Rachold and Hubberten: Carbon Isotope Composition of Particulate Organic Material
235
1993). The pH of the main river is about 8 (Zaitsev, pers. comm.) and DIC is mainly formed
from HC03-. Summer water temperatures are cold to moderate, averaging 10 to J3°C in the
Lena delta during September (Utolle et aI., 1993), with maximum temperatures of 18°C in the
upper reaches during July. In September 1994, the DIC concentrations reached 700 Jlmolll,
corresponding to ol3C values of -6 %0 (Erlenkeuser, 1995). Similar DIC concentrations (about
500 to 600 Jlmolll) were reported for September 1989 and 1991 by Cauwet and Sidorov
(1996). The DIC data refer to the lowermost Lena and the Lena delta. There are no data
available for the upper Lena. The heavy isotope ratio of Lena DIC may be attributed to the
dominance of carbonate weathering. Moreover, CO2 exchange with the atmosphere may further
increase the ol3C values of DIC downriver. No DIC data are available for the Yana and
Khatanga. However, since ol3C values of POM do not significantly differ from the Lena, we
assume similar isotopic ratios for the DIC (although carbonates are less common in the
Khatanga and Yana basins).
The isotope fractionation between plankton and the various fractions of DIC depends on their
concentrations and on temperature, species and growth rates. The fractionation between
dissolved CO2 and plankton amounts to -10 %0 to -15 %0, depending on temperature, which is
equivalent to a fractionation between HC03- and plankton of -20 to -25 %0 (Tan and Strain,
1983). For this reason, the ol3C values of riverine plankton are generally lighter than -30 %0. At
very light ol3C values of the DIe, the autochthonous plankton can be isotopically very light, up
to -40 %0 (Cai et aI., 1988; Mariotti et aI., 1991; Tan and Edmond, 1993).
Based on ol3C values of -6 %0 for Lena DIC, the dominance of HC03 -, and on a fractionation
of ca. -15 %0 between dissolved CO2 and plankton in the cold to moderate temperatures of the
Lena, ol3C values of the autochthonous POM are expected to be around -31.0 %0. This isotope
ratio is in agreement with the postulated endmember [a] indicated in Figure Sa and 5b.
Unfortunately, no data are available on the seasonal variation of plankton development in the
Lena. However, chemical data indicate that seasonal variations may be significant. For this
reason, our data represent the summer situation only.
Organic carbon flux to the Laptev Sea
Based on the average organic C concentrations of SPM in the Lena, Yana, and Khatanga,
which were 5.4 %, 2.1 %, and 3.1 %, respectively and on the annual sediment discharge as
given by Rachold et al. (1996), we are able to evaluate the POC flux to the Laptev Sea. Since
seasonal variations in C concentrations are not included in this calculation, the results must be
regarded as rough extrapolations. By adding the dissolved organic carbon (DOC) flux of the
Lena reported by Lara et al. (in press), we can also give an estimate of the TOC export of the
Lena. Table 2 presents the results and several other estimates. While TOC, POC, and DOC data
are available for the Lena, only very incomplete studies exist for the Yana and Khatanga. The
available data, however, underline the importance of the riverine carbon fluxes of the Siberian
rivers for the Laptev Sea, and especially the contribution of the Lena.
Conclusions
Carbon isotope data indicate that POM of Siberian rivers is generally formed from two
endmembers. The first endmember [d) represents detrital organic material and is characterized
by a ol3C value of ca. -25.0 %0. The second endmember [a] is isotopically lighter, averaging
ca. -31.0 %0, and may be attributed to autochthonous riverine phytoplankton. Mixing
235
1993). The pH of the main river is about 8 (Zaitsev, pers. comm.) and DIC is mainly formed
from HC03-. Summer water temperatures are cold to moderate, averaging 10 to J3°C in the
Lena delta during September (Utolle et aI., 1993), with maximum temperatures of 18°C in the
upper reaches during July. In September 1994, the DIC concentrations reached 700 Jlmolll,
corresponding to ol3C values of -6 %0 (Erlenkeuser, 1995). Similar DIC concentrations (about
500 to 600 Jlmolll) were reported for September 1989 and 1991 by Cauwet and Sidorov
(1996). The DIC data refer to the lowermost Lena and the Lena delta. There are no data
available for the upper Lena. The heavy isotope ratio of Lena DIC may be attributed to the
dominance of carbonate weathering. Moreover, CO2 exchange with the atmosphere may further
increase the ol3C values of DIC downriver. No DIC data are available for the Yana and
Khatanga. However, since ol3C values of POM do not significantly differ from the Lena, we
assume similar isotopic ratios for the DIC (although carbonates are less common in the
Khatanga and Yana basins).
The isotope fractionation between plankton and the various fractions of DIC depends on their
concentrations and on temperature, species and growth rates. The fractionation between
dissolved CO2 and plankton amounts to -10 %0 to -15 %0, depending on temperature, which is
equivalent to a fractionation between HC03- and plankton of -20 to -25 %0 (Tan and Strain,
1983). For this reason, the ol3C values of riverine plankton are generally lighter than -30 %0. At
very light ol3C values of the DIe, the autochthonous plankton can be isotopically very light, up
to -40 %0 (Cai et aI., 1988; Mariotti et aI., 1991; Tan and Edmond, 1993).
Based on ol3C values of -6 %0 for Lena DIC, the dominance of HC03 -, and on a fractionation
of ca. -15 %0 between dissolved CO2 and plankton in the cold to moderate temperatures of the
Lena, ol3C values of the autochthonous POM are expected to be around -31.0 %0. This isotope
ratio is in agreement with the postulated endmember [a] indicated in Figure Sa and 5b.
Unfortunately, no data are available on the seasonal variation of plankton development in the
Lena. However, chemical data indicate that seasonal variations may be significant. For this
reason, our data represent the summer situation only.
Organic carbon flux to the Laptev Sea
Based on the average organic C concentrations of SPM in the Lena, Yana, and Khatanga,
which were 5.4 %, 2.1 %, and 3.1 %, respectively and on the annual sediment discharge as
given by Rachold et al. (1996), we are able to evaluate the POC flux to the Laptev Sea. Since
seasonal variations in C concentrations are not included in this calculation, the results must be
regarded as rough extrapolations. By adding the dissolved organic carbon (DOC) flux of the
Lena reported by Lara et al. (in press), we can also give an estimate of the TOC export of the
Lena. Table 2 presents the results and several other estimates. While TOC, POC, and DOC data
are available for the Lena, only very incomplete studies exist for the Yana and Khatanga. The
available data, however, underline the importance of the riverine carbon fluxes of the Siberian
rivers for the Laptev Sea, and especially the contribution of the Lena.
Conclusions
Carbon isotope data indicate that POM of Siberian rivers is generally formed from two
endmembers. The first endmember [d) represents detrital organic material and is characterized
by a ol3C value of ca. -25.0 %0. The second endmember [a] is isotopically lighter, averaging
ca. -31.0 %0, and may be attributed to autochthonous riverine phytoplankton. Mixing