bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparison of MGS Phase 1 aerobraking radio occultation data and the NASA Ames Mars GCM
Authors: Joshi Manoj , Haberle Robert , Hollingsworth Jeffery , Hinson David
Published: February 2004

The NASA Ames Mars general circulation model (MGCM) is used to investigate how different phenomena in the Martian climate system combine to produce temperature profiles measured by the Radioscience Instrument (RS) aboard Mars Global Surveyor (MGS) in early 1998. When integrated with dust amounts consistent with Thermal Emission Spectrometer (TES) observations, and with a topography data set determined from the Mars Orbiting Laser Altimeter (MOLA), modeled temperature profiles are within 5–10 K of observations. Modeled and measured profiles in the winter subtropics are warmer than pure radiative‐convective considerations suggest, providing further evidence of strong dynamical warming associated with descent in the Hadley cell. South of 40°S, the best fit to the data is obtained by confining airborne dust to below 2 scale heights in this region. The differences in the strength of low‐level inversions found in the MGCM and in the RS data indicate the presence of either a low‐level radiative cooling mechanism such as isolated dust clouds or water‐ice clouds, or a dynamical mechanism such as low‐level gravity wave drag. When initialized with a mean surface pressure that gives the best fit to Viking lander measurements, the model matches RS surface pressure data to within 0.1 mbar. This consistency is a further validation of the MGCM and shows how GCMs can be used to interpret measurements of surface pressure taken at different locations using different methods.
Mars rotation variations induced by atmosphere and ice caps
Authors: Defraigne P. , de Viron O. , Dehant V. , Van Hoolst T. , Hourdin F.
Published: February 2004

Because of the conservation of angular momentum, the atmospheric winds and the mass exchange between the Martian ice caps and atmosphere, associated with the sublimation/condensation process (mainly CO2), induce seasonal effects on Mars' polar motion, nutation, and length of day (LOD). These effects are computed using the output of a global circulation model of the Martian atmosphere, providing atmospheric pressure fields, ice cap surface pressure fields, and zonal as well as meridional winds. For the LOD variations, total amplitudes (CO2 and wind effects) of 0.22 ms for the annual wave and of 0.38 ms for the semiannual wave are obtained. These amplitudes are more than one order of magnitude larger than the LOD variations induced by the zonal tides, which are at the level of 10 μm. For the induced polar motion the annual amplitude is ∼11 milliarcseconds (mas), and the semiannual amplitude is ∼3 mas. The effect on the nutations, related to the diurnal forcing, is at the level of 0.1 mas. The differences between the results for a liquid and for a solid core are examined and shown to be <1% of the total effects.
Sea surface paleotemperature errors in UK′37 estimation due to alkenone measurements near the limit of detection
Authors: Grimalt Joan O. , Calvo Eva , Pelejero Carles
Published: February 2004

Major errors (> 0.5°C) are produced in paleotemperature estimation with the C37 alkenone method when the amount of the diunsaturated or triunsaturated species, C37:2 and C37:3, respectively, approaches the limit of detection. These errors are more commonly encountered with C37:3 because of its higher adsorption to gas chromatographic (GC) columns. They are particularly relevant at low sedimentary C37 alkenone concentrations and may easily increase to temperature differences of 2°–4°C depending on the UK′37 values and the ratio between the C37:3 signal and limit of detection. Mathematical expressions for the description of the analytical constraints derived from this detection limit problem are given. A quality assurance guideline based on sample dilution (three times) and calculation of the sea surface temperature error is proposed for assessment of reliable measurements. Reevaluation of literature data in the light of the equations reported in the present study suggest that some previously reported alkenone geochemical effects, such as variation in UK′37 temperatures due to postdepositional oxidation, may reflect deviations in the GC measurements due to changes in C37 alkenone concentration.
Changes in Arctic Ocean paleoproductivity and hydrography during the last 145 kyr: The benthic foraminiferal record
Authors: Wollenburg Jutta E. , Kuhnt Wolfgang , Mackensen Andreas
Published: February 2004

The benthic foraminiferal record of two sediment cores at 900 and 2500 rn water depth in the marginal Arctic Ocean reflects fluctuations in paleoproductivity, ice sheet extent, and Atlantic water inflow. Highest paleoproductivity is observed in interglacial periods, and at the termination of interstadials to stadials within oxygen isotope substages (OIS) 6.3, 5.3, 5.1, 3.1, and Termination Ia. Stable and high paleoproductivity at the ice‐edge during terminations is indicated by theMelonis zaandamibenthic foraminiferal association. Periods of glacial ice sheet advance or retreat from the core site are dominated by benthic foraminiferal associations related to seasonal organic carbon flux. Temperate saline Atlantic water entered the Arctic Ocean within OIS 6.3, 5.5 and the Holocene. Coincident paleoproductivity maxima are related to extended seasonal ice retreat. During glacial periods of stagnating Atlantic water advection unusual low paleoproductivity values are observed in the Arctic Ocean, indicating increasing ice coverage. Deep water was poorly ventilated during these periods and the deeper site became exposed to corrosive bottom waters.
Formation of Holocene sedimentary laminae in the Black Sea and the role of the benthic flocculent layer
Authors: Pilskaln Cynthia H. , Pike Jennifer
Published: February 2004

