Role of mantle carbonation in trench outer‐rise region in the global carbon cycle

A nearly balanced carbon budget between subduction input and degassing output has likely controlled the long‐term surface environment and habitability of Earth throughout geological history. However, the ongoing extensive hydration and carbonation of the mantle in trench outer‐rise regions may affect the global carbon budget. In this study, we show that the carbon content of the lithospheric mantle can be inferred from geophysical data and thermodynamic modeling. Based on the seismic velocity anomaly in trench outer‐rise regions, we estimated that the total carbon flux due to mantle carbonation is 7–31 Mt C/year, with possible fluid‐to‐rock mass ratios of 250–1000. These values are similar to the carbon uptake by altered oceanic crust, indicating that mantle carbonation has a significant effect on the subduction carbon budget. Although there are large uncertainties on the estimates of the subduction and degassing carbon fluxes, secular cooling of the mantle leads to the development of outer‐rise faults associated with bending of the oceanic lithosphere and increased mantle carbonation, which may disrupt the self‐regulating system of the global carbon cycle on geological timescales.


| INTRODUCTION
The long-term carbon cycle in Earth's interior has regulated its surface climate and habitability throughout geological history (e.g., Berner et al., 1983;Walker et al., 1981).Subduction of organic carbon and carbonate with oceanic plates transports carbon into the mantle, and such carbon is released by metamorphic decarbonation reactions as temperatures and pressures rise during subduction, and is eventually returned to the atmosphere by volcanic degassing (Figure 1).The global carbon flux into and out of the mantle has been studied from a variety of perspectives (e.g., the review by Plank & Manning, 2019).
The current understanding is that the carbon budget is nearly balanced between carbon inputs and outputs (e.g., Dasgupta & Hirschmann, 2010;Jarrard, 2003;Müller et al., 2022;Plank & Manning, 2019).Because the carbon reservoirs in the solid Earth are substantially larger than those in the atmosphere and ocean, even a small perturbation in the subduction and degassing fluxes can change the size of the surface carbon reservoir and affect surface climate and habitability on geological timescales (e.g., Galvez & Pubellier, 2019).
Carbon transported into Earth's interior is hosted mainly by the sedimentary, oceanic crustal, and mantle layers of the subducting plate.The sedimentary layer contains abundant microfossil oozes and carbonates, and thus has a dominant role in the subducted carbon flux (e.g., Rea & Ruff, 1996;Plank & Langmuir, 1998;Clift, 2017).In the altered oceanic crust, carbonates are formed in the permeable upper few hundred meters of volcanic sections during low-temperature alteration on the seafloor (e.g., Alt & Teagle, 1999;Coogan & Gillis, 2018;Staudigel et al., 1996).
In addition to these reservoirs, mantle peridotite can also contain a significant amount of carbon when seawater penetrates into the mantle along fracture zones and extensive interactions occur with the surrounding rocks.The hydration and carbonation reactions of peridotites are geologically rapid, such that the infiltration of CO 2 -bearing fluids can pervasively transform large volumes of mantle rock (e.g., Kelemen & Matter, 2008;Sieber et al., 2018).Serpentinization is common at slow to ultra-slow spreading ridges associated with detachment faults, where the exposed peridotites interact with seawater (e.g., Alt et al., 2013;Früh-Green et al., 2004).Subduction has not yet begun in the slow to ultra-slow spreading plates, such that carbon uptake in these environments may have a small contribution to the present-day carbon budget, although they will be important carbon carriers when such oceanic plates begin to subduct.In relatively old oceanic lithosphere, extensional faulting associated with plate bending in trench outer-rise region creates pathways for seawater penetration into the mantle, resulting in extensive serpentinization (Figure 1).A significant decreases in the seismic velocity in these regions reflects mantle fracturing and hydration, where the degree of serpentinization has been estimated to be up to 30 vol% in the uppermost mantle (e.g., Grevemeyer et al., 2018).Infiltration of carbon-bearing fluids along the fracture zones can lead to carbonate precipitation along with mantle serpentinization, as evident in exposed seafloor serpentinites (e.g., Alt et al., 2013;Früh-Green et al., 2004;Kelley et al., 2005), although the extent of mantle carbonation in the oceanic lithosphere is unclear.The amount of carbonate precipitation in the mantle is likely controlled by fluid fluxes in fracture systems.In this study, we estimated the mantle carbonation in trench outer-rise regions using recently acquired geophysical data along with thermodynamic constraints.

