Marine phosphorus and atmospheric oxygen were coupled during the Great Oxidation Event

Domain: earth_science
Published: 2025-10-15
Editorial outcome: major_revision

Abstract
The Great Oxidation Event (GOE) represents a major increase in atmospheric O₂ concentration between ca. 2430 and 2060 million years ago, culminating in the permanent shift to an oxygenated atmosphere. It’s causes remain debated. Here we use the carbonate-associated phosphate (CAP) proxy to reconstruct oceanic phosphorus concentrations during the GOE from globally distributed sedimentary rocks. We find that the CAP and the inorganic carbon isotope composition of marine sediments co-varied during the GOE, suggesting synchronous fluctuations in marine phosphorus, biological productivity, and atmospheric O₂. Biogeochemical modelling shows that transient increases in P bioavailability can raise oxygenic primary production and organic carbon burial, yielding isotopically heavy seawater inorganic carbon and reproducing the observed patterns. Consequently, geochemical and modelling data together suggest that P availability was a likely contributor to the rapid oxygenation of Earth during the GOE.

Introduction
The Great Oxidation Event (GOE) encapsulates the initial permanent rise of atmospheric O₂. Accordingly, this was a major event in the evolution of Earth’s habitability, which paved the way to Earth’s oxygenated atmosphere, the emergence of eukaryotic life and geochemical transformations of Earth’s surface and interior. While multiple lines of evidence exist for the GOE, its beginning has often been defined by the disappearance of mass-independent fractionation of sulphur isotopes (MIF-S) in sedimentary sulphides and sulphates, which represents the transition to O₂ concentrations above 10⁻⁶ of the present atmospheric level (PAL). By this definition, the GOE started at ca. 2430 million years ago (Myr), however, atmospheric O₂ is thought to have continued to rise even more rapidly after this initial transition, overshooting to between 1 and 40% of the PAL, until an inferred collapse in atmospheric O₂ concentrations before 2060 Myr (Fig. 1a-b). Consequently, the GOE is currently viewed as a protracted event during which O₂ levels may have fluctuated on a large scale, with potential superimposed shorter and low-level fluctuations, allowing for the possible oscillatory loss and return of MIF-S during the early half of the GOE. These shorter-term fluctuations were likely coupled to the occurrence of global glaciations and changes in seawater sulphate levels between ca. 2430 and 2220 Myr (Fig. 1). Henceforth in this study, the term ‘GOE’ refers to the protracted geological interval between ca. 2430 and 2060 Myr.

A unique feature of the later stage of the GOE is the largest and longest-lived positive excursion of sedimentary carbonate-C isotopic composition (δ¹³C_carb) in Earth history, with an average δ¹³C_carb value of +8‰, but highest values reaching up to +28‰. This δ¹³C_carb excursion is commonly referred to as the Lomagundi Excursion or Lomagundi Event (LE), which is globally recorded in carbonate sediments deposited between ca. 2220 and 2060 Myr ago (Fig. 1). The canonical interpretation for the LE is enhanced organic relative to inorganic carbon burial, and consequently oxygenation of the surface environment. This may have been driven by increased availability of the bio-limiting nutrient, phosphorus (P). This interpretation of the LE might be supported by the permanent disappearance of MIF-S at the onset of, or during the LE, which potentially implicates the LE as the ultimate driver to permanent oxygenation of the atmosphere. While it is uncertain whether a net gain in atmospheric O₂ during the LE was responsible for the ultimate disappearance of MIF-S, several lines of evidence suggest that atmospheric O₂ concentrations peaked during the LE and did not reach such levels again until some 1–1.5 billion years later. Other interpretations for the LE have suggested that increases in primary productivity and organic matter burial were less extreme than inferred by the canonical model, or the δ¹³C_carb excursion was not related at all to changes in productivity and organic matter burial, and in turn to P availability (see ref. 18 for a detailed review). Given the number of competing models, whether changes in seawater P availability and associated organic carbon burial are responsible for the rise and fall of atmospheric O₂ level during the later part of the GOE remains uncertain. In this study, we use the Carbonate-Associated Phosphate (CAP), a new proxy, to reconstruct variability in seawater P levels across the GOE, allowing for the examination of whether changing marine P availability drove the Earth’s permanent oxygenation 2 billion years ago.

Carbonate minerals incorporate elements into their crystal structure proportionally to the elemental concentrations in the seawater. Based on this premise, it is possible to estimate ancient marine P concentrations using the concentration of phosphate in carbonate minerals, when factoring in ambient seawater chemistry (e.g. pH, alkalinity and temperature) and mineralogy, which also play a role in the uptake of elements by carbonate minerals. In marine sediments, P is predominantly found in phosphate minerals, with additional amounts bound to iron, sorbed to mineral surfaces, or contained within organic matter or carbonate minerals. These P pools, with the exception of carbonates generally, are largely controlled by redox conditions and biological activity, and not directly attributable to dissolved seawater P concentration. In contrast, phosphate bound in carbonate minerals (i.e. CAP) can be attributed to seawater dissolved P concentration. In order to extract and measure CAP, a leaching protocol that has been previously developed and validated was used in this study (see ‘Methods’). This leaching protocol was applied to sediments deposited during the GOE in order to reconstruct relative marine P variations across the GOE. The extracted P was then normalised to Ca and Mg in the leachate to give CAP as mmol of P per mol of Ca and Mg.

We analysed fourteen carbonate formations from four continents (see Table 1 and Supplementary Information 1) to capture whether global seawater phosphate availability changed during the GOE. These carbonate formations capture the LE as well as other shorter-duration positive δ¹³C_carb excursions during the GOE. This provides an opportunity to study coupled changes in δ¹³C_carb and CAP and, therefore, determine whether P-driven productivity could have triggered the significant rises and falls in atmospheric O₂ during the GOE. All of the studied formations were deposited in shallow-water (sabkha and peritidal, platform) environments, with the exception of the Silverton Formation and the lower and upper Duitschland and Nash Fork formations, respectively, which were deposited in upper-slope and deep-basin settings. From prior work, it was found that dolomite generally hosts larger concentrations of CAP compared to calcite, which can bias CAP records via changes in the dominant style of carbonate mineral preserved. However, carbonate mineralogy is almost exclusively dolomite in our sample set, with the exception of calcite in the lower part of the Espanola Formation (see Supplementary Information 1), which limits the degree to which mineralogy affects our CAP trends. The degree of metamorphism is generally uniform across each individual formation, with the exception of the Duitschland Formation, which has experienced local, thermal contact metamorphism in its lower part. Given metamorphism varies amongst formations, we have grouped formations into three metamorphic categories (Table 1), to avoid potential issues regarding CAP preservation at different degrees of metamorphism.

Results
Among all the measured formations, CAP values are on average higher in carbonates with higher δ¹³C_carb values, when comparing formations from a singular basin, except for the Saunders Formation of the Chocolay Group (ca. 2.3 Ga) and the Espanola Formation (ca. 2.4 Ga) of the Huronian Supergroup (Fig. 1c; Table 1 and Supplementary Information Table 1). To fairly compare CAP values, we evaluate studied formations based on their metamorphic grade, which yields a statistically significant (p < 0.01) positive Pearson correlation between CAP and δ¹³C_carb, for every metamorphic grade (Fig. 2). This correlation is also supported by continuous chemostratigraphic trends for the FC (ca. 2.1 Ga) and Nash Fork formations (ca. 2.1 Ga) and Wooly Dolomite (ca. 2.0 Ga), which show positive trends in δ¹³C_carb and CAP for carbonates deposited during the LE and Wooly Dolomite excursion, respectively (Fig. 3). The carbonates from the LE carry the highest CAP values in the dataset reaching values of 0.408 mmol/mol for the Juderina Formation (ca. 2.1 Ga) compared to carbonate deposited prior to the LE with values as low as 0.005 mmol/mol for the Carawine dolomite (ca. 2.6 Ga). On average, CAP is 4, 3.1, 9.2, 2 and 1.6 times higher in carbonates deposited during the LE relative to those that bracket it in South Africa, Brazil, West Australia, Gabon and Wyoming, respectively (Fig. 1c, d). Similarly, carbonates deposited during the shorter-duration, positive, pre- and post-LE δ¹³C_carb excursions identified in the Wooly Dolomite, Duitschland and Gordon Lake formations and their equivalents have average CAP values that are 2, 2.3 and 4.5 times higher in samples, respectively, compared to carbonates with near-to-zero C isotope values within these units or stratigraphically close to them (Figs. 1 and 3). For example, for the Wooly Dolomite, CAP reaches 0.13 mmol/mol during the peak of the δ¹³C_carb excursion and drops as low as 0.011 mmol/mol in carbonates not recording the δ¹³C_carb excursion.

Methods
Total inorganic carbon (TIC) analysis
TIC was measured using a FOGL Digital Soil Calcimeter. TIC was measured directly by weighing out ~0.2 g of rock powder into gas-tight 250 mL glass bottles and 4 mL of 6M HCl in a plastic cuvette was carefully added to the bottle. Once a bottle was sealed, it was tipped to mix carbonate and HCl and periodically shaken until a stable reading was achieved on the calcimeter. A pure calcium carbonate standard was analysed after every 10 samples to check reproducibility (±2% CaCO₃; n=30).

Carbonate carbon- and oxygen-isotope analysis
Carbonate carbon- and oxygen-isotope analysis was performed in the West Australian Biogeochemistry Centre at the University of Western Australia and Nanjing University. About 60–300 μg of sample powder was loaded into a vial after drying the powders at 70 °C for 24 h in an argon atmosphere. The samples were then reacted with 100% phosphoric acid under a vacuum at 70 °C using a Gas Bench II or a Kiel IV device. The resulting CO₂ was subsequently introduced into a Thermo-Fisher Scientific Delta XL mass spectrometer at the University of Western Australia or MAT 253 mass spectrometer at Nanjing University for isotopic measurements. Delta values were calibrated relative to IAEA reference standards NBS19 and NBS18, and Chinese national standard GBW04416. Carbon-isotope and oxygen-isotope data for carbonates are reported relative to Vienna Pee Dee Belemnite with a precision better than ±0.1‰ and ±0.15‰ (1σ) for carbon and oxygen isotope values, respectively, based on duplicate analyses of standards and unknowns.

Carbonate-associated phosphate (CAP) analysis
CAP was measured following previously established protocols. Only samples comprising >50% carbonate were chosen for analysis. Based on sample TIC and carbonate mineralogy, sufficient sample powder was weighed out, corresponding to ~1 mmol of carbonate. Samples were then repeatedly washed for 24 h, each time using 4 mL of 10% NaCl solution buffered to pH 8 with NaHCO₃ to remove adsorbed P. Once adsorbed P was below 0.1 ppm in the wash solution, an appropriate amount of 2% vol/vol acetic acid was added to the powder to dissolve up to 70% of the carbonate. After 30 min the sample was centrifuged, and the leachate was extracted using 0.2-micron polyethersulfone membrane filters. An aliquot of leachate was taken for major and trace element analysis and measured using a Thermo Fisher Element XR Inductively Coupled Plasma Mass Spectrometer at the Centre for Microscopy, Characterisation and Analysis at the University of Western Australia. Analytical errors were better than ±2% for all elements based on duplicate analyses of two internal standards. Another aliquot of the leachate was taken, and the P concentration was determined spectrophotometrically using the malachite green method at 663 nm and a Perkin-Elmer EnSight® plate reader at the University of Western Australia and Chengdu University of Technology, with a relative standard deviation of less than ±5%. The sample residue was then washed with 4 mL of 10% NaCl solution buffered to pH 8 with NaHCO₃ for 24 h, and P in the wash was measured using the malachite green method. This sequence was repeated until P in the wash was below 0.1 ppm. The P in the leachate and washes was summed together to give CAP, which was then normalised to the Ca and Mg concentrations in the leachate. Three Ediacaran dolomite sediments with varying TIC, total organic carbon and clay contents were run alongside unknowns to check reproducibility for quality assurance, with duplicate CAP measurements differing by <10%.

Biogeochemical modelling
A four-box biogeochemical model after Slomp and Van Cappellen (2007) and Alcott et al. was used to model biogeochemical feedbacks among the P, C and O₂ cycles during the GOE. The model has previously been used to explore a long-term shift in Earth’s redox state, driven by the cooling mantle and the emergence and carbon enrichment of the continental crust. This model is solved in MATLAB using the Ordinary Differential Equation suite. Here, we provide a key description of our model work.

In this model the ocean is divided into proximal, distal, surface and deep ocean reservoirs. The proximal zone comprises the part of the coastal ocean directly influenced by river input and includes large bays, the open-water part of estuaries, deltas, inland seas and coastal sabkhas. The distal zone is where element recycling and upwelling are quantitatively more important than river input, and which includes the open-continental shelves (average water depth is ~130 m). The remaining two boxes include the surface layer of the open ocean (average depth 150 m) and the deep ocean. In the model setup from Alcott et al., a number of forcings are applied to guide the secular rise of atmospheric O₂. The model uses these forcings to progressively increase the delivery of riverine P to the oceans, and/or decrease the reducing gas flux from Earth’s interior to surface; both directly translate to increasingly higher atmospheric O₂ level over time. For our study, we induce an atmospheric O₂ level below 10⁻⁵ PAL using a pre-GOE reduced gas flux (Alcott et al.) before the LE (2220 to 2060 Myr ago). A riverine P input forcing is then applied to create a rise in atmospheric O₂ level during the LE that is consistent with geochemical redox proxies for atmospheric O₂. Some previous use of this model has included an Fe scavenging flux following prior assertion that Fe-oxides may have removed P from Precambrian oceans. This flux adds a redox-dependent negative flux to the marine P pool. We did not apply this Fe scavenging flux in our model run because it is not used in the most recent model version, which has demonstrated a reasonable fit to long-term C and P cycle data. However, as we force the model by adding P to the global ocean and do not rely on redox changes as the driver of P availability in the model, consideration of this flux would not change our conclusions.

