Late Paleozoic oxygenation of marine environments supported by dolomite U-Pb dating

Domain: earth_science
Published: 2024-04-03
Editorial outcome: minor_revision

Abstract
Understanding causal relationships between evolution and ocean oxygenation hinges on reliable reconstructions of marine oxygen levels, typically from redox-sensitive geochemical proxies. Here, we develop a proxy, using dolomite U–Pb geochronology, to reconstruct seawater U/Pb ratios. Dolomite samples consistently give U–Pb dates and initial 207Pb/206Pb ratios lower than expected from their stratigraphic ages. These observations are explained by resetting of the U–Pb system long after deposition; the magnitude of deviations from expected initial 207Pb/206Pb are a function of the redox-sensitive U/Pb ratios during deposition. Reconstructed initial U/Pb ratios increased notably in the late-Paleozoic, reflecting an increase in oxygenation of marine environments at that time. This timeline is consistent with documented shifts in some other redox proxies and supports evolution-driven mechanisms for the oxygenation of late-Paleozoic marine environments, as well as suggestions that early animals thrived in oceans that on long timescales were oxygen-limited compared to today.

Introduction
The oxygenation of Earth’s atmosphere and oceans and the emergence and evolution of life are pivotal episodes in Earth history1–5. Two step-wise increases in atmospheric oxygen level at ~2.4-2 and 0.8-0.55 Ga (billions of years) are generally agreed upon and supported by an abundance of geochemical and sedimentological evidence2. However, the timing in which different marine habitats became oxygenated, and therefore the causal relationship between such oxygenation and evolutionary steps are intensely debated6,7. Hypothesized timelines of marine oxygenation range from a step-wise increase in O2 between the Neoproterozoic and Cambrian parallel to the radiation of animal life4,8–11 (possibly reaching values as high as modern oceans by the early Cambrian8); to minor (0.5–10% of the modern ocean levels12–14), local, and/or episodic oxygenation15–20, prior to the late Paleozoic-early Mesozoic eras, when evolutionary steps such as the expansion of vascular plants or the proliferation of phytoplankton drove major global redox changes17–23. To test these hypotheses, reconstructions of the timeline and extent of oxidation of marine environments from redox sensitive geochemical proxies are imperative.

Interpretations of concentrations and isotope ratios of redox-sensitive elements measured in sedimentary rocks as primary signals, reflecting the conditions in ancient depositional environments, have been criticized for the possibility of post-depositional signal alteration24–26. This problem is accentuated in carbonate rocks and minerals, which are notorious for element and isotope exchange in diagenetic and metamorphic environments22,27. Dolomite, a common alteration product of Ca-carbonate minerals through interaction with seawater in diagenetic environments, is often seen as an unlikely candidate to preserve primary redox-sensitive signals27. Nevertheless, here we demonstrate that such information can be carried beyond the ’veil’ of post-depositional alteration by the U and Pb isotope compositions of dolomites. We examine new and previously published U and Pb isotope analyses of dolomites from Mesoproterozoic to Cretaceous marine strata, and show that (1) dolomite U–Pb dates are commonly reset during burial diagenesis 10–100’s of Ma (millions of years) after deposition; (2) in this process, Pb isotope values are homogenized in a closed-system environment; and (3) calculated initial Pb isotope ratios can be used to infer the redox-sensitive 238U/206Pb ratio in the dolomite-precursor carbonate mineral phase, recorded in marine depositional and early diagenetic environments.

Marine carbonate minerals are desirable targets for U–Pb dating because they are prevalent authigenic phases throughout the geological record28. U–Pb dating is based on the decay of U parent nuclides, 238U and 235U, to their ultimate daughter isotopes, 206Pb and 207Pb, respectively. In an ideal scenario, the initial Pb-isotope composition of a sample is homogeneous whereas the U/Pb ratio is spatially heterogenous. Analyses from a sample will plot along a negative linear trend in 207Pb/206Pb vs. 238U/206Pb composition space (also known as the ‘Tera-Wasserburg’ or ‘inverse’ concordia diagram)29. The intercept of this trend on the 207Pb/206Pb axis (238U/206Pb = 0, therefore [U]=0) defines the whole-rock 207Pb/206Pb ratio at the time of (re)crystallization (“initial 207Pb/206Pb”). The lower intercept between this trend and the concordia curve will indicate the U–Pb date of the sample, interpreted as the time that passed since the closure of the system29,30.

Results
We analyzed Paleozoic and Meso- to Neoproterozoic dolomite samples from the Colorado Plateau and the Death Valley regions, determining their U–Pb dates and initial 207Pb/206Pb using the Tera-Wasserburg concordia scheme (see Methods for detailed description). Using in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), U and Pb isotope measurements were made on a variety of replacive dolomite mineral fabrics associated with depositional and early diagenetic environments (fabric-retentive) or possibly with late diagenetic environments (fabric-destructive), while avoiding void-filling epigenetic dolomite (e.g., veins). We combined these results with U–Pb data from similar dolomite fabrics in the Rocky Mountains31, Eastern Levant32, Arabian Platform33 and Northwest Sichuan Basin34 to create a more globally representative dataset. These data represent six different basins and span a period of ~1.2 billion years, from the Mesoproterozoic Eon to the Late Cretaceous Epoch (Supplementary Dataset S1).

Carbonate minerals forming in marine depositional and shallow diagenetic environments are expected to inherit the initial 207Pb/206Pb ratios of seawater. In turn, the 207Pb/206Pb value of pre-industrial seawater is expected to follow the composition of terrestrial Pb as it evolved over time35, with minor scatter around the average value reflecting the heterogeneity in terrestrial Pb sources and the limited mixing of Pb in the ocean. We estimate the scatter around 207Pb/206Pb ratios of terrestrial Pb to be <0.02, based on the observed range of 207Pb/206Pb values in modern deep ocean water masses, which preserve pre-industrial Pb isotope values36–42. If the U–Pb system in dolomites closed during or shortly after deposition, the U–Pb date would correspond to the depositional age of the sediment, and the initial 207Pb/206Pb would plot within the uncertainty-envelope of the terrestrial Pb evolution trend. However, with few exceptions, calculated U–Pb dates are significantly younger (by 107–8 years, Fig. 1a) than the depositional ages of the sedimentary strata, and calculated initial 207Pb/206Pb ratios of dolomites plot well below the terrestrial and seawater Pb evolution trend (Fig. 1b).

Younger than expected U–Pb dates and lower than expected 207Pb/206Pb initial values in dolomite samples indicate a significant modification to the U–Pb system long after deposition of the original carbonate strata, possibly associated with dolomitization or dolomite-recrystallization during late-stage burial diagenesis. Alteration in deep burial environments through open-system isotope exchange with seawater has been suggested to modify the carbonate oxygen and clumped isotope compositions of massive dolomites in general, and specifically, the set of Paleozoic dolomites from the Colorado Plateau analyzed here43. However, if during such modification the Pb isotope system was similarly open to exchange with seawater, then initial 207Pb/206Pb values would have been reset back to terrestrial values corresponding to the time of modification, contrary to our observation (Fig. 1b). Other external sources for Pb, such as river waters or deep-seated groundwater are also unlikely, as 207Pb/206Pb ratios in these waters are also expected to plot close to terrestrial values44,45. The most likely source for the lower initial 207Pb/206Pb is the dolomite-precursor rocks themselves, which have higher U/Pb ratios than average crust and therefore their bulk 207Pb/206Pb values diverge from the terrestrial trend as U decays (Fig. 2a, b). During the late-stage diagenetic modification, Pb was redistributed among dolomite minerals and isotopically homogenized in a system effectively closed to external Pb isotopic exchange, resulting in a new, lower “initial” 207Pb/206Pb value (Fig. 2b). In this process, U concentrations could also have been re-distributed and partly homogenized in either open or closed system conditions, but some U/Pb heterogeneity was maintained allowing U-Pb discordia to be defined. Following this diagenetic Pb homogenization, further U decay in a closed system ultimately resulted in a younger than depositional U-Pb date, reflecting the time of modification (Fig. 2c). Supporting this interpretation of dolomite U-Pb systematics, a recent study demonstrated that the isotopic composition of U in dolomites is acquired by equilibration with the final diagenetic fluid at the time of alteration, or soon after, and has evolved in a closed system since46. The extent to which the bulk 207Pb/206Pb ratios were lowered between the time of deposition and latest stage of diagenesis depends on the bulk U/Pb ratio of the dolomite-precursor rock, which in turn depends on the redox state of the environment in which it precipitated47. Larger bulk U/Pb ratios will result in larger deviations from the average marine 207Pb/206Pb ratio as U decays.

Dolomite samples in our data compilation include fabric-retentive dolomites that have been interpreted as products of early dolomitization48,49. Yet, many of these samples apparently underwent a closed-system homogenization of Pb isotope values, 107–8 years after deposition (Fig. 1A). This seeming contradiction can be explained if the resetting of the U-Pb system was associated with recrystallization of (proto)dolomite rather than initial dolomitization, at least for the fabric-retentive dolomites in our compilation. Recrystallization of fabric-retentive and -destructive dolomites would be in line with studies included in our dataset, which indicate open-system isotope exchange of carbonate oxygen, and re-setting of clumped isotope compositions in deep burial environments33,43. Therefore, we hypothesize that during late-stage diagenesis there was closed-system homogenization of Pb, while oxygen and clumped isotope compositions were altered under open-system behavior, as oxygen is a highly abundant and mobile element compared to Pb, and therefore more easily buffered by diagenetic fluids. Consistent with this explanation, late recrystallization has been recently suggested to reset the U-Pb dates, and oxygen and clumped isotope compositions within a set of fabric-retentive dolomites33. Alternatively, we cannot rule out the possibility that oxygen and Pb isotope systems have responded to two separate late-stage alteration events. For example, oxygen and clumped isotopes were altered in dolomite samples during contact metamorphism around the Alta Stock (Utah, USA), while the U–Pb dates were not31.

