Marine Incursion Events in the Late Cretaceous Songliao Basin: Constraints from Sulfur Geochemistry Records

Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 152–161

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Marine incursion events in the Late Cretaceous Songliao Basin: Constraints from sulfur geochemistry records

Huang Yongjian a,b,⁎, Gansheng Yang c,⁎⁎, Gu Jian a,b, Wang Pingkang f, Huang Qinghua c, Feng Zihui d, Feng Lianjun e a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China b School of the Earth Science and Resources, China University of Geosciences, Beijing 100083, China c School of Engineering and Technology, China University of Geosciences, Beijing 100083, China d Institute of Oil Exploration and Development, Daqing Oilﬁeld Company, Daqing 163712, China e Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China f Oil and Gas Survey, China Geological Survey, Beijing, 100029, China article info abstract

Article history: Songliao Basin in NE China developed the most productive oilﬁeld in the world sourced from terrestrial rocks. Received 26 December 2011 1 The main source rock of the basin includes member 1 of the Qingshankou (K2qn ) and members 1 and 2 of Received in revised form 5 March 2013 1–2 the Nenjiang Formation (K2nj ). However the exact reasons for the formation of the source rock, especially Accepted 19 March 2013 1 the K2qn are still controversial. Former paleontological and organic geochemical research suggested that organic Available online 13 April 2013 1 matter was deposited during marine incursion events of K2qn but further geochemical evidence is needed. This paper explores the distinct sulfate levels that distinguish marine from fresh waters of the Songliao paleo-lake. We Keywords: Marine incursion events undertook a systematic investigation the sulfur geochemistry of the sediments from top of member 4 of the 4 1 Songliao Basin Quantou Formation (K1q ) to top of member 1 of the Qingshankou (K2qn ). The ratio of organic carbon to the 1 Cretaceous pyrite sulfur (TOC/PYS) proved the previously suggested saline conditions during the deposition of K2qn ;and 1 Sulfur geochemistry the pyrite sulfur isotope indicated that marine incursion may not only have occurred for K2qn , but also for top 4 of K1q . The exact time for the beginning of marine incursion is to be determined. The marine incursion within 1 K2qn is not strong and partially controlled by the ﬂuctuation of paleo-lake level. The present study will be helpful for understanding the contribution of marine incursion events to the burial of organic carbon in the Songliao paleo-lake, although further studies are still needed. © 2013 Elsevier B.V. All rights reserved.

1. Introduction deposited in the Songliao Basin, is still unknown (Li and Pang, 2004). Prior studies of the formation of source rock in the Songliao Basin AsoneofthelargestlateMesozoic–Cenozoic continental sedimentary have found that the hydrocarbon source rock of Songliao was deposited basins in East Asia, the Songliao Basin in NE China developed an almost under brackish-saline conditions (Hou et al., 2000 and references complete Cretaceous succession (Chen, 1987, 2003). During the Creta- therein), despite the fact that the Songliao paleo-lake was mainly a ceous, the basin was a large rift basin that hosted a long-lived deep lake freshwater lake during most of its life cycle. Paleontological and organic (Chen, 1987). This history enabled the basin to become the largest oil geochemical evidences suggested that salinity conditions may be due to and gas producing basin in China, with China's largest oilfield, Daqing, an influx of seawater during a brief marine connection and lake trans- being situated in the central part of the basin (Zhou and Littke, 1999). gression (Huang et al., 1999; Hou et al., 2000). For example the identi- Most of the source rocks in the Daqing oil field are terrestrial fication of marine dinophyceae, planktonic foraminifera, shark teeth, deposits; this may be the most productive oilfield in the world sourced and biomarkers such as C30 4-desmethylsteranes in these strata from terrestrial rocks (Yang et al., 1985). However the exact reason for would suggest conditions with a strong marine influence (Zhang and the formation of the mass hydrocarbon source rock, including the mem- Zhou, 1976; Zhang et al., 1977; Gao et al., 1994a,b; Huang et al., 1998; ber 1 of Qingshankou and member 1 and 2 of Nenjiang Formation Gao and Zhao, 1999; Hou et al., 2000; Huang, 2007), however contro- versy questions the validity of these evidences (Ye and Zhong, 1990; ⁎ Correspondence to: Y. Huang, State Key Laboratory of Biogeology and Environmental Huang and Huang, 1998; Huang et al., 1999; Li et al., 2000; Ye et al., Geology, China University of Geosciences, Beijing 100083, China. Tel.: +86 10 8233 2422; 2002). New evidences of the marine incursion events are still needed. fax:+861082322171. Different levels of sulfate distinguish fresh from marine water ⁎⁎ Corresponding author. Tel.: +86 10 8233 2422; fax: +86 10 8232 2171. μ E-mail addresses: [email protected] (Y. Huang), [email protected] (~10 to hundreds of M vs. 28 mM). The sulfate concentration of (G. Yang). lake water would have increased and the geochemical behavior of

