Transient fertilization of a post-Sturtian Snowball ocean margin with dissolved phosphate by clay minerals

Domain: earth_science
Published: 2023-12-18
Editorial outcome: minor_revision

Abstract
Marine sedimentary rocks deposited across the Neoproterozoic Cryogenian Snowball interval, ~720-635 million years ago, suggest that post-Snowball fertilization of shallow continental margin seawater with phosphorus accelerated marine primary productivity, ocean-atmosphere oxygenation, and ultimately the rise of animals. However, the mechanisms that sourced and delivered bioavailable phosphate from land to the ocean are not fully understood. Here we demonstrate a causal relationship between clay mineral production by the melting Sturtian Snowball ice sheets and a short-lived increase in seawater phosphate bioavailability by at least 20-fold and oxygenation of an immediate post-Sturtian Snowball ocean margin. Bulk primary sediment inputs and inferred dissolved seawater phosphate dynamics point to a relatively low marine phosphate inventory that limited marine primary productivity and seawater oxygenation before the Sturtian glaciation, and again in the later stages of the succeeding interglacial greenhouse interval.

Introduction
The secular rise of phosphorus (P) in Cryogenian marine sediments has been linked to post-Snowball Earth deglaciation resulting in enhanced phosphate (PO 4 3-) supply to seawater, greater marine primary production, ocean-atmosphere oxygenation, and ultimately the evolution of metazoans1–3. The inference of higher dissolved seawater PO 4 3- concentrations across the Neoproterozoic glaciations comes from Fe-rich deposits and shales containing significantly higher P concentrations relative to older sediments of similar depositional settings1,2. However, except for a broad association to mechanical weathering caused by erosional action of end-Snowball melting ice sheets, it remains unclear how PO 4 3- was sourced from the continents and transported in quantitatively dissolvable forms to seawater to enable pervasive global ocean-atmosphere oxygenation.

In the modern world, most PO 4 3- is transferred from land to the oceans by riverine-transported clay and metal oxide particles rather than in solution4–6. This view is consistent with the suggestion that detrital clay minerals associated with more acidic rivers during the Great Oxygenation Event (GOE) played a significant role in conveying PO 4 3- from land to Paleoproterozoic seawater5. At times of less acid generation on land, highly reactive iron oxyhydroxides (Fe-ox HR) instead served as efficient particulate sorbents for dissolved phosphate5. As a corollary, we hypothesize that extensive weathering of continental landmasses by melting ice sheets at the end of the Cryogenian Snowball Earth glaciations7 drove up production, transport, and supply of fine-grained detrital clays and Fe-ox HR to the oceans, which served as vectors of marginal seawater enrichment with PO 4 3-. Indeed, melting of the modern Greenland ice sheet shows that up to 97% PO 4 3- exported by glacial meltwater is associated with suspended sediment particles8. This relationship is predictable, given that clay minerals, which tend to form rapidly upon the retreat of glaciers with climate warming9, together with Fe-ox HR, possess large surface-to-volume ratios. Together, they contain positive charged surfaces at the pH range of rivers that make them efficient adsorbents of dissolved PO 4 3- anions4–6,10–17.

To assess the potential of clay minerals and Fe-ox HR minerals as vectors of dissolved PO 4 3- to post-Snowball seawater, we collected marine sediments exposed on the Isles of Islay and the Garvellachs in the Dalradian Supergroup, Scotland, spanning pre-glacial, deglacial, and post-glacial phases of the Neoproterozoic Sturtian Snowball glaciation7 (Fig. 1). The underlying pre-Snowball Tonian Lossit Limestone Formation (LLF) in the >4-km-thick Appin Group18–23 is characterized by organic-rich mudstones, sandstones, carbonate, and shallow seawater stromatolites. The glaciogenic tillite cap of the LLF passes conformably21 into the overlying ~1.1 km-thick Port Askaig Tillite Formation (PATF), correlated worldwide to Sturtian Snowball deposits based on geochronology, lithostratigraphy, and chemostratigraphy18–23. On the Garvellachs Island, the PATF appears as twelve discrete diamictite units containing dolostone clasts overlain by the ~40-m-thick Great Breccia, comprised of deformed sediment bedrock rafts transported by icebergs from land to the ocean margin, and the 29 to 40-m-thick disrupted beds dominated by Fe-rich siltstones and dolostone concretions. Up-section, ~30 diamictite layers with interlayered sandstone units grade upwards into transported granitic clasts, which are in turn conformably overlain by the post-Snowball Bonahaven Dolomite Formation18,19,21 (BDF). The BDF consists of ~300-m-thick fine-grained clastic rocks composed of siltstones and mudstones, with occasional stromatolitic dolostone and evaporitic deposits indicating deposition in arid greenhouse climate times18–23.

In this study, we use a combination of geochemical techniques to demonstrate a causal relationship between continental erosion and the supply of dissolved PO 4 3- to continental margin waters and associated temporal controls on seawater oxygenation dynamics across the Sturtian Snowball glaciation deposits of the Dalradian Supergroup.

Results
Bulk facies chemostratigraphy and mineralogy
Careful collection of outcrop samples took place in the field to avoid weathered and metamorphosed lithologies (Table S1). Comparative chemostratigraphic correlation of bulk facies geochemistry and δ13C carbonate and δ18O carbonate compositions (Figs. 2 & S1-S2) with previous studies of the same section18–24, together with sediment mineralogy (Fig. 3 & Table S2), was used to evaluate data quality and to account for potential post-depositional alteration of primary geochemical composition. In addition, cross plots of Fe/Ti vs. Al/(Al+Fe+Mn) (ref. 25) were used to constrain the potential influence of hydrothermal fluids (Fig. S3). Our δ13C carbonate and δ18O carbonate values fall within the range reported in previous studies for the pre-glacial, glacial and post-glacial lithologies18,19,21–23, which have been correlated by geochronology and chemo/lithostratigraphy to the Cryogenian-Sturtian interval in Australia, Mongolia, Siberia, China, Oman, Namibia and Canada7,18,19. The pre-/late-Snowball samples are marked by higher δ13C org, C org and δ13C carbonate excursions, compared to persistently lower tillite and immediate post-Snowball values (Fig. 2a-c). Although characterized by lower carbonate content relative to concentrations in tillite samples analyzed for C isotope distribution (Fig. 2d), X-ray diffraction (XRD) and thin-sectioned Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) data are consistent with reported higher and more homogenous carbonate enrichment in the pre-/late-Snowball facies18,19,21,22 (Fig. 3 & Table S2). Associated δ18O carbonate trends generally decline up sequence (Fig. 2e), while the absence of δ13C carbonate and δ18O carbonate correlation suggests negligible diagenetic alteration of primary δ13C carbonate signal (Fig. S2).

Lithologies affected by greenschist metamorphism, confined to the margins of metabasaltic sills24, were excluded. Consistent with this sampling strategy, XRD and thin-sectioned SEM-EDS mineralogical analysis, revealed a major dominance of no to low-grade metamorphic clay minerals (Fig. 3 & Table S2). Typical clay minerals like kaolinite that are often associated with strong terrestrial weathering by chemical processes are rare in the samples, while muscovite and illite clay minerals believed to be most abundant in the modern upper continental crust (UCC)17, occur more commonly across the lithofacies (Fig. 3 & Table S2). Chlorite, which also tends to be enriched in the UCC, was detected mainly in the deglaciating tillites, while the presence of albite (a tectosilicate mineral) in all samples regardless of lithology (Fig. 3), suggests an association with the erosion of crystalline bedrocks19.

