bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct atmospheric deposition of water‐soluble nitrogen to the Gulf of Maine
Authors: Jordan C. E. , Talbot R. W.
Published: February 2004

Measurements were made at New Castle, New Hampshire, on the shore of the Gulf of Maine from 1994 to 1997 to assess direct atmospheric deposition of water‐soluble nitrogen to the surface waters of the gulf. Daily dry deposition was highly variable and ranged from ∼ 1 to 144 μmol N m−2 d−1 (median 16 μmol N m−2 d−1). Wet deposition dominated dry deposition, contributing 80‐90% of the total flux annually. Wet deposition was also highly variable and ranged from 3 to 4264 μmol N m−2 d−1 (median 214 μmol N m−2 d−1). Fog water nitrogen deposition could contribute as much as large precipitation nitrogen deposition events, in excess of 500 μmol N m−2d−1. Dissolved organic nitrogen (DON) in precipitation constituted only a small fraction (3%) of the total precipitation nitrogen flux most of the year, except in spring where it comprised 14%, on average, of the total. The total atmospheric direct nitrogen (ADN) deposition numbers reported here do not include the contributions of fog and DON as they were not sampled regularly over the course of this study. The total ADN flux ranged from 1 to 4262 μmol N m−2 d−1 (median 23 μmol N m−2 d−1), depositing 52 mmol N m−2 yr−1 to the surface waters of the Gulf of Maine, 3% of the total N input to those waters annually. However, this deposition was highly episodic with events over 500 μmol N m−2 d−1 occurring in 8% of the days sampled but contributing 56% of the total measured flux and events in excess of 1000 μmol N m−2 d−1 occurring in 2% of the samples and contributing 22% of the total measured flux. It is these large events that may influence biological productivity of the Gulf of Maine. The annual wet deposition of inorganic N measured at New Castle exceeded that reported by two National Atmospheric Deposition Program (NADP) sites by 42% on average of that reported from Cape Cod, Massachusetts, and by 69% ofthat at Mt. Dessert Island, Maine. Estimates of the episodic atmospheric nitrogen flux to the surface waters of the Gulf of Maine suggest large deposition events could be sufficient to support substantial chlorophyll α production, especially under calm conditions.
Blooms of Emiliania huxleyi are sinks of atmospheric carbon dioxide: A field and mesocosm study derived simulation
Authors: Buitenhuis Erik T. , van der Wal Paul , de Baar Hein J. W.
Published: February 2004

During field measurements in a bloom of Emiliania huxleyi in the North Sea in 1993, an apparently inconsistent combination of observations was measured: (1)fCO2 was lower in the center of the bloom than in the surrounding nonbloom areas and undersaturated with respect to the atmosphere in both cases, (2) within the bloom, enhanced sedimentation of coccoliths‐containing fecal pellets was observed, (3) a large atmospheric sink of 1.3 mol C m−2 was derived, and (4) in the same bloom a positive correlation between CaCO3 and fCO2 was observed, which was interpreted as an increase of fCO2 during production of CaCO3. In order to resolve the inconsistency between observations (1, 2, 3) and 4 a one‐dimensional three‐layer model was constructed. A positive correlation between CaCO3 and fCO2 was obtained when the model was parameterized with data obtained from field and mesocosm studies. The correlation is a feature of the decay phase of a bloom and represents a decrease of fCO2 with a decrease Of CaCO3. Thus it represents the dissolution and sedimentation of CaCO3 rather than its production. Having resolved the ambiguity within the field data by adding the dimension of time in the model, blooms of E. huxleyi can be identified as sinks for atmospheric carbon dioxide. This sink is a function of the calcification to photosynthesis (C:P) ratio of the nitrate‐using phytoplankton and is maximal when the C:P ratio is 0.42 (that is, E. huxleyi constitutes 97% of the nitrate‐using phytoplankton). Rather than using the model for making accurate predictions about the magnitude of this sink, a sensitivity analysis was performed to give a range of magnitudes for the range of parameter values that were obtained during previous studies. Furthermore, gaps were identified in the current knowledge of carbon fluxes within blooms of E. huxleyi.
European continental shelf as a significant sink for atmospheric carbon dioxide
Authors: Frankignoulle Michel , Borges Alberto V.
Published: February 2004

The concentration of carbon dioxide was measured during 18 cruises in the surface waters of the North Atlantic European Shelf (Galician sea, Gulf of Biscay, Armonican Sea, Celtic Sea, English Channel, North Sea), covering all four seasons (9 out of 12 months) at interannual scale. This is the very first intensive field study of continental shelves, in terms of source/sink for atmospheric CO2, which allows to integrate fluxes on an annual basis and over a large surface area. Here we show that European continental shelves are a sink of 90–170 million tons of carbon per year, which is an additional appreciable fraction to the presently proposed flux for the open North Atlantic Ocean (∼ 45%). The air‐sea fluxes of CO2 we obtained are similar to those recently reported in the East China Sea, allowing us to conclude that the coastal ocean plays a considerable role in the global oceanic carbon cycle.
Decreasing marine biogenic calcification: A negative feedback on rising atmospheric pCO2
Authors: Zondervan Ingrid , Zeebe Richard E. , Rost Björn , Riebesell Ulf
Published: February 2004

