Magnetization of Sediments and the Environment - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 106

Fig. 8.7 ARM of material fi ltered from Lake Ely's water column at the oxic-anoxic interface (left) and TEM images of the
fi ltered material showing magnetosomes created by magnetotactic bacteria. TEM images from Kim et al . (2005) . B - Y Kim, KP
Kodama and RE Moeller, Bacterial magnetite produced in water column dominates lake sediment mineral magnetism: Lake
Ely, USA, Geophysical Journal International , 163, 26-37, 2005, Blackwell Publishing. (See Colour Plate 17)

a zero magnetic fi eld. Magnetite goes through a transi-
tion in crystal structure at about 110 K that affects its
magnetism. The magnetization of a sample decreases
sharply when it is warmed up through the 110 K mag-
netic transition, called the Verwey transition. If it has
been cooled in a strong magnetic fi eld it drops more
than if it was cooled in a zero strength fi eld. The ratio
of magnetization decrease is 2 or greater for magneto-
some chains from samples of magnetotactic bacteria
cultured in the laboratory. The test was much less suc-
cessful for the Lake Ely sediments (Kim et al. 2005 ). The
dark organic-rich layers from the lake sediment had
weak Verwey transitions at 110 K while the light-
colored silt layers saw virtually no Verwey transitions.
The Moskowitz et al . (1993) ratios of fi eld - cooled to
zero - fi eld-cooled magnetization decreases were only
about 1.5, suggesting that magnetosome chains were

not present in the samples despite TEM observations of
the chains.
Unmixing of the low-temperature magnetic behav-
ior using numerical modeling (Carter-Stiglitz et al.
2001) however suggests that there are about twice as
many magnetosome chains in the dark organic-rich
layers as in the silt layers ( c. 60% compared to c. 30%).
The failure of the test is primarily due to the additional
magnetic minerals, besides magnetite, in the lake sedi-
ments (antiferromagnetics from the watershed and
greigite produced biogenically).
Moskowitz et al . (1993) also produced a diagnostic
test for magnetosomes that could be conducted at
room temperature, thus not requiring the specialized
equipment needed to cool samples to nearly absolute
zero and simultaneously measure their magnetization
while a strong fi eld is applied. This test involves the plot