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Difficulty: MediumStrengthening Arguments

Geochemists analyzing drill core samples from subglacial Lake Mercer in Antarctica discovered elevated concentrations of dissolved methane alongside specialized methanotrophic bacteria. Because these bacterial populations exhibit peak metabolic rates strictly within thermal gradients matching geothermal vents detected beneath the ice sheet, researchers concluded that geothermal activity, rather than surface organic runoff trapped during ice sheet formation, is the primary energy source sustaining the microbial ecosystem in the subglacial lake. Which of the following, if true, most strengthens the researchers' conclusion?

  1. Subglacial lakes in the region that lack geothermal activity contain trapped surface organic runoff comparable to Lake Mercer but show virtually no dissolved methane or active methanotrophic bacteria.Answer
  2. B
    Isotopic analysis of the dissolved methane in Lake Mercer reveals chemical signatures identical to methane produced by ancient surface organic matter decomposing in anaerobic conditions.
  3. C
    Geothermal vents located in deep oceanic trenches support methanotrophic bacterial species with metabolic genes similar to those found in subglacial Lake Mercer.
  4. D
    Water temperatures in Lake Mercer have fluctuated slightly over the past decade in correlation with seasonal changes in Antarctic surface atmospheric conditions.
  5. E
    Deep-ice drilling equipment used to obtain the core samples requires specialized thermal heating elements to prevent sample contamination during extraction.

Answer

Subglacial lakes in the region that lack geothermal activity contain trapped surface organic runoff comparable to Lake Mercer but show virtually no dissolved methane or active methanotrophic bacteria.
The correct answer provides a comparative control scenario. By showing that other subglacial lakes with trapped surface organic runoff but no geothermal activity fail to produce elevated methane or bacterial activity, it rules out surface runoff as the primary cause and confirms that geothermal activity is the key driving factor.

Step-by-Step Solution

1
Identify the core argument structure
Premise: Methanotrophic bacteria in Lake Mercer thrive within thermal gradients matching geothermal vents. Conclusion: Geothermal activity, rather than trapped surface organic runoff, is the primary energy source sustaining the ecosystem.
Isolating the conclusion and the competing alternative explanation (surface organic runoff) is necessary to determine what evidence strengthens the claim.
2
Evaluate the strengthening requirement
To strengthen the claim that geothermal activity (and not surface runoff) is the source, evidence should demonstrate that surface runoff alone is insufficient to produce methane/bacteria without geothermal vents.
Ruling out an alternative cause or demonstrating a control case where the alternative cause exists without the effect reinforces a causal argument.
3
Assess the correct choice against alternative explanations
Showing that nearby subglacial lakes with surface runoff but no geothermal vents lack methane and bacteria demonstrates that surface runoff alone cannot sustain the ecosystem, thereby strengthening the geothermal conclusion.
Establishing a comparative control group isolates geothermal activity as the critical missing factor.

Key Concept

Strengthening a Causal Argument by Ruling Out an Alternative Cause
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Topics

Strengthening Arguments

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