Marine redox variations and nitrogen cycle of the early Cambrian southern margin of the Yangtze Platform, South China: Evidence from nitrogen and organic carbon isotopes

“Molybdenum geochemistry The trace element molybdenum, preserved in black shales, can be a useful redox proxy of the ancient ocean (Algeo and Maynard, 2004; Dahl et al., 2011; Scott et al., 2008; Scott and Lyons, 2012; Tribovillard et al., 2006).”

“The marine nitrogen cycle Nitrogen is essential for life because this element is an impor- ant constituent of amino acids, nucleic acids, pigments such as hlorophyll, and other biomolecules (Canfield et al., 2010; Godfrey nd Glass, 2010).”

“Mo concentrations and TOC contents exhibit co-variation in odern euxinic marine sediments as well as in many ancient black hales (Algeo and Lyons, 2006; Algeo and Maynard, 2004; Holland, 984; Robl and Barron, 1988).”

“…nitrogen bond, N2-fixation can nly occur when fixed-N (nitrate and ammonia) concentrations are ow, and nutrient phosphorus is in excess supply (Tyrrell, 1999). his means that fixed-N (nitrate and ammonia) in the surface ater most likely had been consumed by intense denitrification nd/or…”

“This ammonium is then rapidly oxidized to nitrite (NO2−) and subsequently to nitrate (NO3−) through nitrification with negligible fractionation under modern oxic conditions or with a significant fractionation under oxygen-deficient conditions (Sigman et al., 2009; Wang et al., 2013).”

“In the modern oxygenated ocean, Mo is the most abundant transition metal (∼105 nM) (Collier, 1985; Lyons et al., 2009) and exists primarily as molybdate (MoO42−), with a residence time of ∼0.8 Myr (Emerson and Huested, 1991; Morford and Emerson, 1999).”

“In ulfidic silled basins, aqueous Mo concentrations can be lowered s a result of sedimentary Mo uptake without adequate resuply through deep water renewal (Algeo and Lyons, 2006; Algeo nd Rowe, 2012).”

“Molybdenum geochemistry The trace element molybdenum, preserved in black shales, can be a useful redox proxy of the ancient ocean (Algeo and Maynard, 2004; Dahl et al., 2011; Scott et al., 2008; Scott and Lyons, 2012; Tribovillard et al., 2006).”

“Some geochemical studies suggest that cean basins were well oxygenated by the late Ediacaran (Canfield t al., 2007; Fike et al., 2006; Scott et al., 2008).”

“…species through abiotic processes (Hansen et al., 1996), or converted to NO2-/N2/NH4+ mediated by anaerobic nitrate-dependent Fe(II)-oxidizing microorganisims under dominantly ferruginous conditions (Busigny et al., 2013; Shen et al., 2013; Straub et al., 1996; Weber et al., 2006a,b) (Fig.”

“Diagenetic alteration under oxic conditions usually increases the 15N composition of organic matter (Gaye et al., 2009; Lehmann et al., 2002; Möbius et al., 2011; Nakanishi and Minagawa, 2003).”

“Based on published Fe speciation data (Canfield et al., 2008; hen et al., 2015; Feng et al., 2014; Jin et al., 2014; Och, 2011; Och t al., 2013; Wang et al., 2012a), the Liuchapo and Niutitang forations (and correlative formations) were mostly deposited under noxic conditions.”

“In the modern oxygenated ocean, Mo is the most abundant transition metal (∼105 nM) (Collier, 1985; Lyons et al., 2009) and exists primarily as molybdate (MoO42−), with a residence time of ∼0.8 Myr (Emerson and Huested, 1991; Morford and Emerson, 1999).”

“Meanwhile, low Mobulk concentrations and Mo/TOCbulk ratios (averaging 3 ppm and ∼8, respectively) at Sancha section indicate that euxinic conditions did not spread to the shelf marginal regions (Scott and Lyons, 2012) (Fig.”

