The Pleistocene

Causes of Glaciation

• 1) Why is there ice anywhere on Earth?

• 2) If there is ice, what makes it contract and expand?

Before 3.5 MaAfter 3.5 Ma

Milankovitch Cycles

Milutin Milankovitch(1879-1958)

Precession of the equinoxes. The direction of the long axis of the ellipse itself rotates slowly.

It has the effect that at certain times aphelion will occur during the northern

hemisphere winter and at other times aphelion will occur during the northern hemisphere summer.

The period of precession of the Earth's orbit is between 19,000 and 21,000 years.

Changes in obliquity. The Earth's axis of rotation is tilted

relative to the plane in which the Earth orbits around the sun

(the plane of the ecliptic). Currently the tilt (obliquity) is 23.5o, but

it varies between 21.5o and 24.5o with a period of about 41,000 years.

Changes in eccentricity of the orbit. The Earth's orbit around the sun

is elliptical with the sun at one focus of the ellipse. The eccentricity

changes cyclically with a period of about 96,000 years.

The End-Pleistocene Mass-Extinction of Large

Mammals

Pleistocene of Alaska

The La BreaTar Pits

The La BreaTar Pits

The La BreaTar Pits

Columbian mammoth

American mastodon

The La BreaTar Pits

Dire wolf

The disappearing mammals in North America included:

Mammoths Dire wolves

Mastodons Giant jaguar

Horses Saber-tooth cat

Tapirs Ground sloths

Peccaries Four-horned antelopes

Giant beaver Camels

More than 200 genera of very large herbivores (2 tons or more)

go extinct, including 95 % in North America

The Pleistocene Mass-Extinction

The following disappeared from America,

Europe and Australia:

All herbivores > 1000 kg

75% of herbivores 100-1000 kg

41% of herbivores 5-100 kg

< 2% of herbivores < 5kg

The Pleistocene Mass-Extinction

It occurred at different times on different land masses:

Time of start of major extinction episodes(years before present)

Africa and S.E. Asia 50,000

Australia 50,000

North Eurasia 13,000

North America 11,000

South America 10,000

West Indies 4,000

New Zealand 900

Madagascar 800

The Pleistocene Mass-Extinction

Possible Causes of the Pleistocene Mass Extinction

1) Climate

2) Human Hunting

Clovis Projectile Points

UF14616Mammuthus columbi (Columbian mammoth)thoracic vertebra spine with cut marksGilchrist County, Florida

Asian (L) African (R)