Orbital control on cyclical organic matter accumulation in Early Silurian Longmaxi Formation shales

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Research Paper

Orbital control on cyclical organic matter accumulation in Early Silurian Longmaxi Formation shales Siding Jin a, b, Hucheng Deng b, *, Xing Zhu c, Yan Liu b, Sibing Liu b, Meiyan Fu b a b c

State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China College of Energy, Chengdu University of Technology, Chengdu 610059, China College of Environment and Civil Engineering, Chengdu University of Technology, Chengdu 610059, China

A R T I C L E I N F O

A B S T R A C T

Handling Editor: Nick M W Roberts

High resolution (939 samples) total organic carbon content (TOC) analyses were conducted on the Shuanghe Section of ~152.6 m in the Changning area, Sichuan Basin. The sampling section was divided into two units considering the distinct-different deposit environment and sediments accumulation rate. The lower part (Unit 1) and the peer part (Unit 2) with high resolution sample spacing (0.08–0.4 m) enables the identification of the precession cycle in two sedimentary sequences with distinct different sedimentary accumulation rate. MTM Power spectral analyses on untuned TOC series reveals significant peaks exceeding above the 95% confidence level and shows that both Unit 1 and Unit 2 have recorded Milankovitch cycles of 405 kyr long eccentricity, short eccentricity, obliquity and precession. The floating astronomical time scale (ATS) was constructed on the Shuanghe Section in the Early Silurian (~439.673–444.681 Ma), and which was calibrated by 405 kyr long eccentricity cycles. The total duration of the Wufeng and Longmaxi shales is 5.01 Myr. The floating ATS used for estimating the duration of the graptolite zones and each stage in the study interval. Finally, we postulated two models that could verify the linkage between orbital cycle and organic accumulation. To make sure whether productivity or preservation is the main factor that under long eccentricity control, the phase correlation between the obtained filtered signal and the theoretical orbital solution should be made clear in the further research.

Keywords: Cyclostratigraphy Floating astronomical time scale Early Silurian Organic matter accumulation

1. Introduction The temporal and spatial insolation distribution, climate variations and sea-level changes on the Earth's surface system over the past 4.5 billion years are affected by the Earth's astronomical parameters including eccentricity, obliquity and precession, and this theory has been widely accepted in recent years (Strasser et al., 2006; Boulila et al., 2011; Jin et al., 2018). While the stratigraphic record of orbital forcing is well investigated and documented in the Mesozoic and Cenozoic, cyclostratigraphy of the Palaeozoic remains quite a challenge (Hinnov, 2013; Huang, 2014). Due to lacking accurate astronomical solutions in deep time, only the 405 kyr long eccentricity cycle are regarded to be constant all along the geological time (Laskar et al., 2004; Ma et al., 2014). The Late Ordovician to Early Silurian Period is marked by great environmental changes, second most devastating mass extinction as a sign and also associated with widespread organic-rich black shale deposits (Webby et al., 2004; Delabroye and Vecoli, 2010; Yan et al., 2015).

In order to understand these critical geological events, also the controlling factors of the Early Silurian organic matter abundance, a precise time framework is required to be established primary for these “hot shale” strata. The long-running debate “preservation” (Pratt, 1984; Betts and Holland, 1991; Junium et al., 2018) versus “productivity” (Muller and Suess, 1979; Pedersen and Calvert, 1990; Sageman et al., 2003) over the origin of organic-rich sediments attracted much attention in both modern and ancient sediments. Milankovitch scale redox cycle (Curiale et al., 1992; Courtinat, 1993; Kim et al., 2018) may evaluate the former argues, that means palaeoenvironment controlled oxic/anoxic benthic water condition corresponds to low/high TOC value. Orbital control on cyclical primary productivity (Wang, 2009, 2015; Gambacorta et al., 2018) may explain the latter suggest, the formation of monsoons caused nutrient supply increases/declines corresponds to low/high TOC value under the palaeoclimate forcing. Besides, several factors such as sediment granulometry and sediment accumulation rate also play important roles in

* Corresponding author. E-mail address: [email protected] (H. Deng). Peer-review under responsibility of China University of Geosciences (Beijing). https://doi.org/10.1016/j.gsf.2019.06.005 Received 22 September 2018; Received in revised form 28 January 2019; Accepted 8 June 2019 Available online xxxx 1674-9871/© 2019 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: Jin, S. et al., Orbital control on cyclical organic matter accumulation in Early Silurian Longmaxi Formation shales, Geoscience Frontiers, https://doi.org/10.1016/j.gsf.2019.06.005

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Fig. 1. (A) Global paleogeography of the Late Ordovician (after Scotese and Mckerrow, 1991; Young et al., 2008). Note that South China Craton was rotated ~90 counter-clockwise relative to its modern orientation. (B) Paleogegraphy of the South China paleoplate, modified from Zhang et al. (2010). (C) Wufeng- Longmaxi organic-rich shale distribution. (D) Location of the Shuanghe Section in the Sichuan Basin.

