Reliability Of Paleoenvironmental Information Recorded By Calcareous Root Tubes In The Tengger Desert,NW China

The mid-latitude arid desert region is very sensitive to regional and even global climate changes.Paleoenvironmental evolutions in the desert region will be conducive to further understand the evolution of the regional and global environmental response.However,due to the paleoenvironmental reconstruction materials in the desert hinterland are difficult to acquire,the existing paleoenvironmental reconstruction results cannot fully evaluate the response of arid deserts to global changes both in time and space.Therefore,exploring the new paleoenvironmental reconstruction materials to improve the reliability of the reconstruction results is expected to promote in-depth research on climate and environmental changes in arid desert areas where geological records are relatively scarce.The Tengger Desert,as an important part of the deserts of northern China with strong sandstorm activity,is situated at the Asian monsoon margin and its climate is influenced by the combined effect of the Asian summer monsoon and westerlies.Hence,this desert is a critical area for the study of millennial-scale Asian monsoon changes at the northwestern margin of the monsoon region.In this area,calcareous root tubes(CRTs)contain abundant information about palaeoenvironmental conditions and have beenused for palaeoenvironmental studies in the desert hinterlands of arid regions.However,as a kind of secondary carbonates,CRTs may be suffered recrystallizaiotn after their formation due to changes of soil chemical and physical properties.Hence,the reliability of paleoenvironmental information recorded by CRTs may be confused.Unfortunately,whether further recrystallization occurred after CRT formation and whether the recrystallization affected the paleoenvironmental reconstruction results in this area is ambiguous so far.In addition,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,and it remains uncertain whether variations in chemical element concentrations amongst different subclasses of the CRTs can affect palaeoenvironmental reconstructions.Therefore,to clarify above problems will be conducive to improve the reliability of CRTs for Holocene paleoenvironmental evolution studies in desert areas.Hence,in this study,we selected the Tengger Desert in northern China as the study area,and a total of 127 CRT samples were collected from 40 sites the eastern,southern,and western portions of the Tengger Desert hinterland.The petrographic analysis,AMS¹⁴C dating,?¹³C and?¹⁸O values analyses,element analyses,as well as the Sr/Ca and Mg/Ca ratios between calcareous sheaths and rhizocretions during the Holocene millennial-scale.The results show as follows:(1)In this study,the precipitated carbonate layers collected from samples obtained in the Tengger Desert located in northwestern China were not affected by recrystallization,whereas most of the CRT detrital layers were affected by recrystallization due to the direct contact with soil environment.According to the luminescent characteristics and positions,four recrystallization characteristic types were identified in the detrital layer:no recrystallization,inner recrystallization,outer recrystallization and outer and inner recrystallization.The CRT detrital layer recrystallization is caused by the introduction of calcium-bearing meteoric waters after CRT formation.Moreover,CRTs from the same sampling site contained different recrystallization modes and there were no distinct recrystallization spatial patterns in the samples studied.This recrystallization characteristic is consistent with the occurrence of similar recrystallization characteristics reported in other regions with seasonal soil moisture deficits.(2)Recrystallization occurring in the CRT detrital layer can result in geochemical changes in the bulk CRT samples and detrital layers,making them unsuitable for ¹⁴C dating at the century scale.Moreover,?¹³C and?¹⁸O values obtained from the CRT detrital layer can also be altered.Carbonate from the precipitated carbonate layer in the CRT samples should be selected,because they can be suitable materials for ¹⁴C dating and?¹³C and?¹⁸O analyses.Paleoenvironmental reconstruction results derived from changes in the Sr/Ca and Mg/Ca ratios in CRTs should not be affected by recrystallization.Therefore,CRT detrital layers can be used in the Sr/Ca and Mg/Ca ratio determinations for paleoenvironmental reconstructions.(3)The calcareous sheath and rhizocretion samples contained the same chemical elements with varying concentrations.The rhizocretions had a high concentration of mobile elements(Ca,Mg and Sr)and P,whereas the calcareous sheaths had a high concentration of stable elements,including Al,Si,Ti,Zr,Rb and Ba.These differences are due to the different formation processes of the two subclasses of CRTs.Sr/Ca and Mg/Ca ratios from both calcareous sheaths and rhizocretions can be used as proxies for palaeo-effective moisture changes.Although the ratios of Sr/Ca and Mg/Ca in the calcareous sheathswere higher than those in the rhizocretions during the same period,they had little effect on the palaeo-effective moisture reconstruction at the millennial scale during the Holocene.The reconstruction results were not influenced by the various CRT subclasses.(4)Holocene millennial-scale moisture changes in the Tengger Desert,as revealed by the Sr/Ca and Mg/Ca ratios,indicate that there was an arid period during the Early Holocene(before 7 cal.ka BP),a humid period during the Middle Holocene(7–4 cal.ka BP)and a humid to arid period during the Late Holocene(after 4 cal.ka BP).This study clarifies the characteristics of recrystallization in CRTs and the influence on the results of paleoenvironmental reconstruction.Then,evaluates the differences of paleoenvironmental reconstructions between calcareous sheaths and rhizocretions(the subclasses of CRTs)as well as the outer clastic particle layer and the inner carbonate sedimentation layer.The results further confirmed that the CRTs is a new paleoenvironmental reconstruction material in arid desert areas.