Phytate as a novel phosphorus-specific paleo-indicator in aquatic sediments   总被引：1，自引：0，他引：1  

Benjamin L. Turner  Kaarina Weckström 《Journal of Paleolimnology》2009,42(3):391-400

A reliable geochemical paleo-indicator for phosphorus remains elusive, despite the importance of understanding historical changes in the nutrient status of aquatic ecosystems. We assessed the potential of phytate (salts of myo-inositol hexakisphosphate) as a novel phosphorus-specific paleo-indicator by measuring its concentrations in dated sediments from an embayment in Helsinki, Finland, with a known 200-year history of trophic changes. Phytate was extracted in a solution containing sodium hydroxide and EDTA and detected by solution ³¹P NMR spectroscopy with spectral deconvolution. Concentrations varied markedly with sediment depth and paralleled previously determined changes in diatom assemblages and geochemical indicators linked to trophic status. In contrast, total sediment phosphorus did not reflect phosphorus inputs to the embayment, presumably due to the mobilization of inorganic phosphate under anoxic conditions during periods of high pollutant loading. Importantly, phytate appeared to be stable in these brackish sediments, in contrast to other organic and inorganic phosphates which declined abruptly with depth. We therefore conclude that phytate represents a potentially important indicator of historical changes in phosphorus inputs to water bodies, although additional studies are required to confirm its stability under conditions likely to be encountered in lakes and coastal ecosystems.  相似文献   

Merging eruption datasets: building an integrated Holocene eruptive record for Mt Taranaki,New Zealand     

Michael B. Turner  Mark S. Bebbington  Shane J. Cronin  Robert B. Stewart 《Bulletin of Volcanology》2009,71(8):903-918

Acquiring detailed eruption frequency datasets for a volcano system is essential for realistic eruption forecasts. However, accurate datasets are inherently difficult to compile, even if one or more well-dated eruption records are available. A single record typically under-represents the eruption frequency, while combining two or more records may result in an overrepresentation. Although glass compositions have proven to be successful in tephrochronological studies of dominantly rhyolitic tephras; microlitic growth and thin glass shards inhibit their application to andesitic tephras. A method consisting of a combination of two techniques for correlating syn-eruptive deposits is demonstrated on data from the typical andesitic stratovolcano of Mt. Taranaki, New Zealand. Firstly, tentative matches are identified using the radiocarbon age and associated error of each event. Secondly, the compositions of titanomagnetite micro-phenocrysts are used as an independent check, and shown to be a useful correlation tool where age data is available. Using two lake-core records containing tephra layers in an overlapping time-frame, the radiocarbon age-correlation procedure suggested 31 tephra matches. Geochemistry data were available for 15 of these pairs. In three of these cases, the titanomagnetite compositions did not match. Hence, these “paired” tephras were from compositionally distinct magmas and therefore likely represent separate events. An additional three matches were reassigned within the temporal uncertainty limits of the dating procedure, based on better geochemical pairing. The final combined dataset suggests that there have been at least 138 separate ash fall-producing eruptions between 96 and 10 150 years B.P. from Taranaki. Using the combined dataset the mixture of Weibulls renewal model forecasts a probability of 0.52 for an eruption occurring in the next 50 years at this volcano. The present annual eruption probability is estimated at 1.6%. This likelihood is almost double that obtained when relying on a single stratigraphic record.  相似文献   

The price of spice: Ethnic minority livelihoods and cardamom commodity chains in upland northern Vietnam     

Claire Tugault‐Lafleur  Sarah Turner 《Singapore journal of tropical geography》2009,30(3):388-403

The northern uplands of Vietnam are witness to rapidly developing commodity markets for a range of forest and agricultural products. Since the early 1990s the cultivation of black cardamom (Amomum aromaticum) has become a source of cash income for a growing number of ethnic minority households in these uplands. Drawing on ethnographic fieldwork, and utilizing a commodity chain and livelihoods approach, this article investigates the contemporary dynamics surrounding the cultivation, harvesting and trade of dried cardamom fruit from Hmong cultivators in Lao Cai province to consumers elsewhere in Vietnam and beyond to China and the global market. We analyse the spatial and social structures of these commodity chains, drawing attention to a range of mechanisms of access. We find that these mechanisms are reliant upon specific ethnic relations as well as different forms of capital that together sustain economic advantages for particular actors. While focusing on the ethnic minority cultivators active in these commodity chains, we also call for more nuanced understandings of how the trade in forest products fits within local agricultural systems and household livelihood decision making processes in the Vietnam uplands.  相似文献   

