Sedimentary records from the North Atlantic can be
correlated with a very good approximation to records
obtained in ice cores, by using anchor points common to
both types of records, such as levels of volcanic ash or
melting events (Bond et al. 1993). However, the correlation
of ice cores with sedimentary records or sedimentary records
between themselves in very diverse parts of the world, is a
far more complex problem: a direct correlation, based on the
recognition of climate signals may not take into account the
possibility of a time difference between the occurrence of a
particular climatic event in two distant regions or by to
different media. And yet an accurate calculation of these
phase shifts (leads and lags) is of central concern to paleoclimatologists, since it is essential to understand the mechanisms of global climate change, and to the highlight of
chains of causality and the spatio-temporal spread of a climate event.
At the scale of a basin, the magnetic susceptibility measured continuously in sediment cores, directly on-board
research ocean vessels, usually allows the correlation of the
different cores with a resolution of about one centimeter,
which is sufficient for most studies. This physical measure
allows splicing of records obtained from different cores,
taken from the same site or in neighboring sites, to join
together the different sections of cores obtained by the
hydraulic piston corer, aboard the Joides Resolution within
the ODP international program. Magnetic susceptibility
reflects the ability of the sediment to acquire a magnetization
induced by a weak magnetic field. Measurement of magnetic
susceptibility is commonly used because it has the advantage
of being non-destructive, as the induced magnetization
measured disappears as soon as the imposed field is switched
off. It varies depending on the magnetic content of the
sediment, and particularly on the concentration of magnetic
particles, their nature and size. As changes in these physical
and chemical parameters are generally highly dependent on
the paleo-environmental context, these are the same within
the same basin or water mass, and so the magnetic susceptibility can be used as a local (or regional) correlation tool.
Over long distances, oxygen isotopic ratios are the most
commonly used stratigraphic tool. However, these ratios also
include regional climate components, precisely those that
researchers are trying to fix in time relative to each other.
Correlating these ratios between distant locations could
therefore mask the phase shifts which may really exist and
that climatic studies attempt to quantify. Independent time
constraints of climate variations with a global value are
required for this exercise.
Initially proposed a decade ago, a new method, based on
relative variations in the intensity of the geomagnetic field in
the past recorded in sediments, is now being recognized as
capable of revealing phase shifts over thousands of years at
most, in climate records obtained from sites geographically
very distant from each other. In this method, the relative
changes in the paleointensity curve obtained from a specific
sedimentary core is compared with a reference curve
depicting the relative variations of the geomagnetic dipole
field. The latter is, in fact, the only component of the field
which varies synchronously across the globe. After synchronization of two magnetic profiles with the reference
curve, the phase shifts and/or synchronicity of the two
paleoclimate records from these two distant locations can be
evaluated.
This method needs to be applied with some caution.
Firstly, everywhere in the world, the local field is the
superposition of the dipole field on multipolar components,
which are much more variable in space and have shorter
time constants. To plot the dipole field variations curve, the
authors compiled records obtained from different parts of the
world. These compilations eliminate the non-dipolar components which are averaged out in space. Compilations of
relative paleointensity variations in the field for the last
800,000 years (Sint 800) (Guyodo and Valet 1999) and the
last 1.5 Ma (PISO-1500) (Channell et al. 2009) show synchronization of characteristic events globally and the widespread attenuation of the non-dipolar field. However, only
characteristics with time constants of the order of 10
4
–
10
5 years are apparent on these two compilations, as some of
the individual records have been obtained from sediment
with accumulation rates of a few centimeters per thousand
years, insufficient to record short-term characteristics and
rapid changes.
These rapid characteristics are, however, clear in other
compilations characterized by a higher sediment accumulation rate. Initially proposed for the North Atlantic Ocean, the
NAPIS-75 compilation (North Atlantic Paleointensity Stack
for the last 75,000 years) (Laj et al. 2000), then SAPIS (South
Atlantic Paleointensity Stack) (Stoner et al. 2002) and finally
GLOPIS-75 (Global Paleointensity Stack) (Laj et al. 2004)
have shown that rapid components of the variations in the
intensity of the geomagnetic dipole field can be recognized
globally in sedimentary records (Fig. 7.8). The outer limit of
temporal resolution appears to be of the order of 400 years,
which is the time constants of the dipolar field. GLOPIS-75 is
currently the reference for the last 75,000 years because it is
precisely placed on the accurate ice age model (Cf. Section “A Correlation Between Sediment and Polar Ice”). This
new age scale for Greenland ice was developed by counting
annual levels (Greenland Ice Core Chronology or GICC05)
(Andersen et al. 2006; Svensson et al. 2006). For earlier
periods, up to 1.5 Ma, PISO-1500 is used.
