Strontium isotope geochemistry of Icelandic geothermal systems and implications for sea water chemistry     

Henry Elderfield  Mervyn J Greaves 《Geochimica et cosmochimica acta》1981,45(11):2201-2212

Chemical and Sr isotopic analyses have been made of waters from 16 geothermal sites in Iceland with particular reference to the systems at Reykjanes and Svartsengi for which compositions of geothermal sea water and fresh and hydrothermally-altered rocks have been compared. The alkalies display mixing relationships indicating a hydrothermal input of Rb and K to local meteoric and sea waters as do results for Sr and Ca involving high-temperature fluids. ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the geothermal waters of meteoric origin parallel those of associated rocks but are higher. Ratios for geothermal sea waters are 0.7042 (Reykjanes) and 0.7040 (Svartsengi), lower than for normal sea water (0.7092) because of leaching of Sr from rocks followed by partial removal into alteration minerals, of which epidote and chlorite may be most important. Consequently, associated hydrothermally-altered rocks have been subject to significant Sr isotopic contamination by sea water Sr raising ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios from 0.7032 for fresh rock to 0.7038–0.7042 (Reykjanes) and to 0.7039–0.7041 (Svartsengi). Altered basalt is only ~50% equilibrated isotopically with geothermal sea water, at a water/rock ratio of ~2, but is internally equilibrated whereas palagonitized rocks (water/rock ratio of 3 to 4) are close to Sr isotopic equilibrium with associated sea water but show significant internal Sr disequilibrium. Hydrothermal input is unlikely to be important in the oceanic mass balance of Sr but is likely to be highly significant in controlling the strontium isotopic composition of sea water.  相似文献   

The isolation and ecological implications of the microvertebrate fauna of a Lower Cretaceous lignite bed     

E.F. Freeman 《Proceedings of the Geologists' Association. Geologists' Association》1975,86(3):307-312

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The role of upstream ULF waves in the generation of quasi-periodic ELF-VLF emissions     

K. Morrison  M. P. Freeman 《Annales Geophysicae》1995,13(11):1127-1133

Recent work suggests that the quasi-periodic (QP) modulation \sim10-50 s of naturally occurring ELF-VLF radio emissions (\sim0.5-5 kHz) is produced by the compressional action of Pc3 magnetic pulsations on the source of the emissions. Whilst it is generally accepted that these magnetic pulsations have an exogenic source, it is not clear what the mechanism of their generation is. A study of QP emissions observed during 1988 at Halley, Antarctica, in conjunction with IMP-8 satellite solar wind data, shows that the occurrence and modulation frequency of the emissions are strongly dependent upon the direction and strength of the IMF, respectively. The observed relationships are very similar to those previously reported for Pc3 pulsations associated with upstream ion-cyclotron resonance, involving proton beams reflected at the bowshock. In comparing the observed QP modulation frequencies with upstream wave theory, agreement was found by considering wave excitation exclusively associated with a proton beam reflected from a position on the bowshock at which the shock normal is parallel to the ambient IMF direction. Other geometries were found to be either impropitious or uncertain. The work indicates the useful diagnostic role QP emissions could play in the study of compressional ULF waves in the upstream solar wind and in monitoring the IMF conditions responsible for their generation.  相似文献   

Displacement-based design of RC bridge columns in seismic regions     

Mervyn J. Kowalsky  M. J. Nigel Priestley  Gregory A. MacRae 《地震工程与结构动力学》1995,24(12):1623-1643

This paper presents a seismic design philosophy based on displacements rather than forces. By inverting the seismic design process, a rational method is established where member strength and stiffness depend on the target displacement. A comprehensive procedure for displacement-based design of cantilever bridge columns is presented and verified by dynamic inelastic time history analysis. Parameter studies are used to examine the influence of several variables within the possible design solution space.  相似文献   

Energetic lunar nighttime ion events     

H. E. Schneider  J. W. Freeman 《Earth, Moon, and Planets》1975,14(1):27-33

The Rice University Suprathermal Ion Detector Experiment regularly observes ion events normally ranging from 250 eV q^?1 to 1000 eV q^?1 all through the lunar night. These ion events occur most often 2 to 3 days prior to the sunrise terminator. There is also a secondary activity peak 3 to 4 days after local sunset on the Moon. The events are normally of 4 hr or less in duration and the integral flux is 10⁶ ions cm^?2 s^?1 ster^?1. This article discusses the character of these events and presents the preliminary findings of a detailed study begun on this subject.  相似文献   

Compound-specific isotopic analyses: a novel tool for reconstruction of ancient biogeochemical processes   总被引：10，自引：0，他引：10  

Hayes JM  Freeman KH  Popp BN  Hoham CH 《Organic Geochemistry》1990,16(4-6):1115-1128

