Distribution and sources of rare earth elements in sediments of the Chukchi and East Siberian Seas

Distribution and sources of rare earth elements in sediments of the Chukchi and East Siberian Seas

Polar Science 20 (2019) 148–159 Contents lists available at ScienceDirect Polar Science journal homepage: www.elsevier.com/locate/polar Distributio...

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Polar Science 20 (2019) 148–159

Contents lists available at ScienceDirect

Polar Science journal homepage: www.elsevier.com/locate/polar

Distribution and sources of rare earth elements in sediments of the Chukchi and East Siberian Seas

T

A.S. Astakhova, V.V. Sattarovaa,∗, Shi Xuefab, Hu Liminb, K.I. Aksentova, A.V. Alatortseva, O.N. Kolesnika, A.A. Mariasha a b

V.I.Il'ichev Pacific Oceanological Institute (POI), Far Eastern Branch of Russian Academy of Sciences (FEB RAS), Vladivostok, 690041, Russia First Institute of Oceanography, State Oceanic Administration, Qingdao, 266071, China

ARTICLE INFO

ABSTRACT

Keywords: Rare earth elements Marine sediment Grain size Chukchi sea East Siberian Sea

Using inductively coupled plasma mass spectrometry (ICP-MS), the distribution of rare-earth elements (REE) at the surface and Holocene sediments of the Chukchi and East Siberian Seas was studied in two cores of up to 8.5 ka in age. The total REE concentration of the surface sediments of the Chukchi Sea varied from 62 mg kg−1 to 169 mg kg−1. The NASC-normalized REE patterns were relatively similar to each other and are characterized by a slight enrichment in the middle lanthanides. The total REE concentrations in surface sediments from the East Siberian Sea ranged from 123 mg kg−1 to 200 mg kg−1. The normalized patterns showed a strong predominance of light REE, in particular, La and Ce. The main concentrators of REE are the sand and silt fractions of the sediment. The East Siberian Sea is characterized by REE association with elements contained in heavy stable clastic minerals (Zr, Nb, Hf, Th, and Ti). REE in this region are derived from the erosion of the mainland coast and the New Siberian Islands ice complex, as well as from river discharge, primarily from the Lena River, the basin of which comprises ancient crystalline shield and magmatic rocks enriched in light REE. The sediments in the eastern and southern Chukchi Sea have low REE contents, indicating that the terrigenous flux supplying the Chukchi Sea is through the Bering Strait.

1. Introduction The study of the Arctic Basin has emerged as a recent priority for the international scientific community. In particular, the study of the East Siberian Shelf, the least studied area in the Arctic, is essential due to its significance in understanding several processes that determine global climate change. Several previous studies have indicated that large amounts of methane and carbon dioxide have been released due to degradation (melting) of the permafrost on the shelf which has subsequently affected the global balance of greenhouse gases in the atmosphere (Vonk et al., 2012; Shakhova et al., 2015; Tesi et al., 2016). This area is also the site of formation of ice involved in the “transarctic drift” that is transported through the central part of the Arctic to Greenland (Spielhagen et al., 1997; Parkinson and Cavalieri, 2008). Melting of this ice takes place along the entire route. Several previous studies have reconstructed the intensity and direction of this ice transport by studying the number and composition of sediments deposited on the seafloor (Polyak et al., 2016), which is necessary for identifying rearrangements in the global ocean-atmosphere system over time (Wang et al., 2013; Polyak et al., 2016; Cronin et al., 2017). ∗

Data regarding the composition of sedimentary material transported by ice and currents from the coast and shelf of Eastern Siberia to the central Arctic are limited. However, a few studies on the general composition of surface sediments and local processes of modern sedimentation have been conducted (Kosheleva and Yashin, 1999; Astakhov et al., 2013, 2015; Dudarev et al., 2016). In recent years, a series of studies on seafloor sediment cores and sediment sources have emerged from studies focusing the paleoclimatic reconstructions. Additionally, these studies have compared the compositions of clay minerals, rock fragments, and specific minerals with source data collected from the coast (Spielhagen et al., 1997; Wang et al., 2013; Stein et al., 2017; Kobayashi et al., 2016; Yamamoto et al., 2017). Nevertheless, the available information regarding sedimentation processes and sources of sediment input to the East Siberian Shelf as well as its specific sediment composition is too limited to comprehensively understand the role of this region in Arctic sedimentation processes. The study of rare earth elements (REE) in sediments can be used as a tool for elucidating substance sources as well as the physicochemical characteristics of sedimentation processes. The lanthanide group is unique since two lanthanide series elements (Ce and Eu) change their

Corresponding author. E-mail address: [email protected] (V.V. Sattarova).

https://doi.org/10.1016/j.polar.2019.05.005 Received 21 September 2018; Received in revised form 13 May 2019; Accepted 17 May 2019 Available online 29 May 2019 1873-9652/ © 2019 Elsevier B.V. and NIPR. All rights reserved.

