Asymmetric mantle dynamics in the MELT region of the East Pacific Rise     

D. R. Toomey  W. S. D. Wilcock  J. A. Conder  D. W. Forsyth  J. D. Blundy  E. M. Parmentier  W. C. Hammond 《Earth and Planetary Science Letters》2002,200(3-4):287-295

The mantle electromagnetic and tomography (MELT) experiment found a surprising degree of asymmetry in the mantle beneath the fast-spreading, southern East Pacific Rise (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Scheirer et al., Science 280 (1998) 1221–1224; Evans et al., Science 286 (1999) 752–756). Pressure-release melting of the upwelling mantle produces magma that migrates to the surface to form a layer of new crust at the spreading center about 6 km thick (Canales et al., Science 280 (1998) 1218–1221). Seismic and electromagnetic measurements demonstrated that the distribution of this melt in the mantle is asymmetric (Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Evans et al., Science 286 (1999) 752–756) at depths of several tens of kilometers, melt is more abundant beneath the Pacific plate to the west of the axis than beneath the Nazca plate to the east. MELT investigators attributed the asymmetry in melt and geophysical properties to several possible factors: asymmetric flow passively driven by coupling to the faster moving Pacific plate; interactions between the spreading center and hotspots of the south Pacific; an off-axis center of dynamic upwelling; and/or anomalous melting of an embedded compositional heterogeneity (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Evans et al., Science 286 (1999) 752–756). Here we demonstrate that passive flow driven by asymmetric plate motion alone is not a sufficient explanation of the anomalies. Asthenospheric flow from hotspots in the Pacific superswell region back to the migrating ridge axis in conjunction with the asymmetric plate motion can create many of the observed anomalies.  相似文献   

Authigenic uranium: Relationship to oxygen penetration depth and organic carbon rain   总被引：3，自引：0，他引：3  

James McManus  William M. Berelson  Douglas E. Hammond 《Geochimica et cosmochimica acta》2005,69(1):95-108

We have measured U in benthic incubation chambers, sediment pore waters, and in sediments along the California continental margin. Sedimentary U uptake rates, based on a combination of sediment pore water profiles and benthic incubation chambers, generally agree with those predicted from sediment accumulation rate data. This agreement supports the view that most of the continental margin sedimentary U is delivered by diffusion across the seawater-sediment boundary. The average rate of authigenic U accumulation for all the sites examined here is ∼−0.2 nmol cm^-2 y^-1, which is consistent with published global estimates of sedimentary U uptake. In addition, the accumulation rate of U in sediments exhibits a nonlinear relationship with the oxygen penetration depth and a linear relationship with the organic carbon rain rate. These relationships highlight the potential utility for the U accumulation rate as a proxy for these processes.  相似文献   

Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis   总被引：2，自引：0，他引：2  

P.N. Froelich  G.P. Klinkhammer  M.L. Bender  N.A. Luedtke  G.R. Heath  Doug Cullen  Paul Dauphin  Doug Hammond  Blayne Hartman  Val Maynard 《Geochimica et cosmochimica acta》1979,43(7):1075-1090

Pore water profiles of total-CO₂, pH, PO^3?₄, NO^?₃ plus NO^?₂, SO^2?₄, S^2?, Fe²⁺ and Mn²⁺ have been obtained in cores from pelagic sediments of the eastern equatorial Atlantic under waters of moderate to high productivity. These profiles reveal that oxidants are consumed in order of decreasing energy production per mole of organic carbon oxidized (O₂ > manganese oxides ~ nitrate > iron oxides > sulfate). Total CO₂ concentrations reflect organic regeneration and calcite dissolution. Phosphate profiles are consistent with organic regeneration and with the effects of release and uptake during inorganic reactions. Nitrate profiles reflect organic regeneration and nitrate reduction, while dissolved iron and manganese profiles suggest reduction of the solid oxide phases, upward fluxes of dissolved metals and subsequent entrapment in the sediment column. Sulfate values are constant and sulfide is absent, reflecting the absence of strongly anoxic conditions.  相似文献   

