Isotope Geochemistry Provides fundamental observational constraints on Earth and solar system processes through time

A few examples: Quantitative constraints on

1. Timing of age of solar system formation 2. Formation of the Earth 3. Genesis of terrestrial units: continents, ocean, mantle, core 4. Climate variability over Earth history 5. Quantitative timescales a. Hominid evolution b. Landscape evolution Ice Sheet Retreat c. 6. Cultural evolution: 14C dating

What has isotope geochemistry done for you lately?

Isotope Geochemistry Provides fundamental observational constraints on Earth and solar system processes through time

A few examples: Quantitative constraints on

1. Timing or age of solar system formation 2. Formation of the Earth 3. Genesis of terrestrial units: continents, ocean, mantle, core 4. Climate variability over Earth history 5. Quantitative timescales a. Hominid evolution b. Landscape evolution Ice Sheet Retreat 6. Cultural evolution: 14C dating

4.55x109 years

Patterson (1956)

Questions: What physics is involved? What chemistry is involved? What assumptions?

Quantify the antiquity of our solar system: 238U 206Pb 235U 207Pb } knowledge of relative decay kinetics linear correlation between systems

(i.e., meteorites and Earth)

Origin of our moon:

Terrestrial isotopic Fractionation Line

Earth

Common correlation between 17O/16O ratios and 18O/16O ratios measured in lunar rocks and Earth rocks

Common source planetary impact formed the moon

What chemistry/physics is behind this logic?

Wiechert et al., 2001 after Clayton & Mayeda, 1975

Evolution of Earths mantle and crust:

147Sm 143Nd

DePaolo, 1988

Constraints on: Large scale chemical differentiation in mantle Timescales Models of Earth evolution

When?

Global climate records benthic Foraminifera

After Molnar and England, 1990; Small et al. 1999; Whipple et al, 1999

Isotopic proxy records of long-term global climate variability:

Questions: How do observable 18O/16O ratios relate to temperature?

What chemistry/physics is involved?

What assumptions are involved?

800 600 400 200 0

Age (103 years before present)

Augustin et al., 2004

Antartic Vostok ice core:

D/H ratio in ice

18O/16O ratio in carbonate sediments

Isotopic proxy records of (pre-industrial) global climate variability:

18O/16O of carbonate is function of: temperature and 18O/16O of water global ice volume Epstein (1953)

Temp

800 600 400 200 0

Age (103 years before present)

Augustin et al., 2004

Antartic Vostok ice core:

D/H ratio in ice

18O/16O ratio in carbonate sediments

Isotopic proxy records of (pre-industrial) global climate variability:

Temp

IPCC Climate Change, 2007

Where is this headed?

} Is this normal?

Infe

rred

rela

tive

tem

pera

ture

(o

C)

Isotopic records allow us to place more recent records into a larger context: Q What is natural variability? What is forced by industrial activity?

Multiple temperature proxy records from: tree rings, isotopic composition of snow, coral, stalactites and other sources

Timescale of Hominid evolution:

Koobi Fora Formation, Lake Turkana McDougall and Harrison, 1999

Geochronology of volcanic rocks

40K 40Ar Knowledge of decay kinetics Timing of volcanic eruptions Quantify sediment scale Antiquity of hominid fossils

Homo erectus: ~1.8 million years ago

Radiocarbon dating

14C 14N

Quantify the age of C-containing organic materials up to ~62,000 years:

Archaeological sites

14N + n 14C + p nuclear reaction in atmosphere:

radioactive decay in dead organic matter:

West Antartica Ice Sheet Holocene Deglaciation Stone (2003)

Exposure ages from erratic cobbles stranded on nunataks in West Antarctica that record steady Holocene ice surface lowering, from Stone(2003). Image at left shows Mt. Darling, a nunatak in the Ford Ranges of Marie Byrd Land (77 15' S, 143 20 W). Center image shows a typical erratic cobble from the Ford Ranges. Right panel shows exposure age - elevation relationship for erratics on Mt Darling. Exposure ages increase with elevation, recording steady Holocene ice surface lowering and emergence of the nunatak.

Ice surface lowering over time

Glacial erratic Mt. Darling, Antarctica

Text books:

Chart of the Nuclides, Lockheed Martin (Required): $25 check give to D. Shuster Check payable to UC Regents, memo = EPS 124/224

Radiogenic Isotope Geochemistry, A.P.Dickin (Required): ~$50 used paperback (Amazon) Introductory Nuclear Physics, K.S.Krane (Recommended): ~$75 used hardback (Amazon) Principles of Stable Isotope Distribution, R.E.Criss (Recommended): ~$20 used (Amazon)

Isotope Geochemistry Iso-topos (opos) = same place First use: Soddy (1913)

Neutrons

Pro

tons

Chart of Nuclides

Introduction to Nuclear Physics (an historical perspective)

Henri Becquerel (1896) Potassium uranyl sulfate K2UO2(SO4)2 H2O Some kind of radiation from the salt blackened photographic plate

(which penetrated paper, glass and other solid matter) Intensity of radiation amount of uranium

Marie Sklodowska Curie & Pierre Curie (1898)

Rays are generated by uranium Natural U-ore is more radio-active than pure Uranium Intensity of radiation shown to be a function of time Isolated other radioactive elements: polonium, radium, actinium First use of the word radio-active (Curie & Curie, 1898) Samples with concentrated Ra heated up (100 cal/hour/g Ra) Thorium is radioactive

