Aerobic nitrogen cycle 100 My before permanent atmospheric oxygenation.

Area of Science:

• Geochemistry
• Paleoclimatology
• Microbial Evolution

Background:

• The Great Oxidation Event (GOE) marked a significant shift in Earth's redox balance, impacting microbial evolution and biogeochemistry.
• Previous understanding placed the aerobic marine nitrogen cycle's transition in sync with the GOE's final phase around 2.33 billion years ago (Ga).
• The GOE's dynamic, oscillatory nature and limited study scope create ambiguity regarding the nitrogen cycle's evolution before permanent oxygenation.

Purpose of the Study:

• To investigate the drivers and tempo of the GOE and its impact on Earth's biogeochemical cycles.
• To characterize the marine nitrogen cycle's evolution in the lead-up to the GOE using stable isotope geochemistry.
• To refine the timeline of early aerobic conditions and their influence on nitrogen cycling.

Main Methods:

• Analysis of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios.
• Sampling from the ~2.43 Ga Duitschland and ~2.33 Ga Rooihoogte formations across multiple drill cores.
• Correlation of isotopic data with paleoclimatic events and redox-sensitive element cycling.

Main Results:

• A significant negative carbon isotope excursion (6–8‰) in the Duitschland Formation indicates massive organic carbon oxidation linked to a snowball Earth event and an early oxygen pulse at 2.43 Ga.
• Consistently positive δ¹⁵N values (≤ +20.3‰) in the Duitschland Formation suggest an aerobic nitrogen cycle.
• A temporal shift in global δ¹⁵N records indicates an aerobic nitrogen cycle approximately 100 million years earlier than previously established.

Conclusions:

• The findings push back the timeline for an aerobic marine nitrogen cycle to at least 2.43 Ga, predating the commonly accepted GOE timeline.
• This study provides crucial evidence for an earlier and potentially more complex oxidation of Earth's surface.
• The results refine our understanding of the interplay between atmospheric oxygenation, paleoclimatic events, and the evolution of major biogeochemical cycles.