Magnetic Analysis of Soils from the Wind River Range, Wyoming

Publication Year:
2011
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Repository URL:
https://digitalrepository.trincoll.edu/theses/14
Author(s):
Quinton, Emily
Tags:
Wind River Range; soils; chronosequence; hematite; magnetism; Environmental Sciences
thesis / dissertation description
In order to constrain the rate of magnetic enhancement in glacial fluvial sediments, we investigated modern soils from five fluvial terraces in the eastern Wind River Range, Wyoming. Profiles up to 1.2 m deep were sampled in five cm intervals from hand-dug pits or natural riverbanks exposures. These profiles include soils from fluvial terraces correlated to Sacagawea Ridge, Bull Lake and Pinedale-age glacial advances and one Holocene profile. Soil ages range from approximately >500 ka to modern. To characterize changes in magnetic properties we performed a variety of rock magnetic analyses. Abundance and grain size of magnetic minerals were estimated through measurements of magnetic susceptibility, anhysteretic remanent magnetization and isothermal remanent magnetization. We also examined the absolute and relative contributions of ferrimagnetic magnetite/maghemite and antiferromagnetic hematite to the magnetic signal through measurements of “hard” isothermal remanent magnetization, S-ratios and magnetic coercivity distributions. Magnetic enhancement of the A-horizon as well as an increase in fine-grained material occurred mostly in and not prior to the Bull Lake profiles. A loss of ferrimagnets and an increase in antiferromagnetic minerals occurred in older soil profiles, suggesting the conversion of ferrimagnetic magnetite or maghemite to weakly magnetic hematite with progressing soil age. Absolute and relative hematite abundance increases with age, making it a useful proxy for soil age. All coercivity proxies are consistent with each other, which suggests that observed changes are representative of real changes in hematite abundance rather than shifts in coercivity distributions, even though the modified L-ratio varies widely.