Dynamic redox and nutrient cycling response to climate forcing in the Mesoproterozoic ocean

Domain: earth_science
Published: 2023-10-20
Editorial outcome: minor_revision

Abstract
Controls on Mesoproterozoic ocean redox heterogeneity, and links to nutrient cycling and oxygenation feedbacks, remain poorly resolved. Here, we report ocean redox and phosphorus cycling across two high-resolution sections from the ~1.4 Ga Xiamaling Formation, North China Craton. In the lower section, fluctuations in trade wind intensity regulated the spatial extent of a ferruginous oxygen minimum zone, promoting phosphorus drawdown and persistent oligotrophic conditions. In the upper section, high but variable continental chemical weathering rates led to periodic fluctuations between highly and weakly euxinic conditions, promoting phosphorus recycling and persistent eutrophication. Biogeochemical modeling demonstrates how changes in geographic allocation relative to global atmospheric circulation cells could have driven these temporal changes in regional ocean biogeochemistry. Our approach suggests that much of the ocean redox heterogeneity apparent in the Mesoproterozoic record can be explained by climate forcing at individual locations, rather than specific events or step-changes in global oceanic redox conditions.

Introduction
Our understanding of the chemical evolution of the Precambrian ocean has evolved markedly over recent years. For the Mesoproterozoic (1.6–1.0 billion years ago; Ga) in particular, a stratified ocean redox model has emerged, with oxic surface waters commonly being underlain by euxinic (sulfidic) mid-depth waters in productive areas, with deeper waters dominantly being ferruginous (anoxic and iron-bearing)1–3. Emerging geochemical evidence suggests that Mesoproterozoic ocean redox chemistry was, however, considerably more variable than this generalized model implies. For example, while long-term paleo-records in some areas document dominantly ferruginous conditions with oxygenation of only very shallow waters4, other studies suggest either a deepening of the oxycline5–7, or the development of dysoxic8,9 or oxic deeper waters10,11. In addition, transitions between ferruginous and euxinic conditions have been documented within the same stratigraphic succession on multi-million-year timescales12.

The factors that governed this variability in Mesoproterozoic ocean redox chemistry are essentially unknown. These uncertainties are at least partly due to inherent difficulty in distinguishing temporal from spatial variability when comparing limited, and often poorly-dated, Mesoproterozoic records from different settings and time periods. This also limits understanding of potential oxygenation feedbacks related to the availability of essential nutrients such as phosphorus, which was strongly impacted by variability in the precise redox state of the Mesoproterozoic water column13–16.

One factor that has largely been overlooked in terms of its impact on ocean redox chemistry and nutrient cycling in the Mesoproterozoic is the role of climate, both on longer (e.g., millions of years) and shorter (e.g., orbital) timescales, and in terms of the climatic regime at the paleogeographic location of individual study sites. Recently, a climatic control on orbital and longer (tens of million of years) timescales has been documented in relation to the geochemistry of marine sediments from the ~1.4 Ga Xiamaling Formation on the North China Craton (NCC)12,17. However, detailed links between changes in climate, oceanic redox conditions and nutrient cycling have not specifically been explored, which limits understanding of controls on regional-scale ocean redox heterogeneity and hence links to global-scale oxygenation dynamics.

Here, we focus on two sections sampled at cm-scale, which comprise fine-grained siliciclastic rocks from the Mesoproterozoic Xiamaling Formation. We combine multiple redox proxies (Fe speciation, redox-sensitive trace element systematics, pyrite S isotopes) with elemental weathering indicators (K/Al, Ti/Al), and we utilize a phase partitioning approach to reconstruct P cycling. Our focus on two distinct high-resolution sections allows a detailed evaluation of climatic controls on ocean redox and nutrient cycling, both on orbital timescales (within sections) and over million of years (between sections). We then utilize a biogeochemical multi-box model to explore how distinct regional climatic regimes may have instigated local variability in ocean biogeochemistry, thus providing a compelling explanation for the extensive ocean redox heterogeneity observed in the Mesoproterozoic paleo-record.

Results
Geological setting
The investigated Xiamaling Formation succession was deposited in the Xiahuayuan region, NCC (Fig. 1)12,17. The Xiamaling Formation is commonly divided into six units, with the first four units (note that these units are numbered from the top of the section downwards) comprising fine-grained siliciclastic rocks that represent deep-water deposition near or below storm wave base12,17. High-precision zircon dating of a tuff layer in unit 2 and a bentonite layer at the top of unit 3 gave ages of 1384.4 ± 1.4 million years ago (Ma) and 1392.2 ± 1.0 Ma, respectively, which constrain deposition of the Xiamaling Formation to within the Ectasian Period of the Mesoproterozoic Era17. The Xiamaling Formation rocks in the study area are of very low thermal maturity, with burial temperatures dominantly below 90°C17. Paleogeographic reconstructions place the NCC at low latitudes (~15°N) during deposition of the Xiamaling Formation18,19. Further details of the geological setting are provided in the Supplementary Information.

