636
Bleil and von Dobeneck
from remanent magnetization records of both sediments and volcanics. The latest comprehensive reexamination of the relevant literature by Nowaczyk
et al. (1994) once again emphasized the lack in precise dating of many rock units precluding to verify
the global synchroneity of geomagnetic events found
in different parts ofthe world (e.g., Jacobs 1984;
Champion et al. 1988; Hanna and Verosub 1989;
L0vlie 1989). Even more puzzling is the fact that
little or no evidence for their recording could be
detected in many sediment series even with elaborate paleomagnetic investigations. Apparently these
features are too short-lived for certain types of
marine sedimentary deposits to acquire respective
stable permanent remanence components.
Paleomagnetic and rock magnetic studies of
marine sediments from the polar North Atlantic and
the Arctic Ocean between about 70° and 90 0 N
(e.g., L0Vlie et al. 1986; Bleil 1987; Thouveny 1988;
Bleil and Gard 1989; Nowaczyk 1991; Nowaczyk
and Baumann 1992; Nowaczyk et al. 1994; Frederichs 1995) have recently provided several new
data sets. While they confirm the existence of
multiple late Quaternary geomagnetic events, the
age control is mostly of limited resolution, because
biostratigraphic and oxygen isotope analyses typically face severe problems at these high latitudes.
Neither the number of Brunhes Chron geomagnetic events*) nor their ages and durations are
hence adequately known at present. It remains an
open question whether they are global or only
regional features. Due to these restrictions they
have practically not yet been used as references in
high-resolution late Quaternary magnetostratigraphies. Likewise, their potential implications for
the temporal stability ofthe Earth's main field and
processes in the Earth's core still await to be fully
explored.
Another approach to achieve more insights to
the generation and driving forces of the Earth's
*) As there is insufficient evidence in many cases to
discriminate between field excursions and true short
reversal episodes (e.g., Merrill and McFadden 1994),
the more general tenn 'geomagnetic event' is used in
this paper. In the equatorial latitude late Quaternary
sediments of the Ceara Rise a recording of geomagnetic
events is identified, where stable remanent declinations
depart by 90° or more from north.
magnetic field are reconstructions of its former
intensity variations. Theoretically and experimentally recognized techniques to determine absolute
paleo field intensities require detailed analyses
of the thermoremanent magnetization (TRM) of
magmatic rocks (Thellier and Thellier 1959). As an
alternative it has become common practice to use
the natural remanent magnetization intensity of
marine sediments normalized to the concentration
of magnetic minerals as a proxy for the past geomagnetic field strength (e.g., Kent and Opdyke
1977; Tauxe and Wu 1990; Tricetal. 1992; Yamazaki and Ioka 1994; Tauxe and Shackleton 1994;
Stoner et al. 1995; Lehman et al. 1996). After
moderate altemating field demagnetization to eliminate spurious overprints, ratios of the remaining
NRM and either susceptibility or anhysteretic
remanent magnetization (ARM) define so-called
relative paleointensities. Opdyke et al. (1973) first
noticed at the Brunhes / Matuyama boundary that
conspicuous minima of relative paleointensity
coincide with polarity transitions. This effect was
also demonstrated in laboratory experiments (L0Vlie
1976) and is best illustrated in an extended natural
record for the last about 4 million years acquired
from combined Ocean Drilling Program (ODP)
sites in the equatorial Pacific Ocean (Valet and
Meynadier 1993). The authors also tentatively correlated several remarkable intensity drops with
Quaternary geomagnetic events although none of
them had actually been identified in their magnetostratigraphy. In fact, prior to this paper Brunhes
geomagnetic events were only once reported from
marine sediments together with relative paleointensities (Nowaczyk and Antonow 1997;
Nowaczyk 1997).
