Limited long-term cooling effects of Pangaean flood basalt weathering

Domain: earth_science
Published: 2025-05-23
Editorial outcome: minor_revision

Abstract
The emplacement of large igneous provinces (LIPs) is known to be a driver of climate change in Earth’s past. However, the balance of climate warming through CO2 emission and cooling through weathering is poorly understood. To better understand the role of LIP emplacement on long-term climate change, here we utilize the SCION coupled climate-biogeochemical model which considers the impact of LIPs through degassing of CO2 and enhancement of local continental weathering rates. This approach results in better correspondence between the modelled output and proxy reconstructions of the period (especially for seawater Sr isotope composition) when compared to previous modelling studies. Of the seven LIPs during the breakup phase of Pangaea (approximately 300–150 Ma), only the Central Atlantic Magmatic Province (CAMP) drives noticeable long-term cooling in the model, a minor effect (between 1-2°C) despite emplacement of a very large surface area in the humid tropics. Similarly, only the CAMP imparts a significant change in the long-term Sr isotope record whereas the other LIPs of this period do not. Due to limited areal extents, and emplacement outside tropical weathering zones, we conclude most LIPs have no significant global cooling effect on multi-million year timescales.

Introduction
The emplacement of large igneous provinces (LIPs), systems of voluminous mafic magmatism related to processes other than seafloor spreading, has occurred regularly through Earth’s history. The release of large quantities of greenhouse gases (up to 1.7 × 105 Gt CO2 in the case of the Siberian Traps), and the emplacement of considerable volcanic terranes associated with LIPs are known to impact many global biogeochemical cycles. Greenhouse gas release may cause rapid warming, with large-scale sulfate release driving short-term cooling. As a result, linkages between LIP emplacement and large-scale changes in the Earth system (e.g., environmental and climatic shifts), are often made, convincingly linking LIP emplacement to a number of mass extinctions. In addition to their potential importance in geologically rapid perturbations of the Earth system such as those observed during mass extinctions, LIPs may be important for setting the climate state of Earth over longer time periods, potentially as drivers of long-term cooling.

The period between 300 and 150 million years ago (Ma), when the supercontinent Pangaea began to rift and break apart, initiated many of the Earth’s systematic and evolutionary upheavals that led to the planet’s current configuration. During this time some of the largest LIPs in Earth’s history were emplaced, sometimes coinciding with mass extinctions. These include the Siberian Traps (252 Ma), which is the largest continental LIP by volume, and widely thought to be the driver of the End-Permian Mass Extinction. Later in the Mesozoic, the Central Atlantic Magmatic Province (CAMP; 201 Ma), the largest continental LIP by area, was linked to the end-Triassic extinction. Further, the Karoo and Ferrar LIPs (183 Ma) have been implicated in the end-Pliensbachian extinction, and Toarcian anoxic event, respectively.

It is generally assumed that one of the most damaging impacts of LIPs on the biosphere is carbon-rich volatile release (especially CO, CO2, and CH4) enhancing the greenhouse effect and resulting in catastrophic global warming. This is especially true for examples of LIPs that were emplaced into organic carbon-rich sediment, such as the Siberian Traps. However, research has also shown how large volcanic episodes can lead to long-term global cooling. This cooling can occur via the supply of mantle- or crustal-derived nutrients to the oceans leading to enhanced carbon sequestration via the biological pump, and through an enhanced silicate weathering cycle fuelled by highly weatherable volcanic rocks. Consequently, the full impact of LIP emplacement on climate is uncertain, and very few modelling studies consider concurrently the likely cooling and warming impact of LIPs together. As such, the exact balance and interplay between LIP-driven cooling and warming mechanisms are unclear. For example, recent work that compiled the area of LIPs within low latitudes over the Phanerozoic concluded that they were not a primary control on greenhouse–icehouse cycles, but other recent work argues that the Neoproterozoic Franklin LIP was a critical factor in initiating snowball Earth.

To determine if major LIPs could have led to cooling on multimillion-year timescales, and to investigate the cumulative impact of numerous LIP emplacements, we use a long-term climate-biogeochemical model (SCION) integrated with the record of LIP emplacement between 300 and 150 Ma. SCION uses a 3D emulated climate, which allows us to move beyond simple consideration of latitude bands to consider intersections of LIPs with local temperature, relief and hydrology, which is essential for estimating weathering—especially considering the prevalence of extensive aridity in Pangaea. Furthermore, SCION produces a range of isotopic tracers, as well as predictions for global temperature and CO2 concentration which can be compared to the geological record to test the validity of its predictions. By constraining the model against multiple proxy systems it is possible to make clear quantitative tests of hypotheses. As well as incorporating volumes of degassed CO2 during emplacement, these LIPs are added to the 2D model land surface grid as basaltic terranes and interact with local temperature, relief and hydrology to amplify silicate weathering rates (Fig. 1, see the “Methods” section).

Here, we use an updated version of the SCION model, which considers the high weatherability of LIPs alongside their degassing impact for the period 300–150 Ma. Our approach allows us to simultaneously consider the warming and cooling potential of each individual LIP on the Earth system over long timescales. This is especially important during this period of climatic and evolutionary upheaval.

Results
The addition of LIPs to the SCION model impacts the carbon cycle and therefore changes the model reconstruction of climate between 300 and 150 Ma (Figs. 2 and 3). By calculating a total carbon balance between the input of carbon from a LIP minus the removal of carbon via LIP weathering (see the “Methods” section, Fig. 2a), we can disentangle the impact of individual LIPs on carbon cycling. To do this, we run a range of model scenarios (Table S1, see the “Methods” section), these include a baseline (the unaltered SCION code), a model with only LIP weathering considered, and scenarios with only degassing, and with weathering and degassing considered. Unless otherwise stated, our discussion below refers only to the output of the scenario ‘Weathering & Degassing High’ (Table S1, see the “Methods” section).

Our model results identify that on multimillion year timescales, LIP weathering is the dominant driver of LIP-related carbon cycling perturbations, with the majority of the studied timeframe characterized by LIP-induced carbon drawdown, as indicated by carbon balance values below zero (Fig. 2a). This makes sense as LIPs become a permanent feature of the weathering environment, whereas CO2 release is short-lived. The suggestion that LIPs may lead to global cooling has been made previously, but it has been challenging to quantify the exact balance between LIP carbon emission and removal. At 300 Ma LIPs are a net CO2 sink in the model (Fig. 2a), reflecting the weathering of previously-established terranes. Following the emplacement of the Siberian Traps (252 Ma) there is a slight enhancement of net LIP-related CO2 removal in the model (Fig. 2c), reflecting the weathering of Siberian Traps material (Fig. 3c). However, the location of this LIP in the high latitudes (Fig. 1, Fig. S1) means the silicate weathering feedback is muted, and weaker than if it were emplaced in the tropics. The signature of this weathering is also noticeable in the seawater 87Sr/86Sr, which in the weathering-only scenario, declines slightly in the period following the emplacement of the Siberian Traps (Fig. 4b). From 220 Ma onwards, global cooling, driven by pCO2 reduction in the period 220–200 Ma is reconstructed by SCION (Fig. 4d). However, this is also a feature of the ‘baseline’ model scenario, so is not related to LIP activity.

The CAMP, emplaced from 200 Ma onwards in the model, leads to the highest LIP-related carbon burial values in the Mesozoic (Fig. 2c). The location of the CAMP across the equatorial region (Figs. 1 and 3) and adjacent to an incipient ocean basin, means it is subjected to the most intense chemical weathering regime. The impact of this weathering is most clearly seen in the model seawater 87Sr/86Sr record for the ‘weathering only’ scenario (Fig. 4b), which diverges considerably from the baseline SCION run (scenario 'baseline' in Fig. 4b) and is much closer to proxy reconstructions (Fig. 4b). Intense LIP weathering leads to effective CO2 drawdown through silicate weathering (Fig. 3c) and associated carbonate deposition, and also through nutrient delivery to oceans coupled with a strong biological pump, resulting in higher levels of organic carbon burial in the model. The action of both these carbon sinks leads to slightly cooler climate between 200 and 180 Ma relative to baseline SCION (Fig. 4), but it is noticeable that this shift is only around 1°C (Figs. 3a and 4c), echoed by a small decrease in pCO2. This shift may go a small way to reconciling the early Jurassic cooling, an event which has proven enigmatic to explain. However, cooling of up to 5°C, as seen in some proxy records in the early Jurassic is not reproduced by our model. It is possible that we underestimate the level of weatherability enhancement represented by basaltic LIP emplacement, meaning we underestimate the cooling impact of LIPs. However, the good correspondence between the seawater 87Sr/86Sr record and our model results for the scenario ‘weathering and degassing high’ suggests the amount of additional unradiogenic material being input to the oceans is of the correct order of magnitude, at least for the period 250–180 Ma (Fig. 4b). Indeed, to reach 5°C of cooling following the CAMP emplacement, the weathering enhancement value must be raised to 30-fold (Fig. S2b), a value which invalidates the Sr isotope curve, resulting in values well below the reconstructed proxy data (Fig. S3). A weathering enhancement of 15-fold remains possible given the Sr isotope curve across the CAMP (Fig. S3), suggesting it may be a plausible maximum value for this LIP (and one which results in 2°C of cooling), but this would require considerable uplift of basaltic material. A weathering enhancement value of seven-fold, as used by previous studies, therefore appears suitable, suggesting LIPs—even the areally extensive CAMP—are unable to cause considerable cooling on multimillion year timescales. From 200 to 150 Ma, the ongoing influence of CAMP basalt weathering is evident in the modelled outputs, keeping global temperatures and pCO2 low despite the emplacement and degassing of two other major LIPs. These LIPs (the Karoo-Ferrar and the NW Australian) have only short-term impacts on temperatures and pCO2 (Fig. 4).

One of the clearest impacts of the addition of LIPs to the SCION model is on the Sr isotope system. Prior to the emplacement of the Siberian Traps, the model is unable to reproduce most of the Mesozoic trends in oceanic Sr isotope composition (Fig. 4). This offset has previously been linked to the poor availability of surface lithological data, and so the inclusion of LIPs as sources of Sr with igneous signatures more strongly reconciles model outputs with the geological record. In particular, the long-term drop in seawater 87Sr/86Sr after 200 Ma, after the CAMP emplacement, is well produced by our updated SCION model. Here, the drop is linked to enhanced basaltic weathering from the emplacement of the LIP at this time (Fig. 4b), where this basalt has a mantle-like Sr isotope composition and therefore acts to reduce seawater 87Sr/86Sr. Further, Sr input resulting from enhanced continental weathering under high global temperatures also improves the comparability of the data to the model outputs. This is particularly clear across the P–T boundary, where 87Sr/86Sr rises markedly. This rise is driven by the warming impact of the Siberian Traps and proportional increase in weathering of radiogenic continental crustal (non-LIP) material—as the LIP was emplaced at higher latitudes away from major weathering zones. In general, the model reconstruction of carbonate δ13C is within the range of proxy reconstructions (Fig. 4a), although as a result of the single-ocean box in SCION (meaning marginal environments are not considered), there is little variability outside of the major LIP emplacement periods.

Outside of the Siberian Traps and the CAMP, for most of the time between 300–150 Ma, there is little evidence of LIPs driving large scale, multimillion year cooling through enhanced weathering in our model, contrary to suggestions that LIP weathering is a long-term driver of cool climate. Our results agree with work that finds no correlation between LIP area and ice sheet size, and suggests that any correspondence between emplacement and long-term cooling may not be LIP-weathering related. One potential explanation is that the deposition of volcanic ash, known to drive periods of transient cooling, may be more important than basalt emplacement as a driver of CO2 removal on long timescales during LIP episodes. For example, during the late Ordovician, ash supply was sufficient to drive up to 3°C of cooling for up to a million years in a global biogeochemical model. There was no LIP associated with the late Ordovician, but many LIPs had periods of intense explosive volcanism alongside the large-scale effusive eruptions. For example, the North Atlantic Igneous Province in the Cenozoic had an extended episode of basaltic ash deposition. This hypothesis requires further testing, but would explain how LIPs may still be linked with global cooling, despite the apparently small impact of their weathering on global climate.

Despite our focus on the cooling impact of LIPs, to consider overall impact we also model contributions to solid-Earth carbon degassing. The SCION model uses the baseline degassing rate of ref. 35 and in this work we add an additional component for contributions from LIPs in three scenarios (see Table S1, see the “Methods” section). Here we reconstruct LIP degassing rates using the timing of LIP emplacement unless the exact period of degassing is known (see the “Methods” section). As such, if pulsed degassing occurred over extremely short timescales such as centuries, as proposed for the CAMP, our model would not capture this short-term warming.

As expected, we see transient warming events during the largest LIP emplacements in our study period (Fig. 4c). At the end of the Permian and beginning of the Jurassic periods, the impact of massive, rapid carbon release from LIPs is clear (Fig. 4c, d). Compared to other LIPs in the Mesozoic, the rate and scale of carbon released during the emplacement of the Siberian Traps is nearly an order of magnitude higher (Fig. 2b, ref. 15). In the ‘baseline’ SCION scenario, transient carbon cycle perturbations from LIPs are not included (Fig. 4, black dashed lines), but in scenario ‘weathering and degassing high’, the P–Tr boundary is represented by a shift to pCO2 as high as 10,000 ppm (Fig. 4d), reflective of proxy data from the period. This is reflected in the Global Average Temperature (GAT) as modelled in our ‘weathering and degassing high’ scenario, which rises roughly 7°C across the P–Tr. However, this rise is well below what has been reconstructed using proxy data, which show up to 15°C rise in tropical temperatures (and thus presumably more at the global scale) across the P–Tr. This disconnect is potentially related to the low climate sensitivity and relatively high pre-event CO2 within SCION. Later in the Mesozoic, we also see the warming impact of the emplacement of the Central Atlantic Magmatic Province (201 Ma), with a rise of 4°C in our model (Fig. 4c). This contrasts with reconstructions of above 5°C change or up to 16°C change in some proxy data. For the Karoo-Ferrar (182 Ma), we see a rise of 3°C. Again, this is below proxy reconstructions of between 4 and 7°C shift across this interval. Our model underestimates early Mesozoic (250–200 Ma) temperatures (Fig. 4c), a feature of the original SCION configuration, potentially linked to non-carbon aspects of the Earth system such as albedo, or arelated to low climate sensitivity of the climate model used (FOAM).

