Nannofossils: the smoking gun for the Canarian hotspot          下载免费PDF全文

Valentin R. Troll  Frances M. Deegan  Steffi Burchardt  Kirsten Zaczek  Juan‐Carlos Carracedo  Fiona C. Meade  Vicente Soler  Mario Cachao  Jorge Ferreira  Abigail K. Barker 《Geology Today》2015,31(4):137-145

The origin of volcanism in the Canary Islands has been a matter of controversy for several decades. Discussions have hinged on whether the Canaries owe their origin to seafloor fractures associated with the Atlas Mountain range or to an underlying plume or hotspot of superheated mantle material. However, the debate has recently come to a conclusion following the discovery of nannofossils preserved in the products of the 2011–2012 submarine eruption at El Hierro, which tell us about the age and growth history of the western‐most island of the archipelago. Light coloured, pumice‐like ‘floating rocks’ were found on the sea surface during the first days of the eruption and have been shown to contain fragments of pre‐island sedimentary strata. These sedimentary rock fragments were picked up by ascending magma and transported to the surface during the eruption, and remarkably retained specimens of pre‐island Upper Cretaceous to Pliocene calcareous nannofossils (e.g. coccolithophores). These marine microorganisms are well known biostratigraphical markers and now provide crucial evidence that the westernmost and youngest island in the Canaries is underlain by the youngest sediment relative to the other islands in the archipelago. This finding supports an age progression for the onset of volcanism at the individual islands of the archipeligo. Importantly, as fracture‐related volcanism is known to produce non‐systematic age‐distributions within volcanic alignments, the now‐confirmed age progression corroberates to the relative motion of the African plate over an underlying mantle plume or hotspot as the cause for the present‐day Canary volcanism.  相似文献   

Defining trace-element alteration halos to skarn deposits hosted in heterogeneous carbonate rocks: Case study from the Cu–Zn Antamina skarn deposit,Peru     

A. Escalante  G.M. DippleS.L.L. Barker  R. Tosdal 《Journal of Geochemical Exploration》2010

The presence of geochemical anomalies, defining haloes around hydrothermal ore deposits, can be used to vector towards mineralization, or identify ore bodies buried at depth. Several important types of ore deposits, including skarn deposits, are often hosted within carbonate-rich sedimentary rocks. Identifying anomalous trace-element concentrations in carbonate rocks is complicated by variable lithology (i.e. siliciclastic component) and volume loss during hydrothermal alteration. In this study of the world-class Antamina skarn deposit in Peru, we use the ratio of metals:immobile elements (e.g. La, Al₂O₃) to differentiate genuine and false geochemical anomalies in limestones and marbles surrounding the skarn deposit. Unaltered limestones are used to define threshold values for metal:immobile element ratios (through use of the median value ± 2 median absolute deviations). Genuine anomalies are identified when metal concentrations exceed those predicted using median + 2 median absolute deviations. In addition, comparison of “four acid” and lithium-borate fusion analytical techniques reveals that the lower cost four-acid techniques give reliable results. Our approach can be used to identify geochemical anomalies and halos related to hydrothermal alteration of carbonate-rich rocks, which have direct application to skarn deposits.  相似文献   

Effective anisotropic velocity model from surface monitoring of microseismic events   总被引：1，自引：0，他引：1  

Yang Zhang  Leo Eisner  William Barker  Michael C. Mueller  Kevin L. Smith 《Geophysical Prospecting》2013,61(5):919-930

We develop a methodology to obtain a consistent velocity model from calibration shots or microseismicity observed on a buried array. Using a layered 1D isotropic model derived from checkshots as an initial velocity model, we invert P‐wave arrival times to obtain effective anisotropic parameters with a vertical axis of symmetry (VTI). The nonlinear inversion uses iteration between linearized inversion for anisotropic parameters and origin times or depths, which is specific to microseismic monitoring. We apply this technique to multiple microseismic events from several treatments within a buried array. The joint inversion of selected events shows a largely reduced RMS error indicating that we can obtain robust estimates of anisotropic parameters, however we do not show improved source locations. For joint inversion of multiple microseismic events we obtained Thomsen anisotropic parameters ε of 0.15 and δ of 0.05, which are consistent with values observed in active seismic surveys. These values allow us to locate microseismic events from multiple hydraulic fracture treatments separated across thousands of metres with a single velocity model. As a result, we invert the effective anisotropy for the buried array region and are able to provide a more consistent microseismicity mapping for past and future hydraulic fracture stimulations.  相似文献   

