The paleoredox context of early eukaryotic evolution: insights from the Tonian Mackenzie Mountains Supergroup, Canada

Tonian (ca. 1000–720 Ma) marine environments are hypothesised to have experienced major redox changes coinciding with the evolution and diversification of multicellular eukaryotes. In particular, the earliest Tonian stratigraphic record features the colonisation of benthic habitats by multicellular macroscopic algae, which would have been powerful ecosystem engineers that contributed to the oxygenation of the oceans and the reorganisation of biogeochemical cycles. However, the paleoredox context of this expansion of macroalgal habitats in Tonian nearshore marine environments remains uncertain due to limited well‐preserved fossils and stratigraphy. As such, the interdependent relationship between early complex life and ocean redox state is unclear. An assemblage of macrofossils including the chlorophyte macroalga Archaeochaeta guncho was recently discovered in the lower Mackenzie Mountains Supergroup in Yukon (Canada), which archives marine sedimentation from ca. 950–775 Ma, permitting investigation into environmental evolution coincident with eukaryotic ecosystem evolution and expansion. Here we present multi‐proxy geochemical data from the lower Mackenzie Mountains Supergroup to constrain the paleoredox environment within which these large benthic macroalgae thrived. Two transects show evidence for basin‐wide anoxic (ferruginous) oceanic conditions (i.e., high FeHR/FeT, low Fepy/FeHR), with muted redox‐sensitive trace metal enrichments and possible seasonal variability. However, the weathering of sulfide minerals in the studied samples may obscure geochemical signatures of euxinic conditions. These results suggest that macroalgae colonized shallow environments in an ocean that remained dominantly anoxic with limited evidence for oxygenation until ca. 850 Ma. Collectively, these geochemical results provide novel insights into the environmental conditions surrounding the evolution and expansion of benthic macroalgae and the eventual dominance of oxygenated oceanic conditions required for the later emergence of animals.

Fossil evidence supports the evolution and ecological expansion of complex, benthic macroscopic algae in the Tonian (Maloney et al., 2021;Tang et al., 2020), following a protracted middle Proterozoic interval for which the eukaryotic fossil record remains sparse and ambiguous (Cohen & Kodner, 2021;Cole et al., 2020;Knoll & Nowak, 2017).Benthic macroalgae play an important role in shaping modern nearshore marine ecosystems and have profoundly affected local carbon and nutrient cycling throughout Earth's history.However, the drivers of this apparent increase in eukaryotic complexity and expansion of habitable environments continue to be debated, and the role of environmental change (e.g., oxygenation, nutrient availability) in driving these biological innovations is unclear.Furthermore, the cause-and-effect relationship between the evolution of complex life and these purported redox transformations remains elusive, with continued debate over the timing and significance of a potential stepwise increase in O 2 (see reviews in Cole et al., 2020;Lyons et al., 2021).
A recent empirical study suggests that the early diversification of microbial eukaryotes could have been facilitated by even a small rise in atmospheric oxygen (2%-3% of modern, Mills et al., 2023).
Alternatively, oxygen may not have been a critical factor if atmospheric O 2 was already above this threshold when crown-group eukaryotes first appeared.
The fossiliferous strata of the Mackenzie Mountains Supergroup (MMS; Yukon and Northwest Territories) allow for the investigation of ca.1000-800 Ma redox conditions coincident with multicellular eukaryotic evolution.In particular, MMS features a diverse macroalgal assemblage including large (cm-scale) green macroalgae Archaeochaeta, found in the Hematite Creek Group (Maloney et al., 2023), the carbonaceous macrofossils Chuaria and Tawuia (Hofmann, 1985;Hofmann & Aitken, 1979) and purported poriferan body fossils (Turner, 2021) within reefal facies in the Little Dal Group.Previous redox studies on the Cryogenian to Ediacaran Windermere Supergroup in the Wernecke Mountains and equivalent strata in the Mackenzie Mountains have provided evidence for a generally anoxic, ferruginous basin (Johnston et al., 2013;Miller et al., 2017;Shen et al., 2008;Sperling et al., 2016).Studies of sections in northwestern Canada that host large and structurally complex Ediacara biota and metazoan traces (Carbone et al., 2015;Narbonne et al., 2014) suggest that the appearance of macrofossils does not coincide with clear evidence for a significant marine increase in O 2 levels (Johnston et al., 2013;Miller et al., 2017;Sperling et al., 2016).Iron paleoredox studies (iron speciation and iron isotopes) have detected a possible redox change to oxygenated surface waters in the late Tonian Fifteenmile Group in the Ogilvie Mountains (Gibson et al., 2020;Sperling et al., 2013) and equivalent Tatonduk inlier of Alaska (Sperling et al., 2013), which has been stratigraphically correlated with the MMS in the Wernecke and Mackenzie Mountains (Halverson et al., 2012;Macdonald et al., 2012).It remains unclear if this change in O 2 in surface waters at ca. 800 Ma is a regional trend or whether evidence of a stratified water column can be found in other inliers (such as the Wernecke Mountains) and older successions (Hematite Creek Group), suggesting a more widespread phenomenon.The MMS in the Wernecke Mountains records clear evidence of changes in the global biosphere through a diverse fossil record and represents a promising target for understanding Tonian marine ecosystems.
Here, we present results of a multi-proxy geochemical investigation that includes iron speciation data, redox-sensitive trace element abundances and Nd-Sm data from ca. 1000-850 Ma shales in the lower MMS in the Wernecke Mountains, including rocks from which macroalgal fossils have been reported (Maloney et al., 2021(Maloney et al., , 2023)).
Geochemical characterisation of these fossiliferous sections aids in reconstructing the paleoenvironmental conditions in which these primary producers diversified and provides insight into the relationship between eukaryotic expansion and environmental conditions during this critical transition in Earth's history.
The MMS sediments and other coeval strata in northwestern Canada were accommodated by episodic extension resulting in an intracratonic rift basin (Macdonald et al., 2012).The MMS in the Wernecke Mountains consists of the Hematite Creek Group at its base, which transitional upwards into the Katherine Group.The contact between the Katherine and the Little Dal Group is also transitional, but the latter is heavily truncated in the Wernecke Mountains such that only the lower part of the Stone Knife Formation is preserved (Figures 1 and 2; Macdonald et al., 2018).The Hematite Creek Group comprises, in ascending stratigraphic order, the Dolores Creek, Black Canyon Creek and Tarn Lake formations (Turner, 2011).The Dolores Creek Fm. is characterized by bright orange-weathering microbial dolomite with stromatolitic intervals and dark grey to black siltstone and shale.The Dolores Creek Fm.
is typically ~300 m thick; however, the section in the southern part of the exposure belt where the multicellular macroalgae were recovered extends to nearly 1 km in thickness (Maloney et al., 2021).
The informal lower Dolores Creek Fm. consists of ~600 m of shale and siltstone with minor debrites coarsening upward with increasing carbonate content including minor microbially laminated beds, blocks of stromatolites (olistoliths) and finally in-place stromatolite bioherms.The upper Dolores Creek Fm. includes shales and biostromes of columnar stromatolites interpreted to record a proximal, southward-prograding shelf margin over what is thought to represent a fault escarpment that formed in response to an extensional episode that initiated subsidence and formed the Hematite Creek Basin (Turner, 2011).The basin was filled as the stromatolites on the shelf margin prograded southward (in present coordinates) and shed debris.The fossiliferous part of the Dolores Creek Fm. is interpreted to record a shallowing-upward succession of upper slope to shelf margin deposits.The stromatolitic bioherms represent the photic zone where the macroalgae likely lived before transport and burial on the upper slope (Maloney et al., 2022).
The Katherine Group in the Wernecke Mountains is subdivided into seven informal units (K1-K7) of fluvial-deltaic sandstones and shales, which likely correspond to the seven formation-scale units of the Katherine Group as defined in the Mackenzie Mountains (Northwest Territories) (Long & Turner, 2014).These sandstone-and shale-dominated intervals are interpreted to represent alternating periods of deposition in braided-meandering rivers and shallow marine environments, respectively (Aitken et al., 1978;Long et al., 2008).However, only the Shattered Range (K5), McClure (K6) and Abraham Plains (K7) formations are exposed in the study area at SW Profeit.The Eduni Formation (K1) is recognized elsewhere in the Wernecke Inlier (Long & Turner, 2012).The Little Dal Group is confined to a small area near SW Profeit, where it is only ~250 m thick as compared to 2.0-2.5 km thick in the Mackenzie Mountains (Aitken, 1981;Halverson, 2006;Long et al., 2008;Turner, 2011;Turner & Long, 2012).This discrepancy is likely due to a combination of pre-Cryogenian uplift and folding related to the Corn Creek Orogeny (Thorkelson et al., 2005) and a deep unconformity, which places Cryogenian conglomerates atop the Stone Knife Formation at SW Profeit (Figure 1; Eisbacher, 1981;Macdonald et al., 2013Macdonald et al., , 2018)).

