Category: Extreme Earth Phenomena
Carbon is the primary chemical regulator of terrestrial climate over million-year timescales. While surface carbon cycling between the atmosphere, biosphere, and surface hydrosphere operates over short timescales (days to millennia), over 99.8% of Earth’s total carbon inventory resides sequestered within the solid silicate mantle and core. The continuous exchange of carbon between surface reservoirs and the deep planetary interior—the Deep Carbon Cycle—is governed by the balance between tectonic subduction of oceanic carbonates and volcanic arc outgassing. Resolving whether Earth is currently net-sequestering or net-outgassing carbon represents one of the foremost questions in planetary geodynamics.
1. Subduction Influx: Sedimentary vs. Altered Oceanic Crust
Carbon enters subduction trenches through two primary geological vectors on the downgoing oceanic plate:
- Pelagic Carbonate Sediments: Calcite (CaCO₃) and aragonite oozes deposited on pelagic seafloor, delivering an estimated 20 to 35 megatons of carbon per year (Mt C/yr).
- Carbonated Altered Oceanic Crust (AOC): Hydrothermal circulation along mid-ocean ridges precipitates carbonate minerals (calcite, dolomite, siderite) within veins, vesicles, and breccias throughout basaltic lavas and sheeted dikes, delivering an estimated 30 to 45 Mt C/yr.
Total global carbon subduction influx is estimated between 55 and 80 Mt C/year.
2. Subduction Metamorphism and Decarbonation Barriers
As the downgoing slab sinks and heats, carbonates encounter progressive metamorphic pressure-temperature regimes. Unlike hydrous minerals (which dehydrate readily below 100 km), pure calcite and dolomite remain thermodynamically stable along cold subduction geotherms to depths exceeding 200–300 km.
Silicate-Carbonate Reactions and Fluid-Fluxed Dissolution
Decarbonation occurs primarily through reaction with quartz and hydrous fluids released from underlying serpentinites and basalts:
CaCO₃ (calcite) + SiO₂ (quartz) → CaSiO₃ (wollastonite) + CO₂ (fluid)
In hot subduction zones (e.g., Cascadia, SW Japan), fluid-fluxed dissolution and silicate-carbonate reactions strip 40% to 70% of subducted carbon, channeling it into the overlying mantle wedge. In cold subduction zones (e.g., Tonga, Mariana), over 80% of carbonate carbon survives sub-arc depths, descending into the deep convective mantle.
3. Volcanic Arc Outgassing and the Mantle Carbon Budget
Carbon mobilized into the mantle wedge lowers the peridotite melting temperature, generating volatile-rich basaltic and andesitic arc magmas. As magmas ascend and decompress into the crust, carbon dioxide exsolves rapidly due to its exceptionally low solubility in silicate melts relative to water.
The Deep Earth Carbon Flux Balance
Global volcanic arc outgassing discharges approximately 18 to 44 Mt C/year. Mid-ocean ridge basalt (MORB) degassing contributes an additional 12 to 24 Mt C/year, and intraplate ocean island plumes vent 3 to 10 Mt C/year. Comparing the total volcanic efflux (~35–75 Mt C/yr) against the subduction influx (~55–80 Mt C/yr) indicates that the modern solid Earth is operating in near-equilibrium or acting as a mild net sink of carbon into the deep mantle.
The Planetary Scale Metric
Global Subduction Influx: 55–80 Mt C/year | Total Volcanic Outgassing: 35–75 Mt C/year | Deep Mantle Carbon Residence Time: > 1.0–2.5 billion years
| Deep Carbon Reservoir / Flux | Estimated Carbon Inventory | Annual Flux Rate | Primary Mineral Host |
|---|---|---|---|
| Subduction Ingress (AOC + Sediment) | Dynamic flux | 55–80 Mt C / yr | Calcite, aragonite, dolomite |
| Arc Volcanic Degassing | Dynamic flux | 18–44 Mt C / yr | Exsolved gaseous CO₂ |
| Convective Upper Mantle | ~ 10⁸ Mt C | Internal reservoir | Carbonatitic melts, magnesite |
| Deep Lower Mantle / D” | ~ 10⁹–10¹⁰ Mt C | Internal reservoir | Diamond, iron carbide (Fe₇C₃) |
4. Key Takeaways and Geophysical Summary
- Subduction Vectors: Ocean floor sediments and hydrothermally altered basaltic crust convey 55–80 Mt C/yr into subduction zones globally.
- Thermal Partitioning: Hot slabs lose carbon via fluid dissolution, while cold slabs transport stable carbonates past arc depths into the transition zone and lower mantle.
- Volcanic Return Flux: Arc volcanism, mid-ocean ridges, and mantle plumes return 35–75 Mt C/yr to the atmosphere, maintaining long-term climate stability.
- Deep Mantle Storage: Carbon surviving past sub-arc depths precipitates as diamonds or iron carbides, remaining sequestered in the deep mantle for billions of years.
