Enhanced erosion by continental arc volcanism as a driver of the Cambrian Explosion

Domain: earth_science
Published: 2025-10-16
Editorial outcome: minor_revision

Abstract
Nutrient availability and rising oxygen levels are important for the Cambrian Explosion (~540–515 Ma), both of which can be satisfied via enhanced weathering driven by climate and tectonics. Although increased subduction and continental arc volcanism coincided with the Cambrian Explosion, their weathering-nutrient feedback role remained unclear. Here we integrate Li-Os isotopes with previously published Sr isotope to investigate connections between subduction-driven weathering dynamics and animal diversification. Our results reveal a shift from enhanced erosion of arc-related juvenile rocks (ca. 540–525 Ma) to clay-forming weathering of old crust (525–515 Ma). We propose that subduction-driven warming and uplift may have accelerated erosion of phosphorus-rich juvenile rocks, releasing nutrients that boosted marine productivity and subsequent oxygen buildup through organic burial. This process ultimately fueled early animal diversification, providing additional evidence for tectonic control of biogeochemical cycles during the Cambrian Explosion.

Introduction
Continental arcs, where oceanic lithosphere is subducted beneath continental lithosphere, are thought to release more CO2 than other types of volcanoes due to decarbonation of sedimentary carbonates stored in the upper continental plate1,2. As such, the rise and fall of continental arcs pace with Earth’s greenhouse and icehouse intervals since at least the Cryogenian2, strengthening their role in modulating atmospheric CO2 levels and climate change. Elevated arc-related CO2 degassing and associated uplift of continental arcs would then promote silicate weathering and the drawdown of atmospheric CO2, stabilizing the climate system on multimillion-year timescales3–5.

During the Ediacaran–Cambrian transition (E–C), subduction along the Proto-Pacific and Proto-Tethyan margin of Gondwana became active following the final breakup of the Rodinian supercontinent and the initial formation of Gondwana6,7. An increased continental arc length and fraction of young zircon grains indicate enhanced continental arc volcanism during the E–C transition2,8,9, consistent with an increase in atmospheric CO2 and climatic warming2,10. At this time, the rapid rise in animal diversity and abundance (the “Cambrian Explosion”, ca. 540–515 Ma) also occurred11,12. To sustain complex life, nutrient availability (e.g. P, Ca) and oxygen are considered as two important limiting factors13–16, both of which could be affected by enhanced weathering of mountains linked to collision-dominated tectonics during the assembly of Gondwana17,18. However, a large part of the Gondwana assembly-related collisions (mainly before 610 Ma)7 occurred too early to be responsible for the Cambrian Explosion. In contrast, increased subduction-dominated continental arc volcanism along the margin of Gondwana temporally overlapped with the Cambrian Explosion2,8,9. Fast-eroding features of continental arcs, characterized by crustal thickening and intense mantle involvement, may fuel a more efficient feedback loop via CO2 degassing and climatic warming, enhanced weathering of mantle-derived juvenile rocks, and hence the rapid delivery of nutrients to the oceans3,8,19. Along with this feedback process, removal of long-term subduction-related CO2 would result in increased oxygen levels through efficient organic carbon burial that impedes O2 back-reactions10,20. Juvenile rocks from large igneous provinces (LIPs) or arc volcanism typically contain 1.5–2 times higher phosphorus content than continental crust21,22, and are highly susceptible to weathering23. Their rapid weathering would release large amounts of P into ocean, favoring primary productivity, efficient organic burial and O2 production21,22. However, it is not clear whether subduction-driven climatic warming accompanied by enhanced weathering of arc-related juvenile rocks may have played a significant role in the Cambrian biogeochemical cycle and its animal radiation. Additional geochemical evidence is thus required to explore their potential relationship.

Given the fast-eroding features of continental arcs developed upon either a mantle-involved unradiogenic or an ancient continental radiogenic basement, they play an important role in controlling the strontium (Sr) isotopic composition (87Sr/86Sr) of seawater through chemical weathering over geological times4,19. Like Sr, the Os isotope compositions (187Os/188Os) of seawater reflect a balance between two highly distinctive end-member fluxes: unradiogenic sources of 187Os/188Os~0.13 (e.g. extraterrestrial flux, hydrothermal flux and mantle-derived juvenile material weathering) and radiogenic sources of 187Os/188Os~1.4 (e.g. weathering of continental crust)24,25. Thus, variations in seawater 187Os/188Os over geological times can be applied to trace changes in the weathering of juvenile rocks and continental crust24. Besides the Os-Sr isotopes used to trace weathering sources, seawater Li isotope records could provide unique information on silicate weathering rates and regimes in response to climatic changes26,27. δ7Li values of modern rivers vary significantly (3–42‰; average 23‰)27 and depend on the silicate weathering intensity denoted as W/D, which is the ratio of the chemical weathering (W) rate to the total denudation rate (D, consisting of the erosion (E) + chemical weathering (W) rates)27. Low W/D regimes are characterized by high primary rock dissolution, known as congruent weathering with a low dissolved δ7Li and high Li flux, while moderate W/D regimes are linked to secondary mineral (largely clay) formation driving high dissolved δ7Li and a lower Li flux, known as incongruent weathering27,28. In high W/D regimes, there is little rock dissolution and pre-formed secondary clays redissolve, resulting in low dissolved δ7Li and a very low Li flux27,28. The riverine Li makes up over 50% of the modern ocean Li input, with the remainder from mid-ocean ridge hydrothermal inputs and balanced by Li sinks into marine authigenic clays and low-temperature alteration of the oceanic crust28,29. Given the fairly long residence time of Li-Os-Sr isotopes in the modern ocean, they can provide insights into the relationship between weathering dynamics and major paleo-environmental fluctuations25,26,30.

Here, we measured Li isotopes in carbonates and Re-Os isotopes of black shales and cherts that potentially record seawater isotope compositions from two drillcores covering the main interval of the Cambrian Explosion (ca. 560–515 Ma) in the Yangtze Block, South China. With compiled Sr isotope ratios, our work provides critical evidence of subduction-driven weathering dynamics and discusses their role in seawater chemistry and the Cambrian Explosion.

Results
The E-C transition witnessed widespread continental arc volcanism along the Proto-Pacific and Proto-Tethyan margin of Gondwana7 (Fig. 1a), such as the Avalonia–Cadomian arcs (750–500 Ma)31, the Ross and Delamerian arcs (590–480 Ma)6,32, and Proto-Tethyan arc (550–490 Ma)33. Some newly generated Large Igneous Provinces (LIPs) were also emplaced around the Central Iapetus on the Laurentian and Baltican cratons (ca. 535–533 Ma)34. The South China, consisting of the Yangtze Block and the Cathaysia Block, was surrounded by an open ocean during the E–C transition period7.

Based on biostratigraphy, radiometric ages (U-Pb and Re-Os), carbon isotope stratigraphy, and marker layers, we have reconstructed a regional stratigraphic framework across the shallow carbonate platform, transitional belt and deepwater slope-basin regions of the Yangtze Block (Fig. 1b, c). The regional black shale (~10 m) of the Doushantuo Member IV is constrained by a U-Pb age of 551.1 ± 0.6 Ma from an ash bed at its top35. Above this layer, the E–C boundary is clearly defined by the globally recognizable basal Cambrian negative carbon isotopic excursions (BACE, N1), and the appearance of small shelly fossils (SSFs) in the shallow shelf Meishucun and Xiaotan sections36 (Fig. 1c). The Liuchapo Formation slope-basin regions traddles the E–C boundary, supported by two U–Pb ages of 542.1 ± 5.0 Ma and 542.6 ± 3.7 Ma from ash beds in the Ganziping and Bahuang sections37 (Fig. 1c), corresponding to the boundary of the Dengying/Yanjiahe Formations in shallow sections. Widespread black shales over the whole Yangtze Block act as a regional marker layer38, constrained by Re-Os ages of 521 ± 5 Ma and 520.3 ± 9.1 Ma for Ni-Mo-V ore layers at the bottom39,40. Thus, the depositional age of the Yanjiahe Formation in the shallow shelf is constrained at ca. 540–521 Ma, time-equivalent to the middle-upper Liuchapo Formation in deep water. Phosphorites are widely distributed in the carbonate platform, with a depositional age of ca. 541–525 Ma based on the U-Pb age of 526.5 ± 1.1 Ma above the phosphorite deposits41,42. Trilobites and more complex, large-bodied animals (e.g., Chengjiang and Qingjiang Biota) also first appeared in the shelf sections at ca. 521–518 Ma38,43 (Fig. 1c).

Core samples covering late Ediacaran to early Cambrian (ca. 560–521 Ma) were collected from carbonates of the Dengying and Yanjiahe Formations in the shallow shelf of drill core ZK4803 (31°28′22“N and 111°11′25“E) and black cherts and shales of the Doushantuo Member IV and Liuchapo Formations in the slope region of drill core ZK4411 (28°02′32“N and 109°04′33“E) (Fig. 2a, b). We measured and calculated Re-Os isochron ages of 557.3 ± 9.1 Ma (2σ, n=9, MSWD=1.6) for the Doushantuo member IV black shales with a high initial 187Os/188Os ratio (Osi) of 0.825 ± 0.086, and 529.1 ± 6.0 Ma (2σ, n=8, MSWD=2.6) for the Liuchapo cherts with evidently low Osi value of 0.341 ± 0.014 (Fig. 2b, c). The continuous Osi records of 560–523 Ma is presented based on their Re-Os data and estimated depositional ages. In general, Osi values stay at a high level with an average of 0.82 at 557 Ma, and 0.62–0.68 at ca. 541–540 Ma, and then rapidly decrease to ~0.34 at ca. 540–525 Ma. After this, they return to high values of 0.76–0.94 at ca. 525–523 Ma (Fig. 2b and Supplementary Data 1). Interestingly, δ7Li values of leached carbonate samples show the same varying trends as Osi, i.e., a general decreasing trend of δ7Li from ~18‰ to ~14‰ at ca. 550–540 Ma, and further to the lowest value of 6.3‰ at ca. 540–525 Ma and then back to a high value of ~17‰ at ca. 525–515 Ma (Fig. 2a and Supplementary Data 2).

Following the typical diagenetic characteristics of modern marine carbonates44,45, additional elemental ratios (e.g., Al/(Mg+Ca), Li/(Mg+Ca) and Sr/(Mg+Ca)) and their relationships with δ7Li are used to evaluate the possible overprint of δ7Li values of ancient carbonates by detrital contamination during carbonate leaching, carbonate mineralogy, and diagenetic alterations. These observations suggest that carbonates in our drill core are most likely formed during marine fluid-buffered diagenesis (Supplementary Figs. S1 and S2), and their δ7Li values are considered to approach the ambient seawater Li isotopic signals with a minor Li isotope fractionation of –0.5‰ to –2‰44,45. More details are given in Supplementary Information.

Methods
Sample preparation
Twenty-one fresh core samples of drill core ZK4411 and forty-two carbonate samples of drill core ZK4803 covering 560–515 Ma were selected for Li-Os isotope analysis, and thirty-seven core samples are selected for rare-earth elemental analysis. Core samples were cut into fresh chips and then manually ground to 200 mesh size using an agate mortar and pestle to avoid any metal contaminant. The powered samples were dried and then preserved in a desiccator before instrumental analysis.

