Selenium speciation in Lower Cambrian Se-enriched strata in South China and its geological implications

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Geochimica et Cosmochimica Acta 75 (2011) 7725–7740 www.elsevier.com/locate/gca

Selenium speciation in Lower Cambrian Se-enriched strata in South China and its geological implications Haifeng Fan, Hanjie Wen ⇑, Ruizhong Hu, Hui Zhao State key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China Received 19 March 2010; accepted in revised form 1 September 2011; available online 22 September 2011

Abstract To understand the impact of Selenium (Se) into the biogeochemical cycle and implications for palaeo-redox environment, a sequential extraction method was utilized for samples including black shales, cherts, a Ni–Mo–Se sulfide layer, K-bentonite and phosphorite from Lower Cambrian Se-enriched strata in southern China. Seven species (water-soluble, phosphate exchangeable, base-soluble, acetic acid-soluble, sulfide/selenide associated, residual Se) and different oxidation states (selenate Se(VI), selenite Se(IV), organic Se, Se (0) and mineral Se(-II)) were determinated in this study. We found that the Ni–Mo–Se sulfide layer contained a significantly greater amount of Se(-II) associated with sulfides/selenides than those in host black shales and cherts. Furthermore, a positive correlation between the degree of sulfidation of iron (DOS) and the percentage of the sulfide/selenide-associated Se(-II) was observed for samples, which suggests the proportion of sulfide/selenide-associated Se(-II) could serve as a proxy for palaeo-redox conditions. In addition, the higher percentage of Se(IV) in K-bentonite and phosphorite was found and possibly attributed to the adsorption of Se by clay minerals, iron hydroxide surfaces and organic particles. Based on the negative correlations between the percentage of Se(IV) and that of Se(-II) in samples, we propose that the K-bentonite has been altered under the acid oxic conditions, and the most of black shale (and cherts) and the Ni–Mo–Se sulfide layer formed under the anoxic and euxinic environments, respectively. Concerning Se accumulation in the Ni–Mo–Se sulfide layer, the major mechanism can be described by (1) biotic and abiotic adsorption and further dissimilatory reduction from oxidized Se(VI) and Se(IV) to Se(-II), through elemental Se, (2) contribution of hydrothermal fluid with mineral Se(-II). Ó 2011 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Recent studies (e.g. Morford and Emerson, 1999; Algeo and Maynard, 2004; Tribovillard et al., 2006) have inferred that some trace elements such as Mn, Cd, Cr, Mo, Re, U, and V can provide important clues to how the redox states of depositional environments and biological productivity have changed over time. Due to their sensitivity to changes under different redox conditions, palaeo-redox environments can be interpreted from variations in concentrations and ratios of these metals in sedimentary rocks (e.g. Lyons et al., 2003; Algeo, 2004). In these elements, selenium (Se)

⇑ Corresponding author. Tel.: +86 851 5891723.

E-mail address: [email protected] (H. Wen). 0016-7037/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.09.027

has been studied in the last few decades as a redox-sensitive element in modern water and sedimentary environments, and has showed the great potential to trace the redox processes (Cutter, 1982; Cutter and Church, 1986, Thomson et al., 2001). In natural environments, Se can exist in four different oxidation states (-II, 0, IV, and VI). Under oxidizing conditions, Se(VI) is favored thermodynamically and forms the selenate (SeO24 ) anion, and is highly soluble and not strongly adsorbing (Neal and Sposito, 1989); In comparison with Se(VI), Se(IV) is more strongly adsorbed to sedimentary constituents such as Fe-, Mn-, and Aloxyhydroxides, because the adsorption process is largely pH- and Eh-dependent (Mikkelsen et al., 1989; Balistrieri and Chao, 1990). The adsorption and subsequent precipitation of Se(VI) and Se(IV) can strongly reduce

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dissolved Se in contaminated water bodies (Zhang and Frankenberger, 2003a,b; Chen et al., 2009), which is an important pathway for removing Se from water to sediment. Another important pathway for Se removal from the water column to sediments is microbial reduction of Se(VI) and/or Se(IV) to insoluble Se species as both elemental Se(0) (Oremland et al., 1989; Zhang and Moore, 1996; Zhang and Frankenberger, 2003a,b; Chen et al., 2006) and metal-selenides such as FeSe and FeSe2 (Belzile et al., 2006). The formation of elemental Se(0) could be from either Se(IV) (Oremland et al., 1989; Kenward et al., 2006; Lee et al., 2007) or Se(VI) (Garbisu et al., 1996; Zhang and Moore, 1996; Hockin and Gadd, 2003), which are commonly involved with different microbial metabolic processes. The reduction sequence in biotic systems is from Se(VI) to Se(IV), and then to Se(0). Besides microbial reduction, the formation of elemental Se can also take place in abiotic processes (Myneni et al., 1997; Charlet et al., 2007; Chen et al., 2009). The most reduced form of Se, selenide(Se(-II)), forms either from reduction of Se(IV), Se(VI) and Se(0) in anoxic sediments and seems to require the presence of bacteria such as Bacillus selenitreducens (Herbel et al., 2003) and Desulfovibrio desulfuricans (Zehr and Oremland, 1987; Nelson et al., 1996). Subsequently, Se(-II) is immobilized on the sediment surface, also within the sediments by free metal ions (Zhang and Frankenberger, 2003b). Different Se species of sedimentary can be determined using the chemical properties of Se, including solubility, exchangeability, volatility, and redox reactivity (Kulp and Pratt, 2004). Chao and Sanzolone (1977, 1989) firstly applied a sequential extraction procedure to discriminate between bio-available and unavailable forms of Se in soils. Lipton (1991) proposed a more comprehensive method to separate the soluble, adsorbed, acid-soluble and organic forms of Se in soils (Lipton, 1991), and the method was also applied by Martens and Suarez (1997a,b) for black shales, the proposed contamination source of soils and wetlands in California. Recently, a more accurate speciation method was proposed to quantify Se species in wetland sediments (Zhang and Moore, 1996; Zhang et al., 1999; Zhang and Frankenberger, 2003a), which permits the separation of the soluble, ligand-exchangeable (or adsorbed), organic matter (OM)-associated, elemental Se(0), and other resistant forms of Se. This method was also utilized to study Se speciation on sediments of the evaporation basins in Tulare Lake Drainage District (TLDD) of the San Joaquin Valley, California (Gao et al., 2007). Furthermore, Kulp and Pratt (2004) argued that previous extraction methods could not distinguish the insoluble Se species, and proposed a method to discriminate seven Se species in upper Cretaceous chalks and shales, including water-soluble, ligandexchangeable, base-soluble, elemental, acetic acid-soluble, sulfide/selenide, and organic fractions. In this study, the sequential-extraction method proposed by Kulp and Pratt (2004) was modified and used to quantify Se chemical speciation in Lower Cambrian Se-enriched strata from Zunyi region, South China, on the basis of seven operationally defined Se fractions. The Se-enriched