Calculation of proximal and distal ocean inorganic carbon isotope values
The biogeochemical model used here includes an inorganic carbon cycle with the inclusion of a one-box inorganic carbon reservoir. This allows the computation of the average isotopic composition of DIC in the oceans (see Alcott et al. for further details). This simplified one-box component, however, cannot capture the heterogeneity of the inorganic carbon cycle within the oceans, which we will briefly describe below.

In the modern ocean, the C-isotopic composition of DIC (δ¹³C_DIC) is modulated by the effects of photosynthesis and respiration. In the shallow portions of the ocean (<100 m depth), rates of photosynthesis outweigh those of respiration, resulting in the production of high δ¹³C_DIC values. At progressively greater depths, rates of respiration increase over those of photosynthesis, resulting in lower δ¹³C_DIC values.

In order to capture the heterogeneity of the ocean inorganic carbon cycle, we follow the assumption that seawater δ¹³C_DIC values are related to seawater P concentrations, as is the case in the modern ocean. This assumption is based on the fact that photosynthesis in shallow waters results in the uptake of P and hence lower P concentrations in shallow waters. At depth, greater rates of respiration result in the concomitant release of P and isotopically depleted C to the oceans. This innate connection between dissolved inorganic C and P results in what is referred to as the ‘Redfield slope’. Consequently, assuming that seawater, inorganic P and C concentrations are mainly controlled by photosynthesis and respiration, the δ¹³C_DIC can be related to seawater phosphate concentration as follows:

δ¹³C_DIC = δ¹³C_A.O. + Δ_photo * (C_org/P) * (PO₄_A.O. - PO₄)

In this equation, A.O. denotes average ocean. Therefore δ¹³C_A.O. in our model refer to the global seawater δ¹³C value (termed A_iso in the model code). Δ_photo is the isotopic offset between organic and inorganic carbon, ΣCO₂_A.O. corresponds to the average seawater DIC concentration, C_org/P is the ratio of C/P in biomass, PO₄ is the concentration of P in the ocean region for which δ¹³C_DIC is being calculated for and PO₄_A.O. is the average seawater P concentration, which in our model corresponds to the deep-ocean P concentration.

Monte Carlo parameters
As shown by the sensitivity areas (green shading) in Fig. 6, several parameters were varied across 1000 model runs for a Monte Carlo analysis. In our model runs, we varied the C/P ratio of biomass, the mean seawater DIC concentration, and the magnitude of P input to the ocean.

The C/P of biomass is known to vary with the abundance of nutrients in water bodies. The ratio is constrained between the Redfield ratio (106) and previously speculated values of up to 400. The DIC concentration in the Paleoproterozoic oceans is uncertain, but current best estimates suggest that <30 mmol/L is most likely. With this in mind, we varied the parameter between the modern seawater value of 2.2 mmol/L and 30 mmol/L as estimated by geochemical data for the Paleoproterozoic.

We varied the magnitude of the additional P input in our model to explore how an order of magnitude increase in P input between 10⁹ and 10¹⁰ mol P/yr would affect atmospheric O₂ level.

For the full model description, including model equations and parameters, see Alcott et al.

Data availability
The geochemical data generated and used in this study have been deposited in the Figshare database at https://doi.org/10.6084/m9.figshare.29380727.

Code availability
Model code can be downloaded at GitHub (https://github.com/lalcott/d13Ctemp_2023).

