Nitrogen and organic carbon isotope stratigraphy of the Yangtze Platform during the Ediacaran–Cambrian transition in South China

    Nitrogen and organic carbon isotope stratigraphy of the Yangtze Platform during the Ediacaran-Cambrian transition in South China Lorenzo Cremonese, Graham A. Shields-Zhou, Ulrich Struck, Hong-Fei Ling, Lawrence M. Och PII: DOI: Reference:

S0031-0182(13)00546-4 doi: 10.1016/j.palaeo.2013.12.016 PALAEO 6696

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Revised date: Accepted date:

2 January 2013 5 December 2013 10 December 2013

Please cite this article as: Cremonese, Lorenzo, Shields-Zhou, Graham A., Struck, Ulrich, Ling, Hong-Fei, Och, Lawrence M., Nitrogen and organic carbon isotope stratigraphy of the Yangtze Platform during the Ediacaran-Cambrian transition in South China, Palaeogeography, Palaeoclimatology, Palaeoecology (2013), doi: 10.1016/j.palaeo.2013.12.016

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ACCEPTED MANUSCRIPT Title: Nitrogen and organic carbon isotope stratigraphy of the Yangtze Platform during the

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Ediacaran-Cambrian transition in South China

Authors: Lorenzo Cremonese(1), Graham A. Shields-Zhou(2, 3), Ulrich Struck(4), Hong-Fei Ling (5),

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Lawrence M. Och(6)

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(1) Corresponding author. Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK. Email: [email protected]. Present address: Institute for Advanced

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Sustainability Studies, Berliner Straße 130, 14467 Potsdam, Germany. Telephone: +493068326158; fax: +493020938565.

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UK. Email: [email protected].

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(2) Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT,

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(3) State Key Laboratory of Paleontology and Stratigraphy, NIGPAS, Chinese Academy of Sciences, Nanjing, 39 East Beijing Road, Nanjing 210008, China. Email: [email protected]. (4) Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity,

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Invalidenstraße 43, 10115 Berlin, Germany. Email: [email protected]. (5) State Key Laboratory for Mineral Deposits Research, Nanjing University, 163 Xianlin Road, Nanjing 210023, China. Email: [email protected]. (6) EAWAG, Swiss Federal Institute of Aquatic Science and Technology, CH-6047 Kastanienbaum, Switzerland. Email: [email protected]

ACCEPTED MANUSCRIPT Abstract N and Corg isotope results are presented from six sections along a West-East transect in the

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South China Basin (SCB) covering both shallow and deeper domains, in order to investigate

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biogeochemical cycling, stratigraphic correlation and isotope systematics over the crucial

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Ediacaran-Cambrian transition. 15N bulk values range between -3‰ and +7‰, while 13Corg values range between -21‰ and -39‰. Similar isotopic trends have been identified for both these proxies

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across the basin, although hiatuses, differences in depositional setting and syn-depositional bacterial fermentation may have caused some inconsistencies. A trend towards negative N isotope values can

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be recognized above the PC-C boundary in both shallow and deeper basin realms across the Yangtze platform. This negative 15N excursion is probably a response to photic zone anoxia and

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intense nitrogen fixation/assimilation by diazotrophic cyanobacteria and Green/Purple Sulfur

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Bacteria (GSB and PSB). The Xiaotan section and a composite section from the Yangtze Gorges area show meaningful similarities in their nitrogen isotope trends, interpreted as chemocline

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fluctuations in the water column that testify to rapid mixing of water overlying the shallow platform. Using carbon isotope stratigraphy, we correlate boundary strata across the platform to test

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the wider significance of nitrogen isotopic variations. Increased bioturbation and food-chain complexity across the Ediacaran-Cambrian transition probably led to more frequent variations in nitrate isotope composition and related pool dimension during the early Cambrian, reflecting the beginning of a new more biologically controlled era.

Keywords: Ediacaran, Cambrian, nitrogen isotopes, organic carbon isotopes, Yangtze platform, marine biogeochemistry.

ACCEPTED MANUSCRIPT 1. Introduction The geological record of the Yangtze Platform (South China Block or SCB) possesses a

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remarkable sedimentary record of the Precambrian-Cambrian transition, characterized by relatively

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continuous and unaltered successions that are ideally suited to high-resolution investigations of

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paleoseawater and environmental conditions. Compared to the Proterozoic, this transition witnessed the appearance of new, more complex marine ecosystems characterized by more rapid chemical

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reactions that facilitated ocean pool and isotope perturbations in a more productive marine environment (Falkowski et al., 2008; Logan et al., 1995). The onset of a reinvigorated nitrogen

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cycle and a greater diversity of chemical species, possibly related to contemporaneous increases in seawater oxygen and the onset of bioturbation, characterizes the beginning of a more biologically

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controlled geological era (Falkowski et al., 2008; Grotzinger et al., 1995; Shen et al., 2008).

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Today, the Ediacaran-Cambrian (Precambrian-Cambrian) boundary is defined by the first

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occurrence of the trace fossil Treptichnus (formerly Phycodes) pedum, as described by Landing (1994). The Global Standard Stratotype Section and Point (GSSP) is located at Fortune Head, Newfoundland in Canada. Despite the global correlation potential of trace fossils, biostratigraphic

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records from the Yangtze Platform still exhibit facies bias, as for example Treptichnus pedum has only been found in China in the Meishucun Section in Eastern Yunnan (Zhu, 1997; Zhu et al., 2005). Another obstacle to the use of this trace fossil is a widespread hiatus which characterizes the basal Cambrian throughout South China (Zhu et al., 2003). The most continuous successions covering the Ediacaran-Cambrian boundary are situated in northeastern Yunnan, Hubei, eastern Guizhou and central Hunan, the latter two regions recording more distal environmental settings with low fossiliferous contents. On the Yangtze Platform, the passage to Cambrian strata is commonly marked by the appearance of phosphorites rich in small phosphatic remains (endoskeletons, exoskeletons, shells of brachiopods and mollusks) called Small Shelly Fossils (SSFs; Matthews and Missarzhevsky, 1975),

ACCEPTED MANUSCRIPT which occur above a stratigraphic unconformity. SSFs, widely distributed across South China, characterize the lower Cambrian in other regions of the world, too (Siberia, Mongolia, Australia,

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etc.), and represent a fundamental tool for Cambrian Series and Stage subdivision. An additional

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criticism concerning the use of a trace fossil is the impossibility to relate it to a specific species and

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then strictly to a First Appearance Datum (FAD). Metazoan diversification was accompanied by a change from horizontal (e.g. Planolites, Paleophycus) to vertical burrows (Treptichnus pedum-type

involving a variety of species (Jensen, 1997).

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traces) and so undoubtedly marks an evolutionary step from Ediacaran to Cambrian fauna, likely

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Although the Ediacaran-Cambrian boundary is defined biostratigraphically, several attempts have been made to date Ediacaran-Cambrian sections directly using U/Pb methods (Bowring et al., 2007; Grotzinger et al., 1995), resulting in a widely accepted age of 541 Ma for the beginning of the

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Cambrian Period.13Ccarb chemostratigraphy also constitutes a valid tool for stratigraphic correlation. Over the last twenty years, outcrop investigations carried out mostly in Canada

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(Narbonne et al., 1994), California (Corsetti and Hagadorn, 2000), northeastern Siberia (Fedonkin, 1985), Mongolia (Brasier et al., 1996) and South China (Weber et al., 2007; Zhu et al., 2001) show

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how the first appearance of Treptichnus pedum occurs just above a profoundly negative 13Ccarb excursion, from 0‰ to -2‰ down to -7‰ in some locations. The disappearance of such negative carbon isotope excursions has been put down to increased oxygenation. It has been argued that successive episodes of oxidation of a large organic carbon pool (OCP), characterized by low 13C, caused such negative carbon excursions during the late Neoproterozoic and early Cambrian (Swanson-Hysell et al., 2010). The onset of negative 13Ccarb values has been widely used to locate the Ediacaran-Cambrian boundary in sections where marker fossils are not preserved, such as Oman and Morocco (Amthor et al., 2003; Maloof et al., 2005). Using our geochemical results, we will discuss several aspects of the Ediacaran-Cambrian transition with reference to the bathymetric profiles across the Yangtze Platform proposed by Jiang et al. (2011) and Zhu et al. (2013). The water column chemistry will be characterized with

ACCEPTED MANUSCRIPT particular attention to aquatic nitrogen, oxygen concentrations (chemocline depth) and biodiversification in surface waters as well as deeper domains and communication between the

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platform and the basin. One remarkable advantage of nitrogen isotopes as a geochemical tool is its

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sensitivity to specific local and regional biochemical processes. As a consequence, the 15N study

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outlined here may be used to identify the predominant biological processes that characterize past oceans, such as dinitrogen fixation, assimilation or denitrification. Our outcomes will be compared to iron speciation data from other studies in order to strengthen isotopic interpretations. With this

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work we aim to contribute to a better understanding of lateral geochemical variability between time-

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equivalent stratigraphic units and describe in detail the marine environment across the Pc-C in

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South China.

