4.2. Sediment Deposition, Sedimentation Rate, and Transition Duration
The M/B event occurred during the interglacial period (MIS 19) following the glacial period (MIS 20) (Cohen and Gibbard, 2019). In the case of cave sediments, we see coarser grains at a greater depth (below 12 cm depth; Bed No. 2 in Fig. 2c; Matuyama section) and finer grains at a shallower depth. Since cave sedimentation took place at the time of glaciation, the cave itself was not completely frozen, which means that seasonal variation induced a thaw-freeze cycle that typically generates physical weathering and a source of coarser sediment. This phenomenon is observed in our sediment. The change from glacial to interglacial is supported by finer grain size sediment due to the lesser influence of the thaw-freeze cycle. Therefore, our observations support frozen surfaces and later warming with smaller sediment availability for sedimentation, supporting the transition from glacial to interglacial periods.
According to Lundberg et al. (2014), the cave was filled with water during sedimentation, which was continuously active without any significant color change or hiatus with the exception of a slight change toward finer grains. This provides the continuous magnetic record of the reversal in the cave sediment and allows the sediment to acquire and keep the primary magnetization without the possibility of secondary mineralization. Furthermore, there are no obvious signs of breaks in deposition (e.g., lithological boundaries or desiccation cracks) in the studied sediment section, other than a slight change in grain size.
Our sedimentation rate estimation (0.7±0.2 cm/kyr) seems to be similar to the sediments from other European cave studies (Table 1). While the duration of the M/B transition was reported to last between 4 and 13 kyr (Suganuma et al., 2010; Valet et al., 2014; Okada et al., 2017), the average sedimentation rate of 0.7±0.2 cm/kyr in this study suggests a transition duration of 8.1±0.2 kyr (7.1-12.8 cm transition section) and thus supports the reliability of our sedimentation rate and paleomagnetic record estimates. King and Channell (1991) suggested that large lock-in-depths are associated with interparticle rigidity and strength, characteristic of clayey low accumulation rate sediments (<1 cm/kyr), which results in delays of magnetic acquisition. This shows that magnetic polarity reversal could have a large (25 kyr) apparent age offset between sediments with high and very low accumulation rates (King and Channell, 1991).