Palaeomagnetic field intensity variation recorded in a Brunhes epoch deep-sea sediment core

Abstract

DEEP-SEA sediments have been shown to possess a natural remanent magnetisation (NRM) that often can be attributed to the statistical alignment of detrital magnetic grains in the Earth's magnetic field at or shortly after the time of their deposition^1,2. In favourable circumstances this remanence can be interpreted as a record of palaeomagnetic field behaviour^3,4. In the study reported here we have attempted to describe relative variations in palaeomagnetic field intensity on a time scale of 10⁴–10⁵ yr, during the past 700,000 yr, using the palaeomagnetic record of deep-sea sediment piston core RC10-167 (33°2′N, 150°23′E), which has an exceptionally thick section of sediment deposited during the Brunhes normal polarity epoch⁵ (Fig. 1). After subtracting the stratigraphic contribution of several distinct volcanic ash layers interspersed with the otherwise uniform pelagic sediment⁶, we calculate an average deposition rate of 2.1cm kyr⁻¹ between the adjusted level (1,470cm) of the Brunhes–Matuyama boundary (t=700,000 yr) and the top of the core, assumed t=0 yr. A 2-cm thick sample, representing about 1,000 yr of deposition, was taken at an average interval of 3.3 cm (representing about 1,600 yr). This sampling placed a theoretical limit of 3,000–4,000 yr on the period of a resolvable sinusoidal variation.