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2. Section locations and geological setting

2.1 The Yangtze Gorges area

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2.1.1 Litho- and bio-stratigraphy

The Yangtze Gorges area (or Three Gorges area) in the Yichang area of western Hubei possesses some of the best preserved Ediacaran outcrops on the SCB (Lee and Chao, 1924; Liu and Sha, 1963). The entire geological complex overlies nonconformably a granitic body, which outcrops at the core of the Huangling anticline and was emplaced during the early Neoproterozoic following the break-up of the supercontinent Rodinia. The excellent Ediacaran rock outcrops were created by the Yangtze River valley that cuts through the Huangling anticline in its southern part shaping the famous Yangtze Gorges (Fig. 1). This area has been intensively investigated over the last 30 years (Ishikawa et al., 2013, 2008; Jiang et al., 2011, 2006; Liu et al., 2013; McFadden et al., 2009, 2008; Wang et al., 2002; Yin et al., 2011; Zhou et al., 2007; Zhu et al., 2013, 2007),

ACCEPTED MANUSCRIPT supplying many geochemical and biostratigraphic data to investigate the paleogeography and stratigraphy of the Sinian System in South China.

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The earliest Paleozoic (early Cambrian) is recorded in the Yanjiahe, Shuijingtuo and Shipai

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formations although the succession is significantly condensed with respect to NE Yunnan (see

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Cremonese et al., 2013). The uppermost Ediacaran in this area is represented by the Dengying Fm. (Fig. 2e), which conformably overlies the Doushantuo Fm. The total thickness in the Yichang area, where it crops out in towering cliffs is about 100 meters (Wuhe section), but can reach 800 or 900

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meters in northern Yunnan and eastern Sichuan (Steiner et al., 2005). In the stratigraphy analyzed

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its thickness is always less than 100 meters, often measuring only few tens of meters. The Dengying Fm. in this area is commonly composed of three parts (in ascending order): the Hamajing, Shibantan and Baimatuo members. Samples through this formation have been labeled WH 1→WH

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Oolitic textures and oncolites in the intraclastic dolomitic grainstone of the Hamajing Mb.

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are organized in medium-thick bedded, grey sparitic and arenitic fine-crystalline dolomites. Bird’s eyes structures, cross bedding and chert interbeds are also present in this member. Sedimentological

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structures are characteristic of a high-energy environment following black shale deposition at the top of the underneath Doushantuo Fm. Its thickness is about 200m on the eastern limb of the Huangling anticline at the type section and changes to ca. 10m in the western limb, with similar values on the South China Platform. The fossiliferous content for this member consists of sphaeromorph acritarchs (Wang et al., 1998), with lithological features indicating a shallowingupward succession. The Shibantan Mb. is composed of dark greyish, thin to medium-bedded laminated micritic limestone with trace fossils, rich fragments of vendotaenids (algal or bacterial colonies), generally rich in organic material, chert bands and concretions. At the Wuhe section (Three Gorges area) this member appears as decimetric to centimetric beds, darker in color and with chert nodules in its

ACCEPTED MANUSCRIPT upper unit. Its total thickness ranges between 100m and 140m in the Yichang area (Zhu et al., 2007), while as much as 170m in Hunan Province (Wang et al., 1998).

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The uppermost Baimatuo Mb. usually consists of massive micritic and recrystallized

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dolomite with an erosional surface at the top that is overlain by various Cambrian successions (Zhu

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et al., 2003). It is medium- to thick-bedded, grey and with chert beds and bird’s eye structures, probably indicating intertidal or supratidal conditions (Wang et al., 1998). This member also shows some karstification with abundant dissolution structures or voids occurring in many localities as far

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as southern Shaanxi Province. The tubular fossil Sinotubulites, which may represent the earliest

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shell-producing metazoan, was found in shelly beds in the lower part of the Baimatuo Mb. (Chen et al., 1981; Ding et al., 1992). Also present are the trace fossil Skolithos and sphaeromorph acritarchs (Zhao et al., 1988). In our area of study, the thicknesses attributed to these members are 10m for the

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Hamajing Mb., 49m for the Shibantan Mb. and 29m for the Baimatuo Mb. The fossiliferous content in the middle and upper parts of the Dengying Fm. is referred to as

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the “Xilingxia Biota” (Chen et al., 1981; Ding et al., 1992), and its depositional environment is interpreted as a widespread prograding platform accompanied by sea level fall. Towards the

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southeast, the carbonate successions become gradually more condensed, ultimately changing into slope and basinal facies of the corresponding Liuchapo Fm. The lower Cambrian Yanjiahe and Shuijingtuo formations have been sampled at the Wuhe (YS100→YS116; YS0→YS4) and Jijiawan (JJL1→ JJL11) sections. The locations of these outcrops in the Yangtze Gorges area are reported in Fig. 1. In this area, the Yanjiahe Fm. overlies the upper Baimatuo Mb. of the Dengying Fm. discontinuously, registering a wavy contact about 1m in amplitude, reasonably describing a paleokarstic surface (in contrast to Ding et al., 1992). Its basal part is composed of several meters of cherty dolostones organized in thin layers or nodules in a brownish, occasionally sandy, calcareous matrix. About seven meters above, the lithology becomes phosphatic and chert-banded (Fig. 2g). The last 20 meters are composed of grey dolostones, sandy at the base and with cherts in beds and nodules higher up with a phosphatic interbed. The last 5

ACCEPTED MANUSCRIPT meters contain large dark carbonate concretions, similar to those described by Cremonese et al. (2013) at the top of the Shiyantou Fm. in the Xiaotan section (diameter~1m; Fig. 2f). These

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structures probably formed by microbial degradation in organic-rich sediments during early

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diagenesis, favoring calcite precipitation. In the field, it was observed that these concretions are still

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present for the first 20 meters of the Shuijingtuo Fm. The boundary with the Shuijingtuo Fm. (Fig. 2h) is conglomeratic with some elongated dark, rounded components and framboidal pyrite. The blade-like components appear to be rip-up clasts within a phosphatic layer and likely represent a

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hard-ground as also supposed by Ding et al. (1992). This portion was sampled at higher resolution;

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the sample YSOB being 2 meters below the contact, YSOC from the phosphatic layer, YSO from the conglomeratic/clastic layer and YS1B being about 20 cm above the contact. The total thickness of the Yanjiahe Formation ranges between 54m (Wang et al., 2002) and 43m (Ishikawa et al.,

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2008).

The Shuijingtuo Fm. records at its base grey carbonates organized in decimetric layers with large

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dark-grey carbonate concretions. Another break in the section did not permit us to sample the middle part, nor to measure its complete thickness, although this formation should cover in total

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about 70 meters (Ishikawa et al., 2008). The upper part presents medium-bedded grey dolomite with carbonate concretions (their occurrence is probably continuous from the basal part of the formation), while in the last 10 meters the sandy/organic fraction increases and concretions disappear, passing transitionally into the upper silty-sandy Shipai Fm.

Biostratigraphy

of

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basal Cambrian in the Three Gorges area is characterized by relative paucity of fossils. Subdivision of the Yanjiahe and Shuijingtuo formations based on SSF Assemblages and Cambrian Series/Stages is required to test chemostratigraphic correlations with other sections investigated on the Yangtze Platform. Analysis of early Cambrian SSF occurrence and fossil associations are reported by Chen (1984) and Guo et al. (2008) and suggest that the beginning of the cherty-phosphorites assemblage about 14m from the formation base corresponds to the FAD of SSFA1, while a few meters above, the SSFA2 begins. This biozone continues until the other phosphatic horizon at the very top of the

ACCEPTED MANUSCRIPT formation. Three meters before the passage to the Shuijingtuo Fm., SSFs belonging to the assemblage 3 have been recovered, passing to assemblage 4 somewhere through the formation. The

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occurrence of the first trilobites in the upper part of the Shuijingtuo Fm. (Zhu et al., 2003) is in

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accordance with the SSF assemblages, and suggests an ‘Atdabanian’ (Cambrian Stage 3) or younger

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age for much, if not all of the Shuijingtuo Formation, and highlights either condensation or missing strata between the Yanjiahe and Shuijingtuo formations. A reconstruction of Cambrian biozones is reported in Fig. 3a and 10 alongside the geological column. The Ediacaran-Cambrian boundary at

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this section is placed at the Dengying-Yanjiahe contact by Wang et al. (2002), but at the beginning

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2.2 The Longbizui section

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of SSF1 appearance by Chen, (1984).

