Formation and burial of pyrite and organic sulfur in mud sediments of the East China Sea inner shelf: Constraints from solid-phase sulfur speciation and stable sulfur isotope

Continental Shelf Research 54 (2013) 24–36

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Formation and burial of pyrite and organic sulfur in mud sediments of the East China Sea inner shelf: Constraints from solid-phase sulfur speciation and stable sulfur isotope Mao-Xu Zhu n, Xiao-Ning Shi, Gui-Peng Yang, Xiao-Chen Hao Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 June 2012 Received in revised form 31 December 2012 Accepted 3 January 2013 Available online 10 January 2013

Solid-phase sulfur speciation and stable sulfur isotopic compositions are used to elucidate the formation and burial of pyrite-sulfur (Spy) and organic sulfur (OS) at three selected sites in mud sediments of the East China Sea (ECS) inner shelf, and to infer potential factors influencing the preservation of Spy and OS in the sediments. Our results in combination with previous studies show that the overall reactivity of sedimentary organic matter (OM) is low, while OM at the site impacted by frequent algal-bloom events displays somewhat enhanced reactivity. We observed characteristically low contents of acid volatile sulfide (AVS) and Spy in the sediments, which can be attributed to low sulfate reduction rate due to high redox potential together with limited availability of labile OM. Several geochemical features, for example, persistent occurrence of S0, good coupling among the profiles of AVS, S0 and Spy, and large 34Spy depletion, all suggest that the polysulfide pathway and disproportionation are likely involved in the pyrite formation. Organic sulfur amounts in the sediments are at the lower end of OS contents reported in many other marine sediments around the world. The sources of OS are both biosynthetic and diagenetic, with the biosynthetic OS being the major share (59–73%). In one site studied (C702), enhanced accumulation of OS within the upper layers (14 cm) is believed to be associated with frequent algal-bloom events. Net burial fluxes of Spy and OS in the three sites studied range from 0.27 to 0.82 mmol/m2/d and from 0.22 to 0.74 mmol/m2/d, respectively. Sedimentation rate and algal-bloom events are two important factors influencing the spatial variability of Spy and OS burial fluxes in the whole shelf. & 2013 Elsevier Ltd. All rights reserved.

Keywords: East China Sea Organic sulfur Pyrite Elemental sulfur Acid volatile sulfide Stable sulfur isotope

1. Introduction Pyrite-sulfur (Spy) is quantitatively the most important form of reduced sulfur eventually buried in marine sediment (Berner and Raiswell, 1983), although the pathways for its formation have long been a matter of debate (see Goldhaber, 2004 and Schoonen, 2004, for a review). The final burial of pyrite means permanent preservation of both reduced iron and sulfur and an end of their early diagenetic cycling. The preservation not only exerts an important influence on the biogeochemical cycling of sulfur, carbon and iron (Berner, 1985; Morse and Berner, 1995; Goldhaber, 2004), but also regulates atmospheric O2 and CO2 concentrations over geological timescales (Holland, 1984; Berner, 1999; Canfield et al., 2000). Organic sulfur (OS) is the second most important sulfur pool in marine sediments (Francois, 1987; Anderson and Pratt, 1995;

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Vairavamurthy et al., 1995a; Werne et al., 2004). It is formed via two basic pathways. One is assimilatory sulfate reduction with subsequent formation of sulfur-requiring cellular components, that is, biosynthetic OS (bio-OS) (Francois, 1987; Canfield et al., 2005). Most bio-OS such as proteins and amino acids is highly labile and rapidly decomposes during very early diagenesis, and consequently only a small fraction of the sulfur species can survive the diagenesis and be eventually preserved in the sediments. Based primarily on sulfur isotope mass-balance modeling, bio-OS generally accounts for  10–25% of the total OS in most marine sediments (Anderson and Pratt, 1995; Werne et al., 2003), while larger fractions of bio-OS (as much as 450–87%) have also ¨ been reported in many marine settings (Bruchert and Pratt, 1996; ¨ Bruchert, 1998; Canfield et al., 1998; Passier et al., 1999). To date what remains still poorly understood are how some bio-OS molecules could survive early diagenesis, whether they have been subject to modifications prior to the final preservation, and on what conditions a large fraction of bio-OS has been preserved. The other pathway for OS formation is diagenetic sulfurization of organic matter (OM), that is, diagenetic OS (diag-OS). In this

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cannot be quantitatively evaluated because the organic compounds that can be analyzed represent only a minute fraction of total OS in the sediments. Application of X-ray absorption nearedge structure spectroscopy (XANES) promises to offer considerable information on identification and quantification of various types of OS compounds (Vairavamurthy et al., 1994, 1995b; Morgan et al., 2012), but still cannot provide any details on sulfur sources and mechanisms for the OM sulfurization. The East China Sea (ECS) inner shelf is characteristic of high terrestrial input (Liu et al., 2007; Wang et al., 2008), highly dynamic fluidized surface mud (Aller et al., 1985; DeMaster et al., 1985), and the impact of algal-blooms in some areas (Yu and Cao, 2002; Wang et al., 2004). Thus, this area provides an excellent opportunity to look at how the above factors have influenced diagenetic geochemistry of inorganic and organic sulfur in these sediments and in other analogous large-sized shelf sediments in general. Up to date, however, no efforts, to best of our knowledge, have been devoted to study on sedimentary OS geochemistry of the sea. As for inorganic sulfur, only limited efforts have been made to quantify acid volatile sulfide (AVS) and pyrite in the southern ECS (south of 26.81N, see Fig. 1) (Huang and Lin, 1995; Lin et al., 2000, 2002a), where sedimentation rates are much lower in comparison with the northern areas (DeMaster et al., 1985; Huh and Su, 1999), however, potential limiting factors and mechanism/pathway for pyrite formation and burial in the ECS sediments as a whole have not been well documented.

2. Study area and sampling 2.1. Background of study area The ECS is one of the largest shelf seas in the world (Fig. 1). It receives annually 5  108 t of terrestrial particulates (Liu et al., 2007; Wang et al., 2008) including 2–5  106 t of particulate OM (Wu et al., 2007), mainly from the Yangtze River, the largest river 35° N 34°

