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Difficulty: MediumLogical Inferences

In paleoclimatology, the ratio of oxygen isotopes (δ18O\delta^{18}\text{O}) in stalagmites is used to reconstruct historical rainfall, as wetter periods typically result in lower δ18O\delta^{18}\text{O} values in the precipitated calcium carbonate. However, researchers analyzing a stalagmite from Spain’s system of caves noticed that during a documented multi-decade drought in the thirteenth century, the stalagmite’s δ18O\delta^{18}\text{O} values remained persistently low. They hypothesized that the cave's drip water during this dry spell was fed by a massive, deep aquifer that takes decades to filter water downward, rather than by immediate precipitation. If this hypothesis is correct, it suggests that the Spanish stalagmite ______

Which choice most logically completes the text?

  1. was formed from groundwater that had fallen as precipitation decades prior to the thirteenth-century drought.Answer
  2. B
    will exhibit a dramatic increase in δ18O\delta^{18}\text{O} values if the drought continues for another fifty years.
  3. C
    proves that subterranean aquifers worldwide consistently delay climatic signals recorded in geological formations.
  4. D
    recorded rapid, year-to-year shifts in Spain's rainfall patterns throughout the thirteenth century.

Answer

the option stating that the stalagmite was formed from groundwater that had fallen as precipitation decades prior to the thirteenth-century drought
The correct option logically completes the argument by identifying the source of the low oxygen isotope values. Since the aquifer requires decades to transport water, the mineral deposits laid down during the dry spell reflect the wetter climate of decades past.

Step-by-Step Solution

1
Identify the core premises of the passage.
Lower isotope values correspond to wet periods. However, during a drought, the stalagmite registered low values. The researchers hypothesize that the cave's drip water came from a deep aquifer with a decadal delay.
This establishes the relationship between precipitation levels, isotope signals, and the geological delay mechanism.
2
Connect the delay mechanism to the anomalous signal.
Because the aquifer takes decades to filter water, the water feeding the stalagmite during the thirteenth-century drought represents rain that fell decades before.
This bridges the cause (slow filtration of old rainwater) to the effect (low isotopic values indicating past wet periods).
3
Select the option that matches this logical completion.
The correct answer explains that the stalagmite was formed from rainwater that entered the ground prior to the drought.
This completes the argument without making ungrounded assumptions or over-generalizations.

Key Concept

Logical Inferences
Estimated Time:1m 35s
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Logical Inferences

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