bluuebunny/crossref_metadata_2025 huggingface dataset:
Neotectonics of Puerto Rico and the Virgin Islands, northeastern Caribbean, from GPS geodesy
Authors: Jansma Pamela E. , Mattioli Glen S. , Lopez Alberto , DeMets Charles , Dixon Timothy H. , Mann Paul , Calais Eric
Published: February 2004

The boundary between the North American and Caribbean plates is characterized primarily by left‐lateral motion along predominantly east‐west striking faults. Seismicity and marine geophysical survey data are consistent with at least two, and possibly three, microplates in the diffuse boundary zone in the northeastern Caribbean: (1) the Gonave, (2) the Hispaniola, and (3) the Puerto Rico‐northern Virgin Islands (PRVI). We discuss results from GPS geodetic measurements acquired since 1994 to test the microplate hypothesis, define PRVI translation and rotation within the boundary zone, and constrain PRVI neotectonics. GPS‐derived velocities are analyzed with respect to both North American and Caribbean plate reference frames. Integrated displacements across PRVI are limited to a few millimeters per year, consistent with a rigid PRVI and permitting calculation of an average velocity for PRVI. The motions of PRVI relative to North America and the Caribbean are 16.9±1.1 mm/yr toward N68°E±3° (1σ) and 2.4±1.4 mm/yr toward S79°W±26° (1σ), respectively. In contrast with some recent models, ongoing rotation of PRVI about a nearby (< 25° distant) vertical axis is not supported by the geodetic data. In addition, we argue against eastward tectonic escape of PRVI and favor a simple, progressive increase in velocity across the plate boundary zone, requiring that the summed magnitude of strike‐slip fault slip rates will equal the total plate motion rate between the Caribbean and North America. GPS data are consistent with components of left‐lateral strike‐slip faulting along the Muertos trough south of Puerto Rico and shortening across the Puerto Rico trench. Comparison of GPS velocities for PRVI with respect to North America with total North America‐Caribbean relative motion suggests up to 85% of North American‐Caribbean plate motion is accommodated by the Puerto Rico trench and offshore faults north of Puerto Rico. Differences in GPS‐derived velocities from Hispaniola and PRVI yield east‐west extension across the N‐S trending Mona rift of a few millimeters per year when estimated elastic strain accumulation effects along the north Hispaniola deformed belt and the Septentrional fault zone are considered. The opening rate implies an age of the Mona rift of 2–3 million years, agreeing with marine geophysical data that support a young age for the structure.
A 1°×1° resolution data set of historical anthropogenic trace gas emissions for the period 1890–1990
Authors: van Aardenne J. A. , Dentener F. J. , Olivier J. G. J. , Goldewijk C. G. M. Klein , Lelieveld J.
Published: February 2004

An anthropogenic emissions data set has been constructed for CO2, CO, CH4, nonmethane volatile organic compounds, SO2, NOx, N2O, and NH3spanning the period 1890–1990. The inventory is based on version 2.0 of the Emission Database for Global Atmospheric Research (EDGAR 2.0). In EDGAR the emissions are calculated per country and economic sector using an emission factor approach. Calculations of the emissions with 10 year intervals are based on historical activity statistics and selected emission factors. Historical activity data were derived from the Hundred Year Database for Integrated Environmental Assessments (1890–1990) supplemented with other data and our own estimates. Emission factors account for changes in economical and technological developments in the past. The calculated emissions on a country basis have been interpolated onto a 1°×1° grid. This consistent data set can be used in trend studies of tropospheric trace gases and in environmental assessments, for example, the analysis of historical contributions of regions and countries to environmental forcing like the enhanced greenhouse gas effect, acidification, and eutrofication. The database focuses on energy/industrial and agricultural/waste sources; for completeness, historical biomass‐burning estimates where added using a simple and transparent approach.
Carbon to phosphorus ratios in sediments: Implications for nutrient cycling
Authors: Anderson L. D. , Delaney M. L. , Faul K. L.
Published: February 2004

Defining the transformations of phosphorus (P) in sediments is critical to assessing the geochemical behavior of sedimentary P and the behavior of reactive P (Preactive= Poxide‐associated + Pauthigenic + Porganic) relative to organic carbon (Corganic). This study includes the determination of P components and Corganic concentrations on samples from sites in the California Current region (Ocean Drilling Program (ODP) leg 167, sites 1010, 1011, 1012, 1014, 1016, and 1021), in the Benguela Current region (ODP leg 175, sites 1082, 1084, and 1085), and on Blake Nose in the western Atlantic (ODP leg 17 1B, sites 1050 and 1052). Sample depths range up to 650 m, and ages range up to 65 Ma. Our data demonstrate the transformation of labile forms of P (Porganic and Poxide‐associated) to Pauthigenic throughout the sediment column in all redox states and at all sedimentation rates investigated, until a substantial portion of reactive P is in the authigenic form (>80%). (C/P)organic ratios are always greater than or equal to the Redfield Ratio in all of the sediments we studied. Sediments with Corganic concentrations ≤2 wt % have Corganic/Preactive ratios less than the Redfield Ratio, and Corganic/Preactive ratios increase linearly with Corganic, consistent with degradation of Corganic with age. In general, Corganic/Preactive ratios for sediments with Corganic concentrations ≥2 wt %, from a variety of redox states and sedimentation rates, center around the Redfield Ratio, although some ratios are as high as 3 times the Redfield Ratio. However, Corganic/ Preactive ratios (up to 400) do not approach the high ratios observed for (C/P)organic ratios (up to 4500). Because of the efficient transfer of labile forms of P to Pauthigenic, Corganic/Preactive ratios better describe the geochemical behavior of sedimentary P.
Carbon isotope geochemistry of the Santa Clara River
Authors: Masiello Caroline A. , Druffel Ellen R. M.
Published: February 2004

