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The decision in 1988 by the Rangitikei-Wanganui Catchment Board to allocate flows away from use for electricity generation into recreation use brought into focus the conflict between market and non-market valuation of river uses. The travel cost method (TCM) of non-market valuation is used to estimate the in-stream recreation values of the Upper Wanganui and Whakapapa rivers. 相似文献
13.
ABSTRACT. Numerous avifauna species face extinction on New Zealand's two main islands, owing largely to forest clearance and to introduced mammals. In response, New Zealand selects certain offshore islands for the relocation of threatened native birds, first purging them of mammalian predators. Over the past few decades, this procedure has evolved to become fairly successful. Protected habitat “islands” within mainland forested areas are also being created, but with success less certain. As historic habitats are lost, small biopreservation islands may become a standard mechanism for protecting threatened species, a process that can be termed the “miniaturization” of nature. 相似文献
14.
CHRIS A. McLINDEN D. J. CHARTRAND E. GRIFFIOEN J. C. McCONNELL C. T. McELROY 《Journal of Atmospheric Chemistry》1997,26(1):29-64
We have developed models of physically-based cloud and ocean surfacesfor use in photochemical models. These surface models are described in termsof a flux albedo and a normalized reflection function.Through these, the dependence of albedo on wavelength, solar zenithangle, cloud optical depth (cloud surfaces) and surface windspeed (ocean surfaces) are allowed for. In addition, the non-Lambertian nature of these surfaces is accounted for.We have integrated these surfacemodels into a multiple scattering radiative transfer model to assess their effects on the stratospheric radiation field and J-values. This was accomplished by comparison with results obtainedusing Lambertian, constant albedo surfaces. Comparisons of stratospheric radiation fields revealed that boththe wavelength and directional dependences of the cloud and oceansurfaces could be large effects.Differences between calculated J-values varied from 0 to 12% depending upon species, solar zenith angle, andheight.The J-values were then used as input for a chemical box model to examine the effects these surfaces had on stratospheric chemistry. Comparisons were made against box model runs using J-values fromconstant surfaces. Overall, the effect was on the order of 10%.Differences in number densities using these different surfacesvaried with latitude, height and species.Runs were made with and without heterogeneous chemistry. 相似文献
15.
Upper-regime parallel lamination as the result of turbulent sediment transport and low-amplitude bed forms 总被引:3,自引:0,他引:3
A series of laboratory experiments has been carried out in which parallel-laminated deposits were produced from an upper-regime plane bed. The laminae had thicknesses of a few mm and could be traced continuously over distances up to the length and width of the depositional area (0–3 m by 1–5m). Fluctuations in bed elevation were measured both during deposition and at equilibrium; much of the bed fluctuation occurs at time scales that are too long to be due directly to turbulence, as most theories for lamina formation would require. We suggest instead that extremely low-amplitude bed forms are present even on apparently flat beds and that the migration of these bed forms produces laterally continuous lamination. All the lamination produced in the laboratory experiments was normally graded. Using high-speed photography it was observed that the normal grading results from rapid deposition of a layer of loosely packed coarse sand several grain-diameters thick followed by the slow sieving-out of a well packed surface layer of finer sand. The initial deposition is the result of small-scale turbulent fluctuations in boundary shear stress. The sieving-out that follows results in a smooth surface whose low friction coefficient temporarily inhibits further deposition; we term this process ‘glazing’. The alignment of small-scale turbulent scour-and-fill structures along the paths traced by migrating bedform troughs produces laterally continuous parallel lamination. 相似文献
16.
A Quantitative Study of Magmatic Inclusions in the Plutonic Ejecta of Ascension Island 总被引:1,自引:2,他引:1
Plutonic blocks in the volcanic and pyroclastic rocks of AscensionIsland contain a variety of silicate melt and fluid inclusions,mainly in the intermediate (monzonite-syenite) and granite types.The silicate melt inclusions consist of trapped granitic meltwith a small contraction bubble. The fluid inclusions are: (i)highly saline (40 to > 67 wt. per cent NaCI) aqueous inclusions;(ii) low density CO2 + H2O inclusions, predominantly vapourand (iii) high density approximately pure H2O inclusions. Mixedsilicate melt-saline aqueous fluid inclusions also occur showingthat the original magma became saturated with respect to salineaqueous fluid and a magmatic origin is thus indicated for thesaline fluid inclusions. Microprobe analysis of the silicatemelt inclusions suggests that the original magma may have containedgreater than 5 wt. per cent H2O before saturation was reachedand homogenization temperatures show the trapping of the silicatemelt took place between 715-790?C. Separated amphiboles fromthe granites have D values which are significantly differentfrom the whole rock D values of the dry but otherwise chemicallysimilar obsidians. Sheppard & Harris (1985) interpret thisdifference as reflecting a sea water source for the water inthe gramtes. Qualitative and quantitative study of the fluidinclusions (extending the earlier work of Roedder & Coombs,1967) suggests that at least one episode of fluid immiscibility(into aqueous liquid+vapour, i.e. boiling) occurred.This resulted in a loss of H2O via the vapour and increasedthe salinity of the remaining liquid. The initial fluids belongedto the system H2O-CO2-NaCl-KCl and the CO2 was strongly partitionedinto the vapour phase during fluid unmixing, hence Xco2 decreasedas the fluid evolved. A suite of gabbroic blocks from Dark SlopeCrater contain pure CO fluid inclusions of just higher thancritical density. Assuming a trapping temperature of 10001200?Cthe pressure must have been > 18 kb28 kb or 7-11 kmin depth, i.e. well below the base of the Ascension Cone. Thepure CO2 inclusions in this suite of gabbros and the fluidsin the H2O-CO2NaCl-KCl system would seem to require entirelydifferent origins and it may be significant that these gabbrosshow a different isotopic Pb signature than do the volcanicsand other blocks. 相似文献
17.
