Explosive volcanic eruptions can act as carbon sinks

Domain: earth_science
Published: 2025-05-08
Editorial outcome: minor_revision

Abstract
Volcanic soils, covering only ~1% of the Earth’s land, store over 5% of the global soil organic C stock. The frequent burial of these soils by tephra fallout from explosive volcanic eruptions is a critical but poorly quantified C storage process in soils from volcanically active regions. Using field measurements, we demonstrate that single eruptions can bury substantial amounts of stable organic carbon in soils. We develop a modelling framework and estimate that, in Ecuador alone, at least 1.1 Pg C has been stored in volcanic soils repeatedly affected by tephra deposition during the Holocene. This stock of tephra-buried soil organic carbon exceeds the cumulative CO2 emissions from the source eruptions. Here, we show that explosive volcanism, through the repeated burial of organic C in volcanic soils, acts as a significant regional C sink over time, ultimately averaging to net C-negative events.

Introduction
Tephra emissions associated with explosive volcanic eruptions serve as the primary substrate for volcanic soil formation. Volcanic soils exhibit a unique capacity for SOC accumulation, particularly during the first few thousand years of pedogenesis1,2. This property is attributed to abundant reactive surfaces, the creation of organo-mineral associations and the development of microaggregation which protect SOC from microbial decomposition3–5. In addition, being co-located with Late Pleistocene and Holocene terrestrial volcanoes2, organic C-rich volcanic soils are prone to repeated and widespread burial by tephra fallout from explosive eruptions. A tephra deposit just a few tens of centimetres thick, forming in a few hours to a few days, can halt the development of pre-existing soils while providing fresh parent material for new pedogenesis. The recurrence of this process, occurring at intervals ranging from a few years to several thousand years, is responsible for the formation of multi-layered volcanic soils, typically featuring several subsurface horizons with high SOC content2,6. Importantly, due to unfavourable environmental conditions for decomposers and a lower accessibility to decomposers and limited input of fresh organic matter5,7–10, the buried SOC can be stable over millennial timescales5,7,11,12. The combination of high SOC accumulation rates and repeated burial result in volcanic soils generally containing more organic C than other mineral soil types1,2.

Although it has long been suggested that volcanic soil burial by tephra increases SOC content2, the causal impact of explosive eruptions on SOC stocks in volcanically active regions has still not been unequivocally demonstrated2,12,13. The difficulty lies in quantitatively accounting for both the magnitude-frequency relationship of volcanic explosive eruptions and the accumulation of SOC over time in soils repeatedly affected by tephra deposition. Measuring total C stocks in volcanic soils limits our understanding of how explosive volcanism affects SOC storage over time, as stocks give only a single value of C inventory in a system for a sampling year. Here, we hypothesise that repeated tephra fallout from explosive eruptions plays a major role in the build-up of SOC stocks in volcanically active regions by sequestering substantial amounts of organic C in volcanic soils.

The accumulation of SOC in soils ultimately sequesters atmospheric C that was initially captured by plants through photosynthesis. In the soil-plant system, some C is diverted from the short-term (daily to annual timescale) photosynthesis–respiration cycle into inorganic C phases during chemical weathering of silicates present in the soil parent material. However, in the first few thousand years of soil formation, organic C accumulation typically far exceeds CO2 removal by silicate weathering14. Therefore, we posit that the burial of organic C by tephra in volcanic soils is the primary mechanism through which an explosive eruption may act as a C sink, that is, when more C is sequestered in the buried soils than is released into the atmosphere as magmatic CO2 during the eruption.

Based on field data, we first demonstrate that soil burial by tephra fallout from single eruptions can lead to significant storage of stable organic C in the subsurface soils. We then develop a modelling framework to quantify the spatio-temporal accumulation of the tephra-buried SOC stock in a region repeatedly affected by explosive eruptions (Fig. 1) and compare this stored C to the total mass of magmatic CO2 emissions directly associated with these eruptions.

Results
SOC stock buried by tephra from a single explosive eruption
We estimated the SOC stock stored in the soil intercalated between the tephra deposits from the last two eruptions of the Atacazo-Ninahuilca Volcanic Complex, located in the Western Cordillera of Ecuador, 10 km southwest of Quito (Fig. 2). These Plinian events (Volcanic Explosive Index (VEI)15 5), known as N5 (4400±35 BP) and N6 (2270±15 BP), produced ≥1 km3 of dacitic tephra (ref. 16). We sampled 35 soil profiles at altitudes ranging from 1273 to 3772 m within a ~1560 km2 area corresponding to the 10-cm contour of the N6 tephra deposit (Fig. 2; “Methods”). The SOC content (CSOC) is significantly lower (P < 0.001) in the soil buried by the N6 tephra fallout (CSOC = 10–36 g C kg−1) compared to the surface soil (CSOC = 23–68 g C kg−1) formed from this deposit (Supplementary Table 1). Both soils developed from tephra of the same composition (dacite), over comparable time periods of surface exposures of ~2100 and 2270 years16, respectively, and under similar climatic conditions; accordingly, it is reasonable to assume that SOC would have accumulated at roughly the same rate17,18. However, human-induced variations in vegetation cover may have occurred locally over time, which could partly account for the difference in soil C between the buried and surface soils. Alternatively, a more likely explanation is partial loss of SOC after burial due to mineralisation of its most labile fraction7,9. The remaining stock of the tephra-buried SOC is stable on a millennial time scale, as suggested by C-14 ages ranging from 1800±60 to 2365±55 yr (Supplementary Table 2).

We found that SOC density (in g C m−2) in soils buried by the N6 tephra is predictable based on altitude (Supplementary Fig. 1), consistent with the idea that temperature and precipitation (which typically decreases and increases with altitude, respectively) are the primary factors driving SOC decomposition and storage17–19. By applying this relationship to a 30-m digital elevation model20, we estimated the SOC stock buried by the N6 tephra ~2340 years ago, yielding a total mass of ~18 (3–33) Tg C (Methods). This is a conservative result because soil burial by tephra can occur for deposits thinner than 40 cm, hence covering a larger surface area and increasing the buried SOC stock. Additionally, tephra redistribution following deposition may increase further the surface area impacted by burial. We also calculated that the magmatic CO2 emitted into the atmosphere by the N6 eruption amounted to 5.7–8.7 (median = 7.1) Tg of C (Methods). Thus, the SOC stock buried by the N6 tephra is up to ~6 times larger than the mass of magmatic C released by the source eruption. Therefore, we contend that soil burial by tephra can turn an explosive eruption into a negative-C emission event.

SOC accumulation in soils repeatedly buried by tephra over time
To quantitatively assess how much SOC can be sequestered over time in a region repeatedly affected by tephra fallout from explosive volcanoes, we developed a modelling framework to predict the spatio-temporal distribution of tephra deposits on the ground and the subsequent accumulation of SOC in the buried soils (Fig. 1 and Supplementary Fig. 2). Additionally, the model enables estimation of the magmatic CO2 released by the explosive eruptions within a given time interval. We applied our model ('Methods') using the Ecuadorian segment of the Andean volcanic belt, ~30% of which is covered with volcanic soils formed from tephra21,22 (Fig. 2). We considered only eruptions of VEI 4, 5 and 6. This region has experienced 42 eruptions with a VEI ≥ 4 originating from ten volcanoes during the Holocene23. The spatial distribution of tephra accumulation on the ground was calculated using a probabilistic modelling approach24 ('Methods'). To account for uncertainties, we considered tephra accumulations associated with 10, 50 and 90% exceedance probabilities. We also evaluated the minimum tephra thickness necessary for effective soil burial, testing three values of 10, 30 and 50 cm. Using the eruptive record of the ten identified volcanoes and correcting for record completeness, we estimated for continental Ecuador an average recurrence interval of eruptions of VEI 4, 5 and 6 and generated 1000 synthetic eruption catalogues simulating the Holocene eruptive history. Our modelling framework can be conceptualised as a three-dimensional matrix with a spatial grid covering the Ecuadorian Andes at a resolution of 1 km and a temporal resolution of 1 month (Fig. 1d). For each timestep of a given catalogue, we first tested if an eruption occurs, in which case a VEI and a source volcano were sampled based on the eruption record.

Our model provides constraints, at any location in Ecuador, on the time between two consecutive eruptions that generated tephra deposits thick enough to bury surface soil. Where burial occurs, this time interval represents the duration of accumulation of organic C in the new surface soil that forms from the tephra deposit (Fig. 1b and c). Volcanic soils exhibit a unique capacity for SOC storage, with ~31 kg C m−2 on average, which is second only to organic soils1 (Histosols). According to refs. 12,25,26, the rate of organic C accretion in volcanic soils decreases as a power function of soil age, regardless of the climate zone. We refitted this power law with additional data points and described uncertainties (Supplementary Fig. 4). Using this relationship, we estimated the cumulative amount of organic C stored in volcanic soils across Ecuador, with each grid cell’s SOC stock being the result of 1000 synthetic eruptive catalogues for the Holocene. To account for the potential loss of labile organic C after tephra burial7,9, we adopted a cautious approach and divided the computed SOC stock by two. This is consistent with our field observations at Atacazo-Ninahuilca and a modelling study9, which shows that the burial of topsoil can lead to the preservation of up to half of the initial CSOC.

Explosive eruptions drive SOC storage and act as C sinks
Based on the median time-dependent SOC accrual rate and assuming a 50% probability of soil burial by 30 cm of tephra, our model indicates that ~1.1 Pg of organic C has accumulated in soils covered by tephra from Holocene eruptions in Ecuador (Fig. 3). When considering the 10th and 90th percentile of the SOC accumulation rate-time regression model (Supplementary Fig. 4), the estimated tephra-buried SOC stock values are lower (~0.3 Pg) and slightly higher (~1.2 Pg), respectively (Fig. 3 and Supplementary Table 6). Higher SOC storage in soils is predicted for conditions that decrease the minimum tephra thickness and/or tephra isomass probability, and vice versa. Due to conditions that are unfavourable for microbial respiration7,8, the residence time of buried SOC stocks is likely to be on the order of centuries to millennia, as also already indicated by our C-14 analyses at Atacazo-Ninahuilca (Supplementary Table 2) and other studies5,11,27–29.

