The Development of the Baltic Sea Basin During the Last 130 ka - The Baltic Sea Basin - page 86

Fig. 4.6 Paleogeographic
map showing the Yoldia Sea
at the end of the brackish
phase ca. 11.1 ka BP
areas around Lake Vänern led to a situation where the outlets west of Vänern shal-
lowed up and could not “swallow” all outflowing water from the Baltic. This marks
the end of the Yoldia Sea.
4.3.3 10.7-9.8 ka BP
When the shallowing up of the outlets west of Lake Vänern forced the water level
inside the Baltic to rise, the next stage, the Ancylus Lake (AL), began. The sedi-
ments of this large freshwater lake are usually poor in organic material, which is
partly a consequence of the melt water input to the Baltic from the final deglacia-
tion of the Scandinavian ice sheet and the fairly pristine soils of the mainly recently
deglaciated drainage area. Together, this resulted in an aquatic environment with
low nutrient input and hence low productivity. The absence of a halocline in the AL
led to a well-mixed oxygenated water body and the fairly common sulfide-banded
sediments of this stage can probably be explained by H 2 S diffusion from younger
sediments (Sohlenius et al. 2001 ) .
The onset of the AL is displayed by a simultaneous switch in relative water
level change in the areas south of Stockholm-Helsinki (corresponding to the mean
isobase of the outlets west of Lake Vänern), which had previously experienced
regression since deglaciation: transgression now took over. This is documented not
only by an array of shore displacement curves, but also by simultaneous flooding