Modeling fractional crystallization of group IVB iron meteorites     

Richard J. Walker  William F. McDonough  Nancy L. Chabot  Richard D. Ash 《Geochimica et cosmochimica acta》2008,72(8):2198-2216

A ¹⁸⁷Re-¹⁸⁷Os isochron including data for all twelve IVB irons gives an age of 4579 ± 34 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os of 0.09531 ± 0.00022, consistent with early solar system crystallization. This result, along with the chemical systematics of the highly siderophile elements (HSE) are indicative of closed-system behavior for all of the HSE in the IVB system since crystallization.Abundances of HSE measured in different chunks of individual bulk samples, and in spot analyses of different portions of individual chunks, are homogeneous at the ±10% level or better. Modeling of HSE in the IVB system, therefore, is not impacted by sample heterogeneities. Concentrations of some other elements determined by spot analysis, such as P, Cr and Mn, however, vary by as much as two orders of magnitude and reflect the presence of trace phases.Assuming initial S in the range of 0 to 2 wt.%, the abundances of the HSE Re, Os, Ir, Ru, Pt, Rh, Pd and Au in bulk IVB irons are successfully accounted for via a fractional crystallization model. For these elements, all IVB irons can be interpreted as being representative of equilibrium solids, liquids, or mixtures of equilibrium solids and liquids.Our model includes changes in bulk D values (ratio of concentration in the solid to liquid) for each element in response to expected increases in S and P in the evolving liquid. For this system, the relative D values are as follow: Os > Re > Ir > Ru > Pt > Rh > Pd > Au. Osmium, Re, Ir and Ru were compatible elements (favor the solid) throughout the IVB crystallization sequence; Rh, Pd and Au were incompatible (favor the liquid). Extremely limited variation in Pt concentrations throughout the IVB crystallization sequence requires that D(Pt) remained at unity.In general, D values derived from the slopes of logarithmic plots, compared with those calculated from recent parameterizations of D values for metal systems are similar, but not identical. Application of D values obtained by the parameterization method is problematic for comparisons of the compatible elements with similar partitioning characteristics. The slope-based approach works well for these elements. In contrast, the slope-based approach does not provide viable D values for the incompatible elements Pd and Au, whereas the parameterization method appears to work well. Modeling results suggest that initial S for this system may have been closer to 2% than 0, but the elements modeled do not tightly constrain initial S.Consistent with previous studies, our calculated initial concentrations of HSE in the IVB parent body indicate assembly from materials that were fractionated via high temperature condensation processes. As with some previous studies, depletions in redox sensitive elements and corresponding high concentrations of Re, Os and Ir present in all IVB irons are interpreted as meaning that the IVB core formed in an oxidized parent body. The projected initial composition of the IVB system was characterized by sub-chondritic Re/Os and Pt/Os ratios. The cause of this fractionation remains a mystery. Because of the refractory nature of these elements, it is difficult to envision fractionation of these elements (especially Re-Os) resulting from the volatility effects that evidently affected other elements.  相似文献   

Environmental Refugees? A tale of two resettlement projects in coastal Papua New Guinea     

John Connell  Nancy Lutkehaus 《The Australian geographer》2017,48(1):79-95

ABSTRACT

Environmental change in small islands may be associated with migration as a means of adaptation. Both Manam and the Carteret Islands in Papua New Guinea (PNG) have experienced rapid- and slow-onset changes, respectively. These have been accompanied by the forced migration and ‘temporary’ resettlement of the Manam population and attempts at resettlement by Carteret Islanders. Neither has proved successful, thwarted by ‘host’ landowners, the impossibility of gaining adequate access to land and land rights, and government inactivity. Settlers have been perceived as outsiders and rival claimants to valuable coastal resources. Inability to resettle in national contexts raises issues of ambiguity, identity and citizenship. The problems experienced by quite small population groups moving short distances in similar cultural contexts are indicative of the potential future problems facing environmental migrants in other contexts.  相似文献   

Climate change impacts on ecosystems and ecosystem services in the United States: process and prospects for sustained assessment     

Nancy B. Grimm  Peter Groffman  Michelle Staudinger  Heather Tallis 《Climatic change》2016,135(1):97-109

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Pasture enclosures increase soil carbon dioxide flux rate in Semiarid Rangeland,Kenya     

Collins O. Oduor  Nancy Karanja  Richard Onwong’a  Stephen Mureithi  David Pelster  Gert Nyberg 《Carbon balance and management》2018,13(1):24

