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131.
Collen JD Baker JA Dunbar RB Rieser U Gardner JP Garton DW Christiansen KJ 《Marine pollution bulletin》2011,62(2):251-257
Anthropogenic lead (Pb) inputs to the atmosphere increased greatly over the past century and now dominate Pb supply to the oceans. However, the Pb content of sediments across the equatorial Pacific region is relatively unknown, and data exist only for deep sea sites where Pb deposition lags surface water inputs by up to a century. Here we present ICP-MS analyses of Pb of a core from a lagoon at Palmyra Atoll, northern Line Islands, that spans approximately the past 160 years. The non-bioturbated sediments of the euxinic lagoon, coupled with rapid rates of deposition, provide a unique fine-scale record of atmospheric Pb supply at a remote Pacific location. These first observations of historic Pb sedimentation in an atoll lagoon reveal a 63-fold increase in Pb flux to sediments during the past century and correlate directly with the North American consumption of leaded gasoline that began in 1926. 相似文献
132.
Connectivity clues from short-term variability in settlement and geochemical tags of mytilid mussels
F. Joel Fodrie Bonnie J. Becker Lisa A. Levin Kristen Gruenthal Pat A. McMillan 《Journal of Sea Research》2011,65(1):141-150
The use of geochemical tags in calcified structures of fish and invertebrates is an exciting tool for investigating larval population connectivity. Tag evaluation over relatively short intervals (weeks) may detect environmental and ecological variability at a temporal scale highly relevant to larval transport and settlement. We collected newly settled mussels (Mytilus californianus and M. galloprovincialis) weekly during winter/spring of 2002 along the coast of San Diego, CA, USA, at sites on the exposed coast (SIO) and in a protected coastal bay (HI), to investigate temporal patterns of geochemical tags in mussel shells. Analyses of post-settlement shell via LA-ICP-MS revealed statistically significant temporal variability for all elements we examined (Mg, Mn, Cu, Sr, Cd, Ba, Pb and U). Despite this, our ability to distinguish multielemental signatures between sites was largely conserved. Throughout our 13-week study, SIO and HI mussels could be chemically distinguished from one another in 78–87% of all cases. Settlement varied between 2 and 27 settlers gram-byssus?1 week?1 at SIO and HI, and both sites were characterized by 2–3 weeks with “high” settlement. Geochemical tags recorded in early larval shell of newly settled mussels differed between “high” and “low” settlement weeks at both sites (MANOVA), driven by Mg and Sr at SIO (p = 0.013) and Sr, Cd, Ba and Pb at HI (p < 0.001). These data imply that shifts in larval sources or transport corridors were responsible for observed settlement variation, rather than increased larval production. In particular, increased settlement at HI was observed concurrent with the appearance of geochemical tags (e.g., elevated Cd), suggesting that those larvae were retained in upwelled water near the mouth of the bay. Such shifts may reflect short-term changes in connectivity among sites due to altered transport corridors, and influence the demography of local populations. 相似文献
133.
We have examined the effects of oblateness up to J 4 of the less massive primary and gravitational potential from a circum-binary belt on the linear stability of triangular equilibrium points in the circular restricted three-body problem, when the more massive primary emits electromagnetic radiation impinging on the other bodies of the system. Using analytical and numerical methods, we have found the triangular equilibrium points and examined their linear stability. The triangular equilibrium points move towards the line joining the primaries in the presence of any of these perturbations, except in the presence of oblateness up to J 4 where the points move away from the line joining the primaries. It is observed that the triangular points are stable for 0 < μ < μ c and unstable for \(\mu_{\mathrm{c}} \le \mu \le \frac {1}{2},\) where μ c is the critical mass ratio affected by the oblateness up to J 4 of the less massive primary, electromagnetic radiation of the more massive primary and potential from the belt, all of which have destabilizing tendencies, except the coefficient J4 and the potential from the belt. A practical application of this model could be the study of motion of a dust particle near a radiating star and an oblate body surrounded by a belt. 相似文献
134.
Rachel S. Somerville Joel R. Primack S. M. Faber 《Monthly notices of the Royal Astronomical Society》2001,320(4):504-528
Using semi-analytic models of galaxy formation set within the cold dark matter (CDM) merging hierarchy, we investigate several scenarios for the nature of the high-redshift ) Lyman-break galaxies (LBGs). We consider a 'collisional starburst' model in which bursts of star formation are triggered by galaxy–galaxy mergers, and find that a significant fraction of LBGs are predicted to be starbursts. This model reproduces the observed comoving number density of bright LBGs as a function of redshift and the observed luminosity function at and with a reasonable amount of dust extinction. Model galaxies at have star formation rates, half-light radii, colours and internal velocity dispersions that are in good agreement with the data. Global quantities such as the star formation rate density and cold gas and metal content of the Universe as a function of redshift also agree well. Two 'quiescent' models without starbursts are also investigated. In one, the star formation efficiency in galaxies remains constant with redshift, while in the other, it scales inversely with disc dynamical time, and thus increases rapidly with redshift. The first quiescent model is strongly ruled out, as it does not produce enough high-redshift galaxies once realistic dust extinction is accounted for. The second quiescent model fits marginally, but underproduces cold gas and very bright galaxies at high redshift. A general conclusion is that star formation at high redshift must be more efficient than locally. The collisional starburst model appears to accomplish this naturally without violating other observational constraints. 相似文献
135.
