A study of the radiolaria from the tops of more than 80 cores spaced around the Antarctic continent between 35° and 77°S. has revealed two distinct faunas, the boundary between them corresponding closely with the position of the South Polar Front (Antarctic Convergence).

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An examination of samples taken at 40-cm. intervals down cores from both north and south of the Polar Front shows that several cores located north of the Polar Front contain alternating layers of the two faunas, whereas cores located at the south show no such faunal fluctuations. If the alternating faunal zones observed in the northern cores are caused by a shift in the position of the Polar Front, then the data collected so far suggest that the Polar Front today may be as far south as it has been during the time period represented by the cores sampled.

Other changes are exhibited by 10 cores from the area. In these cores there is an abrupt change from a fauna composed predominantly of recent species, the majority of which are not found living in Antarctic waters today. This change in the radiolarian assemblage is accompanied in 80 cores by a large decrease in other siliceous or calcareous organisms. In two cores the carbonate content drops from 40-60 per cent to zero. In one core the older assemblage is associated with abundant discoasters and on this basis is tentatively assigned a Late Tertiary age.

The cause of the change in planktonic assemblages from Late Tertiary to Quaternary is not known, but it is suggested that it may be due to a change from a relatively stable stratification of the Antarctic Ocean during Late Tertiary to a stronger degree of vertical convection in the Quaternary associated with the cooling of this part of the world during the growth of the Antarctic ice cap.

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