission rates and rapidly changing land use make regional estimation and future prediction of biogenic trace gas emission particularly difficult. This study estimates contemporary, historical, and future N2O and NO emissions from 0.5 million ha of northeastern Costa Rica, a well‐documented region in the wet tropics undergoing rapid agricultural development. Estimates were derived by linking spatially distributed environmental data with an ecosystem simulation model in an ensemble estimation approach that incorporates the variance and covariance of spatially distributed driving variables. Results include measures of variance for regional emissions. The formation and aging of pastures from forest provided most of the past temporal change in N2O and NO flux in this region; future changes will be controlled by the degree of nitrogen fertilizer application and extent of intensively managed croplands.
Pulse‐labeling studies of carbon cycling in Arctic tundra ecosystems: The contribution of photosynthates to methane emission
Authors: King J. Y. , Reeburgh W. S. , Thieler K. K. , Kling G. W. , Loya W. M. , Johnson L. C. , Nadelhoffer K. J.
Published: November 2002

We investigated a possible mechanism underlying observed correlations between net ecosystem carbon dioxide exchange and methane emission. Using the technique of 14C pulse‐labeling, we traced the movement of carbon fixed by photosynthesis as it moved through wet sedge and moist tussock tundra plant‐soil mesocosms and was emitted as methane to the atmosphere. The 14C tracer provided a definitive way of quantifying the fate of recently fixed carbon. Carbon fixed by photosynthesis was measured as emitted methane from both moist tussock and wet sedge tundra mesocosms within 2 hours after labeling. Integration of time series measurements of methane emission showed that recent photosynthates are an important source of carbon for methane production. Approximately 2% of carbon fixed by photosynthesis at peak growing season was subsequently emitted as methane from moist tussock tundra, and approximately 3% was emitted as methane from wet sedge tundra. Measurements of soil pore water carbon pools demonstrate rapid transfer of 14C from plant carbon to dissolved forms and subsequently to the atmosphere as carbon dioxide or methane.
Planktic foraminiferal sedimentation and the marine calcite budget
Authors: Schiebel Ralf
Published: November 2002

The vertical flux and sedimentation rate of planktic foraminiferal tests are quantified and a global planktic foraminiferal CaCO3 budget is presented. Test and calcite flux rates are calculated according to the distribution of species obtained from multinet and sediment trap samples. Modern planktic foraminiferal population dynamics are discussed as a prerequisite for the quantification of the calcite budget, highlighting the importance of ecological, autecological (e.g., reproduction), and biogeochemical conditions that determine the presence or absence of species. To complete the open‐marine, particulate CaCO3 inventory, the contribution of coccolithophores, pteropods, and calcareous dinophytes is discussed. Based on the studied regions, the global planktic foraminiferal calcite flux rate at 100 m depth amounts to 1.3–3.2 Gt yr−1, equivalent to 23–56% of the total open marine CaCO3 flux. The preservation of tests varies on a regional and temporal scale, and is affected by local hydrography and dissolution. During most of the year (off‐peak periods), many tests dissolve above 700‐m water depth while settling through the water column, with on average only 1–3% of the initially exported CaCO3 reaching the deep‐seafloor. Pulsed flux events, mass dumps of fast settling particles, yield a major contribution of tests to the formation of deep‐sea sediments. On average, ∼25% of the initially produced planktic foraminiferal test CaCO3 settles on the seafloor. The total planktic foraminiferal contribution of CaCO3 to global surface sediments amounts to 0.36–0.88 Gt yr−1, ∼32–80% of the total deep‐marine calcite budget.
Assimilating atmospheric data into a terrestrial biosphere model: A case study of the seasonal cycle
Authors: Kaminski T. , Knorr W. , Rayner P. J. , Heimann M.
Published: November 2002

