Lacustrine facies dependence of highly 13C-depleted organic matter during the global age of methanotrophy

• We examine the Neoarchean stable carbon isotope record.
• We find a correlation between highly 13C-depleted organic matter and restricted depositional environments.
• Some Neoarchean lakes may have been deficient in oxidants.
• Oxidant-deficient lakes may have selected for methane cycling microbial communities.

Highly 13C-depleted organic matter reported from Neoarchean formations worldwide has led to the concept of a “Global Age of Methanotrophy” (GAM) in the Neoarchean. A temporal peak in the GAM is suggested by values as low as −61‰ that are reported from rocks deposited at ∼2.7 Ga. Here we analyse previously reported values, report new field observations and isotope data, and re-evaluate the depositional settings of several units of this age. We find a statistically significant lowering of δ13Corg values in units of Neoarchean age compared to values reported from other Precambrian intervals, both older and younger, confirming the existence of the GAM. However, we also report a correlation between very low δ13Corg values and lacustrine units deposited during the Neoarchean. We hypothesize methanogenesis may have been promoted in some Neoarchean lakes due to local deficiencies of oxidants, specifically Fe3+ and SO4, relative to the Archean oceans. Lower availability of these oxidants could have limited higher energy yield metabolisms such as sulfate and iron reduction and provided an ecological niche for methanogens, ultimately resulting in the local burial of biomass highly depleted in 13C. We conclude that the exceptionally low δ13Corg values reported from formations deposited at ∼2.7 Ga could represent the prevalence of closed basin depositional environments preserved in the limited outcrop available, rather than a peak in the global age of methanotrophy at this time.