From continental extension to seafloor spreading: crustal structure of the Goban Spur rifted margin, southwest of the UK     

Andrew D. Bullock  Timothy A. Minshull 《Geophysical Journal International》2005,163(2):527-546

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Mantle components in Iceland and adjacent ridges investigated using double-spike Pb isotope ratios   总被引：2，自引：0，他引：2  

M.F. Thirlwall  M.A.M. Gee  R.N. Taylor  B.J. Murton 《Geochimica et cosmochimica acta》2004,68(2):361-386

High precision Sr-Nd isotope ratios together with Pb isotope ratios corrected for mass fractionation using a double spike are reported for an extensive suite of late Quaternary to Recent lavas of Iceland, the Kolbeinsey and Reykjanes Ridges, and a small number of basalts from further south on the Mid-Atlantic Ridge. Compared with global MORB, the Icelandic region is distinguished by having low ²⁰⁷Pb/²⁰⁴Pb for any given ²⁰⁶Pb/²⁰⁴Pb, expressed by negative Δ²⁰⁷Pb (−0.8 to −3.5) in all but four Icelandic samples. Most samples also have elevated ²⁰⁸Pb/²⁰⁴Pb (strongly positive Δ²⁰⁸Pb), which combined with their negative Δ²⁰⁷Pb is very unusual in MORB worldwide. The negative Δ²⁰⁷Pb is interpreted as a consequence of evolution in high-μ mantle sources for the last few hundred Ma. The region of negative Δ²⁰⁷Pb appears to correspond with the region of elevated ³He/⁴He, suggesting that both lithophile and volatile elements in melts from the whole region between 56 and 70°N are dominantly sourced in a plume that has incorporated recycled Palaeozoic ocean crust and unradiogenic He, probably from the deep mantle. At least four mantle components are recognized on Iceland, two with an enriched character, one depleted and one that shows some isotopic affinities to EM1 but is only sampled by highly incompatible-element-depleted lavas in this study. Within restricted areas of Iceland, these components contribute to local intermediate enriched and depleted components that display near binary mixing systematics. The major depleted Icelandic component is clearly distinct in Pb isotopes from worldwide MORB, but resembles the depleted mantle source supplying the bulk of the melt to the Kolbeinsey and southern Reykjanes Ridges. However, an additional depleted mantle source is tapped by the northern Reykjanes Ridge, which with very negative Δ²⁰⁷Pb and less positive Δ²⁰⁸Pb is distinct from all Icelandic compositions. These components must mostly mix at mantle depths because a uniform mixture of three Icelandic components is advected southward along the Reykjanes Ridge.Despite strong covariation with isotope ratios, incompatible trace element ratios of Icelandic magmas cannot be representative of old mantle sources. The observed parent-daughter ratios in depleted and enriched Icelandic lavas would yield homogeneous Sr, Nd, Hf and ²⁰⁶Pb isotope signatures ∼170 Ma ago if present in their sources. The heterogeneity in ²⁰⁷Pb/²⁰⁴Pb is not however significantly reduced at 170 Ma, and the negative present day Δ²⁰⁷Pb cannot be supported by the low μ observed in depleted lavas from Iceland or the adjacent ridges. Since μ is higher in melts than in their sources, it follows that all the depleted sources must be residues from <170 Ma partial melting events. These are thought to have strongly affected most incompatible trace element ratios.  相似文献   

Mobility of authigenic rhenium, silver, and selenium during postdepositional oxidation in marine sediments     

