Geochemical characteristics of the lower Silurian Longmaxi Formation on the Yangtze Platform, South China: Implications for depositional environment and accumulation of organic matters

Journal of Asian Earth Sciences 184 (2019) 104003

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Geochemical characteristics of the lower Silurian Longmaxi Formation on the Yangtze Platform, South China: Implications for depositional environment and accumulation of organic matters

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Yu Liua, , Bin Wub, Qisen Gongb, Haiyang Caoa a b

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China Chongqing Mineral Resources Development Co. Ltd., Chongqing 400060, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Early Silurian Paleo-climate Water redox condition Primary productivity Organic accumulation

The organic-rich lower Silurian Longmaxi Formation is now regarded as the favorable target for shale gas exploration and production. However, the mechanism of organic matter accumulation in these deposits remains highly controversial. In this study, we investigate the total organic carbon (TOC) content and the major and trace element abundances of a newly recovered core from the Yangtze Platform to reconstruct the paleo-environment and to elucidate the factors that control organic matter accumulation. Our results suggest that the greenhouse conditions during the early Silurian were punctuated by several cooling events. Bottom water redox conditions improve gradually upsection with a transition from anoxic water conditions in the lower Longmaxi Formation to suboxic-oxic water conditions in the upper Longmaxi Formation. The primary productivity on the Yangtze Platform appears to have been high during deposition of the Longmaxi Formation, especially in its lower part, due to an enhanced phosphorus recycling. The strong correlations between TOC and copper and nickel imply that the productivity was especially critical to the organic matter accumulation in the lower Longmaxi Formation. Furthermore, the rapid consumption of dissolved oxygen in bottom water favored the establishment of anoxic water conditions, which facilitated organic matter preservation. The poor correlations between TOC and barium and phosphorus in the lower Longmaxi Formation likely reflect intense recycling of nutrients. The evolution of water redox conditions and accumulation of organic matters in the Longmaxi Formation can be related to global fluctuations in sea level and regional tectonic uplift of the Yangtze Platform.

1. Introduction The Ordovician-Silurian transition (OST) witnessed a series of significant environmental and biotic perturbations, including a protracted cooling trend during the Katian stage, large-scale glaciation and mass extinction in the Hirnantian, and climatic amelioration and biotic recovery in the early Silurian (Brenchley et al., 2003; Fan et al., 2009; Finnegan et al., 2011; Harper et al., 2014; Algeo et al., 2016). It has long been suggested that the early Silurian represents an interval of stable greenhouse conditions and faunal compositions (Fischer, 1983). However, recent geochemical and stratigraphic evidence indicates that the early Silurian climate fluctuated (Munnecke et al., 2010; Trotter et al., 2016). Sedimentological investigations in South America (Page et al., 2007) and North Africa (Moreau, 2011) suggest that at least three glacial advance-retreat cycles occurred during the early Silurian (DíazMartínez and Grahn, 2007; Munnecke et al., 2010), which is consistent

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with the elevated δ18O values of Llandovery carbonate fossils (Finnegan et al., 2011). Such fluctuating climatic conditions may have had significant implications for the evolution of oceanic chemistry and biotic recovery. The accumulation of widespread black shale during the early Silurian is another matter of debate. In view of the fact that black shale deposited in this interval accounts for the generation of 9% of the world’s petroleum reserves (Klemme and Ulmishek, 1991), the determination of the sedimentary environment during black shale deposition is of great significance for oil-gas exploration. A variety of explanations, including high primary productivity (Lüning et al., 2000; 2006), anoxic bottom water conditions (Armstrong et al., 2009; Yan et al., 2015; Cao et al., 2018a; Shi et al., 2018; Yan et al., 2019), topographic factors (Le Heron and Dowdeswell, 2009; Moreau, 2011, Cao et al., 2018b; Hu et al., 2019), sea-level changes (Hu et al., 2017), orbital cyclicity (Armstrong et al., 2009), and combinations of these

Corresponding author. E-mail address: [email protected] (Y. Liu).

https://doi.org/10.1016/j.jseaes.2019.104003 Received 30 December 2018; Received in revised form 26 August 2019; Accepted 30 August 2019 Available online 30 August 2019 1367-9120/ © 2019 Elsevier Ltd. All rights reserved.