Yellow JC

Cheju Island

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Yangtze River YDW

31° Qiantang River

Zhoushan Fishery

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process reduced sulfur, originally produced by microbial sulfate reduction in anoxic sediments, is incorporated intramolecularly and/or intermolecularly into unsaturated and functionalized organic molecules (lipids) via a range of possible mechanisms to build large three-dimensional sulfur-bearing macromolecules connected through mono-, di- and trisulfide bridges (Francois, 1987; Kohnen et al., 1989; Zaback and Pratt, 1992; Wakeham et al., 1995; Werne et al., 2000). Dissolved sulfide and intermediate sulfur such as elemental sulfur (S0) and polysulfides are possible sulfur sources for OM sulfurization (Francois, 1987; Vairavamurthy and Mopper, 1987), while lines of evidence indicate that polysulfides appear to be the most likely sources for the sulfurization (Mossmann et al., 1991; Raiswell et al., 1993; Werne et al., 2003, 2008). Sulfurization has a bearing on OM burial because macromolecules formed through di- and polysulfide crosslink appear to be resistant to microbial degradation (Sinninghe Damste´ et al., 1989; Ferdelman et al., 1991), providing a mechanism for preservation of sedimentary OM (see Sinninghe Damste´ and de Leeuw, 1990, for a review). Since sulfurized OM is also less amenable to the traditional methods (Kohnen et al., 1991a,b), our knowledge of biomarker distributions and paleoenvironmental conditions based on the traditional chemical extraction analysis may have been biased (Kohnen et al., 1991c; Werne et al., 2004). Several lines of evidence indicate that pyrite formation is kinetically favored relative to OM sulfurization, suggesting that OM sulfurization does not start before reactive iron pool has been depleted and, therefore, iron-limited environments are conducive to OM sulfurization (Sinninghe Damste´ et al., 1989; Canfield et al., 1998; Zaback and Pratt, 1992; Raiswell et al., 1993). However, simultaneous occurrence of pyrite formation and OM sulfurization without obvious competition has also been observed in many marine sediments (Ferdelman et al., 1991; Bates et al., 1995; ¨ Bruchert and Pratt, 1996; Hsieh and Shieh, 1997). OM sulfurization is also of significance in numerous lacustrine sediments (Rudd et al., 1986; Urban et al., 1999), particularly in terrestrial peat forming environments (Coulson et al., 2005; Bartlett et al., 2009), in spite of relatively low sulfate concentrations. Thus, limited availability of reactive iron may not be absolutely required for the sulfurization, as suggested by Werne et al. (2004). As a matter of fact, the formation and persistence of reactive intermediate sulfur are likely to be more important than the availability of reactive iron in controlling the timing and extent of OM sulfurization during diagenesis (Raiswell et al., 1993). Thus, in some cases rapid input or enrichment of iron oxides may actually promote both OM sulfurization and pyrite formation through rapid generation of reactive intermediate sulfur species (Ferdelman et al., 1991; Filley et al., 2002). Detailed speciation and isotopic composition measurement of sedimentary sulfur are an efficient approach to OS research, whereas pathway and timing of OM sulfurization are still very difficult to constrain due to the wide variety of OS compounds found in marine sediments, the complexity of biological and abiological sulfur cycling, and the analytical difficulties in identification and quantification of extremely reactive intermediate inorganic sulfur species (see Werne et al., 2004, for a review; Werne et al., 2008). Under most circumstances, one can only exclude some highly unlikely mechanisms based on sulfur species and their isotopic compositions, but cannot unambiguously ¨ identify the actual sources/pathways (Bruchert and Pratt, 1996; Werne et al., 2003). Establishing precursor–product relationship of OM sulfurization (Werne et al., 2000) and sulfur isotope analysis of the products (compound-specific sulfur isotope analysis) (Werne et al., 2008) are promising methods to put more precise limits on sources and timing of OM sulfurization. However, the relative importance of the specific sources/pathways

25

HL

25° 118° 119° 120° 121° 122° 123° 124° 125° 126° 127° 128°E Fig. 1. Regional ocean circulation patterns and sampling stations (702, 802, and 803) in the East China Sea (ECS). Sites F and H were stations of Duan and Chen (1993). The dashed-line area labeled ‘‘HL’’ in the southern ECS was study area of Huang and Lin (1995) and Lin et al. (2000, 2002a). The location of Zhoushan Fishery is indicated by the dashed-line rectangle. JCC: Jiangsu Coastal Current; KC: Kuroshio Current; TWC: Taiwan Warm Current; ZFCC: Zhejiang–Fujian Coastal Current; YDW: Yangtze Diluted Water.

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in China and one of the largest in the world. A large amount of fine-grained particulates from the Yangtze Estuary has been transported southward along the coast driven by the Jiangsu, Zhejiang–Fujian Coastal Currents, and trapped in the inner shelf by the blocking of the northward Warm Taiwan Current offshore, developing an elongated inner-shelf mud wedges from the Yangtze River mouth into the Taiwan Strait (Xu et al., 2009). In this dispersal system, sedimentation rate decreases rapidly from the Yangtze Estuary southward along the inner shelf and eastward offshore (DeMaster et al., 1985; Huh and Su, 1999). Accordingly, OM distribution in the sediments follows a similar spatial pattern (Kao et al., 2003; Zhu et al., 2012). The ECS shelf sediment is an important site for OM accumulation and its mineralization as well. Sulfate reduction is an important anaerobic pathway for the mineralization (Aller et al., 1985; Huang and Lin, 1995; Lin et al., 2000), while dissimilatory iron reduction also plays a role (Bao, 1989; Lu¨ et al., 2011; Zhu et al., 2012). Algal blooms have been reported in coasts of the ECS since the 1960s, but frequent bloom events, which were reported only after the 1980s (Yu and Cao, 2002; Wang et al., 2004), occurred mainly around the Yangtze Estuary and Zhoushan Fishery. The site of C702 for our present study (Fig. 1) was within the direct impact of the frequent algal-blooms around the Zhoushan Fishery, and the impact on the sediments is estimated to reach a depth of at least 30 cm on basis of a steady-state sedimentation rate of 1.1 cm/a at this site (Huh and Su, 1999), while the sites of C802 and C803 are beyond the reach of the direct impacts. Up to date, there have been no any reports on potential impacts of the blooms on formation and burial of pyrite and OS in the sediments. 2.2. Sampling methods Three sediment cores, C702 (122.451E, 29.211N), C802 (121.441E, 27.751N) and C803 (121.651E, 27.641N) (Fig. 1), were collected in the mud area of the ECS inner shelf during the spring cruise (late April to mid May) of 2009 using a box corer. Upon retrieval of the box cores, PVC tubes (inner diameter 8 cm) were vertically inserted for tube coring. The cores were frozen at 20 1C immediately after the two ends were covered leaving no headspace. During transportation to our home laboratory (within 2 h), the cores were covered with ice to keep them cool. After thawing at room temperature, the sediment cores were extruded and sectioned at 2 or 3 cm interval in N2 glove-box for further handling.

3. Analytical methods 3.1. Wet/dry weight ratio and organic carbon Accurately preweighed wet sediment samples in duplicate were dried at 105 1C to constant weight for determination of wet/dry weight ratios. For organic carbon (OC) analysis, sediment samples of known weight ( 0.5 g) in duplicate were treated with 1 M HCl overnight to remove carbonates. After washing and then drying at 60 1C for 12 h, the samples were ground to  100 mesh for OC analysis by a CHN element analyzer (PerkinElmer 2400II, USA) with variability between duplicates better than 5%. 3.2. Sulfur speciation 3.2.1. Acid volatile sulfide (AVS) An attempt was made to determine porewater sulfide in the three cores via centrifuge (4800 rpm) under N2, however sulfide concentrations in most samples were undetectable ( o1 mM), as reported off the Yangtze Estuary (Aller et al., 1985) and in the

