bluuebunny/crossref_metadata_2025 huggingface dataset:
Cutover peatlands: A persistent source of atmospheric CO2
Authors: Waddington J. M. , Warner K. D. , Kennedy G. W.
Published: October 2002

Peatlands represent an important component of the global carbon cycle, storing 23 g C m−2 yr−1. Peatland mining eliminates the carbon sink function of the peatland. In this paper we measure the total ecosystem respiration in a natural, 2 and 3 year (young) and 7 and 8 year (old) postcutover peatland near Sainte‐Marguerite‐Marie, Québec, during the summers of 1998 and 1999. Although the natural site was a source of CO2 during the dry 1998 study season (138 g C m−2), CO2 emissions were between 260 and 290% higher in the cutover sites (363 and 399 g C m−2 for young and old, respectively). Cutover site CO2 emissions were only 88 and 112 g CO2‐C m−2 at the young and old sites during the wet 1999 study season. Total ecosystem respiration was more dependent on the water table position than on changes in the thermal regime or the labile carbon of the peat in a dry summer, but the opposite was the case in a wet summer. CO2 emissions increased with postharvest time regardless of a decrease in labile carbon, demonstrating that cutover peatlands are a large persistent source of atmospheric CO2. Direct measurement of the net ecosystem CO2 exchange in cutover peatlands, as opposed to determining the loss of carbon from bulk density determinations, provides a better understanding of how peat drainage and harvesting operations affect the carbon balance in peatlands.
Growth rate dependence of Sr incorporation during calcification of Emiliania huxleyi
Authors: Rickaby R. E. M. , Schrag D. P. , Zondervan I. , Riebesell U.
Published: October 2002

Reconstructing the dissolved carbon dioxide of surface waters from the δ13C of organic carbon relies on accurate determination of the growth rate of phytoplankton contributing to sedimentary organic matter. We demonstrate that the Sr/Ca ratio in the coccoliths of Emiliania huxleyi is correlated with the rates of both organic carbon fixation and calcification. An investigation of biomineralization models suggests that these three factors may be linked by the differential pumping rates of calcium relative to strontium ions across the cellular membranes which supply ions for photosynthetic carbon fixation and calcification and derives energy from photosynthetic products. A quantitative relationship is derived between calcification, organic carbon fixation, and Sr/Ca such that a combined geochemical proxy approach using εp and Sr/Ca can be applied to the paleoceanographic record.
Carbon isotopic composition of the C37:2 alkenone in core top sediments of the South Atlantic Ocean: Effects of CO2and nutrient concentrations
Authors: Benthien Albert , Andersen Nils , Schulte Sonja , Müller Peter J. , Schneider Ralph R. , Wefer Gerold
Published: October 2002

We have analyzed the stable carbon isotopic composition of the diunsaturated C37 alkenone in 29 surface sediments from the equatorial and South Atlantic Ocean. Our study area covers different oceanographic settings, including sediments from the major upwelling regions off South Africa, the equatorial upwelling, and the oligotrophic western South Atlantic. In order to examine the environmental influences on the sedimentary record the alkenone‐based carbon isotopic fractionation (εp) values were correlated with the overlying surface water concentrations of aqueous CO2 ([CO2(aq)]), phosphate, and nitrate. We found εp positively correlated with 1/[CO2(aq)] and negatively correlated with [PO43−] and [NO3−]. However, the relationship between εp and 1/[CO2(aq)] is opposite of what is expected from a [CO2(aq)] controlled, diffusive uptake model. Instead, our findings support the theory of Bidigare et al. [1997]that the isotopic fractionation in haptophytes is related to nutrient‐limited growth rates. The relatively high variability of the εp−[PO4] relationship in regions with low surface water nutrient concentrations indicates that here other environmental factors also affect the isotopic signal. These factors might be variations in other growth‐limiting resources such as light intensity or micronutrient concentrations.
Separating methane production and consumption with a field‐based isotope pool dilution technique
Authors: von Fischer Joseph C. , Hedin Lars O.
Published: October 2002

Despite the importance of methane for climate, it has remained difficult to measure gross rates of methane production and consumption without inducing artifacts. To remedy this, we have developed, tested, and applied a field‐based 13CH4 pool dilution technique. Laboratory tests, sensitivity analyses, and field data indicate that this technique is robust for measuring gross rates of methane production and consumption. In our analyses of 130 soil cores from 17 field sites of differing environmental conditions, we encountered a wide range of gross methane production rates (0.04–930 mg CH4‐C m−2 day−1), but encountered a narrower range of consumption rates (0.1–9.2 mg CH4‐C m−2 day−1). Unexpectedly, we found that gross production of methane was common (mean = 0.15 mg CH4‐C m−2 day−1) even in dry, oxic soils where average soil conditions cannot support methane producers. Through improved measurement of methane turnover in soils, this technique can offer a more fine‐grained understanding of how productive and consumptive processes are linked to soil‐atmosphere trace gas balances.
Patchy productivity in the open ocean
Authors: Martin Adrian P. , Richards Kelvin J. , Bracco Annalisa , Provenzale Antonello
Published: October 2002

