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Volume 31 Issue 1
Jan.  2020
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Zinc Isotope Characteristics in the Biogeochemical Cycle as Revealed by Analysis of Suspended Particulate Matter (SPM) in Aha Lake and Hongfeng Lake, Guizhou, China

  • Zn isotope is a useful tool for tracing biogeochemical processes as zinc plays important roles in the biogeochemistry of natural systems. However, the Zn isotope composition in the lake ecosystems has not been well characterized. In order to resolve this problem, we investigate the Zn isotope compositions of suspended particulate matter (SPM) and biological samples collected from the Aha Lake and Hongfeng Lake, and their tributaries in summer and winter, aiming to explore the potential of this novel isotope system as a proxy for biogeochemical processes in aqueous environments. Concentration of dissolved Zn ranges from 0.65 to 5.06 μg/L and 0.74 to 12.04 μg/L for Aha Lake and Hongfeng Lake, respectively, while Zn (SPM) ranges from 0.18 to 0.70 mg/g and 0.24 to 0.75 mg/g for Aha Lake and Hongfeng Lake, respectively. The Zn isotope composition in SPM from Aha Lake and its main tributaries ranges from -0.18‰ to 0.27‰ and -0.17‰ to 0.46‰, respectively, and it varies from -0.29‰ to 0.26‰ and -0.04‰ to 0.48‰, respectively in Hongfeng Lake and its main tributaries, displaying a wider range in tributaries than lakes. These results imply that Zn isotope compositions are mainly affected by tributaries inputting into Aha Lake, while adsorption process by algae is the major factor for the Zn isotope composition in Hongfeng Lake, and ZnS precipitation leads to the light Zn isotope composition of SPM in summer. These data and results provide the basic information of the Zn isotope for the lake ecosystem, and promote the application of Zn isotope in biogeochemistry.
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Zinc Isotope Characteristics in the Biogeochemical Cycle as Revealed by Analysis of Suspended Particulate Matter (SPM) in Aha Lake and Hongfeng Lake, Guizhou, China

    Corresponding author: Lili Liang, lianglily99@126.com
    Corresponding author: Cong-Qiang Liu, liucongqiang@vip.skleg.cn
  • 1. School of Environmental Studies, China University of Geosciences, Wuhan 430074, China
  • 2. State Key Laboratory of Environmental Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China
  • 3. Key Laboratory of Isotopic Geology of the Ministry of Land and Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
  • 4. School of Geosciences, University of Edinburgh, Edinburgh EH9 3FE, UK
  • 5. School of Geographic and Environmental Sciences, Tianjin Normal University, Tianjin 300387, China
  • 6. College of Water Sciences, Beijing Normal University, Beijing 100875, China

Abstract: Zn isotope is a useful tool for tracing biogeochemical processes as zinc plays important roles in the biogeochemistry of natural systems. However, the Zn isotope composition in the lake ecosystems has not been well characterized. In order to resolve this problem, we investigate the Zn isotope compositions of suspended particulate matter (SPM) and biological samples collected from the Aha Lake and Hongfeng Lake, and their tributaries in summer and winter, aiming to explore the potential of this novel isotope system as a proxy for biogeochemical processes in aqueous environments. Concentration of dissolved Zn ranges from 0.65 to 5.06 μg/L and 0.74 to 12.04 μg/L for Aha Lake and Hongfeng Lake, respectively, while Zn (SPM) ranges from 0.18 to 0.70 mg/g and 0.24 to 0.75 mg/g for Aha Lake and Hongfeng Lake, respectively. The Zn isotope composition in SPM from Aha Lake and its main tributaries ranges from -0.18‰ to 0.27‰ and -0.17‰ to 0.46‰, respectively, and it varies from -0.29‰ to 0.26‰ and -0.04‰ to 0.48‰, respectively in Hongfeng Lake and its main tributaries, displaying a wider range in tributaries than lakes. These results imply that Zn isotope compositions are mainly affected by tributaries inputting into Aha Lake, while adsorption process by algae is the major factor for the Zn isotope composition in Hongfeng Lake, and ZnS precipitation leads to the light Zn isotope composition of SPM in summer. These data and results provide the basic information of the Zn isotope for the lake ecosystem, and promote the application of Zn isotope in biogeochemistry.

0.   INTRODUCTION
1.   STUDY BACKGROUND AND SAMPLE COLLECTION
  • Aha and Hongfeng lakes are artificial river interception reservoirs located in about 8 and 31.5 km southwest of Guiyang City, in a subtropical humid monsoon climate zone. The catchments are characterized by low rainfall and river discharge during winter and spring, whereas high temperatures in summer and autumn bring more rainfall and high river flow. They are both seasonally anoxic reservoirs.

    Aha Lake covers an area of 4.5 km2, with a total water volume of 4.45×107 m3. The average and maximum depths are 13 and 24 m, respectively. The residence time of lake water is about 0.44 year. The watershed area is 190 km2 with an average annual precipitation of 1 109 mm, and the average annual temperature is 13.8–15.5 ℃. Previously, more than 200 coal mines were widely distributed in the watershed, where significant amount of acid mining drainages and dump filtrates were produced. There are six main rivers flowing through the watershed area including five inflowing tributaries, Youyu River (YYR), Caichong River (CCR), Lannigou River (LNR), Baiyan River (BYR) and Sha River (SR), and only one draining river, Xiaoche River (XCR) (Fig. 1). The YYR and BYR are mainly polluted by coal mines, CCR and LNGR are mainly polluted by domestic sewerage, and SR polluted by industrial and domestic sewerage. The surface of lake water is colonized by sparse diatoms and causes eutrophication in summer.

    Figure 1.  Sketch map showing the locations, discharges of tributaries and the sampling sites of Hongfeng and Aha lakes, Southwest China. The sampling locations were HFDB (Daba) and HFHW (Howwu) profiles in Hongfeng Lake, AHDB (Daba) and AHLJK (Liangjiangkou) profiles in Aha Lake.

    Hongfeng Lake is much bigger than Aha Lake. It covers an area of 57.2 km2, its reservoir storage capacity is 6.01×108 m3, with a drainage area of 1 596 km2, the water residence time is about 0.33 year. The average and maximum water depth is 10.52 and 45 m, respectively. Hongfeng Lake consists mainly of two areas: the north lake and the south lake, and there are six main tributaries flowing through the watershed area, including five inflowing tributaries, YCR (Yangchang River), MXR (Maxian River), HLR (Houliu River), MBR (Maibao River), THYR (Taohuayuan River), and one draining river, MTR (Maotiao River) (Fig. 1). The discharges of YCR and THYR are larger than others among these tributaries. The industrial wastewater pollution constitutes a more serious impact on the water quality of Hongfeng Lake. In particular, the fertilizer plant of Guizhou is the most serious polluting enterprise, it discharges lots of N, P into the lake every year. Accordingly, the lake becomes eutrophic in spring and summer, as evident from the presence of cyanobacteria and algae.

