The Supercontinent, Rodinia

Transcription

1 The Supercontinent, Rodinia The supercontinent, Rodinia, as it appeared about 1.1 b.y. ago. The reddish band down the center of the globe is the location of continental collisions and orogeny, including the Grenville orogeny.

2 The Supercontinent, Rodinia Rodinia formed as the continents collided during the Grenville Orogeny. Rodinia persisted as a supercontinent for about 350 million years. It was surrounded by an ocean called Mirovia.

3 Rifting in Rodinia Rodinia began to rift and break up about 750 million years ago, forming the proto-pacific Ocean, Panthalassa, along the western side of North America.

4 Rifting in Rodinia An early failed attempt at rifting began in eastern North America about 760 m.y. ago, with the deposition of sediments of the Mount Rogers Formation in a fault-bounded rift valley. Felsic and mafic volcanic rocks are interlayered with the sedimentary rocks of the Mount Rogers Formation.

5 Neoproterozoic Era The Neoproterozoic (or "new" Proterozoic) ranges from about 1.0 b.y. to b.y. (542 m.y.).

6 Highlights of the Neoproterozoic Extensive continental glaciations Sediments deposited in basins and shelf areas along the eastern edge of the North American craton. Most of these rocks were deformed during Paleozoic orogenies.

7 Glacial deposits - Neoproterozoic Glacial deposits formed roughly m.y. ago. Evidence for glaciation: Glacial striations (scratched and grooved pebbles and boulders) Tillites (lithified, unsorted conglomerates and boulder beds) found nearly worldwide Glacial dropstones (chunks of rocks released from melting icebergs) Varved clays from glacial lakes

8 Rifting in Rodinia Around 570 million years ago, rifting began again, and South America began to separate from North America, forming the Iapetus Ocean (or proto-atlantic Ocean). The rift ran along what is now the Blue Ridge province. Basaltic lava flows formed the Catoctin Formation. As the Iapetus Ocean opened, sands and silts were deposited in the shelf areas.

9 Glacial deposits - Neoproterozoic This time is referred to as "snowball Earth" because glacial deposits are so widespread. Varangian glaciation (named after an area in Norway). Late Proterozoic ice age lasted about 240 m.y.

10 Plate Tectonics and Glaciation Plate tectonics may have had a role in cooling the planet. Continents were located around the equator about 600 to 700 m.y. ago. No tropical ocean.

11 Plate Tectonics and Glaciation Heat lost by reflection from the rocks on the surface of the continents may have caused global cooling. (Land plants had not yet appeared.) As continental glaciers and ice caps formed, reflectivity of snow and ice caused further temperature decrease.

12 Atmospheric Gases and Glaciation Glaciation was associated with: Decrease in CO 2 and Increase in O 2. CO 2 causes the greenhouse effect and global warming. Decrease in CO 2 may have caused cooling. Decrease in CO 2 was probably caused by increase in the number of photosynthetic organisms (cyanobacteria, stromatolites).

13 Limestones and Glaciations Limestones are associated with glacial deposits, which is unusual, since limestones generally form in warm seas, not cold ones. Association of limestones with glacial deposits suggests that times of photosynthesis and CO 2 removal alternated with times of glaciation. Limestones (made of CaCO 3 ) are a storehouse of CO 2, which was removed from the atmosphere.

14 Limestones and Glaciations Glacial conditions may have inhibited photosynthesis by stromatolites. As a result, CO 2 may have accumulated periodically and triggered short episodes of global warming. This produces the paradox of glaciers causing their own destruction.

15 Proterozoic Rocks South of the Canadian Shield Extensive outcrops of Precambrian rocks are present in the Canadian Shield. Precambrian rocks are also present in other areas, including: Rocky Mountains Colorado Plateau (Grand Canyon)

16 Events Recorded in Proterozoic Rocks 1. Collision of an Archean terrane with volcanic island arc, 1.7 or 1.8 b.y.a. (Wyoming and western Colorado) 2. Extensive magma intrusion in Mesoproterozoic, b.y.a. (California to Labrador) 3. Widespread rifting 4. Rifts with thick sequences of shallow water Neoproterozoic sedimentary rocks, b.y.a. Belt Supergroup (Glacier National Park, Montana, Idaho, and British Columbia).

17 Precambrian rocks of the Grand Canyon Vishnu Schist metasediments and gneisses, intruded by Zoroaster Granite about 1.4 b.y. to 1.3 b.y.a. during the Mazatzal orogeny. Top of Vishnu Schist is an unconformity.

18 Precambrian rocks of the Grand Canyon Grand Canyon Supergroup overlies unconformity. Neoproterozoic sandstones, siltstones, and shales. Correlates with Belt Supergroup. Unconformably overlain by Cambrian rocks.

19 Life at the beginning of Proterozoic was similar to that of Archean 1. Archaea in deep sea hydrothermal vents 2. Planktonic prokaryotes floated in seas and lakes 3. Anaerobic prokaryotes in oxygen-deficient environments 4. Photosynthetic cyanobacteria (prokaryotes) constructed stromatolites (algal filaments) 5. Eukaryotes (as indicated by molecular fossils)

20 Other forms of life appeared during Proterozoic 1. More diverse eukaryotes including acritarchs 2. Metazoans or multicellular animals with soft bodies 3. Metazoans with tiny calcium carbonate tubes or shells 4. Metazoans that left burrows in the sediment

21 Microfossils of the Gunflint Chert First definitive Precambrian fossils to be discovered (in 1953) were in the 1.9 b.y. old Gunflint Chert, NW of Lake Superior (Paleoproterozoic).