2. Geological background

abundance of organic matter in sediments (Tyson, 2001; Rimmer et al., 2004). In order to decode which factors predominate the accumulation of organic matter in Early Silurian sediments, here, we conducted cyclostratigraphy research on the total organic carbon content (TOC) series from a ~152.4 m section of the Wufeng and the Lower Longmaxi formations, Changning area, Sichuan Basin. Three main scientific targets of this study are as follows: (1) identifies orbital forcing of climate driving mechanisms in the Early Silurian, South China; (2) supplies an optional method for the precise geological time scale calibration in the poorly investigated Paleozoic; (3) investigates the eccentricity period during Early Silurian and its possible relationship with “preservation” or “productivity” cycle in the accumulation of organic-rich sediments.

2.1. The Shuanghe Section in the Changning area, Sichuan Basin The Sichuan Basin developed on the basis of the South China Craton as superimposed basin. It was located in a tectonically active region along the Gondwana northwestern margin, which was close to the paleoequator during the Early Paleozoic (Fig. 1A and B) (Metcalfe, 1994; Wang et al., 2015). It is a diamond-shaped basin located in the eastern of the Longmen Mountain, west to the Qiyue Mountain, southern of Micang Mountain- Daba Mountain, and north to the Daliang Mountain- Loushan Mountain. The basin covered an area of 180  103 km2, and can be divided into three tectonic units: high-steep tectonic area lied to the east 2

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2.3. Sedimentary environment and shale distribution characteristics

of the Huayingshan fault belt (including high-steep folded belt of eastern Sichuan and low-steep folded belt of southern Sichuan) in the southeastern Sichuan slope, low-steep tectonic area lied to the west of the Longquan Mountain in the western Sichuan depression, and low-gentle tectonic area between Huaying Mountain and Longquan Mountain in the central Sichuan uplift (Fig. 1C) (Zhai, 1989; Wang et al., 2002). The Hirnantian-Rhuddanian shales in Wufeng to Lower Longmaxi formations are deposited in a dysoxic/anoxic environment in the Sichuan Basin, and which are mainly consisted of black calcareous shale on account of favorable preservation condition for organic matter (Chen et al., 2004; Zhao et al., 2017). The Guanyinqiao Member formed during Hirnantian glaciation that is on the top of the Wufeng Formation and usually contains lime mudstone and a thin shell-rich marl (Chen et al., 2005). The upper Longmaxi Formation is mainly consisted of grey silty shale corresponding to an oxic/dyoxic environment with quite strong organic matter metabolizable (Guo and Zhang, 2014). Abundant graptolite fossils saved in the Wufeng and Longmaxi formations span the uppermost Ordovician and Lower Silurian that show evidence of “high and stable organic primary productivity” sedimentary environment (Chen et al., 2000). Geologically, the Shuanghe Section in the Changning area is located in the Weiyuan-Changning Shale gas field of the southern part of the Yangtze Block, which is in Changning Town, Yibin City of Sichuan Province (Fig. 1C and D). The Shuanghe Section consists of 152.4 m shale that can be divided into two depositional units which show a greatly different depositional environment from the upper Ordovician to Lower Silurian strata. The black shale at the upper Wufeng–Longmaxi Formation and the bottom of the Longmaxi Formation (Unit 1)(Fig. 2A) was simply regarded as deposits of deep-water continental shelf facies, and the grey shale at upper Longmaxi Formation (Unit 2)(Fig. 2B) was regarded as shallow-water shelf facies (Chen et al., 2015a, b).