southern ESC (Huang and Lin, 1995; Lin et al., 2000). Thus our further efforts were devoted only to AVS determination. Twenty milliliters of HCl (final concentration 4 N) together with 1 mL ascorbic acid (0.1 M) was employed to release AVS in sealed Erlenmeyer reactors containing preweighed wet sediments (  5 g) under N2. Ascorbic acid was used to inhibit oxidation of sulfide by concomitantly extracted Fe3 þ (Hsieh et al., 2002; Burton et al., 2008). Evolving sulfide was absorbed using the cool diffusion method (Hsieh et al., 2002; Burton et al., 2008) and trapped as ZnS precipitates by 15 mL alkaline ZnAc solutions (20% ZnAcþ2 M NaOH) in a separate vial attached to the inner wall of each the reactor. After 24 h (stirring once a while), sulfide trapped was measured by colorimetry (Hewlett–Packard 8453 UV–VIS spectrophotometer) (Cline, 1969) with variability between duplicates in most cases better than 8.8%, but up to 18% when AVS contents are extremely low in the upper 10 cm of C702 and C803. 3.2.2. Elemental sulfur (S0) Residual sediment pellets after the AVS extraction above were washed twice using deionized water and then immediately treated with 20 mL acetone to extract S0 under stirring for 24 h. After centrifuge (4800 rpm) and filtration (0.2 mm) the resulting filtrates were left to evaporate to near dryness at room temperature. The extracted S0 was reduced to sulfide by a cold acid Cr(II) solution (48 h) (Kallmeyer et al., 2004; Burton et al., 2008). The evolving sulfide was collected and measured by the same methods for the AVS analysis above with variability between duplicates in most cases better than 8.0%, but up to 21% when S0 contents are extremely low in the upper 10 cm of C702. 3.2.3. Pyrite Residual sediment pellets after the S0 extraction above were rinsed twice using deionized water and then immediately treated with the cold acid Cr(II) solution to reduce Spy to sulfide (48 h, stirring once a while) (Kallmeyer et al., 2004; Burton et al., 2008). Evolving sulfide was collected and measured by the same methods for the AVS analysis above. The variability between duplicates in most cases is better than 7.6%, but up to 17% when pyrite contents are very low in the upper 10 cm of C702. For analysis of stable sulfur isotopic composition, ZnS precipitates were washed carefully with 2 M NaOH solution once and then alkaline water (pH 8.0) twice, the purified ZnS was converted to Ag2S by adding excess 0.1 M AgNO3 solution, followed by rinse with 1 M NH3  H2O to dissolve Ag2O which might have formed in the conversion. After dryness at 60 1C the Ag2S was saved for determination of sulfur isotopic compositions using a Finnigan MAT-252 mass spectrometer in State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences. Sulfur isotopic composition is denoted, according to 34 32 S/ S ratio, as:  0 1 34 S=32 S C sample d34 S ð%Þ ¼ B 1A  1000 ð1Þ @ 34 32  S= S CDT

34

32

34

where ( S/ S)sample is S/32S of samples and (34S/32S)CDT 34S/32S of the Canyon-Diablo Troilite (CDT) standard. The standard deviation for the d34S analysis was better than 70.2% (n¼5). 3.2.4. Organic sulfur Residual sample pellets after the pyrite extraction above were washed three times and then dried and ground for OS analysis. Organic sulfur in the pellets was converted to sulfate at 800 1C following the Eschka’s procedure. After dissolved in deionized water, the sulfate was quantitatively precipitated as BaSO4 for determination by gravimetry. The mean variability for the

M.-X. Zhu et al. / Continental Shelf Research 54 (2013) 24–36

OC(%) 1

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Fig. 2. Depth profiles of organic carbon (OC) in C702, C802, and C803.

AVS or S0(μmol/g) 1

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Fig. 3. Depth profiles of acid volatile sulfide (AVS) and elemental sulfur (S0) in C702, C802, and C803.

determinations is 6.5% (0.27–9.5%). The BaSO4 was also used for analysis of OS isotopic composition. Because of loss of a scant fraction of hydrolysable and Cr-reducible OS during the previous extraction steps (Canfield et al., 1998), OS determined in this study represent not the total OS in the sediments, but actually non-Cr-reducible OS (non-CROS) according to Canfield et al. (1998). For simplicity, non-CROS in this paper is referred simply to as OS hereafter, unless otherwise stated. 3.3. Solid-phase iron speciation 3.3.1. Extraction of Fe(II) Under N2, 25 mL of NaAc–HAc buffer solution (pH 4.5) was added to preweighed wet sediments (  1 g) (in duplicate) for Fe(II) extraction (Poulton and Canfield, 2005. After shaken in water bath at 50 1C for 24 h, the supernatants were collected for Fe analysis after centrifuge (4800 rpm) and filtration (0.2 mm). The extracted Fe was regarded as being mainly AVS–Fe(II) and Fe(II)–carbonates (if any), since the NaAc–HAc solution is capable of extracting carbonate–Fe(II) and AVS–Fe(II) while essentially leaving iron (oxyhydr)oxides and pyrite unaffected (Poulton and Canfield, 2005). Iron(II) was measured using the ferrozine colorimetry (Hewlett–Packard 8453 UV–VIS spectrophotometer) (Stookey, 1970) with a mean variability of 3.0% (0.25–7.5%) between duplicates. 3.3.2. Highly reactive Fe(III) extraction Residual sediment pellets after the Fe(II) extraction above were washed twice and then subject to extraction by 0.2 M oxalate (pH 3.2) (6 h) using the same procedure as the F(II) extraction above. Acid oxalate is capable of extracting AVS–Fe(II), Fe(II)–carbonates,

amorphous and poorly crystalline iron oxides such as ferrihydrite and lepidocrocite (Kostka and Luther, 1994; Poulton and Canfield, 2005). Since Fe(II) was moved prior to the Fe(III) extraction and also Fe2 þ -catalytic Fe(III) dissolution by oxalate (Suter et al., 1988; Phillips et al., 1993; Poulton and Canfield, 2005) was expected to be minimized following our present extraction procedure, the oxalate extraction can be used as an estimate of highly reactive iron(III) oxides [Fe(III)HR]. The extracted Fe(III) was determined by flame atomic absorption spectroscopy (Thermo Electron Co., USA) with a mean variability of 5.6% (0.02–19.0%) between duplicates.

4. Results 4.1. Organic carbon Organic carbon contents are 0.6–0.9% (average: 0.77%) in C702, 0.6–0.7% (average: 0.67%) in C802, and 0.67–0.86% (average: 0.77%) in C803 (Fig. 2). The OC contents in C702 increase gradually from the surface to a depth of 14 cm, below which the contents decrease gradually to a minimum of 0.6% at the bottom. In spite of some fluctuations in the upper 15 cm of C802, the OC contents remain generally constant over the entire core. In C803, a small decrease in OC contents was observed in the upper 10 cm, below which the contents remain nearly constant except an abnormally high value at a depth of 12 cm. Average OC contents in the three cores are slightly higher than that for the inner shelf sediments (0.61%), but obviously higher than those for the middle-outer shelf (0.28%) and the entire ECS shelf sediments (0.34%) (Kao et al., 2003). Larger OC content in the inner shelf sediments is associated with high clay fraction in comparison

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δ34Spy (‰)

Spy (μmol/g) 0

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Fig. 4. Depth profiles of pyrite-sulfide (Spy) (a) and sulfur isotopic compositions (d34Spy) of Spy (b) in C702, C802, and C803.

