Secular variation as recorded in lake sediments: a discussion of North American and European results

Abstract

Type-curves depicting secular variations in declination and inclination through Holocene time have been constructed by stacking data from individual cores of lake sediment from east-central North America and the U.K. There is a strong positive correlation in declination with zero phase shift froiti 0 to 4500 ¹⁴ C years b.p. but from 4500 to 8500 ¹⁴ C years b.p. the correlation is strongly negative , which we interpret in terms of fluctuations in intensity of standing geomagnetic sources. A source located under the Atlantic Ocean appears to have been dominant before ca . 4500 14C years b.p., and then another similar source located either to the east or to the west of both observation areas gained dominance. However, the inclination type-curves show optimum positive correlation for a phase shift of ca. 400 years, suggesting that the geomagnetic sources that caused them were drifting westward at ca. 0.2 degrees a year. There is no way of adjusting the timescales so as to obviate this apparent paradox. Secular variations of the total geomagnetic vector have been investigated by noting the sense (clockwise, anticlockwise) of looping of the v.g.p. paths, though this cannot be interpreted uniquely in terms of westward or eastward drifting sources in the core, as demonstrated by specific radial dipole models.