Crustal Controls on Seafloor Weathering and Climate Regulation

The upcoming generation of extremely large ground-based telescopes and space observatories promises to transform our understanding of rocky exoplanets within the habitable zones of their stars. Observations from the James Webb Space Telescope are already challenging current models of exoplanetary atmospheres and interiors (e.g., Foley 2024). Conventionally, a habitable zone exoplanet orbits within a circumstellar region where liquid water could potentially exist on its surface (e.g., Hart 1979; Kasting et al. 1993). It is generally assumed that silicate weathering regulates atmospheric carbon dioxide (CO₂) to levels supporting liquid water, providing a stabilizing feedback on climate (Walker et al. 1981); otherwise, planetary habitability would be a matter of luck. This negative feedback underpins the concept of the circumstellar habitable zone (CHZ) and may play a critical role in climate regulation on water-bearing, tectonically active rocky exoplanets. Identifying evidence for a carbon cycle on exoplanets and verifying the validity of the habitable zone concept are key objectives for future research (e.g., Bean et al. 2017)—efforts that would benefit from a deeper understanding of the carbonate–silicate cycle. In particular, the interplay between global climate, atmospheric CO₂, and silicate weathering rates is not fully understood. While the role of continental weathering in this negative feedback has been extensively studied, seafloor weathering has received comparatively less attention despite its potential to be equally significant. For instance, during the Late Mesozoic—known for its hothouse climate state—seafloor weathering fluxes were comparable in magnitude to those of continental weathering (e.g., Coogan & Gillis 2013). Here, I explore the factors controlling basalt dissolution by applying the more mechanistic weathering model of Maher and Chamberlain (2014) to both the modern and Late Mesozoic upper oceanic crust.

The oceanic crust, through its formation, alteration, and subduction, is a key component of this geochemical cycle. Carbon is transferred from Earth’s mantle to surface reservoirs (e.g., the atmosphere and oceans) via volcanic outgassing. Concurrently, basalt dissolution and carbonate precipitation recapture dissolved carbon from seawater, storing it within the oceanic crust. Over geological timescales, subducted tectonic plates carry these carbonates back into the mantle reservoir, ultimately supplying carbon to volcanoes. Nonetheless, the primary controls on seafloor weathering rates remain debated. Most models have historically focused on the dependence of mineral dissolution kinetics on temperature and CO₂ concentrations, yet observational data suggest that both kinetic and thermodynamic factors govern global weathering fluxes, underscoring the need for models that can incorporate this dual control. Kinetic weathering models (e.g., Walker et al. 1981) fail to account for the changes in Earth’s weatherability through time (e.g., West et al. 2005). To address this, Maher & Chamberlain (2014) developed a solute transport model that integrates hydrological and tectonic influences and imposes a thermodynamic limit on weathering rates. However, this model has only occasionally been applied to continental weathering in exoplanet climate studies (e.g., Graham & Pierrehumbert 2020, 2024), and its relevance to seafloor weathering remains largely unexplored (Hakim et al. 2021).

In this work, I assessed the model’s sensitivity to key parameters by comparing its predictions with observed age-dependent carbon content in the upper oceanic crust (Gillis & Coogan 2011). I then extended the model into two dimensions to represent the evolving age distribution of Earth’s seafloor and examine its impact on global weathering fluxes. Simulations using this supply-limited seafloor weathering model successfully reproduce observed age-related trends in carbon content within Earth’s upper oceanic crust and provide further evidence that over 80% of carbonate formed within 20 Myr of crust formation (Gillis & Coogan, 2011; Albers et al. 2023). This model can also capture the higher CO₂ concentrations in Late Mesozoic crust compared to Cenozoic crust. Our results indicate that crustal age, permeability, porosity and fluid flow strongly influence weathering rates. These findings challenge the prevailing view that elevated temperatures primarily drove enhanced carbon uptake during the Late Mesozoic, emphasizing further the importance of incorporating geologic and hydrologic processes into climate models. Consequently, seafloor weathering emerges as a necessary process not only for understanding Earth’s past climate but also for interpreting future observations of potentially habitable rocky exoplanets.