A new publication by Rajdeep Dasgupta of Rice University  provides constraints on the origin of Earth’s carbon, and modulation and re-distribution of initial, planet-scale budget of carbon set by accretion by various differentiation processes.
The processes that had impact on the carbon cycle through time included core formation, magma ocean-atmosphere interactions, magma ocean crystallization, addition of late-veneer, plate-tectonic cycles of Archean and Proterozoic, and subduction and mantle melting of the Phanerozoic eon.
Dasgupta argues that during the first few tens of millions of years of the Hadean Eon, deep carbon partitioned strongly into the core, making core the largest terrestrial reservoir of carbon. Replenishing carbon to the silicate Earth required special processes such as late veneer addition, ingassing from a CO2-rich atmosphere, or later core-mantle interactions among others. Mechanisms of carbon cycling in a matured Earth involved in-gassing of carbon via sinking of altered crustal and mantle lithospheric rocks plus sediments in subduction zones and out-gassing by erupting volcanoes.
However, the author shows that the flux of carbon being in-gassed versus out-gassed has not remained constant throughout Earth’s history. Lack of systematic subduction cycles in the Archean Eon and hotter mantle made deep subduction of carbon less efficient, but at the same time made arc volcanism more powerful in releasing carbon. This efficient CO2 out-gassing in the Archean arcs might have provided the necessary dose of greenhouse gasses to offset the faint young Sun and keep liquid water at Earth’s surface. On the contrary, the deep in-gassing of carbon by subduction during the Phanerozoic Eon appears extremely efficient and Earth at the present-day may be losing surface carbon to the interior via this process. Dasgupta also synthesizes constraints on storage of carbon in various carbon-bearing phases in Earth’s shallow to deep mantle, starting from carbonated silicate melt, carbonatite, diamond, and C-bearing Fe-Ni alloy, Fe-Ni carbide, and Fe-Ni-C-S alloy/ sulfide melt.
Figure: Cartoon illustrating the range of deep-Earth processes from magma ocean stage of the Hadean Eon to plate tectonic framework of the modern world that likely influenced the long-term carbon cycle. Early Earth processes cartoon (left), although shown together in the cartoon, may be temporally separated. For example, if carbon exchange between the proto-atmosphere and the magma ocean had an important role in the later evolution of the mantle, such a process was required to continue after the core formation had ceased.