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Fig. 5. Depth profiles of organic sulfur (OS) (a) and sulfur isotopic compositions (d34Sorg) of OS (b) in C702, C802, and C803.

with the middle-outer shelf (Lin et al., 2002b; Kao et al., 2003; Fang et al., 2009; Zhu et al., 2012). 4.2. AVS and S0 Both the AVS and S0 in C702 maintain extremely low in the upper 10 cm (Fig. 3a), below which there is an obvious increase in AVS contents. An obvious increase in S0 contents occurs only within a narrow interval from 10 to 12 cm, below which, the S0 contents remain nearly constant and are much lower than the AVS contents. Both the AVS and S0 contents in C802 and C803 increase from the upper layer to a depth of 25–28 cm, below which the contents start to decrease (Fig. 3b, and c). The S0 contents in C802 are comparable to the AVS contents in the upper 16 cm, below which the S0 contents are lower than the AVS contents. The elemental sulfur contents in C803 are similar to the AVS contents over the entire interval. The AVS contents in the three cores are within the ranges of AVS in sediments off the Yangtze Estuary (0–5 mmol/g) (Aller et al., 1985), but lower than the peak value in the southern ECS slope sediments (0–25 mmol/g) (Lin et al., 2002a).

4.3. Pyrite and pyrite-sulfur isotope Pyrite-sulfur contents in C702 range from 2.70 to 17.9 mmol/g (Fig. 4a) and remain extremely low (o5 mmol/g) in the upper 20 cm, below which the contents increase gradually to the bottom. The core of C803 has similar Spy contents and depth pattern to C702. The Spy contents in C802 (5.10–38.3 mmol/g) increase from the topmost layer to a depth of 20 cm, and then remain nearly constant to the bottom. With the exception of the upper 5 cm, the Spy contents in C802 are markedly higher than in C702 and C803. The Spy contents in the three cores are very similar to those (6.3– 69 mmol/g) previously reported (Duan and Chen, 1993 for two sites nearby (F and H, see Fig. 1) and for the southern ECS shelf sediments (0–60 mmol/g) (Huang and Lin, 1995; Lin et al., 2000), but the contents are much lower than the peak value for the southern ECS slope sediments (range: 0–240 mmol/g) (Lin et al., 2002a). Stable isotopic compositions of the Spy (d34Spy) for C702 are within a wide range from  21.0 to  31.6% (average: 25.6%) (Fig. 4b), and display a gradual decrease with depth. The d34Spy values for C802 are within a narrow range from  29.0 to  33.6% (average:  32.2%), but display a gradual decrease with depth in the upper 10 cm, below which the values remain almost constant.

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FeHR (μmol/g)

Fe(III)HR (μmol/g) 0

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Fig. 6. Depth profiles of highly reactive iron(III) (Fe(III)HR) (a) and total highly reactive iron (FeHR) (b) in C702, C802, and C803.

The d34Spy values for C803 maintain almost constant (from 31.0 to  33.7%, average: 32.0%) in the upper 22 cm, below which a small decrease was observed. 4.4. Organic sulfur and sulfur isotope Organic sulfur contents in C802 and C803 are relatively constant (16.5–19.7 mmol/g) over the entire cores (Fig. 5a). In C702 the contents (38.1–48.0 mmol/g) display a slow downcore decrease over the upper 14 cm, then a rapid decrease to a depth of 18 cm, below which the contents remain almost constant (17.1–22.7 mmol/g), and are only slightly higher than those in C802 and C803. Stable isotopic compositions of the OS (d34Sorg) for the three cores are within a narrow range from 5.84 to 7.88% (average: 6.98%) in C702, from 5.46 to 6.75% (average: 5.96%) in C802, and from 5.39 to 6.82% (average: 6.09%) in C803 (Fig. 5b), but all exhibit a downcore decrease. 4.5. Highly reactive iron(III) and total highly reactive iron The contents of Fe(III)HR are 30.0–47.6 mmol/g (average: 40.2 mmol/g) in C702, 16.0–38.0 mmol/g (average: 24.9 mmol/g) in C802, and 39.5–48.5 mmol/g (average: 44.2 mmol/g) in C803 (Fig. 6a), and all exhibit a downcore decrease. The contents of total highly reactive iron (FeHR ¼ S[Fe(III)HR þFe(II)]) are 103–114 mmol/g (average: 108 mmol/g) in C702, 48.0– 96.4 mmol/g (average: 77.1 mmol/g) in C802, and 91.3–109 mmol/g (average: 103 mmol/g) in C803 (Fig. 6b). The contents in C702 and C803 remain roughly constant over the entire intervals, while the contents in C802 are lower and decrease downcore. The average FeHR contents for the three cores are roughly comparable to the onestep oxalate extractable Fe (80–120 mmol/g) in the southern ECS shelf sediments (Huang and Lin, 1995).

5. Discussion 5.1. Organic matter reactivity and diagenetic degradation Almost constant OC contents over the entire intervals of C802 and C803 indicate that OM mineralization has proceeded to only a limited extent due probably to refractory nature of the OM. In comparison with C802 and C803, the slightly higher OC contents

in the upper 14 cm of C702 and a continuous decrease from this depth to the bottom may suggest somewhat higher reactivity of the OM. This feature is probably associated with the frequent algal blooms around this site (Yu and Cao, 2002; Wang et al., 2004), which is expected to have enhanced labile OM accumulation and thus to enable persistent degradation to a greater depth. High fraction of terrestrial OM in the ECS inner shelf sediments (up to 54%) estimated by OC isotope mass balance (Zhu et al., 2012) may have imparted the bulk OM with low reactivity. Fluidized mud layer of the surface sediments, which is a highly dynamic batch reactor for OM decomposition (Aller 1998, 2004), may have intensified the OM degradation and thus left finally buried OM much less degradable. Sulfate reduction rate (SRR) is a good indicator of OM reactivity. The rates in the present study were not measured, however integrated SRRs (to a depth of 20 cm) in the southern ECS shelf sediments were 1–6 mmol m2/ d (Lin et al., 2000), considerably lower than in OM-rich sediments (Chanton et al., 1987; Crill and Martens, 1987). It is reasonable to assume that OM in the ECS inner shelf sediments as a whole are characteristic of low degradability, since the sediments have high terrestrial OM fraction and the OM shares a common source, that is, mainly the Yangtze River input. As has been demonstrated earlier by the persistent OC decrease with depth in C702, SRR and OM degradability in this core is expected to be somewhat higher than in C802 and C803. 5.2. Diagenetic transformation of AVS and S0 Characteristically low AVS contents in the upper layers of the three cores, particularly in C702, are consistent with the generally low degradability of the OM, dynamic conditions (DeMaster et al., 1985; Aller, 1998) and high redox potential ( þ100 mV) of the ECS subsurface sediments (Aller et al., 1985) (note that the three factors may not be completely independent), which may have not only resulted in low SRR, but also facilitated upward diffusion and oxidation of dissolved sulfide. Gradual AVS accumulation with depth is largely due to a decrease in O2 availability and an increase in activity of sulfate reducer. Deeper in the cores, the decrease in AVS could be credited to decreasing SRR due to gradual depletion of labile OM and to pyrite formation at the expense of AVS. In comparison with C802 and C803, the higher OM content and degradability in C702 seemingly has facilitated accumulation of OS, but not of AVS and Spy, in the upper 14 cm as shown in Fig. 5a.

