Onset of persistent surface ocean oxygenation during the Great Oxidation Event

Area of Science:

• Geochemistry
• Paleoceanography
• Earth Science

Background:

• The Great Oxidation Event (GOE) marked the rise of atmospheric oxygen (O2) after the Archean-Proterozoic transition.
• Understanding surface ocean oxygenation during the GOE is crucial for planetary oxygenation thresholds.
• Geochemical records offer insights into global ocean redox conditions during this critical period.

Purpose of the Study:

• To investigate surface ocean oxygenation during the Great Oxidation Event.
• To quantify the nature of ocean redox conditions shortly after atmospheric O2 accumulation.
• To utilize vanadium (V) isotope geochemistry to reconstruct past ocean conditions.

Main Methods:

• Analysis of vanadium (V) isotope ratios in 2.32-2.26-billion-year-old shales.
• Stratigraphic examination of samples from the Transvaal Supergroup, South Africa.
• Interpretation of V isotopes as a proxy for seawater redox conditions and V drawdown.

Main Results:

• Shales indicate a unidirectional transition in global ocean redox conditions.
• Around 2.32 Ga, anoxic environments dominated, leading to significant seawater V drawdown.
• A positive shift in V isotopes signifies the expansion of marine settings with ≥10 μM dissolved O2 in bottom waters.

Conclusions:

• Early ocean oxygenation during the GOE was likely restricted to shallow-water environments.
• The observed changes suggest widespread equilibration between shallow oceans and an oxygenated atmosphere.
• Vanadium isotopes provide a valuable tool for reconstructing paleoceanographic redox conditions.