In laboratory experiments with the coccolithophore species Emiliania huxleyi and Gephyrocapsa oceanica, the ratio of particulate inorganic carbon (PIC) to particulate organic carbon (POC) production decreased with increasing CO2 concentration ([CO2]). This was due to both reduced PIC and enhanced POC production at elevated [CO2]. Carbon dioxide concentrations covered a range from a preindustrial level to a value predicted for 2100 according to a “business as usual” anthropogenic CO2 emission scenario. The laboratory results were used to employ a model in which the immediate effect of a decrease in global marine calcification relative to POC production on the potential capacity for oceanic CO2 uptake was simulated. Assuming that overall marine biogenic calcification shows a similar response as obtained for E. huxleyi or G. oceanica in the present study, the model reveals a negative feedback on increasing atmospheric CO2 concentrations owing to a decrease in the PIC/POC ratio.
Methane oxidation in a peatland core
Authors: Pearce D. M. E. , Clymo R. S.
Published: February 2004

We made an experiment on a 30 cm diameter core of Sphagnum‐dominated vegetation and peat to estimate the parameters controlling methane oxidation during movement to the ambient air: 13CH4 was added at the water table, and excess 13CO2 appeared in the gas space above the core. At 20°C in otherwise undisturbed conditions, ∼22% of CH4 was oxidized to CO2 during passage up through the overlying 10‐cm thick unsaturated peat and plants. We simulated the experiment, with seven parameters: transfer coefficients in water, in the gas phase, and through the container wall; the rate of CH4 and of CO2 generation; and the two parameters of a hyperbolic relation between CH4 concentration and the rate of CH4 oxidation. We optimized these parameters to fit the experimental results, and then were able to generalize to any temperature (0°−25°C) and any depth (0‐55 cm) of water table. Changing temperature has important effects on the proportion of CH4 oxidized.
Carbon accumulation in bay of fundy salt marshes: Implications for restoration of reclaimed marshes
Authors: Connor Richard F. , Chmura Gail L. , Beecher C. Beth
Published: February 2004

Transformation of agricultural land to natural terrestrial vegetation has been suggested as a means to increase soil carbon storage. However, the capacity for carbon storage in terrestrial soils is limited as compared to soils of tidal salt marshes, the original vegetation of many coastal agricultural lands. In a number of countries, tidal salt marshes have been “reclaimed,” that is drained and diked to prevent tidal flooding and create suitable conditions for dry land agriculture. In this study we examine spatial and temporal patterns of carbon accumulation in tidal salt marshes of the Bay of Fundy and estimate the carbon storage potential of the bay's extensive area of reclaimed marsh. Rates of carbon accumulation vary from the upper to the outer Bay, over which there is a gradient of decreasing tidal range and suspended sediment supply. In the outer bay, high‐marsh densities are highest (0.042 ± 0.009 g C cm−3), but carbon accumulation rates over the past 30 years are lowest (76 g C m−2 yr−1). The reverse pattern occurs in the upper bay where carbon densities in the high‐marsh environment are lowest (0.036 ± 0.002 g C cm−3), but carbon accumulation rates over the past 30 years may be as high (184 g C m−2 yr−1). Compared to other ecosystems, the rates of carbon accumulation presented in this study were similar over timescales of years to centuries. Increases in relative sea level (over time) and suspended sediment supply (across the bay) positively affect the marsh soil accumulation rate and the rate of carbon sequestration. Parameters such as %C are not useful predictors of a marsh's potential for carbon sequestration. Soil carbon densities of functioning marshes and reclaimed marsh soils are similar, but marsh soils have a storage capacity that increases with rising sea level, while agricultural soils, such as those in reclaimed marshes, have a fixed (or possibly decreasing in reclaimed marshes) volume over time.
Factors controlling the rate of CaCO3 precipitation on Great Bahama Bank
Authors: Broecker Wallace S. , Langdon Chris , Takahashi Taro , Peng Tsung‐Hung
Published: February 2004

Measurements by Langdon et al. [2000] in the man‐made mesocosm coral reef at Biosphere 2's ocean reveal a strong dependence of calcification rate on the degree of supersaturation of CaCO3 in seawater. A similar trend was previously encountered on the Bahama Banks, where Halimeda and other calcifiers are likely responsible for aragonite precipitation [Broecker and Takahashi, 1966]. In this paper we compare these two sets of results and conclude that the dependence on saturation state is significant but less strong in the Bahamas. However, it must be kept in mind that to some extent, the reduction in CaCO3 precipitation on the Bahama Banks may be due to impact of higher salinity on the growth of the calcifying algae. However, if, as many sedimentologists are convinced, the precipitation of CaCO3 on the Bahama Banks is inorganic [Macintyre and Reid, 1992; Milliman et al., 1993], then the comparison of the Bahamas and Biosphere 2 results for dependence of calcification rate on saturation state is telling us something quite different.
Influence of atmospheric CO2 enrichment on nitrous oxide flux in a temperate forest ecosystem
Authors: Phillips Rebecca L. , Whalen Stephen C. , Schlesinger William H.
Published: February 2004

