bluuebunny/crossref_metadata_2025 huggingface dataset:
SOMARE‐99: Observations of tropospheric scattering layers using multiple‐frequency range imaging
Authors: Palmer Robert D. , Chilson Phillip B. , Muschinski Andreas , Schmidt Gerhard , Yu Tian‐You , Steinhagen Hans
Published: February 2004

Results from an experimental implementation of multiple‐frequency range imaging (RIM) are presented. The technique exploits the benefits of frequency diversity to improve range resolution of atmospheric radar systems. The theory has been described in the literature, and simulations have proven its usefulness. Nevertheless, experimental results have been extremely limited. Over a 5‐day period in May 1999 we conducted experiments using RIM on the sounding system (SOUSY) radar in northern Germany to observe the layered structure in the troposphere. The experiment is referred to as the SOUSY Multifrequency Atmospheric Radar Experiment 1999 (SOMARE‐99). Estimates of range brightness produced by the RIM analysis provide insight about the layered structure of the atmosphere. The RIM results show distinct similarities to previous in situ measurements, which have shown sharp refractive index discontinuities to exist throughout the troposphere and stratosphere. Examples from selected time periods show layers modulated by possible short‐period gravity waves or advection of periodic structures as well as other layers with apparent downward motion possibly caused by the progression of a warm front or large‐scale subsidence.
Scintillation modeling for GPS‐Wide Area Augmentation System receivers
Authors: Hegarty Christopher , El‐Arini M. Bakry , Kim Taehwan , Ericson Swen
Published: February 2004

A scintillation signal model and a Global Positioning System (GPS)‐Wide Area Augmentation System (WAAS) receiver model are developed. The scintillation signal model is based on a Nakagami‐m distribution for intensity and a Gaussian distribution with zero mean for phase. The GPS‐WAAS receiver model includes Link 1 (L1) GPS and WAAS carrier‐and C/A‐code‐tracking loops, as well as semicodeless Link 2 (L2) carrier and Y‐code tracking capabilities. The results show that noncoherent delay locked loops (DLLs) typically used for code tracking are very robust to both amplitude and phase scintillation. Carrier‐phase‐tracking loops are much more susceptible to scintillation, and the signal‐to‐noise threshold for reliable carrier tracking is very dependent on the scintillation strength. Fortunately, it appears that the worst case scintillation encountered at midlatitudes, including the United States, does not significantly impact L1 carrier‐tracking performance. Semicodeless tracking of the L2 carrier is shown to be very fragile. Even weak scintillation can cause loss of L2 carrier lock for low‐elevation satellites.
Active continental rifting in southern Baja California, Mexico: Implications for plate motion partitioning and the transition to seafloor spreading in the Gulf of California
Authors: Fletcher John M. , Munguía Luis
Published: February 2004

Three major earthquake series have occurred during the past 30 years on fault systems that are spatially and kinematically distinct from oceanic spreading centers and transform faults in the southern Gulf of California. Focal mechanism solutions for earthquakes of the April 1969 and June 1995 series indicate nearly pure normal displacement across NNW striking planes. This seismicity falls along different strands of a major system of normal faults that extends at least 300 km along strike and defines the western limit of the Gulf Extensional Province. The dominant normal faults control distribution of Neogene basins and have been active since middle to late Miocene time. Epicenters of the August 1969 earthquake series lie south of the Baja California peninsula and focal mechanism solutions indicate strike‐slip displacement. This seismic activity is likely related to active WNW striking faults in the continental borderland, which isolate Baja California as a microplate within the Pacific‐North American plate boundary. Therefore, modern Pacific‐North America plate motion is partitioned into at least three distinct fault systems: oceanic spreading centers and transforms in the Gulf of California, normal faults along the western margin of the Gulf of California, and faults along the Pacific margin of the Baja California peninsula. Kinematic partitioning of plate motion is required to accommodate the strongly three dimensional strain of transtensional rifting. Continental rifting and seafloor spreading have overlapped in time for the past 3.6 Myr., which also is likely attributed to the highly oblique kinematics of rifting.
Structural evolution of the Makkovik Province, Labrador, Canada: Tectonic processes during 200 Myr at a Paleoproterozoic active margin
Authors: Culshaw Nicholas , Ketchum John , Barr Sandra
Published: February 2004

In the Makkovik Province, the structures and plutonic rocks of the reworked southern edge of the Archean Nain craton (parautochthon) and an adjacent juvenile domain form a spatially concentrated record of ∼ 200 Ma at a Paleoproterozoic active margin. D1 thin‐ and thick‐skinned structures, formed during terrane collision before ca. 1896 Ma, were nearly erased by D2 transpressive structures present across the entire parautochthon that had been thermally softened by emplacement of arc plutons at ca. 1895–1870 Ma. D3 transpressive inversion of a back arc rift and D4 strike‐slip shearing formed structures at the oceanward margin of the parautochthon and were responses to events in the active arc that was now widely separated from the parautochthon by accreted terranes. The tectonic isolation of the parautochthon ended by the time of D5 (ca. 1740–1710 Ma) and D6 with the reappearance there of granite and low‐grade shear zones. Structural and plutonic style varied at each stage, primarily controlled by the crustal level attained.
Fallow season straw and water management effects on methane emissions in California rice
Authors: Fitzgerald G. J. , Scow K. M. , Hill J. E.
Published: February 2004