Our results unambiguously show that, in volcanically active regions, the combination of rapid organic C accumulation during volcanic soil accretion and recurrent soil burial by tephra deposits leads to substantial storage at depth of organic C. For Ecuador, the SOC stock in volcanic soils covered repeatedly by tephra during the Holocene may make up as much as one-third of the country’s total stock (~3.2 Pg of C to a depth of 1 metre beneath the surface30). Over a similar period, tephra fallout in Ecuador has led to the sequestration of as much organic C in soils as the regional-scale Bignell loess deposition episode that could have buried ~0.7–2.7 Pg C in a paleosoil (the Brady soil in Nebraska, USA) 10,500–9,000 years ago31. However, comparing SOC stocks on a per square kilometre basis reveals that Ecuadorian explosive volcanism may have resulted in ~4.5 times greater organic C storage (Supplementary Methods), highlighting recurrent tephra deposition as a particularly efficient geological process for sequestering C at depth. Our analysis reinforces the idea that soil burial mediated by volcanic materials constitutes a significant contribution to the total soil C stock13,32,33.

Repeated tephra fallout events during the Holocene may have led to the storage of ~1.9 Mg C km−2 yr−1, or ~11 Tg C yr−1, in Ecuadorian soils. This estimate, while slightly lower, remains within a comparable range to the CO2 drawdown associated with rock chemical weathering in the Ecuadorian Andes and sub-Andes (2.3–4 Mg C km−2 yr−1 (ref. 34)). Basalt weathering is recognised as the primary mechanism by which volcanic activity generates a strong CO2 sink, accounting for ~49 Tg C yr−1, or 30–35% of the flux derived from continental silicate weathering35. Notably, organic C sequestration in tephra-buried soils in Ecuador alone represents up to ~23% of the CO2 consumption flux from basalt weathering. Given the extensive distribution of volcanic soils in continental arc systems with explosive volcanism, such as the Cascades, Central America, the Andes and Kamchatka, this contribution is likely much greater at a global scale. In volcanic island arcs, a variable proportion of tephra emissions from explosive activity is deposited offshore, thereby reducing the potential for organic C preservation through soil burial. Nonetheless, explosive eruptions in these regions can still result in significant SOC sequestration, as suggested by the high CSOC observed in multi-layered volcanic soils in Japan36,37 and New Zealand38,39. We suggest that, over millennial timescales, soil burial by tephra worldwide may constitute a C sink as efficient as basalt weathering. However, while inorganic C derived from silicate weathering is transported to the oceans and effectively removed from terrestrial cycling for over 108 years40, the sequestration of SOC in volcanic soils occurs over shorter timescales, typically spanning 103–104 years5,7,11,12. Except for the most restrictive modelling scenarios (e.g., thick tephra deposit with high probability of deposit formation, or low percentile in the SOC accumulation rate-time regression model), our estimates (Fig. 3) highlight that the SOC stock buried by tephra fallout during the Holocene dominantly exceeds, by a factor of ~3–15, the cumulative magmatic C emissions (0.08–0.4 Pg C) from all VEI ≥ 4 events in our synthetic catalogues of Ecuadorian eruptions (Methods). While likely minor compared to C storage in volcanic soils developing over a few thousand years40, atmospheric CO₂ consumption during tephra weathering would further contribute to the C sink. Our findings lead to the counterintuitive conclusion that Holocene explosive eruptions of VEI ≥ 4 in Ecuador, through repeated burial of organic C in volcanic soils, average to C-negative events. This result stands in sharp contrast to the prevailing view that explosive volcanic activity is a net source of C emissions to the atmosphere41–43.

Our modelling framework provides a spatially and temporally explicit estimate of the capacity of explosive eruptions to store organic C in volcanic soils and can be used to assess the C balance of explosive volcanism. It can be readily applied to other volcanic arc regions to better constrain SOC storage in the subsurface soils, thereby helping improve understanding of the role of volcanic activity in the terrestrial C cycle.

Methods
SOC stock buried by the N6 tephra from Atacazo-Ninahuilca
We sampled 35 volcanic soil outcrops around Atacazo-Ninahuilca volcano in 2016 and 2017. The outcrops were located within the 10-cm isopach of the tephra fallout from the 2,270 BP N6 eruption (Fig. 2). All outcrops contain the N5 and N6 tephra layers and a buried soil intercalated between them. The locations and altitudes of the 35 outcrops and the main properties measured for the surface and tephra-buried soils are provided in Supplementary Table 1. At each sampling site, we measured the thickness of the buried soil, i.e. the soil intercalated between the N5 and N6 tephra fallout from the 4,400 BP and 2,270 BP eruptions of Atacazo-Ninahuilca volcano16 (Fig. 2). Approximately 100 g of material from the surface and tephra-buried soils were collected using a 7-cm soil corer. A soil sample was obtained by compositing three individual samples collected at different soil depths. For the buried soil, the soil corer was inserted at least 1-m deep horizontally. In doing so, we ensured as much as possible that the sample corresponded to a soil material that had remained isolated from the atmosphere after burial by tephra. The samples were stored in sealed plastic bags at 4 °C, then air dried prior to analysis. Plant roots and microfauna were removed manually before sieving through a 2-mm mesh. The <2 mm soil fraction was homogenised by milling with a vibratory disk mill (Retsch RS200).

The bulk density (Dbsoil, in g cm−3) of the tephra-buried soils was estimated from triplicates of 100 cm3 Kopecky ring samples44. The coarse material fraction (solid material >2 mm, CF, in weight %) present in the soil was separated and weighed. In general, the CF in the soil buried under the N6 tephra is low (<1%) but increases (up to 10%) in outcrops located closer to the volcano. Volcanic soils usually have acidic pH values2 and carbonates are absent within them. Thus, the C present in our soils is composed entirely of organic C. The total C content (CSOC in weight % dry soil) in the surface and tephra-buried soil samples was measured by dry combustion with an elemental analyser (Elementar Vario MACRO Cube, precision <0.1%).

For each of the 35 soil outcrops, we calculated the SOC density (dSOC in kg C m−2) in the N6 tephra-buried soils as follows:

dSOC = CSOC × 1000 × Dbsoil × z × (1 − CF) / 100 / 100 (1)

where z (in m) is the soil thickness.

We related the variations in dSOC to altitude using a linear regression model (Supplementary Fig. 1). This allowed us to predict a dSOC value for each grid cell within the ~790 km2 area delineated by the 40 cm-isopach of the N6 tephra (Fig. 1). The SOC stock (in Tg) in the buried soils was then computed as:

SOCstock = Σn i=1 dSOC,i × Acell (2)

where n is the total number of grid cells (n = 878,014) and Acell the surface area of a cell (Acell = 900 m2).

Magma volume of Atacazo-Ninahuilca’s N6 eruption
The estimate of the magma volume released by an explosive eruption can be constrained by determining the volume of tephra fallout deposit associated with it44. This is obtained from field reconstructions that rely on the integration of isopach maps, i.e. hand-drawn contours representing equal deposit thickness, inferred from punctual observations at the outcrop level. Typically, for most Holocene eruptions, only the medial part of the tephra deposit is preserved in the landscape. To account for the missing contribution of distal tephra fallout to the total deposit volume, the thinning of the tephra deposit with distance from the volcanic vent must be described mathematically. For the N6 eruption of Atacazo-Ninahuilca, ref. 16 applied an exponential method and estimated a total volume of tephra fallout of 1.3 km3, to which an additional 0.8 km3 of pyroclastic density current (PDC) material was added. However, the exponential method is known to underestimate the volume of tephra deposits for which only the medial part is preserved45. As an alternative, we recomputed the tephra fallout volume of the N6 eruption with a power-law model45 to extrapolate the thinning trend to missing parts of the deposit. Using a conservative distal integration limit of 300 km ± 50% and incorporating an uncertainty of 20% on both tephra thickness and isopach area, a stochastic estimation of the tephra volume24 results in a median value of 5.0 km3 (3.9–6.5 km3 for the 10–90th percentiles), or 5.8 km3 (4.7–7.3 km3) when accounting for the PDC volume. Assuming a bulk density of 1100 kg m−3 for tephra deposits46 and a density of ~2500 kg m−3 for a dacitic magma47 such as that involved in the N6 eruption16, the estimated median volume of erupted magma is ~2.6 km3 (2.1–3.2 km3).

Magmatic CO2 emission from Atacazo-Ninahuilca’s N6 eruption
Explosive volcanic eruptions release substantial quantities of CO2 into the atmosphere41–43. These emissions originate from degassing of C originally dissolved in the magma. The paroxysmal phase of the 1991 eruption of Mt. Pinatubo, Philippines, emitted 10 Tg of CO2 per km3 of magma42. A similar rate of magmatic CO2 degassing was used for estimating the CO2 output of the eruptions of Tambora 1815 and Krakatau 1883 in Indonesia, Rabaul 1994 in Papua New Guinea, as well as Bishop Tuff, Katmai-Novarupta 1912; Mt. St. Helens 1980 and Redoubt 1989-1990 in the USA (refs. 41,48). This rate is considered representative of paroxysmal phases of andesitic, dacitic and rhyolitic explosive eruptions41. The CO2 emissions during an explosive eruption can also be inferred by considering a primary silicate melt with a typical CO2 content ranging from 0.2 to 0.4 weight % (refs. 49,50) and a 50% crystallisation rate during magma differentiation. Such conditions will lead to a CO2 content in the residual melt in the range of 0.4–0.8 weight % and therefore, 1 km3 of magma with 50% crystals by volume will contain 1.3 Tg of melt (assuming a density of ~2500 kg m−3 (ref. 46)) and approximately 5–10 Tg of CO2. Assuming complete CO2 release from the magma during an explosive eruption, a conservative estimate for the magmatic CO2 degassing rate (FCO2) is 10 Tg km−3, an estimate fully consistent with field-based measurements41,42,48.

Knowing the magma volume (Vmagma) of an explosive eruption (see above), the corresponding mass of magmatic C emitted (MC, in Tg) is calculated using:

MC = Vmagma × FCO2 × mC / mCO2 (3)

where mC and mCO2 are the molar masses of C and CO2, respectively, and FCO2 = 10 Tg km−3. Applying equation [3] to the N6 eruption of Atacazo-Ninahuilca indicates that 5.7–8.7 Tg (median = 7.2 Tg) of magmatic C were released into the atmosphere.

