Transcript Methods of Dating Ice Cores - University of Colorado Boulder

Glaciers as records of climate
• Ice cores:
– Detailed records of
temperature,
precipitation, volcanic
eruptions
– Go back hundred of
thousands years
Past climate reconstructions
• Instrumental records
– Global air temperature: limited records
• Proxy records of climate
– “proxy” = a measure of climate conditions of
the past
– clues such as temperature, precipitation
– EX: ICE-CORES
Global temperature- instrumental
record
Global temperature- reconstructed
Methods of Dating Ice Cores
• Counting of Annual Layers
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–
–
Temperature Dependent
Marker: ratio of 18O to 16O
find number of years that the ice-core accumulated over
Very time consuming; some errors
• Using volcanic eruptions as Markers
– Marker: volcanic ash and chemicals washed out of the
atmosphere by precipitation
– use recorded volcanic eruptions to calibrate age of the icecore
– must know date of the eruption
How do we reconstruct past climate
from ice cores?
• Oxygen Isotope analysis:
– Examining type of water isotopes contained
in ice
– Gives clues about temperature at time of
deposition
Isotopes Defined
Isotope = atoms of the same element with a
different number of neutrons (different
mass)
Example: Oxygen Isotopes
Name
Electrons
Protons
Neutrons
Abundance
16O
8
8
8
99.76%
18O
8
8
10
0.20%
Stable Isotopes
16O
(Light Element)
18O
(Heavy Element)
Chemical and Biological processes
can sort the light elements from the
heavy elements
Fractionation
Change in d18O value
Oxygen isotopes
• Three isotopes: 18O,17O and 16O
• Important isotopes: 18O and 16O
• Modern 18O/16O ratio:
• 1:500
Isotopic change
16
• Evaporation: lighter O evaporates more easily
from a water body resulting atmospheric H2O
vapor is poorer in 18O than oceanic water
• Condensation: heavier 18O are precipitated faster
than lighter 16O;
• So: coldest snow is lightest (less heavy
16
isotopes, more lighter O isotopes)
18
O
Evaporative Fractionation
Water vapor d18O?
Precipitation will be
depleted in 18O relative
to the standard (ocean
water)
Negative d18O
18
O/ O ratio: “delta 18O”
16
• Stable isotope ratios are expressed as
parts per thousand (per mil – ‰) relative
to a standard
18
• Ratio expressed as deviation of O from
the Standard Mean Ocean Water ratio
(SMOW)
Delta O18 and temperature
• Temperature affects 18O/16O ratio:
– colder temperatures  more negative values
for the delta 18O
– warmer temperatures  delta 18O values that
are less negative (closer to the standard ratio
of ocean water)
Delta 18O and temperature
Seasonal variations
• Summer (warmer) and winter (colder)
periods in glacial ice can be observed as
long as the delta 18O ratio is locked into
the ice
Continental effect
Altitudinal effect
• As water molecules
travel up on an ice
sheet  water
changes from vapor
to liquid18O is
precipitated first
• So: ice on the top of
the glacier has less
18O than at the base
of the mountain
How far back do records go?
• Greenland: the last 100,000 years
• Antarctica: record going back 400,000
years has been reconstructed
Temperature reconstructed from
Greenland Ice core