This paper demonstrates a new method of assimilating atmospheric concentration data into terrestrial biosphere models. Using a combination of adjoint and tangent linear models of both the underlying biosphere model and the atmospheric transport model, we directly infer optimal model parameters and their uncertainties. We also compute biospheric fluxes and their uncertainties arising from these parameters. We demonstrate the method using the Simple Diagnostic Biosphere Model (SDBM) and data on the seasonal cycle of CO2 from 41 observing sites. In the model, the light‐use efficiency for several biomes is well‐constrained by concentration observations. Optimal values generally increase with latitude as required to match the seasonal cycle. Modeled Q10 values are poorly constrained unless local flux measurements are also used. Values also increase with latitude but are less than the commonly assumed value of 2.
Increasing terrestrial carbon uptake from the 1980s to the 1990s with changes in climate and atmospheric CO2
Authors: Cao Mingkui , Prince Stephen D. , Shugart Hank H.
Published: November 2002

Atmospheric measurements suggest that the terrestrial carbon sink increased from the 1980s to the 1990s, but the causes of the increase are not well understood yet. In this study we investigated the responses of global net primary production in (NPP), soil heterotrophic respiration (HR), and net ecosystem production (NEP) to atmospheric CO2increases and climate variation in the period 1981–1998. Our results show that the unusual climate variability in this period associated with strong warming and El Niño caused high interannual variations in terrestrial ecosystem carbon fluxes; nevertheless NPP and NEP increased consistently from the 1980s to 1990s. Annual global NPP and HR varied with a similar magnitude and contributed about equally to the interannual variations in NEP. Global NEP fluctuated between −0.64 and 1.68 Gt C yr−1with a mean value of 0.62 Gt C yr−1, its decadal means increased from 0.23 Gt C yr−1in the 1980s to 1.10 Gt C yr−1in the 1990s. Total and vegetation carbon storage increased with increases of NPP, but soil carbon storage declined because of higher HR than litter inputs. The tropics (20°N–20°S) had higher mean NEP than the north (>20°N), however, they contributed similarly to the global NEP increase from the 1980s and 1990s. Our estimated terrestrial ecosystem carbon uptake, in response to climate variation and atmospheric CO2increase, accounted for only about 15 to 30% of the total terrestrial carbon sink but contributed 73% of its increase from the 1980s to the 1990s.
Isotopic composition of suspended particulate nitrogen (δ15Nsus) in surface waters of the Atlantic Ocean from 50°N to 50°S
Authors: Mino Yoshihisa , Saino Toshiro , Suzuki Koji , Marañón Emilio
Published: November 2002

Isotopic composition of suspended particulate nitrogen (δ15Nsus) has been studied in near‐surface waters along a transect extending from 50°N to 50°S in the Atlantic Ocean. The δ15Nsus values ranged from −0.8 to +5.4‰ and showed a significant correlation with depth of nitracline (δ15Nsus = −0.047 × + 5.706; R2 = 0.81, n = 15) in nutrients‐depleted regions (45°N–40°S). This inverse relationship implied that higher uptake of new NO3− during mesotrophic conditions, characterized by shallower derived relatively enriched δ15Nsus. On the other hand, lower 15N signature of the remineralized NH4+ would be mainly imprinted on particulate nitrogen pool in the oligotrophic regions characterized by deep The observed inverse relationship can be modeled by a simple isotopic mass balance equation concerning with the N utilizability of phytoplankton (in terms of f‐ratio). The model results that the variations in δ15Nsus reflect the regional differences in nitrogen cycle in the upper ocean. In the region between equator and 15°N, where N2‐fixing cyanobacteria (Trichodesmium spp.) were found, the measured δ15Nsus values were about 2‰ lower than those expected from the regression with the indicating that over 38% of the suspended particulate nitrogen came through N2‐fixation in this region. Regional variations in δ15Nsus in surface waters also correlated with those in water column productivity implying that the δ15Nsus signature can provide an estimate of in situ algal production.
Nitrogen and oxygen isotopomeric constraints on the origins and sea‐to‐air flux of N2O in the oligotrophic subtropical North Pacific gyre
Authors: Popp Brian N. , Westley Marian B. , Toyoda Sakae , Miwa Tatsuya , Dore John E. , Yoshida Naohiro , Rust Terri M. , Sansone Francis J. , Russ Mary E. , Ostrom Nathaniel E. , Ostrom Peggy H.
Published: November 2002

