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?
【GRE阅读机经:地球冷暖周期】相关文章:
最新
2016-03-01
2016-03-01
2016-03-01
2016-03-01
2016-03-01
2016-03-01