Holocene Black Sea sediments recovered in 1988 and 1993 from box cores and gravity cores were analyzed geochemically, microscopically, and with backscattered electron imagery (BSEI) in order to determine the temporal, geochemical, and sedimentological relationships between the benthic flocculent layer (often referred to as the fluff layer) and the formation of underlying laminated unit I sediments. Existence of a permanent benthic fluff layer in the Black Sea basin is suggested, acting as a geochemical transition layer within which all sedimentary particles are hydraulically sorted and particles subject to dissolution or organic remineralization are altered prior to accumulation. We propose that particle residence time within the benthic fluff layer is a key factor in determining sedimentary microfabric and geochemical composition of laminated unit I sediments. We present a schematic model depicting the above relationships and use it to propose a paleoflux scenario for laminae formation in the unit II sapropel.
An organic tracer for surface ocean radiocarbon
Authors: Pearson A. , Eglinton T. I. , McNichol A. P.
Published: February 2004

The Δ14C of surface water dissolved inorganic carbon (DIC) in the Southern California Bight was compared to Δ14C as recorded by the sterols in Santa Monica and Santa Barbara Basin sediments. All of the C26, C27, C28, and C29 sterols as well as dinosterol had 14C concentrations equal to surface water DIC, indicating that all of the major sterols were derived from phytoplanktonic production. There is no detectable terrestrial component. Their tracer capability was confirmed by comparing the “bomb 14C”‐derived change in surface water Δ14CDIC with the change in Δ14Csterol. The “prebomb” Δ14CDIC was −82‰, and prebomb sterols averaged −75±19‰. The Δ14C value in 1996 was +71‰. Eighteen measurements representing eight different sterols from the sediment‐water interface of both Santa Monica and Santa Barbara Basins averaged +62±23‰. When three of these values were eliminated because of suspected contamination, the remaining data averaged +71 ±12‰. The entire compound class could serve as an excellent proxy for the 14C concentration of ocean surface waters.
Glacial sea surface temperatures in the subtropical North Pacific: A comparison of U37k′, δ18O, and foraminiferal assemblage temperature estimates
Authors: Lee Kyung E. , Slowey Niall C. , Herbert Timothy D.
Published: February 2004

We have investigated glacial‐interglacial differences in sea surface temperature (SST) near Hawaii using two relatively high deposition rate, shallow‐water piston cores collected near Oahu, Hawaii. Modern hydrographic data show that local surface water temperatures are broadly consistent with the regional pattern of SSTs in the southern subtropical North Pacific. Past SSTs were estimated on the basis of three independently measured parameters: (1) U37k′ values of alkenones, (2) δ18O of Globigerinoides ruber, and (3) assemblages of planktonic foraminifera using the modern analog technique (MAT). The two cores yield similar SST records, and if differences in the ecology of foraminifera and coccolithophores are considered, the three different approaches to estimating SSTs yield consistent results. U37k′‐based temperatures, which may represent winter values at this location, were ∼2.5°C colder during the Last Glacial Maximum than today, which is consistent with the February MAT estimates. The δ18O‐based temperature estimates, likely biased toward summer temperatures, indicate that the glacial SSTs were at least 1°C cooler than today, which is comparable to the results of MAT August estimates.
Stagnant slabs in the upper and lower mantle transition region
Authors: Fukao Yoshio , Widiyantoro Sri , Obayashi Masayuki
Published: February 2004

We made a region‐by‐region examination of subducted slab images along the circum‐Pacific for some of the recent global mantle tomographic models, specifically for two high‐resolution P velocity models and two long‐wavelength S velocity models. We extracted the slab images that are most consistent among different models. We found that subducted slabs tend to be subhorizontally deflected or flattened in the upper and lower mantle transition region, the depth range of which corresponds roughly to the Bullen transition region (400–1000 km). The deflected or flattened slabs reside at different depths, either above or across the 660‐km discontinuity as in Chile Andes, Aleutian, Southern Kurile, Japan, and Izu‐Bonin; slightly below the discontinuity as in Northern Kurile, Mariana, and Philippine; or well below it as in Peru Andes, Java, and Tonga‐Kermadec. There is little indication for most of these slabs to continue “directly” to greater depths well beyond the transition region. Mantle downflow associated with present slab subduction appears to be blocked strongly to turn into predominantly horizontal flow in the transition region. Recent global tomographic models show also a group of lithospheric slabs deeply sinking through the lower mantle, typically the presumed Farallon slab beneath North and Central America and the presumed Indian (Tethys) slab beneath Himalaya and the Bay of Bengal. These remnant slabs are not connected to the surface plates or to the presently subducting slabs and appear to sink independently from the latter. The presence of these deeply sinking slabs implies that the pre‐Eocene subduction occurred in much the same way as in the present day to accumulate slab bodies in the transition region and that the consequent unstable downflow occurred extensively through the transition region in the Eocene epoch to detach many of the surface plates from the subducted slabs at depths and hence to cause the reorganization of global plate motion.
Statistical addition method for external noise sources affecting HF‐MF‐LF systems
Authors: Neudegg David
Published: February 2004

