Anaerobic oxidation of methane by Mn oxides in sulfate-poor environments

Chunfang Cai,Lei Jiang,Lianjun Feng,Kaikai Li,Cedric M John,Bin Fu,Hong He,Dawei Liu,Daowei Wang,Mojtaba Fakhraee

doi:10.1130/g48553.1

Abstract

Abstract Strongly 13C-depleted authigenic carbonates (e.g., δ13CVPDB &lt;−30‰; VPDB—Vienna Peedee belemnite) in nature are generally believed to form by sulfate-dependent anaerobic oxidation of methane (AOM). However, we demonstrate using geochemical data and thermodynamic calculation that such calcites are most likely derived from biogenic oxidation of methane in sulfate-poor, nonmarine environments during early diagenesis, as observed in the Triassic sandy conglomerates from the Junggar Basin, northwestern China. This process operated through preferential oxidation of 13C-depleted methane by Mn oxides in closed conditions, producing calcites with higher Mn contents and δ13C values in association with more 13C-enriched residual methane as a result of kinetic isotope fractionation. Thus, the Mn-rich and 13C-depleted carbonates are proposed as tracers of Mn-dependent AOM, which should have served as an important sink of greenhouse methane in low-sulfate early Earth's oceans.

Highlights

Methane is a greenhouse gas that is closely related to the global carbon cycle and climate change (Etiope et al, 2008)
13C-depleted authigenic carbonate is commonly interpreted to result from SO42−dependent anaerobic oxidation of methane (AOM; Drake et al, 2015)
Mn- and Fe-dependent AOM has been suggested to have the oxidative potential to oxidize a large amount of biogenic methane produced in the Proterozoic oceans (e.g., Beal et al, 2009)

Summary

Introduction

Methane is a greenhouse gas that is closely related to the global carbon cycle and climate change (Etiope et al, 2008). The sulfate ion is commonly thought to be the dominant electron acceptor, with >90% of total CH4 production from modern marine sediments consumed through a biogeochemical process involving consortia of methanotrophic archaea and sulfatereducing bacteria at the sulfate-methane transition zones (Boetius et al, 2000; Egger et al, 2016). 13C-depleted authigenic carbonate is commonly interpreted to result from SO42−dependent anaerobic oxidation of methane (AOM; Drake et al, 2015). Such a process might not have been efficient in removing methane during the Precambrian, owing to low seawater sulfate concentration (Bristow and Grotzinger, 2013; Fakhraee et al, 2019). Various additional electron acceptors such as Fe and Mn oxides may have been involved in AOM, CH4 + 4Mn2O3 + 15H+. 12C- and Mn-rich carbonates in the rock records (e.g., Neoproterozoic and Mesoarchean Mn-rich rocks) were widely described to be a result of the organoclastic Mn-oxide reduction (Liu et al, 2006; Ossa Ossa et al, 2018) or of meteoric water diagenesis in nonmarine sandstones as seen in both the San Joaquin (California, USA) and Junggar (northwestern China)

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Discussion

Conclusion