Relationships between CO₂, thermodynamic limits on silicate weathering, and the strength of the silicate weathering feedback

Journal Article · 27 January 2018 · Earth and Planetary Science Letters

DOI: https://doi.org/10.1016/j.epsl.2018.01.005 · OSTI ID:1419011

Winnick, Matthew J.  ^[1]; Maher, Kate ^[1]

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1. Stanford Univ., CA (United States). Dept. of Geological Sciences

Recent studies have suggested that thermodynamic limitations on chemical weathering rates exert a first-order control on riverine solute fluxes and by extension, global chemical weathering rates. As such, these limitations may play a prominent role in the regulation of carbon dioxide levels (pCO₂) over geologic timescales by constraining the maximum global weathering flux. In this study, we develop a theoretical scaling relationship between equilibrium solute concentrations and pCO₂ based on equilibrium constants and reaction stoichiometry relating primary mineral dissolution and secondary mineral precipitation. Here, we test this theoretical scaling relationship against reactive transport simulations of chemical weathering profiles under open-and closed-system conditions, representing partially and fully water-saturated regolith, respectively. Under open-system conditions, equilibrium bicarbonate concentrations vary as a power-law function of pCO₂(y =kxⁿ)where nis dependent on reaction stoichiometry and kis dependent on both reaction stoichiometry and the equilibrium constant. Under closed-system conditions, bicarbonate concentrations vary linearly with pCO₂ at low values and approach open-system scaling at high pCO₂. To describe the potential role of thermodynamic limitations in the global silicate weathering feedback, we develop a new mathematical framework to assess weathering feedback strength in terms of both (1) steady-state atmospheric pCO₂ concentrations, and (2) susceptibility to secular changes in degassing rates and transient carbon cycle perturbations, which we term 1st and 2nd order feedback strength, respectively. Finally, we discuss the implications of these results for the effects of vascular land plant evolution on feedback strength, the potential role of vegetation in controlling modern solute fluxes, and the application of these frameworks to a more complete functional description of the silicate weathering feedback. Most notably, the dependence of equilibrium solute concentrations on pCO₂ may represent a direct weathering feedback largely independent of climate and modulated by belowground organic carbon respiration.

• Research Organization: Stanford Univ., CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER); National Science Foundation (NSF)
• Grant/Contract Number: SC0014556; DOE-DESC0014556; EAR-1254156
• OSTI ID: 1419011
• Alternate ID(s): OSTI ID: 1548838
• Journal Information: Earth and Planetary Science Letters, Vol. 485, Issue C; ISSN 0012-821X
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English