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Question

Difficulty: HardNecessary Assumption

Paleoecologist: Marine sediment cores from the Subarctic Pacific show that during historical epochs characterized by high atmospheric dust deposition, the ratio of preserved giant virus DNA fragments to diatom microfossils was more than four times higher than during dust-poor epochs. Because atmospheric dust delivers bioavailable iron that accelerates diatom reproduction, researchers concluded that iron enrichment intensifies the rate of viral infection among diatoms.

Which one of the following is an assumption on which the researchers' conclusion depends?

  1. A
    Atmospheric dust deposition is the only mechanism by which bioavailable iron is introduced into the Subarctic Pacific.
  2. B
    Any marked increase in the rate of viral infection among diatoms in subpolar regions is caused by an influx of atmospheric dust.
  3. The giant virus DNA fragments detected in the sediment cores did not predominantly originate from viruses that infect marine organisms other than diatoms.Answer
  4. D
    Chemical conditions in the sediment did not cause any degradation whatsoever of the diatom microfossils deposited during dust-rich epochs.
  5. E
    Diatom reproduction in dust-poor epochs was constrained primarily by temperature rather than by a lack of bioavailable iron.

Answer

The argument depends on the assumption that the giant virus DNA fragments detected in the sediment cores did not predominantly originate from viruses that infect marine organisms other than diatoms.
The conclusion attributes the quadrupled ratio of viral DNA fragments to diatom microfossils specifically to an intensified rate of viral infection among diatoms. For this inference to hold, the viral DNA must represent infections occurring within diatoms rather than other marine taxa that leave no mineralized fossils. Negating this statement establishes that the viral DNA predominantly came from viruses infecting non-diatom organisms, which completely severs the link between the sediment ratio and diatom viral infections.

Step-by-Step Solution

1
Analyze the argument's structure and identify premises and conclusion
Premise 1: Dust-rich epochs feature a fourfold increase in the ratio of giant virus DNA fragments to diatom microfossils in sediment cores. Premise 2: Atmospheric dust delivers bioavailable iron that accelerates diatom reproduction. Conclusion: Iron enrichment intensifies the rate of viral infection among diatoms.
Deconstructing the argument isolates the logical leap between sediment fossil ratios and diatom-specific infection rates.
2
Identify the unstated gap (Defender Assumption)
The researcher assumes that the viral DNA found in the sediment actually corresponds to infections of diatoms, rather than infections of other co-occurring marine organisms whose populations or viral dynamics also responded to dust.
If the viral DNA comes from non-diatom hosts, the ratio of viral DNA to diatom frustules in the sediment cannot prove that diatoms experienced higher viral infection rates.
3
Apply the Negation Test to confirm necessity
Negating the correct statement gives: 'The giant virus DNA fragments detected in the sediment cores DID predominantly originate from viruses that infect marine organisms other than diatoms.' This directly invalidates the inference about diatom infection rates.
A necessary assumption must fatally undermine the conclusion when logically negated.

Key Concept

Necessary Assumption (Defender Assumption and Negation Test)
Estimated Time:1m 30s
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Topics

Necessary Assumption

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