The Hadean (Eoarchean) and Archean. h"p://

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1 The Hadean (Eoarchean) and Archean h"p://

2 The Precambrian Eons The Precambrian Eon contains three subdivisions: Proterozoic Eon (2.5 Ga 0.54 Ga) Archean Eon (4.0 Ga 2.5 Ga) Hadean (4.5 Ga 4.0 Ga) The Hadean frequently doesn t appear on geologic times scales because there are no Earth rocks of this age. The beginning of the Archean is defined as the age of the oldest known rock.

3 The Precambrian Eons The Hadean, Archean and Proterozoic Eons represent ~88% of Earth s history. There are no Hadean rocks left on Earth, but there are some detrital grains found in Archean rocks.

4 The Hadean 4,600,000,000 4,000,000,000 The Hadean is not an official geologic eon because no rocks of this age remain on Earth. A combination of weathering, metamorphism, subduction and the massive bombardment of Earth at ~4,100,000,000 have recycled all Earth s Hadean rocks. Major Events of the Hadean include: Formation of the Earth Several very large collisions (and many smaller collisions) Formation of Earth s Moon Differentiation of the Earth into core, mantle and crust

5 The Hadean 4,600,000,000 4,000,000,000 The Earth formed in the cloud of material left over from the formation of the Sun. This disk rotated around the young star and the larger pieces were drawn together by mutual gravitational attraction. Eventually, these protoplanets swept up most of the debris in their orbital path. The Earth collided at least once (and perhaps half a dozen) with a very large protoplanet. One of these collisions is thought to have resulted in the formation of Earth s Moon.

6 The Big Whack Theory of Lunar Formation ~4,500,000,000 years ago Earth s Moon has a very low density compared to Mercury, Venus, Earth and Mars. The Big Whack theory of lunar formation was originally proposed to explain this oddity. Instead of being formed from detritus in the early Solar System, the Moon was derived from the Earth and a big impactor. The theory assumes that the Earth had already differentiated into crust and mantle. An oblique collision smeared mantle material into orbit around Earth, which coalesced into Earth s Moon. The Earth may have absorbed the iron core of the collider. Video: h)p://

7 The Hadean 4,600,000,000 4,000,000,000 The constant bombardment prevented formation of solid crust until ~4,400,000,000 years ago. The first crust would have been ultramafic basically a frozen crust of the mantle. Differentiation into a thicker, more silica-rich crust would have begun as soon as plate tectonics started. Recent discoveries indicated that oceans were present on this early frozen crust so the early Earth was much cooler than previously suspected.

8 A Cool Early Earth 4,400,000,000 4,000,000,000 Zircon (ZrSiO 4 ) crystals form in various igneous environments and are nearly impervious to most chemical processes. They are also physically tough and thus make very sturdy sedimentary clasts. Zircons also tend to incorporate metals like uranium that can be used for radiometric dating and other chemical analyses. h)p://

9 A Cool Early Earth 4,400,000,000 4,000,000,000 Evidence for this cool period in Earth s early history was found in detrital zircon crystals from Australia s Jack Hills metaconglomerate (a metamorphosed sedimentary rock). Radiometric dating indicates that the oldest part of one zircon crystal is 4,400,000,000 years old. The chemistry of the crystal indicates that liquid water oceans must have been present on Earth s surface. h)p://

10 A Cool Early Earth 4,400,000,000 4,000,000,000 Multiple samples were taken from this tiny crystal using a very small drill. Uraniumlead dating gave an age of 4.4 Ga for the oldest part of the crystal h)p://

11 A Cool Early Earth 4,400,000,000 4,000,000,000 The ratio of the two stable oxygen isotopes (which are sensitive to temperature) indicates that the surface temperature during this early phase of Earth s history was similar to Archean temperatures an eon for which we have direct evidence of rocks deposited in oceans. h)p://

12 A Cool Early Earth 4,400,000,000 4,000,000,000 Whatever differentiation of the crust occurred at this time was at least partially destroyed by the ~100,000,000 year late heavy bombardment of the inner solar system by a massive number of asteroids starting at ~4.0 Ga. The early oceans and early atmosphere of Earth were also lost in this event. h)p://

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14 Acasta Gneiss h"p:// The beginning of the Archean Eon is defined by the age of the world s oldest known rock formation ~4.0 Ga.

15 Early Archean Eon After the late heavy bombardment, the Earth s atmosphere was reformed primarily from outgassing from volcanoes. Magma contains dissolved gases that would have been liberated when the magma came to the surface. Water (H 2 O), carbon dioxide (CO 2 ), nitrogen (N 2 ) and hydrogen (H 2 ) gas are the major volcanic gases. Methane (CH 4 ) and ammonia (NH 3 ) would form from chemical reactions in the atmosphere. Some free oxygen gas (O 2 ) might be produced when solar radiation blew apart water in the upper atmosphere, but it remained a very minor trace element until the onset of photosynthesis at ~3.5 Ga by cyanobacteria.

16 Archean Earth Conditions Atmosphere - CO 2, H 2 O, N 2, methane, ammonia Very little free free oxygen (O 2 ) No stratospheric ozone layer (O 3 ) Weather - storms dump acid rain, UV light shines down Surface - newly cooled Earth has thin crust, volcanic activity, frequent bombardment by space junk, rapid weathering of exposed rocks Temperature - drops below 100 o C everywhere on surface, allowing liquid oceans Oceans - filled with dissolved minerals

17 The Earth is approximately 4.5 billion years old, but the oldest preserved rocks are half a billion years younger. The oldest undoubted fossils are more than 3 billion years old. 1,000,000,000 years is a loooong time for a biochemistry experiment to run.