The Santa Clara River is a prototypical small mountainous river, with a headwater height greater than 1000 m and a basin area smaller than 10,000 m 2. Although individual small mountainous rivers export trivial amounts of sediment and carbon to the ocean, as a group these rivers may export a major fraction (as much as 50%) of the total global river sediment flux [Milliman and Syvitski, 1992], making their geochemistry relevant the study of the ocean's carbon cycle. In addition, many small rivers export sediment in a few high flux events, causing massive, sporadic discharge of carbon onto coastal shelves, discharge conditions very different from those of large rivers. This class of rivers is an end‐member of the river‐ocean carbon exchange system,. opposite the Earth's largest river, the Amazon. The carbon mass and isotopic properties of the Santa Clara River are significantly different from previously studied large rivers. During the 1997–1998 winter, all Santa Clara carbon pools were old, with flux‐weighted average Δl4C values of−428±76‰ for particulate organic carbon, −73±31‰ for dissolved organic carbon, and−644±58‰ for black carbon. The age of exported carbon is primarily due to the deep erosion of old soils and not to inclusion of fossil fuel carbon. Additionally, the δ13C signatures of exported carbon pools were high relative to terrestrial carbon, bearing a signature quite similar to marine carbon (average particulate organic carbon (POC) δ13C = −22.2±0.8‰). The Santa Clara's estuary is small and drains onto the narrow eastern Pacific coastal margin, exporting this old soil organic matter directly into the ocean. If the Santa Clara export patterns are representative of this class of rivers, they may be a significant source of refractory terrestrial carbon to the ocean.
Carbon dynamics in peat bogs: Insights from substrate macromolecular chemistry
Authors: Kuder Tomasz , Kruge Michael A.
Published: February 2004

The macromolecular compositions of subfossil plants from boreal Sphagnum bogs and restiad bogs (New Zealand) have been studied by pyrolysis‐gas chromatography/mass spectrometry to evaluate the extent of degradation in the anoxic zone (catotelm) of a peat bog. Degradation of vascular plant polysaccharides was apparent only into the upper catotelm. Sphagnum was degraded more slowly than vascular plants, but no cessation of degradation was observed. The inferred rate of degradation varied depending on type of plant, extent of aerobic, precatotelmic degradation, and mode of litter deposition (rooting versus at the surface). Environmental forcing on anaerobic carbon dynamics would potentially be largest if the hydrology was disturbed at a wet and vascular plant‐rich site. Peat deposited under a dry regime would be relatively inert in anaerobic conditions. Although catotelmic degradation is usually not extensive, in some cases, if labile organic matter is retained in the aerobic phase (e.g., restiad bogs) a major fraction of peat is degraded in catotelm, potentially resulting in a delayed major export of 14C‐old methane.
Iron supply and demand in the upper ocean: Is extraterrestrial dust a significant source of bioavailable iron?
Authors: Johnson Kenneth. S.
Published: February 2004

Interplanetary dust particles accrete on the Earth at a rate of ∼40 ktons yr−1. Some 90% of this material evaporates in the atmosphere, producing a bioavailable iron flux of 3×10−7 mol Fe m−2 yr−1. This extraterrestrial Fe flux is 30–300% of the eolian flux of bioavailable iron transported from terrestrial sources in remote marine regions and ∼20% of the upwelled Fe flux in the Southern Ocean. Extraterrestrial Fe may play an important role in regulating the marine carbon cycle in these regions.
Estimating net air‐sea fluxes from ocean bulk data: Methodology and application to the heat cycle
Authors: Gloor Manuel , Gruber Nicolas , Hughes Tertia M. C. , Sarmiento Jorge L.
Published: February 2004

