Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis   总被引：10，自引：0，他引：10  

Clark M. Johnson  Brian L. Beard  Nic J. Beukes 《Geochimica et cosmochimica acta》2008,72(1):151-169

The voluminous 2.5 Ga banded iron formations (BIFs) from the Hamersley Basin (Australia) and Transvaal Craton (South Africa) record an extensive period of Fe redox cycling. The major Fe-bearing minerals in the Hamersley-Transvaal BIFs, magnetite and siderite, did not form in Fe isotope equilibrium, but instead reflect distinct formation pathways. The near-zero average δ⁵⁶Fe values for magnetite record a strong inheritance from Fe³⁺ oxide/hydroxide precursors that formed in the upper water column through complete or near-complete oxidation. Transformation of the Fe³⁺ oxide/hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of δ⁵⁶Fe values: (1) addition of marine hydrothermal , (2) complete reduction by bacterial dissimilatory iron reduction (DIR), and (3) interaction with excess that had low δ⁵⁶Fe values and was produced by DIR. Most siderite has slightly negative δ⁵⁶Fe values of ∼ −0.5‰ that indicate equilibrium with Late Archean seawater, although some very negative δ⁵⁶Fe values may record DIR. Support for an important role of DIR in siderite formation in BIFs comes from previously published C isotope data on siderite, which may be explained as a mixture of C from bacterial and seawater sources.Several factors likely contributed to the important role that DIR played in BIF formation, including high rates of ferric oxide/hydroxide formation in the upper water column, delivery of organic carbon produced by photosynthesis, and low clastic input. We infer that DIR-driven Fe redox cycling was much more important at this time than in modern marine systems. The low pyrite contents of magnetite- and siderite-facies BIFs suggests that bacterial sulfate reduction was minor, at least in the environments of BIF formation, and the absence of sulfide was important in preserving magnetite and siderite in the BIFs, minerals that are poorly preserved in the modern marine record. The paucity of negative δ⁵⁶Fe values in older (Early Archean) and younger (Early Proterozoic) BIFs suggests that the extensive 2.5 Ga Hamersley-Transvaal BIFs may record a period of maximum expansion of DIR in Earth’s history.  相似文献   

Microbial mass-dependent fractionation of chromium isotopes   总被引：1，自引：0，他引：1  

Eric R. Sikora  Thomas M. Johnson  Thomas D. Bullen 《Geochimica et cosmochimica acta》2008,72(15):3631-3641

Mass-dependent fractionation of Cr isotopes occurs during dissimilatory Cr(VI) reduction by Shewanella oneidensis strain MR-1. Cells suspended in a simple buffer solution, with various concentrations of lactate or formate added as electron donor, reduced 5 or 10 μM Cr(VI) to Cr(III) over days to weeks. In all nine batch experiments, ⁵³Cr/⁵²Cr ratios of the unreacted Cr(VI) increased as reduction proceeded. In eight experiments covering a range of added donor concentrations up to 100 μM, isotopic fractionation factors were nearly invariant, ranging from 1.0040 to 1.0045, with a mean value somewhat larger than that previously reported for abiotic Cr(VI) reduction (1.0034). One experiment containing much greater donor concentration (10 mM lactate) reduced Cr(VI) much faster and exhibited a lesser fractionation factor (1.0018). These results indicate that ⁵³Cr/⁵²Cr measurements should be effective as indicators of Cr(VI) reduction, either bacterial or abiotic. However, variability in the fractionation factor is poorly constrained and should be studied for a variety of microbial and abiotic reduction pathways.  相似文献   

Leaching mechanisms of oxyanionic metalloid and metal species in alkaline solid wastes: A review     

Geert Cornelis  C. Anette Johnson  Tom Van Gerven  Carlo Vandecasteele 《Applied Geochemistry》2008

An overview is presented on possible mechanisms that control the leaching behaviour of the oxyanion forming elements As, Cr, Mo, Sb, Se, V and W in cementituous systems and alkaline solid wastes, such as municipal solid waste incinerator bottom ash, fly ash and air pollution control residues, coal fly ash and metallurgical slags. Although the leachability of these elements generally depends on their redox state, speciation measurements are not common. Therefore, experimental observations available in the literature are combined with a summary of the thermal behaviour of these elements to assess possible redox states in freshly produced alkaline wastes, given their origin at high temperature. Possible redox reactions occurring at room temperature, on the other hand, are reviewed because these may alter the initial redox state in alkaline wastes and their leachates. In many cases, precipitation of oxyanions as a pure metalate cannot provide a satisfactory explanation for their leaching behaviour. It is therefore highly likely that adsorption and solid solution formation with common minerals in alkaline waste and cement reduce the leachate concentration of oxyanions below pure-phase solubility.  相似文献   

