Anything that is under fifty thousand years old uses a potassium argon (P/K) test. It consists of using
a piece of a bone and measuring the amount of P/K being emmitted from the bone."
I don't particularly enjoy ragging on people, but I am tired of reading the garbage that the resident "anthropologist" passes off as scientific truth. The above post appeared on another board and represented Rob Anderson's views on the radiometric dating of fossilized bone. These comments reflect utter ignorance. I offer the following support for my harsh statement.
(1)If we are going to use chemical symbols, let's at least get them right. P is phosphorus, not potassium. K is posassium. Ar is argon. Pass high school chemistry, Mr. Anthro?
(2)Radiometric dating relies on the fact that certain elements have radioactive isotopes which decay to stable isotopes of other elements and that the process has a known half-life, i.e. time during which 1/2 of the original atoms of the parent isotope will decay to the daughter isotope. Among these are Carbon-14 which decays to Nitrogen-14 with a half life of about 5370 years. Radiocarbon dating NOT Potassium-Argon dating is the preferred method of dating organic material (read "bones") which are have not undergone complete mineralization and which are under 50,000 years old. The atmosphere contains a constant ratio of Carbon-14 and Carbon-12 (the more common stable Carbon isotope) atoms. While alive, respiration causes all creatures to maintain the same ratio in their systems. At death, Carbon-14 is no longer replenished. By determining the ratio between the two isotopes in an organic sample we can therefore estimate date of death. The relatively short half-life of Carbon-14 limits usefulness of this method to samples no older than 50,000-70,000 years.
(3)Another radiometric method is Potassium-Argon dating, which relies on the fact that Potassium-40 decays to Argon-40 with a half-life of about 1.25 million years. The great utility of this method is that the long half-life allows much OLDER not younger samples to be dated. BUT for more reasons than I have time to go into, and CONTRARY to what Rob Anderson says, K-Ar dating can NEVER be used to directly date the age of a fossil. For present purposes, all we need to know is that this method, unlike C-14 dating, requires us to count the numbers of atoms of the daughter isotope. Ar-40 under normal earthbound conditions is an inert gas. This means that in porous materials (like bone) the decay product, Ar-40, is lost to the atmosphere and can't be counted. It is retained only where the the original K-40 is locked in a hard, crystaline structure. Where does this occur? Most notably in unweathered volcanic ash. By melting the crystals, the retained Ar-40 atoms are released and can be counted. How do we use this to date fossils? If we are lucky (and this often occurs in the Great Rift Valley of East Africa) we can find "bracketing" strata of suitable volcanic ash above and below the level where the fossil was found. These dates when the ash was produced give upper and lower limits for the age of the fossil.
So I ask you again Mr. Anderson, please talk about stuff you understand or at least admit that you're not an expert and you're shooting from the hip. That way, you might save some nice people from absorbing and believing some serious misinformation. BS is BS whether it comes from a hot-shot anthropologist or not.
Posted on May 10, 2002, 12:55 PM from IP address 64.6.105.6