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

Abstract

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 µm. 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.

The Potential use of Silicon Isotope Composition of Biogenic Silica as a Proxy for Environmental Change

“…An alternative/complementary approach to heavy liquid separation of biogenic silica from minerogenic grains is gravitational split-flow thin fractionation (SPLITT) [79,85,86]. SPLITT has only currently been used for diatom material in this context.…”

“…A number of methodologies have been developed to separate biogenic silica in sediments (both recent and geological) cores where biogenic silica materials are commonly intermixed with silt, clay, tephra, carbonates and organic matter [72][73][74][75][76][77][78][79][80][81]. Methods have also been developed which allow the simultaneous extraction of phytoliths, pollen and spores from sediment samples [82].…”

Transfer and early diagenesis of biogenic silica oxygen isotope signals during settling and sedimentation of diatoms in a temperate freshwater lake (Lake Holzmaar, Germany)

“…Pure diatomaceous silica samples from trap material and from the bottom sample were obtained by SPLITT fractionation (Rings et al, 2004). This method is based on the different sinking velocities of particles in a liquid suspension that is gently pumped through a narrow channel in a laminar flow.…”

Oxygen isotope ratios of sedimentary biogenic silica reflect the European transcontinental climate gradient

“…However, in most cases, standard silica extraction procedures (Morley et al, 2004) and split-flow thin fractionation (SPLITT), whereby particles are separated by passage through a laminar flow (Rings et al, 2004;Leng and Barker, 2006), failed to produce pure biogenic silica for isotope analysis. These methods rely on discriminating diatoms from contaminating minerals based on the different size and density of the particles; however, in many cases contaminating minerals are of equal size and density to diatom frustules.…”