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11.
Implicit in all spatial mismatch models is the concept of accessibility. However, there is an immense variety of measures utilized in the literature to index the concept, and it is not clear how much of the disparity in findings between studies is attributable to alternative measures. This research note attempts to provide some evidence on the impact of alternative measures of accessibility in employment probability models. 相似文献
12.
Nancy L. Chabot 《Geochimica et cosmochimica acta》2004,68(17):3607-3618
Magmatic
iron meteorites are thought to be samples of the central metallic cores
of asteroid-sized parent bodies. Sulfur is believed to have been an
important constituent of these parental cores, but due to the low
solubility of S in solid metal, initial S-contents for the magmatic
groups cannot be determined through direct measurements of the iron
meteorites. However, experimental solid metal-liquid metal partition
coefficients show a strong dependence on the S-content of the metallic
liquid. Thus, by using the experimental partition coefficients to model
the fractional crystallization trends within magmatic iron meteorite
groups, the S-contents of the parental cores can be indirectly
estimated. Modeling the Au, Ga, Ge, and Ir fractionations in four of
the largest magmatic iron meteorite groups leads to best estimates for
the S-contents of the parental cores of 12 ± 1.5 wt% for
the IIIAB group, 17 ± 1.5 wt% for the IIAB group, and 1
± 1 wt% for the IVB group. The IVA elemental
fractionations are not adequately fit by a simple fractional
crystallization model with a unique initial S-content. These S-content
estimates are much higher than those recently inferred from
crystallization models involving trapped melt. The discrepancy is due
largely to the different partition coefficients that are used by the
two models. When only partition coefficients that are consistent with
the experimental data are used, the trapped melt model, and the low
S-contents it advocates, cannot match the Ge and Ir fractionations that
are observed in IIIAB iron
meteorites. 相似文献
13.
Derek G. Turner Brent C. Ward Duane G. Froese Michel Lamothe Jeffrey D. Bond Nancy H. Bigelow 《Boreas: An International Journal of Quaternary Research》2016,45(3):521-536
At least five Middle to Late Pleistocene advances of the northern Cordilleran Ice Sheet are preserved at Silver Creek, on the northeastern edge of the St Elias Mountains in southwest Yukon, Canada. Silver Creek is located 100 km up‐ice of the Marine Isotope Stage (MIS) 2 McConnell glacial limit of the St Elias lobe. This site contains ~3 km of nearly continuous lateral exposure of glacial and non‐glacial sediments, including multiple tills separated by thick gravel, loess and tilted lake beds. Infrared‐stimulated luminescence (IRSL) and AMS radiocarbon dating constrain the glacial deposits to MIS 2, 4, either MIS 6 or mid‐MIS 7, and two older Middle Pleistocene advances. This chronology and the tilt of the lake beds suggest Pleistocene uplift rates of up to 1.9 mm a?1 along the Denali Fault since MIS 7. The non‐glacial sediment consists of sand, gravel, loess and organic beds from MIS 7, MIS 3 and the early Holocene. The MIS 3 deposits date to between 30–36 14C ka BP, making Silver Creek one of the few well‐constrained MIS 3‐aged sites in Yukon. This confirms that ice receded close to modern limits in MIS 3. Pollen and macrofossil analyses show that a meadow‐tundra to steppe‐tundra mosaic with abundant herbs and forbs and few shrubs or trees, dominated the environment at this time. The stratigraphy at Silver Creek provides a palaeoclimatic record since at least MIS 8 and comprises the oldest direct record of Pleistocene glaciation in southwest Yukon. 相似文献
14.
