Origin of modern dolomite in surface lake sediments on the central and western Tibetan Plateau

Journal Pre-proof Origin of modern dolomite in surface lake sediments on the central and western Tibetan Plateau Jiao Li, Liping Zhu, Minghui Li, Junbo Wang, Qingfeng Ma PII:

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DOI:

https://doi.org/10.1016/j.quaint.2020.02.018

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Quaternary International

Received Date: 20 October 2019 Revised Date:

27 December 2019

Accepted Date: 12 February 2020

Please cite this article as: Li, J., Zhu, L., Li, M., Wang, J., Ma, Q., Origin of modern dolomite in surface lake sediments on the central and western Tibetan Plateau, Quaternary International, https:// doi.org/10.1016/j.quaint.2020.02.018. 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. © 2020 Published by Elsevier Ltd.

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Origin of modern dolomite in surface lake sediments on the central and

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western Tibetan Plateau

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Jiao Lia, Liping Zhua,b,c, Minghui Lia, Junbo Wanga,b, Qingfeng Maa

4

a

5

Research, Chinese Academy of Sciences (CAS), Beijing 100101, China

6

b

CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China

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c

University of Chinese Academy of Sciences, Beijing, China

8

Abstract

Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau

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Modern dolomite was found in the sediments of 20 lakes on the central and western Tibetan

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Plateau during lake surveys, in August to November of 2012-2015. We analyzed the mineralogy of

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authigenic carbonates and the chemical composition of lake water to investigate the environmental

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factors affecting dolomite formation and to determine the mechanisms of dolomite formation.

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X-ray diffraction analysis suggested that the dolomite content ranged from 3% to 10%, with a

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mean of 5%, and that the majority of the dolomite in the surface lake sediments was Ca-dolomite.

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Dolomite formed in a wide range of salinity environments. There was a positive linear relationship

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between salinity and Mg/Ca ratio, whereas the pH and sulfate (SO42-) concentration are not

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significantly correlated with Mg/Ca ratio. High Mg/Ca ratio, high salinity, high sulfate

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concentration, and high pH are important environmental factors in favor of dolomite precipitation.

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Evaporation and microbial activity are potential mechanisms for dolomite formation in the central

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and western Tibetan Plateau. The positive relationship between δ18O and δ13C indicates that the

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dolomite formed in lakes has a long residence time and undergoes continuous evaporation. The

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crystal morphologies of dolomite were scattered euhedral grains with diameters of 2‒10 µm, as

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well as sub-spherical and spherical grains (< 2 µm) which were present either as dispersed crystals

24

or were aggregated into dolomite clusters. Dolomite crystals were identical both in shape and size

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to those of microbial dolomite previously discovered in modern lake environments and culture

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experiments. The dolomite precipitation temperature range calculated using the low-temperature

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microbial equation was close to the measured temperature, further indicating that microbial

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activity may be involved in dolomite formation. This study provides natural locations with

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different water salinities and other environmental factors for dolomite precipitation, which has

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important significance for studying the mechanism of dolomite formation.

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Keywords: dolomite, mechanisms of dolomite formation, surface lake sediments, Tibetan Plateau

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1 Introduction

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Dolomite (CaMg[CO3]2 ) is a common carbonate mineral in the geological record, but very rare

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in modern carbonate environments (Warren, 2000). The inability to precipitate dolomite

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inorganically in laboratory condition under earth surface temperatures and pressures (Land, 1998)

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makes it difficult to identify the mechanisms of dolomite formation. Controversy over the origin

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of dolomite has generally been referred to as the “dolomite problem”, which is a complex issue

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involving a large number of interacting factors such as thermodynamics, chemical kinetics,

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hydrology, host-rock mineralogy and texture, and microbial activity (Fairbridge, 1957; Hardie,

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1987; Vasconcelos and McKenzie, 1997). Over the last few decades, dolomite formation has been

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identified in different environments and settings, such as marine sediments (Saller, 1984; Jose

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Carballo et al., 1987; Compton, 1988; Chellie S. Teal et al., 2000), marginal marine lagoon and

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lakes (Aharon et al., 1977; Warren, 1990; Vasconcelos and McKenzie, 1997; Van Lith et al., 2002;

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Sánchez-Román et al., 2009b), sabkha (Bontognali et al., 2010). In recent years there has been an

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increasing number of reports of dolomite in lakes, and a number of landmark studies of the origins

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of dolomite have been proposed based on lacustrine sediments (Talbot and Kelts, 1986; Deckker

47

and Last, 1988; Rosen and Coshell, 1992; Colson and Cojan, 1996; Jiang and Liu, 2007;

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BrÉHÉRet et al., 2008; Deng et al., 2010; Meister et al., 2011). The formation mechanisms of

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lacustrine dolomite are diverse. Therefore, lakes are ideal large-scale laboratories that offer a wide

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range of depositional environment to study the kinetics and define the conditions that promote

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dolomite formation (Last, 1990).

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Studies of sedimentary dolomite formation in modern environments have greatly increased our

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understanding of the physicochemical conditions required for dolomite formation. Kinetic barriers

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are believed to be the cause of the failure to form dolomite inorganically in laboratory at low

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temperatures, which may be a sufficient explanation for the paucity of dolomite in modern marine

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environments (Land, 1998). High salinity, pH, Mg/Ca ratio, and temperature are conducive to

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dolomite formation due to the long-term replacement of pre-existing calcite (Krauskopf and Bird,

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1995). The primary precipitation of dolomite can occur in aqueous solutions which are associated

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with saline evaporate deposits and organic-rich sediments (Compton, 1988; Deckker and Last,

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1988; Vasconcelos and McKenzie, 1997; Wright, 1999; Wright and Oren, 2005). Although many

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dolomite formation models have been established, e. g., the seepage refluxion model (Adams and

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Rhodes, 1960), the mixing-zone model (Badiozamani, 1973), sulfate reduction model (Baker and

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Kastner, 1981), organogenic model (Slaughter and Hill, 1991), and microbial dolomite model

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(Vasconcelos and McKenzie, 1997), a large number of culture experiments have proved that

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microbial activity is the key to dolomite formation at low temperatures (Sánchez-Román et al.,

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2008).