The calculated initial 207Pb/206Pb values in dolomite samples reflect the homogenized bulk-average 207Pb/206Pb evolved in the dolomite-precursor rock at the time of closed-system alteration. This value is expected to be driven further below the terrestrial/marine initial 207Pb/206Pb trend with a longer interval between deposition and alteration time, and higher bulk-average initial 238U/206Pb in the dolomite precursor rock. Considering that the time gap between original deposition and alteration is known from the difference between stratigraphic ages and U–Pb dates, we can estimate the initial 238U/206Pb in the dolomite precursor. To do that, we assume that the precursor mineral’s initial 207Pb/206Pb ratio was the terrestrial 207Pb/206Pb ratio during the deposition of the dolomite-precursor carbonate minerals35. We then calculate the corresponding initial bulk-averaged 238U/206Pb ratio that is required to drive 207Pb/206Pb from that value to the observed initial value in the dolomite after a time interval defined by the difference between stratigraphic ages and U–Pb dates. We repeat this calculation for all samples in which the initial 207Pb/206Pb ratio is lower and the U-Pb date is younger than the expected marine value (54 out of 65 dolomite fabrics examined), and consider the reconstructed values to approximate bulk averaged 238U/206Pb ratios in the dolomite-precursor rock.

Post depositional alteration of geochemical signals is common in ancient rock records. The approach we develop here, to reconstruct redox sensitive 238U/206Pb ratio in depositional and early diagenetic environments from the redox insensitive initial 207Pb/206Pb ratio, is likely applicable to other authigenic mineral records (e.g., calcite, apatite, and iron oxides). In fact, ‘younger-than-depositional’ U-Pb dates and ‘lower-than-expected’ initial 207Pb/206Pb values have been recently reported in bio-apatite50.

Reconstructed 238U/206Pb values remain below 14 during the Proterozoic Eon and early Paleozoic Era, and show a distinct increase in range, reaching maximum values of ~25-376 after ~400 Ma, following the reconstructed trajectory of atmospheric pO2 (Figs. 3a, 4). One possible explanation for intervals with elevated 238U/206Pb is that these periods have been dominated by aragonite precursor for dolomite, in which U is more compatible than in calcite51. However, reconstructions of the primary carbonate minerals deposited in shallow marine environments suggest that alterations between dominantly aragonite and calcite precipitates during the Neoproterozoic and Phanerozoic eons do not correlate to the step variation in the range of reconstructed 238U/206Pb shown here52,53, thus contradicting this explanation as the major driver of the observed variability.

An alternative explanation for the increase in the range of reconstructed 238U/206Pb is that the availability of dissolved U and Pb species in carbonate crystallization environments has changed dramatically. It is not likely that such a change is the result of variation in the supply of U and Pb to the ocean, as oxidative continental weathering of these elements has been dominant since the Paleoproterozoic1,54. Instead, this change could have resulted from rising levels of oxygen in the water column and pore fluids of the upper sediment pile, which would increase the supply of dissolved U6+ and decrease that of Pb (now scavenged by Fe-Oxyhydroxides e.g.55) to the crystallization site and thereby drive the 238U/206Pb of dolomite-precursor carbonate minerals higher47. Supporting this hypothesis, lower concentrations of U have been recorded in carbonates during intervals of expanded oceanic anoxia within the Phanerozoic56,57.

The above interpretation is based on the observed initial 207Pb/206Pb ratio, which is relatively impervious to alteration after dolomite (re)crystallization, and specifically, to false-positive oxygenation signals (higher 238U/206Pb) recorded as samples were approaching the surface during exhumation. However, it may still record signals less-oxic (lower 238U/206Pb) than surface environments, if the dolomite precursor minerals formed in anoxic pore fluids beneath the sediment-water interface. Much of the observed scatter in reconstructed 238U/206Pb values among samples from the same rock units can be explained as reflecting variability in pore-water redox conditions. Such could also be the case for dolomites associated with the degradation of microbial mats in the shallow sediment and/or with local anoxia e.g., 33,49. Nevertheless, the first order <400 Ma oxygenation signal is larger than this scatter (Figs. 3a, 4). An example of this might be seen in the Jurassic microbial-associated dolomites which show a moderately lower range of reconstructed 238U/206Pb values, compared to slightly older and younger fabric-destructive (massive) dolomites: the values from these microbial-associated dolomites are still higher than the ranges observed in >400 Ma samples (Fig. 3a), suggesting that a signal of elevated U/Pb might still be recorded in such environments. Other non-microbial-associated dolomite textures, record significantly higher values of reconstructed 238U/206Pb that are expected under more oxygenated depositional or early diagenetic environments only after ~400 Ma (Figs. 3a, 4). Notably, the variation in reconstructed 238U/206Pb values postdates the Neoproterozoic-Cambrian transition between fabric-retentive and -destructive dolomite textures, hypothesized to have resulted from the reduction of the extent of microbial mats capping the sediment48,49. However, given that microbial mats played a persisting role in the carbonate factory throughout early Paleozoic marine depositional environments e.g.58, it is possible that in some settings microbial mats continued to modulate 238U/206Pb of carbonate precipitates during this interval.

Accepting the hypothesis that the higher reconstructed 238U/206Pb ratios represent an increase in marine dissolved O2, the question remains: Which parts of the ocean were oxygenated? Opposite redox sensitivities of dissolved U and Pb mean that the U/Pb ratio may be affected by both global trends (e.g., the extent of oceanic anoxia) or more local redox conditions (e.g., shallow marine and pore water environments). These possibilities are not mutually exclusive, and both are supported by independent datasets (Fig. 3b–d). After ~420 Ma, increased oxygenation of deep marine basins is supported by the increase in Fe3+/ΣFe in hydrothermally-altered submarine basalts23. At approximately the same time, oxygenation of shallow marine and early diagenetic environments is supported by decreasing Ce/Ce* and increasing I/Ca ratios in marine carbonate samples17,22, as well as by the observation of limited bioturbation extending at least 120 Ma into the Phanerozoic eon59. Thus, reconstructed 238U/206Pb values are broadly consistent with data sets that show increases in oxygenation within both water and/or sediment in shallow and deep ocean environments in the late Paleozoic.

Proxy-based reconstructions of the oxygenation of marine environments typically follow one of three patterns leading to persistent oxygenation of the modern ocean: (1) A step increase during the NOE (0.8–0.55 Ga) e.g, 6,11,60; (2) Episodic oxygenation between the NOE and Cambrian e.g. 15,16; and (3) gradual or episodic increase in oxygenation during the mid–late Paleozoic e.g. 17,21–23. These differences may reflect the complex structure of the early Paleozoic oxycline, in which shallow remineralization of organic material reduced continental shelves and left deeper basins relatively oxidized17, or possibly different sensitivities of elements and the records that contain them to post-depositional alteration25. In this context, the fact that reconstructed 238U/206Pb ratios, which are resistant to the effects of ‘false positive oxygenation’, do not appear to record oxygenation signals during the Neoproterozoic and Cambrian within multiple samples from different formations and different paleoenvironments, at a first order, supports the model that persistent oxygenation of shallow and deep marine environments was delayed until the Devonian (Fig. 3). This scenario does not necessarily preclude the possibilities of minor (<10% or the modern oxic ocean), local, and/or episodic (1–10 Ma) oxygenation events during the NOE, which did not register in the compiled 238U/206Pb and Ce/Ce* records shown here, but could have had critical ecological impacts vital for the Cambrian radiation12.

Several independent datasets now support the oxygenation of marine environments at or after ~400 Ma (Fig. 3). Possible driving mechanisms for this process include: (1) The Devonian expansion of vascular plants, ultimately bringing atmospheric pO2 to peak value towards the end of the Paleozoic18,21,61; and (2) early Mesozoic increase in the size of eukaryote plankton, leading to more efficient export of organic carbon to the deep ocean, and thus removing reducing power from the upper water column17,62. Regardless of the specific mechanism, the persistent oxygenation of marine habitats long after the Neoproterozoic Era is consistent with changes in organic carbon burial efficiency resulting from novel evolutionary steps. It is also consistent with previous suggestions that several major evolutionary and ecological steps in the diversification of animal life took place within a largely oxygen-limited ocean, in which O2 concentrations were sufficient to overcome functional and ecological thresholds, but insufficient to register in a series of redox proxies, possibly since they were minor (<10%) or existed temporary and/or locally relative to the modern oxic ocean7,14,20.

Methods
We collected 15 dolomite rock samples from localities around the Death Valley region. In addition, we used 14 dolomite rock samples from the Paleozoic sedimentary section from the Colorado Plateau, originally collected by Ryb and Eiler43. Sample details are available in Supplementary Dataset S2. U–Pb analyses were performed on specific dolomite fabrics (e.g., matrix and various allochems) on 100 μm thick thin-sections. Eighteen dolomite fabrics in samples from the Colorado Plateau samples were analyzed at UCSB using a Nu Plasma 3D multi-collector ICP-MS coupled with a Photon Machines Analyte 193nm ATLEX-SI 193nm ArF excimer laser ablation system with a double volume HelEx II cell. Seventeen dolomite fabrics in samples from the Death Valley region were measured at JHU Tectonics, Metamorphic Petrology, and Orogenesis (TeMPO) Laboratory using a Teldyne-Cetac Analyte G2 193nm Excimer laser ablation system, with a double volume HelEx II cell, coupled to an Agilent 8900 quadrupole LA-ICP-MS with no gas in the collision cell. Each analyzed fabric includes 15–59 measured spots. Baseline-subtraction and instrument drift corrections were made with the commercially available Iolite software, using the U–Pb Geochronology data reduction scheme and the NIST 612 (JHU) or NIST 614 (UCSB) glass standard as the primary reference material. Following initial data processing in Iolite, a mass-bias correction to the 238U/206Pb ratios of each analysis was made such that the lower-intercept date of the Tera-Wasserburg discordia for limestone secondary reference materials measured simultaneously with the unknowns were accurately reproduced. At JHU, reference material limestones WC-163 and Duff Brown Tank64 were used. At UCSB, WC-163, ASH1565, and an in-house secondary reference travertine Whitepine were used. The accuracy of carbonate U–Pb dates (probably no better than ~±3% at 95% C.I.) is currently limited by heterogeneity of available reference materials, such as WC-1 which exhibits ≥2.5% scatter about best fit discordia63, as well as uncertainties surrounding differences in the ablation characteristics of different carbonate materials63,66. This limitation in method accuracy does not influence the results of this study, for which calculated date uncertainties (analytical and regression) are often quite large and possible error in calculated U–Pb dates on the order of a few million years are unimportant for the overall interpretations.

