- Email: [email protected]
Geochemical evidence for sources of surface dust deposited on the Laohugou glacier, Qilian Mountains
Accepted Manuscript Geochemical evidence for sources of surface dust deposited on the Laohugou glacier, Qilian Mountains Ting Wei, Zhiwen Dong, Shichang Kang, Xiang Qin, Zhilong Guo PII:
S0883-2927(16)30428-0
DOI:
10.1016/j.apgeochem.2017.01.024
Reference:
AG 3816
To appear in:
Applied Geochemistry
Received Date: 23 October 2016 Revised Date:
21 January 2017
Accepted Date: 30 January 2017
Please cite this article as: Wei, T., Dong, Z., Kang, S., Qin, X., Guo, Z., Geochemical evidence for sources of surface dust deposited on the Laohugou glacier, Qilian Mountains, Applied Geochemistry (2017), doi: 10.1016/j.apgeochem.2017.01.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT
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Ting Wei a, Zhiwen Dong a*, Shichang Kang a, b, Xiang Qin a, c, Zhilong Guo a
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a
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Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000,
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China.
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b
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c
State
Key
Laboratory
of
Cryospheric
Sciences,
Northwest
Institute
of
CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China
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Geochemical evidence for sources of surface dust deposited on the Laohugou Glacier, Qilian Mountains
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Qilian Shan Station of Glaciology and Ecologic Environment, Chinese Academy of
Sciences, Lanzhou 730000, China.
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*Corresponding Author. Address: State Key Laboratory of Cryospheric Sciences,
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Northwest Institute of Eco-Environment and Resources, Chinese Academy of
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Sciences, Lanzhou 730000, China.
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Email: [email protected]
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Abstract
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Atmospheric dust deposited on glacier surfaces can decrease snow albedo by
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enhancing lighting absorption and by forming cryoconites via microbial activity,
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which accelerates glacier melt. Using snow and cryoconite sampled from the
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Laohugou Glacier No.12 in the Qilian Mountains during spring 2012 and summer
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2014, this work investigates the Sr-Nd isotopic and rare earth element (REE)
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geochemistry of dust and its environmental significance. Results demonstrate that
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dust REEs possess lower Eu/Eu* and L/HREE, and higher (Gd/Yb)N values compared
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with that of the Mu Us and Hobq Deserts but yield higher (La/Yb)N, L/HREE,
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(La/Sm)N and (La/Yb)N ratios than that of the Taklimakan Desert. The REE
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composition of glacier surface dust is similar to the material from nearby arid regions,
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such as the Qaidam Basin and the Tengger and Badain Jaran Deserts surrounding the
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eastern Tibetan Plateau. The εNd(0) values of glacial dust resemble the isotopic ratios
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of the Qaidam, Badain Jaran, Tengger Deserts and local dust (εNd(0) value of -13.6)
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but differ from that of the Taklimakan Desert, which has higher
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Analysis of air mass trajectory also indicates a potential dust input from the
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surrounding areas to the alpine glaciers in the Qilian Mountains. These results
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strongly indicate that the dust source from the arid, northern Tibetan Plateau region
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was the dominant contributor to atmospheric dust deposition on the glacier surface,
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rather than dust originating from the long-range transport of Taklimakan Desert
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material.
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Keywords: Surface dust; REEs; Sr-Nd isotopes; Dust source; glacier melt
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1. Introduction
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Mineral dust exerts a significant influence on the climate system and biosphere by
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affecting the chemical composition of the troposphere (Bauer et al., 2004; Dentener et
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al., 1996; Zhang et al., 2015), potentially reducing precipitation by increasing
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cloudiness with smaller droplets (Rosenfeld et al., 2001) and changing the radiative
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balance of the atmosphere by absorbing solar radiation and scattering terrestrial
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radiation (Sokolik and Toon, 1999; Sullivan et al., 2007).
Sr/86Sr ratios.
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The Cryosphere plays an important role in the Earth’s climate system, affecting the
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hydrological cycle and energy balance (Di Mauro et al., 2015), and is sensitive even
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to small climate changes. Glaciers, the largest component of the Cryosphere, are
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currently experiencing dramatic melting attributed to increasing temperatures and
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decreasing snow accumulation. Mineral dust, one type of Light Absorbing Impurity
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(LAI) deposited on glaciers, is an important factor influencing glacier melt (Kaspari et
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al., 2015; Wang et al., 2013). Specifically, iron oxides in dust such as goethite and
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hematite enhance absorption of shortwave spectra (ultraviolet and visible wavelengths)
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in snow and thus lead to reduced snow albedo and glacier melt (Zhang et al., 2015;
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Wu et al., 2015). Previous studies indicated that snow albedo and radiative forcing of
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dust have a greater influence on climate warming than does black carbon when
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impurity contents are low (Kaspari et al., 2014; Ramanathan and Carmichael, 2008).
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Mineral dust in the Solu-Khumbucan decrease snow albedo by 40-42% (Kaspari et al.,
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mean annual albedo of snow by less than 0.01 (Gabbi et al., 2015). Iron oxide mineral
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dust has a significant effect on the melting of snow and glaciers. The mineralogical
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properties of surface dust on glaciers have a strong effect on glacier ablation.
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However, many previous studies focused more on biological properties and less on
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mineralogy, which depends on dust source and transport. Therefore, to constrain the
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influence of dust on glacier ablation, the provenance of dust deposited onto glaciers
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must be identified.
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Rare earth elements (REEs) and strontium and neodymium isotopic compositions
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are powerful tools for determining dust origin (Piper et al., 2013). Like isotopes,
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REEs possess identical external electronic configurations and therefore display
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essentially identical chemical properties. Owing to their low solubility, REEs are
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generally in the particulate phase during transport (Henderson, 1984). These
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properties impose limitations on REE fractionation during weathering and diagenesis.
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Similarly,
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properties and less on alteration due to surficial processes, including weathering,
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transport and deposition. These isotopes are therefore utilized as fingerprints for dust
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sources (Grousset and Biscaye, 2005; Nie et al., 2012). Using REE compositions and
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Sr-Nd isotopes, we can both determine dust composition and trace the source of
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mineral dust deposited onto glaciers (Dong et al., 2015; Dong et al., 2016; Du et al.,
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2015; Wu et al., 2015; Tepe et al., 2015). We therefore employ a multi-parameter
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method to address provenance (Nie et al. 2012).
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Nd/144Nd isotopic signatures mainly depend on geological
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Laohugou (LHG) glacier No.12 is a mountain glacier in northern China that has
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gradually retreated over the last 50 years (Du et al., 2008). This dramatic glacier melt
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is attributed to a rise in temperature, a reduction in snow accumulation and the
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deposition of LAIs such as mineral dust (Kaspari et al., 2015). Therefore, tracing the
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provenance of dust on the glacier is important for understanding both glacial change,
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and regional atmospheric circulation. Previous studies have mainly focused on tracing
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sources and analyzing the physicochemical composition of snow dust, as well as the
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microbial process of cryoconite formation in the upper portion of the glacier (Dong et 3
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of glacier surface dust, including on snow dust and cryoconites from LHG glacier
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No.12. In addition, very limited Sr-Nd isotopic and REE composition data have been
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reported for dust on local glacier surfaces. Therefore, this study aims for a
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comprehensive understanding of dust geochemical composition over the entire glacier
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and discusses the potential dust transport mechanism in the larger region of western
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and northern China.
