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Earth History 3. The Proterozoic Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States. What to Look For: • Proterozoic plate tectonics involved true continental drift. By the end of the Archaean there were large enough cratons to behave like fullsized continents, by the end of the Proterozoic the modern cratons were recognizable and incorporated all the modern types of plate margins and stable interiors in their structure. • Oxygen production and build-up, which had begun in the late Archaean was quickly finalized in the early Proterozoic. Ever since there has been free O2 in the atmosphere. • Sedimentary rocks were consequently characterized by minerals with completely oxidized metals. Hematite is particularly conspicuous in redbeds and ironstones. • This abundance of free oxygen also allowed blossoming of aerobic life: more stromatolites, aerobic bacteria, and finally eukaryotic protists. By the latest interval of the Eon (the Ediacaran) there were multicellular organisms whose relationship to later life is uncertain. Proterozoic History 1 – Plate Tectonics By the end of Archaean time there were large areas of aggregated island arcs that began to behave as continental crust rather than island arcs. You are familiar with this type of tectonics from earlier in the course, and we will elaborate on how it actually progressed when we get to the Phanerozoic. The Proterozoic Eon saw continued aggregation of these cratons into even bigger landmasses. For example, early in the Eon the various provinces of the Canadian Shield collided and built mountain chains between them (next slide). They have remained as one cratonic unit ever since. The green bands represent metamorphic rocks dated at 1.8 to 2.0 by – in the middle Early Proterozoic. The Late Archaean provinces are also labeled. The green rocks were metamorphosed as the small cratons accreted during this interval, continuing the growth of the craton seen in the Archaean. Rae Province Greenland Province Slave Province Hearne Province Superior Province Wyoming Province Of course, another way to think of this is that by a little less than 2 by ago North America looked roughly like the red area on this map. This area covers all the rocks in the continent 1.8 by in age or older and the youngest of these are sediments and/or volcanics deformed as the older pieces accreted at just prior to about 1.8 by or so. With the passage of more time these continental cores grew even larger, by both arc volcanism on their edges and by accretion of offshore arcs and small continents. Between about 1.8 and 1.75 by, accretionary events added the material shown in green on this map. Then between about 1.75 and 1.6 by the region in blue was similarly added. This is the rough outline of our continent at the end of the Middle Proterozoic. Precambrian continental accretion culminated with a series of collisions among essentially all the continents in the neighborhood of a billion years ago – Late Proterozoic but pre-Ediacaran. The region in yellow on this map was the consequence of that collision in North America – the Grenville Mountains. The Grenvilles were a Himalayan-type chain of great size. The sediments that came off them as they eroded away were later incorporated into younger mountains, in part. The rocks of the Cohutta Mountains in GA, the Great Smokeys in TN/NC, and the other high chains of the western Blue Ridge are mainly built of these rocks, metamorphosed. This was our continent about 900 my ago. Georgia was mostly not there yet. It would form, in part, during a series of additional volcanic arcs and a collision in the Paleozoic, and from a piece of Africa left behind when the Atlantic Ocean rifted. It is still forming as its rivers bring sediment out of the stubs of the Appalachians and deposit it along the coast and the continental shelf edge. X You are here. Within the landmass that resulted from the mass pile-up, the modern cratons were recognizable, though they were aggregated into that one large landmass. We call that “supercontinent” Rhodinia. Look over the map on the next page and pick out where they are. Note that some continents have more than one craton (or shield) and so are not always recognizable as their modern shapes. Ediacaran paleogeography (modified from Scotese) sChn nChn Aust Arab Panthalassia Ind Ant Rhodinia nAf sAf N Asia NAm wAf Grn nSAm Eur Grenville Mts By the very end of the Ediacaran, Rhodinia had rifted to produce the Cambrian continents whose