Carbonate crusts of Paleolake Zhuyeze, Tengeri Desert, China: Formation mechanism and paleoenvironmental implications

Journal Pre-proof Carbonate crusts of Paleolake Zhuyeze, Tengeri Desert, China: Formation mechanism and paleoenvironmental implications Qingfeng Sun, Kazem Zamanian, Yanrong Li, Hong Wang, Christophe Colin, Haixia Sun PII:

S1040-6182(19)30875-4

DOI:

https://doi.org/10.1016/j.quaint.2019.11.030

Reference:

JQI 8063

To appear in:

Quaternary International

Received Date: 17 March 2019 Revised Date:

12 November 2019

Accepted Date: 13 November 2019

Please cite this article as: Sun, Q., Zamanian, K., Li, Y., Wang, H., Colin, C., Sun, H., Carbonate crusts of Paleolake Zhuyeze, Tengeri Desert, China: Formation mechanism and paleoenvironmental implications Quaternary International, https://doi.org/10.1016/j.quaint.2019.11.030. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd and INQUA. All rights reserved.

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Carbonate crusts of Paleolake Zhuyeze, Tengeri Desert, China:

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Formation mechanism and paleoenvironmental implications

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Qingfeng Suna,*, Kazem Zamanianb, Yanrong Lic, Hong Wangd,e, Christophe Colinf, Haixia Suna

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a

Faculty of Geography, Northwest Normal University, Lanzhou 730070, China

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b

Department of Soil Science of Temperate Ecosystems, Georg-August University of Goettingen, Buesgenweg 2,

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37077 Goettingen, Germany

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c

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d

Department of Earth Sciences and Engineering, Taiyuan University of Technology, Taiyuan 030024, China Interdisciplinary Research Center of Earth Science Frontier, Beijing Normal University, Beijing, 100875, PR

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China

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e

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Champaign, IL 61820, USA

Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign,

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f

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Bâtiment 504, 91405 Orsay Cedex, France

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……………………………………………………………………………………………………………………………………………………………………

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Laboratoire Geosciences Paris-Sud, UMR 8148, CNRS-Université de Paris-Sud, Université Paris-Saclay,

*Corresponding author: Faculty of Geography, Northwest Normal University, Lanzhou 730070, China E-mail address: [email protected] (Q. Sun)

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Abstract

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Carbonate crusts with unusual morphologies are scattered on the Holocene sand

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surface, i.e., dunes, which cover the Paleolake Zhuyeze bed in the Tengeri Desert,

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China. Two types of the crusts were identified. The first type contained fossilized

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wrinkles of plant root cortex, while the second type consisted of hollow chambers

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without the wrinkle features. Optical microscopy, scanning electron microscopy and

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cathodoluminescence analyses were performed to investigate the mineralogy and

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geochemistry of the crusts. Fossilized cortex texture imprints on the first type of

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crusts suggested that the crusts formed around the deceased rhizomes of reeds

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(Phragmites communis) as the nuclei of encrustation. Weakly oxidizing

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soil–sediment with rhizome was a prerequisite, which was favorable for rhizome

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decomposition to produce sufficient CO2 and HCO3− and generate carbonate

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minerals of encrustation on rhizome surfaces, with lake water as the main Ca2+

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supply. A conceptual formation mechanism of the encrustation was conjectured.

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Similar to the formation mechanism of the first type, another type of crust with

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chambers and without fossilized imprints of the cortex textures formed around the

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spherical rhizomes of Scirpus maritimus. The crust characteristics and other

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evidence suggested that the lake water was fresh; water plants, such as P. communis

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and S. maritimus, and snails previously inhabited the water; and encrustation

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reflected a stable, slow-oxidizing soil–sediment environment. These desert crusts

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provided insights to study paleoecology, paleohydrology and water-soil/sediment

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interface environment in the paleolake.

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Key words

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Carbonate crusts;

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Aquatic plant roots;

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Soil and sediment;

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Encrustation mechanism;

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Paleoenvironment;

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Paleolake

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1. Introduction

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The carbonate features of sedimentary petrology display special morphologies,

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such as concretions (e.g., McCoy et al., 2016, 2001; Loyd & Berelson, 2016;

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Yoshida et al., 2015), nodules (e.g., Yang et al., 2014; Achyuthan et al., 2012, 2010;

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Asikainen & Werle, 2007; Shankar & Achyuthan, 2007; Il'Yashuk, 2001), crusts (e.g.,

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Pronin et al., 2014; Jonkers et al., 2012), hardpan calcretes or petrocalcic horizons

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(e.g., Shankar & Achyuthan, 2007), calcified rootlets, filaments (e.g., Achyuthan et

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al., 2012; Achyuthan, 2003) and rhizoliths (e.g., Sun et al., 2019; Bojanowski et al.,

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2015; Alonso-Zarza et al., 2008; Jones & Ng, 1988; Klappa, 1980), as well as cutans

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and coatings (e.g., Pustovoytov et al., 2007; Pustovoytov, 1998). These carbonate

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features usually form primarily around nuclei from decaying or living organisms,

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shells or shell fragments (e.g., Zamanian et al., 2016a,b; Yoshida et al., 2015), in

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soils, sediments and rocks (e.g., Zhao et., 2016), and they are usually isolated,

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centred on fossils and typically separated from the surrounding matrices by sharp

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boundaries. Their morphology, mineralogy and geochemistry are important

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indicators in paleogeography, paleoclimate, paleoecology and paleohydrology (e.g.,

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Zhu et al., 2019; Gao et al., 2019; Li et al., 2018; Gocke et al., 201a, b; Kraus &

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Hasiotis, 2006). However, no detailed systemic work has been undertaken to

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understand the formation mechanisms and formation environment, which directly

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affects the paleo-environmental interpretations based on such features. In most cases,

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the cementing minerals that compose the crystallize matrix around a nucleus, e.g., an

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organic core like plant roots, or typically from shell fragments or other remains of

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71

plant bodies, lead to encrustation. These crusts can form in soils with air on ground

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surface (Finstad et al., 2016) and in bed sediments of lakes (Pronin et al., 2016) or

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seas (Jakubowicz et al., 2014; Luci & Cichowolski, 2014; Borszcz et al., 2013; Zatoå

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& Borszcz, 2013) through geological time to the present day. Thus, the external

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morphology and the formation mechanisms of such crusts, formed by carbonate

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encrustation, are considerably similar to some degree (e.g., Sun et al., 2019; Yoshida

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et al., 2015), thus leading to misinterpretation about their formation environment.

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Carbonate crusts (e.g., Pronin et al., 2014; Jonkers et al., 2012; Liutkus, 2009;

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Verrecchia et al., 1995) or coatings (e.g., Pustovoytov et al., 2007; Pustovoytov,

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2003) have been observed in soil and less often in lacustrine environments.

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Carbonate crusts or coatings mostly belong to pedogenic carbonate types, and their

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formation mechanisms have been discussed (e.g., Zamanian et al., 2016c;

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Pustovoytov, 2003). However, the unique characteristics of carbonate crusts, such as

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ball-like shape with hollows or nodules of varying degrees in this case, the origins of

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their contrasting chemical compositions and the sedimentary environments in which

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they have formed remain unreported and poorly understood. In this work, we

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investigate the unusual crusts with rhizome trace imprints and hollow chambers

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found on the bed of Paleolake Zhuyeze of Tengeri Desert, China, which is now

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covered by dune sands - and aim at (1) describing their morphology, petrology,

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mineralogy and geochemistry; (2) discussing their formation mechanisms; and (3)

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inferring the ecology and hydrology of their formation environment. This study will

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highlight the importance of carbonate crusts as a paleoenvironmental tool in the

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sedimentological, pedological and ecological analysis of ancient and modern lake

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systems.

