Fróðskaparrit - 01.01.1998, Side 187

HOLOSENAR TEFRAFLÁIR í FØROYUM
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sizes generally < 100 |am (Persson, 1968).
On the basis of radiocarbon dating, tenta-
tive identifications were made of three of
the tephras; the ‘Landnam Ash’ from the
Vatnaoldur eruptions of the ninth century
AD or Hekla 1104 AD, H-3 and either H-4
or H-S. Persson had no geochemical data
on the tephras, so precise identification of
these Faroese horizons is uncertain and dis-
puted (Waagstein and Jóhansen, 1968).
The key breakthrough of Persson’s work
was to prove that discrete horizons of
tephras had been deposited across the Faroe
Islands on a number of occasions during
the Holocene. He showed that tephro-
chronology could be applied on Faroe, even
though the necessary data for the precise
identification of the tephras was lacking.
In the course of palynological studies in
the Faroe Islands, Jóhannes Jóhansen iden-
tified a number of localities where a well-
developed (cm-scale) layer of basaltic
tephra was present within the early
Holocene stratigraphy (Jóhansen, 1975;
1981). Subsequently, Saksunarvatn became
the type site for the definition of the ‘Sak-
sunarvatn tephra’ (Jóhansen, 1981;
Mangerud et al., 1986). Dated to c. 9,100
l4C yrs BP this tephra has been traced
southwards to the British Isles and Ger-
many (Bennett et al, 1992, Merkt et al.,
1993) and north to early Holocene, ice free
areas of northern Iceland (Bjork et al.,
1992). From its source within the Grims-
votn volcanic system of southern Iceland
this tephra spread across much of the NE
Atlantic region, where it forms a most sig-
nificant isochrone, constraining studies of
the rapid environmental changes of the ter-
mination of the last glacial cycle (Kvamme
etal, 1989).
The identification of the Saksunarvatn
tephra in Faroe highlights three particularly
important issues. Firstly, it established the
presence of basaltic tephra from Iceland in
stratigraphic sequences in Faroe. Persson
had focused on the silicic tephras because
of their contrast to the basaltic sediments
that form the Faroe islands. With the defin-
ition of the Saksunarvatn tephra, it was
shown that basaltic tephras from exotic
sources could be identified, and traced
through many different sites on Faroe. A
second issue related to the identifícation of
the Saksunarvatn tephra is the strengths and
limitations of grain-specific major element
analysis in the definition of individual
tephras. A strength is that without this geo-
chemistry, correlations between Faroe,
seabed sites, NW Europe and Iceland
would not have been convincing, and sepa-
ration from other basaltic components of
the North Atlantic Ash Zone One would
not have been possible. Critical limitations
of major element data on basaltic tephra are
also highlighted, as, while the source vol-
cano can be established with certainty, the
specific eruption cannot (cf. Bennett et al.,
1992). This important caveat, reinforces the
conclusions of Westgate and Gordon
(1981) on the need for a rigorous, multi-
strand approach to tephra identifícation and
correlation. Finally, despite the importance
of the Saksunarvatn tephra, it has seen little
use as an isochrone in palaeoenvironmental
study within Faroe (Edwards et al., 1994).
This does, however, also reflect the com-
paratively limited work undertaken on en-