Carrier free 10Be/9Be measurements with low-energy AMS: Determination of sedimentation rates in the Arctic Ocean

Abstract

Using the TANDY AMS facility (600kV) at ETH Zurich the seawater-derived (authigenic) 10Be/9Be ratio of marine sediment samples is measured without the addition of 9Be carrier. This novel method reduces systematic uncertainties because the 10Be/9Be ratio of a sample is determined in only one (AMS) measurement. A challenge of carrier-free AMS is to avoid any contamination of the sample with 9Be during the chemical preparation. Further, the leaching procedure has to be reproducible and ideally should attack the authigenic Be of the sediments only, leaving the detrital Be untouched. The low amount of stable 9Be in the unspiked samples causes low currents during the AMS measurement. This requires a good stability and sensitivity of the AMS setup. Our first results show that the new preparation method is reliable and that background from stable 9Be is avoided.For a comparison study, sediment samples from two cores located in the Arctic Ocean (HLY0503-09JPC, HLY0503-14JPC) were used. The authigenic 10Be/9Be ratio of these samples had been determined previously applying the conventional method where 10Be and 9Be concentrations are measured separately by AMS and ICP-MS, respectively. The resulting sedimentation rates are in discrepancy with values derived from biomarkers. To cross check the 10Be/9Be based age model two samples from each core were measured again with the new carrier-free method. The carrier-free results show systematically higher authigenic 10Be/9Be ratios. The calculated sedimentation rates of about 0.2cm/kyr, however, are consistent for the carrier free and the conventional method.