Global and regional controls on carbon-sulfur isotope cycling during SPICE event in south China

Abstract

The positive S-isotopic excursion of carbonate-associated sulfate (δ³⁴S_CAS) is generally in phase with the Steptoean positive carbon isotope excursion (SPICE), which may reflect widespread, global, transient increases in the burial of organic carbon and pyrite sulfate in sediments deposited under large-scale anoxic and sulphidic conditions. However, carbon-sulfur isotope cycling of the global SPICE event, which may be controlled by global and regional events, is still poorly understood, especially in south China. Therefore, the δ¹³C_PDB, δ¹⁸O_PDB,δ³⁴S_CAS, total carbon (TC), total organic carbon (TOC) and total sulfate (TS) of Cambrian carbonate of Waergang section of Hunan Province were analyzed to unravel global and regional controls on carbon-sulfur cycling during SPICE event in south China.

The δ³⁴S_CAS values in the onset and rising limb are not obviously higher than that in the preceding SPICE, meanwhile sulfate (δ³⁴S_CAS) isotope values increase slightly with increasing δ¹³C_PDB in rising limb and near peak of SPICE (130–160 m). The sulfate (δ³⁴S_CAS) isotope values gradually decrease from 48.6‰ to 18‰ in the peak part of SPICE and even increase from 18‰ to 38.5% in the descending limb of SPICE. The abnormal asynchronous C-S isotope excursion during SPICE event in the south China was mainly controlled by the global events including sea level change and marine sulfate reduction, and it was also influenced by regional events such as enhanced siliciclastic provenance input (sulfate), weathering of a carbonate platform and sedimentary environment. Sedimentary environment and lithology are not the main reason for global SPICE event but influence the δ¹³C_PDB excursion-amplitude of SPICE. Sea level eustacy and carbonate platform weathering probably made a major contribution to the δ¹³C_PDB excursion during the SPICE, in particularly, near peak of SPICE. Besides, the trilobite extinctions, anoxia, organic-matter burial and siliciclastic provenance input also play an important role in the onset, early and late stage of SPICE event.

Graphical abstract

Keywords

sulfate isotope excursion / terrigenous matter / carbonate platform weathering / sea level change / transitional slope environment / Waergang section

1 Introduction

The global Steptoean positive carbon isotope excursion (SPICE) during late Cambrian has been identified around the world, in North America (Laurentia), Kazakhstan, China, Australia (Gondwana) and England (Peng, 1992; Peng and Robison, 2000; Saltzman et al., 2000; Peng et al., 2004a, 2004b; Saltzman et al., 2004; Runnegar et al., 2010; Woods et al., 2011; Dahl et al., 2014). The SPICE is recorded as a 4 ‰−6 ‰ excursion in δ¹³C_carb during about 2–4 million years and its onset defines the lower boundary of the Upper Cambrian (Furongian) at about 499 million years ago (Saltzman et al., 2000, 2004; Gill et al., 2011a). The SPICE marks a global oceanographic event and may be related with the global trilobite extinctions, organic-matter burial, sea level change and seawater temperature (Saltzman et al., 2000; Elrick et al., 2011; Dahl et al., 2014), which could be mostly confirmed by corresponding widespread positive excursion of sulfur isotope due to the carbon-sulfur cycling (Gill et al., 2011a; Thompson and Kah, 2012; Liu et al., 2016).

The positive S-isotopic excursions of sedimentary pyrite (δ³⁴S_Pyrite) and carbonate-associated sulfate (δ ³⁴S_CAS) is generally in phase with the SPICE, although the δ³⁴S_CAS excursion amplitude varies in different SPICE sections (Hurtgen et al., 2002; Gill et al., 2007; Gill et al., 2011a). These carbon-sulfate isotope synchronous excursions were thought to reflect global transient increases in the burial of organic carbon and pyrite sulfate in sediments deposited under large-scale anoxic and sulphidic (euxinic) conditions (Saltzman et al., 1998, 2000, 2004; Hurtgen et al., 2009; Gill et al., 2011a; Thompson and Kah, 2012). However, controls on the carbon-sulfur cycling during SPICE event, especially the regional factors, was still poorly understood.

The SPICE events were identified in three typical sections of south China including the Waergang Section, Yongshun Section and Yankong Section, which were deposited in different environments (Peng, 1992; Peng and Robison, 2000; Peng et al., 2004a, 2004b; Zuo et al., 2008a, 2008b; Wang et al., 2011). The SPICE event was related to upwelling, the fast burial of organic materials, sea level eustacy, Trilobite extinctions and the weathering of carbonate rocks in Yongshun and Yankong Sections of the south China (Zuo et al., 2008a, 2008b; Wang et al., 2011). However, the carbon-sulfur cycling of SPICE event is still research gap in the south China (Peng, 1992; Peng and Robison, 2000; Peng et al., 2004a, 2004b; Zuo et al., 2008a, 2008b; Wang et al., 2011). As one of typical sections related to SPICE event of south China, the Waergang Section, which was deposited in the transitional slope environment and very close to the Yangtze Platform and deep shale basin, was sensitive to the sea level change, organic carbon burial, regional weathering and siliciclastic provenance input.

To reveal the global and regional controls on the carbon-sulfur cycling during SPICE event, we measured isotopic data (δ¹³C, δ¹⁸O, δ³⁴S_CAS) and elemental abundance data (TOC, TC, and TS) in samples from the Waergang Section. On the basis of previously published data from Yankong Section of Yangtze Platform and Yongshun Section of slope and the new data of Waergang SPICE Section, we aim to explain the carbon-sulfur cycling mechanism and reveal the controls on the sulfur isotope excursion during the SPICE event.

