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doi:10.2204/iodp.proc.322.204.2013

Methods

When placed in an alkaline solution, amorphous silica (e.g., glass) dissolves more quickly than structured silicates (e.g., clay minerals). This difference in dissolution rates allows us to use the amount of dissolved silica leached from a sediment sample over time to determine the original amount of amorphous silica in the sample.

The amorphous silica contents of 36 samples were measured using an alkaline leaching method modified from DeMaster (1981). Freeze-dried, crushed sediment samples were dissolved in a 1% sodium carbonate solution at 85°C. We used a leachate with pH 11.4 in order to minimize digestion of clay minerals (Schlüter and Rickert, 1998). Aliquots of the leachate were collected 15, 30, 60, 90, 120, 200, and 300 min after digestion began. The silica concentration in each aliquot was determined by spectrophotometry (e.g., Grasshoff et al., 1983).

The concentration of dissolved silica in the leachate increases over time. A rapid initial increase occurs, largely caused by the dissolution of amorphous silicates, followed by a gradual increase in dissolved silica concentration resulting from the slow dissolution of structured silicates (Fig. F1). The dissolution rate of the structured silicates is assumed to be constant throughout the entire dissolution process (DeMaster, 1981). We fit a line to the late, low-slope portion of the concentration curve. Projecting this line to the time when digestion began accounts for the contribution of structured silicate dissolution to the dissolved silica concentration during early time, when the signal is dominated by amorphous silicate dissolution (Fig. F1). The y-intercept of this line indicates the total contribution from amorphous silicate to the dissolved silica in the leachate. Error estimates for the amorphous silica content are determined from the 95% confidence interval on this y-intercept value. Commonly, it takes longer for samples with more amorphous silica to reach the steadily increasing dissolved silica concentration trend (i.e., longer for all amorphous silica to dissolve), relative to samples with less amorphous silica. Therefore, for samples with more amorphous silica, the line fit to the low-slope portion of the concentration curve must be projected farther from the data used to define the line to the y-intercept. As a result, there is more uncertainty in the y-intercept value (and the amorphous silica content) of samples with more amorphous silica. For replicates of samples from the Middle America margin (Spinelli and Underwood, 2004) the maximum error is 12%. In addition, some uncertainty in calculating the weight percent of amorphous silica in the bulk samples derives from potential variability in the molecular weight of amorphous silica (SiO₂·nH₂O). We assume the amorphous silica is 11% water by weight, which is consistent with hydrated glass shards in Shikoku Basin sediment (White et al., 2011) and global estimates for opal (Keene, 1976).

This technique uses physiochemical differences between amorphous and structured silicates to quantify the amorphous silica content of sediment samples. With this technique, we cannot discern the type of amorphous silica present in the sediment (e.g., volcanic glass, biogenic opal, etc.). All sediment samples analyzed in this study are hemipelagic mud (Underwood et al., 2010; Expedition 333 Scientists, 2012). Previous electron microprobe image analyses of similar clay-dominated Shikoku Basin sediment found disseminated volcanic glass shards more common than biogenic opal (White et al., 2011).

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