VSF 2010 Report - Nabo

Previous tephra studies in Vestfirðir and at Vatnsfjörður
There has been a limited amount of research into the tephrostratigraphy of the Vestfirðir
region. A few visible tephra layers have been identified in soils in the region; the majority of
those described relate to the early Holocene (for example, Andrews et al., 2002). Late
Holocene tephra layers originating from the Snæfellsjökull volcanic system have been found
in soils in the southern part of Vestfirðir, but these date to before the historical settlement of
Iceland (Sigurgeirsson, 2006). Written sources document three tephra fallout events in
northwest Iceland: the Hekla eruptions of 1693 and 1766 and the Katla eruption of 1721
(Thorarinsson, 1955a; Thorarinsson, 1967). Contemporary descriptions record that, at one
farm in the Vatnsfjörður region during the Hekla 1693 eruption, so much ash fell that “in the
time needed to write a short letter”, traceable footprints were left on the ground. Like many
Icelandic eruptions, the events often went on for months, e.g., the Hekla 1693 eruption began
on 13 February and ended sometime in the autumn (Thorarinsson, 1967). Sigurgeirsson
suggests that 90% of the tephra fell within the first few hours of the eruption (2006: 79).
At the Vatnsfjörður site, the stratigraphy throughout the Farm Mound, as well as
above the Viking Age area’s human occupation layers and in several test trenches, contains a
number of visible tephra layers. However, it was unknown until this study if they represented
ash fall from the same or multiple eruptions. Many of these layers have a very similar
appearance in the stratigraphy: a layer that is ~1-3cm thick, grey-green in colour, and
composed uniformly of fine grains. During the 2006 field season similar tephra was sampled
near the site and geochemically identified to be from the Hekla system, and most likely from
the 1693 eruption (Sigurgeirsson, 2006). The SiO 2 content of the glass shards varied from
58.5-61.2%, meaning they came from a volcano with an intermediate composition.
Methodology
Sample Collection
Four possible tephra samples were collected for geochemical study from open stratigraphic
sections by various excavators during the 2009 field season:
1. Sample labelled VSF09 Viking Age (OxT-5163) is of the tephra layer visible in
the section exposed on the edge of excavation area 14.
2. Sample labelled VSF09 Area 32 (OxT-5164) was taken from the section exposed
on the edge of excavation area 32, which is next to the Viking Age house (the
skáli).
3. Sample labelled VSF09 Farm Mound (OxT-5166) was taken from within a section
of turf in a turf collapse layer in the Farm Mound.
4. It was thought that the sample labelled VSF09 7157 (OxT-5165) may have
represented a different ash fall event from the others. It was from a black sand that
was part of the turf that formed the walls of structure 7 (context 7157). The turf
was reddish and may have been from a bog off-site, so there was some question
about whether it was in fact tephra, or a black sand.
Laboratory Methods
The samples from Vatnsfjörður were treated as microtephra, despite being from a visible
tephra layer. This was for two reasons: first, because the grain size of the ash was so small,
210