Localized upwelling is a strong driver of primary production in many regions of the open ocean. This is achieved through providing an increased vertical flux of nutrients to otherwise starved surface waters. The impact of such upwelling hot spots on regional production is investigated with particular emphasis on three parameters: the fraction of the region experiencing upwelling, the increase in nutrient flux within upwelling regions with respect to ambient waters, and the rate of horizontal mixing between upwelled and ambient waters. It is demonstrated that although independently increasing one of these parameters has a limited effect on the production of the region, increasing them simultaneously can dramatically increase production. Furthermore, it is shown that the spatial distribution of upwelling regions can have a strong influence on productivity. Numerous small upwelling regions increase production significantly more than one large upwelling hot spot even if the total rate of upwelling is constant for the two cases. The magnitude of this discrepancy is shown to be influenced by the presence of coherent structures such as eddies in the surface ocean. The ocean components of global carbon cycle models are still far from being able to resolve mesoscale features fully. These results imply that such models may be greatly underestimating primary production in large parts of the ocean as a consequence.
Interannual growth rate variations of atmospheric CO2 and its δ13C, H2, CH4, and CO between 1992 and 1999 linked to biomass burning
Authors: Langenfelds R. L. , Francey R. J. , Pak B. C. , Steele L. P. , Lloyd J. , Trudinger C. M. , Allison C. E.
Published: October 2002

High‐precision, multispecies measurements of flask air samples since 1992 from CSIRO's global sampling network reveal strong correlation among interannual growth rate variations of CO2 and its δ13C, H2, CH4, and CO. We show that a major fraction of the variability is consistent with two emission pulses coinciding with large biomass burning events in 1994/1995 and 1997/1998 in tropical and boreal regions, and observations of unusually high levels of combustion products in the overlying troposphere at these times. Implied pulse strengths and multispecies emission ratios are not consistent with any other single process, but do not exclude possible contributions from covarying processes that are linked through climatic forcing. Comparison of CO2 with its δ13C indicates that most of the CO2 variation is from terrestrial exchange, but does not distinguish forcing by biomass burning from imbalance in photosynthesis/respiration of terrestrial ecosystems. Partitioning of terrestrial CO2 fluxes is constrained by H2, CH4, and CO, all of which are products of biomass burning but which have no direct link to net respiration of CO2. While CO is a strong indicator of biomass burning, its short lifetime prevents it from usefully constraining the magnitude of CO2 emissions. If the H2 and CH4 variations were dominated by biomass burning, they would imply associated carbon emissions in excess of mean annual levels of other years, of 0.6–3.5 and 0.8–3.7 Pg C for 1994/1995 and 1997/1998, respectively. The large range in emission estimates mainly reflects uncertainty in H2/CO2 and CH4/CO2 emission ratios of fires in these years.
Biogeochemical constraints on the Triassic‐Jurassic boundary carbon cycle event
Authors: Beerling D. J. , Berner R. A.
Published: October 2002

The end‐Triassic mass extinctions represent one of the five most severe biotic crises in Earth history, yet remain one of the most enigmatic. Ongoing debate concerns the environmental effects of the Central Atlantic Magmatic Province (CAMP) eruptions and their linkage with the mass extinction event across the Triassic‐Jurassic boundary. There is conflicting paleo‐evidence for changes in atmosphericpCO2during the extrusion of the CAMP basalts. Studies on sediments from European and Pacific localities have, however, identified a substantial negative isotopic anomaly (up to −3.5‰) across the TR‐J boundary, providing an important indicator of changes in the operation of the ancient global carbon cycle. We sought to explain the paleo‐evidence by utilizing a carbon cycle model for the “hothouse” world of the end‐Triassic that emphasizes the chemical weathering of silicate and carbonate rocks and the ocean carbonate chemistry. We find that volcanic CO2outgassing fails to fully account for either a sufficient rise in atmosphericpCO2(indicated by the stomata of fossil leaves) or the sedimentary isotopic fingerprint. Instead, the scenario that best fits all of the geologic evidence is a positive feedback loop in which warming, due to a buildup of volcanically derived CO2, triggers destabilization of seafloor methane hydrates and the catastrophic release of CH4[Pálfy et al., 2001]. We calculate that this carbon cycle perturbation was huge, involving the release of ∼8000–9000 Gt C as CO2during the CAMP basaltic eruptions and ∼5000 Gt C as CH4. In the model the initial isotopic excursion is assumed to take place over ∼70 kyr, while complete reequilibration of the ocean‐atmosphere system with respect to CO2is accomplished over 700–1000 kyr. Our results thus provide a preliminary theoretical explanation for the bioevents, estimatedpCO2changes, and isotopic excursions observed in marine and continental sediments at this time.
Regional water balance trends and evaporation‐transpiration partitioning from a stable isotope survey of lakes in northern Canada
Authors: Gibson J. J. , Edwards T. W. D.
Published: October 2002

Regional variations in evaporation losses and water budget are interpreted from systematic isotopic patterns in surface waters across a 275,000 km2 region of northern Canada. Differential heavy isotope enrichment in a set of >255 nonheadwater lakes sampled by floatplane during 1993 and 1994 is strongly correlated to varying hydroclimatic conditions across the region. Calculated catchment‐weighted evaporation losses typically range from ∼10–15% in tundra areas draining into the Arctic Ocean to as high as 60% in forested subarctic areas draining to the Mackenzie River via Great Bear or Great Slave Lakes. Because of the diversity in drainage order and the ratio of catchment to surface area, lakes in the region may inherit as little as 30% to as much as 99% of their isotopic enrichment signal from upstream water bodies. Open‐water evaporation generally decreases with increasing latitude and accounts for 5–50% of total evapotranspiration. Coupling of meteorological and isotopic data permits a novel assessment of regional evaporation‐transpiration flux partitioning in the three major ecoclimatic zones (high‐boreal forest, subarctic forest‐tundra, and low‐arctic shrub tundra), while the differing frequency distributions of lake water balance in these zones provides a new index of landscape‐scale hydroclimatology that may have significant potential for investigating ongoing (or past) changes in response to high‐latitude climate change.
Near mantle solidus trace element partitioning at pressures up to 3.4 GPa
Authors: Salters Vincent J. M. , Longhi John E. , Bizimis M.
Published: October 2002