According to the graptolite zone division of the upper Ordovician to lower Silurian, the Wufeng Formation to Longmaxi Formation could be divided into two intervals with distinct sedimentary environments. (1) During the period of WF4, LM1 to LM5, the sedimentary environment was a deep shelf caused by global sea level rise except WF4 graptolitic zone (a significant sea level decreasing during the early Hirnantian)(Nie et al., 2017; Jin et al., 2018). The shale lithofacies of WF4 was dominated by black thin-layered carbonaceous shale, and the LM1 to LM5 was also composed of graptolites-bearing black organic-rich shales with lithofacies association of siliceous shale, thin-layered carbonaceous shale with bentonite interlayer. The lower intervals mainly contained black shales with abundance graptolites, a small amount of silty shale or siltstone laminae (Fig. 2). (2) During the period of LM6 graptolitic zone, the sedimentary environment was transgression and the water column was relatively shallow and turbulent. The lithology was mostly dominated by grey to grey-dark shale intercalated with silty-fine sandstone, with the characteristics of fast sediment accumulation rate and low TOC (Fig. 2). A detailed δ13Corg of organic matter curve was provided from the Changning block (Rong et al., 2011), and the composite δ13C records from the Shuanghe Section (Duan, 2011) in complement to the biostratigraphyic framework shows evidences for sedimentary environment and also the source of organic matter (Fig. 2). Under the influence of tectonic movements and the closure of the palaeo-marine, palaeo-productivity of marine in the southern area of the Sichuan Basin presented a high trend in the early stage and low in the late stage, and the sediment accumulation rate was also low in the early stage and high in the late stage. The main controlling factor of the extremely low sedimentation rate in Hirnantian and Rhuddanian Stage is the stable tectonic setting in the period of the Sichuan Basin. An intense tectonic activity during Aeronian resulted in a higher sedimentation rate especially in the southeastern Sichuan Basin, Changning area, whereas close to the southeast provenance. The δ13C values in the Wufeng–Longmaxi Formation shales ranged from 31.2‰ to 29.4‰ (Chen and Pi, 2009) in the Sichuan Basin. The relative sea level falling together with nutrients input decreasing, induced the weak preservation condition and the primary productivity decreasing. TOC values decreased and δ13C values increased. A slightly positive shift δ13C values in the Hirnantian glaciation age reflected sea level fell rapidly (Wang et al., 2017). A negative shift δ13C values in the early to middle Rhuddanian demonstrated a warming climate and sealevel rise. A positive shift δ13C values at the transition from Unit 1 to Unit 2 (Late Rhuddanian to early Aeronian) indicated a continues decreasing of the global sea level change. A continuously decline sea level changes lead to a positive shift δ13C values in the late Rhuddanian to Aeronian (Zhou et al., 2015).

2.2. Biostratigraphy of the Shuanghe Section Graptolite is the most abundant zooplankton in the Wufeng and Longmaxi shales in the Yangtze area, a detailed biostratigraphy work has been done by Chen et al. (2015c), and it has been used as the stratigraphic division standard in South China (Fig. 2A and B). The first graptolites appear in upper Ordovician Katian which are the WF1 (Foliomena-Nankonolithus), WF2 (Dicellograptus copmplexus), WF3 (Paraorthograptus pacificus) biozones and above them are the WF4 (N. extraordinarius), LM1(Persculptograptus persculptus) biozones in Hirnantian. The LM2 (Akigograptus ascensus), LM3 (Parakidograptus acuminatus), LM4 (Cystograptus vesiclosus) and LM5 (Coronograptus cyphus) biozones are in Llandovery of the lower Silurian, the LM6 (Demirastrites triangulatus), LM7 (Lituigraptus convolutus) and LM8 (Stimulograptus sedgwickii) biozones are in Aeronian and the LM9 biozones is in Telychian from bottom to top. In the Changning Section, biostratigraphy based on graptolite has been established by Mu et al. (1978, 1993), and Duan (2011). The local section encompasses the Dicellograptus complanatus, Dicellograptus complexus, Paraorthograptus pacificus, Normalograptus extraordinarius in the Wufeng Formation, Persculptograptus persculptus zone in Hirnantian, Akidograptus ascensus, Parakidograptus acuminatus, Cystograptus vesiculosus, Coronograptus cyphus zones in Rhuddanian, Demirastrites triangulatus, Demirastrites pectinatus-Monograptus argenteus, Lituigraptus convolutus, and Stimulograptus sedgwickii zones in Aeronian of the lower Longmaxi Formation. Here, we used the standard biozonation established by Chen et al. (2015c), the sampling strata from WF4 to LM6 are annotated in Fig. 2. The first appearance datum (FAD) of Akidograptus ascensus have been proposed to be the bottom boundary of the Early Silurian (Chen et al., 2006), and the boundary of Ordovician– Silurian is passing through the bottom of the Longmaxi black graptolite shale (Fig. 2).

3. Materials and methods 3.1. Selected sections and sampling intervals Due to the distinct difference of the sediment accumulation rates and also the obviously difference of the shale color (black in lower and grey in upper) between the upper Longmaxi Formation and the lower Longmaxi Formation (Wang et al., 2017), the sample collection is divided into two deposit units which are the top Wufeng Formation and lower Longmaxi Formation and the upper Longmaxi Formation (Fig. 2). Notably, the Wufeng Formation strata is well-exposed in the Shuanghe Section, but we only sampled top of the Wufeng Formation about 10.6 m that developed with Normalograptus extraordinarius zone. In order to obtain true high-frequency orbital signals, the sampling intervals should be less than a quarter of precession cycle thickness (Weedon, 2003). Because the sediment accumulation rates are constantly rising during the target interval in this study, a further division was optimized for separated statistical analyses which are Unit 1 (WF4, LM1 to LM4), Unit 2 (LM6). The 3

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Fig. 2. Lithostratigraphy, biostratigraphy and data (TOC, δ13C) series in the stratigraphic domain of the Wufeng-Longmaxi Formation in Shuanghe Section, Changning area, Sichuan Basin. The δ13C of organic matter curve is modified from Duan (2011). WF4, Metabolograptus extraordinarius Zone; LM1, Metabolograptus persculptus Zone; LM2, Akidograptus ascensus Zone; LM3, Parakidograptus acuminatus Zone; LM4, Cystographtus vesiculosus Zone; LM5, Coronograptus cyphus Zone; LM6, Demirastrites triangulatus Zone. Graptolite zone labels after Chen et al. (2015c).

