Fission-track retention age of the Bondoc mesosiderite     

R.K. Bull  S.A. Durrani 《Earth and Planetary Science Letters》1980,49(2):181-187

Whitlockites have been extracted from the Bondoc mesosiderite and found to contain ～(0.7–2.7) × 10⁶ cm^?2 of spontaneous-fission tracks. Thermal-neutron irradiation of these crystals along with terrestrial apatite age standards, and measurement of the track densities so induced, lead to the determination of a whitlockite track-retention age of 2.68 (±0.30) Gyr for Bondoc, which is essentially independent of the initial (Pu/U) ratio.This young age is in qualitative, and, within the experimental uncertainties, in quantitative, agreement with the slow cooling rates determined for many mesosiderites [1,2] and specifically for Bondoc [6] from Ni distributions in the metal phase.  相似文献   

Carbon isotopic fractionation in lipids from methanotrophic bacteria II: the effects of physiology and environmental parameters on the biosynthesis and isotopic signatures of biomarkers     

Jahnke LL  Summons RE  Hope JM  Des Marais DJ 《Geochimica et cosmochimica acta》1999,63(1):79-93

Controls on the carbon isotopic signatures of methanotroph biomarkers have been further explored using cultured organisms. Growth under conditions which select for the membrane-bound particulate form of the methane monooxygenase enzyme (pMMO) leads to a significantly higher isotopic fractionation than does growth based on the soluble isozyme in both RuMP and serine pathway methanotrophs; in an RuMP type the delta delta 13Cbiomass equaled -23.9% for pMMO and -12.6% for sMMO. The distribution of biomarker lipids does not appear to be significantly affected by the dominance of one or the other MMO type and their isotopic compositions generally track those of the parent biomass. The 13C fractionation behaviour of serine pathway methanotrophs is very complex, reflecting the assimilation of both methane and carbon dioxide and concomitant dissimilation of methane-derived carbon. A limitation in CH4 availability leads to the production of biomass which is 13C-enriched with respect to both carbon substrates and this occurs irrespective of MMO type. This startling result indicates that there must be an additional fractionation step downstream from the MMO reaction which leads to incorporation of 13C-enriched carbon at the expense of dissimilation of 13C-depleted CO2. In these organisms, polyisoprenoid lipids are 13C-enriched compared to polymethylenic lipid which is the reverse of that found in the RuMP types. Serine cycle hopanoids, for example, can vary anywhere from 12% depleted to 10% enriched with respect to the CH4 substrate depending on its concentration. Decrease in growth temperature caused an overall increase in isotopic fractionation. In the total biomass, this effect tended to be masked by physiological factors associated with the type of organism and variation in the bulk composition. The effect was, however, clearly evident when monitoring the 13C signature of total lipid and individual biomarkers. Our results demonstrate that extreme carbon isotopic depletion in field samples and fossil biomarker lipids can be indicative of methanotrophy but the converse is not always true. For example, the hopanoids of a serine cycle methanotroph may be isotopically enriched by more than 10% compared to the substrate methane when the latter is limiting. In other words, hopanoids from some methanotrophs such as M. trichosporium would be indistinguishable from those of cyanobacteria or heterotrophic bacteria on the basis of either chemical structure or carbon isotopic signature.  相似文献   

Chemical textures on quartz grains: An experimental approach using salts     

A. W. Magee  P. A. Bull  A. S. Goudie 《地球表面变化过程与地形》1988,13(8):665-676

Two groups of fresh crushed Brazilian quartz grains (0.4–0.6 mm) were placed in 10 ml of various saturated salt solutions (sodium sulphate, sodium chloride, magnesium sulphate, sodium carbonate, and sodium carbonate and soil). One group was placed in an environmental cabinet programmed to simulate summer diurnal temperature and relative humidity values recorded in Wheeler Valley, a dry valley in southern Victoria Land, Antarctica. The other group was allowed to remain at normal laboratory conditions. Quartz grains from both groups were removed at pre-selected intervals for examination using the scanning electron microscope. After 50 hours chemical surface textures were formed on the quartz grains in all but the sodium sulphate solution. At the 140 hour interval all the salt solutions used were producing chemical surface textures on the quartz grains. This paper demonstrates that chemical surface textures can be produced on quartz grain surfaces by saturated salt solutions in a short period of time and may prove to be representative of chemical surface textures produced in a saturated saline environment.  相似文献   

The design of precast concrete pavements on low bearing capacity subgrades     

R. F. Ackroyd  J. W. Bull 《Computers and Geotechnics》1985,1(4):279-291

By using the finite element method and combining it with Westergaard's analysis for concrete pavements, a new design method using 2 metre by 2 metre precast concrete pavement units is presented for heavy duty pavements on low bearing capacity subgrades. One design example is included.  相似文献   

Concentrations and isotope ratios of carbon,nitrogen and sulfur in ocean-floor basalts     

H. Sakai  D.J.Des Marais  A. Ueda  J.G. Moore 《Geochimica et cosmochimica acta》1984,48(12):2433-2441

Fresh submarine basalt glasses from Galapagos Ridge, FAMOUS area, Cayman Trough and Kilauea east rift contain 22 to 160 ppm carbon and 0.3 to 2.8 ppm nitrogen, respectively, as the sums of dissolved species and vesicle-filling gases (CO₂ and N₂). The large range of variation in carbon content is due to combined effect of depth-dependency of the solubility of carbon in basalt melt and varying extents of vapour loss during magma emplacement as well as in sample crushing. The isotopic ratios of indigenous carbon and nitrogen are in very narrow ranges,?6.2 ± 0.2% relative to PDB and +0.2 ± 0.6 %. relative to atmospheric nitrogen, respectively. In basalt samples from Juan de Fuca Ridge, however, isotopically light carbon (δ¹³C = around ?24%.) predominates over the indigenous carbon; no indigenous heavy carbon was found. Except for Galapagos Ridge samples, these ocean-floor basalts contain 670 to 1100 ppm sulfur, averaging 810 ppm, in the form of both sulfide and sulfate, whereas basalts from Galapagos Ridge are higher in both sulfur (1490 and 1570 ppm) and iron (11.08% total iron as FeO). The δ³⁴S values average +0.3 ± 0.5%. with average fractionation factor between sulfate and sulfide of +7.4 ± 1.6%.. The sulfate/sulfide ratios tend to increase with increasing water content of basalt, probably because the oxygen fugacity increases with increasing water content in basalt melt.  相似文献   

