Controls on the evolution of Ediacaran metazoan ecosystems: A redox perspective

Abstract A growing number of detailed geochemical studies of Ediacaran (635–541 Ma) marine successions have provided snapshots into the redox environments that played host to the earliest known metazoans. Whilst previous compilations have focused on the global evolution of Ediacaran water column redox chemistry, the inherent heterogeneity evident in palaeogeographically distinct environments demands a more dissected approach to better understand the nature, interactions and evolution of extrinsic controls on the development of early macrobenthic ecosystems. Here, we review available data of local‐scale redox conditions within a palaeogeographic and sequence stratigraphic framework, to explore the mechanisms controlling water column redox conditions and their potential impact on the record of metazoans. The openly connected Laurentian margin, North America (632–540 Ma) and Nama basin, Namibia (550–538 Ma), and the variably restricted Yangtze Block, South China (635–520 Ma), show continued redox instability after the first fossil evidence for metazoans. This may support opportunistic benthic colonisation during periods of transient oxygenation amidst episodic upwelling of anoxic waters beneath a very shallow, fluctuating chemocline. The first skeletal metazoans appeared under conditions of continued redox stratification, such as those which characterise the Dengying Formation of the Yangtze Block and the Kuibis Subgroup of the Nama basin. Current data, however, suggests that successful metazoan reef‐building demanded more persistent oxia. We propose that cratonic positioning and migration throughout the Ediacaran Period, in combination with gradually increasing dissolved oxygen loading, may have provided a first‐order control on redox evolution through regulating circulation mechanisms in the Mirovian Ocean. Some unrestricted lower slope environments from mid‐high latitudes benefited from sustained oxygenation via downwelling, whilst transit of isolated cratons towards more equatorial positions stifled pervasive ventilation either through ineffective surface ocean mixing, Ekman‐induced upwelling, elevated surface ocean productivity or a combination of these processes.

estimating molecular divergence times and the patchiness of the fossil record, this date is in broad agreement with the earliest evidence for Metazoa, as interpreted from demosponge sterols at ~713 Ma (Love et al., 2009). Beyond these biomarker traces, the archive of animal life remains absent until the appearance of credible animal fossils in the Ediacaran Period (635-541 Ma), which reveals diverse ecosystems preserved via a number of taphonomic pathways (Liu, 2016;Narbonne, 2005).
Modern marine environments show differing ecological distributions which correlate with local water column dissolved oxygen concentration. Furthermore, controls on local redox conditions include the degree of productivity as well as the influence of local hydrodynamics.

Dominant redox environment of habitation
The earliest bilaterian trace fossil occurrence is observed in middle Ediacaran (>585 ± 3.3 Ma) strata of the Tacuarí Formation, Uruguay . Subsequent deposits of the White Sea area (Russia) and Ediacara member (South Australia) reveal trace fossil evidence for motility alongside co-preservation of the culprit molluscan trace maker, Kimberella quadrata Gehling, Runnegar, & Droser, 2014;Martin et al., 2000).
The adoption of biomineralisation as a life habit is thought to have required not only the environmental availability of biologically exploitable compounds (e.g., carbonate ions and calcium), but also an external stimulus, with some suggesting the rise of predators as a candidate pressure (Wood, 2011). Earliest evidence for active predation has been documented from organic walled microorganisms within facies of the late Tonian (~780-740 Ma) Chuar Group (Porter, 2016;Shields-Zhou et al., 2016), whilst suggested predatory borings in Cloudina have been reported from the ~550 Ma Dengying Formation, South China (Bengtson & Zhao, 1992), and the Kuibis Subgroup of the Nama Group, Namibia (Brain, 2001).

| Redox and metazoan ecology
The importance of oxygen provision in enabling high energy yields through aerobic respiration has driven a long-standing debate on the possibility of an increase in marine dissolved oxygen (beyond a threshold concentration) as a primary factor enabling the rise of animals (Nursall, 1959;Runnegar, 1991;Sperling, Knoll, & Girguis, 2015).
Studies on the colonisation and structuring of modern marine ecosystems under variably reducing conditions have shown that welloxygenated, nutrient-rich environments permit sustained habitation by larger organisms, in addition to the potential for biomineralisation (Sperling, Knoll, et al., 2015). Contrastingly benthic metazoan trophic structure in suboxic/anoxic waters is limited to low diversity and is usually characterised by small, unmineralised organisms (Levin et al., 2009;Sperling, Knoll, et al., 2015).
With substantial contention remaining as to the phylogenetic affinity of the majority of soft-bodied Ediacara biota, it is unclear what environmental requirements may have facilitated their diversification . In the light of this, hereon the discussion of physiological oxygen requirements within diverse fossil assemblages of the Ediacaran can only be considered to represent end-member taxa whose presence required elevated levels of dissolved oxygen in the water column, rather than the conditions which dictated establishment of assemblages as a whole.
Recent investigation into one of the most basal modern diploblastic organisms has revealed oxygen concentration requirements between 0.5% and 4% of present atmospheric levels (PAL) (Mills et al., 2014).
However, atmospheric oxygen concentrations during the Meso-Neoproterozoic are poorly constrained and widely debated, with the latest estimates from modelling of proxy data ranging from <0.1% to >4% PAL until at least 800 Ma (Cole et al., 2016;Gilleaudeau et al., 2016;Planavsky et al., 2014;Zhang et al., 2016). By contrast, gas inclusion in ~815 Ma halite from the Officer Basin, South Australia, has been interpreted to suggest an atmospheric oxygen concentration of >10% PAL (Blamey et al., 2016).
Despite such instability, however, marine redox conditions during the Cryogenian Period are thought to have been locally permissive for the evolutionary origin of Metazoa .
Significantly, however, high-resolution investigations of local-scale redox within fossiliferous terminal Ediacaran successions indicate continued dynamism between deposition under anoxic and oxic water column conditions in both deep marine and shallow shelf settings (Och et al., 2015;Sperling, Carbone, et al., 2015;. Whilst low atmospheric oxygen concentration in the Neoproterozoic may have been a principal reason for sustained local deep-water oxygen deficiency, evidence for substantial and long-standing spatial marine redox heterogeneity within approximately time-equivalent Ediacaran sections demands consideration of more subtle physical mechanisms.
Many now consider that successful early ecosystems required stable O 2 above a threshold concentration that was maintained for an ecologically significant timescale (Johnston et al., 2012. Under this hypothesis, it was local redox instability which may have delayed proliferation of early animal ecosystems and resulted in the paucity of fossil Metazoa until the late Ediacaran. In summary, the advent of animals is thought to represent the culmination of inherent prerequisite genomic development, physical and chemical change in the marine environment and predation, which together helped drive the evolutionary step towards skeletonisation.

| CONTROLS ON LOCAL REDOX IN MODERN AND ANCIENT ENVIRONMENTS
Global atmospheric oxygen concentration is ultimately controlled by the balance between oxygen supply via photosynthetic primary production and long-term burial of reduced elements (e.g., pyrite iron and organic carbon), and oxygen consumption resulting from oxidative weathering of reduced elements within continental sediments and oxidation of reduced gases delivered through volcanic emissions (Canfield, 2014). The primary mechanisms responsible for subsequent atmospheric oxygen delivery to the global ocean include diffusion and efficient physical ventilation of surface waters due to wave agitation and dispersive mesoscale eddies, oxic riverine influx to coastal waters and downwelling of oxygenated surficial water masses Broecker, 1997;Kershaw, 2015;Petsch, 2003). Finally, the concentration and spatial distribution of dissolved oxygen in the marine environment may fluctuate depending on local circulation and the rate and extent of primary production and remineralisation (Helly & Levin, 2004;Petsch, 2003).