An XMM‐Newton survey of broad iron lines in AGN     

K. Nandra  P. M. O'Neill  I. M. George  J. N. Reeves  T. J. Turner 《Astronomische Nachrichten》2006,327(10):1039-1042

We report on the iron Kα line properties of a sample of Seyfert galaxies observed with the XMM‐Newton EPIC pn instrument. Using a systematic and uniform analysis, we find that complexity at iron‐K is extremely common in the XMM‐Newton spectra. Once appropriate soft X‐ray absorption, narrow 6.4 keV emission and associated Compton reflection are accounted for, ∼75% of the sample show an improvement when a further component is introduced. The typical properties of the broad emission are both qualitatively and quantitatively consistent with previous results from ASCA. The complexity is in general very well described by relativistic accretion disk models. In most cases the characteristic emission radius is constrained to be within ∼50R _g, where strong gravitational effects become important. We find in about 1/3 of the sample the accretion disk interpretation is strongly favoured over competing models. In a few objects no broad line is apparent. We find evidence for emission within 6R _g in only two cases, both of which exhibit highly complex absorption. Evidence for black hole spin based on the X‐ray spectra therefore remains tentative. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Raman spectroscopy of olivine in dunite experimentally shocked to pressures between 5 and 59 GPa     

S. Farrell‐Turner  W. U. Reimold  M. Nieuwoudt  R. M. Erasmus 《Meteoritics & planetary science》2005,40(9-10):1311-1327

Abstract— Previous Raman investigations on experimentally shocked ingle‐crystal olivine indicated that the olivine Raman bands seemingly shift to a higher wave number with increasing shock pressure. If this effect could be confirmed, Raman analysis of natural shock‐metamorphosed minerals could potentially provide an important shock barometric tool. We carried out a Raman spectroscopic study on olivine in a series of natural dunite samples experimentally shocked to pressures between 5 and 59 GPa. In addition, we analyzed olivine grains in a sample of the Cold Bokkeveld C1 meteorite. We studied samples of several dunites with olivine of 90.64–92.00 mole% Fo to determine Raman effects in the region from 200 to 900 cm^?1. Several olivine grains per sample/shock pressure stage were analyzed. Raman analysis, however, showed little or no shift with increasing shock pressure. The shifts to higher or lower frequencies observed were not specific for a given pressure stage, with some grains within a sample showing more shift than others. This finding is unrelated to the crystallographic orientation of analyzed grains and cannot be related systematically to the different degrees of optically determined shock metamorphism of the analyzed grains. We identified an increase in full width at half maximum (FWHM) for the 824 cm^?1 band with increased shock pressure in the shocked Åheim samples above 45 GPa and, to a lesser extent, for the 856 cm^?1 band. Evaluation of band broadening of olivine in the Cold Bokkeveld meteorite showed FWHM values that were much greater (9–20 cm^?1) than those of olivine in the shocked dunite samples (7–12 cm^?1). We concluded that these differences in FWHM are due to differences in chemical composition between the meteoritic and the experimentally shocked olivine. Therefore, using Raman spectroscopy to detect small shifts in wave numbers to higher frequencies with increased shock pressure does not yield consistent effects for polycrystalline dunite. An extra band at 650 cm^?1 was identified in the Raman spectra of the unshocked Mooihoek dunite and the Åheim dunite samples shocked to 5, 29.3, and 59 GPa, as well as another at 696 cm^?1 in all the spectra of the 59 GPa Åheim sample. The cause of these extra bands is not known. Comparison of these results with Raman spectra of olivine from the Cold Bokkeveld C1 meteorite did not allow us to determine shock pressures for the meteoritic olivine.  相似文献   

Comment On Leighly*     

B.L. Turner 《The Professional geographer》1987,39(4):415-416

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Iodine-xenon studies of Bjurböle and Parnallee using RELAX     

J. D. Gilmour  R. D. Ash  R. Hutchison  J. C. Bridges  I. C. Lyon  G. Turner 《Meteoritics & planetary science》1995,30(4):405-411

Abstract— Iodine-xenon analyses of chondrules from the Bjurböle L4 and Parnallee LL3.6 meteorites have been made using a continuous wave laser microprobe and the resonance ionisation mass spectrometer RELAX. The excess ¹²⁹Xe content released from the Bjurböle chondrule is lower than previous stepped-heating studies have found, suggesting that the technique does not completely degas the samples. Nonetheless, clear isochrons were produced, and data for initial ¹²⁹Xe/¹³⁰Xe are consistent with earlier work. A correlation is evident in each chondrule between ¹³¹Xe^* and ¹²⁸Xe^* perhaps indicating a common host-phase for their parent nuclides, a condition possibly fulfilled by a Te- and I-bearing sulfide. The I-Xe system of a Parnallee macrochondrule exhibits no excess ¹²⁹Xe, possibly as a result of thermal alteration or deformation before accumulation of the meteorite. A cristobalite-bearing chondrule depleted in ¹⁶O yields an I-Xe age of 4.5 ± 0.5 Ma after the mean Bjurböle age.  相似文献   

