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Geologic evidence for major environmental change. Geologic evidence for major environmental change. Banded Iron Formations. Geologic evidence for major environmental change. Banded Iron Formations.

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geologic evidence for major environmental change3
Geologic evidence for major environmental change

Banded Iron Formations

Banded iron formation is iron rich chert (cryptocrystalline silica (SiO2). The banded colours, usually on a cm scale are due to differing amounts and oxidation states of Fe-containing minerals: haematite, magnetite, grunerite, limonite, siderite and sometimes pyrite. BIF is not forming today and although it can be found in the Archean, most deposits of BIF were formed around 2 billion years ago.

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Banded Iron Formations are a distinctive type of rock often found in old sedimentary rocks. The structures consist of repeated thin layers of iron oxides. The oldest known rock formations dated around 3,800,000,000 years before present -- 3800MA -- include banded iron layers, and the banded layers are a common feature in sediments for much of the Earth's history. Banded iron beds are less common after 1800MA although some are known that are much younger.

The conventional concept is that the banded iron layers are the result of oxygen released by photosynthetic organisms (i.e. "plants" -- although the actual agents were likely bacteria), combining with dissolved iron in Earth's oceans to form insoluble iron oxides. The banding is assumed to result from cyclic peaks in oxygen production. It is unclear whether these were seasonal or followed some other cycle. It is assumed that initially the Earth started out with vast amounts of iron dissolved in the world's seas. Eventually, as "plants" pumped out oxygen, all the available iron in the Earth's oceans was precipitated out as iron oxides. The atmosphere became oxygenated. It is assumed that the rare later banded iron deposits represent unusual conditions where oxygen was depleted locally and iron rich waters could form then come into contact with oxygenated water.

The total amount of iron [oxygen?]locked up in the banded iron beds is estimated to be perhaps 20 times the volume of oxygen present in the modern atmosphere. Banded iron beds are an important commercial source of iron ore.

Retrieved from "http://en.wikipedia.org/wiki/Banded_iron_formation"

geologic evidence for major environmental change5
Geologic evidence for major environmental change

Banded Iron Formations

Banded iron formation is iron rich chert (cryptocrystalline silica (SiO2). The banded colours, usually on a cm scale are due to differing amounts and oxidation states of Fe-containing minerals: haematite, magnetite, grunerite, limonite, siderite and sometimes pyrite. BIF is not forming today and although it can be found in the Archean, most deposits of BIF were formed around 2 billion years ago.

huronian iron formations

banded iron formation consists of finely alternating layers of hematite-rich chert and grey iron-poor chert

Huronian iron formations

  • likely mechanism of precipitation was in response to cyanobacterial blooms
  • releases large quantities of oxygen
  • rapidly oxidises sea water, causing precipitation of Fe3+ as hematite
  • iron formations overlain by shale – BIFs were probably only deposited on basin margins, where water was agitated and oxidised.

http://ace.acadiau.ca/SCIENCE/GEOL/rraeside/backup/work/Courses%203-5/4003/7

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Stromatolites

The first traces of life appear nearly 3.5 billion years ago, in the early Archaean. However, clearly identifiable fossils remain rare until the late Archaean, when stromatolites, layered mounds produced by the growth of microbial mats, become common in the rock record.

Stromatolite diversity continued to increase through most of the Proterozoic.

Until about 1 billion years ago, they flourished in shallow waters throughout the world.

Their importance for understanding Proterozoic life is tremendous; stromatolites that have been silicified (forming a type of rock known as stromatolitic chert) often preserve exquisite microfossils of the microbes that made them. Shown here is a sample of stromatolitic chert from the Bitter Springs Formation of central Australia, about 850 million years old. Note the typical fine banding patterns.

Stromatolites began to decline in abundance and diversity about 700 million years ago. A popular theory for their decline (though certainly not the only possible explanation) is that herbivorous eukaryotes, perhaps including the first animals, evolved at about this time and began feeding extensively on growing stromatolites. Stromatolites are rare fossils after about 450 million years ago. Today, they are found only in restricted habitats with low levels of grazing, such as the shallow, saline waters of Shark Bay, Australia.

