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Difficulty: Very hardIdentifying Underlying Assumptions and Premises

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 (CO2CO_2) through the reaction:

CaSiO3+CO2→CaCO3+SiO2CaSiO_3 + CO_2 \rightarrow CaCO_3 + SiO_2

Because the Franklin LIP erupted in a warm, humid equatorial region, weathering rates were exceptionally high. This process sequestered CO2CO_2 into marine carbonates at a rate that far exceeded volcanic outgassing, causing atmospheric CO2CO_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 (SO2SO_2) gas during the explosive phases of the Franklin LIP eruptions. Once in the stratosphere, SO2SO_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∘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?

  1. The rate of chemical weathering did not decrease with falling temperatures quickly enough to halt the drawdown of CO2CO_2 before the ice-albedo feedback became self-sustaining.Answer
  2. B
    The outgassing of CO2CO_2 from the Franklin LIP eruptions was entirely suppressed, ensuring that no greenhouse warming occurred prior to the weathering phase.
  3. C
    Sulfate aerosols from volcanic eruptions are incapable of causing significant global cooling over short timescales.
  4. D
    The Franklin LIP was located at high polar latitudes during the time of eruption, maximizing the rate of physical weathering.

Answer

The rate of chemical weathering did not decrease with falling temperatures quickly enough to halt the drawdown of CO2CO_2 before the ice-albedo feedback became self-sustaining.
The correct answer correctly identifies the geoclimatic assumption behind Geologist 1's hypothesis. Silicate weathering is a negative feedback loop; as temperatures fall, chemical reactions slow down and precipitation decreases, which reduces weathering rates and stops further cooling. For Geologist 1's mechanism to successfully plunge the Earth into a global glaciation, the geologist must assume that the weathering rate did not decrease so rapidly with cooling that the carbon dioxide drawdown was arrested before the ice sheets expanded enough to trigger the runaway ice-albedo feedback.

Step-by-Step Solution

1
Identify the primary mechanism proposed by Geologist 1.
Geologist 1 proposes that the chemical weathering of the equatorial Franklin LIP basalt drew down atmospheric CO2CO_2, reducing the greenhouse effect and cooling the Earth.
This establishes the core sequence of events leading to the glaciation according to the first geologist's viewpoint.
2
Analyze the feedback systems inherent in the proposed mechanism.
Chemical weathering of silicates is a temperature-dependent chemical reaction that slows down as temperatures drop, which acts as a stabilizing negative feedback loop (the silicate weathering thermostat).
Understanding the physical constraints of the proposed mechanism is necessary to identify implicit assumptions.
3
Determine the necessary condition for a runaway glaciation to occur via this mechanism.
For the temperature drop to continue to the point of a runaway ice age, the CO2CO_2 drawdown must continue despite the cooling temperatures. Thus, the weathering rate must not have dropped so rapidly that it halted the drawdown before the ice sheets reached the threshold for self-sustaining ice-albedo feedback.
This reveals the underlying assumption required for Geologist 1's hypothesis to hold true under basic laws of geochemistry.

Key Concept

Identifying implicit geological and physical assumptions in scientific hypotheses
Estimated Time:2m 0s
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Topics

Identifying Underlying Assumptions and Premises

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