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2. Sampling and Methodology
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2.1. Sampling glacier surface dust
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The Qilian Mountains, located in the northwestern Tibetan Plateau (TP), are
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surrounded by several large sand deserts. To the northwest and west are the
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Gurbantunggut and the Taklimakan Deserts, respectively, the largest two deserts in
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central Asia. The Qaidam Desert is located immediately to the southwest of the Qilian
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Mountains, and to the southeast are the Badain Jaran and Tengger Deserts. The LHG
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Glacier No.12 (39.438N, 96.568E), with a length of 10.1 km and an area of 21.9 km2,
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is the largest glacier in the LHG basin and is separated into two branches at 4560 m
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above sea level (asl). A typical continental climate characterizes the northern Qilian
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Mountains, and the southeasterly winds prevailing in the area are mainly controlled
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by Westerlies.
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In June 2012 and August 2014, we collected snow pit and cryoconite samples at
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LHG glacier No.12 (Figure 1, Table 1). Thirteen samples were acquired, including
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seven cryoconite samples from 5 cm in depth and six snow pit samples from 95 and
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120 cm in depth. Cryoconite samples were collected between 4386 and 4850 m asl
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and snow pit samples at 5040 m asl. Samples were collected using a pre-cleaned
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stainless-steel shovel and polyethylene gloves, then put into pre-cleaned low-density
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polyethylene (LDPE) bottles (Thermo Scientific). All samples were kept frozen until
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they were analyzed at the Analytical Laboratory of Beijing Research Institute of
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Uranium Geology.
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2.2. REE parameters and Sr-Nd isotopes REEs are a group of 14 elements ranging from La to Lu with similar ionic radii and
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valance states. According to their mass number, REEs are divided into light REEs
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(LREE: La, Ce, Pr, Nd, Sm, Eu) and heavy REEs (HREE: Gd, Tb, Dy, Ho, Er, Tm,
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Yb, Lu). Chondrite-normalization of REEs was used to eliminate the odd-even effect.
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In this study, several REE composition parameters were applied to determine the
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potential sources of REEs in the dust of both the study region and the potential source
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areas. Ratios of L/HREE and (La/Yb)N can reflect the differential degree of REEs.
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The (La/Sm)N ratio represents the differential degree of LREEs, with a positive
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correlation between the ratio and the differential degree. Alternatively, (Gd/Yb)N
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indicates the differential degree of HREE and is negatively correlated with differential
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degree. Additionally, Eu/Eu*(=EuN/(SmN ∗ GdN)1/2) values can not only indicate
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sedimentary environment but also parent rock characteristics (Zhang, 1997).
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Radiogenic Sr and radiogenic Nd isotopic compositions are expressed in ratios of
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Sr/86Sr and 143Nd/144Nd. In general, 143Nd/144Nd ratios possess little variation and are
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represented as a ten thousand part deviation from the
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chondrites
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143
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affected by surficial processes, the cycling of crustal material or grain size. In contrast,
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Sr isotopes are easily altered by weathering, wind sorting and grain size effects (Chen
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et al., 2011). Moreover, Sr isotopes are weakly affected by the reaction of dust with
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acid aerosols during atmospheric transport, and acid leaching during pretreatments
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(Meyer et al., 2011; Revel-Rolland et al., 2006). Therefore, our discussion will
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principally focus on the comparison of εNd (0) values of dust samples and the deserts.
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2.3. Geochemical analyses
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(0.512638):
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Nd/144Nd ratio of modern
εNd=((143Nd/144Nd)sample/143Nd/144Nd)CHUR)−1)×104.
The
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Nd/144Nd ratio is mainly controlled by crust-mantle differentiation age and is not
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REEs in dust samples were measured by inductively coupled plasma-mass
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spectrometry (ICP-MS, Thermo Scientific Element/XR). To improve data accuracy
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and ensure the credibility of results, non-powder vinyl clean room gloves and masks
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were worn during sample analysis to avoid potential contamination. Experimental 5
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blank analyses showed that contamination produced during sampling, transportation
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and analysis was negligible. This methodology has been described in detail by Dong
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et al. (2015). Thermal ionization mass spectrometry (TIMS) measurement of Sr-Nd isotopes was
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performed at the same laboratory as REE analysis. Dissolution was carried out in
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PTFE screw-top bombs with a mixture of ultra-pure HF + HClO4 at 120℃for seven
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days on a hot plate. Then, Sr-Nd isotopes were isolated in quartz columns by
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ion-exchange chromatography. Isotopes of Sr were measured using single Re
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filaments and a Ta activator in static mode. The 87Sr/86Sr and
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corrected for internal mass bias to
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87
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which has a recommended value of 0.710248. The 143Nd/144Nd ratio for the reference
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material ShinEtsu was 0.512095±9 (2σ, n=10), which has a recommended value of
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0.512110. For convenience, the
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εNd(0)=[(143Nd/144Nd) /0.512638–1] x104.
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3. Results and Discussion
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3.1 REE composition of dust deposited on the glacier surface
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The LHG glacier No.12 is located near the central Asian dust sources and may be
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significantly affected by dust emissions from these deserts. However, the deserts
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differ in their influence on dust transport to the study site because of geographic
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location, meteorological conditions and mineralogical properties. The deserts have
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distinct mineral compositions, such as goethite and hematite, which are LAIs on
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glacier and snow surfaces (Maher et al., 2009). The sources of dust transported to
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LHG Glacier No.12 should be identified in order to establish iron oxide indicators and
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to determine the influence of iron oxides on glacier and snow melt.
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Nd/144Nd data were
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Sr/86Sr =0.1194 and
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Nd/144Nd =0.7219. The
Nd/144Nd ratios were normalized and denoted as
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Sr/86Sr ratio for the reference material NBS 987 was 0.710229±13 (2σ, n=10),
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The REE characteristics of glacier surface dust from LHG Glacier No.12 vary
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widely but are different from the Hobq and Mu Us Deserts in their lower Eu/Eu* and
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L/HREE values, and higher (Gd/Yb)N values (Figure 2 and Figure 3). The Taklimakan
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Desert, the main Asian dust source, can be roughly distinguished from samples 6
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and Figure 3). However, several samples yield REE data that are similar to those of
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the northern and southern Taklimakan Desert, indicating a potential contribution of
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Taklimakan Desert dust to the Laohugou basin (Figure 2 and Figure 3). Furthermore,
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the samples have lower (La/Yb)N and L/HREE ratios, but higher (Gd/Yb)N ratios,
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than the western Tibetan Plateau (TP), suggesting that it contributes little to the
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glacier surface dust loading at the study site. However, REE composition of glacier
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surface dust is similar to the Qaidam Basin, Tengger and Badain Jaran Deserts and the
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eastern TP with respect to L/HREE, (Gd/Yb)N, (La/Sm)N and (La/Yb)N. Additionally,
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one of the glacial dust samples possesses a distinct REE signature compared to the
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above deserts and the TP, which potentially implies other unknown dust sources.
