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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. | 