A novel method to estimate annual mean heat, water, and gas exchange fluxes between the ocean and the atmosphere is proposed that is complementary to the traditional approach based on air‐sea gradients and bulk exchange parameterization. The new approach exploits the information on surface exchange fluxes contained in the distribution of temperature, salinity, and dissolved gases in the ocean interior. We use an Ocean General Circulation Model to determine how the distribution in the ocean interior is linked to surface fluxes. We then determine with least squares the surface fluxes that are most compatible with the observations. To establish and test the method, we applyit to ocean temperature data to estimate heat fluxes across the air‐sea interface for which a number of climatological estimates exists. We also test the sensitivity of the inversion results to data coverage, differences in ocean transport, variations in the surface flux pattern and a range of spatial resolutions. We find, on the basis of the World Ocean Circulation Experiment (WOCE) data network augmented with selected high‐quality pre‐WOCE data, that we are able to constrain heat exchange fluxes for 10–15 regions of the ocean, whereby these fluxes nearly balance globally without enforcing a conservation constraint. Our results agree well with heat flux estimates on the basis of bulk exchange parameterization, which generally require constraints to ensure a global net heat flux of zero. We also find that the heat transports implied by our inversely estimated fluxes are in good agreement with a large range of heat transport estimates based on hydrographic data. Increasing the number of regions beyond the 10–15 regions considered here is severely limited because of modeling errors. The inverse method is fairly robust to the modeling of the spatial patterns of the surface fluxes; however, it is quite sensitive to the modeling of ocean transport. The most striking difference between our estimates and the heat flux climatologies is a large heat loss of 0.64 PW to the atmosphere from the Southern Ocean and a large heat gain by the subpolar South Atlantic of 0.56 PW. These results are consistent with the large gain of carbon dioxide called for by Takahashi et al. [1999] in his recent analysis of the air‐sea flux of carbon dioxide but inconsistent with the large loss of oxygen and carbon dioxide such as those of Stephens et al. [1998].
Radium 228 based nitrate fluxes in the eastern Indian Ocean and the South China Sea and a silicon‐induced “alkalinity pump” hypothesis
Authors: Nozaki Yoshiyuki , Yamamoto Yoshiyuki
Published: February 2004

The activities of 228Ra and 226Ra in the seawaters of the eastern Indian Ocean and its adjacent seas were measured during the 1996/1997 “Piscis Austrinus” Expedition using R/V Hakuho‐Maru. The surface Ra data are in good agreement with those in literature and indicate that the continental shelves of the southeast Asian Seas are strong sources for 228Ra and, to much lesser extent, for 226Ra which are largely transported by the surface currents into the Indian Ocean. The vertical profiles of 228Ra obtained at six stations from the different oceanic regimes along the cruise track generally showed an almost exponential decrease from the surface to the middepths of 1200‐ 1500 m. Coupling the vertical profiles of 228Ra and nitrate and using the vertical one‐dimensional model developed by Ku et al., [1995], we estimated the nitrate fluxes into the euphotic zone ranging from 1.5 mmol N m−2 d−1 (or 0.55 mol m−2 yr−1) in Andaman Sea to 4.0 mmol N m−2 d−1 (1.5 mol m−2 yr−1) in the Southern Ocean. In steady state, these nitrate fluxes correspond to the new production of 11–26 mmol C m−2 d−1 (or 3.9–9.6 mol C m−2 yr−1). In the sub‐Antarctic region between 30° and 40°S, north of the polar front, dissolved Si is depleted relative to nitrate in the upper water column as compared to their ∼1:1 molar ratio demanded by diatoms. In such high‐nitrate but low‐silicate regions, growth of diatoms is likely to be limited by available silicate. It also favors calcareous coccoliths to grow by utilization of nitrate, as deduced from a decrease in alkalinity in the surface waters. During the last glacial period, supply of atmospheric dust containing silicon and iron to the surface ocean was much more intense than that during the interglacial period, and hence, diatom production might have been enhanced in the Si‐depleted regions. Diatoms would utilize nitrate readily and inhibit growth of coccoliths, resulting in increases in alkalinity and pH in the surface water. This species change of phytoplankton may account for the lowered atmospheric CO2 level during the glacial period without causing a large‐scale depletion of dissolved oxygen in the deep sea. The records of opal and carbonate contents in the Antarctic sediment cores studied by Charles et al. [1991] seem to be consistent with this silicon‐ induced alkalinity pump hypothesis.
Carbon gas exchange at a southern Rocky Mountain wetland, 1996–1998
Authors: Wickland Kimberly P. , Striegl Robert G. , Mast M. Alisa , Clow David W.
Published: February 2004

Carbon dioxide (CO2) and methane (CH4) exchange between the atmosphere and a subalpine wetland located in Rocky Mountain National Park, Colorado, at 3200 m elevation were measured during 1996–1998. Respiration, net CO2flux, and CH4flux were measured using the closed chamber method during snow‐free periods and using gas diffusion calculations during snow‐covered periods. The ranges of measured flux were 1.2‐526 mmol CO2m−2d−1(respiration), −1056−100 mmol CO2m−2d−1(net CO2exchange), and 0.1–36.8 mmol CH4m−2d−1(a positive value represents efflux to the atmosphere). Respiration and CH4emission were significantly correlated with 5 cm soil temperature. Annual respiration and CH4emission were modeled by applying the flux‐temperature relationships to a continuous soil temperature record during 1996–1998. Gross photosynthesis was modeled using a hyperbolic equation relating gross photosynthesis, photon flux density, and soil temperature. Modeled annual flux estimates indicate that the wetland was a net source of carbon gas to the atmosphere each of the three years: 8.9 mol C m−2yr−1in 1996, 9.5 mol C m−2yr−1in 1997, and 9.6 mol C m−2yr−1in 1998. This contrasts with the long‐term carbon accumulation of ∼0.7 mol m−2yr−1determined from14C analyses of a peat core collected from the wetland.
Estimating methane production rates in bogs and landfills by deuterium enrichment of pore water
Authors: Siegel D. I. , Chanton J. R. , Glaser P. H. , Chasar L. S. , Rosenberry D. O.
Published: February 2004

