Carbon and oxygen isotopes of calcified root cells, carbonate nodules and total inorganic carbon in the Chinese loess–paleosol sequence: The application of paleoenvironmental studies

The continuous loess-paleosol sequence on the Chinese Loess Plateau (CLP) is a major terrestrial archive of paleoclimate changes. The carbon and oxygen isotopes of soil carbonate in the sequence have long been applied to reconstruct Quaternary climate changes, yet which soil carbonates has the integrity to signify the climate variability still remains a controversy. Hence, in this study, we compared carbon and oxygen isotopic compositions of soil inorganic carbon (TIC), carbonate nodules, and calcified root cells (CRCs) found in the Xifeng loess profile on the CLP to test their significance as paleoclimate indicators. The X-ray diffraction and scanning electron microscope analysis performed on CRCs prove that they are pure micro-scale calcite minerals with distinct epidermal cell structures of plant rootlets. The results of ¹⁴C dating show that CRCs and nodules have evident translocation of younger carbon while the TIC appears more or less consistent with the model ages. The δ¹³C_CRC values vary from −18.2‰ to −11.6‰, while the δ¹⁸O_CRC values range from −13.7‰ to −9.2‰ on the V-PDB scale, spanning 6.6‰ and 4.5‰, respectively. The contrast shows that δ¹³C_CRC values are more negative than those of nodules and TIC, and the δ¹³C_CRC values disperse within a large spectrum, which reflects a rapid germination of root cell calcification with little or even no time for diffusion process within individual C₃ plant roots. We argue that pure secondary micro carbonates like CRCs are closely associated with the plant-genic carbon. Their isotope values carry seasonal or even single event-based climate signals. Our study also suggests that stable isotope compositions of carbonate nodules and TIC are interfered by primary carbonate minerals. Thus, it should be used with caution when applying those materials for researches of paleoclimate changes.