Overall, the SEM-EDS mineral maps together with XRD mineralogical analysis, show a variable prevalence of siliciclastic detrital material in all lithologies, with a dominance of sheet silicates and quartz, even in carbonate-rich facies. These findings agree with past observations18–23, evidenced here by fine-grained siliciclastic particles in post-Snowball sediments and the prevalence of coarser quartz grains in tillite and pre-Snowball samples. The tillites are noted for hosting abundant detrital grains likely associated with illite, muscovite, albite, and, to a lesser extent, kaolinite (Fig. 3d-e & Table 2S). Illite and muscovite are quantitatively more abundant in the post-Snowball facies. Although detected in the pre-Snowball rocks, illite is not always present in readily quantifiable amounts. Typically, chemical weathering of most rock-forming silicates promotes secondary formation of clay minerals and Fe(III)- and Al-oxides, with an accumulation of residual quartz, heavy minerals, and sheet silicates like muscovite and biotite26. Sediments and soils originating from this process can predictably contain >50% sheet silicates and Fe oxyhydroxides by volume, depending on the nature of the source rocks and prevailing climatic conditions17,26. The variable occurrence of illite and muscovite across all sampled lithologies could be due to varying degrees of detrital supply or distinct local diagenetic pathways, while the largely low kaolinite content may be the result of possible transformation because kaolinite reacts with K-feldspar to form illite during burial diagenesis17 and with alkaline fluids during metasomatism. Although chlorite can also form as a low-grade metamorphic mineral, with further transformation to muscovite at higher temperatures, the mineralogy of the facies is consistent with their suggested low metamorphic grade and observed preservation of primary features18–23. Further, the notable prevalence of quartz and sheet silicates (mainly clay minerals), including co-occurrence with tectosilicate albite, hint at the dominant supply of clastic debris to the primary sediments, derived from physical erosion of intermediate-to-felsic continental rocks (Fig. S4).

Bulk sediment geochemistry
A wider range of geochemical changes emerge along the studied lithostratigraphic section (Fig. 4 & Table S3), distinguished by persistently elevated bulk sediment P2O5, TiO2, Al2O3, Fe2O3, Cr, V, and Ni concentrations in the PATF and the lower BDF, with much lower and more variable concentrations recorded in the PS-16 section of the Portant-Struthain rocks in the upper BDF (Fig. 4a-c & f-h). Notable Al2O3 and TiO2 peaks, especially in the PATF and lower BDF, agree with the prevalence of various mineral phases pointing to substantial detrital contribution to sediment accumulation across the sequence. To test this idea, elemental normalization to Ti—a detrital indicator of authigenic enrichment27,28—shows subtle enrichment consisting of two broad patterns across the stratigraphic profile (Fig. 4j-p). This relationship is highlighted by two observations. First, a reasonable stratigraphic correlation exists between P/Ti and Fe/Ti and to some degree with Mn/Ti ratios (Fig. 4n-p), although generally, MnO does not show significant lithostratigraphic variations (Fig. 4d). Second, a broad minimal correlation is observed for Cr/Ti, V/Ti, Ni/Ti, and Co/Ti (Fig. 4j-m), suggesting that their accumulation was co-regulated by the same sedimentary processes. Interestingly, Cr/Ti is anomalous in the sense that in the lower BDF, when the other elements display multiple peaks, Cr/Ti distribution remains uniform, but becomes noticeable when enrichment in most elements is muted in the upper BDF Portant-Struthain section (i.e., the PS-16 samples). To add to these observations, the near static Al2O3 to TiO2 ratios (Fig. 4q) are interpreted to either reflect stable detrital contribution and/or potential long-term sediment accumulation from a similar UCC rock source.

Comparable Fe2O3 to TiO2 and P2O5 to TiO2 ratios, together with Loss on Ignition (LOI) being inversely correlated with P2O5 and Fe2O3 content throughout the sampled section (Fig. 4r & Fig. S5), suggest bulk sediment P2O5 and Fe2O3 are most likely preserved in inorganic instead of organic and carbonate mineral phases. Where LOI values are elevated, they presumably predict higher combustible sedimentary organic and/or carbonate content, while lower values indicate the contrary, due to higher susceptibility of carbonates and organic matter to complete combustion to CO2 under elevated temperatures29. Indeed, the lowest LOI values are recorded in the carbonate-poor PATF and lower BDF section where sustained peak P2O5 and Fe2O3 concentrations are found, compared to the minor and more variable P2O5 content in the carbonate-rich LLF and upper BDF PS-16 facies (Figs. 4c, e & 3r). Higher LOI values that tend to relate inversely with P2O5 and Fe2O3 concentrations across the section (Fig. S5), are unique to the carbonate-rich LLF and PS-16 upper BDF samples. Measured trace metals are generally enriched across the sequence relative to UCC values (Fig. 4s-y). Notably, significant co-enrichment of P is also observed in the tillites and upwards relative to the UCC, compared to pre-Snowball times (Fig. 4t). Despite their overall similar redox properties, Mn enrichment patterns relative to the UCC are dissimilar to those of Fe, particularly in some tillite and immediate post-Snowball samples (Fig. 4s & u), pointing perhaps to different enrichment pathways or the known differences in sensitivity to reduction. Chromium shows two anomalous enrichment peaks relative to UCC, one in the LLF that is also seen for V and Fe and a second corresponding to samples shown above to have high Cr/Ti ratios in the upper BDF (Fig. 4j, s, x & y).

The generally low C org levels across the sampled sequence (Fig. 2b) are considered to be of little importance to sedimentary P and Fe preservation. Positive correlations are revealed for P2O5 and Fe2O3 and poorly mobile TiO2 and Al2O3 across lithostratigraphy, although they are much weaker with respect to Al2O3 for the pre-Snowball Earth LLF samples (Fig. 5a-c). A similar relation appears for Fe2O3 versus TiO2 and Al2O3 plots, but like P2O5 the correlation is unsupported for Al2O3 in the LLF section (Fig. 5d-e). When considered in light of the strong positive correlation between Al2O3 and TiO2 (Fig. 5f) and Fe2O3 and P2O5 (Fig. 5a) in all three lithological sections, the observations suggest some level of decoupling of pre-Snowball P2O5 and Fe2O3 enrichment pathways with respect to Al2O3 sources (Fig. 5d-f). The strong correlations between TiO2, P2O5 and Fe2O3 for tillites and post-Snowball facies may consequently be the result of sediment source homogenization by the indiscriminate erosional action of melting ice sheets. Regardless, these observations provide further evidence for across sequence P2O5 and Fe2O3 enrichment by detritus.

The low C org and negative δ13C carbonate distribution in the facies (Fig. 2 & S1-S2) suggest microbial oxidation of C org likely prevailed at the sediment-water interface, which would have promoted the release of organic-bound P into sea and sediment pore water, influencing dissolved aqueous P content and eventual incorporation into various sedimentary minerals. This would have been particularly relevant in the immediate post-Snowball interval where apatite formation became more pronounced, compared to the rest of the section (Fig. 3 & Table S2). It is also possible that prevailing environmental conditions influenced ongoing microbial transformation of P-rich C org and Fe-ox HR minerals to trigger enough diagenetic sediment porewater P supersaturation30. For example, the geochemical data suggest that bulk primary sediment P was predominantly associated with non-calcium bearing minerals such as Fe-ox HR and unreactive detrital silicate phases (Figs. S6-S9), which would have starved primary sediments of dissolved P. Except for some immediate post-Snowball samples, across-sequence CaO and P2O5 inverse correlations, being up to 79% for the tillites (Fig. S6a-b), corroborate limited potential P preservation in non-calcium bearing mineral phases. As discussed above, this view is supported by low LOI, low-carbonate facies associated with bulk high P2O5/Fe2O3 ratios and high LOI, high-carbonate lithologies with bulk low-P2O5/Fe2O3 ratios (Fig. S5). Overall decreasing P2O5 concentration coincides with C org content that increases from post-Snowball to pre-Snowball facies (Fig. S6c), signalling either possible diagenetic P enrichment or loss in primary sediments through microbial oxidation of organic-rich P biomass originating from the water column. Indeed, our lowest δ13C org and C org values in the tillites and immediate post-Snowball interval (Fig. 2a-b) coincide with P2O5 enrichment compared to the low P2O5 pre-Snowball samples (Fig. S6c-d). Based on these observations, microbial recycling of organic-rich P at the sediment-water interface could explain the increasing prevalence of apatite in the immediate post-Snowball interval. However, the data suggest that the P supersaturation required to spontaneously precipitate large amounts of apatite precursor phases and significant substitution in carbonate minerals30,31 was limited, especially in the pre-Snowball interval. Instead, our results indicate that dominant P supply and enrichment in the primary sediments mostly reflect detrital loading with non-apatite minerals. Moreover, as we show below, the synthesis of Ca-rich P minerals was perhaps limited by abundant Fe-ox HR particles acting as efficient scavengers of dissolved PO 4 3- at the sediment-seawater interface.

