GEOL 102 Historical Geology

Spring Semester 2012
The Proterozoic Eon I: Birth of Modern Geology

Proterozoic Eon: 2.5 - 0.542 Ga

Proterozoic is distinct from Archean in:

The 2.5 Ga boundary is somewhat arbitrary: shift from "Archean" to "Proterozoic"-style crust begins at 2.95 Ga in southern Africa, but not until 2.6 in North Ameirca, and after 2.45 in some other regions.

Lithologies of the Proterozoic:

The Great Oxidation Event: Prokaryotic photosynthesizers, included the newly-evolved cyanobacteria (and eventually eukaryotic algae) release more and more oxygen into atmosphere. Between 2.7 and 2.4 Ga, most of this oxygen got absorbed by the copious levels of dissolved iron in the ocean water. This produced "rust", which accumulated on the sea floors as Banded Iron Formations (or BIFs).

When the dissolved iron was all used up, BIF production stopped and the oxygen began to add to the atmosphere (and as dissolved oxygen in the water). Atmospheric levels of oxygen rises to about 10% of modern levels. The Great Oxidation Event produced the modern oxygen-nitrogen atmosphere:

The Great Oxidation Event would have devastated anaerobic organisms, which from that point onward would survive only in "extreme" environments.

Paleoproterozoic saw the suturing together of many small Archean cratons to form much larger continents: the formation of the large cratons of today:

Recent work has suggested the following Paleoproterozoic continents and supercontinents had formed by accretion and suturing of Archean provinces:

The Paleoproterozoic Era has recently been subdivided into the Siderian ("iron", after banded iron: 2.50 - 2.30 Ga), Rhyacian ("streams of lava": 2.30 - 2.05 Ga), Orosirian ("mountain building": 2.05 - 1.80 Ga), and Statherian ("stabilization": 1.80 - 1.60 Ga) Periods. The boundaries are arbitrarily defined, but their names are derived from prominent geologic processes occurring at that time.

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Last modified: 19 January 2012