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711.
21st century climate change in the Middle East 总被引:1,自引:0,他引:1
Jason P. Evans 《Climatic change》2009,92(3-4):417-432
This study examined the performance and future predictions for the Middle East produced by 18 global climate models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report. Under the Special Report on Emission Scenarios A2 emissions scenario the models predict an overall temperature increase of ~1.4 K by mid-century, increasing to almost 4 K by late-century for the Middle East. In terms of precipitation the southernmost portion of the domain experiences a small increase in precipitation due to the Northward movement of the Inter-Tropical Convergence Zone. The largest change however is a decrease in precipitation that occurs in an area covering the Eastern Mediterranean, Turkey, Syria, Northern Iraq, Northeastern Iran and the Caucasus caused by a decrease in storm track activity over the Eastern Mediterranean. Other changes likely to impact the region include a decrease of over 170,000 km2 in viable rainfed agriculture land by late-century, increases in the length of the dry season that reduces the length of time that the rangelands can be grazed, and changes in the timing of the maximum precipitation in Northern Iran that will impact the growing season, forcing changes in cropping strategy or even crop types. 相似文献
712.
Faruk Aydin Richard M. Thompson Orhan Karsli Hinako Uchida Jason B. Burt Robert T. Downs 《Contributions to Mineralogy and Petrology》2009,158(1):131-147
Chemical and structural data are reported for C2/c pyroxene phenocrysts collected from three potassic series (Group A: basanite-tephrite, Group B: tephrite-phonolitic tephrite,
Group C: alkaline basalt-trachybasalt) of the Neogene alkaline volcanics (NAVs) in northeastern Turkey, in order to investigate
the evolution of the magmatic plumbing system and the location of magma chamber(s) with crystallization conditions. The rock
series hosting the clinopyroxene phenocrysts show generally porphyritic texture and have a variable phenocryst-rich nature
(20–58%), with phenocryst assemblages characterized by cpx ± ol ± plag ± foid ± amp ± bio. The clinopyroxene phenocrysts can
be chemically classified as Ti- and Fe3+-rich Al-diopsides for Groups A and B (AB-cpxs) and Ti- and Fe3+-poor Al-diopsides for Group C (C-cpxs). They have poorly variable composition, clustering in the diopside field. Structurally,
the diopside groups have nearly similar a (ranging from 9.73 to 9.75 ?), V
cell (437.2–440.9 ?3), and 〈beta〉 angle values (106.01°–106.23°), but some differences in polyhedral parameters and geometries of the AB-cpxs and C-cpxs have
been observed. For example, the AB-cpxs are characterized by larger c (5.27–5.30 vs. 5.25–5.28 ?), V
T (2.27–2.30 vs. 2.23–2.28 ?3), and V
M2 (25.53–25.72 vs. 25.41–25.59 ?3) values and smaller b (8.87–8.88 vs. 8.88–8.91 ?) and V
M1 (11.49–11.63 vs. 11.64–11.83 ?3) values with respect to the C-cpxs. In addition, the AB-cpxs show higher values of V
M2/V
M1 (2.20–2.23) due to large V
M2 and small V
M1 compared to the V
M2/V
M1 ratios of the C-cpxs (<2.19). Such differences in the crystal structure of the AB-cpxs and C-cpxs from the NAVs are partly
related to different crystallization pressures, but mostly related to variation in melt composition and, possibly, the influence
of other crystallizing mineral phases. In particular, R(M2-O1) and R(M1-O2) (i.e. bond lengths) differences in the clinopyroxenes
of different groups support the presence of evolved host rocks with different alkaline character (i.e. silica-undersaturated
Groups A–B and silica-saturated Group C). Based on the cpx-geothermobarometry, the crystallization pressures for the C-cpxs
are lower than 4.5 kbars, but the AB-cpxs have relatively high-pressure values (5.6–10.6 kbars), suggesting that the AB-cpxs
crystallized in higher pressure environments. The relatively higher crystallization temperatures of the AB-cpxs also indicate
higher cooling rates. The P–T estimates suggest that the source regions of the clinopyroxene phenocrysts from the NAVs were crustal magma chambers in a
closed plumbing system at a moderate- to low-pressure regime. 相似文献
713.
714.