strata, located at the late Neoproterozoic/Cambrian boundary, host numerous rock types such as black shales, cherts, phosphorite and K-bentonite, and include an economically significant Ni–Mo–Se sulfide ore layer with up to 2000 ppm Se (Orberger et al., 2007). This study will lead to a better understanding of Se biogeochemical cycling, under specific redox environments, during this critical geological period. 2. GEOLOGICAL SETTING The terminal Neoproterozoic to Early Cambrian successions were well preserved over the Yangtze platform in South China with different paleo-environmental settings. According to current sedimentological studies, four distinct sedimentary facies can be distinguished: carbonate platform, protected basin, Jiangnan uplift and deep oceanic basin (Steiner et al., 2001; Wang and Li, 2003). In several areas these were exposed in the transition from platform to basin, offering an opportunity to study the marine palaeo-environment and biological evolution during the Precambrian to Cambrian interval (Guo et al., 2007). The Lower Cambrian Se-enriched strata, containing unusual metal enrichments including Mo, Ni, Hg, Sb, Ag, Au, PGE and organic materials, was formed along a major deep fault zone in the transition between a Neoproterozoic back-arc basin and Yangtze platform (Steiner et al., 2001; Pasava et al., 2008). The Se-enriched strata of the Lower Cambrian Niutitang Formation discontinuously across a broad region of southern China, throughout the belt of provinces extending from Yunnan in the west through parts of Guizhou, Sichuan, Hunan and Jiangxi, to Zhejiang (Fig. 1). It is interesting that there are a series of economic Ni–Mo–Se sulfide ore deposits formed in a protected basin, and sub-economic barite deposits formed in deeper basins (See Fig. 1, Wang and Li, 1991; Zeng, 1998; Steiner et al., 2001; Emsbo, 2004; Jiang et al., 2007). The Lower Cambrian Niutitang Formation in Zunyi region has been described in detail (Steiner et al., 2001; Luo et al., 2003). The basal Cambrian Se-enriched strata are composed of thin phosphorites of an intertidal facies (Pasava et al., 2008), overlain by volcanic tuff alterated to K-bentonite(Luo et al., 2005). The upper section consists of cherts, intercalated with black shales hosting the Ni–Mo–Se sulfide layer (Fig. 1). The Ni–Mo– Se sulfide layer varies in thickness that can be as much as 30 cm, and consists largely of MoSC phase (a complex mixture of MoS2 and carbonaceous matter nanocrystallites; Orberger et al, 2007), Ni-sulfides (such as vaesite, gersdorffite), pyrite, apatite, quartz, carbonate and clay minerals, i.e., illite (Fan, 1983; Kao et al., 2001). The Ni–Mo–Se sulfide layer commonly contains 2–4 wt.% Mo (the highest value be up to 7 wt.%), 2 wt.% Ni, 2000 ppm Se (Orberger et al., 2007) and 1 ppm of total PGE (in particularly, Os, 137  10 9; Pt, 424  10 9 and Pd, 411  10 9, Mao et al., 2002). The sulfide layer was initially dated to Late Ediacaran and/or Early Cambrian based on arthropods and sponges (Steiner et al., 2001), consistent with Pb–Pb isochron ages of 531 ± 24 Ma (Jiang et al., 2006) and ReOs (541 ± 16 Ma; Mao et al., 2002; 560 Ma; Horan et al.,

Selenium speciation in lower Cambrian Se-enriched strata

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Fig. 1. Distribution of Se-enriched strata and Ni–Mo–Se sulfide layer within the Lower Cambrian Niutitang Formation or it’s Stratigraphic equivalents on the Yangtze Platform, southern China (modified after Steiner et al., 2001) and Stratigraphic column of Xiaozhu section from Zunyi region in Guozhou Province.

1994). The newest zircon U-Pb age of K-bentonite (532.3 ± 0.7 Ma) defined the age of sulfide layer to be Early Cambrian(Jiang et al., 2009). These studies also suggested that the age of the sulfide layer is similar to the host black shales. However, the formation mechanism for the Ni–Mo– Se sulfide ore layer is still strongly debated, with its possible origins being sorption from normal seawater (e.g. Mao et al., 2002; Lehmann et al., 2003, 2007) or from seawater enriched in metals through hydrothermal fluid (Lott et al., 1999; Steiner et al., 2001; Coveney, 2003; Jiang et al., 2007; Orberger et al., 2007; Pasava et al., 2008).

3. SAMPLING AND METHODS 3.1. Sampling The Xiaozhu section in Zunyi region was selected for this study, which represent the typical Lower Cambrian formation in south China. Thirty-one samples were collected from this section, including phosphorite, K-bentonite, black shales, carbonaceous cherts, and the Ni–Mo–Se sulfide ore layer (Fig. 1). Prior to geochemical analyses, the weathered surface was removed from each sample, and cleaned with distilled water and dried, then crushed to 200 meshes. To prevent oxidation, the sample powder was preserved in airtight glass bottles. All preparation and instrumental analyses were conducted at the State key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry of Chinese Academy of Sciences.

3.2. Determination of Se, organic carbon (Corg) and Sulfur (S) Selenium abundance was determined by continuous flow hydride-generation Atomic Fluorescence Spectrometer (HG-AFS810) under a linear standard range between the detection limits of 1 mg L 1 and 100 mg L 1. Solutions of Se were prepared using a modified protocol proposed by Marin et al. (2003) which was estimated for standard reference material and geological samples in previous study (Fan et al., 2008). First, 0.2 g of sample was weighted in a polyfluortetraethylene beaker, and dissolved in mixture solution of 10 mL HNO3, 5 mL HF and 2 mL HClO4. The solution was then heated at less than 90 °C until brown NO2 fumes were replaced by white perchloric acid fumes for 20 min. Second, 2 mL of 30% H2O2 was added after cooling and the beaker was then reheated until the white perchloric acid fumes again appeared. This process ensures that the organic materials are fully digested, and all of the added HF, H2O2, and HNO3 are evaporated to initial drying. In addition, these two steps also convert all of the dissolved Se species to Se(IV) or Se(VI). Finally, the solutions were cooled to room temperature and diluted to 25 mL with distilled water. 0.5–1 mL aliquot was mixed with same the volume of HCl (12 mol L 1) in PFA sealed bottle. Then the mixture solution was heated for 1 h at 100 °C in a water bath in order to convert all of Se(VI) to Se(IV) (Fan et al., 2008), and then diluted to 10 mL. This step is necessary for measuring Se concentration by HG-AFS, which is only sensitive to the Se(IV) fraction.

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Corg and S content were measured by a C–S element analytical instrument (CS-314). 3.3. Determination of degree of sulfidation The degree of sulfidation (DOS) is defined as the ratio of sulfidic iron (FeS) to total reactive iron (FeR), is the latter being the sum of FeS and HCl soluble Fe (FeA)(Kao et al., 2004a,b). The index is similar to the degree of pyritization (DOP) used by Goldberg et al. (2005, 2007), and has been used widely as a useful proxy for evaluating the redox conditions of iron sulfide mineral formation (Kao et al., 2004a,b). In this study, the procedure by Kao et al. (2004a) was used. FeA was extracted from the samples by 1 mol L 1 HCl for 16 h at room temperature. The residue was further treated with a mixture of hydroxylamine hydrochloride and acetic acid for 6 h at 75 °C, followed by a mixture of hydrogen peroxide and nitric acid for 2 h, and finally was stirred overnight with ammonium acetate solution to extract sulfidic iron (FeS). The concentrations of these iron species were determined by the Atomic Absorption Spectrometry. 3.4. Sequential extraction method We developed a seven-step procedure for Se extraction modified from Kulp and Pratt (2004) outlined as follows: (1) extraction of water-soluble Se with distilled water. This includes Se in the forms of Se(VI)O24 and Se(IV)O23 , and some water-soluble organic Se; (2) extraction of exchangeable Se from the residue using a mixture of 0.1 mol L 1 K2HPO4–KH2PO4 (pH = 7.0) buffer (P-buffer). This is the Se associated with oxides and clay particles; (3) extraction of base-soluble Se using 0.1 mol L 1 NaOH; (4) extraction of elemental Se (0) by 1 mol L 1 Na2SO3 (adjusted to pH of 7.0 with HCl); (5) extraction of Se in carbonate using 15% acetic acid; (6) extraction of sulfide/selenide Se(-II) using a mixture of 0.5 g KClO3 solid powder and 20 mL of 12 mol L 1 HCl (modified from Chao and Sanzolone, 1977), which is compared with volatilization of the sulfide/selenide Se(-II) by acidified CrCl2 (according Kulp and Pratt, 2004; see Supplementary materials); and finally (7) extraction of Se from the final residue using a mixture of HNO3, HF and HClO4. 3.5. Speciation treatments After the sequential extraction, Se in different oxidation states and organic forms in the water-soluble, phosphate exchangeable and base-soluble fractions was further separated following the standard procedure given by Kulp and Pratt (2004) and the other workers (Martens and Suarez, 1997a,b; Zhang et al. 1999; Zhang and Frankenberger, 2003a,b; Gao et al., 2007). The concentration of Se(IV) was determined directly using HG-AFS-810 after diluting one aliquot of the extract with 4% HCl. The concentration of Se(VI) was then calculated from the difference between that of Se(IV) and the sum of Se(IV) and Se(VI), which was measured after heating another aliquot mixing with 12 mol L-1 HCl for one hour, reducing the Se(VI) to Se(IV). Finally, the amount of org-Se(-II) in the extracts was

