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Journal of the Geological Society

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Further comments on using carbon isotopes in palaeosols to estimate the CO₂ content of the palaeo-atmosphere

T. E. Cerling writes: Wright & Vanstone (1991) raise some issues concerning the use of carbon isotopes in palaeosols to estimate the CO₂ content of the palaeo-atmosphere. I take this opportunity to clarify some of the points made in our previous research (Cerling 1984, 1991; Cerling et al. 1989, 1991; Quade et al. 1989a, b Quade et al. in press) and to discuss some of the potential problems in using palaeosols in this way. The issues raised by Wright & Vanstone include: (1) the model to describe the isotopic composition of pedogenic carbonate in soils; (2) the problem of identifying pedogenic carbonate in the geological record; and (3) the problem of the presence of C₄ or CAM plants in the geological record.

Model for the isotopic composition of pedogenic carbonate in the vadose (unsaturated) zone. Wright & Vanstone (1991) have considerably simplified the diffusion-reaction model of Cerling (1984, 1991) and present it as an empirical model with some elements of the model of Salomons et al. 1978 which was slightly modified by Salomons & Mook (1986). The diffusion-reaction model is critical to the discussion because it makes a number of specific predictions concerning modern soils which can be tested; it further predicts that isotopic composition of pedogenic carbonate has ¹³C-enriched values at high atmospheric CO₂ levels, which is the basis for the paper by Cerling (1991).

Some of the specific predictions that are clear from the work of Cerling (1984) are that: (a) there is an isotopic gradient in the uppermost part of

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