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The Tibetan Plateau has a large group of lakes with rich mineral resources. Modern dolomite

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has also been found in lakes on the Tibetan Plateau. Xia and Li (1986) suggested that dolomite

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that formed in the Xiaochaidan Salt Lake originated from organic processes during cyanobacterial

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activity rather than from replacement or pure chemical precipitation. Sub-spherical and spherical

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dolomite in Bayinchagan Lake and Qinghai Lake are identical both in size and in shape to

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microbially precipitated dolomite, suggesting a biogenic origin (Jiang and Liu, 2007; Yu et al.,

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2007; Deng et al., 2010). Low-content dolomite was universal in all selected typical lakes in the

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Qaidam basin and Central Tibetan Plateau; however, the origin of dolomite has not been discussed

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(Wang et al., 2008).

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In this study, we obtained numerous lacustrine surface sediment samples based on field surveys

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of lakes on the Tibetan Plateau. Among them, dolomite occurred in 20 lakes with different

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salinities on the central and western Tibetan Plateau (Table 1). The objectives of this study were,

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therefore, to: (a) investigate the environmental factors affecting dolomite formation, such as the

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Mg/Ca ratio, salinity, pH, and the SO42- concentration, and; (b) determine the mechanisms of

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dolomite formation. This study provides new locations of modern dolomite in lacustrine

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environments which has important significance for understanding dolomite formation.

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2 Geological setting

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The study area (30.8‒35.0 °N, 80.1‒89.0 °E) is located in the central and western Tibetan

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Plateau (Fig. 1). It is surrounded by the Kunlun Mountains to the north, and the Gangdese

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Mountains and Nyainqentanglha Mountains to the south. All the investigated lakes are located in

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the Tibetan Plateau endorheic drainage basin. The lake area of these endorheic drainage basins

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accounts for 70.7% of the total lake area of the Tibetan Plateau (Wan et al., 2016). The elevations

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of these lakes are ~4500‒5000 m above-sea-level (a.s.l.), while the surrounding mountains reach a

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mean altitude of ~5500‒6000 m (Guan et al., 1980).

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The area has been subjected to arid to semi-arid climatic conditions in modern times, with mean

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annual precipitation (MAP) of < 50 to 300–400 mm, and mean annual temperatures (MAT)

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varying from < −8 to 2 °C (Institute of Geography, 1990). Lakes in the endorheic drainage basins

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are mainly inland saltwater lakes or saline lakes (Guan et al., 1980). Bedrock of the lake basins is

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primary composed of clastic, metamorphic, and igneous rocks (Fig. 1a-f) consisting mainly of

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conglomerate, sandstone, siltstone, mudstone, limestone, shale, pyroclastic rock, granite, gneiss,

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diabase, and ophiolite (Bureau of Geology and Mineral resources of Xizang Autonomous Regions,

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1993). Sixteen types of carbonate minerals are found in the Tibetan Plateau, of which calcite,

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dolomite, aragonite, and magnesite are the most abundant (Zhang and Zheng, 2017).

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3 Methods

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Sediment and water sampling was mainly carried out in late summer and autumn (August to

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November) of 2012‒2015. Surface sediment samples were sampled from 55 lakes using an Ekman

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Bottom Sampler. To reduce the possible influence of lakeshore detrital carbonate, surface lake

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sediments were sampled from the centers of the lakes. The top 2 cm of sediments were collected

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from each site. Lake water samples were collected from lake water surfaces. A multi-parameter

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water detector (HYDROLAB DS5, Hach and EXO2, YSI) was used to measure water quality

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parameters (i.e., temperature, pH, conductivity, salinity and total dissolved solids [TDS])

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simultaneously at the water sampling positions. Climate data of the 55 lake sites were obtained

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from the China Meteorological Forcing Dataset (Yang et al., 2010; Chen et al., 2011). The

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horizontal resolution of climate data is 0.1°. The MAP and MAT for each site were the 30-year

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average from 1981 to 2010.

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Prior to mineralogical analysis and microscopic observation, all sediment samples were sieved

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and the < 40 µm fractions were collected for further analysis, as the carbonate in the < 40 µm

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fraction of lake sediment was considered to be authigenic origin (Fontes et al., 1996; Jiang and Liu,

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2007). Mineral compositions were analyzed using X-ray diffraction (XRD) in the Beijing Micro

117

Structure Analytical Laboratory. Carbonate contents were calculated using the results of TIC,

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which were measured using a Shimadzu TOC-VCPH analyzer with a solid sample module

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SSM-5000 at the Institute of Tibetan Plateau Research, Chinese Academy of Sciences. A total of

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20 samples containing dolomite from 20 lakes (Table 1) on the central and western TP were taken

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for analyses of the crystal morphology. Scanning electron microscope (SEM) observations with an

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accompanying energy dispersive spectroscopy (EDS) was performed in the SINOPEC Research

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Institute of Petroleum Processing. The MgCO3 mole% of dolomite was calculated using the

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following formula:

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NMgCO3 mol% =-333.33d 104 +1011.99,

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where d 104 is the interplanar spacing which can be calculated based on the 2θ value of the (104)

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face’s diffraction peak from the XRD profiles (Solotchina and Solotchin, 2014).

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A total of 20 carbon and oxygen isotope sediment samples were performed on an IsoPrime

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100 mass spectrometer equipped with a MultiPrep system at the Institute of Earth Environment,

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Chinese Academy of Sciences. The international standard NBS19 and the laboratory standard TB1

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were included in the analysis to check for homogeneity and reproducibility of results. The

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analytical results for the isotope values are presented using the δ-notion relative to Vienna Pee Dee

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Belemnite (VPDB). The standard deviations of both δ13C and δ18O were < 1‰ (2σ).

