Magnetic Properties of a Soil Chronosequence from the Eastern Wind River Range, Wyoming

1. Magnetic Properties of a Soil Chronosequence from the Eastern Wind River Range, Wyoming Emily Quinton1, Christoph Geiss1, Dennis Dahms2, 1 Environmental Science Program, Trinity College, Hartford, CT, 2 Department of Geography, University of Northern Iowa, Cedar Falls, IA GP13B-0782 Abstract In order to constrain the rate of magnetic enhancement in glacial fluvial sediments, we sampled modern soils from eight fluvial terraces in the Eastern Wind River Range in Wyoming. Soil profiles up to 1.2 meters deep were described in the field and sampled in five cm intervals from a series of hand-dug pits or natural river-bank exposures. The ages of the studied profiles are estimated to range from >600 ka to modern. They include Sacagawea Ridge, Bull Lake and Pinedale-age fluvial terraces as well as one Holocene profile. To characterize changes in magnetic properties we measured low-field magnetic susceptibility, anhysteretic remanent magnetization, isothermal remanent magnetization and S-ratios for all, and hysteresis loops for a selected sub-set of samples. Our measurements show no clear trend in magnetic enhancement with estimated soil age. The observed lack of magnetic enhancement in the older soils may be due to long-term deflation, which continuously strips off the magnetically enhanced topsoil. It is also possible that the main pedogenic processes, such as the development of well-expressed calcic horizons destroy or mask the effects of long-term magnetic enhancement Soil Lithology Legend Figure 6: Variations in mass-normalized magnetic susceptibility (m3/kg) for all studied profiles. Figure 1: Magnetic properties of WIN – 10 B (Holocene). • Methods • The following magnetic parameters were used to characterize the amount and size of magnetic materials present in the soils. • Mass normalized magnetic susceptibility (χ) was measured using a KLY-4 Kappabridge susceptibility meter. • Anhysteric remanent magnetization (ARM) was acquired in a peak AF field of 100mT combined with a 50 μT bias field using a Magnon International AFD 300 alternating magnetic field demagnetizer. • Isothermal remanent magnetization (IRM) was acquired through three pulses of a 100 mT field using an ASC-Scientific IM-10-30 pulse magnetizer. • Magnetic coercivity distributions of SIRM (acquired in tree field pulses of 1200 mT) were determined through stepwise AF-demagnetization in fields up to 300 mT.Coercivity data were fitted to cumulative log normal distributions (Geiss et al., 2008). • The abundance of high-coercivity minerals was estimated from the hard remanence remaining after demagnetization of an SIRM in a 300 mT AF field. Red Canyon Figure 2: Magnetic properties of WIN 10 – A (Pinedale). Figure 7: Variations in ARM (Am2/kg) for all studied profiles. Figure 10: The first plot shows absolute abundance of high-coercivity minerals (determined by IRM) and the second plot shows the ratio of high and low-coercivity minerals, versus age. Study Area In August 2010 we collected samples from eight soil profiles on fluvial terraces near Lander, Wyoming, as well as two samples from the Red Canyon shale. Figure 1 shows our sampling locations. In this study we focus on the five profiles located in Red Canyon. Based on stratigraphic relationships, these profiles are assumed to represent soil ages as young as the Holocene (WIN 10-B), as well as soil profiles that have developed since the Pinedale (WIN 10-A), Late Bull Lake (WIN 10-D), Early Bull Lake (WIN 10-E) and Sacagawea Ridge (WIN 10-C) glacial periods. Two of the remaining three soil profiles were collected north of Lander. These sites represent soils that have developed since the Pinedale (WIN 10 – F) and Bull Lake (WIN 10 – G) glacial periods. The last profile was collected in the town of Lander and is estimated to represent the Sacagawea Ridge glacial period (WIN 10 – H). The soils from these sites developed in different parent materials and have therefore been excluded from our soil chronosequence. Results We observed no clear trend in increase of soft ferrimagnetic minerals with age as it is observed elsewhere (e.g., Geiss and Zanner, 2006). Overall, the magnetic enhancement of our calcite and hematite-rich soil profiles is weak. We did observe an absolute increase of high-coercivity minerals as well as a consistent increase in the ratio between high and low-coercivityminerals in the top sol horizons (Figure 10). These ratios indicate an increase in the amount of hematite with age, suggesting that the end-member of soil development processes is hematite, as proposed by Torrent et al.(2006). Figure 3: Magnetic properties of WIN 10 – D (Late Bull Lake) Figure 8: Variations in IRM (Am2/kg) for all studied profiles. References Dahms, D. E. in Quaternary Glaciations – Extent and Chronology, Part II: North America. Editors J. Ehlers and P.L. Gibbard. (2004) Glacial limits in the middle and southern Rocky Mountains, U.S.A., south of the Yellowstone Ice Cap. 275-288. Published by Elsevier. Dahms, D. E. (2004) Relative and numeric age data for Pleistocene glacial deposits and diamictons in and near Sinks Canyon, Wind River Range, Wyoming, U.S.A. Arctic, Antarctic, and Alpine Research. 36(1): 59-77. Geiss, C. E. and C. W. Zanner. (2006). How abundant is pedogenic magnetitite? Abundance and grain size estimates for loessic soils based on rock magnetic analyses. Journal of Geophysical Research. 111, B12 S21, doi:10.1029/2006JB004564. Geiss, C. E., R. Egli and C. W. Zanner. (2008). Direct estimates of pedogenic magnetite as a tool to reconstruct past climates from buried soils. Journal of Geophysical Research. 113, B11102, doi:10.1029/2008JB005669. Torrent, J. V. Barrón and Q. Liu. (2006) Magnetic enhancement is linked to and precedes hematite formation in aerobic soil. Geophysical Research Letters. 33, L02401, doi:10.1029/2005GL024818. A E B Figure 4: Magnetic properties of WIN 10 – E (Early Bull Lake) D Figure 9: Variations in ARM/IRM for all studied profiles. C Figure 1: Locations of our eight soil profiles near Lander, Wyoming Figure 5: Magnetic properties of WIN 10 – C (Sacagawea Ridge). Figure 2: Location of Lander and our eight sampling locations in Fremont County, Wyoming Soil Profile Sampling in Red Canyon Soil Profile at WIN 10 - E