| REACTION PROGRESS OF MANTLE HYDRATION AND CARBONATION
Along outer-rise faults in an extensional stress field, seawater could penetrate into the oceanic mantle, driven by sub-hydrostatic or even negative pressure gradients in the permeable fault zones (e.g., Faccenda et al., 2009).Once aqueous fluids reach the mantle, fluid-rock interactions promote serpentinization according to the following reaction: This reaction induces intense fracturing due to volume expansion, which in turn facilitates fluid transport and accelerates the reaction rate by increasing the reactive surface area with a resultant positive feedback system (e.g., Jamtveit et al., 2008;Kelemen & Hirth, 2012;Shimizu & Okamoto, 2016).During mantle hydration, carbon dioxide dissolved in aqueous fluids can be precipitated as carbonate veins, as evident in highly altered sub-oceanic mantle and ophiolite sections (e.g., Kelemen & Matter, 2008;Menzel et al., 2022;Ternieten et al., 2021).Chemical reactions between the mantle and carbonbearing fluids include the following: These are simplified carbonation reactions for olivine and serpentine.Harzburgitic compositions and other naturally present components, such as Fe and Ca, can modify the mineral reactions during mantle carbonation.In trench outer-rise region, where relatively old oceanic lithosphere is being flexed, the half-space cooling model predicts temperatures as low as 100 C in the uppermost mantle (e.g., Turcotte & Schubert, 2002), and brittle deformation releases accumulated flexural stress in the lithosphere.Lizardite is the dominant serpentine polymorph under such low-temperature conditions, whereas antigorite is stable during high-temperature alterations associated with the hydration of subducted oceanic lithosphere and the mantle wedge (e.g., Reynard, 2013;Scambelluri et al., 2004).
The magnesite-quartz assemblage (listvenite) represents the endproduct of the interaction between carbon-bearing fluids and peridotite.However, the reaction products of mantle carbonation can vary widely depending on the solid and fluid chemistry, and the fluidto-rock mass ratio during alteration (e.g., Klien & Garrido, 2011;Okamoto et al., 2021).
The reaction kinetics of mantle serpentinization and carbonation are geologically rapid, even at low temperatures, when carbon-bearing fluids penetrate into the mantle along fracture zones.Both the hydration and carbonation reactions have a bell-shaped temperature

Outer-rise faults
Crust Weathering F I G U R E 1 Schematic illustration of the formation and subduction of oceanic lithosphere.In trench outer-rise regions, extensional faulting associated with plate bending can facilitate seawater penetration, serpentinization, and possible carbonation in the mantle.Mantle hydration and carbonation may occur along fracture zones associated with detachment faults and mid-ocean ridges, but this study focuses on the processes of fluid infiltration in outer-rise regions.
dependence, but the maximum temperature of the reaction rate varies with the process: 260 C for serpentinization (e.g., Martin & Fyfe, 1970) and 185 C for carbonation (Kelemen & Matter, 2008).This indicates that these reactions are highly temperaturedependent, and we calculated the reaction rate based on the relationship reported by Kelemen and Matter (2008).Figure 2 shows the calculation results for mantle hydration and carbonation rates at 100 C with P CO2 = 1 and 5 bar, which indicate that mantle carbonation occurs more rapidly than serpentinization during lowtemperature alteration, with both reactions being completed within ca. 100 years.The carbonation reaction of serpentinite may be slower than that of unaltered peridotite, but is still geologically rapid (e.g., Kelemen et al., 2019;Klein & McCollom, 2013).On the ocean floor, serpentinite-hosted carbonate chimneys and veins form as a result of extensive fluid-rock interactions, which creates a remarkable submarine ecosystem (e.g., Kelley et al., 2005;Oyanagi et al., 2021).
At the reaction front, mantle hydration is controlled by a sluggish process, either by reaction kinetics or water availability, which indicates that the fluid transport flux may be a primary factor that controls serpentinization in the oceanic mantle (e.g., MacDonald & Fyfe, 1985).Hatakeyama et al. (2017) measured the permeability of natural serpentinites and suggested that the lateral extent of serpentinization can be limited to $1 km around fault zones in the mantle.Although seawater has access to the uppermost mantle along fracture systems, mantle carbonation requires a flux of CO 2 dissolved in aqueous fluids to reach the reaction front, such that carbonate precipitation may be localized to the fluid channels where relatively high fluid-to-rock mass ratios occur (Figure 3).

