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111.
112.
H.J. Hirche K.N. Kosobokova B. Gaye-Haake I. Harms B. Meon E.-M. Nthig 《Progress in Oceanography》2006,71(2-4):288
After a short introduction to the physical setting and the history of biological research the pelagic ecosystem of the Kara Sea is described. Main emphasis is on regional aspects of the plankton communities and their seasonal dynamics using mostly data collected between 1996 and 2001. In the zooplankton, for which most data were available, four regional aggregations were separated: (1) the rivers and estuaries of the Southern Kara Sea, (2) the south-western and (3) the central Kara Sea, and (4) the northern troughs and slope. The phytoplankton communities had a similar distribution. To provide components for detailed carbon budgets the regional dynamics of bacterial, phytoplankton and zooplankton biomass and production are described and carbon requirements of bacteria and zooplankton are estimated. For completeness a short literature review on higher trophic levels is included. Finally, recent observations of the pelago-benthic coupling are considered. Estimates of the carbon requirements from the plankton and benthos reveal a large underestimation of primary production, which to date, together with seasonal aspects, shows the largest gap in our knowledge. 相似文献
113.
Grit Steinhoefel Friedhelm von Blanckenburg Ingo Horn Nicolas J. Beukes 《Geochimica et cosmochimica acta》2010,74(9):2677-5891
To investigate the genesis of BIFs, we have determined the Fe and Si isotope composition of coexisting mineral phases in samples from the ∼2.5 billion year old Kuruman Iron Formation (Transvaal Supergroup, South Africa) and Dales Gorges Member of the Brockman Iron Formation (Hamersley Group, Australia) by UV femtosecond laser ablation coupled to a MC-ICP-MS. Chert yields a total range of δ30Si between −1.3‰ and −0.8‰, but the Si isotope compositions are uniform in each core section examined. This uniformity suggests that Si precipitated from well-mixed seawater far removed from its sources such as hydrothermal vents or continental drainage. The Fe isotope composition of Fe-bearing mineral phases is much more heterogeneous compared to Si with δ56Fe values of −2.2‰ to 0‰. This heterogeneity is likely due to variable degrees of partial Fe(II) oxidation in surface waters, precipitation of different mineral phases and post-depositional Fe redistribution. Magnetite exhibits negative δ56Fe values, which can be attributed to a variety of diagenetic pathways: the light Fe isotope composition was inherited from the Fe(III) precursor, heavy Fe(II) was lost by abiotic reduction of the Fe(III) precursor or light Fe(II) was gained from external fluids. Micrometer-scale heterogeneities of δ56Fe in Fe oxides are attributed to variable degrees of Fe(II) oxidation or to isotope exchange upon Fe(II) adsorption within the water column and to Fe redistribution during diagenesis. Diagenetic Fe(III) reduction caused by oxidation of organic matter and Fe redistribution is supported by the C isotope composition of a carbonate-rich sample containing primary siderite. These carbonates yield δ13C values of ∼−10‰, which hints at a mixed carbon source in the seawater of both organic and inorganic carbon. The ancient seawater composition is estimated to have a minimum range in δ56Fe of −0.8‰ to 0‰, assuming that hematite and siderite have preserved their primary Fe isotope signature. The long-term near-zero Fe isotope composition of the Hamersley and Transvaal BIFs is in balance with the assumed composition of the Fe sources. The negative Fe isotope composition of the investigated BIF samples, however, indicates either a perturbation of the steady state, or they have to be balanced spatially by deposition of isotopically heavy Fe. In the case of Si, the negative Si isotope signature of these BIFs stands in marked contrast to the assumed source composition. The deviation from potential source composition requires a complementary sink of isotopically heavy Si in order to maintain steady state in the basin. Perturbing the steady state by extraordinary hydrothermal activity or continental weathering in contrast would have led to precipitation of light Si isotopes from seawater. Combining an explanation for both elements, a likely scenario is a steady state ocean basin with two sinks. When all published Fe isotope records including BIFs, microbial carbonates, shales and sedimentary pyrites, are considered, a complementary sink for heavy Fe isotopes must have existed in Precambrian ocean basins. This Fe sink could have been pelagic sediments, which however are not preserved. For Si, such a complementary sink for heavy Si isotopes might have been provided by other chert deposits within the basin. 相似文献
114.
