10 Influence of Geochemical Processes on Stable Isotope Distribution in Marine Sediments
340
melting, adsorption or desorption, biologically
mediated reactions, and diffusion.
In general, light isotopes are more mobile and
more affected by such processes than heavy isotopes. The isotope fractionation that occurs during these processes is indicated by the fractionation factor α which is defined as the ratio R A of
the heavy to the light isotopes in one compound
or phase A divided by the corresponding isotope
ratio R B for the compound or phase B:
α A-B = R A / R B
(10.1)
For example, the fractionation factor for the exchange of 18 O and 16 O between water and calcium
carbonate is expressed as:
H 2
18 O + 1/3 CaC 16 O 3 ⇔
H 2
16 O + 1/3 CaC 18 O 3
(10.2)
with the fractionation factor α CaCO3 – H2O defined
as:
α CaCO3 – H2O =
( 18 O / 16 O) CaCO3 / ( 18 O / 16 O) H2O =
1.031 at 25°C
(10.3)
Because isotopic fractionation factors are close to
1, they can be expressed in ‰ with the introduction of the ε-value defined as
ε A-B = (α A-B - 1) · 1000
(10.4)
For geochemical purposes, the dependence of
isotope fractionation factors on temperature is the
most important property. In principle, fractionation
factors for isotope exchange reactions are also
slightly pressure-dependent, but experimental
studies have shown the pressure dependence to
be of no importance within the outer earth environments (Hoefs 2004). Occasionally, the fractionation factors can be calculated by means of partition functions derivable from statistical mechanics. However, the interpretation of observed variations of the isotope distribution in nature is
largely empirical and relies on observations in
natural environments or experimental results obtained in laboratory studies. A brief summary of
the theory of isotope exchange reactions is given
by Hoefs (2004).
10.2.2 Analytical Procedures
Stable isotope measurements on light elements are
made on gases, i.e. H 2 for hydrogen, CO 2 for carbon and oxygen, N 2 for nitrogen, and SO 2 or SF 6
for sulfur isotopes. A variety of techniques is
used to convert samples to a compound suitable
for analysis. The most important aspect of sample
preparation is to avoid isotopic fractionation.
Since molecules with different isotopic masses
have different reaction rates, procedures with less
than a quantitative (i.e. 100%) output may produce a reaction product that does not have the
same isotopic composition as the original sample.
Furthermore, a pure gas is necessary to avoid interference by contaminants in the mass spectrometer. Contamination may result from incomplete
evacuation of the vacuum preparation system or
degassing of the sample, as well as from unwanted
side reactions in the preparation procedures. In
general, the error attributable to sample preparation is greater than the instrumental analysis of
the product gas.
Isotopic abundance measurements for geochemical research are determined using mass
spectrometry. A mass spectrometer separates
and detects ions based on their motions in magnetic or electrical fields. For detailed information
on mass spectrometry and the according analytical techniques we refer the reader to the comprehensive review by Hoefs (2004). This volume also
gives an introduction in new microanalytical techniques, such as laser-assisted ablation, which allows the on-line transfer of submilligram quantities of mineral into a standard gas-source spectrometer (Kyser 1995), and the gas chromatography combined with mass spectrometry, which allows the determination of the isotopic composition of single compounds previously separated
by means of a gas chromatograph (Hayes et al.
1990; Merrit and Hayes 1994).
Boron isotope ratios are determined by positive or negative thermal ionization mass spectrometry (TIMS). Whereas the positive ionisation
technique requires microgram quantities of boron
producing Na 2 BO 2
+
or Cs 2 BO 2
+
ions (Spivack and
Edmond 1986), the negative ionisation technique
using BO 2
-
ions allows for analyzing nanogram
quantities of boron (Hemming and Hanson 1994).
Recently, Lecuyer et al. (2002) described the use
of multiple collector ICP-MS technique for boron
isotopic measurements of waters, carbonates,
340
melting, adsorption or desorption, biologically
mediated reactions, and diffusion.
In general, light isotopes are more mobile and
more affected by such processes than heavy isotopes. The isotope fractionation that occurs during these processes is indicated by the fractionation factor α which is defined as the ratio R A of
the heavy to the light isotopes in one compound
or phase A divided by the corresponding isotope
ratio R B for the compound or phase B:
α A-B = R A / R B
(10.1)
For example, the fractionation factor for the exchange of 18 O and 16 O between water and calcium
carbonate is expressed as:
H 2
18 O + 1/3 CaC 16 O 3 ⇔
H 2
16 O + 1/3 CaC 18 O 3
(10.2)
with the fractionation factor α CaCO3 – H2O defined
as:
α CaCO3 – H2O =
( 18 O / 16 O) CaCO3 / ( 18 O / 16 O) H2O =
1.031 at 25°C
(10.3)
Because isotopic fractionation factors are close to
1, they can be expressed in ‰ with the introduction of the ε-value defined as
ε A-B = (α A-B - 1) · 1000
(10.4)
For geochemical purposes, the dependence of
isotope fractionation factors on temperature is the
most important property. In principle, fractionation
factors for isotope exchange reactions are also
slightly pressure-dependent, but experimental
studies have shown the pressure dependence to
be of no importance within the outer earth environments (Hoefs 2004). Occasionally, the fractionation factors can be calculated by means of partition functions derivable from statistical mechanics. However, the interpretation of observed variations of the isotope distribution in nature is
largely empirical and relies on observations in
natural environments or experimental results obtained in laboratory studies. A brief summary of
the theory of isotope exchange reactions is given
by Hoefs (2004).
10.2.2 Analytical Procedures
Stable isotope measurements on light elements are
made on gases, i.e. H 2 for hydrogen, CO 2 for carbon and oxygen, N 2 for nitrogen, and SO 2 or SF 6
for sulfur isotopes. A variety of techniques is
used to convert samples to a compound suitable
for analysis. The most important aspect of sample
preparation is to avoid isotopic fractionation.
Since molecules with different isotopic masses
have different reaction rates, procedures with less
than a quantitative (i.e. 100%) output may produce a reaction product that does not have the
same isotopic composition as the original sample.
Furthermore, a pure gas is necessary to avoid interference by contaminants in the mass spectrometer. Contamination may result from incomplete
evacuation of the vacuum preparation system or
degassing of the sample, as well as from unwanted
side reactions in the preparation procedures. In
general, the error attributable to sample preparation is greater than the instrumental analysis of
the product gas.
Isotopic abundance measurements for geochemical research are determined using mass
spectrometry. A mass spectrometer separates
and detects ions based on their motions in magnetic or electrical fields. For detailed information
on mass spectrometry and the according analytical techniques we refer the reader to the comprehensive review by Hoefs (2004). This volume also
gives an introduction in new microanalytical techniques, such as laser-assisted ablation, which allows the on-line transfer of submilligram quantities of mineral into a standard gas-source spectrometer (Kyser 1995), and the gas chromatography combined with mass spectrometry, which allows the determination of the isotopic composition of single compounds previously separated
by means of a gas chromatograph (Hayes et al.
1990; Merrit and Hayes 1994).
Boron isotope ratios are determined by positive or negative thermal ionization mass spectrometry (TIMS). Whereas the positive ionisation
technique requires microgram quantities of boron
producing Na 2 BO 2
+
or Cs 2 BO 2
+
ions (Spivack and
Edmond 1986), the negative ionisation technique
using BO 2
-
ions allows for analyzing nanogram
quantities of boron (Hemming and Hanson 1994).
Recently, Lecuyer et al. (2002) described the use
of multiple collector ICP-MS technique for boron
isotopic measurements of waters, carbonates,