The Development of the Baltic Sea Basin During the Last 130 ka - The Baltic Sea Basin - page 88

Darss and Møn, was inundated by Ancylus waters. Through Mecklenburg Bay and
Fehmarn Belt, along the eastern side of Langeland and out through the Great Belt
to Kattegat, a large river was established through Denmark, i.e., the Dana river (von
Post 1929 ) . The idea of a sudden and large drainage of AL was proven impossible
by Lemke et al. ( 2001 ) but due to initial erosion of the riverbed of soft Quaternary
deposits this might have caused an initial lowering of the AL in the order of 5 m,
followed by a period of rising base level, i.e., the sea in the north. This resulted
in a complex river system through Denmark with river channels, levées, and lakes
(Bennike et al. 2004 ) with a gradually smaller fall-gradient as the sea level was
rising. When sea level in Kattegat had reached the level of the AL inside the Baltic
basin we think it is possible that saltwater could penetrate all the way through the
long river system into the Baltic, at least during periods when the Baltic region
had been under influence of a long-lasting high-pressure suddenly followed by deep
low pressure systems and strong westerly winds. We therefore place the end of the
Ancylus Lake when it was at approximate level with the sea and we see the first
signs of marine influence since the YS.
4.3.4 9.8-8.5 (8) ka BP
According to independent evidence in the Blekinge archipelago (Berglund et al.
2005 ) and from the Bornholm basin (Andrén et al. 2000b ) , the first, though weak,
signs of saline water entering the Baltic basin after the AL have been 14 C dated to
9.8 ka BP. Also in the eastern Gotland basin is an increase in brackish freshwater
diatom taxa recorded at ca. 9.8 ka BP (Andrén et al. 2000a ) . This period with very
low saline influence has been named the Initial Littorina Sea (Andrén et al. 2000b ) ,
which is very appropriate considering the fact that the Baltic was at level with the
sea. The Scandinavian ice sheet finally melted during the early part of this stage,
and although most of the Baltic Sea coastline still experienced regression due to
the rebound, the “0-line,” i.e., the areas where eustasy and isostatic uplift balanced,
moved north. The 0-line during the first part of this stage was possibly along a
line from SE Sweden to Estonia, i.e., all areas south of such a line would have
experienced a transgression.
In comparison with the AL sediments the organic content often rises gradu-
ally throughout this stage. In the coastal zone, a diverse brackish diatom flora, the
so-called Mastogloia flora, was established (Miettinen 2002 ) . In the open basin,
however, a very low diatom abundance characterizes the Initial Littorina Sea (e.g.,
Andrén et al. 2000a , b , Paabo 1985 , Sohlenius et al. 1996 , 2001 , Thulin et al. 1992 ,
Westman and Sohlenius 1999 ) . Pigment biomarkers indicate that cyanobacteria
were abundant during the Initial Littorina Sea (Bianchi et al. 2000 , Borgendahl and
Wes tman 2007 ) . Stable nitrogen isotopes which are indicative of the origin of nitro-
gen have been used to show that these early blooming cyanobacteria were actually
nitrogen fixers (Borgendahl and Westman 2007 , Vossetal. 2001 ) .