Palaeoceanographic controls on spatial redox distribution over the Yangtze Platform during the Ediacaran–Cambrian transition

The Ediacaran–Cambrian interval was an eventful transitional period, when dynamic interactions between the biosphere and its physical environment allowed the Earth System to cross into a new state, characterized by the presence of metazoans, more equable climates and more expansive oxygenation of the oceans. Due to the retreat of widespread sulphidic conditions, redox‐sensitive trace‐metals could accumulate to a greater extent in ‘black shales’ deposited in localized anoxic/euxinic environments, such as highly productive ocean margins. This study investigates the concentrations of the redox‐sensitive trace‐metals molybdenum and vanadium in organic‐rich sedimentary rocks from seven sections of the Yangtze Platform, slope and basin. Iron speciation analyses were carried out in order to distinguish oxic, anoxic‐ferruginous and anoxic‐sulphidic settings, while sulphur and nitrogen isotope ratios were measured to gain insight into sulphate and nitrate availability, respectively, in the context of changing redox conditions. The data herein demonstrate an overall increase in redox‐sensitive trace‐metal contents in black shales across the Ediacaran–Cambrian transition, but with marked temporal and spatial variability. Euxinia is evident in South China before 551 Ma in the Ediacaran, and again in the early Cambrian. However, some time‐equivalent sections are not enriched in redox‐sensitive trace‐metals, and also exhibit contrasting S‐isotope and N‐isotope systematics. A more complex configuration of the Yangtze Platform, for example with vast intra‐shelf basins, together with changing (generally rising) eustatic sea‐level may account for this variability. In this regard, it is proposed that a mid‐depth sulphidic wedge, caused by nutrient upwelling over the south‐east platform margins, migrated over time (but generally landward), leading to spatially variable redox conditions determined by sea‐level, currents and bathymetric constraints. The changing extents of anoxia and euxinia appear to have limited the distribution of emerging Ediacaran and Cambrian ecosystems.


INTRODUCTION
Over its 4Á6 billion year history, the Earth System evolved in several broad steps and experienced profound modifications, until a surface environment suitable for the diverse biosphere of today emerged after ca 4 billion years. Rarely, amid these seemingly slow and gradual changes, a threshold was crossed when major events followed in such quick succession that cause and effect relationships are difficult to distinguish. Such a 'revolution' happened during the Archean-Proterozoic transition when tectonic reconfigurations (Campbell & Allen, 2008;Reddy & Evans, 2009), major glaciations (Kirschvink et al., 2000;Kasting & Ono, 2006), the emergence of eukaryotes (Brocks et al., 1999;Knoll et al., 2006) and a significant rise in atmospheric oxygen levels, termed the Great Oxidation Event (GOE) by Holland (2002), changed the face of the Earth. This first Oxygenation Event (e.g. Cloud, 1972;Holland, 1984Holland, , 2006Des Marais et al., 1992;Bekker et al., 2004;Canfield, 2005) was originally believed to have led to at least moderate ventilation of the deep ocean, ending large-scale precipitation of banded iron formations ca 1Á8 Ga (Holland, 1984(Holland, , 2002Isley & Abbott, 1999). Alternatively, Canfield (1998) proposed that the deep ocean developed widespread euxinia (anoxic-sulphidic water column conditions) after the GOE, because rising oxygen levels would have enhanced sulphate delivery to the ocean which then fuelled bacterial sulphate reduction (Canfield & Raiswell, 1999;Habicht et al., 2002;Strauss, 2004). Widespread euxinia would, in turn, have resulted in titration of dissolved iron from the water column.
Such an anoxic and at least intermittently euxinic ocean after the GOE has been substantiated by several studies (Fig 1; Shen et al., 2002Shen et al., , 2003Arnold et al., 2004;Poulton et al., 2004;Brocks et al., 2005;Sarkar et al., 2010;Poulton & Canfield, 2011) and euxinia was probably an intermittent feature even across the Neoproterozoic-Cambrian boundary (Canfield et al., 2008;Gill et al., 2011). However, a growing body of evidence suggests that ocean redox gradients were more complex, with anoxic-sulphidic waters restricted to mid-water depths along productive continental margins, overlying anoxicferruginous deeper waters (Li et al., 2010;Poulton et al., 2010;Poulton & Canfield, 2011). A second oxidation event, termed the Neoproterozoic Oxygenation Event or NOE Och & Shields-Zhou, 2012), may have occurred during the Neoproterozoic-Cambrian interval when, with a few notable exceptions (see Meyer & Kump, 2008, for a review), the deep ocean became pervasively oxygenated. The hypothesis of a NOE largely rests on several lines of geochemical evidence. Carbon isotope studies indicate that the fractional burial of reduced organic carbon was unusually high after ca 800 Ma (Des Marais et al., 1992;Mills et al., 2014). Although fractional burial estimates do not allow estimation of redox fluxes, higher proportional burial would most probably have resulted in atmospheric oxygenation during periods of enhanced chemical weathering, for example during the Ediacaran Period following the Cryogenian glaciations (e.g. Derry et al., 1992). Apart from low stratigraphic resolution chromium isotope studies (Frei et al., 2009), most other relevant geochemical data relate to changes in ocean redox and, in particular, a 'ventilation' of the deeper ocean realms during the Ediacaran Period (Canfield & Teske, 1996;Johnston et al., 2005;Le Guerrou e et al., 2006;Canfield et al., 2007Canfield et al., , 2008Scott et al., 2008;Sahoo et al., 2012). For example, Sahoo et al. (2012) used high levels of Mo enrichment in marine sediments deposited under anoxic conditions to demonstrate ocean (and possibly atmospheric) oxygenation in the wake of the end-Cryogenian 'Marinoan' glaciations at ca 632 Ma (Fig. 1).  have shown that Mo/TOC (total organic carbon) ratios probably represent a better proxy for the actual Mo budget in marine basins, and applying this ratio still shows an increase of at least one order of magnitude during the Ediacaran-Cambrian transitional interval ( Fig. 1; Scott et al., 2008;Sahoo et al., 2012). A similar pattern is observed for other redox-sensitive trace-metals that are influenced minimally by detrital sources, such as vanadium and uranium ( Fig. 1; Tribovillard et al., 2006;Och & Shields-Zhou, 2012;Sahoo et al., 2012;Partin et al., 2013).
Due to predictable differences between the behaviour of different palaeoredox proxies  Canfield, 2005) with concomitant changes in deep ocean redox conditions whereby anoxic-ferruginous (red) and euxinic (yellow) conditions prevail during the Precambrian before a dominantly oxygenated (blue) ocean characterizes the Phanerozoic (major uncertainty is shaded lightly and marked by a question mark; modified after Lyons & Gill, 2010, and extended after Shen et al., 2002Shen et al., , 2003Arnold et al., 2004;Poulton et al., 2004;Brocks et al., 2005;Canfield et al., 2008;Meyer & Kump, 2008;Reinhard et al., 2009;Scott et al., 2011;Partin et al., 2013). Compilations of the redox-sensitive trace-metals Mo, V and U contents in anoxic, organic-rich sediments is shown below (filled circles) with the data collected from samples on the Yangtze Platform depicted in red (mostly from the present study but also from Wallis, 2006;Guo et al., 2007;Scott et al., 2008;Sahoo et al., 2012). The Mo/TOC and V/TOC ratios, illustrated by the yellow shaded envelope, further indicate a major increase in the ocean trace-metal budget during and after both oxygenation events but, in particular, during the Neoproterozoic-Cambrian interval. A few ratios exceed the upper limit shown here and are likely to represent artefacts due to very low TOC (see Table S1). (B) Same dataset as in (A) but only for the Ediacaran and Cambrian period. The thick black bars represent the range of absolute Mo and V concentrations, while the thin grey bars show the range of Mo/TOC and V/TOC ratios. The red arrowheads indicate where Mo/TOC and V/TOC ratios exceed the depicted scale.
under particular redox conditions, the use of a multiproxy approach is crucial to obtain a more complete picture of the NOE. This is especially true considering that past ocean and atmospheric composition is difficult to quantify directly, while trace-metal budgets may have been significantly different from those of the modern ocean ( Fig. 1; Scott et al., 2008). In the present study a multiproxy approach has been applied involving trace element (Mo and V) concentrations, Fe speciation and stable isotopes (d 14 N and d 34 S), to investigate redox variability, and controls on such variability, across the Yangtze Platform, slope and basin during the Ediacaran-Cambrian transition. The well-preserved organic-rich sedimentary successions on the Yangtze Platform offer an excellent opportunity to study changes in biogeochemical cycling during the Ediacaran-Cambrian transition. Some of the earliest and best preserved Ediacaran and Cambrian fossil assemblages are found on the Yangtze Platform, whereby phosphatized, possible animal embryos from the Ediacaran Doushantuo Formation (Xiao & Knoll, 2000), and the Early Cambrian Chengjiang Fauna, one of the earliest Burgess-shale type fossil occurrences (e.g. Babcock et al., 2001;Hagadorn, 2002), are among the most intensely studied. Redox geochemistry during the putative NOE has mainly been inferred from single sedimentary successions deposited on the Yangtze Platform (Canfield et al., 2008;Scott et al., 2008;Sahoo et al., 2012), while a relatively simple palaeobathymetry has been assumed in multiproxy palaeoredox studies (e.g. Li et al., 2010; see also Cui et al., 2015, for a critical re-evaluation). The present study aims to consolidate previously documented changes in redox geochemistry, with estimates of redox reservoir sizes based on S isotopes, and nutrient/redox dynamics inferred from N isotopes and Fe speciation. Furthermore, this multi-proxy approach is used to shed light on the complex inter-relationships between biogeochemical cycling and the broad morphological and bathymetric changes that occurred on the Yangtze Platform during the Ediacaran-Cambrian transition.
Although Mo and V are not dominantly delivered to the sediment in association with organic matter, concentrations often track TOC in anoxic non-sulphidic environments. However, under euxinic conditions, concentrations tend to be decoupled from TOC, resulting in higher Mo/TOC and V/TOC ratios (Tribovillard et al., 2006, and references therein). Molybdenum and, in a less efficient way, V also represent key bio-nutrients which are mainly involved in nitrogenase, an enzyme used in nitrogen-fixing bacteria (Stiefel, 1997;Anbar & Knoll, 2002;Eady, 2003;Zerkle et al., 2006;Canfield et al., 2010b). It has been hypothesized that such trace-metals might have been the limiting nutrients in the Precambrian ocean, as opposed to phosphorus during the Phanerozoic (Anbar & Knoll, 2002;Planavsky et al., 2010;Zhang et al., 2014; see also Godfrey et al., 2013). Today, however, the budgets of Mo and V are not controlled by marine plankton, i.e. by their roles as micronutrients (e.g. Ho et al., 2003;Tribovillard et al., 2006), and Mo does not exhibit a nutrient-like distribution (concentrations remain relatively constant through the water column), while V exhibits only minor depletion in the photic zone (Collier & Edmond, 1984;Collier, 1985;Piper, 1994).
A complication with the use of Mo-based redox proxies is that the importance of euxinia on a global scale is often assessed from the level of Mo enrichment (or Mo isotope value) in euxinic black shales on a local scale (e.g. Scott et al., 2008). Local Mo enrichment in black shales is generally taken to indicate a lack of Mo sequestration on a global scale, while low Mo concentrations are taken to indicate that the global ocean Mo reservoir has been reduced due to widespread Mo sequestration. However, the absolute level of Mo-enrichment may also reflect the degree of euxinia, or other factors such as basin restriction or sedimentation rate. Therefore, for Mo to be an effective predictor of global euxinia, local redox conditions must be established using independent means, such as Fe speciation, while the nature of the basin and sedimentation characteristics need to be considered.