| Geochronology
The age of the Dolores Creek Fm. in the Wernecke Mountains is constrained by a direct depositional Re-Os isochron age of 898 ± 68 Ma from the upper Dolores Creek Formation (Maloney et al., 2021) and a maximum depositional detrital muscovite ( 40 Ar/ 39 Ar) age of 1033 ± 9 Ma (Thorkelson, 2000).These ages agree with a detrital zircon maximum depositional age ( 206 Pb/ 238 U) of ca.1000 Ma from presumed equivalent strata in the Hart River Inlier to the west (Rainbird et al., 1997).Detrital zircon ages of 1081 ± 2 Ma (Rainbird, Villeneuve, et al., 1996), and 1005 Ma ± 1 Ma (Leslie, 2009) have also been reported from the Katherine Group in the Mackenzie Mountains.
Based on the ages of detrital zircons, Katherine Group sandstones could have been fed by an extensive river system associated with the Grenville Orogen present around 1 Ga (Rainbird et al., 1997(Rainbird et al., , 2017)).
The minimum age of the MMS is provided by a U-Pb zircon Isotope Dilution-Thermal Ionization Mass Spectrometry (ID-TIMS) age of 775.10 ± 0.54 Ma on a diabase that crosscuts the units in the neighbouring Mackenzie Mountains (Milton et al., 2017).This diabase is considered part of the Gunbarrel magmatic event, which includes the Little Dal Basalt that caps the MMS in the Mackenzie Mountains (Jefferson & Parrish, 1989).Based on these collective dates and stratigraphic framework, the age of the MMS is constrained to ca. 1000-775 Ma, with the Dolores Creek fossils estimated to be ca.950-900 Ma (Maloney et al., 2021).

| RED OX PROXIE S
Multi-proxy redox framework studies are strengthened by precise geochronological constraints, and are most reliable when they demonstrate consistency between independent proxies (Raiswell et al., 2018;Raiswell & Canfield, 1998).Although multi-proxy approaches can lead to more complex results, they are necessary to develop robust interpretations and to avoid false signals, in particular in outcrop samples (Gibson et al., 2020;Raiswell et al., 2018).
In this study, we employed a combination of iron speciation and

| Iron speciation
Iron-based redox proxies are widely used to understand modern and ancient environmental redox conditions (Lyons & Severmann, 2006;Poulton & Canfeld, 2011;Raiswell et al., 2018).These methods are fundamentally based on observations in modern environments that show that highly reactive iron (Fe HR ), which refers to iron that is geochemically and biologically active during early diagenesis, is enriched when deposited in sediments under an anoxic water column (Poulton & Canfeld, 2011).The abundance of sulfide in the environment can also be estimated based on the extent to which highly reactive iron is converted to pyrite (Raiswell & Canfield, 1998).Poulton and Canfield (2005) developed a procedure to extract iron sequentially into operationally defined iron pools.These iron pools are defined based on their extraction method: Fe carb (carbonate-associated iron; e.g., siderite and ankerite), Fe ox1 (easily reducible oxides; e.g., ferrihydrite and lepidocrocite), Fe ox2 (reducible oxides, e.g., goethite, hematite and akaganéite), Fe mag (magnetite), Fe PRS (poorly reactive sheet silicate Fe), Fe py (pyrite Fe) and Fe U (unreactive silicate Fe).Highly reactive iron is the sum of Fe carb , Fe ox , Fe mag and Fe py (Poulton & Canfield, 2005).The ratio of highly reactive to total iron (Fe HR /Fe T ) provides insight into whether an ancient water column was oxic or anoxic based on the threshold value observed in modern environments: FeHR/FeT < 0.22 indicates oxic conditions, FeHR/FeT > 0.38 indicates possible anoxic water column, and 0.22 < FeHR/FeT > 0.38 is regarded as equivocal (Poulton, 2021;Raiswell & Canfield, 1998).
The degree of pyritisation (e.g., Fe py /Fe HR ratio; Raiswell et al., 1988;Canfield et al., 1992) can help determine whether an anoxic environment was euxinic (anoxic and sulfidic) or ferruginous (iron rich) (Poulton et al., 2004;Poulton & Canfeld, 2011).The ratio reflects the amount of Fe HR converted to Fe py , with this ratio tending to be higher in sediments deposited in euxinic environments.Euxinic environments are classified as those with Fe py /Fe HR > 0.8 and Fe HR / Fe T > 0.38 (Canfield et al., 2008;Poulton et al., 2004;Poulton & Canfeld, 2011).However, it is important to note that increased Fe py /Fe HR ratios are also common in oxic continental margins where sulfide accumulates in porewaters at depths (Raiswell et al., 2018).
As with all redox proxies, these thresholds should be critically examined during each study in new depositional environments and geologic settings (Raiswell et al., 2018) with consideration for the influence of diagenesis (Pasquier et al., 2022).
Iron speciation is influenced by several factors including depositional rates (Canfield et al., 1996), hydrothermal inputs (Raiswell et al., 2018), weathering (Ahm et al., 2017;Slotznick et al., 2020;Wei et al., 2021), and the iron content of source sediments (Clarkson et al., 2014).Fe HR itself is relatively immune to weathering since Fe 2+ phases become oxidized to insoluble Fe 3+ phases, which are still captured as highly reactive iron in the extractions (Canfield et al., 2008).However, weathering can make it challenging to differentiate deposition under euxinic (anoxic and sulfidic) versus ferruginous (anoxic and iron-rich) water columns as the original pyrite weathers to iron (oxyhydr)oxides, thus lowering Fe py /Fe HR while leaving Fe HR /Fe T broadly unchanged.Dilution of highly reactive Fe due to rapid sedimentation (e.g., by turbidites) can result in false oxic signals (Lyons & Severmann, 2006;Raiswell & Canfield, 1998), while false anoxic signals can occur in certain depositional environments, such as estuaries, where large amounts of iron oxides may be trapped (Poulton & Raiswell, 2002).

| Total iron enrichments
Based on the assumption that total iron will be enriched against a detrital baseline in anoxic environments due to iron shuttling (Lyons et al., 2003;Severmann et al., 2008;Werne et al., 2002), iron enrichments in sediments can also be assessed by considering the ratios of total iron-to-aluminum (Fe T /Al; Lyons et al., 2003;Raiswell et al., 2018) and titanium (Fe T /Ti; Werne et al., 2002).One challenge with the total iron-to-aluminum proxy is identifying the appropriate Fe T /Al baseline.A value of ~0.5 representing the average value in shales (Taylor & McLennan, 1985) is one option.However Fe T / Al studies on Paleozoic marine sediments (0.53 ± 0.11; Raiswell et al., 2008), modern marine sediments (0.55 ± 0.11; Clarkson et al., 2014) and soils (0.47 ± 0.30; Cole et al., 2017) suggest a range of average values and do not account for regional variability in source lithology.This variation highlights the importance of establishing a baseline for each geological setting when looking to define thresholds to interpret redox proxy data.Unusually low Fe T /Al values (mean = 0.34; Sperling et al., 2013) have been reported from the Fifteenmile Group (Gibson et al., 2020;Sperling et al., 2013), as well as younger strata of the Windermere Supergroup (Sperling et al., 2016).Here, we follow Gibson et al. (2020) who proposed a detrital baseline of Fe T /Al = 0.3 for early Neoproterozoic strata regionally in the Proterozoic inliers based on integrated data from previous studies by Sperling et al. (2013Sperling et al. ( , 2016)).