We consider these data together with published 207Pb/206Pb and 235U/238U ratios from the eastern Levant region (n=7)32, the northwest Sichuan Basin (n=5)34, the Arabian Platform33 (n=11) and the Wasatch Range, Utah (n=8)31. For consistency, we calculated 238U/206Pb and 207Pb/206Pb ratios and (re)plotted all data in Tera-Wasserburg concordia plots to determine the U–Pb age of the samples and the initial 207Pb/206Pb ratio30. We used an in-house Python based code to find the best fit line (discordia) through the data, and determine the U–Pb date, initial 207Pb/206Pb, and their uncertainties67. For the latter, we used York least square method to calculate the uncertainty in the slope and intercept of isochrons that derives from analytical errors and natural scatter68, and propagated these values to determine the uncertainty in the age and intercept. Raw data of new measurements and Tera-Wasserburg plots of all analyzed data are available in Supplementary Dataset S3 and Fig. S1, respectively. Average 238U/206Pb of the dolomite precursor mineral were calculated for all data (n=65). Derivation for this calculation is given in Note S1, and the code is available online at69. We reported for all 238U/206Pb values greater than zero (n=54). Negative average 238U/206Pb values emerge from samples having older than stratigraphic U–Pb ages or initial 207Pb/206Pb ratios above the terrestrial evolution trend.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "See attached review, which I signed.", "reviewer_2": "Review of Late Paleozoic Oxygenation of Marine Environments Supported by Dolomite U-Pb Dating\nIn this paper, the authors present a dataset that captures U-Pb dates of dolomites of various ages and\nvarious precursor sediment ages, in order to reconstruct their initial 207Pb/206Pb ratios. The data\nsuggest that the ages of the dolomite are younger than expected and the 207Pb/206Pb ratios are\nlower than expected, which is interpreted to reflect resetting of the U-Pb system coinciding with U\ndecay, allowing for reconstruction of initial 238U/206Pb ratios of the rock. The study finds that\n238U/206Pb ratios did not dramatically increase until the late Paleozoic, which they attribute to\noxygenation of seawater, and ultimately providing evidence in support of the hypothesis that early\nanimal life evolved within an oxygen-limited ocean until the late Paleozoic (a hotly debated topic).\nI find this study interesting and of broad interest to the scientific community—appropriate for a Nature\nCommunications publication. However, I have some comments and suggestions that the authors might\ntake into consideration prior to acceptance for publication. I will be upfront and admit that I am no\nexpert on the U-Pb system and especially as it applies to carbonates (which appears to be a relatively\nnewer but intriguing development for the carbonate discipline). Rather, my review focuses on the\naspects of dolomitization, dolomite textures, and dolomite recrystallization—for which my background\nis rooted in. I hope the authors find my comments useful to help improve their manuscript, I would be\nhappy to provide any further clarification.\nMajor Comments:\nLine 74 the authors state “Using in-situ laser ablation inductively coupled plasma mass spectrometry,\nU and Pb isotope measurements were made on a variety of dolomite mineral fabrics, associated with\ndepositions and early diagenetic environments (fabric-retentive cements) or with late diagenetic\nenvironments (fabric-destructive cements), while avoiding void-filling epigenetic dolomite (e.g.,\nveins).”\nHowever, my question is: Are they actually cements? Or are they replacement products? An important\ndistinction, since in the former case they would NOT be inheriting precursor sediment geochemistry\n(they would be reflecting the fluids responsible for cementation), and would not accurately capture the\ninterpretations that underpin the importance of this manuscript. However, in the latter case (i.e. the\ndolomites are replacive) there is potential to retain the precursor mineral proxy records. If the latter,\nyou could replace these clarifiers with \"(replacive and fabric-retentive)\" and “(replacive and fabric-\ndestructive)”.\nAs an aside, fabric-destructive dolomite is not always indicative of \"late diagenetic environments\"--\nnumerous examples of fabric-destructive dolomite that has been interpreted to form in early\ndiagenetic environments exist (planar-e sucrosic dolomite that does not retain the precursor fabric is\nextremely common).\nLine 131 the authors state “Dolomite samples in our data compilation include fabric-retentive\ndolomites, which are though to be associated with early dolomitization43,44.”\nI recommend replacing \"which are thought\" with \"that have been interpreted\". I suggest this change\nso the reader does not presume that you are suggesting that fabric retentive-dolomites are thought to\nbe associated with early dolomitization. Rather, the fabric-retentive dolomites in your compilation have\nbeen studied by others who subsequently interpreted that they are associated with early\ndolomitization.\nIf you do mean the former (a generalization about fabric retentive-dolomites), I will refer you to\nnumerous high-temperature dolomitization experiments in which precursor fabrics were preserved--\nwhich would somewhat refute the idea that early diagenetic environments (i.e., near-surface\ntemperatures and conditions) are required for preservation (e.g., Bullen and Sibley 1984\nhttps://doi.org/10.1130/0091-7613(1984)12>655:DSAMR<2.0.CO;2 or Zempolich and Baker 1993\nhttps://doi.org/10.1306/D4267B86-2B26-11D7-8648000102C1865D).\nLines 136-137 The authors state “Within this sample set, there are also indications of open-system\nisotope exchange of carbonate oxygen, and re-setting of clumped isotope compositions in deep burial\nenvironments39.”\nWhen I first read this sentence, I was a bit confused because this is the first instance in which you are\ninterpreting the early diagenetic fabric-retentive dolomites have been recrystallized in deep burial\nenvironments (which should apparently not be confused with fabric-destructive dolomites that also\ncrystallized in deep burial environments), but you do not clearly state as such. I would recommend\nchanging this sentence to something of the like: \"Recrystallization of the fabric-retentive dolomites\nwould be in line with studies included in our dataset, which indicate open-system isotope exchange of\ncarbonate oxygen, and re-setting of clumped isotope compositions in deep burial environments 39.\"\nLines 202-203 the authors state “Other dolomite textures, associated with precursor mineral\ndeposition at the surface (ooids, peloids, and fossil fragments) or that are fabric-destructive\n(massive), record significantly higher values of reconstructed 238U/206Pb that are expected under\nmore oxygenated depositions environments only after ~400 Ma (Fig. 3a).”\nI think that you are here specifically trying to refer to non-microbial-associated dolomites, which you\nshould explicitly say. Because what you say here \"other dolomite textures associated with precursor\nmineral deposition at the surface, or that are fabric-destructive\" means very little considering this\ndescribes nearly all textures with a broad stroke that is this sentence. I'm not sure whether you are\ntrying to pivot here to non-microbial-associated dolomite, or to replacive dolomite, but you should\nmake it clear rather than saying \"other dolomite textures\" and describing majority of the dolomite\ntextures that exist.", "reviewer_3": "This manuscript by Ben-Israel and colleagues presents new U-Pb in dolomite data, which provides new\ninsight into U-Pb geochronology in carbonates and the oxygenation history of the Earth. Although I am\nnot an expert in these methods, the data appear analytically sound and the paper is well-written and\ninformative.\nMy primary comment on the paper concerns its overall pitch towards the Earth’s oxygenation record.\nAlthough this topic is near and dear to my heart, I found the implications for U-Pb dating in dolomites\n(for instance, being able to discern the time of alteration) much more interesting, and likely the most\nuseful part of the study. I agree with the authors that their results do support late Paleozoic\noxygenation, but looking at the Figure 3, there are already a large and diverse set of datasets that\nsupport this conclusion, while the results here are relatively widely spaced in time and space. So\nwithout trying to take anything away from the study, I found the implications for oxygenation to be\ninteresting (and supportive of what I believe is the correct trajectory) but not that conclusive\ncompared to the other available datasets. I also took from statements in the paper that the main thing\nthat affects the U-Pb trend is the seawater U concentration, and if so, one could develop a much larger\nand better-coverage dataset simply by looking at [U] in carbonates. It is up to the authors, but my\npersonal feeling was that the implications for dolomite geochronology were the more exciting aspect,\nand it might be better pitched as a geochronology paper.\nNo matter how the authors want to focus their paper, I think the questions of oxygenation should be\nbolstered by simple statistical analyses and data reporting regarding the primary hypothesis that\n238U/206Pb ratios increased at 400 Ma. This should include reporting the average, median and 95%\nCI of the pre-400 and post-400 Ma interval, and the correct parametric or non-parametric statistical\ntest (based on normality) of whether the younger samples are higher. Just eyeballing the data they\nlook significantly and substantially larger but that needs to be demonstrated.\nIn the abstract and to some extent the introduction I think that, as written, the authors have set up a\nfalse dichotomy between Neoproterozoic oxygenation/animal evolution and later Paleozoic\noxygenation. I believe there are two separate questions here: did an oxygen increase influence or\n‘trigger’ the Cambrian radiation? And when did the ocean/atmosphere system become fully\noxygenated to modern levels? The critical ecological thresholds that would have been important for\nthe Cambrian radiation are likely in the 3-10% PAL O2 range (discussed in (Sperling et al., 2022,\n2015; not exhorting citation of these papers, rather explaining where my thoughts are published). A\nsustained change in atmospheric O2 from say 4% to 10% PAL would likely be difficult to detect in\nmost global datasets, especially when they are just presented in a ‘shotgun blast’ form like Fig. 3. And\nit would clearly not be the full oxygenation of the ocean-atmosphere system. But it would have had a\ndramatic effect on early animal ecosystems, including the advent of predators, and could have\ntriggered the Cambrian radiation. So, I think it is very possible that a muted rise in oxygen\ninfluenced Cambrian evolution and that fully oxygenation occurred in the Devonian. Changing the\nwriting will not require much, but I do think it is important to get this distinction right.