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

Abstract

Abstract. Biogenic silica (BSi) is used as a proxy by soil scientists to identify biological effects on the Si cycle and by paleoecologists 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 nano-crystalline weathering products are hypothesized to overlap those of BSi, however, data to support this behavior are lacking. The understanding that the Si-bearing fractions that dissolve in alkaline media (SiAlk) do not necessarily correspond to BSi, question the applicability of BSi as a proxy. 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 shard (between 0.2–4 wt.% SiO2), the main constituent of distal tephra, during alkaline dissolution can be substantial depending on the total SiAlk. 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.

Comparing volcanic glass shards in unfertilised and fertilised Andisols derived from rhyolitic tephras, New Zealand: Evidence for accelerated weathering and implications for land management

“…An alternative mechanism for the formation of the silica polymorphs could be through the uptake and biocycling of Si by plants (Lowe, 1986;Henriet et al, 2008;Churchman and Lowe, 2012;Clymans et al, 2015). Farmer et al (2005) postulated that plant phytoliths provide a sink, hence short-term nutrient uplift, for Si from soil solutions during the growing season that is released back to solutions during winter and spring.…”

The Last Termination in the South Indian Ocean: A unique terrestrial record from Kerguelen Islands (49°S) situated within the Southern Hemisphere westerly belt

“…During measurement it became clear that the amount of dissolved Si was high in most samples. High dissolved Si concentration indicated a negligible contribution of mineral fractions and therefore the mean of the three subsamples (3, 4 and 5 h) were retained as the amount of BSi (Conley and Schelske, 2001 but see also Clymans et al, 2015).…”