We present new experimental partitioning data for a range of petrogenetically important elements at pressures of up to 3.4 GPa. The experiments are designed to mimic low degrees of anhydrous melting beneath mid‐ocean ridges. The available data indicate that the partition coefficients are pressure, temperature, and composition dependent. Therefore partitioning behavior over the appropriate range of pressure, temperature, and composition must be quantified, in order to model continuous extraction of melt during the adiabatic rise of mantle material. For this purpose, we have parameterized the partitioning behavior of the REE, Hf, Zr, U, and Th based on a simple thermodynamic model. Although these parameterizations cannot be used for retrieving thermodynamic constants yet, they do yield accurate descriptions of the partitioning behavior that are useful for modeling decompression melting. Our parameterizations show that the partitioning of trace elements is strongly dependent on the Ca and Al‐content of the clinopyroxene (cpx) and REE are always incompatible in cpx on the peridotite solidus at pressures up to 3.4 GPa. For garnet the data indicate that the heavy REE partition coefficients decrease with increasing pressure. Our data also indicates that Pb is more incompatible than Ce in clinopyroxene; Ce and Pb have similar partition coefficients in garnet. Therefore the presence of a residual phase with high Pb partition coefficients is required to produce the near‐constant Ce/Pb ratios in MORB and OIB. Sulfides are the most likely phase to buffer the Pb content in the melt. Except at small porosities (<0.3%), clinopyroxene on the peridotite solidus is unable to fractionate U from Th significantly (15% 230Th‐excess), whereas garnet can fractionate U from Th effectively at porosities up to 1%. Therefore if the 230Th‐excesses in mid‐ocean ridge basalts are melting phenomena, then melting with garnet residual is required in order to be compatible with physical observations on porosities and upwelling rate at mid‐ocean ridges. New model calculations that include the compositional dependent partitioning of the trace elements show that the predicted physical characteristics (depth and extent of melting, upwelling rate, porosity) of the MORB melting regime are similar for the Lu/Hf, Sm/Nd, and U‐Th systems.
An intensity ratio calibration method for the accurate determination of Mg/Ca and Sr/Ca of marine carbonates by ICP‐AES
Authors: de Villiers Stephanie , Greaves Mervyn , Elderfield Henry
Published: October 2002

An inductively coupled plasma‐atomic emission spectroscopy (ICP‐AES) method for the accurate and precise simultaneous measurement of the Mg/Ca and Sr/Ca content of carbonates was established. While a precision of <0.3% (1σ standard deviation (SD)) is easily obtainable for both Mg/Ca and Sr/Ca analysis, a Ca matrix effect complicates achieving similar levels of accuracy with conventional calibration procedures. An alternative ratio calibration procedure is proposed which overcomes the Ca matrix effects and ensures the accuracy of the Mg/Ca and Sr/Ca analysis of marine carbonates to <0.3%, almost an order of magnitude better than conventional calibration methods. The longer‐term precision is <0.1% if the batch run average values are corrected for longer‐term drift. The method is suitable for analysis of foraminiferal calcite and coral aragonite and can easily be adjusted for the analysis of other carbonates or microsamples.
The Line Islands revisited: New 40Ar/39Ar geochronologic evidence for episodes of volcanism due to lithospheric extension
Authors: Davis A. S. , Gray L. B. , Clague D. A. , Hein J. R.
Published: October 2002

New 40Ar/39Ar ages of mineral separates and whole rock samples from nine volcanic edifices in the northern Line Islands region, between latitudes 20°N and 6°N, are incompatible with single or multiple hot spot models. Instead, two major episodes of volcanism, each lasting ∼5 Ma and separated by ∼8 Ma, occurred synchronously over long distances, not just along the main chain but also at nonaligned edifices. Volcanism during the older episode (81–86 Ma) extended over a distance of at least 1200 km along the eastern part of the complex seamount chain. Volcanism during the younger episode (68–73 Ma) was concentrated in the western part of the chain and may have extended over a distance of >4000 km. Chemical analyses of 68 samples represent a compositionally diverse suite, including tholeiitic, transitional, and alkalic basalt, strongly alkalic basanite and nephelinite, and alkalic differentiates ranging from hawaiite to trachyte. The most diverse assemblage of rocks was recovered from a cross‐trending seamount chain south of Johnston Atoll. Although compositions of rocks from the two volcanic episodes overlap, compositions from the younger episode generally are more alkalic and include a larger proportion of highly differentiated compositions. None of the samples from the older episode, but many from the younger one, contain hydrous mineral phases such as amphibole and biotite. Extensive coeval volcanism along major segments of the chain is compatible with decompressional melting of heterogeneous mantle due to diffuse lithospheric extension along pre‐existing zones of weakness. Episodes of volcanism are probably related to broad upwarping of the Superswell region in the eastern South Pacific, where these lavas originated.
Insights into volcanic conduit flow from an open‐source numerical model
Authors: Mastin Larry G.
Published: October 2002

