Isotope stratigraphy uses measurements of isotopic ratios in biogenic minerals like calcite and phosphate to determine the relative ages of sediments. Different isotopes, like oxygen, carbon, and strontium, are incorporated into these minerals in proportions that vary over time due to environmental and geological conditions. While this primary signal can be altered by sediment diagenesis, analyzing isotopic ratios from fossils like foraminifera has provided insights into paleoenvironmental changes like ocean temperatures and ice sheet growth/decay over geologic history.

1. Isotope stratigraphy
 Isotope stratigraphy is a method of
determining relative ages of sediments
based on measurement of isotopic ratios
of a particular element.
 It works on the principle that the
proportions of some isotopes
incorporated in biogenic minerals (calcite,
aragonite, phosphate) change through
time in response to fluctuating
palaeoenvironmental and geological
conditions.

2.  However, this primary signal is often masked
by diagenetic alteration of sediments which
have secondarily altered the isotopic ratios.
 Disentangling primary and secondary
components of measured isotopic ratios is a
difficult and frequently controversial subject.
 Although isotopes of many elements have
been studied oxygen, carbon and strontium,
are of particularly wide application.

3. Oxygen isotopes.
 The ratios in which the two stable isotopes of
oxygen (16O and 18O) are precipitated in
carbonates and phosphates depends upon
the oxygen isotopic composition of the fluid
from which the mineral precipitated and also
on the temperature at which this took place.
 However, some organisms incorporate
oxygen isotopes that are out of equilibrium
with temperature and seawater composition.

4.  In addition, primary isotopic values may
commonly be altered by diagenetic
recrystallisation of carbonate sediments.
 Oxygen isotopes can record detailed changes in
ocean temperature and ice volume.
 The most extensive use of oxygen isotopes has
been in deep-sea cores of Cenozoic, especially
Quaternary sediments, where data from calcitic
microfossils, notably foraminifera, record
fluctuating temperatures and the growth and
decay of ice-sheets, allowing the recognition of
oxygen isotope stages.

5.  The separate effects of temperature and ice
volume are distinguished by comparing
isotope ratios in coeval planktonic and
benthonic microfossils, mainly foraminifera.
 Because both parameters were driven by
Milankovitch climatic cycles, it has been
possible to identify and correlate oxygen
isotope stages in detail across the globe, and
18O curves provide a very refined (20 ka
resolution) time-scale for Quaternary to
Neogene time.

6.  In pre-Cenozoic sediments the use of
oxygen isotopes in both stratigraphy
and palaeoenvironmental studies has
been much more limited because much
of the carbonate is recrystallised, and
only rarely reflects secular changes in
oxygen isotope ratios.

7. Carbon isotopes.
 The two stable isotopes of carbon, 12C and 13C,
vary in relative abundance through time in both
carbonate minerals and organic matter.
 The fluctuations in 13C are brought about by
changes in the balance of fluxes of the carbon cycle,
including inputs of terrestrial carbon and oxidation
of marine organic matter, and outputs by production
and burial of marine carbonate and organic matter.
 Because the residence time in the carbon cycle is
brief (10 ka), changes in flux are recorded accurately
and globally in the sedimentary record.
Furthermore, carbon isotopes are relatively robust
and resistant to diagenesis.

8. Foraminiferal Oxygen and Carbon Isotopes as
palaeoenvironmental Proxies
• Oxygen and carbon isotopes of foramininfera
have been used in paleoclimatic and
paleoceanography studies for decades.
• Oxygen and carbon exist in nature in various
stable isotopic species.
• There are three stable isotopes of oxygen:
16O, 17O, and 18O, with relative natural
abundances of 99.76%, 0.04% and 0.20%,
respectively.

9. • Because of the higher abundances
and the greater mass difference
between 16O and 18O, research on
oxygen isotopic ratios normally
concerns 18O/16O ratios.
• Carbon occurs as two stable
isotopes: 12C and 13C, with relative
natural abundances of 98.89% and
1.11%, respectively.

10.  The application of oxygen isotope
techniques to foraminifera was
developed with the aim of
determining the past temperature
of the sea floor, which had
previously been regarded as a
constant environment.
 The most successful applications
of isotope paleoclimatology have
been in the study of foraminifera
from deep-sea sediments.

11. • Foraminifera, a group of unicellular
eukaryotic protozoa, have widely been
used in palaeoclimatic and
palaeoceanographic reconstructions.
• Most foraminifera are marine, and many
secrete a test (or shell) made of
calcium carbonate (CaCO3; generally
low-Mg calcite, but high-Mg calcite in
porcelaneous species and aragonite in
some groups) and test-morphology
itself records their preferred
environmental conditions.

12. • The stable carbon (12C and 13C) and
oxygen (16O and 18O) isotopes present
within the calcitic tests of foraminifera
are widely used as key proxies for
palaeoenvironmental interpretations.
• Some foraminifera live among the
ocean plankton distributed in the
upper part of the water column; others
are benthic, living directly on the sea
floor or at shallow depths in the
sediment.

13.  Foraminifera are heterotrophs,
feeding on organic matter rather
than photosynthesizing,
although many species live in
symbiotic association with
photosynthetic algae.
• Foraminiferal tests can occur in
large numbers, and in many
places, they form a significant
component of the sea-floor.

14. • The oxygen in foraminiferal
calcite derives from the seawater
in which the organism lived.
• Hence, the isotope ratios can
provide information about the
composition and history of that
water, and the environmental
conditions in which the test was
secreted.

15.  Foraminifera and other organisms can
potentially preserve their original
isotope ratio for many millions of years.
 Planktonic foraminifer oxygen isotopes
are used to investigate the history of
past sea surface temperatures,
revealing the extent of past
'greenhouse' warming and global sea
surface temperatures.

16. Strontium isotopes.
 Strontium isotope stratigraphy relies on
measurement of 87Sr/86Sr in marine biogenic
carbonate or phosphate.
 Precipitation of these minerals involves
incorporation of strontium from seawater, which
will have an 87Sr/86Sr identical to that of oceanic
values, which is of the same value globally at any
point in time.
 The 87Sr/86Sr ratio changed systematically
through time and it is therefore possible to date
samples by placing them on a standard curve.

17.  The method works best for periods of
time over which there was a long-term
unidirectional shift in ratios, as during
the Tertiary.
 Strontium isotope stratigraphy gives a
maximum time-resolution of about 1
ma.

18. Strontium in seawater is derived from three
sources:
 fluvial input of material weathered
from
continental crust;
 hydrothermal leaching of oceanic
basalts
 at mid-ocean ridges; and
 recrystallisation of carbonate
minerals.

19.  Changing strontium values
reflect global changes in these
geological processes.
 Isotope geochemistry is an
aspect of geology based upon
study of the natural variations
in the relative abundances of
isotopes of various elements.

20.  Variations in isotopic abundance are
measured by isotope ratio mass
spectrometry, and can reveal
information about the ages and origins
of rock, air or water bodies, or
processes of mixing between them.