[B-CG06] Decoding the history of Earth: From Hadean to the present

[BCG06-08] Subglacial weathering during a snowball event could affect the carbon cycle

*Shintaro Kadoya¹, Mohit Melwani Daswani² (1.Earth-Life Science Institute, Institute of Science Tokyo, 2.Jet Propulsion Laboratory, California Institute of Technology)

Keywords:Snowball Earth, Carbon cycle

Earth has been globally ice-covered at least three times, which are known as snowball Earth events. Under global glaciations, CO₂ uptake via continental silicate weathering has traditionally been considered to cease due to the absence of liquid water, leading to an accumulation of atmospheric CO₂ and, eventually, the melting of the global ice cover. However, recent studies suggest that continental weathering could occur under global glaciation conditions. This syn-glacial (sub-glacial) weathering could prevent the atmospheric CO₂ level from rising as expected, influencing the duration of the global glaciation, though its impact is still less understood. In this study, we modeled chemical interactions between minerals and water under limited water and rock supply conditions to investigate the key controlling factors and processes of discharging and precipitations associated with sub-glacial weathering. Our calculations indicate that dissolved concentrations and mineral precipitations reached at their steady state over a timescale of ~10⁵ years. Since this timescale is shorter than the duration of snowball glaciations (~ 10⁶ years), the sub-glacial weathering during a global glaciation would also be expected to reach a steady state condition under a global glaciation. Under steady-state conditions, a given ratio between the water supply and fresh rock supply results in the same dissolved concentrations and precipitated mineral compositions. Thus, this ratio would be a key controlling factor for sub-glacial weathering. Additionally, the discharge flux of dissolved species depends on the water supply rate, while the precipitation flux of a mineral depends on the rock supply rate. Assuming the precipitation rate (10—100 mm/yr) and the erosion rate (~ 0.01 mm/yr), based on modern Antarctica, and considering an entirely ice-covered continent, the global CO₂ consumption rate via sub-glacial weathering is estimated to be on the order of 0.1—1 Tmol C/yr. A smaller ice-covered continental area results in a lower CO₂ consumption rate, while a higher precipitation rate would lead to greater CO₂ consumption. Given that the global CO₂ degassing rate is ~ 10 Tmol/yr, syn-glacial weathering could influence the carbon cycle during a global glaciation, potentially delaying deglaciation. Ice-covered rock is a common planetary condition, observed on ancient Mars and icy satellites such as Titan and Ganymede. Therefore, this study provides insights into the chemical evolution of these celestial bodies.