Seasonality of C4 plant growth and carbonate precipitation in the Chinese Loess Plateau may cause positive carbon isotope anomalies in pedogenic carbonates

Abstract Carbon isotope analysis of pedogenic carbonate (δ 13 C Carb ) and soil organic matter (δ 13 C TOC ) is widely applied in reconstructions of terrestrial paleovegetation. The δ 13 C of different archives is considered well matched and equally reflects the proportion of C 3 /C 4 plant biomass covering the soil profile. However, modern soil and paleosol sequences provide substantial evidence that δ 13 C Carb and δ 13 C TOC do not always match, raising doubts about the accuracy of quantitative C 4 plant reconstructions. Here we report paired δ 13 C records of pedogenic carbonates and organic matter occluded within carbonate nodules from the Shaozhai section in the central Chinese Loess Plateau (CLP). The δ 13 C Carb record exhibits a positive anomaly and exceeds the theoretical fractionation range with the coexisting δ 13 C TOC record during the expansion of C 4 plants. The possibility of contamination by detrital carbonates and atmospheric CO 2 affecting δ 13 C Carb was ruled out based on the morphological features, mineral fractions, and geochemical composition of carbonate nodules. Our study suggests that the enhanced respiration of C 4 plants during pedogenic carbonate precipitation may have caused positive shifts in δ 13 C Carb records, supporting the hypothesis that the discrepancy in carbon sources explains the δ 13 C Carb positive anomaly. Thus, the δ 13 C Carb could reflect the maximum relative abundance of C 4 plants during their metabolic peaks.