Biogenic Silica

The measurement of the biogenic silica (BSi) content of sediments is a chemical estimate of the siliceous microfossil abundance. Briefly, sediments are leached with a weak base, usually Na₂CO₃, for a period of time (2–5 hours), and aliquots withdrawn over time. The aliquots are then measured for the amount of Si extracted and a least-squares regression is made on the increase in concentration with time to separate the Si extracted from amorphous Si compounds, e.g. diatoms, sponges, etc., from that of mineral silicates. Comparison of chemical estimates of BSi with diatom microfossil point counts demonstrate that the extraction techniques provide a valid proxy for the abundance of diatom microfossils in sediments. However, the exact choice of methodology will depend upon the type of siliceous components in the sediments and the ability of the digestion solution to dissolve those components. Therefore, both the strength of the digestion solution used and the time over which subsamples are taken should be adjusted for depending upon the type of sediment used. Application of these techniques as a proxy for siliceous microfossil abundance have been instrumental in unraveling the response of aquatic systems to nutrient enrichment and has provided important information on paleoproductivity in particular in studies of paleoclimate.