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Water samples were filtered and diluted based on the salinity prior to water chemistry analysis.

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Concentrations of major cations (Na+, K+, Ca+, Mg2+) and anions (Cl- and SO42-) were determined

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using an ICS-2000 ion chromatography system (DIONEX Company, USA). HCO3- concentrations

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were estimated by the balance between cations and anions (Wang et al., 2010b). Oxygen isotope

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analyses of lake water from six lakes were prepared and analyzed using a Wavelength Scanning

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Cavity Ring-down Spectrometer (WS-CRDS, Picarro-L2120i). Each sample was measured six

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times and the first three analyses were not used to avoid memory effects from previous samples.

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All oxygen isotope compositions are reported in standard δ-notion relative to Standard Mean

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Ocean Water (SMOW). The standard results showed that the external precision of δ18O analysis

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was better than 0.1‰. All analyses were performed at the Institute of Tibetan Plateau Research,

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Chinese Academy of Sciences.

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4 Results

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4.1 Mineral composition of the surface lake sediments

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The XRD results showed that the mineralogy of the surface lake sediments was characterized

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mainly by carbonate, clay, and detrital minerals. Carbonate contents ranged from 18.89‰ to

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60.71‰, with a mean value of 43.24‰. Carbonates consisted of calcite (CaCO3), aragonite

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(CaCO3), dolomite (CaMg(CO3)2), and trace hydromagnesite [Mg5(CO3)4(OH)2·H2O]. Ankerite

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[Ca(Fe, Mg)(CO3)2] only appeared in Serling Co. Clay and detrital minerals were composed of

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illite and chlorite, quartz, and feldspar, respectively. The XRD patterns of carbonate minerals are

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shown in Fig. 2.

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Dolomite contents varied from 3‒10%, with a mean of 5.1% (Table 1). The MgCO3 content in

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dolomite was approximately 45.10‒50.60% as shown by the XRD analysis (d 104 = 2.8842‒

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2.9007 Å). Generally, the mol% MgCO3 in Ca-dolomite was > 40%, whereas the MgCO3 content

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in stoichiometric dolomite is > 50% (Van Lith et al., 2002). Therefore, most dolomite in the

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surface lake sediments was Ca-dolomite, except for that from Sumxi Co and Dong Co (MgCO3

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contents of 50.13% and 50.60%, respectively).

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4.2 Morphology of carbonate minerals

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Calcite crystals appeared in the form of rhombohedral or blocky crystals with the grains size

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< 15 µm as shown by SEM observations (Fig. 3a, b). The low magnesium peak in the EDS chart

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of calcite crystals indicated that these crystals were magnesium carbonate (Mg-calcite). Aragonite

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crystals showed prismatic morphology (~ 5 µm) (Fig. 3c). Dolomite exhibited two principal forms

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in the surface lake sediments. Rhombic dolomite appeared as well-formed euhedral crystals (2‒10

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µm) and was mostly scattered in sediments (Fig. 3d, e). Sub-spherical and spherical dolomite

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occurred either as dispersed crystals or were attached to the surface of other minerals with grains <

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2 µm (Fig. 3f, g) and aggregated into clusters (Fig. 3h, i). In some lakes, dolomite was not

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identified under SEM due to the low content or the inhomogeneity of the mineral distribution.

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4.3 Chemistry of lake water

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The water chemistry of dolomitic lakes is quite different from those of lakes without

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dolomite, especially in terms of Mg/Ca ratio, salinity, and SO42- concentration (Table 2). The

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salinity of the dolomitic lake water ranged from 0.11 to 135 g/L, with a mean of 25.87 g/L (Fig.

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4a); and the pH ranged from 7.61 to 10.56, with a mean of 9.02 (Fig. 4b). The major ionic

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composition showed distinct variability in lake water. For example, the concentrations of K+, Na+,

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Mg2+, and Ca2+ cations were 0.88‒4853.58 mg/L, 5.85‒74165.60 mg/L, 0‒10690.26 mg/L, and 1‒

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5172.51 mg/L, respectively. The concentrations of Cl-, SO42-, and HCO3- anions also had large

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ranges (0‒128777.8 mg/L, 10.36‒51160.79 mg/L, and 0‒13429.05 mg/L, respectively). The

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resulting Mg/Ca (in mol) ratios ranged between 0.04‒4519.54, with an average of 535.94 (Table 2,

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3).

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4.4 Isotopic composition of lake water and sediments

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The oxygen isotope results of water taken from six lakes showed that all the δ18OSMOW values

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were negative, spanning a wide range of -4.12‰ to -0.22‰. The δ13CPDB values for carbonates are

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shown in Fig. 5 (plotted against δ18O) and ranged from 1.7‒8.92‰. The δ18OPDB values for

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carbonates ranged from -7.82‰ to 0.54‰, and on the SMOW scale were 22.85‰ to 31.47‰.

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These δ18O values are very similar to those reported previously for Qinghai Lake (from -9.00‰ to

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-1.59‰, Deng et al., 2010), and fall into the field of normal sedimentary rocks (Zheng and Chen,

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2000). There was a significantly positive linear relationship between δ13CPDB and δ18OPDB (r =

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0.70, P < 0.01).

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5 Discussion

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The exposed rocks in the study area are mainly composed of clastic, metamorphic, and igneous

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rocks, however, a few limestone and marine carbonatites are also distributed around some lakes,

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such as Serling Co, Gyeze Caka, Lagkor Co and Bura Co (Fig 1a, b, c, and f). To preclude the

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presence of detrital carbonate, all sediment samples of the < 40 µm fraction were selected for

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analyses, which are considered authigenic carbonate (Fontes et al., 1996; Jiang and Liu, 2007).

