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came from sanidine crystals in volcanic ash from the Lenggong Valley in Malaysia,
located only 350 km from the source (Storey et al.,
2012
). The very high proportion
of pure ash in this alluvial deposit (Gatti et al.,
2012
) suggests that little time had
elapsed between the initial deposition of the primary airfall ash and its subsequent
re-deposition in the Lenggong Valley.
6.4.2 Geomagnetic dating
The earth's magnetic polarity has changed periodically by 180
, with the polarity
direction sometimes normal, that is, similar to the present, and sometimes reversed.
Periods when the magnetic polarity remains stable for a long time (100 ka to 10 Ma)
are called
polarity epochs
, with three recognised during the last 3 Ma (
Figure 6.2
).
The current epoch of normal polarity began 0.78 Ma ago (Spell and McDougall,
1992
;
Brown et al.,
1994
; Pillans,
2003
) and is termed the Brunhes Normal Chron. The pre-
ceding polarity epoch (the Matuyama Reversed Chron) lasted from 2.6 to 0.78 million
years (Ma) ago, and it encompasses the base of the Pleistocene as presently defined
(Gibbard et al.,
2010
). Shorter intervals of global polarity change (
polarity events
),
lasting 10-100 ka, are also apparent in both terrestrial and marine sequences, enabling
the record to be used worldwide to establish the age of major environmental changes
on land and sea. More short-lived secular geomagnetic fluctuations, termed excursions
and often discernible only at a regional scale, have allowed the geomagnetic time scale
(
Figure 6.2
) to be further refined and have been used, for instance, to establish the
timing of fluctuations in the level of Holocene and Upper Pleistocene sediments from
a lake in Cameroon (Thouveny and Williamson,
1988
) and from Pleistocene Lake
Bonneville in Utah (Liddicoat and Coe,
1998
).
The method is based on the fact that certain rocks and sediments are able to acquire
the prevailing direction of the earth's magnetic field, in effect becoming natural
magnets (King and Peck,
2001
). For example, as lavas cool and start to solidify,
the ferromagnetic minerals within them become oriented according to the magnetic
field that is prevalent at that time. Likewise, ferromagnetic minerals settling to the
ocean floor become similarly oriented, so that long marine cores will show alternating
phases of normal and reversed polarity. In the absence of independent dating, this
method would only provide relative ages. However, volcanic rocks containing an
excellent record of polarity changes spanning the entire Cenozoic have been dated by
potassium-argon dating (see
Section 6.5.1
), so that the geomagnetic record of dated
polarity epochs and events provides an imprecise but still useful means of obtaining
absolute ages for marine and terrestrial sediments. Examples include the Pliocene
hominid-bearing deposits of the Afar Desert (Taıeb,
1974
; White et al.,
1994
; White
et al.,
2006
) and theMiocene fossils of the Siwalik rocks in northern India and Pakistan
(Pillans et al.,
2005
).
°
