Formation Mechanism And Paleoenvironmental Significance For Holocene Calcareous Root Tubes In The Alashan Desert

Calcareous root tubes,which are also called rhizoliths or calcified roots,are defined as secondary carbonate crusts formed by biological deposition of terrestrial higher plants.They are widespread in arid and semi-arid areas and is expected to become a new paleoenvironmental information carrier.And there are some controversies about the formation mechanism of Calcareous root tubes in different areas,which restricts the further study of the paleoenvironmental significance of Calcareous root tubes.Therefore,clarifying the formation mechanism and paleoenvironmental significance of the Calcareous root tubes can provide a new information carrier for the paleoenvironment reconstruction.Further extraction of paleoenvironmental signals and reconstruction regional paleoenvironmental change with Calcareous root tubes can not only recognize the reliability and representativeness of the existing environmental reconstruction results,but also expand the research field of the Holocene environmental evolution in this region,and provide new evidences for the study of the Holocene environmental evolution research in this region.The Alashan Desert,located in an arid area of northwestern China,is a very sensitive area about climate change and it's an important region for the study of paleoenvironment.Holocene Calcareous root tubes were widely distributed across the hinterlands of the Alashan Desert,but it is unclear what the formation mechanism and what the specific environmental significance of Holocene Calcareous root tubes in the Alashan Desert.In this study,we collected 32 Holocene Calcareous root tubes samples and 23 surface aeolian sand samples from the Badain Jaran desert,the Tengger Desert and the Ulan Buh Desert.Based on conventional ¹⁴C dating results of 32 Holocene calcareous root tubes samples,the microstructure of Calcareous root tubes was analyzed by scanning electron microscope?SEM?;the elemental composition of Calcareous root tubes and surface aeolian sand samples was determined by X-ray fluorescence;then the mineral composition was measured by X-ray diffraction.The formation mechanism and the environmental significance of the Calcareous root tubes would be discussed in this study.SEM results showed that the Holocene Calcareous root tubes in the Alashan Desert can be divided into two subgroups:rhizocretion and calcareous sheath.There was an obvious difference in microstructure between rhizocretion and calcareous sheath.The rhizocretion's inner side secondary carbonate cemented texture was uniform,and the hard crust on the outer side was cemented tightly;the morphology of secondary carbonate cementation in the inner part of the calcareous sheath was affected by the lateral quartz sand particles,and the outer part of the cement was covered with different dissolution pits.There were a large number of grain-coating calcite needle mats,calcified filaments,and needle-fiber calcite,which was formed by plant root activity and microbial activity of rhizosphere.The need-fiber calcite can be divided into M-type needle-fiber calcite,MB-type needle-fiber calcite,and P-type needle-fiber calcite.The M-type and MB-type needle-fiber calcite originate from the calcification of bacteria and fungi,while the P-type needle-fiber calcite was formed by the dissolution and precipitation process of the soil solution.The results of X-ray fluorescence spectrometry showed that the constant elements in the Holocene Calcareous root tubes and surface Aeolian sand samples include Fe₂O₃?Al₂O₃?SiO₂?K₂O?Na₂O?MgO and CaO.And trace elements include Cl?S?P?As?Ba?Bi?Ce?Co?Cr?Cu?Hf?La?Mn?Nb?Nd?Ni?Rb?Sr?Ti?V?W?Y?Zn and Zr.The highest content of constant elements in Calcareous root tubes was CaO and SiO₂.And the highest content of trace elements in Calcareous root tubes is S,Ti,Sr,Ba and Mn.The highest element content of surface aeolian sand samples is SiO₂;the highest content of trace elements in the surface aeolian sand is Ti,W and Ba.The results show that S,CaO and Sr with a high degree of enrichment in the Calcareous root tubes,Cl,MgO,P,Mn,Ni,Cu have a low or no enrichment in calcareous root tube.Ti,Fe₂O₃,Al₂O₃ and SiO₂ have a low degree of enrichment in the Calcareous root tubes.The cluster results show that when the critical value is equal 2,the Calcareous root tubes samples of the Alashan Desert can be subdivided into 5 major categories.The soil parent material and the climate factor were the main factors that causing elements differentiation of Holocene Calcareous root tubes in the Alashan Desert.X-ray diffraction results show that Holocene Calcareous root tubes contain silicate mineral,mineral oxides and hydroxides mineral,and carbonate mineral.Moreover,calcite and silicon are the main minerals in the Calcareous root tubes.Compared to surface aeolian sand samples,the Holocene Calcareous root tubes with a high secondary carbonate content?about 5²4 times than surface aeolian sand samples?,and silicate mineral was enriched with a low degree or not enrich in Holocene Calcareous root tubes.The correlation analysis results show that the secondary carbonate content in Holocene Calcareous root tubes have a significantly positive correlation with Sr,CaO,S,have a negative correlation withAl₂O₃?P?Ba?Bi?Ce?Hf?La?Nb?Rb?Ti?W?Y?Zr;the silicon minerals content in Holocene Calcareous root tubes have a significantly negative correlation with Sr?CaO and S,and have a positive correlation with Bi?Ce?Hf?Nb?Rb?Ti?W?Y and Zr.After Calcareous root tubes formed,the secondary carbonate layer wouldn't exchange element with the external soil,so the Calcareous root tubes inner secondary carbonate cementation layer can save high-resolution paleoenvironmental information.In this paper,the change of the Sr/Ca ratio in the Calcareous root tubes of the Tengger Desert in Holocene was used to reconstruct paleo-effective moisture.The results show that highest value of Sr/Ca ratio occurred in 8⁵ cal ka B.P.,and then decreases during5² cal ka B.P.The results of the reconstruction are as follows:the climate was relatively dry before 8 cal ka B.P.;relatively humid in 8⁵ cal ka B.P.;increasingly drier in 5² cal ka B.P.The reconstruction results are consistent with other records in adjacent areas.The above results show that:?1?Holocene Calcareous root tubes in the Alashan Desert was formed by the interaction of the rhizosphere calcification mechanism and evaporation of soil solution,which is affected by root activity,rhizosphere microbial activity and evaporation effect of the soil solution.Therefore,Holocene Calcareous root tubes in the Alashan Desert forming process would be divided into two stages.In the first stage:which during plant roots alive and decay period,the rapid accumulation of calcium carbonate minerals caused by root activity and rhizosphere microbial activity in the rhizosphere of plants,and then the soil particles,namely the quartz sand particles attachment around the plant roots surface.In the second stage:when after the plant root decayed completely,calcareous cementation in the outer concrete sediment.Compared to calcareous sheath at the first stage,the rhizocretion's have higher soil moisture in rhizosphere,or with a stronger root activity and more rhizosphere microbial activity,and the higher content of soluble carbonate mineral in the soil around the rhizocretion;in the second stage,compared to calcareous sheath,the rhizocretion had a stronger diagenesis or a weak weathering.The formation of the Calcareous root tubes in the Alashan Desert can indicate relatively humid climates.In addition,the Sr/Ca ratios of Calcareous root tubes can indicate the change of regional effective humidity.Therefore,reliable paleoenvironmental signals can be extracted from the Calcareous root tubes in the Alashan Desert that can be implemented to paleoenvironmental reconstruction,and the study of Calcareous root tubes can expand the research field of paleoclimate change in arid areas.In this study,the formation mechanism of Calcareous root tubes in the Alashan Desert was clarified,and the research field of Holocene environmental evolution in the region was expanded,which also provided a new evidence for the study of Holocene environmental evolution in this region.