Milankovitch proposed in the early twentieth century

that the ice ages were caused by variations in the Earth’s

orbit around the Sun. For sometime this theory was

considered untestable, largely because there was no suffi-

(5) ciently precise chronology of the ice ages with which

the orbital variations could be matched.

To establish such a chronology it is necessary to

determine the relative amounts of land ice that existed

at various times in the Earth’s 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

behind 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 Earth’s 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 Earth’s 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 Earth’s orbit

can be calculated and dated by applying LACE>NewtonLACE>’s 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.

54. It can be inferred from the passage that calcium carbonate shells

(A) are not as susceptible to deterioration as rocks

(B) are less common in sediments formed during an ice age

(C) are found only in areas that were once covered by land ice

(D) contain radioactive material that can be used to determine a sediment’s isotopic composition

(E) reflect the isotopic composition of the water at the time the shells were formed

constructed with “oxygen atoms drawn from the surrounding ocean.”

passage, “becomes progressively enriched in oxygen 18”with the onset of an ice age (lines 19-27).

The author states that “The degree of enrichment can be determined by analyzing ocean

sediments…composed of calcium carbonate shells of marine organisms (lines 27-31)”. Thus, it