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Reverse weathering as a long-term stabilizer of marine pH and planetary climate

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Abstract

For the first four billion years of Earth’s history, climate was marked by apparent stability and warmth despite the Sun having lower luminosity1. Proposed mechanisms for maintaining an elevated partial pressure of carbon dioxide in the atmosphere (\({p}_{{{\rm{CO}}}_{{\rm{2}}}}\)) centre on a reduction in the weatherability of Earth’s crust and therefore in the efficiency of carbon dioxide removal from the atmosphere2. However, the effectiveness of these mechanisms remains debated2,3. Here we use a global carbon cycle model to explore the evolution of processes that govern marine pH, a factor that regulates the partitioning of carbon between the ocean and the atmosphere. We find that elevated rates of ‘reverse weathering’—that is, the consumption of alkalinity and generation of acidity during marine authigenic clay formation4,5,6,7—enhanced the retention of carbon within the ocean–atmosphere system, leading to an elevated \({p}_{{{\rm{CO}}}_{{\rm{2}}}}\) baseline. Although this process is dampened by sluggish kinetics today, we propose that more prolific rates of reverse weathering would have persisted under the pervasively silica-rich conditions8,9 that dominated Earth’s early oceans. This distinct ocean and coupled carbon–silicon cycle state would have successfully maintained the equable and ice-free environment that characterized most of the Precambrian period. Further, we propose that during this time, the establishment of a strong negative feedback between marine pH and authigenic clay formation would have also enhanced climate stability by mitigating large swings in \({p}_{{{\rm{CO}}}_{{\rm{2}}}}\)—a critical component of Earth’s natural thermostat that would have been dominant for most of Earth’s history. We speculate that the late ecological rise of siliceous organisms8 and a resulting decline in silica-rich conditions dampened the reverse weathering buffer, destabilizing Earth’s climate system and lowering baseline \({p}_{{{\rm{CO}}}_{{\rm{2}}}}\).

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Acknowledgements

This research was supported by the NASA Astrobiology Institute under Cooperative Agreement number NNA15BB03A issued through the Science Mission Directorate. We thank R. Zeebe for access to LOSCAR v.2.0.4. We also thank M. Zhao, K. Daviau and D. Pennman for model discussions.

Reviewer information

Nature thanks L. Coogan and Y. Godderis for their contribution to the peer review of this work.

Author information

Affiliations

  1. Department of Geology and Geophysics, Yale University, New Haven, CT, USA

    • Terry T. Isson
    •  & Noah J. Planavsky

Authors

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Contributions

T.T.I. conceived the research ideas, developed and analysed the model, and wrote the paper. N.J.P. contributed to discussion and writing.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Terry T. Isson.

Extended data figures and tables

  1. Extended Data Fig. 1 Conceptual model for reverse weathering as a stabilizer of the long-term global carbon cycle and climate.

    Reverse weathering regulates atmospheric \({p}_{{{\rm{CO}}}_{{\rm{2}}}}\) through the establishment of a stabilizing feedback with marine pH (a pH thermostat). This feedback operates in conjunction with the silicate weathering feedback (dependence of chemical weathering and erosion on temperature and rainfall) to stabilize climate. A more potent reverse-weathering buffer, which is to be expected under high-silica Precambrian conditions, greatly strengthens the pH buffering capacity of oceans.

  2. Extended Data Fig. 2 Saturation index of some common rock forming silicates in modern and Precambrian bottom waters.

    The baseline conditions of the dissolved species are Na = 0.48 M; Fe = 540 × 10−12 M; Mg = 52.7 mM; K = 10.2 mM; Al = 1 × 10−15 M; Ca = 10.3 mM. Dissolved silica and iron concentrations are varied between modern-like (blue) and estimated Precambrian-like conditions (yellow and red). Thermodynamic database is Geochemists Workbench 10.0 (https://www.gwb.com/pdf/GWB10/GWBessentials.pdf).

