Nield reported in the popular press on the work of Farsang and 8 co-authors, who reported in mid-2021 that Earth’s deep carbon cycle affects atmospheric CO2, climate, and habitability. Owing to the extreme solubility of CaCO3, aqueous fluids released from the subducting slab could extract all carbon from the slab. However, recycling efficiency is estimated at only around 40%.
Data from carbonate inclusions, petrology, and Mg isotope systematics indicate Ca2+ in carbonates is replaced by Mg2+ and other cations during subduction. The authors determined the solubility of dolomite [CaMg(CO3)2] and rhodochrosite (MnCO3), and put an upper limit on that of magnesite (MgCO3) under subduction zone conditions. Solubility decreases at least two orders of magnitude as carbonates become Mg-rich. This decreased solubility, coupled with heterogeneity of carbon and water subduction, may explain discrepancies in carbon recycling estimates.
Over a range of slab settings, the authors found that aqueous dissolution is responsible for mobilizing 10 to 92% of slab carbon. Globally, aqueous fluids mobilize 35+20−1735−17+20% (27+16−1327−13+16 Mt/yr) of subducted carbon from subducting slabs.
Two modes of tectonic carbon cycling are associated with subduction zones1. The first is the shallow accretionary carbon cycle that includes the accretion of shelf, oceanic island, and minor seafloor sedimentary carbon to continents, the incorporation and transport of this carbon by ascending fluids and magmas, followed by its release to the atmosphere or oceans.
The second is the deeper carbon cycle that includes the subduction of carbon, the devolatilization, aqueous dissolution, or melting of carbon-bearing phases, the transport of liberated CO2 to the mantle wedge and volcanic arc followed by active or passive degassing. Finally, carbon may be transferred to the deep mantle in high-pressure carbon-bearing phases. The extent to which each of these processes operates is contentious (Fig. 1).
Managed & Built by I2M Consulting, LLC | © 2010-
