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Quaternary Dating Methods by Mike Walker

Quaternary Dating Methods (edition 2005)

by Mike Walker

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Title:Quaternary Dating Methods
Authors:Mike Walker
Info:Wiley (2005), Edition: 1, Paperback, 304 pages
Collections:Your library
Tags:nonfiction, science, paleoclimatology, climate change, Quaternary

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Quaternary Dating Methods by Mike H. C. Walker



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Not about the singles scene in the Mesolithic. Quaternary Dating Methods covers the range of techniques used for Quaternary chronology: radiometric dating; radiation exposure dating; annual bands; relative methods; and chronological equivalence methods. (No archeological methods – pottery sequence dating, for example – are included; these justify a book of their own). An excellent, readable (though technical) discussion by author Mike Walker; a lot of things I thought I knew turned out to be wrong, which is always good.

Before the emergence of physical methods the Quaternary was dating with old-fashioned sequence stratigraphy. In the 1970s I took a course in Pleistocene archaeology at the University of Chicago; we had to memorize the glacial sequences – Wisconsin, Illinois, Kansas, Nebraska for North America and Wūrm, Gūnz, Riss, Mindel for Europe. Alas, just about the time I was memorizing it all went away; marine coring and oxygen isotope analysis demonstrated that the old four glacial sequence was false and there were actually somewhere around 106; sequence stratigraphy is now pretty much confined to “Wisconsin” and “Pre-Wisconsin” (sometimes with a little fine tuning within – “Pinedale” and “Bull Lake” are the last two glacials within the Wisconsin. I have no idea what the European equivalent is.

The get an actual date for something the pre-eminent method is, of course, carbon-14. Walker gives a careful explanation of how this works, and a long discussion of what could go wrong; this last could be woowoo fodder if taken out of context since there are a whole lot of pitfalls. The most obvious is contamination with younger material; since the accelerator mass spectroscopy method can work with microgram quantities, even a skin flake from the archeologist collecting the sample can screw things up. I was surprised to find that the original method of beta-counting is still used; although it takes a much larger sample than AMS and take much longer (about a week) it also gives much more precision (±20 years is possible). I knew of a couple of the error factors; changes in solar activity and the reservoir effect. This last notes that the assumption that all the carbon in an organic sample is from the atmosphere; this is mostly true for terrestrial but can be seriously wrong for aquatic or marine life, since carbon in the hydrosphere can also come from the dissolution of carbonate rocks, which are usually infinitely old as far as C14 is concerned. I wasn’t aware of another interesting problem with marine samples; it takes much longer (up to 400 years) for carbon input from the atmosphere to mix with deep marine water. It was also interesting to discover that radiometric dating deliberately uses the wrong half-life for C14; the original value used by Libby when the method was invented was 5568 years; this was corrected to 5730 years in the 1960s but uncorrected dates still use the 5568 year half-life to allow comparison to earlier results (uncorrected dates also don’t take into account the marine reservoir effect, isotopic fractioning in living material, and past solar activity; various calibration programs are available to convert raw dates to actual years before present; for C14 purposed “present” is 1950). Some non-obvious things can be C14 dated; these include lime mortar (the assumption is the material is in equilibrium with the atmosphere until it hardens) and iron (which picks up a significant amount of carbon during working).

Other radiometric methods include uranium series and potassium argon. These are different in fundamental concept for C14 dating; with C14 you’re trying to figure out how much C14 is left from a presumed original amount while with the others you’re trying to see how much daughter product has formed. I was surprised to discover that potassium-argon dating has largely been replaced by argon-argon. With potassium-argon dating, you need two samples; one to measure potassium-40 and one to measure argon-40. Although you can take the samples side-by-side there’s still a possibility that they might have different original potassium 40 content. The argon-argon method uses irradiation to convert potassium-39 (which is to most common isotope) to argon-39. Then both argon isotopes (argon-39 and argon-40) get measured by mass spectrometry. Since the isotopic ration of potassium-39 to potassium-40 is well-known, the argon-39 acts as a surrogate for the original potassium-40 concentration.