| Productivity and the biological pump
Marine redox on a local scale is subject to substantial variation resulting from the balance between oxygen supply and biological oxygen consumption through energy-yielding organic matter oxidation (remineralisation). The initial concentration of organic matter production is primarily dictated by nutrient availability in the surface ocean (Li, Meng, Algeo, & ShuCheng, 2015). Organic matter remineralisation follows a predictable electron acceptor utilisation pathway dictated by the associated energy yield per mole of organic carbon derived from each oxidation reaction, with the highest energy yield achieved through oxidation of free O 2 during aerobic respiration (Canfield & Thamdrup, 2009). As dissolved O 2 concentration decreases with organic matter sinking, the preferred electron acceptor first becomes nitrate within the nitrogenous zone followed by a manganousferruginous layer with reduction of continentally derived iron and manganese oxides (Canfield & Thamdrup, 2009;Cheng et al., 2016;Li, Meng, et al., 2015). The underlying euxinic zone is defined by sulphate reduction and build-up of H 2 S aq . The presence or absence of euxinia in an anoxic ocean is partly controlled by the relative fluxes of highly reactive Fe minerals and sulphate (Poulton & Canfield, 2011), in addition to the efficiency of organic carbon delivery from productive surface waters. Furthermore, it has been suggested that longstanding euxinia demands nitrate depletion as a consequence of the higher free energy yield associated with denitrification over dissimilatory sulphate reduction, and new production must accordingly be sustained via nitrate provision in addition to anoxic N 2 -fixation (Boyle et al., 2013;Canfield, 2006). In this way, excess bioavailable nitrogen is able to support organic matter production required for oxidation during sulphate reduction, after quantitative denitrification.
Recent model investigations of the biological pump suggest that enhanced efficiency of surface water organic matter oxidation through reduced rates of sinking and/or enhanced rates of respiration is able to effectively lift the oxycline to shallower depths (Meyer, Ridgwell, & Payne, 2016). This supports previous studies invoking oxycline deepening resulting from faster sinking of organic matter due to the consequent increase in remineralisation depth (Butterfield, 2009;Lenton, Boyle, Poulton, Shields-Zhou, & Butterfield, 2014).

| Upwelling and downwelling in the open ocean
Superimposed upon biochemical processes, which locally act to consume oxygen, are environmental factors which influence dissolved oxygen and nutrient distribution. These include changes in salinity and water temperature, alongside hydrodynamic mechanisms that are subject to local variation as a function of intrinsic factors such as palaeobathymetry, and local water column circulation patterns (Petsch, 2003).
Effective downwelling occurs in areas subject to elevated surface density resulting from high salinity and low temperature. In such areas, the dissolved oxygen concentration of surface waters is elevated due to the effect of higher oxygen saturation solubility at lower seawater temperatures (Petsch, 2003). Downwelling in the modern ocean is therefore principally controlled by palaeolatitude, with lower temperature at higher latitudes promoting the formation of oxygenated deep water (Broecker, 1997;Tomczak & Godfrey, 2001). However, regions prone to fresh water dilution as a consequence of low surface evaporation, such as the modern north Pacific, suffer from less efficient downwelling (Bruce, 1983).
By contrast, persistent upwelling systems ordinarily form in midlatitudes as a consequence of equatorial current activity related to displacement of surficial water masses via Ekman transport and equatorial divergence (Fuenzalida, Schneider, Garcés-Vargas, Bravo, & Lange, 2009). Upwelling may develop seasonally or interannually as an effect of differential wind stress and climatic conditions, respectively (Helly & Levin, 2004;Paulmier & Ruiz-Pino, 2009). Nutrient-rich upwelling waters lead to stimulated surface ocean productivity driving oxygen depletion of subsurface waters through organic matter oxidation and often resulting in shoaling of the oxycline (Fuenzalida et al., 2009).

| Restricted environments
Modern marine environments which lack influence from extensive physical mixing by open ocean current activity include the Black Sea and the Cariaco Basin on the Venezuelan continental shelf (Ho et al., 2004). In these settings well-mixed oxic surface waters transition to anoxic, commonly euxinic (sulphidic) deeper layers due to strong salinity-induced density gradients and the absence of efficient physical ventilation mechanisms at depth . The maintenance of euxinia in restricted environments is a consequence of high sulphate supply from oxidative continental weathering of reduced sulphur species (e.g., pyrite) alongside hydrogen sulphide production via bacterial sulphate reduction (BSR) in the oxygen-depleted subsurface water column . Under these conditions, available water column ferrous iron delivered to the deep anoxic layer through reductive dissolution of ferric oxides is sulphidised and deposited as pyrite.
Drainage basin area of the enclosing landmass and regional precipitation rate influences the concentration of oxic riverine discharge to the restricted basin which may episodically be of sufficient volume to overcome salinity-induced stratification and result in short-term lowering of the oxycline at basin margins (Kershaw, 2015). Furthermore, the relative stability of chemical stratification in restricted basins is a function of the rate of deep-water renewal related to the degree of basin connectivity with the open ocean as indicated by variations in chemocline depth and depth of the basin margin sill relative to total basin depth Konovalov, Murray, Luther, & Tebo, 2006). More effective restriction and reduced mixing of deep basin waters are implied by lower chemocline and sill depth ratios, respectively .
Mechanisms for deep-water oxygenation include extensive cooling above the oxycline and convective overturn of the stratified basin due to density inversion, in addition to submarine mass wasting brought on by slope instability (Anderson & Devol, 1973;Kershaw, 2015). Additionally, enhanced basin connectivity through eustatic sealevel rise may result in overflow and breach of well-mixed, higher density oxygenated waters into the underlying anoxic zone (Konovalov et al., 2006). However, this hyperpycnal incursion may be accompanied by nutrient replenishment and contrastingly result in consumption of oxygen through short-term elevated organic carbon production and remineralisation (Li, Taylor, Astor, Varela, & Scranton, 2012). The efficiency of these mechanisms towards effective oxygenation of the subsurface is dependent upon their frequency and magnitude with respect to basin volume ).

