Enhanced ocean oxygenation during Cenozoic warm periods

Area of Science:

• Paleoceanography
• Marine Biogeochemistry

Background:

• Dissolved oxygen (O2) is vital for marine ecosystems and nutrient cycling.
• Ocean oxygen-deficient zones (ODZs) are thought to be expanding due to global warming, but model predictions are uncertain.
• Paleoclimate records offer insights into potential ODZ dynamics under sustained warmth.

Observation:

• Nitrogen isotopes from foraminifera indicate reduced water-column denitrification in the eastern tropical North Pacific during the Middle Miocene Climatic Optimum and Early Eocene Climatic Optimum.
• Denitrification is a process confined to oxygen-poor waters.

Findings:

• During past warm periods (MMCO and EECO), ODZs were contracted, not expanded.
• This contraction may be linked to reduced biological productivity in the tropical Pacific, lowering oxygen demand.
• Alternatively, enhanced deep-water ventilation could have increased deep-ocean oxygen levels.

Implications:

• Cenozoic ODZ behavior suggests that sustained global warming might eventually lead to ODZ contraction, challenging near-term projections.
• Understanding the mechanisms behind past ODZ changes is crucial for predicting future oceanographic shifts.
• The rate of ODZ contraction could vary from decades to millennia, depending on the underlying oceanic processes.