2 Geological setting

The sedimentary environment of south China mainly comprises the exposed Kangtien old Land, a shallow-water Platform (Yangtze Platform), a slope belt (Xiangxi Basin), deep-water Basin (Jiangnan Basin) and the Southeast Clastic Platform during the late Cambrian (Peng and Robison, 2000; Feng et al., 2001, 2002; Peng et al., 2004a, 2004b; Zuo et al., 2008a, 2008b) (Fig.1). The carbonate was mainly deposited in the Yangtze Platform (block) and Slope (Xiangxi Basin) but clastic sediment accumulated in the Jiangnan Basin and South-east block (Feng et al., 2001, 2002; Peng et al., 2004a,2004b; Zuo et al., 2008) (Fig.1). The Kangtien land area, which was a peneplained, only provided limited fine-grained clastic sediment for the Yangtze Platform (block) (Feng et al., 2001). The provenance of clastic rock within Jiangnan Basin and Southeast Clastic Platform mainly comes from the Cathaysia, which was located roughly in the southeast of the southeast Block, provided much fragmentary material (Feng et al., 2001; Feng et al., 2002). The Kangtien Land area expanded in the Late Cambrian (Feng et al., 2001). Compared with the Middle Cambrian, the Yangtze Platform was still a shallow-water carbonate platform evolving to a gypsum-salt lake in the Late Cambrian, suggesting the sea-level rise in the Late Cambrian (Feng et al., 2001). Carbonate deposition increased within Jiangnan Basin in the Late Cambrian (Feng et al., 2001). The gravity flow deposits were more developed within the slope in the Late than Middle Cambrian, indicating the sea-level rise (Feng et al., 2001). The Paibi Stage and Furongian Series are derived from localities and the Global Standard Stratotype-section and Point (GSSP) of south China, and the Furongian Series, which is equal to the Middle-late of Late Cambrian, replaced the old Late Cambrian (Peng et al., 2004a, 2004b, 2004c). Therefore, after larger-scale transgression of the early Wuling Series, the south China area was characterized by sustained regression and sea level fall in the Early Furongian (New Late Cambrian) (Wang et al., 2011).

Both Waergang Section and Yongshun Section are situated within the slope belt, which represents transitional facies between shallow-water carbonate of Yangtze Platform and deep basinal deposits of Jiangnan Basin (Peng, 1992; Wang et al., 2011; Liang et al., 2015; Li, 2017) (Fig.1). The Yongshun Section was at the Yangtze Platform-Slope boundary, but the Waergang Section was on the boundary of Slope-Jiangnan shale Basin (Fig.1). The Middle-Upper Cambrian lithological unit mainly consists of Aoxi, Huaqiao, Chefu, Bitiao and Shenjiawan Formations in the Waergang Section of Hunan Province, which mainly comprised carbonate rock, mudstone and argillaceous carbonate rock (Fig.2) (Peng, 2009; Liang et al., 2015). The Middle-Upper Cambrian lithological unit of Yongshun Section comprises Aoxi Formation, Huaqiao Formation, Chefu Formation, Bitiao Formation and Zhuitun Formation in western Hunan Province (Fig.2) (Wang et al., 2011; Liang et al., 2015). Besides, the carbonate sediment Yankong Section (Guizhou) was deposited in the Yangtze Platform (Zuo et al., 2008a, 2008b). The lithological unit consist of Loushanguan Group in the Yankong Section of Guizhou Province (Fig.2) (Zuo et al., 2008a, 2008b; Liang et al., 2015). Macro fossils are quite rare in the Late Cambrian (Furongian) carbonate in the Yangtze Platform (Zuo et al., 2008a).

The trilobite fauna, which occurred in the slope environment of south-eastern Yangzte Platform during Cambrian, generally contain many global Agnostida trilobites and some Polymerid trilobites that had global ranges (Peng and Robison, 2000; Peng, 2009). The trilobite fauna, which is characterized by the broad distribution in geography, rapid evolution and short stratigraphical age range, contributes to the local and global division of biostratigraphy and chronostratigraphy (Peng, 2009). The Cambrian biostratigraphic succession and chronostratigraphic system of south China has been summarized by Peng based on the Waergang, Paibi, Yongshun and Luoyixi Sections (Fig.3) (Peng and Robison, 2000; Peng, 2009).

3 Materials and methods

This study mainly focused on carbonate samples of the Upper Cambrian (Furongian Series) in the Slope, south China, from the Waergang (Fig.1). Carbon and oxygen isotope data of Yongshun and Yankong Sections were referenced and adopted from the previous results (Zuo et al., 2008a; Wang et al., 2011).

The Waergang Section was sampled (av. 5 m/sample) in fresh exposures of Upper Cambrian carbonate rock in the field. Sixty-four large samples (>200 g) were carefully trimmed to remove possible weathered surfaces, post-depositional veins and any visible pyrite nodules or bands, and then cut into small pieces. The freshest pieces were chosen and further crushed to powder (200 mesh) and used for carbon and oxygen isotope, TOC, TC and TS analyses.

Carbon (δ¹³C) and oxygen (δ¹⁸O) isotope data of 64 carbonate samples were obtained by using a Finnigan MAT 251 gas source mass spectrometer fitted with an elemental analyzer at the State Key Laboratory of Biogeology and Environmental Geology in China University of Geosciences. The analytic error is less than 0.1‰.

Carbonate associated sulfate (CAS) was extracted using the following method (Gill et al., 2008; Gill et al., 2011a, 2011b). Approximately 3–6 g remaining fine powder from each sample were treated with two deionized water rinses lasting 24 h each. After each rinse, the overlying water was carefully decanted. All samples were then treated with a 4% sodium hypochlorite solution for 48 h to remove any metastable sulfides and organically bound sulfur (for example, ester bound sulfates). Two additional deionized water rinses followed before the samples were dissolved using trace-metal clean 4 N HCl. The resulting sample was then vacuum filtered (45 μm) to remove any insoluble materials. In each case, the insoluble fraction was minor (<0.01 mg), so that concentrations reported for the bulk rocks can be treated as concentrations within the carbonate. We added approximately 25 mL of a saturated BaCl₂ solution (250 g/L) to the remaining solution to precipitate sulfate as BaSO₄. We let the samples sit for at least 3 days to ensure complete precipitation. The BaSO₄ powders were filtered, dried, and homogenized, and combined with an excess of V₂O₅ and combusted online for measuring the sulfur isotopic composition of carbonate-associated sulfate (δ³⁴S_CAS) then homogenized and loaded into tin capsules with excess V₂O₅ and analyzed for their ³⁴S/³²S isotope ratios at China University of Geosciences using a Thermo Fisher Scientific Delta V Plus isotope ratio mass spectrometer (IRMS) coupled with a Flash elemental analyzer. All sulfur isotope compositions are reported in standard delta notation as per mil (&) deviations from Vienna Canyon Diablo Troilite (V-CDT) with analytical errors of less than 0.2‰.

