Combining Nitrogen Isotopes and Redox Proxies Strengthens Paleoenvironmental Interpretations: Examples From Neoproterozoic Snowball Earth Sediments

Abstract

The history of the nitrogen cycle on Earth is linked to the redox evolution of the surface environment. Many nitrogen cycle fluxes are microbially mediated, and the particular fluxes operating at any given time in an ecosystem depend on the presence, absence or abundance of oxygen. However, interpreting this relationship is complicated as several isotopic fractionations associated with N-cycling are not diagnostic of a particular redox state. Thus, linking nitrogen isotopic analyses with redox-sensitive proxies is essential when interpretating past environments. Specifically, we use concentrations of U, V and Mo, along with Fe-speciation, to augment and contextualize nitrogen isotopic measurements. As an example, we consider samples from the Neoproterozoic Cryogenian period to suggest that there was oxygenated water, with associated aerobic N cycle fluxes. This interpretation is based on positive δ¹⁵N values between 4 to 8⁰/₀₀, Fe-speciation data consistent with anoxic bottom water during the Snowball ocean and oxygenated after, and redox-sensitive trace metals indicative of oxic weathering and surface water. Typically, high δ¹⁵N values are interpreted to reflect enhanced denitrification. We propose potential causes including a post-Snowball freshwater melt lid that suppressed deep water ventilation and that denitrification occurred more rapidly at high temperatures after the Snowball. These interpretations are buttressed by combined N isotope and redox analyses. This approach is especially useful during times of dynamic redox in the ocean-atmosphere system to interpret biologic isotopic signals.

1 Nitrogen and Redox Biogeochemistry

Nitrogen is a key nutrient for life, a major component of the atmosphere, and tracks connections between the biosphere, atmosphere, and geosphere over long time scales (Zerkle and Mikhail, 2017). It is possible to investigate changes in the evolution and operation of the ancient nitrogen cycle via stable isotope geochemistry¹ as nitrogen undergoes a number of biologically mediated, redox sensitive transformations, each of which is associated with an isotopic fractionation (see Stüeken et al., 2016a).

Atmospheric N₂ enters the biosphere via nitrogen fixation by diazotrophic organisms. Such fixing splits the triple bond in N₂ using the nitrogenase enzyme. The most common enzyme (Fe - Mo nitrogenase) produces a minimal isotopic effect (Zhang et al., 2014). Nitrogen which is then remineralised from diazotrophic biomass can be stable and bioavailable in the environment as or, in the presence of oxygen, is transformed by microbes to via nitrification. Nitrification has a significant isotopic effect, with product being enriched by 13 to 38⁰/₀₀ compared to substrate (e.g., Casciotti et al., 2003, 2011). Typically, in modern oxygenated waters, nitrification is complete, such that all available is converted, and thus there is no net isotopic effect recorded in the resultant .

Denitrification and anammox, which are microbially mediated respiration reactions that transform to N₂ (with minor N₂O), imparts a large net isotope effect where product N₂ is about 25⁰/₀₀ depleted compared to reactant . In the modern ocean, water column denitrification occurs primarily in oxygen minimum zones, but is only partial as the wider ocean is oxygenated, leaving a residual pool of with an average δ¹⁵N of 5 to 7⁰/₀₀ (Sigman et al., 2009). By contrast, in modern marine sediments, there is no net isotopic effect on the bioavailable N pool, as here conversion to N₂ is quantitative (Sigman et al., 2009). The same isotopic pattern would result from anaerobic ammonium oxidation (anammox), especially in the geologic record (Brunner et al., 2013).

We can observe in modern oceans that sedimentary N-isotope values reflect the integrated signal of nitrogen cycling by organisms in the water column (Tesdal et al., 2013). However, interpretation of N-isotope values is complicated by the fact that fractionations produced by transitions between N compounds are not diagnostic (Busigny et al., 2013; Ader et al., 2014, 2016; Stüeken et al., 2016b). For example, a bulk ecosystem δ¹⁵N value of 0⁰/₀₀ typically indicates anaerobic conditions (e.g., Garvin et al., 2009), but could also reflect quantitative nitrification without any denitrification. The latter case could occur in a completely aerobic environment, and thus the same bulk δ¹⁵N value could be produced under completely opposite redox conditions in the water column. These issues are amplified in ancient sediments and rocks, since we cannot measure contemporaneous water conditions during sediment deposition.

