bluuebunny/crossref_metadata_2025 huggingface dataset:
Fading timescales associated with GPS signals and potential consequences
Authors: Kintner Paul M. , Kil Hyosub , Beach Theodore L. , de Paula Eurico R.
Published: February 2004

The effect of equatorial ionospheric scintillations on the operation of GPS receivers is investigated, with special attention given to the effect of scintillation timescales on the code division multiple access (CDMA) protocol used by GPS. We begin by examining the timescales of scintillation fades modeled as a horizontally drifting pattern whose timescales are determined by the Fresnel length and the drift speed. The model is tested by comparing the speed, determined by dividing the Fresnel length by the autocorrelation time (width), with the speed estimated using spaced receivers, and the two independent estimates of speed are shown to possess a linear relationship. Next we show that the scintillation pattern drift speed is given by the difference of the ionospheric drift and the speed of the GPS signal F region puncture point. When the ionosphere and GPS signal puncture point speeds match, the fade timescales lengthen. Additionally, if the fade depth is adequate, during periods of longer fade times the loss of receiver lock on GPS signals is more likely, as shown in several examples; that is, both larger fade depths and longer fade timescales are required to produce loss of tracking. We conclude by demonstrating that speed matching or resonance between the ionosphere and receiver is most likely when the receiver is moving from west to east at speeds of 40–100 m/s (144–360 km/h). This is in the range of typical aircraft speeds.
A self‐consistent impedance method for electromagnetic surface impedance modeling
Authors: Thiel David V. , Mittra Raj
Published: February 2004

A two‐dimensional, self‐consistent impedance method has been derived and used to calculate the electromagnetic surface impedance above buried objects at very low frequencies. The earth half space is discretized using an array of impedance elements. Inhomogeneities in the complex permittivity of the earth are reflected in variations in these impedance elements. The magnetic field is calculated for each cell in the solution space using a difference equation derived from Faraday's and Ampere's laws. It is necessary to include an air layer above the earth's surface to allow the scattered magnetic field to be calculated at the surface. The source field is applied above the earth's surface as a Dirichlet boundary condition, whereas the Neumann condition is employed at all other boundaries in the solution space. This, in turn, enables users to use both finite and infinite magnetic field sources as excitations. The technique is shown to be computationally efficient and yields reasonably accurate results when applied to a number of one‐ and two‐dimensional earth structures with a known surface impedance distribution.
Extremely low ionospheric peak altitudes in the polar hole region
Authors: Benson Robert F. , Grebowsky Joseph M.
Published: February 2004

Vertical electron density Ne profiles, deduced from newly available ISIS 2 digital ionospheric topside sounder data, are used to investigate the altitude structure of localized ionization depletions within the winter, nighttime polar cap ionosphere during solar minimum. The region investigated, called the polar hole‐auroral cavity region, is located just after midnight magnetic local time near the poleward side of the auroral oval. Earlier investigations had at times found very low Ne values in this region (down to 200 cm−3 near 300 km). In the present study, latitudinal Ne depletions were found, but such low Ne values (≈ 100 cm−3) were only found near the ISIS 2 altitude of 1400 km. The peak ionospheric concentration below ISIS 2 remained fairly constant (∼105 cm−3) across the Ne‐depleted region, but the altitude of the peak dropped dramatically to the vicinity of 100 km. The observations, in combination with other data, indicate that the absence of an F layer peak may be a frequent occurrence at high latitudes and stress the importance of a knowledge of the vertical Ne distribution in high‐latitude investigations of horizontal Ne structures.
Real‐time characterization of the ionosphere using diverse data and models
Authors: Ganguly Suman , Brown Andrew
Published: February 2004

Complete descriptions of spatial and temporal distributions of the ionospheric plasma are necessary for scientific understanding and for several practical applications involving communication, radar, navigation, etc. Ionospheric diagnostic techniques, on the other hand, provide spatially and temporally incomplete information. A framework for data fusion is needed which would be self‐consistent in terms of various input data and would also provide spatial‐temporal extrapolations. Incorporation of a physical ionospheric model as a framework for assimilation of ionospheric (ionosonde) data was demonstrated by Ganguly [1990, 1991]. The free parameters of the physical model were adjusted using the observational data, and then the adjusted model was used to predict the state of the ionosphere at a different place and time. Most recently, the approach has been extended to incorporate total electron content (TEC) observation from ground and/or satellite‐borne receivers and beacons, as well as data from a variety of other sources such as oblique and vertical sounders, radar, radio propagation, incoherent scatter, optical observations, in situ measurements, etc. This TEC‐based reconstruction utilizes novel tomographic techniques and allows integration of any satellite constellation including GPS. Data from other sensors are integrated into the complete system, and the fusion is performed in conjunction with some physical/empirical ionospheric model. This provides a self‐consistent four‐dimensional ionosphere. The system can accept any reasonable ionospheric model and any amount of diverse input data covering a wide range of space, time, and observational methods.
Ionospheric tomography: Issues, sensitivities, and uniqueness
Authors: Brown A. , Ganguly S.
Published: February 2004