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degree of oxidation and differ from other REE in oxidizing and reducing conditions. The concentrations of lanthanides in the seafloor sediments of the Arctic Sea have been studied locally, usually with an incomplete REE pattern (Yang et al., 2001; Chen et al., 2003; Viscosi-Shirley et al., 2003; Shakirov et al., 2010). Data regarding the REE composition of rivers, especially during abrasion flow, are limited to a single study which demonstrated the unique role of the Lena River in the supply of lanthanides to the sediments of the Arctic Shelf (Rachold, 1999). The REE concentration of suspended particulate material (SPM) in the Lena River was 211 mg kg−1, while REE values in the SPM of rivers flowing westward (Khatanga) and eastward (Yana) were 119 and 161 mg kg−1, respectively (Rachold, 1999). That particular study also concluded that relatively high REE concentrations in the SPM of the Lena River could be attributed to the presence of sections of the East Siberian platform in the drainage basin. The East Siberian platform contains the oldest crust on Earth (Aldan and Anabar cratons) as well as alkaline rocks and their derivatives such as lamproites, kimberlites, carbonatites, and other light REE-enriched rocks. For example, a unique Tomtor deposit of niobium-rare earth ores, located 250 km from the coast of the Laptev Sea, was formed by the weathering of rocks from the largest global carbonatite massif associated with the Anabar craton (Vladykin and Torbeeva, 2005) (Fig. 1). Monazite was the main ore mineral in this deposit, with a predominant size of ∼50 μm (Lazareva et al., 2017). Erosion of rare-earth metal-containing shield minerals over geological timescales has resulted in the accumulation of REE in Phanerozoic sedimentary deposits, including Quaternary sediments, which are presently the main source of sedimentary material to the continental shelf (Astakhov et al., 2018). This study presents a new high-precision REE data-set to elucidate their distribution in the sediments of the Chukchi and East Siberian Seas. Additionally, REE distribution patterns in the surface sediments in relation to water mass dynamics and localization of supply sources, the effects of dynamic sediment sorting, and variations in REE concentrations of Holocene sediments were also evaluated.

(Oxford INCA Energy, Great Britain). Mineral fractions of 10–160 μm were wet-sieved from the surface sediments. Mineral grains were pasted on strips of electroconductive tape. A silicon-lithium detector (INCA Xsight) was used to determine the concentrations of elements from boron to uranium in the range from 0.01 to 100 wt %. The energy resolution of the detector for Mn (Kα) was 137 eV, the accelerating voltage was 20 kV, the current was 10−8 Å, and the X-ray radiation takeoff angle was 45°. The scanning zone diameter was no more than 5 μm. The distribution of individual elements over the area was established by recording the intensity of the characteristic X-ray lines of these elements during the scanning of the electron beam. The concentrations of elements were calculated from the ratios of the intensities of lines emitted by the sample to the intensities of the same lines obtained for standards (Oxford Instruments Analytical, United Kingdom). The correction of matrix effects was performed based on the Phi-Rho-Z method. The measurement error did not exceed 2 rel. %. Chemical analysis was performed at the Analytical Center of the Far East Geological Institute, FEB RAS. Major elements were analyzed by inductively coupled plasma atomic-emission spectrometry (ICP-AES) using an ICAP6500 Duo spectrometer (Thermo Electron Corporation, USA). To correct for instrumental drift and matrix effects, 10 mg kg−1 of added cadmium solution was used as an internal standard. Other trace and the rare earth elements were determined by inductively coupled plasma mass-spectrometry (ICP-MS), using an Agilent 7500с quadrupole mass-spectrometer (Agilent Technologies, USA). To correct for instrumental drift and matrix effects, 10 mg kg−1 of added 115In was used as an internal standard. For a detailed description of the methodology and accuracy determination, see Strekopytov and Dubinin (1997). Geological Samples of sediment standards MAG-1 (marine sediment) and ООPЕ 201 (volcano-terrigenous clay) were used to check the data quality. The results of standard sample analyses are presented in Table 1. The total silica (SiO2) was determined using the gravimetric (weight) method. For a detailed description of methodology, see Popova and Stolyarova (1974). The organic carbon (Corg) in bottom sediments was measured by a TOC-VCPN analyzer (Shimadzu, Japan) with an SSM-5000A attachment for the incineration of solid samples. The relative standard deviations for determination of carbon content were 1.5% and 2.0% for total carbon and inorganic carbon, respectively. REE results were normalized on the North American Shale Composite (NASC) (Gromet et al., 1984). Extents of europium and cerium anomalies were calculated by the formulas Euan = 2 × Eu/EuN/ (Sm/SmN + Gd/GdN); Cean = 2 × Ce/CeN/(La/LaN + Nd/NdN), respectively (Dubinin, 2006). The ratio of light REE to heavy REE was considered as LREE/HREE = (La/LaN + 2 × Pr/PrN + Nd/NdN)/(Er/ ErN + Tm/TmN + Yb/YbN + Lu/LuN). This version of the LREE/HREE is based on a large number of elements and less to the influence of analysis errors in the determination of individual lanthanides. To identify the forms of occurrence and the sources of REE from different regions, correlation and factor analyses were performed using StatSoft STATISTICA software package (version 10), including analyses for data regarding concentrations of chemical elements normalized for aluminum and grain size fractions (see Tables A.3, A.4). The analyses were carried out separately for areas with sharply contrasted REE distributions, i.e., the western East Siberian Sea and the Chukchi Sea. Aluminum normalization is usually performed to remove the influence of grain size distribution of sediments and to address issues associated with the identification of sedimentary matter sources (McKay and Pedersen, 2008). This method has been successfully implemented previously for the analysis of sedimentation conditions in the Chukchi Sea (Astakhov et al., 2015).

2. Materials and methods Surface sediment sampling was performed using a box-corer and multicorer during cruises 46 and 52 of the R/V “Professor Khromov” in 2002 and 2004, a cruise of the R/V “Sever” in 2006, and cruise 77 of the R/V “Akademik Lavrentiev” in 2016 (Figs. 1 and 2). The published data (Shakirov et al., 2010) were added to detail the REE distribution maps. In addition to surface sediments, two sediment gravity cores were used in this study (Fig. 2). Core HC-11 (111 cm in length) was collected from the southwestern part of the Chukchi Sea on the traverse of Kolyuchinskaya Bay, approximately 100 km from the coast (67°52.00′ N, 172°34.90′ W) at a water depth of 49 m. The age model for bottom sediments was constructed based on radioisotopic dating (210Pb, accelerated mass spectrometry (AMS) 14С) (Tsoy et al., 2017). Core LV7736 (376 cm in length) was collected from the southwestern part of the East Siberian Sea in the flooded paleovalley of the Indigirka River (74°05.28′ N, 155°38.19′ W) at a water depth of 36 m. Radiocarbon dating was conducted at the AMS radiocarbon laboratory of BETA Analytic Inc. Calibration of 14C dates to calendar ages was carried out using the CALIB 3.0 software (Stuiver and Reimer, 1993; Bronk Ramsey, 2009) with the Marine13 calibration curve (Reimer et al., 2013). The reservoir effect with ΔR = 42 ± 60 years was applied according to Keskitalo et al. (2017). A laser particle size analyzer “Analysette 22 NanoTec” (Fritsch, Germany) was used for grain size distribution. A classification of the fraction size < 4 μm (clay), 4–63 μm (silt) and > 63 μm (sand) was used. Shepard (1954) scale was adopted for grain size classification. The mineral composition of sediments was determined using a procedure outlined in Kolesnik and Kolesnik (2015) using a JXA-8100 (JEOL Ltd., Japan) microprobe with an energy-dispersive system 149