In the Precambrian, most measured δ¹⁵N values from sediments are positive (Stüeken et al., 2016a). During this time, anoxic water and potential oxyclines were more widespread. As a result there are multiple scenarios which could lead to positive δ¹⁵N values. For example, in a study of the Archean Mount McRae Shale and Brockman Iron Formations Busigny et al. (2013) measured variable but consistently positive δ¹⁵N values. Such positive values could reflect partial assimilation of enriched in ¹⁵N due to partial oxidation to . Such oxidation could occur in a fully anoxic water column via reactions with Fe or mediated biologically in local oxygen “oases” or in pervasively oxygenated shallow water (Busigny et al., 2013).

Previous nitrogen isotope studies of Neoproterozoic sedimentary rocks observed mostly positive δ¹⁵N values (Ader et al., 2014; Wang et al., 2017; Stüeken et al., 2019a). Such values have mostly been interpreted to reflect an aerobic nitrogen cycle, due to the similarity of measured δ¹⁵N to the modern ocean. As discussed, however, the relationship between N-isotopes as recorded in sediments and water column redox state is not straightforward. There are multiple potential environmental redox structures that can produce similar δ¹⁵N values in sediments.

Thus, in order to interpret nitrogen isotope data it is essential to independently establish redox conditions in the depositional environment. This ideally includes both water column and sediment-water interface proxy data. Previous work has combined redox proxies with carbon isotopes (e.g., Kunzmann et al., 2015), and some have combined redox proxies with nitrogen isotopes (Quan et al., 2008, 2013; Zerkle et al., 2017; Stüeken et al., 2019b). This is crucial, given the strong connection between nitrogen cycling and oxygen. In this contribution, we combined redox proxies - redox sensitive trace elements and Fe-speciation - and nitrogen isotopes. Specifically, we apply this approach to sediments from the Neoproterozoic Snowball Earth ocean with new measurements from the Mineral Fork and Kelley Canyon Formations in Utah, United States. We further compare our results to previous data from the Ghaub Formation (Johnson et al., 2017b) and find that syn-glacial sediments have consistently positive δ¹⁵N values which increase after the glaciation. Iron speciation suggests mostly anoxic bottom water during the glaciation and oxygenated bottom water after. Redox sensitive trace element concentrations suggest oxic weathering and an oxygenated upper water column throughout both the glaciation and afterwards. Combining all data, we suggest persistent oxygenated shallow water during Snowball glaciations which expands in depth after the glaciations. This finding supports previous interpretations of the Neoproterozoic ocean redox structure, and provides additional evidence for oxygenated water in the syn-glacial ocean.

2 Geologic Setting and Sample Descriptions

Samples presented in this study are from the Marinoan Ghaub Formation, Namibia (Johnson et al., 2017b) and the Mineral Fork Tillite and Kelley Canyon Formation, Utah, USA (Figure 1). Detailed description of the Ghaub samples are in Johnson et al. (2017b), but briefly these units are interpreted to be glacial outwash deposited below the ice-grounding line. The sequence was deposited during the Marinoan ( Ma), though exactly when during this interval is unclear. The sampled unit unconformably overlies the Naarchams Member, and conformably grades upwards into the overlying cap carbonate, the Keilberg Member. In detail, the Ghaub Formation is predominantly detrital carbonate (Hoffman, 2016), as the glaciers in this area were eroding an underlying carbonate platform. Specifically, samples were taken from the Fransfontein Homocline (Hoffman, 2011).

FIGURE 1

The Mineral Fork Formation is found in the central Wastach Range and on Antelope Island in Utah, United States. The unit comprises interbedded diamictites, sandstones and mudstones (Ojakangas and Matsch, 1980; Christie-Blick, 1982). The diamictite is typically massive, while the sandstone is subfeldspathic to quartzose, and includes laminated and cross-bedded facies; mudstones contain rare dropstones, several Fe-rich layers (Young, 2002), and carbonaceous layers with microfossils (Knoll et al., 1981). The unit is interpreted as having been deposited in a marginal marine setting, with dynamic glacial activity and periodic connection to the open ocean (Crittenden et al., 1983). Further, the overall geologic setting is consistent with a rifted margin creating accommodation space, but the depositional rate of the diamictite could have been more rapid than the accumulation of space due to tectonic activity (Hoffman P. F. et al., 2017). The relationship between glaciation, rifting, and deposition during the Sturtian in western North America may be complex (e.g., Kennedy and Eyles, 2021). We assume, however, for this study, that the Mineral Fork Formation is the result of glacial deposition in a marginal marine environment.

