Jökull - 01.01.2015, Blaðsíða 78

Jónsdóttir et al.
Table 1. Age model information for core A2010-10-586. – Upplýsingar um aldur kjarna A2010-10-586.
Depth Laboratory Material 14C age BP ±1σ Cal age BP
cm No (∆R=400)
36 CURL-16067 marine bivalve shell 555 20 1756
164 CURL-16081 marine bivalve shell 905 20 1437
305 CURL-16073 marine bivalve shell 1395 20 1002
426 Katla tephra ∼700
512 CURL-16078 marine bivalve shell 2020 20 356
Zone 1 (ca. 350–800 CE)
Cibicides lobatulus dominates the oldest part of the
core, with up to 76% of the individuals in the
bottom samples (Figure 4). The Arctic species
C. reniforme and E. excavatum f. clavata have low val-
ues, 5 to 10% near the bottom part, increasing up to
ca. 60% at around 500 CE. Conversely, C. lobatulus
decreases over the same interval. Cassidulina reni-
forme decreases towards the upper part of this zone
but E. excavatum increases. Quinqueloculina stalkeri
amounts to 7% at the lower part of the zone but de-
creases near the top. Other species do not have high
abundance. The diversity of the foraminiferal sample
ranges from 11–19 in each sample.
Zone 2 (ca. 800–1200 CE)
In this zone C. lobatulus still dominates with up to
59% of the faunal composition. Elphidium excavatum
peaks (41%) in the lowest part of this unit but then
decreases in abundance, reaching a minimum around
1000 CE (Figure 4). Cassidulina reniforme slowly in-
creases from 15% up to 40%. Both Astrononion gal-
lowyi (11%) and Buccella calida (5%) peak in this
unit. Quinqueloculina stalkeri has a sudden increase
(11%) in one sample near the upper part of the zone.
Zone 3 (ca. 1200–1845 CE)
In this zone the most dominant species is C. reniforme
(53%) along with E. excavatum (25%). Cibicides lo-
batulus decreases towards the top, fluctuating from
50% down to 8% (Figure 4). At the top of this unit
Q. stalkeri peaks (11%). The diversity in this zone
ranges from ∼10 to 20 species but three samples have
the lowest count of only 8 to 9 species in each sample.
Buccella hannai arctica (10%) and Buccella frigida
reach their highest abundance in the core.
Temperature reconstruction
The BWTTF estimates range from 1.5◦C to 4.75◦C
for the time period ca. 350–1850 years CE (Figure 5).
The temperature estimate is highest around 350 CE
but decreases steadily, reaching one of its lowest val-
ues ∼800 CE. An interval of increased but fluctuating
temperature is seen from 800 to 1100 CE. After that
the temperature starts to decrease towards the top with
distinctive lows around 1300, 1500 and 1700 CE.
Physical and organic properties of the sediments
Zone 1 has fairly stable MS and density values. No
coarse grained material was detected between 350 and
700 CE (Figure 6). At ca. 750 CE the first sign of
coarser material in the core is found. The bottom
part of the core has a fairly stable content of car-
bonate. The TC% record from A2010-10-586 indi-
cates an overall decreasing trend towards the top of
the core, where the bottom part has higher and more
stable values. MS values increase in the middle zone
of the core with a broad peak around 1000 CE. The
X-radiographs also show coarser grained material in
the core at the same time. The carbonate values
have a significant decrease in this zone and reach a
low around 1050 CE. The TC% stays fairly stable.
Two broad peaks in the MS record are observed at
1050 CE and between 1650 and 1750 CE. The density
has a similar trend as the MS. The most abundant of
coarse grained material occurs between ca. 1250 and
1300 CE and again between ca. 1450 and 1600 CE.
The top part of the core between 1600 and 1850 CE
shows considerable variations in the amount of coarse
grained material. The carbonate values increase to-
wards the top of the core and the TC% has a fairly
stable trend, however, both show a distinct low in their
measurements between ca. 1550 and 1750 CE.
78 JÖKULL No. 65, 2015