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Fig. 1. Schematic structural map showing the distribution and the lithologies of the different terraines adjacent to the Arctic Ocean (Fagel et al., 2014). Structural units are identified by color: shield in blue, orogenic foldbelt in green, sedimentary platform in yellow and volcanic province in pink. Blue arrows indicate surface current distribution (Fagel et al., 2014). BG: Beaufort Gyre, TPD: Transpolar Drift. Black star: Tomtor massif. For the East Siberian Sea we divided the region into two parts (eastern and western) conditionally along 170°E longitude. . (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3. Results

area. Areas in the East Siberian Sea with depths greater than 50 m are characterized by silts (20–50%), whereas the inner shelf features mixed fine-grained sediments with different ratios of clay and silt. The chemical composition of sediments in the Chukchi Sea is characterized by high Corg values with the maximum (up to 2.7%) at a station located in the southern part of the sea where a strong influence of the Pacific waters penetrating through the Bering Strait was noted (Table A.4). The Corg concentrations in the East Siberian Sea vary from 0.34 to 1.89% and correspond to sediment grain size (Table A.3). Sediments in the coastal zone are represented by silty clays with Corg

3.1. Surface sediments The Chukchi Sea sediments are classified from coarse-grained sands to silts based on their grain-size composition. In general, the distribution of sediments in the study area according to grain size is consistent with previous studies (Kosheleva and Yashin, 1999). Sands are common in the littoral shelf and on the Herald and Hanna shoals. Silts cover the outer shelf, the continental slope, and the deep-water part of the study 150

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Fig. 2. REE concentrations in bottom sediments of the Chukchi and East Siberian Seas. 1–3: locations of sediment samples; 1, 3: surface sediments (1: present study, 3: from Shakirov et al. (2010)); 2: sediment cores; 4: actively eroded areas of the coast (> 0.5 m/year) (from Lantuit, 2012); 5: direction of surface currents (Arctic Atlas, 1985); 6: coastal areas containing ice complex deposits (yedoma) (Ershov, 1989).

the total lanthanide concentrations is observed in sediments from the Chukchi Sea to the western part of the East Siberian Sea (Fig. 1). In general, the total REE concentrations in sediments from the East Siberian Sea ranges from 123 mg kg−1 to 200 mg kg−1, which is considerably higher than REE concentrations estimated for modern marine sediments in the continental regions (Anikiev et al., 1997) (Table 2). The LREE/HREE ratio ranges from 1.31 to 1.80 (Table 2). The Cean values range from 0.91 to 1.01 and the Euan values range from 0.88 to 1.07. The normalized patterns show a strong predominance of light REE, in particular, La and Ce (Fig. 2а).

Table 1 Results of standard sample analyses, mg kg−1. Elements

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y

MAG-1

OOPE 401

Measured

Certified

Measured

Certified

42 85 10.0 36 7.0 1.40 6.09 0.80 4.22 0.80 2.25 0.30 2.11 0.30 23

43 88 9.3 38 7.5 1.55 5.80 0.96 5.20 1.02 3.00 0.43 2.60 0.40 28

26 53 n.d. 31 7.3 2.14 7.37 1.10 6.27 1.20 3.29 0.44 2.83 0.41 33

25 50 n.d. 40 9.0 3.00 8.00 n.d. n.d. n.d. n.d. n.d. 3.00 0.80 33

3.2. Holocene sediments 3.2.1. Core HC-11 The grain size composition, age, and diatom complexes of sediments in Core HC-11 have been studied previously (Tsoy et al., 2017). The sediments are characterized by olive-green clayey mud with an admixture of diatomaceous residues with concentrations up to 5–8%. The 0–7 cm interval is characterized by liquefied clay with distinct color and consistency at the lower boundary. Below this depth, the sediment is characterized by dense clayey mud. The bottom of the core is characterized by fragments of shells which were used for radiocarbon dating. The dating results suggest that the sediments accumulated over the last 2300 years (Tsoy et al., 2017). Three zones were identified based on changes in the diatomaceous content (I: 0–41 cm, II: 41–73 cm, and III: 73–111 cm). The diatoms (Th. nordenskioeldii) inhabiting the Pacific water masses entered through the Bering Strait and prevailed in zones II and III. The diatoms (Th. antarctica) inhabiting the cold waters of the Siberian coastal current prevailed in zone I and correspond to the Little Ice Age (Fig. 4). The two groups of chemical elements were distinguished by changes in their concentrations. The concentrations of biogenic elements (Corg, P), carbonate elements (Mg and partially Ca, without accounting for abnormal emissions), and elements accumulating in anoxic conditions (V, Sc) decrease toward the core bottom. Additionally, some association with the diatom zones was also observed (Fig. 4). Moreover, the concentrations of elements accumulated in the terrigenous clastic components of the sediments (REE, Zr, Hf, Ti, and Rb) (Astakhov et al., 2015) increase toward the bottom of the core. An abnormal decrease in the

n.d. – no data.