Age constraints are relatively poor. The Mineral Fork overlies the Big Cottonwood Formation, which is stratigraphically correlated with the Ma Uinta Mountain Group (Link and Christie-Blick, 2011). High Fe content supports association with Sturtian glaciation successions in North America (Young, 2002). The cap carbonate above the Mineral Fork Formation, however, is visually and texturally similar to the Keilberg Member in Namibia, supportive of a Marinoan age. Detrital zircons from the Kelley Canyon Formation yield maximum deposition ages for the till of 703 ± 6 Ma and an overlying graywacke of 667 ± 5 Ma (Balgord et al., 2013). There are no strong minimum age constraints. Thus, while existing geochronology does not rule out a younger, Marinoan age, these data more strongly support that the Mineral Fork tillite is Sturtian in age.

The paleogeographic location and tectonic setting of the Mineral Fork Formation is also not well constrained, due to its uncertain age. If the Mineral Fork is Sturtian, it is likely that it was deposited in a rift basin (Li et al., 2013; Merdith et al., 2017). Such a setting is more likely to have been a restricted basin, and thus would not record global oceanic geochemical conditions. If, however, the Mineral Fork is Marinoan, it was more likely deposited in a basin that was better connected to the global ocean (Li et al., 2013; Merdith et al., 2017). Regardless of this under-constrained geological setting, the main purpose of this study is to demonstrate that interpreting N isotopes in the geologic record is strengthened by combined consideration of independent redox proxies. Providing better constraints on the age of the Mineral Fork Formation, and whether or not these samples represent local or global signals, are beyond the scope of this paper.

We also analyzed samples from the Kelley Canyon Formation, which includes a post-Marinoan cap dolomite and a slate member (Table 1). Specifically, samples are from the slate member and from three distinct facies. The three facies contain layers that are purple and resistant, tan and resistant, and green-gray recessive. Bedding in all facies is thin (∼1–3 cm), supporting a depositional setting that is below wave-base (Table 1).

3 Analytic Techniques

For all samples, we collected specimens free of obvious weathering or alteration. Prior to analyses, weathered edges of hand samples were trimmed with a rock saw. Samples were broken into smaller pieces with a rock hammer, and crushed to a powder in a motorized agate mortar and pestle. As detailed later, we argue that analyzed values represent primary values. Regional metamorphism was greenschist facies or lower (Yonkee et al., 2014), and measured values are very similar to previous work (Johnson et al., 2017b).

3.1 Nitrogen Concentration and Isotopes

Nitrogen isotopic techniques for the Namibian samples are presented in (Johnson et al., 2017b), and are similar for Mineral Fork and Kelley Canyon Formation samples. Nitrogen concentration and isotope ratios for the Mineral Fork Formation were characterized at the University of Colorado Boulder Earth Systems Stable Isotope Lab (CUBES-SIL), and values for the Kelley Canyon Formation were determined, using identical protocol, at the Earth System Evolution Lab (EaSEL) at Iowa State University. Carbon concentrations were obtained during the same runs as N concentration and isotopic analyses, using the same standards.

Sample powders were decarbonated via reaction with 5 ml of 6 N HCl. Samples were then sonicated for 30 min. All tubes were then placed in a 60°C oven overnight. The next day, samples were centrifuged to settle all undissolved material. Acid was poured off, fresh acid was added as before, and samples sat in the oven overnight. This acid refresh was repeated once more. To clean samples, all were rinsed three times with DI H₂O, centrifuging between each rinse. Sample powders then dried at 60 °C for 2 days; all vials containing multiples of the same sample powder were combined and homogenized after drying.

Samples were then analyzed on a Thermo Delta V after combustion in a Thermo Elemental Analyzer at CUBES-SIL and on a Thermo Delta V Plus following combustion in a Thermo Isolink EA at EaSEL. Between 50 and 100 mg of sample powder was weighed into a Sn capsule, as well as standards: two acetanilides (act1, δ¹⁵N = 1.18⁰/₀₀ ± 0.02 and act2, δ¹⁵N = 19.56⁰/₀₀ ± 0.03) and pugel (δ¹⁵N = 5.45⁰/₀₀ ± 0.1) at CUBES-SIL and urea (internal, δ¹⁵N = −2.60 ± 0.2⁰/₀₀) and caffeine (USGS62, δ¹⁵N = 20.17 ± 0.1⁰/₀₀) at EaSEL. All samples were flash-combusted with an excess of O₂ at 1,020° C in a combustion column packed with cobaltous oxide (combustion aid) and silvered cobaltous oxide (sulphur scrubber). Combustion products were passed over a reduced copper to reduce all N to N₂ and absorb excess O₂. Finally, sample gas was passed through a magnesium perchlorate trap to absorb water and a 3 m gas chromatography column to separate N₂ from CO₂. All analyses were quantified using IsoDat software and the Isoreader pipeline (Kopf et al., 2021). Errors reported are standard deviations from repeated analyses.