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2. Materials and Methods

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2.1 Study area

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The crusts were found on the bed of the present Qingtu Lake, Minqin Basin,

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Central Tengeri Desert in Northwest China (Figs. 1a, b, c). A mega-lake called

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Paleolake Zhuyeze was present in the basin during the Quaternary (e.g., Mischke et al.,

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2016; Long et al., 2011; Li et al., 2008; Chen et al., 1999, 2003; Shi et al., 2002). The

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extant salt Qingtu and Baijian lakes are relicts of the paleolake (Fig. 1c). Shiyang

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River in the basin drains a vast catchment from its upper reaches. Due to the recent

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diversion of the river water for irrigation and other uses in the upstream area, the

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terminal lakes have dried up since the 1950s (Fig. 1c; Chen et al., 1999). Moreover

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our long-term investigation found that no fish, snail and water-plant species are living

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in the remaining salt lakes at the present time. The inferences of pollen data in

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previous studies, however, indicated that there were fresh- and shallow-water plant

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species such as Typha latifolia, Scirpus maritimus, Carex stenophylla, and

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Phragmites communis, which were living in marginal or palustrine places during

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Holocene (Zhang et al., 2002; Zhu et al., 2002; The Editorial Board of Chinese

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Vegetation, 1980). Because of the local desertification under intensive erosion and

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weathering, dunes and salt marshes have colonized much of the bed of the paleolake

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area. According to our field exploration and previous works (Peng et al., 2004; Chang

5

115

et al., 2007), only sparse vegetation have survived, such as Phragmites communis,

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Haloxylon ammodendron (C.A.Mey.) Bunge, Nitraria sibirica Pall., Reaumuria

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songarica (Pall.) Maxim., Kalidium foliatum (Pall.) Moq., Salsola collina Pall,

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Peganum harmala L., Agriophyllum squarrosum (L.) Moq., Salicornia europaea,

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Rumex acetosa L., Suaeda glauca (Bunge) Bunge, Caragana arborescens Lam.,

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Eruca sativa Mill., Hemerocallis citrina Baroni, Plantago asiatica L., Limonium

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bicolor (Bag.) Kuntze, Glycyrrhiza uralensis Fisch, Saussurea subgen Theodorea and

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Iris lactea Pall. var. chinensis (Fisch.) Koidz.

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2.2 Samples

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The crusts were found on the surface of the lake bed where the lake sediments are

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now mixed with dune sands due to desertification. We found all the samples in the

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small studied area over a couple of meters at formerly lake margins. The crusts were

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not present in the deep sediments (Long et al., 2011; Li et al., 2008) and not in

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numerous pits (~50–100 cm deep) that were dug during field investigation. Six crusts

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were collected from the bed of the paleolake (Fig. 1c). We believe that these samples

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are representative of at least the shallow parts of the paleolake margins, which were

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inhabited by similar plant species in the past. The crusts were hard, broken and

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littered sparsely on the surface of the lacustrine dark greenish silt and silty clay. Large

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quantities of small white snail shells were found in some shallow dried depressions,

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which were small ponds on the lake bed.

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The 14C ages of the superficial lake sediments from 2000 years BP to 27000 years

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calBP increased with depth from 1.1 m to 8.2 m downward (Long et al., 2010; Chen

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et al., 2003; Chen et al., 1999; Wang & Gao, 1999; Pachur et al., 1995). Based on

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stratigraphic sequence law (Harris, 1989) and depth–age data, the ages of the

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uppermost sediments should not be more than approximately 2000 years calBP. The

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lake sediments were neither dated with radiocarbon dating method nor suitable for

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this method because of the old carbon effect (e.g., Wang et al., 2012; Kuzyakov et al.,

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2006) of the carbonate cement within the crusts. The lake sediment contains a mixture

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of carbonate minerals from lithogenic limestone fragments with dead

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secondary carbonate minerals of various ages formed through in situ dissolution and

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re-precipitation.

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2.3 Methods

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C ages and

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These crust samples were studied by the methods as mentioned in Sun et al.

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(2019) for rhizoliths. After classification based on the crust morphologies, the samples

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were impregnated with resin, cut and polished into 4.8 cm × 2.8 cm slices along

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transverse

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cathodoluminescence (CL) analyses were performed under a polarizing Leica

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microscope and an Olympus BX41-P microscope equipped with a Qicam Fast 1394

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camera at the Laboratory Geosciences Paris-Sud (GEOPS), Paris-Sud University,

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France. Ultra-microscale observations were employed on carbon-coated thin sections

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with a Philips XL30 at the Laboratory GEOPS and an ULTRA Plus scanning

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electronic microscope (SEM) integrated with a FEI Quanta 450 at the Chemistry

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Department of Northwest Normal University, China.

or

longitudinal

sections.

Petrological,

mineralogical

and

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3. Results

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Two types of crusts were identified based on the morphology, color and fossil

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imprint. Type I crusts are black and contain fossil imprints, while type II crusts are

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grey and lack fossil imprints.

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3.1 Characteristics of type I crusts

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3.1.1 External morphology

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Type I crusts are black, broken carbonate pieces with external forms resembling

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flattened spheres (Figs. 2a, b) and half-hollow bead-like chambers (Fig. 2c). The inner

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chambers of the spheres and bead can be shallow (Figs. 2a, 2b) or deep (Fig. 2c) and

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are partly filled with a peace of snail shell fragment (Fig. 2a) and fine loose

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grey-white sand grains (Fig 2c). Parallel strip-like cortex texture imprints which are

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fossilized are visible on the inner surfaces (Figs. 2a, b, c). Two circular white-grey

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traces occur at the ends of the parallel cortex texture imprints (Fig. 2b).

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3.1.2 Microscopic characteristics

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Under the microscope, the crusts with cortex texture imprints show the

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following traits. The clastic constituents are mainly fine grains of quartz and feldspar

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with a long piece of mollusc shell and other heavy minerals, which are probably

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titanite and garnet or piedmonite. The clastic grains are isolated from one another (Fig.

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2d). The mineral composition of the cement is carbonating mineral, and the cementing

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pattern is basal as indicated by the red CL (Fig. 2e). Some of the feldspar particles

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were transformed into black-grey cement, which is composed of clay minerals most

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likely against most of the orange cement (Fig. 2e).

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3.1.3 SEM characteristics

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Under a SEM, the crusts with cortex texture imprints illustrate the following

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characteristics. The cement minerals are long, thin, rod-like crystals, which are

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aragonite based on their morphology (Fig. 2f, Sandberg, 1985; Light, 1985).

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Intergrown networks in the cement are smectite (Fig. 2g, Sun et al., 2019). Hyphae or

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fibrous textures that appear like extensive aerenchyma fibers of root occur in the

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cement (Fig. 2h, Sun et al., 2019).

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3.2 Characteristics of type II crusts

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3.2.1 External morphology

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Type II crusts are grey-white with inner isolated hollow chambers (Fig. 3a).