Long‐term exposure of native vegetation to elevated atmospheric carbon dioxide (CO2) is expected to increase the water content and the input of labile carbon (C) to soil, which could stimulate nitrification and denitrification and enhance nitrous oxide (N2O) emissions. We measured N2O fluxes for 2 years in a Pinus taeda forest that was continuously enriched 200 μL L−1 CO2 above the ambient atmospheric CO2 concentration (∼560 μL L−1) beginning 16 months prior to our study. Soil treated with elevated CO2 showed higher N2O emissions at low winter temperatures than the ambient CO2 control. Conversely, soil treated with elevated CO2 showed lower N2O emissions at high summer temperatures than the control soil. Annual N2O fluxes, however, were similar between treatments (∼6600 μg m−2). Factors that influence denitrification and N2O production were investigated in the laboratory using intact soil core incubations. Nitrate additions (0.17 mg KNO3−N g−1 ) to intact soil cores during laboratory incubations stimulated total N2O production as well as denitrification in both treatments, whereas glucose additions lowered N2O production in both treatments. These experiments demonstrated that N2O production is strongly limited by available nitrogen (N) and that the addition of labile C is likely to reduce the amount of N2O produced by nitrification. Our results collectively suggest that CO2 enrichment of this N‐limited ecosystem may reduce N2O flux during the growing season, when soil C inputs and plant‐microbial competition for NH4+ are high. Alternatively, elevated CO2 may enhance N2O flux in the winter, when conditions are moist and cold and plants are less active. The potential indirect effects of CO2 enrichment (greater soil moisture and labile C inputs) could reduce N2O flux from nitrification in summer and enhance N2O flux from denitrification in winter, resulting in no net change in total ecosystem N2O flux at the soil‐atmosphere interface.
Emission of trace gases and aerosols from biomass burning
Authors: Andreae M. O. , Merlet P.
Published: February 2004

A large body of information on emissions from the various types of biomass burning has been accumulated over the past decade, to a large extent as a result of International Geosphere‐Biosphere Programme/International Global Atmospheric Chemistry research activities. Yet this information has not been readily accessible to the atmospheric chemistry community because it was scattered over a large number of publications and reported in numerous different units and reference systems. We have critically evaluated the presently available data and integrated these into a consistent format. On the basis of this analysis we present a set of emission factors for a large variety of species emitted from biomass fires. Where data were not available, we have proposed estimates based on appropriate extrapolation techniques. We have derived global estimates of pyrogenic emissions for important species emitted by the various types of biomass burning and compared our estimates with results from inverse modeling studies.
Calibration of pressure‐dependent sensitivity and discrimination in Nier‐type noble gas ion sources
Authors: Burnard P. G. , Farley K. A.
Published: February 2004

The efficiency of many noble gas mass spectrometers to ionize gas species is known to be a function of the pressure of gas in the spectrometer. This work shows how the half plate voltage for maximum He or Ar signal depends on the spectrometer pressure and shows that the half plate voltage for maximum 4He sensitivity does not coincide with the half plate voltage for maximum 3He sensitivity. In addition, half plate voltage has a greater control on sensitivity at higher spectrometer pressures. Variations in He and Ar sensitivity and isotopic discrimination as a function of pressure are due, at least in part, to these variations in the position of maximum sensitivity with respect to half plate voltage. The maximum sensitivity settings shift to lower half plate voltage at high spectrometer pressures, irrespective of if the pressure increase is due to the gas being investigated or a different species. Therefore noble gas mass spectrometers should always be tuned at the maximum possible pressure; measurements at higher pressures should be avoided. Significant errors in the spectrometer sensitivity and discrimination can result from improper tuning and calibration of noble gas mass spectrometers.
Evidence from 10Be and U series disequilibria on the possible contamination of mid‐ocean ridge basalt glasses by sedimentary material
Authors: Bourdon B. , Goldstein S. J. , Bourlès D. , Murrell M. T. , Langmuir C. H.
Published: February 2004

It has often been suggested that U series disequilibria measured in mid‐ocean ridge basalts (MORB) can be perturbed by contamination with sedimentary or hydrothermal material found near the ridge axis where the basalts are erupted. Here we provide an independent way of estimating the maximum degree of contamination by sediment using constraints from 10Be. Since 10Be is mostly a cosmogenic nuclide and has a half life of 1.5 × 106 years, any 10Be found in a MORB glass must result from contamination by sedimentary material where cosmogenic 10Be is enriched. Four MORB samples with a wide range in Th concentrations (87–550 ppb) were measured for U decay series and 10Be. No 10Be above the detection limit (6 × 104 to 1 × 105 atoms) was measured in the glasses except for one sample that was not picked. Considering that the sedimentary contaminant contains 109 to 1010 atoms/g, the maximum fraction of contaminant in the picked glasses is 10−4 to 10−5. Mass balance calculations between a surficial contaminant and a hypothetical pristine glass with (231Pa/235U) = 1 and (10Be/9Be) = 0 show that only for the unpicked sample, nearly all the measured excess 231Pa can be accounted for by contamination. For the other samples, <15% to <1% of the excess 231Pa and 230Th can be explained by incorporation of sedimentary material. This study reemphasizes the need for careful sample preparation for U series measurements in MORB, especially at low U levels (<50 ppb). It also confirms that the large U series excesses in carefully picked and cleaned MORB samples are magmatic in origin.
Comparison of continental and oceanic mantle electrical conductivity: Is the Archean lithosphere dry?
Authors: Hirth Greg , Evans Rob L. , Chave Alan D.
Published: February 2004

