Chronostratigraphy of a loess-palaeosol sequence in Biafy Kosciol, Poland using OSL and radiocarbon dating

摘要

Optically stimulated luminescence (OSL) is particularly suitable for providing a numerical timescale necessary for loess to be useful in climatic reconstructions. It is widely accepted that quartz OSL dating using the single-aliquot regenerative dose (SAR) procedure usually yields reliable ages. The current work presents OSL ages derived from coarse (90-125 mu m) and medium (45-63 mu m) quartz grain size fractions, and post-IR IR290 for polymineral fine grains (4-11 mu m). In addition to dating, analyses of the magnetic susceptibility, grain-size distribution, carbonate and organic carbon content were conducted. Previously the profile was dated using single aliquot optically stimulated luminescence (OSL) applied to coarse grained quartz fraction (90-125 mu m) only, using the central age model. This analysis yielded unexpectedly young ages for the deepest part of the Bialy Kosciol loess profile. In the current study, we investigate the differences between the previously obtained results and new results obtained for medium grained quartz and polymineral fine grain fractions. Ages obtained for coarse and medium fractions show no differences up to the equivalent dose of about 150 Gy. In contrast to the younger samples, for the five oldest samples we observe that the results obtained for coarse grained quartz yield ages lower that the medium grained quartz. Those differences reach 25 for the two oldest samples. In addition, the polymineral fine grain fraction yields results very similar to the previously obtained coarse grained quartz ages with the exception of the two oldest samples where the polymineral fraction gives much older results. Those differences between the ages of the two grain size fractions can arise from post-depositional (grain size dependent) sediment translocation processes due to pedo- and bio-turbation and a different efficiency of bleaching of the post-IR signal. Those results, obtained for lithostratigraphic units II and I correspond to MOIS 4 and 5, respectively, confirming the earlier established MOIS chronology and strengthen the chronological correlation of the loess profile at Bialy Kosciol with other loess profiles in Europe.