4. Discussion
Our data indicate that Matuyama Brunhes transition boundary constitutes
5.7 cm, between 7.1-12.8 cm depth of the sampled sedimentary section, of
the Za Hajovnou cave sediment. Magnetic reversal is characterized by
frequent fluctuations of inclination angle (Figure 6a) and VGP latitude
(Figure 7). Maximum angular deviation (MAD) values of this study are
within the error limit as seen in Figure 5. Similarities in comparisons
of this data set with other studies indicate that Za Hajovnou cave
sediment dates to Matuyama-Brunhes magnetic transition.
Because of low coercivity in most of the samples with demagnetization
generally at 20 mT, some large fluctuations in the data may be
considered as unstability of remanent magnetization. This shows that
minerals with low coercivity is responsible for the magnetization of the
cave sediments in our study. On the other hand, similar fluctuations
which are seen in previous studies (Figure 6) show reliability of the
data.
Although the data in this study and Okada et al. 2017 belong to
geographically different locations and sediment types, this similarity
during polar migration shows that the reversal was a dipole transition,
and non-dipole field component was less significant (Simon et al. 2019;
Mochizuki et al. 2011; Oda et al., 2000).
Our estimate of the Za Hajovnou cave’s sedimentation rate seems to be
significantly lower than the sediments from other studies (Jin and Liu.,
2011; Okada et al., 2017; Sagnotti et al., 2010; Sagnotti et al., 2014;
Giaccio et al., 2013; Suganuma et al., 2010; Bella et al., 2019; Liu et
al., 2016). This is likely due to contrasting sediment types.
Analyzes of cave sediments by paleomagnetism carried out in different
locations around the earth such as in Western Europe (Pares et al.,
2018), South Africa (Nami et al., 2016), South America (Jaqueto et al.,
2016), North America (Stock et al., 2005), Southern Europe (Pruner et
al., 2010), Eastern Asia (Morinaga et al., 1992) showed that cave
sediments recorded magnetic reversals. Morinaga et al. (1992) suggested
low sedimentation rate for the Western Japan cave sediments 1.6 cm/kyr
which shows similar rate with our Central European cave sediment
estimation. King and Channel (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 Channel, 1991).
Glass and Heezen (1967) claimed that a meteorite impact could result in
a magnetic reversal. Large meteorite impacts may provide the sufficient
moment to the exterior of the outer core to cause motion relative to the
outer core. In this way, the angular difference disrupts the convection
and the position of the magnetic poles. Changing in the convection
pattern would affect the electric currents in the liquid core to have a
new core convection with modified Coriolis forces. In addition the inner
core’s angular momentum change due to the new core convection dynamics,
in respect to the mantle, would change the heat transfer in the inner
core and have an effet on dynamo reset. This process may cause
variations in the currents producing the earth’s magnetic field and
cause a geomagnetic reversal. In our study, the inclination and VGP
change which have an anomaly about 12 cm depth just before the reversal
(Figure 6, 7) could be a reason of a meteorite impact which was shown in
a study in Indonesia from marine sediments (Hyodo et al., 2011) by
micro-tektite level formed due to cosmic impacts. This can be supported
with oscillations in VGP’s latitude approximately at the same depth (1.5
m) below the reversal in the study of Yamazaki and Oda (2001) in South
Atlantic. Both of the studies (Hyodo et al., 2001; Yamazaki and Oda,
2001) have very similar sedimentation rates between 8-10 cm/kyr. The
same anomaly can be seen in sediments from other studies (Sagnotti et
al., 2014; Giaccio et al., 2013; Valet et al., 2014; Jin and Liu, 2011;
Liu et al., 2016; Okada et al., 2017) (Figure 6). Migration of the pole
during Matuyama polarity is towards to South America (Figure 8) which
may be caused by the angular momentum and convection pattern change
because of the northward directed meteorite impact in Asia (Sieh et al.,
2020).