July 9th

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Geoscience Time Scales Paleoclimate
Teacher workshop

• July 9th 10th, 2008

Dr. Norlene Emerson
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The Age of the Earth How do we know how old
rocks are?
3.96 Billion Year Old Gneiss
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Age of the Earth
Buddhist TraditionInfinite Age (Cyclic)
Han Chinese Tradition 23 Million Year Cycle
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Archbishop James Ussher (1654)
(1625-1656)
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Most scientific attempts are based on principle
that

• Requires
• Natural Process
• Occurs at a Constant Rate
• Leaves a Geologic Record

Age (Time) Amount of Change Rate
of Change
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William Thomson, Lord Kelvin (1862)
(1824-1907)
20-400 Million yrs
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John Joly (1899)
80-100 Million yrs
Saltiness of the Oceans
(1857-1933)
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John Phillips (late 1800s)
About 100-500 Million yrs
Accumulation of Sedimentary Rocks
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George Darwin (late 1800s)
56 Million yrs
Evolution of the Moon tidal drag
(1845-1912)
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The Discovery of Radioactivity (1896)
Antoine Henri Becquerel
Marie and Pierre Curie
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Arthur Holmes
Bertram Boltwood
1904-1907 Dated first rocks 250 million to 1.3
billion years Earth's age - 2.2 billion years
1913 Earth about 1.6 billion years
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Radioactive DecayParent Isotope --gtDaughter
Isotope Decay Particle Energy
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Alpha Decay
Daughter Isotope Atomic Number -2 Atomic Weight
-4
Uranium-238 --gt Thorium-234 Alpha Particle
Energy
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Beta Decay
Daughter Isotope Atomic Number 1 Atomic Weight
0
Carbon-14 --gt Nitrogen-14 Beta Particle Energy
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Decay of U-238 to Pb-206
Alpha Decay
Beta Decay
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Dating Radioactive Decay
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Information Required for Radiometric Dating

• Initial Parent Isotope Content
• Half Life of Isotope
• Current Parent Isotope Concentration
• Closed System

Remember Age Amount of Change Rate
of Change
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Radioactive Isotopes Used for Absolute Dating
parent daughter half life (years)
235U 207Pb 4.50 billion 238U 206Pb 710
million 40K 40Ar 1.25 billion 87Rb 87Sr 47
billion 14C 14N 5,730
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Mass Spectrometer
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When does a system become Closed?(i.e., What
are you dating?)
Cooling of Igneous Rock
Metamorphism
Death of Organic Material
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GeologicTimeScale
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Back to the Age of the Earth
Oldest Rocks on Earth(Acasta Gneiss, Northern
Canada) - about 3.96 Billion Years
Age of the Earth - 4.56 Billion Years
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Type Number Method Age (Gyr)) Chondrites (CM,
CV, H, L, LL, E) 13 Sm-Nd 4.21 /-
0.76 Carbonaceous chondrites 4 Rb-Sr 4.37 /-
0.34 Chondrites (undisturbed H, LL,
E) 38 Rb-Sr 4.50 /- 0.02 Chondrites (H, L, LL,
E) 50 Rb-Sr 4.43 /- 0.04 H Chondrites
(undisturbed) 17 Rb-Sr 4.52 /- 0.04 H
Chondrites 15 Rb-Sr 4.59 /- 0.06 L
Chondrites 6 Rb-Sr 4.44 /- 0.12 L
Chondrites 5 Rb-Sr 4.38 /- 0.12 LL Chondrites
(undisturbed) 13 Rb-Sr 4.49 /- 0.02 LL
Chondrites 10 Rb-Sr 4.46 /- 0.06 E Chondrites
(undisturbed) 8 Rb-Sr 4.51 /- 0.04 E
Chondrites 8 Rb-Sr 4.44 /- 0.13 Eucrites
(polymict) 23 Rb-Sr 4.53 /- 0.19 Eucrites 11
Rb-Sr 4.44 /- 0.30 Eucrites 13 Lu-Hf 4.57 /-
0.19 Diogenites 5 Rb-Sr 4.45 /- 0.18 Iron
(plus iron from St. Severin) 8 Re-Os 4.57 /-
0.21 ---------------------------------------------
--------------------------- After Dalrymple
(1991, p. 291) duplicate studies on identical
meteorite types omitted.
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Other Forms of Absolute Dating
Dendrochronology
Fission Tracks
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GeologicTimeScale
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Relative AgeThe study of the relationship and
order of rock layers (Strata).
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• Correlation - establishing equivalence or
  matching rocks of similar age in different
  regions
• Based on lithology, fossils, key beds, polarity
  reversals,
• Lithostratigraphy
• Biostratigraphy
• Magnetic stratigraphy
• Isotopic stratigraphy
• Event Stratigraphy