0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.03.017 Y. Huang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 152–161 153 sulfur changed if the lake was connected with the open ocean incursion events of this lake. Previous data have shown that the re- (Holmer and Storkholm, 2001). This effect is best illustrated by the duced sulfur was relatively enriched in member 1 of the Qingshankou modern Black Sea, which was formerly a fresh-water lake but was and members 1 and 2 of the Nenjiang Formation deposited in the connected with the Mediterranean not long ago; sulﬁdic conditions Songliao Basin, which makes such investigation possible (Huang, prevail in the present water column (Yücel et al., 2010 and reference 2007). This work concentrates on the isotopic geochemistry of sulfur therein). A complete investigation of sulfur geochemistry of the sedi- to elucidate if marine incursion events actually occurred during the ments, i.e., the species and isotope ratio would be of help to under- deposition of member 1 of the Qingshankou Formation. The present stand the details of the sulfur cycle in an aquatic system (Holmer study will also be of value in answering the long-debated question and Storkholm, 2001), and thus revealing the potential marine of whether the marine incursion really contributed to the organic

Fig. 1. (a) Location of the Songliao Basin showing major structure zones and oil ﬁelds in the basin (Zhou and Littke, 1999). SK-1(N) and SK-1(S) refers to the north and south hole of the SK-1 drilling program. (b) Stratigraphic column of the Mesozoic successions of the Songliao Basin, NE China. 1-andesite; 2-rhyolite; 3-mudstone; 4-argillaceous siltstone; 5-siltstone; 6-sandstone; 7-gravel; 8-conglomerate; 9- fossil (after Wang and Liu (2001) with revision). 154 Y. Huang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 152–161

Fig. 1 (continued). carbon accumulation in the Songliao paleo-lake, because the sulfur (WRR), northern plunge region (NPR), central depression region cycle regulates the redox condition, nutrient status and affects the pro- (CDR), northeastern uplift region (NUR), southeastern uplift region ductivity of both the lake (Fleet et al., 1988; Caroca et al., 1993; Cohen, (SUR), and southwestern uplift region (Fig. 1)(Yang et al., 1985; Gao 2003) and marine systems (Adams et al., 2010). et al., 1994a, 1994b). The basin is ﬁlled predominantly with alluvial fan, ﬂuvial and lacustrine sediments of Late Jurassic, Cretaceous and 2. Geological setting Paleogene ages on a pre-Mesozoic basement (Fig. 1b). The oldest sedimentary cover within the basin is the Upper Jurassic Huoshiling