estimated from the difference between the concentration of total Se and the sum of Se(IV) and Se(VI). Total Se in each sequence extraction was measured after an aliquot of the solution was first oxidized to convert all Se to Se(VI) and then reduced to Se(IV). The procedure was also used to determine the concentrations of Se(-II) in sulfide/selenide and elemental Se(0) from the other extracts and the final residues. 4. RESULTS 4.1. Se, Corg, S and Fe The results for total Se, Corg, S, and different iron species contents are presented in Table 1 for the thirty-one samples. Se was mostly enriched in the Ni–Mo–Se sulfide layer (average, 2081 ppm), similar to values of 1000–2000 ppm reported by Orberger et al. (2007), the black shales (84.19 ppm) and K-bentonite (131.18 ppm). There was no obvious difference between carbonaceous cherts (28.86 ppm) and phosphorite (20.36 ppm). Similar to Se, the S content is highest in the Ni–Mo–Se sulfide layer (21.3%), and the other samples contain equivalent S content (0.1–1.5%). The ratios of Se/S from all samples (0.6– 46.5  10 3) are several orders of magnitude higher than that of seawater (10 7) (Measures and Burton, 1980) and apt to that of hydrothermal fluid (1.5  10 4) (Huston et al., 1995). The Corg content varied from 3.3% to 20.2% for black shales (average 11.5%), 3.4–8.9% for carbonaceous cherts (average, 6.7%), 9.2–11.6% for the Ni–Mo–Se sulfide layer, 0.1% for K-bentonite and 1.0% for phosphorite. The ratios of Corg/S are lower for the Ni–Mo–Se layer (average 0.5) and K-bentonite (0.2) than for the normal marine value (2.8, defined by Berner and Raiswell, 1983), and are higher for other sample types (XZ-30 only excluded). In addition, the highest concentration of total iron was also found in the Ni–Mo–Se sulfide layer, whereas the black shales contained a greater proportion iron (2.1%) than cherts (0.7%). Excess reactive iron and higher DOS values are also present in the Ni–Mo–Se sulfide layer. 4.2. Sequential extraction For sequential extraction, total Se recovery of all seven fractions ranged from 81.15–114.94% (102.91% on average). The distributions of Se species along the studied section are shown in Tables 2–4. Se fractions extracted by water and the P-buffer extractions should represent the most bio-available forms of Se, including Se(VI) from evaporate salts, Se(IV) from surface sites on charged particles, and soluble Se(-II) from proteins (Martens and Suarez, 1997a). In our samples a lower percentage of water-soluble Se was found in K-bentonite (0.97%) and phosphorite (2.76%).The percentage of water-soluble Se in black shales (varying greatly from 1.13% to 11.80%, average 5.04%), cherts (7.32% average), and the Ni–Mo–Se sulfide layer (5.02% average) were similar. Table 3 shows that the forms of Se mobilized by the Pbuffer extraction included both absorbed Se(IV, VI), and organic Se. Se mobilized by the P-buffer extraction

Table 1 Results of major elements (SiO2, Fe, TOC, S, Se, Mo, Ni) and degree of sulfidation (DOS). Lithology

SiO2 (%)

TOC (%)

Total S (%)

Se (ppm)

Mo(ppm)

Ni (ppm)

Fe (%)

FeA (%)

FeS (%)

DOS

Corg/S

Se/S (10 3)

XZ-01 XZ-03 XZ-05 XZ-07 XZ-081 XZ-082 XZ-083 XZ-084 XZ-085 XZ-086 XZ-09 XZ-10 XZ-11 XZ-12 XZ-13 XZ-14 XZ-15 XZ-16 XZ-17 XZ-18 XZ-19 XZ-20 XZ-21 XZ-25 XZ-26 XZ-27 XZ-28 XZ-29 XZ-30 XZ-31 XZ-32

Black shale Black shale Black shale Black shale Ni-Mo ore Ni-Mo ore Ni-Mo ore Ni-Mo ore Ni-Mo ore Ni-Mo ore Black shale Black shale Black shale Black shale Carbonaceous Black shale Black shale Carbonaceous Black shale Carbonaceous Black shale Carbonaceous Black shale Black shale Carbonaceous Black shale Carbonaceous Carbonaceous Black shale K-Bentonite Phosphorite

63.7 61.6 60.7 58.4 24.8 18.4 27.2 10.8 8.6 11.3 67.7 70.1 65.3 60.8 80.9 57.9 53.5 91.6 50.1 89.3 51.7 83.1 47.2 45.6 80.4 48.1 91.7 84.58 55.7 56.14 14.54

4.5 4.2 16.9 17.5 11.1 9.2 11.5 11.3 11.6 10.7 7.7 8.1 10.8 13.5 8.9 13.8 14.1 4.8 15.1 5.7 15.9 8.9 16.8 18.6 8.5 20.2 3.4 4.2 0.2 0.1 1.0

1.2 1.1 0.6 0.2 12.8 22.5 20.7 23.2 26.7 22.2 1.5 1.4 1.2 1.1 0.2 1 1.2 0.2 0.4 0.2 0.3 0.2 0.3 0.5 0.2 0.3 0.1 0.1 0.3 0.7 0.2

7.14 4.92 22.05 18.13 1403 2457 1479 1700. 2849 2599 97.14 117.94 107.08 143.52 49.36 164.44 141.00 15.52 105.96 25.39 116.22 36.03 135.50 98.02 20.01 53.00 8.87 47.00 14.80 131.18 20.36

74 44 159 259 30030 65950 52299 45624 65823 58368 237 133 152 646 218 121 329 33 681 71 845 172 737 2103 241 1411 131 689 394 121 65

182 147 154 234 27570 36420 36546 59017 23437 23467 221 188 193 232 82 173 157 34 185 44 210 88 213 775 81 188 33 89 64 22 196

3.4 2.7 1.2 1.5 7.9 10.6 5.8 8.8 4.7 6.9 2.2 1.9 2 2 1.3 1.5 2.2 0.4 2.8 0.4 2.6 0.7 3.3 2.5 0.7 1.8 0.7 1.9 1.3 0.8 3.1

0.4 0.4 0.2 0.4 0.4 0.2 0.8 0.9 1 1.4 0.3 0.3 0.3 0.4 0.3 0.8 1.1 0.1 1 0.2 0.8 0.2 1.7 0.5 0.1 0.7 0.1 0.1 0.6 0.3 1.6

1.8 1.3 0.6 1.1 2.9 3.7 3.9 2.9 3.5 4.6 1.5 0.9 1.1 1.2 0.6 0.6 1 0.1 1.4 0.3 1.4 0.1 1.2 1.8 0.3 0.7 0.1 0.4 0.3 0.2 1.5

0.83 0.81 0.58 0.75 0.89 0.94 0.8 0.79 0.78 0.79 0.83 0.8 0.77 0.73 0.63 0.44 0.48 0.58 0.6 0.64 0.65 0.45 0.42 0.62 0.68 0.5 0.36 0.78 0.32 0.39 0.48

3.6 3.9 66.5 72.3 0.9 0.4 0.6 0.5 0.4 0.5 5.3 6 9.2 11.9 49.2 14.3 11.7 28.1 35 26.8 63 57.9 52.4 37.2 55.2 72.5 38.4 30.9 0.5 0.2 5.7

0.6 0.5 8.8 7.6 11 10.9 7.1 7.4 10.7 11.7 6.7 8.7 9.1 12.8 27.4 17 11.8 9.1 24.6 12.1 46.5 24 42.3 19.6 13.3 18.9 9.9 36.2 4.5 19 12

chert

chert chert chert

chert chert chert

Selenium speciation in lower Cambrian Se-enriched strata

Sample ID

FeA: HCl soluble Fe; FeS: sulfidic iron.