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these uplifts, the Yangtze Platform evolved into a semirestricted basin system (Chen et al., 2004; Su et al., 2007). The QD-2 well is located in the Qianjiang district, Chongqing municipality (Fig. 1B). The core includes the Upper Ordovician Baota, Linxiang, Wufeng Formations and the Kuanyinchiao Bed and the lower Silurian Longmaxi and Xintan Formations in ascending order (Fig. 1C). The gray muddy limestones of the Baota and Linxiang Formations are overlain by the graptolite-rich black shale of the Wufeng Formation. The Kuanyinchiao Bed is composed of calcareous mudstone and limestone and records a drop in eustatic sea level during the glacial maximum (Fan et al., 2009). The Longmaxi Formation conformably overlies the Kuanyinchiao Bed and is overlain by mudstone of the Xintan Formation (Fig. 1C). Although there is no constraint on the graptolite zones, the similar thickness of the stratum and lithologic variation between the QD-2 well (this study) and the JY-1 well (Liu et al., 2017) suggest that the Longmaxi Formation in the QD-2 well was deposited in Rhuddanian and Aeronian (Fig. 1C). According to the TOC contents, Li et al. (2017) subdivided the Longmaxi Formation into two intervals. The lower interval is composed of graptolitic black shales and is considered to be a favorable horizon for the development of shale gas in China (Pang et al., 2018). The upper interval is dominated by organiclean gray muddy siltstone. The conformable contact between the Kuanyinchiao bed and Longmaxi Formation and the consistent lithology and TOC contents of the Longmaxi Formation across the Yangtze Platform provide a good stratigraphic framework for geochemical comparison among disparate sections.

factors (Zhao et al., 2016; Li et al., 2017), have been proposed to explain the accumulation of organic matters. However, many debates still focus on the relative dominance of productivity versus preservation in the accumulation of organic matters (Sageman et al., 2003; Mort et al., 2007). The former stresses that high primary productivity not only provides abundant organic matter but also favors the establishment of anoxic water conditions that facilitate the preservation of organic matter (Murphy et al., 2000; Sageman et al., 2003). The latter argues that the anoxic bottom water, which is related to specific paleogeographic settings, may be more important than productivity (Arthur and Sageman, 1994; Mort et al., 2007). It is worth noting that, as a complex biogeochemical process, the sedimentary facies and sedimentation rate may also have certain impacts on the accumulation of organic matters (Rimmer, 2004; Katz, 2005; Ma et al., 2016). The well-preserved OST sedimentary record on the Yangtze Platform provides an excellent opportunity to assess the interactions between oceanic chemical conditions and organic matter accumulation. However, most research primarily focuses on a limited stratigraphic interval (organic-enriched Upper Wufeng-Lower Longmaxi Formation) and cannot adequately assess the mechanism of organic matter accumulation (Yan et al., 2009, 2012; Zhou et al., 2012; Chen et al., 2016; Liu et al., 2016; Zou et al., 2018). Moreover, the geochemical data obtained from outcrop samples may also be biased by contamination and weathering (Li et al., 2017). Based on cores obtained from shale gas wells, recent studies have revealed that the accumulation of organic matters in the Longmaxi Formation is a comprehensive outcome of anoxic water conditions, high productivity and low detrital influx input (Y.X. (Yuxuan) Wang et al., 2019; Zhang et al., 2019). However, there are significant differences in the recognition of the key factor that controlled the accumulation of organic matters on the Yangtze Platform. For example, some authors have proposed that the accumulation of organic matters was dependent on the anoxic water conditions and was little affected by primary productivity (Yan et al., 2015; Chen et al., 2016; Li et al., 2017; Zhao et al., 2017), whereas others have suggested that primary productivity was more important than anoxic water conditions (X.Q. (Xiaoqi) Wang et al., 2019; Yan et al., 2019). Furthermore, Lu et al. (2019) proposed that preservation and productivity may have been dominant in different locations on the Yangtze Platform. Obviously, the geochemical characteristics (e.g., productivity, redox condition and terrestrial input) are closely related to local/regional paleogeographic settings. To better understand the relationship between environmental evolution and organic accumulation, it is necessary to conduct further research on longer time scales in different regions and then compare the differences in geochemical characteristics between them. In this study, core samples that cover the entire Longmaxi Formation were collected from the QD-2 well in Qianjiang, Chongqing municipality. TOC and major and trace element geochemistry are used to reconstruct the paleoenvironmental conditions that accompanied deposition of the Longmaxi Formation and to elucidate the mechanism (s) that influenced organic matter accumulation in the lower Longmaxi Formation.