The Longbizui section is located in Guzhang County, western Hunan. The sampled outcrop

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spans the upper Liuchapo Fm. to the upper Niutitang Fm., covering the late Ediacaran and early Cambrian for a total stratigraphic height of 92 meters, cut at the base and top by a fault system. The

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upper Liuchapo Fm. consists mainly of cherts with variable organic matter content indicated by the grey-scale in the legend (Fig. 3), and cherty limestone in its lower part with sporadically interbedded organic- and clay-rich shales. The base of the section is composed of dolomitic layers and shales, possibly corresponding either to the underlying Doushantuo Fm. or to a stratigraphic equivalent to the generally dolomitic Dengying Fm. The transgressive passage to the Niutitang Fm. is marked by phosphoritic and silicified phosphoritic layers before a long succession of organic-rich shales. Several pyritic horizons have been recognized in this section of likely biogenic (sedimentary) origin (Goldberg et al., 2007), as well as volcanic ash beds (Guo et al., 2013). This rock series formed on the slopes of the South China basin, along a NE-SW striking transitional belt defining the passage to more basinal settings.

ACCEPTED MANUSCRIPT The fossil content in this section is limited, and composed merely of sponge spicules probably belonging to Cambrian stages 2-3, confirming the existence of condensed sedimentation

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above the Liuchapo-Niutitang boundary (Xiao et al., 2005).

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2.3 The Maoshi-Zhongnan section

The Maoshi-Zhongnan section here described is a composite section of two different outcrops in Guizhou Province (Zunyi County) called respectively Maoshi and Zhongnan. At

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Maoshi, the late Ediacaran is recorded through the Dengying Fm., where the lithology changes from

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grey-brownish sandy dolostones in its lower part to silty dolostones in its upper part, for a total length of about 14 meters. In the same area, the late Ediacaran tubular fossil Cloudina was found in

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the Dengying Fm. (Bengtson and Zhao, 1992).

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The transgressive contact with the Niutitang Fm. was observed at the Zhongnan outcrop 12 meters from the base of the stratigraphic column (Fig. 2c-d). As also reported by Och et al., (2013),

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this contact is structured in the following way: the first 40 centimeters show a phosphatic and siliceous rock with a relatively low TOC (mostly <1%), followed by ~20cm of a Fe-V-Mo enriched

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sulfide layer (respectively with maximum values of 16%, 6732ppm and 218ppm) also containing significant amounts of Zn and Ni. Black shales at the base of the Niutitang Fm. have been dated at 53710 Ma by a Re-Os isochron (Jiang et al., 2003). In the remaining 10 meters, strata switch to typical black shale lithology, being thinly bedded and dark grey.

2.4 The Huanglian section The outcrop labeled Huanglian was sampled at a road cutting in Guizhou Province, on the Songtao anticline nearby the city of the same name. A description of the late Ediacaran-early Cambrian succession in this area was published by Guo et al. (2007), Yang et al. (2004) and Zhao et al. (2006). This outcrop is located in a tectonized area and covers the Liuchapo Fm. for the

ACCEPTED MANUSCRIPT Ediacaran and the basal Jiumenchong Fm. for the early Cambrian. All formations were deposited on the Yangtze basin slope, more proximal to the shelf rim compared with the Longbizui section. The

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section was partially inaccessible during the sampling campaign especially in its lower half, limiting

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the number of samples to only 11.

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Deposition in the Ediacaran terminates with 12 meters of pale grey thin- and mediumbedded shale of the Liuchapo Fm. The Jiumenchong Fm. is highly silicified (SiO2 ~80%), with high carbonaceous content and supposedly marks the onset of Cambrian rocks. We only sampled the first

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2 meters of the Jiumenchong Fm. due to vegetation cover. Zhao et al. (2006) found bivalved

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arthropods (Sunella) and tubular fossils (Sphenothallus) 15 meters above the LiuchapoJiumenchong boundary as well as trilobite-bearing limestone approximately 50 meters higher.

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2.5 The Lijiatuo section

The section Lijiatuo is located in NW Hunan, in the south-eastern part of the Yangtze

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Platform. According to sedimentary textures and structures, strata were formed below the storm wave base in relatively deep waters, probably the deepest of our sections (Jiang et al., 2007). The

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Liuchapo Fm. displays cherty beds with carbonaceous silty cherts interbedded at its top for a total length of 70 meters. Phosphatic-siliceous shales also occur occasionally in the formation. Fossil sponges are reported to appear at ca. 44 meters from the base of the Liuchapo Fm., with richer layers in the above Xiaoyanxi Fm. (Guo et al., 2007). The onset of Cambrian black shales is marked by nodular phosphate and sulfide rock, continuing with cherty black shales as already observed in previous sections. The silicate content varies across the formation with rare chert.

3. Methods

ACCEPTED MANUSCRIPT Analyses of rock samples have been performed in the geochemical laboratories of the Museum für Naturkunde, Berlin, Germany, using a Thermo Finnigan Elemental Analyzer Mass

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Spectrometer (continuous flow). Rock samples have been washed, cut into small chips and ground

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after removing altered and recrystallized fragments. An amount of rock powder between 10mg and

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200mg has been used for nitrogen isotope analyses depending on the expected nitrogen concentration. The error associated with 15N and 13Corg is 0.50‰ as discussed in Supplementary Information. The large number of rock samples with no reported 15N value arose

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from the difficulties associated with analyzing nitrogen concentrations and isotopes in pure

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carbonates or due to incomplete combustion of the organic matter. This latter case was evident from a peak in the nitrogen curve (sometimes pronounced) in blanks following normal measurements.

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For this reason, many samples needed to be analyzed repeatedly, in order to ensure a meaningful

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nitrogen isotope signal. In some few samples, repeat analyses were performed in order to test analytical reproducibility, although here we publish only the highest-fidelity values based on the

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signal background and peak intensity. 15N values have been calibrated during each session using the minor internal standard drift experienced by the EA. Samples with a total nitrogen content

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below 0.012mg (<0.006%) have also been considered for calculating the C/N ratio, while the 15N values from the same samples have not been taken into account here because of low accuracy, as testified by our fidelity analyses reported in Supplementary Information. Vanadium oxide treatments, although claimed to overcome low 15N accuracy for samples poor in nitrogen, were not used as they failed to provide satisfactory results. The TN has been calculated during the isotopic measurements according to the nitrogen’s curve area. Specifically, the sum of the areas depicted by 28N and 29N has been transformed into TN after comparison with the nitrogen standard’s area (known nitrogen amount and concentration). The area composed by the Gaussian-shape curve is in fact directly related to the total amount of nitrogen contained in the sample analyzed. The nitrogen amount originally calculated in mg has been then transformed into percentages when needed. 13Corg has been measured on a variable amount of

ACCEPTED MANUSCRIPT rock sample (2-40mg) after removal of carbonate in the rock. In fact, fine-grained sample previously prepared for nitrogen measurements has been decalcified using 2N HCl at room

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temperature.

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Total Organic Carbon (TOC) analyses were performed at the Wolfson Laboratories of

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University College London on a selected range of samples. Depending on the anticipated organic carbon content, an amount ranging between 150 and 350 mg was weighed and then attacked in ceramic crucibles by 3N HCl at room temperature until total elimination of carbonate minerals.

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Afterwards, the residuals were burned in a LECO CS 200 by microwave at 1000°C in pure oxygen

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catalyzed by iron-platinum-silica grains, oxidizing the residue completely to carbon dioxide. The gas produced after combustion undergoes purification in order to eliminate HCl and water before it is sent to the detector, where the TOC is measured. The reference standard used consists of 1g

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standard steel rings. To obtain additional TOC data on samples of particular interest that were excluded from the first round of measurements, we used C/N values determined during analyses of

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decalcified samples and TN measured in the bulk during 15N analyses. Assuming the same C/N ratio in the bulk as well as in decalcified samples, we can easily recalculate the TOC originally

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contained in the bulk. However, considering minor nitrogen and carbon losses during rinsing, these values should be considered indicative and potentially affected by low accuracy (red values in Table 1). All the results and values here discussed are reported in Table 1.

4. Isotopic results

4.1 The Yangtze Gorges area 15N profile The nitrogen isotope profile from the composite section Wuhe-Jijiawan is shown in Fig. 3a. The variability of data through the section is wide, spanning from -2‰ to +6‰ at the top of the

ACCEPTED MANUSCRIPT Yanjiahe Fm. The Dengying Fm. shows discontinuous and extreme values that have been omitted from the results because of low precision and accuracy due to the very low nitrogen concentration

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as these are predominantly pure carbonates (%TN <0.006%, TN <0.012mg for samples of 200mg

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which represents the maximum amount measurable with our device (see Supplementary

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Information). The Cambrian Yanjiahe and Shuijingtuo formations exhibit significant fluctuations: two negative shifts, one at the SSFA2-SSFA3 passage and one in the middle Shuijingtuo Fm., separated by a large positive spike. The nitrogen concentrations along the section vary as well,

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yielding high values in the Shuijingtuo Fm. (0.12%-0.24%), low in the Yanjiahe Fm. (<0.10%) and

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very low at other stratigraphic levels.

