Ice Core Dating. Could we have dinner sometime? I've bee

1
Ice Core Data
2
Vostok team, Antarctica NOAA
3
Ice Core Dating
Ice cores have a number of structural and
chemical features from which we can gain
information trapped gas bubbles trapped
dust and particulates trace substances
layering conductance
Air bubbles trapped in ice cores can be analyzed
using a number of methods, including gas
chromatography, mass spectrometry, and infrared
spectroscopy
4
Ice Core Dating
Measure hydrogen peroxide (H2O2) concentration vs
depth remember O2 hv (? lt 254nm) ? 2 O O3
hv (? lt 325nm) ? O2 O O H2O ? H2O2 H2O2 is
a source of hydroxyl radical ( OH ) The O-O bond
is weak (142 kJ/mol) In summer, during the
abundance of sunlight, photodissociation
reactions are promoted. Leads to higher
concentration of reactive oxygen atoms (O) ? more
H2O2. The result is alternating layers of
H2O2-poor and H2O2-rich ice, with each pair of
layers representing one year.
Dr. T.H. Jacka, Glaciology Program, Antarctic
Cooperative Research Centre and Australian
Antarctic Division
5
Ice Core Dating - Isotopes
Atoms are made up of three types of particles
proton, neutron, electron The nucleus of the
atom contains most of its mass. This is
where the protons and neutrons are located.
Electrons reside in cloud-like orbitals around
the nucleus, and govern the atoms chemistry
The number of protons in the nucleus determines
the type of element ex an atom with 11 protons
in the nucleus is a sodium atom ex an atom
with 36 protons in the nucleus is a krypton
atom Neutral atoms have equal numbers of
protons and electrons otherwise it is an ion.
Ex 11 protons, 10 electrons is a sodium(1)
cation Na The number of neutrons in the
nucleus can vary. Thus different atoms that
contain equal numbers of protons and different
numbers of neutrons are called isotopes. ex an
atom with 11 protons, 11 neutrons is sodium-22
22Na ex an atom with 11 protons, 12 neutrons
is sodium-23 23Na The mass number of the
element is the sum of protons and neutrons ex
the atomic mass of 23Na is 23 atomic mass units
(amu) The atomic mass of an element as seen on
the periodic table is the average of the atomic
masses of all of the naturally occurring isotopes
of that element. ex atomic mass of Na is
22.98977 amu (mostly 23Na in nature) more
precisely proton mass 1.0073 amu neutron mass
1.0087 amu. 1 amu 1/12 mass of 12C
6
Ice Core Dating - Isotopes
Some isotopes are stable indefinitely, and
exist in high abundance in nature Some
isotopes decay spontaneously, resulting in the
emission of energetic particles and high energy
radiation. These are radioactive
isotopes. note all elements heavier than
Bismuth only occur in nature as radioactive
isotopes! Some radioactive isotopes decay very
slowly, others very rapidly. ex Uranium-238 (
238U ) is very stable half-life 4.47 x 109
years ex Lawrencium-260 ( 260Lr ) is unstable
half-life 3 minutes Radioactive isotopes
decay in a steady fashion. The term half-life
is a convenient measure of the time that passes
for 50 of the isotope to decay. Because of
the steady radioactive decay of isotopes, the
ratio of isotopes in a sample can be used to
learn how old the sample is.
http//www.colorado.edu/physics/2000/isotopes/radi
oactive_decay3.html
7
Ice Core Dating

• Lets say we are given an ice core section that
  contains trapped bubbles of CO2
• The CO2 contains 12C and 14C
• Carbon-12 is stable, the half-life of carbon-14
  is 5470 years
• The natural abundance of 14C is 0.00000000012
• 14C is naturally generated by solar irradiation
  in the stratosphere
• n 14N ? 14C 1H
• We measure the 14C abundance in the sample is 12
  natural abundance
• How old is the sample?
• 0.12 0.5(age/half-life) ? 0.12 0.5(x/5470)
• log 0.12 (x/5470)log 0.5
• 5470 x (log 0.12)/(log 0.5) x
• 5470 x (-.92)/(-.30) x
• x 16,730 years

8
Ice Core Dating - Layers
Ice core - U.S. National Ice Core Laboratory
9
Ice Core Dating
Comparison of ice core information
Plots of ? 18O against age and depth (from EPICA
and Vostok)
Note that the Vostok core is deeper, but does not
extend so far back in time. Note also that the
offset between the EPICA and Vostok cores in ?
18O is real Vostok is a colder site, hence has
more negative ? 18O.