Frequency of Holocene explosive eruptions in Ecuador
We focused our study on eruptions with a VEI of 4, 5 and 6, which are sub-Plinian/Plinian events producing tephra volumes ranging from 0.1 to 100 km3 (ref. 15). Eruptions with a lower VEI are disregarded because the spatial dispersion of their tephra is limited, are either under-reported23 or missing51 from existing databases, and may feature a variety of eruption dynamics that do not necessarily follow the relationship between magma volume and CO2 emission used in this study (see above). We used the Global Volcanism Program (GVP)23 database to infer the eruptive history of all Andean Ecuadorian volcanoes during the Holocene. The database records 42 eruptions ≥ VEI 4 from a total of ten volcanoes, including 27 VEI 4 (65%), 14 VEI 5 (33%) and one VEI 6 (2%) eruptions (Supplementary Table 3). Cotopaxi volcano dominates the eruptive record, with 14 eruptions of VEI 4 and 7 eruptions of VEI 5. Only Quilotoa volcano produced a VEI 6 eruption.

Under the assumption that eruptions of VEI ≥ 4 are independent events52,53, we estimated a monthly average recurrence interval (ARI) over the Holocene. The monthly ARI (Supplementary Table 3) was computed as the number of eruptions of a given VEI normalised by the time period over which the eruption record is complete (i.e. 520 years for VEI 4 and 12,020 years for VEI 5 and 6). Supplementary Fig. 5 shows the cumulative number of eruptions recorded in the GVP catalogue for the ten selected Ecuadorian volcanoes. Since the record completeness varies with eruption magnitude53, the eruption frequency obtained from the GVP catalogue must be interpreted with care. Following previous studies52,53, we assumed that the catalogue of VEI 4 eruptions is only complete after 1500 AD and estimated an ARI for the ten volcanoes in continental Ecuador of 1.62 × 10−3 VEI 4 eruptions per month. However, there was a transition from rhyolitic to andesitic volcanism ~4000 years ago54, after which only one VEI 5 eruption is recorded. Evidence for such changes in eruptive dynamics led us to adopt a conservative approach: we considered the catalogue of VEI 5 eruptions as complete and estimated an ARI of 9.71 × 10−5 VEI 5 eruptions per month. Finally, we included only one eruption of VEI 6 for the Holocene, resulting in an ARI of 6.93 × 10−6 VEI 6 eruptions per month.

Using the ARI values, we generated 1,000 synthetic eruption catalogues spanning the Holocene. The stochastic sampling procedure was performed in Python 3.9 with the random.choices() function. Firstly, we sampled whether an eruption of any of VEI 4, 5 or 6 occurred over the 144,240 monthly timesteps between the beginning of the Holocene and 2020 using the sum of the total ARI for each VEI. Secondly, for timesteps when an eruption occurred, we used the total ARI of each VEI as relative weights to sample the VEI of the eruption. Thirdly, with sampled VEI, we used the ARI of each volcano as relative weights to sample a volcano source. Supplementary Fig. 6 summarises the distributions of eruptions of all VEI sampled over the 1,000 synthetic eruption catalogues. On average, each catalogue contains 248 eruptions (ranging from 200 to 309), with most of the discrepancy between the observed 42 eruptions in the GVP database attributed to under-representation of VEI 4 eruptions.

Spatial extent of tephra deposits from VEI ≥ 4 eruptions in Ecuador
The spatial distribution of tephra fallout resulting from VEI ≥ 4 explosive eruptions was estimated using a scenario-based probabilistic approach55. We modelled three eruption scenarios of VEI 4, 5 and 6 using Atacazo-Ninahuilca volcano as a source with the Tephra2 advection-diffusion model within the TephraProb probabilistic framework24. Each scenario consists of 5,000 runs of Tephra2, where each run samples a set of eruption source parameters (ESPs; Supplementary Table 4) and a wind condition stochastically. Wind conditions for Ecuador were obtained from the ERA5 database from the European Centre for Medium-Range Weather Forecasts56, using 16 years of four-daily data from 2000 to 2015 (Supplementary Fig. 3). Outputs were compiled into probabilistic isomass maps, which show the spatial distribution of tephra fallout accumulation associated with a specific exceedance probability. For instance, for a given VEI and a given spatial coordinate, a tephra accumulation of 10 kg m−2 at a 25% probability threshold implies that a deposit ≥ 10 kg m−2 was obtained in 1250 of the 5000 simulations.

Magmatic CO2 emissions from VEI ≥ 4 Holocene eruptions in Ecuador
Similar to the approach discussed above for the N6 eruption of Atacazo-Ninahuilca, the mass of CO2 emitted by an explosive eruption is obtained by considering a degassing rate of 10 Tg CO2 per km3 of magma erupted. For each of the 5000 sets of ESPs sampled for each VEI value in the probabilistic modelling, we converted the tephra volume (Vtephra) into a mass of magmatic C emitted (MC, in Tg) using:

MC = Vtephra × Dbtephra × 10 × mC / (Dmagma × mCO2) (4)

where Dbtephra is the tephra deposit bulk density (1100 kg m−3 (ref. 46)), Dmagma is the magma density (2500 kg m−3 (ref. 47)), mC is the molar mass of C and mCO2 is the molar mass of CO2. For a given VEI, we express the 10th, 50th and 90th percentiles of MC.

Spatio-temporal evolution of Holocene tephra deposits in Ecuador
Following the methodology described above, every synthetic eruption catalogue records if an eruption occurs at each timestep and, when it does, the corresponding VEI and source volcano are identified. Conceptually, the spatio-temporal reconstruction of tephra accumulation over the Holocene in the Ecuadorian Andes can be schematised as a three-dimensional matrix of coordinates [y, x, t], with a spatial domain at a 1 km resolution and a temporal domain at a monthly resolution. At every timestep of each synthetic eruption catalogue where an eruption occurs, the tephra accumulation modelled for Atacazo-Ninahuilca volcano for the relevant VEI is spatially translated to the source volcano and stored in the three-dimensional matrix.

The translation of tephra accumulations modelled for a VEI 4-, 5- or 6-eruption of Atacazo-Ninahuilca volcano to other volcanoes in the region can be justified as follows. Firstly, from a computation perspective, modelling each eruption (average of 248 eruptions per catalogue) for each catalogue (n = 1000) in a probabilistic way would require 1.24 × 109 individual runs of the Tephra2 model with TephraProb24. Our approach reduces this to a more manageable 15,000 simulations. Secondly, Tephra2 relies on an analytical solution of advection-diffusion equation, which significantly decreases computation time compared to numerical models, and also allows probabilistic modelling. Since no local topographic effect on ground tephra accumulation can be resolved by Tephra2, we used a 1 km spaced computation grid set at a constant elevation of 2500 m above sea level, which we assume to be representative for all volcanoes in Ecuador. Finally, Tephra2 works with two-dimensional wind profiles. Existing analyses of wind data over Ecuador57 suggest that the Andean arc experiences similar general wind patterns with a dominant westward direction. Since i) we consider the general wind direction to be of first order importance in our study compared to small-scale orographic wind effects and ii) general wind directions inferred from the ERA5 database56 are similar along a north-south profile across Ecuadorian volcanoes (Supplementary Fig. 3), we assumed wind conditions at Atacazo-Ninahuilca volcano to be representative for the studied region.

SOC accumulation rate in volcanic soils
In general, climatic conditions, namely temperature and precipitation, are key drivers of SOC accumulation, affecting both C input into the soil and SOC decomposition17,18. Parent material also influences SOC accumulation due to its control on soil mineralogy, texture and fertility, which affect net primary productivity and the stabilisation of organic matter18,58,59. At the regional and continental scales in various climatic zones, vegetation type is another factor that modulates SOC storage. Volcanic soils have a large potential for retaining organic C1,2. This is due to abundant reactive surfaces, the formation of organo-mineral associations and the development of microaggregation which protect SOC from decomposition and enhance its residence time.