Materials availability
All samples were collected and exported in a responsible manner and in accordance with relevant permits and local laws. Global coordinates and/ or location information and drill core names are given for all samples collected in the supplementary information files. Requests for materials should be addressed to M.S.D., C.L., A.B.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "General Comments\nIn this paper, Dodd and coauthors present interesting data showing that the abundance of carbonate-associated phosphate\nis higher in rocks preserving the Lomagundi Carbon isotope excursion (LE). They claim that this is evidence that marine\nphosphate levels were higher during the LE, and consequently argue for a mechanistic link between P delivery to the ocean\nand the oxygenation of the atmosphere at the time.\nI do think the data are interesting, and the idea is an enticing one. However, there are several issues with the manuscript that\ndecrease my confidence in the conclusions drawn by the authors:\n1) The authors too often make strong assertions without literature support and confuse interpretations with facts.\n2) The authors offer a simplistic view of the sulfur cycle that is outdated with respect to the literature.\n3) The authors gloss over important recent literature demonstrating the facies-dependence of the LE, which has major\nimplications for how it might be interpreted with respect to oxygenation.\n4) Throughout, there is a missing detailed analysis of how early stages of diagenesis might influence the main data\ndiscussed in the paper (CAP and d13C-carb).\n5) A major and obvious alternative explanation for both elevated CAP and d13C values (evaporative conditions) is ruled out\nvery quickly by the authors, without much justification.\nI have outlined these concerns and others in detailed comments below, which I hope will be of use to the authors. I believe\nthis work will be suitable for publication in this journal after major revisions.\nThank you for the opportunity to review this interesting work.\nRoger Bryant\nLine-by-line comments\nLine 30: “Great Oxidation Episode” does not correspond to most usage of “GOE” in the community. Suggest the authors stick\nto Event.\nLine 34: I’m not sure there is such a thing as a ‘direct’ proxy. In this case, phosphate is measured but as a proxy for oceanic\nphosphorus (not orthophosphate?). That seems like quite a complex, certainly indirect proxy.\nLine 37: Why a phosphorus control on the global carbon cycle and not the other way around? The authors don’t need me to\nremind them that correlation need not imply causation.\nLine 50: See comment on Line 30.\nLine 60-61: I think the community agrees that the GOE was protracted, but the O2 fluctuations and oscillations are less well-\nagreed upon, due in complexities in proxy behavior (e.g., the sensitivity of metal speciation to both depositional and post-\ndepositional effects). Nevertheless, if this statement represents the authors’ view, that can be stated explicitly rather than\npresuming to speak for the broader community.\nLine 62-64: “Oscillating seawater sulfate level” is an interpretation, not a fact. Here, and throughout, more careful language is\nneeded to distinguish between observations and interpretations. In this case, there are numerous reasons why the\ninterpretation of oscillating seawater sulfate level might be an over-interpretation. To give one example, in reference 10\n(Planavsky et al., 2012), the sulfur isotopic composition sedimentary sulfate is a piece of evidence used to argue for changes\nin the seawater sulfate concentration. However, one can imagine several scenarios by which the primary d34S of marine\nsulfate could change in the absence of changes in the size of the marine sulfate reservoir. Then, there is the issue of\ndiagenetic alteration of carbonate-associated sulfate, which has been argued to afflict much younger rocks (see, e.g.,\nPresent et al., 2019; Murray et al., 2021), but has yet to be evaluated for most Paleoproterozoic rocks, with some recent\nexceptions (Bryant et al., 2024).\nT. M. Present, M. Gutierrez, G. Paris, C. Kerans, J. P. Grotzinger, J. F. Adkins, Diagenetic controls on the isotopic\ncomposition of carbonate-associated sulphate in the Permian Capitan Reef Complex, West Texas. Sedimentology. 66,\n2605–2626 (2019).\nS. T. Murray, J. A. Higgins, C. Holmden, C. Lu, P. K. Swart, Geochemical fingerprints of dolomitization in Bahamian\ncarbonates: Evidence from sulphur, calcium, magnesium and clumped isotopes. Sedimentology. 68, 1–29 (2021).\nR. N. Bryant, J. P. Todes, J. A. Richardson, T. C. Kalia, A. R. Prave, A. Lepland, K. Kirsimäe, C. L. Blättler, Local\nsedimentary effects shaped key sulfur records after the Great Oxidation Event. Earth Planet. Sci. Lett. 648, 119113 (2024).\nLines 70-71: It seems strange to mention only the canonical interpretation of the LE here, despite recent work highlighting\nthe facies-dependence of the magnitude of the LE (Prave et al., 2022; Hodgskiss et al., 2023), which clearly has implications\nfor how the LE is interpreted (i.e., the LE might have required less oxidation/oxygenation than canonically assumed).\nLines 93-94: Yes, but the proportionality for a given element is often a function of the concentrations of other things in\nsolution (e.g., Ca, Mg, carbonate ions), kinetics, etc. See, e.g., Barkan et al. (2020). One could decrease CAP for the same\northophosphate concentration by increasing the activity of CO32- in solution.\nY. Barkan, G. Paris, S. M. Webb, J. F. Adkins, I. Halevy, Sulfur isotope fractionation between aqueous and carbonate-\nassociated sulfate in abiotic calcite and aragonite. Geochim. Cosmochim. Acta. 280, 317–339 (2020).\nLines 94-95: P, or phosphate? Also, why ‘relative concentration of P’?\nLines 99-101: This is quite the statement – needs literature support or further justification. To make a rough analogy with\nCAS, the concentration of a non-stoichiometric element in carbonate can change after burial and need not bear ‘direct’\nrelevance to the concentration of that element in the primary fluid.\nLines 119-121: This sentence is confusingly structured – consider revising for clarity.\nLines 123-138: The results section strikes me as being very short. One thing I would like to see here as a reader is some\nactual CAP abundances in the text, in addition to the comparisons with bracketing carbonates. If CAP works how it is\nsupposed to, the relative changes in CAP should be less meaningful than the absolute changes. I am also wondering how\none chooses how much of the bracketing carbonates to include in this relative CAP assessment.\nLine 141: This should be in the results rather than the discussion, in my opinion. The analysis in the following clause can\nstay in the discussion.\nLine 143: Here and throughout, I think the use of ‘CAP values’ rather than ‘CAP abundance’ is perhaps unhelpful. Nowhere\nin the text is CAP assigned a unit. One is left with a sense of CAP values being an abstract statistical measure rather than a\nmeasured quantity.\nLines 143-147: This is a lengthy sentence and complex argument. I had to read this several times to grasp the intention. I\nwould suggest splitting the sentence up and laying out the argument more systematically without skipping logical steps.\nLine 159: I think the authors have done a good job of alleviating fears about metamorphism influencing the data in a\nmeaningful way. However, they allude here to other ‘post-depositional processes’. Missing in the text is a discussion of\nthese other post-depositional processes and how those might affect CAP abundance. Reading between the lines, the\nprocesses in question could be early marine diagenesis of carbonates. How might one screen for the influence of such\nprocesses? What about meteoric diagenesis? I would be worried about both styles of diagenesis, but particularly the latter,\nbecause it could conceivably decrease both CAP abundance and d13C-carb. Can the authors demonstrate that the\nbracketing units (which they say are the same sabkha facies as the LE carbonates) are not simply more influenced by\nmeteoric diagenesis than those in the LE carbonates? Does mineralogy have a role in the preservation of CAP?\nLine 163: I disagree with diagenetic and metamorphic alteration being lumped together here. The authors have\ndemonstrated that metamorphic grades did not drive the signals they report. They have not conducted a similar analysis for\nany style or phase of diagenesis (although I see some work to this end further into the document).\nLines 165-167: This information should be in the results section.\nLine 178: I appreciate this analysis by the authors. I would also be interested to know if there are correlations between CAP\nand CAS content. There are several ways one could interpret such a correlation. The first would be high orthophosphate and\nsulfate concentrations in the water column. High alkalinity and/or carbonate ion activity would be another.\nLines 188-189: This information should be in the results. Here, it would be useful to explain why these diagenetic indices\nwere chosen, and what they specifically help to screen for. To my mind, Mg/Ca and Mn/Sr are necessary, but not sufficient to\nrule out the influence of some types of diagenesis.\nLines 194-196: This section needs references. My view is that the effect of the diurnal cycle on CAP need not necessarily be\nas simple as orthophosphate concentrations being drawn down throughout the water column. What if the locations of\ncarbonate formation and photosynthesis were decoupled?\nLines 197-201: This section is extremely important but is glossed over rather quickly. These evaporitic conditions could be\ndriving many of the signals presented in this paper. The existence of one LE-bearing Formation with a putative deeper water\norigin does not in my opinion make evaporative conditions an unlikely mechanism for the coupling between CAP and d13C.\nBear in mind also that elevated d13C values in deeper water settings are uncommon during the LE (Prave et al., 2022;\nHodgskiss et al., 2023), and in some cases have been linked to remobilization and transport of shelf sediments, following\nWalther’s Law. The authors need to do more here to justify this argument.\nLine 205: Why pH and not alkalinity or carbonate ion concentration?\nLines 206-208: This sentence needs a reference. Also, what direction would pH have needed to shift?\nLines 209-211: This sentence is an oversimplification. Ruling out pH changes is not sufficient to argue that background\nocean chemistry (which includes many parameters other than pH) was not an overarching control.\nLine 226: This information should be in the results section.\nLines 221-227: This sentence is far too long and should be broken up for clarity.\nLine 260: Given that the CIE is facies-dependent and is preserved mostly in shallow water settings, it is not necessary to\nmake the entire ocean DIC pool 8 per mil heavier, as I assume was done in this modeling exercise.\nLines 281-287: Okay this section resolves my previous comment.\nLines 326-329: These lines are too strong for my liking. In my opinion, the data presented in this paper, while interesting,\ndo not conclusively demonstrate that marine phosphate levels were higher during the LE, let alone that there was a mechanistic\nlinkage between high phosphate levels and oxygenation. There is a disconnect between the complicated picture painted in\nthe discussion and the certainty with which the conclusions are delivered.\nFigure 1, panel B: This panel is very out of date, and most egregiously ignores sulfate evaporite and CAS data from the\nOnega Basin. See, e.g., Blattler et al. (2018). This is an important omission because there is strong evidence from that paper\nthat seawater sulfate d34S was around 5-6 per mil right before the end of the LE.\nC. L. Blättler, M. W. Claire, A. R. Prave, K. Kirsimäe, J. A. Higgins, P. V. Medvedev, A. E. Romashkin, D. V. Rychanchik, A.\nL. Zerkle, K. Paiste, T. Kreitsmann, I. L. Millar, J. A. Hayles, H. Bao, A. V. Turchyn, M. R. Warke, A. Lepland, Two-billion-year-\nold evaporites capture Earth’s great oxidation. Science. 360, 320–323 (2018).", "reviewer_2": "With interest I have read “Increased phosphorus bioavailability triggered Earth’s Permanent Surface Oxygenation”, in which\nthe authors explore the role of P bioavailability during the Great Oxidation Episode (GOE), ~2,000 Ma). Across ancient\nsediments with varying degrees of diagenetic and metamorphic alteration, the authors present a positive correlation between\na specific sedimentary P phase that is assumed to reflect the ambient seawater PO4 concentration (carbonate-associated P)\nand the δ13CCARB, for which positive excursions may reflect increased marine primary productivity (PP) which sequesters\nlight δ13CCARB in organic matter. This is interpretated as evidence that PO4 bioavailability controlled marine PP, i.e.\noxygenic photosynthesis, and thereby drove oxygenation of Earth’s atmosphere. A coupled biogeochemical model is\nsubsequently used to explore the feedbacks between P input, carbon cycling and atmospheric O2. While I find the presented\ndata interesting, I think the storyline needs further development. Here, I also notice that little attention is given by the authors\nto previous studies (some I mention below) that have attempted to reconstruct ancient oceanic P cycling. In my opinion, a\nbroader consideration of ancient ocean chemistry and P cycling would strongly benefit this submission, which is too\nnarrowly focused on one correlation at the moment. On a personal note, from experience, chemical methods to determine P\nspecies are always uncertain, so actual ‘direct’ evidence of P retention mechanisms and CAP would be a big step forward in\nthis field.\nPaleoceanographers have to on nuanced lines of evidence to reconstruct an ancient, unknown world. This is especially true\nwhen trying to reconstruct P cycling and Earth oxygenation some 2 billion years ago. Careful conclusions have to be drawn\nfrom limited data, further complicated by long-term alteration of available sediment records. Such work can fuel new,\ncontestable insights into the evolution of the global P cycle; recent(ish) examples are Reinhard et al., 2017 (Nature, 2016)\nCanfield et al., 2020 (Earth-Science Reviews), two papers that are not mentioned in this manuscript, to my surprise. The\nmanuscript by Dodd and others that lies before me, leaves me excited yet unfulfilled. I will explain why I find the manuscript\ninteresting but not fully developed below.\nMy assessment is that the authors provide some interesting data regarding P cycling around the GOE, but fail to provide an\nadequate framework for interpretation of the data. A recently developed proxy for seawater PO4 (carbonate-associated P, CAP) is used well beyond the time frame for which it was “validated” (Dodd et al., 2021, GCA), subsequently a correlation\nbetween two proxies (δ13C and CAP) is presented as a direct (this word is used again and again; a proxy is inherently\nindirect and correlation is not causation) evidence that the GOE was driven by increased P bioavailability. I am not\nconvinced that a P-driven increase in PP is the only way to increase CAP; there is so little attention to carbonate formation\nmechanisms and P incorporation. As it stands, I could not discount that an increase in P fluxes to the sediment by any\nmechanism could result in an increase in P incorporation into carbonate, particularly because there is no other data\npresented whatsoever to illustrate the diverse depositional settings (other than shallow or not). The overselling of the data\ndoes not sit well with me; it also contrasts starkly with the consideration of numerous complicating factors regarding data\ninterpretation that the authors mention themselves (e.g. interpretation of the δ13C excursion, impact of long-term sediment\nalteration on CAP).\nOverall:\n• Proxies are not direct evidence and correlation is not causation; the δ13C-CAP correlation is interesting and suggests\nlinks/feedbacks between PO4 availability/burial and primary productivity (that have been established in numerous previous\nexperimental and modelling studies on modern and ancient sediment), however more caution in the wording, specifically\nproclaiming direct evidence of P as driver of the GOE, is advisable.\n• The caption to Fig. 3 states that “the high δ13Ccarb values are generally accompanied by the high CAP values” and that is\nmore or less the extent of the evidence provided for far-reaching conclusions about drivers of the GOE. That is an interesting\nstarting point but rather meager to build the complete story on. Using a coupled biogeochemical model in which primary\nproductivity is controlled by PO4 and increasing PO4 obviously will return the desired result, but fails to provide much insight\ninto the cascade of drivers and effects and feedbacks in the very different ocean 2 billion years ago.\n• The study fails to provide a useful context for the results; there are no other data (total P, TOC, redox-sensitive elements,\n…) that illustrate P cycling and the depositional settings and evolution thereof in the investigated sections. The ancient\nocean may have had very specific chemistry that would impact P availability (e.g. Fe-P coupling), none of this is considered\nin the paper. The model apparently was parametrized with modern-ocean fluxes? It is difficult from the information in the\nmanuscript to assess the model’s suitability to explore coupled biogeochemical cycling in the ocean 2 billion years ago.