Late Quaternary Climate Variations Reflected in Baltic Sea Sediments - The Baltic Sea Basin - page 119

of zone B1 show still some ventilation of the bottom water (dysoxic), whereas a
lower subzone of B4 is deposited under suboxic conditions. The change of lami-
nated and homogeneous sediments in Gotland Basin sediments and its relation to
changing oxygen supply to the bottom water has been discussed already by Ignatius
et al. ( 1981 ) (see also Conley et al. 2002 ) , but there is still a debate ongoing whether
the ventilation of the bottom water is caused by inflow events of higher saline water
or by vertical convection during fresher phases of the water body (see, e.g. Meier
and Kauker 2003 , Zillén et al. 2008 ) . To answer this question we have used diatom
analyses for the reconstruction of paleosalinity. Westman and Sohlenius ( 1999 ) ,
Sohlenius et al. ( 2001 ) , Emeis et al. ( 2003 ) , and among others had already shown
the potential of diatoms for paleosalinity studies in the Baltic. Figure 5.12 shows
the result of a corresponding paleo-environmental study for core 303610-12 where
dominant species are displayed.
Based on physico-stratigraphic zonation and changes in the distribution of the
dominant species two diatom assemblage zones (zones A and B) were distinguished.
In addition, in zone B, six subzones (B1-B6) were distinguished.
Zone A (520-417 cm)
In this sediment interval freshwater forms, the so-called large lake species
predominate and they attain over 80% of the diatom assemblage. Among them
Aulacoseira islandica (O. Muller) Simonsen, Aulacoseira subarctica (Muller)
Fig. 5.12 Diatomological paleo-environmental indicators and physico-stratigraphic zonation in
sediments of master station core 313610-12 (Eastern Gotland Basin)