Despite showing significant warming, our temperature results for LIP-induced degassing in the ‘weathering and degassing high scenario’ tend to be lower than previously suggested. This may be because our degassing rates are controlled by the length of LIP emplacement, and so may result in underestimation of rates if pulsed degassing during the LIP occurred. By running comparative models which reduce the length of time the degassing occurred for to 50 kyr for each event, we can reconstruct changes in climate which are much closer to proxy reconstructions (Fig. S2a). In this scenario (‘Weathering & Degassing 50kyr’), the temperature rise across the P–T is 9°C, and for the CAMP it is 8°C, much closer to the reconstructions of 15°C for the P–T and >5°C for the CAMP emplacement (ref. 49 Fig. S2), suggesting carbon volume estimates may be correct, but the periods of degassing may have been much shorter than the lifetime of the LIP. A further model scenario which fully considers the role of cryptic degassing (Weathering & Degassing Cryptic), using the approach of ref. 53, is also able to reconstruct up to 15°C warming for the P–T (Fig. S2a).

Most studies linking LIPs to global climatic change have focussed on the warming impact of LIPs. However, other studies claim that weathering of highly reactive basaltic terranes associated with LIP emplacement is a driver of global cooling. While we reconstruct the short-term warming in our model, we see little evidence of Mesozoic LIPs driven cooling. The one exception is the emplacement of the CAMP, which drives global cooling (~1°C) associated with enhanced organic carbon and carbonate burial and pCO2 reduction. We conclude, therefore, that LIPs do not have a major cooling impact on the Earth’s climate, even when a spatially-extensive LIP is emplaced in the tropics such as the CAMP (Fig. S1).

A possible link between emplacement of LIPs and glaciation and/or climate cooling has been made in previous studies, but these studies are predominantly correlative, and so testing the feasibility of a causative link over long time periods has not been completed. Our work suggests it is unlikely the Tarim, Panjal, Emeishan or Choiyoi LIPs had important roles in the successive glacial periods of the early Permian (the so-called P1–P4 events), as has been previously proposed. We also consider it unlikely that the Karoo-Ferrar LIP drove the late Pleinsbachian cooling and the Jurassic icehouse episodes. Indeed, the emplacement of the largest tropical LIP and the one LIP able to drive global cooling in our model, the CAMP, is not implicated in any global cooling events. However, further tests considering important aspects of the SCION model such as climate sensitivity and the topographic changes related to LIP emplacement must be completed to be certain of these conclusions.

Our findings may also have implications for earlier periods of Earth’s history. The emplacement of the Franklin LIP has been suggested as a driver of runaway cooling and eventual Snowball Earth development around 720 Ma. However, our results suggest its role could have been minor, even if the Franklin LIP was emplaced as a high-relief terrane in a humid and tropical Neoproterozoic world.

Our model framework has its limitations. It uses a relatively low-resolution climate emulator based on the FOAM climate model at 4.5° × 7.5° (latitude by longitude), it also does not explicitly include the process of soil formation or more nuanced aspects of LIP emplacement such as variations in crustal thickness, dynamic topography or extension at the location of emplacement. We argue here that these limitations do not greatly affect our conclusions. As noted above, the model reproduces the drop in the 87Sr/86Sr record coincident with CAMP emplacement, indicating that it is broadly correctly solving the mass balance of weathered felsic and mafic material at this time. If we were to assume that additional mafic weathering and cooling could result from improved climate model resolution or more accurate treatment of soils or relief, then this would imply a further drop in 87Sr/86Sr because mafic weathering produces unradiogenic strontium. This further drop in 87Sr/86Sr would then not be in agreement with the geological record, as can be seen in the ‘weathering & degassing × 30’ scenario (Fig. S3).

As demonstrated by this study, the location of the LIP material is key to determining the importance of its weathering in global biogeochemistry. As a result, accurate plate and palaeogeographic reconstructions are vital, as the LIP placement is reliant upon them. In particular, paleolatitude and paleotopography is central to the determination of a LIP’s location within or outside of a tropical or mid-latitude rain belt. For example, the muted topographical variation of our plate reconstructions between 220 and 200 Ma may be a factor in the limited weathering of LIP material at this time (Fig. 3c). Therefore, to determine exactly how LIPs drive cooling, work must be completed to improve plate reconstructions in deeper time and couple them to improved reconstructions of exact extent and location of LIPs. For estimating rates of carbon degassing from LIPs, it is vital for accurate reconstructions of original LIP volume and rate, as well as any cryptic degassing component of LIP emplacement. Considerable work has been completed to understand these features, but due to subsequent erosion and subduction, volume estimates will always be associated with higher errors (see Methods and Table S2 for the range of degassing estimates used here). Approaches to reconstruct emplacement rates are improving, and with ongoing methodological improvements in radiometric dating, we anticipate future work to further improve and constrain these estimates.

Methods
We use the SCION (Spatial Continuous Integration) Earth Evolution model version 1.1 (ref. 35) in this work but with a number of additions as outlined below. SCION combines a long-term biogeochemical box model with a 3D interpolated steady-state climate and a 2D continental lithology and weathering module. We run the simulation forwards for the whole Phanerozoic but only display the output of the period 300–150 Ma as we intend to focus on the impact of the largest LIPs in Earth history in detail, rather than a general investigation of the entire Phanerozoic. For ease of comprehension, we focus on the outputs of atmospheric CO2, global average temperature, global ocean δ13C and 87Sr/88Sr. Model code is available at https://github.com/jlongman1/SCION_LIPs.

As in previous work, we include LIPs as weatherable terranes in the SCION GCM interpolation stacks (see Supplementary Table 1). For this work, we mapped the locations of each LIP onto the palaeogeographic land–sea masks used in the SCION model (Fig. 1). We used the compilation of ref. 33, which is based on two prior compilations. In the study of ref. 33, the authors reconstruct the original, full extent of LIPs when and where they were emplaced as a series of digital polygons. They then determined a statistical relationship using the present-day aerial extent of LIPs and age (i.e. time since emplacement) and found an exponential relationship between the data. Reference 33 calculated a LIP half-life of 29 Myr (Eqs. (1) and (2)), allowing us to calculate a fractional area of each LIP at a time after its emplacement (Adecay):

Adecay = A e−λt (1)

λ = log(2) / T1/2 (2)

With A being the original areal content (= 1); λ, the decay constant; t, time since emplacement, and; T1/2, the calculated half-life (= 29 Ma). Note that the half-life increases to 36 Ma if one accounts for the burial of LIPs, but this is not taken into account here. Therefore, at each GCM data-stack time slice we rasterize the LIP polygons from ref. 33 onto a 40 × 48 grid (which is the resolution of the underlying FOAM GCM in SCION) at their correct location. This gives us a grid of ‘0s’ and ‘1s’ representing cells of either non-LIP or LIP respectively. We then calculate the fractional content of each LIP cell that has eroded according to Eq. (1) after ref. 33 and update the relevant cell to reflect this (so after emplacement LIP cells will be between 0 and 1, with 1 representing 100% LIP extent). Finally, because the palaeogeographic model used in the SCION model is slightly different from that used by ref. 33, some manual manipulation of the grid cells was required to ensure they were placed correctly with respect to the palaeogeography. We complete this exercise for each individual LIP, so for the Mesozoic, we have 7 individual LIP maps that vary through time (e.g. Siberian Traps Fig. S4). SCION uses a ‘double keyframing’ approach to calculate continental processes in between the GCM climate simulation times. For example, at 20 Ma the model is looking at both the 30 Ma and 15 Ma GCM simulations and averaging between them. As most LIPs are emplaced between the keyframe times, we include a ‘switch-on’ in the code. This checks the time of emplacement for each LIP, and when the model has reached that point, the LIP appears as a weatherable terrane in both the previous and next GCM palaeogeography. See Supplementary Text for further information on the LIP digitization approach.

The model considers all land other than LIPs to be a homogenous mixture, which is calculated to be 7 times less weatherable for silicates than LIP basalts. This estimate is based on previous research indicating the weatherability of mafic rocks to be around seven times greater than that of silicates and is the same factor as used in previous work. As we are adding large areas of weatherable material to the model, we remove the original global basalt weathering curve outlined in ref. 35. As such, we assume that LIPs represent most of the total basalt emplaced through the period. The equations governing the relationship between temperatures, runoff and erosion are those used in ref. 35, and we refer the reader to that publication for more detail. The LIPs themselves have no topographic height so the model topography is not altered by LIP emplacement.

As with previous work, we also consider the potential for LIP emplacement to release significant quantities of CO2 and CH4 into the atmosphere. To estimate the amount of carbon emitted from each LIP, we use published estimates of the volume (in km3) of magma emplaced and the duration of the LIP (in kyrs) to produce an estimate of magma emplacement in km3/kyr. We then convert these values to an estimate of carbon degassing using a conversion factor. For silicic LIPs, we use the same approach as 29, which calculates an emission rate of 1011 g C per km3 of magma emplacement, based upon an estimate of 500 ppm CO2 in the pre-eruptive magma. For basaltic LIPs, we assume pre-eruptive CO2 content of 0.5 wt%, and so an emission rate of 1012 g C per km3. For modelling purposes, these values are converted to molar carbon flux. For each LIP, we derive two degassing scenarios, ‘low’ (calculated using the lowest volume and longest duration in the literature) and ‘high’ (calculated from the highest volume and shortest duration in the literature). Some LIPs are emplaced into carbon-rich rocks, a process that leads to additional carbon degassing through thermogenic alteration. Where estimates of this process are available, for the CAMP, Karoo and Siberian Traps, we use those values for our ‘high’ scenario (see Table S1 for further information).

For each LIP, the rate estimate is used to emit CO2 into the model atmosphere. We use Gaussian curves to complete this, with the midpoint of the LIP activity and the peak of carbon emissions used to construct the function (see Table S2). We set the width of the Gaussian function to be related to the period of activity known for the LIP. For the ‘low’ scenario, we construct the curve based on the longest possible duration, and for ‘high’ the shortest. For example, the Siberian Traps are taken to have degassed in 2 Myr in the ‘low’ scenario and in 0.8 Myr in the ‘high’ scenario, see Supplementary Table 1 for all LIP details. In addition to the ‘low’ and ‘high’ scenarios, we run a further configuration with the full impact of cryptic degassing of the Siberian Traps included, using the approach of ref. 53. This configuration (‘high & cryptic’) considers a more complex approach to estimating the total degassing of a LIP including degassing that continues after the emplacement of lava has finished. For carbon isotopic mass balance, we include a new ‘LIP CO2 δ13C’ estimate in the model, which we take to be −5‰, and is the isotopic composition of the degassed CO2. All modelled configurations are detailed in Table S1, and the ranges of input data relating to these configurations are in Table S2.

The aim of this work is to investigate the holistic role of LIP emplacement on the Earth system. However, we also test the dependence of the model upon each individual factor associated with LIP emplacement. For this, we construct several model scenarios. The first, which we term ‘Baseline’ is SCION version 1.1 without any model additions. Second is the model with no enhanced degassing, but with the enhanced weatherability of LIPs included (‘Weathering Only’). Additionally, we run three scenarios, with only degassing and no enhanced weatherability (LIP weatherability set to the same as homogeneous silicates). Degassing in these scenarios varies with the first utilizing the ‘low’ estimates (‘Degassing Low Only’), the second using the ‘high’ estimates (‘Degassing High Only’), and the third with the ‘high and cryptic’ estimates (‘Degassing High & Cryptic Only’). We also run these three scenarios again, but with LIP weatherability on (‘Weathering & Degassing Low’, ‘Weathering & Degassing High’ and ‘Weathering & Degassing High & Cryptic’). An additional test runs a model where all degassing episodes are assumed to have only 50 kyr duration (an arbitrarily short time to drive rapid carbon degassing rates), to investigate the ability of rapid carbon release to drive climatic warming (scenario “Weathering & Degassing 50kyr”). All model scenarios are outlined in Table S1.

For the ‘Weathering & Degassing High’ scenario we perform sensitivity tests, completed via the running of 1000 simulations with random variation of a number of variables following previous work. Specifically, the weathering enhancement from plant evolution is varied between no enhancement and 7-fold enhancement, the carbon isotope fractionation associated with microbial sulfate reduction between 20–40‰ and the fractionation associated with photosynthesis between 20–30‰ (land) and 25–35‰ (marine). See ref. 35 for further details on SCION sensitivity tests. Results of the sensitivity analysis can be found in Fig. S5.

To validate the model results, we compare a range of geological and proxy data. We generally select the data based on two primary factors. Firstly, the record covers the entirety of our modelled period, and secondly that it is global in scale. Using these criteria, we compare our model outputs to the global average temperature reconstruction of ref. 48, the global seawater 87Sr/88Sr curve of ref. 67 and compilations of atmospheric CO2 (refs. 68, 69) and carbonate δ13C (ref. 70).

