GEOL 102 Historical Geology

Spring Semester 2012
The Proterozoic Eon III: Snowball Earth and the Garden of Ediacara

The Neoproterozoic Era has recently been divided into three Periods: Tonian ("stretching", after continued expansion of the platform covers": 1000 - 850 Ma), Cryogenian ("ice origins", after the Snowball Earth glaciations: 850 - 630 Ma), and Ediacaran (After Ediacara Springs, Australia: 630 - 542 Ma). The boundaries of the Tonian are arbitrarily defined, while the uppermost boundary of the Cryogenian is the the end of the Marinoan glaciation (and hence may be shifted to about 635.5 Ma pending further revisions), and the uppermost boundary of the Ediacaran is the first appearance of the trace fossil Trichophycus pedum (and thus the oldest biostratigraphically-determined boundary).

Climates of the Proterozoic

Good evidence of rising levels of oxygen in atmosphere (and hence oceans):

Suggests atmosphere of about 1% of atmosphere was oxygen at end of Archean, rising to 10% (or half of modern level) by end of Proterozoic.

During Paleoproterozoic, good evidence of widespread glaciation (tillites, striations, carbon shifts).
During Neoproterozoic (in particular, from c. 750-580 Ma), evidence for superglaciations:

Strong evidence for a Neoproterozoic Snowball Earth

The reason Neoproterozoic superglaciations stop and haven't been seen again may be due to the rise of complex animals, which were able to liberate the carbon in the sediment through bottom-feeding (previously was simply trapped in sediment). However, another factor may be the ever-increasing brightness of the Sun.

Proterozoic Life: Stromatolites still common, becoming even more common c. 2.2 Ga, becoming more complex c. 1.2 Ga.

Origin of eukaryotes by endosymbiosis:

Eukaryotes remain unicellular for most of the Proterozoic. Life remains entirely aquatic, but food chains get more complex with diversifying levels of heterotroph consumers and detritivores and phototroph producers.

Fossils from Bitter Springs Formation of Australia (1 Ga) looks as if they show cells dividing.

Oldest known acritarchs (fossils of uncertain origin: some likely cysts of some kind of fossil photosynthesizers, possibly dinoflagellates, others are eggs of animals) at 1.4 Ga; become complex around 600 Ma; the main index fossils for the Proterozoic

Metabionts: multicellular organisms include:

Some of these groups have records back to the Proterozoic:

Animal record is somewhat better, but still much debate:

The Ediacaran fauna is preserved only as impressions in sediment. Indicate a variety of organisms. Originally were pigeon-holed into modern groups, but new evidence suggests that a number of types of animals are present:

None of the Ediacaran fauna apparently burrowed into the sediment, nor had preservable hard parts.

Interestingly, almost all the evidence for multicellular organisms comes AFTER the Gaskiers Glaciation. Some speculate that selective pressures from these hard times led to development of complex life; others that the appearance of creatures able to mobilize the carbon in the sea sediments kept atmospheric carbon dioxide from getting low enough to trigger the super ice ages.

Recent studies suggest that the size (and complexity) increases seen with the rise of endosymbiosis and of multicellularity both coincide with major increases in the amount of oxygen available.

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Last modified: 19 January 2012