Daniel Rothman, MIT: Slow Closure of Earth's Carbon Cycle

The carbon cycle's production and consumption of organic carbon must ultimately balance. Microbes close the loop, but as organic matter ages, microbial degradation becomes slower and microbial populations decrease at distinct powers of age. Mechanisms that quantitatively predict this slowdown remain unknown. Here I show that these and other observations follow from the assumption that the decay of organic matter is limited by progressively rare extreme fluctuations in the free energy available to microbes. The theory successfully predicts not only observed scaling exponents, but also a previously unobserved crossover to a different scaling regime in old (>1--2 Myr) sediments underlying unproductive regions. The resulting picture suggests that the carbon cycle's age-dependent dynamics are analogous to the slow approach to equilibrium in disordered systems. The impact of these slow dynamics is profound: they require the accumulation of unoxidized organic carbon in deep sediments, thereby freeing molecular oxygen to accumulate in the atmosphere.