| SEISMIC VELOCITY DURING MANTLE HYDRATION AND CARBONATION
Serpentinite is characterized by significantly lower seismic velocities as compared with peridotite, indicating a systematic change in the seismic velocity during serpentinization (e.g., Christensen, 2004;Watanabe et al., 2007).This relationship is commonly used to estimate the degree of mantle serpentinization (e.g., Carlson & Miller, 2003;Hatakeyama & Katayama, 2020).
Figure 4 shows the relationship between P-wave velocity and the density of serpentinites and listvenites from the Oman Drilling Project with the mineral end-members (Table A1).Most serpentinites are plotted along the olivine-serpentine line with some scatter due to the effects of porosity and the presence of minor minerals.Listvenites have a higher velocity and density than serpentinites due to its magnesite-quartz assemblage.In the Oman ophiolite, serpentinization occurred mainly during the sub-oceanic hydrothermal alteration, whereas listvenites formed during the later stages of obduction and fluid infiltration, which suggests that carbonation of ultramafic rocks occurred after serpentinization (e.g., Falk & Kelemen, 2015;Menzel et al., 2022;Okazaki et al., 2021).Figure 4 shows that the velocity and density of listvenites are not plotted along the olivine-magnesite line, but are plotted between serpentine and magnesite, which is indicative of carbonation from serpentinite and consistent with the structural relationships described above.
Figure 5 shows the relationship between P-and S-wave velocities of peridotite (Vp = 8.0 km/s, Vs = 4.5 km/s), serpentinite (Vp = 5.2 km/s, Vs = 2.6 km/s), and listvenite (Vp = 7.2 km/s, Vs = 4.4 km/s).Seismic velocities decrease during both mantle serpentinization and carbonation, but the velocity changes are smaller during carbonation.If carbonation reactions occur after serpentinization, the seismic velocities tend to increase, but the similar velocities of peridotite and listvenite make it difficult to distinguish between mantle hydration/dehydration and carbonation.In contrast, Vp/Vs ratio is markedly different for serpentinite, listvenite, and peridotite (Figure 5), and this parameter can be used to identify mantle hydration and carbonation.Reaction fractions (X) were calculated from the Avrami's relation, where K is the reaction rate from Kelemen and Matter (2008) with n = 1.The carbonate reaction also depends on the CO 2 partial pressure in the aqueous fluid, where P CO2 = 1 and 5 bar were used for the calculations.The serpentinization and carbonation processes can lead to the development of cracks due to reaction-induced dilation, which can affect the seismic velocity of the mantle.Although this effect depends on the crack shape, serpentinites contain cracks with relatively large aspect ratios, which have little effect on the seismic velocity (e.g., Hatakeyama & Katayama, 2020;Katayama et al., 2021).