Ionospheric tomography offers several challenges. The tomographic reconstructions are dependent on geometries, algorithms, accuracies, noise, bias, and multipath, among other things. Relative roles of different factors are not clearly understood, and the “objectivity” of tomographic reconstruction is generally uncertain. A series of simulations has been performed to understand and obtain quantitative descriptions of various factors in tomographic reconstruction. Simulations include different geometries, beam divergences, angular coverage, algorithms, and initial guesses, etc. Simulations have been performed under the most general circumstances, and the results can be used for generalized tomographic reconstruction. On the basis of these simulations we provide guidelines for objective reconstruction of the ionosphere using ground‐based and satellite‐based receivers. These guidelines allow determination of uniqueness of reconstruction under different situations.
Finite difference time domain tests of random media propagation theory: 2. Signal correlation length and channel bandwidth
Authors: Nickisch L. J. , Franke P. M.
Published: February 2004

The finite difference time domain method (FDTD) has been extended for application to frequency‐dependent (dispersive) media such as the ionospheric plasma. This formulation of FDTD can be applied to the study of electromagnetic propagation in the plasma of the ionosphere. In the current work, we consider propagation in randomly structured ionization. The results of numerical FDTD computations of HF fields propagated through realizations of ionospheric structure are compared to the predictions of the standard random media propagation theory, which is based on the parabolic wave equation. The motivation for this study is to expose the way in which the approximate theory breaks down when applied outside of its regime of strict applicability. An earlier published analysis by the authors was limited to spatial signal decorrelation as expressed in ℓ0, the signal correlation length. In the current work, this spatial decorrelation analysis is supplemented with a similar analysis of signal frequency decorrelation (or delay spread) as expressed in ƒ0, the channel bandwidth. We find that the standard propagation theory for the calculation of ƒ0 predicts larger values than those produced in the FDTD computation. This appears to be due to a polarization coupling effect that is ignored in the standard propagation theory. FDTD‐computed correlation lengths are found to be in substantial agreement with the propagation theory at higher frequencies. Agreement degrades in a graceful and understandable way at lower frequencies where the limitations of the approximations used in the theory are violated.
Verification of ionospheric sensors
Authors: Coker Clayton , Kronschnabl Glenn , Coco David S. , Bust Gary S. , Gaussiran Thomas L.
Published: February 2004

Ionospheric products from sensors and models were compared to investigate strengths and limitations of each. Total electron content data from computerized ionospheric tomography (CIT) and TOPEX sensors in the Caribbean region in 1997 were compared to estimates produced by models Parameterized Ionospheric Model (PIM) and Raytrace/ICED‐Bent‐Gallagher (RIBG) and global maps from GPS. A 5 total electron content unit (TECU) bias was observed in TOPEX. CIT and TOPEX confirmed the location and structure of the equatorial anomaly. A GPS map confirmed the location of the anomaly but did not reproduce structure less than 1000 km in latitude and 1500 km in longitude and underestimated TEC by at least 11 TECU or 25%. PIM positioned the anomaly 13° equatorward of its observed location and greatly underestimated (∼50%) the rise in content over 5°‐25°N range. RIBG overestimated the latitudinal extent of the anomaly and underestimated TEC at the peak by 40%. Additional comparisons were made using CIT and ionosonde sensors at midlatitude during the summer of 1998. Fourteen days of TEC, hmF2, NmF2, and half‐thickness comparisons showed reasonable agreement between CIT and ionosonde for TEC and NmF2. The hmF2 and half‐thickness comparisons were contaminated by noise, which accounted for a significant portion of the ionospheric variation. Daytime cases where CIT overestimated maximum density were attributed to underestimating layer thickness. Finally, TOPEX and multiple GPS sensors were compared to verify regional ionospheric conditions associated with occurrence of nighttime ionospheric depletions in the Caribbean during Combined Ionospheric Campaigns in June of 1998. From 0300 to 0800 UT on June 26, GPS and TOPEX showed elevated nighttime content over the entire Caribbean region. Vertical TEC approached 25 TECU in some places with interspersed depletions, which in some cases evacuated nearly the entire ionospheric content.
Computerized ionospheric tomography analysis of the Combined Ionospheric Campaign
Authors: Bust G. S. , Coco D. S. , Gaussiran T. L.
Published: February 2004

Results from the June 1998 Combined Ionospheric Campaign (CIC) will be presented. The CIC represents an attempt to focus a large number of different instruments on one interesting geophysical region. One of the objectives of these campaigns is to develop suitable data sets for ingestion into data assimilative models and also to serve as validation sets for these models. The Center for Ionospheric Research (CIR) at Applied Research Laboratories, University of Texas at Austin, has had several computerized ionospheric tomography (CIT) receivers deployed in the Caribbean region since July 1997. Analysis of the CIT data from the first CIC will be presented, and comparison with other data sets will be made. Analysis will initially focus on examining the total electron content (TEC) data from the CIT receivers and ground‐based GPS TEC data and correlating it with other data sets. Subsequently, the analysis will shift to performing four‐dimensional electron density estimations using the Ionospheric Data Assimilation 3D (IDA3D) algorithm developed at CIR. The resulting electron density estimates will be compared with other data sources both for accuracy of the technique and scientific investigations.
Intracratonic, strike‐slip partitioned transpression and the formation and exhumation of eclogite facies rocks: An example from the Musgrave Block, central Australia
Authors: Camacho Alfredo , McDougall Ian
Published: February 2004

