Pechora Lake, northern Kamchatka
(photo: K. Bennett), located at 42 m a.s.l.
and 200 km from the nearest active volcanic
system, Shiveluch
SEM of a tephra shard from Pechora lake
Tephra concentration in one section of the
Pechora Lake sediment. Only the major
spike in tephra stands out as a visible tephra layer.
Advances in Kamchatkan tephrochronology: age-modelling and micro-tephrochronology studies at QUB
Located on the northwestern rim of the Pacific's Ring of Fire, Kamchatka is one of the most actively volcanic regions in the world. Sedimentary sequences throughout the region frequently feature distinct beds of tephra produced by explosive eruptions that range in thickness from several millimetres to several meters. As key-marker horizons, these tephras have aided the dating of palaeoenvironmental change on the Peninsula and centimetre-thick ash beds have been identified in sequences from the Sea of Okhotsk and the Russian mainland. Research has until now focused on visible tephra beds, characterised mainly by physical properties and bulk chemistries, and dated using bulk samples.
Researchers in GAP have been collaborating with Russian volcanologist Dr Vera Ponomareva to further the understanding of Holocene volcanism in Kamchatka. Dr Maarten Blaauw is enhancing the precision and reliability of available age estimates for Kamchatkan tephras, through compiling hundreds of published radiocarbon dates using state-of-the-art statistical methods.
Dr Gill Plunkett and Dr Sarah Coulter have initiated the first search for micro-tephra horizons in Kamchatkan sedimentary deposits through the analysis of a sediment from Pechora Lake. The lake is one of four Kamchatkan sites (three lakes, one bog) that were cored in 2005 as part of an international palaeoenvironmental project supported by the Swedish Polar Secretariat and the Swedish Research Council and led by Prof. Keith Bennett. In the Pechora Lake sequence, seven visible tephras can be seen, and ITRAX core scanning by postgraduate Andrea Klimaschewski indicates at least 11 more such horizons. Microscopic examination of loss-on-ignition residues from two 0.5 m-long sections of the core, however, shows a consistently high background of tephra that is not otherwise evident in the sequence. Electron microprobe analysis of tephra from the visible layers shows that they originate mainly from Shiveluch, the nearest volcanic system located 200 km to the southwest of the lake, but ash from eruptions of Khangar (central Kamchatka) and Ksudach (southern Kamchatka) is also present. Results from the microtephras have so far indicated Shiveluch as the source. These results illustrate that microtephra horizons are indeed present in Kamchatkan sequences. Further work is needed to establish whether the continuous micro-tephra record is due to many, frequent eruptions in the North Pacific region, if there are taphonomic reasons for its vertical distribution and if this is typical of lake sequences in Kamchatka.
