Paleomagnetic overprints in ocean sediment cores and their relationship to shear deformation caused by piston coring

Abstract

We use paleomagnetic data from cores collected at Ocean Drilling Program Site 1062 on the Bahama Outer Ridge, as well as observations from prior cruises, to quantify the nature and to assess the causes of drilling overprints. The largest overprint arises from an isothermal remanent magnetization that is imparted to the core during coring or core retrieval. A second type of overprint is apparent only in soft sediments collected with a piston corer and has coercivities and unblocking temperatures that completely overlap those of the primary remanent magnetization, which means it cannot be removed by standard demagnetization methods. This demagnetization‐resistant overprint can produce significant biases in paleomagnetic observations, particularly those from split core sections. We hypothesize that shearing, which occurs as a result of friction on the inside wall of the piston corer as it rips through sediments and cuts a core, deflects the ancient magnetization. We develop and test a model that predicts how shear deformation can rotate remanence‐carrying grains and deflect the paleomagnetic remanence of a split core section. Using the shear model predictions and directions measured on split core sections and U channel samples from the undeformed core center, we estimate the average deformation sustained and then correct the split core data for biases caused by shear deformation. The reduction in the deviations between corrected split core data and U channel data is statistically significant, indicating that the shear model is capable of accounting for a significant portion of the systematic biases observed in paleomagnetic observations obtained from piston cores.