| Unrestricted continental shelf
Open ocean shelf settings lack bathymetric restriction from the wellmixed ocean and as a result may be locally subject to vertical and lateral mixing through ocean current activity at mid-depths, Ekman transport and baroclinic transport of surface waters (Fuenzalida et al., 2009).
Some shelf and continental slope areas experience oxygen depletion through local enhancement of the biological pump stimulated by upwelling of nutrient-rich bottom water (Fuenzalida et al., 2009;Helly & Levin, 2004). This differs from restriction-induced maintenance of subsurface anoxia, in that oxygen minimum zones (OMZs) on the open shelf exhibit variation in the vertical and lateral positioning of upper and lower boundaries, which are generally maintained through relatively sluggish local circulation (Fuenzalida et al., 2009;Helly & Levin, 2004). Volumetric changes in oxygen minima along continental margins of the modern ocean occur on glacial-interglacial timescales and are thus identifiable through palaeoredox proxy methods.
Four major OMZ settings in the modern ocean, with dissolved oxygen concentrations <20 μmol/kg, include the eastern south Pacific, the eastern tropical and subtropical north Pacific, the Arabian Sea, and the Bay of Bengal in the northern Indian ocean (Helly & Levin, 2004;Paulmier & Ruiz-Pino, 2009). A further, weaker OMZ (≥20 μmol/ kg dissolved O 2 ) is related to upwelling of the Benguela current and occurs off the coast of Walvis Bay, Namibia, in the eastern tropical south Atlantic (Helly & Levin, 2004).
Differential thickness and spatial extent of the OMZ off the coast of Peru in the south-eastern tropical Pacific correlates well with nutrient input via Ekman-induced upwelling, whilst the positional offset of the OMZ is an effect of the disconnect between the physical process of upwelling and regional migration of biological activity (Fuenzalida et al., 2009). Transient adjustment of the mixed layer depth along continental margins such as the Namibian shelf and Arabian Sea is induced through strong seasonal differences in wind stress and sea surface temperature, resulting in shoaling of oxygen-depleted water from depth, in addition to convective mixing Kumar & Narvekar, 2005). Variations in areal extent, thickness and intensity of an OMZ may occur on interannual timescales associated with cyclic changes in sea surface temperature and circulation. An example of this in the modern ocean is the El Niño Southern Oscillation, which is responsible for shrinking the OMZ in the eastern tropical south Pacific during periods of enhanced surface warming (Fuenzalida et al., 2009;Helly & Levin, 2004).  Sahoo et al., 2012Sahoo et al., , 2016. In this way, extreme enrichments of RSE within organic-rich shales are indicative of local euxinia, whilst maximum values may aid interpretation of the global seawater elemental inventory and thus the degree of global marine anoxia (Kendall et al., 2015;Sahoo et al., 2012;Tribovillard, Algeo, Lyons, & Riboulleau, 2006 3.2 | Local/regional proxies

| Iron speciation
Iron speciation via the technique developed by Poulton and Canfield (2005) allows for localised redox reconstruction through evaluation of the concentration of iron phases considered highly reactive (Fe HR ) to biological/abiological reduction under anoxic conditions, relative to total iron (Fe T ). Under oxic conditions, soluble Fe 2+ is almost entirely oxidised to insoluble Fe 3+ , whilst anoxic conditions can allow transport of Fe 2+ until water column precipitation is induced (Raiswell & Canfield, 1998). Water column Fe 2+ may precipitate as pyrite when transported to euxinic settings or may be precipitated as a range of non-sulphidised minerals (including Fe carbonates and oxides) under anoxic, non-sulphidic (ferruginous) conditions (Poulton, Fralick, & Canfield, 2004). This augments the detrital influx of Fe HR, potentially giving enrichments in the deposited sediment. The technique of Poulton and Canfield (2005) subdivides these minerals into operationally defined phases, including iron carbonates (e.g., ankerite and siderite), ferric oxyhydroxides (e.g., goethite, lepidocrocite, ferrihydrite and haematite), magnetite and sulphide-associated iron phases (e.g., pyrite and mackinawite). The sum of Fe HR plus iron bound in poorly reactive or unreactive silicates (geochemically inert on early diagenetic timescales) encompasses the total iron (Fe T ) content of modern sediments and ancient marine shales (Raiswell & Canfield, 1996. Sediments deposited under oxic water column conditions record suppressed Fe HR /Fe T (commonly below 0.22) due to the lack of highly reactive iron accumulation in the water column, whereas under anoxic water column conditions, ratios of Fe HR /Fe T are typically elevated above 0.38 (Poulton & Canfield, 2005). Where samples have 0.22 < Fe HR /Fe T > 0.38, redox interpretation is problematic due to the potential for physical processes such as rapid sedimentation to reduce the rate of Fe HR enrichment under anoxic depositional conditions (Lyons & Severmann, 2006;Poulton & Canfield, 2011). Furthermore, alteration of Fe HR to unreactive iron (Fe U ) may result in reduced Fe HR / Fe T and false oxic interpretation (Poulton and Raiswell, 2002;Raiswell et al., 2008). In these cases, additional consideration of Fe T /Al ratios (see below) and poorly reactive Fe contents may allow oxic and anoxic samples to be distinguished (see Cumming, Poulton, Rooney, & Selby, 2013;Poulton, Fralick, & Canfield, 2010).
The iron speciation proxy has the additional advantage of being able to distinguish between euxinic and ferruginous conditions.
Under euxinic conditions, the build-up of water column hydrogen sulphide (H 2 S aq ) results in sulphidation of iron oxides and formation of iron pyrite (FeS 2 ; Fe py ), leading to elevated Fe py /Fe HR (Poulton et al., 2004). Enrichments in Fe HR with low Fe py are considered indicative of ferruginous anoxia (Poulton & Canfield, 2005

| Rare earth elements and cerium anomalies
Distributions of rare earth elements (REEs) within authigenic minerals (e.g., carbonates, phosphates and chert) represent contemporaneous equilibrium between solution complexes and solid phase surface complexes (metal (oxyhydr)oxides, clay and organic matter) provided that there has been no deep-burial diagenetic modification (McArthur & Walsh, 1984). Cerium is the only REE prone to substantial transformation as a function of ambient seawater E h , due to the relatively reduced solubility of oxidised Ce 4+ and consequent scavenging by Fe-Mn (oxyhydr)oxides, which leaves the seawater REE pool comparatively depleted in Ce in oxic settings (German & Elderfield, 1990). Characteristic REE profiles with associated anomalous Ce depletion (Ce/Ce*) can therefore be a good indicator of oxia, on condition that there has been no signal modification by later reducing fluids (Bau & Dulski, 1996;Shields, Kimura, Yang, & Gammon, 2004). As such, the entire REE profile must display a distinguishing pattern of diagnostic relative depletions and enrichments indicative of average seawater, from which depletion of Ce relative to the light rare earth elements (LREE: praseodymium to gallium), lanthanum and neodymium indicates likely deposition under oxic water column conditions (Shields et al., 2004;.

| Trace fossils
Studies of modern benthic macrofaunal diversity and complexity under different dissolved oxygen levels imply that traces indicative of motility or active bioturbation, such as T. pedum, are restricted to formation by organisms with active metabolic lifestyles that most likely require elevated dissolved oxygen concentrations (Chang, Chronis, Karow, Marletta, & Bargmann, 2006;Wilson et al., 2012).
The absence of trace fossil evidence for active motility (Aceñolaza, Germs, & Aceñolaza, 2009), in addition to a lack of evidence for extensive carnivory and predation (Sperling, Knoll, et al., 2015), may support geochemical evidence for widespread anoxic, or low oxygen

| Redox proxy limitations
A shortfall of most redox proxies has traditionally been that they can only be applied to a limited range of lithologies, with most originally calibrated to target fine-grained siliciclastic sediments such as shale.
As discussed above however, recent calibration of iron speciation (Clarkson et al., 2014), alongside redox proxy extraction processes targeting Ce/Ce* (German & Elderfield, 1990;Shields et al., 2004; within carbonate-rich sediments can significantly aid redox interpretation of mixed carbonate-siliciclastic palaeoenvironments. Differing lithological requirements and proxy sensitivity to different reducing conditions are summarised in Table 2. is related to the rate of deep-water renewal Gilleaudeau & Kah, 2015;Konovalov et al., 2006). A number of studies have explored the potential for the ratio of molybdenum to total organic carbon, and changes in RSE concentration and size of the local seawater sulphate reservoir as geochemical proxies for basin restriction in anoxic sulphidic settings .
Previous studies have also stressed the relative insensitivity of bulk rock techniques to record rapid fluctuations in water column redox conditions, such as those potentially associated with individual fossil occurrences, as samples often represent a significant period of time. Thus, periods of very fleeting oxia and rapid colonisation of substrate by opportunistic biota may be preserved as an overall signature of pervasive anoxia, which in fact may only represent the dominant redox condition during sedimentation of the bulk sample Sperling, Carbone, et al., 2015;Sperling, Knoll, et al., 2015;.