18 Could the basic building blocks of life form under those conditions? Miller-Urey Experiment, 1953 Mixture of methane, hydrogen, and water sparked by electrical current to simulate lightning. Produced organic compounds including amino acids.

19 H 2 O N 2 H 2 NH 3 CH 4 CO 2

20 Cyanogen Formaldehyde H 2 O N 2 H 2 NH 3 CH 4 CO 2

21 Hydrogen cyanide Cyanogen Cyanoacetylene Formaldehyde Acetaldehyde Propionaldehyde Other Compounds Glycol acid N-Methylalanine Lactic acid Aminobutyric acids Formic acid Acetic acid Propionic acid Urea Aspartic acid And others Some organic and nonorganic chemicals produced in Miller-Urey s experiment. Many of these molecules are basic building blocks of more complex organic molecules.

22 Amino Acids Glycine Two amino acids (the building blocks of proteins) were also synthesized in this very simple experiment. Making the building blocks of biochemistry is relatively easy. Alanine

23 M-U s gases were not representative of current theories of the Earth s Hadean atmosphere. However, basic organic compounds form under all hypothesized early Earth chemical system as well as on other planets and space bodies. All that is required is the raw materials, reducing conditions, and an energy sources (e.g., sunlight, electricity, radioactivity, and plain old heat). Simple proteins have also been produced in these experiments.

24 Earliest Traces of Life The earliest of evidence of life on Earth primarily comes from stable isotope excursions preserved in 3,800,000,000 sedimentary rocks. Organic chemistry tends to enriched in the light carbon isotope (12C) compared to the heavy carbon isotope (13C). Rosing, M.T C-depleted carbon microparticles in >3700-Ma sea-floor sedimentary rocks from West Greenland. Science 283: Turbiditic and pelagic sedimentary rocks from the Isua supracrustal belt in west Greenland [more than 3700 million years ago (Ma)] contain reduced carbon that is likely biogenic. The carbon is present as 2- to 5micrometer graphite globules and has an isotopic composition of δ13c that is about 19 per mil (Pee Dee belemnite standard). These data and the mode of occurrence indicate that the reduced carbon represents biogenic detritus, which was perhaps derived from planktonic organisms.

25 Earliest Body Fossils These microfossils are preserved in chert of the Swartkoppie Formation. Figure from Knoll and Barghoorn (1977). The have been caught in the act of dividing just like living bacteria. Age: 3,500,000,000 years

26 Phototrophs Cyanobacteria In addition to archaebacteria, some eubacteria ( true bacteria ) also evolved early in the history of life on Earth. Cyanobacteria (blue-green algae) fossils are found in 3.0 billion year old rocks.

27 Modern Cyanobacteria Large bloom due to pollution of Bedetti Lake, Argentina enlarged 2500X

28 Green Layer - filamentous blue-green algae (cyanobacteria) Pink Layer - Purple sulfur bacteria Black Layer - Iron sulfide, produced by abundant sulfur-reducing bacteria Grey Layer - color change due to formation of mineral pyrite, bacteria less abundant copyright 1997 Rolf Schauder Cross section of a microbial mat in a salt marsh. Approximately 5 cm deep.

29 Stromatolites Stromatolites are sedimentary structures formed by the activity of microbial mats. They are found in rocks over 3.0 billion years old.

30 Modern Stromatolites Hyper-saline bay exposed at low tide, Shark s Bay, Australia

31 Ancient Stromatolites Hakati Shale, Middle Proterozoic Grand Canyon NP, Arizona Characteristic laminated structure

32 Archean Ecology O 2 O 2 Photosynthesis by blue-green algae (cyanobacteria) in shallow water releases oxygen into the water Communities of bacteria and archaebacteria

33 Archean Plate Tectonics Archean plate tectonic differed from modern plate tectonics: Plates likely moved faster Magma was likely generated more rapidly Continents likely grew more rapidly along their margins as plates collided with island arcs and other plates Continental crust was thinner than it is today 33

34 Cratons, Shields, and Platform Terminology Craton: Precambrian core of continental crust Shield: exposed craton Platform: A Precambrian sedimentary sequence covering part of a craton Both Archean and Proterozoic rocks are present in the cratons. All the Archean rocks record evidence of episodes of mountain building. Most cratons have experienced very little deformation since the Precambrian 34

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36 The Canadian Shield The Canadian shield comprises most of northeastern Canada a large part of Greenland parts of Minnesota, Wisconsin, Michigan, and New York Topography is subdued and thinly covered in places by Pleistocene glacial deposits. Unmetamorphosed Proterozoic sedimentary and volcanic rocks overlie Archean metamorphic rocks. Greenstone belts in Archean shields in the other continents have the same stratigraphic relationship as those within the Canadian Shield

37 Greenstone Belts and Granulites Greenstones are metamorphosed ultramafic and mafic volcanic rocks and associated clastic sedimentary rocks. Archean-age greenstone belts are present in all cratons Granulites are strongly metamorphosed granites and gneisses that intruded and underlie the greenstones 37

38 Island Arc Model for Formation of Greenstone Belts Subduction of oceanic crust formed volcanic island arcs Greenstone belt began forming in the back-arc basin Magma underplated the back-arc basin Compression caused the basin to be folded and intruded by magma 38

39 Intracratonic Model for Formation of Greenstone Belts Greenstone belts (dark green) Granite-gneiss complexes (light green The intracratonic model for evolution of part of the Superior craton 39