Redox-sensitive Mn shows a weak negative correlation with TiO2 and Al2O3 across all facies (Fig. S10), suggesting that Mn enrichment pathways are to some extent decoupled from those of P2O5 and Fe2O3, particularly given the considerable positive correlation between P2O5, TiO2, and Al2O3 (Fig. 5). Except for the short-lived spike in Cr enrichment in the upper BDF relative to Ti, overall, our data do not indicate that significant oxidative/chemical weathering of terrestrial rocks was a predominant mechanism for PO 4 3- delivery to seawater throughout the sampled section. This observation is consistent with independent findings pointing to primary sediment detrital inputs of continental provenance18,20–23, reinforced by our stronger physical rather than chemical weathering indicators. This relationship is further strengthened by the association of average UCC profiles (Fig. S11) with the continental margin location of the sedimentary basin.

High Cr/Ti ratios recorded during the GOE were previously ascribed to mobilization of dissolved Cr from land to the ocean by acid rock drainage via the activity of acidophilic aerobic bacteria following oxygenation of the atmosphere27. A similar but previously unreported acidification event could explain the brief spike in Cr/Ti ratios in the upper BDF. However, a close look at the data indicates that total P2O5 and Fe2O3 contents immediately before, during and after the transient increase in Cr/Ti ratios remained low, suggesting that even if this event was triggered by acidic weathering of terrestrial rocks, it cannot explain the behaviour, source, and mechanism of P2O5 enrichment in our samples.

Further, the highly soluble properties of ferrous and ferric Fe under extreme acidic conditions characteristic of acid rock drainage32 should translate to relative co-enrichment of Fe and Cr in the sediments with respect to poorly mobile Ti. This relationship is not seen in the studied section. Moreover, the broad expression of similar but conservative Fe/Ti and P/Ti profiles throughout the BDF and the tillite formation are consistent with reported low to moderate chemical weathering indices in the tillites33. Thus, the combined data better reflect terrestrial detritus contributing to high bulk P2O5 and Fe2O3 enrichment in the PATF and the lower BDF section, assuming continental erosion by melting ice sheets was the principal source of detrital inputs into the sediment pile. Moreover, our bulk P data are consistent with historical records showing a substantial spike in the P content of Cryogenian fine-grained marine siliciclastic rocks2 (Fig. 6).

Highly reactive Fe and dissolved P chemistry
In light of the above discussions, we pose the question of whether P was associated with Fe in the primary sediments and, if so, whether such a relationship can provide insights into seawater-dissolved P profiles. The distribution of leachable highly reactive Fe (Fe HR) phases in the form of Fe-ox HR + pyrite Fe and poorly reactive sheet silicate Fe (Fig. 7a) in representative samples was therefore explored according to references34–36 (see methods). Intriguingly, the Fe-ox HR phases decline by a factor of two when emerging from the Snowball ice sheet melting stage and transitioning into post-Snowball greenhouse state (Fig. 7b). The ratio of Fe-ox HR over total Fe content, suggest that Fe-ox HR constitutes only a small proportion of total Fe across the sequence, with distinct fluctuations that parallel low pre-Snowball P2O5, immediate post-Snowball high P2O5 and late post-Snowball low P2O5 intervals (Fig. 7c). A similar pattern is noted when Fe-ox HR concentrations are compared to unreactive sheet silicate Fe content, with the data suggesting that the majority of Fe throughout the sampled stratigraphy is largely present as unreactive silicates. These data are consistent with the above observations pointing to significant detrital contribution to sediment Fe enrichment, as opposed to chemical sedimentation.

Bulk sediment P analysis, as shown by our results, provides important clues on the overall behaviour of P in marine waters1,2,37,38. However, bulk sediment P content is a poor differentiator of reactive and unreactive P composition and therefore is an unreliable indicator of P bioavailability. Instead, we consider the linear correlation between Fe-ox HR-bound P and dissolved P (refs. 1,37–39) in modern marine systems as the best estimator of dissolved P behaviour during sediment deposition, following the recommendation of Thompson and others40 (see methods). We found 0% of measurable leached P associated with the Fe carbonates, compared to ~9.1% for the combined goethite, akageneite and hematite phases (all post-Snowball); ~45.5% for magnetite (42.5% post-Snowball and 3.0% pre-Snowball); ~6.1% for hematite (all pre-Snowball); and 39.4% for the tillites and post-Snowball sheet silicates (Figs. S7 & S8).

The sheet silicates in the tillites and immediate post-Snowball, record the highest leachable P content. Overall, the distribution of P between the sheet silicates and the Fe-ox HR reservoir indicate that high P enrichment in association with sheet silicates at the terminal Snowball immediate post-Snowball greenhouse transition, is marked by redistribution of P to a putative magnetite sink (Fig. 7d). A corresponding ~twofold reduction in the size of the leachable Fe-ox HR inventory at this time would have reduced the potential removal efficiency of dissolved PO 4 3- from seawater by an equally similar magnitude, since Fe-ox HR is otherwise a strong P sink37–39. This process alone would have enabled substantial build-up of dissolved PO 4 3- in immediate post-Snowball seawater relative to the period before and after, both distinguished by a higher Fe-ox HR reservoir (Fig. 7b). Furthermore, the peaking of putative sheet silicate P in the tillites, which is hundreds of times above baseline detection concentrations of 5.47 ppb in the pre-Snowball interval, is consistent with rock flour production expected with the mechanical grinding action of melting ice sheets on the bedrock. This supposition is supported by SEM-EDS imaging showing elevated fine-grained sheet silicate clasts in the immediate post-Snowball samples (Fig. 3d).

We assume that the coincidental rise of appreciable P enrichment in immediate post-Snowball magnetite grains relative to the tillites interval, points to increasing immediate post-Snowball seawater conditions, probably related to the balance between seawater oxygen content and local Fe sedimentary mechanisms. For instance, modern turbid Arctic glacial outflows generally display high dissolved Fe content of up to 20 µM, which decreases to nanomolar concentrations at the shelf-fjord water mixing boundary58. This trend results from rapid flocculation of dissolved Fe at this interface and the binding of particle surfaces because of increasing salinity and pH, triggering the loss of up to 98% total dissolved Fe (refs. 58,59). Similar rapid precipitation of particulate Fe with increasing oxygenation at Snowball meltwater-seawater interfaces would have accelerated the scavenging of dissolved P released by clay minerals along the gradients of rising salinity and pH in the continental margin waters5,6.

Magnetite precipitation and seawater dissolved P behaviour
Magnetite, a common Fe mineral in Precambrian Fe-rich sedimentary rocks, often forms through a combination of biotic and abiotic processes involving ferrihydrite and green rust precursors44–48. In this light, green rust and ferrihydrite are thought to sequester and bury dissolved seawater trace nutrients in the sedimentary pile49,50. Particularly, the ability for both green rust and ferrihydrite to bind aqueous PO 4 3- has been demonstrated experimentally37–39,51–54, with green rust reported to possess a greater propensity to bind PO 4 3- than ferric Fe53. It is thus reasonable to assume that the incorporation of P into magnetite crystals scaled proportionally with the concentration of primary seawater green rust and ferrihydrite magnetite precursors.

Nonetheless, although prograde metamorphism exerts a negligible effect on magnetite transformation, it can become significant in the presence of olivine55,56. Mineralogical appraisal of our samples by XRD and SEM-EDS mapping, however failed to identify olivine as an important mineral phase throughout the sequence, while the influence of hydrothermal fluids on Fe distribution appear to be insignificant (see Fig. S3). Moreover, a lack of covariation between carbonate Fe and magnetite Fe reservoirs across sequence stratigraphy, further suggests little or no thermal breakdown of Fe carbonates to magnetite by burial metamorphism57. Importantly, spontaneous binding of PO 4 3- to magnetite occurs by physical adsorptive intermolecular attractive forces and by chemical transfer of electrons between magnetite and PO 4 3- molecules, promoting an adsorption capacity of ~57.8 mg of PO 4 3- per gram and resulting in a positive correlation between magnetite-bound and dissolved PO 4 3- (ref. 13). This binding of dissolved PO 4 3- by magnetite enhances magnetite’s stability12,13 and therefore enhancement of possible long-term preservation in sediments.