| EDIACARAN REDOX SYNTHESIS
Apparently conflicting evidence has been published for the extent of open ocean ventilation during the late Neoproterozoic. Molybdenum T A B L E 2 Summary of major palaeoredox proxies, the redox potentials at which they record transformation, and lithologies targeted for their application. (a) Examples of elements which, in addition to providing information on local basin-scale redox, also enable inference of the nature and extent of global ocean redox and (b)  Whilst iron speciation inherently reflects local/regional redox conditions, extensive compilations from globally distributed shales deposited below wave base can be considered to provide a global redox perspective. Compilations of this style suggest that the majority of the Neoproterozoic (but with important exceptions; see below) was characterised by anoxic ferruginous conditions, which persisted into the Neoproterozoic (Canfield et al., 2008;Guilbaud et al., 2015;Sperling, Wolock, et al., 2015).

| Local redox record
To allow direct comparison between sections, iron speciation data have been compiled herein based on calibrated iron phase and major element ratios for depositional conditions. We have employed a conservative framework whereby oxic conditions are indicated by Fe HR /Fe T < 0.22, anoxic ferruginous by Fe HR /Fe T > 0.38 and Fe py / Fe HR < 0.7, and euxinic conditions by Fe HR /Fe T > 0.38 and the upper limit of Fe py /Fe HR > 0.8. Importantly, where analyses include both siliciclastic and carbonate lithologies, redox variations are shown to be primary and not lithologically determined (Clarkson et al., 2014;. Iron speciation is used herein as a redox proxy baseline, but where available additional proxy data is discussed. We consider 44 sections with accompanying Fe speciation data with the aim of reviewing local water column redox within platform to basin environments bordering the Yangtze Block, Laurentia, Kalahari Craton, Avalonia, the East European Platform (EEP) and Río de la Plata   are key to deciphering possible environmental requirements of earliest animal ecosystems (Cai, Hua, Schiffbauer, Sun, & Yuan, 2014;Chen et al., 2014;Hua et al., 2003;Van Iten et al., 2013;Zhu et al., 2008).

| Yangtze block
Intense study of Ediacaran to early Cambrian sections of the Yangtze Block has allowed unparalleled detail in palaeoredox reconstruction across an array of palaeodepth profiles, despite difficulty in coherent determination of lateral equivalence between some formations and members ( Figure 3). Basin reconstruction and tentative sequence stratigraphic correlation have been made possible by concerted studies of ash bed dating and detailed δ 13 C chemostratigraphy ( Figure 3) (Sahoo et al., 2016;Zhu et al., 2007). Abundant riftrelated and block faulted grabens were gradually incorporated into a broad passive continental margin during deposition of Ediacaran sediments, with changing relative sea level enabling affecting restriction of intrashelf basin environments (Jiang et al., 2011;Zhu et al., 2007).
In the Yangtze Gorges area, the Doushantuo Formation has classically been subdivided into four lithostratigraphic members which, in ascending order, comprise the thin basal cap dolostone of member I, 80-120 m of shale with occasional medium-bedded dolostone and chert nodules of member II, 40-60 m of banded and lenticular chert interbeds and dolostone of member III, and a locally absent 10-m-thick black shale unit of member IV which commonly exhibits large dolomite concretions (Liu, Yin, Chen, Tang, & Gao, 2013).
Doushantuo member IV is thus commonly referred to as the Miaohe member after the distinctive "Miaohe biota" assemblage at the type locality (Xiao et al., 2002). This repeated sequence is permitted through long-term eustatic sealevel rise across the Ediacaran-Cambrian boundary (Haq & Schutter, 2008).
Importantly, recent integrated chemostratigraphic investigation of the Doushantuo Formation suggests regionally variable completeness of δ 13 C profiles and infers either truncation in shallow marine settings or a diachronous Doushantuo-Dengying boundary (Cui et al., 2015).
Consequently, it is expected that regional palaeoenvironmental reconstruction based on equivalence across platform to basin sections may be reinterpreted through future chemostratigraphic studies.
The spatial distribution of local redox observed within the Doushantuo Formation ( Figure 3) is schematically illustrated in . Sulphate limitation in a closed system during extended periods of basin restriction or effective disconnect between pore water and seawater will tend to drive the sulphur isotope composition of sedimentary pyrite (δ 34 S py ) towards heavier values, thus reducing the offset between the isotopic composition of seawater sulphate (preserved in carbonate associated sulphate) and pyrite (Δ 34 S CAS-py ) through continued BSR of the increasingly isotopically enriched sulphate reservoir (Gomes & Hurtgen, 2013). Sporadic euxinia inferred from intermittently elevated Fe py /Fe HR at Jiulongwan (Li et al., 2010) is thought to be a consequence of episodically low Fe HR supply in the dominantly ferruginous depths of the intrashelf basin during deposition of Doushantuo members II-III (Och et al., 2015). Yet more proximal settings on the shallow platform (e.g., Baiguoyuan; Figure 3, section 0) show evidence for infrequent deposition within well-mixed, oxic shallow waters which punctuate dominantly ferruginous deposition (Fan et al., 2014).
Variations in thickness of the euxinic zone are thought to be functionally equivalent to spatial variability observed in modern open marine OMZs related to high productivity stimulated through nutrient upwelling (Li et al., 2010;Och et al., 2015;Sahoo et al., 2016).
Sustained euxinic conditions in unrestricted settings such as those seen to have characterised the open slope at Wuhe (Han & Fan, 2015;Sahoo et al., 2012Sahoo et al., , 2016 require both high levels of organic matter supplied by surface water productivity and at least locally elevated influxes of marine sulphate capable of supporting build-up of H 2 S aq , after quantitative pyritisation by available highly reactive iron (Poulton & Canfield, 2011).
Uranium and molybdenum isotope data from euxinic shales of the Yangtze Block have been interpreted to indicate a global increase in oceanic dissolved oxygen concentrations throughout the Ediacaran (Chen, Ling, et al., 2015;Kendall et al., 2015). However, secular organic matter-normalised RSE enrichment and depletion within euxinic shales of the Doushantuo Formation have been interpreted to support limited global ocean Mo scavenging and temporarily widespread ocean oxygenation (Kendall et al., 2015;Sahoo et al., 2012Sahoo et al., , 2016Scott et al., 2008). for a corresponding lack of RSE enrichment and elevated δ 34 S py at this time (Och et al., 2015). Additional iron speciation and RSE data collected at Jiuqunao and nearby Miaohe sections corroborate ferruginous anoxia for the lowermost deposits of Doushantuo member IV, but also suggest a trend towards more euxinic conditions within overlying shales (Li, Planavsky, et al., 2015). Importantly, however, inception of euxinic conditions at Miaohe is seen to post-date fossil occurrence of the Miaohe biota (Li, Planavsky, et al., 2015). Limited Mo enrichment within both Jiuqunao and Miaohe sections (Li, Planavsky, et al., 2015) is consistent with the model of Och et al. (2015) for continued partial restriction of intrashelf sections, and accompanying δ 15 N data at Jiuqunao have been interpreted as evidence for restriction-induced nitrate limitation which may have precluded the maintenance of euxinia (Och et al., 2015).
Published Ce/Ce* data of the Doushantuo Formation are in broad agreement with iron speciation at Jiulongwan, supporting deposition beneath a redox stratified water column with some evidence for a trend towards more persistent anoxia up-section (Cui et al., 2015;Ling et al., 2013;Shields et al., 2004;Zhou, Jiang, Xiao, Chen, & Yuan, 2012).
Basinal open ocean deposition at Xiangtan is defined by dominantly ferruginous conditions where organic matter was depleted (Han & Fan, 2015;Li, Meng, et al., 2015). Under this model, nearshore and distal Fe sources are thought to have been distinct, with the shallow manganous-ferruginous zone permitted through reductive dissolution of detrital Fe and Mn oxides (Li, Meng, et al., 2015). In contrast, anoxic deep waters were typically enriched in soluble reduced Fe 2+ derived from long-term hydrothermal build-up, in addition to reduction of iron oxides and mobilisation of Fe to depth (Li, Meng, et al., 2015;Lyons & Severmann, 2006;Severmann et al., 2008).
A number of samples in the basinal Lantian section indicate low Fe HR /Fe T , possibly corresponding to sedimentation under oxic water column conditions. However, the extraction procedure used by Shen et al. (2008) at this locality does not isolate carbonate-bound iron Poulton & Canfield, 2005;Sperling, Carbone, et al., 2015), and given that inferred oxic samples show Fe HR /Fe T bordering the upper calibrated threshold for identification of oxic con-  Hua et al., 2003). To date, no evidence has been presented for reef-building by Cloudina in Dengying Formation carbonates and all specimens appear to occupy a "matsticker" mode of life . Possible borings have also been described in specimens of Cloudina hartmannae from shallow platform carbonates of the upper Gaojiashan member, which may represent the earliest evidence of metazoan predation in the fossil record (Bengtson & Zhao, 1992).

