A diverse Ediacara assemblage survived under low-oxygen conditions

The study addresses whether Ediacaran macro-organisms, including rangeomorphs and related taxa, required oxygen and had metazoan affinities with aerobic metabolisms, or whether they belonged to a distinct group potentially reliant on chemoautotrophy or symbiosis. Previous hypotheses linked the evolution and demise of Ediacaran taxa to changing oxygen levels along continental shelves, whereas alternative views suggest independence from oxygen availability. To test the redox sensitivity and metabolic implications for these organisms, the authors couple local and global redox proxies measured directly in carbonate strata that preserve in situ Ediacaran fossils from the Olenek Uplift, arctic Siberia. Local redox is constrained using carbonate rare-earth element patterns and cerium anomalies (Ce/Ce*), while global marine redox (extent of euxinia) is inferred from uranium isotopes (δ238U) in coeval carbonates. By integrating geochemistry with fossil occurrences in the same succession and refining chronostratigraphy with U–Pb zircon ages, the study aims to determine whether diverse Ediacaran assemblages lived under anoxic conditions and to place their ecology within the dynamic Ediacaran–Cambrian redox landscape.

The paper synthesizes multiple lines of prior work: (1) Ediacaran biology and ecology, including interpretations of rangeomorph affinities as stem eumetazoans and proposed feeding/metabolic strategies ranging from osmotrophy, chemosymbiosis with sulfur-oxidizing bacteria, to extracellular digestion in modular fronds. Other studies posit aerobic metabolism and link Ediacaran diversification and extinction to changing oxygenation states. (2) Ediacaran ocean redox heterogeneity through time and across facies, with sections showing variable proportions of oxic vs anoxic seafloor conditions and redox gradients controlling organism distributions. (3) Chemostratigraphy using carbon (δ13C) and sulfur (δ34S) isotopes, including positive excursions and the occurrence of 'superheavy pyrite' (δ34Spyr ≥ δ34SCAS) in multiple post-Shuram successions (South China, Namibia, Oman, NW Canada, NW China), interpreted as reflecting shifts in ocean redox and sulfate budgets. (4) Constraints from redox proxies: Ce anomalies in carbonates as indicators of local oxygenation relative to Mn redoxclines, and U isotopes in carbonates as a global proxy for the extent of euxinia, with modern baseline δ238Useawater ~ −0.39‰ and carbonate offsets (~ −0.15‰), and much lighter values in ancient intervals indicating expanded euxinic sinks. (5) Regional and global stratigraphic correlations using δ13C, fossil biostratigraphy (e.g., small shelly fossils, Treptichnus pedum), and high-precision radiometric dates across Ediacaran–Cambrian sections in Siberia, South China, Namibia, and elsewhere.

• Field area and stratigraphy: Studied the Olenek Uplift (NE Siberia) encompassing Maastakh (peritidal dolostone), Khatyspyt (bituminous limestone, fossiliferous with Ediacaran nodules and compressions), Turkut (dolostone/limestone with breccias), Syhargalakh (siliciclastics/volcanics), and Mattaia (mixed siliciclastics and allodapic limestones) formations. Chronostratigraphy refined using U–Pb zircon ages (detrital and ash beds).
• Sampling: 56 carbonate-rich samples (>75 wt.% carbonate) spanning 497.5 m of composite section from the Khorbusuonka and Olenek river areas (collected 2009–2010). Hand samples cut and powdered in Russia; analyzed in the USA. Selected based on prior geochemical screening.
• Diagenetic assessment: Petrography (micrite/microspar textures; organic laminations in Khatyspyt; dolomicrite/dolomicrosparite in Turkut; local coarsely recrystallized dolomite spar). Geochemical screens included Mn/Sr ratios (<10 indicating good preservation; coarse upper Turkut dolomites more altered), oxygen isotopes (δ18O between −0.65‰ and +8.38‰; heavy values consistent with evaporative conditions), lack of covariation between δ13C and δ18O, and no correlation of Mn/Sr or δ18O with δ238U or Ce anomaly.
• Major and trace elements: Sequential digestions in HNO3; supernatants analyzed by Q-ICP-MS (Thermo iCAP) at ASU for major/trace elements; U concentrations by Element-2 ICP-MS at UMD. Precision typically <6% RSD for elements; U better than 3%.
• Rare-earth element protocol: Multi-step leaching to minimize detrital contamination and target seawater-like REE signatures. For calcite: discard first 20% dissolution, analyze subsequent 40% fraction; for dolomite: analyze first 20% dissolution fraction. REE+Y concentrations normalized to PAAS; Y/Ho ratios used to screen for authigenic signals (Y/Ho >36). Ce anomaly calculated as Ce/Ce* = [Ce]/([Pr]2/[Nd]) using PAAS-normalized concentrations.
• Uranium isotopes (δ238U): Carbonate δ238U measured on carbonates; interpreted relative to modern seawater and carbonate offsets (modern Bahamas median carbonate δ238U ≈ −0.15‰). Analytical uncertainty ~±0.08‰ (2 s.d.) based on replicates. Interpretation framework considers preferential removal of heavy 238U into euxinic sediments, leaving seawater lighter; diagenetic and local redox offsets evaluated with reference to modern calibrations.
• Correlative chemostratigraphy: Integrated δ13C records for regional/global correlation to South China (Dengying Formation, Gaojiashan Member), and considered δ34S of pyrite from Khatyspyt residues to place sulfur cycle anomalies in context of redox shifts.

