Helium isotope geochemistry of mid-ocean ridge basalts from the South Atlantic     

David W. Graham  William J. Jenkins  Jean-Guy Schilling  Geoffrey Thompson  Mark D. Kurz  Susan E. Humphris 《Earth and Planetary Science Letters》1992,110(1-4):133-147

We report new helium isotope results for 49 basalt glass samples from the Mid-Atlantic Ridge between 1°N and 47°S.³He/⁴He in South Atlantic mid-ocean ridge basalts (MORB) varies between 6.5 and 9.0 R_A (R_A is the atmospheric ratio of1.39 × 10⁻⁶), encompassing the range of previously reported values for MORB erupted away from high³He/⁴He hotspots such as Iceland. He, Sr and Pb isotopes show systematic relationships along the ridge axis. The ridge axis is segmented with respect to geochemical variations, and local spike-like anomalies in³He/⁴He, Pb and Sr isotopes, and trace element ratios such as(La/Sm)_N are prevalent at the latitudes of the islands of St. Helena, Tristan da Cunha and Gough to the east of the ridge. The isotope systematics are consistent with injection beneath the ridge of mantle “blobs” enriched in radiogenic He, Pb and Sr, derived from off-axis hotspot sources. The variability in³He/⁴He along the ridge can be used to refine the hotspot source-migrating-ridge sink model.

MORB from the 2–7°S segment are systematically the least radiogenic samples found along the mid-ocean ridge system to date. Here the depleted mantle source is characterized by⁸⁷Sr/⁸⁶Sr of 0.7022, Pb isotopes close to the geochron and with²⁰⁶Pb/²⁰⁴Pb of 17.7, and³He/⁴He of 8.6–8.9 R_A. The “background contamination” of the subridge mantle, by radiogenic helium derived from off-ridge hotspots, displays a maximum between 20 and 24°S. The HePb and HeSr isotope relations along the ridge indicate that the³He/⁴He ratios are lower for the hotspot sources of St. Helena, Tristan da Cunha and Gough than for the MORB source, consistent with direct measurements of³He/⁴He ratios in the island lavas. Details of the HeSrPb isotope systematics between 12 and 22°S are consistent with early, widespread dispersion of the St. Helena plume into the asthenosphere, probably during flattening of the plume head beneath the thick lithosphere prior to continental breakup. The geographical variation in theHe/Pbratio deduced from the isotope systematics suggests only minor degassing of the plume during this stage. Subsequently, it appears that the plume component reaching the mid-Atlantic ridge was partially outgassed of He during off-ridge hotspot volcanism and related melting activity.

Overall, the similar behavior of He and Pb isotopes along the ridge indicates that the respective mantle sources have evolved under conditions which produced related He and Pb isotope variations.  相似文献   

The mechanisms of finite brittle strain   总被引：1，自引：0，他引：1  

Geoffrey C. P. King  Charles G. Sammis 《Pure and Applied Geophysics》1992,138(4):611-640

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Introduction     

Geoffrey C. P. King 《Pure and Applied Geophysics》1992,138(4):529-529

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Internally consistent gahnitic spinel-cordierite-garnet equilibria in the FMASHZn system: geothermobarometry and applications   总被引：8，自引：0，他引：8  

Geoffrey T. Nichols  Ron F. Berry  David H. Green 《Contributions to Mineralogy and Petrology》1992,111(3):362-377

The equilibrium (Mg, Fe, Zn)₃Al₂Si₃O₁₂+2Al₂SiO₅=3(Mg, Fe, Zn)Al₂O₄+5SiO₂ garnet + sillimanite/kyanitc = spinel + quartz was calibrated in the piston-cylinder apparatus between 11 and 30 kbar, and over the temperature range of 950 to 1200°C. Three experimental mixes of Mg no. [100^*MgO/(MgO+FeO)] 40, 47 and 60, in the FeO –MgO–Al₂O₃–SiO₂–ZnO (FMASZn) system were used under low oxygen fugacities and anhydrous conditions. We derive a ternary Fe–Mg–Zn symmetric mixing model for aluminous spinels in equilibrium with garnet, to quantify the increase in gahnitic end-member of spinel with increasing pressure and descreasing temperature. Further experiments in the spinel-cordieritequartz-sillimanite field were combined with garnet-cordierite data from the literature to produce a consistent set of equations describing the exchange reactions in FMASHZn relevant to quartz-sillimanite bearing rocks at granulite facies conditions. As spinel is an important mineral participating in many rocks of aluminous composition at granulite-facies conditions, and as zinc contributes to an enlargement of spinel's stability field towards higher pressures and lower temperatures, the thermobarometric calibrations presented here will be most significant in delineating the prograde and retrograde trajectory of P-T paths.  相似文献   

The Baltic press and the environment: A study of the coverage of environmental problems in Estonian and Latvian newspapers 1992–1993     

Geoffrey D. Gooch 《Geoforum》1995,26(4):429-443

For most people, both in the West and the former Soviet empire, the mass media is an important source of information about the environment, but little has been known about the ways in which environmental issues are treated in the mass media in the former communist states of East Europe. In this paper, articles dealing with environmental issues in the Estonian and Latvian press during 1992 and 1993 were analysed, and the type of problems dealt with in the articles, the sources of information used and the locations of the places referred to were examined. The results of this study show that a majority of the articles in the Estonian and Latvian press were concerned with environmental issues that were concentrated in a few places within these countries, and that when international issues were treated, these were localized to nearby European countries. Despite the existence of problems with air pollution, especially in the north-east of Estonia, these were seldom mentioned in the press. The main sources of information used in the articles were the authorities.  相似文献   

