[问答] PP II 第一套长阅读 sediment in the North Atlantic and Bond's Hypothesis [复制链接]

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发表于 2013-10-8 08:19:34 |显示全部楼层

本帖最后由 crazyrobin 于 2013-10-8 15:01 编辑

帮你把标题改了，以后发帖请不要点名，谢谢。另外发帖请注意标明题目来源。

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 the red-stained grains were generallycloser to the North Pole than were the places yielding a high proportion of“clean” 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.

第四题不懂~
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
答案是选C。
不过我不知道为什么要更接近17percent 一些，各个论坛都看了下，都没找到很满意的答案！！