In response to legislative mandate to reduce postharvest straw burning and environmental concerns to restore wetland habitat for Pacific flyway waterfowl, California rice growers are incorporating straw into soil and flooding rice fields in winter. These changes were hypothesized to alter soil carbon cycling pathways across the region. The principal objective of this study was to determine how various winter fallowed straw and water management changes would affect year‐round methane emissions. Main plots were winter flood and nonflood, and subplots had straw treatments: burned, soil incorporated, or rolled (partially soil incorporated). Results showed the principal factor controlling methane emissions was the interaction of flooding and straw amendments. The presence of either water or straw alone led to low emissions. Winter emissions accounted for 50% of annual totals in straw‐amended treatments despite lower temperatures and the presence of plants in summer. Summer emissions were significantly influenced by winter straw amendments but not by winter flood. Postdrain peaks after winter drain accounted for 10–13% of annual emissions in treatments with amended straw. Although rolled and incorporated treatments had similar straw inputs, methane fluxes from rolled treatments were higher than from incorporated treatments. Measurements of methane should be conducted year‐round to capture fallow and postdrain fluxes and improve global emission estimates. Regional emission estimates showed that 2.6 times more methane was emitted after flooding plus incorporation was implemented than before the legislative mandate was enacted.
Estimates of anthropogenic carbon uptake from four three‐dimensional global ocean models
Authors: Orr James C. , Maier‐Reimer Ernst , Mikolajewicz Uwe , Monfray Patrick , Sarmiento Jorge L. , Toggweiler J. R. , Taylor Nicholas K. , Palmer Jonathan , Gruber Nicolas , Sabine Christopher L. , Le Quéré Corinne , Key Robert M. , Boutin Jacqueline
Published: February 2004

We have compared simulations of anthropogenic CO2 in the four three‐dimensional ocean models that participated in the first phase of the Ocean Carbon‐Cycle Model Intercomparison Project (OCMIP), as a means to identify their major differences. Simulated global uptake agrees to within ±19%, giving a range of 1.85±0.35 Pg C yr−1 for the 1980–1989 average. Regionally, the Southern Ocean dominates the present‐day air‐sea flux of anthropogenic CO2 in all models, with one third to one half of the global uptake occurring south of 30°S. The highest simulated total uptake in the Southern Ocean was 70% larger than the lowest. Comparison with recent data‐based estimates of anthropogenic CO2 suggest that most of the models substantially overestimate storage in the Southern Ocean; elsewhere they generally underestimate storage by less than 20%. Globally, the OCMIP models appear to bracket the real ocean's present uptake, based on comparison of regional data‐based estimates of anthropogenic CO2 and bomb 14C. Column inventories of bomb 14C have become more similar to those for anthropogenic CO2 with the time that has elapsed between the Geochemical Ocean Sections Study (1970s) and World Ocean Circulation Experiment (1990s) global sampling campaigns. Our ability to evaluate simulated anthropogenic CO2 would improve if systematic errors associated with the data‐based estimates could be provided regionally.
Soil CO2 dynamics, acidification, and chemical weathering in a temperate forest with experimental CO2 enrichment
Authors: Andrews Jeffrey A. , Schlesinger William H.
Published: February 2004

Soils constitute a major component of the global carbon cycle that will be affected by anthropogenic additions of CO2 to the atmosphere. As part of the Duke Forest Free‐Air CO2 Enrichment (FACE) experiment, we examined how forest growth at elevated (+200 ppmv) atmospheric CO2 concentration affects CO2 dynamics in the soil. Soil respiration and the concentration of CO2 in the soil pore space to a depth of 200 cm were measured over a 3‐year period. Soil CO2 production was linked to soil acidification and mineral weathering by measuring changes in the composition of the soil solution, including alkalinity, Si, and major cations. The total flux of dissolved inorganic carbon to groundwater was then calculated from field measurements. The FACE fumigation gas contained a unique 13C signature that labeled newly fixed carbon, which was monitored in the soil system. As a result of CO2 enrichment, annual soil respiration increased by 27% and was accompanied by higher CO2 concentrations in the soil pore space. These changes to soil CO2 dynamics were most likely the result of increased root and rhizosphere respiration, as suggested by the changes to the δ13C of soil CO2. Increased soil CO2 under FACE accelerated the rates of soil acidification and mineral weathering. Thus an increase of 55% in atmospheric CO2 concentration over 2 years resulted in a 271% increase in soil solution cation concentration, a 162% increase in alkalinity, and a 25% increase in Si concentration at 200–cm depth. The flux of dissolved inorganic carbon to groundwater increased by 33%, indicating a negative feedback to changes in atmospheric CO2 that could regulate the global carbon cycle over geological time.
Linking remote‐sensing estimates of land cover and census statistics on land use to produce maps of land use of the conterminous United States
Authors: Hurtt George C. , Rosentrater Lynn , Frolking Steve , Moore Berrien
Published: February 2004

