Geochemical characteristics in the Longmaxi Formation (Early Silurian) of South China: Implications for organic matter accumulation

Marine and Petroleum Geology 65 (2015) 290e301

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Research paper

Geochemical characteristics in the Longmaxi Formation (Early Silurian) of South China: Implications for organic matter accumulation Detain Yan a, *, Hua Wang a, Qilong Fu b, Zhonghong Chen c, Jin He a, Zhan Gao a a

Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China Bureau of Economic Geology, University of Texas at Austin, Austin 78758, USA c School of Geosciences, China University of Petroleum, Qindao 266580, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 September 2014 Received in revised form 13 April 2015 Accepted 20 April 2015 Available online 29 April 2015

The organic-rich Early Silurian sediments (Longmaxi Formation) on the Yangtze platform are considered to be one of the main source rocks in South China. In order to investigate the mechanism of organic accumulation, multiple geochemical proxies, including redox parameters (S/C and trace-element ratios), productivity indices (P and Ba contents) and clastic influx indicator (Ti/Al ratios), are presented here from the Hehua section, Yichang, Hubei province. The Longmaxi sediments have high TOC contents (3.00 e10.85 ％, avg. 5.63 ％), low Pyrite sulfur contents (0.42e1.14 ％, avg. 0.59 ％), high P (0.02e0.44 ％, avg. 0.12 ％) and Ba contents (576.65e2542.43 ppm, avg. 1592.17 ppm). Calculated S/C, Th/U, Ni/Co, V/Cr and V/(V þ Ni) ratios are 0.05e0.24 (avg. 0.12), 0.23e9.86 (avg. 2.30), 2.65e46.24 (avg. 15.46), 0.27e14.42 (avg. 3.75) and 0.36e0.96 (avg. 0.75), respectively. S/C and traceeelement ratios, together with pyrite morphology, suggest dysoxic/anoxic conditions prevailed during deposition of the Longmaxi intervals, which was mainly caused by a worldwide marine transgression after the melting of Gondwana glaciation. Productivity proxies indicate that the organic-rich Longmaxi sediments were deposited in moderate to high primary productivity water columns. The consistent Ti/Al ratios suggest a rather homogeneous nature for the detrital supply. Geochemical data provide a good constraint on the accumulation of organic matter and relevant oceanic environment. Anoxic conditions, together with relatively high productivity, were likely responsible for the accumulation of organic-rich sediments during the Longmaxi interval. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Organic matter Redox condition Primary productivity Early Silurian Yangtze platform

1. Introduction The accumulation of organic matter in marine sediments is one of the most fundamental biogeochemical processes on Earth, which not only influences the temperature and oxygenation of the oceans and atmosphere, but also comprises the primary source of hydrocarbons and other economic mineral deposits (Arthur and Sageman, 1994; Burdige, 2007). Although much work has been conducted on these organic-rich sediments, their genesis is still a matter of debate. The debate concerning the origin of these sediments polarized many workers toward “preservation” (e.g., Demaison and Moore, 1980; Arthur and Sageman, 1994; Mort et al., 2007) versus “productivity” (e.g., Pedersen and Calvert, 1990; Sageman et al., 2003) models. The former argues that bottom

* Corresponding author. E-mail address: [email protected] (D. Yan). http://dx.doi.org/10.1016/j.marpetgeo.2015.04.016 0264-8172/© 2015 Elsevier Ltd. All rights reserved.

water anoxia due to physical isolation of the benthic environment beneath a permanent pycnocline, as in the modern Black Sea, enhances organic matter preservation owing to the diminution or absence of aerobic decomposition. The latter suggests that bottom water anoxia is not the cause of high organic matter content but instead the consequence of high surface water productivity, which subsequently drives benthic oxygen demand to exceed its advective resupply by water column mixing (Murphy et al., 2000). In the last decades, it has been documented that other factors, such as the clastic influx of terrigenous elements, linked to sea-level fluctuations and/or climatic conditions, or oceanic circulation, may also influence the sequestration of organic matter (Sageman et al., 2003; Riquier et al., 2006). Therefore, there is no single control can explain organic accumulation in all sediments, and that each sedimentary setting may have several factors that contribute to the accumulation of organic-rich sediments (Rimmer et al., 2004). The Early Silurian is characterized by drastic environmental changes associated with widespread organic matter burial and