30

M.-X. Zhu et al. / Continental Shelf Research 54 (2013) 24–36

Elemental sulfur is quantitatively the most important intermediate product of biotic/abiotic oxidation of sulfide in marine sediments, but is generally very low in content (Troelsen and Jørgensen, 1982; King et al., 1985). In C802 and C803, the S0 contents are comparable to, or slightly lower than, the AVS contents at the corresponding depths (Fig. 3); the S0 contents in C702 were low but persisted to the bottom. The presence of S0 at a measurable level in the three cores suggests the occurrence of active sulfide oxidation in the anoxic environment. It is known that beyond a depth of O2 available, reactive iron oxides usually serve as the main electron acceptor for sulfide oxidation to intermediate sulfur (including S0) (Dos Santos Alfonso and Stumm, 1992; Mortimer et al., 2011). Excess Fe(III)HR (Fig. 6a) in the three cores renders persistent sulfide oxidation by the Fe(III)HR highly possible. The presence of S0 is believed to favor pyrite formation via the polysulfide pathway (Eqs. (2) and (3)) (e.g., Rickard, 1975; Luther, 1991). FeS þS0 ¼FeS2

(2)

 FeS(s)þS2n  ¼FeS2 þ S2n  1

(3)

In C802 and C803 a simultaneous decrease in S0 and AVS, coupled to an increase in Spy at greater depths, implies that pyrite formation has most likely proceeded via the polysulfide pathway. Note that the low S0 contents deep in C702 do not necessarily exclude this pathway. Potential reasons for the much higher Spy in C802 than in C702 and C803 remain unresolved. However, a more obvious downcore decrease in the FeHR contents in C802 compared with the two others (Fig. 6b) may suggest that pyritiztion at the expense of FeHR has proceeded to a greater extent. 5.3. Pyritization and stable sulfur isotope constraints on pyrite formation 5.3.1. Pyritization The Spy depth patterns of the three cores suggest that pyritization in C802 is restricted to the upper  20 cm, while the process in C702 and C803 has proceeded to a greater depth in spite of the lower Spy contents in comparison. The Spy contents in the three cores are substantially low relative to many coastal sediments around the world (Chambers et al., 2000; Neumann et al., 2005; Morse et al., 2007; A´lvarez-Iglesias and Rubio, 2012). Both the low AVS and Spy contents imply that pyritization through AVS consumption is not the main reason for the low AVS. As has been highlighted earlier, the factors limiting AVS formation, that is, the high redox potential and low SRR, have also been limiting pyrite formation and its accumulation, resulting in both the low Spy and AVS contents. The extent to which potentially reactive iron minerals have been transformed to pyrite can be quantitatively characterized by the degree of pyritization (DOP) (Berner, 1970; Raiswell et al., 1994), which is defined as Eq. (4): DOPð%Þ ¼

Fepy  100 Fepy þ FeR

ð4Þ

where Fepy is pyrite-Fe and FeR is ‘‘residual’’ reactive iron that has not been pyritized. Chemical extractions using HCl of various concentrations (1, 6, or 12 N) (Berner, 1970; Lin and Morse, 1991; Mossmann et al., 1991; Chambers et al., 2000), citrate-buffered sodium dithionite (Morse et al., 2007), acid oxalate (Huang and Lin, 1995), and hydroxylamine hydrochloride (Kao et al., 2004) have been used as a measure of FeR. In the present study, acid oxalate extractable iron was used as a measure of FeR for the convenience of a comparison with literature DOP data for the southern ESC. The calculated DOPs for the three cores range from 0.61 to 18%, comparable to the characteristically low DOP (o16%)

for the southern ECS shelf sediments (Huang and Lin, 1995), and even within the range of DOP calculated using boiling, 12 N HClextractable iron as a measure of FeR for many oxic shelf sediments around the world (Lyons and Severmann, 2006). The results indicate that the availability of metabolizable OM, but not of reactive iron, has been limiting the pyritization.

5.3.2. Sulfur isotope constraint on pyrite formation The Spy in the three cores is substantially 34S-depleted relative ¨ to modern seawater sulfate (d34Ssulfate  þ21%) (Bottcher et al., 2007), and the isotopic compositions of Spy (d34Spy) do not exhibit a downcore increase at depth as commonly observed in organicrich coastal sediments due to the reservoir effect (Goldhaber, 2004), indicating that microbial sulfate reduction has proceeded in a relatively open system, i.e., downward diffusion of sulfate has been in excess of its consumption by microbial reduction. Although porewater sulfate was not measured in the present study, two sites (F and H, see Fig. 1) near C802 and C803 were previously reported to have almost constant sulfate concentrations ( 28 mM) in the upper 60 cm (Duan and Chen, 1993), indicating that sulfate is not in depletion. In addition, sediments off the Yangtze Estuary, where both sedimentation rate and OM content are higher than at the sites of this study, had sulfate concentrations more than 15 mM in the upper 60 cm, although obvious sulfate consumption was observed (Aller et al., 1985). Based on information available (Aller et al., 1985; DeMaster et al., 1985; Lin et al., 2000), the relatively open system could be maintained by low SRR, strong physical mixing in the surface layer, and bioturbation in the interior. Biogenic reworking was found to be as deep as 1 m below the sediment-water surface at some sites off the Yangtze Estuary (Aller et al., 1985). It has been established that sulfur isotope fractionation during sulfate reduction is inversely proportional to cell-specific SRR. Theoretically, where labile OM is readily available for sulfate reducer in a relatively open system, SRR is generally high and thus sulfur isotope fractionation small, and vice versa (Canfield et al., 2005). Sulfate reduction rate generally decreases rapidly with depth in the subsurface layers (Burdige, 2006), while it is possible that small amount of metabolizable OM can maintain low-rate sulfate reduction to a great depth resulting in a large sulfur isotope fractionation. This paradigm is seemingly confirmed by almost constant OC contents (Fig. 2) and low d34Spy (Fig. 4b) over the entire intervals of C802 and C803. It should be pointed out, however, that sulfides (including pyrite) in natural sediments represent an integrated isotopic signal over history of sulfide accumulation, and the isotopic signal is largely dependent of the isotope fractionations in sulfate reduction and disproportionation of intermediate sulfur produced by sulfide reoxidation (Canfield and Thamdrup, 1994; Habicht et al., 1998; Habicht and Canfield, 2001; Bottrell et al., 2009). Thus the low d34Spy in C802 and C803 may be a combined result of low SRR and disproportionation, as will be documented later. High reactivity of OM usually leads to a high but rapidly decreasing SRR with depth, which is commonly accompanied by enhanced d34Spy below the bioturbated zone due to the reservoir effect. Relative to C802 and C803, the larger d34Spy values in the upper layer of C702 may indicate a greater SRR and/or a lower extent of disproportionation, while a downcore decrease to the bottom may be a combined result of the preservation of a larger proportion of isotopically light pyrite formed early in these layers in an open system and the accumulation of later-formed pyrite at a low SRR owing to gradually increasing resistance of the OM to sulfate reducer after these layers reaching a greater depth, but not the result of disproportionation since the availability of intermediate sulfur would become progressively limited with the

