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排序方式: 共有210条查询结果,搜索用时 15 毫秒
201.
Seth G. John Olivier J. Rouxel Paul R. Craddock Alison M. Engwall Edward A. Boyle 《Earth and Planetary Science Letters》2008,269(1-2):17-28
Many of the heaviest and lightest natural zinc (Zn) isotope ratios have been discovered in hydrothermal ore deposits. However, the processes responsible for fractionating Zn isotopes in hydrothermal systems are poorly understood. In order to better assess the total range of Zn isotopes in hydrothermal systems and to understand the factors which are responsible for this isotopic fractionation, we have measured Zn isotopes in seafloor hydrothermal fluids from numerous vents at 9–10°N and 21°N on the East Pacific Rise (EPR), the TAG hydrothermal field on the Mid-Atlantic Ridge, and in the Guaymas Basin. Fluid δ66Zn values measured at these sites range from + 0.00‰ to + 1.04‰. Of the many physical and chemical parameters examined, only temperature was found to correlate with fluid δ66Zn values. Lower temperature fluids (< 250 °C) had both heavier and more variable δ66Zn values compared to higher temperature fluids from the same hydrothermal fields. We suggest that subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite (Zn sulfide), and that this process is a primary cause of Zn isotope variation in hydrothermal fluids. Thermodynamic calculations carried out to determine saturation state of sphalerite in the vent fluids support this hypothesis with isotopically heaviest Zn found in fluids that were calculated to be saturated with respect to sphalerite. We have also measured Zn isotopes in chimney sulfides recovered from a high-temperature (383 °C) and a low-temperature (203 °C) vent at 9–10°N on the EPR and, in both cases, found that the δ66Zn of chimney minerals was lighter or similar to the fluid δ66Zn. The first measurements of Zn isotopes in hydrothermal fluids have revealed large variations in hydrothermal fluid δ66Zn, and suggest that subsurface Zn sulfide precipitation is a primary factor in causing variations in fluid δ66Zn. By understanding how chemical processes that occur beneath the seafloor affect hydrothermal fluid δ66Zn, Zn isotopes may be used as a tracer for studying hydrothermal processes. 相似文献
202.
Alison M. Shaw Mark D. Behn Susan E. Humphris Robert A. Sohn Patricia M. Gregg 《Earth and Planetary Science Letters》2010,289(3-4):311-322
We present new analyses of volatile, major, and trace elements for a suite of glasses and melt inclusions from the 85°E segment of the ultra-slow spreading Gakkel Ridge. Samples from this segment include limu o pele and glass shards, proposed to result from CO2-driven explosive activity. The major element and volatile compositions of the melt inclusions are more variable and consistently more primitive than the glass data. CO2 contents in the melt inclusions extend to higher values (167–1596 ppm) than in the co-existing glasses (187–227 ppm), indicating that the melt inclusions were trapped at greater depths. These melt inclusions record the highest CO2 melt concentrations observed for a ridge environment. Based on a vapor saturation model, we estimate that the melt inclusions were trapped between seafloor depths (~ 4 km) and ~ 9 km below the seafloor. However, the glasses are all in equilibrium with their eruption depths, which is inconsistent with the rapid magma ascent rates expected for explosive activity. Melting conditions inferred from thermobarometry suggest relatively deep (25–40 km) and cold (1240°–1325 °C) melting conditions, consistent with a thermal structure calculated for the Gakkel Ridge. The water contents and trace element compositions of the melt inclusions and glasses are remarkably homogeneous; this is an unexpected result for ultra-slow spreading ridges, where magma mixing is generally thought to be less efficient based on the assumption that steady-state crustal magma chambers are absent in these environments. All melts can be described by a single liquid line of descent originating from a pooled melt composition that is consistent with the aggregate melt calculated from a geodynamic model for the Gakkel Ridge. These data suggest a model in which deep, low degree melts are efficiently pooled in the upper mantle (9–20 km depth), after which crystallization commences and continues during ascent and eruption. Based on our melting model and the assumption that CO2 is perfectly incompatible, we show that the highest CO2 concentrations of the melt inclusions (~ 1600 ppm) are consistent with the calculated CO2 concentrations of primary undegassed melts. The highest measured CO2/Nb ratio (443) of Gakkel Ridge melt inclusions predicts a mantle CO2 content of 134 ppm and would result in a global ridge flux of 2.0 × 1012 mol CO2/yr. 相似文献
203.
Abstract: This study investigates why New Zealanders move to Australia. A survey of New Zealanders living in Australia found 'pull' factors, notably greater opportunities and better climate, were the most common reasons. While desire for change or sense of adventure was important, dissatisfaction with life in New Zealand was also a factor. Results indicated a form of transnationalism as migrants felt at home in both countries. They maintained links with New Zealand evident in support for New Zealand sporting teams, expressions of New Zealand nationality, emotional connection and regular contact with New Zealand. 相似文献
204.
Dissolved metal contamination in the East River-Long Island sound system: potential biological effects 总被引:3,自引:0,他引:3
A suite of dissolved trace metals (Ag, Cd, Cu and Pb), inorganic nutrients (NO(3), PO(4)), and chlorophyll a was measured along a 55 mile transect from the East River into western and central Long Island Sound. The main objectives of this study were to determine the relative levels of contamination from sewage, and to assess its possible biological impact on local waters. The East River-Long Island Sound system receives large volumes of treated sewage and industrial effluent as a result of the heavy urbanization of the area. Despite these strong environmental pressures, this study is among the first to report dissolved metal levels from that region. Consistent with the locations of anthropogenic sources, a strong east-west concentration gradient was observed for Ag, Pb, NO(3) and PO(4) with the highest levels found in the East River. In contrast, dissolved Cd and Cu were relatively constant throughout the area of study, suggesting that sewage sources have a more limited influence on the levels of those metals. Remobilization from contaminated sediments may represent the primary source of Cd and Cu to the Long Island Sound under low-runoff conditions in summer. Chlorophyll a concentrations, used as an indicator of total biomass, were also low in the East River. These low chlorophyll concentrations could not be explained by nutrient or light limitation, water column stratification, or to advection of phytoplankton out of the river during tidal flushing. These preliminary results suggest a potential toxic effect of sewage on the biological communities of the East River. 相似文献
205.
