Ocean sediment data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 168

According to Monica Bruckner:

''Foraminifera, also known as forams, and diatoms are commonly used
microbial climate proxies. Forams and diatoms are shelled microorganisms
found in aquatic and marine environments. There are both planktic (floating
in the water column) and benthic (bottom dwelling) forms. Foram shells are
made up of calcium carbonate (CaCO 3 ) while diatom shells are composed of
silicon dioxide (SiO 2 ). These organisms record evidence for past environmental
conditions in their shells. Remains of foram and diatom shells can be found by
taking sediment cores from lakes and oceans, since their shells get buried and
preserved in sediment as they die. The chemical make-up of these shells reflects
water chemistry at the time of shell formation. Stable oxygen isotope ratios
contained in the shell can be used to infer past water temperatures. These oxygen
isotopes are found naturally in both the atmosphere and dissolved in water.
Warmer water tends to evaporate off more of the lighter isotopes, so shells grown
in warmer waters will be enriched in the heavier isotope. Measurements of stable
isotopes of planktic and benthic foram and diatom shells have been taken from
hundreds of deep-sea cores around the world to map past surface and bottom
water temperatures.
Researchers may also use foram and diatom population dynamics to infer
past climate. Relative abundance as well as species composition in particular
areas may indicate environmental conditions. Typically, warmer weather will
cause organisms to proliferate. In addition, since each species has a particular
set of ideal growing conditions, species composition at a particular site at a
particular time may indicate past environmental conditions.''

A steady rain of shells from small, surface-dwelling animals falls continually,
eventually building up hundreds of meters of sediment. These sediments preserve
the shells of these small animals for millions of years. The most important of these
animals—foraminifera (or forams for short)—construct their tiny shells from a
form of calcium carbonate (CaCO 3 ). Carbonate originally dissolved in the oceans
contains oxygen whose atoms exist in two naturally occurring stable isotopes, 18 O
and 16 O. The ratio between these two isotopes is dependent on past temperatures.
When carbonate solidifies to form a shell, d 18 O varies slightly, depending on the
temperature of the surrounding water. Unfortunately, there are complications.
While the value of d 18 O in forams changes from its mean value as the water
temperature changes from its mean value, the mean value of d 18 O in the oceans
varies widely with location. This variability arises because, when water evaporates,
the lighter molecules of water (those with 16 O atoms as compared with those with
18 O) tend to evaporate first. Therefore, water vapor is more depleted (fewer H 2 18 O
molecules) than the ocean from which it evaporates. Thus, the ocean has more 18 O
in places where water evaporates heavily like the sub-tropics and less

18 O where it

rains a good deal like the mid-latitudes (Schmidt, 1999).
Similarly, when water vapor condenses (to produce rain, for instance), the
heavier molecules (H 2 18 O) tend to condense and precipitate first. So, as water