To describe the rate of organic C accumulation (in g m−2 yr−1) in volcanic soils of different age, we revisited a previous study12 by adding measurements, including our own samples of volcanic soils collected in the Ecuadorian Andes (Supplementary Table 5). The revised dataset includes 76 measurements covering volcanic soils of varying age (~70 to 16,000 yr), formed from parent material composition ranging from basalt to rhyolite and located in contrasted climate zones (polar to tropical equatorial). As shown in Supplementary Fig. 4, the relationship between SOC accumulation rate and soil age in volcanic soils can be described by a power function. The decrease in the rate of SOC accumulation over time is attributed to changes in reactive iron- and aluminium-bearing soil constituents, which result in a reduced efficiency of organic matter stabilisation25,26. We used quantile regression to estimate prediction intervals based on the 10th, 50th and 90th percentiles.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Delmelle et al. examined the quantitative importance of organic C burial by tephra from volcanic eruptions at a regional scale using field measurements and the modeling of both (i) temporal changes in soil OC and (ii) spatio-temporal evolution of tephra. I am not a geologist. But, from soil’s side, I think the authors did a good job carefully estimating the C burial using a reasonable set of approaches (more detail comments below). It’s been known that volcanic soils accumulate OC at faster rates compared to non-volcanic soils. But, in my knowledge, no study estimated this OC sink capacity in a meaningful way. Thus I think this is an important and high-impact work as we need to better understand C cycle facing climate change. This work is also timely and significant because both geology and agriculture/soil science communities are actively working on different ways to increase C sink on land by various means.\nKey assumptions the authors had to make to estimate the C sink capacity are the following four.\n1. the rate of organic C accretion in volcanic soils decreases as a power function of soil age.\n2. a half of SOC is lost during the burial due to microbial decomposition.\n3. 1,000 synthetic eruptions for the Holocene.\n4. a 50% probability of soil burial by 30 cm of tephra.\nGiven the spatio-temporal complexity of the OC accumulation, burial, and tephra deposition processes, I think these assumptions are reasonable (esp. 1 and 2 given my background). In my view, their estimate was reasonably conservative and the actual C sink is possibly greater when we account for re-distribution of tephra on the land surfaces. While the authors account for the elevation, tephra re-distribution after deposition was not accounted for. Finer tephra tend to redistribute to lower land by wind and water. The lowland tend to have higher soil OC and the tephra input is likely to slow down OC decomposition via sorption and aggregation type processes. Accounting for such process in the model is too complex but, I think, it is worth mentioning in the manuscript that such process would increase the soil C even further.\nA few minor points:\nFig. 1. The conceptual representation of soil development (Fig. 1b) is not really correct. After each deposition of relatively thick tephra, vegetation cover, OM input, and chemical weathering all start on the top part of the tephra. Thus, during the soil development, the surface remains the same or goes downward, in contrast with the current figure which shows the rising of the surface. The surface rise can happen due to the repeated deposition of small amounts of tephra from less explosive tephra (e.g., < a few mm deposition per year). If that is assumed, the authors should mention this. Otherwise, it makes no sense that soil mass increase upward over time between the explosive eruption.\nFig. S4. These plots showed about 40 eruptions over 10 k years. But the authors assumed 1,000 synthetic eruptions for the Holocene, which appeared to be too high. Probably I am missing something.", "reviewer_3": "Overview\nThis is an interesting and timely paper that evaluates a simple mechanistic idea of C sequestration versus C delivery during and after a volcanic eruption in an elegant quantified framework, and then places it within the very current discussion of C movement in regional (and possibly global) systems. It uses a combination of modelling regarding CO2 fluxes to/from the atmosphere, and thorough field-based sampling of an Ecuadorian volcanic tephra successions and their intercalated soils to determine SOC. The volcanic gas yield and likely C sequestration is then compared to arrive at the apparently surprising conclusion that net CO2 drawdown (through C sequestration), rather than CO2 delivery causing atmospheric increase/accumulation is the net result of such volcanic eruption types.\n‘Explosive eruptions have the potential to act as net C sinks’: I like the idea that SOC buried by tephra exceeds, over time, that initially emitted by the eruption. Yes, this is counter-intuitive to the layman, but implicitly understood by the wider Earth Sciences community. It does need to be said, and this paper could be a worthy attempt to do this through case-based study.\nHowever, I have a couple of problems with the structure of this m/s. First, it seems a little strange that the methods section comes later in the paper than might be expected, and effectively after the ‘conclusions’ are delivered. It would be more appropriate this is delivered before the discussion on SOC storage?\nSecond, the conclusions section itself is disappointingly weak. It is poorly flagged around Lines 185 – 198. This needs restructuring with a clearly flagged Discussion and Conclusions section. I would then go further and deliver the key findings (i.e., that C drawdown exceeds C release for many volcanic eruptions of this type) as three or four clear and hard-hitting bullet points. In my opinion this could turn the paper from a useful incremental interest to a specific (volcanological) audience into a (probably) well-cited article that appeals to a wider geo-climate scientific community.\nTo summarise, I enjoyed reading the m/s, and feel it offers a worthy addition to the study of volcano - climate interaction.\nGeneral\nPerhaps better frame the ‘time frames’ over which the authors wish to discuss (again a simple table may clarify). We’re dealing with several processes here all operating at different rates and scales: atmospheric exchange of CO2, the exchange between substrate and atmosphere and organic and inorganic chemistries within the soil. Most of these are further mediated by climate (T) and soil moisture (precipitation).\nA little more on the background of the ‘example’ volcano may be helpful– just to provide context. Add 2 or 3 descriptive sentences in text, or in appropriate figure caption?\nOne of the few weaknesses of the paper is it doesn’t explicitly address the varying timescales of ‘instantaneous’ CO2 delivery (during eruption, and the differing time scales (and half-lives) of silicate weathering and C sequestration through SOC. Perhaps these could be outlined in a table – just as example – since it isn’t the focus of this paper to directly address silicate weathering, but rather the role of soils and their organic C composition (through vegetation breakdown?). That being the case, then perhaps this ‘weathering’ aspect should be more thoroughly/explicit stated in the introductory section.\nUnderstandably, not all aspects of this complex volcanic – weathering – soil profile and C content system can necessarily be addressed in one short paper. So, I’m not arguing for an in-depth treatment because I can see the authors’ approach is focussed around the SOC quantification, and this approach is timely since it helps better quantify part of this complex system, and one aspect that is less commonly tackled.\nWeathering of tephra isn’t really directly addressed. It seems likely that net CO2 drawdown would be (substantially?) higher that that determined by the authors because the nascent ash would be subject to aggressive silicate weathering in the years immediately after deposition.\nEruption output – volcanic product weathering approach has also been attempted wrt to flood basalt eruptions in several papers. These cover huge areas, then themselves break down at the surface and in developing weathering profiles thus entrapping SOC and acting as CO2 sink through the silicate weathering chain and may be capable of delivering global climate modification. Most recently:\nBlack, B.A., Karlstrom, L., Mills, B.J.W. et al. Cryptic degassing and protracted greenhouse climates after flood basalt events.\nNat. Geosci. (2024). https://doi.org/10.1038/s41561-024-01574-3\nGiven several approaches combine into the ‘method’ – why not use a simple box/flow diagram/chart to show the stages from data collection to the output conclusion? That would be a simple device and be enormously helpful to the reader.\nSpecific\nFigures\nFigure 1 is good – a nice straight forward figure. Please add ‘oldest and youngest labels to Y axis (I read it upside down initially – with T= 0 at top). Also add simple key for the coloured layers (e.g. yellow = tephra (Tp1, purple Tp2 etc.,). Also use a thick brown line to show soils on each tephra deposit. Scales c and d are very good and convey the authors key point regarding spatial variation of SOC very effectively. Nice figure, overall.\nFigure 2 is good, but would prefer lats and longitude (b) in Degree – Minute format.\nFigure 3. I can see this is a decent statistical treatment of the SOC stock versus C release on eruption. This is fine. However, it needs to be additionally clarified with a simpler figure that shows a more readily accessible comparison of SOC with an eruption output. Many reading this paper will need a straight-forward illustration – for instance using an exemplar eruption to demonstrate that C drawdown/storage exceeds that of the C ion eruption gases, and by how much (i.e., double, five times, a magnitude?). This part of the paper is pivotal to the authors’ arguments and at present, this figure represents too complex a treatment (maybe put this into the supplement, and replace with a accessible one that better delivers/illustrates will deliver the authors’ statement (Lines 158 – 160) ‘Our results unambiguously show that, in volcanically active regions, the combination of rapid organic C accumulation during volcanic soil accretion and recurrent soil burial by tephra deposits leads to significant storage at depth of organic C’.\nText\nLines 74 – 75: SOC ‘stable over millennial timescales’ Need to better quantify, of model/speculate upon SOC degradation post-entombment, and the likelihood of rate of exhumation by erosion. Also, microbial breakdown will release both CO2 and methane.\nLine 69. Alter to: ‘over comparable time periods of surface exposure’\nLine 70. Alter to: ‘and under similar climatic conditions; accordingly, it is reasonable to assume SOC would have accumulated at roughly the same rate’.\nLine 73. ‘Alternatively, a more likely explanation may be partial loss of SOC…’\nLine 88 ‘We found that SOC density’ – should this read: ‘We found that SOC content’?\nStated/determined change in SOC with elevation may likely be due to change in type and amount of vegetation cover. Perhaps worth stating more explicitly.\nLines 172 - 174. ‘For comparison, the mean CO2 drawdown by rock chemical weathering in the Ecuadorian Andes, at ~0.25 Mg C km-2 yr-1 (ref.32), is much lower’. Fine but this is a general measurement of silicate weathering from the given reference. However, using this, or the Dessert et al., work may not be fully appropriate. Tephra is very easily weathered due to is huge granular surface area: rock, such as basalt, presents only a surface interface for weathering, or else a weathering profile/critical zone as alteration progresses. This can be self-limiting as alterd materials accumulate. In essence, my suspicion is that neo-formed tephra will be a very efficient CO2 sink – more than ‘rock’. This need to be considered in the discussion of C weathering drawdown versus C emissions.