“…MoS42−) forms and is rapidly scavenged by pyrite and organic mater, resulting in near-quantitative removal of Mo (Algeo and Lyons, 006; Erickson and Helz, 2000; Helz et al., 1996, 2011; Huerta-Diaz nd Morse, 1992; Tribovillard et al., 2004; Vorlicek and Helz, 2002; orlicek et al.,…”

“…for the persistence of ferruginous nd/or intermittently euxinic deep waters into the early Camrian, particularly over the Yangtze block of South China (Canfield t al., 2008; Goldberg et al., 2007; Li et al., 2010; Och et al., 2013; oulton and Canfield, 2011; Wille et al., 2008; Xu et al., 2012).”

“The Yuanjia section in Zhongfang County which is a basinal section (Jiang et al., 2011) consists of the upper part of the Liuchapo Formation and the overlying Niutitang Formation.”

“The largest modern oceanic sink for Mo is co-precipitation and adsorption with Mn-oxyhydroxides in oxic environments (Barling et al., 2001; McManus et al., 2006; Siebert et al., 2003).”

“Furthermore, the higher Mo/TOCbulk ratios than those (∼ 1–14) of the Stage 2 transgression imply a larger global Mo reservoir (Algeo and Rowe, 2012; Sahoo et al., 2012; Scott et al., 2008), which could probably mainly be due to more oxygenated open ocean, although enhanced weathering during the…”

“Furthermore, the higher Mo/TOCbulk ratios than those (∼ 1–14) of the Stage 2 transgression imply a larger global Mo reservoir (Algeo and Rowe, 2012; Sahoo et al., 2012; Scott et al., 2008), which could probably mainly be due to more oxygenated open ocean, although enhanced weathering during the…”

“The key to this positive loop is the transfer of phosphorus from deep anoxic waters to the photic zone (Calvert et al., 1996; Ingall and Jahnke, 1994) (Fig.”

“Schematic depiction for a positive feedback loop between anoxia, phosphoru 1994), Ingall and Van Cappellen (1990) and Murphy et al. (2000). sing nitrite as the electron acceptor (NH4+ + NO2− = N2 + 2H2O) Kuypers et al., 2003).”

“…(−4‰ to −2‰) may result from partial assimilation of ammonia with a large isotopic fractionation effect (Crowe et al., 2008b; Higgins et al., 2012; Kappler et al., 2005), while high 15Nbulk values around +2‰ would mainly be due to the assimilation of 15N-enriched ammonia by anaerobic…”

“In his case, water-column denitrification and anammox would have esulted in a positive fractionation and a 15N-enriched nitrate pool Cline and Kaplan, 1975; Liu and Kaplan, 1988) (Fig.”

“…that fixed-N (nitrate and ammonia) in the surface ater most likely had been consumed by intense denitrification nd/or anammox processes (Junium and Arthur, 2007; Luo et al., 011) while phosphorus was efficiently recycled under anoxic onditions (Ingall et al., 1993; Van Cappellen and Ingall, 1994).”

“In these anoxic environments, microbial egradation is suggested to retain the initial 15N value or only lightly decrease the 15N value (Altabet et al., 1999; Higgins et al., 010; Junium and Arthur, 2007; Möbius et al., 2010).”

“The correlation between Ni–Mo enrichment layers of the basal Niutitang Formation in Hunan and Guizhou provinces and the upper Shiyantou Formation at Xiaotan section in Yunnan by Och et al. (2013) is supported by a U–Pb age of 526.5 ± 1.1 Ma for zircons from a volcanic ash bed within the lower part of the Shiyantou For…”

“The largest modern oceanic sink for Mo is co-precipitation and adsorption with Mn-oxyhydroxides in oxic environments (Barling et al., 2001; McManus et al., 2006; Siebert et al., 2003).”

“In modern oceans, suspended organic particles in surface waters have the lowest 15N due to N2 fixation or assimilation of low- 15N ammonium released by heterotrophic organisms (ammonium recycling), but their 15N values increase below the euphotic zone (Altabet, 1988; Sigman et al., 2009).”

“…succession overlying the Yangtze lock of South China was deposited in various paleoenvironmental ettings from large platformal areas with a transitional zone nterpreted as outer shelf, into slope and then deeper more basinal reas (Jiang et al., 2012; Steiner et al., 2007; Zhu et al., 2007).”