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The chambers are similar to those of one specimen (type I) presented in Fig. 2c. The

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inner surfaces of these chambers are smooth (Fig. 3a), whereas the outer surfaces are

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knobby (Fig. 3b). A long thin piece of mollusc shell was cemented onto the outer

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surface of one specimen (Fig. 3b).

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3.2.2 Microscopic characteristics

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Similar to the type I crusts, the crusts without cortex texture imprint demonstrate

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the following microscopic characteristics. The clastic constituents are particles of

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feldspar and quartz (Fig. 3c). Some of the feldspar particles were transferred into

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black-grey cement, which are clay minerals in all probability (Fig. 3d). The cement is

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micritic carbonate mineral, and the cementing pattern is basal as indicated by the red

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CL (Fig. 3d).

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3.2.3 SEM characteristics

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Under a SEM, the crusts without cortex texture imprints illustrate the following

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characteristics. The cement comprises grain-like crystals, which are calcite based on

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their morphology (Fig. 3e). Micritic calcite cement crystallized on the surfaces of

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quartz grains (Fig. 3f), and the uniform imprints of hyphae or fungus and a long

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filament of fungus formed on the surface of an individual quartz grain (Fig. 3g).

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Flattened tubes of root hairs are observed between the layers of calcite cement (Fig.

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3h).

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4. Discussion

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The formation of crusts usually requires two basic components: a nucleus and

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suitable environmental conditions around the nucleus (Jones et al., 1998). The former

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usually initially triggers and provides composition of CaCO3 for mineral precipitation,

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and then continues for the sustainable generation and supply of Ca2+ and CO32- . These

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are the key factors affecting encrustation and reflecting the sedimentary environment.

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4.1 Formation of type I crusts

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4.1.1 Nuclei of encrustation

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Crust formation always requires a nucleus at first, which generates a geochemical

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environment and promotes the precipitation of authigenic minerals (e.g., Coleman &

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Raiswell, 1993; Raiswell & Fisher, 2000). In this study, the nuclei of the crusts (Figs.

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2a, b, c) were lost due to geological erosion. Inferring the nuclei constituents are

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currently difficult and challenging because of the limited known characteristics.

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However, we determined that the parallel wrinkles textures inside the crusts with

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hollow chambers (Figs. 2a, b, c) are the traces of the inner nucleus and the lost nuclei

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were previously located at the two hollow chambers. Based on their appearance, the

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parallel wrinkled textures are similar to the cortex wrinkles of the plant roots. After

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intensively comparing the cortex textures surrounding the living roots of all plants in

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the dunes and the marshes, we found that only the dried cortex of the reed (P.

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communis) rhizomes is similar to the fossil imprints, and the two circular trace

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imprints also match very well with the node traces of the sub-branches in reed

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rhizome at the joints (Fig. 2i, a, b, c). These findings further confirm the relationship

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between the rhizomes and the fossil imprints.

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The presence of aerenchyma in the cement (Fig. 2h) is similar to the aerenchyma

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of reed. Considering that the root aerenchyma of various plant species in the

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waterlogged soil varies widely and sediment their capacity for internal aeration also

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differs, this similarity again emphasizes on the role of reed in the formation of studied

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crusts.

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Reeds are among the main vegetation around deserts and lakes in the studied area.

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We investigated numerous living reed roots and found that no crusts are presently

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forming around them. Thus, we believed that the crusts formed through diagenesis

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and fossilization, meaning that they did not form by physiological processes, after the

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death of reed rhizomes (Verrecchia et al., 1995; Yoshida et al., 2015). First, the fresh

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rhizomes die; they lose water, weaken physically, shrivel and diminish in size, leading

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to the wrinkling of the cortex (Fig. 2i). Then, the rhizomes are flattened by the

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pressure of the overlying water body and soil/sediment. Finally, the wrinkled cortex

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247

textures are saved later through diagenesis and fossilization of the carbonate mineral

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cementing. Therefore, the parallel striation-like textures of the crusts are records of

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the dried cortex wrinkles characteristics of P. communis living in the lake before

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because calcite accumulates around plant tissues (Borowitza, 1984; Roux, 1985).

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Thus, we conclude that the crusts that display imprints of cortex texture are

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formed along the rhizomes and sub-branches of reed roots, which are their nuclei.

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This reasoning is thus far the best logical explanation for the parallel striation-like

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fossil imprints and the nuclei origin of the crusts based on our observations and the

255

used techniques.

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4.1.2 CO32− and Ca2+ sources

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Aside from nuclei, which first trigger carbonate crystallization, a continuous

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supply of two basic compositions, CO32− and Ca2+, are required for the sustainable

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growth of carbonate encrustation.

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4.1.2.1 CO32− sources

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For cement, CO32− originates from three sources in this case. Firstly, external

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CO32− of the pre-dissolved carbonate can be obtained in soil/sediment pore water from

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lake water and upstream. Secondly, primary CO32− is formed from the dissolution of

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pre-existing lithogenic carbonate and shell particles (Figs. 2d, e) of the soil/sediment.

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Thirdly, new CO32− is produced in situ by CO2 gas release of organic nuclei

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decomposition of reed roots (Figs. 2a, b, c, i). Due to differences in CO32− sources, the

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cement contains carbon from different origins and so, different 14C ages.

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Since the crusts are formed around reed root nuclei, the CO2 from reed root

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decomposition plays an important role in encrustation. Organic decomposition results

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mainly in considerable CO2 production (Brown, 1985; Colberg, 1988; Hatakka et al.,

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2000) and favored HCO3− production and further triggers carbonate dissolution and

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then crystallization for encrustation. The degradation of root organic matter enhances

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carbonate cement precipitation by transferring carbon from organic substances to

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inorganic carbon via CO2 dissolution in pore waters (i.e. dissolved inorganic carbon)

275

(Bojanowski et al., 2015). Therefore, a semi-closed, slow-oxidizing soil/sediment

276

environment at an appropriate depth is a prerequisite for crust formation. Otherwise, if

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the deceased roots are in a tightly closed reducing soil and sediment environment

278

without enough oxygen, for instance, in a very deep closed soil, CH4 gas and not CO2

279

gas will be mainly produced. Furthermore, if the deceased roots are located in a very

280

open, oxidizing soil and sediment environment, for instance, in a very shallow soil

281

and sediment, the CO2 gas will rapidly be released from the soil and sediment and not

282

enough HCO3− or CO32− will be produced around the roots. Both cases are not

283

favorable for sufficient CO2 gas production (Luo & Zhou, 2006). When favorable

284

conditions for HCO3− production are disrupted, for example, by losing water and CO2

285

degassing due to seasonal increase in temperature and evaporation, carbonates will

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crystallize to form a crust around the deceased rhizomes.

