GRE阅读高频机经原文及答案：地球冷暖周期_GRE阅读

GRE阅读高频机经原文及答案：地球冷暖周期

　　21. 地球冷暖周期通过石头含铁量推定新题

　　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 the red-stained grains were generallycloser to the North Pole than were the places yielding a high 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.