We use the mean values of the ‘Weathering & Degassing High’ scenario to calculate an overall carbon balance. That is the total impact of LIP emplacement on carbon cycling. For this we subtract C associated with basalt weathering (basw in the model) from the LIP degassing rate (LIP_CO2 in the model outputs), resulting in a value of carbon perturbation associated with LIP emplacement at each time step (Fig. 1a).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review Longman et al.: Limited long-term cooling effects of flood basalt emplacements\nThe concept of the study is interesting and novel, and based on that, I would recommend publication. However, I do think the\nauthors could improve the manuscript, and I have a series of questions and comments that I believe the authors should\naddress. To summarize, the list below represents my main concerns:\n1. The original pre-erosive extent/volume of any LIP is highly uncertain, and so is the magnitude of degassed carbon. I can’t\nsee that the authors really discuss this, and take these uncertainties into account in their model. As it is very difficult to\nconstrain carbon release from LIPs, a range of potential values for each LIP is often shown by different studies/publications.\nThe same is true of most data shown in your supplementary Table S1 (i.e. the LIP volume, carbon degassing rate and\ntiming, but also original/pre-erosive LIP volume). I can’t really see that authors have taken any ranges into account, but have\ninstead picked one value per LIP. There is no discussion as to why they picked those specific values. Do they consider that\nvalue to be more realistic? If so, why is it more realistic? How would your choice of value affect your model results? For\nexample, if you consider minimum and maximum degassing values for each LIP, how would that impact your results? I\nunderstand that you have to make a choice, but I believe it would improve the paper if you include a discussion as to why\nyou chose what you did, and how this could impact your model. For example, you could do additional minimum and\nmaximum degassing scenarios (based on the range of published estimates) to test the impact. There are many ways to do\nthis. You can think about it. Even if there is a minor difference, such comparisons could strengthen your study because you\nshow that you have taken different possible scenarios into account.\n2. The authors don’t take thermogenic carbon release into account. Why not? For LIPs emplaced in carbon-rich host rocks,\nthe magnitude of thermogenic degassing could be equivalent or even larger than the magmatic carbon. Surely this could\naffect your model results?\n3. I find your discussion a bit hard to follow. I think this is partially due to your figures, especially Figure 2 and 4, which I\nbelieve could be improved (see specific comments below). Further, I believe your discussion would improve if you could be\nmore specific when linking the descriptions of your results with your figures. For example, please refer specifically to the\nappropriate figure panel when you discuss your results (for example 2a instead of just Fig. 2). It would also help to specify\nwhich curve/data point you are referring to (e.g., yellow line, Fig. 2c).\n4. Parts of the discussion include comparisons of your model results to proxy data that are not present in your figures. Why is\nthis? Why not include all available proxy data in your figure? Please discuss why you chose the specific proxy data to\ncompare to, while leaving out others. Also, I feel like there is a lack of thorough discussion regarding why your model results\ndon’t always match up with the proxy records shown in Figure 4. There is some discussion regarding the Sr curve, but what\nabout pCO2, temperature and the C isotopes?\n5. It would be very helpful to see an overview of all the different model runs; I suggest to create a summary table. Further,\nplease be consistent with model run names throughout the text and in all figures, and make sure the same model run always\nhas the same plot color/style/type. Combined, this will make it much easier for the reader to follow along.\nSee specific comments below:\n13: I know you are focusing on long-term cooling effects, but perhaps you could at least mention (in the introduction) the\npotential short-term cooling effect of sulfur release from LIPs. See for example Landwehrs et al. (2020) or Black et al. (2018).\n19: Please specify what kind of model you used.\n19: Could you please quantify this “minor effect” in your abstract? Include specific quantitative results from your study.\n25: I suggest this change: “The emplacement of large igneous provinces (LIPs1), systems of voluminous mafic magmatism\nrelated to processes other than seafloor spreading, has occurred regularly through Earth’s history2-4”.\n27: I would replace the word “enormous”. Also, please quantify the carbon release; e.g., up to xxx Gt; in the range of xx-xxx\nGt C.\n29: I suggest this change: “As a result, linkages between LIP emplacement and large-scale changes in the Earth system\n(e.g., environmental and climatic shifts) are often made3,7-8, convincingly linking LIP emplacement to a number of mas\nextinctions2,3,9-11”.\n43: I would use Davies et al. (2017) instead of Schaller et al. (2012) in this case. Marzoli et al. (1999) is a classical paper for\nlinking CAMP and the ETE.\n44: I would also cite Greber et al. (2020).\n46: I don’t agree with this statement – the consensus is that carbon release is important, but cannot explain the mass\nextinctions alone. I would thus moderate this statement, and avoid saying that C release is a primary kill mechanism,\nbecause we actually don’t know that.\n48: LIP emplacement into carbon-rich sediment is not really that rare; some other examples besides Siberian Traps are\nCAMP, Karoo, HALIP, Emeishan.\n53: I would replace the word “holistic”.\n69: I would replace “make sense”.\n70-71: Please rewrite this sentence.\n74-77: Please rewrite this sentence.\n80-81: How do Figs 2-3 clearly show that the inclusion of LIPs alters the model reconstruction of climate? What am I\nsupposed to be looking at? Please be more specific.\n81-82: The input of carbon minus the removal from weathering - is this actually shown anywhere? Is this supposed to be\nwhat we see in Fig. 2b?\n81-84: This sentence seems out of place and like doesn’t belong there. It’s very broad. I would move to introduction or\ndelete.\n85-87: I don’t think Fig. 2 highlights the fact that weathering is the dominant driver. There is a lot going on in this figure. Am I\nsupposed to be looking at 2b or c here? Please be more specific.\nSome other comments on Figure 2:\n• I would make the legend font size bigger.\n• What do you mean by “modern area”?\n• What do you mean by relative LIP area versus present?\n• What is the difference between the 0.05 Ma and 5 Ma degassing curves? This is somewhat explained further down in the\ndiscussion/methods, but you refer to Fig. 2 already in the beginning of the discussion, so the reader has no idea what these\ncurves are supposed to represent. Either move them to the supplement, or include a description in the figure legend, or refer\nto specific text/table where you describe what these are.\n• What does “Degassing Data” mean?\n• I suggest to use different units of the y-axis for panel b and c. These values are difficult to read and they also make the\nfigure look too busy, and it’s hard to see where panel b/c starts/ends. Do you have to present your results in mol C?\n• I don’t completely understand what panel b is supposed to show. Is this supposed to be carbon input minus carbon\nremoval? So, everything in the yellow area represents net removal and the opposite for the purple area? Can you perhaps\nthink of another way to show this?\n• Due to the range of y-axis in panel c, it looks like the carbon removal from weathering is constant (yellow line). This is an\nimportant part of your results. I think you should “zoom in” so we can see the variability.\n• I suggest these following changes to Fig. 2 so that your most important results are highlighted: Move panel a to the\nsupplement. Only show the degassing curve that you focus on in the main text and show the other curves in the supplement.\nChange the y-axis units of panel b and c. For the yellow curve, show a more appropriate range of the y-axis. For panel b,\nplace y-axis ticks on for example the right side, and then for panel c, show the ticks on the opposite side. Or, could you\ninstead combine the carbon balance in panel b and degassing curve in panel c into the same plot? And then show the\ncarbon removal in a separate panel instead.\n92-93: Please refer to a figure to back up this statement.\n93-94: Where do I see that there is slight enhancement of net LIP-related CO2 drawdown this in Figure 2?\n93-94: I suggest this change: “Following the emplacement of the Siberian Traps (252 Ma), there is slight enhancement of net\nLIP-related CO2 drawdown in the model (Fig. 2), reflecting the weathering of Siberian Traps material36. However, the\nlocation of this LIP in the high latitudes (Fig. 1) suggests that the silicate weathering feedback is much weaker than if it were\nemplaced in the tropics35”.\n97-98: If the reader does not read the methods section first, they don’t what you mean by “weathering-only scenario”. I think\nyou should inform the reader about your different scenarios at the beginning of the “Result and Discussion” section. I\nsuggest that you make a summary table that describes your different scenarios. Then you can just mention that you have x\nnumber of scenarios and refer to the table, and the reader can get more information there.\n99: Specify which panel in Fig. 4 that show these results.\n99-100: Which figure(s) show this? What do you mean by “original model”?\n102-103: I can’t really see that CAMP leads to the highest LIP-related carbon burial values in Fig. 2.\n103-104: Figure 2 doesn’t show the location of CAMP across the equatorial regions.\n110-112: What are you comparing to the baseline here? The weathering only scenario (blue line)? Please be specific. Also,\nplease refer to the specific panel in Fig. 4 that you are describing (in this case 4c I think). It’s really hard to see a 1°C shift\nbecause of the range of the y-axis in Fig. 4c; this is also the case for Fig. 3: I don’t see a shift of 1°C in this figure. Please\nrefer to Fig. 4d at the end of this sentence when you are referring to pCO2.\n114-115: “as seen in some proxy records in the early Jurassic37”: Here you refer to Dera et al. (2011), but Fig 4c show proxy\ndata from Scotese et al. (2021). Why do you only show proxy records from Scotese in your figure? But then compare to\nDera in the text? This question also goes for the rest of the panels of Fig. 4. They all have just one reference for the different\nproxy data. Why did you choose these proxy data as opposed to other records in other publications? I understand that you must\nmake a decision, but it would be nice if you could discuss your choices.\n117-118: Again – which lines are you comparing? Which model results? Please be specific. Also, I am not sure if I agree\nwith the general statement that “there is good correspondence between the seawater 87Sr/86Sr record and our model\nresults”. I see some correspondence for the green and blue lines between ca. 250 and 180, but that period represents only\nparts of your full study period.\n118-119: I don’t understand how Fig. 3 shows that “the amount of additional unradiogenic material being input into the\noceans is of the correct order of magnitude”. Where in Fig. 3 am I supposed to see this?\n128-129: You state that the model is unable to reproduce most of the Mesozoic trends in oceanic Sr isotope composition\nprior to LIP addition and refer to Fig. 4. But don’t you add LIP (Tarim/Skagerak) already at 300 Ma, so in the beginning of\nyour study period? So, what do you mean by prior to LIP addition then? I would argue that, generally, your model is\nunable to reproduce Sr record prior to ca. 250 Ma and after 170 Ma.\n130: delete “a”.\n136-140: Please rewrite this sentence.\n146-148: Based on your results, how can you conclude/propose that volcanic ash is more important than basalt\nemplacement as a driver of CO2 removal? What is the evidence behind this statement?\n151-152: which model run uses this baseline degassing rate? If you make the summary table I suggested above, you could\ninclude that information there.\n157-158: Please be specific when referring to the figures here; I assume you are referring to Fig. 4c, but which panel in Fig.\n2?\n158-159: Again, is this Fig. 4c?\n159-161: How do I see that the scale of C release during ST emplacement is larger in Fig. 3?\n160: Please replace the word “speed”.\n161-163: There is no red line in Fig. 3. Also, the baseline run should be a black dashed line right (not red)? What do you\nmean by the “new model scenario”? Please give all model scenarios clear names (list these in the table I suggested), and\nconsistently use the scenario names throughout the text so that it is always clear which scenario/model run you are talking\nabout.\n163-164: I think you are referring to Fig. 4d here? Also, here you refer to proxy data from Wu et al. (2021), but panel d in Fig.\n4 includes proxy data from Saltzman & Thomas (2012). Again, why do you compare to proxy data that is not in the figure,\nand why is this proxy data not in the figure? Why did you choose to present only Saltzman & Thomas (2012) in your\nfigure?\n164-165: Are you referring to your own model results here or to another study? Please specify.\n170: Again, it is very hard to see a 4°C rise in Fig 4c due to the range of the y-axis.\n174-175: Which results? Please be specific. Also, you refer to “previous proposals”, but mention only one reference.\n177-179: Please add a more thorough description of this additional run in the methods section. Again, if you make a\nsummary table, you can include information there. You say “reduce the degassing window to 50 ka for each even”; what is\nthe original “degassing window”? And where is this described? I can barely see the dashed cyan line in Fig. 4c (again,\nplease refer to the correct panel). I would suggest to change the color, and/or “zoom in” and change the range of the y-axis.\nAlso, please be consistent, use either 50 ka or 0.05 Ma (as shown in Fig. 2 or in the methods; line 302).\nSome other comments on Figure 4:\n• Add grey dots and shading to the legend and name it “proxy data”.\n• I don’t think panel a is discussed at all in the main text? So, you could move this to the supplement (or discuss it, if you\nthink these results are important).\n• It’s really hard to see the different model results, and compare the different runs, because the range of the proxy data is so\nmuch larger than your results (y-axis). I understand that you want to show the range of the proxies, but your model results are\nthe most important. I suggest that you “zoom in” and focus on a y-axis range that is more appropriate for showing your\nresults. This would cut off the proxy data, but you could show the full range in the supplement.\n• I recommend to change the cyan color because it is very hard to see, although this problem may be solved if you change\nthe range of the y-axis.\n• From looking at this figure, the difference between the two “Weathering & Degassing” scenarios is not clear. If you make a\nsummary table, you could refer to that in the figure caption.\n• In the figure caption you say upper panel, below, third and final panel, but why not refer to a,b,c,d?\n• Please thoroughly describe in the discussion: 1) why you chose the specific proxy data shown in this figure, and 2) why\nyour model curves don’t match the proxy data in all four panels.\n• Are the green and cyan lines here equivalent to the red dashed and red dotted lines in Figure 2? Same question for the\nblue (weathering only) line in Fig. 4 and the yellow line (weathering) in Figure 2. If so, please be consistent with both the\ncolor/type of line and the name of the model run.\n180: What do you mean by “previous reconstructions”?\n183: I would replace the word “purport”.\n190: Another important limitation is the uncertainty from original LIP volume/extent and magnitude of carbon degassing from\nthe LIPs, but this is not discussed. Please add that to this part of the discussion.