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3.2 Sr-Nd compositions
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Isotopes of Sr-Nd are considered to be the most effective and reliable indicators for
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tracing dust source. Generally, Sr isotopes are easily altered by weathering and wind
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sorting during transportation and deposition (Chen et al., 2011). The Sr isotopes are
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also weakly affected by the reaction of dust with acid aerosols during atmospheric
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transportation and by acid leaching during pretreatments (Meyer et al., 2011;
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Revel-Rolland et al., 2006). In this work, an acid leaching pretreatment was used for
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mineral cryoconites formed on the glaciers of the TP in order to remove carbonates
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(Wu et al., 2010; Schettler et al., 2009). According to a previous study on the TP (Wu
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et al., 2010), the carbonate content of dust deposited onto glaciers is very low, which
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suggests that the effect of acid leaching on Sr isotopic composition is negligible. It is
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noteworthy that Sr isotopic compositions are heavily dependent on grain size (Dasch,
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1969; Chen et al., 2007; Feng et al., 2009; Újvári et al., 2012). The grain size effect
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has a significant influence on the
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size fractions have similar
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2007). Chen et al. (2007) divided dust from potential source areas into <75µm and
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<5µm fractions, and Dong et al. (2016) found that most LHG glacier surface dust is
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<75µm in diameter. Therefore, the studied grain sizes of the potential dust sources and
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Sr/86Sr ratio of particles <2µm, while other grain
Sr/86Sr compositions to the bulk size range (Chen et al.,
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Nd/144Nd isotopes
glacier surface dust are similar. In a contrast with Sr isotopes,
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experience limited alteration during the processes of transportation, deposition and
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eluviation (Chen et al., 2011). Therefore, our discussion will principally focus on the
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comparison of εNd(0) values of dust samples and the deserts. The Sr-Nd isotopic
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signatures vary widely, with 87Sr/86Sr ratios of 0.715324 to 0.723112 and εNd(0) values
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of -14.96 to -9.09 (Figure 4). The isotopic composition of glacier surface dust is more
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radiogenic than that of the Ordos Plateau (the Hobq and Mu Us Deserts) with εNd(0)
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values from -16.3 to -10.5, but is lower than the Gurbantunggut Desert with εNd(0)
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values from -6.3 to -1.2. Some samples from the LHG Glacier No.12 are
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geochemically similar to the deserts on the northern margin of the TP (the Qaidam,
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Badain Jaran, and Tengger Deserts), which have εNd(0) values ranging from -12.8 to
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-7.4 and
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similar to the local soil, indicating that this could be the primary source of glacier dust.
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It is noteworthy that different regions within a desert have dramatically different
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provenance and geochemical characteristics. Nie et al. (2015) and Stevens et al. (2013)
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revealed different provenances for the western and eastern Mu Us Desert. Therefore, a
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list of sampling sites of the potential source areas is presented in Table 2. The upper
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reach of the Yellow River shares similar geochemical properties to the western Mu Us
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Desert. Thus, we infer that upper reach Yellow River sediments are a potential dust
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source for the glacier (Nie et al., 2015).
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3.3 Principal component analysis (PCA) for REE composition in surface dust
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Principal component analysis (PCA) is a well-established multivariate statistical
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method that reduces dimensionality of the data to produce a new set of variables while
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retaining the features of the dataset (Jolliffe, 2002). This is useful for further analysis
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of the geochemical composition of dust with a relatively complex source. A
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mathematical algorithm in SPSS 22.0 was used to describe the similarities between
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dust sink and potential source areas (Du et al., 2015). In this study, three principle
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components (PCs) were extracted using SPSS 22.0, yielding a 56.0%, 21.2% and 17.4%
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variance of PC1, PC2 and PC3, respectively, and an eigenvalue higher than 1 (Figure
Sr/86Sr ratios from 0.713 to 0.722. However, the remaining samples are
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loadings of (La/Sm)N, (La/Yb)N, L/HREE and Ce/Yb are higher than 0.839, except for
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(Gd/Yb)N and Eu/Yb in the PC1 column. The (La/Sm)N, L/HREE and Ce/Yb loadings
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for PC2 are negative, and the absolute values of (Gd/Yb)N and Eu/Yb are significantly
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greater than 0.653 (Table 4). Using these results, the first two PCs are identified as
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possessing the original characteristics of both the studied samples and potential source
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regions. The PCA1 scores are negative for the Qaidam, Tengger, Badain Jaran Deserts
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and eastern TP, indicating that these deserts and the eastern TP are the major sources
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of surface dust deposited onto LHG glacier No.12. In contrast, the PCA1 scores are
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positive for the Taklimakan Desert, suggesting that it is a minor contributor to the
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glacier surface dust (Figure 5).
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3.4 Discussion of the possible sources of glacier surface dust
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Geochemical composition (Sr-Nd isotope and REEs) is generally a highly effective
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and reliable dust source tracer. Samples from LHG glacier No.12 are distinct from the
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Mu Us and Hobq Deserts, with very different REE parameters (higher (Gd/Yb)N and
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lower Eu/Eu* and L/HREE) (Figure 2 and Figure 3). The Sr-Nd isotopic compositions
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of the samples show variable εNd(0) values that are higher than the εNd(0) values
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(-16.3 to -10.9) of the Mu Us and Hobq Deserts (Honda et al., 2004; Rao et al., 2008;
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Yang et al., 2007) (Figure 4). Both deserts are located to the northeast of LHG glacier
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No.12 and are perennially dominated by northwesterly and westerly winds. It is
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therefore inferred that the Mu Us and Hobq Deserts could not be the source of surface
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dust in the study area.
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Deserts on the northern margin of the TP include the Qaidam, Badain Jaran and
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Tengger Deserts. Figure 3 shows that some or all samples have very similar REE
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signatures to the Qaidam, Badain Jaran and Tengger Deserts, but only near or just
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within the Taklimakan Desert field. Furthermore, Sr-Nd isotopic compositions show
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that the samples collected at LHG glacier No.12 resemble the Qaidam (-10.2 on
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average εNd(0) values), Badain Jaran (-9.31 on average εNd(0) values) and Tengger
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(-8.56 in average εNd(0) values) Deserts in εNd(0) values but can be distinguished from 9
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Sr/86Sr ratios (Figure 4). The study region is
the Taklimakan Desert by its higher
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close to the Qaidam, Badain Jaran and Tengger Deserts (Figure 1). Additionally, the
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aeolian deposits in the Badain Jaran Desert are predominantly derived from the Qilian
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Mountain via fluvial processes, which further supports the Badain Jaran Desert as a
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surface dust source for the study site (Hu and Yang, 2016). Although the Tengger and
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Badain Jaran Deserts are generally located downwind of LHG glacier No.12, local
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circulation in both deserts may transport dust to the glacier. Back trajectory analyses
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of air masses support this interpretation. These results confirm that the Qaidam,
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Badain Jaran and Tengger Deserts are the major dust sources for LHG glacier No.12.
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However, Figure 4 and Figure 5 show that the Taklimakan Desert is a minor
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contributor to dust composition. The LHG glacier No.12 is located at a high altitude
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(>4200 m asl) in the northeastern TP, whereas the Taklimakan desert is adjacent to the
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northwestern TP. Dust storm events in the Tarim Basin (Taklimakan Desert) occur
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frequently and with a longer duration than in other areas, with dust emissions from the
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desert accounting for 21% of all Asian dust emissions (Zhang et al., 2003; Wang et al.,
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2005). Satellite observations indicate that dust layers emerge most frequently at
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approximately 4 to 7 km a.s.l. Portions of dust storms occurring in the Taklimakan
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Desert can be lifted to the level of the TP by westerly winds, which are transported to
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LHG glacier No.12, while the remaining storm components are blocked by the TP
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(approximately 3 to 6 km a.s.l.). However, dust emissions from the Qaidam, Badain
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Jaran and Tengger Deserts amount to 31% of total Asian dust emissions, which is
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greater than that of the Taklimakan Desert (Zhang et al., 2003). Therefore, the Qaidam,
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Badain Jaran and Tengger Deserts are major sources for dust deposited on the study
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glacier, and the Taklimakan Desert is a minor source. The eastern TP and local soil are
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also major dust sources, judging from their similarity to REE parameters (Eu/Eu*,
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L/HREE, (Gd/Yb)N, (La/Sm)N) and to Sr-Nd isotopic compositions of samples
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collected from LHG glacier No.12 (Figure 4 and Figure 5). Although the Tengger and
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the Badain Jaran Deserts are located downwind of LHG glacier No.12, the
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near-surface atmospheric circulation in both deserts may transport dust to the alpine
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glacier area. Air mass back trajectory confirms this result. The Hysplit4 back
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is mainly from the Qaidam, Badain Jaran, and Tengger Deserts as well as the
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Taklimakan Desert. This is confirmed by air mass back trajectory from the north and
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from local wind circulation (Figure 6).