| Redox-sensitive trace elements
The processes that control the relative distribution of oxidising agents across depositional and diagenetic gradients can be investigated using redox-sensitive elements (Raiswell et al., 2018;Tribovillard et al., 2006).Little to no enrichment in Mo, V and U occurs in depositional settings with permanent or temporary exposure to oxygen (e.g., seasonal), while significant enrichments are observed in euxinic basins.Under reducing conditions in anoxic environments, Mo and V are enriched in shales because these elements become less soluble and form complexes with sulfur, or are scavenged by organic and inorganic particles (Tribovillard et al., 2006).Further, the greatest enrichments of these elements occur in reducing settings that are connected to a large, oxygenated body of water, which provides a reservoir of redox-sensitive trace elements (Algeo, 2004).
Redox-sensitive elements in shales can be enriched through authigenic or detrital sources (Bennett & Canfield, 2020;Brumsack, 2006;Cole et al., 2017;Van Der Weijden, 2002).As detrital redox-sensitive elements input is directly linked to the provenance of the sediments, it can be difficult to identify an effective detrital cut-off based simply on the average crustal composition.Cole et al. (2017) analysed over 4000 soil samples and demonstrated large variations between the previously accepted averages for redox-sensitive metals compared to detrital tracers and recommend using confidence intervals and element ratios that normalize for detrital influence (ex.Fe/Al, V/Al, U/ Th).On the contrary, Bennett and Canfield (2020) proposed that deriving threshold values from modern marine sediments in different depositional environments would be the most accurate method to determine trace metal enrichment.However, this method requires caution because settings analogous to the dominant anoxic conditions in the Proterozoic, including ferruginous basins, are not wellrepresented in modern analogues.The VUMoRe database (Bennett & Canfield, 2020) was compiled to identify potential thresholds that are calibrated using the geochemical behaviour of trace metals in modern sedimentary environments including continental margin upwelling zones, euxinic basins and oxic settings.We follow Bennett and Canfield (2020) using the TM/Al approach for trace metal normalisation where TM enrichment is calculated as: The TM enrichments quantified for the MMS in the Wernecke Mountains can then be compared to the threshold values for different redox settings (Table 1).Van Der Weijden (2002) identified limitations associated with this trace metal normalisation technique, including the possibility of introducing spurious correlations and that TM enrichment does not address the problem of the closure effect (i.e., induced correlations when there are limited variables; Rietjens, 1995), nor does it quantify contributions by other sediment components not associated with the detrital fraction.The influence of diagenesis on trace metal enrichments also requires further investigations to address how these influences vary between depositional settings (Bennett & Canfield, 2020).

| Interpretative framework
In our multi-proxy redox framework, samples are interpreted to have been deposited under an oxic water column if Fe HR /Fe T < 0.22, Fe T /Al < 0.30, and little to no enrichment in redox-sensitive trace elements is seen.We apply a threshold limit of Fe HR /Fe T > 0.38 (Poulton & Canfeld, 2011), Fe T /Al > 0.30 and measurable enrichment in redox-sensitive trace elements for deposition under an anoxic water column.Intermediate values are interpreted as ambiguous.
Samples with a high proportion of highly reactive iron that has been sulfidized (Fe py /Fe HR > 0.70) are interpreted to indicate deposition under a euxinic water column (Lyons & Severmann, 2006;Poulton & Canfeld, 2011).
Our interpretations consider the results with respect to any evidence of post-depositional alteration documented through analytical microscopy (e.g., reactive pyrite altered to pyrrhotite; Poulton et al., 2010;Poulton & Raiswell, 2002) and evidence of post-depositional Fe and S mobilisation (pyrite weathering; Ahm et al., 2017;Gibson et al., 2020;Raiswell et al., 2018;Slotznick et al., 2020) in our samples.Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were employed to examine textures and mineralogy in shale samples at the μm-scale.

| Sample collection and preparation
As part of the stratigraphic logging of MMS in the Wernecke Mountains of Yukon, Canada (Figures 1 and 2), fine-grained siliciclastic rock samples were excavated and collected every 4 m (where available) from two measured sections.A total of 46 shale samples were collected from seven logged stratigraphic sections.The first section was a transect of the Mackenzie Mountains Supergroup with samples from the Hematite Creek Group (Dolores Creek Fm. [n = 14] (1) TM enrichment = Trace metal concentration ( g ∕ g) Aluminum concentration (wt.% ) Correlations between stratigraphic sections in the Hematite Creek Group, Mackenzie Mountains Supergroup, Wernecke Mountains incuding the type sections for the Dolores Creek and Black Canyon Creek formations (Turner, 2011).Wavy red lines indicate an unconformity, and dashed red lines inferred correlations of formation boundaries.& Canfield, 2020).et al., 2015;Sperling et al., 2013).Neoproterozoic black shales from Svalbard were analyzed along with the samples in this study as quality control standards (Kunzmann et al., 2015).A Thermo Scientific iCAP 6000 series ICP-OES was used to analyse the leachates for each of the three extraction steps.Pyrite associated iron (Fe py ) was extracted using a chromium chloride distillation technique (Canfield et al., 1996).The amount of iron in pyrite was calculated stoichiometrically based on the assumption that all extracted sulfur was pyrite.The Fe py contents can be biased if there are high amounts of acid-volatile sulfur.However, previous studies on similar Neoproterozoic shales have not found evidence of acid-volatile sulfur (Kunzmann et al., 2015;Sperling et al., 2013).
Four samples were run with three replicates and 75% of the replicates were within 5% standard error while the remaining samples were within 15%.

| Trace elements, major elements and TOC
Major and trace elements were measured following a modified protocol from Kunzmann et al. (2015).To oxidize organic complexes and determine the loss on ignition, each sample (~2.5 g) was first weighed, combusted at 1000°C for 2 h, then weighed again.Approximately 0.5 g of combusted material was weighed into a Savillex™ Teflon beaker and taken up by 1 mL of HNO 3 7 N.Samples were then digested using the following acids: (1) 29 N HF at ≥80°C for 5 days, (2) aqua regia (3 mL 6 N HCl + 1 mL 7 N HNO 3 ) ≥80°C for 48 h, (3) reverse aqua regia (1 mL 6 N HCl + 3 mL 7 N HNO 3 ) and (4) HNO 3 3 N at ≥80°C for 3 h.Bulk rock total digest stock samples were diluted, and standards were prepared with 2% HNO 3 .A Thermo Finnigan iCAP Q ICP-MS was used to measure trace element abundances, and a Thermo Scientific iCAP 6000 series ICP-OES was used to measure major element abundances at McGill University.
Whole rock analysis was performed by ACTlabs to quantify major (oxide) elements using Fusion XRF (Norrish & Hutton, 1969).Major element concentrations were calculated in percent weight oxide using oxide alpha-influence coefficients to account for matrix effects, and  2) groups.
Fe HR /Fe T in the MMS ranging from 0.10 to 0.69 (mean = 0.34).
Thirteen samples (28%) show characteristically oxic Fe HR /Fe T ratios <0.22, and 18 (39%) samples exhibit anoxic Fe HR /Fe T ratios >0.38. 3) for this basin from this study (see redox framework).(d) Ratio of highly reactive iron to total iron (Fe HR /Fe T ).Samples below a lower threshold suggests deposition under oxic conditions (Fe HR /Fe T < 0.22) and samples above a higher threshold indicates deposition under anoxic conditions (Fe HR / Fe T > 0.38) (Poulton & Canfield, 2005;Poulton & Raiswell, 2002;Raiswell & Canfield, 1998).The samples in the grey area between 0.22 and 0.38 remain ambiguous where samples could have been deposited under oxic or anoxic conditions (Poulton & Canfeld, 2011;Sperling et al., 2013).(e) Ratio of pyrite iron to highly reactive iron (Fe py /Fe HR ).Samples with ratios >0.8 reflect deposition within a euxinic water column (Canfield et al., 2008;Poulton et al., 2004;Poulton & Canfeld, 2011).(f) Ratio of iron oxy(hydr)oxide to total iron (Fe ox /Fe T ).(g-i) Bulk Mo, V and U contents in ppm (black circles) and calculated trace metal enrichments in ppm/wt.%normalized to Al (red circles, (Bennett & Canfield, 2020)).Black lines represent thresholds for anoxia and red lines are trace metal enrichment thresholds for anoxia.(j) Preliminary interpretation of redox column where green is ferruginous, light green is possibly ferruginous, blue is oxic and grey remains unconstrained.See the redox proxy framework for a detailed description of interpretations.
reactive iron in each sample is in the form of Fe (III) oxy(hydr)oxides, making up an average of 69% based on Fe ox /Fe HR values.

