424
S. Wakeham
ments. A second means of inferring source and ecological setting is through light
(C, N, H, S) stable isotope tracers. Carbon fixation during photosynthesis results in a
differential partitioning (fractionation) of isotopes of different atomic masses, yielding different ratios of isotope abundances (for definitions and background, see Degens
1969; Fogel and Cifuentes 1993; Hayes 1993). Isotope ratios can be indicative of physiological processes occurring during photosynthetic fixation of inorganic carbon into
organic matter and during assimilation of organic compounds up the food chain. The
stable isotopic composition of bulk organic matter is an integrative signal. It will reflect isotope fractionation resulting from the range of metabolic processes and environmental conditions that occurred over the life of the organism or multiple organisms that biosynthesized the organic matter and over the time frame during which
the organism(s) lived and the sediments deposited. Interpreting isotope ratios for bulk
organic material can thus be ambiguous. Isotopic compositions of individual compounds, on the other hand, have the potential of isolating specific biosynthetic sources
and metabolic events in the life of an organism, although the biochemical mechanisms
and environmental factors determining isotopic fractionation are complex and the
subject of continuing research. Carbon isotopes have received the widest attention,
because potentially all organic matter and compounds carry a carbon isotopic signal.
Isotopes of nitrogen, sulphur, oxygen, and hydrogen have generally been less widely
used because of difficulties in analytical procedures (Fogel and Cifuentes 1993), but
this is rapidly changing.
Early studies of carbon isotopes found that there were metabolic controls on the
photosynthetic isotope fraction (lOp) that could be used to elucidate organic carbon
(OC) sources in geochemistry. As a consequence of different carbon fixation pathways
and fractionations, there is a wide range of isotopic compositions in various types of
living organisms (Fig. 19.1) and in different biochemical constituents (Degens 1969).
Most plants use the enzyme ribuiose-l,s-biphosphate-carboxyiase (RuBP-carboxylase)
during photosynthesis to produce a 3-carbon compound, hence the Crpathway. It is
the RuBP-carboxylase reaction that is responsible for the fractionation of carbon isotopes during photosynthesis, which is about 20-30%0. To a first approximation, land
plants are isotopically lighter (depleted in l3C relative to 12C with del-values (ol3C) of
about -25 to -30%0 according to the nomenclature; Degens 1969) than marine plankton (013C -18 to -22%0). This difference results in part from isotopic fractionation and
input from isotope differences for the source carbon, atmospheric CO2 for land plants
vs. dissolved CO2 for plankton. Recent work has shown that photosynthetic isotope
fractionation in the ocean is in fact quite complicated and the range of isotopic compositions considerably wider (ol3C -18 to -28%0). Photosynthtic isotope fractionation
by marine plankton depends of the concentration of dissolved CO2, the phytoplankton species physiology (do the cells take up CO2 by passive diffusion across the cell
membrane or do they actively pump CO2 or HC0 3 ), cellular volume, and cellular growth
rate, to name a few (see Popp et al. 1999 and references therein).
A few plants (e.g. grasses, sugarcane, corn) utilize the C 4 pathway that employs an
alternate enzyme, phosphoenol-pyruvate-carboxylase (PEP carboxylates), and produces a 4-carbon product. This reaction has a smaller isotope fractionation (-2%0).
Land plants fixing carbon by the C4-pathway are isotopically heavier (enriched in l3C;
ol3C -12 to -16%0) than plants employing the Crpathway.
S. Wakeham
ments. A second means of inferring source and ecological setting is through light
(C, N, H, S) stable isotope tracers. Carbon fixation during photosynthesis results in a
differential partitioning (fractionation) of isotopes of different atomic masses, yielding different ratios of isotope abundances (for definitions and background, see Degens
1969; Fogel and Cifuentes 1993; Hayes 1993). Isotope ratios can be indicative of physiological processes occurring during photosynthetic fixation of inorganic carbon into
organic matter and during assimilation of organic compounds up the food chain. The
stable isotopic composition of bulk organic matter is an integrative signal. It will reflect isotope fractionation resulting from the range of metabolic processes and environmental conditions that occurred over the life of the organism or multiple organisms that biosynthesized the organic matter and over the time frame during which
the organism(s) lived and the sediments deposited. Interpreting isotope ratios for bulk
organic material can thus be ambiguous. Isotopic compositions of individual compounds, on the other hand, have the potential of isolating specific biosynthetic sources
and metabolic events in the life of an organism, although the biochemical mechanisms
and environmental factors determining isotopic fractionation are complex and the
subject of continuing research. Carbon isotopes have received the widest attention,
because potentially all organic matter and compounds carry a carbon isotopic signal.
Isotopes of nitrogen, sulphur, oxygen, and hydrogen have generally been less widely
used because of difficulties in analytical procedures (Fogel and Cifuentes 1993), but
this is rapidly changing.
Early studies of carbon isotopes found that there were metabolic controls on the
photosynthetic isotope fraction (lOp) that could be used to elucidate organic carbon
(OC) sources in geochemistry. As a consequence of different carbon fixation pathways
and fractionations, there is a wide range of isotopic compositions in various types of
living organisms (Fig. 19.1) and in different biochemical constituents (Degens 1969).
Most plants use the enzyme ribuiose-l,s-biphosphate-carboxyiase (RuBP-carboxylase)
during photosynthesis to produce a 3-carbon compound, hence the Crpathway. It is
the RuBP-carboxylase reaction that is responsible for the fractionation of carbon isotopes during photosynthesis, which is about 20-30%0. To a first approximation, land
plants are isotopically lighter (depleted in l3C relative to 12C with del-values (ol3C) of
about -25 to -30%0 according to the nomenclature; Degens 1969) than marine plankton (013C -18 to -22%0). This difference results in part from isotopic fractionation and
input from isotope differences for the source carbon, atmospheric CO2 for land plants
vs. dissolved CO2 for plankton. Recent work has shown that photosynthetic isotope
fractionation in the ocean is in fact quite complicated and the range of isotopic compositions considerably wider (ol3C -18 to -28%0). Photosynthtic isotope fractionation
by marine plankton depends of the concentration of dissolved CO2, the phytoplankton species physiology (do the cells take up CO2 by passive diffusion across the cell
membrane or do they actively pump CO2 or HC0 3 ), cellular volume, and cellular growth
rate, to name a few (see Popp et al. 1999 and references therein).
A few plants (e.g. grasses, sugarcane, corn) utilize the C 4 pathway that employs an
alternate enzyme, phosphoenol-pyruvate-carboxylase (PEP carboxylates), and produces a 4-carbon product. This reaction has a smaller isotope fractionation (-2%0).
Land plants fixing carbon by the C4-pathway are isotopically heavier (enriched in l3C;
ol3C -12 to -16%0) than plants employing the Crpathway.