A continuous record of inclination and intensity of the Earth's magnetic field during the last 2250 kyrs was obtained from a marine sediment core of 42m long. This record reveals the presence of 100-kyr periodicity in inclination as well as intensity. Geomagnetic field is produced by fluid flows within the Earth's outer core, which is called the geodynamo. The geodynamo was thought to be a self-sustained system within the core, but the discovery of the 100-kyr periodicity, which is longer than the electrical diffusion time of the core, indicates the presence of external energy sources: orbital eccentricity and paleoclimatic changes.