\nLines 277 – 278: ‘the N6 eruption of Atacazo-Ninahuilca indicates that ~7.0 Tg of magmatic C were released into the atmosphere’. Yes, that’s a fair ‘average’ but why not run the carbon balance determinations using max and min values of each of the key components outlined in the methodology (or does this fall out of the percentile argument?). This will then provide a probabilistic envelope of determinations which more is likely to contain the ‘real’ value. In either case, a little clearer description/outline could help.\nMethods\nMethods are comprehensive: Primary data uses multiple soil profiles associated with two eruptions Atacazo-Ninahuilca volcano (given as Atacazo in figure 2) in Ecuador (VEI 5?). Sampling appears thorough and robust, and the density and TOC determinations are good.\nThe calculations under methods look logical and are straight forward in explanation. It is good to see such thorough field sampling underpinning the SOC determinations that are carried forward into the modelling.\nMagmatic C is always challenging to estimate, but the authors’ approach is honest – using known eruptions with measure releases as the guideline for estimating the CO2 release. I can see no major objection to this approach.\nIt is inevitable that ‘estimates’ need to be generated for CO release etc. There are actual measurements (FTIR etc.,) for modern eruptions, and these can be judiciously applied for estimates of ancient eruptions Estimation of CO2 – methods?\nInitially, the main text seems to suggest this to based only upon Iacovino and Till (2019), but then Supplementary material reveals the authors’ use of recent historic values (good idea) for CO2 release. Nevertheless, there are several approaches to this estimation.\nAnother method is to actually determine the actual (measured) gas release per unit volume of magma, then scale this appropriately. Different magma compositions can contain different quantities of the key gases SO2 and CO2 – generally basaltic magmas can contain the highest. Other authors have attempted this approach (e.g., Self et al., 2008, 2009; Elements and EPSL), and used gas contents preserved in basalt as inclusions. Presumably there are comparable determinations for dacite?\nAdditionally, an ‘envelope’ of estimate may be determined by offering the min and max yield using the variability inherent in key input parameters. See similar types of argument in Jolley and Widdowson (2005) and those by Self et al., (above).\nUse of percentiles as a range of estimates to evaluate SOC in soils – this is good statistics. But this type of approach then isn’t clearly followed through to CO2 eruption volumes (unless I’ve missed something). Again, we are dealing with an ‘estimate’ here, so it seems logical to provide an ‘envelope of probability’ to these values too (see previous statement).\nIn effect, there are assumption made about the coverage of tephra (magma volume), the though I was surprise not to see the Pyle et al., research cited here as well as the Bonadonna and Houghton (2005). Perhaps this work is worth a look during any revision to m/s.\nStock of SOC – though this is well-constrained by the authors’ field sampling and subsequent analytical work.\nWhy does rate of C accumulation decrease in volcanic soils with age? I assume this is due to primary colonisation and early vegetation delivering the main biomass, then subsequent degradation through breakdown? Perhaps a line of explanation to this statement (maybe delivered in the reference material, but could do summarising here).\nUse of the SI magnitude system is correct – see SOC values in Pg (petagramme). However, in volcanic studies its more usual to relate values in tonnes or megatonnes. Somewhow a ‘tonne’ and its derivatives (e.g., megatonne) are a more readily understood quantity for ‘human’ scale understanding than that a value of 10 to the power 6 (i.e., megagram) and 12 (i.e., terragram) respectively. I leave this to the authors to consider and decide, but this paper makes some very interesting statements about CO2 delivery and drawdown and it would help increase its impact to make it more readily ‘scalable’ in a relative way for the reader.\nFor instance: 1.1 Pg SOC (i.e., = 1100 megatonnes)\nSpatial extent of tephra fallout is elegantly approached by using an accepted modelling programme an inputting modern wind condition record. In the absence of ‘real historic data’, we can reasonably assume these will be representative of the earlier Holocene.\nSupplementary Material\nThis is well-delivered and clearly labelled/described."}, "author_response": "We are grateful to both Reviewers for their positive evaluation of our manuscript and for their\nconstructive comments. We have carefully addressed all of them, and our detailed\nresponses are provided below in blue text.\nThe revised version of the manuscript is attached, with modifications to the original\nsubmission highlighted in pink text. We also attach the revised version of the Supplementary\nInformation, which includes a new Figure.\nReviewer #1 (Remarks to the Author):\nDelmelle et al. examined the quantitative importance of organic C burial by tephra from\nvolcanic eruptions at a regional scale using field measurements and the modeling of both (i)\ntemporal changes in soil OC and (ii) spatio-temporal evolution of tephra. I am not a\ngeologist. But, from soil’s side, I think the authors did a good job carefully estimating the C\nburial using a reasonable set of approaches (more detail comments below). It’s been known\nthat volcanic soils accumulate OC at faster rates compared to non-volcanic soils. But, in my\nknowledge, no study estimated this OC sink capacity in a meaningful way. Thus I think this is\nan important and high-impact work as we need to better understand C cycle facing climate\nchange. This work is also timely and significant because both geology and agriculture/soil\nscience communities are actively working on different ways to increase C sink on land by\nvarious means.\nKey assumptions the authors had to make to estimate the C sink capacity are the following\nfour.\n1. the rate of organic C accretion in volcanic soils decreases as a power function of soil age.\n2. a half of SOC is lost during the burial due to microbial decomposition.\n3. 1,000 synthetic eruptions for the Holocene.\n4. a 50% probability of soil burial by 30 cm of tephra.\nGiven the spatio-temporal complexity of the OC accumulation, burial, and tephra deposition\nprocesses, I think these assumptions are reasonable (esp. 1 and 2 given my background). In\nmy view, their estimate was reasonably conservative and the actual C sink is possibly\ngreater when we account for re-distribution of tephra on the land surfaces. While the authors\naccount for the elevation, tephra re-distribution after deposition was not accounted for.\nFiner tephra tend to redistribute to lower land by wind and water. The lowland tend to have higher\nsoil OC and the tephra input is likely to slow down OC decomposition via sorption and\naggregation type processes.\nAccounting for such process in the model is too complex but, I think, it is worth mentioning in\nthe manuscript that such process would increase the soil C even further.\nWe have included a sentence in the revised manuscript which mentions this additional\nprocess.\nLines 99-100\nAdditionally, tephra redistribution following deposition may increase further the surface area\nimpacted by burial.\nA few minor points:\neach deposition of relatively thick tephra, vegetation cover, OM input, and chemical\nweathering all start on the top part of the tephra. Thus, during the soil development, the\nsurface remains the same or goes downward, in contrast with the current figure which shows\nthe rising of the surface. The surface rise can happen due to the repeated deposition of\nsmall amounts of tephra from less explosive tephra (e.g., < a few mm deposition per year). If\nthat is assumed, the authors should mention this. Otherwise, it makes no sense that soil\nmass increase upward over time between the explosive eruption.\nThe Reviewer is correct. Figure 1 has been modified accordingly.\nFig. S4. These plots showed about 40 eruptions over 10 k years. But the authors assumed\n1,000 synthetic eruptions for the Holocene, which appeared to be too high. Probably I am\nmissing something.\nFig. S4 is correct; however, the original figure caption lacked clarity. We have revised the\nfigure caption for better understanding. Fig. S4 is now Supplementary Fig. 5.\nReviewer #3 (Remarks to the Author):\nOverview\nThis is an interesting and timely paper that evaluates a simple mechanistic idea of C\nsequestration versus C delivery during and after a volcanic eruption in an elegant quantified\nframework, and then places it within the very current discussion of C movement in regional\n(and possibly global) systems. It uses a combination of modelling regarding CO2 fluxes\nto/from the atmosphere, and thorough field-based sampling of an Ecuadorian volcanic\ntephra successions and their intercalated soils to determine SOC. The volcanic gas yield\nand likely C sequestration is then compared to arrive at the apparently surprising conclusion\nthat net CO2 drawdown (through C sequestration), rather than CO2 delivery causing\natmospheric increase/accumulation is the net result of such volcanic eruption types.\n‘Explosive eruptions have the potential to act as net C sinks’: I like the idea that SOC buried\nby tephra exceeds, over time, that initially emitted by the eruption. Yes, this is counter-\nintuitive to the layman, but implicitly understood by the wider Earth Sciences community. It\ndoes need to be said, and this paper could be a worthy attempt to do this through case-\nbased study.\nHowever, I have a couple of problems with the structure of this m/s. First, it seems a little\nstrange that the methods section comes later in the paper than might be expected, and\neffectively after the ‘conclusions’ are delivered. It would be more appropriate this is delivered\nbefore the discussion on SOC storage?\nWe appreciate the Reviewer’s feedback. However, we would like to clarify that the structure\nof the manuscript adheres to the formatting guidelines provided by Nature journals.\nAccording to these guidelines, the Methods section is positioned towards the end of the\nmanuscript, following the Results and Discussion sections.\nSecond, the conclusions section itself is disappointingly weak. It is poorly flagged around\nLines 185 – 198. This needs restructuring with a clearly flagged Discussion and Conclusions\nsection. I would then go further and deliver the key findings (i.e., that C drawdown exceeds\nC release for many volcanic eruptions of this type) as three or four clear and hard-hitting\n2\nbullet points. In my opinion this could turn the paper from a useful incremental interest to a\nspecific (volcanological) audience into a (probably) well-cited article that appeals to a wider\ngeo-climate scientific community.\nThis comment echoes that made above. In accordance with the formatting guidelines\nprovided by Nature journals, the Conclusions are integrated within the Discussion section.\nThey are typically presented as a brief concluding paragraph or set of paragraphs towards\nthe end of the Discussion, rather than as a standalone section. We have slightly reorganized\nthe conclusions, ensuring each paragraph delivers a clear conclusive statement.\nTo summarise, I enjoyed reading the m/s, and feel it offers a worthy addition to the study of\nvolcano - climate interaction.