Human use of the land has a large effect on the structure of terrestrial ecosystems and the dynamics of biogeochemical cycles. For this reason, terrestrial ecosystem and biogeochemistry models require moderate resolution (e.g., ≤0.5°) information on land use in order to make realistic predictions. Few such data sets currently exist. To create a land use data set of sufficient resolution, we developed models relating land cover data derived from optical remote sensing and a census database on land use for the conterminous United States. The land cover product used was from the International Geosphere‐Biosphere Programme DISCover global product, derived from 1 km advanced very high resolution radiometer imagery, with 16 land cover classes. Land use data at state‐level resolution came from the U.S. Department of Agriculture's Major Land Uses database, aggregated into four general land use categories: Cropland, Pasture/Range, Forest, and Other. We developed and applied models relating these data sets to generate maps of land use in 1992 for the conterminous United States at 0.5° spatial resolution.
Air‐sea flux of oxygen estimated from bulk data: Implications For the marine and atmospheric oxygen cycles
Authors: Gruber Nicolas , Gloor Manuel , Fan Song‐Miao , Sarmiento Jorge L.
Published: February 2004

We estimate the annual net air‐sea fluxes of oxygen for 13 regions on the basis of a steady state inverse modeling technique that is independent of air‐sea gas exchange parameterizations. The inverted data consist of the observed oceanic oxygen concentration after a correction has been applied to account for biological cycling. We find that the tropical oceans (13°S−13°N) emit ∼212 Tmol O2 yr−1, which is compensated by uptake of 148 Tmol yr−1 in the Northern Hemisphere (>13°N) and by uptake of 65 Tmol yr−1 in the Southern Hemisphere (<13°S). These results imply that the dominant feature of oxygen transport in the combined ocean‐atmosphere system is the existence of a closed circulation cell in each hemisphere. These two cells consist of O2 uptake by the ocean in the middle and high latitudes of both hemispheres and transport in the ocean toward the tropics, where O2 is lost to the atmosphere and transported in the atmosphere back toward the poles. We find an asymmetry in the two cells involving O2 uptake in the temperate regions of the Northern Hemisphere versus loss of O2 in the temperate regions of the Southern Hemisphere. There is an additional asymmetry between the Atlantic basin, which has a net southward transport at all latitudes north of 36°S, in agreement with independent transport estimates, versus the Indian and Pacific Oceans, which have a strong equatorward transport everywhere. We find that these inverse estimates are relatively insensitive to details in the inversion scheme but are sensitive to biases in the ocean general circulation model that provides the linkage between surface fluxes and ocean interior concentrations. Forward simulations of O2 in an atmospheric tracer transport model using our inversely estimated oxygen fluxes as a boundary condition agree reasonably well with observations of atmospheric potential oxygen (APO ≈ O2 + CO2). Our results indicate that the north‐south asymmetry in the strength of the two hemispheric cells coupled with a strong asymmetry in fossil fuel emissions can explain much of the observed interhemispheric gradient in APO. Therefore it might not be necessary to invoke the existence of a large southward interhemispheric transport of O2 in the ocean, such as proposed by Stephens et al. [1998]. However, we find that uncertainties in the modeled APO distribution stemming from seasonal atmospheric rectification effects and the limited APO data coverage prevent the currently available APO data from providing strong constraints on the magnitude of interhemispheric transport.
Vegetation succession and carbon sequestration in a coastal wetland in northwest Florida: Evidence from carbon isotopes
Authors: Choi Yonghoon , Wang Yang , Hsieh Yuch‐Ping , Robinson Larry
Published: February 2004

Measurements of stable carbon isotopic ratios as well as carbon (C), nitrogen (N), and phosphorus (P) contents in soils and plants were made along a chronovegetation sequence stretching from high marsh to low marsh in a coastal wetland in northwest Florida. The wetland is dominated by Juncus roemerianus , which is a C3 plant and has an average δ13C of −27‰. Lesser amounts of other species, including C4 plants, are also present in the area. The δ13C values of soil organic matter from low and middle marshes range from −24 to −27 ‰, which are consistent with the current plant community. However, the δ13C values of soil organic matter from high marsh show significant variations, from −23‰ in the surface soil to −17‰ at depth. This large C isotopic variation within soil profiles indicates a shift in local vegetation, from a C4‐dominated community to the current C3 plant‐dominated marsh, as a result of landward expansion of the wetland due to sea level rise. Radiocarbon dates on soil organic matter indicate that this ecological change occurred in the past hundred years or so as a result of sea level rise presumably due to global warming. Soil organic carbon inventory was ∼29 ± 3.6 kg m−2 in low marsh (the oldest part of the wetland), 15 ± 3.6 kg m−2 in middle marsh, and 13 ± 6.0 kg m−2 in high marsh (the youngest and most inland part of the wetland). N and P inventories are also higher in low marsh than in high marsh and seem to correlate directly with aboveground productivity in the marshes. The much higher C storage in low marsh than in high marsh indicates that carbon sequestration increased significantly as coastal wetland evolves from high marsh (initial stage) to low marsh (steady state). This has important implications to the global C cycle. As sea level rises owing to global warming, coastal wetlands are expected to expand landward in many areas where topography is gentle, which would provide a significant sink for atmospheric carbon dioxide.
Role of the California Undercurrent in the export of denitrified waters from the eastern tropical North Pacific
Authors: Castro C. G. , Chavez F. P. , Collins C. A.
Published: February 2004

