O4 NATIONAL OCEANOGRAPHIC PROGRAM—1965 
lection and preparation, counting techniques, and the interpretation of results. 
A Conference on the Geochronology of Marine and Fluvial Sediments was held 
at the Oak Ridge Institute of Nuclear Studies, October 17 to 19, 1963, with the 
purpose of discussing these questions, in order to establish items of agreement, 
to seek explanations of points of disagreement, and to discuss the validity of 
the basic assumptions on which various dating methods rest. The meeting, 
limited to 50 participants, was dedicated to Hans Pettersson (Goteborg, Sweden) 
who was in attendance. In 1937 he suggested that Th*” is precipitated (unsup- 
ported by its parent uranium) from the ocean onto the bottom, thus laying the 
conerstone for thorium dating of the sediments. Emphasis was put on discus- 
sions and free intercourse of ideas. Six invited papers and 15 contributed papers 
were presented by scientists from leading institutions in this field. 
The method of dating by the ratio of Th™ to Pa™ is based on the assumption 
that Th* (commonly called ionium) and Pa™ are removed simultaneously from 
sea water soon after formation (authigenic materials) and are thus separated 
from their respective parents, U™’ and U**. In the sediments, each isotope de- 
cays with its own half-life of 76,000 years and 32,500 years, respectively. Pro- 
vided the ratio of the uranium parents in sea water has remained constant for 
at least several hundred thousand years, the ratio of Th*” to Pa™ ean be used for 
estimating deposition rates over periods of at least 200,000 years. The validity 
of this method rests on a few assumptions which were considered by the 
participants. 
The longest and most involved discussions centered around the problem 
of separating authigenic thorium and protactinium isotopes from those isotopes 
of allogenic origin. It is obvious that some of. these isotopes originate from 
allogenie minerals, such as volcanic ash, meteorites and continental detritus. 
EB. D. Goldberg (Scripps Institution of Oceanography) presented data on samples 
prepared by leaching with 6N HCl and compared these results with those 
obtained by bringing the whole sample into solution by fusion methods. He 
found that the ratios of Th* to: Th were higher in the leached than in the 
fused samples. This is an indication that a harsher treatment brings the detrital 
thorium into solution. F. F. Koezy (Marine Laboratory, Miami) and most of the 
other participants believed that leaching (even mild leaching) dissolves some 
detrital thorium. Paul Antal (Marine Laboratory, Miami) found that Th” 
was more refractory to acid leaching in deeper segments of a sediment core. 
He attributed this to a diagenetic induration, a process in which amorphous 
precipitates are converted into chemically resistant minerals. If this diagenesis 
causes a reduced leaching yield of Th*’, a large error in dating will result. 
Less leaching indicates apparently older ages for the sample. 
The detrital contribution of thorium is most serious if ratio of Th® to Th” 
is used for sedimentation rates; the detrital contribution of Th” is hard to 
assess. Goldberg showed, however, that dating methods with the ratios of Th” 
to Pa* and Th*° to Th’ gave the same results if proper geological considera- 
tions are observed. Sackett has shown, however, that the ratio of Pa™ to Th” 
in the top of the cores is seldom equal to the predicted ratio. He attributes this 
result to the mixing of the sediments, which would upset the ratio of Th to 
Th’, but less seriously affect the ratio of Th” to Pa™. 
The general consensus was that the best method is the dissolution of the 
total sample and subsequent correction for the isotopes of detrital origin. Dif- 
ferent approaches for arriving at this correction were discussed. Elizabeth 
Rona et al. (Oak Ridge Institute of Nuclear Studies) are investigating the 
validity of using the ratio of Th*”’ to Pa™ for dating in the Gulf of Mexico. 
They reported on the analysis of samples (thorium, protactinium, and uranium) 
along the Mississippi River, the principal contributor of sediment material to 
the gulf. Using the ratios Th to Th’ and Th to Pa™, they hope to make 
corrections on the sediments of the open gulf for terrestrial contributions of 
these isotopes. Thurber found activity ratios of Th*° to Th™ in the Mississippi 
and other gulf coast waters to be about 1, although this ratio varied considerably 
in other bodies of water. 
Aaron Kaufman (Lamont Geological Observatory) discussed this problem 
from a different aspect. He determined the isotopic ratios of thorium, radium, 
and uranium on 25 selected samples of the fluvial carbonate sediments in the 
Great Basin. These sediments were also dated by C™“ age procedures. In 
nearly every case, in which the Ra“ and Th*® were in equilibrium, the data 
lead to the assumption that the initial activity ratio of Th?’ to This 2.2. Thus, 
one can use the Th™ data for these selected samples to convert Th to authigenie 
