During Leg 202, we found that one significant source of magnetic overprints in ODP cores involves the time a core spends in contact (through the liner) with the steel (magnetic) core barrel. This is not the only source of overprints, but it is one that can be mitigated. In a series of experiments we have found that the use of a nonmagnetic core barrel, cutting shoe, and flapper valve is helpful in reducing magnetic overprints in marine sediments, especially in continental margin lithologies with a substantial silty siliciclastic component. Most importantly, we found that VRM acquired from a steel core barrel may be difficult or impossible to remove with stepwise AF demagnetization, whereas the weaker VRM acquired when the nonmagnetic core barrel was used was more easily removed at sea. Given the success of the experiments during Leg 202, we recommend the use of nonmagnetic core barrels for all future ODP legs that include paleomagnetic objectives. The nonmagnetic core barrel may reduce the magnetic overprints even at sites containing pelagic carbonate-rich sediment and will enhance the prospects of paleomagnetic success in many sediment types.
Although more expensive than steel core barrels, the use of nonmagnetic core barrels will overall prove its worth in terms of better sampled and better understood records of magnetic field variability. Considering the cost of labor that over the years has gone into the often futile efforts to remove magnetic overprints from ODP cores, and the even larger cost of lost scientific opportunities caused by magnetic overprints, the incremental cost of purchasing and maintaining nonmagnetic drilling tools seems to us like money well spent.