Quaternary Dating Methods Numerical age, Calibrated age, Relative age, Stratigraphic correlation Four General Categories –Radioisotopic (atomic disintegration) –Paleomagnetic Correlation –Organic/Inorganic Chemical –Biological

Radioisotopic Method

Parent Daughter Half-Life Samarium-147 Neodymium billion yrs Rubidium-87 Strontium billion yrs Thorium-232 Lead billion yrs Uranium-238 Lead billion yrs Potassium-40 Argon billion yrs Uranium-235 Lead billion yrs Beryllium-10 Boron million yrs Chlorine-36 Argon ,000 yrs Carbon-14 Nitrogen yrs Uranium-234 Thorium ,000 yrs Thorium-230 Radium ,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 42x10 12 tons of 12 C 47x10 10 tons of 13 C 62 tons of 14 C 14 N + 1 n – 14 C + 1 H 14 C 14 N + B + neutrinos Radiocarbon formed by cosmic flux neutron ~15 km altitude Polar concentration greater Related to geomagnetic field ~constant global amount (not really) CO 2 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 14 C is worst source Sample age relative to event age respiration, photosynthesis, tissue, aqueous, organic, inorganic states

Difference due to past variations in C 14 production and possibly CO 2 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 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.”

Paleomagnetism

Paleomagnetism 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 centered on 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 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, dating based on K/Ar dates of lavas Dipole component

Secular Variation – regional in extent ( km)

Amino 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 Acid Racemization EDT-estimated diagenetic temperature

Biological 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