Geoscience Reference
In-Depth Information
Table2.1
Isotopicspeciesofwater
Composition
Natural abundance (%)
1
H
1
H
16
O
99.732
1
H
1
H
18
O
0.200
1
H
1
H
17
O
0.038
1
H
2
H
16
O
0.030
2
H
2
H
16
O
0.020
1
H
2
H
18
O
0.006
1
H
2
H
17
O
0.001
2
H
2
H
18
O
0.0004
2
H
2
H
17
O
0.00008
In 1986 another robust index was established, called the TEX 86 index, named
after the Dutch island of Texel on which it was developed. TEX 86 is based on
molecules that have 86 carbons. However, these molecules come from far less com-
mon single-celled Archaea, but because such Archaea have been found in sediments
spanning the past 100 million years they represent a powerful complement to the U
K
37
index.
2.3 Non-bioticindicators
Whereas (formerly) living organisms are most useful as an aid to estimating palaeo-
climates because they either live solely within usefully narrow climatic restrictions or
because in the course of their lives they somehow incorporate a palaeobiotic indicator
into their bodies, there are other indicators of past climates that operate completely
independently of living creatures. Of these, one of the most useful in terms of helping
determine climate change hemispherically (if not globally) in the past few hundred
thousand years has been the isotopic analysis of water ice.
2.3.1 Isotopicanalysisofwater
Water is not just H
2
O or HHO. As discussed in preceding sections, oxygen exists
as two principal isotopes,
16
O and
18
O, together with another,
17
O. Further, there is
also a second stable isotope of hydrogen in addition to the common
1
H and this is
2
H, or deuterium (sometimes portrayed as D). Only a tiny proportion of hydrogen
and oxygen is not of the respective principal isotopic forms, but nonetheless these
rarer isotopes do exist. Consequently water is not just HHO, as in
1
H
1
H
16
O, but has
a variety of isotopic variations (see Table 2.1).
As has previously been noted the take-up by photosynthetic organisms of water
containing
18
O for photosynthesis is affected by temperature and is complicated
by preferential evaporation of water containing lighter oxygen isotopes. Yet such
preferential evaporation of lighter isotopes with temperature can also be of use as a
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