Causes and consequences of mid-Proterozoic anoxia

Abstract

Evidence for low pO2 and a ferruginous ocean characterize the mid‐Proterozoic (1.8–0.8 Ga). Considerations of redox sources and sinks imply that generation of O2 via organic carbon (Corg) burial must be low to maintain a low pO2 atmosphere for geologically long intervals, yet low oxygen should result in increased Corg preservation. Marine export production must therefore be low to limit Corg burial and O2 generation. Formation of ferrous phosphate can buffer deepwater phosphate (Pi) to levels an order of magnitude or more below those in the modern ocean, limiting export production. Low deepwater Pi is consistent with the hiatus in sedimentary phosphorite deposits during the mid‐Proterozoic, and low pO2 limits formation of sedimentary iron deposits (BIF). We propose that low pO2 was maintained by P limitation resulting from ferrous phosphate buffering. The near‐absence of BIF and phosphorite deposition is direct and indirect consequences of the low pO2, respectively.

Earth’s oxygen cycle and the evolution of animal life

“…4). Relatively modest [Fe 2+ ] levels well below some recent predictions for ferruginous Proterozoic oceans (32) can significantly depress seasonal minima and maxima in dissolved [O 2 ], even at very high rates of biological productivity (Fig. 4B).…”

“…In addition, even modest amounts of Fe 2+ relative to some predictions for the Proterozoic ocean interior (18,32) would have been sufficient to depress seasonal minima in dissolved O 2 to levels that would be expected to have inhibitory metabolic effects on emerging metazoan organisms for much of Proterozoic time. Such temporal variability in redox structure, particularly in light of the spatial complexity of the long-term mean state oxygen landscape discussed above, would have provided an additional environmental constraint on the early evolution of animal life.…”

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…From a modelling perspective, Laakso and Schrag (2014) and Derry (2015) argued for low P levels in the Proterozoic through the establishment of an Fe trap -either through scavenging by Fe(III) particles or precipitation of ferrous phosphates, such as vivianite, Fe 3 (PO 4 ) 2 . Similarly, Reinhard et al (2017) presented a large compilation of P data from siliciclastic sediments and identified a four-fold increase in P burial at the start of the Cryogenian (~720 Ma), which they linked to progressive oxygenation of the Earth and loss of the deep-sea Fe trap.…”

“…Recently, greenalite has also been suggested as a primary precipitate that formed in the water column (Rasmussen et al, 2015(Rasmussen et al, , 2016 Europium anomalies in REE+Y patterns have been central in tracing Fe sources. Europium enrichment in chemical sedimentary rocks that precipitated from seawater indicates a strong influence of high-temperature hydrothermal fluids on the seawater dissolved REE+Y load (e.g., Klinkhammer et al, 1983;Derry andJacobsen, 1988, 1990;Bau and Dulski, 1996;Viehmann et al, 2015). Positive Eu anomalies in hydrothermal fluids are linked to the breakdown of plagioclase minerals within the volcanic rocks underlying hydrothermal vents, as plagioclase is intrinsically enriched in Eu relative to other REE+Y during igneous crystallisation (Schnetzler and Philpotts, 1970;Graf, 1977Graf, , 1978Fryer et al, 1979).…”

Food for early animal evolution

“…An increase in oxygen levels would cause this phosphate trap to disappear, resulting in a substantially increased phosphate flux into the oceans. At the same time, the low-oxygen and iron-rich waters of Proterozoic oceans constituted major sinks for seawater phosphate that would have been attenuated when oxygen levels increased 13 . Some models 11 predict that an increase in oxygen during global glaciation causes a selfsustaining state change in nutrient availability.…”