\nI found Figure 2 to be very useful in my understanding and had a few suggestions that might help\nmake it even clearer to people (like me) from outside this system. I would note in the first line of the\nfigure caption that this is a 400 million year old sample that was altered at 200 million years ago.\nI would also drop the “~” on each of these, and just work the example such that it is 400 and 200,\nas the ‘approximate’ introduces unneeded confusion. On panel a I would also say that the deposition is at\nIs that the ‘ideal’ isochron that would result if there was no alteration or homogenization?\nCould that be clarified?\nI have some additional comments below, but otherwise I very much enjoyed reading this manuscript.\nSincerely,\nErik Sperling\nStanford University\nLine 15- Reference 7 and 10 are inappropriate here for this statement on oxygenation and\nNeoproterozoic-Cambrian animal evolution, as they are focused on the GOE.\nThe authors may have meant (Planavsky et al., 2014) Science instead of Nat. Geosci, which would be\nmore appropriate here.\nLine 18 and also line 41- (Dahl et al., 2010; Wallace et al., 2017) should be cited here with respect to\na later Paleozoic rise in atmospheric oxygen. Although they have been superseded in terms of the size\nof the datasets available, these two papers were really important in putting that idea out there.\nFigure 1- Have the authors plotted the age offset as a function of absolute age? It doesn’t really look\nlike there is a trend, which is presumably interesting in and of itself for carbonate geochron.\nLine 145—please very briefly give a little more geological detail. Presumably these are the Wasatch\nrange samples, and you should say (Utah, USA) for those not familiar with the Alta stock.\nLine 185- Modeling in (Johnson et al., 2014), which to my knowledge is the most recent on this topic,\ndemonstrates that U is remarkably sensitive to atmospheric oxygen. These results suggest U oxidation\nhas been quantitative even farther back than the Neoproterozoic, to the GOE.\nLine 190- (Lau et al., 2016) should also be cited here in addition to the Elrick paper, especially as\nthere is excellent modeling of how U concentrations relates to global anoxia in the supplement to that\npaper.\nFigure 3- Plotting the Berner GEOCARBSULF results is fine, but those are a little out of date. I would\nsuggest the updated curves from (Mills et al., 2023) as more realistic.\nLine 224- I am not too familiar with Pb redox cycling, could that be explained a little further?\nLine 241- Lu et al. is cited twice with different numbers\nLine 247-248: I think the possibility of minor or muted oxygenation is just as important, from a\nbiological standpoint, if not more so, than ‘short lived’\nLine 289- calculated\nReferences:\nDahl, T.W., Hammarlund, E.U., Anbar, A.D., Bond, D.P.G., Gill, B.C., Gordon, G.W., Knoll, A.H., Nielsen,\nA.T., Schovsbo, N.H., Canfield, D.E., 2010. Devonian rise in atmospheric oxygen correlated to the\nradiation of terrestrial plants and large predatory fish. Proceedings of the National Academy of\nSciences, U.S.A. 107, 17911–17915.\nJohnson, J.E., Gerpheide, A., Lamb, M.P., Fischer, W.W., 2014. O2 constraints from Paleoproterozoic\ndetrital pyrite and uraninite. Geological Society of America Bulletin B30949.1.\nhttps://doi.org/10.1130/B30949.1\nLau, K.V., Maher, K., Altiner, D., Kelley, B.M., Kump, L.R., Lehrmann, D.J., Silva-Tamayo, J.C., Weaver,\nK.L., Yu, M., Payne, J.L., 2016. Marine anoxia and delayed Earth system recovery after the end-\nPermian extinction. PNAS 113, 2360–2365. https://doi.org/10.1073/pnas.1515080113\nMills, B.J.W., Krause, A.J., Jarvis, I., Cramer, B.D., 2023. Evolution of Atmospheric O2 Through the\nPhanerozoic, Revisited. Annual Review of Earth and Planetary Sciences 51, 253–276.\nhttps://doi.org/10.1146/annurev-earth-032320-095425\nPlanavsky, N.J., Reinhard, C.T., Wang, X., Thomson, D., McGoldrick, P., Rainbird, R.H., Johnson, T.,\nFischer, W.W., Lyons, T.W., 2014. Low mid-Proterozoic atmospheric oxygen levels and the delayed rise\nof animals. Science 346, 635–638. https://doi.org/10.1126/science.1258410\nSperling, E.A., Boag, Thomas H., Duncan, M.I., Endriga, C.R., Marquez, J.A., Mills, D.B., Monarrez,\nP.M., Sclafani, J.A., Stockey, R.G., Payne, J.L., 2022. Breathless through time: oxygen and animals\nthrough Earth’s history. Biological Bulletin 243, 184–206.\nSperling, E.A., Knoll, A.H., Girguis, P.R., 2015. The ecological physiology of Earth’s second oxygen\nrevolution. Annual Review of Ecology, Evolution, and Systematics 46, 215–235.\nhttps://doi.org/10.1146/annurev-ecolsys-110512-135808\nWallace, M.W., Hood, A., Shuster, A., Greig, A., J. Planavsky, N., Reed, C., 2017. Oxygenation history\nof the Neoproterozoic to early Phanerozoic and the rise of land plants. Earth and Planetary Science\nLetters 466, 12–19. https://doi.org/10.1016/j.epsl.2017.02.046", "reviewer_4": "I appreciate that the authors took data that might otherwise be considered a failure and tried to do\nsomething creative with it. I like that they took a systematic approach to looking at their own data and\nalso published work from the literature. They have provided their code and others might use that to\nconsider similar problems. They have done a nice job of tying this to other studies that seek to\nconsider oxygenation of the atmosphere/ocean through time. They certainly have as much or more\ndata and are on no shakier ground that these other studies.\nI do however have a number of reservations about the thought process that went into getting to the\nplace where they think their calculated U/Pb ratio has something to do with the oxygenation of the\nocean.\nFirst, the assumption that good fabric preservation probably required early diagenetic stabilization is\nreasonable, but poor fabric preservation could be early or late, there is no way to know that. Second,\nthere appears to be an underlying assumption that these were originally dolomite and have been\nrecrystallized to a secondary dolomite. I don’t see any justification for the original dolomite. I think\nit could have been any carbonate mineral. I believe the assessment of mineralogy and diagenesis needs\nto be carefully reworded to avoid making unsubstantiated statements.\nSecond, there is an assumption that the U is in the oxidized state. Based on a number of studies on\nUXANES it is not at all clear that this is the case. For sure dolomite is almost never a primary mineral\nforming directly from seawater. That means it is buried and altered in a different environment than the\nopen ocean. We know from pore water studies across the ocean, that pore waters quickly become\nreducing. Reduced U appears to have a much higher Kd for carbonates that oxidized U.\nThird, the U/Pb ratio is only part of the story. What are the concentrations? The paper seems to imply\nthat redox has some control on Pb. This is not likely in a near surface environment. The U/Pb is not\njust a U concentration problem, it is a Pb concentration problem.\nFinally, while it is reasonable to assume that the initial Pb isotope ratio of seawater has changed in a\nsystematic way that follows the Pb growth curve, fluids that have interacted with other rocks along the\npath to dolomitization could be more radiogenic. Certainly the fluids don’t have to be homogenizing\nthe local carbonate (dolomite) as implied by the model."}, "author_response": "Point-by-point reply to reviewers’ comments (replies in green)\nReviewer #1 (Remarks to the Author):\nThe notion of long-delayed comprehensive oxygenation of the biosphere is relatively new and exciting.\nResearch has moved that landmark event/interval from the GOE to the NOE and now well into the\nPaleozoic. It is remarkable to think that overall marine oxygen deficiencies persisted across an interval\nmarked by the most significant steps in innovation and diversification of animal life. The model is gaining\nacceptance, although the evidence for that long protraction of oxygenation is still embryonic and demands\nmore work and additional approaches. For example, relationships and decouplings between shallow and\ndeep waters are critical and harder to assess.\nThe authors have taken this challenge on. The idea of looking at U-Pb relationships in carbonates for this\npurpose is absolutely novel, and, in a first-order sense, seems to line up with other\napproaches/arguments. Importantly, it does so in very independent ways that confirm but also broaden\nour understandings through mechanistic underpinnings that speak to global inventories of redox sensitive\nspecies rather being a proxy for local shallow or deep conditions. As such, I support the work very much\nand hope it will be published. But I have questions/comments:\n(1) The approach taken is complicated. The authors have worked hard to make the details accessible,\nbut they will not be easy or even possible to follow for many readers outside the field. I very much\nencourage rereads and edits with an eye toward even greater clarity. For example, unless I missed\nit, we are never given the essential overarching detail that 238U decays to stable 206Pb. I know,\nbasic stuff, but still useful to include. And this is just one example of where clarity could be\nmassaged.\nWe went over the paper and made several modifications aiming to make it more accessible to non-expert\nreaders. Edits include: 1) U nuclides decay to Pb daughters is now explained (lines 64-65); 2) Figure 2 was\nmodified for clarity; 3) We added some details on location and geological context for non-experts (lines\n154-155); 4) We added a geological time scale to figure 3; 5) we spell out ‘billions of years’ and ‘millions\nof years’ for Ga and Ma, respectively.\n(2) Those who follow this field will tell you that carbonate-hosted proxies in old rocks are the most\nvulnerable to alteration and corresponding push back from the community. This paper not only focuses\non carbonate but specifically acknowledges and incorporates the effects/signatures of primary signal\nresetting in the arguments. These relationships are addressed through extensive conversations about the\nhow, when, and the significance of age resetting and specifically ingrowth of 206Pb through decay\nfollowing resetting that scales with 238U availability in the sample. With the added ability to look at the\ndifferences between stratigraphic ages and U-Pb dates, we can in theory back out initial U inventory (U/Pb\nfor seawater specifically) in ways that speak directly to the global ocean redox state. I think I get all this\n(maybe not), and we are given many discussions about Pb exchange and potentially confounding factors\ntied to varying primary carbonate mineralogies, dolomite fabrics/generations, etc. But I am still missing,\nand probably this is my fault, why fundamentally the U inventory is also not reset during alteration, making\nit very hard to solve for the initial. What am I missing? We are told about resetting of the U/Pb system,\nbut I am struggling to see how that overprint is resolved for U too. Too many variables? Much of the\ndiscussion focuses on Pb controls. Does the approach allow for resolvable loss and addition of both\nisotopes? What about diagenesis (recrystallization) in pore waters selectively striped of U? Again, there\nare a lot of pieces to this story, and we are taken on a journey through multiple alternative hypothesis\nfollowed by arguments that challenge those. This is good. I suppose what makes me most excited about\nthis paper is that while the details do make the arguments complicated and even suspicious, they do not\nmake the first order U/Pb predictions in Fig. 3 unbelievable. In other words, I see the scope for mess, but\nI also see a first-order jump well into the Paleozoic that generally jibes with the other data types\nsummarized in that figure. Again, it is this consistency that is tantalizing and elegant because of the very\ndifferent approach taken for these rocks. Although I would like to hear more about what specifically this\nmethod brings to the table relative to other data types.