The total carbon (TC), total inorganic carbon (TIC) and total sulfur (TS) were obtained at the China University of Geosciences using a Jena multi-EA 4000 carbon-sulfur analyzer through online combustion at 1350°C and acidification with ~30%−40% phosphoric acid, respectively. Analytical errors are better than ±0.2% for TC and TIC based on replicate analyses of Alpha Resources standards AR 4007 and AR 1034. Then total organic carbon (TOC) was calculated by difference between total carbon (TC) and total inorganic carbon (TIC). Total sulfur (TS) was analyzed with the relative standard deviations of 0.15%−3.7% (n = 3).

4 Results

4.1 Lithology and sedimentary environment of Waergang Section

The Bitiao Formation of Waergang Section mainly consists of argillaceous limestone, micrite and shale (Fig.4). The micrite and argillaceous limestone indicate shallow sea and open shelf environment in SPICE rising limb of Waergang Section (Fig.4). The laminal limestone and argillaceous banded limestone can be interpreted into the end of slope in the peak of SPICE section (Fig.4). The micrite, shale, laminal limestone and some argillaceous limestones suggest the open shelf and basin environment in the descending limb of the SPICE (Fig.4). The increasing shale indicate that the shelf evolved into the basin environment in the top of SPICE (Fig.4).

4.2 Carbon isotope and petrology of Waergang Section

The Bitiao Formation calcites at Waergang Section exhibit a total δ¹³C_PDB range from −0.38 to +4.53‰ with discrete positive anomaly within 180 m thickness (110–290 m), which can be recognized as the carbon isotope excursion (SPICE) (Tab.1, Fig.2 and Fig.4). The SPICE shows maximum δ¹³C_PDB value of 4.53‰ with the pre-excursion baseline values of 1‰ and excursion amplitude of 3.53‰ (Tab.1, Fig.4). The δ¹³C_PDB values of SPICE rising limb vary from baseline values (1‰) to peak part (4.34‰) over a thickness of about 45 m (110–155 m) (Fig.4). The δ¹³C_PDB values of SPICE peak part vary from 3.5‰ to 4.53‰ with small amplitude variation within 80 m carbonate rock (155–235 m) (Fig.4). The descending limb of the SPICE displays a more gradual return to baseline δ¹³C_PDB values in the carbonate rocks of 60 m (235–290 m) (Fig.4).

The strata of pre-SPICE (0–110 m) comprise major argillaceous limestone (~45 m) interbedded with thick-layer limestone (10 m and 12 m) and 15 m micrite (Fig.4). The strata of SPICE rising limb mainly consist of argillaceous banded limestone (interbedded with two layers of micrite) and micrite from bottom to top (Fig.4). The peak part of the SPICE mainly includes the major laminal limestone, micrite, two thin layers of argillaceous banded limestone and little shale (Fig.4). The dark shale interbedded with thin-layer limestone mainly constitutes the descending limb strata of the SPICE (Fig.4).

4.3 Sulfur isotope and total sulfur (TS) of the Waergang section

The sulfate (δ³⁴S_CAS) isotope values mainly vary from 30‰ to 50‰ with a small amplitude variation in the preceding SPICE (0–110 m) and rising limb of SPICE (110–155 m), but the δ³⁴S_CAS values in the onset and rising limb of SPICE are not obviously more than that preceding SPICE (Fig.4). The δ³⁴S_CAS values obviously decrease from 48.6‰ (about 160 m) to 18‰ (about 240 m) in the peak part of SPICE (Fig.4). The δ³⁴S_CAS values of SPICE peak part are less than that in the pre-SPICE and rising limb of SPICE (Fig.4). The sulfate (δ³⁴S_CAS) isotope value just slightly increases with increasing δ¹³C_PDB in rising limb and near peak of SPICE (130–160 m) (Fig.4). The sulfate (δ³⁴S_CAS) isotope values gradually decease from 48.6‰ to 18.2‰ with stable δ¹³C_PDB values in the peak of SPICE (160–240 m) (Fig.4). The sulfate (δ³⁴S_CAS) isotope values gradually increase from 18.2‰ to 38.5‰ in the descending limb of the SPICE (Fig.4). Besides, the cross plot of δ¹³C_PDB and ³⁴S_CAS indicates no significant correlation (R² = 0.11) of carbon and sulfate isotope during SPICE event (Fig.5. There are just slight positive correlations between Carbon (δ¹³C_PDB) and Sulfate (δ³⁴S_CAS) isotope in the rising limb (Fig.5) and peak part of SPICE (Fig.5). However, there is obvious negative correlation between Carbon (δ¹³C_PDB) and Sulfate (δ³⁴S_CAS) isotope in descending limb of SPICE (Fig.5). The slight sulfate (δ³⁴S_CAS) isotope excursion was not completely in phase with SPICE (Fig.4 and Fig.5).

The total sulfur (TS) values mainly range from 0.2% to 0.4% in the Waergang Section (Fig.4). The total sulfur (TS) values are broadly invariant in the preceding-SPICE, whole SPICE and after-SPICE, but suddenly and abnormally increase in the argillaceous limestone (sample W41) near dark mudstone during descending limb of the SPICE (Fig.4).

4.4 Oxygen isotope, total carbon (TC) and total organic carbon (TOC) of Waergang Section

The content of total carbon (TC) and the oxygen isotope co-vary with carbon isotope excursion (SPICE) marked in the middle of Bitiao Formation of Wangergang Section (Fig.4). During the SPICE, the δ¹⁸O_PDB values ranging from −9.85‰ to −13.44‰ show a general negative δ¹⁸O_PDB excursion with several fluctuations, but the total carbon (TC) values varying from 6.21% to 11.93% indicate a positive TC excursion (Fig.4). The δ¹⁸O_PDB maintain a steady high value beneath the onset of the SPICE event, but abruptly decrease at the onset of the SPICE (Fig.4). Besides, the δ¹⁸O_PDB show negative excursion and low values in the rising limb of SPCIE (Fig.4). The lowest δ¹⁸O_PDB values and the highest total carbon (TC) values coincide approximately at the peak of the δ¹³C_PDB SPICE (Fig.4). Total carbon (TC) values at and beneath the onset of the SPICE of SPICE are generally lower than that in the rising limb and peak part of SPICE (Fig.4). The TC values show a general positive excursion coincide with SPICE event (Fig.4). Besides, the total carbon (TC) shows negative excursion and low values in the descending limb of the SPICE (Fig.4). Total organic carbon (TOC) is invariant with slight fluctuation beneath SPICE, whole SPICE and upper SPICE (SPICE) (Fig.4). However, the total organic carbon (TOC) sharply increased to 0.99% and rapidly decreased to the normal value at the rising limb (sample No. w22) of the SPICE (Fig.4).

