232
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
Discussion
The relationship between TOC concentrations of the SPM and total sediment load (Figure 3)
indicates that the observed variations in TOC can be explained by a two-component mixing
model. In the following the stable carbon isotope composition of the paM will be discussed in
order to identify endmembers and to quantify their contribution to the paM.
Two-component mixing model
Figure 5a presents the relationship between ()13 C values of the paM and reciprocal organic C
concentrations in the SPM. In general, low C concentrations correspond to heavy ()13C values
and the data for each river studied plot linearly. The Yana is characterized by intermediate ()13C
values while the Lena exhibits slightly heavier and the Khatanga slightly lighter ()13C values.
However, the differences between the three rivers are small. CIN ratios and reciprocal organic
C concentrations of the SPM exhibit a similar relationship. An increase in C concentrations is
paralleled by a decrease in CIN ratios.
The linear relation between ()13C values and reciprocal C concentrations of the SPM suggests
that the variations in carbon isotope composition of paM can be attributed to mixing of two
approximately constant endmembers (Faure, 1986). The hypothetical endmembers are indicated
in Figure 5a. Endmember [dJ, which was extrapolated from the lowest observed C
concentration, is characterized by a ()13C value of -25 %0. Most probably this endmember,
which is almost constant within the three studied rivers, represents a continuous background
organic C component. Endmember [a], which was extrapolated from 1/C -> 0, has a value of
ca. -31 %0. However, the isotope composition of endmember [a] varies slightly among the three
rivers. As shown in Figure 5a, each river has a noticeably individual character. In order to fit all
rivers into one parameter configuration of the model, the relationship between ()13C values of
paM and reciprocal pac concentrations in the water volume (as calculated from the sediment
load and its TOC concentration: pac [mg/l] = SPM [mg/I]· TOC [%] • 10- 2 ) must be studied
(Figure 5b). Figure 5b clearly shows that the variations in carbon isotope composition can be
explained by two-component mixing of the hypothetical endmembers [a] and [dJ, except for
one sample (station 5, 1996). Station 5, 1996 is located close to a coal mine at the Kotuy, a
tributary of the Khatanga. The ()13C value of -25.0 %0 observed at station 5, 1996 is very
similar to the carbon isotope ratio of the Kotuy coal, which averages -24.8 %0. It thus seems
apparent that the paM at this station is formed mainly from coal.
Quantification of endmember contributions
Before discussing possible sources for the hypothetical endmembers of our mixing model, we
present a mixing calculation that enables us to quantify the contribution of the two endmembers
in each sample. The relative proportions can be evaluated from
()13C (PaC) = f· ()13C (endmember [a]) + (l-f) • ()13C (endmember [d])
where f is the relative amount of endmember [a].
The results show that the studied paM is generally dominated by endmember [d], which
comprises more than 50 % of most of the samples. The highest proportions of endmember [d),
ranging from 79 to 96 % (average 87 %), are observed in the Yana POM, where total SPM
concentrations are> 100 mg/1. In the Lena paM, endmember [d] ranges from 37 to 89 %
(average 65 %), while in the POM of the Lena delta, at total SPM concentrations of < 10 mg/l,
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
Discussion
The relationship between TOC concentrations of the SPM and total sediment load (Figure 3)
indicates that the observed variations in TOC can be explained by a two-component mixing
model. In the following the stable carbon isotope composition of the paM will be discussed in
order to identify endmembers and to quantify their contribution to the paM.
Two-component mixing model
Figure 5a presents the relationship between ()13 C values of the paM and reciprocal organic C
concentrations in the SPM. In general, low C concentrations correspond to heavy ()13C values
and the data for each river studied plot linearly. The Yana is characterized by intermediate ()13C
values while the Lena exhibits slightly heavier and the Khatanga slightly lighter ()13C values.
However, the differences between the three rivers are small. CIN ratios and reciprocal organic
C concentrations of the SPM exhibit a similar relationship. An increase in C concentrations is
paralleled by a decrease in CIN ratios.
The linear relation between ()13C values and reciprocal C concentrations of the SPM suggests
that the variations in carbon isotope composition of paM can be attributed to mixing of two
approximately constant endmembers (Faure, 1986). The hypothetical endmembers are indicated
in Figure 5a. Endmember [dJ, which was extrapolated from the lowest observed C
concentration, is characterized by a ()13C value of -25 %0. Most probably this endmember,
which is almost constant within the three studied rivers, represents a continuous background
organic C component. Endmember [a], which was extrapolated from 1/C -> 0, has a value of
ca. -31 %0. However, the isotope composition of endmember [a] varies slightly among the three
rivers. As shown in Figure 5a, each river has a noticeably individual character. In order to fit all
rivers into one parameter configuration of the model, the relationship between ()13C values of
paM and reciprocal pac concentrations in the water volume (as calculated from the sediment
load and its TOC concentration: pac [mg/l] = SPM [mg/I]· TOC [%] • 10- 2 ) must be studied
(Figure 5b). Figure 5b clearly shows that the variations in carbon isotope composition can be
explained by two-component mixing of the hypothetical endmembers [a] and [dJ, except for
one sample (station 5, 1996). Station 5, 1996 is located close to a coal mine at the Kotuy, a
tributary of the Khatanga. The ()13C value of -25.0 %0 observed at station 5, 1996 is very
similar to the carbon isotope ratio of the Kotuy coal, which averages -24.8 %0. It thus seems
apparent that the paM at this station is formed mainly from coal.
Quantification of endmember contributions
Before discussing possible sources for the hypothetical endmembers of our mixing model, we
present a mixing calculation that enables us to quantify the contribution of the two endmembers
in each sample. The relative proportions can be evaluated from
()13C (PaC) = f· ()13C (endmember [a]) + (l-f) • ()13C (endmember [d])
where f is the relative amount of endmember [a].
The results show that the studied paM is generally dominated by endmember [d], which
comprises more than 50 % of most of the samples. The highest proportions of endmember [d),
ranging from 79 to 96 % (average 87 %), are observed in the Yana POM, where total SPM
concentrations are> 100 mg/1. In the Lena paM, endmember [d] ranges from 37 to 89 %
(average 65 %), while in the POM of the Lena delta, at total SPM concentrations of < 10 mg/l,