Electrical conductivity profiles derived from magnetotelluric and geomagnetic sounding methods provide a means of constraining upper mantle properties that is complementary to seismic studies. We analyze conductivity profiles for an Archean craton and an oceanic setting, in conjunction with independent constraints on mantle geotherms, to estimate the water content of the upper mantle in these very different geologic environments. Results from this analysis indicate that the Archean lithosphere contains less water than the oceanic mantle in the depth range between ∼150 and ∼250 km. Below ∼250 km these oceanic and continental environments show similar conductivities, suggesting that the water content of the mantle does not vary significantly between ∼250 km and the 410‐km discontinuity. These observations indicate that the Archean lithosphere may be stabilized against convective instabilities partly because it has a high viscosity associated with a dry composition.
Status of alkenone paleothermometer calibration: Report from Working Group 3
Authors: Prahl F. , Herbert T. , Brassell S. C. , Ohkouchi N. , Pagani M. , Repeta D. , Rosell‐Melé A. , Sikes E.
Published: February 2004

The alkenone unsaturation index, U37K′, has now proven its worth as a tool for paleothermometry. U37K′ measured in most modern sediments throughout the World Ocean can be translated using established calibrations into realistic, seemingly reliable estimates of mean annual temperature at the sea surface (SST). However, it remains mysterious why water temperature estimates based on this biotic index correspond to “mean annual” SST and whether such estimates apply back in geological time. Solving these mysteries is imperative and will require thoughtful, concerted research effort by the biological, chemical, and geological oceanographic community. This report summarizes what is now known about alkenones, in particular, their use in estimating the growth temperature of specific haptophyte algae that synthesize and export these compounds to the marine sediment record. It highlights future field and laboratory research directions that should be taken to clarify and bolster utility of alkenones as a generally valued paleoceanographic tool.
Partitioning of rare earth elements, Y, Th, U, and Pb between pargasite, kaersutite, and basanite to trachyte melts: Implications for percolated and veined mantle
Authors: Tiepolo Massimo , Vannucci Riccardo , Bottazzi Piero , Oberti Roberta , Zanetti Alberto , Foley Stephen
Published: February 2004

A new set of partitioning data for rare earth elements (REE: La, Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb), Y, Th, U, and Pb has been obtained for 25 calcic amphiboles (pargasites and kaersutites) crystallized from alkali‐basaltic and basanitic bulk rock compositions at pressure P = 1.4 GPa, and temperature T between 950° and 1075°C. The variations of amphibole/liquid partition coefficients and of their ratios relevant to petrogenetic studies are discussed with reference to the major element composition of the amphiboles and of the coexisting melt, and to the crystal chemical mechanisms for trace element incorporation. Our results support the conclusions that REE and actinides are incorporated into the M4 cavity in calcic amphiboles and distributed between the two available sites within that cavity and that Pb is incorporated into the A site. In our sample population, REE patterns are systematically enriched in heavy REE (HREE), as expected from the presence of significant cummingtonite component. No significant fractionation is observed between Th and U. The major factor controlling the amount of trace element incorporation is the SiO2 content of the melt. The major implication of this study is that HREE can become compatible in amphibole in systems with SiO2 content greater than ∼50 wt %, whereas LREE always remain incompatible. We use the new DREEamph/l values to calculate the effects of amphibole crystallization during melt migration in the upper mantle by reactive porous flow as well as fractional crystallization of amphibole during melt migration in veined systems. We show that both processes will lead to residual liquids and solids with extremely variable LaN/YbN ratios.
Evidence from oceanic gabbros for porous melt migration within a crystal mush beneath the Mid‐Atlantic Ridge
Authors: Coogan L. A. , Saunders A. D. , Kempton P. D. , Norry M. J.
Published: February 2004

The evolution of a gabbroic crystal mush beneath the Mid‐Atlantic Ridge has been investigated using evidence from gabbros recovered from Ocean Drilling Program (ODP) Hole 923A (Leg 153). Lithological variations occur on a vertical scale of meters and correlate with mineral compositions. This defines a detailed chemical stratigraphy in which variations in olivine, plagioclase, and clinopyroxene solid‐solution component compositions correlate with each other. The volumetrically dominant lithology (plagioclase + clinopyroxene ± olivine ± orthopyroxene gabbros) has variable grain size, grain shape, and mineral compositions. These variations correlate, such that coarser grained samples have more granular textures and lower mafic phase Mg/Fe ratios than adjacent finer grained samples. Clinopyroxene trace element systematics, determined by ion probe, cannot be explained by growth from a melt that evolved along either an equilibrium or a fractional crystallization trend. Clinopyroxene crystals are strongly zoned and enriched in Zr with respect to rare earth elements (more to less incompatible elements). These textural and geochemical characteristics are not expected from simple crystal accumulation processes or the crystallization of trapped melt. Instead, melt migration within a crystal mush is suggested as the most likely process to explain them. The meter‐scale mineral compositional variations, which correlate between phases (e.g., olivine forsterite content and plagioclase anorthite content), suggest that the porous melt flux after the formation of this layering was insufficient to destroy this correlation.
Osmium isotopes in the Wiedemann Fjord mantle xenoliths: A unique record of cratonic mantle formation by melt depletion in the Archaean
Authors: Hanghøj Karen , Kelemen Peter , Bernstein Stefan , Blusztajn Jerzy , Frei Robert
Published: February 2004