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7730 Table 2 Se concentration (ppm) among various extraction of water soluble(A), P-buffer solution(B), NaOH(C), Na2SO3(D),acetic acid(E), mixture of 0.5 g KClO3 solid powder and 20 mL of 12 mol L 1 HCl (F) and mixture of HNO3, HF and HClO4 soluble forms(G). Recovery percentages represent the sum of sequentially extracted values compared to total Se values obtained by whole rock digestion. P Sample ID A B C D E F G Se recov Total Se recovery Se(VI)

org-Se

Se(IV)

Se(VI)

org-Se

Se(IV)

org-Se

Se(0)

mg kg

0.26 0.2 1.41 0.12 16 14.77 3.23 2.41 6.46 2.78 0.7 0.86 0.7 0.62 1.61 0.56 2.02 0.57 1.42 0.36 1.59 1.163 3.11 2.86 0.87 0.42 0.35 2.93 0.20 0.22 0.37

0.19 0.11 0.69 0.09 34.93 38.06 85.8 52.04 35.41 34.19 3.57 4.82 3.28 2.8 1.26 3.25 2.64 0.73 0.69 0.53 0.94 0.58 0.78 4.87 0.45 1.92 0.22 2.08 0.94 1.07 0.17

0.14 0.08 0.81 0.01 6.68 15.22 30.35 77.72 50.34 30.16 1.23 0.13 0.96 0.53 1.04 0.42 1.03 0.6 1.01 0.34 0.65 0.007 0.75 0.84 0.32 0.06 0 0.34 0.07 0 0.04

0.71 0.52 4.03 2.69 82.74 92.1 41.25 110.28 110.13 163.74 8.12 5.62 4.23 12.03 9.15 11.95 11.73 1.47 8.38 1.34 6.15 2.14 9.4 13.48 1.26 11.72 0.66 15.02 5.51 11.36 0.72

0 0.2 0 0.75 0 0 0 0 33.63 0 2.04 1 0.97 1.18 2.83 4.31 2.73 0.28 2.26 0.13 0 0.51 2.06 4.77 0.23 4.18 0.09 1.34 0.00 3.8 0

0.06 0 0.31 1.93 10.98 23.88 45.51 119.28 81.08 54.36 5.96 4.83 2.87 6.81 3.58 6.59 4.1 0.53 3.99 2.47 0.73 0.91 6.24 0.00 0.91 5.26 0 1.29 0.63 1.37 0

1.11 1.04 9.34 8.45 86.66 115.54 34.26 168.95 307.22 284.83 11.12 17.06 17.89 35.14 11.85 75.51 65.3 6.13 15.28 3.41 27 9.84 64.34 18.47 6.58 26.48 4.48 21.73 7.97 11.03 12.67

0.03 0 0.72 0.76 92.19 90.04 158.92 96.38 101.91 153.56 8.82 1.71 0.92 8.35 0.88 12.36 13.07 0 10.84 0.46 10.37 0 3.03 9.49 0 0.19 0.3 0.63 0.00 97.26 0.28

0.006 0.006 0.15 0.12 19.29 33.93 24.82 27.65 78.3 36.33 3.44 3.98 3.96 3.96 0.13 2.75 1.46 0.05 1.19 0.22 0.49 0.09 0.94 2.00 0.04 0.14 0 0.12 0.00 3.14 0.22

0.21 0.08 0.52 0.29 20.24 22.68 19.31 13.25 3.1 5.08 2.22 1.38 1.36 2.1 0.1 2.15 1.17 0 0.9 0.3 0.37 0.02 0.34 1.11 0.1 0 0 0 0.00 0.19 0

1

1

Se(-II)

org-Se

mg kg

4.09 3.54 3.22 4.16 901.96 1754.03 811.07 848.2 1294.32 1432.24 51.78 62.13 58.88 60.13 5.88 37.77 19.57 4.41 56.18 10.42 64.32 12.4 40.79 21.57 7.03 5.02 2.41 5.14 1.26 3.1 6.16

0.57 0.35 3.47 0.06 82.9 203.46 180.59 139.31 210.36 245.97 4.67 10.09 5.76 11.88 15.03 22.01 12.07 2.57 13.36 6.92 15.39 9.56 11.49 14.34 5.21 2.82 0.67 1.03 0.36 0.53 0.41

7.376 6.126 24.67 19.43 1354.57 2403.71 1435.11 1655.47 2312.26 2443.24 103.67 113.61 101.78 145.53 53.34 179.63 136.89 17.34 115.5 26.9 128 37.22 143.27 93.8 23 58.21 9.18 51.65 16.9 133.07 21.04

mg kg 7.14 6.71 22.05 18.13 1403.21 2457.49 1479.34 1700.05 2849.27 2599.36 97.14 117.94 107.08 143.52 49.36 164.44 141.00 15.52 105.96 25.39 116.22 36.03 135.50 98.02 20.01 53.00 8.87 47.00 14.80 131.18 20.36

1

(%) 103.31 91.30 111.88 107.17 96.53 97.81 97.01 97.38 81.15 93.99 106.72 96.33 95.05 101.40 108.06 109.24 97.09 111.73 109.00 105.95 110.14 103.30 105.73 95.69 114.94 109.83 103.49 109.89 114.19 101.44 103.34

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XZ-01 XZ-03 XZ-05 XZ-07 XZ-0801 XZ-0802 XZ-0803 XZ-0804 XZ-0805 XZ-0806 XZ-09 XZ-10 XZ-11 XZ-12 XZ-13 XZ-14 XZ-15 XZ-16 XZ-17 XZ-18 XZ-19 XZ-20 XZ-21 XZ-25 XZ-26 XZ-27 XZ-28 XZ-29 XZ-30 XZ-31 XZ-32

Se(IV)

Table 3 Average proportion of total Se mobilized in each step of the sequential extraction method. Black shale

Ni-Mo ore

Bentonite

Phosphorite

Average (mg kg 1)

Mean Percent

Percent range

Average (mg kg 1)

Mean percent

Percent range

Average (mg kg 1)

Mean percent

Percent range

Average (mg kg 1)

Mean Percent

Average (mg kg 1)

Mean percent

3.59 12.07 30.15 1.64 1.01 30.96 8.04

5.04 16.47 33.28 1.37 1.15 34.58 8.12

1.13–11.80 5.38–36.35 15.46–48.91 0.08–3.32 0.00–2.85 7.43–57.85 0.31–15.68

2.34 6.59 9.47 0.11 0.13 6.81 5.86

7.32 17.04 32.00 0.29 0.26 25.04 18.05

4.57–10.96 9.56–34.17 14.39–54.07 0.00–0.81 0.00–1.11 10.00–38.74 2.00–28.18

87.62 189.82 278.83 36.72 13.94 1151.97 177.10

5.02 9.82 14.48 1.85 0.82 60.32 9.12

2.83–8.31 4.82–15.55 8.55–17.94 1.41–3.39 0.21–1.49 51.24–72.97 6.12–12.58

1.29 16.53 108.29 3.14 0.19 3.1 0.53

0.97 12.42 81.37 2.36 0.14 2.33 0.39

0.58 0.72 12.95 0.22 0 6.16 0.41

2.76 3.42 61.55 1.05 0 29.28 1.95

Table 4 Average proportions of each chemical species and type of organic Se mobilized during sequential extraction. Black shale

Se(IV) Se(VI) Se(0) Mineral Se(-II) Extractable org-Se Kerogen org-Se

Carbonaceous chert

Ni-Mo ore

Bentonite

Phosphorite

Average (mg kg 1)

Mean percent

Percent range

Average (mg kg 1)

Mean percent

Percent range

Average (mg kg 1)

Mean percent

Percent range

Average (mg kg 1)

Mean percent

Average (mg kg 1)

Mean percent

33.43 3.63 1.64 30.96 9.62 8.04

41.21 4.45 1.37 34.58 9.95 8.12

19.23–80.75 0.73–10.48 0.08–3.32 7.43–57.85 2.24–17.58 0.31–15.68

14.7 1.61 0.11 6.81 2.16 5.86

45.47 4.55 0.29 25.04 6.6 18.05

18.99–59.80 1.78–7.67 0.00–0.81 10.00–38.74 2.51–13.27 2.00–28.18

273.89 52.34 36.72 1151.97 220.37 177.10

13.8 3.04 1.85 60.32 12.35 9.12

6.03–18.47 1.40–6.57 1.41–3.39 51.24–72.97 6.31–19.47 6.12–12.58

109.21 0.66 3.14 3.1 13.92 0.53

82.06 0.49 2.36 2.33 10.46 0.39

13.76 0.17 0.22 6.16 0.32 0.41

65.4 0.81 1.05 29.28 1.52 1.95

Selenium speciation in lower Cambrian Se-enriched strata

Water soluble Ligand exchangeable Base soluble Elemental Acetic Acid soluble Sulfide/Selenide Associated Se Residual

