442
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
Unit T3
The top of unit T3 is marked by a strong reflector near the deepest part of the lake. This
reflector is oblique to the overlying unit T2 and correlates with a strong decrease in water
content and an discontinuous change from mud to poorly bedded sands in core PG 1227 (Figure
5). There is no information on the geometry, thickness and backscatter characteristics of unit T3
in the profundal area of the lake because all acoustic energy is reflected at the top of T3 in
Profile 4 (Figure 5). As a result of the limited sound penetration, T3 is not seen in the deeper
part of profile 11 below 25 m. Above 40 m T3 appears to be condensed to a strong reflector
forming the T4ff2 boundary along an unconformity (Figure 4).
UnitT4
Unit T4 is defined only in profiles from relatively shallow water (Figures 4 and 6). T4 is well
stratified and the sediments drape subbottom topography (Figures 4 and 6). The reflection
geometry indicates an increase in unit thickness towards shallower water. In the shallow part of
profile 10 (Figure 6), T4 is characterised by the local occurrence of strong reflectors
discontinuous to the stratification and by numerous small-scale diffraction hyperbolae. Both
patterns are indicative of upwardly migrating sedimentary gas, locally migrating even into the
overlying unit Tl, where a pock mark is observed on the lake bottom (Figure 6). The shallow
part of profile 10 is located near the delta of a stream draining a catchment to the north of the
lake.
Reflectors of T4 are truncated at the top of the unit, which defines a distinct unconformity
(Figure 4, Figure 6). Thus, the original thickness of unit T4 cannot be reconstructed. It might
have been well in excess of 20 m thick. The lower boundary of T4, however, is clearly seen in
shallow water profiles (e.g. profile 11, Figure 4), where it is marked by a strong reflector
below 22 m, which defines a hummocky surface. Diffuse reflections from below this strong
reflector indicate that a further sedimentary unit of unknown thickness may be underlying T4.
Lake Levinson Lessing
In Lake Levinson-Lessing maximum sound penetration was observed to sediment depths of
about 60 m in high-penetration profiles (2-8 kHz). The relatively uniform, well-stratified
sediment fill thins toward the southern end of the lake (Figure 7) so that a strong basal reflector
becomes visible if the sediment fill is less than 40 m thick (Figure 9). Above the basal reflector
there are four seismic units which are best seen in water depths between 40 and 20 m (Figure
9). A characterisation of seismic stratigraphy is not possible in the northern part of the lake
because sound penetration is limited to only a few metres, in particular near the Krasnaya delta.
Because the entire south-north profile (no. 31, Figure 3) cannot be presented for geometric
reasons, unit thicknesses were measured at 22 section intervals between 106 m water depth in
the central part (coring location PG 1228, Figure 7) and 18 m water depth near the outlet in the
most southerly part of the lake (Figure 9). The results are presented in Figure 10.
UnitLl
Unit Ll is well stratified in deeper water and slightly diffuse in the shallower parts of the profile
near the outlet (Figure 8 and Figure 9). The reflectors show strong backscatter so that Ll
appears as a dark horizon overlying the deeper part of the fill without unconformity (Figure 7).
The unit thickness steadily decreases from 8.5 m at the coring location to 5 m at a distance of
4.5 km to the south of the coring location (Figure 7). A strong decrease to less than 2 m
thickness is observed in shallow water at the southern end of the profile (Figure 9 and Figure
10). Subbottom topography is draped with sediment below 35 m b.p.l.l .. For unit Ll, the
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
Unit T3
The top of unit T3 is marked by a strong reflector near the deepest part of the lake. This
reflector is oblique to the overlying unit T2 and correlates with a strong decrease in water
content and an discontinuous change from mud to poorly bedded sands in core PG 1227 (Figure
5). There is no information on the geometry, thickness and backscatter characteristics of unit T3
in the profundal area of the lake because all acoustic energy is reflected at the top of T3 in
Profile 4 (Figure 5). As a result of the limited sound penetration, T3 is not seen in the deeper
part of profile 11 below 25 m. Above 40 m T3 appears to be condensed to a strong reflector
forming the T4ff2 boundary along an unconformity (Figure 4).
UnitT4
Unit T4 is defined only in profiles from relatively shallow water (Figures 4 and 6). T4 is well
stratified and the sediments drape subbottom topography (Figures 4 and 6). The reflection
geometry indicates an increase in unit thickness towards shallower water. In the shallow part of
profile 10 (Figure 6), T4 is characterised by the local occurrence of strong reflectors
discontinuous to the stratification and by numerous small-scale diffraction hyperbolae. Both
patterns are indicative of upwardly migrating sedimentary gas, locally migrating even into the
overlying unit Tl, where a pock mark is observed on the lake bottom (Figure 6). The shallow
part of profile 10 is located near the delta of a stream draining a catchment to the north of the
lake.
Reflectors of T4 are truncated at the top of the unit, which defines a distinct unconformity
(Figure 4, Figure 6). Thus, the original thickness of unit T4 cannot be reconstructed. It might
have been well in excess of 20 m thick. The lower boundary of T4, however, is clearly seen in
shallow water profiles (e.g. profile 11, Figure 4), where it is marked by a strong reflector
below 22 m, which defines a hummocky surface. Diffuse reflections from below this strong
reflector indicate that a further sedimentary unit of unknown thickness may be underlying T4.
Lake Levinson Lessing
In Lake Levinson-Lessing maximum sound penetration was observed to sediment depths of
about 60 m in high-penetration profiles (2-8 kHz). The relatively uniform, well-stratified
sediment fill thins toward the southern end of the lake (Figure 7) so that a strong basal reflector
becomes visible if the sediment fill is less than 40 m thick (Figure 9). Above the basal reflector
there are four seismic units which are best seen in water depths between 40 and 20 m (Figure
9). A characterisation of seismic stratigraphy is not possible in the northern part of the lake
because sound penetration is limited to only a few metres, in particular near the Krasnaya delta.
Because the entire south-north profile (no. 31, Figure 3) cannot be presented for geometric
reasons, unit thicknesses were measured at 22 section intervals between 106 m water depth in
the central part (coring location PG 1228, Figure 7) and 18 m water depth near the outlet in the
most southerly part of the lake (Figure 9). The results are presented in Figure 10.
UnitLl
Unit Ll is well stratified in deeper water and slightly diffuse in the shallower parts of the profile
near the outlet (Figure 8 and Figure 9). The reflectors show strong backscatter so that Ll
appears as a dark horizon overlying the deeper part of the fill without unconformity (Figure 7).
The unit thickness steadily decreases from 8.5 m at the coring location to 5 m at a distance of
4.5 km to the south of the coring location (Figure 7). A strong decrease to less than 2 m
thickness is observed in shallow water at the southern end of the profile (Figure 9 and Figure
10). Subbottom topography is draped with sediment below 35 m b.p.l.l .. For unit Ll, the