\n204-222: These sections seem out of place. You kind of jump back and forth between limitations and implications (the first\ntwo and last paragraphs discuss limitations). Perhaps re-structure this section into “implications” and “limitations” separately.\n231: Please add a reference.\n238: What do you mean by “whole ocean”?\n248: I suggest this change: “Ref.30 calculated a LIP half-life of 29 Ma (Equations 1 and 2), allowing us to calculate a\nfractional area of each LIP at a time after its emplacement (Adecay):”. Then after “… the calculated half-life (=29 Ma).” add\n“Note that the half-life increases to 36 Ma if one accounts for burial of LIPs30, but this is not taken into account here”.\n260-262: Just to clarify: does this mean that 1 = the maximum extent of the LIP, and 0 = 100% of the LIP has eroded?\n284-287: Instead of using Siberian Traps as an example, list all the LIPs where this is true and you used a value from\nprevious literature. And again, explain why you chose that specific degassing value (as opposed to other estimates, or for\nexample the mean of a range).\n287: Also list the LIPs where you calculated this yourself. I know that this information is provided in Supplementary Table 1,\nbut you can easily just list the different LIPs here, and then refer to the table for more information.\n298-299: Does all literature really agree that the Siberian Traps and Karoo degassed in 1 and 0.47 Myrs, respectively?\nThere are more recent papers than those cited, for example Burgess et al., (2017), Kasbohm et al. (2021), Greber et al.\n(2020), Gaynor et al. (2022). The same question goes for the other LIPs. The rate of degassing is important when\nconstraining climatic and environmental effects. Please explain why you chose these specific references/values.\n300-302: What do you base the duration of 0.05 Ma on? Why do you refer to Fig. 1 here? Give this model run a specific\nname, and be consistent throughout the manuscript and in the figures.\n315-318: please describe this sensitivity test more thoroughly. What does “random variation of a number of variables\nfollowing previous work” mean? Which variables? Are the results from this sensitivity test only shown in Sup. Fig 2? What do\nthe results tell us? Sup.Fig.2 is lacking a legend, so it’s hard to understand what I’m looking at here.\n332: Mean of what? The 1000 simulations? Does the green line shown in Figure 4 represent a mean of the 1000\nsimulations?\nFigure 3: This figure is barely mentioned. Maybe move to supplement? Also, add a,b,c to the different panels.\nFig. S1: Add legend. I would change the yellow color – it’s quite difficult to see.\nSupplementary Text: “This value is then multiplied by the LIP multiplies (in this case 7)”. Note it should be “is” and not “in”.\nAlso, could you please remind the reader why 7?", "reviewer_2": "The impact of Large Igneous Provinces (LIPs) on climate is widely acknowledged in scientific research, yet it remains\nlargely untested. With the potential to cause both warming and cooling effects, LIPs serve as an ideal mechanism to explain\npuzzling climate shifts. At least two reasons make this study valuable and interesting:\n1. It specifically tests the effect of LIPs on the long-term carbon cycle and climate change during a period with multiple LIP\nepisodes.\n1. It demonstrates a limited but significant effect, with results validated by strontium (Sr) simulations that align with geological\nrecords, providing constraints on its reliability.\nThis manuscript tackles an essential and engaging topic, numerically assessing the impact of LIPs on both the degassing\nand weathering components of the carbon cycle. The study's scientific importance is significant, especially given the widely\nhypothesized but still underexplored role of LIPs in climate shifts and mass extinctions. Furthermore, it addresses the current\nknowledge gaps regarding the true climatic potential of LIP events.\nIn terms of scientific quality, the study is rigorous and well-executed. The authors thoroughly test the reliability of their\nmethods and provide detailed discussions of their approach. While additional tests, such as reverse modeling and climate\nsensitivity analyses, could further solidify the conclusions, the current methodology is robust and well-supported. The\nscientific reproducibility of the study is strong, with the methods clearly explained and the data presented in a way that\nallows for replication.\nThe data and methodology used in this study are solid, with justifications for the time interval and LIP selection, models\nemployed, and acknowledged limitations. This thorough approach enhances the credibility of the results, making the study a\nreliable source for further exploration of LIPs' climate impact.\nThe analytical approach is equally strong, providing a clear framework for understanding the relationship between LIPs and\nthe long-term carbon cycle. However, some clarity is needed in the methods section, particularly regarding the link between\nthe General Circulation Model (GCM) simulations and the LIP effects, which remains somewhat unclear. Additionally, the\neffect of the Siberian Traps could benefit from a more nuanced treatment, especially in light of evidence suggesting that its\ninfluence may not strictly align with its location outside the tropical zone, indicating a potentially more complex, non-\nlatitudinal distribution of its climatic effects.\nThe validity of this study is well-established, with its methods, findings, and conclusions all supported by thorough analysis.\nThe presentation is effective, with figures that enhance the text and aid comprehension.\nAddressing the minor points mentioned—particularly in improving the clarity and comprehensiveness of the manuscript—\nwill strengthen the overall contribution. In conclusion, I recommend this manuscript for publication with only minor revisions.\nSpecific comments: (the numbering applies to the version given to the reviewer file)\n(abstract) It would be beneficial to highlight that the implementation of LIPs in the model results in a better alignment\nbetween simulated Sr levels and the geological record, suggesting that this approach is plausible and reliable.\n(65-67) You mention the prevalence of low-latitude aridity in Pangea, yet none of the maps in Figure 3 show significant arid\nregions developing in the tropics. Is this discrepancy intentional?\n(72-74) It remains unclear to me, even after reviewing the entire study, whether changes in topography related to the\nimplementation of the LIPs were incorporated. If so, what is the magnitude of these changes? Additionally, were any GCM\nsimulations rerun for the time slices where LIPs were included? As we know, all the volatiles associated with LIPs could\nalso influence the hydrological cycle.\n(81-84) Regarding Fig. 2, how do you define tropical land? Are specific latitudinal boundaries used? It's important to note\nthat during the Pangea period, latitude-based estimates of tropical land may not be accurate due to the continental\nconfiguration of central Pangea. Could you elaborate on how this was addressed in your analysis?\n(93-96) This assertion holds true only if we assume that climatic belts were similar to those of the present, which may not\nalways have been the case. For instance, examining between 270 & 250 Ma the distribution of bauxites and laterites, which\nare indicators of 'tropical conditions,' reveals a shift towards higher latitudes, as well as a tendency for these deposits to be\nlocated on isolated terranes. On the other hand, evaporites tend to cover most of the tropical region. Could you discuss how\nthis perspective was considered in your analysis? (similar to previous comment I guess).\n(115-117) This presents quite a challenge, as you are attempting to define this effect. Could you try to constrain this\nunderestimation? Specifically, how much would you need to increase the weatherability to achieve the 5°C cooling? Some\nreverse modeling could help clarify this.\n(117-119 + 129-131) This is great and needs to be also said in the abstract.\n(140) Higher altitudes ? I guess you meant latitudes?\n(140-148) While I agree with your conclusions, I think there might be more to elaborate on in this paragraph. I noticed that the\ntopographical variation pose a problem for the simulations and potentially impact the effects of LIP weathering?\nYou also suggest volcanic ash as a cooling driver. Is it possible to test this in the model as well?\n(167-168) Why not conduct additional simulations with varying climate sensitivity to determine if this is indeed the cause of\nthe disconnect?\n(174-180) Similar to the approach with climate sensitivity, it would be valuable to assess whether the degassing required to\ninduce such a temperature rise is plausible. This is somewhat addressed by reducing the degassing window, as it effectively\nincreases the rate of degassing per time unit.\n(197-199) While you emphasize this point here, it's unfortunate that nothing regarding this aspect is mentioned in the\nabstract, as it just makes the study’s reliability stronger.\n(207-212) However, the tests conducted here do not account for changes in climate sensitivity, and some of the maps exhibit\nvery little or no topography. This point may need to be addressed with more nuance.\n(218-219) Very true ! it is a very important statement to make.\n(223-227) Yet, It is very unlikely this belt remained located ±10° NS latitudes (see 270-250 Ma evaporites distribution).\n(262-265 +272) Yes, manipulation is required; however, how are temperature and runoff modeled in relation to the LIPs?\nThis aspect is not very clear in the methods section.\n(Figure 1) This is a very nice figure, but it feels a bit crowded with names. Perhaps consider using just the first letter in the\nfigure itself and providing the full names in the figure description.\n(Figure 3) I have questions primarily regarding the 260 Ma maps. When surface air temperatures reach 30+ degrees Celsius\nacross nearly ±30° N/S, you observe very high runoff and weathering. How is this possible?\n(Figure S1) What do the yellow and orange colors represent? I assume they indicate the mean and the upper/lower limits\nfrom the sensitivity test?\n(Figure S2) This is a fantastic figure! It significantly helps in understanding the weathering implementation.\nI sincerely appreciated the opportunity to read this study, which I believe makes a significant\ncontribution. Thank you for allowing me to review it."}, "author_response": "RESPONSE TO REVIEWERS\nPlease find our responses to the comments below, highlighted in blue. We attach a\nversion of the manuscript with changes tracked. For our responses below, however, we\nuse line numbers pertaining to the updated (changes accepted) manuscript.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nReview Longman et al.: Limited long-term cooling effects of flood basalt emplacements\nThe concept of the study is interesting and novel, and based on that, I would recommend\npublication. However, I do think the authors could improve the manuscript, and I have a\nseries of questions and comments that I believe the authors should address. To summarize,\nthe list below represents my main concerns:\nWe thank reviewer 1 for their detailed and constructive comments, and have attempted\nto address all their concerns in the updated version of the manuscript.\n1. The original pre-erosive extent/volume of any LIP is highly uncertain, and so is the\nmagnitude of degassed carbon. I can’t see that the authors really discuss this, and take\nthese uncertainties into account in their model. As it is very difficult to constrain carbon\nrelease from LIPs, a range of potential values for each LIP is often shown by different\nstudies/publications. The same is true of most data shown in your supplementary Table S1\n(i.e. the LIP volume, carbon degassing rate and timing, but also original/pre-erosive LIP\nvolume). I can’t really see that authors have taken any ranges into account, but have instead\npicked one value per LIP. There is no discussion as to why they picked those specific values.\nDo they consider that value to be more realistic? If so, why is it more realistic? How would\nyour choice of value affect your model results? For example, if you consider minimum and\nmaximum degassing values for each LIP, how would that impact your results? I understand\nthat you have to make a choice, but I believe it would improve the paper if you include a\ndiscussion as to why you chose what you did, and how this could impact your model. For\nexample, you could do additional minimum and maximum degassing scenarios (based on\nthe range of published estimates) to test the impact. There are many ways to do this. You\ncan think about it. Even if there is a minor difference, such comparisons could strengthen\nyour study because you show that you have taken different possible scenarios into account.\nWhilst the focus of this work was on the development of a model which considers the impact\nof LIP basalt erosion on the Earth system, as we include degassing in the approach we\nappreciate the importance of estimating these values correctly.\nIn line with the reviewer’s comment, we have adjusted the degassing aspect of the model in\nthe following ways:\n1. Rather than a single degassing gaussian for each LIP, we now model three\nscenarios. One for ‘low’ degassing, one for ‘high’ degassing’ and one for ‘high’\ndegassing with a further ‘cryptic’ component. For clarification of these model\nconfigurations (and in response to a following comment), we detail their setup in a\nnew Table S2.\n2. To develop these scenarios, we completed a new literature review to determine the\nhighest and lowest published estimates for LIP volume and duration. From these\nvalues we then calculate the lowest possible LIP degassing rate (scenario ‘low’) and\nhighest possible (scenario ‘high’) and show these numbers in an updated Table S1.\n3. For the ‘high’ scenarios, wherever estimates of total degassing rates including\nthermogenic or cryptic degassing are available, we use these values (see footnotes\nb-e in Table S1).\n4. For the Siberian Traps, we also consider the estimates of cryptic degassing as\npublished in a recent study using the SCION model (Black et al., 2024). This\napproach is used in combination with ‘high’ degassing estimates for the other LIPs, in\nthe scenario ‘high & cryptic’ (Table S2. Figure S2).\nThe inclusion of three estimates for LIP degassing allow us to include envelopes for Earth\nsystem response, and so we have updated all relevant figures and text. None of the data\nalter the findings substantially, but in the ‘high & cryptic’ scenario, the warming post-Siberian\nTraps is considerably longer. The main conclusion of limited cooling post-LIP emplacement\nis not changed.\nWe detail our approach in an updated Methods section (L327-355):\n“To estimate the amount of carbon emitted from each LIP, we use published estimates of the\nvolume (in km3) of magma emplaced and the duration of the LIP (in kyrs) to produce an\nestimate of magma emplacement in km3/kyr. We then convert these values to an estimate of\ncarbon degassing using a conversion factor. For silicic LIPs, we use the same approach\nas26, which calculates an emission rate of 1011 g C per km3 of magma emplacement, based\nupon an estimate of 500 ppm CO in the pre-eruptive magma58. For basaltic LIPs, we\n2\nassume pre-eruptive CO content of 0.5 wt%, and so an emission rate of 1012 g C per km3.\nderive two degassing scenarios, ‘low’ (calculated using the lowest volume and longest\nduration in the literature) and ‘high’ (calculated from the highest volume and shortest\nduration in the literature). Some LIPs are emplaced into carbon-rich rocks, a process which\nleads to additional carbon degassing through thermogenic alteration. Where estimates of\nthis process are available, for the CAMP59, Karoo60 and Siberian Traps61, we use those\nvalues for our ‘high’ scenario (see Table S1 for further information).