GRE阅读：地球冷暖周期

　　Recent studies of sediment in the North Atlantics deepwaters reveal possible cyclical patterns in the history of Earths 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. Bonds 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 Earthsclimate. 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 oceans bottom.

　　During some periods,the bottom layer of the worlds 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 oceans 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 ,which followed the Medieval Warm Period of the eighth through tenth centuries,may represent the most recent time that the oceansdynamic changed in this way. If ongoing climate-history studies support Bondshypothesis of 1,500-year cycles, scientists may establish a major natural rhythmin Earths temperatures that could then be extrapolated into the future.Because the midpoint of the Medieval Warm Period was about A.D. 850, anextension of Bonds 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 inEarths 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 ,the author is concerned primarily with

　　A. answering a question about Earths climatic history

　　B. pointing out a potential flaw in Bonds hypothesis

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

　　D. tracing the general history of Earths climate

　　E. discussing possibleimplications of Bonds 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 Atlantics 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