Chemostratigraphy
Chapter 17
Isotopes
• Atoms of the same chemical element
having the same number of protons in the
nucleus, but differing from one another by
having a different number of neutrons (ie.
Different atomic weight).
Chemostratigraphy
• Unstable isotopes- are used for
determining age.
• Stable Isotopes- isotopes that are stable
exist in well-define ratios in the ocean and
in the atmosphere.
• When oceanographic and climatic changes
occur, some isotopes can become more or
less abundant with respect to others due to
FRACTIONATION.
Fractionation
• Separation of isotopes by their differences
in atomic weight.
• These fluctuations of certain isotope ratios
have become a powerful tool for
stratigraphy.
Oxygen Isotopes
• Most of the oxygen in the Earth (99.756%)
is in the form of 16O, which has eight
protons and eight neutrons; a slightly
heavier stable isotope, 18O, has two more
neutrons and is relatively rare (0.205%)
Usually these two isotopes are present in
the ocean in this ratio.
Oxygen Isotopes
• Water that is richer in 16O evaporates more
readily than water with more 18O.
• In a world without glacials this water rains
over land and travels back to the oceans, so
there is no net change in the ratio of the
oxygen isotopes.
Oxygen isotope fractionation
during glacial-interglacial
cycles. In A water carrying the
lighter isotope 16O is
preferentially evaporated to
form clouds. As the clouds
move landward and rain out,
they become even more 18Odepleted. During interglacial
periods, however, this 18O-poor
waters returns to sea, and there
is no net change. In B, during
glacial periods, the 18O-depleted
water is trapped in the ice caps,
which have 18O/16O ratios of -30
parts per thousand (‰). The
ocean as a consequence, is
relatively enriched in 18O
(+1.6‰).
Oxygen Isotopes
• Oxygen isotopes are measured with respect
to an arbitrary laboratory standard called
PDB, after the Pee Dee belemnite.
• Calcite from this abundant cephalopod in
the Cretaceous Pee dee Formation of South
Carolina is used to calibrate the mass
spectrometer.
Oxygen Isotopes
• The ratio is calculated by the following equation:
δ18O = [(18O/16O) sample - (18O/16O)standard] x 1000
(18O/16O)standard
Oxygen Isotopes
• A shell that has a δ18O value of 3 ‰ to
PDB means that the CO2 derived from that
shell is 3 ‰ richer in that isotope than PDB.
• Positive δ18O values are enriched in 18O ,
indicating increased ice volume and cooling
• Negative values are enriched in 16O,
indicating decreased ice volume and
warming.
Carbon Isotopes
• Like oxygen, carbon has more than one
stable isotope in the Earth’s Ocean and
atmosphere.
• Most of the carbon (98.89%) is in the form
of 12C, which has six protons and six
neutrons. However 1.11% is the heavier
isotope 13C, with one extra neutron.
Carbon Isotopes
• These two isotopes circulates through the
ocean and are incorporated into the calcite
organisms in much the same way as
oxygen isotopes.
• Typically both are measured together
during an isotopic analysis, and the formula
for calculating 13C is the same as the
formula fro 18O, with appropriate
substitutions.
Carbon Isotopes
• The carbon system is not controlled by ice
volume and temperature but by oceanic
circulation.
• Organic material tend to be low in 13 C, so
when they decay,they release not only
excess 12C but also less 13C , so the value
of 13C decreases in the water. Deep ocean
water, in particular, are traps for organic
nutrients and CO2, which are relatively
depleted in surface water as a result of
photosynthesis, the sinking of organic
debris, and respiration by bottom-dwelling
organism.
Carbon Isotopes
• When major changes in oceanic circulation
occurs, these bottom waters exchange with
ocean surface waters and release their 12C,
which makes the 13C value more negative.