2a. stratigraphic vs. SED
* Explain basic distinction
* Earth & Archaeological Sciences based on stratigraphic.
Almost a paradigm in the methodology.
Affects sampling, interpretation, funding, future work.
b. Numerical/Correlated/Calibrated/Relative
3. Dendrochrology
Basic idea: in seasonal climates of mid- and high-latitudes yearly
growth rings are
prominent. Each ring is formed by larger cells and softer wood formed in
spring and smaller/denser cells in summer and autumn.
5. Radiocarbon Dating
Carbon-14 atoms form in the earth atmosphere from cosmic ray
bombardment,
mostly protons from elsewhere in the galaxy.
- radiometric OK. Relates to spontaneous radioactibe decay by loss of alpha (helium atom, 2 protons & 2 neutrons) or Beta (electrons) particles. Rate of decay a physical constant. It does not change in different enironments. Measured in 1/2 lives
At Death all there
5730 yrs 1/2 left
11,460yrs 1/4 left
17,000 yrs 1/8 left
Too low to Detect, about 40,000
6. K-Ar Dating
* Used to date volcanic events. Important to Quaternary studies,
because can calibrate other methods (paleomag, in situ, varnish) and
because may be stratigraphically related to key deposits.
* Potassium (K) common in minerals. 39K and 41K are stable. 40K is in small amounts (0.012%) and decays to 40Ca or 40Ar with a 1/2 life of 1.3 billion years. Ca more common than Ar, so Ar used.
* Age is 40K vs 40Ar. Assume when rock is molten, argon is driven out. So volcanic rocks contain no argon. (Can have excess argon problem). With time Ar-40 produced and retained.
* Younger limit is about 100,000 for basalts and 10,000 for rocks rich in potassium. Older limit is over a billion.
7. Uranium-Series
Encompasses the range of radiometric techniques based on the declay of
Uranium-238 and Uranium-235 by emission of alpha particles.
Ideal Sample:
Chemically precipitated from a low temperature solution
and not redissolved. Typically precipitated as calcium
carbonate with Uranium in Solution.
Assumptions:
(1) At the time of formation of a dated sample, the parent
isotope(s) was
present, but the daughter in question was either absent or present at a
known activity level
(2) After a long time, the activity of the daughter becomes equal
to the
parent...reaches radiocactive equilibrium
(3) Sample has not been chemically disturbed since it was formed.
Materials Dated:
9. Paleomagnetism
The earth's magnetic field varies. The history of fluctuations
are recorded
by magnetic particles in rocks and sediments when they are deposited.
By dating (e.g. K-Ar, fission track) these deposits, can assign dates to these fluctuations. Therefore, can use these paleomagnetic fluctuations are correlation tools to date deposits elsewhere.
10. Obsidian (and tephra) Hydration Dating
* Obsidian glass will react with water and hydrate. The rind is seen
in thin
sections by a change in the refraction index in a microscope. The
thickness of
the rind increases with time and temp.
* Hydration starts when a fresh surface is exposured, by glacial abrasion? By new obsidian flow? By artifact manufacturing
* Hydration is NOT LINEAR; more log.
* Depends on temp, moisture, physical structure of glass,chemistry of glass
* Needs to be calibrated in each local area for each microsite
* Best use in archaeological dating and glacial events in volcanic areas. Obsidian pebbles in moraines in Yellowstone provide dating of glacial deposits to 140 ka (Bull Lake) and 25 ka (Pinedale), using Lava flows 114 ka, 179 ka to calibrate obsidian hydration
11. Thermoluminescence (TL)
* TL is the light emitted from a mineral crystal when it is heated
* TL is derived from exposure to ionizing radiation. Free electrons are produced by the decay of radioisotopes in the matrix.
* Free electrons are trapped in the crytal defects. The longer the crystal is exposed to radiation, the higher the trapped content, and the more TL when the sample is heated.
* Ideally, amount of light is a funcation of the length of time the sample has seen since it was last heated to the point where it would zero the TL.
USE:
*Pottery in archaeology
*sediments baked by lava
*calcite precipitation
* marine and terrestrial sediments, if the TL is "zero'd" by exposure
to
sunlight
during weathering, detachment, and erosion.
12. Amino Acid Racemization (AAR)
* Chemical Dating Method, dependent on time and temperature
* Amino acids are among the basic structures required by living organisms. Have amino groups (NH2) and carboxylic acid (COOH).
* Some amino acids are stable. Occur in similar amounts in recent and tertiary sheels of the same species.
* Others undergo RACEMIZATION, which is the transformation of a substance (with an asymmetric structure) to its mirror side, like racemic acid to tartaric acid)
* Rate of racemization is controlled by temp and time. If can guess at temperature, can estimate time. or establish correlative chronology
* need about 10 mg and measure in gas or liquid chromatography. Different AA labs typically complete inter-lab comparisons.
* The ratio of D/L in modern environments are pretty much zero, but increases with increasing age of fossils.
* AAR used in:
13. Soils as a Dating Tool Birkeland - Soil Stratigraphy
Field Properties: Depth of Solum, horizonation, color, texture consistency, structure, pH, clay films, carbonate
Lab Properties: particle size distribution, % OM loss on ignition, pH, dry color
PROBLEMS: Local Environment Erosion of soil horizons Aeolian input
Harden- Soil Index
BASIC APPROACH: one or more properties of a series of soils of differing ages are keyed to relative age values to produce a chronosequence. Then non-dated soils are assigned ages by comparison with the reference sequence
HARDEN Combines Soil Field (& can use lab) properties:
Assign points to each of these properties (like clay films, abundance, thickness, locaiton from 0-10-20-30-40.
Then normalize the point value for each property on scale 0-10
Found soil development has a semi-log relationship. Soil index from 0 to 1 (max) arithmatic and age log.
Ca-carbonate: Model, C-14, U-trend, U-series
14. Rock Varnish
15. In Situ Cosmogenic Isotopes
* Over 30 elements have radioisotopes with half-lives in the range of 1 ka to 100 myr. Many are within the range of interest to the Quaternary, 103 to 106.
* Five have been measured with AMS (Accelerator Mass Spectrometry) at natural levels. The idea is to use milligram samples of isotopes: C-14, 10-Be, Al-26, Cl-36, I-129
BASIC IDEA OF AMS: Each atomic species has a unique mass
-formation of a charge ion
- accelerate through electrostatic potential of MeVs
(conventional mass spect uses KeVs)
- separate of ions based on mass-to-charge ratio
- measurement of number of ions in detectors
EXAMPLES OF USE:
AMS C-14:
CHLORINE-36 (Fred Phillips)
* idea isotope for geomorphic studies BC 300,000 yr half-live
In situ build-up can provide dating:
* hydrology studies -- used for trace movements in Great Artesian Basin in Australia, high level nuclear waste siting, geothermal systems, bomb trace in infiltration studies
* continental saline sediments, Searles Lake, provide new reconstruction of climatic history
Al-26 * Use quarts from natural surfaces, because low AL
concentration allows measurement of Al-25 buildup
after about 1000 years.
* glacial polish in Sierra Nevada
Be-10 (in soils)
* depth profiles of Be-10 in ferromanganese nodules yield growth
rates of 1-4 mm/million years. Also old ages for nodules
* can help determine long-term erosion history of landscapes
EXAMPLE OF USE: Merced River Terraces (Pavich, Harden) Be-10 retained in clays in top few meters. Decays there. Good technique until about 100 to 200 ka.