\nGeneral\nPerhaps better frame the ‘time frames’ over which the authors wish to discuss (again a\nsimple table may clarify). We’re dealing with several processes here all operating at different\nrates and scales: atmospheric exchange of CO2, the exchange between substrate and\natmosphere and organic and inorganic chemistries within the soil. Most of these are further\nmediated by climate (T) and soil moisture (precipitation).\nOne of the few weaknesses of the paper is it doesn’t explicitly address the varying\ntimescales of ‘instantaneous’ CO2 delivery (during eruption, and the differing time scales\n(and half-lives) of silicate weathering and C sequestration through SOC. Perhaps these\ncould be outlined in a table – just as example – since it isn’t the focus of this paper to directly\naddress silicate weathering, but rather the role of soils and their organic C composition\n(through vegetation breakdown?).\nWe thank the Reviewer for their comment. While the original manuscript included some\ninformation on timescales, we agree that further clarification is needed. However, providing a\nsingle representative timescale for tephra weathering is not feasible. Tephra consists of\nsilicate minerals and glass in various proportions. In natural settings, the weathering rate of\nsilicate minerals typically decreases as a power function of time (White and Brantley 2003).\nKinetics of silicate weathering reactions depends both on processes intrinsic to the silicate\nmineral/glass and processes extrinsic to the weathering environment. Temporal and spatial\nvariability in these processes lead to significant variations in weathering rates. Long-term\ntephra weathering studies suggest volcanic glass fragment half-lives of 1650 to 7000 years\n(Kirkman and McHardy 1980; Ruxton 1988, Shoji et al. 1993). Rather than adding a new\ntable, we have integrated additional details into the revised text where relevant.\nLines 15-16\nVolcanic soils exhibit a unique capacity for SOC accumulation, particularly during the\nfirst few thousand years of pedogenesis1,2.\nLines 21-25\nA tephra deposit just a few tens of centimetres thick, forming in a few hours to a few\ndays, can halt the development of pre-existing soils while providing fresh parent material\nfor new pedogenesis. The recurrence of this process, occurring at intervals ranging from\na few years to several thousand years, is responsible for the formation of multi-layered\nvolcanic soils, typically featuring several subsurface horizons with high SOC content2,6.\nImportantly, due to unfavourable environmental conditions for decomposers and a lower\naccessibility to decomposers and limited input of fresh organic matter5,7–10, the buried\nSOC can be stable over millennial timescales5,7,11,12.\nThe timescales for eruption, deposition of tephra layer and development of soil and C\naccumulation are now indicated in Figure 1.\nA little more on the background of the ‘example’ volcano may be helpful– just to provide\ncontext. Add 2 or 3 descriptive sentences in text, or in appropriate figure caption?\nWe appreciate the Reviewer’s suggestion. We have provided additional information,\nincluding the location of the volcano and the Volcanic Explosive Index (VEI) for the N5 and\nN6 eruptions.\nLines 72-76\nWe estimated the SOC stock stored in the soil intercalated between the tephra deposits\nfrom the last two eruptions of the Atacazo-Ninahuilca Volcanic Complex, located in the\nWestern Cordillera of Ecuador, 10 km southwest of Quito (Fig. 2). These Plinian events\n(Volcanic Explosive Index (VEI)15 5), known as N5 (4,400 ± 35 BP) and N6 (2,270 ± 15\nBP), produced ≥ 1 km3 of dacitic tephra (ref.16).\nThat being the case, then perhaps this ‘weathering’ aspect should be more\nthoroughly/explicit stated in the introductory section. Understandably, not all aspects of this\ncomplex volcanic – weathering – soil profile and C content system can necessarily be\naddressed in one short paper. So, I’m not arguing for an in-depth treatment because I can\nsee the authors’ approach is focussed around the SOC quantification, and this approach is\ntimely since it helps better quantify part of this complex system, and one aspect that is less\ncommonly tackled.\nWe acknowledge that tephra weathering can contribute to atmospheric CO removal.\nfalls beyond the scope of our study. Furthermore, as outlined in our reply to an earlier\ncomment, quantifying the spatio-temporal dynamics of tephra weathering across varying\nclimatic and physico-chemical conditions would be highly complex and require a\ncomprehensive new study. That said, during the initial few thousand years of soil formation,\norganic C accumulation generally far exceeds CO removal via silicate weathering\n2\n(Chadwick et al. 1994). In line with the Reviewer’s suggestion, we have added a new\nparagraph in the Introduction section to clarify those points.\nLines 42–50\nThe accumulation of SOC in volcanic soils ultimately sequesters atmospheric C that was\ninitially captured by plants through photosynthesis. In the soil-plant system, some C is\ndiverted from the short-term (daily to annual timescale) photosynthesis–respiration cycle\ninto inorganic C phases during the chemical weathering of tephra. However, in the first\nfew thousand years of soil formation, organic C accumulation typically far exceeds CO\n2\nremoval by silicate weathering14. Therefore, we posit that the burial of organic C by\ntephra in volcanic soils is the primary mechanism through which an explosive eruption\nmay act as a C sink, that is, when more C is sequestered in the buried soils than is\nreleased into the atmosphere as magmatic CO during the eruption.\n2\n2\nwould be (substantially?) higher that that determined by the authors because the nascent\nash would be subject to aggressive silicate weathering in the years immediately after\ndeposition. Eruption output – volcanic product weathering approach has also been\nattempted wrt to flood basalt eruptions in several papers. These cover huge areas, then\nthemselves break down at the surface and in developing weathering profiles thus entrapping\nSOC and acting as CO2 sink through the silicate weathering chain and may be capable of\ndelivering global climate modification. Most recently: Black, B.A., Karlstrom, L., Mills, B.J.W.\net al. Cryptic degassing and protracted greenhouse climates after flood basalt events. Nat.\nGeosci. (2024). https://doi.org/10.1038/s41561-024-01574-3\nThis comment echoes the previous one. Black et al. (2024) did not address the silicate\nweathering feedback associated with Large Igneous Provinces (LIP) volcanism, but we are\nfamiliar with studies that have sought to quantify this process. For example, the Deccan\nTraps eruption initially increased atmospheric CO levels, but emplacement of fresh basaltic\n2\nrocks across an area of 500,000 km2 accelerated weathering over the next million years,\nultimately leading to a net CO drawdown (Dessert et al., 2003). Similarly, continental silicate\n2\nweathering increased by a factor of 1.5 following the eruptions of Central Atlantic Magmatic\nProvince basalts in eastern North America ~201.5 million years ago. The eruption lasted <1\nmillion year but was followed by a 1.5-million-year decline in atmospheric CO , eventually\n2\ndecreasing concentrations below pre-eruptive levels (Schaller et al., 2012). A key distinction\nbetween these LIP events and explosive eruptions lies in their timescales. Recurrent\nexplosive eruptions with a VEI of 4 to 6 (such as those considered in our studies) occur at\nintervals of a few thousand years. During his period, organic carbon accumulation in soils\ndeveloping from the tephra deposits typically far outstrips CO removal by weathering\n2\n(Chadwick et al., 1994).\nWe have revised the conclusions to emphasise organic C accumulation in soils as the\nprimary mechanism by which explosive eruptions act as C sinks.\nLines 197-202\nWe suggest that, over millennial timescales, soil burial by tephra worldwide may\nconstitute a C sink as efficient as basalt weathering. However, while inorganic C derived\nfrom silicate weathering is transported to the oceans and effectively removed from\nterrestrial cycling for over 108 years40, the sequestration of SOC in volcanic soils occurs\nover shorter timescales, typically spanning 103–104 years5,7,11,12.\nLines 210-214\nWhile likely minor compared to C storage in volcanic soils developing over a few\nthousand years40, atmospheric CO₂ consumption during tephra weathering would further\ncontribute to the C sink. This result stands in sharp contrast to the prevailing view that\nexplosive volcanic activity is a net source of C emissions to the atmosphere41–43.\nGiven several approaches combine into the ‘method’ – why not use a simple box/flow\ndiagram/chart to show the stages from data collection to the output conclusion? That would\nbe a simple device and be enormously helpful to the reader.\nWe have generated a new Figure in the Supplementary Information (Supplementary Figure\n2) which outlines the different steps of the method used to quantify organic C accumulation\nin soils repeatedly buried by tephra from VEI ≥ 4 eruptions over time, as well as the\ncumulative magmatic CO emissions associated with these events.\n2\nFigures\nFigure 1 is good – a nice straight forward figure. Please add ‘oldest and youngest labels to Y\naxis (I read it upside down initially – with T= 0 at top). Also add simple key for the coloured\nlayers (e.g. yellow = tephra (Tp1, purple Tp2 etc.,). Also use a thick brown line to show soils\non each tephra deposit. Scales c and d are very good and convey the authors key point\nregarding spatial variation of SOC very effectively. Nice figure, overall.\nWe thank the Reviewer for their suggestion. Figure 1 has been modified accordingly.\nFigure 2 is good, but would prefer lats and longitude (b) in Degree – Minute format.\nWe prefer to keep UTM coordinates as this format offers higher precision and is more\npractical for mapping and spatial analysis. UTM coordinates provide a consistent, grid-based\nreference system.\nFigure 3. I can see this is a decent statistical treatment of the SOC stock versus C release\non eruption. This is fine. However, it needs to be additionally clarified with a simpler figure\nthat shows a more readily accessible comparison of SOC with an eruption output.\nMany reading this paper will need a straight-forward illustration – for instance using an\nexemplar eruption to demonstrate that C drawdown/storage exceeds that of the C in eruption\ngases, and by how much (i.e., double, five times, a magnitude?).\nThis part of the paper is pivotal to the authors’ arguments and at present, this figure\nrepresents too complex a treatment (maybe put this into the supplement, and replace with a\naccessible one that better delivers/illustrates will deliver the authors’ statement (Lines 158 –\n160) ‘Our results unambiguously show that, in volcanically active regions, the combination of\nrapid organic C accumulation during volcanic soil accretion and recurrent soil burial by\ntephra deposits leads to significant storage at depth of organic C’.\nWe thank the Reviewer for their comment to improve Figure 3 comparing SOC storage and\nmagmatic C emissions. In response, we have created a new figure to address the concern\nregarding accessibility. However, we would like to emphasise that the purpose of this figure\nis not to compare SOC storage and magmatic C emissions from a single eruption. Instead, it\nhighlights the primary finding of our study: the SOC stock in volcanic soils repeatedly buried\nby tephra from VEI ≥4 eruptions in Ecuador during the Holocene surpasses the total\nmagmatic C mass emitted by these eruptions. We have simplified the presentation of the\nmodelling results to enhance the figure’s readability and accessibility. The revised figure now\ncontains a single graph, while still providing sufficient detail for readers to understand the\nkey findings. We believe this revised figure better communicates our findings and supports\nour argument that, in volcanically active regions, the rapid accumulation of organic C during\nvolcanic soil accretion and its recurrent burial by tephra deposits lead to significant C storage\nat depth.