Thermodynamic and Energetic Limits on Continental Silicate Weathering Strongly Impact the Climate and Habitability of Wet, Rocky Worlds

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In this paper, we demonstrate that incorporating previously-neglected chemical and physical effects into an idealized coupled model of continental silicate weathering and global-mean planetary climate leads to predictions of novel behavior in the inorganic carbon cycles of Earth-like planets.

All previous global weathering models applied to understanding Earth-like exoplanet climate have used models of silicate weathering that assume global weathering is limited by the kinetic rate of silicate mineral dissolution. However, as noted in Maher & Chamberlain 2014, the precipitation of secondary minerals (clays) during the weathering process can lead to chemical equilibration of weathering reactions, eliminating the influence of reaction kinetics and imposing a “thermodynamic limit” to weathering. In this regime, global-mean weathering becomes a function of runoff instead of temperature. Our paper is the first to apply this insight to the weathering of temperate exoplanets, and we find that it leads to dramatic changes in the sensitivity of planetary weathering to parameters like land fraction and CO2 outgassing rate. Specifically, because of the weakening of the feedback between climate and weathering in the thermodynamic regime compared to the kinetic regime, planets with thermodynamically-limited weathering must go through larger climate changes to re-equilibrate their carbon cycle in response to changes in boundary conditions.

We also impose an energetic limit to precipitation in our climate model that arises from the fact that global-mean evaporation (which is equal to global-mean precipitation in steady-state) cannot exceed the rate at which the global latent heat of evaporation equals the total absorption of stellar irradiation falling on a planet. Because continental weathering is driven by the precipitation and runoff of water, this energetic limit on rainfall can strongly affect the efficacy of a planet’s weathering feedback by changing the relationship between global-mean precipitation and planetary climate. When precipitation becomes energetically-limited, it no longer changes in response to global-mean temperature, which eliminates the main feedback between climate and weathering rates under the thermodynamic regime. Planets in the energetically-limited regime may display a counter-intuitive anti-correlation between global-mean surface temperature and instellation.