5 Discussion

5.1 Evaluation of diagenetic alteration

The diagenetic alteration of marine carbonate generally reduces δ¹³C_PDB and δ¹⁸O_PDB values together and enhances the positive correlation between them (Bathurst, 1975; Knauth and Kennedy, 2009; Derry, 2010; Chen et al., 2014; Peng et al., 2016; Li, 2017). There is only slight correlation between the δ¹³C_PDB and δ¹⁸O_PDB values of the Waergang Section (correlation index: 0.15), (Fig.6). Although many negative δ¹⁸O_PDB values of the Waergang Section were less than −10‰ (Fig.4 and Fig.6) suggesting that the carbonate experienced slight diagenetic alteration, the abundant carbon (TC varies 6.21% to 11.93%) might have protected the δ¹³C_PDB values from obvious influence of diagenetic fluids (Fig.4 and Fig.6). Lithology of Waergang SPICE section mainly consist of micrite and argillaceous limestone (Fig.4). The argillaceous limestone and micrite were not susceptible to remineralization by diagenetic fluids because of their extremely low permeability (Poulson and John, 2003; Chen et al., 2014). Therefore, these indicate that the δ¹³C_PDB values of the Waergang Section carbonate, which were slightly altered during diagenesis, may almost represent the original information of seawater during the deposition process (Young et al., 2016). The lithology of the Waergang Section SPICE mainly comprises micrite, argillaceous limestone and mudstone and lacks crystal grain limestone and dolomite (Fig.4) (Peng, 1990), which indicate limited influence of diagenesis on the carbonate. Therefore, the δ¹⁸O_PDB values might reflect the main change trend of paleo-seawater to some extent (Elrick et al., 2011). There are many research papers covering the diagenetic effects on δ¹³C_PDB and δ¹⁸O_PDB values in marine carbonates, but few investigations have focused on meteoric influences on δ³⁴S_CAS (Gill et al., 2008; Sim et al., 2015). Meteoric diagenesis significantly lowers CAS concentrations but have little to no effect on δ³⁴S_CAS values recorded within the strata (Gill et al., 2008; Sim et al., 2015). The absence of δ¹⁸O_carb and δ³⁴S_CAS correlation of Waergang Section (Fig.6) suggests that primary marine δ³⁴S_CAS values have not been significantly reset during diagenesis.

5.2 Controls on carbon isotope (δ¹³C_PDB) excursion during the SPICE event

5.2.1 Sedimentary environment and lithology (regional factor)

The SPICE events of south China mainly occurred in the three typical sections (Fig.1 and Fig.2). Both Waergang Section and Yongshun Section were deposited in Jiangnan slope environment but Yankong Section (Guizhou) in the Yangtze Platform (Fig.1). These indicate that the sedimentary environment and lithology may not be the main reason for the global SPICE event. However, the δ¹³C_PDB excursion amplitude is obviously different in the three sections (Fig.2). Besides, the global SPICE recorded in the rocks should vary as a function of facies and carbonate platform geometry (Schiffbauer et al., 2017). The water depth of sedimentary environment has an obvious negative correlation with δ¹³C_PDB value of SPICE (Fig.4). Therefore, the sedimentary facies (Sedimentary environment and lithology) and carbonate platform geometry may influence the δ¹³C_PDB excursion amplitude of SPICE event.

5.2.2 Sea level eustacy (global factor) and carbonate platform weathering (regional factor)

The lithological evidence suggests that sea level fall in the rising limb and peak of SPICE and sea level rise in the descending limb of the SPICE (Fig.4), which was consistent with the regression and sea level fall in the early Furongian (Wang et al., 2011). The lower TC values of carbonate indicates that lower relative contents of carbonate mineral due to higher contents of argillaceous matter and higher sea level in at and beneath the onset of the SPICE (Fig.4). The increasing and high TC values supports higher relative contents of carbonate mineral due to lower contents of argillaceous matter and sea level fall in the rising limb of SPICE (Fig.4). The total carbon (TC) shows negative excursion and low values may be related to the higher content of argillaceous matter and sea level rise in the descending limb of the SPICE (Fig.4). Besides, the peak of SPICE may coincide with maximum regression (Fig.4) (Saltzman et al., 2000). Therefore, the general positive excursion of total carbon (TC), which coincide with SPICE, may be related to the sea level eustacy (Fig.4).

The oxygen isotope of carbonate can nearly reflect the seawater temperature during deposition, because of the slightly diagenetic alteration on the carbon and oxygen isotope values (Fig.6) (Elrick et al., 2011). Although the limited diagenetic alteration interfered the absolute temperature, the trend of δ¹⁸O values across the SPICE generally reflects seawater temperature change (Elrick et al., 2011). Besides, the ¹³C-depleted carbon release operated as positive feedback to temperature (Elrick et al., 2011). The higher δ¹⁸O values at or beneath the onset of the SPICE suggests seawater cooling and supports earlier hypotheses of upwelling of cool waters and benthic extinction onto the shallow slope at the onset of the SPICE (Fig.4) (Lochman-Balk, 1970; Stitt, 1975; Taylor, 1977; Palmer, 1984; Saltzman et al., 2000; Elrick et al., 2011), which may be related to a large-scale transgression (Wang et al., 2011).

