Paleomagnetic Study of Antarctic Deep-Sea Cores

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Abstract
The magnetic inclinations and intensities of about 650 samples from seven deep-sea cores taken in the Antarctic were measured on a spinner magnetometer. This series of measurements provided a magnetic stratigraphy, based on zones of normally or reversally polarized specimens for each core, which was then correlated with the magnetic stratigraphy of Cox et al. One core (V16-134) gave a continuous record of the paleomagnetic field back to about 3.5 million years. When selected samples were subjected to alternating-field demagnetization, most were found to have an unstable component that was removed by fields of 150 oersteds; all samples from two cores were partially demagnetized in a field of 150 oersteds. The average inclination in these two cores was then in good agreement with the average inclination of the ambient field for the latitude of the core site. It was also found that the intensities of the samples decreased at the points of reversal; this finding is to be expected if, as has been postulated by the dynamo theory, the intensity of the dipole field decreases to zero and builds again with opposite polarity. We believe that the magnetization of the cores results from the presence of detrital magnetite, although other magnetic minerals also may be present. Four faunal zones (φ, χ, ψ, and ω) have been recognized in these Antarctic cores on the basis of upward sequential disappearance of Radiolaria. The faunal boundaries and reversals consistently have the same relations to one another, indicating that they are both time-dependent phenomena. Using previously determined times of reversal, one may date the following events in the cores: 1) Radiolarian faunal boundaries: φ-χ, 2 million years; χ-ψ, 0.7 million years; ψ-ω, 0.4 to 0.5 million years. These dates are in good agreement with ages previously extrapolated from radiometric dates. 2) Initiation of Antarctic diatom-ooze deposition, approximately 2.0 million years ago. 3) First occurrence of ice-rafted detritus, approximately 2.5 million years ago. One can also calculate rates of sedimentation, which vary in the cores studied from 1.1 to about 8.0 millimeters per 1000 years. Sedimentation rates for the Indian Ocean cores are higher than for the Bellingshausen Sea cores. The near coincidence of faunal changes and reversals in the cores suggests but does not prove a causal relation. We conclude from this study that paleomagnetic stratigraphy is a unique method for correlating and dating deep-sea cores, and that future work with such cores may provide a complete or nearly complete record of the history of the earth's magnetic field beyond 4 million years.