Ambiguity of luminescence ages from periglacial loess in Poland – “As you like it” or honesty with users?

Last glacial loess in Poland was deposited under periglacial conditions nearby to the regional maximum advance of the Scandinavian Ice Sheet during the Weichselian glaciation. Its glacier advance and retreat stages are expected to be mirrored in Loess-Paleosol Sequences (LPS) calling for reliable and exact dating. In the Bayreuth luminescence laboratory (BT) established modern SAR protocols of optical dating (OSL, pIRSL) were applied to the 4-11µm fine grain fraction of 38 loess samples from four LPS, distributed from Silesia to the Volhynian upland. Results were compared to published results from the Gliwice luminescence laboratory. Pleasing basic agreement even for surprising results is overshadowed however by somewhat differing apparent ages complicating a sound palaeoclimatic interpretation. A comparison of OSL and pIRIR ages obtained in BT poses a similar problem. OSL ages from quartz are prone to dose-dependent age underestimates but the critical absorbed dose may be grainsize-dependent. Experiences in BT resulted in favoring the fine grains from quartz, allowing for higher credible ages. Nevertheless, our quartz ages > ca. 50 ka are possibly but not necessarily underestimated, obviously dependent on the site and the providence of the quartz grains. In contrast, pIRIR ages turn out to be prone to overestimates due to incomplete bleaching leaving an unknown residual dose at deposition, a serious problem for beds with strong periglacial reworking.

Loess stratigraphy in Poland is well-based mainly on lithostratigraphy, palaeopedology, and periglacial geomorphology. Apart from – often problematic – radiocarbon dating and contradictory previous TL and OSL ages there is a lack however of independent physical dating methods and also of tephrostratigraphic markers. But at least the age of a prominent interstadial soil (or soil complex) now labelled L1SS1 at the end of the Middle Pleniglacial can now be fixed between ca. 30 and ca. 40 ka. Interpretation of dating results from samples older than L1SS1 is challenging. The observed diverging ages (OSL, pIRIR) are critical for the accurate time bracketing of geomorphologic and pedostratigraphic features such as ice wedging, thermokarst erosion events and interstadial soil formations and for their attribution to marine isotope stages. Alternative interpretations are discussed including possible periglacial mirroring of pre-LGM regional ice advances (Ristinge and Klintholm advances) in the southwestern Baltic Sea area.

I suggest that the strange behavior of quartz ages from different grain sizes is caused by various sources (ranging from Paleozoic crystalline rocks to Quaternary glacial drift) with very different geological and thermal histories, stored due to the very high resistance of quartz against weathering. Local and remote sources contribute to individual sedimentary beds of LPSs to varying extents. Thus, fine grains and coarser grains from an individual sample may be derived from diverse sources. In spite of the addressed uncertainties, for honesty reasons it is so far recommended aiming at age bracketing as narrowly as possible, simultaneously using OSL from different quartz fractions and pIRIR from fine polymineral grains. A refinement of this approach remains challenging as far as the sole reliable dating protocol is not ensured.