Young basalts of the central Washington Cascades, flux melting of the mantle, and trace element signatures of primary arc magmas   总被引：3，自引：0，他引：3  

Peter W. Reiners  Paul E. Hammond  Juliet M. McKenna  Robert A. Duncan 《Contributions to Mineralogy and Petrology》2000,138(3):249-264

Basaltic lavas from the Three Sisters and Dalles Lakes were erupted from two isolated vents in the central Washington Cascades at 370–400 ka and 2.2 Ma, respectively, and have distinct trace element compositions that exemplify an important and poorly understood feature of arc basalts. The Three Sisters lavas are calc-alkaline basalts (CAB) with trace element compositions typical of most arc magmas: high ratios of large-ion-lithophile to high-field-strength elements (LILE/HFSE), and strong negative Nb and Ta anomalies. In contrast, the Dalles Lakes lavas have relatively low LILE/HFSE and no Nb or Ta anomalies, similar to ocean-island basalts (OIB). Nearly all Washington Cascade basalts with high to moderate incompatible element concentrations show this CAB or OIB-like compositional distinction, and there is pronounced divergence between the two magma types with a large compositional gap between them. We show that this trace element distinction can be easily explained by a simple model of flux-melting of the mantle wedge by a fluid-rich subduction component (SC), in which the degree of melting (F) of the peridotite source is correlated with the amount of SC added to it. Distinctive CAB and OIB-like trace element compositions are best explained by a flux-melting model in which dF/dSC decreases with increasing F, consistent with isenthalpic (heat-balanced) melting. In the context of this model, CAB trace element signatures simply reflect large degrees of melting of strongly SC-fluxed peridotite along relatively low dF/dSC melting trends, consistent with derivation from relatively cold mantle. Under other conditions (i.e., small degrees of melting or large degrees of melting of weakly SC-fluxed peridotite [high dF/dSC]), either OIB- or MORB (mid-ocean ridge basalt)-like compositions are produced. Trace element and isotopic compositions of Washington Cascade basalts are easily modeled by a correlation between SC and F across a range of mantle temperatures. This implies that the dominant cause of arc magmatism in this region is flux melting of the mantle wedge. Received: 2 March 1999 / Accepted: 18 August 1999  相似文献   

Square Kilometre Array: The radio telescope of the XXI century     

K. Grainge  B. Alachkar  Shaun Amy  D. Barbosa  M. Bommineni  P. Boven  R. Braddock  J. Davis  P. Diwakar  V. Francis  R. Gabrielczyk  R. Gamatham  S. Garrington  T. Gibbon  D. Gozzard  S. Gregory  Y. Guo  Y. Gupta  J. Hammond  D. Hindley  U. Horn  R. Hughes-Jones  M. Hussey  S. Lloyd  S. Mammen  S. Miteff  V. Mohile  J. Muller  S. Natarajan  J. Nicholls  R. Oberland  M. Pearson  T. Rayner  S. Schediwy  R. Schilizzi  S. Sharma  S. Stobie  M. Tearle  B. Wang  B. Wallace  L. Wang  R. Warange  R. Whitaker  A. Wilkinson  N. Wingfield 《Astronomy Reports》2017,61(4):288-296

The Square Kilometre Array (SKA) will be the world’s largest and most sensitive radio telescope. It will address fundamental unanswered questions about our Universe including how the first stars and galaxies formed after the Big Bang, how dark energy is accelerating the expansion of theUniverse, the role of magnetism in the cosmos, the nature of gravity, and the search for life beyond Earth. This project envisages the construction of 133 15-m antennas in South Africa and 131072 log-periodic antennas in Australia, together with the associated infrastructure in the two desert sites. In addition, the SKA is an exemplar Big Data project, with data rates of over 10 Tbps being transported from the telescope to HPC/HTC facilities.  相似文献   

Capturing magma intrusion and faulting processes during continental rupture: seismicity of the Dabbahu (Afar) rift   总被引：2，自引：0，他引：2  

C. J. Ebinger  D. Keir  A. Ayele  E. Calais  T. J. Wright  M. Belachew  J. O. S. Hammond  E. Campbell  W. R. Buck 《Geophysical Journal International》2008,174(3):1138-1152

