Comparison of lake and land tephra records from the 2015 eruption of Calbuco volcano, Chile

Abstract

Tephra layers in lake sediment cores are regularly used for tephrostratigraphy as isochronous features for dating and recording eruption frequencies. However, their value for determining volcanic eruption size and style may be complicated by processes occurring in the lake that modify the thickness and grain size distributions of the deposit. To assess the reliability of data from lake cores, we compare tephra deposited on land during the 2015 eruption of Calbuco volcano in Chile to records in sediment cores from three lakes of different sizes that are known to have received primary fall deposits. In general, the thickness and granulometry of the deposit in lake cores and nearby terrestrial sections are very similar. As anticipated, however, cores sampled close to (here, within 300 m of) fluvial inflows were affected by sediment deposition from the lake's catchment; they differed from primary deposits not only in their greater thickness and organic content but also in poor sorting and lack of grading. Cores 850 m away from the inlet were not affected. We consider our results in the context of the particle settling regime as well as each lake's location, bathymetry and catchment area. We find that the particle settling regime is important in more distal settings where the ash particles are small and particle settling occurs in density plumes rather than as individual particles. We conclude that lake cores can be useful for physical volcanology providing consideration is given to eruption parameters such as particle size and mass flux, as well as lake features such as bathymetry and catchment area.

Sources of uncertainty in the Mazama isopachs and the implications for interpreting distal tephra deposits from large magnitude eruptions

“…To avoid the challenges of redeposited terrestrial tephra deposits, numerous studies have used lake sediments to Adams et al (1997) reconstruct volcanic histories as lake cores can provide excellent records of tephra stratigraphy (e.g. Lowe et al 1980;Hopkins et al 2015;Fontijn et al 2016;McNamara et al 2019). However, we find that lake cores, as with land sections, do not always provide reliable records of tephra thickness.…”

“…The effectiveness of transferring a tephra deposit to the stratigraphic record will depend on the background sedimentation, vegetation cover and slope angle (Cutler et al 2018;Dugmore et al 2018). For this reason, peat bogs and lakes are often favoured depositional settings for studying tephra stratigraphy (Watson et al 2016;McNamara et al 2019). The Mazama tephra is recorded not only in peat bogs (Harward and Youngberg 1969) and lakes (Long et al 2014) but also in dry land sections (Young 1990; this study) and aeolian sediments (Sweeney et al 2005).…”

“…Factors independent of the lake setting may also influence the preservation of tephra in lake cores. For example, the rate of tephra deposition and the quantity of accumulated tephra will influence how the tephra is transported from the lake surface to the lake bottom (Engwell et al 2014;McNamara et al 2019). The density and grain size of the tephra could be another external factor affecting tephra preservation in lakes.…”