At three sample sites where there are good exposures of the upper 15 m of the 1912 ash-flow sheet in the Valley of Ten Thousand Smokes (VTTS), Alaska, 18O/16O studies indicate that fumarolic activity produced a very wide range of δ18O values (?0.1 to +12.6; n=32) in the groundmass adjacent to fossil fissure fumaroles. This contrasts sharply with the uniformity of δ18O in the groundmass away from fumarolic conduits (δ18O=+5.9 to +7.1; n=7) and in all of the feldspar phenocrysts (δ18O=+6.11 to +7.5 1 for 11 samples from this study and Hildreth 1987), independent of whether these were collected from fossil fumaroles or from unaltered tuff. Only one sample contained feldspars that were even slightly 18O-enriched relative to the others (cloudy plagioclase δ18O=+8.45). and this sample also contained the most 180-enriched groundmass of any of those analyzed (δ18O=+12.6). This preservation of primary magmatic δ18O values in the VTTS feldspar phenocrysts is clearly a consequence of the extremely short time span (i.e., 1912 to ≈1923) of vigorous, high-temperature, fumarolic activity in the 1912 ash-flow sheet. These 18O/l6O systematica are strikingly similar to those discovered in the 2.8-Ma intracaldera Chegem Tuff (Gazis et al. 1996) and in the fossil fumaroles in the outflow sheet of the 0.76 Ma Bishop Tuff (Holt and Taylor 1998), thus confirming that a similar type of fumarolic meteoric-hydro-thermal activity occurred above the zone of intense welding in all three of these ash-flow tuffs. This is particularly important, because it provides a direct linkage between the older tuffs and the actual observations at the VTTS of steam chemistry, water/rock interaction, circulation geometry, flow velocities, and fumarolic temperatures (up to 645°C). The 18O/l6O effects in the VTTS can all be explained in terms of a two-stage history: (a) an early, 10- to 15-year-long, high-temperature (τ;450°C), fumarolic 18O-depletion event (groundmass δ18O=?0.1 to +4.8); and (b) a subsequent, much longer-lived, low-temperature (<150°C), 18O-enrichment episode (groundmass as high as δ18O=+12.6). Steam in these low-temperature fumaroles probably passed through various parts of the same hydrothermal system associated with the earlier, higher-temperature, fumarolic activity, and a weakened form of this low-temperature hydrothermal circulation continues to the present day (Keith et al. 1992; Lowell and Keith 1991). This low-temperature 18O/16O exchange probably occurred in combination with mineralogical alteration of both the groundmass and the calcium-rich portions of feldspar phenocrysts during the waning (<150°C) stages of fumarolic activity (Spilde et al. 1993). The slight 18O enrichment of apparently pristine, transparent feldspar phenocrysts (δ18O=+7.51) in one of the 18O-depleted, meteoric-hydrothermally altered fumarolic samples (whole-rock δ18O=+4.8) probably indicates that this sample was incipiently altered at low temperatures as fumarolic activity waned, and thus may have had a whole-rock δ18O value much lower than +4.87‰ prior to 1923. 相似文献
715.
716.
A computer correlation technique was used to deduce the spreading history of the Mid-Atlantic Ridge from 5 magnetic profiles between 28°S and 43°S. In general, several possible histories are indicated for each profile involving changes of spreading rate and faulting, some of which are easily overlooked by the visual method. The only spreading history that was consistent will all the profiles required spreading at approximately 2.2 cm yr-1 from 11 m.y.b.p. to approximately 5.5 m.y.b.p., followed by a decrease in rate to 1.7 cm yr-1 relative to the Vine (1966) magnetic reversal model based on the South Pacific. Comparison of the data with other reported spreading rate discontinuities suggests that the South Pacific may be reponsible for the reported spreading rate changes. 相似文献
717.
Translating ecosystem indicators into decision criteria 总被引:3,自引:1,他引:3
718.
Solute-based geochemical mass balance methods are commonly used in small-watershed studies to estimate rates of a variety of geochemical processes at the Earth’s surface, including primary-mineral weathering and soil formation, and the quantitative contribution of these elemental transfer processes to cation budgets, nutrient cycling, and landscape susceptibility to acid deposition. Weathering rates of individual minerals in watershed mass-balance studies are determined by solving a system of simultaneous linear geochemical mass-balance equations with constant (stoichiometric) coefficients. These equations relate the measured net fluxes to the (known) stoichiometries and (unknown) rates of weathering reactions for multiple minerals in the weathering profiles. Solving the system of equations requires petrologic, mineralogic, hydrologic, botanical, and aqueous geochemical data. The number of mineral-weathering rates that can be determined is limited by the number of elements for which solute mass-balance equations can be written. In addition to calculating mineral weathering rates, elemental transfer into or out of the biomass may also be calculated. Elemental uptake by aggrading forest vegetation can act as an intrawatershed sink for at least some mineral-derived cations, producing mineral weathering rates higher than would be estimated from solute fluxes alone; similarly, element release from decaying forest biomass can result in higher solute fluxes than are produced by weathering alone. The mathematics of, significant contributions from, role of biomass in, and recent advances in, watershed geochemical mass-balance methods are discussed using examples from the Appalachian headwaters watersheds of the Coweeta Hydrologic Laboratory in the southern Blue Ridge Physiographic Province of North Carolina, USA. 相似文献
719.
Cave sediments collected from Reflection Cave on the Vaca Plateau, Belize show variations in the δ13C values of their fulvic acids (FAs), which indicate periods of vegetation change caused by climatic and Maya influences during the late Holocene. The δ13C values range from − 27.11‰ to − 21.52‰, a shift of ∼ 5.59‰, which suggests fluctuating contributions of C3 and C4 plants throughout the last 2.5 ka, with C4 plant input reflecting periods of Maya agriculture. Maya activity in the study area occurred at different intensities from ∼ 2600 cal yr BP until ∼ 1500 cal yr BP, after which agricultural practices waned as the Maya depopulated the area. These changes in plant assemblages were in response to changes in available water resources, with increased aridity leading to the eventual abandonment of agricultural areas. The Ix Chel archaeological site, located in the study area, is a highland site that would have been among the first agricultural settlements to be affected during periods of aridity. During these periods, minimal water resources would have been available in this highly karstified, well-drained area, and supplemental groundwater extraction would have been difficult due to the extreme depth of the water table. 相似文献
720.
Countries can use both mitigation and adaptation strategies to protect their citizens from catastrophic risk posed by climate
change (e.g., shift in the jet stream). A nation can mitigate by reducing CO2 emissions, which reduces the probability of a catastrophic event; it can adapt by altering the infrastructure so that damages
can be reduced in the event a catastrophe is realized. Herein we add to the current literature by extending the endogenous
risk framework into a dynamic framework permitting analysis of both mitigation and adaptation while allowing for the dynamic
process of global climate change. Our results suggest adaptation to catastrophe is a small fraction of the national climate
protection budget relative to mitigation when nations cooperate fully, when damages are both continuous and catastrophic,
and when nations have a short planning horizon. Adaptation becomes more important relative to mitigation when nations are
unlikely to cooperate, when damages are mainly catastrophic, or when the nation’s planning horizon increases. 相似文献