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.

Absolute Paleolatitude of Northern Zealandia From the Middle Eocene to the Early Miocene

“…These values indicate a particularly low degree of compaction. This is consistent with an absence of AMS fabric, as the anisotropy degree is particularly low, with an average value of 1.005 (Dallanave & Chang, 2020), also indicating the absence of drilling disturbance (Acton et al, 2002;Yang et al, 2019).…”

Greigite Formation Modulated by Turbidites and Bioturbation in Deep‐Sea Sediments Offshore Sumatra

“…Some samples have a (sub)vertical, downward‐oriented NRM component, which is easily removed by thermal demagnetization to 100°C–150°C (Figures 8b and 8c) or AF demagnetization to 5–10 mT (Figures 8e–8g and 8i). Such a component is common in drill cores and is considered a drilling‐induced overprint (McNeill, Dugan, Petronotis, et al., 2017; Yang et al., 2019). Thermal decay curves of the NRM usually unblock gradually with temperature, and most samples are fully demagnetized at ∼350°C (Figures 8a–8d), which further confirms the presence of greigite, as expected from thermomagnetic curves (Figure 6 and Figure S1 in Supporting Information S1).…”

Persistent Influence of Non‐Dipole Geomagnetic Field on East Asia Over the Past 4,000 Years

“…The minimum susceptibility axes ( k min ) of most samples are nearly perpendicular to the horizontal plane (with a range of 70° and 90°), and the inclination of intermediate and maximum susceptibility axes ( k int and k max ) tends to be horizontal, suggesting a typical normal depositional fabric (Hrouda & Kahan, 1991). The shallow k min (<65°) of some sporadic samples may result from mechanical disturbances during sampling (T. Yang et al., 2019). Hence, these samples with shallow k min were not used to reconstruct the paleomagnetic records.…”