Site Formation Processes
What is aeolian deposition?
Wind deposition
True of False: When reporting radiocarbon dates, "BP" stands for Before Present, with "Present" set at 1950.
True.
What is absolute dating?
Absolute dating is dating which provides specific calander age estimates. It allows specific time difference between dates to be understood.
● Note: Varies in precision, requires a robust chronometric reference.
What is radiometric dating?
Bonus: What radiometric dating method is discussed most in this class?
A dating method which uses rate of decay of radioactive isotopes to measure time. The range of time is based on an isotope's half-life; the decay rate is unaffected by environment.
● Requires an event that "sets the clock to zero".
Bonus: C14 dating
Under which specific conditions does organic material preserve well?
● Very dry conditions.
● Very cold conditions.
● Oxygen starved conditions.
● Chemically 'special' conditions.
What is the difference between fluvial and lacustrine deposition?
Fluvial deposition relates to moving water (rivers) while lacustrine deposition relates to stopped water (lakes).
True of false: Inorganic materials do not degrade.
False. Some inorganic materials degrade.
ex: Metal rusts (chemical weathering), abrasion or erosion cause mechanical weathering.
What is relative dating?
Dating which does not require a specific chronometric reference and can be done based on stratigraphy; that is, dating which requires good context. Useful to order things (ie: artifacts) into sequences (seriation and cross-dating). Orders remains in chronological sequence.
How does C14 (radiocarbon) dating work?
● Cosmic rays (high energy particles) interact with Nitrogen (14N) in the atmosphere, which gains a neutron to become 14C
●Living beings continuously take in atmospheric carbon– They continuously replenish 14C
●When a living organism dies, it stops replenishing 14C, and the 14C radioactive decay begins
● 14C decays at a constant rate: The half-life of 14C is 5730 years
● 12C and 13C are stable; they do not decay
● By comparing the amounts of 14C to 12C and 13C in a sample we can learn how much C14 has decayed.
How is calibration done for radiocarbon dating?
Bonus: Why is this necessary?
Calibration is done using very long-lived trees (for the last 11,800 yrs) and deep sea marine corals (dated using a different method: Uranium-Thorium dating) for pre-11,800 yrs. Generally, the older the true age of the material, the greater the difference between radiocarbon and calendar dates.
Bonus: Changes in the earth’s magnetic field and in solar activity result in variations in the level of atmospheric C-14 - these levels have always naturally fluctuated over time.
What is colluvial desposition?
Deposition influenced by gravity such as landslides or rockslides.
True or False: Radiocarbon dates are always provided with an error range (ie: +/- 100 BP) due to the random nature of radioactive emission and other factors that can affect the measurement.
True.
What are direct dates?
Dates derived from the actual artifact, ecofact, or feature itself to determine the age of that material.
How are radiocarbon dates reported?
● Radiocarbon results are reported as years `before present' (BP)
● ‘ Present’ is defined as 1950.
● Fixed “present” date allows comparison of radiocarbon dates without having to know the year when dates were calculated.
● Atmospheric carbon is no longer reliable. Nothing later than 1950 can be dated.
What are three complications to the calibration of radiocarbon dating?
● Old wood problem– Trees are alive, but “wood” is dead. Only the cambium layer is replenishing its protein. – Each growth ring is its own age – a tree’s C14 age includes the entire time range of its life.
● Marine reservoir effect– Carbon circulates slowly through the water column -> lots of old carbon in there – Marine organisms absorb carbon from the water rather than from the atmosphere– Dates might be too old
● Contamination – Natural or cultural contaminants can affect samples
What is volcanic deposition?
Deposition influenced by volcanic activity. ie: layers of tephra (ash, lava, etc.)
True of False: Radiometric dating is a form of relative dating.
False. Radiometric dating is a form of absolute dating.
What are indirect dates?
Dates based on date-able material in associate with the archaeological subject of interest.
What is Radiocarbon AMS Dating and how is it different from Radiocarbon Dating?
Radiocarbon Accelerator Mass Spectrometry (AMS) dating method has been used to directly count radiocarbon atoms in a sample, rather than measuring the rate of radioactive decay. AMS allows much smaller samples outside of the range of conventional radiocarbon dating to be dated, with higher levels of precision and a reduced error range.
What is the significance of radiocarbon dating?
The significance of the radiocarbon dating method to archaeology in the 20th century is equal to the impact that the establishment of human antiquity had in 19th century thought. (In other words, the specificity of the dating is revolutionary.)
What is anthropogenic deposition?
Human-caused/influenced deposition.
True of False: Fossils are organic.
False. Fossils are not organic.
C14 dating is accurate to _____ - _____ years.
50,000 to 70,000
What are 4 dating methods other than radiocarbon dating?
● Potassium-Argon (K/Ar; 100,000 to billions of yrs range)
● Paleomagnetism (millions of yrs range) & Archaeomagnetism (~10,000 yrs range)
● Obsidian hydration (100s to millions of yrs range)
● Luminescence (TL/OSL; a few yrs to millions of yrs range)
What is the half-life of C14?
5730 years.