Magnetization of Sediments and the Environment - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 107

0.40

Water filtered, Oct 2000

0.35

Water filtered, Apr 2002

0.30

Water filtered, Sep 2002

0.25

Gravity core sediments

magnetosomes

0.20

Trap, retrieved in Mar 01

0.15

Trap, retrieved in Sep 01

0.10

Inorganic
magnetites

Trap, retrieved in Apr 02

0.05

Trap, retrieved in Sep 02

0.00

0.2 0.4 0.5 0.6

0.3

R af

Fig. 8.8 ARM/SIRM versus R af for Lake Ely sediments, sediment trap material and material fi ltered from the water column.
All the results show that this measurement is a good test for detecting magnetosomes made by magnetotactic bacteria. Figure
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.

of the ARM/SIRM ratio of a sample versus a quantity
denoted R af , the crossover between a sample's IRM
acquisition curve and the alternating fi eld demagneti-
zation of its SIRM. It was initially proposed by Cisowski
(1981) as a test for magnetic interactions between
particles; values less than 0.5 indicated magnetic
interactions. However, the magnetosome chains act
like big non-interacting single-domain particles and R af
values greater than 0.5 are diagnostic of magneto-
some chains, as are ARM/SIRM ratios greater than c.
0.15. Samples containing magnetosome chains there-
fore plot in the upper right-hand part of the ARM/
SIRM versus R af plot (Fig. 8.8) and detrital magnetite
particles in the lower left. This test worked beautifully
for the Lake Ely samples, showing magnetosome chains
for the material fi ltered from the water column and the
lake sediment collected by the gravity cores. It is inter-
esting that the material caught in the sediment trap
plots between the magnetosome chain region and the
region for inorganic magnetite particles, based on the
data of Moskowitz et al . (1993) .
All the work discussed up to this point was based on
measurements of material fi ltered from the water
column, collected in a sediment trap at the bottom of

the lake and in gravity cores from the top 30 cm of the
sediment column. Since it was demonstrated that mag-
netosomes dominated the magnetism of the lake sedi-
ments and that the concentration of magnetosomes
appeared to record historic variations in rainfall, it
was important to determine whether the magneto-
some concentration variations would be preserved in
the sedimentary record over longer time periods than
just 100 - 200 years.
To answer that question, Robert Moeller collected a
125 cm long Livingstone piston core through the ice in
the winter of 2005. Standard environmental magnetic
measurements were made on detailed sampling of the
core. SIRM and χ were measured every centimeter and
ARM and the S - ratio every 3 cm. In addition to these
magnetic measurements, the concentration of organic
material was quantifi ed by loss - on - ignition (LOI) meas-
urements downcore. The main fi nding of this work was
that the magnetic concentration, presumably of the
magnetosomes in the lake's sediment, decreased sig-
nifi cantly to a depth of 75 cm in the core (Fig. 8.9).
S -ratio measurements show a greater contribution
from high-coercivity antiferromagnetic minerals as
the overall magnetic mineral concentration decreases