\n• CAP is used as direct (?) measure of seawater PO4 concentration based on a previous study that ‘validated’ (with various\ncaveats) the use of this P fraction as proxy for dissolved PO4 during formation of carbonates for sediments much younger\nthan those investigated here. I appreciate the authors’ focus on the broad co-occurrence of elevated δ13C and CAP, but\nsome discussion on the suitability of CAP in these ancients sediments, linked to mechanisms of carbonate formation, would\nbe useful.\n• It would be very interesting if the large scatter in Fig. 3, perhaps indicative of other processes acting upon CAP?) was\ndiscussed more seriously, now the reader is pushed in a certain direction with thick grey lines (what is “locally weighted\nscatter plot smoothing?) that brush aside what seems to me high uncertainty introduced by large scatter in low-resolution\nsediment records. I understand that there are statistically significant correlations presented in Fig. 2, but the scatter is large\nand because there is no other data (besides S isotopes to support timing of oxygenation) it is just difficult to appreciate what\nthe plots are really telling us, and particularly difficult to conclude from this that direct evidence of P availability as driver of\nthe GOE is provided.\nSome specific comments:\nL34-35. You are not reconstructing oceanic PO4 concentrations (this suggests that you provide information on\nconcentrations), you are qualitatively evaluating whether the sedimentary CAP record shows evidence for variations in\nseawater PO4 concentrations.\nL41. Is the positive correlation between CAP and d13C direct evidence that PO4 availability drove the GOE? Isn’t correlation\nbetween two proxies by definition indirect evidence?\nL50. The time bracket indicated for the GOE in Fig. 1 actually succeeds the initial permanent rise, starting at the first peak.\nL51-52. As the GOA was the oxygenation of the atmosphere, “paved the way” seems awkward phrasing because it implies\nanother subsequent process was responsible for that.\nL59. Indicate collapse in Figure and perhaps add indication of shift from 10-1 to 10-3 PAL\nL60.In Figure 1, it’s Great Oxygenation Event.\nL71. “and consequently net oxidation of the surface environment” may require some additional clarification for the broad\nreadership.\nL88-89. A proxy is by definition indirect, now it’s being sold almost as a PO4 sensor.\nL92. A proxy is not direct.\nL102. Validated for sediments up to Ediacaran (630-530 Ma), not 2,000 Ma\nL110-11. Well, whether changes in P availability may have occurred during the GOE\nL159-161. This is the actual observation. From this to ‘direct evidence that P availability is the direct driver of the GOE’ is a\nlong way for me.\nL165-167. Also valid for these billions-years-old rocks?\nL207-210. Please elaborate a bit on the required pH shift. How big could the pH shift be?\nL239. Please explain the feedbacks a bit more.\nL255-258. The model is parametrized for the modern ocean?\nL311-312. I think more insight into the global, ‘full-ocean’ nature of this event would be very useful; global increase in ocean\nprimary productivity is needed to drive the isotopic event?", "reviewer_3": "Note: all line numbers refer to manuscript file without tracked changes\nReviewer #1 (Remarks to the Author):\nGeneral Comments\nIn this paper, Dodd and coauthors present interesting data showing that the abundance of\ncarbonate-associated phosphate is higher in rocks preserving the Lomagundi Carbon\nisotope excursion (LE). They claim that this is evidence that marine phosphate levels were\nhigher during the LE, and consequently argue for a mechanistic link between P delivery to\nthe ocean and the oxygenation of the atmosphere at the time.\nI do think the data are interesting, and the idea is an enticing one. However, there are\nseveral issues with the manuscript that decrease my confidence in the conclusions drawn by\nthe authors:\n1) The authors too often make strong assertions without literature support and confuse\ninterpretations with facts.\n2) The authors offer a simplistic view of the sulfur cycle that is outdated with respect to the\nliterature.\n3) The authors gloss over important recent literature demonstrating the facies-dependence\nof the LE, which has major implications for how it might be interpreted with respect to\noxygenation.\n4) Throughout, there is a missing detailed analysis of how early stages of diagenesis might\ninfluence the main data discussed in the paper (CAP and d13C-carb).\n5) A major and obvious alternative explanation for both elevated CAP and d13C values\n(evaporative conditions) is ruled out very quickly by the authors, without much justification.\nI have outlined these concerns and others in detailed comments below, which I hope will be\nof use to the authors. I believe this work will be suitable for publication in this journal after\nmajor revisions.\nThank you for the opportunity to review this interesting work.\nRoger Bryant\n1.11\nWe thank the reviewer for their support of this work and time spent and comments\noffered. Our detailed responses to the major comments raised by Dr. Bryant above\nare provided below in our responses to related lined comments.\nLine-by-line comments\nLine 30: “Great Oxidation Episode” does not correspond to most usage of “GOE” in the\ncommunity. Suggest the authors stick to Event.\n1.12\nChanged to Great Oxidation Event\nLine 34: I’m not sure there is such a thing as a ‘direct’ proxy. In this case, phosphate is\nmeasured but as a proxy for oceanic phosphorus (not orthophosphate?). That seems like\nquite a complex, certainly indirect proxy.\n1.13\nWe agree that all proxies are indirect and have removed the wording ‘direct’ proxy.\nLine 37: Why a phosphorus control on the global carbon cycle and not the other way\naround? The authors don’t need me to remind them that correlation need not imply\ncausation.\n1.14\nWe acknowledge this important consideration which was also raised by the other\nreviewers. We have expanded on this in lines 318-329 and have tempered the\nlanguage in the abstract.\nLine 50: See comment on Line 30.\n1.15\nChanged wording.\nLine 60-61: I think the community agrees that the GOE was protracted, but the O2\nfluctuations and oscillations are less well-agreed upon, due in complexities in proxy behavior\n(e.g., the sensitivity of metal speciation to both depositional and post-depositional effects).\nNevertheless, if this statement represents the authors’ view, that can be stated explicitly\nrather than presuming to speak for the broader community.\n1.16\nAgreed and has been reworded to reflect this.\nLine 62-64: “Oscillating seawater sulfate level” is an interpretation, not a fact. Here, and\nthroughout, more careful language is needed to distinguish between observations and\ninterpretations. In this case, there are numerous reasons why the interpretation of oscillating\nseawater sulfate level might be an over-interpretation. To give one example, in reference 10\n(Planavsky et al., 2012), the sulfur isotopic composition sedimentary sulfate is a piece of\nevidence used to argue for changes in the seawater sulfate concentration. However, one\ncan imagine several scenarios by which the primary d34S of marine sulfate could change in\nthe absence of changes in the size of the marine sulfate reservoir. Then, there is the issue of\ndiagenetic alteration of carbonate-associated sulfate, which has been argued to afflict much\nyounger rocks (see, e.g., Present et al., 2019; Murray et al., 2021), but has yet to be\nevaluated for most Paleoproterozoic rocks, with some recent exceptions (Bryant et al.,\n2024).\n1.17\nWe have adjusted the manuscript wording to clearly distinguish between\nobservational data and the interpretation of \"oscillating seawater sulfate levels.\" We\nfurther acknowledge possible scenarios where δ34S changes might not necessarily\nreflect fluctuations in sulfate reservoir size, citing additional literature highlighting\nthese complexities.\nT. M. Present, M. Gutierrez, G. Paris, C. Kerans, J. P. Grotzinger, J. F. Adkins, Diagenetic\ncontrols on the isotopic composition of carbonate-associated sulphate in the Permian\nCapitan Reef Complex, West Texas. Sedimentology. 66, 2605–2626 (2019).\nS. T. Murray, J. A. Higgins, C. Holmden, C. Lu, P. K. Swart, Geochemical fingerprints of\ndolomitization in Bahamian carbonates: Evidence from sulphur, calcium, magnesium and\nclumped isotopes. Sedimentology. 68, 1–29 (2021).\nR. N. Bryant, J. P. Todes, J. A. Richardson, T. C. Kalia, A. R. Prave, A. Lepland, K. Kirsimäe,\nC. L. Blättler, Local sedimentary effects shaped key sulfur records after the Great Oxidation\nEvent. Earth Planet. Sci. Lett. 648, 119113 (2024).\nLines 70-71: It seems strange to mention only the canonical interpretation of the LE here,\ndespite recent work highlighting the facies-dependence of the magnitude of the LE (Prave et\nal., 2022; Hodgskiss et al., 2023), which clearly has implications for how the LE is\ninterpreted (i.e., the LE might have required less oxidation/oxygenation than canonically\nassumed).\n1.18\nWe have acknowledged other interpretations of the Lomagundi Excursion on lines 80-\n82. We also show in our biogeochemical model that a facies-dependent expression of\nthe LE aligns with our data and conclusions if the Paleoproterozoic carbon gradient\nvaried with depth like in the modern ocean, see lines 365-368.\nLines 93-94: Yes, but the proportionality for a given element is often a function of the\nconcentrations of other things in solution (e.g., Ca, Mg, carbonate ions), kinetics, etc. See,\ne.g., Barkan et al. (2020). One could decrease CAP for the same orthophosphate\nconcentration by increasing the activity of CO32- in solution.\nY. Barkan, G. Paris, S. M. Webb, J. F. Adkins, I. Halevy, Sulfur isotope fractionation between\naqueous and carbonate-associated sulfate in abiotic calcite and aragonite. Geochim.\nCosmochim. Acta. 280, 317–339 (2020).\n1.19\nYes, this is true, we have clarified that phosphate incorporation into carbonate\nminerals is influenced by multiple environmental parameters including ion\nconcentrations and kinetics on lines 92-94 and lines209-230 and acknowledge this as\na limitation.\nLines 94-95: P, or phosphate? Also, why ‘relative concentration of P’?\n1.20\nWe have now clarified P as phosphate and removed the word “relative”.\nLines 99-101: This is quite the statement – needs literature support or further justification. To\nmake a rough analogy with CAS, the concentration of a non-stoichiometric element in\ncarbonate can change after burial and need not bear ‘direct’ relevance to the concentration\nof that element in the primary fluid.\n1.21\nWe have reworded to “…while phosphate bound inside carbonate minerals can be\nattributable to ambient dissolved P concentrations.”.\nLines 119-121: This sentence is confusingly structured – consider revising for clarity.\n1.22\nWe have reworded on lines 118-121\nLines 123-138: The results section strikes me as being very short. One thing I would like to\nsee here as a reader is some actual CAP abundances in the text, in addition to the\ncomparisons with bracketing carbonates. If CAP works how it is supposed to, the relative\nchanges in CAP should be less meaningful than the absolute changes. I am also wondering\nhow one chooses how much of the bracketing carbonates to include in this relative CAP\nassessment.\n1.23\nWe have added CAP values to show the range of absolute values on lines 134-135 and\n143-144. We used a cut-off of 3 per mil to compare CAP values across carbon isotope\nexcursions, we do not expect this cut-off to play a major factor in the comparison as\nFigure 2 shows the correlation of CAP and d13C is continuous and not a phenomenon\ndeveloping from discrete data binning. Regarding relative vs. absolute change it is\nimportant to note that absolute values could range between localities if some ocean\nregions had higher local phosphate availability as discussed on lines 232-234 or\nvarying degrees of metamorphism lower CAP values. Take the Nash fork location for\nexample, it shows a change in absolute CAP values across the LE, but if you compare\nthese absolute values to the Francevillian section they are considerably lower.\nTherefore, we use relative to describe the fact that despite differences in absolute\nvalues between sections they show the same trends relative to the CAP values from\nthe individual sections.\nLine 141: This should be in the results rather than the discussion, in my opinion. The\nanalysis in the following clause can stay in the discussion.\n1.24\nYes, this sentence contains both a result descriptor but also an interpretive clause.\nWe have chosen to leave it as a bridge between sections.\nLine 143: Here and throughout, I think the use of ‘CAP values’ rather than ‘CAP abundance’\nis perhaps unhelpful. Nowhere in the text is CAP assigned a unit. One is left with a sense of\nCAP values being an abstract statistical measure rather than a measured quantity.\n1.25\nWe have now detailed the expression of CAP units on lines 102-104\nLines 143-147: This is a lengthy sentence and complex argument. I had to read this several\ntimes to grasp the intention. I would suggest splitting the sentence up and laying out the\nargument more systematically without skipping logical steps.\n1.26\nAgreed we have removed as it is discussed later in the text around basin preservation\nover time on lines 231-244.\nLine 159: I think the authors have done a good job of alleviating fears about metamorphism\ninfluencing the data in a meaningful way. However, they allude here to other ‘post-\ndepositional processes’. Missing in the text is a discussion of these other post-depositional\nprocesses and how those might affect CAP abundance. Reading between the lines, the\nprocesses in question could be early marine diagenesis of carbonates. How might one\ns one\nscreen for the influence of such processes? What about meteoric diagenesis? I would be\nworried about both styles of diagenesis, but particularly the latter, because it could\nconceivably decrease both CAP abundance and d13C-carb. Can the authors demonstrate\nthat the bracketing units (which they say are the same sabkha facies as the LE carbonates)\nare not simply more influenced by meteoric diagenesis than those in the LE carbonates?\nDoes mineralogy have a role in the preservation of CAP?\n1.27\nWe appreciate the support of our discussion of metamorphic influences on CAP.\nWe have moved the metamorphic discussion so it now follows the expanded discussion\non diagenetic processes on lines 234-261. Yes, mineralogy does affect CAP\npreservation and is discussed on lines 93 and 116.\nLine 163: I disagree with diagenetic and metamorphic alteration being lumped together here.\nThe authors have demonstrated that metamorphic grades did not drive the signals they\nreport. They have not conducted a similar analysis for any style or phase of diagenesis\n(although I see some work to this end further into the document).\n1.28\nWe have expanded our discussion on diagenesis to cover a wide array of hypothetical\ndiagenetic scenarios on lines 146-190 and include new data statistics which compare\ncarbonate trace element chemistry and CAP to discuss diagenetic influences.\nLines 165-167: This information should be in the results section.\n1.29\nAgreed.\nLine 178: I appreciate this analysis by the authors. I would also be interested to know if there\nare correlations between CAP and CAS content. There are several ways one could interpret\nsuch a correlation. The first would be high orthophosphate and sulfate concentrations in the\nwater column. High alkalinity and/or carbonate ion activity would be another.\n1.30\nWe did not measure CAS contents of the sample analysed for CAP, however we can\nuse CAS values from the literature to give a broad overview. Based on the literature,\nCAS is generally elevated in carbonate from the LE relative to those carbonates\ndeposited outside the LE. This would mean there would be a general positive\ncorrelation of CAP and CAS across the LE. We would favour the first interpretation\nput forward by the reviewer, given exceptionally high alkalinity increases would be\nrequired to explain the CAP and CAS increases on a solely alkalinity-controlled basis.\nFor example, Barkan et al., 2018 found a roughly 1:1 relative increase in alkalinity to\nCAS concentration which based on data in Planavsky et al., 2012 which show a rough\nchange from ca. 50ppm to 250ppm across the LE, which would require a ca. >5x\nincrease in marine alkalinity to explain the CAS concentration change across the LE.\nThis would result in an increase in ocean pH if not matched with a corresponding\nincrease in atmospheric CO , which is unlikely for the GOE interval, based on existing\n2\nevidence for glaciations.\nLines 188-189: This information should be in the results. Here, it would be useful to explain\nwhy these diagenetic indices were chosen, and what they specifically help to screen for. To\nmy mind, Mg/Ca and Mn/Sr are necessary, but not sufficient to rule out the influence of some\ntypes of diagenesis.\n1.31\nWe have now included a description of why the diagenetic indices were used and\nother possible digenetic indices on lines 176-178.