D. Yan et al. / Marine and Petroleum Geology 65 (2015) 290e301

black shale deposition (Schlanger and Jenkyns, 1976; Brenchley et al., 1994; Yan et al., 2010). Deposition of the Early Silurian organicerich sediments took place in many parts of the world, such as North Africa, Arabian Peninsula, Russian platform and the interior basins of the US, and in a variety of palaeobathymetric settings, ranging from deep ocean basins to shallow shelfal seas (Klemme and Ulmishek, 1991; Luning et al., 2000). Globally, Early Silurian organicerich sediments account for the generation of 9% of the world's petroleum reserves (Klemme and Ulmishek, 1991). In South China, the Lower Silurian succession contains thick (~30 m), organicerich (up to 10% total organic carbon) black shales and mudstones, which is an important source rock for conventional oil and gas, and has recently been considered as a potential shale gas reservoir (Zou et al., 2010; Liang et al., 2012). Although there are important, the mechanisms of organic matter accumulation during the Early Silurian in South China have not been well understood. In order to assess which factors controlled the accumulation of organic matter in Early Silurian sediments, a systematic geochemistry investigation at the Hehua section, Yichang, Hubei province was conducted. As there is no single parameter that could conclusively indicate the conditions of organic-rich sediments, we have chosen a multi-proxy approach. Pyrite morphology and multiple geochemical parameters (i.e. C/S, U/Th, V/Cr, Ni/Co and V/ (V þ Ni)) are used to determine the redox state of bottom waters. The contents of phosphorus (P), Barium (Ba), biogenic Barium (Babio) and the ratios of P/Ti, C/P are used as proxies for primary productivity. The role of clastic input was evaluated using the ratios of Ti/Al (Bertrand et al., 1996; Murphy et al., 2000). The implications of these results will be considered at both local and regional scales to assess possible mechanism concerning the organic matter accumulation during Early Silurian in South China. 2. Geological setting South China was a separate plate during the Paleozoic, and one of the most important paleogeographic features on the South China Plate was the Yangtze platform, which was covered by a broad epeiric sea and bordered to the southeast by the deep Pearl River Sea, which was probably connected to the world oceans (Wang et al., 1993) (Fig. 1). Continuous, stable sedimentation from the Sinian through the Paleozoic to the Late Triassic was widespread on the Yangtze platform (Wang, 1985; Wang et al., 1993). During the OrdovicianeSilurian transition, drastic environmental changes took place; the most remarkable of these, in the context of a longterm greenhouse climate, was the abrupt glaciation in Gondwana (Brenchley et al., 1994; Saltzman and Young, 2005; Yan et al.,

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2009a). Regional palaeogeographical reorganization was coincided with eustatic sea-level changes associated with the development and demise of Gondwanan glaciation, which indicated that a rapid Early Silurian transgression followed the Late Ordovician glacial regression (Chen et al., 2004). The Hehua Section lies in Yichang Area, Hubei Province, which is 75 km north of Yichang City (Fig. 2). The Hehua Section occurs on the east flank of the Huangling Anticline, which has a core of Precambrian rocks (Wang et al., 1999, Fig. 2). The OrdovicianeSilurian boundary succession, in ascending order, includes the Wufeng, Guanyinqiao, and Longmaxi Formations (Fig. 2). The Longmaxi Formation, generally several hundreds of meters thick, is mainly composed of black shales, siliceous mudstones, and locally siltstone with thin sandstone beds. The graptolitic black shale facies at the bottom of Longmaxi Formation is the most organic-rich part of the Lower Silurian. Exact descriptions about the stratigraphy and palaeogeography of Longmaxi can be seen in Wang et al. (1999) and Chen et al. (2004). 3. Methods Twenty-seven samples collected from a ca. 8.0em thick sequence at the Hehua section have been provided for the present study. All samples, pulverized to <200 mesh sizes, were split in several parts for organic carbon, pyrite sulfur, major and trace elements analysed. Organic carbon analyses were determined by the combustion method of Krom and Berner (1983). In this method, one subsample is analysed for total (inorganic plus organic) carbon (by combustion) and another is analysed for inorganic carbon (by acid evolution-gravimetry). The difference represents organic carbon. The reproducibility of TOC measurements is better than 0.1%. Pyrite sulfur was extracted by the Cr-reduction method of Canfield et al. (1986). Precisely weighed powered sample is reacted with CrCl2 to extracted sulfur as H2S under a stream of purified nitrogen gas. The liberated H2S is quantitatively precipitated as ZnS in a zincacetate solution, then converted to Ag2S by adding a silver-nitrate solution. The Ag2S precipitate is quantitatively filtrated, dried in an oven, and weighed precisely. The reproducibility of replicate analyses was generally better than 0.1%. The pyrite sulfur content is calculated by stoichiometry from the extracted pyrite. Major elements were analysed using an automatic X-ray fluorescence spectrometer (Shimadzu XRFe1500; Shimadzu Corporation, Kyoto, Japan) using fusion glasses made from a mixture of powdered sample and flux (Li2B4O7) in the proportion of 1: 8. The precision of the major element data are 3%. The sample splits (40 mg) for trace elements analysis were toasted in an oven at 105  C for 1e2 h and cooled to room temperature, then digested in a tightly sealed Teflon screwecap beaker with ultrapure 0.5 ml HNO3þ2.5 ml HFþ0.5 ml HClO4, then dried. The dried sample was digested again with 1 ml HNO3þ3 ml H2O until a clear solution was obtained. The solution was diluted to 1:1000 by mass and analysed on a VG PQ2 Turbo inductively coupled plasma source mass spectrometer (ICP-MS) at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Analytical precision for elemental concentrations was generally better than 4%. 4. Results

Figure 1. Early Silurian palaeogeographic map with the distribution of litho- and biofacies of the Yangtze area. Study area (+):1 ¼ Hehua, Yichang, Hubei Province (after Chen et al., 2004).

At Hehua, the TOC value varies generally between 3.00 and 10.85% (avg. 5.63%), with the lowest abundance in the Upper parts of Longmaxi (Fig. 3; Table 1). The pyrite sulfur content is variable, but is generally lower than 1.0% throughout the entire sequence. The S/C ratios of the samples are consistently low (avg. 0.12). It is noted that there is no covariance between the TOC and sulfur contents (Fig. 3).