\nFor each LIP, the rate estimate is used to emit CO into the model atmosphere. We use\ncarbon emissions used to construct the function (see Supplementary Table 1). We set the\nwidth of the Gaussian function to be related to the period of activity known for the LIP. For\nthe ‘low’ scenario, we construct the curve based on the longest possible duration, and for\n‘high’ the shortest. For example, the Siberian Traps is taken to have degassed in 2 Myr62 in\nthe ‘low’ scenario and in 0.8 Myrs63 in the ‘high’ scenario, see Supplementary Table 1 for all\nLIP details. In addition to the ‘low’ and ‘high’ scenarios, we run a further configuration with\nthe full impact of cryptic degassing of the Siberian Traps included, using the approach of\nref.64. This configuration (‘high & cryptic’) considers a more complex approach to estimating\nthe total degassing of a LIP including degassing which continues after the emplacement of\nlava has finished64. For carbon isotopic mass balance, we include a new ‘LIP CO δ13C’\n2\nestimate in the model, which we take to be -5 ‰, and is the isotopic composition of the\ndegassed CO . All modelled configurations are detailed in Table S2.”\n2\n2. The authors don’t take thermogenic carbon release into account. Why not? For LIPs\nemplaced in carbon-rich host rocks, the magnitude of thermogenic degassing could be\nequivalent or even larger than the magmatic carbon. Surely this could affect your model\nresults?\nWe did consider thermogenic degassing in our original work but appreciate our methodology\nwas unclear. In the updated manuscript we have attempted to clarify this and refer the\nreviewer back to our response to the previous comment.\n3. I find your discussion a bit hard to follow. I think this is partially due to your figures,\nespecially Figure 2 and 4, which I believe could be improved (see specific comments below).\nFurther, I believe your discussion would improve if you could be more specific when linking\nthe descriptions of your results with your figures. For example, please refer specifically to the\nappropriate figure panel when you discuss your results (for example 2a instead of just Fig.\n2). It would also help to specify which curve/data point you are referring to (e.g., yellow line,\nFig. 2c).\nThese suggestions and the detailed ones below are very helpful, and we have revised the\nfigures accordingly. Wherever suitable, we also now refer to panels when mentioning figures\nin-text.\n4. Parts of the discussion include comparisons of your model results to proxy data that are\nnot present in your figures. Why is this? Why not include all available proxy data in your\nfigure? Please discuss why you chose the specific proxy data to compare to, while leaving\nout others. Also, I feel like there is a lack of thorough discussion regarding why your model\nresults don’t always match up with the proxy records shown in Figure 4. There is some\ndiscussion regarding the Sr curve, but what about pCO2, temperature and the C isotopes?\nAs discussed below, we select the proxy data based on a pair of criteria – firstly that they are\nglobal in their representation, and secondly that they cover the entirety of the studied period.\nWe only present single proxy records to ensure the presentation of data is as uncluttered as\npossible. This means many of the studies we cite would not be suitable for comparison, and\nso we only discuss them in text.\nWe feel the results we present have all been discussed in text – we focus in depth on\ntemperature and CO2 change, for example across major LIP emplacement periods. We do\nnow however include further discussion of why temperatures may be underestimated outside\nof LIP periods (L198-201):\n“Our model underestimates early Mesozoic (250 – 200 Ma) temperatures (Fig. 4c), a feature\nof the original SCION configuration36, potentially linked to non-carbon aspects of the Earth\nsystem such as albedo, or a related to low climate sensitivity of the climate model used\n(FOAM).”\nAnd the d13C data in lines 154-157:\n“In general, the model reconstruction of carbonate δ13C is within the range of proxy\nreconstructions (Fig. 4a), although as a result of the single-ocean box in SCION (meaning\nmarginal environments are not considered), there is little variability outside of the major LIP\nemplacement periods36.”\n5. It would be very helpful to see an overview of all the different model runs; I suggest to\ncreate a summary table. Further, please be consistent with model run names throughout the\ntext and in all figures, and make sure the same model run always has the same plot\ncolor/style/type. Combined, this will make it much easier for the reader to follow along.\nWe have now included a summary table in the supplementary information detailing the\nconfigurations of the runs. Please see Table S2. We have also updated the manuscript\nsection ‘Model Scenarios’ (L357-380):\n“The aim of this work is to investigate the holistic role of LIP emplacement on the Earth\nsystem. However, we also test the dependence of the model upon each individual factor\nassociated with LIP emplacement. For this, we construct several model scenarios. The first,\nwhich we term ‘Baseline’ is SCION version 1.1 without any model additions. Second is the\nmodel with no enhanced degassing, but with the enhanced weatherability of LIPs included\n(‘Weathering Only’). Additionally, we run three scenarios, with only degassing and no\nenhanced weatherability (LIP weatherability set to the same as homogeneous silicates).\nDegassing in these scenarios varies with the first utilising the ‘low’ estimates (‘Degassing\nLow Only’), the second using the ‘high’ estimates (‘Degassing High Only’), and the third with\nthe ‘high & cryptic’ estimates (‘Degassing High & Cryptic Only’).We also run these three\nscenarios again, but with LIP weatherability on (‘Weathering & Degassing Low’, ‘Weathering\n& Degassing High’ and ‘Weathering & Degassing High & Cryptic’). An additional test runs a\nmodel where all degassing episodes are assumed to have only 50 kyr duration (an arbitrarily\nshort time to drive rapid carbon degassing rates), to investigate the ability of rapid carbon\nrelease to drive climatic warming (scenario “Weathering & Degassing 50kyr”). All model\nscenarios are outlined in Table S1.\nFor the ‘Weathering & Degassing High’ scenario we perform sensitivity tests, completed via\nthe running of 1000 simulations with random variation of a number of variables following\nprevious work 32,36. Specifically, the weathering enhancement from plant evolution is varied\nbetween no enhancement and 7-fold enhancement, the carbon isotope fractionation\nassociated with microbial sulfate reduction between 20–40‰ and the fractionation\nassociated with photosynthesis between 20–30‰ (land) and 25–35‰ (marine). See ref. 36\nfor further detail on SCION sensitivity tests. Results of the sensitivity analysis can be found\nin Figure S6.”\nSee specific comments below:\n13: I know you are focusing on long-term cooling effects, but perhaps you could at least\nmention (in the introduction) the potential short-term cooling effect of sulfur release from\nLIPs. See for example Landwehrs et al. (2020) or Black et al. (2018).\nWe have included these references and a sentence to detail this in the updated manuscript\n(L32-34):\n“Greenhouse gas release may cause rapid warming7, with large-scale sulfate release driving\nshort-term cooling8.”\n19: Please specify what kind of model you used.\nChange made specifying the SCION model (L15).\n19: Could you please quantify this “minor effect” in your abstract? Include specific\nquantitative results from your study.\nWe quantify this in the updated version (up to 1°C of change).\n25: I suggest this change: “The emplacement of large igneous provinces (LIPs1), systems of\nvoluminous mafic magmatism related to processes other than seafloor spreading, has\noccurred regularly through Earth’s history2-4”.\nChange made.\n27: I would replace the word “enormous”. Also, please quantify the carbon release; e.g., up\nto xxx Gt; in the range of xx-xxx Gt C.\nWe have included an estimate of CO2 degassing from the Siberian Traps here.\n29: I suggest this change: “As a result, linkages between LIP emplacement and large-scale\nchanges in the Earth system (e.g., environmental and climatic shifts) are often made3,7-8,\nconvincingly linking LIP emplacement to a number of mas extinctions2,3,9-11”.\nChange made.\n43: I would use Davies et al. (2017) instead of Schaller et al. (2012) in this case. Marzoli et\nal. (1999) is a classical paper for linking CAMP and the ETE.\nChange made, and Marzoli et al. (1999) also added here.\n44: I would also cite Greber et al. (2020).\nReference added here.\n46: I don’t agree with this statement – the consensus is that carbon release is important, but\ncannot explain the mass extinctions alone. I would thus moderate this statement, and avoid\nsaying that C release is a primary kill mechanism, because we actually don’t know that.\nWe have tempered this sentence here (L49-50):\n“It is generally assumed that one of the most damaging impacts of LIPs on the biosphere is\ncarbon-rich volatile release…”\n48: LIP emplacement into carbon-rich sediment is not really that rare; some other examples\nbesides Siberian Traps are CAMP, Karoo, HALIP, Emeishan.\nWe have removed ‘rare’ here.\n53: I would replace the word “holistic”.\n‘Holistic’ replaced with ‘full’ here.\n69: I would replace “make sense”.\nSentence adjusted ‘to test the validity of its predictions’.\n70-71: Please rewrite this sentence.\nSentence rewritten: “By constraining the model against multiple proxy systems it is possible\nto make clear quantitative tests of hypotheses.”\n74-77: Please rewrite this sentence.\nSentence broken into two and repetitive aspects removed.\n80-81: How do Figs 2-3 clearly show that the inclusion of LIPs alters the model\nreconstruction of climate? What am I supposed to be looking at? Please be more specific.\nWe have adjusted this sentence to be more specific – this section focusses on the carbon\ncycling aspect of the model.\n81-82: The input of carbon minus the removal from weathering - is this actually shown\nanywhere? Is this supposed to be what we see in Fig. 2b?\nIn the updated figure 2 this is now panel A, which we state in the updated manuscript.\n81-84: This sentence seems out of place and like doesn’t belong there. It’s very broad. I\nwould move to introduction or delete.\nWe have adjusted to make this less broad, and focus on the carbon-related aspect of the\nEarth system here.\n85-87: I don’t think Fig. 2 highlights the fact that weathering is the dominant driver. There is a\nlot going on in this figure. Am I supposed to be looking at 2b or c here? Please be more\nspecific.\nWe have made it clearer in the updated manuscript that we mean here for the majority of the\ntime period the carbon balance of LIPs is below zero, indicative of a net carbon drawdown of\nLIPs (L94-95):\n“…with the majority of the studied time frame characterised by LIP-induced carbon\ndrawdown, as indicated by carbon balance values below zero (Fig. 2a)”\nSome other comments on Figure 2:\n• I would make the legend font size bigger.\n• What do you mean by “modern area”?\n• What do you mean by relative LIP area versus present?\n• What is the difference between the 0.05 Ma and 5 Ma degassing curves? This is somewhat\nexplained further down in the discussion/methods, but you refer to Fig. 2 already in the\nbeginning of the discussion, so the reader has no idea what these curves are supposed to\nrepresent. Either move them to the supplement, or include a description in the figure legend,\nor refer to specific text/table where you describe what these are.\n• What does “Degassing Data” mean?\n• I suggest to use different units of the y-axis for panel b and c. These values are difficult to\nread and they also make the figure look too busy, and it’s hard to see where panel b/c\nstarts/ends. Do you have to present your results in mol C?\n• I don’t completely understand what panel b is supposed to show. Is this supposed to be\ncarbon input minus carbon removal? So, everything in the yellow area represents net\nremoval and the opposite for the purple area? Can you perhaps think of another way to\nshow this?\n• Due to the range of y-axis in panel c, it looks like the carbon removal from weathering is\nconstant (yellow line). This is an important part of your results. I think you should “zoom in”\nso we can see the variability.\n• I suggest these following changes to Fig. 2 so that your most important results are\nhighlighted: Move panel a to the supplement. Only show the degassing curve that you focus\non in the main text and show the other curves in the supplement. Change the y-axis units of\npanel b and c. For the yellow curve, show a more appropriate range of the y-axis. For panel\nb, place y-axis ticks on for example the right side, and then for panel c, show the ticks on the\nopposite side. Or, could you instead combine the carbon balance in panel b and degassing\ncurve in panel c into the same plot? And then show the carbon removal in a separate panel\ninstead.\nThese comments are very useful and we have updated figure 2 as suggested. Briefly, we\nhave simplified the figure to present only carbon drawdown in panel C, and carbon release in\npanel B (with only two scenarios – high and low degassing). The updated panel A now\ndisplays the carbon balance value through time from the ‘high’ degassing scenario. All other\ninformation from this figure is now presented in the supplementary information.\n92-93: Please refer to a figure to back up this statement.\nFigure reference added here.\n93-94: Where do I see that there is slight enhancement of net LIP-related CO2 drawdown\nthis in Figure 2?\nReference to figure 2c now added here.\n93-94: I suggest this change: “Following the emplacement of the Siberian Traps (252 Ma),\nthere is slight enhancement of net LIP-related CO2 drawdown in the model (Fig. 2),\nreflecting the weathering of Siberian Traps material36. However, the location of this LIP in\nthe high latitudes (Fig. 1) suggests that the silicate weathering feedback is much weaker\nthan if it were emplaced in the tropics35”.\nSentences updated according to the suggestions.\n97-98: If the reader does not read the methods section first, they don’t what you mean by\n“weathering-only scenario”. I think you should inform the reader about your different\nscenarios at the beginning of the “Result and Discussion” section. I suggest that you make a\nsummary table that describes your different scenarios. Then you can just mention that you\nhave x number of scenarios and refer to the table, and the reader can get more information\nthere.\nWe have now added a table with all scenarios included in the supplementary information\n(Table S1). We have also added the following text to the manuscript (L87-91):\n“To do this, we run a range of model scenarios (Table S1, Methods), these include a\nbaseline (the unaltered SCION code), a model with only LIP weathering considered, and\nscenarios with only degassing, and with weathering and degassing considered. Unless\notherwise stated, our discussion below refers only to the output of the scenario ‘Weathering\n& Degassing High’ (Table S1, Methods).”\n99: Specify which panel in Fig. 4 that show these results.\nPanel 4b highlighted here.\n99-100: Which figure(s) show this? What do you mean by “original model”?\nFigure reference added (Fig. 4d) and clarification that we mean the baseline model added\nhere.\n102-103: I can’t really see that CAMP leads to the highest LIP-related carbon burial values in\nFig. 2.\nWe have adjusted Fig. 2 as suggested above, and so variability of this value is now much\neasier to see in Fig. 2c.\n103-104: Figure 2 doesn’t show the location of CAMP across the equatorial regions.\nAdjustment made – we now only refer to Fig. 1 here.\n110-112: What are you comparing to the baseline here? The weathering only scenario (blue\nline)? Please be specific. Also, please refer to the specific panel in Fig. 4 that you are\ndescribing (in this case 4c I think). It’s really hard to see a 1°C shift because of the range of\nthe y-axis in Fig. 4c; this is also the case for Fig. 3: I don’t see a shift of 1°C in this figure.\nPlease refer to Fig. 4d at the end of this sentence when you are referring to pCO2.\nWe have included specific references to the model scenarios and figure panels we mean in\nthe updated manuscript. As discussed below, we have also adjusted the axes for Fig. 4 so it\nis easier to see temperature variability.\n114-115: “as seen in some proxy records in the early Jurassic37”: Here you refer to Dera et\nal. (2011), but Fig 4c show proxy data from Scotese et al. (2021). Why do you only show\nproxy records from Scotese in your figure? But then compare to Dera in the text? This\nquestion also goes for the rest of the panels of Fig. 4. They all have just one reference for\nthe different proxy data. Why did you choose these proxy data as opposed to other records\nin other publications? I understand that you must make a decision, but it would be nice if you\ncould discuss your choices.