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Lithostratigraphy
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Lithostratigraphic ExampleGrand Canyon
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Biostratigraphy
based on the stratigraphic range of fossils
Defined by first and last appearance of fossils
and/or fossil assemblages
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Magnetic Stratigraphy
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Lava Flows Some igneous and rocks capture the
Earths magnetic field as they cool or are
deposited. They record the location and polarity
of the magnetic pole.
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Lava Flows At least 9 separate flows
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Isotopic Stratigraphy

• Ratios of certain isotopes of elements found in
  mineral grains or fossils can be used for
  correlation
• Most commonly used with Sr86 / Sr87, O16 / O18,
  or C12 / C13

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Event Stratigraphy

• A geologic event that occurred for a short time
  and covers a wide region
• The record is preserved in the rock in a Key or
  Marker bed (one that has some unique, easily
  recognizable characteristic)
• Examples volcanic ash eruption, iridium layer,
  glacial tills, evaporite beds

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K-bentonite Locust, IA 454 Ma
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West TX Permian Evaporites
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• Geologic Time Scale
• Time units
• Eons
• Eras
• Periods
• Epochs Ages

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Palaeoclimatology
Teacher workshop

• (Earths Climate History)

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Earths climate history Timescale - Millions
Years
Pleistocene ice ages (2-4)
Eocene hothouse (warm, wet climate) (55-45)
Late Paleozoic ice age (300)
Early Paleozoic ice age (440)
Proterozoic ice age (600)
Proterozoic ice age ( 2,300)
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Climate History of the Earth Timescale in
Millions of Years

• Warm climates indicated by
• Fossil reefs, limestones
• Al ore- bauxite (tropical soils)
• Evaporite minerals
• Certain fossil organisms
• Cold climates indicated by
• glaciers
• Certain fossil organisms

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Causes of Climate Change

• Long-term
• Plate Tectonics
• Mountain Building
• CO2 Cycle
• Medium-term
• Milankovitch Cycling
• Short-term
• Solar Forcing
• Volcanic Forcing
• Anthropogenic Forcing

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The concept of climate proxies

• A climate proxy is something that records or
  reflects a change in temp or rainfall but does
  not DIRECTLY measure temperature or precipitation
• For example

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Principle sources of proxy data for
palaeoclimatic reconstructions

• Historical              meteorological
  records              parameteorological records
  (droughts, floods,)              phenologica
  l records
• (migration dates, fall color dates, )
• Biological              Tree rings (width,
  density, isotope analysis)              Pollen
  (species, abundances)              Insects
• Glaciological (Ice Cores)              Oxygen
  isotopes              Physical
  properties              Trace element
  concentrations

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proxy data for Palaeoclimate

• Geological  A. Sediments    1. Marine (ocean
  sediment cores)      i) Organic sediments
  (fossils)              Oxygen isotopes          
      Faunal floral abundances              Morph
  ological variations      ii) Inorganic
  sediments              Mineralogical
  (composition texture)              Distribution
  of terrigenous sed.              Ice-rafted
  debris              Geochemistry   