Mountain belts developed in an intracratonic setting during the Neoproterozoic in the center of Australia. Mesoproterozoic granulite and amphibolite facies gneisses of the Musgrave Block were overprinted during the Petermann Orogeny (∼550 Ma) with zones of high strain concentrated along broadly east‐west trending shear zones. These zones formed under eclogite facies (T ∼650°C and P ∼12 kbars) to greenschist facies conditions and may form part of a strike‐slip‐related, crustal‐scale flower‐type structure. Granulite facies gneisses preserve ages older than 700 Ma, suggesting that these rocks did not experience temperatures greater than ∼350°C at ∼550 Ma. For these rocks not to have been thermally equilibrated to temperatures of >350°C at a depth of ∼12 kbars requires either an extraordinarily low geothermal gradient (<9°C km−1) or, if the geothermal gradient was more typical (∼20°C km−1), then transport to that depth and subsequent exhumation occurring in a short time interval (≤40 Myr).
Changes in ecosystem processes induced by land use: Human appropriation of aboveground NPP and its influence on standing crop in Austria
Authors: Haberl Helmut , Erb Karl‐Heinz , Krausmann Fridolin , Loibl Wolfgang , Schulz Niels , Weisz Helga
Published: February 2004

Human land use influences important properties of terrestrial ecosystems, such as energy flow, standing crop, and biomass turnover. Human interference with ecological energy flows may be studied by calculating the “human appropriation of NPP” (HANPP), defined as the difference between the net primary production (NPP) of potential vegetation and the actual NPP remaining in ecosystems after harvest. Comparing the standing crops of the potential vegetation and actually prevailing vegetation, we demonstrate the human impact on the amount of carbon stored in living vegetation. We discuss these concepts using empirical results for aboveground vegetation in Austria calculated from statistical data and from land use and land cover models derived from remote‐sensing data. According to our calculations the human appropriation of aboveground NPP in Austria today amounts to ∼50%. The aboveground standing crop (biomass stock) of the vegetation prevailing in Austria today is ∼64% lower than that of the potential vegetation.
Sphagnum peatland distribution in North America and Eurasia during the past 21,000 years
Authors: Gajewski K. , Viau A. , Sawada M. , Atkinson D. , Wilson S.
Published: February 2004

The distribution and abundance of Sphagnum spores in North America and Eurasia are mapped for the past 21 ka. The present‐day distribution of abundant Sphagnum spores corresponds closely to areas with peatland development, with maximum Sphagnum abundance between 630 and 1300 mm annual precipitation and between −2° and 6°C mean annual air temperature. During the Wisconsin glaciation, there were apparently not large areas of peatland in North America, except in Alaska. High Sphagnum spore percentages were found in eastern North America during deglaciation. Major peatland development occurred in boreal North America after 9 ka and there was a southward movement of high Sphagnum spore abundance after 5 ka in the western Great Lakes region. Major peatland development began after 9 ka in Europe and Asia. On the basis of maps of the area supporting peatlands, carbon accumulation in peatlands is estimated to be low prior to 11 ka, increased slightly between 11 and 5 ka, and greatly increased during the past 5 ka.
A time series study of silica production and flux in an eastern boundary region: Santa Barbara Basin, California
Authors: Shipe Rebecca F. , Brzezinski Mark A.
Published: February 2004

A high‐resolution data set describing the annual silica cycle between January 1994 and August 1999 in the Santa Barbara Basin (SBB) captures processes and events occurring in both surface and deep basin waters. Increases in biogenic silica (bSi) concentrations in the surface waters indicated strong bloom events in the early spring of each year and sporadic blooms during the summer and fall. Lithogenic silica (lSi) was present at elevated concentrations in surface waters in the winter and spring. At times, the siliceous biomass in the SBB is comparable to that in major global upwelling regions. However, anomalously low annual depth‐integrated bSi concentrations were observed during the E1 Niño conditions of 1997–1998. A 1.5‐year time series of biogenic silica production rate measurements was used to calculate the highest resolution estimate of annual silica production to date, an estimated 5.5 mol Si m −2 yr−1. This silica production rate approaches that estimated for the major global upwelling zones (8.3 mol Si m−2 yr−1). Eastem boundary regions outside of the major coastal upwelling zones would account for 30–40% of global silica production if the Santa Barbara Basin is representative of these coastal regions. Since the sediments of the SBB contain a high‐resolution climate record, an understanding of the coupling between surface processes and deep basin sedimentation events is of value. Fluxes of bSi and lSi to 470 m were not strongly coupled to the seasonal cycle of particulate concentrations or production rates in surface waters. Continuous sediment trap collections of sedimenting particles indicate that both lSi and bSi tend to be exported at the highest rates during the spring and summer. As particles sink, lSi becomes enriched relative to bSi and consistently dominates the mass flux of silica at 470 m. Brief increases in the percent contribution of bSi to Si flux may create microlaminae within larger‐scale laminations which are thought to be annual varves. A comparison of annual bSi sedimentation rates with annual silica production estimates reveal that ∼30% of the annual silica production is exported to a depth of 470 m. This export rate is the highest observed to date and suggests the potential for large quantities of silica burial in coastal sediments.
Controls on carbon and energy exchange by a black spruce–moss ecosystem: Testing the mathematical model Ecosys with data from the BOREAS Experiment
Authors: Grant R. F. , Jarvis P. G. , Massheder J. M. , Hale S. E. , Moncrieff J. B. , Rayment M. , Scott S. L. , Berry J. A.
Published: February 2004