Radiation exposure measures how long some material has been exposed to radiation. For cosmogenic exposure dating, the radiation is cosmic rays and the material is rock surfaces; you look for the amount of various isotopes (usually Be-10 or Al-26) that have accumulated since the rock surface was exposed. For fission track dating, you’re using a crystalline mineral that contains uranium; when it spontaneously fissions the fragments leave a damage track in the crystal lattice that can be enhanced and measured by etching (what you’re measuring here is the time since the material solidified from a melt). With luminescence dating, the radiation comes from exposure to naturally occurring radioactive material that contains uranium, thorium, or potassium; electrons get kicked out of the mineral crystal lattice and trapped in “holes”. The can be released by heating or exposure to sunlight and cause the material to emit light as they move back into the crystal; thus what you’re measuring is the last time the material was exposed to light or heat; luminescence dating is often used on pottery.

The most obvious annual banding is tree rings – dendrochronology. Counting rings – which is now more or less automated – has been used to calibrate all the other methods. By matching rings, the method can be extended back to around 11000 ybp or so. One of the confounding factors here is reuse of old wood, a particular problem in Egypt and other wood-poor areas (annoyingly, these are also the same climates that are likely to preserve old wood samples). Varve counting uses regular cycles in glacial lakes; in winter when the lake is frozen over little sediment accumulates while spring melting produces a lot, resulting in alternating light and dark bands. I noted that unlike dendrochronology, where distinct tree annual rings can be correlated regionally and even sometimes across species, varve counting is limited to individual lakes. Annual banding in molluscs, corals, and speleothems can also be used for dating.

Walker issues cautions about a couple of previously popular methods; obsidian hydration and amino acid racemization (both relative chronology techniques). Obsidian hydration depended on the formation of perlite from obsidian on exposure to water. Proponents originally made a number of dating claims that turned out to be unfounded; the hydration “rind” turned out to be very difficult to measure accurately. At best, the method can be used to establish relative chronology between two obsidian samples. Amino acid racemization was used to claim 60000-year-old human bones from California; it turns out that bone is a very unsuitable material for the method, porous enough to allow bacterial and chemical action to distort enantiomer levels. This put the method in a bad light for a while; however, it turns out mollusc shell is much “tighter” and racemization had been used on both marine and terrestrial molluscs with success.

Chronological equivalence is the process of finding some sort of marker in separated localities that establishes contemporaneity. The most common marker is volcanic ash layers; these have distinct trace element concentrations so identical beds can be identified even if far apart. The ash beds themselves don’t provide a date, but if some other method can be used in any of the locations with a marker bed than can all be assigned the same age.

There’s a lot of fascinating material here; this is a very terse and incomplete summary. Walker provides examples, often multiple examples, for every method mentioned, and provides references to the original literature. There are numerous graphs, tables and maps. Highly recommended if you have any interest in the area; especially handy to explain to woowoos why (for example) a house mouse might C14 date as 300 years old. ( )
  setnahkt | Dec 16, 2017 |
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Amazon.com Product Description (ISBN 0470869275, Paperback)

This introductory textbook introduces the basics of dating, the range of techniques available and the strengths and limitations of each of the principal methods.

Coverage includes:the concept of time in Quaternary Science and related fieldsthe history of dating from lithostratigraphy and biostratigraphythe development and application of radiometric methodsdifferent methods in dating: radiometric dating, incremental dating, relative dating and age equivalence

Presented in a clear and straightforward manner with the minimum of technical detail, this text is a great introduction for both students and practitioners in the Earth, Environmental and Archaeological Sciences.

Praise from the reviews:

"This book is a must for any Quaternary scientist." SOUTH AFRICAN GEOGRAPHICAL JOURNAL, September 2006

“…very well organized, clearly and straightforwardly written and provides a good overview on the wide field of Quaternary dating methods…” JOURNAL OF QUATERNARY SCIENCE, January 2007

(retrieved from Amazon Thu, 12 Mar 2015 18:16:05 -0400)

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