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  • 1
    ISSN: 0012-821X
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0392-6737
    Keywords: Mössbauer effect ; other γ-ray spectroscopy ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Sediment cores with different sub-bottom depths (I: 45 cm and II: 700 cm) from the Peru Basin have been investigated. From the depth profile of the relative amount of Fe(II) a redox zone is obtained which correlates with the organic carbon flux into the sediment (core I). Mössbauer parameters suggest that the iron in the sediments is mainly contained in clay minerals and to varying extent also in goethite.
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  • 3
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 314 (1985), S. 87-90 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] On core retrieval, the liners were immediately cut into 1-m sections and 500 g of wet sediment removed to a freezer and kept at -80 C until analysed (within 24 h). The analytical procedure for the molecular and stable isotope composition of the hydrocarbon gases is similar to that given elsewhere4. ...
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  • 4
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract  Mössbauer spectroscopy was applied to characterize the valence states Fe(II) and Fe(III) in sedimentary minerals from a core of the Peru Basin. The procedure in unraveling this information includes temperature-dependent measurements from 275 K to very low temperature (300 mK) in zero–field and also at 4.2 K in an applied field (up to 6.2 T) and by mathematical procedures (least-squares fits and spectral simulations) in order to resolve individual spectral components. The depth distribution of the amount of Fe(II) is about 11% of the total Fe to a depth of 19 cm with a subsequent steep increase (within 3 cm) to about 37%, after which it remains constant to the lower end of the sediment core (at about 40 cm). The steep increase of the amount of Fe(II) defines a redox boundary which coincides with the position where the tan/green color transition of the sediment occurs. The isomer shifts and quadrupole splittings of Fe(II) and Fe(III) in the sediment are consistent with hexacoordination by oxygen or hydroxide ligands as in oxide and silicate minerals. Goethite and traces of hematite are observed only above the redox boundary, with a linear gradient extending from about 20% of the total Fe close to the sediment surface to about zero at the redox boundary. The superparamagnetic relaxation behavior allows to estimate the order of magnitude for the size of the largest goethite and hematite particles within the particle-site distribution, e.g. ∼170 Å and ∼50 Å, respectively. The composition of the sediment spectra recorded at 300 mK in zero-field, apart from the contributions due to goethite and hematite, resembles that of the sheet silicates smectite, illite and chlorite, which have been identified as major constituents of the sediment in the 〈2 μm fraction by X-ray diffraction. The specific “ferromagnetic” type of magnetic ordering in the sediment, as detected at 4.2 K in an applied field, also resembles that observed in sheet silicates and indicates that both Fe(II) and Fe(III) are involved in magnetic ordering. This “ferromagnetic” behavior is probably due to the double-exchange mechanism known from other mixed-valence Fe(II)–Fe(III) systems. A significant part of the clay-mineral iron is redox sensitive. It is proposed that the color change of the sediment at the redox boundary from tan to green is related to the increase of Fe(II)–Fe(III) pairs in the layer silicates, because of the intervalence electron transfer bands which are caused by such pairs.
