A model of diffuse degassing at three subduction-related volcanoes     

Glyn Williams-Jones  John Stix  Martin Heiligmann  Anne Charland  Barbara Sherwood Lollar  N. Arner  Gustavo V. Garzón  Jorge Barquero  Erik Fernandez 《Bulletin of Volcanology》2000,62(2):130-142

2 and δ¹³C in soil gas were measured at three active subduction-related stratovolcanoes (Arenal and Poás, Costa Rica; Galeras, Colombia). In general, Rn, CO₂ and δ¹³C values are higher on the lower flanks of the volcanoes, except near fumaroles in the active craters. The upper flanks of these volcanoes have low Rn concentrations and light δ¹³C values. These observations suggest that diffuse degassing of magmatic gas on the upper flanks of these volcanoes is negligible and that more magmatic degassing occurs on the lower flanks where major faults and greater fracturing in the older lavas can channel magmatic gases to the surface. These results are in contrast to findings for Mount Etna where a broad halo of magmatic CO₂ has been postulated to exist over much of the edifice. Differences in radon levels among the three volcanoes studied here may result from differences in age, the degree of fracturing and faulting, regional structures or the level of hydrothermal activity. Volcanoes, such as those studied here, act as plugs in the continental crust, focusing magmatic degassing towards crater fumaroles, faults and the fractured lower flanks. Received: 16 December 1997 / Accepted: 27 January 2000  相似文献   

BOOK REVIEWS     

D. L. Nicol  M. S. HALL  W. B. LANGBEIN 《水文科学杂志》2013,58(1):160-162

Abstract

A distributed eco-hydrological model based on soil—vegetation—atmosphere transfer processes is applied to estimate actual evapotranspiration (ET) and gross primary production (GPP) over the Wuding River basin, Loess Plateau, China, based on digital elevation model, vegetation and soil information between 2000 and 2003 over three grid sizes: 250 m, 1 km and 8 km. The spatial patterns of annual ET and GPP are related to precipitation variability and land-use/cover conditions. The grid size is shown to affect the spatial patterns of annual ET and GPP, the effect on GPP being more significant than that on ET. Geostatistical and regression analyses demonstrate that precipitation and vegetation influence the scaling effect of ET and GPP in a complex way. When precipitation is high, the scaling effect of ET is more dependent on precipitation. The scaling effect of ET and GPP from 1-km to 8-km grid size is much larger than that from 250-m to 1-km grid size, showing the 1-km grid size to be a feasible choice for simulation of their spatial patterns. Although the annual GPP is more sensitive to the grid size than annual ET, both daily ET and daily GPP averaged over the whole basin seem to be insensitive to the grid size, illustrating that the coarse grid size can be used to simulate spatially-averaged variables without losing much accuracy.  相似文献   

The impact of natural versus anthropogenic aerosols on atmospheric circulation in the Community Atmosphere Model     

Robert J. Allen  Steven C. Sherwood 《Climate Dynamics》2011,36(9-10):1959-1978

The equilibrium response of atmospheric circulation to the direct radiative effects of natural or anthropogenic aerosols is investigated using the Community Atmosphere Model (CAM3) coupled to two different ocean boundary conditions: prescribed climatological sea surface temperatures (SSTs) and a slab ocean model. Anthropogenic and natural aerosols significantly affect the circulation but in nearly opposite ways, because anthropogenic aerosols tend to have a net local warming effect and natural aerosols a net cooling. Aerosol forcings shift the Intertropical Convergence Zone and alter the strength of the Hadley circulation as found in previous studies, but also affect the Hadley cell width. These effects are due to meridional gradients in warming caused by heterogeneous net heating, and are stronger with interactive SST. Aerosols also drive model responses at high latitudes, including polar near-surface warming by anthropogenic aerosols in summer and an Arctic Oscillation (AO)-type responses in winter: anthropogenic aerosols strengthen wintertime zonal wind near 60°N, weaken it near 30°N, warm the troposphere, cool the stratosphere, and reduce Arctic surface pressure, while natural aerosols produce nearly opposite changes. These responses are shown to be due to modulation of stratospheric wave-driving consistent with meridional forcing gradients in midlatitudes. They are more pronounced when SST is fixed, apparently because the contrast in land-ocean heating drives a predominantly wavenumber-2 response in the northern hemisphere which is more efficient in reaching the stratosphere, showing that zonal heating variations also affect this particular response. The results suggest that recent shifts from reflecting to absorbing aerosol types probably contributed to the observed decadal variations in tropical width and AO, although studies with more realistic temporal variations in forcing would be needed to quantify this contribution.  相似文献   

Application of integrated vitrinite reflectance and FAMM analyses for thermal maturity assessment of the northeastern Malay Basin,offshore Vietnam: Implications for petroleum prospectivity evaluation     

H.I. Petersen  N. Sherwood  A. Mathiesen  M.B.W. Fyhn  N.T. Dau  N. Russell  J.A. Bojesen-Koefoed  L.H. Nielsen 《Marine and Petroleum Geology》2009