136.
The rise of atmospheric O3 as a function of the evolution of O2 has been investigated using a one-dimensional steady-state photochemical model based on the chemistry and photochemistry of Ox(O3, O, O(1D)), N2O, NOx(NO, NO2, HNO3), H2O, and HOx(H, OH, HO2, H2O2) including the effect of vertical eddy transport on the species distribution. The total O3 column density was found to maximize for an O2 level of 10?1 present atmospheric level (PAL) and exceeded the present total O3 column by about 40%. For that level of O2, surface and tropospheric O3 densities exceeded those of the present atmosphere by about an order of magnitude. Surface and tropospheric OH densities of the paleoatmosphere exceeded those of the present atmosphere by orders of magnitude. We also found that in the O2-deficient paleoatmosphere, N2O (even at present atmospheric levels) produces much less NOx than it does in the present atmosphere. 相似文献
137.
Joel C. Hoffman John R. Kelly Greg S. Peterson Anne M. Cotter Matthew A. Starry Michael E. Sierszen 《Estuaries and Coasts》2012,35(6):1453-1467
There is growing interest in applying ??15N in biota as an indicator of anthropogenic nutrient inputs to coastal environments because changes in ??15N correlate to inputs of land-based nutrients. In complex coastal receiving waters, however, land-use effects on biota ??15N may be masked by local hydrologic processes, especially exchange with coastal waters of different geochemical character. We examined ??15N differences among larval fish, a novel biotic indicator, in coastal receiving waters at both among and within watershed scales. Our goal was to characterize how hydrologic processes within coastal river mouths and embayments mediate the effect of land-based N sources on larval fish ??15N. We sampled three Lake Superior river-embayment systems from watersheds that span a large population density gradient. Over all stations, mean fish ??15N ranged from 2.7??? to 10.8???. Within each system, we found a different pattern in ??15N across the river?Clake transition zone. Correlations between fish ??15N and water quality, particularly NH 4 + and total nitrogen, were highly significant and corresponded to known differences in sewage waste water inputs. A multivariate model that included both watershed-based population density and NH 4 + was found to provide the best fit to the ??15N data among a series of multi- and univariate candidate models. These results demonstrate that: (1) fish larvae ??15N responded at within watershed scales, and (2) within coastal receiving waters, fish larvae ??15N was related to waste water inputs at the watershed scale; however, expression at specific locations within a coastal system was strongly influenced by local hydrologic processes. 相似文献
138.
Research on hunger and food security in the Global South and the Global North has often emphasized different factors and scales
of analyses. Unlike newer monitoring systems in the Global South, which evolved substantially following critiques by Amartya
Sen, US food security research has rarely combined the two dimensions of food availability and food access. Furthermore, this
research has paid scant attention to household coping strategies. This study responds to this lacuna in US hunger research
by developing a spatial model for predicting risk to food insecurity based on proxy measures for access (three demographic
variables) and availability (grocery store density). The study then employs qualitative methodologies (surveys and semi-structured
interviews) to understand household coping strategies in two ethnically distinct areas in Minneapolis-Saint Paul at risk to
food insecurity. One neighborhood is dominated by Southeast Asian and East African immigrants and the other by African-Americans.
This approach should allow for better targeting of food aid and programs that help alleviate food insecurity. 相似文献
139.
Polycyclic aromatic hydrocarbons (PAH) have been measured in mussel tissues in early spring and summer since 1993 throughout Prince William Sound (PWS) and the Gulf of Alaska (GOA). Season-specific thresholds were established at reference sites to identify ‘above background’ total PAH levels. Thresholds were estimated using one-sided 99% tolerance limits. Thresholds were similar across reference sites but differed by an order of magnitude across seasons. Trends in total PAH since 1998 were assessed for sites impacted by the 1989 Exxon Valdez oil spill or the Alyeska Marine Terminal. Summer samples exhibited no trends; early spring samples declined. In early spring, all sites were judged ‘recovered’ by 2004; in summer, one site in western Prince William Sound and two in the western GOA exceeded thresholds by 11 ng/g dry weight or less. Robust estimation methods prevented bias from observations affected by unknown releases or laboratory errors. 相似文献
140.
Influence of the construction process and nonstructural components on the modal properties of a five‐story building
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Rodrigo Astroza Hamed Ebrahimian Joel P. Conte José I. Restrepo Tara C. Hutchinson 《地震工程与结构动力学》2016,45(7):1063-1084
A full‐scale five‐story reinforced concrete building was built and tested on the NEES‐UCSD shake table during the period from May 2011 to May 2012. The purpose of this test program was to study the response of the structure and nonstructural components and systems (NCSs) and their dynamic interaction during seismic base excitation of different intensities. The building specimen was tested first under a base‐isolated condition and then under a fixed‐based condition. As the building was being erected, an accelerometer array was deployed on the specimen to study the evolution of its modal parameters during the construction process and placement of major NCSs. A sequence of dynamic tests, including daily ambient vibration, shock (free vibration) and forced vibration tests (low‐amplitude white noise and seismic base excitations), were performed on the building at different stages of construction. Different state‐of‐the‐art system identification methods, including three output‐only and two input‐output methods, were used to estimate the modal properties of the building. The obtained results allow to investigate in detail the effects of the construction process and NCSs on the dynamic parameters of this building system and to compare the modal properties obtained from different methods, as well as the performance of these methods. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献