history of glaciation

History of Glaciation

There have been several periods of glaciation in Earth's history. Evidence supports the conclusion that the Earth had 2 (perhaps up to 5) periods of glaciation that the were so intense, they produced a "Snowball Earth". They occurred:

Around 2400 million years ago - Related link here
As many as four times between 750 million and 580 million years ago. Related link here or here or here

This idea is still debatable since this is was first proposed by a Caltech geobiology professor, Joe Kirschvink in 1992. But evidence is mounting and this once "crackpot" idea is becoming mainstream thought.

Produced a "Snowball Earth" - most severe ice age where it iced all the way to the equator
Each episode lasted about 10 million years
Caused major mass extinctions

Simple life could still survive in deep, hot geothermal vents on the ocean floor

Was brought on quickly ... perhaps only decades

Computer models suggest that if methane (a greenhouse gas) levels drop (perhaps by an increase in oxygen levels in the atmosphere), the earth would rapidly cool. If ice sheets advances to within 30 degrees of the equator (about Florida), the high reflectivity of the ice would limit absorption by solar radiation and the earth would spiral into a deep freeze - Snowball Earth! This is known as the "albedo effect".

Ended quickly ... what would bring the earth out of this deep freeze?

Volcanoes! Volcanoes release carbon dioxide (another greenhouse gas). Once in the atmosphere, rapid atmospheric heating melts the ice.

When it ended (first event), oxygen producing algae (Cyanobacteria) flourished ... a Green Earth resulted... enriching our atmosphere with oxygen and "rusting" lots of iron and manganese. This produced huge 45 meter deposits of manganese in the Kalahari and huge oxygen "spikes" in the geologic records.

A later snowball event could have triggered the "Cambrian Explosion" ..the proliferation of soft bodied, multi cellular marine life just after a period of glaciation 540 million years ago

It also explains the thick carbonate deposits found in sedimentary rock layers immediately following the snowball event.

We will study the carbon cycle in this test unit. You will see that carbon can move from the atmosphere, to the sea and then to rocks.

Other Ice events:

350 - 250 million years ago

Pangaea (the super continent) when a lot of land was situated near the pole!

2 million years - present Click here for more information

Pleistocene Ice Age - we will call this period "the ice age"
The earth actually started cooling down much earlier (about 40 million years ago) ... but why?
at the maximum, about 8% of the earth's water was ice (currently it is 2%)
at the maximum, about 30% of the earth's surface was ice covered (10% now).
there have been about 20 periods of glacial advance/retreat during this period

Why did ice sheets go through cycles of advance and retreat? (click here)

Orbital changes and ice ages?

In 1912, Serbian scientist Milutin Milankovitch offered a theory to explain why we have been going through cycles of "ice ages" over the past several million years. He suggests that changes in the eccentricity (shape of the ellipse) of the earth's orbit along with changes in our tilt (currently 23½ degrees) could produce the proper conditions to trigger an "ice age". Recently some scientists offered another astronomical hypothesis but not the one suggested by Milankovitch. This new hypothesis suggest that the earth "bobs" up and down slightly off the ecliptic plane (as the sun orbits around our galaxy) ... thereby encountering more space dust and debris ... which effects the climate. Either way, the manner in which the earth moves as it orbits the sun (or the way the sun orbits the galaxy) may be the cause of these cycles of glaciation.

If Pleistocene ice cycles are an ongoing pattern, we may not be out of the woods yet ... we may be in for another glacial advance in the future.

What caused the latest ice age in the first place? This is a big mystery and the answer could come from a young scientist, Maureen Raymo. In-class students will view a video - Cracking the Ice Age (get the handout here). The main points are:
- Plate tectonics cause a great uplift - the Tibetan plateau.
- This produces much more rain in the local region (something covered in unit 4).
- The rain absorbs carbon dioxide from the atmosphere, forming a weak acid solution.
- This acid chemically weathers the exposed rocks ... (something already covered in unit 1).
- Vast quantities of carbon dioxide are washed to the ocean. Since carbon dioxide is a greenhouse gas, less carbon dioxide in the atmosphere means the earth gets colder.
What other variables effect global cooling (on other time scales)?

Meteor impacts. This most likely occurred 65 million years ago and is known as a "nuclear winter" event. The debris from the impact kicks up so much dust into the atmosphere that it blocks sunlight from penetrating the surface. You can guess why this could also cause a mass extinction event.
The sunspot cycle (or lack of) Maunder Minimum - mini ice age 1645 -1715. During this period, no solar activity was noticed and during the same time northern Europe reported record cold temperatures ... coincidence?
Intense volcanic activity - Tambora 1815 (the year without a summer). Like a small version of nuclear winter.
El niño-La niña - changes in global wind patterns induced by changes in ocean currents.

History of Glaciation

Orbital changes and ice ages?