Redox stratification and anoxia of the early Precambrian oceans: Implications for carbon isotope excursions and oxidation events

An updated compendium of δ13C data offer compelling evidence that δ13C positive excursions of unsurpassed magnitude in the recorded Earth history (>8‰, and up to 18‰ PDB) occurred in the early Proterozoic (the Lomagundi event). Questions whether or not these unprecedented positive δ13C shifts were contemporaneous in various basins and represent local or global events remain unresolved. The framework of major geologic events that occurred in the Paleoproterozoic argues against a Snowball Earth scenario as a backdrop to these exceptional 13C-enrichments. Substantial increases of carbon flux ratio (Fo/Fc, eight times the Phanerozoic average) and organic carbon burial rate (Fo, three times the Phanerozoic average) are required to account for the observed 13C-enrichments under steady-state and dynamic equilibrium modes, respectively. These enhanced ratios and rates are conditional on the availability of a flux of nutrients to the contemporaneous biota producers in excess of the riverine flux, and a decoupling of the P and C cycles. It is argued that these two conditions were met between 2.25 and 2.11 Ga in a redox-stratified ocean with weak upwelling and sluggish meridional surface circulation. The alternative, that the major disturbance in the terrestrial carbon cycle occurred in a rapidly ventilated ocean, is assessed as being unlikely. A large pulse of O2 equivalent to six to seven times the present terrestrial oxygen budget was rapidly scavenged and spent in the oxidation of reduced Fe and S transported to the shelves by rivers. This contention is consistent with data inferring low oxygen levels (10−5 to 10−2 PAL) in the Paleoproterozoic atmosphere and the occurrence of ocean anoxia until the late Precambrian.