contents between 1.05% and 1.10%. Silty sand is distributed in the eastern part of Kolyma Bay at depths ranging from 15 to 20 m (Dudarev et al., 2016) with a Corg concentration of 0.34%. Relatively high Corg levels are observed in areas adjacent to the deltas of the Kolyma River (1.06%) and Indigirka River (1.0–1.16%). The Corg concentration in samples from the submerged shelf of the East Siberian Sea at depths ranging from 300 to 800 m is relatively high (up to 1.6%), and the clay fraction ranges from 50 to 70% (Table A.3). The total REE concentration of the surface sediments of the Chukchi Sea varies from 62 mg kg−1 to 169 mg kg−1 (Table A.1), which is lower than in the North American Shale composite (NASC) (172 mg kg−1: Gromet et al., 1984). The NASC-normalized REE patterns are relatively similar to each other and are characterized by a slight enrichment in the middle lanthanides (Fig. 3). The LREE/HREE varies from 1.27 to 1.76 (Table A.1). The Cean values vary from 0.89 to 1.08 and the Euan values range from 0.96 to 1.18. Sediments from the eastern and southern regions of the Chukchi Sea show low REE concentrations. An increase in 151

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Fig. 3. NASC-normalized REE patterns of riverine SPM, seafloor sediments, and sediment cores HC-11 and LV77-36.

Table 2 Average REE contents in the suspended particulate material and sediments, mg kg−1. Element

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ∑REE Cean Euan LREE/HREE a

Yana River suspension (Rachold, 1999)

Lena River suspension (Rachold, 1999)

Laptev Sea (Astakhov et al., 2018)

East Siberian Sea Western part

Eastern parta

31 68 7.8 29 5.8 – 5.9 0.80 4.4 0.87 2.5 0.36 2.4 0.42 161 0.95 1.57 1.23

43.2 94.8 10.08 36.9 6.58 1.49 6.21 0.81 4.50 0.85 2.54 0.32 2.47 0.32 211 0.99 1.03 1.77

45.5 90.1 10.0 36.6 6.3 1.4 5.6 0.8 4.4 0.8 2.5 0.3 2.4 0.344 207 0.92 1.05 1.75

36.4 75.0 8.0 29.7 5.9 1.18 4.89 0.75 4.02 0.78 2.27 0.32 2.19 0.32 172 0.96 0.97 1.52

22.8 48.2 5.7 22.4 4.7 0.96 4.16 0.59 3.28 0.74 1.82 0.26 1.71 0.24 120 0.93 0.96 1.37

The published data (Shakirov et al., 2010) were added for the calculation of average contents.

152

Chukchi Sea

Gulf of Anadyr, Bering Sea (Anikiev et al., 1997)

21.9 50.5 5.4 20.3 4.1 0.89 3.55 0.55 2.94 0.56 1.63 0.23 1.51 0.21 113 1.01 1.02 1.44

16.7 19.2 3.3 12.3 3.4 0.9 3.5 0.6 3.3 0.74 1.98 0.3 1.6 0.28 68 0.6 1.14 0.76

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Fig. 4. Variations in chemical composition of sediment core HC-11.

REE concentrations appeared at the 40 cm horizon, likely corresponding to a sharp change in the mineral composition of the sediment. Additionally, the concentrations of Al, Y, and Zr decrease while Ca and Corg concentrations increase at this boundary. A distinct increase in the REE concentrations at the bottom of the core is also demonstrated by the distribution of the NASC-normalized REE (Fig. 3b). The results also indicate that changes in the concentrations occurred simultaneously for all REE. Additionally, relatively high Euan values are noted for the lower part of the core which is also characterized by sharp variability in the LREE/HREE ratio (Table A.2).

while silt is present in slightly smaller amounts (44–52.5%), and a small admixture of sand is observed for some horizons in the core top (0.1–0.6%). The upper 100 cm and lower 80 cm of the core were dated (Fig. 5). These intervals are characterized by different sedimentation rates. In the top of the core the sedimentation rate is approximately 0.2 mm yr−1, but in the bottom section of the core is more than 2 mm yr−1. The dating results of shells (Table 3) indicate that nearly all sediments in core LV77-36 accumulated during the Holocene (the last 8 thousand years). The homogeneous composition of the sediments is confirmed by the distributions of Al, Fe, Ti, Corg, and S, which did not vary significantly along the length of the core. Additionally, a stable insignificant increase in the concentrations of these elements is characteristic toward the bottom of the core (Fig. 5). In general, this trend coincides with an increase in the clay fraction and a reduction in the amount of sand present in the sediments. Sulfur concentration gradually increases

3.2.2. Core LV77-36 The core sediments are represented by homogeneous gray and dark gray clayey-silts that gradually become more dense toward the bottom of the core (from 1.54 g/cm3 at the top of the core to 1.72 g/cm3 at the bottom). The predominant grain size fraction is clay (46.2–57.8%),

Fig. 5. Variations in chemical composition of sediment core LV77-36. 153

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Table 3 Radiocarbon (14C) ages of the mollusc shells retrieved from sediment core LV77-36. Lab. No.

Depth, cm

Type

Beta-478652 Beta-478657 Beta-478653 Beta-478654 Beta-478655 Beta-466792 Beta-466793

12–13 36–37 60–61 80–81 92–93 322 354

Shell, Shell, Shell, Shell, Shell, Shell, Shell,

14

fragment fragment fragment fragment fragment fragment fragment

C age, year (BP)

1210 2450 3090 3690 4130 7660 7780

toward the bottom of the core and its concentrations in the 356–375 cm interval are higher than the background content. The total REE concentration in the core varies slightly from 158 mg kg−1 to 181 mg kg−1, increasing toward the base of the core, with maxima in the 110–170 cm and 280–370 cm intervals. Such a distribution is characteristic for Zr, Y, Nb, Sc, Cr, and LREE/HREE. The base of the core (310–370 cm) is also characterized by a sharp increase in Zr and Y concentrations. The NASC-normalized REE patterns are similar throughout the core (Fig. 3b) and Cean and Euan values vary slightly (Table A.2).

