NATIONAL OCEANOGRAPHIC PROGRAM—1965 95 
Th™, Applying this correction, the ages derived from the ratio of Th™ to U*™ are 
found to be equal to 80 percent of the C™ ages. 
Disequilibrium of members of the uranium family can be used to date fossil 
marine carbonates. The principle of the method is that uranium is in low, 
but measurable, concentrations in modern shells, whereas Th™ is below the 
limit of detection. As the shells get older, Th grows in them, and the ratio of 
Th” to U** is then a measure of the age of the fossil shell. Results of analysis 
of 12 fossil sediments were presented by Herbert Potratz (Washington Univer- 
sity, St. Louis) and Richard Blanchard (U.S. Public Health Service). Ages, 
based on the ratio of Th*” to U*", agreed in most instances with ages estimated 
on stratigraphy. Wallace Broecker (Lamont Geological Observatory) found 
that fossil coral without calcite shows uranium concentrations close to the level 
contained in living coral. However, recrystallized coral samples, compared 
to their living equivalents, are deficient in uranium. Broecker reported that 
unrecrystallized material could apparently be dated with reliability to 300,000 
years, and by using the ratio of U™ to U™ even beyond this range. 
Until recently, the ratio of U™ to U** has been assumed to be equal to 1. 
Several cases of disequilibrium between these two isotopes have been reported 
recently. To assess the authigenic origin of Th and to correct for uranium- 
produced Th™, the concentration of its parent U™ must be known. Deviation 
of the uranium ratio from 1.0 was discussed. Goldberg reported that the ratio 
for water from the Atlantic, Pacific, and Indian Oceans is 1.14+0.01: a Red 
Sea sample showed a ratio of 1.18+0.01. Ratios of U™ to U** were determined 
on lake and river samples by David Thurber (Lamont Geological Observatory). 
He found isotope fractionation resulted in activity ratios of U™ to U™ that 
were greater than 1, in nearly all cases being greater than 1.2 and ranging as 
high as 6. 
Considerable isotope fractionation was reported by John Rosholt (U.S. Geo- 
logical Survey, Denver) in roll features (uranium-bearing fluvial sandstone) in 
Shirley Basin, Wyo. In these samples, the U™ is found to be deficient in altered 
ore sands by as much as 70 percent, while calcite ore sand shows only a slight 
deficiency. A small amount of uranium in unaltered sand, above and below the 
roll feature, does not show isotopic fractionation. The reasons for fractionation 
in the samples were discussed by Rosholt, and several mechanisms were pro- 
posed. Any environment which contains intimately mixed sediment and hydro- 
logical phases for a sufficient length of time may lead to the enrichment or deple- 
tion of U** in one phase or another. Thurber suggested that the recoil of the 
nuclides during alpha decay makes the daughter U™ nuclide more available to 
weathering than the parent U**. Subsequent preferential leaching leads to the 
deficiency or enrichment of U*. 
Methods of chemical separation of nuclides in dating were discussed in some 
detail. Two approaches to the determination of the ratio of Th” to Pa™ were 
presented. William Sackett (Lamont Geological Observatory) used a direct 
chemical separation of protactinium and thorium. Goldberg in a few experi- 
ments and Ronas group in all experiments used an indirect method which involves 
Separating thorium chemically and then counting the Po” a daughter of Th”. 
This isotope (Po) can be well characterized by its alpha emission of 7.35 mil- 
lion electric-volts energy in alpha spectroscopy. In both cases, a thorough knowl- 
edge of thorium and protactinium chemistry is necessary. Harold Kirby (Mon- 
santo Research Corp., Miamisburg, Ohio) discused the chemical similarities 
and differences of these two elements; the latter far outnumbered the former. 
Protactinium can be separated from thorium by the hydrolysis of protaetinium in 
strong hydrochloric acid, in which process the thorium remains in solution. 
There different behavior in ion exchange and solvent extraction techniques is also 
exhibited. Kirby concluded by stating that the wet chemical behavior of tho- 
rium and protactinium denies the existence of an actinide series of elements 
because protactinium and thorium have little in common with each other or with 
the elements of higher atomic number, from uranium on. This dissimilarity of 
the chemistry of thorium and protactinium raises the speculation as to whether 
the two isotopes, once reaching the ocean bottom, behave in the same way in the 
sediments. Richard Ku suggested that uranium and possibly thorium may mi- 
grate in the sediments. 
The use of isotopes as tracers for yield determinations was also discussed, and 
it was pointed out that the lack of exchange between the tracer and the isotope, 
whose yield has to be determined, may lead to serious errors. 