Re-Os isotope analysis
The Re-Os chemical separation and analytical methods followed the procedure described by Yin et al. (ref. 59). Approximately 0.2–0.6 g of powdered sample was digested together with 185Re and 190Os enriched spikes in sealed Carius tubes. Prior to sealing, the tubes were frozen in an ethanol–liquid nitrogen freezing bath (–50 to –80°C) and then sealed using an oxygen–propane torch. Digestion was carried out at 220°C for 48 h. Following digestion, osmium was isolated via CCl4 solvent extraction, back-extracted into concentrated HBr, and further purified by microdistillation60. The remaining solution, containing Re, was processed and purified through anion-exchange chromatography. Rhenium concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS; Thermo Elemental X2 Series). Osmium isotopic measurements were conducted by negative thermal ionization mass spectrometry (N-TIMS) as OsO3⁻ ions loaded onto Pt filaments, using a Thermo-Finnigan Triton instrument operated in electron multiplier mode at the State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. Reproducibility and accuracy of the Os isotopic measurements were monitored by repeated analysis of the DROsS standard solution, which yielded a mean 187Os/188Os ratio of 0.16094 ± 0.00007 (2σ, n=4), consistent with previously reported values25.

Total procedural blanks (TPBs) averaged 0.41 ± 0.22 pg (2σ; n=4) for Os and 2.9 ± 0.4 pg (2σ; n=4) for Re, with an average 187Os/188Os ratio of 0.286 ± 0.082 (2σ; n=4). All sample data were corrected for the corresponding procedural blank of each analytical batch. The contribution from the blanks was generally negligible. Analysis of the black shale reference material SGR-1b yielded mean values of 187Os/188Os = 1.7857 ± 0.0098, Os = 0.444 ± 0.026 ppb, and Re = 34.67 ± 0.34 ppb (2σ; n=2), which are consistent with certified and literature values59. Regression analyses were performed using IsoplotR61 (https://www.ucl.ac.uk/~ucfbpve/isoplotr/home/). Initial 187Os/188Os values (Osi) were calculated from the measured 187Os/188Os and 187Re/188Os values, and the estimated ages (Supplementary Data 1), with a 187Re decay constant of 1.666 × 10−11/yr62. Total uncertainties include measurement, blank, decay constant, and spike calibration uncertainties, which are reported at the 2σ level.

Li isotope analysis
We targeted the Li isotopic composition of the carbonate core samples. A chemical leaching method using 0.3 M dilute acetic acid was adopted63. After cleaning using deionized water, approximately 200 mg of bulk carbonate sample was leached with 0.3 M acetic acid for at least one hour. After centrifuging, the supernatants were dried and re-dissolved in 1 mL 8 M HCl and dried again. The samples were then dissolved in 2 mL 0.2 M HCl for column purification. Purification of Li for isotope analysis was achieved using an organic solvent-free single-step liquid chromatography procedure in a super-clean lab at Hefei University of Technology. Large columns (4 mL AG 50W-X12 resin, 100–200 mesh) were used to ensure that the column was not saturated for calcium and other cations, which is important for low-Li samples. Separations were monitored with ICP-MS analysis to guarantee both high Li yield [>99.8% recovery] and low Na/Li molar ratio (<0.5)64. The Li yield was calculated using the following equation: Yield (%) = collected Li / (collected Li + pre-yield Li + after-yield Li) × 100%.

The lithium isotopes were measured on a Neptune plus MC-ICP-MS at Hefei University of Technology. The set-up was optimized to achieve both high sensitivity (7Li signal > 6V for 50 ng/g Li solutions, Jet+X cones) and high stability. Solutions for MC-ICP-MS analysis were introduced through a PFA nebulizer (50 μL/min) coupled with a quartz Scott-type spray chamber and were matrix-matched to 10 or 50 ng/g according to the bracketing standards to ensure the best precision and accuracy. The total procedure blanks determined for the combined sample digestion and column procedure were about 0.03–0.14 ng Li. Compared with the ~10–100 ng Li used for our analysis, the blank correction is not significant. Each measurement was bracketed with a L-SVEC standard. The in-run precision of 7Li/6Li measurements is ≤0.2‰ for one block of 30 ratios. The external precision, based on long-term monitoring of two in-house standards, Li = +8.8 ± 0.3‰ (2SD, n>500) and Li = −19.3 ± 0.2‰ (2SD, n>200), is better than 0.5‰. Analysis of international rock reference materials yield δ7Li values of +4.3 ± 0.4‰ (2SD, n=8) for BHVO-2, −0.8 ± 0.2‰ (2SD, n=29) for GSP-2 and +5.9 ± 0.4‰ (2SD, n=9) for AGV-1, which are within the uncertainty of previously published values64.

