Magnetic Properties of a Soil Chronosequence from the Eastern
Wind River Range, Wyoming Emily Quinton 1, Christoph Geiss 1,
Dennis Dahms 2, 1 Environmental Science Program, Trinity College,
Hartford, CT, 2 Department of Geography, University of Northern
Iowa, Cedar Falls, IA GP13B-0782 Red Canyon Soil Profile Sampling
in Red CanyonSoil Profile at WIN 10 - E 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 Methods The following
magnetic parameters were used to characterize the amount and size
of magnetic materials present in the soils. Mass normalized
magnetic susceptibility () 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 T 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.
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.
Barrn 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. Figure 2: Magnetic
properties of WIN 10 A (Pinedale). Figure 1: Magnetic properties of
WIN 10 B (Holocene). Figure 5: Magnetic properties of WIN 10 C
(Sacagawea Ridge). Figure 3: Magnetic properties of WIN 10 D (Late
Bull Lake) Figure 4: Magnetic properties of WIN 10 E (Early Bull
Lake) Figure 9: Variations in ARM/IRM for all studied profiles.
Figure 6: Variations in mass-normalized magnetic susceptibility (m
3 /kg) for all studied profiles. 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. Figure 8: Variations in IRM (Am 2 /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. Figure 7: Variations in ARM (Am 2 /kg) for all studied
profiles. Figure 2: Location of Lander and our eight sampling
locations in Fremont County, Wyoming Figure 1: Locations of our
eight soil profiles near Lander, Wyoming 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-coercivity minerals 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). Soil Lithology Legend A B C D E