Jökull - 01.01.2012, Síða 51

The use of tephrochronology in geomorphology
3-D ENVIRONMENTAL
RECONSTRUCTION
A novel use of tephrochronology in Iceland has been
the detailed mapping of the spatial variation of sedi-
ment accumulation. This has been facilitated by the
predominantly aeolian origin of soils and the gener-
ally high rates of sediment accumulation that have
resulted in rapid (decadal) formation of stratigraphic
units that may be mapped in the field (e.g. Streeter et
al., 2012).
Following on from Thórarinsson’s pioneering
studies of soil erosion (e.g. Thórarinsson, 1961), oth-
ers have used the presence of multiple isochrons to
define the variation of soil accumulation across land-
scapes developed during discrete periods of time (e.g.
Dugmore and Buckland, 1991; Dugmore and Ersk-
ine, 1994; Dugmore et al., 2000, 2009; Simpson et
al., 2001; Ólafsdóttir and Guðmundsson, 2002; Mairs
et al., 2006; Streeter et al., 2012). Spatial variation
of sediment accumulation has been used to infer the
scale of first settlement impacts, vegetation change,
land use and climate change. A key question is how
to best combine data from multiple profiles. Useful
patterns have been revealed through the combination
of profiles in similar landscape units (e.g. Mairs et al.,
2006). When large numbers (>30) of high resolution
measurements (within 1mm) are made in individual
profiles, then in-profile variation becomes a powerful
indicator of change (Streeter et al., 2012).
CONCLUSIONS
The most common use of tephra layers is to define
isochrons and use them to date environmental records.
Of all the Holocene tephras produced in Iceland, few
form ’classic’ isochrons with four key characteris-
tics; a known extensive spatial distribution, distinctive
properties that are well-characterised, good indepen-
dent dating and an occurrence at times of widespread
interest.
The most detailed applications of tephrochronol-
ogy, and those with the greatest practical utility in
geomorphology, involve the use of all tephras within
a deposit, including unprovenanced and remobilised
units; the effective use of ’total tephrochronology’
requires multiple stratigraphic sections and the clear
identification of primary and re-mobilised deposits
even in complex stratigraphic sequences that record
geomorphological, environmental and archaeological
change.
Terrestrial sequences, despite their generally
poorer individual quality than lake cores, offer the
greatest potential for understanding spatial variations
in the lived environment. Spatially extensive, large
scale replication of stratigraphic sequences through
the use of multiple profiles is possible (and desirable)
with terrestrial sediments and peats, and can offer de-
tailed understanding of land surface and environmen-
tal processes.
An undisturbed layer of primary tephra fallout
will be isochronous and the surface it covers will also
be isochronous. Post-depositional modification of the
geometry of the tephra can, however, mean that al-
though stratigraphical relationships may remain unal-
tered, the new surface defined by the tephra may not
relate to the time of the tephra-forming eruption. Re-
shaped tephra horizons, while presenting chronologi-
cal complications, can contain key records of environ-
mental processes.
Despite current limits to our knowledge of the spa-
tial distribution of pre-Landnám tephras in Iceland
and the inherent spatial and temporal variability of
the surviving record, it is possible to use local strati-
graphic sequences of unprovenanced tephras as a ’bar-
code’ to enhance local correlations and refine the ap-
plication of well-known marker horizons to environ-
mental records.
High frequency and high resolution measurement
of both the units between tephra layers and the tephra
layers themselves can give valuable insight and iden-
tify subtle shifts in landscape stability and land use.
An enduring legacy of Thórarinsson’s great vision of
tephrochronology is its utility within geomorphology
and human-environment interactions and uses that go
beyond the identification of isochrons (Thórarinsson,
1944, 1981b).
In Iceland, tephra layers in deep stratigraphic se-
quences created by near continuous sediment accu-
mulation and exhibiting little if any re-mobilisation
have been used to develop remarkable insights into
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