Aging and Slow Dynamics in Earth's Carbon Cycle

Carbon near the Earth's surface cycles between the production and

consumption of organic carbon; the former sequesters carbon dioxide

while the latter releases it. Microbes attempt to close the loop, but

the longer organic matter survives, the slower microbial degradation

becomes. This aging effect leaves observable quantitative signatures:

organic matter decays at rates that are inversely proportional to its

age, while microbial populations and concentrations of organic carbon

in ocean sediments decrease at distinct powers of age. Yet mechanisms

that predict this collective organization remain unknown. 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 energy available to microbes for decomposition.

The theory successfully predicts not only observed scaling exponents,

but also a previously unobserved scaling regime that emerges when

microbes subsist on the minimum energy flux required for survival.

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 preclude complete oxidation of organic carbon in sediments,

thereby freeing molecular oxygen to accumulate in the atmosphere.