Rare-earth elemental analysis
Concentrations of rare earth elements were measured by inductively coupled plasma mass spectrometry (ICP-MS, Thermo Scientific Element XR) at the State Key Laboratory of Isotope Geochemistry of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. About 40 mg of powdered samples (200 mesh) were digested with a mixture of HNO3 + HCl + HF + HClO4 in high-pressure Teflon cups. The organic-rich samples were firstly combusted in a muffle oven at 650°C for 4 h to completely remove organic matter before dissolution by acids. The analytical standards GSR1 (granite), GSR3 (basalt), SGR-1b (oil shale) plus replicate samples were jointly used to monitor analytical precision, which was better than 7% for rare earth elements.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I find the manuscript itself is good, and it will benefit from some revisons, but I do not find it suitable for nature. In brief, my main critique is the lack of novelty, and missing literature.\nIn more detail about my decision:\nThe authors present this as a novel approach. They present their own Li data, and do a box model calculation with it.\nHowever, if they draw globally valid conclusions, they should use all of the available Li data, or at least explain why they dismiss that. They did use for example Sr global composite, but this is also missing some key data. I am less familiar with Os isotopes, but again a quick search revealed other Os isotope data from that time episode.\nThe outcome of that study, is that they can show, that only continental arc volcanism could have been the source of volcanism, effectively driving the Cambrian explosion. That is also not very novel, as they themselves state in the introduction, line 61ff. The novelty seems to be the quantification of the activity, which is a good result but not entirely exciting.\nMoreover, but not overly relevant, there are some unnecessary self-citations, e.g. line 240 ff.; or 255ff. They use the comparison to other events in different times, merely to state what would have happened if the conditions were different.\nI belief the author contributions are missing the info what Sui Ji contributed, and sample collection is mentioned twice in the author contributions.", "reviewer_2": "In this revision, the authors clarified the citation for the compiled continental arc length during the E-C transition, and added more reasoning to exclude the contribution from the LIPs to emphasize the arc effect. I am fine with their clarification for the previously miss-cited data compilation, and I do not have much problem with their data and modeling results, but I do think this manuscript would benefit from a more quantitative discussion on their enhanced P flux part.\nAs reviewer 1 mentioned, continental arc volcanism driving the Cambrian explosion is not really a new idea. The authors’ main contribution in this study is to provide some quantitative constraints on the major contributor to the Li-Sr-Or isotopic system, that there was indeed enhanced arc derived erosion during the E-C transition. Note that this is the only direct conclusion from their data. All the further discussions on how the continental arc activity drive the Cambrian explosion is based on a 240% increase in F_hyd and a 3-4 fold increase of P flux derived through box modeling. My question is whether the increase of continental arc magmatism during the E-C transition could be able to provide such increase in the erosion flux and P flux that match the authors modeling result. Since the authors already have the compiled data of arc duration and length, they should be able to make some estimation of the erosion and elemental flux.", "reviewer_3": "The authors state the objective of this paper is to explore whether the subduction-driven climatic warming and enhanced weathering of arc-related juvenile rocks played a significant role in the Cambrian Explosion (lines 83-86). From my reading of the ms, the sequence of cause and effect is as follows and differs for the two periods [540-525 Ma] and [525-515 Ma]. Validating the connections in the sequence is important, but I wasn’t fully convinced.\n540-525 Ma\n1. Tectonic activity: subduction; early stage of continental arc volcanism\n2. A: CO2 degassing -> warm climate\nB: Uplift of continental arcs -> silicate weathering (drawdown of CO2)\njuvenile crust; congruent weathering\n3. A: primary productivity; organic matter burial; O2 production\nB: Nutrient delivery to the ocean\n4. Cambrian Explosion\n525-515 Ma\n1. Tectonic activity: subduction; later stage of continental arc volcanism\n2. A: Decreased CO2 degassing -> warm climate\nB: Uplift of continental arcs -> silicate weathering (drawdown of CO2)\nold crust; incongruent weathering (clay formation)\n3. A: Decreased primary productivity; organic matter burial; O2 production\nB: Decreased Nutrient delivery to the ocean\n4. Cambrian Explosion slows down?\nThe difference in the two time periods from my reading of the ms has to do with the stage of continental arc evolution. The first phase (540-525 Ma) involves juvenile crust during initial subduction and the second phase (525-515 Ma) involves older crust. It would help the readers if this distinction was more clearly stated.\nTo support Step 1, the arc length (Fig. 3F) is given, but the dramatic increase in the arc length predates by ~10 myrs any significant changes in the three isotope systems.\nRegarding Step 2A, It would be helpful to have the “sedimentary carbonate stored I the upper continental plate”(line 50) drawn in on Figure 5A and B to show clearly how the release of CO2 is greater for the 540-525 Ma period than the 525-515 Ma period. Is this the platform carbonate on the margin of the continental plate or something interior to the continental crust (grey or green part of Fig 5A and B)? Also, the meaning of different coloring (green and grey) in Fig. 5 is not explained in the caption.\nTo support increased weathering of juvenile rocks (low Sr and Os isotope ratios 540-525 Ma) and later of older crustal material (high Sr and Os isotope ratios 525-515 Ma), Sr and Os isotope ratio data are used (Fig 3C and E) (Step 2B). Yet, there is a period before 340 Ma where the initial 187Os/188Os and 87Sr/86Sr move in opposite directions. Granted there is a lack Os data points for this period; still, this relationship should be explained. Also, the general statement (lines 87-90) that there is a positive relationship between continental arc length and 87Sr/86Sr does not hold (Fig. 3E and F).\nTo support congruent weathering of juvenile rocks for the 540-525 Ma period and incongruent weathering and clay formation for the 525-515 Ma period (Step 2B), the authors use lithium isotopes. Over these periods, there has been a shift in carbonate mineralogy from aragonite to calcite. The two mineral types involve different fractionation factors between seawater and carbonate. Shouldn’t this be considered in discussing the d7Li record in Fig 2 and 3D?\nThe elemental ratios (e.g. Al/(Ca+Mg), Mn/Sr, etc) that are used to test clay contamination or diagenetic alteration for the Li isotope data are a little higher (lenient) than what recent publications use. Also, data points that do not satisfy the cutoff are retained based on the argument that there is no relationship between the isotope ratio and the elemental ratio proxy. This is adopting a rather lenient condition.\nTo support the increase in nutrients delivered by weathering (Step 3B), the authors give phosphate distribution (Fig. 3B). The data is only qualitative (rare vs. abundant) yet the figure is drawn as if there is a drastic and quantifiable change. This could be misleading. The Ca2+ concentrations changes (Fig. 3B) is based on only two data points and yet the curve is drawn as if there is some structure to it (Fig 3B). I think detailed figure caption or supplementary explanation is necessary.\nAuthors use multiple isotopes to provide a more robust interpretation (lines 174-176). The lithium and osmium isotope data covering the period of the Cambrian Explosion are new and of interest to the community. The interpretation connecting it to the Cambrian Explosion is interesting and plausible, but the logic needs to be tightened and carefully stated. I realize that the authors may be limited by the length imposed by the journal. Usually, other authors make full use of the Supplement to supply those details.\nMinor editorial comments\nThe material used for isotope ratio measurements should be separately given and the meaning stated briefly (Line 119-123). Are both carbonate and black shales intended to represent seawater values at the time?\nIn the box model, only a few cases are explored. It would be more persuasive if the authors presented some other combinations of parameter changes (scenarios) they tested in the Supplementary material.\nState what the REE pattern for “typical hydrothermal fluids” (line 186) would look like.\nState a brief description of the box model used (line 191). For example, it is a dynamic box model.\nConsider rewriting lines 194-198, as the meaning does not come through well. The authors are not arguing necessarily for a 50% shorter residence time, per se, but only intend to give this as a extreme scenario. Correct?\nLine 61: Add (Fig. 1A).\nLine 63: Add (Fig. 3F).\nLine 66: Add (Fig. 3A).\nLine 69: via -> by\nLine 73: mantle-involved -> mantle-derived\nLine 302: Cox et al 2016 reference is missing from the list\nFig. 1C: Does [blue star + red age] notation mean something different from [blue star + blue age]?\nFig. 3 Please state in the caption what the pink lines in C, D, E are. Are they rigorously drawn?\nF: The numbers given for each of the different arcs don’t match with the ages given in Table S1."}, "author_response": "Responses to Reviewer #1\nComments: I find the manuscript itself is good, and it will benefit from some\nrevisions, but I do not find it suitable for nature. In brief, my main critique is the lack\nof novelty, and missing literature. In more detail about my decision: The authors present\nthis as a novel approach. They present their own Li data, and do a box model calculation\nwith it. However, if they draw globally valid conclusions, they should use all of the\navailable Li data, or at least explain why they dismiss that. They did use for example Sr\nglobal composite, but this is also missing some key data. I am less familiar with Os\nisotopes, but again a quick search revealed other Os isotope data from that time episode.\nThe outcome of that study, is that they can show, that only continental arc volcanism\ncould have been the source of volcanism, effectively driving the Cambrian explosion.\nThat is also not very novel, as they themselves state in the introduction, line 61. The\nnovelty seems to be the quantification of the activity, which is a good result but not\nentirely exciting. Moreover, but not overly relevant, there are some unnecessary self-\ncitations, e.g. line 240 or 255. They use the comparison to other events in different times,\nmerely to state what would have happened if the conditions were different. I belief the\nauthor contributions are missing the info what Sui Ji contributed, and sample collection\nis mentioned twice in the author contributions.\nResponse: Although we feel your decision is biased partly due to the confusion of\nour Nature communications manuscript with Nature, we are still grateful for your\ncomments which have helped improve our work. We have investigated published Li-Os-\nSr isotope data in literature, as clearly shown in Fig. 3. It is worthy to note that only the\ncarbonate Li isotope may reflect the seawater signal and the Li isotopes reported for the\nshales of this time interval (down to -5‰; Wei et al., 2024) are therefore not adopted in\n1\nour study. Moreover, some evidently low carbonate Li isotopes for weathered outcrop\nsamples at ca. 550 Ma (1.7–6.9‰; Kalderon-Asael et al., 2021) were also not used in our\nstudy, because we think weathering influences the Li isotopes that prevent them from\nreflecting the seawater signal. To the best of our knowledge, this is the first time to present\nstrikingly coupled Li-Os isotopic shift at 540–525 Ma that represents global seawater\nsignals due to their longer residence time than seawater mixing time. Therefore, our\nmodelling results and interpretations are representative of weathering dynamics and\nmajor paleo-environmental fluctuations during the Cambrian Explosion. We have revised\nother small mistakes following your comments, such as author contributions and some\nunnecessary self-citations.\nResponses to Reviewer #2\nMajor concerns 1: Continental arc volcanism as the primary source for the\nenhanced juvenile material.\nIn the manuscript Figure 3, there is a devastating misuse of Cao et al. (2017)’s\ncontinental arc length data (see Figure 1 below). In Cao’s original Figure 4d, continental\narc length decreased after 550 Ma, stayed plateaued, then briefly increased after 500 Ma.