22 Microfossils of the Gunflint Chert The fossils are well-preserved, abundant and diverse and include: String-like filaments Spherical cells Filaments with cells separated by septae (Gunflintia) Finely separate forms resembling living algae (Animikiea) Star-like forms resembling living iron- and magnesium-reducing bacteria (Eoastrion)

23 Microfossils of the Gunflint Chert A = Eoastrion ( = dawn star), probably iron- or magnesium-reducing bacteria B = Eosphaera, an organism or uncertain affinity, about 30 micrometers in diameter C = Animikiea (probably algae) D = Kakabekia, an organism or uncertain affinity

24 Microfossils of the Gunflint Chert Gunflint fossil organisms resemble photosynthetic organisms. The rock containing these organisms contains organic compounds that are regarded as the breakdown products of chlorophyll. The Gunflint Chert organisms altered the composition of the atmosphere by producing oxygen.

25 The Rise of Eukaryotes The appearance of eukaryotes is a major event during the history of life. Eukaryotes have the potential for sexual reproduction, which increases variation through genetic recombination.

26 The Rise of Eukaryotes Genetic recombination provides greater possibilities for evolutionary change. Diversification of life probably did not occur until after the advent of sexual reproduction, or until oxygen levels reached a critical threshold.

27 Eukaryotic cells can be differentiated from prokaryotic cells on the basis of size. Eukaryotes tend to be much larger than prokaryotes (larger than 60 microns, as compared with less than 20 microns).

28 The Rise of Eukaryotes Eukaryotes appeared by Archean (as determined by molecular fossils or biochemical remains). Larger cells begin to appear in the fossil record by 2.7 b.y. to 2.2 b.y. Eukaryotes began to diversity about 1.2 to 1.0 b.y. ago.

29 Acritarchs 1. Eukaryotes 2. Single-celled, spherical microfossils 3. Thick organic covering 4. May have been phytoplankton 5. First appeared 1.6 b.y. ago (at Paleoproterozoic- Mesoproterozoic boundary) 6. Some resemble cysts or resting stages of modern marine algae called dinoflagellates.

30 Acritarchs 7. Reached maximum diversity and abundance 850 m.y. ago 8. Declined during Neoproterozoic glaciation 9. Few acritarchs remained by 675 m.y. ago 10.Extinction during Ordovician 11.Useful for correlating Proterozoic strata

31 The First Metazoans (Multicellular Animals) Metazoans are multicellular animals with various types of cells organized into tissues and organs. Metazoans first appeared during Neoproterozoic, about 630 m.y. ago (0.63 b.y.). Preserved as impressions of softbodied organisms in sandstones.

32 Examples of metazoan fossils in Proterozoic rocks Ediacara fauna - Imprints of soft-bodied organisms, first found in Australia during the 1940s Metazoan eggs and embryos in uppermost Neoproterozoic Doushantuo Formation, South China Trace fossils of burrowing metazoans in rocks younger than the Varangian glaciation. Tiny shell-bearing fossils (small shelly fauna)

33 Ediacara fauna Ediacara fauna is an important record of the first evolutionary radiation of multicellular animals. Some were probably ancestral to Paleozoic invertebrates. Oldest Ediacara-type fossils are from China. Youngest Edicara-type fossils are Cambrian (510 m.y., Ireland).

34 Types of Ediacara fossils Discoidal Frondlike Elongate or ovate

35 Ediacara fauna Because the Ediacara creatures are not really similar to animals that are living today, this has led to the suggestion that they be placed in a separate taxonomic category or new phylum. The name proposed for this new category is Vendoza (named after the Vendian, or the latest part of the Neoproterozoic in Russia).

36 Small Shelly Fauna: The Origin of Hard Parts Small fossils with hard parts or shells appeared during Neoproterozoic.

37 Small Shelly Fauna: The Origin of Hard Parts Cloudina, an organism with a small, tubular shell of calcium carbonate (CaCO 3 ). Resembles structures built by a tube-dwelling annelid worm. Earliest known organism with a CaCO 3 shell. Found in Namibia, Africa.

38 Small Shelly Fauna: The Origin of Hard Parts Other latest Proterozoic and earliest Cambrian small fossils with shells include: Possible primitive molluscs Sponge spicules, Tubular or cap-shaped shells, and Tiny tusk-shaped fossils called hyoliths Some early shelly material is made of calcium phosphate.

39 Precambrian Trace Fossils Trails, burrows, and other trace fossils are found in Upper Neoproterozoic rocks. In rocks deposited after Neoproterozoic Varangian glaciation. Mostly simple, shallow burrows. Trace fossils increase in diversity, complexity, and number in younger (Cambrian) rocks.

40 What stimulated the appearance of metazoans? May be related to the accumulation of sufficient oxygen in the atmosphere to support an oxygen-based metabolism. Ancestral metazoans may have lived in "oxygen oases" of marine plants. Ediacaran life may have evolved gradually from earlier forms that did not leave a fossil record.

41 Review of Proterozoic Events