Martin Oeser Stefan Weyer Ingo Horn Stephan Schuth 《Geostandards and Geoanalytical Research》2014,38(3):311-328
In this study, a technique for high precision in situ Fe and Mg isotope determinations by femtosecond‐laser ablation‐multi collector‐ICP‐MS (fs‐LA‐MC‐ICP‐MS) was developed. This technique was employed to determine reference values for a series of common reference glasses that may be used for external standardisation of in situ Fe and Mg isotope determinations in silicates. The analysed glasses are part of the MPI‐DING and United States Geological Survey (USGS) reference glass series, consisting of basaltic (BIR‐1G, BCR‐2G, BHVO‐2G, KL2‐G, ML3B‐G) and komatiitic (GOR128‐G and GOR132‐G) compositions. Their Fe and Mg isotope compositions were determined by in situ fs‐LA‐MC‐ICP‐MS and by conventional solution nebulisation multi‐collector ICP‐MS. We determined δ56Fe values for these glasses ranging between ‐0.04‰ and 0.10‰ (relative to IRMM‐014) and δ26Mg values ranging between ‐0.40‰ and ‐0.15‰ (relative to DSM‐3). Our fs‐LA‐MC‐ICP‐MS results for both Fe and Mg isotope compositions agreed with solution nebulisation analyses within analytical uncertainties. Furthermore, the results of three USGS reference glasses (BIR‐1G, BHVO‐2G and BCR‐2G) agreed with previous results for powdered and dissolved aliquots of the same reference materials. Measurement reproducibilities of the in situ determinations of δ56Fe and δ26Mg values were usually better than 0.12‰ and 0.13‰ (2s), respectively. We further demonstrate that our technique is a suitable tool to resolve isotopic zoning in chemically‐zoned olivine crystals. It may be used for a variety of different applications on isotopically‐zoned minerals, e.g., in magmatic or metamorphic rocks or meteorites, to unravel their formation or cooling rates. 相似文献
115.
Abstract— Here we present the first purely physical model for cosmogenic production rates in iron meteorites with radii from 5 cm to 120 cm and for the outermost 1.3 m of an object having a radius of 10 m. The calculations are based on our current best knowledge of the particle spectra and the cross sections for the relevant nuclear reactions. The model usually describes the production rates for cosmogenic radionuclides within their uncertainties; exceptions are 53Mn and 60Fe, possibly due to normalization problems. When an average S content of about 1 ± 0.5% is assumed for Grant and Carbo samples, which is consistent with our earlier study, the model predictions for 3He, 21Ne, and 38Ar are in agreement. For 4He the model has to be adjusted by 24%, possibly a result of our rather crude approximation for the primary galactic α particles. For reasons not yet understood the modeled 36Ar/38Ar ratio is about 30–40% higher than the ratio typically measured in iron meteorites. Currently, the only reasonable explanation for this discrepancy is the lack of experimentally determined neutron induced cross sections and therefore the uncertainties of the model itself. However, the new model predictions, though not yet perfect, enable determining the radius of the meteoroid, the exposure age, the sulphur content of the studied sample as well as the terrestrial residence time. The determination of exposure ages is of special interest because of the still open question whether the GCR was constant over long time scales. Therefore we will discuss in detail the differences between exposure ages determined with different cosmogenic nuclides. With the new model we can calculate exposure ages that are based on the production rates (cm3STP/(gMa)) of noble gases only. These exposure ages, referred to as noble gas exposure ages or simply 3,4He, 21Ne, or 36,38Ar ages, are calculated assuming the current GCR flux. Besides calculating noble gas ages we were also able to improve the 41K‐40K‐and the 36Cl‐36Ar dating methods with the new model. Note that we distinguish between 36Ar ages (calculated via 36Ar production rates only) and 36Cl‐36Ar ages. Exposure ages for Grant and Carbo, calculated with the revised 41K‐40K method, are 628 ± 30 Ma and 841 ± 19 Ma, respectively. For Grant this is equal to the ages obtained using 3He, 21Ne, and 38Ar but higher than the 36Ar‐ and 36Cl‐36Ar ages by ?30%. For Carbo the 41K‐40K age is ?40% lower than the ages obtained using 3He, 21Ne, and 38Ar but equal to the 36Ar age. These differences can either be explained by our still insufficient knowledge of the neutron‐induced cross sections or by a long‐term variation of the GCR. 相似文献
116.