287

4.1.2.2 Ca2+ sources

288

The cement minerals are aragonite by their morphological traits (Fig. 2f). They

289

are acicular and radiating crystals, which look like spherulites to some degree (e.g.,

290

Verrecchia & Verrecchia, 1994; Verrecchia et al., 1995). However, the typical

13

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structure of spherulites (Verrecchia & Verrecchia, 1994; Verrecchia et al., 1995)

292

does not occur in the thin microspore sections (Figs 2d, e). However, the formation

293

mechanisms of spherulites are still debated until today (Chafetz et al., 2018;

294

Mercedes-Martín et al., 2017; Zhong & Chu, 2010; Sanchez-Navas et al., 2009;

295

Mees, 1999; Wright et al., 1996). Hence, the aragonite here only indicated that it

296

formed solely in a soil/sediment environment here. Calcium ions of aragonite could

297

have been provided from two sources in the soil and sediment, namely, soil and

298

sediment pore water with the pre-dissolved carbonates derived from the lake or river

299

waters and the in situ dissolution and weathering of carbonate minerals. Carbonates

300

dissolved in river and lake water bodies come from the dissolution of pre-existing

301

lithogenetic carbonate out of the paleolake. Because the cementing pattern is basal

302

(Figs. 2d, e), the dissolution and weathering on the outer surfaces of the particles

303

have at least occurred (Durand et al., 2010). In addition, the huge amount of fine

304

clastic particles that exhibit an immense total amount of spherical areas resulted in

305

dissolution and weathering that are too little and trackless to be observed under

306

normal microscopy (Figs. 2d,e). Deceased root decomposition, which provides a

307

considerable amount of organic acids (Bennett et al., 2001), and CO2 (aquatic)

308

producing HCO3− and CO32− may have caused the weathering of the feldspars and

309

led to the formation of clay minerals and carbonate minerals (Fig. 4) (Kojima et al.,

310

1997; Wendt et al., 1998; Andri, 2001).

311

Aragonite is commonly found in the shells of aquatic organisms but rare in soil

312

profiles because it transforms gradually to ordered calcite at earth-surface conditions

14

313

(Durand et al., 2010). The occurrence of aragonite in the cement (Fig. 2f) indicates

314

that they are newly formed without diagenesis conversions and that the crust ages are

315

not very old, indicating the previous presence of fresh water shells (Fig. 2a, d). These

316

findings are the evidence of a fresh water environment of the paleolake (e.g., Mischke

317

et al., 2016; Long et al., 2011; Li et al., 2008), which is now turned a saline water

318

body that is no longer a habitat for snails.

319

4.2 Formation of type II crusts

320

Similar to type I crusts, type II crusts also formed possibly around the root hairs

321

branching out from the main rhizomes of reed. However, the lost unidentified cores or

322

nuclei (Figs. 3a, b) may also originate from the roots of other plants. Holocene pollen

323

data (Zhu et al., 2002; Zhao et al., 1983; The Editorial Board of Chinese Vegetation,

324

1980) and shell evidence (Fig. 3b) from this case indicate that the paleolake water was

325

fresh and inhabited by Scirpus maritimus. This plant has small spherical rhizomes

326

with hair roots connected by main thread taproots (Fig. 3i). The chamber

327

morphologies of this crust type are similar to the spherical rhizomes of S. maritimus

328

(Figs 3i, a). The crusts may have formed around the spherical rhizomes as nuclei.

329

However, this conjecture needs further confirmation once additional evidence of this

330

plant root relict in the field is acquired.

331

The clastic particles and cementing pattern of this type crust (Figs 3c, d) are same

332

as those of type I crust. The cementing carbonate mineral of this type crust is most

333

possibly calcite (Fig. 3e), which is similar to aragonite of type I crust, which shows

334

that both types have been formed via a similar diagenesis mechanism because

15

335

aragonite could form in a surface environment with similar atmospheric pressures and

336

temperatures and is unstable and easily be transformed into calcite after diagenesis

337

(Zhang et al., 2014; Frisia et al., 2002). Aragonite precipitation is related to dissolved

338

Mg2+ and/or to high CaCO3 supersaturation in evaporative-concentrated pore waters

339

(Ostermann, et al., 2007), which is consistent with the changes of soil-sediments from

340

paleolake to desert during the Holocene in this area (Sun et al., 2019, Mischke et al.,

341

2016; Long et al., 2011).

342

The fungal imprints in type II crust (Figs. 3f, g) indicate that microorganisms

343

played a role in calcite cement formation (Verrecchia et al., 1995) because the

344

complex organic matter of roots in the rhizosphere induce microorganism population

345

and activity (Frankel & Bazylinski, 2003; Kuzyakov, 2006). The microorganisms also

346

cause carbonate precipitation within their tissue (Ehrlich & Newman, 2009;

347

Verrecchia & Verrecchia, 1994). However, based on our results, the role of fungi or

348

other organisms in crust formation cannot further be discussed.

349

Tiny hair root tubes in cement (Fig. 3h) show that they likely are root hairs of

350

aquatic plants, such as S. maritimus, in an ancient lake environment (Fig .3i).

351

4.3 Conceptual formation mechanism of encrustation

352

At the early stage (Fig. 4a), reed rhizomes died and lost their rigidity, shrank and

353

were flattened under the sediment and water weight. The decompositions of the reed

354

roots mainly released CO2 gas, and microorganisms were involved in organic

355

decomposition. As CO2 gas accumulated, gas pressure increased within the

356

rhizosphere. CO2 gas, which was converted to HCO3－, caused the weathering of

16

357

feldspar particles and the dissolution of pre-existing lithogenetic carbonate. New

358

carbonate minerals and clay minerals were also formed in the pores (Fig.4).

359

Because of gas pressure differences from inside to outside around the rhizosphere,

360

an initial crystallization of calcite and aragonite was encrusted along the outer surface

361

of the rhizosphere, which acted as a nuclei (Coleman & Raiswell, 1993; Raiswell &

362

Fisher, 2000). The cortex wrinkles were preserved as fossil traces.

363

At the later stage (Fig. 4b), slow and stable weakly oxidizing conditions

364

(Verrecchia et al., 1995) in the soil/sediment sustained and prolonged this

365

equilibrium reaction and the crust wall became progressively thicker. The porosity of

366

the walls was reduced until most of the pores were full of cement. The sealing

367

cement strongly decreased CO2 diffusion outward and segregated the contact of

368

HCO3－ inside and Ca2+ outside of the wall. Ultimately, the wall growth of the crusts

369

ceased. The fibrous roots and rhizomes decomposed thoroughly and the gas diffused

370

out through the two end holes of the rhizome tubes. The sediment particles inside the

371

rhizosphere were not yet cemented. The hollow chambers were formed by losing the

372

loose, non-cemented fragments.

373

Subsequently, the lake dried up and the soil and sediment were deflated (Sun et

374

al., 2019; Mischke et al., 2016; Long et al., 2011; Li et al., 2008; 2003; Shi et al.,

375

2002; Chen et al., 1999), causing the crusts to be broken, cropped out, and littered on

376

the surface of the soil and sediment.

377

As for the formation of type II crusts, as the nuclei decomposed, the deceased

378

roots of P. communis triggered carbonate mineral dissolution and precipitation

17

379

around the roots. No cortex wrinkles were fossilized because its root characteristics

380

were different from those of reeds (Figs. 2i, 3i).

381

4.4 Paleoenvironment implications

382

Crust formation occurred in a waterlogged soil/sediment at the earth surface

383

environment. The parallel cortex texture, aerenchyma and hair root relicts indicate

384

that reeds previously lived in the paleolake. The snail shells indicate that the paleolake

385

consisted of fresh water (Mischke et al., 2016; Long et al., 2011). The fresh aquatic

386

plant S. maritimus lived in the paleolake (Zhang et al., 2002; Zhu et al., 2002). The

387

plant remains (i.e., roots) were decomposed in semi-closed slow oxidation conditions

388

(see section 4.3) to acidify the soil/sediment water and trigger calcite and aragonite

389

crystallization around the roots. Finally, the crusts indicated a semi-closed slow

390

oxidation condition in soil/sediment with roots as the prerequisite for encrustation.