Patterns of isotopic fractionation in biogeochemical processes are reviewed and it is suggested that isotopic fractionations will be small when substrates are large. If so, isotopic compositions of biomarkers will reflect those of their biosynthetic precursors. This prediction is tested by consideration of results of analyses of geoporphyrins and geolipids from the Greenhorn Formation (Cretaceous, Western Interior Seaway of North America) and the Messel Shale (Eocene, lacustrine, southern Germany). It is shown (i) that isotopic compositions of porphyrins that are related to a common source, but which have been altered structurally, cluster tightly and (ii) that isotopic differences between geolipids and porphyrins related to a common source are equal to those observed in modern biosynthetic products. Both of these observations are consistent with preservation of biologically controlled isotopic compositions during diagenesis. Isotopic compositions of individual compounds can thus be interpreted in terms of biogeochemical processes in ancient depositional environments. In the Cretaceous samples, isotopic compositions of n-alkanes are covariant with those of total organic carbon, while delta values for pristane and phytane are covariant with those of porphyrins. In this unit representing an open marine environment, the preserved acyclic polyisoprenoids apparently derive mainly from primary material, while the extractable, n-alkanes derive mainly from lower levels of the food chain. In the Messel Shale, isotopic compositions of individual biomarkers range from -20.9 to -73.4% vs PDB. Isotopic compositions of specific compounds can be interpreted in terms of origin from methylotrophic, chemautotrophic, and chemolithotrophic microorganisms as well as from primary producers that lived in the water column and sediments of this ancient lake.  相似文献   

The effect of aromatization on the isotopic compositions of hydrocarbons during early diagenesis     

Freeman KH  Boreham CJ  Summons RE  Hayes JM 《Organic Geochemistry》1994,21(10-11):1037-1049

Polycyclic aromatic hydrocarbons with varying degrees of aromatization were isolated from the Eocene Messel Shale (Rheingraben, Germany). The high abundances of these compounds and their structural resemblances to cyclic triterpenoid lipids are consistent with derivation from microbial rather than thermal processes. Compounds structurally related to oleanane contain from five to nine double bonds; those within a series of aromatized hopanoids contain from three to nine. All are products of diagenetic reactions that remove hydrogen or methyl groups, and, in several cases, break carbon-carbon bonds to open rings. Aromatized products are on average depleted in 13C relative to possible precursors by l.2% (range: l.5% enrichment to 4% depletion, n = 9). The dependence of 13C content on the number of double bonds is not, however, statistically significant and it must be concluded that there is no strong evidence for isotopic fractionation accompanying diagenetic aromatization. Isotopic differences between series (structures related to ursane, des-A-ursane, des-A-lupane, des-A-arborane, and possibly, des-A-gammacerane are present) are much greater, indicating that 13C contents are controlled primarily by source effects. Fractionations due to chromatographic isotope effects during HPLC ranged from 0.1 to 2.8%.  相似文献   

Methylhopane biomarker hydrocarbons in Hamersley Province sediments provide evidence for Neoarchean aerobiosis   总被引：1，自引：0，他引：1  

Jennifer L. Eigenbrode  Katherine H. Freeman  Roger E. Summons 《Earth and Planetary Science Letters》2008,273(3-4):323-331

Bacteriohopanepolyols with A-ring methyl substituents are bacterial lipid biomarkers that are presently known to originate from just a few specific taxa. When preserved in ancient rocks, the fossilized hydrocarbon skeletons of these molecules have the potential to provide biogeochemical records of those taxa and their respective physiologies. Considering both their occurrences in modern organisms, and their Proterozoic and Phanerozoic sedimentary distributions, hopanes carrying 2-methyl or 3-methyl substituents are proposed to be derived from cyanobacteria and oxygen-respiring methanotrophs, respectively. Here we report the distribution of methylhopanes in 2.72–2.56 billion-year-old, Neoarchean rocks from the Hamersley Province on the Pilbara Craton. The relative abundance of C₃₁-3β-methylhopane, but not that of C₃₁-2α-methylhopane, shows a strong correlation to the carbon isotopic composition of co-occurring kerogen (insoluble particulate organic matter). The unanticipated nature of this relationship provides evidence for a shallow-water locus of carbon cycling through aerobic oxidation of CH₄ and, coincidentally, a means to demonstrate biomarker syngenicity. 2α-Methylhopanes are most abundant in both shale and carbonate from shallow-water sediments, and a positive correlation to carbonate abundance, contrasts to variable, but generally lower, 2α-methylhopane abundances of deeper water facies. These observations are consistent with their origin from cyanobacteria which were likely the cornerstone of microbial communities in shallow-water ecosystems providing molecular oxygen, fixed carbon, and possibly fixed nitrogen.  相似文献   

Searching for periodicities in the MACHO light curve of LMC X-2     

C. Alcock  R. A. Allsman  D. R. Alves  T. S. Axelrod  A. C. Becker  D. P. Bennett  P. A. Charles  K. H. Cook  A. J. Drake  K. C. Freeman  M. Geha  K. Griest  P. Groot  †  M. J. Lehner  S. L. Marshall  K. E. McGowan  ‡  D. Minniti  C. A. Nelson  B. A. Peterson  P. Popowski  M. R. Pratt  P. J. Quinn  W. Sutherland  A. B. Tomaney  T. Vandehei  J. van Paradijs 《Monthly notices of the Royal Astronomical Society》2000,316(4):729-733

Using the exceptional long-term monitoring capabilities of the MACHO project, we present here the optical history of LMC X-2 for a continuous 6-yr period. These data were used to investigate the previously claimed periodicities for this source of 8.15 h and 12.54 d: we find upper limits of 0.10 mag and 0.09 mag, respectively.  相似文献   

The greenhouse gas (GHG) emissions associated with aquatic carbon removal during drinking water treatment     

Timothy G. Jones  Chris D. Evans  Chris Freeman 《Aquatic Sciences - Research Across Boundaries》2016,78(3):561-572

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