Magnetization of Sediments and the Environment - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 111

genesis dissolution allows the observation of the BS
and BH components at greater depths. This obser-
vation also suggests that different magnetosome
morphologies are resistant to diagenesis by different
degrees.
Chen et al . (2007) studied magnetosomes from labo-
ratory cultures and from natural lake sediment with
FORC diagrams to better determine the signature of
magnetotactic bacteria. Subsequently, Egli et al . (2010)
studied a sample from shallow in the sediment column
of Lake Ely (6 cm depth in the piston core). The result-
ing FORC diagram shows no evidence of interactions
between the particles. The FORC diagram shows a very
tight distribution around the H c - axis (where H c is coer-
civity) and the magnetosome chains, demonstrated to
be present from the earlier TEM observations of mag-
netic extracts, act as if they were non-interacting
single-domain particles. The FORC diagram for a
sample deeper in the sediment column (26 cm depth)
shows a greater spread around the H c - axis, indicating
magnetic interaction between grains, possibly due to
the disruption of magnetosome chains as reduction
diagenesis proceeds.
Bob Kopp made ferromagnetic resonance (FMR)
absorption spectra measurements on samples from the
Lake Ely piston core. Since FMR measures the effective
magnetic fi eld in a sample, including magnetic particle
anisotropy and magnetic interactions, it is perfect for
detecting the presence of magnetosome chains pro-
duced by magnetotactic bacteria. Kopp et al . (2006)
made FMR measurements of laboratory-grown mag-
netotactic bacteria, but measurements of natural lake
sediment samples with demonstrated magnetosomes
would be a good demonstration of the applicability
of the technique. The FMR absorption spectra from
samples collected at different depths from the Lake Ely
piston core (Fig. 8.12) show the presence of magneto-
somes, particularly in the top of the core. The ampli-
tude of the spectra decreases downcore as reduction
diagenesis affects the samples to a greater degree. The
effective fi eld ( g eff ) has a value of c. 2.0, down slightly
from that of pure magnetite (2.12); the anisotropy
factor ( A factor) degrades downcore. These spectra are
consistent with the known presence of magnetosomes
in the samples.
Kopp et al . (2007) demonstrated the presence
of magnetosomes in the sediments recording the
Paleocene-Eocene thermal maximum collected from a
core drilled in coastal plain sediments from southern

New Jersey. For this work they showed that in a plot
of FMR parameters Δ B FWHM (an empirical parameter
measured from the FMR absorption factor, defi ned as
the fi eld B at full - width at half - maximum amplitude
of the spectrum) versus A , magnetosome chains had
lower values of these parameters and detrital magnet-
ite had higher values. When the Lake Ely data are
plotted on a Δ B FWHM versus A plot (Fig. 8.13), the data
fall close to the range of likely magnetofossils. Finally,
modeling of the FMR spectra by Kopp shows that the
spectra are consistent with the two different coercivity
components for magnetosomes, BH and BS, originally
observed by Egli (2004) .
The environmental magnetic study of Lake Ely
presented in detail here shows that environmental
magnetic parameters can detect the presence of mag-
netosomes generated by magnetotactic bacteria, even
the two different coercivity components (BH and BS)
initially observed by Egli (2004). Modeling of IRM
and ARM acquisition data is particularly sensitive to
detection of magnetosomes. FMR absorption spectra
are an entirely different way to detect magnetosome
chains in natural lake sediment containing magneto-
somes. Reduction diagenesis removes the magneto-
some record almost completely by about 1200 years
after deposition and suggests that reduction diagenesis
can occur relatively soon after the deposition of lake
sediments. The diagenesis at Lake Ely occurs within
about 1 m of the sediment-water interface in the
topmost part of the sediment column.
For Lake Ely, lake sediments dominated by magne-
totactic bacteria magnetosomes could provide a
record of paleorainfall variations, at least until the
magnetosomes are completely destroyed by reduction
diagenesis. In the topmost post-settlement sediments,
the changes in the concentration of the magneto-
somes follows the historic record of local rainfall. The
ARM intensity of sediment trap material collected
once a year in the fall over a three year period shows
a good correlation with the rainfall that occurs
after the ice leaves the lake in the spring, further
bolstering the potential paleoclimate usefulness of
magnetosome - dominated sediment (Fig. 8.14 ). The
magnetic mineral concentration variations may pos-
sibly be caused by the amount of nutrients washed into
the lake by different amounts of rainfall.
The environmental magnetic study of Lake Ely
shows one way that magnetic mineral measurements
can provide powerful indicators of past environmental