\nThe U reservoir in our model is free to reset in open or closed system reactions during the alteration event\nin which age is reset. Considering the high mobility of this element – it is likely altered in the process.\nThis is stated in lines 125-128: “In this process, U concentrations could also have been re-distributed and partly\nhomogenized in either open or closed system conditions, but some U/Pb heterogeneity was maintained\nallowing U-Pb Discordia to be defined”. Importantly this does not interfere with the interpretation of\ninitial 238/206 in the dolomite precursor, as this specific ‘memory’ is carried by two values in the dolomite:\n1) the difference between the measured initial 207/206 and the expected initial at the time of deposition\n(Based on Stacey and Kremers, 1975 model); and 2) the time gap between depositional and U-Pb age. The\nU-Pb system is assumed to remain closed after modification. This assumption is supported by the fact that\nwe obtain an isochron – that is, that lower 207/206 values are supported by higher 238/206 – meaning\nthat U has decayed to lead and both parent and daughter remained in a closed system.\nRegarding the possibility of U (and Pb) ratio altering in sediment/pore water profiles – this is certainly an\noption that is likely responsible to much of the scatter we see among samples from the same rock units.\nIn the revised paper, we now acknowledge that U/Pb signal may be recorded in sediment under less-oxic\nconditions then the water column (lines 209-232, 246, 252-254). This issue has been mentioned in the\nprevious version with respect to some samples. In the revised paper we now broaden the argument to\ninclude all samples – with the point that, regardless of the specific crystallization site (sediment or surface)\nsuggested by dolomite texture, we observe high 238U/206Pb values only after 400 Ma – suggesting a rise in\nthe levels of oxygen of marine surface and sediments at that time.\n(3) At the end of the day, I think this is an argument about an increased U inventory in the ocean in the\nlate Paleozoic. Fair enough. But I was very surprised by the absence of any mention of the Partin et al.\n(2013, EPSL) paper, which convincingly shows a U inventory increase recorded in black shales much\nearlier—in the latest Proterozoic. Something needs to be said about the apparent disagreement of these\ndata types and what it might mean.\nGood points, thanks you for directing us to this paper. Different patterns of redox-sensitive elements,\nincluding the step-wise increase during the Neoproterozoic observed by Partin et al. (2013) are briefly\ndiscussed in lines 255-270 – we made sure to include a reference to this paper in the revised version. One\npossible reason for the differences between the black shale and carbonate records is that these records\ncapture different marine environments, which were possibly oxidized at different times. Specifically, Lu\net al. (2018) suggested that shallow remineralization depth of organic material has kept marine shelves\noxygen-poor well into the Phanerozoic, while deeper marine basins have been relatively oxygenated\n(Figure 2b in their paper). Another possibility is that rock records have been affected by post depositional\nalteration – e.g., recent oxidative weathering (Albut et al., 2018; Planavsky et al., 2020). These ideas were\nimplied in the previous version of the paper and are now made explicit on lines 259-262: “These\ndifferences may reflect the complex structure of the early Paleozoic oxycline, in which shallow\nremineralization of organic material reduced continental shelves and left deeper basins relatively\noxidized (Lu et al., 2018), or possibly different sensitivities of elements and the records that contain\nthem to post-depositional alteration (Planavsky et al., 2020).”\n(4) Line 47, the authors seem content to put a cloud of doubt over a massive amount of past data by\nsimply citing the Slotznick et al. challenge to a much early hypothesized whiff of oxygen. That challenge is\nnot very substantial (see recent Anbar et al. rebuttal), and to suggest that this (the only cited paper)\nthrows into doubt so much other work is just silly. In other words, the authors might work to more\nconvincingly make a case for why their very complex approach applied to dolomite is better or at least\nneeded—as in providing additional or different constraints, etc. Ironically, the most convincing part of\ntheir story may be that it agrees with other data types. (I noted the absence the Dahl et al. Mo isotope\npaper that many years ago set the stage for protracted oxygenation arguments.)\nThe possibility that redox proxies are altered by post depositional processes is certainly not limited to\nSlotznick et al. or the Archean “whiffs” of oxygen. Probably the most suitable example is the recent paper\nby Planavsky et al. (2020), in which post depositional alteration of redox signal, and specifically secondary\noxidative weathering are considered (and demonstrated) to have affected Neoproterozoic and Early\nPaleozoic records. We are not saying that such alteration is necessarily the case, but that it should be\nconsidered as one way to explain the differences between records. In the introduction, we simply point\nout that this issue has been a major source for concern in the community. We agree with the reviewer\nthat the conflicting schedules of Neoproterozoic-Paleozoic oxygenation warrant new approaches to this\nproblem – and our approach is certainly a new one – but think that its greatest strength and source for\njustification is that it “…is based on the observed initial 207Pb/206Pb ratio, which is relatively impervious\nto alteration after dolomite (re)crystallization, and specifically, to false-positive oxygenation signals\n(higher 238U/206Pb) recorded as samples were approaching the surface during exhumation.”. We made\nsure to refer to the paper of Dahl et al. (2010).\n(5) In my view, the authors way overstep the implications of their results. First, the U-Pb data show lots\nof scatter, wide error bars, and poor temporal coverage. There is no way the hypothesized late Paleozoic\nstep, particularly as recorded in their data, can be used to infer that most of the major innovation in the\nhistory of animal life (the GOBE, infaunal revolution, predation and largeness, complexly layered\necologies, skeletonization, motility, etc.) happened under persistently very low oxygen levels in surface\noceans. And even if they did, the U-Pb data say very little in this regard. The idea of persistently very low\noxygen surface waters is hard to grasp and is out of phases with recent redox studies for the Ordovician\ncoming out of the Florida State group, etc. (not cited in this paper).\nWe agree with the reviewer that our approach has several notable disadvantages (low temporal\nresolution, large error bars etc.). However, with these caveats, it can still constrain an oxidation schedule\nthat is consistent with some higher-resolution records, and inconsistent with others – namely,\nNeoproterozoic step-wise major oxygenation of marine environments. Considering the main advantage\nof this approach (impervious to ‘false-positive’ oxygenation signals), this results lends credibility to the\nformer, and thereby support hypotheses suggesting the persistent oxygenation of marine environments\nhappened only after the Early Paleozoic (which is different from saying that anoxia was persistent before\nthe Early Paleozoic). We revise the text to make this point clearer: “…In this context, the fact that\nreconstructed 238U/206Pb ratios, which are resistant to the effects of ‘false positive oxygenation’,\ndo not appear to record oxygenation signals during the Neoproterozoic and Cambrian within multiple\nsamples from different formations and different paleoenvironments, at a first order, supports the model\nthat persistent oxygenation of shallow and deep marine environments was delayed until the Devonian (Fig.\n3).”\nA bigger question is the relationship between the deep and shallow ocean that best fits all the\nobservations, including the certain role of oxygen in driving early-mid Paleozoic evolution. The U-Pb data\nsimply cannot speak to long-term persistence and prevalence of low oxygen in Paleozoic surface waters\nor its variation on shorter time scales. The required resolution for temporal, spatial, and persistence\npatterns is missing. Shorter-term variation could be huge part of the story but would go undetected in\nthis approach. If I am not mistaken, the U-Pb data, like many other approaches, capture whole ocean\nrelationships that can be controlled by evolution of the deep ocean with poorly understood relationships\nto the surface ocean where biological innovation took place.\nWe agree with the reviewer. Considering the limitations of the U-Pb and other records, the argument we\nnow make is more nuanced than ‘major innovation in the evolution of animal life have occurred under\nvery low oxygen levels’. As mentioned above, we do not argue for persistently low oxygen levels before\n“This scenario does not necessarily preclude the possibilities of minor (<10 % or the modern oxic ocean),\nlocal, and/or episodic (1-10 Ma) oxygenation events during the NOE, which did not register in the\ncompiled 238U/206Pb and Ce/Ce* records shown here, but could have had critical ecological impacts vital\nfor the Cambrian radiation (Sperling et al., 2015)”. Animal life could have evolved in response to elevated\noxygen levels in these episodes or environments, or possibly under minor rise in oxygen that remained\n‘muted’ in some of the redox records.