Establishing the Scale of Magnetic Polarities
First Coupled Measurements: Magnetization
of Volcanic Rocks—K/Ar Dating; the McDougall
and Tarling Scale and the Mankinen
and Dalrymple Scale for the Plio-Pleistocene
In the 1960s, the first geomagnetic polarity time scales (GPTS)
were developed using studies coupling magnetic polarity with
radiometric dating (K/Ar, Chap. 5). These include the first
studies conducted by Cox et al. (1963) in California and by
McDougall and Tarling (1963a, b) on the Hawaiian Islands.
Identification of the different zones of polarity of the
Plio-Pleistocene was based solely on their radiometric age, as
none of the volcanic sequences studied (apart from the Icelandic sequences studied by Watkins and Walker) had the
continuity and duration to form a long, more or less continuous, stratigraphic sequence. The scale of polarities was
therefore constructed by combining results from locations
often very far from each other. So, there is no ‘stratotype’
(typical locality), as is the case for sedimentary outcrops.
The studies that marked this first phase of development of
the scale were conducted from Jaramillo, the Cobb Mountain, in the Olduvai Gorge, Reunion, Mammoth, Cochiti and
Nunivak (see Opdyke and Channell 1996). All these studies
led to the gradual establishment of the magnetostratigraphic
scale for the Plio-Pleistocene, shown in Fig. 7.2.
It was at this time that the custom of naming the long
periods of polarity (polarity chrons) after the great geophysicists (Brunhes, Matuyama, Gilbert, Gauss) was introduced, while the shorter periods (polarity subchrons) were
named after the place where they were discovered (Jaramillo, Mammoth, etc.).
The magnetic polarities scale by Mankinen and Dalrymple (1979) (Fig. 7.3), based on studies coupling magnetic polarity and K/Ar dating of volcanic rocks, is the last
stage of this development phase of the magnetostratigraphic
scale and remained the standard in the discipline covering
the last five million years, for over ten years.
Further back than 5 million years, however, analytical
uncertainties in radiometric dating become of the order of
the duration of polarity periods, making their identification
unreliable. Other methods are required to extend this scale to
more ancient times.
Magnetic Stratigraphy in Pliocene-Pleistocene
Sedimentary Series
It was the studies of magnetism of deep ocean sediments
that first enabled this extension (Opdyke et al. 1974). The
magnetic particles contained in the sediments were
deposited with a preferred orientation along the lines of
the geomagnetic field prevailing at the time of deposition.
In many cases, this orientation was preserved over time:
just as for lava, sediments have a “magnetic” memory.
Although the precise mechanisms by which sediments
acquire magnetism are still poorly understood and this may
limit their use in some applications, sediments, on the other
hand, have the enormous advantage of registering the
direction of the geomagnetic field almost continuously
unlike lava, where the continuity of the signal is conditioned by the sporadic nature of volcanic eruptions.
Figure 7.4 shows an example of magnetic polarity
recorded in the core RC12-65 collected by Opdyke et al.
(1974), on the research vessel R. Conrad of the Lamont
Doherty Earth Observatory, Columbia University, USA.
While some details may differ, the sequence of polarity
periods from the Mankinen and Dalrymple scale can be seen
in the record (from the Brunhes to the Gilbert period).
However, the sediment core goes back further in time; the
lowest level is from around 10 million years ago, based on
paleontological estimates.
Magnetic Anomalies at Sea and the Heirtzler
Scale of Magnetic Polarities
The study of magnetic anomalies at sea constituted the major
step towards establishing the sequence of magnetic polarities,
from the middle of the Mesozoic era through to recent times.
The hypothesis by Vine and Mathews (1963) of the
spreading of the seafloor directly explains the formation of these
magnetic anomalies on both sides of oceanic ridges (Fig. 7.5).
