Timing and magnitude of the Lomagundi–Jatuli carbon isotope excursion

The rise of atmospheric oxygen during the Great Oxidation Event (GOE) (ca. 2.5 to 2.1 billion years ago) permanently transformed Earth's biogeochemical cycles. The chemistry of contemporaneous marine carbonates provides a window into operation of the carbon cycle across this transition. Specifically, carbonate rocks co-eval with the GOE preserve a large and long-lived positive carbon isotope ([Formula: see text]C) excursion, the Lomagundi-Jatuli excursion (LJE), that canonically is interpreted as an increase in organic matter burial linked to the oxygenation of Earth's surface. However, the cause, synchroneity, and global nature of the LJE remain contentious due to significant uncertainties in the excursion's timing and magnitude. These uncertainties stem from the incomplete, time-uncertain, and spatially variable nature of the shallow-water sedimentary record. Here, we use Bayesian inference to reconstruct Paleoproterozoic [Formula: see text]C from globally distributed stratigraphic observations. Our inference reaffirms that the LJE is a global excursion, although its expression varies locally, and provides revised estimates for its timing and magnitude. We find that [Formula: see text]C most likely began to increase at 2,445 Ma, subsequently returning to baseline values at 2,018 Ma. The most likely excursion peak occurs at 2,130 Ma, and it is very unlikely (5% probability) that peak [Formula: see text]C values exceeded 9.1[Formula: see text]. Altogether, our results indicate the LJE has an earlier onset, longer duration, and lower magnitude than previously thought. The initial [Formula: see text]C increase occurs before or contemporaneously with both the earliest rise of atmospheric O₂ and Paleoproterozoic "snowball" glaciations, hinting at a mechanistic link among the LJE, the GOE, and climate.