| Redox-sensitive trace elements and total organic carbon
Redox-sensitive trace elements are normalised to their average upper crustal values and total organic carbon contents (TOC;

| Petrographic and SEM analyses
Petrographic analyses of the MMS siliciclastic rocks indicate a range of sedimentary microstructures including dark wavy laminations and "domes" attributed to a microbial influence (see Appendix S1).They show limited to no pyrite in thin section.
Additional imaging using SEM-EDS targeted three thin sections and corresponding thick sections.One sample had well-defined pyrite (Fe p y/Fe HR = 0.19) disseminated throughout, although most pyrite crystals were small (e.g., ~4 to 50 microns; Figure 8a-f).
Framboids were visible in thin section, as were likely framboid pluck-out structures (Figure 8c,d,f).In thick sections, the framboids were detectable while pluck-out structures were not visible.Evidence for iron (oxyhydr)oxide pseudomorphs after pyrite
To address the influence of post-depositional alteration within our samples, we conducted petrographic and analytical microscopy analyses.Samples were carefully selected to document textures in samples with a variety of Fe py content (ranging from 0 to 2217 ppm).Samples from the Dolores Creek Fm. retained primary pyrite (e.g., cubic to framboidal structures with Fe and S) with more evidence of pyrite alteration in samples from the southern part of the basin (e.g., SW Profeit compared to north of Tarn Lake; Figures 8-10).The samples with the highest preserved pyrite content showed limited to no evidence of alteration.
However, post-depositional alteration of pyrite to iron oxyhydroxides was documented in other samples, where pyrite pseudomorph "ghosts" were observed along with some primary pyrite.
These pyrite "ghosts" maintain their original crystal structure and can be identified based on their composition (e.g., iron-rich, lacking sulfur).Similar observations of framboidal pyrite ghosts have been found in outcrop samples from the Fifteenmile Group (Gibson et al., 2020).These findings align with experimental studies by Mahoney et al. (2019) that simulated oxidative weathering in shales and found the influence of the weathering was negligible on Fe HR /FeT (difference of <<0.03%), but significant for Fe py /Fe HR (differing up to 32.5%).Since original textures can be destroyed, the documented pyrite ghosts represent only a minimum qualitative gauge of the pyrite that was lost.However, abundant pyrite does not necessarily support an euxinic interpretation for the water column during deposition; for example, Long Island Sound has highly sulfidic pore waters beneath the oxic water bottom with almost 1% pyrite sulfur (~2% pyrite; Canfield et al., 1992).This scenario can occur when H 2 S accumulates in the pore water because the rate of reaction between sulfide and iron is slower than the rate of sulfate reduction.The size of the pyrite can provide insight into its formation, as pyrite formed in the water column is typically <5 μm in diameter and uniform in shape, whereas pyrite from pore waters tends to be larger and more variable in shape (Wilkin et al., 1996;Wilkin & Barnes, 1997).
We have considered the extent of oxidative weathering within our samples and found samples from the northern part of the subbasin that show limited alteration.Because all sections demonstrate similar trends, we propose that iron proxy data in our samples are reliable when interpreted within the redox proxy framework except for the Fe py /Fe HR proxy, which is considered at least partially overprinted by oxidative weathering.This limits our ability to distinguish euxinic versus ferruginous anoxic conditions.However, large Fe mag enrichments observed in the samples could be related to the formation of Fe-rich clays (e.g., berthierine and chamosite) as proposed by Slotznick et al. (2020).These would provide independent evidence for Fe 2+ -rich pore water supporting a ferruginous interpretation.

| Detrital FeT/Al baseline
Total Fe normalized to Al provides a baseline for understanding the roles of detrital input and the iron shuttle in influencing redox proxies (Lyons et al., 2003;Raiswell et al., 2018).et al., 1983;Taylor & McLennan, 1985), while the average Fe (2.51 wt.%) is lower than the average in the upper crust of 3.50% (McLennan et al., 1983;Taylor & McLennan, 1985).The average Figures 3-6).Seven samples from the Dolores Creek Fm. have Fe HR / Fe T > 0.38 while Fe T /Al < 0.30, yielding their interpretation ambiguous.Nevertheless, a total of 7 samples meets both criteria (Fe HR / Fe T > 0.38 and Fe T /Al > 0.30) for anoxia, as compared with 12 samples that are definitively below the oxic thresholds Fe HR /Fe T < 0.22 and Fe T /Al < 0.30.These data suggest that much of the MMS was deposited in an anoxic ocean during dominantly ferruginous conditions with brief oxic intervals, which is consistent with the occurrence of ironstones in shallow shelf settings in the upper Katherine Group, as well as data from other early Tonian basins that indicates ferruginous and anoxic waters in the early Tonian (Guilbaud et al., 2015).
Interpreted independently, the trace metal enrichments observed in the MMS suggest the sediments were deposited in oxic waters beneath the core of a perennial oxygen minimum zone (OMZ) based on the thresholds Mo < 5 ppm/w.%,V < 23 ppm/w.%and U > 1 ppm/w.%(Bennett & Canfield, 2020).However, the average V enrichment in the Little Dal Group exceeds 23 ppm/w.%with a mean value of 24.19 ppm/w.%.The depositional environment could also be interpreted as seasonal OMZ, but these environments remain poorly constrained by paleoredox tracers.
Overall, we interpret our results to indicate a degree of redox instability and suggest the most plausible explanation of the data is that sediment deposition occurred close to the redoxcline (e.g., seasonal variations in wave intensity).Alternati4vely, longer-term changes in relative sea level could have caused iron speciation values to change significantly (e.g., from anoxic to oxic).Similar scenarios are recorded by transgressive shale deposits in the Reefal assemblage of the Fifteenmile Group (Gibson et al., 2020;Sperling et al., 2013) and in some modern basins influenced by seasonal variability (Bennett & Canfield, 2020;Böning et al., 2005Böning et al., , 2009;;Brumsack, 1989).These two interpretations represent end members, with redox instability occurring over geologic time scales while seasonal trends occur on biological timescales.Based on our redox proxy framework, twelve samples are interpreted as deposited beneath an oxic water column.
These samples are from transgressive shales that display similar trends to those observed by Gibson et al. (2020).The fluctuating signal could also represent periodic incursions of oxygenated water from the global ocean in a restricted, anoxic basin.