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Figure 2. (a) Sketch map of main tectonic elements and stratigraphic units in Yichang area, Hubei province (modified from Wang et al., 1999); (b) Lithological column and sample locations across the OrdovicianeSilurian boundary in the Hehua Section, South China. FM.eFormation; H.eHirnantian; G.eGuanyinqiao.

Figure 3. Vertical variations of TOC, S, Ti, Al, P contents, and S/C, C/P, Ti/Al, P/Ti ratios in the sediments from Hehua section. See Fig. 2 for rock legends.

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Table 1 Variations of multiple geochemical parameters at Hehua section, Yichang, Hubei Province, South China. EF means enrichment factors: EFelement X ¼ (X/Al)sample/(X/Al)average shale (Brumsack, 1989; Calvert and Pedersen, 1993; Morford and Emerson, 1999; Piper and Perkins, 2004). Samples

H01

H02

H03

H04

H05

H06

H07

H08

H09

H10

H11

H12

H13

H14

Height (m) TOC (%) S (%) P (%) Si (%) Al (%) Ti (%) K (%) Th (ppm) V (ppm) Cr (ppm) Co (ppm) Ni (ppm) Cu (ppm) Zn (ppm) U (ppm) Ba (ppm) Babio (ppm) S/C C/P Ti/Al P/Ti Th/U V/(V þ Ni) V/Cr Ni/Co EFBa EFNi EFP EFV EFU EFCr EFZn EFCu EFCo

0.06 8.66 0.51 0.43 32.75 4.28 0.26 1.20 10.19 527.92 42.14 2.56 77.41 173.68 301.07 32.54 1373.51 1052.68 0.06 19.96 0.06 1.69 0.31 0.87 12.53 30.23 5.35 1.81 33.81 8.23 28.17 0.90 8.28 4.06 0.26

0.25 10.29 0.63 0.43 31.29 4.84 0.29 1.17 10.52 635.51 49.98 1.74 66.97 172.84 306.42 32.77 1404.59 1083.76 0.06 23.81 0.06 1.52 0.32 0.90 12.72 38.60 5.47 1.57 33.68 9.90 28.37 1.06 8.43 4.04 0.18

0.40 10.85 0.62 0.44 29.67 5.57 0.27 1.79 16.01 589.79 40.89 1.12 51.56 67.04 181.73 46.08 2029.36 1611.34 0.06 24.69 0.05 1.64 0.35 0.92 14.42 46.24 6.07 0.93 26.28 7.05 30.62 0.67 3.84 1.20 0.09

0.55 8.04 0.45 0.03 32.42 7.77 0.48 3.26 17.93 433.67 98.49 1.18 51.90 55.65 23.48 18.51 2445.93 1862.95 0.06 260.09 0.06 0.06 0.97 0.89 4.40 43.87 5.24 0.67 1.33 3.72 8.82 1.15 0.36 0.72 0.07

0.75 5.08 0.43 0.02 34.43 6.95 0.42 2.63 13.50 615.37 93.84 1.31 26.40 40.49 23.48 14.35 2173.76 1652.81 0.08 210.78 0.06 0.06 0.94 0.96 6.56 20.16 5.22 0.38 1.16 5.91 7.65 1.23 0.40 0.58 0.08

1.00 5.34 0.42 0.02 34.34 7.11 0.40 2.79 14.50 250.13 80.17 1.20 18.05 78.52 22.29 15.53 2542.43 2009.04 0.08 256.40 0.06 0.05 0.93 0.93 3.12 15.05 5.96 0.25 0.98 2.34 8.09 1.02 0.37 1.10 0.07

1.30 6.00 0.62 0.02 38.09 4.25 0.26 1.72 11.05 111.53 66.31 1.16 10.24 22.15 12.97 7.30 1261.85 943.18 0.10 271.58 0.06 0.09 1.51 0.92 1.68 8.83 4.95 0.24 1.73 1.75 6.36 1.42 0.36 0.52 0.12

1.55 6.92 0.61 0.02 38.66 4.82 0.27 1.69 11.08 112.14 66.49 1.17 12.56 24.74 12.89 7.34 1282.14 963.47 0.09 316.98 0.06 0.08 1.51 0.90 1.69 10.75 5.03 0.30 1.71 1.76 6.40 1.42 0.36 0.58 0.12

1.80 6.06 0.55 0.03 36.12 5.38 0.33 2.21 14.98 189.49 88.29 3.84 38.66 10.46 122.69 9.38 1603.01 1199.64 0.09 238.48 0.06 0.08 1.60 0.83 2.15 10.07 4.97 0.72 1.57 2.35 6.46 1.49 2.68 0.19 0.31

2.05 6.54 0.54 0.02 35.52 6.00 0.36 2.33 12.30 173.79 80.53 1.45 13.67 3.30 15.91 8.55 1765.34 1315.45 0.08 421.94 0.06 0.04 1.44 0.93 2.16 9.42 4.90 0.23 0.86 1.93 5.28 1.22 0.31 0.05 0.11

