Pedogenic carbonate recrystallization assessed by isotopic labeling: a comparison of ¹³C and ¹⁴C tracers

The C isotopic composition (δ 13 C) of pedogenic carbonates reflects the photosynthetic pathway of the predominant local vegetation because pedogenic (secondary) CaCO 3 is formed in isotopic equilibrium with soil CO 2 released by root and rhizomicrobial respiration. Numerous studies show the importance of pedogenic carbonates as a tool for reconstructing paleoecological conditions in arid and semiarid regions. The methodological resolution of these studies strongly depends on the time scale of pedogenic carbonate formation, which remains unknown. The initial formation rate can be assessed by 14 C labeling of plants grown on loess and subsequent incorporation of 14 C from rhizosphere CO 2 into newly formed carbonate by recrystallization of loess CaCO 3 . We tested the feasibility of 14 C and 13 C tracers for estimating CaCO 3 recrystallization rates by simultaneous 14 C and 13 C labeling and comparison with literature data. 14 C labeling was more efficient and precise in assessing recrystallization rates than 13 C labeling. This is connected with higher sensitivity of 14 C liquid scintillation counting when compared with δ 13 C measurement by IRMS. Further, assessment of very low amounts of incorporated tracer is more precise with low background signal (natural abundance), which is true for 14 C, but is rather high for 13 C. Together, we obtained better reproducibility, higher methodological precision, and better plausibility of recrystallization rates calculated based on 14 C labeling. Periods for complete CaCO 3 recrystallization, extrapolated from rates based on 14 C labeling, ranged from 130 (125–140) to 240 (225–255) y, while it was ≈ 600 (365–1600) y based on the 13 C approach. In terms of magnitude, data from late‐Holocene soil profiles of known age provide better fit with modeled recrystallization periods based on the 14 C approach.