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Difficulty: HardLogical Inferences

To reconstruct ancient ocean pH, scientists measure boron isotopes in fossil shells of marine organisms. However, these organisms alter their internal chemistry during shell growth—a phenomenon known as the 'vital effect'���which skews isotope ratios away from seawater equilibrium. Scientists correct for this skew by using calibration curves based on modern specimens of the same species grown in controlled environments. A recent study of the benthic foraminifer *Marginopora vertebralis* revealed that its vital effect is heavily influenced by the photosynthetic activity of its symbiotic algae, which varies with light exposure. Therefore, applying a calibration curve to fossil *M. vertebralis* specimens without accounting for the light levels they experienced is likely to ______

Which choice most logically completes the text?

  1. A
    lead to the incorrect conclusion that ancient *M. vertebralis* specimens lived in much deeper waters than modern specimens do.
  2. B
    demonstrate that the chemical composition of seawater has remained completely stable over geological time.
  3. yield inaccurate reconstructions of ancient ocean acidity levels.Answer
  4. D
    prevent researchers from cultivating modern foraminifera specimens under controlled laboratory conditions.

Answer

The correct answer is the option stating that applying the calibration curve without accounting for light levels is likely to yield inaccurate reconstructions of ancient ocean acidity levels.
The passage explains that researchers reconstruct ancient ocean pH (acidity) by measuring boron isotopes in fossil shells, but must correct for the 'vital effect' using calibration curves. Since a study revealed that the vital effect in *Marginopora vertebralis* fluctuates based on light exposure, applying a standard calibration curve without accounting for the light levels experienced by the organism would fail to properly correct for the vital effect. This would logically lead to inaccurate reconstructions of ancient ocean acidity levels.

Step-by-Step Solution

1
Analyze the premises regarding boron isotope measurements and the vital effect.
Boron isotopes estimate ancient ocean pH, but the 'vital effect' skews these measurements, requiring corrections using species-specific calibration curves.
To understand the relationship between the measured isotopes, the actual pH, and the calibration curves.
2
Examine the new finding regarding *Marginopora vertebralis*.
The vital effect in *Marginopora vertebralis* varies with the photosynthetic activity of its symbiotic algae, which depends directly on light exposure.
To identify the variable (light exposure) that introduces inconsistency in the vital effect.
3
Determine the logical consequence of neglecting light levels when applying calibration curves to fossil specimens.
Failing to account for light levels means the calibration curve does not accurately correct the isotopic skew, resulting in faulty estimates of ancient pH (acidity).
To draw the only logically sound conclusion supported by the text's premises.

Key Concept

Logical Inferences
Estimated Time:2m 30s
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Logical Inferences

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