The contribution of tephra constituents during biogenic silica determination: implications for soil and palaeoecological studies

Abstract

Abstract. Biogenic silica (BSi) is used as a proxy by soil scientists to identify biological effects on the Si cycle and by palaeoecologists to study environmental changes. Alkaline extractions are typically used to measure BSi in both terrestrial and aquatic environments. The dissolution properties of volcanic glass in tephra deposits and their nanocrystalline weathering products are hypothesized to overlap those of BSi; however, data to support this behaviour are lacking. The potential that Si-bearing fractions dissolve in alkaline media (Si Alk ) that do not necessarily correspond to BSi brings the applicability of BSi as a proxy into question. Here, analysis of 15 samples reported as tephra-containing allows us to reject the hypothesis that tephra constituents produce an identical dissolution signal to that of BSi during alkaline extraction. We found that dissolution of volcanic glass shards is incomplete during alkaline dissolution. Simultaneous measurement of Al and Si used here during alkaline dissolution provides an important parameter to enable us to separate glass shard dissolution from dissolution of BSi and other Si-bearing fractions. The contribution from volcanic glass shards (between 0.2 and 4 wt % SiO 2 ), the main constituent of distal tephra, during alkaline dissolution can be substantial depending on the total Si Alk . Hence, soils and lake sediments with low BSi concentrations are highly sensitive to the additional dissolution from tephra constituents and its weathering products. We advise evaluation of the potential for volcanic or other non-biogenic contributions for all types of studies using BSi as an environmental proxy.

Silicon Mobilization in Soils: the Broader Impact of Land Use

“…NaOH chemical continuous extraction in parallel with Al showed that clay minerals and other pedogenic Si fractions display initial fast dissolution dynamics, very similar to amorphous silica. This was shown in aquatic samples [30] and in soil samples of different origins [25,28,31]. Other studies using different methods have also hinted that non-biogenic fractions occurring in the soil show similar dynamics to the biogenic Si fractions in alkaline environments [19,21,32].…”

Late-Holocene ultra-distal cryptotephra discoveries in varved sediments of Lake Żabińskie, NE Poland

“…The selected sediment sections were sampled in contiguous 1 cm intervals with a volume of 1 cm 3 and chemically pre-treated with 30% H 2 O 2 overnight in order to remove organic matter. Subsequently, a 7-10% HCl solution was added for a maximum of 1 hour to dissolve carbonates, and a 2M Na 2 CO 3 solution was used for 6-7 hours in a 70-80°C water bath to dissolve biogenic silica (diatoms) instead of more aggressive NaOH (Rose et al, 1996;Davies et al, 2003;Clymans et al, 2015). The remaining material was wet-sieved, and the 20-100 µm grain size fraction was transposed to transparent plastic boxes for microscopic inspection of volcanic glass.…”