\nFor simplicity, and to ensure we have records which cover the whole period, we have\ndecided to present only single proxy records in Fig. 4. The Scotese et al. (2021) record is\nselected as it covers the whole Mesozoic and provides an estimate of global temperature\nchange. We feel it is suitable to compare to specific studies such as the one by Dera et al.\n(2011) in the text rather than clutter the figure with more data. We include a line in the\nMethods to explain our choice of proxy records (L381-386)\n“To validate the model results, we compare to a range of geological and proxy data. We\ngenerally select the data based on two primary factors. Firstly, that the record covers the\nentirety of our modelled period, and secondly that it is global in scale. Using these criteria,\nwe compare our model outputs to the global average temperature reconstruction of ref.49 ,\nthe global seawater 87Sr/88Sr curve of ref.70 and compilations of atmospheric CO (ref.71,72)\n2\nand carbonate δ13C (ref.73).”\n117-118: Again – which lines are you comparing? Which model results? Please be specific.\nAlso, I am not sure if I agree with the general statement that “there is good correspondence\nbetween the seawater 87Sr/86Sr record and our model results”. I see some correspondence\nfor the green and blue lines between ca. 250 and 180, but that period represents only\nparts of your full study period.\nWe have clarified here the findings (L127-132):\n“However, the good correspondence between the seawater 87Sr/86Sr record and our model\nresults for the scenario ‘weathering and degassing high’ suggests the amount of additional\nunradiogenic material being input to the oceans is of the correct order of magnitude, at least\nfor the period 250 – 180 Ma (Fig. 3).”\n118-119: I don’t understand how Fig. 3 shows that “the amount of additional unradiogenic\nmaterial being input into the oceans is of the correct order of magnitude”. Where in Fig. 3 am\nI supposed to see this?\nWe now point the reader to the location of these data: Fig 4b.\n128-129: You state that the model is unable to reproduce most of the Mesozoic trends in\noceanic Sr isotope composition prior to LIP addition and refer to Fig. 4. But don’t you add\nLIP (Tarim/Skagerak) already at 300 Ma, so in the beginning of your study period? So, what\ndo you mean by prior to LIP addition then? I would argue that, generally, your model is\nunable to reproduce Sr record prior to ca. 250 Ma and after 170 Ma.\nWe have adjusted this sentence here, to reflect our findings better. Instead of saying ‘Prior to\nLIP addition’, we now state ‘Prior to the emplacement of the Siberian Traps…”.\n130: delete “a”.\nChange made.\n136-140: Please rewrite this sentence.\nWe have rewritten (and broken up into 3) the sentence as follows:\n“Further, Sr input resulting from enhanced continental weathering resulting from high global\ntemperatures also improves the comparability of the data to the model outputs. This is\nparticularly clear across the P–T boundary, where 87Sr/86Sr rises markedly. This rise is\ndriven by the warming impact of the Siberian Traps and weathering of generally radiogenic\ncrustal (non-LIP) material – as the LIP was emplaced at higher latitudes away from major\nweathering zones.”\n146-148: Based on your results, how can you conclude/propose that volcanic ash is more\nimportant than basalt emplacement as a driver of CO2 removal? What is the evidence\nbehind this statement?\nWe have now updated this sentence. Rather than relying on our model results here, we are\nputting forward a separate mechanism to explain the link between cooling and LIPs, if\nweathering is unlikely to be a primary driver (the main finding of this work here) of global\ncooling, lines 163-172:\n“One potential explanation is that the deposition of volcanic ash, known to drive periods of\ntransient cooling15, may be more important than basalt emplacement as a driver of CO\n2\nremoval31 on long time scales during LIP episodes. For example, during the late Ordovician,\nash supply was sufficient to drive up to 3°C of cooling for up to a million years in a global\nbiogeochemical model31. There was no LIP associated with the late Ordovician, but many\nLIPs had periods of intense explosive volcanism alongside the large-scale effusive\neruptions. For example, the North Atlantic Igneous Province in the Cenozoic had an\nextended episode of basaltic ash deposition46. This hypothesis requires further testing, but\nwould explain how LIPs may still be linked with global cooling, despite the apparently small\nimpact of their weathering on global climate.”\n151-152: which model run uses this baseline degassing rate? If you make the summary\ntable I suggested above, you could include that information there.\nClarified here as follows:\n“The SCION model uses the baseline degassing rate of ref.36 and in this work we add an\nadditional component for contributions from LIPs in three scenarios (see Table S1,\nMethods).”\n157-158: Please be specific when referring to the figures here; I assume you are referring to\nFig. 4c, but which panel in Fig. 2?\nWe remove the reference to figure 2 here, and specify figure 4c.\n158-159: Again, is this Fig. 4c?\nCorrection made.\n159-161: How do I see that the scale of C release during ST emplacement is larger in Fig.\n3?\nAdjusted to make clear we mean Fig 2b (carbon release from LIPs).\n160: Please replace the word “speed”.\n‘Speed’ replaced with ‘rate’.\n161-163: There is no red line in Fig. 3. Also, the baseline run should be a black dashed line\nright (not red)? What do you mean by the “new model scenario”? Please give all model\nscenarios clear names (list these in the table I suggested), and consistently use the scenario\nnames throughout the text so that it is always clear which scenario/model run you are talking\nabout.\nUsing the new scenario table we have clarified this here (and changed ‘red’ to ‘black dashed’\nline for the baseline run, lines 185-188):\n“In the ‘baseline’ SCION scenario, transient carbon cycle perturbations from LIPs are not\nincluded (Fig. 3, black dashed line), but in scenario ‘weathering and degassing high’, the P–\nTr boundary is represented by a shift to pCO as high as 10,000 ppm (Fig. 4d), reflective of\n2\nproxy data from the period17.”\n163-164: I think you are referring to Fig. 4d here? Also, here you refer to proxy data from Wu\net al. (2021), but panel d in Fig. 4 includes proxy data from Saltzman & Thomas (2012).\nAgain, why do you compare to proxy data that is not in the figure, and why is this proxy data\nnot in the figure? Why did you choose to present only Saltzman & Thomas (2012) in your\nfigure?\nAs discussed above, for figure simplicity and clarity, we chose to present only long-term\nglobal synthesis data in Figure 3. We think it is sometimes useful to discuss our findings in\nthe light of other work, even if we do have space to present that other work in our figure.\n164-165: Are you referring to your own model results here or to another study? Please\nspecify.\nSpecified here that it is our modelling results.\n170: Again, it is very hard to see a 4°C rise in Fig 4c due to the range of the y-axis.\nAs suggested, we have altered the y axes on this figure.\n174-175: Which results? Please be specific. Also, you refer to “previous proposals”,\nbut mention only one reference.\nWe now specify we mean our results relating to temperature change. We have also now\nchanged this sentence, and removed ‘previous proposals’ to make it clearer.\n177-179: Please add a more thorough description of this additional run in the methods\nsection. Again, if you make a summary table, you can include information there. You say\n“reduce the degassing window to 50 ka for each even”; what is the original “degassing\nwindow”? And where is this described? I can barely see the dashed cyan line in Fig. 4c\n(again, please refer to the correct panel). I would suggest to change the color, and/or “zoom\nin” and change the range of the y-axis. Also, please be consistent, use either 50 ka or 0.05\nMa (as shown in Fig. 2 or in the methods; line 302).\nWe have removed the results of this modelling from Figure 4 and moved them to the\nsupplementary information.\nSome other comments on Figure 4:\n• Add grey dots and shading to the legend and name it “proxy data”.\n• I don’t think panel a is discussed at all in the main text? So, you could move this to the\nsupplement (or discuss it, if you think these results are important).\n• It’s really hard to see the different model results, and compare the different runs, because\nthe range of the proxy data is so much larger than your results (y-axis). I understand that you\nwant to show the range of the proxies, but your model results are the most important. I\nsuggest that you “zoom in” and focus on a y-axis range that is more appropriate for showing\nyour results. This would cut off the proxy data, but you could show the full range in the\nsupplement.\n• I recommend to change the cyan color because it is very hard to see, although this\nproblem may be solved if you change the range of the y-axis.\n• From looking at this figure, the difference between the two “Weathering & Degassing”\nscenarios is not clear. If you make a summary table, you could refer to that in the figure\ncaption.\n• In the figure caption you say upper panel, below, third and final panel, but why not refer to\na,b,c,d?\n• Please thoroughly describe in the discussion: 1) why you chose the specific proxy data\nshown in this figure, and 2) why your model curves don’t match the proxy data in all four\npanels.\n• Are the green and cyan lines here equivalent to the red dashed and red dotted lines in\nFigure 2? Same question for the blue (weathering only) line in Fig. 4 and the yellow line\n(weathering) in Figure 2. If so, please be consistent with both the color/type of line and the\nname of the model run.\nWe have taken on board all comments listed above in the updated figure 4, and updated the\nfigure caption to reflect the changes. We appreciate the suggestions and think it’s a much\nclearer figure now.\n180: What do you mean by “previous reconstructions”?\nAdjustment made (L208-214):\n“In this scenario (‘Weathering & Degassing 50 kyr’), the temperature rise across the P–T is\n9°C, and for the CAMP it is 8°C, much closer to the reconstructions of 15°C for the P-T 48,49\nand >5°C for the CAMP emplacement (ref. 50 Fig. S4), ), suggesting carbon volume\nestimates may be correct, but the periods of degassing may have been much shorter than\nthe lifetime of the LIP.”\n183: I would replace the word “purport”.\n‘Purport’ replaced by ‘claim’.\n190: Another important limitation is the uncertainty from original LIP volume/extent and\nmagnitude of carbon degassing from the LIPs, but this is not discussed. Please add that to\nthis part of the discussion.\nWe now include a discussion of this in the relevant section limitations (L263-270):\n“For estimating rates of carbon degassing from LIPs, it is vital for accurate reconstructions of\noriginal LIP volume and rate, as well as any cryptic degassing component of LIP\nemplacement54. Considerable work has been completed to understand these features this,\nbut due to subsequent erosion and subduction, volume estimates will always be associated\nwith high errors (see Methods and Table S2 for the range of degassing estimates used\nhere). Approaches to reconstruct emplacement rates are improving, and with ongoing\nmethodological improvements in radiometric dating, we anticipate future work to further\nimprove and constrain these estimates.”\n204-222: These sections seem out of place. You kind of jump back and forth between\nlimitations and implications (the first two and last paragraphs discuss limitations). Perhaps\nre-structure this section into “implications” and “limitations” separately.\nWe agree, and have split this section into two, moving all paragraphs to the correct\nlocations.\n231: Please add a reference.\nReference added (Mills et al., 2021).\n238: What do you mean by “whole ocean”?\n“Whole” changed to “Global” here.\n248: I suggest this change: “Ref.30 calculated a LIP half-life of 29 Ma (Equations 1 and 2),\nallowing us to calculate a fractional area of each LIP at a time after its emplacement\n(Adecay):”. Then after “… the calculated half-life (=29 Ma).” add “Note that the half-life\nincreases to 36 Ma if one accounts for burial of LIPs30, but this is not taken into account\nhere”.\nAdjustment made.\n260-262: Just to clarify: does this mean that 1 = the maximum extent of the LIP, and 0 =\n100% of the LIP has eroded?\nYes – we clarify this in lines 303-304 now:\n“…and update the relevant cell to reflect this (so after emplacement LIP cells will be between\n0 and 1, with 1 representing 100% LIP extent).”\n284-287: Instead of using Siberian Traps as an example, list all the LIPs where this is true\nand you used a value from previous literature. And again, explain why you chose that\nspecific degassing value (as opposed to other estimates, or for example the mean of a\nrange).\nAs discussed in response to the major comment above, we have adjusted this section of the\nmethodology in lines 334-355:\n“For modelling purposes, these values are converted to molar carbon flux. For each LIP, we\nderive two degassing scenarios, ‘low’ (calculated using the lowest volume and longest\nduration in the literature) and ‘high’ (calculated from the highest volume and shortest\nduration in the literature). Some LIPs are emplaced into carbon-rich rocks, a process which\nleads to additional carbon degassing through thermogenic alteration. Where estimates of\nthis process are available, for the CAMP65, Karoo66 and Siberian Traps67, we use those\nvalues for our ‘high’ scenario (see Table S1 for further information).\nFor each LIP, the rate estimate is used to emit CO into the model atmosphere. We use\ncarbon emissions used to construct the function (see Table S2). We set the width of the\nGaussian function to be related to the period of activity known for the LIP. For the ‘low’\nscenario, we construct the curve based on the longest possible duration, and for ‘high’ the\nshortest. For example, the Siberian Traps is taken to have degassed in 2 Myr68 in the ‘low’\nscenario and in 0.8 Myrs69 in the ‘high’ scenario, see Supplementary Table 1 for all LIP\ndetails. In addition to the ‘low’ and ‘high’ scenarios, we run a further configuration with the full\nimpact of cryptic degassing of the Siberian Traps included, using the approach of ref.54. This\nconfiguration (‘high & cryptic’) considers a more complex approach to estimating the total\ndegassing of a LIP including degassing which continues after the emplacement of lava has\nfinished54. For carbon isotopic mass balance, we include a new ‘LIP CO δ13C’ estimate in\n2\nthe model, which we take to be -5 ‰, and is the isotopic composition of the degassed CO .\nthese configurations are in Table S2.”\n287: Also list the LIPs where you calculated this yourself. I know that this information is\nprovided in Supplementary Table 1, but you can easily just list the different LIPs here, and\nthen refer to the table for more information.\nWe refer the reviewer to our response to the previous comment here, and include all\nnecessary data in the updated supplementary table (now Table S2).\n298-299: Does all literature really agree that the Siberian Traps and Karoo degassed in 1\nand 0.47 Myrs, respectively? There are more recent papers than those cited, for example\nBurgess et al., (2017), Kasbohm et al. (2021), Greber et al. (2020), Gaynor et al. (2022). The\nsame question goes for the other LIPs. The rate of degassing is important when\nconstraining climatic and environmental effects. Please explain why you chose these specific\nreferences/values.\nAgain, we refer the reviewer to our initial response to their comments earlier in this\ndocument. We have used the literature cited and a more detailed search to update all the\nestimates of degassing.\n300-302: What do you base the duration of 0.05 Ma on? Why do you refer to Fig. 1 here?\nGive this model run a specific name, and be consistent throughout the manuscript and in the\nfigures.