Deep Carbon Emissions from Volcanoes

Over long periods of time (~Ma), we may consider the oceans, atmosphere and biosphere
as a single exospheric reservoir for CO2. The geological carbon cycle describes the inputs to
this exosphere from mantle degassing, metamorphism of subducted carbonates and outputs
from weathering of aluminosilicate rocks (Walker et al. 1981). A feedback mechanism relates
the weathering rate with the amount of CO2 in the atmosphere via the greenhouse effect (e.g.,
Wang et al. 1976). An increase in atmospheric CO2 concentrations induces higher temperatures,
leading to higher rates of weathering, which draw down atmospheric CO2 concentrations (Berner
1991). Atmospheric CO2 concentrations are therefore stabilized over long timescales by this
feedback mechanism (Zeebe and Caldeira 2008). This process may have played a role (Feulner
et al. 2012) in stabilizing temperatures on Earth while solar radiation steadily increased due to
stellar evolution (Bahcall et al. 2001). In this context the role of CO2 degassing from the Earth is
clearly fundamental to the stability of the climate, and therefore to life on Earth. Notwithstanding
this importance, the flux of CO2 from the Earth is poorly constrained. The uncertainty in our
knowledge of this critical input into the geological carbon cycle led Berner and Lagasa (1989)
to state that it is the most vexing problem facing us in understanding that cycle.