How to measure Si isotopic signatures in specific phases of suspended particles?
To be clear, this presentation will not answer the question posed in the title. Rather, its aim is to open the discussion on different practices and perspectives focusing on Si isotopes and the specificities of suspended particles in marine environments close to the continental margin or in freshwater. These particles are characterised by a highly variable mixture of amorphous (biogenic or not) and crystalline Si phases, while Si adsorption adds a level of complexity to such matrices. For decades, the silicon community has debated the best way(s) to measure amorphous silicon concentrations in water, soils or sediments, and the jury is still out. Silicotopists have now successfully applied a combination of physical and chemical separation steps to measure Si isotopes in sediments or soils. However, due to the limited amount of material available in suspended particles, these methods are generally not directly applicable to such samples. Recently, some studies have reported promising Si isotopic compositions of suspended amorphous phases, mostly in glacial environments where mechanical erosion tends to produce fine particles with a high contribution of amorphous silicon. Here we review different methods used and propose some perspectives to start a constructive discussion with the IBIS community in order to identify best practices.