Iron speciation
Iron speciation analyses of marine sediments are increasingly being used to infer the redox conditions that prevailed in the water column above a depositional environment; for example in modern , 1998, Phanerozoic (Raiswell et al., 2001;Poulton & Raiswell, 2002), Proterozoic (Shen et al., 2002(Shen et al., , 2003Poulton et al., 2004Poulton et al., , 2010Canfield et al., 2007Canfield et al., , 2008Sahoo et al., 2012) and Archean (Reinhard et al., 2009;Kendall et al., 2010) settings. The Fe speciation scheme identifies several operationally derived iron pools (Poulton & Canfield, 2005), whereby iron species are characterized as either being unreactive (mostly silicate Fe) or highly reactive towards dissolved sulphide (Fe HR ). Highly reactive Fe includes four pools, namely pyrite Fe (Fe py ), magnetite Fe (Fe mag ), reducible-oxide Fe (Fe ox ) and carbonate-associated Fe (Fe carb ). The Fe Py /Fe HR ratio must be applied in combination with the Fe HR /Fe T ratio in order to determine whether the deposition of a given marine sediment occurred under oxic, anoxic-ferruginous or anoxic-sulphidic conditions. In modern marine sediments deposited under an oxic water column, the Fe HR /Fe T ratio does not exceed 0Á38 (Raiswell & Canfield, 1998;Canfield et al., 2008;Poulton & Canfield, 2011). A higher proportion of highly reactive iron would indicate an anoxic water column and, for these samples, the Fe Py /Fe HR ratio separates ferruginous (<0Á8) from sulphidic (>0Á8) bottom waters. Ratios between 0Á7 and 0Á8, however, are equivocal and do not necessarily exclude sulphidic conditions (M€ arz et al., 2008;Poulton & Canfield, 2011).

Stable isotopes
The removal of sulphur from the ocean is controlled by pyrite formation (which is often accompanied by a large isotopic fractionation), and the precipitation of evaporites (accompanied by negligible fractionations (e.g. Canfield, 2001). Today, the most important catalyst for sulphur isotope fractionation is the sulphur metabolism of microbes, especially during, but not restricted to, the process of sulphate reduction (Jones & Starkey, 1957;Harrison & Thode, 1958;Kaplan & Rittenberg, 1964) which has been active since the Archean (Shen et al., 2001;Shen & Buick, 2004;Archer & Vance, 2006). The following major controls on isotope fractionation during sulphate reduction can be formulated (Canfield, 2001): (i) when organic electron donors are used, lower specific rates of sulphate reduction lead to higher fractionations; (ii) lower fractionations (3 to 16&; Kaplan & Rittenberg, 1964;Kemp & Thode, 1968) are achieved when H 2 is used as electron donor, particularly at low specific rates of sulphate reduction; (iii) small fractionations (<4&) occur under sulphate-limiting conditions (ca 200 lM, e.g. Habicht et al., 2002); and (iv) high fractionations reaching up to 70& occur when sulphate is more abundant (>1 mM; Canfield & Teske, 1996;Canfield et al., 2010a). Controls (iii) and (iv) are commonly used to estimate marine sulphate concentrations.
The stable isotope signature of nitrogen, d 15 N, can potentially give additional insights into the prevailing redox conditions in the water column, as well as the biochemical reactions involving nitrogen (Quan & Falkowski, 2009;Sigman et al., 2009;Boyle et al., 2013;Quan et al., 2013;Ader et al., 2014). Among all water column nitrogen reactions, incomplete denitrification gives the highest positive isotope fractionation, leaving the residual nitrate pool enriched in 15 N, which is subsequently utilized by autotrophs to produce 15 N-enriched organic matter (Sigman et al., 2009). Denitrification occurs at the suboxic/oxic boundary in the water column, and its intensity depends on the size of the nitrate pool and oxidative conditions. On the other hand, N 2 fixation by Mo-nitrogenase is carried out by diazotrophic cyanobacteria transferring atmospheric nitrogen into the oceanic pool, whereby the d 15 N of newly fixed N is ca À1& (e.g. Zhang et al., 2014). Furthermore, as shown in a recent study by Zhang et al. (2014), the d 15 N value resulting from nitrogen fixation by alternative (i.e. the V-only and Fe-only) nitrogenases can lead to significantly lower d 15 N values (below À2&). Generally speaking, vigorous denitrification will increase the nitrogen pool isotopic signature, while intense nitrogen fixation (for example, in the case of sea water nitrogen depletion) will stabilize values at ca 0&. For these reasons, under anoxic conditions nitrification (and thus denitrification) is inhibited, while under euxinic conditions, including in the photic zone, N 2 fixation becomes more highly favoured. In the latter scenario, nitrogen isotopic values can be negative (Higgins et al., 2012;Johnston et al., 2009;Meyers et al., 2012;Struck, 2012;Godfrey et al., 2013). In addition, higher d 15 N values in well-preserved sediments can indicate the transition between anoxic and oxic conditions (in both directions), while lower values suggest either an oxic or anoxic environment (Quan et al., 2008). Because denitrification during early diagenesis does not lead to significant isotopic fractionation in comparison to water column denitrification (Brandes & Devol, 2002), the d 15 N signature can be a reliable redox proxy when used in combination with independent measures, such as the redox-sensitive tracemetals Mo and V described above. The present study discusses published d 15 N profiles within this wider redox context (Cremonese et al., , 2014. In addition, it has been hypothesized that euxinic conditions, evidenced by high Mo/TOC and high Fe Py /Fe HR ratios, will affect the nitrogen isotope composition of organic matter in a systematic way, lowering d 15 N to atmospheric values (Boyle et al., 2013). However, this idea can only be tested properly in integrated geochemical studies (e.g. Godfrey et al., 2013).