The current statistical method for the addition of external component noise sources in the LF, MF, and lower HF band (100 kHz to 3 MHz) produces total median noise levels that may be less than the largest‐component median in some cases. Several case studies illustrate this anomaly. Methods used to sum the components rely on their power (decibels) distributions being represented as normal by the statistical parameters. The atmospheric noise component is not correctly represented by its decile values when it is assumed to have a normal distribution, causing anomalies in the noise summation when components are similar in magnitude. A revised component summation method is proposed, and the way it provides a more physically realistic total noise median for LF, MF, and lower HF frequencies is illustrated.
Decay of whistler‐induced electron precipitation and cloud‐ionosphere electrical discharge Trimpis: Observations and analysis
Authors: Dowden Richard , Rodger Craig , Brundell James , Clilverd Mark
Published: February 2004

There are two distinctly different causes of phase and amplitude perturbations of subionospheric VLF transmissions (termed “Trimpis”): (1) ionization enhancement in the ionospheric D region due to whistler‐induced electron precipitation (WEP) and (2) modifications to the ionosphere induced directly by lightning. The latter appear to be subdivided into ionization anomalies produced by cloud‐ionosphere electrical discharge (CID), or “red sprites,” and heating anomalies which may not involve ionization at all. Here we consider only Trimpis (WEP and CID) produced by ionization and find that the magnitude of the Trimpi perturbation (which includes both the amplitude and phase perturbations) of both Trimpi types decays logarithmically rather than exponentially with time. While this has been previously shown for CID Trimpis, the decay of WEP Trimpis was previously thought to be exponential.
New generation topside sounder
Authors: Ganguly Suman , Wickwar Vincent , Goodman John M.
Published: February 2004

Having ionospheric electron density distributions as a function of height, latitude, longitude, and time under different conditions is essential for scientific, technical, and operational purposes. A satellite‐based, swept‐frequency, HF sounder can obtain electron density profiles on a global scale. We are developing a new generation HF sounder that employs recent developments in technology, electronics, and processing capabilities. It will provide global‐scale electron density distributions, contours of fixed densities, maps of ƒoF2, hmax, etc. It will allow us to map irregularities, estimate anomalous propagation and conditions for ducting, determine angles of arrival, etc. It will also be able to perform various plasma diagnostics and, because of new flexibility, will be programmable from the ground to perform a variety of experiments in space. Need for such a system exists through the Department of Defense and several civilian agencies. Some of the novel features of the system include software‐based design, direction of arrival estimation and synthetic aperture radar‐type operation, onboard processing, and reconfigurable and flexible architecture with multimission capabilities.
Uncertainties in the temperature sensitivity of decomposition in tropical and subtropical ecosystems: Implications for models
Authors: Holland Elisabeth A. , Neff Jason C. , Townsend Alan R. , McKeown Becky
Published: February 2004

Tropical ecosystems play a central role in the global carbon cycle. Large changes in tropical temperature over geologic time and the significant responses of tropical ecosystems to shorter‐term variations such as El Niño/La Niña argue for a robust understanding of the temperature sensitivity of tropical decomposition. To examine the responsiveness of heterotrophic respiration to temperature, we measured rates of heterotrophic respiration from a wide range of tropical soils in a series of laboratory incubations. Under conditions of optimal soil water and nonlimiting substrate availability, heterotrophic respiration rose exponentially with rising temperature. The meanQ10measured across all temperature ranges in these short‐term incubations was 2.37, but there was significant variation inQ10s across sites. The source of this variation could not be explained by soil carbon or nitrogen content, soil texture, site climate, or lignin to nitrogen ratio. At the beginning of the incubation, heterotrophic respiration increased exponentially with temperature for all sites, despite the fact that the fluxes differed by an order of magnitude. When substrate availability became limiting later in the incubation, the temperature response changed, and heterotrophic response declined above 35°C. The documented changes in temperature sensitivity with substrate availability argue for using temperature relationships developed under optimal conditions of substrate availability for models which include temperature regulation of heterotrophic respiration. To evaluate the significance of this natural variation in temperature control over decomposition, we used the Century ecosystem model gridded for the areas between the tropics of Cancer and Capricorn. These simulations used the mean and upper and lower confidence limits of the normalized exponential temperature response of our experimental studies. We found that systems with the lowest temperature sensitivity accumulated a total of 70 Pg more carbon in soil organic carbon and respired 5.5 Pg yr−1 less carbon compared to the systems with the highest sensitivity.
Estimation of new production in the tropical Pacific
Authors: Aufdenkampe Anthony K. , McCarthy James J. , Rodier Martine , Navarette Claudie , Dunne John , Murray James W.
Published: February 2004

