The rise of oxygen and siderite oxidation during the Lomagundi event

The Paleoproterozoic Lomagundi Event is an interval of 130-250 million years, ca. 2.3-2.1 billion years ago, in which extraordinarily ¹³C enriched (>10‰) limestones and dolostones occur globally. The high levels of organic carbon burial implied by the positive δ¹³C values suggest the production of vast quantities of O₂ as well as an alkalinity imbalance demanding extremely low levels of weathering. The oxidation of sulfides has been proposed as a mechanism capable of ameliorating these imbalances: It is a potent sink for O₂ as well as a source of acidity. However, sulfide oxidation consumes more O₂ than it can supply CO₂, leading to insurmountable imbalances in both carbon and oxygen. In contrast, the oxidation of siderite (FeCO₃ proper, as well as other Fe²⁺-bearing carbonate minerals), produces 4 times more CO₂ than it consumes O₂ and is a common - although often overlooked - constituent of Archean and Early Proterozoic sedimentary successions. Here we propose that following the initial rise of O₂ in the atmosphere, oxidation of siderite provided the necessary carbon for the continued oxidation of sulfides, burial of organic carbon, and, most importantly, accumulation of free O₂. The duration and magnitude of the Lomagundi Event were determined by the size of the preexisting Archean siderite reservoir, which was consumed through oxidative weathering. Our proposal helps resolve a long-standing conundrum and advances our understanding of the geologic history of atmospheric O₂.