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| Titel |
The Mono Lake geomagnetic excursion recorded in loess: Its application as time marker and implications for its geomagnetic nature |
| VerfasserIn |
U. Hambach, M. Hark, C. Zeeden, B. Reddersen, L. Zöller, M. Fuchs |
| 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 |
250029362
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| Zusammenfassung |
| One of the youngest and worldwide documented geomagnetic excursions in the Brunhes
Chron is the Mono Lake excursion (MLE). It has been detected in marine and terrestrial
sedimentary archives as well as in lavas. Recent age determinations and age estimates
for the MLE centre around an age interval of approximately 31 – 34 ka. Likewise
the Laschamp excursion the MLE goes along with a distinct peak in cosmogenic
radionuclides in ice cores and sedimentary archives. It provides therefore an additional
geomagnetic time marker for various geoarchives to synchronise different climate
archives.
Here we report on a detailed record of the MLE from a loess site at Krems, Lower
Austria. The site is situated on the southern slope of the Wachtberg hill in the vicinity of the
old city centre of Krems. The archive comprises Middle to Upper Würmian (Late
Pleistocene) loess in which an Upper Palaeolithic (Early Gravettian) cultural layer is
embedded. The most spectacular finds are a double infant burial found in 2005 and a single
burial discovered in 2006 (Einwögerer et al., 2006). Generally, archaeological findings show
an extraordinarily good preservation due to embedding in rapidly sedimented loess (Händel
et al., 2008).
The about 10 m thick loess pile consists of calcareous sandy, coarse silt which is rich in
mica indicating local sources. It is well stratified with brownish horizons representing
embryonic soils pointing to incipient pedogenesis. Some of the pedo-horizons show
occasionally indications of minor erosion and bedding-parallel sediment transport, but no
linear erosional features. Pale greyish horizons are the result of partial gleying under
permafrost conditions. No strong pedogenesis including decalcification and clay formation is
present. The cultural layer is still covered by more than 5 m of loess, and dated by
radiocarbon to ~27 ka 14C BP (Einwögerer et al., 2006). Below this layer up to 2.5 m of
loess resting on Lower Pleistocene fluvial gravels are preserved. Thus, the loess
section represents a palaeoclimatic record of alternating cold-dry and warm-humid
conditions on millennial scale. Optical stimulated luminescence dating of aeolian
loess around the cultural layer reveals ages of 30 to 32 ka which is supported by
thermoluminesence dating of burnt loess from a hearth belonging to the archaeological living
floor.
In summer 2005 and 2006, two overlapping sections were continuously sampled in for
palaeomagnetic investigations. The sampled sections are located outside the centre of the
main archaeological occupation in the northwestern corner of the excavation pit. Sample
spacing is strictly 2.1 cm, measured from centre to centre of the specimens. In total,
432 individually oriented specimens were recovered from the almost 8 m thick
section.
Magnetic susceptibility (MS) as function of depth resembles generally the lithology. Low
MS-values represent pure unaltered or weakly gleyed loess, whereas higher values represent
the enhancement of magnetic minerals caused by incipient soil formation. Anhysteretic
remanent magnetisation (ARM) versus MS reveals an enhancement of super-paramagnetic
particles where MS is increased. Consequently, the rock magnetic variations with depth can
be taken as a palaeoclimatic record representing the climatic variations between drier and
slightly more humid conditions at the transition from Middle to Upper Pleniglacial. Based
on the ARM/MS record a correlation of the geoarchive at the Krems-Wachtberg
site with the NORTH-GRIP isotopic record (NGRIP Members, 2004) and with
sedimentological data from Maar-lake sediments of the Eifel area (ELSA; Schaber and
Sirocko, 2005), Germany can be established. The general correlation suggests the dating
of the loess at the excavation site to a time interval between approx. 20 to 40 ka,
covering Greenland interstadials (GI) 2 to 8 and Heinrich Events 3 and 4 (top). The
Gravettian living floor is assigned to the base of GI 5 and thus to an age of 32 to 33
ka.
The directional palaeomagnetic record is of high quality and shows variations in the
bandwidth of secular variation in the upper and in the lower part of the section, whereas in the
central part shallow (-¤ 30Ë ) and oversteep inclinations reveal the record of a geomagnetic
excursion just above the find horizon. The shallow inclinations are preceded by and
go along with westerly declinations, whereas the steep inclinations are preceded
by easterly declinations. This directional pattern is similar to what was found at
the Mono Lake in California (e.g. Liddicoat and Coe, 1979; Lund et al., 1988). A
relative palaeointensity (RPI) record was constructed by using MS and ARM as
normalisers. This record corresponds quite well to the GLOPIS (Laj et al., 2004) and
thus provides additional dating. The peak of the directional excursion coincides
with a relative minimum of RPI. The average RPI during the excursional interval,
however, is significantly higher than during normal periods, contrary to what is
usually reported. Furthermore, just before and after the directional excursion the
highest values of RPI occur. The largest amplitude of the directional excursion
does not correspond to the well defined minimum in RPI preceding this interval
which is usually taken for the MLE in the marine RPI records. This offset between
the RPI and the directional record may indicate the presence of strong non-dipole
components and may also explain the blur in dating of the MLE. The calculated VGPs
of the directional excursion lie over North America but do not correspond to the
looping behaviour as reported from the Mono Lake VGPs itself (Liddicoat and Coe,
1979).
The cultural layer at the Krems-Wachtberg site is located in the centre of the RPI
minimum which is slightly older than the peak of the directional excursion. The radiocarbon
ages from the cultural layer (~27 ka 14C age BP = ~32 ka calendric age calBP) fit well to
the age estimates of the MLE at the Mono Lake based on radiocarbon dating and
tephrochronology (31.5 – 33.3 ka; Benson et al., 2003). Furthermore, the recently published
40Ar/39Ar ages of one excursional group (Auckland cluster 1: 31.6 ± 1.8 ka) from the
Auckland volcanic field, New Zealand correspond to the ages discussed above. Thus, the
MLE is a perfect time marker occurring globally but is probably dominated by strong
non-dipole components.
Benson et al. (2003). Quaternary Science Reviews, 22,135–140; Cassata et al.
(2008). Earth and Planetary Science Letters, 268, 76–88; Einwögerer et al. (2006).
Nature, 444, 285; Händel et al. (in press). Quaternary International; Laj et al. (2004).
Geophysical Monograph Series, 145, 255-265; Liddicoat and Coe (1979). Journal of
Geophysical Research, 84, 261–271; Lund et al. (1988). Geophysical Research
Letters,15,10, 1101-1104; North Greenland Ice Core Project Members (2004). Nature
431, 147-151; Schaber and Sirocko (2005). Mainzer geowiss. Mitt., 33, 295-340. |
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