Humboldt State University ® Department of Chemistry

Richard A. Paselk

	Science 331
Fall 2004	Lecture/Activity	Office: SA560a
Notes: 29 November		Phone: x 5719 Home: 822-1116
		e-mail: rap1

Deep Time and The History of the Earth

How do we know ages?

Universe

1. Stellar Life Cycle: Stars burn hydrogen at particular rates depending on how big (bright) they are. Oldest globular clusters have only stars dimmer than the Sun (< 0.7 solar mass) implying ages between 11 By and 18 By (Sun will run out of hydrogen and become a Red Giant at about 10 By).
2. Hubble Constant: Universe is "expanding." That is all galaxies are going away. If calculate how long it has been since they were in same place, then Universe 12-14 By old by recent calculations (range broader in past, narrowed by increasing quality of measurements).
3. WMAP Satellite: Measures of microwave background pattern, etc. Complex calculations give an age of 13.7 By ±1%.

Earth and Solar System

1. Rock layers. Layers in sedimentary rock generally will be seasonal, like tree rings. Again can extend range by looking for overlaps. Determined Earth must be 100's of millions of years old in nineteenth century. (Rock thicknesses for various eras tend to be km to miles thick.)
   • Different biological organisms are characteristic of each time period, thus can determine relationships of geological deposits by comparing characteristic organisms.
2. Glacial Ice. Count layers, like in tree rings (below). In very deep ice-fields (Greenland and Antarctica) have gone back over 200,000 years. Includes info on atmosphere etc.
3. Radiometric dating. Depends on rate of decay of radioisotopes.
   • All isotopes decay at constant rates characteristic of the isotope.
   • These rates are independent of environment (not affected by submersion in water, heating or cooling, etc.)
   • The decays are 'first order,' with characteristic half-lives (at any time 1/2 of whatever remains will disappear in the next half-life).
   • k = 0.693/t_1/2; t = ln(x/x_o)/k, where x/x_o is the remaining fraction at any given time.
   • Some important half-lives include: ²³⁸U = 4.5 Gy (decay series gives ²⁰⁶Pb as end product), ⁴⁰K = 1.28 Gy, ²³²Th = 14.05 Gy

Archeology

1. Tree rings. Note that can go past oldest living tree if older wood which overlaps is available (e.g. in old buildings, preserved in bogs, at archeological sites). Limited to about 10,000 y so far.
2. Radiometric dating. Depends on rate of decay of radioisotopes.
   • All isotopes decay at constant rates characteristic of the isotope.
   • These rates are independent of environment (not affected by submersion in water, heating or cooling, etc.)
   • The decays are 'first order,' with characteristic half-lives (at any time 1/2 of whatever remains will disappear in the next half-life).
   • k = 0.693/t_1/2; t = ln(x/x_o)/k, where x/x_o is the remaining fraction at any given time.
   • Example: ¹⁴C is continuously produced in the atmosphere by cosmic rays impacting nitrogen in the upper atmosphere: ¹₀n + ¹⁴₇N -> ¹⁴₆C + ¹₁H
     It is then incorporated into living organisms via carbon dioxide. All organisms will then have a ¹⁴C/¹²C the same as the atmosphere until they die, at which time the ¹⁴C will gradually decay. The half-life of ¹⁴C is 5670 y. In a piece of charcoal from an archeological site the fraction of ¹⁴C is 1/4 its initial concentration. How old is the charcoal? Solution ­ 1/4 = two half-lives, thus the charcoal is 2(5670y) = 11,340y.

Overview of Earth History

The Solar system is almost exactly one-third of the age of the Universe! (4.57 By vs. 13.7 By)

Time line of early Earth: from origin of solar system (4.57 Ga) to present (5 meter model).

Major occurrences of Precambrian (approx. 90% of Earth history):

• 4.57 Ba
  Formation of Solar System
  Oldest known meteorites.
  • 4.47 Ba
    Earth's accretion complete.
  • 4.4 Ba
    Jack Hills
    Zircons of apparent continental origin ­ evidence of early (Hadean) crust and surface water
• 4.0 Ba
  Hadean-Archean Boundary: Archean eon begins.
  Oldest known rocks.
  • 3.96 Ba
    Acasta Gneiss
    Oldest known rock.
  • 3.9 Ba
    End of intense bombardment.
  • 3.85 Ba
    Isua Supergroup
    Banded iron & black chert with graphite having a high ¹²C/¹³C characteristic of life.
  • 3.5 Ba
    Warrawonna Group
    Banded iron & black chert containing stromatolites & filamentous structures
  • 3.2 Ba
    Deep-sea volcanic rock
    Filamentous "fossils."
  • 3.0 Ba
    Fig Tree Group
    Banded iron & black chert containing stromatolites & dividing cells; high ¹²C/¹³C, pristane and phytane.
  • 2.7 Ba
    Steranes, a eucaryotic chemical marker, appear.
• 2.5 Ba
  Archean-Proterozoic Boundary: Proterozoic eon begins
  Extensive global metamorphism, modern tectonic geology begins? Large, extensive stromatolitic reefs, oxygenic photosynthesis well established.
  • 2.2 Ba
    First extensive glaciation in geological record.
  • 2.2-2.0 Ba
    Oxygen increases 0.0001-0.1 atm.
  • 2.1 Ba
    Gunflint Chert
    Stromatolites, black cherts & chemical fossils. Dramatically increased diversity vs. earlier sites.
  • 2.0 Ba
    Banded iron formations cease.
  • 2.0-1.8 Ba
    Earliest fossil eucaryotes (Green algae?). Distinguished by larger cell size (<10um vs >100um)
  • 1.75 Ba
    Assured eucaryote algae-acritarchs
  • 1.25 Ba
    Oldest known metaphyte (green & brown algae) fossils. Ornamented acritarchs.
  • 1.0 Ba
    First fossils of branched algae.
  • 0.9-0.675 Ba
    Planktonic acritarchs collapse
  • 0.8-0.75 Ba
    Bitter Springs
    Diverse assemblage with cyanobacteria, bacteria and green algae. Some cells in meiotic? Tetrads, possible organelles. Black chert, stromatolites, high ¹²C/¹³C, pristane and phytane.
  • 0.65 Ba
    Ediacaran megabiota
    First fossils of large multicellular organisms other than algae.
  • 0.6 Ba
    "Snowball Earth"
    Extensive glaciation in tropics, most of ocean covered with sea-ice. Oxygen increases to near modern level.
  • 0.575 Ba
    Small shelly faunas
    First fossils with hard parts (<2mm). At least some represent scales and spines from armored worms.
• 0.542 Ba
  Cambrian begins -Phanerozoic Eon begins.
  Trace fossils of Phycodes pedum and small shelly organisms mark end of Precambrian.

Phanerozoic time ­ HSU NHM exhibits website

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© R A Paselk

Last modified 29 November 2004