Dynamic deep marine oxygenation during the Early and Middle Paleozoic.

Area of Science:

• Geoscience
• Paleoceanography
• Isotope Geochemistry

Background:

• The Early Paleozoic era saw significant animal diversification, often linked to well-ventilated oceans.
• However, the precise oxygenation history of deep Paleozoic oceans remains a subject of scientific debate.

Purpose of the Study:

• To reconstruct the deep marine oxygenation history from approximately 485 to 380 million years ago.
• To investigate the dynamic ventilation patterns of ancient oceans using thallium (Tl) isotopes.

Main Methods:

• Utilized thallium (Tl) isotope ratios in deep-marine mudrocks as a proxy for ocean oxygenation.
• Applied Tl isotope analysis to a global dataset of mudrocks, with a specific focus on the Road River Group in Yukon, Canada.
• Interpreted Tl isotope variations as indicators of seafloor manganese oxide burial, which is sensitive to bottom water oxygen levels.

Main Results:

• Reconstructed an oscillatory pattern in seawater Tl isotope ratios, indicating a dynamic ocean ventilation history.
• Identified a prolonged episode of deep ocean oxygenation between approximately 405 and 386 million years ago.
• Observed short-term oxygenation dynamics superimposed on a gradual, muted positive trend in ocean oxygenation throughout the Early and Middle Paleozoic.

Conclusions:

• Deep ocean ventilation during the Early and Middle Paleozoic was not constant but exhibited significant fluctuations.
• Sustained oxygen accumulation in global marine bottom waters likely occurred after the studied period, around 380 million years ago.
• The findings provide crucial insights into the environmental conditions supporting early animal life and evolution.