. The Earth beneath the sea : History . Fig. 5. Decay of radioactive substance. The ordinate gives tlie latio of the original amount of the radio-nuchde to the amount left after a certain time. The abscissa gives the non-dimensional units indicating the time as multiples of mean lifetime. 3. Geochemical Considerations The basic condition for all age determinations is the elimination or addition of radio-nuclides to a closed system which, after the completion of the processes, does not exchange with the surroundings. Age determination in marine sedi- ments is based on the same principle. Becaus
. The Earth beneath the sea : History . Fig. 5. Decay of radioactive substance. The ordinate gives tlie latio of the original amount of the radio-nuchde to the amount left after a certain time. The abscissa gives the non-dimensional units indicating the time as multiples of mean lifetime. 3. Geochemical Considerations The basic condition for all age determinations is the elimination or addition of radio-nuclides to a closed system which, after the completion of the processes, does not exchange with the surroundings. Age determination in marine sedi- ments is based on the same principle. Because the sediments are a mixture of many components of different origins and different histories during the trans- port to their final burying ground on the ocean floor, many difficulties arise. In the following sections, some of the most important processes that have a bearing on age determination are discussed. A. Thorium As early as 1908, Joly had found a high radium content in sediment samples he had received from the Challenger Expedition. Hans Pettersson gave the right explanation in 1937 by suggesting that ionium (230Th) is precipitated in the ocean and added to the sediment. His hypothesis was confirmed by Koczy et al. (1957) and later reconfirmed by Sackett, Potratz and Goldberg (1958). The yield of separation of sso^h from sea-water was more than 98%. It is believed that it is as high as If we assume that the uranium content of ocean water is 3 x 10"^ g U/ml and that the produced ionium is evenly spread over the ocean floor, about x 10~i3 g saoTh is precipitated per square centi- meter per year on the ocean floor. The first assumption, based on several
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