2nd Order Climate Changes: RECAP

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2nd Order Climate Changes RECAP

• Timescales of tens of thousands to hundreds of
  thousands of years.
• Global temperature changes of 8-12 C
• Driven by changes in the earths orbit
• Eccentricity, obliquity (tilt), precession
• This sets up a positive feedback cycle that leads
  to glaciation

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Recording 2nd Order Climate Changes

• Stable-Isotope Geochemistry
• Stable isotopes atoms with extra NEUTRONS
• Example Oxygen
• regular oxygen is O16
• Isotopes are O17 and O18

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Abundances of Oxygen Isotopes

• Oxygen-16 (8 neutrons) 99.759 of all oxygen
• Oxygen-17 (9 neutrons) 0.0374
• Oxygen-18 (10 neutrons) 0.2039
• Oxygen 17 and 18 are, literally, heavier than O16

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Why oxygen isotopes are important

• O18/O16 distributed all over the planets oceans
  (H2O)
• Isotopes are separated (FRACTIONATED) by
  temperature (see the next 2 slides)
• In other words, an ocean will have more or less
  O18 depending on whether the climate is warm
  (non-glacial) or cold (glacial)
• We can measure the RATIO of O18/O16 in water and
  in fossils to find evidence for climate changes

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How measuring O18/O16 ratios tells us about the
past climate

• Climate changes lead to changes in ocean water
  chemistry
• Animals that live in the ocean and make their
  shells out of CaCO3 get the Ca, C, O from the
  water
• Any changes in the abundance of O18/O16 in ocean
  water are reflected in the chemistry of shells

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How does fractionation work?How does temperature
affect the abundance of O18/O16?

• Lets start with fractionation

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Fractionation

• Since O16 has less mass than O18, it evaporates
  more readily, leaving O18 behind
• In times of warm climate (nonglacial),
  precipitation puts back O16 into the oceans
• In times of cold climates (glacial), the
  evaporated water containing O16 is put into
  glaciers, instead of going back to the oceans

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Non-glacial (warm) climates
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Glacial Climates
O16 doesnt get back to the oceans, but is
instead locked up in the glacier
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• Glaciers are made from water that is evaporated
  from the oceans, precipitated on land, and frozen
  into ice sheets
• As the glaciers get bigger, the oceans get more
  and more ENRICHED with O18 and DEPLETED with O16
• This is because more and more O16 is being taken
  out
• So by drilling into the ocean sediments and
  studying the chemistry of the fossil shells, we
  can identify glacial/nonglacial times

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Using O18/O16 ratios

• 1. You must physically measure the amount of O18
  and O16 in sediment intervals
• 2. Then divide the 18 value by the 16 value for
  each interval
• 3. Then plot on a graph to find enriched and
  depleted intervals
• 4. Interpret the results

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1. Measuring the amount of O18 and O16
Interval 1 O18 value151970, O16 value2171 Age
10,000 years
Interval 2 O18 value1143820, O16 value7191 Age
20,000 years
Interval 3 O18 value646290, O16 value7181 Age
30,000 years
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2. Computing the ratios

• 151970 / 2171 70
• 143820 / 7191 20
• 646290 / 7181 90

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3. Plotting
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4. Interpretation
ENRICHED IN O18
DEPLETED IN O18
ENRICHED IN O18
A level of 50 in this graph would be normal
(not enriched or depleted)
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4. Interpretation, continued

• Enriched O18 levels indicate COLD climates
• Depleted O18 levels indicate WARM climates
• Climate was GLACIAL 10,000 BP and 30,000 BP
• Climate was NON-GLACIAL 20,000 BP