“…above and below this Ni–Mo ore layer have high 98Mo values (∼1.9–2.4‰) (Chen et al., 2015) (modern seawater 98Mo = +2.34‰, Goldberg et al., 2013), suggesting that hydrothermal input of Mo may have contributed only to the Ni–Mo ore layer, but not to the adjacent black shales (Chen et al., 2015).”

“However, NH4+ generated beneath the seafloor can e captured by clay minerals, in which case 15Nbulk values may how little or no change (Freudenthal et al., 2001).”

“Today phototrophic organisms are able to utilize various forms of dissolved nitrogen species such as dinitrogen, nitrate, ammonia and urea to synthetize organic matter, which is then transported to the sediments and preserved through geological time (Sigman et al., 2009; Waser et al., 1998).”

“In the modern oxygenated ocean, Mo is the most abundant transition metal (∼105 nM) (Collier, 1985; Lyons et al., 2009) and exists primarily as molybdate (MoO42−), with a residence time of ∼0.8 Myr (Emerson and Huested, 1991; Morford and Emerson, 1999).”

“…might only increase to small extent, depending on metamorphic grade, with increases f ∼1–2‰ at greenschist facies and of ∼3–4‰ at amphibolite facies Bebout and Fogel, 1992; Jia, 2006; Mingram and Bräuer, 2001). s the 15Nbulk values from our studied successions are unlikely o have been…”

“…MoS42−) forms and is rapidly scavenged by pyrite and organic mater, resulting in near-quantitative removal of Mo (Algeo and Lyons, 006; Erickson and Helz, 2000; Helz et al., 1996, 2011; Huerta-Diaz nd Morse, 1992; Tribovillard et al., 2004; Vorlicek and Helz, 2002; orlicek et al., 2004).”

“2b), but these data may have not ully recorded the biogeochemical signature of the deep water ealm because of the extremely condensed nature of sedimentary eposition due to a rapid rise in sea level (Goldberg et al., 2007; chütter, 1998; Steiner et al., 2001, 2007).”

“However, organic particles are mostly produced in the euphotic zone, and they usually go through biotic alterations such as microbial digestion or deamination during sinking and deposition (Robinson et al., 2012).”

“…species through abiotic processes (Hansen et al., 1996), or converted to NO2-/N2/NH4+ mediated by anaerobic nitrate-dependent Fe(II)-oxidizing microorganisims under dominantly ferruginous conditions (Busigny et al., 2013; Shen et al., 2013; Straub et al., 1996; Weber et al., 2006a,b) (Fig.”

“…reservoir is likely to have been small, and was either reduced to ammonia by mixed Fe(II)/(III) oxy(hydr)oxide species through abiotic processes (Hansen et al., 1996), or converted to NO2-/N2/NH4+ mediated by anaerobic nitrate-dependent Fe(II)-oxidizing microorganisims under dominantly…”

“In the modern oxygenated ocean, Mo is the most abundant transition metal (∼105 nM) (Collier, 1985; Lyons et al., 2009) and exists primarily as molybdate (MoO42−), with a residence time of ∼0.8 Myr (Emerson and Huested, 1991; Morford and Emerson, 1999).”

“The early Cambrian Ni–Mo enrichment layers can be tracked at a 1600 km belt along the whole southern margin of the Yangtze Platform and can be used as a correlation marker throughout the region (Jiang et al., 2012; Och et al., 2013; Zhu et al., 2003).”

“2b) that could result from the assimilation of 12C-enriched inorganic carbon by anaerobic organisms (Küspert, 1982; Schouten et al., 2000; Struck et al., 2001) (Fig.”

“A ‘sandwich’-like model for Precambrian ocean margins has ecently been proposed, with mid-depth euxinic waters in between xic surface waters and ferruginous deep waters (Li et al., 2010, 012a; Lyons et al., 2012; Poulton and Canfield, 2011; Poulton et al., 010; Reinhard et al., 2009).”