A synthesis of field data from nine cruises and 121 stations in the tropical Pacific (15°N–16°S by 135°–W 167°E) was used to develop a statistical model relating areal new production rates (based on 15NO3 uptake incubations) to other measured biological and chemical water properties. The large dynamic range of f ratios (new to primary production) measured in the region (0.01–0.46, with a mean of 0.16±0.08) could not be described by any simple function of any of the more than three dozen measured variables tested. Thus the commonly used approach of extrapolating new production using mean f ratios is likely to lead to large uncertainties when used in the tropical Pacific. An alternative approach is examined in which new production is estimated directly by multiple linear regression (MLR) of measured properties. Nearly 80% of variability in new production could be explained with a MLR of four variables together (rates of primary production (or chlorophyll inventories), inventories of ammonium and nitrate, and temperature) better than any single variable alone or any other combination of variables. Each of these variables exhibited effective linearity with respect to new production for this data set, and the robustness of this MLR method to predict new production for other data sets was confirmed by cross validation. These results thus provide a robust, simple tool to extend new production estimates to locations and times where it is not measured directly, using ship‐based measurements and potentially remotely sensed data from moorings and satellites.
Modeling the variation of δ13C in atmospheric methane: Phase ellipses and the kinetic isotope effect
Authors: Allan W. , Manning M. R. , Lassey K. R. , Lowe D. C. , Gomez A. J.
Published: February 2004

We use the TM2 three‐dimensional atmospheric tracer model with a methane source‐sink budget based on existing literature to simulate small spatial and temporal variations in the 13C/12C ratio of atmospheric methane. The results show that δ13C varies markedly with wind direction everywhere outside the extratropical Southern Hemisphere (ETSH). Within the ETSH, both methane mixing ratio and δ13C have regular seasonal cycles with differing and latitude‐dependent phases. Phase diagrams constructed from these seasonal cycles, showing changes in δ13C versus changes in mixing ratio, have elliptical shapes. The slope of the major axis of these ellipses is determined by the kinetic isotope effect (KIE) of the single atmospheric methane removal process used in the model. The ellipse eccentricity is determined by seasonal variation in the source δ13CH4, which is dominated by the biomass burning source because of its isotopic enrichment relative to other sources. Comparison of the model results, for a KIE based on CH4 + OH oxidation, with observations in the South Pacific region shows significant discrepancies in both the ellipse major axis slopes and eccentricities. We suggest that this is an indicator of an additional sink process that discriminates strongly against 13CH4. Such a sink could be active chlorine in the marine boundary layer.
Carbon accumulation in West Siberian Mires, Russia Sphagnum peatland distribution in North America and Eurasia during the past 21,000 years
Authors: Turunen Jukka , Tahvanainen Teemu , Tolonen Kimmo , Pitkänen Aki
Published: February 2004

The rates of carbon (C) accumulation and the role of fires in the C dynamics of the major types of boreal West Siberian mires were investigated. Detailed analysis of dry bulk density, C, and N concentration and age of the peat layers were used to determine C accumulation rates throughout the Holocene. The average long‐term apparent rate of carbon accumulation (LORCA) at 11 studied sites was 17.2±1.0 (SE) g m−2 yr−1, ranging from 12.1 to 23.7 g m−2 yr−1, and the total apparent carbon sink 11.8 Tg yr−1 (1 Tg = 1012 g) for Russian raised string bogs (68.5 million hectares). These estimates of C accumulation in West Siberian mires are roughly a half of the earlier estimates for these boreal mires. Differences in LORCA for three major mire types in the study area, the ridge‐hollow pine bogs, Sphagnum fuscum pine bogs, and dwarf‐shrub pine bogs, were not significant. The age versus depth (measured as cumulative carbon from the surface downward) curve was slightly convex, indicating a general declining trend in LORCA with decreasing age. About 55% of the present carbon store was already accumulated about 6000 cal. BP. The most intensive expansion phase of the study area occurred between 7000 and 8000 cal. BP. The subsequent lateral expansion has been very slow in the later Holocene. The charcoal data indicated that these mires have burned only 2–3 times during the past 7000–8000 cal. BP, and only a strip of a few meters along the mire margins has burned relatively frequently. No evidence of significant carbon losses due to fires could be found. The charcoal layers at the mire margins suggest a declining trend in burning rates during the later Holocene.
Seasonal biogeochemical fluxes of 234Th and210Po in the Upper Sargasso Sea: Influence from atmospheric iron deposition
Authors: Kim Guebuem , Church Thomas M.
Published: February 2004

The naturally occurring radionuclide tracers, 234Th‐238U and 210Po‐210Pb pairs, were measured bimonthly at the Bermuda Atlantic Time‐Series Study (BATS) site in the Sargasso Sea from October 1996 to August 1997. Biological production and biogeochemical fluxes of thorium, polonium, and particulate organic carbon were highest in December 1996 and August 1997. Although the enhanced production and fluxes in December 1996 could be due to winter nitrate inputs from the subsurface ocean, those in August 1997 were not expected, on the basis of the nutrient mass balance and extreme stratification of the Sargasso Sea during this summer. The fixed‐nutrient inputs from the atmosphere, eddy intrusion, or deepwater upwelling were unlikely sources of such elevated summer productivity. Alternatively, the unusually high atmospheric “wet” deposition of Fe, which occurred during this summer period, appears to have fueled high nitrogen fixation and thus greater carbon and reactive‐element export. Also as important, ocean production did not respond to significant “dry” deposition of Fe. This seems to be due to the reported lower solubility of Fe dust in seawater versus rainwater.
The biological pump in the subtropical North Pacific Ocean: Nutrient sources, Redfield ratios, and recent changes
Authors: Emerson Steven , Mecking Sabine , Abell Jeffrey
Published: February 2004

