Although the ocean is full of carbon—dissolved as bicarbonate and in the shells and bodies of marine organisms—very little carbon makes it to the deep seafloor to be conveyed to a trench. The carbon that enters subduction zones includes calcium carbonate and reduced organic carbon that exist within the sedimentary, oceanic crust and mantle layers of the incoming plate. Each trench makes unique selections from the carbon menu.

Starting with the lowermost layer, mantle peridotite that forms the bulk of the subducting lithosphere readily hydrates and carbonates if exposed to seawater, forming carbonated serpentinites22. However, most peridotite usually resides at least 6 km beneath the seafloor and is not in direct contact with seawater. Faulting and fracturing are necessary to bring mantle rocks to the sea floor or seawater to the mantle. Carbonated serpentinites may form near spreading centres or near trenches. Near spreading centres, extensional faulting is linked to hydration and carbonation reactions, as well as to the precipitation of magnesium and calcium carbonate veins in mantle peridotites22. Carbon isotopes in oceanic peridotites reflect mixing between seawater-derived carbonate and reduced carbon23. Serpentinization is most pronounced at slow-spreading mid-ocean ridges, but most subducting lithosphere is not formed there, because of the low plate production rate and the preferential existence of subduction zones and fast-spreading crust in the Pacific. Oceanic carbonated serpentinites therefore make a minor contribution to the global carbon input flux to subduction zones3, although they may be locally important. The volcanism at the South Sandwich margin, where slow-spreading crust is preferentially consumed, is notable for having some of the highest 11B/10B ratios among island arcs, a feature that could be derived from the high 11B/10B ratio that is typical of seafloor serpentinized peridotites24. The South Sandwich margin may thus represent a rare case, in which subducting carbon predominantly resides in oceanic carbonated serpentinite (Box 2).

Near trenches, the subducting plate deforms and fractures because of bending. The resulting faults have been seismically imaged to penetrate into the mantle of the incoming plate, and seismic velocities decrease towards the trench25, leading to speculation that the extent of hydration due to ingress of seawater could consume an ocean every billion years26,27. However, outer-rise serpentinites have never been sampled, and other factors besides hydration—such as fracturing and anisotropy—may explain this reduction in seismic velocity. The extent of carbonation in unsampled outer-rise serpentinites is also unknown, and fluid pathways longer than 5 km may lead to low fluid-to-rock ratios and low carbon transport into the mantle section of the downgoing plate23. Indeed, electrical-resistivity imaging does not support extensive reaction of seawater with the mantle section of the incoming plate at the Central America trench28. Most global flux estimates for carbonated peridotite are low (Box 1).

In contrast to peridotite carbonation, there is abundant evidence for pervasive carbonation of the near-ridge oceanic crust. Seawater-derived fluids circulate predominantly in the higher-permeability upper-crustal volcanic section, leading to low-temperature precipitation of carbonate minerals. Because these carbonates are largely seawater-derived, they are isotopically heavy (δ13C ≈ 0‰, where δ13C is the deviation of the ratio 13C/12C relative to that of Pee Dee belemnite), although some biotic and abiotic processes also lead to CO 2 reduction and precipitation of isotopically light carbon (δ13C < −20‰29,30; Fig. 2a). Nonetheless, the dominant form of carbon in altered oceanic crust (AOC) is calcium carbonate (calcite and aragonite) that precipitates in veins and vugs, as has been found in samples from a small number (about 15) of drill sites. Carbon uptake occurs near the ridge axis in crust that is 20 Myr old, but surprisingly, AOC older than 80 Myr has higher carbonate content31. This may be due to higher bottom-water temperatures in the Cretaceous period promoting greater abiogenic carbonate precipitation32. Although low in carbonate, young AOC (less than 10 Myr old) is isotopically light owing to the intense bio-alteration of young crust30. Thus, an important prediction for carbon inputs is that old plates (for example, Marianas and Tonga) will have greater AOC carbonate concentrations and higher average δ13C, whereas young plates (for example, Cascadia and Central America) will have little AOC carbonate with lower δ13C (Fig. 2a).