| Paleoenvironmental analysis
Iron speciation data can provide insight into local conditions while trace metals can yield context about the broader redox landscape (Gilleaudeau et al., 2020;Lyons et al., 2021;Sperling et al., 2015).
Portable X-ray fluorescence analysis (pXRF) in the Hematite Creek Group north of Tarn Lake documented elevated Zn, Pb and Ni at some stratigraphic levels in the Dolores Creek Fm. (Turner, 2011).However, limited enrichments of redox-sensitive trace metals Mo, V and U are observed in the present study.Subtle trace metal enrichments have previously been recorded in the Proterozoic inliers of northwestern Canada in the Fifteenmile Group (Gibson et al., 2020;Sperling et al., 2013) and the Windermere Supergroup of the Wernecke and Mackenzie Mountains (Miller et al., 2017).Here, we present three possible scenarios to explain the muted trace metal enrichments.False anoxic signals (i.e., Fe HR /Fe T ratios > 0.38) can be caused by nearshore trapping of detrital iron oxides under oxic conditions (Poulton & Raiswell, 2002), a large detrital reactive-iron input, or a proportionally small flux of detrital unreactive iron (Raiswell et al., 2018).However, it is more likely that the high Fe ox values are the product of iron oxides precipitated in the water column, although they could also be a result of weathering of other highly reactive iron pools, specifically iron pyrite, which is also within the Fe HR pool.
A study of the Little Dal Group east of our study site in the Mackenzie Mountains reported an average Fe T /Al of 0.51 (ranging from 0.29 to 0.85; O'Hare, 2014).Although most of these samples suggest ferruginous, anoxic to possibly oxic bottom water conditions, there is also evidence for an authigenic iron signal overwhelmed by siliciclastic input in siltier mudstones with low Fe T /Al.The dilution of authigenic Fe can result in a low Fe T /Al when overwhelmed by a relatively high-siliciclastic supply (Lyons & Severmann, 2006).
Evidence for increased sediment supply in the MMS is found in the 450 m of shale below the fossil interval in the Dolores Creek Fm.
likely deposited rapidly during basin extension (Figures 1 and 2).Therefore, we propose that redox interpretations based on our geochemical data are generally robust, though likely unable to differentiate consistently between euxinic and ferruginous conditions.

| Scenario 2: Restricted basin
The presence of black siltstone and mudstone in the Dolores Creek Fm. has previously been invoked as evidence that the early MMS in the Wernecke Mountains was anoxic and at least partially restricted (Turner, 2011).This could explain the limited trace metal enrichment observed in our study because the trace metal reservoir in a restricted basin would be diminished compared to a basin open to the global ocean (Algeo & Lyons, 2006;Algeo & Rowe, 2012).In the northern part of the basin, the bright orange stromatolitic and other microbialite layers are rich in bitumen, indicating the presence of organic-rich sediments.Three transgressive intervals were identified at the type section north of Tarn Lake (Turner, 2011).Samples interpreted as oxic in our study correlate to some of these shoaling up sections from Turner (2011), and the overall trends are similar to those observed in equivalent units from the Ogilvie Mountains in a basin proposed to be hydrographically restricted (Gibson et al., 2020).However, a tidal influence has been documented in the Black Canyon Creek Fm.Fm. (Maloney et al., 2021) is also consistent with a robust marine influence at this time (Azmy et al., 2008;Cohen et al., 2017;Geboy et al., 2013;Kendall et al., 2009;Rooney et al., 2010Rooney et al., , 2014Rooney et al., , 2018;;Sperling et al., 2014;Strauss et al., 2014;VanAcken et al., 2013).
Precambrian successions interpreted to have been deposited in restricted marine to lacustrine environments typically have Os i > 0.8 due to the strong influence of inputs from evolved continental crust (Cumming et al., 2013;Rooney et al., 2018;Tripathy & Singh, 2015).reservoirs, which is supported by the observation of muted trace metal enrichment in coeval units in Canada (Johnston et al., 2013) and Svalbard (Kunzmann et al., 2015).Specifically, Mo inventories could have been severely depleted in Proterozoic oceans because of widespread euxinia (Partin et al., 2013;Scott et al., 2008).
In addition, it is important to consider that the average shale values used to determine whether a trace metal enrichment is present are based on comparisons to average Phanerozoic and modern shales (Tribovillard et al., 2006;Bennett & Canfield, 2020, Table 1).
There are no known "true" modern ferruginous basins or widely anoxic oceans with a low-trace metal reservoir that allow us to calibrate trace metal enrichments, which increases the uncertainty of this paleoredox proxy in suspected ferruginous and globally anoxic settings (Miller et al., 2017).Subtle enrichments of redox-sensitive trace metals can be challenging to separate from background levels with no modern analogue for comparisons (Scott & Lyons, 2012).
Thus, we propose that a poorly oxygenated global ocean during the deposition of the MMS with apparent instability in the local paleoenvironment is most consistent with our data.

| Evidence from other Proterozoic Inliers
Interpretations of redox data from several studies in the Proterozoic inliers of Canada provide support for redox instability in dominantly ferruginous waters (see Appendix S1).Studies have concentrated on the late Neoproterozoic when the earliest large complex fossils emerged (Canfield et al., 2008;Johnston et al., 2013;Miller et al., 2017;Shen et al., 2008;Sperling et al., 2016) and early Neoproterozoic strata that record the diversification of eukaryotes and their ecosystems (Gibson et al., 2020;O'Hare, 2014;Sperling et al., 2013;Thomson et al., 2015).There is evidence for an anoxic, ferruginous global ocean throughout the Neoproterozoic with brief pulses of oxic and scarcer euxinic conditions, possibly representing the expansion and contraction of an OMZ.However, these oxic conditions rarely align with significant fossil deposits, such as biomineralized scale microfossils (Sperling et al., 2013), "Twitya discs" (Sperling et al., 2016), Ediacara biota (Johnston et al., 2013;Sperling et al., 2016), bilaterian traces (Sperling et al., 2016) and green macroalgae (this study).Samples from the Shaler Supergroup of Victoria Island are noticeably different from the other sites, displaying strong evidence for euxinic conditions during the Bitter Springs interval in an otherwise mostly oxic setting (Thomson et al., 2015).These results highlight regional differences in ocean redox conditions between the Proterozoic inliers that exert local controls on basin dynamics.
Based on comparisons with other Proterozoic inliers and evi- This inference aligns with a recent investigation that found nitrate limitation in the early Tonian, followed by a stepwise increase in δ 15 N sed values at ca. 800 Ma, suggesting increased nitrate availability (Kang et al., 2023) and ultimately a stepwise increase in O 2 at around 800 Ma (e.g., Cole et al., 2017;Guilbaud et al., 2015;Kang et al., 2023;Planavsky et al., 2022;Wang et al., 2022).

| Requirements for habitable macroalgal environments
The Earth experienced dramatic environmental change during the Neoproterozoic, which may have played an important role in the evolution of life by altering the availability of macroalgal habitats and bioessential trace elements (Anbar & Knoll, 2002;Erwin et al., 2011;Knoll & Nowak, 2017).It has been suggested that the availability of nutrients may have delayed the rise of green algae as the dominant primary producers in early Tonian oceans, with eukaryotes unable to outcompete prokaryotes until conditions became more favourable (Brocks et al., 2017;Kang et al., 2023;Maloney et al., 2021;Nguyen et al., 2019;Zumberge et al., 2020).However, fossil discoveries have demonstrated that chlorophytes were able to thrive in benthic habitats and support diverse communities by ca.1000 Ma (Maloney et al., 2021;Tang et al., 2020).Considering the factors that influence the habitability of eukaryotic ecosystems can aid in elucidating trends in macroalgal expansion and the influence of environmental conditions (e.g., redox setting).