\nText\nLines 74 – 75: SOC ‘stable over millennial timescales’ Need to better quantify, of\nmodel/speculate upon SOC degradation post-entombment, and the likelihood of rate of\nexhumation by erosion. Also, microbial breakdown will release both CO2 and methane.\nage of the tephra-buried SOC ranges from 1,800 ± 60 to 2,365 ± 55 yr, suggesting that this\nC has remained stable on at least a millennial time scale. This stability is due to inefficiency\nof microbial decomposition once the surface soil is isolated from the atmosphere by burial\nunder tephra. Several studies5,7,11,12 have similarly shown that buried C can remain stable\nover millennial timescales (Lines 25-28).\nLine 69. Alter to: ‘over comparable time periods of surface exposure’\nThe sentence has been modified.\nLine 70. Alter to: ‘and under similar climatic conditions; accordingly, it is reasonable to\nassume SOC would have accumulated at roughly the same rate’.\nThe sentence has been modified.\nLine 73. ‘Alternatively, a more likely explanation may be partial loss of SOC…’\nThe sentence has been modified.\nLine 88 ‘We found that SOC density’ – should this read: ‘We found that SOC content’?\n“SOC density” (in g C m-2) is correct.\nStated/determined change in SOC with elevation may likely be due to change in type and\namount of vegetation cover. Perhaps worth stating more explicitly.\nThe relationship between SOC and altitude primarily reflects the impact of temperature and\nprecipitation on the rate of SOC decomposition rather than changes in the type and amount\nof vegetation cover.\nWe have clarified this and rephrased Lines 100-103.\nWe found that SOC density (in g C m-2) in soils buried by the N6 tephra is\npredictable based on altitude (Supplementary Fig. 1), consistent with the idea that\ntemperature and precipitation (which typically decreases and increases with\naltitude, respectively) are the primary factors driving SOC decomposition and\nstorage17,18,20.\nLines 172 - 174. ‘For comparison, the mean CO2 drawdown by rock chemical weathering in\nthe Ecuadorian Andes, at ~0.25 Mg C km-2 yr-1 (ref.32), is much lower’. Fine but this is a\ngeneral measurement of silicate weathering from the given reference. However, using this,\nor the Dessert et al., work may not be fully appropriate.\nWe are unsure why the Reviewer finds the comparison perhaps inappropriate. Our objective\nis to highlight the importance of C sequestration rate through soil burial by tephra in the\nEcuadorian Andes relative to rate estimates of atmospheric C removal via rock weathering in\nthe same region. Upon revising our original submission, we found a minor error in the\nreported weathering value for the Andes, which was underestimated. However, this\ncorrection does not affect our main argument, and the comparison remains valid, as it helps\nto contextualise our findings in a regional C cycle context. Similarly, comparing C storage\nthrough tephra-induced soil burial with atmospheric C removal via basalt weathering\n(Dessert et al., 2003) further highlights the significance of the former process.\nWe have revised the following lines in the manuscript:\nRepeated tephra fallout events during the Holocene may have led to the storage of ~1.9\nMg C km-2 yr-1, or ~11 Tg C yr-1, in Ecuadorian soils. This estimate, while slightly lower,\nremains within a comparable range to the CO₂ drawdown associated with rock chemical\nweathering in the Ecuadorian Andes and sub-Andes (2.3–4 Mg C km-2 yr-1 (ref.34)).\nBasalt weathering is recognised as the primary mechanism by which volcanic activity\ngenerates a strong CO sink, accounting for ~49 Tg C yr-1, or 30–35% of the flux derived\n2\nfrom continental silicate weathering35 Notably, organic C sequestration in tephra-buried\n.\nsoils in Ecuador alone represents up to ~23% of the CO consumption flux from basalt\n2\nweathering. Given the extensive distribution of volcanic soils in continental arc systems\nwith explosive volcanism, such as the Cascades, Central America, the Andes and\nKamchatka, this contribution is likely much greater at a global scale.\nTephra is very easily weathered due to is huge granular surface area: rock, such as basalt,\npresents only a surface interface for weathering, or else a weathering profile/critical zone as\nalteration progresses. This can be self-limiting as alterd materials accumulate. In essence,\nmy suspicion is that neo-formed tephra will be a very efficient CO2 sink – more than ‘rock’.\nThis need to be considered in the discussion of C weathering drawdown versus C\nemissions.\nWe agree with the Reviewer that tephra weathering likely contributes to the C balance of an\nexplosive eruption. We have already answered the Reviewer’s concern in a previous\ncomment.\nLines 277 – 278: ‘the N6 eruption of Atacazo-Ninahuilca indicates that ~7.0 Tg of magmatic\nC were released into the atmosphere’. Yes, that’s a fair ‘average’ but why not run the carbon\nbalance determinations using max and min values of each of the key components outlined in\nthe methodology (or does this fall out of the percentile argument?). This will then provide a\nprobabilistic envelope of determinations which more is likely to contain the ‘real’ value.\nWe agree with this remark and have improved our estimate of the mass of magmatic C\nreleased explosively by the N6 eruption of Atacazo-Ninahuilca. We performed a stochastic\nestimation of the tephra volume, applying a conservative distal integration limit of 300 km ±\n50% and incorporating an uncertainty of 20% for both tephra thickness and isopach area.\nThis yielded a median erupted magma volume of 2.8 km³ (2.1–3.2 km³). Using equation [3],\nwe then calculated that 7.0 Tg (5.7–8.7 Tg) of magmatic C were released into the\natmosphere.\nThe text has been modified as follows:\nLines 274-280\nUsing a conservative distal integration limit of 300 km ± 50% and incorporating an\nuncertainty of 20% on both tephra thickness and isopach area, a stochastic estimation of\nthe tephra volume24 results in a median value of 5.0 km3 (3.9–6.5 km3 for the 10–90th\npercentiles), or 5.8 km3 (4.7–7.3 km3) when accounting for the PDC volume. Assuming a\nbulk density of 1,100 kg m-3 for tephra deposits46 and a density of ~2,500 kg m-3 for a\ndacitic magma47 such as that involved in the N6 eruption16, the estimated median volume\nof erupted magma is ~2.6 km3 (2.1–3.2 km3).\nLines 295-303\n𝑚𝑎𝑔𝑚𝑎\ncorresponding mass of magmatic C emitted (𝑀 , in Tg) is calculated using:\n𝐶\n𝑀 = 𝑉 ×𝐹 ×\n𝑚𝐶\n[3]\n𝐶 𝑚𝑎𝑔𝑚𝑎 𝐶𝑂2\n𝑚𝐶𝑂2\nwhere 𝑚 and 𝑚 are the molar masses of C and CO , respectively, and 𝐹 = 10 Tg km-3.\n𝐶 𝐶𝑂2 2 𝐶𝑂2\nApplying equation [3] to the N6 eruption of Atacazo-Ninahuilca indicates that 5.7–8.7 Tg\n(median = 7.2 Tg) of magmatic C were released into the atmosphere.\nMethods\nMethods are comprehensive: Primary data uses multiple soil profiles associated with two\neruptions Atacazo-Ninahuilca volcano (given as Atacazo in figure 2) in Ecuador (VEI 5?).\nSampling appears thorough and robust, and the density and TOC determinations are good.\nThe calculations under methods look logical and are straight forward in explanation. It is\ngood to see such thorough field sampling underpinning the SOC determinations that are\ncarried forward into the modelling.\nMagmatic C is always challenging to estimate, but the authors’ approach is honest – using\nknown eruptions with measure releases as the guideline for estimating the CO2 release.\nI can see no major objection to this approach.\nIt is inevitable that ‘estimates’ need to be generated for CO release etc. There are actual\nmeasurements (FTIR etc.,) for modern eruptions, and these can be judiciously applied for\nestimates of ancient eruptions Estimation of CO2 – methods?\nInitially, the main text seems to suggest this to based only upon Iacovino and Till (2019), but then Supplementary material\nreveals the authors’ use of recent historic values (good idea) for CO2 release.\nThere appears to be some confusion from the Reviewer. Iacovino and Till (2019) (ref.45 in the\noriginal submission, ref.47 in the revised version) is cited solely for calculating the density of a\ndacitic melt. Melt density is required to infer the mass of CO it contains and, subsequently,\n2\nto derive an estimate of the magmatic CO degassing rate.\nAnother method is to actually determine the actual (measured) gas release per unit volume of magma, then scale this\nappropriately. Different magma compositions can contain different quantities of the key\ngases SO2 and CO2 – generally basaltic magmas can contain the highest. Other authors\nhave attempted this approach (e.g., Self et al., 2008, 2009; Elements and EPSL), and used\ngas contents preserved in basalt as inclusions. Presumably there are comparable\ndeterminations for dacite?\nThe Reviewer likely refers to Self et al. (2005, 2006), not Self et al. (2008, 2009). In these\nstudies, the authors estimated sulfur gas release from flood basalt eruptions using the\npetrological method (Devine et al. 1984), which compares sulfur contents in pre-eruption\nmelt inclusions within crystals to post-eruption concentrations in glassy lava. Strictly\nspeaking, Self et al. (2005; 2006) did not use this method to estimate CO degassing as they\n2\ndid not have measurements of CO concentrations in phenocryst-hosted melt inclusions.\n2\nand considered complete degassing, resulting in a CO degassing rate of ~13 Tg km⁻³.\n2\nmagma crystallisation and differentiation, a process that increases the concentration of\ndissolved CO in the melt fraction. In general, the solubility of CO in silicate melts decreases\n2 2\nfrom mafic to felsic compositions. Holocene explosive eruptions in Ecuador correspond to\nandesitic, dacitic and more rarely, rhyolitic magmas (Hall and Mothes, 2008). Experimental\ndata indicate that CO solubility in andesitic, dacitic and rhyolitic melts (prior to\n2\ncrystallisation) varies in the ranges 0.3–0.4, 0.08–0.3 and 0.002–0.005 wt.%, respectively\n(Blank et al., 1993; King and Holloway, 2002; Behrens et al., 2004). Measurements of CO\n2\nconcentrations in phenocryst-hosted andesitic/dacitic/rhyolitic melt inclusions are scarce.\nRecently, Howe et al. (2025) reported values (total concentration including CO in bubbles)\n2\nin the range 0.01–0.3 wt.%. for the andesitic magma associated with the 1995–2010 eruptive\nactivity of Soufrière Hills volcano, Montserrat. We are not aware of similar measurements for\ndacitic or rhyolitic magmas. For our calculations, we used the highest CO solubility value\n2\n(0.4 wt.%) in the initial melt. The low solubility of CO in silicate melts at atmospheric\n2\npressure means that fully degassed magmas retain a negligible fraction of their CO and\n2\ntherefore, we assumed complete degassing. We are confident that a degassing rate of 10 Tg\nCO km-3, such as calculated in our study, is a highly conservative value, thus representing\n2\nthe most unfavourable scenario for the C balance calculation.\nAdditionally, an ‘envelope’ of estimate may be determined by offering the min and max yield\nusing the variability inherent in key input parameters. See similar types of argument in Jolley\nand Widdowson (2005) and those by Self et al., (above).\nUse of percentiles as a range of estimates to evaluate SOC in soils – this is good statistics. But this type of approach then\nisn’t clearly followed through to CO2 eruption volumes (unless I’ve missed something).\nAgain, we are dealing with an ‘estimate’ here, so it seems logical to provide an ‘envelope of\nprobability’ to these values too (see previous statement).