The broader tectonic context
The Neoproterozoic sedimentary successions of the Yangtze Platform were greatly influenced by the tectonic history of the South China craton, one of three major tectonic cratons in China (Fig. 2). The South China craton consists of the Yangtze and Cathaysia blocks, which amalgamated during the Sibao-Jinning orogeny at ca 900 Ma (Li et al., 1995(Li et al., , 2002(Li et al., , 2003b(Li et al., , 2005. During the break-up of Rodinia, whereby a plume-centre was possibly located under South China, inducing widespread granite intrusions (such as the 819 AE 7 Ma Huangling Granite in the Three Gorges Area) around the Yangtze block (Li et al., 1999(Li et al., , 2003a, major rift basins formed along the south-eastern and western margins of the South China craton (Li et al., 2003b;Wang & Li, 2003). The subsequent thermal subsidence created the necessary accommodation space for the Cryogenian-Silurian sediments that unconformably overlie Mesoproterozoic metamorphic rocks or early Neoproterozoic rift-related bimodal magmatic rocks, reflecting the different rifting phases (Wang & Li, 2003). The Yangtze Platform can be described schematically as comprising a large platformal area, transitioning into a slope and basin (Fig. 2). However, more precise palaeobathymetric reconstructions have shown that a generally shallow-water platform was characterized by numerous intra-shelf basins and carbonate barriers on its margins (Zhu et al., 2003Vernhet, 2007;Vernhet & Reijmer, 2010;Jiang et al., 2011).
The Neoproterozoic sedimentary successions of the Yangtze Platform, which despite the complex tectonic history of China remained relatively undeformed, can be subdivided into three main intervals: pre-glacial predominantly volcano-siliciclastic rocks (for example, the ca 750 Ma Liantuo Formation on the platform and the approximately contemporaneous more fine-grained Banxi Group in the basin), two Cryogenian glacial diamictite intervals (the Gucheng/Tiesiao/Chang'an formations and Nantuo Formation, respectively) separated by an interglacial unit (the Datangpo/Xiangmeng formations) and post-glacial Ediacaran marine carbonates and shales (the Doushantuo Formation and the Dengying/Liuchapo formations). The Early Cambrian successions are found over both the platform and basin areas, character-ized by black shales with abundant carbonate concretions overlying phosphatic strata of the lowermost Cambrian; they can be very condensed on the platform margin and very thick on the platform , in particular in the south-west (Yunnan & Guizhou provinces).

The Doushantuo Formation
The Doushantuo Formation is among the most extensively studied Neoproterozoic formations worldwide, notably because it accommodates the richest fossil record of this crucial time period, including acritarchs, multicellular algae and controversial fossil embryos Zhang et al., 1998;Xiao & Knoll, 2000;Condon et al., 2005;Jiang et al., 2006;Ling et al., 2007;Yin et al., 2007;McFadden, 2008;McFadden et al., 2008;Ohno et al., 2008;Lu et al., 2013;Zhu et al., 2013). Overlying the glacial diamictites of the Nantuo Formation, the basal Ediacaran cap carbonate of the Doushantuo Formation represents the aftermath of the end-Cryogenian 'Snowball Earth' glaciation (Hoffman et al., 1998). Based on U-Pb age constraints (Condon et al., 2005) and an extremely negative d 13 C excursion of arguable duration between ca 565 Ma and 550 Ma, the upper Doushantuo Formation has been correlated with the Johnnie Formation of Death Valley (USA), the Wonoka Formation of the Adelaide rift complex (Australia), the Shuram Formation in Oman, the post-Marinoan Windermere Supergroup (Canada), the Nama and Tsumeb groups of Namibia and south-east Siberia (Le Guerrou e et al., 2006;Lu et al., 2013).
Although the Doushantuo Formation was deposited over the time span from 635 to 551 Ma (i.e. encompassing ca 90% of the Ediacaran Period; Condon et al., 2005), it nowhere exceeds a thickness of ca 320 m. Whether this reflects a condensed succession or major undetected breaks in sedimentation is presently unclear, although the former possibility is favoured . Vernhet (2007) identified three different depositional environments in a study of several sections in Hunan, Guizhou and Hubei provinces, spanning the shallow, subtidal shelf environment, through intertidal or shoal settings, to deep-water basinal environments, illustrating the wide bathymetric range under which sedimentation of the Doushantuo Formation took place on the Yangtze Platform. Two and a half stratigraphic sequences (i.e. transgressive-regressive cycles), have been identified McFadden et al., 2008); they are, however, clearest in the shallower water facies and not easily discernible in the Yangtze Gorges Area .
In the Yangtze Gorges Area, the type locality of the Ediacaran System (known as the Sinian in China), the Doushantuo Formation can be subdivided into four lithological members (Wang et al., 1998). Member I consists of a succession of cap carbonates (dolostone) which extend throughout the central and southern Yangtze Platform; they are characterized by tepee-like structures, sheet cracks, macropeloids, barite crystal fans and negative d 13 C values (Jiang et al., 2003a(Jiang et al., , 2006Zhou et al., 2004). A precise U-Pb age of 635Á2 AE 0Á6 Ma has been determined from an ash layer within the cap carbonate at one locality (Condon et al., 2005). In the Three Gorges Area, the cap carbonates have a thickness of ca 5 m and are thus relatively thin compared to some basal Ediacaran cap carbonates around the world (Hoffman et al., 2007). The overlying second member is between 80 m and 140 m thick and is composed of interbedded black shale, organic-rich calcareous mudstone and thinly bedded dolomicrite. An ash layer dating from 632Á5 AE 0Á5 Ma is situated a few metres above the cap carbonate (Condon et al., 2005). Abundant centimetre-sized chert nodules, decreasing up-section, occur in the lower-middle part of Member II and contain numerous acanthomorphic acritarchs and multicellular algae Yin et al.,  The yellow dots on both maps indicate the sections analysed for the present study whereas the Baiguoyuan section (white dot) has been considered from earlier analysis (Wallis, 2006). Localities investigated during recent studies by Li et al. (2010) and Sahoo et al. (2012) have also been included for comparison. Shp = Shipai Formation; Cr = Cryogenian Period.