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Calcite appeared as rhombohedral or blocky crystals, aragonite appeared as prismatic crystals, and

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dolomite appeared as rhombic or sub-spherical and spherical crystals, which were similar in

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morphology to those formed from natural environments and precipitation experiments and argue

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against a detrital origin (Talbot and Kelts, 1986; Vasconcelos and McKenzie, 1997; Jiang and Liu,

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2007; Gu et al., 2015). The close relationships between the δ18O of bulk carbonate, calcite, and

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dolomite and environment indictors (the δ18O of lake water, temperature, salinity,

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precipitation/evaporation, altitude and latitude) in typical lakes on the Tibetan Plateau also

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indicate that carbonate minerals are formed directly through chemical precipitation in lake water

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(Wang et al., 2008). In addition, the cold climate on the Tibetan Plateau leads to weak physical

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weathering and a limited contribution of detrital carbonate in lake sediments (Li and Kang, 2007).

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This evidence suggests that the contribution of detrital carbonate can be ignored in most lakes.

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5.1 Environmental factors affecting the dolomite formation

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Under low temperature and pressure conditions, there are three main kinetic inhibitors of

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dolomite formation: (1) the high hydration energy of Mg2+ ions (Lippmann, 1973); (2) the

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extremely low concentration and activity of CO32- (Garrels and Thompson, 1962), and; (3) the

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presence of SO42-. Previous studies have suggested that dolomite precipitation is governed by

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geochemical factors, such as the Mg/Ca ratio, salinity (Folk and Land, 1975; Sibley et al., 1987;

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Deckker and Last, 1988; Van Lith et al., 2002), pH (Slaughter and Hill, 1991; Hammes and

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Verstraete, 2002), and the SO42- concentration (Baker and Kastner, 1981; Vasconcelos and

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McKenzie, 1997). Compared with non-dolomitic lakes, dolomitic lakes are characterized by high

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Mg/Ca ratio, high salinity, high sulfate concentration, and slightly higher pH (Table 2).

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In general, a high Mg/Ca ratio and high salinity are regarded as important factors affecting

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dolomite precipitation (Müller et al., 1972; Wright and Oren, 2005). Mg2+ ions are strongly

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hydrated by polar water shells, which makes it difficult for them to enter the dolomite lattice, and

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hinders dolomite precipitation from supersaturated solution (Lippmann, 1973). A high Mg/Ca ratio

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(i.e. high Mg2+ concentrations of hypersaline waters) means that the hydration barrier is more

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easily overcome (Warren, 2000). High salinity favors hydrated ions losing their water shell and

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decreasing the hydration energy of Mg2+, due to direct binding as a function of ion distribution,

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repartition of fixed charges, and the anions present (Wright and Oren, 2005). In addition, above

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dolomite saturation, increasing salinity clearly favors dolomite formation because supersaturation

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increases (Machel and Mountjoy, 1986).

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Previous studies suggested that hypersaline dolomite begins to precipitate only when the

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Mg/Ca weight ratio exceeds 5:1 to 10:1(Kinsman, 1966; Folk and Land, 1975). In the lakes of the

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East Asian monsoon region, a Mg/Ca molar ratio of 1.7 is the threshold for dolomite formation

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(Gu et al., 2015). With methanogenic metabolic activity, dolomite can precipitate in waters with

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very low Mg/Ca molar ratios (< 1) (Roberts et al., 2004; Kenward et al., 2009). In our study, only

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the Mg/Ca (in mol) ratios of Hongshan Lake and Matou Lake were < 1.

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Our investigation revealed a positive linear relationship between salinity and the Mg/Ca ratio

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(r = 0.56, P < 0.05; Fig. 4a). The high Mg/Ca ratio and high salinity of most lakes would favor

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dolomite precipitation. However, in some cases, the salinity was as low as 0.11 mg/L in the

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freshwater lake where dolomite formed. Although high salinity favors the breakdown of hydration

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shells, Folk and Land (1975) argued that high salinity results in a more rapid crystallization rate

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and an increase in the number of interfering ions (e.g. Ca2+), which is not conducive to dolomite

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formation because of the precise Ca-Mg ordering required. Higher salinity also means higher

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sodium, which means less free CO32- due to the formation of soluble NaCO30 (Garrels and

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Thompson, 1962). Taking Nganggun Co as an example, although the salinity was only 1.89 g/L,

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the Mg2+ concentration in the lake water can reach more than ten times the Ca2+ concentration,

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which is beneficial for dolomite formation. Therefore, a high Mg/Ca ratio may be more important

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than high salinity for dolomite precipitation in Tibetan lakes.

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High pH is important because dolomitization is partly controlled by the activity of carbonate

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anions in solution. This activity increases the CO32- concentration relative to HCO3- concentration

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with increased pH (Slaughter and Hill, 1991). pH was not significantly correlated with the Mg/Ca

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ratio in this study (Fig. 4b), which was revealed by a low correlation coefficient of -0.18 (P =

249

0.454, n = 20). Compared with Qinghai Lake (pH = 9.3, Deng et al., 2010), Coorong Region (pH

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= 6.82‒9.11, Wright and Wacey, 2005; Wacey et al., 2007), and Lagoa Vermelha (pH = 8.0‒8.5,

251

Vasconcelos and McKenzie, 1997), the pH range of lakes in this study is similar to that of

252

previous studies (Table 3). The high pH (=10.56) of Zhari Namco may be caused by the

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consumption of large amounts of CO2 in the upper lake due to photosynthesis by aquatic

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organisms (Wang et al., 2010a).

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SO42- is regarded as a dolomite formation inhibitor (Baker and Kastner, 1981; Kastner, 1984).

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Even very low SO42- concentrations can cause the formation of strongly bonded neutral MgSO40

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ion pairs, which significantly increase Mg2+ solubility (Slaughter and Hill, 1991; Wright and Oren,

258

2005). In contrast, Van Lith et al. (2002) considered that during dolomite precipitation, SO42- does

259

not decrease to zero, therefore the accompanying alkalization due to bacterial SO42- reduction is

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more important for dolomite formation than the low SO42- concentrations. As for pH, there was no

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simple linear relationship between the SO42- concentration and the Mg/Ca ratio (Fig. 4c), however

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it is closely related to the water chemistry of lake water on the Tibetan Plateau (Zheng and Liu,

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2010). Our results suggest that SO42- need not be eliminated in order to precipitate dolomite.

