Ar-39/Ar-40 and K-40/Ca-40 methods are similar to K/Ar methods, and K/Ca and Sm/Nd methods are similar to the Rb/Sr methods, subject to most of the same uncertainties and alternate interpretations. Many other methods are now generally regarded as still experimental, obsolete, or of limited use for determining the age of life on earth. These include: Kr/Kr and U/Xe, U/He, Pb/alpha, thermoluminescence, cosmic ray-induced isotopes (Mn-53, Cl-36, Kr-81, I-129, Al-26/Be-10), obsidian hydration, N content of bone, and collagen content of bone. Some of these methods may be affected by heat, pressure, moisture, pH, solutes, radiation dosage, and many other factors, as well as by time.
2010 Arthur V. Chadwick, Ph.D.