| Laurentia
Sediments along the Canadian Cordillera were deposited in a rift setting, with evolution to a passive continental margin and associated

| Nama Group
Exceptional exposure along two shelf-to-basin transects has allowed for sequence stratigraphic reconstruction, geochemical analysis and fossil distribution of a substantial portion of the Nama Group down to parasequence level ( Figure 5) (Dibenedetto & Grotzinger, 2005;Saylor, 2003;Saylor, Grotzinger, & Germs, 1995;Saylor, Kaufman, Grotzinger, & Urban, 1998;. The Nama foreland basin formed on the Kalahari Craton as a consequence of convergence along the Damara and Gariep orogenic belts to the present northeast and southwest, respectively, due to closure of the Brazilides Ocean during amalgamation of southwest Gondwana (Gaucher, Frimmel, & Germs, 2009). The Nama basin was subdivided into northern Zaris and southern Witputs sub-basins by a zone of depositional thinning across the "Osis Arch" palaeobathymetric high (Germs, 1983). Correlative formations of fluvial to shallow marine siliciclastic and carbonate sediments within both sub-basins have been mapped extensively across the Osis Arch and support basin connectivity during deposition, with a general palaeodepth increase to the northwest in the Zaris sub-basin and southwest in the Witputs sub-basin (Germs, 1983).  (Germs, 1983;Saylor et al., 1995). Shuram-Wonoka negative δ 13 C carb anomaly (Kaufman, Hayes, Knoll, & Germs, 1991;. Uranium-lead chronology of four volcanic ash beds has constrained the duration of Nama Group deposition, with a lower  (Grotzinger, Bowring, Saylor, & Kaufman, 1995;Jensen & Runnegar, 2005;Schmitz, 2012).

Strata of the Dabis Formation and lower Zaris
Formation which were deposited prior to the lowermost ash bed rest atop crystalline basement along a dramatic, readily identifiable angular unconformity with an contact age inferred at ~550-553 Ma, whilst conglomeratic and fluvial to shallow marine siliciclastic facies of the Nomtsas Formation unconformably overlie the Urusis Formation with an intercalated ash bed dated at 538.18 ± 1.11 Ma Schmitz, 2012). Therefore, total stratigraphic coverage of the Nama Group below the Cambrian Fish River Formation spans the final 10-12 million years of the Ediacaran Period ( Figure 5). Member at Arasab and Grens Farm sections (Hall et al., 2013;. Nama assemblage fossils are recorded throughout the overlying Schwarzrand Subgroup of the Witputs sub-basin and include the Erniettamorpha Pteridinium, Rangea and Paramedusium, in addition to discoidal Cyclomedusa in the lower Schwarzrand Subgroup (Germs, 1995). Pteridinium, Swartpuntia, Aspidella and Bradgatia are also recorded within the Spitskop Member at Swartpunt section Narbonne et al., 1997). The published range of soft-bodied fossils within contemporaneous deposits of the Zaris subbasin is restricted to the Nudaus Formation of the lower Schwarzrand

Subgroup and includes recently documented occurrences of Aspidella
and Shaanxilithes , in addition to mention of possible Pteridinium .  (Grotzinger, 2000;Saylor et al., 1995). The earliest documented occurrences of C. hartmannae, Cloudina riemkeae and N. hermanastes in the Nama Group are found in association with thrombolitic-stromatolitic microbial reefs of the lower Omkyk Member, and the first appearance of active reef-building by a metazoan is found in the high energy mid-ramp setting at Driedoornvlagte (Grotzinger, 2000;Penny et al., 2014;Wood & Curtis, 2015). Driedoornvlagte also exhibits the only known occurrence of the neptunian dyke-dwelling, robust skeletal Namapoikia rietoogensis of probable poriferan affinity (Wood et al., 2002). Associated Cloudina and Namacalathus have also been found within shallower facies of the upper Omkyk and lower Hoogland members at Zwartmodder  and ichnofossil-rich strata immediately overlying a soft-bodied Nama assemblage horizon in the middle Spitskop Member at Swartpunt (Darroch et al., 2015;Narbonne et al., 1997;. A wealth of trace fossil evidence recorded throughout the Nama Group initially enabled its interpretation as a terminal Ediacaran/ Vendian succession (Crimes & Germs, 1982 (Jensen & Runnegar, 2005;Jensen, Saylor, Gehling, & Germs, 2000;Wilson et al., 2012). In addition, an array of enigmatic tubular compression fossils have been noted from the lowermost Nudaus Formation and Feldschuhhorn member of the Witputs sub-basin (Cohen et al., 2009), and a diverse assemblage of organic walled microfossils has been noted from the Schwarzrand
Recent, extensive redox analyses utilising iron speciation of shales, silts and carbonates and Fe T /Al ratios of nine study sections within the Nama Group in association with palaeoecological data, has enabled reconstruction of three distinct time-equivalent shelfto-basin transects, thus allowing for interpretation of the relationship between redox hospitability and sustained ecological presence ( Figure 5) (Sperling, Wolock, et al., 2015;. The first transect incorporates data of the Dabis Formation from three sections of the Witputs sub-basin at Arasab, Grens and Zuurburg, and two sections of the Zaris sub-basin at Zwartmodder and Brak. Shallow water oxia persisted throughout deposition of the Kanies member (Zwartmodder) and lower Mara member (Zuurburg) coincident with considerable redox heterogeneity between the moderately deeper Arasab and Grens sections which record probable oxic conditions (inferred from extremely low Fe T of carbonate sediments) and ferruginous anoxia . Deep-water deposition within the Zaris sub-basin (Brak) is seen to have been exclusively anoxic and ferruginous .  .
The third transect incorporates three sections of the Feldschuhhorn and Spitskop Members of the upper Urusis Formation at the mid-ramp pinnacle reefs locality, outer ramp FSH section and variable depth deposits at Swartpunt (Sperling, Wolock, et al., 2015;Wilson et al., 2012;. Additional redox data of Nomtsas Formation deposits at the distal Sonntagsbrunn section are also considered herein (Sperling, Wolock, et al., 2015;Wilson et al., 2012). cessation of reef growth is seen to coincide with repeated drowning by siliciclastic deposits of the Feldschuhhorn Member (Grotzinger, 2000;Saylor et al., 1995).
Recent complimentary data supporting redox stratification of the Nama Basin have enabled nuanced interpretation of intermediate redox states through identification of unusual REE(+Y) profiles . This has also allowed infilling of data gaps where Fe T < 0.5 wt% of some carbonate samples previously impeded analysis via Fe speciation. Where iron speciation indicates surface water oxia, these data are corroborated by negative Ce anomalies. However, where highly reactive iron enrichments indicate anoxic ferruginous conditions, REE patterns either show the absence or expression of positive Ce anomalies . In the latter case, positive Ce anomalies are interpreted to indicate the presence of an intermediate layer of dissolved oxygen ≥~10 μm immediately overlying ferruginous deeper waters . Within this layer, the reductive dissolution of Mn (oxyhydr) oxides likely resulted in release to the water column of Ce(IV), leading to Ce accumulation and resultant enrichment in carbonate sediments relative to neighbouring REEs .
In the Nama Group, Ce anomaly data indicate episodic incursion of the manganous zone at shallow water Arasab, Grens, Zwartmodder and Omkyk sections and intermediate depth at Zebra River. However, an absence of positive Ce anomalies at Driedoornvlagte, the Pinnacle Reefs or Swartpunt sections supports sediment deposition at these localities under predominantly oxic conditions (Figure 7a) .

