Carbon permanence of biochar; a lesson learned from the geologically preserved charcoal in carbonaceous rocks

Fusain is a macroscopic component, or lithotype, of coal. Microscopic fragments of fusain are referred to as “inertinite” maceral, which is commonly found in all carbonaceous and most sedimentary rocks. Fusain/inertinite is formed by carbonization of biomass in oxygen deficient paleo-wildfires and subsequent transportation into peatbogs and sedimentary basins. Substantial quantity of fusain/inertinite is commonly found in most post-Devonian coal and sedimentary rocks. Inertinite fragments are often microscopically characterized by their intricate vacuole structures inherited from the original cell lumens, which attests to their remarkable high preservability during geological processes.

Fusain/Inertinite is generally believed by geologists to be chemically highly inert due to its intense degree of aromatization and ordering of carbon molecular structure and cannot be degraded by shallow surface processes including oxidation and biodegradation. The “selective diagenesis” processes continue to preferentially degrade the more labile organic carbon fractions while preserving the most refractory fractions that are thermodynamically least favored to breakdown. The long-term, geological evolution of organic carbon in the Earth’s crust through three main stages of diagenesis, catagenesis, and metagenesis, is studied by organic petrology and organic geochemistry methods. This presentation provides results of these methods for a set of 20 synthetic biochars produced from different feedstocks to compare their geochemistry and optical characteristics with the commonly preserved geological fusain/inertinite in coal and other carbonaceous rocks.

The results show that biochars that have been produced at maximum pyrolysis temperature of over 600°C, resemble properties of the most refractory fusain/inertinite in the Earth’s crust and would not be degraded as long as other more thermodynamically labile organic carbon compounds are in existence. Any degradation of these biochars can only be perceived under intense geological burial heat in the Earth’s mantle or contact metamorphism by igneous intrusions and hydrothermal processes. The claim of short-term carbon permanence for biochars contradicts co-existence of fusain/inertinite with labile organic carbon commonly observed in carbonaceous rocks. Degradation of refractory fusain/inertinite would not be thermodynamically favored while large quantity of labile organic carbon is readily and commonly available in carbonaceous rocks. The results of this study highlight the need for re-thinking carbon permanence of biochars within the context of the deep geological carbon cycle.