The Development of the Baltic Sea Basin During the Last 130 ka - The Baltic Sea Basin - page 80

NW Finland at ca. 64 ◦ N (Salonen et al. 2007 ) , the first advance into northern
Denmark at 65-60 ka BP blocking any Baltic outlets through Kattegat (Larsen et al.
2009 ) , while the first Baltic ice lobe advance into southeastern Denmark is dated
to ca. 55-50 ka BP (Houmark-Nielsen 2007 ) . It is likely that freshwater lakes cov-
ered the deeper subbasins of the central and southern BSB until at least 60 ka BP
when sea level was >50 m lower than today (Lambeck and Chappell 2001 , Siddall
et al. 2003 ) . In the Hanö Bay, Bornholm and Landsort Deep basins were probably
sediments deposited during several tens of millennia through the first half of the last
glacial.
Based on detailed correlations and dating of the southwestern Baltic glacial strati-
graphies, Houmark-Nielsen and Kjær ( 2003 ) and Houmark-Nielsen ( 2007 , 2008 )
conclude that the SW Baltic may have experienced two major ice advances during
MIS 3, at ca. 50 and 30 ka BP. The latter is being vividly discussed (Wohlfarth 2010 )
as well as the general asynchroneity of MIS 3 ice advances at the western margin
of the Scandinavian ice sheet (SIS) (Mangerud 2004 ) compared to the ice margins
in the south (Houmark-Nielsen et al. 2005 ) . This less well-known period between
ca. 50 and 25 ka BP with its partly incompatible records is followed by a complex
glaciation in the southern BSB (Houmark-Nielsen and Kjær 2003 ) leading up to the
LGM. Previous off-shore studies in the southern Baltic have documented the pres-
ence of marine brackish sediments, dated to MIS 3 or older, that were overridden by
a glacier (Klingberg 1998 ) at Kriegers Flak and, e.g., two varved clay sequences -
the upper one dates from the last deglaciation - separated by an organic-rich layer
dated to >35 ka 14 C BP (bulk date) in Hanö Bay (Björck et al. 1990 ) .
Recently, 40 cores were obtained from drillings for the planning of the Kriegers
Flak wind-mill park, of which 9 indicate that complex yet incomplete stratigraphies
occur in this shallow part of the BSB. The shallow Kriegers Flak area shows a sur-
prisingly complex stratigraphy with a variety of lithologic units, gravel-sand-silt,
clays of glaciolacustrine and brackish origin, interstadial lacustrine, and terres-
trial organic sediments with five 14 C dates between 36 and 41 ka BP, sandwiched
between several glacial diamicts (Fig. 4.3 ) (Anjaretal. 2010 ) .
The geographic location and altitude (in relation to sea level) of the critical
threshold, or “gateway,” between the open ocean and the BSB are a key factor for
the BSB history, as it controls if, and how much, water can flow in or out of the
BSB. Presently, the two main thresholds are the Öresund Strait (-7 m) and the Store
Belt (ca. -20 m).
However, during earlier stages in the history of the BSB, a bedrock threshold
situated 60 m below sea level, the buried Esrum/Alnarp bedrock valley running
through SW Skåne in Sweden and northernmost Sjaelland in Denmark, 120 km
long and 6 km wide, has been suggested as a possible connection to the oceans
(Lagerlund 1987 , Andrén and Wannäs 1988 ) . Deep corings in the 1970s of this
main aquifer recovered fluvial and lacustrine sediment units with an organic carbon
content that made radiocarbon dating possible. The ages presented by Miller ( 1977 )
indicate that the valley was sediment-filled during the later part of MIS 3. The valley
may thus have served as the outlet route for the entire BSB until it later was filled
up by sediments.