be dated, can be particularly challenging. Volcanic ash
beds may provide the best dates for a sedimentary
sequence.
It is important to be able to identify the source of
clastic minerals in sedimentary rocks so that the areas
of erosion and sedimentation can be reconstructed.
Minerals like zircon can be characterised and dated,
based on their isotopic composition (Nd isotopes and
U-Pb).
3.4.1 Light stable isotopes
The fact that isotopes have different masses causes
fractionation to take place through both chemical and
biological processes. The simplest example is water,
H 2 O, which contains two oxygen isotopes and two
hydrogen isotopes. The evaporation of water to vapour
causes fractionation and concentrates the light water
molecule with
16 O. This is because the
18 O isotope has
greater mass and a phase change from fluid to vapour
therefore requires more energy. H
16
2 O has higher
vapour pressure than H
18
2 O. This is the reason why
rainwater and ice contain less
18 O than seawater.
Isotope fractionation is a function of temperature,
however, and is much more effective with evaporation
at low temperatures than at high ones. The explanation
for this is that at high temperatures the energy of the
molecules is so great that the difference in mass
between
18 O and
16 O is of less importance. At low
temperatures the isotopic separation evaporation is
much more selective so that the water evaporated is
more enriched in
16 O. When water vapour condenses
to rainwater, molecules with
18 O will condense faster.
Rain and snow becomes enriched in the heavier isotope (
18 O), so that the water vapour remaining in the
air becomes more enriched in
16
O. Most of the evaporation takes place at low latitudes and the water vapour
in the air has a progressively lower
18 O-content
towards higher latitudes as the air cools and it rains.
The concentration of oxygen isotopes is expressed in
relation to a standard:
δ
18 O ¼ R sample =R std À 1
À
Á Á 1000
R ¼
18 O=
16 O
The concentration of stable isotopes in minerals or
water is given as a ratio compared with certain
standards. The standards can then be analysed on a
mass spectrometer together with the samples. The δ
value thus represents the deviation from the isotopic
ratio in the standard.
This standard may be the average composition of
seawater, called SMOW (Standard Mean Ocean
Water). Another commonly used standard is PDB
(Pee Dee Belemnite), which is the composition of
calcite in a Cretaceous belemnite. The calcite
(CaCO 3 ) was precipitated in the sea and its composition was in equilibrium with the seawater at normal
temperatures (15–20
C). There is more
18 O in calcite
than in the water (positive fractionation), but with
higher temperatures the less effective fractionation of
oxygen lowers the δ
18 O values. The relationship
between the two standards is:
δ
18 O SMOW ¼ 1:031 Á δ
18 O PDB þ 30:8
PDB values are preferred for carbon isotopes in
carbonate minerals but also for oxygen in carbonates
while the SMOW scale is mainly used for water
samples and silicate minerals.
Hydrogen has two stable isotopes,
1 H and
2 H (deuterium), and an unstable one,
3 H (tritium), which has a
half-life of 12 years. The hydrogen isotopes are even
more strongly fractionated than oxygen isotopes during evaporation. Water molecules with deuterium
(heavy water) have lower vapour pressure than water
molecules with hydrogen and during evaporation the
heavier deuterium atom is depleted in the evaporated
water and concentrated in the remaining water which
also has higher salinity.
In meteoric (rainwater) water there is a linear relation between the deuterium/hydrogen ratio (D/H) and
the δ
18 O.
The isotopic composition of seawater has varied
through geological time, though not so much during
the last 200–300 million years. During glacial periods,
seawater acquires more positive δ
18 O values because
the water bound as ice has more negative δ
18 O values.
Rainwater (meteoric water) has normal δ
18 O values
from –2 to –15. The values become more negative
towards higher latitudes, and near the poles one can
3 Sedimentary Geochemistry
97
beds may provide the best dates for a sedimentary
sequence.
It is important to be able to identify the source of
clastic minerals in sedimentary rocks so that the areas
of erosion and sedimentation can be reconstructed.
Minerals like zircon can be characterised and dated,
based on their isotopic composition (Nd isotopes and
U-Pb).
3.4.1 Light stable isotopes
The fact that isotopes have different masses causes
fractionation to take place through both chemical and
biological processes. The simplest example is water,
H 2 O, which contains two oxygen isotopes and two
hydrogen isotopes. The evaporation of water to vapour
causes fractionation and concentrates the light water
molecule with
16 O. This is because the
18 O isotope has
greater mass and a phase change from fluid to vapour
therefore requires more energy. H
16
2 O has higher
vapour pressure than H
18
2 O. This is the reason why
rainwater and ice contain less
18 O than seawater.
Isotope fractionation is a function of temperature,
however, and is much more effective with evaporation
at low temperatures than at high ones. The explanation
for this is that at high temperatures the energy of the
molecules is so great that the difference in mass
between
18 O and
16 O is of less importance. At low
temperatures the isotopic separation evaporation is
much more selective so that the water evaporated is
more enriched in
16 O. When water vapour condenses
to rainwater, molecules with
18 O will condense faster.
Rain and snow becomes enriched in the heavier isotope (
18 O), so that the water vapour remaining in the
air becomes more enriched in
16
O. Most of the evaporation takes place at low latitudes and the water vapour
in the air has a progressively lower
18 O-content
towards higher latitudes as the air cools and it rains.
The concentration of oxygen isotopes is expressed in
relation to a standard:
δ
18 O ¼ R sample =R std À 1
À
Á Á 1000
R ¼
18 O=
16 O
The concentration of stable isotopes in minerals or
water is given as a ratio compared with certain
standards. The standards can then be analysed on a
mass spectrometer together with the samples. The δ
value thus represents the deviation from the isotopic
ratio in the standard.
This standard may be the average composition of
seawater, called SMOW (Standard Mean Ocean
Water). Another commonly used standard is PDB
(Pee Dee Belemnite), which is the composition of
calcite in a Cretaceous belemnite. The calcite
(CaCO 3 ) was precipitated in the sea and its composition was in equilibrium with the seawater at normal
temperatures (15–20
C). There is more
18 O in calcite
than in the water (positive fractionation), but with
higher temperatures the less effective fractionation of
oxygen lowers the δ
18 O values. The relationship
between the two standards is:
δ
18 O SMOW ¼ 1:031 Á δ
18 O PDB þ 30:8
PDB values are preferred for carbon isotopes in
carbonate minerals but also for oxygen in carbonates
while the SMOW scale is mainly used for water
samples and silicate minerals.
Hydrogen has two stable isotopes,
1 H and
2 H (deuterium), and an unstable one,
3 H (tritium), which has a
half-life of 12 years. The hydrogen isotopes are even
more strongly fractionated than oxygen isotopes during evaporation. Water molecules with deuterium
(heavy water) have lower vapour pressure than water
molecules with hydrogen and during evaporation the
heavier deuterium atom is depleted in the evaporated
water and concentrated in the remaining water which
also has higher salinity.
In meteoric (rainwater) water there is a linear relation between the deuterium/hydrogen ratio (D/H) and
the δ
18 O.
The isotopic composition of seawater has varied
through geological time, though not so much during
the last 200–300 million years. During glacial periods,
seawater acquires more positive δ
18 O values because
the water bound as ice has more negative δ
18 O values.
Rainwater (meteoric water) has normal δ
18 O values
from –2 to –15. The values become more negative
towards higher latitudes, and near the poles one can
3 Sedimentary Geochemistry
97