\nLines 194-196: This section needs references. My view is that the effect of the diurnal cycle\non CAP need not necessarily be as simple as orthophosphate concentrations being drawn\ndown throughout the water column. What if the locations of carbonate formation and\nphotosynthesis were decoupled?\n1.32\nThe diurnal cycle explicitly requires carbonate formation in-situ in order for carbonate\nto capture changing carbon isotopic composition from local photosynthetic activity\n(Geyman and Maloof, 2019). Maybe future works may be able to build a more complex\ncycle but based on the current state of knowledge we can confidently rule this out.\nLines 197-201: This section is extremely important but is glossed over rather quickly. These\nevaporitic conditions could be driving many of the signals presented in this paper. The\nexistence of one LE-bearing Formation with a putative deeper water origin does not in my\nopinion make evaporative conditions an unlikely mechanism for the coupling between CAP\nand d13C. Bear in mind also that elevated d13C values in deeper water settings are\nuncommon during the LE (Prave et al., 2022; Hodgskiss et al., 2023), and in some cases\nhave been linked to remobilization and transport of shelf sediments, following Walther’s Law.\nThe authors need to do more here to justify this argument.\n1.33\nWe have discussed this phenomenon in more detail on lines 204-208 and include new\nY/Ho data to develop this point.\nLine 205: Why pH and not alkalinity or carbonate ion concentration?\n1.34\nThese effects have been previously documented in experiments and pH was found to\nexert the biggest influence on CAP incorporation (Dodd et al., 2023; Dodd et al., 2021).\nWe have now noted earlier in the manuscript that alkalinity/ carbonate ion\nconcentration also effects CAP.\nLines 206-208: This sentence needs a reference. Also, what direction would pH have\nneeded to shift?\n1.35\nReference now added on line 227 (Haley and Bachan 2017). The pH change direction\ndoes not matter unless we are specifically talking about which way the CAP values\nchanged. Here we simply state the change whether it be higher or lower was too large\nto be likely explained by pH alone.\nLines 209-211: This sentence is an oversimplification. Ruling out pH changes is not sufficient\nto argue that background ocean chemistry (which includes many parameters other than pH)\nwas not an overarching control.\n1.36\nWe agree there are many chemical effects which can impact CAP incorporation into\ncarbonate and have expanded discussion on this on lines 209-230.\nLine 226: This information should be in the results section.\n1.37\nThese lines are a discussion element with reference to the results. We have added a\ndescription of what the sample mineralogy is in the results section.\nLines 221-227: This sentence is far too long and should be broken up for clarity.\n1.38\nAgreed and has been modified.\nLine 260: Given that the CIE is facies-dependent and is preserved mostly in shallow water\nsettings, it is not necessary to make the entire ocean DIC pool 8 per mil heavier, as I\nassume was done in this modeling exercise.\nLines 281-287: Okay this section resolves my previous comment.\nLines 326-329: These lines are too strong for my liking. In my opinion, the data presented in\nthis paper, while interesting, do not conclusively demonstrate that marine phosphate levels\nwere higher during the LE, let alone that there was a mechanistic linkage between high\nphosphate levels and oxygenation. There is a disconnect between the complicated picture\npainted in the discussion and the certainty with which the conclusions are delivered.\n1.39\nWe have adopted a more conservative concluding statement in line with caveats and\nlimitations of the CAP proxy and connection with carbon isotope values on lines 415-\n433\nFigure 1, panel B: This panel is very out of date, and most egregiously ignores sulfate\nevaporite and CAS data from the Onega Basin. See, e.g., Blattler et al. (2018). This is an\nimportant omission because there is strong evidence from that paper that seawater sulfate\nd34S was around 5-6 per mil right before the end of the LE.\nC. L. Blättler, M. W. Claire, A. R. Prave, K. Kirsimäe, J. A. Higgins, P. V. Medvedev, A. E.\nRomashkin, D. V. Rychanchik, A. L. Zerkle, K. Paiste, T. Kreitsmann, I. L. Millar, J. A. Hayles,\nH. Bao, A. V. Turchyn, M. R. Warke, A. Lepland, Two-billion-year-old evaporites capture\nEarth’s great oxidation. Science. 360, 320–323 (2018).\n1.40\nWe have added the new data from Blattler to the Fig. 1 panel B.\nReviewer #2 (Remarks to the Author):\nWith interest I have read “Increased phosphorus bioavailability triggered Earth’s Permanent\nSurface Oxygenation”, in which the authors explore the role of P bioavailability during the\nGreat Oxidation Episode (GOE), ~2,000 Ma). Across ancient sediments with varying\ndegrees of diagenetic and metamorphic alteration, the authors present a positive correlation\nbetween a specific sedimentary P phase that is assumed to reflect the ambient seawater\nPO4 concentration (carbonate-associated P) and the δ13CCARB, for which positive\nexcursions may reflect increased marine primary productivity (PP) which sequesters light\nδ13CCARB in organic matter. This is interpretated as evidence that PO4 bioavailability\ncontrolled marine PP, i.e. oxygenic photosynthesis, and thereby drove oxygenation of\nEarth’s atmosphere. A coupled biogeochemical model is subsequently used to explore the\nfeedbacks between P input, carbon cycling and atmospheric O2. While I find the presented\ndata interesting, I think the storyline needs further development. Here, I also notice that little\nattention is given by the authors to previous studies (some I mention below) that have\nattempted to reconstruct ancient oceanic P cycling. In my opinion, a broader consideration of\nancient ocean chemistry and P cycling would strongly benefit this submission, which is too\nnarrowly focused on one correlation at the moment. On a personal note, from experience,\nchemical methods to determine P species are always uncertain, so actual ‘direct’ evidence\nof P retention mechanisms and CAP would be a big step forward in this field.\nPaleoceanographers have to on nuanced lines of evidence to reconstruct an ancient,\nunknown world. This is especially true when trying to reconstruct P cycling and Earth\noxygenation some 2 billion years ago. Careful conclusions have to be drawn from limited\ndata, further complicated by long-term alteration of available sediment records. Such work\ncan fuel new, contestable insights into the evolution of the global P cycle; recent(ish)\nexamples are Reinhard et al., 2017 (Nature, 2016) Canfield et al., 2020 (Earth-Science\nReviews), two papers that are not mentioned in this manuscript, to my surprise. The\nmanuscript by Dodd and others that lies before me, leaves me excited yet unfulfilled. I will\nexplain why I find the manuscript interesting but not fully developed below.\n2.11 We thank the reviewer for their time spent and comments offered and agree the\ninclusion of further works pertaining to ancient P cycling can help frame the\nimportance of this study.\nMy assessment is that the authors provide some interesting data regarding P cycling around\nthe GOE, but fail to provide an adequate framework for interpretation of the data. A recently\ndeveloped proxy for seawater PO4 (carbonate-associated P, CAP) is used well beyond the\ntime frame for which it was “validated” (Dodd et al., 2021, GCA), subsequently a correlation\nbetween two proxies (δ13C and CAP) is presented as a direct (this word is used again and\nagain; a proxy is inherently indirect and correlation is not causation) evidence that the GOE\nwas driven by increased P bioavailability. I am not convinced that a P-driven increase in PP\nis the only way to increase CAP; there is so little attention to carbonate formation\nmechanisms and P incorporation. As it stands, I could not discount that an increase in P\nfluxes to the sediment by any mechanism could result in an increase in P incorporation into\ncarbonate, particularly because there is no other data presented whatsoever to illustrate the\ndiverse depositional settings (other than shallow or not). The overselling of the data does not\nsit well with me; it also contrasts starkly with the consideration of numerous complicating\nfactors regarding data interpretation that the authors mention themselves (e.g. interpretation\nof the δ13C excursion, impact of long-term sediment alteration on CAP).\nOverall:\n• Proxies are not direct evidence and correlation is not causation; the δ13C-CAP correlation\nis interesting and suggests links/feedbacks between PO4 availability/burial and primary\nproductivity (that have been established in numerous previous experimental and modelling\nstudies on modern and ancient sediment), however more caution in the wording, specifically\nproclaiming direct evidence of P as driver of the GOE, is advisable.\n2.12\nWe agree that all proxies are indirect and have removed the wording ‘direct’ proxy.\nWe acknowledge this important consideration which was also raised by the other\nreviewers. We have expanded on this in lines 318-329 and have tempered the\nlanguage in the abstract.\n• The caption to Fig. 3 states that “the high δ13Ccarb values are generally accompanied by\nthe high CAP values” and that is more or less the extent of the evidence provided for far-\nreaching conclusions about drivers of the GOE. That is an interesting starting point but\nrather meager to build the complete story on. Using a coupled biogeochemical model in\nwhich primary productivity is controlled by PO4 and increasing PO4 obviously will return the\ndesired result, but fails to provide much insight into the cascade of drivers and effects and\nfeedbacks in the very different ocean 2 billion years ago.\n2.13\nWe do not see this result as “meagre”, considering that higher ocean P levels during\nthe LE were a key testable prediction of the P-driven productivity model of the GOE.\nNevertheless, we have now elaborated on the comparison of the biogeochemical\nmodel results and observations of fluctuating redox conditions during the GOE on\nlines 386-396. Our new model results show that if the GOE occurs because of a\nreduction in oxygen sinks, a stepwise transition from low to high atmospheric O2\noccurs, whereas if an increase in P-driven productivity is a driver for the GOE,\natmospheric O2 level rises and falls, which is more consistent with redox proxies.\n• The study fails to provide a useful context for the results; there are no other data (total P,\nTOC, redox-sensitive elements, …) that illustrate P cycling and the depositional settings and\nevolution thereof in the investigated sections. The ancient ocean may have had very specific\nchemistry that would impact P availability (e.g. Fe-P coupling), none of this is considered in\nthe paper. The model apparently was parametrized with modern-ocean fluxes? It is difficult\nfrom the information in the manuscript to assess the model’s suitability to explore coupled\nbiogeochemical cycling in the ocean 2 billion years ago.\n2.14\nIndeed, it would be interesting to have other tracers of P cycling during the GOE,\nhowever diving further into what was controlling P level during the GOE is beyond the\nscope of this study. This study provides evidence for how ocean P level was\nchanging, but not what was causing it to change. We agree that further details should\nbe added to provide a better overview of what may have been controlling ocean P\nlevel on lines 265-306.\nThe biogeochemical model only uses modern fluxes to recreate the present day.\nWhen it is run over Earth history (Alcott et al., 2019; Alcott et al., 2024) its fluxes are\ncomputed dynamically based on elemental abundances, climate and tectonics, and\nthe overall model output compares reasonably well to a suite of long-term proxies. It\nis probably the best tested model of this type currently available. We have\nemphasised this now in the methods description.\n• CAP is used as direct (?) measure of seawater PO4 concentration based on a previous\nstudy that ‘validated’ (with various caveats) the use of this P fraction as proxy for dissolved\nPO4 during formation of carbonates for sediments much younger than those investigated\nhere. I appreciate the authors’ focus on the broad co-occurrence of elevated δ13C and CAP,\nbut some discussion on the suitability of CAP in these ancients sediments, linked to\nmechanisms of carbonate formation, would be useful.\n2.15\nThe CAP proxy was validated principally through experimental work, which\nsynthesised carbonates under analogous conditions to modern seawater. This abiotic\nprecipitation style would be similar to the Precambrian world. Others have also tested\nCAP incorporation using analogues of Proterozoic seawater (Roest-Ellis et al., 2023)\nand achieve similar results. The biggest unknown would be CAP incorporation into\ndolomite which was a pervasive carbonate with debated origin in the Precambrian.\nWe have added further information on lines 114-116, which detail the effects of\nmineralogy on CAP.\n• It would be very interesting if the large scatter in Fig. 3, perhaps indicative of other\nprocesses acting upon CAP?) was discussed more seriously, now the reader is pushed in a\ncertain direction with thick grey lines (what is “locally weighted scatter plot smoothing?) that\nbrush aside what seems to me high uncertainty introduced by large scatter in low-resolution\nsediment records. I understand that there are statistically significant correlations presented\nin Fig. 2, but the scatter is large and because there is no other data (besides S isotopes to\nsupport timing of oxygenation) it is just difficult to appreciate what the plots are really telling\nus, and particularly difficult to conclude from this that direct evidence of P availability as\ndriver of the GOE is provided.\n2.16\nWhile there is indeed significant point-to-point scatter in the CAP profiles, which is an\ninherent feature of low-resolution sampling in deep time carbonate archives, we argue\nthat the observed stratigraphic trends are unlikely to be primarily diagenetic. First, we\nnote that the CAP excursions are observed consistently across distinct basins\n(Francevillian and Snowy Pass), each with different diagenetic and burial histories,\nyet exhibiting broadly synchronous patterns when normalized to formation thickness.\nThis stratigraphic alignment supports a common paleoenvironmental driver affecting\nboth carbon and phosphorus systems, rather than localized diagenetic overprinting.\nThe 3 sections in Figure 3 record the Lomagundi and Wooly Dolomite carbon isotope\nexcursion and show continuous chemostratigraphic trends of correlated δ13C and\nCAP which show continuous correlation of δ13C and CAP across the Lomagundi\nEvent, as opposed to comparing individual formations from discrete time points.\nGiven the Lomagundi Event is tied to the rise and fall in O levels via other studies,\n2\nthe CAP trends across these sections supports their connection with higher and\nlower O levels (Canfield et al., 2013). We have also added extra discussion on the\n2\npossible impacts of post-depositional alteration as a mechanism of scatter in the data\non lines 187-192.\nLocally weighted scatterplot smoothing (LOWESS) is a common technique applied in\nstratigraphic geochemical analysis. It is a non-parametric regression technique that\nfits local polynomial functions to the data, allowing for visualization of broader trends\nwhile minimizing the influence of short-wavelength noise. We have now clarified this\nin the figure caption. The smoothing curve is included not to obscure scatter, but to\nguide the reader in identifying stratigraphic trajectories that are difficult to interpret in\nnoisy datasets typical of Precambrian carbonates.\nWe have added the following clarifying text in the revised figure 3 caption: “Dark-grey\nlines represent locally weighted scatter plot smoothing”\nSome specific comments:\nL34-35. You are not reconstructing oceanic PO4 concentrations (this suggests that you\nprovide information on concentrations), you are qualitatively evaluating whether the\nsedimentary CAP record shows evidence for variations in seawater PO4 concentrations.\n2.17\nAgreed, qualitatively we can say concentrations were potentially higher or lower\nwithout putting an absolute value to them.\nL41. Is the positive correlation between CAP and d13C direct evidence that PO4 availability\ndrove the GOE? Isn’t correlation between two proxies by definition indirect evidence?\n2.18\nWe agree that despite there being evidence for temporally correlated increases in\nocean P, O2 and biomass burial we cannot conclude that P-driven biomass burial was\nthe sole driver of the GOE. We have added this caveat on lines 320-321 and 420-444.\nL50. The time bracket indicated for the GOE in Fig. 1 actually succeeds the initial permanent\nrise, starting at the first peak.\n2.19\nWe have extended the arrow to capture that.\nL51-52. As the GOA was the oxygenation of the atmosphere, “paved the way” seems\nawkward phrasing because it implies another subsequent process was responsible for that.\n2.20\nWe have reworded to “which established a permanent oxygen atmosphere on Earth”\nL59. Indicate collapse in Figure and perhaps add indication of shift from 10-1 to 10-3 PAL\n2.21\nAdded.\nL60.In Figure 1, it’s Great Oxygenation Event.