Redox geochemistry
Oceanic redox conditions were reconstructed for continuous sections from unit 1 (Section A) and unit 4 (Section B) of the Xiamaling Formation (Fig. 1; Fig. S1; see Methods for analytical procedures and Supplementary Information for details of the geochemical proxies used). In Section B, total organic carbon (TOC) concentrations are very low (~0.06–0.09 wt%), with slight peaks occurring in the green mudstone intervals (Fig. 2B). Highly reactive iron over total iron (FeHR/FeT) ratios show clear cyclicity through this section, with values below 0.22 in the green mudstone intervals, and above 0.38 in the red mudstone intervals (Fig. 2B). However, uranium enrichment factors (UEF; see Methods) show the opposite trend, with mild enrichments (i.e., >1) in the green mudstone intervals, and values below 1 in the red mudstone intervals. Very low Fepy/FeHR ratios demonstrate negligible pyritization of the FeHR pool throughout Section B (Fig. 2B), and this is accompanied by molybdenum enrichment factors (MoEF) that are also very low (≤2.0; Supplementary Data 1). Rhenium enrichment factors (ReEF) are elevated throughout the section, while manganese enrichment factors (MnEF) are persistently low (Fig. 2B).

The geochemistry of Section A contrasts greatly with that of Section B. In Section A, TOC concentrations range from 2.43 wt% to 4.96 wt%, with clear cyclicity between relatively TOC-rich and TOC-poor intervals (Fig. 2A; Supplementary Data 1). All samples in this section have FeHR/FeT ratios above 0.38, with UEF values that are consistently elevated (i.e., >1). In almost all cases, Fepy/FeHR ratios are above 0.6, while MoEF values are also high, but with distinct cyclicity that broadly matches the TOC cyclicity (Fig. 2A; note that some geochemical profiles exhibit slightly offset responses, which we explore further in the Supplementary Information). Pyrite δ34S data also show general cyclicity, whereby higher values tend to occur in the intervals where TOC concentrations and MoEF values are lower (Fig. 2A).

Nutrient and weathering proxies
In Section B, total P concentrations (expressed in terms of P/Al in ppm/wt%) are lower than the average upper continental crust (UCC) value (P/Al = 87 ppm/wt%)20, with distinct peaks in the upper parts of the green mudstone horizons (Fig. 2B). Reactive P concentrations (P phases that are potentially bioavailable, expressed as Preac/Al; see Methods) show similar trends to P/Al ratios, suggesting that reactive P exerts the dominant control on variability in the P/Al profile. Molar TOC/organic P (TOC/Porg) and TOC/Preac ratios are both considerably lower than the Redfield ratio (TOC/P ratio = 106:1), with no apparent trends through the section (Fig. 2B). K/Al ratios are consistently higher than the UCC average (0.3), with no clear trends through the section, while Ti/Al ratios are also consistently above the UCC average (0.05), with slight peaks in the middle of green mudstone intervals, and lower values dominating in red mudstone intervals (Fig. 2B).

P/Al ratios are above the UCC average throughout Section A, but distinct troughs occur in the low TOC intervals, and these intervals are also characterized by similar troughs in Preac/Al ratios (Fig. 2A). Compared to the Redfield ratio, TOC/Porg ratios in Section A are highly elevated, with distinct cyclicity that matches trends in TOC (Fig. 2A). TOC/Preac ratios are lower than TOC/Porg ratios and show opposite cyclicity, but all values remain above the Redfield ratio (Fig. 2A). K/Al ratios are consistently above the UCC average and reach peaks in the intervals characterized by lower TOC, followed by a general progressive decrease to lower values (Fig. 2A). Ti/Al ratios are below the UCC average with no distinct change through the section.

Redox cyclicity
For Section B, we reconcile the contrasting redox signals from FeHR/FeT ratios and UEF values (Fig. 2B) by additionally considering the phase partitioning of the FeHR pool, which is dominated by oxic phases (e.g., Fe(oxyhydr)oxides) in the red mudstone intervals, and reduced FeHR phases (e.g., siderite) in the green mudstone intervals (Supplementary Data 1). Together, these geochemical signals suggest that the green mudstones were deposited under anoxic bottom water conditions (giving elevated UEF values)21,22, but extensive reductive dissolution of Fe(oxyhydr)oxide minerals during early diagenesis promoted recycling of a proportion of the dissolved Fe2+ to the water column (giving lower FeHR/FeT ratios)3,12,23. This would promote ferruginous, rather than euxinic, water column anoxia, which is supported by very low Fepy/FeHR ratios (Fig. 2B) and low MoEF values (Supplementary Data 1).

By contrast, the geochemistry of the red mudstones suggests deposition under less reducing conditions, where Fe2+ was oxidized in the water column and precipitated as Fe(oxyhydr)oxide minerals. An oxygen minimum zone (OMZ) setting has previously been invoked for this part of the Xiamaling Formation12. Our high-resolution data support this scenario, with the green mudstones representing deposition within the ferruginous OMZ, and the red mudstones representing deposition beneath the OMZ during regular repeated intervals when the spatial extent of the OMZ contracted (Fig. 3; see discussion in the Supplementary Information). A fully oxygenated deeper ocean (i.e., below the OMZ) has previously been invoked11,12. However, elevated ReEF and depleted MnEF values occur throughout the section (Fig. 2B). Observations from modern marine sediments24,25 demonstrate that Re is enriched under intermediate redox conditions, where the oxygen penetration depth below the sediment-water interface is shallow (~1 cm). Such conditions may occur in sediments deposited beneath an oxic water column if sufficient TOC is available to promote oxygen depletion close to the sediment-water interface26. However, our red mudstone samples are characterized by particularly low TOC concentrations (0.070 ± 0.006 wt%), and hence elevated Re concentrations in the absence of elevated U (Fig. 2B) would be consistent with dysoxic bottom waters24,25. This is supported by very low MnEF values (Fig. 2B), which suggests mobilization of Mn under oxygen-depleted conditions, in contrast to the Mn retention that might be expected in well-oxygenated settings (note that oxidation of Mn(II) to Mn(IV) requires a higher redox potential compared to Fe(II) oxidation27).

