Carbon isotope evidence for large methane emissions to the Proterozoic atmosphere

Pierre Cadeau,Carine Chaduteau,Magali Ader,Emilie Le Floc’H,Sophie D’Amore,Gérard Sarazin,Amandine Katz,Didier Jézéquel,Julia Guélard,Eric Fouilland,Christophe Leboulanger,Cécile Bernard,Vincent Milesi

doi:10.1038/s41598-020-75100-x

Abstract

The Proterozoic Era records two periods of abundant positive carbon isotope excursions (CIEs), conventionally interpreted as resulting from increased organic carbon burial and leading to Earth’s surface oxygenation. As strong spatial variations in the amplitude and duration of these excursions are uncovered, this interpretation is challenged. Here, by studying the carbon cycle in the Dziani Dzaha Lake, we propose that they could be due to regionally variable methane emissions to the atmosphere. This lake presents carbon isotope signatures deviated by ~ + 12‰ compared to the modern ocean and shares a unique combination of analogies with putative Proterozoic lakes, interior seas or restricted epireic seas. A simple box model of its Carbon cycle demonstrates that its current isotopic signatures are due to high primary productivity, efficiently mineralized by methanogenesis, and to subsequent methane emissions to the atmosphere. By analogy, these results might allow the reinterpretation of some positive CIEs as at least partly due to regionally large methane emissions. This supports the view that methane may have been a major greenhouse gas during the Proterozoic Era, keeping the Earth from major glaciations, especially during periods of positive CIEs, when increased organic carbon burial would have drowned down atmospheric CO2.

Highlights

Resulting from processes involving methanogenesis a ctivity[11,12], challenging the idea that they record a global C-cycle perturbation
We focus on this mechanism because, as previously suggested, it might be relevant for understanding the spatial and temporal variability of Proterozoic carbon isotopic excursions (CIEs), and because it is one of the potential sources of greenhouse gas to the atmosphere of the early Earth
We examine these questions through the lens of a unique thalasso-haline lake that exhibits extremely positive δ13C values both in its organic and inorganic carbon pools, and analogous in many ways to some Proterozoic restricted settings

Summary

Introduction

Resulting from processes involving methanogenesis a ctivity[11,12], challenging the idea that they record a global C-cycle perturbation. The methane carbon isotopic signature (δ13CCH4) was very negative with an average value of − 68 ± 3‰ and − 71 ± 6‰ for dissolved methane within the pore water sediment and the water column, respectively (Fig. 1, Table S2 and S3), and − 65‰ for methane degassing from the lake to the atmosphere (Table S4).

Results

Conclusion