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4. Conclusions
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Using REEs and Sr-Nd isotope geochemical tracers, this study investigates the
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provenance of surface dust on LHG glacier No.12, which is located in a dust source
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area of east-central Asia. The REE characteristics of the surface dust vary widely but
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are distinguished from the Mu Us and Hobq Deserts by the lower Eu/Eu* and
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L/HREE and higher (Gd/Yb)N of those deserts. One sample from the study site is
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similar to the Taklimakan Desert, while the remainder differ in that they possess lower
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(La/Yb)N, L/HREE and (La/Sm)N ratios. However, the REE characteristics of glacier
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surface dust are similar to that of the Qaidam Basin, Tengger, and Badain Jaran
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Deserts and the eastern TP. The Sr-Nd isotopic signatures of the samples possess great
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variability, ranging from 0.715324 to 0.723112 in 87Sr/86Sr ratios and -14.96 to -9.09
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in εNd(0) values. These values are more radiogenic than the Ordos Plateau, which has
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εNd(0) values from -16.3 to -10.5 but are lower than the Gurbantunggut Desert, with
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εNd(0) values from -6.3 to -1.2.
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Our data clearly show that dust sample characteristics from LHG Glacier No.12 are
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somewhat close to those of deserts on the northern margin of the Tibetan Plateau
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(including the Qaidam, Badain Jaran, and Tengger Deserts), ranging from -12.8 to
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-7.4 in εNd(0) values and 0.713 to 0.722 in
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geochemistry can be attributed to the local soil surrounding LHG glacier No.12. It is
313
therefore inferred that the glacier surface dust may be derived from the Qaidam,
314
Badain Jaran, Tengger Deserts and eastern TP crust, as well as local soil, while the
315
Taklimakan Desert may have contributed little dust.
316
Acknowledgments
317
This work was funded by the National Natural Science Foundation of China
318
(41421061, 41671062), the Chinese Academy of Sciences (KJZD-EW-G03-04), and
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Sr/86Sr. Much of the remaining dust
ACCEPTED MANUSCRIPT the State Key Laboratory of Cryosphere Sciences (SKLCS-ZZ-2016). The authors
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also would like to thank the summer 2014 field work team on the Tibetan Plateau for
321
their logistical field work. We also thank anonymous reviewers and the Editor, Dr.
322
Michael Kersten, for their helpful comments and suggestions.
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Tables Table 1 Sampling of snow and cryoconite dust from Laohugou Glacier No.12. Altitude Sample no.
Sample data
Depth (cm)
Sample type
(m a.s.l.) 2014/7/29
5
cryoconite
LHG30-6
4592
2014/7/30
5
cryoconite
LHG29-4
4450
2014/7/29
5
cryoconite
LHG29-3
4386
2014/7/29
5
cryoconite
LHG30-4
4698
2014/7/30
5
LHG30-1
4849
2014/7/30
5
LHG8-4
4850
2014/8/8
5
LHG-01
5040
2012/6/21
120
LHG-02
5040
2012/6/21
120
LHG-03
5040
2012/6/21
LHG-04
5040
2012/6/21
LHG-05
5040
2012/6/21
LHG-06
5040
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4590
cryoconite cryoconite cryoconite snow pit
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snow pit snow pit
95
snow pit
95
snow pit
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2012/6/21
95
snow pit
Table 2 Sampling sites for dust Sr-Nd isotopic and REE compositions in the arid dust source regions of China. Location
Parameters
References
Gurbantunggut Desert
west of the desert
Sr-Nd isotope/REE
Honda et al., 2004
45.06-45.6N, 87.46-89.11E
Sr-Nd isotope
Chen et al., 2007
37.45-37.68N,104.97-105.5E
Sr-Nd isotope
Chen et al., 2007
38.35-38.64N, 102.33-105E
Sr-Nd isotope
Honda et al., 2004
39N,103.5E
REE
Ferrat et al., 2011
38.34-39N, 102.5-102.54E
Sr-Nd isotope
Nakano et al., 2004
39.4-42.02N, 100.63-103.23E
Sr-Nd isotope
Chen et al., 2007
39.2N,103E
REE
Ferrat et al., 2011
36.8-37.99N, 93.75-97.69E
Sr-Nd isotope
Chen et al., 2007
36N, 95E
REE
Ferrat et al., 2011
whole range
Sr-Nd isotope/REE
Honda et al., 2004
38.5-39.15N, 107.56-108.5E
Sr-Nd isotope
Nakano et al., 2004
38.22-40.33N,108.08-111.35E
Sr-Nd isotope
Chen et al., 2007
40.05-41.45N, 106.77-109.7E
Sr-Nd isotope
Chen et al., 2007
39.65-41.45N, 107.02-111.35E
Sr-Nd isotope
Rao et al., 2008
whole range
REE
Kwon et al., 2004
36.41-39.69N, 81.84-94.35E
Sr-Nd isotope
Chen et al., 2007
37.5-40N, 77.5-94.8E
Sr-Nd isotope
Honda et al., 2004
Gobi
40.43-43.7N, 105-114.5E
Sr-Nd isotope
Nakano et al., 2004
western Tibetan Plateau
29.5-37.5N, 87.5-79.5E
REE
Li et al., 2009
Tengger Desert
EP
Badain Jaran Desert
TE D
Sources
Qaidam Basin
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481 482 483
LHG29-1
Mu Us Desert
Hobq Desert
Taklimakan Desert
18
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Ferrat et al., 2011
Northern Taklimakan Desert
42N,83E
REE
Yang et al., 2007b
Southern Taklimakan Desert
38N,81.5E
REE
Ferrat et al., 2011
Table 3 Total variance explained in principal component analysis. Eigenvalue
Variance%
Cumulative%
1.000
3.362
56.033
56.033
2.000
1.270
21.167
3.000
1.043
17.390
4.000
0.228
3.805
5.000
0.079
1.317
6.000
0.017
0.286
RI PT
Component
77.200 94.591
98.396 99.713
SC
486 487
32N,101E
100.000
Table 4 Loadings of each element upon each of three significant principal components.
M AN U
484 485
East of the Tibetan Plateau
Component
Index 1
3
-0.284
0.192
0.842
-0.492
0.653
0.739
0.206
0.019
0.884
(Gd/Yb)N
0.130
Eu/Yb
0.077
(La/Yb)N
0.970
L/HREE
0.956
-0.108
0.186
Ce/Yb
0.839
-0.006
-0.428
TE D
(La/Sm)N
EP
488 489 490 491
2
Figure Captions
493
Figure 1 Location of Laohugou Glacier No.12, sampled for surface dust deposition.
494
Figure 2 The REE composition (correlation between Gd/Yb and Eu/Eu*, and La/Yb)
495
of surface dust from LHG glacier No.12 and from potential source regions, including
496
the northern and southern Taklimakan Desert, Junggar Basin, eastern and western TP,
497
Qaidam Basin, Tengger Desert, Badain Jaran Desert, Mu Us Desert and Hobq Desert.
498
Figure 3 The REE signatures (correlations between La/Sm and L/HREE, La/Yb and
499
L/HREE) of surface dust from LHG glacier No.12.
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Glacier No.12 with the Gobi Desert and deserts from northern and northwestern
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China. Dotted areas represent d<5 µm fractions of the Chinese deserts, and the
503
remaining data are the d<75 µm fractions of dust particles in arid regions. Some data
504
are from Xu et al., 2012.
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Figure 5 The PCA scores of each site in three principal components, indicating
506
potential dust sources.
507
Figure 6 Three-day backward trajectories of LHG Glacier No.12 during the
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high-frequency dust storm season (March to July) in 2012 and 2014, showing the
509
potential dust transport to the Qilian Mountains glacier area from surrounding regions.