Negative δ¹⁸O_PDB excursion in the rising limb of SPICE also suggests that the SPICE event might be related to seawater temperature (Fig.4 and Fig.5). Descending limb of δ¹⁸O values suggests seawater warming, which responds to increasing TC values, may be caused by the drop of thermocline due to sea level fall (Saltzman et al., 2000; Wang et al., 2011) (Fig.4). Yangtze carbonate-platform (Fig.1), which was susceptible to weathering due to the sea level fall, can result in the positive carbonate isotope excursion (Kump et al., 1999; Kump and Arthur, 1999; Chen et al., 2016). Besides, the facies and carbonate platform geometry, which were mainly controlled by sea level, may influence the δ¹³C_PDB excursion amplitude but not the excursion trend of SPICE event. Therefore, the increasing δ¹³C_PDB of carbonate may be associated with enhanced weathering of carbonate platform (Yangtze Platform) due to regression (sea level fall) in the rising limb and peak of SPICE (Feng et al., 2001, 2002; Peng et al., 2004a, 2004b; Saltzman et al., 2004; Zuo et al., 2008a, 2008b; Wang et al., 2011).

The deeper sea water is generally more ¹³C-enriched than shallow sea water (Schiffbauer et al., 2017). However, the δ¹³C values of upper SPICE with high sea level are much less than peak of SPICE, which indicated that transgression was not the critical reason for SPICE. The decreasing carbonate and total carbon (TC), increasing shale deposition, indicate reduced weathering of carbonate platform and enhanced siliciclastic provenance in descending limb of SPICE (Fig.4). Therefore, the decreasing and low δ¹³C_PDB of upper SPICE limestone may be related to reduce weathering of carbonate platform due to sea level rise and enhanced siliciclastic provenance in the descending limb of the SPICE (Fig.4 and Fig.7).

5.2.3 Trilobite extinctions, anoxia and organic-matter burial (global factor)

The start of SPICE excursion nearly coincides with a worldwide mass extinction of trilobite species at the base of the Glyptagnostus Reticulatus Zone (Palmer, 1965; Öpik, 1966; Saltzman et al., 2000; Ahlberg et al., 2009) (Fig.3 and Fig.7). The special widespread and rapidly evolving trilobite fauna mainly occurred in the Jiangnan slope environment during the Cambrian (Fig.3) (Peng, 2009). The abrupted increase and abnormally high value of total organic carbon (TOC), which coincides with the suddenly remarkable decrease of δ¹⁸O values at the onset of the SPICE δ¹³C curve (Fig.4). The mid-depth water transitioned from anoxic condition during Cambrian Age 2 into stable oxic condition during Cambrian Age 4 (Li et al., 2017). These suggest that rapidly enhanced production and mass extinction might have ever existed at the onset or early rising limb of SPICE excursion, and due in part to seawater warming and the drop of thermocline caused by sharp fall of sea level (Saltzman et al., 2000; Wang et al., 2011; Elrick et al., 2011) (Fig.4). The killing mechanism of extinctions may be attributed to a disruption of food chain that occurred because essential nutrients were depleted by the enhanced production and reduction of dissolved O₂ owing to the seawater warming and sea level fall in the rising limb of SPICE (Paul and Mitchell, 1994; Saltzman et al., 2000; Elrick et al., 2011). Besides, anoxia and organic-matter burial may be caused by reduction in dissolved O₂ and trilobite extinctions due to seawater warming (Arthur et al., 1987; Elrick et al., 2011). Therefore, the increasing δ¹³C_PDB of carbonate may be also related to trilobite extinctions, anoxia and organic-matter burial in the rising limb of SPICE.

5.3 Controls on Sulfate isotope (δ³⁴S_CAS) cycling during the SPICE event

The burial of carbon and sulfate as organic matter and pyrite (FeS₂) resulting in the removal of carbon and sulfate from the ocean during SPICE, which generally display the globally parallel excursion behavior between the carbon (δ¹³C_PDB) and sulfate isotope (δ³⁴S_CAS and δ³⁴S_Pyrite) during SPICE event (Gill et al., 2011a). The isotopic composition of CAS is an important paleoceanographic proxy, because the primary δ³⁴S for carbonates can be strongly preserved during diagenetic alteration (Fig.6) (Gill et al., 2008). Therefore, the positive excursions of sulfate isotope (δ³⁴S_CAS) corresponding to SPICE event may reflect transient increases in the burial of organic carbon and pyrite sulfate in sediment deposited under large-scale anoxic and sulphidic (euxinic) conditions (Saltzman et al., 1998, 2000, 2004; Hurtgen et al., 2009; Gill et al., 2011a; Thompson and Kah, 2012). The δ³⁴S_CAS values should have an obvious excursion consistent with the SPICE in the Waergang Section of south China, which can be assumed as the dashed blue line (Fig.4), although the magnitude of the δ³⁴S_CAS excursion of SPICE varies from 0‰ to ~10‰ to 15‰ and even 30‰ (Hurtgen et al., 2002; Gill et al., 2007; Gill et al., 2011a). However, the slight sulfate (δ³⁴S_CAS) isotope excursion was not in phase with SPICE of Waergang Section in the south China (Fig.4 and Fig.5).

5.3.1 Sulfate isotope (δ³⁴S_CAS) cycling mechanism in the onset and rising limb of SPICE

The δ¹³C positive excursion may be related to trilobite extinctions, anoxia and organic-matter burial in the onset and rising limb of SPICE. Therefore, the anoxia, abundant sulfate and organic matter provided material basis and environment condition for marine sulfate reduction, which should produce pyrite and result in positive excursion of CAS (Fig.7) (Johnston et al., 2010; Li et al., 2015). However, the δ³⁴S_CAS values in the onset and rising limb of SPICE are similar or just slightly elevated relative to that in preceding SPICE (Fig.4 and Fig.7).

The major input of sulfur to ocean reservoir from runoff derived from the weathering of continental sulfides and sulfate minerals with sulfur isotope values (δ³⁴S) between 0 and 10‰ (Holser et al., 1988; Gill et al., 2007). The siliciclastic provenance of clastic rock within Jiangnan Basin and South-east clastic Platform mainly come from the South-eastern Cathaysia (Feng et al., 2001; Feng et al., 2002). The gravity flow deposit was more developed within the slope in the Late rather than Middle Cambrian (Feng et al., 2001). The weathering of continental sulfides and sulfate minerals with low δ³⁴S_CAS, which mainly derived from weathering product of the Yangtze Platform and south-eastern siliciclastic provenance, was the major input of sulfur to ocean (Holser et al., 1988; Gill et al., 2007). Therefore, the very slight δ³⁴S_CAS excursion (130–160 m) may be related to the enhanced input of terrigenous matter (sulfate) with low δ³⁴S_CAS in the rising limb of SPICE due to sea level fall, which could significantly reduce the δ³⁴S_CAS excursion (Fig.4).