Observations from the Hawaii Ocean Time series indicate that a large fraction of the nitrogen supply to the euphotic zone is presently from nitrogen fixation. There is growing evidence, some presented here, that the main phosphorus flux is from the upper thermocline via eddy‐induced mixing processes. These mechanisms of supply create a non‐steady state nutrient flux in which the N:P ratio in particulate organic matter exiting the upper ocean (N:P = 23) is greater than the ratio of dissolved nitrogen to phosphorus that is remineralized in the upper thermocline (N:P = 13–14). Carbon to nitrogen ratios in the particulate export flux have remained between 7 and 8 in spite of these N:P changes, suggesting that the biological carbon export may have increased along with nitrogen fixation even though Redfield Ratio arguments would suggest phosphorus should be limiting. A simple model of the upper ocean circulation and carbon export demonstrates the sensitivity of apparent oxygen utilization (AOU) in the upper thermocline, atmospheric pCO2 and atmospheric pO2 to changes in the carbon pump. Comparison of AOU distributions from four meridional transects through the northeast subtropical Pacific between 1980 and 1997 suggests there has been an increase of 20–25% over the past two decades. These results cannot be explained without invoking changes in the biological pump or upper‐ocean ventilation by as much as 30–50%. Estimated atmospheric pCO2 and pO2 responses to this change are <10% and ∼15%, respectively, of the observed trends in the 1990s. Taken together, changes in the organic matter C:N:P ratios and in the AOU within the upper thermocline of the eastern North Pacific indicate that the onset of nitrogen fixation has resulted in an increase in the biological pump over that expected from nutrient delivery via ocean circulation. This suggests that at least on decadal timescales, the biological pump is not limited by phosphate concentrations in the Redfield ratio (N:P = 15–16).
Mass spectra of sn‐2‐hydroxyarchaeol, a polar lipid biomarker for anaerobic methanotrophy
Authors: Hinrichs Kai‐Uwe , Pancost Richard D. , Summons Roger E. , Sprott G. Dennis , Sylva Sean P. , Sinninghe Damsté Jaap S. , Hayes John M.
Published: February 2004

Extremely 13C‐depleted sn‐2‐hydroxyarchaeol is widely observed at marine methane seep sites. It is the first biosynthetic product that provides information about a previously unknown microorganism involved in anaerobic methanotrophy, a process that occurs widely in marine sediments. The mono‐ and di‐TMS derivatives of this compound yield sharply differing and distinctive mass spectra. Depending on reaction conditions, either of these products can be formed in the course of conventional analyses.
Cooling of the Earth: A parameterized convection study of whole versus layered models
Authors: McNamara Allen K. , van Keken Peter E.
Published: February 2004

Compositionally layered mantle models have often been invoked in order to explain the geochemistry observed at the Earth's surface, specifically the discrepancy between ocean island basalt and mid‐ocean ridge basalt compositions. One disadvantage of layered models is the reduction in cooling efficiency compared to whole‐mantle convection as a direct result of the insulating nature of the thermal boundary layer that develops between the two convecting layers. This may pose a significant problem for layered models in which the bottom layer is enriched in heat‐producing radioactive elements with respect to the top layer. One may expect that the bottom layer would become superheated over the lifetime of the Earth. We perform this study in order to test whether layered models are thermally feasible. We are interested in discovering whether it was possible, within Earth‐like constraints, to produce a bottom layer temperature that remains below the solidus without simultaneously producing a top layer that is too cool. We use parameterized convection to explore a wide parameter range of input values for several layering configurations. We study both a whole mantle convection model and the recently proposed layered convection model, which places the boundary between the layers at 1600‐km depth [Kellogg et al., 1999]. We use the present‐day heat flow and mantle viscosity as primary constraints and use the resulting average temperature as a test for the feasibility of the models. Our results reveal that for whole mantle convection, a wide parameter range produces results that satisfy our constraints. This is in contrast to the layered convection model in which we find that the parameter range that satisfies constraints is significantly reduced or perhaps, nonexistent.
Thermodynamics of pressure release melting of a veined plum pudding mantle
Authors: Phipps Morgan Jason
Published: February 2004

