Geoscience Reference
In-Depth Information
of deep-sea cores, a way out of this impasse was discovered. During glacial cycles,
the lighter isotope of oxygen (
16
O) is preferentially evaporated from the oceans and
concentrated in the growing ice sheets, while the oceans become relatively enriched
in the heavier isotope (
18
O). With the interglacial melting of the ice sheets, this lighter
16
O isotope is released once more into the oceans. The ratio of
18
O)
serves to distinguish foraminifera living under glacial conditions from those living
during times of minimum ice volume, according to the expression:
16
Oto
18
O(
18
O
18
O
16
O
18
O
16
O
18
O
16
O
∂
=
(
−
)
sample
−
(
−
)
standard
/(
−
)
standard
×
1000
(6.1)
18
O units are in parts per thousand (
The
, or 'per mil'). The standard used for
foraminifera is PDB, a Cretaceous belemnite from the Pee Dee Formation in North
Carolina. (
Foraminifera
are single-celled, mostly marine, planktonic animals with a
chalky shell.)
Apairof
marine isotope stages
thus brackets each glacial-interglacial cycle. These
stages are numbered from the most recent (MIS 1) backwards. Even numbers denote
glacial stages; uneven numbers denote interglacial stages. Analysis of the oxygen
isotopic composition of marine microfossils, especially
foraminifera
, has revealed
eight glacial-interglacial cycles during the last 800 ka and more than fifty in the last
2.5 Ma (Shackleton et al.,
1990
;Walker,
2005
). The earlier cycles were of shorter
magnitude and duration than were those of the last 800 ka, as discussed in
Chapter 3
.
MIS records correspond closely with the earth's orbital (or Milankovitch) cycles,
and they can therefore be calibrated ('orbitally tuned') with respect to the latter. The
orbital cycles reflect regular fluctuations in the distance of the earth from the sun
and the tilt of the earth's axis (see
Chapter 3
). For present purposes, all we need
note is that the three main orbital cycles have a duration of approximately 100,000,
41,000 and 23,000 to 19,000 years, and act as the pacemakers of the ice ages (Hays
et al.,
1976
; Imbrie and Imbrie,
1979
). In many instances, the record of environmental
changes preserved in marine cores off the coast of desert regions complements the
fragmentary terrestrial record and provides a more continuous archive of the climatic
changes experienced in deserts and their margins.
‰
6.5 Radiometric dating: radioisotope parent - stable daughter
6.5.1 Potassium-argon and argon-argon dating of volcanic rocks
Potassium is common in many minerals in igneous rocks, particularly feldspars. There
are three naturally occurring isotopes of potassium, of which
39
K is the most abundant,
followed by
41
K, with
40
K present in trace amounts (0.00118 per cent). The potassium-
argon (
40
K/
40
Ar) dating method was developed in the 1960s and involves measuring
the ratio of radioactive
40
K to the relatively inert gas
40
Ar which is a daughter product
in its radioactive decay chain, both natural isotopes. Because
40
K has a very long
