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).