domain: sdu sde

text:
Qingfeng Sun

Kazem Zamanian

Arnaud Huguet

Keyu Fa

Hong Wang

Characterization and formation of the pristine rhizoliths around Artemisia roots in dune soils of Tengeri Desert, NW China

Keywords: Pristine rhizoliths, Dune soil, Redox condition, Artemisia roots, Soil moisture, Carbonatization

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Introduction

Rhizoliths are defined as organo-sedimentary structures produced through accumulation and/or cementation of mineral matters such as carbonates around or within vascular plant roots [START_REF] Klappa | Rhizoliths in terrestrial carbonates: classification, recognition, genesis, and significance[END_REF]. They are products of interaction between roots and soils/sediments, and occur mostly as trace fossils without showing root morphological structure [START_REF] Genise | Ichnoentomology: Insect Traces in Soils and Paleosols[END_REF][START_REF] Buatois | Ichnology: Organism-substrate interactions in space and time[END_REF]. But, occasionally rhizoliths occur in old strata as body fossils for petrified roots [START_REF] Klappa | Rhizoliths in terrestrial carbonates: classification, recognition, genesis, and significance[END_REF]. In paleontology and taphonomy, rhizoliths are also called fossil roots formed through fossilization, which comprises the processes leading to the preservation of trace of life in the geological record [START_REF] Li | The link between biomineralization and fossilization of bacteria: insights from field and experimental studies[END_REF]. Rhizoliths are important materials that can be used to study paleoclimate (Li et al., 2015a, b), paleoecology [START_REF] Nascimento | The paleoecological and paleoenvironmental importance of root traces: Plant distribution and topographic significance of root patterns in Upper Cretaceous paleosols[END_REF][START_REF] Khechai | Characterization and origin of gypsum rhizoliths of Ziban Oases soil-Algeria[END_REF] and paleohydrology [START_REF] Bojanowski | Root-related rhodochrosite and concretionary siderite formation in oxygen-deficient conditions induced by a ground-water table 31 / 51 rise[END_REF][START_REF] Kraus | Significance of different modes of rhizolith preservation to interpreting paleoenvironmental and paleohydrologic settings: examples from paleogene paleosols, Bighorn Basin, Wyoming[END_REF], as well as paleosols identification [START_REF] Barta | Secondary carbonates in loess-paleosoil sequences: A general review[END_REF][START_REF] Retallack | Soils of the Past: An Introduction to Paleopedology[END_REF] and carbon cycle [START_REF] Alonso-Zarza | Study of a modern calcrete forming in Guadalajara, Central Spain: An analogue for ancient root calcretes[END_REF].

Although many types of carbonate rhizoliths have been found since the Silurian geological period, their formation mechanisms have been widely debated with many hypotheses brought forward (Sun et al., 2019a,b;[START_REF] Li | Formation of calcareous nodules in loess-paleosol sequences: Reviews of existing models with a proposed new "per evapotranspiration model[END_REF][START_REF] Alonso-Zarza | Study of a modern calcrete forming in Guadalajara, Central Spain: An analogue for ancient root calcretes[END_REF]. Several related questions remain open, such as the relationship between vegetation types and rhizolith formation, the chemical and morphological differences between carbonate encrustations around living and/or deceased roots and the relationship between modern rhizosheaths and rhizoliths.

So far, no pristine rhizoliths around living or recently deceased roots have been found in dune soils, except in the Early Holocene dune sands of the Belaya River Valley (Upper Angara Region) of Russia [START_REF] Golubtsov | Carbonate Rhizoliths in Dune Sands of the Belaya River Valley (Upper Angara Region)[END_REF]. In the present study, we characterized pristine rhizoliths in dune soils of the Tengeri Desert (Northwest China), which are formed around deceased roots of Artemisia. The factors controlling their formation mechanisms were investigated for the first time in relationship with botanical, pedological and geological parameters. This study provides an excellent opportunity to better constrain the mechanisms of ancient rhizolith formation in dune soils, especially the nature of the interactions between sand minerals and roots in such settings.

Geographical setting of the study site

The investigated rhizoliths were found in dune surface soil overlying the bed of the Salt Lake Baijian in the Minqin Basin, central Tengeri Desert, NW China (Fig. 1a).

Previous studies [START_REF] Mischke | Holocene climate and landscape change in the northeastern Tibetan plateau foreland inferred from the Zhuyeze Lake record[END_REF] indicated that a Quaternary mega-lake, Zhuyezhe, was located between the terminal of the Shiyang River and the Laifu and Tangjia Ebo mountains. The present Salt Lake Baijian is the remnant of the Zhuyezhe paleolake (Fig. 1b; [START_REF] Chen | Environmental changes documented by 32 / 51 32 sedimentation of Lake Yiema in arid China since the Late Glaciation[END_REF]. Due to the diversion of river water from the upper reach for irrigation and other purposes, the lake has been dried up since the 5 / 51 1950s. Many parts of the paleolake bed have been buried by aeolian dunes and salty marsh deposits (Fig. 1b). The main vegetation around the paleolake currently comprises a shrubbery or semi-shrubbery xerophytic community [START_REF] Chang | A Primary Study on the Process of Formation and Succession of Desert Vegetation in Minqin (in Chinese with English abstract)[END_REF].

The annual precipitation (110 mm) is dominated by summer monsoon rainfall, which accounts for most of the annual precipitation, and a cold and dry continental airmass that is prevalent in winter. The mean annual air temperature is 7.8°C and the potential evaporation is over 2,600 mm [START_REF] Long | Holocene climate variations from Zhuyeze terminal lake records in East Asian monsoon margin in arid northern China[END_REF].

Methods and materials

Two types of rhizoliths were found in an area of 100 m x 100 m at the study site (Fig. 1): (i) deflated-out/weathered rhizoliths, out of dune soil (Fig. 2) and (ii) pristine rhizoliths, inside the dune soil profile (Figs. 3,4,5,6,7). Intensive investigations on rhizoliths, soils and plant roots were done with digging where the pristine rhizoliths had been found. Surface soil (20-40 cm thickness) was removed in the locations where the weathered rhizoliths were found, with the aim of finding intact pristine rhizoliths. Many pits (30-50 cm depth) were dug. Preserved pristine rhizoliths were observed in four locations, where they were generally buried under sand soil of ca. 10-30 cm thickness.

Eighteen specimens among the myriad deflated-out/weathered rhizoliths were collected randomly for lab analyses. We paid special attentions to the rhizoliths with root relicts within them when collecting the samples. Yet, only three rhizoliths with black curly threads or hair-like root relicts were found. Four pristine rhizoliths were only found and collected, which were air-dried at room temperature before laboratory analyses. Some roots of modern Artemisia were also collected for element analysis, and one soil sample of ~20cm depth was collected for analyzing bulk carbonate content.

The selected deflated-out/weathered and pristine rhizoliths were analyzed in China Petroleum University (Qingdao, China) using a Zeiss Scope A1 microscope (Zeiss, Carl Zeiss, Germany) and an FEI Quanta 450 FEG (Frequency Electronics, Inc., USA) scanning electronic microscope (SEM) coupled to energy dispersive X-ray spectrometer (EDS). The samples were impregnated with resin, cut into 4.8 cm × 2.8 cm slices and polished to create transverse or longitudinal thin sections to study their micromorphology under a polarizing microscope. Rhizolith fragments were gold coated for conductivity measurements, and then ultra-microscale observations and chemical compositions of the cement by SEM and EDS were acquired. A mixture of Alizarin red S and potassium ferricyanide was used to distinguish among carbonate minerals under the microscope [START_REF] Dickson | A modified staining technique for carbonate in thin section[END_REF][START_REF] Dickson | Carbonate identification and diagenesis as revealed by staining[END_REF]. One sample of pristine rhizoliths containing roots relicts (#Dong I) and one sample of deflated-out/weathered rhizoliths (#Dong II) were radiocarbon dated by accelerator-mass spectrometry (AMS; Beta Analytical Inc., Miami, Florida, USA). This laboratory uses standard procedures for different types of unknown samples (information can be found online at https://www.radiocarbon.com/beta-AMS-lab.htm). The measured 14 C ages were calibrated to calendar ages using the IntCal13 curve in Calib 7.1 (Talma & Vogel, 1993;Reimer et al., 2013). The δ 13 C and δ 18 O values of the dating samples were 7 / 51 measured in the laboratory separately using an isotope ratio mass spectrometer. Eight deflated-out/weathered and pristine rhizolith samples were also analyzed for δ 13 C and δ 18 O of carbonate cement using a Thermo Fisher Gasbench II-MAT 253 stable isotope mass spectrometer at the State Key Laboratory of Biogeology and Environmental Geology (China University of Geosciences, Wuhan) and the Institute of Geology (Chinese Academy of Geological Science, China), respectively. The detail procedures were previously reported by [START_REF] Song | Large vertical δ 13 C DIC , gradients in Early Triassic seas of the South China craton: Implications for oceanographic changes related to Siberian Traps volcanism[END_REF]Song et al ( , 2014)). In addition, the carbonate contents of the bulk soils were determined in Beta Laboratory (Miami, Florida, USA) using phosphoric acid dissolution.

Results

Field observation: rhizoliths, soil and Artemisia roots

Rhizoliths

The deflated-out/weathered rhizoliths were observed abundantly at the surface of the sandy soil in several locations around the site. These crumbly rhizoliths lie horizontally on the ground and are randomly scattered. They present short to long branched tube-like structures, with irregular morphology due to intense erosion and weathering (Fig. 2). Cross-section observations show that these rhizoliths have mostly circular shapes, with sizes from several millimeters to centimeters. Most of them are hollow and devoid of plant roots, some others being filled with loose dune sand particles and/or showing the presence of dark fiber roots. Shrubs of Artemisia sphaerocephala Krasch are still living sparsely in this site (Fig. 2; Sun et At the first location, the pristine rhizolith was thick and branched with two lateral crotches along its taproot (Fig. 3). This rhizolith likely originates from the deceased roots of Artemisia, consistent with the fact that (i) Artemisia sphaerocephala Krasch shrubs live there and that (ii) remains of black plant roots were observed within the rhizolith. The rhizolith, shortly after being exposure to the air and sun, became hard and fragile.

At the second location, a pristine rhizolith, a little harder than the surrounding wet soil, was observed under the surface (Fig. 4). After a few minutes of air contact both the rhizolith and the soil became dry and white under the sunshine and the blowing winds. This rhizolith, containing deceased root remains, was not connected to any nearby Artemisia plant bodies, either living or deceased.

At the third location, a pristine rhizolith, harder than the ambient soil, was discovered just under the surface at 3-5cm depth (Fig. 5). Dense, curly deceased hair roots binding sand particles together have been noticed inside this rhizolith (Fig. 6).

At the fourth location, a little harder, fragile, horizontally lying rhizolith was found in superficial loose soil (Fig. 7). A thread of main root was visible at the two ends of the pristine rhizolith.

Soils

The paleolake sediments are derived from the upstreams of Shiyang River (Qilian Mountains) and the dunes nearby (Fig. 1), and correspond to local in situ lacustrine deposits [START_REF] Mischke | Holocene climate and landscape change in the northeastern Tibetan plateau foreland inferred from the Zhuyeze Lake record[END_REF][START_REF] Long | Holocene climate variations from Zhuyeze terminal lake records in East Asian monsoon margin in arid northern China[END_REF]. Small low dunes are 9 / 51 being formed on the dry paleolake bed. From a pedological perspective, the soils correspond to dune sandsleptic regosols (FAO, 2014), formed in desert arid climate.