In Section A, elevated FeHR/FeT ratios and UEF values are entirely consistent with persistent deposition beneath anoxic bottom waters3,21–23. Elevated Fepy/FeHR ratios (>0.6) suggest possible euxinia28, and a sulfidic water column is supported by persistently elevated MoEF values, requiring significant sulfide availability to convert the molybdate anion to particle-reactive thiomolybdate29–31. However, Fepy/FeHR ratios are at the lower end of the range typically taken to indicate euxinia (Fig. 2A), and Mo concentrations are relatively low compared to modern euxinic basins32. These results may relate to relatively low sulfide availability compared to modern euxinic basins, which would be an expectation given the low sulfate concentrations proposed for the Mesoproterozoic ocean33. Indeed, while oceanic Mo concentrations would also have been impacted by widespread Mo sequestration due to expansive ocean anoxia34, the distinct cyclicity in MoEF values (Fig. 2A) suggests that the redox state of the water column likely varied between a weakly and more strongly sulfidic state (see Supplementary Information for more detailed discussion). Since TOC concentrations are relatively high throughout the section (and thus microbial sulfate reduction would not have been limited by TOC availability), the cyclicity between weakly and more strongly sulfidic conditions likely relates to changes in sulfate availability. This is supported by pyrite δ34S data, whereby higher values tend to occur during the weakly euxinic intervals, which implies either enhanced limitation of sulfate reduction rates12,35, or a relatively increased proportion of the pyrite forming under closed system conditions in the sediment pile36, both of which are consistent with lower sulfate concentrations in the water column (see Supplementary Information).

Nutrient cycling dynamics
Phosphorus bioavailability likely exerted a major control on primary productivity during the Mesoproterozoic Era, which in turn may have influenced longer- and shorter-term oxygenation dynamics via a variety of redox-linked feedbacks13–16. In Section B, TOC/Porg and TOC/Preac ratios are well below the Redfield ratio (Fig. 2B). Such ratios can arise due to extensive oxidation of a limited supply of organic matter and the increased ability of bacteria to store P under better oxygenated conditions (giving low TOC/Porg ratios)37, coupled with additional P drawdown in association with Fe minerals (giving lower TOC/Preac ratios than TOC/Porg)15. In the case of the green mudstone intervals, the low TOC/Preac ratios suggest that Fe-bound P (along with organic P) was largely retained in the sediment (following “sink-switching” to authigenic carbonate fluorapatite; Supplementary Data 2), while Fe2+ was more extensively recycled back to the water column as discussed above. During deposition of the red mudstones, the less reducing conditions also promoted retention of P in the sediment, dominantly as either authigenic carbonate fluorapatite or organic P, with only a minor post-diagenetic contribution from Fe-bound P (Supplementary Data 2). Ultimately, this limited degree of recycling, coupled with the potential for initial drawdown of P in association with Fe minerals precipitating in the water column, would have helped promote persistent oligotrophic conditions.

In Section A, molar TOC/Porg ratios (Fig. 2A) are above both the Redfield ratio and the predicted ratio of ~300:1 for organic matter buried under the P-limited conditions sometimes inferred for the Mesoproterozoic13. The elevated TOC/Porg ratios demonstrate extensive preferential release of P during anaerobic organic-matter remineralization, which is commonly particularly enhanced during microbial sulfate reduction37. Indeed, consistent with our suggestion of cyclicity in terms of sulfide availability (as constrained by cyclicity in MoEF and pyrite δ34S values), TOC/Porg ratios are distinctly lower under weakly sulfidic conditions, relative to more highly sulfidic intervals (Fig. 2A).

Phosphorus that is recycled back to the water column, following organic matter remineralization and the reductive dissolution of Fe(oxyhydr)oxide minerals, may promote a positive productivity feedback38. However, the extent of this recycling depends on the degree to which the P released during diagenesis is fixed as authigenic phases in the sediment38–40. As with Section B, TOC/Preac ratios are lower than TOC/Porg ratios throughout Section A (Fig. 2), demonstrating ‘sink-switching’ and/or additional drawdown of water column P in association with other phases, such as Fe(oxyhydr)oxides. Recycling of P back to the water column is, however, apparent throughout Section A, as indicated by TOC/Preac ratios that are above the Redfield ratio. However, despite a lower extent of P release from organic matter in weakly euxinic intervals (indicated by lower TOC/Porg ratios; Fig. 2A), a higher proportion of the recycled P was mobilized back to the water column (indicated by higher TOC/Preac ratios) relative to highly sulfidic intervals. Nevertheless, because more TOC was deposited during highly sulfidic intervals, a similar flux of P was recycled under both weakly and highly euxinic conditions (assuming a relatively constant sedimentation rate; see Supplementary Information). Despite this P recycling, sediments in Section A are characterized by persistently elevated P/Al ratios and consistently high TOC concentrations (Fig. 2A), which suggests an overall productive setting throughout deposition of Section A. However, TOC and Preac (Preac/Al profile; Fig. 2A) concentrations are higher during the highly euxinic intervals, despite the relatively constant recycling flux of P throughout Section A. This implies enhanced bioavailability of phosphate and more productive conditions during the more intensely euxinic intervals, which we explore below in terms of potential variability in the weathering influx of bioavailable phosphate.

