Neoarchaean oxygen-based nitrogen cycle en route to the Great Oxidation Event

Area of Science:

• Geochemistry
• Paleobiology
• Early Earth History

Background:

• Sedimentary nitrogen isotopes (δ¹⁵N) track redox processes and early oxygenation.
• The Great Oxidation Event (~2.45 Ga) shows no major δ¹⁵N shift, suggesting decoupled nitrogen and oxygen cycles.
• Positive δ¹⁵N values (2.8–2.6 Ga) indicate a precursor to oxygenation, but their origin is unclear.

Purpose of the Study:

• Investigate the origin of positive δ¹⁵N values in Precambrian rocks.
• Determine the timing and implications of early nitrogen cycle evolution.
• Identify evidence for cryptic oxygen cycles before atmospheric oxygen accumulation.

Main Methods:

• Analysis of nitrogen isotopic composition (δ¹⁵N) in 2.68-Gyr-old sedimentary rocks.
• Geochemical study of the Serra Sul Formation, Amazonian Craton.
• Interpretation of nitrogen cycling under early Earth redox conditions.

Main Results:

• Discovery of strongly positive δ¹⁵N values (>30‰) in 2.68-Ga marine sediments.
• Findings attributed to regional ammonium oxidation linked to a cryptic oxygen cycle.
• Evidence suggests oxygenic photosynthesis was active around 2.7 Ga.

Conclusions:

• The 'Nitrogen Isotope Event' marks early biogeochemical reorganization.
• This event preceded significant atmospheric oxygenation.
• It indicates the initial steps towards the Great Oxidation Event.