We can only speculate that the prevalence of putative magnetite grains, particularly in the immediate post-glacial P2O5-rich interval, compared to the low P2O5 intervals, reflects nascent post-glacial seawater conditions, probably related to the balance between seawater oxygen content and local Fe sedimentary mechanisms. For instance, modern turbid Arctic glacial outflows generally display high dissolved Fe content of up to 20 µM, which decreases to nanomolar concentrations at the shelf-fjord water mixing boundary58. This trend results from rapid flocculation of dissolved Fe at this interface and the binding of particle surfaces because of increasing salinity and pH, triggering the loss of up to 98% total dissolved Fe (refs. 58,59). Similar rapid precipitation of particulate Fe with increasing oxygenation at Snowball meltwater-seawater interfaces would have accelerated the scavenging of dissolved P released by clay minerals along the gradients of rising salinity and pH in the continental margin waters5,6.

Although we are unable to quantitatively distinguish the extent of Fe(II) oxidation at the end-Snowball post-Snowball transition, predominantly positive δ56Fe bulk sedimentary values (Table S4) indicate that partial Fe(II) oxidation60 was common throughout the deposition of the studied sequence. This process would have favoured preferential formation of magnetite as allowed by local thermodynamic conditions of redox, electrical conductivity, pH, and temperature61,62. For instance, magnetite precipitation and stability may have been favoured at the lower temperatures and pH conditions that accompanied the immediate post-glacial CO2-rich world61,62. It is also possible that the reduction of ferrihydrite by biomass to magnetite could have been common45,47. However, the absence of strong negative δ56Fe signatures across the studied profile is consistent with overall negligible diagenetic production of magnetite and Fe carbonates via dissimilatory microbial reduction of Fe-ox HR. Thus, considering the linear relationship existing between magnetite-bound and aqueous PO 4 3- (ref. 12), PO 4 3- bioavailability in the immediate post-Snowball waters potentially increased by at least 20-fold compared to pre-Snowball conditions. Unlike the immediate post-Snowball interval, putative immediate pre-Snowball magnetite-bound P remained below the instrument detection limit of ~5 ppb, with the data indicating that unreactive silicate-bound P dominated bulk P burial before and during the Sturtian Snowball (Fig. S7).

Implications for seawater redox
Carbonate-rich samples with <5% Fe content tend to produce anomalously high Fe HR/Fe T ratios34. The reconstructed Fe-based redox trends were therefore evaluated for deviations as a function of Fe and carbonate content. The absence of significant differences regardless of total Fe and carbonate content resulting in consistent trends along sequence stratigraphy, was taken to imply a negligible impact of carbonate content on the reliability of the Fe-based redox proxy. We further interpret the general lack of significant deviations in the Fe-based redox proxy profiles as a reflection of the significant siliciclastic and Fe-rich content of the studied lithologies18–23, including those containing carbonates, as established by our geochemical analyses. The impact of sedimentation rates was determined to be negligible, considering that estimated values are four times lower than those expected to compromise the sensitivity of the Fe-based redox proxy34 (see supplementary information).

Average Fe HR/Fe T ratios are 0.55±0.21, 0.16±0.04, and 0.68±0.30 for the pre-Snowball and tillites, the P2O5-rich immediate post-Snowball, and the P2O5-poor late post-Snowball intervals, respectively (Fig. 8a). Their corresponding average Fe Py/Fe T ratios are 0.21±0.22, 0.31±0,34, and 0.42±0.31, respectively (Fig. 8b). The Fe HR/Fe T ratios of <0.8 suggest deposition of the entire sediment profile in mainly ferruginous-like conditions, although the Fe HR/Fe T ratios of <0.22 in the immediate post-Snowball interval are consistent with oxygenated waters, while the >0.38 Fe HR/Fe T ratios indicate anoxic bottom water depositional conditions for the pre-Snowball, tillite, and late post-Snowball samples34–36. Average Fe HR/Fe T values for samples straddling the peak post-glacial P2O5 interlude correlate negatively with their associated magnetite-bound P values, implying rising magnetite-bound P values coincide with lower Fe HR/Fe T ratios associated with oxic waters (Fig. 8c). No such correlation is observed for the pre-Snowball, tillites and late post-Snowball samples. These observations provide substantial evidence that the transitory rise in sedimentary P2O5 content in the immediate post-Snowball sediments was likely attended by transient seawater oxygenation and that magnetite indeed records dissolved seawater PO 4 3- dynamics at this time.

The oxygenation trends are further highlighted by temporal Fe/Al profiles across the sequence34,41,42,63 (Fig. 8d). In this regard, the >0.5 pre-Snowball Fe/Al ratios are generally consistent with higher sediment pyrite enrichments, compared to tillites samples and the immediate post-Snowball facies, which are depleted in pyrite (Figs. 7b & 8b). Such high Fe/Al values have been linked to the deposition of anoxic sediments beneath modern sulphide-enriched seawater, and lower values characterize oxic settings with limited C org supply to sediments41,42. Due to persistent supply of continental detritus as shown above, together with overall sedimentation rates that do not compromise the Fe HR/Fe T proxy as discussed in the supplementary information, the >0.5 Fe/Al ratios are best explained by syngenetic changes in anoxic pyrite precipitation. The <0.5 Fe/Al ratios, however, do not effectively delineate the pyrite-poor anoxic tillites from the apparently oxic immediate post-Snowball deposit (Fig. 8a), possibly due to increase input of Al-rich detritus during deposition of the tillites and immediate post-Snowball facies (Fig. 4b). Similarly, <0.5 Fe/Al ratios in the anoxic pyrite-poor late post-Snowball interval, as also suggested by the Fe HR/Fe T redox proxy, are followed by a small >0.5 Fe/Al spike corresponding to anoxic conditions (Fig. 8a) and pyrite enrichment at the top of the section (Figs. 7b, 8b & 8d).

Furthermore, a numerical biogeochemical model64,65 (see methods) replicate the observed temporal rise and fall in seawater PO 4 3- content and the transient immediate-post Snowball oxygenation event (Fig. 8e-j). The model which considers abrupt large input of riverine PO 4 3- to near continental margin waters and the deep ocean, suggests that the initial considerable introduction of PO 4 3- to continental margin waters could have briefly triggered seawater deoxygenation due to eutrophication (Fig. 8i). We note that the early rise in sedimentary P2O5 content was not immediately accompanied by seawater oxygenation. Moreover, the model shows that rising dissolved seawater PO 4 3- concentration would have promoted increased PO 4 3- burial, with subsequent decline in supply resulting in seawater deoxygenation (Fig. 8k-n), similar to our observations.

Our data reveal bulk P content comparable to previously published concentrations for fine grain siliciclastic Cryogenian facies. These data suggest that Cryogenian continental seawater P bioavailability before, during, and after the Sturtian glaciations may have been limited by persistent detrital and variable Fe-ox HR loading. Further, microbial recycling of organic-rich P at the primary sediment-water interface was insufficient to generate sediment porewater P saturation to spontaneously trigger vast precipitation of calcium-bearing P minerals across the sequence. This observation limits potential diagenetic interference with primary sediment P content. Production of dissolved sheet silicate bound PO 4 3- would have been facilitated by grinding of the bedrock by thawing ice sheets, with the generation of sub-glacial acidity66. These subglacial acidic conditions, combined with acidic water produced by the immediate post-Snowball high CO2 atmosphere67, would have sustained chemical leaching of P from rocks, including apatite minerals68. The sheet silicate clay minerals that are expected to more easily bind PO 4 3- in more acidic conditions4–6 transported and liberated bound PO 4 3- to seawater following contact with higher marine pH5,6,10 and salinity11,17. The sudden decline in leachable sheet silicate P entering the immediate post-Snowball state, followed by abrupt rise in magnetite P content by at least a factor of 200 compared to Pre-Snowball hints at a potential switch in P sink from clays to seawater. The increase in dissolved seawater P promoted primary productivity and oxygenation, with the resultant recycled biomass P captured together with dissolved inorganic P and preserved by Fe-ox HR minerals generated in the oxidized water column (Fig. 9). Taken together, the data indicate a major switch from marine waters with low dissolved PO 4 3- content to an enlarged inventory created by a deglaciating Cryogenian Snowball clay factory.