3. Samples and methods 3.1. Samples Thirty-eight samples were collected from the QD-2 well for TOC and major and trace element analysis. All samples were washed in deionized water to remove surface contamination and then pulverized to powders less than 200 mesh in an agate mortar. 3.2. Analytical methods Approximately 0.1 g of each powdered sample was analyzed for TOC. The samples were first reacted with 6 M HCL to remove carbonate. The residues were then rinsed at least six times until deemed neutral. TOC contents were determined using a LECO CS 230 elemental analyzer. Approximately 1 g of each powdered sample analyzed for trace elements was first combusted at 800 °C to remove organic matter. The mass lost during this procedure was recorded for correction. The fusion glasses were made by mixing an ashed sample with Li2B4O7 at a proportion of 1:8. Major element concentrations were measured on a Rigaku ZSK 100e X-ray fluorescence instrument. Analytical errors were better than 5%. A PE Elan 6000 standard inductively coupled plasma mass spectrometer (ICP-MS) was used to measure trace element concentrations. Prior to analysis, approximately 1 g of powder sample was combusted in a muffle furnace at 800 °C for 4 h to remove volatiles and carbonate. Then, ~50 mg residual was dissolved in 2 ml mixed HF, HNO3 and HClO4 at 200 °C for 12 h. After drying, each sample was mixed with 3 ml HNO3 for complete digestion. Finally, each sample was diluted to 1/1000 with 1% HNO3 for further trace element analysis. Two USGS standards (W-2 and G-2) and two Chinese national standards (GSR-5 and GSR-6) were used to monitor data quality, yielding an analytical precision better than 5%.

2. Geological setting South China during the OST was a separate plate but still connected to the northwestern margin of Gondwana (Fig. 1A) (Metcalfe, 1994). It was composed of the Cathaysia block in the southeast and the Yangtze block in the northeast (Chen et al., 2004; Su et al., 2009). During earlymiddle Ordovician time, the Yangtze Platform was covered by a broad epeiric sea that was bordered to the southeast by the Pearl River Sea, which may have been connected to the global ocean (Wang et al., 1993, 1997). The regional Kwangsian Orogeny during late Ordovician time resulted in the formation of the Chengdu uplift to the northwest of the Yangtze Platform, the Dianqian uplift to the south of the Yangtze Platform, and the Hunan-Hubei submarine highs to the east of the Yangtze Platform (Liang et al., 2009; Chen et al., 2014). Surrounded by

3.3. Data calculations The trace element concentration of sediment is a mixture of both authigenic and detrital components, but only the former reflects water 2

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Fig. 1. (A) Global paleogeography during the Ordovician-Silurian transition (modified from Melchin et al., 2013). (B) Paleogeographic map of the Yangtze Platform during the Ordovician-Silurian transition (modified from Chen et al., 2004). The stars represent the location of the studied QD-2 well. The black dots represent the locations of other wells and outcrops displaying the OST: 1. Wangjiawan section, Yichang, Hubei; 2. Hehua section, Yichang, Hubei; 3. Shizhu section, Chongqing; 4. Shuanghe section, Yibing, Sichuan; 5. QQ-1 well, Xiushui, Chongqing. (C) The lithostratigraphic units in QD-2 well and the eustasy variation in the studied interval (modified from Liu et al., 2017). Sys.: System; Sta.: Stage; Fm.: Formation; Lith.: Lithology. Hirn.: Hirnantian; Rhud.: Rhuddanian.

calculating the chemical index of alteration (CIA*) using the following formula (Nesbitt and Young, 1982; Price and Velbel, 2003):

redox conditions. A customary method of assessing elemental concentrations of specific redox-sensitive trace elements is the calculation of the enrichment factor (EF) (Tribovillard et al., 2006):

EFX = (X/Al)sample /(X/Al)AUCC

CIA∗ = Al2O3 /(Al2O3 + Na2 O+ K2O)

(1)

Although CaO is included in the denominator of the original formula of Nesbitt and Young (1982), it is omitted in the present study because of difficulty in measuring the CaO content of calcareous sedimentary rocks (Liu et al., 2017).

where the subscript AUCC represents the composition of the average upper continental crust (McLennan, 2001). The numerator and denominator represent the concentrations of the element and Al in the given sample and in AUCC, respectively. In general, EFX < 1 represents a depleted concentration of an element relative to that of AUCC; EFX > 3 and 10 represent moderate and strong enrichment, respectively (Tribovillard et al., 2006). The concentration of biogenic barium (Ba) is calculated by subtracting the detrital fraction from the measured Ba concentration as per the following equation:

Babio = Batotal − [Al sample × (Ba/Al)detr]

(3)

4. Results 4.1. Total organic carbon contents The TOC contents display obvious stratigraphic variation through the QD-2 core (Fig. 2). The lower Longmaxi Formation is characterized by the highest TOC contents, ranging from 0.24 wt% to 4.13 wt% (mean = 2.17 wt%). The TOC contents of the upper Longmaxi Formation vary between 0.14 wt% and 1.27 wt% (mean = 0.63 wt%). The described TOC profile is similar to those reported in previous studies of the Longmaxi Formation (Yan et al., 2015; Chen et al., 2016; Li et al., 2017).

