Measuring biogenic silica in marine sediments

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

“…Existing studies have demonstrated a linear relationship between the alkaline release of Si and Al from clay minerals, except during the first 5-10 minutes of digestion when Si release is greater than Al (Kamatani and Oku 2000;Koning et al . 2002).…”

“…An alternative and potentially more accurate method for performing non-BSi correction in wet-alkaline digestions may lie in an examination of the Al concentration within the digested solution (Eggimann et al 1980). In alkaline digestions, strong linear relationships have been observed over time between the release of Si and Al from a range of clays including allophane, kaolinite, illite and montmorillonite (r = 0.988, 0.998, 0.994, 0.997, respectively) (Kamatani and Oku 2000). Higher-resolution, one-second measurements indicate that these relationships are established within the first 10 minutes of digestion (Koning et al 2002).…”

“…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.…”