• Local redox from Ce anomalies: Among samples with Y/Ho >36, Ce/Ce* values range 0.99–1.55 (median 1.30), with no negative Ce anomalies recorded across formations, indicating persistent anoxic water-column conditions during deposition throughout the Maastakh, Khatyspyt, Turkut, and Mattaia formations. Mn/Fe ratios are low overall (median 0.09), suggesting ferruginous rather than manganous conditions; formation medians: Maastakh, Khatyspyt, and Turkut <0.11; Mattaia elevated at 0.28 (less reducing, possibly manganous).
• Global redox from U isotopes: Median carbonate δ238U values by formation are Maastakh −0.58‰, Khatyspyt −0.64‰, Turkut −0.57‰, Mattaia −0.53‰ (overall median −0.58‰; ±0.08‰, 2 s.d.). These values are significantly lighter than modern carbonates (Bahamas median ≈ −0.15‰), indicating widespread marine euxinia in the terminal Ediacaran and persisting into Cambrian Stage 2. Olenek values align with coeval sections from Namibia (median −0.77‰) and South China (median −0.87‰), with observed offsets attributable to local redox conditions affecting carbonate-seawater fractionation.
• Paleontological context: In situ diverse Ediacaran macrofossils (e.g., Nenoxites curvus, Charnia masoni, Khatyspytia grandis, macrophytes, holdfast structures) occur within Khatyspyt carbonates that record anoxic local conditions, demonstrating coexistence of diverse Ediacaran assemblages with low-oxygen environments.
• Chemostratigraphic integration: δ13C trends correlate Olenek formations to South China equivalents (e.g., positive excursions comparable to Dengying/Gaojiashan; negative shifts near or older than BACE). Pyrite δ34S in Khatyspyt shows a step from −20‰ to +30‰, paralleling post-Shuram 'superheavy pyrite' anomalies globally, interpreted here within a framework of expanded euxinia and sulfate distillation.
• Chronostratigraphic constraints: A maximum depositional age of 550 Ma near the top of the Maastakh constrains the overlying Khatyspyt as younger than 550 Ma (terminal Ediacaran). A tuff in Syhargalakh yields U–Pb zircon age 542.8 ± 1.3 Ma (reinterpreted), and an ash above the Mattaia platform yields 529.56 ± 0.24 Ma, constraining SSF zones and the onset of the Cambrian explosion.

The co-occurrence of in situ, diverse Ediacaran macrofossils with carbonate Ce/Ce* values ≥1 indicates these organisms inhabited environments lacking evidence for Mn oxidation and thus with oxygen concentrations below submicromolar thresholds that typically generate negative Ce anomalies. This suggests at least some Ediacaran taxa tolerated, or were adapted to, low-oxygen to anoxic conditions. Possible interpretations include: (1) facultative anaerobic lifestyles or specialized metabolic adaptations (e.g., hydrogenosome-like organelles) potentially supported by chemoautotrophic symbionts; and/or (2) colonization during brief oxygenation episodes not fully captured by time-averaged carbonate redox proxies, analogous to episodic oxygenation recorded in otherwise anoxic successions and rapid ecological recovery observed in modern hypoxic systems. Global δ238U records from multiple terminal Ediacaran basins converge on light values, indicating expanded euxinia along productive margins during Ediacaran diversification. Despite widespread euxinia, Ediacaran communities persisted, implying habitation of oxic refugia above anoxic waters, tolerance of low oxygen, or both. Integration of δ13C, δ34S, and δ238U supports a post-Shuram interval characterized by expanded anoxia/euxinia, enhanced organic carbon burial (positive δ13C), and sulfur cycle distillation producing 'superheavy pyrite'. Redox states oscillated through the Ediacaran–Cambrian transition, with evidence for re-oxygenation across the boundary and renewed euxinia during Cambrian Stage 2. These fluctuations may have modulated the tempo of early animal diversification or, alternatively, indicate that redox exerted only modest control on early complex life relative to other ecological and evolutionary factors.

By directly pairing local (Ce anomalies) and global (U isotopes) redox proxies with in situ Ediacaran fossils in Siberian carbonates, the study shows that a diverse Ediacara assemblage inhabited environments that were frequently anoxic, while global oceans experienced expanded euxinia during the terminal Ediacaran and into Cambrian Stage 2. This integrated evidence strongly suggests that some Ediacaran macrobiota were tolerant of low oxygen and potentially capable of facultatively anaerobic metabolisms, possibly leveraging chemolithoautotrophy directly or via symbionts. Alternatively, brief, unrecorded oxygenation pulses may have allowed episodic colonization. The work refines terminal Ediacaran chronostratigraphy (placing the Khatyspyt Formation younger than 550 Ma), aligns redox-sensitive isotopic records across basins, and highlights an oscillatory redox landscape tied to major evolutionary events. Future research should aim to: (1) obtain higher temporal resolution of local redox proxies to resolve short-lived oxygenation events; (2) expand paired fossil–proxy datasets across additional basins and facies; (3) integrate biomarker, microfossil, and genomic constraints to test hypotheses of symbiosis and metabolic strategies; and (4) refine modeling of coupled C–S–U cycles to quantify feedbacks between productivity, redox, and early animal ecosystems.

• Time averaging and proxy response: Carbonate Ce anomalies may integrate over durations longer than ecological timescales, potentially missing brief oxygenation events that could support benthic communities.
• Local vs global signals: δ238U reflects global euxinic sink size but can be offset by local pore-water redox or basin hydrography; interpreting absolute seawater values requires assumptions about fractionation and local conditions.
• Diagenetic uncertainties: Although multiple screens suggest good preservation, some dolostones (e.g., coarse dolomite spar in upper Turkut) show signs of recrystallization that could subtly affect trace element distributions.
• Facies bias: Ediacaran soft-bodied fossils are rarely preserved in carbonates; results may not represent the full range of Ediacaran environments or taxa preserved primarily in sandstones.
• Metabolic inferences: Geochemical proxies constrain redox conditions but cannot directly determine organismal metabolism or symbioses; interpretations remain inferential.