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ACCEPTED MANUSCRIPT Highlights： ： 1. We present new geochemical data of surface dust on glacier in Qilian Mountains 2. Glacier dust were mainly from the Qaidam, Badain Jaran and Tengger Deserts
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S0883-2927(16)30428-0
DOI:
10.1016/j.apgeochem.2017.01.024
Reference:
AG 3816
To appear in:
Applied Geochemistry
Received Date: 23 October 2016 Revised Date:
21 January 2017
Accepted Date: 30 January 2017
Please cite this article as: Wei, T., Dong, Z., Kang, S., Qin, X., Guo, Z., Geochemical evidence for sources of surface dust deposited on the Laohugou glacier, Qilian Mountains, Applied Geochemistry (2017), doi: 10.1016/j.apgeochem.2017.01.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT
4
Ting Wei a, Zhiwen Dong a*, Shichang Kang a, b, Xiang Qin a, c, Zhilong Guo a
5
a
6
Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000,
7
China.
8
b
9
c
State
Key
Laboratory
of
Cryospheric
Sciences,
Northwest
Institute
of
CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China
SC
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Geochemical evidence for sources of surface dust deposited on the Laohugou Glacier, Qilian Mountains
1
Qilian Shan Station of Glaciology and Ecologic Environment, Chinese Academy of
Sciences, Lanzhou 730000, China.
11
*Corresponding Author. Address: State Key Laboratory of Cryospheric Sciences,
12
Northwest Institute of Eco-Environment and Resources, Chinese Academy of
13
Sciences, Lanzhou 730000, China.
14
Email: [email protected]
15
Abstract
16
Atmospheric dust deposited on glacier surfaces can decrease snow albedo by
17
enhancing lighting absorption and by forming cryoconites via microbial activity,
18
which accelerates glacier melt. Using snow and cryoconite sampled from the
19
Laohugou Glacier No.12 in the Qilian Mountains during spring 2012 and summer
20
2014, this work investigates the Sr-Nd isotopic and rare earth element (REE)
21
geochemistry of dust and its environmental significance. Results demonstrate that
22
dust REEs possess lower Eu/Eu* and L/HREE, and higher (Gd/Yb)N values compared
23
with that of the Mu Us and Hobq Deserts but yield higher (La/Yb)N, L/HREE,
24
(La/Sm)N and (La/Yb)N ratios than that of the Taklimakan Desert. The REE
25
composition of glacier surface dust is similar to the material from nearby arid regions,
26
such as the Qaidam Basin and the Tengger and Badain Jaran Deserts surrounding the
27
eastern Tibetan Plateau. The εNd(0) values of glacial dust resemble the isotopic ratios
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of the Qaidam, Badain Jaran, Tengger Deserts and local dust (εNd(0) value of -13.6)
29
but differ from that of the Taklimakan Desert, which has higher
30
Analysis of air mass trajectory also indicates a potential dust input from the
31
surrounding areas to the alpine glaciers in the Qilian Mountains. These results
32
strongly indicate that the dust source from the arid, northern Tibetan Plateau region
33
was the dominant contributor to atmospheric dust deposition on the glacier surface,
34
rather than dust originating from the long-range transport of Taklimakan Desert
35
material.
36
Keywords: Surface dust; REEs; Sr-Nd isotopes; Dust source; glacier melt
37
1. Introduction
38
Mineral dust exerts a significant influence on the climate system and biosphere by
39
affecting the chemical composition of the troposphere (Bauer et al., 2004; Dentener et
40
al., 1996; Zhang et al., 2015), potentially reducing precipitation by increasing
41
cloudiness with smaller droplets (Rosenfeld et al., 2001) and changing the radiative
42
balance of the atmosphere by absorbing solar radiation and scattering terrestrial
43
radiation (Sokolik and Toon, 1999; Sullivan et al., 2007).
Sr/86Sr ratios.
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The Cryosphere plays an important role in the Earth’s climate system, affecting the
45
hydrological cycle and energy balance (Di Mauro et al., 2015), and is sensitive even
46
to small climate changes. Glaciers, the largest component of the Cryosphere, are
47
currently experiencing dramatic melting attributed to increasing temperatures and
48
decreasing snow accumulation. Mineral dust, one type of Light Absorbing Impurity
49
(LAI) deposited on glaciers, is an important factor influencing glacier melt (Kaspari et
50
al., 2015; Wang et al., 2013). Specifically, iron oxides in dust such as goethite and
51
hematite enhance absorption of shortwave spectra (ultraviolet and visible wavelengths)
52
in snow and thus lead to reduced snow albedo and glacier melt (Zhang et al., 2015;
53
Wu et al., 2015). Previous studies indicated that snow albedo and radiative forcing of
54
dust have a greater influence on climate warming than does black carbon when
55
impurity contents are low (Kaspari et al., 2014; Ramanathan and Carmichael, 2008).
56
Mineral dust in the Solu-Khumbucan decrease snow albedo by 40-42% (Kaspari et al.,
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58
mean annual albedo of snow by less than 0.01 (Gabbi et al., 2015). Iron oxide mineral
59
dust has a significant effect on the melting of snow and glaciers. The mineralogical
60
properties of surface dust on glaciers have a strong effect on glacier ablation.
61
However, many previous studies focused more on biological properties and less on
62
mineralogy, which depends on dust source and transport. Therefore, to constrain the
63
influence of dust on glacier ablation, the provenance of dust deposited onto glaciers
64
must be identified.
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Rare earth elements (REEs) and strontium and neodymium isotopic compositions
66
are powerful tools for determining dust origin (Piper et al., 2013). Like isotopes,
67
REEs possess identical external electronic configurations and therefore display
68
essentially identical chemical properties. Owing to their low solubility, REEs are
69
generally in the particulate phase during transport (Henderson, 1984). These
70
properties impose limitations on REE fractionation during weathering and diagenesis.
71
Similarly,
72
properties and less on alteration due to surficial processes, including weathering,
73
transport and deposition. These isotopes are therefore utilized as fingerprints for dust
74
sources (Grousset and Biscaye, 2005; Nie et al., 2012). Using REE compositions and
75
Sr-Nd isotopes, we can both determine dust composition and trace the source of
76
mineral dust deposited onto glaciers (Dong et al., 2015; Dong et al., 2016; Du et al.,
77
2015; Wu et al., 2015; Tepe et al., 2015). We therefore employ a multi-parameter
78
method to address provenance (Nie et al. 2012).
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Laohugou (LHG) glacier No.12 is a mountain glacier in northern China that has
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gradually retreated over the last 50 years (Du et al., 2008). This dramatic glacier melt
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is attributed to a rise in temperature, a reduction in snow accumulation and the
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deposition of LAIs such as mineral dust (Kaspari et al., 2015). Therefore, tracing the
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provenance of dust on the glacier is important for understanding both glacial change,
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and regional atmospheric circulation. Previous studies have mainly focused on tracing
85
sources and analyzing the physicochemical composition of snow dust, as well as the
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microbial process of cryoconite formation in the upper portion of the glacier (Dong et 3
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of glacier surface dust, including on snow dust and cryoconites from LHG glacier
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No.12. In addition, very limited Sr-Nd isotopic and REE composition data have been
90
reported for dust on local glacier surfaces. Therefore, this study aims for a
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comprehensive understanding of dust geochemical composition over the entire glacier
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and discusses the potential dust transport mechanism in the larger region of western
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and northern China.