(2)

where Batotal and Alsample are the measured elemental concentrations in the sample. We used a (Ba/Al)detr ratio of 55 based on the Ba-versus-Al crossplot derived from the Nanbazi section (Zhou et al., 2015). Paleoclimate and weathering conditions were assessed by 3

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Fig. 2. Profiles of TOC contents, detrital proxies, redox proxies and productivity proxies in the QD-2 well. KY: Kuanyinchiao. Other abbreviations are shown in Fig. 1. Ranges for V/Cr are from Jones and Manning (1994), those for Th/U are from Wignall and Twitchett (1996) and those for C/P are from Algeo and Ingall (2007).

4.2. Major element geochemistry

4.3. Paleoenvironmental proxies

SiO2, Al2O3 and CaO are traditionally regarded as the three endmembers of mudstone (Ross and Bustin, 2009). In this study, SiO2 is the dominant oxide of all studied samples, ranging from 52.3 wt% to 69.79 wt% (mean = 63.20 wt%). Al2O3 varies between 8.1 wt% and 18.76 wt% (mean = 13.13 wt%). CaO ranges from 0.46 wt% to 12.43 wt% (mean = 3.73 wt%). The Fe2O3, FeO, MgO and K2O concentrations have average values of 1.45 wt%, 2.57 wt%, 2.02 wt%, and 3.53 wt%, respectively (Table S1). The remaining major elements show relatively low contents in the studied samples (Table S1). The positive covariance of Al2O3 with K2O and TiO2 suggests a detrital source of these major elements (Fig. 3).

The Zr/Al ratio and CIA* have been employed to elucidate paleoclimate conditions (Nesbitt and Young, 1982; Bahlburg, 2009). The Zr/ Al ratios of the studied core range from 9.1 to 77.6 (mean = 36.3) (Fig. 2). These ratios display an increasing trend from 34.82 at 872.41 m to 58.14 at 853 m through the lower Longmaxi Formation. The Zr/Al ratio of the upper Longmaxi Formation decreases gradually from 77.6 at 833.32 m to 9.2 at 794.81 m. The CIA* values vary between 0.68 and 0.77 (mean = 0.72) and display an inverse trend relative to that of the Zr/Al ratio (Fig. 2). V/Cr, Th/U, MoEF, UEF, and Corg/P are commonly used as water redox proxies (Algeo and Ingall, 2007; Algeo and Tribovillard, 2009;

Fig. 3. Correlations between Al2O3 contents and K2O and TiO2 contents in the QD-2 well and other sections and cores on the Yangtze Platform, including Shuanghe section, Shizhu section, Xingwen section and QQ-1 well (Li et al., 2017), Cangling section (Chen et al., 2016), Wangjiawan section (Yan et al., 2009, 2012), Hehua section (Yan et al., 2015) and JY-1 well (Liu et al., 2017). 4

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conditions (Ross and Bustin, 2009), we employed V/Cr, Th/U, Corg/P, MoEF, and UEF to reconstruct the redox conditions associated with deposition of the Longmaxi Formation. Uranium (U) and vanadium (V) are redox-sensitive elements that are preferentially enriched in sediments deposited under anoxic conditions (Kimura and Watanabe, 2001). Chromium (Cr) and thorium (Th), however, are little affected by redox conditions and remain insoluble in the marine environment (Tribovillard et al., 2006; Ross and Bustin, 2009). Thus, the V/Cr and Th/U ratios can be used to evaluate oceanic redox conditions. In general, the V/Cr ratios are < 2 in oxic conditions, 2.0–4.25 in dysoxic conditions and > 4.25 in anoxic conditions (Jones and Manning, 1994). For Th/U, it is widely accepted that a range of 0–2 represents anoxic water conditions and a ratio up to 10 indicates a strongly oxidizing environment (Wignall and Twitchett, 1996; Kimura and Watanabe, 2001; Chang et al., 2012). The molybdenum (Mo) concentration in sediment is strongly dependent on the water redox conditions (Tribovillard et al., 2006). Modern sediment deposited under nonsulfidic conditions commonly contains Mo contents less than 25 ppm, whereas sediment that has accumulated under intermittently/seasonally euxinic water mass conditions may contain 25 to 100 ppm Mo, and permanently euxinic bottom water conditions may result in Mo concentrations of more than 100 ppm (Scott and Lyons, 2012). The recycling of phosphorus (P) in marine environments is closely related to water redox conditions (Kraal et al., 2010; März et al., 2014). The dissolution of Fe-oxyhydroxides and reduction of P assimilation by microbes under reducing conditions can lead to effective diffusion of P from sediments into the overlying water column (Filippelli, 2001; März et al., 2014; Shen et al., 2014a). In contrast, the adsorption of P by Fe-oxyhydroxides under oxic conditions may facilitate the segregation of P in sediments (Ingall and Jahnke, 1997; Shen et al., 2014b). The results of modern marine sediments show that the Corg/P ratios are < 50 in oxic conditions, 50–100 in suboxic conditions, and > 100 in anoxic conditions (Algeo and Ingall, 2007). Because of the inconsistency in the water redox conditions interpreted by the thresholds of different redox proxies, it is generally best to assess the trend of a redox proxy rather than the absolute values (Rimmer, 2004). Although there are some slight differences in absolute redox conditions interpreted by different redox proxies (Fig. 2), the similar trends of all redox proxies record significant changes in water redox conditions in the early Silurian. Except for sample QD-25 at 848.27 m in the lower Longmaxi Formation, relatively high V/Cr, Corg/P and low Th/U ratios indicate the existence of anoxic bottom water conditions during accumulation of the lower Longmaxi Formation. Furthermore, moderate to strong correlations of TOC with U/Al (except for sample QD-38), V/Al and Mo/Al suggest that noneuxinic (ferruginous) water conditions dominated the time period represented by the lower Longmaxi Formation (Fig. 4; Algeo and Maynard, 2004; Tribovillard et al., 2006). This interpretation is consistent with the moderate Mo (average = 33.85 ppm), U (average = 15.23 ppm), and V (average = 373.8 ppm) concentrations in these deposits. The low TOC content (0.24 wt%), V/Cr and Corg/P values (1.9 and 11.83, respectively), and the concentration of redox-sensitive elements (Mo = 8.00 ppm; U = 7.26 ppm; V = 74.4 ppm) in sample QD-25 (848.27 m) may imply that the stable anoxic bottom water conditions were interrupted by a transient suboxic episode at the boundary of the lower and upper Longmaxi Formations. We suggest that the formation of suboxic water redox conditions may be a result of low primary productivity (Fig. 2). In contrast, the low TOC contents, element ratios (except Th/U ratios) and concentrations of redox-sensitive elements in the upper Longmaxi Formation indicate stable oxic to suboxic bottom water conditions (Fig. 2). The gradual amelioration of redox conditions during deposition of the Longmaxi Formation may be a common feature of the depositional history of the Yangtze Platform. Indeed, lower Longmaxi samples collected from various sections and cores are characterized by high MoEF and UEF values as well as Mo/U ratios of 1–3 times that of modern