\nThe duration is an arbitrary value used to represent a short degassing period (and therefore\nrapid degassing rate) for each LIP. We include a new description of this scenario here (L368-\n372):\n“An additional test runs a model where all degassing episodes are assumed to have only 50\nkyr duration (an arbitrarily short time to drive rapid carbon degassing rates), to investigate\nthe ability of rapid carbon release to drive climatic warming (scenario “Weathering &\nDegassing 50kyr”). All model scenarios are outlined in Table S1.”\n315-318: please describe this sensitivity test more thoroughly. What does “random variation\nof a number of variables following previous work” mean? Which variables? Are the results\nfrom this sensitivity test only shown in Sup. Fig 2? What do the results tell us? Sup.Fig.2 is\nlacking a legend, so it’s hard to understand what I’m looking at here.\nWe now include a better description of the sensitivity analysis here (L375-380):\n“Specifically, the weathering enhancement from plant evolution is varied between no\nenhancement and 7-fold enhancement, the carbon isotope fractionation associated with\nmicrobial sulfate reduction between 20–40‰ and the fractionation associated with\nphotosynthesis between 20–30‰ (land) and 25–35‰ (marine). See ref. 36 for further detail\non SCION sensitivity tests. Results of the sensitivity analysis can be found in Figure S6.”\n332: Mean of what? The 1000 simulations? Does the green line shown in Figure 4 represent\na mean of the 1000 simulations?\nFigure 4 displays single simulations not ensemble runs. Here we clarify this point, and point\nthe reader to where these results are found – Figure S4.\nFigure 3: This figure is barely mentioned. Maybe move to supplement? Also, add a,b,c to the\ndifferent panels.\nAs we believe this is a nice demonstration of the spatial variability of climate we retain its use\nin the text. We now refer to it more in the discussion.\nFig. S1: Add legend. I would change the yellow color – it’s quite difficult to see.\nLegend added here. Colours of lines have also been adjusted.\nSupplementary Text: “This value is then multiplied by the LIP multiplies (in this case 7)”. Note\nit should be “is” and not “in”. Also, could you please remind the reader why 7?\nChange made, and addition here to clarify why we use a value of sevenfold – it is based on\nprevious estimates (Dessert et al., 2003) and previous work (Longman et al., 2022, Lefebvre\net al., 2013).\nReviewer #2 (Remarks to the Author):\nThe impact of Large Igneous Provinces (LIPs) on climate is widely acknowledged in\nscientific research, yet it remains largely untested. With the potential to cause both warming\nand cooling effects, LIPs serve as an ideal mechanism to explain puzzling climate shifts. At\nleast two reasons make this study valuable and interesting:\n1. It specifically tests the effect of LIPs on the long-term carbon cycle and climate change\nduring a period with multiple LIP episodes.\n1. It demonstrates a limited but significant effect, with results validated by strontium (Sr)\nsimulations that align with geological records, providing constraints on its reliability.\nThis manuscript tackles an essential and engaging topic, numerically assessing the impact\nof LIPs on both the degassing and weathering components of the carbon cycle. The study's\nscientific importance is significant, especially given the widely hypothesized but still\nunderexplored role of LIPs in climate shifts and mass extinctions. Furthermore, it addresses\nthe current knowledge gaps regarding the true climatic potential of LIP events.\nIn terms of scientific quality, the study is rigorous and well-executed. The authors thoroughly\ntest the reliability of their methods and provide detailed discussions of their approach. While\nadditional tests, such as reverse modeling and climate sensitivity analyses, could further\nsolidify the conclusions, the current methodology is robust and well-supported. The scientific\nreproducibility of the study is strong, with the methods clearly explained and the data\npresented in a way that allows for replication.\nThe data and methodology used in this study are solid, with justifications for the time interval\nand LIP selection, models employed, and acknowledged limitations. This thorough approach\nenhances the credibility of the results, making the study a reliable source for further\nexploration of LIPs' climate impact.\nThe analytical approach is equally strong, providing a clear framework for understanding the\nrelationship between LIPs and the long-term carbon cycle. However, some clarity is needed\nin the methods section, particularly regarding the link between the General Circulation Model\n(GCM) simulations and the LIP effects, which remains somewhat unclear. Additionally, the\neffect of the Siberian Traps could benefit from a more nuanced treatment, especially in light\nof evidence suggesting that its influence may not strictly align with its location outside the\ntropical zone, indicating a potentially more complex, non-latitudinal distribution of its climatic\neffects.\nThe validity of this study is well-established, with its methods, findings, and conclusions all\nsupported by thorough analysis. The presentation is effective, with figures that enhance the\ntext and aid comprehension.\nAddressing the minor points mentioned—particularly in improving the clarity and\ncomprehensiveness of the manuscript—will strengthen the overall contribution. In\nconclusion, I recommend this manuscript for publication with only minor revisions.\nWe appreciate the reviewer’s support for our work and their helpful comments. We have\nattempted to address all concerns in the updated manuscript.\nSpecific comments: (the numbering applies to the version given to the reviewer file)\n(abstract) It would be beneficial to highlight that the implementation of LIPs in the model\nresults in a better alignment between simulated Sr levels and the geological record,\nsuggesting that this approach is plausible and reliable.\nWe have included the following in the updated abstract (L17-19):\n“This approach results in better correspondence between the modelled output and proxy\nreconstructions of the period (especially for seawater Sr isotope composition).”\n(65-67) You mention the prevalence of low-latitude aridity in Pangea, yet none of the maps in\nFigure 3 show significant arid regions developing in the tropics. Is this discrepancy\nintentional?\nYes, it is more correct to say subtropical aridity. We have revised the paper to simply state\nthat aridity is extensive, as that is the main point of the section.\n(72-74) It remains unclear to me, even after reviewing the entire study, whether changes in\ntopography related to the implementation of the LIPs were incorporated. If so, what is the\nmagnitude of these changes? Additionally, were any GCM simulations rerun for the time\nslices where LIPs were included? As we know, all the volatiles associated with LIPs could\nalso influence the hydrological cycle.\nWe have made it clearer in the revision that we do not adjust the topography in this study in\nresponse to LIP emplacement. We update the manuscript to clarify this (L74-78):\n“As well as incorporating volumes of degassed CO during emplacement, these LIPs are\n2\nadded to the 2D model land surface grid as basaltic terranes and interact with local\ntemperature, relief and hydrology to amplify silicate weathering rates (Fig. 1), although the\nLIPs themselves have no topographic height so the model topography is not altered by LIP\nemplacement.”\nGCM simulations were not rerun for timeslices associated with LIP emplacement, but this\nwill be tackled in future work. Indeed, the role of volatiles is not properly addressed in this\nstudy, beyond the simple degassing of carbon into the atmosphere box.\nOne simple way of considering the impact of LIPs on the physical topography (i.e. increased\nheight due to basaltic emplacement) is to enhance the weatherability of LIP material, as a\nproxy for this process. In the SI we now include the results of a 30x enhancement run, which\ndemonstrates that at this level the Sr isotope record is invalidated. We also discuss the\nimplications of this in the updated manuscript.\n(81-84) Regarding Fig. 2, how do you define tropical land? Are specific latitudinal boundaries\nused? It's important to note that during the Pangea period, latitude-based estimates of\ntropical land may not be accurate due to the continental configuration of central Pangea.\nCould you elaborate on how this was addressed in your analysis?\nIn light of the comments of reviewer 1, we have now moved the panel relating to tropical land\nto the supplementary information (Fig. S1). We do now include a definition (up to 20° from\nthe equator) in the new figure caption. We also highlight this estimate is purely based upon\nthe total number of gridsquares which fall in this region and contain LIPs.\n(93-96) This assertion holds true only if we assume that climatic belts were similar to those\nof the present, which may not always have been the case. For instance, examining between\n270 & 250 Ma the distribution of bauxites and laterites, which are indicators of 'tropical\nconditions,' reveals a shift towards higher latitudes, as well as a tendency for these deposits\nto be located on isolated terranes. On the other hand, evaporites tend to cover most of the\ntropical region. Could you discuss how this perspective was considered in your analysis?\n(similar to previous comment I guess).\nAs discussed above, we use a very simple definition of what is ‘tropical’ in this study. The\nreviewer is quite correct to point out that this is not fully suitable for the period studied, but\nfor simplicity, and for continuity through our modelled period we retain the definition of <20°\nfrom the equator.\n(115-117) This presents quite a challenge, as you are attempting to define this effect. Could\nyou try to constrain this underestimation? Specifically, how much would you need to increase\nthe weatherability to achieve the 5°C cooling? Some reverse modeling could help clarify this.\nThis is a great suggestion, and we have completed some extra modelling runs with\nenhanced weatherability to test it in a simple manner (reverse modelling is a good\nsuggestion for follow up work). To reach the full 5°C of cooling, we require 30-fold\nweatherability enhancement. We include a discussion of this point in the updated manuscript\n(L130-136):\n“Indeed, to reach 5°C of cooling following the CAMP emplacement, the weathering\nenhancement value must be raised to thirty-fold (Fig. S2b), a value which invalidates the Sr\nisotope curve, resulting in values well below the reconstructed proxy data (Fig. S3b). A\nweathering enhancement of fifteen-fold remains possible given the Sr isotope curve across\nthe CAMP (Fig.S3), suggesting it may be a plausible maximum value for this LIP (and one\nwhich results in 2°C of cooling), but this would require considerable uplift of basaltic\nmaterial”\n(117-119 + 129-131) This is great and needs to be also said in the abstract.\nAs suggested above, we now mention the correspondence of the Sr curve in the updated\nabstract.\n(140) Higher altitudes ? I guess you meant latitudes?\nChange made.\n(140-148) While I agree with your conclusions, I think there might be more to elaborate on in\nthis paragraph. I noticed that the 220 Ma and 200 Ma maps included in SCION show little to\nno topography compared to the other maps. Could this lack of topographical variation pose a\nproblem for the simulations and potentially impact the effects of LIP weathering?\nYou also suggest volcanic ash as a cooling driver. Is it possible to test this in the model as\nwell?\nThis is a good point. We have noted that the more muted topography in these maps may be\na factor in the limited weathering of LIP basalts in the limitations section (L259-261):\n“For example, the muted topographical variation of our plate reconstructions between 220 –\nThe question of ash impacting climates is one we intend to tackle fully in upcoming work. A\nsingle example of the approach can be seen in Longman et al. (2021), and we note this in\nthe revised paper, and include a little more detail (L162-169):\n“One potential explanation is that the deposition of volcanic ash, known to drive periods of\ntransient cooling15, may be more important than basalt emplacement as a driver of CO\n2\nremoval31 on long time scales during LIP episodes. For example, during the late Ordovician,\nash supply was sufficient to drive up to 3°C of cooling for up to a million years in a global\nbiogeochemical model31. There was no LIP associated with the late Ordovician, but many\nLIPs had periods of intense explosive volcanism alongside the large-scale effusive\neruptions. For example, the North Atlantic Igneous Province in the Cenozoic had an\nextended episode of basaltic ash deposition46.”\n(167-168) Why not conduct additional simulations with varying climate sensitivity to\ndetermine if this is indeed the cause of the disconnect?\nThis is not something we can test easily. The climate sensitivity referred to here is that of the\nFOAM model used for the GCM runs. As such, to test variations in climate sensitivity, we\nwould need to perform a large number of runs using an alternative high-sensitivity GCM,\nwhich is a difficult project in itself. We do now clarify that this sensitivity issue relates to the\nGCM, not to SCION per se in the updated manuscript.\n(174-180) Similar to the approach with climate sensitivity, it would be valuable to assess\nwhether the degassing required to induce such a temperature rise is plausible. This is\nsomewhat addressed by reducing the degassing window, as it effectively increases the rate\nof degassing per time unit.\nAs discussed in the response to reviewer 1, we have updated the manuscript to clarify that in\nthe models with shorter degassing windows temperature rises in line with proxy data can be\nreconstructed for the P-T and the CAMP. Further, we now also present the results of a model\nwhich considers in detail the role of cryptic degassing (from Black et al., 2024), which allows\nus to reconstruct the full warming of the Siberian Traps. We have added this information to\nthe updated manuscript (L205-214) and in two new figures (Fig. S2, S3):\n“By running comparative models which reduce the length of time the degassing occurred for\nto 50 kyr for each event, we can reconstruct changes in climate which are much closer to\nproxy reconstructions (Fig. S4). In this scenario (‘Weathering & Degassing 50 kyr’), the\ntemperature rise across the P–T is 9°C, and for the CAMP it is 8°C, much closer to the\nreconstructions of 15°C for the P-T 48,49 and >5°C for the CAMP emplacement (ref. 50 Fig.\nS4), suggesting carbon volume estimates may be correct, but the periods of degassing may\nhave been much shorter than the lifetime of the LIP. A further model scenario which fully\nconsiders the role of cryptic degassing (Weathering & Degassing Cryptic), using the\napproach of ref.54, is also able to reconstruct up to 15°C warming for the P-T (Fig. S2a).”\n(197-199) While you emphasize this point here, it's unfortunate that nothing regarding this\naspect is mentioned in the abstract, as it just makes the study’s reliability stronger.\nAs mentioned above, this is now included in the updated abstract.\n(207-212) However, the tests conducted here do not account for changes in climate\nsensitivity, and some of the maps exhibit very little or no topography. This point may need to\nbe addressed with more nuance.\nWe include a qualifying statement at the end of this paragraph in the updated manuscript\n(233-235):\n“However, further tests considering in more detail important aspects of the SCION model\nsuch as climate sensitivity must be completed to be certain of these conclusions.”\nWe also include a sentence on topography as a limitation (L259-261):\n“For example, the muted topographical variation of our plate reconstructions between 220 –\n(218-219) Very true ! it is a very important statement to make.\nThank you for agreeing!\n(223-227) Yet, It is very unlikely this belt remained located ±10° NS latitudes (see 270-250\nMa evaporites distribution).\nWe agree with this point, and that is why this section is now included in a specific ‘limitations’\nsection of the manuscript. Further work should be focussed on improving questions relating\nto this sort of question.