264

Compared with lakes where microbially-mediated dolomite formation, the SO42- concentration in

265

this study is close to the ranges reported in previous studies (Table 3). In the microbial dolomite

266

model, an abundant and continuous SO42- supply is also considered a necessary factor for

267

maintaining microbial activity to produce dolomite (Vasconcelos and McKenzie, 1997).

268

Many experiments and field cases have demonstrated that metabolic activities of

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microorganisms, such as SO42--reducing bacteria (SRB) (Compton, 1988; Wright, 1999;

270

Warthmann et al., 2000; Wright and Wacey, 2005), methanogenic Archaea (Kenward et al., 2009),

271

and halophilic bacteria (Sánchez-Román et al., 2009a; Deng et al., 2010) played key roles in

272

mediating low-temperature dolomite precipitation. Metabolic activities of microbial communities

273

in lakes can raise the pH and carbonate and magnesium ion concentrations, and reduce the SO42-

274

concentration, thereby forming a microenvironment that help overcome the kinetic inhibitors of

275

dolomite precipitation (Wright, 1999; Wright and Wacey, 2005). Microbial cell surfaces and

276

excreted extracellular polymeric substances (EPS) carry a net negative electric charge, easily

277

adsorbing metal cations (Mg2+ and Ca2+) on its surface and forming a microenvironment on its

278

surface that facilitates dolomite precipitation (Sánchez-Román et al., 2008). It has also been

279

suggested that microbes such as bacteria, nano-bacteria, and EPS can act as nuclei for dolomite

280

precipitation (Warthmann et al., 2000; Bontognali et al., 2008). Therefore, ubiquitous

281

microorganisms may play an important role in the dolomite formation.

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5.2 Potential mechanism of dolomite formation on the Tibetan Plateau

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5.2.1 The role of evaporation

284

Modern authentic dolomite was observed mostly in high salinity environments such as

285

lagoons and saline lakes, and was usually accompanied by strong evaporation. Jiang and Liu

286

(2007) suggested that in the lake environment, the primary dolomite can be used as a proxy for

287

climate drying. Therefore, these dolomite formations have been interpreted as typically

288

evaporative in origin (Hsü and Siegenthaler, 1969; Deckker and Last, 1988; Warren, 1988). In

289

hypersaline environments, crystallization of minerals is commonly rapid and accompanied by high

290

concentrations of competing ions, and it is difficult for Ca and Mg to segregate into monolayers to

291

form stoichiometric dolomite (Folk and Land, 1975). In such conditions, dolomite is usually

292

highly disordered Ca- dolomite (Warren, 2000), as found in this study.

293

The positive correlation between δ18O and δ13C (r=0.7) in the carbonates suggests that

294

carbonates precipitated in lake water have long-term residence in closed lakes (Talbot, 1990). Arid

295

to semi-arid climates accelerate the continuous evaporation of lake water, along with the

296

precipitation of calcite and aragonite. Quantitative Ca2+ was consumed, increasing the Mg/Ca ratio.

297

With further evaporation, the death and degradation of organic components would provide large

298

amounts of magnesium for dolomite formation. The metabolism of organic matter by SRB or

299

other microbes would also elevate the pH values of lake water (Wright, 1999; Wacey et al., 2007).

300

However, evaporation alone could not explain dolomite formation in lakes with low Mg/Ca

301

ratios. Evaporation experiments of lake waters from Coorong Lake, a classic location for the study

302

of modern dolomite, show that aragonite is precipitated instead of dolomite (Warren, 1988; Wacey

303

et al., 2007). Lippmann (1973) argued that evaporation alone could not favor dolomitization

304

because CO32--free ions were decreasing with the increase of Mg/Ca ratios. In freshwater lakes,

305

the evaporation and concentration of lake water is weak and the Mg/Ca ratio of lake water is low

306

(< 3), which is insufficient to form the large amount of magnesium-rich brine that is required for

307

dolomite precipitation. Generally, freshwater dolomite formation typically appears in the mixture

308

of dissolved inorganic carbon (DIC)-rich meteoric water with magnesium-rich groundwater

309

(Kenward et al., 2009). Therefore, in addition to evaporation, for lakes on the Tibetan Plateau,

310

there should be other mechanisms involved in the formation of dolomite. The most likely

311

mechanism would be that of microbial activity.

312

5.2.2 The role of microbial activity

313

Under the regulation of microbial metabolism, dolomite can be formed not only in high

314

salinity environments (Vasconcelos and McKenzie, 1997; Wright, 1999; Van Lith et al., 2002), but

315

also in low salinity lake waters (Deng et al., 2010) or even fresh water (Roberts et al., 2004;

316

Kenward et al., 2009).

317

In this study, dolomite was present in two distinct forms. One type is characterized by single,

318

well-formed euhedral and rhombic crystals range in length from 2‒10 µm. These dolomite rhombs

319

with micron grain size are found mostly in modern dolomite as a replacement of aragonite or

320

calcite, such as the Qatar Sabkha (Illing and Taylor, 1993), and the permanent hypersaline

321

environment of Lake Hayward (Rosen and Coshell, 1992). In high-temperature (≥ 175 ℃)

322

synthesis experiments, rhombic dolomite formed by dolomitization of CaCO3 can nucleate on the

323

corners of calcite reactants (Sibley et al., 1987). However, replacement dolomite appears to

324

require long reaction times (≥ 104 yr) at low temperatures (Hardie, 1987). Although Tibetan lakes

325

are generally characterized by a low recent sedimentation rate, such as Serling Co (0.25 mm a-1,

326

Gu et al., 1994), Nam Co (0.43‒0.98 mm a-1, Wang et al., 2011), Mapam Yumco (0.31 mm a-1,

327

Wang et al., 2013), Taro Co (0.49‒0.58 mm a-1, Ma et al., 2014) and Tangra Yumco (0.18‒0.64

328

mm a-1, Wang et al., 2017), lake sediments accumulated faster than the time required for formation

329

of replacement dolomite. Furthermore, no evidence of replacement textures from petrographic

330

analysis suggests that dolomite is most likely directly deposited from dolomite-saturated brines.

