O01_P01

Isochron-burial dating of the oldest glaciofluvial sediments in the northern Alpine Foreland

Broś E¹, Ivy-Ochs S¹, Grischott R², Kober F³, Vockenhuber C¹, Christl M¹, Maden C⁴, Synal H¹

¹Laboratory of Ion Beam Physics, ETH Zurich, Zurich, Switzerland, ²BTG Büro für Technische Geologie AG, Sargans, Switzerland, ³NAGRA, Wettingen, Switzerland, ⁴Geochemistry and Petrology, ETH Zurich, Zurich, Switzerland

During the Middle and Early Pleistocene, the northern Alpine Foreland was glaciated several times and witnessed numerous phases of alternating incision and deposition, shaping the landscape that can be seen today. High elevated plateaus separated by deeply incised valleys, create a topography with relief of several hundreds of meters. On top of these plateaus can be found the oldest Quaternary glaciofluvial sediments in the northern Alpine Foreland. They have traditionally been divided into two gravel units: older Höhere (Higher, HDS) and younger Tiefere (Lower, TDS) Deckenschotter. The HDS is located topographically higher by ~100-150 m than the TDS. The Deckenschotter consist mainly of glaciofluvial sediments intercalated with glacial and/or overbank deposits and form gravel terraces located up to about 250 m above the modern valley bottom. Their exact time of deposition is an important source of information for establishing erosion and incision scenarios and quantification of landscape evolution in the northern Alpine Foreland during the Middle and Early Pleistocene. Our focus is placed on similar and complementary Deckenschotter deposits outcropping in several sites across the northern Alpine Foreland. In selected six sites, we implement isochron-burial dating technique with a pair of cosmogenic nuclides ²⁶Al and ¹⁰Be, to further examine and refine the question of the age of the Deckenschotter. The first preliminary age estimates point to deposition in the latter part of the Early Pleistocene. With the aim to determine the age of these sediments, the results will also complement our understanding of landscape change during and after Deckenschotter times.

O01_P02

New evidence for the persistence of the Ilanzersee (Flims rockslide)

Grischott R^1,2, Wacker L², von Poschinger A³, Gilli A⁴

¹Büro Für Technische Geologie, Sargans, Switzerland, ²Laboratory for Ion Beam Physics, Zurich, Switzerland, ³Private adress, Kempfenhausen, Germany, ⁴Department of Earth Sciences, Zurich, Switzerland

The Flims rockslide is one of the largest known rockslide in the Alps and had a strong influence on the landscape evolution in the Vorderrhein-Valley. The Flims rockslide (volume 9–12 km³) has been dated to 9400 cal yr BP with the radiocarbon method [2]. The Vorderrhein was completely blocked by a more than 600 m-thick landslide dam and a lake, Ilanzersee, formed upstream [1]. Its maximum level obviously did not reach higher up than 930 m a.s.l. After the breach of the dam, an important sediment transport down the Rhine valley had occurred. Nevertheless, a relict lake existed for a longer time, probably for centuries. A level of about 820 m a.s.l that was held for fairly long time. The duration of this second lake level, until the lake was finally emptied, has not been clear so far. Drill cores retrieved along a transect of 4 km on top of the former delta plain of Ilanzersee revealed the continuous presence of typical fine-grained delta sediments overlain by recent fluvial sediments. A wood fragment embedded in delta sediment was dated to 8900-9000 cal yr BP and supports geological evidence found elsewhere, that the lake persisted during quite a long time. More samples from other drillings will be analysed to underline this first evidence.

[1]: Von Poschinger 2005

[2]: Deplazes et al., 2007

O01_P03

Comparison of two ¹⁰Be purification methods for AMS measurement

Loftfield J¹, Lachner J², Malter M¹, Stübner K², Adolphi F¹

¹Alfred Wegener Institute, Bremerhaven, Germany, ²Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany

The cosmogenic radionuclide ¹⁰Be is a powerful tool in paleosciences. Its applications include the dating of rocks and sediments, reconstruction of past changes in solar activity and the geomagnetic field strength, and the synchronization of climate archives.

However, sample preparation and especially purification of Beryllium from environmental samples can be time consuming and expensive. Typically, purification of sediment samples is achieved by a combination of hydroxide precipitations and ion exchange chromatography. Here, we test a method of successive hydroxide precipitations at different pH-values in combination with precipitation in NaHCO₃, to purify samples of marine sediments from the Norwegian Sea and the Lomonosov Ridge for ¹⁰Be-analysis. We compare the performance of this method to an ion chromatography-based method (Simon et al., 2016) with respect to the Beryllium yield, purity of the resulting Be(OH)₂, blank, and performance in the AMS for both protocols. We discuss the advantages and challenges of the protocols, their applicability, and their capacity in terms of sample throughput.