While the dominant mineralogy of the Ghaub at this location is carbonate, the geochemical results presented here, and in Johnson et al. (2017b), reflect analysis of the siliciclastic portion of the Ghaub sediment. Carbonate was removed via dissolution, so that the clay minerals could be analyzed directly. Clays record both the N signal (Ader et al., 2016) and the trace metal budget (Tribovillard et al., 2006; Johnson et al., 2017b).

Mineral Fork and Kelley Canyon samples were analyzed in the same run as previously measured glacial till samples (Timeball Hill and Blaubeker) from published work (Gaschnig et al., 2016; Johnson et al., 2017b). Measured N and δ¹⁵N for Timeball (250 ppm, 5.06⁰/₀₀) and Blaubeker (158 ppm, 4.1⁰/₀₀) are similar to published values: 305 ± 5 ppm and 4.9 ± 0.3⁰/₀₀ for Timeball Hill; and 90 ± 20 ppm and 4.4 ± 0.2⁰/₀₀ for Blaubeker. We note that agreement in δ¹⁵N values is quite good, though there are differences in N concentration outside uncertainty. Further, we also ensured that organic standards (urea, acetanilide, pugel, and caffeine) used to calculate N concentration included very small standard amounts (g total mass) to capture relatively low N content in these rocks. The overall N concentrations are in the 100s of ppm, which is a reasonable N content suitable for EA-IRMS analysis (Bräuer and Hahne, 2005; Boocock et al., 2020).

3.2 Redox Proxies

The Fe speciation method targets operationally defined Fe pools, including carbonate-associated Fe (Fe_carb), ferric oxide Fe (Fe_ox), magnetite Fe (Fe_mag) and pyrite Fe (Fe_py). Extractions were performed according to well-established protocols (Poulton and Canfield, 2005, 2011; Poulton, 2021), with subsequent analysis via atomic absorption spectroscopy (AAS) for Fe_carb, Fe_ox and Fe_mag at the University of Leeds. Fe_py was determined gravimetrically following chromous chloride distillation at the University of St Andrew’s. Previous studies have shown reproducibility of better than ±2% (Mettam et al., 2017). Total Fe (Fe_T) was determined after HF–HClO₄–HNO₃ dissolution via AAS. All Fe extractions gave a relative standard deviation (RSD) of based on replicate analyses, and sequential extraction analyses were within 5% of the international Fe-speciation reference material WHIT (Alcott et al., 2020). Total dissolution of international sediment standards (USGS; SGR-1bl; USGS SBC-1) gave an Fe recovery of .

The sum of Fe_carb, Fe_ox, Fe_mag and Fe_py defines a highly reactive (Fe_HR) pool, which is considered to represent Fe that is biogeochemically reactive during deposition and early diagenesis (Raiswell and Canfield, 1998; Poulton et al., 2004). Anoxic waters commonly have Fe_HR/Fe_T ratios 0.38, in contrast to oxic depositional conditions where ratios are generally 0.22 (Poulton and Canfield, 2011). Elevated Fe_HR/Fe_T ratios in anoxic settings arise from the additional water column formation of pyrite in euxinic (sulphidic) settings, or unsulphidized Fe_HR minerals in ferruginous (Fe-containing) settings. Thus, for anoxic samples (i.e., Fe_HR/Fe_T > 0.38) the ratio of Fe_py/Fe_HR distinguishes euxinic (Fe_py/Fe_HR > 0.8) from ferruginous (Fe_py/Fe_FR < 0.6) water column conditions (Poulton and Canfield, 2011; Benkovitz et al., 2020; Poulton, 2021). Fe_HR/Fe_T ratios of 0.22–0.38 are considered equivocal, and may occur due to the masking of water column enrichments via rapid sedimentation (e.g., during turbidite deposition Canfield et al., 1996), or due to transformation of unsulphidized Fe_HR to clay minerals during diagenesis and metamorphism (e.g., Poulton et al., 2010). This second possibility can be evaluated by considering Fe/Al ratios, since Fe_T is preserved even if Fe_HR is lost to clay minerals. In this case, normal oxic marine shales tend to have Fe/Al ratios of 0.55 ± 0.11, and thus Fe/Al 0.66 is considered to provide a robust indication of water column anoxia (Clarkson et al., 2014).