The Songliao Basin is an intra-cratonic Cretaceous rift basin, as dem- Formation (J3h). Overlying the Jurassic strata are the Lower Cretaceous onstrated by an abundance of data generated by the petroleum industry Shahezi (K1sh), Yingcheng (K1y), Denglouku (K1d), and Quantou (K1q) regarding its subsidence and geothermal histories, sedimentary facies, formations; these are overlain by the Upper Cretaceous Qingshankou crustal underpinnings, and structural style (Song, 1997; Einsele, 2000). (K2qn), Yaojia (K2y), Nenjiang (K2n), Sifangtai (K2s), and Mingshui The evolution of the Songliao Basin consists of five stages: 1) a pre-late (K2m) formations. The sedimentary sequence is capped by the Jurassic pre-rift doming, 2) the latest Jurassic extensional fracturing, Paleogene–Neogene Yi'an (E2–3ya), Da'an (N2da) and Taikang (N2tk) 3) the earliest Cretaceous full graben, 4) late Early to mid-Cretaceous formations. downwarping, and 5) Late Cretaceous basin shrinkage and regional The Qingshankou Formation was defined at the surface section compression (Song, 1997). Based on tectonic evolution of the basin exposed in the Qingshankou village of Nong'an County, Jilin Province and regional tectonics, the Songliao Basin can be divided into the of China. It also crops out along the banks of the Songhuajiang River following 6 first-order tectonic units (Fig. 1a): western ramp region throughout the Jilin and Heilongjiang Province (Chen, 1987). The Y. Huang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 152–161 155 thickness of the strata is approximately 300–500 m, which can be divid- 4. Results 1 2–3 ed into 3 members, i.e., member 1 (K2qn ) and members 2–3(K2qn ) according to their lithological and biological assemblages (Huang, The analytical results of the TOC, pyrite sulfur and sulfur isotope 1 2007). The K2qn conformably overlies member 4 of Quantou Forma- are presented in Table 1. In member 4 of the Quantou Formation, tion and is dominated by dark gray, olive-gray, and olive-black the average content of pyrite sulfur is 0.55% with a range from mudstones from 25 to 164 m thick. This unit is considered to be one 0.003 to 0.96%, and the sulfur isotope ratio (δ34S) varies from 17.29 of the most favorable source rocks deposited during the first largest to 21.51‰ and has an average of 19.47‰. In member 1 of the lake transgression of the Songliao Basin (Huang, 2007). The Qingshankou Formation, the pyrite sulfur is at the level from 0.03 to 1 34 basin-wide laminated oil-shale developed in the lower part of K2qn 1.74% with the average of 0.59%; and the sulfur isotope ratio (δ S) bearing thin-layered siderite bands (lens). In the deeper part of the ranges from 14.4 to 24.06% with an average of 18.48%. The TOC 2–3 basin the K2qn is composed of intercalated black mudstone and content ranges from 1.02 to 8.63% with an average of 3.25%, and gray, green thinly layered siltstone, which becomes red silty mudstone from 0.003 to 0.97% with an average of 0.35%, in member 1 of the 1 in the marginal area. Underlying the K2qn is member 4 of the Quantou Qingshankou Formation and member 4 of the Quantou Formation, Formation characterized by a series of positive rhythmic units com- respectively. posed of green-gray or white-gray siltstones and fine sandstones, and The features of these geochemical indexes in the two formations red mudstone and silty mudstone. At the center and eastern parts of can be better explored by plotting them with the depth of the sam- the basin, the color of mudstone in the top part of the strata is black. ples (Fig. 2). The TOC content remains at a low level of approximately 4 Previous evidence suggested that the K1q was deposited in a 0.35% in member 4 of the Quantou Formation, but rises dramatically 1 fluvial-lacustrine environment. However, the K2qn was deposited in member 1 of the Qingshankou Formation, which shows an increasing during the first great lake transgression when the lacustrine basin trend, albeit with fluctuations, from the lowest sample at about 1782 m was rapidly subsiding and the subsidence rate in the basin was greater to the peak of 8.63‰ at the depth of 1769.87 m. The level remains high than sediment supply (Gao et al., 1994a, 1994b). The depo-center up to 1750 m, and then decreases slowly through the upper part of migrated with the subsidence center, and both are located in the member 1 of the Qingshankou Formation. However the sulfur isotope Qijia-Gulong and Sanzhao regions. Semi-deep to deep lacustrine facies ratio maintains a high level of approximately 19.47‰ in member 4 of 1 of K2qn were deposited in the Gulong Sag (Wang et al., 1994). the Quantou Formation, and forms a “valley” from 1782 to 1770 m in the lower part of the Qingshankou Formation (with a minimum of 3. Material and methods 14.40‰ at the 1782 m). The sulfur isotope curve maintains at a relatively high level at the middle part of strata (1770 to 1750 m) and then The SK-1 drilling program, supported by the Ministry of Science decreases through the rest of member 1 of the Qingshankou Formation. and Technology of China (MOST) and the Daqing Oilfield, was initiated The pyrite sulfur is low in member 4 of the Quantou Formation but in- and conducted to recover complete cores from the upper Quantou to creases dramatically to form a “plateau” from 1782 to 1770 m in the Taikang Formations in the Songliao Basin (Fig. 1a, b). A “one well-two lower part of the Qingshankou Formation, then decreases to 0.5% at holes” plan was implemented from August 18, 2006 to October 20, 1768 m and maintains at low level of 0.5% through the rest of the strata. 2007 according to the site selection strategy (Huang, 2007). The northern hole, SK-1(N), recovered 1541.66 m of continuous core from Knj1 through the Yi'an Formation of oldest Tertiary with a recovery ratio of 5. Discussion 94.56%; and the southern hole, SK-1(S), recovered 944.23 m of core 3 2 5.1. Variation of the total organic carbon to pyrite sulfur ratios in from K1q through the bottom of K2nj , with a recovery ratio of 99.73%. The two holes, 77.35 km apart, can be correlated via the member 4 of the Quantou Formation and member 1 of the Qingshankou 2 1 Formation basin-wide oil-shale in K2nj . For K2qn , the depth is 1701.52– 1782.93 m. The lithology of the cores is dominated by dark gray and black mudstone, grayish black silty mudstone, yellowish gray marl, Previous paleontological and organic geochemical evidences sug- and brownish black oil shale (Wan et al., 2013). We have taken 51 sam- gested that, although the paleo-Songliao lakes primarily hosted fresh ples, at an average spacing of 1.8 m per sample, from the top of the waters during most of its lifetime, member 1 of the Qingshankou For- Quantou Formation, to the bottom of member 2 of the Qingshankou mation was deposited under (semi-)saline conditions (Zhang and Formation in the cores of SK-1(S). Zhou, 1976; Zhang et al., 1977; Gao et al., 1994a,b; Huang et al., 1998; fi Sulfur isotope analysis was conducted in the Stable Isotope Lab of Gao and Zhao, 1999; Hou et al., 2000; Huang, 2007). This nding the State Key Laboratory of Lithospheric Evolution situated in the could also be further tested by the variation of total organic carbon to Institute of Geology and Geophysics, Chinese Academy of Sciences. pyrite sulfur ratio of the sedimentary rock (Goldhaber and Kaplan, The analytical process included the extraction of the pyrite sulfur and 1974; Berner, 1984; Berner and Raiswell, 1984). The rationale lies in fi the analysis of pyrite sulfur isotopes. The pyrite sulfur was extracted the formation process of sedimentary pyrite. The simpli ed reactions by reduction of chromous chloride and precipitated as silver sulfide, are as follows: e.g., Ag S, based on the method of Canfield et al. (1986). The silver sul- 2 2− fide was weighted to compute the pyrite sulfur content of the sample, CH2O þ SO4 →CO2 þ H2S and then burned as sulfur dioxide in the Delta-S mass spectrometer to H S þ Fe2þ →Hþ þ FeS analyze the sulfur isotope content. The sulfur isotope ratio is reported 2 2 based on the V-CDT international standard. The analysis of total organic carbon (TOC) was conducted in the According to the above reactions, the formation of sedimentary Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. pyrite was determined by the interactions of the sulfate, organic matter, The samples were firstly ground to 80 meshes and reacted with 5% and dissolved iron (Sweeney, 1972; Sweeney and Kaplan, 1973; Bein hydrochloride to eliminate the inorganic carbonates. Then the and Nielsen, 1988; Bloch and Krouse, 1992; Leventhal, 1995; Canfield, processed sample was burnt at 20 °C and 50% relative humidity 2001; Watanabe et al., 2004; Ryu et al., 2006; Nara et al., 2010), and conditions to transform the TOC into carbon dioxides (CO2), which obviously under redox conditions of the depositional environment was measured using the CS-400 Carbon–Sulfur Analyzer. The results (Morse and Berner, 1995). The most favorable conditions for the forma- of CO2 measurement were then computed and reported as the TOC tion of pyrite, as a whole, include the high organic carbon and sulfate contents of the sample. concentration, and anoxic condition, however the controlling factors 156 Y. Huang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 152–161

Table 1 Pyrite sulfur, sulfur isotope and total organic carbon contents.