Raised bogs and municipal waste landfills harbor large populations of methanogens within their domed deposits of anoxic organic matter. Although the methane emissions from these sites have been estimated by various methods, limited data exist on the activity of the methanogens at depth. We therefore analyzed the stable isotopic signature of the pore waters in two raised bogs from northern Minnesota to identify depth intervals in the peat profile where methanogenic metabolism occurs. Methanogenesis enriched the deuterium (2H) content of the deep peat pore waters by as much as +11‰ (Vienna Standard Mean Sea Water), which compares to a much greater enrichment factor of +70‰ in leachate from New York City's Fresh Kills landfill. The bog pore waters were isotopically dated by tritium (3H) to be about 35 years old at 1.5 m depth, whereas the landfill leachate was estimated as ∼17 years old from Darcy flow calculations. According to an isotopic mass balance the observed deuterium enrichment indicates that about 1.2 g of CH4 m−3 d−1 were produced within the deeper peat, compared to about 2.8 g CH4 m−3 d−1 in the landfill. The values for methane production in the bog peat are substantially higher than the flux rates measured at the surface of the bogs or at the landfill, indicating that deeper methane production may be much higher than was previously assumed.
An Examination of the “Continental shelf pump” in an open ocean general circulation model
Authors: Yool Andrew , Fasham Michael J. R.
Published: February 2004

In a recent study of the shelf region of the East China Sea, Tsunogai et al. [1999] estimated that a combination of air‐sea exchange and biological and physical transport processes could transfer carbon from the shelf region into the open ocean at a rate of 35 g C m−2 yr−1. Contrasting with the solubility and biological pumps of the open ocean, they described this collective activity as the “continental shelf pump” and suggested that if this pump operated throughout the world's shelf regions, it could be responsible for ocean uptake of ∼1 Gt C yr−1 (≈50% current ocean uptake of anthropogenic CO2). In this work a general circulation model (GCM) is used to explore the potential strength of this pump across the world's shelves. Since the GCM does not represent the continental shelf regions explicitly, a parameterization of the pump has been used. Results of simulations find modeled pump activity very variable between shelf regions, with the East China Sea shelf behaving very similarly to the global average. Storage of pump carbon is particularly high in the Atlantic Ocean and other regions where deep water is formed. A considerable reservoir of pump carbon becomes trapped under the Arctic ice sheet. Simple extrapolations from the results suggest that should shelf regions absorb CO2 at the rate of the East China Sea, the pump would account for a net oceanic uptake of 0.6 Gt C yr−1.
Fractionation of oxygen isotopes by respiration and diffusion in soils and its implications for the isotopic composition of atmospheric O2
Authors: Angert Alon , Luz Boaz , Yakir Dan
Published: February 2004

The 18O content of atmospheric O2 is an important tracer for past changes in the biosphere and has been used to estimate changes in the balance between terrestrial and marine productivity. Its quantitative use depends on knowledge of the isotopic fractionations associated with the various O2 production and consumption processes. Here we monitored oxygen concentration and δ18O of O2 in sandy and clayey soils to evaluate in situ 18O fractionation associated with soil respiration. In the clayey soil, O2 concentrations decreased as low as 1% at 150 cm depth, and δ18O values ranged from 0‰ to – 1.6‰ relative to atmospheric O2. In the sandy soil the O2 concentration was 20.38−20.53‰, and δ18O values were −0.06 ± 0.015‰ to 0.06 ± 0.015‰ relative to atmospheric O2. Using the observed [O2] and δ18O profiles and their change with time, together with a one‐box analytical model and a five‐box numerical model, a mean discrimination of 12 ± 1% was estimated for the two sites (including effects of concentration and temperature gradients). This low discrimination was consistent with that determined in closed‐system soil incubation experiments (8.4–16.9‰). The current understanding of the composition of air O2 attributes the magnitude of the fractionation in soil respiration to biochemical mechanisms alone (about 18‰ and 25–30‰ in cyanide‐sensitive and cyanide‐resistant respiration, respectively). The low discrimination we report is significantly less than in dark respiration and is explained by diffusion limitation in soil aggregates and root tissues that results in low O2 concentration in the consumption site. Soil respiration is a major component of the global oxygen uptake, and the potential contribution of low discrimination, such as observed here, to the global Dole effect should be considered in global‐scale studies.
Global warming feedbacks on terrestrial carbon uptake under the Intergovernmental Panel on Climate Change (IPCC) Emission Scenarios
Authors: Joos Fortunat , Prentice I. Colin , Sitch Stephen , Meyer Robert , Hooss Georg , Plattner Gian‐Kasper , Gerber Stefan , Hasselmann Klaus
Published: February 2004