\nHow come in authors’ Figure 3F, they show a sharp increase of continental arc length\nright after 550 Ma (which is completely opposite to the trend in Cao’s original graph)?\nInterestingly, there appears to be increased total subduction zone length (Mills et al., 2017)\nand young zircon proportion (McKenzie et al., 2016) between 560 and 510 Ma. Combined\nwith the decreasing/steady trend in continental arc length from Cao et al. (2017), it implies\nan increase in island arc activities during this period, which, unfortunately, makes the\nmanuscript’s title and emphasis on continental arcs suspicious (I do not deny on the\n2\npotential influence from the subduction zone activities, though).\nI want to point out that in either Cao et al., 2017, McKenzie et al., 2016, or Mills et\nal., 2017 compilation, age constraints are quite loose. For example, McKenzie’s\ncompilation only has two controlling points for the young zircon proportion between 600\nand 500 Ma. Cao’s compilation was based on occurrence of arc-related igneous rocks,\nand at least 10–20 Ma uncertainty is expected. Mills’ subduction zone length is largely\nbased on paleomap plate tectonic reconstructions. None of these original compilations\nhas the resolution to support for the ~15 Ma decrease in the Os-Sr-Li isotopic values. I\nam at odds with the authors stretching data/trends compiled for a much longer time scale\n(i.e. the entire Phanerozoic) to “match” their isotopic variations in a small time window,\nnot mention they did not even copy correctly (similar complaints on their presentation of\nthe seawater Ca2+ concentration in the manuscript Figure 3B).\nIn fact, the duration of the observed decrease in the isotopic values (~15 Ma) is a bit\ntoo short for arc erosion. Continental arc erosion usually lasts for ~50 Myr, considering\nthe deposition duration in back-arc and fore-arc basins. For example, forearc deposition\n(with arc igneous rocks as primary source) for the North American Cordilleran arc lasted\nfrom the Late Jurassic to Early Eocene (>40 Ma) (Jiang and Lee, 2017; Kimbrough et al.,\n2001; Paterson and Ducea, 2015; Sharman et al., 2015). Similar duration is also\ndocumented in other circum-Pacific and Neo-Tethyan continental arc systems (Wang et\nal., 2012). In addition, eroded continental arc materials are in most cases dominated by\nandesitic–felsic composition, especially these associated to major magmatic flare-ups\n(Dü rr, 1996; Jiang and Lee, 2019; Surpless, 2015), instead of the mafic composition\nemphasized by the authors.\nThe negative Os-Li-Sr isotopic shifts obtained by the authors looks clearly event\n3\ndriven, as the shifts only occur in a 15 Myr time window, prior and after which the isotopic\nvalues shift back to the background. The entire 700–500 Ma period witnessed an overall\nenhanced subduction zone activities (including continental arc) (Cao et al., 2017; Mills et\nal., 2017). Unless the authors could provide solid evidence that there were intensified arc\nmagmatic flare-ups right around the 540–525 Ma time window, and the associated\nmagmatic/tectonic thickening could be able to produce the expected increase in erosion\nflux that matches the authors’ modeling results, it is difficult to convince that continental\narc volcanism plays as the primary contributor to the observed isotopic variation. The\nauthors should take consideration of other possible tectonic events or mechanisms that\nwould contribute to the enhanced input from more juvenile material to the ocean. Except\nof enhanced subduction zone activities, the Early Cambrian is also characterized by\nenhanced mid-ocean ridge production rate and development of continental rifting and\nplume related intermediate-mafic magmatism along both the Laurentian and Gondwana\nmargin (Brueseke et al., 2016; Chichorro et al., 2008; Ernst et al., 2021; Mills et al., 2017;\nProkopiev et al., 2016). Each of the above could possibly contribute to the Os-Sr-Li\nisotopic shifts. The authors only attempt to rule out the contribution from the LIPs with a\nsimple statement without further investigation: “Although the weathering of flood\nbasalt around the Central Iapetus also explains our observed negative Li-Os-Sr shifts, it only\noccurred in local regions and might support short-lived shifts, perhaps on timescales of <\nOceanic Anoxic Event 2 (Pogge von Strandmann et al., 2013), but not for a duration of\nconvergent to divergent plate boundaries. Rift-related magmatism and erosion products\ndeveloped between 540 and 500 Ma are well documented (Brueseke et al., 2016;\nwith the isotopic shifts, but completely ignored by the authors.\nResponse 1: We appreciate your insightful comments. We try to find a reasonable\nexplanation for primary weathering source of the juvenile material that drive our Li-Os-\nSr shifts. We are sorry to not clearly explain why our continental arc length curves are\ndifferent from Cao et al. (2017). In Cao’s paper, the duration of Avalonia-Cadomian arcs\n(avg. 6750 km) are at 660–550 Ma based on early papers, but new age constraints of\nAvalonia-Cadomian arcs reveal a longer duration of 750–500 Ma (Oriolo et al., 2021;\nMoghadam et al., 2024). Therefore, the addition of 6750 km of continental arc after 550\nMa in our study leads to a longer continental arc length during 550–510 Ma that is\nevidently different from the one reported in Cao’s paper. To avoid any misunderstanding,\nwe have updated continental arc length data (see details in Table R1 below) based on\nCao’s paper and other publications (Cawood and Buchan, 2007; Zhu et al., 2012;\nBrown et al., 2020; Casquet et al., 2018; Oriolo et al., 2021; Moghadam et al., 2024). We\nagree that there are multimillion-year uncertainty of continental arc length, but they at least\nprovide an overall increasing trend since 550 Ma.\nTable R1 Compiled continental arc length during the E-C transition\nthe small time window of negative Li-Os-Sr shifts at 540–525 Ma. We do not conclude\nthat continental arcs could only cause mantle-like unradiogenic Os-Sr inputs. Generally,\ncontinental arc system evolves through time during subduction process (Cawood and\nBuchan, 2007). When subduction-related crustal thickening with limited intermediate-\nmafic magmatism flare-ups, continental arcs could provide radiogenic Os-Sr source.\nWhen intermediate-mafic magmatism flare-ups, they could become the dominated\nunradiogenic Os-Sr source (Jagoutz et al., 2016; Gernon et al., 2021; Macdonald et al.,\n2021; Cao et al., 2022). Given the fastest-eroding surface features of continental arcs on\nEarth (Lee et al., 2015; Gernon et al., 2021), evolved continental arcs exert strong controls\non either global radiogenic or unradiogenic Os-Sr weathering fluxes. In this scenario,\nevolved crustal and juvenile compositions of continental arc could likely explain our\nlong-term rise and fall of Os-Sr isotope through 560–510 Ma (~50 Ma), and perhaps\nextend a longer duration of the Ediacaran- early Cambrian.\nThe E-C continental arc system, characterized by external orogenic system like\nCircum-Pacific, could become isotopically juvenile following successive juvenile\nadditions during ongoing arc magmatism since 550 Ma, as reflected by complied Hf\nisotopic trend toward mantle-like values (Collins et al., 2011). For example, arc\nmagmatism activities were recorded in Avalonia-Cadomian arcs (avg. 6750 km) at ca.\n550–525 Ma (Oriolo et al., 2021; Moghadam et al., 2024), Gondwana proto-Tethyan\nmargin arc (avg. 5775 km) at ca. 547–522 Ma (Zhu et al., 2012), Ross arc (avg. 3400 km)\nat ca. 540–520 Ma (Brown et al., 2020), and Pampeanas arc (avg. 700 km) at ca. 545–\ncompositions may co-exist in continental arcs, highly weatherable arc-related juvenile\n6\nrocks and their fastest-eroding feature still make them become a dominant control of\nunradiogenic Os-Sr input to sedimentary system, as evidenced by the correlation between\nincreasing continental arc length and low seawater Sr isotope over the past 400 Ma\n(Gernon et al., 2021; Cao et al., 2022).\nInitially, We try to interpret our negative Li-Os-Sr shifts by LIPs flood basalt\nweathering because it seems a common way to produce such Li-Os-Sr shifts, such as Pre-\nSturtian snowball Earth (Rooney et al., 2014), the Permian-Triassic transition (Sun et al.,\n2018) and Oceanic Anoxic Event 2 (Pogge von Strandmann et al., 2013). However, there\nare some problems for this interpretation of our results. Firstly, LIPs flood basalt (535–\n(Ernst et al., 2021), and such volumes of flood basalt without new replenishment from\nmantle source like long-term volcanic arc volcanism, is not enough to sustain a 15 Ma\nweathering Os-Sr flux. Secondly, flood basalts were typically flat-lying rather than\nextensive uplifts like continental arcs, which may not sustain an increased erosion rate\nover multimillion timescales as evidenced by our low δ7Li . Although rift-related\nriv\nmagmatism (~540-500Ma) may match the time window of our isotopic shift (Brueseke\net al., 2016; Prokopiev et al., 2016), they commonly have limited area and short duration\nin local regions of Central Iapetus and northeastern Siberian Craton, which are also not\nenough to produce global seawater Li-Os-Sr shifts like LIPs basalt. Therefore, except for\nflood basalt weathering, other unradiogenic Os-Sr sources are required to produce our\nnegative Li-Os-Sr shifts. As discussed above, climatic warming by efficient subduction-\nrelated CO outgassing, extensive uplifted arc-related juvenile rocks, and high erosion\n2\nrates could thus be realistically invoked to explain the consistent negative Li-Os-Sr shifts\nat ca. 540–525 Ma. Compared with other tectonic events (e.g. limited area and duration\n7\nof LIPs at 535–533 Ma and pre-dating collisional orogens within Gondwana mainly >550\nMa; Zhao et al., 2018), enhanced continental arc volcanism seem to match the whole time\nwindow of the Cambrian Explosion, despite complex compositions and timescales\nuncertainty of evolved continental arc.\nMajor concern 2: Enhanced weathering of juvenile materials drives the\nCambrian Explosion. Apparently, all the links between “enhanced juvenile material\nweathering” and the Cambrian explosion provided from the manuscript are from\ndeduction and based on co-occurrence of various isotopic excursion, increased\nphosphours deposits, organic carbon burial and the biota evolution. I am no expert in\npaleontology, so below are just a few questions that confused me after reading this\nmanuscript. Granted, the enhanced nutrition input resulting from enhanced weathering of\njuvenile material could be promoting biota development, but is it a necessary factor for\nsuch a great biological transition in Earth history? Is the enhanced nutrition input and/or\norganic carbon burial enough to drive oxygenation and biological explosion? Enhanced\nsubduction zone activities are not rare phenomena in geologic history, then why this one\nparticularly be the trigger for such an important biological evolution?\nThere is also a timing mismatch between the Cambrian negative carbon isotopic\nexcursions vs Li and Os isotopic shifts. From Figure 3 in the manuscript, the basal\nCambrian carbon isotope excursion (BACE) occurred earlier than the decrease (and the\nlowest values) in Li and Os isotopes. The BACE is suggested to be linked to increased\nbiological productivity, potentially due to the rapid evolution and diversification of early\nCambrian animals leading to a large uptake of carbon dioxide from the atmosphere. It is\nalso suggested that diversification starts before and extends across the\nexcursion and Li, Os isotopic shifts seem to suggest that the biota diversification took\nplace earlier than the intensified weathering. In addition, it is questionable that a 15-Myr-\nonly enhancement of chemical weathering/nutrition influx would be able to sustain the\nbiological explosion that lasted through the Early Cambrian?\nResponse 2: Thanks for your comments. Recent hypotheses for the Cambrian\nexplosion fall into a complex interplay of biotic and abiotic processes such as\ndevelopmental/genetic, ecologic, and abiotic/environmental interactions (Cook, 1992;\nBrennan et al., 2004; Smith and Harper, 2013; Wood et al., 2019; Wei et al., 2024).