A transect off Point Sur, California (36°20'N), was occupied bimonthly from April 1988 to May 1990 to resolve spatial and temporal patterns in the physics and chemistry of the California Current system. Alongshore velocity fields and the chemical parameter N* [Gruber and Sarmiento, 1997] show coherence. Subarctic waters, conveyed by the California Current (CC), were characterized by high N* values, while in the region of poleward flow of the California Undercurrent (CU) a minimum of N* (N*< −5 μmol kg−1) was observed. Low N* is a signature of denitrification, and its possible sources are considered. The distribution of N* on isopycnal surfaces, using data from the World Ocean Circulation Experiment Hydrographic Programme, suggests that northward advection from the eastern tropical North Pacific by the CU is the primary source of low N*. Local benthic and water column denitrification rates cannot account for the observed values. From nitrate deficits and velocity fields an annual nitrate transport deficit off Point Sur of 4.6 Tg N yr−1 was estimated for the study period. Maximum offshore extent of the low N* signal was observed during the upwelling period, probably associated with an expansion of the CU and mixing due to mesoscale eddies detached from it. This mixing propagates the denitrification signal into the CC and the subtropical gyre. When the loss from the poleward flow due to horizontal mixing is calculated, an export out of the eastern tropical North Pacific of 8.3 Tg N yr−1 by the CU is estimated. The bulk of the denitrified waters off Point Sur were below 400 m and did not affect primary production in the central California upwelling ecosystem. However, the export of nitrate‐deficient waters into the subtropical gyre may play a role in the biogeochemical cycling of this region.
Self‐consistent generation of tectonic plates in time‐dependent, three‐dimensional mantle convection simulations
Authors: Tackley Paul J.
Published: February 2004

Presented here are self‐consistent, three‐dimensional simulations of mantle convection, some of which display an approximation of plate tectonic behavior that is continuous in space and time. Plate behavior arises through a reasonable material description of silicate deformation, with a simple yield stress being sufficient to give first‐order plate‐like behavior; however, the required yield strength or fault frictional coefficient is much less than experimentally determined values. Toroidal:poloidal ratios are within geologically observed limits. The sensitivity of the system to yield strength and the form of strength envelope is systematically investigated. Optimum plate character is obtained in a narrow range of yield strength, below which diffuse boundaries, and above which episodic behavior, and eventually a rigid lid, are observed. Models with mobile lids develop very long wavelength horizontal structure, the longest wavelength possible in the domain. Two‐dimensional models display much greater time dependence than three‐dimensional models.
Distribution of carbonate ion in the deep ocean: Support for a post‐Little Ice Age change in Southern Ocean ventilation?
Authors: Broecker Wallace S. , Sutherland Stewart
Published: February 2004

The distribution of CO3= ion concentrations in the deep sea proves difficult to reconcile with a steady state thermohaline circulation. The problem is that assuming steady state, we cannot come up with any satisfactory explanation for the 28 μmol/kg deficiency of CO3= ion concentration in deep Pacific and Indian Ocean water relative to that for the source water mix. However, following Broecker et al. [1999], if we assume that during the Little Ice Age, deep water formation in the Southern Ocean was not only much stronger than now but also had lower PO4* and CO3= concentrations, then this anomaly can be explained.
Controls on the molecular distribution and carbon isotopic composition of alkenones in certain haptophyte algae
Authors: Laws Edward A. , Popp Brian N. , Bidigare Robert R. , Riebesell Ulf , Burkhardt Steffen , Wakeham Stuart G.
Published: February 2004

Although the biochemical functions and biosynthetic pathways of alkenones are still largely unknown, alkenone unsaturation ratios are now used extensively to infer ancient sea surface temperature, and their isotopic compositions have been used to reconstruct ancient atmospheric CO2levels. The inferred relationships between alkenone unsaturation ratios, isotopic compositions, and growth conditions are based on empirical laboratory and field studies and, in the case of isotope fractionation, on simple models of carbon acquisition and fixation. Significant uncertainty still exists concerning the physiological and ecological factors affecting cellular production of alkenones, unsaturation ratios, and isotopic composition. Phytoplankton culture conditions have been shown to affect alkenone unsaturation (U37K′), cellular alkenone content, intracellular isotopic compositions (Δδ), and changes in fractionation (εP) as a function of the quotient of algal growth rate and aqueous carbon dioxide concentration (μ/CO2). Such studies imply that plant physiology can affect the interpretation of environmental signals. The factor(s) controlling cellular alkenone concentrations and unsaturation ratios are reviewed, as well as the carbon isotopic composition of the alkenone‐producing algae. A new technique is presented to determine growth rates of the alkenone‐containing algae in natural settings that will facilitate testing laboratory‐based hypotheses concerning the carbon isotopic fractionation and its relationship to growth rate/growth status of alkenone‐producing algae in the field.
Alkenones as paleoceanographic proxies
Authors: Sachs Julian P. , Schneider Ralph R. , Eglinton Timothy I. , Freeman Katherine H. , Ganssen Gerald , McManus Jerry F. , Oppo Delia W.
Published: February 2004

