Ebel et al.: Lamiooted Sediments from Levinsoll-Lessillft Lake
433
The sedimentary processes influencing both the chemical composition and the physical
properties of the sediment have undergone changes dependent on natural climatic variations.
The total organic carbon, nitrogen, and sulfur contents show distinct increases at the
Pleistocene-Holocene transition (Figure 5). Whilst the values of organic carbon and nitrogen
remain at high levels closer to the sediment surface, sulfur is clearly enriched in the Preboreal
and Boreal pollen zones, which are believed to represent the Holocene climatic optimum in this
area (Hahne and Melles, this volume). Varying contents of all three parameters within the
Weichselian sediments are less pronounced, showing somewhat higher values in horizons
which exhibit an interstadial character according to pollen data (Interstadial I and 2).
The increasing contents of organogenic components in warmer periods are not exclusively the
result of enhanced aquatic biogenic accumulation. The total organic carbon contents comprise
autochthonous aquatic plant material as well as allochthonous terrestrial plant detritus and fossil
coal. Higher C/N ratios during warmer times clearly evidence increasing proportions of
vascular plant detritus in the total organic fraction (Meyers and Ishiwatari, 1995). At least for
the Holocene, this was probably due to both enhanced availability of plants in the lake's
catchment and enhanced river supply to the lake, as indicated by a denser vegetation cover and
the supposedly higher amounts of summer precipitation during that period (Hahne & Melles,
this volume).
The higher amounts of organic material, starting at the Pleistocene-Holocene transition, led to
chemical changes in the interstitial waters of the sediment. The occurrence of pyrite (FeS)
framboids in parts of the post-Oldest Dryas deposits indicates anoxic conditions within the pore
water, as well as sufficient sulfate supply for that time, whereas iron oxides are ubiquitous
throughout the entire sequence. Autochthonous vivianite (Fe3(P04h • 8H20) points to a
complete pyritic fixation of sulfide at these depths, allowing post-sedimentary formation of iron
phosphates (Zolitschka, 1990). The anoxic pore waters and increased organic contents are
related to increased biological methane formation. Horizons with highest methane contents are
rendered macroscopically visible by enrichments of lenticular gas inclusions (Figure 5). Whilst
methane remains dissolved and is partly consumed in surface sediments and the water column
by methanotrophic bacteria at site PG 1228, in the central, deepest part of the lake, in shallow
waters it is released to the atmosphere in significant amounts (Samarkin, 1997).
The high gas contents in parts of sediment core PG 1228 strongly affected the whole-core
measurements of the sediments physical properties - i.e. wet bulk density (WBD) and
susceptibility (Figure 5) - resulting in scattered values and under-estimations of both
parameters. Nevertheless, their general trends, similarly to that of the water content, show good
(positive or negative) correlations with geochemical data. Significant decreases in WBD and
susceptibilities towards the upper, Holocene part of the sediment core can be traced back to
higher water and methane contents as well as to a higher concentration of organic material with
lower density, and related lower concentrations of minerogenic components, including
magnetically active mineral grains.
Conclusions
From the description and first analyses of core PG 1228 the following conclusions can be
drawn on the sedimentation history recorded in the Levinson-Lessing Lake basin since the late
Middle Weichselian:
A continuous lacustrine sediment record was recovered from Levinson-Lessing Lake
covering at least the last 30,000 years. Sedimentation rates with a mean value of 0.7 mm/a were
relatively constant throughout this period.
The sedimentation was dominated (ca. 80% of the whole record) by seasonal fluvial sediment
433
The sedimentary processes influencing both the chemical composition and the physical
properties of the sediment have undergone changes dependent on natural climatic variations.
The total organic carbon, nitrogen, and sulfur contents show distinct increases at the
Pleistocene-Holocene transition (Figure 5). Whilst the values of organic carbon and nitrogen
remain at high levels closer to the sediment surface, sulfur is clearly enriched in the Preboreal
and Boreal pollen zones, which are believed to represent the Holocene climatic optimum in this
area (Hahne and Melles, this volume). Varying contents of all three parameters within the
Weichselian sediments are less pronounced, showing somewhat higher values in horizons
which exhibit an interstadial character according to pollen data (Interstadial I and 2).
The increasing contents of organogenic components in warmer periods are not exclusively the
result of enhanced aquatic biogenic accumulation. The total organic carbon contents comprise
autochthonous aquatic plant material as well as allochthonous terrestrial plant detritus and fossil
coal. Higher C/N ratios during warmer times clearly evidence increasing proportions of
vascular plant detritus in the total organic fraction (Meyers and Ishiwatari, 1995). At least for
the Holocene, this was probably due to both enhanced availability of plants in the lake's
catchment and enhanced river supply to the lake, as indicated by a denser vegetation cover and
the supposedly higher amounts of summer precipitation during that period (Hahne & Melles,
this volume).
The higher amounts of organic material, starting at the Pleistocene-Holocene transition, led to
chemical changes in the interstitial waters of the sediment. The occurrence of pyrite (FeS)
framboids in parts of the post-Oldest Dryas deposits indicates anoxic conditions within the pore
water, as well as sufficient sulfate supply for that time, whereas iron oxides are ubiquitous
throughout the entire sequence. Autochthonous vivianite (Fe3(P04h • 8H20) points to a
complete pyritic fixation of sulfide at these depths, allowing post-sedimentary formation of iron
phosphates (Zolitschka, 1990). The anoxic pore waters and increased organic contents are
related to increased biological methane formation. Horizons with highest methane contents are
rendered macroscopically visible by enrichments of lenticular gas inclusions (Figure 5). Whilst
methane remains dissolved and is partly consumed in surface sediments and the water column
by methanotrophic bacteria at site PG 1228, in the central, deepest part of the lake, in shallow
waters it is released to the atmosphere in significant amounts (Samarkin, 1997).
The high gas contents in parts of sediment core PG 1228 strongly affected the whole-core
measurements of the sediments physical properties - i.e. wet bulk density (WBD) and
susceptibility (Figure 5) - resulting in scattered values and under-estimations of both
parameters. Nevertheless, their general trends, similarly to that of the water content, show good
(positive or negative) correlations with geochemical data. Significant decreases in WBD and
susceptibilities towards the upper, Holocene part of the sediment core can be traced back to
higher water and methane contents as well as to a higher concentration of organic material with
lower density, and related lower concentrations of minerogenic components, including
magnetically active mineral grains.
Conclusions
From the description and first analyses of core PG 1228 the following conclusions can be
drawn on the sedimentation history recorded in the Levinson-Lessing Lake basin since the late
Middle Weichselian:
A continuous lacustrine sediment record was recovered from Levinson-Lessing Lake
covering at least the last 30,000 years. Sedimentation rates with a mean value of 0.7 mm/a were
relatively constant throughout this period.
The sedimentation was dominated (ca. 80% of the whole record) by seasonal fluvial sediment