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Difficulty: HardStrengthen

Paleoclimatologist: Deep-sea core samples from the subpolar Southern Ocean reveal sediment strata from approximately 430,000 years ago containing concurrent spikes in windborne mineral dust and dense accumulations of fossilized Fragilariopsis diatoms. Because bioavailable iron is a documented limiting nutrient for phytoplankton in these waters, researchers concluded that episodic increases in atmospheric dust deposition directly triggered massive surface diatom blooms during that glacial epoch.

Statement: The researchers' conclusion is strengthened if geochemical analysis demonstrates that during the periods when these strata formed, deep-sea bottom currents were not vigorous enough to laterally transport mineral grains and diatom microfossils from distant shallow shelves into the sampling basin.

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Answer

True: Ruling out the lateral transport of minerals and microfossils by deep ocean currents eliminates a plausible rival explanation, thereby substantiating the claim that the sediment spike reflects local surface diatom blooms stimulated by atmospheric iron deposition.
The conclusion asserts a specific causal relationship: airborne iron dust triggered local surface diatom blooms. The sediment data is consistent with this hypothesis, but it is also consistent with an alternative scenario in which deep currents aggregated materials from distant regions. Showing that bottom currents could not have transported these materials eliminates this counter-explanation, thereby strengthening the argument's central causal claim.

Step-by-Step Solution

1
Identify the argument's premises and central conclusion
Premises: Deep-sea sediment cores exhibit simultaneous peaks in windborne mineral dust and fossilized diatom density, and iron is a limiting nutrient in the region. Conclusion: Episodic airborne iron dust deposition directly caused local surface diatom blooms.
Deconstructing the argument isolates the evidentiary basis from the causal inference.
2
Examine the logical gap and identify potential rival hypotheses
The author assumes that the physical accumulation of dust and fossils at the sampling site accurately reflects local surface productivity rather than post-mortem lateral transport from elsewhere by ocean currents.
Strengtheners frequently operate by eliminating unstated alternative explanations that could account for the observed evidence without the author's proposed cause.
3
Evaluate the logical effect of the proposed statement
Confirming that deep bottom currents were insufficient to transport sediment from distant shelves rules out lateral advection, reinforcing the inference of local in situ biological production.
Eliminating a major competing explanation strengthens the validity of the primary causal conclusion, making the statement logically true.

Key Concept

Strengthening Causal Inferences by Eliminating Alternative Hypotheses
Estimated Time:1m 15s
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