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proxy data for Palaeoclimate

•  2. Terrestrial              Periglacial
  features               Glacial deposits
  erosional features              Glacio-eustatic
  features (shorelines)              Aeolian
  deposits (sand dunes)              Lacustrine
  deposits/varves (lakes)  B. Sedimentary
  Rocks              Facies analysis              
  Fossil/microfossil analysis              Mineral
  analysis 
• Isotope geochemistry

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Carbon Isotope Proxy

• Carbon reservoirs
• biosphere (in plants animals)
• atmosphere (CO2 gas)
• geosphere (carbonate rocks, fossils)
• hydrosphere (bicarbonate dissolved in rivers and
  oceans)
• Carbon flux carbon transfer
• from the atmosphere to the biosphere when plants
  use CO2 to make plant tissue and then animals eat
  the plants.

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Carbon flux

• CO2 combines with water in the Atmosphere to form
  Carbonic Acid (H2CO3).
• H2CO3 attacks limestones and silicate rocks
  (dissolves them) releasing Ca and HCO3-
  (bicarbonate)
• Ca and HCO3- travel to the ocean in the
  dissolved state where they form CaCO3.

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GIC Giga ton Carbon
http//earthobservatory.nasa.gov/Library/CarbonCyc
le/carbon_cycle4.html
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Animal Plant Respiration cycle
Plants are either Eaten Decompose Buried
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Burial of Plant Debris Atmospheric Chemistry
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Carbon Cycle
Rate of burial balances rate of weathering
Rate of burial is greater, decreases atmospheric
CO2
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Detecting past changes

• Carbon isotopes
• Two stable isotopes of C
• 13C and 12C (12C is most abundant)
• Plants prefer 12C
• Effect is increase 13C in atmosphere

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Isotopic ratio of 12C13C
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Carbon Isotopes Limestones Organic rich
sediments

• Limestone is composed of CaCO3
• The Carbon can be organic or inorganic carbon but
  mostly inorganic
• At times when organic matter burial is increased,
  limestones become rich in 13C (removing 12C from
  the sea water)

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Formation of Deep water Carbonates
Destroyed by subduction, release of CO2
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Shallow water carbs dont subduct Take longer to
weather
Passive Margin
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The Importance of Weathering

• Changes in the rate of weathering affect the amt
  of atmospheric carbon
• weathering of Ca and Mg silicate rocks removes
  CO2 from the atmosphere.
• Mountain building accelerates weathering
• Warm Temp accelerates weathering
• Rain accelerates weathering
• Vegetation accelerates weathering pumping CO2 and
  water

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Phanerozoic Trends
Atmospheric CO2 through time
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Negative Feedbacks CO2 held in Check

• 1. Temperature accelerates weathering which
  consumes atmospheric CO2. Thus the negative
  feedback occurs is reverse greenhouse effect or
  cooling.
• Precipitation Adding CO2 to the atmosphere
  increases Precipitation. Precipitation allows
  forests to expand and accelerates weathering
  which consumes atmospheric CO2.

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Oxygen Isotopes and Climate

• 1. Two stable Isotopes 18O and 16O. 16O is more
  abundant (99.8). (d18O)
• 2. The ratio in shells is a function of the ratio
  in the water and temperature.
• 3. Temperature has an inverse effect - warmest
  temperature cause a decrease in the d18O values
  such that for every 1C increase there is a 0.2
  decrease.

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High rate of evap increase in 18O in sea water
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Using d18O we can infer times of increased
glaciation and therefore cooler global temp
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Curve of average O18 over the past 2my based on
analysis of deep sea sediment
The curve illustrates changes in global ice
volume in successive glacial (blue) and
interglacial (green) cycles of the Quaternary
Period.
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http//www.worldviewofglobalwarming.org/pages/pale
oclimate.htm
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