13Corg profile

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The 13Corg profile shows a more linear data pattern, with a range between -38‰ and -24‰. From the base of the Dengying Fm. the signal registers values between -33‰ and -26‰, with a

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negative shift towards the Yanjiahe Fm. The top of this formation shows a negative spike (-38‰), returning afterwards to values around -33‰ with only one sample measuring -30‰ at the beginning

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of SSFA3. Data resemble values from Ishikawa et al. (2013), who explain the lack of significant similarities between the two carbon isotope parameters by invoking oxidation of a large organic carbon pool still present in the early Cambrian. The organic carbon concentrations along the column are variable: the Shuijingtuo Fm. presents the highest concentration (up to 14%), while the upper Yanjiahe has a TOC up to 3%. The other samples record values lower than 1%, with the lowest values registered in the Shibantan Mb. of the Dengying Fm.

13Ccarb profile The carbonate carbon isotopic profile (Ling H.-F., unpublished data) for the same section is reported in Fig. 3a. In the Dengying Fm. we observe a wide and smooth positive shift running all

ACCEPTED MANUSCRIPT through the formation reaching a maximum value of +6‰, and then a new negative excursion close to -4‰ in the lower Yanjiahe Fm. After a return of values to +5‰ in the upper part of the

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formation, new negative signatures have been registered at the boundary with the Shuijingtuo Fm.

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This isotopic profile resembles results documented by Jiang et al., (2006) and McFadden et al.

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(2008), and from sections located in other areas collected in Zhou et al. (2007). Regarding the Cambrian part of the section, results shown in Ishikawa et al. (2013, 2008) look similar to the

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results presented here, although collected from drill-core.

4.2 The Longbizui section

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Nitrogen and organic carbon isotope results together with TN and TOC are shown in Fig.

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3b. 13Corg and TOC data from this section have already been presented by Guo et al. (2013), and

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we observe similar features. The nitrogen isotope proxy exhibits a general increase through the Liuchapo Formation from +1‰ to +4‰, and very stable negative values (-2‰) in the Niutitang Fm. Following a positive spike in the upper Liuchapo Fm. (+6‰), values fall smoothly to the negative

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Cambrian signatures. The TN is low in the Liuchapo Fm. (<0.1%), but increases from the base of the Niutitang Fm. to 0.2%, although the highest values are reached within the first several meters of black shale that marks the beginning of the Niutitang Fm. (up to 0.4%). The discontinuous organic carbon isotope trend shows a narrow range of values: the Liuchapo Fm. is characterized by a large positive shift, interrupted by negative values in shales at the base of the section. Another clear positive excursion is registered across the Liuchapo-Niutitang boundary, and another one in the next 20 meters, although less pronounced. The measured TOC is generally low at the bottom of the section (<3%) and high in the Cambrian (up to 7%), with the most pronounced spike between the two phosphorite layers just above the formation contact. Our profiles show similar trends to those presented by Guo et al. (2013).

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4.3 The Lijiatuo section

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Isotopic results from the Lijiatuo section are reported in Fig. 3c. 15N records significant

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fluctuations in a range between +7‰ and -2‰: always positive in the Liuchapo Fm., while mostly negative in Cambrian black shales. A pronounced negative excursion is observed in the upper

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Liuchapo Fm. coeval to cherty black shale deposition and sponge FAD. Nitrogen concentrations across the section may be separated into two major trends: the Ediacaran strata, where

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concentrations are always below 0.1% (apart from black shales at the top of the Liuchapo) and Cambrian layers, where they reach a peak of 0.4% at the beginning of the Xiaoyanxi Fm.

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decreasing upward to values around 0.1%.

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Organic carbon isotopes display a relatively smooth trend from the base of the section to the middle Liuchapo, falling from -27‰ to -35‰. Values increase again upward reaching -33‰ in the

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first few meters of Cambrian black shales. Further up, the same proxy records two separate positive excursions covering 20 meters of black shale, returning to values between -33‰ and -34‰ at the

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top. TOC varies greatly across this section; the highest values are registered in the Xiaoyanxi Fm. with peaks up to 20% and in the Liuchapo Fm. in interbedded silty cherts.

4.4 The Maoshi-Zhongnan section Nitrogen isotope values from the Maoshi-Zhongnan section (Fig. 3d) register relatively high values in the first half of the Dengying Fm., ranging between +7 and +9‰. Towards the end of the formation, values start decreasing reaching +4‰ at the very top. From the first few centimeters of the basal Niutitang Fm., isotope values register an important drop, reaching -1.5‰ just above the early Cambrian sulfidic layer. In the following 4 meters, values return to between 1‰ and 2‰ with

ACCEPTED MANUSCRIPT two spikes down to 0‰. Total nitrogen concentration results remain constant at ~0.1% through the Dengying Fm., also exhibiting low TOC. In Cambrian strata, despite very low concentrations in the

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phosphatic layer values rise until the range 0.1%-0.3%. Organic carbon isotope values in this

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section range between -35‰ in the Niutitang black shales and -25‰ in the upper Dengying Fm. In

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the latter, strata register 13Corg values ~-30‰, increasing in the first part of the formation, while across the Dengying-Niutitang boundary they decrease smoothly registering a short but marked positive excursion in the phosphate-sulfide layer. The TOC contents are low in the Dengying

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dolostones and high in the Niutitang black shales (up to 10%), with the sulfide layer exhibiting a

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4.5 The Huanglian section

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TOC content lower than 0.7%.

Both nitrogen and organic carbon measured at the Huanglian section do not show a wide

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range (Fig. 3e). Nitrogen data in the Ediacaran strata are constant around 2‰, smoothly decreasing until 0‰ in the Jiumenchong black shales. Nitrogen concentration varies according to the TOC

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showing very low values in the Liuchapo Fm., and increasing until ca. 0.2% in the Cambrian black shales. Organic carbon isotope data displays values between -31‰ and -36‰ and resembles results from Guo et al. (2007), although at lower resolution. Organic matter content strongly correlates with mineralogy, with concentrations lower than 3% in Precambrian strata (principally carbonates) and up to 9% in the Cambrian black shales at the top of the stratigraphy.

5. Data analysis

5.1 The Yangtze Gorges area

ACCEPTED MANUSCRIPT 5.1.1 Total Nitrogen/Total Organic Carbon vs. 15N In Fig. 4a-b available TN, TOC and 15N data for the Wuhe-Jijiawan section have been

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plotted excluding some extreme values and the few data available for the Dengying Formation. The

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lack of covariation between isotopic values and organic carbon/total nitrogen contents implies that

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the results are meaningful and that 15N values are not altered by secondary processes such as early/late diagenesis or weathering. The Shuijingtuo Fm. records low 15N variability between -2‰

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and +1‰ against a wide range of TOC and TN contents. By contrast, the Yanjiahe Fm. contains low amounts of TOC and TN (TN<0.03% and %Corg<0.1%), but the associated isotopic values

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show high variability. Similar features are showed by the 15N-TOC plot (Fig. 4b).

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5.1.2 Total Nitrogen vs. Total Organic Carbon

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A plot of TN vs. TOC (Fig. 4c) for all the formations analyzed can be used to identify the source of the nitrogen in the samples and its relation to organic carbon. The co-variation of these

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two parameters is similar for almost all the formations, which indicates similarities in the type and origin of the organic matter. The Yanjiahe Fm. shows an intercept at the origin that can be

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explained by a lack of clay-bound nitrogen, in contrast to the intercept on the y-axis shown by the Shuijingtuo Fm., which is known to contain illite (Bristow et al., 2009). This mineral, together with montmorillonite, is the one most able to fix or adsorb ammonia molecules into its structure (Krooss et al., 2006). The regression coefficient of trend lines quantifies the variability in dilution between the organic fraction and the silicates (in this member composed of illite and smectite; Bristow et al., 2009) due to carbonate or other debris (Calvert, 2004). Our data show moderate regression values for the Yanjiahe Fm. (~0.6) and low values for all the others (~0.15), testifying to high variability in fraction dilution across the section. This is also related to the high mineralogical variability within individual formations.

5.1.3 Total Organic Carbon vs. Carbon/Nitrogen

ACCEPTED MANUSCRIPT In Fig. 4d the TOC is plotted against the C/N ratio. It is evident that the Shuijingtuo Fm. shows a different pattern with a much lower slope when compared to the Yanjiahe Fm., which

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records values between 30 and 100. We relate this difference to nitrogen retention within the rock

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through fixation and/or adsorption after diagenesis. The Shuijingtuo Fm. contains high amounts of

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silicate minerals that are capable of forming bonds with nitrogen, explaining why the C/N ratio remains below 40 while registering values approaching 100 for the Yanjiahe Fm. Accordingly, during thermal history an unconstrained fraction of the total nitrogen would have been transferred to

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the clay fraction where present (Shuijingtuo Fm.) or lost (Yanjiahe Fm.).

