Tracing the stepwise oxygenation of the Proterozoic ocean

Area of Science:

• Geochemistry
• Paleoceanography
• Sedimentary geology

Background:

• Earth's atmosphere oxygenation occurred in two major steps during the Proterozoic eon.
• The Proterozoic ocean's oxidation state and deep-ocean oxygenation timing are crucial for life's evolution but poorly understood.

Observation:

• Authigenic molybdenum accumulation in black shales offers insights into ocean oxygenation.
• Molybdenum enrichments indicate weak continental weathering before 2,200 million years ago.
• Vigorous oxidative weathering began around 2,150 million years ago, after initial atmospheric oxygen rise.

Findings:

• Molybdenum accumulation in shales began by 2,650 million years ago.
• Persistent oxidative weathering commenced approximately 2,150 million years ago.
• Mid-Proterozoic molybdenum reservoir remained below 20% of modern levels due to euxinic conditions.

Implications:

• Limited molybdenum availability may have constrained eukaryotic evolution and expansion.
• Deep-ocean oxygenation by 551 million years ago expanded the oceanic molybdenum reservoir.
• Changes in ocean chemistry profoundly impacted early life's evolutionary trajectory.