Pleistocene variations of beryllium isotopes in central Arctic Ocean sediment cores

Neogene marine sediments can be dated via decay of the cosmogenic radionuclide 10Be. Two cores from the Alpha and Mendeleev Ridges in the Arctic Ocean have been analyzed for seawater-derived beryllium (Be) isotopes in order to date the sediments and to calculate sedimentation rates. The decrease of 10Be concentration in the cores was used to calculate first order sedimentation rates. To eliminate the dilution effect of beryllium caused by short-term changes in sedimentation rate and grain size, the 10Be concentrations were normalized to the terrigenous stable isotope 9Be determined in the same sample aliquot. The measured 10Be concentrations yield low average sedimentation rates for the Alpha and Mendeleev Ridges of 2.3 mm ka− 1 and 2.7 mm ka− 1, respectively. Sedimentation rates calculated from the 10Be/9Be ratios result in similarly low values, ranging from 0.2 to 6.8 mm ka− 1 for the Alpha Ridge core and from 1.9 to 6.9 mm ka− 1 for the Mendeleev Ridge core. However, amino acid racemization dating for the past 150 ka of a core adjacent to the Mendeleev Ridge core studied here indicates significantly higher sedimentation rates than calculated from the downcore decrease of 10Be and 10Be/9Be. If such higher rates also prevailed at the locations of our cores, for which there is biostratigraphic evidence, either the supply of 10Be was much lower than assumed or that of 9Be was much higher. This could imply that the signature of the deep waters in this part of the Arctic Ocean compared to today was largely different for most of the past approximately one million years with a significantly lower 10Be/9Be ratio. Our study also addresses the variability of beryllium isotopes in sediment cores across the Arctic Ocean through a comparison of previously published results. Calculated 10Be fluxes reveal low values in the Amerasian Basin and highest values in the Eurasian Basin, near the Fram Strait. The decrease of Be isotopes in the two studied Amerasian Basin cores may thus have been caused by environmental factors such as significantly reduced inflow of Atlantic waters in the past, reduced input of 10Be and/or increased input of 9Be from the shelves, combined with a more efficient sea ice shielding in this part of the Arctic Ocean.