Testing cave ice deposits as archives of past atmospheric ¹⁰Be deposition

INTRODUCTION
Depositional records of atmospheric cosmogenic radionuclides play an important role in the reconstruction of fluctuations of the solar activity over millennial timescales (Beer, 2000). However, sedimentary ¹⁰Be records reflect partly the local depositional conditions such as precipitation patterns. Therefore, a cross-check of ¹⁰Be records obtained from different geographical locations with distinct precipitation regimes is important. To meet this demand, as polar ice cores proved to be invaluable archives of atmospheric ¹⁰Be deposition, increasing scientific interest turned to ¹⁰Be records of mid-latitude glaciers (Inceoglu et al., 2016). Conversely, while presently surface glaciation is mostly absent at mid-latitudes, subterranean glaciation (i.e., ice caves) is a common feature, even on low-elevation karstic areas. Once it forms, cave ice can preserve the deposition record of ¹⁰Be similarly to surface ice bodies, so it has the potential to be a useful complementary archive providing comparable records of past atmospheric ¹⁰Be deposition.
We present here a record of atmospheric ¹⁰Be locked in the millennial old ice deposits from Scărișoara Ice Cave, Romania. To our knowledge, our project is the very first in measuring atmospherically-produced ¹⁰Be in cave ice deposits.

SAMPLING STRATEGY and METHODOLOGY
A ~6 m long, 10 cm diameter ice core was extracted from the ice block of the Scărișoara Ice Cave (Apuseni Mts, Romania, Fig. 1) in 2015 in segments each between 5 and 30 cm long. The outer surface of the core was immediately cleaned in the field using sterilized plastic knives, subsequently wrapped in clean plastic bags and stored at temperatures between -20°C and -40°C prior to analysis. The ice cores were transported frozen to the Cosmogenic Nuclide Sample Preparation Laboratory in Budapest (http://www.geochem.hu/kozmogen/Lab_en.html) in 2018. Nine ice core sections, each weighing ~300 g, were selected for a pilot study. Radiochemical sample processing including addition of defined amounts of stable ⁹Be followed the methodology of Zipf et al. (2016) and was carried out at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). Accelerator Mass Spectrometry (AMS) measurements of the ¹⁰Be/⁹Be ratio of the samples were performed also there. Data were normalized to SMD-Be-12 (Akhmadaliev et al., 2013), which is traceable to the NIST4325 standard.
The processing and measurement of these pilot samples was successful: all samples provided measurable and distinct ¹⁰Be/⁹Be ratios. The performance of five out of nine samples was excellent. Although the chemical yield of four samples was lower than expected (except for one sample) uncertainties remained below 5% (range between 2.3 and 5.1%; mean 3.5%).
An additional set of nine samples was selected for analysis in 2019, with the aim of using a slightly modified radiochemistry method to achieve increased and more stable chemical yield for all samples and to provide more details of the variations of atmospheric ¹⁰Be concentrations along the core. This sample set was radiochemically processed by the same people but at the University of Vienna. These samples were investigated by AMS out at the Vienna Environmental Research Accelerator (Steier et al., 2019). The data were again normalized to the secondary standard SMD-Be-12 to allow direct comparability between the two datasets.
The age of the ice core was determined by transferring the depth-age model of the Perșoiu et al. (2017) record, based on 26 ¹⁴C ages, to the present core. Four ¹⁴C measurements of this new core were used as anchor points for the older chronology. The chronological framework has been assigned to the cave ice derived ¹⁰Be results following the synchronization of the depth-scales of the two cores.

RESULTS and DISCUSSION
Due to successfully-improved chemical preparation, the chemical yield could be increased for all samples, hence, leading to smaller overall uncertainties of the ¹⁰Be data of the second sample set (1.9-3.6% (mean 2.7%). The measured ¹⁰Be/⁹Be ratio of the samples and processing blanks are in the same range for both sample sets (Fig. 2).
The ¹⁰Be concentrations range from (0.52±0.02)×10⁴ at/g_ice to (4.17±0.16)×10⁴ at/g_ice in the combined dataset (Fig. 2). This concentration range is comparable to those found in polar ice cores (Berggren et al., 2009, von Albedyll et al., 2017) but slightly lower than in the high-elevation Asian mountains (Inceoglu et al., 2016).
Based on the ¹⁴C measurements, the maximum age of the 6 m core is estimated to be 900 years. The ¹⁰Be concentrations of the studied section covers the upper 1.5 m of the ice core and corresponds to the ~1630 AD to ~1850 AD time interval.
The main trend in the cave ice derived ¹⁰Be concentration mirrors quite well the ¹⁰Be concentration profiles obtained from polar ice cores for the same period (von Albedyll et al., 2017, Berggren et al., 2009). The ¹⁰Be concentration peak (3.96±0.20)×10⁴ at/g_ice, Fig. 2) in the Dresden data found at the depth range of ~97-103 cm below surface corresponding to the late 1680s AD might reflect the Maunder Minimum documented as peak concentration both in the Akademii Nauk ice core (von Albedyll et al., 2017) and the NGRIP ice core (Berggren et al., 2009).
The data looks very promising, but further data evaluation and interpretation is still needed.

ACKNOWLEDGEMENTS
This research was funded by the National Research, Development and Innovation Office of Hungary grant OTKA FK 124807 (ZsRR), and UEFISCDI Romania through grant number PN-III-P1-1.1-TE-2016-2210 (AP). Parts of this research were carried out at the Ion Beam Centre (IBC) at the Helmholtz-Zentrum Dresden-Rossendorf e. V., a member of the Helmholtz Association. AMS measurements at VERA facility (University of Vienna) were supported by the Radiate Transnational Access 19001687-ST. This is contribution No.71 of the 2ka Palæoclimatology Research Group.

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