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

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

Laboratory and field investigations on freeze and gravity core sampling and assessment of coring disturbances with implications on gas bubble characterization†

The quantification of greenhouse gas emissions from aquatic ecosystems requires knowledge about the spatial and temporal dynamics of free gas in sediments. Freezing the sediment in situ offers a promising method for obtaining gas‐bearing sediment samples, unaffected by changes in hydrostatic pressure and sample temperature during core withdrawal and subsequent analysis. This article presents a novel freeze coring technique to preserve the in situ stratigraphy and gas bubble characteristics. Nondestructive X‐ray computed tomography (CT) scans were used to identify and characterize coring disturbances of gravity and freeze cores associated with gassy sediment, as well as the effect of the freezing process on the gas bubble characteristics. Real‐time X‐ray CT scans were conducted to visualize the progression of the freezing process. Additional experiments were conducted to determine the freezing rate to assess the probability of sediment particle/bubble migration, and gas bubble nucleation at the phase transition of pore water to ice. The performance of the freeze coring technique was evaluated under field conditions in Olsberg and Urft Reservoir (Germany). The results demonstrate the capability of the freeze coring technique for the preservation of gas‐bearing sediments and the analysis of gas bubble distribution pattern in both reservoirs. Nevertheless, the obtained cores showed that nearly all gravity and freeze cores show some degree of coring disturbances