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ABSTRACT. The prototype mini carbon dating system (MICADAS) at ETH Zurich has been in routine operation for almost 2 yr. Because of its simple and compact layout, setting up a radiocarbon measurement is fast and the system runs very reliably over days or even weeks without retuning. The stability of the instrument is responsible for the good performance in highestprecision measurements where results of single samples can be reproduced within less than 2‰. The measurements are described and the performance of MICADAS is demonstrated on measured data.

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There is growing interest in geochronological applications of terrestrial in situ-produced cosmogenic nuclides, with the most commonly measured being 10 Be and 26 Al in quartz. To extract and then separate these radionuclides from quartz and prepare them in the oxide form suitable for accelerator mass spectrometry (AMS) requires extensive and careful laboratory processing. Here we discuss the suitability of a crushed, sieved and etched, sub-aerially exposed vein quartz specimen (CoQtz-N) to act as a reference material for chemical laboratory preparation and AMS measurements. Splits of CoQtz-N were distributed to eleven target preparation laboratories. The CoQtz-N 10 Be targets were then measured at seven different AMS facilities and five of the preparation laboratories had their 26 Al targets measured at four different AMS facilities. We show that CoQtz-N splits are sufficiently homogeneous with regard to nuclide concentrations, that it has been cleaned of any atmospheric derived (i.e. meteoric) 10 Be and that it has low concentrations of the major elements that can interfere with Be and Al extraction chemistry and AMS measurements. We derive preliminary concentrations for 10 Be and 26 Al in CoQtz-N as 2.53 ±

The first dataset of 236 U/ 238 U in the water column of the Arctic Ocean (AO) is presented and shows the widest range of ratios reported so far in the open ocean, from (5±5) to (3840±260) ×10-12. Surface samples and depth profiles were collected during two GEOTRACES expeditions in 2011-2012 and analyzed for the concentrations of 236 U and 129 I, with the aim of investigating whether the combination of 236 U/ 238 U and 129 I/ 236 U can be used as a new oceanographic tool in the AO. Results show that the distributions of the 236 U/ 238 U and 129 I/ 236 U atomic ratios are consistent with the different water masses in the AO. High 236 U/ 238 U and 129 I/ 236 U ratios in the upper water column (>2000 ×10-12 and >200, respectively) illustrate the penetration of Atlantic waters (AW) into the AO. Lower values were found in Pacific waters (PW) and deep waters of the AO. Rivers seem to represent a temporally and spatially-constrained third anthropogenic source of 236 U but more data is needed to confirm this. In a simple

This study presents the data on 129 I and 236 U concentrations in seawater samples and sea ice cores obtained during two expeditions to the Arctic Ocean that took place onboard R/V Polarstern (PS94) and R/V Lance (N-ICE2015) in summer 2015. Carbon-14 was also measured in the deep water samples from the Nansen, Amundsen, and Makarov Basins. The main goal was to investigate the distribution of 129 I and 236 U in a transect from the Norwegian Coast to the Makarov Basin to fully exploit the potential of combining 129 I and 236 U as a dual tracer to track Atlantic waters throughout the Arctic Ocean. The use of the 129 I/ 236 U and 236 U/ 238 U atom ratios allowed identifying a third Atlantic branch that enters the Arctic Ocean (the Arctic Shelf Break Branch) following the Norwegian Coastal Current that carries a larger proportion of the European reprocessing plants signal compared to Fram Strait Branch Water and Barents Sea Branch Water. The combination of 129 I and 236 U also allowed quantifying the different proportions of the La Hague stream, the Scottish stream, and Atlantic waters forming the three Atlantic branches of the Arctic Ocean Boundary Current. The results show that the 129 I/ 236 U atom ratio can now be used to identify the different Atlantic branches entering the Arctic Ocean. New input functions for 129 I, 236 U, and 129 I/ 236 U have also been described for each branch, which can be further used for calculation of transit time distributions of Atlantic waters. Plain Language SummaryIn this work we present results of artificial radionuclides that were measured in seawater samples collected in the Arctic Ocean. We measured the long-lived artificial radionuclides 129 I and 236 U to track the different water masses. These two radionuclides are present in the marine environment after the nuclear weapon tests (1950s-1960s) and from two European nuclear reprocessing plants (from 1960s until today) located in France and United Kingdom. In particular, the input of 129 I from these two reprocessing plants changed over time and can therefore also be used to estimate travel times of water masses. In this study, we collected about 150 seawater samples from 20 different stations in the Arctic Ocean in summer 2015, onboard the R/V Polarstern. Our results reveal that the mixing ratios of the two reprocessing plant effluents are different for the Fram Strait Branch and the Barents Sea Branch, contrasting with previous studies. The 129 I/ 236 U was used as a new tool to identify the characteristics of these two Atlantic branches entering the Arctic Ocean and even prove the existance of a more surface branch (Arctic Shelf Break Branch) carrying a significant proportion of reprocessing plant-derived radionuclides.

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