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HR: 1330h
AN: GP13B-13
TI: Magnetic Grain-size Proxies in Loessic Soils and Their Potential use in Paleorainfall Reconstruction
AU: * Machac, T A
EM: tamara.machac@trincoll.edu
AF: Trinity College, 300 Summit St., Hartford, CT 06106 United States
AU: Geiss, C E
EM: christoph.geiss@trincoll.edu
AF: Trinity College, 300 Summit St., Hartford, CT 06106 United States
AU: Zanner, C W
EM: bzanner2@unl.edu
AF: University of Nebraska at Lincoln, 133 Keim Hall School of Natural Resources Conservation and Survey Division, Lincoln, NE 68583-0915 United States
AB: As part of our ongoing rock-magnetic study of loessic soil profiles we sampled over 70 in Nebraska, Iowa, Missouri and Illinois. Our sampling sites are located in stable upland positions and extend along a rainfall gradient which ranges from an average annual precipitation of less than 500 mm/year in southwestern Nebraska to almost 1000 mm/year in central Missouri. Soil cores were obtained with the aid of a hydraulic soil probe, described and subsampled into small plastic bags. Samples were air-dried in the laboratory and the < 2mm fraction was used for magnetic analyses. We measured magnetic susceptibility X and several remanence parameters (ARM, IRM) for all samples. Hysteresis measurements, IRM acquisition curves and time dependence of IRM acquisition were measured for a subset of samples. All samples show magnetically enhanced A- and B-horizons, which results in increased values of X, ARM and IRM. Changes in the ratio of ARM/IRM suggest an increase in the relative abundance of stable single domain (SSD) particles. VRM analyses show that the upper soil horizons are enhanced in ultrafine superparamagnetic (SP) ferrimagnets as well. Changes in the relative abundance of SP and SSD ferrimagnets along our transsect correlates well with the modern precipitation gradient, suggesting the use of grain-size dependent magnetic parameter as a potential paleorainfall proxy when analyzing paleosols.
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
DE: 3344 Paleoclimatology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly

HR: 09:50h
AN: GP41B-06
TI: Chemical, morphological, and magnetic properties of loessic soils and the development of a multiproxy model to reconstruct paleorainfall
AU: * Zanner, C W
EM: bzanner2@unl.edu
AF: School of Natural Resources, University of Nebraska 133 Keim Hall, Lincoln, NE 68583-0915 United States
AU: Geiss, C E
EM: christoph.geiss@trincoll.edu
AF: Trinity College, 300 Summit Street, Hartford, CT 06106 United States
AB: In recent publications, we have been able to show that changes in the relative abundance of ferrimagnets along a rainfall gradient from less than 500 mm/year in southwestern Nebraska to almost 1000 mm/year in western Illinois correlates well with the modern precipitation gradient. This suggests that we can use a grain-size dependent magnetic parameter as a potential paleorainfall proxy when analyzing paleosols in central North America. Because prior attempts of others to develop a paleoclimate proxy based on magnetics alone have proved challenging, we are proposing that a multiproxy approach using other soil properties in addition to ferrimagnets will be more fruitful. Soil carbon, nitrogen, color, and depths to the top of the B horizon are examples of soil properties that have been shown to correlate to climate. We have sampled loessic soils with a truck-mounted soil probe from more than 70 stable undisturbed upland positions from Nebraska to Illinois. We described and subsampled the cores in the field. Laboratory measurements of soil carbon and nitrogen show a strong correlation to the rainfall gradient. Soil color and other morphological properties also show promise as climatic indicators. When combined with our measured magnetic parameters, such as magnetic susceptibility, anhysteretic remanent magnetization, hysteresis loops, and viscous remanent magnetization, these soil parameters will result in a robust model that tracks modern precipitation and in a promising tool that can be applied to paleosols for reconstructing paleoprecipitation.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1512 Environmental magnetism
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 9360 South America
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly

HR: 09:35h
AN: GP41B-05
TI: Multi-proxy Analysis of Lake Sediments from Western Connecticut Reveals Changes in Past Environments and Land-use
AU: * Geiss, C E
EM: christoph.geiss@trincoll.edu
AF: Trinity College, 300 Summit St, Hartford, CT 06106 United States
AU: Abrams, J L
EM: jeffrey.abrams@trincoll.edu
AF: Trinity College, 300 Summit St, Hartford, CT 06106 United States
AU: Sultan, R S
EM: ryan.sultan@trincoll.edu
AF: Trinity College, 300 Summit St, Hartford, CT 06106 United States
AB: Mudge Pond (lat. 41.90 deg. N, long 73.48 deg. W) is a small glacial lake in the highlands of western Connecticut. Our analyses of a transect of eight piston cores aim at reconstructing the combined effects of past climatic changes and human land use practices on the chemical, biological and physical properties of lake sediments. Magnetic analyses (magnetic susceptibility, hysteresis loops, remanence parameters and low-T analyses) characterize the terrigenous components of all cores and allow for correlation between sites. Sediments deposited prior to 10 ka B.P. are poor in organic carbon and its magnetic properties are dominated by paramagnetic minerals, leading to low remanence values and high values of magnetic susceptibility. Low lake levels during part of the Holocene, as reconstructed from sedimentary facies changes along our transect of cores, led to erosion and partial redeposition of these sediments in the deepest part of the lake. Late Holocene sediments are composed of Marl, which is only weakly magnetic and displays a magnetic remanence carried by large MD and PSD-sized ferrimagnets. The onset of Euroamerican settlement in New England led to increased erosion rates, which are reflected in higher concentrations of magnetic minerals. Analyses of macroscopic charcoal show that, despite extensive logging activities in the watershed, the severity of forest fires did not change noticeably compared to pre-settlement times. Our magnetic and sedimentological studies are supplemented by a palynological analyses, which confirm our reconstruction of past environmental change and help to clarify the onset of Euroamerican settlement and it's influence on the watershed of Mudge Pond.
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 3344 Paleoclimatology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly