Fróðskaparrit - 01.01.1998, Blaðsíða 330

336 ICELANDIC TEPHRA IN LATE-GLACIAL SEDIMENTS OF SCOTLAND (14-9,000^0 BP)
Tephra Deposit Locations Discovered 14C age (years BP)
Saksunarvatn Tephra Icelandbe, NAMC, Faroe Islandsc- s, Shetlande-u, Orkneye-v, 8700», 8900L 9000 - 9100», 9140=1
Germany3, Greenlands
l-Thol-l Icelandb 9200b
Vedde Ash Iceland (Skogar Tephra)b-C- 9, NAMCh-°, Norwaye>P, Swedenr, Greenlands 10,300=
l-Thol-2 Icelandb 10,800b
Borrobol Tephra Scotlandf 12,500f
Table 1: Components and radiocarbon ages ofNorth Atlantic Ash Zone One (NAAZO). NAMC - North Atlantic
Marine Cores.
Talva 1: Tilfar og kolevni 14-aldur á norðuratlantsøskuumráði eitt (NAAZO = North Atlantic Ash Zone One).
NAMC - North Atlantic Marine Cores, t.e. norðuratlantshavskjamar.
Source information: a Merkt et al. (1993); b Bjorck et al. (1992);c Mangerud et al. (1986);d Jóhansen (1975);e
Birks et al. (1996): fTurney et al., (1998); s Jóhansen (1985); h Bard et al. (1994), • Serjup et al. (1989); i
Kvamme et al. (1989); k Sigurdsson and Loebner (1981);1 Fillon etal. (1981); m Ruddiman and Glover (1972);
" Long et al. (1986); °Austinet al. (1995) and (1996); p Mangerud et al. (1984);‘t Nordaahl and Haflidson
(1992);r Wastegárd et al. (1998) and Wastegárd et al. (submitted);s Grønvold et al. (1995);' Sjøholm et
al.(1991);u Bennett et al. (1992);v Bunting (1994).
of the accuracy and consistency of radio-
carbon dates associated with time-equiva-
lent horizons. However, tephra deposition
is somewhat irregular at sites distal from
the volcanic system and evidence from sev-
eral sites is required when attempting to es-
tablish a reliable tephrochronological
framework in such areas.
By applying a density-separation techni-
que to remove unwanted minerogenic par-
ticles, two horizons of measurable concen-
trations of volcanic ash have been detected
in deposits of Late Devensian Late-glacial
age (13 - 10,000 l4C BP) at six sites from in
Scotland (Turney, 1998a). On the basis of
major element shard geochemistry, an up-
per micro-tephra layer is equated with the
well-known Vedde Ash, the principle com-
ponent of North Atlantic Ash Zone One
(NAAZO) (Table 1). It has also been found
in a variety of oceanic, cryogenic and ter-
restrial repositories of the North Atlantic
region and has been dated to 10,300 l4C BP
(Birks et al., 1996). The lowermost tephra
horizon, the Borrobol Tephra (-12,500 14C
BP; Turney et al., 1997) has only been de-
tected so far at sites in Scotland, for few
sites elsewhere have been investigated to
determine whether an equivalent ash hori-
zon can be detected at around this time. No
source has yet been identified for the Bor-
robol Tephra, but its geochemical composi-
tion also indicates an Icelandic origin.
A clear spatial pattern of maximum
shard concentration is evident for both
tephras. The highest number of shards per
cm3 occur in sites in the North and West,
but this pattern is not simply related to lo-
cation (Figure 1; Table 2). Atmospheric
and post-depositional sedimentation pro-
cesses appear to be the most important fac-
tors controlling the stratigraphic distribu-
tion of tephra shards in lake sediments. Sin-
gle peaks tend to reflect air-fall, though in