2.30 6.42 0.57 0.03 33.96 6.94 0.43 2.86 18.07 377.67 94.85 1.41 17.42 8.41 17.82 10.38 2018.43 1497.77 0.09 244.25 0.06 0.06 1.74 0.96 3.98 12.35 4.85 0.25 1.26 3.63 5.54 1.24 0.30 0.12 0.09

2.60 5.94 0.48 0.03 34.52 5.86 0.32 1.97 12.39 138.30 86.18 11.89 41.97 49.02 33.54 9.07 1464.43 1024.60 0.08 175.09 0.05 0.11 1.37 0.77 1.60 3.53 4.16 0.72 1.93 1.57 5.73 1.34 0.67 0.84 0.88

2.90 5.10 0.55 0.04 33.69 7.32 0.43 3.25 19.03 151.51 64.02 2.32 21.09 74.22 38.23 6.08 2274.28 1725.55 0.11 131.24 0.06 0.09 3.13 0.88 2.37 9.08 5.18 0.29 1.77 1.38 3.08 0.80 0.61 1.01 0.14

3.35 4.96 0.51 0.04 33.15 7.18 0.41 3.32 18.51 145.89 63.42 2.31 22.76 75.69 36.92 5.94 2447.95 1899.22 0.10 126.22 0.06 0.10 3.12 0.87 2.30 9.85 5.58 0.31 1.79 1.33 3.00 0.79 0.59 1.03 0.14

Samples

H15

H16

H17

H18

H19

H20

H21

H22

H23

H24

H25

H26

H27

Height (m) TOC (%) S (%) P (%) Si (%) Al (%) Ti (%) K (%) Th (ppm) V (ppm) Cr (ppm) Co (ppm) Ni (ppm) Cu (ppm) Zn (ppm) U (ppm) Ba (ppm) Babio (ppm) S/C C/P Ti/Al P/Ti Th/U V/(V þ Ni) V/Cr Ni/Co EFBa EFNi EFP EFV EFU EFCr EFZn EFCu EFCo

3.75 5.06 0.89 0.05 31.15 6.63 0.33 2.46 13.98 146.72 89.69 21.45 63.67 57.60 122.63 6.71 1427.16 929.73 0.18 106.52 0.05 0.14 2.08 0.70 1.64 2.97 3.59 0.96 2.39 1.47 3.75 1.23 2.18 0.87 1.41

4.10 4.82 0.63 0.24 40.73 2.14 0.12 0.37 4.03 669.47 145.63 3.77 86.20 47.50 105.26 15.73 2011.24 1850.67 0.13 20.33 0.06 1.93 0.26 0.89 4.60 22.86 15.66 4.03 36.91 20.85 27.21 6.18 5.78 2.22 0.77

4.50 4.81 1.14 0.05 31.83 6.25 0.33 2.07 13.18 267.78 93.69 14.45 80.60 50.35 176.12 6.29 1287.02 818.04 0.24 95.96 0.05 0.15 2.09 0.77 2.86 5.58 3.43 1.29 2.67 2.85 3.73 1.36 3.31 0.81 1.00

4.85 3.40 0.59 0.03 32.91 6.97 0.43 2.53 17.74 60.60 56.66 18.39 53.06 31.12 56.01 1.80 966.05 443.39 0.17 130.22 0.06 0.06 9.86 0.53 1.07 2.89 2.31 0.76 1.25 0.58 0.96 0.74 0.95 0.45 1.15

5.20 3.20 0.63 0.02 31.50 2.97 0.19 1.20 8.24 28.74 32.58 9.98 26.47 28.08 38.43 0.88 1465.88 1242.81 0.20 188.89 0.06 0.09 9.39 0.52 0.88 2.65 8.21 0.89 1.90 0.64 1.09 1.00 1.52 0.94 1.46

5.60 3.46 0.61 0.04 29.57 8.48 0.49 3.37 22.23 92.71 86.04 14.29 68.64 102.67 125.17 3.47 1521.84 885.86 0.18 87.95 0.06 0.08 6.41 0.57 1.08 4.80 2.99 0.81 1.55 0.73 1.52 0.92 1.74 1.21 0.73

5.95 4.50 0.58 0.04 30.31 9.03 0.53 3.23 24.18 158.54 87.83 43.06 168.34 117.51 348.04 4.00 1697.62 1020.06 0.13 121.54 0.06 0.07 6.04 0.49 1.80 3.91 3.13 1.86 1.37 1.17 1.64 0.88 4.53 1.30 2.07

6.30 3.00 0.62 0.03 34.14 7.38 0.46 3.25 18.39 146.88 73.09 2.34 28.88 64.50 41.47 5.87 1877.31 1323.48 0.21 88.54 0.06 0.07 3.14 0.84 2.01 12.35 4.24 0.39 1.53 1.33 2.94 0.90 0.66 0.87 0.14

6.70 4.20 0.53 0.02 43.95 0.86 0.05 0.41 2.30 40.50 147.60 2.30 17.47 37.52 38.33 1.55 889.65 825.24 0.13 262.11 0.06 0.30 1.48 0.70 0.27 7.61 17.26 2.03 6.22 3.14 6.67 15.62 5.25 4.37 1.16

