U–Pb ages of Neoarchean granitoids from the Black Hills, South Dakota, USA: implications for crustal evolution in the Archean Wyoming province     

James A. McCombs  Peter S. Dahl  Michael A. Hamilton 《Precambrian Research》2004,130(1-4):161-184

Zircons in basement rocks from the eastern Wyoming province (Black Hills, South Dakota, USA) have been analyzed by ion microprobe (SHRIMP) in order to determine precise ages of Archean tectonomagmatic events. In the northern Black Hills (NBH) near Nemo, Phanerozoic and Proterozoic (meta)sedimentary rocks are nonconformably underlain by Archean biotite–feldspar gneiss (BFG) and Little Elk gneissic granite (LEG), both of which intrude older schists. The Archean granitoid gneisses exhibit a pervasive NW–SE-trending fabric, whereas an earlier NE–SW-trending fabric occurs sporadically only in the BFG, which is intruded by the somewhat younger LEG. Zircon crystals obtained from the LEG and BFG exhibit double terminations, oscillatory zoning, and Th/U ratios of 0.6±0.3—thereby confirming a magmatic origin for both lithologies. In situ analysis of the most U–Pb concordant domains yields equivalent ²⁰⁷Pb/²⁰⁶Pb ages (upper intercept, U–Pb concordia) of 2559±6 and 2563±6 Ma (both ±2σ) for the LEG and BFG, respectively, which constrains a late Neoarchean age for sequential pulses of magmatism in the NBH. Unzoned (in BSE) patches of 2560 Ma zircon commonly truncate coeval zonation in the same crystals with no change in Th/U ratio, suggesting that deuteric, fluid-assisted recrystallization accompanied post-magmatic cooling. A xenocrystic core of magmatic zircon observed in one LEG zircon yields a concordant age of 2894±6 Ma (±2σ). This xenocryst represents the oldest crustal material reported thus far in the Black Hills. Whether this older zircon originated as unmelted residue of 2900 Ma crust that potentially underlies the Black Hills or as detritus derived from 2900 Ma crustal sources in the Wyoming province cannot be discerned. In the southern Black Hills (SBH), the peraluminous granite at Bear Mountain (BMG) of previously unknown age intrudes biotite–plagioclase schist. Zircon crystals from the BMG are highly metamict and altered, but locally preserve small domains suitable for in situ analysis. A U–Pb concordia upper intercept age of 2596±11 Ma (±2σ) obtained for zircon confirms both the late Neoarchean magmatic age of the BMG and a minimum age for the schist it intrudes. Taken together, these data indicate that the Neoarchean basement granitoids were emplaced at 2590–2600 Ma (SBH) and 2560 Ma (NBH), most likely in response to subduction associated with plate convergence (final assembly of supercontinent Kenorland?). In contrast, thin rims present on some LEG–BFG zircons exhibit strong U–Pb discordance, high common Pb, and low Th/U ratios—suggesting growth or modification under hydrothermal conditions, as previously suggested for similar zircons from SE Wyoming. The LEG–BFG zircon rims yield a nominal upper intercept date of 1940–2180 Ma, which may represent a composite of multiple rifting events known to have affected the Nemo area between 2480 and 1960 Ma. Together, these observations confirm the existence of a Paleoproterozoic rift margin along the easternmost Wyoming craton. Moreover, the 2480–1960 Ma time frame inferred for rifting in the Black Hills (Nemo area) corresponds closely to a 2450–2100 Ma time frame previously inferred for the fragmentation of supercontinent Kenorland.  相似文献   

Holocene climate variability   总被引：17，自引：0，他引：17  

Paul A. Mayewski  Eelco E. Rohling  J. Curt Stager  Wibjrn Karln  Kirk A. Maasch  L. David Meeker  Eric A. Meyerson  Francoise Gasse  Shirley van Kreveld  Karin Holmgren  Julia Lee-Thorp  Gunhild Rosqvist  Frank Rack  Michael Staubwasser  Ralph R. Schneider  Eric J. Steig 《Quaternary Research》2004,62(3):243-255

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Geochemical Correlation Between Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province, South Africa and the Ahlmannryggen Group of the Grunehogna Province, Antarctica     

Ian J. Basson  Samantha Perritt  Michael K. Watkeys  Andrew H. Menzies   《Gondwana Research》2004,7(1):57-73