Depth Sample Lithology δ34S‰ Pyrite sulfur Total organic carbon (%) (%)

Unit 1 of Qingshankou 1704.27 Q1-4 Dark gray mudstone 18.71 0.52 3.25

Formation 1709.27 Q1-14 Dark gray mudstone 16.88 0.35 2.29

1711.27 Q1-18 Dark gray mudstone 17.59 0.77 17.59

1713.27 Q1-22 Dark gray mudstone 18.11 0.55 2.98

1715.27 Q1-26 Dark gray mudstone 18.88 0.52 2.32

1717.27 Q1-30 Dark gray mudstone 15.39 0.55 1.53

1719.27 Q1-34 Dark gray mudstone 16.13 0.55 3.34

1721.27 Q1-38 Dark gray mudstone 17.31 0.52 1.84

1723.27 Q1-42 Dark gray mudstone 19.47 0.16 3.77

1725.27 Q1-46 Dark gray mudstone 19.08 0.66 2.45

1727.27 Q1-50 Black mudstone 20.22 0.23 2.87

1730.37 Q1-56 Black mudstone 16.25 0.35 2.13

1733.37 Q1-62 Black mudstone 16.22 0.65 3.29

1735.37 Q1-66 Black mudstone 19.73 0.52 3.59

1737.37 Q1-70 Black mudstone 18.23 0.39 5.21

1739.27 Q1-74 Black mudstone 18.9 0.81 2.99

1739.27 Q1-78 Black mudstone 18.18 0.52 4.29

1741.37 Q1-82 Black mudstone 19.01 0.45 4.33

1743.37 Q1-88 Black mudstone 16.42 0.35 4.41

1746.37 Q1-93 Black mudstone 20.73 0.45 5.11

1750.87 Q1-97 Black mudstone 22.62 0.45 3.42

1752.87 Q1-101 Black mudstone 17.92 0.32 2.76

1754.87 Q1-105 Black mudstone 22.61 0.13 2.61

1756.87 Q1-109 Black mudstone 17.68 0.52 4.73

1758.87 Q1-113 Black mudstone 20.56 0.1 6.7

1760.57 Q1-117 Black mudstone 20.38 1.7 3.94

1761.37 Q1-122 Black mudstone 15.98 0.51 3.88

1763.37 Q1-128 Black mudstone 18.17 0.39 4.11

1766.37 Q1-134 Black mudstone 19.27 0.03 1.63

1768.37 Q1-138 Black mudstone 24.06 0.13 3.21

1769.87 Q1-141 Black mudstone 19.83 1.1 8.63

1770.67 Q1-145 Black mudstone 16.9 0.9 3.15

1772.47 Q1-150 Black mudstone 17.58 0.77 2.95

1774.87 Q1-156 Black mudstone 14.53 0.45 1.07

1775.47 Q1-158 Black mudstone 14.8 0.97 2.48

1777.47 Q1-165 Black mudstone 17.53 1.06 2.65

1778.47 Q1-169 Black mudstone 19.11 1.45c 3.85

1779.32 Q1-172 Black mudstone 20.4 0.29 1.42

1779.47 Q1-173 Black mudstone 18.52 0.68 4.38

1781.47 Q1-179 Black mudstone 17.96 0.9 2.28

1781.97 Q1-180 Gray mudstone 20.39 0.94 0.825

1782.47 Q1-181 Gray mudstone 14.4 1.26 1.02

Unit 4 of Quantou Formation 1782.93 Qt1-182 Gray mudstone 21.04 0.45 0.499

1784.43 Qt4-184 Gray mudstone 19.82 0.42 1.19

1785.43 Qt4-185 Brown siltstone 21.51 0.48 0.761

1786.43 Qt4-186 Dark gray mudstone 19.01 0.58 0.505

1788.43 Qt4-188 Green gray siltstone 18.01 0.19 0.201

1790.43 Qt4-190 Green mudstone 21.46 0.97 0.275

1794.43 Qt4-194 Green gray mudstone 17.29 0.07 0.245

1794.43 Qt4-198 Gray, brown siltstone 18.56 0.003 1.07

1802.44 Qt4-202 Light gray siltstone 20.17 0.003 0.175 responsible for the pyrite formation might have changed in diverse In modern normal marine sediments, TOC/PyS ratios are 2.8 ± 0.8 deposition environments. In the oxic marine bottom waters, the (varying between 2.0 and 3.6, Berner, 1984). Berner (1984b) found an sulfate is usually adequate for sulfate reduction to occur below the inverse relationship between the TOC/PyS ratio and water salinity sediment-water interface (SWI). The formation of the pyrite is con- when the water salinity is less than 18‰ in the Chesapeake Bay. trolled by the supply of organic matter, which results in the almost The TOC/PyS is between 3.6 and 11.1 when the salinity is stable at linear relationship between the TOC and pyrite sulfur (PyS) content of 14‰; and in fresh or least saline water (b1‰), the ratio is higher the marine sediment and rocks (Sweeney, 1972; Bein and Nielsen, than 16.8. Due to its sensitivity in reflecting the limiting factors for 1988; Bloch and Krouse, 1992; Leventhal, 1995; Morse and Berner, the formation of the pyrite, the TOC/PyS ratio of the sediments has 1995; Watanabe et al., 2004; Ryu et al., 2006; Nara et al., 2010). While been successfully applied to distinguish geochemically different sedi- in the euxinic and usually stratified oceans, pyrite can form in the mentary environments (Berner and Raiswell, 1984; Bein and Nielsen, water column due to the rise of sulfate reduction zone above the SWI 1988; Tuttle and Goldhaber, 1991; Bloch and Krouse, 1992; Leventhal, (Wilkin and Arthur, 2001; Schenau et al., 2002; Schoonen, 2004). The 1995; Morse and Berner, 1995; Wilkin and Arthur, 2001; Schenau et ratio of organic carbon to pyrite sulfur (TOC/PyS) in the sediments al., 2002; McKay and Longstaffe, 2003). would decrease dramatically. In lakes the productivity and supply of In addition to the TOC/PyS ratio, the size distribution and mor- iron oxides are usually much higher than in marine systems (Dean phology of sedimentary pyrite also changes in different depositional and Gorham, 1998; Nara et al., 2010), therefore sulfate would be in environment. Pyrite framboids formed in sulfidic/anoxic conditions shortage for pyrite formation in fresh water lakes resulting in a very are usually smaller (b10 μm on average) than those in oxic/dysoxic high TOC/PyS ratio in the sediment. bottom waters (>10 μm on average, Wilkin et al., 1996; Wilkin et Y. Huang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 152–161 157