Whilst almost exclusive oxia recorded within the upper Urusis
Formation of the Witputs sub-basin may suggest progressive oxygenation of the Nama Group towards the Ediacaran-Cambrian boundary , additional sampling of Urusis Formation strata of the Zaris sub-basin appears to complicate this development. Finegrained, olive green and purple-red mudstones are interbedded with channelised sandstones of the Schwarzrand Subgroup in the Zaris sub-basin and yield iron speciation data which indicate exclusive formation under anoxic ferruginous water column conditions. Ratios of Fe T /Al suggest normal marine deposition within the calibrated range of 0.53 ± 0.11, with the exception of 5 outliers indicating significant iron enrichment and supporting deposition under an anoxic water column ( Figure 5).

Redox evolution accompanying the earliest reef-builders
The majority of sampled sections within the Kuibis Subgroup represent shallow to mid-ramp marine facies above storm wave base (with the exception of the most distal section at Brak), and as such introduction of oxygen via diffusion and surficial mixing from the overlying atmosphere is expected to have been pervasive. Riverine input of oxic freshwater may also have been an important mechanism for introduction of dissolved oxygen to the nearshore environment, with palaeocurrent data supporting dominantly westward-directed effluent sediment transport from the Kalahari Craton, evident from trough cross-stratification within siliciclastic lowstand systems tracts of the Kanies and lower Kliphoek members of the Dabis Formation (Germs, 1983;Saylor et al., 1995). Throughout the overlying Zaris Formation, frequent occurrence of anoxia in proximal, shallow environments has been interpreted as a consequence of upwelling anoxic ferruginous deep water, which is supported by progressively decreasing Fe T /Al within shallower sections . Although shallow waters of the exposed mid-ramp are thought to have been subject to active physical mixing and effective oxygenation, frequent incursions of anoxia are also thought to correspond to upwelling. However, relatively quiescent conditions at proximal Zwartmodder and Omkyk sections conducive to microbial mat growth may represent an environment prone to high surface water productivity fuelled by riverine nutrient input. Efficient remineralisation of the resultant elevated organic matter via aerobic respiration within shallow waters may have reduced dissolved oxygen concentration, followed by reduction of readily available shallow water iron oxides leading to thickening of a shallow water zone of Fe-Mn reduction ( Figure 7a) . dominantly oxic/probable oxic water column . It has been shown that unconsolidated seafloor conditions relating to increased siliciclastic influx from the Damara orogen to the north may have prevented early cementation conducive to formation of relief-structures during platform development of the Hoogland member (Dibenedetto & Grotzinger, 2005) and cessation of Cloudina reef growth at Driedoornvlagte is seen to locally occur simultaneously with a transition to ferruginous conditions which accompanied siliciclastics of the Urikos member (Figures 5 and 7b). This may imply a favourable  (Germs, 1983;Grotzinger et al., 1995). Analysis of the Niederhagen Member, which locally overlies Hoogland and Urikos sediments, indicates provenance from a relict volcanic island arc within the Damara Belt, which divided the Congo and Kalahari Cratons to the present-day north/northwest of the Nama Group (Blanco et al., 2011;Germs, 1983). The axis of the carbonate platform within the overlying Urusis Formation was shifted to the deeper, shallow marine environment to the southwest of Osis, and equivalent facies of the smaller Zaris sub-basin represent proximal siliciclastic deposition (Germs, 1983). The depocenter of the Zaris sub-basin gradually shifted southwards associated with diminishing influence of the Osis Arch throughout deposition of the upper Schwarzrand Subgroup (Germs, 1983). A potential scenario for some elevated Fe HR /Fe T within the Nudaus and Urusis Formations in the Zaris sub-basin may follow a simple Fe-trap mechanism, whereby Fe HR is effectively retained within this proximal deltaic environment (Figure 7b) . Such a setting would subsequently act as a source for Fe HR via reductive remobilisation to the subjacent water column via oxic or anoxic iron shuttle processes (Lyons & Severmann, 2006;Severmann et al., 2008). This may be supported by a significantly greater contribution of oxide-bound iron within the highly reactive iron pool of the Schwarzrand subgroup north of Osis, when compared to contemporaneous siliciclastic deposits of the Witputs sub-basin, and accompanied by "normal shale" values of Fe T /Al. Future detailed sampling and associated sedimentological and relative palaeodepth assessment within the Zaris sub-basin is required to support either an anoxic or false anoxic water column interpretation.

Metre
Whilst diverse soft-bodied and skeletonising biota thrived within the upper Urusis Formation, as well as motile metazoans evident from the diverse ichnofossil record at the oxic Swartpunt locality, the Schwarzrand Subgroup in the Zaris sub-basin lacks evidence for a comparatively significant biotic presence but for Pteridinium  and recently recorded Aspidella and Shaanxilithes within the Nudaus Formation . This may simply be a consequence of comparatively poor time-equivalent stratal exposure north of Osis farm, which has, until recently, impeded extensive body fossil recognition. If the interpretation of an episodic prodeltaic irontrap within this formation is correct, then original water column conditions may in fact have been at least intermittently oxic.
Whilst the rapid emplacement of turbiditic sediments may innately result in reduced Fe HR accumulation beneath an anoxic water column, the upper layers of each turbidite were sampled and define the finest sediment emplaced at the slowest rate (Canfield et al., 2007). Shale of the Conception and St. Johns Groups are confidently regarded to record oxic deposition within and above the Drook Formation (Canfield et al., 2007).
Lower rates of BSR indicated by low C and S concentrations and persistently elevated δ 34 S py nearing the approximated contemporaneous composition of seawater sulphate are seen to have preceded the Gaskiers Formation (Canfield et al., 2007;Fike et al., 2006). A decrease to predominantly negative δ 34 S py values is subsequently observed in the Drook Formation, coincident with inferred oxygenation of the deep marine environment, followed by a return to more elevated δ 34 S py during deposition of the Fermeuse Formation.