Contrasting pyroclast density spectra from subaerial and submarine silicic eruptions in the Kermadec arc: implications for eruption processes and dredge sampling     

Simon J. Barker  Melissa D. Rotella  Colin J. N. Wilson  Ian C. Wright  Richard J. Wysoczanski 《Bulletin of Volcanology》2012,74(6):1425-1443

Pyroclastic deposits from four caldera volcanoes in the Kermadec arc have been sampled from subaerial sections (Raoul and Macauley) and by dredging from the submerged volcano flanks (Macauley, Healy, and the newly discovered Raoul SW). Suites of 16–32?mm sized clasts have been analyzed for density and shape, and larger clasts have been analyzed for major element compositions. Density spectra for subaerial dry-type eruptions on Raoul Island have narrow unimodal distributions peaking at vesicularities of 80–85%, whereas ingress of external water (wet-type eruption) or extended timescales for degassing generate broader distributions, including denser clasts. Submarine-erupted pyroclasts show two different patterns. Healy and Raoul SW dredge samples and Macauley Island subaerial-emplaced samples are dominated by modes at ~80–85%, implying that submarine explosive volcanism at high eruption rates can generate clasts with similar vesicularities to their subaerial counterparts. A minor proportion of Healy and Raoul SW clasts also show a pink oxidation color, suggesting that hot clasts met air despite 0.5 to >1?km of intervening water. In contrast, Macauley dredged samples have a bimodal density spectrum dominated by clasts formed in a submarine-eruptive style that is not highly explosive. Macauley dredged pyroclasts are also the mixed products of multiple eruptions, as shown by pumice major-element chemistry, and the sea-floor deposits reflect complex volcanic and sedimentation histories. The Kermadec calderas are composite features, and wide dispersal of pumice does not require large single eruptions. When coupled with chemical constraints and textural observations, density spectra are useful for interpreting both eruptive style and the diversity of samples collected from the submarine environment.  相似文献   

Physico‐chemical features of lake Pupuke,Auckland     

M. A. Barker 《新西兰海洋与淡水研究杂志》2013,47(4):406-430

Summary

Lake Pupuke, situated in the North Island of New Zealand near Auckland (Latitude 36° 53'S, Longitude 174° 52'E), is a small, circular body of water (maximum width 1.2 km, maximum depth 55 m). From November 1966 to November 1967 observations on its physico‐chemical factors were made twice monthly.

The range of temperatures in the year was 12.0–24.2°c at the surface and 11.3–14.5°c at 40 m. A thermocline farmed between early October and early January, when it became strongly developed at about 17 m and persisted until late April. Between early May and late June the stratification was breaking down and between mid July and mid September the lake was isothermal.

The values for light transmitted per metre through the water were high from mid August to mid December (53–76% per metre), very low from mid December until mid February (23–49% per metre), high in March (75% per metre) and relatively constant over the winter months (53–64% per metre). The amount of light transmitted depended closely on the density of phytoplankton. Changes in the depth at which a Secchi disc disappeared paralleled the light transmission values, and the mean Secchi depth was 2.2 m, range 1.0–5.2 m.

Oxygen concentrations down to 30 m were highest at all depths from mid November to early January (112–134% saturation). From early January to late April oxygen was strongly stratified (epilimnion 55–100% saturation, hypolimnion 24–77% saturation). During winter oxygen values gradually increased at all depths.

The range of pH was 7.8–9.5 units at the surface and 7.8–8.6 units at 30 m. From early January to late April pH values were strongly stratified but from mid July to mid August they were nearly uniform. The range of alkalinity (mainly HCO₃ ^? ions) was 76.0–80.8 ppm CaCO₃ and values showed stratification in summer. Carbonate ion was present from early October to mid June, but free carbon dioxide was only found in trace amounts.

