The lower Salzach Valley (LSV) extends from the south, from the town of Golling, to the north, where it meets the city of Salzburg and terminates in the northern Alpine Foreland. This part of the Salzach Valley is characterized by a wider alluvial plain which is in stark contrast to the steep flanks composed of carbonate rocks from the northern Calcareous Alps south of Golling. The LSV is mainly cut into the northern Calcareous Alps in the south and the Rhenodanubian Flysch in the north, with a Quaternary infill containing lake deposits that are regionally described as Salzburger Seeton (SST). Prey (1959) described the upper part of this succession as coarsening upward sequences of silts and sands (not clay-dominated, as the German name would suggest), which can be identified throughout the LSV (Herbst and Riepler, 2006). Pomper et al. (2017) characterized the infill of the LSV as a succession of silts and sands, intercalated and overlain by coarser-grained sediments. Herbst and Riepler (2006) reported peat deposits at depths of between 13 and 20 m, intercalated into the SST. The focus of the investigation presented here is on the depositional ages of a 30 m long research percussion drill core (diameter: 10 cm) retrieved in 2013 from the orographic right bank of the Salzach River (47°44^′1.1^′′ N, 13°5^′1.5^′′ E; 435 m a.s.l.) in the direct vicinity of the Urstein hydroelectric power plant (Fig. 1), in close vicinity (∼ 50 m) to the drill cores investigated in Herbst and Riepler (2006).

Figure 1Overview map of the lower Salzach Valley (Copernicus DEM used as a basemap).

A detailed log of the investigated core was obtained to produce a sedimentological profile. Two dating techniques were applied to the core. Two plant debris samples were taken for radiocarbon dating at depths of 13.8–13.6 m (URS-C1) and 2.35 m (URS-C8) and were sent to BETA Analytic for dating. Multiple samples were taken for luminescence-dating purposes.

Luminescence dating

Single-grain luminescence measurements were carried out at the Vienna Laboratory for Luminescence dating (VLL) using a Risø TL/OSL DA-20 luminescence reader with a single-grain measurement attachment (calibrated ⁹⁰Sr $/$ ⁹⁰Y beta source (Bøtter-Jensen et al., 2000) delivering a dose rate of ∼ 0.09 Gy s⁻¹ and an infrared (IR) laser diode emitting at 830 nm) on six samples distributed across the entire depth of the drill core. Sample preparation followed the approach described in detail by Lüthgens et al. (2017). The well-proven pIRIR measurement protocol (Buylaert et al., 2009) adjusted for single-grain measurements (Firla et al., 2024) was used to detect two luminescence signals from potassium-rich feldspar. Before the detection of the luminescence signals used for age calculation, a preheat of 250 °C was applied. The first signal was detected at 50 °C (IRSL₅₀), and the second one was detected after the first at 225 °C (pIRIR₂₂₅). The dose response curves were fitted to a single saturating exponential function. Dose recovery tests revealed that the applied measurement protocol was able to best reproduce a given dose when using 20 % as a threshold for recycling, recuperation, and test dose error (average dose recovery ratio of 0.96 ± 0.02 for the IRSL₅₀ signal and 0.96 ± 0.03 for the pIRIR₂₂₅ signal). Stricter thresholds did not improve the dose recovery ratio but decreased the number of accepted grains. All measured equivalent dose distributions exhibited right-skewed distributions, common for poorly bleached glacially associated sediments. Therefore, the three-parameter Minimum Age Model (MAM-3) was applied (Galbraith et al., 1999). For the IRSL₅₀ distribution of VLL-0314-L, the low-outlier correction approach of Firla et al. (2024) was applied. The parameters used for age calculation are listed in Table 1. Luminescence signals from potassium-rich feldspar often exhibit athermal signal loss (anomalous fading). If not addressed, the ages based on these anomalous fading-affected signals would underestimate the age of deposition. We therefore conducted anomalous fading experiments (Auclair et al., 2003 – modified to include the pIRIR₂₂₅ signal). These revealed that the pIRIR₂₂₅ signal is not significantly affected by anomalous fading (average g-value: 0.2 ± 0.3). The IRSL₅₀ fading test measurements from this core were averaged with fading measurements from a core (ICDP-DOVE core 5068_4_A Neusillersdorf – Fiebig et al., 2014; Firla et al., 2024) west of the city of Salzburg (average g-value based on nine fading measurements: 2.1 ± 0.3). Ages based on the IRSL₅₀ signal were corrected for anomalous fading according to Huntley and Lamothe (2001), while the pIRIR₂₂₅ signal based ages were not corrected for fading. The activity of naturally occurring radionuclides (²³⁸U, ²³²Th, and ⁴⁰K) was measured using high-resolution, low-level gamma spectrometry (Baltic Scientific Instruments HP-Ge p-type detector with ∼ 40 % efficiency). No radioactive disequilibria were detected. For a detailed description of the applied measurement approach and age calculation, we refer to Firla et al. (2024).

Table 1Luminescence-dating dose rate, equivalent dose, and age results.

^a Overall effective dose rate caused by environmental and cosmogenic radiation. Calculated using an alpha attenuation factor of 0.07 ± 0.02, an internal K content of 12.5 ± 0.5 %, and an estimated average water content throughout the burial period of 20 ± 10 %. Details on dose rate calculation are given in Firla et al. (2024). ^b Number of accepted grains after synthetic aperture radar (SAR) rejection criteria and low-outlier correction after Firla et al. (2024). ^c Overdispersion derived from Central Age Model (Galbraith et al., 1999) calculations. ^d Bootstrapped (Cunningham and Wallinga, 2012) three-parameter Minimum Age Model (Galbraith et al., 1999) equivalent dose calculated using the R luminescence package (Kreutzer et al., 2012). σ_b = 30 ± 10 %. ^e Ages calculated using the ADELE software (Kulig, 2005). Fading correction applied after Huntley and Lamothe (2001) using a g value of 2.1 ± 0.3. ^f Ages calculated using the ADELE software (Kulig, 2005). No fading correction applied.

