Balancing Earth's Deep Carbon Cycle

Abstract The deep carbon cycle of the Earth remains poorly understood in spite of its importance to the evolution of our planet. Here we consider estimated carbon fluxes for plume‐related volcanism from the lower mantle as an independent constraint on the deep carbon cycle, avoiding uncertainties regarding high pressure mineralogy. We perform parameterized carbon cycle models using plume and other volcanic fluxes as constraints, including both an upper and lower mantle reservoir, and employ Monte Carlo sampling to discover the distribution of unconstrained parameters that satisfy our target constraints. For a nominal global carbon budget of kg, models satisfying these constraints require that less than 5% of carbon involved in mantle magmatic processes is degassed to the surface, unless undegassed carbon is recycled to the lower mantle which then permits up to 50% degassing efficiency. 100% degassing is only achieved when global carbon budgets are below kg, although higher budgets may be permitted if there exist additional carbon sinks not included in our model. Achieving a higher degassing efficiency requires upper mantle carbon concentrations below ∼1 kg /. The balance of carbon in the upper and lower mantle is mainly determined by the fate of undegassed carbon, and our results suggest that the lower mantle contains a significant fraction of the Earth's carbon. We also find that variations in insolation, temperature‐dependent weathering, initial conditions, and early tectonics have negligible influence on the long‐term deep carbon cycle.