Orbital forcing of climate change
The short-term redox and nutrient cyclicity we document is consistent with the suggestion of an orbital climate forcing control during deposition of the Xiamaling Formation17. However, given the inherent difficulties in estimating the dominant orbital timescales for these cycles, our main aim here is rather to evaluate particular environmental controls on the observed cyclicity. To this aim, we evaluate potential weathering controls on redox-nutrient dynamics by considering different elemental weathering proxies. In Section B, consistently high K/Al ratios suggest that chemical weathering rates in the sediment source region were likely low and relatively stable, due to the diminished solubility of K as weathering intensity decreases (Fig. 2B)41,42. However, high Ti/Al ratios, while also reflecting the primary chemical nature of the source material, imply a significant contribution of sediment from aeolian sources43–45. Indeed, since there is no apparent reason to assume large-scale, repetitive changes in the geographic area being weathered (i.e., the sediment source area) during deposition of Section B, peaks in Ti/Al around the mid-point of green mudstone intervals likely document maxima in the aeolian source flux. These combined observations imply a wind-driven orbital control in an arid environment12. We suggest that this control likely drove OMZ dynamics via a combination of changes in aeolian sediment fluxes and possible changes in the intensity of upwelling (and hence P supply).

In Section A, peaks in K/Al ratios in weakly euxinic intervals (Fig. 2A) are consistent with a lower intensity of chemical weathering, and gradual transitions to lower values in more highly sulfidic intervals suggest progressively more intense chemical weathering, due to the enhanced solubility of K as weathering intensity increases41,42. In addition, the low and relatively constant Ti/Al ratios (Fig. 2A) support a limited and stable wind-driven aeolian contribution43–45, with a dominant weathering influx from continental runoff. The intervals of more intense chemical weathering would be expected to result in increased delivery of dissolved phosphate and sulfate in runoff (Fig. 3A), the latter of which is supported by the lower pyrite δ34S values (Fig. 2A), despite more intense sulfide generation. In this overall eutrophic setting, this would have promoted enhanced productivity and TOC burial, along with the development of more intense euxinia, all of which would have been exacerbated by P recycling from the sediments (Fig. 3A). By contrast, periods of less intense chemical weathering would have resulted in a lower influx of both phosphate and sulfate. Whilst a lower sulfate influx would have promoted less intense euxinia, the overall flux of recycled P from sediments remained similar. Therefore, lower phosphate availability in surface waters, which occurred largely due to the lower weathering influx, would have led to relatively lower rates of productivity and TOC burial (Fig. 2A).

Mesoproterozoic ocean redox heterogeneity driven by regional climate dynamics
Our ocean redox and nutrient data document a highly dynamic response to orbital climate forcing in the Mesoproterozoic Era. More broadly, since our samples originate from the deepest part of the Xiamaling basin with no clear change in water depth between the two sections6 (and hence a sea-level control on the biogeochemical dynamics is highly unlikely), the distinct contrasts in ocean biogeochemistry between sections A and B are consistent with the suggestion of longer-term climate forcing driven by shifts in the position of the intertropical convergence zone (ITCZ) and related atmospheric circulation cells12,17,46. This climatic variability may also have been impacted by a possible southward shift of the NCC during the depositional period from ~1.44 Ga to 1.35 Ga18,19. Paleogeographic reconstruction places the study site at about 15˚N at 1.4 Ga (Fig. 4A; Supplementary Information), which according to climate modeling, has been the approximate position of the variable northern limit of the ITCZ, at least over Phanerozoic time47. Thus, the arid, aeolian conditions documented by our geochemical data for Section B indicate deposition outside the influence of the ITCZ, probably near the northern limb of an ancient Hadley Cell. By contrast, Section A appears to have been deposited in a region with a strong contribution from continental runoff, consistent with a low latitude, full tropical setting under direct influence of the ITCZ.

We explore this further by running a multi-box biogeochemical model that considers the mixing of water between the surface ocean, deep ocean and a globally representative continental shelf, alongside changes in weathering inputs (Fig. 4B)48. The model is modified to include two different surface ocean boxes that overlied different continental shelves and have unique weathering inputs, to explore whether these ocean margins can maintain different carbon cycle dynamics simultaneously, depending on ITCZ influence (see Methods). Shelf 1 is subject to a high weathering influx and high upwelling rate, while Shelf 2 is subject to a much lower weathering influx and upwelling rate, both taken to be two orders of magnitude lower. We alter both the weathering and upwelling supply of P because in this relatively simple model, all deep-water recycling transits through the continental shelves and open-ocean upwelling is not considered. Thus, the model shelves are likely overly connected to the deep ocean phosphate supply which artificially limits the effects of changes in weathering rates. We allow weathering fluxes to oscillate at periods of 15 kyr and 60 kyr, respectively, consistent with our broad estimates for the duration of the orbital cycles (see Supplementary Information), but note that while this affects the frequency of the modeled cycles at each site, it does not affect the contrasting million-year-scale biogeochemical dynamics observed between the two study sections.

