Magnetization of Sediments and the Environment - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 103

Environmental processes and DRM
accuracy

CASE STUDY: FORMATION OF
BIOGENIC MAGNETIC MINERALS AT
LAKE ELY

Understanding the effects of environmental conditions
and changes on the magnetic minerals in a sediment
has important implications for the robustness and
accuracy of the paleomagnetic signal, the overriding
focus of this topic. The source and primacy of the mag-
netic minerals are an important control on the age of
a sediment's paleomagnetism. For instance, if the mag-
netic minerals are biogenic, they are secondary but
probably formed soon after deposition of the sediment.
If the magnetic minerals are clearly fl uvially derived
or from eolian sources, they are primary depositional
magnetic minerals. If astronomically driven global
climate changes are recorded by variations in magnetic
mineral concentration, these variations could leak into
the relative paleointensity records in marine sedi-
ments. This could happen because variations in mag-
netic mineral concentration, usually detected by ARM
or IRM, are used to normalize the relative paleointen-
sity record in sediments, assumed to be a record of
geomagnetic fi eld intensity changes.
Some workers have done detailed work to check
whether astronomical variations in the paleointensity
normalizer have affected relative paleointensity records
from marine sediments (e.g. Channell et al. 2004 ).
Such care is needed in all relative paleointensity studies
to ensure that geomagnetic fi eld variations are all that
is being observed. Another consideration for the accu-
racy of a sedimentary paleomagnetic record is whether
changes in paleoproductivity caused variations in
the amount of organics in a sediment, and therefore
affected the degree of reduction diagenesis and dissolu-
tion of magnetic minerals. Tarduno (1994) observed
this effect in Pacifi c Ocean sediments. Dissolution due
to redox fronts are seen with a 100 kyr periodicity, but
offset in time from the 100 kyr glacial-interglacial
intervals by 30-40 cm due to the lag between deposi-
tion and reduction diagenetic dissolution in the sedi-
ment column.
A fi nal example of how environmental changes could
affect the paleomagnetic signal comes from Chapter 4 on
inclination shallowing. It was shown that clay content
could affect the degree of compaction-caused inclina-
tion shallowing and possibly the post-compaction inten-
sity of the NRM. These changes in clay content, driven by
environmental changes, could therefore show up as
cyclicities in inclination or paleointensity not related to
geomagnetic fi eld variations.

There are many examples of biomineralization, but it
is beyond the scope of this topic to attempt a compre-
hensive review. For the purposes of this chapter we're
interested in a small subset of organisms that produce
minerals, magnetic minerals that are iron oxides and
sulfi des formed by bacteria in lake and marine sedi-
ments. These bacterially produced minerals have been
the focus of much study in the last several decades
since they were fi rst described (Blakemore 1975).
Recently, a multi-cellular prokaryote from a coastal
lagoon in the Yellow Sea region has been reported to
produce magnetic minerals (Zhou et al. 2011 ), so it ' s
apparent that the production of biogenic magnetic
minerals is not restricted to bacteria. Since the 1980s,
magnetoreception and biomineralization have been
reported for fi sh, pigeons, bees and other higher
animals (Kirschvink et al . 1985 ).
There are two types of bacterially produced mag-
netic minerals: those due to biologically organized min-
eralization (BOM) processes and those due to bacterially
induced mineralization (BIM) processes. BOM pro-
cesses are also called bacterially controlled minerali-
zation (BCM). In BOM or BCM bacteria, chains of
single-domain magnetite or greigite particles are
created to act as magneto-receptors. The chains of
small magnetic particles allow these micro-aerophilic
bacteria to sense the Earth's weak magnetic fi eld and
use it to swim up and down along the magnetic fi eld
lines. At intermediate latitudes where most of these
bacteria are studied, the Earth's magnetic fi eld lines are
steeply inclined to the horizontal. This ability allows
the bacteria to stay near the oxic-anoxic interface
(OAI). The OAI is typically in the uppermost part of the
sediment column, but can be in the water column of
lakes with anoxic bottom waters. The focus of the case
study presented here is the production, preservation
and environmental magnetic detection of magneto-
somes in the freshwater environment of Lake Ely.
BIM minerals are produced extra-cellularly when
bacteria use Fe for respiration. While the magnetosome
chains of SD grains generated by magnetotactic bacte-
ria are very stable magnetically, the magnetite pro-
duced in the BIM process are much smaller and are not
stable magnetically; they are superparamagnetic in
grain size (< 20 - 30 nm for magnetite). The magnetiza-
tion of superparamagnetic grains is caused by the