John Crusius  John Thomson 《Geochimica et cosmochimica acta》2003,67(2):265-273

Sedimentary records of redox-sensitive trace elements hold significant potential as indicators of paleoceanographic environmental conditions. Records of Re can reveal the intensity of past reducing conditions in sediments at the time of deposition, whereas records of Ag may record the magnitude of past diatom fluxes to the seafloor. Confidence in paleoenvironmental reconstruction from records of either metal, however, requires it to have experienced negligible redistribution since deposition. This study examines diagenetic rearrangements of Re and Ag that occur in response to exposure to bottom-water O₂ in environments of low sedimentation rate, including Madeira Abyssal Plain turbidites and eastern Mediterranean basin sapropels. Authigenic Re was remobilized quantitatively by oxidation but poorly retained by the underlying sediments. All records are consistent with previous work demonstrating that only a limited reimmobilization of Re occurs preferentially in C_org-rich, reducing sediments. Silver was also mobilized quantitatively by oxidation, but it was subsequently immobilized more efficiently in all cases as sharp peaks immediately into anoxic conditions below active oxidation fronts, and these peaks remain immobile in anoxic conditions during long-term burial. Comparison of Ag, S, and Se records from various cores suggests that Ag is likely to have been immobilized as a selenide, a mechanism previously proposed for Hg in similar situations (Mercone et al., 1999). Coexisting narrow peaks of Ag and Hg with Se offer a means of assessing whether oxidative burndown has ever occurred at the top of C_org- and sulfide-rich sedimentary units. Although these results suggest that caution must be used when inferring paleoenvironmental information from records of Ag and Re in cores with low sediment accumulation rates (<5 cm ka⁻¹), they should not affect the promise that authigenic Ag and Re records hold for paleoenvironmental reconstruction in sediments with higher accumulation rates and where anoxic conditions have been maintained continuously.  相似文献   

On the Role of Wind Direction in ERS SAR Signatures of Internal Waves on the Iberian Shelf     

I. B. Araú  jo  J.C.B. da Silva  S. A. Ermakov  I. S. Robinson 《Journal of Atmospheric & Ocean Science》2002,8(4):269-281

The relationship between internal wave (IW) signatures in Synthetic Aperture Radar (SAR) images and wind velocity is investigated. The effect of the wind velocity relative to the IW propagation direction on the IW signature is studied by means of a defined signature mode parameter ( S m ). S m is the parameter that quantifies the signature of the IW intensity profile in relation to the mean backscatter of the image background.

A wind contrast model based on a simple first-order Bragg scattering theory is combined with hydrodynamic modulation theory to explain the modulation of IW signatures. It takes into account the modulation of short-scale surface waves by the effect of the relative variations of wind velocity and the surface currents generated by the IWs.

It is shown that the signature mode parameter increases with the angle between the wind velocity and the IW propagation direction so that IWs propagating against the wind direction are imaged mostly as positive sign signatures, while those propagating in the wind direction are mostly negative sign signatures.  相似文献   

A late diagenetic (syn-folding) magnetization carried by pyrrhotite: implications for paleomagnetic studies from magnetic iron sulphide-bearing sediments     

Richard Weaver  Andrew P. Roberts  Andrew J. Barker 《Earth and Planetary Science Letters》2002,200(3-4):371-386

Paleomagnetic, rock magnetic, and sedimentary micro-textural data from an early Miocene mudstone sequence exposed in Okhta River, Sakhalin, Russia, indicate the presence of pyrrhotite and magnetite at different stratigraphic levels. Sites that contain only magnetite have a reversed polarity characteristic remanent magnetization (ChRM) with a low-coercivity overprint, which coincides with the present-day geomagnetic field direction. Pyrrhotite-bearing sites have stable normal polarity ChRMs that are significantly different from the present-day field direction. After correction for bedding tilt, the ChRM data fail a reversals test. However, the normal polarity pyrrhotite ChRM directions become antipodal to the tilt-corrected magnetite ChRM directions and are consistent with the expected geocentric axial dipole field direction at the site latitude after 40% partial unfolding. These data suggest that the pyrrhotite magnetization was acquired during folding and after lock-in of the magnetite remanences. Electron microscope observations of polished sections indicate that fluid-associated halos surround iron sulphide nodules. Pyrrhotite is present in randomly oriented laths in and around the nodules, and the nodules do not appear to have been deformed by sediment compaction. This observation is consistent with a late diagenetic origin of pyrrhotite. Documentation of a late diagenetic magnetization in pyrrhotite-bearing sediments here, and in recent studies of greigite-bearing sediments, suggests that care should be taken to preclude a late origin of magnetic iron sulphides before using such sediments for geomagnetic studies where it is usually crucial to establish a syn-depositional magnetization.  相似文献   

