<<

The () and

hp://www.arcadiastreet.com/cgvistas/earth/01_precambrian/earth_01_precambrian_2200b.htm The “ 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 rocks of this age. The beginning of the Archean is defined as the age of the oldest known . The Precambrian Eons

The Hadean, Archean and 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. 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 , 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 Differentiation of the Earth into core, mantle and crust 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 . 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. The “Big Whack” Theory of Lunar Formation ~4,500,000,000 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: hp://www.pbs.org/wgbh/nova/tothemoon/origins2.html 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 started.

Recent discoveries indicated that were present on this early frozen crust – so the was much cooler than previously suspected. A Cool Early Earth 4,400,000,000 – 4,000,000,000

Zircon (ZrSiO4) 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 and other chemical analyses.

hp://www..wisc.edu/zircon/cool_early/cool_early_home.html 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 ’s metaconglomerate (a metamorphosed ).

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 oceans must have been present on Earth’s surface.

hp://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html 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. Uranium- lead dating gave an age of 4.4 Ga for the oldest part of the crystal

hp://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html A Cool Early Earth 4,400,000,000 – 4,000,000,000

The ratio of the two stable 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.

hp://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html 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 of the inner solar system by a massive number of asteroids starting at ~4.0 Ga.

The early oceans and early were also lost in this event.

hp://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html hp://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html Acasta

hp://www.nmnh.si.edu/

The beginning of the Archean Eon is defined by the age of the world’s oldest known rock formation ~4.0 Ga. Early Archean Eon

After the late heavy bombardment, the Earth’s atmosphere was reformed primarily from out- gassing from volcanoes.

Magma contains dissolved gases that would have been liberated when the magma came to the surface. Water (H2O), dioxide (CO2), nitrogen (N2) and hydrogen (H2) gas are the major volcanic gases.

Methane (CH4) and ammonia (NH3) would form from chemical reactions in the atmosphere.

Some free oxygen gas (O2) 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 at ~3.5 Ga by . Archean Earth Conditions

Atmosphere - CO2, H2O, N2, methane, ammonia

Very little free free oxygen (O2)

No stratospheric ozone layer (O3) 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 100oC everywhere on surface, allowing liquid oceans Oceans - filled with dissolved minerals The Earth is approximately 4.5 old, but the oldest preserved rocks are half a billion years younger.

The oldest undoubted are more than 3 billion years old.

1,000,000,000 years is a loooong time for a biochemistry experiment to run. Could the basic building blocks of 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.

http://www.dc.peachnet.edu/~pgore/geology/geo102/precamb.htm H2O

N2

H2

NH3

CH4

CO2 Cyanogen

Formaldehyde

H2O

N2

H2

NH3

CH4

CO2 Hydrogen cyanide

Cyanogen

Formaldehyde Acetaldehyde Propionaldehyde Cyanoacetylene

Glycol acid N-Methylalanine Some organic and non- Lactic acid organic chemicals produced Aminobutyric acids Formic acid in Miller-Urey’s experiment. Acetic acid Many of these are Propionic acid basic building blocks of more Urea complex organic molecules.

Other Compounds Compounds Other Aspartic acid And others… Amino Acids

Glycine

Alanine 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. 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.

http://www.dc.peachnet.edu/~pgore/geology/geo102/precamb.htm 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. 1999. 13C-depleted carbon microparticles in >3700-Ma sea-floor sedimentary rocks from West . Science 283: 674-676.

“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 5- micrometer 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 .” Earliest Body Fossils

These are preserved in of the Swartkoppie Formation. Figure from Knoll and Barghoorn (1977).

The have been caught in the act of dividing just like living .

Age: 3,500,000,000 years

http://www.syslab.ceu.hu/corliss/4-HadArchEarth.html Phototrophs

Cyanobacteria

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

http://www.ucmp.berkeley.edu/ Modern Cyanobacteria

Large bloom due to pollution of Bedetti Lake, Argentina

enlarged 2500X

http://www-cyanosite.bio.purdue.edu/ Green Layer - filamentous blue-green algae Pink Layer - Purple sulfur (cyanobacteria) bacteria

Black Layer - Iron sulfide, Grey Layer - color change produced by abundant due to formation of mineral sulfur-reducing bacteria pyrite, bacteria less abundant

copyright 1997 Rolf Schauder

Cross section of a in a salt marsh. Approximately 5 cm deep. http://www.rz.uni-frankfurt.de/~schauder/mats/microbial_mats.html

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

http://www.wf.carleton.ca/ Modern Stromatolites

Hyper-saline bay exposed at low tide, ’s Bay, Australia

http://www.ldeo.columbia.edu/~small/personal/Images/Stromatolites.jpg Ancient Stromatolites

Hakati Shale, Middle Proterozoic Grand Canyon NP, Arizona

Characteristic laminated structure

http://www-cyanosite.bio.purdue.edu/ Archean Ecology

O2 O2

Photosynthesis by blue-green algae (cyanobacteria) in shallow water releases oxygen into the water

Communities of bacteria and archaebacteria

http://www.nature.com/nature/journal/v405/n6787/fig_tab/405625a0_F1.html Archean Plate Tectonics

Archean plate tectonic differed from modern plate tectonics: Plates likely moved faster Magma was likely generated more rapidly 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 , Shields, and Platform Terminology

Craton: Precambrian core of continental crust

Shield: exposed

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 35 The Canadian

The comprises most of northeastern a large part of Greenland parts of Minnesota, Wisconsin, Michigan, and New York

Topography is subdued and thinly covered in places by 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 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 and that intruded and underlie the greenstones 37 Model for Formation of Greenstone Belts

Subduction of oceanic crust formed volcanic island arcs

Greenstone belt began forming in the back-arc basin

Compression caused the basin to be Magma underplated the 38 back-arc basin folded and intruded by magma Intracratonic Model for Formation of Greenstone Belts

Greenstone belts (dark green) -gneiss complexes (light green

The intracratonic model for of part of the

39