Carbonaceous chert

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accounted for 5.38–36.35% of the Se budget for black shales, 9.56–34.17% for cherts, 4.82–15.55% for the Ni– Mo–Se sulfide layer, 12.42% for K-bentonite and 3.42% for phosphorite. The percentage of base-soluble Se extracted by NaOH solution varied greatly (8.55–81.37%). Black shales contain 15.46–48.9% of the recovered Se inventory as base-soluble forms, compared to 14.39–54.07% for cherts. The lowest percentage (8.55–17.94%) of the base-soluble forms was in the Ni–Mo–Se sulfide layer, and the highest value was in K-bentonite (81.37%) and phosphorite (61.55%). Black shales (except sample XZ-09, 17) and cherts contain a low concentration of Se extracted by NaOH solution as base-soluble organic compounds. Compared to cherts and black shales, the Ni–Mo–Se sulfide layer contains higher proportion of org-Se associated with dissolved humic compounds (on average 45.65% of the total base-soluble forms, Table 2). The percentage of elemental Se extracted by sodium sulfite solution was similar for all samples. Elemental Se accounts for 0.08–3.32% for black shales, 0.00–0.81% for cherts, 1.41–3.39% for the Ni–Mo–Se sulfide layer, 2.36% for K-bentonite, and 1.05% for phosphorite. Similar to sodium sulfite solutions, relatively little Se was found as the acetic acid-extractable form in all sample types. Black shales contain <2.85% of Se recovered by acetic acid extraction, cherts contain <1.11%, Ni-Mo sulfide layer contains 0.21–1.49%, K-bentonite contains 0.14%, and phosphorite contains none. The Ni–Mo–Se sulfide layer contains a greater proportion (60.32% average) of Se in the sulfide/selenide phase than all other sample types. Black shales contain 7.43– 57.85% of the recovered Se in sulfide/selenide fraction, cherts contain 10.00–38.74%, and K-bentonite and phosphorite contain 2.33% and 29.28% of Se substituted for S in sulfide or diselenide minerals. Se associated with the last residual material accounted for 0.31–15.68% of the black shales, 2.00–28.18% of the cherts, 6.12–12.58% of the Ni–Mo–Se sulfide layer samples, and a small fraction of K-bentonite (0.4%) and phosphorite (1.95%). Se associated with residual kerogen is the predominant form of organic Se in cherts (73.23% on average) and phosphorite (56.20%). By contrast, black shales, the Ni– Mo–Se sulfide layer and K-bentonite contain a lower average proportion of residual Se than extracted organic-Se. Black shales and cherts contain a larger proportion of Se(IV) (41.21% and 45.47% on average, respectively) than the Ni–Mo–Se sulfide layer samples(13.8%, average). However, the Ni–Mo–Se sulfide samples contain a larger 60.32% of the sulfide/selenide Se(-II) than all other samples. Compared to all samples, K-bentonite contains the highest Se(IV) content (81.79%) and lowest Se(-II) as the sulfide/ selenide form (2.33%). The phosphorite contains 65.40% of Se(IV) and 29.28% of sulfide/selenide Se(-II). 5. DISCUSSION 5.1. Distribution of Se speciation and chemical state The percentage of water-soluble Se has been used as an index of the degree of weathering (Martens and Suarez, 1997a;

Kulp and Pratt, 2004). The lower percentage of water-soluble Se in all samples (0.97–11.80%) is consistent with the results from chalks and shales obtained by Kulp and Pratt (2004) and from black shales obtained by Zhu et al. (2007), suggesting no weathering in our samples. In water-souble Se, Se(IV) and Se(VI) are dominated. The oxidized speciation is dominated by Se(IV) in most of studied samples which suggests that adsorption of Se(IV) by clay minerals is as important to its sequestration as adsorption by organic matter. This was also identified by Bruggeman et al. (2005) who found that the Se speciation in Boom clay minerals is dominated by sorption of Se(IV). We also found that there are greater proportions of the sum of oxidized species Se(IV) and Se(VI) in K-bentonite (100%) and phosphorite (93.10%) compared to Ni-Mo sulfide layer (average 63.72%), which is explained by different contents of clay minerals in these samples. It is reasonable to interpret these observations as resulting from Se speciation under different sedimentary conditions. Se fractions extracted by the water-soluble and P-buffer extractions should represent the most bio-available forms of Se (Martens and Suarez, 1997a). For our samples, the bio-available forms of Se account for 21.51% and 24.37% of the total recovered Se inventory for black shales and cherts, respectively. In addition, the proportion (66.98% for the black shale, 65.70% for the chert and 64.55% for the Ni-Mo sulfide layer) of Se(IV) to total Se for the P-buffer extracted solution in this study is similar to the value of reduced sediments determined by Gao et al. (2000), where 70% of total extracted Se was identified as Se(IV). The quantitative estimate of Se(IV) in the Se-enriched strata indicated that reduction of Se(VI) to Se(IV) under reducing conditions followed by adsorption onto the sediment and/ or incorporation into organic matter is an important pathway for removing Se from water to being immobilized in sediment (Gao et al., 2007). The amount of Se in the NaOH extraction for black shales (20.3–57.3%, except for XZ-09, -11) and cherts (23.9–52.1%, except for XZ-18) is consisted with previous results of the Moreno and Kreyenhagen shales from California (48.9–63.6%) (Martens and Suarez, 1997a), Smoky Hill and Sharon Springs Member shales from South Dakota and Wyoming (average 34.9%) (Kulp and Pratt, 2004), black shales and cherts from the Yutangba (34.6–42.1%, 20.7–30.1%) and Zunyi areas (26.5–29.8%, 45.7–50.9%) in South China (Zhu et al., 2007), and sediments from two operating evaporation basins located in Tulare Lake Drainage District in the San Joaquin valley (33–49%) (Gao et al., 2007). This observation implies that the extent of bio-absorption/adsorption is quite stable for modern and ancient sediments. For black shales and cherts, the basesoluble organic-Se determined by NaOH extraction is one small component of total organic-Se inventory, most of which is likely assimilatory (Table 2). This differs from previous studies in which most of the Se extracted by NaOH solution was found to be present in association with base-soluble organic compounds in shale, chalk and soil samples (Martens and Suarez, 1997a; Kulp and Pratt, 2004). The reason still remains unclear, but possibly is related to the thermally mature degree of the Se-rich organism during diagenetic processes.

Selenium speciation in lower Cambrian Se-enriched strata

The percentage of base-soluble Se was lowest (average 14.48%) for the Ni–Mo–Se sulfide layer. However, the samples contain predominantly base-soluble Se as organic compounds, which suggest that more humic compounds from thermally alteration of the Se-rich kerogen are present. This interpretation is supported by Kribek et al (2007) who reported that there are higher amounts of extractable organic materials and a higher occurrence of migrabitumens in the Ni–Mo–Se sulfide layer compared to black shales. In their study, the Ni–Mo–Se sulfide layer is interpreted as representing a remnant of phosphate- and sulfide-rich subaquatic hardground supplied with organic matter derived from plankton and benthic communities as well as algal/ microbial oncolite-like bodies. By contrast, the black shales contain only organic particles interpreted as remnant of in situ bacterially-reworked organic matter derived from cyanobacteria/algae (Krı´bek et al., 2007). This might also explain why there is higher residual Se concentration in the Ni–Mo–Se sulfide layer than that of black shales, although the percentage of residual Se is equivalent in the two sample types. The Se form in the final residue should be kerogen-bound organic Se (Kulp and Pratt, 2004) and/ or Se associated with silicate minerals (Zhang et al., 1997). The lower amount of silicate mineral (SiO2 < 27.2%) and higher contents organism (TOC > 9.2%) in the Ni–Mo–Se sulfide layer suggested that most of the residual Se is associated with kerogen. The residual Se associated with kerogen would represent assimilatory incorporation into algae or microbes. There is a good correlation between the percentage of residual Se and TOC, except that the slope of the relation is different for the black shales and cherts (Fig. 2). In general, the average percentage of residual Se is higher for carbonaceous cherts when compared to black shales with the higher TOC concentrations. The distinction may be explained by different kinds of organic matter in the two sample types, and that more Se maybe associated with biogenic silicon in the carbonaceous cherts. This interpretation needs to be tested with further research. The lower residual Se

Fig. 2. Relationship between the percentage of total organic carbon(TOC) and residual Se associated with kerogen. The samples XZ-07, 27 were removed.

