Constraining the climate and ocean pH of the early Earth with a geological carbon cycle model

Abstract

The early Earth's environment is controversial. Climatic estimates range from hot to glacial, and inferred marine pH spans strongly alkaline to acidic. Better understanding of early climate and ocean chemistry would improve our knowledge of the origin of life and its coevolution with the environment. Here, we use a geological carbon cycle model with ocean chemistry to calculate self-consistent histories of climate and ocean pH. Our carbon cycle model includes an empirically justified temperature and pH dependence of seafloor weathering, allowing the relative importance of continental and seafloor weathering to be evaluated. We find that the Archean climate was likely temperate (0-50 °C) due to the combined negative feedbacks of continental and seafloor weathering. Ocean pH evolves monotonically from [Formula: see text] (2σ) at 4.0 Ga to [Formula: see text] (2σ) at the Archean-Proterozoic boundary, and to [Formula: see text] (2σ) at the Proterozoic-Phanerozoic boundary. This evolution is driven by the secular decline of pCO₂, which in turn is a consequence of increasing solar luminosity, but is moderated by carbonate alkalinity delivered from continental and seafloor weathering. Archean seafloor weathering may have been a comparable carbon sink to continental weathering, but is less dominant than previously assumed, and would not have induced global glaciation. We show how these conclusions are robust to a wide range of scenarios for continental growth, internal heat flow evolution and outgassing history, greenhouse gas abundances, and changes in the biotic enhancement of weathering.

Keywords: Precambrian; carbon cycle; ocean pH; paleoclimate; weathering.

Fig. 1.

Schematic of carbon cycle model used in this study. Carbon fluxes (Tmol C y⁻¹) are denoted by solid green arrows, and alkalinity fluxes (Tmol eq y⁻¹) are denoted by red dashed arrows. The fluxes into/out of the atmosphere–ocean system are outgassing, $F_{\text{out}}$, silicate weathering, $F_{\text{sil}}$, carbonate weathering, $F_{\text{carb}}$, and marine carbonate precipitation, $P_{\text{ocean}}$. The fluxes into/out of the pore space are basalt dissolution, $F_{\text{diss}}$, and pore-space carbonate precipitation, $P_{\text{pore}}$. Alkalinity fluxes are multiplied by 2 because the uptake or release of one mole of carbon as carbonate is balanced by a cation with a 2+ charge (typically Ca²⁺). A constant mixing flux, $J$ (kg y⁻¹), exchanges carbon and alkalinity between the atmosphere–ocean system and pore space.

Fig. 2.

Gray shaded regions are ranges assumed for selected model input parameters. (A) Range of continental growth curves assumed in our nominal model, $f_{\text{land}}$. Various literature estimates are plotted alongside the model growth curve (SI Appendix A). (B) Range of continental growth curves for an endmember of no Archean land; (C) range for biological enhancement of weathering histories, $f_{\text{bio}}$; and (D) range of internal heat flow histories, $Q$, compared with literature estimates (SI Appendix A).

Fig. 3.

Nominal model outputs. Gray shaded regions represent 95% confidence intervals, and black lines are the median outputs. (A) Ocean pH with the 95% confidence interval from Halevy and Bachan (29) plotted with red dashed lines for comparison. Our model predicts a monotonic evolution of pH from slightly acidic values at 4.0 Ga to slightly alkaline modern values. (B) Atmospheric pCO₂ plotted alongside proxies from the literature. (C) Global outgassing flux. (D) Mean surface temperature plotted alongside glacial and geochemical proxies from the literature. Our model predicts surface temperatures have been temperate throughout Earth history. (E) Continental silicate weathering flux. (F) Seafloor weathering flux plotted alongside flux estimates from Archean altered seafloor basalt. dep, deposit.

Fig. 4.

No Archean land endmember scenario. Panels A–F, lines, and, shadings are the same as in Fig. 3. (E) Continental weathering drops to zero in the Archean, but (F) seafloor weathering increases due to its temperature dependence to balance the carbon cycle. This causes an increase in surface temperature in the Archean, (D) but conditions are still temperate throughout Earth history. The evolution of (A) ocean pH and (B) pCO₂ are similar to the nominal model. dep, deposit.

Fig. 5.

Default continental growth range with imposed 100 ppm methane in the Proterozoic and 1% methane in the Archean. Panels A–F, lines, and, shadings are the same as in Fig. 3. (D) Temperature increases sharply in the Archean due to methane, but by less than what would be expected if pCO₂ were unchanged. In practice, there is a compensating decrease in atmospheric pCO₂, (B) which must occur to balance the carbon cycle. Otherwise temperatures would be too high and weathering sinks would exceed outgassing sources. Because pCO₂ is lower, Archean pH values are closer to circumneutral (A). dep, deposit.