Numerical models that calculate the fluid dynamics of explosive volcanic eruptions have been used with increasing frequency to understand volcanic processes and evaluate volcanic hazards. Yet those who develop such models rarely make them publicly available so that they can be verified, used, and possibly improved by other scientists. In this paper I present a visual, interactive, open‐source numerical model that calculates steady state flow of magma and gas in vertical eruptive conduits and contains user‐friendly utilities for quickly determining physical, thermodynamic, and transport properties of silicate melts, H2O gases, and melt‐gas‐crystal mixtures. The model represents an advance over previously published conduit models by incorporating a non‐Arrhenian viscosity relation for hydrous silicate melts, a relation between viscosity and volume fraction of gas that depends on Capillary number, and adiabatic temperature changes in the mixture using established thermodynamic relations for melts and H2O gas, respectively. Volcanologists who have not had access to conduit models have frequently approximated conduit flow using an analytical equation for incompressible, laminar, Newtonian pipe flow, which predicts that the mass flux is proportional to the fourth power of conduit radius and inversely proportional to mixture viscosity. The model presented here, which is not much more difficult to use than a back‐of‐the‐envelope calculation, shows that the pipe‐flow approximation significantly overestimates the sensitivity of mass flux to both conduit radius and mixture viscosity. Results from the model also show that viscous heating in the lower conduit, which is not considered in most other models, may increase the mass flux of large silicic eruptions by several percent and decrease the viscosity of the mixture at the fragmentation depth by a few tens of percent.
Core top calibration of Mg/Ca in tropical foraminifera: Refining paleotemperature estimation
Authors: Dekens Petra S. , Lea David W. , Pak Dorothy K. , Spero Howard J.
Published: October 2002

Optimal use of Mg/Ca as a paleotemperature proxy requires establishing calibrations for different species of foraminifera and quantifying the influence of dissolution. To achieve this goal, we have measured Mg/Ca and δ18O in a series of tropical and subtropical core tops, including four depth transects: the Ceara Rise, the Sierra Leone Rise, and the Rio Grande Plateau in the Atlantic, and the Ontong Java Plateau in the Pacific, focusing on spinose mixed layer dwelling species Globigerinoides ruber and Globigerinoides sacculifer, and nonspinose thermocline dwelling Neogloboquadrina dutertrei. Shell Mg/Ca in G. sacculifer is 5–15% lower than in G. ruber, while N. dutertrei Mg/Ca is 49–55% lower than in G. ruber. This statistically significant offset has allowed us to establish different calibrations for each species. Multilinear regression analysis was used to develop calibration equations that include a correction term for the dissolution effect on Mg/Ca in foraminiferal calcite. Presented in this paper are two sets of calibrations; one set using core depth as a dissolution correction and another using ΔCO32− as a dissolution parameter. The calibrations suggest that G. ruber is the most accurate recorder of surface temperature, while G. sacculifer records temperatures below the surface at 20–30 m. The depth habitat of N. dutertrei is more uncertain, owing to the wide range in habitat depths depending on hydrographic conditions, but on average, Mg/Ca and δ18O data suggest it is at ∼50 m. Of the three species, N. dutertrei is the most sensitive to dissolution (up to 23% decrease in shell Mg/Ca per km), while G. sacculifer is the most resistant.
Physical interpretation of hysteresis loops: Micromagnetic modeling of fine particle magnetite
Authors: Tauxe Lisa , Bertram H. Neal , Seberino Christian
Published: October 2002

Hysteresis measurements have become an important part of characterizing magnetic behavior of rocks in paleomagnetic studies. Theoretical interpretation is often difficult owing to the complexity of mineral magnetism and published data sets demonstrate remanence and coercivity behavior that is currently unexplained. In the last decade, numerical micromagnetic modeling has been used to simulate magnetic particles. Such simulations reveal the existence of nonuniform remanent states between single and multidomain, known as the “flower” and “vortex” configurations. These suggest plausible explanations for many hysteresis measurements yet fall short of explaining high saturation remanence, high coercivity data such as those commonly observed in fine grained submarine basalts. In this paper, we review the theoretical and experimental progress to date in understanding hysteresis of geological materials. We extend numerical simulations to a greater variety of shapes and sizes, including random assemblages of particles and shapes more complex than simple rods and cubes. Our simulations provide plausible explanations for a wide range of hysteresis behavior.
A data brief on the Smithsonian Abyssal Volcanic Glass Data File
Authors: Melson William G. , O'Hearn Timothy , Jarosewich Eugene
Published: October 2002