Zhongling
2007; . The sparse sedimentary structures include parallel laminations, crinkle laminations and rare intraformational breccias, indicating that wave and current activity were mostly absent during sedimentation of member II . This is likely to have resulted from the Yangtze Gorges Area being: (i) a protected basin; and/or (ii) overlain by a water depth exceeding ca 100 m (i.e. below 'stormwave base'; Zhu et al., 2013). Member III is between 30 m and 60 m thick and comprises medium to thick-bedded dolostone with thin chert horizons and irregular chert nodules that grade up section into thinbedded limestone and dolomitic interbeds (i.e. ribbonites). Although most chert layers are diagenetic in origin, some contain well-preserved microfossils Xiao, 2004;Yin et al., 2007). Sedimentary structures include scour marks, crinkle laminations and low angle cross-bedding, while sandy layers capping limestone and dolomite bedding surfaces are common, indicating that deposition occurred at shallower water depths than the underlying Member II, possibly in shallow subtidal environments . Member IV, commonly referred to as the Miaohe Member, comprises a succession of black shales and organic-rich mudstones. Sedimentary structures are absent apart from the fine lamination and abundant, sometimes huge (diameter >1 m), carbonate concretions of diagenetic origin (e.g. Lu et al., 2013;Zhu et al., 2013). Pyrite and barite are also common features of this member. An ash bed at the top of the Miaohe Member yields a U-Pb age of 551Á1 AE 0Á7 Ma, and a 3 to 4 cm thick bentonite bed that in some sections separates the Doushantuo Formation from the Dengying Formation has yielded an age of 550Á4 AE 0Á8 Ma (Condon et al., 2005).

Studied sections
Black shale successions of the Doushantuo Formation were studied, predominantly from Members II and IV, at several locations on the Yangtze Platform: Jiulongwan (Jiulongwan and Xiangdangping) and Jiuqunao (Jijiawan) in the Yangtze Gorges Area, Hubei Province, and at Maoshi (north-west of Zunyi, Guizhou Province). At the Jiulongwan section, a substantial part of Member II was sampled from directly above the cap carbonate throughout a tectonically disrupted succession of massive grey dolomite beds with interbedded black shales. Further outcrops sampled at Jiulongwan comprise the >10 m thick Miaohe Member, consisting of laminated black shales, barite and abundant huge carbonate nodules which lie between Member III and the wavy, shaly horizon with a pyrite-rich layer that constitutes the contact to the overlying Dengying Formation Below the Miaohe Member, the top of Member III consists of dark grey dolomite which transitions downwards into paler carbonate ribbonites, followed by dolomite beds with some intercalated chert beds. The boundary between the Doushantuo Members II and III is obscured by poor outcrops and dense vegetation in this region.
At the Jiuqunao section, the lower black shales of the Doushantuo Member II and the black shales of the Miaohe (Doushantuo Member IV) have been investigated. Parts of Member III, particularly towards the top, consist of massive dolostone, which is considered to represent allochthonous blocks, due to either slumping and/or faulting .
The upper Doushantuo Formation, including the transition to the overlying Dengying Formation was also sampled at the Maoshi section, Guizhou Province, where it was presumably deposited in a lagoonal setting close to the margin of the Yangtze Platform (e.g. Jiang et al., 2011). Here, the upper Doushantou Formation is characterized by carbonate-rich black shales that can tentatively be correlated with the Miaohe Member (Doushantuo Member IV). The black shale unit at the top of the Doushantuo Formation occurs in almost all palaeoenvironments of the Yangtze Platform and its sharp onset represents a flooding surface  rather than a major sequence boundary . Similar to the cap carbonates, it can be used as a regional stratigraphic marker. However, due to lithostratigraphic variability within the Doushantuo Formation throughout South China, it is unclear to what extent the subdivision into four members can be applied away from the southern limb of the Huangling granite in the Yangtze Gorges Area .

The Dengying/Liuchapo formations
In contrast to the underlying Doushantuo Formation, a relatively short time interval, from ca 551 to 541 Ma, is represented by the Dengying Formation (Zhao et al., 1988). In places, this formation is much thicker (240 to 850 m), and can be subdivided into three members. In the Three Gorges Area these are: (i) the Hamajing Member at the base (20 to 190 m thick), consisting of light-grey, medium to thick-bedded dolomite with intercalated thin chert layers; (ii) the overlying Shibantan Member (100 to 160 m), characterized by dark grey, thin-bedded limestone; and (iii) the Baimatuo Member (60 to 570 m), which is composed of light-grey, thickly bedded/massive dolomite. The tripartite subdivision of the Dengying Formation can be recognized in many places in South China, although a different terminology is sometimes used (Ding et al., 1992;Zhu et al., 2003).
A few samples near the top of the Dengying Formation, just below the Early Cambrian successions, notably the Jijiapo (Hubei Province) section, were collected and analysed but, due to their trace-metal content close to detection limit, they will not be discussed in the present study (see Och, 2011, for details). At the Maoshi section, the lower part of the Dengying Formation comprises a succession of alternating black carbonates, which get paler up-section, and sandy carbonates. At the Jijiapo section, Three Gorges Area, the base of the Hamajing Member consists of wavy beds of massive dolomite. The base of the Shibantan Member is characterized by thinly bedded limestone followed by a succession of dark, macrocrystalline limestone beds, a few intercalated chert layers and chert nodules. The top of the Shibantan Member is rich in chert nodules and possibly grades into the Baimatuo Member with low amplitude wavy bedding of the carbonates.
The depositional environment of the Dengying Formation is interpreted in terms of a prograding platform. Oolitic textures and oncolites in the dolomitic Hamajing Member are characteristic of a shallow, high-energy environment following black shale deposition at the top of the Doushantuo Formation, indicating a sea-level drop from the Doushantuo to the Dengying Formation . Towards the south-east, the carbonate successions become gradually thinner, ultimately changing into the slope and basinal facies of the corresponding Liuchapo Formation, which is mainly composed of black silicified shales that in places reach into the early Cambrian (e.g. Wang et al., 1998;Chang et al., 2009).

Studied sections
At the Huanglian section, Guizhou Province, stratigraphic control is rather poor, but an equivalent level there to the Liuchapo Formation was sampled, consisting of black silicified shale below a black shale succession. The nearby Longbizui section, Hunan Province, on the other hand, offers an excellent transitional succession from cherty shales and bedded cherts of the predominantly Late Ediacaran Liuchapo Formation, to the black shales of the Early Cambrian Niutitang Formation.

Early Cambrian formations
The global standard stratotype-section and point (GSSP) for the base of the Cambrian System is the first appearance datum (FAD) of the trace fossil Trichophycus (Treptichnus) pedum (Brasier et al., 1994;Landing, 1994;Babcock & Peng, 2007). However, due to a lack of convincing evidence for the occurrence of this trace fossil in Cambrian sediments in South China and absolute age constraints (Compston et al., 1992(Compston et al., , 2008Yang et al., 1996;Jenkins et al., 2002;Jiang et al., 2009;Och et al., 2013), correlation of the Precambrian-Cambrian boundary focuses instead on the biostratigraphy of small shelly fossils (Steiner et al., 2007), as well as C-isotope stratigraphy (for example, at the Meishucun section sampled for this study; Cowie, 1985;Luo et al., 1992;Shields et al., 1999;Zhu et al., 2003). Because of the strongly varying abundance of small shelly fossils on the Yangtze Platform, phosphorite horizons occurring in the lowermost Cambrian have also been taken into account in the present study, where the Precambrian-Cambrian boundary was not obvious stratigraphically.

The Niutitang Formation
The Niutitang Formation, which is locally strongly condensed, comprises mainly black shales, siltstones, intercalated chert, organic-rich carbonates and phosphate nodules. It predominantly crops out within the transitional belt at the platform margin and, often uncomformably, overlies the Dengying/Liuchapo formations. Phosphorite beds at the base of the Niutitang Formation and organic-rich black shales above can be used as marker horizons across the slope and basin of the Yangtze Platform; these have predominantly been studied because of an extreme enrichment of Ni and other metal sulphides, in particular Mo with concentrations up to several per cent (e.g. Och et al., 2013, and references therein).