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2. Sampling and Methodology
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2.1. Sampling glacier surface dust
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The Qilian Mountains, located in the northwestern Tibetan Plateau (TP), are
97
surrounded by several large sand deserts. To the northwest and west are the
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Gurbantunggut and the Taklimakan Deserts, respectively, the largest two deserts in
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central Asia. The Qaidam Desert is located immediately to the southwest of the Qilian
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Mountains, and to the southeast are the Badain Jaran and Tengger Deserts. The LHG
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Glacier No.12 (39.438N, 96.568E), with a length of 10.1 km and an area of 21.9 km2,
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is the largest glacier in the LHG basin and is separated into two branches at 4560 m
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above sea level (asl). A typical continental climate characterizes the northern Qilian
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Mountains, and the southeasterly winds prevailing in the area are mainly controlled
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by Westerlies.
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In June 2012 and August 2014, we collected snow pit and cryoconite samples at
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LHG glacier No.12 (Figure 1, Table 1). Thirteen samples were acquired, including
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seven cryoconite samples from 5 cm in depth and six snow pit samples from 95 and
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120 cm in depth. Cryoconite samples were collected between 4386 and 4850 m asl
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and snow pit samples at 5040 m asl. Samples were collected using a pre-cleaned
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stainless-steel shovel and polyethylene gloves, then put into pre-cleaned low-density
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polyethylene (LDPE) bottles (Thermo Scientific). All samples were kept frozen until
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they were analyzed at the Analytical Laboratory of Beijing Research Institute of
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Uranium Geology.
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2.2. REE parameters and Sr-Nd isotopes REEs are a group of 14 elements ranging from La to Lu with similar ionic radii and
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valance states. According to their mass number, REEs are divided into light REEs
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(LREE: La, Ce, Pr, Nd, Sm, Eu) and heavy REEs (HREE: Gd, Tb, Dy, Ho, Er, Tm,
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Yb, Lu). Chondrite-normalization of REEs was used to eliminate the odd-even effect.
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In this study, several REE composition parameters were applied to determine the
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potential sources of REEs in the dust of both the study region and the potential source
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areas. Ratios of L/HREE and (La/Yb)N can reflect the differential degree of REEs.
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The (La/Sm)N ratio represents the differential degree of LREEs, with a positive
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correlation between the ratio and the differential degree. Alternatively, (Gd/Yb)N
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indicates the differential degree of HREE and is negatively correlated with differential
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degree. Additionally, Eu/Eu*(=EuN/(SmN ∗ GdN)1/2) values can not only indicate
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sedimentary environment but also parent rock characteristics (Zhang, 1997).
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Radiogenic Sr and radiogenic Nd isotopic compositions are expressed in ratios of
128 87
Sr/86Sr and 143Nd/144Nd. In general, 143Nd/144Nd ratios possess little variation and are
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represented as a ten thousand part deviation from the
131
chondrites
132
143
133
affected by surficial processes, the cycling of crustal material or grain size. In contrast,
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Sr isotopes are easily altered by weathering, wind sorting and grain size effects (Chen
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et al., 2011). Moreover, Sr isotopes are weakly affected by the reaction of dust with
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acid aerosols during atmospheric transport, and acid leaching during pretreatments
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(Meyer et al., 2011; Revel-Rolland et al., 2006). Therefore, our discussion will
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principally focus on the comparison of εNd (0) values of dust samples and the deserts.
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2.3. Geochemical analyses
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(0.512638):
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Nd/144Nd ratio of modern
εNd=((143Nd/144Nd)sample/143Nd/144Nd)CHUR)−1)×104.
The
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Nd/144Nd ratio is mainly controlled by crust-mantle differentiation age and is not
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REEs in dust samples were measured by inductively coupled plasma-mass
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spectrometry (ICP-MS, Thermo Scientific Element/XR). To improve data accuracy
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and ensure the credibility of results, non-powder vinyl clean room gloves and masks
143
were worn during sample analysis to avoid potential contamination. Experimental 5
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blank analyses showed that contamination produced during sampling, transportation
145
and analysis was negligible. This methodology has been described in detail by Dong
146
et al. (2015). Thermal ionization mass spectrometry (TIMS) measurement of Sr-Nd isotopes was
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performed at the same laboratory as REE analysis. Dissolution was carried out in
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PTFE screw-top bombs with a mixture of ultra-pure HF + HClO4 at 120℃for seven
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days on a hot plate. Then, Sr-Nd isotopes were isolated in quartz columns by
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ion-exchange chromatography. Isotopes of Sr were measured using single Re
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filaments and a Ta activator in static mode. The 87Sr/86Sr and
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corrected for internal mass bias to
154
87
155
which has a recommended value of 0.710248. The 143Nd/144Nd ratio for the reference
156
material ShinEtsu was 0.512095±9 (2σ, n=10), which has a recommended value of
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0.512110. For convenience, the
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εNd(0)=[(143Nd/144Nd) /0.512638–1] x104.
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3. Results and Discussion
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3.1 REE composition of dust deposited on the glacier surface
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The LHG glacier No.12 is located near the central Asian dust sources and may be
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significantly affected by dust emissions from these deserts. However, the deserts
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differ in their influence on dust transport to the study site because of geographic
164
location, meteorological conditions and mineralogical properties. The deserts have
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distinct mineral compositions, such as goethite and hematite, which are LAIs on
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glacier and snow surfaces (Maher et al., 2009). The sources of dust transported to
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LHG Glacier No.12 should be identified in order to establish iron oxide indicators and
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to determine the influence of iron oxides on glacier and snow melt.
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Nd/144Nd data were
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Sr/86Sr =0.1194 and
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Nd/144Nd =0.7219. The
Nd/144Nd ratios were normalized and denoted as
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Sr/86Sr ratio for the reference material NBS 987 was 0.710229±13 (2σ, n=10),
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The REE characteristics of glacier surface dust from LHG Glacier No.12 vary
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widely but are different from the Hobq and Mu Us Deserts in their lower Eu/Eu* and
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L/HREE values, and higher (Gd/Yb)N values (Figure 2 and Figure 3). The Taklimakan
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Desert, the main Asian dust source, can be roughly distinguished from samples 6
ACCEPTED MANUSCRIPT collected at the glacier by higher (La/Yb)N, L/HREE and (La/Sm)N ratios (Figure 2
174
and Figure 3). However, several samples yield REE data that are similar to those of
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the northern and southern Taklimakan Desert, indicating a potential contribution of
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Taklimakan Desert dust to the Laohugou basin (Figure 2 and Figure 3). Furthermore,
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the samples have lower (La/Yb)N and L/HREE ratios, but higher (Gd/Yb)N ratios,
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than the western Tibetan Plateau (TP), suggesting that it contributes little to the
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glacier surface dust loading at the study site. However, REE composition of glacier
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surface dust is similar to the Qaidam Basin, Tengger and Badain Jaran Deserts and the
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eastern TP with respect to L/HREE, (Gd/Yb)N, (La/Sm)N and (La/Yb)N. Additionally,
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one of the glacial dust samples possesses a distinct REE signature compared to the
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above deserts and the TP, which potentially implies other unknown dust sources.