This study explores the thermodynamics of adiabatic decompression melting of peridotitic mantle containing pyroxenite veins that have lower solidi than the peridotite. When a vein of lower solidus‐temperature material melts adjacent to more refractory material, additional heat will flow into the melting region to increase its melting productivity. If pyroxenite veins have a solidus‐depletion gradient ((∂Tm/∂F)P) like that of olivine or peridotite, then the melting of the veins is enhanced by up to a factor of 4 by this heat. However, the solidus‐depletion gradient of pyroxenites is apparently lower than that of peridotites; thus pyroxenite melting would be even more enhanced. If pyroxenite veins have a gentler solidus‐pressure (T‐P) dependence (i.e., lower (∂Tm/∂P)F) than that of peridotite solidi, then although these veins will experience enhanced melting while they are the only melting assemblage, they will stop melting soon after their peridotite matrix begins to melt. During large‐scale peridotite melting the material ascends along aT‐Ppath close to that of the peridotite solidus, so that the mixture's temperature remains lower than the solidus of the residual pyroxenite, and pyroxenite melting ceases throughout the shallower sections of the melting column. If pyroxenitic material makes up a large fraction of the mantle mixture (∼20%), then the heat consumed by deep pyroxenite melting cools the ascending mantle mixture enough so that peridotite melting begins ∼5–10 km shallower than it would in the absence of precurser pyroxenite melting. After recycling into the mantle, the melt extraction residue will again melt if it is reheated to ambient mantle temperatures and rises again to shallow depths.
Primary signal: Ecological and environmental factors—Report from Working Group 2
Authors: Bijma J. , Altabet M. , Conte M. , Kinkel H. , Versteegh G. J. M. , Volkman J. K. , Wakeham S. G. , Weaver P. P.
Published: February 2004

Alkenone‐based proxies are routinely used for paleoceanographic and climatologic reconstructions despite the fact that their calibration to environmental parameters is still a matter of debate. Significant differences are observed among culture calibrations using different strains of alkenone producers, between regional field calibrations within the euphotic zone, and between water column‐ and core top‐based calibrations. The reasons for these discrepancies are not well understood and call for further evaluation. The possibility is raised that several aspects of genetics, physiology, and ecology of alkenone‐producing haptophytes may be responsible, and suggestions of future research are proposed.
Alteration processes of alkenones and related lipids in water columns and sediments
Authors: Harvey H. Rodger
Published: February 2004

Alkenones produced by the haptophyte algae are currently being used as indices of sea surface temperature in recent and past ocean environments, but limited information is available concerning the impact of biotic and abiotic processes on the integrity of these long chain lipids. This synthesis provides selected background information on major alteration processes that must be considered before such indices can be used with confidence. A number of processes in the water column and surface sediments have the potential to impact the structural integrity of alkenones and compromise their ability as temperature markers. Processes discussed include the alteration of alkenone structure during early diagenesis, direct biotic and abiotic impacts, and the effect of digestive processes by grazers. Current literature suggests that despite substantial changes in concentration from biological processing, the temperature signal is preserved. For each of these processes, information on the integrity of the alkenone isotopic signature is also needed and limited information available is reviewed. In addition to the alkenones, related lipids including the long chain alkadienes and akyl alkenoates that might serve as ancillary markers are discussed.
Alkenones and multiproxy strategies in paleoceanographic studies
Authors: Mix A. C. , Bard E. , Eglinton G. , Keigwin L. D. , Ravelo A. C. , Rosenthal Y.
Published: February 2004

We assess multiproxy strategies involving alkenone‐based and other paleoceanographic indicators in an effort to define uncertainties and to advance understanding of past oceanic and climatic changes. Looking to the future, we recommend a parallel strategy of applying proxies to the geologic record while continuously seeking refinements in methods and understanding of mechanisms that control each proxy. A multiproxy strategy increases confidence in results and provides oceanographic context for more effective interpretation of processes. A combination of proxies allows derivation of additional properties, such as upper ocean paleosalinity or paleo‐pCO2, for a more complete view of the oceanographic and climate mechanisms involved in changing Earth's environment.
Osmium abundance and isotope variations in mafic Mexican volcanic rocks: Evidence for crustal contamination and constraints on the geochemical behavior of osmium during partial melting and fractional crystallization
Authors: Lassiter John C. , Luhr James F.
Published: February 2004

Primitive lavas and scoriae from the western Mexican Volcanic Belt (MVB) and the Mexican Basin and Range Province (MBRP) display a large range in Os concentrations and Os isotopes, with [Os] ranging from ∼10 to 320 ppt and 187Os/188Os ranging from ∼0.128 to 0.207. Os concentrations and Os isotopes are correlated with abundances of MgO and Ni and with other indices of fractional crystallization, indicating that Os behaved as a compatible element during fractional crystallization. The high oxygen fugacity of subduction‐related calc‐alkaline and lamprophyric magmas from the western MVB precludes significant sulfide fractionation. Therefore Os must be compatible in at least one phase other than sulfide (e.g., olivine and/or Cr‐spinel). The similarity of Os abundance trends in arc and nonarc suites suggests that Os concentrations are controlled primarily by equilibrium partitioning between melt with residual sulfide and silicate or oxide phases, and that DOs (sulfide/melt) is less than ∼2 × 105. Three chemically distinct Mexican volcanic suites, calc‐alkaline basalts and lamprophyres from the western MVB and intraplate‐type alkalic basalts from the MBRP, all display similar ranges in Os isotopes. Assimilation/fractional crystallization modeling suggests that the observed isotopic variations can be generated by minor (<12%) assimilation of Triassic age mafic lower crust. The least radiogenic samples still possess Os isotopes that are elevated relative to normal mantle peridotite. This difference may reflect variable input of radiogenic slab‐derived fluids or melts into the source region of the Mexican magmas, with the slab signature strongest in the lamprophyres and calc‐alkaline basalts and weakest in the intraplate‐type alkalic basalts. Correlations between Os isotopes and Os concentrations in other arc suites and mid‐ocean ridge basalts are unlikely to reflect mixing of distinct mantle components but instead reflect pervasive crustal (or seawater) contamination of magmas with low Os abundances.
Record of seafloor CaCO3 dissolution in the central equatorial Pacific
Authors: Broecker Wallace S. , Anderson Robert , Clark Elizabeth , Fleisher Martin
Published: February 2004