4 1 Fig. 2. Correlations of organic carbon, pyrite sulfur content, sulfur isotope ratio and accommodation curve from top of K1q to K2qn . Accommodation curve is from Wang (2010). See the text for the details.

al., 1997; Wilkin and Barnes, 1996, 1997). According to the size distri- investigation for the same core material, acritarchs Granodiscus as- bution data of member 1 of the Qingshankou Formation (Wang et al., semblage were recovered indicating fresh-brackish water conditions 2013), the average diameter of pyrite framboids is in the range of (Wan et al., 2013). The sulfur content of the samples (0.41 on average) 4.3–6.2 μm in the middle part (1770–1750 m) of member 1 which is not low when compared to that of modern lake sediments suggests anoxic (if not sulfidic?) bottom water conditions. While in suggesting possibly higher content of sulfate content in the lake 1 the lower (1770–1782 m) and upper parts (1704–1751 m) of K2qn , water than that of modern fresh lakes (Holmer and Storkholm, the pyrite framboids are bigger with average diameters at 6.7–10.4 μm 2001). The low organic carbon of the rock might be due to the oxic and 7.1–18.5 μm, respectively, indicating both oxic–dysoxic bottom condition of the shallow water as suggested by the layers of siltstone waters environment (Wang et al., 2013). in the strata, and/or the low productivity (Huang, 2007). The Songliao 4 The TOC/PyS profile was constructed to show the evolution of the paleo-lake during deposition of K1q might be brackish, but not fresh ratio along the studied core (Fig. 3a). The TOC/PyS ratio shows various or saline. 4 1 features at different depth intervals. In the K1q , it decreases linearly Samples from 1781.47–1772.47 m in the lower part of K2qn (ex- from 70 to less than 2 in 1800–1790 m and then fluctuates within cept outlier Q1–173) fall into region B, i.e., the modern normal marine 0–2.0 in the 1790–1782 m. From 1782 to 1770 m, TOC/PyS ratio environment (Fig. 3b). The nearly positive relationship between the varies between 2.0 and 3.6. Dramatic variation of TOC/PyS ratio (3.6 TOC and pyrite sulfur contents of the samples is the main feature of to 70) occurs in 1770–1750 m, and then the TOC/PYS ratio changes this region, which is more enriched in both carbon and sulfur when in the range 3.6–16.8 from 1750 m to the top of member 1 of the compared with the A area. As the sediments of Songliao Basin during Qingshankou Formation. this period are obviously of lacustrine origin, such TOC/PyS features The TOC and pyrite sulfur contents of different intervals (labeled as may indicate that the interactions of the sulfur cycle and organic car- A, B, C and D for the depth of 1802–1782 m, 1782–1770 m, 1770– bon accumulation during this time was similar to that of modern nor- 4 1 1750 m, 1750 m above, respectively) in the K1q and K2qn ,werealso mal marine environments, which require increased salinity of the projected to the TOC vs. pyrite sulfur diagram of Berner (1984) water accompanied by a higher sulfate concentration (Berner and (Fig. 3b), to elucidate the possible salinity change at every step of depo- Raiswell, 1984). It seems that the higher pyrite sulfur content of the sition within this period. sediment and the dysoxic–oxic condition of the depositional environ- 4 In the A region (corresponding to the K1q ), the TOC content of ment support further a saline lake during the deposition of interval of 1 most samples (except Q1–181) is less than or only approximately 1772.47–1781.47 m in the lower part of K2qn (Wan et al., 2013; 1%. This method is invalid to discern the salinity condition for the Wang et al., 2013). organic-carbon poor sedimentary rocks (b1%, Berner and Raiswell, At first glance most of the C and D regions are located in the 1984). Former investigations for the Quantou Formation reported non-marine zone, which suggests the brackish-saline environment the brackish-water dinoflagellate cysts discovered in certain segment as a whole, but variations do exist (Fig. 3a). Region C is more enriched of the formation, suggesting the possible salinization of the paleo-lake in organic carbon but depleted in pyrite sulfur than area B (3.75% vs. (Wang et al., 1994; Sha et al., 2008). In a paralleled paleontological 2.9% and 0.46% vs. 0.83% on average, respectively), which leads to an 158 Y. Huang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 152–161