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The retrieved sedimentary sequence was divided into three different units (Fig. 2). The basal sedimentary unit from 30 to 14.6 m consists of fine-grained-silt-dominated deposits which show a coarsening-up trend (unit A) and exhibit a blue-greyish colour. The middle unit (unit B) ranges from 14.6 to 3.0 m and is dominated by sandy sediments and showed a trend of coarsening up. The top unit from 3.0 m up to the landscape surface (unit C) is dominated by coarser, gravel-dominated sediments that showed signs of anthropogenic reworking in the top section of unit C, such as roof tile or brick fragments.

Figure 2Left: lithological log showing litho-facies associations, as well as luminescence-dating and radiocarbon dating sampling locations. A, B, and C refer to the general sedimentological units. Dcm: diamicton, clast supported, massive. Sm: sand, massive. Gcm: gravel, clast supported, massive. Scu: sand, inverse grading trend. Fl(d): fines (silt dominated), laminated, outsized clasts. Fm: fines (silt dominated), massive. Fl: fines (silt dominated), laminated. Fcl: fines (silt dominated), convolute lamination. Right: age overview, with the IRSL₅₀ age in red and the pIRIR₂₂₅ age in dark red. The IRSL₅₀ ages represent the last depositional event; the pIRIR₂₂₅ can be considered to denote the maximum ages because of incomplete bleaching. The radiocarbon ages (URS-C1: > 43.5 ka; URS-C8: late 1960s) are presented in purple. The Marine Isotope Stages after Lisiecki and Raymo (2005) and the Last Glacial Maximum (LGM) (24 ± 2 ka) are given as references.

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The basal radiocarbon sample (13.8–13.6 m) was beyond the reliable dating limit of this method, resulting in a minimum age of > 43.5 ka. Organic material from the top sample (2.35 m) was dated to be of recent age (1966–1967 CE).

Units A and B are interpreted to be glacio-lacustrine deposits of a pro-glacial lake. The description of the SST as blue-greyish silt and sands with coarsening-up trends provided by Prey (1959) and Herbst and Riepler (2006) matches the retrieved sediments of unit A and unit B. Basal deposits from unit C are interpreted to be of fluvial origin.

Interestingly, radiocarbon ages from unit B (> 43.5 ka) and from Herbst and Riepler (2006) (> 38.6 and > 43.3 ka) do not agree with the fading-corrected IRSL₅₀ ages of the SST (18.6 ± 0.9 ka).

A radiocarbon age determines the last time organic material was in metabolic exchange with the atmosphere. As sediments are susceptible to reworking in glacially associated environments, this age might be considered to be a minimum age because any later redeposition will not be recorded in the radiocarbon content. In general, the radiocarbon ages of a sample taken from a sedimentary deposit only give limited insights into the last depositional event. In comparison to radiocarbon dating, luminescence dating provides the time when the sediment was last exposed to sunlight. Additionally, potential reworking was identified by investigating the shape of the equivalent dose distributions (which were right-skewed, suggesting poor bleaching). These radiocarbon-dating results, together with the luminescence-dating depositional ages, suggest the presence of older, reworked organic material. A potential source for the redeposited organic material is described by Ebers et al. (1966) as interglacial deposits in the vicinity of Adnet, southeast of the drill site.

The sedimentary succession of units A and B is the top of a fine-grained lake sediment section that starts beyond the depth of the drill core. It captures the end of the lake depositional phase during the late glacial in the LSV and the transition into a fluvial environment (topset). Complementarily, Firla et al. (2024) examined an overdeepened basin (ICDP-DOVE core 5068_4_A Neusillersdorf) to the north of the drill site and found that infilling occurred over a comparatively short period. They found no discernible age trend from the bottommost sample at the base of the basin (∼ 115 m) to the topmost sample of fine-grained basin fill (∼ 28 m). We therefore suspect that the Urstein fine-grained lake sediments below the retrieved section could be of similar age compared to the section investigated here.

Unit C was deposited during the Holocene. Based on radiocarbon dating and signs of anthropogenic activity at its top, it was partially reworked, potentially during the construction of the hydroelectric power plant in the late 1960s.

Core 5068_4_A Neusillersdorf (Firla et al., 2024) retrieved an overdeepened MIS 6 fine-grained basal section and a non-overdeepened MIS 3 coarse-sediment-dominated top section. It is challenging to answer whether the basal section is part of the SST because the SST is not strictly defined. However, it is generally associated with a more proximal position within the Salzach glacier than the more distal position observed here. Pomper et al. (2017) argued for a pre-LGM (Last Glacial Maximum) deposition of the SST, at least around Vigaun. Van Husen (1979) postulated the depositional age of the SST to be of late Würmian (MIS 2) to Holocene origin. With the luminescence-dating results presented here, we can support this interpretation for the shallower section of the SST with numerical evidence.

Code and data are made available on reasonable request from the corresponding author.

GF: writing (original draft), visualization, methodology, investigation, formal analysis, data curation, software. MF: validation, supervision, investigation, resources, funding acquisition, conceptualization. TR: formal analysis, data curation. CL: validation, supervision, resources, investigation, methodology, project administration, conceptualization.

At least one of the (co-)authors is a member of the editorial board of E&G Quaternary Science Journal. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

This research has been supported by the Austrian Science Fund (grant no. P23138).

This paper was edited by Bernhard Salcher and reviewed by Markus Fuchs.