  • Samples were mainly collected in Aha Lake and Hongfeng Lake and their tributaries (Fig. 1). For the Aha Lake, samples were collected at AHLJK (Liangjiangkou) as the upstream site and AHDB (Daba) as the downstream site. For Hongfeng Lake, samples were collected along the flow direction from south to north, with HFHW (Houwu) site of the south lake and HFDB (Daba) site of the north lake. The samples were collected with stratified collection at each site; sampling interval with water depth in each site is slightly different, but generally ranged between 3 and 5 m. Samples of all tributaries were collected at sites near the lake but far away from living areas. All samples were collected in August 2006 (summer) and January 2007 (winter). The algae samples were collected using nylon net from the surface of the Maxian River (MXR).

    All collection wares used in the field were carefully cleaned. Polyethylene bottles, tubes for sample collectors were all soaked in 6 N HCl (GR) for more than three days and then rinsed with 18.2 Ω Milli-Q water. Bottles for sampling were pre-rinsed with the corresponding water samples three times prior to sampling. A multi-parameter sensor was used for determining the pH, water temperature (T), and DO (dissolved oxygen). Water samples for measurement of Zn isotope compositions of SPM were collected in 10 L polyethylene barrels; water samples for determining the concentration of SPM, concentration of Zn and Al in SPM, and speciation of SPM were collected in 1.5 L polyethylene bottles; water samples for analyzing chlorophyll were collected in 50 mL brown glass bottles with two drops of HgCl2 to prevent metabolic activity. Samples for analyzing the dissolved Zn were filtered with 0.45 μm millipore membrane filter in the field and acidified to pH < 2 with ultra-pure HNO3. All samples were transported to laboratory as soon as possible after collection.

2.   SAMPLE PREPARATION FOR Zn ISOTOPE ANALYSIS
  • The sample preparation work was carried out in a clean room. All the critical work including sample filtration, digestion and purification was completed in class 100 laminar flow hoods. Hydrochloric acid (HCl) was distilled twice in quartz sub-boiling still, hydrofluoric acid (HF) and nitric acid (HNO3) were distilled with teflon two-bottle setup. Milli-Q water (18.2 MΩ) was used throughout the procedures. The filters were treated three times with 1 N HCl (double-distilled), rinsed with Milli-Q water (18.2 MΩ), and then dried at 50 ℃ in an oven and weighted. After those processes, the blank of filters is as low as 0.001 μg/L and can be negligible.

  • The SPM for measurement of Zn isotope composition was isolated by collecting SPM both deposited either on millipore HA membrane filter (100 mm, 0.45 μm) or particulate matter that settled at the bottom of the container. The filters with SPM were stored in polyethylene tubes in a fridge. The sample for measuring the concentration of SPM, the speciation of SPM and concentration of Zn and Al in SPM was also filtered through millipore HA membrane filter (45 mm, 0.45 μm), then dried at 50 ℃ in the oven, and weighed. The volume of water filtered was recorded to calculate the concentration of SPM. Samples for determination of chlorophyll were filtered and chlorophyll quantified following the acetone extraction spectrophotometric method (Barnes et al., 1992). Algae samples were cleaned and dried in a freeze dryer, and then ground to 50 meshes for digestion and δ66Zn analysis.

    The speciation of SPM was determined following a sequential extraction procedure (Tessier et al., 1979). For this, we only extracted three fractions, including adsorption, exchangeable and carbonate bound (AEC) fraction using pH=2 HCl, bound to organic matter fraction using 30% H2O2 (pH=2), and residual fraction. The extracted solution was evaporated on a hot plate and the solid residue was digested, and then all of them were dissolved in 2% HNO3 for analysis.

  • All SPM and algae samples for Zn isotope measurement, concentration of Zn, Al, and the residual fraction of SPM were digested. These samples were soaked with 3 mL aqua regia and 0.5 mL concentration HF for 48 h in acid-cleaned teflon beakers (7 mL, Savillex). The beakers were placed on a hot plate and dried at 80 ℃. Another 3 mL aqua regia and 0.5 mL concentration HF were added and the closed beaker was placed on a hot plate for 72 h at 140 ℃ for digestion. The procedure was repeated until the particles were thoroughly digested. After samples were digested thoroughly, solutions of sample were left on the hot plate to dry at 80 ℃. For the Zn isotope measurement samples, the last step was sequentially repeated three times with 0.5 mL concentrated HCl to eliminate HNO3 and HF, and then the residue re-dissolved in 7 N HCl+0.001% H2O2 for chemical purification. Other samples were just re-dissolved in 2% HNO3 for analysis.

  • Chemical purification was carried out using procedures similar to those of Maréchal et al. (1999), Ding et al. (2006) and Tang et al. (2006), with slight modifications. Details are as follows: Anion-exchange chromatography was performed with polypropylene column (Bio-Rad, diameter: 6.8 mm, height: 4.3 cm) filled with AG MP-1 resin (Bio-Rad, 100–200 mesh, chloride form). The resin was first cleaned with 2 mL 0.5 M HNO3 alternating with 10 mL 18.2 MΩ Milli-Q water three times. Then 5 mL Milli-Q water was used to ensure that the HNO3 was thoroughly removed. The resin was then continuously pre-conditioned with 5 mL 7 N HCl+0.001% H2O2 and 4 mL 7 N HCl+0.001% H2O2. Then the prepared samples were loaded on the resin and the matrix were striped with 35 mL 7 N HCl+0.001% H2O2; Fe was eluted with 20 mL 2 N HCl+0.001%H2O2, and Zn was eluted with 10 mL 0.5 N HNO3. The Zn eluate was evaporated to dry on a hot plate at 80 ℃ and dissolved in 0.1 N HNO3 to a concentration of 100 to 200 μg/L for isotope analysis. The recoveries of Zn for all samples were nearly 100%, so the Zn isotope fractionation can be avoided during the purification process (Maréchal and Albarède, 2002). The procedural blanks including digestion, column purification and evaporation were always less than 0.11% of the total Zn extracted from the samples.

  • The concentration of dissolved Zn was analyzed on quadrupole ICP-MS (GV Instruments), and the concentration of Zn and Al in SPM was analyzed on ICP-OES (Varian vista MPX). The Zn isotope composition was analyzed on Nu Plasma instrument HR MC-ICP-MS at the Laboratory of Isotope Geology, MLR, Institute of Geology, CAGS, Beijing, China. The Zn samples and standard Zn sample, with concentrations ranging from 100 to 200 μg/L in 0.1 N HNO3, were introduced to the argon plasma via a desolvation nebulizer DSN-100 system, with gas flow rates of 50–100 μL/min. The typical ion beams for 200 μg/L Zn solutions of both standards and samples were 4–6 V on 64Zn and the blanks were always below 0.005 V. The standard-sample bracketing (SSB) method has been used throughout the study to minimize the instrumental mass bias and the standard-sample concentrations matched within 5%. The performance of the instrument was assessed by repetitive measurements of an internal lab standard (GSB-Zn) relative to the Zn isotope reference material Romil. The average Zn isotope values for GSB-Zn is δ66Zn= 6.96‰±0.11‰, δ67Zn=10.4‰±0.23‰, δ68Zn=13.2‰±0.22‰ (2SD) in high resolution mode under optimized conditions. The long-term instrumental reproducibility defined from the 7 monthsʼ replicate analyses are 0.11‰ for δ66Zn, 0.23‰ for δ67Zn and 0.22‰ for δ68Zn. The detailed conditions and the performance of isotope measurements were described in Li et al. (2008) and Gao and Zhu (2014).