391

The crusts formed through exodiagenesis, and the paleoecology and paleohydrology

392

of the paleolake was identified, which is agreement with the previous works on the

393

paleolake (Sun et al., 2019; Mischke et al., 2016; Long et al., 2011; Li et al., 2008;

394

Chen et al., 1999, 2003; Shi et al., 2002). Carbonate crusts should indicate the

395

soil/sediment environment of the paleolake. However, due to intensive erosion and

396

weathering on the paleolake bed (Sun et al., 2019; Mischke et al., 2016; Long et al.,

397

2011), loss of original information of crusts limited the sedimentological and

398

paleoenvironmental interpretations. Nonetheless, understanding the formation

399

mechanisms of carbonate features (i.e., crusts) is necessary before these features can

400

be used for paleo-environmental interpretations. This requirement is also

18

401

generalizable to all carbonate features and any matrices where such features might be

402

found. As we showed here, the crusts that are now present in dune sands have actually

403

been formed in a lake environment and by encrustation of roots of water-plant

404

species.

405 406

5. Conclusions

407

The crusts discovered in Paleolake Zhuyeze displayed fossilized cortex wrinkles

408

that resemble those of modern dried reed (P. communis) roots. The paleolake

409

consisted of fresh water as indicated by snails and aquatic plants that previously lived

410

there, such as P. communis and S. maritimus. After the death of the plants, the

411

decomposition of their roots produced CO2 gas under a semi-closed weak oxidation

412

condition in the shallow lake water and soil/sediments and then formed carbonic acid

413

in the pore water of the soil and sediment surrounding the roots. The roots provided

414

the nuclei for encrustation. The pore water derived from the lake water with the

415

dissolution of carbonate and weathering of feldspar grains around the roots through

416

the action of carbonic acid were the main sources of Ca2+, which enabled the cement

417

formation of the encrustation. Microorganisms, such as fungi, might also have played

418

a role in the crust formation which is however, difficult to discuss based on our

419

technique and image resolution. The chambers of these crusts were the nuclei, which

420

were the sub-branch root hairs of P. communis and the spherical rhizomes of S.

421

maritimus. Because of lake strata erosion, some original information of the crusts is

422

lost, which makes the sedimentological and paleoenvironmental interpretations based

19

423

on crust difficult and challenging. Nonetheless, the crusts can be used as an index to

424

infer the paleohydrological and paleoecological environment of the studied area.

425 426

Acknowledgements

427

We would like to thank Dr Yaoxia Yang, Northwest University, for providing

428

assistance on SEM. The authors are greatly indebted to the anonymous reviewers and

429

the Editor-in-Chief, Professor Kolfschoten, for their constructive comments that

430

improved the manuscript. The research was supported by the Natural Science

431

Foundation of China (Grant No. 41561046).

432 433

References

434

Achyuthan, H., 2003. Petrologic analysis and geochemistry of the Late

435

Neogene-Early Quaternary hardpan calcretes of Western Rajasthan, India. Quat.

436

Int. 106–107(02), 3–10.

437

Achyuthan, H., Flora, O., Braida, M., Shankar, N., & Stenni, B., 2010. Radiocarbon

438

ages of pedogenic carbonate nodules from Coimbatore Region, Tamil Nadu. J.

439

Geolo. Soc. India 75(6), 791–798.

440

Achyuthan, H., Shankar, N., Braida, M., & Ahmad, S. M., 2012. Geochemistry of

441

calcretes (calcic palaeosols and hardpan), Coimbatore, Southern India:

442

Formation and Paleoenvironment. Quat. Int. 265, 155–169.

443

Alonso-Zarza, A.M., Genise, J.F., Cabrera, M.C., Mangas, J., Martín-Pérez, A.,

444

Valdeolmillos, A., Dorado-Valiño, M., 2008. Megarhizoliths in Pleistocene

445

aeolian deposits on Gran Canaria (Spain): ichnological and palaeoenvironmental

446

significance. Palaeogeogr. Palaeoclimatol. Palaeoecol. 265, 39–61.

447

Andri, S., 2001. Dissolution of primary minerals of basalt in natural waters. I.

448

Calculation of mineral solubilities from 0oC to 350oC. Chem. Geol. 172,

20

449

225–250.

450

Asikainen, C.A., & Werle, S.F., 2007. Accretion of ferromanganese nodules that

451

form pavement in Second Connecticut Lake, New Hampshire. PNAS 104(45),

452

17579–81.

453 454

Bennett, P.C., Rogers, J.R., Choi,W.J., Hiebert, F.K., 2001. Silicates, silicate weathering, and microbial ecology. Geomicrobiol. 18(1), 3–19.

455

Bojanowski, M. J., Jaroszewicz, E., Kosir, A., Lozinski, M., Marynowski,

456

L.,Wysocka, A., Derkiwski, A., 2015. Root-related rhodochrosite and

457

concretionary siderite formation in oxygen-deficient conditions induced by a

458

ground-water table rise. Sedimentology, 63(3), 523–551.

459

Borszcz, T., Kuklinski, P., & Zatoń, M., 2013. Encrustation patterns on Late

460

Cretaceous (Turonian) echinoids from Southern Poland. Facies, 59(2), 299–318.

461 462

Borowitza, M. A., 1984. Calcification in aquatic plants. Plant Cell Environ. 7(6), 457–466.

463

Brown, A., 1985. Review of lignin in biomass. Appl. Biochem.7, 371–383.

464

Chen F., Shi Q., Wang J., 1999. Environmental changes documented by

465

sedimentation of Lake Yiema in arid China since the Late Glaciation. J.

466

Paleolimn. 22, 159–169.

467

Chang, Z. F., Liu, H. J., Zhao, M., Han, H.G., Zhong, S. N., & Tang, J. N., 2007. A

468

primary study on the process of formation and succession of desert vegetation in

469

Minqin. J. Arid Land Resources and Environment. 21(7), 116–124 (In Chinese

470

with English abstract).

471

Chafetz, H., Barth, J., Cook, M., Guo, X., Zhou, J., 2018. Origins of carbonate

472

spherulites: Implications for Brazilian Aptian pre-saltreservoir. Sedi. Geol. 365,

473

21–33.

474

Chen F., Wu W., Holmes J.A., Madsen D.B., et al., 2003. Amid-Holocene drought

475

interval as evidenced by lake desiccation in the Alashan Plateau, Inner Mongolia,

21

476

China. Chi. Sci. Bull. (48)14, 1401–1410

477

Colberg P.J., 1988. Anaerobic microbial degradation of cellulose, lignin oligolonols,

478

and monoaromatic lignin derivates. In Zehnder A J B. Biology of Anaerobic

479

Microorganisms. New York, USA: John Wiley and Sons: 333–372.

480

Coleman, M.L., & Raiswell, R., 1993. Microbial mineralization of organic matter:

481

mechanisms of self-organization and inferred rates of precipitation of diagenetic

482

minerals [and discussion]: Royal Society of London, Philosophical Transactions

483

Series A: Physical and Engin. Sci. 344, 69–87.

