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Difficulty: MediumStrengthen

Marine Biologist: Deep-sea bamboo corals incorporate trace elements from surrounding seawater into their calcite skeletons as they grow. Recent chemical analyses of fossilized bamboo coral specimens collected from the Tasmanian Seamounts revealed that specimens dating to the Younger Dryas cooling period contain significantly higher barium-to-calcium ratios than specimens from adjacent warmer epochs. Because elevated barium concentrations in seawater are primarily associated with the upwelling of nutrient-rich, deep polar waters, researchers concluded that a major northward shift in sub-Antarctic deep ocean currents occurred during the Younger Dryas.

Which of the following, if true, most strengthens the marine biologist's argument?

  1. A
    Under laboratory conditions, lower ambient water temperatures significantly accelerate the rate at which bamboo corals absorb dissolved barium, even when seawater barium concentrations remain constant.
  2. B
    All marine invertebrates that build calcite skeletons absorb trace elements in exact proportion to the concentrations of those elements in the surrounding water column.
  3. Variations in seawater temperature do not significantly alter the ratio at which living bamboo corals incorporate barium relative to calcium from seawater of a given chemical composition.Answer
  4. D
    Deep-sea bamboo corals are known to live for several centuries, allowing individual colonies to record multidecadal climatic shifts within single continuous skeletal bands.
  5. E
    Localized hydrothermal vents near the Tasmanian Seamounts intermittently discharge mineral-rich fluids containing high concentrations of barium into nearby deep waters.

Answer

The statement establishing that seawater temperature variations do not significantly alter the ratio at which living bamboo corals incorporate barium relative to calcium from seawater of a given chemical composition.
The conclusion asserts that a northward shift of sub-Antarctic polar waters occurred based on higher barium-to-calcium ratios found in corals from a cold period. A major potential flaw is that the cold water temperatures themselves might have caused the corals to absorb more barium regardless of ambient water chemistry. The correct answer eliminates this alternative explanation by confirming that temperature does not alter barium uptake, ensuring the observed ratio reflects actual changes in water composition.

Step-by-Step Solution

1
Analyze the argument's structure, premises, and conclusion.
Premise: Younger Dryas coral fossils have higher barium-to-calcium ratios than corals from warmer epochs. Premise: High seawater barium indicates deep polar water upwelling. Conclusion: A major northward shift in sub-Antarctic deep ocean currents occurred during the Younger Dryas.
Identifying the premises and conclusion clarifies the logical leap between skeletal chemistry and ocean current movement.
2
Identify potential vulnerabilities or unstated assumptions in the reasoning.
The author assumes that the elevated barium-to-calcium ratio in the corals reflects actual higher environmental barium concentrations in the water, rather than a physiological response to the colder temperatures of the Younger Dryas period.
Strengthening questions often require ruling out plausible rival explanations or validating the reliability of a physical proxy.
3
Evaluate the answer choices to identify the statement that best defends the argument against alternative explanations.
The statement confirming that temperature changes do not alter barium uptake ratios eliminates temperature as a confounding cause, thereby strengthening the conclusion that ambient polar water upwelling caused the observed ratio.
Ruling out an alternative hypothesis directly supports the causal attribution made by the researcher.

Key Concept

Ruling out an alternative cause / validating an empirical proxy
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