    Zn isotope data was reported in δxZn (δ66Zn, δ68Zn) as parts per thousand deviations relative to JMC 3-0749. All the δ64Zn and δ66Zn values obtained in this study followed the theoretical mass-dependent fractionation line, with a formula of δ68Zn=1.976×δ66Zn+0.000 5 (R2=0.999 8), where

3.   RESULTS
  • Environment parameters are summarized in Table 1, and plotted in Figs. 2 and 3. In summer thermal stratification was observed in August with a temperature gradient of ca. 10 ℃ in Aha Lake. The thermoclines were located at a water depth ca. 10 m for AHDB and ca. 6 m at for AHLJK station. Dissolved oxygen declined sharply below the thermoclines, with average concentration ca. 1.2 mg/L in Aha Lake, and there was also a marked decrease in pH of ca. 0.5 units below the thermocline for AHDB profile. However, there were no clear depth- dependent variations in temperature, DO and pH in the winter both for AHDB and AHLJK profiles (Fig. 2). Moreover the temperature gradient was ca. 6 ℃ from surface water to thermocline for Hongfeng Lake, and the thermocline was located at a water depth ca. 12 m for HFHW station in summer. DO was also almost depleted under the thermocline, with average concentration of 2.0 mg/L, and hypoxic conditions prevailed in summer. There was also marked decrease of ca. 2 units in the deep layers at Hongfeng Lake. However, there were also no clear depth-dependent variations in temperature, dissolved oxygen and pH for Hongfeng Lake in winter (Fig. 3).

    Sample site Sampling date Depth
    (m)
    pH T
    (℃)
    DO
    (mg/L)
    Chlorophyll
    (μg/L)
    SPM
    (mg/L)
    Zn (DIS)
    (μg/L)
    AHDB Aug 0 8.19 25 7.00 11.08 2.27 1.28
    AHDB Aug -4 8.32 25.2 7.04 6.64 2.13 2.21
    AHDB Aug -8 7.93 24.6 5.11 8.06 1.53 0.77
    AHDB Aug -12 7.7 22.3 5.32 7.26 1.33 1.97
    AHDB Aug -16 7.53 17.4 2.09 4.86 2.00 1.94
    AHDB Aug -20 7.5 14.9 1.98 2.05 1.73 1.70
    AHDB Aug -23 7.6 14 2.00 2.20 0.65
    AHDB Jan 0 7.49 7.9 8.43 3.79 1.19 2.99
    AHDB Jan -5 7.49 7.4 7.70 2.97 1.31 2.60
    AHDB Jan -10 7.97 7.3 8.14 3.05 0.96 2.64
    AHDB Jan -15 7.95 7.1 8.76 1.46 0.80 2.66
    AHDB Jan -20 7.92 7.23 8.80 3.21 1.05 2.19
    AHDB Jan -23 7.9 7.3 8.33 2.56 0.82 2.67
    AHLJK Aug 0 8 25.1 7.80 6.08 3.00 1.65
    AHLJK Aug -3 7.8 22.1 6.51 5.82 3.20 1.78
    AHLJK Aug -6 7.6 19.2 2.33 5.75 2.40 1.29
    AHLJK Aug -9 7.5 18.6 1.65 6.11 2.50 1.20
    AHLJK Aug -13 7.6 14.2 1.60 2.53 1.05
    AHLJK Jan 0 7.95 7.95 9.00 2.80 0.86 2.01
    AHLJK Jan -3 7.8 7.8 8.88 3.00 1.26 5.06
    AHLJK Jan -6 7.87 7.87 8.60 2.50 0.91 1.98
    AHLJK Jan -10 7.46 7.46 9.20 2.00 0.89 2.32
    HFHW Aug 0 9.37 28.1 8.10 42.10 3.73 1.48
    HFHW Aug -3 8.49 26.6 7.50 29.23 3.00 0.75
    HFHW Aug -6 8.42 26.3 5.20 21.06 3.00 1.55
    HFHW Aug -9 8.21 25.9 1.20 11.74 3.27 1.80
    HFHW Aug -12 7.66 24.1 1.10 7.50 2.60 1.67
    HFHW Aug -15 7.54 23.3 1.00 5.23 1.73 1.29
    HFHW Aug -19 7.44 22.6 1.20 2.20 1.22
    HFHW Jan 0 8.31 8 15.00 9.91 2.59 2.13
    HFHW Jan -5 7.99 7.8 14.20 10.61 2.48 2.58
    HFHW Jan -10 8.13 8.1 14.00 10.52 1.40 2.15
    HFHW Jan -15 7.94 8 13.00 8.36 1.94 2.83
    HFHW Jan -20 8.18 7.8 12.00 8.34 1.87 5.69
    HFHW Jan -25 8.08 7.5 12.50 5.58 1.49 12.04
    HFDB Jan 0 7.75 8.1 12.10 7.30 1.09 2.44
    HFDB Jan -5 7.84 7.7 11.90 5.30 1.72 2.20
    HFDB Jan -10 7.81 7.8 11.80 6.50 1.27 3.94
    HFDB Jan -15 7.79 7.7 11.90 5.20 1.82 1.95
    HFDB Jan -20 7.51 7.7 12.10 4.90 1.54 3.21
    HFDB Jan -30 7.7 7.6 11.80 5.10 1.58 2.83

    Table 1.  Data of environment parameters, concentrations of dissolved Zn and SPM in summer and winter for Aha Lake and Hongfeng Lake, Southwest China

    Figure 2.  Plots of temperature, pH, chlorophyll, DO (dissolved oxygen), concentrations of SPM, dissolved Zn, Zn (SPM), and δ66Zn of SPM for AHDB and AHLJK profile of Aha Lake. For both profiles, green and red triangles refer to the data of August 2006 and January 2007 for AHDB, respectively; while green and red squares refer to the data of August 2006 and January 2007 for AHLJK, respectively.

    Figure 3.  Plots of temperature, pH, chlorophyll, DO (dissolved oxygen), concentrations of SPM, dissolved Zn, Zn (SPM), and δ66Zn of SPM for HFHW and HFDB of Hongfeng Lake. For both profiles, green and red circles refer to the data of August 2006 and January 2007 for HFHW, respectively, while red diamonds refer to the data of January 2007 for HFDB.

