Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 43

but, of the remaining proportion, the next most common isotope of oxygen is 18 O.
Because lighter 16 O evaporates preferentially to 18 O the ratio between the two varies
in atmospheric water vapour and in rain water depending on climate. The oxygen
in water is then incorporated into wood through photosynthesis and so a record of
the isotopic ratio of the rain at the time the wood was growing can be determined.
This should give an indication of the climate. However, there are several problems
with this form of analysis. These include rain-out effects: the first precipitation from
a cloud has a greater composition of the heavier 18 O. There are also altitude and
ice/snow meltwater effects. Consequently isotopic dendrochronology has only met
with limited success and not all such work has borne equivocal results. This form of
palaeoclimatic analysis needs some further development. Yet, despite considerable
limitations there have been some interesting possibilities. For example in 2006 the
Swiss and German team led by Kerstin Treydte reported that a dendrochronological
isotope record from the mountains in northern Pakistan correlated well with winter
precipitation and that this suggested that the 20th century was the wettest of the past
1000 years, with intensification coincident with the onset of industrialization and
global warming (Treydte et al., 2006).
Despite the extra caution required with isotopic dendrochronology, as we shall
see using 16 O: 18 O ratios in another biological context, Foraminifera (an order of
marine plankton) can be employed as a kind of climate proxy but more exactly as an
indication of the global amount of water trapped in ice caps.
Because dendrochronology gives precise annual rings, it has also been used as
one of the means to calibrate radioactive 14 C dating (as discussed in Chapter 1). In
1947, the US chemist Willard Libby explained how the radioactive isotope 14 Cwas
produced, for which he received the Nobel Prize for Chemistry in 1960. 14 C, with its
half-life of 5730 years, is used for dating biological samples from several decades to
thousands of years old beyond the last glacial maximum about 20 000 years ago: the
limit of 14 C dating is usually considered to be about 45 000 years (and we will return
to this when considering human migration during the last glacial in Chapter 4). 14 C
is continually being generated in the atmosphere due to cosmic rays, mainly from the
Sun, hitting nitrogen and carbon atoms. 14 C so formed is then incorporated into plants
through photosynthesis. The common 12 C and the radioactive 14 C are in equilibrium
with that in the atmosphere up until the plant dies, and at that point the various carbon
isotopes become fixed. From then on the fixed reservoir of 14 C in the plant begins to
decay and so the ratio between 12 C and 14 C changes. This ratio enables dating.
However, as mentioned, all this assumes that the rate of 14 C production in the
atmosphere is constant. This is not precisely true as more 14 C is produced in the
atmosphere at times of, for instance, exceptional solar activity, such as when there are
many sunspots. Carbon dating has therefore had to be verified by comparison with
other dating methods, including dendrochronology. (In recent times 12 C: 14 C ratios
have been upset by atmospheric tests of atomic bombs in the mid-20th century and by
the Chernobyl nuclear power plant disaster.) Notwithstanding these difficulties, 14 C
decays at an immutable rate of 1% every 83 years. If the rate of decay is measured
to obtain an accuracy of, for instance, plus or minus 0.5%, then the observation of
around 40 000 decaying carbon atoms would be required. This would, in principle,
allow the sample to be dated to an accuracy of ± 40 years and would require a sample