The Magnetic View on the Marine Paleoenvironment
593
added to gather additional information, e.g., on
diagenetic precipitation of greigite (Roberts and
Turner 1993; Roberts 1995).A typical trait of such
magnetic analyses is their regional character. The
specific significance of rock magnetic proxy parameters has to be carefully evaluated and calibrated
for each individual depositional setting.
Example 3: Diagenetic Dissolution o/Biogenic
Magnetite in a Reducing Marine Environment
Early diagenetic processes in marine sediments are
primarily driven by microbially mediated degradation of organic matter. Oxidation of organic compounds provides energy for the microorganisms to
maintain their metabolism. Froehlich et al. (1979)
identified a systematic depth succession ofterminal electron acceptors in the sediment column from
oxygen in the top layer to nitrate, Mn(lV)-oxides,
Fe(III)-oxides and sulphate at depth. This sequence
of oxidants reflects the accessible amount of energy per mole of organic carbon. The depth extent
of the early diagenetic zonation critically depends
on organic matter supply, scdimcntation rate and
the availability of reactants (Berner 1981).
It is well known that diagenetic processes also
affect thc magnctic mineral assemblage. In reducing environmcnts cven well crystallized ferrimagnetic oxides may be unstable, e.g., dissolution of
magnetite and/or its transformation to iron sulphides
is a common phenomenon in sub oxic to anoxic
sediments (Canfield and Berner 1987; Karlin 1990).
As a result, systematic changes in magnetic properties are observed (Karlin and Levi 1983). The
present cxample is to demonstratc the potential of
high-resolution rock magnetic analyses to contribute proxy parameters for geoehemically induced
alterations in near-surface layers of organic-rich
sedimentary deposits.
Reducing conditions in the sediment column are
widespread, where an intense primary biologic productivity in the surface waters supplies high fluxes
of organic material to the sea floor. A representative area for such a depositional regime is the
Benguela upwelling system in the eastern South
Atlantic. In this region, high abundances of living
magnetotactic bacteria have been observed in the
uppermost sedimentary layers (Petermann and Bleil
1993). The magnetite crystals synthesized intracellularly by these bacteria are preserved after
decomposition oftheir organic constituents (Petersen et al. 1986). They have been identified as the
by far dominant carrier of thc magnetic signal in
recent sediments at the upper continental slope off
southwest Africa, also because the concentration
of lithogenic, terrigenous ferrimagnetic mineral
components is relatively low compared to other
hemipelagic environments. Within a few centimeters sub-bottom an unusually sharp decrease in
magnetic susceptibility is observed in numerous
cores from this area (Fig. 13a).
A high-rcsolution rock magnetic study of an
18 cm long sediment sequence, which was recovered with a multicorer at 23°13'S /13°01 'E from
about 600 m water depth (core GeoB 1713-5) in
the center of the Namibian upwelling cell, aimed
at quantifying in full detail this conspicuous feature.
The parameters determined provide information
about the concentration of ferrimagnetic minerals
(X, O'ip O'ap O's), their grain-size (O'ar/O'ip O'rs/O's,
Ber /Bc) and also contributions of non-ferromagnetic compounds of the sedimentary matrix
(Knf/K). The objectivc of supplementary electron
microscopy was to identify the magnetic minerals
on the basis of their morphology and grain-size as
well as corrosion phenomena indicating potential
dissolution effects.
Both concentration dependent parameters, the
isothermal remanent magnetization O'ip (acquired
in a 0.1 T direct field) and the anhysteretic remanent magnetization O'ar (0.1 T alternating field,
0.04 mT constant biasing field) convincingly tcstify
that the drastic drop in specific susceptibility X
results from a diminishing content of ferrimagnetic
components in the uppermost centimeters of corc
GeoB 1713-5 (Fig. 13b, c). In the narrow depth
interval between about 3 and 7 cm contributions of
the paramagnetic sediment matrix to the susceptibility signal rise from approximately 20 to 80%
(Fig. 13e) at the expense of the ferrimagnetic
mineral fraction. Concurrently, the grain-size sensitive parameter O'ar/O'ir (Fig. 13d) reflccts a downward coarscning offerrimagnetic grains from primarily single-domain particles in the top centimeters
to multi-domain particles in deeper strata. High
O'ar /O'ir ratios in the upper 6 cm and particularly
593
added to gather additional information, e.g., on
diagenetic precipitation of greigite (Roberts and
Turner 1993; Roberts 1995).A typical trait of such
magnetic analyses is their regional character. The
specific significance of rock magnetic proxy parameters has to be carefully evaluated and calibrated
for each individual depositional setting.
