Eight times within the past million years, something in the Earth’s cl

1999-04 SECTION A 17-24

Eight times within the past million years, something in the Earth’s climatic equation has changed, allowing snow in the mountains and the northern latitudes to accumulate from one season to the next instead of melting away. Each time, the enormous ice sheets resulting from this continual buildup lasted tens of thousands of years until the end of each particular glacial cycle brought a warmer climate. Scientists speculated that these glacial cycles were ultimately driven by astronomical factors: slow, cyclic changes in the eccentricity of the Earth’s orbit and in the tilt and orientation of its spin axis. But up until around 30 years ago, the lack of an independent record of ice-age timing made the hypothesis untestable.

Then in the early 1950’s Emiliani produced the first complete record of the waxings and wanings of past glaciations. It came from a seemingly odd place, the seafloor. Single-cell marine organisms called “foraminifera” house themselves in shells made from calcium carbonate. When the foraminifera die, sink to the bottom, and become part of seafloor sediments, the carbonate of their shells preserves certain characteristics of the seawater they inhabited. In particular, the ratio of a heavy isotope of oxygen (oxygen-18) to ordinary oxygen (oxygen-16) in the carbonate preserves the ratio of the two oxygens in water molecules. It is now understood that the ratio of oxygen isotopes in seawater closely reflects the proportion of the world’s water locked up in glaciers and ice sheets. A kind of meteorological distillation accounts for the link. Water molecules containing the heavier isotope tend to condense and fall as precipitation slightly sooner than molecules containing the lighter isotope. Hence, as water vapor evaporated from warm oceans moves away from its source, its oxygen-18 returns more quickly to the oceans than does its oxygen-16. What falls as snow on distant ice sheets and mountain glaciers is relatively depleted of oxygen-18. As the oxygen-18-poor ice builds up, the oceans become relatively enriched in the isotope. The larger the ice sheets grow, the higher the proportion of oxygen-18 becomes in seawater—and hence in the sediments.

Analyzing cores drilled from seafloor sediments, Emiliani found that the isotopic ratio rose and fell in rough accord with the Earth’s astronomical cycles. Since that pioneering observation, oxygen-isotope measurements have been made on hundreds of cores. A chronology for the combined record enables scientists to show that the record contains the very same periodicities as the orbital processes. Over the past 800,000 years, the global ice volume has peaked every 100,000 years, matching the period of the orbital eccentricity variation. In addition, “wrinkles” superposed on each cycle—small decreases or surges in ice volume—have come at intervals of roughly 23,000 and 41,000 years, in keeping with the precession and tilt frequencies of the Earth’s spin axis.

1.​​Which of the following best expresses the main idea of the passage?

(A) Marine sediments have allowed scientists to amass evidence tending to confirm that astronomical cycles drive the Earth’s glacial cycles.
(B) The ratio between two different isotopes of oxygen in seawater correlates closely with the size of the Earth’s ice sheets.
(C) Surprisingly, single-cell marine organisms provide a record of the Earth’s ice ages.
(D) The Earth’s astronomical cycles have recently been revealed to have an unexpectedly large impact on the Earth’s climate.
(E) The earth has experienced eight periods of intense glaciation in the past million years, primarily as a result of substantial changes in its orbit.


​2. ​​The passage asserts that one reason that oceans become enriched in oxygen-18 as ice sheets grow is because

(A) water molecules containing oxygen-18 condense and fall as precipitation slightly sooner than those containing oxygen-16
(B) the ratio of oxygen-18 to oxygen-16 in water vapor evaporated from oceans is different from that of these isotopes in seawater
(C) growing ice sheets tend to lose their oxygen-18 as the temperature of the oceans near them gradually decreases
(D) less water vapor evaporates from oceans during glacial periods and therefore less oxygen-18 is removed from the seawater
(E) the freezing point of seawater rich in oxygen-18 is slightly lower than that of seawater poor in oxygen-18


3. ​​​​According to the passage, the large ice sheets typical of glacial cycles are most directly caused by

(A) changes in the average temperatures in the tropics and over open oceans
(B) prolonged increases in the rate at which water evaporates from the oceans
(C) extreme seasonal variations in temperature in northern latitudes and in mountainous areas
(D) steadily increasing precipitation rates in northern latitudes and in mountainous areas
(E) the continual failure of snow to melt completely during the warmer seasons in northern latitudes and in mountainous areas


4. ​​​​​​​​​​It can be inferred from the passage that which of the following is true of the water locked in glaciers and ice sheets today?

(A) It is richer in oxygen-18 than frozen water was during past glacial periods.
(B) It is primarily located in the northern latitudes of the Earth.
(C) Its ratio of oxygen isotopes is the same as that prevalent in seawater during the last ice age.
(D) It is steadily decreasing in amount due to increased thawing during summer months.
(E) In comparison with seawater, it is relatively poor in oxygen-18.


5. ​​​​​​​The discussion of the oxygen-isotope ratios in paragraph three of the passage suggests that which of the following must be assumed if the conclusions described in lines 49-58 are to be validly drawn?

(A) The Earth’s overall annual precipitation rates do not dramatically increase or decrease over time.
(B) The various chemicals dissolved in seawater have had the same concentrations over the past million years.
concentrations over the past million years.
(C) Natural processes unrelated to ice formation do not result in the formation of large quantities of oxygen-18.
(D) Water molecules falling as precipitation usually fall on the open ocean rather than on continents or polar ice packs.
(E) Increases in global temperature do not increase the amount of water that evaporates from the oceans.


6. ​​​​​​​​​​The passage suggests that the scientists who first constructed a coherent, continuous picture of past variations in marine-sediment isotope ratios did which of the following?

(A) Relied primarily on the data obtained from the analysis of Emiliani’s core samples.
(B) Combined data derived from the analysis of many different core samples.
(C) Matched the data obtained by geologists with that provided by astronomers.
(D) Evaluated the isotope-ratio data obtained in several areas in order to eliminate all but the most reliable data.
(E) Compared data obtained from core samples in many different marine environments with data samples derived from polar ice caps.


7. ​​​​​​​​The passage suggests that the scientists mentioned in line 8 considered their reconstruction of past astronomical cycles to be

(A) unreliable because astronomical observations have been made and recorded for only a few thousand years
(B) adequate enough to allow that reconstruction’s use in explaining glacial cycles if a record of the latter could be found
(C) in need of confirmation through comparison with an independent source of information about astronomical phenomena
(D) incomplete and therefore unusable for the purposes of explaining the causes of ice ages
(E) adequate enough for scientists to support conclusively the idea that ice ages were caused by astronomical changes