| East European Platform
Whilst no direct fossil identification beyond microfossil evidence is possible from drill core samples, the stratigraphic succession from which the Kel'tminskaya-1 core was extracted is known to host a  Gehling et al., 2014;Martin et al., 2000). The equivalent Ust'-Pinega Formation on the Onega River also hosts soft-bodied Swartpuntia, Vendoconularia triradiata and Ventogyrus (Ivantsov & Fedonkin, 2002;. Redox evolution of the unrestricted EEP (Figure 6b) margin has been evaluated through application of Fe speciation, alongside δ 34 S py , δ 13 C and major element analyses of the Kel'tminskaya 1 drillcore (Johnston et al., 2012). The lower 2,000 m of the drillcore comprises the Vapol' Formation, with age constraint dictated by microfossil biostratigraphy of basal Vychegda (Vorob'eva et al., 2009) and U-Pb dating within the upper and lower Redkino Formation (Grazhdankin, 2003;Martin et al., 2000). The lower boundary of the fossiliferous Redkino Formation has since been re-interpreted by Grazhdankin, Marusin, Meert, Krupenin, and Maslov (2011) to be in the range 570-600 Ma, thereby placing it in approximate stratigraphic equivalence with the Drook Formation of Newfoundland. The underlying Vychegda Formation occupied a mid-shelf depositional environment and iron speciation measurements suggest predominantly oxygenated conditions during deposition with minor incursions of ferruginous anoxia (Johnston et al., 2012). Additional δ 34 S py data from the Ediacaran Vychegda Formation reveal generally depleted values, supporting the existence of an oxidative water column sulphur cycle (Johnston et al., 2012). The overlying 1,000 m siliciclastic succession of the Redkino and Kotlin Formations exhibits exclusive oxia, recorded by low Fe HR /Fe T . Higher δ 34 S py observed in deposits of the Redkino and Kotlin successions is considered to be a consequence of localised sedimentary pore water sulphate limitation (Johnston et al., 2012). These data have been interpreted to represent a shift towards oxygen stability of the local environment reflected in reduced variation of Fe HR /Fe T about the mean up-section (Johnston et al., 2012).
This is comparable to deep-water sediments of Avalonia which may indicate that oxygen concentration allowed for effective suppression of anoxia in the local water column from as early as 570-600 Ma (Canfield et al., 2007).

| Arroyo del Soldado Group
Outcropping Ediacaran stratigraphy of the Río de la Plata craton in Uruguay constitute the Arroyo del Soldado Group, which was unconformably deposited over Archean and Proterozoic units of the Nico Perez Terrane and includes the Barriga Negra, Yerbal,
Fossils of the Arroyo del Soldado Group include two distinct acritarch assemblages, in addition to the biomineralising probable animal Cloudina (Gaucher, 2000). Examples of in situ haematised C. riemkeae predominantly outcrop in upper Yerbal Formation siltstones and reworked fragments of C. riemkeae have been reported from within storm deposits of the overlying Polanco Formation (Gaucher, 2000;Gaucher & Poiré, 2009).
Iron speciation analyses of the Yerbal and Polanco Formations ( Figure 6c) indicate the predominance of anoxic ferruginous water column conditions with some evidence for occasional deposition in oxic waters (Frei, Gaucher, Stolper, & Canfield, 2013). Additional RSE and Ce/Ce* anomaly data of the Arroyo del Soldado Group are consistent with iron speciation data and are interpreted to indicate a water column with suboxic to anoxic non-sulphidic depths, overlain by an oxygenated surface layer Pecoits, 2010).

| Palaeogeographic controls on local redox
The  (Li, Planavsky, et al., 2015;Och et al., 2015). (c) Dengying to early Cambrian Formations (<551 Ma to <520 Ma): Deposition during continued eustatic sea-level rise resulted in reduced restriction of proximal intrashelf basins. Whilst euxinia continued to intermittently characterise platform and slope settings, there is some evidence to suggest deepening of the chemocline with first appearance of episodic oxia recorded in deposits of the upper Niutitang Formation at basinal Longbizui. (d) Kuibis Subgroup (550 Ma to <547 Ma): Deposits of the Witputs sub-basin initially record heterogeneous redox with dominantly anoxic ferruginous conditions followed by subsequent reef growth confined to transgressive systems tract of the Zaris sub-basin and skeletal metazoan ecology influenced by incursions of anoxia during shoaling of the chemocline. Manganous zone suggested after regional Ce/Ce* study of . ( Sperling, Carbone, et al., 2015). Cooling of surface water in this region may have stimulated density-induced deep-water development in the aftermath of the regional Gaskiers deglaciation (Laflamme et al., 2013;. Under these conditions, oxygen-rich water from the wellmixed surface ocean would be drawn to depth in a similar manner to present-day North Atlantic deep-water formation (Broecker, 1997).
Importantly, deep-water formation in the modern ocean is seen to be location specific and dependent on factors including local bathymetry, freshwater input and evaporation (Broecker, 1997;Bruce, 1983).
Whilst the key conditions conducive to deep-water formation con- In summary, whilst a gradual increase in global ocean oxygenation may have occurred during the Ediacaran Period (e.g., Chen, Ling, et al., 2015;Kendall et al., 2015), cratonic positioning likely influenced mechanisms for local oxygenation resulting in regions characterised by continued dominance of anoxia.  . In this scenario, initially more oxidising conditions supported soft-bodied and bioturbating communities, whilst subsequently enhanced continental weathering associated with elevated water column alkalinity and carbonate saturation not only led to reinforced water column redox stratification but may, alongside the advent of predation, have promoted the necessity for biocalcifying communities inhabiting shallow oxic waters through instigating the requirement for a mode of Ca removal from newly developed circulatory systems .

| The distribution of biomineralising biota
Shallow, inner-ramp carbonate and siliciclastic sediments of the Omkyk and lower Hoogland members of the Nama Group host Cloudina and Namacalathus within transient oxic/manganous/ferruginous conditions attributed to the short-lived incursion of anoxic deeper water ( Figure 7d) . However, water column anoxia induced via primary productivity and organic matter oxidation, as may have occurred in shallowest settings influenced by riverine nutrient influx, results in the by-product of substantial dissolved CO 2 and lowered pH conducive to enhanced CaCO 3 dissolution (Sperling, Knoll, et al., 2015). Therefore, under lowered pO 2 of the Ediacaran Period, long-lived productivity-induced anoxia as seen in modern OMZs is unlikely to have supported immobile, benthic, strongly-calcifying organisms such as Namapoikia. Intervals associated with protracted oxia both throughout, above and below biomineralising fossil horizons, in shallow to mid-ramp settings are accompanied by thicker walled and larger individuals reflective of the ease of metabolically demanding calcification within these stable oxic, oligotrophic, carbonate-saturated waters .

