Geoscience Reference
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that populations of different species may respond synchronously to global climate
change over large regions.
But terrestrial species are not the only ones to be used as palaeoclimatic indicators.
Marine species are too (see section 2.2) and even those that can live both in the sea and
on land. In 2004 it was reported that the migration of fish that can live for extended
periods on land can be used as a palaeoclimate indicator, when Madelaine Bohme
from the University of Munich used fossil snakeheads, a group of predatory fish
(Channidae) native to Africa and south-east Asia, from the Miocene to Pliocene (23-
5 mya; Bohme, 2004). The species provides a good indicator of summer precipitation.
Its present-day distribution is limited to climates with at least 1 month of rainfall of
150 mm and a mean temperature of 20 C. Bohme's study suggests that the most
extended migration events (17.5 mya and between 8 and 4 mya) must have been
linked to changes in northern-hemisphere air circulation, resulting in formerly dry
summers having higher rainfall.
2.2 Marinebioticclimaticproxies
Marine proxies for palaeoclimates have several advantages over many terrestrial
indicators, especially biological ones. The sea surface is subject to fewer variables
affecting biotic proxies compared to their terrestrial counterparts. Surface topography
is constant and so there is no need to worry about whether the proxy originated on
a long-since-gone slope. Vagaries in water availability for the proxy are also not a
problem. Finally, the sea temperature (because of the sheer volume of water being
affected by the climate) is more applicable to the regional climate (albeit affected by
heat transport by ocean currents) than the local climatic factors that affect terrestrial
biotic indicators. Marine biotic indicators are therefore particularly important when
attempting to ascertain palaeoclimates. There are a number of marine biotic indicators
of climatic proxies. These include terrestrial counterpart methods such as the use of
species. However, of all the methods used, one in particular has generated much
information as to climate change, both generally over several tens of millions of
years and more recently in greater detail over hundreds of thousands of years. This
method employs 18 O analysis.
2.2.1 18 OIsotopeanalysisofforamsandcorals
For decades now, the concentrations of the heavy isotope 18 O in the microshells of
Foraminifera - an order of plankton - and also corals have been widely used to
reconstruct the temperature profiles of ancient seas. Originally it was in 1947 that
Harold Urey (who previously won a Nobel Prize for Chemistry for his discovery of
deuterium) pointed out that the oxygen isotope composition of fossil seashells could
serve as a palaeothermometer of past sea-surface temperatures. The stable heavy
isotope 18 O accounts for about 0.2% of the oxygen in sea water H 2 O compared to the
most common 16 O isotope. The greater proportion of 18 O that a shell incorporated,
Urey showed, the colder the water in which it was formed. The slightly heavier nucleus
of
18 O also affects evaporation and precipitation, but here it is crystal formation
 
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