Alkenone‐derived estimates of temperature and CO2 are contributing to our understanding of Earth's climate history. In order to increase confidence in alkenone‐based climate proxies we recommend the following actions. First, the sedimentary component or fraction containing alkenones should be identified in order to assess the impact of horizontal advection and vertical mixing on alkenone‐derived temperature and CO2 estimates. Second, differential mixing rates of alkenone‐containing particles and sand‐sized foraminifera should be quantified by independent dating of the two phases. Until that is accomplished, apparent temporal offsets of climate proxies in the two phases should be interpreted cautiously. Third, the stability of the unsaturation ratio and carbon isotopic compositions of alkenones during all phases of diagenesis should be confirmed. Both field and laboratory observations are required. Fourth, future alkenone investigations should be coupled with other paleoclimate proxy measurements at high‐deposition‐rate‐sites in a variety of oceanographic settings. In upwelling regions and in the vicinity of river plumes, salinity and nutrient proxies should be measured since changes in these parameters may affect alkenone biosynthesis.
Results of the Basin and Range Geoscientific Experiment (BARGE): A marine‐style seismic reflection survey across the eastern boundary of the central Basin and Range Province
Authors: Brady Robert , Wernicke Brian , McNutt Marcia , Mutter John , Correa Gustavo
Published: February 2004

Approximately 120 km of marine‐style deep seismic reflection data were shot during a survey on the waters of Lake Mead in southeastern Nevada. The survey extends from near the abrupt eastern edge of the Basin and Range Province (BRP) to a point ∼80 km into the extended domain. Data quality throughout the survey ranged from fair to poor; the recorded data include significant towing noise and occasionally problematic diffractions and sideswipe from canyon walls. The upper 2–4 s of the data shows well‐defined reflections from sedimentary fill, but below that point, reflectivity is weak. Lower crustal reflectivity is generally absent under the eastern part of the survey, with a slight increase in reflectivity to the west. The reflection Moho appears as a series of weakly defined, discontinuous reflections, most of which occur at 10–11 s. A particularly interesting feature of the data set is the relative lack of reflectivity from the lower crust, which is a region of strong reflectivity on other seismic reflection data sets from the BRP.
Carbonaceous films in midcrustal rocks from the KTB borehole, Germany, as characterized by time‐of‐flight secondary ion mass spectrometry
Authors: Mogk D. W. , Mathez E. A.
Published: February 2004

Amphibolites from 4.6 and 9.1 km depth in the Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland (KTB) borehole have developed extensive, interconnected networks of carbonaceous films on cracks and grain boundaries as determined by electron probe and time‐of‐flight secondary ion mass spectrometry. In the sample from 9.1 km depth, the carbon is dominantly to completely elemental. In contrast, the carbonaceous matter in the samples from 4.6 km depth comprises a mixture of elemental carbon and simple hydrocarbons such as alkanes, and possibly C‐O‐H compounds. The microcracks within the 4.6 km sample also contain a retrograde micro‐assemblage consisting of ferri‐oxy‐hydroxide, calcite, and possibly clay minerals, suggesting that the carbonaceous matter and retrograde minerals formed together at relatively high crustal levels and at a time much later than the peak metamorphism. Because the carbon films likely influence the electrical conductivity of the rocks in situ, it is proposed that production of hydrocarbons during retrograde metamorphism of grain boundary and microcrack carbon tends to increase the resistivity of the rocks. This chemical destruction of the interconnectivity of electrical pathways may contribute to the observed diminished conductivity of the shallow crust relative in relation to thto the deep crust.
New experimental observations on the anhydrous solidus for peridotite KLB‐1
Authors: Herzberg Claude , Raterron Paul , Zhang Jianzhong
Published: February 2004

New multianvil experimental data are reported for the anhydrous solidus for peridotite KLB‐1 at 5 to 9.7 GPa. The solidus is unaffected by variable experimental run durations, but it is lowered in temperature when rhenium capsules are replaced with graphite. Using data reported previously, the anhydrous solidus for KLB‐1 can be described by T = 1086 − 5.7P + 390 ln P (±68°C 2σ) at pressures that range from 2.7 to 22.5 GPa. At lower pressures the effect of composition on the temperature of the spinel peridotite solidus is small and dominated by alkali abundances; variations in Mg number have almost no effect on solidus temperature.
The δ44Ca‐temperature calibration on fossil and cultured Globigerinoides sacculifer: New tool for reconstruction of past sea surface temperatures
Authors: Nägler T. F. , Eisenhauer A. , Müller A. , Hemleben C. , Kramers J.
Published: February 2004

We report direct δ44Ca‐temperature calibration on cultured and fossil calcite foraminifera, showing that Ca isotopes are potentially a new proxy for past sea surface temperatures (SST). Samples have been analyzed using a 43Ca‐48Ca double spike and thermal ionization mass spectrometry (TIMS). In order to avoid species‐dependent isotope fractionation we focused our investigations on a single foraminifera species (Globigerinoides sacculifer), which is known to inhabit shallow euphotic waters in tropical and subtropical oceans. Ca isotope measurements were performed on cultured G. sacculifer that grew in seawater kept at temperatures of 19.5°, 26.5°, and 29.5°C. A δ44Ca change of 0.24 ± 0.02 per 1°C is defined by the weighted linear regression through reproduced δ44Ca data of the three temperatures (95% confidence level). Application of this new method to fossil G. sacculifer of an Equatorial East Atlantic sediment core (GeoB1112; 5°46.7′S, 10°45.0′W, 3125 m) indicates that the δ44Ca difference between marine isotope stage 1 (MIS‐1) and MIS‐2 is 0.71 ± 0.24. According to the current δ44Ca‐temperature calibration this value corresponds to a temperature difference between MIS‐1 and MIS‐2 of ∼3.0 ± 1.0°C.
Mixing properties in the Earth's mantle: Effects of the viscosity stratification and of oceanic crust segregation
Authors: Ferrachat Sylvaine , Ricard Yanick
Published: February 2004

