Jökull - 01.01.2012, Blaðsíða 41

Reviewed research article
Isochrons and beyond: maximising the use of tephrochronology
in geomorphology
Andrew J. Dugmore1,2 and Anthony J. Newton1
1Institute of Geography and the Lived Environment, School of GeoSciences,
University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, Scotland, UK
2Human Ecodynamics Research Centre and Doctoral Program in Anthropology,
The Graduate Center, City University of New York, 365 Fifth Avenue, NY 10016-4309, USA
Corresponding author: andrew.dugmore@ed.ac.uk
Abstract – This paper reflects on the application of tephrochronology in geomorphology. A common use of
tephra layers is to define isochrons and use them to date environmental records. Applications of tephrochronol-
ogy with the greatest practical utility, however, involve both classic isochrons (layers with an extensive distri-
bution, distinctive well-characterised properties and good independent dating) and all other tephras present,
including poorly-identified, unprovenanced and re-mobilised units that define time transgressive horizons. The
effective use of this ’total tephrochronology’ requires replication across multiple sites, the clear identification
of primary tephra deposits and re-mobilised deposits, combined with a good understanding of when tephra
deposits truly define isochrons. Large scale replication of tephra stratigraphy is possible (and desirable) with
terrestrial sequences, and can offer a detailed understanding of both geomorphological processes and human
interactions with the environment. It is possible to use sequences of unprovenanced tephras as a ’barcode’
to undertake local correlations and refine the application of well-known marker horizons to environmental
records. High frequency and high resolution measurement of both the units between tephra layers and the
tephra layers themselves can identify subtle shifts in landscape stability and land use.
INTRODUCTION
Tephrochronology is based on the utilisation of
isochrons defined by tephra layers formed by the
undisturbed fallout from volcanic eruption clouds
(Thórarinsson, 1944). The identification of the tephra
produced by a specific eruption permits the correla-
tion of separate tephra deposits formed at the same
time, the recognition of a contemporaneous surface
and the definition of an isochron. This isochron
may be traced to the most suitable location or source
for dating. Tephra isochrons have great utility as
they provide precise and accurate correlation be-
tween different records. They are especially use-
ful if their age is also known with both accuracy
and precision. While the utilisation of distinctive
widespread isochrons is perhaps the best known as-
pect of tephrochronology, there are other very useful
attributes of tephras and different ways in which they
may be used to gain a better understanding of past
environments (Lowe, 2011). In this paper we focus
on some of the less-commonly utilised uses of tephra
layers in environmental reconstruction and different
ways of thinking about tephrochronology that go be-
yond the straightforward identification and utilisation
of well-constrained isochrons. Figure 1 shows the lo-
cation of places mentioned in the text.
JÖKULL No. 62, 2012 39