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Difficulty: MediumIdentifying Core Claims and Hypotheses

### Tectonic Gateways vs. Atmospheric Carbon Dioxide

About 34 million34\text{ million} years ago, Earth's climate transitioned from a warm "greenhouse" state to a cold "icehouse" state, resulting in the rapid glaciation of Antarctica. Two scientists debate the primary cause of this cooling event.

Scientist 1
The primary driver of the Eocene-Oligocene transition (EOT) cooling was the opening of tectonic gateways, specifically the Drake Passage and the Tasmanian Gateway. As Australia and South America drifted away from Antarctica, a continuous Antarctic Circumpolar Current (ACC) was established. This current thermally isolated Antarctica by preventing warm tropical waters from reaching high southern latitudes. The resulting drop in regional temperature initiated the growth of the Antarctic ice sheet, which increased Earth's albedo and triggered global cooling. Atmospheric CO2\text{CO}_2 drawdown occurred as a consequence, rather than the primary driver, of this glaciation due to changes in ocean circulation patterns.

Scientist 2
The opening of tectonic gateways played a negligible role in the EOT cooling. Instead, the transition was driven primarily by a significant decline in global atmospheric CO2\text{CO}_2 concentrations, which fell below a critical threshold of approximately 600 ppm600\text{ ppm} (parts per million). This decline was caused by increased silicate weathering of continental rocks, which absorbed carbon dioxide from the atmosphere and deposited it in marine sediments. As CO2\text{CO}_2 levels dropped, the greenhouse effect weakened globally, allowing temperatures at both poles to fall sufficiently for continental glaciation to begin. The establishment of the ACC was an effect of the cooling and ice sheet growth, which rearranged surface winds and ocean currents, rather than its cause.

Based on the passage, which of the following statements best describes the core claim of Scientist 1 regarding the factor that directly initiated Antarctic glaciation?

  1. The tectonic separation of Antarctica from other landmasses, which established the Antarctic Circumpolar Current and thermally isolated the continent.Answer
  2. B
    A decrease in atmospheric carbon dioxide levels below 600 ppm600\text{ ppm} driven by the weathering of continental silicate rocks.
  3. C
    An increase in Earth's albedo that resulted from altered global wind patterns and ocean circulation changes.
  4. D
    A global reduction in the greenhouse effect that caused both polar regions to cool simultaneously.

Answer

The tectonic separation of Antarctica from other landmasses, which established the Antarctic Circumpolar Current and thermally isolated the continent.
Scientist 1 explicitly states that the primary driver of the cooling was the opening of tectonic gateways, which established the Antarctic Circumpolar Current. This current thermally isolated Antarctica from warm tropical waters, directly initiating regional temperature drops and ice sheet growth. Thus, the tectonic separation and resulting circumpolar current represent Scientist 1's core claim.

Step-by-Step Solution

1
Identify the specific scientist being questioned and locate their viewpoint in the passage.
The question asks for the core claim of Scientist 1. Scientist 1's section describes the tectonic gateways (Drake Passage and Tasmanian Gateway) opening as South America and Australia drifted away.
This narrows the focus to the arguments presented only by Scientist 1.
2
Analyze the causal chain proposed by Scientist 1 to find the primary initiating factor.
Scientist 1 claims: Opening of tectonic gateways -> Establishment of Antarctic Circumpolar Current (ACC) -> Thermal isolation of Antarctica -> Regional temperature drop -> Ice sheet growth (glaciation).
This chain shows that the physical separation and resulting circumpolar current are the primary drivers in Scientist 1's hypothesis.
3
Evaluate the answer choices against this identified core claim.
The option describing tectonic separation, the circumpolar current, and thermal isolation matches Scientist 1's primary claim.
It correctly identifies the initiating event in Scientist 1's causal explanation.

Key Concept

Identifying Core Claims and Hypotheses
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Topics

Identifying Core Claims and Hypotheses

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