331

The widespread occurrence of primary-dolomite grains in the Upper Cretaceous sandstones of

332

the Western Interior and Alaska are single rhombic crystals (< 0.3 mm) formed within the

333

depositional basin prior to final settling-down and burial of the sediment (Sabins, 1962).

334

Experiments have shown that rhombic dolomite can be induced by L. sphaericus at 30 °C under

335

20 MPa pressure and micro-aerobic conditions (Song et al., 2014). Dolomite rhombs also occur in

336

pelagic, organic-rich sediments with active degradation of organic matter (Baker and Kastner,

337

1981; Compton, 1988). Huang et al. (1997) suggests that the primary rhombic dolomite in

338

algae-rich dolostones is characterized by crystal forms precipitated and deposited directly from

339

seawater. The euhedral-subhedral micritic dolomite crystals from Junggar Basin are rapidly

340

crystallized in the contemporaneous-penecontemporaneous period and are closely related to the

341

metabolic activities of methanogens (Zhang et al., 2018). Microbial activity may also be involved

342

in the formation of dolomite rhombs in this study.

343

A series of growth experiments have been carried out with selected bacterial cultured from

344

natural samples at low temperatures (Vasconcelos et al., 1995; Sánchez-Román et al., 2008;

345

Sánchez-Román et al., 2009b; Warthmann et al., 2000; Deng et al., 2010). The results showed that

346

under the mediation of microbial activity, dolomite formed in lakes has the same morphology as

347

dolomite synthesized in the laboratory, which manifested as dumbbell, sub-spherical, and

348

spherical crystals.

349

Under SEM, dolomite appeared as sub-spherical and spherical crystals, dispersed in

350

sediments, or adhered to the surface of feldspar to form a knobbly dolomite coating (Fig. 3f, g).

351

Individual dolomite crystals were fairly uniform in size, and < 2 µm in diameter. In some lakes,

352

sub-spherical or spherical crystals were aggregated into ~20 µm assemblages (Fig. 3h, i). The

353

characteristics of these dolomites are very similar to those of microbial origin reported in Coorong

354

Lake (Wright, 1999), Lagoa Vermelha (Vasconcelos et al., 1995; Vasconcelos and McKenzie, 1997)

355

and culture experiments (Vasconcelos et al., 1995; Sánchez-Román et al., 2008). The occurrence

356

of spherical dolomites even in low-salinity lakes (such as Serling Co and Co Ngoin) further proves

357

the possibility of microbial origin. This discovery is consistent with the precipitation of biogenic

358

dolomite in freshwater from both field and laboratory experiments and suggests a pivotal role of

359

microbial processes in dolomite formation across a wide range of environmental conditions

360

(Roberts et al., 2004).

361

For equilibrium mineral precipitation, δ18O of lacustrine carbonate are controlled by the

362

temperature and isotopic composition of the lake water δ18O (Leng and Marshall, 2004). Using

363

δ18O-lakewater and δ18O-dolomite results, the range of precipitation temperature of dolomite can

364

be calculated, assuming equilibrium. The δ18O of carbonate, however, is a result of the mixture of

365

the oxygen isotopic compositions in different carbonate minerals, which is expressed as follows

366

(Wang et al., 2008): =

×

+

×

+

× + ⋯,

367

where δ18OA, δ18OB, and δ18OC are δ18Odolomite, δ18Ocalcite, and δ18Oaragonite, respectively; and a, b,

368

and c stand for the proportion of dolomite, calcite, and aragonite to the bulk carbonate. Previous

369

studies have suggested that under the same conditions, δ18O of aragonite and dolomite are about

370

0.6‰ and 3‰ more positive than calcite, respectively (Land, 1980). Based on this relationship,

371

six lakes which precipitated dolomite, calcite, and aragonite were used to calculate the isotope of

372

dolomite (Fig. 6).

373 374 375

Based on high-temperature experiments, Northrop and Clayton (1966) proposed the following equilibrium fractionation relationship for dolomite: 1000 ln

! "#$% − ' $%( = 3.20 × 10, - ./ − 1.50. (1)

376

Here, T is the temperature in Kelvin, and α is the fractionation factor for oxygen isotope between

377

dolomite and lake water. Due to the failure of low temperature synthetic dolomite experiments,

378

Vasconcelos et al. (2005) proposed a new oxygen isotope fractionation equation based on

379

microbial experiments at a low temperature:

380

1000 ln

! "#$% − ' $%( = 2.73 × 10, - ./ + 0.26. (2)

381

Since the field sampling was conducted in late summer and autumn, the water temperature of

382

the lake had a large range (3‒17 °C). Using the equation obtained from the high-temperature

383

experiment (Eq. 1), the temperature range of dolomite formation was calculated to be 21‒58 ℃

384

(Fig. 6a). However, according to the microbial-mediated low-temperature equation (Eq. 2), the

385

temperature favoring dolomite precipitation is approximately 10 ℃ in most lakes and 42 ℃ in

386

Hongshan Lake (Fig. 6b). Compared to the temperature range derived by Eq. 1, the range

387

calculated using Eq. 2 is closer to the temperature measured in the field, further suggesting that

388

microbial mediation is likely mechanism for dolomite formation.