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concentration in K-bentonite and phosphorite at the bottom of the Niutitang Formation corresponds with lower silicate mineral and kerogen fractions in the two sample types. In a number of previous studies (Oremland et al., 1989; Garbisu et al., 1996; Claudia et al., 2003; Siddique et al., 2006), large amounts of Se(0) were found in modern sediments from Benton lake on western edge of the Northern Great Plains (up to 60%, Zhang and Moore, 1996) and the Tulare Lake Drainage District in California (up to 46%, Gao et al., 2007), primarily as the product of the microbial respiration of Se oxyanions during the degradation of organic matter under anoxic conditions (Oremland et al., 1989; Garbisu et al., 1996). This is consistent with the experimental observation that the rate of the reduction from Se(VI) and Se(IV) to elemental Se(0) is twice that for the transformation of Se(VI) to org-Se(-II) (Zhang and Moore, 1997). Kulp and Pratt (2004), however, found that the amount of Se(0) is lower (17%) in sedimentary rocks in Smoky Hill and Sharon Springs Member from South Dakota. There is less (average 1.24%) elemental Se (0) in our samples. The absence of Se(0) within the Niutitang Formation could be interpreted as elemental Se(0) being precipitated initially in sediments, and then further reduced to stable speciation as Se(-II) by the microbial action in anaerobic sediments (Herbel et al., 2003). However, it is difficult to determine if formation of elemental Se(0) resulted from biotic or abiotic redox processes. We found a positive correlation between the percentages of Se(0) and the total S content in our samples (Fig. 3), which might reflect the role played by sulfate-reducing bacteria in reducing Se(IV) and Se(VI) to elemental Se(0). This interpretation appears to be supported by other studies in which high amounts of SO42- accelerated reduction of Se(VI) to Se(0) (Zhang and Frankenberger, 2003c). The deviation from the trend for the Ni–Mo–Se sulfide layer reflects the excess input of other S sources, possibly hydrothermal or other(Taavitsainen et al., 1998; Wiberg et al., 2001). A high content of Se(-II) associated with sulfides and selenides was present in the Ni–Mo–Se sulfide layer. At present, it is difficult to determine whether these high concentrations resulted from biological processes or abiotic redox transformations. Although Se(VI) can be reduced to Se(-II) by the sulfate-reducing bacterium D. desulfuricans, this reaction was thought to be nonsignificant in the biogeochemical cycling of Se in anoxic sediments (Zehr and Oremland, 1987). Recently, Herbel et al. (2003) observed that certain anaerobic bacteria can reduce Se(0) to Se(-II) during the metabolism of organic matter, and this may provide an explanation for the greater proportion of Se(-II) with sulfides/selenides in our samples. Conversely, Myneni et al. (1997) argued that high-valence Se forms can also be reduced to elemental Se(0) under suboxic environments and eventually to Se(-II) under anoxic condition by the inorganic Fe(II, III) oxides such as green rust (GR) present in sediments. Although only the reduction of Se(VI) to Se(IV) and Se(0) was observed in laboratory experiments by Myneni et al. (1997), the possibility for a dominant Se(-II) species under a higher concentration of Fe(II) was not ruled out because the experiment was only conducted at low Fe(II) concentration over a limited time (several

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5.2. Se geochemical cycling and Early Cambrian palaeoredox conditions

Fig. 3. Relationship between the percentage of elemental Se(0) and total S content. XZ-01, 03 were removed.

hours). This reaction in solution, under anoxic conditions and relatively low pH values with Se(-II) as the stable species, has been also predicted from thermodynamic calculations (Seby et al., 1998). However, there was no obviously correlation between the Se(-II) proportion and the Fe (II)/Fe (III) ratios observed. We therefore proposed that the abiotic redox processes play a minor role in enrichment of Se(-II) in Ni–Mo–Se sulfide layer. The distribution of Se(-II) with sulfides/selenides is consistent with the results analyzed by electron microprobe (Pan et al., 2005; Orberger et al., 2007). Electron microprobe analyses of the Ni–Mo–Se sulfide layer by Orberger et al. (2007) suggested that the Se concentration of MoSC (0.44 wt.%) and Ni-Fe sulfides (0.46 wt.%) is higher than pyrite (0.09 wt.%), which is consistent with the results of the Ni–Mo–Se sulfide layer from western Hunan province by Pan et al. (2005). A similar result was also observed in the mineralized black shales of Devonian strata at the Mackenzie platform on the continental margin of the Selwyn Basin, where Se concentration increases systematically from Fe-sulfides (0.05 wt.%) to Fe-Ni-sulfides (0.3 wt.%), Ni-Fesulfides (1.04 wt.%) and Ni-sulfides (1.6 wt.%) (Orberger et al., 2003). It is also noted that the positive correlation between Ni and Se in Ni-Fe-sulfides and the increasing Ni and Se concentrations from Fe-sulfides to Fe-Ni-sulfides, NiFe-sulfides and Ni-sulfides indicate the influence of Niand Se-bearing hydrothermal fluid on those sulfides. This genetic model of the Ni–Mo–Se sulfide layer is also supported by several researchers who suggested that fractions of Se could be derived from submarine hydrothermal fluids (Steiner et al., 2001; Orberger et al., 2005, 2007; Jiang et al, 2007; Pasava et al., 2008). Selenium contents in hydrothermal sulfide minerals are highly variable, but are universally higher. For example, at 13°N, high Se values (i.e. up to 2500 ppm) occur in high-temperature mineral assemblages in the middle of the deposit (Auclair et al., 1987). High Se contents in pyrite (200 ppm) in volcanic-hosted Cu-rich massive sulfide deposits also were reported by Huston et al. (1995). This indicates that hydrothermal fluid can provide a large amount of Se(-II) associated with sulfides/selenides.

The global biogeochemical Se cycle is very complex. Studies of Se speciation and distribution (Cutter, 1985; Cutter and Cutter, 1995; Zhang and Moore, 1996, 1997; Zhang and Frankenberger, 2003a; Gao et al., 2003, 2007) have shown that the transport of Se from seawater to sediments can take a number of different, highly selective pathways, depending on the pH and the redox state of the environment. In general these include (1) removal of Se from the water column to sediments by marine organism uptake, adsorption onto clay minerals, and incorporation into organo-Se compounds by plankton and algae; (2) dissimilatory reduction to insoluble elemental Se and selenide minerals during anaerobic microbial respiration, and coprecipitation with pyrite and other metallic sulfides. Conversely, there is an important multi-step regeneration for Se geochemical cycling that dissolves organic selenides from the organic fraction and oxidizes them. Cutter and Church (1986) suggested that Se speciation might be a useful tracer for oxidation-reduction equilibrium and kinetics. In aqueous solutions, Se occurs mainly in two oxidation states: Se(VI) in the form of SeO42- under highly oxidized conditions; and Se(IV) in the forms of H2SeO3, HSeO3-, or SeO32- under less oxidized or neutral conditions. Elemental Se(0), selenides and Se-sulfides, on the other hand, are only stable in reducing environments, and in principle they cannot be absorbed by plants and animals (Kulp and Pratt, 2004). Thermodynamic calculations also predict that the most reduced environments should be dominated by Se(-II), with HSe- the major Se species in solutions (Seby et al., 1998). In studying the Se species for Saanich Inlet along the southeastern part of Vancouver Island in Canada, Cutter (1982) found both selenate and selenite in the oxic surface waters, although Se(VI) is the only stable form predicted from thermodynamic modeling. Excepting Se(IV), large proportion Se(0) and organic-Se has also been observed in soils and wetlands sediments under reducing conditions, and it has been argued that the reduction of Se(VI) to [Se(IV)+Se(0)] followed by subsequent adsorption into sediment is an important pathway for immobilizing Se from water (Zhang and Frankenberger, 2003a,b). For sedimentary rocks, Kulp and Pratt (2004) and Bruggeman et al. (2005) suggested that the clay-rich nature of the shale matrix might also lead to the enrichment of Se(IV) in shales. These observations suggest that the incorporation of Se(IV) into clay minerals and organic particulates at the surface, vertical transport should be important for accumulating Se(IV) in sediments. The model maybe explain enrichment of Se(IV) in our samples. The more oxygen-depleted condition with amount valid reducing agents would lead to reduction of Se(IV) to lower valance (such as Se (0) or Se(-II))(eg. Herbel et al., 2003). The presence of Se(-II) in sulfides/selenides from biotic and abotic reduction processes may be more suitable for reconstructing palaeo-redox conditions for sedimentary rocks under a closed oceanic environment. For our samples, we observed that the percentage of Se(-II) is related

Selenium speciation in lower Cambrian Se-enriched strata

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Fig. 4. Covariation diagram of Se(-II) percentage associated with sulfide/selenide and the degree of sulfidation (DOS) for our sample types. Only one sample (XZ-29) was removed.