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Gowgonda Formation - The formation outcrops in Canada, just north of Michigan. This rock is called a tillite, it was deposited many millions of years ago as a glacial till. Millions of years later, when the glaciers moved south during the last ice age, they picked up chunks of Gowgonda rock and carried them into lower Michigan. This is a good example of how the rock cyles works. First, the original till was deposited, buried, and became lithified. Then the rock was uplifted, exposed, and incorporated into another glacial till. Gowganda rocks are quite easy to find in the glacial till deposits (any gravel pit) around Grand Rapids.

http://www.angelfire.com/rock3/michael/Gowgonda.html

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The Paleoproterozoic glaciation is difficult to time exactly because it is a terrestrial sedimentary deposit and does not contain any datable materials; however, the tillites are constrained in their age by the granites they overlay (2.6 Ga), and the dikes that cut through them (2.1 Ga). Paleoproterozoic tillites of southern Ontario comprise part of the Gowganda Formation and are classic in many ways. As a matter of fact, your humble instructor had a chance to appreciate them directly while a student at McMaster University in the early 1980’s. Our undergraduate field school was located near Sudbury and we spend many a fine spring day examining dropstones (rocks released from icebergs that drop down and distort layered sediment in marine environments), and scratched and striated boulders in the tillites. The sample I collected from the Gowganda Formation was one of the first rocks that I ever collected as a “geologist”. It was also one of the first rocks that my mother “borrowed” for her rock garden. The Gowganda tillites provide clear evidence of a major glaciation in the early Proterozoic and some refer to it as the Huronian Glaciation. The Neoproterozic event is known as the Varangian Glaciation and seems to have involved at least 4 distinct advances and retreats of glacial ice. The last one (600 Ma) may have been the grand-daddy of all glaciations. As your text book states, tillites of this age are found on all continents with the exception of Antarctica, including those that lay near the equator. Many geologists have concluded that world temperatures must have been much colder than anything experienced before or after this time period. Several geologists have also suggested that the Earth may have experienced true icehouse conditions. Sea water over much of the glob was frozen and in some areas (the poles?), the oceans may have been frozen right down to the bedrock. If this did occur (and not everyone believes that it did), we are lucky to have liquid water on our planet today. Frozen oceans and frozen land masses would have greatly increased the amount of sunlight reflected back to space (i.e., the Earth’s albedo would greatly increase), and world temperatures would have plummeted even more. Computer models of an icehouse Earth suggest that it would never warm up. Skeptics to the icehouse Earth hypothesis point out that other rocks do occur in the Neoproterozic besides tillites, including a lot of tropical sedimentary rocks. Like tillites, these rocks appear to be distributed all over the globe. There is clearly something stuffed up with the Neoproterozoic, or at least with our interpretation of the rocks formed during GY 112 Lecture Notes D. Haywick (2004) 5 this time. Maybe the glaciations across the planet did not occur at the same time. Remember, there is an error with radiometric dating. The older the rocks you are dating, the greater the error. Perhaps one continent experienced a glaciation 500 Ka or 1 Ma before the next. This is plenty of time for an advance and retreat of glacial ice (we’ve experienced a couple full blown glaciations during the past million years in the Quaternary), but we could not adequately resolve this difference in radiometric dates. However, even if this were the case, and the Earth was characterized by severe climatic belts like today back in the Neoproterozoic, it still could not explain the juxtapositioning of tropical and glaciated continental areas unless plate motion was much faster in the past. A plate moving 50 cm per year (about 10 times faster than today) would move 500 km over a million years. This might just do it. Then again, maybe we are just pain wrong about the origin of some of these “tillites”. The Neoproterozoic glaciation offers plenty of opportunities for quality graduate research projects should you be tempted to major in geology.

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Gowganda tillite

Gowganda glacial conglomerate

http://www.amnh.org/nationalcenter/online_field_journal/cp/cprk/cprkmain.html

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Penrith Sandstone, Permo-Triassic desert sandstone from England

http://www.stoneroof.org.uk/permo.html#Top