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Difficulty: MediumIdentifying Core Claims and Hypotheses

### The Younger Dryas Cooling

The Younger Dryas was a period of abrupt cooling that occurred approximately 12,90012,900 years ago, temporarily reversing the warming trend after the last glacial maximum. Scientists debate the primary trigger of this event.

Hypothesis 1
Around 12,90012,900 years ago, a large cometary body fragmented and collided with Earth's atmosphere. This extraterrestrial impact caused widespread wildfires, rapid thermal shocks, and the sudden melting of large portions of the Laurentide Ice Sheet. The massive influx of freshwater into the North Atlantic Ocean reduced the salinity of the surface water. This dilution prevented the sinking of cold, dense water, shutting down the Atlantic Meridional Overturning Circulation (AMOC) and triggering global cooling.

Hypothesis 2
The cooling was initiated solely by terrestrial hydrological shifts, without any extraterrestrial influence. As the Laurentide Ice Sheet naturally retreated due to gradual solar warming, a massive ice dam holding back Lake Agassiz collapsed. This burst released an immense volume of freshwater directly into the Arctic Ocean and North Atlantic Ocean. The sudden decrease in ocean salinity disrupted the thermohaline circulation (AMOC), causing the rapid onset of the Younger Dryas cooling.

Hypothesis 3
The primary trigger was a period of intense volcanic activity. Multiple large-scale volcanic eruptions injected high volumes of sulfur dioxide (SO2\text{SO}_2) gas into the stratosphere. These gases reacted with water vapor to form highly reflective sulfate aerosols, which blocked incoming solar radiation and cooled the Northern Hemisphere. This atmospheric cooling led to increased sea ice extent, which secondarily altered ocean circulation currents like the AMOC, sustaining the cold period.

According to Hypothesis 2, the freshwater influx that disrupted the AMOC was directly caused by which of the following events?

  1. A
    The sudden melting of the Laurentide Ice Sheet caused by the thermal shock of a cometary impact
  2. The collapse of an ice dam holding back a large freshwater lake during natural glacial retreatAnswer
  3. C
    A sequence of massive volcanic eruptions injecting sulfur dioxide gas into the stratosphere
  4. D
    A gradual, natural decrease in solar radiation that directly cooled the Atlantic Ocean

Answer

The collapse of an ice dam holding back a large freshwater lake during natural glacial retreat
According to Hypothesis 2, the Younger Dryas cooling was triggered by natural glacial retreat causing the collapse of an ice dam holding back Lake Agassiz, which released massive volumes of freshwater into the oceans and disrupted the AMOC. This is a purely terrestrial hydrological explanation.

Step-by-Step Solution

1
Locate the portion of the passage describing Hypothesis 2.
Hypothesis 2 attributes Younger Dryas cooling to terrestrial hydrological shifts, specifically the retreat of the Laurentide Ice Sheet and the collapse of an ice dam holding back Lake Agassiz.
To identify the core mechanism proposed by the target hypothesis.
2
Compare the mechanisms described in the options with the core claims of Hypothesis 2.
The collapse of the ice dam matches the description in Hypothesis 2. The other options describe mechanisms from Hypothesis 1 (cometary impact) and Hypothesis 3 (volcanic sulfur dioxide).
To determine which option accurately represents the trigger identified in Hypothesis 2 and rule out distractors.

Key Concept

Identifying the central claim and causal mechanism of a specific scientific hypothesis.
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Topics

Identifying Core Claims and Hypotheses

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