The Wiedemann Fjord mantle xenoliths from Tertiary basanitic dykes in east Greenland contain extremely depleted spinel harzburgites. The harzburgites are unusual because of their highly forsteritic olivine and low modal orthopyroxene, consistent with an origin as residues of up to 40% melting. Also present are lherzolites with olivine contents as low as 50% and with up to 25% clinopyroxene, and there is a continuous range of compositions between the most depleted harzburgites and the lherzolites. Osmium isotopic data show that the depleted harzburgites were formed by melt depletion in the Archaean. In contrast to most other Archaean xenoliths (e.g., from the Kaapvaal and Siberian cratons) the composition of the Wiedemann Fjord harzburgites is consistent with depletion by polybaric melting. The lherzolites have less refractory mineral compositions than the harzburgites (lower bulk Mg # and Cr #, higher aluminum contents in orthopyroxene (opx)) and also have more radiogenic Os isotopic compositions. The data suggest that the Wiedemann Fjord xenoliths formed as restites from high degrees of melting in the Archaean and that some of the restites were partly refertilized by metasomatism involving addition of pyroxene and reequilibration of mineral and isotopic compositions.
Distribution of magma beneath the Toba caldera complex, north Sumatra, Indonesia, constrained by three‐dimensional P wave velocities, seismicity, and gravity data
Authors: Masturyono  , McCaffrey R. , Wark D. A. , Roecker S. W. , Fauzi  , Ibrahim G. , Sukhyar
Published: February 2004

We estimate the one‐ and three‐dimensional P wave velocity structure beneath the Toba caldera complex, a 30 × 100 km topographic depression in North Sumatra, using arrival time data of local earthquakes recorded by a 40‐station seismic network that operated for 4 months. Inversions reveal the presence of P velocities up to 37% below normal that likely map the distribution of magma within this subduction‐related volcanic system, considered the world's largest. In the upper 10 km of crust the largest low‐velocity region underlies the southern two thirds of the depression and coincides with a gravity low centered over the resurgent dome. A smaller volume of low velocities is observed in the upper crust under the north end of the depression. Separating the two regions is a zone of locally high velocities, indicating that the shallow, subcaldera magma system is composed of two separate reservoirs, not a single one that extends the entire length of the caldera complex. Above each low‐velocity region is a postcollapse volcano that erupted mostly mafic lavas after the last major caldera collapse ∼74 kyr ago. A low‐velocity column below one of these volcanoes can be traced into the uppermost mantle and corresponds with a planar distribution of low‐frequency earthquakes in the 20‐ to 40‐km‐depth range. The low‐frequency earthquakes apparently record the migration of melt in the mafic roots of this large‐volume, crustal magma system.
Role of a low‐viscosity zone in stabilizing plate tectonics: Implications for comparative terrestrial planetology
Authors: Richards Mark A. , Yang Woo‐Sun , Baumgardner John R. , Bunge Hans‐Peter
Published: February 2004

Earth's near‐surface layer, its lithosphere, is broken into quasi‐rigid plates that form the upper thermal boundary layer for mantle convection. Since the discovery of plate tectonics, it has been widely conjectured but only recently demonstrated that this peculiar style of convection may be facilitated by an upper mantle low viscosity zone (LVZ) over which the plates glide easily. The LVZ, or “asthenosphere,” concept dates from 19th century investigations of isostatic support of mountain belts and is supported by modern evidence for a seismic low velocity zone and by studies of postglacial rebound and dynamic compensation of the Earth's gravity field. Here we show in both two‐dimensional (2‐D) Cartesian and 3‐D spherical Earth models that combining a pronounced LVZ and a plastic yield stress to allow localized weakening of the cold thermal boundary layer results in a distinctly plate tectonic style of convection, with ∼30% toroidal surface motion for the 3‐D case. Recycling of water into the upper mantle at subduction zones is a plausible cause of Earth's LVZ, whereas Venus is dry and lacks both an LVZ and plate tectonics.
A statistical analysis of magnetic fields from some geodynamo simulations
Authors: McMillan D. G. , Constable C. G. , Parker R. L. , Glatzmaier G. A.
Published: February 2004

We present a statistical analysis of magnetic fields simulated by the Glatzmaier‐Roberts dynamically consistent dynamo model. For four simulations with distinct boundary conditions, means, standard deviations, and probability functions permit an evaluation based on existing statistical paleosecular variation (PSV) models. Although none closely fits the statistical PSV models in all respects, some simulations display characteristics of the statistical PSV models in individual tests. We also find that nonzonal field statistics do not necessarily reflect heat flow conditions at the core‐mantle boundary. Multitaper estimates of power and coherence spectra allow analysis of time series of single, or groups of, spherical harmonic coefficients representing the magnetic fields of the dynamo simulations outside the core. Sliding window analyses of both power and coherence spectra from two of the simulations show that a 100 kyr averaging time is necessary to realize stationary statistics of their nondipole fields and that a length of 350 kyr is not long enough to fully characterize their dipole fields. Spectral analysis provides new insight into the behavior and interaction of the dominant components of the simulated magnetic fields, the axial dipole and quadrupole. Although we find spectral similarities between several reversals, there is no evidence of signatures that can be conclusively associated with reversals or excursions. We test suggestions that during reversals there is increased coupling between groups of spherical harmonic components. Despite evidence of coupling between antisymmetric and symmetric spherical harmonics in one simulation, we conclude that it is rare and not directly linked to reversals. In contrast to the reversal model of R. T. Merrill and P. L. McFadden, we demonstrate that the geomagnetic power in the dipole part of the dynamo simulations is either relatively constant or fluctuates synchronously with that of the nondipole part and that coupling between antisymmetric and symmetric components occurs when the geomagnetic power is high.
Magmatic processes and segmentation at a fast spreading mid‐ocean ridge: Detailed investigation of an axial discontinuity on the East Pacific Rise crest at 9°37′N
Authors: Smith Matthew C. , Perfit Michael R. , Fornari Daniel J. , Ridley W. Ian , Edwards Margo H. , Kurras Gregory J. , Von Damm Karen L.
Published: February 2004