A review of available data and their compilation
in a synthetic stack for the last 200.000 years (' Sint200' , Fig. 6) published by Guyodo and Valet (1996)
shows large paleointensity fluctuations but also an
extensive scatter between results from different
oceans. In dealing with sediment magnetization
intensities evidently some ofthe same intrinsic difficulties are encountered which hinder a simple
identification and interpretation of geomagnetic
events. The fundamental hypothesis in both cases
is a high fidelity recording of ancient geomagnetic
field characteristics in the remanent magnetiza-
Bleil and von Dobeneck
from remanent magnetization records of both sediments and volcanics. The latest comprehensive reexamination of the relevant literature by Nowaczyk
et al. (1994) once again emphasized the lack in precise dating of many rock units precluding to verify
the global synchroneity of geomagnetic events found
in different parts ofthe world (e.g., Jacobs 1984;
Champion et al. 1988; Hanna and Verosub 1989;
L0vlie 1989). Even more puzzling is the fact that
little or no evidence for their recording could be
detected in many sediment series even with elaborate paleomagnetic investigations. Apparently these
features are too short-lived for certain types of
marine sedimentary deposits to acquire respective
stable permanent remanence components.
Paleomagnetic and rock magnetic studies of
marine sediments from the polar North Atlantic and
the Arctic Ocean between about 70° and 90 0 N
(e.g., L0Vlie et al. 1986; Bleil 1987; Thouveny 1988;
Bleil and Gard 1989; Nowaczyk 1991; Nowaczyk
and Baumann 1992; Nowaczyk et al. 1994; Frederichs 1995) have recently provided several new
data sets. While they confirm the existence of
multiple late Quaternary geomagnetic events, the
age control is mostly of limited resolution, because
biostratigraphic and oxygen isotope analyses typically face severe problems at these high latitudes.
Neither the number of Brunhes Chron geomagnetic events*) nor their ages and durations are
hence adequately known at present. It remains an
open question whether they are global or only
regional features. Due to these restrictions they
have practically not yet been used as references in
high-resolution late Quaternary magnetostratigraphies. Likewise, their potential implications for
the temporal stability ofthe Earth's main field and
processes in the Earth's core still await to be fully
explored.
Another approach to achieve more insights to
the generation and driving forces of the Earth's
*) As there is insufficient evidence in many cases to
discriminate between field excursions and true short
reversal episodes (e.g., Merrill and McFadden 1994),
the more general tenn 'geomagnetic event' is used in
this paper. In the equatorial latitude late Quaternary
sediments of the Ceara Rise a recording of geomagnetic
events is identified, where stable remanent declinations
depart by 90° or more from north.
magnetic field are reconstructions of its former
intensity variations. Theoretically and experimentally recognized techniques to determine absolute
paleo field intensities require detailed analyses
of the thermoremanent magnetization (TRM) of
magmatic rocks (Thellier and Thellier 1959). As an
alternative it has become common practice to use
the natural remanent magnetization intensity of
marine sediments normalized to the concentration
of magnetic minerals as a proxy for the past geomagnetic field strength (e.g., Kent and Opdyke
1977; Tauxe and Wu 1990; Tricetal. 1992; Yamazaki and Ioka 1994; Tauxe and Shackleton 1994;
Stoner et al. 1995; Lehman et al. 1996). After
moderate altemating field demagnetization to eliminate spurious overprints, ratios of the remaining
NRM and either susceptibility or anhysteretic
remanent magnetization (ARM) define so-called
relative paleointensities. Opdyke et al. (1973) first
noticed at the Brunhes / Matuyama boundary that
conspicuous minima of relative paleointensity
coincide with polarity transitions. This effect was
also demonstrated in laboratory experiments (L0Vlie
1976) and is best illustrated in an extended natural
record for the last about 4 million years acquired
from combined Ocean Drilling Program (ODP)
sites in the equatorial Pacific Ocean (Valet and
Meynadier 1993). The authors also tentatively correlated several remarkable intensity drops with
Quaternary geomagnetic events although none of
them had actually been identified in their magnetostratigraphy. In fact, prior to this paper Brunhes
geomagnetic events were only once reported from
marine sediments together with relative paleointensities (Nowaczyk and Antonow 1997;
Nowaczyk 1997).
A review of available data and their compilation
in a synthetic stack for the last 200.000 years (' Sint200' , Fig. 6) published by Guyodo and Valet (1996)
shows large paleointensity fluctuations but also an
extensive scatter between results from different
oceans. In dealing with sediment magnetization
intensities evidently some ofthe same intrinsic difficulties are encountered which hinder a simple
identification and interpretation of geomagnetic
events. The fundamental hypothesis in both cases
is a high fidelity recording of ancient geomagnetic
field characteristics in the remanent magnetiza-