The whole-rock geochemistry of metamorphosed greywackes, arenites and arkoses within the Mesoproterozoic Namaqua-Natal-Maudheim Province is interpreted with the aim of establishing geochemical correlations and defining common sediment source terrains. Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province were sampled from their type area in the Mfongosi Valley. Metagreywackes from the northern limits of the Mfongosi Valley, directly adjacent to the Kaapvaal Craton, show ocean island arc signatures while metagreywackes from the southern limits of the Mfongosi Valley, near the contact with the Madidima Thrust of the Natal nappe zone, show mainly active continental margin signatures. Interleaved, geochemically distinct low-Ca+Na, high-K metamorphosed arkoses to lithic arkoses indicate a minor passive margin sediment component. Geochemical classification of low-grade Ahlmannryggen Group greywackes, arenites and arkoses of the Grunehogna Province, Antarctica, indicates both active and passive continental margin sediment sources. An oceanic island arc signature is not evident in Ahlmannryggen Group data. The active continental margin signature in both Natal Sector and Grunehogna Province metasediments potentially provides for a common link between these terranes. Discriminant Function Analysis, using three pre-defined provenance sub-sets within the Mfongosi Group and two pre-defined provenance sub-sets within the Ahlmannryggen Group, indicate that metasediments with active continental margin signatures from both groups are geochemically identical, implying that the active continental margin of the Grunehogna Province shed immature sediments westwards (African azimuths) into the developing, narrow or restricted Mesoproterozoic ‘Mfongosi Basin.’ This was accompanied by minor sediment influx from a stable continental platform, potentially the Kaapvaal Craton. Oblique and diachronous collision, initiated in the southwestern portions of the combined Natal Sector/Grunehogna Province system produced a laterally variable Mfongosi Group, which formed in the ‘Mfongosi Basin’. Coarse-grained sediments dominated in its eastern portions while basalts with thin sapropelite units dominated in its western portions.  相似文献   

L'archivage des activités humaines par les neiges et glaces polaires : le cas du plomb     

Claude Boutron  Kevin Rosman  Carlo Barbante  Michael Bolshov  Freddy Adams  Sungmin Hong  Christophe Ferrari 《Comptes Rendus Geoscience》2004,336(10):847-867

The investigation of the occurrence of lead in dated snow and ice from Greenland and Antarctica has played a major role in our understanding of the history of the pollution of the atmosphere of our planet by this metal. Such studies have however proved to be very demanding, mainly because of the extreme purity of polar snow and ice. Reliable measurements can be obtained only if ultra-clean and highly sensitive procedures are used, as pioneered by Clair Patterson. The Greenland data show evidence of large-scale pollution of the atmosphere of the Northern Hemisphere for lead as early as two millennia ago during Greco–Roman times, especially because of mining and smelting activities in southern Spain. It peaked at the end of the 1960s, with lead concentrations in snow about 200 times higher than natural values, before declining during recent times because of the fall in the use of leaded gasoline. Lead pollution in Antarctica was already significant at the end of the 19th century as a consequence of whaling activities, the traffic of coal-powered ships crossing the Cape Horn, and mining activities in South America, South Africa and Australia. After declining because of the opening of the Panama Canal, the great economic depression and World War II, it reached a maximum during the 1980s, with lead concentrations 20 times higher than natural values. Other studies focus on past natural variations of lead in ancient ice dated from the last climatic cycles. To cite this article: C. Boutron et al., C. R. Geoscience 336 (2004).  相似文献   

Anaerobic methane oxidation and a deep H₂S sink generate isotopically heavy sulfides in Black Sea sediments     

Bo Barker Jørgensen  Michael E. Böttcher  Lev N. Neretin 《Geochimica et cosmochimica acta》2004,68(9):2095-2118