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Editorial     

Andrew Rivkin  Nancy Chabot  Neil Dello Russo 《Meteoritics & planetary science》2009,44(12):1823-1823

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Effect of silicon on trace element partitioning in iron‐bearing metallic melts     

Nancy L. CHABOT  Trevor M. SAFKO  William F. McDONOUGH 《Meteoritics & planetary science》2010,45(8):1243-1257

Abstract– Despite the fact that Si is considered a potentially important metalloid in planetary systems, little is known about the effect of Si in metallic melts on trace element partitioning behavior. Previous studies have established the effects of S, C, and P, nonmetals, through solid metal/liquid metal experiments in the corresponding Fe binary systems, but the Fe‐Si system is not appropriate for similar experiments because of the high solubility of Si in solid metal. In this work, we present the results from 0.1 MPa experiments with two coexisting immiscible metallic liquids in the Fe‐S‐Si system. By leveraging the extensive available knowledge about the effect of S on trace element partitioning behavior, we explore the effect of Si. Results for 22 trace elements are presented. Strong Si avoidance behavior is demonstrated by As, Au, Ga, Ge, Sb, Sn, and Zn. Iridium, Os, Pt, Re, Ru, and W exhibit weak Si avoidance tendencies. Silicon appears to have no significant effect on the partitioning behaviors of Ag, Co, Cu, Cr, Ni, Pd, and V, all of which had similar partition coefficients over a wide range of Si liquid concentrations from Si‐free to 13 wt%. The only elements in our experiments to show evidence of a potentially weak attraction to Si were Mo and Rh. Applications of the newly determined effects of Si to problems in planetary science indicate that (1) The elements Ni, Co, Mo, and W, which are commonly used in planetary differentiation models, are minimally affected by the presence of Si in the metal, especially in comparison to other effects such as from oxygen fugacity. 2) Reduced enstatite‐rich meteorites may record a chemical signature due to Si in the metallic melts during partial melting, and if so, elements identified by this study as having strong Si avoidance may offer unique insight into unraveling the history of these meteorites.  相似文献   

An ~11,200?year paleolimnological perspective for emerging archaeological findings at Quartz Lake, Alaska     

Matthew J. Wooller  Joshua Kurek  Benjamin V. Gaglioti  Les C. Cwynar  Nancy Bigelow  Joshua D. Reuther  Carol Gelvin-Reymiller  John P. Smol 《Journal of Paleolimnology》2012,48(1):83-99

Wetlands and lakes in the Tanana Valley, Alaska, have provided important resources for prehistoric humans who inhabited this region. We examine an ~11,200?cal?yr BP record of environmental and paleolimnological changes from Quartz Lake in the middle Tanana Valley. Our data are also presented in the context of recent archaeological findings in the lake??s general vicinity that have 18 associated AMS ¹⁴C dates. We analyzed the stable-carbon and nitrogen isotope composition of total organic matter from the core, coupled with oxygen and carbon isotope analyses of Pisidiidae shells (fingernail clams), in addition to chironomid assemblage changes. Lacustrine sediments began to accumulate at ~11,200?cal?yr BP. Initially, autochthonous production was low and allochthonous organic input was negligible between 11,000 and 10,500?cal?yr BP, and were associated with relatively cool conditions at Quartz Lake at ~10,700?cal?yr BP. After 10,500?cal?yr BP, autochthonous production was higher coincident with a shift to chironomid assemblages dominated by taxa associated with warmer summer climates. A decrease in ??¹³C values of total organic carbon (TOC) and organic content of the sediment between 9,000 and 4,000?cal?yr BP may indicate declining autochthonous primary production. This period ended with an abrupt (~7???) decrease in the ??¹⁸O values from Pisidiidae shells at ~3,000?cal?yr BP, which we hypothesize represented an episodic connection (flood) of the lake with flow from the nearby (~6?km) Tanana River. Our findings coincide with evidence for major flooding at other locations connected to the Tanana River and further afield in Alaska. From ~3,000?cal?yr BP Quartz Lake subsequently appeared to become a relatively closed system, as indicated by the ??¹⁸O_Pisidiidae and ??¹³C_Pisidiidae data that are positively correlated and generally higher, which also correlates with a shift to moderately higher abundances of littoral chironomids. The cause of the transition to closed-basin conditions may have been geomorphic rather than climatic. This evidence of a progressively stronger evaporative influence on the lake??s closed hydrology after ~3,000?cal?yr BP is consistent with our modern ??¹⁸O and ??D water data from Quartz Lake that plot along a regional evaporative line we base on isotopic measurements from other local lakes and rivers.  相似文献   