Although a large quantity of geochemical data on oceanic basalts has been collected, it is not sufficient to characterize the recycling processes between the Earth's mantle, on the one hand, and the crust and the lithosphere, on the other hand. In particular, it remains unclear why mid‐oceanic ridge basalts (MORBs) are relatively homogeneous throughout the world, while oceanic island basalts (OIBs) present a wide spectrum of heterogeneities. Assuming whole mantle convection, it is often argued that the increase of viscosity with depth could be responsible for some stratification of the mixing properties of the mantle and consequently could generate the observed geochemical and isotopic differences between MORBs and OIBs. In this study we test this assumption by means of two‐dimensional circulation models where passive tracers are advected. First, we quantify the ability of the upper and lower mantles to erase heterogeneities. Second, we simulate the evolutions of U, 4He, and 3He concentrations, taking into account the magmatic processes at ridges (differentiation, outgassing, and recycling). We conclude that the viscosity layering does not induce any vertical stratification of the mixing properties of the mantle. On the contrary, the partial segregation of the oceanic crust in the D″ layer would explain the generation of large zones of high 3He/4He ratios on top of D″. In our simulations these zones have a primitive He signature, although they have already been processed at ridges. Trapping oceanic crust in D″ would not only explain the presence of recycled components in hotspots and particularly in the high μ type hotspots [Hofmann and White, 1982] but would also suggest that the high 3He/4He ratios of volcanoes like Loihi could have their origin in ancient subducted lithosphere.
Geodetic constraints on postseismic deformation following the 1990 Ms 7.8 Luzon earthquake and implications for Luzon tectonics and Philippine Sea plate motion
Authors: Beavan John , Silcock David , Hamburger Michael , Ramos Emmanuel , Thibault Catherine , Feir Renato
Published: February 2004

We have obtained three epochs of high‐quality dual‐frequency GPS data at 15 stations in the Luzon, Philippines, region since the July 16, 1990, Ms 7.8 earthquake, from which we generate horizontal velocity fields at the Earth's surface for the 1993–1996 and 1996–1998 periods. We use these velocities, plus 1996–1998 GPS velocity results reported elsewhere from 13 additional stations, to show that present‐day deformation in Luzon is dominated by strike‐slip motion along the Philippine fault system. The measured strike‐slip rate across central Luzon is faster than the expected long‐term slip rate on the fault, which we attribute to postseismic deformation following the 1990 earthquake. The 1993–1996 and 1996–1998 deformation patterns cannot be statistically distinguished within the ∼10% uncertainties of the data. If the observed deformation is interpreted using an elastic half‐space model with uniform slip below a subsurface locking depth, we find a fault slip rate of 40 mm/yr and a locking depth of ∼15 km. This is too fast to be a steady interseismic rate, as it disagrees with what is known from paleoseismic and other evidence about earthquake recurrence and long‐term slip rate on the Philippine fault system in central and northern Luzon. Also, a 15 km locking depth for steady interseismic slip is probably too shallow compared to the depth of rupture in the 1990 earthquake. If the deformation is instead interpreted with two types of viscoelastic model, we find that the observed velocities can be fit well for a range of values of lower lithosphere viscosity and long‐term slip rate. Using a two dimensional (2‐D) viscoelastic coupling model, the minimum allowable lower lithosphere viscosity is 0.5 × 1019 Pa s, with an associated long‐term slip rate of 15–22 mm/yr. Faster long‐term slip rates also fit well, with correspondingly higher values of viscosity. With a possibly more realistic model that includes 3‐D viscoelastic effects we find preferred long‐term velocities in the range 20–35 mm/yr and viscosities in the range 2–6 × 1019 Pa s. Our preferred viscosities correspond to Maxwell times on the order of 7–20% of the typical earthquake recurrence interval and are several times higher than values found for a number of Californian strike‐slip earthquakes. Other implications of our 1993–1998 surface velocities are that the Philippine Sea‐Eurasia (PH‐EU) Euler vector of Seno et al. [1993] is a better description of the PH‐EU convergence rate than a Euler vector recently estimated from GPS velocities in the northern PH and that the great majority of the normal component of PH‐EU plate convergence is taking place west of Luzon, presumably largely at the Manila Trench. We also find that Luzon is rotating counterclockwise relative to PH at 1°–2°/Myr, only ∼25% of the rate recently suggested by other workers on the basis of limited GPS data.
Age of the Batoka basalts, northern Zimbabwe, and the duration of Karoo Large Igneous Province magmatism
Authors: Jones D. L. , Duncan R. A. , Briden J. C. , Randall D. E. , MacNiocaill C.
Published: February 2004