\nWe appreciate the Reviewer’s comment and would like to clarify the context regarding the\nso-called \"envelope\" of estimates for sulfur degassing during flood basalt eruptions. To the\nbest of our understanding, Self et al. (2005, 2006) did not provide such an \"envelope.\" The\nReviewer probably refers to Figure 4 in Jowley and Widdowson (2005), which illustrates an\nenvelope-based approach to relate volcanic sulfur emissions with climate cooling effects.\nThis approach was implemented by varying a mean value to establish upper and lower\nboundaries. However, we do not endorse this method, as it relies on an arbitrary decision\nand does not incorporate statistical modelling. In contrast, our study adopts a stochastic\napproach, allowing us to better constrain the tephra volume and the associated release of\nmagmatic carbon. We generated a plausible range for magmatic CO emissions (mean: 7.0\ndemonstrate that even under an unfavourable scenario of magmatic CO degassing, the\n2\namount of carbon stored in tephra-covered soils significantly exceeds the amount released\ninto the atmosphere during the N6 eruption of Atacazo-Ninahuilca. Given the reliability of our\nstochastic estimation, we believe the simpler envelope approach does not add significant\nvalue to our analysis. We hope this explanation highlights the strength of our methodological\nframework and addresses the Reviewer’s concerns.\nIn effect, there are assumption made about the coverage of tephra (magma volume), the\nthough I was surprise not to see the Pyle et al., research cited here as well as the\nBonadonna and Houghton (2005). Perhaps this work is worth a look during any revision to\nm/s.\nWe do reference Bonadonna and Houghton (2005) in the original manuscript (ref.43; now\nref.45 in the revised version) to recalculate the tephra volume of the N6 eruption using a\n10\npower-law model. Additionally, we cite Hidalgo et al. (2008) for previous calculations of the\ntephra volume. Hidalgo et al. applied the exponential model (following Pyle et al. 1989 and\nother studies), which is known to underestimate the tephra volume. Since we have opted not\nto use the exponential method in our study, it is not necessary to cite Pyle et al., or similar\nworks.\nStock of SOC – though this is well-constrained by the authors’ field sampling and\nsubsequent analytical work.\nWhy does rate of C accumulation decrease in volcanic soils with age? I assume this is due\nto primary colonisation and early vegetation delivering the main biomass, then subsequent\ndegradation through breakdown? Perhaps a line of explanation to this statement (maybe\ndelivered in the reference material, but could do summarising here).\nThe rate of carbon accumulation in volcanic soils decreases with age due to the reduced\nefficiency of stabilisation mechanisms as alumino-organo complexes become unstable and\nthe secondary mineralogy evolves toward more crystalline and less reactive solid phases.\nThis trend has been well-documented (e.g., Torn et al. 1997; Egli et al. 2008; Pena-Ramirez\net al. 2009).\nWe have added the following text in the Method section:\nLines 413-415\nThe decrease in the rate of SOC accumulation over time is attributed to changes in\nreactive iron- and aluminium-bearing soil constituents, which result in a reduced\nefficiency of organic matter stabilisation25,26.\nUse of the SI magnitude system is correct – see SOC values in Pg (petagramme). However,\nin volcanic studies its more usual to relate values in tonnes or megatonnes. Somewhow a\n‘tonne’ and its derivatives (e.g., megatonne) are a more readily understood quantity for\n‘human’ scale understanding than that a value of 10 to the power 6 (i.e., megagram) and 12\n(i.e., terragram) respectively. I leave this to the authors to consider and decide, but this\npaper makes some very interesting statements about CO2 delivery and drawdown and it\nwould help increase its impact to make it more readily ‘scalable’ in a relative way for the\nreader.\nFor instance: 1.1 Pg SOC (i.e., = 1100 megatonnes)\nWe prefer to adhere to SI units in accordance with the guidelines of Nature journals.\nSpatial extent of tephra fallout is elegantly approached by using an accepted modelling\nprogramme an inputting modern wind condition record. In the absence of ‘real historic data’,\nwe can reasonably assume these will be representative of the earlier Holocene.\nSupplementary Material\nThis is well-delivered and clearly labelled/described.\nReviewer #3 (Remarks on code availability):\nI'm not able to comment with authority on computational code.\nBehrens, H., Ohlhorst, S., Holtz, F. & Champenois, M. CO2 solubility in dacitic melts\nequilibrated with H2O-CO2 fluids: Implications for modeling the solubility of CO2 in silicic\nmelts. Geochimica et Cosmochimica Acta 68, 4687-4703,\ndoi:https://doi.org/10.1016/j.gca.2004.04.019 (2004).\nBlank, J. G., Stolper, E. M. & Carroll, M. R. Solubilities of carbon dioxide and water in\nrhyolitic melt at 850°C and 750 bars. Earth and Planetary Science Letters 119, 27-36,\ndoi:https://doi.org/10.1016/0012-821X(93)90004-S (1993).\nChadwick, O. A., Kelly, E. F., Merritts, D. M. & Amundson, R. G. Carbon dioxide consumption\nduring soil development. Biogeochemistry 24, 115-127, doi:10.1007/BF00003268 (1994).\nDessert, C., Dupré, B., Gaillardet, J., François, L. M. & Allègre, C. J. Basalt weathering laws\nand the impact of basalt weathering on the global carbon cycle. Chemical Geology 202,\n257-273, doi:https://doi.org/10.1016/j.chemgeo.2002.10.001 (2003).\nDevine, J. D., Sigurdsson, H., Davis, A. N. & Self, S. Estimates of sulfur and chlorine yield to\nthe atmosphere from volcanic eruptions and potential climatic effects. Journal of\nGeophysical Research: Solid Earth 89, 6309-6325,\ndoi:https://doi.org/10.1029/JB089iB07p06309 (1984).\nEgli, M., Nater, M., Mirabella, A., Raimondi, S., Plötze, M. & Alioth, L. Clay minerals,\noxyhydroxide formation, element leaching and humus development in volcanic soils.\nGeoderma 143, 101-114, doi:http://dx.doi.org/10.1016/j.geoderma.2007.10.020 (2008).\nHall, M. & Mothes, P. The rhyolitic–andesitic eruptive history of Cotopaxi volcano, Ecuador.\nBulletin of Volcanology 70, 675-702, doi:10.1007/s00445-007-0161-2 (2008).\nHowe, T. A., Christopher, T. E., Moune, S., Tuffen, H. & Schiavi, F. Melt inclusion bubbles\nprovide new insights into crystallisation depths and CO systematics at Soufrière Hills\n(2025).\nJolley, D. W. & Widdowson, M. Did Paleogene North Atlantic rift-related eruptions drive early\nEocene climate cooling? Lithos 79, 355-366,\ndoi:https://doi.org/10.1016/j.lithos.2004.09.007 (2005).\nKing, P. L. & Holloway, J. R. CO2 solubility and speciation in intermediate (andesitic) melts:\nthe role of H2O and composition. Geochimica et Cosmochimica Acta 66, 1627-1640,\ndoi:https://doi.org/10.1016/S0016-7037(01)00872-9 (2002).\nKirkman, J. H. & McHardy, W. J. A comparative study of the morphology, chemical\ncomposition and weathering of rhyolitic and andesitic glass. Clays and Clay Minerals 15,\n163-173 (1980).\nPeña-Ramirez, M. V., Vazquez-Selem, L. & Siebe, C. Soil organic carbon stocks and forest\nproductivity in volcanic ash soils of different age (1835-30,500 years BP) in Mexico.\nGeoderma 149, 224-234, doi:10.1016/j.geoderma.2008.11.038 (2009).\nRuxton, B. P. Towards a weathering model of mount lamington ash, Papua New Guinea.\nEarth-Science Reviews 25, 387-397, doi:https://doi.org/10.1016/0012-8252(88)90006-2\n(1988).\nAtlantic Magmatic Province as a net sink for CO2. Earth and Planetary Science Letters\n323-324, 27-39, doi:https://doi.org/10.1016/j.epsl.2011.12.028 (2012).\nSelf, S., Thordarson, T. & Widdowson, M. Gas Fluxes from Flood Basalt Eruptions. Elements\n1, 283-287, doi:10.2113/gselements.1.5.283 (2005).\nSelf, S., Widdowson, M., Thordarson, T. & Jay, A. E. Volatile fluxes during flood basalt\neruptions and potential effects on the global environment: A Deccan perspective. Earth\nAnd Planetary Science Letters 248, 518-532 (2006).\nShoji S., Nanzyo M., Shirato Y. & Ito T. Chemical kinetics of weathering in young Andisols\nfrom northeastern Japan using 10°C-normalized soil age. Soil Science 155, 53–60\n(1993).\nTorn, M. S., Trumbore, S. E., Chadwick, O. A., Vitousek, P. M. & Hendricks, D. M. Mineral\ncontrol of soil organic carbon storage and turnover. Nature 389, 170-173 (1997).\nWhite, A. F. & Brantley, S. L. The effect of time on the weathering of silicate minerals: why do\nweathering rates differ in the laboratory and field? Chemical Geology 202, 479-506,\ndoi:https://doi.org/10.1016/j.chemgeo.2003.03.001 (2003).\n13"}, {"round": 2, "reviewer_comments": {"reviewer_3": "Comments to the Authors\nThis is an interesting paper and offers insightful conclusions regarding the role of volcanism in C sequestration. It makes considerable effort to build a model based upon measurable parameters and tests/builds this upon well-executed field work and analysis. There are necessarily a series of assumptions built into the authors’ model, but these are clearly outlined and the limitations appropriately flagged. We do perhaps differ in approach to determining an ‘envelope of probability’ regarding likely outcomes of such modelling, but I fully accept that the authors’ approach does offer an elegant statistical approach.\nI take the point that exploring how tephra breaks down is not the purpose here, but rather the measurable effect of C sequestration associated with its deposition, exposure and/or entombment. However, I have a suspicion there is much to yet be discovered regarding mineralogical transformation of tephra, the rates of alteration, and the associated effects of its silicate weathering on CO2 drawdown.\nThe point regarding CO2 in magmas is well-tackled. It is certainly difficult to obtain CO2 inclusion data – but it does represent that trapped in the melt – so is ‘primary’ rather than modelled data. I am pleased that the manner by appropriate measurement is determined is further clarified. It adds robustness to the authors’ argument.\nI referred to CFB literature partly because this is what I’m most familiar with regarding degassing budgets. The conundrum there is that even massive individual eruptive events in the building of CFBP stacks likely contribute negligible change to the overall atmospheric CO2 reservoir content: Of course, there may be a cumulative effect. The Ecuadorian eruptions in themselves are not on this scale, but from the authors’ work the clear message that in such eruptions (whatever their CO2 release may be) will result in net C sequestration. If that effect is repeated elsewhere globally and persistently over geological time (e.g., Pacific ‘ring of fire’) it then presents a (newly recognised?) additional/key component of global C cycling. This latter point is well made by these authors’ work.\nUpdates to figures are welcome, and usefully support and explain some of the complexity in the ideas and models outlined. The new figure offers a nice touch.\nI do still prefer to see lats and Longs rather than UTM because I think they easily help the reader place/conceptualise unfamiliar localities broadly on the globe; but I am content to accept that opinions differ.\nLikewise the usage of the informal (non-SI) megatonne still seems a more ‘understandable’ value in common parlance. I would have like to see it included (even in brackets following the actual SI value). Again, a matter of stylistic choice perhaps.\nIn summary, the authors have substantively engaged with the comments and suggestions of my first review. I have re-read the m/s in detail and find helpful improvement and clarity to many of the minor and some of the more major points raised. Those that the authors have chosen not to tackle with alterations are otherwise provided with a detailed rationale and explanation."}, "author_response": ""}]

Decision letter