\nAlthough the cause of the Cambrian explosion is complex and still under hot debate,\nenvironmental change (e.g., nutrient availability (P, Ca) and oxygen) has been considered\nas an important factor of the Cambrian explosion with respect to biocalcification and high\noxygen requirement for large-body, skeletonized animals (Brennan et al., 2004; Wood et\nal., 2019; Wei et al., 2024). Although it is not easy to claim that nutrient availability (P,\nCa) and rising oxygen via enhanced weathering of juvenile material are necessary for the\nCambrian explosion, this weathering process at least may have created unprecedented\nfavorable conditions for the Cambrian Explosion. For example, small shelly fossils were\nmainly recorded in phosphorus deposits of South China (Jin et al., 2016). The COPSE\nbiogeochemical models reveal that continental arc-driven CO outgassing and weathering\n2\nassociated with increased rate of organic carbon burial are capable of causing increasing\natmospheric pO levels to ~0.25 of the present atmospheric level (PAL), which exceeds\n2\nthe oxygen requirement threshold of large-body animals during the Cambrian Explosion\n(Williams et al., 2019). Animal diversification started since Ediacaran (perhaps ~580 Ma)\nand reached to the peak during the early Cambrian, coupled with the second pulse of\n9\nglobal phosphorus deposits, the Neoproterozoic oxygenation event after GOE, and\nclimatic warming (Lyons et al., 2014; Reinhard et al., 2017). Enhanced continental arc\nvolcanism fully match the whole time window of these environmental-biological events,\nalthough we focus on the peak of animal diversification during the early Cambrian. Once\nnutrient availability (P, Ca) and oxygen by subduction-driven weathering already fulfilled\nthe requirement threshold of the first animal evolution in the oceanic system at the dawn\nof the Phanerozoic, favorable conditions by other subduction-related weathering may\nbecome no particularly important in later Phanerozoic.\nThe transient basal Cambrian carbon isotope excursion (BACE, <1 Ma) may be\nlinked to tectonic-driven release of isotopically light CO , but the Li-Os-Sr shifts are\n2\ncaused by weathering dynamics, which is different from the driving mechanism of BACE\nevent. The weathering generally has a lagged response to tectonic events, and the\nresidence time of Li and Sr in seawater (>1 Ma) is enough to make them insensitive to\nBACE event. Our work provides not only subduction-driven enhanced erosion of uplifted\narc-related juvenile rocks at ca. 540–525 Ma (mainly the time of small shell fossil), but\nalso a switch to continental clay formation at 525–515 Ma (mainly the time of large-body\nanimals), and the latter could have increased efficient clay-bounded organic carbon burial,\nthus further facilitating oxygen accumulation required by large-body animals (Wei et al.,\n2024).\nMinor concerns\nMost are issues in grammar, typo, misleading statement etc. I only listed the ones\nthat caught my eye, there might be more.\nLine 277: “… but is significantly higher than in the preceding period” to “… but is\nsignificantly than that in the preceding period”.\nLine 300: “…nearly four times as high as the average crust” is a confusing statement.\nDid the authors mean four times as high as the background flux (as their setup)? Or higher\nthan the average flux assuming average erosion rate and average composition from the\ncontinental crust?\nFigure 5A and B: Typo. “Evolved continental mental” should be “Evolved\ncontinental mantle”. Figure 5B is also confusing. I would suggest the authors modify this\nfigure to show a dead continental arc instead of a completely disappeared arc.\nResponse: Small concerns have been revised following your comments."}, {"round": 2, "reviewer_comments": {"reviewer_2": "I can see that the authors have made a sincere effort to address my previous comments. Most of the other revisions concerning Li isotopes fall outside my area of expertise. However, for the sections that are technically within my domain, I do not have any major remaining concerns, apart from a few comments below, which are intended solely to help the authors avoid potential misunderstandings. At this stage, I believe the manuscript is suitable for publication.\nThe authors still make special emphasis on eroding of mafic rocks here and there in the manuscript. However, during the majority of continental arc evolution—and particularly during the main phase of intense erosion—the dominant lithologies are typically of andesitic to felsic composition. I already raised this issue in my initial review. While the authors responded using Os-Sr isotopic signatures, they did not consider the lithologic record, which is far more reliable than isotopic evidence alone. I got a feeling that the authors being persistent may be driven by a desire to support the argument for enhanced P and Ca fluxes to the ocean, due to the higher concentrations of these elements in mafic rocks. In arc magmas, there is a strong trend of P saturation towards andesitic composition, and the reason is discussed in Lee and Bachmann (2014). elevated P and Ca fluxes can naturally result from higher denudation rates and runoff, regardless of whether the eroded rocks are mafic. To be clear, I fully agree with the emphasis on \"juvenile\" or \"mantle-derived\" rocks. However, I strongly encourage the authors to refrain from repeatedly highlighting “basalt and arc-related mafic rocks” (e.g., lines 84, 281–282), as this is both unnecessary and potentially misleading. Notably, juvenile character and non-mafic arc-related lithology are not mutually exclusive.\nMinor issue:\nLine 93-94, “Given the fast-eroding features of continental arcs developed upon either an mantle-involved unradiogenic or ancient continental radiogenic basement…” should be “….either a mantle-involved unradiogenic or an ancient continental radiogenic basement…”.\nLine 327-329, “It is the enhanced erosion of arc-related juvenile rocks that might have accelerated nutrient availability (e.g., P, Ca) and then raised oxygen levels, promoting Cambrian animal diversity.” Accelerated nutrient availability and raised oxygen levels is not directly related, a link is missing here. Better rephrase this sentence.\nReference\nLee, C.-T. A., and Bachmann, O., 2014, How important is the role of crystal fractionation in making intermediate magmas? Insights from Zr and P systematics: Earth and Planetary Science Letters, v. 393, p. 266-274.", "reviewer_4": "The authors have made substantial improvements in response to earlier reviews. Quantifying phosphorus flux from continental arcs, clarifying the three-stage arc evolution model, and expanding discussion of diagenesis and contamination controls for Li isotopes strengthen the manuscript. The additional calculations linking arc dimensions, erosion rates, and P concentrations to modelled flux increases are a clear step forward.\nHowever, several areas still need clarification and tightening before publication:\n1. Geochemical mechanisms\nThe transition from congruent weathering of juvenile rocks to incongruent weathering of continental crust is central to the argument but not fully explained. The authors should briefly describe why juvenile mafic/intermediate lithologies weather congruently (rapid dissolution, minimal secondary clay retention, low δ⁷Li) and why old felsic crust tends to weather incongruently (feldspar → clay transformation, high δ⁷Li).\nThe statement that continental arcs can deliver unradiogenic ⁸⁷Sr/⁸⁶Sr fluxes needs context. Most arcs are radiogenic; unradiogenic flux occurs only when dominated by newly accreted, mantle-derived mafic rocks. This distinction should be made explicit.\n2. Terminology and clarity\nThe phrase “continental clay factory” is used without prior definition; introduce it on first use (e.g., large-scale clay production from incongruent weathering of continental crust).\nReplace awkward expressions such as “it is feasibly inferred” with “it is reasonable to infer” or similar.\n3. Interpretation issues\nOrganic-rich shale deposition is typically indicative of low-oxygen marine environments. If the argument is that burial of organic matter led to atmospheric oxygen accumulation, this causal link must be stated explicitly to avoid confusion.\nThe suggestion that Ca²⁺ flux from juvenile rock erosion “triggered the advent of biocalcification” is speculative, particularly given the sparse seawater Ca²⁺ data. Recast more cautiously as a potential contributing factor coincident with early Cambrian calcifiers.\nEvidence for increased continental arc length after 550 Ma should be clearly summarised in the main text, not only shown in Fig. 3F.\n4. Figures and data presentation\nPhosphate abundance data remain coarse; tonnage estimates help, but measured vs. inferred values should be clearly distinguished in figures.\nSparse Os data before 540 Ma and the limited Ca²⁺ dataset should be flagged prominently in captions to avoid overinterpretation.\nImprove figure captions for clarity (e.g., explain colour schemes in Fig. 5; clarify meaning of lines in Fig. 3C–E).\n5. Writing quality\nSeveral sentences require tidying for grammar and concision. For example:\n“A significant increase of seawater Sr isotope” → “A significant increase in seawater ⁸⁷Sr/⁸⁶Sr”.\n“LIPs only occurred…” → “LIPs during this interval were restricted to the Central Iapetus, covering only ~0.18 × 10⁶ km² and lasting ~2 Myr (535–533 Ma; Fig. 3F; Ernst et al., 2021), making them unlikely to explain a ~15 Myr negative shift in seawater Li–Os–Sr isotopes.”\n“It is illustrated that…” → “Both the expansion of active subduction zones along Gondwana’s margin and the increase in continental arc length after ~550 Ma (Fig. 3F) coincide with rising seawater ⁸⁷Sr/⁸⁶Sr.”\nRecommendation\nThe revision has addressed most substantive concerns and improved the robustness of the conclusions. However, the above clarifications, especially regarding geochemical mechanisms, data limitations, and writing precision, should be addressed before acceptance. I recommend minor revision focused on improving clarity, tightening the prose, and making the causal arguments more explicit.\nAnthony Dosseto"}, "author_response": "Response to reviewers\nResponses to Reviewer #2\nComment 1: In this revision, the authors clarified the citation for the compiled\ncontinental arc length during the E-C transition, and added more reasoning to exclude the\ncontribution from the LIPs to emphasize the arc effect. I am fine with their clarification\nfor the previously miss-cited data compilation, and I do not have much problem with their\ndata and modeling results, but I do think this manuscript would benefit from a more\nquantitative discussion on their enhanced P flux part. As reviewer 1 mentioned,\ncontinental arc volcanism driving the Cambrian explosion is not really a new idea. The\nauthors’ main contribution in this study is to provide some quantitative constraints on the\nmajor contributor to the Li-Sr-Or isotopic system, that there was indeed enhanced arc\nderived erosion during the E-C transition. Note that this is the only direct conclusion from\ntheir data. All the further discussions on how the continental arc activity drive the\nCambrian explosion is based on a 240% increase in Fhyd and a 3-4 fold increase of P flux\nderived through box modeling. My question is whether the increase of continental arc\nmagmatism during the E-C transition could be able to provide such increase in the erosion\nflux and P flux that match the authors modeling result. Since the authors already have the\ncompiled data of arc duration and length, they should be able to make some estimation of\nthe erosion and elemental flux.\nResponse 1: It is an inspiring idea to match our modelling results with geological\nestimation. We have estimated phosphorus (P) flux based on compiled data of arc length\nand P weathering laws (see details in the second part of Discussion section in the revised\nmanuscript). Briefly, assuming a continental area of 9 × 107 km2 (Godderis et al., 2003),\nan average chemical weathering rate of 2 t/km2/yr (reasonable for granitic catchments;\nMillot et al., 2002) and an average granitic P concentration of 650 ppm, the total\nbackground reactive P flux would be 1.5×109 mol P/yr (Horton, 2015), if 40% of\nweathered P become bioavailable. Then, we estimated a continental arc area of 6 × 106\nkm2 (~6.7% of continental area) based on the arc length exceeding 20,000 km and mean\n1\nwidth of 300 km (50–600 km of arc system; Capaldi et al., 2021). As shown in Fig. R1,\nsilicate minerals in mafic igneous rocks contain 1.5–2 times of higher P concentration\nthan granitic rocks because P5+ readily substitutes for Si4+ and P will be concentrated in\neasily weathered volcanic glass (Horton, 2015; Cox et al., 2016).\nFig. R1 A compilation of P O concentrations in continental flood basalts and volcanic arcs from\n2 5\n~81,000 samples (Cox et al., 2016). Peak P O concentrations are closely associated with basalt-\n2 5\nand arc-related mafic rocks.