Ross and Bustin, 2009). Furthermore, the degrees of covariance between TOC and Mo/Al, V/Al and U/Al are used to distinguish ferruginous and euxinic water columns (Tribovillard et al., 2006, and references therein). Except for Th/U, all profiles display higher values and enrichment factors in the lower Longmaxi Formation (Fig. 2). The V/Cr ratios range from 1.1 to 12.8 (mean = 3.3). The Th/U ratios vary between 0.50 and 5.33 (mean = 2.49). MoEF varies between 1.22 and 56.41 (mean = 14.18). UEF ranges from 1.12 to 10.45 (mean = 4.36). The Corg/P ratios vary between 5.92 and 182.92 (mean = 54.33). The range and average of Mo/Al are 0.23–10.52 and 2.65, respectively. The range and average of V/Al are 7.1–152.0 and 33.8, respectively. Finally, the range and average of U/Al are 0.39–5.19 and 1.52, respectively (Fig. 2, Table S1). The Babio and P/Ti ratios are used as paleoproductivity proxies (Dymond and Collier, 1996; Paytan et al., 1996; Algeo et al., 2011; Latimer and Filippelli, 2002). The Babio contents of the studied samples range from 709.5 ppm to 1821.8 ppm (mean = 975.3 ppm). P/Ti ratios range from 0.11 to 0.20 (mean = 0.15). It is noteworthy that the Babio profile displays little variation through the studied core, while the P/Ti appears to be slightly higher in the lower Longmaxi Formation (Fig. 2). 5. Discussion 5.1. Terrigenous fluxes and climate change The Al contents of sediment are mainly sourced from aluminosilicate clay minerals and are rarely affected by weathering or diagenesis, thus providing a robust proxy of the terrigenous flux (Tribovillard et al., 2006; Calvert and Pedersen, 2007; Lézin et al., 2013). The increasing Al content upsection through the QD-2 core indicates a protracted increase in the terrigenous input into the studied area of the Yangtze Platform. The concentration of Zr in the heavy mineral zircon (Rachold and Brumsack, 2001) makes the Zr/Al ratio a useful proxy of depositional energy and/or eolian input of coarser grains in sediments (Young and Nesbitt, 1998; Straub et al., 2013). Increasing Zr/Al ratios upward through the lower Longmaxi Formation suggest increasing grain size, perhaps related to more active eolian processes. Diminishing Zr/Al through the upper Longmaxi Formation suggests a reduction in grain size, perhaps related to reduced environmental energy or eolian influence (Fig. 2). The CIA can be used as a proxy for the intensity of chemical weathering in the provenance (Young and Nesbitt, 1998). In general, sediments deposited in hot and humid tropical climates have CIA values of 0.8–1, whereas those deposited in temperate and cold, arid climates have CIA values of 0.7–0.8 and less than 0.7, respectively (Nesbitt and Young, 1982, 1989; Bahlburg, 2009). It is important to bear in mind, however, that CIA values can also be influenced by the chemical composition of the source rock (Bahlburg, 2009). Thus, it is recommended to interpret the climatic fluctuations using the variation trend of the CIA profile rather than absolute threshold values (Liu et al., 2017). In the studied core, the CIA profile displays an almost mirrored relationship to the Zr/Al profile (Fig. 2), with a monotonous descending trend in the lower Longmaxi Formation and a fluctuating increasing trend in the upper Longmaxi Formation. This change in the CIA profile indicates a transition from cold and dry conditions during the lower Longmaxi period to warm and humid conditions associated with deposition of the upper Longmaxi Formation. 5.2. Paleo-oceanic redox conditions A variety of geochemical proxies have been used to reconstruct paleo-oceanic redox conditions, including trace element ratios (Rimmer, 2004; Ross and Bustin, 2009), enrichment factors of trace elements (Algeo and Tribovillard, 2009; Scott and Lyons, 2012; Tribovillard et al., 2012), and mineral proxies (Algeo and Ingall, 2007). Because no signal proxy can conclusively illuminate water redox 5