\n2.22\nWe have reworded to Event throughout the manuscript.\nL71. “and consequently net oxidation of the surface environment” may require some\nadditional clarification for the broad readership.\n2.23\nWe have changed to “oxygenation of the surface environment.”\nL88-89. A proxy is by definition indirect, now it’s being sold almost as a PO4 sensor.\n2.24\nWe agree that all proxies are indirect and have removed the wording ‘direct’ proxy.\nL92. A proxy is not direct.\n2.25\nWe agree that all proxies are indirect and have removed the wording ‘direct’ proxy.\nL102. Validated for sediments up to Ediacaran (630-530 Ma), not 2,000 Ma\n2.26\nThe CAP proxy was validated principally through experimental work, which\nsynthesised carbonates under analogous conditions to modern seawater. This abiotic\nprecipitation style would be similar to the Precambrian world. Others have also tested\nCAP incorporation using analogues of Proterozoic seawater (Roest-Ellis et al., 2023)\nand achieve similar results. The biggest unknown would be CAP incorporation into\ndolomite which was a pervasive carbonate with debated origin in the Precambrian.\nWe have added further information on lines 114-116, which detail the effects of\nmineralogy on CAP.\nL110-11. Well, whether changes in P availability may have occurred during the GOE\n2.27\nReworded to “…whether global seawater phosphate availability changed during the\nGOE”\nL159-161. This is the actual observation. From this to ‘direct evidence that P availability is\nthe direct driver of the GOE’ is a long way for me.\n2.28\nWe agree that despite there being evidence for temporally correlated increases in\nocean P, O2 and biomass burial we cannot conclude that P-driven biomass burial was\nthe sole driver of the GOE. We have added this caveat on lines 320-321 and 420-444.\nL165-167. Also valid for these billions-years-old rocks?\n2.29\nThe CAP proxy was validated and tested in modern and Proterozoic-like seawater\n(Dodd et al., 2021; Ingalls et al., 2020; Ingalls et al., 2022; Ren et al., 2025; Richardson\net al., 2022; Roest-Ellis et al., 2023) and achieve similar results.\nL207-210. Please elaborate a bit on the required pH shift. How big could the pH shift be?\n2.30\nWe have now referenced a study which modelled pH changes during the GOE, with\n95% of modelled results returning a pH drop of < 1 unit (Halevy and Bachan, 2017).\nL239. Please explain the feedbacks a bit more.\n2.31\nWe have added further explanation of P cycling in the Paleoproterozoic on lines 267-\n331\nL255-258. The model is parametrized for the modern ocean?\n2.32\nAs above, the model is developed around the modern ocean but the fluxes are\nallowed to vary dynamically through Earth history, so during the GOE the model is\nvery different to the modern.\nL311-312. I think more insight into the global, ‘full-ocean’ nature of this event would be very\nuseful; global increase in ocean primary productivity is needed to drive the isotopic event?\n2.33\nWe agree and have added additional discussion on lines 404-419.\nReviewer #3 (Remarks to the Author):\nReview of Increased phosphorus bioavailabilty triggered Earth’s permanent surface\noxygenation by Dodd et al.\nOverview: The authors present carbonate-associated phosphate (CAP) data from globally\ndistributed marine sediments across the GOE and demonstrate a positive covariation with\ncarbonate carbon isotopes. The primary finding of this analysis is that CAP values are high\nduring the Lomagundi positive carbon isotope excursion. These data are used in a\nbiogeochemical box model to support how increased P availability would lead to a rapid\noxygenation of the atmosphere by increased primary productivity (photosynthesis).\nComments:\n1. In several places, including a section title in the Introduction, the authors refer to CAP\nas a direct proxy for seawater phosphate concentrations. However, the first author’s\n2021 paper demonstrates a strong dependence of phosphate incorporation into\ncarbonate minerals on pH and mineralogy. I recommend tempering the language\naround direct proxy if the evaluation is based on qualitative comparisons between\nCAP datasets across time without quantifying seawater [P]. I want to make clear that\nI think there is significant value in these qualitative comparisons.\n3.11\nWe agree with tempering the language around CAP being a direct proxy and have\nincluded discussion of this on lines 33-35, 85-87, and 101-104.\n2. I appreciate the approach of considering metamorphic grade in your diagenetic\nassessment of these samples. However, I question whether comparing across\nsimilar metamorphic grade is valid. Presumably, alteration of CAP values is more\nrelated to the phosphate contents of the diagenetic fluid rather than the P-T\nregime of the alteration environment (other than potentially extent of water-rock\nexchange). I recommend performing some amount of petrography on samples from each\nformation and analyzing a clear secondary fabric for CAP, such as a secondary vein.\nThis will provide some information about the phosphate contents of altering fluids\nwhich will then provide a means of assessing whether CAP values in your fabrics\ninterpreted as primary provide a minimum or maximum cap on CAP. A few analyses\nof secondary fabrics would really bolster your argument for accepting CAP values as\nprimary, or at least usable for comparison across geographic regions.\n3.12\nWe analysed CAP over a P-T range because prior work has shown that progressively\nincreasing metamorphism can lead to increasing CAP loss (Ingalls et al., 2022). We\nagree that the degree of alteration will be unique for each case and could explain\nsome spread in data but is highly unlikely to recreate the observed correlation in CAP\nand d13C across such a huge range of metamorphic grades seen in this study. Our\nsamples were carefully selected to avoid visible grains before selecting a sample for\ncrushing and analysis. There are however instances where we do not need to\nconsider metamorphism to support the conclusions such as the FC Formation, Wooly\nDolomite and Nash Fork Formation, which preserved continuous sections of\nconsistent metamorphic grade and all preserve a positive correlation of CAP and\nd13C suggesting metamorphism is an unlikely cause of the correlation.\nPetrography has been performed on these samples in prior published studies\n(Kalderon-Asael et al., 2021; Ossa Ossa et al., 2018). We subsampled the samples\npreviously used in these other studies. When subsampling we avoided visible\nalteration or veining.\n3. Rationale for spread in CAP values: In the very first sentence of the Discussion, the\nauthors state “There is a large spread in CAP values of formations deposited during\nthe LE, which is expected given the heterogeneous nature of phosphorus\nconcentration in the surface ocean.” Can the spread realistically be explained by a\nreasonable range of surface seawater [P]? I think it is more likely pointing to variable\ndegrees of water-rock alteration and/or phosphate contents of altering fluids in the\nmetamorphic environment. To be clear, I don’t think this takes away from your main\nfinding that there is a positive covariance in CAP and d13C across the LE, but I\nrecommend casting this within a more cautious light with respect to primacy of CAP\nvalues.\n3.13\nWe agree with the reviewer that the results should be discussed in a more cautious\nlight and have added this discussion throughout the text. We keep the original\nsentence as the spread could indeed be attributed to the heterogenous nature of\nsurface ocean P concentrations. In the modern ocean CAP varies by more than an\norder of magnitude in the surface ocean (Martiny Adam et al., 2019).\n4. Has any petrography been performed on any of these samples? How were fabrics\nconsidered in sub-sampling for chemical analyses? Whether you are sampling a\nprimary marine cement vs a late diagenetic cement vs an ooid that formed on the\nplatform top can significantly change how CAP values should be interpreted, and by\nhaving data from the different formation and diagenetic environments, one can\nleverage these data to build a more complete picture of the environment across the\nLE.\n3.14\nPetrography has been performed on these samples in prior published studies\n(Kalderon-Asael et al., 2021; Ossa Ossa et al., 2018) before samples were selected for\ngeochemical analysis. We subsampled the samples previously used in these other\nstudies. When subsampling we avoided visible alteration or veining. In these prior\nstudies detailed environmental reconstructions of the studied samples were\nperformed. Despite widely varying depositional environments and a likely mix of\nprimary and diagenetic cements the positive trend of d13C and CAP holds true. On\nlines 245-263, we discuss how this broad mix of environmental factors support a\nprimary origin of the positive trend.\n5. I appreciate how the authors investigated the CAP-d13C covariation from various\nangles in the SI. In my opinion, this paper would be stronger if it was published in a venue that\nallowed much of this detailed analysis and consideration to be moved into the main text,\nbut that is just my opinion.\n3.15\nThe SI file is intended for experts in the field who would find such information\ninteresting but we believe the broad appeal of the findings beyond geochemists make\nthis journal choice appropriate. We kept detailed diagenetic discussion in the SI so\nthe general reader can easily digest the key takeaways.\nSummary: I found this paper incredibly interesting and would like to see it published. My\ncomments are intended to primarily serve to enrich the diagenetic assessment and boldness\nof some claims, but overall, I think the main finding is made on solid footing. Placing it within\nthe context of a bit more diagenetic assessment would just ensure it stands the test of time.\n3.16 We thank the reviewer for their support of this study and time spent reviewing\nand providing comments on this manuscript.\nAlcott, L. J., Mills, B. J. W., and Poulton, S. W., 2019, Stepwise Earth oxygenation is\nan inherent property of global biogeochemical cycling: Science, v. 366, no.\n6471, p. 1333-1337.\nAlcott, L. J., Walton, C., Planavsky, N. J., Shorttle, O., and Mills, B. J. W., 2024,\nCrustal carbonate build-up as a driver for Earth’s oxygenation: Nature\nGeoscience, v. 17, no. 5, p. 458-464.\nCanfield, D. E., Ngombi-Pemba, L., Hammarlund, E. U., Bengtson, S., Chaussidon,\nM., Gauthier-Lafaye, F., Meunier, A., Riboulleau, A., Rollion-Bard, C., Rouxel,\nO., Asael, D., Pierson-Wickmann, A.-C., and El Albani, A., 2013, Oxygen\ndynamics in the aftermath of the Great Oxidation of Earth’s atmosphere:\nProceedings of the National Academy of Sciences, v. 110, no. 42, p. 16736-\n16741.\nDodd, M. S., Shi, W., Li, C., Zhang, Z., Cheng, M., Gu, H., Hardisty, D. S., Loyd, S.\nJ., Wallace, M. W., vS. Hood, A., Lamothe, K., Mills, B. J. W., Poulton, S. W.,\nand Lyons, T. W., 2023, Uncovering the Ediacaran phosphorus cycle: Nature,\nv. 618, no. 7967, p. 974-980.\nDodd, M. S., Zhang, Z., Li, C., Algeo, T. J., Lyons, T. W., Hardisty, D. S., Loyd, S. J.,\nMeyer, D. L., Gill, B. C., Shi, W., and Wang, W., 2021, Development of\ncarbonate-associated phosphate (CAP) as a proxy for reconstructing ancient\nocean phosphate levels: Geochimica et Cosmochimica Acta, v. 301, p. 48-69.\nGeyman, E. C., and Maloof, A. C., 2019, A diurnal carbon engine explains 13C-\nenriched carbonates without increasing the global production of oxygen:\nProceedings of the National Academy of Sciences, v. 116, no. 49, p. 24433-\n24439.\nHalevy, I., and Bachan, A., 2017, The geologic history of seawater pH: Science, v.\n355, no. 6329, p. 1069-1071.\nIngalls, M., Blättler, C. L., Higgins, J. A., Magyar, J. S., Eiler, J. M., and Fischer, W.\nW., 2020, P/Ca in Carbonates as a Proxy for Alkalinity and Phosphate Levels:\nGeophysical Research Letters, v. 47, no. 21, p. e2020GL088804.\nIngalls, M., Grotzinger, J. P., Present, T., Rasmussen, B., and Fischer, W. W., 2022,\nCarbonate-Associated Phosphate (CAP) Indicates Elevated Phosphate\nAvailability in Neoarchean Shallow Marine Environments: Geophysical\nResearch Letters, v. 49, no. 6, p. e2022GL098100.\nKalderon-Asael, B., Katchinoff, J. A. R., Planavsky, N. J., Hood, A. v. S., Dellinger,\nM., Bellefroid, E. J., Jones, D. S., Hofmann, A., Ossa, F. O., Macdonald, F. A.,\nWang, C., Isson, T. T., Murphy, J. G., Higgins, J. A., West, A. J., Wallace, M.\nW., Asael, D., and Pogge von Strandmann, P. A. E., 2021, A lithium-isotope\nperspective on the evolution of carbon and silicon cycles: Nature, v. 595, no.\n7867, p. 394-398.\nMartiny Adam, C., Lomas Michael, W., Fu, W., Boyd Philip, W., Chen Yuh-ling, L.,\nCutter Gregory, A., Ellwood Michael, J., Furuya, K., Hashihama, F., Kanda, J.,\nKarl David, M., Kodama, T., Li Qian, P., Ma, J., Moutin, T., Woodward, E. M.\nS., and Moore, J. K., 2019, Biogeochemical controls of surface ocean\nphosphate: Science Advances, v. 5, no. 8, p. eaax0341.\nOssa Ossa, F., Eickmann, B., Hofmann, A., Planavsky, N. J., Asael, D., Pambo, F.,\nand Bekker, A., 2018, Two-step deoxygenation at the end of the\nPaleoproterozoic Lomagundi Event: Earth and Planetary Science Letters, v.\n486, p. 70-83.\nRen, C., Li, Y., Wang, S., Wang, J., Ji, J., Teng, H. H., Phillips, B. L., Kwon, K. D.,\nand Li, W., 2025, Coprecipitation of phosphate with calcite: Molecular-scale\nevidence for incorporation and inclusion mechanisms: Geochimica et\nCosmochimica Acta, v. 399, p. 1-17.\nRichardson, J. A., Roest-Ellis, S., Phillips, B. L., Strauss, J. V., Webb, S. M., and\nTosca, N. J., 2022, Characterization and Geological Implications of\nPrecambrian Calcite-Hosted Phosphate: Geophysical Research Letters, v. 49,\nno. 17, p. e2022GL100328.\nRoest-Ellis, S., Richardson, J. A., Phillips, B. L., Mehra, A., Webb, S. M., Cohen, P.\nA., Strauss, J. V., and Tosca, N. J., 2023, Tonian Carbonates Record\nPhosphate-Rich Shallow Seas: Geochemistry, Geophysics, Geosystems, v.\n24, no. 5, p. e2023GC010974.\nReviewer #1 (Remarks to the Author):\nI commend the authors on doing a good job responding to my earlier concerns, and I congratulate\nthem on a thought-provoking paper. It will be interesting to see how work on the CAP proxy and the\nGOE evolves in the coming years.\nReviewer #2 (Remarks to the Author):\nThe authors took care to incorporate the comments from 3 reviewers into this revised version of the\nmanuscript. With the altered wording, the authors to my opinion mostly addressed the main criticism\nof all three reviewers, namely the caution required when using a sedimentary proxy to reconstruct\nancient seawater.\nOf course, in paleoceanography large uncertainties inherently exist and all three reviewers agreed\nthat Dodd et al. provide valuable new data that help shed further light on the co-evolution of the\nmarine P cycle and atmospheric oxygenation.\nIn the revised discussion, Dodd et al. now include existing work regarding ancient marine P cycling\n(e.g. Reinhard et al., Canfield et al.) and paint a more nuanced picture of their results, which I\nappreciate. What I do note that is that the new level of caution in interpretation of results is not\nincluded in the title; I understand the wish for a snappy title, but I would recommend a less definitive\nstatement, considering the amount of caution now expressed in the manuscript.\nRegarding the link between CAP and the size of the oceanic dissolved P reservoir, I remain of the\nopinion that additional data such as TOC, total P, trace metals would contribute significantly to our\nunderstanding of the sedimentary and depositional context (and P cycling) that helps shape the CAP\nrecord as well. The authors consider this 'beyond the scope of this study' and it likely complicates the\nstory, but I find it a missed opportunity to more fully develop our understanding of the P cycle at that\ntime. Furthermore, the authors mention that \"This study provides evidence for how ocean P level was\nchanging, but not what was causing it to change\" but my point is more that additional data would help\nbetter understand sedimentary P cycles that helps shape the CAP record. That being said, also\nconsidering the feedback from the other reviewers, I appreciate that Dodd et al. included more\nelaborate discussion on ancient P cycling from other studies.\nReviewer #3 (Remarks to the Author):\nI recommend publication of the revised version of this manuscript.\nReview of Increased phosphorus bioavailabilty triggered Earth’s permanent\nsurface oxygenation by Dodd et al.\nOverview: The authors present carbonate-associated phosphate (CAP) data from\nglobally distributed marine sediments across the GOE and demonstrate a positive\ncovariation with carbonate carbon isotopes. The primary finding of this analysis is that\nCAP values are high during the Lomagundi positive carbon isotope excursion. These\ndata are used in a biogeochemical box model to support how increased P availability\nwould lead to a rapid oxygenation of the atmosphere by increased primary productivity\n(photosynthesis).\nComments:\n1. In several places, including a section title in the Introduction, the authors refer to\nCAP as a direct proxy for seawater phosphate concentrations. However, the first\nauthor’s 2021 paper demonstrates a strong dependence of phosphate\nincorporation into carbonate minerals on pH and mineralogy. I recommend\ntempering the language around direct proxy if the evaluation is based on\nqualitative comparisons between CAP datasets across time without quantifying\nseawater [P]. I want to make clear that I think there is significant value in these\nqualitative comparisons.