Evaluation of fractionation effects during the early stages of primary migration     

Cs. Sajgo  J.R. Maxwell  A.S. Mackenzie 《Organic Geochemistry》1983,5(2):65-73

A stepwise extraction technique, based on that of Beletskaya (1972), has been aplied to sedimentary rock samples of variable grain size and maturity from a Neogene sequence in the Pannonian Basin (S.E. Hungary). The resulting chloroform extracts, claimed by Beletskaya (1978) to sample “open” and mineralogically “closed” pores, have been analysed by gas chromatography and gas chromatography-mass spectrometry. Differences in the concentrations of the two extracts and the concentrations of their hydrocarbon fractions, and in the distributions of $\text{n}$-alkanes and steroid hydrocarbons suggest that either the “open” pores are impregnated with mature oil which has migrated from depth, or that movement of organic material from the “closed” to “open” pores occurs with considerable fractionation based on both polarity and molecular size. The implications of these differences for source rock-oil correlation studies are discussed and an assessment of mechanisms for primary migration is given.  相似文献   

On the mechanisms for H and Al incorporation in stishovite     

Geoffrey D. Bromiley  Fiona A. Bromiley  David W. Bromiley 《Physics and Chemistry of Minerals》2006,33(8-9):613-621

The solubility and incorporation mechanisms of hydrogen in synthetic stishovite as a function of Al₂O₃ content have been investigated. Mechanisms for H incorporation in stishovite are more complex than previously thought. Most H in stishovite is incorporated via the Smyth et al. (Am Mineral 80:454–456, 1995) model, where H docks close to one of the shared O–O edges, giving rise to an OH stretching band in infrared (IR) spectra at 3,111–3,117 cm⁻¹. However, careful examination of IR spectra from Al-stishovite reveals the presence of an additional OH band at 3,157–3,170 cm⁻¹. All H is present on one site, with interstitial H both coupled to Al³⁺ substitutional defects on adjacent octahedral (Si⁴⁺) sites, and decoupled from other defects, giving rise to two distinct absorption bands. Trends in IR data as a function of composition are consistent with a change in Al incorporation mechanism in stishovite, with Al³⁺ substitution for Si⁴⁺ charge-balanced by oxygen vacancies at low bulk Al₂O₃ contents, and coupled substitution of Al³⁺ onto octahedral (Si⁴⁺) and interstitial sites at high bulk Al₂O₃ contents. Trends in OH stretching frequencies as a function of Al₂O₃ content suggest that any such change in Al incorporation mechanism could alter the effect that Al incorporation has on the compressibility of stishovite, as noted by Ono et al. (Am Mineral 87:1486–1489, 2002).  相似文献   

Biotic Feedbacks in the Warming of the Earth   总被引：16，自引：0，他引：16  

G. M. Woodwell  F. T. Mackenzie  R. A. Houghton  M. Apps  E. Gorham  E. Davidson 《Climatic change》1998,40(3-4):495-518

A positive correlation exists between temperature and atmospheric concentrations of carbon dioxide and methane over the last 220,000 years of glacial history, including two glacial and three interglacial periods. A similar correlation exists for the Little Ice Age and for contemporary data. Although the dominant processes responsible may be different over the three time periods, a warming trend, once established, appears to be consistently reinforced through the further accumulation of heat-trapping gases in the atmosphere; a cooling trend is reinforced by a reduction in the release of heat-trapping gases. Over relatively short periods of years to decades, the correspondence between temperature and greenhouse gas concentrations may be due largely to changes in the metabolism of terrestrial ecosystems, whose respiration, including microbial respiration in soils, responds more sensitively, and with a greater total effect, to changes in temperature than does gross photosynthesis. Despite the importance of positive feedbacks and the recent rise in surface temperatures, terrestrial ecosystems seem to have been accumulating carbon over the last decades. The mechanisms responsible are thought to include increased nitrogen mobilization as a result of human activities, and two negative feedbacks: CO₂ fertilization and the warming of the earth, itself, which is thought to lead to an accumulation of carbon on land through increased mineralization of nutrients and, as a result, increased plant growth. The relative importance of these mechanisms is unknown, but collectively they appear to have been more important over the last century than a positive feedback through warming-enhanced respiration. The recent rate of increase in temperature, however, leads to concern that we are entering a new phase in climate, one in which the enhanced greenhouse effect is emerging as the dominant influence on the temperature of the earth. Two observations support this concern. One is the negative correlation between temperature and global uptake of carbon by terrestrial ecosystems. The second is the positive correlation between temperature and the heat-trapping gas content of the atmosphere. While CO₂ fertilization or nitrogen mobilization (either directly or through a warming-enhanced mineralization) may partially counter the effects of a warming-enhanced respiration, the effect of temperature on the metabolism of terrestrial ecosystems suggests that these processes will not entirely compensate for emissions of carbon resulting directly from industrial and land-use practices and indirectly from the warming itself. The magnitude of the positive feedback, releasing additional CO₂, CH₄, and N₂O, is potentially large enough to affect the rate of warming significantly.  相似文献   