It is well known that the compositions of glasses quenched by contact with seawater are a good guide to unmodified erupted liquid compositions. We present here an updated version of the Smithsonian Abyssal Volcanic Glass Data File (AVGDF) of the major and minor element compositions of such glasses mainly from seafloor spreading centers determined by electron microprobe analyses. These data help define the major and minor element compositions of the most abundant terrestrial volcanic rock. The AVGDF contains 9050 individual glass analyses along with the sample donor, latitude, longitude, depth, tectonic setting, ship‐cruise‐dredge, and the major and minor elements SiO2, Al2O3, FeO*, MgO, CaO, Na2O, K2O, TiO2, P2O5, and MnO (for some samples). The samples are from 664 Atlantic, 28 Caribbean, 89 Indian, and 1304 Pacific localities. These include dredge, submersible, DSDP and ODP sites. The data serve to (1) map out and model processes associated with magma generation and fractionation in the asthenosphere and lithosphere beneath seafloor spreading centers and (2) as an educational aide for exercises in the use of such data at the graduate and undergraduate levels in geochemistry, geophysics, and igneous petrology. The extensive polished sample collection is available on loan for additional petrographic (in reflected light) or microbeam analyses.
Lead isotope systematics of sulfide minerals in the Middle Valley hydrothermal system, northern Juan de Fuca Ridge
Authors: Cousens Brian L. , Blenkinsop John , Franklin James M.
Published: October 2002

The sources of metals for the sediment‐hosted massive sulfide deposits currently forming at the sediment‐filled Middle Valley segment of the northern Juan de Fuca Ridge have been investigated using Pb isotopes. Leg 139 of the Ocean Drilling Program drilled three sites in Middle Valley at which basaltic rocks, sediments, and sulfide mineralization were recovered. At site 856, massive sulfides composed of pyrite, pyrrhotite, minor chalcopyrite, and sphalerite have isotopic compositions intermediate between Juan de Fuca basalts and Middle Valley turbiditic sediments, with the exception of two samples that are relatively nonradiogenic. Primary pyrrhotite‐dominated and secondary pyrite‐dominated sulfides overlap in isotopic composition, indicating that both high‐ and low‐temperature hydrothermal fluids have interacted with the sediment pile. Drilling at sites 857 and 858 intersected hydrothermally altered basaltic sill‐sediment complexes containing disseminated aggregates and veins of secondary hydrothermal sulfides. The sulfide minerals have highly variable, continental crust‐like Pb ratios with Stacey‐Kramers model ages ranging from zero to 1.5 Ga (206Pb/204Pb ∼ 16.2). Some massive sulfide samples from site 856 also include an “old” sedimentary Pb component. The source of this old Pb is most likely Proterozoic detrital sulfides from turbidites within Middle Valley, even though the dominant source of detritus is thought to be the Mesozoic accreted terranes of Vancouver Island. The relative abundances of Cu and Zn in Middle Valley massive sulfides do not correlate with Pb isotopic composition, probably due to similar Cu/Zn in basalt and sediment components.
Precambrian marine carbonate isotope database: Version 1.1
Authors: Shields Graham , Veizer Ján
Published: October 2002

We present a compilation of strontium, carbon, and oxygen isotope compositions of roughly 10,000 marine carbonate rocks of Archean ‐ Ordovician age (3800 Ma – 450 Ma). The Precambrian Marine Carbonate Isotope Database (PMCID) has been compiled from 152 published and 3 unpublished articles and books of the past 40 years. Also included are 30 categories of relevant “metadata” that allow detailed comparisons and quality assessments of the isotope data to be made. The PMCID will be updated periodically as new data and better age constraints come to light. Here we outline the structure of the first published version of the database and its inherent merits and limitations.
Composition, age, and provenance of organic matter in NW African dust over the Atlantic Ocean
Authors: Eglinton T. I. , Eglinton G. , Dupont L. , Sholkovitz E. R. , Montluçon D. , Reddy C. M.
Published: October 2002

Eolian dust deposited on a meteorology buoy over the boreal winter and spring of 1992/1993 in the Northeast Atlantic off Northwest Africa (18°N, 22°W) afforded sufficient material for detailed assessment of its biogeochemical characteristics and provenance. The sample was subjected to microscopic examination and bulk, elemental, isotopic, black carbon, and molecular (lignin, lipid) analyses. The bulk elemental composition and organic carbon (OC) content (1.02% dry weight) establishes the dust's origin as continental upper crust and is typical of dusts that emanate from West Africa. These data are in accord with the extensive satellite imagery documenting the transit of dust clouds from the Sahara and Sahel regions (e.g., NASA TOMS aerosol index). Microscopic examination reveals that charcoal‐like particles from vegetation fires (“black carbon” (BC)) are the most abundant morphologically distinct organic components. Accelerator mass spectrometry (AMS) 14C analysis yields conventional 14C ages for bulk OC of 1260 ± 40 years and BC of 2070 ± 35 years. Taken together with corresponding stable carbon isotopic compositions (δ13C, −18.9 and −15.1‰, respectively), these results suggest the presence of biomass and burning residues derived from predominantly C4 vegetation that accumulated in soils over the late Holocene. Molecular‐level measurements are also consistent with this scenario. For example, lignin‐derived phenols in the dustfall sample indicate a significant contribution of angiosperm grass‐type tissue. The altered nature of the lignin suggests storage as residues in soils. Solvent extractable (lipid) constituents include a marked leaf wax type component with a molecular and stable carbon isotopic composition similar to those observed in surface marine sediments in this region. The hydrocarbon fraction contained some diesel‐type contamination but the n‐alkanes (C23–C33) retained a prominent odd/even distribution, with δ13C ranging from −26.7 to −28.5‰. The suite of even carbon numbered n‐alkanols (C22–C32) is 14C dated at 647 ± 150 years, with δ13C values ranging from −23.9 to −30.4‰. The long‐chain n‐alkanoic acids (C22–C32) exhibited δ13C values ranging from −22.6 to −27.4‰. These waxes are evidently a mixed signal derived from contemporary C3 and C4 vegetation and from ablated soils and desiccated lake sediments of middle to late Holocene age. This molecular approach shows promise as a tool for continental paleoenvironmental assessment, particularly with respect to past vegetation cover, regional aridity, and wind systems.
A simple evaluation of cleaning procedures on fossil benthic foraminiferal Mg/Ca
Authors: Martin Pamela A. , Lea David W.
Published: October 2002

