GRE Question of the Day (September 10)

Recent studies of sediment in the North Atlantic’deep waters reveal possible cyclical patterns in the history of Earth’s climate. The rock fragments in these sediments are too large to have been transported there by ocean currents, they must have reached their present locations by traveling in large icebergs that floated long distance from their point of origin before melting. Geologist Gerard Bond noticed that some of the sediment grains were stained with iron oxide, evidence that they originated in locales where glaciers had overrun outcrops of red sandstone. Bond’s detailed analysis of deep-water sediment cores showed changes in the mix of sediment sources over time: the proportion of these red-stained grains fluctuated back and forth from lows of 5 percent to highs of about 17 percent, and these fluctuations occurred in a nearly regular 1,500-year cycle.

Bond hypothesized that the alternation cycles might be evidence of changes in ocean-water circulation and therefore in Earth’s climate. He knew that the sources of the red-stained grains were generally closer to the North Pole than were the places yielding a high proportion of“clean” grains. At certain times, apparently, more icebergs from the Arctic Ocean in the far north were traveling south well into the North Atlantic before melting and shedding their sediment.

Ocean waters are constantly moving. And water temperature is both a cause and an effect of this movement. As water cools, 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 in the far North Atlantic. This causes the warm surface waters of the Gulf Stream to be pulled northward. Bond realized that during such periods, the influx of these warm surface waters into northern regions could cause a large proportion of the icebergs that bear red grains to melt before traveling very far into the North Atlantic. But sometimes the ocean’s dynamic changes, and waters from the Gulf stream do not travel northward in this way. During these periods, surface waters in the North Atlantic would generally be colder, permitting icebergs bearing red-stained grains to travel farther south in the North Atlantic before melting 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’s dynamic changed in this way. If ongoing climate-history studies support Bond’s hypothesis of 1,500-year cycles, scientists may establish a major natural rhythm in Earth’s temperatures that could then be extrapolated into the future. Because the midpoint of the Medieval Warm Period was about AD.850, an extension of Bond’s cycles would place the midpoint of the next warm interval in the twenty-fourth century.