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Difficulty: MediumAligning Data and Predictions with Viewpoints

Students debate the extent of global ice cover during the Cryogenian period.

Student 1 (Hard Snowball Model)
During the glaciation, the entire Earth was encased in ice, including the equatorial oceans. Global ocean temperatures dropped drastically, and ice sheets reached thicknesses of up to 1 kilometer. The thick ice cover completely sealed the oceans, preventing any gas exchange between the seawater and the atmosphere and blocking sunlight from reaching the marine water column. Consequently, marine photosynthesis ceased, and biological activity was restricted to isolated hydrothermal vents on the deep seafloor.

Student 2 (Slushball Model)
While ice sheets covered the continents and mid-to-high latitude oceans, the equatorial region remained free of thick, continuous ice. Instead, the equator featured a dynamic band of open ocean water and thin, seasonal ice. This allowed sunlight to penetrate the surface waters and permitted continuous gas exchange between the ocean and the atmosphere. Marine photosynthetic organisms survived in these open equatorial environments throughout the glacial epoch.

A geochemist analyzes equatorial marine sediment samples deposited during the peak of the Cryogenian glaciation. The sediment shows continuous deposition of organic carbon derived from photosynthetic organisms, indicating active surface-ocean photosynthesis and uninterrupted atmospheric gas exchange. This finding, if true, would best support the viewpoint of which student?

  1. A
    Student 1, because a sealed ocean would facilitate the continuous deposition of photosynthetic carbon.
  2. B
    Student 1, because thick equatorial ice sheets would allow for uninterrupted gas exchange with the atmosphere.
  3. Student 2, because open equatorial waters would allow sunlight and atmospheric gases to reach marine organisms.Answer
  4. D
    Student 2, because a dynamic band of open water would prevent photosynthetic organisms from receiving nutrients.

Answer

The correct answer states that the finding supports Student 2 because open equatorial waters would allow sunlight and atmospheric gases to reach marine organisms.
The correct answer shows that the geochemist's finding supports Student 2. The sediment data indicates that active photosynthesis and atmospheric gas exchange occurred continuously. This directly aligns with Student 2's assertion that open equatorial waters and thin ice allowed sunlight and atmospheric gases to reach marine organisms, whereas it contradicts Student 1's claim that global thick ice completely blocked sunlight and prevented gas exchange.

Step-by-Step Solution

1
Identify the core claims of both student viewpoints regarding ice cover, photosynthesis, and gas exchange.
Student 1 claims the global ocean was completely covered in thick ice, preventing gas exchange and halting marine photosynthesis. Student 2 claims the equator had open water and thin ice, allowing gas exchange and sustaining photosynthesis.
Establishing the specific claims of each viewpoint is necessary to determine how new evidence aligns with them.
2
Analyze the new geological finding to determine what physical conditions and biological activities it indicates.
The sediment finding indicates active surface-ocean photosynthesis and continuous gas exchange between the ocean and the atmosphere during the peak of the glaciation.
Translating the new evidence into concrete physical parameters allows comparison with the students' models.
3
Compare the finding with each viewpoint to determine support or contradiction.
The finding contradicts Student 1's model (which predicts no photosynthesis or gas exchange) and supports Student 2's model (which predicts open water enabling photosynthesis and gas exchange).
This step identifies the correct alignment between the data and the competing hypotheses.

Key Concept

Aligning Data and Predictions with Viewpoints
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Topics

Aligning Data and Predictions with Viewpoints

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