    The concentration of chlorophyll was measured for both Aha Lake and Hongfeng Lake. In summer, the concentration of chlorophyll was very high, reaching 42.1 μg/L at surface water, with marked decrease to 5.2 μg/L at the bottom for HFHW station, while the concentration of chlorophyll varied from 11.1 to 2.1 μg/L for AHDB station. It is apparent that eutrophication occurred at the surface of Hongfeng Lake in summer (Figs. 2 and 3).

    Temperature, DO and pH were also measured for all tributaries. The temperatures of most tributaries were similar to that of the thermocline of the lake, and concentrations of DO for most rivers in summer were lower than in winter.

  • The average concentration of SPM was 1.88 and 2.73 mg/L in summer for AHDB and AHLJK profiles, respectively, it was higher than in winter (average is 1.02 and 0.98 mg/L for AHDB and AJLJK, respectively), and it decreased with increasing water depth in summer. Similarly, the average concentration of SPM was 2.79 mg/L in summer for HFHW, it was also higher than in winter (average is 1.96 mg/L), and the concentration was higher at the surface than bottom in summer (Tables 1 and 2, Figs. 2 and 3).

    Sample site Date Depth
    (m)
    Al2O3 (SPM)
    (%)
    Zn (SPM)
    (mg/g)
    AEC-Zn
    (SPM) (%)
    Organic-Zn
    (SPM) (%)
    Residual-Zn
    (SPM) (%)
    δ66ZnJMC
    (‰)
    Zn/Al
    AHDB Aug 0 4.44 0.26 70.23 16.05 13.72 0.10 0.011 0
    AHDB Aug -4 0.37 0.30 74.26 19.04 6.69 -0.05 0.152 9
    AHDB Aug -8 3.11 0.32 68.00 23.90 8.10 -0.03 0.019 5
    AHDB Aug -12 1.57 0.40 86.87 9.04 4.09 0.02 0.048 1
    AHDB Aug -16 0.80 0.48 78.59 18.49 2.92 0.01 0.113 5
    AHDB Aug -20 1.57 0.70 66.67 23.25 10.08 0.02 0.084 0
    AHDB Aug -23 1.23 0.32 86.80 9.73 3.47 0.19 0.049 0
    AHDB Jan 0 5.14 0.36 86.42 4.04 9.55 0.16 0.013 1
    AHDB Jan -5 5.24 0.57 79.07 15.46 5.47 0.23 0.020 5
    AHDB Jan -10 6.45 0.27 83.66 8.42 7.92 0.27 0.008 0
    AHDB Jan -15 1.28 0.25 87.69 5.43 6.89 0.21 0.037 2
    AHDB Jan -20 4.01 0.40 94.72 3.86 1.42 0.55 0.018 7
    AHDB Jan -23 3.05 0.27 0.20 0.016 7
    AHLJK Aug 0 0.62 0.38 59.37 18.38 22.25 -0.18 0.114 9
    AHLJK Aug -3 0.47 0.18 70.89 12.92 16.20 0.05 0.073 8
    AHLJK Aug -6 1.46 0.24 86.92 12.24 0.83 0.12 0.030 7
    AHLJK Aug -9 2.39 0.26 83.21 8.99 7.80 0.16 0.020 3
    AHLJK Aug -13 1.17 0.32 57.36 19.50 23.14 0.09 0.051 4
    AHLJK Jan 0 2.15 0.21 81.94 4.76 13.30 0.13 0.018 7
    AHLJK Jan -3 2.65 0.69 76.91 8.83 14.26 0.03 0.048 8
    AHLJK Jan -6 2.99 0.40 71.59 7.31 21.10 0.20 0.025 4
    AHLJK Jan -10 2.85 0.32 75.64 5.56 18.80 0.14 0.021 3
    HFHW Aug 0 2.15 0.39 54.69 9.10 36.21 0.20 0.034 4
    HFHW Aug -3 6.44 0.48 52.01 11.26 36.73 0.01 0.014 0
    HFHW Aug -6 3.28 0.34 29.13 53.14 17.73 -0.11 0.019 3
    HFHW Aug -9 5.13 0.60 75.72 7.59 16.69 -0.19 0.022 0
    HFHW Aug -12 6.58 0.35 80.71 5.82 13.47 -0.29 0.010 1
    HFHW Aug -15 5.17 0.29 84.00 5.65 10.35 -0.20 0.010 7
    HFHW Aug -19 5.89 0.52 81.80 7.69 10.50 -0.11 0.016 7
    HFHW Jan 0 4.75 0.40 83.39 6.12 10.48 0.15 0.015 8
    HFHW Jan -5 4.29 0.24 78.95 10.84 10.21 0.10 0.010 4
    HFHW Jan -10 5.22 0.31 86.33 5.26 8.40 0.12 0.011 2
    HFHW Jan -15 8.77 0.30 81.88 4.74 13.38 0.22 0.006 5
    HFHW Jan -20 10.41 0.75 84.66 5.97 9.38 0.26 0.013 6
    HFHW Jan -25 12.02 0.48 75.27 12.77 11.95 0.13 0.007 5
    HFDB Jan 0 4.76 0.30 89.07 6.01 4.92 0.11 0.012 1
    HFDB Jan -5 4.56 0.34 86.79 5.40 7.80 0.20 0.014 1
    HFDB Jan -10 4.30 0.38 89.36 4.64 6.00 0.07 0.016 6
    HFDB Jan -15 5.37 0.40 90.08 4.63 5.29 0.17 0.014 2
    HFDB Jan -20 5.02 0.35 82.15 12.06 5.79 0.12 0.013 2
    HFDB Jan -30 4.70 0.34 88.66 5.08 6.26 0.22 0.013 6
    Algae MXR 0.41
    Aglea MXR 0.40
    Plant MXR 0.21

    Table 2.  Data of Zn isotope composition of SPM, and other parameters related to SPM in summer and winter for Aha Lake and Hongfeng Lake

    Dissolved Zn ranges from 0.65 to 5.06 μg/L and 0.74 to 12.04 μg/L for Aha Lake and Hongfeng Lake respectively, the Zn (SPM) ranges from 0.18 to 0.70 mg/g and 0.24 to 0.75 mg/g (Figs. 2 and 3). Generally speaking, the concentration of Zn in Hongfeng Lake is higher than that in Aha Lake, but dissolved Zn concentration does not exceed regulatory limits in both lakes, in contrast to the Yellow River and Greece Kalloni Bay (Hong et al., 2006; Gavriil and Angelidis, 2005). Dissolved Zn is slightly higher in winter than in summer, but there are no significant variations with water depth for Aha Lake. Meanwhile, average of Zn (SPM) in summer is very similar to that in winter. Dissolved Zn in winter is higher than that in summer for Hong Lake, which is similar to Aha Lake; while the average of Zn (SPM) in summer was slightly higher than in winter for Hongfeng Lake (Figs. 2 and 3).