Its texture is loose fine to medium size sands (Fig. 3,4,5,6,7,8). The upper soil horizon (ca. 0-20 cm depth) is dry (Fig. 2,3,5,6,7) and the deeper soil is occasionally moist (Fig. 4,8). A lamellar crust (cemented sand) occurs in some rare parts of the soil surface (Fig. 8).

Roots of Artemisia

Numerous lateral roots, deceased and alive, were found within the dry soil horizon (ca. 10 cm depth) but were absent within deeper wet soil horizons (Fig. 8). Hair-like (Fig. 6) and lateral roots (Figs. 3,4,5,7) were observed within the pristine rhizoliths.

They are likely derived from Artemisia shrubs considering their external appearance and inner texture (Fig. 8) because the Artemisia shrubs are the only species growing around the sampling site (Fig. 2,3). Nevertheless, the modern living roots of the Artemisia do not show any evidence of carbonate accumulation and rhizolith formation.

Laboratory results

Mineralogical and petrological features of pristine rhizoliths

The thin section of rhizoliths (Fig. 9a) shows that the main clastic particles within the carbonate-cemented tubes are quartz, feldspars and lithic fragments. Quartz particles do not show clear evidence of weathering, but feldspar particles exhibit weathered rims, along which bright carbonate precipitates. The lithic fragments are motley-colored with stains and speckles. Staining confirmed that there are lithogenic carbonate fragments in the clastic particles that the cement is calcite with an aphanitic-crystalline texture (Fig. 9b). The lithogenic carbonate occurs as coarse fragments and pedogenic carbonate exhibits commonly precipitates in the silt-and clay-size fractions [START_REF] Kraimer | Mineralogical distinctions of carbonates in desert soils[END_REF].

The SEM images show that feldspar particles have partially been dissolved or eroded (Fig. 10). The cement, calcium carbonate, doesn't show fiber calcite as a biologically-produced mineral such as calcified filaments and calcified fungal hyphae (Sun et al., 2019a;[START_REF] Khormali | Experimental micropedology -A technique for investigating soil carbonate biogenesis along a desert-grassland-forest transect, New Mexico[END_REF]. There are pores, cavities and voids in the cement (Fig. 11,12a), which is mainly calcium carbonate (calcite) (Fig. 12b).

Radiocarbon ages

The AMS 14 C dates of two rhizolith samples (Fig. 13a,b; a pristine and a deflated-out/weathered, respectively) and the surrounding soil are listed in Table 1.

The deceased root relicts are dated as modern, with fractions of modern carbon-14 (F 14 C) of 1.0000  0.0050 and 1.0138  0.0038. The calibrated ages obtained using the CALIBomb program [START_REF] Hua | Atmospheric radiocarbon for the period 1950-2010[END_REF] are AD 1825130 and AD 19561, whereas the co-existing rhizoliths cements yield ages of 6355 and 6905 cal year BP (the error is 90 years; Reimer et al., 2013; Table 1). The age of the bulk soil carbonate is 18,670  220 years cal BP. While the age of the bulk soil carbonate is 18,670  220 year calBP.

Carbon and oxygen isotopes

The δ 13 C of the carbonate cement varies from 2.05‰ to 3.27‰ (mean = 2.55‰) within the pristine rhizoliths and from 3.20‰ to 6.80‰ (mean = 5.05‰) within the deflated-out/weathered rhizoliths (Table 2). The relative contributions of the different carbon sources in rhizoliths were estimated using mixing model based on stable carbon isotopic composition (δ 13 C). Results suggested that only 9.40% of carbon from decomposed plant roots and 90.6% of carbon from soil carbonates contributed to the rhizolith formation (Sun et al., 2019b). The δ 18 O of carbonate cement varies from 6.01‰ to 7.40‰ (mean = 6.8‰) within pristine rhizoliths and from 2.32‰ to 9.30‰ (mean = -4.54 and 3.56‰ with and without δ 18 O values of Dong II, respectively) within the deflated-out/weathered rhizoliths. The δ 18 O value of the carbonate cement (9.30‰) in the sample Dong II is the lowest among deflated-out/weathered rhizoliths (Table 2).

Carbonate contents of the bulk soil

The carbonate content of the bulk soil is 10.5%. Previous work showed that the average carbonate content in Tengeri Desert is 2.57% [START_REF] Wang | Carbonate content and carbon and oxygen isotopic composition of surface soil in the dust source regions of China[END_REF].

Interpretation

Natural conditions of pristine rhizolith formation

The fresh pristine rhizoliths showed that the carbonate cementing was taking 12 / 51 place but was incomplete within shallow soil, with semi-open/semi-closed redox conditions. Comparatively, the deeper wet-saturated soil was in a closed reduction environment. Only the moist soil environment favored complete rhizolith formation in these leptic regosols.

The deflated-out/weathered rhizoliths and pristine rhizoliths occur only at the places where Artemisia bushes are growing, suggesting that carbonatization encrustation are formed when this type of vegetation is present. However, the pristine rhizoliths occur only around the deceased Artemisia roots, suggesting that the decomposition of deceased roots likely provides organic acids required for carbonate formation, not the respiration of the living roots. Nevertheless, the deceased and living roots of Artemisia certainly have rather different physiological and biochemical mechanisms and products.

Cementing mineral

Microscopy analyses (Fig 9,10,11,12) showed that the cementing mineral, calcite, is homogenous, implying that calcite precipitation occurred in a stable soil environment around the roots. No petrified root texture was observed inside the rhizoliths. This indicated that syngenetic metasomatism and diagenesis did not occur within the shallow soils, the latter always resulting in fossil formation through permineralization, recrystallization, dissolution and replacement in closed redox environments such as sediments and rocks [START_REF] Brett | Comparative taphonomy: A key to paleoenvironmental interpretation based on fossil preservation[END_REF]. The calcite cement of rhizoliths may partly originate from bulk soil carbonates.

Ages of the rhizoliths

13 / 51

Radiocarbon dating (Table 1) indicates that the root relicts are of modern age, whereas the calcite cements are prehistoric, ranging from 6355 to 6905 cal years BP and the bulk soil carbonate is 18,670 cal years BP. Age discrepancies between root relicts and rhizolith cements suggest that the latter incorporated a significant amount of old carbon originating from primary and secondary carbonate minerals within the clastic debris of the soil matrix (Sun et al., 2019b). Such a significant incorporation also indicates the relatively fast formation rates of rhizoliths in the Tengeri Desert leading to the entrapment of old carbon [START_REF] Wang | Dryland ecohydrology and climate change: Critical issues and technical advances[END_REF]Gocke et al., 2011b;[START_REF] Kuzyakov | Carbonate re-crystallization in soil revealed by 14 C labeling: experiment, model and significance for paleo-environmental reconstructions[END_REF]) instead of complete recrystallization and re-equilibration with respired CO 2 of living roots or the atmosphere. Simple mass balance calculation (Sun et al., 2019b;[START_REF] Kuzyakov | Carbonate re-crystallization in soil revealed by 14 C labeling: experiment, model and significance for paleo-environmental reconstructions[END_REF] demonstrated that contamination of secondary CaCO 3 of Holocene age with even a small portion of primary CaCO 3 can entail strong overestimation of the true radiocarbon age. Nonetheless, the bulk carbonate content of soil is still sufficiently large to cause the reservoir impact on the radiocarbon ages of the dune sediment (X. [START_REF] Yang | Recharge to the interdune lakes and Holocene climatic changes in the Badain Jaran Desert, Western China[END_REF].

Therefore, the ages of root relicts suggest that the rhizoliths were formed in a very recent time and that the calcite cement ages are biased. Caution should be taken when 14 C ages are used to decipher strata, climate and environmental changes (Sun et al., 2019b).

Rhizoliths formation environment inferred by C and O isotopes

The δ 13 C and δ 18 O values of pedogenic carbonates in deserts and paleosols are good indices to reconstruct paleoclimate and paleoenvironment (Gocke, 2011b; 14 / 51 [START_REF] Monger | Scale and the isotopic record of C 4 plants in pedogenic carbonate: from the biome to the rhizosphere[END_REF].Two distinct morphologies of rhizoliths (Fig. 2, 3, 4, 5, 7) with specific δ 18 O and δ 13 C values (Table 2, Fig. 14) suggest the deflated-out/weathered rhizoliths on the one hand and pristine rhizoliths on the other hand have been formed under contrasting environmental conditions. The deflated-out/weathered rhizoliths, which have more enriched δ 18 O values (4.0 to 2.5 ‰) should be formed in a relatively more evaporative condition, comparing to the pristine rhizoliths with δ 18 O values of 9 to 6‰. The deflated-out/weathered rhizoliths hence have gone through high temperatures close to the soil surface [START_REF] Wright | Spherulites in calcrete laminar crusts: biogenic CaCO 3 precipitation as a major contributor to crust formationreply[END_REF] due to strong solar radiation or during the warmest seasons i.e. shortly after summer monsoonal rains.

The deflated-out/weathered rhizoliths should also have been formed over relatively short periods e.g. over one growing season when water was available for organisms' activity and root decomposition. This is also evident due to more depleted δ 13 C values (7.0 to 3.5 ‰) of the deflated-out/weathered rhizoliths referring to higher incorporation of respired carbon than carbon from the entrapped lithogenic carbonates (Table 2). The size, occurrence and δ 13 C values of the pristine rhizoliths (-3.5 to -2 ‰; Table 2) suggest that they have been formed via a mixture of biogenic and non-biogenic processes at deep soil depths [START_REF] Verrecchia | Needle-fiber calcite: A critical review and a proposed classification[END_REF], Verrecchia et al. 1995a). As water is nearly available during whole year in deep soil horizons (Fig. 4, 8, 15), δ 13 C of the pristine rhizoliths should be in equilibrium with isotopic composition of HCO 3 -in soil water. The formation of the rhizoliths likely stops at some points because of severe drought, surface soil erosion and denudation by wind and solar radiation, explaining in turn the enriched δ 13 C values of the 15 / 51 pristine vs. weathered rhizoliths (Table 2). These processes might also have led to the exposition of the deceased roots at the surface and/or the end of the root decay, as it most probably occurred for Dong II specimen (Fig. 2,13b).

6. Discussion: Carbonatization around the deceased roots of Artemisia

Factors controlling the formation of pristine rhizolith

The presence of Artemisia deceased roots and soil moisture are the main factors controlling rhizolith formation. Results showed that the pristine rhizoliths were formed by cementing dune sand particles through CO 2 released from decaying Artemisia roots. This triggered mineral weathering and calcite crystallization around the roots acting as nuclei. Thereafter, the rhizoliths were deflated out of the soil and weathered under wind and solar radiation. This mechanism of rhizolith formation can be argued as follows.

