Post - Depositional Diagenesis and Chemical Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 69

remanence. We will however see that studies in the lit-
erature are evenly divided about whether the secondary
diagenetic remanence occurs so soon after deposition
that it can be considered 'syn-depositional' or much
later after deposition. Certainly, the organic matter dia-
genesis in marine and lake sediments is observed to
occur near to the top of the sediment column (in about
the top meter) so the reactions that create greigite
should occur soon after deposition. In various papers
(Roberts & Weaver 2005; Rowan et al . 2009 ), Roberts
provides evidence of much later stage greigite magneti-
zations including SEM observations of greigite grow-
ing on pyrite (Roberts et al . 2005 ; Sagnotti et al . 2005 ;
Jiang et al . 2001a ), suggesting late - stage diagenesis
forming greigite. Mackinawite (Fe 1+ x S) is also one of the
precursor minerals formed in the reactions that eventu-
ally lead to pyrite. It is not ferromagnetic and is there-
fore not of interest to paleomagnetists.
One question that should be addressed is: why
doesn't all organic matter diagenesis lead to the fi nal
product pyrite, which is not ferromagnetic? Why is
greigite observed in so many anoxic sedimentary rocks
if it is only a precursor mineral on the way to pyrite?
Roberts & Weaver (2005) point out that the conditions
for the preservation of greigite in the sedimentary
record are abundant ferrous iron in solution, but
limited availability of sulfi de in solution. For this
reason, lake waters that have limited amounts of
sulfate tend to have greigite preserved during lake sedi-
ment diagenesis. Another reason that greigite is more
likely to be preserved in the sedimentary record is a
relatively fast sediment accumulation rate that will
push sediments through the diagenetic zone before the
reactions can run to completion and form pyrite from
the dissolved magnetite. For this reason greigite is often
observed in hemipelagic near-shore marine deposi-
tional environments. Plentiful greigite has also been
observed in paleomagnetic studies of both marine and
lake sediments (Table 6.1) so its preservation is not
unusual. When greigite was fi rst observed as an impor-
tant ferromagnetic mineral several decades ago, it was
thought that it would not be a signifi cant remanence
carrier because it was metastable and would oxidize
easily after sampling, during transport and storage of
the paleomagnetic samples. However, it has now been
observed in many paleomagnetic studies (see Table
6.1) and has even been observed in Cretaceous marine
sedimentary rocks (Reynolds et al . 1994 ).
Paleomagnetists also recognize that pyrrhotite is a
potentially important iron sulfi de remanence carrier in

sedimentary rocks. Like greigite, pyrrhotite shows a
decrease in magnetic intensity at temperatures between
300 and 350°C during thermal demagnetization. It
comes in monoclinic and hexagonal forms; monoclinic
pyrrhotite (Fe 7 S 8 ) is ferrimagnetic, due to missing Fe
cations in its crystal lattice, and strongly magnetic like
greigite and magnetite. Hexagonal pyrrhotite (Fe 9 S 10 )
is antiferromagnetic and is magnetic, like the mono-
clinic form, if there are missing cations in the crystal
lattice. Pyrrhotite is usually found in igneous and
metamorphic rocks. It can form during sulfate diagen-
esis in sediments, but it forms exceedingly slowly at
temperatures less than 180°C, making it less impor-
tant in sediment diagenesis (Horng & Roberts 2006).
If it is identifi ed in sediments, it is more likely to be
a depositional magnetic mineral (Horng & Roberts
2006 ).
The critical question for paleomagnetists is how
soon after deposition and over what time interval is the
CRM, carried by the secondary iron sulfi de minerals,
created by reduction diagenesis. Table 6.1 lists 29
studies from the literature that have identifi ed greigite
or secondary iron sulfi des as an important remanence
carrier. For 10 localities, the CRM carried by greigite or
iron sulfi des is observed to be 'syn-depositional' in age.
This observation is consistent with the observation
that sulfate and iron reduction diagenesis usually
occur in about the top meter of the sediment column.
For 10 localities evidence is however given of late-
stage diagenesis of the CRM-carrying greigite. In some
cases there is direct observation of greigite growing on
secondary pyrite grains, clear indication of post-early
diagenesis greigite formation. Two studies show evi-
dence for both early and late-stage greigite formation
(Table 6.1). Almost all these studies are of marine sedi-
ments; just two studies concern lake sediments and one
is of terrestrial gravels and clays. In the Rowan et al .
(2009) study of Pleistocene-Holocene marine sedi-
ments from northern California and Oman, some
actual time constraints are put on greigite formation.
The sediments are hemipelagic with moderate sedi-
ment accumulation rates of 7 cm/kyr. The age of the
greigite CRM is offset from the depositional age by tens
of thousands of years and the record is smoothed over
by tens-hundreds of thousand years.
This sad picture should however be contrasted with
the study of Kodama et al . (2010) of the Eocene marine
marls of the Arguis Formation in Spain which observed
precessional-scale (20 kyr) Milankovitch cycles in the
concentration of magnetic minerals that included both