Stable Carbon Isotopes in Palaeosol Carbonates
- Médard Thiry and
- Régine Simon-Coinçon
Published Online: 14 APR 2009
Copyright © 1999 The International Association of Sedimentologists
Palaeoweathering, Palaeosurfaces and Related Continental Deposits
How to Cite
Cerling, T. E. (1995) Stable Carbon Isotopes in Palaeosol Carbonates, in Palaeoweathering, Palaeosurfaces and Related Continental Deposits (eds M. Thiry and R. Simon-Coinçon), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304190.ch2
- Published Online: 14 APR 2009
- Published Print: 26 MAY 1995
Print ISBN: 9780632053117
Online ISBN: 9781444304190
- steady-state diffusion-production equation;
- diffusion-production model;
- evapotranspiration, driving carbonate precipitation in soils;
- homogenization, indicating bioturbation;
- carbon isotopes, for studying C3 C4 plants
The stable carbon isotope composition of carbonates formed in soils can be modelled using a steady - state diffusion-production equation. This model predicts an enrichment of 13C in soil CO2 compared with soil-respired CO2, which results from the difference in diffusion coefficients of 12CO2 and 13CO2 and from the influence of the atmosphere. Carbon isotope studies of modern soils show that the specific predictions of the diffusion-production model are fulfilled, giving confidence to predictions made by the model that are not readily testable.
The diffusion-production model was developed for soils where mass transport is controlled by gaseous diffusion and should not be applied to other conditions. It assumes isotopic equilibrium between all oxidized carbon species, which means it cannot be applied to soils (or palaeosols) where there is an inherited detrital component. For modern soils, the conditions of diffusion-controlled mass transport are met in those soils with high free-air porosity but not with those that are saturated with water. Identification of palaeosols meeting these conditions is more difficult than simply identifying palaeosols, so that care must be taken in establishing the character of palaeosols where this model is used.
The palaeoenvironmental interpretations of carbon isotopes in palaeosols include estimates of the fraction of C4 biomass in soils, which is very useful in the Neogene. An example from Pakistan shows the change from a C3-dominated to a C4-dominated biomass between about 7 and 5 Ma. Another important application of carbon isotopes in palaeosols is to the study of paleo-pCO2 levels of the atmosphere. Because the solution to the diffusion-production model includes atmospheric CO2 as one of the boundary conditions, studies of palaeosol carbonate can give estimates of ancient atmospheric CO2 levels. Preliminary studies of palaeosols indicate that atmospheric CO2 levels in the Mesozoic were between 2000 and 3000p.p.m.