Methods
Sampling and sample preparation
Outcrop sampled lithologies and their locations are described in Table S1. Only a subset of representative rocks were analyzed for this study, but the overall collection was used to establish the placement of samples along sequence stratigraphy. Prior to geochemical analysis, the outer exposed layers of samples were removed and only the unweathered portions were used for analysis. A fraction was milled into a fine powder for geochemical, chemostratigraphic and XRD mineralogical analyses. Slices of whole rock samples were thin sectioned and polished for scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS).

Mineralogical analysis
X-ray diffraction (XRD) analysis for bulk semi-quantitative qualitative mineralogy analysis was undertaken with a PANalytical Xpert-pro and a Bruker D8 ADVANCE diffractometers at Cardiff University and University of Poitiers, respectively, as previously described69–71. SEM-EDS analysis was performed with an Oxford FEI-XL30 fitted Environmental SEM (ESEM) connected to an EDS system at Cardiff University. Analyses were conducted on polished thin sections coated with carbon to a thickness of 14 nm with a BIO-RAD SC500 sputter. The SEM-EDS analyses were run at a working distance of 8.9 mm and a 15 kV accelerating voltage. Backscattered elemental maps, point-specific atomic composition and spectra were combined to identify putative mineral phases. Semi-quantitative XRD analyses of the relative abundance of the mineral assemblages was calculated using the integrated area ratios of the principal peaks after decomposition by the FYTIK program72.

Elemental analysis
An approximate 250 mg of bulk rock powder were dissolved using reagent grade acids in closed screw-top Teflon vessels (Savillex) at 90°C for one day in concentrated 3 ml 40% HF, 3 ml 32% HCl, and 1 ml 65% HNO3. Excess HF was neutralised with 93 ml H3BO3 (20 g/L) aqueous solution. Elements were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a Horiba Jobin Yvon® Ultima 2 spectrometer with boron as an internal standard. Calibrations were made with ACE, JB2, and WSE international standards, with relative standard deviation of ≤1% for SiO2, ≤2% for the other major elements, and ≤5% for traces elements.

Iron speciation analysis
Chemical extraction of sediment Fe phases was conducted using the sequential Fe extraction protocol of Poulton and Canfield and Fe measured using the ferrozine method34–36. Redox trends were further constrained using the distribution of Fe/Al ratios across the studied section, where average ratios of ~0.55±0.11 are common for oxic waters, with much greater values for anoxic settings34–36. Sediment Al/(Al+Fe+Mn) content was used to demonstrate that hydrothermal contribution25 of Fe to the sediments was negligible and that observed high Fe/Al ratios cannot be associated with hydrothermal activity. The sequential Fe extraction procedure resulted in the quantification of eight operationally defined Fe phases: (1) Total bulk rock Fe (Fe T) from whole rock dissolved powder using hydrofluoric acid (HF); (2) Poorly reactive sheet silicates Fe or sheet silicates (Fe SS) extracted with boiling 12 M HCl; (3) Reducible oxyhydr(oxides), including goethite, akageneite and hematite Fe (Fe-oxy) with sodium dithionite, and magnetite Fe with ammonium oxalate (Fe Mag); (4) Carbonate-associated Fe (Fe Carb) extracted with sodium acetate; (5) pyrite Fe (Fe Py) extracted using the chromium reduction method. (6) Unreactive silicate Fe (Fe URS) was determined as Fe URS = (Fe T) – (Fe SS + Fe-oxy + Fe Mag + Fe Carb + Fe Py).

Chemically extractable Fe-bound P analysis
Co-extracted P associated with the Fe phases was approximated following the optimized Fe-P speciation extraction procedure of Thompson and others40 for ancient sediments and Fe and P co-measured by ICP-AES. These included Fe carbonate bound-P (Fe-P carb), Fe oxyhydr(oxides) associated-P (Fe-ox HR-P), magnetite bound-P (P Mag) and sheet silicate Fe associated P (P SS). The remaining P was assumed tied up mainly with unreactive silicates (P URS), first verified by a lack of correlation between total organic carbon and P. We use multiple lines of geochemical evidence, together with Loss on Ignition (LOI) to show that most P trapped in the sediments is bound to inorganic mineral phases. Further, SEM-EDS and XRD analyses were used to show that apatite is not a major mineral phase in the sediments. The potential P URS fraction was then estimated from P URS = (P T) – (Fe-P carb + Fe-ox HR-P + P Mag + P SS). Because of the linear correlation between dissolved P and particulate Fe-ox HR in marine sediments37–39, we further evaluate the distribution of P in the Fe-ox HR phases to approximate the general behaviour of dissolved P across the studied section.

Stable isotope analysis
For analysis of δ13C carbonate and δ18O carbonate, powdered samples weighed into septum vials were flushed with helium, acidified with 99% ortho-phosphoric acid, and left to react for 24 hours at 60°C to ensure complete dolomite dissolution. The long-term precision of in-house Carrara marble standard was 0.05‰ for both δ18O and δ13C (1SD). Organic carbon samples were acidified in 10% HCl and left to react for up to two days to remove all inorganic carbon, prior to δ13C org measurement. Residues were rinsed three times, dried and weighed into tin capsules. The aperture of the autosampler determined the maximum amount of sediment that could be analysed for C org to be 60-80 mg. Because of low C org content, total analysable minimum C org was estimated to be 10 µg. To allow calibration of such small samples, three standards (IAEA-CH6 [δ13C = −10.449], IAEA-6oo [δ13C = −27.771] and in-house caffeine [δ13C = −33.30]) were dissolved in de-ionised water to improve homogeneity and allow accurate dosing of small aliquots using a micropipette. Results for IAEA-CH6 and the in-house caffeine were used to estimate a correction function for sample size and size-dependent two-point normalization, which was applied to IAEA-600 as independent standard. The resultant precision was dependent on sample size. The long-term precision for δ13C was estimated to be 0.09 (1SD) for routine samples containing ≥100 µg C. However, the standard deviation increases with decreasing sample size to 0.38 for aliquots of 10–30 µg C for the present study (IAEA-600, n=15). Iron isotope analysis was conducted on rock powders as previously described71.