± ± ± ± ± ± ±

30 30 30 30 30 30 30

Delta ΔR, year

Age, cal. year (BP)

42 42 42 42 42 42 42

751 ± 133 2065 ± 188 2847 ± 144 3552 ± 175 4142 ± 207 8082 ± 147 8192 ± 153

± ± ± ± ± ± ±

60 60 60 60 60 60 60

are present in the form of REE-bearing minerals and as admixtures in carrier minerals (Fig. 6; Table 4). Angular and complex shaped mineral grains are predominant. In general, their size does not exceed 10–20 μm but larger individuals with diameters up to 100–120 μm are also found. The REE minerals are accessory minerals. In the fraction greater than 10 μm, they are dispersed among the mass of rock-forming minerals, mainly feldspars and quartz. Their frequency of occurrence, calculated from scan area of one sample, is 10 grains/mm2 for Ce, 21 grain/mm2 for La, and 3 grains/mm2 for Y (Fig. A1). In general, sediments with elevated concentrations of rare-earth minerals are also enriched in ilmenite, rutile, zircon, barite, and fluorapatite. Most rare-earth minerals are composed of light lanthanides with a predominance of cerium, whose concentration reached up to 40% (Table 4). Lanthanum and neodymium (almost always occurring with

3.3. Mineralogy According to electron probe microanalysis, the REE in the sediments

Fig. 6. Back-scattered electron images of rare earth minerals in surficial sediments of the Chukchi Sea (10–160 μm fraction). The number near the rare earth mineral grain corresponds to the row in Table 4 which presents data for the chemical composition of the grain. Ap: fluorapatite (Ca5[PO4]3F); Brt: barite (BaSO4); Bt: biotite; Grt: mineral of the garnet group; Ilm: ilmenite (FeTiO3); Ttn: titanite (CaTiSiO5). Most of the unmarked grains are rock-forming minerals (feldspars, quartz). 154

Sr

Mineral grain

155

21.69 22.21

18.47

Zr

2.38

7.85

0.43

F

3.7

6.3 5.5 4.6

12.8 9.3 3.5 6.4 8.6 11.9 12.7 6.4

La

0.34

1.99 0.30

0.50

0.32

0.48

1.34

0.60

0.35

Mg

2.92

0.73

Na

24.7 21.8 7.9 12.2 21.6 14.2 22.6 13.0 7.0 12.5 11.8 9.5 5.8 7.7 7.6 2.0 0.9 0.5

Ce

0.26

0.25 1.75

0.92 0.37 9.86 9.64 1.58 6.02 1.52 1.34 4.33 1.67 4.90 12.98 16.99 1.82 9.08

Al

2.5 1.0

2.4

Pr

1.75 3.08 16.11 14.07 6.34 10.85 5.40 5.39 7.58 3.88 15.82 2.20 7.23 7.41 24.50 4.20 1.24 11.65 1.92 5.18 4.43 13.94

Si

0.6

9.5 10.0 2.9 4.9 11.3 3.3 7.3 4.7 3.3 5.1 4.0 2.4 2.7 3.5 2.7 2.4 0.7

Nd

14.61

17.07

10.09 11.04 3.85 4.43

14.18 12.93 0.51 3.80 12.46 10.16 11.34 9.11 7.35 14.16

P

0.67 0.71

1.48

2.08

Sm

0.32 0.23 7.92

0.54

S

0.42

0.27

Eu

0.81

0.15

0.43

0.48

Cl

2.74 2.93 1.89

0.94

1.25

Gd

0.11 0.21

0.38 0.45 7.02

0.16

0.91

0.17 2.25

0.13

K

0.60

Tb

1.86

4.46 3.47 3.67

Dy

0.83

1.24

Ho

0.35

0.23

4.11

Er

0.47

1.03

2.57

16.91

0.58 10.26 5.65

Fe

14.90

4.14

Ti

2.47 35.32 17.68

0.47

Sc

2.23 0.67 1.03 3.75 0.57 0.90 0.28 0.40 1.83 1.78 6.82

0.21 6.60 6.79 0.30 3.44 0.96 3.10 1.72 10.28 7.61 0.63 0.64 8.76

Ca

2.37

3.58

Yb

0.69 0.92

Co

0.52

Lu

0.82

1.74

1.53

Br

1.25 1.10 0.52 1.01 1.29 1.34 1.41 0.59

1.81 1.25 0.68 1.28 1.52 1.38 1.42 0.59

0.98

5.64 31.76 24.22 10.26

2.41

7.98 1.06

Th

0.87 0.69

1.46

U

1.26

1.11 1.61 1.17 1.37 2.31

1.39 1.89 1.31 1.88 2.51

1.28 0.44

0.92

Zna

1.34

Cua

105.5 105.0 100.4 107.1 103.5 99.3 106.2 114.5 106.1 102.7 104.7 84.2 108.4 84.8 103.0 102.1 94.0 84.1 100.4 101.3 105.0 80.6

Total

31 32 33 4

Y

REEs are shown in boldface type. Blank cells mean “the element is not identified”. Deviations in the total content of 100 wt % are due to specifics of the analysis (see “Methods”, including references). a Most of the sediments considered contained impurities of Cu and Zn. The impurity noted was most steadily registered in Hope Sea Valley sediments and on adjacent areas of the sea floor. Most likely, these were subcolloidal Cu–Zn particles (< 1 μm) accumulating in negative forms of the topography under lithodynamic processes. The chemical composition of such small particles cannot be reliably detected by electron microprobe X-ray analysis.

4.01

41.67 30.32 40.49 41.04 30.44 30.27 30.68 33.28 44.12 34.95 40.43 35.97 60.68 35.75 39.19 44.98 34.78 33.05 31.22 52.11 34.13 34.66

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 15 16 17 18 19 20 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 15 16 17 18 19 20 21

O

Mineral grain

Table 4 Chemical compositions of REE-bearing minerals in surficial sediments from the Chukchi Sea (10–160 μm fraction), wt. %.