Many coupled ocean–atmosphere general circulation models (GCMs) suffer serious biases in the tropical Atlantic including a
southward shift of the intertropical convergence zone (ITCZ) in the annual mean, a westerly bias in equatorial surface winds,
and a failure to reproduce the eastern equatorial cold tongue in boreal summer. The present study examines an ensemble of
coupled GCMs and their uncoupled atmospheric component to identify common sources of error. It is found that the westerly
wind bias also exists in the atmospheric GCMs forced with observed sea surface temperature, but only in boreal spring. During
this time sea-level pressure is anomalously high (low) in the western (eastern) equatorial Atlantic, which appears to be related
to deficient (excessive) precipitation over tropical South America (Africa). In coupled simulations, this westerly bias leads
to a deepening of the thermocline in the east, which prevents the equatorial cold tongue from developing in boreal summer.
Thus reducing atmospheric model errors during boreal spring may lead to improved coupled simulations of tropical Atlantic
climate. 相似文献
117.
Abstract— Cosmogenic He, Ne, and Ar were measured in the iron meteorites Grant (IIIAB) and Carbo (IID) to re‐determine their preatmospheric geometries and exposure histories. We also investigated the influence of sulphur‐ and/or phosphorus‐rich inclusions on the production rates of cosmogenic Ne. Depth profiles measured in Grant indicate a preatmospheric center location 117 mm left from the reference line and 9 mm below bar B, which is clearly different (?10 cm) from earlier results (?165 mm left from the reference line on bar F). For Carbo the preatmospheric center location was found to be 120 mm right of the reference line and 15 mm above bar J, which is in agreement with literature data. The new measurements indicate a spherical preatmospheric shape for both meteorites and, based on literature 36C1 data, the radii were estimated to be about 32 cm and 70 cm for Grant and Carbo, respectively. We demonstrate that minor elements like S and P have a significant influence on the production rates of cosmogenic Ne. In our samples, containing on average 0.5% S and/or P, about 20% of 21Ne was produced from these minor elements. Using measured 21Ne concentrations and endmember 22Ne/21Ne ratios for Fe + Ni and S + P, respectively, we show that it is possible to correct for 21Ne produced from S and/or P. The thus corrected data are then used to calculate new 41K‐40K exposure ages—using published K data—which results in 564 ± 78 Ma for Grant and 725 ± 100 Ma for Carbo. The correction always lowers the 21Ne concentrations and consequently decreases the 41K‐40K exposure ages. The discrepancies between 36Cl‐36Ar and 41K‐40K ages are accordingly reduced. The existence of a significant long‐term variation of the GCR, which is based on a former 30–50% difference between 41K‐40K and 36Cl‐36Ar ages, may warrant re‐investigation. 相似文献
118.
119.
120.
Post-collisional potassic granitoids from the southern and northwestern parts of the Late Neoproterozoic East African Orogen: a review 总被引:10,自引:0,他引:10
Potassic metaluminous granitoids with enrichments of HFS elements constitute part of widespread post-collisional magmatism related to the Late Neoproterozoic Pan-African orogeny in northeastern Africa (Sudan, Ethiopia, Somalia) and Madagascar. The plutons were emplaced between 580 and 470 Ma and comprise both subsolvus and hypersolvus biotite–granite, biotite–hornblende–granite, quartz–monzonite and quartz–syenite. Pyroxene-bearing granitoids are subordinate. Basic dikes and enclaves of monzodioritic composition are locally associated with the granitoid plutons. Granitoids emplaced in pre-Neoproterozoic crust have Sri-ratios between 0.7060 and 0.7236 and Nd(t) values between −15.8 and −5.6 while those emplaced in, or close to the contact with, juvenile Neoproterozoic crust have lower Sri-ratios (0.7036–0.7075) and positive Nd(t) values (4.6). However, it is unlikely that the potassic granitoids represent products of crustal melting alone. The association with basic magmas derived from subduction-modified enriched mantle sources strongly suggests that the granitoids represent hybrid magmas produced by interaction and mixing of mantle and crust derived melts in the lower crust. The most intense period of this potassic granitoid magmatism occurred between 585 and 540 Ma, largely coeval with HT granulite facies metamorphism in Madagascar and with amphibolite facies retrogression in northeastern Africa (Somalia, Sudan). Granitoid magmatism and high-grade metamorphism are probably both related to post-collisional lithospheric thinning, magmatic underplating and crustal relaxation. However, the emplacement of potassic granites continued until about 470 Ma and implies several magmatic pulses associated with different phases of crustal uplift and cooling. The potassic metaluminous granites are temporally and spatially associated with post-collisional high-K calc-alkaline granites with which they share many petrographical, geochemical and isotopical similarities, except the incompatible element enrichments. The resemblance indicates a strongly related petrogenesis of both granite associations. 相似文献