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

• Published in: Precambrian research Vol. 137; no. 3; pp. 207 - 222
• Main Author: Aharon, Paul
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.06.2005
• Subjects: Anoxia; Carbon isotopes; Precambrian oceans; Redox stratification; Anoxia; Precambrian oceans; Carbon isotopes; Redox stratification
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2005.03.008

An updated compendium of δ 13C data offer compelling evidence that δ 13C 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 δ 13C 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 ( F o/ F c, eight times the Phanerozoic average) and organic carbon burial rate ( F o, 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 O 2 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.