Several exploration wells have intersected a Cenozoic coal-bearing, fluvial-deltaic mudstone and sandstone succession in the northeastern Vietnamese part of the Malay Basin, and have successfully tested seismically identified direct hydrocarbon indicators (DHIs). The oil and gas/condensate discovery well 46-CN-1x encountered a ∼55 m thick section of lacustrine mudstones having considerable potential as an oil source. Vitrinite reflectance (VR) measurements from these alginite-bearing rocks introduce several problems in thermal maturity evaluation, including associated VR suppression and delineation of cavings and bitumens. Reliable thermal maturity gradients, however, may be established using a combination of conventional VR measurements and ‘equivalent VR’ (EqVR) values derived from the fluorescence alteration of multiple macerals (FAMM) technique. These measurements, performed on dispersed organic matter (DOM) in cuttings from 46-CN-1x, allow separation of low-reflecting bitumens and vitrinite in cavings from indigenous vitrinite and the FAMM results indicate VR suppression of 0.14% in an alginite-bearing mudstone with a high Hydrogen Index value. On the basis of available ‘raw’ VR data, a highly irregular maturity trend is determined, with the deepest sample (2675–2680 m) having a VR of ∼0.4%R_o. The EqVR value, however, for the deepest sample is 0.70%. The maturity trend determined from the FAMM data (and VR data, omitting samples having suppressed VR) indicates that the top of the oil window (VR of 0.75%R_o) is located at about 2800 m depth. Modelling the geothermal gradient using the EASY%R_o algorithm yields ∼40 °C/km for both of the two maturity profiles; this is in the low end of the range for the Malay Basin. Modelled temperature histories indicate onset of hydrocarbon generation for the uppermost Oligocene source rocks between 2 Ma and present-day, which post-dates trap formation. Seismic facies patterns suggest that lacustrine oil-prone units are in the oil window in the same graben complex a few km NW of the investigated well, and these rocks are likely to be the source of the hydrocarbons found in the well. A more widespread occurrence of hydrocarbons sourced from this kitchen is indicated by other discoveries and mapping of DHIs in the area.  相似文献   

Quantifying turbulent mixing and oxygen fluxes in a Mediterranean-type,microtidal estuary     

Jonathan?Sharples  Michael J.?CoatesEmail author  John E.?Sherwood 《Ocean Dynamics》2003,53(3):126-136

Experiments were carried out on an intermittent estuary during its closed (summer) and open (winter) states to identify the physical processes responsible for vertical mixing across the halocline, and to quantify vertical fluxes of oxygen and salt between water layers. During the blocked phase a two-layer structure was observed, with a brackish surface layer overlying old seawater. Within a deep basin the wind-driven turbulent mixing was consistent with the measured surface-layer turbulent dissipation, but the dissipation in the bottom layer appeared to be driven by internal seiching. In the shallow regions of the estuary vertical fluxes of dissolved oxygen were indicative of oxygen demand by respiration and remineralization of organic material in bottom water and sediments. During the estuary's open phase a three-layer structure was observed, having a fresh, river-derived surface layer, a middle layer of new seawater, and a bottom layer of old seawater. In the shallower regions surface-layer turbulent diffusion was consistent with the strong, gusty winds experienced at the time. The dissolved oxygen of the incoming seawater decreased to very low values by the time it reached the upstream deep basin as a result of the low cross-pycnocline oxygen flux being unable to compensate for the oxygen utilization. At least 50 % of the cross-pycnocline salt fluxes in the shallow reaches of the open estuary are suggested to be driven by Holmboe instabilities. Responsible Editor: Hans Burchard  相似文献   

H<Subscript>2</Subscript>-rich and Hydrocarbon Gas Recovered in a Deep Precambrian Well in Northeastern Kansas     

K. David Newell  John H. Doveton  Daniel F. Merriam  Barbara Sherwood Lollar  William M. Waggoner  L. Michael Magnuson 《Natural Resources Research》2007,16(3):277-292

In late 2005 and early 2006, the WTW Operating, LLC (W.T.W. Oil Co., Inc.) #1 Wilson well (T.D. = 5772 ft; 1759.3 m) was drilled for 1826 ft (556.6 m) into Precambrian basement underlying the Forest City Basin in northeastern Kansas. Approximately 4500 of the 380,000 wells drilled in Kansas penetrate Precambrian basement. Except for two previous wells drilled into the arkoses and basalts of the 1.1-Ga Midcontinent Rift and another well drilled in 1929 in basement on the Nemaha Uplift east of the Midcontinent Rift, this well represents the deepest penetration into basement rocks in the state to date. Granite is the typical lithology observed in wells that penetrate the Precambrian in the northern Midcontinent. Although no cores were taken to definitively identify lithologies, well cuttings and petrophysical logs indicate that this well encountered basement metamorphic rocks consisting of schist, gneiss, and amphibolitic gneiss, all cut by aplite dikes. The well was cased and perforated in the Precambrian, and then acidized. After several days of swabbing operations, the well produced shows of low-Btu gas, dominated by the non-flammable component gases of nitrogen (20%), carbon dioxide (43%), and helium (1%). Combustible components include methane (26%), hydrogen (10%), and higher molecular-weight hydrocarbons (1%). Although Coveney and others [Am. Assoc. Petroleum Geologists Bull., v. 71, no, 1, p. 39–48, 1987] identified H₂-rich gas in two wells located close to the Midcontinent Rift in eastern Kansas, this study indicates that high levels of H₂ may be a more widespread phenomenon than previously thought. Unlike previous results, the gases in this study have a significant component of hydrocarbon gas, as well as H₂, N₂, and CO₂. Although redox reactions between iron-bearing minerals and groundwater are a possible source of H₂ in the Precambrian basement rocks, the hydrocarbon gas does not exhibit the characteristics typically associated with proposed abiogenic hydrocarbon gases from Precambrian Shield sites in Canada, Finland, and South Africa. Compositional and isotopic signatures for gas from the #1 Wilson well are consistent with a predominantly thermogenic origin, with possible mixing with a component of microbial gas. Given the geologic history of uplift and rifting this region, and the major fracture systems present in the basement, this hydrocarbon gas likely migrated from source rocks and reservoirs in the overlying Paleozoic sediments and is not evidence for abiogenic hydrocarbons generated in situ in the Precambrian basement.  相似文献   

