Earth History GEOL 2110

The Pleistocene EpochThe Ice Age

Major Concepts• The gradual cooling evident in the mid to late Cenozoic

reached a critical threshold which triggered extensive glaciation around 1.8 Ma.

• Causes for this general cool down are not generally agreed upon but seem to be related to the dispersal and enlargement of the continents (relative to ocean areas) and thermal isolation of the polar regions.

• Multiple cycles of advance and retreat (with periods of about 100,000 yrs most recently) appear to be related to earth’s orbital motions (orbital eccentricity & axial wobble.

• These glacial cycles have had a profound global effect on climate, sea level changes, land elevation, distribution of lakes and rivers, and the ecology of the land.

The Great Cenozoic Cool Down

Thermal Threshold

Recognition of Continental Glaciation

Swiss Alps

Jura Mtns

Swiss Plains

Drift, Erratics, and Moraines• Hutton (1795) suggests Alpine

glaciers more extensive than today• Esmark (1824) first to propose

continental glaciation of N Europe• Venetz-Sitten & de Charpentier

(1830) confirm Alpine glaciers extended to the Swiss Plain

Louis Agazzi – Father of Continental Glaciation Theory (1840)

The Swiss-born naturalist came to the U.S. in the mid-1800’s and founded the Museum of Comparative Zoology at Harvard. Ichthyologist, paleontologist, geologist, Agassiz mounted the first significant scientific expedition on Lake Superior in the summer of 1848, delivering 12 papers describing his findings at the first annual meeting of the American Association for the Advancement of Science that fall.

Louis Agassiz (1807-1873)

The Last Peak of Glaciation20,000ybp

Tropical Belt Shrinks, but same climate

Continental Shelves Exposed

Siberian Desert

The Last Peak of Glaciation20,000ybp

Missouri and Ohio – Pro-glacial Drainage Pathways of Glacial Meltwaters

Continental Shelves Exposed

Semi-arid desert - Loess

Effects of GlaciationIsostatic Rebound

Effects of GlaciationSea Level Change

Minimum – 140mMaximum + 65m

Effects of GlaciationSea Level Change

River Terraces Develop as Base Levels Drop in Steps

Effects of GlaciationLarge Meltwater Lakes

LavaDam

IceDam

Channeled Scablands of the Columbia PlateauCarved 18,000ybp by catastrophic outletting of Glacial Lake Missoula. Water velocities likely topped 50 mph

Potholes

Effects of GlaciationLarge Meltwater Lakes

Effects of GlaciationWind-blown Silt - Loess

Palouse Loess of Southeastern

Washington

Loess Hills of Western Iowa

Dating Glacial Episodes Techniques: C14 isotopic dating (<80,000y)Fission Track in obsidianK-Ar isotopic dating (>100,000y)Th-Pr dating of clays (<300,000)

The Current Glacial Stratigraphy

Correlating Glacial Episodes

Correlating Glacial EpisodesIce Cores from Greenland and Antarctica

Correlating Glacial EpisodesIce Cores from Greenland and Antarctica

Correlating Glacial EpisodesLake Sediment Cores

Piston Coring

Greenland ice core

warm coldIce rafted debris

North Atlantic sediment core% cold water foram species

Last 650,000 yrs of Glacial CyclesAntarctic Record

Current CO2 level

380!

Why the Cycles?

Possible Factors• Changes in Solar Radiation• Orbital Effects• Changes in Atmospheric Transparency• Changes in Reflectivity – Albedo• Paleogeographic Controls of Climate• Changes to Ocean Currents

Can’t Test

Volcanism not apparently more prevalent in Pleistocene

Why the Cycles?Orbital Effects

Milankovitch Cycles

Why the Cycles?Orbital Effects

What Brought us to the Thermal Threshold?

Thermal Threshold

What Brought us to the Threshold?Changes in Reflectivity - Albedo

•Land more reflective than Oceans

•Land area has been growing

•Land getting higher – more cloud cover

•Ice caps increase albedo - feedback

What Brought us to the Threshold? Changes in Paleogeography

The Thermohaline Conveyer BeltBrings warm waters to the Arctic

The Younger DryasSmall Scale (1,000 y) Cycles

Fresh Meltwaters Shut Down the Conveyor

CatastrophicGlacial Calving

Next Lecture

The Pleistocene EpochThe Ice Age in Minnesota

Read MN at a Glance – Quaternary Geology