Closure of Constraints in the Earth System: Planetary Biological Organisation Across Biogeochemical Cycles

Alejandro Merlo 

Closure of constraints characterises the distinctive causal regime of biological systems: a set of constraints, each conserved with respect to the process it constrains and maintained by processes running at another scale, mutually dependent for their maintenance. The framework has been extended above the organism principally to ecosystems, where individuating the systems involved has proved difficult. We argue that the Earth system realises closure of constraints, and is therefore biologically organised in the technical sense defined by the framework. The argument does not locate closure in any biogeochemical cycle. Cycles are regularly dismissed as instances of merely physical circularity, and for any single cycle that dismissal is justified. Closure is realised across a set whose members belong to different cycles: among them, the populations of protein complexes that catalyse the planet’s redox transformations, the atmospheric oxygen inventory they sustain, and the surface water inventory the organisation recruits rather than produces. The dependencies among them run in both directions and in both temporal orders. Constraints of this kind are individuated by the transformation they harness and are planetary in extent, so no boundary need be drawn within the system to specify the set. The membership of the set has changed over geological time, the Quaternary being one regime among several. This opens the question of whether the Earth system has norms of its own.