Nonequilibrium between the radioactive elements in the surface of the ocean bottom provides a method of measuring time in the past 300,000 years. The principles of this method are not to be confused with those underlying the radioactive methods of determining geologic time measured in millions of years. The well-preserved geological and biological history of the ocean basins has been worked out for many of the cores of ocean sediments procured by Piggot, and the above method of measuring time intervals in years has been applied to these cores. The results indicate: (1) that the effects of glaciation on the continents are contemporaneous with equivalent effects on the type of deposit in the ocean bottom; (2) that the effects of glaciation on the type of ocean sediment are widespread, extending in the northern hemisphere at least to the Caribbean Sea; (3) that within short intervals of time there is, in the ocean bottom, a considerable variation in the type of deposit attributable directly or indirectly to climatic changes which, probably because of the continuous effacement of the evidence on land, has not been reported in studies of continental glaciation.
This method cannot compete in accuracy with the use of varved clays, but it possesses the advantages of (1) application to a considerable area of the earth's surface, (2) reference to the present time, and (3) applicability to at least half of the Pleistocene epoch.
The rate of deposition of ocean sediments is treated as a subsidiary problem. The obvious step from the determination of time intervals in a core to the rate of deposition of the sediment represented by the core is complicated by the present incomplete knowledge of the distortion of the sediment in the process of taking the sample. The results in general agree with previous estimates by other methods, some of which, however, are open to the same criticism. It is possible to discuss within limits the variations of the rate of deposition of ocean sediments in the past.