The model outputs (Fig. 4C–F) show that changes in phosphate input result in substantially different (about tenfold) phosphorus concentrations and unique cyclic variations in each area, despite their mutual connectivity to the deep ocean interior. This drives a similar degree of variability in the amount of organic carbon (Corg) reaching the sediment in each area. The remineralization pathways in the model are simplified. We assume a simple burial efficiency of 10%, and that iron reduction processes a fixed fraction of sedimentary Corg, with the remainder being remineralized via sulfate reduction. In the oligotrophic Shelf 2 setting, Corg is less available and is significantly depleted via Fe reduction, and little sulfide is produced. Furthermore, Fe reduction rates fluctuate, which would affect the size of the ferruginous OMZ, as documented by our data (Fig. 3C, D).

In the eutrophic Shelf 1 setting, sulfate reduction rates are high and variable, which would be expected to lead to the observed oscillations between weakly and strongly euxinic waters (Fig. 3A, B). Although highly simplified, our model confirms that variations in weathering inputs should be able to drive substantially different carbon cycle and redox signatures on different parts of the continental shelf. We additionally explore the model outputs when upwelling rates are evenly split between boxes, and these also show high and variable sulfate reduction rates in Shelf 1 coexisting with low sulfate reduction rates in Shelf 2, although the strong connections to the deep ocean synchronize the timescales of these changes in this model version (Fig. S7). We thus hypothesize that variations in the ITCZ position over the period of deposition (Fig. 4A) resulted in the Xiamaling Formation recording first our “Shelf 2” signal (i.e., Section B) and then our “Shelf 1” signal (i.e., Section A), with both signals likely coexisting in multiple locations throughout the time period.

Our combined approach demonstrates how ocean redox and nutrient cycling varied as a function of location and specific positioning relative to the ITCZ and global atmospheric circulation cells, which drove large-scale variability in ocean redox conditions and associated nutrient cycling, and more subtle, regional-scale orbital timescale variability. Unlike more recent time periods, this influence of shorter- and longer-term climate forcing has rarely been factored into studies evaluating Mesoproterozoic ocean redox chemistry and nutrient cycling, which instead often assume that the biogeochemistry of one region reflects the global ocean, and that any apparent “heterogeneity” reflects an “event” or “transition” to a new redox state. Future studies should therefore directly consider the paleogeographic location of study sites and the potential role of climate forcing, when expanding regional observations to the global scale, which in turn will impact understanding of Mesoproterozoic oxygenation dynamics and its role in biological evolution.

Methods
Elemental analyses
TOC concentrations were analyzed on a LECO carbon analyzer, with TOC determined after inorganic carbon removal using 20% HCl. Replicate analyses of 0.717 ± 0.027 wt% and 10.80 ± 0.26 wt% carbon reference materials yielded relative standard deviations (RSDs) of 3% and 5%, respectively, with an accuracy of >95%. Major and trace element concentrations were measured by inductively coupled plasma–optical emission spectrometry (Fe, P, Al, K, Ti) and inductively coupled plasma–mass spectrometry (U, Mo, Re, Mn) following an HNO3-HF-HClO4-H3BO3 digest on ashed (550°C for 8 h) samples49. Precision and accuracy were monitored by analyzing international sediment standard SGR-1b, with a RSD of <5% for all elements except Mo, which had a RSD of 12%. Accuracy was within 10% for all elements. Enrichment factors for each element (XEF) were calculated relative to average UCC20,50, as:

XEF = (X/Al)sample / (X/Al)UCC (1)

Fe-S systematics
Sequential Fe extractions were performed following standard operationally defined protocols28,51. A sodium acetate solution at pH 4.5 for 48 h at 50°C was used to target carbonate-associated Fe (Fecarb); a room-temperature sodium dithionite solution (pH 4.8 for 2 h) was used to target Fe (oxyhydr)oxide minerals (Feox); and a room-temperature ammonium oxalate (6 h) was used to target magnetite (Femag). Iron concentrations in the extraction solutions were measured via atomic absorption spectrometry. Pyrite concentrations (Fepy) were determined via gravimetric precipitation of Ag2S following a standard chromous chloride extraction, with samples first checked for acid volatile sulfide (not present) using boiling 6N HCl52. Accuracy and precision were monitored relative to international Fe speciation standard, WHIT49, with a RSD of <5% and accuracy >94% for all phases. Highly reactive Fe (FeHR) was calculated as the sum of Fecarb, Feox, Femag and Fepy53. Further details on the application of Fe speciation to paleoredox studies are provided in the Supplementary Information.

Pyrite-S isotope (δ34S) analyses were performed on Ag2S precipitates (no Ag2S was recovered for Section B samples) using an Elementar PYRO cube coupled to an IsoPrime continuous flow mass spectrometer. Pyrite δ34S values are reported relative to the international standard V-CDT (δ34S = 0‰). International reference standard IAEA S-1 (δ34S = -0.3‰), laboratory BaSO4 (δ34S = -4.56‰), and international reference standard IAEA S-3 (δ34S = -32.06‰) were used to check precision and accuracy. Replicate analyses of a laboratory BaSO4 standard yielded an accuracy of <0.33‰, and a precision of <0.3‰ (1σ) for δ34S.

