Relative Geomagnetic Intensity during the Last 4 m.y. from the Equatorial Pacific

Abstract

A complete sequence of relative geomagnetic field intensity was obtained for the last 4 m.y. from multiple holes drilled at Sites 848, 851, and 852 (Leg 138) in the east equatorial Pacific Ocean. The time vs. depth correlation has been derived by tuning the gamma-ray attenuation porosity evaluator (GRAPE) density record (and, in some cases, the susceptibility) to the insolation curve, yielding one calibration point every 25,000 yr. Relative paleointensities were determined after normalizing the remanent magneti-zation of U-channels, single samples, and long core measurements by anhysteretic remanent magnetization (ARM) and low-field susceptibility (K). Identical results are obtained with every technique. The composite records obtained at the three sites are very similar within their temporal resolution, which varies as a function of the deposition rates and the different sampling methods. The most noticeable features are large decreases in field intensity accompanying the field reversals. The process is clearly asymmetrical with a long-term decrease before a transition and a rapid field recovery immediately after, thereby suggesting two different processes. Once the field has reached a maximum, it starts relaxing until the occurrence of the next reversal. Thus, the duration of the polarity intervals appears to be controlled by the field regeneration prevailing during very short episodes after major geomagnetic events, which appear themselves to result from a progressive degradation of the field. Shorter intensity minima observed during the last 2 m.y. are correlated to geomagnetic excursions and/or short events already noticed in other data sets. This record provides a framework for a stratigraphy based on the intensity variations of the geomagnetic field.