HR: 08:45h
AN: GP51A-04 INVITED    [Abstracts]
TI: Rock Magnetic Detection of Two Coercivity Components of Magnetosomes in Lake Ely Sediments
AU: * Kodama, K P
EM: kpk0@lehigh.edu
AF: Lehigh University, Department of Earth and Environmental Sciences, 31 Williams Drive, Bethlehem, PA 18015, United States
AU: Chen, A
EM: chen0653@umn.edu
AF: University of Minnesota, Dept of Geology and Geophysics, 310 Pillsbury Drive SE, Minneapolis, MN 55455, United States
AU: Egli, R
EM: egli@geophysik.uni-muenchen.de
AF: Ludwig-Maximilians University, Dept of Earth and Environmental Siences, Theresienstr. 41, Munich, 80333, Germany
AU: Vavrek, J
EM: jkv209@lehigh.edu
AF: Lehigh University, Department of Earth and Environmental Sciences, 31 Williams Drive, Bethlehem, PA 18015, United States
AB: The modeling of IRM acquisition curves has become an important tool used by environmental magnetists to characterize a sample's magnetic mineralogy. We have applied IRM modeling to lake sediments from Lake Ely, a small, post-glacial lake located in northeastern Pennsylvania. The lake is unique in having a strong sedimentary magnetization dominated by the magnetosomes produced by magnetotactic bacteria. Measurements of Fe, S, and O₂ at different depths in the lake's water column, as well as the magnetization of material filtered from the water, indicate that magnetotactic bacteria live in the water column at the oxic-anoxic interface. Magnetic measurements and TEM observations of lake sediments and material caught in a sediment trap indicate that magnetosomes are the major constituent of the magnetic minerals in the lake sediments. IRM modeling shows two separate coercivity components at about 30 mT and 60 mT that are consistent with Egli's (2004) observations of BS (biogenic soft) and BH (biogenic hard) coercivity components for magnetosomes in lake sediments. However, the peaks are poorly determined because only 6-7 data points constrain the 0-100 mT part of the IRM coercivity spectrum. We used pARMs (97 μT DC field) applied in 5 mT alternating field steps between 0 and 100 mT to better resolve the BH and BS coercivity peaks. pARM spectra collected from lake sediment samples reveal one strong coercivity peak at approximately 30 mT in the 0-100 mT range. A second higher coercivity peak at about 80 mT is weakly evident in some samples. Alternating field demagnetization in 104 steps of an ARM applied to one sample also suggests the presence of two coercivity peaks but at 20 and 40 mT. First order reversal curves (FORCs) measured at the Institute for Rock Magnetism show that the magnetic grains behave as non-interacting single domain grains, implying that the magnetosome chains are relatively far apart and probably still intact. The FORC diagrams also show a large coercivity peak at 30-40 mT and a weak peak at about 70 mT consistent with the pARM spectra and the IRM modeling. Although subsequent independent coercivity measurements support the IRM acquisition modeling, direct measurement of coercivity spectra by application of pARMs appears to be the best way to resolve the two different coercivity components of magnetosomes present in lake sediments and to determine their relative size.
DE: 1505 Biogenic magnetic minerals
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting