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ACT Reading: Natural Science (Drill 2)

Drill 2 · Reading · Natural Science

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About This Drill

ACT Reading: Natural Science (Drill 2) is a Reading practice drill covering Natural Science. It contains 5 original questions developed by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.

Natural Science passages cover topics in biology, chemistry, physics, earth science, and related fields. As you read, follow the central scientific claim and the evidence used to support it. Pay attention to how researchers conducted their investigations and how findings relate to earlier theories. Questions may ask about main ideas, specific details, inferences, vocabulary in context, or the function of particular information.

Passage

NATURAL SCIENCE: This passage is adapted from the article "The Memory of Water: How Oceans Record Climate History" by Soren Nielsen (©2022, Earth Science Perspectives). The ocean is an archive. Over millions of years, the shells of microscopic marine organisms called foraminifera have accumulated on the seafloor, building up sediment layers that encode a detailed record of past ocean temperatures, sea levels, and atmospheric composition. By extracting cores of this sediment and analyzing the chemical signatures preserved in foraminifera shells, paleoceanographers can reconstruct climates that predate any human record by tens of millions of years. The primary tool for this reconstruction is oxygen isotope analysis. Oxygen occurs naturally in two stable forms, or isotopes: the lighter oxygen-16 and the heavier oxygen-18. When foraminifera build their calcium carbonate shells, they incorporate both isotopes from the surrounding seawater, but the ratio of oxygen-18 to oxygen-16 they incorporate reflects the temperature of the water at the time. Cold water contains a higher proportion of oxygen-18; warm water contains relatively more oxygen-16. By measuring this ratio in fossil shells, researchers can determine how warm or cold the ocean was when those organisms were alive. The same ratio also records information about ice volume. During glacial periods, when large amounts of water are locked in continental ice sheets, the ocean becomes enriched in oxygen-18 because water molecules containing lighter oxygen-16 evaporate more readily and fall as precipitation over land, where they are stored as ice. When glaciers melt, this isotopically light water returns to the ocean, reducing the oxygen-18 ratio. These complementary signals allow researchers to distinguish between temperature changes and ice volume changes, though the two are often correlated. One of the most significant findings from foraminifera records is that past climate transitions have sometimes occurred much more rapidly than previously assumed. The end of the last glacial maximum, approximately 11,700 years ago, involved large temperature shifts over periods of decades rather than centuries. This finding has influenced contemporary climate science by demonstrating that the climate system is capable of rapid, threshold-crossing transitions rather than only gradual change.

Questions & Explanations

Question 1. The main purpose of the passage is to:

  • A) argue that ocean sediment cores are more reliable climate records than ice cores.
  • B) explain how researchers use chemical signatures in marine fossils to reconstruct past climates. ✓
  • C) describe the life cycle of foraminifera and their role in building ocean sediment.
  • D) warn that rapid climate transitions similar to those in the past may occur again in the future.

Explanation: Choice B is correct. The passage explains foraminifera shells as climate archives, describes oxygen isotope analysis and what it reveals about temperature and ice volume, and presents a key finding about rapid climate transitions. Choice A introduces a comparison to ice cores not made in the passage. Choice C overemphasizes the biology of foraminifera; they are discussed as a tool, not as the main subject. Choice D draws a warning about the future that is not the author's stated purpose.

Question 2. According to the passage, a higher ratio of oxygen-18 to oxygen-16 in foraminifera shells indicates that the surrounding seawater was:

  • F) warmer than average.
  • G) colder than average. ✓
  • H) less salty than average.
  • J) at a lower depth than average.

Explanation: Choice G is correct. The passage states that 'cold water contains a higher proportion of oxygen-18', so shells formed in cold water will have a higher oxygen-18 to oxygen-16 ratio. Choice F is the opposite of what the passage states. Choices H and J introduce salinity and depth, neither of which is discussed in the passage's description of the oxygen isotope ratio.

Question 3. The passage most strongly suggests that during glacial periods, the oxygen-18 ratio in ocean water increases because:

  • A) foraminifera shells dissolve more slowly in colder water, preserving more oxygen-18.
  • B) water containing lighter oxygen-16 evaporates and becomes trapped in ice on land. ✓
  • C) volcanic activity on the seafloor releases higher concentrations of oxygen-18.
  • D) the reduced ocean volume during glacial periods concentrates oxygen-18 in a smaller area.

Explanation: Choice B is correct. The passage explains that during glacial periods, 'water molecules containing lighter oxygen-16 evaporate more readily and fall as precipitation over land, where they are stored as ice', leaving the ocean enriched in the heavier oxygen-18. Choice A introduces shell dissolution rates not mentioned in the passage. Choice C introduces volcanic activity as a mechanism not discussed. Choice D introduces reduced ocean volume as a concentration mechanism, which is not the explanation given in the passage.

Question 4. As it is used in the passage, the word 'complementary' most nearly means:

  • F) mutually supportive and together more informative than either alone. ✓
  • G) similar enough to be used interchangeably.
  • H) contradictory in ways that require further analysis.
  • J) flattering or favorable in their implications.

Explanation: Choice F is correct. The passage describes the temperature signal and the ice volume signal as 'complementary'; they work together to give researchers more information than either signal alone would provide, allowing them to 'distinguish between temperature changes and ice volume changes.' Choice G suggests interchangeability, which is the opposite of the point, the signals are different and work together. Choice H suggests contradiction, which is also not the meaning here. Choice J uses a social meaning of 'complementary' (as in giving a compliment) that does not apply.

Question 5. The information about the end of the last glacial maximum primarily functions in the passage to:

  • A) provide a specific example of how rapidly the climate system can change. ✓
  • B) argue that the current period of climate change is unprecedented in Earth's history.
  • C) explain why oxygen isotope analysis was first developed approximately 11,700 years ago.
  • D) demonstrate that paleoceanography is a more reliable discipline than other climate sciences.

Explanation: Choice A is correct. The final paragraph uses the end of the last glacial maximum as evidence that 'the climate system is capable of rapid, threshold-crossing transitions', the specific example (large temperature shifts over decades, not centuries) supports this broader point. Choice B makes a comparative claim about current climate change the passage does not make. Choice C misidentifies when oxygen isotope analysis was developed; the passage does not connect the technique's origin to this event. Choice D introduces a comparison to other climate sciences not made in the passage.