Fig. 2: Carbon isotopes in subducted input, diamonds and arc volcanic gas. a, Subducted carbon input, including that in the upper volcanic layer of AOC older than 10 Myr (refs 30,102), carbonate sediments from ODP site 765 (ref. 41), total organic carbon, C org , in Bengal Fan sediments103. b, Diamonds, including 319 eclogitic diamonds (orange; from ref. 30) and Juina SLD (green; ref. 104). c, Volcanic output per kilometre of arc from ref. 92, where error bars are one standard deviation about the mean value. Yellow shading encompasses mantle values. Diamond populations appear skewed towards light carbon isotopes74, whereas arc volcanic C is skewed towards high δ13C values92. It is unclear whether this reflects preferential recycling of carbonate to the arc and preferential subduction of more refractory organic carbon to the deeper mantle source of diamonds, or whether ancient diamonds reflect more-reduced sources in Earth’s past14. Full size image

The sedimentary layer that is deposited on top of the oceanic crust contains dramatically different forms of carbon than the largely inorganic precipitates of the oceanic crust and peridotite. Sediments are the graveyard of marine organisms and the resting place of terrestrial organic remains. Organisms that grow a carbonate shell, such as nannoplankton coccoliths and bottom-dwelling foraminifera, are the richest source of carbon deposits on the seafloor. For example, a 100-m section of nannofossil ooze may contain as much carbon as the entire oceanic crust below it (using average values given in ref. 3). Marine sediments also contain the organic remains of marine and terrestrial organisms. Although most sediments have less than 1 wt% organic carbon, deep-sea fans can dominate the input flux at some margins. For example, a 1.5-km section of terrigenous turbidites with 0.35 wt% organic C (Fig. 2a) contains more carbon than the average oceanic crust. The balance of marine carbonate (δ13C = 0‰ to +3‰) versus organic carbon (δ13C = −22‰ to −27‰) has a very large effect on the isotopic composition of subducting carbon (see ref. 33 and Fig. 2a). Carbonate sediments can approach 100% CaCO 3 and therefore may contain more than 10 times the carbon of a sediment rich in organic carbon (1% C; Fig. 2a). This is offset by a greater flux of organic carbon-bearing sediments approaching trenches (in thick fans), so the proportion of organic to inorganic carbon subducted globally may be about 20% (ref. 34).

Although sedimentary carbon has the potential to dominate global input fluxes, it may be entirely absent from some subducting sections. Indeed, the odds are stacked against carbon burial, as most carbonate dissolves and organic carbon oxidizes in the water column before reaching the seafloor. The ocean’s cold and corrosive bottom waters are particularly challenging to carbonate survival. The calcite compensation depth (CCD), which marks the transition between carbonate-bearing and carbonate-absent sediments, is about 5,000 m deep in today’s oceans. The CCD increases locally if the carbonate flux is high, as occurs in regions of high biological productivity, but it was generally shallower (less than 3,500 m) earlier in the Cenozoic era35 and the Cretaceous period36. Much of the oceanic crust subducting today is old (average age of about 70 Myr)37 and the combination of a shallower CCD and thermal subsidence with age means that carbonate is rare on the seafloor near trenches. For example, essentially zero sedimentary carbonate is subducted along the Tonga, Central Aleutian and Kuriles–Kamchatka trenches. On the other hand, abundant carbonate is subducted at the Central American margin, where the seafloor is beneath regions of high biological productivity, and at the New Zealand margin, where the seafloor is shallow38 (Box 2).

Organic carbon is also consumed in the oxic ocean and in the sediments themselves by microbially mediated reactions, so its preservation in sediments requires rapid supply and burial. These conditions are met beneath regions of high biological productivity and in deep-sea fans, where rivers deliver high fluxes of carbon-bearing sediment to the ocean from regions of active uplift and erosion39. On the other hand, vast expanses of the ocean are deserts owing to low biological productivity and to the challenge of survival in the harsh oxic sediments40. Seafloor currently subducting in the western Pacific spent most of its lifetime traversing the central gyres and is therefore devoid of organic carbon; essentially no organic carbon is subducted at Tonga or Honshu34. By contrast, turbidite sediments in the Bengal and Indus fans, which derive from and fringe India’s collision zone with Asia, constitute the largest fluxes of organic carbon into trenches. Other margins with notable piles of sediment containing organic carbon include those of Nankai, Cascadia, Alaska, South Chile and the Southern Antilles34.

Thus, subducting carbon depends on geologic happenstance at locations where a deep-sea fan (high sedimentary organic carbon) or shallow seafloor (high sedimentary carbonate) happen to be near a trench, or where the subducting oceanic plate happens to be created by slow spreading (favouring carbonated serpentinites) or was formed in the Cretaceous period (favouring carbonated oceanic crust). There is exceptionally wide global variation in carbon input to a subduction zone; each downgoing plate has a distinct formation, evolution and sedimentation history (Box 2 and ref. 5), with potentially large along-strike variations41. Global averaging obscures these underlying factors controlling recycling efficiency, which are essential for constructing the long-term history of the deep carbon cycle. This affects not only the amount and distribution of subducting carbon but also its reactivity and isotopic composition, and contributes to heterogeneity in the deep Earth. The former affects the fate of carbon in the subduction zone (discussed in the next section) and the latter serves as a useful tracer for the source of volcanic gases and diamonds (discussed in subsequent sections).