Analysis of the Batoka Basalts exposed in the Zambezi Gorge some 40 km east of Victoria Falls characterizes them as high Fe, moderately high Ti, and low K, P, and Zr tholeiites. The 40Ar‐39Ar age determinations are tightly clustered at 180–179 Ma. All of the lavas that were sampled have normal paleomagnetic polarity, and the corresponding pole position is 63.9°N, 260.6°E, A95 = 14.9°. In South Africa, Lesotho, and Namibia the vast majority of Karoo basalts were extruded at 183 ± 1 Ma with some as young as 179 Ma [Duncan et al., 1997]. Paleomagnetic and geochemical correlation of the ∼179 Ma rocks between Zimbabwe and Lebombo supports the conclusion that the age difference is real and hence confirms the estimate of ∼5 Myr for the duration of emplacement of the Karoo Large Igneous Province.
Contrasted interactions between plume, upper mantle, and lithosphere: Foundation chain case
Authors: Maia Marcia , Hémond Christophe , Gente Pascal
Published: February 2004

The Foundation Seamounts are a volcanic chain formed during the last 21 m.y. by the action of a hot spot, presently located near the axis of the Pacific‐Antarctic Ridge. The western part of the chain is formed by volcanoes with ages ranging from 21 to 16 m.y., spatially distributed along two parallel lines roughly 200 km apart. The shape and distribution of the edifices are strongly suggestive of an emplacement along preexisting fissures or fractures, resulting from lithospheric deformation during the major kinematic reorganization of 25 m.y. Their chemical and isotopic composition becomes enriched toward young volcano ages, resulting from the progressive contamination of the upper mantle by the arrival of a plume pulse. The eastern part of the chain is formed by volcanoes younger than 5 m.y., also distributed along two subparallel lines. The distance between these two lines diminishes toward the Pacific‐Antarctic Ridge axis. A geoid high is located very near the Pacific‐Antarctic Ridge axis, over the volcanoes of the north line, which lies in the prolongation of the older Foundation Seamounts, and possibly marks the location of the Foundation hot spot. Both the geoid anomaly and the morphology of the edifices show that the north line is the main locus of the volcanism. The southern line was probably formed on top of the flexural arch resulting from the emplacement of the north line. The chemical and isotopic compositions of the basalts reveal a growing influence of the ridge on the off‐axis plume magmatism. The pattern is coherent with a mixing between two sources, occurring when the two melting zones merge and overlap.
Olivine‐poor sources for mantle‐derived magmas: Os and Hf isotopic evidence from potassic magmas of the Colorado Plateau
Authors: Carlson Richard W. , Nowell Geoffrey M.
Published: February 2004

The contribution of olivine‐poor lithologies (pyroxenite, eclogite, etc.) in the mantle to the composition of mantle‐derived melts is a growing topic of discussion in light of results suggesting that some magmatic characteristics are not easily explained by melt derivation solely from peridotitic sources. To investigate this issue, we have examined the Hf and Os isotopic composition of potassic magmas from the Navajo Volcanic Field of the Colorado Plateau. Most of the magmas have Os isotopic compositions much more radiogenic than observed for any uncontaminated oceanic basalt. The Os isotope compositions of these samples overlap those measured for crustal/upper mantle eclogitic xenoliths carried by nearby intrusions. Compositional and Os isotope systematics, however, suggest that the characteristics of the magmas are not determined by crustal contamination, but rather are the result of melting of pyroxene‐ and mica‐rich veins/layers in the lithospheric mantle beneath the plateau. Once generated, the melts of olivine‐poor lithologies interacted with surrounding metasomatized peridotite to create the compositional spectrum from katungite to minette. One component of this compositional variation is the appearance of strong relative depletions of high field strength elements in the minette end‐member, suggesting that this chemical signature, common in convergent margin magmas, may be caused by melt‐wall rock interaction.
Petrogenesis of the crust‐mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite
Authors: Koga Kenneth T. , Kelemen Peter B. , Shimizu Nobumichi
Published: February 2004

We studied trace element geochemistry and petrology of the crust‐mantle transition zone (MTZ) in the Samail massif of the Oman ophiolite to constrain the location where different primitive magmas mix beneath an oceanic spreading ridge. The MTZ is the deepest location where crystallization took place and thus is an ideal place to determine the compositional diversity of melts leaving the mantle, with various sources and degrees of depletion. We have reached three main conclusions: (1) More than 90% of our samples record equilibration with compositionally indistinguishable parental melts, similar to mid‐ocean ridge basalts (MORB) and the melts that formed the crust in Oman. This suggests that mixing of diverse, polybaric partial melts of mantle peridotite occurred at or below the depth of the MTZ. The presence of distinct heterogeneity in less than 10% of our samples is similar to the nature and frequency of heterogeneity observed in melt inclusions in olivines from MORB. (2) Among the samples recording trace element equilibrium with MORB‐like liquids are wehrlitic rocks, previously suggested to be cumulates from an unusual parental melt on the basis of petrological observation. (3) Systematics of Eu distribution among plagioclase and clinopyroxene in “impregnated peridotites” demonstrate that these minerals did not crystallize from “trapped melt.” As a consequence, it is not possible to use the modal proportion or texture of plagioclase + clinopyroxene impregnations to estimate the instantaneous melt porosity or the shape of melt pores at any time during the formation of these rocks.
High field strength element/rare earth element fractionation during partial melting in the presence of garnet: Implications for identification of mantle heterogeneities
Authors: van Westrenen Wim , Blundy Jonathan D. , Wood Bernard J.
Published: February 2004

