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Difficulty: Very hardNecessary Assumption

Paleoclimatologist: Amber—fossilized tree resin—often contains trapped microscopic air bubbles. Analysis of these bubbles in amber from the mid-Cretaceous period reveals an absolute concentration of argon-40 that is vastly higher than that found in the modern atmosphere. On Earth, argon-40 is almost exclusively produced by the radioactive decay of potassium-40 in the crust, and it enters the atmosphere primarily through tectonic outgassing. Because the mid-Cretaceous was an era of unprecedented continental rifting, the elevated argon-40 levels in these amber bubbles must be the result of a massive global spike in tectonic outgassing during that epoch.

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

  1. A
    The concentration of argon-36 in the mid-Cretaceous atmosphere was largely identical to the concentration of argon-36 in the modern atmosphere.
  2. B
    No other geological epoch in Earth's history experienced a greater volume of tectonic outgassing than the mid-Cretaceous period.
  3. The amount of argon-40 present in the trapped air bubbles was not significantly augmented by the decay of potassium-40 within the amber or its surrounding sediments after fossilization.Answer
  4. D
    Amber is perfectly impermeable to all atmospheric gases, ensuring that the trapped bubbles never undergo any chemical or isotopic exchange with their environment.
  5. E
    The global rate of tectonic outgassing is the only mechanism that can substantially alter the concentration of argon-40 in Earth's atmosphere.

Answer

The assumption that the argon-40 levels in the trapped air bubbles were not significantly augmented by local radioactive decay after the amber fossilized.
The argument concludes that a global atmospheric event (massive tectonic outgassing) caused the extremely high argon-40 levels found in the amber bubbles. For this leap to be valid, the author must assume that the argon-40 in those bubbles was actually trapped from the atmosphere when the resin originally formed, rather than being produced locally by radioactive decay and permeating the bubbles millions of years later. If we negate this correct answer (i.e., assume the argon-40 WAS significantly augmented by post-fossilization decay within the surrounding soil or the amber itself), the argument's core causal link collapses, proving this is a necessary assumption.

Step-by-Step Solution

1
Identify the main conclusion of the argument.
The elevated argon-40 levels in mid-Cretaceous amber bubbles must be the result of a massive global spike in tectonic outgassing during that epoch.
Isolating the conclusion is essential for determining what the argument fundamentally relies upon.
2
Analyze the premises provided as evidence.
The author cites the high concentration of argon-40 in the bubbles, notes that argon-40 is produced by crustal decay and released via outgassing, and points out that the mid-Cretaceous featured unprecedented continental rifting.
Understanding the evidence reveals the logical leap the author is making from the data to the conclusion.
3
Locate the logical vulnerability (the gap) in the reasoning.
The author assumes that the high argon-40 found in the amber accurately reflects the global atmosphere at the time of formation, ignoring the possibility of localized contamination over millions of years of burial.
Arguments relying on geological or fossil evidence often overlook post-depositional changes, which must be defended against.
4
Apply the Negation Test to evaluate the options.
Negating the correct option reveals that if local decay DID augment the argon-40 post-fossilization, the argument that it came from global tectonic outgassing collapses entirely.
The Negation Test is the most reliable method for proving an assumption is strictly necessary.

Key Concept

Defender Assumptions and Alternative Explanations
Estimated Time:2m 30s
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Topics

Necessary Assumption

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