field. The field acts as a screen for cosmic radiation (consisting essentially of charged particles), and this reduction
causes a significant increase in the amount of cosmonuclides
(
14 C,
10 Be,
26 Al…) formed in the upper atmosphere through
the impact of cosmic radiation on the various particles air.
As
10 Be can be measured in polar ice cores, this opens up
new opportunities for ice-sediment correlation.
A Scale of Geomagnetic Instabilities?
The use of geomagnetic excursions as precise temporal tie points
could be particularly useful in at least two important areas of
Earth sciences. This is especially useful to understand the
operating mechanisms of the terrestrial dynamo which is the
origin of the field itself. Recent models, theoretical and numerical, of the Earth’s dynamo (Glatzmaier and Roberts 1995)
indeed, give accurate assessments of time constants, frequencies
and geometries of the transitional field during excursions. It is of
prime importance to characterize the role of the solid inner core
and the lower mantle in the mechanism of excursions and
reversals. It is also important in paleoclimatology studies, where
an independent chronology of climatic/environmental phenomena needs to be of higher resolution that the scale of
polarities to allow evaluation of the synchronicity and phase
shifts, either early or late, of climate events in different parts of
the globe (examples are described below).
Currently, the major obstacle to the widespread use of
this method is probably the difficulty of integrating sedimentary data and volcanic data into a single unified scale of
geomagnetic instabilities. The brevity of the excursions is
both an advantage (giving very precise temporal tie points),
and paradoxically, it also represents an obstacle to the creation of this scale. In fact, a specific excursion is not systematically recorded in all sequences, including those with a
medium to high sedimentation rate. Studies to develop a
high-resolution chronology of geomagnetic instabilities,
especially of excursions, are often based on accurate dating,
using the K/Ar and
40 Ar/
39 Ar methods, of as large a number
as possible of lava flows recording either an abnormal
direction, or a geomagnetic field with a very low intensity, or
both characteristics together. It is important to keep in mind
that
40 Ar/
39 Ar datings are obtained by reference to standards
whose ages were defined based on astro-chronological calibrations. So, several ages have been proposed for the same
standard (e.g. Fish Canyon Sanidine, commonly used for
dating in the Quaternary). It sometimes appears that none of
them provides good agreement between the
40 Ar/
39 Ar ages
and the glaciological or astronomical scales. Further work is
therefore necessary to “reconcile” these various approaches.
Today, at least seven geomagnetic excursions from the
Brunhes period have been inventoried in detail: the excursions of Mono Lake (34 ka), Laschamp (41 ka), Blake
(120 ka), Iceland Basin (188 ka) Pringle Falls (211 ka), Big
Lost (560–580 ka) and Stage 17 (670 ka) (Laj and Channell
2007). Other excursions from the same period are being
studied. Studies are also underway for earlier periods, such
as the Matuyama period during which at least eleven
excursions seem to have occurred. All of these excursions
act as specific temporal tie points, that greatly increase the
temporal resolution of the magnetostratigraphic scale.
Magnetostratigraphy Based on Variations
in the Intensity of the Geomagnetic Field
Introduction
Over recent years, the stratigraphy of climate records has
undergone a major change, particularly due to the discovery
of rapid and precise markers, both lithostratigraphic (Heinrich events in marine sediments in the North Atlantic) and
climatic (Dansgaard-Oeschger events in Greenland ice
cores).
Fig. 7.7 Magnetic polarities observed in the Siwalik cut correlated
with the Mankinen and Dalrymple (1979) polarities scale, the most
complete and reliable at the time
7 Magnetostratigraphy: From a Million to a Thousand Years
109
causes a significant increase in the amount of cosmonuclides
(
14 C,
10 Be,
26 Al…) formed in the upper atmosphere through
the impact of cosmic radiation on the various particles air.
As
10 Be can be measured in polar ice cores, this opens up
new opportunities for ice-sediment correlation.
A Scale of Geomagnetic Instabilities?
The use of geomagnetic excursions as precise temporal tie points
could be particularly useful in at least two important areas of
Earth sciences. This is especially useful to understand the
operating mechanisms of the terrestrial dynamo which is the
origin of the field itself. Recent models, theoretical and numerical, of the Earth’s dynamo (Glatzmaier and Roberts 1995)
indeed, give accurate assessments of time constants, frequencies
and geometries of the transitional field during excursions. It is of
prime importance to characterize the role of the solid inner core
and the lower mantle in the mechanism of excursions and
reversals. It is also important in paleoclimatology studies, where
an independent chronology of climatic/environmental phenomena needs to be of higher resolution that the scale of
polarities to allow evaluation of the synchronicity and phase
shifts, either early or late, of climate events in different parts of
the globe (examples are described below).
Currently, the major obstacle to the widespread use of
this method is probably the difficulty of integrating sedimentary data and volcanic data into a single unified scale of
geomagnetic instabilities. The brevity of the excursions is
both an advantage (giving very precise temporal tie points),
and paradoxically, it also represents an obstacle to the creation of this scale. In fact, a specific excursion is not systematically recorded in all sequences, including those with a
medium to high sedimentation rate. Studies to develop a
high-resolution chronology of geomagnetic instabilities,
especially of excursions, are often based on accurate dating,
using the K/Ar and
40 Ar/
39 Ar methods, of as large a number
as possible of lava flows recording either an abnormal
direction, or a geomagnetic field with a very low intensity, or
both characteristics together. It is important to keep in mind
that
40 Ar/
39 Ar datings are obtained by reference to standards
whose ages were defined based on astro-chronological calibrations. So, several ages have been proposed for the same
standard (e.g. Fish Canyon Sanidine, commonly used for
dating in the Quaternary). It sometimes appears that none of
them provides good agreement between the
40 Ar/
39 Ar ages
and the glaciological or astronomical scales. Further work is
therefore necessary to “reconcile” these various approaches.
Today, at least seven geomagnetic excursions from the
Brunhes period have been inventoried in detail: the excursions of Mono Lake (34 ka), Laschamp (41 ka), Blake
(120 ka), Iceland Basin (188 ka) Pringle Falls (211 ka), Big
Lost (560–580 ka) and Stage 17 (670 ka) (Laj and Channell
2007). Other excursions from the same period are being
studied. Studies are also underway for earlier periods, such
as the Matuyama period during which at least eleven
excursions seem to have occurred. All of these excursions
act as specific temporal tie points, that greatly increase the
temporal resolution of the magnetostratigraphic scale.
Magnetostratigraphy Based on Variations
in the Intensity of the Geomagnetic Field
Introduction
Over recent years, the stratigraphy of climate records has
undergone a major change, particularly due to the discovery
of rapid and precise markers, both lithostratigraphic (Heinrich events in marine sediments in the North Atlantic) and
climatic (Dansgaard-Oeschger events in Greenland ice
cores).
Fig. 7.7 Magnetic polarities observed in the Siwalik cut correlated
with the Mankinen and Dalrymple (1979) polarities scale, the most
complete and reliable at the time
7 Magnetostratigraphy: From a Million to a Thousand Years
109