Comparisons between redox conditions in the Tonian
Longfengshan Biota-bearing and non-fossiliferous shales in North China have shown oxic conditions in the non-fossil intervals, while fossil units record a dominantly ferruginous water column with limited trace metal enrichment (Wang et al., 2021).These results are interpreted to reflect benthic oxygen oases where the macroalga Longfengshaniaceae regulated the stratified redox water column through photosynthesis and O 2 consumption.A Chuaria-Tawuia-Longfengshaniacea assemblage has also been observed in the Little Dal Group in the Mackenzie Mountains (Hofmann, 1985).Our data suggest a geographic trend in the Hematite Creek Group, with northern samples mostly anoxic (11/16 includes possibly ferruginous) while the southern redox transect provides more evidence for oxic conditions (7/16 includes possibly oxic) proximal to the macroalgal habitat.These data suggest that the southern part of the basin may have been more favourable for macroalgal life.Alternatively, it could also suggest that macroalgae were bioengineering seafloor habitats by providing a source of oxygen that contributed to benthic oxygen oases similar to trends demonstrated by Wang et al. (2021).It is also possible that benthic macroalgae would influence the sequestration of trace metals in addition to regulating O 2 .Modern macroalgae are used in reclamation to remove heavy metals from wastewater (Arumugam et al., 2018), and heavy metals have been documented in dried seaweeds used for human consumption (Besada et al., 2009;Chen et al., 2018).As such, Tonian macroalgae may also have bioaccumulated metals, though experimental studies are necessary to provide an empirical framework for interpreting the ancient record.
Benthic macroalgal communities would have also had a transformative effect on organic carbon burial (LoDuca et al., 2017), further influencing long-term oxygen accumulation and trace metal inventories.

| Implications for early Neoproterozoic Eukaryotic Ecosystems
The cause-and-effect relationship between the evolution of complex life and oxygenation remains controversial (Cole et al., 2020;Lenton et al., 2014;Lyons et al., 2021;Planavsky et al., 2014).Specifically, did increasing oxygen levels in the Neoproterozoic drive, or at least facilitate the emergence of the first large complex organisms, the Ediacara Biota (~571-539; Knoll, 1992;Canfield et al., 2007;Sahoo et al., 2012)?Or did the rise of complex life drive the oxygenation of the ocean by restructuring the flow of carbon into sediments (Lenton et al., 2014)?In any case, the appearance of new species of benthic macroalgae in ca. 1 Ga strata from Yukon (Maloney et al., 2021) and North China (Tang et al., 2020) raises new questions about the habitability and redox stability of ancient ecosystems.
The structure of modern seaweed communities is heavily influenced by competition for resources including light, substrate and nutrients (Carpenter, 1990).Lyons et al. (2021) proposed that certain intervals in the Proterozoic were dominated by hostile environmental conditions that restricted the ecological expansion of eukaryotes, whereas heterogeneity in the environment during critical transitions permitted biological adaptions that allowed life to thrive.Our study supports this hypothesis, with large macroalgae able to colonize some shallow marine environments that previously would have been dominated by cyanobacteria in an extensively anoxic ocean.These macroalgal fossils only occur in specific sedimentary facies, and their distribution is at least partially controlled by taphonomy (Maloney et al., 2022).However, the observation of more than one species representing diverse size ranges indicates a relatively complex algal ecosystem (Maloney et al., 2023).
Our geochemical results suggest that these organisms inhabited environments with fluctuating redox conditions.It is possible that the shallow water settings were oxygenated, but these fleeting moments when macroalgae inhabited the outer shelf are not captured by our analyses (Sperling et al., 2016), especially when considering that the average life span of modern macroalgae (e.g., Kelp, Macrocystis pyifera) is several months to a few years (North, 1961;Tussenbroek, 1989).This trend has also been observed in younger Ediacaran communities where soft-bodied metazoans episodically colonize shallow marine environments in anoxic basins (Bowyer et al., 2020;Sperling et al., 2016;Wood et al., 2015).Future investigations of similar Tonian paleoenvironments will provide more insight into whether this opportunistic colonisation  et al., 2020).

| CON CLUS ION
The diversification of macroalgae and expansion of eukaryotic ecosystems likely influenced the oxygenation of shallow marine environments.Here, we provide insight into regional ocean oxygenation

CO N FLI C T O F I NTER E S T S TATEM ENT
Authors declare no conflicts of interest.

F
Stratigraphic log and geological map of study area in the Wernecke Mountains.(a) Mackenzie Mountains Supergroup (MMS) stratigraphy with age constraints and fossiliferous units.(b) Geological map of the Tonian MMS in the Wernecke Mountains, showing location of the fossil locality and camps where stratigraphic data and samples were collected.Inset map of Canada showing location of geological map with red rectangle.Map is modified from the Yukon Geological Survey Bedrock Geology Dataset (Yukon Geologic Survey, 2018).Gp., Group; Fm., Formation; Sta., Statherian Period; Eta , Etagochile Formation; Sh.Ran., Shattered Range Formation; Abr.Pl., Abraham Plains Formation; Cryo, Cryogenian Period; E, Ediacaran Period; Winder., Windermere supergroup; Mt.Land., Mount Landreville Formation; Pass Mtn., Pass Mountain Formation; SG, supergroup.redox-sensitive trace element analyses, complemented by petrographic and Nd isotope data to provide insight into weathering and sediment provenance.
and mainly carbonate Black Canyon Creek Fm. [n = 2]), the Katherine Group (n = 7) and the Little Dal Group (n = 7).The second section was a transect of the Hematite Creek Group with 16 samples from the Dolores Creek Fm. and one sample from the Black Canyon Creek Fm.Geochemical sampling targeted horizons with fine-grained material and no visible evidence of post-depositional alteration (see Appendix S1).4.2 | Iron speciation and pyrite extractionSequential iron extraction and analysis was performed in the Department of Earth and Planetary Sciences at McGill University (QC, Canada) following the procedure developed by Poulton and Canfield (2005) with minor modifications (see Kunzmann standard G-16 was used for quality control (provided by Dr. K. Norrish of Commonwealth Scientific and Industrial Research Organisation[CSIRO], Australia).The total organic carbon (TOC) concentrations were determined by ACTlabs using an ELTRA instruments C-S analyser.4.4 | Petrographic and SEM analysesThin sections (n = 12) were cut at Queen's University (ON, Canada) from a subset of samples selected to compare samples based on their Fe py and analysed using a petrographic microscope.Thin sections (n = 3) and bulk rock samples (n = 3) were further analysed to investigate the distribution of minerals and provide insight into post-depositional processes using a Zeiss Sigma 500 variablepressure SEM equipped with dual, co-planar Bruker XFlash EDS units at the University of Missouri X-ray Microanalysis Core.SEM imaging and EDS analyses used identical beam and chamber conditions including: 20 keV beam accelerating voltage, 40 nA current, beam apertures of 60 μm (imaging) and 120 μm (EDS), a working distance of 16 mm (±0.2 mm; flat samples allowed for minimal variation) and 20 Pa chamber pressure with a 99.999% nitrogen atmosphere.Z-contrast backscattered (BSE) imaging was conducted using a high-definition 5-segment backscatter detector, and EDS elemental analyses were conducted using both spectrometers in tandem.4.5 | Sm-Nd isotopesAliquots of combusted sample (~0.1-0.95g) were first leached in acetic acid to remove carbonates and then spiked with an enriched 150 Nd-149 Sm tracer.Samples were then digested according to the following steps to dissolve silicates: (1) HF (4.5 mL; ~29 N) and HNO 3 (1 mL; ~15 N); (2) aqua regia (3:1, 6 N HCl:7 N HNO); (3) 6 N HCl.A three-stage chromatography process was applied to the samples to first remove iron and rare earth elements, and then isolate Nd and Sm for analysis.Iron was removed first by passing the sample though columns filled with 200-400 mesh AG1X8 anion exchange resin.The second step targeted the rare earth elements by passing the sample through columns filled with Eichrom TRU Resin SPS 50-100 μm twice.Third, Sm and Nd were isolated using columns filled with ~600 mg of Eichrom LN Resin 50-100 μm.Nd and Sm separates were taken up by 3 mL of 2% HNO 3 .Nd and Sm isotope ratios were then measured on a Nu Plasma II MC-ICP-MS (Multicollector Inductively Coupled Plasma Mass Spectrometer) at Geotop/Université du Québec à Montréal.Nd isotope ratios are reported in εNd notation, where The εNd values are commonly presented as a function of age (i.e.εNd(t)), where both the sample and CHUR 143 Nd/ 144 Nd ratios (2) Nd = (143Nd∕144Nd) sample (143Nd∕144Nd) CHUR − 1 × 10000 are corrected for 143 Nd ingrowth since the time the rocks were deposited.In the models presented here, t = 900 Ma for the Hematite Creek Group, t = 875 for Black Canyon Creek and the Katherine Group, and t = 850 for the Little Dal Group.5 | RE SULTS 5.1 | Iron speciation and major elements All results presented here are based on 46 samples (see Appendix S1).The Katherine Group samples have the highest average abundance of Fe (4.80 wt.%) and Al (9.60 wt.%), while the Hematite Creek Group (Fe = 1.92 wt.%, Al = 8.76 wt.%) and Little Dal Group (Fe = 2.90 wt.%, Al = 8.64 wt.%) samples had lower concentrations.Fe T /Al ratios range from 0.06 to 1.16 (mean = 0.31) with means varying between the Hematite Creek (mean = 0.23), Katherine (mean = 0.59) and Little Dal (mean = 0.37, Figures 3-6; Table The remaining 15 samples (33%) are ambiguous, with Fe HR /Fe T falling between 0.22 and 0.38.Fe py /Fe HR ratios are low throughout the sample set, ranging from 0 to 0.49 (mean = 0.06), with no values within the possible euxinic range (0.6-0.8;Poulton, 2021).Ten samples (22%) meet both threshold values of Fe HR /Fe T (>0.38) and Fe T /Al (>0.30) for deposition under anoxic conditions, and 12 samples (26%) fit our criteria for an oxic depositional environment (i.e., Fe HR /Fe T < 0.22 and Fe T /Al < 0.30; Figure 6).Seven samples (7%) are interpreted as possibly oxic by meeting one threshold value of Fe HR /Fe T ratios <0.22, but demonstrate high (FeT/Al > 0.30) while 20 samples are interpreted as possibly anoxic (i.e., Fe HR /Fe T > 0.38 or Fe T /Al > 0.30).Most of the F I G U R E 3 Stratigraphic column and geochemical data from shale intervals in the Hematite Creek Group, Mackenzie Mountains Supergroup, Wernecke Mountains.(a) Stratigraphic section at SW Mt.Profeit (T1820/T1821) legend in Figure 2. (b) Total organic carbon content (wt.%).(c) Ratio of total iron to aluminum (Fe T /Al).The threshold line is the inferred detrital baseline (Fe T /Al ~ 0.

