Carbon isotopic evidence for ecological restructuring of carbonate platforms across the Great Oxidation Event

The Lomagundi-Jatuli Event (LJE; ca. 2.2−2.0 Ga) is the largest-magnitude and longest-duration positive carbonate carbon isotope excursion recorded in the sedimentary record. Of the globally deposited locations that express the LJE, exceptionally high δ13Ccarb values (up to +28‰) in the Nash Fork Formation in Wyoming (United States) have been interpreted as evidence for extreme organic carbon burial and possible atmospheric O2 overshoot during the Great Oxidation Event (GOE; ca. 2.4 Ga). We present new high-resolution, phase-specific isotope data from shallow-water dolostones of the Nash Fork Formation. In the basal 22 m, δ13Ccarb values vary from 21.9‰ to 30.5‰ with no systematic stratigraphic trends, and exhibit texturally controlled heterogeneity of up to 5‰ at sub-decimeter scale. Up section, both the magnitude and spatial variability of δ13Ccarb values decrease, despite consistent shallow-water peritidal carbonate facies. These patterns are inconsistent with a well-mixed global carbon reservoir and instead indicate strong local controls on dissolved inorganic carbon composition. We interpret these extreme and heterogeneous δ13Ccarb values to reflect microbial carbon cycling dominated by methanogenesis during the LJE, rather than exceptionally high rates of global organic carbon burial. Subsequent dampening of both δ13Ccarb values and variability within similar facies may reflect a secular increase in the contribution of oxic respiration across the GOE.