Although the oceans are a significant source of the greenhouse gas nitrous oxide (N2O) to the atmosphere, the magnitude and characteristics of this source are poorly constrained. We present here stable isotope and isotopomer (intramolecular distribution of 15N within the linear NNO molecule) results for N2O and oxygen stable isotopic data for dissolved O2 from Station ALOHA in the subtropical North Pacific gyre near Hawaii. The results indicate shallow (∼100–300 m) in situ N2O production. Results of isotope mass balance models constrain the rate of N2O production and the sea‐to‐air flux of N2O. Results of an isotope mass balance model that takes into account the ratios of the vertical gradients in the isotopic abundances of N, O, Nα (central N) and Nβ (terminal N) of N2O and the measured gradients of N2O concentration through the thermocline indicate that shallow in situ production contributed 40% to 75% of the sea‐to‐air flux of N2O. This model also indicates that the net sea‐to‐air flux of N2O was at least 0.4 μmol m−2 d−1 and could be as high as 1.0 μmole m−2 d−1. These model results are not statistically different from the sea‐to‐air flux of N2O calculated using an empirical relationship between wind speed and gas transfer rate (1.1 ± 0.7 μmole m−2 d−1) derived from measured values for wind speed, temperature and the surface mixed layer concentration of N2O. These results can be used to better constrain the global N2O budget.
Emissions of N2O and NO from fertilized fields: Summary of available measurement data
Authors: Bouwman A. F. , Boumans L. J. M. , Batjes N. H.
Published: November 2002

Information from 846 N2O emission measurements in agricultural fields and 99 measurements for NO emissions was summarized to assess the influence of various factors regulating emissions from mineral soils. The data indicate that there is a strong increase of both N2O and NO emissions accompanying N application rates, and soils with high organic‐C content show higher emissions than less fertile soils. A fine soil texture, restricted drainage, and neutral to slightly acidic conditions favor N2O emission, while (though not significant) a good soil drainage, coarse texture, and neutral soil reaction favor NO emission. Fertilizer type and crop type are important factors for N2O but not for NO, while the fertilizer application mode has a significant influence on NO only. Regarding the measurements, longer measurement periods yield more of the fertilization effect on N2O and NO emissions, and intensive measurements (≥1 per day) yield lower emissions than less intensive measurements (2–3 per week). The available data can be used to develop simple models based on the major regulating factors which describe the spatial variability of emissions of N2O and NO with less uncertainty than emission factor approaches based on country N inputs, as currently used in national emission inventories.
Upper ocean carbon fluxes in the Atlantic Ocean: The importance of the POC:PIC ratio
Authors: Koeve Wolfgang
Published: November 2002

The mean depth distribution of the POC:PIC ratio of sinking particles, measured with particle interceptor traps deployed in the Atlantic Ocean, is fitted by an exponential function (POC:PIC = 64.3Z−0.56; r2 = 0.69) The function is successfully evaluated by comparison with (a) estimates of the POC:PIC ratio of export production, computed from seasonal changes of nitrate and alkalinity and (b) estimates of the POC:PIC ratio of remineralization on shallow isopycnals. The basin mean POC:PIC ratio of export production is 4.2–4.37. The POC:PIC‐depth function is combined with empirical relationships between the flux of particulate organic matter, primary production and depth, satellite derived primary production data sets, and the regional distribution of ψ (the ratio of released CO2:precipitated carbonate during CaCO3 formation) in order to estimate the effective carbon flux (Jeff) in the Atlantic Ocean. Remineralization of organic carbon above the winter mixed layer (11–17%) and CaCO3 sequestration from the winter mixed layer (13–16%), which is the balance between CaCO3 production and shallow dissolution, are the two main processes which control the difference between export production (0.9 and 2.9 GT C yr−1) and Jeff (0.64 and 2.2 GT C yr−1) on the basin scale (65°N to 65°S). CaCO3 sequestration is the dominant process modulating effective carbon export in the tropics, while shallow POC remineralization dominates in temperate and polar waters. Observed regional patterns like polarward increases of the POC:PIC export ratio and of ψ counteract each other largely when Jeff is computed.
An integrated model of soil, hydrology, and vegetation for carbon dynamics in wetland ecosystems
Authors: Zhang Yu , Li Changsheng , Trettin Carl C. , Li Harbin , Sun Ge
Published: November 2002