Nancy England April L. James Krystopher J. Chutko Richard S. Pyrce Huaxia Yao 《水文研究》2019,33(6):905-919
Stable water isotope surveys have increasingly been integrated into river basins studies, but fewer have used them to evaluate impact of hydropower regulation. This study applies hydrologic and water isotope survey approaches to a Canadian Shield river basin with both regulated and natural flows. Historical streamflow records were used to evaluate the influence of three hydroelectric reservoirs and unregulated portions of the basin on downstream flows and changes in water level management implemented after an extreme flood year (1979). In 2013, water isotope surveys of surface and source waters (e.g., rainfall, groundwater, snowmelt) were conducted to examine spatial and temporal variation in contributions to river flow. Seasonal changes in relative groundwater contribution were assessed using a water‐isotope mass balance approach. Within the basin, two regulated reservoirs exhibited inverted hydrographs with augmented winter flows, whereas a third exhibited a hydrograph dominated by spring snowmelt. In 2013, spatial variation in rain‐on‐snow and air temperatures resulted in a critical lag in snowmelt initiation in the southern and northern portions of the basin resulting in a dispersed, double peak spring hydrograph, contrasting with 1979 when a combination of rain‐on‐snow and coincident snowmelt led to the highest flood on record. Although eastern basin reservoirs become seasonally enriched in δ18O and δ2H values, unregulated western basin flows remain less variable due to groundwater driven baseflow with increasing influence downstream. Combined analysis of historical streamflow (e.g., flood of 1979, drought of 2010) and the 2013 water isotope surveys illustrate extreme meteorological conditions that current management activities are unable to prevent. In this study, the influence of evaporative fractionation on large surface water reservoirs provides important evidence of streamflow partitioning, illustrating the value of stable water isotope tracers for study of larger catchments. 相似文献
15.
16.
Sibel Bargu Melissa M. Baustian Nancy N. Rabalais Ross Del Rio Benjamin Von Korff R. Eugene Turner 《Estuaries and Coasts》2016,39(5):1345-1356
The presence of domoic acid (DA) toxin from multiple species of Pseudo-nitzschia is a concern in the highly productive food webs of the northern Gulf of Mexico. We documented the Pseudo-nitzschia presence, abundance, blooms, and toxicity over three years along a transect ~100 km west of the Mississippi River Delta on the continental shelf. Pseudo-nitzschia were present throughout the year and occurred in high abundances (>104 cells l?1) in the early spring months during high Mississippi River (MSR) flow (~20,000 m3 s?1) but were most abundant (>106 cells l?1) when MSR discharge was relatively lower among the spring months. A high particulate toxin production (maximum reaching 13 μg DA l?1) was associated with the high cell abundances and exceeded, by an order of magnitude, prior reports of particulate DA concentrations in Louisiana coastal waters. Differences in Pseudo-nitzschia peak times and its toxicity were correlated mainly with the timing and magnitude of MSR discharge and changes in associated parameters such as nutrient stoichiometry and salinity. A negative relationship between high MSR discharge and Pseudo-nitzschia and particulate DA concentrations was documented. These riverine dynamics have the potential to influence DA contamination in pelagic and benthic food webs in the coastal waters of the northern Gulf of Mexico. 相似文献
17.
It is often argued that the growth of major emerging powers, such as China, India, Russia, and Brazil, will have a transformative effect on the world economy and politics, but the implications of their rise on sustainable development diplomacy have remained understudied. Do these new powers have the potential to exercise leadership and address the stalled performance on the sustainable development agenda? Drawing on theories of leadership in multiparty negotiations and the empirical study of two major emerging powers’ coalitions, BRICS (Brazil, Russia, India, China, South Africa) and BASIC (Brazil, South Africa, India, China), this article assesses their leadership potential in sustainable development diplomacy. It finds that emerging powers are already engaging in policy coordination in the sustainable development realm, have begun to use their influence to create incentives for other actors in the system and have been conducting skillful diplomacy to develop their coalitions and forge consensus in global policy-making. However, exerting actual global leadership would require building a stronger policy agenda. One area with particular potential for emerging powers to exert leadership is the clean energy arena of sustainable development diplomacy. 相似文献
18.