Fig. 5. Relationship between the percentage of Se(-II) associated with sulfide/selenide and that of total Se(IV) for natural samples(Han, 2006; Kulp and Pratt, 2004 and this study)

with the DOS independent of the rock type (Fig. 4), which has been used widely to estimate the redox condition under which the iron sulfides were formed (Kao et al., 2004a,b). A particularly interesting feature is that the Ni–Mo–Se sulfide layer was formed under euxinic conditions, with notable differences to the suboxic/anoxic conditions under which the overlying and underlying black shales and cherts were deposited. This suggests that redox conditions should play an important role for the enrichment of redox-sensitive elements in the Ni–Mo–Se sulfide layer, consistent with the argument of previous studies (Steiner et al., 2001; Mao et al., 2002; Krı´bek et al., 2007; Lehmann et al., 2007; Orberger et al., 2007; Jiang et al., 2007). In these studies, black shales are interpreted as sediments formed in an anoxic environment protected from oceanic surface currents. The Ni–Mo–Se sulfide layer have been interpreted to represent moderate depth, most likely close to the limit of light penetration, and close to the oxic-anoxic and sulfide-saturated water interface (Steiner et al., 2001; Krı´bek et al., 2007). In addition, the higher Ni–Mo–Se signature in the sulfide layer reflects more anoxic sulfate-reducing conditions when compared to the black shales (Mao et al., 2002). In comparison with other rock types, only very less Se(-II) was found in the K-bentonite. Considering origin of K-bentonite, it could reflect the ambient oxic seawater, rather than redox condition of parent volcanic ash. Both DOS and the percentage of Se(-II) indicate that the phosphorite formed in a suboxic environment, which is consistent with the result of Goldberg et al. (2007) and Pasava et al. (2008). In their studies, the phosphorite overlying on Dengying dolomite is interpreted as extremely shallowwater sediments during transgression. Although elemental Se(0) can be produced by anaerobic dissimilatory metal reduction, we proposed that Se(-II) in sulfide/selenide is the major production of dissimilatory species resulting from metal reduction due to the absence of elemental Se in our samples. There is a good correlation (Fig. 5) between the percentage of Se(-II) with sulfide/selenide and the sum of Se(IV) from the water-soluble, exchangeable, and base soluble fractions, not only for our

samples but also for the black shales and cherts from the Yutangba region formed in the Permian (Han, 2006). As shown in Fig. 5, the Yutangba region was formed under only slightly reducing conditions, in comparison to the Niutitang Formation which was deposited under different redox conditions, ranging from the dysoxic for the K-bentonite to more anoxic sulfate-reducing conditions for the Ni–Mo–Se sulfide layer. Compared to selenite (Se(IV)), the formation of elemental Se is expected under more reducing conditions. Inorganic selenides and volatile H2Se analogous to H2S are formed under strongly reducing conditions (Lenz and Lens, 2009). Unlike the biological sulfur cycle in which sulfate is reduced to sulfide, selenate-reducing bacteria such as Sulfurospirillum barnesii, Seleniuhalanaerobacter shriftii form elemental Se as an end product (Chen et al., 2009). However, there is little elemental Se in our sample types, which suggests that dissimilatory reduction from oxidized Se(IV) species to elemental Se(0) and finally to inorganic Se(-II) with sulfides/selenides by the selenite-respiring B. selenitreducens (Herbel et al., 2003), Shewanella sp. HN-41(Lee et al., 2007), and an organic carbon source (Zhang et al., 2008) are an important pathway for Se transportation in the Early Cambrian (except K-bentonite). We found that the K-bentonite has a much higher Se(IV) content (81.79%), which is distinct from those obtained from the Upper Cretaceous chalks, shales and bentonites by Kulp and Pratt (2004), who pointed out that the lower Se(IV) content in their samples is likely the result of weathering under alkaline oxidizing conditions. Their hypothesis is supported by Sun et al. (2004), who reported that Se(IV) was not adsorbed by sediments and soils under alkaline conditions. Therefore, the greater proportion of Se(IV) in the K-bentonite can be explained by leaching of Se(-II) in parent volcanic ash and adsorption of Se(IV) on second minerals(i.e., illite, an important mineral in K-bentonite) under acid oxic conditions, which is consistent with the observation by Hunter (2007) who found that the reduction of Se(IV) to Se(0) takes place under microaerophilic and denitrifying but not aerobic conditions. As such, the difference between these two trends in Fig. 5 might

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indicate a subtle change in redox condition and pH between Cambrian and Cretaceous seawater. The acidic deposition environment of the Ni–Mo–Se sulfide layer is also confirmed by the significant increase of the Mo solubility in solutions at pH > 6 and Mo-fixation in acid environments (Orberger et al., 2007). Assimilatory metabolism refers to metabolic processes wherein elements are incorporated into cellar structures, whereas dissimilatory metabolism refers to processes wherein elements are oxidized or reduced, and the organism utilizes the energy released (Liermann et al., 2007). Under Se-rich sedimentary environments, assimilatory processes can be represented by the fraction of organic Se in the water-soluble, P-buffer exchange, base-soluble extraction and final residual fractions. Dissimilatory processes result in the reduction of elemental Se(0) and Se(-II) with sulfides/selenides minerals (Kulp and Pratt, 2004). In Fig. 6, these two fractions are plotted for all of the available data, including those of Lower Cambrian Se-enriched strata from Zunyi region, those of Permian black shales and cherts from Yutangba region in Southern China (Han, 2006), and those of Upper Cretaceous chalks and shale from Western USA. (Kulp and Pratt, 2004). Several unusual trends are evident. Based on the proportion of disulfide- and diselenide-associated Se, Kulp and Pratt (2004) suggested that assimilatory microbial Se absorption was more prevalent in shales than chalks from the Upper Cretaceous strata in western United States. They also suggested that direct algal Se assimilation was the primary mechanism for its presence in both chalk and shale. Their samples (exclude bentonite) are located in I zone defined as assimilation dominated processes (Fig. 6). For our Ni– Mo–Se sulfide layer and part of black shales(in II zone) however, the lower proportion of Se associated with organic matter and the largest proportion of Se associated with

Fig. 6. Relationship between assimilatory and dissimilatory processes for natural samples(Kulp and Pratt, 2004; Han, 2006 and this study). The percentage of organic Se in the water-soluble, Pbuffer exchange, NaOH extraction and final residual fractions, reflect assimilatory processes. However, the percentage of reduction of elemental Se(0) and Se(-II) with sulfides/selenide minerals represent dissimilatory processes (Kulp and Pratt, 2004). In the Fig., three different zone was identified as I assimilatory dominant, II dissimilatory dominant and III aboitic redox dominant.