Discounting and uncertainty: a non-economist’s view     

Steven Sherwood 《Climatic change》2007,80(3-4):205-212

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Adakite-like Lavas from Antisana Volcano (Ecuador): Evidence for Slab Melt Metasomatism Beneath Andean Northern Volcanic Zone   总被引：6，自引：1，他引：6  

BOURDON  ERWAN; EISSEN  JEAN-PHILIPPE; MONZIER  MICHEL; ROBIN  CLAUDE; MARTIN  HERVE; COTTEN  JOSEPH; HALL  MINARD L. 《Journal of Petrology》2002,43(2):199-217

Extensive sampling of the Antisana volcano in Ecuador (NorthernVolcanic Zone of the Andes) has revealed the presence of adakite-likerocks throughout the edifice, i.e. rocks with geochemical characteristicsclose, but not identical, to those of slab melts. Two main volcanicgroups have been distinguished, characterized by two distinctevolutionary trends. The AND group, mostly composed of andesites,shows the clearest adakitic characteristics such as high La/Yband Sr/Y ratios and low heavy rare earth element (HREE) contents.The CAK group, composed of high-K andesites and dacites, displaysless pronounced adakitic-like characteristics. Although themore basic rocks of each group are difficult to distinguishon many geochemical diagrams, a geochemical study shows thatthe evolution of the AND and CAK groups is dominated by differentpetrogenetic processes. The isotopic characteristics of theCAK rocks suggest that evolution of this group is dominatedby a limited assimilation–fractional crystallization processwithin the granitic continental basement of the cordillera.In the AND group, the abundances of incompatible elements, suchas Nb or HREE, suggest that the series was produced by a partialmelting process in a mantle rich in garnet, amphibole and/orclinopyroxene. Such a mantle source has been demonstrated (experimentallyand by exhumed mantle xenoliths) to be produced in subductionzones where slab melts react with and metasomatize the mantlewedge. In Ecuador, magmas erupted in the Western Cordillera(trenchward relative to Antisana volcano) are true adakites,suggesting that slab melts can be responsible for the metasomatismof the mantle wedge beneath the NVZ in Ecuador. If mantle convectioncan drag down this modified mantle beneath Antisana volcano,destabilization of metasomatic amphibole at appropriate pressuresin this modified garnetiferous mantle can adequately explainthe formation and the geochemical features of Antisana lavas. KEY WORDS: subduction; adakite; metasomatism; Ecuador; AFC; Sr and Nd isotopes  相似文献   

Between a Hotspot and a Cold Spot: Isotopic Variation in the Southeast Indian Ridge Asthenosphere, 86{degrees}E-118{degrees}E     

MAHONEY  J. J.; GRAHAM  D. W.; CHRISTIE  D. M.; JOHNSON  K. T. M.; HALL  L. S.; VONDERHAAR  D. L. 《Journal of Petrology》2002,43(7):1155-1176

Glasses from a 2600 km section of the Southeast Indian Ridgewest of the Australian–Antarctic Discordance all possessNd–Pb–Sr isotopic signatures typical of Indian Oceanridge basalt. The boundary between Pacific- and Indian-Ocean-typeridge basalt within the Discordance thus marks the westernmostextent of shallow Pacific-type asthenosphere beneath the ridge.Along-axis He, Nd, Pb, and Sr isotopic patterns are largelyindependent of ridge segmentation, but a weak tendency is evidentfor the most strongly Indian-Ocean-type mantle to be relativelyfusible and for shallower asthenosphere to have lower ³He/⁴He.On average,  相似文献   

The DMS-cloud albedo feedback effect: Greatly underestimated?     

Sherwood B. Idso 《Climatic change》1992,21(4):429-433

There are a number of ways by which the biosphere may counter any impetus for global warming that might be produced by the rising CO₂ content of earth's atmosphere. Evidence for one of these phenomena, the DMS-cloud feedback effect, is discussed in light of recent claims that it is not of sufficient strength to be of much importance.  相似文献