Martin Morlidge Alison Pawley Giles Droop 《Contributions to Mineralogy and Petrology》2006,152(3):365-373
The pressure–temperature conditions of the reactions of the double carbonates CaM(CO3)2, where M = Mg (dolomite), Fe (ankerite) and Mn (kutnohorite), to MCO3 plus CaCO3 (aragonite) have been investigated at 5–8 GPa, 600–1,100°C, using multi-anvil apparatus. The reaction dolomite = magnesite + aragonite is in good agreement with the results of Sato and Katsura (Earth Planet Sci 184:529–534, 2001), but in poor agreement with the results of Luth (Contrib Mineral Petrol 141:222–232, 2001). The dolomite is partially disordered at 620°C, and fully disordered at 1,100°C. All ankerite and kutnohorite samples, including the synthetic starting materials, are disordered. The P–T slopes of the three reactions increase in the order M = Mg, Fe, Mn. The shallower slope for the reaction involving magnesite is due partly to its having a higher compressibility than expected from unit-cell volume considerations. At low pressures there is a preference for partitioning into the double carbonate of Mg > Fe > Mn. At high pressures the partitioning preference is reversed. Using the measured reaction positions, the P–T conditions at which dolomite solid solutions will break down on increasing P and T in subduction zones can be estimated. 相似文献
206.
207.
Previous measurements from cool microtidal temperate areas suggest that microphytobenthic incorporation of nitrogen (N) exceeds
N removal by denitrification in illuminated shallow-water sediments. The present study investigates if this is true also for
fully nontidal sediments in the Baltic Sea., Sediment-water fluxes of inorganic (DIN) and, organic nitrogen (DON) and oxygen,
as well as denitrification, were measured in early autumn and spring, in light and dark, at four sites representing different
sediment types. All sediments were autotrophic during the daytime both in the autumn and spring. On a 24-h time scale, they
were autotrophic in the spring and heterotrophic in early autumn. Sediments funcitoned as sources of DIN and DON during the
autumn and sinks during the spring, with DON fluxes dominating or being as important as DIN fluxes. Microphytobenthos (MPB)
activity controlled fluxes of both DIN and DON. Significant differences between sites were found, although sediment type (sand
or silt) had no consistent effect on the magnitude of MPB production or nutrient fluxes. The clearest effect related to sediment
type was found for denitrification, although only in the autumn, with higher rates in silty sediments. Estimated N assimilation
by MPB, based on both net primary production (0.7–6.5 mmol N m−2 d−1) and on 80% of gross primary production (1.9–9.4 mmol N m−2 d−1) far exceeded measured rates of denitrification (0.01–0.16 mmol N m−2 d−1). A theoretical calculation showed that MPB may incorporate between 40% and 100% of the remineralized N, while denitrification
removes, <5%. MPB assimilation of N appears to be a far more important N consuming process than denitrification in these nontidal,
shallow-water sediments. 相似文献
208.
209.
Selenium’s (Se) protective effects against mercury (Hg) toxicity have been demonstrated; however, this is seldom considered in health assessments, where dietary exposure is still evaluated by Hg concentration alone. Se:Hg ratios and selenium health benefit values (Se HBVs) offer a more comprehensive seafood safety model. Here we describe total mercury (THg), methylmercury (MeHg) and Se concentrations in fish from a Hg-polluted estuary. Spatial variation in THg, MeHg and Se was evident, though all regions maintained Se:Hg ratio values >1. Se HBV varied between regions and in one region mean negative values (−5.17) were evident. This study provides the first evidence that quoting a single all-encompassing Se HBV is not appropriate when species demonstrate strong site fidelity. It highlights the need for research into Se–Hg relationships in environments with established Hg pollution and reinforces the assertion that Se concentration be considered in assessments of human health risk to Hg exposure. 相似文献
210.
Bronfenbrenner’s bioecological theory for modelling community resilience to natural disasters 总被引:1,自引:1,他引:0
Helen J. Boon Alison Cottrell David King Robert B. Stevenson Joanne Millar 《Natural Hazards》2012,60(2):381-408
This paper advocates the use of Bronfenbrenner’s bioecological theory as a framework to analyse resilience at diverse scales. Bronfenbrenner’s bioecological theory can be employed to (a) benchmark
social resilience, (b) target the priority interventions required and (c) measure progress arising from these interventions
to enhance resilience to natural disasters. First, the paper explores resilience to natural disasters in the context of climatic
change as building resilience is seen as a way to mitigate impacts of natural disasters. Second, concepts of resilience are
systematically examined and documented, outlining resilience as a trait and resilience as a process. Third, issues arising
in relation to the measurement of resilience are discussed. Fourth, Bronfenbrenner’s bioecological systems theory is described
and proffered to model and assess resilience at different scales. Fifth, studies are described which have supported the use
of the bioecological systems theory for the study of resilience. Sixth, an example of the use of Bronfenbrenner’s theory is
offered and the paper concludes with suggestions for future research using Bronfenbrenner’s theory. 相似文献