地球冷暖周期推断
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 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.
