Dating 2:7:19 - Lecture notes 8 PDF

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relative dating, absolute dating, radiometric dating...

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Ordering in Time Critical o Relative dating: established the relative (age) relationship between things o Absolute dating: assigns an age in year, expressing as a range in year (a +/symbol) 1. Calendrical (historical) dating  Must be careful with the context 2. Dendrochronology and varve  D: A dating method that links the growth rings in trees to calendrical time  Tree-ring dating o Each year trees gain a growth ring o Uneven thickness of rings  Narrower with the increasing age of the tree  Annual climate changes affect tree growth o The bark (outermost) of a tree is the most important o Method: 1. Use drill to get a core 2. Count tree rings 3. Produce a diagram indicating the thickness of successive rings in an individual tree 4. Dating from present up to 10,000 B.P. (con)  Pros: o Useful for calibration of radioactive dating  Cons: o Not a universal method: applicable to trees outside the tropics  Trees people used in the past  Uneven thickness of rings o No precise date without the outermost rings o Cutting date does not equal the construction date  Extended use  Reuse  Replacement of old timbers  Use of old wood  V: Cores (sea, lake, and ice) o Forms levels of sediments every year o Up to 52,800 B.P. o Limitations:  Requires a specific environmental condition such as smooth ocean/lakes, surrounded by mountains, deep water depth, etc. in order to find data from remote times 3. Radiometric dating: dating methods that rely on calculations of a temporal length in which one half of a unstable isotope becomes a stable isotope  Isotopes: elements that possess the same number of protons but vary in neutrons

Radioactive decay: transformation of unstable radioactive isotopes into stable isotopes  Half-life concept: the time it takes for radioactive elements to decay Radiocarbon dating 14C  First radiometric technique (1949)  Developed by William Libby 1. Organic remains 2. Half decay of 14C is about 5730 years  Accelerator Mass Spectrometry (AMS) o Directly counts 14C in the sample  Limitations: o Calibration: production rate of 14C on earth not constant over time. It needs to calibrate with tree-ring dating. 14C dates must be calibrated to obtain calendar dates. The results always include estimate of possible error o Date up to 50,000 years B.P. o Poor choice of samples o What is dated is the sample, not necessarily the context o Contamination o Statistical uncertainty Potassium-Argon dating  Decay of Potassium (40K) into Argon (40Ar)  Use of volcanic rock sample (10g)  From 100,000 B.P. (half-life is 1.3 billion)  Refined method is 40Ar-39Ar dating  Limitations: o Use volcanic ricks; indirect date o Useful only for areas buried by volcanic activity o Cannot date after 100,000 B.P. o Error range is 30,000 years (big) o Nevertheless, this is the most useful method for early humans (hominins) Uranium series dating  A radiometric technique decay of uranium isotope (234U) into a thorium isotope (230Th)  Dates calcium carbonates  The range is 1000 – 500,000 B.P.  Limitations: o Useful only in a limited environment such as caves with calcite layers or buried teeth o The correct order of deposition in a cabe is ambiguous o But, it fills the gap between 14C dating and 40K – 40Ar dating Fission track dating 

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 The decay of 238U leaves pathways called fission tracks  Use volcanic rocks and glasses  Count the number of fissions an under optical microscope  From 1 M to 2 B years ago  Used for cross-checking  Each absolute dating has limitations Direct and Indirect Dating o Matrix, association, context, etc. are important when figuring out indirect dating Reading the Past o Stratigraphy: the study and interpretation of the sequentially layered deposits of a site  Terminus ante quem: limit before which  Terminus post quem: limit after which o Stratification: the observed layering of matrices and features Architecture o Abutted walls: those are built up against existing construction o Bonded walls: those are built at the same time Construction Sequence Archaeoastronomy Seriation: idea that style changes over time o Ex. Sir Flinders Petrie in Egypt found different styles of pots o Ex. Battleship curves  Ideal shape of a seriation curve o Ex. Northeastern headstones Climatostratigraphy Glottochronology: idea of language evolving as they spread farther away from the origin language...