    The speciation of Zn in SPM also was determined. It was found that AEC-bound Zn ranged from 57.4% to 94.7% and 29.1% to 90.1% for Aha and Hongfeng lakes, respectively (Table 2 and Fig. 4). The percentage of organic bound Zn averaged 17.1% in summer, higher than the average of 7.4% in winter for AHDB station. In addition, the percentage of organic bound Zn reached 53.1% at subsurface water and averaged 14% in summer for HWFW station, higher than the average 7.4% in winter at Hongfeng Lake, which is analogous to HFHW station.

    Figure 4.  The proportion of Zn different speciation of SPM. Plot A is the proportion of Zn different speciation of SPM for Aha Lake, while plot B is for Hongfeng Lake. For both of the two lakes, the open columns refer to AEC-bound Zn, green columns refer to organic-bound Zn, and slash columns refer to the residual-bound Zn.

    Concentration of SPM, dissolved Zn and Zn (SPM) varies significantly in time and space (Table 3). For Aha Lake, SR and YYR have the higher SPM (78.03 and 65.93 mg/L) and YYR (18.07 μg/L) has the highest dissolved Zn content than other rivers. The concentrations of SPM and dissolved Zn in summer are higher than those in winter for most rivers, and concentration of Zn (SPM) varies between summer and winter. In addition, average dissolved Zn (5.3 μg/L) and Zn (SPM) (0.51 mg/g) of Aha tributaries are higher than those of Aha Lake (2.03 μg/L and 0.36 mg/g, respectively). For Hongfeng Lake, THYR has the highest SPM (9.4 mg/L) and dissolved Zn concentrations (14.78 μg/L than other rivers. The concentrations of dissolved Zn in winter are higher than those in summer for most rivers, and THYR also has the highest SPM-Zn (2.9 mg/g). Moreover, average dissolved Zn (4.63 μg/L) in Aha Lake is similar to that in Hongfeng Lake (5.73 μg/L), but average Zn (SPM) (0.70 mg/g) is higher than that in the Hongfeng Lake (0.40 mg/g).

    Sampling site Sampling date Average discharge
    (m3/S)
    Draining In/out pH T
    (℃)
    DO
    (mg/L)
    SPM
    (mg/L)
    Zn (DIS)
    (μg/L)
    Zn (SPM)
    (mg/g)
    Al2O3
    (%)
    δ66ZnJMC
    (‰)
    Zn/Al
    Tributaries of Aha Lake
    XCR Aug 0.65 Out 7.21 12 5.19 0.87 1.07 0.84 1.35 0.20 0.116 6
    XCR Jan 0.65 Out 7.66 7.7 8.45 1.03 2.48 0.62 5.57 0.11 0.021 2
    BYR Aug 0.90 In 8.14 21.8 7.47 4.53 1.53 0.26 5.64 -0.17 0.008 8
    BYR Jan 0.90 In 8.36 7.4 10.32 2.21 5.68 0.26 6.84 0.05 0.007 3
    CCR Aug 0.21 In 7.66 21.6 5.19 6.27 1.42 0.93 4.99 0.34 0.035 1
    CCR Jan 0.21 In 7.71 9.3 8.8 1.71 7.54 0.43 1.27 0.10 0.063 5
    SR Aug 0.83 In 8.28 22.6 7.6 78.03 1.03 0.00
    SR Jan 0.83 In 8.28 8.7 10.4 3.29 5.13 1.27 8.02 0.29 0.029 9
    YYR Aug 1.18 In 8.18 21.6 8.17 65.93 18.07 0.49 1.24 -0.09 0.074 4
    YYR Jan 1.18 In 7.88 8.6 9.28 21.53 4.78 0.49 1.24 0.46 0.074 4
    LNGR Aug 0.15 In 7.61 21 1.6 27.40 3.56 0.00 4.38
    LNGR Jan 0.15 In 7.14 8.7 5.11 14.00 11.30 0.51 3.17 -0.04 0.030 3
    Turbutaries of Hongfeng Lake
    MTR Aug 14.74 Out 7.49 20.7 5.98 0.93 1.48 0.19 1.21 0.22 0.028 8
    MTR Jan 14.74 Out 7.42 6.9 6.91 1.48 2.95 0.48 5.38 0.10 0.017 0
    THYR Aug 4.14 In 7.13 24.5 7.8 9.40 14.78 2.90 10.28 0.40 0.053 3
    THYR Jan 4.14 In 7.29 5.6 9.23 4.91 5.31 1.83 7.49 0.25 0.046 2
    YCR Aug 12.67 In 7.37 26.5 7.2 3.00 2.59 0.70 9.61 0.40 0.013 7
    YCR Jan 12.67 In 7.08 3.4 10.2 3.42 12.42 0.43 5.79 0.04 0.014 1
    MXR Aug 5.31 In 8.04 26.3 8.17 3.47 0.24 0.17 6.85 0.30 0.004 8
    MXR Jan 5.31 In 7.67 4.2 10.38 0.91 2.46 0.25 6.06 0.48 0.007 9
    HLR Aug 1.86 In 8.07 24.9 7.38 1.20 0.65 0.00 0.03
    HLR Jan 1.86 In 9.72 3.9 9.76 1.74 4.13 0.25 16.70 0.13 0.002 8
    MBR Jan 0.32 In 7.44 9 8.32 1.93 3.94 0.46 12.44 0.14 0.006 9

    Table 3.  The pH, temperature, DO (dissolved oxygen), discharges, concentration of dissolved Zn, SPM and SPM Zn, and Zn isotope composition in tributaries of Aha Lake and Hongfeng Lake

  • The Zn isotope composition of SPM varies significantly in time and space. Generally speaking, δ66Zn of SPM ranges from -0.29‰ to 0.55‰ for the samples collected from Aha Lake and Hongfeng Lake and their tributaries, the variation is about 9–10 times compared to precision of determination. This falls largely within the previously determined isotope range of particle Zn from Greifen Lake (-0.66‰ to 0.21‰) and Seine River (-0.08‰ to 0.30‰), but slightly lighter than terrestrial geological material (0.4‰ to 1.4‰) (Little et al., 2016; Chen et al., 2009; Peel et al., 2009; Cloquet et al., 2006). All the δ66Zn data for SPM from Aha Lake and Hongfeng Lake are given in Table 1 and δ66Zn data for SPM from tributaries are given in Table 3.

    The Zn isotope composition of SPM for Aha Lake range from -0.18‰ to 0.27‰, is slightly lighter than Aha tributaries. The Zn isotope composition in summer (-0.18‰ to 0.19‰) is lighter than in winter (0.03‰ to 0.27‰) for Aha Lake. The Zn isotope composition of SPM for Hongfeng Lake ranges from -0.29‰ to 0.26‰, and is also slightly lighter than the Zn isotope composition of SPM in Hongfeng tributaries (-0.04‰ to 0.48‰). Similarly, the δ66Zn varies from -0.29‰ to 0.20‰ for Hongfeng Lake in summer, is also slightly lighter than the δ66Zn in winter (Figs. 2, 3, Table 1).

