Detection of middle to late Holocene Icelandic cryptotephras in the Netherlands: tephra versus biogenic silica

Abstract

Every volcano erupts ash that is characteristic for that volcano. When tephra is produced, prevailing winds can transport the tephra over large distances, where they are then deposited due to gravitational and/or precipitational fallout. The goal of this paper is to detect and chemically analyse middle to late Holocene cryptotephras in the Netherlands and trace these tephras back to their source. The Rheinberg and Zijderveld cores are sampled and a separation method based on heavy liquid separation was adapted for use on clay-rich material. Instead of ashing the samples in a furnace, which would result in producing bricks from the clay, H2O2 is used to dissolve all the organic material. The material is then subjected to heavy liquid separation. The particles remaining were then analysed with the optical microscope and a tephra shard count was done. The particles were deemed tephra based on their morphology and optical properties. However, after chemical analysis using the SEM, all of these supposed tephra particles turned out to be biogenic silica or mica. No Icelandic tephras were present. The biogenic silica was mistaken to be tephra due to its tephra-like morphology and the same optical properties. The absence of tephras can be explained in three possible ways: 1) the ashcloud did not extend as far as the Rheinberg and Zijderveld localities, 2) the deposition occurred by wet fallout and since there was no precipitation in the Netherlands there was no tephra deposition and 3) the localities were not suitable for deposition of tephra due to the possible presence of running water.