Azimuthal seismic anisotropy in a zone of exhumed continental mantle, West Iberia margin     

P. B. Cole  T. A. Minshull  R. B. Whitmarsh 《Geophysical Journal International》2002,151(2):517-533

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Settling velocities and entrainment thresholds of biogenic sands (shell fragments) under unidirectional flow   总被引：3，自引：0，他引：3  

D. Paphitis  M. B. Collins  L. A. Nash†  & S. Wallbridge‡ 《Sedimentology》2002,49(1):211-225

Settling velocities and entrainment thresholds of biogenic sedimentary particles, under unidirectional flow conditions, are derived on the basis of settling tower and laboratory flume experiments. Material consisting predominantly of equant blocks (shell fragments of Cerastoderma edule , density, ρ _s=2800 kg m⁻³) or of mica-like flakes and elongate rods ( Mytilus edulis fragments, ρ _s=2720 kg m⁻³) are used in separate series of experiments. Differences in the measured settling and threshold properties are related primarily to particle shape. The selection of a characteristic length scale for non-spherical grains is investigated by comparing two approaches used to define the grain size ( D ) of the sediment samples: grain settling and sieve analysis that are used to derive data for the threshold criteria, in terms of the Shields and Movability diagrams. The empirical curves effectively predict the threshold conditions for any grain shape, provided that grain size is defined in terms of grain settling velocity. However, a functional distinction is made between the characteristic `hydraulic' grain size, defined by grain settling for grain transport applications, and the actual (physical) grain size defined by sieve analysis.  相似文献   

Propagation and reflection of long equatorial waves in the Indian Ocean from TOPEX/POSEIDON data during the 1993–1998 period     

J.-L. Le Blanc  J.-P. Boulanger 《Climate Dynamics》2001,17(7):547-557

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High-resolution stratigraphy and turbidite processes in the Seine Abyssal Plain, northwest Africa     

T. L. Davies  B. Van Niel  Robert B. Kidd  P. P. E. Weaver 《Geo-Marine Letters》1997,17(2):147-153

 Seine Abyssal Plain sediments consist of turbidites interbedded with pelagic oozes, marls, and clays. The pelagics produce a high-resolution integrated stratigraphy combining oxygen isotope, carbonate curves, palaeotemperature analyses, and nannofossil acme zone stratigraphy. Individual turbidites could then be correlated. Isogram and isopach maps of the turbidites maximum median grain size and thickness, respectively, were used to identify turbidity current pathways and study their depositional processes. Nannofossil fingerprints were used to study possible sedimentary links between the Seine and Madeira abyssal plains. Received: 8 February 1996 / Revision received: 9 May 1996  相似文献   

Aspects of the distribution, population structure and reproduction of the gastropod Tibia delicatula (Nevill, 1881) inhabiting the oxygen minimum zone of the Oman and Pakistan continental margins     

Eva Ramirez-Llodra  Celia Olabarria 《Journal of Sea Research》2005,54(4):299-306

The present study describes some aspects of the distribution and biology of Tibia delicatula (Nevill), a gastropod belonging to the family Strombidae. This species has been found in large numbers in the upper oxygen minimum zone (OMZ) of the Oman margin, and has also been collected from the OMZ of the Pakistan margin. The highest abundance of adult specimens in the Oman OMZ was found between 300 and 450 m. Numbers dropped rapidly below 450 m, to zero below 500 m depth. Similarly dense populations were not observed in the Pakistan OMZ. Multiple regression with oxygen concentration and depth indicates that depth (and its related variables) is the main factor explaining the variation in abundance of T. delicatula. The populations from the Oman and Pakistan OMZs were dominated by juveniles. This suggests a unimodal size structure with evidence of a marked recruitment event. Basic reproductive aspects were analysed. All specimens had a penis and sperm groove. The gonad wall consisted of reticular tissue that might be used for nutrient storage or as an irrigation system. Only vitellogenic oocytes were present. The large oocyte sizes observed (200–300 μm) suggest a lecithotrophic larval development.  相似文献