    For Aha Lake, there are no discernible trends with increasing water depth both in AHDB and HALJK stations in winter, but it is apparent that the heavier δ66Zn appears at the surface for AHDB, which is similar to the HFHW in summer. Whereas, there is also a clear increase of δ66Zn with water depth for AHLJK station in summer. For the Hongfeng Lake, there are no clear trends with depth in winter for both HFHW and HFDB sites. However, a pronounced decrease of δ66Zn was observed with increasing water depth to -0.29‰ at a depth of 12 m in summer at HFHW station. Lower δ66Zn appears at the thermocline while the higher δ66Zn appears at the surface. This trend is similar to that of particles collected from Atlantic and Pacific oceans (Maréchal et al., 2000).

    The Zn isotope of SPM in tributaries also varies significantly in time and space. For Aha Lake, the δ66Zn values of YYR and BYR are -0.09‰ and -0.17‰ in summer, respectively, which are isotopically light relative to sphalerite (0.02‰ to 0.44‰), but are the same as the pyrite (-0.19‰ to -0.19‰) (Maréchal et al., 1999). In addition, YYR and BYR have lighter δ66Zn in summer than in winter. However, the δ66Zn value of CCR is 0.34‰ in summer, which is isotopically heavier than in winter (0.10‰). For the only draining river of the Aha Lake, XCR has similar δ66Zn value in summer and winter.

    For Hongfeng Lake, THYR and YCR have slightly heavier Zn isotope of 0.40‰ in summer than in winter (0.25‰ and 0.04‰), and the δ66Zn value of MXR in summer (0.30‰) is slight lighter than in winter (0.48‰) in contrast. Moreover the MTR and HLR have similar δ66Zn value in summer and winter. Two algae samples have similar Zn isotope compositions of 0.41‰ and 0.40‰, respectively, and the δ66Zn of algae collected from MXR is 0.21‰.

4.   DISCUSSION
  • The SPM in lake water is mainly supplied by fluvial input, plankton and inorganic materials produced within the lake (autochthonous material), and sediment resuspension (Reimann and Caritat, 2005; Håkanson and Peters, 1995; Sigg et al., 1995). Aha Lake and Hongfeng Lake have surface area of 4.5 and 57.2 km2, water depth of 14 to 24 m and 10 to 45 m, respectively, as well as temperature gradients of > 10 ℃ (Figs. 2 and 3), which implies that wind induced resuspension of sediment will have limited contributions. Furthermore, Aha and Hongfeng both are seasonal anoxic lakes, therefore our discussion focuses on fluvial, plankton and seasonal anoxic controls.

  • The Zn contents were normalized to Al to determine Zn enrichment due to non-detrital inputs since Al concentration is a good indicator of detrital input (Chen et al., 2009). Here we investigate the relation between δ66Zn and Zn/Al (Fig. 5). The Zn/Al ratio of Aha Lake ranges from 0.007 to 0.153 and the average is 0.045, it is much higher than the Zn/Al in Hongfeng Lake (average 0.014 5), and indicating Zn (SPM) is more enriched in Aha Lake than in that Hongfeng Lake.

    Figure 5.  Relation between Zn isotope composition of SPM and Zn/Al ratio for Aha Lake (a) and Hongfeng Lake (b). (a) Green triangles represent data points for SPM of AHDB in summer, red triangles are AHDB in winter, green squares are AHLJK in summer, red squares are AHLJK in winter; in addition, the data of YYR, SR and BYR are plotted, as their discharges are bigger than other rivers. (b) Green circles are data points for SPM of HFHW in summer, red circles are HFHW in winter, red diamonds are HFDB in winter, and data of YCR, THYR and MXR are plotted as they are the main tributaries of Hongfeng Lake.

    For the Aha Lake, including AHDB and AHLJK profiles in summer, a clear negative relationship between δ66Zn and Zn/Al can be observed (Fig. 5). Samples in summer show higher Zn/Al ratio and lighter Zn isotope composition, whereas samples in winter show lower Zn/Al ratio and heavier Zn isotope composition. As the discharge of YYR, BYR and SR is relatively bigger than that of other rivers, the δ66Zn of SPM for Aha Lake may be controlled by these rivers. The discharge of YYR (1.18 m3/s) is the largest of any other rivers, and it is mainly contaminated by coal mine and with bigger discharge and high SPM concentration, displays a higher Zn/Al and lighter Zn isotope in summer, and represents the detrital input from the coal mine, thus the δ66Zn of SPM may be affected by inputting of the YYR with the coal mine. By contrast, SR displays lower Zn/Al ratio and heavier Zn isotope, and represents input from domestic and industrial activities, its δ66Zn (0.05‰ and 0.29‰) are close to anthropogenic samples, ranging from 0.08‰ to 0.31‰ (Chen et al., 2009). Comparing the discharge and concentration of SPM of SR (0.83 m3/s and 78.03 mg/L, respectively) river with BYR (0.90 m3/s and 4.53 mg/L) (Table 2), it is shown that the discharge of them is similar, but the concentration of SPM of SR is almost 18 times higher than BYR. Therefore the main SPM source is likely to be SR, and δ66Zn of SPM is likely to be affected by the inputting of SR with domestic and industrial waste water. Consequently, δ66Zn of SPM for Aha Lake should mainly be affected by mixing of YYR and SR process (Fig. 5).

    We further investigated the relationship between δ66Zn of SPM and the residual Zn of SPM (Fig. 6), since the residual fraction of metals comes mainly from primary and secondary minerals in which trace metals are not expected to be released in solution over a reasonable time under natural conditions (Tessier et al., 1979). Thus, the residual form of SPM may represent the material from background or terrigenous sediment (Turner and Millward, 2002; Ödman et al., 1999; Tessier, 1979).

    Figure 6.  The δ66Zn versus to residual bound Zn of SPM. (a) and (b) refer to AHDB and AHLJK profiles respectively, and (c) and (d) refer to HFHW and HFDB profiles. The green triangle and red triangle refer to the data in summer and winter respectively for AHDB respectively, and the green square and red square refer to the data in summer and winter for AHLJK respectively. The green circle and red circle refer to the date in summer and winter for HFHW respectively and red diamond refer to the date in winter for HFDB.