Studied sections
The base of the Niutitang Formation has been sampled at the basinal Huanglian and Longbizui sections, and mainly consists of an uninterrupted black shale succession. In the more extensively sampled Longbizui section, two distinct layers of pyrite and carbonate concretions have been found above the Liuchapo-Niutitang boundary.

The Yanjiahe and Shuijingtuo formations
The Yanjiahe Formation, which covers the Precambrian-Cambrian transition south of the Huangling anticline in the Three Gorges Area, overlies pale, massive dolomites of the Dengying Formation (Baimatuo Member) above a sharp, wavy contact. The Yanjiahe Formation is ca 35 m thick and consists of dolomitic muddy limestone, calcareous black shale and some sandstone and chert (Ishikawa et al., 2008). The overlying Shuijingtuo Formation is ca 100 m thick and mainly consists of black shale with prominent carbonate concretions (Ishikawa et al., 2008). Mainly based on carbon isotope stratigraphy, it is likely that the upper part of the Yanjiahe Formation is equivalent to the Dahai Member of the Early Cambrian Zhujiaqing Formation Cremonese et al., 2014), while the presence of sponge spicules and trilobite fragments suggests that the overlying Shuijingtuo Formation could be considerably younger, possibly even Cambrian Stage 3.

Studied sections
The sampling of the Yanjiahe and Shuijingtuo formations was conducted at the Jijiapo and Jiuqunao section where the base of the Yanjiahe Formation is characterized by grey carbonate beds with thin chert intercalations. Abundant phosphatic cherts were recognized in the middle part of the formation, whereas massive dolomite beds, followed by organic-rich black dolomite interbedded with siltstone, including regularly shaped silicified carbonate nodules, occur in the upper part below the boundary to the Shuijingtuo Formation The boundary zone is characterized by a thin (ca 10 cm) phosphorite bed, followed by a conglomeratic layer with rip-up clasts and framboidal pyrite. The basal Shuijingtuo Formation consists of black shales with abundant, metre-sized dolomite concretions and some intercalated grey massive dolomite beds.

MATERIALS AND METHODS
One hundred forty-three well-preserved predominantly black shale samples and some more or less organic-rich carbonate samples were collected at seven localities during two field seasons (2008 and 2009) on the Yangtze Platform, from the Doushantuo Formation Member II to the early Cambrian Shuijingtuo Formation. Sampling steps vary considerably between highly resolved segments of decimetre-scale to broad steps of over 10 m. All of the present samples were converted into powders using a tungsten carbide mill.
Geochemical analyses included X-ray fluorescence carried out on a Philips PW2400 spectrometer (PANalytical, Almelo, The Netherlands) for major and trace elements on 54 samples collected during the first field season (Maoshi, Huanglian and Longbizui). The 89 samples collected during the second field season in the Three Gorges Area (Jiulongwan, Jiuqunao, Yangjiahe and Jijiapo) were dissolved after ashing at 550°C before analysis for bulk elemental composition using ICP-MS and Fe speciation. In detail, 5 ml of nitric acid was added to the ashed powder, before adding 2 ml of hydrofluoric acid and a few drops of HClO 4 . The sample was then dissolved on a hot plate. After evaporation, 2Á5 ml of boric acid (50 g l À1 ) acid was added and evaporated to dryness on a hot plate to dissolve any aluminium hexafluorates. The samples were then taken up in 5 ml of hot 50% HCl. Iron speciation analysis was carried out on all samples according to the method outlined by Poulton & Canfield (2005). The present authors measured the concentrations of four different highly reactive iron phases (Fe HR ): pyrite iron (Fe Py ), carbonate-associated iron (Fe carb ), ferric oxides (Fe ox ) and magnetite iron (Fe mag ), and total iron content (Fe T ); Fe Py was determined via the chromous chloride technique of Canfield et al. (1986), while AVS (FeS) was only detected in trace amounts. The sequential extraction procedure of Poulton & Canfield (2005) was used to determine the other highly reactive iron phases, specifically Fe carb , Fe ox and Fe mag . To determine Fe T , ca 100 mg sample powder was dissolved using the above-mentioned method for total dissolution and then diluted 50 times. All solutions were analysed for their respective iron contents using an atomic absorption spectrometer (AAS), with a relative standard deviation (RSD) of <5% for all stages. The same sample solutions were then evaporated to dryness and redissolved in nitric acid in order to measure the trace-metal contents. The final solution, containing 0Á15 ml HNO 3 , 0Á1 ml 500 ppb Re for instrument calibration and 3Á75 ml H 2 O, was analysed at the State Key Laboratory for Mineral Deposits Research, Nanjing University, using a Finnigan Element II inductively coupled plasma mass spectrometer (ICP-MS; Thermo Fisher Scientific Inc, Waltham, MA, USA). The precisions were generally better than 5% for the analysed elements based on long-term uncertainty of the laboratory measurement on a standard carbonate sample.
Samples which yielded enough Ag 2 S residue (>0Á03 g) after the sulphide extraction procedure were analysed for sulphide isotopes using a Finnigan MAT DeltaPlus plumbed to a Carlo Erba elemental analyser through a Conflo II interface (CE Elantech Inc, Lakewood, NJ, USA). All analytical work for sulphur isotopes was carried out at the Institute for Geology and  The evaluation of TOC, total carbon (TC) and total sulphur (TS) was carried out using a Leco C/S analyser (Leco Corporation, St Joseph, MI, USA) at the Wolfson Laboratory, University College London. The TC and TS were measured directly after weighing between 100 mg and 300 mg of sample powder. The TOC content was determined after dissolving each sample with 10% HCl at room temperature until no effervescence was observed, followed by filtering and rinsing with H 2 O prior to introducing the dried remains into the C/S analyser.

The Doushantuo Formation
The extensive Jiulongwan sections (JLW) in the Three Gorges Area, Hubei Province, span from Doushantuo Member II (DST II) to Doushantuo Member IV (DST IV). For these sections, Mo and V concentrations mostly remain well below average shale values, until the onset of DST IV (Figs 3 and 4). In this late Ediacaran black shale succession, Mo concentrations reach ca 370 ppm, with concomitant V concentrations of over 2400 ppm found just at the onset of black shale deposition in DST IV. Molybdenum correlates only moderately with V, with a more significant correlation within DST IV (R 2 = 0Á53) than DST II (R 2 = 0Á25). Total organic carbon shows no significant correlation with Mo, but is weakly correlated with V (R 2 = 0Á34) within the lower DST II, where TOC contents are generally below 1% but with a few peaks of up to 2Á4%. Within DST IV, no significant correlation is found between trace-metals and TOC contents, where TOC rises smoothly after the DST III/DST IV boundary, stabilizes between 4 wt% and 6 wt %, and reaches a maximum concentration of 14Á9% towards the top. Total sulphur and pyrite sulphur correlate well throughout the section, but in particular within DST II, where less nonpyrite sulphur is found within DST IV. The Mo/ TOC ratios within DST II are highly variable, ranging up to 4 9 10 À4 . The Mo/TOC ratio reaches a maximum of 180 9 10 À4 at the bottom of DST IV and rapidly declines to oscillate between 0 and slightly above 40 9 10 À4 . Iron speciation analysis suggests deposition under anoxic conditions, with DST II sediments indicating a ferruginous water column (Fe HR /Fe T > 0Á38; Fe py /Fe HR < 0Á7 to 0Á8) and the overlying DST IV indicating at least intermittently euxinic conditions Fe HR /Fe T > 0Á38; Fe py /Fe HR > 0Á7 to 0Á8). The pyrite d 34 S profile is very variable but initially positive with values between 0& and 23& within DST II (Fig. 3). Further up in the Doushantuo Formation (Fig. 4), a decline occurs from ca À7& within the carbonates of DST III, to À19& within the upper part of the Miaohe black shale (DST IV), which is followed by a steep increase back to À7& below the Doushantuo/ Dengying boundary. Within DST II, d 15 N values are also positive and vary between 1Á9% and 6Á7& without any particular trend (Fig. 3). In the upper part of DST III the d 15 N profile exhibits an increase from 1Á3 to 4Á3& towards the Miaohe Member, at which value they remain approximately except for a single, minor excursion down to 3Á2& at the top (Fig. 4).
Throughout the Doushantuo Formation at the Jiuqunao section (JJ), Hubei Province, very low redox-sensitive trace-metal concentrations close to average shale were measured (Fig. 5). Anoxic conditions can, however, be inferred from iron speciation analysis throughout the section but, although relatively high Fe Py /Fe HR ratios of between 0Á6 and 0Á7 are common within the black shale of DST IV, there is no convincing evidence for euxinic water column conditions. The d 34 S values for pyrite show a sharp increase from À10 to +10& within DST II and remain at this approximate level. In the Miaohe Member, d 34 S values are ca +15& and exhibit a decrease down to +10& 1 m below the Doushantuo/ Dengying boundary. The d 15 N profile basically shows a decrease from values between 5& and 6& within DST II, to values that decrease from 2Á5 to 0Á6& in the Miaohe Member.
The Maoshi section (MS), in Guizhou Province spans the boundary between the late Ediacaran Doushantuo and Dengying formations, and exhibits average shale Mo and V concentrations of ca 3 ppm and 100 ppm, respectively (McLennan, 2001; Fig. 6). The TOC contents are high in the upper Doushantuo Formation (1 to 3Á4%) and fall below 1% in the overlying Dengying Formation, with only a weak correspondence between peaks in TOC and TS. The correlation between the trace-metals and TOC and TS is weak but the highest Mo and V con-