F
Figure 8g-l).Cross-plots comparing the difference between pyrite (Fe py ) and Fe-oxide (Fe ox ) with oxidative weathering documented

F
Stratigraphic column and geochemical data from shale intervals in the Hematite Creek Group, Mackenzie Mountains Supergroup, Wernecke Mountains.(a) Stratigraphic section north of Tarn Lake (TN1, TN2, T1827) legend in Figure 2. (b) Total organic carbon content (wt.%).(c) Ratio of total iron to aluminum (Fe T / Al ).Thresholds described in Figure 3.(d) Ratio of highly reactive iron to total iron (Fe HR /Fe T ).(e) Ratio of pyrite iron to highly reactive iron (Fe py /Fe HR ).(f) Ratio of iron oxy(hydr)oxide to total iron (Fe ox /Fe T ).(g-i) Bulk Mo, V and U contents in ppm (black circles) and calculated trace metal enrichments in ppm/wt.%normalized to Al (red circles, (Bennett & Canfield, 2020)).(j) Preliminary interpretation of redox column where green is ferruginous, light green is possibly ferruginous, blue is oxic and grey remains unconstrained.See the redox proxy framework for a detailed description of interpretations.F I G U R E 6 Iron proxies.(a) Cross-plot of Fe HR /Fe T and Fe py /Fe HR .Grey dashed lines are threshold values defined in Figure 3.(b) Crossplot of Fe T /Al and Fe HR /Fe T .The Red dashed line is the inferred detrital baseline from this study (Fe T /Al ~ 0.3).TA B L E 2 Shale total organic carbon (TOC), major and minor elemental concentration and iron speciation data.
Raiswell et al. (2018) emphasized the importance of targeting fresh material when collecting geochemical samples from outcrop to limit the oxidative weathering of pyrite.To test this approach,Ahm et al. (2017) compared trace and major element geochemistry and iron speciation in paired samples from outcrop and drill cores.They documented that pyrite in outcrop samples had been oxidised and remobilised by weathering, while the majority of reactive iron from core samples was preserved as pyrite.This behaviour highlights the difficulties of working from outcrop samples as Fe py can weather out of the sample or is converted to the Fe ox operational pool in iron speciation analyses, which could skew an interpretation between euxinic and ferruginous conditions.Loss of total reactive iron and depleted Fe HR /Fe T were also observed in outcrop samples, whereas no significant difference was documented in Fe T .This suggests that remobilized iron could have accumulated in the unreactive phases, such as authigenic Trace metal enrichments cross plots with enrichments normalized to Al reported in ppm/wt.%plotted with TOC in wt.%.See Appendix S1 for the full data set.TA B L E 3 Sm-Nd results.

F
Analytical microscopy of pyrite and weathered pyrite textures where red = iron and yellow = sulfur.(a, c) SEM maps of TN1-124 thin section.(b, e) EDS elemental map on SEM image in A with pyrite retaining sulfur and iron.(d, f) EDS elemental map on SEM image in C with white arrowheads pointing to sulfur enrichment.(g, i) SEM maps of T1820-214 thin section.(h, k) EDS elemental map on SEM image in G with white arrowheads pointing to evidence for pyrite (relative enrichment of Fe).(j, l) EDS elemental map on SEM image in I with white arrowheads pointing to limited sulfur enrichment compared to iron.Samples from Tarn Lake N (a-f); SW Profeit (g-l); White scale = 50 microns, Black and grey scales = 5 microns.Additional microscopy images are available in the Appendix S1.
is important to note that Ahm et al. (2017) conducted their study in Nevada, where the surficial weathering conditions are different than what would be expected in Yukon based on climate, and there are currently no constraints on exposure time for either site although both would be important factors.It is also necessary to consider the influence of post-depositional alteration on the iron speciation analysis.For example, goethite is a common alteration mineral in sedimentary rocks that forms during surficial oxidative weathering and can increase the highly reactive iron values because it is extracted with the reducible oxides pool (Fe ox ) (Slotznick et al., 2020).Mixed-valance Fe clays

F
Possible evidence for oxidative weathering.(a) Fe speciation for the Mackenzie Mountains Supergroup in the southern part of the basin with Fe py /Fe HR plotted against Fe ox /Fe HR .(b)The percentage of highly reactive iron represented by each of the reactive pools is shown above and includes carbonate (Fe carb ), pyrite (Fe py ), oxy(hydr)oxides (Fe ox ) and magnetite (Fe mag ).The sections include the Hematite Creek Group (Dolores Creek (T1820) and Black Canyon Creek (T1821) Formations), Katherine Group (G1806) and Little Dal Group (LD).The section and sample number are indicated on the y-axis.The trend of oxidative weathering of pyrite is indicated by the arrow(Ahm et al., 2017).
The average detrital flux into sedimentary basins is often approximated as the average value for the continental crust, or the upper continental crust (Fe T / Al ~0.48; McLennan, 2001), and thus a Fe T /Al baseline of ~0.5 is assumed.However, the Fe T /Al varies widely in detrital fluxes based on their source composition, as demonstrated by a recent study by Cole et al. (2017) on modern environments that quantified this heterogeneity in detrital flux.The average Al observed in the Wernecke shale samples (8.79 wt.%) is comparable to 8.04 wt.% in the upper crust (McLennan

