Fróðskaparrit - 01.01.1998, Page 251

FLOGSÁÐFRØÐILIGAR OG GRÓÐRARLIGAR BROYTINGAR f SAMBANDI
VIÐ BOTNSETING AV SAKSUNARVATNSØSKU í FØROYUM
257
Site Thickness of ash layer (cm)
Saksunarvatn, Streymoy 45.0
Saksunardalur, Streymoy 1.0
Hoydalur, Streymoy < 1.0
Ovaru Hoydalar, Streymoy 1.6
Hovi A, Suðuroy < 1.0
Hagimýra, Suðuroy 6.7
Table 1. Thickness ofthe the Saksunarvatn ash layer at pollen sites in the Faroe Islands Talva 1. Tjúkt á saksunarvatnsøskuflónni á
flogsáðstøðum í Føroyum.
drainage area (~ 13 km2) relative to its small
current surface area (0.078 km2), and its
steep gulleyed sides, would have experi-
enced considerable surface runoff (Mange-
rud et al, 1986; cf. Jóhansen, 1978).
Tephra deposited as ash-fall on the slopes
of the catchment would have been rede-
posited into the lake basin, adding perhaps
44 cm to the ~1 cm of tephra which seems
to have represented the aerial component as
indicated at nearby Saksunardalur. Similar
processes of redeposition into lake bodies
is evident at Hagamýra and to a lesser ex-
tent at Ovaru Hoydalar. These sites have the
added evidence of diffuse boundaries and a
lessening upwards of tephra densities as
ash becomes mixed and/or combined with
organic accumulations. The erosional in-
puts of pollen and spores is also demon-
strated by the increased values for damaged
and damaged unidentifiable pollen in spec-
tra at or after the ash layer.
It is not possible to infer the time period
involved in tephra deposition with any ac-
curacy; it is likely to vary greatly according
to lake catchment properties. Some 5 radio-
carbon years only, based on straight-line
extrapolations from dates either side of the
ash layer, and ignoring problems of dating
imprecision, would apply to the 45 cm of
ash at Saksunarvatn. A period of 1000 l4C
was suggested for 37.5 cm of Saksunarvatn
ash (basal 20 cm of massive ash overlain by
a tephra/gyttja admixture) at Torfadalsvatn,
northern Iceland, although it is inferred that
tephra there caused a transitory (c. 100 l4C
year) change in vegetational succession,
with expansions in Salix and Poaceae
(Rundgren, 1997).
Palynological, and by inference vegeta-
tional changes consequent upon the fall of
ash within the pollen catchment areas of the
Faroese sites are mooted, with the strongest
apparent effects at Hagamýra, where wetter
habitats may have become ‘choked’ with
ash, thus diminishing the pollen contribu-
tions of associated flora. Similarly, the
aquatic taxa are also reduced. Palynologi-
cally at least, and arguably vegetationally,
this process favours dry land and taller taxa.
Major palynological changes in spectra ei-
ther side of the ash layer, not necessarily
those within or apparently adjacent to it,
may be indicating the vegetational changes
in the catchment area. Mixing of sediment
may thus be causing difficulties in infer-
ence and it may be necessary to accept the
more circumstantial evidence of gross
changes from sediments not in intimate
contact with the ash.
Acknowledgements
The Leverhulme Trust are thanked for the financial sup-
port which enabled fieldwork to be carried out at the
sites of Hagamýra and Upper Hoydalur. Sampling took
place in 1991 with the assistance of Jon Hunt and Jon
Sadler. Sampling at Saksunardalur was carried out in