Fig. 7.2 The first scales of geomagnetic polarity for the
Plio-Pleistocene period. After McDougall (1979). In black: normal
periods, in white: inverse periods
7 Magnetostratigraphy: From a Million to a Thousand Years
103
First Coupled Measurements: Magnetization
of Volcanic Rocks—K/Ar Dating; the McDougall
and Tarling Scale and the Mankinen
and Dalrymple Scale for the Plio-Pleistocene
In the 1960s, the first geomagnetic polarity time scales (GPTS)
were developed using studies coupling magnetic polarity with
radiometric dating (K/Ar, Chap. 5). These include the first
studies conducted by Cox et al. (1963) in California and by
McDougall and Tarling (1963a, b) on the Hawaiian Islands.
Identification of the different zones of polarity of the
Plio-Pleistocene was based solely on their radiometric age, as
none of the volcanic sequences studied (apart from the Icelandic sequences studied by Watkins and Walker) had the
continuity and duration to form a long, more or less continuous, stratigraphic sequence. The scale of polarities was
therefore constructed by combining results from locations
often very far from each other. So, there is no ‘stratotype’
(typical locality), as is the case for sedimentary outcrops.
The studies that marked this first phase of development of
the scale were conducted from Jaramillo, the Cobb Mountain, in the Olduvai Gorge, Reunion, Mammoth, Cochiti and
Nunivak (see Opdyke and Channell 1996). All these studies
led to the gradual establishment of the magnetostratigraphic
scale for the Plio-Pleistocene, shown in Fig. 7.2.
It was at this time that the custom of naming the long
periods of polarity (polarity chrons) after the great geophysicists (Brunhes, Matuyama, Gilbert, Gauss) was introduced, while the shorter periods (polarity subchrons) were
named after the place where they were discovered (Jaramillo, Mammoth, etc.).
The magnetic polarities scale by Mankinen and Dalrymple (1979) (Fig. 7.3), based on studies coupling magnetic polarity and K/Ar dating of volcanic rocks, is the last
stage of this development phase of the magnetostratigraphic
scale and remained the standard in the discipline covering
the last five million years, for over ten years.
Further back than 5 million years, however, analytical
uncertainties in radiometric dating become of the order of
the duration of polarity periods, making their identification
unreliable. Other methods are required to extend this scale to
more ancient times.
Magnetic Stratigraphy in Pliocene-Pleistocene
Sedimentary Series
It was the studies of magnetism of deep ocean sediments
that first enabled this extension (Opdyke et al. 1974). The
magnetic particles contained in the sediments were
deposited with a preferred orientation along the lines of
the geomagnetic field prevailing at the time of deposition.
In many cases, this orientation was preserved over time:
just as for lava, sediments have a “magnetic” memory.
Although the precise mechanisms by which sediments
acquire magnetism are still poorly understood and this may
limit their use in some applications, sediments, on the other
hand, have the enormous advantage of registering the
direction of the geomagnetic field almost continuously
unlike lava, where the continuity of the signal is conditioned by the sporadic nature of volcanic eruptions.
Figure 7.4 shows an example of magnetic polarity
recorded in the core RC12-65 collected by Opdyke et al.
(1974), on the research vessel R. Conrad of the Lamont
Doherty Earth Observatory, Columbia University, USA.
While some details may differ, the sequence of polarity
periods from the Mankinen and Dalrymple scale can be seen
in the record (from the Brunhes to the Gilbert period).
However, the sediment core goes back further in time; the
lowest level is from around 10 million years ago, based on
paleontological estimates.
Magnetic Anomalies at Sea and the Heirtzler
Scale of Magnetic Polarities
The study of magnetic anomalies at sea constituted the major
step towards establishing the sequence of magnetic polarities,
from the middle of the Mesozoic era through to recent times.
The hypothesis by Vine and Mathews (1963) of the
spreading of the seafloor directly explains the formation of these
magnetic anomalies on both sides of oceanic ridges (Fig. 7.5).
Fig. 7.2 The first scales of geomagnetic polarity for the
Plio-Pleistocene period. After McDougall (1979). In black: normal
periods, in white: inverse periods
7 Magnetostratigraphy: From a Million to a Thousand Years
103