282-1 The Proterozoic Earth: A Perfect World for Metamorphic Decarbonation?

Session: Crustal Petrology, Part II

Presenting Author:

Authors:

Abstract:

When carbonate-bearing sediments are heated during metamorphism, they have the potential to release CO₂ and, in turn, interact with the long-term carbon cycle, surface climate, and planetary habitability. A number of processes can enhance the efficiency of metamorphic carbon release: (1) decarbonation reactions are driven forward at high temperatures but suppressed by high pressures. Therefore, a higher T/P ratio should favor a greater degree of decarbonation. (2) Most decarbonation reactions require both carbonate and silicate material as reactants, so a higher silicate to carbonate ratio will allow a higher percentage of the reactant carbonate to be consumed. Finally, (3) dolomite-consuming reactions tends to occur at lower temperatures and require less silicate reactant than calcite-consuming reactions; thus, we expect dolomitic sediments to decarbonate more readily than comparable calcitic rocks.

The Proterozoic Earth likely possessed (1) higher metamorphic T/P ratios (Brown 2006), (2) more mixed carbonate-silicate sediments (e.g.,Grotzinger and James 1986; Cantine et al. 2020), and (3) more dolomitic carbonates (e.g., Ronov 1969) when compared with Earth today. We can speculate that the Proterozoic Earth was a perfect world for maximum metamorphic decarbonation.

We undertook thermodynamic modeling of representative carbonate sediments at realistic P-T conditions across the Archean, Proterozoic, and Cambrian. Holding all else equal, the predicted change in metamorphic decarbonation efficiency during the Mesoproterozoic would drive a metamorphic carbon flux ~1.7x greater than on the modern Earth. Based on numerical modeling (the carbon cycle program PreCOCIOUS from Penman & Rooney, 2019), corresponding Mesoproterozoic atmospheric CO₂ concentrations would be increased by a factor of four or more (Stewart & Penman 2024). We suggest that enhanced metamorphic decarbonation could explain observations of a warm, deglaciated Earth from ~1 to ~2 billion years ago, despite the lower luminosity of a faint-ish young-ish sun. Future work could explore this hypothesis with petrologic, field-based approaches.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10575

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