The Essex Naturalist
15
less evaporation during colder periods and in colder latitudes. At these times
and in these places, less of the heavier 18O gets into the atmosphere and more
remains in the sea. Thus the ratio between the two gives an indication of
temperature (or more correctly the relative deviation of 18O from the mean ratio,
818O). The ratios of the two types of oxygen can be measured in the shells of
marine molluscs and in air bubbles in ice, giving a measure of the sea-water and
atmospheric ratios respectively. The variations in the ratios between the types of
oxygen can be plotted and an oxygen isotope curve drawn up. The swings on the
curve are numbered, the colder periods being given even numbers and the
warmer periods odd; oscillations within a warm or cold period are given letters,
a,c,e being warmer, b,d,f colder. Thus 5d, 5e would indicate an oscillation from
cooler to warmer within a warm period. This technique gives us a measure of
global climatic change and increasingly it is being taken as the standard to
which to correlate deposits and events. The Oxygen Isotope Stages are listed in
Table 1.
AMINO ACID testing of molluscs is helping date the interglacial material.
Proteins in bones and shells undergo molecular changes on death of the
individual. The amino acids in the protein exist in two molecular forms, termed
the D- and L-isomeric forms. Only L-isomers occur in living proteins. D-
isomers can occur in a free state, for instance in fossil bone as a result of the
breakdown of proteins. When exposed, e.g. to the atmosphere, shell and bone
degrade rapidly, but if they are in a protected, closed environment, much
slower reactions occur. The L-isomers convert to D-isomers at a known rate,
given certain conditions, and equilibrium is reached when the two are equal
(i.e. the D/L ratio is 1.0). Thus the ratio between the two can give an
indication of the age of the bone or shell. The typical amino acid ratios (AAR)
are:
OI Stage 5e AAR 0.09
7 0.17
9 0.25
11 0.30
The D/L ratios (Table 2) satisfactorily separate out the Aveley interglacial
deposits from those at Trafalgar Square, the latter having ratios around 0.10,
in keeping with those from the typesite at Bobbitshole, Ipswich, and the
former having ratios in the range 0.15-0.19. The next oldest interglacial
deposits, at Ilford, Purfleet and Thurrock, again have higher D/L ratios, in the
range 0.23-0.38. While this satisfactorily separates out the terrace from the
younger ones below, the range encompasses ratios as old as Stage 13 or earlier,
so the technique does not successfully separate the deposits from the next
oldest ones. It would appear that the technique may lose reliability with older
deposits.