The effect of Ni on element partitioning during iron meteorite crystallization     

Nancy L. Chabot  Sarah A. Saslow  William F. McDonough  Timothy J. McCoy 《Meteoritics & planetary science》2007,42(10):1735-1750

Abstract— ‐Iron meteorites exhibit a large range in Ni concentrations, from only 4% to nearly 60%. Most previous experiments aimed at understanding the crystallization of iron meteorites have been conducted in systems with about 10% Ni or less. We performed solid metal/liquid metal experiments to determine the effect of Ni on partition coefficients for 20 trace elements pertinent to iron meteorites. Experiments were conducted in both the end‐member Ni‐S system as well as in the Fe‐Ni‐S system with intermediate Ni compositions applicable to high‐Ni iron meteorites. The Ni content of the system affects solid metal/liquid metal partitioning behavior. For a given S concentration, partition coefficients in the Ni‐S system can be over an order of magnitude larger than in the Fe‐S system. However, for compositions relevant to even the most Ni‐rich iron meteorites, the effect of Ni on partitioning behavior is minor, amounting to less than a factor of two for the majority of trace elements studied. Any effect of Ni also appears minor when it is compared to the large influence S has on element partitioning behavior. Thus, we conclude that in the presence of an evolving S‐bearing metallic melt, crystallization models can safely neglect effects from Ni when considering the full range of iron meteorite compositions.  相似文献   

Denitrification in a Laurentian Great Lakes coastal wetland invaded by hybrid cattail (Typha × glauca)     

Shane C. Lishawa  KathiJo Jankowski  Pamela Geddes  Daniel J. Larkin  Andrew M. Monks  Nancy C. Tuchman 《Aquatic Sciences - Research Across Boundaries》2014,76(4):483-495

Wetland ecosystems maintain and improve water quality through the process of denitrification, an increasingly important ecosystem service due to global N pollution. Invasive plants have the potential to disrupt denitrification by altering the environmental conditions that facilitate this process. Great Lakes coastal wetlands are experiencing widespread invasion by highly productive hybrid cattail with largely uncertain biogeochemical effects. Through field and controlled mesocosm studies, we sought to determine the effects of cattail invasion through time on denitrification rates and associated environmental factors in a Great Lakes coastal wetland. In the field, we found that cattail density correlated with increased denitrification and a suite of environmental and plant community characteristics and denitrification rates were positively correlated with NH₄ ⁺, sediment organic matter, reduced water levels, and cattail stand age. Through our controlled mesocosm study, we documented conditions 1- and 5-year following invasion and found that denitrification rates and soil organic matter increased in year 5, and cattail and year-since-invasion altered plant communities and soil NH₄ ⁺. Only a weak correlation between denitrification rates and cattail treatments was noted, however, owing to high replicate variability. Our results indicate that with increasing cattail residence time, one ecosystem service, biodiversity, was negatively impacted, while two other services, denitrification and sediment carbon accumulation, were enhanced. Thus, this highly invaded wetland still provides valuable services to aquatic ecosystems and to society. A holistic perspective is therefore critical when evaluating invasive species impacts in which negative impacts are weighed against other ecosystem services, which may be stimulated.  相似文献   

The significance of the personal within disability geography     

Nancy Worth 《Area》2008,40(3):306-314

Drawing on in-depth qualitative data, this article critically examines disability geography as a subfield where the personal is highly valued. The value and the risks inherent in this personal approach will be evaluated, including the usefulness of being an 'insider' and the difficulties of being reflexive and critically making use of one's positionality. The article concludes with reflections regarding how disability geography can confront its marginal status, appealing to researchers who claim no experience of disability while also supporting and encouraging those with personal experiences of disability to participate in the field.  相似文献   

THE MARBLE BEACHES OF TUSCANY*     

Karl F. Nordstrom  Enzo Pranzini  Nancy L. Jackson  Massimo Coli 《Geographical review》2008,98(2):280-300