6.85 4.30 0.53 0.44 39.14 1.72 0.11 0.40 4.71 66.55 60.29 6.80 120.35 49.73 263.96 13.04 576.65 447.31 0.12 9.75 0.06 4.01 0.36 0.36 1.10 17.70 5.57 6.98 85.24 2.57 28.00 3.18 18.01 2.88 1.71

7.05 5.20 0.56 0.09 35.59 4.12 0.22 1.67 11.73 140.07 91.96 5.72 100.75 134.25 250.15 10.97 1125.65 816.74 0.11 60.46 0.05 0.39 1.07 0.58 1.52 17.62 4.55 2.45 6.96 2.27 9.87 2.03 7.15 3.26 0.60

7.40 4.90 0.53 0.25 37.19 1.65 0.09 0.64 3.37 83.71 58.62 5.56 62.18 203.27 199.53 14.90 696.96 573.07 0.11 19.95 0.06 2.61 0.23 0.57 1.43 11.19 7.03 3.76 49.57 3.38 33.41 3.23 14.21 12.31 1.46

7.90 4.92 0.56 0.35 35.19 2.59 0.15 0.38 5.98 133.98 64.79 6.40 123.06 202.43 228.07 14.49 1358.56 1164.68 0.11 14.03 0.06 2.34 0.41 0.52 2.07 19.22 8.76 4.76 45.21 3.46 20.76 2.28 10.38 7.83 1.08

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Stratigraphic variations of Ti, Al and P contents and Ti/Al and P/ Ti ratios are shown as representatives of major elements in Figure 3. The Ti contents of Longmaxi sediments are relatively variable, ranging from 0.05 % to 0.53 % (avg. 0.31%) and lower than a normal shale value (0.48%). The stratigraphic variation of Al content shows a similar trend of Ti variation. The Al contents vary generally between 0.86% and 9.03% (avg. 5.37%), mostly less than North America Shale Composite (NASC) (Gromet et al., 1984). The P contents are highly variable (Fig. 3), but samples from the Middle Longmaxi deposits are characterized by low P contents except one sample with relative high P content (0.24%). The Ti/Al profile shows relatively consistent values between 0.048 and 0.064 (avg. 0.058) throughout the section. The stratigraphic variation of P/Ti ratios shows a similar image of P variation, and most samples from the middle Longmaxi sediments yield relative low P/Ti ratios (Fig. 3). The concentrations of Ba range between 576.65 and 2542.43 ppm (avg. 1592.17 ppm), and are generally higher in the lower part of the section, and fluctuated decrease upward (Table 1). The Longmaxi sediments yield U contents (2.70e46.08 ppm; avg.11.98 ppm), V contents (28.74e669.47 ppm; avg. 240.33 ppm), Ni contents (10.24e168.34 ppm; avg. 54.46 ppm), Cr contents (32.58e147.60 ppm; avg. 77.93 ppm) and Th contents (2.30e24.18 ppm; avg. 12.97 ppm). The poor correlations of these redox-sensitive trace elements with Th indicate that U, V, Ni and Cr in the sediments concentrated through authigenic enrichment rather than detrital input (Fig. 4). Trace elements generally show systematic enrichment throughout the Longmaxi sequence.

Compared to NASC, Ba shows 2.31e17.26 (avg. 6.18) times enrichment; P shows 0.86e85.24 (avg. 15.13) times enrichments; U shows 0.96e33.41 (avg. 11.36) times enrichments; V shows 0.58e20.85 (avg. 4.09) times enrichments. However, Ni, Cr, Zn, Cu and Co show minor enrichment or even slight depletions (Fig. 5). 5. Discussion 5.1. Redox conditions Seafloor oxygenation can qualitatively be evaluated from observations of total organic carbon combined with descriptions of sedimentary structure (laminated or bioturbated) and benthic faunal abundance (Schlanger and Jenkyns, 1976; Arthur and Sageman, 1994). The Longmaxi Formation at the Hehua section dominated by black, laminated fabric, organic-rich shale and mudstone. Although nektonic graptolites are abundant and well preserved (Chen et al., 2004), there is no evidence of bioturbation. So, these can be interpreted as signatures of dysoxic/anoxic conditions at the seafloor during the Longmaxi period. Pyrite in sedimentary rock can occur in two general forms, cubic to pyritohedral crystals, and spheroidal aggregates of pyrite microcrystals (framboids) (Powell et al., 2003). The exact reaction mechanism for framboid formation is debated (e.g., Wilkin and Barnes, 1996; Richard, 1997) but is generally considered to be an inorganic multi-stage process that occurs under predominantly anoxic conditions (Powell et al., 2003). Studies of pyrite framboid

Figure 4. Binary cross-correlation plots of TheV (A), TheCr (B), TheNi (C), and TheU (D).