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In Fig. 4e we delete the N-clay fraction (represented by the intercept with the y-axis in Fig. 4c) from the C/N calculation so that the fixed nitrogen (constant) does not affect the final results. The outcome shows that the Yanjiahe Fm. and the Shuijingtuo Fm. exhibit different C/N vs. TOC

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trends, which might be related to a minor loss of organic nitrogen in the Shuijingtuo Fm. due to a different thermal history, or more probably to mineral associations that are variably protective

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towards nitrogen. It is noteworthy that, when high C/N ratios occur, a significant fraction of the initial nitrogen might be lost since rock formation. This has been also testified in several studies

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dealing with nitrogen loss during early diagenesis, burial diagenesis and low-grade metamorphism. In all three cases a significant portion of the nitrogen can experience a transfer mostly due to bacterial reworking (early diagenesis), ammonia-rich fluid escape (burial diagenesis) or thermal devolatilization of ammonium phyllosilicates (low-grade metamorphism). While the last case represents the only serious concern in preserving the original nitrogen signatures (Händel et al., 1986; Jia and Kerrich, 2004), early and burial diagenesis do not cause significant or appreciable nitrogen isotope variations except for in extreme cases (Ader et al., 2009, 2006; Altabet et al., 2002, 1999; Godfrey and Falkowski, 2009; Smith et al., 2002; Thomazo et al., 2011). Considering also the likely absence of isotopic fractionation during nitrogen transfer between kerogen and silicates (Freudenthal et al., 2001; Godfrey and Falkowski, 2009; Scholten, 1991; Williams et al., 1995), the nitrogen isotopic value would not have been significantly altered in the Yanjiahe Fm. Moreover, the

ACCEPTED MANUSCRIPT linearity of the isotopic profiles and their reproducibility across different sections may act as a significant test of data fidelity. In conclusion, we believe that the nitrogen isotopic values presented

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here represent the original oceanic values across the Precambrian/Cambrian boundary and that post-

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sedimentary processes able to modify the original nitrogen signature can be ruled out.

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5.2 Longbizui section

Four data analysis diagrams have been plotted in order to assess possible diagenetic effects

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and overall meaningfulness of the data from the Longbizui section. A plot of 15N vs. TN shows no significant correlation (Fig. 4f), and similar results are obtained from the 13Corg vs. TOC plot (Fig.

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4g).

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Diagram (h) exhibits TOC-TN correlations that deserve special attention: the Liuchapo data

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trend line shows a negative slope testifying to elevated nitrogen concentrations at low TOC. This indicates that rock samples containing less organic matter are instead enriched in other minerals (i.e. phyllosilicates) containing nitrogen. At the same time the organic fraction seems to be strongly

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impoverished in nitrogen, probably the result of post-depositional loss during diagenesis. Nevertheless, the very low R2 shown by the Liuchapo Formation data raises doubts about its statistical significance. The Niutitang Fm. dataset, although showing positive correlation, presents an intercept on the TOC axis (~1%), indicating again a possible significant loss of nitrogen from the organic fraction. Both trend lines indicate a much higher C/N ratio with respect to primary organic matter, indicating that the whole section has been subjected to thermal diagenesis. In the last diagram (i), the C/N ratio plotted against TOC confirms previous outcomes: the high slope of the Liuchapo Fm. and the greatly scattered data from the Niutitang Fm. indicates enhanced thermal maturity which may potentially affect the fidelity of these results, especially for the Liuchapo Fm. From a sedimentological point of view, the low C/N ratio (20 to 60) recorded in high TOC samples

ACCEPTED MANUSCRIPT (>5%) might be the result of enhanced OM rain down and conditions on the seafloor that did not encourage significant bacterial reworking (Altabet and Francois, 1994; Altabet et al., 1999), lending

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these samples higher isotopic data confidence.

5.3 Lijiatuo section

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In diagrams (a) and (b) in Fig. 5, concentrations and isotopic ratios for the nitrogen and organic carbon proxies do not exhibit any linear variations. High organic H/C ratios measured by

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Guo et al. (2007) suggest also that these rocks preserved primary isotope values in the samples. Taken together, these two pieces of evidence tend to rule out significant diagenetic overprinting.

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Plotting TN vs. TOC in Fig. 5(c), only the Liuchapo Formation displays a positive correlation due

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to organically-bound nitrogen, although a small amount of clay-bound nitrogen (intercept on the

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vertical y-axis) is also evident. Samples belonging to the Xiaoyanxi Fm. are dispersed over the entire area, with a cluster concentrated in the upper left belonging to the lower part of the formation. These values record very low C/N ratios with a TOC around 1%, and could represent an episode of

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almost complete nitrogen transfer from the organic phase to silicate mineral lattices in response to considerable OM loss. Such a high nitrogen fraction may indicate negligible loss of organic nitrogen despite low carbon contents, further supporting the fidelity of our N-isotopic results. 15N at the base of the Xiaoyanxi Fm. records similar values when compared to the upper part and other sections investigated (e.g. Longbizui section in its Cambrian part), indicating negligible fractionation linked to the passage between host phases. Absence of isotopic discrimination can also be extended to the organic carbon profile given the independence of 13C with TOC in Fig. 5b. The last diagram (d) shows similar rock maturity among formations as well as small variations (Xiaoyanxi Fm.) in dilution between different N-bound lithological components.

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5.4 The Maoshi-Zhongnan section

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In Fig. 5e isotope and elemental concentration data for the composite Maoshi-Zhongnan

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section show no correlation for the Niutitang Fm. The apparent correlation between 13Corg and

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TOC in diagram (f) for the Niutitang samples may reflect contributions from enhanced production of chemoautotrophic biomass in TOC-enriched sediments. These bacteria are responsible for OM

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remineralization, lowering the total sedimentary organic pool (Strauss et al., 1992). In diagram (g) no relation between 15N and TN is observed for any formation. Data for the Niutitang Fm. belong

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to three different lithologies: phosphorites, sulfide layer and black shales (fields in diagram g). It is evident that they follow different patterns: low TN and TOC for the phosphorite because of the lack

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of minerals suitable to host nitrogen, high TN but low TOC for the sulfide layer, and high TN and

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TOC for the organic-rich shales, containing N in organic remains as well as embedded in clays, as

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can also be inferred from the high data dispersion in diagrams (g) and (h) (see also Calvert, 2004). Whether nitrogen is bound in the metallic-rich layer is unknown, but it is likely linked to the

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peculiar mineralogical composition of this horizon (Wille et al, 2008).

5.5 The Huanglian section Given the small number of samples, the Jiumenchong Fm. cannot be meaningfully represented in Fig. 5 limiting the discussion to the Liuchapo Fm. only. In plot (i) no correlation can be discerned between nitrogen concentration and isotopic signal, nor between TOC and 13Corg in plot (l). This lack of relationship testifies to negligible fractionation during diagenetic maturation, although further analyses would be required to confirm such an assumption. High TOC, but large TN ranges exclude nitrogen from being organic matter-bound, as also confirmed by plot (m) where TOC and TN show independent patterns.

ACCEPTED MANUSCRIPT The last plot in Fig. 5(n) displays wide range of C/N ratios for the Liuchapo Fm., with values in a range between 20 and 400. Despite not knowing the precise clay concentrations in our

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samples, high C/N ratios may be attributed to strong N loss during diagenesis, as already reported

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for this formation in the discussion. In conclusion, a significant portion of nitrogen might be clay-

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bound, although isotopic overprinting is not obvious from our data analyses.

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6. Discussion

6.1 Stratigraphic correlation

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6.1.1 Yangtze Platform

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In Fig. 6 we have produced a stratigraphic correlation scheme for the Yangtze Platform

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between the sections presented in this study including Xiaotan section (Cremonese et al., 2013) based mainly on nitrogen and organic carbon chemostratigraphic profiles. The scarcity of fossil remains has resulted in the widespread use of lithostratigraphic marker layers such as the sulfide ore

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layer for correlation. Generally speaking, basinal deposits record more complete sequences without significant hiatus although with low sedimentation rates, but the lack of SSF fossils and precise radiometric age determinations rarely provide biostratigraphic markers across the early Cambrian. Poor reproducibility linked to diagenetic overprinting, local effects and insufficient sampling resolution result in a wide margin of error in this reconstruction. However, greater accuracy in the light of available results can scarcely be achieved at this stage. In Ediacaran strata, low 13Corg and 15N variability do not permit precise identification of stratigraphic markers across sections. Based on a nitrogen isotope decrease in the upper Liuchapo Formation at Lijiatuo section, we placed the Ediacaran-Cambrian transition before the beginning of the Xiaoyanxi Fm., as already postulated by Shields-Zhou and Zhu (2013) and Zhu et al. (2003).

ACCEPTED MANUSCRIPT Unfortunately, lack of SSF biostratigraphy from these layers cannot confirm our hypothesis. In contrast, greater accuracy was probably achieved across the Precambrian-Cambrian boundary at

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Xiaotan and Jijiawan sections thanks to fossil records (SSFs) and clearer isotopic variations.

