Late Pleistocene and Holocene tephrostratigraphy of interior Alaska and Yukon: Key beds and chronologies over the past 30,000 years

Glass composition-based correlations of volcanic ash (tephra) traditionally rely on extensive manual plotting. Many previous statistical methods for testing correlations are limited by using geochemical means, masking diagnostic variability. We suggest that machine learning classifiers can expedite correlation, quickly narrowing the list of likely candidates using well-trained models. Eruptives from Alaska's Aleutian Arc-Alaska Peninsula and Wrangell volcanic field were used as a test environment for 11 supervised classification algorithms, trained on nearly 2000 electron probe microanalysis measurements of glass major oxides, representing 10 volcanic sources. Artificial neural networks and random forests were consistently among the top-performing learners (accuracy and kappa > 0.96). Their combination as an average ensemble effectively improves their performance. Using this combined model on tephras from Eklutna Lake, south-central Alaska, showed that predictions match traditional methods and can speed correlation. Although classifiers are useful tools, they should aid expert analysis, not replace it. The Eklutna Lake tephras are mostly from Redoubt Volcano. Besides tephras from known Holocene-active sources, Holocene tephra geochemically consistent with Pleistocene Emmons Lake Volcanic Center (Dawson tephra), but from a yet unknown source, is evident. These tephras are mostly anchored by a highly resolved varved chronology and represent new important regional stratigraphic markers.

Section: Methodsmentioning

confidence: 99%

Machine learning classifiers for attributing tephra to source volcanoes: an evaluation of methods for Alaska tephras

Bolton

Jensen

Wallace

et al. 2019

“…Glass major–minor element (wt%) data used to generate Fig. include: (B‐Tm) this study; Hughes et al ., (); McLean et al ., (); (WRAe), Pyne‐O'Donnell et al ., (); Jensen et al ., (); Davies et al ., (); (Saksunarvatn ash) Wastegård et al ., (). Figure is available in colour at wileyonlinelibrary.com.…”

Section: Methodsmentioning

confidence: 99%

Effects of the peat acid digestion protocol on geochemically and morphologically diverse tephra deposits

Monteath

Teuten

Hughes

et al. 2019

Tephra shards for electron probe microanalysis are most efficiently extracted from peat using acid digestion, which removes organic material that hinders density separation methods. However, strong acids are known to alter glass chemical compositions, and several studies have examined how acid digestion affects rhyolitic volcanic glass. The focus on rhyolitic tephra in these studies leaves considerable uncertainty, as the dissolution rates of natural glasses (including tephra) are determined by the chemical composition and surface area/volume ratio, both of which vary in tephra deposits. Here, we use duplicate samples of basaltic, trachydacitic and rhyolitic tephra to examine physical and geochemical alteration following acid digestion. Scanning electron microscope imagery reveals no discernible degradation of glass surfaces, and electron probe microanalysis results from duplicate samples are statistically indistinguishable. These findings suggest the acid digestion protocol for organic peats does not significantly alter glass geochemistry regardless of shard morphologies or geochemical compositions.

“…The dating of the Aniakchak CFE II tephra has proved to be contentious, as the age estimate based on radiocarbon dating conflicts with the Greenland ice‐core age (Davies et al ., ). However, Pearce et al .…”

Section: Discussionmentioning

confidence: 98%

Traces of volcanic ash from the Mediterranean, Iceland and North America in a Holocene record from south Wales, UK

Jones

Davies

Staff

et al. 2019

A tephra record is presented for a sediment core from Llyn Llech Owain, south Wales, spanning the early‐ to mid‐Holocene. Seven cryptotephra deposits are discovered with three thought to correlate with known eruptions and the remaining four considered to represent previously undocumented events. One deposit is suggested to correlate with the ~6.9 cal ka bp Lairg A tephra from Iceland, whereas more distant sources are proposed as the origin for two of the tephra deposits. A peak of colourless shards in early‐Holocene sediments is thought to tentatively correlate with the ~9.6 cal ka bp Fondi di Baia tephra (Campi Flegrei) and a second cryptotephra is tentatively correlated with the ~3.6 cal ka bp Aniakchak (CFE) II tephra (Alaska). The Fondi di Baia tephra has never been recorded beyond proximal sites and its discovery in south Wales significantly extends the geographical distribution of ash from this eruption. The remaining four cryptotephra deposits are yet to be correlated with known eruptions, demonstrating that our current understanding of widespread tephra deposits is incomplete. This new tephra record highlights the potential for sites at more southerly and westerly locations in northwest Europe to act as repositories for ash from several volcanic regions.