Example 3: Diagenetic Dissolution o/Biogenic
Magnetite in a Reducing Marine Environment
Early diagenetic processes in marine sediments are
primarily driven by microbially mediated degradation of organic matter. Oxidation of organic compounds provides energy for the microorganisms to
maintain their metabolism. Froehlich et al. (1979)
identified a systematic depth succession ofterminal electron acceptors in the sediment column from
oxygen in the top layer to nitrate, Mn(lV)-oxides,
Fe(III)-oxides and sulphate at depth. This sequence
of oxidants reflects the accessible amount of energy per mole of organic carbon. The depth extent
of the early diagenetic zonation critically depends
on organic matter supply, scdimcntation rate and
the availability of reactants (Berner 1981).
It is well known that diagenetic processes also
affect thc magnctic mineral assemblage. In reducing environmcnts cven well crystallized ferrimagnetic oxides may be unstable, e.g., dissolution of
magnetite and/or its transformation to iron sulphides
is a common phenomenon in sub oxic to anoxic
sediments (Canfield and Berner 1987; Karlin 1990).
As a result, systematic changes in magnetic properties are observed (Karlin and Levi 1983). The
present cxample is to demonstratc the potential of
high-resolution rock magnetic analyses to contribute proxy parameters for geoehemically induced
alterations in near-surface layers of organic-rich
sedimentary deposits.
Reducing conditions in the sediment column are
widespread, where an intense primary biologic productivity in the surface waters supplies high fluxes
of organic material to the sea floor. A representative area for such a depositional regime is the
Benguela upwelling system in the eastern South
Atlantic. In this region, high abundances of living
magnetotactic bacteria have been observed in the
uppermost sedimentary layers (Petermann and Bleil
1993). The magnetite crystals synthesized intracellularly by these bacteria are preserved after
decomposition oftheir organic constituents (Petersen et al. 1986). They have been identified as the
by far dominant carrier of thc magnetic signal in
recent sediments at the upper continental slope off
southwest Africa, also because the concentration
of lithogenic, terrigenous ferrimagnetic mineral
components is relatively low compared to other
hemipelagic environments. Within a few centimeters sub-bottom an unusually sharp decrease in
magnetic susceptibility is observed in numerous
cores from this area (Fig. 13a).
A high-rcsolution rock magnetic study of an
18 cm long sediment sequence, which was recovered with a multicorer at 23°13'S /13°01 'E from
about 600 m water depth (core GeoB 1713-5) in
the center of the Namibian upwelling cell, aimed
at quantifying in full detail this conspicuous feature.
The parameters determined provide information
about the concentration of ferrimagnetic minerals
(X, O'ip O'ap O's), their grain-size (O'ar/O'ip O'rs/O's,
Ber /Bc) and also contributions of non-ferromagnetic compounds of the sedimentary matrix
(Knf/K). The objectivc of supplementary electron
microscopy was to identify the magnetic minerals
on the basis of their morphology and grain-size as
well as corrosion phenomena indicating potential
dissolution effects.
Both concentration dependent parameters, the
isothermal remanent magnetization O'ip (acquired
in a 0.1 T direct field) and the anhysteretic remanent magnetization O'ar (0.1 T alternating field,
0.04 mT constant biasing field) convincingly tcstify
that the drastic drop in specific susceptibility X
results from a diminishing content of ferrimagnetic
components in the uppermost centimeters of corc
GeoB 1713-5 (Fig. 13b, c). In the narrow depth
interval between about 3 and 7 cm contributions of
the paramagnetic sediment matrix to the susceptibility signal rise from approximately 20 to 80%
(Fig. 13e) at the expense of the ferrimagnetic
mineral fraction. Concurrently, the grain-size sensitive parameter O'ar/O'ir (Fig. 13d) reflccts a downward coarscning offerrimagnetic grains from primarily single-domain particles in the top centimeters
to multi-domain particles in deeper strata. High
O'ar /O'ir ratios in the upper 6 cm and particularly