484

Durand, N., Monger, H. C., & Canti, M. G. 2010. 9–Calcium Carbonate Features.

485

Interpretation of Micromorphological Features of Soils & Regoliths, 149–194.

486

Ehrlich H. L. and Newman D. K., 2009. Geomicrobiology, 5th edition. Taylor &

487

Francis Group, LLC. CRC Press.

488

Finstad, K., Pfeiffer, M., Mcnicol, G., Barnes, J., Demergasso, C., & Chong, G.,

489

Amundson R., 2016. Rates and geochemical processes of soil and salt crust

490

formation in Salars of the Atacama Desert, Chile. Geoderma 284, 57–72.

491 492

Frankel, R. B., & Bazylinski, D. A., 2003. Biologically induced mineralization by bacteria. Re. Mineral. Geochemi. 54(1), 95–114.

493

Frisia, S., Borsato, A., Fairchild, I. J., Mcdermott, F., & Selmo, E. M., 2002.

494

Aragonite-calcite relationships in speleothems (grotte de clamouse, france):

495

environment, fabrics, and carbonate geochemistry. J. Sedi. Res. 72(5), 687–699

496

Gao, Y., Li, Z., Wang, N., & Li, R., 2019. Controlling factors and the

497

paleoenvironmental significance of chemical elements in Holocene calcareous

498

root tubes in the Alashan Desert, Northwest China. Quat. Res. 91(1), 149–162.

499

Gocke, M., Pustovoytov, K., Kühn, P., Wiesenberg, G.L.B., Löscher, M., Kuzyakov, Y.,

500

2011a.

Carbonate

rhizoliths

in

loess

and

their

implications 13

for 14

501

paleoenvironmental reconstruction revealed by isotopic composition: δ C, C.

502

Chem. Geol. 283, 251–260.

503

Gocke, M., Pustovoytov, K., Kuzyakov, Y., 2011b. Carbonate recrystallization in

22

504

root-free soil and rhizosphere of Triticum aestivum and Lolium perenne

505

estimated by C-14 labeling. Biogeochem. 103, 209–222.

506 507 508 509 510

Hatakka, A., Tuomela, M., Vikman, M., & Itävaara, M., 2000. Biodegradation of lignin in a compost environment: A review. Biores. Tech. 72(2), 169–183. Harris, E. C., 1989. Principles of Archaeological Stratigraphy (Second Edition). 475 London & New York: Academic Press. p136. Il'Yashuk, B. P., 2001. Ferromanganese nodules in lake sediments as a limiting factor

511

in the development of zoobenthic communities. Rus. J. Ecolo. 32(6), 444–446.

512

Jakubowicz, M., Belka, Z., & Berkowski, B., 2014. Frutexites, encrustations on

513

rugose corals (Middle Devonian, Southern Morocco): complex growth of

514

microbial microstromatolites. Facies 60(2), 631–650.

515

Jonkers, L., Nooijer, L. J. D., Reichart, G. J., & Zahn, R., 2012. Encrustation and

516

trace element composition of Neogloboquadrina dutertrei assessed from single

517

chamber analyses, implications for paleotemperature estimates. Biogeosci.

518

Discu. 9(8), 4851–4860.

519 520

Jones, B., Ng, K.C., 1988. The structure and diagenesis of rhizolites from Cayman Brac, British West Indies. J. Sed. Petro. 58, 457–467.

521

Jones, B., Renaut, R. W., Rosen, M. R., & Klyen, L., 1998. Primary siliceous

522

rhizoliths from Loop Road Hot Springs, North Island, New Zealand. J. Sed. Res.,

523

68(1), 115–123.

524 525 526 527

Klappa, C.F., 1980. Rhizoliths in terrestrial carbonates: classification, recognition, genesis, and significance. Sedimentol. 26, 613–629. Kojima, T., Nagamine, A., Ueno, N., & Uemiya, S., 1997. Absorption and fixation of carbon dioxide by rock weathering. Energ. Convers. Manage. 38(36), 237–242.

528

Kraus, M.J. and Hasiotis, S.T., 2006. Significance of different modes of rhizolith

529

preservation to interpreting paleoenvironmental and paleohydrological setting:

530

Examples from Paleogene paleosols, Bighorn Basin, Wyoming, U.S.A. J. Sed.

531

Res. 76,633–646.

23

532 533 534

Kuzyakov, Y., 2006. Sources of CO2 efflux from soil and review of partitioning methods. Soil Biol. Biochem. 38(3), 425–448. Kuzyakov, Y., Shevtzova, E., Pustovoytov, K., 2006. Carbonate re-crystallization in 14

535

soil revealed by

C labeling: Experiment, model and significance for

536

paleo-environmental reconstructions. Geoderma 131 (1–2), 45–58.

537

Li, Y., Wang, N., Cheng, H., Long, H. & Zhao, Q., 2008. Holocene environmental

538

change in the marginal area of the Asian monsoon: a record from Zhuye Lake,

539

NW China. Boreas 38, 349–361.

540

Li, Y.R., Zhang, W.W., Aydin, A., Deng, X.H., 2018. Formation of calcareous

541

nodules in loess–paleosol sequences: Reviews of existing models with a proposed

542

new "per evapotranspiration model". J Asian Earth Sci. 154, 8–16.

543

Light R.G.1985. Preservation of internal reef porosity and diagenetic sealing of

544

submerged early holocence barrier reef, SouthFlorida Shelf. In: Scheneiderman

545

N., harries P.M.(eds), Society of Economic Paleontologists and Mineralogists.

546

Special Publication 36,123–151.

547

Liutkus, C.M., 2009. Using petrography and geochemistry to determine the origin

548

and formation mechanism of calcitic plant molds; rhizolith or tufa? J. Sed. Res.

549

79(12), 906–917.

550

Long H., Lai Z. P., Wang N. A., Li Y., 2010. Holocene climate variations from

551

Zhuyeze terminal lake records in East Asian monsoon margin in arid northern

552

China. Quat. Res. 74(1), 46–56.

553

Long, H., Lai, Z.P., Wang, N.A., Zhang, J.R., 2011. A combined luminescence and

554

radiocarbon dating study of Holocene lacustrine sediments from arid northern

555

China. Quat. Geochron. 6(1), 1–9.

556

Loyd, S.J., Berelson, W.M., 2016. The modern record of “concretionary” carbonate:

557

Reassessing a discrepancy between modern sediments and the geologic record.

558

Chem. Geol. 420,77–87.

559

Luci, L., & Cichowolski, M., 2014. Encrustation in Nautilids: a case study in the

24

560

Cretaceous species Cymatoceras Perstriatum, Neuquén Basin, Argentina. Palaios

561

29(3), 101–120.

562

Luo, Y., & Zhou, X., 2006. CHAPTER 3 - Processes of CO2 production in soil, in

563

Soil respiration and the environment. Academic Press, an imprint of Elsevier.

564

35–59.

565 566

McCoy, V.E., Young, R. T., & Briggs, D. E. G., 2001. Factors controlling exceptional preservation in concretions. Palaios 30, 272–280.

567

McCoy, V. E., Young, R. T., & Briggs, D. E. G., 2016. Sediment permeability and the

568

preservation of soft-tissues in concretions: an experimental study. Palaios 30(8),

569

608–612.