Deceased Artemisia roots

After intensive comparison of the morphological features of the rhizoliths and those of the modern roots of Artemisia and other regional vegetation, it appears that the decayed root relicts have the same characteristics as the dried residues of the modern Artemisia roots. The general morphologies of the weathered rhizoliths are also mostly similar to those of modern Artemisia roots. Therefore, Artemisia roots can be considered as the major host material (i.e. nucleus) for rhizoliths formation. The decay of deceased roots may have triggered rhizolith formation, via release of CO 2 16 / 51

during root degradation and production of both HCO 3 -and H + , creating acidic conditions, Ca-silicate weathering, carbonate weathering, and driving precipitation of carbonates [START_REF] Fa | Abiotic carbonate dissolution traps carbon in a semiarid desert[END_REF][START_REF] Wang | Carbon accumulation in arid croplands of northwest China: pedogenic carbonate exceeding organic carbon[END_REF][START_REF] Berg | Carbon dioxide mediated dissolution of Ca-feldspar: implications for silicate weathering[END_REF]. This is consistent with the two conditions necessary for the precipitation of carbonate concretions: supersaturation of dissolved carbonate and a nucleus for carbonate precipitation [START_REF] Raiswell | Mudrock-hosted carbonate concretions: A review of growth mechanisms and their influence on chemical and isotopic composition[END_REF]. However, it is unclear why rhizoliths are only formed around the deceased roots of Artemisia, rather than any other kind of vegetation.

Soil moisture

The pristine rhizoliths with sub-branches containing deceased hair-like roots (Fig. 6, 7) were discovered horizontally in relatively moist soil and/or upper shallow dry soil horizon but not in deeper continually wet layers (Fig. 3, 4, 5, 7). Thus, besides the deceased roots, the soil moisture content also plays a key role in rhizolith formation [START_REF] Alonso-Zarza | Initial stages of laminar calcrete formation by roots: examples from the Neogene of central Spain[END_REF]Verrecchia et al., 1995b). Nevertheless, if the moisture is too high in the deep soil, the deceased root will be in a reducing environment without enough air exchange, resulting in closed fermentation producing methane rather than carbon dioxide. On the other hand, if the soil is dry without occasional wetness, the root decay will not take place, or will be too slow to release CO 2 and generate HCO 3 - (Sun et al., 2019a). In our sampling site the 10-30 cm depth horizon is the most favorable one for rhizolith formation because of its occasional wetness (Fig. 3, 4, 5, 7, 8) due to climatic and seasonal precipitation variability.

Other factors influencing rhizolith formation

As discussed above, soil moisture and vegetation type (i.e. Artemisia shrubs) are the main factors controlling rhizolith formation. Nevertheless, soil moisture is dependent on other factors, such as soil depth, porosity, permeability, landscape position and climate. Based on field exploration the only water sources in Tengeri Desert soil are rainfall and snowfall, as no groundwater traces were observed in the sampling site. The sub-branches of the Shiyang River and the Salt Baijian Lake are fed by the upriver streams from the Qilian Mountains. In addition, the Asian monsoon provides rainfall water to the desert area in summer and the westerlies transport snowfall in winter [START_REF] Zhao | Holocene vegetation and climate change from a lake sediment record in the Tengger Sandy Desert, northwest China[END_REF]. The rainfall and snowfall accumulate into the swales and depressions among the small dunes. Soil moisture varies along the slopes of the dunes, from the crescent peaks to the beds of the swales and depressions. No

Artemisia is living in the upper slopes of the dunes where soil moisture is very low or in the beds of lakes because of high salinity. Artemisia only lives in the beds of the swales and depressions among the small dunes. Roots and rhizoliths can only be observed there (Fig. 2), showing that the morphology of landforms influences the water catchment of rainfall and snowfall and consequently soil moisture required for rhizolith formation in this area.

HCO 3 -and Ca +2 sources for rhizoliths formation

Carbon sources

There are potentially eight C sources for the formation of rhizolith carbonates in desert soils (Sun et al., 2019b): 1) CO 2 from the respiration of living roots; 2) CO 2 from the decomposition of deceased roots; 3) CO 2 from decay and respiration of 18 / 51 organic matter (other than decaying roots) in soil; 4) dissolved ancient pedogenic carbonates; 5) dissolved old lithogenic carbonates; 6) rainfall with carbonate dust; 7) dissolved inorganic carbon from groundwater; and 8) atmospheric CO 2 penetration.

Since no rhizoliths were formed around living roots, CO 2 respiration from such roots cannot be considered as a viable C source. All rhizoliths were observed around deceased roots. The decomposition of the latter provides some carbons necessary for the rhizolith formation, as during such process a fraction of the organic matter is volatilized as carbon dioxide, while residues are increasingly functionalized, with carboxyl, phenol, or hydroxyl groups [START_REF] Oste | Solid solution partitioning of organic matter in soils as influenced by an increase in pH or Ca concentration[END_REF][START_REF] Peinemann | Soil organic matter and its lignin component in surface horizons of salt affected soils of the Argentinian Pampa[END_REF].

HCO 3 -can also be derived from the dissolved old lithogenic carbonates and ancient pedogenic carbonates (Fig. 9b). The contribution of dissolved air-CO 2 in rainfall (carbonate) dust can be considered as negligible due to extremely low annual precipitation [START_REF] Zhang | Local circulation maintains the coexistence of lake-dune pattern in the Badain Jaran Desert[END_REF][START_REF] Dong | Geomorphology of the megadunes in the Badain Jaran Desert[END_REF], although after rainfall events CO 2 dissolved in rain is sequestered, at least temporarily, from exchange processes between soil and atmosphere, but it is then quickly released to the atmosphere under drier conditions [START_REF] Serranoortiz | Hidden, abiotic CO 2 flows and gaseous reservoirs in the terrestrial carbon cycle: Review and perspectives[END_REF]. Last, dissolved inorganic carbon in groundwater is not a likely source of carbon in Tengeri Desert where groundwater was not detected. Mass balance calculation suggests that degradation of plant roots and soil carbonates are the two main carbon sources of rhizolith cement (Sun et al., 2019b;[START_REF] Jin | Geochemistry of sediment moisture in the Badain Jaran Desert: implications of recent environmental changes and water-rock interaction[END_REF].

The δ 18 O of the pristine rhizoliths was remarkably lower than the one of deflated-out/weathered rhizoliths (Table 2), which may be due to oxygen isotope (H 2 18 O) fractionation during soil water evaporation. Indeed, as the evaporation rate of H 2 18 O is lower than that of H 2 16 O [START_REF] Merlivat | Molecular diffusivities of H 2 16 O, HD 16 O, and H 2 18 O in gases[END_REF], the 18 O can be enriched in the deflated-out/weathered rhizoliths which are exposed to the atmosphere, in contrast with the pristine rhizoliths preserved in a wet environment. In addition, the δ 13 C of the pristine rhizoliths was significantly higher than the one of weathered rhizoliths (p<0.05; Table 2). This suggests that the transformation of pristine to weathered rhizoliths when the former are exposed to atmosphere and wind erosion leads to carbon isotope fractionation processes.

The δ 13 C of pristine rhizoliths (3.27‰ ~ 2.05‰) is much higher than those of the atmosphere (about 8‰) and modern roots (about 18.9‰, Table 2). The fact that no rhizoliths were found around living roots at the investigated site implies that 13 C from atmosphere, living root respiration and secondary metabolic processes (living roots of Artemisia can secrete organic acid, [START_REF] Deng | Allelopathy of Artemisia Ordosica community in the process of plant succession[END_REF] which are 13 C-enriched [START_REF] Bowling | Carbon isotopes in terrestrial ecosystem pools and CO 2 fluxes[END_REF] cannot directly participate in the rhizolith formation process.

Although mass balance calculation suggests that the lithogenic carbonates and pedogenic carbonates are the main carbon sources (Sun et al., 2019b;[START_REF] Jin | Geochemistry of sediment moisture in the Badain Jaran Desert: implications of recent environmental changes and water-rock interaction[END_REF] for rhizolith formation, soil CO 2 remains an important and direct source of carbon.

When the pristine rhizoliths are transformed into weathered rhizoliths, the water is evaporated resulting in enrichment in 18 O and decomposition of H 13 CO 3 -. Ultimately, the deflated-out/weathered rhizoliths are 13 C-depleted and 18 O-enriched compared to pristine rhizoliths.

Ca 2+ sources

20 / 51 20 Ca 2+ originates from the weathering of both primary and secondary minerals [START_REF] Likens | The biogeochemistry of calcium at Hubbard Brook[END_REF] and has therefore typically been thought to persist or accumulate chiefly in semi-arid to arid environments [START_REF] Rowley | Calcium-mediated stabilisation of soil organic carbon[END_REF]. There are several potential Ca 2+ sources for CaCO 3 rhizolith cement: 1) authigenic carbonates from in situ chemical weathering of soil primary silicate minerals such as feldspar, mica, amphibole, and pyroxene, favored by acidic conditions in relationship with organic matter decomposition [START_REF] Monger | Soils as generators and sinks of inorganic carbon in geologic time[END_REF][START_REF] Durand | 9-Calcium carbonate features[END_REF] and enhanced by pressure-solution of crystallizations (Monger & Daugherty, 1991a); 2) dissolution of in situ primary lithogenic carbonate rock debris such as limestone and dolomite; 3) dissolution of ancient pedogenic carbonates after many generations of recyclable carbonatization [START_REF] Monger | Sequestration of inorganic carbon in soil and groundwater[END_REF]; 4) dissolved Ca 2+ from groundwater; 5) dissolved Ca 2+ in rainfall and snowfall; 6) decomposition of Ca 2+ -rich organic materials [START_REF] Ranjbar | Calcium, magnesium, sodium, and potassium release during decomposition of some organic residues[END_REF].

First, the in situ chemical weathering of soil primary silicate minerals can provide authigenic carbonates. Nevertheless silicate weathering in surface soils is expected to be extremely slow [START_REF] Nyachoti | Insight into factors controlling formation rates of pedogenic carbonates: A combined geochemical and isotopic approach in dryland soils of the US Southwest[END_REF][START_REF] Serranoortiz | Hidden, abiotic CO 2 flows and gaseous reservoirs in the terrestrial carbon cycle: Review and perspectives[END_REF] due to both the limited amount of rainfall and the relatively neutral soil water pH, with mean lifetimes of 102 to 107 years for carbonate generation [START_REF] Serranoortiz | Hidden, abiotic CO 2 flows and gaseous reservoirs in the terrestrial carbon cycle: Review and perspectives[END_REF]. The rates of plagioclase dissolution are expected to be even lower in dryland soils, where < 10% of soil Ca is present in silicate minerals and the dissolution kinetics is further limited by mineral surfaces [START_REF] Nyachoti | Insight into factors controlling formation rates of pedogenic carbonates: A combined geochemical and isotopic approach in dryland soils of the US Southwest[END_REF]. For example, it was suggested that the dissolution of silicate rocks contributes to a low extent only to the overall loading of Ca in pedogenic carbonates of the US Southwest [START_REF] Machette | Calcic soils of the southwestern United States. In: Soils and Quaternary Geology of the Southwestern United States (Author(s): David L. Weide)[END_REF][START_REF] Capo | Sources of strontium and calcium in desert soil and calcrete[END_REF]. Our bulk microscopy and SEM observations show that the phenomena of silicate weathering and dissolution within the particles are weak and difficult to be seen, with only bright crystalline colour around the clastic particles (Figs. 9, 10). Therefore, chemical weathering of silicate might represent a minor source of calcium for rhizolith formation in the Tengeri Desert.