\nAn average chemical weathering rate of 40 t/km2/yr is then assumed based on\nmodern weathering rates for basaltic watersheds with moderate runoff (Millot et al., 2002;\nDessert et al., 2003). In high-runoff areas, such as Reunion Island, chemical weathering\nrates may be as high as 63–170 t/km2/yr (Louvat and Allegre, 1997). Although physical\n2\nerosion rates in modern settings can be many times greater than chemical weathering rates\n(Millot et al., 2002), we conservatively adopted an estimated physical erosion rate of ~80\nt/km2/yr, which is equal to the chemical weathering rate of Reunion island. With a\ncontinental arc area of 6 × 106 km2, physical erosion rate of ~80 t/km2/yr, P concentration\nof 975 ppm (1.5× granitic P concentration of 650 ppm), the total reactive P flux from\nphysical erosion of continental arc would be 6×109 mol P/yr, which is nearly four times\nof background P flux and also consistent with a 3.75× weathering flux as suggested by\nour models. Furthermore, based on seismic and gravity data from volcanic arcs in the\nwestern Pacific region, estimated arc magmatic addition rates of 30–95 km3/km/Myr\n(volume per unit width along the strike direction of arc; Dimalanta et al., 2002), which\nare converted to ~8.4×104–2.7×105 t/km/yr assuming an average mafic rock density of\n2.8 g/cm3, are capable of supporting long-term physical erosion. Therefore, the increase\nof continental arc magmatism during the E-C transition could be able to provide such\nincrease in the erosion flux of P that match our modeling result.\nResponses to Reviewer #3\nComment 1: The authors state the objective of this paper is to explore whether the\nsubduction-driven climatic warming and enhanced weathering of arc-related juvenile\nrocks played a significant role in the Cambrian Explosion (lines 83-86). From my reading\nof the ms, the sequence of cause and effect is as follows and differs for the two periods\n[540–525 Ma] and [525–515 Ma]. Validating the connections in the sequence is important,\nbut I wasn’t fully convinced.\n 540–525 Ma\n1. Tectonic activity: subduction; early stage of continental arc volcanism.\n2. A: CO degassing -> warm climate; B: Uplift of continental arcs -> silicate\n2\nweathering (drawdown of CO ) of juvenile crust; congruent weathering.\n2\n3. A: primary productivity; organic matter burial; O production; B: Nutrient\n2\ndelivery to the ocean.\n3\n4. Cambrian Explosion.\n 525–515 Ma\n1. Tectonic activity: subduction; later stage of continental arc volcanism\n2. A: Decreased CO degassing -> warm climate; B: Uplift of continental arcs ->\n2\nsilicate weathering (drawdown of CO ) of old crust; incongruent weathering (clay\n2\nformation)\n3. A: Decreased primary productivity; organic matter burial; O production; B:\n4. Cambrian Explosion slows down?\nThe difference in the two time periods from my reading of the ms has to do with the\nstage of continental arc evolution. The first phase (540–525 Ma) involves juvenile crust\nduring initial subduction and the second phase (525–515 Ma) involves older crust. It\nwould help the readers if this distinction was more clearly stated. To support Step 1,\nthe arc length (Fig. 3F) is given, but the dramatic increase in the arc length predates by\n~10 myrs any significant changes in the three isotope systems. Regarding Step 2A, it\nwould be helpful to have the “sedimentary carbonate stored in the upper continental plate”\n(Line 50) drawn in on Figure 5A and B to show clearly how the release of CO is greater\n2\nfor the 540–525 Ma period than the 525–515 Ma period. Is this the platform carbonate on\nthe margin of the continental plate or something interior to the continental crust (grey or\ngreen part of Fig 5A and B)? Also, the meaning of different coloring (green and grey) in\nFig. 5 is not explained in the caption. To support increased weathering of juvenile rocks\n(low Sr and Os isotope ratios 540–525 Ma) and later of older crustal material (high Sr\nand Os isotope ratios 525–515 Ma), Sr and Os isotope ratio data are used (Fig 3C and E)\n(Step 2B). Yet, there is a period before 540 Ma where the initial 187Os/188Os and 87Sr/86Sr\nmove in opposite directions. Granted there is a lack Os data points for this period, still\nthis relationship should be explained. Also, the general statement (lines 87–90) that there\nis a positive relationship between continental arc length and 87Sr/86Sr does not hold (Fig.\n3E and F).\nhighlights our viewpoints of the two distinct stages. As shown in Fig. R2B (Lee et al.,\n2015), continental arc evolution is typically divided into three stages at a timescale of\n40–50 Ma. Early stage of subduction is characterized by crustal thickening as the oceanic\nlithosphere initially subducted beneath continental lithosphere. The middle stage of\nsubduction is marked by extension of upper plate and intense juvenile magma addition.\nLate stage of subduction is defined by reduced juvenile magma addition. Following your\ncomments and suggestion, we have re-interpreted the links among continental arc\nevolution, sedimentary Li-Os-Sr isotope records (550–515 Ma; 35 Ma), seawater\nchemistry, and the Cambrian Explosion in three distinct stages as explained below\n(also see details in the Discussion section of the revised manuscript and Fig. 5):\n 550–540 Ma\n1. Tectonic activity: early stage of subduction (increased arc length at ca. 550 Ma);\ncontinental crustal thickening.\n2. CO degassing: gradual climatic warming.\n2\n3. Sedimentary isotope variations: decreased δ7Li (erosion of relatively P-poor\nuplifted crust); possible high Os (insufficient Re-Os isotopic analysis because of\ni\ndominated carbonate and limited shale deposition in South China and other basins) and\nincreased Sr isotope (weathering of continental crust)\n4. A: Low level of primary productivity; organic matter burial; O production; B:\n2\nlimited nutrient delivery (P) to the ocean\n 540–525 Ma\n1. Tectonic activity: middle stage of subduction; intense juvenile magma addition\nfollowing upper plate extension (see Fig. 3F in the revised manuscript, intense arc magma\nrecords of this stage) and uplift.\n2. Efficient CO degassing: warm climate.\n2\n3. Sedimentary isotope variation: decreased δ7Li to low values (enhanced erosion of\nuplifted P-rich juvenile rocks (1.5–2 times P of granitic crust); low seawater Os and Sr\ni\n5\nisotopic composition (preferential weathering of easily weathered juvenile rocks).\n4. A: High level of primary productivity; organic matter burial; O production;\n2\npositive δ13C values; B: Nutrient delivery to the ocean and global-scale Phosphorite\ncarb\n(significant change from Precambrian P-limited to Phanerozoic P-rich seawater\ncondition?)\n5. Appearance of small shell fauna (first stage of Cambrian Explosion)\n 525–515 Ma\n1. Tectonic activity: late stage of subduction; weak juvenile magma addition and/or\nuplifted juvenile rocks are consumed through preferential weathering at 540–525 Ma.\n2. Slow CO degassing and less efficient drawdown of CO because of incongruent\n2 2\nweathering: stabilized climate?\n3. Sedimentary isotope variation: increased δ7Li (incongruent weathering, clay\nformation); high Os and Sr isotopes (back to weathering of continental crust).\ni\n4. A: After the onset of enhanced phosphorus (P) flux around 540–525 Ma, P is no\nlonger limited in seawater and facilitates more efficient P cycling through positive\nfeedback between P and ecosystem; increased primary productivity; organic matter burial;\nO production; B: efficient delivery of continental clays to the oceans, increased clay-\n2\nbounded organic carbon burial, and further oxygen accumulation (Wei et al., 2024) as\nevidenced by organic-rich and high K/Al ratios of shale deposition over the whole\nYangtze Block following the large-scale transgression (Fig. 1C in the revised manuscript;\nJin et al., 2016).\n5. Appearance of large-body animals with high-oxygen requirements.\nbatholith); B: Total volumetric magma production for the Sierra Nevada batholith. Red curve includes\nmafic cumulates (down to 70 km) and is thus more representative of the total magmatic input;\nC: Paleo-depth estimates in the batholith versus time based on Al-in-hornblende barometry and U–Th/He\nand apatite fission track thermochronological results; D: Paleo-elevation of surface markers. See\ndetails in the Fig.5 of Lee et al. (2015).\nComment 2: To support congruent weathering of juvenile rocks for the 540–525 Ma\nperiod and incongruent weathering and clay formation for the 525–515 Ma period (Step\n2B), the authors use lithium isotopes. Over these periods, there has been a shift in\ncarbonate mineralogy from aragonite to calcite. The two mineral types involve different\nfractionation factors between seawater and carbonate. Shouldn’t this be considered in\ndiscussing the δ7Li record in Fig 2 and 3D? The elemental ratios (e.g. Al/(Ca+Mg), Mn/Sr,\n7\netc) that are used to test clay contamination or diagenetic alteration for the Li isotope data\nare a little higher (lenient) than what recent publications use. Also, data points that do not\nsatisfy the cutoff are retained based on the argument that there is no relationship between\nthe isotope ratio and the elemental ratio proxy. This is adopting a rather lenient condition.\nResponse 2: It is an inspiring comment about the detrital contamination, carbonate\nmineralogy, and diagenetic alterations that may overprint δ7Li values of ancient\ncarbonates as an archive of seawater. Following your suggestion, we have re-evaluated\nthe possible influence of these processes on our δ7Li values of carbonates (see details in\nSuppl. text).\nThe cutoff of carbonate Al/(Ca+Mg) ratios for clay contamination are variable\namong different researchers, such as 0.45 mmol/mol (Dellinger et al., 2020), 0.8\nmmol/mol (Pogge von Strandmann et al., 2013), and 1.2 mmol/mol (Cao et al., 2023).\nFor dolostones, higher Al/(Ca+Mg) ratios (even > ~1 mmol/mol) are not indicative of\nsilicate contamination because a higher proportion of Al in dolomites relative to other\ncarbonate phases (Dellinger et al., 2020). As shown in Fig. R3b, the δ7Li values of\ndolostones (~28–30‰) with high Al/(Ca+Mg) ratios (up to 1.2 mmol/mol) are still\nrecording seawater Li isotopic signals (~31‰), further indicating a limited silicate\ncontamination with this cutoff (Liu et al., 2023). More recently, carbonate Al/(Ca+Mg)\nratios of <1.2 mmol/mol are suggested as the threshold for silicate contamination using\ndifferent carbonate dissolution methods (Fig. R4; Cao et al., 2023). Given the dominance\nof dolostones in our section, Al/(Ca+Mg) ratios of <1.2 mmol/mol were adopted to\nindicate little or no silicate contamination, following which all our samples are subject to\nno or little silicate contamination.\nelemental ratios (Liu et al., 2023)\nFig. R4 Cross plots of δ7Li values VS Li/Ca ratios (a) and Li/Ca ratios VS Al/Ca ratios (b) in ancient\nmarine carbonates using different dissolution methods (Cao et al., 2023). Dashed lines represent the\nthreshold for carbonate-bounded Li isotope signal, i.e., Li/Ca<60 μmol/mol and Al/Ca < 1.2 mmol/mol.\nAncient carbonates are indeed thought to undergo multi-stage complex diagenetic\nprocesses and generally hard to understand post-depositional transformation of carbonate\nmineralogy (from aragonite or high-Mg calcite to low-Mg calcite or dolostone) and\ndiagenetic alterations in ancient carbonates. By comparison with typical diagenetic\ncharacteristics of modern marine carbonates (Fig. R5), we evaluate the possible influence\nof diagenetic processes on our δ7Li values of carbonates. Meteoric diagenetic carbonates\n9\nhave significantly lower δ7Li values than ambient seawater (Δ7Li ~ –\nmeteoric diagenetic–seawater\n8.0%) and also tend to decrease Li/(Mg + Ca) ratios (< 6 μmol/mol) because the Li/(Mg\n+ Ca) ratios of meteoric fluids are much lower than that of seawater (Dellinger et al.,\n2020). However, as illustrated Fig. R5, all of our carbonate samples have high Li/(Mg +\nCa) values greater than 6 μmol/mol, indicating little or minor meteoric interference.\nFig. R5 Cross plot of Li/(Mg + Ca) and Sr/(Mg + Ca) ratios for our carbonates. The stars represent\nthe typical compositions of aragonite, high-Mg calcite, low-Mg calcite and dolomite from marine\ncarbonates of South China Sea and Bahamas (Dellinger et al., 2020; Wei et al., 2023). The decreasing\nLi/(Mg + Ca) trend of meteoric diagenesis is adopted from Dellinger et al. (2020).\nOriginal aragonite displays a greater Li fractionation factor (Δ7Li\nSeawater-\n~12‰) and typically has higher Sr/(Ca+Mg) ratios (~9 mmol/mol; Dellinger et al.,\nAragonite\n2020), and both of them can be lowered by fluid-buffered diagenetic alteration of\naragonite (Dellinger et al., 2020; Wei et al., 2023). Low Sr/(Ca+Mg) ratios of our\ncarbonate samples are consistent with those of fluid-buffered diagenetic marine\ncarbonates in South China Sea and Bahamas rather than primary aragonite (Fig. R5;\ncontents of the Dengying Formation dolomites and Yanjiahe Formation dolomites and\nlimestones of a nearby Gaojiaxi–Yanjiahe section (~100 km away from our drillcore;\nFig. R6) further suggest early marine fluid-buffered diagenesis (Wei et al., 2022). δ7Li\nvalues of marine limestones and dolostones formed during marine fluid-buffered\ndiagenesis, are considered to approach the ambient seawater Li isotopic signals with a\nminor Li isotope fractionation of –0.5‰ to –2‰ (Dellinger et al., 2020; Wei et al., 2023).