ACT Reading — Natural Science — Drill 2

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.