D. Yan et al. / Marine and Petroleum Geology 65 (2015) 290e301

295

Figure 5. Comparison of enrichment factor of some trace metals for the Longmaxi horizon of the Hehua section. The extent of the boxes corresponds to the range of values (minemax) and the inner line to the average value. The dashed line indicates the value for which there is no enrichment/depletion with regards to average shale composition.

diameters from a variety of modern and past environments have provided a potential tool to distinguish anoxic conditions from oxicdysoxic conditions (Wilkin et al., 1997; Wignall and Newton, 1998; Riquier et al., 2006). The mean size of pyrite framboids which formed in anoxic water columns is generally less than 4.7 ± 0.5 mm (Wilkin et al., 1997; Wignall and Newton, 1998). Our SEM observations of the Hehua samples (Fig. 6AeD) reveal the presence of dispersed pyrite framboids. All of the framboid diameters in these samples are less than 5 mm, which are rather constant and small sizes. The framboid size distribution data indicate the persistence of anoxic water column during the Longmaxi intervals. The Organic carbonepyrite sulfur (CeS) relationship is commonly a quick method to assess the oxygen level of bottom

waters (Leventhal, 1987; Hofmann et al., 2000). This method is based on the covariance of organic carbon and pyrite sulfur, which is resulted from the catabolism of organic carbon and concomitant reduction of sulfate by sulfate reducing bacteria to form hydrogen sulfides that reacts with iron to form pyrite in the sediments (Leventhal, 1987). The S/C ratio of 0.36 is commonly used as a reference value for Phanerozoic normal marine shales (Berner and Raiswell, 1984). Anoxic marine environments are either characterized by S/C ratios higher than 0.36 with positive intercepts on the Saxis in SeC plots (Leventhal, 1983; Berner, 1984), or by a fairly constant sulfur content independent of the organic carbon content (Hofmann et al., 2000). The latter case occurs mainly in anoxic environments where sulfur fixation via pyrite formation is limited

Figure 6. Scanning electron microscope (SEM) photomicrographs of pyrite from studied samples: (A) Pyrite framboids in sample H03 from Hehua section; (B) Framboidal pyrite made up of uniform microcrystals in sample H05 from Hehua section; (C) Cluster of well developed octahedral crystals in sample H07 from Hehua section; (D) Pyrite framboids in sample H14 from Hehua section.

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by the availability of reactive iron, hence, pyrite precipitation ceases after the reactive iron is exhausted (Raiswell and Berner, 1985; Dean and Arthur, 1989). Most samples from the Longmaxi Formation at Hehua section, although yield fairly low S/C ratios, have a fairly constant sulfur content independent of the organic carbon content. This indicates Longmaxi sediments mainly deposited under anoxic marine conditions (Fig. 3). However, several high S/C ratios in the mid Longmaxi samples, close to the normal marine values (S/C ¼ 0.36), likely resulting from the relative oxygenated environments (Fig. 3). Redox-sensitive trace element concentrations or ratios are the most widely used in geochemistry to estimate the degree of bottom water oxygenation during sedimentation (e.g., Calvert and Pedersen, 1993; Jones and Manning, 1994; Dean et al., 1997; Cruse and Lyons, 2004). Redox-sensitive trace element commonly exhibit considerable enriched in laminated, organic-rich facies, especially those deposited under anoxic conditions and, conversely, little if any enrichment in bioturbated, organic-poor facies (Algeo and Maynard, 2004). In accordance with this, relatively high trace element concentrations were previously reported from Low Silurian Longmaxi black shales of Fengxiang and Wangjiawan sections in the Yichang area and were attributed to enhanced trace metal scavenging in an anoxic water column (Fig. 2; Wang et al., 1993; Yan et al., 2009b). To estimate the effects of dilution in sedimentary rocks, the enrichment factors were calculated by normalizing trace element concentrations to Al (Brumsack, 1989; Calvert and Pedersen, 1993; Morford and Emerson, 1999; Piper and Perkins, 2004). The average enrichment factors of U, V, and Cr in the Longmaxi shales are much higher than 2 (Fig. 5), indicating authigenic enrichment of these redox-sensitive trace elements and an anoxic depositional setting. Further information on water column redox conditions can be gained from some calculated redox indices, such as Th/U, V/Cr, Ni/Co and V/(V þ Ni).

The actinide metals, uranium (U) and thorium (Th), exhibit similar geochemical behaviour, except under oxidizing conditions (Liu et al., 1984). Thorium is unaffected by redox conditions and remains insoluble as Th4þ. Uranium, however, exists as insoluble U4þ under highly reducing conditions, which leads to U enrichment in sediments, whereas it exists as soluble U6þ under oxidizing conditions, leading to U loss from sediments (Wignall and Twitchett, 1996; Kimura and Watanabe, 2001). Th/U ratios typically vary from 0 to 2 in anoxic environments and up to 10 in a strongly oxidizing environment (Wignall and Twitchett, 1996), and this general guideline has been followed in subsequent studies (e.g., Kimura and Watanabe, 2001; Chang et al., 2012). At Hehua section, most samples from the Longmaxi Formation are dominated by low Th/U ratios, suggesting that sediments deposited in a predominantly anoxic depositional environment, but the water column might have been oxygenated in some short-term interval (Fig. 7). Those samples from the mid Longmaxi interval are dominated by high Th/U ratios, indicating marine was dominated by oxygenated water column. Under anoxic conditions, Ni and V is preferentially enriched (Lewan and Maynard, 1982; Calvert and Pedersen, 1993), and higher Ni/Co (>5.0), V/Cr (>2.0) and V/(V þ Ni) (>0.6) ratios have been used to indicate dysoxic/anoxic conditions (Jones and Manning, 1994; Rimmer, 2004). Ni/Co, V/Cr and V/(V þ Ni) ratios of the Longmaxi samples are 2.65e46.24 (avg. 15.46), 0.27e14.42 (avg. 3.75) and 0.36e0.96 (avg. 0.75) (Table 1; Fig. 7), respectively, and most of them are higher than the threshold values (5.0, 2.0 and 0.6, respectively). So, these signatures indicate that bottom water environments fluctuate between long-term dysoxic/anoxic and transient oxidation in the Longmaxi interval. To summarize, the present data (C/S, Th/U, V/Cr, Ni/Co and V/ (V þ Ni)) and pyrite morphology as stated above indicate that sediments from the Longmaxi formation, even though existing

