Page 388 - Earth's Climate Past and Future

364 APPENDIX 2 • Isotopes of Carbon

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surface water enriched in C, with δ C values of 1‰ of photosynthesis called the C4 pathway, which pro-
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or slightly higher. duces vegetation with less negative δ C values, ranging
Overall, organic carbon forms a small fraction of the between –11‰ and –15‰ and averaging –13‰. By
total carbon reservoir in the ocean, where inorganic far the largest amount of organic biomass on Earth’s
carbon predominates. The combined effect of the two land surfaces resides in C3 trees, and as a result the
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reservoirs (a small amount of organic carbon with a average δ C of vegetation on land (and of litter in soils)
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δ C value near –22‰ and a large amount of inorganic is –25‰.
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carbon with a δ C value near +1‰) yields a mean δ C For purposes of reconstructing past climates, the
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value of ~0‰ for all the carbon in the ocean. δ C composition of a wide range of fossilized materials
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Inorganic CO in the atmosphere with a δ C value contain can be analyzed on mass spectrometers. The
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of –7‰ is the carbon source for photosynthesis by same CaCO shells of marine foraminifera that yield oxy-
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plants. All trees as well as most shrubs and cool-climate gen for δ O analyses also provide carbon for δ C analy-
grasses use a type of photosynthesis called the C3 path- ses. Acid can be used to dissolve the shells and produce
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way, which produces organic tissue with δ C values in CO for analysis. Living vegetation or its organic residue
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the range of –21‰ to –28‰ and an average value of in soils as well as organic matter in plankton and ocean
–25‰. Some shrubs and most grasses that grow in hot sediments can also be analyzed on mass spectrometers
climates during the summer season use a different kind after the organic carbon is burned to produce CO gas.
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