A coupled physical‐biogeochemical climate model that includes a dynamic global vegetation model and a representation of a coupled atmosphere‐ocean general circulation model is driven by the nonintervention emission scenarios recently developed by the Intergovernmental Panel on Climate Change (IPCC). Atmospheric CO2, carbon sinks, radiative forcing by greenhouse gases (GHGs) and aerosols, changes in the fields of surface‐air temperature, precipitation, cloud cover, ocean thermal expansion, and vegetation structure are projected. Up to 2100, atmospheric CO2 increases to 540 ppm for the lowest and to 960 ppm for the highest emission scenario analyzed. Sensitivity analyses suggest an uncertainty in these projections of −10 to +30% for a given emission scenario. Radiative forcing is estimated to increase between 3 and 8 W m−2 between now and 2100. Simulated warmer conditions in North America and Eurasia affect ecosystem structure: boreal trees expand poleward in high latitudes and are partly replaced by temperate trees and grasses at lower latitudes. The consequences for terrestrial carbon storage depend on the assumed sensitivity of climate to radiative forcing, the sensitivity of soil respiration to temperature, and the rate of increase in radiative forcing by both CO2 and other GHGs. In the most extreme cases, the terrestrial biosphere becomes a source of carbon during the second half of the century. High GHG emissions and high contributions of non‐CO2 agents to radiative forcing favor a transient terrestrial carbon source by enhancing warming and the associated release of soil carbon.
Numerical models of the geodynamo and observational constraints
Authors: Dormy Emmanuel , Valet Jean‐Pierre , Courtillot Vincent
Published: February 2004

The past few years have seen the emergence of a large number of numerical simulations of the geodynamo. In parallel, both new and old geomagnetic, archeomagnetic, and paleomagnetic observations have been interpreted as actual geomagnetic features and used as constraints for dynamo models. Naturally, model predictions should be tested against actual characteristics of the geomagnetic field. Despite huge differences (sometimes in excess of a billion) between the values of parameters used in the simulations and those estimated for the Earth, it is intriguing that many available simulations succeed in producing largely axial dipolar magnetic fields with weaker nondipolar structures, in agreement with the first‐order characteristics of the geomagnetic field. Yet, when considering finer characteristics, there are significant differences, and failures to actually produce a number of fundamental characteristic features. In this presentation, we first review numerical results obtained to date, then we attempt to summarize which field characteristics derived from observational data sets can be considered robust. On the basis of simple criteria used to evaluate the degree of confidence that can be placed in each datum, we sort presumably characteristic geomagnetic features into three categories (robust, controversial, and unlikely). We conclude that numerical models should be illustrated with a number of key “predictions,” averaged over at least 10 dipole diffusion times. These predictions should be tested against the subset of robust observations only. Controversial observations should await additional confirmation.
Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites
Authors: Shervais John W.
Published: February 2004

Suprasubduction zone (SSZ) ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites. This sequence includes the following: (1) birth, which entails the formation of the ophiolite above a nascent or reconfigured subduction zone; this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex; (2) youth, during which is continued melting of refractory asthenosphere (depleted during birth) occurs in response to fluid flux from the subducting slab, with extensional deformation of the older plutonic suite, eruption of refractory lavas, and the intrusion of wehrlite‐pyroxenite; (3) maturity, with the onset of semistable arc volcanism, typically calc‐alkaline, as the subduction zone matures and stabilizes, and the intrusion of quartz diorite and eruption of silicic lavas; and (4) death, which is the sudden demise of active spreading and ophiolite‐related volcanism, which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis; expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites; (5) resurrection, with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction. The early stages (1–3) may be diachronous, and each stage may overlap in both time and space. The existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the SSZ tectonic setting.
Continental crust formation by crustal delamination in subduction zones and complementary accumulation of the enriched mantle I component in the mantle
Authors: Tatsumi Yoshiyuki
Published: February 2004

A possible mechanism for continental crust formation, which includes slab‐dehydration‐induced mantle melting, basalt magma generation, remelting of the initial basaltic crust, followed by delamination of the residue, is examined by geochemical modeling of dehydration, partial melting, and fluid‐solid reactions. The results suggest that such processes can explain the major and trace element compositions of the andesitic bulk continental crust. Isotopic modeling suggests that a pyroxenitic delaminated component formed at 3–4 Ga possesses Sr‐Nd‐Pb isotopic compositions typical of the EMI component, one of the enriched geochemical endmembers in the mantle. Continental crust formation and complementary accumulation of the EMI reservoir in the deep mantle may have taken place simultaneously in Archean subduction zones.
Geologic evidence for a mantle superplume event at 1.9 Ga
Authors: Condie Kent C. , Des Marais David J. , Abbott Dallas
Published: February 2004

Both preserved and restored areal distributions of Proterozoic marine intracratonic, passive margin, and platform sediments show a prominent peak at ∼1.9 Ga, indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time. The chemical index of alteration in shales deposited at this time was high, suggesting warm climates, possibly due to enhanced CO2levels in the atmosphere. High sea level and warms climate may also explain an abundance of black shale, banded iron formations, and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1.9 Ga. All of these observations are consistent with a 1.9‐Ga superplume event. The occurrence of only a minor positive carbon isotope shift in marine carbonates at 1.9 Ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present. Slightly low87Sr/86Sr isotopic ratios in seawater at 1.9 Ga reflect increased mantle input of Sr from the proposed superplume event, whereas higher ratios at 1.85–1.75 Ga may reflect increased input of continental Sr from a growing supercontinent. The first massive sulfate evaporites in the geologic record at 1.8–1.6 Ga follow the possible 1.9‐Ga superplume event. This may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume‐related volcanism wanes.
Across‐arc geochemical trends in the Izu‐Bonin arc: Contributions from the subducting slab
Authors: Hochstaedter Alfred , Gill Jim , Peters Robert , Broughton Phil , Holden Pete , Taylor Brian
Published: February 2004

