Jökull - 01.12.2006, Síða 33

The stable isotopic (C and N) composition of modern plants and lichens from northern Iceland
MATERIALS AND METHODS
A total of twenty-eight plant and lichen samples were
collected during our field season. Three plant sam-
ples were aquatic plants while the rest were terres-
trial. Surface lake sediment samples were collected
using a Glew sampler. Plant and lichen samples were
collected into Whirlpack bags and kept at 4◦C or
frozen and then transported to the Alaska Stable Iso-
tope Facility (ASIF), where they were freeze-dried
and ground into a homogeneous powder. Between
0.4 and 0.6 mg of each sample was weighed into a
tin capsule. Surface sediment samples were also sub-
sampled (2–3 mg) for carbon and nitrogen stable iso-
tope analysis of the TOC and Total Organic Nitro-
gen (TON). These surface sediments were tested first
with 6M HCL acid to examine any presence of car-
bonates (Harris et al., 2001). Even though no visi-
ble reactions were detected, sediment samples were
still acid fumed before freeze-drying. The δ13C and
δ
15N values of the samples were determined using a
Costech ECS4010 Elemental Analyzer (EA) attached
via a Conflo III to a continuous flow Isotope Ratio
Mass Spectrometer (IRMS) (Thermo Finnigan Delta-
plus XL) at the ASIF. Most samples were run in dupli-
cate. The δ13C and δ15N values are calculated using
the equation:
δX =
(
Rsample
Rstandard
− 1
)
× 1000
where X is the 13C or 15N and R is 13C/12C or
15N/14N. Results are expressed in standard per mil
(‰) units relative to the international standards [Vi-
enna Peedee Belemnite (VPDB) for C and atmo-
spheric nitrogen for N]. The analytical precision was
established by measuring peptone as a calibrated ref-
erence standard (n=7). The analytical precisions for
%C, %N, δ13C and δ15N were 2.1%, 0.6%, 0.2‰
and 0.1‰, respectively. Some plant and lichen sam-
ples had low %N (∼0.2-1%) and six of these types of
plants were re-run using a larger sample mass (∼2 mg)
and the δ15N values produced from the greater mass
were not significantly different from those produced
from the lower mass.
RESULTS
Table 1 presents the elemental (N and C) and stable
isotope (δ15N and δ13C) data from the analyses of
plants, lichens and surface sediments from the four
study sites. Figure 3 illustrates the wide range of
stable isotope values of organic material (e.g. plants
and lichens) from the four sites. The %N of plants
and lichens from Lake Litla-Viðarvatn ranged from
0.2% to 1.7% with a mean of 0.8% while the %C
ranged from 42.1% to 58.2% with a mean of 49.3%.
The plants and lichens showed some relatively low
δ15N values ranging from -9.3‰ to -0.7‰ with a
mean value of -6.0‰. The δ13C value for plants from
this site ranged from -29.0‰ to -23.3‰ with a mean
of -27.0‰. The %N and %C of total organic matter
(TOM) from the surface sediment sample were 2.2%
and 10.2%, respectively. The δ15N and δ13C of this
surface sediment sample were -1.2‰ and -19.8‰, re-
spectively with a C/N value of 4.7 (Figure 3, Table 1).
Compared to Lake Litla-Viðarvatn, the %N of
plants and lichens from Lake Torfadalsvatn site were
higher with a mean value of 1.3%, ranging from 0.2%
to 4.8% (Table 1), although this apparent difference
was within analytical precision. The %C value of
plants and lichens had a mean of 42.3% (ranging
from 28.0% to 52.6%). Similar to those from Lake
Litla-Viðarvatn, the δ15N values of plants and lichens
from Lake Torfadalsvatn showed a wide range from
-8.2‰ to 4.3‰ with a mean of -3.4‰ (Figure 3).
Only an aquatic plant (Potamogeton sp.) had a positive
δ15N value (duplicates of 4.3‰ and 4.2‰), while
the δ15N values of the terrestrial plants and lichens
were all negative, and less negative compared to the
plants from Lake Litla-Viðarvatn. The δ13C values
of plants and lichens showed a very wide range from
-29.5‰ to -11.5‰. The δ13C values of terrestrial
plants and lichens were between 29.5‰ to -26.1‰
(Figure 3) while the aquatic plants had values between
-14.2‰ and -11.6‰. The %N and %C of TOM from
the surface sediment sample were 1.1% and 12.8%,
respectively, with a δ15N value of -1.5‰ and δ13C
value of -20.6‰. The C/N of this bulk sediment
sample was 11.4.
The two herbs collected from Lake Stífluvatn
yielded very different %N and %C from each other.
JÖKULL No. 56 31