Shell weights for Globigerina sacculifer, Pulleniatina obliquiloculata, and Neogloboquadrina dutertrei from cores raised from a depth of 4.36 km (at 2°S, 140°W) reveal large dissolution‐related shifts at Termination I and at the marine isotope stage 5‐4 boundary. These shifts suggest that the carbonate ion concentration at this depth was ∼27 μmol kg−1 higher during glacial time than during marine isotope stages 1 and 5 and that the boundaries separating these regimes were quite sharp (in other words, the dissolution chronology has a rectangular character). However, when compared to results for a much shallower core, there are disconcerting aspects to this data set. One is that the initial shell wall thickness may depend on growth conditions. Hence the estimates of paleocarbonate ion concentrations presented here remain tentative.
Thick ice layers in snow and frozen soil affecting gas emissions from agricultural soils during winter
Authors: van Bochove E. , Thériault G. , Rochette P. , Jones H. G. , Pomeroy J. W.
Published: February 2004

We investigated soil and snow cover gas concentrations at two agricultural sites (St‐Lambert; Chapais) in Quebec, Canada, during winter 1998–1999. Both sites showed frozen and unfrozen soils and complex snow cover structure. At St‐Lambert we measured higher average concentrations of N2O (35 to 62 μl L−1) and CO2 (3 to 19 mL L−1) below the frozen soil surface of plots subjected to a treatment of pig slurry than in the control plot (N2O, 9 to 30 μL L−1; CO2, 3 to 7.5 mL L−1). The lack of vertical gaseous concentration gradients in the snowpack was due to the trapping of accumulating gas below the impermeable frozen soil layer. Soil gas concentrations decreased sharply when soil warmed to the freezing point. At the same time, the snow cover was isothermal. N2O could have been lost at spring thaw through gaseous emissions and/or dissolved in meltwaters and leached to the drainage system. High N2O fluxes were measured using closed chambers (215 ng m−2 s−1, slurry treatment; 55 ng m−2 s−1, control) as soon as snow ablation was completed, but became negligible 2 days later, suggesting that emissions were the result of passive degassing rather than of increased biological activity. At Chapais, N2O and CO2 accumulated in the unfrozen soil surface below a thick (0.1 m) basal ice layer. The basal ice layer and the continuous ice layer above it were impermeable to gas diffusion, as demonstrated by the accumulation of a tracer gas (Ar, >50 mL L−1) introduced by a diffuser into the soil. The existence of a basal ice layer is uncommon in eastern Canada. The occurrence of such a phenomenon may increase with climate change due to more frequent rain events during the cold season and affect the dynamics of winter gas emissions from soils.
Chemistry‐climate interactions in the Goddard Institute for Space Studies general circulation model: 2. New insights into modeling the preindustrial atmosphere
Authors: Grenfell J. Lee , Shindell Drew T. , Koch Dorothy , Rind David
Published: February 2004

We investigate the chemical (hydroxyl and ozone) and dynamical response to changing from present‐day to preindustrial conditions in the Goddard Institute for Space Studies general circulation model. We identify three main improvements not included by many other works. First, our model includes interactive cloud calculations. Second, we reduce sulfate aerosol which impacts NOx partitioning hence Ox distributions. Third, we reduce sea surface temperatures and increase ocean ice coverage which impact water vapor and ground albedo, respectively. Changing the ocean data (hence water vapor and ozone) produces a potentially important feedback between the Hadley circulation and convective cloud cover. Our present‐day run (run 1, control run) global mean OH value was 9.8×105 molecules cm−3. For our best estimate of preindustrial conditions run (run 2) which featured modified chemical emissions, sulfate aerosol, and sea surface temperatures/ocean ice, this value changed to 10.2×105 molecules cm−3. Reducing only the chemical emissions to preindustrial levels in run 1 (run 3) resulted in this value increasing to 10.6×105 molecules cm−3. Reducing the sulfate in run 3 to preindustrial levels (run 4) resulted in a small increase in global mean OH (10.7×105 molecules cm−3). Changing the ocean data in run 4 to preindustrial levels (run 5) led to a reduction in this value to 10.3×105 molecules cm−3. Mean tropospheric ozone burdens were 423, 306, 305, 305, and 310 Tg for runs 1–5, respectively.
Relative climatic effects of landcover change and elevated carbon dioxide combined with aerosols: A comparison of model results and observations
Authors: Chase T. N. , Pielke R. A. , Kittel T. G. F. , Zhao M. , Pitman A. J. , Running S. W. , Nemani R. R.
Published: February 2004

