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Titel |
The Matuyama-Brunhes geomagnetic reversal in the Stari Slankamen loess section (Vojvodina, Serbia): Its detailed record and its stratigraphic position |
VerfasserIn |
U. Hambach, M. Jovanović, S. B. Marković, N. Nowaczyk, C. Rolf |
Konferenz |
EGU General Assembly 2009
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 11 (2009) |
Datensatznummer |
250029407
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Zusammenfassung |
The Matuyama-Brunhes geomagnetic reversal is the most important time marker in
Quaternary sedimentary sequences. It provides unambiguous correlation between marine,
lacustrine, loess and even fluvial deposits because its recognition does ideally not depend
directly on the sediment properties but only on their recording capability. The recognition of
the Matuyama-Brunhes boundary (MBB) in the Chinese Loess Plateau (CLP) by Friedrich
Heller and Tung-Sheng Liu (HELLER & LIU 1982) started the success story of magnetic
polarity dating in continental Quaternary sequences. They assigned the Chinese loess
to the Quaternary time table and correlated it with the global ice volume record
using the magnetic susceptibility signal as palaeoclimate proxy (HELLER & LIU
1984).
At the western edge of the Eurasian loess belt, in the southern Carpathian basin in the
region called Vojvodina we find several decametres thick loess deposits dating back to the
Middle Pleistocene at least (Marković et al., 2008). Like in the CLP in the Vojvodina one can
find true loess plateaus reaching thicknesses of more than 50 m. The exposure at Stari
Slankamen is located in the north-eastern part of Srem Loess Plateau, on the right bank of the
Danube River. The loess sequence is comprised of multiple couplets of loess and
palaeosol units totaling approximately 40 m thickness of sediments. 8 palaeosol units
can be distinguished separated by several metres thick loess layers. The lower 5
palaeosol units are developed as strongly rubified forest soils with decreasing degree of
pedogenesis from old to young. In contrary, the younger palaeosol units including the
recent soil are developed as steppe soils. This strongly suggests that a considerably
longer palaeoclimatic record is preserved at the site than is found elsewhere in
Europe.
Magnetic susceptibility (MS) of the whole section was measured in the field as well as in
the laboratory at 10 cm intervals in the palaeosol units and at 15 cm intervals in the loess. In
2005, high resolution sampling was carried out covering almost 11 m from the lowest part of
the profile and involving palaeosol units V(ojvodina)S8 to VS6. A total of 434 samples were
collected from 2 parallel columns using oriented tubes with sampling step every 5 cm.
Measurements of standard rock-magnetic parameters were performed and detailed
palaeomagnetic analyses including thermal and AF demagnetization on parallel samples were
carried out.
The cyclicity of alternating high and low MS values with palaeosols and loess units
respectively reflects magnetic enhancement via varying degrees of pedogenesis, similar to
that observed in Chinese and Central Asian loess deposits and corresponding perfectly to
recently published records from the same region (Marković et al., 2008). MS values in
interglacials decrease through time. This supports the previous assertion, based on
palaeopedological observations by Bronger (2003), that climate over the Pleistocene has
become progressively more arid in the region. A stratigraphic correlation based only on the
MS pattern reveals an unambiguous chronology assigning palaeosol units down to VS5 to
marine isotope stages (MIS) 1 to 15. For the lower units, however, such a correlation neither
to the Chinese loess nor to loess sections in south-eastern Europe is possible (e.g. Sartori et
al., 1999; Jordanova et al., 2007).
Palaeomagnetic analyses of the loess and palaeosol units VS6 to VS8 reveal a
pattern of reversed, normal and mixed polarity. About 1 m of normal polarity at the
section base is followed by 3.5 m of reversed polarity which in turn is overlain
by about 1.5 m of mixed polarity. Above this interval exclusively normal polarity
was found. Within the reversed interval a short (3 - 3.5 m) normal interval occurs.
The normal interval at the section base belongs to the palaeosol unit VS8 which
is in total about 4 m thick. The mixed polarity interval as well as the transition
to normal polarity occurs inside the loess unit VL8 just below the palaeosol unit
VS7 which consist of two individual soils separated by a 0.2 m thin loess layer.
Root channels stemming from this double soil penetrating several metres down into
VL8.
Taking into account the palaeosol-loess stratigraphy, the course of MS variations with
depth and the polarity pattern we interpret the double soil VS7 as the equivalent of MIS
19 and 21 corresponding to S7 and S8 in the Chinese loess stratigraphy. Strong
pedogenesis and biological activity (root channels) transferred the MBB to the
loess below VS7 which corresponds to L9 in China. Consequently, the normally
magnetised VS8 complex at the base of the section is the amalgamated equivalent of S10
and S11 in China which span the Jaramillo subchron (Sun et al. 2006). The loess
sites at Stari Slankamen reveal probably the oldest loess of the region found so far
and open the perspective for palaeoclimatc reconstructions going back 1 Ma at
least.
Bronger (2003). Quaternary International, 106-107, 11-31; Heller and Liu (1982). Nature,
300, 431-433; Heller and Liu (1984). Geophys. J. Royal Astron. Soc., 77, 125-141: Jordanova
et al. (2007). Geophysical Journal International, 171, 1036-1047; Marković et al. (2008).
Quaternary International, doi:10.1016/j.quaint.2008.12.004; Sartori et al. (1999). Physics of
the Earth and Planet. Interiors, 116, 53-64; Sun et al. (2006). Quaternary Science Reviews,
25, 33-48. |
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