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  • 5
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The distribution of the forms of Fe in the solid phases in core samples of sediments from the Peru Basin has been investigated by Mössbauer spectroscopy with special attention to the cause of the sharp color transition between an upper green colored and a lower tan colored part. An important part of sample handling includes strict exclusion of oxygen during preparation of absorbers and measurements at cryogenic temperatures. The measurement strategy includes measurements between 77 K and 300 mK in zero external magnetic field, supplemented by measurements in external magnetic fields at 4.2 and 300 mK (up to 6.2 and 1 T, respectively). The temperature scans allow detection, identification and quantification of superparamagnetic iron oxides (goethite and hematite). The oxides are only present in samples from the upper tan-colored part of the core. The major part of the Fe(II) and Fe(III) (〉80%) is present in a magnetic structure similar to that of layer silicates. The relative Fe(II) content of the layer silicates is practically identical to that determined from the paramagnetic components measured at liquid nitrogen temperature. This shows that the color transition in the sediment coincides with a change in the relative Fe(II) content in layer silicates from 11 to 37%. The color change can thus be explained by an increase in occurrence of Fe(II)–Fe(III) pairs exhibiting absorption bands due to intervalence electron transfer.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    ISSN: 1437-3262
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1437-3262
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Description / Table of Contents: Abstract The effects of the Mg2+ ion concentration and the ionic products of carbonate upon the induction time for the onset of precipitation and the different mineralogies of calcium carbonates were studied. It was shown that Mg2+ ions delay the spontaneous precipitation of calcium carbonate from supersaturated solutions (e.g. seawater) with respect to calcium carbonate mineral to such an extent that only biogenic removal of skeletal calcium carbonate is possible from the open ocean. Low concentrations of magnesium ions in solution favor calcite formation while aragonite is formed at high magnesium concentrations. The mole% of MgCO3 in magnesian calcite increases with the increase of (Mg2+) in solution and with the increase of (CO3 2−) in the presence of (Mg2+) in solution. Therefore, one would expect that high Mg-calcite is formed in wormed coastal regions, where high temperature and or the increase of photosynthesis activities tend to expel CO2 and increase supersaturation, and low-magnesian calcite is favored in meteoric-vadose environment where low concentration of magnesium ions or in burial environment where respiration and oxidation is high and decrease supersaturation.
    Abstract: Résumé Une étude a été menée sur l'effet de la concentration en ions Mg2+ et des produits ioniques des carbonates sur le temps d'induction du début de la cristallisation et sur la nature minéralogique du carbonate de calcium formé. On a pû montrer que les ions Mg2+ retardent la précipitation spontanée du carbonate de Ca à partir de solutions sursaturées par rapport aux minéraux de CaCO3 (par exemple l'eau de mer) et ce à un point tel que seule la précipitation biogénique du carbonate de Ca dans les squelettes est possible à partir de l'eau de mer. Les faibles concentrations en ions Mg2+ dans les solutions favorisent la formation de calcite, l'aragonite se formant pour des concentrations plus élevées. Le pourcentage molaire de MgCO3 dans la calcite magnésienne augmente avec la teneur, dans la solution, de Mg2+ et de Co3 2− enprésence de Mg2+. On présume donc que la calcite riche en Mg se forme dans des milieux côtiers chauds dans lesquels la haute température et/ou la forte activité photosynthétique provoquent la libération de CO2, ce qui augmente la sursaturation. La calcite pauvre en Mg se forme de préférence dans des milieux météoriques vadoses à faible concentration en ions Mg2+ ou dans des milieux d'enfouissement où la respiration et l'oxydation importantes abaissent le degré de sursaturation.
    Notes: Zusammenfassung Die Wirkung der Mg2+-Ionenkonzentration und des Ionenprodukts von Karbonaten wurde in Abhängigkeit von der Induktionszeit für den Beginn der Ausfällung und der unterschiedlichen Mineralogie von Kalziumkarbonaten untersucht. Es konnte gezeigt werden, daß Mg2+-Ionen die spontane Ausfällung von Kalziumkarbonat aus einer übersättigten Lösung (z. B. Meerwasser), im Vergleich zu Kalziumkarbonatmineralien derart verzögert, daß nur noch der biogene Entzug für skelettbildendes Kalziumkarbonat aus dem Meerwasser möglich ist. Niedrige Konzentrationen von Magnesiumionen in der Lösung fördern die Kalzitbildung, während Aragonit bei hohen Magnesiumkonzentrationen gebildet wird. Die Mol% MgCO3 in Magnesiumkalzit steigen mit der Zunahme von (Mg2+) und mit der Zunahme von (CO3 2−) bei Gegenwart von (Mg2+) in der Lösung. Daher würde man erwarten, daß Hochmagnesiumkalzit in erwärmten Küstengebieten gebildet wird, wo hohe Temperaturen, und/oder die Zunahme der Photosyntheseaktivität dazu führen, daß CO2 frei wird, und damit die Übersättigung ansteigen lassen. Niedrigmagnesiumkalzit wird bevorzugt in meteorische-vadosen Gebieten mit niedriger Magnesiumionenkonzentration oder in Versenkungsgebieten gebildet, wo die Respiration und die Oxidation hoch ist, bei sinkender Übersättigung.
