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Difficulty: HardNecessary Assumption

Paleoclimatologist: Stalagmites preserve variations in oxygen isotope ratios that reflect local precipitation levels, with lower ratios of oxygen-18 to oxygen-16 indicating periods of heavier rainfall. A geochemical analysis of stalagmites from a cave in the Karst Plateau revealed a substantial decline in this isotopic ratio dating between 2200 BCE and 2000 BCE. Because several nearby agricultural settlements were abruptly abandoned during this exact period, historians have claimed that regional desiccation and prolonged drought caused the societal collapse. However, the isotopic evidence indicates that this historical claim is incorrect.

Which one of the following is an assumption required by the paleoclimatologist's argument?

  1. A
    An increase in regional precipitation was the primary factor that caused the inhabitants of the Karst Plateau to abandon their settlements.
  2. B
    Any region that experiences a significant decline in oxygen isotope ratios will inevitably maintain productive agricultural yields.
  3. The decline in the oxygen isotope ratio in the stalagmites was not caused by atmospheric or hydrological factors that operate independently of an increase in local rainfall.Answer
  4. D
    No prolonged droughts occurred in any geographic region adjacent to the Karst Plateau between 2200 BCE and 2000 BCE.
  5. E
    Archaeological records do not accurately document the precise historical timeframe during which the agrarian settlements were abandoned.

Answer

The argument depends on assuming that the decline in the oxygen isotope ratio in the stalagmites was not caused by atmospheric or hydrological factors that operate independently of an increase in local rainfall.
The conclusion relies on the geochemical premise that a lower oxygen-18 to oxygen-16 ratio indicates heavier rainfall, which contradicts the historians' drought hypothesis. If the observed decline in the isotope ratio were actually produced by other confounding atmospheric or hydrological mechanisms (such as changes in moisture source trajectories or condensation temperatures) rather than an actual increase in rainfall, the data would not disprove the occurrence of a drought. Therefore, ruling out such confounding factors is strictly required for the argument's conclusion to stand.

Step-by-Step Solution

1
Identify the premises and the main conclusion of the stimulus
Premises establish that lower oxygen isotope ratios reflect heavier rainfall, that stalagmites in the plateau showed a sharp decline in this ratio between 2200 BCE and 2000 BCE, and that settlements were abandoned during this timeframe. The conclusion asserts that historians are incorrect in claiming that drought caused the societal collapse.
Clarifying the logical core exposes the gap between the geochemical data and the refutation of the drought claim.
2
Determine the necessary gap between the evidence and the conclusion
The paleoclimatologist infers from the isotopic decline that the region experienced wet conditions rather than drought. For this inference to hold, the isotopic ratio must genuinely reflect higher precipitation rather than an alternative confounding factor.
A necessary assumption must hold true for the premise to serve as valid counterevidence against the drought claim.
3
Apply the Negation Test to candidate statements
Negating the statement—supposing that the isotopic decline was driven by unrelated atmospheric or hydrological factors—breaks the link between lower ratios and increased rainfall, thereby destroying the author's refutation of drought.
If the negation of a statement causes the argument to fail completely, that statement is strictly necessary to the argument.

Key Concept

Identifying unstated necessary defender assumptions by applying the Negation Test to eliminate alternate explanations for evidence
Estimated Time:1m 45s
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Topics

Necessary Assumption

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