Part of the cement is likely derived from authigenic carbonates. Precipitation of authigenic carbonates in soil is accompanied by the formation of Mg-rich clay minerals after the weathering of silicates [START_REF] Diazhernandez | Textural and isotopic evidence for Ca-Mg carbonate pedogenesis[END_REF]. In desert with high temperature and salinity, clay minerals like palygorskite and sepiolite are usually formed, favored by the arid to semi-arid conditions and a calcium and magnesium-rich environment [START_REF] Verrecchia | Occurrence and genesis of palygorskite and associated clay minerals in a Pleistocene calcrete complex, Sde Boquer, Negev Desert, Israel[END_REF][START_REF] Garcia-Romero | On the chemical composition of sepiolite and palygorskite[END_REF][START_REF] Galán | Palygorskite and sepiolite deposits in continental environments. Description, genetic patterns and sedimentary settings[END_REF]. Nevertheless, no authigenic clay minerals were observed in the thin sections of rhizoliths (Fig. 9, 10). Hence, it is unlikely that the processes operating today were also taking place in the past when rhizoliths were formed [START_REF] Diazhernandez | Textural and isotopic evidence for Ca-Mg carbonate pedogenesis[END_REF].

Second, limestone and dolostone clastic particles do not show obvious dissolution phenomena based on microscopy observations (Fig. 9b). It is unclear whether carbonate rocks provided enough calcium for rhizolith cementation.

Third, re-dissolution of pedogenic carbonates is impossible to be observed by microscopy and SEM (Fig. 9, 12) because bio-weathering processes in arid soils with predominance of calcite precipitation are complex [START_REF] Monger | Sequestration of inorganic carbon in soil and groundwater[END_REF].

Fourth, no groundwater was found at the sampling site, implying that groundwater dissolved carbonate is not a likely source of Ca 2+ .

Fifth, rainfall and snowfall are formed through condensation of cloud fresh water vapor or by evaporation of ocean and land water. They do not contain large amounts of salt and cannot provide large amounts of Ca 2+ , except within rare cases of dust storm rain of carbonate powders [START_REF] Duan | Modeling atmospheric transport and deposit ion 33 / 51 of calcium in China[END_REF].

The last potential source of calcium is related to the oxalate-carbonate pathway taking place during root decomposition [START_REF] Verrecchia | Litho-diagenetic implications of the calcium oxalate-carbonate biogeochemical cycle in semiarid calcretes, Nazareth, Israel[END_REF]. Artemisia is among plant species with high oxalate content (13.40 g kg -1 dry matter ; Huang et al., 2015). Hence, the release of oxalate during root decomposition might be a first step towards carbonate precipitation (Verreccchia 1990;Verreccchia et al., 2006). Altogether, our results

suggest that in the Tengeri Desert, rhizolith formation is primarily connected with root decomposition rather than water uptake by living root. This should be confirmed by further field investigations and lab analyses of roots from different types of vegetation.

The roles of other chemicals during root decomposition

Besides the acidity produces by CO 2 from decomposition of Artemisia roots, other minor chemical compositions of the root are S, N, P, etc, which produce acidic water with the negative ions of SO 4 2-, NO 3 -, PO 4 2-, etc (Fig. 15). They could enhance and accelerate the weathering of silicate minerals and the dissolution of the primary and secondary carbonate minerals in the soil horizon [START_REF] Heydari | Massive recrystallization of low-Mg calcite at high temperatures in hydrocarbon source rocks: implications for organic acids as factors in diagenesis[END_REF] Z. [START_REF] Yang | Recharge to the interdune lakes and Holocene climatic changes in the Badain Jaran Desert, Western China[END_REF], where abundant carbonate minerals in soil were also found 23 / 51 [START_REF] Wang | Pore fluid evolution, distribution and water-rock interactions of carbonate cements in red-bed sandstone reservoirs in the Dongying Depression, China[END_REF]. The carbon dioxide with high gas pressures certainly produced much acidity leading to much carbonate fragments of soil to be dissolved. Indeed, the CaCO 3 solubility in pure H 2 O at 25 °C is 0.013 g/L, whereas in weakly acid conditions (e.g. in presence of carbonic acid), the solubility increases up to five times [START_REF] Aylward | SI Chemical Data[END_REF]. The function of the inorganic acids above will cause a lower pH soil environment and further enhance particles dissolution and transformation, for instance from feldspar to carbonate. The salts formed by SO 4 2-, NO 3 -, and PO 4 2-with Na + and K + can be leached away and deposited in the lowest ponds and lakes of the desert [START_REF] Yang | Chemical weathering of the loess deposits in the lower Changjiang Valley, China, and paleoclimatic implications[END_REF][START_REF] Buggle | An evaluation of geochemical weathering indices in loess-paleosol studies[END_REF][START_REF] Liu | Holocene moisture change revealed by the Rb/Sr ratio of aeolian deposits in the southeastern Mu Us Desert[END_REF], as we observed in the Tengeri Desert. Therefore, the relationship between the chemical weathering of silicate minerals and CO 2 released by root decay is not straightforward as acidic conditions may be induced by microbial organic acids and sulfuric and nitric acids [START_REF] Karim | Weathering processes in the Indus River Basin: implications from riverine carbon, sulfur, oxygen and strontium isotopes[END_REF]Spence & Telmer, 2005) derived from the decomposition of Artemisia roots (Fig. 15) in addition to CO 2 .

Redox environment around Artemisia roots in soil

The main factors controlling the dissolution and precipitation of CaCO 3 (Gocke et al., 2011a;[START_REF] Drever | The geochemistry of natural waters[END_REF] in soil are: 1) CO 2 partial pressure in pore space; 2) pH of soil solution; 3) temperature and 4) mass flow of dissolved carbon species (H 2 CO 3 , HCO 3 ). Soil CO 2 partial pressure depends on root respiration and decomposition of organic compounds which subsequently affects soil pH. At the same time, temperature impacts root decomposition rates to produce CO 2 and influences the above mentioned factors [START_REF] Schlesinger | An evaluation of abiotic carbon sinks in deserts[END_REF], including the mass flow of acids (H 2 CO 3 , HCO 3 ).

The impact of these different factors vary between the soil horizons, with the formation of open, semi-closed or closed systems with respect to soil gas exchange with the atmosphere (Fig 15; [START_REF] Deines | Stable carbon isotope ratios and the existence of a gas phase in the evolution of carbonate groundwaters[END_REF]. As previously discussed, soil water content and gas exchanges with the atmosphere are vital factors for root decomposition and then carbonate formation in dune soils (Fig. 15). In the superficial horizon corresponding to an open O 2 -CO 2 system, desert soils have unlimited supply of dissolved inorganic carbon from the atmosphere and are characterized by low organic matter content due to spare vegetation. This is in turn influences the reactivity and solubility of carbonate minerals [START_REF] Williams | The carbonate system geochemistry of shallow groundwater/surface water systems in temperate glaciated watersheds (Michigan, USA): significance of open system dolomite weathering[END_REF][START_REF] Williams | The carbonate system geochemistry of shallow groundwater/surface water systems in temperate glaciated watersheds (Michigan, USA): significance of open system dolomite weathering[END_REF].

The rate of root decomposition is high at depths where water content is optimal [START_REF] Nizami | Dynamics of root and leaf decomposition in chronosequence of rubber plantation (hevea brasilensis) in 38 / 51 SW China[END_REF], optical means water content is most favorable to form a weak redox soil environment without less or too much water. This controls in turn the redox conditions (oxygen content from air) of the rhizosphere and the mineral phase formation (Fig. 15). Soil water fluctuations are controlled by rainfall, water vapor condensation and vertical movement of groundwater. Seasonal climate or diurnal weather changes, including increasing air temperature, evaporation intensity, and faster wind speed, may cause a water vapor deficit across the dunes. Some landscapes, such as interdunal swales, are characterized by optimal moisture conditions for carbonate precipitation by evaporation and rhizolith formation compared to the summits and slopes of dunes or interdunal lakes and ponds. The variable soil moisture conditions across the Tengeri Desert likely explains why rhizoliths were only found in some, but not all, places around Artemisia shrubs.

Carbonate epidiagenesis in semi-closed desert soil

25 / 51

Rhizoliths form only in semi-wet shallow desert soils, when Ca 2+ and HCO 3 become supersaturated around deceased roots of Artemisia. This corresponds to a dewatering (evaporative) phase [START_REF] Breecker | Seasonal bias in the formation and stable isotope composition of pedogenic carbonate in modern soils from central New Mexico[END_REF], concomitant with a decrease of pCO 2 [START_REF] Robbins | The CaCO 3 -CO 2 -H 2 O system in soils[END_REF][START_REF] Salomons | Isotope geochemistry of carbonate dissolution and reprecipitation in soils[END_REF]. During this transition phase from wet to dry conditions, as Ca 2+ cations are generally sufficiently abundant in the dune soils, they form carbonate minerals with HCO 3 -, which is confirmed by the carbonate content of the soils. Furthermore, during the decomposition of the roots, the decrease in soil water pH leads to the geochemical weathering of feldspars and other rock debris (Fig. 15) and in turn to the release of Ca 2+ and other alkali earth metal cations, such as K + , Na + , Ca 2+ and Mg 2+ . As salts based on K + and Na + are highly soluble in water, they can be leached out and transferred to the ponds and lakes in the desert area.

In contrast, Mg 2+ and Ca 2+ cations mostly remain in situ. Thus, geochemical weathering continuously and sustainably provides Ca 2+ cations for the formation of CaCO 3 in arid desert . Laboratory and field studies showed that only weeks or months are needed for fresh, undersaturated waters to reach equilibrium of calcite dissolution, the duration of the process depending mainly on pH and temperature conditions [START_REF] Serranoortiz | Hidden, abiotic CO 2 flows and gaseous reservoirs in the terrestrial carbon cycle: Review and perspectives[END_REF]. The transformations through time of clastic particles and soil carbonate minerals (i.e. diagenesis) will likely affect rhizolith formation [START_REF] Rossi | Facies-related diagenesis and multiphase siderite cementation and dissolution in the reservoir sandstones of the Khatatba Formation, Egypt's Western Desert[END_REF][START_REF] Mansour | Geochemical constraint on the origin of the multi-mineralogic carbonate cements in the subsurface middle Jurassic sandstones, Central Sinai, Egypt[END_REF]. This process should be accurately called epidiagenesis because it takes place in shallow loose sandy soil, without high overlying strata pressure and in connection with the atmosphere (Fig. 2, 3, 4, 5, 6, 7, 8;

Table 1).

