Effect of pH on the measurement of biogenic silica

A comparison of the Si/Al and Si/time wet-alkaline digestion methods for measurement of biogenic silica in lake sediments

“…Within the water column, levels of diatom-bound Al are minimal with Al/Si ratios of 8.3x10 -3 to 7.0x10 -5 (Lewin 1961;Martin and Knauer 1973;Kamatani 1974;van Bennekom et al 1989;Schlüter and Rickert 1998;Beck et al 2002;Gehlen et al 2002). Although additional Al can become incorporated into the diatom frustule at the sediment-water interface (van Bennekom et al 1988(van Bennekom et al , 1991Dixit et al 2001;Dixit and van Cappellen 2002;van Cappellen et al 2002;Gehlen et al 2002;Rickert et al 2002;Koning et al 2007), the overall amount of Al originating from diatoms is likely to be negligible relative to contributions from non-BSi, particularly when sample BSi concentrations are low.…”

“…Although van Bennekom et al (1989) reported fossilised diatom Al concentrations of up to 6 wt.% in sediments from the Zaire/Congo deep-sea fan, a wealth of other data indicates that fossilised diatom Al concentrations actually peak at c. 1.0 wt.%, with typical concentrations <0.3 wt.% and many concentrations c. 0.01 wt.% (e.g. Shemesh et al 1988;Schlüter and Rickert 1998;Ellwood and Hunter 1999;2000;Dixit et al 2001;Dixit and van Cappellen 2002;Lin and Chen 2002;Gehlen et al 2002;van Cappellen et al 2002;Kryc et al 2003;Lal et al 2006). …”

“…As such, by assuming that concentrations of Al in BSi are negligible, measurements of digested Al can be used to directly account for non-BSi leached during the alkaline digestion. This may be particularly important in accounting for the rapid parabolic release of non-BSi that occurs during the first two hours of an alkaline digestion (Schlüter and Rickert 1998;Kamatani and Oku 2000;Koning et al 2002), which may lead to an over-estimation of the true amount of BSi, particularly in samples low in BSi in which the relative contributions from non-BSi may be high.…”

“…Recent work has also focused on a single, sequential Si/Al extraction technique, in which a linear regression to the y-intercept is used to separate out the non-BSi phase on aliquots collected following complete BSi digestion (Fig. 1b) (Schlüter and Rickert 1998;Kamatani and Oku 2000;Koning et al 2002). One advantage of this particular approach is the speed and ease with which samples can be analysed relative to other Si/Al methods.…”

“…To date, the sequential Si/Al method has been applied primarily to surface sediments (Kamatani and Oku 2000;Koning et al 2002;Ragueneau et al 2005), with comparisons failing to indicate a significant difference between conventional Si-only and sequential Si/Al measurements of BSi (Schlüter and Rickert 1998;Kamatani and Oku 2000). However, the sequential Si/Al method has yet to be applied and compared to conventional BSi techniques in a long core section.…”