“The early Cambrian Ni–Mo enrichment layers can be tracked at a 1600 km belt along the whole southern margin of the Yangtze Platform and can be used as a correlation marker throughout the region (Jiang et al., 2012; Och et al., 2013; Zhu et al., 2003).”

“The 15N values of these particles do not change with depth in regions of high sediment accumulation such as continental margins, but show a slightly decreasing trend in regions of low organic N depositional flux (Altabet et al., 1991; Sigman et al., 2009).”

“When heterotrophic organisms such as sulfate reducing microbes (BSM) decompose the organic matter sinking into the stratified anoxic deep water, an isotopically light DIC repository forms (Fry et al., 1991; Velinsky and Fogel, 1999), and at the same time ammonia, free H2S and phosphorus are released as well [sulfate r…”

“…in the ratios range from 3.0 to 293.9 with a mean value of 61.5 in the Sancha section and from 35.8 to 133.5 with a mean value of 76.7 in the Yuanjia section, similar to other typically high Precambrian C/N ratios (>100) (Beaumont and Robert, 1999; Garvin et al., 2009; Papineau et al., 2009).”

“…for the persistence of ferruginous nd/or intermittently euxinic deep waters into the early Camrian, particularly over the Yangtze block of South China (Canfield t al., 2008; Goldberg et al., 2007; Li et al., 2010; Och et al., 2013; oulton and Canfield, 2011; Wille et al., 2008; Xu et al., 2012).”

“A Ni–Mo enrichment layer along the transitional zone between the platform and basin was deposited under these euxinic conditions (Lehmann et al., 2007; Steiner et al., 2001).”

“…in the ratios range from 3.0 to 293.9 with a mean value of 61.5 in the Sancha section and from 35.8 to 133.5 with a mean value of 76.7 in the Yuanjia section, similar to other typically high Precambrian C/N ratios (>100) (Beaumont and Robert, 1999; Garvin et al., 2009; Papineau et al., 2009).”

“…reducing microbes (BSM) decompose the organic matter sinking into the stratified anoxic deep water, an isotopically light DIC repository forms (Fry et al., 1991; Velinsky and Fogel, 1999), and at the same time ammonia, free H2S and phosphorus are released as well [sulfate reduction:…”

“…presence of free 2S in the photic zone (Ohkouchi et al., 2005) can either participate n N2-fixation or assimilate ammonia and/or nitrite with a large nd variable fractionation depending on the seawater ammonia oncentration (Ohkouchi et al., 2005; Pennock et al., 1996), producng very low 15N values.”

“The major source of Mo to the ocean is oxidative weathering of crustal sulfides and black shales (Bertine and Turekian, 1973).”

“Heavier 13Corg values than before are consistent with the deepening of he chemocline and organic synthesis by oxygenic photosynthesis sing CO2 in the atmosphere as the dominant process for carbon ycling (Goldberg et al., 2007) (Fig.”

“Our two studied sections are both located in Hunan Province where the sedimentary facies dramatically changes from platform margin to slope and deep basinal facies (Vernhet, 2007).”

“2b) that could result from the assimilation of 12C-enriched inorganic carbon by anaerobic organisms (Küspert, 1982; Schouten et al., 2000; Struck et al., 2001) (Fig.”

“Under anoxic and non-sulphidic (ferruginous) conditions, dissolved Mo can be transported to the sediments via absorption to Fe (oxyhydr)oxides as has been observed in the modern ferruginous Matano Lake, Indonesia (Crowe et al., 2008a; Goldberg et al., 2009).”

“The key to this positive loop is the transfer of phosphorus from deep anoxic waters to the photic zone (Calvert et al., 1996; Ingall and Jahnke, 1994) (Fig.”

“Recently, a growing number of nitrogen isotope studies on marine sediments have demonstrated the potential of N isotope systematics in reconstructing the environmental and biogeochemical history of the oceans (Robinson et al., 2012; Tesdal et al., 2013).”

“Based on published Fe speciation data (Canfield et al., 2008; hen et al., 2015; Feng et al., 2014; Jin et al., 2014; Och, 2011; Och t al., 2013; Wang et al., 2012a), the Liuchapo and Niutitang forations (and correlative formations) were mostly deposited under noxic conditions.”

