Abstract

The marine nitrogen cycle is dominated by redox-controlled biogeochemical processes and, therefore, is likely to have been revolutionised in response to Earth-surface oxygenation. The details, timing, and trajectory of nitrogen cycle evolution, however, remain elusive. Here we couple nitrogen and carbon isotope records from multiple drillcores through the Rooihoogte–Timeball Hill Formations from across the Carletonville area of the Kaapvaal Craton where the Great Oxygenation Event (GOE) and its aftermath are recorded. Our data reveal that aerobic nitrogen cycling, featuring metabolisms involving nitrogen oxyanions, was well established prior to the GOE and that ammonium may have dominated the dissolved nitrogen inventory. Pronounced signals of diazotrophy imply a stepwise evolution, with a temporary intermediate stage where both ammonium and nitrate may have been scarce. We suggest that the emergence of the modern nitrogen cycle, with metabolic processes that approximate their contemporary balance, was retarded by low environmental oxygen availability.

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.

Title: Nitrogen fixation sustained productivity in the wake of the Palaeoproterozoic Great Oxygenation Event

Authors: Genming Luo, Christopher Junium, Gareth Izon, Shuhei Ono, Nicolas Johannes Beukes, Thomas J. Algeo, Ying Cui, Shucheng Xie, Roger E. Summons

Source: Nature Communications 9(1):978

Published: March 2018