Stomatal limitations to mass and energy exchange over boreal black spruce forests may be caused by low needle N concentrations that limit CO2 fixation rates. These low concentrations may be caused by low N uptake rates from cold boreal soils with high soil C:N ratios and by low N deposition rates from boreal atmospheres. A mathematical model of terrestrial ecosystems ecosys was used to examine the likelihood that slow N cycling could account for the low rates of mass and energy exchange measured over a 115‐year old boreal spruce/moss forest as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). In the model, net N mineralization was slowed by the high C:N ratios measured in the forest floor and by high lignin contents in spruce litterfall. Slow mineralization caused low N uptake rates and hence high C:N ratios in spruce and moss leaves that reduced specific activities and areal densities of rubisco and chlorophyll. Consequent low CO2 fixation rates caused low stomatal conductances and transpiration rates which in turn caused high soil water contents. Wet soils, in conjunction with large accumulations of surface detritus generated by slow litter mineralization, caused low soil temperatures that further slowed mineralization rates. Model outputs for ecosystem N status were corroborated by low needle N concentrations (< 10 mg g−1), stomatal conductances (< 0.05 mol m−2 s−1) and CO2 fixation rates (< 6 μmol m−2 s−1), and by high canopy Bowen ratios (1.5–2.0) and low canopy net CO2 exchange rates (< 10 μmol m−2 s−1) measured over the black spruce/moss forest at the BOREAS site. Modeled C accumulation rates of 60 (wood) + 10 (soil) = 70 g C m−2 yr−1 were consistent with estimates from aggregated CO2 fluxes measured over the spruce canopy and from allometric equations developed for black spruce in Canadian boreal forests. Model projections under IS92a climate change indicate that rates of wood C accumulation would rise and those of soil C accumulation would decline from those under current climate. Because these rates are N‐limited, they would be raised by increases in atmospheric N deposition.
Effects of shifting growth stage and regulating temperature on seasonal variation of CH4 emission from rice
Authors: Watanabe Akira , Yamada Hiromi , Kimura Makoto
Published: February 2004

Seasonal variations in CH4 emission rates from rice paddies have been reported to have one or more maxima during the middle and late periods of rice growth. The factor affecting an appearance of CH4 emission maxima was examined in three types of pot experiments. In the experiment 1, four rice cultivars with difference in length of the period from transplanting to heading were transplanted on the same days. For the experiment 2, a cultivar was transplanted 4 times with interval of two weeks. In these experiments, the heading differed about a month between the earliest and latest treatments, respectively. However, shifting growth stage of rice plants did not shift the CH4 emission maxima, and the CH4 emission maxima often matched the maxima of daily mean air temperature. The effect of variation in temperature on CH4 emission rate was further investigated in the experiment 3 by placing the rice‐planted pots under regulated temperature. Besides the first emission peak of CH4 attributable to rice straw (RS) carbon, three emission peaks corresponding to the peaks of air temperature were detected for the RS‐applied pots placed outdoors. These three peaks were not observed or much less conspicuous for the RS‐applied pots in a phytotron at 30°C. Temporal decreases in CH4 emission were detected both for the pots placed in the phytotron and outdoors just after the topdressing of (NH4)2SO4, which was considered to be a major cause of irregular disagreement between the variations in CH4 emission rates and in air temperature during the middle period of rice growth.
Spatial and seasonal variations of Q10 determined by soil respiration measurements at a Sierra Nevadan Forest
Authors: Xu Ming , Qi Ye
Published: February 2004

We examined the spatial and seasonal variation of Q10 as an indicator of the temperature sensitivity of soil respiration based on field measurements at a young ponderosa pine plantation in the Sierra Nevada Mountains in California. We measured soil CO2 efflux and soil temperature and moisture in two 20 m × 20 m plots from June 1998 to August 1999. The Q10 values calculated from soil temperature at 10‐cm depth ranged spatially from 1.21 to 2.63 among 18 chamber locations in the plots. Seasonally, the Q10 values calculated on the basis of the average soil CO2 efflux and temperature (10 cm) across the sites could vary from 1.05 to 2.3. Q10 and soil temperature are negatively correlated through a simple linear relationship with R2 values of 0.45, 0.40, and 0.54 for soil temperature at 5−, 10−, and 20−cm depth, respectively. However, Q10 and soil moisture are positively correlated with R2 values of 0.81, 0.86, and 0.51 for soil temperature at 5−, 10−, and 20−cm depth, respectively. Q10 values derived from temperatures at different soil depths also showed considerable variation along the vertical dimension. Q10 had a large seasonal variation with the annual minimum occurring in midsummer and the annual maximum occurring in winter. Seasonal values of Q10 depended closely on both soil temperature and moisture. Soil temperature and moisture explained 93% of the seasonal variation in Q10. The spatial variation of Q10 had significant influences on the estimation of soil CO2 efflux of the ecosystem. These variations tended to affect the seasonality of the soil CO2 efflux more than the annual average. The variations of Q10 and its dependence on soil moisture and temperature have important implications for regional and global ecosystem carbon modeling, in particular for predicting the responses of terrestrial ecosystems to future global warming.
Influence of soils on oxygen isotope ratio of atmospheric CO2
Authors: Stern Libby A. , Amundson Ronald , Baisden W. Troy
Published: February 2004

