An interlaboratory comparison for the measurement of biogenic silica in sediments

An interlaboratory comparison was made to assess the variability in the mea surement of biogenic silica (BSI) in sediments among aquatic scientists. Th e purpose of this study was to (I) assess the present state of BSi analysis by aquatic scientists, (2) establish the range of variability in the measu rement of BSi across the community of aquatic scientists (paleolimnologists , paleooceanographers, limnologists, estuarine scientists, and oceanographe rs), and (3) to determine if patterns in the measurement were related to sp ecific methodologies used or to treatments. This study included 30 laborato ries primarily using a variety of different wet chemical extraction techniq ues. The six samples used in the interlaboratory comparison were from moder n freshwater and coastal marine depositional environments. The samples were chosen to cover a wide range of BSi concentrations, contain no sponge spic ules or radiolarians, and were easy to digest relative to relict samples. I ndependently measured BSi concentrations of the same sediment ranged widely . BSi concentrations determined by X-ray diffraction were significantly hig her than concentrations determined by wet chemical methods. The percent sta ndard deviation of the mean in samples analyzed by wet chemical digestion t echniques was highest in the samples with the lowest BSi concentration (Tab le 3) with 67.5% variation in sample 6 (1.31 +/- 0.88 wt.% as SiO2), and lo west at the highest BSI concentration with 21.2% variation in sample 2 (44. 3 +/- 9.38 wt.% as SiO2,). An Analysis of Variance (ANOVA) was used to test for significant differences between treatments for the following variables : (1) pH of digestion solution, (2) digestion solution (Na,CO, or NaOH), an d (3) whether a mineral correction was made. Although significant differenc es were found, inspection of the raw data suggests that several anomalous p oints may be responsible for the significant differences obtained. Therefor e, the ANOVA was rerun following removal of 7 laboratories from the compari son. These laboratories were removed if they measured BSi concentrations mo re than Fl standard deviation from the mean in at least three of the six sa mples used in the intercomparison. For this analysis no significant differe nces were found between laboratories. Finally, no significant differences w ere found between the two most widely used techniques to measure BSi in sed iments (DeMaster, D.J., 1979. The marine budgets of silica and Si-32. PhD D issertation, Yale University, 308 pp.; Mortlock, R.A. and Froelich, P.N., 1 989. A simple method for the rapid determination of biogenic opal in pelagi c marine sediments. Deep-Sea Res., 36, 1415-1426.). The importance of optim izing the technique for the type of sample and of making a determination of slope for the mineral correction are explored. (C) 1998 Elsevier Science B .V. All rights reserved.