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10 Influence of Geochemical Processes on Stable
Isotope Distribution in Marine Sediments
TORSTEN BICKERT
10.1 Introduction
Stable isotope geochemistry has become an essential part of marine geochemistry and has
contributed considerably to the understanding
of the ocean’s changing environment and the
processes therein. In some fields, such as
biogeochemistry and paleoceanography, the
application of stable isotopes is still growing
due to new microanalytical techniques, permitting a relatively precise analysis of very small
samples or single compounds, which allows the
investigation of a new generation of problems.
Stable isotopes are useful tracers for reconstructing past temperatures, salinities, productivity, pCO 2 , nutrients, etc. However, limitations
exist on the application of these tracers because
of diagenetic processes which may considerably
alter the primary signals, due to preferential
preservation, decomposition or relocation of
particular tracers. On the other hand, stable isotope geochemistry offers the chance to iden-tify
and better understand such diagenetic processes, which is essential for the interpreta-tion
of isotope variability in marine sediments of the
past.
Within this chapter, we focus on four elements (C, O, N, S), which participate in most marine geochemical reactions and which are important elements in the biological system. Some
recent developments in the use of B isotopes
are added to this chapter. We summarize the
influence of geochemical processes on the stable isotope distribution of those elements in
ocean water and marine sediments. After a short
review on the fundamentals of stable isotope
fractionation and mass spectrometry, the most
important fractionation mechanisms for each element within the marine environment are described, followed by the identification of geochemical processes responsible for syn- or postsedimentary changes in isotope signals.
10.2 Fundamentals
Excellent summaries of the use of stable isotopes
in the study of sediments and the environment in
which they are deposited are given by Hoefs
(2004), and by Faure and Mensing (2005). The
fundamentals of stable isotope fractionation for
selected elements (esp. H, C, O, S) are also
presented in a special volume edited by Valley
and Cole (2001).
10.2.1 Principles of Isotopic Fractionation
The stable isotopic compositions of elements having low atomic numbers (e. g. H, C, N, O, S) vary
considerably in nature as a consequence of the
fact that certain thermodynamic properties of molecules depend on the masses of the atoms of
which they are composed. The partitioning of isotopes between two substances or two phases of
the same substance with different isotope ratios is
called isotopic fractionation. In general, isotopic
fractionation occurs during several kinds of
physical processes and chemical reactions:
• Isotope exchange reactions involving the redistribution of isotopes of an element among
different molecules containing that element.
• Kinetic effects, which are associated with unidirectional and incomplete processes such as
condensation or evaporation, crystallization or
10 Influence of Geochemical Processes on Stable
Isotope Distribution in Marine Sediments
TORSTEN BICKERT
10.1 Introduction
Stable isotope geochemistry has become an essential part of marine geochemistry and has
contributed considerably to the understanding
of the ocean’s changing environment and the
processes therein. In some fields, such as
biogeochemistry and paleoceanography, the
application of stable isotopes is still growing
due to new microanalytical techniques, permitting a relatively precise analysis of very small
samples or single compounds, which allows the
investigation of a new generation of problems.
Stable isotopes are useful tracers for reconstructing past temperatures, salinities, productivity, pCO 2 , nutrients, etc. However, limitations
exist on the application of these tracers because
of diagenetic processes which may considerably
alter the primary signals, due to preferential
preservation, decomposition or relocation of
particular tracers. On the other hand, stable isotope geochemistry offers the chance to iden-tify
and better understand such diagenetic processes, which is essential for the interpreta-tion
of isotope variability in marine sediments of the
past.
Within this chapter, we focus on four elements (C, O, N, S), which participate in most marine geochemical reactions and which are important elements in the biological system. Some
recent developments in the use of B isotopes
are added to this chapter. We summarize the
influence of geochemical processes on the stable isotope distribution of those elements in
ocean water and marine sediments. After a short
review on the fundamentals of stable isotope
fractionation and mass spectrometry, the most
important fractionation mechanisms for each element within the marine environment are described, followed by the identification of geochemical processes responsible for syn- or postsedimentary changes in isotope signals.
10.2 Fundamentals
Excellent summaries of the use of stable isotopes
in the study of sediments and the environment in
which they are deposited are given by Hoefs
(2004), and by Faure and Mensing (2005). The
fundamentals of stable isotope fractionation for
selected elements (esp. H, C, O, S) are also
presented in a special volume edited by Valley
and Cole (2001).
10.2.1 Principles of Isotopic Fractionation
The stable isotopic compositions of elements having low atomic numbers (e. g. H, C, N, O, S) vary
considerably in nature as a consequence of the
fact that certain thermodynamic properties of molecules depend on the masses of the atoms of
which they are composed. The partitioning of isotopes between two substances or two phases of
the same substance with different isotope ratios is
called isotopic fractionation. In general, isotopic
fractionation occurs during several kinds of
physical processes and chemical reactions:
• Isotope exchange reactions involving the redistribution of isotopes of an element among
different molecules containing that element.
• Kinetic effects, which are associated with unidirectional and incomplete processes such as
condensation or evaporation, crystallization or