Phosphorus phase partitioning
Phosphorus phase partitioning was performed with an established sequential extraction scheme54 adapted for ancient sedimentary rocks (Table S1)55. Four operationally-defined phosphorus pools were targeted, including iron-bound P (PFe), authigenic carbonate fluorapatite-associated P (Pauth), detrital apatite (Pdet), and organic-bound P (Porg). Phosphorus concentrations were determined spectrophotometrically using the molybdate-blue method on a Spectronic GENESYS™ 6 at 880 nm, except for steps I, IV and V (Table S1), where the reagents interfere with the molybdate complex. For these steps, P was measured by inductively coupled plasma–optical emission spectrometry. Replicate analyses gave RSDs of <5% for each step, apart from PFe, where the RSD was 18% due to the low concentrations of P in this phase.

Biogeochemical modeling
The model developed here is modified from a simple 4-box atmosphere-ocean carbon-alkalinity system48, which is based on a combination of similar systems in the literature56–58. All changes made for the present study are documented in this section and the full model equations are presented in the Supplementary Information.

The model environment is expanded from a single surface box with a high latitude deep water cycle, to include two surface boxes, Shelf 1 and Shelf 2, while retaining the high latitude deep-water box. An upwelling fraction (Ufrac) determines the fraction of deep water that enters each box, and this was also used to set the other circulation parameters for steady state. A weathering fraction (Wfrac) controls how the globally-calculated weathering inputs are split between the two surface boxes. For our experiments, which test an end-member case, both Ufrac and Wfrac were set to 0.99 for Shelf 1 and 0.01 for Shelf 2, which mean that Shelf 1 receives 99% of the global upwelling water flux and 99% of the global weathering flux, with Shelf 2 receiving only 1% of these. As noted in the main text, this 99:1 split in nutrient inputs drives a more muted ~9:1 split in the calculated nutrient inventory and carbon export, due to the interconnectedness of the model ocean. A smaller 9:1 split in P inputs drives only a ~2:1 ratio in the shelf nutrient inventories and carbon export. Carbonate burial in each surface box is driven by the saturation state in that box, while organic carbon burial is scaled by the phosphorus concentration in the box, with P being delivered through weathering and circulated as a model tracer and buried in association with organic matter.

To replicate the orbital forcing, a sinusoidal multiplier was applied to the weathering fluxes into Shelf 1 and Shelf 2. These were set to oscillate on approximately 15 kyr and 60 kyr timeframes, where t is time in model years (upwelling fluxes are not modified in this way):

W15 = 1 + sin(2πt / 15000) (2)

W60 = 1 + sin(2πt / 60000) (3)

A simplified remineralization scheme was applied to the model sediment-water interface to demonstrate how altered carbon supply in each shelf could drive cyclicity in the severity of euxinic conditions or in the extent of a ferruginous OMZ. We first calculated the available organic carbon from the overall model Corg burial flux at the present day (kmocb), a relationship to local shelf P concentration relative to the present day, and a simple burial efficiency (BE) set to 10%.

Csed_i = kmocb * (Pi / P0) * BE (4)

For our test case we assumed a constant fraction of the available carbon is processed through Fe(III) reduction and the remainder is processed via SO4 reduction:

remin_Fe_i = 0.5 * Csed_i * (1 - BE) (5)

remin_S_i = Csed_i * (1 - BE) * (1 - remin_Fe_i) (6)

Organic carbon burial is calculated as:

mocb_sed_i = Csed_i * BE (7)

Data availability
The original data generated in this study are provided in the Supplementary Data Files 1, 2.

