The stubborn silica: Why do diatom frustules withstand a sequential leaching protocol?

Description

Biogenic silica (bSi) in lake and marine sediments is commonly quantified using sequential alkaline extraction protocols (DeMaster, 1981; Conley and Schelske, 2001). This is based on the faster dissolution rate of highly reactive (amorphous) bSi compared to slowly reacting (semi)crystalline silicon-bearing phases (e.g., feldspar, quartz and phyllosilicates) of the sediment. The standard method involves a 0.1M Na2CO3 solution (pH ~ 11) at 85°C with multiple samplings over 1-3-hour intervals to monitor potential clay mineral dissolution. This method was applied to sediments from the (hydro)thermally affected Goose Lake in the Lower Geyser Basin of Yellowstone National Park (Wyoming, USA), but it was proved to be ineffective.

The well-preserved diatom frustules from sediments with bSi ranging from 26-68wt%
remained undissolved even after 5h hours of sequential leaching. Scanning electron
microscopy and energy-dispersive X-ray spectroscopy analyses revealed no significant
compositional changes in the diatom frustules.

The lack of bSi dissolution can be explained by:
1. Early Diagenetic Effects: Early diagenetic processes, such as re-crystallization of
biogenic opal, may have altered the bSi frustules, making them more resistant to
alkaline extraction.
2. Sequential Leaching Effects: Bulk sediment composition retards bSi dissolution due
to pH buffering and diatom coating phenomena.
3. Species dependency: Mineralogy and chemistry of diatom species (Al/Si ratio)
affecting the dissolution behaviour/kinetics.

This resistance of diatom frustules to dissolution highlights a potential limitation of the current bSi quantification method in complex sedimentary environments like
(hydro)thermally affected lakes. Further research is needed to understand the factors
influencing bSi preservation in such settings.

Period	29 May 2024 → 31 May 2024
Event title	Isotopen in Biogenic Silica
Event type	Conference
Location	Louvain-La-Neuve, BelgiumShow on map