| FLUID-MANTLE INTERACTIONS AND CARBON UPTAKE
The carbonation reaction is rate-limited by the supply of CO 2 to the reaction front, such that mantle carbonation is highly dependent on the fluid-to-rock mass ratio (hereafter, fluid/rock ratio).Consequently, the degree of mantle carbonation may be highly localized and expected to be overall low, and thus it is difficult to constrain the degree of mantle carbonation solely from the seismic velocity.Therefore, we undertook thermodynamic modeling of the interactions between seawater and peridotite at variable fluid/rock ratios.
The calculations were undertaken at 200 MPa and 100 C, corresponding to the conditions of the uppermost mantle in the outer-rise region, using CHIM-xpt software (Palandri & Reed, 2004;Reed, 1982).The mineral dataset and equations of states were taken from Holland and Powell (2011).The initial solid composition was forsterite, Mg 2 SiO 4 , and the solid phases were treated as the Mg endmember and solid solution was not taken into account.The initial fluid composition is listed in Table A2.The thermodynamic data (slop07. dat) for aqueous species including bicarbonic acids such as Ca(HCO 3 ) + and Mg(HSiO 3 ) + was taken from SUPCRT92 (Johnson et al., 1992;Shock et al., 1992), which was last updated in 2017.In the model, we calculated the mineral-fluid equilibria by the incremental addition of olivine to 1 kg of seawater with a composition identical to the mineral saturation.A decrease in the fluid/rock ratio corresponds to seawater penetration from the fractures into the interiors of the mantle rocks.
The model calculations indicate that lizardite and brucite form at relatively low fluid/rock ratios, and that magnesite appears instead of brucite at higher fluid/rock ratios (Figure 6).This suggests that carbonate is preferentially formed in the fluid-dominant system near dominant fluid pathways (i.e., fractures), and that its precipitation can be localized in the vein networks of serpentinized mantle.Although temperature has little effect on the mineral assemblages at <250 C, fluid chemistry is important because carbonate formation occurs even at low fluid/rock ratios when hydrothermal fluids are enriched in mafic components (Malvoisin, 2015).We modeled the fluid-mantle interactions with seawater, because the fluid chemistry in the  F I G U R E 4 Relationship between the P-wave velocity and density of serpentinites and listvenites from the Oman Drilling Project (Kelemen et al., 2020) with the mineral end-members (Table A1).
uppermost mantle is poorly constrained.Figure 6 also shows changes in the water and carbon concentrations in the rocks obtained from the mineral assemblages, which indicate a systematic decrease in water and increase in carbon with increasing fluid/rock ratios.
Based on thermodynamic modeling of the fluid-mantle interactions, we calculated the seismic velocity from the mineral abundances using the Voigt-Reuss-Hill average (Table A3).Lizardite and brucite are the dominant minerals formed during mantle hydration at relatively low fluid/rock ratios, which results in significantly lower seismic velocities and high Vp/Vs ratios as compared with unaltered peridotite.Carbonate is only sparsely precipitated at the reaction front, but it is expected to be abundant along fracture zones where the fluid/ rock ratio is high (Figure 6).Given that carbonate minerals are characterized by higher seismic velocities than serpentine and brucite, carbonate precipitation causes an increase in seismic velocities and decrease in Vp/Vs ratios.Although the mineral assemblages and seismic velocities exhibit large variations during mantle alteration, the fluid flow can be highly heterogeneous and fluid-mantle interactions can vary spatially due to a wide range of fluid/rock ratios.
Carbonates in natural serpentinites occur as veins associated with fluid channels, and their abundance is up to 2 vol% in drill-core samples from the Atlantic Massif (e.g., Ternieten et al., 2021).Although the distribution of carbonates in seafloor serpentinites is highly heterogeneous, the mean carbon content of low-temperature serpentinites collected from ODP Holes 1272A and 1274A near the fracture zone on the Mid-Atlantic Ridge is 0.32 wt% (Alt et al., 2013), which is almost comparable with the total carbon concentration inferred from the vein abundance.Based on our thermodynamic calculations, these natural occurrences of mantle carbonatation suggest that the overall fluid/rock ratio during low-temperature serpentinization is $500, although the fluid/rock ratio may be locally heterogeneous and lower for deeper mantle hydration.
Figure 7 shows the relationship between seismic velocity and carbonate-bearing serpentinite for fluid/rock ratios of 250-1000, which is almost insensitive to the fluid/rock ratio because of the limited carbonate abundance.As such, the effects of carbonate on the seismic velocity of the mantle appear to be relatively small, making it difficult to constrain the mantle carbonation, and carbonate precipitation in the mantle is strongly dependent on fluid/rock ratios.

| MANTLE HYDRATION AND CARBONATION IN THE OUTER-RISE REGION
The low seismic velocity in the uppermost mantle in trench outer-rise regions is likely due to serpentinization, but the penetration of carbon-bearing fluids into the mantle could also result in the precipitation of carbonate minerals.Although the degree of mantle carbonation is difficult to constrain from the seismic velocity, the carbon concentration can be estimated by combining the seismic data and thermodynamic modeling (Figure 6).In the Middle America Trench, the seismic velocity in the mantle decreases significantly near the trench, which is interpreted to reflect serpentinization along bendingrelated faults (e.g., Ranero et al., 2003;Grevemeyer et al., 2007;Ivandic et al., 2008;Lefeldt et al., 2009).We applied our model calculation to the P-wave velocity structure in this region to infer the water and carbon concentrations in the mantle (Figure 8).Water and carbon contents were calculated from the mineral assemblages, and the seismic velocities were calculated using the Voigt-Reuss-Hill average.We used a dunite composition for the interaction with seawater, because it is the simplest mantle lithology.Note that quartz was absent in our calculations, and thus it was not appropriate to model listvenite formation.
The volume of mantle hydration and carbonation can be estimated from a depth integration of the serpentinized mantle, but one-dimensional velocity structures are not always available for every subduction system.Therefore, we calculated the serpentinite volume from the velocity reduction in the uppermost mantle using the Middle America Trench data as a reference value.The subduction flux of serpentinite (i.e., volume rate) was then multiplied by the trench length and orthogonal convergence rate (Table 1).Even within a given subduction system, the seismic velocity in the outer-rise region is spatially variable (e.g., Grevemeyer et al., 2018).We used the average value for the calculations, but these spatial variations result in $50% uncertainty in the flux estimates, particularly for the north Chile arc and Carbon content (wt%) (Lefeldt et al., 2012) F I G U R E 8 Velocity structure in the Middle America Trench (Lefeldt et al., 2009), which shows a marked velocity reduction in the uppermost mantle.Serpentinization, water contents, and carbon contents were calculated based on thermodynamic modeling with variable fluid/rock ratios.
Middle America Trench.