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Fig. 7. R-mode factor analysis of chemical and grain-size compositions of the western East Siberian Sea samples with Varimax rotation. Lines indicate the main positive correlations between the elements and the fractions (solid line: > 0.8, dashed line: 0.6–0.8). I–IV: multi-element associations; I–II: clastic elements; I: detrital; II: clayey; III: biogenic elements; IV: redox sensitive elements. Grain-size fractions: sand (> 63 μm), silt (4–63 μm), and clay (< 4 μm).

cerium) followed next with regard to prevalence and quantitative presence in the minerals. The main lanthanide-bearing mineral in the sediments is monazite (Се, La, Nd …)PO4, which usually contains an isomorphic admixture of thorium (up to 10% and higher) and silicon (usually 3–4%). In addition to monazite, allanite (REE, Ca)2(Al, Fe)3[Si2O7][SiO4]O[O,OH] is also distributed in the sediments, occasionally with small amounts of magnesium, and rarely manganese and titanium. Allanite is also enriched with phosphorus.

The proportion of yttrium-bearing minerals (phosphates, oxides, and presumably silicates) among the rare-earth minerals recorded in the sediments does not exceed 15%. The main yttrium-bearing mineral was xenotime (YPO4). The yttrium concentration in minerals reached 30–33 wt %. Overall, REE in the sediments are also present as admixtures of other minerals, and these forms are particularly characteristic of medium and heavy lanthanides in the crystalline phases of yttrium.

Fig. 8. R-mode factor analysis of chemical and grain-size compositions of the Chukchi Sea samples with Varimax rotation (n = 26, r = 0.70). Lines indicate the main positive correlations between the elements and the fractions (solid line: > 0.85, dashed line: 0.75–0.85). 156

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various sources. In the western East Siberian Sea, the REE association with niobium and other elements of stable heavy minerals (characteristic of alkaline rocks and niobium-rare-earth deposits of the East Siberian platform) is well expressed, in addition to the correlation with sand and silt sediment fractions. It can be assumed that the REE enrichment in the western East Siberian Sea results from the supply of silty-sandy clastic material from orebody sources or intermediate ore aggregates of the East Siberian platform. These inputs could be attributed to the Lena River discharge to the east of the delta as well as to the erosion of coastal deposits (see Fig. 1). However, REE in the Chukchi Sea do not show a significant correlation with niobium, as in the western East Siberian Sea. In addition, they indicate negative correlations with the silt and clay fractions and a have significant positive correlation with sand (Table A.6). The REE distribution in surface sediments (Fig. 2) along with mineralogical, geochemical, and grain size data indicate that REE enrichment in sediments from the western East Siberian Sea results from inputs of sandy-silty clastic material from the Lena River and/or from coastal erosion. The coastlines of the East Siberian islands as well as the adjacent mainland are composed of ice complex (yedoma) sediments (Schirrmeister et al., 2017). These are loess-like, well-sorted deposits of predominantly alluvial origin including veinlets, veins, and ice massifs ranging from 50 to 90% of the strata volume. The 10–50 μm sediment fraction constitutes 42.5–68.9% of these deposits (Vtyurin et al., 1984). It has been suggested that these deposits are the main sources of terrigenous material and organic carbon for the southwestern East Siberian Sea (Dudarev et al., 2016).

Fig. 9. LREE/HREE values in the surface sediments of the Chukchi and East Siberian Seas. Also shown are schematic circulation pathways such as the Bering Sea Water, the Alaskan Coastal Water, and the Siberian Coastal Current. Black arrows are from AMAP (1998), and blue arrows are from Corlett and Pickart (2017). . (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

4. Discussion 4.1. Statistical analyses of geochemical and grain size data The statistical analysis results for the East Siberian Sea sediments indicates a significant positive correlation between REE concentrations and the sand and silt fractions (Table A.5). This result is in agreement with the predominant occurrence of REE in compositions of clastic minerals (monazite etc.) and does not contradict the assumption regarding their supply from eroded ancient crystalline rocks of the East Siberian platform and Phanerozoic intermediate reservoirs (Astakhov et al., 2018). Fig. 7 presents the occurrence of several associations of chemical elements. The variability in their sediment concentrations is determined by the presence and composition of certain groups of minerals or sediment components: clastic (I), clay (II), biogenic (III), and authigenic (IV) (Fig. 7). REE are associated with elements found in clastic minerals. Almost all these elements, including REE, are characterized by a significant positive correlation with the contents of the sand and silt fractions (Table A.5). The maximum values of factor II are characteristic to a group of elements (REE, Zr, Hf, Nb, Th, Ti) typical for the stable heavy minerals. A significant positive correlation between the elements in this group is also noted (Table A.5). The elemental associations and correlation links in sediments of the Chukchi Sea are considerably different from the East Siberian Sea. The factor analysis results (Fig. 8) clearly distinguish only the elemental associations related to clay and authigenic sediment components (II and IV). Elements of clastic minerals and biogenic components (I and III), including REE, show only poorly expressed associations (Fig. 8). Calcium and strontium indicate biogenic associations in the East Siberian Sea but are associated with clastic mineral elements in the Chukchi Sea. Elements that are part of stable heavy minerals indicate weaker correlations between themselves and with coarser sediment fractions (sand and silt). Significant positive correlations with sand are noted only for REE/Al and Th/Al, whereas negative correlations were observed with silt (Table A.6). REE indicate a significant positive correlation only with Zr, Th, and Y. These correlations between chemical elements and grain size fractions in the Chukchi Sea sediments can be explained by the specifics of sedimentation. The role of biogenic sedimentation in the Chukchi Sea is significant in contrast to most Arctic seas (Chen et al., 2003; Zhao and Yan, 1994). The observed REE distribution in sediments of the East Siberian and Chukchi Seas can be explained by the terrigenous sediment supply from

