Soru

Initiation of the Sturtian Glaciation

The Sturtian glaciation, which occurred approximately 717 million years ago, represents one of the most extreme ice ages in Earth's history, resulting in a "Snowball Earth" where ice covered nearly the entire planet. Two geologists discuss competing hypotheses for the trigger of this event.

Geologist 1
The Sturtian glaciation was initiated by the eruption of the Franklin Large Igneous Province (LIP), a massive volcanic field located in the tropics of the supercontinent Rodinia. The primary driver of cooling was the rapid chemical weathering of the freshly erupted, highly reactive basaltic rocks. Silicate weathering consumes atmospheric carbon dioxide ($CO_2$) through the reaction:

Because the Franklin LIP erupted in a warm, humid equatorial region, weathering rates were exceptionally high. This process sequestered $CO_2$ into marine carbonates at a rate that far exceeded volcanic outgassing, causing atmospheric $CO_2$ levels to plummet. The resulting reduction in the greenhouse effect cooled the planet, allowing polar ice sheets to expand and ultimately trigger a runaway ice-albedo feedback.

Geologist 2
Silicate weathering is a slow process that operates over millions of years, which is too gradual to trigger the rapid onset of a global glaciation. Instead, the glaciation was triggered by the stratospheric injection of sulfur dioxide ($SO_2$) gas during the explosive phases of the Franklin LIP eruptions. Once in the stratosphere, $SO_2$ reacted with water vapor to form highly reflective sulfate aerosols. Because these aerosols block incoming solar radiation, they caused immediate global cooling. This cooling allowed polar ice sheets to rapidly advance to a critical latitude of approximately $30^\circ$. At this point, the ice-albedo feedback became self-sustaining, driving the Earth into a global glaciation before the sulfate aerosols could settle out of the atmosphere.

Based on the passage, Geologist 1’s explanation of how the chemical weathering of the Franklin LIP initiated global cooling relies on which of the following underlying assumptions?