| EFFECT OF MANTLE CARBONATION ON THE GLOBAL CARBON CYCLE
Although direct samples of the mantle from beneath the oceanic crust are not currently available, the carbon content of the mantle and its flux can be inferred from geophysical data and thermodynamic calculations.We have shown that the mantle carbon flux in trench outer-rise regions is 7-31 Mt C/year, which is broadly equivalent to the carbon flux due to the altered oceanic crust (e.g., Plank & Manning, 2019).This range is similar to a previous estimate of 4-15 Mt C/year for the outerrise mantle (Kelemen & Manning, 2015), although their model assumed 10% serpentinization of the upper 10 km of mantle and an average carbon concentration for altered peridotite.
In addition to trench outer-rise regions, serpentinites occur at slow to ultra-slow spreading ridges where they are enriched in carbon T A B L E 1 Summary of subduction characteristics, calculated water and carbon fluxes in various subduction zones.b Velocity reduction at the uppermost parts of mantle from geophysical observations: North Chile (Ranero & Sallarès, 2004), Middle America (Lefeldt et al., 2009), Alaska-Aleutian (Shillington et al., 2015), Kuril-Kamtchatka (Fujie et al., 2013), Northeast Japan (Obana et al., 2019), Izu-Bonin-Mariana (Cai et al., 2018), Tonga-Kermadec (Contreras-Reyes et al., 2011), Java (Planert et al., 2010).(e.g., Cannat et al., 1995;Früh-Green et al., 2004).Alt et al. (2013) estimated a carbon uptake of 1.4-1.9Mt C/year for low-temperature serpentinites associated with detachment faults at slow and ultra-slow spreading ridges.Subduction has not been initiated in these oceanic plates, and thus their contribution to the present-day subduction flux is small, but will increase when such plates begin to subduct.Serpentinized peridotites have also been found in association with propagating rifts and fracture zones in the oceanic basement formed at fast spreading ridges where hydrothermal circulation leads to serpentinization at temperatures of 200-400 C (e.g., Früh-Green et al., 1996).
The carbon contents of high-temperature serpentinites are lower than those of low-temperature serpentinites.However, the higher rate of plate production at fast-spreading ridges may result in a significant contribution to the global carbon sink (Alt et al., 2013).
Previous estimates of the carbon budget have suggested that the carbon input from incoming sediments and altered oceanic crust is nearly balanced by the carbon output due to volcanic degassing (e.g., Dasgupta & Hirschmann, 2010;Jarrard, 2003;Müller et al., 2022;Plank & Manning, 2019).However, extensive mantle serpentinization can affect the global carbon budget, and an excess of the carbon input, relative to output, enhances carbon transport and storage in Earth's interior (Figure 10).During subduction, most carbon is released by mechanical removal, decarbonation reactions, and melting as the temperature and pressure increase with depth (e.g., Plank & Manning, 2019).Metamorphic decomposition is strongly dependent on the subduction geotherm, and carbonate minerals become unstable in warm subducting plates (e.g., Eberhard et al., 2023;Kerrick & Connolly, 2001a, 2001b).However, mantle hydration and carbonation are expected to occur in cold lithosphere and, therefore, subducted mantle-trapped carbon may survive metamorphic decomposition and be recycled into the deeper mantle.
Although there are large uncertainties in estimating carbon fluxes over time, the long-term carbon cycle controls atmospheric CO 2 levels and Earth's surface climates (e.g., Berner et al., 1983;Walker et al., 1981).Volcanic degassing increases the amount of carbon in the atmosphere, but this carbon is trapped in sedimentary and crustal layers and returned to the mantle by subduction.This self-regulating system may be disrupted by extensive hydration and carbon uptake in oceanic mantle.The development of trench outer-rise faults is related to the stress field of the bending plate (e.g., Craig et al., 2014), and a cold and rigid lithosphere can develop extensive fractures that facilitate penetration of seawater into the mantle and possible carbonate precipitation.The subduction geotherm has changed throughout geological history due to the continual loss of heat source from Earth's interior (e.g., Brown, 2007;Maruyama & Liou, 2005), which is likely to have enhanced mantle hydration and carbonation.
Although it is uncertain when the global carbon cycle was affected by mantle carbonation, the input and output fluxes appear to be in disequilibrium in the present-day Earth.The degassing rate is expected to increase as large amounts of carbon are transported into the interior, but secular cooling could lead to pervasive mantle carbonation associated with lithospheric flexure, resulting in disruption of the global carbon cycle.

