Jökull - 01.01.2012, Síða 47

The use of tephrochronology in geomorphology
Across the soil-covered lands of Iceland, the po-
tential for the repeated analysis of the local tephra
sequence in many different stratigraphic sections
makes a vitally important contribution to the rigor of
tephrochronological applications. Not every soil pro-
file will contain every tephra layer to have fallen in a
region; so ideally profiles are added to the analysis un-
til it can be shown that adding more profiles does not
add more new primary tephra deposits; at that point
it is possible to be confident that all possible tephras
have been identified and the potential omissions from
any individual profile can be established.
Lake cores can preserve a much more detailed
tephra record than surrounding terrestrial deposits
(e.g. Björck et al., 1992; Haflidason et al., 1992;
Caseldine et al., 2003; Hardardóttir et al., 2009).
There can, however, be significant stratigraphical
variation across a lake bed, sediments may be re-
worked by currents, the record affected by earth-
quakes and tephra deposits may be so thick that suc-
cessful coring represents a real challenge (Boygle,
1999). While lakes can preserve excellent multiple
proxy indicators of environmental conditions and a
homogenised record of catchment processes, they are
one step removed from the landscapes that people in-
teract with on a daily basis. Within a catchment-scale
lake record, geographical patterns of the environment
at a moment in time cannot be resolved with accuracy
and it is not possible to differentiate between differ-
ent in-catchment landholdings, or components of the
landscape (e.g. Mairs et al., 2006). It is notable that
recent key works on the volcanic histories of Katla,
Grímsvötn, Bárdarbunga and Kverkfjöll have utilised
terrestrial sites (Óladóttir et al., 2005, 2011a, 2011b).
Isochrons and primary tephra deposits
To be confident that all significant episodes of vol-
canic fallout across a specific area have been identi-
fied, it is necessary to clearly identify primary tephra
deposits, remobilised layers that still define isochrons
and reworked tephra that form time-transgressive de-
posits. This is not always a straight-forward task,
especially when seeking to utilise tephrochronology
in fields such as geomorphology, environmental re-
construction and archaeology. In these applications,
the stratigraphy under consideration is often com-
plex, present in short vertical sequences and spatially
fragmented; tephra layers are often intercalated with
many other types of deposit, from cultural materials
such as midden and artificial structures to natural fea-
tures such as fluvial deposits and glacial till. Tephra
layers often lie within soils formed from aeolian sed-
iments, but they may also lie within very different
materials such as cultural deposits or diamictons.
Complex sequences produced by a shifting interplay
of episodes of deposition, transport and erosion may
contain both tephra deposits that have been disturbed
in situ, yet still define an isochronous horizon, and
tephra deposits that have been remobilised, lost their
isochronous status and yet appear to be primary de-
posits because of their lack of exotic admixtures, lim-
ited grain modification and the presence of ambiguous
sedimentary structures. Where there has been a lim-
ited or non-existent contemporaneous movement of
other sediments, redistributed deposits of tephra may
be essentially similar in character to those of primary
undisturbed fallout. The presence of exotic materials
or distinctive sedimentary structures can be definitive
evidence of remobilisation and re-working of tephra
(Óladóttir et al., 2011a); but their absence does not
necessarily mean that there has been no mobilisa-
tion and post-eruption thickening of the tephra layer.
Likewise, reworked layers may have both sharp upper
and lower contact and laterally continuous sedimen-
tary structures. This may, for example, happen when
tephra layers are re-deposited across snow beds – and
so be a key concern when considering upland areas
or winter eruptions. In these circumstances, the key
field observations of tephra layer colour and contacts,
grain size and shape, and layer thickness identified
by Óladóttir et al., (2011a) can be usefully expanded
to include an assessment of the spatial distribution
and regional stratigraphic patterns. Detailed mapping
of each tephra layer in relation to the geomorphol-
ogy, probable contemporaneous vegetation cover and
land use can show the degree to which modification
is likely, or not. This can effectively identify both
isochrons defined by internally modified layers and
tephra deposits that may be uncontaminated.
JÖKULL No. 62, 2012 45