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Fig. 4. Correlations between TOC contents and U/Al, V/Al and Mo/Al ratios in lower Longmaxi Formation.

Fig. 5. Crossplot of MoEF versus UEF for the Longmaxi Formation in the QD-2 core and other sections and cores of the Yangtze Platform, including the Shuanghe section, Shizhu section, and QQ-1 well (Li et al., 2017) and the Wangjiawan section (Zhou et al., 2015). (A) Lower Longmaxi Formation; (B) Upper Longmaxi Formation. Particulate shuttle and redox fields are from Algeo and Tribovillard (2009).

the upper Longmaxi rather than the basin-reservoir effect. Moreover, the different distribution patterns of samples on the UEF-MoEF crossplot suggest that the water redox structure on the Yangtze Platform was heterogeneous during the lower Longmaxi period. Samples from the QQ-1 well display a similar distribution with the stratigraphically equivalent samples of the present study, suggesting anoxic and ferruginous water conditions. The distribution of samples of the Shuanghe section, however, suggests deposition under euxinic conditions. Some

seawater, reflecting authigenic enrichment of redox-sensitive elements in anoxic water columns (Fig. 5A). Upper Longmaxi samples, however, display obvious low enrichment of redox-sensitive elements and Mo/U ratios of 0.3–1 times that of modern seawater (Fig. 5B). In view of the weak to moderately restricted water mass conditions during the Longmaxi period (Li et al., 2017; Liu et al., 2017), we suggest that the weak enrichment of redox-sensitive elements reflects the existence of oxicsuboxic water conditions on the Yangtze Platform during deposition of

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Fig. 6. Correlations of TOC contents and Ti/Al ratios for samples from the QD-2 well and other sections and cores on the Yangtze Platform, including the Shuanghe section, Shizhu section and QQ-1 well (Li et al., 2017), Wangjiawan section (Yan et al., 2009, 2012) and Hehua section (Yan et al., 2015). (A) Lower Longmaxi Formation; (B) Upper Longmaxi Formation.

probably reflect low primary productivity due to the more effective sequestration of P in sediments under oxic conditions. The Babio content of sediment is another useful proxy for evaluating primary productivity (Dymond et al., 1992; Eagle et al., 2003; Tribovillard et al., 2006). Biogenically produced barite appears to originate in decayed organic matter produced by surface productivity (Dymond et al., 1992; Eagle et al., 2003). However, barite is unstable under anoxic (euxinic) conditions, resulting in the recycling of Ba into the overlying water columns via bacterial sulfate reduction (BSR) (Dymond et al., 1992; Torres et al., 1996). Sediments deposited in oxic bottom-water conditions below highly productive regions in the modern equatorial Pacific Ocean contain 1000–5000 ppm Babio (Murray and Leinen, 1993; Schoepfer et al., 2015). The Babio concentrations in the studied Longmaxi Formation samples range from 709.5 to 1821.8 ppm, suggesting moderate to high primary productivity. However, given the strongly anoxic bottom-water conditions that prevailed during accumulation of the lower Longmaxi Formation, the original Babio concentration of these deposits may have been strongly enriched relative to those of the present study.

samples of the Shizhu section are distributed in the particulate shuttle field, suggesting that the accumulation of Mo was partially controlled by the particulate shuttle mechanism associated with an unstable chemocline (Algeo and Tribovillard, 2009). We suggest that sea-level fluctuations during the early Silurian exerted a first-order control on the evolution of water redox conditions on the Yangtze Platform. Elevated sea level following Hirnantian glaciation favored the formation of a stable stratified water column that facilitated the formation of anoxic water conditions. The regression associated with deposition of the upper Longmaxi Formation may have promoted the ventilation of the Yangtze Sea, resulting in the establishment of oxygenated water conditions (Li et al., 2017). However, the obvious spatial heterogeneity in the water redox structure may be a reflection of regional factors, including water depth and local primary productivity.