Although, in principle, the method is simple, its implementation is far from it. Firstly, a linear relationship between
the intensity of the geomagnetic field existing at the time of
deposition and the magnetization of the sediment only exists if
a single magnetic mineral carries the magnetization and the
110
C. Laj et al.
correlated with a very good approximation to records
obtained in ice cores, by using anchor points common to
both types of records, such as levels of volcanic ash or
melting events (Bond et al. 1993). However, the correlation
of ice cores with sedimentary records or sedimentary records
between themselves in very diverse parts of the world, is a
far more complex problem: a direct correlation, based on the
recognition of climate signals may not take into account the
possibility of a time difference between the occurrence of a
particular climatic event in two distant regions or by to
different media. And yet an accurate calculation of these
phase shifts (leads and lags) is of central concern to paleoclimatologists, since it is essential to understand the mechanisms of global climate change, and to the highlight of
chains of causality and the spatio-temporal spread of a climate event.
At the scale of a basin, the magnetic susceptibility measured continuously in sediment cores, directly on-board
research ocean vessels, usually allows the correlation of the
different cores with a resolution of about one centimeter,
which is sufficient for most studies. This physical measure
allows splicing of records obtained from different cores,
taken from the same site or in neighboring sites, to join
together the different sections of cores obtained by the
hydraulic piston corer, aboard the Joides Resolution within
the ODP international program. Magnetic susceptibility
reflects the ability of the sediment to acquire a magnetization
induced by a weak magnetic field. Measurement of magnetic
susceptibility is commonly used because it has the advantage
of being non-destructive, as the induced magnetization
measured disappears as soon as the imposed field is switched
off. It varies depending on the magnetic content of the
sediment, and particularly on the concentration of magnetic
particles, their nature and size. As changes in these physical
and chemical parameters are generally highly dependent on
the paleo-environmental context, these are the same within
the same basin or water mass, and so the magnetic susceptibility can be used as a local (or regional) correlation tool.
Over long distances, oxygen isotopic ratios are the most
commonly used stratigraphic tool. However, these ratios also
include regional climate components, precisely those that
researchers are trying to fix in time relative to each other.
Correlating these ratios between distant locations could
therefore mask the phase shifts which may really exist and
that climatic studies attempt to quantify. Independent time
constraints of climate variations with a global value are
required for this exercise.
Initially proposed a decade ago, a new method, based on
relative variations in the intensity of the geomagnetic field in
the past recorded in sediments, is now being recognized as
capable of revealing phase shifts over thousands of years at
most, in climate records obtained from sites geographically
very distant from each other. In this method, the relative
changes in the paleointensity curve obtained from a specific
sedimentary core is compared with a reference curve
depicting the relative variations of the geomagnetic dipole
field. The latter is, in fact, the only component of the field
which varies synchronously across the globe. After synchronization of two magnetic profiles with the reference
curve, the phase shifts and/or synchronicity of the two
paleoclimate records from these two distant locations can be
evaluated.
This method needs to be applied with some caution.
Firstly, everywhere in the world, the local field is the
superposition of the dipole field on multipolar components,
which are much more variable in space and have shorter
time constants. To plot the dipole field variations curve, the
authors compiled records obtained from different parts of the
world. These compilations eliminate the non-dipolar components which are averaged out in space. Compilations of
relative paleointensity variations in the field for the last
800,000 years (Sint 800) (Guyodo and Valet 1999) and the
last 1.5 Ma (PISO-1500) (Channell et al. 2009) show synchronization of characteristic events globally and the widespread attenuation of the non-dipolar field. However, only
characteristics with time constants of the order of 10
4
–
10
5 years are apparent on these two compilations, as some of
the individual records have been obtained from sediment
with accumulation rates of a few centimeters per thousand
years, insufficient to record short-term characteristics and
rapid changes.
These rapid characteristics are, however, clear in other
compilations characterized by a higher sediment accumulation rate. Initially proposed for the North Atlantic Ocean, the
NAPIS-75 compilation (North Atlantic Paleointensity Stack
for the last 75,000 years) (Laj et al. 2000), then SAPIS (South
Atlantic Paleointensity Stack) (Stoner et al. 2002) and finally
GLOPIS-75 (Global Paleointensity Stack) (Laj et al. 2004)
have shown that rapid components of the variations in the
intensity of the geomagnetic dipole field can be recognized
globally in sedimentary records (Fig. 7.8). The outer limit of
temporal resolution appears to be of the order of 400 years,
which is the time constants of the dipolar field. GLOPIS-75 is
currently the reference for the last 75,000 years because it is
precisely placed on the accurate ice age model (Cf. Section “A Correlation Between Sediment and Polar Ice”). This
new age scale for Greenland ice was developed by counting
annual levels (Greenland Ice Core Chronology or GICC05)
(Andersen et al. 2006; Svensson et al. 2006). For earlier
periods, up to 1.5 Ma, PISO-1500 is used.
Although, in principle, the method is simple, its implementation is far from it. Firstly, a linear relationship between
the intensity of the geomagnetic field existing at the time of
deposition and the magnetization of the sediment only exists if
a single magnetic mineral carries the magnetization and the
110
C. Laj et al.