Carbon Carbon isotope compositions (expressed as δ C) in sediments have been interpreted to reflect the balance 13 between carbon burial as carbonate and as organic matter. For most of Earth's history, the δ C of carbonates has varied little around 0‰, which is representative of 20% of carbon buried as organic matter and the 4 13 remainder as carbonate . In comparison, the δ C of carbonate rocks is 8 -15‰ across the Lomagundi Event. These anomalously high values require an enormous amount of organic carbon burial relative to carbonate burial. However, as organic carbon production generates oxygen, burial of this quantity of organic carbon would mean oxygen production in volumes that are far greater than the concentrations even in today's relatively oxygen-rich atmosphere. A number of hypotheses have been proposed to address this seemingly massive 5 6 oxygen imbalance. These range from the weathering of sulfides and/or siderites to globally distributed pulses 13 7 of high- δ C carbonate production in sediments ; however, these hypotheses either treat the Lomagundi Event as mechanistically unrelated to the Great Oxidation Event (GOE) or rely on a slowly growing atmospheric O 2 reservoir that triggers large positive feedbacks in surface oxidation reactions: triggers and feedbacks that remain speculative. 13 13 In using δ C as a tracer of organic and carbonate carbon burial, a central assumption is that the δ C composition of volcanic carbon released to the ocean and atmosphere is invariant at -5‰. However, a compilation of carbon isotope data from volcanic CO suggests that this value may change depending on the 2 source of volcanic CO , which can vary between decarbonized carbonates, oxidized organic matter and 2 8 primitive mantle . 13 Eguchi and colleagues build this potential for variable volcanic δ C composition into a carbon cycle model. Their model tracks how differential cycling of subducted carbonate and organic carbon after the GOE would 13 modify the δ C of volcanic carbon in the following hundreds of millions of years. They posit that a major tectonic trigger - such as a switch from stagnant-lid to plate tectonics - dramatically increased the volcanic 9 flux of CO to the atmosphere , resulting in a nearly immediate, large increase in the flux of weathered 2 materials to the ocean and increased carbonate burial. Concomitantly, these weathered materials delivered phosphorus to the ocean, spurring primary productivity and increasing the flux of organic carbon that is buried in marine sediments. These carbonate- and organic-carbon-rich sediments would have experienced vastly different trajectories through the mantle as they were subducted (Fig. 1). In the hotter mantle of the Paleoproterozoic Earth, 13 carbonates would have been efficiently decarbonized, with the resulting high- δ C CO returned to the 2 atmosphere along island and continental arcs, thereby explaining the anomalously high carbon isotope values of the Lomagundi event. By contrast, the remaining organic carbon in the slab would be converted to refractory graphite or diamond and transported deeper into the mantle. This carbon would only return to the atmosphere much later, once this slab material has been entrained in deep-seated mantle plumes and the carbon oxidized and emitted at ocean is land volcanoes. In Eguchi and colleagues' framework, the relatively quick return of 13 carbonate carbon in arc volcanoes causes the prolonged period of high δ C in the ocean - atmosphere carbon 13 13 reservoir; the δ C Lomagundi excursion only comes to an end once the lower δ C organic carbon that is stored in the lower mantle is returned to the atmosphere (on the hundred-million-year timescale of whole mantle mixing). Although simple, the framework proposed by Eguchi and colleagues links the Great Oxidation and Lomagundi events with no assumption regarding the amount of carbonate or organic carbon burial; however, as with any simple model, there remain questions. The envisioned tectonic forcing is enormous and requires a one- 10 thousand-fold increase in volcanic CO emissions. Even with an efficient weathering feedback , such an 2 increase would have dramatically increased the radiative forcing due to CO , potentially conflicting with 2 evidence for large-scale glaciations shortly after the GOE. Furthermore, the change in radiative forcing during the switch from a reducing to oxidized atmosphere would have probably modified the scaling between silicate weathering and climate, which is difficult to capture in a simple model of the silicate weathering response to 13 climate. Even with this large tectonic forcing, the maximum δ C excursion that is estimated by this carbon 13 cycle model is 5‰, only one -third of the maximum observe d δ C change that is recorded in sediments. Perhaps 13 additional processes that are not represented in this model further modified δ C. Finally, the residence time of carbonate and organic carbon in the mantle is treated as a single number, whereas the return of these carbon reservoirs to the atmosphere is probably more accurately described as a distribution that would change the shape 13 of the modelled δ C excursion.