M.-X. Zhu et al. / Continental Shelf Research 54 (2013) 24–36

increase in depth. This speculation appears to be confirmed by our argument that the OM in C702 is more labile than in the two other cores. Larger d34Spy was also observed in Watson Bayou sediments (St. Andrew Bay, USA) with high OM content due to contamination in comparison with uncontaminated Callaway ¨ ¨ Bayou sediments of the same bay (Bruchert, 1998; Bruchert and Pratt, 1999). The differences of d34S between seawater sulfate (d34Ssulfate  þ21%) and Spy (i.e., D34Ssulfate-pyrite) for the three cores range from 42 to 55%, and, except in the upper 10 cm of C702, all the D34Ssulfate-pyrite values are in excess of 46%. A value greater than 46% is commonly attributed to additional isotope fractionation associated with disproportionation of intermediate sulfur, because such large D34Ssulfate–sulfide in a single-step sulfate reduction of pure culture has previously never been observed (Kaplan and Rittenberg, 1964; Rees et al., 1978; Habicht and Canfield, 2001). Precipitation of iron sulfide and its conversion to pyrite, and also biotic/abiotic oxidation of dissolved sulfide and metal sulfides yield only a minor sulfur isotope fractionation ( 0.5– 5%) (Nakai and Jensen, 1964; Price and Shieh, 1979; Fry et al., 1988a,b), while disproportionation can result in an significant isotope fractionation. For example, the disproportionation of S0, S2O23  , and SO23  can produce isotope fractionations between product SO24  and H2S of 23%, 3–15% and  28%, respectively (Canfield and Thamdrup, 1994; Habicht et al., 1998; Habicht and Canfield, 2001). Therefore, D34Ssulfate-pyrite greater than 46% was commonly used as a proxy to infer the involvement of disproportionation pathways in pyrite formation ¨ ¨ (Bruchert, 1998; Bruchert and Pratt, 1999; Werne et al., 2003), although disproportionation as a sole explanation for D34Ssulfate– sulfide greater than 46% is increasingly challenged because theoretical modeling (Wortmann et al., 2001; Brunner and Bernasconi, 2005) and recent culture experiments (Canfield et al., 2010; Sim et al., 2011) point to the possibility that a single-step sulfate reduction, particularly at extremely low SRR, could induce such a high D34Ssulfate–sulfide. Given the persistent presence of low but measurable S0 (and probably other intermediate sulfur as well) in the three cores studied (Fig. 3) and the dynamic conditions favoring their continuous formation at the redox interface, pyrite formation involving disproportionation is thus highly possible in the ECS inner shelf. Note that the involvement of the disproportionation in pyrite formation in the upper 10 cm of C702 cannot be excluded although the D34Ssulfate-pyrite is less than 46%, and S0 (and probably other intermediate sulfur as well) concentrations were very low (Fig. 3a), since if original sulfate reduction with

31

larger SRR in C702 have generated less 34S-depleted sulfide (and thus pyrite), resultant D34Ssulfate-pyrite might not be greater than 46% even though the involvement of the disproportionation. 5.4. Sulfur isotope constraints on diagenetic variations and sources of OS Diag-OS usually accounts for the majority of total sedimentary OS, but OM sulfurization is kinetically less favorable than pyritization. Therefore, total OS content in many marine sediments is often much less than Spy content. C702 and C803 have higher OS contents than Spy contents at most depths; C802 also has higher OS contents in the upper 12 cm, although the opposite was observed below this depth. The feature of higher OS contents than Spy contents is, however, not unique to the ECS inner shelf. A similar feature was also observed in northwestern the Arabian Sea sediments where Spy content was relatively low (Passier et al., 1997), and at the Miocene Monterey Formation (Zaback and Pratt, 1992). As will be demonstrated later, the feature above in the ECS inner shelf sediments is not a result of enhanced OS preservation and/or OM sulfurization, but a result of relatively low Spy formation and burial. Base (0.1–0.5 N NaOH) extraction has been commonly used to quantitatively characterize OS in soils and sediments (Francois, ¨ ¨ 1987; Bruchert and Pratt, 1996; Yucel et al., 2010), but some portions of OS are base non-extractable (Ferdelman et al., 1991; Passier et al., 1999). The sum of base extractable OS (i.e., humic-S) plus base non-extractable one is believed to represent total OS in sediment, and it has been demonstrated that the sum is comparable to non-CROS (Passier et al., 1999), although the losses of acid hydrolysable and Cr-reducible OS during the pretreatment for the non-CROS determination (Canfield et al., 1998). It follows that non-CROS can be used as an approximate estimate of total OS and should be larger than base extractable OS to some extent. Keeping this caveat in mind, a comparison of our OS results (i.e., nonCROS) with literature data of humic-S and non-CROS may offer some insights, in a relative sense, into OS burial in the ECS shelf. As shown in Table 1, the OS contents in the three cores are at the lower ends of both humic-S and non-CROS in marine sediments around the world, implying low OS burial in the ECS inner shelf. The isotopic compositions of OS (d34Sorg) (from þ5.39 to þ7.88%) for the three cores are much larger than the d34Spy (from  21.0% to 33.7%), but much lower than the d34S of bio-OS (d34Sbio), which is assumed to be similar to d34Ssulfate ( þ21%) of modern seawater sulfate because of only a small sulfur isotope

Table 1 Comparison of organic sulfur (OS) contents in ancient and modern marine sediments of different areas. Area

OS content (mmol/g)

Form of OS

Source

Black Sea Salt marsh, Delaware Marine sapropel, Mangrove Lake, Bermude St. Andrew Bay, Florida Jervis Inlet, British Columbia Mediterranean sapropel Oman Margin, Arabian Sea Loch Duich, Scottish fjord Peru Margin Monterey Formation, Santa Maria Basin Jurassic marine shales Eastern Mediterranean Sea ECS inner shelf

o6–80 98–211 640–790 3.4–463 403–681 0.9–516 5.1–12 13–22 25–181 39–285 16–1091 13–140 16.5–22.7

Humic-S Humic-S Non-CROS Humic-S Humic-S Humic-S Humic-S Non-CROS# Non-CROS Non-CROS Non-CROS# Humic-S Non-CROS

(1) (2) (3) (4), (5) (6) (7) (8) (9) (10) (11) (12) (13) This study

¨ ¨ ¨ (1) Yucel et al. (2010), (2) Ferdelman et al. (1991), (3) Canfield et al. (1998), (4) Bruchert and Pratt (1996), (5) Bruchert (1998), (6) Francois (1987), (7) Passier et al. (1999), (8) Passier et al. (1997), (9) Bottrel et al. (2009), (10) Mossmann et al. (1991), (11) Zaback and Pratt (1992), (12) Raiswell et al. (1993), (13) Henneke et al. (1997). # For a approximate comparison, organic sulfur determined after pyrite removal is defined as non-CROS, even though pyrite is extracted using other extractants, rather than acid Cr(II) solution.