Trace element composition was determined by ActLabs following their Ultratrace 4 protocol (https://actlabs.com/geochemistry/exploration-geochemistry/4-acid-near-total-digestion/). This is a total digestion using HF, HNO₃ and HCl, followed by analysis via ICP-MS. All measured values are above stated detection limits which are 0.1, 1 and 0.05 ppm for U, V and Mo, respectively. Reported accuracy for all elements are within 1–5% compared to certified standards (GXR-4, SDC-1, DNC-1a, and SBC-1). Reported precisions are ±0.3 ppm for U, ± 2 ppm for V, and ±0.1 ppm for Mo.

4 Results and Interpretation

We will first discuss the Fe-speciation and U, V and Mo redox proxy data. These establish the geochemical boundary conditions that govern the nitrogen cycle, and within which N isotope data can be interpreted. Then, we place the N isotopic data within this geochemical context. We also include data from Johnson et al. (2017b).

4.1 Redox Proxies

Iron speciation data from the Ghaub Formation are characteristic of anoxic, ferruginous bottom water (Figure 2), with Fe_pyrite/Fe_HR ratios below 0.1 and Fe_HR/Fe_total above 0.38. The Mineral fork and Kelly Canyon Formations, however, have Fe speciation data ranging from ferruginous to oxic conditions. In both Utah and Namibia, there appears to be minor fluctuations between anoxic and possibly oxic conditions, but in the samples from the Kelley Canyon Formation reflect oxygenated bottom waters after the Snowball glaciation (Table 2).

FIGURE 2

In all Utah samples, Mo is low, between detection limit of 0.05 ppm and 0.24 ppm, with the exception of one sample at 4.8 ppm. Vanadium ranges from 20–72 ppm, and U ranges from 0.4 to 6 ppm, with most values above 2.5 ppm (Table 2). These variations are only partially due to differential detrital input, as seen by plotting U normalized to Zr as a function of Al (Figure 3). Zirconium is geochemically similar to U, but Zr is non redox-active, and when plotted against Al (a proxy for detrital input), it is apparent that there is a weak correlation (r² = 0.36). This indicates that some variation in trace metal abundance may be related to detrital input into the basin, but it is likely not the primary control. In contrast, we observe high U values at low Al concentration in Ghaub samples, indicating authigenic enrichment of U in this location.

FIGURE 3

There is no clear correlation, however, between the trace element concentration (U and Mo), normalized to Post-Archean Average Shale (PAAS), and total organic carbon (TOC) (Figure 4). By normalizing to PAAS, we are showing whether or not each trace element is enriched compared to the average over time. Such an “enrichment factor”, when plotted as a function of TOC, is an effective redox proxy (Algeo and Liu, 2020). Specifically, this indicates that scavenging by organic carbon during burial is unlikely to be the primary control on trace element abundance. Rather, the abundance of trace elements indicate oxic weathering and delivery of soluble forms of these elements to the water column.

FIGURE 4

Johnson et al. (2017b) interpreted the Mo, U and V data from Namibia to reflect oxic weathering on the continents, followed by delivery to the ocean and scavenging in ferruginous bottom waters. While U supply is controlled by detrital input, by normalizing to Al (Figure 3), Johnson et al. (2017b) demonstrated that changing U concentrations were not due to changing detrital input alone, and were consistent with iron speciation data. Similarly, these redox-sensitive elements from Utah are present in similar concentrations (Figure 5). Uranium, specifically, also displays a negative correlation with Fe_HR/Fe_Total, consistent with higher U concentrations reflecting a more pervasively oxygenated water column.

FIGURE 5

There is no enrichment, however, in Mo or V observed from the Mineral Fork or Kelley Canyon formation. Recent work from the early Cambrian of South China has suggested that in settings with a well developed, dynamic redox-cline, the behavior of Mo, V, and U may be decoupled (Han et al., 2018). Specifically, the presence of a zone of Fe-Mn particulate formation can preferentially enrich sediments in V. If such a zone is absent, or sediments sampled are from shallower water, such V enrichment could be missing. It is likely that the Mineral Fork samples were deposited near the ice-grounding line (Christie-Blick, 1982), and perhaps the water column at this location did not contain a Fe-Mn particulate shuttle, leading to low V concentrations.