“…low 15Nbulk values (−4‰ to −2‰) may result from partial assimilation of ammonia with a large isotopic fractionation effect (Crowe et al., 2008b; Higgins et al., 2012; Kappler et al., 2005), while high 15Nbulk values around +2‰ would mainly be due to the assimilation of 15N-enriched ammonia…”

“In these anoxic environments, microbial egradation is suggested to retain the initial 15N value or only lightly decrease the 15N value (Altabet et al., 1999; Higgins et al., 010; Junium and Arthur, 2007; Möbius et al., 2010).”

“…the Daibu Member of the Zhujiqing Formation at Xiaotan section, Yunnan Province (Cremonese t al., 2013; Li et al., 2013a; Zhou et al., 1997) and in the lower art of the Yanjiahe Formation at Wuhe section, Three Gorges area Cremonese et al., 2014; Ishikawa et al., 2008; Kikumoto et al., 2014) Fig.”

“…to have been deposited under widespread noxic, or even euxinic conditions based on numerous lines of eochemical evidence (Canfield et al., 2008; Goldberg et al., 2005, 007; Guo et al., 2007a; Kimura and Watanabe, 2001; Lehmann t al., 2007; Och et al., 2013; Wang et al., 2012a; Wille et al.,…”

“…has been suggested to be placed within the upper Liuchapo Formation where there occurs a negative 13Corg excursion that could be correlated with the negative 13Ccarb excursion in the Daibu Member of the Zhujiaqing Formation at Xiaotan section (Guo et al., 2013; Li et al., 2013a; Zhou et al., 1997).”

“…produced by green or purple sulfur bacteria participating in ammonia assimilation with a large fractionation (Ohkouchi et al., 2005; Pennock et al., 1996) when upwelling currents were vigorous enough for some ammonia to reach the surface water before being converted to N2 (Ader et al., 2014) (Fig.”

“…black shale along a NE-striking 1600 km belt, which can be used as a correlation marker (Jiang et al., 2012; Och et al., 2013; Zhu et al., 2003), but mineable ore layer is usually less than half meters in thickness and exposed only within Guizhou and Hunan provinces (Jiang et al., 2006, 2007).”

“…sediments ormally increase with decreasing TNdecarb content due to the referential loss of 14N during alteration, thus causing negative corelations (Möbius, 2013; Pinti et al., 2007; Thomazo et al., 2011). nstead, the positive correlation between 15Nbulk and TNdecarb ontents could reflect two…”

“Simplified geological map of the Yangtze block (after Jiang et al., 2012; Stein after Li et al., 2012b and Guo et al., 2013). ecause amino acids are much more easily degraded and therefore elease more 14N under oxic than anoxic conditions (Nguyen and arvey, 1997).”

“On the other and, although thermal metamorphism can increase C/N ratios to undreds and even thousands, 15N values might only increase to small extent, depending on metamorphic grade, with increases f ∼1–2‰ at greenschist facies and of ∼3–4‰ at amphibolite facies Bebout and Fogel, 1992; Jia, 2006; Mingram and Bräuer, 2…”

“The correlation between Ni–Mo enrichment layers of the basal Niutitang Formation in Hunan and Guizhou provinces and the upper Shiyantou Formation at Xiaotan section in Yunnan by Och et al. (2013) is supported by a U–Pb age of 526.5 ± 1.1 Ma for zircons from a volcanic ash bed within the lower part of the Shiyantou For…”

“…sampled at three mine sites at the Sancha, Dazhuliushui and Maluhe sections (Xu et al., 2011) and also with the U-Pb age of 532 ± 0.7 Ma for zircons from a volcanic ash bed beneath the Ni–Mo sulfide marker layer in the lowermost Niutitang Formation in Guizhou province (Jiang et al., 2009).”