Circulating subbeam systemsand the physics of pulsar emission     

Joanna?M.?RankinEmail author  Geoffrey?A.?E.?Wright 《Astronomy and Astrophysics Review》2003,12(1):43-69

The purpose of this paper is to suggest how detailed single-pulse observations of slow radio pulsars may be utilized to construct an empirical model for their emission. It links the observational synthesis developed in a series of papers by Rankin in the 1980s and 90s to the more recent empirical feedback model of Wright (2003a) by regarding the entire pulsar magnetosphere as a non-steady, non-linear interactive system with a natural built-in delay. It is argued that the enhanced role of the outer gap in such a system indicates an evolutionary link to younger pulsars, in which this region is thought to be highly active, and that pulsar magnetospheres should no longer be seen as being driven by events on the neutron stars polar cap, but as having more in common with planetary magnetospheres and auroral phenomena.Received: 8 May 2003, Published online: 14 November 2003 Correspondence to: Joanna M. Rankin. On leave from: Physics Department, University of Vermont, Burlington, VT 05405, USA  相似文献   

Trees as riparian engineers: the Tagliamento river,Italy     

Angela Gurnell  Geoffrey Petts 《地球表面变化过程与地形》2006,31(12):1558-1574

Pristine river corridors were characterized by island and floodplain development driven by the inter‐play of flows, sediments and woody vegetation. Here we explore these relationships within topographically controlled settings within the upper, middle and lower reaches of a large, semi‐natural alpine to mediterranean river. These reaches have expanding or contracting valley floors within which we show that there are more or less predictable patterns of stream power and rates of vegetation growth, reflecting water availability during dry periods and also the availability of sand and finer sediment. We relate these to the pattern of island distribution that is repeated within the three reaches and is indicative of the engineering role of riparian trees. Islands are shown to develop within thresholds defined by stream power, rates of woody vegetation growth and rates of sedimentation, and to develop most quickly where riparian species include those capable of sprouting from driftwood. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Water clarity modeling in Lake Tahoe: Linking suspended matter characteristics to Secchi depth     

Theodore J. Swift  Joaquim Perez-Losada  S. Geoffrey Schladow  John E. Reuter  Alan D. Jassby  Charles R. Goldman 《Aquatic Sciences - Research Across Boundaries》2006,68(1):1-15

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Linking vulnerability, adaptation, and resilience science to practice: Pathways, players, and partnerships   总被引：7，自引：1，他引：7  

Coleen Vogel  Susanne C. Moser  Roger E. Kasperson  Geoffrey D. Dabelko 《Global Environmental Change》2007,17(3-4):349-364

Vulnerability, adaptation and resilience are concepts that are finding increasing currency in several fields of research as well as in various policy and practitioner communities engaged in global environmental change science, climate change, sustainability science, disaster risk-reduction and famine interventions. As scientists and practitioners increasingly work together in this arena a number of questions are emerging: What is credible, salient and legitimate knowledge, how is this knowledge generated and how is it used in decision making? Drawing on important science in this field, and including a case study from southern Africa, we suggest an alternative mode of interaction to the usual one-way interaction between science and practice often used. In this alternative approach, different experts, risk-bearers, and local communities are involved and knowledge and practice is contested, co-produced and reflected upon. Despite some successes in the use and negotiation of such knowledge for ‘real’ world issues, a number of problems persist that require further investigation including the difficulties of developing consensus on the methodologies used by a range of stakeholders usually across a wide region (as the case study of southern Africa shows, particularly in determining and identifying vulnerable groups, sectors, and systems); slow delivery of products that could enhance resilience to change that reflects not only a lack of data, and need for scientific credibility, but also the time-consuming process of coming to a negotiated understanding in science–practice interactions and, finally, the need to clarify the role of ‘external’ agencies, stakeholders, and scientists at the outset of the dialogue process and subsequent interactions. Such factors, we argue, all hinder the use of vulnerability and resilience ‘knowledge’ that is being generated and will require much more detailed investigation by both producers and users of such knowledge.  相似文献   

Mechanical shredding of water hyacinth (<Emphasis Type="Italic">Eichhornia crassipes</Emphasis>): Effects on water quality in the Sacramento-San Joaquin River Delta,California     

Ben K. Greenfield  Geoffrey S. Siemering  Joy C. Andrews  Michael Rajan  Stephen P. Andrews  David F. Spencer 《Estuaries and Coasts》2007,30(4):627-640

Management actions to control invasive aquatic species can have significant ecosystem-scale effects. We evaluated the water chemistry and nutrient effects of mechanical shredding to control water hyacinth (Eichhornia crassipes) in an agricultural slough and a tidal wetland on the Sacramento-San Joaquin River Delta, California. Shredding was conducted with two types of shredder boats in fall of 2003 and another boat in spring of 2004. Shredding measurably affected water quality, but specific effects varied as a function of shredding site and season. Significant increases were observed for total Kjeldahl nitrogen and total phosphorus for all experiments. Dissolved oxygen effects varied by site, decreasing after shredding at the agricultural slough but increasing at the tidal wetland. The increase in dissolved oxygen likely resulted from tidal incursions from the adjacent river. A year-long time series of dissolved oxygen data indicated a negative relationship between hyacinth abundance and dissolved oxygen concentrations. Hyacinth contained similar tissue concentrations of mercury to underlying sediments, suggesting that plant harvesting could aid mercury remediation efforts. Simple mass calculations indicated that Delta-wide shredding operations could cause between 0.1% and 9.6% increases in the overall abundance of carbon, nitrogen, and phosphorus in the Delta water column. Results suggest that local effects of management actions to control invasive aquatic plants will vary widely as a function of site-specific hydrology, but that estuary-wide effects would be limited.  相似文献