Code availability
The code for the multi-box biogeochemical model is freely available at github.com/bjwmills/CARMER.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_2": "Song and colleagues present extremely-high-resolution geochemical investigation on the Xiamaling\nFormation, and demonstrate the control of orbital climate forcing on oceanic redox and nutrient\ndynamics based on the redox and weathering proxies, phosphorus phase and biogeochemical modelling.\nThe study is interesting, important, and valuable, and the manuscript is well organized and easy to read.\nI suggest publication after a moderate revision. My detailed comments are as follows.\nMajor comments:\n1. In this study, the authors used redox-sensitive elements, Fe speciation, and S isotopes as tracers for\noceanic redox condition, K/Al and Ti/Al ratios for chemical weathering intensity, and P speciation for P\ncycling. I suggest to provide more backgrounds of these proxies at least in Supplementary Information,\nespecially for weathering proxies (K/Al and Ti/Al ratios).\n2. In this study, authors used K/Al ratio to trace chemical weathering intensity, based on the enhanced\nsolubility of K as an indicator of increased weathering intensity (Line 208–209), and thus higher K/Al\nratios in sediments indicate higher intensity of chemical weathering. I think this application is on the\nbasis that detrital clay minerals and rock debris in sediments are reliable archives for terrestrial inputs.\nHowever, all the K/Al ratios of the Xiamaling Formation reported in this study are higher than that of the\nUCC (~0.3), seemingly indicating that there was seawater-derived K in these siliciclastic rocks. Please\nclarify this clearly (also see Major comment 1).\n3. Tables S5 and S6 only present raw geochemical data. As shown in Fig. 2 and discussed in main text,\nthe calculated values of FeHR/FeT, Fepy/FeHR, UEF, MoEF, ReEF, MnEF, P/Al, Preac/Al, TOC/Porg,\nTOC/Preac, K/Al, and Ti/Al are better to be presented in tables.\nMinor comments:\nLine 21–26. I would suggest to introduce the reconstruction result from the lower section first, to the\nupper section then.\nLine 41. What does “similarly” mean?\nLine 69. I suggest that at least briefly introduce the background information of the studied area,\nstratigraphy, and samples, before the geochemical results in the main text, although details have been\npresented in Supplementary Information.\nLine 70–71. As the authors only investigate two short intervals of the Xiamaling Formation,16 cm of Unit\n1 (Section A) and 70 cm of Unit 4 (Section B), thus the words “through unit 1 (Section A) and unit 4\n(Section B)” is not appropriate here.\nLine 76. As enrichment factors are calculated relative to average UCC, thus “relative to upper\ncontinental crust (UCC)” is not needed and should be deleted here.\nLine 80. “values” cannot be “heavier”.\nLine 81. How about Re and Mn enrichment factors of Section A?\nLine 88. Fepy/FeHR ratios and MoEF values of Section B are extremely low, which should be also\nmentioned here.\nLine 106. Add “those” between “than” and “in”.\nLine 112. “ratios” and “values” cannot be “enrichments”.\nLine 119. You just stated that a sulfidic water column is supported by persistently elevated MoEF values\nrequiring significant sulfide availability in Line 115-116, which seems inconsistent with the sentence\nhere. Please clarify this inconsistence.\nLine 126. “higher” rather than “heavier”.\nLine 128. It is better to present the contrasting reconstruction results of FeHR/FeT ratios and UEF values\nfirst.\nLine 131–134. I think the high TOC contents in the green mudstone intervals can also be an important\npiece of evidence for anoxic bottom water conditions.\nLine 143–144. A fully oxygenated deeper ocean was proposed based on the geochemical evidence of the\nUnit 3 (Zhang et al., 2016).\nLine 160–161. The P/Al ratios of Section A are not low, inconsistent with the supposed P-limited\nconditions.\nLine 164. δ34S values can also be included.\nLine 171. (1) \"lower than” rather than “below”. (2) the Preac includes Porg, PFe, and Pauth, and thus\nTOC/Preac ratios should be usually lower than TOC/Porg ratios. Why not directly present the PFe and\nPauth enrichments instead?\nLine 180–182. Reorganize this sentence.\nLine 190. See the comments on Line 171.\nLine 206. See Major comment 2.\nLine 207. Add “of” between “intensity” and “chemical”.\nLine 213. “lower” rather than “lighter”.\nLine 216–219. Reorganize this sentence. The lower sulfate input would have promoted less intense\neuxinia, which ultimately lower the flux of recycled P from sediments.\nLine 220. See Major comment 2.\nLine 227. Cite Wang et al. (2017) here.\nLine 228. Is there evidence for the changes in the upwelling intensity?\nLine 232–233. How do you know / What is the evidence, for the two sections in this study being “the\ndeepest part of the Xiamaling basin”?\nLine 238. Add “Ga” between “1.44” and “to”\nLine 240–244. Unit 4 of Section B was deposited before Unit 1 of Section A, so I suggest to reorganize\ntwo sentence.\nLine 267. “the mixed layer” of what, chemocline?\nLine 273. Need to change to “approaches demonstrate”.\nLine 273–276. Merge two sentences.\nLine 276. “shorter-”.\nLine 315–316. Move this sentence before the sentence of “Laboratory standard SWS-3A (BaSO4, δ34S =\n+20.3‰), inter-laboratory standard CP-1 (chalcopyrite, δ34S = –4.56‰), and international reference\nstandard IAEA S-3 (δ34S = –32.06‰) were used to check precision and accuracy”, since you had\nreported δ34S values before.\nLine 326. “RSDs”.\nLine 533. Remove “K/Al = 0.3\".\nFigure 2. (1) I suggest to make items of the geochemical profiles of Sections A and B consistent, namely,\nadd ReEF and MnEF profiles for Section A and add MoEF profile for Section B; (2) the line representing\nthe K/Al ratio of the UCC has not been shown and needs to be added.\nFigure 4. Marker Shelf 1 and 2 in (B).\nComments on Supplementary Information:\nLine 601. “High precision”.\nLine 607–608. The Xiamaling Formation is divided into four members, Member 1 to 4 for bottom to top,\nor into six units, Unit 1 to 6 from top to bottom.\nLine 610. “siliciclastic rocks”.\nLine 612–614. The Xiamaling Formation is more than 400 m in thickness. You should explain why you\nonly focused on two short intervals of Unit 1 (Section A) and Unit 2 (Section B).\nFigure S1. Please add plotting scales for Sections A and B.", "reviewer_3": "General comments:\nSong et al. present a variety of high resolution data exploring the influence of climate fluctuations on\ngeochemical cycling within a Mesoproterozoic environment. Data include a variety of analyses (Fe spec,\nP spec, S isotopes, major and trace elements, and TOC) and are of good quality. Interpretations of redox\nstate are very solid; authors do an excellent job of integrating findings from each analysis into a\ncoherent geochemical story. The authors propose orbital scale climate variation in the form of ITCZ and\nHadley cell changes as drivers of redox and nutrient fluctuation in their study environment. These\nappear to be reasonable hypotheses. The article's supplementary information is very useful and goes\ninto great detail about aspects of the paper. Additionally, the manuscript is very well written and\npleasant to read.