We propose that across‐arc differences in the geochemistry of Izu‐Bonin arc magmas are controlled by the addition of fertile‐slab fluids to depleted mantle at the volcanic front, and residual‐slab fluids to fertile mantle in the back arc without slab melting or contemporaneous back arc spreading. The arc consists of a volcanic front, an extensional zone, and seamount chains (the Western Seamounts) that trend into the Shikoku Basin. Each province produces a distinct suite of arc‐like volcanic rocks that have relative Nb depletions and high ratios of fluid‐mobile elements to high field strength elements. The volcanic front has the lowest concentrations of incompatible elements and the strongest relative enrichments of fluid‐mobile elements (high U/Nb, Ba/Nb, Pb/Zr, Th/Nb, 206Pb/204Pb, ɛNd, and 87Sr/86Sr). A fluid derived from both sediment and altered oceanic crust explains most of the slab‐related characteristics of the volcanic front. The Western Seamounts and some of the extensional zone rocks have lower ɛNd, 87Sr/86Sr, 206Pb/204Pb, Ba/Th, and U/Th; moderate Ba/Nb and U/Nb; and similar or higher Th/Nb and Th/Nd. Although the lower ɛNd and higher Th/Nd tempt a sediment melt explanation, a lack of correlation between the strongest sediment proxies, such as ɛNd, Th/Nb, and Ce/Ce*, precludes sediment melts. The subduction component for the Western Seamounts is probably a fluid dehydrated from a residual slab that was depleted in fluid‐mobile elements beneath (as well as trenchward of) the volcanic front. This depleted fluid is added to elementally and isotopically more enriched mantle beneath the Western Seamounts.
A three‐plate kinematic model for Lau Basin opening
Authors: Zellmer Kirsten E. , Taylor Brian
Published: February 2004

We present revised compilations of bathymetry, magnetization, acoustic imagery, and seismicity data for the Lau Basin and surrounds. We interpret these data to more precisely locate the plate boundaries in the region and to derive a three‐plate kinematic model for the opening of the Lau Basin during the Brunhes Chron. Our tectonic model includes the Niuafo'ou microplate, which is separated from the Australian Plate along the Peggy Ridge‐Lau Extensional Transform Zone‐Central Lau Spreading Center and from the Tongan Plate along the newly discovered Fonualei Rift and Spreading Center. Our model shows that Australia‐Tonga plate motion between 15.5°S and 19°S is partitioned across the microplate, and it resolves the former apparent conflict between geodetic versus seafloor spreading velocities. Regionally, this implies that the angular rate of opening of the Lau Basin has been faster than that of the Havre Trough to the south and that the rapid current rates of Pacific subduction at the Tonga Trench (∼240 mm/yr at 16°S) have been sustained for at least 0.78 m.y.
Relationships between the trace element composition of sedimentary rocks and upper continental crust
Authors: McLennan Scott M.
Published: February 2004

Estimates of the average composition of various Precambrian shields and a variety of estimates of the average composition of upper continental crust show considerable disagreement for a number of trace elements, including Ti, Nb, Ta, Cs, Cr, Ni, V, and Co. For these elements and others that are carried predominantly in terrigenous sediment, rather than in solution (and ultimately into chemical sediment), during the erosion of continents the La/element ratio is relatively uniform in clastic sediments. Since the average rare earth element (REE) pattern of terrigenous sediment is widely accepted to reflect the upper continental crust, such correlations provide robust estimates of upper crustal abundances for these trace elements directly from the sedimentary data. Suggested revisions to the upper crustal abundances of Taylor and McLennan [1985] are as follows (all in parts per million): Sc = 13.6, Ti = 4100, V = 107, Cr = 83, Co = 17, Ni = 44, Nb = 12, Cs = 4.6, Ta = 1.0, and Pb = 17. The upper crustal abundances of Rb, Zr, Ba, Hf, and Th were also directly reevaluated and K, U, and Rb indirectly evaluated (by assuming Th/U, K/U, and K/Rb ratios), and no revisions are warranted for these elements. In the models of crustal composition proposed by Taylor and McLennan [1985] the lower continental crust (75% of the entire crust) is determined by subtraction of the upper crust (25%) from a model composition for the bulk crust, and accordingly, these changes also necessitate revisions to lower crustal abundances for these elements.
Rise of atmospheric oxygen and the “upside‐down” Archean mantle
Authors: Kump Lee R. , Kasting James F. , Barley Mark E.
Published: February 2004

The establishment of an oxygen‐rich atmosphere dramatically altered the evolution of life on Earth. Most of the recent discussion of the topic has been focused on the timing of the event rather than on its mechanism. Here we draw upon recent developments in the understanding of Earth's interior to propose that the rise of oxygen followed a geologically abrupt period of mantle overturn and/or intense plume activity near the Archean‐Proterozoic transition, 2470–2450 million years ago. The magmatic event has already been linked to the widespread deposition of oxide‐facies banded iron formation, and the rise of oxygen has been implicated as the trigger for iron deposition and Earth's first major glaciation. We argue that these events are all related to a change in redox state of volcanic gases brought about by deep‐seated Late Archean and earliest Paleoproterozoic magmatism.
Vibrations from the “Perfect Storm”
Authors: Bromirski Peter D.
Published: February 2004