The soil component of atmospheric CO2 oxygen isotope budget is evaluated in light of the recent recognition of abiotic oxygen isotope exchange of atmospheric CO2 and soil water in excess of soil respiratory CO2 flux. By using variations of published analytical models, we estimate that the amount of atmospheric CO2 that undergoes oxygen isotope exchange with soil water, exclusive of soil‐respired CO2, to be approximately 0.2–0.7 μmol m−2 s−1 for representative unsaturated soils from a range of biomes. Globally, the amount of atmospheric CO2 that undergoes oxygen isotope exchange with soil water through purely nonbiological processes is probably significantly larger than the current annual fossil fuel combustion, yet this process has been neglected in all recent global 18O‐CO2 budgets. Furthermore, abiotic oxygen isotope exchange with soils will occur roughly equally in soils with low and high respiration rates, suggesting that soils with low respiration rates are currently under‐represented disproportionately in the existing global CO2 oxygen isotope budgets. Because soils with low respiration rates tend to have the most extreme soil water δ 18O values, their underrepresentation may have a large and heretofore unsuspected impact on the global atmospheric C18 O16O budget. Finally, soil carbon is no longer in steady state due to land use practices, and this additional source of CO2 to the atmosphere may contribute to the decreasing trend in atmospheric CO2‐δ18O values through time.
Basin‐wide particulate carbon flux in the Atlantic Ocean: Regional export patterns and potential for atmospheric CO2 sequestration
Authors: Antia Avan N. , Koeve Wolfgang , Fischer Gerhard , Blanz Thomas , Schulz‐Bull Detlef , Schölten Jan , Neuer Susanne , Kremling Klaus , Kuss Joachim , Peinert Rolf , Hebbeln Dirk , Bathmann Ulrich , Conte Maureen , Fehner Uwe , Zeitzschel B.
Published: February 2004

Particle flux data from 27 sites in the Atlantic Ocean have been compiled in order to determine regional variations in the strength and efficiency of the biological pump and to quantify carbon fluxes over the ocean basin, thus estimating the potential oceanic sequestration of atmospheric CO2. An algorithm is derived relating annual particulate organic carbon (POC) flux to primary production and depth that yields variations in the export ratio (ER = POC flux/primary production) at 125 m of between 0.08 and 0.38 over the range of production from 50 to 400 g C m−2 yr−1. Significant regional differences in changes of the export ratio with depth are related to the temporal stability of flux. Sites with more pulsed export have higher export ratios at 125 m but show more rapid decreases of POC flux with depth, resulting in little geographic variation in fluxes below ∼3000 m. The opposing effects of organic carbon production and calcification on ΔpCO2 of surface seawater are considered to calculate an “effective carbon flux” at the depth of the euphotic zone and at the base of the winter mixed layer. POC flux at the base of the euphotic zone integrated over the Atlantic Ocean between 65°N and 65°S amounts to 3.14 Gt C yr−1. Of this, 5.7% is remineralized above the winter mixed layer and thus does not contribute to CO2 sequestration on climatically relevant timescales. The effective carbon flux, termed Jeff, amounts to 2.47 Gt C yr−1 and is a measure of the potential sequestration of atmospheric CO2 for the area considered. A shift in the composition of sedimenting particles (seen in a decrease of the opal:carbonate ratio) is seen across the entire North Atlantic, indicating a basin‐wide phenomenon that may be related to large‐scale changes in climatic forcing.
Evaluation of a protocol for the quantification of black carbon in sediments
Authors: Gustafsson Örjan , Bucheli Thomas D. , Kukulska Zofia , Andersson Mette , Largeau Claude , Rouzaud Jean‐Noël , Reddy Christopher M. , Eglinton Timothy I.
Published: February 2004

Formation of highly condensed black carbon (BC) from vegetation fires and wood fuel combustion presumably transfers otherwise rapidly cycling carbon from the atmosphere‐biosphere cycle into a much slower cycling geological form. Recently reported BC fractions of total organic carbon (TOC) in surficial marine sediments span a wide range (2‐90%), leaving it presently unclear whether this variation reflects natural processes or is largely due to method differences. In order to elucidate the importance of BC to carbon burial the specificity of applied methods needs to be constrained. Here the operating range and applicability of a commonly used chemothermal oxidation (CTO) method is evaluated using putative BC standards, potentially interfering substances, and natural matrix standards. Test results confirm the applicability of the method to marine sediments. Integrity tests with model substrates suggest applicability to low‐carbon soils but only with a lower specificity to seawater particulate matter. The BC content of marine sediment samples in a set of studies employing the CTO method proved to be consistent with associated geochemical information. The radiocarbon content of the BC isolate in an environmental matrix standard was shown to be similar to the radiocarbon signature of pyrogenic polycyclic aromatic hydrocarbons (PAHs), here serving as molecular markers of combustion (fraction modern fM of BC was 0.065 ± 0.014 and of PAHs 0.056 ± 0.020), while being clearly distinct from the radiocarbon content of the bulk TOC (fM = 0.61 ± 0.08). Urgent questions such as the global accumulation rate of black carbon in soils and sediments may prove approachable with the chemothermal oxidation technique of BC quantification.
Comparison of alkenone estimates with other paleotemperature proxies
Authors: Bard Edouard
Published: February 2004