Recent studies suggest that benthic foraminiferal Mg/Ca may be useful in reconstructing deep water temperature changes during the Quaternary and on longer timescales [Rosenthal et al., 1997; Lear et al., 2000; Martin et al., 2002]. Because changes in deep water temperature may be small, sample preparation could play an important role in reconstructing accurate temperatures from shell Mg/Ca. A simple examination of changes in measured shell chemistry of benthic foraminifera from two intervals subjected to increasingly rigorous cleaning reveals that fossil benthic foraminiferal samples cleaned by oxidation yield lower Mg/Ca values than samples only rinsed and weak acid leached. Reductive cleaning further lowered the shell Mg/Ca of one of the two benthic species examined. Lower Mg/Ca values following oxidation and reduction appear to reflect removal of contaminant Mg associated with remnant organic matter and adsorbed phases.
Geotectonic setting of hydrothermal activity on the summit of Lucky Strike Seamount (37°17′N, Mid‐Atlantic Ridge)
Authors: Humphris Susan E. , Fornari Daniel J. , Scheirer Daniel S. , German Christopher R. , Parson Lindsay M.
Published: October 2002

We have investigated the relations between volcanic, tectonic, and hydrothermal activity on Lucky Strike Seamount (37°17′N, Mid‐Atlantic Ridge) using a nested survey strategy involving collection of data from different deep‐sea mapping systems. The highly tectonized seamount summit consists of three volcanic cones surrounding a relatively flat depression with a young lava lake in its center. Hydrothermal activity is focused mainly within the summit depression with most of the vents located proximal to the lava lake. Isolated active and inactive chimneys and mounds are widespread throughout the summit depression and occur on both volcanic (pillow lava) and hydrothermal (sulfide rubble and hydrothermally cemented breccias) substrates. The large volume of sulfide rubble, together with the nature of the sulfide structures, indicates that hydrothermal activity has been episodic but ongoing for a long period of time (hundreds to thousands of years). On the basis of the distribution of hydrothermal deposits, we propose a model of alternation between tectonic and volcanic control on hydrothermalism at Lucky Strike Seamount. Midsegment melt focusing produces a spatially and temporally stable heat source that sustains focused high‐temperature hydrothermal activity over long time periods. During periods of amagmatic extension, active faulting within the summit depression provides multiple, near‐surface fluid flow pathways for discharge of high‐temperature fluids and widespread deposition of massive sulfides. During eruptive events, rapid effusion of very hot lava creates a lava lake and hyaloclastite deposits. The new sheet flows form a cap on the hydrothermal system, and fluid upflow is reorganized. Discharge of high‐temperature fluids is restricted to isolated sites with relatively high permeability, for example, the edges of the lava lake. Much of the upwelling hydrothermal fluid pools in the subsurface, conductively cools, and mixes with entrained seawater before discharging as widespread low‐temperature diffuse flow. Hyaloclastites become cemented, further augmenting the sealing of the system. Present‐day activity at Lucky Strike Seamount represents this locally volcanically controlled phase of activity, despite the segment as a whole being dominantly amagmatic.
Microbial fixation of methane carbon at 2.7 Ga: Was an anaerobic mechanism possible?
Authors: Hinrichs Kai‐Uwe
Published: October 2002

In Earth's history, the most extreme episode of 13C‐depletion in sedimentary organic matter occurred about 2.7 billion years ago. This isotopic excursion is widely interpreted as resulting from increased burial of organic carbon derived from microbial fixation of 13C‐depleted methane. Modern microbial physiologies involve either oxygen or sulfate as electron acceptors in such processes. Until now, the isotopic signal has been exclusively assigned to growth of aerobic, methanotrophic bacteria and consequently interpreted as an indication for the presence of molecular oxygen in Late Archaean surface environments. The alternative anaerobic pathway involving sulfate as the electron acceptor was dismissed because this process has been considered inefficient in producing biomass sufficient to account for significant fractions of 13C‐depleted carbon in some Archaean rocks. Calculations based on recent findings at two modern sites now show, however, that anaerobic processes could account for the Archaean signal.
Observationally based assessment of polar amplification of global warming
Authors: Polyakov Igor V. , Alekseev Genrikh V. , Bekryaev Roman V. , Bhatt Uma , Colony Roger L. , Johnson Mark A. , Karklin Valerii P. , Makshtas Alexander P. , Walsh David , Yulin Alexander V.
Published: October 2002

Arctic variability is dominated by multi‐decadal fluctuations. Incomplete sampling of these fluctuations results in highly variable arctic surface‐air temperature (SAT) trends. Modulated by multi‐decadal variability, SAT trends are often amplified relative to northern‐hemispheric trends, but over the 125‐year record we identify periods when arctic SAT trends were smaller or of opposite sign than northern‐hemispheric trends. Arctic and northern‐hemispheric air‐temperature trends during the 20th century (when multi‐decadal variablity had little net effect on computed trends) are similar, and do not support the predicted polar amplification of global warming. The possible moderating role of sea ice cannot be conclusively identified with existing data. If long‐term trends are accepted as a valid measure of climate change, then the SAT and ice data do not support the proposed polar amplification of global warming. Intrinsic arctic variability obscures long‐term changes, limiting our ability to identify complex feedbacks in the arctic climate system.
Fluid‐driven seismicity in a stable tectonic context: The Remiremont fault zone, Vosges, France
Authors: Audin Laurence , Avouac Jean‐Philippe , Flouzat Mireille , Plantet Jean‐Louis
Published: October 2002