5.3.2 Sulfate isotope (δ³⁴S_CAS) cycling mechanism during peak period of SPICE

The Sulfate isotope (δ³⁴S_CAS) started to slightly decrease in early peak period of SPICE and continue to gradually decrease until the descending period of SPICE, which are obviously less than the rising limb of SPICE (Fig.4). Marine sulfate reduction, which should produce pyrite and result in positive excursion of CAS, was determined by organic matter supply, sulfate availability and the anoxic condition (Johnston et al., 2010; Li et al., 2015). However, the organic matter supply and the anoxic condition could not meet the requirements for strong marine sulfate reduction in the slope (Waergang Section) during peak period of SPICE, because of maximum regression during the peak of SPICE (Fig.4) (Saltzman et al., 2000). Besides, the weathering of Yangtze Platform carbonate with low δ³⁴S_CAS, which probably made a major contribution to the δ¹³C_PDB excursion, reduced the δ³⁴S_CAS of slope carbonate (Waergang Section) as the important terrigenous matter (sulfate) near the peak of SPICE due to the low sea level (Fig.7). Therefore, the decreasing and lowest δ³⁴S_CAS value during peak period of SPICE may be locally controlled by the absence of marine sulfate reduction, enhanced weathering of Yangtze platform carbonate and low sea level in the slope of south China (Fig.1, Fig.4, and Fig.7).

5.3.3 Sulfate isotope (δ³⁴S_CAS) cycling mechanism in descending limb of SPICE

The δ³⁴S_CAS decreased to the lowest value (18.2‰) in the bottom and gradually increase to 38.5‰ in the top of SPICE descending limb (Fig.4). The sulfate leaving the ocean through marine sulfate reduction are enriched in ³²S via isotope fractionations, leaving seawater correspondingly enriched in ³⁴S (Gil et al., 2011a, 2011b). Besides, the dark shale with the slight positive excursion of TOC and TS occurred in the descending limb of the SPICE (Fig.4). Therefore, the gradual increasing δ³⁴S_CAS values may be related to marine sulfate reduction due to sea level rise in the descending limb of the SPICE (Fig.4 and Fig.7). However, the thickness of limestone gradually increased and shale decreased from bottom to top in the descending limb of SPICE (Fig.4). Consequently, the lowest δ³⁴S_CAS value may be related to the enhanced input of siliciclastic provenance with low δ³⁴S_CAS in the bottom of SPICE descending limb (Fig.4).

6 Conclusions

The SPICE event was well discovered in the Waergang Section of the south China. The δ¹³C_PDB, δ¹⁸O_PDB, and δ³⁴S_CAS values of the Waergang Section carbonates may have ever been altered by the diagenetic alteration, but they can indicate some original information and paleo-temperature change of seawater during the deposition process.

The weathering rate change of carbonate platform due to sea level eustacy probably made a major contribution to the δ¹³C_PDB excursion during the SPICE. Besides, the trilobite extinctions, anoxia, organic-matter burial and siliciclastic provenance input (mudstone deposition) may also play an important role in the onset and rising limb of SPICE event. Sedimentary facies and carbonate platform geometry may influence the δ¹³C_PDB excursion amplitude of SPICE event.

Very slight positive excursion of δ³⁴S_CAS in the rising limb of SPICE may be mainly related to enhance terrigenous matter (sulfate) input with low δ³⁴S_CAS due to sea level fall, which mainly derived from weathering product of Yangtze Platform and south-eastern siliciclastic provenance.

The decreasing and lowest δ³⁴S_CAS value during peak period of SPICE may be locally controlled by the absence of marine sulfate reduction, enhanced weathering of Yangtze platform carbonate and low sea level in the slope of south China. The gradual increasing δ³⁴S_CAS values may be related to marine sulfate reduction due to global sea level rise in the descending limb of the SPICE.

The total sulfate (TS) of argillaceous limestone or mudstone content sharply increased due to the enhanced siliciclastic provenance (sulfate) and sea level rise in the descending limb of SPICE. The obvious lower δ³⁴S_CAS values of the descending limb than rising limb of the SPICE may be associated with enhanced siliciclastic provenance input (mudstone deposition), sea level rise, relatively limited organic matter and marine sulfate reduction in the descending limb of the SPICE.

References

[1] Ahlberg P,, Axheimer N,, Babcock L E,, Eriksson M E,, Schmitz B,, Terfelt F. ( 2009). Cambrian high-resolution biostratigraphy and carbon isotope chemostratigraphy in Scania, Sweden: first record of the SPICE and DICE excursions in Scandinavia. Lethaia, 42( 1): 2– 16

[2] Arthur M A,, Schlanger S O,, Jenkyns H. ( 1987). The Genomanian-Turonian oceanic anoxic event, II. Palaeoceanographic controls on organic-matter production and preservation. Spec Publ Geol Soc Lond, 26( 1): 401– 420

[3] Bathurst R G C ( 1975). Carbonate Sediments and Their Diagenesis (2nd ed). Amsterdam: Elsevier

[4] Chen Z,, Wang X,, Hu J,, Yang S,, Zhu M,, Dong X,, Tang Z,, Peng P,, Ding Z. ( 2014). Structure of the carbon isotope excursion in a high-resolution lacustrine Paleocene–Eocene Thermal Maximum record from central China. Earth Planet Sci Lett, 408: 331– 340

[5] Chen Z,, Ding Z,, Yang S,, Zhang C,, Wang X. ( 2016). Increased precipitation and weathering across the Paleocene-Eocene Thermal Maximum in central China. Geochem Geophys Geosyst, 17( 6): 2286– 2297

[6] Dahl T W,, Boyle R A,, Canfield D E,, Connelly J N,, Gill B C,, Lenton T M,, Bizzarro M. ( 2014). Uranium isotopes distinguish two geochemically distinct stages during the later Cambrian SPICE event. Earth Planet Sci Lett, 401: 313– 326

