Luminescence dating and 3D modelling of a Mid Pleistocene inner-alpine alluvial-talus succession

Sedimentary archives from inner-Alpine settings predating the Last Glacial Maximum (LGM) are rare due to the strong erosional impact of LGM glaciers on these landscapes. In the Eastern Alps, a key site for reconstructing pre-LGM inner-Alpine environmental conditions is located near Innsbruck (Austria): the so-called Höttinger Breccia. This >1300m -thick sedimentary succession consists of basal lodgement till overlain by a thick sequence of alluvial to talus-slope deposits with intercalated aeolian and lacustrine sediments, capped by LGM moraine. Over a century ago, this succession also provided some of the first evidence for the multiple, cyclical nature of the ice ages.

Despite the significance of this site for reconstructing pre-LGM inner-Alpine environmental conditions, chronological constraints on these sediments remain limited. Previous optically stimulated luminescence (OSL) dating of quartz provided only minimum age estimates for the Höttinger Breccia because the quartz signals approached saturation. To overcome this limitation, this study applies infrared-stimulated luminescence (IRSL) dating to feldspar, as the IRSL signal saturates at much higher doses than the quartz OSL signal, thereby enabling dating further back in time. We exploit a range of post-IR IRSL (pIRIR) feldspar signals at stimulation temperatures from 110°C to 290°C to systematically investigate feldspar-related issues of anomalous fading and partial bleaching. We (i) present an optimized dating protocol that identifies the IRSL signal providing the best trade-off between signal stability and bleachability; (ii) report preliminary age estimates obtained from samples taken at various stratigraphic depths within the Höttinger Breccia succession; and (iii) integrate these age estimates into a 3D model of the sedimentary succession.

This combined chronological and spatial framework provides a basis for reconstructing Mid Pleistocene paleoenvironmental conditions in an inner-Alpine valley that otherwise preserves almost no geomorphic record of pre-LGM paleoclimate variability.