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M.-X. Zhu et al. / Continental Shelf Research 54 (2013) 24–36

Fractions of diag-OS (%) 30

34

38

Fractions of diag-OS (%)

42

26 0

30

34

38

Fractions of diag-OS (%)

42

26 0

5

5

10

10

15 20 25 30

Depth (cm)

5 10 Depth (cm)

Depth (cm)

26 0

15 20 25

35

35

C802

38

42

15 20 25

35

C803

40

40 Lower boundary Upper boundary

34

30

30 C702

30

Lower boundary Upper boundary

Lower boundary Upper boundary

Fig. 7. The lower and upper boundaries of diagenetic organic sulfur (diag-OS) fractions based on sulfur isotope mass-balance modeling in C702, C802, C803. For details see the text.

fractionation in assimilatory sulfate reduction ( 1–3%) (Kaplan and Rittenberg, 1964; Peterson and Howarth, 1987; Fry et al. 1988a). Thus the OS in the three cores could not be derived solely from bio-OS. Diag-OS could neither be the sole source because the incorporation of reduced sulfur alone (including partially oxidized intermediate sulfur) into the OM cannot yield sulfur isotopic compositions comparable to the observed d34Sorg, given a maximum 34 Sorg enrichment of  þ10% relative to the coexisting pyrite (Werne et al., 2008), which includes 34S enrichment (4–5%) associated with polysulfide incorporation into OM (Amrani and Aizenshtat, 2004) and additional enrichment (2–4%) in polysufides relative to sulfide at equilibrium (Amrani et al., 2006). Like in many other ancient and modern marine environments (Raiswell et al., 1993; Anderson and Pratt, 1995; Canfield et al., 1998), OS (i.e., nonCROS) in the ECS shelf sediments is of a mixed source involving both diag- and bio-OS. Given the quite different depth patterns of the OS and d34Sorg in C702 from those in C802 and C803, further discussion on depth-dependent OS features for C702 will be made separately in the following subsections.

5.4.1. C802 and C803 In C802 and C803 the OS becomes progressively 34Sorg-depleted with depth, whereas the OS contents remain almost constant. A similar phenomenon was also observed in Scottish fjord sediments (Bottrell et al., 2009). If the constant OS content simply means that there is neither sulfur addition nor loss, sulfur isotope exchange between 34S-depleted dissolved reduced sulfur species and the moieties of the OS is therefore required to interpret the observed depth pattern of d34Sorg (Bottrell et al., 2009). However, Canfield et al. (1998) have demonstrated that there is no obvious isotope exchange between porewater sulfide and non-CROS. Therefore, the progressive 34Sorg depletion without sulfur addition and/or loss in the two cores is highly impossible. Diagenetic sulfur addition and/or bio-OS mineralization loss could result in a decrease in d34Sorg, while either process alone could not maintain a constant OS content. We are forced here to conclude that the sulfur-bound OM is open to simultaneous diagenetic sulfur addition and bio-OS mineralization loss. Bottrell et al. (2009) also argued that simultaneous occurrence of the two processes without a net sulfur gain/loss is possible.

eutrophication was the decisive factor for enhanced OS in the upper layers of lacustrine sediments studied. Compared with C802 and C803, the larger OS contents and d34Sorg compositions in C702 could result from enhanced bio-OS burial, while enhanced diagenetic sulfur addition could also result in the same result since sulfide in C702 is isotopically much heavier than in the two other cores. So it is hard to unambiguously distinguish whether enhanced bio-OS burial or enhanced sulfurization has been responsible for the much higher OS contents in C702. The large downcore decrease in the OS contents at 14–18 cm (Fig. 5a), which is accompanied by a small decrease in d34Sorg (Fig. 5b), is also hard to interpret, because several possibilities exist: (i) sole bio-OS mineralization loss, (ii) simultaneous diagenetic sulfurization and bio-OS mineralization loss, or (iii) a large bio-OS mineralization loss with a small diag-OS loss, which all could result in a simultaneous decrease in OS contents and d34Sorg. As will be demonstrated later, sulfur isotope mass-balance modeling could put more constraints on the potential likelihoods. If the depth of OM burial impacted by the algal blooms (up to 30 cm), as estimated earlier, is correct at the site of C702, the rapid decrease in OS in the 10–20 cm interval to values only slightly higher than those in C802 and C803 may suggest that the OS enrichment in upper the sediment has not substantially ¨ contributed to its final burial at depth. Yucel et al. (2010) also found that an enrichment of humic-S in the uppermost euxinic sediments of the Black Sea has not significantly contributed to the final burial of OS due to its decomposition in the upper 10 cm. Urban et al. (1999) argued, on basis of the study of seven lacustrine sediments, that OS enrichment in the upper sediments caused by eutrophication did not necessarily translate into a large enhancement of OS at depth. It should be pointed out that, for C702, the notion above should be taken with caution because of the uncertainties in age-depth relation estimated on basis of a steady-state sedimentation rate at the site and of the great variability of bloom intensity from year to year. The almost constant OS contents and d34Sorg but progressively decreasing d34Spy below 20 cm in C702 reconfirm the conclusion reached by Canfield et al. (1998) that there is no significant sulfur isotope exchange between non-CROS and the porewater sulfides. 5.5. Relative contributions of diag-OS vs. bio-OS and net burial fluxes of Spy and OS

5.4.2. C702 Relative to C802 and C803, the obviously enhanced OS contents in the upper  14 cm of C702 (Fig. 5a) should be associated with algal-blooms around this site. Urban et al. (1999) also observed that increasing primary production caused by

5.5.1. Relative contributions of diag-OS vs. bio-OS As has been demonstrated above, both bio- and diag-OS are the sources of the OS (i.e., non-CROS) in the three cores. A simple two end-member sulfur isotope mass balance model (Eq. (5))

M.-X. Zhu et al. / Continental Shelf Research 54 (2013) 24–36

permits a quantitative estimation of the relative contributions of ¨ ¨ the two sources (Bruchert and Pratt, 1996, Bruchert, 1998; Passier et al., 1999). p  d34Sdiag þ(1  p)  d34Sbio ¼ d34Sorg

(5)

34

34

where p is fraction of diag-OS (%), d Sdiag and d Sbio are isotopic compositions of diag- and bio-OS, respectively. To a first-order approximation, d34Sbio is assumed to be equal to d34Ssulfate (  þ21%) of modern seawater sulfate. We assume the lower boundary of d34Sdiag to be equal to d34Spy of coexisting pyrite and P 34 the upper boundary to be equal to the sum of (d Spy þ10%) 34 given a  þ10% of maximum Sdiag enrichment relative to d34Spy of the coexisting pyrite (Werne et al., 2008). The calculated lower boundaries of diag-OS fractions for the three cores range from 27 to 32% (Fig. 7), and the upper ones from 33 to 41%. The calculated diag- and bio-OS contents (Fig. 8) based on the averaged fractions of the lower and upper boundaries (for simplicity) indicate that due to the algal-bloom events, both the diag- and bio-OS in the upper  14 cm of C702 have been enriched relative to C802 and C803, and the rapid decrease in the OS from 14 to 18 cm (Fig. 5a) has been due to mineralization loss of both the diag- and bio-OS. Note that we have argued earlier based primarily on the depth-dependent d34Sorg that the sulfur-bearing OM in C802 and C803 is open to simultaneous diagenetic sulfide addition (i.e., sulfurization) and bio-OS mineralization loss. However, the calculated results do not clearly reflect the depth-dependent variability (Fig. 8). This is probably because the approximate mass-balance calculation was unable to distinguish the small variations in the diag- and bio-OS due to the limited d34Sorg changes in the two cores. For C802 and C803, the calculated diag-OS contents (Fig. 8a) are lower than the corresponding Spy contents (Fig. 4a), probably indicating that pyritization has outcompleted OM sulfurization; however, the calculated contents for the upper 14 cm of C702 are larger than the Spy contents, probably indicating a absence of the competition due to the impact of algal-blooms. Even though large uncertainty in the estimated diag-OS fractions above, one thing is certain from the three cores that the bio-OS fractions (lower boundaries: 59–67%; upper boundaries: 68–73%) in