5.3. Paleo-oceanic productivity The supply of P is known to be critical to oceanic primary productivity (Schmitz et al., 1997; Rimmer et al., 2004; Shen et al., 2014a). The concentrations of P in marine sediments principally reflect the contribution of organically bound P and P adsorbed onto Fe-oxyhydroxides (Algeo and Ingall, 2007; Stein et al., 2011). The recycling of P in the marine environment is controlled by redox conditions. Specifically, oxic conditions favor retention of P by adsorption onto Fe-oxyhydroxides and subsequent transformation into phosphate minerals (Algeo et al., 2011; Shen et al., 2014b). However, the mineralization of organic matter and dissolution of Fe-oxyhydroxides under anoxic bottom water conditions lead to the diffusion of P from sediments into the overlying water columns (März et al., 2014). This positive feedback loop between P recycling and anoxic water conditions favors the maintenance of high surface water primary productivity (Saltzman, 2005). The P/Ti ratio has been proposed as an indicator of primary productivity (Algeo et al., 2011). In this study, the average P/Ti ratio of 0.15 in the Longmaxi Formation (Fig. 2) approaches that of post-Archean Australian shale (PAAS) (0.13) but is far less than the values (2–8) observed in sediment deposited beneath high-productivity regions in the modern equatorial Pacific (Murray and Leinen, 1993). The low P/Ti values of the Longmaxi Formation may suggest low productivity during its depositional history. As mentioned above, however, the low P/Ti ratios of the lower Longmaxi Formation may be a reflection of redox-induced P recycling, as suggested by high Corg/P ratios associated with intense P recycling in the ocean (Li et al., 2017). In contrast, the low P/Ti ratios observed in the upper Longmaxi Formation

5.4. Controls on organic matter accumulation on the Yangtze Platform The accumulation of organic matters is controlled by multiple factors, including primary productivity (Pedersen and Calvert, 1990; Sageman et al., 2003), anoxic bottom water conditions (Arthur and Sageman, 1994; Mort et al., 2007), and the influx of clastic materials (Murphy et al., 2000). It is worth noting, however, that these geochemical parameters are not entirely independent variables (Tyson, 1995). Previous studies have suggested that organic accumulation on the Yangtze Platform during the OST was principally controlled by anoxic water conditions based on stronger covariance of TOC contents and redox proxies (Yan et al., 2015; Chen et al., 2016; Li et al., 2017). In this study, we explore the role of primary productivity on organic accumulation on the Yangtze Platform. The weak correlations between TOC and Ti/Al in the Longmaxi Formation suggest that the sedimentation rate did not significantly influence organic matter accumulation on the Yangtze Platform (Fig. 6, Murphy et al., 2000; Yan et al., 2015). Furthermore, except for the Wangjiawan section, the lack of covariance between TOC and Al contents documented from analyzed lower Longmaxi samples from multiple sites suggests that the terrigenous detritus did not dilute the organic flux severely on the Yangtze Platform (Fig. 7A). The positive relationship of TOC contents and Al contents documented in the QD-2 core and Shuanghe section (Fig. 7B) suggests that the fine-grained clay 7

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Fig. 7. Correlations of TOC contents and Al contents for samples from the QD-2 well and other sections and cores on the Yangtze Platform. (A) Lower Longmaxi Formation; (B) Upper Longmaxi Formation. Other details are shown in Fig. 6.

studies (Wu et al., 2019; Yan et al., 2019). Thus, the moderate to strong covariances of TOC and redox proxies in the lower Longmaxi Formation suggest that organic matter accumulation likely contributed to the formation of anoxic water conditions by drawing down dissolved oxygen, thus facilitating further organic accumulation (Fig. 4; Algeo and Maynard, 2004; Yan et al., 2015; Chen et al., 2016). It is worth noting that the negative relationship between P/Ti and TOC identified in the upper Longmaxi Formation samples (Fig. 9B) may reflect preferential dissolution of Fe-oxyhydroxides (Fe (II)-Fe (III)) to barite (bacterial sulfate reduction) at higher redox potential; that is, P is more sensitive to minor fluctuations in the water redox conditions. We suggest that the fluctuations in sea level and regional tectonic movement are two important factors controlling the environmental changes and accumulation of organic matters in the Longmaxi Formation (Fig. 1C, Chen et al., 2014; X.Q. (Xiaoqi) Wang et al., 2019; Y.X. (Yuxuan) Wang et al., 2019). During the lower Longmaxi period, the high sea level caused by the global transgression after the Hirnantian glaciation facilitated the formation of stratified water columns on the Yangtze Platform. Moreover, the high nutrient flux caused by upwelling could further have promoted primary productivity (Chen et al., 2004; Yan et al., 2012, 2015). The high organic flux combined with weak mixing of the water columns facilitated the establishment of anoxic bottom-water conditions, which further promoted the preservation of organic matter. It is worth noting that the high primary productivity could have been sustained by intense P recycling once the anoxic water conditions had been established (Werne et al., 2002). In addition,

