Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 55

18 O in marine calcite is an indicator of the extent of these terrestrial ice caps, and,
commensurately, sea level, but not nearly so much of temperature.
Because 18 O concentration in forams and corals is determined primarily by two
factors - temperature in the absence of terrestrial ice and sea level in the presence of
terrestrial ice - a number of global palaeotemperature graphs have two temperature
scales, one relating to before 35 mya, when Antarctic glaciers began forming, and
one after, when terrestrial ice became a major factor. This is because of the problems
discussed above about this form of palaeoclimatic determination over periods of
many million years. For this reason Figure 2.3 should actually be considered to be the
merging of two 18 O graphs relating to temperature, one covering the period before 35
mya and one after. Alternatively, it could have been a single 18 O concentration curve
but necessitating two temperature-related scales, again one before 35 mya and one
after. Figure 2.3 was drawn the way it was to clarify matters simply.
But forams are not the only biological sources of 18 O that can be analysed. Coral
analyses have certain advantages over foram analysis. Unlike forams, which are
more subject to the vagaries of water movement, coral reefs are fixed. Consequently,
problems with changes in temperature with depth are far less important.
Corals are animals in the cnidarian (coelenterate) phylum, order Scleractinia, class
Anthozoa. Their basic body plan consists of a polyp containing unicellular endosym-
biotic algae known as zooxanthellae overlaying a calcium carbonate exoskeleton and
it is this calcium carbonate (calcite) that is used for 18 O analysis. Of particular value
is that many corals outside of the tropics also have annual growth rings: actually
they have daily rings that are further apart in the summer and narrower in the winter
and during their monthly (tide-driven) breeding cycle. (This last has been used to
determine the distance between the Earth and the Moon millions of years ago but that
is a separate story.)
2.2.2 Alkenoneanalysis
Long-chain (C 37 -C 39 ) alkenones are cell membrane lipids that are organic molecular
'fossils' from phytoplankton and represent about 8% of total phytoplankton carbon.
They can be used to indicate the sea-surface temperature. The principle behind this
is not exactly known, although the correlation between the unsaturated proportion
(that with double and triple bonds in the chain) and temperature is undeniable. It is
speculated that as temperature changes so does the rigidity, or some other physical
property, of phytoplankton cell membranes. What is known is that, in culture, living
algae respond to changing temperature conditions in a matter of days. Because of this
they are particularly good for recording rapid temperature change subject to accurate
dating. Also, alkenones are robust molecules that are capable of being preserved for
many hundreds of thousands of years and so are useful in charting more than our
current glacial-interglacial cycle.
One of the longest established alkenone-based proxies was devised at the Uni-
versity of Bristol in 1985. Known as the U K 37 index, it is based on the number of
double bonds in alkenones with 37 carbon atoms that are found in many unicellular
eukaryotic marine algae, including Emiliania huxleyi , one of the most common spe-
cies of such algae.