/ 51

The cement texture is micritic-aphanitic . In general, micrite crystals of calcite indicate initial crystalline stages [START_REF] Zhou | The genesis of late Quaternary caliche nodules in Mission Bay, Texas: stable isotopic compositions and palaeoenvironmental interpretation[END_REF] and rapid precipitation [START_REF] Alonso | Carbonization in palaeosols formed on 30 / 51 terraces of the Tormes river basin (Salamanca, Spain)[END_REF]. This is consistent with the environmental conditions in desert, with a quick alternation of day-night temperatures and short shower events. Therefore, strongly alternating wet and dry conditions promote dissolution and precipitation of CaCO 3 and consequently formation of secondary carbonates [START_REF] Becze-Deák | Small scale secondary CaCO 3 accumulations in selected sections of the European loess belt: morphological forms and potential for paleoenvironmental reconstruction[END_REF][START_REF] Borchardt | Pedogenic calcite as evidence for an early Holocene dry period in the San Francisco Bay area, California[END_REF] in desert. In the CaCO 3 -CO 2 -HCO 3 -system, at higher temperatures, the low solubility of CO 2 decreases the CaCO 3 solubility, and thus promotes CaCO 3 precipitation [START_REF] Arkley | Calculation of carbonate and water movement in soil from climatic data[END_REF]. On the other hand, a large amount of CaCO 3 has to be in the dissolved state before precipitation, which can be promoted by increasing amounts of dissolved CO 2 from the decomposition of the deceased roots of Artemisia. During the day temperatures in the desert are high due to solar radiation on the soil surface without vegetation cover. In these conditions, formation and recrystallization of secondary CaCO 3 require less time (<10 2 -10 3 years) than at lower temperatures (Gocke & Kuzyakov, 2011). These processes occur in shorter periods of time if secondary CaCO 3 concretions form around the roots, preventing further dissolution and reprecipitation of CaCO 3 .

The cement, calcium carbonate, does not contain any biologically-produced fibers such as calcified filaments and calcified fungal hyphae (Sun et al., 2019a;[START_REF] Khormali | Experimental micropedology -A technique for investigating soil carbonate biogenesis along a desert-grassland-forest transect, New Mexico[END_REF], which might be caused fast by the high temperature in desert.

The higher calcium content of the cement compared to the soil matrix can be 27 / 51

explained by the dissolution of grains due to lithostatic pressure, which is restricted to grain-to-grain contacts [START_REF] Robins | Conceptual mineral genesis models for calcic pendants and petrocalcic horizons, Nevada[END_REF]. The force of crystallization which is caused by the growth of secondary minerals subsequently increases the solubility of interacting grains (Monger & Daugherty, 1991a;[START_REF] Robins | Conceptual mineral genesis models for calcic pendants and petrocalcic horizons, Nevada[END_REF]. The pressure is also generated by the precipitation of calcite crystals that grow and push against silicate grains (Monger & Daugherty, 1991b). In this case, silicate grains do not connect with each other within the calcite matrix, with mainly a basal cement pattern.

However, only a vague dissolution of silicate grains occurred at the calcite-silicate connection, with silicate grains matching the shapes of calcite crystals impacted against them (Fig 9, 10;[START_REF] Maliva | Diagenetic replacement controlled by force of crystallization[END_REF]. As a result, pressure-solution might have provided part of Ca source for the carbonate cement formation. Low pH caused by root decay of Artemisia (Fig. 15) might be operating in combination with pressure-solution (Monger & Daugherty, 1991a). Altogether, these two processes certainly accelerate chemical weathering of clastic particles and represent major Ca sources. The kinetics of cement formation is also related to the large specific surface area of minerals in sand dunes [START_REF] Whitfield | Predicting surface area of coarse-textured soils: Implications for weathering rates[END_REF][START_REF] White | Long-term flow-through column experiments and their relevance to natural granitoid weathering rates[END_REF], which can enhance chemical reactions [START_REF] Jung | Physical heterogeneity control on effective mineral dissolution rates[END_REF]. Mineral-water interfacial area controls the rates of many heterogeneous reactions [START_REF] Brantley | Surface area of primary silicate minerals. Growth, dissolution and pattern formation in geosystems[END_REF]. For instance, weathering rates of feldspars are proportional to their exposed surface areas [START_REF] Holdren | Reaction rate-surface area relationships during the early stages of weatheringinitial observations[END_REF]. Reaction rates are also proportional to the number of surface complexes with H, OH, or ligands [START_REF] Bloom | Kinetics of dissolution of oxide and primary silicate minerals[END_REF]. The mechanisms of feldspar dissolution and weathering are still under discussion [START_REF] Yuan | A review of feldspar alteration and its geological significance in sedimentary basins: from shallow aquifers to deep hydrocarbon reservoirs[END_REF]. In the present study, decomposition of Artemisia roots led to more acidic pH conditions and to feldspar transformation to other minerals such as carbonates.

In conclusion, the formation of rhizolith cement in semi-open desert soil is jointly favored by the acidic conditions induced by the decomposition of Artemisia root, the pressure-solution of silicate particles, and the large specific surface area of clastic fragments of soil.

Conclusions

Well-preserved pristine rhizoliths were found only along the deceased Artemisia deceased roots and microbial diversity in the rhizosphere [START_REF] Liu | Desert soil sequesters atmospheric CO 2 by microbial mineral formation[END_REF]Tang et al., 2020).

Figures and captions

  The cluster distribution of δ 13 C and δ 18 O values of the carbonate cement of rhizoliths gave two scopes (Fig.14): (i) the deflated-out/weathered rhizoliths fall in the same part of the diagram, with δ 13 C values < 3.5‰ and δ 18 O values > 5‰, except for Dong II with root relicts and (ii) the pristine rhizoliths fall in the other part of the diagram.

  roots in shallow sandy soil horizons in the swales and depressions among small dunes, of the Tengeri Desert, Northwest China. Rhizolith formation occurred in soils with intermediate moisture, high porosity and permeability, and semi-closed redox conditions in burial depths. These favorable factors firstly facilitated the production of CO 2 from root decomposition where dissolved CO 2 in the soil solution decreased the pH and triggered dissolution of primary and secondary rock debris as well as carbonate minerals. Thus, the decay of deceased roots and minerals weathering provided the carbon sources for the cement formation of pristine rhizoliths. Further, weathering of minerals like feldspars with the influence of pressure-solution and specific surface area of clastic mineral particles were involved in Ca release for carbonate formation. Soil texture and its associated permeability, rainfall fluctuations and its effect on soil moisture content, as well as wind speed variability and dune surface denudation played roles in locations where rhizoliths are formed. The ages of root relicts within the rhizoliths indicated that latter were formed over a relatively 29 / 51 short period of time in the recent decades. The age discrepancies between the carbonate rhizolith tubes and the root fabric relicts illustrated that caution should be taken when interpreting related data and discussing the significance of the rhizolith formation on the evolution of paleogeography, paleoclimate changes, past ecosystems and hydrological variations, as well as when characterizing paleosol development scales. Finally, it remains unknown why rhizoliths are absent around the roots of plant species other than Artemisia in the studied area. Further studies are needed to answer this question and especially investigating the biochemical composition of Artemisia

Fig. 1a .

 1a Fig. 1a. Location of the study area in Tengeri Desert. Fig. 1b.Sampling site (red dots) in the Paleolake Zhuyezhe.

Fig. 2 .

 2 Fig. 2. Fragments of whitish, tubular rhizoliths deflated/weathered on the sandy soil

Fig. 14 .

 14 Fig. 14. Cross-plot cluster diagram of δ 13 C and δ 18 O values of the carbonate cement of

  Fig.15. Conceptual diagram showing pristine rhizolith formation. Rhizoliths are

Acknowledgements

We express special gratitude to the anonymous reviewers and the editors for their constructive and valuable comments to improve the paper. We thank Wenhui Xue and Wentao Pei for assistant in field exploration; Darden Hood, Ronald Hatfield, Christopher Patrick for radiocarbon analysis; Pu Wang for microscopy and SEM work; Professor Lijun Ling for photographing; Professor Huyue Song for analyzing isotopes; Professor Xuelin Chen for identifying vegetation species; Professor Keyu Liu, Professor Guangui Yuan, Professor Viktor Golubtsov, and Doctor Eron Raines for their comments. We also thank Professor Fahu Chen for suggestions on this study. This work is supported by National Natural Science Foundation of China (No. 41561046) and Sino-German Mobility Project (M-0069).

Song, H., Tong, J., Algeo, T. J., Song, H., Qiu, H., Zhu, Y., Tian, L., Bates, S. M., Lyons, T.W., Luo, G., Kump, L. R., 2014. Early Krasch (the length of the ruler is 20cm). Fig. 3. A well-preserved pristine rhizolith were observed after removing the surface sand layer. The small shrubs are Artemisia sphaerocephala Krasch (the length of the ruler besides the pristine rhizolith is 20cm.).

Tables and captions


domain: sde sdu

text:
Qingfeng Sun

Arnaud Huguet

Kazem Zamanian

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

Keywords: Rhizoliths, Dune soil, Deflation and weathering, Formation factors, Soil environment

Carbonate rhizoliths and related features are environmental and ecological indicators of not only palaeosols, but also modern soils. They designate subaerial vadose and pedogenic diagenesis environments. Rhizoliths are commonly used as a tool for paleoenvironmental reconstructions, but uncertainty remains regarding their formation mechanisms. Field characteristics of the dune rhizoliths in the Badain Jaran Desert, NW China were analyzed to investigate formation mechanisms and interpret paleoenvironmental significance. Systematic field sampling and intensive examination showed that the tube-like rhizoliths without morphological structures of plant roots have been placed at the dune soil surface following erosion and weathering but the underground rhizoliths are still connected to the dead Artemisia roots. Rhizoliths occurred only along the windward, long gentle slopes of mega-dunes. Their distribution patterns of both types in connection with plant distribution and landscapes have been studied. Their petrological and mineralogical characters were performed in detail using microscopic and spectroscopic techniques (cathodoluminescence, scanning electronic microscopy and energy dispersive X-ray spectroscopy) in laboratory. Altogether, field and laboratory analyses showed that rhizolith cement is calcium carbonate and despite the high porosity of soil and high aridness of climate, *Revision, Unmarked Click here to download Revision, Unmarked: Rhizoliths outcrops of Badain Jaran Desert 2020 09 21 Revision Unmarked.doc 2 soil moisture plays an important role in sustaining Artemisia growth and rhizolith formation. Desert rhizoliths can therefore be used as a proxy to infer local soil moisture and plant communities. Moreover, as soil moisture content is related to climate and landscape position, rhizoliths could indirectly be indicators of aboveground environmental conditions, e.g. annual precipitation or humidity.

Introduction

Rhizolith, an umbrella term, has been used widely and includes many sub-terms, such as root tubules, root casts, root moulds, rhizocretions, root petrifactions, rhizolites, rhizomorphs, hypocoatings, rhizohalo, calcified root cells, and Microcodium (e.g. [START_REF] Klappa | Rhizoliths in terrestrial carbonates: classification, recognition, genesis and significance[END_REF][START_REF] Košir | Microcodium revisited: root calcification products of terrestrial plants on carbonate-rich substrates[END_REF][START_REF] Kraus | Significance of different modes of rhizolith preservation to interpreting paleoenvironmental and paleohydrological setting: Examples from paleogene paleosols, Bighorn Basin, Wyoming[END_REF][START_REF] Barta | Secondary carbonates in loess-paleosoil sequences: A general review[END_REF].

All these sub-terms are somewhat defined by the morphological features associated with their strata, sediments, or soils where they were persevered. Hence, morphological features of rhizoliths provide important evidence for rhizolith research.

