15. CORING-INDUCED MAGNETIC OVERPRINTS AND LIMITATIONS OF THE LONG-CORE PALEOMAGNETIC MEASUREMENT TECHNIQUE: SOME OBSERVATIONS FROM LEG 160, EASTERN MEDITERRANEAN SEA1

Major fluctuations in paleomagnetic directions were commonly observed from long-core measurements of sediments recovered during ODP Leg 160. The recovered sediments display large-scale variations in magnetic remanence intensity partly because of the presence of magnetically "enhanced " sapropels. The pass-through long-core measurement is highly sensitive to large changes in remanence intensities. Significant discrepancies between the results obtained from discrete samples and long-core measurements indicate that it is necessary to determine whether the measured magnetic signal arises from geomagnetic field behavior, coring-induced overprints, or artifacts of the long-core measurement. Results of experiments designed to distin-guish between these possibilities suggest that there are several factors responsible for the observations. Coring-induced over-prints do not appear to be a significant problem: they are usually easily removed at low alternating fields (AFs). In some cases, particularly in the lower part of the record at Site 963, remagnetization is evident in intervals where sediment suck-in occurred during piston coring. The sediments in these intervals have stable and intense nearly vertical magnetizations, probably acquired within the core barrel or drill string. These zones are relatively rare, however, and do not account for many of the observed problems. Discrepancies between the paleomagnetic directions obtained from discrete samples and long-core measurements indicate that major intensity changes are a likely cause of some of the anomalous paleomagnetic directions in the long-core data. Our experiments therefore verify that large-scale remanence intensity changes reduce the usefulness of the pass-throug