“…TNdecarb content due to the referential loss of 14N during alteration, thus causing negative corelations (Möbius, 2013; Pinti et al., 2007; Thomazo et al., 2011). nstead, the positive correlation between 15Nbulk and TNdecarb ontents could reflect two end-members of nitrogen source in he…”

“Recent studies demonstrated that the Ni–Mo sulfide ore layer, which is usually less than half meters in thickness (Jiang et al., 2006), has relatively lower 98Mo values (<1.6‰) (Lehmann et al., 2007; Xu et al., 2012) (low-temperature hydrothermal fluids 98Mo = +0.8‰, McManus et al., 2002), while…”

“3). pward, the 13Corg curve exhibits positive excursions in the Dahai ember of the Zhujiaqing Formation, Yunnan and in the upper Yan- iahe Formation, Three Gorges area (Cremonese et al., 2013, 2014; ikumoto et al., 2014) (Fig.”

“After the death of organisms, organic-N is converted to ammonium (NH4+) through ammonification with little fractionation (ε = 15Nreactant − 15Nproduct < 3‰) (Prokopenko et al., 2006; Sigman et al., 2009).”

“Molybdenum geochemistry The trace element molybdenum, preserved in black shales, can be a useful redox proxy of the ancient ocean (Algeo and Maynard, 2004; Dahl et al., 2011; Scott et al., 2008; Scott and Lyons, 2012; Tribovillard et al., 2006).”

“The correlation between Ni–Mo enrichment layers of the basal Niutitang Formation in Hunan and Guizhou provinces and the upper Shiyantou Formation at Xiaotan section in Yunnan by Och et al. (2013) is supported by a U–Pb age of 526.5 ± 1.1 Ma for zircons from a volcanic ash bed within the lower part of the Shiyantou For…”

“Our two studied sections are both located in Hunan Province where the sedimentary facies dramatically changes from platform margin to slope and deep basinal facies (Vernhet, 2007).”

“2b) that could result from the assimilation of 12C-enriched inorganic carbon by anaerobic organisms (Küspert, 1982; Schouten et al., 2000; Struck et al., 2001) (Fig.”

“Heavier 13Corg values than before are consistent with the deepening of he chemocline and organic synthesis by oxygenic photosynthesis sing CO2 in the atmosphere as the dominant process for carbon ycling (Goldberg et al., 2007) (Fig.”

“Our two studied sections are both located in Hunan Province where the sedimentary facies dramatically changes from platform margin to slope and deep basinal facies (Vernhet, 2007).”

“Previous studies argued that the Ni–Mo enrichments in the sediments could originate from hydrothermal input (Lott et al., 1999; Steiner et al., 2001), enrichment from seawater (Lehmann et al., 2007; Mao et al., 2002), or more likely the combination of both processes (Pašava et al., 2008).”

“In addition, the lower Shuijingtuo Formation in Hubei province contains the Rhombocorniculum cancellatum taxon-range Zone as well as trilobites and so already belongs to Stage 3 (Qiongzhusian Stage) (Guo et al., 2014; Steiner et al., 2007) (Fig.”

“3a) (Guo et al., 2014; Li and Xiao, 2004; Yang et al., 2014).”

“…(2015) 209–226 s m e a v e i e e b t s u a f s t p t i g u 5 p t s a i t 2 z d to modern-like levels, evidenced by high Mo values (∼+2.4‰) t al., 2014; Kikumoto et al., 2014), and (c) Sancha, (d) Yuanjia sections in Hunan, S lope-basin areas without reaching the shelf margin area in the iddle and…”

“In these anoxic environments, microbial egradation is suggested to retain the initial 15N value or only lightly decrease the 15N value (Altabet et al., 1999; Higgins et al., 010; Junium and Arthur, 2007; Möbius et al., 2010).”

“Pennock, J.R., Velinsky, D.J., Ludlam, J.M., Sharp, J.H., Fogel, M.L., 1996.”