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3.2 Sr-Nd compositions
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Isotopes of Sr-Nd are considered to be the most effective and reliable indicators for
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tracing dust source. Generally, Sr isotopes are easily altered by weathering and wind
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sorting during transportation and deposition (Chen et al., 2011). The Sr isotopes are
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also weakly affected by the reaction of dust with acid aerosols during atmospheric
189
transportation and by acid leaching during pretreatments (Meyer et al., 2011;
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Revel-Rolland et al., 2006). In this work, an acid leaching pretreatment was used for
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mineral cryoconites formed on the glaciers of the TP in order to remove carbonates
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(Wu et al., 2010; Schettler et al., 2009). According to a previous study on the TP (Wu
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et al., 2010), the carbonate content of dust deposited onto glaciers is very low, which
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suggests that the effect of acid leaching on Sr isotopic composition is negligible. It is
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noteworthy that Sr isotopic compositions are heavily dependent on grain size (Dasch,
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1969; Chen et al., 2007; Feng et al., 2009; Újvári et al., 2012). The grain size effect
197
has a significant influence on the
198
size fractions have similar
199
2007). Chen et al. (2007) divided dust from potential source areas into <75µm and
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<5µm fractions, and Dong et al. (2016) found that most LHG glacier surface dust is
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<75µm in diameter. Therefore, the studied grain sizes of the potential dust sources and
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Sr/86Sr ratio of particles <2µm, while other grain
Sr/86Sr compositions to the bulk size range (Chen et al.,
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Nd/144Nd isotopes
glacier surface dust are similar. In a contrast with Sr isotopes,
203
experience limited alteration during the processes of transportation, deposition and
204
eluviation (Chen et al., 2011). Therefore, our discussion will principally focus on the
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comparison of εNd(0) values of dust samples and the deserts. The Sr-Nd isotopic
206
signatures vary widely, with 87Sr/86Sr ratios of 0.715324 to 0.723112 and εNd(0) values
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of -14.96 to -9.09 (Figure 4). The isotopic composition of glacier surface dust is more
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radiogenic than that of the Ordos Plateau (the Hobq and Mu Us Deserts) with εNd(0)
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values from -16.3 to -10.5, but is lower than the Gurbantunggut Desert with εNd(0)
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values from -6.3 to -1.2. Some samples from the LHG Glacier No.12 are
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geochemically similar to the deserts on the northern margin of the TP (the Qaidam,
212
Badain Jaran, and Tengger Deserts), which have εNd(0) values ranging from -12.8 to
213
-7.4 and
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similar to the local soil, indicating that this could be the primary source of glacier dust.
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It is noteworthy that different regions within a desert have dramatically different
216
provenance and geochemical characteristics. Nie et al. (2015) and Stevens et al. (2013)
217
revealed different provenances for the western and eastern Mu Us Desert. Therefore, a
218
list of sampling sites of the potential source areas is presented in Table 2. The upper
219
reach of the Yellow River shares similar geochemical properties to the western Mu Us
220
Desert. Thus, we infer that upper reach Yellow River sediments are a potential dust
221
source for the glacier (Nie et al., 2015).
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3.3 Principal component analysis (PCA) for REE composition in surface dust
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Principal component analysis (PCA) is a well-established multivariate statistical
224
method that reduces dimensionality of the data to produce a new set of variables while
225
retaining the features of the dataset (Jolliffe, 2002). This is useful for further analysis
226
of the geochemical composition of dust with a relatively complex source. A
227
mathematical algorithm in SPSS 22.0 was used to describe the similarities between
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dust sink and potential source areas (Du et al., 2015). In this study, three principle
229
components (PCs) were extracted using SPSS 22.0, yielding a 56.0%, 21.2% and 17.4%
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variance of PC1, PC2 and PC3, respectively, and an eigenvalue higher than 1 (Figure
Sr/86Sr ratios from 0.713 to 0.722. However, the remaining samples are
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loadings of (La/Sm)N, (La/Yb)N, L/HREE and Ce/Yb are higher than 0.839, except for
233
(Gd/Yb)N and Eu/Yb in the PC1 column. The (La/Sm)N, L/HREE and Ce/Yb loadings
234
for PC2 are negative, and the absolute values of (Gd/Yb)N and Eu/Yb are significantly
235
greater than 0.653 (Table 4). Using these results, the first two PCs are identified as
236
possessing the original characteristics of both the studied samples and potential source
237
regions. The PCA1 scores are negative for the Qaidam, Tengger, Badain Jaran Deserts
238
and eastern TP, indicating that these deserts and the eastern TP are the major sources
239
of surface dust deposited onto LHG glacier No.12. In contrast, the PCA1 scores are
240
positive for the Taklimakan Desert, suggesting that it is a minor contributor to the
241
glacier surface dust (Figure 5).
242
3.4 Discussion of the possible sources of glacier surface dust
243
Geochemical composition (Sr-Nd isotope and REEs) is generally a highly effective
244
and reliable dust source tracer. Samples from LHG glacier No.12 are distinct from the
245
Mu Us and Hobq Deserts, with very different REE parameters (higher (Gd/Yb)N and
246
lower Eu/Eu* and L/HREE) (Figure 2 and Figure 3). The Sr-Nd isotopic compositions
247
of the samples show variable εNd(0) values that are higher than the εNd(0) values
248
(-16.3 to -10.9) of the Mu Us and Hobq Deserts (Honda et al., 2004; Rao et al., 2008;
249
Yang et al., 2007) (Figure 4). Both deserts are located to the northeast of LHG glacier
250
No.12 and are perennially dominated by northwesterly and westerly winds. It is
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therefore inferred that the Mu Us and Hobq Deserts could not be the source of surface
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dust in the study area.
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Deserts on the northern margin of the TP include the Qaidam, Badain Jaran and
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Tengger Deserts. Figure 3 shows that some or all samples have very similar REE
255
signatures to the Qaidam, Badain Jaran and Tengger Deserts, but only near or just
256
within the Taklimakan Desert field. Furthermore, Sr-Nd isotopic compositions show
257
that the samples collected at LHG glacier No.12 resemble the Qaidam (-10.2 on
258
average εNd(0) values), Badain Jaran (-9.31 on average εNd(0) values) and Tengger
259
(-8.56 in average εNd(0) values) Deserts in εNd(0) values but can be distinguished from 9
ACCEPTED MANUSCRIPT 87
Sr/86Sr ratios (Figure 4). The study region is
the Taklimakan Desert by its higher
261
close to the Qaidam, Badain Jaran and Tengger Deserts (Figure 1). Additionally, the
262
aeolian deposits in the Badain Jaran Desert are predominantly derived from the Qilian
263
Mountain via fluvial processes, which further supports the Badain Jaran Desert as a
264
surface dust source for the study site (Hu and Yang, 2016). Although the Tengger and
265
Badain Jaran Deserts are generally located downwind of LHG glacier No.12, local
266
circulation in both deserts may transport dust to the glacier. Back trajectory analyses
267
of air masses support this interpretation. These results confirm that the Qaidam,
268
Badain Jaran and Tengger Deserts are the major dust sources for LHG glacier No.12.
269
However, Figure 4 and Figure 5 show that the Taklimakan Desert is a minor
270
contributor to dust composition. The LHG glacier No.12 is located at a high altitude
271
(>4200 m asl) in the northeastern TP, whereas the Taklimakan desert is adjacent to the
272
northwestern TP. Dust storm events in the Tarim Basin (Taklimakan Desert) occur
273
frequently and with a longer duration than in other areas, with dust emissions from the
274
desert accounting for 21% of all Asian dust emissions (Zhang et al., 2003; Wang et al.,
275
2005). Satellite observations indicate that dust layers emerge most frequently at
276
approximately 4 to 7 km a.s.l. Portions of dust storms occurring in the Taklimakan
277
Desert can be lifted to the level of the TP by westerly winds, which are transported to
278
LHG glacier No.12, while the remaining storm components are blocked by the TP
279
(approximately 3 to 6 km a.s.l.). However, dust emissions from the Qaidam, Badain
280
Jaran and Tengger Deserts amount to 31% of total Asian dust emissions, which is
281
greater than that of the Taklimakan Desert (Zhang et al., 2003). Therefore, the Qaidam,
282
Badain Jaran and Tengger Deserts are major sources for dust deposited on the study
283
glacier, and the Taklimakan Desert is a minor source. The eastern TP and local soil are
284
also major dust sources, judging from their similarity to REE parameters (Eu/Eu*,
285
L/HREE, (Gd/Yb)N, (La/Sm)N) and to Sr-Nd isotopic compositions of samples
286
collected from LHG glacier No.12 (Figure 4 and Figure 5). Although the Tengger and
287
the Badain Jaran Deserts are located downwind of LHG glacier No.12, the
288
near-surface atmospheric circulation in both deserts may transport dust to the alpine
289
glacier area. Air mass back trajectory confirms this result. The Hysplit4 back
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291
is mainly from the Qaidam, Badain Jaran, and Tengger Deserts as well as the
292
Taklimakan Desert. This is confirmed by air mass back trajectory from the north and
293
from local wind circulation (Figure 6).
