Ocean Drilling Program, United States

During Leg 202, we observed and tested the effects of drilling on the magnetic remanence of sediments in a variety of lithologies. We found substantial magnetic overprints superimposed on the sediment natural remanent magnetization because of the use of steel core barrels and the time the sediment spends within them. The use of the advanced piston corer temperature (APCT) tool, which extends the duration of the coring process by 15 min for each core, induced a substantially larger overprint as compared to cores not using this tool. Laboratory experiments indicated that this large overprint is not APCT tool specific, but rather due to the extra time the sediment rests in the core barrel. Alternating use of a nonmagnetic core barrel on even-numbered cores (e.g., on Cores 2H, 4H, 6H, etc.) with a normal magnetized (steel) core barrel on odd-numbered cores (e.g., on Cores 1H, 3H, 5H, 7H, etc.) within a hole, at three sites, demonstrated a reduced overprint with the nonmagnetic core barrel. The improvement of the magnetic record due to the use of the nonmagnetic core barrel was most dramatic in relatively coarse siliciclastic sediments from the continental margin of Chile. The effect, though less dramatic, was still noticeable in fine-grained, open-ocean pelagic carbonate sediments. Based on these experiments and observations, we recommend that nonmagnetic core barrels, cutting shoes, and associated coring equipment that is in close contact with the sediment be made available for full-time use in coring with the advanced piston corer where a paleomagnetic record is of interest.