Volcanic CO₂ Triggered Ocean Acidification and Cooling During Ocean Anoxic Event 2 (OAE2)
Two complementary studies led by Northwestern University geoscientists shed new light on Ocean Anoxic Event 2 (OAE2), a major mid-Cretaceous perturbation of the global carbon cycle first co-discovered over 40 years ago. By analyzing preserved planktonic microfossils and bulk sediments from multiple sites worldwide, researchers gathered direct evidence that massive volcanic CO₂ emissions caused ocean acidification during the earliest stages of OAE2.
Key Findings
- Ocean Acidification and Plenus Cold Event: Acidification triggered by volcanic CO₂ may have caused a temporary cooling, known as the Plenus Cold Event, interrupting the otherwise extreme greenhouse conditions.
- Malformed Microfossils: Foraminifera fossils from Gubbio, Italy, showed “dwarfing” and other shell abnormalities, with calcium isotope analysis confirming stress from acidification.
- Deep-Sea Core Evidence: Sediments offshore southwest Australia revealed a sudden absence of carbonate minerals, indicating disrupted biomineralization and confirming ocean acidification.
- Volcanic Link: Shifts in osmium isotope ratios pinpointed massive submarine volcanic activity (large igneous provinces) as the primary source of CO₂, establishing a cause-and-effect relationship.
- Biological Feedback: Reduced calcification by plankton buffered ocean acidity and enhanced CO₂ absorption, partially explaining temporary cooling.
Implications
These studies illustrate how ocean chemistry, biology, and volcanism interact to shape Earth’s climate and carbon cycle. While temporary feedbacks helped stabilize OAE2 conditions, they came at a significant ecological cost, including marine extinctions. Researchers caution that current human-driven CO₂ emissions may trigger similar acidification and anoxia without the ocean’s natural buffering mitigating impacts in time.
“These geological analogs provide crucial insights for understanding how modern oceans might respond to large-scale CO₂ emissions,” said Northwestern’s Brad Sageman and Andrew Jacobson. “While the Earth system has stabilizing feedbacks, they come with a cost, reminding us that active mitigation is essential for today’s climate crisis.”
