Variations in chemical element compositions in different types of Holocene calcareous root tubes in the Tengger Desert, NW China, and their palaeoenvironmental significance

Calcareous root tubes (CRTs) contain abundant information about palaeoenvironmental conditions and have been used for palaeoenvironmental studies in the desert hinterlands of arid regions. However, as subclasses of CRTs,calcareous sheaths and rhizocretions are formed by different processes, and it remains unclear whether these differences produce variations in the chemical element composition. Furthermore, it remains uncertain whether variations in chemical element concentrations amongst different subclasses of the CRTs can affect palaeoenvironmental reconstructions. In this study, we collected 54 CRT samples from the Tengger Desert of northwestern China. All samples were dated by accelerator mass spectrometry (AMS) C-14 dating, and the chemical element composition and concentration differences of the two CRT subclasses were determined using X-ray fluorescence spectrometry. The CRT samples were dated to the Holocene. The calcareous sheath and rhizocretion samples contained varying concentrations of the same chemical elements. The rhizocretions had high concentrations of mobile elements (Ca, Mg and Sr) and P, whereas the calcareous sheaths had high concentrations of stable elements, including Al, Si, Ti, Zr, Rb and Ba. These differences were due to the different formation processes of the two subclasses of CRTs. Moreover, the Sr/Ca and Mg/Ca ratios in the calcareous sheaths were higher than those in the rhizocretions from the same period, but these ratio differences had little effect on palaeo-effective moisture reconstructions at the millennial scale during the Holocene. The reconstructions were not influenced by the various CRT subclasses. The Holocene millennial-scale moisture changes in the Tengger Desert revealed by the Sr/Ca and Mg/Ca ratios showed that there was an arid period during the Early Holocene, a humid period during the Middle Holocene and a humid to arid period during the Late Holocene.