Determination of the Zn/Si ratio in diatom opal: a method for the separation, cleaning and dissolution of diatoms

A procedure is presented for the separation and the purification of diatoms from marine sediments prior to chemical analysis of the opal. The method h as been optimised for the measurement of the Zn/Si ratio of the frustule. T he separation procedure eliminates artifacts due to the presence of clay co ntaminants in the sample and the adsorption of clays onto the frustule surf ace, The concentration of trace elements (Fe, Al, Ba and Ge) in the sample digest were low indicating that the samples were almost pure biogenic opal. For a number of samples, Zn and Si were measured as a function of time as the opal is dissolved, Both Zn and Si were released at similar rates, there by indicating a frustule source. On the other hand, Fe and Al were released to solution at a constant rate, even after all of the opal had dissolved, indicating a non-opal source. However, the amount of Zn from clay contamina nts was small and likely negligible. The Zn/Si ratios from four core top sa mples were found to be good agreement (3.4 +/- 0.1 mu mol/mol) with each ot her. These results were also close to Zn/Si ratios reported for biogenic op al collected in plankton tows from the South Atlantic sector of the Souther n Ocean and the Pacific Ocean. (C) 1999 Elsevier Science B.V. All rights re served.