The range of nitrate ion concentrations was 0.05–0.31 ppm; nitrate became obviously depleted in summer. Only one nitrite value was significant, 0.25 ppm. other nitrite values never exceeded 0.10 ppm. The concentration of ammoniacal nitrogen built up to a summer maximum of 0.95 ppm in the hypolimnion (annual range 0–0.95 ppm); values for albuminoid ammonia (0.016–1.03 ppm of nitrogen) varied with the quantity of living matter present.

The annual mean for total ion concentration (3.30 meq./1) was high and the order of ions was Na⁺>Mg²⁺>Ca²⁺>K⁺ and HCO₃ ^?>CI^?>SO₄ ^2?. Such dissolved salts may have arrived as airborne particles, dissolved in rain, or as sea spray.  相似文献   

Variation in sunscreen compounds (mycosporine‐like amino acids) for marine species along a gradient of ultraviolet radiation transmission within doubtful sound,New Zealand     

Miles D. Lamare  Michael P. Lesser  Mike F. Barker  Thomas M. Barry  Kate B. Schimanski 《新西兰海洋与淡水研究杂志》2013,47(5):775-793

We examined the response of four species of New Zealand marine algae (Ecklonia radiata, Apophlaea lyallii, Rhodymenia spp., Ulva lactuca) and a sea urchin (Evechinus chloroticus) to spatial variation in ultraviolet radiation (UV‐R) by examining the concentration of UV‐R absorbing compounds known as mycosporine‐like amino acids (MAAs). The purpose was to understand how, and the degree to which, local marine species could potentially respond to any future increases in incident UV‐R in the New Zealand marine environment. The research was undertaken in Doubtful Sound, where we observed a gradient of water column UV‐R transmission along the 40 km length of the fiord. We examined spatial differences in MAAs along the UV‐B gradient in the macrophytes and temporal changes in MAAs in sea urchin gonads. Among the algae, thallus MAA concentrations (nmol mg^–1 protein) ranged from 12.5 to 87.8 in E. radiata, from 433.1 to 1446.4 in A. lyallii, 12.7 to 103.4 in Rhodymenia spp., but were not detected in U. lactuca. For E. chloroticus, gonadal MAA concentrations ranged from 83.9 to 224.3 nmol mg^–1 protein spatially, and over the year from 1.85 to 14.12 nmol mg^–1 dry weight (DW) depending on site and gametogenic cycle. Laboratory manipulations indicated that concentrations of MAAs in E. chloroticus gonads and eggs are influenced by diet. MAA concentration could be correlated with UV‐B intensities in two of the algal species. E. chloroticus MAA concentrations could also be correlated with UV‐B transmission, which we concluded was a reflection of the greater ingestion and accumulation of MAA‐rich macrophytes at those sites where higher ambient UV‐R induced greater MAA concentrations to occur in the algae. Given this, we suggest that one response of marine species to increases in UV‐B would be an increase in the synthesis and/or accumulation of MAAs for photoautotrophs and a dietary accumulation of those MAAs in E. chloroticus, an important herbivore in this system.  相似文献   

Geochemical Stratigraphy of Submarine Lavas (3-5 Ma) from the Flamengos Valley, Santiago, Southern Cape Verde Islands   总被引：1，自引：0，他引：1  

Barker  Abigail K.; Holm  Paul M.; Peate  David W.; Baker  Joel A. 《Journal of Petrology》2009,50(1):169-193

