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

Abstract

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) 14C 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.

Outcrop distribution and formation of carbonate rhizoliths in Badain Jaran Desert, NW China

“…Previous studies in the desert suggested that rhizoliths could be used as an indicator of precipitation intensity (Gao et al, 1993;Yang, 2000;Yang et al, 2003;Chen et al, 2004;Li et al, 2015a, b;Li et al, 2017;Zhu et al, 2019). Therefore, this work combined with previous research (Sun et al, 2019b) confirmed that such application is still uncertain.…”

“…Carbonate rhizoliths (Sun et al, 2019b) or carbonate root tubes (Li et al, 2017) have been previously investigated in the dunes of the deserts of the Alashan Plateau, especially in the Badain Jaran Desert, Northwest China, (Gao et al, 1993;Yang, 2000;Yang et al, 2003;Chen et al, 2004;Li et al, 2015a, b;Li et al, 2017;Zhu et al, 2019; 4 / 28 Sun et al, 2019b). However, it is still unclear whether the rhizolith records can be effectively used to reconstruct the paleovegetation.…”

“…We further examined the macro-and micromorphological characteristics of the cementing minerals of the collected rhizoliths from the desert in laboratory. Altogether, these characterizations aim to achieve a better understanding of rhizoliths formation mechanisms and help in ascertaining the paleoenvironmental interpretations (Li et al, 2015a, b;Li et al, 2017 ;Zhu et al, 2019) and 14 C dating previously made by using these objects (Sun et al, 2019b) .…”

Holocene Millennial‐Scale Precipitation Variations in the Asian Summer Monsoon Margin of Northwest China and Their Relation to Migrations of Monsoon Northern Boundary via Oxygen Isotope Analysis of Calcareous Root Tubes in Deserts

“…(2015), Zhu et al. (2019). The CRTs in the four deserts were scattered horizontally or vertically on the surface of the sand layers (Figure 2).…”

Holocene climate background for lake evolution in the Badain Jaran Desert of northwestern China revealed by proxies from calcareous root tubes

“…Yang (2000) first used radiocarbon methods to date the CRTs and discussed the landscape evolution and precipitation changes in the desert. In contrast to lakes in the desert hinterland, which are affected by groundwater flows, the formation of CRTs is mainly controlled by changes in soil moisture, which is affected by precipitation and evaporation but not by groundwater (Li et al, 2015a(Li et al, , 2015bGao et al, 2019;Zhu et al, 2019). Moreover, 14 C carbonate dating for Holocene CRTs is reliable at the millennial scale (Kuzyakov et al, 2006;Pustovoytov et al, 2007;Gocke et al, 2011;Li et al, 2020b).…”