210
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
to both the geology and the weathering regime of the catchment, is assumed to remain constant
during the short period of riverine transport. Furthermore, previous studies clearly documented
that the Lena is not affected by pollution (Martin et aI., 1993; Guieu et aI., 1996).
Additional variations in the chemical composition of the SPM could be related to changes in
transport energy. An increase in flow velocity may result in the resuspension of coarse-grained
bottom sediments whereas a reduction in transport energy may correspond to the deposition of
fine-grained SPM. Two prominent changes in transport energy occur in the Lena. North of the
settlement Kyusyur, the Lena cuts through the Verkhoyansk range and narrows from a width of
approximately 20 km to less than 2 km in the so-called Lena pipe. The narrow channel of the
Lena pipe opens directly into the wide delta plain. The position of the Lena pipe and the Lena
delta are indicated in Figure 1 and Figure 4. Although the transitions from the broad Lena to the
Lena pipe and from the Lena pipe to the Lena delta represent drastic changes in transport
energy, the chemical composition of the SPM was almost unaffected (Figure 4). Compared to
variations in U/AI and ThiAI ratios in the upper and middle Lena, the slight increase in the Lena
delta appears negligible.
The evolution observed in the course of the Lena is therefore largely attributed to the input of
SPM by larger tributaries, i.e. the Chara (Ll), the Aldan (L2), and the Vilyuy (L3). The values
of the tributaries L l-L3 are indicated in Figure 4 according to the position of their confluences.
Based on the U and Th data, the Lena can be subdivided into three sections: the upper Lena
(south of the Aldan confluence), the middle Lena (between Aldan and Vilyuy confluence), and
the lower Lena (north of the Vilyuy). The upper Lena and the Chara were characterized by the
highest U/AI and ThlAI ratios, under the influence of the Baikal and Mongolo-Okhotsk folded
region (Figure 1). North of the confluence of the Aldan, which drains Archean and Proterozoic
crystalline and metamorphic rocks of the Aldan shield, a decrease in U/AI ratios was seen,
while Thl Al ratios remained constant. North of the confluence of the Vilyuy, which drains the
sedimentary basin of the Siberian platform and the Siberian Trap basalts, a pronounced
decrease in both U/AI and ThiAI ratios was observed. Unfortunately, accurate data on the SPM
discharge of the Aldan and the Vilyuy are not available. It is estimated that the SPM discharge
of the Aldan accounts for about 60 % and the Vilyuy for about 10 % of the Lena's discharge
(Zaitsev, pers. comm.). On the basis of these data and using average U/AI and ThlAI ratios of
the upper Lena and of the Aldan, U/AI and ThlAI ratios resulting from the mixing of these two
sources could be calculated and compared to the observed values in the middle Lena.
Accordingly, based on the observed ratios of the middle Lena and of the Vilyuy, the expected
U/AI and ThlAI ratios of the lower Lena can be estimated. Observed and estimated U/AI and
ThlAI ratios of the middle and upper Lena are displayed in Figure 4a and 4b. The observed
values are in general agreement with the calculated data but the decreases in UI Al and Thl Al
ratios in both the middle and the upper Lena were more pronounced than calculated. However,
since the sediment discharge data are not very accurate, it seems reasonable that the chemical
evolution along the Lena is controlled mainly by material input through tributaries. Alteration
processes within the river and variations in transport energy playa minor role in comparison.
Since U and Th are affected in the same way, local differences in the weathering regime can
also be neglected.
REE patterns of river SPM are sensitive to the drainage basin geology. Rivers draining
basaltic island arcs are characterized by flat or heavy REE-enriched, shale-normalized patterns,
while rivers draining Archean crustal rocks show a strong enrichment in light REE (Goldstein
and Jacobsen, 1988). Recent studies, however, have shown that the REE pattern of source
rock and its weathering product are not identical (Braun et aI., 1993; Condie et aI., 1995). The
SPM of major rivers draining large basins of heterogeneous geology display similar REE
patterns characterized by a slight enrichment in light REE and a deficiency of heavy REE
(Martin et aI., 1976; Goldstein and Jacobsen, 1988). The enrichment in light REE relative to
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
to both the geology and the weathering regime of the catchment, is assumed to remain constant
during the short period of riverine transport. Furthermore, previous studies clearly documented
that the Lena is not affected by pollution (Martin et aI., 1993; Guieu et aI., 1996).
