Abstract
Abstract Weathering of silicate rocks at a planetary surface can draw down CO2 from the atmosphere for eventual burial and long-term storage in the planetary interior. This process is thought to provide essential negative feedback to the carbonate-silicate cycle (carbon cycle) to maintain clement climates on Earth and potentially similar temperate exoplanets. We implement thermodynamics to determine weathering rates as a function of surface lithology (rock type). These rates provide upper limits that allow the maximum rate of weathering in regulating climate to be estimated. This modeling shows that the weathering of mineral assemblages in a given rock, rather than individual minerals, is crucial to determine weathering rates at planetary surfaces. By implementing a fluid-transport-controlled approach, we further mimic chemical kinetics and thermodynamics to determine weathering rates for three types of rocks inspired by the lithologies of Earth's continental and oceanic crust, and its upper mantle. We find that thermodynamic weathering rates of a continental crust-like lithology are about one to two orders of magnitude lower than those of a lithology characteristic of the oceanic crust. We show that when the CO2 partial pressure decreases or surface temperature increases, thermodynamics rather than kinetics exerts a strong control on weathering. The kinetically and thermodynamically limited regimes of weathering depend on lithology, whereas the supply-limited weathering is independent of lithology. Our results imply that the temperature sensitivity of thermodynamically limited silicate weathering may instigate a positive feedback to the carbon cycle, in which the weathering rate decreases as the surface temperature increases.
Highlights
Greenhouse gases such as CO2 are essential in raising Earth’s surface temperature (Kasting et al 1993; Kopparapu et al 2013)
We find that thermodynamic weathering rates of a continental crust-like lithology are about one to two orders of magnitude lower than those of a lithology characteristic of the oceanic crust
We show that when the CO2 partial pressure decreases or surface temperature increases, thermodynamics rather than kinetics exerts a strong control on weathering
Summary
Greenhouse gases such as CO2 are essential in raising Earth’s surface temperature (Kasting et al 1993; Kopparapu et al 2013). An important feature of the carbon cycle on Earth is the negative feedback of silicate weathering (e.g., Walker et al 1981; Berner et al 1983; Kump et al 2000; Sleep & Zahnle 2001; Abbot et al 2012; Foley 2015; KrissansenTotton & Catling 2017; Graham & Pierrehumbert 2020) This feedback buffers the climate against changes in stellar luminosity and impacts the extent of the habitable zone (e.g., Kasting et al 1993; Kopparapu et al 2013). We develop a silicate weathering model of the inorganic carbon cycle with key inclusion of lithology by applying the fluid-transport model of Maher & Chamberlain (2014) to fluid-rock reactions: CHILI (CHemical weatherIng model based on LIthology). The key philosophy behind this study is to investigate the extent to which the silicate weathering model may be generalized, beyond its Earthcentric origins, in order to apply it to rocky exoplanets with secondary atmospheres
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