Hadean Ocean Carbonate Geochemistry     

John W. Morse  Fred T. Mackenzie 《Aquatic Geochemistry》1998,4(3-4):301-319

Relatively soon (0.2 Ga) after the Earthformed, it is likely that major oceans appeared in ahot (100°C) reducing environment where carbondioxide was probably the dominant atmospheric gas,with PCO2, values reaching perhaps in excess of 10atm. During the Hadean Eon between 4.3 and 3.8 Ga BP,major changes in the concentration of atmosphericCO2 and associated temperature changes had aprofound influence on the carbonate geochemistry ofthe Hadean Ocean. Although no rocks are known to havesurvived prior to the Archean Eon, it is stillpossible to calculate approximate values for importantseawater parameters during the Hadean Eon based onother sources of information and reasonableassumptions about processes such as weatheringreactions.Our calculations are based on a linear temperaturechange from 100°C to 70°C and logPCO2 change from 1 to -1.5 over the Hadean Eon. Over this range in temperature and P CO2, theinfluence of T is relatively small, but changes inP CO2 result in large compositional variations inthe carbonate chemistry of Hadean seawater. In theearly Hadean, seawater pH was probably about5.8 ± 0.2, DIC may have reached close to 130 mM,and alkalinity was perhaps close to 30 mM. By thelate Hadean, seawater pH probably had changed to closeto neutral (6.8), and DIC and alkalinity were closerto present-day values. Even large uncertainties inNa+ + Cl-, K+ and Mg2+concentrations produce relatively small uncertaintiesin our calculated values for the carbonic acid system. However, larger uncertainties result from reasonableranges for Ca2+ concentrations and the saturationstate of Hadean seawater with respect to calcite.Our calculations support the hypothesis that acarbonate chemistry of seawater roughly similar tothat of modern oceans could have been acquired veryearly in Earth history. If seawater composition werebuffered by reactions involving carbonates andsilicates, then the composition of late Hadean-earlyArchean seawater was not vastly different from that oftoday. Thus, by the conclusion of the Hadean Eon, ifnot before, environmental conditions at the Earth's surface, including temperature and seawatercomposition, were sufficiently equable for theevolution of life, including the Archaebacteria: theextreme halophiles and thermophiles and methanogens.Contrary to the hypothesis of an early Na-bicarbonateocean, our calculations suggest the possibility thatthe early oceans of Earth were a NaCl-dominatedaqueous solution, with somewhat higher DIC andalkalinity concentrations, higher saturation state,and the possibility of lower calcium concentrations.The time course of approach of Hadean seawater to acarbonate composition closer to that of today isdifficult to predict. It is distinctly possible thatthe concentration of calcium in seawater did not reachlevels like that of modern seawater until the latePrecambrian and thus constrained the timing of the"Big Bang" of organic evolution, the emergence of theshelled invertebrates at the beginning of thePhanerozoic.  相似文献   

Evidence that polycyclic aromatic hydrocarbons in two carbonaceous chondrites predate parent-body formation     

Fiona L Plows  Richard N Zare  Peter R Buseck 《Geochimica et cosmochimica acta》2003,67(7):1429-1436

