in Figure 2B, where bomb14 C has invaded everywhere except for the deep basin waters and older
sedimentary deposits.
In general, the global distribution of organic
14 C is
complicated by these interreservoir mixing and exchange processes. The more end-member sources
contributing organic carbon to a sample, the more
complicated it is to interpret a measured D
14 C value,
especially when trying to translate that value to
chronological time. Source-specific
14 C dating is
needed, and this requires isotopic measurements at
the molecular level.
Compound-specific
14
C Analysis:
Methods
The ability to perform natural-abundance
14
C measurements on individual compounds has only recently
been achieved. This capability arose from refinements
in the measurement of
14
C by AMS that allow increasingly small samples to be measured, and from
methods that resolve the complex mixtures encountered in geochemical samples into their individual components. Here we describe the methods
that are currently used for this purpose.
Selection of Compounds for
14 C Analysis
The organic matter in marine sediments consists of
recognizable biochemical constituents of organisms
(carbohydrates, proteins, lipids, and nucleic acids) as
well as of more complex polymeric materials and
nonextractable components (humic substances,
kerogen). Among the recognizable biochemicals, the
lipids have a diversity of structures, are comparatively easy to analyze by gas chromatographic and
mass spectrometric techniques, and are resistant to
degradation over time. These characteristics have
resulted in a long history of organic geochemical
Syringyl
Vanillyl
Isorenieratene
Peridinin
Fucoxanthin
Bacteriochlorophylls
Chlor ophyll- c
Chlor ophyll- b
Chlor ophyll- a
>C 24 Aldehydes
Ether Lipids
C 37 Alkenones
Poly--OH-Butyrate
Cyclopr opane Acids
C 18:1 7c
iC 17:1
10Me16:0
-am yrin
Friedelanol
Bacteriohopanepolyol
C 28-32 Alkanediols
4-Methyl Sterols
Dinosterol
9,10,18-TriOH-n-C18
9,16-DiOH-n -C 16
Abietic
3-Hydroxy
Iso and Anteiso F As
n -C 22:6
>C 22 n -Alkanoic
Pimaradiene
Heneicosahexaene
>C 24 n-Alkane
Alcohols
Polar
lipids
Other
lipids
Pigments
Lignin
ARCHAEBACTERIA
Methanog ens
Halophiles
PROCARYOTES
Other bacteria
Cyanobacteria
Sulfur-bacteria
EUCARYOTES
Fungi
Algae
green
diatoms
dinoflagellates
prymnesiophytes
brown
Mosses
Vascular plants
gymnosperms
angiosperms
eustigmatophytes
Carboxylic acids
Hydrocarbons
Figure 3 Common source assignments of lipid biomarkers (Modified from Hedges and Oades (1997).)
254 SINGLE COMPOUND RADIOCARBON MEASUREMENTS
sedimentary deposits.
In general, the global distribution of organic
14 C is
complicated by these interreservoir mixing and exchange processes. The more end-member sources
contributing organic carbon to a sample, the more
complicated it is to interpret a measured D
14 C value,
especially when trying to translate that value to
chronological time. Source-specific
14 C dating is
needed, and this requires isotopic measurements at
the molecular level.
Compound-specific
14
C Analysis:
Methods
The ability to perform natural-abundance
14
C measurements on individual compounds has only recently
been achieved. This capability arose from refinements
in the measurement of
14
C by AMS that allow increasingly small samples to be measured, and from
methods that resolve the complex mixtures encountered in geochemical samples into their individual components. Here we describe the methods
that are currently used for this purpose.
Selection of Compounds for
14 C Analysis
The organic matter in marine sediments consists of
recognizable biochemical constituents of organisms
(carbohydrates, proteins, lipids, and nucleic acids) as
well as of more complex polymeric materials and
nonextractable components (humic substances,
kerogen). Among the recognizable biochemicals, the
lipids have a diversity of structures, are comparatively easy to analyze by gas chromatographic and
mass spectrometric techniques, and are resistant to
degradation over time. These characteristics have
resulted in a long history of organic geochemical
Syringyl
Vanillyl
Isorenieratene
Peridinin
Fucoxanthin
Bacteriochlorophylls
Chlor ophyll- c
Chlor ophyll- b
Chlor ophyll- a
>C 24 Aldehydes
Ether Lipids
C 37 Alkenones
Poly--OH-Butyrate
Cyclopr opane Acids
C 18:1 7c
iC 17:1
10Me16:0
-am yrin
Friedelanol
Bacteriohopanepolyol
C 28-32 Alkanediols
4-Methyl Sterols
Dinosterol
9,10,18-TriOH-n-C18
9,16-DiOH-n -C 16
Abietic
3-Hydroxy
Iso and Anteiso F As
n -C 22:6
>C 22 n -Alkanoic
Pimaradiene
Heneicosahexaene
>C 24 n-Alkane
Alcohols
Polar
lipids
Other
lipids
Pigments
Lignin
ARCHAEBACTERIA
Methanog ens
Halophiles
PROCARYOTES
Other bacteria
Cyanobacteria
Sulfur-bacteria
EUCARYOTES
Fungi
Algae
green
diatoms
dinoflagellates
prymnesiophytes
brown
Mosses
Vascular plants
gymnosperms
angiosperms
eustigmatophytes
Carboxylic acids
Hydrocarbons
Figure 3 Common source assignments of lipid biomarkers (Modified from Hedges and Oades (1997).)
254 SINGLE COMPOUND RADIOCARBON MEASUREMENTS