To a certain degree, all sea surface temperature (SST) proxies suffer from uncertainty due to signal preservation problems and to perturbations by other environmental variables. However, a multiproxy reconstruction is justified because most biases should, in principle, cancel out as they are specific to the biological groups and/or to the analyzed chemical species. Such a multiproxy strategy has proven crucial for calibrating these proxies and is also useful in assessing downcore profiles. Until now there have been only a dozen deep‐sea cores where it is possible to compare directly alkenone‐based SSTs with other paleotemperature records. From this still limited data set the broad picture is of general agreement for the amplitude of changes at low and middle latitudes. The few observed discrepancies suggest that alkenone‐based temperatures are more comparable to those based on Mg/Ca ratios than those obtained through statistical analysis of the foraminiferal distribution. More systematic differences are noted at higher latitudes, where alkenones indicate warmer temperatures during the glacial period than transfer functions based on foraminifera or diatoms. Several reasons have been invoked to resolve the mismatch, but more “ground truthing” work is needed before deciding which temperature changes are the most accurate.
Effects of biosynthesis and physiology on relative abundances and isotopic compositions of alkenones
Authors: González Elma L. , Riebesell Ulf , Hayes John M. , Laws Edward A.
Published: February 2004

The number of double bonds in long‐chain, unsaturated ketones (alkenones) produced by some members of the Haptophyceae is correlated with the ambient temperature at the time of synthesis. For these same organisms the depletion of carbon‐13 in biosynthetic products relative to dissolved inorganic carbon is related directly to the specific growth rate and inversely to the concentration of dissolved carbon dioxide. This report summarizes issues relating to the physiology, metabolism, and biochemistry of alkenone producers and how they affect the abundances and isotopic compositions of alkenones. These considerations show that an understanding of cellular responses to parameters governing uptake of inorganic carbon (Ci), isotopic fractionation, growth under diverse nutrient conditions, and genetic variability, both in the field and in culture, is necessary for developing a conceptual understanding of the biological significance of the ɛPand U37K′indices.Emiliana huxleyiis the best known alkenone producer and can serve as a model organism for these studies. This report identifies knowledge gaps and appropriate objectives for both field‐ and laboratory‐based research.
Modifications of the C37 alkenone and alkenoate composition in the water column and sediment: Possible implications for sea surface temperature estimates in paleoceanography
Authors: Grimalt Joan O. , Rullkötter Jürgen , Sicre Marie‐Alexandrine , Summons Roger , Farrington John , Harvey H. Rodger , Goñi Miguel , Sawada Ken
Published: February 2004

The literature pertaining to C37 alkenone and C37 and C38 alkenoate production and diagenesis has been reviewed and evaluated for issues that might jeopardize their usefulness in paleotemperature estimation. We also examined the use of the C37 alkenones as paleoproductivity indicators, the stability of their δ13C isotopic compositions, and their incorporation into the nonsolvent extractable organic matter fraction. Biological transformation of organic matter by bacteria and zooplankton does not appear to cause significant changes to the ratio of C37:2 and C37:3 alkenones, but there are major alterations in the relative composition of alkenoates. Studies of water column processes and postdepositional sedimentary changes indicate overall stability in the C37:2/(C37:2 + C37:3) ratio, leading to effective preservation of the paleotemperature signal. This is not the case for alkenoates, and there also appear to be some doubts about the dependence between sea surface temperature and the sedimentary abundance of the C37:4 alkenone. C37 alkenones may provide useful qualitative estimates of marine paleoproductivity. Although these compounds are apparently only derived from Haptophycea, changes in their quantitative composition might reflect changes in nutrient availability that on sedimentary timescales should also be generally significant of other algal species. One caveat is that in highly productive upwelling systems, productivity may be expressed mostly in diatom growth and thus not measurable by the C37 alkenones. No changes in δ13C C37 alkenone composition are observed upon zooplankton ingestion. Although this is expected from a theoretical perspective, further studies are encouraged in order to fully assess the stability of the isotopic signal during C37 alkenone diagenesis. Finally, alkenones bound in the nonextractable fraction of sedimentary organics have not been adequately studied and further work is required to clarify any possible correspondence with a temperature signal.
Mantle solidus: Experimental constraints and the effects of peridotite composition
Authors: Hirschmann Marc M.
Published: February 2004

