The Development of the Baltic Sea Basin During the Last 130 ka - The Baltic Sea Basin - page 84

The uplift of Öresund continued to be greater than the sea level rise, which meant
that the altitudinal difference increased further between the closed-in BIL and the
open sea (Fig. 4.4 ) . At the end of Younger Dryas, we have ample evidence for milder
conditions in NW Europe (e.g., Bakke et al. 2009 ) , which triggered a retreat of the
ice sheet even before the Younger Dryas cold period ceased (Björck and Digerfeldt
1984 , Johnson and Ståhl 2010 ) . Although we do not know the details about the final
drainage of the BIL, we know frommany independent evidence that there was a sud-
den lowering of the Baltic level of ca. 25 m down to sea level, and it occurred over a
time period of 1-2 years just prior to the onset of the Holocene (Björck et al. 1996 ,
Jakobsson et al. 2007 ) , which dates it to ca. 11.7 ka BP (Walker et al. 2009 ) . The
effects both inside and outside the Baltic basin have been described in more detail
by Björck ( 1995 ) , but it must have had a huge impact on the whole circum-Baltic
environment, with large coastal areas suddenly subaerially exposed, large changes
in fluvial systems, considerable reworking of previously laid down sediments as
well as the establishment of a large land-bridge between Denmark and Sweden.

4.3.2 11.7-10.7 ka BP

Hence, the onset of the next Baltic Sea stage, the Yoldia Sea (YS), coincides more
or less exactly with the base/start of the Holocene Series/Epoch (Walker et al. 2009 )
and the rapid warming connected with that. In fact, varved clay thicknesses in north-
western Baltic Proper and

18 O values in the GRIP ice core display a strikingly
similar pattern over a 150 year long Younger Dryas-Preboreal transition period
(Andrén et al. 1999 , 2002 ) , showing a distinct increase in sedimentation rate as the
ice sheet began to melt and rapidly retreat (Fig. 4.5 ) . Apart from being characterized
by the rapid deglaciation of the Scandinavian ice sheet, the relative sea level changes
of the YS played an important role and were a combination of rapid regression in
the recently deglaciated regions and normal regression rates in southern Sweden
(1.5-2 m/100 years).
Although the YS were at level with the sea, it would take ca. 300 years before
saline water could enter through the fairly narrow straits of the southcentral Swedish
lowland. This brackish phase has been documented by the varve lithology, geochem-
istry, and marine/brackish fossils, such as Portlandia (Yoldia) arctica . Occasionally
this phase shows up as sulfide banding, implying a halocline, and the maximum
duration of this brackish phase has been estimated to 350 years (Andrén et al.
2007 ) , although some records indicate it only lasted some 70-120 years (Andrén
and Sohlenius 1995 , Wastegård et al. 1995 ) . Due to the high uplift rate in south-
central Sweden, the strait area shallowed up rapidly, which together with the large
outflow prevented saline water to enter the Baltic (Fig. 4.6 ) . This turned the Yoldia
Sea into a freshwater basin again although there was an open contact with the sea in
the west through Lake Vänern and valley systems further west.
At the end of this stage the ice sheet had receded far north and most of today's
Baltic Sea basin was deglaciated, with the exception of the Bothnian Bay. This

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