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 has been optimised for the measurement of the Zn/Si ratio of the frustule. The separation procedure eliminates artifacts due to the presence of clay contaminants in the sample and the adsorption of clays onto the frustule surface. 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, thereby 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 contaminants was small and likely negligible. The Zn/Si ratios from four core top samples were found to be good agreement (3.4±0.1 μmol/mol) with each other. These results were also close to Zn/Si ratios reported for biogenic opal collected in plankton tows from the South Atlantic sector of the Southern Ocean and the Pacific Ocean.