A review of experiments on natural peridotites allows improved constraints on the location of the mantle solidus. Available constraints on the location of the nominally dry peridotite solidus show considerable scatter, owing to interlaboratory uncertainties and the effects of bulk composition. When experiments on enriched peridotite are filtered from the database, the best fit to the solidus between 0 and 10 GPa is given by T(°C) = aP2 + bP + c where a = −5.104, b = 132.899, and c = 1120.661 and P is in gigapascals. Compared to previous models, the solidus in this parameterization is at lower temperature between 2 and 6 GPa, with the largest differences near 4 GPa, where it is 30°–60°C cooler. Consideration of experimental constraints on the peridotite solidus and of a theoretical model of melting in a simple analogue system suggests that a key variable affecting peridotite solidus temperature is the near‐solidus liquid alkali concentration. The effect of alkalis on the solidus increases with bulk concentration in the peridotite but decreases with bulk partition coefficient. Thus small bulk concentrations of K can have a significant influence on the peridotite solidus, and the effect of Na diminishes with increasing pressure, as it becomes more compatible in the solidus residua. Mg # [=100 × MgO/(MgO + FeO)] variations are subordinate to alkali variations in controlling solidus temperature at lower pressures but may increase in relative importance as alkalis become more compatible in peridotite residua with increasing pressure. Increased clinopyroxene mode has the effect of making Na more compatible in residual solids and so diminishes the solidus‐lowering tendencies of alkalis. As a consequence, experiments performed on a range of peridotite compositions may not reflect the likely effect of variable mantle composition on solidus temperature if they do not match the appropriate correlation between alkali content and clinopyroxene mode.
Barium geochemistry in sediment pore waters and formation waters of the oceanic crust on the eastern flank of the Juan de Fuca Ridge (ODP Leg 168)
Authors: Monnin Christophe , Wheat C. Geoffrey , Dupre Bernard , Elderfield Henry , Mottl Mike M.
Published: February 2004

Sediment pore waters collected on the eastern flank of the Juan de Fuca Ridge at 10 sites drilled during Ocean Drilling Program (ODP) Leg 168 and basement fluids discharging at the seafloor on a basaltic outcrop called Baby Bare located at ∼100 km east of the Juan de Fuca Ridge were analyzed for barium. The data show an overall symmetrical behavior between Ba and sulfate, suggesting that the pore water Ba content is controlled by equilibrium with barium sulfate. Barite saturation indices calculated with a thermodynamic model of the Na‐K‐Ca‐Mg‐Ba‐Sr‐Cl‐SO4‐H2O system show that barite does not equilibrate with the pore waters at the shipboard conditions (20°C, 1 bar) but reflects an overall equilibrium between pore waters and barite at in situ temperatures and pressures, for depths below ∼20 m below the seafloor. Barite supersaturation is found for conditions in the upper 20 m of the sediment column. Ba concentrations in pore waters at the base of the sediment section exceed those in the underlying formation fluids, based on the direct sampling of these fluids in the open borehole at ODP Sites 1026 and the hydrothermal springs at Baby Bare. This suggests that Ba is supplied to basement by the sediment. Despite this, the formation fluids are undersaturated with respect to barite. These fluids, which contain ∼2–3 times more Ba than bottom seawater, vent at the seafloor resulting in a net flux of Ba to bottom seawater. The calculated flux from warm (63°C) ridge flank hydrothermal systems is insignificant to the oceanic budget for Ba, but the flux associated from cool (less than 25°C) ridge flank hydrothermal systems, which dispense most of the heat and fluid flux through the oceanic crust, is unconstrained.
Changes in dissolved oxygen in the Southern Ocean with climate change
Authors: Matear R. J. , Hirst A. C. , McNeil B. I.
Published: February 2004

Climate models predict that increasing greenhouse gases in the atmosphere will change our climate. However, the range of observations available to assess these model predictions is limited. We have utilized an ocean biogeochemical model to project the impact of climate change on the oxygen cycle in the ocean and the air‐sea exchange of oxygen. With climate change, our model predicts an outgassing of oxygen from the ocean. In the model, the Southern Ocean is the region that displays the largest change in the air‐sea exchange of oxygen with climate change. The reduced oxygen uptake in the Southern Ocean is dominated by a reduction in convective mixing and in the subduction rate of bottom water which decreases the amount of dissolved oxygen in the Southern Ocean. Our model results suggest that dissolved oxygen is sensitive to changes in Southern Ocean circulation revealing the potential to use dissolved oxygen to test model predictions of climate change. From Southern Ocean observations collected 28 years a part we observed a decrease in dissolved oxygen that was consistent with model climate change simulation. This provides credibility to the climate model predictions. Additional analysis and observations of dissolved oxygen are needed to substantiate our climate model simulations.
Late Pleistocene sea level variations derived from the Argentine Shelf
Authors: Guilderson T. P. , Burckle L. , Hemming S. , Peltier W. R.
Published: February 2004

