Rachold and Hubberten: Carbon Isotope Composition of Particulate Organic Material
233
the contributions of endmember [d] only amount to 0 to 76 % (average 46 %). In the Khatanga
POM, endmember [d] averages 55 %, ranging from 44 to 70 %.
Possible endmember sources
The riverine POC reservoir originates from two different sources: (1) detrital organic carbon
which is formed from soil-derived terrigeneous material, eroded coal deposits, and reworked
peat and (2) in situ produced riverine plankton.
In general, autochthonous phytoplankton is characterized by C/N ratios averaging ca. 6 to 7,
while the C/N ratio of terrestrial plants is generally higher than 10. C/N ratios are controlled by
several other parameters and therefore cannot be applied alone to identify the origin of organic
material (Wetzel, 1983; Lerman et aI., 1995). However, the general trend observed from our
data suggests that samples of low organic C concentrations and high turbidity are dominated by
terrestrial organic material whereas samples of high organic C concentrations and low turbidity
are additionally influenced by autochthonous phytoplankton.
The carbon isotope composition of terrestrial plants making use of the C3 photosynthetic
pathway (trees are C, plants) represents an average of -25 to -26 %0. Aerial plants utilizing the
C4 cycle, which are mainly grasses, are less depleted in l3 C and exhibit an average carbon
isotope composition of -12 %0 (O'Leary, 1981). Almost the entire catchment areas of the
studied rivers are covered by Siberian taiga. Only the northernmost regions, i.e. the coastal
zone and the Lena delta, are located within the tundra zone. For this reason, it is suggested that
the detrital organic material within the rivers is strongly dominated by C3 plants with an average
b13C of -25 to -26 %0. The bl3C values of Lena coal exhibit a mean value of -25.7 %0 and the
Khatanga coal of -24.8 %0. Peat samples from the Lena and Yana deltas are characterized by a
carbon isotope composition of ca. -27 %0. Peat deposits, however, are only observed in the
river deltas and do not contribute to the detrital organic material in the upper reaches of the
rivers. Terrestrial organic material and coal have very similar b 13 C values. Pooling these two
components, the average carbon isotope composition of the detrital organic material in the
catchment areas amounts to ca. -25 %0. It therefore seems reasonable to regard endmember [d]
of the mixing model indicated in Figure 5a and 5b as the detrital organic carbon fraction.
The carbon isotope composition of autochthonous riverine paM depends on the isotope
fractionation between phytoplankton and the various fractions of dissolved inorganic carbon
(DIC), i.e. dissolved CO2, HCO" and CO/·. The relative amounts of CO2, HCOj , and cot
dissolved in water are related to temperature and pH. The major sources of carbon contributing
to the DIC pool are (a) CO2 derived from the decay of organic material in soils and (b) CO 2
released during the dissolution of carbonates. In general, the uptake of atmospheric CO 2 in
surface waters is negligibly small; in contrast, CO2 loss by degassing is more common (Mook
and Tan, 1991).
The carbon isotope composition of soil derived CO2 yields values of around -26 %c. For this
reason, bl3C values of DIC as low as -26 %0 have been observed in drainage areas that are
characterized by very low limestone content, large amounts of vegetation and a low pH
(Longinelli and Edmond, 1983). However, most rivers belong to the Ca bicarbonate type and
their isotope compositions are detennined by the reaction of limestone and soil derived CO2 that
produces DIC b DC values of about -12 %0 (Tan and Edmond, 1993). In the absence of
vegetation, on the other hand, exchange with atmospheric CO2 may affect the DIC and its b13C
values can be as heavy as -7 %0. The degassing of CO2 may cause a further increase in b I3 C
values (Mook and Tan, 1991).
During summer, the Lena belongs to the Ca bicarbonate type of river and its geochemistry is
controlled mainly by weathering of limestones and by groundwater (Gordeev and Sidorov,
233
the contributions of endmember [d] only amount to 0 to 76 % (average 46 %). In the Khatanga
POM, endmember [d] averages 55 %, ranging from 44 to 70 %.
Possible endmember sources
The riverine POC reservoir originates from two different sources: (1) detrital organic carbon
which is formed from soil-derived terrigeneous material, eroded coal deposits, and reworked
peat and (2) in situ produced riverine plankton.
In general, autochthonous phytoplankton is characterized by C/N ratios averaging ca. 6 to 7,
while the C/N ratio of terrestrial plants is generally higher than 10. C/N ratios are controlled by
several other parameters and therefore cannot be applied alone to identify the origin of organic
material (Wetzel, 1983; Lerman et aI., 1995). However, the general trend observed from our
data suggests that samples of low organic C concentrations and high turbidity are dominated by
terrestrial organic material whereas samples of high organic C concentrations and low turbidity
are additionally influenced by autochthonous phytoplankton.
The carbon isotope composition of terrestrial plants making use of the C3 photosynthetic
pathway (trees are C, plants) represents an average of -25 to -26 %0. Aerial plants utilizing the
C4 cycle, which are mainly grasses, are less depleted in l3 C and exhibit an average carbon
isotope composition of -12 %0 (O'Leary, 1981). Almost the entire catchment areas of the
studied rivers are covered by Siberian taiga. Only the northernmost regions, i.e. the coastal
zone and the Lena delta, are located within the tundra zone. For this reason, it is suggested that
the detrital organic material within the rivers is strongly dominated by C3 plants with an average
b13C of -25 to -26 %0. The bl3C values of Lena coal exhibit a mean value of -25.7 %0 and the
Khatanga coal of -24.8 %0. Peat samples from the Lena and Yana deltas are characterized by a
carbon isotope composition of ca. -27 %0. Peat deposits, however, are only observed in the
river deltas and do not contribute to the detrital organic material in the upper reaches of the
rivers. Terrestrial organic material and coal have very similar b 13 C values. Pooling these two
components, the average carbon isotope composition of the detrital organic material in the
catchment areas amounts to ca. -25 %0. It therefore seems reasonable to regard endmember [d]
of the mixing model indicated in Figure 5a and 5b as the detrital organic carbon fraction.
The carbon isotope composition of autochthonous riverine paM depends on the isotope
fractionation between phytoplankton and the various fractions of dissolved inorganic carbon
(DIC), i.e. dissolved CO2, HCO" and CO/·. The relative amounts of CO2, HCOj , and cot
dissolved in water are related to temperature and pH. The major sources of carbon contributing
to the DIC pool are (a) CO2 derived from the decay of organic material in soils and (b) CO 2
released during the dissolution of carbonates. In general, the uptake of atmospheric CO 2 in
surface waters is negligibly small; in contrast, CO2 loss by degassing is more common (Mook
and Tan, 1991).
The carbon isotope composition of soil derived CO2 yields values of around -26 %c. For this
reason, bl3C values of DIC as low as -26 %0 have been observed in drainage areas that are
characterized by very low limestone content, large amounts of vegetation and a low pH
(Longinelli and Edmond, 1983). However, most rivers belong to the Ca bicarbonate type and
their isotope compositions are detennined by the reaction of limestone and soil derived CO2 that
produces DIC b DC values of about -12 %0 (Tan and Edmond, 1993). In the absence of
vegetation, on the other hand, exchange with atmospheric CO2 may affect the DIC and its b13C
values can be as heavy as -7 %0. The degassing of CO2 may cause a further increase in b I3 C
values (Mook and Tan, 1991).
During summer, the Lena belongs to the Ca bicarbonate type of river and its geochemistry is
controlled mainly by weathering of limestones and by groundwater (Gordeev and Sidorov,