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Fig. 3. (A) and (C) The MTM power spectra of the TOC series of Unit 1 (Wufeng and lower Longmaxi formations) and Unit 2 (upper Longmaxi Formation) in the Shuanghe Section. The red, green, blue and purple dashed curves represent the red noise spectra, at the 90%, 95%, 99% confidence levels, respectively. (B) and (D) Evolutionary FFT spectrogram of the TOC values of Unit 1 with a 10 m sliding window, and Unit 2 with a 10 m sliding window. The white dashed lines labelled with E, e1, e2, O1, O2, P1 and P2 represent the long eccentricity, short eccentricity, obliquity, and precession cycles, respectively.

study, the TOC value was optimized as a proxy for cyclostratigraphic analysis of the stratigraphic interval in the Wufeng Formation to the lower Longmaxi Formation. To remove the long-term trend, the TOC series is detrended by subtracting a 25% weighted average using the software Kaleidagraph™ (Cleveland, 1979). The kSpectra toolkit software is used for spectral analysis on the untuned TOC series, which was performed by multitaper method (MTM)(Thomson, 1982). The peaks above the 90%, 95%, and 99% confidence interval were selective considered (Mann and Lees, 1996). Matlab scripts of evofft.m (Kodama and Hinnov, 2015) is utilized for fast Fourier transform (FFT) spectrograms to identify the frequency changes caused by sediment accumulation rate variation (Weedon, 2003). Because the precise age constraints are unavailable for this period, the average spectral misfit (ASM) method adopted in this study (Meyers and Sageman, 2007) for testing the orbital signals that preserved in older strata. R package “Astrochron” (Meyers, 2014) was used to achieve the ASM in this study. Gaussian band-pass filtering were used to extract the 405 kyr eccentricity cycles, and astronomical calibration were accomplished by free software AnalySeries 2.0.8 (Paillard and Donnadieu, 2015).

5–10 cm interval was adopted for the top Wufeng Formation and lower Longmaxi Formation (WF4, LM1 to LM5) and about 20–50 cm interval was carried out on the upper Longmaxi Formation (LM6). An uneven sampling strategy was followed to ensure that every weathered zones or tuff layer was avoided. A 0.05–0.1 m sampling interval was adopted for Unit 1 shales, and 0.2–0.5 m for upper shale intervals (Unit 2). The TOC series were interpolated to a uniform spacing of 0.08 m for Unit 1 and 0.4 m for Unit 2 respectively. 3.2. Total organic carbon analyses All samples were collected for total organic carbon (TOC), and TOC analyses were carried out in the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, China. First, fresh samples were powdered into 200 mesh size, and then about 100 mg powder were weighed and treated with hydrochloric acid. Next, all the powdered samples were heated under 60  C in order to removing the residual acid. At last, the residue was dried and measured by a Leco CS-200 carbon-sulfur equipment. Instrumental accuracy is 0.5% of reported TOC values, based on replicate analyses of the Chinese national standard GB/T 19145–2003 (Fu et al., 2011).

3.4. Early Silurian orbital parameter Due to the long-term chaotic evolution of the planetary orbits in the Solar System, it is unpredictable to promote the orbital solution beyond 50 Ma (Laskar et al., 2004, 2011). However, the stable and consistent long eccentricity cycle (405 kyr) is supposed to be accurate mainly due to the substantially constant gravitational interaction between Earth and Jupiter (Ma et al., 2017; Li et al., 2018), and it can be used to establish a reliable ATS (Long, 2007; Waltham, 2015). The eccentricity cycle (400 kyr and ~100 kyr), obliquity cycle (54 kyr and 41 kyr) and precession

3.3. Time series analysis Time series analysis has been widely used in cyclostratigraphy research (Wu et al., 2013, 2014; Li et al., 2016), which enables the transformation between the depth series and the time series by an absolute age control. To detect links between sedimentary cycles and orbital cycles, the period ratio was the most common approach in traditional cyclostratigraphy analysis (Mayer and Appel, 1999). In this 5

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Fig. 4. (A, D) Average spectral misfit results, indicating an optimal fit at a sedimentation rate of 1.52 cm/kyr for the Unit 1 interval and 7.52 cm/kyr for the Unit 2 interval. (B, E) Null hypothesis significance levels for ASM results, indicating lowest values at optimal sedimentation rates. (C, F) Tables shown relative data series of target periods and identified orbital frequencies of Unit 1 and Unit 2, respectively.