Neritic‐littoral marine sedimentary deposits from the Argentine Shelf provide a record of late Pleistocene sea level variation. Sediments interpreted to reflect the last glacial maximum low‐stand are ∼150 m below present. Using a simple Airy isostatic model for hydroisostatic compensation and correcting for a minor tectonic component yields a local eustatic sea level lowering of ∼105 m at the last glacial maximum. The deglacial sea level curve records two rapid sea level rises consistent with MWP1‐A and 1‐B as documented by the Barbados coral‐based sea level curve. Comparison with relative sea level variations predicted by the ICE4G VM2 viscoelastic model have highlighted a deficiency in the model's predicted sea level history for this region. Detailed data‐model comparisons of late Pleistocene sea level variations are necessary in the face of climate change induced sea level perturbations to determine regional and or systematic biases in the treatment of lithosphere viscosity and accurate predictions of future sea level.
Geodetic constraints on coseismic rupture during the 1990 Ms 7.8 Luzon, Philippines, earthquake
Authors: Silcock David M. , Beavan John
Published: February 2004

We analyze single‐frequency GPS measurements taken before and after the July 16, 1990, Ms 7.8 Luzon earthquake to constrain the faulting parameters of this major strike‐slip event. The geodetic data imply fairly uniform left‐lateral slip of 5.5–6.5 m along the well‐mapped part of the fault trace, in agreement with field mapping of the rupture and in general accord with seismological estimates of the fault parameters. In addition, we find that substantial slip of about 4–5 m magnitude continued about 40–50 km northward or northwestward into the Cordillera Central beyond the end of the mapped rupture. This fault segment includes a substantial thrust component in addition to left‐lateral slip. The geodetic data favor a faulting depth of at least 20 km along the central part of the rupture, unusually deep for a major continental strike‐slip earthquake.
Evolution of palagonite: Crystallization, chemical changes, and element budget
Authors: Stroncik Nicole A. , Schmincke Hans‐Ulrich
Published: February 2004

The structural and chemical evolution of palagonite was studied as a function of glass composition, alteration environment, and time by applying a range of analytical methods (electron microprobe, infrared photometry, atomic force microscopy, X‐ray fluorescence, and X‐ray diffraction). Palagonitization of volcanic glass is a continuous process of glass dissolution, palagonite formation, and palagonite evolution, which can be subdivided into two different reaction stages with changing element mobilities. The first stage is characterized by congruent dissolution of glass and contemporaneous precipitation of “fresh,” gel‐like, amorphous, optically isotropic, mainly yellowish palagonite. This stage is accompanied by loss of Si, Al, Mg, Ca, Na, and K, active enrichment of H2O, and the passive enrichment of Ti and Fe. The second stage is an aging process during which the thermodynamically unstable palagonite reacts with the surrounding fluid and crystallizes to smectite. This stage is accompanied by uptake of Si, Al, Mg, and K from solution and the loss of Ti and H2O. Ca and Na are still showing losses, whereas Fe reacts less consistently, remaining either unchanged or showing losses. The degree and direction of element mobility during palagonitization was found to vary mainly with palagonite aging, as soon as the first precipitation of palagonite occurs. This is indicated by the contrasting major element signatures of palagonites of different aging steps, by the changes in the direction of element mobility with palagonite aging, and by the general decrease of element loss with increasing formation of crystalline substances in the palagonite. Considering the overall element budget of a water‐rock system, the conversion of glass to palagonite is accompanied by much larger element losses than the overall alteration process, which includes the formation of secondary phases and palagonite aging. The least evolved palagonitized mafic glass studied has undergone as much as 65 wt% loss of elements during palagonite formation, compared to ∼28 wt% element loss during bulk alteration. ABout 33 wt% element loss was calculated for one of the more evolved, in terms of the aging degree, rocks studied, compared to almost no loss for bulk alteration.
Algal and archaeal polyisoprenoids in a recent marine sediment: Molecular isotopic evidence for anaerobic oxidation of methane
Authors: Bian Liangqiao , Hinrichs Kai‐Uwe , Xie Tianmin , Brassell Simon C. , Iversen Neils , Fossing Henrik , Jørgensen Bo Barker , Hayes John M.
Published: February 2004

Analyses of 13C contents of individual organic molecules in a marine sediment show that crocetane, 2,6,11,15‐tetramethylhexadecane, an isomer of phytane, is produced by microorganisms that use methane as their main source of carbon. The sediments lie at a water depth of 68 m in the Kattegat, the strait between Denmark and Sweden. Crocetane appears first 185 cm below the sediment‐water interface, in the zone marking the transition from sulfate reduction to methanogenesis. Its δ13C value is −90 ± 10‰ versus Vienna Pee Dee Belemnite (VPDB). Its structure, which includes four isoprene units arranged symmetrically around a tail‐to‐tail linkage, suggests that it is produced by a member of the archaea. Growing at the intersection of the diffusion gradients for sulfate and methane in sedimentary pore waters, the source organism apparently functions as a methane‐consuming member of the microbial consortium responsible for the anaerobic oxidation of methane [Hoehler et al., 1994], in which, as first demonstrated quantitatively in these sediments [Iversen and Jørgensen, 1985], electrons are transferred from methane to sulfate. The presence of archaeal biomass throughout the sediment section is indicated by significant concentrations of 2,6,10,15,19‐pentamethylicosane (PMI) and of ether‐bound phytane and biphytane. The PMI reaches a minimum δ value of −47‰ well below the transition zone. Its isotopic depletion could reflect either methanogenic or methanotrophic sources. The ether‐bound lipids are isotopically uniform throughout the section and are presumed to derive from archaea that utilize a carbon source unaffected by the oxidation of methane.