Wetland ecosystems are an important component in global carbon (C) cycles and may exert a large influence on global climate change. Predictions of C dynamics require us to consider interactions among many critical factors of soil, hydrology, and vegetation. However, few such integrated C models exist for wetland ecosystems. In this paper, we report a simulation model, Wetland‐DNDC, for C dynamics and methane (CH4) emissions in wetland ecosystems. The general structure of Wetland‐DNDC was adopted from PnET‐N‐DNDC, a process‐oriented biogeochemical model that simulates C and N dynamics in upland forest ecosystems. Several new functions and algorithms were developed for Wetland‐DNDC to capture the unique features of wetland ecosystems, such as water table dynamics, growth of mosses and herbaceous plants, and soil biogeochemical processes under anaerobic conditions. The model has been validated against various observations from three wetland sites in Northern America. The validation results are in agreement with the measurements of water table dynamics, soil temperature, CH4 fluxes, net ecosystem productivity (NEP), and annual C budgets. Sensitivity analysis indicates that the most critical input factors for C dynamics in the wetland ecosystems are air temperature, water outflow parameters, initial soil C content, and plant photosynthesis capacity. NEP and CH4 emissions are sensitive to many of the tested input variables. By integrating the primary drivers of climate, hydrology, soil and vegetation, the Wetland‐DNDC model is capable of predicting C biogeochemical cycles in wetland ecosystems.
High‐resolution models of subduction zones: Implications for mineral dehydration reactions and the transport of water into the deep mantle
Authors: van Keken Peter E. , Kiefer Boris , Peacock Simon M.
Published: November 2002

Arc volcanism is intimately linked to mineral dehydration reactions in the subducting oceanic mantle, crust, and sediments. The location of slab dehydration reactions depends strongly on the temperature and pressure conditions at the top of the subducting plate and hence on the detailed thermal structure of subduction zones. A particularly important physical property of subduction zone thermal models is the viscosity of the mantle wedge. The introduction of an olivine rheology, with appropriate stress and temperature dependence, focuses flow into the tip of the mantle wedge. This leads to a temperature increase of a few hundred degrees in the wedge and the top of the slab as compared to the isoviscous case. Sensitivity tests show that this conclusion is robust under a variety of subduction zone parameters. The new high‐resolution finite element models are used to reevaluate the thermal structure of the Honshu and Cascadia subduction zones using the more realistic olivine rheology. For Honshu, the model predicts slab–mantle interface temperatures of ∼800°C beneath the volcanic front. Deeper parts of the subducting oceanic crust and upper mantle remain relative cool (e.g., the subducted Moho beneath the volcanic front is ∼400°C) because of the rapid subduction of old Pacific lithosphere. High interface temperatures are consistent with Th and Be data, indicating sediment melting, whereas cooler crustal temperatures are consistent with boron evidence, indicating lower temperature dehydration. The strong temperature gradient at the top of the subducting plate may thus reconcile conflicting estimates of slab temperature in subduction zones that are characterized by rapid subduction of mature oceanic lithosphere. Low temperatures persist to great depth in the shallow upper mantle of the subducting plate, which allows for water transport in the form of hydrous minerals to the deep mantle. The thermal structure of the Cascadia subduction zone is markedly different because of the age of the incoming lithosphere and low subduction speed. In this case, shallow dehydration and melting of the subducting crust and lithosphere is predicted.
Simultaneous determination of individual ambient particle size, hygroscopicity and composition
Authors: Buzorius G. , Zelenyuk A. , Brechtel F. , Imre D.
Published: November 2002

A new system for simultaneous determination of individual particle size, hygroscopicity, and composition of internally and externally mixed aerosols is presented. An HTDMA is used to select particles according to their size and hygroscopicity for compositional analysis by a single particle mass spectrometer. The capability of the system is demonstrated on a laboratory generated aerosol consisting of polystyrene latex spheres coated with a layer of ammonium nitrate to represent an internally mixed aerosol with controllable hygroscopicity that is externally mixed with pure ammonium nitrate particles. Results are presented from an ambient study illustrating the sensitivity of the system and the capability to provide detailed information on individual atmospheric particles within a short sampling time.
Crustal structure of the Cocos Ridge northeast of Cocos Island, Panamá Basin
Authors: Walther Christian
Published: November 2002