In this study we examine the possibility that the historical total of human landcover changes have had a comparable effect on climate to that of historical increases in CO2 and aerosols. We compared results from two coupled climate model simulations which investigated transient climate changes produced by observed historical changes of CO2 combined with sulfate aerosol forcing with two other climate model simulations that examined the equilibrium climatic effects of currently observed changes in landcover from its natural state. We found that simulated, near‐surface temperature anomalies due to transient increases in atmospheric CO2 combined with aerosols at the level currently observed are of similar amplitude as simulated temperature anomalies due to the direct and remote (nonlocal) equilibrium effects of historical anthropogenic landcover change in all models. Both effects are of comparable amplitude to observed temperature trends in the past 2 decades, the period of largest global surface warming. These results provide evidence for a confounding influence on surface temperatures and may be an indication that the problem of detection of the radiative warming effect of increased CO2 in the observational record may be more complicated than previously appreciated.
Behavior of OH and HO2 radicals during the Observations at a Remote Island of Okinawa (ORION99) field campaign: 2. Comparison between observations and calculations
Authors: Kanaya Yugo , Matsumoto Jun , Kato Shungo , Akimoto Hajime
Published: February 2004

The OH and HO2 concentrations observed during the Observations at a Remote Island of Okinawa intensive field campaign (ORION99) were compared with those calculated by using the ancillary observations as input parameters. Detailed comparisons were performed for HO2 with the time resolution of 10 min. During daytime, the observed HO2 concentration levels and variations were basically well reproduced by the model calculations without including heterogeneous processes. On average, the model underestimated daytime HO2 by only 20%. The squared correlation coefficient between observed and calculated HO2 was 0.79. The model underestimated HO2 significantly during a morning period with high NOx concentrations by up to a factor of 3 and for sudden surges observed during noontime on two days by a factor of 1.5. The basically good agreement between measured and calculated HO2 at Okinawa Island was in contrast with the significant model's overestimation of midday HO2 by a factor of 2 at Oki Island. The causes for the difference are discussed. For OH, detailed comparisons were not possible owing to the large uncertainties of measurements and calculations. However, diurnal patterns of OH on several days calculated by a simple steady state model were similar to those observed, indicating the observed OH data were statistically significant when they were averaged hourly. During one night, HO2 concentrations were observed to be 2–5 pptv, positively correlating with NO2. The model significantly underestimated HO2 by up to a factor of 4 and did not reproduce the positive correlation. Currently, no explanations can be given for the nocturnal behaviors.
Apparent freezing temperatures modeled for several experimental apparatus
Authors: Hung Hui‐Ming , Martin Scot T.
Published: February 2004

Ice formation by homogeneous nucleation in aqueous atmospheric particles composed of high ionic strength electrolytes is at times believed to be the dominant initiation step in cirrus and polar stratospheric cloud formation. Microphysical models of ice nucleation occurring during atmospheric processes are based upon volume nucleation rates, J(T, x), measured in laboratory experiments where T is temperature and x is mole fraction salt composition. At the present time, there are large discrepancies among apparent freezing temperatures reported by several investigators employing different experimental apparatus. One current hypothesis is that a common J function is expressed differently in these several techniques, which have variations in observation times, system volumes, vapor mass transfer, and so on. In the current paper, we model the several experiments based on a common J function and simulate apparent freezing temperatures of H2SO4/H2O and (NH4)2SO4/H2O particles. We find that the experimental results can be categorized into two groups, one of which is consistent with a function J1 and the other with J2, while J1 and J2 are mutually exclusive. We thus conclude no single J function exists that can simultaneously simulate all reported experimental results, thus refuting the previously proposed “common J” hypothesis for the reconciliation of experimental results.
Volatile organic compound emissions from Scots pine: Mechanisms and description by algorithms
Authors: Shao Min , Czapiewski Kristin V. , Heiden Arnd C. , Kobel Klaus , Komenda Michael , Koppmann Ralf , Wildt Jürgen
Published: February 2004

The mechanisms of volatile organic compound (VOC) emissions from Scots pine (Pinus sylvestris L.) were investigated in laboratory experiments. The plants emitted mainly monoterpenes and acetone. Isoprene was emitted only in small amounts, but the mechanisms of its emissions were similar to those of the other compounds. Isoprene, acetone, and monoterpene emissions from Scots pine could be well described by an algorithm that considers emissions caused by evaporation of VOCs out of pools and emissions in parallel with their biosynthetic production. Monoterpene emissions were mainly affected by temperature. In some cases, monoterpene emissions were also influenced by photosynthetic active radiation implying that monoterpene emissions from Pinus sylvestris occur from storage pools as well as from processes that are linked to monoterpene biosynthesis. The coupling of monoterpene emissions with photosynthesis was confirmed by results of experiments with 13CO2. The 13CO2 exposure resulted in emission of 13C labeled monoterpenes during 13CO2 exposure as well as during the night following the exposure. Similar results were also obtained for isoprene emissions. Scots pine emitted isoprene during illumination as well as in darkness. The emitted isoprene was labeled during 13CO2 exposure and in the night following the exposure. The results obtained for monoterpene emissions in the laboratory were compared to those of outdoor measurements with Scots pine. While the temperature dependencies of emission rates were comparable to those obtained from laboratory experiments, a PAR dependence was not detectable. Temperature variations during outdoor measurements prevented a detection of this dependence.