Organic material in meteorites provides insight into the cosmochemistry of the early solar system. The distribution of polycyclic aromatic hydrocarbons (PAHs) in the Allende and Murchison carbonaceous chondrites was investigated by use of a technique called microprobe laser-desorption laser-ionization mass spectrometry, which permits spatial resolution with approximately a 40-μm spot size. Sharp chemical gradients of PAHs are associated with specific meteorite features. The ratios of various PAH intensities relative to the smallest PAH, naphthalene, are nearly constant across the sample. These findings suggest a common origin for PAHs dating before or during the formation of the parent body, consistent with proposed interstellar formation mechanisms.  相似文献   

Crustal structure across the Canadian High Arctic region from teleseismic receiver function analysis   总被引：1，自引：0，他引：1  

Fiona A. Darbyshire 《Geophysical Journal International》2003,152(2):372-391

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Community Cohesion: Social and Economic Ties in the Personal Networks of Fisherfolk     

Fiona Nunan  Dražen Cepić  Bwambale Mbilingi  Konstantine Odongkara  Ernest Yongo  Monica Owili 《社会与自然资源》2018,31(3):306-319

Social ties influence access to knowledge and cooperation in natural resource management, with the sharing of certain characteristics thought to be positive for social cohesion and participatory forms of management. In this article, a holistic characterisation of fisherfolk personal networks is developed, disaggregating results by the main occupational groups within the fisheries studied, to provide a more nuanced understanding of the personal networks of types of fisherfolk. Links are then made between the characteristics of personal networks and evidence on how fisherfolk benefit from their networks, interpreted as contributing to social cohesion within the communities. The personal networks of fisherfolk (boat crew, boat owners and traders/processors) of Kenya, Tanzania and Uganda bordering Lake Victoria were investigated using personal network analysis, with fisherfolk asked who they discuss their fisheries activities with. The analysis found that networks based on the same occupation were more characteristic of fish traders/processors networks than those of boat crew and boat owners and that shared ethnicity, gender and location were characteristic of boat owner and boat crew networks, though shared ethnicity may reflect the composition of the communities rather than choice. Social and economic interactions within the networks were based on provision of credit, social support and advice, suggesting that these form the basis of social cohesion and should be taken into consideration in working with fishing communities in both development interventions and designing collaborative management approaches.  相似文献   

Development of the HEFT and NuSTAR focusing telescopes     

Fiona A. Harrison  Finn E. Christensen  William Craig  Charles Hailey  Wayne Baumgartner  C. M. H. Chen  James Chonko  W. Rick Cook  Jason Koglin  Kristin-Kruse Madsen  Michael Pivavoroff  Steven Boggs  David Smith 《Experimental Astronomy》2005,20(1-3):131-137

Hard X-ray/soft gamma-ray astrophysics is on the verge of a major advance with the practical realization of technologies capable of efficiently focusing X-rays above 10 keV. Hard X-ray focusing telescopes can achieve orders of magnitude improvements in sensitivity compared to the instruments based on coded apertures and collimated detectors that have traditionally been employed in this energy band. Compact focal planes enable high-performance detectors with good spectral resolution to be employed in efficient, low-background configurations. We have developed multilayer coated grazing incidence optics and solid state Cadmium Zinc Telluride focal plane systems for the High Energy Focusing Telescope (HEFT) balloon-borne experiment, and for the Nuclear Spectroscopic Telescope Array (NuSTAR) Small Explorer satellite. In this paper we describe the technologies, telescope designs, and performance of both experiments.  相似文献   

High‐energy sedimentation,Creag Aoil,Spean Bridge,Scotland: implications for meltwater movement and storage during Loch Lomond Stadial (Younger Dryas) ice retreat     

Andrew J. Russell  Fiona S. Tweed  Tim Harris 《第四纪科学杂志》2003,18(5):415-430

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Radiocarbon dating of charcoal from tropical sequences: results from the Niah Great Cave,Sarawak, and their broader implications     

Thomas F. G. Higham  Huw Barton  Chris S. M. Turney  Graeme Barker  Christopher Bronk Ramsey  Fiona Brock 《第四纪科学杂志》2009,24(2):189-197

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