Jökull - 01.01.2012, Side 69

Holocene marine tephrochronology on the Iceland shelf
core MD99-2275, have been correlated to the Svartár-
kot and Kárahnjúkar soil sections, respectively (Fig-
ure 3) (Gudmundsdóttir et al., 2012). Further de-
tails on correlations of core MD99-2275 with these
soil sections are presented in Gudmundsdóttir et al.,
2012. Tephra layers from Katla volcanic system in
core MD99-2269 between the Hekla 1104 and Hekla
3 tephra markers could possibly be correlated to some
of the Katla tephra layers in core MD99-2275 and
Svartárkot and/or Kárahnjúkar soil sections (Figure
3). A Katla tephra layer with an age of 2450 years
BP in core MD99-2269 could be correlated to two
Katla tephra layers with an age of 2337 years or 2473
years (Table 2). Such a correlation, however, would
be rather weak because the tephra stratigraphy of core
MD99-2269 has not been fully documented, with ev-
ery 10 cm investigated for tephra layers versus every
cm in core MD99-2275. Therefore other potentially
undiscovered Katla tephra layers in core MD99-2269
of similar age could be a better correlation. Good
stratigraphic and chronological control is of utmost
importance when correlating tephra layers, in particu-
lar basaltic tephra layers where appearance and chem-
ical composition is similar within a particular volcanic
system. The same applies for correlation of basaltic
Grímsvötn and Katla tephra layers in the interval be-
tween the Hekla 4 and Saksunarvatn ash marker layers
in core MD99-2269, a more detailed tephra stratigra-
phy is needed to make a robust correlation.
High-resolution and systematic studies of tephra
layers in marine sediments on the Iceland shelf opens
up the possibility of high-resolution layer-by-layer
correlations with centennial to decadal resolution
within certain time intervals (Gudmundsdóttir et al.,
2012). This is especially valuable in areas where
frequent shifts in climate/atmosphere-oceanographic
system may have occurred and where synchronicity
of leads and lags between such events needs to be es-
tablished. Correlating marine tephra stratigraphy with
terrestrial (including ice core) tephra stratigraphy is
critical because dates can be obtained from the ter-
restrial setting. In many cases, this will also add to
the available information on its distribution outside
Iceland.
CONCLUDING REMARKS
Tephra studies on the Iceland shelf show that the ma-
rine sediments are a good preserver of tephra layers
from Icelandic volcanoes facilitating robust correla-
tion and dating of various proxy archives for paleo-
environmental and climatic research. High-resolution
marine tephrochronology, allows detailed layer by
layer correlation were centennial to decadal resolu-
tion may be possible for certain time periods. Tephra
studies on the North Iceland shelf have revealed new
tephra layers, such as the Hrafnkatla, Hekla Ö and
Hekla DH, that can be used as markers or isochrones
on the Iceland shelf and possibly in the North Atlantic
region.
Marine tephrochronology also provides an inde-
pendent control on marine radiocarbon dates, and as-
sessment of marine reservoir age. The marine tephra
stratigraphy on the North Iceland shelf captures gen-
eral trends of explosive volcanic activity, providing
an important supplement to the high-resolution ter-
restrial record on volcanism in Iceland. In addition,
the marine record extends further back in time than
the terrestrial record. Regional identification of spe-
cific tephra layers in marine sediments, ice cores and
soil, will improve understanding of the frequency and
dispersal directions of Icelandic tephra. However, it
is important when working with marine tephra lay-
ers to carefully consider sedimentation processes that
can influence the integrity of a tephra layer such as
reworking by bioturbation, currents and mass move-
ments and the presence of sea ice during a tephra fall.
Development of tephrochronology in marine
archives on the Iceland shelf is still in its infancy.
Much of the work has been carried out on the North
Iceland shelf while the west, south and east Iceland
shelf areas await further investigation. The numer-
ous tephra layers identified on the North Iceland shelf
demonstrate the potential of the marine record around
Iceland for tephra studies.
Acknowledgements
We thank Prof. Haflidi Haflidason, University of
Bergen, Norway, and dr. Peter Abbott, University of
Swansea, UK, for valuable comments and suggestions
on the manuscript and the editors for their patience.
JÖKULL No. 62, 2012 67