Phanerozoic atmosphere and ocean conditions estimated by a continuous climate and multi-box ocean model

Alexander James Krause , Mingyu Zhao, Andrew Merdith, Benjamin Mills

The Phanerozoic Eon has witnessed marked fluctuations in Earth’s climate and the concentrations of elements conducive to life. Understanding what has driven these changes to the Earth system over such a long timeframe has traditionally used ‘box models’ which split the Earth into a small number of chemical reservoirs, with many models utilising a single, combined ocean-atmosphere reservoir. This simplification allowed for very efficient long-term computation, but limited the realistic representation of many important processes.
Here, we present AMMONITE: the Atmosphere and Minimal Multi-box Ocean model for Numerical Integration Through Earth history. The model uses an efficient 5-box ocean scheme with a minimal representation of thermohaline circulation, shelf sea processes, carbonate chemistry and productivity-remineralization cycling. We use AMMONITE to replace the single-box atmosphere-ocean in the existing SCION Earth Evolution Model, forming a new biogeochemical model of the Earth system which can run continuously over Phanerozoic time and dynamically links 3D emulated climate, 2D surface processes and a multi-box ocean. SCION-AMMONITE makes predictions of many facets of the Earth system which will help with understanding habitability over deep time, including a new detailed estimation of the pH and oxygen content of the ocean. In contrast to earlier SCION model outputs, SCION-AMMONITE suggests that O2 concentrations in the atmosphere and deep ocean were very low in the early Paleozoic, owing to lower predicted sulfate levels, better consideration of the large Precambrian sulfide reservoir, and reduced strength of global oxygen-stabilizing feedbacks as a result of phosphorus limitation in a spatially structured ocean.