2.2. Sampling of Section No. 1
The upper part of Section No. 1, from 0 cm to ~12 cm (Fig. 2), which is called Bed No. 1 (Kadlec et al. 2014), is composed of fine backwater sediment of brown clayey silt with white angular clasts of weathered limestone and bone fragments (Bed No. 1) (Kadlec et al., 2014). The lower part of the section, from ~12 to ~35 cm, consists of brown silty clay without white clasts (Bed No. 2) (Kadlec et al., 2014). Although our “Section No. 2” is the same as “Profile 2” of Kadlec et al. (2005) and “Section No. 2” of Kadlec et al. (2014), our “Section No. 1” is not the same as “Profile 1” of Kadlec et al. (2005) but it is the same as “Section No. 1” of Kadlec et al. (2014). The depths in the present study are not the same as those in Kadlec et al. (2005, 2014).
Our sample collection involved the larger sediment part below the transition and the smaller part above the transition according to the position of the reversed polarity in Kadlec et al. (2014). We chose three sets of overlapping boxes (Fig. 2) to completely characterize the transition. The 35.1 cm of exposed sediment at the base of Section No. 1 was flattened to a clean vertical face, and the samples for the paleomagnetism measurements were taken by pushing the plastic boxes (2x2x2 cm; 8 cc) into the sediment (Fig. 2). We used a Brunton geological compass to measure the azimuth and tilt of the boxes. In addition, another 4 samples were collected for rock magnetism measurements (Fig. 2c), which corresponded with the paleomagnetic samples (13_0P, 7_7P, 17_2M, 22_0M).
We examined the sediment structure near the walls of the plastic sediment holder and observed that the process of pushing the box into the sediment created deformation structures along the walls of the boxes. The structure was on the order of 0.05 mm thick. With a box volume of 8000 mm3 (20x20x20 mm) and a structurally modified layer volume of 80 mm3 (4x 0.05x20x20 mm), the volume modified by pushing is 1/100 of a fraction of the unmodified volume. Even if this moment would be organized in (e.g., perpendicular direction), it would only deflect the magnetic remanence by <5%.