Long-term secular variation of the geomagnetic field during the last 200 kyr recorded in sediment cores from the western equatorial Pacific

Abstract

We have conducted a paleomagnetic study of five hemipelagic-clay cores of 4–7 m in length obtained from the West Caroline Basin, western equatorial Pacific. The purpose of this study is to clarify the long-term secular variation of the geomagnetic field during the last 200 kyr, using both intensity and directional changes. One core was dated using oxygen isotope ratios ( δ 18O), and characteristic magnetic susceptibility variations of about a factor of four allowed precise correlation between all cores. The magnetic mineralogy is estimated to be dominated by magnetite of single-domain (SD) and/or pseudo-single-domain (PSD) state. The magnetic mineralogy and grain size are uniform throughout the cores, which enabled us to obtain relative paleointensity variations as well as directional changes. Remanent intensity normalized by anhysteretic remanent magnetization (ARM) indicates that sharp intensity drops (to ≤ 20% of the present field) occurred at about 40 and 190 kyr B.P., with a broader decrease around 110 kyr B.P. The former two are accompanied by anomalous remanent directions, suggesting short polarity reversals or excursions. Their ages are close to those of the previously reported Laschamp excursion and Biwa I event. A prominent intensity peak occurred immediately preceding the possible excursion at 40 kyr B.P. Our results are in general similar to the recently reported datasets spanning the last 80–140 kyr from the Somali Basin, the Mediterranean, and the Sulu Sea [1–3]. The inclination record shows long-term cyclic changes of several degrees in amplitude. The intervals of the recurring variation are 40–50 kyr, which is longer than the core's memory and is close to the Milankovitch frequency associated with the change in obliquity of Earth's rotational axis. We infer that an external force such as orbital forcing could be a cause of the long-term secular variation.