find the orbital forcing imprint on the bottom-water palaeoredox conditions or primary productivity. In the Shuanghe Section, the TOC value ranges from 1.63% to 5.74% in the Wufeng and the lower Longmaxi formations (WF4, LM1 to LM5), and it ranges from 0.86% to 3.11% in the upper Longmaxi Formation(LM6)(Fig. 2). On the basis of color, lithology, we assume that the TOC value is higher in the black shale that deposited under low benthic oxygen levels, while the lower TOC in the grey silty shale that deposited during phases of increased bottom water oxygenation. The sudden increasing or decreasing TOC value indicates that the shift from oxic to anoxic conditions was abrupt, while the gradual transition from black shale to grey silt shale facies suggests a gradual conditions changes. Numerous studies have demonstrated that the organic matter enriched Unit 1 (Wufeng Formation and the lower Longmaxi Formation) is in the continental shelf with a relative deep-water and anoxic/dyoxic environment, and the upper Longmaxi Formation is deposited in the shallow water with the dyoxic/oxic sedimentary environment (Nie et al., 2017; Zhao et al., 2017). Thus, both the sea level fluctuation or detrital input could explain either the cyclical variation of TOC variations and the facies changes.

cycle (23 kyr and 19 kyr) are the most crucial cycles at present (Read et al., 1995). A shorter Earth-Moon distance in the Early Silurian (Longmaxi Formation) would lead a shorter obliquity and precession cycle. The calculations proposed by Waltham (2015) give an accurate predict orbital parameter for Early Silurian (440 Ma), which are 33.4  3.8 kyr for obliquity, and 20.9  1.5 kyr, 19.9  1.3 kyr, 17.1  1.0 kyr, and 17.2  1.0 kyr for precession. In this study, we assume 405:125:95:33.4:20.4:17.15 as the astronomical parameters ratio to verify the orbital forcing sedimentary cycles in the TOC series. 4. Result 4.1. TOC variations Total organic carbon (TOC) is usually the first proxy to be carried out on potential petroleum source rock in oil exploration, which is of great significant to fine-grained sediment research, and it is an important proxy to evaluate the organic matter (OM) abundance which links with primary productivity, bottom water redox conditions, buried rate, and grain size of sediment (Bjørlykke, 2010). There is an equation that calculates the sediment organic matter content using the TOC value on the premise of kerogen type (Tissot and Welte, 1984), and TOC value has been widely used as palaeoclimatic proxy and stratigraphic correlation tool due to the closely connection with OM content (e.g. Sageman et al., 2003; Rimmer et al., 2004; Wagner et al., 2004; Rowe et al., 2008; Liu et al., 2017). In order to verify the controlling factor on organic matter accumulation, here, we attempt to use TOC as the palaeoclimatic proxies for cyclostratigraphy research and

4.2. Cyclostratigraphy of the Shuanghe Section Power spectral analysis of the untuned TOC series in the Shuanghe Section was applied on the two separate intervals (Fig. 3A, C), from 0 m to about 55.4 m of Unit 1, and 55.4 m to about 152.6 m of Unit 2. TOC series of Unit 1 reveals significant sedimentary cycles with wavelengths of 6.345 m, 1.985 m, 1.140 m, 0.645 m, 0.502 m, 0.327 m, 0.288 m 6

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Fig. 5. Integrated floating astronomical time scale of Unit 1 (Wufeng Formation and lower Longmaxi Formation) in Late Ordovician to Early Silurian in the Shuanghe Section. The interpreted 405 kyr (red curve) cycles were extracted with Gauss filters with passbands of 0.250  0.100 cycles/m. The FAD of A. Ascensus at 443.8 Ma (red straight line) (Gradstein et al., 2012) as a reference point. The 405 kyr calibrated TOC series, relative boundaries ages and graptolite zones are also shown in this figure.

4.3. Average spectral misfit (ASM) result

above the 95% confidence level (Fig. 3A and B). The ratios of 6.345 m, 1.140 m, 0.502 m, and 0.327–0.288 m are consistent with the ratio of the Early Silurian orbital cycles (Waltham, 2015). Here we interpret these cycle wavelengths long, short eccentricity, obliquity and precession cycles respectively. TOC series of Unit 2 reveals significant sedimentary cycles with wavelengths of 30.120 m, 9.351 m, 6.522 m, 4.292–3.545 m, 2.564–2.262 m above the 95% confidence level (Fig. 3C and D). They are interpreted as 405 kyr long eccentricity (30.120 m), 125 kyr short eccentricity (9.351 m), obliquity (4.292–3.545 m) and precession cycles (2.564–2.262 m), respectively. For further information, the evolutionary FFT spectrogram reveals the variable sediment accumulation rates during the depositional period (Fig. 3B, D).