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The polymetallic sulfide-rich layer (also named ore-layer) deposited in the early Cambrian

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probably records events many millions of years after the Ediacaran-Cambrian boundary. In the present study, this layer is situated on top of the phosphorite horizon at the Zhongnan section (ZN 4-5-6, Table 1). Several attempts have been made to constrain the age of sulfide layer deposition,

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although we refer to the most recent investigations presented by Jiang et al. (2009) and Xu et al.

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(2011). The former reported a U-Pb-SHRIMP zircon age of 532.30.7 Ma from a volcanic ash bed a few meters beneath the ore layer in Guizhou Province, while new Re-Os analyses from three

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different mines in Guizhou and Hunan reported by Xu et al. fit to an isochron age of 5215Ma for

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the sulfide layer itself. If these data are confirmed, the time elapsed from the Pc-C boundary is ca. 20My.

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Och et al. (2013) measured Mo- and Ni-enrichments in a layer in the upper Shiyantou Fm. at Xiaotan (samples XTY 27 and 28), suggesting correlation with the ore-layer observed at Zhongnan

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and elsewhere. Stratigraphy vs. age reconstruction at Xiaotan shown by Cremonese, (2012) aimed at re-establishing a linear relation between time and thickness of geological strata using dated layers and biomarkers, so to erase bias due to sedimentation rates. Following this reconstruction, samples XTY 27 and 28 are placed at 522 Ma (20 My after the Pc-C boundary), explaining why in our reconstruction in Fig. 6 we envisaged high sedimentation rates at Xiaotan with respect to basinal sections during the early Cambrian. At Zhongnan section, two short-lived positive excursions are recorded across the sulfide layer, apparently comparable to carbon isotope variations from Longbizui and Lijiatuo sections despite their limited offsets. This similarity might call for extremely low sedimentation rates rather than a sedimentary hiatus, in which case muted excursions might be attributed to enhanced sedimentary reworking by bacteria. High-resolution chemostratigraphy of this horizon could

ACCEPTED MANUSCRIPT confirm this. The limited thickness of Cambrian layers together with data invariability did not

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permit reliable correlation of Huanglian strata with other sections.

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6.1.2 Carbon and nitrogen isotope correlation at Jijiawan and Xiaotan sections

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As mentioned in section 2.1, the Dengying-Yanjiahe passage at the Wuhe-Jijiawan section is disconformable and the negative carbon isotope excursion of the basal Cambrian might be partially

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absent. Negative shifts in the early Cambrian are here observed in both carbon isotope proxies in the SSFA1 zone (Fig. 3, see also Ishikawa et al., 2008) and not in the lower unfossiliferous portion

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of this section as registered at the Xiaotan section (Daibu Member; Cremonese et al., 2013). Poorer biostratigraphic resolution at Jijiawan section may play an essential role in accounting for this

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anomaly. Assuming isotopic variations to be synchronous in the basinal domain, an alternative

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explanation would be that the SSFA 1 FAD might be placed at lower layers at Xiaotan, being thus taphonomically biased (Li et al., 2013).

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Because of the low resolution related to the SSF assemblages’ FAD and LAD, we base our correlation on the SSFA2-SSFA3 boundary, which is well constrained at Jijiawan and associated

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with significant nitrogen isotope variations (Fig. 7a). The SSF column in this Figure is time-related to the Xiaotan section, while blue and yellow shading correlates the two sections. The SSF3-4 boundary at Jijiawan Fm. has not been precisely identified within the lithostratigraphy, leaving doubts as to the real SSFA thicknesses. Nevertheless, similarities in dimension and nature of carbonate concretions in the two sections at the passage between the biozones SSFA3/SSFA4 increase the consistency of the stratigraphic correlation here proposed. The nitrogen patterns look very similar, especially at this passage (nitrogen signatures between -1‰ and -2‰) and during the SSFA3, depicting the same positive and negative trends. The high denitrification peak registered in Xiaotan at the end of SSFA2 is missing at Jijiawan, possibly because of low sampling resolution, while absolute values showing differences of up to 1-2‰. This offset could be related to a constant diagenetic overprint through these two biozones, lower sampling resolution, water depth changes,

ACCEPTED MANUSCRIPT or more reasonably to isotopic signature differences in the original marine nitrate considering their considerable distance apart (~700 km). 15N data would reasonably not experience significant

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overprint during diagenesis in siliciclastic successions due to a protection-effect operated by silicate

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structures on nitrogen compounds fixed in the clay layers (Scholten, 1991).

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Different carbonate and organic carbon trends between the two sections are instead depicted in Fig. 7b-c, where 13Ccarb does register a positive excursion during the SSFA3, although shorter and less pronounced than at Xiaotan. The isotopic trends described by the four samples in the

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SSFA3 at Jijiawan section cannot be precisely positioned in the stratigraphy because the SSF3-

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SSF4 boundary is not well defined. For this reason, the termination of the positive excursion at Jijiawan might correspond to the one recorded in the Dahai Mb. at Xiaotan, but at much lower

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sedimentation rates. This possibility would also explain the relative shortness of the positive 13Corg

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excursion in the Yangtze Gorges area (Fig. 7c). The organic carbon isotopic signature at Jijiawan shows the same pattern as for carbonate carbon, although missing the significant negative excursion

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at the base of the Shuijingtuo Fm. as also recorded by Ishikawa et al. (2013). Isotopic carbon patterns differences between the two sections in Hubei and Yunnan may be

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the result of diagenetic signal overprint or regional variations due to reduced circulation. However, due to the similarity between the nitrogen profiles and between the 13Ccarb and 13Corg trends, a purely diagenetic interpretation seems unlikely. Based on such evidence, in our opinion all the proxies here reported register original values that characterize the Nanhua Basin in the Early Cambrian. Nevertheless, high TOC measured at Jijiawan across the SSFA2 and 3 might still be responsible for a decrease in carbon isotope values, through depletion of the 13CDIC in response to the continuous chemocline fluctuations registered by nitrogen isotopes (see also Cremonese et al., 2013) and anoxic OM remineralization (Dickson, 1990; Strauss et al., 1992). In another scenario, differences in nitrogen and carbon isotopic trends at Jijiawan and Xiaotan sections might be related to differences in residence times and chemical pool structure, which led to different responses to perturbations in a restricted environment (Bristow et al., 2009; Jiang et al., 2011).

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6.2 A composite model for the late Ediacaran-early Cambrian in South China

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6.2.1 Ediacaran: the Dengying Fm.

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We consider a bathymetric West-East profile across the Yangtze Platform, valid from the late Ediacaran to the early Cambrian and based on results shown by Jiang et al. (2011), Vernhet

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(2007) and Xiao et al. (2012). For geographic reasons, the Wuhe-Jijiawan section should not appear on this block diagram, being about 700km distant from the transect. Nevertheless, its depositional

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environment (proximal side of shelf lagoon) places it between the sections Xiaotan and MaoshiZhongnan on the transect (Fig. 8). The Xiaotan section is located at the extreme West of the profile,

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at the proximal side of the shelf in an area characterized by tidal flats and shallower waters. Its

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sedimentary aspect during the Cambrian is typical of sedimentary highs (Steiner et al., 2007; Wang and Mo, 1985), showing a diversified stratigraphy that is missing in basinal and lagoon domains.

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For the lithostratigraphy and biogeochemical settings of the Xiaotan section we refer to Cremonese et al. (2013). Nitrogen isotope data from the Liuchapo/Dengying Fm. are only reported from the

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Lijiatuo, Maoshi-Zhongnan, Wuhe-Jijiawan and Huanglian sections. N-outputs and inputs in the oceanic pool are operated through denitrification and dinitrogen fixation, respectively, that also control the isotopic composition of nitrate in seawater (for a review, see Robinson, 2001). Isotopic signatures related to these biochemical processes are transferred to the entire marine pool (Montoya et al., 2002), although marine nitrate values can be buffered to different extents according to the distance from the area where N2-fixation or denitrification occur (e.g. in the modern Arabian Sea; Deutsch et al., 2004; Gaye-Haake et al., 2005). Our isotopic results, discussed in the light of the basin morphology along an E-W profile across South China shown in Fig. 8 indicate that the Maoshi-Zhongnan section was influenced by vigorous denitrification during deposition of the Dengying Fm. (15N between +9‰ and +4‰, Fig. 3d) and

ACCEPTED MANUSCRIPT the Huanglian and Longbizui sections were probably affected by enhanced N2-fixation (15N~23‰). By contrast, the Wuhe-Jijiawan section experienced episodes of phototrophic anoxia (as

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explained more in detailed in the following discussion) and “normal marine production”, with

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nitrogen isotopic ratios between +4‰ and -1‰.