Numerical modelling
A four-box ocean PO 4 3- model was simulated using the equations described in reference64. The model was operated from an initial low oxygen steady state and followed by a pulse of reactive P from a riverine source or from within the ocean. The latter state represents within the ocean the chemical weathering of glacial debris as a source of PO 4 3- to seawater. The initial steady state flux conditions at t=0 assume half the annual amount of current riverine PO 4 3- supply to seawater and increased flux of reductants to the surface ocean equivalent to 1×1013 mol O2, equating to ~5% PAL of O2 producing a slightly oxic ocean surface and anoxic ocean interior and shelf bottom waters, except for ~20% of the proximal shelf. At t=0 a pulse of P was delivered to the system by increasing the riverine PO 4 3- input by 50% for a total of 5 Myr in duration or adding this additional PO 4 3- throughout the ocean interior. In agreement with geological evidence, the model calculates the relationship between PO 4 3- and ocean-atmosphere oxygenation and PO 4 3- burial rates as authigenic calcium phosphate, Fe-ox HR and organic matter bound P (refs. 64,65).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript entitled “Transient fertilization of a post-Sturtian Snowball ocean margin with dissolved phosphate by clay minerals” by Fru et al. studied glaciation deposits of the Dalradian Supergroup. The authors performed sequential extraction, XRD, SEM, and bulk geochemical analysis on carbonate and tillite samples. By analyzing the data, the authors argued that post-glaciation delivered clay minerals to the ocean which fertilized the ocean and afterwards the precipitation of Fe(II) from ocean water column sequestered marine dissolved P in the sediments.\nI think the authors properly interpreted the data and the conclusion is straightforward. The normalization of trace elements over Al or Ti to interpret detrital source is widely applied to study trace elemental enrichment. Also, the separation of different mineral phases through sequential extraction approach provides a powerful tool to study the distribution of P among different reservoirs. And the iron speciation analysis which is commonly used in studying ocean anoxia is applied to study the redox environment. Through analyzing the geochemical data, the authors linked two important conclusions on P cycling from previous experimental studies: clay shuttling P and the co-precipitation of Fe and P.\nOverall, the experimental data support their argument, and their conclusion is interesting. Thus I suggestion acception after minor revision. I only have a couple of comments on the presentation of data. Firstly, I would think scattered plots instead of shadowed figures are better to present the results. Also, a correlation matrix analysis could strength their arguments.\nSpecific comments:\n1. Figure 2: maybe delete the sub-figure nomenclature (e.g., a, b, c ……) because these alphabet letters are more notable than useful information. Also, in the main text, it’s not necessary to use “Fig. 2a”, just “Fig. 2” should be enough. Same as figure 5.\n2. Figure 2,4", "reviewer_5": "maybe plot as scatter instead of shadow might be better?\n3. The authors discussed a lot about the correlation between P, Fe, Ti, Al as well as other trace elements in the “bulk geochemistry section”. I agree with the discussion and figures 2 and 3 show obvious trends. Actually, talking about correlation, the easiest way is to do a correlation matrix analysis among all the studied proxies. In which case “positive correlation” or “no correlation” are more distinguishable and easier to compare.\n4. Figure 4e is the ratio of P over Fe in magnetite phase, and the unit should be 1 instead of ppb.\n5. Fig. S1: I see no data in “Fig.S1a”-“Fig.S1d”.\n6. Line 273: I guess you mean “whether P is associated with Fe”?\n7. Line 279-280: should be “The ratio of Fe-oxHR over total Fe content suggests……?”", "reviewer_2": "Dominic Papineau\nReviewer #2 Attachment on the following page\nReview of Chi Fru et al. 2023 – Nat Comm\nThis manuscript describes new geochemical data from a Sturtian age sequence from Scotland spanning a glaciogenic tillite. The geochemical data (elements, and Fe and P speciation) was collected stratigraphically above, below and within the tillite. This was used to discuss how phosphate was likely bound onto clays, as opposed to Fe-oxide, and delivered as such by rivers. Some modelling is also performed. The conclusion is that phosphate was delivered to seawater during glacial runoff. In my opinion this shows some level of novelty, although the core of this idea (of post-glacial riverine delivery of phosphate to stimulate primary productivity) has been broadly accepted in the community for the past 15 years or so for Proterozoic glacial periods. The manuscript present lots of new data, especially in the supplementary information, some of which is overlooked. As a result, major modifications are needed to incorporate discussion of this new information. This assessment does not question the quality of the work and data, nor the observed correlations between clays and phosphate, but is meant to encourage the authors to give a more accurate picture of what is preserved in these interesting rocks."}, "author_response": "REPLY TO REVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThe manuscript entitled “Transient fertilization of a post-Sturtian Snowball ocean margin\nwith dissolved phosphate by clay minerals” by Fru et al. studied glaciation deposits of\nthe Dalradian Supergroup. The authors performed sequential extraction, XRD, SEM, and\nbulk geochemical analysis on carbonate and tillite samples. By analyzing the data, the\nauthors argued that post-glaciation delivered clay minerals to the ocean which fertilized\nthe ocean and afterwards the precipitation of Fe(II) from ocean water column\nsequestered marine dissolved P in the sediments.\nI think the authors properly interpreted the data and the conclusion is straightforward.\nThe normalization of trace elements over Al or Ti to interpret detrital source is widely\napplied to study trace elemental enrichment. Also, the separation of different mineral\nphases through sequential extraction approach provides a powerful tool to study the\ndistribution of P among different reservoirs. And the iron speciation analysis which is\ncommonly used in studying ocean anoxia is applied to study the redox environment.\nThrough analyzing the geochemical data, the authors linked two important conclusions\non P cycling from previous experimental studies: clay shuttling P and the co-precipitation\nof Fe and P.\nOverall, the experimental data support their argument, and their conclusion is\ninteresting. Thus I suggestion acception after minor revision. I only have a couple of\ncomments on the presentation of data. Firstly, I would think scattered plots instead of\nshadowed figures are better to present the results. Also, a correlation matrix analysis\ncould strength their arguments.\nResponse\nWe want to extend our sincere appreciation to the reviewer and for their helpful\nsuggestions. We have undertaken the corrections as suggested, and below we provide\nexplanations to the specific comments.\nSpecific comments:\n1. Figure 2: maybe delete the sub-figure nomenclature (e.g., a, b, c ……) because these\nalphabet letters are more notable than useful information. Also, in the main text, it’s not\nnecessary to use “Fig. 2a”, just “Fig. 2” should be enough. Same as figure 5.\nResponse\nBecause the letters are required to identify the individual panels for clear referencing\nwithin the text, we have opted to reduce the font size of the letters to allow them to\nbetter blend with the figure without creating distraction. Labelling each panel is also in\nline with Nature Communications’ requirement that each panel on a figure must be\nlabelled, described in the Figure legend, and cited in the text.\n2. Figure 2,4"}, {"round": 2, "reviewer_comments": {"reviewer_2": "This manuscript has moved in the right direction, but still has several short comings. In particular:\n- I have the impression that several comments from both the other reviewer and me were not all addressed properly. The argument raised in response to my comment on a possible biological source of P, based primarily to the low TOC of these rocks, is still not considering the likely possibility that a massive amount of biomass was oxidised during post-glacial thawing, which is a likely interpretation for the observed negative d13Ccarb values of the Bonahaven carbonate. In fact, this observation is almost only used to suggest broad, nameless correlations to other tillite-carbonate bearing sequences of Sturtian age.\n- These correlations are also not demonstrated however, also because the age constraints in the stratigraphy of figure 1 are not shown (only an interpretative age scale is shown instead), and no specific comparison is made to at least one other similar Sturtian chemostratigraphic sequence known.\n- The point raised by the other reviewer about the absence of the measured data points in most of the figures, and to which I agree fully, shows that both reviewers want to see the measured real data points on the figure panel. It is OK to use coloured shaded areas in your chemostratigraphies, however, you still need to distinctly show ALL your data in these chemostratigraphies and histograms\nHere are some other in-text specific problems to address and remediate:\n- line 76: ‘In this regard...’’ does not follow; The first statement should not be what is written and rather should follow with one that is more specific to comparable environments, uniformitarian, and to be returned to for later, broader comparisons, so that this should help to setup the discussion. Currently the statement brings in the GOE out of the blue and without much comparisons to be made anyways, here or later.\n- Line 127: equation there needs to have the denominator in brackets for mathematical correctness.\n- Line 131: it is unclear if you can really talk about a Corg excursion, as you do not report organic -rich rocks and also whether the d13Ccarbonate excursion is the postglacial positive or negative that is referred to, because both exist in your (un-shown) data. And what rocks are included in ‘immediate post-snowball values’ (line) 132?\n- Line 150: here and elsewhere in the text, the expression ‘tectosilicate plagioclase feldspar mineral’ is either over-worded or no point is really ever made about the fact that they are indeed ‘tectosilicate' and ‘mineral’.