The Late Ediacaran (Dengying/Liuchapo formations) and Early Cambrian formations
The Huanglian section, Guizhou Province, which covers the Precambrian-Cambrian transition from the Liuchapo (equivalent) Formation to the Niutitang Formation, exhibits significant variability in trace-metal concentrations (Fig. 7). Maxima of 96 ppm for Mo and 9860 ppm for V at the base of the Niutitang Formation coincide with very high TOC contents of up to 9Á6%, leading to a moderate correlation between Mo, V and TOC. Sulphur contents are relatively low and a moderate correlation between TS and pyrite S prevails, with substantial non-pyrite S at the base of the Niutitang Formation Nevertheless, Mo concentrations correlate relatively well with TS (R 2 = 0Á67), indicating that Mo is efficiently scavenged into sulphidic sediment even when pyrite formation is minor. A relatively high Mo/TOC ratio of 19 9 10 À4 is found within sediments of the upper Doushantuo Formation, while low values prevail within the Liuchapo Formation, increasing again at the base of the Niutitang Formation, where a maximum Mo/ TOC ratio of 10 9 10 À4 is found. An anoxic-ferruginous depositional environment is clearly indicated by Fe speciation analysis, with Fe Py / Fe HR ratios that vary considerably but remain low at between 0Á01 and 0Á56. Due to a lack of pyrite, only two samples could be analysed for d 34 S, yielding À4Á3& and À6Á3&. The d 15 N values are low, but positive, with values no higher than 2Á4& in the Liuchapo Formation, decreasing to ca 0& in the Lower Cambrian Niutitang Formation.
Within the predominantly cherty shales and chert beds of the Liuchapo Formation at the Longbizui section (LBZ), a thin black shale interval is found where TOC and TS concentrations both reach over 3Á1% within a thin horizon, while Mo and U remain depleted and V is only slightly enriched, with concentrations of up to 340 ppm (Fig. 8). The Fe Py /Fe HR ratios exceed 0Á7 within an interval of 15 cm just below and within this organic-rich horizon, below which d 34 S pyrite values range widely between +66& and a minimum of À0Á38&.  (Fig. 9). Good correlation is observed between TS and Mo contents (R 2 = 0Á92), whereas V concentrations are significantly better correlated with TOC (R 2 = 0Á61). While TS correlates with pyrite sulphur, a great excess of nonpyrite sulphur was observed. Iron speciation analysis supports anoxic-ferruginous conditions without evidence for sustained euxinia. The d 34 S values within samples from the organicpoor Yangjiahe Formation are all positive, with an increase from values of ca 5& to over 23& beneath the overlying Shuijingtuo Formation, followed by a subsequent decrease down to À5&. The d 15 N profile shows constant values of slightly below 1& within the Yanjiahe Formation except at the top, where values of between À2Á3& and +5Á6& occur within the lowermost Shuijingtuo Formation.
At the Jiuqunao section (JJL), redox-sensitive trace-metal concentrations are significantly enriched in the lower Cambrian black shales of the Shuijingtuo and Yanjiahe formations (Fig. 10). At the onset of the black shale succession of the Shuijingtuo Formation, Mo peaks at 213 ppm with a concomitant peak in V concentration of 910 ppm, before concentrations decrease to relatively constant values of ca 40 ppm and 150 ppm, respectively, with an overall good correlation between Mo and V (R 2 = 0Á95). The Mo/TOC ratios track the Mo concentration profile, attaining a maximum of 36 9 10 À4 . Total organic carbon, TS and pyrite sulphur also correlate well within the Early Cambrian formations, where a significant amount of non-pyrite sulphur is present. In the Shuijingtuo Formation, frequent euxinic conditions are plausible with Fe Py /Fe HR ratios between 0Á7 and 0Á8. Interestingly, the maximum peaks in Mo and V contents occur at the stratigraphic level where Fe Py /Fe HR ratios are particularly low. In the Shuijingtuo Formation, the one carbonate sample (JJL1) analysed for its d 34 S signature within the lower Shuijingtuo Formation has a value of 18Á6&. Similarly, only a few d 15 N values were measured, but they show a clear decrease from ca 0Á8& within the carbonate samples at the bottom, to ca À0Á5& in the TOC-rich black shale further up, before a slight increase (but remaining negative) for the rest of the section. Figures 11 and 12 summarize the most important geochemical indicators considered in the present study. Figure 11 focuses on the Ediacaran and Cambrian strata found in the Three Gorges Area and also includes Mo/TOC and V/TOC ratios of the Baiguoyuan section north of the Huangliang granite (reported by Wallis, 2006). Figure 12 shows the data for the Precambrian/Cambrian transition at Huanglian and Longbizui which were deposited in a basinal setting. The Xiaotan section, at the boundary between the Yunnan and Sichuan provinces (Fig. 2), contains an unusually expanded Early Cambrian succession to the north-west of the Yangtze Platform. The trace-metal concentrations and iron speciation analyses have previously been reported by Och et al. (2013) and the nitrogen isotope signature by Cremonese et al. (2013). These results are summarized here and include a previously unpublished d 34 S profile (Fig. 13). The most significant geochemical feature is the relatively low Fe HR /Fe T ratio within the Yuanshan Formation, down to 0Á2 at the top, which indicates that the sediments were potentially deposited beneath an oxic water column (see Supporting Information; Poulton & Canfield, 2011).