Jaidong Ko Nancy E. Brown Alexandra Navrotsky Charles T. Prewitt Tibor Gasparik 《Physics and Chemistry of Minerals》1989,16(8):727-733
The phase boundary between MnTiO3 I (ilmenite structure) and MnTiO3 II (lithium niobate structure) has been determined by analysis of quench products from reversal experiments in a cubic anvil
apparatus at 1073–1673 K and 43–75 kbar using mixtures of MnTiO3 I and II as starting materials. Tight brackets of the boundary give P(kbar)=121.2−0.045 T(K). Thermodynamic analysis of this boundary gives ΔHo=5300±1000 J·mol−1, ΔSo = 1.98 ±1J·K−1· mol−1. The enthalpy of transformation obtained directly by transposed-temperature-drop calorimetry is 8359 ±2575 J·mol−1. Possible topologies of the phase relations among the ilmenite, lithium niobate, and perovskite polymorphs are constrained
using the above data and the observed (reversible with hysteresis) transformation of II to III at 298 K and 20–30 kbar (Ross
et al. 1989). The observed II–III transition is likely to lie on a metastable extension of the II–III boundary into the ilmenite
field. However the reversed I–II boundary, with its negative dP/ dT does represent stable equilibrium between ilmenite and lithium niobate, as opposed to the lithium niobate being a quench
product of perovskite. We suggest a topology in which the perovskite occurs stably at low T and high P with a triple point (I, II, III) at or below 1073 K near 70 kbar. The I–II boundary would have a negative P-T slope while the II–III and I–III boundaries would be positive, implying that entropy decreases in the order lithium niobate,
ilmenite, perovskite. The inferred positive slope of the ilmenite-perovskite transition in MnTiO3 is different from the negative slopes in silicates and germanates. These thermochemical parameters are discussed in terms
of crystal structure and lattice vibrations. 相似文献
19.
Richard B. McCammon Warren I. Finch John O. Kork Nancy J. Bridges 《Natural Resources Research》1994,3(2):109-122
An integrated data-directed numerical method has been developed to estimate the undiscovered mineral endowment within a given area. The method has been used to estimate the undiscovered uranium endowment in the San Juan Basin, New Mexico, U.S.A. The favorability of uranium concentration was evaluated in each of 2,068 cells defined within the Basin. Favorability was based on the correlated similarity of the geologic characteristics of each cell to the geologic characteristics of five area-related deposit models. Estimates of the undiscovered endowment for each cell were categorized according to deposit type, depth, and cutoff grade. The method can be applied to any mineral or energy commodity provided that the data collected reflect discovered endowment. 相似文献
20.
Steven L. Huntley Richard J. Wenning Steave H. Su Nancy L. Bonnevie Dennis J. Paustenbach 《Estuaries and Coasts》1995,18(2):351-361
In an attempt to characterize localized rates of sediment accretion, 10 sediment cores were collected from the lower reach of the Passaic River, a major tributary of Newark Bay, New Jersey. Sediments were assayed for 210Pb activity at predetermined depths and the rate of sediment accretion (cm yr?1) was estimated from the least squares regression of the log of unsupported activity versus depth. Sediment accretion rates, derived from 210Pb measurements (RPb) were used to predict the depth interval within the core containing sediments deposited around 1954; subsequent 137Cs analyses were focused on this depth interval. Sediment accretion rates derived from 137Cs measurements (RCs) were extrapolated from the depth of the 1954 horizon. Lead-210 derived sediment accretion rates in cores collected from a sediment bench extending along the inside bend on the southern shore of a meander in the river, ranged from 4.1 cm yr?1 to 10.2 cm yr?1 and averaged 6.8 cm yr?1. The RCs estimates for cores from this area ranged from 3.8 cm yr?1 to 8.9 cm yr?1 and averaged 6.6 cm yr?1. The RCs for cores collected in a more hydrologically dynamic reach of the river upstream of the sediment bench, were only 0.41 cm yr?1 and 0.66 cm yr?1. The results of this investigation indicate that this reach of the lower Passaic River is an area of high sediment accumulation, retaining much of the sediment load deposited from upstream and downstream sources. The rates of sediment accretion in the lower Passaic River are among the highest reported anywhere in the Newark Bay estuary. 相似文献