Cosmic spherules and asteroidal collisions     

D. W. Parkin  R. A. L. Sullivan  R. K. Bull 《Geophysical Journal International》1983,75(2):473-491

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Mass flow sedimentology within the HYC Zn–Pb–Ag deposit,Northern Territory,Australia: evidence for syn-sedimentary ore genesis     

Tim?IrelandEmail author  Stuart?W.?Bull  Ross?R.?Large 《Mineralium Deposita》2004,39(2):143-158

Laterally continuous mass-flow deposits are an important feature of the HYC stratiform sediment-hosted Zn–Pb–Ag deposit, which reveal more about the HYC mineralising system than has been previously recognised. Mass flow deposits are interbedded with sheet-like mineralised lenses in a carbonaceous dolomitic siltstone host rock. Sedimentological processes of mass-flow deposit emplacement are proposed that constrain stratiform mineralisation to the top metre of the sediment pile, based on mass-flow geometry and detailed clast petrology. Four distinct sedimentary facies are identified within the mass-flow units: framework-supported polymictic boulder breccia; matrix-supported pebble breccia; and gravel-rich and sand-rich graded turbidite beds. The boulder breccias are weakly reverse graded and show rapid lateral transition into the other facies, all of which are distal manifestations of the same sedimentary events. The flow geometry and relationships between these facies are interpreted to reflect mass-flow initiation as clast-rich debris flows, with transformation via the elutriation of fines into a subsequent turbulent flow from which the turbidite and matrix-supported breccia facies were deposited. All the mass-flow facies contain clasts of the common and minor components of the in-situ laminated base-metal mineralised siltstone. Texturally these are identical to their in-situ counterparts, and are clearly distinct from other sulphidic clasts that are of unequivocal replacement origin. In the boulder breccias, intraclasts may be the dominant clast type and the matrix may contain abundant fine-grained sphalerite and pyrite. Dark coloured sphaleritic and pyritic breccia matrices are distinct from pale carbonate-siliclastic matrices, are associated with high abundance of sulphidic clasts, and systematically occupy the lower part of breccia units. Consequently, clasts that resemble in-situ ore facies are confirmed as genuine intraclasts that were incorporated into erosive mass flows prior to complete consolidation. Disaggregation and assimilation of sulphidic sediment in the flow contributed to the sulphide component of the dark breccia matrices. The presence of laminated sulphidic intraclasts in the mass-flow facies constrains mineralisation at HYC to the uppermost part of the seafloor sediment pile, where this material was susceptible to erosion by incoming clast-rich mass flows.Editorial handling: N. White  相似文献   

Does sedimentary organic delta 13C record variations in quaternary ocean [CO2(aq)]?     

Rau GH  Froelich PN  Takahashi T  Des Marais DJ 《Paleoceanography》1991,6(3):335-347

Ocean surface water [CO2(aq)] variations based on glacial/interglacial changes in sediment delta 13Corg are shown to compare favorably with reconstructions based on ice core [CO2]. In particular, an approximate 80 microatmospheres increase in atmospheric pCO2 during the last glacial-interglacial transition is calculated to correspond to a 3-4 micromolar increase in ocean surface water [CO2(aq)] at atmospheric equilibrium. A widespread marine delta 13Corg decrease of 1-2% accompanied this event and was not preceded by an equivalent isotopic change in surface water total dissolved inorganic carbon. These observations support the hypothesis that [CO2(aq)] influences photosynthetic isotope fractionation between marine inorganic and organic carbon pools, and therefore that plankton/sediment delta 13Corg may serve as a proxy for surface water [CO2(aq)].  相似文献   

Whiting events: biogenic origin due to the photosynthetic activity of cyanobacterial picoplankton   总被引：3，自引：0，他引：3  

Thompson JB  Schultze-Lam S  Beveridge TJ  Des Marais DJ 《Limnology and oceanography》1997,42(1):133-141

An annual whiting event occurs each year in late May to early June in Fayetteville Green Lake, New York. The initiation of this event correlates with exponential growth of the Synechococcus population within the lake. Synechococcus is the dominant (by approximately 4 orders of magnitude) autotrophic organism owing to the oligotrophic condition of the lake. The delta 13C values of the dissolved inorganic C range seasonally from -9.5% in winter to -6.2% in summer due to photosynthetic activity. Calcite precipitates principally in the microenvironment surrounding Synechococcus because of a photosynthetically driven alkalization process and the availability of the cells as nucleation sites. This calcite has a heavier delta 13C value (>4%) than does the dissolved inorganic C of the lake water owing to the cells' preferential uptake of 12C. A conceptual model suggests that photosynthetic activity and cell surface chemistry, together with the substantial surface area that arises from the great abundance of micron-sized cells, allow Synechococcus to dominate the annual whiting events in Fayetteville Green Lake.  相似文献   

Announcing the launch of HPToday     

Jeff McDonnell  Malcolm G Anderson  Des Walling  Normal E Peters 《水文研究》2000,14(14):2365-2365

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