Additional variations in the chemical composition of the SPM could be related to changes in
transport energy. An increase in flow velocity may result in the resuspension of coarse-grained
bottom sediments whereas a reduction in transport energy may correspond to the deposition of
fine-grained SPM. Two prominent changes in transport energy occur in the Lena. North of the
settlement Kyusyur, the Lena cuts through the Verkhoyansk range and narrows from a width of
approximately 20 km to less than 2 km in the so-called Lena pipe. The narrow channel of the
Lena pipe opens directly into the wide delta plain. The position of the Lena pipe and the Lena
delta are indicated in Figure 1 and Figure 4. Although the transitions from the broad Lena to the
Lena pipe and from the Lena pipe to the Lena delta represent drastic changes in transport
energy, the chemical composition of the SPM was almost unaffected (Figure 4). Compared to
variations in U/AI and ThiAI ratios in the upper and middle Lena, the slight increase in the Lena
delta appears negligible.
The evolution observed in the course of the Lena is therefore largely attributed to the input of
SPM by larger tributaries, i.e. the Chara (Ll), the Aldan (L2), and the Vilyuy (L3). The values
of the tributaries L l-L3 are indicated in Figure 4 according to the position of their confluences.
Based on the U and Th data, the Lena can be subdivided into three sections: the upper Lena
(south of the Aldan confluence), the middle Lena (between Aldan and Vilyuy confluence), and
the lower Lena (north of the Vilyuy). The upper Lena and the Chara were characterized by the
highest U/AI and ThlAI ratios, under the influence of the Baikal and Mongolo-Okhotsk folded
region (Figure 1). North of the confluence of the Aldan, which drains Archean and Proterozoic
crystalline and metamorphic rocks of the Aldan shield, a decrease in U/AI ratios was seen,
while Thl Al ratios remained constant. North of the confluence of the Vilyuy, which drains the
sedimentary basin of the Siberian platform and the Siberian Trap basalts, a pronounced
decrease in both U/AI and ThiAI ratios was observed. Unfortunately, accurate data on the SPM
discharge of the Aldan and the Vilyuy are not available. It is estimated that the SPM discharge
of the Aldan accounts for about 60 % and the Vilyuy for about 10 % of the Lena's discharge
(Zaitsev, pers. comm.). On the basis of these data and using average U/AI and ThlAI ratios of
the upper Lena and of the Aldan, U/AI and ThlAI ratios resulting from the mixing of these two
sources could be calculated and compared to the observed values in the middle Lena.
Accordingly, based on the observed ratios of the middle Lena and of the Vilyuy, the expected
U/AI and ThlAI ratios of the lower Lena can be estimated. Observed and estimated U/AI and
ThlAI ratios of the middle and upper Lena are displayed in Figure 4a and 4b. The observed
values are in general agreement with the calculated data but the decreases in UI Al and Thl Al
ratios in both the middle and the upper Lena were more pronounced than calculated. However,
since the sediment discharge data are not very accurate, it seems reasonable that the chemical
evolution along the Lena is controlled mainly by material input through tributaries. Alteration
processes within the river and variations in transport energy playa minor role in comparison.
Since U and Th are affected in the same way, local differences in the weathering regime can
also be neglected.
REE patterns of river SPM are sensitive to the drainage basin geology. Rivers draining
basaltic island arcs are characterized by flat or heavy REE-enriched, shale-normalized patterns,
while rivers draining Archean crustal rocks show a strong enrichment in light REE (Goldstein
and Jacobsen, 1988). Recent studies, however, have shown that the REE pattern of source
rock and its weathering product are not identical (Braun et aI., 1993; Condie et aI., 1995). The
SPM of major rivers draining large basins of heterogeneous geology display similar REE
patterns characterized by a slight enrichment in light REE and a deficiency of heavy REE
(Martin et aI., 1976; Goldstein and Jacobsen, 1988). The enrichment in light REE relative to