Anisotropy of Magnetic Susceptibility (AMS) of Sediments From Holes U1480E and U1480H, IODP Expedition 362: Sedimentary or Artificial Origin and Implications for Paleomagnetic Studies

Abstract

Recognition of coring-induced disturbance, which is essential for magnetic fabric and paleomagnetic studies of poorly lithified sediments, is generally not straightforward. Here, we report on anisotropy of magnetic susceptibility (AMS) and paleomagnetic data of the sediments from Holes U1480E and U1480H, IODP Expedition 362, west of the Sumatra subduction zone. AMS is characterized by steep minimum principal axes (Kmin) in undisturbed sediments. However, a considerable portion of the recovered sediments are affected by significant coring-induced disturbance. In these cases, we observed three AMS patterns: (1) AMS principal axes are randomly distributed for sediments with mingling and distortion of beds, (2) Kmin axes of sediments with upward-arching beds are deflected out of the splitting face of the working half, and (3) suck-in sediments are characterized by vertical Kmax axes. These deformation-dependent AMS patterns can be attributed to the realignment of mineral particles caused by the coring process and subsequent sampling procedures. Besides a low-coercivity, vertical, drilling-induced overprint, we observed a high-coercivity component that is likely a composite of the primary magnetization with a demagnetization-resistant portion of the drilling overprint. After accounting for the disturbed intervals, several polarity transitions can be identified in the undisturbed sediments which correlate well with the Pleistocene geomagnetic polarity timescale. These observations demonstrate that great caution is required when attributing geological significance to AMS and paleomagnetic data obtained from soft sediment cores, which are highly susceptible to coring-induced disturbance. In addition, AMS measurements provide a potential tool for identifying core deformation for further paleomagnetic studies.