  相似文献   

Geographical variation in abundance of striped and common dolphins of the western Mediterranean     

Jaume Forcada  Philip Hammond 《Journal of Sea Research》1998,39(3-4)

Line-transect data from sighting surveys conducted in the western Mediterranean (in 1991) and the Alboran Sea (in 1992) were analysed to estimate densities and numbers of striped and common dolphins in various areas of the western Mediterranean. Density of striped dolphins in the northwestern Mediterranean was estimated as 0.20 dolphins km⁻² (CV = 0.24; 95% CI = 0.12 and 0.32) and was 41% higher than in the southwestern Mediterranean, where it was estimated as 0.12 dolphins km⁻² (CV = 0.38; 95% CI = 0.05 and 0.25). The highest densities were observed in the Liguro–Provençal basin, with 0.24 dolphins km⁻² (CV = 0.26; 95% CI = 0.14 and 0.40), and the Alboran Sea, with 0.20 dolphins km⁻² (CV = 0.33; 95% CI = 0.10 and 0.36). These areas, and especially the Ligurian Sea, appear to be the most productive in terms of the food consumed by striped dolphins. Common dolphins were abundant only in the Alboran Sea with an estimated density of 0.16 dolphins km⁻² (CV = 0.40; 95% CI = 0.08 and 0.35), scarce in the south Balearic area and almost absent in the northwestern Mediterranean. The magnitude of the dolphin by-catch in fishing operations in the Alboran Sea and other areas stresses the need for further assessment of densities and numbers, notably in the Alboran Sea and the North African Mediterranean waters.  相似文献   

First recorded eruption of Nabro volcano,Eritrea, 2011     

Berhe Goitom  Clive Oppenheimer  James O. S. Hammond  Raphaël Grandin  Talfan Barnie  Amy Donovan  Ghebrebrhan Ogubazghi  Ermias Yohannes  Goitom Kibrom  J- Michael Kendall  Simon A. Carn  David Fee  Christine Sealing  Derek Keir  Atalay Ayele  Jon Blundy  Joanna Hamlyn  Tim Wright  Seife Berhe 《Bulletin of Volcanology》2015,77(10):1-21

  相似文献   

The trade-off between gender,energy and climate change in Africa: the case of Niger Republic     

Antwi  Sarpong Hammond 《GeoJournal》2022,87(1):183-195

GeoJournal - This article examines the role of gender in climate change adaptation and energy access in Africa. Drawing on the energy situation in Niger Republic, it argues that redressing...  相似文献   

Enhanced estimation of sonobuoy trajectories by velocity reconstruction with near-surface drifters     

Y. Chang  D. Hammond  A.C. Haza  P. Hogan  H.S. Huntley  A.D. Kirwan  B.L. Lipphardt  V. Taillandier  A. Griffa  T.M. Özgökmen 《Ocean Modelling》2011,36(3-4):179-197

An investigation to improve trajectory prediction using Lagrangian data is presented. The velocity field of a data assimilating model, EAS-16, is corrected using drifter observations taken during an experiment off Taiwan. The results are tested using another independent Lagrangian data set provided by sonobuoys launched in the same area. The latter have instrument chains that extend well into the water column. Consequently the corrected model velocities were projected into the water column in order to calculate sonobuoy trajectories. The drifter and sonobuoy trajectories both show two distinct regimes in the considered area of approximately 1/2° square. One regime is dominated by shelf dynamics, the other by meandering of the Kuroshio, with a sharp boundary dividing the two. These two regimes are not reproduced by the trajectories of the EAS-16 model. When the drifter data are blended with the model velocities, synthetic sonobuoy trajectories track the observed ones much better, and the two regimes are clearly depicted. Two different methods for the velocity reconstruction are tested. One is based on a variational approach and the other on a normal mode decomposition. Both methods show qualitatively similar improvements in the prediction of sonobuoys trajectories, with a quantitative improvement in the total rms error of approximately 50% and 25%, respectively.  相似文献