Some relocated seismic events, which have small magnitudes (ML < 4.8), are found to align along a 40 km‐long fault zone flanking the southern Vosges Massif to the west. It joins to the south with the epicentral area of the historical 1682 earthquake (Io = VIII MSK). The Remiremont cluster was preceded by a period of seismic coalescence and triggered outward of bilateral seismic migration. The 1984 seismic crisis developed along a well defined 3 km‐long vertical plane. In both cases, migration rates of the order of 5–10 km/yr over 30 km‐long distances are determined. This pattern requires some mechanism of stress interaction which must act over distances of the order of 1 to 20 km within years. Given the low tectonic activity and the magnitudes of the events the stress transfer cannot result from co‐seismic elastic loading or from transient strain at depth. We suggest that the seismic activity reflect rupture of asperities driven by fluid‐flow in a zone of relatively high permeability.
IMAGE and FAST observations of substorm recovery phase aurora
Authors: Mende Stephen B. , Frey Harald U. , Carlson Charles W. , McFadden J. , Gérard Jean‐Claude , Hubert Benoit , Fuselier Stephen A. , Gladstone G. R. , Burch James L.
Published: October 2002

Images from the IMAGE Wide‐band Imaging Camera (WIC) and Spectrographic Imager (SI) channel SI12, were compared to in situ data taken by FAST. The IMAGE data segment began during the expansive phase of a substorm and a double oval configuration evolved, consisting of a set of discrete poleward auroral forms and a separate more diffuse oval. The FAST data showed that a narrow (∼1.5° latitude) region of downward currents separated the two ovals. The SI‐12 optical observations showed a single oval of precipitating protons located on the equatorward side within the diffuse aurora. In agreement with IMAGE, the highest intensity proton flux measured by FAST was concentrated on the equatorward region although low flux protons were present throughout the entire double oval. In the lower latitude diffuse oval occasional structured auroras were embedded. These structured auroras were mostly created by inverted V type electrons but there were narrow regions in which intense beams of accelerated electrons were seen whose energy/pitch angle distribution and accompanying electric field data were consistent with Alfven wave acceleration. The poleward oval consisted of an intense inverted V precipitation event poleward of which a weak region of Alfven wave accelerated electrons was located. From the images it appears that the Alfven wave accelerated electron event in the diffuse auroral regions and the poleward features were part of short lived or rapidly moving auroral forms.
Decrease of river runoff in the upper waters of the Eurasian Basin, Arctic Ocean, between 1991 and 1996: Evidence from δ18O data
Authors: Schlosser Peter , Newton Robert , Ekwurzel Brenda , Khatiwala Samar , Mortlock Rick , Fairbanks Rick
Published: October 2002

Measurements of the H218O/H216O ratio of water from two sections crossing the Eurasian Basin in 1991 and 1996 show that the observed decrease in the freshwater contained in the upper waters of the Eurasian Basin during the 1990s is due to decrease in meteoric water (mainly river runoff). The decrease in meteoric water inventories between 1991 and 1996 calculated from balances of mass, salinity, δ18O, and PO4* is about 3 meters and accounts for basically the entire freshwater change inferred from salinity budgets. Climatological data are used to ensure that the two sections can be considered to belong to the same hydrographic regime. The data also suggest that in 96 the formation of sea ice from the upper waters in the Amundsen Basin was lower by about 1 meter (3 m in 1996 compared to 4 m in 1991).
Did Antarctic sea‐ice expansion cause glacial CO2 decline?
Authors: Morales Maqueda M. A. , Rahmstorf S.
Published: October 2002

Recently, Stephens and Keeling [2000] have put forward an appealing theory for explaining the decrease in glacial atmospheric CO2. They argue that a compact sea‐ice cover extending southward of ∼55° S trapped large amounts of CO2 beneath the sea surface, thus accounting for the lower atmospheric concentrations. An atmosphere‐ocean box model in which sea‐ice area is prescribed allows them to simulate ∼80% of the CO2 drawdown. However, glacial CO2 levels can be attained in their model only when the fraction of ice‐covered area southward of the Antarctic Polar Front rises to 99–100%. We present simulations with a coupled sea ice‐upper ocean model indicating that ice‐area fractions so large might have not prevailed even under rather extreme glacial conditions. The combination of our glacial ice‐coverage estimates with the ice area‐CO2 relation derived by Stephens and Keeling suggests that CO2 sequestration under sea ice could account for at most 15–50% of the total glacial CO2 decline.
MODIS Cloud screening for remote sensing of aerosols over oceans using spatial variability
Authors: Martins José Vanderlei , Tanré Didier , Remer Lorraine , Kaufman Yoram , Mattoo Shana , Levy Robert
Published: October 2002

