Quaternary Dating MethodsQuaternary Dating Methods

Numerical age, Calibrated age, Relative age, Stratigraphic correlation

Four General Categories–Radioisotopic (atomic disintegration)

–Paleomagnetic Correlation

–Organic/Inorganic Chemical

–Biological

Radioisotopic MethodRadioisotopic Method

Parent Daughter Half-LifeSamarium-147 Neodymium-143 106 billion yrsRubidium-87 Strontium-87 48.8 billion yrsThorium-232 Lead-208 14 billion yrs Uranium-238 Lead-206 4.5 billion yrs Potassium-40 Argon-40 1.26 billion yrsUranium-235 Lead-207 0.7 billion yrs Beryllium-10 Boron-10 1.52 million yrs Chlorine-36 Argon-36 300,000 yrs Carbon-14 Nitrogen-14 5730 yrs Uranium-234 Thorium-230 248,000 yrs Thorium-230 Radium-226 75,400 yrs

What is half-life?

Decay is often complicated by production of a radioactive daughter that must decay.The daughter then becomes a parent

-Radiocarbon-

Atmosphere has 42x1012 tons of 12C47x1010 tons of 13C62 tons of 14C

14N + 1n – 14C + 1H

14C 14N + B + neutrinos•Radiocarbon formed by cosmic flux neutron•~15 km altitude•Polar concentration greater•Related to geomagnetic field

~constant global amount (not really)

CO2

Oceans, groundwater,surface water

Plants, animals

Problems •Variations in cosmic ray flux

•Variations in geomagnetic field•Lag exchange between global reservoirs

•Knowledge of original amount in organism•Counting errors in lab

•Sample contamination, modern 14C is worst source•Sample age relative to event age

respiration, photosynthesis, tissue,aqueous, organic, inorganic states

Difference due to past variations in C14 production and possibly CO2

exchange between ocean and atmosphere

Tree ring calibration

Calibrating a date 3000+/- 30 BP

Tree ring ages at same period (+/-1 std. dev.)

Possible ages

Contaminated by older carbon

Contaminated by modern carbon

The oldest map showing North America. 1434 A.D. plus or minus 11 years.

“By God's will, after a long voyage from the island of Greenland to the south toward the most distant remaining parts of the western ocean sea, sailing southward amidst the ice, the companions Bjarni and Leif Eiriksson discovered a new land, extremely fertile and even having vines, ... which island they named Vinland.”

PaleomagnetismPaleomagnetism

PaleomagnetismPaleomagnetism

• Major reversals in geomagnetic field are well-known and well-dated from independent locations

• These reversals are useful as time markers– But you must know approximate age to get proper

reversal

• Inclination – field lines– ~0 deg. at equator– 90 deg. at poles

• Declination – angle between true N. and a needle pointing toward mag. N

a) The main part of Earth’s magnetic field is the dipole field (like a bar magnet centeredon the core. Lines of force represent direction in which a magnetized needle tries to point. Concentration of these lines represents field strength.

b) Declination: departure of field from true north. Inclination: measure of dip from Horizontal (where field lines enter surface).

Dipole is stable but reverses through time

Nondipole (declination and inclination) is Unstable and oscillates through time as secular excursions

Paleomagnetic RecordsPaleomagnetic Records

• Thermoremnant magnetization (TRM) – lavas etc.

• Depositional remnant magnetization (DRM) – sediments, smoothed by bioturbation and water content that dilates grains

• Chemical remnant magnetization (CRM) – remineralization, recrystallization

Normal polarity in black, datingbased on K/Ar dates of lavas

Dipole component

Secular Variation – regional in extent (1000-3000 km)

Amino Acid RacemizationAmino Acid Racemization

• All organic materials carry amino acids in their proteins (20 types of AA)

• Amino acids alter with time and temperature (form of weathering)

• Typically found in two molecular forms (isomeres), “D” and “L”

• Only L type exist in living proteins• D type exist in free state or in collagen as result

of diagenesis• D/L ratio result of time and/or temperature

Amino AcidRacemization

EDT-estimated diagenetic temperature

Biological MethodsBiological Methods

• Lichenometry• Largest lichen is oldest• Maximum lichen age is minimum substrate

age• Maximum age is time substrate stabilized

(boulder stopped moving)• Clusters of ages indicate boulder

movement events – glacial advances or retreats

Lichenometry