A new method for the quantitative separation of diatom frustules from lake sediments

Achieving a purified fraction of diatom frustules from soft sediment samples is an essential requirement for using their biogenic silica as a carrier of paleolimnological and paleoenvironmental information. Because diatom frustules behave hydrodynamically unlike most mineral grains, these types of particles can theoretically be separated in liquids. Based on this principle, a new method has been developed, which employs split-flow thin fractionation (SPLITT) as a tool for separating diatom frustules from other sedimentary particles. The SPLITT channel selected for this study had a length of 20 cm, a breadth of 4 cm, and a height of 371 mum. For optimum results, samples were pretreated, e. g., suspended and wet-sieved prior to any processing with the SPLITT cell. The sample concentrations used in the fractionation process were below 0.1% (w/v) to minimize particle-particle interaction, which affects the quality of the separation. The advantages of SPLITT fractionation over other tested methods are good reproducibility, high throughput by continuous flow, minimum losses of frustules, and minimum contamination of the diatom fraction by minerals or sponge spicules. Separation results from representative sediment samples of different origin, age, and compaction as well as various organic (0.5% to 10% total organic carbon) and biogenic opal contents (1% to 20% biogenic silica) are presented and discussed. They demonstrate that the developed separation technique yields highly purified and, within the processing tolerances, quantitative samples of diatom frustules from bulk lacustrine sediments.