Mercury's spectrophotometric properties: Update from the Mercury Dual Imaging System observations during the third MESSENGER flyby     

Deborah L. Domingue  Scott L. Murchie  Nancy L. Chabot  Brett W. Denevi  Faith Vilas 《Planetary and Space Science》2011,59(15):1853-1872

Presented here are analyses of the photometric measurements acquired by the imaging system on the MESSENGER spacecraft during its three flybys of Mercury, in particular the dedicated sequence of photometric measurements obtained during the third flyby. A concise, analytical approach is adopted for characterizing the effects of scattered light on the images. This approach works well for wavelengths shorter than 700 nm but breaks down at the longer wavelengths where the scattering behavior of the imaging system is more complex. Broadband spectral properties are commensurate with ground-based observations for spectra acquired at phase angles less than 110°; photometric corrections to a common illumination and viewing geometry provide consistent results for those phase angles. No phase reddening is apparent in the image-derived spectra. A bolometric albedo of 0.081 is derived over the wavelength range of the imaging system.  相似文献   

Factors affecting the interpretation of caddisfly assemblages from Quaternary sediments     

Nancy E. Williams 《Journal of Paleolimnology》1988,1(4):241-248

The use of assemblages of caddisflies (Trichoptera) in paleoecology has been explored over the past ca. 12 years. During this time, sites in North America, Great Britain and Europe have been studied and progress has been made in both the mechanics of identification and in the understanding of factors relevant to the interpretation of assemblages of this wholly aquatic group. Quaternary caddisfly fossils are abundant and usually well-preserved in waterlaid sediments. Individual larval sclerites can be identified by reference to shapes, textures, colour patterns muscle scar patterns and setal distributions. The flat frontoclypeus is particularly easily identified.Study of the biological and distributional data relevant to a caddisfly assemblage yields information at two levels. First the probable local habitat and second the climate can be described. This information is derived from both modern collections and the literature as the morphology of species, and hence it is assumed their environmental requirements, have not changed during the Quaternary. Comparisons with fossil assemblages of other plant and animal groups suggest that there are important factors to be considered in interpreting caddisfly assemblages, particularly those from sediments deposited during cold periods: for example, caddisflies may be slower to migrate than certain terrestrial insects, in particular the beetles (Coleoptera), and some glacial assemblages may therefore be dominated by hangers on. Lotic and lentic species also may migrate at different rates.Although caddisflies have not yet been fully exploited as paleoecological indicators, they have already contributed to our understanding of past environments and warrant much greater use.  相似文献   

Settlement patterns of brachyuran megalopae in a Louisiana estuary     

Barbara E. Hasek  Nancy N. Rabalais 《Estuaries and Coasts》2001,24(6):796-807

Artificial substrate collectors were used to sample settled brachyuran megalopae in Terrebonne Bay, Louisiana, northern Gulf of Mexico. Three taxa,Callinectes sapidus, Uca spp., andRhithropanopeus harrisii, were the most common settlers. Settlement occurred almost year-round, although settlement densities were highest in late summer and early fall. In 1990, all three taxa had several simultaneous settlement peaks; in 1991, one simultaneous settlement peak occurred. FewerC. sapidus andUca spp. settled in 1991 than in 1990, but settlement abundance ofR. harrissii was similar between years. Local weather events, such as heavy precipitation, may have influenced the lower abundances ofC. sapidus andUca spp. in 1991. The megalopae ofC. sapidus andUca spp., which re-invade the estuary from higher salinity offshore waters where larval development occurs, may have been more affected by the lower 1991 estuarine salinity values than the larvae ofR. harrissii, which are retained within the estuary throughout larval development. Settlement ofC. sapidus andUca spp. was correlated with maximum tidal height, tidal amplitude, salinity, temperature, and lunar phase. In 1990, settlement ofC. sapidus was correlated with lunar declination cycles; settlement peaks occurred during equatorial minimum amplitude tides. The settlement ofR. harrisii was associated with tidal amplitude and lunar phase. Associations between environmental variables and settlement of megalopae were not consistent between the two years sampled. The opposing or reinforcing effects of various environmental variables on settlement, e.g., tidal height and rainfall, are superimposed on long-term hydrologic cycles so that dramatically altered cycles of settlement occur among years.  相似文献