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vapor makes its way poleward from the tropics, it gradually becomes increasingly
depleted in the heavier isotope. Consequently, snow falling at higher latitudes has
much less H 2 18 O than rain falling in the tropics. Changes in climate that alter the
global patterns of evaporation or precipitation can therefore cause changes to the
background d 18 O ratio at any locality that can interfere with the inference of past
temperature change from isotope ratios.
Palaeoclimate reconstruction from the study of forams has resulted from
basically three types of analysis: (1) oxygen isotope composition of calcium
carbonate, (2) relative abundance of warm- and cold-water species, and (3) mor-
phological variations in particular species resulting from environmental factors.
Most studies have focused on oxygen isotope composition.
If a marine organism's calcium carbonate is crystallized slowly in water, 18 Ois
slightly concentrated in the precipitate relative to that remaining in the water. This
fractionation process is temperature dependent, with the concentrating effect
diminishing as temperature increases. When the organism dies, the external shell
of the organism sinks to the seabed and is laid down, with millions of others as
sea floor sediment (calcareous ooze), thus preserving a temperature signal (in the
form of an oxygen isotopic ratio) from the time when the organism lived. If a
record of oxygen isotope ratios is built up from cores of ocean sediment, the cores
can be dated. Standard techniques used to date oceanic sediment cores include
paleomagnetic analysis and radioisotope studies,
such as
radiocarbon and
uranium series dating methods.
Empirical studies relating the isotopic composition of calcium carbonate
deposited by marine organisms to the temperature at the time of deposition have
demonstrated the following relationship:
2
T ¼ 16
:
9 4
:
2 ðd c d w Þþ 0
:
13 ðd c d w Þ
in which T was the water temperature ( C) in which the sample precipitated, d c is
the departure from current standard seawater of d 18 O in the carbonate sample,
and d w is the departure from current standard seawater of the water in which the
sample precipitated. While d c can be measured accurately, it is dicult to estimate
d w because it pertains to millions of years ago. During glacial times, seawater was
isotopically heavier (i.e., enriched in 18 O) compared with today because large
quantities of isotopically lighter water were landlocked in huge ice sheet forma-
tions. Thus, the expected increase in d c due to colder sea surface temperatures
during glacial times is complicated by the increase in d w.
By analyzing isotopic records of deep-water organisms, one can attempt to
resolve how much of the increase in d c for surface organisms was due to decreases
in surface temperature and how much was due to continental ice sheet formation.
It is expected that bottom-water temperatures ( 1 Cto2 C) have changed very
little since glacial times (the Last Glacial Maximum being about 20,000 kybp ) and
increases in d c for deep-water organisms would reflect mainly changes in the
isotopic composition of the glacial ocean.
In the past, as always, the abundance of any species of planktic (surface-
dwelling) forams depended on the local sea surface temperature. Thus, planktic
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