  3. Extended Data Fig. 3 An occurrence density plot of modern marine porewater dissolved silica levels.

    The plot is based on dissolved silica concentrations (n = 6,245) from 453 sediment cores globally48. The map plots dissolved Si levels with sediment depth (depth = 0 indicates the sediment–water interface). Warmer colours indicate elevated occurrences.

  4. Extended Data Fig. 4 Sensitivity analysis to marine weathering fluxes.

    Steady-state results for \({p}_{{{\rm{CO}}}_{{\rm{2}}}}\) and pH over the estimated range of marine weathering CO2 consumption flux (Fmw) after ref. 27. a, Simulating modern Earth states. Acidity release through reverse weathering is fixed at 1 Tmol yr−1. b, Simulating Precambrian Earth states with elevated reverse-weathering buffer at dissolved silica of 1.33 mM. Contours highlight the variation of volcanic outgassing rates in teramoles per year. Shaded areas indicate parameter space that is unattainable for a steady-state system (marine weathering exceeds the sum of CO2 fluxes from degassing and extent of reverse weathering).

  5. Extended Data Fig. 5 Literature compilation of greenalite (gr), minnesotaite (min) and stilpnomelane (stilp) in marine sedimentary units through time.

    a, Proportion of marine sediment coverage through time92. bf, Raw data for the minerals given, and data normalized to the proportion of marine sediment coverage.

  6. Extended Data Fig. 6 Example porewater profiles of DIC, pH, [H4SiO4] and organic (org) matter at steady state.

    Curves represent results from model runs at varying bottom seawater pH (6.6–7.4) and [H4SiO4] (1.00–2.21 mM) conditions in diffusional exchange with sediment porewaters in deep sea (ac), slope (d) and shelf (e) environments. Subscript ‘sw’ indicates seawater.

  7. Extended Data Fig. 7 Analysis of sensitivity to diagenetic model parameters.

    Baseline conditions (green) are the deep-sea environment, with marine [H4SiO4] = 2.21 mM and marine DIC = 0.030 M. Shown are sensitivity to sedimentation rate ω (yr−1), diffusion coefficient of [H4SiO4] (cm2 yr−1), organic matter rate constant korg (yr−1), reverse-weathering rate constant krw (yr−1), calcite rate constant kcalc (mol cm−3 yr−1) and the efficiency of water column P scavenging.

  8. Extended Data Fig. 8 Steady-state outputs for reverse weathering versus the given parameters.

    a, Atmospheric \({p}_{{{\rm{CO}}}_{{\rm{2}}}}\); b, marine pH; c, marine DIC; d, total alkalinity; e, calcite saturation, Ω. Frw is the total silica consumption flux and frw is the fraction of the consumption flux of the total marine silica input. Within panel e, shallow-water values are plotted in red and global ocean mean values in blue. Contours represent variations to Alk:Si. The strength of the silicate weathering feedback (nsi) is set at 0.3. The root of each curve depicts preindustrial estimates and a reverse-weathering silica flux of about 0.5 Tmol yr−1 (frw = 0.05)29.

  9. Extended Data Fig. 9 Sensitivity analysis to the strength of the silicate weathering feedback.

    nsi = 0.1–0.5. Data are from refs 11,20,46,116,117,118,119. The grey-shaded area highlights the range where nsi = 0.3 (Fig. 1). Contours represent variations to Alk:Si.

  10. Extended Data Fig. 10 The relationship between marine [H4SiO4] and \({p}_{{{\rm{CO}}}_{{\rm{2}}}}\) and marine pH at modern outgassing rates.

    The relationship between marine [H4SiO4] and \({p}_{{{\rm{CO}}}_{{\rm{2}}}}\) and marine pH at modern outgassing rates.

Supplementary information

  1. Supplementary Tables 1-2

    This file contains Supplementary Table 1, a list of Model parameters and Supplementary Table 2 the sedimentary record of authigenic clays.

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DOI

https://doi.org/10.1038/s41586-018-0408-4

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