\nNo obvious correlations between δ7Li and these ratios further suggest limited overprint\nof these processes on our δ7Li values (Fig. S3 in Suppl. materials). Given there are\ncarb\nstill some uncertainties in the diagenesis and mineralogical composition of ancient\ncarbonates in literature, we employ a Locally Weighted Scatterplot Smoothing (LOWESS)\ncurve fitting method with a 95% confidence interval to produce a robust δ7Li curve at\nsw\nca. 560–515 Ma as presented in Fig. 3D.\nFig. R6 Geochemical profiles of the Gaojiaxi–Yanjiahe section, including carbonate carbon and Ca\nisotopes, Mg/Ca ratios and Sr contents (Wei et al., 2022). Noting that although limestones in the\nmiddle part of Dengying Formation have relatively low Ca isotope of < –1‰, they do not appear in\nour drillcore with dominated dolostones of Dengying Formation (Fig. 2 in the revised manuscript).\nComment 3: To support the increase in nutrients delivered by weathering (Step 3B),\nthe authors give phosphate distribution (Fig. 3B). The data is only qualitative (rare vs.\nabundant) yet the figure is drawn as if there is a drastic and quantifiable change. This\ncould be misleading. The Ca2+ concentrations changes (Fig. 3B) is based on only two data\npoints and yet the curve is drawn as if there is some structure to it (Fig 3B). I think detailed\nfigure caption or supplementary explanation is necessary.\nResponse 3: Thanks for pointing out these issues in detail. We have supplemented a\nquantitative metric ton of P O in phosphorites during the E-C transition (see details in\n2 5\nFig. 3B; Cook and Shergold, 1984). The two data points of seawater Ca2+ concentrations\nare from Brennan et al. (2004), while estimated seawater Ca2+ concentrations curve\nmarked by dashed line is from Horita et al. (2002). We have clearly stated these issues in\nfigure captions to prevent any misunderstanding.\nComment 4: Authors use multiple isotopes to provide a more robust interpretation\n(lines 174-176). The lithium and osmium isotope data covering the period of the\nCambrian Explosion are new and of interest to the community. The interpretation\nconnecting it to the Cambrian Explosion is interesting and plausible, but the logic needs\nto be tightened and carefully stated. I realize that the authors may be limited by the length\nimposed by the journal. Usually, other authors make full use of the Supplement to supply\nthose details.\nResponse 4: Thank you for reorganizing and summarizing our scientific\ncontributions. Following your suggestion, we have summarized a three-stage model\nregarding the links among subduction-driven weathering dynamics, seawater chemistry,\nand the Cambrian Explosion spanning 550–515 Ma (similar to the timescales of\ncontinental arc evolution) based on sedimentary Li-Os-Sr isotopic records (see details in\nin the Discussion part of the revised manuscript and Fig. 5), and also re-evaluated the\ninfluence of detrital contamination, carbonate mineralogy, and diagenetic alterations on\nour δ7Li values of carbonates (see details in Response 2). We appreciate your inspiring\nsuggestions again to significantly improve our work.\nComment 5: The material used for isotope ratio measurements should be separately\ngiven and the meaning stated briefly (Line 119-123). Are both carbonate and black shales\nintended to represent seawater values at the time?\nResponse 4: Thanks for your suggestion. We have separately stated “the Li isotopic\ndata of carbonates and Re-Os isotopic data of black shales and cherts that potentially\nrecord seawater isotopic compositions from two drillcores of the Yangtze Block, South\nChina covering the main interval of the Cambrian Explosion”. δ7Li values of marine\nlimestones and dolostones formed during marine fluid-buffered diagenesis are considered\nto approach ambient seawater Li isotopic signals with a minor Li isotope fractionation of\n–0.5‰ to –2‰ (Dellinger et al., 2020; Wei et al., 2023). Initial 187Os/188Os ratios of\norganic-rich black shales and cherts directly record the signals of contemporaneous\nseawater provided that the system is closed without post-depositional external fluid\nintrusion or exposure since sediment deposition, which is not observed in our section.\nComment 6: In the box model, only a few cases are explored. It would be more\npersuasive if the authors presented some other combinations of parameter changes\n(scenarios) they tested in the Supplementary material.\nResponse 6: Thanks for your suggestion. We have discussed different scenarios of\nparameter changes in hydrothermal and juvenile rocks-dominated weathering flux\nassuming different residence time relative to that of the modern ocean (e.g., 1×present\nand 0.5×present) for Sr-Os isotopic shift, and Δ = 5‰ or 8‰ and residence time\nsink\n(1×present and 0.5×present), riverine and hydrothermal flux and their δ7Li endmembers\nfor Li isotopic shift (see details in the discussion part of revised text).\nComment 7: State what the REE pattern for “typical hydrothermal fluids” (line 186)\nwould look like.\nResponse 7: revised as “Most of our Eu/Eu* values are around 1 (Fig. 2) and the\npatterns are distinct from typical hydrothermal fluids that show positive Eu anomaly (Fig.\nS1)”.\nComment 8: State a brief description of the box model used (line 191). For example,\nit is a dynamic box model.\nResponse 8: “dynamic” has been added in the box model description.\nComment 9: Consider rewriting lines 194-198, as the meaning does not come\nthrough well. The authors are not arguing necessarily for a 50% shorter residence time,\nper se, but only intend to give this as an extreme scenario. Correct?\nResponse 9: You are right, and we have revised the description as “a 375% increase\nin weathering flux, in the case of juvenile rocks-dominated weathering (F ), can well\njuv\nexplain the negative shifts of both Os and Sr at ca. 540–525 Ma, considering different\nresidence time relative to that of the modern ocean (e.g., 1×present and 0.5×present)”.\nComment 10: Line 61: Add (Fig. 1A). Line 63: Add (Fig. 3F). Line 66: Add (Fig.\n3A). Line 69: via -> by; Line 73: mantle-involved -> mantle-derived; Line 302: Cox et al\n2016 reference is missing from the list.\nResponse 10: These small issues have been revised following your suggestion.\nComment 11: Fig. 1C: Does [blue star + red age] notation mean something different\nfrom [blue star + blue age]?\nResponse 11: “blue star + red age” represents our new Re-Os ages. We have revised\n“red star + red age” as our new ages to avoid confusion with early published Re-Os/U-Pb\nages (See details in Fig. 1C).\nComment 12: Fig. 3 Please state in the caption what the pink lines in C, D, E are.\nResponse 12: Thanks for your suggestion. In the revised Fig. 3C, D, E, the lines\n(orange for Os, purple for Li and the pink for Sr) represent changes in seawater Li-Os-\ni\nSr isotopic compositions using the Locally Weighted Regression (LOWESS) curve fitting\nto show the. The lack of Os data control is marked by dashed orange line in Fig. 3C.\nGrey\ni\nfields indicate the upper and lower bounds at a 95% confidence interval in Fig. 3D. We\nhave clearly stated the meaning of different lines in figure captions.\nComment 13: F: The numbers given for each of the different arcs don’t match with\nthe ages given in Table S1.\nResponse 13: We have supplemented arc magma records of all continental arcs\nsummarized in Table S1 (see details in Fig. 3F). As there are multi-stage arc magmatic\nrecords in the entire cycle of continental arcs, we only listed arc magmatic records in Fig.\n3F that fall within our target time window of 560–510 Ma.\nReferences\nBrennan, S.T., Lowenstein, T.K., Horita, J., 2004. Seawater chemistry and the advent of\nbiocalcification. Geology 32, 473–476.\nCao, C., Liu, X.M., Wang, X.K., Chen, J., 2023. Effective use of limestones to reconstruct\nseawater Li isotope compositions- A community standard proposal. Chemical\nGeology 626, 121441.\nCapaldi, T.N., McKenzie, N.R., Horton, B.K., Mackaman-Lofland, C., Colleps, C.L.,\nStockli, D.F., 2021. Detrital zircon record of Phanerozoic magmatism in the southern\nCentral Andes. Geosphere 17, 876-897.\nCook, P.J., Shergold, J.H., 1984. Phosphorus, phosphorites and skeletal evolution at the\nPrecambrian-Cambrian boundary. Nature 308, 231–236.\nDellinger, M., Hardisty, D. S., Planavsky, N. J., Gill, B. C., Kalderon-Asael, B., Asael, D.,\nCroissant, T., Swart, P. K., West, A. J., 2020. The effects of diagenesis on lithium\nisotope ratios of shallow marine carbonates. American Journal of Science 320(2),\n150–184.\nDessert, C., Dupré, B., Gaillardet, J., François, L.M., Allègre, C.J., 2003. Basalt\n15\nweathering laws and the impact of basalt weathering on the global carbon cycle.\nChemical Geology 202, 257–273.\nDimalanta, C., Taira, A., Yumul, G.P., Tokuyama, H., Mochizuki, K., 2002. New rates of\nwestern Pacific island arc magmatism from seismic and gravity data. Earth and\nPlanetary Science Letters 202, 105-115.\nGoddéris, Y., Donnadieu, Y., Nédélec, A., Dupré, B., Dessert, C., Grard, A., Ramstein, G.,\nFrançois, L.M., 2003. The Sturtian 'snowball' glaciation: fire and ice. Earth and\nPlanetary Science Letters 211(1), 1–12.\nHorita, J., Zimmermann, H., Holland, H.D., 2002. Chemical evolution of seawater during\nthe Phanerozoic: Implications from the record of marine evaporites. Geochimica et\nCosmochimica Acta 66, 3733–3756.\nLee, C.-T.A., Thurner, S., Paterson, S.R., Cao, W., 2015. The rise and fall of continental\narcs: interplays between magmatism, uplift, weathering, and climate. Earth and\nPlanetary Science Letters 425, 105–119.\nLiu, X.F., Liu, X.M., Wang, X.K., Zhai, S.K., Liu, X.Y., 2023. Dolostone as a reliable\ntracer of seawater lithium isotope composition. Communications Earth &\nEnvironment 4, 58.\nLouvat, P., Allegre, C.J., 1997. Present denudation rates on the island of Reunion\ndetermined by river geochemistry: Basalt weathering and mass budget between\nchemical and mechanical erosions. Geochimica et Cosmochimica Acta 61, 3645–\n3669.\nMillot, R., Gaillardet, J., Dupré, B., Allègre, C.J., 2002. The global control of silicate\nweathering rates and the coupling with physical erosion: new insights from rivers of\nthe Canadian Shield. Earth and Planetary Science Letters 196, 83–98.\nWei, G.Y., Hood, A.V.S., Planavsky, N.J., Li, D., Ling, H.F., Tarhan, L.G., 2022. Calcium\nisotopic constraints on the transition from aragonite seas to calcite seas in the\nCambrian. Global Biogeochemical Cycles 36, e2021GB007235.\nWei, G. Y., Zhang, F., Yin, Y. S., Lin, Y. B., von Strandmann, P. A. P., Cao, M., 2023. A\n13 million-year record of Li isotope compositions in island carbonates: Constraints\non bulk inorganic carbonate as a global seawater Li isotope archive. Geochimica et\nCosmochimica Acta, 344, 59–72.\nWei, G.Y., Zhao, M.Y., Sperling, E.A., Gaines, R.R., Kalderon-Asael, B., Shen, J., Li, C.,\nH.F., Planavsky, N.J., Tarhan, L.G., 2024. Lithium isotopic constraints on the\nevolution of continental clay mineral factory and marine oxygenation in the earliest\nPaleozoic Era. Science Advances 10, eadk2152.\nResponses to Reviewer #2\nComment 1: I can see that the authors have made a sincere effort to address my previous\ncomments. Most of the other revisions concerning Li isotopes fall outside my area of\nexpertise. However, for the sections that are technically within my domain, I do not have\nany major remaining concerns, apart from a few comments below, which are intended\nsolely to help the authors avoid potential misunderstandings. At this stage, I believe\nthe manuscript is suitable for publication.\nResponse 1: We are grateful for your recognition of our effort, and the issues you raised\nhave been addressed point-by-point below.\nComment 2: The authors still make special emphasis on eroding of mafic rocks here and\nthere in the manuscript. However, during the majority of continental arc evolution—and\nparticularly during the main phase of intense erosion—the dominant lithologies are\ntypically of andesitic to felsic composition. I already raised this issue in my initial review.\nWhile the authors responded using Os-Sr isotopic signatures, they did not consider the\nlithologic record, which is far more reliable than isotopic evidence alone. I got a feeling\nthat the authors being persistent may be driven by a desire to support the argument for\nenhanced P and Ca fluxes to the ocean, due to the higher concentrations of these elements\nin mafic rocks. In arc magmas, there is a strong trend of P saturation towards andesitic\ncomposition, and the reason is discussed in Lee and Bachmann (2014). elevated P and Ca\nfluxes can naturally result from higher denudation rates and runoff, regardless of whether\nthe eroded rocks are mafic. To be clear, I fully agree with the emphasis on \"juvenile\" or\n\"mantle-derived\" rocks. However, I strongly encourage the authors to refrain from\nrepeatedly highlighting “basalt and arc-related mafic rocks” (e.g., lines 84, 281–282), as\nthis is both unnecessary and potentially misleading. Notably, juvenile character and non-\nmafic arc-related lithology are not mutually exclusive.