4 1 4 1 Fig. 3. (a) Proﬁle of organic carbon to pyrite sulfur ratio from top of K1q to K2qn ; (b) the relationship of organic carbon to pyrite sulfur content from top of K1q to K2qn .

increased TOC/PyS ratio. So the salinity of the paleo-lake would have shown that fractionations are larger during microbial sulfate reduction decreased during the deposition of 1770–1750 m interval of the core. (up to +46‰; Kaplan and Rittenberg, 1964). Still, larger fractionations The average TOC and pyrite sulfur are almost equal in regions C and D (> + 70‰; Canfield and Teske, 1996)havebeendocumentedin (3.75 vs. 3.9; and 0.46 vs. 0.49%, respectively), but their fluctuations nature, and are most probably caused by repeated cycles of sulfide are far more dramatic in C than in D region. The distribution of sam- oxidation to sulfur compounds of intermediate oxidation states ples from the 1770 to 1750 m interval is far more dispersed than that followed by bacterial disproportionation (Canfield and Thamdrup, of 1750 m above. Several peaks of TOC and pyrite sulfur content 1994; Habicht and Canfield, 2001). But more recent works by the appeared in the C region (1770–1750 m), indicating a more dynamic models (Brunner et al., 2005), on-site survey of marine and lacustrine depositional environment. The TOC/PyS ratio, and therefore the salin- sediments (Rudnicki et al., 2001; Canfieldetal.,2010)showedthat ity of the paleo-lake water, decreased stably though slowly with small such larger fractionations (>+70‰) observed in nature can be 1 fluctuations, throughout the rest strata above the 1750 m in K2qn . explained without the need of alternate pathways involving the oxidative sulfur cycle (Brunner et al., 2005; Sim et al., 2011). The extent of 5.2. Variation of pyrite sulfur isotope in member 4 of the Quantou fractionation during microbial sulfate reduction is mostly related to Formation and member 1 of the Qingshankou Formation the rate of sulfate reduction, with the largest fractionation occurring at lower rates of sulfate reduction (Kaplan and Rittenberg, 1964; Rees, 1 The (semi-)saline condition as a whole for the K2qn suggested by 1973; Habicht and Canfield, 1997; Canfield, 2001; Habicht and the above TOC/PyS ratio data is consistent with previous conclusions Canfield, 2001). When sulfate concentration is high, the rate of sulfate based on paleontological and organic geochemical evidence (Zhang reduction is controlled by the reactivity of organic matter (Goldhaber and Zhou, 1976; Zhang et al., 1977; Gao et al., 1994a, 1994b; Huang and Kaplan, 1974; Berner, 1978) and the sulfate can be fed continuously et al., 1998; Gao and Zhao, 1999; Hou et al., 2000; Huang, 2007). (forming the open system) and therefore the lighter sulfate is enough This raises the intriguing question what was the reason for the salini- for the microbial utilization. The sulfur isotope ratio of the produced 1 fi fi zation of the Songliao paleo-lake during deposition of K2qn . Marine sul de would be much more negative than that of sulfate (Can eld, 1 incursion events were preferred because the K2qn was obviously de- 2001). Experimental studies suggest under such condition that more posited during the lake transgression phase as the condensed sedi- reactive (i.e., more easily metabolized) organic matter is associated ments (Wang et al., 1994). This excludes the possible role played by with greater rates of sulfate reduction, and hence smaller fractionation the paleoclimatic control, e.g., elevated evaporation/precipitation ratio between sulfate and sulfide. But in low sulfate system, the replenishment (Gao et al., 1994a, 1994b). However further evidence of a marine incur- of sulfate is very limited, and sulfate is consumed quickly and probably sion are still needed as it remains a controversial problem questioned completely reduced to form a closed system (Canfield, 2001). The frac- by many researchers (Ye and Zhong, 1990; Huang et al., 1998; Huang tionation would be very limited under this situation, and isotope ratio of et al., 1999; Li et al., 2000; Ye et al., 2002). The sulfur isotope ratio of thesulfatecanbealmost“inherited” by the sulfide due to the complete- sulfides of the sediments, e.g., pyrite, is believed to be a record of sulfur ness of the sulfate reduction (Habicht et al., 2002, 2005; Hurtgen et al., cycle status in the depositional environment that provides important 2002; Kah et al., 2004). information on the origin of the sulfur, the concentration of the sulfate, From recent studies, the evolution of the pyrite sulfur isotope ratio redox conditions of the bottom water, and sedimentary facies (terrestri- through the Proterozoic to Phanerzoic has well displayed the control al vs. marine) of the sedimentary rocks (Canfield, 2001). The pyrite of the sulfate concentration on the sulfur isotope fractionation of a sulfur isotope data presented here will give important information sulfate–pyrite system (Canfield, 2001; Hurtgen et al., 2002; Shen et al., about this enigmatic issue. 