    We can see clearly that there is linear relationship between Zn isotope composition and residual fraction of Zn in Aha Lake (Fig. 6). There are a positive relationship between Zn isotope composition and a residual fraction of Zn in summer for AHDB, and a negative relationship in summer and positive relationship in winter for AHLJK. Although data are insufficient, and they are not significantly correlated with each other, we still can obtain some information from them. As mentioned above, the main input tributary is SR for AHDB profile, therefore the main SPM source of AHDB is likely to be SR. According to the Zn isotope composition of SR (0.29‰) and AHDB profile (-0.05‰ to 0.19‰), the Zn isotopic composition of SR is heavier than AHDB profile in summer, thus it leads to positive relationship between Zn isotope composition and residual Zn at AHDB. Consequently, we can infer the Zn isotope composition of SPM for AHDB was mainly affected by input of SR. Similarly, YYR was the main source of AHLK profile according the Table 3. Comparing the Zn isotope composition of YYR in summer (-0.09‰) to that in winter (0.46‰), we can obtain that YYR had light Zn isotopes in summer and acts as a heavy Zn isotope source in winter, consistent with a negative correlation in summer and positive correlation between δ66Zn and Zn/Al in winter for AHLJK profile (Fig. 6). Accordingly we can deduce that the Zn isotope composition of SPM for AHLJK is mainly affected by inputting of YYR. These conclusions agree with those from the relation between δ66Zn and Zn/Al of SPM, and further approve that the Zn isotope composition of Aha Lake is mainly affected by SR and YYR, and it is a mixing of end member process.

    By contrast, there is no correlation between Zn/Al and Zn isotope composition of SPM in HFHW and HFDB both in summer and winter and there are no significant variations of Zn/Al ratio for all samples in Hongfeng Lake. Furthermore, there is still no clear correlation between δ66Zn and residual Zn of SPM in Hongfeng Lake.

    The δ66Zn values of MXR, THYR and YCR in summer are isotopically heavier than that in Hongfeng Lake, however there is no clear relationship between tributaries and the Hongfeng Lake (Figs. 5 and 6). Therefore there is no significant effect on Zn isotope composition coming from fluvial input in Hongfeng Lake.

    From above discussion, we accordingly draw the conclusion that Zn isotope composition at Aha Lake was mainly controlled by inputting of YYR with coal mine input and SR with the domestic and industrial particulate input, whereas the Zn isotope composition was not necessarily affected by fluvial inputting for Hongfeng Lake.

  • The δ66Zn values of SPM depth profile above the thermocline at HFHW profile in summer show that surface SPM has the heaviest Zn isotope composition, and δ66Zn gradually decreases with depth. Similarly for AHDB profile, the heaviest δ66Zn of SPM appears at the surface in summer, and then there is a drop at the sub-surface. These similar phenomena that δ66Zn decreased with depth were found for particle samples in the Central Atlantic Ocean (Maréchal et al., 2000), and that δ66Zn of seawater decreased with water depth above 100 m in the Northeast Pacific Ocean (Bermin et al., 2006), which were thought to be mainly related to the activity of phytoplankton. In addition, the δ66Zn of seawater increased with water depth in the North Atlantic Ocean, it was also related to phytoplankton and organic matter (John and Conway, 2014).

    In summer, the lake water is stratified, the temperature, pH and dissolved oxygen decrease with depth, and the algal proliferate in the surface water of Hongfeng Lake (Fig. 3). Therefore, the variation in δ66Zn in Hongfeng Lake may be related to the algal activities.

    Zn isotope fractionation by biological processes occurs by preferential adsorption of the heavy Zn isotope onto the surface of diatoms, and by the preferential incorporation of the light isotope into biological material (Gélabert et al., 2006; Weiss et al., 2005). Hence, we examined whether there was a correlation between Zn isotope composition and chlorophyll, as chlorophyll is an important indicator of primary producers of phytoplankton biomass, and it is the main pigment of photosynthetic phytoplankton (Kasprzak et al., 2008; Reynolds, 1984).

    As is shown in Fig. 7, when the concentration of chlorophyll was low in winter, the Zn isotope composition of SPM was heavy; when the concentration of chlorophyll was high in summer, the Zn isotope composition of SPM was light (Δδ66Znwinter-summer=0.17‰ for Aha Lake, Δδ66Znwinter-summer= 0.07‰ for Hongfeng Lake). Moreover, a significant positive relationship is evident between δ66Zn and chlorophyll at HFHW profile in summer, and there is no relationship between δ66Zn and chlorophyll in AHDB and AHLJK profiles in summer. It is notable that the biomass of phytoplankton in Hongfeng Lake is much higher than in Aha Lake in summer (Fig. 7), suggesting that phytoplankton plays a major role in controlling Zn isotope variability for Hongfeng Lake in summer.

    Figure 7.  The δ66Zn of SPM versus to chlorophyll in Aha and Hongfeng lakes for Aha Lake (a) and Hongfeng Lake (b). The green and red triangles refer to the data in summer and winter for AHDB, respectively; the red squares refer to the data in winter for AHLJK; the green circles and red diamonds refer to the data in summer and winter for HFHW, respectively.

    How the algae affects the Zn isotope composition during the biogeochemical process remains unclear. Maréchal et al. (2000) thought δ66Zn of particle decreasing from surface to bottom might be caused by the activity of phytoplankton and remineralization, and John et al. were aware that the incorporation by phytoplankton mainly accounted for the δ66Zn of seawater increasing with the depth of water (John and Conway, 2014). Here, we can discuss from the absorption and the adsorption processes to explain the Zn isotope variation of SPM for Hongfeng Lake in summer, and to make out which one is the major control factor.

    Firstly, are the Zn isotope composition of SPM in Hongfeng Lake in summer whether affected by process of incorporation into algal? On one hand, as algal incorporation is expected to produce lighter Zn isotope composition of SPM in surface water relative to bottom water according to other studies (Gélabert et al., 2006; Weiss et al., 2005). However, our data show that the Zn isotope of SPM at surface water is heavier than bottom water at HFHW and AHDB in summer (Figs. 2 and 3). Hence it is contradictory that absorption is major control factor on the Zn isotope composition. On the other hand, organic- bound Zn is 12.53% on average, which is much lower than AEC-bound Zn (69.87% on average) (Fig. 4), and this further illustrates that effect of algal absorption process on the Zn isotope is minor than the adsorption process.

