Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean

Area of Science:

• Geochemistry
• Marine Biogeochemistry
• Paleoceanography

Background:

• Earth's atmosphere was oxygenated ~2 Gyr ago, but deep oceans remained anoxic until the Neoproterozoic (~0.55 Ma).
• Marine sediments indicate ferruginous (Fe(II)-rich) waters with intermittent euxinia (H2S-rich) on continental margins during this period.
• Modern euxinia is rare, requiring nitrate depletion for sulfate reducers to be outcompeted by denitrifiers, and linked to nitrogen-fixing production.

Purpose of the Study:

• To model a Proterozoic coastal upwelling zone and understand the feedbacks governing ocean chemistry.
• To explain the spatial and temporal separation of different biogeochemical states in the mid-Proterozoic ocean.
• To reconcile varying hydrogen sulfide (H2S) concentrations with existing geochemical data.

Main Methods:

• Utilized a simple box model simulating a generic Proterozoic coastal upwelling system.
• Incorporated biogeochemical feedbacks, including nitrate (NO3-) availability, denitrification, and nitrogen (N2)-fixation.
• Analyzed the model's output to identify distinct ocean states and their transitions.

Main Results:

• The model demonstrated a spatial/temporal separation between two ocean states: nitrate presence with denitrification in deeper waters, and nitrogen-fixation-driven production above euxinic conditions.
• These two states represent a dynamic interplay in the Proterozoic ocean's biogeochemistry.
• The model explains the fluctuating H2S concentrations observed in geological records.

Conclusions:

• The mid-Proterozoic ocean exhibited a complex biogeochemical system with distinct zones driven by nutrient cycling and oxygen levels.
• The interplay between denitrification and nitrogen fixation created conditions that allowed for intermittent euxinia.
• This system persisted until the Neoproterozoic oxygenation event, which fundamentally altered marine biogeochemistry to its modern state.