2 results
Modifications of 2:1 Clay Minerals in a Kaolinite-Dominated Ultisol under Changing Land-Use Regimes
- Jason C. Austin, Amelia Perry, Daniel D. Richter, Paul A. Schroeder
-
- Journal:
- Clays and Clay Minerals / Volume 66 / Issue 1 / February 2018
- Published online by Cambridge University Press:
- 01 January 2024, pp. 61-73
-
- Article
- Export citation
-
Chemical denudation and chemical weathering rates vary under climatic, bedrock, biotic, and topographic conditions. Constraints for landscape evolution models must consider changes in these factors on human and geologic time scales. Changes in nutrient dynamics, related to the storage and exchange of K+ in clay minerals as a response to land use change, can affect the rates of chemical weathering and denudation. Incorporation of these changes in landscape evolution models can add insight into how land use changes affect soil thickness and erodibility. In order to assess changes in soil clay mineralogy that result from land-use differences, the present study contrasts the clay mineral assemblages in three proximal sites that were managed differently over nearly the past two centuries where contemporary vegetation was dominated by old hardwood forest, old-field pine, and cultivated biomes. X-ray diffraction (XRD) of the oriented clay fraction using K-, Mg-, and Na-saturation treatments for the air-dried, ethylene glycol (Mg-EG and K-EG) solvated, and heated (100, 350, and 550°C) states were used to characterize the clay mineral assemblages. XRD patterns of degraded biotite (oxidized Fe and expelled charge-compensating interlayer K) exhibited coherent scattering characteristics similar to illite. XRD patterns of the Mg-EG samples were, therefore, accurately modeled using NEWMOD2® software by the use of mineral structure files for discrete illite, vermiculite, kaolinite, mixed-layer kaolinite-smectite, illite-vermiculite, kaolinite-illite, and hydroxy-interlayered vermiculite. The soil and upper saprolite profiles that formed on a Neoproterozoic gneiss in the Calhoun Experimental Forest in South Carolina, USA, revealed a depth-dependence for the deeply weathered kaolinitic to the shallowly weathered illitic/vermiculitic mineral assemblages that varied in the cultivated, pine, and hardwood sites, respectively. An analysis of archived samples that were collected over a five-decade growth period from the pine site suggests that the content of illite-like layers increased at the surface within 8 y. Historical management of the sites has resulted in different states of dynamic equilibrium, whereby deep rooting at the hardwood and pine sites promotes nutrient uplift of K from the weathering of orthoclase and micas. Differences in the denudation rates at the cultivated, pine, and hardwood sites through time were reflected by changes in the soil clay mineralogy. Specifically, an increased abundance of illite-like layers in the surface soils can serve as a reservoir of K+.
Health and occupation: the limits to older adults' work hours
- Tinh Doan, Christine Labond, Amelia Yazidjoglou, Perri Timmins, Peng Yu, Lyndall Strazdins
-
- Journal:
- Ageing & Society / Volume 44 / Issue 4 / April 2024
- Published online by Cambridge University Press:
- 04 May 2022, pp. 743-771
- Print publication:
- April 2024
-
- Article
- Export citation
-
More people are working into older age, raising questions about how many hours they can work before their health becomes compromised. This paper models work-hour tipping points for mental health and vitality among older Australian workers aged 50–70 years. We use longitudinal data from the Household, Income and Labour Dynamics in Australia (HILDA) survey, 2005–2016 (about 44,900 observations), and bootstrapping Three Stage Least Squares (3SLS) estimation techniques to adjust for reverse and reciprocal relationships between wages, work hours and health. Our approach corrects for heteroscedasticity in the system equation error terms, and we estimate models on the relatively healthy older adults who have remained employed into older age. Among these older workers we observe weekly thresholds of 39–40 hours beyond which mental health and vitality decline. This average, however, hides variability in work-hour limits linked to overall health and occupation. Thus, weekly tipping points for blue- and pink-collar jobs are 7–9 hours lower compared to white-collar jobs, and even wider gaps (11 hours) are apparent for workers with poorer physical functioning, which becomes common as people age. Our modelling reveals that age is not the biggest limiting factor for how many hours older adults can work, rather their health and the types of jobs are critical, and likely widen the gap in who ages successfully or not.