The cementing minerals of rhizoliths can be carbonate, silica, clay, jarosite and rhodochrosite-siderite (e.g., [START_REF] Kraus | Significance of different modes of rhizolith preservation to interpreting paleoenvironmental and paleohydrological setting: Examples from paleogene paleosols, Bighorn Basin, Wyoming[END_REF][START_REF] Owen | Mineralogy and origin of rhizoliths on the margins of saline, alkaline Lake 22 / 28 Bogoria, Kenya Rift Valley[END_REF][START_REF] Bojanowski | Root-related rhodochrosite and concretionary siderite formation in oxygen-deficient conditions induced by a ground-water table rise[END_REF]. Carbonate is the most common one among them (e.g. Sun et al., 2019a).

Carbonate rhizoliths occur frequently in Quaternary terrestrial aeolianites of coastal regions (e.g., [START_REF] Klappa | Rhizoliths in terrestrial carbonates: classification, recognition, genesis and significance[END_REF], dunes (e.g., [START_REF] Cramer | A physiological mechanism for the formation of root casts[END_REF][START_REF] Golubtsov | Carbonate Rhizoliths in Dune Sands of the Belaya River Valley (Upper Angara Region)[END_REF], loess-palaeosol sequences (e.g. Wang et al., 2004;[START_REF] Gocke | Rhizoliths in loessevidence for postsedimentary incorporation of root-derived organic matter in terrestrial sediments as assessed from molecular proxies[END_REF][START_REF] Gocke | Carbonate rhizoliths in loess and their implications for paleoenvironmental reconstruction revealed by isotopic composition: δ 13 C, 14[END_REF][START_REF] Gocke | Lateral and depth variation of loess organic matter overprint related to rhizolithsrevealed by lipidmolecular proxies and X-ray tomography[END_REF][START_REF] Barta | Secondary carbonates in loess-paleosoil sequences: A general review[END_REF][START_REF] Huguet | Branched tetraether membrane lipids associated with rhizoliths in loess: Rhizomicrobial overprinting of initial biomarker record[END_REF][START_REF] Huguet | Root-associated branched tetraether source microorganisms may reduce estimated 20 / 28 paleotemperatures in subsoil[END_REF], eolian rocks and eolian sand bodies (e.g. [START_REF] Loope | Rhizoliths in ancient eolianites[END_REF], as well as lake margins (e.g., [START_REF] Owen | Mineralogy and origin of rhizoliths on the margins of saline, alkaline Lake 22 / 28 Bogoria, Kenya Rift Valley[END_REF]Sun et al., 2019a).

The texture and composition of Quaternary calcareous dune soils and the climatic settings in desert especially provide optimal conditions for the development of rhizoliths (e.g., [START_REF] Loope | Rhizoliths in ancient eolianites[END_REF][START_REF] Golubtsov | Carbonate Rhizoliths in Dune Sands of the Belaya River Valley (Upper Angara Region)[END_REF], which are abundant and of large size in such environments (e.g., [START_REF] Klappa | Rhizoliths in terrestrial carbonates: classification, recognition, genesis and significance[END_REF][START_REF] Rao | Dune associated calcrete, rhizoliths and paleosols from the Western Continental Shelf of India[END_REF][START_REF] Alonso-Zarza | Mega-rhizoliths in Pleistocene aeolian deposits on Gran Canaria (Spain): ichnological and palaeoenvironmental significance[END_REF][START_REF] Roberts | Last interglacial fossil elephant trackways dated by OSL/AAR in coastal aeolianites, Still Bay, South Africa[END_REF][START_REF] Cramer | A physiological mechanism for the formation of root casts[END_REF].

Rhizolith architecture and geometry is highly dependent on root type as well as diagenetic and pedogenic parameters. Even though most rhizoliths do not retain the morphological structures of their source plants [START_REF] Sarjeant | Plant trace fossils[END_REF], they can still be used to make some qualitative and quantitative inferences about the densities, sizes and types of former vegetation ecosystem [START_REF] Pfefferkorn | A field classification of fossil plant substrate interactions[END_REF][START_REF] Rinehart | Plant architecture and spatial structure of an early Permian woodland buried by flood waters, Sangre de Cristo Formation, New Mexico[END_REF]. Moreover, rhizoliths and the related features are also indicators of paleosols and hence of subaerial vadose environments in ancient successions and are widely used as a tool for paleoenvironmental reconstructions (e.g., [START_REF] Klappa | Rhizoliths in terrestrial carbonates: classification, recognition, genesis and significance[END_REF][START_REF] Golubtsov | Carbonate Rhizoliths in Dune Sands of the Belaya River Valley (Upper Angara Region)[END_REF][START_REF] Nascimento | The paleoecological and paleoenvironmental importance of root traces: Plant distribution and topographic significance of root patterns in Upper Cretaceous paleosols[END_REF]Sun et al., 2019a). Yet, the paleoecological and paleoenvironmental studies based on rhizoliths often came across difficulties in interpretation of 14 C ages and the stable isotope composition of carbonate cement (Sun et al., 2019). This is mostly due to the inclusion of lithogenic carbonates from the enclosing deposits and recrystallization of deposited carbonates as rhizolith cement (e.g. [START_REF] Gocke | Carbonate rhizoliths in loess and their implications for paleoenvironmental reconstruction revealed by isotopic composition: δ 13 C, 14[END_REF]Sun et al., 2019b;[START_REF] Golubtsov | Carbonate Rhizoliths in Dune Sands of the Belaya River Valley (Upper Angara Region)[END_REF].

Furthermore, the formation mechanisms of rhizoliths in sedimentary environment, especially deserts, still remain highly uncertain (Sun et al., 2019a).

Carbonate rhizoliths (Sun et al., 2019b) or carbonate root tubes [START_REF] Li | Holocene vegetation signals in the Alashan Desert of northwest China revealed by lipid molecular proxies from calcareous root tubes[END_REF] have been previously investigated in the dunes of the deserts of the Alashan Plateau, especially in the Badain Jaran Desert, Northwest China, [START_REF] Gao | Preliminary study on the Holocene desert evolution in the NW boundary of the Asia monsoon (in Chinese with English abstract)[END_REF][START_REF] Yang | Landscape evolution and precipitation changes in the Badain Jaran Desert during the last 30,000 years[END_REF][START_REF] Yang | Evolution of megadunes and lakes in the Badain Jaran Desert, Inner Mongolia, China during the last 31,000 years[END_REF][START_REF] Gu | Challenge from the appearance of vadose water within the surface layer of megadunes, Badain Jaran dune desert, Inner Mongolia[END_REF]Li et al., 2015a, b;[START_REF] Li | Holocene vegetation signals in the Alashan Desert of northwest China revealed by lipid molecular proxies from calcareous root tubes[END_REF][START_REF] Zhu | Variations in chemical element compositions in different types of Holocene calcareous root tubes in the Tengger Desert, NW China, and their palaeoenvironmental significance[END_REF] 4 / 28 Sun et al., 2019b). However, it is still unclear whether the rhizolith records can be effectively used to reconstruct the paleovegetation. Much remains unknown about the impact of fossilization processes, even though these may introduce biases into palaeobiological interpretations. Systemic studies of rhizolith features are still scarce and rudimentary in this area [START_REF] Li | Holocene vegetation signals in the Alashan Desert of northwest China revealed by lipid molecular proxies from calcareous root tubes[END_REF]Sun et al., 2019b), despite usually large numbers of rhizoliths can be found, especially, on a vast area, which provides good data for rhizoliths study. Most importantly and firstly, rhizoliths study relies on the field description of fossil morphologies to understand the evolutionary history of plant and the fossilization mechanism. Fossil morphologies under field conditions remain poorly understood. Therefore, this project focuses on the intensive analyzing topographical distributions and architectural patterns of rhizoliths in the field. 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;[START_REF] Li | Holocene vegetation signals in the Alashan Desert of northwest China revealed by lipid molecular proxies from calcareous root tubes[END_REF][START_REF] Zhu | Variations in chemical element compositions in different types of Holocene calcareous root tubes in the Tengger Desert, NW China, and their palaeoenvironmental significance[END_REF]) and 14 C dating previously made by using these objects (Sun et al., 2019b) .

Geographical setting of Badain Jaran Desert

The Badain Jaran Desert (39°04′15″N to 42°12′23″N, 99°23′18″E to 104°34′02″E) is located in northwestern China (Fig. 1). This desert comprises the tallest mega-dunes in the world, with interdunal lakes frequently occurring between the dunes. The mega-dunes are especially dense and high in the southeastern part, where they can reach up to 500 m a.s.l [START_REF] Wang | Evolution and variability of the Asian monsoon system: state of the art and outstanding issues[END_REF][START_REF] Zhu | Study on the boundary and the area of Badain Jaran Desert based on the remote sensing imagery[END_REF][START_REF] Yang | Quaternary environmental changes in the drylands of China -A critical review[END_REF]Wang et al., 2015).

/ 28

This desert is an arid desert climate according to the Köppen climate classification [START_REF] Kottek | World Map of the Köppen-Geiger climate classification updated[END_REF]. The annual precipitation is currently 40-120 mm, falling mainly in summer, and the annual potential evaporation is over 2500 mm. The conventional precipitation (below 5 mm) accounts for approximately 90% of rainfall events [START_REF] Wang | A preliminary study of precipitation characteristics in the hinterland of Badain Jaran Desert[END_REF]. The mean annual air temperature ranges between 9.8 and 10.2°C. The prevailing cold-arid winter wind direction is northwest and the strongest winds occur in spring and the mean annual wind speed ranges between 2.8 and 4.6 m/s (Fig. 1, [START_REF] Dong | Geomorphology of the megadunes in the Badain Jaran Desert[END_REF][START_REF] Hu | Surface energy and water vapor fluxes observed on a megadune in the Badain Jaran Desert[END_REF][START_REF] Zhang | Local circulation maintains the coexistence of lake-dune pattern in the Badain Jaran desert[END_REF].

The vegetation of this desert is dominated by shrubs and subshrubs, including Haloxylon, Nitraria, Tamarix, and Reaumuria species (Wang et al., 2015a). Perennial woody Artemisia scoparia and herbaceous Psammochloa villosa and Phragmites australis grow on the mega-dunes due to the existence of moist sand layers.

The grain size of surface sand layers is dominated by well-sorted, rounded, pinky-grey, middle to fine sand, containing only a slight amount of melanocratic minerals (such as hornblende, pyroxene and magnetite), with carbonates minerals being less abundant in the silicate desert sands [START_REF] Gates | Conceptual 19 / 28 model of recharge to southeastern Badain Jaran Desert groundwater and lakes from environmental tracers[END_REF][START_REF] Bai | Internal Structure and Formation Process of Mega-Dunes in the Badain Jaran Desert[END_REF]. The average carbonate content in soils was reported as 4.6 % by Wang et al. (2004) and 6.5 % by Sun et al. (2019b). The thickness of the upper dry soil of the mega-dune is only about 20 cm, with a conspicuous moist soil under it. The water content (by weight) is usually 1-5% between 0.2-1.5 m depths [START_REF] Gu | Challenge from the appearance of vadose water within the surface layer of megadunes, Badain Jaran dune desert, Inner Mongolia[END_REF][START_REF] Zhao | Moisture content of sand layer and its origin in a mega-dune area in the Badain Jaran Desert (in Chinese with English abstract)[END_REF][START_REF] Wang | Optical dating reveals that the height of Earth's tallest megadunes in the Badain Jaran Desert of NW China is increasing[END_REF] and is up to 10% at 90 cm depth with well-preserved sand bedding [START_REF] Liu | Growing pattern of mega-dunes in the Badain Jaran Desert in China revealed by luminescence ages[END_REF]. At 2.0-8.3 m depth, water contents vary between 2-5.0% [START_REF] Zhao | Moisture content of sand layer and its origin in a mega-dune area in the Badain Jaran Desert (in Chinese with English abstract)[END_REF]. The high moisture is in low-lying and flat land, which is benefited from the precipitation infiltration [START_REF] Zhao | Moisture content of sand layer and its origin in a mega-dune area in the Badain Jaran Desert (in Chinese with English abstract)[END_REF].