DISCUSSION
In the last few years, research on biogeochemical cycling on the Yangtze Platform during the Late Ediacaran has intensified. Li et al. (2010), Sahoo et al. (2012) and, more recently, Kikumoto et al. (2014), have contributed greatly to the growing understanding of the processes that prevailed during the Precambrian-Cambrian transition and presented datasets that complement the findings of the present study (Fig. 14). The study by Li et al. (2010)  The present study aims to shed light on a crucial time in Earth history, the Neoproterozoic Oxygenation Event (NOE), when changing redox conditions in the ocean were succeeded by an unprecedented rise in the diversity and architectural complexity of organisms. Within this context and using a multiproxy elemental and isotopic approach, the palaeobathymetric situation and the degree of restriction of the regional and local marine environments on the Yangtze Platform during this transition can better constrain the framework for the NOE.
The Nantuo Formation, deposited during the end-Cryogenian 'Marinoan' Glaciation, represents the last stage of the rifting history of the Yangtze Platform, which left a sea floor with abundant horst and graben structures, onto which sediments of the Doushantuo Formation have been draped (e.g. Vernhet, 2007;Zhu et al., 2013). In addition, the Huangling granite intrusion and its subsequent erosion during multiple phases of glaciations are likely to have had a major impact on the bathymetry of the Three Gorges Area, in particular. A mosaic of different depositional environments can therefore be imagined, which has been confirmed by studies demonstrating significant lateral facies varia- tions, notably around the Three Gorges Area (e.g. Vernhet & Reijmer, 2010;Jiang et al., 2011;Zhu et al., 2013). The stratigraphic and palaeoenvironmental ambiguity, however, has contributed to a wealth of different ideas on the depositional environment of the Doushantuo Formation, i.e. whether it was deposited in a non-marine basin (Bristow et al., 2009), an intrashelf basin (Vernhet & Reijmer, 2010) or a shelf lagoon (Jiang et al., 2011). The sections below begin by evaluating environmental conditions during deposition of the Doushantuo Formation, and then expand this palaeoenvironmental analysis to track environmental conditions into the Early Cambrian.

The Ediacaran
Doushantuo Member II The short interval sampled for Doushantuo Member II (DST II) at Jiuqunao is geochemically similar to the more expanded profile analysed at Jiulongwan (Figs 4 and 5), where no enrichment in redox-sensitive trace-metals, comparable TOC and TS contents, generally anoxic non-sulphidic conditions and mostly positive d 34 S Pyrite values (above 10&) are found. Previous studies of the Jiulongwan section agree with the present results (Bristow et al., 2009;Li et al., 2010) and suggest that a lack of trace-metal enrichment was a characteristic of the Yangtze Platform during the deposition of DST II. However, significantly  Fig. 13. Summary of geochemical results from the Early Cambrian Xiaotan section Och et al., 2013). ZJQ = Zhujiaqing Formation; CLP = Canglangpu Formation.
higher enrichments of Mo and V in DST II have been reported from slope and basin sections, where Mo concentrations can attain values from a few tens of ppm (Wallis, 2006;Guo et al., 2007) up to almost 200 ppm (Sahoo et al., 2012).
Although there are possible indications of intermittent euxinia in DST II at Jiulongwan, suggested by high Fe Py /Fe HR ratios ca 0Á7, no Mo enrichment is apparent. Because Mo/TOC and V/TOC ratios remain very low throughout the lower part of the Doushantuo Formation, it is argued here that limited availability of Mo and V in the water column prevailed, at least locally, during deposition of DST II at Jiulongwan. Independently, palaeogeographic, sedimentological and carbon isotope studies support a rimmed platform margin, which developed soon after deposition of the underlying cap carbonate (Jiang et al., 2011), and the formation of variably restricted basins on the Yangtze Platform (Jiang  Vernhet, 2007;Vernhet & Reijmer, 2010;Zhu et al., 2013). This suggests that geographic barriers and a lack of access to the open ocean might have controlled tracemetal and sulphate availability from the Early Ediacaran onwards. Furthermore, the d 34 S profiles at Jiulongwan and Jiuqunao are variable but generally enriched in d 34 S, possibly resulting from near-quantitative sulphate reduction within a sulphate-poor water column, which is a predictable result of restricted access to the open ocean. Nonetheless, this depends on the isotopic signature of Neoproterozoic sea water sulphate, which, based on sulphate-sulphur isotope studies (see Och & Shields-Zhou, 2012, for a compilation), appears to have been exceptionally high during the Precambrian-Cambrian transition. However, as Li et al. (2010) and Sahoo et al. (2012) have shown, the sulphur isotopic signature within DST II successions deposited under a euxinic water column on the shelf margin and within the deeper basin exhibit negative excursions between À30& and À40&. Along with high concentrations of redox-sensitive trace-metals (Fig. 14), this indicates enhanced availability of sulphate within the open ocean, resulting in an increased potential for rates of sulphide production to overwhelm the influx of highly reactive Fe, thus leading to euxinic conditions (Poulton & Canfield, 2011) and, hence, extensive trace-metal drawdown. This condition is indicated by low Mo/TOC ratios in DST II on the shelf margin (Li et al., 2010; this study) and greatly elevated Mo/TOC ratios in basinal successions (Sahoo et al., 2012). The nitrogen isotope record of DST II, where most values lie between 3& and 6& in the Jiulongwan section (see also Kikumoto et al., 2014), and at 6& at Jiuqunao section, both representing inner shelf sedimentary successions, indicate that denitrification and nitrogen fixation were well-balanced in the water column, consistent with stable nitrate supply (see also Ader et al., 2014). Together with the above mentioned positive d 34 S signature measured in the Jiulongwan and Jiuqunao sections, this suggests that these successions were deposited in a restricted environment with low sulphate availability but a complete nitrogen cycle within a redox stratified water column.

Doushantuo Member IV (Miaohe)
In the Three Gorges Area, the black shales from the Miaohe Member at Jiulongwan were deposited under euxinic conditions, whereas euxinia is not indicated at Jiuqunao (Figs 4 and 5), which lies ca 30 km north-west of the Jiulongwan Formation. Intermittently euxinic conditions are also indicated within the uppermost Doushantuo Formation at Maoshi, which lies ca 530 km south-west of the Three Gorges Area in Guizhou Province, suggesting sporadic widespread euxinic conditions on the platform margins during deposition of the upper Doushantuo black shales. While the highest recorded Precambrian Mo concentrations are found at Jiulongwan (>300 ppm; Mo/TOC >180), the Jiuqunao and Maoshi sections exhibit only average shale Mo and V concentrations. In addition, sulphide isotope values are exclusively and distinctively negative in the Miaohe black shale at Jiulongwan, averaging À11Á2&, while at Jiuqunao and Maoshi, mostly positive values were found averaging +18Á4& and +9Á9&, respectively, with similar trends in both sections (Fig. 11). Therefore, Doushantuo Member IV (DST IV) at Jiulongwan was deposited under euxinic conditions, with seemingly unrestricted availability of sulphate and redox-sensitive trace-metals. Nitrogen isotopic signatures support stable nitrate availability and suggest that 'normal' marine production helped to fuel euxinia in this case. Hence, because a steady source of nitrate is required, and the present authors can envisage water column stratification, with euxinic bottom waters beneath oxic/dysoxic surface waters. Presumably, oxidized S, N and Mo originated from surface currents that flowed over the lip of the rimmed basin from the open ocean. The Miaohe Member sediments at the Jiuqunao and Maoshi sections, from the Three Gorges Area and Guizhou Province, respectively, do not exhibit any similar enrichment of redox-sensitive tracemetals, while pyrite is enriched in d 34 S (Fig. 11), which suggests that the depositional environment of Jiuqunao and Maoshi could represent more restricted intra-shelf basins that remained isolated from oxygenated surface waters within the inner platform. Because the Jiulongwan and Jiuqunao sections are located close to one another, this finding implies considerable spatial redox complexity within the shelf lagoon.
The nitrogen isotopic signatures of these sections are considerably different: while d 15 N values at Jiulongwan are relatively constant at ca 4&, contrasting end-member d 15 N values are found of ca 0Á5& at Jiuqunao and 6Á5& at Maoshi. At Jiuqunao, (de)nitrification seems to have been inhibited, possibly due to insufficient availability of nitrate which would also prevent the development of sustained euxinia. In that case, the low d 15 N values indicate that atmospheric nitrogen fixation fuelled productivity after nitrogen loss through organic matter burial. On the other hand, higher values at Maoshi suggest a more balanced nitrogen cycle in a redox stratified water column and the ability to, at least locally, sustain euxinic conditions (Boyle et al., 2013;Cremonese et al., 2013;Ader et al., 2014). Therefore, a gradually increasing exchange with the open ocean can be envisaged, as shown by the transition of pyrite sulphur isotopes from 20& to À4&, in contrast to an average of 18Á5& at Jiuqunao.
In summary, a rising eustatic sea-level would successively have allowed basins that were restricted or semi-restricted during deposition of DST II to have access to the open ocean, thus increasing the availability of redox-sensitive trace-metals to be scavenged by anoxic and particularly sulphidic bottom waters (Fig. 15). Low sulphate concentrations in the water column during the deposition of DST II were likely to be sufficiently counteracted by overall low Fe(II) concentrations at Jiulongwan, to allow episodic euxinia to develop (Poulton & Canfield, 2011). The euxinic sediments of the Miaohe Member at the Jiulongwan section are accompanied by very high Mo/TOC ratios and negative sulphide isotope values, suggesting a ready supply of redoxsensitive trace-metals and sulphate from the shallow, oxic ocean, probably overlying a sulphidic wedge (Fig. 15), as proposed by Li et al. (2010). The apparently close relationship between euxinic conditions and nitrate availability suggests that exchange with the generally oxygenated open ocean was essential to fuel euxinia during times of high productivity, potentially driven by upwelling of P-rich waters. Lower sea-level and basin restriction in these cases may have self-limited the spread of euxinia by limiting the supply of fixed nitrogen from sources other than microbial nitrogen fixation.

