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

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 δ¹³C_org 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 δ¹³C_org 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 δ¹³C_org 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.