THE USE OF CITATION INDICES IN COMPARING GEOGRAPHY PROGRAMS: AN EXPLORATORY STUDY*     

B. L. Turner  William B. Meyer 《The Professional geographer》1985,37(3):271-278

Citation information is discussed as a means for ranking the research and publication visibility of eleven graduate programs in geography. It is suggested that the ranks produced by this objective method correspond better to the peer-based ranks of the 1982 NRC survey of graduate programs than those derived from publication counts. Citation information also provides a means of evaluating the differences between programs' inter- and intradisciplinary visibility.  相似文献   

Interstellar cyanomethane     

Turner BE  Friberg P  Irvine WM  Saito S  Yamamoto S 《The Astrophysical journal》1990,355(2):546-561

We have made an observational study of the newly identified cyanomethane radical CH2CN and the possibly related species CH3CN with the goals of (1) elucidating the possible role of reactions of the type CnHm(+) + N in astrochemistry, and (2) providing a possible test of Bates's models of dissociative electron recombination. We find a remarkably different abundance ratio CH2CN/CH3CN in TMC-1 and Sgr B2, which we deduce is a result of the large difference in temperature of these objects. Studies of CH2CN and CH3CN in other sources, including two new detections of CH2CN, support this conclusion and are consistent with a monotonic increase in the CH2CN/CH3CN ratio with decreasing temperature over the range 10-120 K. This behavior may be explained by the destruction of CH2CN by reaction with O. If this reaction does not proceed, then CH2CN and CH3CN are concluded to form via different chemical pathways. Thus, they do not provide a test of Bates's conjectures (they do not both form from CH3CNH+). CH2CN is then likely to form via C2H4(+) + N --> CH2CNH+, thus demonstrating the viability of this important reaction in astrochemistry. The T dependence of the CH2CN/CH3CN ratio would then reflect the increasing rate of the C2H4(+) + N reaction with decreasing temperature.  相似文献   

Emerald deposits and occurrences: A review   总被引：3，自引：0，他引：3  

L.A. Groat  G. Giuliani  D.D. Marshall  D. Turner 《Ore Geology Reviews》2008,34(1-2):87

Emerald, the green gem variety of beryl, is the third most valuable gemstone (after diamond and ruby). Although it is difficult to obtain accurate statistics, Colombia supplies most (an estimated 60%, worth more than $500,000,000 per year) of the world's emeralds. However there is speculation that the emerald mines in Colombia are becoming depleted. Brazil currently accounts for approximately 10% of world emerald production. Emeralds have also been mined in Afghanistan, Australia, Austria, Bulgaria, China, India, Madagascar, Namibia, Nigeria, Pakistan, South Africa, Spain, Tanzania, the United States, and Zimbabwe.Because it is difficult to obtain accurate analyses of beryllium, most published analyses of beryl are renormalized on the basis of 18 oxygen and 3 Be atoms per formula unit. The color of emerald is due to trace amounts of chromium and/or vanadium replacing aluminum at the Y site; in most cases the Cr content is much greater than that of V. To achieve charge balance, the substitution of divalent cations at the Y site is coupled with the substitution of a monovalent cation for a vacancy at a channel site.Beryl is relatively rare because there is very little Be in the upper continental crust. Unusual geologic and geochemical conditions are required for Be and Cr and/or V to meet. In the classic model, Be-bearing pegmatites interact with Cr-bearing ultramafic or mafic rocks. However in the Colombian deposits there is no evidence of magmatic activity and it has been demonstrated that circulation processes within the host black shales were sufficient to form emerald. In addition, researchers are recognizing that regional metamorphism and tectonometamorphic processes such as shear zone formation may play a significant role in certain emerald deposits.A number of genetic classification schemes have been proposed for emerald deposits. Most are ambiguous when it comes to understanding the mechanisms and conditions that lead to the formation of an emerald deposit. Studies of individual emerald deposits show that in most cases a combination of mechanisms (magmatic, hydrothermal, and metamorphic) were needed to bring Be into contact with the chromophores. This suggests the need for a more flexible classification scheme based on mode of formation. Stable isotopes can be used to estimate the contribution of each mechanism in the formation of a particular deposit. Such estimates could perhaps be more precisely defined using trace element data, which should reflect the mode of formation.Emerald may be identified in the field by color, hardness, and form. It will tend to show up in stream sediment samples but because its specific gravity is relatively low, it will not concentrate in the heavy mineral fraction. In Colombia, structural geology, the sodium content of stream sediment samples, and the lithium, sodium, and lead contents of soil samples have all been used to find emerald occurrences. Exploration for gem beryl could result in the discovery of new occurrences of non-gem beryl or other Be minerals that could become new sources of Be and Be oxide.Future efforts should go towards creating a comprehensive data base of emerald compositions (including trace elements), determination of the role of metamorphism in the formation of some emerald deposits, improved classification schemes, and more effective exploration guidelines.  相似文献