2002; Kah et al., 2004; Canfield et al., 2010). The pyrite sulfur isotope However, to interpret the sulfur isotopic data for pyrite, it is neces- ratio is usually positive (>+10‰) in the Proterozoic ocean because of sary to understand the controls on sulfur isotopic fractionation of sul- the low sulfate level of marine water (Hurtgen et al., 2002; Shen et al., fate reduction during the syn-depositional phase and early diagenesis 2002; Kah et al., 2004). However most of the marine pyrite sulfur of sediments. Both inorganic and microbial sulfate reductions yield isotope ratio is negative during Phanerozoic because of much higher 34S-depleted sulfide and 34S-enriched residual sulfate. The sulfur sulfate concentration in the ocean accompanied by increased atmo- isotope fractionation between sulfate and sulfide is approximately spheric oxygen level (Hurtgen et al., 2002; Kah et al., 2004). Positive +22‰ during inorganic sulfate reduction, but laboratory studies have pyrite sulfur isotope ratios also appear frequently in the low-sulfate Y. Huang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 152–161 159 lake systems (Sweeney and Kaplan, 1973; Tuttle and Goldhaber, 1993; heavier than that of coeval marine sulfate. The reason is that the marine Fry et al., 1995; Holmer and Storkholm, 2001; Nara et al., 2010). sulfate would be continuously reduced during its way to the central lake The pyrite sulfur isotope ratios of all samples are positive with an av- and preferentially lose its lighter isotope deposited as sulfides. So the erage of +19.47‰ and +18.48% (varying between +17.29–+21.51‰ concentration of sulfate would have decreased but with heavier sulfur and +14.4–+24.06%, Table 1) in member 4 of the Quantou Formation, isotope ratio as the transportation distance of sulfate within the lake and member 1 of the Qingshankou Formation, respectively. The sulfate increased. This may result in less production of pyrite with ‘heavier’ concentration of the Songliao paleo-lake must have been low as a sulfur isotope. According to the data, the ‘heavier’ samples always whole; and more importantly, the sulfate reduction must have been have a higher TOC/PyS ratio, which is consistent with the rationale almost completed so the pyrite sulfur isotope could more or less discussed here. Previous evidence suggested that the marine incursion “inherit” that of coeval sulfate (Canfield, 2001; Hurtgen et al., 2002; was from southeast of the basin (Hou et al., 2000), so the pyrite–sulfur Shen et al., 2002; Eimers et al., 2006) because the reactive iron ratio in the southeastern part of Songliao basin must be lower than that would be enough for the formation of the pyrite as evidenced by in the central basin because the sulfate concentration must be higher 1 the occurrences of the siderite (Huang, 2007). with lighter sulfur isotope. The average pyrite sulfur ratio of K2qn The sulfate of lake water usually comes from an external input from Well Chao-73–87 is 15.74‰ (Huang, 2007 and unpublished originated mostly from the source area, which includes the following data), which is significantly lower than the results reported here (Canfield, 2001): (1) dissolved evaporites (mostly gypsum) and further corroborating such scenario. 34 1 oxidized pyrite (with δ Sof+10–+30‰ and −40–+5‰ respec- The marine incursion scenario of K2qn can be further testified with tively) from the old strata; (2) sulfate of atmospheric precipitation the combination of the pyrite-sulfur ratio record and accommodation (δ34S‰ =+3–+15‰); (3) oxidized sulfur-bearing compounds curve (Fig. 2, Wang, 2010). The accommodation space of the basin emitted from the volcano of the source area or hydrothermal system increased slowly during deposition of the 1782–1770 m interval, and within the lake (δ34S=−10–+15‰); (4) marine sulfate with coe- significantly faster from 1770 to 1763 m, then decreased slowly again val marine sulfate isotope ratio if the lake is connected with the open to 1770 m. So the lake level increased slowly from 1782–1770 m but ocean. maintained high stand from 1770 to 1750 m, which means that the Wang and Liu (2001) measured the sulfur isotope of evaporitic way was shorter for marine sulfate into the central lake during the gypsum in member 3 of the Quantou Formation and found that the deposition of the former than the latter. If the strength of the marine range of the sulfur isotope ratio is +8.1–+13.2‰ with an average incursion is approximately the same, then the concentration of the of +10‰. However, as the sulfur isotope of sulfide is usually less, or sulfate in the central lake would be higher during the deposition of at least no greater than that of sulfate (Canfield, 2001) and there is 1782–1770 m interval with lighter sulfate–sulfur isotope, which must no main variation of clastic source area during this period, the much have resulted in more pyrite in the sediment. Low lake level would higher pyrite sulfur isotope ratios reported here needs an additional have helped to promote this trend due to the lower dilution of marine and more importantly the major change of sulfate source during the source by the lake water. It seems that higher contents of pyrite with 4 1 deposition of the uppermost part of K1q into the overlying K2qn lighter sulfur isotope in the 1782–1770 m interval than in the 1770– (Fig. 2). The results of Wang and Liu (2001) may still represent the 1750 m are reasonable results of this scenario. The rationale can be isotope ratio of background sulfate input for the intervals studied. further indicated by the semi-mirrored symmetrical relation between When considering the proximity of the Songliao Basin to the Eastern the pyrite sulfur content and isotope curve within each of these two Asian Sea, a reasonable option for the source of sulfate might be the intervals (see Fig. 2), and the semi-negative relationship between the