Metazoan reefs
The only documented occurrence of active reef-building by C. hartmannae and C. riemkeae alongside dyke-dwelling Namapoikia is recorded within mid-ramp positions typified by inferred persistent oxygenation of the Nama Group, Namibia (Penny et al., 2014;. Prerequisite conditions for effective, long-lived reef building today include stable substrate, low sediment influx, readily available Ca 2+ and CO − 3 ions and relatively well oxygenated conditions above the contemporaneous carbonate compensation depth (James & Jones, 2015). Under these ideal conditions Ediacaran Cloudina reefs grew and there is some evidence for confinement of reef growth to exclusively oxic intervals of the mid-ramp at Driedoornvlagte. Establishment of a pelagic-benthic link was likely enabled in such ecosystems through proficient suspension feeding by Cloudina and Namacalathus. This may have resulted in rapid and effective redistribution of organic matter to depth thereby supporting a model of biological ventilation of shallow and mid-depth environments via reduced oxygen consumption in surface waters towards the end of the Ediacaran Period (Butterfield, 2009;Lenton et al., 2014;Meyer et al., 2016).

| Soft-bodied macrobiota
Whilst some regions indicate a distinct trend towards local oxygenation accompanied by first fossil appearance of complex multicellular eukaryotes (e.g., Avalonia, the EEP and the Kalahari Craton) assemblages including the Avalon assemblage of the Laurentian passive margin, and Lantian and Miaohe biota of the Yangtze Block, are seen to have occupied dominantly anoxic ferruginous bottom water environments. Therefore, if dissolved oxygen above a threshold concentration was necessary to support such communities, benthic colonisation occurred during short-lived oxic episodes indistinguishable by current proxy methods.
Potential benefits of inhabiting an environment prone to dissolved oxygen depletion may be associated with nutrient demand.
For instance, upwelling or continental nutrient delivery and elevated primary production might conceivably have favoured organisms such as rangeomorphs which may have fed via osmotrophic absorption of labile DOC or active fluid endocytosis during periods of less active vertical mixing (Laflamme, Xiao, & Kowalewski, 2009).
With the exception of Eoandromeda which is recorded from ferruginous deposits of the Miaohe member, soft-bodied fossils considered to represent probable Metazoa including Thectardis and Kimberella have so far been noted only from stratigraphic sections known to have been deposited beneath oxygenated bottom waters. This may be considered to support the inference of relatively high physiological oxygen demand suggested for motile Kimberella (Sperling, Knoll, et al., 2015).

| Benthic sulphide stress: Yangtze Block
Semi-restricted intrashelf basins of the Yangtze Block were prone to protracted anoxic intervals typified by free water column hydrogen sulphide and quantitative trace metal drawdown (Li et al., 2010;Och et al., 2015;Sahoo et al., 2016). Whilst localised hypoxia in the modern ocean reduces macrofaunal biodiversity, the additional deleterious effects of elevated H 2 S aq on aerobic benthic communities is also well documented, with accelerated mortality during anoxic periods in the presence of H 2 S aq due to the enzymatic disruption of oxygen aqueous H 2 S, body plans which exhibit higher surface area to volume ratios are expected to suffer greater losses (Sperling, Knoll, et al., 2015).
Extensive benthic sulphide stress of Yangtze Block environments following transgression and flooding of intrashelf basins during deposition of Doushantuo Member IV is likely to have been severe, with long-lived "patchy" water column euxinia persisting into the early Cambrian when sessile benthic communities were likely restricted to inner shelf platform settings (Och et al., 2015). Despite the extreme environmental conditions, a diverse macrofaunal assemblage developed within basins of the Yangtze Block. Repetitive flooding by reducing waters may have been accompanied by repeated local community die-off, this may also have irrigated shallower depths through nutrient recycling from the deeper water environment in preparation for recolonisation during subsequent ventilation. It may be reasonable to assume that, although we know little about the physiological requirements of the Miaohe biota, fossil representatives of probable animals such as Eoandromeda may have suffered considerably under anoxic conditions with elevated H 2 S. Interestingly, iron speciation data of lower fossiliferous units of the Doushantuo member IV at the type locality of Miaohe (Hubei) show a dominance of ferruginous anoxia, with elevated proportions of pyrite indicative of euxinia restricted to overlying shales devoid of fossils (Li, Planavsky, et al., 2015). Similar high-resolution geochemical sampling may benefit physiological discussions of the fossiliferous Lantian Formation. Whilst the early Cambrian saw continued euxinia in environments of the Yangtze Block, this may have gradually given way to a less stressed inner shelf environment with elevated water column H 2 S largely restricted to the lower slope and basin Och et al., 2015;Wang et al., 2012;Yuan et al., 2014).

| CONCLUSIONS
Through development and application of varied geochemical proxies, the past decade has witnessed a revolution in our understanding of global and local redox heterogeneity which accompanied the evolution of ecosystems containing potential candidates for the earliest animals. Although earlier studies suggested that oxygen stabilisation may have characterised the global ocean as early as the late Ediacaran (Canfield et al., 2007;Johnston et al., 2012), continued local redox heterogeneity is evident from multiple shallow to deep marine environments well into the Cambrian. Whilst increase beyond a low pO 2 concentration threshold within the shallow marine environment may have enabled a step-change in complexity of middle Ediacaran ecosystems, in some areas this facultative threshold may have been surpassed only transiently. Currently available bulk rock redox proxy methods are unable to resolve short-term oxygenation which may have permitted opportunistic colonisation of the substrate by organisms with higher oxygen demand. However, high-resolution geochemical sampling conducted in a comprehensive palaeontological and palaeoecological framework, despite the inherent complications associated with taphonomic and facies biases, enables appreciation of changing provincial ecosystem structure together with the extent of accompanying local water column oxygenation through the Ediacaran Sperling, Carbone, et al., 2015). The importance of placing such studies in relative sea-level and palaeogeographic framework will enable appreciation of the nuances of marine redox heterogeneity which characterised environments on the kilometre scale, similar to those that exist in modern shelf-tobasin environments.
Whilst a decrease in the volumetric proportion of euxinic middepths during the Cryogenian is suggested to have removed a toxic barrier to evolutionary diversification of aerobic eukaryotes , the oxygen concentration of shallow waters was likely sufficient to support evolution of the earliest Metazoa represented by crown group demosponges in the Cryogenian . Subsequent environments of the Ediacaran witnessed the proliferation of probable animals which appear to have opportunistically colonised habitable substrate. This may, in part, have been defined by the availability of dissolved oxygen under oxic/ dysoxic conditions, with the greatest diversity permitted in environments subject to effective oxygenation. Once developmental barriers to biomineralisation were surpassed, local ecosystem feedback associated with biological ventilation of the water column may have initiated towards the end of the Ediacaran and incorporated efficient filtration of rapidly sinking, large organic particles via suspension feeding within high surface area to volume ratio, multitiered reefal ecosystems, alongside substrate bioturbation and efficient phosphorus retention (Lenton et al., 2014;Penny et al., 2014;Wood & Curtis, 2015).
The Ediacaran Earth saw extensive cratonic migration during the formation of Gondwana and associated alterations in available niche space. The geographic positioning of palaeoenvironments within the global ocean, alongside their relative palaeodepth, likely influenced mechanisms for changing redox in open shelf environments. When viewed together, Ediacaran geochemical, palaeogeographic and palaeoenvironmental data suggest local ecosystem dynamics constrained by parameters including dissolved oxygen availability, nutrient provision, stable substrate for colonisation and the evolution of predation.
Continued environmental proxy development and utilisation in high-resolution biostratigraphic and palaeoecological studies across shelf-to-basin transects may aid in clarification of compelling issues associated with Ediacaran ecosystem development. Particularly, the development of novel geochemical proxies which record instantaneous transitions sensitive to minor changes in dissolved oxygen concentration may shed light on the necessary oxygen requirements for shallow marine colonisation at ecologically meaningful timescales in the earliest calcifying invertebrate communities.