\n(262-265 +272) Yes, manipulation is required; however, how are temperature and runoff\nmodeled in relation to the LIPs? This aspect is not very clear in the methods section.\nAs discussed above, the topography of the LIPs is not considered in the GCM runs, and we\nhave clarified this in the updated manuscript.\nFull details of the equations governing the interaction between runoff, temperature and\nerosion may be found in Mills et al. (2021), and we do not feel the need to repeat this\ninformation here. Instead, we now point the reader here to that publication for further\ninformation (L322-324):\n“ The equations governing the relationship between temperatures, runoff and erosion are\nthose used in ref.36, and we refer the reader to that publication for more detail”\n(Figure 1) This is a very nice figure, but it feels a bit crowded with names. Perhaps consider\nusing just the first letter in the figure itself and providing the full names in the figure\ndescription.\nThank you for the suggestion – we have made this change.\n(Figure 3) I have questions primarily regarding the 260 Ma maps. When surface air\ntemperatures reach 30+ degrees Celsius across nearly ±30° N/S, you observe very high\nrunoff and weathering. How is this possible?\nSuch high temperatures at high latitudes would indeed likely impact runoff (with evaporation\nbeing concurrently high). If these temperatures were uniform across the high latitudes we\nagree runoff would likely be low, but temperature across these regions is not uniformly high,\nwith variability, and so the possibility for high runoff.\n(Figure S1) What do the yellow and orange colors represent? I assume they indicate the\nmean and the upper/lower limits from the sensitivity test?\nAs discussed in the response to reviewer 1, we have clarified this in a new figure caption.\n(Figure S2) This is a fantastic figure! It significantly helps in understanding the weathering\nimplementation.\nWe appreciate the comments.\nI sincerely appreciated the opportunity to read this study, which I believe makes a significant\ncontribution. Thank you for allowing me to review it.\nReviewer #1 (Remarks to the Author):\nPlease find our responses to the comments below, highlighted in blue. We attach a version\nof the manuscript with changes tracked. For our responses below, however, we use line\nnumbers pertaining to the updated (changes accepted) manuscript.\nSecond review; Longman et al; Limited long-term cooling effects of Pangaean flood basalt\nweathering.\nThe introduction gives a clear overview of background information, what the authors have\ndone and why. The discussion and description of results is now much easier to follow as the\nauthors have improved both the text and figures. The additional model runs, analyses, table\nand discussion make this study much more robust and convincing. I would recommend\npublication. See some minor comments below (line numbers are from the document\nincluding the tracked changes):\nWe appreciate the reviewer’s comments and have attempted to address all their final\nconcerns in the updated manuscript. Our responses are detailed below.\n18: Could you please specify “better” correspondence compared to what?\nWe have updated this on lines 18-20:\n“This approach results in better correspondence between the modelled output and proxy\nreconstructions of the period (especially for seawater Sr isotope composition) when\ncompared to previous modelling studies.”\n80-81: I suggest moving the following sentence to methods: “although the LIPs themselves\nhave no topographic height so the model topography is not altered by LIP emplacement”\nWe have moved this sentence to the methods (lines 327-328).\n123: Please check if the reference to Fig. 3b and c is correct for this sentence.\nThis has been adjusted to reflect silicate weathering is in panel 3c. We also add panel letters\nto figure 3.\n127: Specify the panel letter of Figs. 3,4\nPanels (Figs. 3a and 4c) now specified here.\n136: The addition of Fig. S2 is great! But I recommend some changes:\n• Adjust the y-axis of panel b of figure S2 (“zoom in”) so that we can more easily see the\ntemperature changes from the baseline. I get it that you want to show the full proxy range,\nbut we can see that in panel a.\n• Combine panel a of S2 and Fig S4 into one figure. That way we can easily compare all\nmodel runs. Get rid of all the white space; we want the focus to be on your model results.\n• Place the black dashed line on top so we can see the full extent of the baseline run (it’s\nkind of hidden behind the solid lines as of now). This applies for all these figures + Figure 4\nin the main manuscript.\n• Combine the a and b panels of figure S3.\nWe have made all suggested changes, thank you for the help refining the figures.\n150: This is from my previous review and your response:\n128-129: You state that the model is unable to reproduce most of the Mesozoic trends in\noceanic Sr isotope composition prior to LIP addition and refer to Fig. 4. But don’t you add\nLIP (Tarim/Skagerak) already at 300 Ma, so in the beginning of your study period? So, what\ndo you mean by prior to LIP addition then? I would argue that, generally, your model is\nunable to reproduce Sr record prior to ca. 250 Ma and after 170 Ma.\nWe have adjusted this sentence here, to reflect our findings better. Instead of saying ‘Prior to\nLIP addition’, we now state ‘Prior to the emplacement of the Siberian Traps…”.\nI agree with your response, but I don’t see this change in the revised manuscript (line 150).\nThanks for noticing – we have made sure this addition is in the updated manuscript.\n156: Please check if the reference to Fig. 4c is correct for this sentence.\nFigure reference updated, no Fig. 4b.\n196: Please check if the reference to Fig. 3 is correct for this sentence.\nFigure reference updated, now Fig. 4.\n220: It’s impossible to compare the ‘weathering°assing high” with the “50 kyr” run because\nthe red line is hidden. Could you please make one of the lines dashed. Also, (I already\nmentioned this above), but please adjust the y-axis and/or get rid of the white space so it’s\neasier to see the change in temperature.\nAs mentioned above, these have all been combined into two panels in Fig. S2, with the\nsuggestions included.\n231-232: This sentence seems strange (i.e., while when and clear driven).\nChanges made.\n247-248: This reads a bit strange, so consider re-writing: “…further tests considering in more\ndetail important aspects…”\nChange made, ‘in more detail removed’ here.\n271: I don’t really understand what you mean by “falsified by the geological record”, so\nconsider re-writing this.\nChanged to ‘would not be in agreement with the geological record’.\n284: Delete “this” after “features”\nChange made.\nFig 3: Could you mark CAMP and the Siberian Traps with a symbol instead of the dotted\nline? You could make the symbol transparent so we can still see the colorbar. I wonder if you\nshould also add letters a,b,c (and update the text accordingly).\nWe do not feel that adding a symbol to these figures would make them clearer, and so retain\nthe current line used to indicate their approximate location. We have, however, added panel\nletters and updated text to indicate which panel is being referred to.\nLine 410: This is from my previous review and your response:\nFig. S1: Add legend. I would change the yellow color – it’s quite difficult to see.\nLegend added here. Colours of lines have also been adjusted.\nI don’t see these change in the revised manuscript (Figure S1 is now S6).\nCorrect figure uploaded in updated submission."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Second review; Longman et al; Limited long-term cooling effects of Pangaean flood basalt. weathering.\nThe introduction gives a clear overview of background information, what the authors have done and why. The discussion\nand description of results is now much easier to follow as the authors have improved both the text and figures. The\nadditional model runs, analyses, table and discussion make this study much more robust and convincing. I would\nrecommend publication. See some minor comments below (line numbers are from the document including the tracked\nchanges):\n18: Could you please specify “better” correspondence compared to what?\n80-81: I suggest moving the following sentence to methods: “although the LIPs themselves have no topographic height so\nthe model topography is not altered by LIP emplacement”\n123: Please check if the reference to Fig. 3b and c is correct for this sentence.\n127: Specify the panel letter of Figs. 3,4\n136: The addition of Fig. S2 is great! But I recommend some changes:\n• Adjust the y-axis of panel b of figure S2 (“zoom in”) so that we can more easily see the temperature changes from the\nbaseline. I get it that you want to show the full proxy range, but we can see that in panel a.\n• Combine panel a of S2 and Fig S4 into one figure. That way we can easily compare all model runs. Get rid of all the white\nspace; we want the focus to be on your model results.\n• Place the black dashed line on top so we can see the full extent of the baseline run (it’s kind of hidden behind the solid\nlines as of now). This applies for all these figures + Figure 4 in the main manuscript.\n• Combine the a and b panels of figure S3.\n150: This is from my previous review and your response:\n128-129: You state that the model is unable to reproduce most of the Mesozoic trends in oceanic Sr isotope composition\nprior to LIP addition and refer to Fig. 4. But don’t you add LIP (Tarim/Skagerak) already at 300 Ma, so in the beginning of\nyour study period? So, what do you mean by prior to LIP addition then? I would argue that, generally, your model is unable\nto reproduce Sr record prior to ca. 250 Ma and after 170 Ma.\nWe have adjusted this sentence here, to reflect our findings better. Instead of saying ‘Prior to LIP addition’, we now state\n‘Prior to the emplacement of the Siberian Traps…”.\nI agree with your response, but I don’t see this change in the revised manuscript (line 150).\n156: Please check if the reference to Fig. 4c is correct for this sentence.\n196: Please check if the reference to Fig. 3 is correct for this sentence.\n220: It’s impossible to compare the ‘weathering°assing high” with the “50 kyr” run because the red line is hidden. Could you\nplease make one of the lines dashed. Also, (I already mentioned this above), but please adjust the y-axis and/or get rid of the\nwhite space so it’s easier to see the change in temperature.\n231-232: This sentence seems strange (i.e., whilewhen and cleardriven).\n247-248: This reads a bit strange, so consider re-writing: “…further tests considering in more detail important aspects…”\n271: I don’t really understand what you mean by “falsified by the geological record”, so consider re-writing this.\n284: Delete “this” after “features”\nFig 3: Could you mark CAMP and the Siberian Traps with a symbol instead of the dotted line? You could make the symbol\ntransparent so we can still see the colorbar. I wonder if you should also add letters a,b,c (and update the text accordingly).\nLine 410: This is from my previous review and your response:\nFig. S1: Add legend. I would change the yellow color – it’s quite difficult to see.\nLegend added here. Colours of lines have also been adjusted.\nI don’t see these change in the revised manuscript (Figure S1 is now S6)."}, "author_response": "Please find our responses to the comments below, highlighted in blue. We attach a version\nof the manuscript with changes tracked. For our responses below, however, we use line\nnumbers pertaining to the updated (changes accepted) manuscript.\n\nWe appreciate the reviewer’s comments and have attempted to address all their final\nconcerns in the updated manuscript. Our responses are detailed below.\n\n18: Could you please specify “better” correspondence compared to what?\nWe have updated this on lines 18-20:\n“This approach results in better correspondence between the modelled output and proxy\nreconstructions of the period (especially for seawater Sr isotope composition) when\ncompared to previous modelling studies.”\n\n80-81: I suggest moving the following sentence to methods: “although the LIPs themselves\nhave no topographic height so the model topography is not altered by LIP emplacement”\nWe have moved this sentence to the methods (lines 327-328).\n\n123: Please check if the reference to Fig. 3b and c is correct for this sentence.\nThis has been adjusted to reflect silicate weathering is in panel 3c. We also add panel letters\nto figure 3.\n\n127: Specify the panel letter of Figs. 3,4\nPanels (Figs. 3a and 4c) now specified here.\n\n136: The addition of Fig. S2 is great! But I recommend some changes:\n• Adjust the y-axis of panel b of figure S2 (“zoom in”) so that we can more easily see the\ntemperature changes from the baseline. I get it that you want to show the full proxy range,\nbut we can see that in panel a.\n• Combine panel a of S2 and Fig S4 into one figure. That way we can easily compare all\nmodel runs. Get rid of all the white space; we want the focus to be on your model results.\n• Place the black dashed line on top so we can see the full extent of the baseline run (it’s\nkind of hidden behind the solid lines as of now). This applies for all these figures + Figure 4\nin the main manuscript.\n• Combine the a and b panels of figure S3.\nWe have made all suggested changes, thank you for the help refining the figures.\n\n150: This is from my previous review and your response:\n128-129: You state that the model is unable to reproduce most of the Mesozoic trends in\noceanic Sr isotope composition prior to LIP addition and refer to Fig. 4. But don’t you add\nLIP (Tarim/Skagerak) already at 300 Ma, so in the beginning of your study period? So, what\ndo you mean by prior to LIP addition then? I would argue that, generally, your model is\nunable to reproduce Sr record prior to ca. 250 Ma and after 170 Ma.\nWe have adjusted this sentence here, to reflect our findings better. Instead of saying ‘Prior to\nLIP addition’, we now state ‘Prior to the emplacement of the Siberian Traps…”.\nI agree with your response, but I don’t see this change in the revised manuscript (line 150).\nThanks for noticing – we have made sure this addition is in the updated manuscript.\n\n156: Please check if the reference to Fig. 4c is correct for this sentence.\nFigure reference updated, no Fig. 4b.\n\n196: Please check if the reference to Fig. 3 is correct for this sentence.\nFigure reference updated, now Fig. 4.\n\n220: It’s impossible to compare the ‘weathering°assing high” with the “50 kyr” run because\nthe red line is hidden. Could you please make one of the lines dashed. Also, (I already\nmentioned this above), but please adjust the y-axis and/or get rid of the white space so it’s\neasier to see the change in temperature.\nAs mentioned above, these have all been combined into two panels in Fig. S2, with the\nsuggestions included.\n\n231-232: This sentence seems strange (i.e., while when and clear driven).\nChanges made.\n\n247-248: This reads a bit strange, so consider re-writing: “…further tests considering in more\ndetail important aspects…”\nChange made, ‘in more detail removed’ here.\n\n271: I don’t really understand what you mean by “falsified by the geological record”, so\nconsider re-writing this.\nChanged to ‘would not be in agreement with the geological record’.\n\n284: Delete “this” after “features”\nChange made.\n\nFig 3: Could you mark CAMP and the Siberian Traps with a symbol instead of the dotted\nline? You could make the symbol transparent so we can still see the colorbar. I wonder if you\nshould also add letters a,b,c (and update the text accordingly).\nWe do not feel that adding a symbol to these figures would make them clearer, and so retain\nthe current line used to indicate their approximate location. We have, however, added panel\nletters and updated text to indicate which panel is being referred to.\n\nLine 410: This is from my previous review and your response:\nFig. S1: Add legend. I would change the yellow color – it’s quite difficult to see.\nLegend added here. Colours of lines have also been adjusted.\nI don’t see these change in the revised manuscript (Figure S1 is now S6).\nCorrect figure uploaded in updated submission."}]

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