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Biogeosciences An interactive open-access journal of the European Geosciences Union
Biogeosciences, 12, 3789-3804, 2015
https://doi.org/10.5194/bg-12-3789-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
22 Jun 2015
The contribution of tephra constituents during biogenic silica determination: implications for soil and palaeoecological studies
W. Clymans1, L. Barão2, N. Van der Putten1, S. Wastegård3, G. Gísladóttir4, S. Björck1, B. Moine5, E. Struyf2, and D. J. Conley1 1Department of Geology, Lund University, Sölvegatan 12, 22362 Lund, Sweden
2Department of Biology, Ecosystem Management, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium
3Department of Physical Geography, Stockholm University, 10691 Stockholm, Sweden
4Institute of Life and Environmental Sciences, and the Nordic Volcanological Center, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland
5Université de Lyon, Magmas et Volcans (UBP-UJM-CNRS-IRD), 23 rue Dr. P. Michelon, 42023 Saint-Etienne, France
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 (SiAlk) 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 % 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.

Citation: Clymans, W., Barão, L., Van der Putten, N., Wastegård, S., Gísladóttir, G., Björck, S., Moine, B., Struyf, E., and Conley, D. J.: The contribution of tephra constituents during biogenic silica determination: implications for soil and palaeoecological studies, Biogeosciences, 12, 3789-3804, https://doi.org/10.5194/bg-12-3789-2015, 2015.
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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. We show the presence of tephra constituents can make measurements erroneous at low BSi concentrations, with repercussions for soil and palaeoecological studies. However, we also show that glass shards do not produce an identical dissolution signal to that of BSi, meaning they can be distinguished with appropriate experimental setups.
Biogenic silica (BSi) is used as a proxy by soil scientists to identify biological effects on...
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