Microstructure and quantitative tracing of carbonate for calcareous root tubes in dune soil from the Tengger Desert of Northwestern China: Implications for its formation age and paleoenvironmental reconstruction

Calcareous root tubes (CRTs), which are formed by the encrustation of roots with pedogenic carbonate, serve as an archive for paleoclimate reconstructions. However, controversies remain regarding the formation age of CRT carbonate and the quantitative tracing of its sources in the hinterland of the desert. In this study, 13 CRT samples were obtained from 11 sites in the Tengger Desert of northwestern China. Samples were described using X-ray micro-tomography Scanning (Micro-CT), and their 14C ages, δ13C, and 87Sr/86Sr values were measured. The results show that more than 99.84 % of pore diameters in the precipitated carbonate layer were less than 1 mm, which was much smaller than the plant roots in this region. Moreover, the root residues from poor cementation CRTs were dated at 151 − 431 cal yr BP, which was significantly younger than the carbonate age of other complete cemented CRT. Thus, most of the root residues found within completely cemented CRTs may invade after the CRTs formation and their 14C ages cannot represent the CRTs formation period, while the carbonate 14C dating results can indicate the CRTs formation period. 87Sr/86Sr values of CRTs occurred at 0.71152 − 0.71189, indicating that the main source of calcium in CRTs was the dissolution of carbonate in rhizosphere soil. δ13C values of CRTs varied between −5.4 ‰ and −3.2 ‰. Using the end-member mixing model, carbon sources from CRTs can be divided into rhizosphere soil respiration CO2 (81 − 82 %) and atmospheric CO2 (18 − 19 %). Hence, δ13C values of CRTs may derive the δ13C signal from soil and record paleoenvironment conditions. This study enhances our understanding of the formation mechanism of pedogenic carbonate and provides an important aspect in assisting paleoenvironmental reconstructions.