Geophysical and petrological boundaries on mid‐ocean ridges provide ideal locations to study the relationships between magmatic, tectonic, and hydrothermal processes. Alvin‐based observational data and geochemical data for basalts and hydrothermal fluids are used to investigate these relationships at an axial discontinuity on the East Pacific Rise (EPR) crest between ∼9°36′N and 9°38′N. This ridge‐crest discontinuity is morphologically expressed by the overlap of an eastern and western axial summit collapse trough (ASCT) that delimits the primary volcanic and hydrothermal loci along the ridge crest in this area. The ASCTs overlap by ∼3 km and are offset in a right‐lateral sense by 0.45 km. Near‐bottom imaging of this area in 1989 and 1991 shows changes in volcanic morphology and increases in hydrothermal and biological activity consistent with the occurrence of a magmatic event during that time interval. When combined with the inferred age and structure of the seafloor, basalt geochemistry, and hydrothermal fluid chemistry, these temporal changes suggest active southward propagation of the eastern ASCT and show that the western ASCT was unaffected by the recent magmatic event. Numerous extinct hydrothermal vents and older‐looking lava flow surfaces suggest waning of magmatic activity in the western ASCT.Young‐looking lava flows within or proximal to the eastern ASCT have anomalously high Mg numbers relative to the regional trend and are chemically similar to lava erupted in 1991 along the 9°46′–52′N EPR region. We propose that the young‐looking lava flows in the eastern ASCT are related to the 1991 eruption. Data show that the 9°37′N axial discontinuity marks a magmatic and hydrothermal boundary along the EPR ridge‐crest, and we argue that it be classified as a third‐order discontinuity. This result is consistent with geophysical evidence suggesting fundamental differences in the crust and upper mantle north and south of the 9°37′N discontinuity.
Variation of cooling rate with depth in lower crust formed at an oceanic spreading ridge: Plagioclase crystal size distributions in gabbros from the Oman ophiolite
Authors: Garrido Carlos J. , Kelemen Peter B. , Hirth Greg
Published: February 2004

Analysis of crystal size distributions (CSD) of plagioclase in gabbros from the Oman ophiolite indicates that cooling rates in the lower crust beneath a medium‐ to fast‐spreading oceanic ridge did not vary smoothly with depth. Cooling rates in the upper half of the gabbro section were approximately 1.5–2 times faster than in the lower half of the gabbro section during igneous crystallization and/or high temperature grain growth processes. Our results are inconsistent with thermal models for oceanic spreading ridges that approximate the effect of hydrothermal convection by using an enhanced thermal conductivity together with a conductive heat transfer mechanism. Instead, the Oman CSD data are consistent with recent seismic and modeling studies that suggest that hydrothermal convection leads to closely spaced, near‐vertical isotherms in the upper half of the plutonic section within a few kilometers of the ridge axis.
Precision of the current methods to measure the alkenone proxy U37K′ and absolute alkenone abundance in sediments: Results of an interlaboratory comparison study
Authors: Rosell‐Melé A. , Bard E. , Emeis K.‐C. , Grimalt J. O. , Müller P. , Schneider R. , Bouloubassi I. , Epstein B. , Fahl K. , Fluegge A. , Freeman K. , Goñi M. , Güntner U. , Hartz D. , Hellebust S. , Herbert T. , Ikehara M. , Ishiwatari R. , Kawamura K. , Kenig F. , de Leeuw J. , Lehman S. , Mejanelle L. , Ohkouchi N. , Pancost R. D. , Pelejero C. , Prahl F. , Quinn J. , Rontani J.‐F. , Rostek F. , Rullkötter J. , Sachs J. , Blanz T. , Sawada K. , Schulz‐Bull D. , Sikes E. , Sonzogni C. , Ternois Y. , Versteegh G. , Volkman J. K. , Wakeham S.
Published: February 2004

Measurements of the U37K′ index and the absolute abundance of alkenones in marine sediments are increasingly used in paleoceanographic research as proxies of past sea surface temperature and haptophyte (mainly coccolith‐bearing species) primary productivity, respectively. An important aspect of these studies is to be able to compare reliably data obtained by different laboratories from a wide variety of locations. Hence the intercomparability of data produced by the research community is essential. Here we report results from an anonymous interlaboratory comparison study involving 24 of the leading laboratories that carry out alkenone measurements worldwide. The majority of laboratories produce data that are intercomparable within the considered confidence limits. For the measurement of alkenone concentrations, however, there are systematic biases between laboratories, which might be related to the techniques employed to quantify the components. The maximum difference between any two laboratories for any two single measurements of U37K′ in sediments is estimated, with a probability of 95%, to be <2.1°C. In addition, the overall within‐laboratory precision for the U37K′ temperature estimates is estimated to be <1.6°C (95% probability). Similarly, from the analyses of alkenone concentrations the interlaboratory reproducibility is estimated at 32%, and the repeatability is estimated at 24%. The former is compared to a theoretical estimate of reproducibility and found to be excessively high. Hence there is certainly scope and a demonstrable need to improve reproducibility and repeatability of U37K′ and especially alkenone quantification data across the community of scientists involved in alkenone research.
Realistic semiannual oscillation simulated in a middle atmosphere general circulation model
Authors: Medvedev Alexander S. , Klaassen Gary P.
Published: February 2004