    Secondly, the Zn isotope composition was possibly affected by the adsorption onto the surface of algae. AEC-bound Zn accounts for 69.87% of the total Zn (SPM) (Table 2) for HFHW profile, which indicates that the Zn isotope composition of SPM was controlled by adsorption process. Generally speaking, the adsorption processes contain abiotic adsorption onto the mineral particle (goethite, hematite and birnessite) and biotic adsorption onto the surface of phytoplankton (Gélabert et al., 2006; Pokrovsky et al., 2005a, b; Weiss et al., 2005). The Zn isotope fractionation exceeds 0.5‰ from surface water to deeper water at HFHW profile, as Zn isotope fractionation does not exceed 0.5‰ during adsorption onto most mineral particles (Guinoiseau et al., 2016; Pokrovsky et al., 2005b), thus adsorption onto abiotic surfaces was not the main cause for the variation in Zn isotope composition, whereas adsorption onto algae can be the major factor. Zn isotope can be fractionated during preferential adsorption heavy Zn onto diatoms and plankton (Balistrieri et al., 2008; Juillot et al., 2008; Gélabert et al., 2006; Pokrovsky et al., 2005a; Maréchal et al., 2000). This occurs because during adsorption onto diatoms surfaces, Zn reduces its coordination number from six (octahedrally coordinated to H2O in bulk solution) to four (oxygen and nitrogen tetracoordinated complexes), so the bond distance becomes shorter while bond strength increases, and the heavy isotope bonds to species with stronger metal bond energy preferentially (Young and Ruiz, 2003; Criss, 1999). Therefore, an increase of algae led to heavier Zn isotope composition at surface of HFHW in summer (Fig. 3). In addition, the δ66Zn of algae in MXR ranged from 0.21‰ to 0.41‰ (Table 2), and Zn isotope composition of MXR was heavier than in Hongfeng Lake, also it can be explained by adsorption process. Consequently, adsorption onto algae is the major effect factor for the Zn isotope composition in Hongfeng Lake in summer.

    For Aha Lake, algal biomass was relatively small in summer, so there was no relationship between Zn isotope composition and chlorophyll. However, this interpretation remains to be confirmed given that our data were reported firstly for lake water column. In the absence of isotope data on dissolved Zn due to the low concentration, it is premature to argue that whether isotope fractionation between biologic particles and lake water takes place at equilibrium or by purely kinetic control. Therefore, much work is still required to develop a full understanding of the use of Zn isotope in lake biogeochemistry and material recycling processes.

  • So far, we can conclude from sections 4.1 and 4.2 that Zn isotope composition was mainly affected by the tributary input for Aha Lake, whereas the Zn isotope composition for Hongfeng Lake was mainly affected by algal adsorption. However, it is shown in Fig. 3 that the Zn isotope composition of SPM in summer is lighter than in winter for both Hongfeng and Aha lakes. This result is very similar to δ66Zn seasonal variation in SPM from Lake Greifen, Switzerland (Peel et al., 2009), and δ56Fe value of SPM were also lower in summer than in winter in Aha Lake (Song et al., 2011). Nevertheless, both of the tributary input and algal adsorption can't account for this phenomenon. Instead, it implied that the Zn isotope composition of Hongfeng and Aha lakes in summer may be affected by another factor.

    As shown in previous studies Zn isotope can fractionate during process of sphalerite precipitation, and sphalerite preferential incorporate light Zn isotope (Fujii and Albarède, 2012; Fujii et al., 2011; Kelley et al., 2009; John et al., 2008; Wilkinson et al., 2005; Archer et al., 2004). The precipitation of ZnS in an anoxic environment at room temperature can fractionate the Zn isotope, and the Δδ66ZnZnS-dissolved=0.36‰ (Archer et al., 2004); it was also found that the rapid sphalerite precipitation from the fluid or ore system resulted in light Zn isotope (Gagnevin et al., 2012; Kelley et al., 2009; Wilkinson et al., 2005); subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite (Zn sulfide), and this process is a primary cause of Zn isotope variation in hydrothermal fluids (John et al., 2008); the δ66Zn in different species, like aqueous sulfide, chloride, and carbonated species using Ab initio methods, and negative δ66Zn down to at least -0.6‰ can be expected in sulfides precipitated from solution with pH > 9 (Fujii and Albarède, 2012; Fujii et al., 2011).

    Aha Lake and Hongfeng Lake both are seasonal anoxic lakes. The concentration of DO (dissolved oxygen) ranges from 1.60 to 7.80 mg/L, and the average is 4.2 mg/L for Aha Lake in summer, which is much lower than in winter (the concentration of DO range from 7.7 to 9.0 mg/L and the average is 8.6 mg/L). For Hongfeng Lake, the concentration of DO ranges from 1.0 to 8.1 mg/L and average is 3.6 mg/L in summer, which is also much lower than in winter (average is 12.7 mg/L). In addition, the concentration of DO deceases from surface to bottom rapidly for all profiles (AHDB, AHLJK, HFHW and HFDB), and the DO is only 1.0 mg/L at depth of 15 m in HFHW in summer, leading to the oxygen depletion at the bottom of lakes in summer. These seasonal anoxic characteristics also appeared in Baihua Lake and Black Sea (Bai et al., 1996; Sun and Wakeham, 1994). At this anoxic condition in summer, SRB (sulfate reducing bacterial) can reduce the SO42- to S2- (Bailey et al., 2017; Sass et al., 1997), thus Zn can be precipitated from the water, and exist as the species of sphalerite (ZnS) in the SPM, and this can be approved by the concentration of dissolved Zn lower in summer than in winter (Figs. 2 and 3). As discussed above, the sphalerite (ZnS) preferential incorporated the light Zn isotope during the precipitation process, therefore the Zn isotope composition of SPM should be light in summer than in winter, and this conclusion is coupled with our δ66Zn data for both Aha Lake and Hongfeng Lake. This maybe account for why the δ66Zn in summer was lower than in winter.

5.   CONCLUSION
  • This study describes seasonal variations of δ66Zn values for Hongfeng and Aha lakes, as well as data for tributaries and biological samples, and arrives at the following conclusions.

    Concentration of dissolved Zn ranged from 0.65 to 5.06 μg/L and 0.74 to 12.04 μg/L for Aha and Hongfeng lakes, respectively, while the Zn (SPM) ranged from 0.18 to 0.70 mg/g and 0.24 to 0.75 mg/g for Aha and Hongfeng lakes, respectively. The δ66Zn of SPM ranges from -0.29‰ to 0.26‰ for the Hongfeng Lake and its tributaries, respectively, the δ66Zn of SPM ranges from -0.18‰ to 0.27‰ and -0.17‰ to 0.46‰ for the Aha Lake and its tributaries, displaying a wider range in tributaries than lakes.

    From the relation of δ66Zn versus Zn/Al and δ66Zn versus residual-bond Zn, we conclude that Zn isotope composition of Aha Lake is mainly affected by SR and YYR, and it is a mixing of endmember process. According to the relation of δ66Zn versus chlorophyll and proportion of AEC-bond Zn, it is suggested that Zn isotope composition of Hongfeng Lake is mainly controlled by the adsorption process of algae. As sphalerite (ZnS) preferential incorporated the light Zn isotope during the precipitation process, this can account for why the δ66Zn in summer is lower than in winter.

ACKNOWLEDGMENTS
  • We would like to thank Xiaolong Liu, Hu Ding and Li Bai for their helps in the field works, and Suohan Tang, Shizhen Li and Xuexian He for the technical support with MC-ICP-MS analysis. This study has benefited greatly from discussion with Prof. Nyekachi Adele at University of Edinburgh. This research was financially supported by the National Natural Science Foundation of China (No. 40903005). The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0957-8.

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