Coupled carbon and oxygen isotope model for pedogenic carbonates

Abstract

Carbon and oxygen isotope values of ancient pedogenic carbonates reflect the paleoenvironmental conditions under which the carbonates formed. Carbon isotope values are commonly used to reconstruct pCO2, C3 vs. C4 plant types, and carbon cycle dynamics. Oxygen isotope values of the same carbonates have been used to reconstruct past temperatures and elevations. In the geologic record, changes in these isotope values can be difficult to interpret because they are affected by a suite of multiple local (e.g. precipitation and temperature) and global variables (e.g. pCO2 and global temperature) that can be difficult to separate. In this study, we develop a semi-mechanistic model of factors that affect carbon and oxygen isotope values of pedogenic carbonate. The model links these factors to a common suite of environmental variables using theoretical and empirical relationships. We test and optimize the model using modern environmental data and a compilation of modern pedogenic carbonate isotope measurements. This comparison indicates that the model best reproduces the observed data when carbonates are assumed to form under conditions where (1) evaporation affects oxygen isotope values of source waters at shallow soil depths only, (2) source waters mix precipitation from prior to and during the period of carbonate formation, and (3) soil CO2 respiration rates are anomalously low. Global predictions from the optimized model suggest strong spatial variation in both carbon and oxygen isotope values of pedogenic carbonate and highlight areas where future measurements from modern samples might provide strong tests of the model. The model also demonstrates coherent patterns of coupled δ13Ccarbonate and δ18Ocarbonate value variation associated with different types of environmental forcing, and offers a new framework for interpreting paired carbon and oxygen isotope data from ancient pedogenic carbonates to constrain paleoenvironmental change.