University of Arkansas

About: University of Arkansas is a education organization based out in Fayetteville, Arkansas, United States. It is known for research contribution in the topics: Population & Poison control. The organization has 17225 authors who have published 33329 publications receiving 941102 citations. The organization is also known as: Arkansas & UA.

Stability of perchlorate hydrates and their liquid solutions at the Phoenix landing site, Mars

Abstract: [1] We studied the low-temperature properties of sodium and magnesium perchlorate solutions as potential liquid brines at the Phoenix landing site. We determined their theoretical eutectic values to be 236 ± 1 K for 52 wt% sodium perchlorate and 206 ± 1 K for 44.0 wt% magnesium perchlorate. Evaporation rates of solutions at various concentrations were measured under martian conditions, and range from 0.07 to 0.49 mm h ―1 for NaClO 4 and from 0.06 to 0.29 mm h ―1 for Mg(ClO 4 ) 2 . The extrapolation to Phoenix landing site conditions using our theoretical treatment shows that perchlorates are liquid during the summer for at least part of the day, and exhibit very low evaporation rates. Moreover, magnesium perchlorate eutectic solutions are thermodynamically stable over vapour and ice during a few hours a day. We conclude that liquid brines may be present and even stable for short periods of time at the Phoenix landing site.

Ungulate stimulation of nitrogen cycling and retention in Yellowstone Park grasslands

Abstract: We studied how ungulates and a large variation in site conditions influenced grassland nitrogen (N) dynamics in Yellowstone National Park. In contrast to most grassland N studies that have examined one or two soil N processes, we investigated four rates, net N mineralization, nitrification, denitrification, and inorganic N leaching, at seven paired sites inside and outside long-term (33+ year) exclosures. Our focus was how N fluxes were related to one another among highly variable grasslands and how grazers influenced those relationships. In addition, we examined variation in soil δ15N among grasslands and the relationships between soil 15N abundance and N processes. Previously, ungulates were reported to facilitate net N mineralization across variable Yellowstone grasslands and denitrification at mesic sites. In this study, we found that herbivores also promoted nitrification among diverse grasslands. Furthermore, net N mineralization, nitrification, and denitrification (kg N ha–1 year–1, each variable) were postively and linearly related to one another among all grasslands (grazed and fenced), and grazers reduced the nitrification/net N mineralization and denitrification/net N mineralization ratios, indicating that ungulates inhibited the proportion of available NH4+ that was nitrified and denitrified. There was no relationship between net N mineralization or nitrification with leaching (indexed by inorganic N adsorbed to resin buried at the bottom of rooting zones) and leaching was unaffected by grazers. Soil δ15N was positively and linearly related to in situ net N mineralization and nitrification in ungrazed grasslands; however, there was no relationship between isotopic composition of N and those rates among grazed grasslands. The results suggested that grazers simultaneously increased N availability (stimulated net N mineralization and nitrification per unit area) and N conservation (reduced N loss from the soil per unit net N mineralization) in Yellowstone grasslands. Grazers promoted N retention by stimulating microbial productivity, probably caused by herbivores promoting labile soil C. Process-level evidence for N retention by grazers was supported by soil δ15N data. Grazed grassland with high rates of N cycling had substantially lower soil δ15N relative to values expected for ungrazed grassland with comparable net N mineralization and nitrification rates. These soil 15N results suggest that ungulates inhibited N loss at those sites. Such documented evidence for consumer control of N availability to plants, microbial productivity, and N retention in Yellowstone Park is further testimony for the widespread regulation of grassland processes by large herbivores.

Explaining employees' health care costs: a prospective examination of stressful job demands, personal control, and physiological reactivity.

Abstract: The authors tested the ability of stressful demands and personal control in the workplace to predict employees' subsequent health care costs in a sample of 105 full-time nurses. Both subjective and objective measures of workload demands interacted with personal control perceptions in predicting the cumulative health care costs over the ensuing 5-year period. Tonic elevations in salivary cortisol, moreover, mediated the effects of demands and control on health care costs. Neither the job demands variables nor physiological reactivity measures, however, explained subsequent mental health. The results support findings from the epidemiological literature that demonstrate an important role for employees' control in explaining occupational inequalities in coronary heart disease and mortality. The authors argue that the results also encourage control-enhancing job design interventions by suggesting that their outcomes can benefit both organizations and their members.

Radar-Enabled Recovery of the Sutter’s Mill Meteorite, a Carbonaceous Chondrite Regolith Breccia

Abstract: Doppler weather radar imaging enabled the rapid recovery of the Sutter’s Mill meteorite after a rare 4-kiloton of TNT–equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand’s parameter = 2.8 ± 0.3). Sutter’s Mill is a regolith breccia composed of CM (Mighei)–type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.