Jökull - 01.01.2012, Síða 44

Dugmore and Newton
tributions, despite their large scale and extensive distal
spread (Thórarinsson, 1956, 1981a). In general, the
years of their formation (1875, 1362 and c. 2,250 BC)
have somewhat less general historical, ecological or
archaeological interest than the Landnám period, al-
though the eruptions themselves are of volcanological
significance.
Although the context and the research questions
being asked will ultimately determine the immedi-
ate worth of any tephra layer, those that are identi-
fied with less certainty, not effectively correlated or
inaccurately dated will generally have less individ-
ual value. Tephrochronology can, however, be far
more than the utilisation of a limited number of out-
standing isochrons, and the sum of parts combined
may be much greater than the tally of individual com-
ponents. Collective worth can be developed in two
ways; firstly through the local utilisation of whole
tephra stratigraphies, including poorly provenanced
layers, re-deposited tephras, patchy deposits, notable
absences and cryptotephras. Secondly, environmental
data may be extracted from the form and local distri-
bution of tephra layers themselves. This development
of ’added value’ to tephrochronology is most straight-
forward when dealing with visible traces of multiple
tephra deposits that can be mapped in the field.
In NW Europe, where Icelandic tephras are
mostly present as cryptotephra deposits (and thus not
visible in the field), tephrochronology has tended to
focus on a limited number of key marker horizons
(e.g. Dugmore et al., 1995; Pilcher et al., 1996; Tur-
ney et al., 1997; van den Bogaard and Schmincke,
2002; Wastegård and Davies, 2009). This is the log-
ical development of tephra studies that began with
the convincing demonstration that identifiable tephra
deposits were present, even though they were hid-
den from view (Persson, 1971; Thórarinsson, 1981a;
Dugmore, 1989a). The significant effort required
to isolate and identify cryptotephras has meant that
their principal contribution has been to provide a lim-
ited number of unambiguous and key dates within
palaeoenvironmental sequences. While this has pro-
duced very effective and high profile developments
of tephrochronology, such as the identification of
the Vedde and Saksunarvatn tephras within the last
glacial-interglacial transition of the British Isles (Tur-
ney et al., 2006), it represents a fraction of the poten-
tial richness of interpretation made possible through
Thórarinsson’s original vision (Thórarinsson, 1944).
To explore the development of this vision in more de-
tail, this paper will focus on geomorphological appli-
cations in the birthplace of modern tephrochronology,
and, we hope, pay tribute to Thórarinsson’s enduring
scientific legacy.
When isochronous tephra layers do not necessarily
define isochronous surfaces
When a tephra layer is formed by in-situ fallout
from an eruption cloud, the contact surface between
the tephra and the underlying landscape forms an
isochron (Figure 2). There are times, however, when
this temporal relationship between the tephra and the
landscape it covers may not be that simple. It is the
surface that is the isochron; the materials that form
the surface may or may not be of the same age. For
example, a landscape formed by patches of eroding
soil, river terraces and exposed glacial sediments will
have a surface that is a mosaic of different aged mate-
rials, but has a common exposure at a moment in time
(i.e. the moment the tephra fell).
This provides the single biggest contrast between
the landscape applications of tephrochronology in ge-
omorphology and archaeology and its more restricted
use in dating sedimentary sequences, such as lake sed-
iments or peat cores. We have established that the
surface on which a tephra falls may be composed of
materials of quite different ages, but the surface ex-
posed to tephra fall is isochronous. This can, however,
change after the deposition of the tephra layer, even if
the stratigraphic location of the tephra within the sedi-
ment sequence remains unchanged. A common exam-
ple of this is cryoturbation of near-surface sediments,
leading to frost hummock (thufur) formation (Figure
3). In these circumstances, the tephra layers affected
by the hummock formation still define isochrons, but
it may be that neither the underlying materials in con-
tact with an individual tephra layer, nor the surface
defined by the base of the tephra layer are of a similar
age; after disturbance the age of the tephra will not be
the same as the cryoturbation structures they define.
42 JÖKULL No. 62, 2012