The Lueders Formation (mid-Permian) in Baylor County, Texas, is an intercalated suite of fluvial siliciclastic, shallow marine siliciclastic, and shallow marine carbonate strata. There are at least two generations of carbonate cements (probably originally composed of calcite) in the fluvial sandstones where fractured grains are observed. These cements represent the initial stages of caliche formation. Cementation is envisioned as a two step process. In the first step, calcite cements form from supersaturated fluids in a freshwater, vadose environment as a meniscus cement at grain contacts. Areas of cement formation are restricted to these sites because fluid distribution is restricted to these sites. Stresses generated by the growth of cements at grain contacts are transmitted through and concentrated at quartz/quartz grain point-contacts until the stress is sufficient to fracture quartz grains, even though the ultimate strength of calcite is less than that of quartz, per unit area. This process occurs too rapidly to be accommodated by pressure solution. In the second phase of cementation, cement nucleation is no longer restricted by vadose conditions. In this phase, calcite growth can no longer result in quartz grain breakage; rather, the quartz grains are dispersed in poikilotopic calcite cement. 相似文献
18.
ROGERS NICK W.; EVANS PETER J.; BLAKE STEVE; SCOTT STUART C.; HAWKESWORTH CHRIS J. 《Journal of Petrology》2004,45(9):1747-1776
A suite of peralkaline trachytes from Longonot volcano, Kenya,which erupted during the last 6000 years, has been analysedfor major and trace elements, Pb and Nd isotopes, and UThRadisequilibria. The lavas are divided into three stratigraphicgroups of trachytes (Lt2a, Lt2b and Lt3), and hybrid lavas,designated LMx1 and LMx2, which, respectively, pre-date andpost-date the Lt2 lavas. Major and trace elements are consistent,with up to 37% within-group fractional crystallization of predominantlyalkali feldspar. The parental magma for the different trachytegroups had a more mafic compositionprobably hawaiitic.Nd and Pb isotopes show minimal variation, both within and betweenmagma groups, and indicate that up to 10% comendite magma fromthe neighbouring Olkaria volcanic field may have intermixedwith the Longonot magma. (230Th/238U) disequilibria indicatethat limited U/Th fractionation occurred during the past 10kyr, whereas (226Ra/230Th) disequilibria reflect the effectof alkali feldspar fractionation >8 kyr ago in the Lt2a lavas,between 3 and 7 kyr ago in the Lt2b lavas and in the past 3kyr for the Lt3 lavas. (226Ra/230Th) disequilibria in the Lt2blavas are interpreted using a model that combines the equationsof radioactive decay and in-growth with Rayleigh crystallizationto give fractionation rates of about 0·2 x 104/yearfor the evolution of hawaiite to trachyte, but more rapid ratesof up to 3 x 104/year for fractionation within the trachytesequence. (226Ra/230Th) from two whole-rockalkali feldsparpairs are interpreted to show the crystals formed at 5800 yearsBP (Lt2b) and 2800 years BP (Lt3), implying that phenocrystformation continued almost up to the time of eruption. The resultsstrongly indicate that fractionated magmas can be stored forperiods on the order of 10002500 years prior to eruption,whereas other magmas were erupted as fractionation was proceeding. KEY WORDS: trachyte; magma chambers; u-series; Kenya 相似文献
19.
Zircons from Syros, Cyclades, Greece--Recrystallization and Mobilization of Zircon During High-Pressure Metamorphism 总被引:27,自引:1,他引:27
TOMASCHEK FRANK; KENNEDY ALLEN K.; VILLA IGOR M.; LAGOS MARKUS; BALLHAUS CHRIS 《Journal of Petrology》2003,44(11):1977-2002
Zircons were studied from high-pressure/low-temperature metamorphosedmeta-igneous lithologies from Syros. These rocks carry severalzircon generations related to each other by dissolutionreprecipitationprocesses. One generation is pristine zircon that shows growthzoning, relatively elevated contents of trivalent cations andhigh Th/U ratios. The other end-member is a skeletal zircongeneration with negligible trivalent cation contents and lowTh/U ratios ( 相似文献
20.