Microseismic vibrations during the famous October 1991 “Perfect Storm” were observed at seismic stations across North America. The extreme wave conditions during this storm, in conjunction with the occurrence of Hurricane Grace to the south, are ideal for studying where such vibrations originate and their inland propagation. High‐amplitude primary and double‐frequency (DF) microseisms were observed at broadband seismic station HRV in eastern Massachusetts. Similar spectral variation observed at seismic station ANMO at Albuquerque, New Mexico, shows transcontinental propagation of vibrations from the Perfect Storm. Cross correlation between wave spectra from widely separated buoy measurements and corresponding DF microseism spectra at HRV give high‐correlation coefficients, R2, from the New England coast to Cape Hatteras. Contours of peak R2 scaled by the magnitude of the lag at the peak, together with similarities between wave and microseism spectral variation, imply that the dominant source area of DF microseisms during the Perfect Storm is near the southern Massachusetts coast, not in the open ocean where the highest waves occurred.
Dynamical influences on the distribution and loading of SO2 and sulfate over North America, the North Atlantic, and Europe in April 1987
Authors: Benkovitz Carmen M. , Miller Mark A. , Schwartz Stephen E. , Kwon O‐Ung
Published: February 2004

The loading of tropospheric aerosols is highly variable spatially and temporally as a consequence of large spatial variability in sources, temporal variability in transport winds, and short residence times (days), and additionally for secondary sulfate, intermittent production associated with aqueous‐phase reaction in clouds, and intermittent removal, mainly by precipitation. We have used a chemical transport and transformation model for atmospheric sulfur driven by observationally derived meteorological data to calculate the geographical distribution of sulfate and sulfur dioxide (SO2) over North America, the North Atlantic, and Europe as a function of time for April 1987. We present the results of these calculations, mainly as animations showing the time dependence of the column burden (vertical integral of concentration) and of sulfate wet deposition, and interpret the temporal evolution in terms of the controlling meteorological phenomena as discerned from synoptic analyzes at 925 and 500 hPa. The analyzes show highly localized and episodic buildup and removal of sulfate and SO2 and circulations under the influence of high‐ and low‐pressure systems as well as instances of rapid meridional and zonal transport over distances of thousands of kilometers. Key meteorological phenomena giving rise to large amounts of sulfate include not only slow moving high‐pressure systems, as previously recognized, but also low‐pressure systems. Whether or not a given low‐pressure system results in large sulfate loading depends strongly on its location relative to sources of sulfur species and of the water vapor that is required for formation of clouds and precipitation. We present a detailed analysis of the evolution of sulfate and SO2 over the eastern North Atlantic April 3–8, 1987, under the influence of a cutoff low‐pressure system during which sources in northern Europe continued to feed a system that repeatedly circulated over source regions. Animations such as these facilitate identification of episodes of large aerosol loading and relating these loadings to the controlling meteorological phenomena.
Tracing a mantle plume: Isotopic and trace element variations of Galápagos seamounts
Authors: Harpp Karen S. , White William M.
Published: February 2004

Isotopic and trace element analyses of basalts dredged from across the Galápagos Platform confirm the previously established east facing horseshoe pattern of depleted geochemical signatures at the center of the archipelago and more enriched signatures along the periphery. Statistical analysis of the isotopic data indicates that geochemical variations in the Galápagos cannot be explained by mixing between only the plume and the depleted asthenosphere. Instead, four isotopically distinct end‐members must be interacting to account for the subtleties of the Sr, Nd, Pb, and He isotopic data. Three of the components are geographically restricted: one in the south, one in the central region, and one in the north. These three plume components then mix with the fourth component, depleted mantle, which is indistinguishable from the MORB source. The central component resembles the high 3He/4He mantle reservoir that may be common to many plumes and has variously been called PHEM, FOZO, and C by others. Whereas this mantle reservoir appears to make a minor contribution to the composition of most hot spot systems, it may constitute the main body of the Galápagos plume. Geographic distribution of the end‐members suggests that the plume is centered near Fernandina ∼92°N but may be significantly diluted by depleted mantle even near the main conduit. The geochemically distinct end‐members trace an eastward mantle flow, manifested as decreasing contributions of the plume in the direction of plate motion. The plume may be tilted by shear in the asthenosphere from plate motion and ambient mantle flow. As it is bent, the plume thermally entrains surrounding upper mantle, resulting in the horseshoe‐shaped distribution of depleted and enriched material. The end‐members may also outline a deep, strong lateral flow of mantle toward the Galápagos Spreading Center, supplying plume material to the ridge system. Overall, our results suggest that the Galápagos hot spot is both compositionally and dynamically complex owing to its tectonic setting adjacent to a mid‐ocean ridge.
Hf isotope geochemistry of the Galapagos Islands
Authors: Blichert‐Toft Janne , White William M.
Published: February 2004