Simon, Q., Thouveny, N., Bourlès, D. L., Nuttin, L., Hillaire-Marcel, C., & St-Onge, G. (2016). Authigenic ¹⁰Be/⁹Be ratios and ¹⁰Be-fluxes (230Thxs-normalized) in central Baffin Bay sediments during the last glacial cycle: Paleoenvironmental implications. Quaternary Science Reviews, 140, 142–162. https://doi.org/10.1016/j.quascirev.2016.03.027

O01_P04

Status report of the in-situ ¹⁴C extraction line at HEKAL AMS laboratory

Buró B¹, Fülöp R², Jull A^1,3, Molnar M¹

¹INTERACT AMS Laboratory, Debrecen, Hungary, ²Australian Nuclear Science and Technology Organisation, Lucas Heights, Australia, ³Department of Geosciences, University of Arizona, Tucson, USA

In this study we presented a new cosmogenic in-situ ¹⁴C extraction line at the ICER laboratory, which is similar as the one published by Fülöp et al.(2019). These extraction system used the phase transformation of quartz to cristobalite on high temperature in order to quantitatively extract the carbon as CO₂. The system consists of three independent components. 1: used for remove the atmospheric and meteoric ¹⁴C, 2: offline high-temperature (1650 °C) oven  for extract and trapped the cosmogenic in-situ ¹⁴C from quartz, 3: CO₂ gas purification and mass measurement line. After the extraction and cleaning, the purified CO₂ sapmles are measured with compact ¹⁴C AMS system (Environ MICADAS) and the gas ion source interface. The extraction line allows for rapid sample throughput of about 6 samples per week. The  sample masses ranging between 4 and 7 g of clean quartz.

Purified quartz samples were sieved and used for analyse the fraction of  250 – 500 µm. The carbon yield from quartz samples are good and we have the expected values.

Our first tests were on the borehole CO₂ blank gas and Cronus-R standards. The blank level of the whole line is quite low. We get similar experiences and results as Fülöp et al. (2019).

O01_P05

³²Si – An alternative radionuclide for dating the recent past?

Schlomberg M¹, Vockenhuber C¹, Synal H¹

¹Laboratory of Ion Beam Physics, ETH Zurich, Zurich, Switzerland

Dating the last few hundred years is challenging with currently used radionuclides: To date with ¹⁴C (half-life of 5700 a) is difficult due to ambiguities in the calibration curve in the last ~400 years, although the last 80 years can be again well dated due to the bomb peak of the ¹⁴C concentration and its subsequent dilution. Dating methods based on shorter-lived nuclides like ³H (12.3 a) and ²¹⁰Pb (22.3 a) can only be used in the last ~100 a. A promising candidate for filling this dating gap is cosmogenic ³²Si with a half-life of ~150 a.

However, the application of ³²Si has so far been limited by the imprecisely known half-life. As part of the SINCHRON collaboration which aims at a redetermination of the half-life of ³²Si, the Laboratory of Ion Beam Physics (LIP) at ETH Zurich will perform the AMS measurements for the determination of the absolute number of ³²Si atoms in samples used for activity measurements.

In this poster, the potential of ³²Si as dating tool is discussed and an overview of the previous half life measurements is given. First results concerning the AMS measurement technique are presented and an outlook for potential ³²Si measurements in environmental samples is given.

O01_P06

Studying ¹⁴C production in meteorites using the Bernese ¹⁴C extraction line and the MICADAS system at LARA, University of Bern

Tauseef M¹, Leya I¹, Szidat S², Gattacceca J³

¹Space Research and Planetary Sciences, University of Bern, , Switzerland, ²Department of Chemistry, Biochemistry and Pharmaceutical Sciences, University of Bern, , Switzerland, ³CNRS, Aix Marseille University, CEREGE,  Aix-en-Provence, France

Carbon-14 dating is the most robust technique for determining the terrestrial ages of meteorites. With our updated ¹⁴C extraction system, we can measure up to 15 samples without breaking the vacuum, thereby achieving low blanks and a high sample throughput (Sliz et al. 2018, 2020). Briefly, we are able to quantitatively and reproducibly extract CO₂ gas from pre-cleaned meteorite samples (typical masses 50 mg). Preheating the samples at 500 °C in a continuous flow of pure oxygen reduces the remaining atmospheric contamination. Gas extraction is at ~1600 °C under an O₂ partial pressure of 30±5 mbar for 10 min. Evolved gases are first purified at 500°C –1000°C using CuO, quartz spherules, and silver wool. Next, gases are cleaned and separated using a water trap at –78 °C and cold fingers (~100 °C). The purified CO₂ gas is then collected in a glass capillary and is subsequently introduced into the gas ion source of the MICADAS AMS system (University of Bern). We finally give our results as specific activity concentrations (dpm/kg), which are then used to determine the terrestrial ages of meteorites. Here we will present new data for ¹⁴C and ¹⁰Be activity concentrations in freshly fallen meteorites to better constrain the ¹⁴C and ¹⁰Be production rates and consequently determine more accurate and more precise terrestrial ages of meteorites.

References

Sliz M.U. et al. 2018. 81st Annual Meeting of The Meteoritical Society 2018 (LPI Contrib. No. 2067)

Sliz M.U. et al. 2020. Radiocarbon Vol 62, Nr 5, p 1371–1388