minerals in terrigenous detritus may facilitate the burial and preservation of organic matter via adsorption (Kennedy and Wagner, 2011; Kennedy et al., 2014). The productivity proxies Babio and P/Ti display contrasting degrees of covariance with TOC in the anoxic and oxic-suboxic intervals of the Longmaxi Formation (Figs. 8 and 9). The lower Longmaxi Formation lacks obvious correlations between productivity proxies and TOC contents (Fig. 8). However, Babio and P/Ti show positive and negative correlations with TOC contents, respectively, in the upper Longmaxi Formation (Fig. 9). The lack of covariance of productivity proxies and TOC in the lower Longmaxi Formation may reflect the influence of bottom water redox conditions (i.e., dissolution and recycling) rather than muted primary productivity. It is worth noting that the decline in TOC accompanied by decreases in Babio and P/Ti in the anoxic lower Longmaxi Formation may imply a lower organic flux caused by lower productivity. In contrast, the stronger correlation of Babio and TOC contents documented in the upper Longmaxi Formation reveals the role of primary productivity in organic accumulation under oxic water conditions. To further illuminate the role of productivity on organic matter accumulation in the Longmaxi Formation, we also plot the correlations of two other productivity proxies (copper and nickel, Table S1) and TOC contents. Due to the lower impact of water redox conditions on the two elements (Tribovillard et al., 2006), the statistically significant correlations between nickel (Ni) and copper (Cu) and TOC suggest an important role of productivity in organic matter accumulation (Fig. 10). This phenomenon has also been observed in previous

Fig. 8. Correlations of TOC contents and productivity proxies for the lower Longmaxi Formation. (A) Babio; (B) P/Ti. Other details are shown in Fig. 6. 8

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Fig. 9. Correlations of TOC contents and productivity proxies for the upper Longmaxi Formation. (A) Babio; (B) P/Ti. Other details are shown in Fig. 6.

although there is no obvious evidence showing that the organic matter was diluted by terrestrial input (Fig. 7), the low terrestrial input in the transgression interval still has the potential to maintain the TOC at a high level. During the upper Longmaxi period, the tectonic uplift and the global regression shallowed the Yangtze Sea (Chen et al., 2014; Li et al., 2017). The low TOC content may reflect the effects of oxic water conditions and diminished primary productivity (see above). However, the sympathetic covariance of Babio and TOC content observed in the upper Longmaxi Formation (Fig. 9A) implies that primary productivity still plays an important role in the accumulation of organic matters.

and P) in the lower Longmaxi Formation may be the result of poor preservation of nutrient elements under anoxic bottom water conditions. In contrast, the strong correlations of TOC contents and productivity (Ni and Cu) and redox proxies displayed by the entire suite of samples and the lower Longmaxi samples indicate that elevated primary productivity not only enhanced organic matter supply but also helped to maintain anoxic water conditions, thus preserving deposited organic matter.

Declaration of Competing Interest 6. Conclusions

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Our analysis of the Longmaxi Formation of the Yangtze Platform suggests that the climate conditions in early Silurian time were highly dynamic, with several cooling events punctuating the long-term greenhouse conditions following the Hirnantian glaciation. Bottom water redox conditions on the Yangtze Platform experienced significant changes, as evidenced by the strongly anoxic water conditions associated with deposition of the lower Longmaxi Formation that gradually became oxic by the time that the upper Longmaxi Formation was accumulating. Furthermore, bottom water redox conditions on the Yangtze Platform display strong spatial heterogeneity during deposition of the lower Longmaxi Formation. The primary productivity associated with the deposition of the Longmaxi Formation was high, especially during deposition of the lower Longmaxi Formation. We suggest that the lack of covariance of TOC contents and productivity proxies (Babio

Acknowledgments This study was supported by the NSFC (grant No. 41802122) to LY. We thank all reviewers for their constructive suggestions.

Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jseaes.2019.104003.

Fig. 10. Correlations of TOC contents and productivity proxies Ni and Cu for the Longmaxi Formation. Other details are shown in Fig. 6. 9

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