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Tiny phosphorites nodules in the Dengying Formation are widespread on the shelf but rare on the slope and in the open basin. Their formation is generally linked to upwelling basinal waters along coastal regions and bacterial sulfate reduction (Arning et al., 2009; Hiatt and Budd, 2001;

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Nelson et al., 2010). The phosphate delivery by any of these processes would then encounter

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favorable chemical conditions for phosphogenesis on the shelf lagoon (Jiang et al., 2011). During the deposition of cherts in the basin and limestone on the shelf lagoon at the end of the Ediacaran,

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15N values do not display significant variations from older strata apart from a non-systematic and

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smooth decrease in its upper part. During this time, the Yangtze platform experienced uplift testified to by several hiatuses and sub-aerial exposure (Zhu et al., 2007). The high silicate

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concentration in the seawater, responsible for deposition of the Liuchapo Fm. in deeper waters, was probably due to multi-episodic hydrothermal silica chimneys in basinal domains as identified by

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Chen et al. (2009).

A dedicated discussion is necessary for the Wuhe-Jijiawan section shown in Fig. 3a. Only few nitrogen isotopic results were obtained from the entire Dengying Fm. at the Wuhe outcrop (in the middle Shibantan Mb.) due to the very low nitrogen concentrations. This member was deposited in a deepening-upward subtidal succession. The negative nitrogen isotopic signatures we observe in the middle Shibantan Mb. (Fig. 3a) could result from a photic zone sulfidic environment (probably transient) responsible for the greater preservation of organic matter at the sediment-water interface (see also Ling et al., 2013). In this scenario the development of Purple and Green Sulfur Bacteria (PSB and GSB, respectively) were probably responsible for the negative shift in the nitrogen isotopic signatures (Higgins et al., 2012; Johnston et al., 2009; Meyers et al., 2009; Ohkouchi et al., 2005; Struck, 2012; Struck et al., 2009). At the end of the Ediacaran Period, the Yangtze Platform

ACCEPTED MANUSCRIPT experienced shallower conditions with subaerial exposure of the shelf lagoon (Jiang et al., 2011) and greater ventilation, being less influenced by deeper upwelling waters as indicated by renewed

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positive carbonate signatures. Nevertheless, continental sulfates might have continued to enrich

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episodically the now shallower Dengying depositional system, promoting sulfidic conditions in

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reducing environments. Lack of significant pyrite or sulfide content in the sediment might find explanation in a rapid oxygenation of the sulfidic pool moving to the more distal area, or with a peculiar seawater stratification establishing sulfidic conditions in the upper part of the water column

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while suboxic in the deeper layer. These settings are plausible if we consider nutrients (in this case

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sulfates) were delivered through riverine streams, and then directly to the seawater upper layer.

6.2.2 The early Cambrian

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Based on field observations, all the sections here investigated record a transgressive event at the beginning of black shale deposition in the early Cambrian. Phosphorite defines the beginning of

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the early Cambrian (named variously depending on the location, see Fig. 6), following a stratigraphic hiatus of several million years that is more pronounced on platform sections (Chen et

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al., 2009; Zhu et al., 2003). The black shale series deposited on top of this horizon is the result of widespread anoxia on the Yangtze platform as widely reported in the literature (Arnold et al., 2004; Banerjee et al., 1997; Goldberg et al., 2007; Kimura and Watanabe, 2001; Schröder and Grotzinger, 2007). The phosphate layer supposedly formed from upwelling of P-rich waters from the basin after platform drowning during the Late Ediacaran (for a review see Föllmi, 1996). Ideal conditions for the formation of a conspicuous phosphorus pool in anoxic oceanic waters could have existed here, with P delivered to the upper shelf lagoon due to reorganization of currents and/or sea-level rise due to platform subsidence. Once in contact with oxygenated waters in the shelf lagoon, phosphorus could precipitate all over the area (Wille et al., 2008), in agreement with phosphorites recorded in the Zhongyicun Mb. at Xiaotan section and in the Yanjiahe Fm. at Jijiawan section. High P

ACCEPTED MANUSCRIPT concentrations in seawater cause a decrease in the N:P ratio from the normal value of 16 during steady-state conditions (Falkowski and Davis, 2004). This geochemical settings favor a peak in N2-

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fixer activity in the upper water column (Tyrrell, 1999), shifting first nitrate 15N, and then the

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entire nitrogen marine pool, towards an atmospheric isotopic signature (~0‰). Low nitrogen

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isotope values measured for the early Cambrian phosphorite episode might reflect this scenario, recording the substantial contribution of dinitrogen fixation among the overall seawater biochemical reactions (Fig. 3).

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Deposition of the sulfide layer has been attributed to a significant upwelling event bringing

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nutrient-rich waters in contact with a sulfidic pool, favoring scavenging of Mo, Ni and other metals from the water column (Lehmann et al., 2007; Wille et al., 2008). This is in agreement with the

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sporadic occurrence of the sulfide layer along a W-E belt covering 1600 km of the Yangtze

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Platform (Zhu et al., 2003), but missing in many basinal sections. Significant sulfate input from continental oxidative weathering is also considered to have been responsible for the demise of the

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hypothetical large DOC pool in the early Cambrian (Fike et al., 2006; Ishikawa et al., 2013; Swanson-Hysell et al., 2010). At this time, oceanic areas that were still anoxic probably coexisted

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with well oxygenated waters, (depending on biological O2 utilization, marine currents and water depth) and the sulfate discharged into the mainly anoxic basin could have been reduced to sulfide (perhaps oxidizing then the DOC) and being recirculated to the shelf platform. In our opinion, sulfate inflow would have replenished the shelf area with sulfide, mixing with continental Mo- and Ni-enriched riverine water flowing to the shelf lagoon and creating ideal conditions for ore layer formation. Our proposed scenario, supported by iron speciation analyses of the ore layer (Fig. 9) and by mostly negative nitrogen isotope signatures is again consistent with a very shallow chemocline in an ocean dominated by GSB and PSB in its uppermost layers, where both nitrogenase co-factors (Fe and Mo) and oxygen concentrations were transiently too low to sustain high levels of dinitrogen fixation and nitrate assimilation (Anbar and Gordon, 2008; Anbar and Knoll, 2002; Glass et al., 2009). In fact, although Mo was delivered to the offshore by enhanced

ACCEPTED MANUSCRIPT continental weathering, its rapid scavenging in the photic zone by sulfides could have been crucial to prevent its biological utilization. This is also consistent with the radiation of eukaryotic green

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algae only later in the Cambrian, perhaps triggered by a rise in ambient Mo levels (Knoll, 2007; 14

N-enriched OM,

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Revill et al., 1994). Although high aqueous Fe concentrations can also lead to

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(Zerkle et al., 2008), this condition is not compatible with widespread sulfidic anoxia where Fe is rapidly scavenged from the seawater. Similar conditions were already proposed by Canfield (1998) to follow the Great Oxidation Event (GOE) at ca. 2.3Ga, and likely persisted until the

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Neoproterozoic (Poulton et al., 2004; Scott et al., 2008). Our envisaged scenario is represented in

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Fig. 10a, and contrasts with Lehmann et al. (2007) who propose instead oxygen-rich conditions in shallow waters fuelling intense biological activity.

After the end of ore layer formation, nitrogen isotopic values still register negative or very

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low signatures in basinal sections. These settings are consistent with prolonged and stable photic zone anoxia lasting until the upper Shiyantou Fm. at Xiaotan section, lower Shuijingtuo at Jijiawan

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section and probably longer at Longbizui and Lijiatuo sections. Alternation of sulfidic and ferruginous conditions in the photic zone are shown by iron speciation data in Fig. 9, promoting

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alternation of PSB/GSB activity and dinitrogen fixation by diazotrophic cyanobacteria according to oxygen levels and co-factor concentrations in the upper ocean. Specifically, the deeper Longbizui section exhibits values close to the euxinic-ferruginous boundary; Jijiawan samples record ferruginous conditions with some values in the area proximal to euxinic conditions; the black shales of the Zhongnan section are located in the ferruginous area (with one sample showing possible oxic conditions) while the sulfide layer sampled at Zhongnan plots in the euxinic area. As there is a significant time-gap or extremely condensed strata at Zhongnan section, we relate the samples here as belonging to strata younger than Cambrian black shales at other locations. Here, we register alternation of ferruginous waters and oxidative conditions, with fluctuations between positive and negative 15N values probably reflecting expansion and contraction of the two biochemical pools: the oxygenated western and the suboxic/anoxic distal lagoon waters. This widespread anoxic setting

ACCEPTED MANUSCRIPT on the Yangtze platform evidenced by nitrogen isotope data might represent the last stage of a steporganized OCP oxygenation (see also discussion on 13Corg later in this chapter). A schematic

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model of the marine environment during this period is reported in Fig. 10b.

