A NON-DIAGENETIC ORIGIN OF THE LOMAGUNDI POSITIVE CARBON ISOTOPE EXCURSION

Carbonates deposited between ~2.3-2.1 Ga have markedly positive carbon isotope values, commonly reaching +10‰ and peaking above +20‰. If these values capture the primary composition of marine dissolved inorganic carbon, this event, referred to as the Lomagundi excursion, represents the most severe and long-lived perturbation to the Earth’s global carbon cycle and likely reflects greatly enhanced organic carbon burial. However, these carbon isotope values have also been attributed to widespread diagenetic carbonate formation associated with methanogenesis. These two models have profoundly different implications for the evolution of the carbon cycle and Earth’s oxygenation. We will provide an overview of the sedimentological and geochemical data that point toward a marine (non-diagenetic) origin of the Lomagundi positive carbon isotope values.

Given that the carbon and sulfur cycles are linked on both global and local scales, we will focus on using records of coeval seawater or porewater sulfate to help distinguish between the two models. Coupled carbon and carbonate-associated sulfate (CAS) sulfur isotope data for samples from Lomagundi-aged carbonate platforms point to enhanced primary productivity rather than carbonate precipitation in the methanic zone as the underlying mechanism for this excursion. CAS concentrations in Lomagundi-aged carbonates typically range from 50 to 500 ppm, within the range of typical ancient marine carbonates and consistent with a sulfate-rich system. Since porewater sulfate is typically depleted via bacterial sulfate reduction before the onset of methanogenesis, and given that CAS concentrations track ambient sulfate levels at the time of carbonate precipitation, our high concentrations argue against carbonate formation in the methanic zone. We have found a narrow range of only moderate ³⁴S-enrichments (δ³⁴S_ave =+14‰) in CAS of Lomagundi-aged carbonates, which is also inconsistent with carbonate precipitation in the methanic zone. Furthermore, CAS δ³⁴S values closely match those from coeval sulfate evaporites, suggesting that the carbonates record primary seawater signals and that C-S isotope systematics can be used to track the global C and S cycles.