Re-deposited cryptotephra layers in Holocene peats linked to anthropogenic activity

Abstract

Tephra layers can form useful age-equivalent stratigraphic markers for correlating palaeoenvironmental sequences and they provide information about the spatio-temporal nature of past volcanic ash fall events. The use of microscopic 'cryptotephra' layers has both increased the stratigraphic resolution of tephra sequences in proximal areas and extended the distal application of tephrochronology to regions of the world situated far from volcanoes. Effective tephrochronology requires the discrimination between in situ tephra deposited directly from volcanic plumes and tephras that have been remobilised since their initial deposition. We present tephrostratigraphic and glass chemistry data from two proximal peat profiles (one lowland, one upland) from the Shetland Islands, UK. Both profiles contain the Hekla-Selsund tephra (deposited c. 1800-1750 cal. bc), whilst the Hekla 4 ash (c. 2395-2279 cal. bc) is present in the upland record. Overlying the Hekla-Selsund tephra are a number of distinct peaks in tephra shard abundance. The geochemistry of these layers shows that they represent re-working of the Hekla 4 and Hekla-Selsund layers rather than primary air-fall deposits. Pollen analysis of the peat sequences illustrates that these re-deposited tephra layers are coincident with a rise in heather-dominated vegetation communities (heath and/ or moorland) and a subsequent intensification of burning in the landscape. We suggest that burning caused increased erosion of peats resulting in the remobilisation of tephra shards. The study demonstrates both the need for caution and the opportunities created when applying tephrochronologies in regions heavily affected by past human activity that contain both reworked tephra layers and in situ fallout.

The interpretative value of transformed tephra sequences

“…Cryptotephras may be particularly prone to movement within a profile, because of their generally small particle size and sparse occurrence (Dugmore and Newton, 1992), although experiments have observed little movement of tephra through peat (Payne and Gehrels, 2010). Tephra grain counts on contiguous samples are a particularly effective method for quantifying layer diffusion and remobilisation and can produce some novel insights, such as the potential impacts of peat burning (Swindles et al, 2013). Within-profile movements of tephra grains can result in cryptotephra layers losing their coherence as discrete horizons related to atmospheric fallout and their original stratigraphic contexts, and can present both interpretative challenges and opportunities.…”

A catalogue of major and trace element data for Icelandic Holocene silicic tephra layers

“…Tephra-derived glass shards have been identified in marine, fluvial, glacial and terrestrial deposits and correlated to Icelandic sources using geochemical analyses. Letters denote which system produced the tephras identified at each location: O: Öraefajökull; T-V: Torfajökull-Vatnaöldur; H: Hekla; A: Askja (Dugmore, 1989;Palais et al, 1991;Bennett et al, 1992;Blackford et al, 1992;Dugmore et al, 1995aDugmore et al, , 1996Hall et al, 1994a;van den Bogaard et al, 1994van den Bogaard et al, , 2002aCharman et al, 1995;Grönvold et al, 1995;Pilcher et al, 1995Pilcher et al, , 2005Oldfield et al, 1997;Wells et al, 1997;Zielinski et al, 1997;Boygle, 1998Boygle, , 2004Caseldine et al, 1998;Dugmore and Newton, 1998;Eiríksson et al, 2000Eiríksson et al, , 2004Wastegård, 2001Wastegård, , 2005Larsen et al, 2002;Van Den Bogaard and Schmincke, 2002;Zillén et al, 2002;Bergman et al, 2004;Chambers et al, 2004;Langdon and Barber, 2004;Plunkett et al, 2004;Davies et al, 2007;Kristjánsdóttir et al, 2007;Swindles et al, 2007Swindles et al, , 2010Swindles et al, , 2013Wastegård et al, 2008;Dörfler et al, 2012;Lawson et al, 2012;…”

Correlating tephras and cryptotephras using glass compositional analyses and numerical and statistical methods: Review and evaluation

“…Shane et al, 2008a;Ukstins Peate et al, 2008;Tomlinson et al, 2012;Westgate et al, 2013c;Pearce et al, 2014a;Abbott et al, 2016;Alloway et al, 2016) or from post-depositional mixing processes (e.g. Óladóttir et al, 2011;Tryon et al, 2011;Swindles et al, 2013;Cerovski-Darriau et al, 2014); and (iv) bulk analysis would generate anomalous values for various elements where glass had undergone postdepositional chemical alteration, such as palagonitization or zeolitization (Thorseth et al, 1991;Kraus and Kurbatov, 2010;McHenry et al, 2011;Churchman and Lowe, 2012), or vesicular infilling by pyritization in marine settings (Nelson et al, 1985;Hunt et al, 1995).…”