[7] Derry L A. ( 2010). On the significant of δ¹³C correlations in ancient sediments. Earth Planet Sci Lett, 296( 3–4): 497– 501

[8] Elrick M,, Rieboldt S,, Saltzman M,, McKay R M. ( 2011). Oxygen-isotope trends and seawater temperature changes across the Late Cambrian Steptoean positive carbon-isotope excursion (SPICE event). Geology, 39( 10): 987– 990

[9] Feng Z, Peng Y, Jin Z, Jiang P, Bao Z, Luo Z, Ju T, Tian H, Wang H ( 2001). Lithofacies palaeogeography of the Cambrian in South China. J Palaeogeogr, 3( 1): 1– 14 (in Chinese)

[10] Feng Z, Peng Y, Jin Z, Jiang P, Bao Z ( 2002). Lithofacies palaeogeography of the Late Cambrian in China. J Palaeogeogr, 4( 3): 1– 10 (in Chinese)

[11] Gill B C,, Lyons T W,, Saltzman M R. ( 2007). Parallel, high-resolution carbon and sulfur isotope records of the evolving Paleozoic marine sulfur reservoir. Palaeogeogr Palaeoclimatol Palaeoecol, 256( 3–4): 156– 173

[12] Gill B C,, Lyons T W,, Frank T D. ( 2008). Behavior of carbonate-associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy. Geochim Cosmochim Acta, 72( 19): 4699– 4711

[13] Gill B C,, Lyons T W,, Young S A,, Kump L R,, Knoll A H,, Saltzman M R. ( 2011a). Geochemical evidence for widespread euxinia in the later Cambrian ocean. Nature, 469( 7328): 80– 83

[14] Gill B C,, Lyons T W,, Jenkyns H C. ( 2011b). A global perturbation to the sulfur cycle during the Toarcian Oceanic Anoxic Event. Earth Planet Sci Lett, 312( 3–4): 484– 496

[15] Holser W T, Schidlowski M, Mackenzie F T, Maynard J B ( 1988). Geochemical cycles of carbon and sulfur. In: Gregor C B, Garrels R M, Mackenzie F T, Maynard J B, eds. Chemical Cycles in the Evolution of the Earth. New York: Wiley, 105– 173

[16] Hurtgen M,, Arthur M,, Suits N,, Kaufman A. ( 2002). The sulfur isotopic composition of Neoproterozoic seawater sulfate: implications for snowball Earth?. Earth Planet Sci Lett, 203( 1): 413– 429

[17] Hurtgen M T,, Pruss S B,, Knoll A H. ( 2009). Evaluating the relationship between the carbon and sulfur cycles in the later Cambrian ocean: an example from the Port au Port Group, western Newfoundland, Canada. Earth Planet Sci Lett, 281( 3–4): 288– 297

[18] Johnston D T,, Poulton S W,, Dehler C,, Porter S,, Husson J,, Canfield D E,, Knoll A H. ( 2010). An emerging picture of Neoproterozoic ocean chemistry: insights from the Chuar Group, Grand Canyon, USA. Earth Planet Sci Lett, 290( 1–2): 64– 73

[19] Knauth L P,, Kennedy M J. ( 2009). The late Precambrian greening of the Earth. Nature, 460( 7256): 728– 732

[20] Kump L R,, Arthur M A,, Patzkowsky M E,, Gibbs M T,, Pinkus D S,, Sheehan P M. ( 1999). A weathering hypothesis for glaciation at high atmospheric pCO₂ during the Late Ordovician. Palaeogeogr Palaeoclimatol Palaeoecol, 152( 1–2): 173– 187

[21] Kump L R,, Arthur M A. ( 1999). Interpreting carbon-isotope excursions: carbonates and organic matter. Chem Geol, 111: 299– 302

[22] Li C,, Cheng M,, Algeo T J,, Xie S C. ( 2015). A theoretical prediction of chemical zonation in early oceans (>520 Ma). Sci China Earth Sci, 58( 11): 1901– 1909

[23] Li C,, Jin C,, Planavsky N J,, Algeo T J,, Cheng M,, Yang X,, Zhao Y,, Xie S. ( 2017). Coupled oceanic oxygenation and metazoan diversification during the early–middle Cambrian?. Geology, 45( 8): 743– 746

[24] Li D ( 2017). The interplay between Cambrian ocean chemistry changes and early animal evolution. Dissertation for the Doctoral Degree. Hefei: University of Science and Technology of China (in Chinese)

[25] Liang W, Mou C, Zhou K, Ge X, Chen C, Xu P ( 2015). Palaeogeography of the Cambrian Epoch 3-Furongian in the Middle and Upper Yangtze region. J Palaeogeogr, 17( 2): 172– 185 (in Chinese)

[26] Liu Y,, Li C,, Algeo T J,, Fan J,, Peng P. ( 2016). Global and regional controls on marine redox changes across the Ordovician-Silurian boundary in south China. Palaeogeogr Palaeoclimatol Palaeoecol, 463: 180– 191

[27] Lochman-Balk C. ( 1970). Upper Cambrian faunal patterns on the craton. Geol Soc Am Bull, 81( 11): 3197– 3224

[28] Öpik A A. ( 1966). The early Upper Cambrian crisis and its Petrocorrelation. J Proc R Soc N S W, 100: 9– 14

[29] Palmer A R. ( 1965). Trilobites of the Late Cambrian Pterocephaliid Biomere in the Great Basin, United States. US Geol Surv Prof Pap, 493: 49– 49

[30] Palmer A R. ( 1984). The biomere problem: evolution of an idea. J Paleontol, 58: 599– 611

[31] Paul C R C,, Mitchell S F. ( 1994). Is famine a common factor in marine mass extinctions?. Geology, 22( 8): 679– 682

[32] Peng S. ( 1992). Upper Cambrian biostratigraphy and trilobite faunas of the Cili-Taoyuan area, northwestern Hunan, China. Memoir of the Association Australasian Palaeontologists, 13: 1– 119

[33] Peng S ( 1990). The Upper Cambrian and trilobite succession in the Taoyuan-Cili area of Hunan Province. J Stratigraphy, 14( 4): 261– 276 (in Chinese)

