Jökull - 01.01.2012, Síða 46

Dugmore and Newton
ered into the ocean and dispersed by ocean currents
to be deposited along contemporaneous high water
lines and storm beaches around the North Atlantic.
Some pumice survives as linear deposits on the stran-
dlines of Svalbard (e.g. Salvigsen, 1984) and Norway
(e.g. Undås, 1942; Newton, 1999); in Scotland, rising
sea levels have overwhelmed many of the original de-
posits, but pumice is a frequent find in archaeological
sites from the Mesolithic to modern times (e.g. New-
ton, 2001; Newton and Dugmore, 2003).
In all of these cases it is important to recognise
that isochron status may only apply to undisturbed pri-
mary deposits of tephra - that is material produced,
distributed and deposited in a very short period of
time, and thus effectively defining that moment of
time. It may also apply to the surface the tephra over-
lies and the sediment in direct contact with the tephra,
but even when a tephra does not define a surface the
same age as the tephra-forming eruption, this still pro-
vides a valuable environmental record of process; for
example, the creation of frost hummocks, glacier flow
or ocean circulation.
DEVELOPING THE USE OF
ISOCHRONS
The NW European focus on the identification of
isochrons within palaeoenvironmental records has
parallels with the use of tephrochronology in Ice-
landic glacial geomorphology (e.g. Thórarinsson,
1956; Dugmore, 1989b; Kirkbride and Dugmore,
2001a, 2008). A common theme between NW Europe
and Iceland is the identification of isochrons in rela-
tion to environmental changes and the correlation of
key marker horizons between different environmen-
tal records. The greater abundance of tephra layers
in Iceland means that more nuanced interpretations
are possible, but these may require an approach to
tephrochronology and a use of tephra deposits which
is quite different to those employed with the key in-
ternational marker horizons (e.g. Kirkbride and Dug-
more, 2001b, 2005, 2006).
In most Icelandic stratigraphic sequences formed
within the last 1200 years, all of the tephra layers
present may be identified and dated to very high lev-
els of precision. This capability is built on Thórarins-
son’s pioneering work and in particular the clear un-
derstanding he developed of the post-Settlement erup-
tions of Öræfajökull, Hekla and Katla (Thórarinsson,
1958, 1967, 1975, 1980). In some sequences, how-
ever, the patchy occurrence of tephra layers towards
the margins of their distributions can introduce am-
biguity. In the Markarfljót valley north of Eyjafjalla-
jökull, for example, the 18th century stratigraphy can
include thin black tephra layers formed by Katla erup-
tions in 1755 and 1721 (Larsen, 2000, Kirkbride and
Dugmore 2008) (Figure 2a). Where both tephra lay-
ers are present, identification is unambiguous. Where
only one tephra layer is present, it could be from ei-
ther of the 18th century Katla eruptions, because they
have very similar major and minor element composi-
tions. Moving to the west of the Markarfljót valley
certainty is re-gained because only the 1755 fallout is
present around Stóramörk.
Within these Markarfljót valley sequences a se-
cure 19th century isochron is formed by the silicic
fallout from the 1821–1823 eruption of Eyjafjalla-
jökull, and effective 16th century markers are formed
by the chemically distinct tephra from Hekla 1597,
and the couplet formed by the tephras from Hekla
1510 and Katla c. 1500 (Haraldsson, 1981). Thus,
while some parts of a stratigraphic record may be se-
cure other, intervening parts may vary in the confi-
dence of their identification, chronological accuracy
or precision.
Trace element data can go a long way to re-
solve tephra layer identification (e.g. Óladóttir et al.,
2011a), but there are still circumstances where current
knowledge of trace elements abundances is unable to
resolve tephra identifications (e.g. the eight tephra
layers with ambiguous plots reported by Óladóttir et
al., 2011a). A similar problem of chemical equifinal-
ity exists with other layers of quite different chemical
composition, such as the Hekla silicic tephras from
1510 and 1947 (Larsen et al., 1999), but again strati-
graphic associations are definitive; for example the
combination of Hekla 1510 and Katla c. 1500 is diag-
nostic and has been used in field mapping (Haralds-
son, 1981; Dugmore et al., 2009; Figures 2, 3).
44 JÖKULL No. 62, 2012