PERSISTENTLY ANOMALOUS PACIFIC GEOMAGNETIC-FIELDS

A new average geomagnetic field model for the past 3kyr (ALS3K) helps bridge a large temporal sampling gap between historical models and mor e traditional paleomagnetic studies spanning the last 5 Myr. A quasi-s tatic feature seen historically in the central Pacific has the opposit e sign in ALS3K; its structure is similar to, but of larger amplitude than, that in the time-averaged geomagnetic field for the last 5 Myr. Anomalous geomagnetic fields exist beneath the Pacific over timescales ranging from 10(2)-10(6) years. It is unlikely that bias over such lo ng time scales arises from electromagnetic screening, but conceivable that the Lorentz force is influenced by long wavelength thermal variat ions and/or localized regions of increased electrical conductivity (as sociated with compositional anomalies and possibly partial melt). This is consistent with recent seismic observations of the lower mantle.