Jökull - 01.01.2012, Blaðsíða 49

The use of tephrochronology in geomorphology
same source, it is most unlikely that the fallout was of
the same scale and blown in the same direction and
distance for every eruption; as a result the thicknesses
of the layers will change in different ways across the
landscape. Add layers from other volcanic sources
and the relative variations in tephra thicknesses across
a landscape will become more pronounced. Over very
short distances, however, fallout will remain roughly
similar and the thickness within each profile will be
exaggerated or inhibited depending on the geomor-
phological setting, and the relative thicknesses will
show common patterns. A profile may, for exam-
ple, have a short sequence of basaltic layers that are
in order thin, thick, thicker, thin and thick; although
the absolute thicknesses will change with variations
in slope position and vegetation, over short distances
the ratio of thicknesses is likely to remain similar. As
a result, the barcode they define may be used in local
correlations even when the provenance of the tephra
is uncertain.
Pre-Landnám rates of non-tephra aeolian sedi-
ment accumulation are much lower, than those of the
recent past, so there is less stratigraphic separation
between individual tephra layers. Non-tephra aeolian
sediment accumulation rates (SeAR) are greater in re-
cent times because of the soil erosion triggered by hu-
man impacts - a key point first proved by Thórarins-
son (1961) in an early application of tephrochronol-
ogy. In southern Iceland, the post-Landnám SeAR
generally increase by more than an order of magni-
tude, but it does have great local variation (Dugmore
and Buckland, 1991; Dugmore et al., 2000, 2009;
Streeter et al., 2012). This means that closely-spaced
pre-Landnám eruptions can produce tephra layers that
have little, if any, intervening aeolian sediments. In
addition, lower aggregation rates in pre-Settlement
stratigraphies allow pedological processes to gener-
ate weathered profiles, a phenomenon that is rare in
historical times post-1500, because of the very rapid
rates of profile aggradation.
In contrast to aeolian sediment sequences and
minerogenic soils, peat sequences may contain far
clearer pre-Settlement tephra records than they do
now - and for essentially the same reason. Low lev-
els of aeolian sediment flux are associated with the
growth of peats with very high organic contents. In
favoured areas in pre-Settlement time these peats did
grow rapidly, and so provided clear stratigraphic sep-
aration for tephra layers. In recent centuries, peats
have been affected by both high levels of non-tephra
minerogenic input derived from soil erosion and the
effects of artificial drainage, both of which make the
identification of tephra layers more difficult. Phys-
ical contrasts between the tephra and the surround-
ing materials are reduced, while episodic waterlog-
ging can result in profile weathering and associated
colour changes.
Weathering can change the macroscopic features
of a tephra layer, most noticeably by turning the
colour of basaltic layers from black into shades of
red/brown and creating consolidated, indurated lay-
ers that are more resistant to erosion than the sur-
rounding sediment. Profile weathering that transforms
tephra layers can be distinguished from the red/brown
colouring of dark basaltic tephra caused by oxida-
tion during eruptions, because profile weathering af-
fects both the tephra layers and the intervening sedi-
ments. Importantly, the stratigraphic patterns, defined
by stratigraphic order, layer thicknesses and particle
sizes, remain unaltered, and so even when there is un-
certainty about provenance, the ’barcode’ defined by
the stratigraphy can still be used with confidence.
Rapid sediment accumulation in the surviving ar-
eas of vegetation cover mean that pre-Landnám layers
frequently lie below the depths easily reached by pits
manually-dug from the surface. As a result, access
to early Holocene sections usually relies on natural
sections such as eroding river banks and gully walls
(e.g. Óladóttir, 2011b). Naturally eroding sections
within post-Landnám sediments will tend to form
near-vertical faces as the greatest resistance to erosion
is provided by the surface vegetation; in pre-Landnám
sequences the presence of more resistant layers mean
that, in the absence of erosion focussed at the base,
sloping exposures will tend to form. This combina-
tion of more and less easily eroded sediment gives
rofabards (eroding slopes of soil) their characteristic
concavo-convex profile (Arnalds, 2000).
Soil cover in early Holocene times was patchy
and became more extensive until the onset of post-
JÖKULL No. 62, 2012 47