\nWe did notice that the methods, results, and some interpretations in this manuscript are similar to\nthose in Wang et al. 2017. We recognize that this manuscript brings new insights, so we just\nrecommend that it be made explicit whether or not any of your data derive from Wang et al. 2017 and\nhow your paper builds upon this previous study.\nAll in all, we believe this manuscript is of high quality and worthy of publication. We propose some\nminor changes. In a few places, we suggest the authors should qualify some of their hypotheses and/or\nacknowledge alternative scenarios. We also suggest they include some additional information in places,\nbut nothing critical to the paper's main points.\nSincerely,\nAmy Hagen and Benjamin Gill\nSpecific comments (also included in attached Word doc):\nLine 118-123: We are not sure you can differentiate their proposed scenario that invokes local changes\nin sulfide availability driving Mo enrichments from one where the enhanced weathering also just\ndelivers more Mo to the basin (with the sulfate). Both would lead to intervals with higher\nFe(pyrite)/FeHR ratios, lower δ34S of the pyrite and higher Mo enrichments. We would recommend the\nauthors also discuss this possibility; it doesn’t greatly change their interpretations.\nLine 126-127: It might be nice to show a cross plot or stratigraphic plot with only these two sets of data\nin the supplement to more clearly illustrate this point.\nLine 129: It might be best to qualify what is meant by “mineralogy” here, since Fe spec targets\noperationally defined pools and therefore we don’t know specific minerals.\nLine 131-134: Given the higher Fe/Al ratios (of the green intervals versus those in Section A), it might\nbe worth mentioning the potential for clay to be a sink of highly reactive Fe.\nLine 175-178: I’m not sure ‘rate’ is the correct word here. To compare rates we need to know the\namount of time it took to produce the TOC/P ratio. ‘Amount ‘might be a better word.\nLine 179-180: This is probably not a realistic assumption. It’s fine that is was assumed (it may be the\nbest we can do), but we would like to see this statement better qualified.\nLine 209-210: It would be better to cite the study/studies which ground truth Ti as a dust proxy rather\nthan these two sources which simply apply the proxy.\nLine 217-218: Shouldn’t P recycling have declined as organic matter production declined?\nLine 220-221: Low chemical weathering rates seem at odds with the elevated Fe/Al ratios for the green\nintervals in Section B compared to all of Section A. May suggest that either there is a difference in the\nweathering flux of iron (source or intensity) between these sections.\nLine 224: The authors should be specific here. What large-scale changes are they referring to: overall\nweathering rates, type of material being weathered, and/or amount of geographic area being\nweathered?\nLine 232-234: I’m not sure you can rule this out. Estimating paleowater depths in difficult, so I don’t\nthink you can simply say it is not a factor. Changes in terrigenous input could be factor.\nLine 235-236: This seems to be a reasonable hypothesis, but are there potentially other climate\ninfluence factors? It may be worth mentioning some alternatives.\nLine 250: It could be useful to mention why you’ve chosen to couple these two variables or what model\noutputs would look like if you changed just one of these factors.\nLine 278-280: This seems like a bit of an overgeneralization, I’d suggest changing the wording a little bit\nhere.\nLine 329: I am not a box model expert so I can’t offer much feedback here, but all looks good and seems\nto be explained well.\nLine 521: I would suggest using something other than just a red arrow to indicate the studied sections.\nPerhaps a curly bracket would be helpful in showing that you’re zooming in on a portion of the big\ncolumn?\nLine 572: I’d suggest adding a reference here so we know why you’ve shown that particular range for\nthe ITCZ.\nLine 678: Again, I’m not sure this is likely to be true. After all, you just discussed turbidites which\nrepresent episodic sedimentation. It could be good to just put in a short note acknowledging this.\nLine 734-735: Since you mention this, it might be useful to elaborate a bit on why you don’t see this\nbeing a problem in your section.\nLine 738-739: This section might be a bit confusing for those who haven’t read Pasquier et al. I’d suggest\nadding a bit about what they claim in the paper before you proceed to counter it. However, I also think\nthat the vast majority of papers support Fe spec as a proxy and you may not need to dedicate so much\nspace to addressing this. I think your last 2 sentences sum up your point nicely. This is just an opinion\nand not critical to your paper so feel free to keep this section as is if you like.\nLine 742-746: As mentioned above, I think these two sentences emphasize an important point: that\nregardless of any doubts in Fe spec, your other data fully support your interpretations.", "reviewer_author_response": "Response: We are grateful for the positive response and the useful comments that we hope we have\nbeen able to address in full.\nMajor comments:\n1. In this study, the authors used redox-sensitive elements, Fe speciation, and S isotopes as tracers for\noceanic redox condition, K/Al and Ti/Al ratios for chemical weathering intensity, and P speciation\nfor P cycling. I suggest to provide more backgrounds of these proxies at least in Supplementary\nInformation, especially for weathering proxies (K/Al and Ti/Al ratios).\nReviewer #3 (Remarks to the Author):\nGeneral comments:\nSong et al. present a variety of high resolution data exploring the influence of climate fluctuations on\ngeochemical cycling within a Mesoproterozoic environment. Data include a variety of analyses (Fe\nspec, P spec, S isotopes, major and trace elements, and TOC) and are of good quality. Interpretations\nof redox state are very solid; authors do an excellent job of integrating findings from each analysis\ninto a coherent geochemical story. The authors propose orbital scale climate variation in the form of\nITCZ and Hadley cell changes as drivers of redox and nutrient fluctuation in their study environment.\nThese appear to be reasonable hypotheses. The article's supplementary information is very useful and\ngoes into great detail about aspects of the paper. Additionally, the manuscript is very well written and\npleasant to read."}, "author_response": ""}]

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