subsequent history we will follow more closely through the Phanerozoic Eon. Proterozoic History 2 – Rock Types and their environmental implications The BIFs that had begun appearing late in the Archaean became more and more abundant through the first half-billion years of the Proterozoic. Most are between 2500 and 2000 my (2.5 and 2 by) in age. After that, sediments are dominated by fully oxidized minerals such a hematite. In other words, redbeds and non-BIF ironstones became prevalent. In other words, besides the BIFs, Proterozoic rocks look almost exactly like Cenozoic rocks. All that is missing is the great diversity of fossil organisms. Proterozoic History 3 – Proterozoic Life These are Proterozoic stromatolites, and they are uncharacteristically small. They are usually much larger, frequently quite a bit taller than you are. 1 cm The increasing abundance of BIFs suggests continued accumulation of free oxygen in the atmosphere. This was accompanied, beginning about 2.3 by ago, by a blossoming of larger and more complex stromatolites of a sort presently constructed by aerobic Cyanobacteria. The oldest known eukaryotic cells (those with nuclei) are about 2.1 by old. These cells are preserved with the nucleus clearly visible, often caught in the act of dividing in preparation for cell division. Because all eukaryotic cells are obligatory aerobes (they must have free O2) their existence necessarily implies that this gas was continuously abundant in the atmosphere by that time. At about 1.4 by ago there is a noticeable increase in size in these protists, to the size of modern algae and protozoans. There is still no sign of plants or animals, not even trace fossils. Two interesting types of fossils (animals?) appear in the Ediacaran rocks of most continents. Body fossils take the form of carbon films of rather large organisms. Their shapes are odd, in comparison to any living organisms we know, and even in comparison to any later fossils we know. They have been interpreted as belonging to nearly all of the kingdoms (except the bacterial ones), or even to their own unique one. The Ediacaran is also where the first trace fossils are found. There are no burrows to any great depth, but things were clearly moving around on and in the uppermost parts of sedimentary beds. Thus both body and trace fossils indicate the beginnings of multicellular life in the latest Proterozoic. The diagram on the next page summarizes in detail the major events in Phanerozoic history along with radiometrically derived ages of those events. You do not need to know this level of detail, but if I ask about the Precambrian time scale you should be able to give a rough outline of the Eons, the main characteristics, and the boundary ages. This summary will help: ~542 my ----- End of Proterozoic and Ediacaran/Beginning of Phanerozoic and Cambrian Ediacaran – multicellular organisms evident. Rhodinia. ~900my – 1 by Proterozoic Eon – Assembly of modern cratons and, finally Rhodinia. Abundant free oxygen in the atmosphere, based on: Abundance of aerobic organisms, including Protista, and Abundance of redbeds and non-BIF ironstones. ~2.5 by ----- End of Archaean and Beginning of Proterozoic. Archaean Eon – Tectonic system operating with mostly oceanic crust and island arcs. Limited accretion of these arcs into larger cratonic pieces. O2-free atmosphere based on abundance of reduced metal minerals and lack of apparent aerobic fossils. ~3.9 by (but subject to “oldward” revision) – Beginning of rock record and Archaean Eon. No rock record. ~4.6 by – Existence of differentiated Earth. Take-Home Message: • Proterozoic plate tectonics involved true continental drift. By the end of the Archaean there were large enough cratons to behave like fullsized continents, by the end of the Proterozoic the modern cratons were recognizable and incorporated all the modern types of plate margins and stable interiors in their structure. • Oxygen production and build-up, which had begun in the late Archaean was quickly finalized in the early Proterozoic. Ever since there has been free O2 in the atmosphere. • Sedimentary rocks were consequently characterized by minerals with completely oxidized metals. Hematite is particularly conspicuous in redbeds and ironstones. • This abundance of free oxygen also allowed blossoming of aerobic life: more stromatolites, aerobic bacteria, and finally eukaryotic protists. By the latest interval of the Eon (the Ediacaran) there were multicellular organisms whose relationship to later life is uncertain.