Figure 7. Stratigraphic distributions of V/(V þ Ni), V/Cr, Ni/Co and Th/U of the Longmaxi sediments from Hehua section. Ranges for V/Cr, U/Th and Ni/Co are from Jones and Manning (1994); ranges for V/(V þ Ni) are from Hatch and Leventhal (1992).

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short-term oxidation, were mainly deposited in anoxic depositional environment. 5.2. Primary productivity Availability of nutrients, especially P, in surface waters is a critical issue in the debate over controls on organic matter accumulation as this controls primary productivity (Rimmer et al., 2004). Because distributions of P in sediments are linked to the supply of organic matter, possibly resulting from high productivity, P was proposed as an indicator for biological productivity (Schmitz et al., 1997). The average crustal abundance of P is 0.01%, but it shows a higher content in most marine sediments and sedimentary rocks (Mackenzie et al., 1993). At Hehua, the P content varies generally between 0.02 and 0.44% (avg. 0.12%), with the highest abundance at the Lower and Upper parts of Longmaxi Formation (Fig. 3; Table 1). P/Ti ratios are highly variable, but show a similar trend of P variation (Fig. 3). Samples from the lower and upper Longmaxi deposits yield high P/ Ti ratios, which close to those (~2e8) associated with regions of elevated productivity in the modern equatorial Pacific (Murray and Leinen, 1993) and may imply relative high-nutrient surface waters

297

during this time interval. C/P ratios of samples from the Hehua section generally vary from 9.75 to 421.94 (avg. 150.33) (Fig. 3). The wide variations of C/P ratios may reflect the change of primary productivity during the Longmaxi times. The weak correlation between P and TOC (R2 ¼ 0.26) implies no consistent relationship between organic matter accumulation and productivity (Fig. 8A). However, at the lower Longmaxi Formation, the TOC maximum, corresponding to higher P content, indicates primary productivity sometimes plays a role in organic matter accumulation. Ba is also considered as an indicator for paleoproductivity since it originates from barite formed in decaying phytoplanktonic organic matter (Dymond et al., 1992; Francois et al., 1995). Barite, the main carrier of particulate barium in the water column and the phase associated with carbon export, has also been suggested as a reliable paleoproductivity proxy (Eagle et al., 2003). Such barite formation has been shown to occur in the well-oxygenated open ocean (Dymond et al., 1992; Francois et al., 1995) as well as in the suboxic to anoxic waters and euxinic basins (Dean et al., 1997; Kuypers and Pancost, 2002). Barium, however, is contained in other phases, some of which are also biogenically related (e.g., organic matter, biogenic silica and biogenic carbonate), and others

Figure 8. Relationships between P and TOC contents (A), Babio and TOC contents (B), Ti/Al ratios and TOC contents (C), K and Al contents (D), Si and Al contents (E), Ti and Al contents (F) for the samples from the Longmaxi sediments at Hehua section.

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that are not directly related to carbon export (e.g., terrigenous silicates, FeeMn oxides and hydroxides) (Eagle et al., 2003). The Babio content, which is estimated by correcting for Ba of detrital origin, is proven to be a good proxy to estimate carbon export (e.g., Eagle et al., 2003; Averyt and Paytan, 2004). Babio is determined from the total Ba concentration in the sediment minus the Ba associated with terrigenous material, which is calculated from total Al and normalization to a constant Ba/Al ratio:

Babio ¼ Batot  ðAltot  Ba=Alalu Þ ¼ Batot  ðAltot  0:0075Þ (1) This equation assumes that all the aluminium in sediments is of aluminosilicate origin. Various compilations of elemental abundances in crustal rocks (Taylor, 1964) suggest that the Ba/Alalu ratios of aluminosilicate detritus in crustal rocks range between 0.005 and 0.01. A Ba/Alalu ratio of 0.0075 is used to calculate a normalised Babio content (see Dymond et al., 1992; Wei et al., 2012). Babio concentrations are commonly elevated (~1000e5000 ppm) in high productivity regions of the modern equatorial Pacific (Murray and Leinen, 1993). The Longmaxi sediments yield mostly moderate to high estimates of Babio, from 443.40 to 2009.04 ppm (avg. 1191.95 ppm). Although the relatively weak correlation between Babio and TOC abundance may imply that some environmental factors (i.e. barite dissolve resulting from BSR) influence the preservation of Babio (Fig. 8B; Brumsack and Gieskes, 1983; Torres et al., 1996), these Babio contents can be interpreted as evidence of moderate to high productivity rate during the Longmaxi times. This result is in agreement with P investigation above. 5.3. Clastic influx Changing the clastic influx may influence organic matter concentration in marine sediments (e.g., Ibach, 1982). It may do so directly, by either serving as a variable dilutant or by providing sites