Euro-politics of scale: competing visions of the region in eastern Germany     

Corey Johnson 《GeoJournal》2008,72(1-2):75-89

As John Agnew (Political geography: a reader, 1997) has argued, political and economic change often occasions competing visions of the scales that are appropriate for organizing particular political and economic activities. Nowhere is this more evident than in the European Union, and eastern Germany offers compelling evidence of the contested nature of contemporary scalar politics. Yet a recent debate in human geography (see, e.g. Marston et al., Transactions of the Institute of British Geographers 30:416–432, 2005) calls into question the very concept of scale and rejects its hierarchical conceptualization. In light of this debate, it is appropriate to draw on real-world case studies to examine the ways in which geography figures into policy. Drawing on field work in Saxony, evidence is offered in the form of competing visions of regionalism in the EU context. The evidence presented complicates both hierarchical and flat notions of scale. The current process of querying space to identify those scales that are best-suited for the globalized economy offers insights into both the socially constructed nature of scale as well as the ways in which scalar lenses help to illuminate the geographical aspects (and consequences) of strategies for coping with structural changes.  相似文献   

贵州遵义牛蹄塘组黑色岩系的硒同位素变化及其环境指示初探   总被引：4，自引：0，他引：4  

朱建明  Johnson Thomas M  罗泰义  朱祥坤 《岩石矿物学杂志》2008,27(4):361-366

西南早寒武世牛蹄塘组是分布在中国南方扬子地台的一套黑色岩系,区域上发育有典型富集多金属元素的硫化物矿化层.选取遵义松林小竹牛蹄塘组下部富硒、钼、镍等元素的黑色岩系剖面,利用Se同位素初步探讨了该剖面岩石的沉积环境和硒的可能来源.结果表明,小竹牛蹄塘组下部剖面黑色岩系的δ82/76SeSRM3149比值变化较大,下部底层含碳斑脱岩与磷块岩的变化范围在-4.35‰～ 4.11‰之间;中间镍钼层及碳质页岩、碳质碳酸盐岩的δ82/76SeSRM3149变化范围窄,平均值为0.9‰±0.23‰(n=4);上层碳质页岩为-1.24‰.结合已发表的钼同位素数据,认为中间岩石沉积于缺氧/无氧环境,但存在盆地海水与热液或充氧水团的混合;下层的岩石曾一度位于充氧与贫氧环境的边界面,局部岩石曾暴露地表经历了较强的风化和蚀变作用,海水中硒有可能来自底部富硒斑脱岩的氧化淋滤或海底热液.据此推测遵义松林小竹牛蹄塘组下部岩石的沉积环境极可能处在局限盆地靠近陆地的边缘部分,经历了充氧→贫氧→缺氧/无氧→贫氧的演化阶段.  相似文献   

Coulometric total carbon dioxide analysis for marine studies: maximizing the performance of an automated gas extraction system and coulometric detector     

K.M. Johnson  K.D. Wills  D.B. Butler  W.K. Johnson  C.S. Wong 《Marine Chemistry》1993,44(2-4)

A global survey of the distribution of dissolved CO₂ taking advantage of sampling opportunities provided by the World Ocean Circulation Experiment: World Hydrographic Program (WOCE-WHP) is being carried out through 1995. Goals include the measurement of oceanic inorganic carbon transport and the development of a data base from which future fossil-fuel CO₂ build-up can be monitored. The analytical method selected for total carbon dioxide (C_T) is gas extraction of acidified seawater with coulometric titration of the acid formed by the resultant carbon dioxide and monoethanolamine. To combine high accuracy and precision (± 1.5 μmol/kg for C_T ≥ 2000 μmol/kg) with a high rate of analysis, we have modified an automated single-parameter system. Following prototype development between 1987 and 1990, an instrument emerged with the acronym Somma standing for single-operator multiparameter metabolic analyzer. Improved functional and operating procedures have integrated electronic calibration, CO₂ gas calibration, and sample analysis with automated pressure, temperature, and conductivity (salinity) sensing into a single convenient transportable package.  相似文献   

Abundance of the introduced seastar, Asterias amurensis, and spatial variability in soft sediment assemblages in SE Tasmania: Clear correlations but complex interpretation     