570 571

Mees, F., 1999. The unsuitability of calcite spherulites as indicators of subaerial exposure. J. Arid Environ. 42(2), 0-154.

572

Mercedes-Martín, R., Brasier, A. T., Rogerson, M., Reijmer, J. J. G., Vonhof, H., &

573

Pedley, M., 2017. A depositional model for spherulitic carbonates associated

574

with alkaline, volcanic lakes. Mar. Petro. Geol. 86,168–191,

575

Mischke, S., Lai, Z., Long, H., & Tian, F., 2016. Holocene climate and landscape

576

change in the Northeastern Tibetan plateau foreland inferred from the Zhuyeze

577

lake record. Holocene 26(4), 643–654

578

Ostermann, M., Sanders, D., Prager, C., & Kramers, J., 2007. Aragonite and calcite

579

cementation in “boulder-controlled” meteoric environments on the fern pass

580

rockslide (Austria): implications for radiometric age dating of catastrophic mass

581

movements. Facies 53(2), 189–208.

582

Pachur, H. J., Wünnemann, B., & Zhang, H., 1995. Lake evolution in the Tengger

583

Desert, Northwestern China, during the last 40,000 years. Quat. Res. 44(2),

584

171–180.

585

Peng, H. J., Fu, B.J., Chen, L. D., & Yang, Z. H., 2004. Study on features of

586

vegetation succession and its driving force in Gansu desert areas—a case study at

25

587

Minqin County. J. Desert Res., 24(5), 628–633(In Chinese with English abstract).

588

Pronin, E., Pełechaty, M., Apolinarska, K., Pukacz, A., & Frankowski, M., 2016.

589

Sharp differences in the δ13C values of organic matter and carbonate

590

encrustations but not in ambient water DIC between two morphologically distinct

591

charophytes. Hydrobiol. 773(1), 177–191.

592

Pronin, E., Pełechaty, M., Apolinarska, K., & Pukacz, A., 2014. Relationship

593

between δ13C values in organic matter, carbonate encrustation and ambient

594

waters: a case study of Chara Tomentosa and Chara Globularis. Meeting of the

595

Group of European Charophytologists.

596

Pustovoytov, K., 1998. Pedogenic carbonate cutans as a record of the Holocene

597

history of relic tundra–steppes of the upper Kolyma Valley (North-Eastern Asia).

598

Catena 34(1–2), 185–195.

599 600

Pustovoytov, K., 2003. Growth rates of pedogenic carbonate coatings on coarse clasts. Quat. Int. 106(3), 131–140.

601

Pustovoytov, K., Schmidt, K., Taubald, H., 2007. Evidence for Holocene

602

environmental changes in the northern Fertile Crescent provided by pedogenic

603

carbonate coatings. Quat. Res. 67, 315–327.

604

Raiswell, R., and Fisher, Q.J., 2000. Mudrock-hosted carbonate concretions: a review

605

of growth mechanisms and their influence on chemical and isotopic composition.

606

J. Geol. Soc. (157), 239–251.

607

Roux, A., 1985. Introduction a l'etude des Algues fossiles paleozoiques(de la bacterie

608

a la tech-tonique des plaques). Bull. Centres de Recherche Exploration

609

Production, Elf-Aquitaine, Paris-La Defense, 9(2), 465–699.

610

Sanchez-Navas, A., Martin-Algarra, A., Rivadeneyra, M. A., Melchor, S., &

611

Martin-Ramos, J. D., 2009. Crystal-growth behavior in Ca-Mg carbonate

612

bacterial spherulites. Crystal Growth & Design, 9(6), 2690–2699.

613

Sandberg, P., 1985. Aragonite cements and their occurrences in limestones. In Nahum

26

614

S., Paul M.H.(eds), Carbonate cement. Society of Economic Paleontologists and

615

Mineralogists. Special Publication 36,33–57.

616

Shankar, N., & Achyuthan, H., 2007. Genesis of calcic and petrocalcic horizons from

617

Coimbatore, Tamil Nadu: Micromorphology and geochemical studies. Quat. Int.

618

175(1), 140–154.

619

Shi, Q., Chen, F., Zhu, Y., Madsen, D., 2002. Lake evolution of the terminal area of

620

Shiyang River drainage in arid China since the last glaciations. Quat. Int. 93–94,

621

31–43

622

Sun, Q., Xue, W., Zamanian, K., Colin, C., Duchampalphonse, S., & Pei, W., 2019.

623

formation and paleoenvironment of rhizoliths of Shiyang River Basin, Tengeri

624

Desert, NW China. Quat. Int. 502, 246-257.

625 626 627 628

The Editorial Board of Chinese Vegetation, Chinese Vegetation (in Chinese), Beijing: Science Press, 1980, 195–197. Verrecchia, E.P., Verrecchia, K E., 1 994. Needle-fiber calcite: a critical review and a proposed classification. J. Sed. Res. A64, 650–664.

629

Verrecchia, E.P., Freytet, P., Verrecchia, K.E., Dumont, J.L., 1995. Spherulites in

630

calcrete laminar crusts: biogenic CaC03 precipitation as a major contributor to

631

crust formation. J. Sed. Res. A65, 690-700.

632

Wang, L., D'Odorico, P., Evans, J.P., Eldridge, D.J., McCabe, M.F., Caylor, K.K.,

633

King, E.G., 2012. Dryland ecohydrology and climate change: critical issues and

634

technical advances. Hydrol. Earth Syst. Sci. 16, 2585–2603.

635

Wang, N.A., Gao, S.W., 1999. A preliminary research on the climatic records of

636

lacustrine deposits of Qingtu Lake in the last 6000 years. Chi. Geogr. Sci. 9(9),

637

335–341.

638

Wendt, C.H., Butt, D.P., Lackner, K.S. i Ziock, H.J., 1998. Thermodynamic

639

calculations for acid decomposition of serpentine and olivine in MgCl2 melts I.

640

Los Alamos National Laboratory, Technical Report No LA-UR-98-4528.

27

641

lib-www.lanl.gov.

642

Wright, V. P., Beck, V. H., Sanz-Montero, M. E., Verrecchia, E. P., & Dumont, J. L.,

643

1996. Spherulites in calcrete laminar crusts: biogenic CaCO3 precipitation as a

644

major contributor to crust formation - reply. J. Sed. Res. 66(5), 1040–1044.

645

Yang, S., Ding, Z., & Gu, Z., 2014. Acetic acid-leachable elements in pedogenic

646

carbonate nodules and links to the East-Asian Summer Monsoon. Catena 117(3),

647

73–80.

648

Yoshida, H., Ujihara, A., Minami, M., Asahara, Y., Katsuta, N., & Yamamoto, K., et

649

al., 2015. Early post-mortem formation of carbonate concretions around

650

tusk-shells over week-month timescales. Sci. Rep. 5(14123).

651

Zamanian, K., Pustovoytov, K., Kuzyakov, Y., 2016a. Recrystallization of shell 14

652

carbonate in soil:

C labeling, modeling and relevance for dating and

653

paleo-reconstructions. Geoderma 282, 87–95.

654

Zamanian, K., Pustovoytov, K., Kuzyakov, Y., 2016b. Cation exchange retards shell

655

carbonate recrystallization: consequences for dating and paleoenvironmental

656

reconstructions. Catena 142, 134–138.

