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

Carbon isotope analysis of pedogenic carbonate (δ13CCarb) and soil organic matter (δ13CTOC) is widely applied in reconstructions of terrestrial paleovegetation. The δ13C of different archives is considered well matched and equally reflects the proportion of C3/C4 plant biomass covering the soil profile. However, modern soil and paleosol sequences provide substantial evidence that δ13CCarb and δ13CTOC do not always match, raising doubts about the accuracy of quantitative C4 plant reconstructions. Here we report paired δ13C records of pedogenic carbonates and organic matter occluded within carbonate nodules from the Shaozhai section in the central Chinese Loess Plateau (CLP). The δ13CCarb record exhibits a positive anomaly and exceeds the theoretical fractionation range with the coexisting δ13CTOC record during the expansion of C4 plants. The possibility of contamination by detrital carbonates and atmospheric CO2 affecting δ13CCarb was ruled out based on the morphological features, mineral fractions, and geochemical composition of carbonate nodules. Our study suggests that the enhanced respiration of C4 plants during pedogenic carbonate precipitation may have caused positive shifts in δ13CCarb records, supporting the hypothesis that the discrepancy in carbon sources explains the δ13CCarb positive anomaly. Thus, the δ13CCarb could reflect the maximum relative abundance of C4 plants during their metabolic peaks.