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791.
Jonghan Ko Lajpat R. Ahuja S. A. Saseendran Timothy R. Green Liwang Ma David C. Nielsen Charles L. Walthall 《Climatic change》2012,111(2):445-472
Agricultural systems models are essential tools to assess potential climate change (CC) impacts on crop production and help
guide policy decisions. In this study, impacts of projected CC on dryland crop rotations of wheat-fallow (WF), wheat-corn-fallow
(WCF), and wheat-corn-millet (WCM) in the U.S. Central Great Plains (Akron, Colorado) were simulated using the CERES V4.0
crop modules in RZWQM2. The CC scenarios for CO2, temperature and precipitation were based on a synthesis of Intergovernmental Panel on Climate Change (IPCC 2007) projections for Colorado. The CC for years 2025, 2050, 2075, and 2100 (CC projection years) were super-imposed on measured
baseline climate data for 15–17 years collected during the long-term WF and WCF (1992–2008), and WCM (1994–2008) experiments
at the location to provide inter-annual variability. For all the CC projection years, a decline in simulated wheat yield and
an increase in actual transpiration were observed, but compared to the baseline these changes were not significant (p > 0.05) in all cases but one. However, corn and proso millet yields in all rotations and projection years declined significantly
(p < 0.05), which resulted in decreased transpiration. Overall, the projected negative effects of rising temperatures on crop
production dominated over any positive impacts of atmospheric CO2 increases in these dryland cropping systems. Simulated adaptation via changes in planting dates did not mitigate the yield
losses of the crops significantly. However, the no-tillage maintained higher wheat yields than the conventional tillage in
the WF rotation to year 2075. Possible effects of historical CO2 increases during the past century (from 300 to 380 ppm) on crop yields were also simulated using 96 years of measured climate
data (1912–2008) at the location. On average the CO2 increase enhanced wheat yields by about 30%, and millet yields by about 17%, with no significant changes in corn yields. 相似文献
792.
Recent studies show that ferrous iron (FeII), which is often abundant in anaerobic soil and groundwater, is capable of abiotically reducing many subsurface contaminants. However, studies also demonstrate that FeII redox reactivity in geochemical systems is heavily dependent upon metal speciation. This contribution examines the influence of hydroxamate ligands, including the trihydroxamate siderophore desferrioxamine B (DFOB), on FeII reactions with nitroaromatic groundwater contaminants (NACs). Experimental results demonstrate that ring-substituted NACs are reduced to the corresponding aniline products in aqueous solutions containing FeII complexes with DFOB and two monohydroxamate ligands (acetohydroxamic acid and salicylhydroxamic acid). Reaction rates are heavily dependent upon solution conditions and the identities of both the FeII-complexing hydroxamate ligand and the target NAC. Trends in the observed pseudo-first-order rate constants for reduction of 4-chloronitrobenzene (kobs, s−1) are quantitatively linked to the formation of FeII species with standard one-electron reduction potentials, (FeIII/FeII), below −0.3 V. Linear free energy relationships correlate reaction rates with the (FeIII/FeII) values of different electron-donating FeII complexes and with the apparent one-electron reduction potentials of different electron-accepting NACs, (ArNO2). Experiments describing a redox auto-decomposition mechanism for FeII-DFOB complexes that occurs at neutral pH and has implications for the stability of hydroxamate siderophores in anaerobic environments are also presented. Results from this study indicate that hydroxamates and other FeIII-stabilizing organic ligands can form highly redox-active FeII complexes that may contribute to the natural attenuation and remediation of subsurface contaminants. 相似文献
793.
Camomilia A. Bright Timothy W. Lyons Michael D. Glascock 《Geochimica et cosmochimica acta》2009,73(6):1609-190
Past workers have used rare-earth element patterns recorded in biogenic apatite as proxies for original seawater chemistry. To explore the potency of this approach, we analyzed Pennsylvanian conodonts from limestones, gray shales, and black shales of the Fort Scott and Pawnee formations (Desmoinesian) and Swope and Dennis formations (Missourian) in Kansas, Missouri, and Iowa, U.S.A. Analysis of individual platform conodonts from seven taxa using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) revealed a consistent enrichment in the middle rare-earth elements (MREE). Analogous MREE enrichment has been observed in authigenic apatite and bulk samples of phosphate-rich black shales from the same formations. Importantly, however, phosphate-depleted shales intimately associated with the P-rich intervals are relatively depleted in MREE. These antithetic patterns argue convincingly for secondary migration from the bulk sediment into the phosphate, and the extent of MREE enrichment in the conodonts is correlated positively with the total REE content. MREE enrichment in conodonts does not vary systematically as a function of lithology, stratigraphic level, conodont genus, geographic location, or with independent estimates of paleoredox conditions in the bottom waters. Collectively, these results argue for postmortem (diagenetic) REE uptake resulting in a pronounced (and progressive) MREE enrichment. Any cerium anomalies, if initially present, were masked by diagenetic uptake of REE. Paleoenvironmental interpretations of conodont REE, particularly for samples exhibiting MREE enrichment, should therefore be viewed with caution. 相似文献
794.