\n2. I appreciate the approach of considering metamorphic grade in your diagenetic\nassessment of these samples. However, I question whether comparing across\nsimilar metamorphic grade is valid. Presumably, alteration of CAP values is more\nrelated to the phosphate contents of the diagenetic fluid rather than the P-T\nregime of the alteration environment (other than potentially extent of water-rock\nexchange). I recommend performing some amount of petrography on samples\nfrom each formation and analyzing a clear secondary fabric for CAP, such as a\nsecondary vein. This will provide some information about the phosphate contents of\naltering fluids which will then provide a means of assessing whether CAP\nvalues in your fabrics interpreted as primary provide a minimum or maximum cap\non CAP. A few analyses of secondary fabrics would really bolster your argument\nfor accepting CAP values as primary, or at least usable for comparison across\ngeographic regions.\n3. Rationale for spread in CAP values: In the very first sentence of the Discussion,\nthe authors state “There is a large spread in CAP values of formations deposited\nduring the LE, which is expected given the heterogeneous nature of phosphorus\nconcentration in the surface ocean.” Can the spread realistically be explained by\na reasonable range of surface seawater [P]? I think it is more likely pointing to\nvariable degrees of water-rock alteration and/or phosphate contents of altering\nfluids in the metamorphic environment. To be clear, I don’t think this takes away\nfrom your main finding that there is a positive covariance in CAP and d13C\nacross the LE, but I recommend casting this within a more cautious light with\nrespect to primacy of CAP values.\n4. Has any petrography been performed on any of these samples? How were\nfabrics considered in sub-sampling for chemical analyses? Whether you are\nsampling a primary marine cement vs a late diagenetic cement vs an ooid that\nformed on the platform top can significantly change how CAP values should be\ninterpreted, and by having data from the different formation and diagenetic\nenvironments, one can leverage these data to build a more complete picture of\nthe environment across the LE.\n5. I appreciate how the authors investigated the CAP-d13C covariation from various\nangles in the SI. In my opinion, this paper would be stronger if it was published in\na venue that allowed much of this detailed analysis and consideration to be moved into\nthe main text, but that is just my opinion.\nSummary: I found this paper incredibly interesting and would like to see it published.\nMy comments are intended to primarily serve to enrich the diagenetic assessment and\nboldness of some claims, but overall, I think the main finding is made on solid footing.\nPlacing it within the context of a bit more diagenetic assessment would just ensure it\nstands the test of time."}, "author_response": "1.11\nWe thank the reviewer for their support of this work and time spent and comments\noffered. Our detailed responses to the major comments raised by Dr. Bryant above\nare provided below in our responses to related lined comments.\nLine-by-line comments\nLine 30: “Great Oxidation Episode” does not correspond to most usage of “GOE” in the\ncommunity. Suggest the authors stick to Event.\n1.12\nChanged to Great Oxidation Event\nLine 34: I’m not sure there is such a thing as a ‘direct’ proxy. In this case, phosphate is\nmeasured but as a proxy for oceanic phosphorus (not orthophosphate?). That seems like\nquite a complex, certainly indirect proxy.\n1.13\nWe agree that all proxies are indirect and have removed the wording ‘direct’ proxy.\nLine 37: Why a phosphorus control on the global carbon cycle and not the other way\naround? The authors don’t need me to remind them that correlation need not imply\ncausation.\n1.14\nWe acknowledge this important consideration which was also raised by the other\nreviewers. We have expanded on this in lines 318-329 and have tempered the\nlanguage in the abstract.\nLine 50: See comment on Line 30.\n1.15\nChanged wording.\nLine 60-61: I think the community agrees that the GOE was protracted, but the O2\nfluctuations and oscillations are less well-agreed upon, due in complexities in proxy behavior\n(e.g., the sensitivity of metal speciation to both depositional and post-depositional effects).\nNevertheless, if this statement represents the authors’ view, that can be stated explicitly\nrather than presuming to speak for the broader community.\n1.16\nAgreed and has been reworded to reflect this.\nLine 62-64: “Oscillating seawater sulfate level” is an interpretation, not a fact. Here, and\nthroughout, more careful language is needed to distinguish between observations and\ninterpretations. In this case, there are numerous reasons why the interpretation of oscillating\nseawater sulfate level might be an over-interpretation. To give one example, in reference 10\n(Planavsky et al., 2012), the sulfur isotopic composition sedimentary sulfate is a piece of\nevidence used to argue for changes in the seawater sulfate concentration. However, one\ncan imagine several scenarios by which the primary d34S of marine sulfate could change in\nthe absence of changes in the size of the marine sulfate reservoir. Then, there is the issue of\ndiagenetic alteration of carbonate-associated sulfate, which has been argued to afflict much\nyounger rocks (see, e.g., Present et al., 2019; Murray et al., 2021), but has yet to be\nevaluated for most Paleoproterozoic rocks, with some recent exceptions (Bryant et al.,\n2024).\n1.17\nWe have adjusted the manuscript wording to clearly distinguish between\nobservational data and the interpretation of \"oscillating seawater sulfate levels.\" We\nfurther acknowledge possible scenarios where δ34S changes might not necessarily\nreflect fluctuations in sulfate reservoir size, citing additional literature highlighting\nthese complexities.\nT. M. Present, M. Gutierrez, G. Paris, C. Kerans, J. P. Grotzinger, J. F. Adkins, Diagenetic\ncontrols on the isotopic composition of carbonate-associated sulphate in the Permian\nCapitan Reef Complex, West Texas. Sedimentology. 66, 2605–2626 (2019).\nS. T. Murray, J. A. Higgins, C. Holmden, C. Lu, P. K. Swart, Geochemical fingerprints of\ndolomitization in Bahamian carbonates: Evidence from sulphur, calcium, magnesium and\nclumped isotopes. Sedimentology. 68, 1–29 (2021).\nR. N. Bryant, J. P. Todes, J. A. Richardson, T. C. Kalia, A. R. Prave, A. Lepland, K. Kirsimäe,\nC. L. Blättler, Local sedimentary effects shaped key sulfur records after the Great Oxidation\nEvent. Earth Planet. Sci. Lett. 648, 119113 (2024).\nLines 70-71: It seems strange to mention only the canonical interpretation of the LE here,\ndespite recent work highlighting the facies-dependence of the magnitude of the LE (Prave et\nal., 2022; Hodgskiss et al., 2023), which clearly has implications for how the LE is\ninterpreted (i.e., the LE might have required less oxidation/oxygenation than canonically\nassumed).\n1.18\nWe have acknowledged other interpretations of the Lomagundi Excursion on lines 80-\n82. We also show in our biogeochemical model that a facies-dependent expression of\nthe LE aligns with our data and conclusions if the Paleoproterozoic carbon gradient\nvaried with depth like in the modern ocean, see lines 365-368.\nLines 93-94: Yes, but the proportionality for a given element is often a function of the\nconcentrations of other things in solution (e.g., Ca, Mg, carbonate ions), kinetics, etc. See,\ne.g., Barkan et al. (2020). One could decrease CAP for the same orthophosphate\nconcentration by increasing the activity of CO32- in solution.\nY. Barkan, G. Paris, S. M. Webb, J. F. Adkins, I. Halevy, Sulfur isotope fractionation between\naqueous and carbonate-associated sulfate in abiotic calcite and aragonite. Geochim.\nCosmochim. Acta. 280, 317–339 (2020).\n1.19\nYes, this is true, we have clarified that phosphate incorporation into carbonate\nminerals is influenced by multiple environmental parameters including ion\nconcentrations and kinetics on lines 92-94 and lines209-230 and acknowledge this as\na limitation.\nLines 94-95: P, or phosphate? Also, why ‘relative concentration of P’?\n1.20\nWe have now clarified P as phosphate and removed the word “relative”.\nLines 99-101: This is quite the statement – needs literature support or further justification. To\nmake a rough analogy with CAS, the concentration of a non-stoichiometric element in\ncarbonate can change after burial and need not bear ‘direct’ relevance to the concentration\nof that element in the primary fluid.\n1.21\nWe have reworded to “…while phosphate bound inside carbonate minerals can be\nattributable to ambient dissolved P concentrations.”.\nLines 119-121: This sentence is confusingly structured – consider revising for clarity.\n1.22\nWe have reworded on lines 118-121\nLines 123-138: The results section strikes me as being very short. One thing I would like to\nsee here as a reader is some actual CAP abundances in the text, in addition to the\ncomparisons with bracketing carbonates. If CAP works how it is supposed to, the relative\nchanges in CAP should be less meaningful than the absolute changes. I am also wondering\nhow one chooses how much of the bracketing carbonates to include in this relative CAP\nassessment.\n1.23\nWe have added CAP values to show the range of absolute values on lines 134-135 and\n143-144. We used a cut-off of 3 per mil to compare CAP values across carbon isotope\nexcursions, we do not expect this cut-off to play a major factor in the comparison as\nFigure 2 shows the correlation of CAP and d13C is continuous and not a phenomenon\ndeveloping from discrete data binning. Regarding relative vs. absolute change it is\nimportant to note that absolute values could range between localities if some ocean\nregions had higher local phosphate availability as discussed on lines 232-234 or\nvarying degrees of metamorphism lower CAP values. Take the Nash fork location for\nexample, it shows a change in absolute CAP values across the LE, but if you compare\nthese absolute values to the Francevillian section they are considerably lower.\nTherefore, we use relative to describe the fact that despite differences in absolute\nvalues between sections they show the same trends relative to the CAP values from\nthe individual sections.\nLine 141: This should be in the results rather than the discussion, in my opinion. The\nanalysis in the following clause can stay in the discussion.\n1.24\nYes, this sentence contains both a result descriptor but also an interpretive clause.\nWe have chosen to leave it as a bridge between sections.\nLine 143: Here and throughout, I think the use of ‘CAP values’ rather than ‘CAP abundance’\nis perhaps unhelpful. Nowhere in the text is CAP assigned a unit. One is left with a sense of\nCAP values being an abstract statistical measure rather than a measured quantity.\n1.25\nWe have now detailed the expression of CAP units on lines 102-104\nLines 143-147: This is a lengthy sentence and complex argument. I had to read this several\ntimes to grasp the intention. I would suggest splitting the sentence up and laying out the\nargument more systematically without skipping logical steps.\n1.26\nAgreed we have removed as it is discussed later in the text around basin preservation\nover time on lines 231-244.\nLine 159: I think the authors have done a good job of alleviating fears about metamorphism\ninfluencing the data in a meaningful way. However, they allude here to other ‘post-\ndepositional processes’. Missing in the text is a discussion of these other post-depositional\nprocesses and how those might affect CAP abundance. Reading between the lines, the\nprocesses in question could be early marine diagenesis of carbonates. How might one\nscreen for the influence of such processes? What about meteoric diagenesis? I would be\nworried about both styles of diagenesis, but particularly the latter, because it could\nconceivably decrease both CAP abundance and d13C-carb. Can the authors demonstrate\nthat the bracketing units (which they say are the same sabkha facies as the LE carbonates)\nare not simply more influenced by meteoric diagenesis than those in the LE carbonates?\nDoes mineralogy have a role in the preservation of CAP?\n1.27\nWe appreciate the support of our discussion of metamorphic influences on CAP.\nWe have moved the metamorphic discussion so it now follows the expanded discussion\non diagenetic processes on lines 234-261. Yes, mineralogy does affect CAP\npreservation and is discussed on lines 93 and 116.\nLine 163: I disagree with diagenetic and metamorphic alteration being lumped together here.\nThe authors have demonstrated that metamorphic grades did not drive the signals they\nreport. They have not conducted a similar analysis for any style or phase of diagenesis\n(although I see some work to this end further into the document).\n1.28\nWe have expanded our discussion on diagenesis to cover a wide array of hypothetical\ndiagenetic scenarios on lines 146-190 and include new data statistics which compare\ncarbonate trace element chemistry and CAP to discuss diagenetic influences.\nLines 165-167: This information should be in the results section.\n1.29\nAgreed.\nLine 178: I appreciate this analysis by the authors. I would also be interested to know if there\nare correlations between CAP and CAS content. There are several ways one could interpret\nsuch a correlation. The first would be high orthophosphate and sulfate concentrations in the\nwater column. High alkalinity and/or carbonate ion activity would be another.\n1.30\nWe did not measure CAS contents of the sample analysed for CAP, however we can\nuse CAS values from the literature to give a broad overview. Based on the literature,\nCAS is generally elevated in carbonate from the LE relative to those carbonates\ndeposited outside the LE. This would mean there would be a general positive\ncorrelation of CAP and CAS across the LE. We would favour the first interpretation\nput forward by the reviewer, given exceptionally high alkalinity increases would be\nrequired to explain the CAP and CAS increases on a solely alkalinity-controlled basis.\nFor example, Barkan et al., 2018 found a roughly 1:1 relative increase in alkalinity to\nCAS concentration which based on data in Planavsky et al., 2012 which show a rough\nchange from ca. 50ppm to 250ppm across the LE, which would require a ca. >5x\nincrease in marine alkalinity to explain the CAS concentration change across the LE.\nThis would result in an increase in ocean pH if not matched with a corresponding\nincrease in atmospheric CO , which is unlikely for the GOE interval, based on existing\n2\nevidence for glaciations.\nLines 188-189: This information should be in the results. Here, it would be useful to explain\nwhy these diagenetic indices were chosen, and what they specifically help to screen for. To\nmy mind, Mg/Ca and Mn/Sr are necessary, but not sufficient to rule out the influence of some\ntypes of diagenesis.\n1.31\nWe have now included a description of why the diagenetic indices were used and\nother possible digenetic indices on lines 176-178.\nLines 194-196: This section needs references. My view is that the effect of the diurnal cycle\non CAP need not necessarily be as simple as orthophosphate concentrations being drawn\ndown throughout the water column. What if the locations of carbonate formation and\nphotosynthesis were decoupled?\n1.32\nThe diurnal cycle explicitly requires carbonate formation in-situ in order for carbonate\nto capture changing carbon isotopic composition from local photosynthetic activity\n(Geyman and Maloof, 2019). Maybe future works may be able to build a more complex\ncycle but based on the current state of knowledge we can confidently rule this out.\nLines 197-201: This section is extremely important but is glossed over rather quickly. These\nevaporitic conditions could be driving many of the signals presented in this paper. The\nexistence of one LE-bearing Formation with a putative deeper water origin does not in my\nopinion make evaporative conditions an unlikely mechanism for the coupling between CAP\nand d13C. Bear in mind also that elevated d13C values in deeper water settings are\nuncommon during the LE (Prave et al., 2022; Hodgskiss et al., 2023), and in some cases\nhave been linked to remobilization and transport of shelf sediments, following Walther’s Law.\nThe authors need to do more here to justify this argument.\n1.33\nWe have discussed this phenomenon in more detail on lines 204-208 and include new\nY/Ho data to develop this point.\nLine 205: Why pH and not alkalinity or carbonate ion concentration?\n1.34\nThese effects have been previously documented in experiments and pH was found to\nexert the biggest influence on CAP incorporation (Dodd et al., 2023; Dodd et al., 2021).\nWe have now noted earlier in the manuscript that alkalinity/ carbonate ion\nconcentration also effects CAP.\nLines 206-208: This sentence needs a reference. Also, what direction would pH have\nneeded to shift?