[34] Peng S,, Babcock L E,, Robison R A,, Lin H,, Rees M N,, Saltzman M R. ( 2004a). Global Standard Stratotype-section and Point (GSSP) of the Furongian Series and Paibian Stage (Cambrian). Lethaia, 37( 4): 365– 379

[35] Peng S, Babcock L E, Lin H, Chen Y, Zhu X, Qi Y ( 2004b). The Paibi Section in Hunan, n orthwestern Hunan: a global standard stratotype – section and point for Cambrian Furongian series and Paibi stage. Collection of Papers on Stratigraphic Paleontology, 28: 11– 25 (in Chinese)

[36] Peng S, Zhu X, Lin H ( 2004c). The first global standard stratotype section and poin t of Cambrian System for Paibian Stage and Furongian Series in China. J Stratigraphy, 28( 1): 92– 94 (in Chinese)

[37] Peng S,, Robison R A. ( 2000). Agnostoid biostratigraphy across the Middle-Upper Cambrian boundary in Hunan, China. J Paleontol, 74( sp53): 1– 10

[38] Peng S ( 2009). The newly-developed Cambrian biostratigraphic succession and chronostratigraphic scheme for s outh China. Chin Sci Bull, 54( 18): 2691– 2698 (in Chinese)

[39] Peng Y,, Peng Y,, Lang X,, Ma H,, Huang K,, Li F,, Shen B. ( 2016). Marine carbon-sulfur biogeochemical cycles during the Steptoean Positive Carbon Isotope Excursion (SPICE) in the Jiangnan Basin, south China. J Earth Sci, 27( 2): 242– 254

[40] Poulson S R,, John B E. ( 2003). Stable isotope and trace element geochemistry of the basal Bouse Formation carbonate, southwestern United States: implications for the Pliocene uplift history of the Colorado Plateau. Geol Soc Am Bull, 115: 434– 444

[41] Qi Y, Wang Z, Bagnoli G ( 2004). Conodont b iostratigraphy of the GSSP OF the b ase of the Furongian Series and Paibi Stage. J Stratigraphy, 28( 2): 114– 119 (in Chinese)

[42] Runnegar B,, Saltzman M R,, Kouchinsky A,, Young S A,, Kump L R,, Gill B,, Lyons T,, Young E D. ( 2010). Cambrian SPICE (Steptoean Positive Carbon Isotope Excursion) as a model for Comparable Proterozoic high-amplitude isotopic events. GSA Abstracts with Programs, 42( 5): 398

[43] Saltzman M R,, Cowan C A,, Runkel A C,, Runnegar B,, Stewart M C,, Palmer A R. ( 2004). The Late Cambrian SPICE (δ¹³C) event and the Sauk II-Sauk III regression: new evidence from Laurentian basins in Utah, Iowa, and Newfoundland. J Sediment Res, 74( 3): 366– 377

[44] Saltzman M R,, Ripperdan R L,, Brasier M D,, Lohmann K C,, Robison R A,, Chang W T,, Peng S,, Ergaliev E K,, Runnegar B. ( 2000). A global carbon isotope excursion (SPICE) during the Late Cambrian: relation to trilobite extinctions, organic-matter burial and sea level. Palaeogeogr Palaeoclimatol Palaeoecol, 162( 3–4): 211– 223

[45] Saltzman M R,, Runnegar B,, Lohmann K C. ( 1998). Carbon isotope stratigraphy of Upper Cambrian (Steptoean Stage) sequences of the eastern Great Basin: record of a global oceanographic event. Geol Soc Am Bull, 110( 3): 285– 297

[46] Schiffbauer J D,, Huntley J W,, Fike D A,, Jeffrey M J,, Gregg J M,, Shelton K L. ( 2017). Decoupling biogeochemical records, extinction, and environmental change during the Cambrian SPICE event. Sci Adv, 3( 3): e1602158

[47] Sim M S,, Ono S H,, Hurtgen M T. ( 2015). Sulfur isotope evidence for low and fluctuating sulfate levels in the Late Devonian ocean and the potential link with the mass extinction event. Earth Planet Sci Lett, 419: 52– 62

[48] Stitt J H. ( 1975). Adaptive radiation, trilobite paleoecology, and extinction, Ptychaspid biomere: Late Cambrian of Oklahoma. Fossils and Strata, 4: 381– 390

[49] Taylor M E ( 1977). Late Cambrian of western North America: trilobite biofacies, environmental significance, and biostratigraphic implications. In: Kauffman E G, Hazel J E, eds. Concepts and Methods of Biostratigraphy. Stroudsburg: Dowden, Hutchinson and Ross, 397– 425

[50] Thompson C K, Kah L C ( 2012). Sulfur isotope evidence for widespread euxinia and a fluctuating oxycline in Early to Middle Ordovician greenhouse oceans. Palaeogeogr Palaeoclimatol Palaeoecol, 313–314: 189– 214

[51] Wang C, Li X, Bai Y, Liu A, Zeng X ( 2011). The Cambrian SPICE event in Yongshun area, Hunan Province, and its significance for stratigraphic correlation. Geol Chin, 38( 6): 1440– 1445 (in Chinese)

[52] Woods M A,, Wilby P R,, Leng M J,, Rushton A W A,, Williams M. ( 2011). The Furongian (Late Cambrian) Steptoean Positive Carbon Isotope Excursion (SPICE) in Avalonia. J Geol Soc London, 168( 4): 851– 862

[53] Young S A,, Gill B C,, Edwards C T,, Saltzman M R,, Leslie S A. ( 2016). Middle–Late Ordovician (Darriwilian–Sandbian) decoupling of global sulfur and carbon cycles: isotopic evidence from eastern and southern Laurentia. Palaeogeogr Palaeoclimatol Palaeoecol, 458: 118– 132

[54] Zuo J, Peng S, Zhu X ( 2008 a). Carbon isotope composition of Cambrian carbonate rocks in Yangtze Platform, south China and its geological implications. Geochemica, 37( 2): 118– 128 (in Chinese)

[55] Zuo J, Peng S, Zhu X, Qi Y, Lin H, Yang X ( 2008 b). Evolution of carbon isotope composition in potential global stratotype section and point at Luoyixi, south China, for the Base of the Unnamed Global Seventh Stage of Cambrian System. Earth Science 19( 1): 9– 22 (in Chinese)

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