PRELIMINARY DETERMINATIONS OF GEOMAGNETIC-FIELD INTENSITY FOR THE LAST 400 KYR FROM THE HAWAII SCIENTIFIC DRILLING PROJECT CORE, BIG ISLAND, HAWAII

Preliminary paleointensity determinations using the Thelliers' method have been carried out on 141 samples of con HSDP (Hawaii Scientific Dr illing Project), drilled into the Mauna Loa and Mauna Kea volcanoes an d spanning the last 400 kyr. Rock magnetic investigations identify pse udo-single-domain magnetite as the main carrier of the natural remanen t magnetization (NRM) all along the core. In the Mauna Kea sequence si milar to 75% of the specimens yielded exploitable results with linear NRM/TRM (Thermoremanent Magnetization) plots and positive partial TRM (pTRM) checks generally up to the highest temperatures (550 degrees C) . A lower percentage of successful experiments (similar to 50%) was ob served for the Mauna Loa samples and the NRM/TRM diagrams are linear o ver a shorter temperature interval than for the Mauna Kea samples. The se preliminary results, based on only one sample per flow, document a marked geomagnetic field intensity low at about 40 kyr probably relate d to the Laschamp event. They also suggest that the geomagnetic field at Hawaii might be characterized by large, sharp intensity fluctuation s spaced by a few thousand years. The average value of the field inten sity suggested by these results for the last 400,000 years is slightly higher than the present field at Hawaii.