Eighth Edition
by Harold L. Levin

Chapter 9 - page 4

The Proterozoic: Dawn of a More Modern World

The Neoproterozoic

The Neoproterozoic (= "new" Proterozoic), also called the Late Proterozoic, lasted from about 1.0 by to 0.542 by (542 my).

Table of time divisions of the Precambrian.

Highlights of the Neoproterozoic:

• Extensive continental glaciations
• Sediments deposited in basins and shelf areas along the eastern edge of the North American craton.
  Most of these rocks were deformed during the Paleozoic orogenies.

Glacial deposits in the Neoproterozoic

Glacial deposits formed roughly 600 to 700 million years ago.

Evidence for glaciation:

• Glacial striations (scratched and grooved pebbles and boulders)
• Tillites (lithified, unsorted conglomerates and boulder beds) found nearly worldwide
• Glacial dropstones (chunks of rocks released from melting icebergs)
• Varved clays from glacial lakes

Glacial deposits are so widespread at this time that geologists refer to it as "snowball Earth."
The formal name for this glaciation is the Varangian glaciation (named after an area in Norway).

The late Proterozoic ice age lasted about 240 my.

Note the several times of glaciation (yellow) in geologic history, including the prolonged glaciation in the Neoproterozoic.

Plate tectonics may have had a role in cooling the planet. The continents were located at low and middle latitudes (around the equator) about 600 to 700 my ago. There was no tropical ocean. Heat lost by reflection from the rocks on the surface of the continents may have caused global cooling. (Note that land plants had not yet appeared.)

As continental glaciers and ice caps formed, the reflectivity of snow and ice caused further temperature decrease.

Glaciation was associated with a decrease in atmospheric CO₂ and increase in O₂.
Recall that CO₂ increases the greenhouse effect and causes global warming. The decline in atmospheric CO₂ may have led to cooling.

Decrease in CO₂ was probably caused by the increase in the number of photosynthetic organisms (such as the cyanobacteria which formed stromatolites).

Limestones are associated with the glacial deposits, which is unusual. Limestones generally form in warm seas, not cold ones. The association of limestones with glacial deposits suggests that times of photosynthesis and CO₂ removal alternated with times of glaciation.

Limestones (made of CaCO₃) are a storehouse of CO₂, which was removed from the atmosphere.

Glacial conditions may have inhibited photosynthesis by stromatolites. As a result, CO₂ may have accumulated periodically and triggered short episodes of global warming. This produces the paradox of glaciers causing their own destruction.