389

In the lakes of the Tibetan Plateau, SO42--reducing bacteria (Deng et al., 2010; Yang et al.,

390

2013), halophilic bacteria (Jiang et al., 2006; Zhu et al., 2017), and cyanobacteria (Dong et al.,

391

2006 Liu et al., 2009; Xing et al., 2009; Liu et al., 2010;) are common microbial communities that

392

can induce dolomite formation. The abundance of the dsrB gene shared by sulfate reducing

393

bacteria ranges from 1. 71 × 108 to 1. 55 × 109 copies per gram of sediments from six lakes on the

394

Tibetan Plateau (Yang et al., 2013). Anaerobic SRB and aerobic halophilic bacteria-mediated

395

dolomite precipitation are not only active in hypersaline waters, but have also been confirmed to

396

have implications for dolomite formation in slightly saline lake environments such as Qinghai

397

Lake (Deng et al., 2010; Wacey et al., 2007). In lakes of the eastern and southern Tibetan Plateau,

398

cyanobacteria are one of the main sources of biological productivity (Dong et al., 2006; Liu et al.,

399

2009; Xing et al., 2009), which are characterized by concentrating Mg preferentially in their

400

sheaths compared to the ambient water throughout their growth, and releasing Mg rapidly during

401

late evaporation (Wright, 1999). Although the mechanisms of microbial dolomite formation

402

appear diverse, different microorganism species exhibit the ability to alter the water chemistry and

403

overcome kinetic inhibitors of dolomite precipitation.

404

It is plausible that in Tibetan Plateau lake sediments, dolomite precipitation in different lakes

405

may be induced by different microorganism and microbial dolomite patterns, such as the sulfate

406

reduction mode, bacterial aerobic oxidation mode, and methanogenesis mode (Jiang et al., 2017).

407

Further studies will demonstrate the above conclusions using sulfur isotopes of lake water and

408

sediments. In addition, it is necessary to strengthen the study of microorganisms in the modern

409

lakes of the Tibetan Plateau to better understand microbial roles in dolomite precipitation.

410 411

6 Conclusion

412

Modern dolomite formed in the sediment of 20 lakes on the central and western Tibetan

413

Plateau. The dolomite content ranged from 3% to 10%, with a mean of 5%. Dolomite exhibited

414

two principal forms in the surface lake sediments: (1) euhedral 2‒10 µm rhombic dolomite

415

(mostly scattered in sediments), and; (2) sub-spherical and spherical dolomite occurring either as

416

dispersed crystals or aggregated into dolomite clusters. The chemical compositions of lake water

417

affecting dolomite precipitation showed high variability, with the salinity ranging from 0.11 g/L to

418

135 g/L, the Mg/Ca molar ratio ranging from 0.04 to 4519.54, the pH ranging from 7.61 to 10.56,

419

and SO42- concentrations of 10.36‒51160.79 mg/L. The results showed a good linear relationship

420

between salinity and the Mg/Ca ratio, while the pH and SO42- concentration were not significantly

421

correlated with the Mg/Ca ratio. The measured δ18O and δ13CPDB values of carbonates in sediment

422

ranged from −7.82‰ to 0.54‰, and from 1.7‰ to 8.92‰, respectively.

423

Environmental factors, such as high Mg/Ca ratio, high salinity, high sulfate concentration,

424

and high pH favor dolomite precipitation. Evaporation and microbial activity play important roles

425

in dolomite formation on the central and western Tibetan Plateau. The positive relationship

426

between δ18O and δ13C indicates that dolomite formed in lakes has a long residence time and

427

undergoes continuous evaporation. Dolomite crystals were identical both in shape and size to

428

those of microbial dolomite discovered in modern lake environments and culture experiments.

429

This was also confirmed by the calculations using temperature-dependent fractionation factors for

430

oxygen isotopes between dolomite and lake water, which indicated the contribution of microbial

431

activities to dolomite precipitation in these lakes.

432 433

Acknowledgments

434

This work was supported by the Strategic Priority Research Program of Chinese Academy of

435

Sciences (XDA20020100), the National Natural Science Foundation of China (41807445), and the

436

Key Project of National Natural Science Foundation of China (41831177). We would like to thank

437

Ping Peng, Jianting Ju, Xiao Lin, Xing Hu, Baojin Qiao, Lei Huang, Chong Liu, Teng Xu, Hao

438

Chen and Jinlei Kai for their participation in the field. We also thank Editage (www.editage.cn) for

439

English language editing.

440

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Table.1 Locations, salinity, mineralogy, carbonate and dolomite and content from the sampled lakes on the central and western Tibetan Plateau Latitude

Longitude

Altitude

(°N)

(°E)

(m a.s.l.)

Sumxi Co

34.588

80.233

5051

Freshwater lake

0.26

2

Hongshan Lake

34.827

80.062

5065

Freshwater lake

0.59

3

Matou Lake

34.644

80.893

5217

Freshwater lake

4

Co Ngoin

31.627

88.726

4547

Freshwater lake

5

Nganggun Co

31.200

85.448

4663

Brackish lake

6

Serling Co

31.801

88.993

4547

7

Bura Co

34.417

85.779

8

Zhangne Co

31.547

9

Gomang Co

10

Larung Co

No.

Lake Name

1

Carbonate

Dolomite

Portion of

content

content

dolomiteb

dolomite, calcite

18.89%

10%

53%

dolomite, calcite

37.77%

4%

11%

0.11

dolomite, calcite

42.92%

10%

23%

0.21

dolomite, aragonite, calcite

41.30%

3%

7%

1.89

dolomite, aragonite, calcite

55.48%

3%

5%

Brackish lake

7.85

dolomite, aragonite, calcite, ankerite

48.50%

7%

14%

5172

Brackish lake

5.62

dolomite, aragonite, calcite

26.07%

7%

27%

87.385

4602

Brackish lake

4.06

dolomite, aragonite, calcite, hydromagnesite

59.45%

3%

5%

31.583

87.282

4658

Brackish lake

6.35

dolomite, aragonite, calcite, hydromagnesite

56.99%

5%

9%

34.337

85.228

4890

Brackish lake

12.01

dolomite, aragonite, calcite

28.91%

4%

14%

c

c

dolomite, aragonite, calcite, hydromagnesite

40.36%

5%

12%

(g/L)