the ECS inner shelf are all substantially higher than in many other continental shelves and euxinic basins ( 10–25%) (Anderson and Pratt, 1995; Werne et al., 2003; Morgan et al., 2012). The high bioOS fraction is, however, not unique to the ECS inner shelf. As a matter of fact, high bio-OS fraction has been reported in many other marine and lacustrine environments. For example, the fractions of bio-OS in total OS were 68–87% in the upper 30 cm depth of the Mediterranean non-sapropel, and up to 60% (20–60%) in the Mediterranean sapropel (Passier et al., 1999), Z50% in marine sapropel of Mangrove Lake, Bermuda (Canfield et al., 1998), and up to 55% in Jurassic marine shales (Raiswell et al., 1993). The fractions of bio-OS fractions in humic-S were 27–49% in estuarine sediments of St. Andrew Bay, USA, and even probably up to 71% if recycled hydrogen sulfide with d34S of  þ5% was assumed to be ¨ ¨ utilized by plants assimilation (Bruchert and Pratt, 1996; Bruchert, 1998). As shown in Table 1 and Fig. 8, both the low contents and fractions of diag-OS in the ECS inner shelf sediments is not unexpected, given the low SRR and accumulation of the main reduced sulfur. It is unlikely for a great proportion of bio-OS to be directly preserved in sediments during early diagenesis because of relatively labile nature of most sulfur-bearing biomolecules. However, some reactive sulfur species produced during breakdown of the biomolecules may be rapidly incorporated into the residual OM such that at least a fraction of the originally assimilated sulfur has been preserved in the sediments ¨ (Anderson and Pratt, 1995; Bruchert and Pratt, 1996). Alternatively, a fraction of bio-OS in vascular plants delivered from the lands may be directly buried in the sediments due to its highly refractory nature. Sulfonates are a typical example of important bio-OS compounds in plant debris buried in eutrophic estuarine sediments (Morgan et al., 2012) and probably in other sediments (Vairavamurthy et al., 1994; Bottrell et al., 2010). Given a generally high fraction of terrestrial OM together with poorly metabolizable nature of the bulk OM in the ECS inner shelf sediments, sulfur-bearing biomolecules, particularly those in vascular plant debris, may have significantly contributed to the bio-OS preserved in the sediments. Further research is needed to confirm this speculation at molecular level using XANES and other state-of-the-art in-situ techniques.

Diag-OS (μmol/g) 3

6

9

12

Bio-OS (μmol/g) 15

18

0

5

5

10

10

15

15 Depth (cm)

Depth (cm)

0

0

20 25 30

40

0

5

10

15

20

25

30

35

20 25

C702

30

C802 35

33

C803

35

C702 C802 C803

40

Fig. 8. Diagenetic organic sulfur (diag-OS) (a) and biosynthetic organic sulfur (bio-OS) (b) contents calculated on basis of averages of the lower and upper boundaries of diag-OS fractions for C702, C802, and C803. For details see the text.

34

M.-X. Zhu et al. / Continental Shelf Research 54 (2013) 24–36

5.5.2. Net burial fluxes of Spy and OS Net burial fluxes of OS and Spy across a sediment horizon at a depth beyond physical mixing and bioturbation can be estimated via Eq. (6) (Lin et al., 2000). In the present study, the net burial fluxes are estimated at a depth of 30 cm. J z ¼ rð1fÞwC

ð6Þ 2

where Jz (mmol/cm /yr) is net burial flux of OS or Spy at a given depth, r is bulk density of dry sediments (2.5 g/cm3), f (%) is porosity, o (cm/yr) is sedimentation rate, and C (mmol/g) is content of OS or Spy at a given depth. Sedimentation rates estimated, on basis of Huh and Su (1999), at the sites of C702, C802, and C803 are 1.1, 0.65 and 0.5 cm/a, respectively, and porosities at the depth of the three cores are 54, 53 and 61%, respectively. The calculated net burial fluxes of Spy for C702, C802 and C803 are 0.62, 0.80 and 0.27 mmol/m2/d, respectively, and the fluxes of OS are 0.74, 0.38 and 0.22 mmol/m2/d, respectively. Compared with C802 and C803, the higher OS flux at the site of C702 could be attributed, at least partially, to the frequent algal-blooms and larger sedimentation rate. The net burial fluxes of Spy for C702, C802, and C803 are substantially higher than average Spy burial flux (0.096 mmol/m2/d; range: 0.052–0.23 mmol/m2/d) for the southern ECS shelf (Lin et al., 2000). The larger fluxes for the three cores should be a combined result of higher OM contents and sedimentation rates relative to the southern ECS shelf due to southward decrease in OC content and sedimentation rate (DeMaster et al., 1985; Huh and Su, 1999). Note that the burial fluxes of Spy for C702 and C802 are comparable to the peak value (0.752 mmol/m2/d) for the southern ECS slope (0.025–0.752 mmol/m2/d, average: 0.316 mmol/m2/d) (Lin et al., 2002a). Higher Spy burial flux for the southern slope relative to the southern shelf has been ascribed to enhanced sedimentation rate of OM delivered from the inner shelf by the Coastal Current, Taiwan Warm Current, and Kuroshio (Lin et al., 2002a,b). Clearly, sedimentation rate and algal bloom events have exerted an important impact on the large variability of the Spy and OS burial fluxes among individual sites in the ECS.

6. Summary and conclusions Organic matter in the ECS inner shelf sediments has generally low reactivity due to high fractions of terrestrial OM as well as decomposition in dynamic surface fluidized mud. In comparison with the sites (i.e., C802 and C803) without direct impact by algal bloom events, OM in site (i.e., C702) impacted by the events displays somewhat enhanced reactivity. Low contents of both AVS and Spy in the ECS inner shelf sediments are due to a combined result of low OM degradability, relatively high redox potential, and intensive physical mixing within the surface layers. Availability of labile OM is the limiting factor for pyritization as indicated by the relatively low DOP. Persistent occurrence of S0 from the subsurface to the bottom and generally good coupling of the depth distributions among AVS, S0 and Spy in the three cores suggest that the pyrite formation via the polysulfide pathway is highly possible, while disproportionation may also be involved in the formation of pyrite, as indicated by the d34Spy. Organic sulfur (i.e., non-CROS) contents in the ECS inner shelf sediments are characteristically low and at the lower ends of humic-S and non-CROS contents reported in many marine sediments around the world. Organic sulfur in the sediments is composed of both bio- and diag-OS, with the former the major fraction accounting for at least 59% and probably up to 73%. The high fraction of bio-OS is likely associated, at least in part, with

direct burial of sulfur-bearing biomolecules in vascular plant debris derived from the lands. Frequent algal bloom events have led to an enhanced burial of OS at the site impacted by the events, but not of inorganic reduced sulfur, within the upper layers (14 cm), due to an increase in both bio- and diag-OS. Net burial fluxes of Spy and OS in the three sites studied range from 0.27 to 0.82 mmol/m2/d and from 0.22 to 0.74 mmol/m2/d, respectively. The fluxes of Spy are substantially higher than the average for the southern ECS shelf, but comparable to the peak value for the southern ECS slope. A synthesis of our results and literature data shows that sedimentation rate and algal blooms have exerted an important impact on the spatial variability of Spy and OS burial fluxes in the whole ECS shelf.

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