HR: 0830h
AN: ED41B-1164    [PDF]
TI: Lost in Loess: Paleomagnetic investigation into loess and tephra deposits in interior Alaska
AU: * Semler, L
EM: ljsemler@mtu.edu
AF: Dept. of Geological Engineering, Michigan Tech. Univ., Houghton, MI 49931
AU: Arnold, K E
EM: karnold@hamilton.edu
AF: Dept. of Geology, Hamilton College, Clinton, NY 13323
AU: Williams, D
EM: dwilliams11@mymail.indstate.edu
AF: Dept. of Geography and Geology, Indiana State Univ., Terre Haute, IN 47809
AU: Morton, J
EM: jxmort@wm.edu
AF: Dept. of Geology, College of William and Mary, Williamsburg, VA 23187
AU: Layer, P W
EM: player@gi.alaska.edu
AF: Geophysical Institute, Univ. of Alaska Fairbanks, Fairbanks, AK 99775
AU: Stone, D B
EM: dstone@gi.alaska.edu
AF: Geophysical Institute, Univ. of Alaska Fairbanks, Fairbanks, AK 99775
AU: Beget, J
EM: ffjeb1@uaf.edu
AF: Geophysical Institute, Univ. of Alaska Fairbanks, Fairbanks, AK 99775
AU: Schaefer, J
EM: Janet_Schaefer@dnr.state.ak.us
AF: Alaska Div. of Geological and Geophysical Surveys, 749 University Ave., Fairbanks, AK 99709
AB: As a part of a NSF-funded Research Experience for Undergraduates (REU) program at the University of Alaska Fairbanks, loess and tephra samples were collected at a road cut near Tok, Alaska, to determine if the site was suitable for paleoclimate reconstruction of the Pleistocene. Oriented cubes and cores were obtained from a section of loess just below the Sheep Creek tephra dated at 190 $\pm$ 20 ka, through the Tetlin tephra (1.2 meters below the Sheep Creek tephra) dated at 630 $\pm$ 50 ka to two meters below the Tetlin tephra. The accumulated amount of loess between dated tephra layers suggests a span of more than one million years for the whole section assuming a constant sedimentation rate for the loess. The samples were measured for natural remanent magnetization (NRM) and magnetic susceptibility before magnetic cleaning techniques (Alternating Field Demagnetization and Thermal Demagnetization) were employed. Samples were analyzed using a cryogenic magnetometer. The demagnetizations show stable magnetic vectors and demonstrate that loess is a reliable paleomagnetic recorder. At Tok, we found that the entire section is of normal polarity and shows no sign of the 780 ka Brunhes-Matuyama polarity reversal, which was expected to be 42 cm below the Tetlin tephra. Absence of the reversal may be due to discontinuities or other changes in the sedimentation rate in the loess, a problem with the sampling methods, or incorrect ages of the tephras. Based on our studies, we feel that changing rates of loess deposition or other unseen discontinuities are the reason the reversal was not found. Because of the uncertainty of the depositional history of the Tok loess, this section is not useful for obtaining a continuous record of ancient climate.
DE: 1520 Magnetostratigraphy
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
DE: 6605 Education
SC: Education and Human Resources [ED]
MN: 2003 Fall Meeting