795.
This study provides an experimental calibration of the equilibrium constant for AuPdFe alloys with Fe-bearing silicate melts.
The ideal metal capsules for H2O-bearing experiments are pure Au, because of its slow hydrogen diffusivity. However, above the melting point of Au, other
materials must be used. The solution to this problem is to use AuPd alloy capsules. However, under most relevant fO2 conditions, this alloy absorbs Fe from the coexisting silicate melt, thus changing the bulk composition of the experimental
charge. This study combines previous work on the Au–Pd, Pd–Fe, and Au–Fe binary systems to develop a ternary thermodynamic
solution model for AuPdFe. This solution model is used with experiments to calculate an equilibrium reaction coefficient for
the FeOmelt → Fealloy + 1/2O2 exchange reaction. Using a non-ideal ternary solution model, the fO2 conditions of hydrous, piston cylinder experiments can be estimated by analyzing the sample capsule alloy and the coexisting
liquid composition. 相似文献
796.
David R. Griffith William R. Martin Timothy I. Eglinton 《Geochimica et cosmochimica acta》2010,74(23):6788-6800
Long-term carbon cycling and climate change are strongly dependent on organic carbon (OC) burial in marine sediments. Radiocarbon (14C) has been widely used to constrain the sources, sinks, and processing of sedimentary OC. To elucidate the dominant controls on the radiocarbon content of total organic carbon (14CTOC) accumulating in surface sediments we construct a box model that predicts 14CTOC in the sediment mixed layer (measured as fraction modern, Fm). Our model defines three distinct OC pools (“degradable,” “semi-labile,” and “refractory”) and assumes that 14CTOC flux to sediments is exclusively derived from surface ocean primary productivity, and hence follows a “generic” surface ocean dissolved inorganic carbon (DIC) bomb curve. Model predictions are compared to a set of 75 surface sediment samples, which span a wide geographic range and reflect diverse water column and depositional conditions, and for which sedimentation rate and mixed-layer depth are well characterized. Our model overestimates the Fm value for a majority (65%) of these sites, especially at shallow water depths and for sites characterized by depleted δ13CTOC values. The model is most sensitive to sedimentation rate and mixed-layer depth. Therefore, slight changes to these parameters can lead to a match between modeled and measured Fm values at many sites. Yet, in some cases, we find that measured Fm values cannot be simulated without large and unrealistic changes to sedimentation rate and mixed-layer depth. These results point to sources of pre-aged OC to surface sediments and implicate soil-derived terrestrial OC, reworked marine OC, and/or anthropogenic carbon as important components of the organic matter present in surface sediments. This approach provides a valuable framework within which to explore controls on sedimentary organic matter composition and carbon burial over a range of spatial and temporal scales. 相似文献
797.
Timothy Garn David A. Green Julia M. Riley Paul Alexander 《Monthly notices of the Royal Astronomical Society》2008,387(3):1037-1044
We present observations of the Lockman Hole taken at 610 MHz with the Giant Metrewave Radio Telescope (GMRT). Twelve pointings were observed, covering a total area of ∼5 deg2 with a resolution of 6 × 5 arcsec2 , position angle +45° . The majority of the pointings have a rms noise of ∼60 μJy beam−1 before correction for the attenuation of the GMRT primary beam. Techniques used for data reduction and production of a mosaicked image of the region are described, and the final mosaic is presented along with a catalogue of 2845 sources detected above 6σ. Radio source counts are calculated at 610 MHz and combined with existing 1.4-GHz source counts, in order to show that pure luminosity evolution of the local radio luminosity functions for active galactic nuclei and starburst galaxies is sufficient to account for the two source counts simultaneously. 相似文献
798.
Vivienne Wild Guinevere Kauffmann Simon White Donald York Matthew Lehnert Timothy Heckman Patrick B. Hall Pushpa Khare Britt Lundgren Donald P. Schneider Daniel Vanden Berk 《Monthly notices of the Royal Astronomical Society》2008,388(1):227-241
Using data from the Sloan Digital Sky Survey data release 3 (SDSS DR3), we investigate how narrow (<700 km s−1 ) C iv and Mg ii quasar absorption-line systems are distributed around quasars. The C iv absorbers lie in the redshift range 1.6 < z < 4 and the Mg ii absorbers in the range 0.4 < z < 2.2. By correlating absorbers with quasars on different but neighbouring lines of sight, we measure the clustering of absorbers around quasars on comoving scales between 4 and 30 Mpc. The observed comoving correlation lengths are r o ∼ 5 h −1 Mpc , similar to those observed for bright galaxies at these redshifts. Comparing correlations between absorbers and the quasars, in whose spectra they are identified, then implies: (i) that quasars destroy absorbers to comoving distances of ∼300 kpc (C iv ) and ∼800 kpc (Mg ii ) along their lines of sight; (ii) that ≳40 per cent of C iv absorbers within 3000 km s−1 of the quasi-stellar object are not a result of large-scale clustering but rather are directly associated with the quasar itself; (iii) that this intrinsic absorber population extends to outflow velocities of the order of 12 000 km s−1 ; (iv) that this outflow component is present in both radio-loud and radio-quiet quasars and (v) that a small high-velocity outflow component is also observed in the Mg ii population. We also find an indication that absorption systems within 3000 km s−1 are more abundant for radio-loud quasars than for radio-quiet quasars. This suggests either that radio-loud objects live in more massive haloes, or that their radio activity generates an additional low-velocity outflow, or both. 相似文献
799.
T. Sako T. Sekiguchi M. Sasaki K. Okajima F. Abe I. A. Bond J. B. Hearnshaw Y. Itow K. Kamiya P. M. Kilmartin K. Masuda Y. Matsubara Y. Muraki N. J. Rattenbury D. J. Sullivan T. Sumi P. Tristram T. Yanagisawa P. C. M. Yock 《Experimental Astronomy》2008,22(1-2):51-66
We have developed a wide-field mosaic CCD camera, MOA-cam3, mounted at the prime focus of the Microlensing Observations in Astrophysics (MOA) 1.8-m telescope. The camera consists of ten E2V CCD4482 chips, each having 2k×4k pixels, and covers a 2.2 deg2 field of view with a single exposure. The optical system is well optimized to realize uniform image quality over this wide field. The chips are constantly cooled by a cryocooler at ??80° C, at which temperature dark current noise is negligible for a typical 1–3 min exposure. The CCD output charge is converted to a 16-bit digital signal by the GenIII system (Astronomical Research Cameras Inc.) and readout is within 25 s. Readout noise of 2–3 ADU (rms) is also negligible. We prepared a wide-band red filter for an effective microlensing survey and also Bessell V, I filters for standard astronomical studies. Microlensing studies have entered into a new era, which requires more statistics, and more rapid alerts to catch exotic light curves. Our new system is a powerful tool to realize both these requirements. 相似文献
800.
Rena L. FORD Gretchen K. BENEDIX Timothy J. McCOY Tracy RUSHMER 《Meteoritics & planetary science》2008,43(8):1399-1414
Abstract— Partial melting experiments at temperatures of 950–1300 °C were conducted on the H6 chondrite Kernouvé under reducing conditions using CO‐CO2 gas mixing and graphite‐buffered sealed silica tubes to examine the effect of reducing conditions during melting of starting materials that are more oxidized relative to the oxygen fugacity conditions of the experiments. The experiments produced a range of mineralogical and compositional changes. Olivine exhibits significant reduction to compositions of Fa2–5 at temperatures of 1300 °C. In contrast, orthopyroxene exhibits only slight reduction until the highest temperatures. Chromite is sometimes consumed by intruding sulfides, and displays increasingly magnesian compositions ranging as low as Fe/Fe + Mg of 0.1 at a constant Cr/Cr + Al ratio. The compositional changes with increasing temperature reflect a complex set of reactions, including oxidation‐reduction. One application of these experiments address whether primitive achondrites could have formed from ordinary chondrite‐like precursors by partial melting under reducing conditions. While changes observed in olivine and troilite compositions might support such an idea, differences in oxygen isotopic composition, Cr/Cr + Al in chromite, orthopyroxene compositions, and thermodynamic evidence against reduction during melting of primitive achondrites (Benedix et al. 2005) firmly refute such an idea. 相似文献