\nWe agree that the last sentence in the abstract of the previous version was over-simplifying our\ninterpretations. We have changed this sentence to say: “…This timeline is consistent with evolution-\ndriven mechanisms for the oxygenation of late Paleozoic marine environments and with suggestions\nthat Neoproterozoic and early Paleozoic animals thrived in oceans that overall and on long time scales\nwere oxygen-limited compared to the modern ocean.” We also revised the discussion to accommodate\nthe range of possible scenarios that explain the lack of oxygenation signal prior to 400 Ma: “(our data,\nconsidered together with consistent records) … It is also consistent with previous suggestions that\nseveral major evolutionary and ecological steps in the diversification of animal life took place within a\nlargely oxygen-limited ocean, in which O concentrations were sufficient to overcome functional and\n2\necological thresholds, but insufficient to register in a series of redox proxies, possibly since they were\nminor (<10%) or existed temporary and/or locally relative to the modern oxic ocean.”\n(6) The error bars in Fig. 1 are huge. And while I see the general relationships suggested by the authors,\nsuch as the tendency for data to fall below the 1:1 line with dates significantly younger than the strat.\nage, the plot suggests that the results are at best a first-order indication of very general relationships.\nThese errors/ranges may not be a problem, but they deserve a comment or two.\nThe uncertainties on several U-Pb ages are indeed significant. This is typical of dolomite samples that\nnaturally have low uranium content and an overall small range of 238/206. This is not a problem in the\nanalysis or interpretation in which the signal is well above the statistical noise (Fig. 3a, and new data\nrepository figure S1). As suggested, we commented on the large uncertainties in this figure caption. “Large\nuncertainties in U-Pb dates are common to carbonate samples that typically suffer from low ranges of\n238U/206Pb.”\n(7) I would include geologic ages and well as absolute ages on the X-axis of Fig. 3.\nAdding the geological timescale is a great idea – done. Each measured dolomite point has both\nstratigraphic and U-Pb age that are separated by 10-100’s of Ma. I’m afraid that Including both (say if we\ndefine the x axis as stratigraphic or U-Pb age and connect the dots with a line) will make this panel and\nthe figure too busy. Mostly, we’re afraid that it will cause confusion, since the reconstructed 238/206\nvalues are attributed to the dolomite precursor mineral forming at or near the stratigraphic age – not the\ntime of alteration.\nAgain, because of the cleverness of the approach and its complementarity to other data types in critical\ndiscussions about delayed biospheric oxygenation, I support eventual publication. But I would hope for a\ndialed-back pitch about the implications of the results with more emphasis and balance placed instead\non the promise of the approach and the questions still remaining. In truth, if it weren’t for the general\nagree with the conceptual models previously informed by the other data types in Fig. 3, we might be\nsaying that altered dolomite with reset U-Pb chemistry is not a good target. But I do applaud the\ncreativity and boldness of the approach and don’t disagree with the overarching conclusions.\nTim Lyons UC Riverside\nReviewer #2 (Remarks to the Author):\nReview of Late Paleozoic Oxygenation of Marine Environments Supported by Dolomite U-Pb Dating\nIn this paper, the authors present a dataset that captures U-Pb dates of dolomites of various ages and\nvarious precursor sediment ages, in order to reconstruct their initial 207Pb/206Pb ratios. The data\nsuggest that the ages of the dolomite are younger than expected and the 207Pb/206Pb ratios are lower\nthan expected, which is interpreted to reflect resetting of the U-Pb system coinciding with U decay,\nallowing for reconstruction of initial 238U/206Pb ratios of the rock. The study finds that 238U/206Pb\nratios did not dramatically increase until the late Paleozoic, which they attribute to oxygenation of\nseawater, and ultimately providing evidence in support of the hypothesis that early animal life evolved\nwithin an oxygen-limited ocean until the late Paleozoic (a hotly debated topic).\nI find this study interesting and of broad interest to the scientific community—appropriate for a Nature\nCommunications publication. However, I have some comments and suggestions that the authors might\ntake into consideration prior to acceptance for publication. I will be upfront and admit that I am no\nexpert on the U-Pb system and especially as it applies to carbonates (which appears to be a relatively\nnewer but intriguing development for the carbonate discipline). Rather, my review focuses on the\naspects of dolomitization, dolomite textures, and dolomite recrystallization—for which my background\nis rooted in. I hope the authors find my comments useful to help improve their manuscript, I would be\nhappy to provide any further clarification.\nMajor Comments:\nLine 74 the authors state “Using in-situ laser ablation inductively coupled plasma mass spectrometry, U\nand Pb isotope measurements were made on a variety of dolomite mineral fabrics, associated with\ndepositions and early diagenetic environments (fabric-retentive cements) or with late diagenetic\nenvironments (fabric-destructive cements), while avoiding void-filling epigenetic dolomite (e.g., veins).”\nHowever, my question is: Are they actually cements? Or are they replacement products? An important\ndistinction, since in the former case they would NOT be inheriting precursor sediment geochemistry\n(they would be reflecting the fluids responsible for cementation), and would not accurately capture the\ninterpretations that underpin the importance of this manuscript. However, in the latter case (i.e. the\ndolomites are replacive) there is potential to retain the precursor mineral proxy records. If the latter,\nyou could replace these clarifiers with \"(replacive and fabric-retentive)\" and “(replacive and fabric-\ndestructive)”.\nAs an aside, fabric-destructive dolomite is not always indicative of \"late diagenetic environments\"--\nnumerous examples of fabric-destructive dolomite that has been interpreted to form in early diagenetic\nenvironments exist (planar-e sucrosic dolomite that does not retain the precursor fabric is extremely\ncommon).\nGood point – The term ‘cement’ was used incorrectly in the previous version. Following the\nnomenclature in Sibely and Gregg (1987) all analyzed fabrics are now defined as replaced matrix or\nallochems components. We made sure to correct that. We tweaked the sentence to account for the\npossibility of fabric destructive dolomites have been forming at in early or later diagenetic\nenvironments. It now reads:\n“Using in-situ laser ablation inductively coupled plasma mass spectrometry, U and Pb isotope\nmeasurements were made on a variety of replacive dolomite mineral fabrics, associated with\ndepositional and early diagenetic environments (fabric-retentive) or possibly with late diagenetic\nenvironments (fabric-destructive), while avoiding void-filling epigenetic dolomite (e.g., veins).”\nHaving said that, as mentioned above our analyses brings into account the possibility of dolomite\nprecursory minerals forming from pore water at the subsurface – as they might do in a cement. It is\npossible that samples in our dataset have been affected by shallow diagenetic redox to various extents.\nThis is what can explain the scatter we see within and among dolomites associated with different\nformation environments and processes (massive vs. surface vs. microbial textures). But, this scatter is\nminor when compared with the signal of post 400 Ma oxygenation, that is apparent within each of these\ngroups.\nLine 131 the authors state “Dolomite samples in our data compilation include fabric-retentive\ndolomites, which are thought to be associated with early dolomitization43,44.”\nI recommend replacing \"which are thought\" with \"that have been interpreted\". I suggest this change so\nthe reader does not presume that you are suggesting that fabric retentive-dolomites are thought to be\nassociated with early dolomitization. Rather, the fabric-retentive dolomites in your compilation have\nbeen studied by others who subsequently interpreted that they are associated with early dolomitization.\nIf you do mean the former (a generalization about fabric retentive-dolomites), I will refer you to\nnumerous high-temperature dolomitization experiments in which precursor fabrics were preserved--\nwhich would somewhat refute the idea that early diagenetic environments (i.e., near-surface\ntemperatures and conditions) are required for preservation (e.g., Bullen and Sibley\n1984 https://doi.org/10.1130/0091-7613(1984)12>655:DSAMR<2.0.CO;2 or Zempolich and Baker 1993\nhttps://doi.org/10.1306/D4267B86-2B26-11D7-8648000102C1865D).\nGood catch – sentence now reads: “Dolomite samples in our data compilation include fabric-retentive\ndolomites that have been interpreted as products of early dolomitization (Corsetti et al., 2006; Nelson\net al., 2021)”.\nLines 136-137 The authors state “Within this sample set, there are also indications of open-system\nisotope exchange of carbonate oxygen, and re-setting of clumped isotope compositions in deep burial\nenvironments39.”\nWhen I first read this sentence, I was a bit confused because this is the first instance in which you are\ninterpreting the early diagenetic fabric-retentive dolomites have been recrystallized in deep burial\nenvironments (which should apparently not be confused with fabric-destructive dolomites that also\ncrystallized in deep burial environments), but you do not clearly state as such. I would recommend\nchanging this sentence to something of the like: \"Recrystallization of the fabric-retentive dolomites\nwould be in line with studies included in our dataset, which indicate open-system isotope exchange of\ncarbonate oxygen, and re-setting of clumped isotope compositions in deep burial environments 39.\"\nDone – with a minor tweak to say that isotope evidence for late crystallization come from both fabric\nretentive and destructive dolomites.\nLines 202-203 the authors state “Other dolomite textures, associated with precursor mineral\ndeposition at the surface (ooids, peloids, and fossil fragments) or that are fabric-destructive\n(massive), record significantly higher values of reconstructed 238U/206Pb that are expected under\nmore oxygenated depositions environments only after ~400 Ma (Fig. 3a).”\nI think that you are here specifically trying to refer to non-microbial-associated dolomites, which you\nshould explicitly say. Because what you say here \"other dolomite textures associated with precursor\nmineral deposition at the surface, or that are fabric-destructive\" means very little considering this\ndescribes nearly all textures with a broad stroke that is this sentence. I'm not sure whether you are\ntrying to pivot here to non-microbial-associated dolomite, or to replacive dolomite, but you should\nmake it clear rather than saying \"other dolomite textures\" and describing majority of the dolomite\ntextures that exist.\n‘Non microbial-associated dolomites’ is indeed a better way of putting it. We changed the text as\nsuggested.