marine (Hou et al., 2000; Huang, 2007). two records from 1750 m above. Paytan (2004) has edited the seawater sulfur isotope curve of As mentioned before, marine incursion events were preferred as Cretaceous ocean based on marine barite, and found that the average the reason for the salinization of the paleo-Songliao lake during the 1 sulfur isotope ratio of Cretaceous marine sulfate is about 19‰, which deposition of K2qn rather than paleoclimatic control, e.g., elevated is quite close to that of pyrite sulfur reported here. The sulfur isotope evaporation/precipitation ratio (Gao et al., 1994a, 1994b), while the of marine sulfate is about 18.55‰ (Paytan, 2004) during the deposition paleoclimate might also have affected the formation of pyrite, therefore 1 of K2qn , i.e., at about 91 Ma (see also Wan et al., 2013), which is almost the pyrite–sulfur isotope in the paleo-Songliao lake. For example the 4 1 equal to the average pyrite sulfur isotope ratio of K1q and K2qn .Given paleo-Songliao lake may have been stratified periodically causing the the low sulfate concentration of Cretaceous ocean after the OAE2 (2–3 euxinia in the bottom water by the climate-forcing factors as docu- vs. 28 mM of modern ocean, see Adams et al., 2010 for details), and mented by numerous case studies (Wang et al., 2013). Based on the py- the gigantic volume of the Songliao Basin (the dilution effect), the sul- rite framboid data, and the rock magnetic evidence, the euxinic fate concentration of the Songliao paleo-lake would still be very low conditions had not periodically occurred and are mostly recorded in even though the marine incursion events occurred during the deposi- the 1770–1750 m interval of SK-1(S), corresponding to the three 4 1 1 tion of uppermost K1q and all of K2qn . Modern experiments suggest oil-shales of K2qn . So again the increase of sulfate concentration during 32 1 that that microbes lose their ability to preferentially remove Sassul- deposition of K2qn might have been mainly due to the marine incur- fate levels fall below ~200 μM(0.2mM,Habicht et al., 2002), so the sul- sion events rather than by the paleoclimate factors. 4 fate concentration of the Songliao paleo-lake would be around this level During deposition of the uppermost interval of K1q , the bottom during this period, which favored almost complete reduction of sulfate water is very shallow and oxic (Wang and Liu, 2001); so sulfate re- to produce the pyrite with the positive and almost “inherited” sulfur duction would have occurred below the sediment-water interface. isotope ratio with that of sulfate. In this condition sulfate reduction was still relatively a closed system Although the average pyrite sulfur isotope ratio is almost equal when the sedimentation rate was high or the sulfate concentration is to that of Cretaceous marine sulfate, clear fluctuations do exist, how- low. It is probable that these two conditions were both proper for the 1 4 ever, especially within the K2qn . Ten samples in the 1782–1770 m upper K1q interval according to its lithology, e.g., siltstone, and salin- and 1770–1750 m interval have pyrite sulfur ratios higher than 19‰ ity of the water (brackish, see above discussions). So the isotope ratio 4 (averaging at 20.0‰ with a range of 19.11–24.06‰,seeTable 1 and of pyrite–sulfur at the top of K1q would have been close to that of re- Fig. 2), which seems highly impossible for the marine incursion scenar- duced sulfate. Based on the analytical results, the pyrite–sulfur ratio 4 io, because the isotope ratio of reduced sulfide is usually less than that of in uppermost K1q ranged between 21.5 and 17.29‰ with an average coeval sulfate due to the preferential reduction of isotopic-light sulfate of 19.65‰, which is quite close to that of coeval marine sulfate also (Canfield, 2001). However since the location of drilling site is located indicating marine incursion. However because the number of samples 4 in the central basin, the isotope ratio of reduced sulfate here must be in the K1q is limited, further work needs to be undertaken to 160 Y. Huang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 152–161 determine the beginning of marine incursion in the stratigraphic Acknowledgments succession. Several scenarios are possible to undermine the robustness of the This study was jointly supported by National Key Basic Research deduction in the preceeding discussion. Our current paradigm assumes Program of China Grant (No. 2012CB822005) and Natural Science that pyrite δ34S will be equal to or less than co-occurring sulfate δ34S Foundation of China (No. 40873022). We thank the staff of Stable due to the kinetic isotope effect of bacterial sulfate reduction (BSR), Isotope Lab, the State Key Laboratory of Lithospheric Evolution situated which favors the lighter sulfur isotope (32S) during sulfur–oxygen in the Institute of Geology and Geophysics, Chinese Academy of Sci- bond breakage. However, Ries et al. (2009) documented that dissemi- ences, for their advice and assistance during stable isotope analysis. nated pyrite is consistently enriched in 34S relative to coeval seawater We also thank Prof. Xuelei Chu and Dr. Michael Formolo for their helpful sulfate as preserved in carbonate-associated sulfate (CAS) in the inter- discussions during the research and manuscript revision. The authors val of the terminal Proterozoic Nama Group. Ries et al. 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