关于地球冷暖周期的GRE阅读：

recent studies of sediment in the north atlantic’s deepwaters reveal possible cyclical patterns in the history of earth’s climate. therock fragments in these sediments are too large to have been transported thereby ocean currents; they must have reached their present locations by travelingin large icebergs that floated long distances from their point of origin beforemelting.geologist gerard bond noticed that some of the sedimentgrains were stained with ironoxide, evidence that they originated in localeswhere glaciers had overrun outcrops of red sandstone. bond’s detailed analysisof deep-water sediment cores showed changes in the mix of sediment sources overtime: the proportion of these red-stained grains fluctuated back and forth fromlows of 5 percent to highs of about 17 percent, and these fluctuations occurredin a nearly regular 1,500-year cycle.bond hypothesized that the alternating cycles might beevidence of changes in ocean-water circulation and therefore in earth’sclimate. he knew that the sources of thered-stained grains were generallycloser to the north pole than were the places yielding a high 30 proportion ofclean grains. at certain times, apparently, more icebergs from the arcticocean in the far north were traveling south well into the north atlantic beforemelting and shedding their sediment. ocean waters are constantly moving, andwater temperature is both a cause and an effect of this movement. as watercools, it becomes denser and sinks to the ocean’s bottom.

during some periods,the bottom layer of the world’s oceans comes from cold, dense water sinking inthe far north atlantic. this causes the warm surface waters of the gulf streamto be pulled northward. bond realized that during such periods, the influx ofthese warm surface waters into northern regions could cause a large proportionof the icebergs that bear red grains to melt before traveling very far into thenorth atlantic. but sometimes the ocean’s dynamic changes, and waters from thegulf stream do not travel northward in this way. during these periods, surfacewaters in the north atlantic would generally be colder, permitting icebergsbearing red-stained grains to travel farther south in the north atlantic beforemelting and depositing their sediment.

the onset of the so-called little ice age (1300-1860),which followed the medieval warm period of the eighth through tenth centuries,may represent the most recent time that the ocean’sdynamic changed in this way. if ongoing climate-history studies support bond’shypothesis of 1,500-year cycles, scientists may establish a major natural rhythmin earth’s temperatures that could then be extrapolated into the future.because the midpoint of the medieval warm period was about a.d. 850, anextension of bond’s cycles would place the midpoint of the next warm intervalin the twenty-fourth century.

21.1. according to the passage, which of the following istrue of the rock fragments contained in the sediments studied by bond?

a. the majority of them are composed of red sandstone.

b. they must have reached their present location over 1,500 years ago.

c. they were carried byicebergs to their present location.

d. most of them were carried to their present location during a warm period inearth’s climatic history.

e. they are unlikely to have been carried to their present location during thelittle ice age.

21.2. in the final paragraph of the passage (lines 27-33),the author is concerned primarily with

a. answering a question about earth’s climatic history

b. pointing out a potential flaw in bond’s hypothesis

c. suggesting a new focus for the study of ocean sediments

d. tracing the general history of earth’s climate

e. discussing possibleimplications of bond’s hypothesis

21.3. according to the passage, bond hypothesized that whichof the following circumstances would allow red-stained sediment grains to reachmore southerly latitudes?

a. warm waters being pulled northward from the gulfstream

b. climatic conditions causing icebergs to melt relatively quickly

c. icebergs containing a higher proportion of iron oxide than usual 31

d. the formation of more icebergs than usual in the far north

e. the presence of coldsurface waters in the north atlantic

21.4. it can be inferred from the passage that in sedimentcores from the north atlantic’s deep waters, the portions that correspond tothe little ice age

a.. differ very little in composition from theportions that correspond to the medieval warm period

b. fluctuate significantly in composition between the portions corresponding tothe 1300s and the portions corresponding to the 1700s

c. would be likely tocontain a proportion of red-stained grains closer to 17 percent than to 5percent

d. show a much higher proportion of red-stained grains in cores extracted fromthe far north of the north atlantic than in cores extracted from further south

e. were formed in part as