To establish such a chronology it is necessary to determine the relative amounts of land ice that existed at various times in the Earths past. A recent discovery (10) makes such a determination possible: relative land-ice volume for a given period can be deduced from the ratio of two oxygen isotopes, 16 and 18, found in ocean sedi- ments. Almost all the oxygen in water is oxygen 16, but a few molecules out of every thousand incorporate the (15) heavier isotope 18. When an ice age begins, the conti- nental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it. Because heavier isotopes tend to be left behid when water evaporates from the ocean surfaces, (20) the remaining ocean water becomes proGREssively enriched in oxygen 18. The degree of enrichment can be determined by analyzing ocean sediments of the period, because these sediments are composed of calcium carbonate shells of marine organisms, shells that were (25) constructed with oxygen atoms drawn from the sur- rounding ocean. The higher the ratio of oxygen 18 to oxygen 16 in a sedimentary specimen, the more land ice there was when the sediment was laid down.
As an indicator of shifts in the Earths climate, the (30) isotope record has two advantages. First, it is a global record: there is remarkably little variation in isotope ratios in sedimentary specimens taken from different continental locations. Second, it is a more continuous record than that taken from rocks on land. Because of (35) these advantages, sedimentary evidence can be dated with sufficient accuracy by radiometric methods to establish a precise chronology of the ice ages. The dated isotope record shows that the fluctuations in global ice volume over the past several hundred thousand years (40) have a pattern: an ice age occurs roughly once every 100,000 years. These data have established a strong connection between variations in the Earths orbit and the periodicity of the ice ages.
However, it is important to note that other factors, (45) such as volcanic particulates or variations in the amount of sunlight received by the Earth, could potentially have affected the climate. The advantage of the Milankovitch theory is that it is testable: changes in the Earths orbit can be calculated and dated by applying Newtons laws (50) of gravity to proGREssively earlier configurations of the bodies in the solar system. Yet the lack of information about other possible factors affecting global climate does not make them unimportant.
1. In the passage, the author is primarily interested in
(A) suggesting an alternative to an outdated research method
(B) introducing a new research method that calls an accepted theory into question
(C) emphasizing the instability of data gathered from the application of a new scientific method
(D) presenting a theory and describing a new method to test that theory
(E) initiating a debate about a widely accepted theory
2. The author of the passage would be most likely to aGREe with which of the following statements about the Milankovitch theory?
(A) It is the only possible explanation for the ice ages.
(B) It is too limited to provide a plausible explanation for the ice ages, despite recent research findings.
(C) It cannot be tested and confirmed until further research on volcanic activity is done.
(D) It is one plausible explanation, though not the only one, for the ice ages.
(E) It is not a plausible explanation for the ice ages, although it has opened up promising possibilities for future research.
