Aridity-driven decoupling of δ¹³C between pedogenic carbonate and soil organic matter

Abstract

Pedogenic carbonate is an invaluable archive for reconstructing continental paleoclimate and paleoecology. The δ¹³C of pedogenic carbonate (δ¹³C_c) has been widely used to document the rise and expansion of C₄ plants over the Cenozoic. This application requires a fundamental presumption that in soil pores, soil-respired CO₂ dominates over atmospheric CO₂ during the formation of pedogenic carbonates. However, the decoupling between δ¹³C_c and δ¹³C of soil organic matter (δ¹³C_(SOM)) have been observed, particularly in arid regions, suggesting that this presumption is not always valid. To evaluate the influence of atmospheric CO₂ on soil δ¹³C_c, here we performed systematic δ¹³C analyses of paleosols across the Chinese Loess Plateau, with the sample ages spanning three intervals: the Holocene, the Late Pleistocene, and the mid-Pliocene warm period. Our paired δ¹³C_c and δ¹³C_(SOM) data reveal broadly divergent trending patterns. Using a two-component CO₂-mixing model, we show substantial incorporations of atmospheric CO₂ (up to 60%) into soil pore space during carbonate precipitation. This result readily explains the enrichment of δ¹³C_c and its divergence from δ¹³C_(SOM). As a consequence, δ¹³C of pedogenic carbonates formed under semiarid and/or arid conditions are largely driven by regional aridity through its control on soil CO₂ composition, and thus cannot be used to evaluate the relative abundance of C₃ versus C₄ plants. Nonetheless, these carbonates can be applied for atmospheric CO₂ reconstructions, even for periods with low CO₂ levels.