9:00 AM - 9:15 AM
[SIT17-01] Challenges to Understanding Carbon Flux at Subduction Zones—Toward Mass-Balancing Inputs and Outputs at Individual Margins (and Globally)
★Invited Papers
Keywords:subduction, carbon, recycling, volcanoes, metamorphism, gases
Physical and chemical processes known to occur at forearc-to-subarc depths of subduction zones can impact the efficiency with which initially subducted C is emitted at arc volcanoes, stored in forearc/subarc reservoirs, or conveyed into the deeper mantle, and the extent to which the compositions of arc volcanic gases truly reflect proportions of carbonate and organic C in subducting slab sections. The amount of C returned to the atmosphere via arc degassing appears smaller than the combined C inputs in sediment, altered oceanic crust (AOC), and variably carbonated ultramafic rocks. Estimated return efficiency, typically in the range of 20-60%, reflects compounding of uncertainties beginning with those for seafloor inputs and shallow forearc dynamics and including uncertainties in estimates of arc gas outputs.
Some important issues complicating assessments of C return efficiency relate to uncertainty in structural factors such as accretion, deeper underplating, and subduction erosion. Ideally it would be possible to see, in arc gas compositions, along-strike change in amounts and isotopic compositions of C entering individual margins, as related to change in such factors. To a limited extent this has been demonstrated. House et al. (2019; Geology) considered arc gas outputs as related to massive and differential sediment accretion (at <20km depths) along the Sunda margin, Indonesia, superimposed on dramatic along-trench change in proportions of organic C and carbonate entering the trench (cf. Lopez et al., 2023; Sci. Adv., a study of the Aleutian/Alaska margin). Additions from AOC (known to be highly variable in C content, largely as carbonate) have tended to be invoked when it appears incoming sedimentary carbonate is insufficient to balance the outputs and explain elevated gas δ13C.
Petrologic and geochemical study of HP/UHP metamorphic suites has documented significant redistribution of C via forearc to subarc fluid-rock interactions. In some cases, loss of C is invoked based on study of carbonate dissolution and decarbonation in zones of enhanced fluid infiltration. Other studies have documented additions of C via carbonation reactions, arguing that such carbonation could generate significant sites of C storage at varying timescales. One problem with applying knowledge from metamorphic suites is the difficulty in scaling up such work at outcrop-scale to the scale of modern margins. Some studies document processes that occurred in a paleo-margin but that, for various reasons, are unlikely to be operating in many modern subduction zones or unlikely to be volumetrically significant. As another uncertainty, it is unclear whether C in the form of organic matter (transformed to graphite) or carbonate is released with equal efficiency beneath arcs (into “fluids” broadly defined). Isotopic exchange between the two reservoirs, which has been documented for some suites (e.g., Schistes Lustrés, French/Italian Alps), could further complicate the delivery to arcs of distinct organic vs. carbonate isotopic signatures. Fluid-melt-rock interactions in the mantle wedge could lead to isotopic fractionation and assimilation of crustal wall-rocks can lead to large contributions to arc gases (leading to lowered 3He/4He).
The fact that we increasingly recognize this array of complexities and uncertainties indicates that we are making progress toward more fully understanding this system. Thankfully, there is some correspondence of the δ13C of arc gases with proportions of sedimentary carbonate and organic C deeply subducting at individual modern margins with smaller amounts of accretion and larger C flux (Plank and Malinverno, 2024; abstract, AGU). Multidisciplinary study of individual margins can provide the most thorough assessment, in part pointing to areas of uncertainty warranting further attention. Recent examples of such studies include those by Epstein et al. (2021; Hikurangi margin, NZ; G-cubed) and Lopez et al. (2023; Aleutians/Alaska).
Some important issues complicating assessments of C return efficiency relate to uncertainty in structural factors such as accretion, deeper underplating, and subduction erosion. Ideally it would be possible to see, in arc gas compositions, along-strike change in amounts and isotopic compositions of C entering individual margins, as related to change in such factors. To a limited extent this has been demonstrated. House et al. (2019; Geology) considered arc gas outputs as related to massive and differential sediment accretion (at <20km depths) along the Sunda margin, Indonesia, superimposed on dramatic along-trench change in proportions of organic C and carbonate entering the trench (cf. Lopez et al., 2023; Sci. Adv., a study of the Aleutian/Alaska margin). Additions from AOC (known to be highly variable in C content, largely as carbonate) have tended to be invoked when it appears incoming sedimentary carbonate is insufficient to balance the outputs and explain elevated gas δ13C.
Petrologic and geochemical study of HP/UHP metamorphic suites has documented significant redistribution of C via forearc to subarc fluid-rock interactions. In some cases, loss of C is invoked based on study of carbonate dissolution and decarbonation in zones of enhanced fluid infiltration. Other studies have documented additions of C via carbonation reactions, arguing that such carbonation could generate significant sites of C storage at varying timescales. One problem with applying knowledge from metamorphic suites is the difficulty in scaling up such work at outcrop-scale to the scale of modern margins. Some studies document processes that occurred in a paleo-margin but that, for various reasons, are unlikely to be operating in many modern subduction zones or unlikely to be volumetrically significant. As another uncertainty, it is unclear whether C in the form of organic matter (transformed to graphite) or carbonate is released with equal efficiency beneath arcs (into “fluids” broadly defined). Isotopic exchange between the two reservoirs, which has been documented for some suites (e.g., Schistes Lustrés, French/Italian Alps), could further complicate the delivery to arcs of distinct organic vs. carbonate isotopic signatures. Fluid-melt-rock interactions in the mantle wedge could lead to isotopic fractionation and assimilation of crustal wall-rocks can lead to large contributions to arc gases (leading to lowered 3He/4He).
The fact that we increasingly recognize this array of complexities and uncertainties indicates that we are making progress toward more fully understanding this system. Thankfully, there is some correspondence of the δ13C of arc gases with proportions of sedimentary carbonate and organic C deeply subducting at individual modern margins with smaller amounts of accretion and larger C flux (Plank and Malinverno, 2024; abstract, AGU). Multidisciplinary study of individual margins can provide the most thorough assessment, in part pointing to areas of uncertainty warranting further attention. Recent examples of such studies include those by Epstein et al. (2021; Hikurangi margin, NZ; G-cubed) and Lopez et al. (2023; Aleutians/Alaska).