LOESSFEST'09 | Aug. 31st – Sept. 3rd, 2009 |Novi Sad-Serbia
A Multiple Magnetic Dating Approach to the Upper Palaeolithic Loess Site of Krems-Wachtberg, Lower Austria
Hambach, U.1
1Chair of Geomorphology, University of Bayreuth, Germany
Here, I report on the results of a detailed environmental and palaeo-magnetic study of a loess site from 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, findings show an extraordinarily good preservation due to embedding in rapidly sedimented loess (Händel et al. 2008).
The up to 9 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. 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 the layer, up to 2.5 m of loess resting on Lower Pleistocene fluvial gravels is preserved. Thus, the loess section represents a palaeoclimatic record of alternating cold-dry and warm-humid conditions on millennial scale.
In summer 2005 and 2006, an almost 8 m thick loess section was continuously sampled in two overlapping sections 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 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.
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 32 – 34 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 (≤ 30°) 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. 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.
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.
Unlike the situation in western Central Europe, where interstadial palaeosols interrupt the loess sequences, the loess at the Krems-Wachtberg site yields a quasicontinuous sedimentary archive of the upper Middle to the Late Würmian. Our magnetic dating approach revealed three independent age estimates for the archaeological horizon:
The general correlation of the environmental magnetic signal to independently dated palaeoclimatic records suggests the dating of the loess at the excavation site to a time interval between 20 to 40 ka. The Gravettian living floor is assigned to the base of GI 5 and thus to an age of 32 to 33 ka.
The directional signal of the past Earth’s magnetic field reveals just above the archaeological layer the record of the MLE which is dated to 32-34 ka. Thus, the age of the Gravettian living floor is only slightly higher (< 1 ka).
The investigation of the intensity of the palaeomagnetic signal results in a reliable and comparable RPI record with a characteristic pattern correlable to the GLOPIS in the time interval from approximately 20 to 40 ka. The loess containing the archaeological horizon as well as the loess below and above reveal a minimum in RPI which is correlated to the intensity minimum assigned to the MLE which in turn centres in North Atlantic marine sediments on Greenland interstadial 6 (33- 34 ka in North-GRIP time scale).
The environmental magnetic and the RPI record allow for the construction of independent age models. Both models see the onset of sedimentation just after 40 ka and the cessation around 20 ka. Sedimentation rates calculated on the basis of these models range from 0.2 to 0.7 m/ka with average values of 0.41 m/ka for the age model based on the environmental magnetic record and of 0.34 m/ka for the age model derived from the RPI record.