Passage V

Scientists discuss 2 possible causes of an event 250 million years ago (Ma) in which 90% of all marine species became extinct.

Scientist 1

The extinctions were caused by continental flood vulcanism, a massive volcanic eruption lasting 1 million years that produced 2 million km³ of lava. The eruption sent large amounts of sulfate-containing (SO₄) aerosols into the air. The aerosols combined with water vapor in the air to produce acid rain that fell worldwide and poisoned many bodies of water. SO₄-containing aerosols also helped break down ozone in the atmosphere, permitting high levels of ultraviolet light to reach the surface.

The eruption also released large amounts of carbon dioxide (CO₂) into the air. The higher-than-typical CO₂ levels in the air raised the CO₂ content of ocean surface waters to levels that were toxic to many marine organisms. The increased levels of CO₂ also caused climatic warming that decreased the temperature difference between the poles and the equator, thus slowing ocean circulation and causing ocean water to become oxygen-poor.

Scientist 2

The extinctions were caused by an overturning of deep, CO₂-rich ocean waters.

No continental ice sheets were present on Earth just prior to 250 Ma. Today, continental ice sheets help cool ocean surface waters, which then sink and drive the ocean’s vertical circulation. Since this circulation was absent 250 Ma, the ocean water was stagnant and oxygen-poor.

Photosynthetic organisms living in ocean surface waters removed CO₂ from the atmosphere, converting it to organic matter. This organic matter sank to the ocean bottom, where it was oxidized to CO_2. As a result, the CO₂ levels in the deep ocean waters rose dramatically, while atmospheric CO₂ levels continued to drop, causing climatic cooling. Glaciers and ice sheets grew rapidly, cooling the ocean surface waters such that vertical circulation of the ocean began. Deep ocean water was brought to the surface, where it released the accumulated CO₂, dramatically increasing atmospheric and surface water concentrations of CO₂. This excess CO₂ was toxic to many marine organisms.