Rachold and Hubberten: Carbon Isotope Composition of Particulate Organic Material
225
three months duration) and short, hot summers (maximum temperature +30 to +35°C for two
months duration). The average annual precipitation in the Lena basin is 330 mm of which 70 to
80 % occurs during the summer (Gordeev and Sidorov, 1993). Due to this extreme climate,
most of the water and sediment is transported during summer, between June to September. The
seasonal variations in water discharge of the Lena, Yana, and Khatanga are presented in Figure
2. Average daily water discharges measured at hydrometeorological stations located in the
lower reaches of the rivers are displayed (Russian Federal Service for Hydrometeorology and
Environmental Monitoring, unpublished data).
The majority of the study area is covered by Siberian taiga. Only the northernmost regions,
mainly the Lena and Yana deltas and the upper reaches of the Khatanga, are located within the
tundra zone.
Samples and methods
Sampling was carried out by the Potsdam branch of the Alfred Wegener Institute, in cooperation with the Geographical Faculty of the Moscow State University (1994 and 1995) and
the Arctic and Antarctic Research Institute, St. Petersburg (1996) on three expeditions to the
Siberian rivers. For detailed information on the expeditions, the reader is referred to the cruise
reports (Rachold et aI., 1995, 1997, and in press). Samples of the Lena and its tributaries were
collected from July 9 to July 28, 1994 and from July 1 to August 2, 1995. At this time of the
year, water level and SPM concentrations of the Lena were still high but lower than the
maximum water and sediment discharge immediately after ice breakup in the beginning of June.
The Yana and its tributaries, as well as the Omoloy, were sampled from August 8 to August 23,
1995. Yana water level and sediment discharge in 1995 reached maximum values in August,
during the sampling period. Samples of the Khatanga were collected at the end of July 1996,
three weeks after ice breakup. The sampling periods are indicated in Figure 2. The results
reported in the present article reflect the summer situation of the Siberian rivers. Due to the
severe climate in the drainage area, the winter season can almost be neglected in terms of mass
fluxes. During the spring flood, on the other hand, water and sediment discharges may increase
dramatically. This is especially true for the Lena whereas the spring flood is less pronounced
for the Yana and Khatanga. However, the increase in total organic carbon (TOC) concentrations
in the Lena is of minor significance. Average TOC concentrations during spring flood
amounted to 1050 /lmolll and during summer and autumn to 620 /lmol/l (Cauwet and Sidorov,
1996). Since our samples were collected in early summer, the data can be regarded as average
for the warm season.
Water samples were obtained by a glass water sampler (1994) and a Teflon water sampler
(1995 and 1996). The sampling locations are indicated in Figure I. In general, water samples
were retrieved from the middle of the river channel at a water depth of 1.5 m. Stations along the
main rivers in 1995 and 1996 were sampled as river cross sections. At these stations, two or
three samples distributed equally across the river were collected. In the following, we refer to
average values calculated for each station. River cross sections are treated as field replicates.
Samples taken at river cross sections exhibited small variations in carbon isotope ratios. The
standard deviation amounted to less than ± 0.4 %0 (Table 1).
The SPM was separated by vacuum filtration through precombusted 0.7 /lm Whatman glassfiber filters (GFIF) immediately after sampling. The filtered water volume varied between I and
4 I, depending on sediment load. After air-drying under dust-free conditions, the filters were
stored dry and cool. A total of 90 samples corresponding to 61 stations were analyzed for total
SPM content, organic C and N concentrations of the SPM, and for the stable carbon isotope
composition of the POM at the Alfred Wegener Institute, Research Department Potsdam. In
225
three months duration) and short, hot summers (maximum temperature +30 to +35°C for two
months duration). The average annual precipitation in the Lena basin is 330 mm of which 70 to
80 % occurs during the summer (Gordeev and Sidorov, 1993). Due to this extreme climate,
most of the water and sediment is transported during summer, between June to September. The
seasonal variations in water discharge of the Lena, Yana, and Khatanga are presented in Figure
2. Average daily water discharges measured at hydrometeorological stations located in the
lower reaches of the rivers are displayed (Russian Federal Service for Hydrometeorology and
Environmental Monitoring, unpublished data).
The majority of the study area is covered by Siberian taiga. Only the northernmost regions,
mainly the Lena and Yana deltas and the upper reaches of the Khatanga, are located within the
tundra zone.
Samples and methods
Sampling was carried out by the Potsdam branch of the Alfred Wegener Institute, in cooperation with the Geographical Faculty of the Moscow State University (1994 and 1995) and
the Arctic and Antarctic Research Institute, St. Petersburg (1996) on three expeditions to the
Siberian rivers. For detailed information on the expeditions, the reader is referred to the cruise
reports (Rachold et aI., 1995, 1997, and in press). Samples of the Lena and its tributaries were
collected from July 9 to July 28, 1994 and from July 1 to August 2, 1995. At this time of the
year, water level and SPM concentrations of the Lena were still high but lower than the
maximum water and sediment discharge immediately after ice breakup in the beginning of June.
The Yana and its tributaries, as well as the Omoloy, were sampled from August 8 to August 23,
1995. Yana water level and sediment discharge in 1995 reached maximum values in August,
during the sampling period. Samples of the Khatanga were collected at the end of July 1996,
three weeks after ice breakup. The sampling periods are indicated in Figure 2. The results
reported in the present article reflect the summer situation of the Siberian rivers. Due to the
severe climate in the drainage area, the winter season can almost be neglected in terms of mass
fluxes. During the spring flood, on the other hand, water and sediment discharges may increase
dramatically. This is especially true for the Lena whereas the spring flood is less pronounced
for the Yana and Khatanga. However, the increase in total organic carbon (TOC) concentrations
in the Lena is of minor significance. Average TOC concentrations during spring flood
amounted to 1050 /lmolll and during summer and autumn to 620 /lmol/l (Cauwet and Sidorov,
1996). Since our samples were collected in early summer, the data can be regarded as average
for the warm season.
Water samples were obtained by a glass water sampler (1994) and a Teflon water sampler
(1995 and 1996). The sampling locations are indicated in Figure I. In general, water samples
were retrieved from the middle of the river channel at a water depth of 1.5 m. Stations along the
main rivers in 1995 and 1996 were sampled as river cross sections. At these stations, two or
three samples distributed equally across the river were collected. In the following, we refer to
average values calculated for each station. River cross sections are treated as field replicates.
Samples taken at river cross sections exhibited small variations in carbon isotope ratios. The
standard deviation amounted to less than ± 0.4 %0 (Table 1).
The SPM was separated by vacuum filtration through precombusted 0.7 /lm Whatman glassfiber filters (GFIF) immediately after sampling. The filtered water volume varied between I and
4 I, depending on sediment load. After air-drying under dust-free conditions, the filters were
stored dry and cool. A total of 90 samples corresponding to 61 stations were analyzed for total
SPM content, organic C and N concentrations of the SPM, and for the stable carbon isotope
composition of the POM at the Alfred Wegener Institute, Research Department Potsdam. In