D. Jeff Ross  Craig R. Johnson  Chad L. Hewitt   《Estuarine, Coastal and Shelf Science》2006,67(4):695-707

The northern Pacific seastar, Asterias amurensis, was first collected in southeast Tasmania in 1986. Mistaken for the endemic asteroid Uniophora granifera, its true identity was not realised until 1992. It is now a conspicuous predator in soft sediment habitats in this region, and is considered a major threat to native assemblages and commercial species. We examined the structure of soft sediment assemblages at different spatial scales in southeast Tasmania, and correlated spatial variation in community composition with seastar abundances. We found that the structure of soft sediment assemblages is highly variable at a range of spatial scales from metres to tens of kilometres. Clear differences in the composition of assemblages and abundances of major taxa were detected between areas with and without seastars and between areas with low and high seastar densities. However, the nature of these patterns suggests that they are more likely due to differences in sediment characteristics than due to impacts of the seastar. Thus, spatial differences in soft sediment assemblages might have been erroneously attributed to seastars without detailed information on important physical factors such as sediment characteristics. A second survey, using larger sampling units (1 m²) but across a more limited spatial extent, targeted bivalves and heart urchins that were identified as important prey of the seastar in observations of feeding and in experimental studies. Large-scale patterns of abundance and size structure were consistent with seastar effects anticipated from small-scale experimental and feeding studies for some, but not all, species. While the field survey ultimately provided evidence about the presence or absence of seastar impacts at large-scales, the identification of key ecological variables in experimental and feeding studies proved crucial to both the design and interpretation of patterns observed in the large-scale surveys. Overall, this work highlighted the necessity to consider multiple lines of evidence rather than relying on a single ‘inferential’ test, in the absence of pre-impact data.  相似文献   

Manganese and iron distributions off central California influenced by upwelling and shelf width   总被引：4，自引：1，他引：4  

Zanna Chase  Kenneth S. Johnson  Virginia A. Elrod  Joshua N. Plant  Steve E. Fitzwater  Lisa Pickell  Carol M. Sakamoto 《Marine Chemistry》2005,95(3-4):235-254

In July 2002, a combination of underway mapping and discrete profiles revealed significant along-shore variability in the concentrations of manganese and iron in the vicinity of Monterey Bay, California. Both metals had lower concentrations in surface waters south of Monterey Bay, where the shelf is about 2.5 km wide, than north of Monterey Bay, where the shelf is about 10 km wide. During non-upwelling conditions over the northern broad shelf, dissolvable iron concentrations measured underway in surface waters reached 3.5 nmol L⁻¹ and dissolved manganese reached 25 nmol L⁻¹. In contrast, during non-upwelling conditions over the southern narrow shelf, dissolvable iron concentrations in surface waters were less than 1 nmol L⁻¹ and dissolved manganese concentrations were less than 5 nmol L⁻¹. A pair of vertical profiles at 1000 m water depth collected during an upwelling event showed dissolved manganese concentrations of 10 decreasing to 2 nmol L⁻¹, and dissolvable iron concentrations of 12–20 nmol L⁻¹ in the upper 100 m in the north, compared to 3.5–2 nmol L⁻¹ Mn and 0.6 nmol L⁻¹ Fe in the upper 100 m in the south, suggesting the effect of shelf width influences the chemistry of waters beyond the shelf.These observations are consistent with current understanding of the mechanism of iron supply to coastal upwelling systems: Iron from shelf sediments, predominantly associated with particles greater than 20 μm, is brought to the surface during upwelling conditions. We hypothesize that manganese oxides are brought to the surface with upwelling and are then reduced to dissolved manganese, perhaps by photoreduction, following a lag after upwelling.Greater phytoplankton biomass, primary productivity, and nutrient drawdown were observed over the broad shelf, consistent with the greater supply of iron. Incubation experiments conducted 20 km offshore in both regions, during a period of wind relaxation, confirm the potential of these sites to become limited by iron. There was no additional growth response when copper, manganese or cobalt was added in addition to iron. The growth response of surface water incubated with bottom sediment (4 nmol L⁻¹ dissolvable Fe) was slightly greater than in control incubations, but less than in the presence of 4 nmol L⁻¹ dissolved iron. This may indicate that dissolvable iron is not as bioavailable as dissolved iron, although the influence of additional inhibitory elements in the sediment cannot be ruled out.  相似文献   

Hydrothermal Metalliferous Sediments and Waters Off the Lesser Antilles     

A. Johnson  D. S. Cronan 《Marine Georesources & Geotechnology》2001,19(2):65-83

Considerable metal enrichments have been found in hydrothermal fluids and metalliferous sediments off the central Lesser Antilles Volcanic Arc. The elements variably enriched in the fluids as a result of the hydrothermal activity are Fe, Mn, As, Si, B, Li and in the sediments, Fe, P, Mo, As, Sb, Hg, Cu and Pb. Variations in the concentrations of these elements in both fluids and sediments along the arc result from a number of factors, the most important of which is the stage that each island's volcano has reached in its eruptive cycle. Although hydrothermal mineralization on the sea floor off the islands is only low grade, phase separation in the hydrothermal fluids at depth could be leading to higher grade stockwork mineralization below the vent fields or the discharge of metal rich brines on the lower flanks of the volcanic islands.  相似文献   

Modelling morphological processes in the vicinity of shore-parallel breakwaters     

Julio A. Zyserman  Hakeem K. Johnson 《Coastal Engineering》2002,45(3-4)

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