\nReviewer #3 (Remarks to the Author):\nThis manuscript by Ben-Israel and colleagues presents new U-Pb in dolomite data, which provides new\ninsight into U-Pb geochronology in carbonates and the oxygenation history of the Earth. Although I am\nnot an expert in these methods, the data appear analytically sound and the paper is well-written and\ninformative.\nMy primary comment on the paper concerns its overall pitch towards the Earth’s oxygenation record.\nAlthough this topic is near and dear to my heart, I found the implications for U-Pb dating in dolomites\n(for instance, being able to discern the time of alteration) much more interesting, and likely the most\nuseful part of the study. I agree with the authors that their results do support late Paleozoic\noxygenation, but looking at the Figure 3, there are already a large and diverse set of datasets that\nsupport this conclusion, while the results here are relatively widely spaced in time and space. So without\ntrying to take anything away from the study, I found the implications for oxygenation to be interesting\n(and supportive of what I believe is the correct trajectory) but not that conclusive compared to the other\navailable datasets. I also took from statements in the paper that the main thing that affects the U-Pb\ntrend is the seawater U concentration, and if so, one could develop a much larger and better-coverage\ndataset simply by looking at [U] in carbonates. It is up to the authors, but my personal feeling was that\nthe implications for dolomite geochronology were the more exciting aspect, and it might be better\npitched as a geochronology paper.\nSee response to point #2 and 4 of reviewer #1. The ability to determine the timing of alteration of the U-\nPb system in carbonates has already been demonstrated before (Gasparrini et al., 2023; Mangenot et\nal., 2018). In the revised version, we now highlight the strength of our approach (being resistant to ‘false\npositive’ oxygenation signals’) and its contribution to our understanding of other proxy records and the\noverall history of marine oxygenation (lines 16-18, 209-212, 262-267); we also added a short paragraph\nat the very end of the paper – in which suggest that the approach developed here is likely to work just as\nwell in other authigenic mineral records (lines 171-176). Previous studies have looked at the\nconcentration of [U] in carbonates directly, and report an increase around the NOE (See compilation in\nChen et al., 2021). The different patterns may reflect some difference in formation environments\nbetween the set of dolomite samples we studied and the limestone included in the compilation of Chen\net al. (2021) – or possibly post depositional alteration of U in the limestone samples. To have a better\nidea, we will need to measure U-Pb dates and [U] concentrations on the same samples. Alteration of U\nin carbonates is a likely scenario. For example, diagenetically altered carbonates have very different [U]\nand [Pb] concentrations and overall lower U/Pb compared with primary biogenic carbonates (Roberts et\nal., 2020).\nNo matter how the authors want to focus their paper, I think the questions of oxygenation should be\nbolstered by simple statistical analyses and data reporting regarding the primary hypothesis that\n238U/206Pb ratios increased at 400 Ma. This should include reporting the average, median and 95% CI\nof the pre-400 and post-400 Ma interval, and the correct parametric or non-parametric statistical\ntest (based on normality) of whether the younger samples are higher. Just eyeballing the data they look\nsignificantly and substantially larger but that needs to be demonstrated.\nPost and pre 400 Ma samples have significantly different mean and median values. To demonstrate this,\nwe added a box plot as a supplementary material figure S1– and included relevant statistics parameters\nand test results in the caption (see below)\nFigure S1 – boxplot summarizing reconstructed 238U/206Pb values in Post- and Pre-400 Ma samples. A) full\nrange of values. B) focusing on values up to 50. Samples with stratigraphic ages <400 Ma average 34.9 and\nhave median of 12.2 with 95% confidence interval of [16.4, 7.9]. Samples with stratigraphic ages >400 Ma\naverage 4.8 have median of 3.4 with 95% confidence interval of [5.0, 1.7]. Mean values of post and pre\nI think that, as written, the authors have set up a false dichotomy between Neoproterozoic\noxygenation/animal evolution and later Paleozoic oxygenation. I believe there are two separate\nquestions here: did an oxygen increase influence or ‘trigger’ the Cambrian radiation? And when did the\nocean/atmosphere system become fully oxygenated to modern levels? The critical ecological thresholds\nthat would have been important for the Cambrian radiation are likely in the 3-10% PAL O2 range\n(discussed in (Sperling et al., 2022, 2015; not exhorting citation of these papers, rather explaining where\nmy thoughts are published). A sustained change in atmospheric O2 from say 4% to 10% PAL would likely\nbe difficult to detect in most global datasets, especially when they are just presented in a ‘shotgun blast’\nform like Fig. 3. And it would clearly not be the full oxygenation of the ocean-atmosphere system. But it\nwould have had a dramatic effect on early animal ecosystems, including the advent of predators, and\ncould have triggered the Cambrian radiation. So, I think it is very possible that a muted rise in oxygen\ninfluenced Cambrian evolution and that fully oxygenation occurred in the Devonian. Changing the\nwriting will not require much, but I do think it is important to get this distinction right.\nWe agree, and revised the introduction (line 38-44) and discussion (lines 267-271, 278-283) to make the\ndistinction clear.\nI found Figure 2 to be very useful in my understanding and had a few suggestions that might help make\nit even clearer to people (like me) from outside this system. I would note in the first line of the figure\ncaption that this is a 400 million year old sample that was altered at 200 million years ago. I would also\ndrop the “~” on each of these, and just work the example such that it is 400 and 200, as the\n‘approximate’ introduces unneeded confusion. On panel a I would also say that the deposition is at 400\nMa. Finally, I was a little confused about the “isochron from ~400 Ma after deposition” in panel c. Is that\nthe ‘ideal’ isochron that would result if there was no alteration or homogenization? Could that be\nclarified?\nWe corrected figure 2 and its caption following the suggestions above.\nI have some additional comments below, but otherwise I very much enjoyed reading this manuscript.\nSincerely,\nErik Sperling\nStanford University\nLine 15- Reference 7 and 10 are inappropriate here for this statement on oxygenation and\nNeoproterozoic-Cambrian animal evolution, as they are focused on the GOE.\nThe authors may have meant (Planavsky et al., 2014) Science instead of Nat. Geosci, which would be\nmore appropriate here.\nGood catch – we indeed meant the science paper. Catling and Clair’s paper removed as suggested.\nLine 18 and also line 41- (Dahl et al., 2010; Wallace et al., 2017) should be cited here with respect to a\nlater Paleozoic rise in atmospheric oxygen. Although they have been superseded in terms of the size of\nthe datasets available, these two papers were really important in putting that idea out there.\nDone.\nFigure 1- Have the authors plotted the age offset as a function of absolute age? It doesn’t really look like\nthere is a trend, which is presumably interesting in and of itself for carbonate geochron.\nGood idea – It didn’t occur to us earlier – below is the plot, of course that with older stratigraphic age\nlarger offsets are possible (the limit is marked by the 1:1 line). Otherwise there is no apparent trend.\nIt would take many more data points and extending the range of dated dolomites to explore this\nquantitatively.\nLine 145—please very briefly give a little more geological detail. Presumably these are the Wasatch\nrange samples, and you should say (Utah, USA) for those not familiar with the Alta stock.\nWe added ‘In dolomite samples’ and “(Utah, USA)” as suggested.\nLine 185- Modeling in (Johnson et al., 2014), which to my knowledge is the most recent on this topic,\ndemonstrates that U is remarkably sensitive to atmospheric oxygen. These results suggest U oxidation\nhas been quantitative even farther back than the Neoproterozoic, to the GOE.\nGood point, we revised the ending of this sentence so that: “…It is not likely that such a change is the\nresult of variation in the supply of U and Pb to the ocean, as oxidative continental weathering of these\nelements has been dominant since the early Proterozoic (Holland, 1984; Johnson et al., 2014)”\nLine 190- (Lau et al., 2016) should also be cited here in addition to the Elrick paper, especially as there is\nexcellent modeling of how U concentrations relates to global anoxia in the supplement to that paper.\nReference to Lau et al. (2016) was added as suggested.\nFigure 3- Plotting the Berner GEOCARBSULF results is fine, but those are a little out of date. I would\nsuggest the updated curves from (Mills et al., 2023) as more realistic.\nWe actually switched from GEOCARBSULF to Mill’s (2023) trend just before submission and forgot to\nupdate the caption. It is updated now. Thank you for noticing.\nLine 224- I am not too familiar with Pb redox cycling, could that be explained a little further?\nIn oxygenated environments, dissolved Pb is scavenged by Fe-Oxyhydroxides and removed from solution\n– for example see paper by (Kalnejais et al., 2015). We now make a note of it in lines 203-207.\nLine 241- Lu et al. is cited twice with different numbers\nGood catch – we got rid of one of the doppelgangers.\nLine 247-248: I think the possibility of minor or muted oxygenation is just as important, from a\nbiological standpoint, if not more so, than ‘short lived’\nWe agree. We rephrased the sentence to: “This scenario does not necessarily preclude the possibilities\nof minor (<10 % or the modern oxic ocean), local, and/or episodic (1-10 Ma) oxygenation events\nduring the NOE, which did not register in the compiled 238U/206Pb and Ce/Ce* records shown here, but\ncould have had critical ecological impacts vital for the Cambrian radiation (Sperling et al., 2015)”. We\nbelieve that “muted” is covered by the part of the sentence saying “which did not register in…“\nLine 289- calculated\nCorrected as suggested by the reviewer.\nReferences:\nDahl, T.W., Hammarlund, E.U., Anbar, A.D., Bond, D.P.G., Gill, B.C., Gordon, G.W., Knoll, A.H., Nielsen,\nA.T., Schovsbo, N.H., Canfield, D.E., 2010. Devonian rise in atmospheric oxygen correlated to the\nradiation of terrestrial plants and large predatory fish. Proceedings of the National Academy of Sciences,\nU.S.A. 107, 17911–17915.