Overall concentrations of these elements observed from Utah and Namibia are similar to other work focused on the Cryogenian. For example, a section spanning both Sturtian and Marinoan from the Zavkhan Terrane, southwest Mongolia finds U concentrations of between 0.05 and 2 ppm, very similar to our measurements (Lau et al., 2017). Trace element measurements from the Doushantuo Formation of South China, deposited after the Marinoan glacial period, display quite high concentrations of U (up to 30 ppm), V (1,000 ppm), and Mo (200 ppm) in the immediate deglacial period, with values decreasing rapidly up-section (Sahoo et al., 2012). Since the Kelley Canyon formation overlies the cap carbonate, it would have been deposited sometime after the deglaciation, and thus likely does not capture the immediate deglaciation.

4.2 Nitrogen Isotopes

We present N concentration and isotope data in Figure 6. δ¹⁵N in both Namibia and Utah are positive, between about 1 to 5⁰/₀₀ from the Ghaub and 4.1 to 7.6⁰/₀₀ in the Mineral Fork (Figures 6, 7). Values from the Kelley Canyon Formation are higher, between 6.7 to 8.9⁰/₀₀. Nitrogen concentration is similar in units from Namibia and Utah, ranging from 100 to 650 ppm. As noted in Section 3.1, analysis of previously measured till samples from the Timeball Hill and Blaubeker formations produced nearly identical δ¹⁵N values but different N concentrations. Previous work has noted that within-sample heterogeneity may be an issue for N measurements (Johnson et al., 2017a). Rock hosted N can potentially exist as different species (e.g., , , N₂) and in different organic or mineral phases. So, it is possible that small scale heterogeneity, or slightly different analytical conditions/techniques may be liberating different pools of N. Bulk N isotope values, however, seem to be robust even if measured N concentrations vary (Bräuer and Hahne, 2005; Boocock et al., 2020).

FIGURE 6

FIGURE 7

To assess the potential influence of metamorphic alteration of N content and isotopic signals, we show δ¹⁵N plotted against N concentration, Rb concentration, and C concentration (Figure 6). We observe no correlation between δ¹⁵N and N concentration, suggesting that higher δ¹⁵N values are not caused by progressive N loss during metamorphism (Bebout and Fogel, 1992). There is a rough correlation between N and Rb concentration, suggesting some variation in N content could be related to detrital input. The lack of correlation, however, between δ¹⁵N and N concentration again indicates that while there might be some lithologic control on total N, this does not influence isotope values. There is not a clear correlation between C and N content, which is different than previous work on the non-glacial Neoproterozoic (e.g., Ader et al., 2014), but similar to previous analyses of syn-glacial units (Johnson et al., 2017b). This suggests that there may be either differential preservation of N and C in these units, or that the N and C cycles are differently coupled during Snowball glaciations than the non-glacial intervals of the Cryogenian.

Assuming atmospheric N₂ was at 0⁰/₀₀ during the Cryogenian (Stüeken et al., 2016b), enriched δ¹⁵N values, given the evidence of persistent O₂ availability provided by redox-sensitive element data, are consistent with the presence of partial, water-column denitrification during the Snowball glaciation. That is, there must have been sufficient O₂ present to support nitrification, with denitrification occurring at boundaries between oxygenated and low-oxygen waters, with a likely chemocline location in bottom waters. The similarity of Mineral Fork δ¹⁵N values to average modern marine values (5 to 7⁰/₀₀) indicates that a similar relative amount of partial denitrification was occurring in the water column (Tesdal et al., 2013).

After the glaciation, as preserved by the Kelley Canyon formation, samples have the highest δ¹⁵N values measured. These values are typically interpreted to indicate more extensive denitrification, and lower water-column O₂, to drive residual N to higher δ¹⁵N values. Yet, Fe-speciation data indicates oxygenated bottom waters during the deposition of the Kelley Canyon Formation (Figures 2, 7). Kelley Canyon units also have the highest U concentrations of any samples presented (Figure 5), suggesting oxic weathering and delivery of U to the site of deposition. Thus, we require an interpretation that can explain both higher δ¹⁵N values and a pervasively oxygenated water column. We propose two potential explanations: oceanographic circulation shifts and biologic responses to deglaciation.

A similar pattern, positive but low and then increasing several permil, in δ¹⁵N values in sediments has been observed in a number of locations during the Pleistocene-Holocene deglaciation (Galbraith and Kienast, 2013). Several sites are near-shore environments, including one off the coast of northern Chile (De Pol-Holz et al., 2006). De Pol-Holz et al. (2006) observed an increase in sedimentary δ¹⁵N values from 8 to 12⁰/₀₀ across the deglaciation, and attributed this increase to melting of the Patagonian Ice Sheet. The influx of freshwater slowed formation of intermediate water, which in turn decreased deep water ventilation. This oceanographic reorganization could have resulted in upwelled N that was enriched in ¹⁵N, due to removal by denitrification, compared to the glacial maximum with a less well developed oxygen minimum zone. The Snowball deglaciation would have produced a large meltwater lid (Hoffman PF. et al., 2017), which could affect ocean circulation in a similar way: limited deep-water ventilation, increased denitrification at a chemocline underlying a well oxygenated upper water column.