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  • 8
    ISSN: 1437-3262
    Keywords: Methane plumes ; Marine methane cycle ; Pockmarks ; Barents Sea
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract A pockmark field extending over 35 km2 at 74°54′N, 27°3′E, described by Solheim and Elverhøi (1993), was re-surveyed and found to be covered with more than 30 steep-sided craters between 300 and 700 m in diameter and up to 28 m deep. The craters are thought to have been formed by an explosive gas eruption. Anomalously high concentrations of methane in the shelf waters around the craters suggest that a strong methane source near this area is still active today. Methane enrichment more than 10 km away from the crater field indicates the large dimensions of a plume and the amount of gas released from sources below the seafloor of the Barents Sea shelf. From the characteristic vertical decrease of methane towards the sea surface, it is concluded that biota are extensively using this energy pool and reducing the methane concentration within the water column by about 98% between 300 m depth and the sea surface. Degassing to the atmosphere is minimal based on the shape of the methane concentration gradient. Nevertheless, the net flux of methane from this area of the Barents Sea is about 2.9 × 104 g CH4 km−2 yr−1 and thus in the upper range of the presently estimated global marine methane release. This flux is a minimum estimate and is likely to increase seasonally when rough weather leads to more effective vertical mixing during autumn and winter. The amount of methane consumed in the water column, however, is about 50 times greater and hence should significantly contribute to the marine carbon inventory.
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  • 9
    Publication Date: 2014-09-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 10
    Publication Date: 2018-03-02
    Description: Measurements of dissolved methane in the surface waters of the western Sea of Okhotsk are evaluated in terms of methane exchange rates and are used to assess the magnitude of seasonal variations of methane fluxes from the ocean to the atmosphere in this area. Methane concentrations northeast of Sakhalin were observed to range from 385 nmol L−1 under the ice cover in winter to 6 nmol L−1 in the icefree midsummer season. The magnitude of supersaturations indicates that this part of the Okhotsk Sea is a significant source for atmospheric methane. From the seasonal variation of the supersaturations in the surface waters it is evident that the air-sea exchange is interrupted during the winter and methane from sedimentary sources accumulates under the ice cover. According to our measurements an initial early summer methane pulse into the atmosphere of the order of 560 mol km−2 d−1 can be expected when the supersaturated surface waters are exposed by the retreating ice. The methane flux in July is approximately 150 mol km−2 d−1 which is of the order of the average annual flux in the survey area. The magnitude of the seasonal CH4 flux variation northeast of Sakhalin corresponds to an amount of 7.3 × 105 g km−2 whereby 74% or 5.4 × 105 g km−2 are supplied to the atmosphere between April and July. For the whole Sea of Okhotsk the annual methane flux is roughly 0.13 × 1012 g (terragrams), based on the assumption that 15% of the entire area emit methane. Variations of long-term data of atmospheric methane which are recorded at the same latitude adjacent to areas with seasonal ice cover show a regional methane pulse between April and July. The large-scale level of atmospheric methane in the northern hemisphere undergoes an amplitudinal variation of about 25 parts per billion by volume (ppbv) which translates into approximately 36 Tg. Thus the estimated 0.6 Tg of ice-induced methane dynamics in northern latitudes can hardly explain this seasonal signal. However, the effects of seasonal ice cover on pulsed release of methane appear strong enough to contribute, in concert with other seasonal sources, to characteristic short-term wobbles in the atmospheric methane budget which are observed between 50°N and 60°N.
    Type: Article , PeerReviewed
    Format: text
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