Carbon in Earth's Interior: Storage, Cycling, and Life
Knowledge of the deep interior, which may contain more than 90% of Earth's carbon, is limited. As a result, little is known about how deep carbon reservoirs form, evolve, and reach the surface. Further, scientists know little of the nature and extent of deep microbial ecosystems, which by some estimates rival the total surface biomass.
Fig. 1. Many questions remain about the cycling, behavior, and storage of Earth’s deep carbon, from crust to core. Sources of the carbon in the crust include (a) possible microbial community structures on a mid-ocean ridge flanks (adapted from images by R. Coggon and K. Nakamura). Other deep biosphere habitats impacting subsurface carbon cycling also exist. (b) An epifluorescence micrograph shows an iron-reducing enrichment culture from a serpentinite-hosted habitat. (courtesy: Matt Schrenk). (c) Diamonds and their inclusions give scientists glimpses into Earth’s deep interior (photo: U.S. Geological Survey). Theoretical and experimental studies allow us to speculate on carbon's role even deeper in Earth. (d) Iron carbide (F7C3) could be a potential constituent of Earth’s solid inner core (courtesy: Y. Nakajima). (e) Experimental studies suggest the existence of magnesium-iron carbon-bearing structures similar to phase II of magnesite at high pressures at depths greater than 1800 kilometers (courtesy: E. Boulard).
Reproduced/modified by permission of American Geophysical Union.