The Development of the Baltic Sea Basin During the Last 130 ka - The Baltic Sea Basin - page 91

paleosalinity of the Baltic Sea is to use the strontium isotopic ratio in carbonate
mollusk shells (Widerlund and Andersson 2006 ) and quantify salinity with a preci-
sion better than 5%. However, this method can only be used when carbonate shells

are present/preserved in the sediments. Donner et al. ( 1999 ) suggest a salinity ca.
4‰ higher than present at the coastal areas of the Gulf of Finland and the Gulf of
Bothnia between ca. 7.5 and 4.5 ka BP based on the 18 O/ 16 O ratio in mollusk shells.

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4.3.7 Nutrient Conditions and Hypoxia

The inflowing marine water in the Baltic Sea 8-7 ka BP probably caused the release
of phosphorus from sediments, enhancing the growth of cyanobacteria (Bianchi
et al. 2000 , Borgendahl and Westman 2007 , Kunzendorf et al. 2001 ) . The salin-
ity stratification together with increased primary production initiated periods of
deepwater hypoxia in the open Baltic basin, evident in the sediment record as
extended periods of laminated sediments (Sohlenius and Westman 1998 , Zillén
et al. 2008 ) . Increased upward transport of nutrients from the anoxic bottom water
has been suggested as an explanation of the enhanced primary productivity at
the Ancylus/Littorina transition (Sohlenius et al. 1996 ) . Between 8 and 4 ka BP,
the Littorina Sea experienced a long sustained period of hypoxia (Zillén et al.
2008 ) . Oxygen conditions improved considerably after ca. 4 ka BP, also as salin-
ity decreased (Gustafsson and Westman 2002 ) . This coincided with the onset of the
neoglaciation in N Europe with a more humid and cold climate (Snowball et al.
2004 ) . The shift to colder and wetter conditions probably increased the net precip-
itation in the watershed leading to increased freshwater supplies to the basin and
decreased salinities (Gustafsson and Westman 2002 ) . Such a freshening, in combi-
nation with increased wind stress over the Baltic Sea, would result in a weakened
halocline and enhanced vertical mixing allowing more efficient exchange of oxygen
across the halocline (Conley et al. 2002 ) . This scenario would promote more oxic
bottom water conditions and explain the diminishing of the hypoxic zone around
4 ka BP (Zillén et al. 2008 ) .
Hypoxia occurred again during the middle-late Littorina Sea (ca. 2-0.8 ka BP).
In contrast to the period of oxygen deficiency during the early and more saline
phase of the Littorina Sea, hypoxia during the late Littorina Sea does not show a
relationship to any known changes in salinity. During this time, the surface salin-
ity in the Baltic Proper probably ranged between 7 and 8‰, i.e., similar to the
last ca. 4-3 ka (Gustafsson and Westman 2002 ) . Hypoxia between 2 and 0.8 ka
BP overlaps with a climate anomaly known as the Medieval Warm Period (Lamb
1965 ) when atmospheric temperatures in NWEurope were ca. 0.5-0.8 ◦ warmer than
today (Snowball et al. 2004 ) . However, temperatures have no proven effects on the
oxygen conditions in the Baltic Sea and the relationship between primary produc-
tion and climate change is not straightforward (Richardson and Schoeman 2004 ) .
Furthermore, the link between phytoplankton abundance and sea surface temper-
ature is only indirectly coupled to temperature. The ecological response to NAO