The ASM approach of Meyers and Sageman (2007) is to evaluate the discrepancy degree between sedimentary cycle and orbital cycle, and then, to provide a range of plausible sediment accumulation rates. Particularly, ASM is appropriate for those situations without any time constraint of biostratigraphy and radiometric dating. Waltham (2015) provided the target orbital periods in the Early Silurian (Section 3.4), and the power spectral analysis gave the stratigraphic frequencies with spectrum peaks. The ASM method produces an optimal sediments accumulation rate and yields a statistical test for rejecting the null hypothesis significance level (H0-SL, no orbital forcing). Good correlation between 7

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confidence level were inputted to implement ASM analysis. Finally, an optimal sediment accumulation rate of 7.56 cm/kyr with an excellent H0-SL of 0.202% was produced (Fig. 4B, D), that means the probability of the null hypothesis rejected is 99.798%. Corresponding to a 405 kyr long eccentricity cycle of sediment accumulation rate of 7.56 cm/kyr, 30.61 m thickness cycles were calculated which matched to 30.12 m from the precedent MTM spectrum (Fig. 3C).

the stratigraphic and orbital frequencies generates a lower H0-SL, and vice versa. The Wufeng and the lower Longmaxi formations in the Unit 1 (55.4 m) of the Shuanghe Section experienced Hirnantian and Rhuddanian stage, which lasts 4.4  1.4 Myr (445.2  1.4 Ma to 440.8  1.2 Ma) in GTS2012 (Gradstein et al., 2012), and it indicates sediment accumulation rates ranged between 0.7 cm/kyr and 3.07 cm/kyr. Thus, we performed a broader interval that ranged between 0 cm/kyr and 5 cm/kyr, and 6 frequencies satisfying the 90% confidence level were inputted to implement ASM analysis. Finally, an optimal sediment accumulation rate of 1.52 cm/kyr with an excellent H0-SL of 0.129% was produced (Fig. 4A, C), that means the probability of the null hypothesis (no orbital signal) rejected reached 99.871%. Based on this interpretation, a 405 kyr long eccentricity cycle of 6.156 m that matched well with 6.345 m from the precedent MTM spectrum (Fig. 3A). Sediment accumulation rate of early Aeronian stage approximates from 1.88 cm/kyr to 9.95 cm/kyr (Wang et al., 2017). Thus, we performed a broader interval that ranged between 0 cm/kyr and 10 cm/kyr, and 6 frequencies satisfying the 90%

4.4. Floating astronomical time scale for the Early Silurian The Ordovician to Silurian of Palaeozoic Era geochronology is poorly constrained with no isotopic data available in the Longmaxi shale. The FAD of A. Ascensus (basal Silurian) has been recognized at 13.8 m of the section (Fig. 2; Liu et al., 2017). Therefore, the FAD of A. Ascensus at 443.8  1.5 Ma (Gradstein et al., 2012) can be used as a reference point to construct a floating ATS using the interpreted 405 kyr eccentricity cycles of the TOC series. However, the error of 1.5 Ma is more than three times grater of 405 kyr periodicity that used to recognize the

Fig. 6. Integrated floating astronomical time scale of Unit 2 (LM6) in Early Silurian in the Shuanghe Section. The interpreted 405 kyr (red line) cycles were extracted with Gauss filters with passbands of 0.022  0.010 cycles/m. The initial age of Unit 2 is as same as the ending age of Unit 1 in Fig. 5. The 405 kyr calibrated TOC series, relative boundaries ages and graptolite zones are also shown in this figure. 8

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Table 1 Comparison of estimated duration of graptolite zones and the Hirnantian, Rhuddanian, Aerolian Stage of the Shuanghe Section with those in the GTS 2012 (Gradstein et al., 2012), Huanghuachang Section (Fang, 2017) and 198 stratigraphic sections (Sadler et al., 2009). Stage

Aerolian Rhuddanian

(Base Silurian) Hirnantian

Graptolites Zone

Demirastrites triangulatus Coronograptus cyphus Cystograptus vesoclosus Parakidograptus acuminatus Akigograptus ascensus Persculptograptus persculptus Normalograptus extraordinarius

Estimated durations (Myr) GTS 2012

Huanghuachang Well Yihuang1

198 stratigraphic sections

Gradstein et al. (2012)

Fang (2017)

Sadler et al. (2009)

1.30 0.80 0.90 0.93 0.43 0.60 0.73

1.15 1.36

1.91 0.81 2.24 0.26 0.73 0.72 0.55

0.22 0.63

This study

1.340 0.923 1.864

0.213 0.668

to 441.013 Ma. Gaussian filters with passband of 0.022  0.010 cycles/m in Unit 2 for the depth interval of 55.4–152.6 m (Fig. 6), and set 441.013 Ma (the ending age of Unit 1) as the initial age of Unit 2, the floating ATS ranging from 441.013 Ma to 439.673 Ma was constructed. The resulting floating ATS was applied to investigate astronomical forcing and time estimation for the geological events recorded in the Wufeng and Longmaxi formations.

