An evaluation of two cryptotephra quantification methods applied on lacustrine sediments with distant Laacher See tephra fallout

Abstract

Detailed quantification of volcanic glass is crucial for improving the resolution of paleoenvironmental reconstructions and facilitating more accurate comparisons between distant sedimentary cryptotephra records. Here we present and evaluate two methods for the quantification of cryptotephra, shown on lake sediments from a site with distant Laacher See tephra fallout. Our methods initiate with delineating the extent and distribution of the cryptotephra layer within the sediments, accomplished through the integration of X-ray fluorescence (XRF) and computed tomography (medical- and μ-CT). The first quantification method involves the well-established process of shard extraction through stepwise density separation, followed by improved and statistically evaluated quantification introducing a new standardized marker. While the method itself is used widely for many years among cryptotephra researchers, we demonstrate how the new marker improves its precision for cryptotephra quantification, providing a robust, straightforward laboratory-based technique. Additionally, we introduce an innovative, software-based method that combines an SEM-based automated mineralogy analysis on thin sections with customized image analysis, which allows to study the area fraction of the glass phase, its depth-dependent variation, particle concentration with a focus on clustering behavior, depth-dependent particle count, total particle count, and particle size distribution within the glass phase. The significance of both methods lies in the efficiency and precision of cryptotephra quantification, enabling a deeper understanding of shard concentration and distribution. This study emphasizes the methodological innovations, offering improved tools for cryptotephra quantification, without focusing on detailed application-based analyses.