\nResponse 2: We fully agree that the weathering of “juvenile or mantle-derived” rocks\nsufficiently explain our Os-Sr-Li isotopic signatures and enhanced P and Ca fluxes,\neliminating the need to specifically emphasize “basalt or arc-related mafic” rocks. To\navoid any misunderstanding, we have replaced “basalt and arc-related mafic rocks” with\n“juvenile or mantle-derived” rocks throughout the revised manuscript.\nComment 3: Line 93-94, “Given the fast-eroding features of continental arcs developed\nupon either an mantle-involved unradiogenic or ancient continental radiogenic\nbasement…” should be “….either a mantle-involved unradiogenic or an ancient\ncontinental radiogenic basement…”.\nResponse 3: We have revised this sentence accordingly.\nComment 4: Line 327-329, “It is the enhanced erosion of arc-related juvenile rocks that\nmight have accelerated nutrient availability (e.g., P, Ca) and then raised oxygen levels,\npromoting Cambrian animal diversity.” Accelerated nutrient availability and raised\noxygen levels is not directly related, a link is missing here. Better rephrase this sentence.\nResponse 4: We have rephrased this sentence to clearly clarify the link between enhanced\nnutrient availability (e.g., P, Ca) and raised oxygen levels: “The enhanced erosion of\njuvenile arc-related rocks increased the input of key nutrients such as phosphorus (P) into\nthe ocean, which stimulated marine primary productivity. This elevated productivity, in\nturn, drove higher export and burial of organic carbon. The sequestration of organic\ncarbon facilitated net accumulation of atmospheric oxygen, thereby increasing oxygen\nlevels that ultimately satisfied the demands of Cambrian animal diversification”.\nResponses to Reviewer #4\nComment 5: The authors have made substantial improvements in response to earlier\nreviews. Quantifying phosphorus flux from continental arcs, clarifying the three-stage arc\nevolution model, and expanding discussion of diagenesis and contamination controls for\nLi isotopes strengthen the manuscript. The additional calculations linking arc dimensions,\nerosion rates, and P concentrations to modelled flux increases are a clear step forward.\nResponse 5: Thank you very much for recognizing our efforts.\nComment 6: The transition from congruent weathering of juvenile rocks to incongruent\nweathering of continental crust is central to the argument but not fully explained. The\nauthors should briefly describe why juvenile mafic/intermediate lithologies weather\ncongruently (rapid dissolution, minimal secondary clay retention, low δ⁷Li) and why old\nfelsic crust tends to weather incongruently (feldspar → clay transformation, high δ⁷Li).\nResponse 6: Thanks for the insightful comment. We have clarified why juvenile\nmafic/intermediate lithologies weather congruently in the revised text. Mineral\ndissolution rates decrease sequentially with olivine > Ca-plagioclase > pyroxene > Na-\nplagioclase > K-feldspar > muscovite > quartz (Kump et al., 2000; Sun et al., 2018).\nConsequently, juvenile mafic/intermediate lithologies—primarily composed of pyroxene,\nplagioclase, olivine, and volcanic glass—weather rapidly and highly congruently relative\nto old felsic crust (Dessert et al., 2003). This behavior reflects their geochemical freshness\nand aluminum-deficient composition, which suppress secondary mineral precipitation\n(e.g., clays; Sun et al., 2018). Furthermore, tectonically uplifted fresh rocks in warm\nclimates promote rapid mineral dissolution, enhancing runoff with low riverine δ⁷Li and\nhigh Li fluxes (Pogge von Strandmann et al., 2021). Intense continental arc volcanism\nsatisfies these conditions through: (i) more efficient CO degassing driving climatic\n2\nwarming (McKenzie et al., 2016), and (ii) subduction-driven uplift of highly weatherable\njuvenile arc rocks.\nComment 7: The statement that continental arcs can deliver unradiogenic 87/86Sr fluxes\nneeds context. Most arcs are radiogenic; unradiogenic flux occurs only when dominated\nby newly accreted, mantle-derived mafic rocks. This distinction should be made explicit.\nResponse 7: We have contextualized the continental arc controls on unradiogenic\n87Sr/86Sr fluxes derived from weathering of accreted, mantle-derived juvenile rocks (see\nthe second part of Discussion in the revised text). Continental arcs achieve isotopic\njuvenility through tectonic processes—including oceanic terrane accretion, upper-plate\nextension, slab breakoff, and ridge subduction (Cao et al., 2022)—which strongly\nregulated unradiogenic Sr fluxes and low seawater 87Sr/86Sr during the mid-Cretaceous\n(125–85 Ma; Cao et al., 2022). It is inferred that Ediacaran-Cambrian continental arcs\nperhaps shared similar characteristics. First, Gondwana-margin arcs constituted external\norogenic systems (e.g., Circum-Pacific), where ongoing arc magmatism could develop\nisotopically juvenile signatures (Collins et al., 2011). Second, as shown in Fig. 3f,\nmagmatic records from disparate continental arcs broadly cluster around 540–525 Ma,\nand their emplacements during upper-plate extension could likewise generate isotopic\njuvenility.\nComment 8: The phrase “continental clay factory” is used without prior definition;\nintroduce it on first use (e.g., large-scale clay production from incongruent weathering of\ncontinental crust). Replace awkward expressions such as “it is feasibly inferred” with “it\nis reasonable to infer” or similar.\nResponse 8: We have revised as suggested.\nComment 9: Organic-rich shale deposition is typically indicative of low-oxygen marine\nenvironments. If the argument is that burial of organic matter led to atmospheric oxygen\naccumulation, this causal link must be stated explicitly to avoid confusion.\nResponse 9: We appreciate this insightful comment. Our revised text now explicitly\nemphasizes the causal link between organic burial and atmospheric oxygen accumulation.\nThe enhanced bioavailable phosphorus delivery to oceans may have significantly\nincreased primary productivity, amplifying both photosynthetic oxygen production and\norganic carbon export. Although oxygen consumption during the remineralization of\nsinking organic matter promotes localized anoxia in facies such as deep-water shelf that\nis favorable for organic-rich shale formation, the persistent burial of organic carbon, as\nevidenced by generally positive δ13C values (Fig. 3g), results in net accumulation of\ncarb\natmospheric oxygen. This occurs because, once oxygen is depleted in subsurface waters,\nfurther organic matter settling and burial can proceed without additional oxygen\nconsumption, while ongoing photosynthesis continues to release oxygen, leading to a net\natmospheric oxygen increase (Berner, 2006). Concurrent with these processes, the\noxygenation of shallow water habitats is also enhanced via air–sea gas exchange,\nfulfilling the oxygen requirement of early animals and accelerating animal diversification.\nWe have articulated this causal relationship in the fourth section of the Discussion to\navoid confusion.\nComment 10: The suggestion that Ca2+ flux from juvenile rock erosion “triggered the\nadvent of biocalcification” is speculative, particularly given the sparse seawater Ca2+ data.\nRecast more cautiously as a potential contributing factor coincident with early Cambrian\ncalcifiers.\nResponse 10: We agree that this claim need to be cautious. Accordingly, we have adopted\na more cautious phrasing “potentially contributed to the advent of skeletonized animal\nbiocalcification” to avoid any overinterpretation.\nComment 11: Evidence for increased continental arc length after 550 Ma should be\nclearly summarized in the main text, not only shown in Fig. 3F.\nResponse 11: We have supplemented the evidence for increased continental arc length\nafter 550 Ma in the main revised text as “Based on compiled timing and average arc\nlength data of continental arcs along the margin of Gondwana (e.g., ~6,750 km of\nAvalonia-Cadomian arc, ~5, 775 km of Gondwana proto-Tethyan margin arc), the total\narc length increased from ~11, 500 km in pre-550 Ma to 21, 100 km in post-550 Ma\n(Suppl. Table S1 and Fig. 3f)”.\nComment 12: Phosphate abundance data remain coarse; tonnage estimates help, but\nmeasured vs. inferred values should be clearly distinguished in figures.\nResponse 12: The curve depicting total P O tonnes in phosphorite was estimated by\n2 5\ncompiling P O content data (distinguishing between measured and inferred values where\n2 5\napplicable) from the global-scale spatial and temporal distribution of phosphorite deposits\nduring the Ediacaran-Cambrian transition. We have clearly stated the meaning of P O\n2 5\ndata curve in Figures 3b caption to avoid confusion.\nComment 13: Sparse Os data before 540 Ma and the limited Ca²⁺dataset should be\nflagged prominently in captions to avoid overinterpretation.\nResponse 13: We agree that the sparsity of Os isotope data before 540 Ma and the limited\nCa2+ dataset warrant prominent caution. Accordingly, we have added a dedicated note in\nthe captions of Figures 3b and 3c to explicitly highlight the sparse data coverage for both\nOs isotopes (>540 Ma) and Ca2+, and also have presented these specific data ranges with\ndashed lines.\nComment 14: Improve figure captions for clarity (e.g., explain colour schemes in Fig. 5;\nclarify meaning of lines in Fig. 3C–E).\nResponse 14: In the revised manuscript, we have clearly clarified colour schemes in Fig.\n5 caption and the meaning of lines in Fig. 3c–e captions.\nComment 15: Several sentences require tidying for grammar and concision. For example:\n“A significant increase of seawater Sr isotope” → “A significant increase in seawater\n87Sr/86Sr”. “LIPs only occurred…” → “LIPs during this interval were restricted to the\nCentral Iapetus, covering only ~0.18 × 10⁶ km² and lasting ~2 Myr (535–533 Ma; Fig. 3F;\nErnst et al., 2021), making them unlikely to explain a ~15 Myr negative shift in seawater\nLi–Os–Sr isotopes.” “It is illustrated that…” → “Both the expansion of active subduction\nzones along Gondwana’s margin and the increase in continental arc length after ~550 Ma\n(Fig. 3F) coincide with rising seawater 87Sr/86Sr.”\nResponse 15: Thanks for your comments, and we have crosschecked these issues\nthroughout the manuscript.\nComment 16: The revision has addressed most substantive concerns and improved the\nrobustness of the conclusions. However, the above clarifications, especially regarding\ngeochemical mechanisms, data limitations, and writing precision, should be addressed\nbefore acceptance. I recommend minor revision focused on improving clarity, tightening\nthe prose, and making the causal arguments more explicit.\nResponse 16: We appreciate your suggestions, which have been addressed in the revised\nversion.\nCited References\nBerner, R. A. GEOCARBSULF: A combined model for Phanerozoic atmospheric O and\nisotopic juvenility of mid-Cretaceous continental arcs: Global compilations and\nimplications for chemical weathering. Earth and Planetary Science Letters 587,\n117550.\nCollins, W.J., Belousova, E.A., Kemp, A.I.S., Murphy, J.B., 2011. Two contrasting\nPhanerozoic orogenic systems revealed by hafnium isotope data. Nature Geoscience\n4, 333–337.\nDessert, C., Dupré, B., Gaillardet, J., François, L.M., Allègre, C.J., 2003. Basalt\nweathering laws and the impact of basalt weathering on the global carbon cycle.\nChemical Geology 202, 257–273.\nKump, L.R., Brantley, S.L., Arthur, M.A., 2000. Chemical, weathering, atmospheric CO ,\n2\nand climate. Annual Review of Earth and Planetary Sciences 28, 611–667.\nMcKenzie, N.R., Horton, B.K., Loomis, S.E., Stockli, D.F., Planavsky, N.J., Lee, C.-T.A.,\n2016. Continental arc volcanism as the principal driver of icehouse–greenhouse\nvariability. Science 352, 444–447.\nPogge von Strandmann, P. A., Jones, M. T., West, A. J., Murphy, M. J., Stokke, E. W.,\nTarbuck, G., Wilson, D., Pearce, C.R., Schmidt, D.N., 2021. Lithium isotope\nevidence for enhanced weathering and erosion during the Paleocene-Eocene\nThermal Maximum. Science Advances, 7(42), eabh4224.\nSun, H., Xiao, Y.L., Gao, Y.J., Zhang, G.J., Casey, J.F., Shen, Y.A., 2018. Rapid\nenhancement of chemical weathering recorded by extremely light seawater lithium\nisotopes at the Permian-Triassic boundary. Proceedings of the National Academy of\nSciences of the United States of America 115, 3782–3787."}]

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