294
4. Conclusions
295
Using REEs and Sr-Nd isotope geochemical tracers, this study investigates the
296
provenance of surface dust on LHG glacier No.12, which is located in a dust source
297
area of east-central Asia. The REE characteristics of the surface dust vary widely but
298
are distinguished from the Mu Us and Hobq Deserts by the lower Eu/Eu* and
299
L/HREE and higher (Gd/Yb)N of those deserts. One sample from the study site is
300
similar to the Taklimakan Desert, while the remainder differ in that they possess lower
301
(La/Yb)N, L/HREE and (La/Sm)N ratios. However, the REE characteristics of glacier
302
surface dust are similar to that of the Qaidam Basin, Tengger, and Badain Jaran
303
Deserts and the eastern TP. The Sr-Nd isotopic signatures of the samples possess great
304
variability, ranging from 0.715324 to 0.723112 in 87Sr/86Sr ratios and -14.96 to -9.09
305
in εNd(0) values. These values are more radiogenic than the Ordos Plateau, which has
306
εNd(0) values from -16.3 to -10.5 but are lower than the Gurbantunggut Desert, with
307
εNd(0) values from -6.3 to -1.2.
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Our data clearly show that dust sample characteristics from LHG Glacier No.12 are
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somewhat close to those of deserts on the northern margin of the Tibetan Plateau
310
(including the Qaidam, Badain Jaran, and Tengger Deserts), ranging from -12.8 to
311
-7.4 in εNd(0) values and 0.713 to 0.722 in
312
geochemistry can be attributed to the local soil surrounding LHG glacier No.12. It is
313
therefore inferred that the glacier surface dust may be derived from the Qaidam,
314
Badain Jaran, Tengger Deserts and eastern TP crust, as well as local soil, while the
315
Taklimakan Desert may have contributed little dust.
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Acknowledgments
317
This work was funded by the National Natural Science Foundation of China
318
(41421061, 41671062), the Chinese Academy of Sciences (KJZD-EW-G03-04), and
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Sr/86Sr. Much of the remaining dust
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also would like to thank the summer 2014 field work team on the Tibetan Plateau for
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their logistical field work. We also thank anonymous reviewers and the Editor, Dr.
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Michael Kersten, for their helpful comments and suggestions.
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Tables Table 1 Sampling of snow and cryoconite dust from Laohugou Glacier No.12. Altitude Sample no.
Sample data
Depth (cm)
Sample type
(m a.s.l.) 2014/7/29
5
cryoconite
LHG30-6
4592
2014/7/30
5
cryoconite
LHG29-4
4450
2014/7/29
5
cryoconite
LHG29-3
4386
2014/7/29
5
cryoconite
LHG30-4
4698
2014/7/30
5
LHG30-1
4849
2014/7/30
5
LHG8-4
4850
2014/8/8
5
LHG-01
5040
2012/6/21
120
LHG-02
5040
2012/6/21
120
LHG-03
5040
2012/6/21
LHG-04
5040
2012/6/21
LHG-05
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2012/6/21
LHG-06
5040
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cryoconite cryoconite cryoconite snow pit
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Table 2 Sampling sites for dust Sr-Nd isotopic and REE compositions in the arid dust source regions of China. Location
Parameters
References
Gurbantunggut Desert
west of the desert
Sr-Nd isotope/REE
Honda et al., 2004
45.06-45.6N, 87.46-89.11E
Sr-Nd isotope
Chen et al., 2007
37.45-37.68N,104.97-105.5E
Sr-Nd isotope
Chen et al., 2007
38.35-38.64N, 102.33-105E
Sr-Nd isotope
Honda et al., 2004
39N,103.5E
REE
Ferrat et al., 2011
38.34-39N, 102.5-102.54E
Sr-Nd isotope
Nakano et al., 2004
39.4-42.02N, 100.63-103.23E
Sr-Nd isotope
Chen et al., 2007
39.2N,103E
REE
Ferrat et al., 2011
36.8-37.99N, 93.75-97.69E
Sr-Nd isotope
Chen et al., 2007
36N, 95E
REE
Ferrat et al., 2011
whole range
Sr-Nd isotope/REE
Honda et al., 2004
38.5-39.15N, 107.56-108.5E
Sr-Nd isotope
Nakano et al., 2004
38.22-40.33N,108.08-111.35E
Sr-Nd isotope
Chen et al., 2007
40.05-41.45N, 106.77-109.7E
Sr-Nd isotope
Chen et al., 2007
39.65-41.45N, 107.02-111.35E
Sr-Nd isotope
Rao et al., 2008
whole range
REE
Kwon et al., 2004
36.41-39.69N, 81.84-94.35E
Sr-Nd isotope
Chen et al., 2007
37.5-40N, 77.5-94.8E
Sr-Nd isotope
Honda et al., 2004
Gobi
40.43-43.7N, 105-114.5E
Sr-Nd isotope
Nakano et al., 2004
western Tibetan Plateau
29.5-37.5N, 87.5-79.5E
REE
Li et al., 2009
Tengger Desert
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Qaidam Basin
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LHG29-1
Mu Us Desert
Hobq Desert
Taklimakan Desert
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Northern Taklimakan Desert
42N,83E
REE
Yang et al., 2007b
Southern Taklimakan Desert
38N,81.5E
REE
Ferrat et al., 2011
Table 3 Total variance explained in principal component analysis. Eigenvalue
Variance%
Cumulative%
1.000
3.362
56.033
56.033
2.000
1.270
21.167
3.000
1.043
17.390
4.000
0.228
3.805
5.000
0.079
1.317
6.000
0.017
0.286
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77.200 94.591
98.396 99.713
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32N,101E
100.000
Table 4 Loadings of each element upon each of three significant principal components.
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East of the Tibetan Plateau
Component
Index 1
3
-0.284
0.192
0.842
-0.492
0.653
0.739
0.206
0.019
0.884
(Gd/Yb)N
0.130
Eu/Yb
0.077
(La/Yb)N
0.970
L/HREE
0.956
-0.108
0.186
Ce/Yb
0.839
-0.006
-0.428
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(La/Sm)N
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2
Figure Captions
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Figure 1 Location of Laohugou Glacier No.12, sampled for surface dust deposition.
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Figure 2 The REE composition (correlation between Gd/Yb and Eu/Eu*, and La/Yb)
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of surface dust from LHG glacier No.12 and from potential source regions, including
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the northern and southern Taklimakan Desert, Junggar Basin, eastern and western TP,
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Qaidam Basin, Tengger Desert, Badain Jaran Desert, Mu Us Desert and Hobq Desert.
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Figure 3 The REE signatures (correlations between La/Sm and L/HREE, La/Yb and
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L/HREE) of surface dust from LHG glacier No.12.
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Glacier No.12 with the Gobi Desert and deserts from northern and northwestern
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China. Dotted areas represent d<5 µm fractions of the Chinese deserts, and the
503
remaining data are the d<75 µm fractions of dust particles in arid regions. Some data
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are from Xu et al., 2012.
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Figure 5 The PCA scores of each site in three principal components, indicating
506
potential dust sources.
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Figure 6 Three-day backward trajectories of LHG Glacier No.12 during the
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high-frequency dust storm season (March to July) in 2012 and 2014, showing the
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potential dust transport to the Qilian Mountains glacier area from surrounding regions.
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4. Near surface circulation may transport arid dust from downwind areas