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Difficulty: HardIdentifying Hypotheses and Beliefs

Onset of the Sturtian Glaciation

The Sturtian glaciation (approximately 717 million years ago) was one of the most severe ice ages in Earth's history, covering almost the entire planet in ice. Two hypotheses are proposed to explain the trigger for this "Snowball Earth" event.

Hypothesis 1
The glaciation was initiated by the rapid chemical weathering of the Franklin Large Igneous Province (LIP), a massive basaltic province erupted near the equator. The warm, wet equatorial climate accelerated the chemical weathering of the newly exposed basaltic rocks. This weathering consumed vast quantities of atmospheric carbon dioxide (CO2CO_2) through the reaction of silicate minerals with acid rain. This process drew down CO2CO_2 levels over several hundred thousand years to a critical threshold, triggering runaway ice-albedo cooling.

Hypothesis 2
The glaciation was initiated by the sulfur-rich explosive eruptions of the Franklin LIP. Because these eruptions occurred near the equator, intense equatorial convection carried massive quantities of sulfur dioxide (SO2SO_2) gas directly into the stratosphere. There, the SO2SO_2 reacted to form highly reflective sulfate aerosols. These aerosols remained suspended in the stratosphere for years, reflecting solar radiation back into space. This rapid, severe reduction in solar radiation caused global temperatures to plunge, initiating the glaciation within years, long before basalt weathering could significantly affect the atmosphere.

Based on the proposed hypotheses, proponents of Hypothesis 2 would most likely agree with which of the following statements regarding the timing and mechanism of the glaciation's onset?

  1. The glaciation began rapidly because sulfate aerosols in the stratosphere immediately reflected solar radiation back into space.Answer
  2. B
    The glaciation began rapidly because the equatorial climate allowed basalt weathering to draw down carbon dioxide levels within a few years.
  3. C
    The glaciation began gradually because the accumulation of sulfur dioxide in the troposphere required hundreds of thousands of years to impact global temperatures.
  4. D
    The glaciation began gradually because the chemical weathering of basaltic rocks slowly reduced the greenhouse effect over geological timescales.

Answer

The glaciation began rapidly because sulfate aerosols in the stratosphere immediately reflected solar radiation back into space.
According to Hypothesis 2, the glaciation was triggered by the sulfur-rich volcanic eruptions of the Franklin LIP. Proponents of Hypothesis 2 believe that the onset was rapid ('within years') because sulfur dioxide gas was injected into the stratosphere and converted to sulfate aerosols, which reflected solar radiation and caused a rapid temperature drop before carbon dioxide drawdown could occur.

Step-by-Step Solution

1
Identify the target hypothesis in the question.
The question asks about the beliefs of the proponents of Hypothesis 2.
Focusing on the correct viewpoint prevents confusing the claims of the two hypotheses.
2
Analyze Hypothesis 2 to determine its proposed timing and mechanism of glaciation.
Hypothesis 2 states the glaciation was initiated 'within years' (rapid) due to sulfur dioxide carrying into the stratosphere and forming reflective sulfate aerosols.
This establishes the core claim of Hypothesis 2 regarding how and how quickly the cooling began.
3
Evaluate the options against the established core claim of Hypothesis 2.
The option stating that the glaciation began rapidly due to sulfate aerosol reflection matches Hypothesis 2, while other options incorrectly attribute chemical weathering or tropospheric processes.
This identifies the correct answer and isolates the distractors based on their specific errors.

Key Concept

Identifying core claims and mechanisms within scientific hypotheses
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Topics

Identifying Hypotheses and Beliefs

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