Fig. 5
Schematic model of the evolving marine nitrogen cycle in response to increasing atmospheric pO2. a shows the pre-GOE to early GOE (dissolved O2 content < 11 μM; first stage of Fennel et al.15), when the rate of nitrate loss via denitrification exceeded its replenishment via nitrification. Here, ammonium was relatively stable and potentially served as the main source of biologically available nitrogen. b depicts the nitrogen cycle after a further increase in environmental oxygen availability (dissolved O2 content > 11 μM; potentially second stage of Fennel et al.15), with quantitative nitrification and denitrification destabilising nitrate and ammonium. Microbial nitrogen fixation served as the main source of biologically available nitrogen. c depicts the contemporary nitrogen cycle dominated by nitrate in a predominantly oxygenated ocean. We hypothesise that a encompasses the upper Rooihoogte Formation to the lower Timeball Hill Formation, b encompasses the lower part of the upper Timeball Hill Formation and c represents the uppermost Timeball Hill Formation and following Lomagundi event. Geochemical evidence suggests that any rise in atmospheric oxygen was transient and confined to the Lomagundi interval, decreasing in its immediate aftermath55. Widespread deep-ocean oxygenation was a much later phenomenon beginning in the Neoproterozoic; therefore, c represents a transient state that was not seen for perhaps 1.5 billion years
