Geol 02C Historical Geology

J Bret Bennington

The Proterozoic Eon

The Proterozoic began about 2.5 billion years ago and is marked by extensive global episodes of metamorphism that suggest a period of increased continental accumulation or ‘cratonization’ as larger continental masses were assembled. In the Proterozoic age Slave Province in north central Canada an essentially modern tectonic sequence of geologic regions is preserved in the rocks of the Wopmay orogen. The Wopmay shows successive belts of igneous intrusives and volcanics, highly metamorphosed rock, and a fold and thrust belt. This sequence is similar to that seen in the much younger Appalachian Orogen along the eastern margin of North America and it implies that a convergent plate tectonic boundary existed along the margin of the Slave microcontinent which was deformed in the same way as modern orogenic systems.

Carbon Dioxide

By the early Proterozoic, large regions of continental shelf existed. These new expanses of shallow water covered with algal mats led to the production of extensive carbonate platforms (such as exist today in tropical regions - e.g. the Bahamas).

Carbonate formation, like photosynthesis, uses atmospheric CO₂ as the raw material. However, unlike the organic matter produced by photosythesis which is rapidly reoxidized, releasing CO₂ back into the atmosphere, carbonate is deposited as sedimentary rock, removing the CO₂ for geological spans of time. Thus the growth of carbonate platforms led to a decline in atmospheric CO₂ and a decrease in the global greenhouse.

2.0 GA Glaciation

Evidence for global cooling comes from the first glacial deposits found in the rock record at about 2.0 Ga (Gowgonda Tillite, Huronian Supergroup, Canada).

Global cooling and the formation of high latitude ice sheets caused cold polar water to begin to flow through the deep ocean, mixing the formerly stratified water column. This mixing and oxygenation of the deep ocean prevented the buildup of large amounts of iron (iron released into the water was oxidized immediately and locally) and shut down the production of BIFs.

Late Proterozoic Glaciation and the Icehouse / Snowball Earth

A second, apparently more extensive series of glaciations occurred in the Late Proterozoic between about 850 and 600 Ma. Glacial deposits from this age are found as a series of formations on all continents but Antarctica, suggesting a widespread and prolonged episode of cooling of the Earthâs climate.

Even more remarkable is the fact that paleogeographic reconstructions for this time suggest that most continents were in low latitude, equatorial positions. Glaciers do not usually form in equatorial regions except at high altitudes. The presence of thick deposits of carbonates (developed in equatorial to subtropical oceans) interlayered with glacial tillites (found in the Rapitan Group of the Canadian Cordillera and in Namibia) argues strongly for a rapidly shifting climate that brought glacial conditions almost to the equator. Such an extreme icehouse climate may have been triggered by the presence of so much highly reflective landmass distributed across the equator (equatorial oceans are much better collectors of solar heat) combined with extensive carbonate shelves drawing CO2 out of the atmosphere.

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Escape from the icehouse may have been triggered by the buildup of CO2 caused by the lack of production of carbonates in equatorial waters during glacial times. Thus, the Earth may have fluctuated for several hundred million years from icehouse to greenhouse conditions. Fortunately, the climate never got cold enough to allow CO2 to freeze at the poles. This could have effectively nullified the buildup of CO2 during glacial episodes, leading to a runaway icehouse and a permanently frozen "snowball" Earth. However, it is possible that for millions of years at a time the Earth was almost completely covered in ice - glaciers on the continents and a thick layer of sea ice over the oceans. Surface temperatures during these extreme "icehouse" conditions may have been as low as -40° C. Carbon isotopes preserved in the carbonate rock above and below the glacial deposits suggest that during the icehouse intervals much of the primary productivity in the oceans was lost. Photosynthesis preferentially removes 12C from ocean water, leaving the water and carbonate that forms within it enriched in 13C. Following the icehouse, carbonates are strongly enriched in 12C, suggesting a drastic reduction in photosynthesis in the world's oceans.

Life in the Proterozoic

Stromatolites greatly increase in abundance and diversity beginning at about 2.7 Ga. It is not clear if this is due to the evolution of a variety of stromatolite-forming species of cyanobacteria or to the increase in shelf environment area and diversity that accompanied the growth of larger continental cratons. Whatever the case, it is clear that cyanobacteria and other procaryotes were abundant in the worlds oceans. 2.1 Ga cherts from the Gunflint Formation in southern Canada include a wide variety of bacteria-like fossils.

Oxygen One line of evidence indicating the presence of photosythetic bacteria is the apparent rise in global oxygen levels into the Proterozoic. By the early Proterozoic detrital pyrite and uraninite disappear from sedimentary rock and red beds - terrestrial sediments rich in oxidized iron - become increasingly common in the rock record.

Eucaryotes Microfossils of larger, more thick-walled cells appear in deposits beginning about 1.8 Ga and may be the remains of the first eucaryotic organisms.

Multicellular Organisms The evolution of multicellular organisms occurred near the end of the Proterozoic, beginning at about 1.0 Ga. We will cover this event in detail in the next lecture.

Major Proterozoic Tectonic Events

Rifting in central and eastern North America

From 1.2 to 1.0 Ga an extensive rift zone formed beneath North America, stretching from Kansas northeastward above Lake Huron to Montreal. This episode of rifting caused extensive downfaulting of the crust and the intrusion of large quantities of mafic lava. Mafic igneous activity also extended northwestward up through central Canada where huge systems of dikes were emplaced along fractures in the craton. For unknown reasons, this epidode of rifting was aborted and did not continue enough to divide the North American craton into two separate continents.

Grenville Orogeny and Pangea I (Rodinia)

At the same time that rifting was occurring in the continental interior, eastern North America was experiencing an extensive continental collision called the Grenville Orogeny. In this event, an enormous belt of metamorphic rock was added to the margin of the continent at approximately 1.1-1.0 Ga. The Grenville Orogeny appears to have been part of a global assembly of continental landmasses near the end of the Proterozoic that led to the formation of a supercontinent called Rodinia.

Terminal Proterozoic Rifting

Faults and volcanic intrusives in Grenville age rocks show that several hundred million years after the end of the Grenville Orogeny, near the end of the Proterozoic, Rodinia began to split apart. It has been suggested that another supercontinent was assembled at the very end of the Proterozoic as continental collisions occurred centered around Africa. However, the current evidence for this is inconclusive. This new supercontinent is tentatively named Pannotia. Near the beginning of the Phanerozoic North America (really North America and Greenland - a continent called Laurentia) rifted away from Europe (Baltica) and the rest of the continents, most of which were to remain together until the Triassic as the southern hemisphere land mass known as Gondwanaland.