Mega-dunes feature that small transverse dunes superimposed on the larger 6 / 28 transverse dunes can be classified into three hierarchical orders (Fig. 2, [START_REF] Dong | Geomorphological hierarchies for complex mega-dunes and their implications for mega-dune evolution in the Badain Jaran Desert[END_REF]. Order III corresponds to an erosion belt, next to the shore of lakes; Order II is a belt of both erosion and deposition above Order III; and Order I is a deposition belt.

Order III and II are situated on the windward gentle slopes and Order I dunes correspond to the leeward slopes (slipface), which are steep and even.

Field survey, examination and sampling

Owing to the vast sand sea area, poor accessibility and the dominance of an extremely arid climate, field explorations are only conducted in zones located in the southeastern part next to the desert margin (Fig. 1). Along the exploration route of about 400 km in the hinterland of the desert, rhizoliths were only found at four sites (Fig. 1, red points A-D), which form a profile from the central part to the margin of the desert (Sites A-B-C-D, Fig. 1). Detailed survey and examinations of landscape, vegetation, modern root, sediment/soil, rhizolith exposure and morphology were carried out around the four sites. At each site the most representative samples of carbonate rhizoliths and their typical features were intensively described and photographed. Typical rhizoliths were delicately handled for field photographing and transported to the laboratory.

Laboratory analyses

Eight rhizolith samples (two from each site) were analyzed in the laboratory. The applied analytical methods were microscopy, cathodoluminescence, staining, scanning electronic microscopy -energy dispersive X-ray spectroscopy, which are all performed at the Institute of Northwest Petroleum Geology, Lanzhou, China. Zeiss Scope A1 microscope was used to examine mineral crystal morphology. A staining method (a mixture of Alizarin red S and potassium ferricyanide) was used to analyze 7 / 28 the carbonate minerals in the cement under the microscope [START_REF] Dickson | A modified staining technique for carbonate in thin section[END_REF][START_REF] Dickson | Carbonate identification and diagenesis as revealed by staining[END_REF]; CITL CL8200 MK5 cathodoluminescence (CL) instrument was used to investigate the luminescence color of cement. The micro-scale morphology and chemical compositions of the rhizolith cement were determined by scanning electron microscopy (SEM; FEI Quanta 450) coupled with Energy dispersive X-ray spectroscopy (EDS). Each rhizolith specimen was first impregnated with and without resin cover respectively, cut and polished to 4.8 cm×2.8 cm slices made from transverse sections, and then described by polarizing microscopy and cathodoluminescence. Fragments of the rhizoliths were gold coated, and then analyzed by SEM coupled with EDS.

Results

Field features of rhizoliths

Geomorphological distributions of rhizolith outcrops

In Site A (Sumujilin Lake, 39 o 48'N, 102 o 26'E), the rhizolith outcrops were found at the foot of the windward gentle slope of the mega-dune, located east of the Sumujilin Lake (Fig. 3a). This site is on the eroded area of Order III, which is flatter than the upper zone along the windward slope (Figs. 2a;3a).

In Site B (Nuoertu Lake, 39 o 46'N, 102 o 27'E), rhizolith outcrops were found along the upper gentle slope (Order II) of the mega-dune on the east side of the Nuoertu Lake (Fig. 4a).

In Site C (Baoritaolegai Lakes, 39 o 35'N, 102 o 28'E), rhizoliths occur along the whole windward gentle slope (Order III,II) of a mega-dune, south of Baoritaolegai Lake(Fig. 5a).

Site D (39 o 27'N, 102 o 26'E) is near to (6 km to the east) the Gate of the Badain 8 / 28 Jaran Desert National Geological Park, which is located at the southern-eastern margin of the desert, where small low dunes dominate (Figs. 1; 6a). Rhizolith outcrops occur within the swales between the dunes. In addition to Psammochloa, Artemisia is the dominant vegetation in the area, usually growing in the gentle windward slopes of the swales (Fig. 6a).

Field characteristics of rhizolith specimen

In Site A, rhizolith fragments are eroded out of the sand, together with caliche fragments (sand soil cemented by carbonates) (Fig. 3b). This slope of the mega-dune covered by fragments of the rhizolith and caliche is purple-brown, obviously different from the beige color of the above dune sand along the whole slope (Fig. 3a). The fragments of typical rhizoliths are hard and varnished brightly. Their centers are tubular cavities (Fig. 3c). Except sparse Psammochloa villosa, no other vegetation grows in the area of the rhizolith outcrops.

In Site B, at low altitude, rhizoliths occurred mostly as enormous coarse brown fragments, similar to Site A. At high altitude, the rhizolith tubes are long and white, branchy or separated (Fig. 4b) in some places. Relicts of fine root hairs were observed.

(Fig. 4c).

In Site C, besides usual fragments of straight short tubes, thin tubes with many lateral fine root hairs and thick tubes with hemp rope-like striations were found (Fig. 5b). Primary rhizoliths were found under semi-arid/moist sandy soil (depth ~ 10 cm) at locations where Artemisia was rooted. They are softer and thicker than those eroded out of soil (Fig. 5c). The eroded-out tubes were constituted of either double cemented layers or often a singular layer (Fig. 5d). In addition, the walls of the rhizoliths containing plant relicts were thicker than those without root relicts (Fig. 5e).

In Site D (39 o 27'N, 102 o 26'E), rhizolith fragments are present on the sand 9 / 28 surface (Fig. 6b) and a few of them are in situ vertically within the sand (Fig. 6b). The vertical rhizoliths below ground correspond to pristine rhizoliths, without any signs of erosion and insolation, and are much thicker than those found aboveground (Fig. 6c).

Once broken, the root remains within the tubes of the pristine rhizoliths appear as surrounded by white haloes of cement (Fig. 6d).

Laboratory results

Two rhizoliths from each site were chosen randomly for laboratory analyses.

Staining and cathodoluminescence analyses confirmed that the cementing minerals of the rhizoliths are carbonate, and the clastic particles are quartz and feldspars (Fig. 7a,b). Cathodoluminescence observations indicated that calcium ions for carbonate cement should come from the chemical weathering of feldspars (Fig. 7b). SEM and EDS analyses showed that the carbonate cement was mainly calcite and low-magnesium calcite, with only a slight amount of NaCl (Figs. 7c,d). The bulk characteristics of the rhizoliths from the four sites were generally similar, with carbonate minerals being the cement of hard rhizoliths formed in the dune sands.

Discussion

Classification and environmental implications of rhizoliths

The rhizoliths from the four investigated sites were classified as tubules, or tubes (Li et al., 2015a, b) based on their field features. Only a few of dead root remains were found in the tubes, which indicate no root petrification or body fossilization took place through diagenesis. The central voids were occasionally filled with loose aeolian sand after weathering. Hypocoating occurred through pedogenesis forming secondary carbonates as hard tube walls, as confirmed by SEM-EDS analyses (calcite and low-Mg calcite being predominant; Fig. 7d).

According to the rhizoliths formation locations, two types of rhizoliths, present in different zones of the soil, were observed: (i) primary ones and (ii) eroded-out-weathered ones. Primary rhizoliths are in situ in the soil, which allowed the preservation of their primary state -vertical but also rarely horizontal, soft and thick (Figs. 5c;6b,c,d). In contrast, the eroded-out-weathered rhizoliths were apparent out of the soil, weathered and eroded at the surface, being hard and thin. This indicates that strong wind deflation eroded the surface soil around the sampling site, likely removing former luxurious vegetation.

The eroded-out-weathered rhizoliths were further sub-classified into two types based on their field features: (i) those with brown-grey colors, hard and varnished (Figs. 3c; 5d); (ii) those with white-grey colors, fragile, and presenting fissures (Figs. 4b,c; 5e; 6b). The two subtypes of rhizoliths were formed and subjected to diagenesis in the same type of environment (mega-dunes) and share similar mineralogical characteristics based on SEM-EDS analyses. Nevertheless, the varnish observed on the first subtype of rhizoliths, hard in nature, indicates that they were formed earlier than the second, more fragile subtype. Indeed varnish formation takes a long time, with a stable geomorphic surface subjected to sunlight radiation [START_REF] Watchman | A review of the history of dating rock varnishes[END_REF][START_REF] Liu | How fast does rock varnish grow?[END_REF][START_REF] Liu | Holocene rock varnish microstratigraphy and its chronometric application in the drylands of western USA[END_REF][START_REF] Xu | Characteristics of desert varnish from nanometer to micrometer scale: A photo-oxidation model on its formation[END_REF]. Of course, varnish might also depend on the type and mineralogy of the sediment and oxidation conditions. The laboratory results confirmed that all rhizoliths are constituted of calcium carbonate. Therefore, the first subtype of rhizoliths were eroded and weathered for a longer time than the second one, consistent with geomorphological processes, i.e. the fact that hard, varnished rhizoliths occurred predominantly in Order III belts.

These different features suggest that the rhizoliths were first formed at shallow 11 / 28 soil depth and later eroded out, with concomitant or subsequent modification of the root structure. The double layer walls (Fig. 5d) and the relicts (Figs. 5c,e) of the former roots further suggest that rhizoliths were likely formed around Artemisia spp.

based on comparison with the characteristics of modern desert Artemisia spp. roots (cf. section 6.3).

Rhizolith distribution pattern

In the inner parts (Site A, B, C, Fig. 1) of the desert, rhizolith outcrops occur only in the windward gentle slopes of mega-dunes (Figs. 3a; 4a; 5a), on Orders II and III belts as the main outcrop areas. On Order I belts, rhizoliths were not found in the leeward slopes of the mega-dunes, which are steep and subjected to higher sedimentation rates than the windward slopes, with vegetation being quickly buried.

Therefore, slipping sands in the leeward slopes do not provide a stable soil environment for vegetation and thus rhizolith formation.

In the southeastern margin of the desert (Fig. 1), rhizolith outcrops also occur, but only in the low dune areas and in the windward slopes of the swales where erosion takes place (Fig. 6a). No vegetation and rhizoliths were also observed in the leeward slopes.

Therefore, wind deflation and landforms are key parameters influencing the formation of rhizolith outcrops in the desert. The outcrops occur only in the windward slopes of dunes or in the swale beds and depressions, where deflation and erosion are transporting surface soil away, resulting in the exposure of the buried rhizoliths at the surface.

Vegetation

Rhizolith outcrops generally occur in the same area as living/dead Artemisia bushes rather than any other plants, or alternatively in bare zones without any plants.

/ 28

The morphologies of the rhizoliths and their texture as well as their surface and inner structures (Figs. 4b,c;5b,c,e) are identical or similar to those of modern living Artemisia roots, which are woody roots with cortex, stele, xylem but without a central hollow. Other kinds of vegetation such as Psammochloa and Phragmites also grow at or near the sampling sites, but the latter have herbaceous roots with a central hollow, in contrast with the collected rhizoliths.