Fe T /
Al values are 0.23 for the Hematite Creek Group, 0.59 for the Katherine Group and 0.37 for the Little Dal Group.The low Fe T /Al ratios observed in this study are consistent with the regional trend observed bySperling et al. (2013; mean = 0.37  wt.%) and Gibson et al. (2020; average 0.35 wt.%) in shales of the correlative Reefal Assemblage of the Fifteenmile Group (Figures 3-6).Thus, with the exception of the Katherine Group (mean = 0.59 wt.%), Tonian strata from northwestern Canada contain Fe T /Al values significantly lower than the upper continental crust.We apply the detrital baseline of Fe T /Al of ~0.3 proposed by Gibson et al. (2020) to account for this regional trend, which is interpreted to reflect low-detrital iron silicate input to the basin (rather than high-aluminum content; Sperling et al., 2013).Ahm et al. (2017) suggested Fe T /Al remained unaffected by weathering in their samples.Since the Proterozoic inliers are known to have low Fe T /Al values (Gibson et al., 2020; F I G U R E 1 0 Possible evidence for oxidative weathering.(a) Fe speciation for Dolores Creek (TN1, TN2) and Black Canyon Creek (T1827) Formations in the northern part of the sub-basin with Fe py /Fe HR plotted against Fe ox /Fe HR .(b) The percentage of highly reactive iron represented by each of the reactive pools is shown above and includes carbonate (Fe carb ), pyrite (Fe py ), oxy(hydr)oxide (Fe ox ) and magnetite (Fe mag ).The section and sample number are indicated on the y-axis.The trend of oxidative weathering of pyrite is indicated by the arrow (Ahm et al., 2017).
6.4.1 | Scenario 1: Detrital and/or post-depositional influences Detrital overprints and post-depositional alteration can affect the interpretation of paleoredox information from iron speciation data.
These strata are ascribed to the informal lower Dolores Creek Fm., whereas the overlying ~300 m interval is equivalent to the Dolores Creek Fm. type section.The samples analysed during this study are from the type section (e.g., northern part of basin) and a section of the upper Dolores Creek Fm. in the southern part of the basin where there is limited evidence for rapid sedimentation.Sediment provenance, grain size and accumulation rate are important factors influencing paleoredox proxies.However, based on detailed assessment of detrital and post-depositional influences (e.g., evidence of limited alteration in northern part of the sub-basin with similar results throughout the basin) in our samples and in consideration with the broader, regional geochemical trends and geology, we rule out discrepancies due to detrital and/or post-depositional influences.
(e.g., bidirectional current structures and reactivation structures), which overlies the Dolores Creek Fm., suggesting that at least the upper strata in MMS (e.g., the Katherine and Little Dal Groups) were deposited in a basin connected to the open ocean, while restriction may have occurred during the deposition of the older Dolores Creek Fm.A low initial 187 Os/ 188 Os of 0.38 from the upper Dolores Creek Our relatively low εNd(t) data confirm that the source area at this time was relatively evolved, with a modest mafic contribution.Therefore, based on sedimentary structures indicating tidal influence and the Osi data, it is unlikely that the basin was generally restricted during the deposition of the MMS strata in the Wernecke Mountains, although it was clearly not a broadly open passive margin.6.4.3 | Scenario 3: Poorly oxygenated global oceanSperling et al. (2013) documented thin black shales with high Fe HR /Fe T in the Reefal Assemblage stromatolite reef core of the Fifteenmile Group, which correlates to the fossiliferous Little Dal Group in the Wernecke Mountains (Macdonald & Roots, 2010).Miller et al. (2017) carefully considered several causes for the lack of Phanerozoic-style redox-sensitive trace element enrichment in northwestern Canada and ultimately proposed that global anoxic conditions resulted in limited marine redox-sensitive trace element dence for a connection to the open ocean, we propose that the ocean was poorly oxygenated resulting in a limited trace metal reservoir during the deposition of the lower MMS in the Wernecke Mountains.Increased trace metal enrichments observed in the Fifteen Group(Gibson et al., 2020) correlate with the basal Little Dal Group at SW Profeit where there is limited evidence of increased trace metal enrichment, but iron speciation suggests oxygenation in the Little Dal Group.In the Mackenzie Mountains, thick sulfate evaporite deposits reported from the Ten Stone Fm. of the Little Dal Group(Turner & Bekker, 2016) provide further support for environmental change around ca. 850-800 Ma.These sulfate evaporites mark an abrupt change from the halite evaporites observed in the lower to middle Little Dal strata, implying an increased sulphate supply to the oceans and marine oxygenation extending below the storm wave base.
of photic zone environments by macroalgae in ancient oceans was a local trend or a more widespread phenomenon.The expansion of macroalgal ecosystems would have global environmental (e.g., oxygenation; Xiao & Tang, 2018), carbon cycle (Krause-Jensen & Duarte, 2016) and ecological (e.g., diversity, competition; Carpenter, 1990) implications, which likely spurred biological change including the stepwise increase in morphological disparity observed in Proterozoic macroalgae (Bykova during a critical period of Earth's history when eukaryotic lineages including diverse macroalgal communities were colonising benthic habitats.A multi-proxy redox framework was utilized to identify the prevailing ocean redox conditions recorded in the MMS.Geochemical evidence implies deposition under a dominantly ferruginous water column (low Fe T /Al, high Fe HR /Fe T ) with muted trace metal enrichments punctuated by brief oxygenated intervals.Three hypotheses are provided to explain the muted trace element enrichments despite evidence for generally anoxic water columns: (1) false positive anoxic conditions from the detrital input, or post-depositional alteration; (2) deposition in a restricted basin; or (3) a globally anoxic ocean with a depleted dissolved trace metal reservoir.We propose that the third scenario is the most likely, with our data reflecting a poorly oxygenated global ocean with some potential influence from basin restriction during the deposition of the Dolores Creek Fm.However, there is more evidence for oxygenation in the ca.850 to 800 Ma Little Dal Group.Additional sampling targeting fossil sections will provide insights into Tonian macroalgal paleoenvironments and future studies should continue to document the influence of seasonal variations on redox conditions that can dramatically influence biological activity.It is likely that eukaryotes living in these heterogeneous environments within a ferruginous ocean were able to thrive opportunistically during brief oxygenated pulses.These results provide insight into increasingly complex algal ecosystems that emerged during the early Tonian Period and enhance our knowledge of basin redox conditions at this critical interval of eukaryotic evolution.ACK N OWLED G M ENTSWe gratefully acknowledge the First Nation of Na-Cho Nyak Dun for permitting our fieldwork on their traditional territory.This study was supported by the National Science and Engineering Research Council of Canada (NSERC) postgraduate and postdoctoral scholarships, the Queen Elizabeth II Graduate Scholarship in Science & Technology (QEII-GSST), the Geological Society of America Graduate Research Grant, and the Northern Scientific Training Program to KMM; NSF IF 1636643 to JDS; NSERC Discovery grants to GH RGPIN2017-04025 and ML RGPIN435402, the Polar Continental Shelf Program and the Agouron Institute.

Table 2 ,
Figures 3-5 and 7).TOC contents measured in the MMS Trace metal enrichment of Mo normalized to Al ranges from 0.09 to 1.18 ppm/w.%with the largest average enrichment in the Katherine Group, while U varies from 0.10 to 1.15 ppm/w.%following the same trend.V enrichment ranges from 7.25 to 58.28 ppm/w.%with the largest average enrichment in the Little Dal Group and smallest in the Katherine Group.

Table 3
). Less negative εNd values are observed in the Black Canyon Creek Fm. (εNd(t) = −3.87),while εNd(t) for the older Dolores Creek Fm. ranges from −9.94 to −4.05 with a mean of −7.00.εNd(t) values for the Katherine and Little Dal groups average −6.83 and −7.18, respectively.