Jökull - 01.01.2012, Page 42

Dugmore and Newton
BUILDING BLOCKS: TEPHRA LAYERS
AND ISOCHRONS
There is a hierarchy of importance amongst individ-
ual tephra deposits when it comes to their use in
tephrochronology. At the pinnacle sit the tephra lay-
ers that have four key characteristics; extensive spatial
distribution in a short period of time, distinctive prop-
erties that are well-characterised, good independent
dating and an occurrence at times of widespread in-
terest. In Iceland, this group of tephras is epitomised
by the Vatnaöldur c. 870 AD tephra also known as
the Settlement Layer (a tephra that marks the settle-
ment of Iceland by the Norse (Larsen, 1984)), and
includes major silicic tephras of Hekla (e.g. Hekla
4), Öræfajökull 1362 AD and Askja 1875 AD (e.g.
Thórarinsson, 1958, 1967; Larsen and Thórarinsson,
1977; Larsen and Eiríksson, 2008). While all of these
tephras are well-dated, distinctive in isolation and
have been described in detail, they vary in their use in
archaeology and palaeoenvironmental studies because
of contrasting spatial distributions and their differing
relevance to contemporary research agendas. Today
the Settlement Layer (also referred to as the Landnám
tephra after the Norse term for ’land-taking’) is ar-
guably the most significant marker horizon in Iceland,
because of both its stratigraphic relationship with ini-
tial phase of human settlement and its widespread dis-
tribution across the island (and beyond) (Figures 1
and 2b). These characteristics combined with ice-
core dating (Grönvold et al., 1995; Zeilinski et al.,
1997) create a truly iconic marker horizon. The con-
trasting status of tephras such as Hekla 4, Öræfajökull
1362 AD and Askja 1875 AD comes from a combina-
tion of their age and distribution: Hekla 4 occurs at a
time of widespread archaeological and environmental
change across the British Isles (e.g. Gear and Huntley,
1991; Blackford et al., 1992), covers much of Iceland
(Larsen and Thórarinsson, 1977), but falls when the
island is uninhabited and forms an isochron of rather
more limited Icelandic interest compared to Vatna-
öldur c. 870 AD. While Hekla 4 can be found across
much of Iceland, Öræfajökull 1362 AD and Askja
1875 AD have comparatively restricted Icelandic dis-
Figure 1. Key locations mentioned in the text. The limits of 1 cm and 10 cm thick fallout from the Katla erup-
tion of c. 1357 AD are taken from Einarsson et al. 1980. – Staðsetning öskulagasniða á 2. og 3. mynd ásamt
þykktargeira öskufalls úr Kötlu 1357 samkvæmt Þ. Einarssyni og fl. 1980.
40 JÖKULL No. 62, 2012