Mineralogy

11

Zhari Namco

30.883

85.539

4638

Brackish lake

13.90

12

Yunbo Co

30.804

84.827

4614

Saltwater lake

16.09

dolomite, aragonite, calcite, hydromagnesite

42.94%

3%

7%

13

Dogze Co

31.866

87.555

4528

Saltwater lake

13.87

dolomite, aragonite, calcite

30.61%

4%

13%

14

Dong Co

32.146

84.750

4396

Saltwater lake

46.25

dolomite, aragonite, calcite

47.19%

7%

15%

15

Bero Zeco

32.423

82.955

4395

Saline lake

27.38

dolomite, aragonite, calcite, hydromagnesite

49.34%

4%

8%

16

Bamgdog Co

34.953

81.536

4909

Saline lake

30.74

dolomite, aragonite, calcite

52.68%

3%

6%

17

Lagkor Co

32.051

84.170

4470

Saline lake

40.27

dolomite, aragonite, calcite

60.71%

5%

8%

18

Mang Co

34.509

80.447

5027

Saline lake

84.11

dolomite, aragonite, calcite, hydromagnesite

42.43%

6%

14%

19

Longmu Co

34.615

80.480

5010

Saline lake

70.74

dolomite, aragonite, calcite

41.73%

4%

10%

Gyeze Caka

33.942

80.881

4524

Saline lake

135.00

dolomite, aragonite, calcite, hydromagnesite

40.61%

5%

12%

20

665 666 667

Salinity

Lake typea

a

Here the lake is classified according to salinity. The salinity is <1g/L, 1-35g/L, 35-50g/L and > 50g/L for freshwater lake, brackish lake, saltwater lake and saline lake, respectively.

b

The portion of dolomite refers to the proportion of dolomite to bulk carbonate.

c

The lake type and salinity are based on Wang and Dou (1998).

668 669

670 671 672 673 674

675 676 677

Table.2 Comparison of water chemistry and climatic conditions between dolomitic lakes and non-dolomitic lakes on the central and western Tibetan Plateau Mg/Ca Salinity pH (in mol) (g/L) Dolomitic lakes (n=20) 535.94 9.02 25.75 Non-dolomitic lakes (n=35) 23.19 8.95 12.56 Here water chemistry and climate parameters are mean values.

SO42(mg/L) 6959 2992

MAP (mm)

MAT (℃)

189.19 264.6

-4.3 -2.76

Table 3 Comparison with precipitation environments of typical lakes which biogenic dolomite formed Salinity Mg/Ca Location pH SO42-(mg/L) Reference (g/L) (in mol) Qinghai Lake (Saline lake) 12.5 61.0 9.3 1718.4 (Deng et al., 2010) Coorong Region (hypersaline dolomitic lakes)

15~141

6.82~9.11

9667~56591

(Wacey et al., 2007; Wright and Wacey, 2005)

Lagoa Vermelha (hypersaline costal lagoon)

6

8.0~8.5

3936~5760

(Vasconcelos and McKenzie, 1997; Warthmann et al., 2000)

0.04~45 19

7.61~10.56

10.36~5116 0.79

This study

Central and western Tibetan Plateau

0.11~135

Fig.1 Map of the sampling locations on the central and western Tibetan Plateau. Lakes are distinguished by salinity and marked with different colors. The sequence number of lakes in the figure is consistent with that in the table 1. To make the bedrock investigation convenient, six regions are divided based on the distance of lakes. Clastic rocks, metamorphic and igneous rocks are the most widely bedrock in the study area(a-f). Marine carbonatite is exposed around Sumxi Co, Longmu Co, Gyeze Caka and Bura Co(a-b). Fig.2. X-ray diffractograms showing the occurrence of detrital minerals (quartz, feldspar, clay), carbonates (calcite, aragonite, dolomite, hydromagnesite), respectively, in lake surface sediments. The peaks of each mineral are indicated by the following letters: Qtz=Quartz, Il=Illite, Ab=Albite, Cal=Calcite, Dol=Dolomite, Arg=Aragonite, Hm=Hydromagnesite, Hl=Halite. Fig.3. Scanning electronic microscope(SEM) images of <40 mm fraction from lake surface sediments: (a) Rhombohedral calcite from Hongshan Lake; (b) Blocky calcite stacked together from Nganggun Co; (c) Prismatic aragonite from Zhangne Co;(d)(e) Rhombohedral dolomite from Bamgdog Co and Bura Co; (f) dispersed spherical dolomite from Mang Co as marked by circles; (g) spherical dolomite precipitated on the surface of feldspar as marked by circles from Serling Co; (h) (i) aggregates of sub-spherical or spherical dolomite from Lagkor Co and Zhari Namco. Yellow circles represent the location of Energy dispersive spectrums (EDS) results acquisition (insert of each figure). Fig.4. Mg/Ca molar ratio plotted against (a) salinity; (b) pH; and (c) sulfate concentration. Fig.5 Relationship between δ18O and δ13C for carbonates from surface lake sediments. Fig.6 Plot of temperature versus δ18O of lake water. The curved lines represent isotopic compositions of dolomite in equilibrium with lake water at the given temperature. The range of δ18O of dolomite is 27.54‰ to 33.80‰ (in SMOW) from this study. (a) Profile of δ18O of dolomite using Eq.1; (b) Profile of δ18O of dolomite using and Eq.2. Take Bamdog Co as an example to show how the precipitation temperature of dolomite is calculated: the dolomite from Bamdog Co has a measured δ18O value of 33.80‰ and is marked as red pentagram on the diagram. The measured δ18O of lake water is -1.76‰ and so the precipitation temperature to which these two isotopic values correspond is read off the x-axis of the diagram.

Declaration of interest

We declare that we have no financial and personal relationships with other people or organizations that could inappropriately influence (bias) our work. There are no potential conflicts of interest include employment, consultancies, stock ownership, honoraria, paid expert testimony, patent applications/registrations, and grants or other funding. Signed by all authors as follows: Jiao Li Liping Zhu Minghui Li Junbo Wang Qingfeng Ma