We report 176Hf/177Hf ratios determined by multicollector inductively coupled plasma mass spectrometry on 49 basalts from the Galapagos Islands. The ɛHf values range from about +8, a value typical of many oceanic island basalts, to about +15, a value typical of mid‐ocean ridge basalt. The ɛHf shows the expected correlation with ɛNd and inverse correlations with Sr and Pb isotope ratios. The range of values and correlations observed is consistent with Galapagos mantle plume‐asthenosphere mixing inferred from previous geochemical studies. While oceanic island basalt isotopic arrays suggest that multicomponent mixing is common in the sources of such basalts, the Galapagos is unusual in that depleted upper mantle appears to be a principal component. Thermal and dynamic interaction between the Galapagos mantle plume and the lithosphere and shallow asthenosphere is the most likely cause of mixing and of the geographic pattern of geochemical variation. The Galapagos Hf‐Nd‐Sr‐Pb isotopic array diverges toward the “plume” end, indicating the plume is heterogeneous. This heterogeneity is geographic, with distinct northern, central, and southern components. The central component and, to a lesser degree, the northern component are compositionally similar to the “common component” of plumes (variously called PHEM, FOZO, and “C”), while the southern component is unique with relatively high ɛHf and 206Pb/204Pb compared to 87Sr/86Sr and ɛNd.
Sources and distributions of dust aerosols simulated with the GOCART model
Authors: Ginoux Paul , Chin Mian , Tegen Ina , Prospero Joseph M. , Holben Brent , Dubovik Oleg , Lin Shian‐Jiann
Published: February 2004

The global distribution of dust aerosol is simulated with the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. In this model all topographic lows with bare ground surface are assumed to have accumulated sediments which are potential dust sources. The uplifting of dust particles is expressed as a function of surface wind speed and wetness. The GOCART model is driven by the assimilated meteorological fields from the Goddard Earth Observing System Data Assimilation System (GEOS DAS) which facilitates direct comparison with observations. The model includes seven size classes of mineral dust ranging from 0.1–6 μm radius. The total annual emission is estimated to be between 1604 and 1960 Tg yr−1 in a 5‐year simulation. The model has been evaluated by comparing simulation results with ground‐based measurements and satellite data. The evaluation has been performed by comparing surface concentrations, vertical distributions, deposition rates, optical thickness, and size distributions. The comparisons show that the model results generally agree with the observations without the necessity of invoking any contribution from anthropogenic disturbances to soils. However, the model overpredicts the transport of dust from the Asian sources to the North Pacific. This discrepancy is attributed to an overestimate of small particle emission from the Asian sources.
What controls the temperature of the Arctic stratosphere during the spring?
Authors: Newman Paul A. , Nash Eric R. , Rosenfield Joan E.
Published: February 2004

The temperature of the polar lower stratosphere during spring is the key factor in changing the magnitude of ozone loss in the Arctic polar vortex. In this paper, we quantitatively demonstrate that the polar lower stratospheric temperature is primarily controlled by planetary‐scale waves. We use National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data covering the last two decades to investigate how these planetary waves are connected to polar lower stratospheric temperatures. In particular, we show that the tropospheric eddy heat flux in middle to late winter (January‐February) is highly correlated with the mean polar stratospheric temperature during March. These planetary waves are forced by both thermal and orographic processes in the troposphere and propagate into the stratosphere in the middle and high latitudes. Strong midwinter planetary wave forcing leads to a warmer spring Arctic lower stratosphere in early spring, while weak midwinter forcing leads to cooler spring Arctic temperatures. In addition, this planetary wave driving also has a strong impact on the strength of the polar vortex.
Indirect sulphate aerosol forcing in a climate model with an interactive sulphur cycle
Authors: Jones Andy , Roberts David L. , Woodage Margaret J. , Johnson Colin E.
Published: February 2004

The effects of anthropogenic sulphate aerosol on cloud albedo and on precipitation efficiency (the first and second indirect effects, respectively) are investigated using a new version of the Hadley Centre climate model. This version includes a new cloud microphysics scheme, an interactive sulphur cycle, and a parameterization of the effects of sea salt aerosol. The combined global mean radiative impact from both indirect effects is estimated to be approximately −1.9 W m−2 in terms of the change in net cloud forcing, with the “albedo” effect dominating: we obtain values of −1.3 and −0.5 W m−2 for the first and second effects, respectively, when calculated separately. The estimate for the combined effect has at least a factor of 2 uncertainty associated with it: for example, alternative assumptions which affect the concentration of natural “background” sulphate aerosol reduce the forcing by over 25%, and different parameterizations of the autoconversion of cloud droplets to rainwater can double the forcing.
A simple parameterization for snowdrift sublimation over Antarctic snow surfaces
Authors: Bintanja Richard , Reijmer Carleen H.
Published: February 2004

The surface heat budget at two sites in the escarpment region of Dronning Maud Land, Antarctica, is evaluated using a surface energy balance model that is applied to meteorological data gathered in austral summer 1997–1998. Vertical profile observations indicate that the effective roughness lengths of momentum, and especially heat and moisture, increase with wind speed. This can be attributed to snowdrift, which in turn affects the budgets of heat and moisture through sublimation of airborne particles. The snowdrift‐induced increases in effective roughness lengths are parameterized in terms of friction velocity, which are used in the model to estimate the total latent heat flux and the contributions of the surface and snowdrift components of sublimation. Snowdrift sublimation increases strongly with wind speed and compares fairly well with results from a more sophisticated method to estimate snowdrift sublimation. Calculations show that snowdrift sublimation is as large as surface sublimation at one site and significantly larger at the other colder and more windy site. The parameterization may be used in atmospheric models to evaluate more accurately total sublimation rates.