for organic matter adsorption in and on aluminosilicates (e.g., Rimmer et al., 2004), or indirectly, by influencing burial rate of organic matter and thereby the duration and completeness of its bacterial decomposition (e.g., Canfield, 1994). The Ti/A1 ratio is a reliable proxy for siliciclastic grain size, and hence sedimentation rate (Bertrand et al., 1996; Murphy et al., 2000). Aluminum occurs only in clay minerals, whereas the Ti occurs both in clays and in sand- and silt-sized grains such as ilmenite, sphene, and augite (Rimmer et al., 2004). Therefore, higher Ti/A1 values represent larger grains. In studied Hehua section, the Ti/Al ratios display relatively consistent values between 0.048 and 0.064 (avg. 0.058) (Fig. 3). The very weak correlations between Ti/A1 and TOC abundances (R2 ¼ 0.09; Fig. 8C) show no consistent relationship between organic matter concentration and sedimentation rate. The good correlations of K vs. Al (R2 ¼ 0.93), Si vs. Al (R2 ¼ 0.49), Ti vs. Al (R2 ¼ 0.97) (Fig. 8DeF), may suggest a rather homogeneous nature for the detrital supply. These data suggest that the high TOC concentration is not simply a function of clastic starvation or of the increased surface area for organic matter adsorption associated with finer grains.

5.4. Controls on the accumulation of organic matter Factors controlling organic matter accumulation include enhanced preservation of primarily marine organic matter under reducing conditions (Demaison and Moore, 1980; Mort et al., 2007), high marine surficial primary productivity (Pedersen and Calvert, 1990; Caplan and Bustin, 1998), or combinations of these models (Arthur and Sageman, 1994; Murphy et al., 2000). Other factors, such as influx of clastic material and supply of terrestrial organic matter, may also play an important role in organic matter accumulation (e.g., Murphy et al., 2000; Sageman et al., 2003). It should be noted, however, that factors such as productivity, oxygenation

Figure 9. Relationships between Th/U ratios and TOC contents (A), V/(V þ Ni) ratios and TOC contents (B), V/Cr ratios and TOC contents (C), Ni/Co and TOC contents (D) for the samples from the Longmaxi sediments at Hehua section.

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Figure 10. Schematic illustration to show factors controlling the accumulation of organic matter in Early Silurian Longmaxi sediments in South China.

level and sedimentation rate are not independent variables, but are directly correlated with each other (Tyson, 1995). Within the studied section, paleoproductivity proxies (P and Ba) indicate that the organic-rich sediments were deposited in the moderate to high nutrient surface water column, which is usually considered favourable for organic matter production. The enhanced fluxes of organic matter to the seafloor due to moderate to high primary productivity likely increased the consumption of dissolved oxygen in water column, promoting establishment of oxygen-deficient deep water conditions and further improving preservation of sedimentary organic matter. Ti/A1 ratios are relatively constant throughout the Hehua section and the weak correlations of Ti/A1 vs. TOC (R2 ¼ 0.09) suggest sedimentation rate may not be important in enhanced sequestration of organic matter. Paleoredox indices (C/S, U/Th, V/Cr, Ni/Co, V/(V þ Ni) and pyrite morphology) suggest that sediments from the Longmaxi formation were mainly deposited in anoxic depositional environment. The relatively good correlations of Ni/Co vs. TOC (R2 ¼ 0.61), V/Cr vs. TOC (R2 ¼ 0.69), V/ (V þ Ni) vs. TOC (R2 ¼ 0.31) and Th/U vs. TOC (R2 ¼ 0.29) indicate that organic matter accumulation mainly resulted from anoxic water column (Fig. 9AeD). This anoxic environment might be mainly caused by a worldwide marine transgression after the melting of the Gondwana glaciation (Chen et al., 2004; Yan et al., 2012). Evidences from sedimentological, faunal, and geochemical data all suggest that the major phase of the Gondwana glaciation occurred in the Guanyinqiao Formation, just below Longmaxi Formation (Fig. 2B; e.g., Chen et al., 2004; Yan et al., 2010). In this case, the swift return to a warm climate in the beginning of Longmaxi time (or terminal Guanyinqiao) could result in glaciation melting and eustatic sea level rose. Berry et al. (1989) suggested that the Early Palaeozoic oceans were oxygen poor and had a nitrogenouscompound-rich water layer beneath the surface wind-mixed layer and anoxic water on the bottom. When sea level rose during warminterval transgressions, anoxic waters may have invaded the outer shelf. Therefore, the expansion of anoxic waters across the shelves during warm intervals may mainly responsible for the spread of organic-rich sediments during the Early Silurian Longmaxi times (Fig. 10). 6. Conclusions Based on new evidences of geochemical indicators for the accumulation of organic matter during Early Silurian in South China, these followings are concluded: (1) Paleoredox proxies (C/S, U/Th, V/Cr, Ni/Co, V/(V þ Ni) and pyrite morphology) indicate that the Longmaxi sediments

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