657 658

Zamanian, K., Pustovoytov, K., & Kuzyakov, Y., 2016c. Pedogenic carbonates: forms and formation processes. Earth-Sci. Rev. 157, 1–17.

659

Zatoå, M., & Borszcz, T., 2013. Encrustation patterns on post-extinction early

660

Famennian (late Devonian) brachiopods from Russia. Hist. Biol. 25(1), 1–12.

661

Zhang, H., Cai, Y., Tan, L., Qin, S., & An, Z., 2014. Stable isotope composition

662

alteration produced by the aragonite-to-calcite transformation in speleothems and

663

implications for paleoclimate reconstructions. Sed. Geol. 309, 1–14.

664

Zhang, H.C., Wünnemann, B., Ma, Y.Z., Peng, J., Pachur, H.-J., Li, J., Qi, Y., Chen,

665

G.J., Fang, H.B., Fenge, Z.D., 2002. Lake level and climate changes between

666

42,000 and 18,000

667

Res. 58 (1), 62–72.

14

C yr B.P. in the Tengger Desert, northwestern China. Quat.

28

668 669 670 671

Zhao, X., Zhao, C., Wang, J., Stahr, K., & Kuzyakov, Y., 2016. CaCO3 recrystallization in saline and alkaline soils. Geoderma 282, 1–8. Zhao, R.L., Hong, B.G., Gao, Z.S., 1983. Summary of Plant Ecology (in Chinese). Nanjing: Jiangsu Science and Technology Press.

672

Zhong, C., & Chu, C. C., 2010. On the origin of amorphous cores in biomimetic

673

CaCO3 spherulites: new insights into spherulitic crystallization. Crystal Growth &

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Design, 10(12), 5043–5049.

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Zhu, R.X., Li, Z.L., Gao, Y.H., Chen, Q.J. Yu, Q.J., 2019. Variations in chemical

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element compositions in different types of Holocene calcareous root tubes in the

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Tengger Desert, NW China, and their palaeoenvironmental significance. Boreas

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48,800-809

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Zhu, Y., Chen, F., Madsen, D., 2002. The environmental signal of an early Holocene

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pollen record from the Shiyang River basin lake sediments, NW China. Chin. Sci.

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Bull. 47, 267–273.

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Figures and Captions a

b

c

695 696 697

Fig. 1. a. Location of study area. b. Paleolake Zhuyeze, Minqin Basin, Tengeri Desert, China. c. Sample site.

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30

i

a

b

c

d

F

e

F F

F

Q

F Q

F

Q

fc

g

he

704 705

Fig. 2. Characteristics of type I crusts and modern reed (P. communis) rhizomes with sub-branches of

706

hair roots. a. Flattened piece of spherical crust with visible parallel fossil cortex texture imprints

707

on its inner surface. The shallow void chamber inside the crust is partly filled with fine loose

31

708

white-grey sands and a piece of white snail shell (red arrow, sample F). b. Parallel cortex texture

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imprints on the inner surface of a flattened piece of crust with two circular grey-white node

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imprints (red arrows) of sub-branches of roots (sample G). c. Crust with two joined hollow

711

chambers and parallel cortex texture imprints. The chambers are partly filled with fine white-grey

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sand (sample E). d. Microscopic clastics consist of feldspar (F), quartz debris (Q), mollusc shell (S)

713

and other heavy mineral particles, which are probably titanite (T, blue) and garnet or piedmonite

714

(G, pink). The directional extinction of the shell minerals along the longitudinal direction of the

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shell shows that the minerals are aragonite. The rims of the feldspars were transformed into bright

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calcite and/or cloudy clay minerals (sample E2-2, polarized light). e. CL image of clastics and

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cement. The cementation pattern is basal. The semi-transparent black cloudy cement with the

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brown-red areas, which maintain the shapes of fragments to some degree, consists of clay minerals

719

(C). Transformation from feldspars (F) into carbonate cement (red orange) occurs because of Ca2+

720

release. The yellow luminescent particles are apatite (A,?), which are visible due to the possible

721

activation of Mn2+, and the blue particles are quartz (Q) (sample E1-08, CL). f. SEM image of

722

cement displaying aggregates of long, thin rod crystals of aragonite (sample E2-07). g. SEM

723

image showing intergrown networks of smectite in cement (sample E2-10). h. SEM image

724

showing root fibrous texture in cement. This texture may result from the extensive presence of

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aerenchyma of reed roots that became distorted and melted when an electronic beam of high

726

voltage was focused on them (white) due to their organic matter (sample E2-15). i. Modern fresh

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white and dried beige reed (P. communis) rhizomes with fibrous roots and root hairs. The parallel

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cortex wrinkles of the dried flattened rhizome are visible. The white arrows indicate parts of the

729

rhizomes and rot hairs correspond to the fossilized imprints in the crusts.

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32

i

a

b

ca

d

e

f

g

h

737 738

Fig. 3. Characteristics of type II crusts and modern roots of S. maritimus. a. Broken hollow

739

grey-white crust with two isolated inner chambers without cortex texture imprints (sample B). b. A

740

piece of white mollusc shell was cemented in the knobby outer surface of a broken grey-white crust

33

741

(sample D). c. Microscopic clastics and cement. The quartz grains have remained clear and clean.

742

Feldspar grains are grey and dirty due to weathering. The cementation pattern is basal (sample B-04,

743

polarized light). d. CL photomicrograph showing clastics (blue-black, quartz; blue, quartz) and

744

cement (red). The cementation pattern is basal. The black cloudy cement most likely represents clay

745

minerals derived from weathered feldspar (sample B-1, CL). e. SEM image showing calcite crystals

746

in cement (sample B-11). f. SEM image showing calcite crystals that have grown on a grain of quartz

747

(sample B-04). g. SEM image showing a magnified view of the enlarged Pa R1 of the last figure d. A

748

long filament of fungus and uniform fungi imprints have formed in calcite cement (sample B-04). h.

749

SEM image of two hair root tubes in cement. The ends of these tubes became distorted under high

750

temperatures during high voltage focusing (sample B-02). i. Scirpus maritimus. The green arrows

751

indicate that the spherical roots correspond to the spherical chambers of the crusts.

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34

768 769

Fig. 4. Conceptual diagrams showing encrustation around the roots of P. communis in an appropriate

770

deep soil/sediment in the Paleolake. a. The deceased rhizome with hair roots were decomposed to

771

release CO2 gas to form HCO3− acid. Super saturation solution of carbonate minerals was formed in

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the pore water outside of the rhizosphere with a supply of Ca2+ from the lake water. The carbonate

773

minerals crystallized around the root nuclei because of the decrease in gas pressure from the inside to

774

the outside around the rhizosphere. The cortex textures of the flattened rhizomes were preserved as

775

trace fossils. Because of cement closure, the central rhizosphere was not cemented for lack of Ca2+

776

supply from the outside. b. As the crust became thicker later, the rhizome with hair roots was decayed

777

away and no acid gas was produced anymore. The particles inside the rhizosphere were not cemented

778

and loose. The crust was eroded out of the soil/sediments after the lake dried. The un-cemented

779

rhizosphere became a hollow chamber.

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35

a

b

c

Fig 1

i

a

b

c

d

e

F

F F

F

Q F

Q

F

Q

fc

he

Fig 2

g

i

a

b

ca

d

e

f

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h

Fig 3