\n- Line 158: why are there ‘;’ and not simply ‘,’?\n- Line 170: yes but kaolinite would also react with alkaline fluids during metasomatism.\n- Line 173: vague, your rocks are not demonstrably below the greenschists facies, especially with these phyllosilicates (chlorite, illite, and muscovite).\n- Line 174: ‘primary features’ is too vague.\n- Line 332: the wording ‘staggered distribution’ and appearance in the data is unclear to me.\n- Lines 345-347: confusing sentence.\n- Line 500: then OK, perhaps your explanation is correct, however there isn't much specific that is discussed about other published concentrations of P content and comments on the relevance of the comparison.\n- Line 507: How does it limit this explanation involving diagenesis? Diagenesis is most likely included in this sequence of events.\n- Line 513: The only clay mineral mentioned is illite and that is not unambiguously detected, as the XRD data show planes that overlap those of muscovite, no polarised light microscopy images (PPL and CP) at high magnification is shown to confirm it is also not sericite, which could also indicate higher metamorphic grades than presumed or later fluid infiltrations that hydrolysed feldspars.\n- Line 523: I think you mean ‘deglaciating’ Cryogenian snowball.\n- Line 563-575: this is all text to be discussed in the discussion section, not in the methods section. It does not make sense to leave out important topics of discussion.\n- Supplementary information section needs much better organisation if it is to be truly useful and the author have to revise it to make it acceptable for publication. First this section needs to be written in the portrait orientation, and then more sub-titles are needed, and more introductory text is needed to more clearly make sense of what this concerns exactly. Only one section currently stands and is supposedly on ‘sedimentary rates and iron-based redox proxy’, but I see many topic that are not all well connected. It is not clear if these four points represent constraints for the model?\n- Table S2: I don't understand the logic behind writing ‘semi-quantitative’ in the title because your table very quantitative indeed. What you mean is low precision or low accuracy data or both.\n- Figure 1: indicate more precisely where the dolomite concretions occur, please. Add specific age constraints in the stratigraphic column and not only the interpreted age scale. Also, please list the key sedimentological markers in the Bonahaven dolomite (especially what makes it ‘evaporitic’ as claimed the main text.\n- Figure 2: EDS maps should be accompanied by area scans to show smaller apatite grains, and also possibly with P + Fe and P + Al maps, to better support the arguments in the main text. This is also needed for clarity and give a better purpose to this figure, which is now in the main text.\n- Figures 3, 4, 7, 8, S4", "reviewer_5": "please add data for a more honest and transparent presentation.\n- Table S3: some sense of the data within its sampled sequence would be useful as an additional row and to give a better organisation and sense to this table.\n- Table S4: In my book, this table suffers from the absence of measurements on standards, controls, duplicates, etc.\nSo great story overall, but the presentation needs to be updated with all new data explicitly shown in figures, provide high-resolution images of the four specimens to show the absence or presence of smaller apatite crystals in figure 2. The rest of the supplementary information is adequate and useful."}, "author_response": "REVIEWER COMMENTS\nReviewer #2 (Remarks to the Author):\nThis manuscript has moved in the right direction, but still has several short comings. In\nparticular:\n- I have the impression that several comments from both the other reviewer and me\nwere not all addressed properly. The argument raised in response to my comment on a\npossible biological source of P, based primarily to the low TOC of these rocks, is still not\nconsidering the likely possibility that a massive amount of biomass was oxidised during\npost-glacial thawing, which is a likely interpretation for the observed negative d13Ccarb\nvalues of the Bonahaven carbonate. In fact, this observation is almost only used to\nsuggest broad, nameless correlations to other tillite-carbonate bearing sequences of\nSturtian age.\nResponse: If we have given the impression that some of your comments were not fully\nexplained, it was unintentionally and often, we have avoided running with speculations if\nthe evidence is not obvious in our dataset. We would like to indicate that we use our\ncarbonate carbon systematics (not organic carbon as suggested), for chemostratigraphic\nplacement of our samples along the studied sequence. We do this by comparing our\ndata with published observations for these facies (see for example Brasiers and Shields\n(2000) and Fairchild et al. (2018). This focus is important to establish the authenticity of\nour samples, which together with geochronology, is the basis on which these rocks have\nbeen broadly correlated chemostratigraphically to Sturtian successions globally.\nFollowing your recommendation, we have revised this statement to clarify this point.\nWe then subsequently discuss the C systematics with respect to biological processes\norg\ninvolved in the recycling of organic matter-rich P at the water-sediment interface and\nassociated apatite and carbonate P sedimentation. We however avoid drawing\ncomparisons with other Sturtian glaciation rocks as such data linking C systematics to\norg\nP cycling, particularly apatite, across Sturtian Snowball deposits is limited to enable\nunambiguous generalization or the naming of specific lithologies elsewhere. We think it\nis important to recognize novelty in our work. In doing so, we have further highlighted\nthe idea that biological C oxidation likely contributed to sedimentary P dynamics in\norg\nthe text, consistent with our conceptual model in Figure 9. Finally, as requested, we have\nprovided all the C systematic (C and carbonates) data in a new Figure S1 as requested.\norg\nIn this figure we have plotted the available data, including C and carbonate\norg\nconcentrations and C and O isotope variations along sequence stratigraphy. We would\nlike to indicate that we have avoided making strong quantitative speculations on the\namount of C that was potentially present or oxidized in the various sections, since we\norg\nsimply cannot make such an inference from the available data with any degree of\ncertainty. We agree that the negative d13C trends present in all the lithologies show that\nC was oxidized across the entire section, but they do not tell us how much C was\norg org\npresent in the water column and sediments or oxidized. Nonetheless, out data are clear\nthat P abundance shows a strong co-relationship between sheet silicates (potential clays)\nand iron oxide fluctuations across the studied sequence, the focus of our paper and that\nthese relationships are directly related to aqueous dissolved P content and oxygenation\ndynamics.\n- These correlations are also not demonstrated however, also because the age\nconstraints in the stratigraphy of figure 1 are not shown (only an interpretative age scale\nis shown instead), and no specific comparison is made to at least one other similar\nSturtian chemostratigraphic sequence known.\nResponse: As explained above, we have plotted the C systematic data along the\nsequence stratigraphy, together with sample names, the sample coordinates provided in\nTable S1. The age constraints for the studied sections have been established by previous\nstudies on lithologies that bracket the sampled interval (see for example Brasiers and\nShield (2000), Fairchild et al. (2018); Prave et al. (2009); and Sawaki et al. (2010) and\nreferences therein. We would like to emphasize that the main placement of our samples\nalong the succession is consistent with established chemostratigraphic variations and\nlithological correlations that show that the distinct Port Askaig Tillite Formation is a\nmarker of the global Sturtian Cryogenian glaciation and the post-glacial carbonates in\nthe Bonahaven Dolomite Formation the end of the glaciation. Our comparisons are\nbased on these defined lithological types known to represent the distinct pre-glacial,\nglacial and post-glacial successions. This approach works well for our study, as evidenced\nby the studies cited above and others, since the point was to study chemostratigraphic\nchanges across these distinct climatic intervals, with respect to P cycling and\noxygenation. As it is almost impossible to require age constraints for every single point\nanalyzed along an established sedimentary succession, our approach is consistent with\naccepted approaches used by the geochemistry community.\n- The point raised by the other reviewer about the absence of the measured data points\nin most of the figures, and to which I agree fully, shows that both reviewers want to see\nthe measured real data points on the figure panel. It is OK to use coloured shaded areas\nin your chemostratigraphies, however, you still need to distinctly show ALL your data in\nthese chemostratigraphies and histograms.\nResponse: The data points have been included in all figures.\nHere are some other in-text specific problems to address and remediate:\n- line 76: ‘In this regard...’’ does not follow; The first statement should not be what is\nwritten and rather should follow with one that is more specific to comparable\nenvironments, uniformitarian, and to be returned to for later, broader comparisons, so\nthat this should help to setup the discussion. Currently the statement brings in the GOE\nout of the blue and without much comparisons to be made anyways, here or later.\n- Line 127: equation there needs to have the denominator in brackets for mathematical\ncorrectness.\nResponse: The sentence has been rephrased taking this recommendation into\nconsideration. See text.\n- Line 131: it is unclear if you can really talk about a Corg excursion, as you do not report\norganic -rich rocks and also whether the d13Ccarbonate excursion is the postglacial\npositive or negative that is referred to, because both exist in your (un-shown) data. And\nwhat rocks are included in ‘immediate post-snowball values’ (line) 132?\nResponse: We have changed excursion to C trends to remove ambiguity.