Late Quaternary Climate Variations Reflected in Baltic Sea Sediments - The Baltic Sea Basin - page 101

Quaternary glaciations created the morphology of the Baltic region. The relief of
the northwest European Caledonides, with elevations up to 2,470 m, and the surficial
topography on the crystalline Precambrian rocks of the Fennoscandian Shield were
shaped by a combination of weathering and glacial erosion, and the lowlands of the
Russian Plate and the west European Platform were covered by glacial sediments.
Glaciers also excavated the Baltic Basin (which has an average water depth of 55 m)
and formed a series of sub-basins (Mecklenburgian Bight, 25 m; Arkona Basin,
45 m; Bornholm Basin, 100 m; Gotland Basin, 250 m; Golf of Bothnia, 120 m)
separated by shallower sills (Figs. 5.1 and 5.3 ) .
The postglacial history of the Baltic Sea Basin is explained in detail by Andrén
et al. ( Chap. 4 ) in this topic. The hydrology of the Baltic Sea can be described as
a typical estuarine current system. One driving force is the positive water balance
resulting from precipitation within the Baltic drainage basin, which belongs to the
European humid climate belt. Westerly winds form the second driving force pushing
the denser marine water from the North Sea into the Baltic close to the bottom.
These winds are the result of atmospheric low-pressure systems tracking from the
central North Atlantic to Europe.
The relation between the Icelandic low-pressure and the Eurasian high-pressure
systems controls whether north-easterlies and a cold atmosphere or westerlies and
relative warm air masses govern the climate in central and northern Europe.
The variation of the system follows a hierarchically superimposed cyclic pat-
tern. The Arctic Oscillation (AO) is the dominant pattern of non-seasonal variations
in the stratospheric air pressure of the Northern Hemisphere. The North Atlantic-
European sector of the AO is represented through the well-known North Atlantic
Oscillation (NAO) at sea level. The NAO describes fluctuations in the strength of
geostrophic westerlies affecting predominantly winter climate in the Baltic area.
Here, according to Alheit and Hagen ( 1997 ) a positive NAO causes a “maritime
mode” with strengthened westerlies transporting warm humid air masses eastward
and producing mild winters over the Baltic Sea. The opposite situation (negative
NAO: continental mode) is determined by strengthened westward transport of cold
and dry Siberian air towards Europe. This is accompanied by severe winters in the
Baltic Sea area. The NAO fluctuates periodically on a decadal time scale (Hurrell,
1995 , Hagen 2006 , Hagen and Feistel 2008 ) . In addition, Justino and Peltier ( 2005 )
report about a so-called Atlantic Multi-decadal Oscillation (AMO) of about 30
years. Hagen and Feistel ( 2005 ) showed that the decadal NAO/AMO periodicity
is obviously superimposed on a century lasting trend. In this study we intend to
show that this periodicity is reflected by the facies variation in the central Baltic
Basin.
Figure 5.1 shows a digital terrain model of the Baltic area. Within the central
Baltic Sea (Baltic Proper) the halocline prevents vertical water exchange and leads
here to anoxic conditions below a permanent redoxcline (Fig. 5.2 ) . The absence
of higher benthic biota prevents the sediments from being bioturbated and causes
laminated sequences that record environmental change with high age resolution
(Sohlenius et al. 1996 , Sohlenius and Westman 1998 , Sohlenius et al. 2001 ) . Within
the shallow Belt Sea the water column is not stratified due to the mixing effect of