Late Quaternary Climate Variations Reflected in Baltic Sea Sediments - The Baltic Sea Basin - page 112

dominant taxa, but also for the ecological groups, were constructed. This was done
with Tilia R and C2 software.

In the studied material, a total of 219 diatom taxa were identified. The qual-
ity of their preservation varied. The best preserved flora was found in laminated,
fine-grained sediments. On the contrary the diatom record in the homogeneous sed-
iments was distinctly poorer. The diatom valves were either affected by dissolution
or mechanical fragmentation. In the homogeneous sediments the intervals of 20-37
and 126-172 cm were barren in diatoms. Hence, to be able to count the recom-
mended 300 valves and be able to perform statistic analysis two slides were merged
for the counting.

5.5 Results

5.5.1 Zonation of Basin Sediments

All three cores sampled at the master station have been used for the litho- and
chronostratigraphic subdivision of the postglacial sediments in the Eastern Gotland
Basin: 211660-1 (to develop the age model), 211660-5 (for the physico-stratigraphic
zonation), and 303610-12 (as start profile for the basinwide lithostratigraphic
cross-section). The cores have penetrated undisturbed sediments mirroring the
development of the basin from the Late Pleistocene to the Holocene.
A lithostratigraphic zonation and correlation between cores taken at the master
station was performed using core photography and downcore measured sediment
physical properties. The principles of the method have been described by Harff et al.
( 1999 ) . In the result we obtain a “composite” of the late Pleistocene to Holocene
sedimentary sequence at the master station referred to the 211660-5 depth scale.
Figure 5.7 shows curves of physical properties of the master station cores used
for the correlation. For cores 211660-5 and 303610-12 wet bulk density and mag-
netic susceptibility were available for the correlation. The physico-facies of core
211660-1 (piston corer) has been described by density values measured from
samples taken in a distance of about 2.5 cm.
The physico-stratigraphic zonation defined by Harff et al. ( 2001a ) for core
211660-5 (using p-wave velocity, wet bulk density, and magnetic susceptibility)
has been transferred to the whole set of cores sampled at the master station and
is marked in Fig. 5.7 with an RGB colour scan of core 211660-5.
Within the lower parts of the cores the varved sediments of the Baltic Ice Lake
(A1/A2 zone) are clearly visible. The short Yoldia Phase is marked by an initial
phase (upper part of zone A3), a 3 cm mud layer enriched in organic matter (A4
zone), and an end phase (lower part of A5 zone). It is overlain by the homogeneous
bioturbated fine-grained sediments of the Ancylus Lake with black Fe-sulphide
spots. The lower part (A5 zone) is grey, whereas the upper part (A6 zone) is brown-
ish in the scan due to oxygenated iron. Diagenetic mineral formation (greigite)
causes typical anomalies in the magnetic susceptibility. The transition from the late