4.2. REE in Holocene sediments The supply of REE from eroded coasts is also reflected in their distribution in the Holocene sediments of core LV77-36. The sea level in the study region during the post-glacial transgression at ca. 8–9 ka was estimated to range from −25 to −10 m (Bauch et al., 2001). These depths are close to the maximum depths of the straits connecting the Laptev Sea and the East Siberian Sea (Dmitri Laptev Strait −16 m and Sannikov Strait −24 m). Thus, during accumulation in the lower part of core LV77-36, the straits were closed or were very shallow and narrow, and the movement of seawater from the Laptev Sea to the East Siberian Sea was considerably limited. Nevertheless, the sediments from this period show similar or even higher REE concentrations than sediments accumulated at the current sea level (Fig. 5). The early Holocene sediments are also characterized by increased contents of Zr, Y, and Sc, a high content of phosphorus, and elevated sulfur content at the core bottom (Fig. 5). The last two elements (P and S) indicate the specific conditions of sedimentation such as those observed in semi-enclosed basins, lagoons, and river deltas. Almost identical patterns of NASCnormalized REE concentrations along the length of the core (Fig. 3b) indicate the same source of sediment for the entire depositional period. These distributions are similar to those of the Yana River SPM (Fig. 3b; Rachold, 1999), which is derived from the yedoma sediments, and dissimilar to the distributions of the Lena River SPM. This pattern suggests that the eroded ice complex on the Novosibirsk Islands and the mainland coast, including the Indigirka River valley, was the main source of the REE for the LV77-36 site. Maximum LREE/HREE values are typical for sediments from the inner shelf of the East Siberian Sea despite the intense erosion observed along the adjacent coast (Indigirka and Kolyma River interfluve) (Fig. 9). This pattern could be attributed to coarser grain compositions of sediments in this region, with silt and sand concentrations of 54.5% and 11.6%, respectively, in comparison to the mean values of 47.8% and 2.5%, respectively, for the overall East Siberian Sea. In the Chukchi Sea sediments with maximum LREE/HREE values are confined to the Herald and Hanna shoals (Fig. 9), which are covered by relic sands resulting from winnowing at lower sea levels (Astakhov et al., 2015). The REE sources could be Quaternary deposits reworked on the shoals 157

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Program (Grant No. 18-1-008). Expedition work was also supported by the National Natural Science Foundation of China (Grants no. U160641 and 41420104005).

or washed away from the Chukchi Sea coasts. A narrow strip of elevated LREE/HREE values is also observed near the southern coast of the Chukchi Sea affected by the Siberian coastal current that transports SPM from the East Siberian Sea (Fig. 9). A sizeable part of the Chukchi Sea affected by the western branch of Bering Sea water flowing in via the Bering Strait, features fine-grained sediments with low REE concentrations as well as low LREE/HREE values. This pattern is consistent with the notion that sedimentation here is significantly affected by the Bering Strait throughflow (Kobayashi et al., 2016; Astakhov et al., 2018; Chen et al., 2003). Low LREE/HREE values are characteristic of bottom sediments from the Gulf of Anadyr (Table 2), which supplies the sediment load for the western Bering Strait branch via the Anadyr Coastal Current. Core НC-11 was collected from the area affected by the Bering Sea waters with low LREE/HREE values (Anadyr Coastal Current) and close to the Siberian coastal current (Fig. 9). A marked decrease in REE concentrations along with decreasing Zr, Hf, Ti, and Rb contents toward the bottom of the core (Fig. 4) could be explained by Holocene circulation changes. Over the past ∼2 kyr the influence of the Siberian Coastal Current has decreased, while the supply of sediment from the Bering Sea, as well as the accumulation rate, has increased. This interpretation is in agreement with an earlier conclusion regarding a westward shift of the Anadyr Coastal Current (Astakhov et al., 2015).

