Encyclopedia of Evolution.pdf - Online Reading Center

America had not been connected since the breakup of Pangaea
during the Mesozoic era. A gap of ocean between
them allowed a vast worldwide circulation of ocean water
to flow into the Atlantic Ocean, forcing warm water into the
Arctic Ocean. Water holds more heat than air, and this movement
of heat from tropics to the arctic caused temperatures
to be sufficiently warm that trees grew near the North Pole,
even though they were in darkness half of the year. However,
the Isthmus of Panama gradually arose, first as islands that
slowed the flow of water, then as a land bridge that stopped
it. Today, the Gulf Stream carries warm Caribbean water to
the North Atlantic, causing the climate of Europe to be much
warmer than the land at the same latitudes in North America
(compare Labrador and England, for example). But the
Gulf Stream is not big enough to warm up the Arctic Ocean.
The planetary movements that produce the Milanković cycle
occurred long before the Pleistocene, but until then they had
not produced cyclical glaciations. In general, scientists know
that continental movements are important in determining
global climatic patterns but are not always sure how. Antarctica,
for example, used to be covered with forests also. After
it moved to be directly over the South Pole, it developed a
cold climate and glaciers began to build up. Only in recent
decades have they begun to melt. There was a 20-million-year
lag time, as yet unexplained, between Antarctica covering the
South Pole and the buildup of the glaciers.
As the glaciers advanced, they pushed piles of rubble
ahead of them. These piles, after the glaciers began to retreat,
formed moraines. Some moraines have resulted in major geographical
landmarks, such as Long Island and Cape Cod. The
type of rock that formed from glacial rubble (or till) is called
tillite. Glaciers also pushed large rocks ahead of them, which
produced scrapes in the underlying rock; these scratches are a
major source of information about where the glaciers moved
and when and were an important clue in the discovery of the
ice ages 0
Oxygen isotope patterns indicate a clear pattern of ice ages about every 00,000 years during the Quaternary period. High percentages of O (high oxygen
isotope ratio) indicate cold, dry periods; lower percentages indicate warm, moist periods. (Based on an illustration in Harold Levin’s The Earth Through
Time © 2003; reprinted with permission of John Wiley & Sons, Inc.)
ice ages themselves. Glaciers sometimes carried loads of till
as they broke off to form icebergs. Anomalous piles of rocks,
of many sizes and far out of place from their parent material,
are called dropstones and occur at surprising places on
the ocean floor. Glaciers tore large chunks of rock from some
mountains, while going around others. The isolated mountains
that rose above the sheet of ice were called nunataks.
Sometimes whole mountain systems, such as the Black Hills
of South Dakota, remained above the glaciers. The temperatures
were sufficiently cold and dry that very few organisms
survived on nunataks.
Glaciations during the Quaternary ice ages influenced
the climate of North America very severely but also affected
the tropical regions of the Earth. Although the tropics were a
bit cooler, they were still suitable for the survival and growth
of tropical grasses and forests. However, since so much of
the water was removed from meteorological circulation, the
global climate was drier. In the dry tropical regions, deserts
and grasslands were extensive, while tropical forests consisted
of isolated groves.
Once the summers began getting longer and warmer,
melting exceeded advance, and the glaciers retreated. The
most recent glacial retreat began about 12,000 years ago.
Retreat was generally faster than advance, sometimes as much
as 30 feet (10 m) a year. Therefore a person would readily
notice the retreat of glaciers during a lifetime, and there
were modern humans in both Eurasia and in North America
to observe this. Many scientists warn that the greenhouse
effect could produce similarly rapid temperature changes
for which humans will be unprepared. The return of water to
the meteorological cycle resulted in a wetter climate in tropical
regions, in some regions even wetter than today. What is
now the Sahara Desert, about 9,000 years ago, had extensive
grasslands and herds of wild animals, which human tribes
observed, carving images of them on rocks.