Tree Rings

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Tree Rings
http//sonic.net/bristlecone/dendro.html
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Carbon-14

• Half-life 5730 years
• Abundance 1ppt
• Reaction
• Supply?
• Limit at about 60,000 years

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Potassium-Argon dating

• 40K has natural abundance of 0.0117 and a
  lifetime of 1.26109 years
• Decay is to 40Ca or to 40Ar, with the latter more
  useful for dating purposes.
• Gas is trapped into the crystal lattice of a rock
• Rock is melted and mass spectrometer detects the
  amount of 40Ar
• Good for objects gt100,000 years old

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Rubidium-Strontium dating

• 87Rb has 27.8 natural abundance
• Undergoes b-decay to 87Sr, t1/2 4.91010 years
• 87Srt 87Rb0 - 87Rbt 87Rb0(1 e-lt)
• 87Rbt(elt - 1)
• 87Sr has 7.0 natural abundance 86Sr has 9.9
  natural abundance
• Precision of 1-2 for rocks 3 billion years old

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Deuterium

• Natural abundance 0.0156

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Deuterium
dD/Temp - 9/C
Measurement accuracy is better than 1
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VSMOW
2H / 1H 155.76 0.1 ppm (a ratio of 1 part per
approximately 6420 parts) 3H / 1H 1.85 0.36
10-11 ppm (a ratio of 1 part per approximately
5.41 1016 parts, ignored for physical
properties-related work) 18O / 16O 2005.20
0.43 ppm (a ratio of 1 part per approximately
498.7 parts) 17O / 16O 379.9 1.6 ppm (a ratio
of 1 part per approximately 2632 parts)
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Into the Arctic
Oxygen Isotopes Ice cores reveal annual
variations within glacial ice. As with trees,
rings can be detected that identify seasonal
variation. By comparing the ratio of oxygen with
a molecular weight of 18 (O18) to oxygen with a
molecular weight of 16 (O16) scientists have
determined that the rings within the ice cores
represent a year long span of time. A greater
amount of O16 than O18 exists in the atmosphere.
Because it is heavier, O18 precipitates before
O16. Consequently, in the summer, there is a
greater amount of O18 in the snow pack because,
with warmer temperatures, more O18 precipitates
out with each snow event. Scientists can
identify when one year ends by looking at the
depletion of O18 in a core sample. As the amount
of O18 increases again, the onset of a new
summer is indicated. By looking at the relative
change of O16 and O18 over a long span of time
(a long ice core) it is possible to also
identify changes in temperature by the relative
changes in the amount of O16 compared to O18.
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Ice age vs. gas age

• Oxygen isotopes (in water/ice sample) give age
  signal
• CO2 amounts in trapped air bubbles
• Migration of gas, ice compression
• firn n. Granular, partially consolidated snow
  that has passed through one summer melt season
  but is not yet glacial ice.

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N. Caillon et al., Science 299, 1728 -1731
(2003)
Figure 1. (A) Vostok records covering Termination
III with respect to depth. The deuterium measured
in ice combines 1-m-resolution published data
(1) (black curve) and a new set of detailed
measurements (every 10 cm corresponding to a
time resolution of 20 years) performed between
2680 and 2800 m (gray curve). The d40Ar of gas
trapped in air bubbles is shown (triangles)
(duplicate measurements were carried out with a
pooled standard deviation of 0.01 ). (B) Vostok
records with respect to the GT4 time scale
temperature deduced from the deuterium record,
the accumulation (1), the d40Ar profile, and the
firn depth estimated using a firn densification
model (20), whose result depends both on
temperature and accumulation rate, with deeper
firn depth for higher accumulation and colder
temperatures. Temperature, accumulation, and
d40Ar are very well correlated (R2  0.85),
which suggests that d40Ar may be used as a
temperature proxy in the gas phase. The model
firn depth and d40Ar data vary in antiphase
during the warming.
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Fig. 4. Vostok records of d40Ar and CO2 with
respect to gas age (1). Atmospheric CO2
concentration is a combination of new data and
published data (1, 44). The age scale for the
CO2 proxy has been shifted by a constant
800 years to obtain the best correlation of the
two datasets.
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Greenland
Abrupt Climate Change at the End of the Last
Glacial Period Inferred from Trapped Air in
Polar Ice Jeffrey P. Severinghaus and Edward J.
Brook
Science 29 October 1999Vol. 286. no. 5441, pp.
930 - 934DOI 10.1126/science.286.5441.930
Fig. 2. GISP2 accumulation (51), oxygen isotopes
(5), methane, and nitrogen isotopes of air in
bubbles during the last deglaciation, with
Taylor Dome (Antarctica) methane shown for
comparison (21). Low-resolution nitrogen data
are from (54).
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The 400-year window encompassing the Bølling
Transition, showing high-resolution measurements
of oxygen isotopes of ice (5) and nitrogen,
argon, and methane measurements made on trapped
air bubbles.