The Magnetic View on the Marine Paleoenvironment
585
transitions between sedimentation areas with fuzzy,
gradual or overlapping boundaries by a membership function for each sample.
Figure 7 illustrates the resulting cluster structure
of surface sediments in the equatorial Atlantic
based on their rock magnetic properties. By its
maximum membership function each sample is
attributed to one cluster.
The five clusters in the working area reveal a
distinct regionality. This strongly suggests a first
order interpretation in terms of sedimentation provinces. It has to be pointed out again that no additional information about the sample locations and
their sedimentation regimes has been included to
attain this result which therefore entirely reflects
the variability in rock magnetic sediment characteristics.
The rock magnetic properties of the five cluster centers are depicted in Figure 8. Samples from
the Congo Fan area exclusively belong to the cluster marked by circles. High amounts of small, down
to SP sized particles and relatively high coercivities
of (titano-)magnetites are its most specific features
compared to all other clusters. As described by Pak
et al. (1984), the Congo detritus consists mainly of
fine grain-sizes. Coarser fractions are settled in
continental catchments. There is also an important
fluvial influx of paramagnetic materials from the
African continent by the Congo River. In contrast
to this 'Congo Cluster' the sample collection from
the Amazon Fan and including the Ceara Rise area
(triangles) comprises the coarsest grain-sizes, but
again relatively high coercivities suggesting that an
elevated coercivity of magnetite is typical of continental sources and perhaps as well for fluvial transport. The 'Amazon Cluster' contains the highest
quantities of hematite. Similar, yet slightly lower
concentrations of hematite occur in the cluster
marked by diamonds. The regional extent of available data for this cluster coincides with the southern portion of the Saharan dust plume. Magnetite
is in the PSDIMD grain-size range and has the lowest coercivities encountered, indicating a large scale
transport of coarse particles by strong wind systems also carrying substantial amounts of hematite.
The cluster which mostly concentrates at the equatorial mid-Atlantic Ridge (squares) represents a
typical pelagic sedimentation environment above
the CCD without discernible eolian influx of continental material. The content of diamagnetic biogenic
carbonate is highest and almost no hematite found.
A fifth cluster (crosses) primarily covers the eastern part of the equatorialAtlantic. The characteristics of its cluster center fall approximately in
between those of the 'Saharan Dust Plume Cluster' and the 'MAR Cluster' implying a pelagic sedimentation with some terrigenous components
which could originate from theAfrican Sahel Zone
and are transported by less intense NE trade winds.
The example demonstrates that rock magnetic
investigations combined with cluster analysis are
an effective tool for empirical classification and
identification of marine sedimentation provinces. As
illustrated by the samples from the northeastern
Brazilian continental margin, sediments collected in
a relatively small area may belong to quite different clusters which have their principal occurrence
in distant regions. It should generally be more
appropriate, therefore, to name clusters according
to their major magnetic characteristics instead of
using geographic terms.
Example 2: The Analytical Concept of 'Partial
Susceptibilities' Applied to a Complex Late
Quaternary Magnetic Recordfrom the Western
Equatorial Atlantic
Most rock magnetic analyses of marine sedimentary deposits are based only on five or six
different bulk sample measurements, each of which
can be performed in a few minutes with standard
magnetic laboratory equipments (Maher 1986;
1988). Combined data of high- and low-frequency
susceptibility (Khf' K), anhysteretic and isothermal
remanent magnetization (Mal> M ir ) of corresponding peak fields (in the present case 0.1 T), as well
as saturation and back field isothermal remanent
magnetization (Msil> M-O.3T) enable a fairly detailed
rock magnetic characterization.
These properties represent concentration measures of partly differing, but overlapping magnetic
mineral and grain-size fractions. They are more or
less modulated by variations of the non-ferromagnetic, biogenic and/or terrigenous major components of the sediment matrix. The generally high
585
transitions between sedimentation areas with fuzzy,
gradual or overlapping boundaries by a membership function for each sample.
Figure 7 illustrates the resulting cluster structure
of surface sediments in the equatorial Atlantic
based on their rock magnetic properties. By its
maximum membership function each sample is
attributed to one cluster.
The five clusters in the working area reveal a
distinct regionality. This strongly suggests a first
order interpretation in terms of sedimentation provinces. It has to be pointed out again that no additional information about the sample locations and
their sedimentation regimes has been included to
attain this result which therefore entirely reflects
the variability in rock magnetic sediment characteristics.
The rock magnetic properties of the five cluster centers are depicted in Figure 8. Samples from
the Congo Fan area exclusively belong to the cluster marked by circles. High amounts of small, down
to SP sized particles and relatively high coercivities
of (titano-)magnetites are its most specific features
compared to all other clusters. As described by Pak
et al. (1984), the Congo detritus consists mainly of
fine grain-sizes. Coarser fractions are settled in
continental catchments. There is also an important
fluvial influx of paramagnetic materials from the
African continent by the Congo River. In contrast
to this 'Congo Cluster' the sample collection from
the Amazon Fan and including the Ceara Rise area
(triangles) comprises the coarsest grain-sizes, but
again relatively high coercivities suggesting that an
elevated coercivity of magnetite is typical of continental sources and perhaps as well for fluvial transport. The 'Amazon Cluster' contains the highest
quantities of hematite. Similar, yet slightly lower
concentrations of hematite occur in the cluster
marked by diamonds. The regional extent of available data for this cluster coincides with the southern portion of the Saharan dust plume. Magnetite
is in the PSDIMD grain-size range and has the lowest coercivities encountered, indicating a large scale
transport of coarse particles by strong wind systems also carrying substantial amounts of hematite.
The cluster which mostly concentrates at the equatorial mid-Atlantic Ridge (squares) represents a
typical pelagic sedimentation environment above
the CCD without discernible eolian influx of continental material. The content of diamagnetic biogenic
carbonate is highest and almost no hematite found.
A fifth cluster (crosses) primarily covers the eastern part of the equatorialAtlantic. The characteristics of its cluster center fall approximately in
between those of the 'Saharan Dust Plume Cluster' and the 'MAR Cluster' implying a pelagic sedimentation with some terrigenous components
which could originate from theAfrican Sahel Zone
and are transported by less intense NE trade winds.
The example demonstrates that rock magnetic
investigations combined with cluster analysis are
an effective tool for empirical classification and
identification of marine sedimentation provinces. As
illustrated by the samples from the northeastern
Brazilian continental margin, sediments collected in
a relatively small area may belong to quite different clusters which have their principal occurrence
in distant regions. It should generally be more
appropriate, therefore, to name clusters according
to their major magnetic characteristics instead of
using geographic terms.
Example 2: The Analytical Concept of 'Partial
Susceptibilities' Applied to a Complex Late
Quaternary Magnetic Recordfrom the Western
Equatorial Atlantic
Most rock magnetic analyses of marine sedimentary deposits are based only on five or six
different bulk sample measurements, each of which
can be performed in a few minutes with standard
magnetic laboratory equipments (Maher 1986;
1988). Combined data of high- and low-frequency
susceptibility (Khf' K), anhysteretic and isothermal
remanent magnetization (Mal> M ir ) of corresponding peak fields (in the present case 0.1 T), as well
as saturation and back field isothermal remanent
magnetization (Msil> M-O.3T) enable a fairly detailed
rock magnetic characterization.
These properties represent concentration measures of partly differing, but overlapping magnetic
mineral and grain-size fractions. They are more or
less modulated by variations of the non-ferromagnetic, biogenic and/or terrigenous major components of the sediment matrix. The generally high