New high-precision Pb–Sr–Nd isotope, major and traceelement and mineral chemistry data are presented for the submarinestage of ocean island volcanism on Santiago, one of the southernislands of the Cape Verde archipelago. Pillow basalts and hyaloclastitesin the Flamengos Valley are divided into three petrographicand compositional groups; the Flamengos Formation lavas (4·6Ma) dominate the sequence, with the younger Low Si and Coastalgroups (2·8 Ma) found near the shoreline. Olivine andclinopyroxene compositions and isotopic data for minerals andtheir host melts indicate disequilibrium between some crystalsand the melt. Intra-sample disequilibrium suggests homogenisationof liquids but eruption before complete equilibration betweencrystals and melt preserves the heterogeneity. Pressures ofcrystallization for clinopyroxene (0·4–1·1GPa) indicate stalling and crystallization of the magmas overa range of depths in the lithosphere. Major element compositionsindicate melting of a carbonated eclogite source. Sr–Nd–Pbisotope data suggest the involvement of FOZO-like and EM1-likecomponents in the mantle source, which are simultaneously availableat all depths in the melting column. The Flamengos Valley lavasdisplay large compositional variations, often between stratigraphicallyadjacent flows; these frequent abrupt changes of magma compositionsuggest stalling and crystallization of discrete magma batcheson transport through the lithosphere. KEY WORDS: Cape Verde; crystal–melt disequilibrium; submarine volcanism; source heterogeneity; Pb–Sr–Nd isotopes  相似文献   

Regulation of bacterial sulfate reduction and hydrogen sulfide fluxes in the central namibian coastal upwelling zone   总被引：1，自引：0，他引：1  

Volker Brüchert  Bo Barker Jørgensen  Daniela Riechmann  Heide Schulz 《Geochimica et cosmochimica acta》2003,67(23):4505-4518

The coastal upwelling system off central Namibia is one of the most productive regions of the oceans and is characterized by frequently occurring shelf anoxia with severe effects for the benthic life and fisheries. We present data on water column dissolved oxygen, sulfide, nitrate and nitrite, pore water profiles for dissolved sulfide and sulfate,³⁵S-sulfate reduction rates, as well as bacterial counts of large sulfur bacteria from 20 stations across the continental shelf and slope. The stations covered two transects and included the inner shelf with its anoxic and extremely oxygen-depleted bottom waters, the oxygen minimum zone on the continental slope, and the lower continental slope below the oxygen minimum zone. High concentrations of dissolved sulfide, up to 22 mM, in the near-surface sediments of the inner shelf result from extremely high rates of bacterial sulfate reduction and the low capacity to oxidize and trap sulfide. The inner shelf break marks the seaward border of sulfidic bottom waters, and separates two different regimes of bacterial sulfate reduction. In the sulfidic bottom waters on the shelf, up to 55% of sulfide oxidation is mediated by the large nitrate-storing sulfur bacteria, Thiomargarita spp. The filamentous relatives Beggiatoa spp. occupy low-O₂ bottom waters on the outer shelf. Sulfide oxidation on the slope is apparently not mediated by the large sulfur bacteria. The data demonstrate the importance of large sulfur bacteria, which live close to the sediment-water interface and reduce the hydrogen sulfide flux to the water column. Modeling of pore water sulfide concentration profiles indicates that sulfide produced by bacterial sulfate reduction in the uppermost 16 cm of sediment is sufficient to account for the total flux of hydrogen sulfide to the water column. However, the total pool of hydrogen sulfide in the water column is too large to be explained by steady state diffusion across the sediment-water interface. Episodic advection of hydrogen sulfide, possibly triggered by methane eruptions, may contribute to hydrogen sulfide in the water column.  相似文献   

Rolling torus structures in astronomy     

Ken Barker 《Astronomy& Geophysics》2001,42(4):4.21-4.23

Ken Barker puts forward an idea that may have a place in astrophysical modelling.
Torus structures that roll and orbit the centre of mass can produce a family of nested, almost elliptical orbits. These orbits, with the centre of mass at one focus, have a range of eccentricities and are inclined to the principal plane of the torus. Individual orbiting particles have an "anti-collision feature" from the nature of the rolling torus. In turn, because the torus can be regarded as a family of nested elliptical orbits, it rolls without needing any additional force. The system is acted upon only by central gravitational attraction and has low dissipation and a long lifetime.  相似文献   

Numerical study of fluid flow of deforming fractured rocks using dual permeability model   总被引：6，自引：0，他引：6  

Xing Zhang  David J. Sanderson  Andrew J. Barker 《Geophysical Journal International》2002,151(2):452-468

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