| CONCLUSIONS
Seawater penetration into the mantle along trench outer-rise faults likely causes carbonate precipitation during serpentinization, but it is difficult to constrain the amount of carbon trapped in the mantle from geophysical data alone.We have shown that the carbon content of serpentinized mantle can be estimated from the seismic velocity and thermodynamic modeling.Although the results depend on several factors, the subduction carbon flux due to fluid-mantle interactions in outer-rise regions is approximated to be 7-31 Mt C/year.This is similar to the carbon uptake by altered oceanic crust, suggesting that carbonated mantle is an important reservoir of subducted carbon.The global carbon budget was previously thought to be balanced by subduction and volcanic degassing.However, the carbon input flux, including carbonated mantle, may exceed the output flux.Secular cooling is likely to have enhanced mantle hydration and carbonation and, as such, the selfregulating system of the carbon cycle may be disrupted in the future or even on the present-day Earth.
Reaction kinetics of mantle hydration (serpentinization) and carbonation at a temperature of 100 C.

FF
lu id p e n e tr a ti o n Schematic illustration of mantle hydration and carbonation along a fault zone.Carbonate precipitation can be localized to the regions with high fluid/rock ratios, because carbonation requires a high flux of carbon-bearing fluids.
Vs = 1.78, ρ = 3.3 g/cm 3 ) F I G U R E 5 Seismic velocities and densities of peridotite, serpentinite, and listvenite.Although mantle hydration and carbonation are difficult to determine from the seismic velocities, the Vp/Vs ratio is significantly different for serpentinite and listvenite and can be used to identify hydration and carbonation.Seismic velocities and densities for peridotite and serpentinite are fromKatayama et al. (2021), and for listvenite are fromOkazaki et al. (2021).
The degree of serpentinization was estimated from the velocity reduction relative to the reference mantle velocity, and is up to 20 vol % in the uppermost parts of the mantle.Mantle serpentinization decreases with increasing depth to $13 km below the crust.Given that the relationship between seismic velocity and serpentinization is less sensitive to the fluid/rock ratio, the serpentinization and water concentration in the mantle are nearly constant over the possible range of fluid/rock ratios.However, carbonate precipitation and carbon content are strongly dependent on the fluid/rock ratio (Figure8), and they decrease at lower fluid/rock ratios due to the limited carbon dioxide supply.Based on the natural abundance of carbonates in seafloor serpentinites, we applied the model calculation at fluid/rock ratios of 250-1000.The results indicate that the maximum carbon concentration is estimated to be 0.02-0.09wt% in the uppermost mantle, which is broadly comparable to the carbon content in Results of the thermodynamic modeling showing the mineral abundances as a function of the fluid/rock ratio.
Changes in Vp and Vp/Vs during serpentinization at variable fluid/rock ratios.The seismic velocity is less sensitive to fluid/ rock ratio because of the limited abundance of carbonate.
Figure 9 shows the global flux of carbon in trench outer-rise regions.Each subduction system has a carbon flux in the range of 0.5-6.3Mt C/year, and the global carbon budget due to mantle carbonation in trench outer-rise regions is estimated to be 7-31 Mt C/year.The water and carbon fluxes were calculated based on the mineral assemblages in the serpentinized mantle, assuming fluid/rock ratios of 250-1000.The calculation of water flux is insensitive to the fluid/rock ratio, but the carbon flux is highly dependent on the fluid/ rock ratio in the system.Our calculations are based on the carbonate abundances in seafloor serpentinites, but fluid flow can vary with depth, and thus the mineral assemblages and fluid chemistry may vary in the uppermost mantle.As such, the fluid/rock ratio is less well constrained, and it is a source of large uncertainty in our estimates of carbon flux in the mantle.

c
Vs instead of Vp.Fluxes are calculated with fluid-rock ratio ranging from 250 to 1000.U R E 9 Carbon subduction flux in various subduction zones where mantle hydration and carbonation are expected to occur in trench outer-rise regions (yellow regions).Plate abbreviations are Ph, Philippine Sea; JF, Juan de Fuca; Car, Caribbean.