“…Lawrence Och, Dr. Ben Yang, Xi Chen for 2013), shallow platform 15Nbulk data are from Cremonese et al. (2013) and outer helf 15Nbulk data are from Cremonese et al. (2014) and Kikumoto et al. (2014). ndicate that the shallow waters of the basin were well connected ith the oxygenated open ocean…”

“…of −4.7‰ at Sancha section may have been produced by green or purple sulfur bacteria participating in ammonia assimilation with a large fractionation (Ohkouchi et al., 2005; Pennock et al., 1996) when upwelling currents were vigorous enough for some ammonia to reach the surface water before being…”

“…(BSM) decompose the organic matter sinking into the stratified anoxic deep water, an isotopically light DIC repository forms (Fry et al., 1991; Velinsky and Fogel, 1999), and at the same time ammonia, free H2S and phosphorus are released as well [sulfate reduction: (CH2O)106(NH3)16H3PO4 +…”

“Previous studies argued that the Ni–Mo enrichments in the sediments could originate from hydrothermal input (Lott et al., 1999; Steiner et al., 2001), enrichment from seawater (Lehmann et al., 2007; Mao et al., 2002), or more likely the combination of both processes (Pašava et al., 2008).”

“…species through abiotic processes (Hansen et al., 1996), or converted to NO2-/N2/NH4+ mediated by anaerobic nitrate-dependent Fe(II)-oxidizing microorganisims under dominantly ferruginous conditions (Busigny et al., 2013; Shen et al., 2013; Straub et al., 1996; Weber et al., 2006a,b) (Fig.”

“Diagenetic alteration under oxic conditions usually increases the 15N composition of organic matter (Gaye et al., 2009; Lehmann et al., 2002; Möbius et al., 2011; Nakanishi and Minagawa, 2003).”

“In addition, the presence of free H2S requires nitrate depletion due to the redox potential O2 > NO3− > SO42− (Boyle et al., 2013), which means that instead of denitrification, anammox could have been the dominant process by which fixed-N was removed from the oceanic system, D. Wang et al. / Precambrian Research 267 (…”

“Diagenetic alteration under oxic conditions usually increases the 15N composition of organic matter (Gaye et al., 2009; Lehmann et al., 2002; Möbius et al., 2011; Nakanishi and Minagawa, 2003).”

“Our two studied sections are both located in Hunan Province where the sedimentary facies dramatically changes from platform margin to slope and deep basinal facies (Vernhet, 2007).”

“…the modern oxygenated oceans, the global mean 15N value of itrate is ∼+5‰ (Sigman et al., 2009), and the average 15N value in ediments is ∼+7‰, which is close to the expected value following omplete assimilation of seawater nitrate and minor modification t the seafloor (Pinti and Hashizume, 2011).”

“Based on published Fe speciation data (Canfield et al., 2008; hen et al., 2015; Feng et al., 2014; Jin et al., 2014; Och, 2011; Och t al., 2013; Wang et al., 2012a), the Liuchapo and Niutitang forations (and correlative formations) were mostly deposited under noxic conditions.”

“Diagenetic alteration under oxic conditions usually increases the 15N composition of organic matter (Gaye et al., 2009; Lehmann et al., 2002; Möbius et al., 2011; Nakanishi and Minagawa, 2003).”

“This ammonium is then rapidly oxidized to nitrite (NO2−) and subsequently to nitrate (NO3−) through nitrification with negligible fractionation under modern oxic conditions or with a significant fractionation under oxygen-deficient conditions (Sigman et al., 2009; Wang et al., 2013).”

“…species through abiotic processes (Hansen et al., 1996), or converted to NO2-/N2/NH4+ mediated by anaerobic nitrate-dependent Fe(II)-oxidizing microorganisims under dominantly ferruginous conditions (Busigny et al., 2013; Shen et al., 2013; Straub et al., 1996; Weber et al., 2006a,b) (Fig.”

“…the modern oxygenated oceans, the global mean 15N value of itrate is ∼+5‰ (Sigman et al., 2009), and the average 15N value in ediments is ∼+7‰, which is close to the expected value following omplete assimilation of seawater nitrate and minor modification t the seafloor (Pinti and Hashizume, 2011).”

“In his case, water-column denitrification and anammox would have esulted in a positive fractionation and a 15N-enriched nitrate pool Cline and Kaplan, 1975; Liu and Kaplan, 1988) (Fig.”

