Geology Reference
In-Depth Information
Concentration of dissolved
pore water species
Authigenic iron-bearing
mineral products
Hematite, goethite
Processes
Aerobic respiration
Denitrification
O
2
−
NO
3
Oxic
Manganese
reduction
Mn
2+
Suboxic
Iron
reduction
FeOOH
Fe
2+
2−
SO
4
Sulphate
reduction
Pyrite
(±mackinawite, greigite)
HS
−
Sulphidic
CH
4
Anoxic
Fe
2+
Non-
sulphidic
Methanogenesis
Siderite
Fig. 6.1
Reduction diagenesis in recent marine sediments from Roberts & Weaver (2005). Reprinted from
Earth and Planetary
Science Letters
, 231, AP Roberts and R Weaver, Multiple mechanisms of remagnetization involving sedimentary greigite
(Fe3S4), 263-277, copyright 2005, with permission from Elsevier.
order) of aerobic oxidation, followed by nitrate reduc-
tion, manganese reduction, Fe oxide reduction (in
which magnetite is dissolved) and fi nally sulfate reduc-
tion (Fig. 6.1; Froelich
et al
. 1979 ; Berner 1981 ; Kasten
et al
. 2003 ).
The importance of Fe oxide reduction to the dissolu-
tion of magnetite in marine sediments was fi rst recog-
nized in pioneering studies by Karlin & Levi (1983) and
Karlin (1990), who observed a decrease in the mag-
netic intensity in hemipelagic marine sediments from
the Gulf of California and from coastal Oregon in the
top meter of the sediment column. Iron oxide reduc-
tion puts Fe
2+
into solution that reacts with the sulfi de
already in solution due to sulfate diagenesis, to form a
sequence of iron sulfi des with the fi nal product being
pyrite. Pyrite does not carry a remanence (it is para-
magnetic) so it is not important in paleomagnetic
studies; however, one of its precursor minerals
(greigite) is ferrimagnetic and the Fe sulfi de analogue
of magnetite. Its formula is Fe
3
S
4
, similar to magnetite
(Fe
3
O
4
), with sulfur taking the place of oxygen in the
crystal lattice. It is an inverse spinel and cubo-
octohedral like magnetite and strongly magnetic like
magnetite, having intrinsic magnetizations that are
about 25% of that of magnetites. Because of its strong
magnetization, greigite has coercivities similar to mag-
netite. This makes greigite diffi cult to distinguish from
magnetite in alternating fi eld demagnetization studies
of marine and lake sediments. The only distinctive way
of distinguishing greigite from magnetite is by heating.
Greigite, like other magnetic Fe sulfi des (e.g. pyrrho-
tite), decreases in intensity at temperatures near to
300-350°C while magnetite will remain strongly mag-
netic up to its Curie temperature of 580°C.
When paleomagnetists notice a decrease in magneti-
zation occurring at
c.
300 - 350 ° C during thermal
demagnetization, their fi rst thought is that magnetic
iron sulfi des are important carriers of magnetic rema-
nence in a sedimentary rock. This observation immedi-
ately raises the specter of a secondary diagenetic
Search WWH ::
Custom Search