Rutherford and Thomson (1899) Studied the nature of rays Radioactivity causes ionization in air Rutherford discovers two types of radiation (Rutherford, 1899) Terms and radiation Both types of radiation absorbed by Al foil

rays stopped by thin foil rays only stopped with much thicker (x100) foil

Bragg (1904) introduces the concept of range for particles, finding that particles

emitted from different elements have different ranges

Intensity (d)

foil thickness (d)

Incident intensity o

d

(d)

foil

Foil absorption experiments

o

0

foil thickness (d)

Incident intensity o

d (d) = o e-d

foil

Where: = intensity through d = incident intensity = absorption coefficient d = foil thickness

Foil absorption experiments

o

0

Intensity (d)

Rutherford and Thomson (1899) Studied the nature of rays Radioactivity causes ionization in air Rutherford discovers two types of radiation (Rutherford, 1899) Terms and radiation Both were absorbed by Al foil

rays stopped by thin foil rays only stopped with much thicker (x100) foil

Rutherford postulates an exponential law for rays absorption Bragg (1904) introduces the concept of range for particles, finding that particles

emitted from different elements have different ranges

deII = 0Where: = intensity through d = incident intensity = absorption coefficient d = foil thickness

and rays were recognized as streams of charged particles: using magnetic and electrostatic deflections

rays are negatively charged: electrons moving with almost c (speed of light)

rays are positively charged: charge/mass ratio is found to be ~1/2 that of H+, velocity is ~ c / 10

rays (Villard, 1900) are not deflected in a magnetic field: found to be high energy electromagnetic radiation, more powerful than rays

rays emitted into an evacuated chamber caused detectable Helium gas to appear after a few days

(it had already been found that U and Th ores were rich in He) = He++

Discovery of 3 types of radiation: , and

Rutherford-Soddy transformation hypothesis Noticed bursts in electrometer readings, other than radiation

Could condense a high atomic weight gas (Radon) then its radioactivity diminished rapidly (time scale)

Decreasing radiation intensity shows characteristic time scale depended on the radioactive substance

Radioactivity is accompanied by the transformation of atoms

- uranium-x was discovered (234Th)

- thorium-x was discovered (224Ra)

Radiation is a by-product of transformations

Transformation is at a subatomic level

Statistical nature of radioactivity (Schweidler, 1899) Disintegration probability = decay constant Probability of survival after t And for a time t=nt (a few t) As t zero and

tptp

decay

decay

=

tpsurvive = 1

nsurvive ttp )1()( =

ttn

t

eentet

==

=

)1()1(

For a large number of radioactive atoms (N(t)): Also the solution to differential equation

)(

1

2 12

)()( ttetNtN =

NdtdN

=decay rate # of parent atoms

)()()(

)()(ln)(ln

ln

121

2

1212

)(

)(

2

1

2

1

ttetNtN

tttNtN

dtNd

dtNdN

tN

tN

t

t

=

=

=

=

decay rate (N) is called activity

parent daughter + (or )

Radioactive Change Rutherford & Soddy, 1903

Discovering the nucleus Thomson discovered the electron (1897) Estimated that # e- /atom atomic weight (false) That e- mass 1/2000 of a H atom mass (false) Concluded that most of the mass resided in the positively charged parts Through 1911 it was believed that the electrons were embedded in a

positively charged mass distributed over the volume of the atom plum pudding model (Thomson, 1910) electron repulsion balanced by attraction to positive charge

Uniform + and charge distribution with an atom

Geiger and Marsden (1909) Scattering experiment not consistent with model

If uniform (+) charge density in an atom:

If concentrated charge:

No deflection of in incident particle Scattering of incident () particle

Predicted observations: The testable hypothesis:

Geiger and Marsden (1909) Scattering experiment not consistent with model

Experimental setup: source

Rutherford scattering experiments

Large scattering angles were too frequently observed inconsistent with the diffuse charge model

Occasionally, = 180o was observed!

particle source

gold foil

scattered ions detected here

It wasas if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. (!)

If positive charge uniformly distributed in foil no scattering is expected. Conversely, if charge is non uniform the more scattering

Rutherford scattering experiments

particle source

gold foil

scattered ions detected here

If positive charge uniformly distributed in foil no scattering is expected. Conversely, if charge is non uniform the more scattering

Rutherford (1911) Positive charge is concentrated at a point (later named nucleus) Predicted scattering angles using this model and classical physics

Rutherford Scattering Formula

=

2sin

1

2116

)(4

2

220

vM

eZZerNtnn a

n0 = # incident particles n() = # particles scattered at angle t = thickness of scattering foil N = # nuclei/unit volume in foil r = distance between the scattering point and the photo plate M = mass of alpha v = velocity of alpha Z = atomic # of scattering material Za = 2 e = unit charge

The Rutherford model Was confirmed by systematic scattering experiments Led to the determination of nuclear charges Led to the realization that the atomic # of an element was equal to its nuclear

charge (and # of electrons)

Rutherford Scattering Formula

Size of the nucleus Electrostatic repulsion predicts that the radius of the nucleus must be smaller the

distance of closest approach of the particles when the is at at the distance d from the nucleus, it has a potential energy of:

for an scattered at 180, the potential energy at its turn around point is equal to its original kinetic energy, T:

dZe22

TZed

dZeT

2

2

2

2

=

=

particles nucleus

distance d

Example: for Z 30; T 10 MeV The volume of a nucleus is about 10-12 that of an atom The density is 1012 times of an atom (roughly 1014g/cm3)

mTZd 151086.2 =

Quantifying Rutherford scattering: Size of the nucleus Constant

e = elementary charge 0 = vacuum permittivity

V = potential energy T = kinetic energy of incident nucleus l = angular momentum

0

2

4e

Breakdown of Rutherford scattering