The submarine Cocos ridge in the northwestern Panamá basin, Pacific ocean, is generally interpreted as the trace of the Galápagos hotspot. A 278 km long seismic wide‐angle transect was carried out across a comparatively narrow ridge segment, 150 km northeast of Cocos Island. The results indicate a huge thickened crust, where crustal thickening is mainly achieved by the lower crust, which makes up 75% of the crustal thickness and is tripled compared to normal oceanic lower crust. The velocities are comparable to normal lower crust and suggest no differences to a gabbroic composition. The Moho deepens from 10 to 18 km depth below the ridge. Similarities to other hotspot related oceanic ridges and plateaus with massive lower crusts and velocities below 7.4 km/s suggest the formation of this Cocos ridge segment near or at the plate boundary.
Annealing of shocked quartz during atmospheric re‐entry
Authors: Croskell Michael , Warner Mike , Morgan Jo
Published: November 2002

The Chicxulub impact launched shocked quartz grains on ballistic trajectories outside the atmosphere, thus enabling their global dissemination. The shocked grains heated up during atmospheric re‐entry and the maximum temperature reached was related to their size, velocity and re‐entry angle. The extreme heating of some shocked quartz grains is likely to have resulted in the destruction of their planar deformation features (PDFs). A significant negative correlation exists between the palaeodistance from Chicxulub of a given K/T boundary locality and the size of the largest shocked quartz crystal found at that locality. The re‐entry annealing process can explain this correlation. The largest shocked quartz grain at a given locality followed the shallowest available trajectory and survived re‐entry without being thermally annealed. Larger crystals and those on steeper trajectories exceeded the annealing point and lost their PDFs.
Sea‐ice thickness from airborne laser altimetry over the Arctic Ocean north of Greenland
Authors: Hvidegaard S. M. , Forsberg R.
Published: November 2002

We present a new method to measure ice thickness of polar sea‐ice freeboard heights, using airborne laser altimetry combined with a precise geoid model, giving estimates of thickness of ice through isostatic equilibrium assumptions. In the paper we analyze a number of flights from the Polar Sea off Northern Greenland, and estimate accuracies of the estimated freeboard values to be at a 13 cm level, corresponding to about 1 m in absolute thickness.
Temperature as a passive isopycnal tracer in salty, spiceless oceans
Authors: Stipa Tapani
Published: November 2002

The decoupling between the gradients of temperature and density in cold, relatively fresh oceanic waters is discussed from a theoretical standpoint and furthermore demonstrated on the basis of observations from the Baltic Sea. Near the temperature of maximum density, the thermal expansion coefficient is effectively zero, hereby de facto making temperature a passive tracer. As a consequence, the baroclinic advection of heat anomalies is determined by freshwater gradients. Recognition of such dynamics could prove helpful for the understanding of a warmer, fresher future Arctic Ocean, as projected by the IPCC.
Fingerprints of collisionless reconnection at the separator, I, Ambipolar‐Hall signatures
Authors: Scudder J. D. , Mozer F. S. , Maynard N. C. , Russell C. T.
Published: November 2002