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The hypothesis of a shallow chemocline during deposition of the Yanjiahe and Shuijingtuo

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formations finds additional confirmations in REE data interpretations by Ling et al. (2013), where Ce anomaly data, a redox proxy with a particular high reduction potential, are used to constrain the redox evolution of the shallow marine environment. These settings can be then reasonably

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envisaged also for the more distal sections here discussed. Based on a dynamic equilibrium on the

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Yangtze shelf lagoon between continental input eastward and upwelling westward, water column conditions at Xiaotan and Wuhe-Jijiawan sections probably experienced several redox shifts

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(chemocline vertical fluctuations) according to the predominant currents, as supported by nitrogen

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isotopic data discussed previously and in Cremonese et al. (2013). The occurrence of widespread and intense photic anoxia in the Early Cambrian in South

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China here presented testified to by nitrogen isotope analyses depict a peculiar geochemical situation, where the ocean oxygen concentrations remained low and stable for most of the Early

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Cambrian, with episodes of phototrophic anoxia. A large portion of the Yangtze Platform experienced blooming of GSB, PSB and diazotrophic cyanobacteria in surface waters, maintaining a constant and high input of biogenic reduced nitrogen in the aquatic pool characterized by an incomplete nitrogen cycle (lack of nitrate). Depending on the local chemistry, the main oxidants present in the water column were sulfate and oxidized forms of iron so to prevent oxidative conditions in surface waters and remineralize the organic remains on the sea bottom, as testified to by low nitrogen isotope values and high TOC. The large input of nitrogen in the aquatic food chain as well as the lack of intense denitrification were probably responsible for high nitrogen concentrations (PON) in seawater, sustaining high organic matter fixation rates acting as a positive feedback on this system (oxidants complete consumption). The occurrence of repeated phosphatic layers in the Early Cambrian also testify to high P concentrations supporting thriving biological

ACCEPTED MANUSCRIPT activity. Additional studies in this direction could better constrain the redox conditions on the Yangtze Platform in the Early Cambrian as well as the biochemical cycles active in a

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ferruginous/anoxic environment to discuss further cause-effect relations with radiation of life

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observed at this time of the Earth’s history.

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Organic carbon isotopes registered in all the sections here investigated show instead different patterns, with values increasing across the Liuchapo-Jiumenchong passage at Huanglian, Lijiatuo and Longbizui sections while decreasing at Zhongnan and Jijiawan (Fig. 3). They seem not

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C sequestration (Guo et al., 2013). Chemoautotrophic activity, well

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increase in OM burial and

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to respond to the DIC isotopic variation, but instead tend to higher values in response to a general

developed in anoxic waters, might also play a decisive role in a partial independence of OM

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isotopic signature from the shallow DIC (Goldberg et al., 2007). Differently, increasing 13Corg at

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Zhongnan might reflect the isotopic signatures of later times as discussed above. At Xiaotan and Jijiawan sections the two carbon proxies show instead covariation during the Cambrian Period

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(more pronounced at Xiaotan). Referring to the model in Fig. 10a where the shelf is dominated by oxidative conditions more developed than in the basin, greater influence of the large DOC pool

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would be registered moving south-eastward on the profile. This might result from higher interdependence between DIC and DOC at Xiaotan and Jijiawan (similar to the modern ocean) than at more distal sections, where 13Corg would still be buffered by a large OCP.

ACCEPTED MANUSCRIPT 7. Conclusions

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Nitrogen and organic carbon isotope records have revealed new insights into past marine

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ecosystems, redox conditions and biogeochemistry in both platform and basinal realms contributing

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to a deeper understanding of the Ediacaran-Cambrian transition in South China. Specifically, nitrogen seems to have retained a primary isotopic signature and thus records past water column

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biogeochemical conditions. It is plausible that the last stage of the hypothesized late Precambrian Organic Carbon Pool was characterized by sulfidic/ferruginous seawater during deposition of lower

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Cambrian strata characterized by intense nitrogen fixation and better articulated biological cycles. The exhaustion of this large OCP and cessation of photic zone anoxia on the Yangtze Platform, if

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confirmed, would likely be placed within the Shuijingtuo Fm. (see also Ishikawa et al., 2013)

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Despite lithological variability across this transition which may have unduly influenced the fidelity of nitrogen isotope data due to, for example, variable detrital input and low-nitrogen

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lithologies, this relatively untested proxy has revealed unexpected insights as a chemostratigraphic tool. Nitrogen isotopic values have shown patterns different from any other geochemical proxy that

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is then specifically linked to nitrogen biogeochemical processes occurring in seawater. Frequent signal variations imply that nitrogen isotopes represent a meaningful biogeochemical tool for facies and environmental basin microanalyses. In the light of the high sensitivity of nitrogen isotopes and according to records from modern sediments, this proxy might also play a significant role in highresolution lateral correlation within the same sedimentary basin.

ACCEPTED MANUSCRIPT 8. ACKNOWLEDGEMENTS

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The present study has been funded by the German Research Foundation (DFG) and the

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National Science Foundation of China (NSFC) as part of a Sino-German Research Project FOR

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736.

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Fig. 1. Map of South China with the sampled sections and geological map of the Huangling anticline in the

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Three Gorges area with positions of the outcrops. JJ= Jijiawan section; W= Wuhe section.

Fig. 2. Geological features and formations in the Yangtze Gorges area. a) Bedded cherts at the top of Liuchapo Fm., Longbizui section; b) Cambrian black shale in the Niutitang Fm. (Cambrian) along the

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sampled outcrop, Longbizui section; c) Dengying-Niutitang contact with the sulfide-layer visible in the upper part, Maoshi-Zhongnan section; d) basal Cambrian at sampling site, Maoshi-Zhongnan section; e) Panoramic

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view of the Dengying Fm. in the Three Gorges Area; f) Carbonatic concretions in the Shuijingtuo Fm., Jijiawan section; g) phosphatic cherts in the Yanjiahe Fm., Jijiawan section; h) Yanjiahe-Shuijingtuo contact,

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Jijiawan section. Abbreviations: HJ=Hamajing Mb.; SBN=Shibantan Mb.; BMT=Baimatuo Mb.;

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YJ=Yanjiahe Fm.; SJT=Shuijingtuo Fm.

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Fig. 3. 15N, %N (TN), 13Corg, %Corg (TOC) and 13Ccarb for the sections investigated. (a) WuheJijiawan section. The SSF assemblages are depicted after Chen (1984). Transgression-regression cycles (red lines) are reported as described in McFadden et al. (2008) and Ling et al. (2013). Age of 551Ma is reported

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after Condon et al., 2005 and age of 632Ma after McFadden et al., 2008; (b) Longbizui section. The asterisks beside the stratigraphy represent high sponge occurrence; (c) Lijiatuo section; (d) Maoshi-Zhongnan section; (e) Huanglian section. JMC= Jiumenchong Fm., HMJ= Hamajing Mb. Data values are reported in Table I.

Fig. 4. Diagnostic data plot for the Wuhe-Jijiawan (diagram a-b-c-d-e) and Longbizui sections (f-g-h-i) using data of 15N, TN, 13Corg and TOC.

Fig. 5. Diagnostic data plot for the Lijiatuo (diagram a-b-c-d), Maoshi-Zhongnan (e-f-g-h) and Huanglian sections (i-l-m-n) using data of 15N, TN, 13Corg and TOC.

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stratigraphies based on lithological features and isotopic profiles.

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Fig. 7. Comparison of 15N (a), 13Ccarb (b) and 13Corg (c) between Xiaotan and Jijiawan sections. The SSF

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column relates to the Xiaotan section, while blue and yellow shading correlates the two sections.

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Fig. 8. West-East cross-section in the Yangtze Platform and location of sections investigated (hypothetical reconstruction). The Wuhe-Jijiawan from the Yangtze Gorges area has been placed in the proximal side of

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the shelf lagoon. After Jiang et al., 2011.

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Fig. 9. Iron speciation diagram for Cambrian samples at the sections at Jijiawan, Zhongnan and Longbizui

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from Och, (2011). FeHR (high reactive iron)=Fecarb+Feox+Femag+Fepy; Fecarb=carbonate iron; Femag= iron from magnetite; FeT=total iron; Fepy=Fe from pyrite. According to Poulton and Canfield (2005), marine sediments

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have demonstrated that FeHR/FeT ratios do not exceed 0.38 during normal deposition under oxic water column conditions. By contrast, values above this threshold commonly represent deposition under anoxic conditions. Ferruginous conditions can be distinguished from euxinic conditions by considering the extent of

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pyritization of the highly reactive iron pool (Fepy/FeHR) in sediments showing clear evidence of anoxic deposition. Observations in Black Sea sediments suggest that Fepy/FeHR =0.8 characterizes the upper limit for ferruginous deposition (Anderson and Raiswell, 2004).

Fig. 10. Paleobasinal reconstruction along the transect depicted in Fig. 8 during (a) the sulfide layer formation in the Early Cambrian; (b) black shale deposition in the Early Cambrian. Blue water= oxygenated; green water= sulfidic; orange water= ferruginous.

Tab I.

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Highlights 1) N and Corg analyses indicate an articulated and periodically complete nitrogen cycle. 2) Nitrogen isotopes are potent geochemical tools to monitor the oxidative conditions of the seawater. 3) We are able to build a paleoseawater circulation system across the Pc-C boundary. 4) Episodically occurrence of green/purple sulfur Bacteria has been envisaged. 5) Nitrogen isotopes preserved in rock sediments retained their original values.