Encyclopedia of Evolution.pdf - Online Reading Center

isotopes
food in developed countries. This has caused human bodies
to have a measurably higher δ 13 C than those of wild animals,
prompting one researcher to call modern humans “corn chips
on legs.”
Since fossil fuel combustion, burning of forests, and
landfills emit methane (CH 4) from biomass that contains a
low δ 13 C, the δ 13 C of methane in the atmosphere can serve
as an indicator of these processes. Methane from ice cores
can therefore indicate the extent of forest fires in the geological
past.
Oxygen and hydrogen isotopes. Most oxygen atoms in
seawater are 16 O, but some are 18 O. Evaporation of water
(H 2O) from the ocean surface preferentially removes H 2 16 O,
as H 2 18 O is heavier and sinks. The evaporated water returns
to the ocean as rain and from rivers. During times of glaciation,
much of the H 2 16 O water is trapped in glaciers, and
the weather is drier. Sediments that have high 18 O: 16 O ratios
were therefore deposited during times of glaciation and aridity
(see figure on page 225). A similar process causes water
that contains deuterium ( 2 H 2O) to accumulate in ice layers
during times of glaciation and aridity.
Nitrogen isotopes. Bones of carnivorous animals tend to
have more of the heavy isotope of nitrogen ( 15 N) relative to 14 N
than do bones of herbivorous animals, since the heavier isotope
accumulates with each level of the food chain. The nitrogen isotope
ratio provides an indication of an animal’s diet.
Sulfur isotopes. Gypsum is calcium sulfate (CaSO 4),
usually of inorganic origin. Therefore it usually has a ratio
of light sulfur ( 32 S) to heavy sulfur ( 34 S) that is the same as
the seawater in which it is formed. However, sulfate-reducing
bacteria produce iron pyrite, which precipitates out of the
water. The bacterial enzymes that do this prefer 32 S to 34 S.
Thus a lower than normal amount of 34 S in a deposit may
suggest biological activity.
Strontium isotopes. Strontium atoms in seawater replace
some of the calcium atoms in limestone. The ratio of heavy Sr
( 87 Sr) to light Sr ( 86 Sr) is always close to 0.71. However, the
continents have relatively more of the heavy isotope. Thus,
when sedimentary deposits show an increase in heavy Sr,
this is taken as an indication of significant erosion from the
continents. An increase in the strontium ratio occurred after
the melting of the ice of Snowball Earth. It has also been
increasing in the sediments around Eurasia for the past 20
million years as a result of the uplift of the Himalayas.
Other uses of isotopes. Geologists have used hydrogen
isotope measurements from geological deposits to determine
the time of mountain uplift. They have also used the
buildup of a beryllium isotope on boulder surfaces to determine
the timing of glacial retreat. Measurement of helium
isotopes in dust allows a determination of how much of the
dust has come from outer space. Isotope ratios can also be
used as indicators of the geographical origin of deposits. Different
oceanic areas have different neodymium ( 143 Nd/ 144 Nd)
ratios. The study of neodymium ratios in deposits from the
late Quaternary period has allowed the reconstruction of
ocean circulation patterns at that time.
Isotopes are not the only molecular markers used in the
fossil record. Other molecular markers include lipids, which
are different in bacteria, archaea, cyanobacteria (which have
2-methylhopanes), and eukaryotes (in which steranes are
derived from membrane cholesterol).
Different isotope ratios may measure the same phenomenon.
Oxygen, hydrogen, and strontium isotope ratios are
indicators of the climate that prevailed at the time the deposit
was formed. The processes that influence oxygen, hydrogen,
and strontium ratios are largely probably independent of one
another. Therefore, in the great majority of cases in which
oxygen, hydrogen, and strontium ratio estimates agree with
one another, they provide independent, therefore believable,
estimates of ancient climate.
Further Reading
Condon, Daniel, et al. “U-Pb ages from the neoproterozoic Doushantuo
Formation, China.” Science 308 (2005): 95–98. Summary by
Kaufman, A. J., in Science 308 (2005): 59–60.
Eglinton, G., and R. Pancost. “Immortal molecules.” Geoscientist 14
(2004): 4–16.
Lata, J. C., et al. “Short-term diet changes revealed using stable carbon
isotopes in horse tailhair.” Functional Ecology 18 (2004):
616–624.
Miller, Kenneth G., et al. “The Phanerozoic record of global sea-level
change.” Science 310 (2005): 1,293–1,298.
Mulch, Andreas, Stephan A. Graham, and C. Page Chamberlain.
“Hydrogen isotopes in Eocene river gravels and paleoelevation of
the Sierra Nevada.” Science 313 (2006): 87–89.
Piotrowski, Alexander M., et al. “Temporal relationships of carbon
cycling and ocean circulation at glacial boundaries.” Science 307
(2005): 1,933–1,938.
Schaefer, Georg M., et al. “Near-synchronous interhemispheric termination
of the last glacial maximum in mid-latitudes.” Science 312
(2006): 1510–1513.
Schaefer, Hinrich, et al. “Ice record of δ 13 C for atmospheric CH 4
across the Younger Dryas-Preboreal transition.” Science 313
(2006): 1109–1112.
Siegenthaler, Urs, et al. “Stable carbon cycle-climate relationship during
the late Pleistocene.” Science 310 (2005): 1,313–1,317.