A comparison of polymineral and K-feldspar post-infrared infrared stimulated luminescence ages of loess from Franconia, southern Germany

Christine Thiel,Neda Rahimzadeh,Manfred Frechen,Birgit Terhorst,Tobias Sprafke

doi:10.5194/egqsj-70-53-2021

Abstract

Abstract. Loess-paleosol sequences (LPSs) are essential records for reconstructing Quaternary paleoenvironments. No previous study has provided numerical chronologies of loess in Lower Franconia, southern Germany; their chronostratigraphic assumptions have relied mainly on German (pedo)stratigraphic schemes. In this study, we provide for the first time a chronology for LPSs in Lower Franconia based on optically stimulated luminescence (OSL) dating using quartz and a comparison of K-feldspar (63–100 µm) and the polymineral fraction (4–11 µm). Our results show that all obtained ages are in stratigraphic order, ranging from Holocene to late Pleistocene, and in general confirm the former stratigraphical interpretations. A good agreement of the obtained ages is observed between both feldspar grain size fractions; they also agree well with the quartz OSL ages up to ∼50 ka. However, a marked difference between the growth pattern of the dose response curves and consequently different saturation characteristics of fine and coarse grains is found. Even though in our samples the discrepancy in ages is not very significant, we suggest the use of coarse-grained K-feldspar whenever possible in order to not be confronted with unknowns such as the mineral composition of the polymineral fraction.

Highlights

Loess-paleosol sequences (LPSs) are complex terrestrial archives of Quaternary landscape evolution that are widely distributed in the temperate zone, allowing for regionalization of past climatic changes and providing information of paleo-geoecological responses to paleoclimatic shifts (Sprafke, 2016)
The non-fading-corrected final pIRIR225 De values determined for CG K-feldspar samples range between 59 ± 1 and 357 ± 10 Gy and 111 ± 1 and 379 ± 11 Gy, with corresponding ages ranging from 15 ± 1 to 100 ± 5 ka and 29 ± 1 to 89 ± 4 ka, for KT and HKH, respectively
The observed difference in fading rate may originate from the Na-feldspar grains in the mineralogical composition of the FG extracts which tend to have lower fading rates (Huntley et al, 2007; Huot and Lamothe, 2012); it has to be noted that none of the associated papers report the detection window that was used in their study

Summary

Introduction

Loess-paleosol sequences (LPSs) are complex terrestrial archives of Quaternary landscape evolution that are widely distributed in the temperate zone, allowing for regionalization of past climatic changes and providing information of paleo-geoecological responses to paleoclimatic shifts (Sprafke, 2016). One to three early glacial (EG; 115– 72 ka) humic horizons (Mosbach Humuszones, MHZs) are superimposed by lower Pleniglacial (LPG; 60–72 ka) colluvial deposits (Niedereschbach Zone, NEZ) and loess. The upper Pleniglacial (UPG; 32– 15 ka), sometimes characterized by reworked horizons at the base, contains the thickest loess deposits with intercalated tundra gley soils (Erbenheim soils, En), in which the late glacial (LG; 15–11 ka) to Holocene (< 11 ka) pedogenesis took place

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