\nJohnson, J.E., Gerpheide, A., Lamb, M.P., Fischer, W.W., 2014. O2 constraints from Paleoproterozoic\ndetrital pyrite and uraninite. Geological Society of America Bulletin\nB30949.1. https://doi.org/10.1130/B30949.1\nLau, K.V., Maher, K., Altiner, D., Kelley, B.M., Kump, L.R., Lehrmann, D.J., Silva-Tamayo, J.C., Weaver,\nK.L., Yu, M., Payne, J.L., 2016. Marine anoxia and delayed Earth system recovery after the end-Permian\nextinction. PNAS 113, 2360–2365. https://doi.org/10.1073/pnas.1515080113\nMills, B.J.W., Krause, A.J., Jarvis, I., Cramer, B.D., 2023. Evolution of Atmospheric O2 Through the\nPhanerozoic, Revisited. Annual Review of Earth and Planetary Sciences 51, 253–\n276. https://doi.org/10.1146/annurev-earth-032320-095425\nPlanavsky, N.J., Reinhard, C.T., Wang, X., Thomson, D., McGoldrick, P., Rainbird, R.H., Johnson, T.,\nFischer, W.W., Lyons, T.W., 2014. Low mid-Proterozoic atmospheric oxygen levels and the delayed rise\nof animals. Science 346, 635–638. https://doi.org/10.1126/science.1258410\nSperling, E.A., Boag, Thomas H., Duncan, M.I., Endriga, C.R., Marquez, J.A., Mills, D.B., Monarrez, P.M.,\nSclafani, J.A., Stockey, R.G., Payne, J.L., 2022. Breathless through time: oxygen and animals through\nEarth’s history. Biological Bulletin 243, 184–206.\nSperling, E.A., Knoll, A.H., Girguis, P.R., 2015. The ecological physiology of Earth’s second oxygen\nrevolution. Annual Review of Ecology, Evolution, and Systematics 46, 215–\n235. https://doi.org/10.1146/annurev-ecolsys-110512-135808\nWallace, M.W., Hood, A., Shuster, A., Greig, A., J. Planavsky, N., Reed, C., 2017. Oxygenation history of\nthe Neoproterozoic to early Phanerozoic and the rise of land plants. Earth and Planetary Science Letters\n466, 12–19. https://doi.org/10.1016/j.epsl.2017.02.046\nReviewer #4 (Remarks to the Author):\nI appreciate that the authors took data that might otherwise be considered a failure and tried to do\nsomething creative with it. I like that they took a systematic approach to looking at their own data and\nalso published work from the literature. They have provided their code and others might use that to\nconsider similar problems. They have done a nice job of tying this to other studies that seek to consider\noxygenation of the atmosphere/ocean through time. They certainly have as much or more data and are\non no shakier ground that these other studies.\nI do however have a number of reservations about the thought process that went into getting to the\nplace where they think their calculated U/Pb ratio has something to do with the oxygenation of the\nocean.\nFirst, the assumption that good fabric preservation probably required early diagenetic stabilization is\nreasonable, but poor fabric preservation could be early or late, there is no way to know that.\nGood point. We now acknowledge the possibility of fabric destructive dolomites have been forming at in\nearly or later diagenetic environments (line 79).\nSecond, there appears to be an underlying assumption that these were originally dolomite and have\nbeen recrystallized to a secondary dolomite. I don’t see any justification for the original dolomite. I think\nit could have been any carbonate mineral. I believe the assessment of mineralogy and diagenesis needs\nto be carefully reworded to avoid making unsubstantiated statements.\nThis is the opposite of the assumption we make – the samples are considered to modify through\ndolomitization (e.g., from Ca-carbonate mineral) or dolomite recrystallization (from protodolomite) (Line\n109-112). Later in the paper (lines 138-146), dolomite crystallization is preferred over dolomitization to\nexplain the seeming contradiction between early dolomitization (suggested in previous publications) and\nlarge gaps between U-Pb and depositional ages. Our model will work equally well if the U-Pb system is\nreset during deep dolomitization rather than recrystallization. Further research and larger data-sets are\nrequired to identify specific alteration pathways and associate them with geochemical signal-alteration\ntrends. In the revised discussion, we made sure to broaden the argument regarding the possibility of\ncrystallization of dolomite-precursor minerals in sediments under conditions less-oxic than overlying\nseawater (lines 212-223, 246, 252-254).\nSecond, there is an assumption that the U is in the oxidized state. Based on a number of studies on\nUXANES it is not at all clear that this is the case. For sure dolomite is almost never a primary mineral\nforming directly from seawater. That means it is buried and altered in a different environment than the\nopen ocean. We know from pore water studies across the ocean, that pore waters quickly become\nreducing. Reduced U appears to have a much higher Kd for carbonates that oxidized U.\nIndeed, recent experiments and observations, support that U4+ is preferably incorporated to calcite\nminerals in hydrothermal and near surface environments (Gabitov et al., 2021; Gulbranson et al., 2022).\nHowever, being particle reactive U4+ tends to be quickly immobilized in environments <100 °C (Gabitov\net al., 2021) – limiting the supply of U to the crystallization site in oxygen-limited environments. In the\ncontext of our study, we argue that the supply of U6+ to the crystallization site (in which it may be\nreduced to U4+ before incorporation to calcite) impose a first order control over the observed variation\nin reconstructed 238U/206Pb values - rather than preferential uptake of U4+ to calcite crystal lattice. This\nis for three reasons:\nFirst, the concentrations of U in carbonates is reported to decrease during global marine anoxia events\n(Elrick et al., 2017; Lau et al., 2016) – These events have been associated with the reduction of U6+ to U4+\nin seawater – based on observed decrease in the δ238U values (Lau et al., 2016). If the concentration of\nuranium in carbonate was set by the U partition factor, then we would expect to see more uranium in\ncalcite at the same intervals. The fact that we don’t, suggests that the supply of dissolved U6+ from\nseawater to the carbonate crystallization site is controlling U concentration in carbonate minerals (lines\n207-208).\nSecond, dolomite samples in our dataset preserve primary (laminar, microbial) textures which have\nbeen associated with crystallization of dolomite precursor minerals under reducing pore water\nenvironments. If the U/Pb ratio in the precursor mineral was controlled by preferential partition of U to\ncalcite, then we would expect to see the highest 238/206 ratios in these samples (as U4+ is preferentially\nincorporated) when compared with dolomite fabrics associated with surface deposits (fossils, ooids\netc.). However, we observe the exact opposite, as such fabrics appear to record relatively low 238/206\nin the Jurassic (fig. 3A in the manuscript) which is consistent with the supply of U and possibly Pb from\nseawater being the determining factor (lines 216-226).\nAnd third, the overall temporal trend we observe is of increasing 238/206 during the Phanerozoic. If\nreconstructed 238/206 ratio in the dolomite precursor mineral was dictated by preferential\nincorporation of U4+ under reducing conditions, then we would expect the reverse trend.\nThird, the U/Pb ratio is only part of the story. What are the concentrations? The paper seems to imply\nthat redox has some control on Pb. This is not likely in a near surface environment. The U/Pb is not just a\nU concentration problem, it is a Pb concentration problem.\nOur approach can only reconstruct the 238U/206Pb ratio (which is proportional to the U/Pb ratio) – not\nabsolute U or Pb concentrations. We now briefly refer to Pb redox sensitivity (see reply to reviewer #3).\nAnd yes, as mentioned above we agree that the U/Pb record reflect carbonate crystallization\nenvironments which can be (and are even likely to be) pore water. While the incorporation of Pb to\ncarbonate mineral lattices is yet another under-constrained question, the fact that U/Pb ratio in\ndolomites associated (by texture) with reducing environments register relatively low reconstructed\n238U/206Pb values, and that the 238/206 signal rise, rather than falls between the Proterozoic and\nPhanerozoic confirms that this ratio is redox sensitive. Throughout the paper we broaden the argument\nregarding formation under reducing pore water environments to explicitly include the possibility that\nthe oxygenation in shallow diagenetic environments may have driven this signal.\nFinally, while it is reasonable to assume that the initial Pb isotope ratio of seawater has changed in a\nsystematic way that follows the Pb growth curve, fluids that have interacted with other rocks along the\npath to dolomitization could be more radiogenic. Certainly the fluids don’t have to be homogenizing the\nlocal carbonate (dolomite) as implied by the model.\nWhen considering single result, we cannot rule out this possibility. But since the terrestrial evolution\ntrend reflects an average of the crust, we would expect to see values both above and below the trend if\nthat was the case. The fact that the initial values are always offset towards lower 207/206 suggests that\nthe source naturally has higher U/Pb compared with the terrestrial average – e.g., it is likely to be\nsiliciclastic or igneous which dominate the upper crust. We briefly refer to this source as ‘river or deep\nseated groundwater’ in lines (118-119).\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nI have carefully read through the response note and looked at the associated edits in the paper. There\nare four sets of thorough and constructive reviews, which address a wide diversity of comments and\npotential improvements. The authors have treated each with care, and I believe a better paper has\nresulted. I could quibble over some details, but there is no point to that. This paper presents a clever\nand novel approach that brings new light to important questions. There is enough here now for\nreaders to process and place the results into their own views of the uncertainties and their overall\nthinking about Earth's protracted oxygenation and the implications for the history of life. This\ncontribution dovetails nicely into that conversation. I am happy to sign off on this version.\nReviewer #2 (Remarks to the Author):\nI commend the authors on adequately considering and addressing the comments I made on the\nprevious version, and I have no further comments to make. I support this publication pending the\ndecisions of the reviewers and their responses.\nBrooks Ryan\nReviewer #3 (Remarks to the Author):\nThe authors have made the changes I suggested or explained why they did not.\nI appreciate the new Fig. S1, but to my reading at least, this is the paper, and it shouldn't be buried in\nthe supplement!\nSincerely,\nErik Sperling\nReviewer #4 (Remarks to the Author):\nI have read the revised version of this manuscript and I feel that the authors have done an adequate\njob of addressing my original questions. I think this is an interesting dataset and idea that should be\npublished and is worthy of publication in Nature Communications.\nReviewer #4 (Remarks on code availability):\nI have logged onto the private connection to the codes and found them easy to use."}]

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