Enhanced metamorphic CO2 release on the Proterozoic Earth

Abstract

Rock metamorphism releases substantial CO₂ over geologic timescales (>1 My), potentially driving long-term planetary climate trends. The nature of carbonate sediments and crustal thermal regimes exert a strong control on the efficiency of metamorphic CO₂ release; thus, it is likely that metamorphic CO₂ degassing has not been constant throughout time. The Proterozoic Earth was characterized by a high proportion of dolomite-bearing mixed carbonate-silicate rocks and hotter crustal regimes, both of which would be expected to enhance metamorphic decarbonation. Thermodynamic phase equilibria modeling predicts that the metamorphic carbon flux was likely ~1.7 times greater in the Mesoproterozoic Era compared to the modern Earth. Analytical and numerical approaches (the carbon cycle model PreCOSCIOUS) are used to estimate the impact this would have on Proterozoic carbon cycling and global atmospheric compositions. This enhanced metamorphic CO₂ release alone could increase pCO₂ by a factor of four or more when compared to modern degassing rates, contributing to a stronger greenhouse effect and warmer global temperatures during the expansion of life on the early Earth.

Keywords: Proterozoic; carbon cycle; metamorphism.

Fig. 1.

Effect of protolith composition and thermal regimes on metamorphic CO₂ release. (A) The degree of CO₂ released as a function of the initial calcite percentage shows that rocks with <~12% calcite by weight can achieve 100% decarbonation at moderate metamorphic grade. (B) A rock on the higher T/P path typical of Precambrian metamorphism (red) begins decarbonation at lower temperatures and achieves more total carbon loss than the same rock along a lower T/P path typical of the Phanerozoic. (C) A dolomitic rock will degas more CO₂ than a calcitic rock along the same low T/P path shown in panel B.

Fig. 2.

Changes in carbonate sedimentation and metamorphic P–T conditions. (A and B) data from ref. A shows the total proportion of mixed carbonate-silicate sediments in the rock record while dotted areas represent the contribution from rocks designated significant carbonate component in a siliciclastic rock only. (B) is the proportion of carbonate sediments which are dominantly dolomitic. (C) Modified after (4), shows the changing metamorphic P–T conditions up to and including the Cambrian period.

Fig. 3.

Precambrian metamorphic decarbonation. (A) The volume % of each sediment type among total carbonate-bearing sediments deposited cumulatively for each time interval (data from ref. 34). (B) The average metamorphic T/P ratio for each time interval (after ref. 35) within the 95% prediction envelope generated by ref. 4). (C) The magnitude of the metamorphic carbon flux. The mean metamorphic value is shown added to a constant volcanic flux. (D) The carbon isotope composition of the total solid Earth degassing flux (including volcanic sources) is shown for the mean simulation.

Fig. 4.

Atmospheric response to elevated CO₂ degassing, calculated for a range of solid Earth degassing rates and silicate weathering feedback strengths. (A) Analytical solutions for equilibrium pCO₂ (expressed as RCO₂, normalized to preindustrial CO₂ of 280 ppm), assuming long-term carbon cycle mass balance (Results). Note the solutions in panel a do not include reverse weathering. (B) Equilibrium RCO₂ calculated in the PreCOSCIOUS model with and without elevated reverse weathering reactions (ref. 41). Colors represent different silicate weathering feedback strengths; dashed lines include reverse weathering and solid lines omit it. Arrows indicate the percent change in solid-earth CO₂ degassing at the lowest (Archean) and highest (Mesoproterozoic) points relative to the Cambrian period in our mean model output.

Fig. 5.

The effect of changing the rate and δ¹³C of solid-earth CO₂ degassing on the δ¹³C isotopic mass balance of the exogenic carbon cycle, as recorded by sedimentary CaCO₃ δ¹³C. (A) the δ¹³C of metamorphic degassing will depend strongly upon the fraction of carbon degassed and weakly upon temperature assuming pure Rayleigh distillation. (B) Sedimentary CaCO₃ δ¹³C for each combination of total solid-earth degassing rate and δ¹³C was calculated by solving steady-state mass balance and isotopic mass balance equations describing the sources and sinks of carbon to the ocean–atmosphere system (e.g., refs. , , and 45). Silicate weathering and carbonate burial were assumed to balance changes in solid earth degassing, and for simplicity, organic carbon weathering and burial fluxes and the photosynthetic fractionation factor were fixed at modern values. Stars represent the mean value for five modeled geologic eras/periods.

Fig. 6.

Summary of enhanced metamorphic degassing model results. Panel A, the period between ~2000 and 1000 Ma is characterized by a higher solid Earth degassing flux (mean value and 25th to 75th percentile contours are shown). Modeled pCO₂ levels are shown for nSi = 0.2 without reverse weathering (short-dashed line) nSi = 0.2 with reverse weathering (solid line) nSi = 0.6 without weathering (dotted line) and nSi = 0.6 with reverse weathering (long-dash). The purple-shaded region shows the entire range of model outcomes at the 25th to 75th percentile range. Periods of elevated pCO₂ correspond to the “boring billion” period notably lacking global glaciations [after (48); Supercontinent assembly after (49)]. Panel B is a block diagram illustrating the proposed relationship between Proterozoic metamorphic and surface processes that may contribute to high pCO₂.