\n1.35\nReference now added on line 227 (Haley and Bachan 2017). The pH change direction\ndoes not matter unless we are specifically talking about which way the CAP values\nchanged. Here we simply state the change whether it be higher or lower was too large\nto be likely explained by pH alone.\nLines 209-211: This sentence is an oversimplification. Ruling out pH changes is not sufficient\nto argue that background ocean chemistry (which includes many parameters other than pH)\nwas not an overarching control.\n1.36\nWe agree there are many chemical effects which can impact CAP incorporation into\ncarbonate and have expanded discussion on this on lines 209-230.\nLine 226: This information should be in the results section.\n1.37\nThese lines are a discussion element with reference to the results. We have added a\ndescription of what the sample mineralogy is in the results section.\nLines 221-227: This sentence is far too long and should be broken up for clarity.\n1.38\nAgreed and has been modified.\nLine 260: Given that the CIE is facies-dependent and is preserved mostly in shallow water\nsettings, it is not necessary to make the entire ocean DIC pool 8 per mil heavier, as I\nassume was done in this modeling exercise.\nLines 281-287: Okay this section resolves my previous comment.\nLines 326-329: These lines are too strong for my liking. In my opinion, the data presented in\nthis paper, while interesting, do not conclusively demonstrate that marine phosphate levels\nwere higher during the LE, let alone that there was a mechanistic linkage between high\nphosphate levels and oxygenation. There is a disconnect between the complicated picture\npainted in the discussion and the certainty with which the conclusions are delivered.\n1.39\nWe have adopted a more conservative concluding statement in line with caveats and\nlimitations of the CAP proxy and connection with carbon isotope values on lines 415-\nevaporite and CAS data from the Onega Basin. See, e.g., Blattler et al. (2018). This is an\nimportant omission because there is strong evidence from that paper that seawater sulfate\nd34S was around 5-6 per mil right before the end of the LE.\nC. L. Blättler, M. W. Claire, A. R. Prave, K. Kirsimäe, J. A. Higgins, P. V. Medvedev, A. E.\nRomashkin, D. V. Rychanchik, A. L. Zerkle, K. Paiste, T. Kreitsmann, I. L. Millar, J. A. Hayles,\nH. Bao, A. V. Turchyn, M. R. Warke, A. Lepland, Two-billion-year-old evaporites capture\nEarth’s great oxidation. Science. 360, 320–323 (2018).\n1.40\nWe have added the new data from Blattler to the Fig. 1 panel B.\n2.11 We thank the reviewer for their time spent and comments offered and agree the\ninclusion of further works pertaining to ancient P cycling can help frame the\nimportance of this study.\n2.12\nWe agree that all proxies are indirect and have removed the wording ‘direct’ proxy.\nWe acknowledge this important consideration which was also raised by the other\nreviewers. We have expanded on this in lines 318-329 and have tempered the\nlanguage in the abstract.\n2.13\nWe do not see this result as “meagre”, considering that higher ocean P levels during\nthe LE were a key testable prediction of the P-driven productivity model of the GOE.\nNevertheless, we have now elaborated on the comparison of the biogeochemical\nmodel results and observations of fluctuating redox conditions during the GOE on\nlines 386-396. Our new model results show that if the GOE occurs because of a\nreduction in oxygen sinks, a stepwise transition from low to high atmospheric O2\noccurs, whereas if an increase in P-driven productivity is a driver for the GOE,\natmospheric O2 level rises and falls, which is more consistent with redox proxies.\n2.14\nIndeed, it would be interesting to have other tracers of P cycling during the GOE,\nhowever diving further into what was controlling P level during the GOE is beyond the\nscope of this study. This study provides evidence for how ocean P level was\nchanging, but not what was causing it to change. We agree that further details should\nbe added to provide a better overview of what may have been controlling ocean P\nlevel on lines 265-306.\nThe biogeochemical model only uses modern fluxes to recreate the present day.\nWhen it is run over Earth history (Alcott et al., 2019; Alcott et al., 2024) its fluxes are\ncomputed dynamically based on elemental abundances, climate and tectonics, and\nthe overall model output compares reasonably well to a suite of long-term proxies. It\nis probably the best tested model of this type currently available. We have\nemphasised this now in the methods description.\n2.15\nThe CAP proxy was validated principally through experimental work, which\nsynthesised carbonates under analogous conditions to modern seawater. This abiotic\nprecipitation style would be similar to the Precambrian world. Others have also tested\nCAP incorporation using analogues of Proterozoic seawater (Roest-Ellis et al., 2023)\nand achieve similar results. The biggest unknown would be CAP incorporation into\ndolomite which was a pervasive carbonate with debated origin in the Precambrian. We\nhave added further information on lines 114-116, which detail the effects of\nmineralogy on CAP.\n2.16\nWhile there is indeed significant point-to-point scatter in the CAP profiles, which is an\ninherent feature of low-resolution sampling in deep time carbonate archives, we argue\nthat the observed stratigraphic trends are unlikely to be primarily diagenetic. First,\nwe note that the CAP excursions are observed consistently across distinct basins\n(Francevillian and Snowy Pass), each with different diagenetic and burial histories,\nyet exhibiting broadly synchronous patterns when normalized to formation thickness.\nThis stratigraphic alignment supports a common paleoenvironmental driver affecting\nboth carbon and phosphorus systems, rather than localized diagenetic overprinting.\nThe 3 sections in Figure 3 record the Lomagundi and Wooly Dolomite carbon isotope\nexcursion and show continuous chemostratigraphic trends of correlated δ13C and\nCAP which show continuous correlation of δ13C and CAP across the Lomagundi\nEvent, as opposed to comparing individual formations from discrete time points.\nGiven the Lomagundi Event is tied to the rise and fall in O levels via other studies,\n2\nthe CAP trends across these sections supports their connection with higher and\nlower O levels (Canfield et al., 2013). We have also added extra discussion on the\n2\npossible impacts of post-depositional alteration as a mechanism of scatter in the data\non lines 187-192.\nLocally weighted scatterplot smoothing (LOWESS) is a common technique applied in\nstratigraphic geochemical analysis. It is a non-parametric regression technique that\nfits local polynomial functions to the data, allowing for visualization of broader trends\nwhile minimizing the influence of short-wavelength noise. We have now clarified this\nin the figure caption. The smoothing curve is included not to obscure scatter, but to\nguide the reader in identifying stratigraphic trajectories that are difficult to interpret in\nnoisy datasets typical of Precambrian carbonates.\nWe have added the following clarifying text in the revised figure 3 caption: “Dark-grey\nlines represent locally weighted scatter plot smoothing”\n2.17\nAgreed, qualitatively we can say concentrations were potentially higher or lower\nwithout putting an absolute value to them.\n2.18\nWe agree that despite there being evidence for temporally correlated increases in\nocean P, O2 and biomass burial we cannot conclude that P-driven biomass burial was\nthe sole driver of the GOE. We have added this caveat on lines 320-321 and 420-444.\n2.19\nWe have extended the arrow to capture that.\n2.20\nWe have reworded to “which established a permanent oxygen atmosphere on Earth”\n2.21\nAdded.\n2.22\nWe have reworded to Event throughout the manuscript.\n2.23\nWe have changed to “oxygenation of the surface environment.”\n2.24\nWe agree that all proxies are indirect and have removed the wording ‘direct’ proxy.\n2.25\nWe agree that all proxies are indirect and have removed the wording ‘direct’ proxy.\n2.26\nThe CAP proxy was validated principally through experimental work, which\nsynthesised carbonates under analogous conditions to modern seawater. This abiotic\nprecipitation style would be similar to the Precambrian world. Others have also tested\nCAP incorporation using analogues of Proterozoic seawater (Roest-Ellis et al., 2023)\nand achieve similar results. The biggest unknown would be CAP incorporation into\ndolomite which was a pervasive carbonate with debated origin in the Precambrian. We\nhave added further information on lines 114-116, which detail the effects of\nmineralogy on CAP.\n2.27\nReworded to “…whether global seawater phosphate availability changed during the\nGOE”\n2.28\nWe agree that despite there being evidence for temporally correlated increases in\nocean P, O2 and biomass burial we cannot conclude that P-driven biomass burial was\nthe sole driver of the GOE. We have added this caveat on lines 320-321 and 420-444.\n2.29\nThe CAP proxy was validated and tested in modern and Proterozoic-like seawater\n(Dodd et al., 2021; Ingalls et al., 2020; Ingalls et al., 2022; Ren et al., 2025; Richardson\net al., 2022; Roest-Ellis et al., 2023) and achieve similar results.\n2.30\nWe have now referenced a study which modelled pH changes during the GOE, with\n95% of modelled results returning a pH drop of < 1 unit (Halevy and Bachan, 2017).\n2.31\nWe have added further explanation of P cycling in the Paleoproterozoic on lines 267-\n331\n2.32\nAs above, the model is developed around the modern ocean but the fluxes are\nallowed to vary dynamically through Earth history, so during the GOE the model is\nvery different to the modern.\n2.33\nWe agree and have added additional discussion on lines 404-419.\n3.11\nWe agree with tempering the language around CAP being a direct proxy and have\nincluded discussion of this on lines 33-35, 85-87, and 101-104.\n3.12\nWe analysed CAP over a P-T range because prior work has shown that progressively\nincreasing metamorphism can lead to increasing CAP loss (Ingalls et al., 2022).\nWe agree that the degree of alteration will be unique for each case and could explain\nsome spread in data but is highly unlikely to recreate the observed correlation in CAP\nand d13C across such a huge range of metamorphic grades seen in this study. Our\nsamples were carefully selected to avoid visible grains before selecting a sample for\ncrushing and analysis. There are however instances where we do not need to\nconsider metamorphism to support the conclusions such as the FC Formation,\nWooly Dolomite and Nash Fork Formation, which preserved continuous sections of\nconsistent metamorphic grade and all preserve a positive correlation of CAP and\nd13C suggesting metamorphism is an unlikely cause of the correlation.\nPetrography has been performed on these samples in prior published studies\n(Kalderon-Asael et al., 2021; Ossa Ossa et al., 2018). We subsampled the samples\npreviously used in these other studies. When subsampling we avoided visible\nalteration or veining.\n3.13\nWe agree with the reviewer that the results should be discussed in a more cautious\nlight and have added this discussion throughout the text. We keep the original\nsentence as the spread could indeed be attributed to the heterogenous nature of\nsurface ocean P concentrations. In the modern ocean CAP varies by more than an\norder of magnitude in the surface ocean (Martiny Adam et al., 2019).\n3.14\nPetrography has been performed on these samples in prior published studies\n(Kalderon-Asael et al., 2021; Ossa Ossa et al., 2018) before samples were selected for\ngeochemical analysis. We subsampled the samples previously used in these other\nstudies. When subsampling we avoided visible alteration or veining. In these prior\nstudies detailed environmental reconstructions of the studied samples were\nperformed. Despite widely varying depositional environments and a likely mix of\nprimary and diagenetic cements the positive trend of d13C and CAP holds true. On\nlines 245-263, we discuss how this broad mix of environmental factors support a\nprimary origin of the positive trend.\n3.15\nThe SI file is intended for experts in the field who would find such information\ninteresting but we believe the broad appeal of the findings beyond geochemists make\nthis journal choice appropriate. We kept detailed diagenetic discussion in the SI so\nthe general reader can easily digest the key takeaways.\n3.16 We thank the reviewer for their support of this study and time spent reviewing\nand providing comments on this manuscript.\nResponse: We added the following sentence on lines 344: Future work\nintegrating CAP with Total Organic Carbon, total P and trace-metal datasets\nwill help further disentangle P cycling controls on CAP and marine P\nvariability.\nResponse: Modified text on lines 331 as follows: P speciation data from this\ninterval support increased recycling efficiency as a result of increasing\nsulphate availability, due to sedimentary sulphur cycling 50, reinforcing a\npositive feedback in which rising atmospheric oxygen levels increased\nsulphate availability in anoxic water bodies amplifying P release and sustained\nhigh productivity."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I commend the authors on doing a good job responding to my earlier concerns, and I congratulate them on a thought-provoking paper. It will be interesting to see how work on the CAP proxy and the GOE evolves in the coming years.", "reviewer_2": "The authors took care to incorporate the comments from 3 reviewers into this revised version of the manuscript. With the altered wording, the authors to my opinion mostly addressed the main criticism of all three reviewers, namely the caution required when using a sedimentary proxy to reconstruct ancient seawater.\nOf course, in paleoceanography large uncertainties inherently exist and all three reviewers agreed that Dodd et al. provide valuable new data that help shed further light on the co-evolution of the marine P cycle and atmospheric oxygenation.\nIn the revised discussion, Dodd et al. now include existing work regarding ancient marine P cycling (e.g. Reinhard et al., Canfield et al.) and paint a more nuanced picture of their results, which I appreciate. What I do note that is that the new level of caution in interpretation of results is not included in the title; I understand the wish for a snappy title, but I would recommend a less definitive statement, considering the amount of caution now expressed in the manuscript.\nRegarding the link between CAP and the size of the oceanic dissolved P reservoir, I remain of the opinion that additional data such as TOC, total P, trace metals would contribute significantly to our understanding of the sedimentary and depositional context (and P cycling) that helps shape the CAP record as well. The authors consider this 'beyond the scope of this study' and it likely complicates the story, but I find it a missed opportunity to more fully develop our understanding of the P cycle at that time. Furthermore, the authors mention that \"This study provides evidence for how ocean P level was changing, but not what was causing it to change\" but my point is more that additional data would help better understand sedimentary P cycles that helps shape the CAP record. That being said, also considering the feedback from the other reviewers, I appreciate that Dodd et al. included more elaborate discussion on ancient P cycling from other studies.\nFor the modelling, appreciate that in the revised manuscript, Dodd et al. emphasize the evidence that suggest a \"P-driven GOE\" (C isotopic record, pO2 reconstructions).\nOverall, Dodd et al. provide a revised manuscript that includes a more nuanced and therefore stronger presentation and discussion of the data.\nA minor point in the revised manuscript with tracked changes is that the phrasing in L789-791 is a bit confusing:\n\"Rising sulphate levels would have stimulated microbial sulphate reduction and enhanced P recycling from organic rich sediments. P speciation data from this interval support increased recycling efficiency under more oxidizing conditions due to sedimentary sulphur cycling\"\nThis feels counterintuitive; P recycling is generally more efficient under more reducing sedimentary conditions, in this case more sulfide generation and subsequent trapping of Fe in sulphides. I think the authors refer to a more oxygenated atmosphere that would result in more sulfate supply to the oceans? It's probably useful to clarify these aspects a bit (both how enhanced sulphate reduction affects P burial efficiency and what part of the atmosphere-ocean water-seafloor system is more oxygenated.", "reviewer_3": "I recommend publication of the revised version of this manuscript."}, "author_response": "Response: We added the following sentence on lines 344: Future work\nintegrating CAP with Total Organic Carbon, total P and trace-metal datasets\nwill help further disentangle P cycling controls on CAP and marine P\nvariability.\nResponse: Modified text on lines 331 as follows: P speciation data from this\ninterval support increased recycling efficiency as a result of increasing\nsulphate availability, due to sedimentary sulphur cycling 50, reinforcing a\npositive feedback in which rising atmospheric oxygen levels increased\nsulphate availability in anoxic water bodies amplifying P release and sustained\nhigh productivity."}]

Decision letter