In the wake of both the Sturtian and the Marinoan glaciations, the ocean appears to have become more oxygen rich and the surface temperature increased (Hoffman PF. et al., 2017; Lechte et al., 2019). There are many studies on contemporary denitrifying bacteria, both in the field and in culture, that demonstrate an increase in denitrification activity at higher temperatures (Nowicki, 1994; Veraart et al., 2011). Thus, we would expect to observe an increase in δ¹⁵N in post-Snowball warm oceans, observed in both the Ghaub and Mineral Fork-Kelley Canyon Formations. While an increase in water column oxygen may seem anathema to increased denitrification, modern denitrifiers do function in oxygenated water. Specifically, by using the Nap reductase enzyme, instead of the Nar enzyme (Berks et al., 2001), organisms can perform aerobic denitrification (Ji et al., 2015). Perhaps, then, the combined effect of high temperatures and aerobic denitrification caused an increase in δ¹⁵N values in the post-Snowball ocean, even in the presence of higher water column O₂.

Alternately, it is possible that enriched δ¹⁵N values could reflect partial nitrification followed by complete denitrification. This scenario was proposed to explain very positive δ¹⁵N values from the 2.7 Ga Tumbiana Formation (Thomazo et al., 2011), though an alternate explanation has been proposed suggesting high values could be caused by alkaline conditions and degassing of NH₃ (Stüeken et al., 2015). Nitrification to nitrite has a pronounced isotopic effect, with product depleted between 13 to 38⁰/₀₀ compared to reactant (Casciotti et al., 2003). If, then, is quantitatively removed via anammox, the residual N pool would be comprised of isotopically enriched . In the modern ocean, this scenario has been observed in sediments of the Bering Sea (e.g., Morales et al., 2014). There is, however, a much wider range in sediment δ¹⁵N values (+2⁰/₀₀ to + 18⁰/₀₀), reflecting the transient nature of the geochemical conditions necessary for this to occur. Since the N isotope data from both Cryogenian sections are more tightly coupled with evidence for persistent oxic weathering, we suggest that they instead reflect less transient redox conditions, and likely complete nitrification followed by partial denitrification.

Additionally, we note that δ¹⁵N values are generally lower in samples with Fe_HR/Fe_total values that are ‘equivocal’ compared to those that clearly plot in the ferruginous field (Figure 2, 7). Since this potential trend is not strongly correlated (r² = 0.13), any interpretations at this point are speculative. It is possible that samples with low Fe_total, and therefore higher Fe_HR/Fe_total, represent upwelling into oxygenated conditions, driving denitrification and increasing δ¹⁵N values. However, iron speciation data does not allow for quantitative interpretation of bottom water conditions within fields, but rather serves to distinguish between overarching redox conditions. Future work, either observational or geochemical modeling, could be valuable to investigate any controls on patterns within Fe-speciation fields.

5 Implications for Snowball Ocean and Redox-Biologic Interpretations

There are two major implications from this work. First, we present further support for periods of oxygenated water during the Snowball Earth glaciations based on Fe-speciation, redox sensitive trace elements and enriched δ¹⁵N values. Second, we suggest corroborating evidence for a relatively rapid increase in oxygen content in the post-Snowball ocean set against a background of variable redox conditions (Kunzmann et al., 2017; Lau et al., 2017) while also providing hypotheses to explain positive δ¹⁵N during the Snowball ocean that increased after the glaciation.

Previous data from Namibia, combining N isotopes and redox proxies, had suggested pockets of oxygenated water with active photosynthesis and aerobic N cycling during the Marinoan (Johnson et al., 2017b). Our work herein records similar geochemical values, consistent with a similar water-column redox/nutrient state. Thus, in at least two locations - during a Snowball ocean - there is geochemical evidence for oxygenated water. What is unclear, though, is whether samples from Namibia and Utah represent contemporaneous time periods. Age control from Namibia is relatively tight (Hoffman et al., 2021), clearly indicating the Ghaub Formation is Marinoan in age.