\norg\n- Line 150: here and elsewhere in the text, the expression ‘tectosilicate plagioclase feldspar\nmineral’ is either over-worded or no point is really ever made about the fact that they are\nindeed ‘tectosilicate' and ‘mineral’.\nResponse: We have revised this to tectosilicate mineral and have highlighted the fact that\ntheir presence indicates an association with the erosion of crystalline bedrocks (see for\nexample Brasiers and Shields, 2000).\n- Line 158: why are there ‘;’ and not simply ‘,’?\nResponse: Replaced as suggested.\n- Line 170: yes but kaolinite would also react with alkaline fluids during metasomatism.\nResponse: We have included this statement in the paragraph.\n- Line 173: vague, your rocks are not demonstrably below the greenschists facies,\nespecially with these phyllosilicates (chlorite, illite, and muscovite).\nResponse: We agree with this observation, which is why we made the statement that\nAlthough chlorite can also form as a low-grade metamorphic mineral, with further\ntransformation to muscovite at higher temperatures, the mineralogy of the facies is\nconsistent with their suggested low metamorphic grade and observed preservation of\nprimary features18-23.\nWe would very much like to emphasize that these rocks show no evidence for high-grade\nmetamorphic alteration. These observations are evident from our mineral composition and\nwith conclusions in the cited papers describing the studied succession and geological\nsetting.\n- Line 174: ‘primary features’ is too vague.\nResponse: We are unsure what is required here as we are simply suggesting that primary\nfeatures of these rocks are preserved as opposed to secondary features resulting from\npost-depositional alteration, based on the cited published observations.\n- Line 332: the wording ‘staggered distribution’ and appearance in the data is unclear to\nme.\nResponse: Deleted.\n- Lines 345-347: confusing sentence.\nResponse: Rephrased to: We assume that the rise of appreciable P enrichment in immediate\npost-Snowball magnetite grains relative to the tillites, points to increasing immediate post-\nSnowball co-precipitation of magnetite and dissolved PO 3- out of seawater.\n4\n- Line 500: then OK, perhaps your explanation is correct, however there isn't much specific\nthat is discussed about other published concentrations of P content and comments on the\nrelevance of the comparison.\nResponse: This observation is shown in Figure 6, where our samples are superimposed on\nEarth’s sedimentary P record across Earth history and cited in the discussion.\n- Line 507: How does it limit this explanation involving diagenesis? Diagenesis is most\nlikely included in this sequence of events.\nResponse: Our statement doesn’t rule our diagenetic formation of apatite via the oxidation\nof P-rich C . However, the fact that apatite only becomes relatively pronounced in the\norg\nimmediate post-Snowball facies, suggests that diagenetic enrichment or loss of P through\nthis process was likely limited across most of the sampled sequence.\n- Line 513: The only clay mineral mentioned is illite and that is not unambiguously\ndetected, as the XRD data show planes that overlap those of muscovite, no polarised light\nmicroscopy images (PPL and CP) at high magnification is shown to confirm it is also not\nsericite, which could also indicate higher metamorphic grades than presumed or later fluid\ninfiltrations that hydrolysed feldspars.\nResponse: There is no sericite in these samples. We have not observed sericite-like phases\nby SEM-EDS analysis, consistent with our XRD analysis. We emphasize again that these\nsamples have not experienced high-grade metamorphism as established by our data and\nsupported by the cited literature. The mineralogical analysis was designed for describing\nthe quality of our samples across the distinct climatic zones and was deemed sufficient for\nthe purpose of this study. This is because the core of our work relies on the exhaustive\nchemical extraction experiments that link sheet silicate P and iron oxides P phases to\noxygenation. The idea of a combination of high resolution imaging techniques is the\nsubject of another study which we have completed for a paper currently written for\nPrecambrian Research which we would be very much happy to share if this reviewer wants\nto see the data. The scale of the analysis includes detailed characterization of the\nindependent mineral grains and their associations with P across the various lithologies\nlinked to authigenic and detrital mineral phases, the unnecessary details which would\ndilute the focus of the present paper. Based on the results, we can say we great certainty\nthat these rocks show no obvious signs of high post-depositional compaction and\npresence of high-grade metamorphic minerals and the transformation of primary\nimportant clay minerals.\n- Line 523: I think you mean ‘deglaciating’ Cryogenian snowball.\nResponse: Revised as suggested.\n- Line 563-575: this is all text to be discussed in the discussion section, not in the\nmethods section. It does not make sense to leave out important topics of discussion.\nResponse: Moved into the main text where redox implications are discussed.\n- Supplementary information section needs much better organisation if it is to be truly\nuseful and the author have to revise it to make it acceptable for publication. First this\nsection needs to be written in the portrait orientation, and then more sub-titles are\nneeded, and more introductory text is needed to more clearly make sense of what this\nconcerns exactly. Only one section currently stands and is supposedly on ‘sedimentary\nrates and iron-based redox proxy’, but I see many topic that are not all well connected. It\nis not clear if these four points represent constraints for the model?\nResponse: Figures have been improved and data points inserted as suggested. The\nsupplementary section is divided into three distinct sections, i.e., supplementary\ninformation where we discuss the sedimentation rates, Supplementary Figures and\nSupplementary sections that are accompanied by descriptive legends. With respect to\nredox, sedimentary rates are important because fast rates can dilute Fe content, especially\nwith respect to rapid detrital overloading. We do this by comparing known sedimentations\nrates for co-eval strata in Svalbard and of known thickness with the thickness of the\nsampled lithologies, but of unknown sedimentary rate.\n- Table S2: I don't understand the logic behind writing ‘semi-quantitative’ in the title\nbecause your table very quantitative indeed. What you mean is low precision or low\naccuracy data or both.\nResponse: Semi-quantitative mineralogical analysis is a widely used standard procedure\nwhere XRD data are used to estimate the relative concentration of mineral fractions\nestimated from the integrated intensities of XRD spectral peaks. These data give a realistic\nrepresentation of the relative content of various mineral fractions present in the samples.\nThis semi-quantitative analysis is done using the software described in Wojdyr, M. 2010.\nFityk: a general-purpose peak fitting program. Journal of applied crystallography 43(5), pp.\n1126–1128. See examples of application below.\n1) Gisèle Lecomte-Nana, Jean-Pierre Bonnet, Nibambin Soro. Influence of iron on the\noccurrence of primary mullite in kaolin based materials: A semi-quantitative X-ray\ndiffraction study. Journal of the European Ceramic Society, 113, 669-677.\n2) Talero R. et al., 2011. Comparative and semi-quantitative XRD analysis of Friedel’s\nsalt originating from pozzolan and Portland cement. Construction and Building\nMaterials 25, 2370-2380.\n3) Schoen R. 1962. Semi-Quantitative analysis of chlorites by X-Ray diffraction.\nAmerican Mineralogist 47 (11-12), 1384–1392.\n- Figure 1: indicate more precisely where the dolomite concretions occur, please. Add\nspecific age constraints in the stratigraphic column and not only the interpreted age scale.\nAlso, please list the key sedimentological markers in the Bonahaven dolomite (especially\nwhat makes it ‘evaporitic’ as claimed the main text.\nResponse: We would like to stress that the provided sequence stratigraphy is drawn\naccording to the accepted stratigraphy of this succession in the published literature,\nshowing key transitional boundaries with respect to lithotype. The dolomite concretions\nmentioned in the disrupted beds in the BDF are a minor component of the Bonahaven\nDolomite Formation to make them a prominent feature on the stratigraphic map.\nMoreover, we have not sampled them. Further, what is important to note is that these\nsamples are mostly dominated by a dolomite cement, especially in the Lossit Limestone\nFormation and the Bonahaven Dolomite Formation where carbonates can make up >50\nwt.% of the sampled facies. This carbonate cement is highlighted by the SEM\nphotomicrographs in Figure 3. There are additional minor sedimentological features which\nsimply cannot be included in the stratigraphy as they do not represent important marker\ntransitional boundaries, for example mud cracks, etc.\nFinally, we would like to emphasize that the mineralogy of these rocks, as referenced in\nthe geological setting section, has been studied extensively. Thus, the focus of this study\nwas not to produce information already established by other studies, but to use our\nmineralogical data to ensure that our sampling strategy is consistent with sequence\nstratigraphy. For example, we have not made any unvalidated claims about evaporites.\nThis information is reported in the provided references. Neither was our focus on high-\nresolution mineralogical characterization of these rocks, but on using chemical extraction\nexperiments to investigation the link between sheet silicates (approximated total clay\ncomposition) and iron oxide phases to P cycling and oxygenation across the sequence.\nWe believe that we succeeded in establishing this relationship.\n- Figure 2: EDS maps should be accompanied by area scans to show smaller apatite grains,\nand also possibly with P + Fe and P + Al maps, to better support the arguments in the\nmain text. This is also needed for clarity and give a better purpose to this figure, which is\nnow in the main text.\nResponse: We have provided a new SEM-EDS Figure S9 showing individual P particles\nand their association with Fe in the glaciogenic tillites and the immediate post-Snowball\ninterval where our samples record co-increase in P enrichment and oxygenation\nfollowing the transition in the immediate post-Snowball.\n- Figures 3, 4, 7, 8, S4"}]

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