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Zorluk: KolayMulti-Sentence Synthesis Inferences

Paleoclimatologists analyzing ocean sediment cores rely on the ratio of oxygen-18 to oxygen-16 isotopes in the fossilized calcium carbonate shells of benthic foraminifera—single-celled organisms residing on the abyssal seafloor—to reconstruct historical global ice volumes. Because lighter oxygen-16 evaporates from ocean surfaces more readily than heavier oxygen-18, atmospheric moisture becomes enriched in oxygen-16. During glacial periods, this lighter isotope becomes trapped in expanding continental ice sheets, leaving ocean water relatively enriched in oxygen-18. Consequently, the calcium carbonate shells formed by benthic organisms during glacial eras incorporate higher proportions of oxygen-18 than those formed during warmer interglacial periods.

For decades, critics argued that this isotopic variation was an unreliable proxy for ice sheet volume because isotopic uptake in carbonate shells is also influenced by ambient water temperature. If deep-ocean waters cooled significantly during glacial periods, that thermal shift alone could account for the observed isotopic changes. However, recent trace-element analyses of deep-sea sediments demonstrate that abyssal ocean temperatures fluctuated by less than 1.5 degrees Celsius between glacial and interglacial extremes. Because abyssal waters remained thermally stable, the thermal effect on shell chemistry was negligible.

Based on the passage, which of the following statements regarding benthic foraminifera shell composition during glacial periods is most strongly supported?

  1. The increased proportion of oxygen-18 in the shells is primarily attributable to changes in global ice volume rather than to temperature shifts in the abyssal seafloor environment.Cevap
  2. B
    Benthic foraminifera shells incorporated higher proportions of oxygen-16 during glacial eras because atmospheric water vapor returned directly to deep-ocean waters through precipitation.
  3. C
    Surface water climate volatility directly caused deep-sea benthic foraminifera to alter their chemical mechanism for shell formation.
  4. D
    Trace-element analyses proved that abyssal water temperatures dropped by more than 10 degrees Celsius during peak glacial transitions.
  5. E
    Benthic foraminifera are the only marine organisms capable of providing reliable data on ancient terrestrial atmospheric temperatures.

Cevap

The increased proportion of oxygen-18 in benthic foraminifera shells during glacial periods is primarily attributable to changes in global ice volume rather than to temperature shifts in their abyssal environment.
The passage establishes in the first paragraph that glacial ice traps oxygen-16, enriching ocean water in oxygen-18, which is then incorporated into benthic foraminifera shells. In the second paragraph, the author addresses critics' concerns about water temperature by presenting evidence that deep-ocean water temperatures varied by less than 1.5 degrees Celsius, rendering thermal impact negligible. Synthesizing these two separate points supports the inference that the increased oxygen-18 ratio is driven by global ice volume changes rather than abyssal thermal shifts.

Adım Adım Çözüm

1
Identify premise 1 regarding isotope ratios and ice volume.
The first paragraph establishes that expanding continental ice sheets trap lighter oxygen-16, enriching ocean water—and consequently benthic foraminifera shells—in oxygen-18.
This establishes global ice volume as a cause of increased oxygen-18 proportions in ocean water.
2
Identify premise 2 regarding abyssal water temperature stability.
The second paragraph notes that deep-sea water temperatures fluctuated by less than 1.5 degrees Celsius, meaning thermal effects on shell chemistry were negligible.
This rules out ambient water temperature shifts as a major alternative explanation for isotopic changes in abyssal shells.
3
Synthesize the non-contiguous premises to draw the inference.
Combining both facts confirms that the elevated oxygen-18 in glacial benthic shells reflects global ice volume changes rather than deep-sea thermal changes.
Eliminating thermal shift as a primary cause leaves ice sheet volume as the supported primary factor.

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Multi-Sentence Synthesis Inferences
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