As previously mentioned, the age of the Mineral Fork is crucial for any interpretation linking this site to broader global patterns of redox structure. If the Mineral Fork is Sturtian, it is likely that it was deposited in a rifting basin (e.g., Balgord et al., 2013). The similarity of N and redox geochemical values from Mineral Fork and the Ghaub suggest that they record broadly similar ocean conditions. We cannot say to what extent this might be a global signal during each glaciation, since at present these are the only sites with N and redox data synchronous with the Snowball ocean. Perhaps, then, marginal marine settings during the Marinoan and Sturtian glaciations were geochemically similar. While it is beyond the scope of this paper to determine global ocean redox, our work does demonstrate that combining proxies from geographically distributed samples can help fill in this picture. We suggest long-lived open water and/or ample photosynthesis to keep the atmosphere and at least parts of the upper ocean oxygenated. There may even have been periodic bottom water oxygenation during the deposition of the Mineral Fork Formation.

Comparison to other geochemical and observational work also indicates the presence of persistent oxygenated environments throughout the Snowball ocean. For example, geochemical analyses of iron formations (IFs) indicate that subglacial meltwater would have provided O₂ to marginal marine settings (Lechte et al., 2019). Measurements of cyclicity in banded IFs are consistent with ice sheets responding dynamically to orbital forcing (Mitchell et al., 2021). A potential consequence of such orbital forcing is that dynamic ice could have facilitated oxygenation of the surface ocean via air-sea gas exchange upon either ice retreat or advancement (Mitchell et al., 2021).

Our work demonstrates the utility of the combined redox proxy-N isotope analytical strategy for characterizing Precambrian units. This is especially useful during times of dynamic fluctuations in oxygen, both in the atmosphere and ocean (Sperling et al., 2015; Wei et al., 2021). For example, previous work has demonstrated that different oceanic redox structures can result in similar ranges of δ¹⁵N values recorded in sediment (Ader et al., 2014). Values above approximately 0⁰/₀₀ could be generated in the water column, and therefore recorded in sediments, either from modern-like conditions or strongly redox stratified conditions. For the former, this is the result of complete nitrification followed by partial denitrification. For the latter, higher than zero δ¹⁵N values can be caused by a combination of ammonification and partial assimilation (Ader et al., 2014). Such a redox-cline can either be shallow or deep, and still produce similar δ¹⁵N values.

Thus, water column redox reconstructions are crucial for interpretation of N isotopes. Specifically, using Fe-speciation for bottom water redox and redox-sensitive trace elements for surface water/atmospheric redox conditions can contextualize N isotopes values. As more work utilizes N isotope measurements as a tool to investigate both biologic and redox activity (e.g., Peng et al., 2020), placing such biologically controlled signals into a redox context will be crucial.

6 Conclusion

The evolution of ocean redox and water column nutrient cycling are closely intertwined over Earth history. There are a number of redox proxies, including redox sensitive trace metal abundance and iron speciation, that can be used to investigate oxygenation of the ancient atmosphere and oceans. Stable isotope ratios of major nutrients, such as carbon and nitrogen, are often used to reconstruct ancient microbial ecosystems and metabolisms. Nitrogen isotopes are commonly measured in ancient sediments, but produce non-diagnostic signals. Indeed, there is more than one pathway or combination of fluxes in the nitrogen cycle that could produce the same bulk δ¹⁵N signal. Due to the tight relationship between the nitrogen cycle and oxygen availability, combining redox proxies with isotopic measurements provides insight and context into the redox environment crucial for interpreting isotopic signals.

We utilized a combined redox and N-isotope approach to specifically highlight the nitrogen cycle during the Neoproterozoic Snowball Earth glaciations. We analyzed deposits from Utah, USA likely preserving the Sturtian (Mineral Fork Formation) and post-Sturtian (Kelley Canyon Formation) time periods, and show previous work from the Ghaub Formation in Namibia for comparison (Johnson et al., 2017b). We find that redox proxies (U, V and Mo concentrations, and Fe speciation) are consistent with the presence of oxygenated water near the surface and potentially near the sediment water interface as well. Nitrogen isotope ratios are all above 0⁰/₀₀, and given redox constraints, are indicative of nitrification followed by partial denitrification in the water column. The geochemical proxies are quite similar between the two field sites, suggesting similar water conditions at different paleogeographic locations and times. That is, we present evidence for oxygenated water during Cryogenian glaciations. More broadly, our work shows that combining these two approaches yields a deeper insight into ancient ecosystems and water conditions over Earth history. We also highlight the importance of considering oceanographic and biologic interpretations of redox data, as these are important factors controlling N isotope signals in biomass and sediments through time.

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