Although several general circulation modelling studies have presented simulations containing a tropical middle atmosphere semiannual oscillation (6 month period), none so far have been able to reproduce the observed amplitude and vertical structure extending into the mesosphere. Here we present simulations with the Canadian Middle Atmosphere Model which utilize a nonlinear broad‐spectrum gravity‐wave parameterization [Medvedev and Klaassen, 1995, 2000] and exhibit a semiannual oscillation in good agreement with observations.
The crystal structures of synthetic Re‐ and PGE‐bearing magnesioferrite Spinels: Implications for impacts, accretion and the mantle
Authors: Righter Kevin , Downs Robert T.
Published: February 2004

Re‐ and PGE‐bearing magnesioferrite spinels were synthesized at oxidizing conditions between 0.1 MPa and 5.0 GPa, and 1150 to 1600 °C in equilibrium with Re‐ or PGE‐bearing metals and/or oxides. Electron microprobe analysis and single crystal X‐ray diffraction techniques were employed to determine the composition, crystal structures and site occupancies, indicating that the magnesioferrite structure can accommodate wt% levels of Re and the platinum group elements (PGE) at oxidized conditions. These results suggest that magnesioferrite spinels found in K‐T boundary samples worldwide could be an important host phase for the Ir, Ru, Os, Re, and Rh anomalies found in the boundary layer. Higher Ru/Ir values in Pacific magnesioferrite‐bearing samples may be a reflection of higher condensation temperatures of the oxides. The distribution of PGE's in mantle and magmatic rocks may depend on the stability of spinel‐structure oxides, especially those with a high magnesioferrite component. Finally, magnetite‐rich meteorite fusion crusts may hold PGE' in oxidized form.
Water vapor in the tropical lower stratosphere during the driest phase of the atmospheric “tape recorder”
Authors: Clark H. L. , Billingham A. , Harwood R. S. , Pumphrey H. C.
Published: February 2004

Data from the Microwave Limb Sounder on the Upper Atmosphere Research Satellite are used to examine sequences of days during the driest phase of the atmospheric “tape recorder” signal in the lower stratosphere. It is found that the Indonesian region is the first place to become dry followed by the western Pacific and Panama. Eventually, dry air is found in a band along the equator with the most northerly extent over Indonesia and Panama. Two‐dimensional trajectories show that winds cannot account for the spread of dry air from Indonesia and Panama across the entire tropics. The patterns are difficult to explain but may result from a combination of widespread ascent of air and the effects of deep convection.
Error analysis and characterization of atmospheric profiles retrieved from GNSS occultation data
Authors: Rieder Markus J. , Kirchengast Gottfried
Published: February 2004

The performance of the Global Navigation Satellite System (GNSS) based radio occultation method for providing retrievals of atmospheric profiles up to the mesosphere was investigated by a rigorous Bayesian error analysis and characterization formalism. Starting with excess phase profile errors modeled as white Gaussian measurement noise, covariance matrices for the retrieved bending angle, refractivity/density, pressure, and temperature profiles were derived in order to quantify the accuracy of the method and to elucidate the propagation of statistical errors through subsequent steps of the retrieval process. We assumed unbiased phase errors (the occultation method is essentially self‐calibrating), spherical symmetry in the occultation tangent point region (reasonable for most atmospheric locations), and dry air (disregarding humidity being of relevance below 10 km in the troposphere only) in this baseline analysis. Because of the low signal‐to‐noise ratio of occultation data at mesospheric heights, which causes instabilities in case of direct inversion from excess phase profiles to atmospheric profiles, a Bayesian approach was employed, objectively combining measured data with a priori data. For characterization of the retrievals we provide, in addition to covariance estimates for the retrieved profiles, quantification of the relationship between the measured data, the retrieved state, the a priori data, and the true state, respectively. Averaging kernel functions, indicating the sensitivity of the retrieval to the true state, contribution functions, indicating the sensitivity of the retrieval to the measurement, and the ratio of retrieval errors to a priori errors are shown. Two different sensor scenarios are discussed, respectively, an advanced receiver (AR) scenario with 2 mm and a standard receiver (SR) scenario with 5 mm unbiased RMS error on excess phase data at 10 Hz sampling rate. The corresponding bending angle, refractivity, pressure, and temperature retrieval properties are shown. Temperature, the final data product, is found to be accurate to better than 1 K below ∼40 km (AR)/∼35 km (SR) at ∼2 km height resolution and to be dominated by a priori knowledge above ∼55 km (AR)/∼47 km (SR), respectively. For all data products the use of a Bayesian framework allowed for a more complete and consistent quantification of properties of profiles retrieved from GNSS occultation data than previous work.
Analysis of integrated cloud liquid and precipitable water vapor retrievals from microwave radiometers during the Surface Heat Budget of the Arctic Ocean project
Authors: Westwater Ed R. , Han Yong , Shupe Matthew D. , Matrosov Sergey Y.
Published: February 2004

