Jökull - 01.01.2009, Page 76

Hannesdóttir et al.
Table 2. cont.
Hestvatn core section cum. depth sample number radiocarbon age cal. age date Reference
(cm) /tephra (14C yr BP) (yr BP) type
SOUTH BASIN CORE
GLAD4-HST03-1A-1H-2 109 K-1500 tephra 360 460 H Þórarinsson, 1975
GLAD4-HST03-1A-1H-2 192 Vö tephra 871±2 AD 1080±2 IC Grönvold et al., 1995
GLAD4-HST03-1A-1H-2 240 KE tephra 2850±10 2975±60 I Róbertsdóttir, 1992
GLAD4-HST03-1A-H-2 248 H3 tephra 2880±30 3066±108 R Dugmore et al., 1995
GLAD4-HST03-1A-1H-2 263 KN tephra 3300±100 3555±120 I Róbertsdóttir, 1992
GLAD4-HST03-1A-2H-1 306 H4 tephra 3830 ±10 4260±20 W Dugmore et al., 1995
GLAD4-HST03-1A-2H-1 334 T tephra 5765±55 6100±100 I Jóhannsdóttir, 2007
GLAD4-HST03-1A-2H-1 427 Th-B1 tephra 7380 8600±100 I Jóhannsdóttir, 2007
GLAD4-HST03-1A-2H-2 486 Saksunarvatn tephra 9000 10,180±60 IC Grönvold et al., 1995
GLAD4-HST03-1A-3H-1 627 AA59081 (s) 9925±56/ 10,599±236 R Hannesdóttir, 2006
9525±56
GLAD4-HST03-1C-3H-2 825 AA59080 (s) *9684±55/ 10,901±260 R Hannesdóttir, 2006
10,084±55
GLAD4-HST03-1A-4H-1 926 AA59079 (s) *9715±54/ 10,994±180 R Hannesdóttir, 2006
10,115±54
GLAD4-HST03-1A-5H-1 1251 Vedde tephra 10,300 11,980±80 IC Grönvold et al., 1995
GLAD4-HST03-1A-5H-1 1316 AA59078 (s) *9992±55/ 11,445±233 R Hannesdóttir, 2006
10,392±55
Core HST03-2A obtained from the north basin
lies on line N6 (Figure 3) - and core HST03-1A from
the south basin on line S1 (Figure 4). Sediment core
HST03-2A penetrated 23 m of sediment, with near
complete sediment recovery, halting on the lowermost
transparent seismic unit I. Sediment core HST03-1A
penetrated 15 m of sediment, into sub-unit IIb, which
consists of silty sediment interbedded with sandy-
gravelly sediments and shell fragments. The sandy-
gravelly section was not penetrable, leaving approxi-
mately 20 m of unrecovered sediment from the south
basin core site.
The chronology of the cores is based on
tephrochronology and radiocarbon dating of marine
molluscs (Table 2). The two most widely distributed
and recognized tephra layers in the North Atlantic,
the Vedde Ash (11.8 ka) and the Saksunarvatn Ash
(10.2 ka) provide key chronological control for the
deglacial sediments (Grönvold et al., 1995). The
Vedde Ash (5 cm thick), is found at 12.5 m depth
in core HST03-1A in the south basin (Figure 4), but
is not present in the duplicate cores from the north
basin. Vedde was also identified on x-radiographs
(Hannesdóttir, 2006; Jóhannsdóttir, 2007). Micro-
probe analyses made on the tephra layer confirmed its
origin as the Vedde Ash and its two phase appearance
indicates primary fallout (Jóhannsdóttir, 2007). The
age model for the marine section is not constrained
by tephrochronology (apart from Vedde Ash), but de-
pends on 14C dates on marine molluscs, which are
in chronological order. In core HST03-1A a date
of 11,445±233 ka, is found 50 cm below the Vedde
Ash, hence the upper limit of this date is 73 cal. years
younger than the accepted age of the Vedde Ash.
A reservoir age of 400 years is used for the mol-
lusc dates. The radiocarbon dates are affected by 14C
plateaux (Becker et al., 1991; Kromer and Becker,
1993; Lowe and Walker, 2000) and as such have their
limitations. Studies in the North Atlantic (i.e. Bard
et al., 1994; Haflidason et al., 1995; Jennings et al.,
2000) indicate that a higher ocean reservoir correc-
tion might be needed for the late glacial/Holocene
transition period than for younger intervals. Svein-
björnsdóttir et al. (1998) compared 14C dates of
76 JÖKULL No. 59