A cloud masking algorithm based on the spatial variability of reflectances at the top of the atmosphere in visible wavelengths was developed for the retrieval of aerosol properties by MODIS. It is shown that the spatial pattern of cloud reflectance as observed from space, is very different from that of aerosols. Clouds show a very high spatial variability in the scale of hundred meters to few kilometers, whereas aerosols in general are very homogeneous. The concept of spatial variability of reflectances at the top of the atmosphere is mainly applicable over the ocean where the surface background is sufficiently homogeneous for the separation between aerosols and clouds. Aerosol retrievals require a particular cloud masking approach since a conservative mask will screen out strong aerosol episodes and a less conservative mask could allow for cloud contamination that tremendously affect the retrieved aerosol optical properties (e.g. aerosol optical depth and effective radii). A detailed study on the effect of cloud contamination on aerosol retrievals is performed and parameters are established determining the threshold value for the MODIS aerosol cloud mask (3X3‐STD) over the ocean. The 3X3‐STD algorithm discussed in this paper is the operational cloud mask used for MODIS aerosol retrievals over the ocean.
Southern Hemisphere Origins of the 1976 Climate Shift
Authors: Giese Benjamin S. , Urizar S. Cristina , Fučkar Neven S.
Published: October 2002

Observations from the tropical Pacific Ocean identify an abrupt climate shift in 1976 with surface temperatures changing from cooler than normal to warmer than normal in the span of about 1 year. Model results show that this climate shift originates with subsurface temperature anomalies in the south tropical Pacific Ocean which propagate first to the western boundary, then northward to the equator, and finally eastward along the equator to 140° W where they rise to the surface. The results suggest that changes in the North Pacific respond to changes in the tropical Pacific.
Cryo‐chamber simulation of stratospheric H2SO4/H2O particles: Composition analysis and model comparison
Authors: Zink Peter , Knopf Daniel A. , Schreiner Jochen , Mauersberger Konrad , Möhler Ottmar , Saathoff Harald , Seifert Marco , Tiede Ralph , Schurath Ulrich
Published: October 2002

The combination of a large aerosol chamber and a recently developed Aerosol Composition Mass Spectrometer (ACMS) was used to investigate sulfuric acid aerosols at low temperatures. Concentrations of condensed phase H2SO4 and H2O were determined with an accuracy of better than 4 wt.%. Simultaneous measurements of temperature, partial pressure of water, total sulfate amount and particle size distribution permit to calculate the particle equilibrium composition. The model description of Carslaw et al. [1995a] for H2SO4/H2O solutions was confirmed in the composition range from 35 to 68 wt.% H2SO4 for temperatures between 188 and 236 K, extending the experimental verification to lower temperatures. Although the sub‐micron particles were up to 25 K super‐cooled with respect to solid sulfuric acid hydrates such as the tetrahydrate (SAT), they remained liquid for days.
Evidence of kilometer‐scale waves in the lower E region from high resolution VHF radar observations over Gadanki
Authors: Patra A. K. , Sripathi S. , Siva Kumar V. , Rao P. B.
Published: October 2002

High resolution VHF radar observations made at Gadanki revealed the kilometer‐scale waves in the low latitude lower E region. These waves are found to occur in a narrow altitude range of 92–97 km and have vertical wavelengths of ∼0.5 km and wave periods of ∼40 seconds. They resemble quite closely to that reported recently for midlatitude using the MU radar. The signal intensities associated with these waves, shown in SNR, are found to be as high as 25 dB, quite comparable to that observed at higher altitudes. It is in contrast to the much weaker midlatitude lower E region echoes reported by the MU radar. The wave structures show both positive and negative altitude rates with values in the range of 15–20 m s−1, which are in good agreement with the observed Doppler velocities. The spectral widths are found to be relatively lower, implying the lower E region wave structures to be less turbulent compared to their counterpart at higher altitudes. The observed wave structures bear remarkable resemblance to that simulated for the equatorial E region through nonlocal nonlinear gradient drift instability.
Adopting drought indices for estimating soil moisture: A North Carolina case study
Authors: Sims Aaron P. , Niyogi Dev dutta S. , Raman Sethu
Published: October 2002

Soil moisture availability has a significant impact on environmental processes of different scales. Errors in initializing soil moisture in numerical weather forecasting models tend to cause errors in short‐term weather and medium range predictions. We study the use of two drought indices: Palmer Drought Severity Index (PDSI) values and Standardized Precipitation Index (SPI) for estimating soil moisture. SPI and PDSI values are compared for three climate divisions: western mountains, central piedmont, and the coastal plain in North Carolina, USA. Results suggest SPI to be more representative of short‐term precipitation and soil moisture variation and hence a better indicator of soil wetness. A regression equation that uses SPI is proposed to estimate soil moisture.
N2O/222Rn ‐ soil flux calibration in the stable nocturnal surface layer
Authors: Conen Franz , Neftel Albrecht , Schmid Martin , Lehmann Bernhard E.
Published: October 2002

Concentration changes of N2O and 222Rn were monitored in the surface layer above a grass sward during the first half of two calm nights. N2O flux on a local or field scale was then estimated from the ratio of the concentration differences between 0.1m and 0.4m height and the directly measured 222Rn flux. This estimate (∼7 g N2O‐N ha−1d−1) was between the N2O flux on a larger scale (∼4.5 g N2O‐N ha−1d−1), as estimated from changes in the N2O/222Rn ratio over time, and the small scale flux on the grass plot (∼50 g N2O‐N ha−1d−1) as measured with closed chambers.