We investigated a variety of factors that influence the determination of precipitable water vapor (V) and integrated cloud liquid (L) by dual‐channel microwave radiometers (MWRs). These factors include radiometric calibration; dry, water vapor, and liquid absorption coefficients; and physical versus statistical retrieval methods. We then applied the analysis to the MWR that was operated by the Atmospheric Radiation Measurement Program (ARM) during the Surface Heat Budget of the Arctic Ocean project. In the work reported here, MWR data taken from April 1 to July 31, 1998, were analyzed. Data acquired in situ did not always agree with the original MWR liquid retrievals, with MWR estimates at times being too large by perhaps a factor of 2. These differences led us to examine in detail several of the assumptions that go into V and L retrievals. The radiometer was carefully examined and found to be well calibrated with a 0.3 K RMS error. The predicted accuracy in the L retrievals, for this 0.3 K RMS radiometric error and a statistical retrieval, was 25 g m−2 RMS. This accuracy improves to 10%, if we use the improved knowledge of cloud temperature, as can be obtained using radiosondes and cloud radar/lidar measurements. We also studied the degree to which different clear air and cloud liquid models have an effect on V and L retrievals. The most significant changes from the original ARM retrievals were due to the dry opacity and the cloud liquid dielectric model. Although nothing was found in the original ARM data that was grossly incorrect, application of these models reduced the original ARM retrievals by roughly 20 to 30%. The change of clear‐air absorption model from the original to a more recent one has little impact on V retrievals except when V is <0.5 cm.
Aerosol optical depth measurements by means of a Sun photometer network in Switzerland
Authors: Ingold T. , Mätzler C. , Kämpfer N. , Heimo A.
Published: February 2004

Within the Swiss Atmospheric Radiation Monitoring program (CHARM) the Swiss Meteorological Institute ‐ MeteoSwiss operates a network of presently six Sun photometer stations. Aerosol optical depths (AOD) at 368, 500, and 778 nm were determined from measurements of the relative direct solar irradiance, primarily to provide climatological information relevant in particular to climate change studies. The six instruments are located at various sites representative of high and low altitudes at the north and south part of the Alps in areas free from urban pollution in Switzerland. AOD time series of recordings back to 1991 are discussed, when data were first collected at Davos. An important aerosol layer is often present over stations at lower sites, showing seasonal variability and regional differences for the observed tropospheric aerosols. A classification scheme for synoptic weather types was applied to separate the AOD data into groups corresponding to different atmospheric transport conditions. On average, lower AODs are measured within advective weather situations than within convective ones. However, at the high Alpine sites such a classification is incomplete for AOD characterization due to orographically induced vertical motion. Monthly averaged values of AOD at 500 nm ranged from 0.05 during winter up to 0.3 in summer. The scale height of the aerosol optical depth is found to be 1–2 km depending on season. The high mountain sites are more suitable to the study stratospheric aerosols, for example, the change of the aerosol content and of its size distribution due to Mount Pinatubo eruption was clearly identified at Davos. In 1996 the aerosol optical depth returned to pre‐Pinatubo values. Minimum AODs of ≈0.004–0.007 measured at 500 nm in 1997 are in good agreement with widely reported aerosol optical depth measurements of the stratospheric background aerosols. Besides the Pinatubo‐affected period aerosol characterization by means of the Angström power law exhibits on average a mean wavelength exponent of 1.3–1.8. A climatology of the AOD data obtained between 1994 and 2000 in both space and time has been established.
Variation in N2/O2 ratio of occluded air in Dome Fuji antarctic ice
Authors: Ikeda‐Fukazawa Tomoko , Hondoh Takeo , Fukumura Taku , Fukazawa Hiroshi , Mae Shinji
Published: February 2004

Ancient atmospheric gases are trapped in polar ice sheets. The gas molecules are stored in air bubbles at shallow depth and are incorporated into clathrate hydrates below a depth at which the hydrostatic pressure becomes greater than the formation pressure of the air clathrate hydrate. Significant gas fractionation has been found from measurements of the depth profile of the N2/O2 composition ratios in clathrate hydrates and air bubbles of Vostok antarctic ice. To investigate the effect of the ice condition on the fractionation process, we measured the N2/O2 ratios in clathrate hydrates and air bubbles from Dome Fuji antarctic ice using Raman spectroscopy. The results showed that the N2/O2 ratios in the clathrate hydrates of the Dome Fuji ice are slightly lower than those of the Vostok ice, although the tendency of the variation of the N2/O2 ratio with depth is similar. The difference in the N2/O2 ratio between the Dome Fuji ice and the Vostok ice for the transition zone is attributed to the difference of the ice temperature and the snow accumulation rate. On the other hand, it is concluded that the difference in the bubble‐free ice zone was caused by gas loss from the ice core after coring. The N2/O2 ratio of clathrate hydrate increases after coring because of higher diffusion rate and lower dissociation pressure of O2 than of N2. Our data suggest that the effect of gas loss in the Dome Fuji ice is relatively small, and so the gas composition in the Dome Fuji ice can be a precise paleoenvironmental indicator.