“…from Cambrian successions in South China (δ15Nsed = 1.1 ± 2.1‰) (Cremonese, ShieldsZhou, Struck, Ling, & Och, 2014; Cremonese et al., 2013; Wang, Shi, Tang, & Zhang, 2013; Wang et al., 2015) and Cryogenian successions F IGURE 9 Spatial variation in δ15Nsed of the Doushantuo/Lantian formations.”

“Similar excess N has previously been reported from the Jiumenchong Formation and its stratigraphic equivalents across the Yangtze Block in sections such as Xiaotan, Yangtze Gorges, Sancha, Nangao, Daotuo, Longbizui, Lijiatuo, and Yuanjia (Cremonese et al., 2013a; Cremonese et al., 2013b; Kikumoto et al., 2014; Wang et…”

“…2007; Lehmann et al., 2007; Schröder and Grotzinger, 2007; Zhu et al., 2007; Guo et al., 2007a, b; Ishikawa et al., 2008; Sawaki et al., 2008; Wille et al., 2008; Guo et al., 2013; Ishikawaa et al., 2013; Pi et al., 2013; Xu et al., 2013; Feng et al., 2014; Och et al., 2015; Wang et al., 2015).”

“…ocean redox [Goldberg et al., 2007; Guo et al., 2007; Lehmann et al., 2007; Wang et al., 2012; Xu et al., 2012; Och et al., 2013; Pi et al., 2013; Wang et al., 2015b], and hydrothermal activities [Lott et al., 1999; Steiner et al., 2001; Jiang et al., 2006, 2007; Chen et al., 2009; Pa sava et…”

“Organic-rich shale and mudstone deposition corresponding with low (< 1‰) δ(15)N values was widespread in South China during Cambrian Stages 1–4 (Cremonese et al., 2013, 2014; Kikumoto et al., 2014; Wang et al., 2015; Zhang et al., 2017), suggesting that these low nitrogen isotope values do not reflect an ecological re…”

“…concentrations of the atmosphere at that time (e.g. Knoll, 1991; Zhu et al. 2003; Fike et al. 2006; Zhu, Strauss & Shields, 2007; Guo et al. 2010a , b, 2013; Maloof et al. 2010; Shields-Zhou & Och, 2011; Jiang et al. 2012; Schrag et al. 2013; Feng et al. 2014; Wang et al. 2015; Och et al. 2016).”

“…Cambrian Stage 3 (Interval III) may have been caused by enhancement of the anoxic extent of the Yangtze shelf margin area as another local anoxic event in this period (D. Wang et al. 2015), which may have resulted in the extinction of Niutitang sponge biota at Cambrian Stage 3 (Zhu et al. 2007).”

“The carbon isotopic variations, consequently, may re C ect the local variations in DIC due to the remineralization of organic matter or isotopic gradient in seawater (e.g., Jiang et al. 2007; Wang et al. 2015; Jin et al. 2016; Absar et al. 2019).”

“a significant negative δ(13)Corg excursion in the V-enriched layer, which corresponds to the SHICE found in Mo–Ni deposits in Hunan and eastern Yunnan [49,59,60].”

“2014; Wang et al. 2015).”

“…and pyrite S-isotopic compositions revealed a stratified oceanic paleoenvironment during the sponge-bearing interval of the Cambrian Niutitang Formation, in which oxic surficial water was underlain by anoxic or euxinic deep water (e.g. Wang et al. 2015; Jin et al. 2016; Yeasmin et al. 2017).”

“Exposed sedimentation in the terminal Ediacaran is found in many places around the world, and vuggy dolomite formed by exposure and leaching is also found in the top part of the Dengying Formation of South Qinling and the South China Block, indicating the occurrence of an extensive global regression during the Precamb…”

“Many studies have estimated sedimentation rates for early Cambrian shales (Wang et al ., 2015a,b; Zhou et al ., 2021; Zhang et al ., 2022a,b).”

“…characterized by the loss of nitrogen and an increase in δ 15 N values, negative correlations between δ 15 N -TN and positive correlations between δ 15 N-C/N would be expected for samples that subjected to significant alteration (Cremonese et al. 2014; Wang et al. 2015, 2018b; Ader et al. 2016).”