The distribution of 10Be and 9Be in the South Atlantic

Abstract

Vertical and surface-water distributions of 10Be and 9Be in the South Atlantic Ocean were studied. The major input of 9Be to the surface waters of the region is from the partial dissolution of eolian dusts, with the extent of the dissolution being about seven times that of aluminum from the dust particles. The gradients in surface-water 10Be concentrations appear to reflect the magnitude of the local precipitation. The imprinting of the surface water signals onto the deep water masses appears rapid, particularly in regions of enhanced productivity. Bottom waters of Antarctic origin have characteristic isotope signatures that can be traced along their advective route into the Guinea Basin of the eastern Atlantic. Elevated 9Be concentrations in the Angola Basin are indicative of diagenetic input in this region of restricted circulation. The corresponding anomalous 10Be in the bottom waters indicates historically lower surface-water 10Be/ 9Be ratios in the region, perhaps as a result of the further southward penetration of the Inter-Tropical Convergence Zone at those times. The budgets of 10Be and 9Be in the South Atlantic were estimated. The results show that crossequator transport of 10Be and 9Be from the North Atlantic is 0.3 ± 10 23 atoms/year and 5.7 ±10 6 mol/year, respectively, accounting for less than 5% of 10Be and about 10% of 9Be entering the North Atlantic. There is a net export across 50°S to the Antarctic/Indian/Pacific Oceans of 0.9 ±10 23 atoms/year for 10Be and 7.5 ± 10 6 mol/year for 9Be. Using 10Be as a tracer, we evaluate accumulation rates of lithogenic minerals to be 1.7 ± 10 14 g/year in the North Atlantic and 4.8 ± 10 13 g/year in the South Atlantic. While the North Atlantic rate agrees with the observed eolian dust input, the estimated lithogenic flux in the South Atlantic is about twice the eolian input, suggesting that the riverine input of 9Be to the open ocean may become non-negligible in areas of low eolian dust flux.