A synthesis of recent garnet‐melt trace element partitioning data for key trace elements (Ti, Hf, Zr, U, Th, Sm, and Yb) is used to compare and contrast the trace element signatures imparted on mantle melts by garnets from peridotitic and eclogitic source rocks. Garnet‐melt partition coefficients DGrt/Melt are very sensitive to changes in garnet major element composition. Specifically, high‐pressure, high‐temperature experimental studies show that high field strength elements (HFSE) Zr, Hf, and Ti are incompatible in garnets with <19 ± 1 mol% Ca on their X site, with DTi < DZr ≤ DHf < 1, while at higher Ca levels, all three become compatible with DZr > DHf > DTi > 1. U and Th also have higher partition coefficients at higher garnet Ca contents, while the amount of fractionation between the two decreases. In contrast, the heavy rare earth element partition coefficients DHREE are hardly affected by a change in garnet Ca content. We provide a semiquantitative explanation for the behavior of the high field strength elements based on a crystal lattice strain model in which Zr and Hf are split between the X and Y sites in Ca‐rich garnet and in which significant changes in garnet elasticity occur as a function of garnet composition. The large variations in both absolute DGrt/Melt values and DGrt/Melt ratios (e.g., DZr/DYb), in conjunction with compositional differences between natural peridotitic (Ca poor) and eclogitic (Ca richer) garnets, allow identification of trace element ratios that may best serve as a fingerprint for the presence of eclogitic garnet. We present simple batch melting calculations for two end‐member melting scenarios (anhydrous garnet peridotite melting and anhydrous bimineralic eclogite melting). Our calculations show that near‐uniform Zr/HREE and Hf/HREE as a function of melt fraction, in combination with Hf/Sm and Zr/Sm ratios that are smaller than the source ratio, could serve as fingerprints for the presence of Ca‐rich garnet in the source of mantle melts. Our calculations show that it is impossible to define one unique “garnet signature” to determine the presence or absence of garnet in basalt sources but rather that different garnet‐bearing sources are likely to produce distinctly different “garnet signatures.”
A first look at paleotemperature prospects from Mg in coccolith carbonate: Cleaning techniques and culture measurements
Authors: Stoll Heather M. , Ruiz Encinar Jorge , Ignacio Garcia Alonso J. , Rosenthal Yair , Probert Ian , Klaas Christine
Published: February 2004

Although coccolith calcite is abundant in carbonate sediments, it has not been previously utilized for Mg/Ca paleothermometry. Cleaning experiments with synthetic composite samples (reagent CaCO3 powder and organic matter from noncalcifying marine algae Chlorella) are used to evaluate which traditional and/or novel cleaning methods permit us to recover the known carbonate Mg/Ca ratio. The most effective cleaning treatment, causing the least effect on carbonate chemistry and most complete and rapid oxidation of algal organic matter, was oxidation in an equal volume mixture of sodium hypochlorite (2.8%) and hydrogen peroxide (30%). However, in some organic‐ rich samples this method may not remove sufficient noncarbonate Mg to permit precise determination of carbonate Mg/Ca. Mg/Ca ratios in small culture samples may be determined precisely (relative standard deviation of 0.65%) using sector field inductively coupled plasma‐mass spectrometry. Measurements of Mg/Ca in coccoliths from several species suggest that temperature may be an important control over Mg partitioning in coccolith calcite, although cleaning issues are likely to be an important limitation on paleoceanographic application.
Thermal fractionation of air in polar firn by seasonal temperature gradients
Authors: Severinghaus Jeffrey P. , Grachev Alexi , Battle Mark
Published: February 2004

Air withdrawn from the top 5–15 m of the polar snowpack (firn) shows anomalous enrichment of heavy gases during summer, including inert gases. Following earlier work, we ascribe this to thermal diffusion, the tendency of a gas mixture to separate in a temperature gradient, with heavier molecules migrating toward colder regions. Summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion. Here we explore and quantify this process further in order to (1) correct for bias caused by thermal diffusion in firn air and ice core air isotope records, (2) help calibrate a new technique for measuring temperature change in ice core gas records based on thermal diffusion [Severinghaus et al., 1998], and (3) address whether air in polar snow convects during winter and, if so, whether it creates a rectification of seasonality that could bias the ice core record. We sampled air at 2‐m‐depth intervals from the top 15 m of the firn at two Antarctic sites, Siple Dome and South Pole, including a winter sampling at the pole. We analyzed 15N/14N, 40Ar/36Ar, 40Ar/38Ar, 18O/16O of O2, O2/N2, 84Kr/36Ar, and 132Xe/36Ar. The results show the expected pattern of fractionation and match a gas diffusion model based on first principles to within 30%. Although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision, relative values of different gas pairs may. At Siple Dome, δ40Ar/4 is 66 ± 2% as sensitive to thermal diffusion as δ15N, in agreement with laboratory calibration; δ18O/2 is 83 ± 3%, and δ84Kr/48 is 33 ± 3% as sensitive as δ15N. The corresponding figures for summer South Pole are 64 ± 2%, 81 ± 3%, and 34 ± 3%. Accounting for atmospheric change, the figure for δO2/N2/4 is 90 ± 3% at Siple Dome. Winter South Pole shows a strong depletion of heavy gases as expected. However, the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities (except for Kr, constant at 34 ± 4%), suggesting the action of some other process that is not currently understood. No evidence for wintertime convection or a rectifier effect is seen.