The main terminal processes of organic matter mineralization in anoxic Black Sea sediments underlying the sulfidic water column are sulfate reduction in the upper 2-4 m and methanogenesis below the sulfate zone. The modern marine deposits comprise a ca. 1-m-deep layer of coccolith ooze and underlying sapropel, below which sea water ions penetrate deep down into the limnic Pleistocene deposits from >9000 years BP. Sulfate reduction rates have a subsurface maximum at the SO₄²⁻-CH₄ transition where H₂S reaches maximum concentration. Because of an excess of reactive iron in the deep limnic deposits, most of the methane-derived H₂S is drawn downward to a sulfidization front where it reacts with Fe(III) and with Fe²⁺ diffusing up from below. The H₂S-Fe²⁺ transition is marked by a black band of amorphous iron sulfide above which distinct horizons of greigite and pyrite formation occur. The pore water gradients respond dynamically to environmental changes in the Black Sea with relatively short time constants of ca. 500 yr for SO₄²⁻ and 10 yr for H₂S, whereas the FeS in the black band has taken ca. 3000 yr to accumulate. The dual diffusion interfaces of SO₄²⁻-CH₄ and H₂S-Fe²⁺ cause the trapping of isotopically heavy iron sulfide with δ³⁴S = +15 to +33‰ at the sulfidization front. A diffusion model for sulfur isotopes shows that the SO₄²⁻ diffusing downward into the SO₄²⁻-CH₄ transition has an isotopic composition of +19‰, close to the +23‰ of H₂S diffusing upward. These isotopic compositions are, however, very different from the porewater SO₄²⁻ (+43‰) and H₂S (−15‰) at the same depth. The model explains how methane-driven sulfate reduction combined with a deep H₂S sink leads to isotopically heavy pyrite in a sediment open to diffusion. These results have general implications for the marine sulfur cycle and for the interpretation of sulfur isotopic data in modern sediments and in sedimentary rocks throughout earth’s history.  相似文献   

Structural variations in derivatives of the bacteriochlorophylls of Chlorobiaceae: impact of stratigraphic resolution on depth profiles as revealed by methanolysis     

Michael A. Wilson  Dominic A. Hodgson  Brendan J. Keely   《Organic Geochemistry》2004,35(11-12):1299

Bacteriochlorophylls c and d, recovered from two sedimentary sequences, were converted to bacteriophaeophorbide methyl esters by methanolysis and analysed by atmospheric pressure chemical ionisation liquid chromatography-multistage mass spectrometry (APCI LC-MSⁿ). The distributions in both settings, a moderately consolidated sediment from Kirisjes Pond, Antarctica, and in a finely laminated microbial mat from Les Salines de la Trinitat, Spain, show significant variations within a narrow depth interval. The overall bacteriophaeophorbide c to d ratios in the two sediments are different, as are the ratios of particular C-3¹ diastereoisomers, indicating distinct differences between the bacterial communities that contributed to each sediment. Furthermore, a shift towards more extensive alkylation in homologues within each sediment is consistent either with changing environmental conditions in the depositional environments, or development-related changes in the structure of the bacterial community, leading to increased competition for light or nutrients.  相似文献   

Application of pyrolysis-GC/MS for rapid assessment of organic contamination in sediments from Barcelona harbor     

Michael A. Kruge  Albert Permanyer   《Organic Geochemistry》2004,35(11-12):1395

Pyrolysis-GC/MS is advantageous as a tool for rapid sediment contamination assessment because of the small sample size required, minimal sample preparation and its ability to detect a wide variety of organic pollutants as well as naturally-occurring biological materials. Py-GC/MS was applied, together with determination of organic carbon, and major and minor element concentrations, to evaluate potentially contaminated sediments in the port of Barcelona (Spain) and the adjacent Llobregat River delta. Detected contaminant markers, most evident in the Old Port (Port Vell) area, included hopanes and alkylated PAHs (petroleum), sterenes (sewage), C₁₆–C₁₉ phenylalkanes (detergents) and parent PAHs (combusted fuels).  相似文献   

Assessing natural and cultural influence on soil in remnant tropical woodlands     

C Y Jim  Michael W H Chan 《Area》2004,36(1):6-18

Rural Hong Kong contains culturally protected enclaves of remnant woodlands that provide sanctuary for vegetation, wildlife and soil. This study focuses on the influence of natural and cultural factors on soil properties in woodlands with different human activities. At each of the nine selected sites, two pits were dug to collect soil samples for analysis of 24 physical and chemical properties. Analyses of variance between soil attributes and site factors suggest the prominence of woodland size, and important contribution of topography, aspect and parent material. Soil indicators of human impacts and the tenacity of woodland soil towards anthropogenic disturbance were demonstrated.  相似文献   

Determining the climatic boundary between the Chinese loess and palaeosol: evidence from aeolian coarse-grained magnetite   总被引：3，自引：0，他引：3  

Qingsong Liu  Subir K. Banerjee  Michael J. Jackson  Fahu Chen  Yongxin Pan  Rixiang Zhu 《Geophysical Journal International》2004,156(2):267-274

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Love's problem     

Janet M. Becker  Michael Bevis 《Geophysical Journal International》2004,156(2):171-178

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