sulfides/selenides indicate that dissimilatory reduction of Se could be the primary mechanism in the unit. For III zone, abiotic processes were main pathways for Se enrichment in those samples, rather than assimilatory and dissimilatory processes. This is supported by Wen et al. (2007) who proposed that abiotic reduction is probably dominant pathway for Se enrichment in Yutangba area, based on significant Se isotopic fractionation( 12.77& to 4.93&). The large range of Selenium isotopes ( 4.35& to 4.11&) also indicated that Se enrichment in K-bentonite and phosphorite (in III zone) was attributed to abiotic redox processes (Zhu et al., 2008). We therefore proposed that the formation of reduced Se(0) and Se(-II) resulted from dissimilatory reduction processes in the Ni–Mo–Se sulfide layer and black shales in the II zone, compared to abiotic reduction processes for the samples in the III zone (Fig. 6). For the Niutitang Formation in Zunyi region, Se transmission can be summarized as follows: (1) oxidized species of Se(VI) and Se(IV) in the seawater column were adsorbed on the surface of clay minerals, Fe-oxides and FeS2 and/or assimilated by algae were transformed to Se(-II) associated with organic materials; (2) the higher valence Se forms were reduced to elemental Se(0), and further to solid Se(-II) associated with sulfides/selenides below the water-sediment interface during biotic and abiotic reduction processes. However injection of Se(-II) from reducing hydrothermal fluid could not be excluded for Ni–Mo–Se sulfide layer. 5.3. Genesis of the Ni–Mo–Se sulfide layer There exists increasing arguments for sources and deposition mechanism of metals in the Ni–Mo–Se polymetallic sulfide layer. The opposing models have been proposed: (1) hot hydrothermal fluid influx, based on fluid inclusion studies on quartz veins cross-cutting the ore (Lott et al., 1999) and the distribution of PGE, Mo, Ni, As, Se, and so on (Steiner et al., 2001; Coveney, 2003; Jiang et al., 2007; Orberger et al., 2007; Pasava et al., 2008); and 2) syn-sedimentation/ sorption from seawater under anoxic or euxinic conditions (Mao et al., 2002; Lehmann et al., 2003). As mentioned above, the Ni–Mo–Se sulfide layer is strongly enriched in Ni (up to 3%), Mo (up to 7%), Se (up to 2800 ppm) in comparison with the host black shales (Ni: 202 ppm, Mo: 529 ppm, Se: 82 ppm, on average) (Table 1) and average Cambrian black shale(e.g. Ni: 56 ppm, Mo: 25 ppm,Yudovich and Ketris, 1994). Mao et al. (2002) and Lehmann et al. (2003) found that the inter-element pattern of the Ni–Mo–Se sulfide layer is similar to seawater, and displays a high enrichment factor (106–108) for most of the redox-sensitive metals. Based on this observation, a model of seawater metal precipitation/scavenging origin was proposed. Furthermore, Lehmann et al. (2007) and Wille et al. (2008) provided evidence of the molybdenum isotope composition to support their model. In addition, a series of studies of elemental distributions in marine basins where present-day sulfate reduction occurs in bottom waters have also shown that the seawater are a major sink for redox-sensitive elements such as Mo, Ni, Se, and U (e.g. Piper, 1994). Especially, the Mo concentration in seawater is significantly higher

Selenium speciation in lower Cambrian Se-enriched strata

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Fig. 7. Schematic representation for Se source and enrichment mechanism in Ni–Mo–Se sulfide layer. The enriching pathway is interpreted to be (1) the biotic dissimilatory reduction from higher valence Se(VI) and Se(IV) to Se(-II) associated with the Ni-sulfides/MoSC phase; and (2) fixation of Se(-II) from hydrothermal fluids under an anoxic environment.

(0.01 ppm) than that in hydrothermal fluids (e.g. from the Juan de Fuca vent fluid 0.002 ppm, Trefry et al., 1994). Therefore, combined with these geochemical clues and detailed mineralogy (Orberger et al., 2007), it seems be more reasonable that the mixing mechanism of euxinic, H2S-rich bottom waters with Mo-rich surface waters in an upwelling regime can explain the extreme Mo enrichment found in the Ni–Mo–Se sulfide ore layer. However, the Se and Ni (also including other metals, As, Cu, Zn, and PGE etc.) concentrations of the Ni–Mo–Se sulfide layer are also several hundred times greater than that of modern euxinic sediments (Passier et al., 1997). Lehmann et al. (2007) interpreted this difference by the sedimentation rate at that stage being several times lower than that of modern euxinic sediments. However, Se and Ni concentrations are several orders of magnitude lower in average seawater than that of Mo (1.6  10 4 ppm and 4.8  10 4 ppm vs. 1.0  10 2 ppm, Nozaki, 1997), but also locally exceeds the Se, Ni value in the sulfide ore layer, apart from Mo. This observation with a significantly lower Corg/S ratio in the sulfide layer than the host black shales, suggests multiple sources of Se and Ni might be possible for this unusual metal assemblage (Orberger et al., 2007; Pasava et al., 2008). In our study, we also found that the ratios of Se/S of the Ni–Mo–Se sulfide layer (9.8  10 3) are consistent with hydrothermal fluid (Se/S: 1.5  10 4), and is several orders magnitude higher than average seawater (Se/S: >10 7). Therefore the extreme enrichment of Se in the sulfide layer may be related to a diffuse hydrothermal system within the black shales having favored the growth of Se-reducing bacteria species (Orberger et al., 2007). Se(0) has been detected in many different environmental systems, particularly in anoxic environments (e.g. Zhang and Moore, 1996; Gao et al., 2007). Many studies have shown that Se(0) formation from either Se(VI) or from Se(IV) is predominantly mediated by various types of bacteria(eg. Oremland et al., 1989; Zhang and Moore, 1996). Only a few studies have identified Se(0) formation under abiotic conditions in pure laboratory simulations (Myneni et al., 1997; Charlet et al., 2007). Howevere, the absence of Se(0) and abundance of Se(-II) associated with sulfides/

selenides in the sulfide layer indicated two possible pathways for Se(-II): (1) the product of the further reduction from Se(VI) and Se(IV) through Se(0) by the presence of bacteria such as B. selenitreducens (Herbel et al., 2003) and D. desulfuricans (Nelson et al., 1996); and (2) injection of reducing hydrothermal fluids with abundant Se(-II). In addition, the much higher proportion of Se(-II) associated with sulfides/selenides compared to host black shales, also indicates that anoxic conditions with abundant organic material dominated enriching Se into the Ni-sulfides/MoSC phase. This view is also supported by Jiang et al. (2007) who suggested that both submarine hydrothermal fluids and anoxic environments with abundant organic material should be responsible for the genesis of the Ni-Mo sulfide layer. On the other hand, Se and S are positively correlated only in MoSC phase, and the Se/S ratios in Ni-Fe sulfides are one order of magnitude higher than that of pyrite, which suggests that sulfide/selenide Se(-II) is incorporated into Ni-Fe phase by the strong affinity of Se to Ni sulfides (Orberger et al., 2003, 2007). We therefore propose that the higher Se concentration in the Ni–Mo–Se sulfide layer is related to different sources, including metal-rich seawater and reducing hydrothermal fluid. The enriching pathway is interpreted to be (1) the biotic dissimilatory reduction from higher valence Se(VI) and Se(IV) to Se(-II) associated with the Ni-sulfides/MoSC phase; and (2) fixation of Se(-II) from hydrothermal fluids under anoxic environment (Fig. 7). 6. CONCLUSION We provided a systematic study of Se speciation in the Lower Cambrian Se-enriched strata within the Niutitang Formation in South China to constrain metal sources and accumulation processes,. There are a small component of elemental Se(0) and higher proportions of Se(-II) within sulfides/selenides in the most of black shales, cherts and the Ni–Mo–Se sulfide layer while compared to modern anoxic sediments. This indicated that the Early Cambrian Niutitang Formation was deposited under a more oxygen depleted environment. The distribution of Se speciation

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may be a useful index for reconstructing palaeo-redox conditions of sedimentary processes. For the Niutitang Formation, the K-bentonite with enrichment of Se(IV) reflects alternation of parent volcanic ash under oxic conditions. The phosphorite depositional environment is identified as shallow suboxic conditions. Simultaneously, in deeper parts of the restricted anoxic basin, most black shales and cherts were deposited. The Ni–Mo–Se sulfide layer is interpreted to represent a moderate deep water horizon rich in metals, which originated from a sediment-starved anoxic seawater environment and mixing with submarine hydrothermal fluid. We suggest that there are several Se sources including Se-rich seawater and reducing hydrothermal fluids. The main mechanisms of Se enrichment of the Ni–Mo–Se sulfide layer are described by (1) dissimilatory reduction from higher valence Se(VI) and Se(IV) to Se(0), finally to Se(-II) associated with sulfides and diselenides, (2) fixation of Se(-II) from hydrothermal fluid by dissociative metal ions under an anoxic environment. ACKNOWLEDGEMENTS The project was funded by the Natural Science Foundation of China (40803016, 40930425) and the Ministry of Science and Technology of the PRC (KCZX20100104). We thank two anonymous reviewers and B. Orberger for improving the manuscript.

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