ABSTRACT. Beach‐nourishment operations designed to replace sediment lost through erosion change the identity and meaning of coastal landscapes. Seven beaches in Tuscany, nourished with marble‐quarry waste, reveal how an industrial byproduct is naturalized by particle rounding and sorting and can become a positive symbol of human‐altered nature. The marble was placed on formerly sandy beaches, resulting in different grain size and color of sediments, beach morphology, and value for human use. The abrasion rate of marble makes the nourished beaches unsatisfactory when viewed solely as protection structures, but the rapid particle rounding and aesthetic appeal of marble increase the acceptability of the beaches for recreation.  相似文献   

Climatic influences on riverine nitrate flux: Implications for coastal marine eutrophication and hypoxia   总被引：2，自引：0，他引：2  

Dubravko?Justi?Email author  R.?Eugene?Turner  Nancy?N.?Rabalais 《Estuaries and Coasts》2003,26(1):1-11

The average nitrate flux of the lower Mississippi River increased 3.3-fold between 1954–1967 and 1983–2000. During the same time period, the average nitrate concentration increased 2.3-fold while the average discharge increased 40%. Partitioning of the observed trend in nitrate flux among the two flux components, nitrate concentration and discharge, revealed that about 80% of the observed increase in flux could be explained by the increase in nitrate concentration. This indicates that a historical increase in the anthropogenic nutrient inputs has had a far greater impact on the lower Mississippi River nitrate flux than a change in climate. The influence of climatic factors on nitrate flux has been significant and may further increase as a result of global climate change. This argument is supported by two lines of evidence. The residual component of nitrate flux, obtained by removing a trend from the time series, is controlled primarily by the variability in discharge, i.e., climatic factors. Also, there is a highly significant relationship between discharge and nitrate concentration at the low end of the discharge spectrum (<13,000 m³ s^?1). The differences in nitrate flux between flood and drought years are significantly larger than the variations in discharge. This makes the Mississippi River nitrate flux potentially sensitive to future changes in the frequency of extreme climatic events. Because of the importance of nitrate for the productivity of coastal phytoplankton, future climate change would likely have important implications for coastal marine eutrophication and hypoxia.  相似文献   

THE DECLINE OF PARTY VOTING: A GEOGRAPHICAL ANALYSIS OF THE 1978 MASSACHUSETTS ELECTION     

Bruce Downing  Tim Hudson  Peter Taylor  Peter Bland  Nancy Villanueva 《The Professional geographer》1980,32(4):454-461

A factor analysis of the 1978 Massachusetts election is undertaken to investigate whether the alleged decline of political parties in the United States can be distinguished at the state scale. Four factors are identified, two of which relate to the voting patterns of state-wide candidates. The main factor is easily seen to represent a “party voting”pattern and the fourth factor suggests a nonparty pattern of support associated with the Democratic candidate for governor, Edward King. It is suggested that this separation of the voting pattern for Democratic candidates may possibly reflect party decomposition in Massachusetts.  相似文献   

Relationships among satellite chlorophylla, river inputs, and hypoxia on the Louisiana Continental shelf, Gulf of Mexico     

