DOES LIFE DRIVE DISEQUILIBRIUM IN THE BIOSPHERE?

Lovelock’s Gaia Hypothesis was apparently born out of his insight that the atmosphere of a lifeless planet would be close to chemical equilibrium, while the robust presence of life would generate measurable disequilibrium. Others have expanded this view to postulate the growing disequilibrium of the Earth’s surface system from life’s influence over geologic time. However, the departure from chemical equilibrium within the atmosphere should be distinguished from that of the crust/atmosphere interface.

We show that the carbonate-silicate geochemical cycle (Urey reaction), the long term control on the steady state atmospheric carbon dioxide level, is far from equilibrium on the present Earth, only approaching equilibrium on a billion year timescale in the future as solar luminosity and surface temperature climb. Moreover, the progressive increase in the biotic enhancement of chemical weathering in the last 4 billion years, culminating in the weathering regime of the forest and grassland ecosystems, has brought the steady state atmospheric carbon dioxide level closer to the Urey reaction equilibrium state. In contrast, the abiotic steady state is always further from this equilibrium state than the biotic, except near the origin of life and for their future convergence. These are counter-intuitive results from a classical gaian view.

Equilibrium is an apparent attractor state in biospheric evolution for the case of the Urey reaction and long term atmospheric carbon dioxide levels, but apparently not for other atmospheric gases, especially oxygen, whose source is biotic. Unlike oxygen, the long term control on carbon dioxide levels involves biotic mediation of the sink intensity, not the source (volcanic and metamorphic outgassing).

Finally, an astrobiological flag: Lovelock’s original insight may still be valid for some cases, but far-from-equilibrium abiotic steady states may arise on Earth and other planets, and should not be taken as a priori evidence for gaian self-regulation.