astronomical cyclical in this study. The initial age control with an uncertainty can only help us to build a floating astronomical time scale and to calculate the durations of biozones and stages. In order to extract the long eccentricity cycles (405 kyr) in Units 1 and 2 of the Shuanghe Section respectively, the Gaussian bandpass filters were performed in this study. The results indicate that Unit 1 records nine long eccentricity cycles and Unit 2 records four long eccentricity cycles, cycle number are counted from 1 to 13 (Figs. 5 and 6). The floating astronomical time scale (ATS) for the Shuanghe Section is built by tuning the filtered 405 kyr sedimentary cycles to the 405 kyr sinusoid theoretical curve, and the age of the base of the Silurian is setting as 443.8  1.5 Ma which is the age control in this section (Fig. 5). Gaussian filters with passbands of 0.250  0.100 cycles/m in Unit 1 for the depth interval of 0–55.4 m (Fig. 5), the floating ATS was built from 444.681 Ma

5. Discussion 5.1. The graptolite zones and stage durations in the Shuanghe Section The calibration of 405 kyr long eccentricity floating ATS of the TOC series was performed to calculate the durations of the graptolite zones of

Fig. 7. Schematic model for the first hypnosis. Long eccentricity control on monsoonal climate and related variation in primary productivity. (A) Intensified monsoons controlled by maxima eccentricity shows an increasing primary productivity characteristic. (B) Reduced monsoons with minima eccentricity shows a decreasing primary productivity feature. 9

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Fig. 8. Schematic model for the second hypnosis. Long eccentricity control on monsoonal climate and related variation in preservation. (A) Intensified monsoons controlled by maxima eccentricity shows intense rainfall during wet season with high productivity, while organic matter oxidation during long dry season. (B) Weakened monsoons with stable climate controlled by minima eccentricity shows moderate productivity but prolonged organic matter burial with no seasonal oxic.

transgressive-regressive sequences in the Hirnantian Stage (Delabroye and Vecoli, 2010). The decreasing duration of the Hirnantian Stage from semi-enclosed marine could be due to the regional regressive differences in the Shuanghe Section, Changning area. The Rhuddanian Stage with a duration of 2.80 Myr in this study are much close to the duration of 3.06 Myr in GTS 2012 (Gradstein et al., 2012) and Sadler et al. (2009) that further support our cyclostratigraphic interpretation in this study.

the Shuanghe Section. The estimated durations of the Normalograptus extraordinarius (0–10.6 m), Persculptograptus persculptus (10.6–13.8 m), Coronograptus cyphus (41.8–55.4 m), Demirastrites triangulatus (55.4–152.5 m) zones are 668 kyr, 213 kyr, 923 kyr and 1340 kyr, respectively (Table 1). Whereas, several durations of the same grapolite zones in the Well Yihuang1 of the Huanghuachang area were quite different (Fang, 2017) (Table 1), it might be caused by the biozones diachrony or the different depositional environment. The Hirnantian Stage with a duration of 0.88 Myr calculated in this study which is much shorter than 1.40 Myr of GTS 2012 and also shorter than 0.94 Myr in Well Yihuang1 (Fang, 2017). The glaciation and first extinctions happened during the Normalograptus extraordinarius Zone, the deglaciation induced transgression and second extinctions took place during the Persculptograptus persculptus Zone, associated with the large

5.2. Cyclical marine redox conditions and productivity Fluctuation of total organic carbon content that mainly depend on the cyclical marine redox condition and rhythmic primary productivity, probably with the additional effects of grain size and sediment accumulation rate variation (Tyson, 1995; Stow et al., 2001). Many 10

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theoretical orbital solution plays a key role in order to identify which model is more credible (Lanci et al., 2010; Laurin et al., 2016). However, it is not possible to get the precise eccentricity solution beyond 250 Ma, due to the chaotic behavior of the motion of the Solar System (Laskar et al., 2004, 2011). Thus, this paper just points out that orbital forcing did exert significant impact on the Early Silurian shale deposition, without the possibility of identifying whether a maximum in the TOC series corresponds to a minimum or a maximum in eccentricity. Existing research suggests that both model can be advocated to explain the observed cyclical organic matter accumulation (Figs. 7 and 8). The orbital forcing not only leads to cyclical marine palaeoredox condition (preservation), “eccentricity-paced enhanced productivity” also plays a part in organic matter accumulation in different degrees. Further analysis needs to be carried out, and requires more geochemical proxies to rebuild the depositional environment and to inspect the orbital forcing cycle impacts on sediments.