The present results suggest that the decomposition of the dead Artemisia roots have triggered the rhizolith formation, where carbonate crystallization took place.

Artemisia root decay or rhizosphere fungi may have provided oxalic acid and calcium oxalate crystals, with the microbial transformation of oxalate into carbonates leading to an increase in soil pH [START_REF] Verrecchia | The oxalate-carbonate pathway in soil carbon storage: the role of fungi and oxalotrophic bacteria[END_REF][START_REF] Dupraz | Processes of carbonate precipitation in modern microbial mats[END_REF]Huang et al., 2015). Alternatively, Artemisia dead roots might have provided a stable channel for continuously evaporation from the subsoil and so, calcification along the roots.

Soil and related environmental conditions

The field investigation suggested that soil moisture and the high porosity of sand provided the water and air required to sustain plant live and rhizolith formation, with in turn redox conditions favoring slow root decomposition and yielding enough CO 2 gas for HCO 3 -production.

Plants are growing in the dunes, which should not be considered as classical soils, but rather as aeolian sediments as defined in sedimentology and petrology.

During their formation, the desert sediments are strongly affected by deflation due to their light texture and low density. The high dust accumulation rates do not let enough time for complete soil development [START_REF] Sycheva | Long-term pedolithogenic rhytms in the Holocene[END_REF]. Consequently, quick denudation, erosion and deposition in the mega-dunes of the Badain Jaran desert prevent soil formation in this area.

13 / 28

The surface soils of the desert are aeolian loose sands with high porosity.

Normally, below ca. 20-40 cm depth, the soils are wet (water content ~10%) [START_REF] Wen | New evidence for the links between the local water cycle and the underground wet sand layer of a mega-dune in the Badain Jaran Desert[END_REF]Wang et al., 2015a;[START_REF] Liu | Growing pattern of mega-dunes in the Badain Jaran Desert in China revealed by luminescence ages[END_REF]. Unfortunately, no systematic soil moisture measurement was performed in the desert, as it is too vast. Nevertheless, previous studies [START_REF] Wen | New evidence for the links between the local water cycle and the underground wet sand layer of a mega-dune in the Badain Jaran Desert[END_REF]Wang et al., 2015b;[START_REF] Liu | Growing pattern of mega-dunes in the Badain Jaran Desert in China revealed by luminescence ages[END_REF] confirmed that the soil moisture in some swales and depressions is sufficient to allow Artemisia growth and rhizolith formation in windward slopes. It should be noted that soil moisture of the lower swales and depressions was likely higher than in the higher dunes based on vegetation distribution. The origin of soil and lake water remains controversial, whether from precipitation, condensation of atmospheric water, or groundwater [START_REF] Ma | Groundwater and lake evolution in the Badain Jaran Desert ecosystem, Inner Mongolia[END_REF][START_REF] Wen | New evidence for the links between the local water cycle and the underground wet sand layer of a mega-dune in the Badain Jaran Desert[END_REF][START_REF] Jiao | Satellite-based estimates of groundwater depletion in the Badain Jaran Desert[END_REF][START_REF] Zhang | Local circulation maintains the coexistence of lake-dune pattern in the Badain Jaran desert[END_REF][START_REF] Yue | Relationship between landform development and lake water recharge in the Badain Jaran Desert[END_REF].

Vegetation often grows in small patches or bands which are defined by the dune landforms. The regional landscape is characterized by mobile dunes and interdune lowlands. Stable and semi-stable eolian sandy soils are the dominant form. Previous results suggested that the effect of soil factors on vegetation characteristics was relatively weak and it is not possible to determine whether climate (e.g., the precipitation or temperature during the growing season) or geomorphology (e.g., parent materials) controls the vegetation communities in the desert (Wang et al., 2015b).

Climate

Modern climate of the desert is characterized by very low precipitation and high evaporation with strong northwestern wind. Comparatively, the sub-landscapes (mega-dunes), soil and vegetation on the vast desert are similar and even; but, the rhizolith outcrops are not evenly distributed everywhere in the desert. This shows that 14 / 28 local factors play a crucial role in rhizolith formation under arid climate. Rhizoliths are formed in loess sequences, under a more humid climate (e.g. [START_REF] Gocke | Rhizoliths in loessevidence for postsedimentary incorporation of root-derived organic matter in terrestrial sediments as assessed from molecular proxies[END_REF] than in deserts, but also in the waterlogged soils of lake environments (Sun et al., 2019a). That implies that rhizoliths do not need specific aboveground climatic conditions. In the Badain Jaran Desert, the arid climate and intensive evaporation may favor carbonate precipitation and crystallization around dead roots in the highly porous sandy soils. This would explain the vast amount of rhizolith outcrops observed in the desert dunes.

Diagenesis and pedogenesis of rhizoliths in the sand-sedimentary environment

The above mentioned field features of rhizoliths from the Badain Jaran desert suggest that they were formed through diagenesis and pedogenesis in loose windblown sand sediments, at shallow depth and with open redox environmental conditions. The root relicts or hollows within the rhizoliths (Figs. 4c;5b,c,e;6d) indicate that the deceased roots served as an organic nucleus for carbonate precipitation within the rhizosphere, leading to the preservation of the root structure.

Based on the theory proposed by [START_REF] Dupraz | Processes of carbonate precipitation in modern microbial mats[END_REF], it can postulated that the organic carbon of the deceased roots was oxidized under optimal oxygen and soil moisture (i.e. weak redox) conditions at suitable depths, leading to the production of CO 2 used for CaCO 3 formation. Free Ca 2+ ions could be derived from authigenic carbonates after in situ chemical weathering of soil primary silicate minerals such as feldspar, mica, amphibole and pyroxene [START_REF] Dupraz | Microbial lithification in marine stromatolites and hypersaline mats[END_REF][START_REF] Liu | Atmospheric CO 2 sink: silicate weathering or carbonate weathering?[END_REF][START_REF] Jin | Geochemistry of sediment moisture in the Badain Jaran desert: Implications of recent environmental changes and water-rock interaction[END_REF] due to low pH caused by organic matter decomposition and force-of-crystallization [START_REF] Durand | 9-Calcium carbonate features[END_REF][START_REF] Monger | Soils as generators and sinks of inorganic carbon in geologic time[END_REF] and from the dissolution of in situ primary lithogenic carbonate rocks debris such as limestone and dolomite 15 / 28 (Sun et al., 2019b). The presence of free Ca 2+ and local alkaline conditions through seasonal upwelling of alkaline water followed by evaporation [START_REF] Arp | Microbialite formation in seawater of increased alkalinity, Satonda Crater Lake, Indonesia[END_REF] could lead to the nucleation of calcium carbonate on the root organic matrix (Arp et al., 1999a, b). The initiation of this process depends on the initial chemical composition of the root organic matter, as some organic functional groups have a high metal binding potential, inhibiting calcium carbonate mineral formation, whereas some others are more prompt to biotic and abiotic degradation, favoring calcium carbonate precipitation [START_REF] Dupraz | Microbial lithification in marine stromatolites and hypersaline mats[END_REF].

Previous studies in the desert suggested that rhizoliths could be used as an indicator of precipitation intensity [START_REF] Gao | Preliminary study on the Holocene desert evolution in the NW boundary of the Asia monsoon (in Chinese with English abstract)[END_REF][START_REF] Yang | Landscape evolution and precipitation changes in the Badain Jaran Desert during the last 30,000 years[END_REF][START_REF] Yang | Evolution of megadunes and lakes in the Badain Jaran Desert, Inner Mongolia, China during the last 31,000 years[END_REF][START_REF] Gu | Challenge from the appearance of vadose water within the surface layer of megadunes, Badain Jaran dune desert, Inner Mongolia[END_REF]Li et al., 2015a, b;[START_REF] Li | Holocene vegetation signals in the Alashan Desert of northwest China revealed by lipid molecular proxies from calcareous root tubes[END_REF][START_REF] Zhu | Variations in chemical element compositions in different types of Holocene calcareous root tubes in the Tengger Desert, NW China, and their palaeoenvironmental significance[END_REF]. Therefore, this work combined with previous research (Sun et al., 2019b) confirmed that such application is still uncertain. Rhizolith formation is strongly dependent on local conditions. Rhizolith formation is not a single function of climate but rather multiple environmental factors, e.g., geomorphology of dunefields, vegetation type, strength of pedogenesis, and existence of diagenesis. They could only be used as a local proxy of soil moisture and vegetation coverage, but cannot be applied to reconstruct past environmental conditions in such vast areas.

Conclusions and outlook

The formation mechanisms of rhizolith in desert remain poorly investigated, despite the large abundance of these objects in such settings and their importance for paleoenvironmental reconstructions. This study combined field and laboratory observations to better characterize the different types of rhizoliths in the Badain Jaran desert and constrain their formation conditions. Primary rhizoliths were shown to be

Figures and Captions

Fig

  soil of Badain Jaran desert before being brought out at the surface of the dunes where they are deflated and weathered due to the loose nature of the sandy soil and strong winds. Most of the rhizoliths presented a tube shape and occurred only in some parts of the desert, along the windward gentle slopes of mega-dunes. In contrast, the environmental conditions along the leeward steep slopes of the mega-dunes do not favor rhizoliths formation. Rhizoliths were formed only around dead roots of Artemisia. Rhizoliths and Artemisia bushes were indeed found to systematically co-exist in the swales and depressions between sub-dunes. Soil moisture plays an important role in Artemisia growth as well as rhizolith formation, taking into account the high porosity of sand soil and aridness of the climate. The results of the present study suggest that rhizoliths can only be used as a proxy for local soil moisture and vegetation cover, but are not able to indicate climate conditions directly above ground in a vast area, although the desert climate and the windward gentle slopes of mega-dunes are homogeneous or similar. Further work is required to better understand the formation mechanism of the rhizoliths in deserts, taking into account the interrelations between vegetation type, environmental and climatic parameters. was provided by National Natural Science Foundation of China [No. 41561046 and M-0069 (Sino-German Mobility)].

  Fig. 1 Exploration route and sample sites in Badain Jaran Desert. Megadunes-ridges; Lakesblack dots; Exploration route-red dashed line; Sampling sites-A,B,C,D. A-Sumujilin Lake; B-Nuoertu Lake; C-Baoritaolegai Lakes; D-The Gate of National Geological Park of Badain Jaran Desert.

Fig. 3 .

 3 Fig. 3. Rhizoliths of Sumujinlin Lake. a-Rhizolith outcrops on the southeast side of the lake. b-Brown purple fragments of caliche and rhizoliths on the slope (the spade length is ~ 60 cm). c-Typical hard varnished rhizoliths and thick caliche fragments (the diameter of the coin is 20mm).

Acknowledgements

We express special gratitude to the anonymous reviewers and the Editor-in-Chief, Professor Karl Stahr for their constructive and valuable comments to improve the paper. We would like to thank Wenhui Xue and Wentao Pei for their assistance during field exploration; Pu Wang for lab work; Professor Lijun Ling for photographing;

Professor Hong Wang and Doctor Hanan Badawy for their detail revisions; Professor Viktor Golubtsov, and Doctor Eron Raines for their comments. Funding for this work