The Baiguoyuan section
Previous studies of the Baiguoyuan section have focussed mainly on the black shale hosted Ag-V ore deposit, generally thought to be of sedimentary-diagenetic origin (Qian et al., 1995;Zhuang et al., 1999). While TOC contents are within the same range as at Jiulongwan, the Miaohe Member at Baiguoyuan is poor in total sulphur (TS; Wallis, 2006). Not surprisingly, although Mo is still slightly enriched at Baiguoyuan with respect to average shale values, Mo concentrations do not exceed a tenth of the maximum content reached at Jiulongwan. On the other hand, V attains concentrations of more than 1 wt% at Baiguoyuan, which is about five times more than the maximum at Jiulongwan ( Fig. 4; Wallis, 2006). The depositional environments at Baiguoyuan and Jiulongwan were presumably subject to similar trace-metal availability, but the former was deposited under an anoxic water column where euxinia never developed. Although neither sulphur nor nitrogen isotopic signatures are available for the Baiguoyuan section, it is difficult to formulate a more detailed explanation for the high V concentrations and further research on the section at Baiguoyuan is required. However, at this point these findings merely further corroborate a highly diversified physico-chemical environment on the Yangtze Platform with strongly varying trace-metal availability and redox conditions. environments in the Ediacaran and the Cambrian on the Yangtze Platform is that euxinia was less widespread and pervasive during deposition of the Doushantuo Formation, when it was established in the form of a sulphidic wedge in upwelling areas, carrying over in places to distal shelf lagoon areas such as Jiulongwan (Li et al., 2010). During the Early Cambrian, on the other hand, euxinic conditions have been reported from both basin and platform sections (Canfield et al., 2008;Och et al., 2013, and references therein). At Jiuqunao and Jijiapo (Figs 9 and 10), after the positive peak in Mo and V, nitrogen isotopic signatures decrease to values between 0& and À1&, consistent with an increase in N 2 -fixation and/or photic zone anoxia, possibly facilitated by high concentrations of nitrogenase co-factors (Mo and V) (e.g. Canfield et al., 2010b;Zhang et al., 2014); N 2fixation could then have contributed significantly to the increase in primary productivity, sustaining anoxia and sulphidic conditions. These features can also be observed at Jiuqunao and Jijiapo, although to a different extent due to the different palaeogeographic settings. Nevertheless, Mo and V peaks seem to be widespread on the Yangtze Platform, and the increase in bio-available Mo and V, in a dominantly ferruginous ocean might have triggered unprecedented primary production and organic matter delivery to the ocean floor (Anbar & Knoll, 2002), which created conditions favourable for intensified sulphate reduction and the emergence of widespread and sustained euxinia in the Early Cambrian, prior to the Cambrian bioradiation.
The effects on ecosystems of changing spatial redox distribution It has been established that many extant animals are facultative anaerobes and so are not wholly dependent on oxygen (Martin & M€ uller, 2007;Budd, 2008); some are even able to withstand sulphidic conditions throughout their life cycle (Danovaro et al., 2010). Nonetheless, motility and other muscular activities of animals are repressed at low oxygen levels. Therefore, the finding of widespread anoxia and spatially extensive euxinia across the Ediacaran-Cambrian boundary seems to be incompatible with the ongoing Cambrian explosion, which is commonly believed to have been triggered by oceanic, and possibly atmospheric, oxygenation.
One consequence of such spatial variability in redox conditions would be to create dynamically changing benthic ecosystems. Although preliminary, the present results may help to shed light on the distribution of benthic and, to a lesser extent, pelagic ecosystems on the Yangtze Platform. The most famous Ediacaran fronds are indisputably benthic but have only been reported in China from the Shibantan Member of the Dengying Formation, which is constrained by Fe speciation and rare earth element ( distributions to have been deposited under largely oxygenated conditions Duda et al., 2014); this is supported by examples of both body and trace fossils . As shown above, more distal equivalents such as the Liuchapo Formation, which are largely devoid of any diagnostic fossils, were deposited under anoxic and in some cases even euxinic conditions, thus restricting benthic ecosystems at this time to the generally shallower, inner Yangtze Platform. The Miaohe biota (Steiner, 1994;Ding et al., 1996) is widely known but generally limited to just a few sections from the inner shelf lagoons close to the Jiuqunao section (the Miaohe section is ca 1 km distant and lithologically very similar) and Maoshi section (Guizhou Province). No Miaohe fossil assemblage has been reported from the equivalent horizon at the Jiulongwan section, which according to the present data experienced persistent sulphidic conditions. One possible interpretation of this is that dissolved sulphide was toxic to the Miaohe biota.

CONCLUSIONS
The multi-proxy approach, including the redoxsensitive trace-metals Mo and V, iron speciation, sulphide and nitrogen isotopes suggest a gradually increasing number of intra-shelf basins on the Yangtze Platform gaining access to the greater availability of trace-metals and sulphate present in the open ocean (Fig. 16). This effect can best be illustrated at the sections around Jiulongwan, where sea-level rise submerged the previously restricted rimmed basin during deposition of the Miaohe Member of the Doushantuo Formation. There, Mo/TOC ratios suggest good communication with the open ocean, while the combination of Fe speciation and S isotopes suggests that euxinic conditions did not limit sulphate (or nitrate) availability. Although it is likely that the trace-metal and sulphate inventory increased gradually during the Precambrian-Cambrian transition, the present authors suggest that changing sea-level played an important role in the expression of the Neoproterozoic Oxygenation Event (NOE) at continental margins. However, although the change in biogeochemical cycling across this large transitional time interval follows a clear trend, the details rely on the current stratigraphic framework that may change with additional age constraints, and the signature of changing ocean chemistry and bathymetry cannot clearly be separated at present.
The apparent switch in nitrogen cycling, whereby d 15 N values remained heavy during sulphidic periods in the Ediacaran, but with light values during euxinia in the Early Cambrian, possibly arises from sulphidic conditions being confined to sulphidic wedges in the upwelling areas during the Ediacaran. In contrast, productivity and subsequent sulphate reduction was probably fuelled by the nitrate reservoir of the open ocean during the early Cambrian. In the Early Cambrian, more widespread euxinic conditions across flooded shelves precluded incomplete denitrification, leading to lower d 15 N values. A highly dynamic environment, both bathymetrically and chemically, including a trend to higher oxygen concentrations in the ocean interrupted by regional euxinic events, might have spurred on the emergence and diversification of animals for which the fossil record of the Cambrian Explosion can only represent a glimpse.

Supporting Information
Additional Supporting Information may be found in the online version of this article: Table S1. Elemental and isotopic data. Table S2. Iron Speciation. Table S3. Xiaotan section, Yunnan Province (see Och et al., 2013 for more details).