researches indicated that redox conditions reveal a major control on the organic matter preserved abundance in organic-rich shale (Raiswell and Berber, 1985; Jones and Manning, 1994), and the high TOC value often indicates a low benthic oxygen levels during the organic matter deposited, and vice versa (Pedersen and Calvert, 1990; Wignall, 1994). A number of previous studies referred to the cyclic marine redox conditions, primary productivity as well as eustatic fluctuations, detrital input and weathering that made contribution to the palaeoclimate and palaeoenvironment research in the Early Silurian Yangtze Sea, which is stratigraphically comparable to the sedimentary series in this study (Yan et al., 2010, 2018; Ma et al., 2016; Liu et al., 2017). On one hand, several previous studies indicated that a long-term stable anoxia existed within the Yangtze Sea following the Hirnantian glaciation (Yan et al., 2010; Zhou et al., 2015); on the other hand, the palaeoredox conditions show a cyclical pattern with rapid shifts toward more intensely reducing at the base of each third-order sequence (Liu et al., 2017; Yan et al., 2018). It can be ensured that the dark-greyish to black shales with a higher TOC value should be deposited under dysoxic/anoxic conditions, and the pale to grey shales with a lower TOC value deposited during increasing bottom water oxygenation. It is certain that under the nutrients distribution and turbulent mixing controlling, plankton cycle of seasonally stratifying is the main controlling factor to the cyclical marine primary productivity and which are controlled by ocean surface water chemical conditions, temperature, palaeoclimate, etc (Beger et al., 1989). Numerous studies oriented toward palaeoproductivity characteristics of the Longmaxi shale in South China. Based on inorganic geochemistry analysing, Ma et al. (2016) found that the changes of palaeoproductivity are controlled by periodically algae blooms in the stratified marine basin, and others (Liang et al., 2017; Yan et al., 2018) indicated that the productivity is highest at the bottom and decreases gradually upward and with some small increases in the interval periods in lower Silurian Longmaxi shale, South China.

6. Conclusion Cyclostratigraphic analysis of high resolution TOC series obtained from the Early Silurian Longmaxi Formation, Shuanghe Section, Sichuan Basin, South China, reveals distinct Milankovitch cycles. Interpreted long eccentricity (405 kyr) orbital cycles were applied as a metronome to establish the floating ATS, and gave the durations of 0.88 Myr for Hirnantian Stage and 2.80 Myr for the Rhuddanian Stage. Moreover, the estimated astronomical graptolite zones durations are as follows: 668 kyr for Normalograptus extraordinarius, 213 kyr for Persculptograptus persculptus, 923 kyr for Coronograptus cyphus, 1340 kyr for Demirastrites triangulatus, we infer that the orbital forcing (eccentricity) variations in the monsoon climate, which led to variations in the weathering intensity, nutrient input results a productivity controlled organic matter accumulation model. Besides, the eccentricity variations can also be interpreted to control the climate (wet-dry) stability that led the variations in marine redox condition (anoxic-oxic), and it results a preservation controlled organic matter accumulation model.

5.3. Astronomical forcing on organic matter accumulation Rachold and Brumsack (2001) summarized a number of the orbital forcing climatic processes, and the relevant effects on the depositional environment. Long eccentricity as one of three orbital elements of the Earth that has crucial effects on the insolation, and then, impact on the sedimentary environment including eolian input, sea-level changes, weathering processes, organic productivity and redox conditions (Yan et al., 2018). Moreover, long eccentricity cycle exerts a significant impact in modulating the long term monsoon cycles (Wang, 2009; Wang et al., 2014), maxima and minima eccentricity corresponding to the largest and weakest amplitude variations of climate precession (Ma et al., 2011; Wang et al., 2014), thus resulting in two distinct-different monsoonal climates (Crowley et al., 1992; Zachos et al., 2010). Two contrasting hypotheses (shown in Fig. 7A and B) are interpreted here to verify the linkage between orbital cycle and organic accumulation. The first one (Fig. 7) regards productivity as the main factor that under long eccentricity control. At the eccentricity maxima with a strongest amplitude variation of climate precession, intensive weathering caused by frequently dry-to-wet climate changes and strong upwelling, which produces of great fine grained sediments which contained a large amount nutrient under stronger monsoonal rainfalls. Thus, seasonal productivity blooms lead to the organic matter accumulation (Rohling, 1994). The second model (Fig. 8) takes preservation for the key factor under long eccentricity controlled orbital forcing instead. Although massive inputs of nutrient promoted high seasonal productivity blooms during eccentricity maxima, the organic matter is degraded in the relatively oxic conditions under long dry season. Conversely, decreased seasonality provides a stable conditions and the organic matter accumulation could be occurred under an undisturbed preservation condition during eccentricity minima (Herbert and Fischer, 1986). The phase correlation between the obtained filtered signal and the

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