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polar.2019.05.005. References AMAP, 1998. AMAP assessment report: arctic pollution issues. In: Arctic Monitoring and Assessment Programme (AMAP), Oslo, Norway, pp. xii+859. Anikiev, V.V., Dudarev, O.V., Botsul, A.I., Kolesov, G.M., Sapozhnikov, D.Yu, 1997. Distribution and sedimentation fluxes of chemical elements in the particulate matterbottom sediments system at the transition from the Anadyr River Estuary to the Bering Sea. Geochem. Int. 35 (3), 274–283. Astakhov, A.S., Bosin, A.A., Kolesnik, A.N., Obrezkova, M.S., 2015. Sediment geochemistry and diatom distribution in the Chukchi sea: application for bioproductivity and paleoceanography. Oceanography 28 (3), 190–201. Astakhov, A.S., Kolesnik, A.N., Shakirov, R.B., Gusev, E.A., 2013. Conditions of the accumulation of organic matter and metals in the bottom sediments of the Chukchi Sea. Russ. Geol. Geophys. 54 (9), 1056–1070. Astakhov, A.S., Semiletov, I.P., Sattarova, V.V., Shi, X., Hu, L., Aksentov, K.I., Vasilenko, YuP., Ivanov, M.V., 2018. Rare earth elements in the bottom sediments of the East Arctic seas of Russia as indicators of terrigenous input. Dokl. Earth Sci. 482, 1324–1327. https://doi.org/10.1134/S1028334X18100021. Arctic Atlas, 1985. Moscow. (in Russia). Bauch, H.A., Mueller-Lupp, T., Taldenkova, E., Spielhagen, R.F., Kassens, H., Grootes, P.M., Thiede, J., Heinemeier, J., Petryashov, V.V., 2001. Chronology of the Holocene transgression at the North Siberian margin. Glob. Planet. Chang. 31, 125–139. Bronk Ramsey, C., 2009. Bayesian analysis of radiocarbon dates. Radiocarbon 51 (1), 337–360. Chen, Z., Gao, A., Liu, Y., Sun, H., Shi, X., Yang, Z., 2003. REE geochemistry of surface sediments in the Chukchi Sea. Sci. China (Series D) 46 (6), 603–611. Corlett, W.B., Pickart, R.S., 2017. The Chukchi slope current. Prog. Oceanogr. 153, 50–65. https://doi.org/10.1016/j.pocean.2017.04.005. Cronin, T.M., O'Regan, M., Pearce, C., Gemery, L., Toomey, M., Semiletov, I., Jakobsson, M., 2017. Deglacial sea level history of the east Siberian Sea and Chukchi Sea margins. Clim. Past 13, 1097–1110. https://doi.org/10.5194/cp-13-1097‒2017. Dubinin, A.V., 2006. Rare Earth Element Geochemistry in the Ocean. Nauka, Moscow (in Russian). Dudarev, O.V., Charkin, A.N., Shakhova, N.E., Mazurov, A.K., Semiletov, I.P., 2016. Modern Lithomorphogenesis on the East Arctic Shelf of Russia. Tomsk Polytechnic University, Tomsk. Ershov, E.D., 1989. Geocryology of the USSR. Eastern Siberia and the Far East. Nedra, Moscow, pp. 515 (in Russia). Fagel, N., Not, C., Gueibe, J., Mattielli, N., Bazhenova, E., 2014. Late Quaternary evolution of sediment provenances in the Central Arctic Ocean: mineral assemblage, trace element composition and Nd and Pb isotope fingerprints of detrital fraction from the Northern Mendeleev Ridge. Quat. Sci. Rev. 92, 140–154. https://doi.org/ 10.1016/j.quascirev.2013.12.011. Gromet, L.P., Dymek, R.F., Haskin, L.A., Korotev, R.L., 1984. The “North American Shale composite”. Its compilation, major and trace element characteristics. Geochem. Cosmochim. Acta 48, 2469–2482. Keskitalo, K., Tesi, T., Bröder, L., Andersson, A., Pearce, Ch, Sköld, M., Semiletov, I.P., Dudarev, O.V., Gustafsson, Ö., 2017. Sources and characteristics of terrestrial carbon in Holocene-scale sediments of the East Siberian sea. Clim. Past 13, 1213‒1226. http://doi.org/10.5194/cp-13-1213-2017. Kobayashi, D., Yamamoto, M., Irino, T., Nam, S.-Il, Park, Yu-H., Harada, N., Nagashima, K., Chikita, K., Saitoh, S.-I., 2016. Distribution of detrital minerals and sediment color in western Arctic Ocean and northern Bering Sea sediments: changes in the provenance of western Arctic Ocean sediments since the last glacial period. Polar Sci. 10, 519–531. https://doi.org/10.1016/j.polar.2016.07.005. Kolesnik, O.N., Kolesnik, A.N., 2015. Rare earth elements in ferromanganese nodules of the Chukchi Sea. Lithol. Miner. Resour. 50 (3), 181–191. Kosheleva, V.A., Yashin, D.S., 1999. Bottom Sediments of the Russian Arctic Seas. VNIIOkeangeologiya, St.Petersburg (in Russian). Lantuit, H., 2012. The Arctic coastal dynamics database: a new classification scheme and statistics on Arctic permafrost coastlines. In: In: Lantuit, H. (Ed.), Estuaries and Coasts, vol 35. pp. 383‒400 2. Lazareva, E.V., Zhmodik, S.M., Karmanov, N.S., Tolstov, A.V., Dar’in, A.V., Baranov, L.N., 2017. Features of the composition and micromorphology of rare earth minerals in the Tomtor. In: Geology and Minerageny of Northern Eurasia. Materials of the Meeting Timed to the 60th Anniversary of the Institute of Geology and Geophysics of the SB RAS USSR, Novosibirsk, pp. 123–124. McKay, J.L., Pedersen, T.F., 2008. The accumulation of silver in marine sediments: a link to biogenic Ba and marine productivity. Glob. Biogeochem. Cycles 22, GB4010.

5. Conclusions In this study we investigate the distribution of rare earth elements in surface and Holocene sediments collected from the shelves of the East Siberian and Chukchi Seas. Results indicate that REE in these sediments predominantly occur in sand- and silt-size detrital mineral grains sourced from the adjacent coasts. The main REE-bearing mineral is monazite, although a significant amount of medium and heavy lanthanides is also present as an admixture in yttrium and thorium minerals. REE are associated with other elements common in stable heavy clastic minerals in bottom sediments, i.e., Zr, Nb, Hf, Th, and Ti. These associations are most typical in the western East Siberian Sea, where terrigenous material is supplied from the East Siberian platform area. The total lanthanide concentration in East Siberian Sea sediments increases from east to west. This pattern indicates their supply is from the erosion of the mainland coast and the New Siberian Islands ice complex, as well as from river discharge, primarily from the Lena River. Shale normalized REE patterns similar to the Lena River SPM, but at low concentrations, persist in the entire East Siberian Sea and traces to the Chukchi Sea. REE distributions from the southeastern Chukchi Sea and the Gulf of Anadyr (Bering Sea) (Anikiev et al., 1997) differ markedly by a depletion in light lanthanides. The area of propagation of the western branch of the Pacific Ocean water entering through the Bering Strait is distinguished by low LREE/HREE values. Changes in the REE concentrations in a core from the SW Chukchi Sea with an age of slightly over 2 kyr may indicate a migration of the current system and a supply of sedimentary material derived from Pacific waters. Acknowledgments The authors are extremely grateful to Dudarev O.V., Gusev E.A., Bosin A.A. and Ivanov M.V. for providing additional samples for this study. Special thanks go Zarubina N.V., Tkalina E.A., Gorbach G.I., Hurkalo N.V., Alekseeva L.I., Kuz'mina T.V and Volkova E.V. from the Laboratory of Analytical Chemistry at the Far East Geological Institute for help with chemical analyses. The reported study was funded by RFBR according to the research Project no. 18-05-60104 and by the governmental Grant of POI FEB RAS (№ АААА-А17-117030110033-0). Analytical studies were partly carried out by according to the Far East

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