Nan D. Walker  Nancy N. Rabalais 《Estuaries and Coasts》2006,29(6):1081-1093

SeaWiFS ocean color measurements were used to investigate interannual, monthly, and weekly variations in chlorophylla (chla) on the Louisiana shelf and to assess relationships with river discharge, nitrate load, and hypoxia. During the study period (2000–2003), interannual changes in shelf-wide chla concentrations averaged over January–July ranged from +57% to −33% of the 4-yr average, in accord with freshwater discharge changes of +20% to −29% and nitrate load changes of +20% to −35% from the Mississippi and Atchafalaya Rivers. Chla variations were largest on the shelf between the Mississippi and Atchafalaya Deltas. Within this region, which corresponds spatially to the area of most frequent hypoxia, lowest January–July mean chla concentrations (5.5 mg m⁻³ over 7,000 km²) occurred during 2000, the year of lowest freshwater discharge (16,136 m³ s⁻¹) and nitrate load (55,738 MT N d⁻¹) onto the shelf. Highest January–July mean chla concentrations (13 mg m⁻³ over 7,000 km²) were measured in 2002, when freshwater discharge (27,440 m³s⁻¹) and nitrate load (101,761 MT N d⁻¹) were highest and second highest, respectively. Positive correlations (R²=0.4–0.5) were found between chla and both fresh water and nitrate loads with 0 to 1 month lags, with the strongest relationships just west of the Mississippi Delta. In 2001, unusually clear skies allowed the identification of distinct spring and summer chla blooms west of the Mississippi Delta 4–5 wk after peaks in river discharge. East of the delta, the chla concentrations peaked in June and July, following the seasonal reversal in the coastal current. A clear linkage was not detected between satellite-measured chla and hypoxia during the 4-yr period, based on a time series of bottom oxygen concentrations at one station within the area of most frequent hypoxia. Clear relationships are confounded by the interaction of physical processes (wind stress effects) with the seasonal cycle of nutrient-enhanced productivity and are influenced by the prior year's nitrate load and carbon accumulation at the seabed.  相似文献   

Aeolian sediment transport on a human‐altered foredune     

Karl F. Nordstrom  Nancy L. Jackson  Jean Marie Hartman  Mark Wong 《地球表面变化过程与地形》2007,32(1):102-115

Changes in wind speed and sediment transport are evaluated at a gap and adjacent crest of a 2 to 3 m high, 40 m wide foredune built by sand fences and vegetation plantings on a wide, nourished fine sand beach at Ocean City, New Jersey. Anemometer masts, cylindrical sand traps and erosion pins were placed on the beach and dune during two obliquely onshore wind events in February and March 2003. Results reveal that: (1) changes in the alongshore continuity of the beach and dune system can act as boundaries to aeolian transport when winds blow at an angle to the shoreline; (2) oblique winds blowing across poorly vegetated patches in the dune increase the potential for creating an irregular crest elevation; (3) transport rates and deflation rates can be greater within the foredune than on the beach, if the dune surface is poorly vegetated and the beach has not had time to dry following tidal inundation; (4) frozen ground does not prevent surface deflation; and (5) remnant sand fences and fresh storm wrack have great local but temporary effect on transport rates. Temporal and spatial differences due to sand fences and wrack, changes in sediment availability due to time‐dependent differences in surface moisture and frozen ground, combined with complex topography and patchy vegetation make it difficult to specify cause–effect relationships. Effects of individual roughness elements on the beach and dune on wind flow and sediment transport can be quantified at specific locations at the event scale, but extrapolation of each event to longer temporal and spatial scales remains qualitative. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

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

  相似文献   

The parameterization of solid metal‐liquid metal partitioning of siderophile elements     

Nancy L. CHABOT  John H. JONES 《Meteoritics & planetary science》2003,38(10):1425-1436

Abstract— Many solar system processes involve a metallic liquid, and the composition of the metallic liquid, such as the liquid's concentrations of S, P, and C, will influence the partitioning of elements during such processes. We present a method for parameterizing solid metal‐liquid metal partition coefficients for siderophile (metal‐loving) elements as a function of the metallic liquid composition. Our parameterization method is based on an older theory of Jones and Malvin (1990), which stated that the metallic liquid is composed of metal and non‐metal‐bearing domains, and the domains are the dominant influence on the partitioning behavior. By revising the means by which the metal domains are calculated, our revised parameterization method is able to match experimental partitioning data from the Fe‐Ni‐S, Fe‐Ni‐P, Fe‐Ni‐S‐P, and Fe‐Ni‐C systems. Mathematical expressions were derived for the solid metal‐liquid metal partitioning of 13 siderophile elements. Elements that are chalcophile (S‐loving), P‐loving, or C‐loving prefer the non‐metal‐bearing domains in the metallic liquid and, consequently, aren't fit by the parameterization method presented here. Possible applications for our parameterization method include modeling the crystallization of iron meteorites, planetary differentiation, and the solidification of Earth's inner core.  相似文献