Plasma, electric, and magnetic field data on the Polar spacecraft have been analyzed for the 29 May 1996 magnetopause traversal searching for evidence of in situ reconnection and traversal of the separator. In this paper we confine our analysis to model‐free observations and intrasensor coherence of detection of the environs of the separator. (1) We illustrate the first documented penetration of the separator of collisionless magnetic reconnection in temporal proximity to successful Walén tests with opposite slopes. (2) We present the first direct measurements of E∥ at the magnetopause. (3) We make the first empirical argument that E∥ derives from the electron pressure gradient force. (4) We document the first detection of the electron pressure ridge astride the magnetic depression that extends from the separator. (5) We provide the first empirical detection of the reconnection rate at the magnetopause with the locally sub‐Alfvénic ion inflow, MAi ≃ 0.1, and trans‐Alfvénic exhaust at high electron pressure of MiA ≃ 1.1–5. (6) We exhibit the first empirical detection of supra‐Alfvénic electron flows parallel to B in excess of 5 in narrow sheets. (7) We illustrate the detection of heat flux sheets indicative of separatrices near, but not always in superposition, with the supra‐Alfvénic parallel electron bulk flows. (8) We present the first evidence that pressure gradient scales are short enough to explain the electron fluid's measured cross‐field drifts not explained by E × B drift but predicted by the measured size of E∥. (9) We illustrate that the size of the observed E∥ is well organized with the limit implied by Vasyliunas's analysis of the generalized Ohm's law of scale length , indicative of the intermediate scale of the diffusion region. (10) We document the first detection of departure from electron gyrotropy not only at the separator crossing but also in its vicinity, an effect presaged by Vasyliunas [1975]. (11) We make the first reports of very large values of electron βe ≃ 680 localized at the separator, which imply that the electron thermal gyroradius exceeds the electron inertial length by more than an order of magnitude there. This clearly delineates that the environs of the reversed field region in this data contain non‐MHD scales. The ambipolar association and the measured E∥ data imply the presence of the nonideal ρs scale in these layers surrounding the null point. The high βe signals the possible demagnetization of the thermal electrons in any structures with spatial scales of the electron skin depth, which is theoretically anticipated to surround the magnetic null line of the separator proper. This possibility is supported by the large number of temporally unaliased spectra at high βe that are inconsistent with gyrotropy.
Upper cutoff energy of the electron plasma sheet as a measure of magnetospheric convection strength
Authors: Thomsen M. F. , Korth H. , Elphic R. C.
Published: November 2002

The sharp upper edge of the nightside electron plasma sheet energy distribution observed at geosynchronous orbit is shown to provide a measure of the effective strength of the convection electric field that transports plasma sheet material into the inner magnetosphere. This association is statistically supported by the Kp and LT dependence of the observed median cutoff energies and yields inferred field strengths consistent with previous electric field measurements in this region. The temporal profile of convection typically consists of a sequence of bursts, where each convection enhancement lasts no more than a fraction of an hour. Simultaneous multipoint measurements show that these convection enhancements are observed first near midnight and then propagate sunward with speeds of ∼5–30 km/s. They also reveal that the plasma sheet does not penetrate as deeply into the premidnight region as would be expected for a Volland‐Stern electric field with a shielding parameter of γ = 2. Routine measurements of the plasma sheet electron cutoff energy by multiple geosynchronous satellites could provide a near‐real‐time monitor of the effective strength of the global convection electric field.
Effects of magnetic clouds on the occurrence of geomagnetic storms: The first 4 years of Wind
Authors: Wu Chin‐Chun , Lepping R. P.
Published: November 2002

We investigate geomagnetic activity associated with magnetic clouds as well as some aspects of the clouds' magnetic field structure (e.g., magnetic field distributions of the clouds). Thirty‐four magnetic cloud events observed by Wind over the years 1995–1998 are investigated in this study. Magnetic clouds are a principal source of strong, long‐lasting, interplanetary, negative Bz fields (in solar magnetospheric coordinates) and hence are a major source of geomagnetic activity (Dst < −50 nT). The region just upstream of a cloud may be a significant source of southward fields instead of, or as well as, the cloud itself. We call this upstream region a “sheath” if it is bounded by a shock (or a pressure pulse) and the cloud. This study helps to identify the major sources (regions of southward Bz following shock passages or within the magnetic clouds) that are most dominant in the generation of geomagnetic storms. It is found that a geomagnetic storm can be induced by (1) a sheath, (2) the leading (i.e., front part) region of a cloud, (3) the trailing part of a cloud, and (4) both sheath and cloud regions. (Because of this complexity a storm with a multistep main phase can occur.) The related occurrence percentages of storms were 17.6% (six events), 44.1% (15 events), 5.9% (two events), and 20.6% (seven events), and the averaged storm intensities (minimum Dst) were −60, −85, −92, and −58 nT, respectively. For the remaining four events (11.8%), there were no storms. The occurrence timing of storm intensity is highly correlated with the occurrence timing of minimum Bz (maximum VBs or ϵ) for a magnetic cloud with the field rotating from southward to northward.