Niessen et al: Hi~h-Resolution Seismic StratigraphY of Lake Sediments on the Taymyr Peninsula
447
show whether the decrease of penetration is related to an increase in grain size and/or
sedimentary gas.
Core PG 1227 from Lake Taymyr has not yet been dated. However, it is assumed that T I and
T2 represent the Holocene because both units are built up of massive muds with relatively high
water contents between 40 and 60% (core PG 1227, Figure 5) similar to water contents
measured in Holocene muds in Lake Levinson Lessing (PG 1228, Figure 8). In contrast,
Levinson Lessing sediments of Late and Middle Weichselian age have water contents of about
30% or less (Figure 8). In Lake Taymyr, a significant decrease in sediment water content is
observed at the T2/T3 transition (Figure 5), which we interpret as the Holocene-Pleistocene
boundary. This assumption is reasonable because preliminary pollen results from two samples
of the basal sand of core PG 1227 (top of unit T3) suggest a Pre-Holocene age (Hahne, pers.
comm.).
From shallow to deeper water (Figure 4) the increase of unit thicknesses of Tl and, in
particular, T2 indicates that sediment focusing towards the central area of the lake is an
important process during the Holocene history of Lake Taymyr. This is explained by the
morphology of the lake which is characterised by large areas of very shallow water. During
storm events the water becomes turbid indicating significant resuspension and lateral transport
of fine-grained material into deeper water (Niessen et aI., 1997). Lateral transport of organic
matter is probably associated with the above process because the entire cere PG 1227 is rich in
gas which led the sediments expand by I to 1.5% after recovery (Overduin et aI., 1996). The
high content of sedimentary gas can explain the diffuse character of the backscatter (Figure 5).
In particular, local occurrence of strong reflectors, rapid lateral shifts in penetration and
subbottom resolution can often be associated with gas bubbles in the sediments (e.g. Niessen et
aI., 1993).
We cannot interpret, based on the study of both lakes, whether the transgressive trend during
the Holocene is controlled by climatic change. However, a general climatic deterioration after
the early Holocene climatic optimum is indicated in the pollen record from Lake Levinson
Lessing (Figure 8), which shows decreasing aboreal pollen since the Preboreal to Boreal
chronozones (Figure 8; Hahne and Melles, this volume). It has to be tested whether the climatic
deterioration also affected the hydrology of the region so that a rise of lake levels was caused.
Middle and Late Weichselian - non-glacial lacustrine deposition during LST and HST
conditions
According to the pollen results from core PG 1228 from Lake Levinson Lessing (Figure 8), unit
L2 was deposited during Middle and Late Weichselian time. This interpretation is based on two
interstadials at 17.5 and 19.5 m core depth indicated by a significant increase in arboreal pollen
(Figure 8; Hahne and Melles, this volume). Thus, the last glacial maximum is located
somewhere between 9 and 17 m core depth which corresponds to the upper part of seismic unit
L2. Lack of erosional events and/or deformation or overconsolidation of sediments is clear
evidence that no glacial erosion affected the lake during the deposition of the entire unit L2
(Figure 7 and Figure 8). Moreover, except for the water content, the pre-Holocene lithology of
PGI228 can hardly be distinguished from Holocene lithology (Ebel et aI., this volume). Thus,
for Late and Middle Weichselian time there is no indication in the sediment core for glacial
activity in the catchment of the lake (such as typical glacial varves; e.g. Leonard, 1985;
Leemann and Niessen, 1994). The discovery of a continuous undisturbed record of Middle and
Late Weichselian age also implies that the input of terrigenous debris was not interrupted.
Therefore, seasonal melting of ice and snow on the lake surface and in the catchment has to be
assumed even for the last glacial maximum in order to provide an input of suspended particles
447
show whether the decrease of penetration is related to an increase in grain size and/or
sedimentary gas.
Core PG 1227 from Lake Taymyr has not yet been dated. However, it is assumed that T I and
T2 represent the Holocene because both units are built up of massive muds with relatively high
water contents between 40 and 60% (core PG 1227, Figure 5) similar to water contents
measured in Holocene muds in Lake Levinson Lessing (PG 1228, Figure 8). In contrast,
Levinson Lessing sediments of Late and Middle Weichselian age have water contents of about
30% or less (Figure 8). In Lake Taymyr, a significant decrease in sediment water content is
observed at the T2/T3 transition (Figure 5), which we interpret as the Holocene-Pleistocene
boundary. This assumption is reasonable because preliminary pollen results from two samples
of the basal sand of core PG 1227 (top of unit T3) suggest a Pre-Holocene age (Hahne, pers.
comm.).
From shallow to deeper water (Figure 4) the increase of unit thicknesses of Tl and, in
particular, T2 indicates that sediment focusing towards the central area of the lake is an
important process during the Holocene history of Lake Taymyr. This is explained by the
morphology of the lake which is characterised by large areas of very shallow water. During
storm events the water becomes turbid indicating significant resuspension and lateral transport
of fine-grained material into deeper water (Niessen et aI., 1997). Lateral transport of organic
matter is probably associated with the above process because the entire cere PG 1227 is rich in
gas which led the sediments expand by I to 1.5% after recovery (Overduin et aI., 1996). The
high content of sedimentary gas can explain the diffuse character of the backscatter (Figure 5).
In particular, local occurrence of strong reflectors, rapid lateral shifts in penetration and
subbottom resolution can often be associated with gas bubbles in the sediments (e.g. Niessen et
aI., 1993).
We cannot interpret, based on the study of both lakes, whether the transgressive trend during
the Holocene is controlled by climatic change. However, a general climatic deterioration after
the early Holocene climatic optimum is indicated in the pollen record from Lake Levinson
Lessing (Figure 8), which shows decreasing aboreal pollen since the Preboreal to Boreal
chronozones (Figure 8; Hahne and Melles, this volume). It has to be tested whether the climatic
deterioration also affected the hydrology of the region so that a rise of lake levels was caused.
Middle and Late Weichselian - non-glacial lacustrine deposition during LST and HST
conditions
According to the pollen results from core PG 1228 from Lake Levinson Lessing (Figure 8), unit
L2 was deposited during Middle and Late Weichselian time. This interpretation is based on two
interstadials at 17.5 and 19.5 m core depth indicated by a significant increase in arboreal pollen
(Figure 8; Hahne and Melles, this volume). Thus, the last glacial maximum is located
somewhere between 9 and 17 m core depth which corresponds to the upper part of seismic unit
L2. Lack of erosional events and/or deformation or overconsolidation of sediments is clear
evidence that no glacial erosion affected the lake during the deposition of the entire unit L2
(Figure 7 and Figure 8). Moreover, except for the water content, the pre-Holocene lithology of
PGI228 can hardly be distinguished from Holocene lithology (Ebel et aI., this volume). Thus,
for Late and Middle Weichselian time there is no indication in the sediment core for glacial
activity in the catchment of the lake (such as typical glacial varves; e.g. Leonard, 1985;
Leemann and Niessen, 1994). The discovery of a continuous undisturbed record of Middle and
Late Weichselian age also implies that the input of terrigenous debris was not interrupted.
Therefore, seasonal melting of ice and snow on the lake surface and in the catchment has to be
assumed even for the last glacial maximum in order to provide an input of suspended particles